Research ArticleDevelopment of Flow-Through Polymeric MembraneReactor for Liquid Phase Reactions Experimental Investigationand Mathematical Modeling
Endalkachew Chanie Mengistie1 and Jean-Franccedilois Lahitte2
1Bahir Dar Institute of Technology (BiT) Faculty of Chemical and Food Engineering Bahir Dar University 1920 Bahir Dar Ethiopia2Laboratoire de Genie Chimique Universite Paul Sabatier 118 route de Narbonne Toulouse France
Correspondence should be addressed to Endalkachew Chanie Mengistie endalk10gmailcom
Received 3 August 2017 Accepted 7 November 2017 Published 29 November 2017
Academic Editor Jose C Merchuk
Copyright copy 2017 Endalkachew Chanie Mengistie and Jean-Francois Lahitte This is an open access article distributed under theCreative Commons Attribution License which permits unrestricted use distribution and reproduction in any medium providedthe original work is properly cited
Incorporating metal nanoparticles into polymer membranes can endow the membranes with additional functions This workexplores the development of catalytic polymer membrane through synthesis of palladium nanoparticles based on the approachesof intermatrix synthesis (IMS) inside surface functionalized polyethersulfone (PES) membrane and its application to liquidphase reactions Flat sheet PES membranes have been successfully modified via UV-induced graft polymerization of acrylicacid monomer Palladium nanoparticles have been synthesized by chemical reduction of palladium precursor loaded on surfacemodified membranes an approach to the design of membranes modified with nanomaterials The catalytic performances of thenanoparticle incorporated membranes have been evaluated by the liquid phase reduction of 119901-nitrophenol using NaBH4 as areductant in flow-through membrane reactor configuration The nanocomposite membranes containing palladium nanoparticleswere catalytically efficient in achieving a nearly 100 conversion and the conversion was found to be dependent on the fluxamount of catalyst and initial concentration of nitrophenol The proposed mathematical model equation represents satisfactorilythe reaction and transport phenomena in flow-through catalytic membrane reactor
1 Introduction
The concept of membrane reactors (MRs) combining amembrane-based separation with a catalytic chemical reac-tion in one unit dates back to the 1960s [1] Since then MRsplayed an important role in improving selectivity and yieldingand enhancing conversion for thermodynamically limitedchemical reactions in many chemical processes of industrialimportance In recent years many approaches have been pro-posed to combine membrane properties with chemical reac-tion in order to intensify a process These include extractortype distributor and contactor type Depending on the typeof membrane reactors the membrane performs differentfunctions [2 3]
(1) Extractor Membrane Reactors One of the products iscontinuously and selectively removed from the reaction
mixture by the membrane If the reaction is limited by thethermodynamic equilibrium the conversion can be increasedby removing one of the product components so that theequilibrium shifts towards the desired product side [4 5] Ifthe reaction rate of the undesired secondary reaction is higherthan that of the primary reaction the reaction selectivity canbe significantly enhanced by removing the desired intermedi-ate species Particularly the advantage of selectively removingthe valuable product lies in avoiding further separationsteps or reducing the separation units by increased productconcentrations Furthermore if one of the products hasinhibition effect as in case of some fermentation remov-ing this product strongly improves the reactor productivity[6]
(2) Distributor Membrane Reactors In this type of MR oneof the reactants is specifically added to the reaction mixture
HindawiInternational Journal of Chemical EngineeringVolume 2017 Article ID 9802073 8 pageshttpsdoiorg10115520179802073
2 International Journal of Chemical Engineering
across a membrane The membrane can act as even distribu-tor of the limiting reactant along the reactor to prevent sidereactions and as upstream separation unit to selectively doseone component from a mixture Controlled addition of oxy-gen in gas-phase partial oxidation of hydrocarbons in whichthe intermediate product reactsmore intensively with oxygenthan the reactants in order to prevent total oxidation [7 8]is the main application of this category of membrane reactor
(3) Contactor Membrane Reactors In this configuration thereacting species are fed at different sides of themembrane andmust diffuse through the catalytic layer to react Thereforethe role of the membrane is to provide an interfacial contactarea for the reacting streams but does not perform any selec-tive separationThe two sides ofmembranes are used to bringreactants into contact and if the reaction rate is fast comparedto the diffusion rates of the reactants the reaction occurs inthe catalytic layer in a way that prevents mixing of reactantsDue to higher surface area of membranes a contactor modecan provide higher contact area between two different phasesParticularly if one phase has lower solubility in the otherphase higher surface area contact between these phases candecrease the need of higher pressure that could have beenapplied to lower soluble component Gasliquid contactorsand flow-through membrane reactors are important class ofmembrane contactors
In flow-through catalytic membrane reactor (FTCMR)configuration unselective porous catalytic membrane eitherinherently catalytic or being made catalytic by impregnationof nanocatalysts is applied in dead-end mode operationThepremixed reactants are forced to pass through the catalyticmembrane The function of the membrane is to create areaction environment with intensive contact between thereactants and catalyst with short and controlled residencetimes and high catalytic activity The main drawback inclassical fixed-bed reactors is that the desired conversion ismainly limited by the pore diffusion However if reactantscan flow convectively through the catalyst sites the resultingintensive contact between reactants and catalyst can resultin a high catalytic activity [9 10] Besides this can avoidthe problems derived from internal or external mass transferresistance that may appear in a conventional fixed-bedreactor In FTCMR the reactants flow convectively throughthe membrane to catalyst sites which in turn results inan intensive contact between the reactants and the catalystthereby leading to higher catalytic activity with negligiblemass transport resistance Furthermore the introduction ofconvective flow can avoid undesired side reactions [1 10]
Catalytic membranes (CMs) either inherently catalyticor being made catalytic by impregnation of nanocatalystsare known for more than a decade In spite of this factthe development of CMs is still a major challenge Majorityof the catalytic membranes used in industries are inorganic(either ceramic or metal) for that reason they can withstandharsh reaction conditions (high temperature pressure con-centration and corrosive chemicals) The main drawbacks ofsuch CMmaterials are high cost and frangibility [5] Becausepolymers are less expensive and more flexible than ceramicsand metals it is possible to use them in CM development
instead of high cost metals and ceramics However majorityof the polymers are only suitable for mild operation condi-tions For this compensation high reactive catalysts should beimpregnated inside the polymer membrane matrix In suchcases active catalysts can compromise the demand of highertemperature Therefore stabilization of active catalysts byencapsulating inside polymer membranes can help to boostand enhance the drawbacks of polymer membranes
Metal nanoparticles (MNPs) have shown a great potentialin different catalytic processes and are well known for theirhigher catalytic performances Particularly MNPs of transi-tion metals are found to be efficient and selective catalystsfor several types of catalytic reactions This is due to higherpercentage of surface atoms and associated quantum effectsHowever metal nanoparticles lack chemical stability andmechanical strength They exhibit extremely high pressuredrop or head loss in fixed-bed column operation and arenot found suitable for such systems [11] Also MNPs tendto aggregate this phenomenon reduces their high surfacearea to volume ratio and subsequently reduces effective-ness By appropriately dispersing metal nanoparticles intosurface functionalized polymer membranes many of theseshortcomings can be overcome without compromising theproperties of nanoparticles ImmobilizingMNPs on polymermembrane support besides providing amechanical strengthit offers an option to maintain their catalytic activities by pre-venting unnecessary growth and aggregation Moreover cat-alytic application of MNPs is the best alternative to efficientlyutilize most expensive metals Immobilization of MNPs onpolymer membrane support is therefore best strategy toovercome the drawbacks of both polymers and MNPs [12]The use of functionalized polymer membrane as a supportand stabilizing media enables synthesizing nanocatalysts atthe desired ldquopoint userdquo and will result in formation of catalyt-ically active polymer membrane Encapsulation of MNPs inpolymers membranes offers also unique possibilities forenhancing accessibility of catalytic sites to reactants [13 14]
In this research we developed catalytic polymer mem-brane for liquid phase reactions by surface modification offlat sheet PES microfiltration membrane using UV-inducedgraft polymerization and synthesizing of stable palladiumnanoparticle via intermatrix synthesis method inside surfacefunctionalized PES membranes The catalytic performanceof the membrane was evaluated using reduction of 119901-nitro-phenol (NP) as a model for liquid phase reaction
2 Materials and Methods
21 Materials The following chemicals and materialswere used during the experiment acrylic acid (AA) N-methyl-2-pyrrolidone (NMP) polyvinylpyrrolidone powder(PVP 119872119908 = 29000) casting knife acrylic acid (AA)NN1015840-methylenebisacrylamide 4-hydroxybenzophenoneand tetra-ammine palladium(II)chloride monohydrate Allcompounds have been used without any purification andsolutions were prepared with deionized water
22 Membrane Preparation Flat sheet PES microfiltrationmembranes were prepared via nonsolvent induced phase
International Journal of Chemical Engineering 3
separation (NIPS) using a solution containing polyether-sulfone (18wt) as polymer and N-methyl-2-pyrrolidone(62wt) as solvent and polyvinylpyrrolidone (PVP 20wt)as a pore formerThe solutionwas casted using a casting knifewith 350 120583m thickness and is precipitated in a coagulationwater bath at 18ndash20∘C
23 Membrane Functionalization Flat sheet PES microfil-tration membranes were immersed for 3 minutes in 30mlaqueous solution ofAAmonomer (25wt) After the immer-sion samples were grafted using a simple photograftingsetup containing quartz UV lamp Exposure time was (5ndash20)minutes Distance from the light source to the sample wasadjusted to aminimum 6 cm in order to avoid a possible heatup of the membrane After grafting samples were washedwith deionizedwater in order to remove unreactedmonomerDried samples were taken for surface analysis surface usingattenuated total reflection Fourier transform infrared spec-troscopy (ATR-FTIR Thermo-Nicolet Nexus)
24 Precursor Loading and Intermatrix Synthesis of Pd-Nanoparticles The synthesis of Pd-NPs inside the func-tionalized flat sheet PES polymer membrane matrix wascarried out via intermatrix synthesis (IMS) method withprocedures consisting of the following (1) palladium salt[Pd(NH3)4Cl2] whichwas loaded to the functionalizedmem-brane which enables cation exchange between carboxylicgroups of functionalized PES with [Pd(NH3)4]
2+ ions to tookplace and (2) subsequent chemical reduction by 01MNaBH4solution the cation exchange was performed by submerginggraftedmembrane in to palladiumprecursor solution (001MPd(NH3)4Cl2) over night at room temperatureThe synthesiscan be summarized by the following sequential equations ofion exchange ((1) and (2)) and chemical reduction ((3) and(4)) [13]
25 Palladium Content Measurement The amount of palla-dium nanoparticle loaded to the functionalized membranewas determined by using inductively coupled plasma opticalemission spectrometry (ICP-OES Ultima 2 Horoba JobinYvon) 1 cm2 sample of Pd loaded membrane was dissolvedin aqua regia which is a highly corrosive mixture of acids fortwo days The acid mixture was prepared by freshly mixingconcentrated nitric acid (65) and hydrochloric acid (35)in a volume ratio of 1 3 It was then diluted in ultra-purewater so as to analyze in ICP
26 Catalytic Performance Evaluation The catalytic perfor-mance of Pd loaded flat sheet PES membrane was evaluatedby the reduction of 119901-nitrophenol to 119901-aminophenol with
UV spectroscopy
FTCMR
Feed tank
Gas
cylin
der
Monochromator
Lens
LampSample
CuvetteAmplier Readout
Detector
Figure 1 FTCMR experimental setup
NaBH4 in FTCMR setup shown in Figure 1 The reductionof 119901-nitrophenol in the presence of nanoparticles has beenfrequently used previously in order to evaluate the catalyticactivity of different metal nanoparticles immobilized onmembranes [15ndash17] In addition this reaction has been usedto test the catalytic activity of metal nanoparticles immobi-lized in other carrier systems like core-shell Previous workdemonstrated that 119901-nitrophenol is reduced to aminophenolonly in the presence of the catalyst no reaction takes place inthe absence of the nanoparticles [15]
In FTCMR configuration shown in Figure 2 a solu-tion containing different concentrations of a mixture of 119901-nitrophenol and NaBH4 was forced to pass through themembrane An excess of NaBH4 was used so that the kineticscan be assimilated to a pseudo first order in terms of con-centration of 119901-nitrophenolThe progress of the reaction wasmonitored by measuring the concentration of 119901-nitrophenolusing UV-visible spectroscopy at (120582 = 400 nm)
27 Mathematical Modeling Convective mass transport istaking place in FTCMR because there exists transmembranepressure difference between the two sides of membrane so asto enforce the reactants to pass through In somemembranessuch as nanofiltration membrane the presence of convectiveflow by pressure difference enhances the diffusive driving
4 International Journal of Chemical Engineering
Pressure
CA0
y
x
CA
Figure 2 Scheme of one-dimensional mass transport across cat-alytic layer in flow-through membrane reactor (red particles arethe catalysts and 1198621198600 and 119862119860 are initial and final concentration ofreactant 119860 resp)
force Hence investigating combined effect of the diffusiveand convective flows is important in surface functionalizedpolymer membranes The model takes into account simulta-neous transport by convective and diffusive mass flow withchemical reaction Emin et al [17] have clearly shown thatpalladium nanoparticles have been only immobilized on thegrafted poly (acrylic acid) layer Analyzing the sample usingenergy-dispersive X-ray spectroscopy (EDX) showed thatNPs were not found deep in the support membraneThis factis considered here in modeling FTCMR
The following assumptions are considered [18](i) Reaction occurs uniformly at every position within
the catalyst layer(ii) Transport of reactants through the catalyst layer
occurs both by diffusion and by convection Howeverthe diffusion term does not contribute much topermeate flow rate
(iii) Chemical reaction occurs on the interface of thecatalytic particles so that the diffusion inside the densnanoparticle is negligible
(iv) The concentration changes only across the catalyticmembrane layer (0198 120583m from SEMmeasurement)and nanoparticles are immobilized and stabilized onthe grafted layer
The differential mass transport across a reactive membranelayer with constant transport parameters through combinedmass transport with chemical reaction which is representedin Figure 2 can be described by a continuity equation forCartesian coordinate system For reactant species 119860 [18]
where 119862 is the concentration 119883119860 is molar fraction 119863 is thediffusion coefficient in the membrane (m2hr) 119903119860 is rate ofreaction and 119881 is the convective velocity (mhr) through themembrane For steady state system with a convective flow in119909 direction the above equation becomes
For pseudo first-order reaction 119903119860 = minus119870app lowast 119862119860 where 119870appis the apparent kinetic constant and 119862119860 is the concentrationof reactant 119860 Substitution of rate of reaction to (2) gives
119863 11988921198621198601198891199092 minus 119881119909 119889119862119860119889119909 minus 119870app lowast 119862119860 = 0 (3)
Defining and introducing the dimensionless length epsilon(120576) = 119909120575 whose value varies from 0 to 1 where 120575 is catalyticmembrane layer (0198 120583m) and 119909 is the catalytic layer at anydistance substituting to (3) gives
11988921198621198601198891205762 minus 119881 lowast 120575119863 lowast 119889119862119860119889120576 minus 119870app1205752
119863 119862119860 = 0 (4)
Two dimensionless groupsrsquo Peclet number (119875119890) and reactionmodulus (02) are defined and inserted to (4) where
119875119890 = 119881 lowast 120575119863
02 = 119870app1205752119863
(5)
The diffusion coefficient was predicted based on the variationof the actual measurement taken from the filtration unit anda model prediction that ignores the diffusion term (as if theentire process was controlled by the convective flow) Thesolution of (4) is
31 Membrane Surface Functionalization Both unmodifiedPES and theUV-inducedmodified flat sheetmembraneswerecharacterized by ATR-FTIR Figure 3 shows the spectra of theunmodified andmodifiedmembranes with AAmonomer As
International Journal of Chemical Engineering 5
Unmodied
1720
Modied
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 3 The ATR-FTIR spectra of unmodified and modified PESflat sheet membranes using UV grafting of AA (25wt) at 20minUV batch irradiation time
1535
16051625
1720(a)
(b)(c)
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 4 The ATR-FTIR spectra of the (a) unmodified flat sheetPES membrane and modified membranes using UV-induced graft-ing with (25 wtAA) (b) 20 and (c) 15minute UV irradiation timerespectively
can be seen UV-induced graftedmembranes exhibit differentATR-FTIR spectra than the unmodified one In addition tothe typical PES bands of the unmodified membrane theIR spectra of modified membrane show additional peak at1720 cmminus1 which corresponds to the carbonyl (C=O) groupbands of COOHwhich indicates the existence of poly(acrylicacid) chains [19] and assured that themonomer is successfullypolymerized on the substrate PES Also with UV modifica-tion some original absorbance peak intensity is decreasedthat can be contributed to increase coverage of the PESsurface by poly(acrylic acid)
Apart from this as can be seen from FTIR spectra ofmodified membrane in Figure 4 the intensity of the newpeak at 1720 cmminus1 increases with photografting reaction timewhich corresponds to the energy received during graft poly-merization The more the energy received by the monomerthe higher the degree of modification In addition to thenew absorption peak corresponding to the carbonyl group(C=O) of COOH there are also other new small absorbancepeaks appearing at 1625 1605 and 1535 cmminus1 for themodified
6 10 12 15 20
Palla
dium
amou
nt (m
g)
Graing time (min)
0
01
02
03
04
05
06
07
08
Figure 5 Palladium amount per membrane (152 cm2) versusgrafting time
membranesThe small absorbance peak for the AA-modifiedmembranes at these regions attributed to the presence ofadditives such as PVP in the original casting solution andcross linker in the monomer solution as reported by Rahim-pour [20] and Bernstein et al [21] Also the disappearance ofthe original peaks at 1640ndash1680 cmminus1 for the modified mem-branes indicates that the modification was successful
Membranes grafted at different timeswere used to synthe-size Pd nanoparticle ICP analysis in Figure 5 showed that theweight of Pd loading is increasing with grafting time (energyreceived) These data are in agreement with expected andFTIR results At higher energy the intensity of the modifiedfunctional group was stronger which can lead to better cationexchange with Pd precursor and thus higher Pd amount fromthe reduction
32 Catalytic Performance Evaluation The effect of thenitrophenol concentration in the feed on the conversionwas investigated using different feed concentrations and thesame amount of Pd at a room temperature A closer look atFigure 6 clearly shows the effect of initial119901-NP concentrationon conversion The conversion is higher for the lower 119901-NP initial concentration In all of initial 119901-NP concentrationranges (ie 0033ndash0128mM) the same trend was obtainedwhich strongly indicates that reaction on Pd nanoparticle ishighly active dependant surface For the same catalyst loadingand permeate flux (ie approximately the same residencetime and applied pressure within a membrane) the catalyticactivity of membranes with lower initial concentration washigher than that of higher initial 119901-NP concentration
Furthermore the inverse proportionality of 119901-NP con-version with permeate flux confirmed that our Pd loadedmembranes were not mass transfer limited rather it wasreaction limited If membranes were mass transfer limitedwewould not observe a decrease of conversionwith increasedfluxes As the applied pressure in flow-throughmode enforcesreactants to have an intensive contact with catalyst thereaction at the surface of the catalyst is the main limitationto such system As a result of increasing pressure (flux)reactants permeate through the membrane without reacting
6 International Journal of Chemical EngineeringC
onve
rsio
n of
NP
()
C0 = 0128 mMC0 = 0066mMC0 = 0033mM
002 004 006 008 01 012091
925
94
955
97
985
100
Permeate flux (m3 hrminus1mminus2)
Figure 6 Conversion of nitrophenol (NP) in Pd loaded PESmembrane versus flux for single pass in dead-end mode of filtration(0283mg of palladium)
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
across a membrane The membrane can act as even distribu-tor of the limiting reactant along the reactor to prevent sidereactions and as upstream separation unit to selectively doseone component from a mixture Controlled addition of oxy-gen in gas-phase partial oxidation of hydrocarbons in whichthe intermediate product reactsmore intensively with oxygenthan the reactants in order to prevent total oxidation [7 8]is the main application of this category of membrane reactor
(3) Contactor Membrane Reactors In this configuration thereacting species are fed at different sides of themembrane andmust diffuse through the catalytic layer to react Thereforethe role of the membrane is to provide an interfacial contactarea for the reacting streams but does not perform any selec-tive separationThe two sides ofmembranes are used to bringreactants into contact and if the reaction rate is fast comparedto the diffusion rates of the reactants the reaction occurs inthe catalytic layer in a way that prevents mixing of reactantsDue to higher surface area of membranes a contactor modecan provide higher contact area between two different phasesParticularly if one phase has lower solubility in the otherphase higher surface area contact between these phases candecrease the need of higher pressure that could have beenapplied to lower soluble component Gasliquid contactorsand flow-through membrane reactors are important class ofmembrane contactors
In flow-through catalytic membrane reactor (FTCMR)configuration unselective porous catalytic membrane eitherinherently catalytic or being made catalytic by impregnationof nanocatalysts is applied in dead-end mode operationThepremixed reactants are forced to pass through the catalyticmembrane The function of the membrane is to create areaction environment with intensive contact between thereactants and catalyst with short and controlled residencetimes and high catalytic activity The main drawback inclassical fixed-bed reactors is that the desired conversion ismainly limited by the pore diffusion However if reactantscan flow convectively through the catalyst sites the resultingintensive contact between reactants and catalyst can resultin a high catalytic activity [9 10] Besides this can avoidthe problems derived from internal or external mass transferresistance that may appear in a conventional fixed-bedreactor In FTCMR the reactants flow convectively throughthe membrane to catalyst sites which in turn results inan intensive contact between the reactants and the catalystthereby leading to higher catalytic activity with negligiblemass transport resistance Furthermore the introduction ofconvective flow can avoid undesired side reactions [1 10]
Catalytic membranes (CMs) either inherently catalyticor being made catalytic by impregnation of nanocatalystsare known for more than a decade In spite of this factthe development of CMs is still a major challenge Majorityof the catalytic membranes used in industries are inorganic(either ceramic or metal) for that reason they can withstandharsh reaction conditions (high temperature pressure con-centration and corrosive chemicals) The main drawbacks ofsuch CMmaterials are high cost and frangibility [5] Becausepolymers are less expensive and more flexible than ceramicsand metals it is possible to use them in CM development
instead of high cost metals and ceramics However majorityof the polymers are only suitable for mild operation condi-tions For this compensation high reactive catalysts should beimpregnated inside the polymer membrane matrix In suchcases active catalysts can compromise the demand of highertemperature Therefore stabilization of active catalysts byencapsulating inside polymer membranes can help to boostand enhance the drawbacks of polymer membranes
Metal nanoparticles (MNPs) have shown a great potentialin different catalytic processes and are well known for theirhigher catalytic performances Particularly MNPs of transi-tion metals are found to be efficient and selective catalystsfor several types of catalytic reactions This is due to higherpercentage of surface atoms and associated quantum effectsHowever metal nanoparticles lack chemical stability andmechanical strength They exhibit extremely high pressuredrop or head loss in fixed-bed column operation and arenot found suitable for such systems [11] Also MNPs tendto aggregate this phenomenon reduces their high surfacearea to volume ratio and subsequently reduces effective-ness By appropriately dispersing metal nanoparticles intosurface functionalized polymer membranes many of theseshortcomings can be overcome without compromising theproperties of nanoparticles ImmobilizingMNPs on polymermembrane support besides providing amechanical strengthit offers an option to maintain their catalytic activities by pre-venting unnecessary growth and aggregation Moreover cat-alytic application of MNPs is the best alternative to efficientlyutilize most expensive metals Immobilization of MNPs onpolymer membrane support is therefore best strategy toovercome the drawbacks of both polymers and MNPs [12]The use of functionalized polymer membrane as a supportand stabilizing media enables synthesizing nanocatalysts atthe desired ldquopoint userdquo and will result in formation of catalyt-ically active polymer membrane Encapsulation of MNPs inpolymers membranes offers also unique possibilities forenhancing accessibility of catalytic sites to reactants [13 14]
In this research we developed catalytic polymer mem-brane for liquid phase reactions by surface modification offlat sheet PES microfiltration membrane using UV-inducedgraft polymerization and synthesizing of stable palladiumnanoparticle via intermatrix synthesis method inside surfacefunctionalized PES membranes The catalytic performanceof the membrane was evaluated using reduction of 119901-nitro-phenol (NP) as a model for liquid phase reaction
2 Materials and Methods
21 Materials The following chemicals and materialswere used during the experiment acrylic acid (AA) N-methyl-2-pyrrolidone (NMP) polyvinylpyrrolidone powder(PVP 119872119908 = 29000) casting knife acrylic acid (AA)NN1015840-methylenebisacrylamide 4-hydroxybenzophenoneand tetra-ammine palladium(II)chloride monohydrate Allcompounds have been used without any purification andsolutions were prepared with deionized water
22 Membrane Preparation Flat sheet PES microfiltrationmembranes were prepared via nonsolvent induced phase
International Journal of Chemical Engineering 3
separation (NIPS) using a solution containing polyether-sulfone (18wt) as polymer and N-methyl-2-pyrrolidone(62wt) as solvent and polyvinylpyrrolidone (PVP 20wt)as a pore formerThe solutionwas casted using a casting knifewith 350 120583m thickness and is precipitated in a coagulationwater bath at 18ndash20∘C
23 Membrane Functionalization Flat sheet PES microfil-tration membranes were immersed for 3 minutes in 30mlaqueous solution ofAAmonomer (25wt) After the immer-sion samples were grafted using a simple photograftingsetup containing quartz UV lamp Exposure time was (5ndash20)minutes Distance from the light source to the sample wasadjusted to aminimum 6 cm in order to avoid a possible heatup of the membrane After grafting samples were washedwith deionizedwater in order to remove unreactedmonomerDried samples were taken for surface analysis surface usingattenuated total reflection Fourier transform infrared spec-troscopy (ATR-FTIR Thermo-Nicolet Nexus)
24 Precursor Loading and Intermatrix Synthesis of Pd-Nanoparticles The synthesis of Pd-NPs inside the func-tionalized flat sheet PES polymer membrane matrix wascarried out via intermatrix synthesis (IMS) method withprocedures consisting of the following (1) palladium salt[Pd(NH3)4Cl2] whichwas loaded to the functionalizedmem-brane which enables cation exchange between carboxylicgroups of functionalized PES with [Pd(NH3)4]
2+ ions to tookplace and (2) subsequent chemical reduction by 01MNaBH4solution the cation exchange was performed by submerginggraftedmembrane in to palladiumprecursor solution (001MPd(NH3)4Cl2) over night at room temperatureThe synthesiscan be summarized by the following sequential equations ofion exchange ((1) and (2)) and chemical reduction ((3) and(4)) [13]
25 Palladium Content Measurement The amount of palla-dium nanoparticle loaded to the functionalized membranewas determined by using inductively coupled plasma opticalemission spectrometry (ICP-OES Ultima 2 Horoba JobinYvon) 1 cm2 sample of Pd loaded membrane was dissolvedin aqua regia which is a highly corrosive mixture of acids fortwo days The acid mixture was prepared by freshly mixingconcentrated nitric acid (65) and hydrochloric acid (35)in a volume ratio of 1 3 It was then diluted in ultra-purewater so as to analyze in ICP
26 Catalytic Performance Evaluation The catalytic perfor-mance of Pd loaded flat sheet PES membrane was evaluatedby the reduction of 119901-nitrophenol to 119901-aminophenol with
UV spectroscopy
FTCMR
Feed tank
Gas
cylin
der
Monochromator
Lens
LampSample
CuvetteAmplier Readout
Detector
Figure 1 FTCMR experimental setup
NaBH4 in FTCMR setup shown in Figure 1 The reductionof 119901-nitrophenol in the presence of nanoparticles has beenfrequently used previously in order to evaluate the catalyticactivity of different metal nanoparticles immobilized onmembranes [15ndash17] In addition this reaction has been usedto test the catalytic activity of metal nanoparticles immobi-lized in other carrier systems like core-shell Previous workdemonstrated that 119901-nitrophenol is reduced to aminophenolonly in the presence of the catalyst no reaction takes place inthe absence of the nanoparticles [15]
In FTCMR configuration shown in Figure 2 a solu-tion containing different concentrations of a mixture of 119901-nitrophenol and NaBH4 was forced to pass through themembrane An excess of NaBH4 was used so that the kineticscan be assimilated to a pseudo first order in terms of con-centration of 119901-nitrophenolThe progress of the reaction wasmonitored by measuring the concentration of 119901-nitrophenolusing UV-visible spectroscopy at (120582 = 400 nm)
27 Mathematical Modeling Convective mass transport istaking place in FTCMR because there exists transmembranepressure difference between the two sides of membrane so asto enforce the reactants to pass through In somemembranessuch as nanofiltration membrane the presence of convectiveflow by pressure difference enhances the diffusive driving
4 International Journal of Chemical Engineering
Pressure
CA0
y
x
CA
Figure 2 Scheme of one-dimensional mass transport across cat-alytic layer in flow-through membrane reactor (red particles arethe catalysts and 1198621198600 and 119862119860 are initial and final concentration ofreactant 119860 resp)
force Hence investigating combined effect of the diffusiveand convective flows is important in surface functionalizedpolymer membranes The model takes into account simulta-neous transport by convective and diffusive mass flow withchemical reaction Emin et al [17] have clearly shown thatpalladium nanoparticles have been only immobilized on thegrafted poly (acrylic acid) layer Analyzing the sample usingenergy-dispersive X-ray spectroscopy (EDX) showed thatNPs were not found deep in the support membraneThis factis considered here in modeling FTCMR
The following assumptions are considered [18](i) Reaction occurs uniformly at every position within
the catalyst layer(ii) Transport of reactants through the catalyst layer
occurs both by diffusion and by convection Howeverthe diffusion term does not contribute much topermeate flow rate
(iii) Chemical reaction occurs on the interface of thecatalytic particles so that the diffusion inside the densnanoparticle is negligible
(iv) The concentration changes only across the catalyticmembrane layer (0198 120583m from SEMmeasurement)and nanoparticles are immobilized and stabilized onthe grafted layer
The differential mass transport across a reactive membranelayer with constant transport parameters through combinedmass transport with chemical reaction which is representedin Figure 2 can be described by a continuity equation forCartesian coordinate system For reactant species 119860 [18]
where 119862 is the concentration 119883119860 is molar fraction 119863 is thediffusion coefficient in the membrane (m2hr) 119903119860 is rate ofreaction and 119881 is the convective velocity (mhr) through themembrane For steady state system with a convective flow in119909 direction the above equation becomes
For pseudo first-order reaction 119903119860 = minus119870app lowast 119862119860 where 119870appis the apparent kinetic constant and 119862119860 is the concentrationof reactant 119860 Substitution of rate of reaction to (2) gives
119863 11988921198621198601198891199092 minus 119881119909 119889119862119860119889119909 minus 119870app lowast 119862119860 = 0 (3)
Defining and introducing the dimensionless length epsilon(120576) = 119909120575 whose value varies from 0 to 1 where 120575 is catalyticmembrane layer (0198 120583m) and 119909 is the catalytic layer at anydistance substituting to (3) gives
11988921198621198601198891205762 minus 119881 lowast 120575119863 lowast 119889119862119860119889120576 minus 119870app1205752
119863 119862119860 = 0 (4)
Two dimensionless groupsrsquo Peclet number (119875119890) and reactionmodulus (02) are defined and inserted to (4) where
119875119890 = 119881 lowast 120575119863
02 = 119870app1205752119863
(5)
The diffusion coefficient was predicted based on the variationof the actual measurement taken from the filtration unit anda model prediction that ignores the diffusion term (as if theentire process was controlled by the convective flow) Thesolution of (4) is
31 Membrane Surface Functionalization Both unmodifiedPES and theUV-inducedmodified flat sheetmembraneswerecharacterized by ATR-FTIR Figure 3 shows the spectra of theunmodified andmodifiedmembranes with AAmonomer As
International Journal of Chemical Engineering 5
Unmodied
1720
Modied
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 3 The ATR-FTIR spectra of unmodified and modified PESflat sheet membranes using UV grafting of AA (25wt) at 20minUV batch irradiation time
1535
16051625
1720(a)
(b)(c)
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 4 The ATR-FTIR spectra of the (a) unmodified flat sheetPES membrane and modified membranes using UV-induced graft-ing with (25 wtAA) (b) 20 and (c) 15minute UV irradiation timerespectively
can be seen UV-induced graftedmembranes exhibit differentATR-FTIR spectra than the unmodified one In addition tothe typical PES bands of the unmodified membrane theIR spectra of modified membrane show additional peak at1720 cmminus1 which corresponds to the carbonyl (C=O) groupbands of COOHwhich indicates the existence of poly(acrylicacid) chains [19] and assured that themonomer is successfullypolymerized on the substrate PES Also with UV modifica-tion some original absorbance peak intensity is decreasedthat can be contributed to increase coverage of the PESsurface by poly(acrylic acid)
Apart from this as can be seen from FTIR spectra ofmodified membrane in Figure 4 the intensity of the newpeak at 1720 cmminus1 increases with photografting reaction timewhich corresponds to the energy received during graft poly-merization The more the energy received by the monomerthe higher the degree of modification In addition to thenew absorption peak corresponding to the carbonyl group(C=O) of COOH there are also other new small absorbancepeaks appearing at 1625 1605 and 1535 cmminus1 for themodified
6 10 12 15 20
Palla
dium
amou
nt (m
g)
Graing time (min)
0
01
02
03
04
05
06
07
08
Figure 5 Palladium amount per membrane (152 cm2) versusgrafting time
membranesThe small absorbance peak for the AA-modifiedmembranes at these regions attributed to the presence ofadditives such as PVP in the original casting solution andcross linker in the monomer solution as reported by Rahim-pour [20] and Bernstein et al [21] Also the disappearance ofthe original peaks at 1640ndash1680 cmminus1 for the modified mem-branes indicates that the modification was successful
Membranes grafted at different timeswere used to synthe-size Pd nanoparticle ICP analysis in Figure 5 showed that theweight of Pd loading is increasing with grafting time (energyreceived) These data are in agreement with expected andFTIR results At higher energy the intensity of the modifiedfunctional group was stronger which can lead to better cationexchange with Pd precursor and thus higher Pd amount fromthe reduction
32 Catalytic Performance Evaluation The effect of thenitrophenol concentration in the feed on the conversionwas investigated using different feed concentrations and thesame amount of Pd at a room temperature A closer look atFigure 6 clearly shows the effect of initial119901-NP concentrationon conversion The conversion is higher for the lower 119901-NP initial concentration In all of initial 119901-NP concentrationranges (ie 0033ndash0128mM) the same trend was obtainedwhich strongly indicates that reaction on Pd nanoparticle ishighly active dependant surface For the same catalyst loadingand permeate flux (ie approximately the same residencetime and applied pressure within a membrane) the catalyticactivity of membranes with lower initial concentration washigher than that of higher initial 119901-NP concentration
Furthermore the inverse proportionality of 119901-NP con-version with permeate flux confirmed that our Pd loadedmembranes were not mass transfer limited rather it wasreaction limited If membranes were mass transfer limitedwewould not observe a decrease of conversionwith increasedfluxes As the applied pressure in flow-throughmode enforcesreactants to have an intensive contact with catalyst thereaction at the surface of the catalyst is the main limitationto such system As a result of increasing pressure (flux)reactants permeate through the membrane without reacting
6 International Journal of Chemical EngineeringC
onve
rsio
n of
NP
()
C0 = 0128 mMC0 = 0066mMC0 = 0033mM
002 004 006 008 01 012091
925
94
955
97
985
100
Permeate flux (m3 hrminus1mminus2)
Figure 6 Conversion of nitrophenol (NP) in Pd loaded PESmembrane versus flux for single pass in dead-end mode of filtration(0283mg of palladium)
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
separation (NIPS) using a solution containing polyether-sulfone (18wt) as polymer and N-methyl-2-pyrrolidone(62wt) as solvent and polyvinylpyrrolidone (PVP 20wt)as a pore formerThe solutionwas casted using a casting knifewith 350 120583m thickness and is precipitated in a coagulationwater bath at 18ndash20∘C
23 Membrane Functionalization Flat sheet PES microfil-tration membranes were immersed for 3 minutes in 30mlaqueous solution ofAAmonomer (25wt) After the immer-sion samples were grafted using a simple photograftingsetup containing quartz UV lamp Exposure time was (5ndash20)minutes Distance from the light source to the sample wasadjusted to aminimum 6 cm in order to avoid a possible heatup of the membrane After grafting samples were washedwith deionizedwater in order to remove unreactedmonomerDried samples were taken for surface analysis surface usingattenuated total reflection Fourier transform infrared spec-troscopy (ATR-FTIR Thermo-Nicolet Nexus)
24 Precursor Loading and Intermatrix Synthesis of Pd-Nanoparticles The synthesis of Pd-NPs inside the func-tionalized flat sheet PES polymer membrane matrix wascarried out via intermatrix synthesis (IMS) method withprocedures consisting of the following (1) palladium salt[Pd(NH3)4Cl2] whichwas loaded to the functionalizedmem-brane which enables cation exchange between carboxylicgroups of functionalized PES with [Pd(NH3)4]
2+ ions to tookplace and (2) subsequent chemical reduction by 01MNaBH4solution the cation exchange was performed by submerginggraftedmembrane in to palladiumprecursor solution (001MPd(NH3)4Cl2) over night at room temperatureThe synthesiscan be summarized by the following sequential equations ofion exchange ((1) and (2)) and chemical reduction ((3) and(4)) [13]
25 Palladium Content Measurement The amount of palla-dium nanoparticle loaded to the functionalized membranewas determined by using inductively coupled plasma opticalemission spectrometry (ICP-OES Ultima 2 Horoba JobinYvon) 1 cm2 sample of Pd loaded membrane was dissolvedin aqua regia which is a highly corrosive mixture of acids fortwo days The acid mixture was prepared by freshly mixingconcentrated nitric acid (65) and hydrochloric acid (35)in a volume ratio of 1 3 It was then diluted in ultra-purewater so as to analyze in ICP
26 Catalytic Performance Evaluation The catalytic perfor-mance of Pd loaded flat sheet PES membrane was evaluatedby the reduction of 119901-nitrophenol to 119901-aminophenol with
UV spectroscopy
FTCMR
Feed tank
Gas
cylin
der
Monochromator
Lens
LampSample
CuvetteAmplier Readout
Detector
Figure 1 FTCMR experimental setup
NaBH4 in FTCMR setup shown in Figure 1 The reductionof 119901-nitrophenol in the presence of nanoparticles has beenfrequently used previously in order to evaluate the catalyticactivity of different metal nanoparticles immobilized onmembranes [15ndash17] In addition this reaction has been usedto test the catalytic activity of metal nanoparticles immobi-lized in other carrier systems like core-shell Previous workdemonstrated that 119901-nitrophenol is reduced to aminophenolonly in the presence of the catalyst no reaction takes place inthe absence of the nanoparticles [15]
In FTCMR configuration shown in Figure 2 a solu-tion containing different concentrations of a mixture of 119901-nitrophenol and NaBH4 was forced to pass through themembrane An excess of NaBH4 was used so that the kineticscan be assimilated to a pseudo first order in terms of con-centration of 119901-nitrophenolThe progress of the reaction wasmonitored by measuring the concentration of 119901-nitrophenolusing UV-visible spectroscopy at (120582 = 400 nm)
27 Mathematical Modeling Convective mass transport istaking place in FTCMR because there exists transmembranepressure difference between the two sides of membrane so asto enforce the reactants to pass through In somemembranessuch as nanofiltration membrane the presence of convectiveflow by pressure difference enhances the diffusive driving
4 International Journal of Chemical Engineering
Pressure
CA0
y
x
CA
Figure 2 Scheme of one-dimensional mass transport across cat-alytic layer in flow-through membrane reactor (red particles arethe catalysts and 1198621198600 and 119862119860 are initial and final concentration ofreactant 119860 resp)
force Hence investigating combined effect of the diffusiveand convective flows is important in surface functionalizedpolymer membranes The model takes into account simulta-neous transport by convective and diffusive mass flow withchemical reaction Emin et al [17] have clearly shown thatpalladium nanoparticles have been only immobilized on thegrafted poly (acrylic acid) layer Analyzing the sample usingenergy-dispersive X-ray spectroscopy (EDX) showed thatNPs were not found deep in the support membraneThis factis considered here in modeling FTCMR
The following assumptions are considered [18](i) Reaction occurs uniformly at every position within
the catalyst layer(ii) Transport of reactants through the catalyst layer
occurs both by diffusion and by convection Howeverthe diffusion term does not contribute much topermeate flow rate
(iii) Chemical reaction occurs on the interface of thecatalytic particles so that the diffusion inside the densnanoparticle is negligible
(iv) The concentration changes only across the catalyticmembrane layer (0198 120583m from SEMmeasurement)and nanoparticles are immobilized and stabilized onthe grafted layer
The differential mass transport across a reactive membranelayer with constant transport parameters through combinedmass transport with chemical reaction which is representedin Figure 2 can be described by a continuity equation forCartesian coordinate system For reactant species 119860 [18]
where 119862 is the concentration 119883119860 is molar fraction 119863 is thediffusion coefficient in the membrane (m2hr) 119903119860 is rate ofreaction and 119881 is the convective velocity (mhr) through themembrane For steady state system with a convective flow in119909 direction the above equation becomes
For pseudo first-order reaction 119903119860 = minus119870app lowast 119862119860 where 119870appis the apparent kinetic constant and 119862119860 is the concentrationof reactant 119860 Substitution of rate of reaction to (2) gives
119863 11988921198621198601198891199092 minus 119881119909 119889119862119860119889119909 minus 119870app lowast 119862119860 = 0 (3)
Defining and introducing the dimensionless length epsilon(120576) = 119909120575 whose value varies from 0 to 1 where 120575 is catalyticmembrane layer (0198 120583m) and 119909 is the catalytic layer at anydistance substituting to (3) gives
11988921198621198601198891205762 minus 119881 lowast 120575119863 lowast 119889119862119860119889120576 minus 119870app1205752
119863 119862119860 = 0 (4)
Two dimensionless groupsrsquo Peclet number (119875119890) and reactionmodulus (02) are defined and inserted to (4) where
119875119890 = 119881 lowast 120575119863
02 = 119870app1205752119863
(5)
The diffusion coefficient was predicted based on the variationof the actual measurement taken from the filtration unit anda model prediction that ignores the diffusion term (as if theentire process was controlled by the convective flow) Thesolution of (4) is
31 Membrane Surface Functionalization Both unmodifiedPES and theUV-inducedmodified flat sheetmembraneswerecharacterized by ATR-FTIR Figure 3 shows the spectra of theunmodified andmodifiedmembranes with AAmonomer As
International Journal of Chemical Engineering 5
Unmodied
1720
Modied
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 3 The ATR-FTIR spectra of unmodified and modified PESflat sheet membranes using UV grafting of AA (25wt) at 20minUV batch irradiation time
1535
16051625
1720(a)
(b)(c)
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 4 The ATR-FTIR spectra of the (a) unmodified flat sheetPES membrane and modified membranes using UV-induced graft-ing with (25 wtAA) (b) 20 and (c) 15minute UV irradiation timerespectively
can be seen UV-induced graftedmembranes exhibit differentATR-FTIR spectra than the unmodified one In addition tothe typical PES bands of the unmodified membrane theIR spectra of modified membrane show additional peak at1720 cmminus1 which corresponds to the carbonyl (C=O) groupbands of COOHwhich indicates the existence of poly(acrylicacid) chains [19] and assured that themonomer is successfullypolymerized on the substrate PES Also with UV modifica-tion some original absorbance peak intensity is decreasedthat can be contributed to increase coverage of the PESsurface by poly(acrylic acid)
Apart from this as can be seen from FTIR spectra ofmodified membrane in Figure 4 the intensity of the newpeak at 1720 cmminus1 increases with photografting reaction timewhich corresponds to the energy received during graft poly-merization The more the energy received by the monomerthe higher the degree of modification In addition to thenew absorption peak corresponding to the carbonyl group(C=O) of COOH there are also other new small absorbancepeaks appearing at 1625 1605 and 1535 cmminus1 for themodified
6 10 12 15 20
Palla
dium
amou
nt (m
g)
Graing time (min)
0
01
02
03
04
05
06
07
08
Figure 5 Palladium amount per membrane (152 cm2) versusgrafting time
membranesThe small absorbance peak for the AA-modifiedmembranes at these regions attributed to the presence ofadditives such as PVP in the original casting solution andcross linker in the monomer solution as reported by Rahim-pour [20] and Bernstein et al [21] Also the disappearance ofthe original peaks at 1640ndash1680 cmminus1 for the modified mem-branes indicates that the modification was successful
Membranes grafted at different timeswere used to synthe-size Pd nanoparticle ICP analysis in Figure 5 showed that theweight of Pd loading is increasing with grafting time (energyreceived) These data are in agreement with expected andFTIR results At higher energy the intensity of the modifiedfunctional group was stronger which can lead to better cationexchange with Pd precursor and thus higher Pd amount fromthe reduction
32 Catalytic Performance Evaluation The effect of thenitrophenol concentration in the feed on the conversionwas investigated using different feed concentrations and thesame amount of Pd at a room temperature A closer look atFigure 6 clearly shows the effect of initial119901-NP concentrationon conversion The conversion is higher for the lower 119901-NP initial concentration In all of initial 119901-NP concentrationranges (ie 0033ndash0128mM) the same trend was obtainedwhich strongly indicates that reaction on Pd nanoparticle ishighly active dependant surface For the same catalyst loadingand permeate flux (ie approximately the same residencetime and applied pressure within a membrane) the catalyticactivity of membranes with lower initial concentration washigher than that of higher initial 119901-NP concentration
Furthermore the inverse proportionality of 119901-NP con-version with permeate flux confirmed that our Pd loadedmembranes were not mass transfer limited rather it wasreaction limited If membranes were mass transfer limitedwewould not observe a decrease of conversionwith increasedfluxes As the applied pressure in flow-throughmode enforcesreactants to have an intensive contact with catalyst thereaction at the surface of the catalyst is the main limitationto such system As a result of increasing pressure (flux)reactants permeate through the membrane without reacting
6 International Journal of Chemical EngineeringC
onve
rsio
n of
NP
()
C0 = 0128 mMC0 = 0066mMC0 = 0033mM
002 004 006 008 01 012091
925
94
955
97
985
100
Permeate flux (m3 hrminus1mminus2)
Figure 6 Conversion of nitrophenol (NP) in Pd loaded PESmembrane versus flux for single pass in dead-end mode of filtration(0283mg of palladium)
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
Figure 2 Scheme of one-dimensional mass transport across cat-alytic layer in flow-through membrane reactor (red particles arethe catalysts and 1198621198600 and 119862119860 are initial and final concentration ofreactant 119860 resp)
force Hence investigating combined effect of the diffusiveand convective flows is important in surface functionalizedpolymer membranes The model takes into account simulta-neous transport by convective and diffusive mass flow withchemical reaction Emin et al [17] have clearly shown thatpalladium nanoparticles have been only immobilized on thegrafted poly (acrylic acid) layer Analyzing the sample usingenergy-dispersive X-ray spectroscopy (EDX) showed thatNPs were not found deep in the support membraneThis factis considered here in modeling FTCMR
The following assumptions are considered [18](i) Reaction occurs uniformly at every position within
the catalyst layer(ii) Transport of reactants through the catalyst layer
occurs both by diffusion and by convection Howeverthe diffusion term does not contribute much topermeate flow rate
(iii) Chemical reaction occurs on the interface of thecatalytic particles so that the diffusion inside the densnanoparticle is negligible
(iv) The concentration changes only across the catalyticmembrane layer (0198 120583m from SEMmeasurement)and nanoparticles are immobilized and stabilized onthe grafted layer
The differential mass transport across a reactive membranelayer with constant transport parameters through combinedmass transport with chemical reaction which is representedin Figure 2 can be described by a continuity equation forCartesian coordinate system For reactant species 119860 [18]
where 119862 is the concentration 119883119860 is molar fraction 119863 is thediffusion coefficient in the membrane (m2hr) 119903119860 is rate ofreaction and 119881 is the convective velocity (mhr) through themembrane For steady state system with a convective flow in119909 direction the above equation becomes
For pseudo first-order reaction 119903119860 = minus119870app lowast 119862119860 where 119870appis the apparent kinetic constant and 119862119860 is the concentrationof reactant 119860 Substitution of rate of reaction to (2) gives
119863 11988921198621198601198891199092 minus 119881119909 119889119862119860119889119909 minus 119870app lowast 119862119860 = 0 (3)
Defining and introducing the dimensionless length epsilon(120576) = 119909120575 whose value varies from 0 to 1 where 120575 is catalyticmembrane layer (0198 120583m) and 119909 is the catalytic layer at anydistance substituting to (3) gives
11988921198621198601198891205762 minus 119881 lowast 120575119863 lowast 119889119862119860119889120576 minus 119870app1205752
119863 119862119860 = 0 (4)
Two dimensionless groupsrsquo Peclet number (119875119890) and reactionmodulus (02) are defined and inserted to (4) where
119875119890 = 119881 lowast 120575119863
02 = 119870app1205752119863
(5)
The diffusion coefficient was predicted based on the variationof the actual measurement taken from the filtration unit anda model prediction that ignores the diffusion term (as if theentire process was controlled by the convective flow) Thesolution of (4) is
31 Membrane Surface Functionalization Both unmodifiedPES and theUV-inducedmodified flat sheetmembraneswerecharacterized by ATR-FTIR Figure 3 shows the spectra of theunmodified andmodifiedmembranes with AAmonomer As
International Journal of Chemical Engineering 5
Unmodied
1720
Modied
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 3 The ATR-FTIR spectra of unmodified and modified PESflat sheet membranes using UV grafting of AA (25wt) at 20minUV batch irradiation time
1535
16051625
1720(a)
(b)(c)
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 4 The ATR-FTIR spectra of the (a) unmodified flat sheetPES membrane and modified membranes using UV-induced graft-ing with (25 wtAA) (b) 20 and (c) 15minute UV irradiation timerespectively
can be seen UV-induced graftedmembranes exhibit differentATR-FTIR spectra than the unmodified one In addition tothe typical PES bands of the unmodified membrane theIR spectra of modified membrane show additional peak at1720 cmminus1 which corresponds to the carbonyl (C=O) groupbands of COOHwhich indicates the existence of poly(acrylicacid) chains [19] and assured that themonomer is successfullypolymerized on the substrate PES Also with UV modifica-tion some original absorbance peak intensity is decreasedthat can be contributed to increase coverage of the PESsurface by poly(acrylic acid)
Apart from this as can be seen from FTIR spectra ofmodified membrane in Figure 4 the intensity of the newpeak at 1720 cmminus1 increases with photografting reaction timewhich corresponds to the energy received during graft poly-merization The more the energy received by the monomerthe higher the degree of modification In addition to thenew absorption peak corresponding to the carbonyl group(C=O) of COOH there are also other new small absorbancepeaks appearing at 1625 1605 and 1535 cmminus1 for themodified
6 10 12 15 20
Palla
dium
amou
nt (m
g)
Graing time (min)
0
01
02
03
04
05
06
07
08
Figure 5 Palladium amount per membrane (152 cm2) versusgrafting time
membranesThe small absorbance peak for the AA-modifiedmembranes at these regions attributed to the presence ofadditives such as PVP in the original casting solution andcross linker in the monomer solution as reported by Rahim-pour [20] and Bernstein et al [21] Also the disappearance ofthe original peaks at 1640ndash1680 cmminus1 for the modified mem-branes indicates that the modification was successful
Membranes grafted at different timeswere used to synthe-size Pd nanoparticle ICP analysis in Figure 5 showed that theweight of Pd loading is increasing with grafting time (energyreceived) These data are in agreement with expected andFTIR results At higher energy the intensity of the modifiedfunctional group was stronger which can lead to better cationexchange with Pd precursor and thus higher Pd amount fromthe reduction
32 Catalytic Performance Evaluation The effect of thenitrophenol concentration in the feed on the conversionwas investigated using different feed concentrations and thesame amount of Pd at a room temperature A closer look atFigure 6 clearly shows the effect of initial119901-NP concentrationon conversion The conversion is higher for the lower 119901-NP initial concentration In all of initial 119901-NP concentrationranges (ie 0033ndash0128mM) the same trend was obtainedwhich strongly indicates that reaction on Pd nanoparticle ishighly active dependant surface For the same catalyst loadingand permeate flux (ie approximately the same residencetime and applied pressure within a membrane) the catalyticactivity of membranes with lower initial concentration washigher than that of higher initial 119901-NP concentration
Furthermore the inverse proportionality of 119901-NP con-version with permeate flux confirmed that our Pd loadedmembranes were not mass transfer limited rather it wasreaction limited If membranes were mass transfer limitedwewould not observe a decrease of conversionwith increasedfluxes As the applied pressure in flow-throughmode enforcesreactants to have an intensive contact with catalyst thereaction at the surface of the catalyst is the main limitationto such system As a result of increasing pressure (flux)reactants permeate through the membrane without reacting
6 International Journal of Chemical EngineeringC
onve
rsio
n of
NP
()
C0 = 0128 mMC0 = 0066mMC0 = 0033mM
002 004 006 008 01 012091
925
94
955
97
985
100
Permeate flux (m3 hrminus1mminus2)
Figure 6 Conversion of nitrophenol (NP) in Pd loaded PESmembrane versus flux for single pass in dead-end mode of filtration(0283mg of palladium)
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
Figure 3 The ATR-FTIR spectra of unmodified and modified PESflat sheet membranes using UV grafting of AA (25wt) at 20minUV batch irradiation time
1535
16051625
1720(a)
(b)(c)
Abso
rptio
n
01
02
03
04
05
1800 1600 1400 1200 1000 8002000Wave numbers
Figure 4 The ATR-FTIR spectra of the (a) unmodified flat sheetPES membrane and modified membranes using UV-induced graft-ing with (25 wtAA) (b) 20 and (c) 15minute UV irradiation timerespectively
can be seen UV-induced graftedmembranes exhibit differentATR-FTIR spectra than the unmodified one In addition tothe typical PES bands of the unmodified membrane theIR spectra of modified membrane show additional peak at1720 cmminus1 which corresponds to the carbonyl (C=O) groupbands of COOHwhich indicates the existence of poly(acrylicacid) chains [19] and assured that themonomer is successfullypolymerized on the substrate PES Also with UV modifica-tion some original absorbance peak intensity is decreasedthat can be contributed to increase coverage of the PESsurface by poly(acrylic acid)
Apart from this as can be seen from FTIR spectra ofmodified membrane in Figure 4 the intensity of the newpeak at 1720 cmminus1 increases with photografting reaction timewhich corresponds to the energy received during graft poly-merization The more the energy received by the monomerthe higher the degree of modification In addition to thenew absorption peak corresponding to the carbonyl group(C=O) of COOH there are also other new small absorbancepeaks appearing at 1625 1605 and 1535 cmminus1 for themodified
6 10 12 15 20
Palla
dium
amou
nt (m
g)
Graing time (min)
0
01
02
03
04
05
06
07
08
Figure 5 Palladium amount per membrane (152 cm2) versusgrafting time
membranesThe small absorbance peak for the AA-modifiedmembranes at these regions attributed to the presence ofadditives such as PVP in the original casting solution andcross linker in the monomer solution as reported by Rahim-pour [20] and Bernstein et al [21] Also the disappearance ofthe original peaks at 1640ndash1680 cmminus1 for the modified mem-branes indicates that the modification was successful
Membranes grafted at different timeswere used to synthe-size Pd nanoparticle ICP analysis in Figure 5 showed that theweight of Pd loading is increasing with grafting time (energyreceived) These data are in agreement with expected andFTIR results At higher energy the intensity of the modifiedfunctional group was stronger which can lead to better cationexchange with Pd precursor and thus higher Pd amount fromthe reduction
32 Catalytic Performance Evaluation The effect of thenitrophenol concentration in the feed on the conversionwas investigated using different feed concentrations and thesame amount of Pd at a room temperature A closer look atFigure 6 clearly shows the effect of initial119901-NP concentrationon conversion The conversion is higher for the lower 119901-NP initial concentration In all of initial 119901-NP concentrationranges (ie 0033ndash0128mM) the same trend was obtainedwhich strongly indicates that reaction on Pd nanoparticle ishighly active dependant surface For the same catalyst loadingand permeate flux (ie approximately the same residencetime and applied pressure within a membrane) the catalyticactivity of membranes with lower initial concentration washigher than that of higher initial 119901-NP concentration
Furthermore the inverse proportionality of 119901-NP con-version with permeate flux confirmed that our Pd loadedmembranes were not mass transfer limited rather it wasreaction limited If membranes were mass transfer limitedwewould not observe a decrease of conversionwith increasedfluxes As the applied pressure in flow-throughmode enforcesreactants to have an intensive contact with catalyst thereaction at the surface of the catalyst is the main limitationto such system As a result of increasing pressure (flux)reactants permeate through the membrane without reacting
6 International Journal of Chemical EngineeringC
onve
rsio
n of
NP
()
C0 = 0128 mMC0 = 0066mMC0 = 0033mM
002 004 006 008 01 012091
925
94
955
97
985
100
Permeate flux (m3 hrminus1mminus2)
Figure 6 Conversion of nitrophenol (NP) in Pd loaded PESmembrane versus flux for single pass in dead-end mode of filtration(0283mg of palladium)
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
Figure 7 Plot of 119901-NP conversion versus flux The curve (withred diamond) represents a first-order reaction model with a rateconstant (119870app) of 300 hrminus1 Feed conditions [119901-nitrophenol] =0514mM [NaBH4] = 1438mM Pd = 0733mg and catalytic layer= 0198 120583m
hence the conversion decreases Similar results were reportedby Dotzauer et al [22] and Crock et al [23]
Figure 7 shows the experimental plot points and modelpredictions according to the model equation (7) developedand it confirms well to a simple first-order kinetic modelof the reaction The proposed mathematical model equation
0 5 10 15 20 25 30Peclet number
00
02
04
06
Con
vers
ion
08
10
= 1
= 2
= 3
= 5
= 7 = 9
Figure 8 Simulation of conversion versus convective flow (Pecletnumber) at different values of reaction modulus
= 1
= 2
= 3
= 6
= 10
02 04 06 08 1000membrane thickness ()
00
02
04
06
08
10
Con
cent
ratio
n di
strib
utio
n (C
AC
A0)
Figure 9 Simulation of concentration distribution across thecatalytic layer at different values of reaction modulus as function ofmembrane thickness
represents satisfactorily the reaction and transport phenom-ena in flow-through catalytic membrane reactor From thesame figure it can be concluded that both convective anddiffusive transport through the catalytic membrane layercontributed to the catalytic activity of the nanoparticle
In order to understand better and check the effect ofdifferent parameters simulations of the FTCMR were per-formed using Wolfram Mathematica 7 at different processparameters such as Peclet number (119875119890) reaction modulus(02) catalytic membrane layer thickness (120575) and concentra-tions
Simulation results of Figures 8 and 9 showed the effectof different process parameters such as Peclet number (119875119890)reaction modulus (02) and catalytic membrane layer thick-ness (120575) on the outlet concentrations of the reactant
4 Conclusion
In this work surface modification of polyethersulfone mem-brane by UV-assisted grafting polymerization of acrylic
International Journal of Chemical Engineering 7
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
acid has been successfully done Catalytically active andefficient Pd nanoparticle has been synthesized via intermatrixsynthesis As a model for liquid phase reaction its catalyticperformance was investigated by the reduction of aqueous 119901-nitrophenol to 119901-aminophenol with sodium borohydride asreductant The catalytic activity of Pd embedded membranewas shown to be directly proportional to the palladiumcontent in the nanocomposite The catalytic activity of flow-through reactor for reduction of nitrophenol outperformedthe batch mode of operation as it was demonstrated byconversion comparison at the same initial nitrophenol con-centration and weight of catalyst This was attributed tothe convective flow of reactants directly to the catalyst siteswhich can provide an intensive contact An important asset inapplication of catalyst embedded membrane is the possibilityof varying the flux to control the conversion At lowerand moderate range of fluxes a complete conversion wasachieved in FTCMR but an increase in the flux decreased theconversion because of insufficient contact time The effect ofinitial nitrophenol concentration at the same flux and catalysthas been investigated In all concentration ranges taken alower initial concentration had higher conversion as higherconcentration poses surface coverage of the catalyst Furtherinvestigation of 119901-nitrophenol reduction reactions related toporosity of the grafted layer has to be done The proposedmathematical model equation represents satisfactorily thereaction and transport phenomena in flow-through catalyticmembrane reactor
Conflicts of Interest
The authors would like to declare that there are no conflictsof interest regarding the publication of this paper
References
[1] A Julbe D Farrusseng and C Guizard ldquoPorous ceramicmem-branes for catalytic reactors - Overview and new ideasrdquo Jour-nal of Membrane Science vol 181 no 1 pp 3ndash20 2001
[2] T Westermann Flow-Through Membrane Microreactor forIntensified Heterogeneous Catalysis 2009
[3] T Westermann and T Melin ldquoFlow-through catalytic mem-brane reactorsmdashprinciples and applicationsrdquo Chemical Engi-neering and Processing Process Intensification vol 48 no 1 pp17ndash28 2009
[4] R Dittmeyer V Hollein and K Daub ldquoMembrane reactors forhydrogenation and dehydrogenation processes based on sup-ported palladiumrdquo Journal of Molecular Catalysis A Chemicalvol 173 no 1-2 pp 135ndash184 2001
[5] S S Ozdemir M G Buonomenna and E Drioli ldquoCatalyticpolymeric membranes Preparation and applicationrdquo AppliedCatalysis A General vol 307 no 2 pp 167ndash183 2006
[6] K K Sirkar P V Shanbhag and A S Kovvali ldquoMembrane ina reactor A functional perspectiverdquo Industrial amp EngineeringChemistry Research vol 38 no 10 pp 3715ndash3737 1999
[7] Y Zhang K Su F Zeng W Ding and X Lu ldquoA novel tubularoxygen-permeable membrane reactor for partial oxidation ofCH4 in coke oven gas to syngasrdquo International Journal ofHydrogen Energy vol 38 no 21 pp 8783ndash8789 2013
[8] M A Al-Juaied D Lafarga and A Varma ldquoEthylene epoxida-tion in a catalytic packed-bed membrane reactor Experimentsand modelrdquo Chemical Engineering Science vol 56 no 2 pp395ndash402 2001
[9] M Pera-Titus M Fridmann N Guilhaume and K FiatyldquoModelling nitrate reduction in a flow-through catalytic mem-brane contactor Role of pore confining effects on water vis-cosityrdquo Journal of Membrane Science vol 401-402 pp 204ndash2162012
[10] E Nagy ldquoMass transfer through a convection flow catalyticmembrane layer with dispersed nanometer-sized catalystrdquoIndustrial amp Engineering Chemistry Research vol 49 no 3 pp1057ndash1062 2010
[11] V V Pushkarev Z Zhu K An A Hervier and G A SomorjaildquoMonodisperse metal nanoparticle catalysts Synthesis charac-terizations and molecular studies under reaction conditionsrdquoTopics in Catalysis vol 55 no 19-20 pp 1257ndash1275 2012
[12] KNa Q Zhang andG A Somorjai ldquoColloidalMetal Nanocat-alysts Synthesis Characterization and Catalytic ApplicationsrdquoJournal of Cluster Science vol 25 no 1 pp 83ndash114 2014
[13] P Ruiz M Munoz J MacAnas C Turta D Prodius and DN Muraviev ldquoIntermatrix synthesis of polymer stabilized inor-ganic nanocatalyst with maximum accessibility for reactantsrdquoDalton Transactions vol 39 no 7 pp 1751ndash1757 2010
[14] J Bastos-Arrieta A Shafir A Alonso M Munoz J MacAnasandDNMuraviev ldquoDonnan exclusion driven intermatrix syn-thesis of reusable polymer stabilized palladium nanocatalystsrdquoCatalysis Today vol 193 no 1 pp 207ndash212 2012
[15] J Macanas L Ouyang M Bruening MMunoz J Remigy andJ Lahitte ldquoDevelopment of polymeric hollow fiber membranescontaining catalytic metal nanoparticlesrdquo Catalysis Today vol156 no 3-4 pp 181ndash186 2010
[16] LOuyangDMDotzauer S RHogg JMacAnas J-F Lahitteand M L Bruening ldquoCatalytic hollow fiber membranes pre-pared using layer-by-layer adsorption of polyelectrolytes andmetal nanoparticlesrdquo Catalysis Today vol 156 no 3-4 pp 100ndash106 2010
[17] C Emin J-C Remigy and J-F Lahitte ldquoInfluence of UVgrafting conditions and gel formation on the loading and stabi-lization of palladium nanoparticles in photografted polyether-sulfonemembrane for catalytic reactionsrdquo Journal of MembraneScience vol 455 pp 55ndash63 2014
[18] C Endalkachew and J-F Lahitte Development of GasLiquidCatalytic Membrane Reactor University of Zaragoza LibraryOnline Thesis Catalog 2014
[19] B Deng J Li Z Hou et al ldquoMicrofiltration membranes pre-pared from polyethersulfone powder grafted with acrylic acidby simultaneous irradiation and their pH dependencerdquo Radia-tion Physics and Chemistry vol 77 no 7 pp 898ndash906 2008
[20] A Rahimpour ldquoUV photo-grafting of hydrophilic monomersonto the surface of nano-porous PESmembranes for improvingsurface propertiesrdquo Desalination vol 265 no 1-3 pp 93ndash1012011
[21] R Bernstein E Anton and M Ulbricht ldquoUV-photo graftfunctionalization of polyethersulfone membrane with strongpolyelectrolyte hydrogel and its application for nanofiltrationrdquoACS Applied Materials amp Interfaces vol 4 no 7 pp 3438ndash34462012
8 International Journal of Chemical Engineering
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
[22] D M Dotzauer J Dai L Sun and M L Bruening ldquoCatalyticmembranes prepared using layer-by-layer adsorption of poly-electrolytemetal nanoparticle films in porous supportsrdquo NanoLetters vol 6 no 10 pp 2268ndash2272 2006
[23] C A Crock A R Rogensues W Shan and V V TarabaraldquoPolymer nanocomposites with graphene-based hierarchicalfillers as materials for multifunctional water treatment mem-branesrdquoWater Research vol 47 no 12 pp 3984ndash3996 2013
