Keywords:Catalytic membrane reactor; Catalytic diffuser; Forced through flow; Multiphase reactions; Catalytic nitrate reduction;Dechlorination of chlorinated hydrocarbons
1. Introduction
1.1. Catalytic diffuser concept
Catalytically active ceramic membranes can be ap-plied for three-phase reactions (one liquid reactant,one gaseous reactant, and solid catalyst). The catalyt-ically active components are deposited on the porewalls of the thin fine-porous membrane layer of an
asymmetrical ceramic membrane that is commerciallyavailable and normally used for micro- or ultrafiltra-tion processes. One reactant is dissolved in the liquidphase and is sucked into the fine-porous catalytic layerby capillary forces. The reactant diffuses through theporous structure of the membrane to the catalytic sites.The gaseous reactant is fed through the support to thecatalytic layer from the other side of the membrane.Thus, the two reactants approach the catalytic layerfrom opposite sides. The gas–liquid phase boundary isdetermined by the pressure difference between the gasand the liquid side. The pressure on the liquid side can
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Fig. 1. Working principle of the catalytic diffuser and the forced through flow concept and concentration/pressure gradients in the membrane.
be atmospheric pressure, on the gas side there is anelevated pressure. The gas pressure is above the bub-ble point of the support so that water in there wouldbe forced out, but does not exceed the bubble point ofthe fine-porous membrane layer. This membrane re-actor concept for gas–liquid reactions is called “cat-alytic diffuser”. Fig. 1 (left) shows an SEM pictureof an asymmetric ceramic (�-Al2O3) membrane witha coarse-porous support and a fine-porous membranelayer on top and demonstrates the working principle ofthe catalytic diffuser. In contrast to a conventional cat-alyst particle, where the two reactants (gas and liquid)have analogous concentration profiles, the concentra-tion profiles in the catalytic diffuser are reverse. Thepressure in the membrane changes from high pressureon the gas side including the support to low or atmo-spheric pressure in the fine-porous catalytic layer andreactor.
The limiting value of 50 mg/l for nitrate in groundwater is exceeded in many regions of Europe. The in-creasing content of nitrate in drinking water involveshealth risks and is mainly caused by agriculturalover-fertilization and emissions of nitrogen oxides.Nitrate and nitrite can be removed from drinking wa-ter by the so called “catalytic nitrate reduction”. Thisprocess was first described by Vorlop and Tacke in1989[2]. Nitrite is an intermediate product in the ni-trate reduction process to nitrogen. Nitrite is reduced
to nitrogen on a supported noble metal catalystswith hydrogen as reducing agent. As an undesiredbyproduct ammonium is formed. For the catalyticnitrite reduction supported palladium as catalyst ispreferably used. For the reduction of nitrate to nitritea second metal, preferably tin, copper or indium isessential.Eqs. (1)–(3)illustrate the stoichiometry ofthese reactions:
NO3− + H2 → NO2
− + H2O (1)
2NO2− + 3H2 → N2 + 2OH− + 2H2O (desired)
(2)
2NO2− + 6H2 → 2NH4
+ + 4OH− (undesired)
(3)
An optimized catalyst for the catalytic nitrate reduc-tion should have bimetallic clusters (e.g. Pd/Sn) andmonometallic palladium clusters side by side on a sup-porting material[3,4]. The activity and selectivity ishighly dependent on the reaction conditions, mainly onthe pH-value of the reaction solution. A high pH-valueleads to a low activity and a high ammonium forma-tion. In a more acidic medium the activity is higherand less ammonium is produced, thus, the selectiv-ity towards nitrogen is higher. Different materials likealumina, zirconia, silica, etc. can be used as support
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for the catalytic metals. The support must provide alarge inner surface for the deposition of the catalyt-ically active metals and should be stable against at-trition and hydrolysis[1]. Furthermore, experimentsby several different research groups have shown thatactivity and selectivity is also strongly influenced bythe particle size of the catalyst[5–7]. The particle sizemust be small enough to enable short diffusion path-ways. If the diffusion inside the catalyst particle istoo slow in comparison to the reaction, the producedhydroxyl ions are not neutralized or transported awayfrom the catalytic sites fast enough. As a consequence,there can be a local pH-increase in the catalyst parti-cle which again results in poor catalyst activity and anincreased ammonium formation. This can be avoidedby using an extremely fine catalyst powder of a fewmicrons. But in a technical process it is very difficultto handle and recover such a fine powder catalyst.
The catalytic diffuser concept was initially devel-oped to solve this problem. On an asymmetric ceramicmembrane the catalytic metals can be deposited in thethin fine-porous top layer of the membrane. Thus, thecatalyst is immobilized and no separating process forthe catalyst from the reaction solution is necessary.
On the one hand, the membrane is the catalyst sup-port, on the other hand it also functions as a dosingdevice for hydrogen. The hydrogen is dosed directly tothe catalyst, where it is consumed, and does not needto be fed to the reaction solution in an additional hy-drogen saturator. This is especially advantageous be-cause the hydrogen solubility in water is very low. Thetwo reactants approach the catalytic membrane layerfrom opposite sides. Thereby, an effective contactingbetween the reactants and the catalyst is achieved.
If tubular membranes with an inorganic membranelayer (Al2O3, ZrO2, TiO2, carbon) on the outside areused, it is possible to work with a high gas pressure in-side the membrane, while the reactor itself can be op-erated at atmospheric pressure. This leads to a simpli-fied process design, and overall, to a safer and cheaperprocess.
By varying the gas pressure inside the membrane itis also possible to control the gas–liquid transition, i.e.the gas uptake into the liquid and the transfer to the cat-alyst and to some extent the position of the gas–liquidphase boundary. Thereby, the catalyst activity can beadjusted to the varying needs of the reaction with theH2-pressure in the membrane.
The membrane enables very short diffusion path-ways, similar to the condition in a powder catalyst, asthe catalytically active metals are deposited in a thinfine-porous membrane layer of about 20�m or less. Inthe case of the catalytic nitrate reduction this is favor-able for a high activity and a high selectivity towardsnitrogen.
Into the membrane a mixture of hydrogen and an-other gas, for example, carbon dioxide, can be fed.The carbon dioxide dissolves physically in water andsome part of the carbon dioxide reacts with water tocarbonic acid. The carbonic acid functions as a bufferand is able to neutralize hydroxyl ions formed duringreaction, pH-gradients in the catalytic membrane layercan thereby be further suppressed.
1.2. Forced through flow concept
In the catalytic diffuser, even with a very thin cat-alytic membrane layer on top of a ceramic support,mass transport limitation by pore diffusion can occurif the reaction is very fast. Furthermore, the additionof carbon dioxide can only decrease pH-gradients inthe membrane layer, but does not change the con-centration gradients of the reactants (nitrate, nitrite,ammonium, hydrogen). By using a very fine catalystpowder it is possible to minimize these concentra-tion gradients, but such fine powders are difficult tobe separated and recovered on a technical scale. An-other way to minimize the pore diffusion is to pumpthe whole reaction solution through an asymmetric ce-ramic membrane or a ceramic support coated with cat-alytically active metals. This idea was also suggestedby Ilinitch et al. [8,9] and Lüdtke[10]. By adjustingthe flow rate of the pump the contact time at the cata-lyst can be varied and very short contact times can beachieved.
If the reaction solution is pumped through the mem-brane fast enough, pore diffusion can be totally elim-inated, so that there are no concentration gradients inthe pore system of the catalytic layer. This is illus-trated inFig. 1 (right).
As the reaction is not fast enough to achieve a to-tal conversion with one pass through the membranea product recycle is necessary, i.e. the reaction solu-tion must be pumped through the membrane severaltimes. As the reactants are transported to the catalystagain and again, a very effective contacting between
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reactants dissolved in the reaction solution and thesolid catalyst on the pore walls is accomplished.
A ceramic membrane or a support coated with cat-alytically active metals behave like a microreactorwith very small channel diameters and thereby rel-atively high contact surface. Moreover, compared toconventional microreactors no expensive microstruc-turing techniques are needed as the membrane or themembrane support already contain small channels.
2. Experimental
2.1. Preparation of catalytically active ceramicmembranes
2.1.1. Palladium-chemical vapor depositionCommercially available ceramic ultra- and microfil-
tration membranes made of different ceramic materialswere used as starting material. In our lab experimentswe mainly used tubular ceramic membranes with anouter diameter of 10 mm, an inner diameter of 7 mmand a length of about 10 cm, and with the membranelayer on the outside of the membrane. The support isusually made of�-alumina and has pores in the rangeof 3�m in diameter. The membrane layer consists of�-Al2O3, �-Al2O3, ZrO2 or TiO2, and the pore diame-ters of the membrane layer vary between 5 and 400 nmdepending on the material. The membranes were ob-tained from HITK e.V., Hermsdorf (Germany). Forhydrogenation processes we coated these membraneswith palladium.
For this purpose we developed a special metal-organic chemical vapor deposition (MOCVD) pro-cess with palladium(II)-hexafluoroacetylacetonate(Aldrich 401471) as palladium precursor. With thisnew technique it is possible to achieve a uniform de-position of palladium in the form of highly dispersedmetal clusters on inert ceramic surfaces that containno active sites. The penetration depth of the palladiuminto a ceramic membrane can be controlled, thus it ispossible to coat the top layer of a membrane only. Alsoceramic supports without any top membrane layer canbe coated by this method. The bonding between sup-port and metal clusters is very stable, thereby any lossof the expensive noble metal palladium by abrasioncan be avoided. A more detailed description of thedeveloped palladium MOCVD preparation technique
will be disclosed soon in a German patent applicationwhich is currently in progress.
2.2. Characterization of catalytically activeceramic membranes
Various catalyst characterization methods are ap-plicable for the characterization of catalytically ac-tive membranes. The pore sizes of the membranelayer are measured with mercury porosimetry andcapillary flow porometry. The cluster sizes of the de-posited catalytic metals can be determined with XRD,CO-pulse-chemisorption and TEM. SEM, TEM andAFM methods are used to analyze pore and mem-brane structure. EPMA (electron probe microanal-ysis)/WDX (wavelength-based detection) is used todetermine the metal distribution over the membranelength and into the membrane layer. Typical resultsof TEM and EPMA/WDX characterization will bedemonstrated here as example.
2.2.1. TEM-characterizationBy TEM-characterization of the coated membranes
it is possible to obtain an impression how the de-posited metals, especially palladium deposited withour CVD-method, are present on the ceramic mem-brane. Thus, we can make a statement about the qual-ity of the developed CVD-technique. For the analysisthe top membrane layer needs to be scratched off. It issuspended in an organic solvent, e.g. ethanol or ace-tone, and then ground in an ultrasonic bath to a particlesize suitable for the investigation in the transmissionelectron microscope. Thereafter, one drop of this sus-pension is given on a copper net that has a carbon filmon one side. The organic solvent evaporates off andthe sample can be investigated in the microscope. Forthe TEM-characterization we used a Philips EM420transmission electron microscope. If the ceramic par-ticles are in the range of 50–100 nm the electron beamis still able to pass through the ceramic material.Fig. 2shows palladium clusters on an�-Al2O3 particle. Theparticle was taken from the top membrane layer of anasymmetric�-Al2O3-membrane that was coated withPd according to the description inSection 2.1.1. Wecan see that the palladium clusters sit on the aluminain the form of hemispheres. The clusters are connectedwith the ceramic and the bond seems to be very strong.No loose particles can be found in the TEM picture.
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Fig. 2. TEM-photograph of palladium clusters on an�-Al2O3-particle. The top membrane layer of an asymmetric membrane wascoated with Pd (MOCVD-process), the layer was scraped off fromthe membrane and prepared for TEM-characterization. Pd-clustersare firmly attached to the�-Al2O3 particle.
Fig. 3. TEM-photograph of Pd-clusters on ZrO2-top membrane layer and appendant cluster size distribution of Pd-clusters. Depositiontechnique: Pd-MOCVD. Pore size of the top ZrO2-layer: 110 nm. Thickness of the top layer: 20�m. Small particles represent Pd-clusters.Large particles represent ZrO2 from top layer. Average palladium cluster size: 7 nm.
In the catalytic experiments in our reactor no abrasionof palladium was observed and no leaching of the pal-ladium should occur during the reaction.
TEM-characterization can also be used to deter-mine the size of the palladium clusters.Fig. 3shows aTEM-photograph of a powder sample scraped off froma top layer made of zirconia that was CVD-coatedwith palladium. The average pore diameter of theZrO2 membrane layer was 110 nm. The larger chunksin the TEM-photograph are ZrO2 particles and thesmall black dots are palladium clusters. A pictureanalysis of the TEM-photograph leads to a clustersize distribution of the Pd-clusters. As a conclusionit can be said that the MOCVD process describedin Section 2.1.1can produce palladium clusters withan average cluster size of about 7 nm on a zirconialayer.
2.2.2. Electron probe microanalysisThe distribution of deposited palladium over the
membrane is determined by EPMA (Cameca SX50)with WDX. Cross-sectional cuts of the membranewere prepared by metallographic methods. The dis-tribution of the catalytic metal is very important as itstrongly influences the catalytic properties of the cat-alytically active membrane. For the catalytic diffuser
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Fig. 4. BSE picture of an asymmetric membrane. Top layer: ZrO2 with 110 nm average pore size. Thickness 30�m. Intermediate layerand support:�-Al2O3. EPMA gives the metal distribution for Zr, Al, and Pd. Pd was deposited into membrane layer with MOCVD. Pd ismainly deposited in the top ZrO2-layer. Deposition of Pd into the ceramic support can be avoided and the Pd-concentration is relativelyconstant over the whole ZrO2-layer.
concept palladium should be deposited only in thetop membrane layer, not in the support. The concen-tration over the whole top layer should be constant.Fig. 4 shows an EPMA for an asymmetric ceramicmembrane consisting of an�-Al2O3-support, an in-termediate�-Al2O3-layer with some smaller particlesand a ZrO2-top layer with an average pore size of110 nm. This structure can also be seen in the backscattered electron (BSE) photograph inFig. 4. Themetal distribution for zirconium and aluminum showswhere the top zirconia layer and the intermediateand support�-alumina layer are located. Palladiumis almost only located in the ZrO2-layer. Only smallconcentrations of Pd can be found in the intermedi-ate layer. The Pd-concentration over the top layer isrelatively constant except for a slight rise towards theZrO2/Al2O3 transition.
2.3. Catalytic membrane and reactor:experimental setup
For our experiments we use asymmetric tubular ce-ramic membranes with an outer diameter of 10 mmand an inner diameter of 7 mm and a length of about10 cm. The fine-porous membrane layer is on the out-side of the membrane. Thus, the outer membrane areaavailable in a typical experiment is about 31.4 cm2.The ends of the coated catalytically active membraneare glued with epoxy resin. It is then built into a mem-brane holder, where the membrane is clamped betweentwo silicon rubbers. The epoxy resin is necessary inorder to harden the ends of the membrane, otherwiseparts of the membrane layer would come off when themembrane is pressed between the two silicon rubbers.The membrane holder has a 6 mm connector for gas
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Fig. 5. Double-walled glass reactor with catalytically active ce-ramic membrane. The tubular membrane is built into a membraneholder clamped between two silicon rubbers.
supply. If the membrane is immersed in water and isfree of defects, a pressure up to 15 bar is possible in-side the membrane with no gas bubbles being visible.
The catalytic properties were tested in a stirred-tankmembrane reactor in a batchwise mode.Fig. 5shows aphotograph of the membrane in the reactor. The mem-brane in the membrane holder is immersed in the re-action solution (VR = 750 ml) which is stirred by amagnetically coupled blade stirrer. Reaction solutionswere made with pure deionized water. Hydrogen and
Fig. 6. Experimental setup for the catalytic diffuser and the forced through flow concept. In the catalytic diffuser setup hydrogen andcarbon dioxide are fed into the membrane at an elevated pressure. In the forced through flow setup the reaction solution is pumped throughthe membrane support and the gases are dosed into the reaction solution. The concentrations of H3O+, H2 and CO2 are measured withelectrochemical sensors. With an ion chromatograph the concentration of the dissolved ions, NO2
− and NH4+, is determined.
carbon dioxide are fed to the membrane with two ther-mal mass flow controllers allowing the H2:CO2-ratioto be set. An electronic upstream pressure controllerkeeps the pressure in the membrane constant. For atypical experiment we have atmospheric pressure inthe reactor and an overpressure of 5 bar in the mem-brane. The reaction scheme for the catalytic diffuserconcept can be seen inFig. 6 (left).
The reactor vessel consists of a double-walledglass jacket fixed between two metal flanges (Buechiglasuster). By a temperature-controlled shell-side liq-uid the temperature in the reactor is regulated. Thereactor is equipped with electrochemical hydrogenand carbon dioxide sensors to detect the concentra-tion of dissolved hydrogen and carbon dioxide inthe liquid. The pH of the solution can be measuredwith a pH-electrode or can be kept constant duringthe reaction by a pH-controller with a hydrochloricacid dosing unit. A small bypass stream is pumpedcontinuously through the injection valves of two ionchromatographs (Metrohm IC 732/733 with columnMetrosep Anion Dual 1 and IC690 with columnMetrosep Cation 1-2) to analyze the anions (nitrate,nitrite) and the cations (ammonium) in the solution.An analysis is made automatically every 20 min.
As this paper wants to make a comparison betweenthe two introduced concepts—catalytic diffuser andforced through flow—we decided to take the reduc-tion of nitrite as test reaction. For the reduction ofnitrite only palladium as catalytic metal is necessary.No bimetallic Pd/Sn sites next to monometallic sitesin a certain ratio and in a certain arrangement on the
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membrane surface are needed. Thus, all influences thatcome from the preparation of bimetallic Pd/Sn areeliminated. To characterize the catalytic performanceof the membranes we will display the concentration ofnitrite over time as a measure for the catalyst activityand the molar selectivity towards ammonium (Eq. (6),index “0” denotes the start of the reaction):
SNH4+ = cNH4
+ − cNH4+,0
cNO2−,0 − cNO2
−(6)
The unwanted byproduct ammonium is measured,since it is very difficult to determine the concentrationof the desired product nitrogen dissolved in water. As-suming that no other products are generated the nitro-gen selectivity can be calculated (SN2 = 1 − SNH4
+ ).Several experiments showed that N2O can be foundas intermediate product but it will finally react to N2.
For the evaluation of the forced through flow con-cept the reaction solution was pumped through themembrane using a gear pump with variable flow rate(Ismatec MS-Z with pump head 181; gear material:graphite). The flow rate through the membrane wasdetermined with a conventional rotameter. Hydrogenand carbon dioxide are dosed to the reaction solutionthrough a sintered metal filter.Fig. 6(right) shows theschematic reaction setup for the forced through flowconcept.
Besides the catalytic nitrite reduction the two mem-brane reactor concepts are also compared for the cat-alytic dechlorination of chlorinated hydrocarbons inthis publication. In order to investigate the dechlorina-tion of chlorinated hydrocarbons on catalytically ac-tive membranes we chose chloroform reduction as atest reaction. To measure the concentration of chloro-form in water we take a 5 ml sample out of the reactorat certain time intervals; the chloroform is extractedwith 5 ml pentane and then an analysis is made in agas chromatograph (Carlo Erba GC6000 with ECD;column DB624 30 m).
First the pH-dependency of the catalytic reductionof nitrite to nitrogen was investigated. The measure-
ment was carried out at 20◦C. Into the membrane hy-drogen was fed at an overpressure of 5 bar. Duringthe reaction hydroxyl ions are released according toEqs. (2) and (3), so that the pH-value would rise if nomeasures are taken against it. In the following exper-iment the pH was kept constant at certain values byautomatic dosing of a 0.1 mol/l hydrochloric acid tothe reaction solution. The nitrite concentration and theselectivity towards ammonium over time for three dif-ferent pH-values are shown inFig. 7. The membraneused was an asymmetric�-Al2O3-membrane with afine-porous membrane layer on the outside that wascoated with the described CVD-method with 23.1 mgpalladium. The average pore diameter of the top mem-brane layer was 100 nm. Activity and selectivity arestrongly dependent on the pH-value of the reaction so-lution. At pH 5 there is still a high selectivity to am-monium of about 64%, at pH 4 the selectivity towardsammonium is suppressed to values below 10%, conse-quently the selectivity towards nitrogen is above 90%.Furthermore, the reactions gets significantly faster. Atthe same time the reaction order seems to change. AtpH 3 the reaction apparently is independent of nitriteconcentration (zero-order reaction).
3.2. Buffering with carbon dioxide—dependencyon H2:CO2-ratio
The hydroxyl ions released during nitrite reductioncan be neutralized by HCl-dosing. Another way tokeep the pH-value of the solution constant is to feeda mixture of hydrogen and carbon dioxide into thetubular membrane instead of pure hydrogen. The CO2dissolves physically in the water and reacts to car-bonic acid that neutralizes the formed hydroxyl ions.Thereby, even without acid dosing the pH of the reac-tion solution can be kept constant. Also the nitrogenselectivity is increased with CO2-dosing from the in-side of the membrane, compared to HCl-dosing fromthe outside, because the carbon dioxide is fed directlyto the catalytic membrane layer where hydroxyl ionsare formed and buffers the solution at the catalyticsites and thus, a local pH-increase in the porous mem-brane layer can be better avoided.
Fig. 8shows the results of an experiment where theH2:CO2 molar ratio was varied over a wide range anddemonstrates what activities and selectivities can beobtained also with extreme ratios (up to H2:CO2 =
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Fig. 7. pH-dependency of catalytic nitrite reduction. Activity and selectivity is strongly dependent on the pH-value of the reaction solution.Membrane: asymmetric, tubular, ceramic�-Al2O3-membrane with membrane layer on the outside. Average pore diameter of membranelayer: 100 nm. Palladium (23.1 mg) deposited (CVD-method) into top membrane layer. Hydrogen in membrane at 5 bar overpressure.Reactor at atmospheric pressure. Temperature of reaction solution: 20◦C. pH kept constant by automatic hydrochloric acid dosing. Reactionvolume: 750 ml.
Fig. 8. Dependency of catalytic nitrite reduction on H2:CO2-molar ratio inside membrane. Membrane: asymmetric, tubular, ceramicmembrane. Support:�-Al2O3 (3�m average pore diameter). Membrane layer: ZrO2 (110 nm average pore diameter) with 18.6 mg Pd;5 bar overpressure in membrane. Temperature: 20◦C. Reaction volume: 750 ml.
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1:700). For this experiment an asymmetric, tubular,ceramic membrane with a top membrane layer madeof ZrO2 (110 nm average pore diameter) was used. Ex-periments showed that zirconia as membrane materialgives slightly higher activity and N2-selectivity for thecatalytic nitrite/nitrate reduction in comparison withalumina. Palladium (18.6 mg) was deposited in the topmembrane layer. The gas inside the membrane wasat 5 bar overpressure and the temperature of the reac-tion solution was 20◦C. On the left side of the graphin Fig. 6 the nitrite concentration, on the right sidethe ammonium selectivity is shown. With increasedCO2-concentration the NH4+-selectivity is more andmore repressed. That means that the nitrogen selectiv-ity increases with increasing CO2-concentration fedto the membrane. Several effects might be responsiblefor this. With increasing CO2-concentration also thepH-value in the bulk changes. At a H2:CO2-ratio of3:2 the pH-value during the reaction stays relativelyconstant at 5.5 after a short initializing period. With aH2:CO2-ratio of 1:7 and 5:70 the pH-value proceedsbetween 4.8 and 4.5, at H2:CO2 = 1:70 the pH-valuesstarts at 4 and rises to 4.5 with proceeding reaction andat H2:CO2 = 1:700 it runs near pH 4. Furthermore, thebuffering within the porous membrane structure mightget better with increasing CO2-concentration. The car-bon dioxide might also change the catalytic palladiumsurface, or the hydroxyl ions might be more quickly re-moved from the catalytic sites with the CO2-transportinto the reaction solution. The reduction of nitrite isat first accelerated with increasing CO2-fraction. Theactivities at the H2:CO2-ratios 1:7 and 5:70 are almostthe same (seeFig. 6). If the hydrogen concentrationis further reduced, the nitrite reduction gets sloweragain, because not enough hydrogen is available anymore for the reaction. But even with a H2:CO2-ratioof 1:700 nitrite is still reduced.
3.3. Forced through flow at different flow ratesand comparison with catalytic diffuser
In our first forced through flow experiment, we tooka regular asymmetric�-Al2O3-membrane with a toplayer on the outside that was coated with palladium.We pumped the reaction solution from the inside to theoutside of the membrane. The initial flow rate throughthe membrane (length: 10 cm) was 54 ml/min. After afew hours the flow decreased gradually until no wa-
ter could be pumped through the membrane any more.Inside the membrane on top of the support, we foundthat small particles plugged the membrane support.During the reaction the filter cake grew until the pres-sure drop through it got too high. Repetition of thisexperiment showed that the pores of the membrane al-ways get very easily plugged, if the reaction mixtureis pumped from the inside to the outside of the mem-brane. In order to solve this problem we decided totake a ceramic support without any membrane layeron top. Thereby, we would also have a higher flow ratethrough the membrane. The surface of the support isstill high enough to deposit 20 mg palladium in a highdispersion. Next, the flow was reversed, i.e. the waterwas sucked from the outside to the inside of the mem-brane (seeFig. 6, right). On the retentate side therewas a turbulent flow from the magnetic stirrer in thereactor. Several experiments showed that the forcedthrough flow operation can be maintained, if the waterflows from the outside to the inside of the membraneand if there is a sufficient cross-flow on the retentateside. Then, no plugging of the membrane pores willoccur.
Fig. 9 demonstrates the performance of the forcedthrough flow with different flow rates through themembrane. For this experiment a ceramic�-Al2O3-support with 3�m average pores in diameter wascoated with 18.6 mg palladium. The stirrer in the re-action solution provided the necessary turbulent flowon the retentate side. A H2/CO2 gas mixture witha H2:CO2 molar ratio of 3:2 was bubbled into thereaction solution.Fig. 7 shows that the activity andselectivity is dependent on the flow rate through themembrane. At very low flow rates the catalyst activ-ity is not so high. With an increased flow rate theactivity gets higher until further increases of the flowdo not change the activity significantly any more.In Fig. 9 it can be seen that for a flow rate of 68and 145 ml/min through the ceramic support (length:10 cm) almost the same activities are obtained. If welook at the selectivities we can see that also the am-monium formation is dependent on the flow rate. Theshorter the contact time is in the porous support, theless ammonium is produced.
The performance of the ceramic support operatedin the forced through flow mode is also comparedwith the catalytic diffuser inFig. 9. For the catalyticdiffuser experiment an asymmetric, tubular, ceramic
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Fig. 9. Forced through flow performance at different flow ratesthrough the membrane in comparison with catalytic diffuser forthe catalytic nitrite reduction. For the forced through flow exper-iment a ceramic support (�-Al2O3) with average pore diameterof 3�m coated with 18.6 mg Pd (MOCVD-technique) was used.Water was externally saturated with gas (H2:CO2-ratio = 3:2)at atmospheric pressure. For the catalytic diffuser experiment anasymmetric, tubular, ceramic membrane with a ZrO2-top mem-brane layer with also 18.6 mg Pd was used; 5 bar overpressurein membrane. H2:CO2-ratio = 3:2. Temperature: 20◦C. Reactionvolume in both experiments: 750 ml.
membrane with a ZrO2-top membrane layer (110 nmaverage pore diameter) with the same amount of pal-ladium (18.6 mg Pd) as in the forced through flow ex-periment was used. A H2/CO2 gas mixture (H2:CO2 =3:2) was fed to the inside of the membrane at an over-pressure of 5 bar.
By quickly passing through the membrane severaltimes, mass transport limitation by pore diffusion canbe avoided and the contact between the reactants dis-solved in the reaction solution and the catalyst in theporous support is intensified. As a consequence, theforced through flow concept shows a higher catalystactivity. The forced through flow concept also shows alower ammonium formation compared to the catalyticdiffuser, because a local pH-increase and concentra-tion gradients in the porous structure can be elimi-nated.
3.4. Forced through flow and catalytic diffuserin comparison for dechlorination of chlorinatedhydrocarbons
Chlorinated hydrocarbons have been widely utilizedin the last few decades as solvent for cleaning pro-cesses, as extraction fluid, as heat transfer fluid, ascoolant, as pesticides and for many other applications.Because of their high stability towards biodegradationand the unconcerned dealing with the problem, toxicchlorinated hydrocarbons can be found nowadays inmany places as contamination in soil and ground wa-ter.
Chlorinated hydrocarbons can be dechlorinatedin a catalytic reaction activated with hydrogen on asupported palladium catalyst[11] in order to reducetheir toxicity and their persistence in the environ-ment.
To investigate the catalytic dechlorination of chlori-nated hydrocarbons on catalytically active membraneswe chose chloroform reduction as a test reaction. Onthe catalytic membranes coated with palladium, chlo-roform is reduced with hydrogen to methane according
Fig. 10. Catalytic diffuser and forced through flow in comparisonfor the dechlorination of chloroform. Catalytic diffuser membrane:tubular, asymmetric ceramic membrane with�-Al2O3 top layerwith 100 nm average pore diameter, 15 mg Pd. Forced throughflow membrane:�-Al2O3-support with average pore diameter of3�m coated with 18.4 mg Pd. Flow of reaction solution throughsupport: 125 ml/min. Initial concentration: 30 mg CHCl3 in 1 lwater, reaction volume: 750 ml.
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to the following equation:
CHCl3 + 3H2 → CH4 + 3H+ + 3Cl− (7)
Hydrochloric acid is formed as side-product.Fig. 10 shows a comparison between the catalytic
diffuser and the forced through flow for the dechlo-rination of chloroform. In the graph the chloroformconcentration over time is displayed for the two exper-iments. Also here the activity for the forced throughflow is significantly higher than with the catalytic dif-fuser, because mass transport effects are eliminatedand by the pumping through the porous catalytic struc-ture the contacting is intensified between catalyst anddissolved reactants and thereby, a process intensifica-tion is achieved.
4. Module design and conclusion
Catalytic diffuser and forced through flow are bothconcepts using catalytically active ceramic membranesfor gas–liquid-reactions. The two concepts have ad-vantages and disadvantages. Forced through flow oper-ation showed a higher catalyst activity for the catalyticnitrite reduction and the dechlorination of chloroformand a higher nitrogen selectivity in the case of catalyticnitrite reduction due to the minimization of transporteffects and an intensified contacting between reactantsand the catalytic metal. On the other hand potentialmembrane blocking and a higher energy consumptionof the forced through flow operation can be a problem.
With different applications the catalytic diffusermight be more advantageous. In a European researchproject the synthesis of hydrogen peroxide from hy-drogen and oxygen with the catalytic diffuser conceptis currently being studied. The hydrogen is fed to theinside of the membrane, and oxygen dissolved in waterapproaches the catalytic layer from the outside. Initialtests showed that with the catalytic diffuser conceptH2O2 can be produced. The forced through flow con-cept in that case is not applicable, as it would probablyonly produce water from hydrogen and oxygen.
The process design for the catalytic diffuser is alsorelatively simple as no cross-flow needs to be es-
tablished in order to avoid membrane blocking. Forthe module design commercially available ceramicmulti-channel filtration elements could be used. Sev-eral of these elements can be combined to a module.
For the forced through flow concept a sufficientlyhigh cross-flow is necessary to avoid pore blocking andmaintain operation. This high turbulent flow involvesa high energy consumption. A suitable module forthe forced through flow concept might be designedwith rotating filter disks as they provide very highcross-flow speeds at a considerably reduced energyconsumption compared to conventional systems thatcreate the necessary cross-flow by recirculating largefluid volumes.
Acknowledgements
The authors greatly appreciate financial support forthis project by the German Federal Ministry of Eco-nomics and Labour provided through the German Fed-eration of Industrial Cooperative Research Associa-tions “Otto von Guericke” under project no. 12413 N.
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