Catalytic Upgrading of Methane to Higher ... - ACS Publications

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Catalytic Upgrading of Methane to Higher Hydrocarbon in a Nonoxidative Chemical Conversion Shaima 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, United States ‡

S Supporting Information *

ABSTRACT: Discovery of large shale gas reserves in recent years resulted in the reduction of natural gas price. In order to convert methane in a direct and energy efficient route, nonoxidative catalytic conversion is a potentially attractive option which includes an activation of methane molecules at low temperature. The oxide of transition metals such as Mo, Fe, V, W, Cr, Zn, and Cu have been studied as catalysts for methane conversion, where usually the conversion is lower than 20% and the operating temperature needed is above 800 °C which causes coking, thus resulting in an early catalyst deactivation. In this work, a noble transition 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 or 3.0 wt % ruthenium loading on zeolite (i.e., ZSM-5) and silica supports separately to compare the effect of metal loading and metal−support combination on the methane conversion reaction, wherein the operating temperature was varied from 500° to 800 °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, a rise in methane conversion took place at 700 °C, where heavy hydrocarbon molecules from C4 to C10 were produced, whereas for the 3.0 wt % Ru/SiO2 catalyst bed, methane conversion was found to be low even at 800 °C and no significant production of higher hydrocarbon molecules was observed. The catalyst bed of 3.0 wt % Ru/ZSM-5 produced some aromatic compounds in liquid product. This could be attributed to the special framework structure in the ZSM-5 catalyst which influenced the formation of 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, the origin 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 negative charge on C atom of methane.

1. INTRODUCTION Methane, the first member of the organic hydrocarbons, possesses very significant chemical properties because of its stable tetrahedral structure, and it is also considered valuable as a combustible gaseous fuel vastly used for electric power generation, for industrial heating, and as a raw material, especially for hydrogen production, domestic cooking, and HVAC systems. Methane, being the main component of both shale gas and biogas (produced from anaerobic digester) and being abundant at present, can be considered as a source of energy other than power generation and industrial utility usage. Despite its abundance, less than 1% of the natural gas resource is being used as vehicle fuel in the U.S. in 2013.1 Therefore, conversion of methane to transportation fuel or value added chemicals has drawn interest both from academics and from industry since its production is increasing in the U.S. Among all the processes of methane conversion to higher hydrocarbon gases and liquids, the most conspicuous ones are reforming for hydrogen or syngas production for Fischer−Tropsch synthesis, direct oxidation to methanol and formaldehyde, oxidative © XXXX American Chemical Society

coupling of methane to ethylene, and nonoxidative for aromatics and hydrogen production.2 In a gas to liquid (GTL) process plant, for example, 60% of the capital cost is associated with the synthesis gas production, which is primarily the major motivation to come up with a cost-effective process of methane conversion to higher hydrocarbon, avoiding the intermediate step via synthesis gas production.3 This study focuses on the direct conversion of methane to higher hydrocarbon gases or liquids by the application of catalysis and heat energy in oxygen-free operating conditions. The nonoxidative conversion can be the most attractive technique for upgrading methane to value added higher hydrocarbons as a single-step conversion process on an industrial scale. Finding out the right catalyst for high conversion of methane and higher yield of aromatics or hydrocarbon products with considerable selectivity and stable Received: November 3, 2015 Revised: March 1, 2016

A

DOI: 10.1021/acs.energyfuels.5b02583 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of experimental setup for direct nonoxidative catalytic upgrading of methane.

producing intermediate carbonaceous species of methylidene and vinylidine, wherein the second step of rehydrogenation of the intermediates at 87 to 107 °C generates ethane as a product with 13 to 15% yield.12 Another study showed that this same catalyst methane underwent dissociative adsorption on the catalyst surface generating three carbonaceous species with an activation energy of 22 kcal/mol, in which the yield of the ethane and propane formed after the hydrogenation of aliphatic hydrocarbons is a function of carbon surface coverage and hydrogenation temperature.13 Additionally, the advantages of using Ru catalyst can be illustrated by the optimum Ru−C bond strength assisting the conversion to surface carbonaceous species or intermediates which ultimately underwent coupling or aromatization processes.12 For the case of ZSM-5 supported Ru catalyst, it was also reported that as high as 80% selectivity of C12 to C20 products were produced via the conversion of synthesis gas of CO and H2.14 One of the studies concerning the Ru catalyst concluded that the addition of Mo/HZSM-5 to the Ru catalyst could promote the activity of the methane conversion reaction while increasing benzene product yield compared to the catalyst without Ru.15 Besides, the strong acid sites of this catalyst decreased with increasing Ru loading, whereas the intermediate and weak acid sites increased when Ru loading was lower than 0.7%.15 Similar study of oxygen-free methane conversion also proposed that when the Mo loading was varied from 1 to 3 wt % while keeping the Ru loading constant at 0.15 wt %, lower coke formation rate was demonstrated with better stability at the operating condition of 600 °C although low methane conversion was achieved.16 Despite the positive effect of Ru, the research on the singlestep methane conversion on ZSM-5 supported Ru catalyst has not yet been studied, and satisfactory reactivity of the catalyst has not been achieved. As a result, this work was performed to examine the activity of the Ru catalysts supported on ZSM-5 zeolite (SiO2/Al2O3 ratio of 23:1) and amorphous silica. The performances of the catalysts were compared at different operating temperature and metal loading for the activity in a single-step methane conversion to higher hydrocarbons. In addition, the activation energies of the first deprotonation step of methane on these catalysts were predicted via the DFT analysis.

catalyst life is a very challenging area of research in chemical engineering as the activation of methane to undergo coupling or ring formation is very challenging as its stable and symmetrical structure results in a high activation energy of 425 kJ/mol together with the lack of functional group to contribute in polarity or magnetic moment making it less vulnerable to be attacked by other molecules or ions.3 Researchers tried to study the activity of different metal catalysts, especially the transition metal catalysts for methane activation reaction including the different metal and support combinations which were found to catalyze the reaction giving a range of end products from ethane and ethylene to liquid aromatic products.2,4 For instance, various ZSM-5 zeolite supported 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 Brønsted acid sites had positive effect on methane conversion.5 Furthermore, other ZSM-5 supported catalysts were being studied as in the case of the 2.5% loading of tungsten metal catalyst on ZSM-5 which showed fair performance of 23% methane conversion with a 96% benzene selectivity when the 1.5% Zn−H2SO4 promoter was applied.6 This high selectivity toward benzene product was also found with other transition metals like Mo supported on ZSM-5 in the dehydroaromatization of methane, wherein the highest conversion achieved was in the range of 18% to 20%.2 These may have been due to the special microporous framework in ZSM-5 which might have enabled ring formation after an activation of methane to methyl radicals,5 combining with its Brønsted acidity.7 Moreover, it was reported that lower coke formation and better conversion in methane conversion could be achieved in the system of 3 wt % Mo on ZSM-5 when compared to other supports. However, at higher 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 the bifunctional catalyst, Pt and Ru were added. Although Pt exhibits satisfactory activity, Ru of comparable activity was more of interest since its bulk price is one-tenth the Pt price, where a small metal loading of 2% to 5% will be economically feasible if a substantial yield of higher hydrocarbons is obtained. Reports on the use of Ru catalyst suggest two-step mechanism on the methane conversion reaction which was pioneered by two research groups since 1991 that studied the kinetic and thermodynamical constraints for the direct conversion of methane to higher hydrocarbons, where they found that the Ru, Co, and Pt outperformed Ni, Mo, W and other transition metals by lowering the activation energy of the exothermic chemisorption process on the catalyst surface, thus increasing methane activation and formation of intermediates at moderate operating conditions.11−13 It was observed that, for the Ru supported on silica in an oxygen-free methane conversion, the first step was the activation of methane at 127 to 527 °C

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Ruthenium catalysts supported on ZSM-5 and silica were prepared, and the detailed description of catalyst preparation along with catalyst characterization is provided in the Supporting Information, S1. 2.2. Experimental Setup and Procedure. A schematic of the apparatus is illustrated in Figure 1. During the operation, methane of chemically pure grade, where the flow rate (mL/min) was controlled by a digital flow meter at a rate of 6 to 8 mL/min, was flowed through the stainless steel tubular reactor (High Pressure Equipment Company, Erie, PA) of 0.8 cm internal diameter and 20.4 mL internal B


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Energy & Fuels volume which was placed inside the electric furnace equipped to monitor the temperature with a thermocouple (Omega Engineering). The input pressure of methane was also recoded. The reactor was packed 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, 3 wt % Ru/SiO2, pure ZSM-5) were tested. Catalysts were reduced while flowing 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 ice water bath followed by a double tube heat exchanger using tap water. After leaving the heat exchanger, the efflux of product gas mixture was fed to a 30 mL Jerguson gauge with sight glass phase separator, in which separated gas product existed at the top embedded with a pressure gauge of 60 psig maximum capacity in order to monitor the product 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 using cyclohexane as a solvent. Operating temperature was varied from 500 to 800 °C with a 100 °C increase with an observed pressure drop that varied 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 keeping the methane flow rate constant in every set of different packing and temperature. 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, where the unconverted methane in the sample was reported in mole fraction, which is then calculated for methane conversion. The samples analyzed were collected at a similar point of the operating time between 15 to 18 h. 2.3. Product Characterization. Gaseous and liquid products were characterized by several analytic instruments such as gas chromatograph (GC), Fourier transform infrared spectroscopy (FTIR), ultraviolet−visible spectroscopy (UV−vis), and gas chromatography−mass spectrometry (GC−MS), and the detailed description is provided 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 simulation package (VASP),17−20 in which the Kohn−Sham equations are solved by self-consistent algorithms. Basis functions were constructed using the projector augmented wave pseudopotentials (PAW)17 to describe the core electrons, while the valence electrons were described by plane wave basis sets with a cutoff energy of 400 eV for all systems. The exchange-correlation functional was described within the generalized gradient approximation (GGA) by the revised Perdew−Burke− Ernzerhof (RPBE).18 Ab initio molecular dynamics simulations (AIMD) were performed employing the NVT ensemble at 800 °C with the Nosé−Hoover thermostat, wherein the Nosé mass parameter of 0.5 and a time step of 1 fs were set. The tritium mass was used instead of hydrogen mass for H atom in all AIMD simulations. The Brillouin zone integration was constructed through a Monkhorst− Pack19 grid of 1 × 1 × 1 (Γ-point) sampling for both DFT and AIMD calculations. The Gaussian smearing method of 0.05 eV smearing width was applied for the partial occupancies. For all systems, the convergence criteria were 10−4 and 10−3 eV for successive electronic and 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 of the 96T cluster ZSM-520 via Materials Studio 6.0 software. In order to represent the real ZSM-5 structure with the SiO2/Al2O3 ratio of 23:1 as in our experiment, Al atoms were substituted into the imported structure aforementioned. The position of Al in the ring was suggested to be on the T12 site of the 10T sinusoidal ring,21 where one Al was placed at T12 position of each 10T sinusoidal ring as illustrated in Figure 2. The Ru/ZSM-5 model was constructed from the optimized ZSM-5 structure by the adsorption of the Ru atom between two oxygen atoms connecting to the substituted Al atom in the 10T ring as

Figure 2. (a) 96T cluster model of ZSM-5; (b) 96T cluster model of Ru/ZSM-5 catalyst. shown in Figure 2. The optimized structure of Ru/ZSM-5 was reconstructed from the 96T cluster down to the 10T cluster as shown in Figure 3. The dangling bonds of the terminal O atoms in the cluster

Figure 3. 10T nanocluster model of Ru/ZSM-5 catalyst.

were saturated by H atoms, where the position of each H atom was designated by the following two-step procedure.22 In the first step, all of the Si atoms connected to the terminal O atoms in the reconstructed 10T cluster were changed into H atoms. The O−H bonding distance in the terminal OH groups was set to 0.95 Å in the second step, wherein the 10T nanocluster model of Ru/ZSM-5 was created as depicted in Figure 3. Prior to the optimization of this cluster, the minimization of cluster boundary effects was achieved by constraining all of the terminal OH groups, whereas the other atoms were relaxed. For the construction of amorphous silica, labeled as SiO2, the following procedure was implemented.23 The slab of amorphous silica surface was first obtained, in which all the undercoordinated Si atoms were saturated with O atoms followed by the saturation of all the undercoordinated O atoms with H atoms. In order to model Ru/ SiO2 catalyst, after the optimization of the SiO2 slab, the Ru atom was adsorbed between two OH groups as shown in Figure 4, where the terminal two H atoms were assumed to desorb as H2 from the Ru atom analogous to previous reports.24 The Ru on amorphous silica system was reconstructed after optimization from the 27-Si-atom surface 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 all of the O−H bond lengths were set to 0.95 Å similar to the construction of Ru/ZSM-5 mentioned above. C


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established. The ab initio molecular dynamics (AIMD)28 technique was applied to initially determine the reactant and product configurations. Methane adsorbed on the catalyst was designated as the reactant configuration, whereas the adsorbed H and CH3 species as products from the first deprotonation step were set as the product configurations as shown in Figure 6. The reactant configuration as observed from the AIMD simulations shows that the C−Ru distances of 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, and 1.684 and 2.041 Å for Ru/SiO2, respectively. The average bond length of CH4 molecule was obtained directly from the simulation by first optimizing the CH4 molecule in vacuum environment; then, the resulting structure was measured for the bond length via Materials Studio 6.0 software. These AIMD configurations were optimized using DFT calculations prior to the calculation of the activation energies by the climbing image nudged elastic band method (cNEB),29,30 wherein four intermediate images were used for this climbing method algorithm. Figure 4. (a) Amorphous silica (SiO2) slab model; (b) Ru/SiO2 catalyst slab model.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterizations. With the impregnation of ruthenium nitrosyl nitrate solution on the ZSM-5, zeolite support, and calcination at 500 °C, new bonds were created in between ruthenium metal ion with Al2O3 and SiO2 in zeolite. Also, presence of ruthenium oxide has been ensured by comparing the peaks of powder X-ray diffraction patterns of 3.0 wt % Ru/ZSM-5 with Ru2O3 XRD spectra from the literature.31 In Figure 7, a comparison of X-ray diffraction patterns is shown between pure ZSM-5 zeolite and ruthenium loaded on ZSM-5 support. The presence of ruthenium oxide can be proven on the catalyst by the characterized peaks at 2θ values of 35° and 54.3° in the spectrum of 3.0 wt % Ru/ZSM-5 which are not present in the pattern of pure ZSM-5. Images of the prepared catalysts at micron level have been taken using a scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDS) is used along with SEM imaging (shown in Figure 8) to estimate the composition of metals 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 (1−5 μm). The dispersed visible white dots on the surface of the support materials in Figures 8a and 8b and the corresponding values from the EDS analysis show that the ruthenium is well dispersed on the surface of support material. The spent catalyst

Figure 5. Nanocluster model of Ru/SiO2. 2.4.3. Analysis of Atomic Charges. Bader charge analysis29−31 was employed to obtain the atomic charges, in which the total electronic charge of an atom is determined by the charge enclosed within the Bader volume defined by zero flux surfaces. The predicted charges were obtained for the Ru, C, and H atoms involved in the first deprotonation step of methane, and the net transferred atomic charges were 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 molecule was built and optimized. It was found that the average bond of the optimized 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 order to obtain the activation energy for the first deprotonation step of methane, the configurations of both reactant and product were

Figure 6. Energy profiles for (a) Ru/ZSM-5 and (b) Ru/SiO2. D


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Figure 7. Comparison of X-ray diffraction spectra of pure ZSM-5 and 3.0 wt % Ru/ZSM-5.

(see Figure 8c) of 3.0 wt % Ru/ZSM-5 was clustered with a much less distinctive dispersed ruthenium metal, which may be due to the excessive residue carbon covering the surface of catalyst particles. In addition, when comparing to the pure ZSM-5 catalyst in Figure 8d the presence of ruthenium in Figure 8a,b becomes more evident and the particle size of ZSM5 was found to be in the same range both as a support material and as pure catalyst. The elemental analysis results showed that fresh catalyst sample of 3 wt % Ru/ZSM-5, spent catalyst sample 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 by weight, respectively. These results also support the statement that the SEM-EDS analysis of the spent catalyst sample of 3 wt % Ru/ZSM-5 showed a lower amount of ruthenium than the actual value due to surface coverage by coke deposition. Bonds formed in the catalysts containing ruthenium were principally between the ruthenium metal or metal oxide with the Al or Si atom in the case of the ZSM-5 support and only with a Si atom for SiO2. In order to compare these bonds formed, the peaks in the spectra generated from the Fourier transform infrared spectroscopy were carried out on the fresh and spent catalysts on support material and also for pure ZSM5 as shown in Figure 9. Significant differences can be noticed for the area under the peaks between fresh and spent catalysts. The ZSM-5 supported catalysts showed higher peak height with a greater area under the peaks for the fresh catalysts compared to the spent ones. The decrease in peak height and peak area implies the deterioration in catalyst activity. No new chemical bonds were formed in the catalyst with either the reactant or product molecules as no new peaks were observed in the spectra of the spent catalyst compared to the spectra of the fresh one. For the SiO2-supported ruthenium metal catalyst, not much change in peak heights and peak area was found which can be due to its low surface activity, thus almost no change in the IR spectra of the 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 to determine the stability of the catalyst since, if the surface area and pore volume change with a significant rate, it can be assumed 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-5 fresh catalysts, it was found that surface area decreases with metal loading. On the other hand, in the case of SiO2 it is evident by the SEM image (Figure 8a) that the support has

Figure 8. SEM images with EDS analysis of (a) 3.0 wt % Ru/SiO2 fresh catalyst, (b) 3.0 wt % Ru/ZSM-5 fresh catalyst, (c) 3.0 wt % RuZSM-5 spent catalyst, and (d) pure ZSM-5 fresh catalyst.

larger particle size, hence, possesses lower surface area than in the case of ZSM-5 as can be seen in Table 1. Significant difference in surface area was observed between fresh and spent 3.0 wt % Ru/ZSM-5 catalysts indicating coke deposition from the methane conversion reaction over an operating time of 60 h. However, the pore volume was observed to increase for the spent catalyst probably due to the transformation of micropores to meso- and macropores. Pore structures and sizes of the catalyst support play a very important role in catalyst activity and conversion. It can be found from the literature that the channel structure in ZSM-5 creates a framework and micropores and mesopores which contribute in shape selective catalysis.32 ZSM-5 supported transition metal catalysts were studied and found to produce aromatics from direct conversion of methane,4,28 however silica E


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CH4 → CH3* +

1 H2 2

(1)

Δ

CH4 → C* + 2H 2

(2)

6CH3* ⎯⎯⎯⎯⎯⎯⎯→ C6H6 + 6H 2

(3)

ZSM − 5

where the asterisk (*) indicates surface species. Methane molecules are first chemisorbed on ruthenium metal, and then dehydrogenation takes place at high temperature.10 The methyl radicals then undergo cyclization in the ZSM-5 pore structures to produce aromatics. 3.3. Effects of Temperature. The catalyst performance in a conversion of methane to higher hydrocarbon products and hydrogen was evaluated based on two criteria: the quality of product (the presence of higher hydrocarbon molecules in the product mixture) and the methane conversion. The procedure was performed by varying the operating temperature while keeping the feed flow rate and catalyst bed pressure at a constant range of 5 to 10 mL/min and 15 to 18 psia, respectively. The product gas analyzed by the FTIR showed no significant changes in the bond structure from the product gas for 1.5 wt % Ru/ZSM-5 catalyst, which implied that no alkenes, alkynes, or aromatic hydrocarbon products were formed even at higher operating temperature. Similar results were also observed in the case of 3.0 wt % Ru/SiO2 catalyst. In contrast, the high metal loading 3.0 wt % Ru/ZSM-5 operated at 700 and 800 °C showed a new spectral peak undetected in either the 1.5 wt % Ru/ZSM-5 or 3.0 wt % Ru/SiO2 catalysts (not shown here) eluted at 1615 cm−1 as shown in Figure 10. It was designated to the unsaturated hydrocarbons or benzene ring found in the product gas mixture, whereas that peak was not observed in lower operating temperatures.

Figure 9. Comparison of FT-IR spectra of fresh and spent catalysts.

Table 1. BET Surface Area and Pore Size Data for Different Catalysts catalyst pure ZSM-5 (fresh)a 3.0 wt % Ru/ZSM-5 (fresh) 3.0 wt % Ru/ZSM-5 (spent) 3.0 wt % Ru/SiO2 (fresh) 3.0 wt % Ru/SiO2 (spent) 1.5 wt % Ru/ZSM-5 (fresh) 1.5 wt % Ru/ZSM-5 (spent) a

surf area (m2/ g)

pore vol (cm3/ g)

pore size (nm)

403 329

0.36 0.21

3.50 1.29

235

0.37

3.12

177 171 334

2.28 1.21 0.31

25.65 14.16 1.88

315

0.20

1.29

Calcined at 500 °C for 5 h; same as for other catalysts.

supported transition metal catalysts were found to produce primarily C2 and C3 hydrocarbon gases due to the difference in pore structure and sizes.10,12 Also other catalysts as Zr(PO)4 supported ruthenium metal produced lighter hydrocarbons (C1 to C5), and it is evident that pore structures and characteristics of support materials play a very important role in selectivity of product33 In order to determine the optimum reduction temperature for the 3.0 wt % Ru/ZSM-5, the calcined sample was analyzed by the H2 temperature-programmed reduction analysis (TPR). The results interpreted from the TCD signal indicated that at the reduction temperature of 110 °C the highest rate of ruthenium oxide reduction has already been reached. The in situ reduction was performed at a much higher temperature in the reactor as previous studies reported that a higher reduction temperature contributes to the increasing in the active metal surface area by the removal of an anion part of the metal salt.34 3.2. Reaction Mechanism. The conversion of methane has been compared among different ruthenium metal catalysts supported on either a ZSM-5 or silica support. The effects of metal loading and metal−support combination on methane conversion mechanism were studied. The proposed reactions taking place when the bifunctional catalyst of ruthenium metal supported on ZSM-5 zeolite support is used in a continuous flow packed bed reactor at operating temperatures higher than 500 °C can be illustrated as

Figure 10. Comparison of FT-IR spectra of product gas mixture at different operating temperatures on 3.0 wt % Ru/ZSM-5 catalyst bed.

Moreover, the characteristics of the product obtained from temperature range of 700 to 800 °C on the 3.0 wt % Ru/ZSM5 were further investigated employing UV−vis spectroscopy (Figure 11). The figure shows pure cyclohexane (solvent) as well as the product profiles. The product exhibits two new peaks between the wavelengths of 220 and 260 nm. Additionally, the liquid product was separately analyzed as can be seen in Figure 12, in which the peak designated to benzene was observed. From both the IR spectra and UV−vis spectroscopic analyses, it was evident that, at the higher F


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5 aid the formation of benzene-like structure from this methyl radical,28,33 without the formation of carbon black from the dissociation of methyl radicals. Additionally, higher hydrocarbons, especially the cyclic structures (e.g., methylcyclohexane, isopropylcyclobutane, dimethylcyclopentane), were also detected by the GC−MS analysis of the solvent-washed product. The total product ranges from hydrocarbons of seven up to as high as 20 carbon atoms. The presence of higher hydrocarbon in the gas phase of product has also been ensured by the chromatograms collected from online GC with the reactor. For 3 wt % Ru/ZSM-5 catalyst packed bed, the product gas mixtures demonstrated chromatograms with significant area under the peaks for hydrocarbons chains or cyclic structures containing higher than four carbons. As the column has been calibrated with calibration gases consisting of C1 to C4 hydrocarbon gases and it was observed in the product gas mixture chromatogram that these peaks were eluted after the retention time of C4 hydrocarbon, it can be assumed that hydrocarbons consisting of five or more carbon atoms were produced. For other catalysts no such high hydrocarbon product peak was found in the chromatogram other than hydrogen and unconverted methane gas. 3.5. Percent Yield of Hydrogen. Hydrogen yield has the same trend as methane conversion where the ZSM-5 supported catalyst held the highest yield at higher temperature, whereas the silica supported possessed lower hydrogen yield. In addition, the yield of hydrogen is directly proportional to the operating 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

Figure 11. Comparison between the UV−vis spectra of the liquid product in solvent from CH4 conversion on 3.0 wt % Ru/ZSM-5 catalyst bed at 800 °C and the pure solvent of cyclohexane.

Figure 12. UV−vis spectrum of product liquid of conversion of CH4 on 3.0 wt % Ru/ZSM-5 catalyst bed at 800 °C.

operating temperature range of 700 to 800 °C, CH4 conversion to 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

Figure 14. Percent yield of hydrogen from methane conversion reaction on different catalyst beds at different operating temperatures.

compared to other catalysts at the same temperature. Although the pure ZSM-5 has highest yield of hydrogen, the conversion of methane was significantly lower than 3.0 wt % Ru/ZSM-5, which implies that if only pure ZSM-5 is used, a much lower amount of higher hydrocarbon will be produced. On the contrary, even though the 3.0 wt % Ru/ZSM-5 exhibited a lower yield of hydrogen, a much higher conversion of methane explained heavier hydrocarbon molecules observed in the product stream as the rest of the hydrogen was contributing to the formation of the heavier hydrocarbons. 3.6. Effect of Operating Time. To find out the effect of operating time on the conversion rate as well as on catalyst life, 60 h of continuous reaction at 800 °C has been conducted on

Figure 13. Conversion of CH4 with operating temperature on different catalysts.

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 the highest conversion among others. This can be explained by the ruthenium metal contributing to the activation of CH4 to methyl free radical, in which the special channel structure and sheets consisting of the chains of five membered rings of ZSMG


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Figure 17. Thermogravimetric analysis results on spent catalyst of 3.0 wt % Ru/ZSM-5 after 60 h of operation.

methane conversion reactions.5 Also coke formation was found to 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 of the catalyst as can be seen from the predicted atomic charges and activation energy for the first deprotonation step of methane. Bader charge analysis predicted that the charges transferred 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 the charges transferred are +1.02|e| for Ru atom, −0.106|e| for C atom, and +0.016|e| for H atom. Thus, it is found that the Ru/ ZSM-5 system induces more negative charge on the C atom and more positive charge on the H atom than the Ru/SiO2 system does. As expected from the Bader charge analysis, it was found that the Ru/ZSM-5 catalyst lowers the activation energy, Ea, for the first deprotonation step of methane more than the Ru/SiO2 catalyst. The predicted activation energy obtained from the climbing image nudged elastic band method (cNEB)3,4 was 19.20 kJ/mol-CH4 for Ru/ZSM-5, while a higher activation energy of 67.55 kJ/mol-CH4 was predicted for the Ru/SiO2, which are illustrated in Figure 6. It is also found that for the first deprotonation step Ru/ZSM-5 has an exothermic heat of reaction of −4.88 kJ/mol-CH4, while for the Ru/SiO2 an endothermic heat of reaction of +67.38 kJ/molCH4 was obtained.

Figure 15. Conversion of methane versus hours of operation on 3.0 wt % Ru/ZSM-5 catalyst packed bed at 800 °C.

to 35 h; then it increased to 55% at 46 h of operation before it slightly decreased to 50% at 60 h of operation, which was still higher 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 the range of 30% to 38% between 26 and 50 h of reaction as shown in Figure 16. Up to 60 h of continuous operation, the conversion of methane went down slightly, whereas the hydrogen yield rose up as high as 47%, which was probably due to methane dissociation to hydrogen and carbon black formation. As a result, the catalyst activity started to deteriorate. Considering both methane conversion and hydrogen yield during the continuous reaction of 60 h on a 3.0 wt % Ru/ZSM5, it can be seen that, during 5 to 35 h of operation, the catalyst performance and product quality were stable and consistent though hydrogen yield increased slowly. These results showed an improvement of the catalyst’s life when using Ru compared to other transition metal catalysts supported on zeolite or silica.1,2 The amount of coke deposition was measured by thermogravimetric analysis. A weight loss of 16.5 wt % was observed from this spent catalyst as the temperature increases from 22 to 900 °C in 135 min (Figure 17). As a result, high stability of the 3.0 wt % Ru/ZSM-5 catalyst was confirmed since a smaller amount of coke (16.5 wt % of catalyst) was formed even after a long operation time of 60 h. Compared to other studies where ZSM-5 supported transition metal catalysts were used, methane conversion started deteriorating after 8 to 10 h of operation, implying early catalyst deactivation for direct

4. CONCLUSION Direct methane conversion to higher hydrocarbon products was studied in an oxygen free route. Ruthenium supported on ZSM-5 and SiO2 catalysts was tested for methane conversion, and the study found that the 3 wt % Ru/ZSM-5 performed the best, both in conversion and in product quality. Above 40% of methane conversion was achieved at 800 °C operating temperature, where C5 to C8 cyclic hydrocarbon were produced

Figure 16. Percent yield of hydrogen versus hours of operation on single catalyst bed of 3.0 wt % Ru/ZSM-5 at 800 °C. H


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(7) Choudhary, T. V.; Aksoylu, E.; Wayne Goodman, D. Nonoxidative Activation of Methane. Catal. Rev.: Sci. Eng. 2003, 45, 151− 203. (8) Wang, D.; Lunsford, J. H.; Rosynek, M. P. Characterization of a Mo/ZSM-5 Catalyst for the Conversion of Methane to Benzene. J. Catal. 1997, 169, 347−358. (9) Amariglio, H.; Saint-Just, J.; Amariglio, A. Homologation of methane under non-oxidative conditions,. Fuel Process. Technol. 1995, 42, 291−323. (10) Belgued, M.; Amariglio, A.; Lefort, L.; Pareja, P.; Amariglio, H. Oxygen-Free Conversion of Methane to Higher Alkanes through an Isothermal Two-Step Reaction on Ruthenium. J. Catal. 1996, 161, 282−291. (11) Belgued, M.; Amariglio, A.; Pareja, P.; Amariglio, H. OxygenFree Conversion of Methane to Higher Alkanes through an Isothermal Two-Step Reaction on Platinum (EUROPT-1). J. Catal. 1996, 159, 441−448. (12) Koranne, M.; Goodman, D.; Zajac, G. Direct conversion of methane to higher hydrocarbons via an oxygen free, low-temperature route. Catal. Lett. 1995, 30, 219−234. (13) Carstens, J.; Bell, A. Methane activation and conversion to higher hydrocarbons on supported ruthenium. J. Catal. 1996, 161, 423−429. (14) Kandaswamy, J.; Kibby, L.; Saxton, J. Zeolite supported ruthenium catalysts for the conversion of synthesis gas to hydrocarbons, and method for preparation and method of use thereof. US Patent and Trademark Office, 15-Dec-2011. (15) Shu, Y.; Xu, Y.; Wong, S.; Wang, L.; Guo, X. Promotional Effect of Ru on the Dehydrogenation and Aromatization of Methane in the Absence of Oxygen over Mo/HZSM-5 Catalysts. J. Catal. 1997, 170, 11−19. (16) Hassan, A.; Sayari, A. Highly active, selective and stable Mo/RuHZSM-5 catalysts for oxygen-free methane aromatization,. Appl. Catal., A 2006, 297, 159−164. (17) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (18) Hammer, B.; Hansen, L.; Nørskov, J. Improved adsorption energetics within density-functional theory using revised PerdewBurke-Ernzerhof functionals. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413−7421. (19) Pack, J. D.; Monkhorst, H. J. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188−5192. (20) Schmidt, W.; Wilczok, U.; Weidenthaler, C.; Medenbach, O.; Goddard, R.; Buth, G.; Cepak, A. Preparation and morphology of pyramidal MFI single-crystal segments. J. Phys. Chem. B 2007, 111, 13538−13543. (21) Olson, D. H.; Khosrovani, N.; Peters, A. W.; Toby, B. H. Crystal Structure of Dehydrated CsZSM-5 (5.8Al): Evidence for Nonrandom Aluminum Distribution. J. Phys. Chem. B 2000, 104, 4844−4848. (22) Gao, J.; Zheng, Y.; Fitzgerald, G. B.; de Joannis, J.; Tang, Y.; Wachs, I. E.; Podkolzin, S. G. Structure of Mo 2 C x and Mo 4 C x Molybdenum Carbide Nanoparticles and Their Anchoring Sites on ZSM-5 Zeolites. J. Phys. Chem. C 2014, 118, 4670−4679. (23) Ewing, C. S.; Bhavsar, S.; Veser, G.; McCarthy, J. J.; Johnson, J. K. Accurate amorphous silica surface models from first-principles thermodynamics of surface dehydroxylation. Langmuir 2014, 30, 5133−5141. (24) Ewing, C. S.; Hartmann, M. J.; Martin, K. R.; Musto, A. M.; Padinjarekutt, S. J.; Weiss, E. M.; Veser, G.; McCarthy, J. J.; Johnson, J. K.; Lambrecht, D. S. Structural and Electronic Properties of Pt 13 Nanoclusters on Amorphous Silica Supports. J. Phys. Chem. C 2015, 119, 2503−2512. (25) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A fast and robust algorithm for Bader decomposition of charge density. Comput. Mater. Sci. 2006, 36, 354−360. (26) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved grid-based algorithm for Bader charge allocation. J. Comput. Chem. 2007, 28, 899−908.

along with hydrogen. However, lower ruthenium loading of 1.5 wt % did not contribute to producing higher hydrocarbon products the same as in the case of 3 wt % Ru/SiO2 catalyst which showed low conversion even at high temperature and no higher hydrocarbon product was observed. For catalyst longevity study, 3 wt % Ru/ZSM-5 started to deteriorate after 40 h of reaction. Compared to other transition metal catalysts, this bifunctional catalyst (ruthenium metal on ZSM-5 support) has certainly shown potential in catalyst performance in terms of methane conversion and stable catalyst life at high operating 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. This may be caused by the more negative charge on C atom induced by the Ru/ZSM-5 structure. This suggested that the ZSM-5 support has an effect of lowering the activation barrier for the first deprotonation step of methane, which is an important initial step toward the formation of higher hydrocarbons.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02583.

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Catalyst preparation, characterization, and product distribution from methane activation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P. B. Balbuena). *E-mail: [email protected]. Tel: +1 334 844 3543; fax: +1 334 844 3530. (S. Adhikari). *E-mail: [email protected] (R. B. Gupta). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

S.N. would like to thank Alabama Agricultural Experiment Station (AAES) for funding this study (ALA014-1-13006). The authors are thankful to Zhouhong Wang for his help in collecting catalyst surface area data. However, only the authors are responsible for any remaining errors in this manuscript.

(1) Chynoweth, D. P.; Owens, J. M.; Legrand, R. Renewable methane from anaerobic digestion of biomass,. Renewable Energy 2001, 22, 1−8. (2) Lunsford, J. H. Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century,. Catal. Today 2000, 63, 165−174. (3) Holmen, A. Direct conversion of methane to fuels and chemicals,. Catal. Today 2009, 142, 2−8. (4) Majhi, S.; Mohanty, P.; Wang, H.; Pant, K. K. Direct conversion of natural gas to higher hydrocarbons: A review,. J. Energy Chem. 2013, 22, 543−554. (5) Weckhuysen, B. M.; Wang, D.; Rosynek, M. P.; Lunsford, J. H. Conversion of Methane to Benzene over Transition Metal Ion ZSM-5 Zeolites. J. Catal. 1998, 175, 338−346. (6) Zeng, J.; Xiong, Z.; Zhang, H.; Lin, G.; Tsai, K. Nonoxidative dehydrogenation and aromatization of methane over W/HZSM-5based catalysts,. Catal. Lett. 1998, 53, 119−124. I


Article

Energy & Fuels (27) Tang, W.; Sanville, E.; Henkelman, G. A grid-based Bader analysis algorithm without lattice bias. J. Phys.: Condens. Matter 2009, 21, 084204. (28) Kresse, G. Ab initio molecular dynamics for liquid metals. J. Non-Cryst. Solids 1995, 192-193, 222−229. (29) Henkelman, G.; Jónsson, H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J. Chem. Phys. 2000, 113, 9978−9985. (30) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. Climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901−9904. (31) Green, M. L.; Gross, M. E.; Papa, L. E.; Schnoes, K. J.; Brasen, D. Chemical Vapor Deposition of Ruthenium and Ruthenium Dioxide Films. J. Electrochem. Soc. 1985, 132, 2677−2685. (32) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Structure of synthetic zeolite ZSM-5,. Nature 1978, 272, 437−438. (33) Cheikhi, N.; Ziyad, M.; Coudurier, G. Conversion of methane to higher alkanes by ruthenium loaded Zr3(PO4)4. Appl. Catal., A 1994, 118, 187−198. (34) Koopman, P. Characterization of ruthenium catalysts as studied by temperature programmed reduction. J. Catal. 1981, 69, 172−179.

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