Catalytic Upgrading of Methane to Higher Hydrocarbon in aNonoxidative Chemical ConversionShaima Nahreen,† Supareak Praserthdam,‡ Saul Perez Beltran,‡ Perla B. Balbuena,*,‡ Sushil Adhikari,*,§
and Ram B. Gupta*,∥
†Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, United States‡Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States§Department of Biosystems Engineering, Auburn University, Auburn, Alabama 36849, United States∥Depertment of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, Virginia 23284, UnitedStates
*S Supporting Information
ABSTRACT: Discovery of large shale gas reserves in recent years resulted in the reduction of natural gas price. In order toconvert methane in a direct and energy efficient route, nonoxidative catalytic conversion is a potentially attractive option whichincludes an activation of methane molecules at low temperature. The oxide of transition metals such as Mo, Fe, V, W, Cr, Zn, andCu have been studied as catalysts for methane conversion, where usually the conversion is lower than 20% and the operatingtemperature needed is above 800 °C which causes coking, thus resulting in an early catalyst deactivation. In this work, a nobletransition metal, ruthenium, has been chosen as the catalyst with the objective to decrease the methane activation temperature,increase the stability, and also achieve higher conversion than other transition metal catalysts. The catalyst was prepared as 1.5 or3.0 wt % ruthenium loading on zeolite (i.e., ZSM-5) and silica supports separately to compare the effect of metal loading andmetal−support combination on the methane conversion reaction, wherein the operating temperature was varied from 500° to800 °C. From online GC and FT-IR analyses of the gas products, it was observed that, on the 3.0 wt % Ru/ZSM-5 catalyst bed, arise in methane conversion took place at 700 °C, where heavy hydrocarbon molecules from C4 to C10 were produced, whereas forthe 3.0 wt % Ru/SiO2 catalyst bed, methane conversion was found to be low even at 800 °C and no significant production ofhigher hydrocarbon molecules was observed. The catalyst bed of 3.0 wt % Ru/ZSM-5 produced some aromatic compounds inliquid product. This could be attributed to the special framework structure in the ZSM-5 catalyst which influenced the formationof cyclic higher hydrocarbon molecules as product after methane is being activated on the surface of the ruthenium metal catalyst.Ruthenium supported on ZSM-5 also produced methyl radicals in a considerable amount at above 700 °C. Furthermore, theorigin of the lower-temperature activation effect on transition metals was examined with the density functional theory analyses,which suggests that the zeolite structure lowers the activation energies more than the silica structure by inducing more negativecharge on C atom of methane.
1. INTRODUCTION
Methane, the first member of the organic hydrocarbons,possesses very significant chemical properties because of itsstable tetrahedral structure, and it is also considered valuable asa combustible gaseous fuel vastly used for electric powergeneration, for industrial heating, and as a raw material,especially for hydrogen production, domestic cooking, andHVAC systems. Methane, being the main component of bothshale gas and biogas (produced from anaerobic digester) andbeing abundant at present, can be considered as a source ofenergy other than power generation and industrial utility usage.Despite its abundance, less than 1% of the natural gas resourceis being used as vehicle fuel in the U.S. in 2013.1 Therefore,conversion of methane to transportation fuel or value addedchemicals has drawn interest both from academics and fromindustry since its production is increasing in the U.S. Among allthe processes of methane conversion to higher hydrocarbongases and liquids, the most conspicuous ones are reforming forhydrogen or syngas production for Fischer−Tropsch synthesis,direct oxidation to methanol and formaldehyde, oxidative
coupling of methane to ethylene, and nonoxidative foraromatics and hydrogen production.2 In a gas to liquid(GTL) process plant, for example, 60% of the capital cost isassociated with the synthesis gas production, which is primarilythe major motivation to come up with a cost-effective processof methane conversion to higher hydrocarbon, avoiding theintermediate step via synthesis gas production.3
This study focuses on the direct conversion of methane tohigher hydrocarbon gases or liquids by the application ofcatalysis and heat energy in oxygen-free operating conditions.The nonoxidative conversion can be the most attractivetechnique for upgrading methane to value added higherhydrocarbons as a single-step conversion process on anindustrial scale. Finding out the right catalyst for highconversion of methane and higher yield of aromatics orhydrocarbon products with considerable selectivity and stable
catalyst life is a very challenging area of research in chemicalengineering as the activation of methane to undergo couplingor ring formation is very challenging as its stable andsymmetrical structure results in a high activation energy of425 kJ/mol together with the lack of functional group tocontribute in polarity or magnetic moment making it lessvulnerable to be attacked by other molecules or ions.3
Researchers tried to study the activity of different metalcatalysts, especially the transition metal catalysts for methaneactivation reaction including the different metal and supportcombinations which were found to catalyze the reaction givinga range of end products from ethane and ethylene to liquidaromatic products.2,4 For instance, various ZSM-5 zeolitesupported metal catalysts exhibit activity in the order Mo > W> Fe > V > Cr for the methane conversion to benzene product,where it was also found that the increasing number of Brønstedacid sites had positive effect on methane conversion.5
Furthermore, other ZSM-5 supported catalysts were beingstudied as in the case of the 2.5% loading of tungsten metalcatalyst on ZSM-5 which showed fair performance of 23%methane conversion with a 96% benzene selectivity when the1.5% Zn−H2SO4 promoter was applied.6 This high selectivitytoward benzene product was also found with other transitionmetals like Mo supported on ZSM-5 in the dehydroaromatiza-tion of methane, wherein the highest conversion achieved wasin the range of 18% to 20%.2 These may have been due to thespecial microporous framework in ZSM-5 which might haveenabled ring formation after an activation of methane to methylradicals,5 combining with its Brønsted acidity.7 Moreover, it wasreported that lower coke formation and better conversion inmethane conversion could be achieved in the system of 3 wt %Mo on ZSM-5 when compared to other supports. However, athigher temperature the conversion increased up to 10−15 h,after which the catalyst’s stability started deteriorating.7,8
To solve the problem regarding the activity of thebifunctional catalyst, Pt and Ru were added. Although Ptexhibits satisfactory activity, Ru of comparable activity wasmore of interest since its bulk price is one-tenth the Pt price,where a small metal loading of 2% to 5% will be economicallyfeasible if a substantial yield of higher hydrocarbons is obtained.Reports on the use of Ru catalyst suggest two-step mechanismon the methane conversion reaction which was pioneered bytwo research groups since 1991 that studied the kinetic andthermodynamical constraints for the direct conversion ofmethane to higher hydrocarbons, where they found that theRu, Co, and Pt outperformed Ni, Mo, W and other transitionmetals by lowering the activation energy of the exothermicchemisorption process on the catalyst surface, thus increasingmethane activation and formation of intermediates at moderateoperating conditions.11−13 It was observed that, for the Rusupported on silica in an oxygen-free methane conversion, thefirst step was the activation of methane at 127 to 527 °C
producing intermediate carbonaceous species of methylideneand vinylidine, wherein the second step of rehydrogenation ofthe intermediates at 87 to 107 °C generates ethane as a productwith 13 to 15% yield.12 Another study showed that this samecatalyst methane underwent dissociative adsorption on thecatalyst surface generating three carbonaceous species with anactivation energy of 22 kcal/mol, in which the yield of theethane and propane formed after the hydrogenation of aliphatichydrocarbons is a function of carbon surface coverage andhydrogenation temperature.13 Additionally, the advantages ofusing Ru catalyst can be illustrated by the optimum Ru−Cbond strength assisting the conversion to surface carbonaceousspecies or intermediates which ultimately underwent couplingor aromatization processes.12 For the case of ZSM-5 supportedRu catalyst, it was also reported that as high as 80% selectivityof C12 to C20 products were produced via the conversion ofsynthesis gas of CO and H2.
14 One of the studies concerningthe Ru catalyst concluded that the addition of Mo/HZSM-5 tothe Ru catalyst could promote the activity of the methaneconversion reaction while increasing benzene product yieldcompared to the catalyst without Ru.15 Besides, the strong acidsites of this catalyst decreased with increasing Ru loading,whereas the intermediate and weak acid sites increased whenRu loading was lower than 0.7%.15 Similar study of oxygen-freemethane conversion also proposed that when the Mo loadingwas varied from 1 to 3 wt % while keeping the Ru loadingconstant at 0.15 wt %, lower coke formation rate wasdemonstrated with better stability at the operating conditionof 600 °C although low methane conversion was achieved.16
Despite the positive effect of Ru, the research on the single-step methane conversion on ZSM-5 supported Ru catalyst hasnot yet been studied, and satisfactory reactivity of the catalysthas not been achieved. As a result, this work was performed toexamine the activity of the Ru catalysts supported on ZSM-5zeolite (SiO2/Al2O3 ratio of 23:1) and amorphous silica. Theperformances of the catalysts were compared at differentoperating temperature and metal loading for the activity in asingle-step methane conversion to higher hydrocarbons. Inaddition, the activation energies of the first deprotonation stepof methane on these catalysts were predicted via the DFTanalysis.
2. EXPERIMENTAL SECTION2.1. Catalyst Preparation. Ruthenium catalysts supported on
ZSM-5 and silica were prepared, and the detailed description ofcatalyst preparation along with catalyst characterization is provided inthe Supporting Information, S1.
2.2. Experimental Setup and Procedure. A schematic of theapparatus is illustrated in Figure 1. During the operation, methane ofchemically pure grade, where the flow rate (mL/min) was controlledby a digital flow meter at a rate of 6 to 8 mL/min, was flowed throughthe stainless steel tubular reactor (High Pressure EquipmentCompany, Erie, PA) of 0.8 cm internal diameter and 20.4 mL internal
Figure 1. Schematic diagram of experimental setup for direct nonoxidative catalytic upgrading of methane.
volume which was placed inside the electric furnace equipped tomonitor the temperature with a thermocouple (Omega Engineering).The input pressure of methane was also recoded. The reactor waspacked either with 11 g of Ru/ZSM-5 (4.5 g) and silica sand (6.5 g)mixture or with 6.4 g of Ru/SiO2 (2.0 g) and silica sand (4.4 g)mixture. Four catalysts (i.e., 3 wt % Ru/ZSM-5, 1.5 wt % Ru/ZSM-5, 3wt % Ru/SiO2, pure ZSM-5) were tested. Catalysts were reduced whileflowing H2 at 10 mL/min for 5 h at 500 °C prior to flowing methane.The outlet line leaving from the reactor was a 316 stainless steel
tubing of 1/16 in. internal diameter and was cooled down with the icewater bath followed by a double tube heat exchanger using tap water.After leaving the heat exchanger, the efflux of product gas mixture wasfed to a 30 mL Jerguson gauge with sight glass phase separator, inwhich separated gas product existed at the top embedded with apressure gauge of 60 psig maximum capacity in order to monitor theproduct gas pressure before entering the gas chromatography analyzer.It was found that a very minute amount of organic liquid product
was collected in the phase separator after 5 h of continuous operation.Therefore, the liquid product was washed out from the separator usingcyclohexane as a solvent. Operating temperature was varied from 500to 800 °C with a 100 °C increase with an observed pressure drop thatvaried from 15 psia to 17 psia, where the weight hourly space velocity(WHSV) was obtained in a fixed range of 0.04 to 0.07 h−1 by keepingthe methane flow rate constant in every set of different packing andtemperature.In order to calculate the conversion of methane, the two-point
calibration line was obtained by first calibrating the GC with 99.9 mol% CH4 followed by the second calibration with 10 mol % CH4, wherethe unconverted methane in the sample was reported in mole fraction,which is then calculated for methane conversion. The samplesanalyzed were collected at a similar point of the operating timebetween 15 to 18 h.2.3. Product Characterization. Gaseous and liquid products were
characterized by several analytic instruments such as gas chromato-graph (GC), Fourier transform infrared spectroscopy (FTIR),ultraviolet−visible spectroscopy (UV−vis), and gas chromatogra-phy−mass spectrometry (GC−MS), and the detailed description isprovided in the Supporting Information, S2.2.4. Computational Details. 2.4.1. General Computational
Details. Spin-polarized periodic density functional theory (DFT)calculations were performed via the Vienna ab initio simulationpackage (VASP),17−20 in which the Kohn−Sham equations are solvedby self-consistent algorithms. Basis functions were constructed usingthe projector augmented wave pseudopotentials (PAW)17 to describethe core electrons, while the valence electrons were described by planewave basis sets with a cutoff energy of 400 eV for all systems. Theexchange-correlation functional was described within the generalizedgradient approximation (GGA) by the revised Perdew−Burke−Ernzerhof (RPBE).18 Ab initio molecular dynamics simulations(AIMD) were performed employing the NVT ensemble at 800 °Cwith the Nose−Hoover thermostat, wherein the Nose mass parameterof 0.5 and a time step of 1 fs were set. The tritium mass was usedinstead of hydrogen mass for H atom in all AIMD simulations. TheBrillouin zone integration was constructed through a Monkhorst−Pack19 grid of 1 × 1 × 1 (Γ-point) sampling for both DFT and AIMDcalculations. The Gaussian smearing method of 0.05 eV smearingwidth was applied for the partial occupancies. For all systems, theconvergence criteria were 10−4 and 10−3 eV for successive electronicand ionic steps, respectively.2.4.2. Models of Ru Supported on ZSM-5 and Amorphous Silica.
Ru/ZSM-5 catalyst was modeled based on the crystallographic data ofthe 96T cluster ZSM-520 via Materials Studio 6.0 software. In order torepresent the real ZSM-5 structure with the SiO2/Al2O3 ratio of 23:1as in our experiment, Al atoms were substituted into the importedstructure aforementioned. The position of Al in the ring was suggestedto be on the T12 site of the 10T sinusoidal ring,21 where one Al wasplaced at T12 position of each 10T sinusoidal ring as illustrated inFigure 2. The Ru/ZSM-5 model was constructed from the optimizedZSM-5 structure by the adsorption of the Ru atom between twooxygen atoms connecting to the substituted Al atom in the 10T ring as
shown in Figure 2. The optimized structure of Ru/ZSM-5 wasreconstructed from the 96T cluster down to the 10T cluster as shownin Figure 3. The dangling bonds of the terminal O atoms in the cluster
were saturated by H atoms, where the position of each H atom wasdesignated by the following two-step procedure.22 In the first step, allof the Si atoms connected to the terminal O atoms in thereconstructed 10T cluster were changed into H atoms. The O−Hbonding distance in the terminal OH groups was set to 0.95 Å in thesecond step, wherein the 10T nanocluster model of Ru/ZSM-5 wascreated as depicted in Figure 3. Prior to the optimization of thiscluster, the minimization of cluster boundary effects was achieved byconstraining all of the terminal OH groups, whereas the other atomswere relaxed. For the construction of amorphous silica, labeled as SiO2,the following procedure was implemented.23 The slab of amorphoussilica surface was first obtained, in which all the undercoordinated Siatoms were saturated with O atoms followed by the saturation of allthe undercoordinated O atoms with H atoms. In order to model Ru/SiO2 catalyst, after the optimization of the SiO2 slab, the Ru atom wasadsorbed between two OH groups as shown in Figure 4, where theterminal two H atoms were assumed to desorb as H2 from the Ruatom analogous to previous reports.24 The Ru on amorphous silicasystem was reconstructed after optimization from the 27-Si-atomsurface down to a nanocluster size of Si2O7Ru as shown in Figure 5,wherein all of the terminal O atoms were saturated by H atoms and allof the O−H bond lengths were set to 0.95 Å similar to theconstruction of Ru/ZSM-5 mentioned above.
Figure 2. (a) 96T cluster model of ZSM-5; (b) 96T cluster model ofRu/ZSM-5 catalyst.
Figure 3. 10T nanocluster model of Ru/ZSM-5 catalyst.
2.4.3. Analysis of Atomic Charges. Bader charge analysis29−31 wasemployed to obtain the atomic charges, in which the total electroniccharge of an atom is determined by the charge enclosed within theBader volume defined by zero flux surfaces. The predicted chargeswere obtained for the Ru, C, and H atoms involved in the firstdeprotonation step of methane, and the net transferred atomic chargeswere compared between Ru/ZSM-5 and Ru/SiO2 systems.2.4.4. Activation Energy Prediction for Methane Deprotonation
Step. In order to model the deprotonation step, the methane moleculewas built and optimized. It was found that the average bond of theoptimized methane molecule is 1.10 Å with atomic charges of −0.130|e| for C atom and an average charge of +0.033|e| per H atom. In orderto obtain the activation energy for the first deprotonation step ofmethane, the configurations of both reactant and product were
established. The ab initio molecular dynamics (AIMD)28 techniquewas applied to initially determine the reactant and productconfigurations. Methane adsorbed on the catalyst was designated asthe reactant configuration, whereas the adsorbed H and CH3 species asproducts from the first deprotonation step were set as the productconfigurations as shown in Figure 6. The reactant configuration asobserved from the AIMD simulations shows that the C−Ru distancesof adsorbed methane on the catalysts were 2.535 and 2.450 Å for Ru/ZSM-5 and Ru/SiO2, respectively. The product configuration has Ru−H and C−Ru bond distances of 1.657 and 2.045 Å for Ru/ZSM-5, and1.684 and 2.041 Å for Ru/SiO2, respectively. The average bond lengthof CH4 molecule was obtained directly from the simulation by firstoptimizing the CH4 molecule in vacuum environment; then, theresulting structure was measured for the bond length via MaterialsStudio 6.0 software. These AIMD configurations were optimized usingDFT calculations prior to the calculation of the activation energies bythe climbing image nudged elastic band method (cNEB),29,30 whereinfour intermediate images were used for this climbing methodalgorithm.
3. RESULTS AND DISCUSSION
3.1. Catalyst Characterizations.With the impregnation ofruthenium nitrosyl nitrate solution on the ZSM-5, zeolitesupport, and calcination at 500 °C, new bonds were created inbetween ruthenium metal ion with Al2O3 and SiO2 in zeolite.Also, presence of ruthenium oxide has been ensured bycomparing the peaks of powder X-ray diffraction patterns of 3.0wt % Ru/ZSM-5 with Ru2O3 XRD spectra from the literature.31
In Figure 7, a comparison of X-ray diffraction patterns is shownbetween pure ZSM-5 zeolite and ruthenium loaded on ZSM-5support. The presence of ruthenium oxide can be proven onthe catalyst by the characterized peaks at 2θ values of 35° and54.3° in the spectrum of 3.0 wt % Ru/ZSM-5 which are notpresent in the pattern of pure ZSM-5.Images of the prepared catalysts at micron level have been
taken using a scanning electron microscope (SEM). Energydispersive X-ray spectroscopy (EDS) is used along with SEMimaging (shown in Figure 8) to estimate the composition ofmetals and oxygen in the catalyst.Comparing Figure 8a and Figure 8b, the particle sizes of SiO2
support were significantly larger (6−15 μm) than ZSM-5 (1−5μm). The dispersed visible white dots on the surface of thesupport materials in Figures 8a and 8b and the correspondingvalues from the EDS analysis show that the ruthenium is welldispersed on the surface of support material. The spent catalyst
(see Figure 8c) of 3.0 wt % Ru/ZSM-5 was clustered with amuch less distinctive dispersed ruthenium metal, which may bedue to the excessive residue carbon covering the surface ofcatalyst particles. In addition, when comparing to the pureZSM-5 catalyst in Figure 8d the presence of ruthenium inFigure 8a,b becomes more evident and the particle size of ZSM-5 was found to be in the same range both as a support materialand as pure catalyst. The elemental analysis results showed thatfresh catalyst sample of 3 wt % Ru/ZSM-5, spent catalystsample of 3 wt % Ru/ZSM-5, and fresh catalyst sample of 3 wt% Ru/SiO2 contain 3.35%, 3.54%, and 2.46% of ruthenium byweight, respectively. These results also support the statementthat the SEM-EDS analysis of the spent catalyst sample of 3 wt% Ru/ZSM-5 showed a lower amount of ruthenium than theactual value due to surface coverage by coke deposition.Bonds formed in the catalysts containing ruthenium were
principally between the ruthenium metal or metal oxide withthe Al or Si atom in the case of the ZSM-5 support and onlywith a Si atom for SiO2. In order to compare these bondsformed, the peaks in the spectra generated from the Fouriertransform infrared spectroscopy were carried out on the freshand spent catalysts on support material and also for pure ZSM-5 as shown in Figure 9.Significant differences can be noticed for the area under the
peaks between fresh and spent catalysts. The ZSM-5 supportedcatalysts showed higher peak height with a greater area underthe peaks for the fresh catalysts compared to the spent ones.The decrease in peak height and peak area implies thedeterioration in catalyst activity. No new chemical bonds wereformed in the catalyst with either the reactant or productmolecules as no new peaks were observed in the spectra of thespent catalyst compared to the spectra of the fresh one. For theSiO2-supported ruthenium metal catalyst, not much change inpeak heights and peak area was found which can be due to itslow surface activity, thus almost no change in the IR spectra ofthe sample taken after 25 h of operation.The BET surface area and pore volume of fresh and spent
catalysts were compared as shown in Table 1 in order todetermine the stability of the catalyst since, if the surface areaand pore volume change with a significant rate, it can beassumed that the stability of the catalyst is low.From Table 1, when comparing among the surface area of
the pure ZSM-5, 1.5 wt % Ru/ZSM-5, and 3.0 wt % Ru/ZSM-5fresh catalysts, it was found that surface area decreases withmetal loading. On the other hand, in the case of SiO2 it isevident by the SEM image (Figure 8a) that the support has
larger particle size, hence, possesses lower surface area than inthe case of ZSM-5 as can be seen in Table 1. Significantdifference in surface area was observed between fresh and spent3.0 wt % Ru/ZSM-5 catalysts indicating coke deposition fromthe methane conversion reaction over an operating time of 60h. However, the pore volume was observed to increase for thespent catalyst probably due to the transformation of microporesto meso- and macropores.Pore structures and sizes of the catalyst support play a very
important role in catalyst activity and conversion. It can befound from the literature that the channel structure in ZSM-5creates a framework and micropores and mesopores whichcontribute in shape selective catalysis.32 ZSM-5 supportedtransition metal catalysts were studied and found to producearomatics from direct conversion of methane,4,28 however silica
Figure 7. Comparison of X-ray diffraction spectra of pure ZSM-5 and3.0 wt % Ru/ZSM-5.
Figure 8. SEM images with EDS analysis of (a) 3.0 wt % Ru/SiO2fresh catalyst, (b) 3.0 wt % Ru/ZSM-5 fresh catalyst, (c) 3.0 wt % Ru-ZSM-5 spent catalyst, and (d) pure ZSM-5 fresh catalyst.
supported transition metal catalysts were found to produceprimarily C2 and C3 hydrocarbon gases due to the difference inpore structure and sizes.10,12 Also other catalysts as Zr(PO)4supported ruthenium metal produced lighter hydrocarbons (C1to C5), and it is evident that pore structures and characteristicsof support materials play a very important role in selectivity ofproduct33
In order to determine the optimum reduction temperaturefor the 3.0 wt % Ru/ZSM-5, the calcined sample was analyzedby the H2 temperature-programmed reduction analysis (TPR).The results interpreted from the TCD signal indicated that atthe reduction temperature of 110 °C the highest rate ofruthenium oxide reduction has already been reached. The insitu reduction was performed at a much higher temperature inthe reactor as previous studies reported that a higher reductiontemperature contributes to the increasing in the active metalsurface area by the removal of an anion part of the metal salt.34
3.2. Reaction Mechanism. The conversion of methane hasbeen compared among different ruthenium metal catalystssupported on either a ZSM-5 or silica support. The effects ofmetal loading and metal−support combination on methaneconversion mechanism were studied. The proposed reactionstaking place when the bifunctional catalyst of ruthenium metalsupported on ZSM-5 zeolite support is used in a continuousflow packed bed reactor at operating temperatures higher than500 °C can be illustrated as
→ * +Δ
HCH CH124 3 2 (1)
→ * +Δ
CH C 2H4 2 (2)
* ⎯ →⎯⎯⎯⎯⎯⎯ +−
6CH C H 6H3 ZSM 5 6 6 2 (3)
where the asterisk (*) indicates surface species.Methane molecules are first chemisorbed on ruthenium
metal, and then dehydrogenation takes place at high temper-ature.10 The methyl radicals then undergo cyclization in theZSM-5 pore structures to produce aromatics.
3.3. Effects of Temperature. The catalyst performance ina conversion of methane to higher hydrocarbon products andhydrogen was evaluated based on two criteria: the quality ofproduct (the presence of higher hydrocarbon molecules in theproduct mixture) and the methane conversion. The procedurewas performed by varying the operating temperature whilekeeping the feed flow rate and catalyst bed pressure at aconstant range of 5 to 10 mL/min and 15 to 18 psia,respectively. The product gas analyzed by the FTIR showed nosignificant changes in the bond structure from the product gasfor 1.5 wt % Ru/ZSM-5 catalyst, which implied that no alkenes,alkynes, or aromatic hydrocarbon products were formed even athigher operating temperature. Similar results were alsoobserved in the case of 3.0 wt % Ru/SiO2 catalyst. In contrast,the high metal loading 3.0 wt % Ru/ZSM-5 operated at 700and 800 °C showed a new spectral peak undetected in eitherthe 1.5 wt % Ru/ZSM-5 or 3.0 wt % Ru/SiO2 catalysts (notshown here) eluted at 1615 cm−1 as shown in Figure 10. It wasdesignated to the unsaturated hydrocarbons or benzene ringfound in the product gas mixture, whereas that peak was notobserved in lower operating temperatures.
Moreover, the characteristics of the product obtained fromtemperature range of 700 to 800 °C on the 3.0 wt % Ru/ZSM-5 were further investigated employing UV−vis spectroscopy(Figure 11). The figure shows pure cyclohexane (solvent) aswell as the product profiles. The product exhibits two newpeaks between the wavelengths of 220 and 260 nm.Additionally, the liquid product was separately analyzed ascan be seen in Figure 12, in which the peak designated tobenzene was observed. From both the IR spectra and UV−visspectroscopic analyses, it was evident that, at the higher
Figure 9. Comparison of FT-IR spectra of fresh and spent catalysts.
Table 1. BET Surface Area and Pore Size Data for DifferentCatalysts
catalystsurf area (m2/
g)pore vol (cm3/
g)pore size(nm)
pure ZSM-5 (fresh)a 403 0.36 3.503.0 wt % Ru/ZSM-5(fresh)
operating temperature range of 700 to 800 °C, CH4 conversionto aromatic hydrocarbon has taken place on the 3.0 wt % Ru/ZSM-5 catalyst bed.3.4. Methane Conversion Analyzed by Online GC. In
Figure 13, it is demonstrated that temperature played a very
significant role in methane conversion. At 500 and 600 °C,methane conversion was lower than 15% for all the catalysts,but a rise in conversion was noticed at 700 °C for 3.0 wt % Ru/ZSM-5 single catalyst bed. At 800 °C, this catalyst showed thehighest conversion among others. This can be explained by theruthenium metal contributing to the activation of CH4 tomethyl free radical, in which the special channel structure andsheets consisting of the chains of five membered rings of ZSM-
5 aid the formation of benzene-like structure from this methylradical,28,33 without the formation of carbon black from thedissociation of methyl radicals. Additionally, higher hydro-carbons, especially the cyclic structures (e.g., methylcyclohex-ane, isopropylcyclobutane, dimethylcyclopentane), were alsodetected by the GC−MS analysis of the solvent-washedproduct. The total product ranges from hydrocarbons ofseven up to as high as 20 carbon atoms. The presence of higherhydrocarbon in the gas phase of product has also been ensuredby the chromatograms collected from online GC with thereactor. For 3 wt % Ru/ZSM-5 catalyst packed bed, the productgas mixtures demonstrated chromatograms with significant areaunder the peaks for hydrocarbons chains or cyclic structurescontaining higher than four carbons. As the column has beencalibrated with calibration gases consisting of C1 to C4hydrocarbon gases and it was observed in the product gasmixture chromatogram that these peaks were eluted after theretention time of C4 hydrocarbon, it can be assumed thathydrocarbons consisting of five or more carbon atoms wereproduced. For other catalysts no such high hydrocarbonproduct peak was found in the chromatogram other thanhydrogen and unconverted methane gas.
3.5. Percent Yield of Hydrogen. Hydrogen yield has thesame trend as methane conversion where the ZSM-5 supportedcatalyst held the highest yield at higher temperature, whereasthe silica supported possessed lower hydrogen yield. Inaddition, the yield of hydrogen is directly proportional to theoperating temperature for all catalysts.From Figure 14, it is found that pure ZSM-5 single catalyst
bed contributed to a higher yield of hydrogen at 800 °C
compared to other catalysts at the same temperature. Althoughthe pure ZSM-5 has highest yield of hydrogen, the conversionof methane was significantly lower than 3.0 wt % Ru/ZSM-5,which implies that if only pure ZSM-5 is used, a much loweramount of higher hydrocarbon will be produced. On thecontrary, even though the 3.0 wt % Ru/ZSM-5 exhibited alower yield of hydrogen, a much higher conversion of methaneexplained heavier hydrocarbon molecules observed in theproduct stream as the rest of the hydrogen was contributing tothe formation of the heavier hydrocarbons.
3.6. Effect of Operating Time. To find out the effect ofoperating time on the conversion rate as well as on catalyst life,60 h of continuous reaction at 800 °C has been conducted on
Figure 11. Comparison between the UV−vis spectra of the liquidproduct in solvent from CH4 conversion on 3.0 wt % Ru/ZSM-5catalyst bed at 800 °C and the pure solvent of cyclohexane.
Figure 12. UV−vis spectrum of product liquid of conversion of CH4on 3.0 wt % Ru/ZSM-5 catalyst bed at 800 °C.
Figure 13. Conversion of CH4 with operating temperature ondifferent catalysts.
Figure 14. Percent yield of hydrogen from methane conversionreaction on different catalyst beds at different operating temperatures.
the 3.0 wt % Ru/ZSM-5 catalyst bed as it exhibited the bestcatalytic performance, and product samples were beinganalyzed every 2 h with the online GC-TCD. It is observedfrom Figure 15 that methane conversion was stable from 5 up
to 35 h; then it increased to 55% at 46 h of operation before itslightly decreased to 50% at 60 h of operation, which was stillhigher than the conversion achieved in the first 35 h.Not only was the conversion high after 40 h of operation but
it is also perceived that hydrogen yield was high as well in therange of 30% to 38% between 26 and 50 h of reaction as shownin Figure 16. Up to 60 h of continuous operation, theconversion of methane went down slightly, whereas thehydrogen yield rose up as high as 47%, which was probablydue to methane dissociation to hydrogen and carbon blackformation. As a result, the catalyst activity started to deteriorate.Considering both methane conversion and hydrogen yieldduring the continuous reaction of 60 h on a 3.0 wt % Ru/ZSM-5, it can be seen that, during 5 to 35 h of operation, the catalystperformance and product quality were stable and consistentthough hydrogen yield increased slowly. These results showedan improvement of the catalyst’s life when using Ru comparedto other transition metal catalysts supported on zeolite orsilica.1,2
The amount of coke deposition was measured bythermogravimetric analysis. A weight loss of 16.5 wt % wasobserved from this spent catalyst as the temperature increasesfrom 22 to 900 °C in 135 min (Figure 17). As a result, highstability of the 3.0 wt % Ru/ZSM-5 catalyst was confirmedsince a smaller amount of coke (16.5 wt % of catalyst) wasformed even after a long operation time of 60 h. Compared toother studies where ZSM-5 supported transition metal catalystswere used, methane conversion started deteriorating after 8 to10 h of operation, implying early catalyst deactivation for direct
methane conversion reactions.5 Also coke formation was foundto be 18 to 30 wt % at 700 °C for Mo/HMCM-22 catalyst.
3.7. Methane Activation on Ru/ZSM-5 and Ru/SiO2.The presence of the support is known to affect the activity ofthe catalyst as can be seen from the predicted atomic chargesand activation energy for the first deprotonation step ofmethane. Bader charge analysis predicted that the chargestransferred to Ru, C, and H atoms in Ru/ZSM-5 are +0.705|e|,−0.226|e|, and +0.024|e|, respectively, whereas for Ru/SiO2 thecharges transferred are +1.02|e| for Ru atom, −0.106|e| for Catom, and +0.016|e| for H atom. Thus, it is found that the Ru/ZSM-5 system induces more negative charge on the C atomand more positive charge on the H atom than the Ru/SiO2system does. As expected from the Bader charge analysis, it wasfound that the Ru/ZSM-5 catalyst lowers the activation energy,Ea, for the first deprotonation step of methane more than theRu/SiO2 catalyst. The predicted activation energy obtainedfrom the climbing image nudged elastic band method(cNEB)3,4 was 19.20 kJ/mol-CH4 for Ru/ZSM-5, while ahigher activation energy of 67.55 kJ/mol-CH4 was predicted forthe Ru/SiO2, which are illustrated in Figure 6. It is also foundthat for the first deprotonation step Ru/ZSM-5 has anexothermic heat of reaction of −4.88 kJ/mol-CH4, while forthe Ru/SiO2 an endothermic heat of reaction of +67.38 kJ/mol-CH4 was obtained.
4. CONCLUSIONDirect methane conversion to higher hydrocarbon productswas studied in an oxygen free route. Ruthenium supported onZSM-5 and SiO2 catalysts was tested for methane conversion,and the study found that the 3 wt % Ru/ZSM-5 performed thebest, both in conversion and in product quality. Above 40% ofmethane conversion was achieved at 800 °C operatingtemperature, where C5 to C8 cyclic hydrocarbon were produced
Figure 15. Conversion of methane versus hours of operation on 3.0 wt% Ru/ZSM-5 catalyst packed bed at 800 °C.
Figure 16. Percent yield of hydrogen versus hours of operation on single catalyst bed of 3.0 wt % Ru/ZSM-5 at 800 °C.
Figure 17. Thermogravimetric analysis results on spent catalyst of 3.0wt % Ru/ZSM-5 after 60 h of operation.
along with hydrogen. However, lower ruthenium loading of 1.5wt % did not contribute to producing higher hydrocarbonproducts the same as in the case of 3 wt % Ru/SiO2 catalystwhich showed low conversion even at high temperature and nohigher hydrocarbon product was observed. For catalystlongevity study, 3 wt % Ru/ZSM-5 started to deteriorateafter 40 h of reaction. Compared to other transition metalcatalysts, this bifunctional catalyst (ruthenium metal on ZSM-5support) has certainly shown potential in catalyst performancein terms of methane conversion and stable catalyst life at highoperating temperature. Computational results also showed that,for the first deprotonation step of methane to methyl(CH3)and hydrogen radicals, the predicted activation energy for Ru/ZSM-5 was found to be lower than that of the Ru/SiO2. Thismay be caused by the more negative charge on C atom inducedby the Ru/ZSM-5 structure. This suggested that the ZSM-5support has an effect of lowering the activation barrier for thefirst deprotonation step of methane, which is an importantinitial step toward the formation of higher hydrocarbons.
■ ASSOCIATED CONTENT
*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.energy-fuels.5b02583.
Catalyst preparation, characterization, and productdistribution from methane activation (PDF)
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
S.N. would like to thank Alabama Agricultural ExperimentStation (AAES) for funding this study (ALA014-1-13006). Theauthors are thankful to Zhouhong Wang for his help incollecting catalyst surface area data. However, only the authorsare responsible for any remaining errors in this manuscript.
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