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Catalytic aromatization of methane Cite this: DOI: 10.1039/c3cs60259a
James J. Spivey*a and Graham Hutchingsb Recent developments in natural gas production technology have led to lower prices for methane and renewed interest in converting methane to higher value products. Processes such as those based on syngas from methane reforming are being investigated. Another option is methane aromatization, which produces benzene and hydrogen: 6CH4(g) - C6H6(g) + 9H2(g) DGor = +433 kJ mol1 DHor = +531 kJ mol1. Thermodynamic calculations for this reaction show that benzene formation is insignificant below B600 1C, and that the formation of solid carbon [C(s)] is thermodynamically favored at temperatures above B300 1C. Benzene formation is insignificant at all temperatures up to 1000 1C when C(s) is included in the calculation of equilibrium composition. Interestingly, the thermodynamic limitation on benzene formation can be minimized by the addition of alkanes/alkenes to the methane feed. By far the most widely studied catalysts for this reaction are Mo/HZSM-5 and Mo/MCM-22. Benzene selectivities are generally between 60 and 80% at methane conversions of B10%, corresponding to net benzene yields of less than 10%. Major byproducts include lower molecular weight hydrocarbons and higher molecular weight substituted aromatics. However, carbon formation is inevitable, but the experimental findings show this can be kinetically limited by the use of
Received 18th July 2013
H2 or oxidants in the feed, including CO2 or steam. A number of reactor configurations involving regeneration
DOI: 10.1039/c3cs60259a
of the carbon-containing catalyst have been developed with the goal of minimizing the cost of regeneration of the catalyst once deactivated by carbon deposition. In this tutorial review we discuss the thermodynamics of
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this process, the catalysts used and the potential reactor configurations that can be applied.
Key learning points 1. Methane aromatization is thermodynamically unfavorable at all conditions of practical interest, but can be made more feasible by the addition of low molecular weight alkanes/alkenes. 2. Carbon deposition is inevitable and presents a significant barrier to practical use of this reaction. Carbon deposition can be kinetically limited by the addition of hydrogen or oxidants such as steam or CO2. 3. By far the most widely studied catalysts are those based on Mo/zeolites. Molybdenum is carbided during the reaction and activates methane, which is oligomerized on the acid sites of the zeolite. 4. A number of reactor designs have been developed for this reaction, all with the goal of regenerating the carbon-containing catalyst and removing hydrogen to drive the reaction to the desired products. 5. A promising area for further study is the application of computational methods to this reaction, especially to understand the role of lower molecular weight compounds in improving the net yield of benzene.
1.0 Introduction Methane is a very abundant fossil fuel resource that is widely distributed around the globe. Together with carbon (in the form of coal) and carbon dioxide, methane is one of the most thermodynamically stable forms in which carbon can be widely found. Its thermodynamic stability represents a challenge to a
Dept. Chemical Engineering, Louisiana State University, Baton Rouge, LA, USA 70803. E-mail: [email protected] b Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, UK CF10 3AT
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chemists and the direct activation of methane to chemicals has been a dream of many chemists for over a century. At present the commercial processes using methane involve an indirect process in which methane is reacted with water (steam reforming) or with oxygen (partial oxidation) to form synthesis gas, a mixture of CO and H2, that is subsequently reacted to form fuels and chemicals. Overall the utilization of methane tends to require high temperatures and high pressures and is energy intensive. In view of this it is not surprising that attempts to use methane in a direct process for the formation of chemicals also require high temperatures. One notable approach using oxidation was heralded by the seminal work of Lunsford
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which showed that methane could be oxidatively coupled to form ethane and ethylene with very high selectivities using oxides at 600 1C.1 However, this reaction involves gas phase radical chemistry and the selectivity is lost at conversions >10%. Subsequently, zeolites were found to be effective for the conversion of propane and higher alkanes to aromatic chemicals. This process, known as the CYCLAR process, has been commercialized and was used as the inspiration for novel catalysis in which methane is converted to aromatic molecules using zeolite catalysts.2 This methane aromatization reaction is the topic of this tutorial review. Ismagilov et al. provide a review of this and related reactions through 2008.3 With the discovery of shale-gas in massive deposits in many locations, the direct utilisation of methane has once again gained research impetus. In this review we will cover all fundamental aspects of this catalysed reaction including the thermodynamic limitations, catalysis and kinetics as well as the possible reactor configurations so that the reader gains a complete insight into this fascinating chemistry.
The conversion of methane to benzene is known as ‘‘dehydrocyclization’’; ‘‘aromatization’’; and ‘‘dehydroaromatization’’. For the purposes of this review, this reaction will be referred to as MA (‘‘methane aromatization’’, which is the most widely used term in the literature). This direct conversion of methane to benzene is as follows:
DHor = +531 kJ mol1
DGor = +433 kJ mol1 (1)
This is an extremely unfavorable reaction thermodynamically.† Fig. 1 shows the result of a calculation of the equilibrium composition‡ of a mixture of an initial 6 kmol of CH4 and allowing † The thermodynamic results shown here are consistent with those published elsewhere.4 ‡ Calculated as a Gibbs free energy minimization in which only methane, benzene, and hydrogen are allowed using HSC 7.1 software (HSC, Houston, TX).
James Spivey is Professor of Chemical Engineering at Louisiana State University. He is the Editor-in-Chief of Catalysis Today, and Editor of the Royal Society of Chemistry’s Catalysis book series. He is a Fellow of the RSC, and author of over 175 publications. He serves as Director of the Center for Atomic-level Catalyst Design, an Energy Frontier Research Center supported by the US Dept. of James J. Spivey Energy. His research focuses on catalysis for clean energy processes, syngas chemistry, and hydrocarbon conversion.
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6CH4(g) + 412O2(g) - C6H6(g) + 9H2O(g) DGor = 1624 kJ mol1
2.0 Thermodynamics of methane aromatization
6CH4(g) - C6H6(g) + 9H2(g)
only benzene and hydrogen in the mixture (solid carbon, C(s), is not allowed). It is clear that benzene formation at equilibrium becomes significant only at impractically high temperatures. A corresponding equilibrium plot for this reaction, with C(s) included in the calculation, shows the following (Fig. 2). At equilibrium, virtually no benzene is formed. Thermodynamically, the reaction goes to solid carbon [C(s)] and hydrogen, as expected. The mechanism can be viewed as follows (Fig. 3), where ‘‘coke’’ would correspond to solid carbon,§ C(s).6 In this mechanism, coke is formed in two ways: (a) from dehydrogenated methane (‘‘CHx’’) in parallel with the desired C–C bond formation step, and (b) from lower MW intermediates (e.g., C2Hy in Fig. 3) in series with the formation of the desired product, benzene (benzene can be viewed as R = 0 in Fig. 3). Two approaches to addressing the equilibrium limitation on benzene formation from MA are: (a) the use of oxygen (or another oxidant),¶ e.g.,:
DHor = 1846 kJ mol1
(2)
Though thermodynamically favorable, the reaction would likely have poor selectivity in practice since benzene would likely decompose to C(s) or be oxidized as soon as it was formed.8 (b) the addition of alkane/alkenes, e.g., n-butane:7 2CH4 ðgÞ þ n-C4 H10 ðgÞ ! C6 H6 ðgÞ þ 6H2 ðgÞ C
DG700 r
¼ 36:2 kJ mol1
C
DHr700
¼ þ396 kJ mol1
(3)
§ In the literature, the terms ‘‘coke’’ and ‘‘carbon’’ are often used interchangeably to denote the carbon-rich deposits formed on catalysts during various reactions. These deposits include a wide range of carbon species, including polynuclear aromatics as well as more graphitic species.5 In this review, no distinction is made among the various types of carbon deposits, and ‘‘coke’’ and ‘‘carbon’’ are used interchangeably. ¶ 1 bar; calculated using HSC 7.1 software. 8 When C(s) is not allowed, the equilibrium composition of a mixture of CH4, C6H6, O2, and H2O (starting from 1 mol CH4) contains 0.167 mol benzene at temperatures between 605 and 1000 1C, 1 bar. When C(s) is included in the corresponding value is o2 1027.
Graham Hutchings
Graham Hutchings, born 1951, studied chemistry at University College London. His early career was with ICI and AECI Ltd where he became interested in gold catalysis. In 1984 he moved to academia and has held chairs at the Universities of Witwatersrand, Liverpool and Cardiff and currently he is Director of the Cardiff Catalysis Institute. He was elected a Fellow of the Royal Society in 2009, and he was awarded the Davy Medal of the Royal Society in 2013.
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Fig. 1
Equilibrium amounts (moles), 1 bar, starting with 6 mol CH4 and allowing only H2 and benzene as components. Calculated using HSC 7.1 software.
Fig. 2
Equilibrium amounts (moles), 1 bar, starting with 6 mol CH4 and allowing C(s), H2, and benzene as components. Calculated using HSC 7.1 software.
Fig. 3 Mechanism for the formation of aromatics and coke in methane aromatization (MA).6
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The conclusion from these various thermodynamic analyses is that the conversion of methane to benzene is thermodynamically favorable at limited conditions, and is likely to be kinetically driven to undesired products such as carbon. The addition of alkanes/alkenes has been shown experimentally to drive the reaction to ‘‘aromatics’’, but the one study exploring this approach did not quantify the selectivity to benzene specifically.7 A qualitative claim of the positive effect of adding C2–C4 hydrocarbons on MA has also been reported.8
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3.0 Catalysts/kinetics
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Mo/ZSM-5 A wide range of catalysts have been investigated for the MA reaction. By far the most widely studied catalysts are those based on Mo/ZSM-5 zeolite. Fig. 4 shows the structure of the ZSM-5 catalyst.** For the MA reaction, various forms of molybdenum are introduced into the zeolite to form a ‘‘bifunctional’’ catalyst – both the Mo site and the acidity/shape selectivity of the zeolite are essential. The Mo site is thought to activate methane, perhaps producing C2H4 that is then oligomerized to C6H6 on the acid sites.9–12 This general mechanism, reflecting the uncertainty in the exact nature of the intermediate formed from methane, is shown in Fig. 5.12 To some extent there is an analogy between the methane aromatization reaction and the conversion of methanol to hydrocarbons over the same zeolite catalyst, i.e. H-ZSM-5. In the conversion of methanol it is believed that carbonaceous deposits within the zeolitic pores act as a scaffold on which the methanol is converted to ethene as a primary product. This process is referred to as the carbon pool mechanism,13 and it is feasible that a similar process operates within the catalyst pores during methane aromatization. In this way methane conversion on the carbonaceous deposits leads to the C2 fragments that oligomerize and aromatize. A number of studies have shown an ‘‘induction period’’ during which the molybdenum is gradually transformed into an active species, (e.g., by the formation of a molybdenum carbide or oxycarbide site upon reduction by methane).14–17 †† The pore size of these catalysts approximates that of the benzene molecule (0.59 nm)4 and provides shape selectivity to the desired product. However, some reports also suggest that migration of the Mo from the channels of the zeolite to the outer surface accompanies the reduction process.11,18 This would presumably limit, if not eliminate, any influence of the zeolite pore on the selectivity. However, it appears that at least some of the molybdenum remains in the channels of the zeolite and increases the benzene yield. Typically, a wide range of higher hydrocarbons are formed on Mo/ZSM-5 catalysts. Fig. 6 shows one example for a 3%Mo/ HZSM-5 catalyst.6 Methane conversion generally decreases with time as carbon forms on the catalyst surface, but this does not always lead to changes in benzene selectivity. An example is shown below for a Mo/HZSM-5 catalyst in which the molybdenum was pre-carbided to form Mo2C (Fig. 7).10 Note the high selectivity to benzene despite the significant decrease in methane conversion with time. Similar results are observed for many of the studies on Mo/HZSM-5 catalysts, e.g. Fig. 8 below.5 Various ‘‘promoters’’ have been investigated with the goal of increasing the selectivity to benzene, overall methane conversion,
Fig. 4 Schematic of ZSM-5 zeolite, consisting of an intersecting twodimensional pore structure. There are two types of pores, both formed by 10-membered oxygen rings. One is straight with an elliptical cross section, while the second type of circular cross-section pores intersect the straight pores at right angles in a zig-zag pattern. The effective pore diameter is 0.59 nm, roughly the kinetic diameter of benzene.
and/or decreasing carbon deposition for Mo/ZSM catalysts. Examples include copper,19 ruthenium,20 and potassium.21 An example showing the effect of copper promotion is shown below, Fig. 9.19 Note in Fig. 9: (a) The induction period (observed on most Mo/ZSM catalysts) during which both methane conversion and benzene selectivity increase. (b) The positive effects of Cu promotion on methane conversion and benzene selectivity. (c) The inevitable decrease in both conversion and selectivity with time, likely due to carbon deposition. Coke deposition is a critical and inevitable process in the MA reaction. On the ZSM-5 catalysts, various approaches have been attempted to minimize carbon formation—e.g., addition of the following along with methane in the feed: (a) addition of CO or CO2 (ref. 22–24). The mechanism by which CO/CO2 reduces carbon formation is postulated to be as shown in Fig. 10.22 (a) H2 (ref. 24–26). Hydrogen can be added to the feed, but is also formed during the MA reaction. It was found that hydrogen minimizes carbon formation,24 particularly on the Bronsted acid sites of the zeolite.25,26 (b) NO27. The addition of NO as an oxidant on the yield of aromatics shows that there is an optimum (Fig. 11), below which presumably coke is not oxidized sufficiently rapidly, and above which oxidation of the active Mo carbide decreases the activity.27 (c) O2 (ref. 27). Similarly for O2 addition, there appears to be an optimum, with a minimum in deactivation rate at an intermediate level of added O2, Fig. 12.27 Other metals for MA on ZSM-5 zeolites
** Image from: http://chemelab.ucsd.edu/methanol/memos/ZSM-5.html †† Other species of molybdenum have been observed during this induction period as well, such as various forms of Mo oxides.15
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Other metals besides Mo have been investigated for MA on ZSM-5, but not as extensively. For example, Re/HZSM-5 is active and (when CO/CO2 is added at a few % level) relatively stable.22,28
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Fig. 5
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Postulated mechanism for methane aromatization on Mo/ZSM-5 catalysts showing the roles of the Mo–C sites and the acid sites in the zeolite.12
Fig. 7 Conversion of methane, rate, and selectivity of the formation of various products on Mo2C/ZSM-5 at 973 K. Mo2C was prepared by decarbonylation and carburization of Mo(CO)6 deposited on ZSM-5.10
Fig. 6 Product distribution for MA over 3%Mo/HZSM-5. Methane conversion at 973 K on 3 wt% Mo/HZSM-5: (a) methane conversion (%) and rates of product formation for benzene, naphthalene, and C2 hydrocarbons on carbon base (nmol s1 g1 cat), (b) selectivities of hydrocarbons, coke, and CO formation (%), and (c) product composition (%) on carbon base for hydrocarbon products versus time on stream (min).6
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Fig. 8 Methane conversion and benzene selectivity for CH4 reaction over 2 wt% Mo/HZSM-5 at 700 1C, 1 atm, and GHSV = 800 h1; K, ’ pretreated in O2 at 700 1C; J, & pretreated in 20% CH4/H2 and then in 10% CH4/H2 at 700 1C, following treatment in O2 at 700 1C for 0.5 h.5
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Fig. 9 Effect of copper promotion of Mo/ZSM-5: methane conversion and benzene selectivity with time on stream at 1023 K: ((a) H-ZSM-5; (b) Cu/H-ZSM-5; (c) 3% MoO3/H-ZSM-5; (d) 3% MoO3-Cu/H-ZSM-5).19
Fig. 12 Effects of oxygen on methane aromatization over 2 wt% Mo/H-ZSM-5 at 770 1C; content (E) 0 vol%, ( ) 1.5 vol%, (n) 2.1 vol%, () 3.2 vol%, () 5.3 vol%, (K) 8.4 vol%.27
Mo/MCM zeolites
Fig. 10 Mechanism of coke formation/removal by CO/CO2.22 In this mechanism, CO/CO2 apparently oxidizes coke produced from the dehydrogenation of methane, with CO2 being reduced by ‘‘CHz’’ formed on the surface (presumably different from ‘‘CHx’’, with x > z).‡‡ CO/CO2 can also oxidize the Mo carbide at higher temperatures and concentrations, forming the inactive oxide, so there are limits to the conditions at which CO/CO2 is beneficial in limiting carbon formation.24
Fig. 11 Effect of nitric oxide on methane aromatization over 2 wt% Mo/H-ZSM-5 at 770 1C; NO content (E) 0 vol%, ( ) 5.6 vol%, ( ) 9.1 vol%, (x) 14.2 vol%.27
In one of these studies,22 a direct comparison to Mo/HZSM-5 under the same conditions shows no significant advantage to Re/HZSM-5 compared to Mo/HZSM-5 (Table 1; references therein can be found in the original paper). ‡‡ However, this mechanism is somewhat unclear, e.g., the fate of [C] formed from CO dissociation is not clear, nor the role of hydrogen.
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Another class of catalysts that have been examined for this reaction are the so-called MCM zeolites. Of particular interest here are the MCM-22 materials, with larger pores than ZSM-5. They have a unique pore structure consisting of two independent pore systems (Fig. 13).29–31 The first is composed of 12-member ring (MR) cages with dimensions of 1.8 0.71 0.71 nm, slightly larger than those in ZSM-5. The cages are connected through 10 MR windows. The second is a two-dimensional 10 MR pore system that does not contain any cages. There is no direct connection between each of the two pore systems. As with the H-ZSM-5 catalysts, the formation of coke on MCM-22 can be affected by the use of an oxidant in the feed gas, such as CO or CO2.23 Though no direct comparison was made to carbon deposition without CO2, the results of this particular study offer valuable insight into the nature and rate of carbon deposition on these catalysts. Specifically, Fig. 14 shows the total carbon deposition with time on stream (left panel) and the TPO characterization§§ of the carbon (right panel). The low-temperature (more reactive) carbon is attributed to carbon on the Mo sites, while the high-temperature carbon is believed to be associated with the acid sites on the zeolite. This is consistent with the conclusions of other investigators, e.g., ref. 32. Note that with time on stream, more of the carbon forms on the acid sites. The loss of the lowtemperature peak with time suggests that coke on the Mo site migrates or converts to coke on adjacent acidic zeolite sites.¶¶
§§ Temperature programmed oxidation (TPO) is a commonly used method to characterize oxidizable carbon on a used catalyst. It consists of ramping the temperature in a stream of dilute oxygen and measuring the CO2 TCD signal in the outlet gas. The peak area in a TPO spectra corresponds to the amount of carbon, while the peak temperature corresponds to the reactivity of the carbon. ¶¶ See Fig. 2 where the general mechanism presented there would correspond to this—with coke from the dehydrogenated/activate CHx species would form on Mo sites while coke produced from the further condensation of the aromatics (in series with the formation of the desired product) would form on the acid sites of the zeolite. Missing from the scheme in Fig. 2 is the possibility that coke formed from CHx could be transformed into coke similar to that formed from the condensation of aromatic products.
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Comparison of Re- and Mo/H-ZSM-5 for methane aromatization22 a
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Selectivity for productc (%) Catalyst
GHSV (h1)
Time (h)
Conv.b (%)
C2’s
Bz
Naph
Coke
Ref.
5% 3% 2% 2% 4%
720 720 720 800 750
2(6) 2(6) 1 2(6) 2(6)
7(6) 9(7) 6 9(5) 10(8)
4(6) 3(5) 4 3(4) 2(5)
48(56) 41(45) 50 57(62) 65(69)
11(10) 20(15) NA 15(9) 18(9)
33(23) 33(32) 43 15(15) 3(1)
This work Our work (14 and 15) 7 12 22
Re Mo MoO3 Mo Mo
a
Experimental conditions: reaction pressure was 1 atm (except for Re of 3 atm), reaction temperature was 973 K (except for 4% Mo of 950 K), space velocity was calculated assuming packed density of 0.5 g cat cm3. b Methane conversion. c C2’s = ethene + ethane, Bz = benzene, Naph = naphthalene, NA = not available.
blockage than comparable catalysts from which aluminum had not been removed.33 Comparison of Mo/HZSM-5 and Mo/MCM-22 In one of the few direct comparisons of these two catalysts, Bai et al. show that the Mo/MCM-22 has less deactivation and maintains higher benzene selectivity than Mo/HZSM-5, at least at the conditions tested (which include 2% CO2 in the methane feed).34 Fig. 16 shows the results. A similar comparison of these two catalysts came to the same conclusion (Table 235). A detailed kinetic study comparing Mo/HZSM-5 and Mo/MCM-22 finds that the mechanism of the MA reaction is essentially the same on the two catalysts31—methane reacts on the Mo sites to produce ethylene, which reacts further on the acid sites of the two catalysts to form aromatics (both benzene and coke precursors). Measurements showed that acid site strengths of the two catalysts were comparable. However, the circular channels and larger pore mouth structures of ZSM-5 led to greater Mo dispersion and thus a methane dimerizak
Fig. 13 Schematic representations of MCM-22 framework structure. MCM-22 consists of an interconnected building unit forming two independent pore systems: two-dimensional, sinusoidal 10-ring interlayer channels of 0.40 0.59 nm and 12-ring interlayer supercages of 0.71 0.18 nm with 0.40 0.59 nm entrance aperture; (a) vertices represent silicon or aluminum atoms and oxygen atoms are omitted for clarity the shaded region shows two 12 MR ‘‘cups’’ back to back, connected by a double six ring;29 (b) the dark lines show the channels.31
Fig. 15 shows the corresponding changes in conversion and selectivity for this same experiment. Note the relatively stable selectivity to benzene, despite the decrease in CH4 conversion. The MCM-22 zeolite can also be modified to reduce carbon formation. For example, the removal of surface aluminum from the zeolite increased benzene selectivity to 80% and also decreased carbon deposition, which was attributed to less pore
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tion rate coefficient (2CH4 ! C2 H4 þ H2 ) two orders of magnitude greater than Mo/MCM-22. Variations on the MCM-22 structure have also been investigated—e.g., Mo/MCM-49, with larger pores than MCM-22 has shown reasonably comparable selectivity, activity and carbon deposition as the Mo/MCM-22.36 As in the case of Mo/ZSM-5 catalysts, various gases have been co-fed with methane in an attempt to minimize carbon deposition on Mo/MCM-22, including: (a) CO2 (ref. 32). The effect of these gases on coke formation is significant. For example, Table 3 shows that CO2 in the feed reduces total coke formation from 199 to 131 mg g1 cat.32 at one set of conditions. In this same study, there was little effect of added CO2 on CH4 conversion or benzene selectivity with time on stream. (b) H2 (ref. 37). A study by the same group using pulses of hydrogen to limit carbon formation found that this procedure decreased the amount of coke formation, especially coke associated with the acid sites, which are thought to catalyze the formation of polynuclear aromatics, which are coke precursors.37 Other zeolites for MA Though not as extensively studied, there are a wide range of other zeolites besides ZSM-5 and MCM-22 that have been investigated for the MA reaction.
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Fig. 14 Left panel: coke deposition on catalysts reacted for different lengths of time onstream; right panel: temperature-programmed oxidation (TPO) of deposited coke on the catalysts reacted for different lengths of time onstream. The four curves in the right panel correspond to TPOs taken after each of the four times on stream in the left panel.23
Fig. 16 Comparison of reaction performance between 6Mo/MCM-22 and 6Mo/ZSM-5. (A) Comparison of the methane conversion and selectivity for CO, coke and hydrocarbons; (B) comparison of the selectivity for benzene and naphthalene in hydrocarbons; in Fig. 1(A) and (B), the full symbols represent the data obtained on 6Mo/MCM-22 and the open symbols represent the data obtained on 6Mo/ZSM-5; reaction conditions: T = 993 K, P = 1 atm, SV = 1500 mL g1 h1.34 Fig. 15 Methane aromatization on 6Mo/MCM-22 catalyst as function of time onstream. (A) The conversion of CH4 and the selectivity of CO, coke and hydrocarbons in CH4 conversion, (B) the selectivity of benzene and naphthalene in hydrocarbons. Reaction conditions: T = 993 K, P = 1 atm, SV = 1500 mL g1 h1, CO2 = 2%.23
Among the highest yields of benzene is work by Choudhary et al., in which various alkanes/alkenes are added to the methane feed,
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using a H–GaAlMFI zeolite (however, the methane concentration in the feed was only 33%).7 This zeolite is bifunctional—with strong acid sites at the framework tetrahedral of the Al and Ga in the zeolite, and dehydrogenation due to the zeolitic protons and extra framework gallium oxide. Table 4 shows the results. Others have reported promising results with variations of these Ga MFI zeolites.38
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Table 2 Comparison of Mo/HZSM-5 and Mo/MCM-22 for methane aromatization at the optimum reaction conditions35 a
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Sample
Selectivity (%) Yields (%) Conversion of methane (%) CO C2 Ben. Nap. Coke Ben. Nap.
2MO/MCM-22 5.7 6MO/MCM-22 10.0 l0Mo/MCM-22 5.8 2Mo/ZSM-5 9.7 6Mo/ZSM-5 10.6 l0Mo/ZSM-5 10.7
0.9 0.2 0.8 0.7 1.0 1.0
4.9 3.4 4.5 4.1 3.3 3.6
67.8 7.9 18.5 80.0 4.4 12.0 61.4 3.6 29.7 47.2 22.0 26.0 57.8 19.8 18.1 47.7 13.2 34.5
3.9 8.0 3.6 4.6 6.1 5.1
0.5 0.4 0.2 2.1 2.1 1.4
a Reaction temperature is 973 K, space velocity is 1500 mL g1 h1. All results were the values at optimum reaction stage, i.e., after 180 and 90 min of running for Mo/MCM-22 and Mo/HZSM-5 catalysts, respectively. There is apparently no CO or CO2 in the feed gas (see p. 69 of the paper), so that the formation of CO must come from either the oxidized form of the catalyst, or the zeolite.
Table 3 Effect of added CO2 on carbon deposition in methane aromatization on Mo/MCM-2232
Amount of carbon deposits by TGA
4.1
Water Carbidic coke Non-carbidic Total coke Coke coke (mg g1) (mg g1) (%) (%) (mg g1) Without CO2 4.5 8.4 With CO2
0.6 —
198.7 131.2
the effects of deactivation by carbon deposition and/or removing hydrogen to drive the reaction toward the desired products. All include some means of regenerating the catalyst, either continuously (typically in a circulating fluid bed) or periodically (e.g., in multiple stationary beds). Heat integration is also a key design objective. It is not the purpose of the discussion here to exhaustively examine and analyze the wide range of reactor designs in the literature. Rather, a representative example is provided for three conceptual reactors of interest: (a) A coupled reactor in which methane is first reacted with substoichiometric oxygen to form primarily ethylene, which then feeds the MA reactor.48,49 (b) Membrane reactors, in which hydrogen produced from the MA reaction is continuously removed to drive the reaction toward benzene.50 (c) A ‘‘two-zone’’ reactor in which both MA and catalyst regeneration take place in a single vessel.51
199.3 131.2
19.9 13.1
Notes: Water is desorbed as ‘‘physisorbed water from the zeolite pores’’. ‘‘Carbidic coke’’ is carbon associate with Mo carbide, the active form of the catalyst, which is oxidized to molybdenum oxide during the TGA procedure. ‘‘Non-carbidic coke’’ is carbon deposited on the catalyst during the run. Run conditions: 700 C at a space velocity of 90 gCH4/ gMo h1; or with the addition of a CO2 stream at a space velocity of 10 gCO2/gMo h1 (CH4/CO2 molar ratio of 25).
A number of studies on other various zeolites such as Mo/ITQ-2,39 Mo/ITQ-13,40 Pt–Ga silicates,41 MFI-supported molybdenum,42 MFI-supported rhenium,43 Mo/HZRP-1,44 metal (Cr, Mo, W)-doped carbon aerogels,45 Ni–Re/Al2O3,46 and various forms of unsupported Mo carbide9,47 show no particular advantage over the ZSM-5,MCM-22, or H–GaAl MFI catalysts discussed above.
4.0 Reactor designs A number of different reactor designs have been proposed, all of which have the goal of maximizing benzene yield by minimizing
Coupled reactor
One reactor design integrates an oxidative coupling of methane (OCM) reactor (which converts methane to ethylene) with a second reactor using Ga/ZSM-548 or 6Mo/HMCM-4949 to convert ethylene to aromatics (Fig. 17). The formation of H2O and CO2 in the OCM reactor then limits carbon formation in the MA reactor downstream.49 4.2
Membrane reactors
These reactors have the goal of removing hydrogen to drive the MA reaction toward the formation of benzene by removing the hydrogen continuously through a selective membrane. In its simplest configuration, it consists of a packed tube containing a solid catalyst, a radially centered tube coated with a selective membrane, through which hydrogen permeates (Fig. 18).50 One study on such a reactor showed a positive effect of hydrogen removal (using a Pd–Ag membrane),50 as shown in Table 5. A closely related study using a similar membrane reactor showed a corresponding increase in benzene yield resulting from hydrogen removal, but also observed more rapid deactivation, presumably due to the lower level of hydrogen in the gas, which can limit carbon formation.52 A theoretical modeling study by the Iglesia group showed the potential for these systems
Table 4 Results for the aromatization of methane over H–GaAl MFI zeolite in the presence of different additives (alkenes or higher alkanes) in the feed (a mixture of CH4, N2, and additive; concentration of methane in feed, 33.3 mol%)7
Conversion (%)
Additive in feed
Additive/CH4 molar ratio
Space velocity (cm3 g1 h1)
Temp. (8C)
CH4
Additive
Selectivity for aromatics (%)
None None n-Butene i-Butene Propene Ethene Propane n-Hexane
0.0 0.0 0.5 0.5 0.8 1.0 0.7 0.8
3100 6200 3100 6200 6200 6200 3100 3100
600 600 600 500 500 600 600 600
0.0 0.0 45.0 44.2 34.7 36.3 12.0 24.1
— — 100 100 99.7 98.8 91.1 100
— — 92.0 93.8 91.8 93.8 84.2 93.8
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SV (mL h1 g1)
H2 separation
Conversion (%)
120
No Yes No Yes No Yes No Yes No Yes No Yes
2.5 7.5 2.4 5.9 2.6 4.9 2.6 4.2 2.4 3.5 2.4 3.3
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180 240 360 720 1440
Selectivity (%) C2
C6H6
C7H8
C10H8
9.6 3.2 5.4 4.1 6.2 5.5 6.9 6.4 8.8 8.3 11.3 8.2
56.0 53.3 70.8 64.4 69.2 65.3 69.2 66.7 70.8 71.4 62.5 63.6
2.4 3.5 2.9 4.4 3.1 4.3 3.8 4.8 4.6 5.4 4.2 5.5
32.0 40.0 20.8 27.1 23.1 24.5 19.2 21.4 16.7 14.6 22.1 22.4
Pressure = 100 kPa; the permeability of H2 through the membrane is ca. 75%.
Fig. 17 Schematic diagram of integrated recycle system for conversion of methane to aromatics. C = mass flow controller; F = flowmeter; P = gas sampling port; R = pressure regulator.48
Fig. 18 Membrane reactor in which hydrogen is continuously removed in the tube side of an annular reactor containing solid catalyst.50
includes almost complete CH4 conversion at B1000 K at practical reactor residence times (o100 s).53 4.3
Two-zone reactor
In this reactor, methane is fed to the middle of the reacting zone, creating two zones in the bed with different environments: an oxidizing atmosphere in the lower part and a reducing atmosphere in the upper part, Fig. 19.51 As the catalyst deactivates by coke formation, this denser catalyst falls to the lower part of the reactor, where it is
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Fig. 19 Two zone fluidized bed reactor.51
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regenerated and rises to the upper (reaction) level. Performance data given in this study using Mo/HZSM-5 show up to 90% benzene + hydrocarbon selectivity, with methane conversion dropping to about 8% after 400 min.88
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5.0 Conclusions Methane aromatization (MA) is of great potential interest as a means of producing both a high-value aromatic compounds as well as pure hydrogen in one reaction. However, the reaction inevitably produces carbon deposits on the catalyst. Carbon formation is thermodynamically favored at all conditions of practical interest, but can be kinetically limited by the addition of hydrogen or oxidants such as CO2 or steam. Thermodynamic calculations suggest that the addition of C2–C4 alkanes/alkenes to the methane feed can improve the selectivity to benzene. However, experimental studies to test this approach are limited, suggesting an area for further study. There are a number of reactor designs that have been evaluated for this reaction, all with the goal of continuous regeneration of the catalyst while maintaining reaction conditions that favor MA in the reaction zone. Although a number of catalysts have been investigated for MA, by far the most well-studied are those based on Mo/HZSM-5 and Mo/MCM-22. These catalysts appear to function by activation of methane on a molybdenum carbide site, formed in the initial stages of the reaction, and oligomerization on the acidic sites of the zeolite, which provides a shapeselective function to maximize benzene selectivity. One area for further study is the application of computational catalysis to this reaction. There are few studies in which DFT methods have been used to follow the reaction mechanism and carbon deposition.54,55 Given the recent advances in these methods, there are likely opportunities to improve the selectivity and kinetically limit carbon formation using the tools of computational catalysis.
Acknowledgements This work was supported by Rubicon LLC (A joint venture of Huntsman and Chemtura).
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7 V. R. Choudhary, A. K. Kinage and T. V. Choudhary, Science, 1997, 275, 1286–1288. 8 L. Devries and P. R. Ryason, US4814534A, 1989. ´, Catal. Lett., 1999, 57, 9 F. Solymosi, L. Bugyi and A. Oszko 103–107. ´nyi, A. Szo ¨ke, T. Ba ´nsa ´gi and A. Oszko ´, 10 F. Solymosi, J. Csere J. Catal., 1997, 165, 150–161. 11 D. Wang, J. H. Lunsford and M. P. Rosynek, J. Catal., 1997, 169, 347–358. 12 R. W. Borry III, E. C. Lu, Y. H. Kim and E. Iglesia, NonOxidative Conversion of Methane with Continuous Hydrogen Removal, US Dept. of Energy/NETL, Morgantown, WV, contract DE-AC03-76SF00098. 13 J. F. Haw, W. Song, D. M. Marcus and J. B. Nicholas, Acc. Chem. Res., 2003, 36, 317–326. 14 D. Ma, Y. Shu, M. Cheng, Y. Xu and X. Bao, J. Catal., 2000, 194, 105–114. 15 H. Jiang, L. Wang, W. Cui and Y. Xu, Catal. Lett., 1999, 57, 95–102. 16 L. Men, L. Zhang, Y. Xie, Z. Liu, J. Bai, G. Sha and J. Xie, Chem. Commun., 2001, 1750–1751. 17 H. Zheng, D. Ma, X. Bao, Z. H. Jian, H. K. Ja, Y. Wang and C. H. F. Peden, J. Am. Chem. Soc., 2008, 130, 3722–3723. 18 R. W. Borry Iii, Y. H. Kim, A. Huffsmith, J. A. Reimer and E. Iglesia, J. Phys. Chem. B, 1999, 103, 5787–5796. 19 S. Li, C. Zhang, Q. Kan, D. Wang, T. Wu and L. Lin, Appl. Catal., A, 1999, 187, 199–206. 20 A. Hassan and A. Sayari, Appl. Catal., A, 2006, 297, 159–164. ¨ke and F. Solymosi, Appl. Catal., A, 1996, 142, 361–374. 21 A. Szo 22 L. Wang, R. Ohnishi and M. Ichikawa, J. Catal., 2000, 190, 276–283. 23 J. Bai, S. Liu, S. Xie, L. Xu and L. Lin, Catal. Lett., 2003, 90, 123–130. 24 Z. Liu, M. A. Nutt and E. Iglesia, Catal. Lett., 2002, 81, 271–279. 25 R. Kojima, S. Kikuchi and M. Ichikawa, Chem. Lett., 2004, 1166–1167. 26 R. Kojima, S. Kikuchi, H. Ma, J. Bai and M. Ichikawa, Catal. Lett., 2006, 110, 15–21. 27 P. L. Tan, Y. L. Leung, S. Y. Lai and C. T. Au, Catal. Lett., 2002, 78, 251–258. 28 L. Wang, R. Ohnishi and M. Ichikawa, Catal. Lett., 1999, 62, 29–33. 29 G. G. Juttu and R. F. Lobo, Microporous Mesoporous Mater., 2000, 40, 9–23. 30 M. E. Leonowicz, J. A. Lawton, S. L. Lawton and M. K. Rubin, Science, 1994, 264, 1910–1913. ¨cker and 31 K. S. Wong, J. W. Thybaut, E. Tangstad, M. W. Sto G. B. Marin, Microporous Mesoporous Mater., 2012, 164, 302–312. 32 A. C. C. Rodrigues and J. L. F. Monteiro, Catal. Lett., 2007, 117, 166–170. 33 A. C. C. Rodrigues and J. L. F. Monteiro, Stud. Surf. Sci. Catal., 2004, 147, 709–714. 34 J. Bai, S. Liu, S. Xie, L. Xu and L. Lin, React. Kinet. Catal. Lett., 2004, 82, 279–286.
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Catalytic aromatization of methane Cite this: DOI: 10.1039/c3cs60259a
James J. Spivey*a and Graham Hutchingsb Recent developments in natural gas production technology have led to lower prices for methane and renewed interest in converting methane to higher value products. Processes such as those based on syngas from methane reforming are being investigated. Another option is methane aromatization, which produces benzene and hydrogen: 6CH4(g) - C6H6(g) + 9H2(g) DGor = +433 kJ mol1 DHor = +531 kJ mol1. Thermodynamic calculations for this reaction show that benzene formation is insignificant below B600 1C, and that the formation of solid carbon [C(s)] is thermodynamically favored at temperatures above B300 1C. Benzene formation is insignificant at all temperatures up to 1000 1C when C(s) is included in the calculation of equilibrium composition. Interestingly, the thermodynamic limitation on benzene formation can be minimized by the addition of alkanes/alkenes to the methane feed. By far the most widely studied catalysts for this reaction are Mo/HZSM-5 and Mo/MCM-22. Benzene selectivities are generally between 60 and 80% at methane conversions of B10%, corresponding to net benzene yields of less than 10%. Major byproducts include lower molecular weight hydrocarbons and higher molecular weight substituted aromatics. However, carbon formation is inevitable, but the experimental findings show this can be kinetically limited by the use of
Received 18th July 2013
H2 or oxidants in the feed, including CO2 or steam. A number of reactor configurations involving regeneration
DOI: 10.1039/c3cs60259a
of the carbon-containing catalyst have been developed with the goal of minimizing the cost of regeneration of the catalyst once deactivated by carbon deposition. In this tutorial review we discuss the thermodynamics of
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this process, the catalysts used and the potential reactor configurations that can be applied.
Key learning points 1. Methane aromatization is thermodynamically unfavorable at all conditions of practical interest, but can be made more feasible by the addition of low molecular weight alkanes/alkenes. 2. Carbon deposition is inevitable and presents a significant barrier to practical use of this reaction. Carbon deposition can be kinetically limited by the addition of hydrogen or oxidants such as steam or CO2. 3. By far the most widely studied catalysts are those based on Mo/zeolites. Molybdenum is carbided during the reaction and activates methane, which is oligomerized on the acid sites of the zeolite. 4. A number of reactor designs have been developed for this reaction, all with the goal of regenerating the carbon-containing catalyst and removing hydrogen to drive the reaction to the desired products. 5. A promising area for further study is the application of computational methods to this reaction, especially to understand the role of lower molecular weight compounds in improving the net yield of benzene.
1.0 Introduction Methane is a very abundant fossil fuel resource that is widely distributed around the globe. Together with carbon (in the form of coal) and carbon dioxide, methane is one of the most thermodynamically stable forms in which carbon can be widely found. Its thermodynamic stability represents a challenge to a
Dept. Chemical Engineering, Louisiana State University, Baton Rouge, LA, USA 70803. E-mail: [email protected] b Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, UK CF10 3AT
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chemists and the direct activation of methane to chemicals has been a dream of many chemists for over a century. At present the commercial processes using methane involve an indirect process in which methane is reacted with water (steam reforming) or with oxygen (partial oxidation) to form synthesis gas, a mixture of CO and H2, that is subsequently reacted to form fuels and chemicals. Overall the utilization of methane tends to require high temperatures and high pressures and is energy intensive. In view of this it is not surprising that attempts to use methane in a direct process for the formation of chemicals also require high temperatures. One notable approach using oxidation was heralded by the seminal work of Lunsford
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which showed that methane could be oxidatively coupled to form ethane and ethylene with very high selectivities using oxides at 600 1C.1 However, this reaction involves gas phase radical chemistry and the selectivity is lost at conversions >10%. Subsequently, zeolites were found to be effective for the conversion of propane and higher alkanes to aromatic chemicals. This process, known as the CYCLAR process, has been commercialized and was used as the inspiration for novel catalysis in which methane is converted to aromatic molecules using zeolite catalysts.2 This methane aromatization reaction is the topic of this tutorial review. Ismagilov et al. provide a review of this and related reactions through 2008.3 With the discovery of shale-gas in massive deposits in many locations, the direct utilisation of methane has once again gained research impetus. In this review we will cover all fundamental aspects of this catalysed reaction including the thermodynamic limitations, catalysis and kinetics as well as the possible reactor configurations so that the reader gains a complete insight into this fascinating chemistry.
The conversion of methane to benzene is known as ‘‘dehydrocyclization’’; ‘‘aromatization’’; and ‘‘dehydroaromatization’’. For the purposes of this review, this reaction will be referred to as MA (‘‘methane aromatization’’, which is the most widely used term in the literature). This direct conversion of methane to benzene is as follows:
DHor = +531 kJ mol1
DGor = +433 kJ mol1 (1)
This is an extremely unfavorable reaction thermodynamically.† Fig. 1 shows the result of a calculation of the equilibrium composition‡ of a mixture of an initial 6 kmol of CH4 and allowing † The thermodynamic results shown here are consistent with those published elsewhere.4 ‡ Calculated as a Gibbs free energy minimization in which only methane, benzene, and hydrogen are allowed using HSC 7.1 software (HSC, Houston, TX).
James Spivey is Professor of Chemical Engineering at Louisiana State University. He is the Editor-in-Chief of Catalysis Today, and Editor of the Royal Society of Chemistry’s Catalysis book series. He is a Fellow of the RSC, and author of over 175 publications. He serves as Director of the Center for Atomic-level Catalyst Design, an Energy Frontier Research Center supported by the US Dept. of James J. Spivey Energy. His research focuses on catalysis for clean energy processes, syngas chemistry, and hydrocarbon conversion.
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6CH4(g) + 412O2(g) - C6H6(g) + 9H2O(g) DGor = 1624 kJ mol1
2.0 Thermodynamics of methane aromatization
6CH4(g) - C6H6(g) + 9H2(g)
only benzene and hydrogen in the mixture (solid carbon, C(s), is not allowed). It is clear that benzene formation at equilibrium becomes significant only at impractically high temperatures. A corresponding equilibrium plot for this reaction, with C(s) included in the calculation, shows the following (Fig. 2). At equilibrium, virtually no benzene is formed. Thermodynamically, the reaction goes to solid carbon [C(s)] and hydrogen, as expected. The mechanism can be viewed as follows (Fig. 3), where ‘‘coke’’ would correspond to solid carbon,§ C(s).6 In this mechanism, coke is formed in two ways: (a) from dehydrogenated methane (‘‘CHx’’) in parallel with the desired C–C bond formation step, and (b) from lower MW intermediates (e.g., C2Hy in Fig. 3) in series with the formation of the desired product, benzene (benzene can be viewed as R = 0 in Fig. 3). Two approaches to addressing the equilibrium limitation on benzene formation from MA are: (a) the use of oxygen (or another oxidant),¶ e.g.,:
DHor = 1846 kJ mol1
(2)
Though thermodynamically favorable, the reaction would likely have poor selectivity in practice since benzene would likely decompose to C(s) or be oxidized as soon as it was formed.8 (b) the addition of alkane/alkenes, e.g., n-butane:7 2CH4 ðgÞ þ n-C4 H10 ðgÞ ! C6 H6 ðgÞ þ 6H2 ðgÞ C
DG700 r
¼ 36:2 kJ mol1
C
DHr700
¼ þ396 kJ mol1
(3)
§ In the literature, the terms ‘‘coke’’ and ‘‘carbon’’ are often used interchangeably to denote the carbon-rich deposits formed on catalysts during various reactions. These deposits include a wide range of carbon species, including polynuclear aromatics as well as more graphitic species.5 In this review, no distinction is made among the various types of carbon deposits, and ‘‘coke’’ and ‘‘carbon’’ are used interchangeably. ¶ 1 bar; calculated using HSC 7.1 software. 8 When C(s) is not allowed, the equilibrium composition of a mixture of CH4, C6H6, O2, and H2O (starting from 1 mol CH4) contains 0.167 mol benzene at temperatures between 605 and 1000 1C, 1 bar. When C(s) is included in the corresponding value is o2 1027.
Graham Hutchings
Graham Hutchings, born 1951, studied chemistry at University College London. His early career was with ICI and AECI Ltd where he became interested in gold catalysis. In 1984 he moved to academia and has held chairs at the Universities of Witwatersrand, Liverpool and Cardiff and currently he is Director of the Cardiff Catalysis Institute. He was elected a Fellow of the Royal Society in 2009, and he was awarded the Davy Medal of the Royal Society in 2013.
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Fig. 1
Equilibrium amounts (moles), 1 bar, starting with 6 mol CH4 and allowing only H2 and benzene as components. Calculated using HSC 7.1 software.
Fig. 2
Equilibrium amounts (moles), 1 bar, starting with 6 mol CH4 and allowing C(s), H2, and benzene as components. Calculated using HSC 7.1 software.
Fig. 3 Mechanism for the formation of aromatics and coke in methane aromatization (MA).6
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The conclusion from these various thermodynamic analyses is that the conversion of methane to benzene is thermodynamically favorable at limited conditions, and is likely to be kinetically driven to undesired products such as carbon. The addition of alkanes/alkenes has been shown experimentally to drive the reaction to ‘‘aromatics’’, but the one study exploring this approach did not quantify the selectivity to benzene specifically.7 A qualitative claim of the positive effect of adding C2–C4 hydrocarbons on MA has also been reported.8
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3.0 Catalysts/kinetics
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Mo/ZSM-5 A wide range of catalysts have been investigated for the MA reaction. By far the most widely studied catalysts are those based on Mo/ZSM-5 zeolite. Fig. 4 shows the structure of the ZSM-5 catalyst.** For the MA reaction, various forms of molybdenum are introduced into the zeolite to form a ‘‘bifunctional’’ catalyst – both the Mo site and the acidity/shape selectivity of the zeolite are essential. The Mo site is thought to activate methane, perhaps producing C2H4 that is then oligomerized to C6H6 on the acid sites.9–12 This general mechanism, reflecting the uncertainty in the exact nature of the intermediate formed from methane, is shown in Fig. 5.12 To some extent there is an analogy between the methane aromatization reaction and the conversion of methanol to hydrocarbons over the same zeolite catalyst, i.e. H-ZSM-5. In the conversion of methanol it is believed that carbonaceous deposits within the zeolitic pores act as a scaffold on which the methanol is converted to ethene as a primary product. This process is referred to as the carbon pool mechanism,13 and it is feasible that a similar process operates within the catalyst pores during methane aromatization. In this way methane conversion on the carbonaceous deposits leads to the C2 fragments that oligomerize and aromatize. A number of studies have shown an ‘‘induction period’’ during which the molybdenum is gradually transformed into an active species, (e.g., by the formation of a molybdenum carbide or oxycarbide site upon reduction by methane).14–17 †† The pore size of these catalysts approximates that of the benzene molecule (0.59 nm)4 and provides shape selectivity to the desired product. However, some reports also suggest that migration of the Mo from the channels of the zeolite to the outer surface accompanies the reduction process.11,18 This would presumably limit, if not eliminate, any influence of the zeolite pore on the selectivity. However, it appears that at least some of the molybdenum remains in the channels of the zeolite and increases the benzene yield. Typically, a wide range of higher hydrocarbons are formed on Mo/ZSM-5 catalysts. Fig. 6 shows one example for a 3%Mo/ HZSM-5 catalyst.6 Methane conversion generally decreases with time as carbon forms on the catalyst surface, but this does not always lead to changes in benzene selectivity. An example is shown below for a Mo/HZSM-5 catalyst in which the molybdenum was pre-carbided to form Mo2C (Fig. 7).10 Note the high selectivity to benzene despite the significant decrease in methane conversion with time. Similar results are observed for many of the studies on Mo/HZSM-5 catalysts, e.g. Fig. 8 below.5 Various ‘‘promoters’’ have been investigated with the goal of increasing the selectivity to benzene, overall methane conversion,
Fig. 4 Schematic of ZSM-5 zeolite, consisting of an intersecting twodimensional pore structure. There are two types of pores, both formed by 10-membered oxygen rings. One is straight with an elliptical cross section, while the second type of circular cross-section pores intersect the straight pores at right angles in a zig-zag pattern. The effective pore diameter is 0.59 nm, roughly the kinetic diameter of benzene.
and/or decreasing carbon deposition for Mo/ZSM catalysts. Examples include copper,19 ruthenium,20 and potassium.21 An example showing the effect of copper promotion is shown below, Fig. 9.19 Note in Fig. 9: (a) The induction period (observed on most Mo/ZSM catalysts) during which both methane conversion and benzene selectivity increase. (b) The positive effects of Cu promotion on methane conversion and benzene selectivity. (c) The inevitable decrease in both conversion and selectivity with time, likely due to carbon deposition. Coke deposition is a critical and inevitable process in the MA reaction. On the ZSM-5 catalysts, various approaches have been attempted to minimize carbon formation—e.g., addition of the following along with methane in the feed: (a) addition of CO or CO2 (ref. 22–24). The mechanism by which CO/CO2 reduces carbon formation is postulated to be as shown in Fig. 10.22 (a) H2 (ref. 24–26). Hydrogen can be added to the feed, but is also formed during the MA reaction. It was found that hydrogen minimizes carbon formation,24 particularly on the Bronsted acid sites of the zeolite.25,26 (b) NO27. The addition of NO as an oxidant on the yield of aromatics shows that there is an optimum (Fig. 11), below which presumably coke is not oxidized sufficiently rapidly, and above which oxidation of the active Mo carbide decreases the activity.27 (c) O2 (ref. 27). Similarly for O2 addition, there appears to be an optimum, with a minimum in deactivation rate at an intermediate level of added O2, Fig. 12.27 Other metals for MA on ZSM-5 zeolites
** Image from: http://chemelab.ucsd.edu/methanol/memos/ZSM-5.html †† Other species of molybdenum have been observed during this induction period as well, such as various forms of Mo oxides.15
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Other metals besides Mo have been investigated for MA on ZSM-5, but not as extensively. For example, Re/HZSM-5 is active and (when CO/CO2 is added at a few % level) relatively stable.22,28
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Fig. 5
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Postulated mechanism for methane aromatization on Mo/ZSM-5 catalysts showing the roles of the Mo–C sites and the acid sites in the zeolite.12
Fig. 7 Conversion of methane, rate, and selectivity of the formation of various products on Mo2C/ZSM-5 at 973 K. Mo2C was prepared by decarbonylation and carburization of Mo(CO)6 deposited on ZSM-5.10
Fig. 6 Product distribution for MA over 3%Mo/HZSM-5. Methane conversion at 973 K on 3 wt% Mo/HZSM-5: (a) methane conversion (%) and rates of product formation for benzene, naphthalene, and C2 hydrocarbons on carbon base (nmol s1 g1 cat), (b) selectivities of hydrocarbons, coke, and CO formation (%), and (c) product composition (%) on carbon base for hydrocarbon products versus time on stream (min).6
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Fig. 8 Methane conversion and benzene selectivity for CH4 reaction over 2 wt% Mo/HZSM-5 at 700 1C, 1 atm, and GHSV = 800 h1; K, ’ pretreated in O2 at 700 1C; J, & pretreated in 20% CH4/H2 and then in 10% CH4/H2 at 700 1C, following treatment in O2 at 700 1C for 0.5 h.5
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Fig. 9 Effect of copper promotion of Mo/ZSM-5: methane conversion and benzene selectivity with time on stream at 1023 K: ((a) H-ZSM-5; (b) Cu/H-ZSM-5; (c) 3% MoO3/H-ZSM-5; (d) 3% MoO3-Cu/H-ZSM-5).19
Fig. 12 Effects of oxygen on methane aromatization over 2 wt% Mo/H-ZSM-5 at 770 1C; content (E) 0 vol%, ( ) 1.5 vol%, (n) 2.1 vol%, () 3.2 vol%, () 5.3 vol%, (K) 8.4 vol%.27
Mo/MCM zeolites
Fig. 10 Mechanism of coke formation/removal by CO/CO2.22 In this mechanism, CO/CO2 apparently oxidizes coke produced from the dehydrogenation of methane, with CO2 being reduced by ‘‘CHz’’ formed on the surface (presumably different from ‘‘CHx’’, with x > z).‡‡ CO/CO2 can also oxidize the Mo carbide at higher temperatures and concentrations, forming the inactive oxide, so there are limits to the conditions at which CO/CO2 is beneficial in limiting carbon formation.24
Fig. 11 Effect of nitric oxide on methane aromatization over 2 wt% Mo/H-ZSM-5 at 770 1C; NO content (E) 0 vol%, ( ) 5.6 vol%, ( ) 9.1 vol%, (x) 14.2 vol%.27
In one of these studies,22 a direct comparison to Mo/HZSM-5 under the same conditions shows no significant advantage to Re/HZSM-5 compared to Mo/HZSM-5 (Table 1; references therein can be found in the original paper). ‡‡ However, this mechanism is somewhat unclear, e.g., the fate of [C] formed from CO dissociation is not clear, nor the role of hydrogen.
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Another class of catalysts that have been examined for this reaction are the so-called MCM zeolites. Of particular interest here are the MCM-22 materials, with larger pores than ZSM-5. They have a unique pore structure consisting of two independent pore systems (Fig. 13).29–31 The first is composed of 12-member ring (MR) cages with dimensions of 1.8 0.71 0.71 nm, slightly larger than those in ZSM-5. The cages are connected through 10 MR windows. The second is a two-dimensional 10 MR pore system that does not contain any cages. There is no direct connection between each of the two pore systems. As with the H-ZSM-5 catalysts, the formation of coke on MCM-22 can be affected by the use of an oxidant in the feed gas, such as CO or CO2.23 Though no direct comparison was made to carbon deposition without CO2, the results of this particular study offer valuable insight into the nature and rate of carbon deposition on these catalysts. Specifically, Fig. 14 shows the total carbon deposition with time on stream (left panel) and the TPO characterization§§ of the carbon (right panel). The low-temperature (more reactive) carbon is attributed to carbon on the Mo sites, while the high-temperature carbon is believed to be associated with the acid sites on the zeolite. This is consistent with the conclusions of other investigators, e.g., ref. 32. Note that with time on stream, more of the carbon forms on the acid sites. The loss of the lowtemperature peak with time suggests that coke on the Mo site migrates or converts to coke on adjacent acidic zeolite sites.¶¶
§§ Temperature programmed oxidation (TPO) is a commonly used method to characterize oxidizable carbon on a used catalyst. It consists of ramping the temperature in a stream of dilute oxygen and measuring the CO2 TCD signal in the outlet gas. The peak area in a TPO spectra corresponds to the amount of carbon, while the peak temperature corresponds to the reactivity of the carbon. ¶¶ See Fig. 2 where the general mechanism presented there would correspond to this—with coke from the dehydrogenated/activate CHx species would form on Mo sites while coke produced from the further condensation of the aromatics (in series with the formation of the desired product) would form on the acid sites of the zeolite. Missing from the scheme in Fig. 2 is the possibility that coke formed from CHx could be transformed into coke similar to that formed from the condensation of aromatic products.
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Comparison of Re- and Mo/H-ZSM-5 for methane aromatization22 a
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Selectivity for productc (%) Catalyst
GHSV (h1)
Time (h)
Conv.b (%)
C2’s
Bz
Naph
Coke
Ref.
5% 3% 2% 2% 4%
720 720 720 800 750
2(6) 2(6) 1 2(6) 2(6)
7(6) 9(7) 6 9(5) 10(8)
4(6) 3(5) 4 3(4) 2(5)
48(56) 41(45) 50 57(62) 65(69)
11(10) 20(15) NA 15(9) 18(9)
33(23) 33(32) 43 15(15) 3(1)
This work Our work (14 and 15) 7 12 22
Re Mo MoO3 Mo Mo
a
Experimental conditions: reaction pressure was 1 atm (except for Re of 3 atm), reaction temperature was 973 K (except for 4% Mo of 950 K), space velocity was calculated assuming packed density of 0.5 g cat cm3. b Methane conversion. c C2’s = ethene + ethane, Bz = benzene, Naph = naphthalene, NA = not available.
blockage than comparable catalysts from which aluminum had not been removed.33 Comparison of Mo/HZSM-5 and Mo/MCM-22 In one of the few direct comparisons of these two catalysts, Bai et al. show that the Mo/MCM-22 has less deactivation and maintains higher benzene selectivity than Mo/HZSM-5, at least at the conditions tested (which include 2% CO2 in the methane feed).34 Fig. 16 shows the results. A similar comparison of these two catalysts came to the same conclusion (Table 235). A detailed kinetic study comparing Mo/HZSM-5 and Mo/MCM-22 finds that the mechanism of the MA reaction is essentially the same on the two catalysts31—methane reacts on the Mo sites to produce ethylene, which reacts further on the acid sites of the two catalysts to form aromatics (both benzene and coke precursors). Measurements showed that acid site strengths of the two catalysts were comparable. However, the circular channels and larger pore mouth structures of ZSM-5 led to greater Mo dispersion and thus a methane dimerizak
Fig. 13 Schematic representations of MCM-22 framework structure. MCM-22 consists of an interconnected building unit forming two independent pore systems: two-dimensional, sinusoidal 10-ring interlayer channels of 0.40 0.59 nm and 12-ring interlayer supercages of 0.71 0.18 nm with 0.40 0.59 nm entrance aperture; (a) vertices represent silicon or aluminum atoms and oxygen atoms are omitted for clarity the shaded region shows two 12 MR ‘‘cups’’ back to back, connected by a double six ring;29 (b) the dark lines show the channels.31
Fig. 15 shows the corresponding changes in conversion and selectivity for this same experiment. Note the relatively stable selectivity to benzene, despite the decrease in CH4 conversion. The MCM-22 zeolite can also be modified to reduce carbon formation. For example, the removal of surface aluminum from the zeolite increased benzene selectivity to 80% and also decreased carbon deposition, which was attributed to less pore
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tion rate coefficient (2CH4 ! C2 H4 þ H2 ) two orders of magnitude greater than Mo/MCM-22. Variations on the MCM-22 structure have also been investigated—e.g., Mo/MCM-49, with larger pores than MCM-22 has shown reasonably comparable selectivity, activity and carbon deposition as the Mo/MCM-22.36 As in the case of Mo/ZSM-5 catalysts, various gases have been co-fed with methane in an attempt to minimize carbon deposition on Mo/MCM-22, including: (a) CO2 (ref. 32). The effect of these gases on coke formation is significant. For example, Table 3 shows that CO2 in the feed reduces total coke formation from 199 to 131 mg g1 cat.32 at one set of conditions. In this same study, there was little effect of added CO2 on CH4 conversion or benzene selectivity with time on stream. (b) H2 (ref. 37). A study by the same group using pulses of hydrogen to limit carbon formation found that this procedure decreased the amount of coke formation, especially coke associated with the acid sites, which are thought to catalyze the formation of polynuclear aromatics, which are coke precursors.37 Other zeolites for MA Though not as extensively studied, there are a wide range of other zeolites besides ZSM-5 and MCM-22 that have been investigated for the MA reaction.
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Fig. 14 Left panel: coke deposition on catalysts reacted for different lengths of time onstream; right panel: temperature-programmed oxidation (TPO) of deposited coke on the catalysts reacted for different lengths of time onstream. The four curves in the right panel correspond to TPOs taken after each of the four times on stream in the left panel.23
Fig. 16 Comparison of reaction performance between 6Mo/MCM-22 and 6Mo/ZSM-5. (A) Comparison of the methane conversion and selectivity for CO, coke and hydrocarbons; (B) comparison of the selectivity for benzene and naphthalene in hydrocarbons; in Fig. 1(A) and (B), the full symbols represent the data obtained on 6Mo/MCM-22 and the open symbols represent the data obtained on 6Mo/ZSM-5; reaction conditions: T = 993 K, P = 1 atm, SV = 1500 mL g1 h1.34 Fig. 15 Methane aromatization on 6Mo/MCM-22 catalyst as function of time onstream. (A) The conversion of CH4 and the selectivity of CO, coke and hydrocarbons in CH4 conversion, (B) the selectivity of benzene and naphthalene in hydrocarbons. Reaction conditions: T = 993 K, P = 1 atm, SV = 1500 mL g1 h1, CO2 = 2%.23
Among the highest yields of benzene is work by Choudhary et al., in which various alkanes/alkenes are added to the methane feed,
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using a H–GaAlMFI zeolite (however, the methane concentration in the feed was only 33%).7 This zeolite is bifunctional—with strong acid sites at the framework tetrahedral of the Al and Ga in the zeolite, and dehydrogenation due to the zeolitic protons and extra framework gallium oxide. Table 4 shows the results. Others have reported promising results with variations of these Ga MFI zeolites.38
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Table 2 Comparison of Mo/HZSM-5 and Mo/MCM-22 for methane aromatization at the optimum reaction conditions35 a
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Sample
Selectivity (%) Yields (%) Conversion of methane (%) CO C2 Ben. Nap. Coke Ben. Nap.
2MO/MCM-22 5.7 6MO/MCM-22 10.0 l0Mo/MCM-22 5.8 2Mo/ZSM-5 9.7 6Mo/ZSM-5 10.6 l0Mo/ZSM-5 10.7
0.9 0.2 0.8 0.7 1.0 1.0
4.9 3.4 4.5 4.1 3.3 3.6
67.8 7.9 18.5 80.0 4.4 12.0 61.4 3.6 29.7 47.2 22.0 26.0 57.8 19.8 18.1 47.7 13.2 34.5
3.9 8.0 3.6 4.6 6.1 5.1
0.5 0.4 0.2 2.1 2.1 1.4
a Reaction temperature is 973 K, space velocity is 1500 mL g1 h1. All results were the values at optimum reaction stage, i.e., after 180 and 90 min of running for Mo/MCM-22 and Mo/HZSM-5 catalysts, respectively. There is apparently no CO or CO2 in the feed gas (see p. 69 of the paper), so that the formation of CO must come from either the oxidized form of the catalyst, or the zeolite.
Table 3 Effect of added CO2 on carbon deposition in methane aromatization on Mo/MCM-2232
Amount of carbon deposits by TGA
4.1
Water Carbidic coke Non-carbidic Total coke Coke coke (mg g1) (mg g1) (%) (%) (mg g1) Without CO2 4.5 8.4 With CO2
0.6 —
198.7 131.2
the effects of deactivation by carbon deposition and/or removing hydrogen to drive the reaction toward the desired products. All include some means of regenerating the catalyst, either continuously (typically in a circulating fluid bed) or periodically (e.g., in multiple stationary beds). Heat integration is also a key design objective. It is not the purpose of the discussion here to exhaustively examine and analyze the wide range of reactor designs in the literature. Rather, a representative example is provided for three conceptual reactors of interest: (a) A coupled reactor in which methane is first reacted with substoichiometric oxygen to form primarily ethylene, which then feeds the MA reactor.48,49 (b) Membrane reactors, in which hydrogen produced from the MA reaction is continuously removed to drive the reaction toward benzene.50 (c) A ‘‘two-zone’’ reactor in which both MA and catalyst regeneration take place in a single vessel.51
199.3 131.2
19.9 13.1
Notes: Water is desorbed as ‘‘physisorbed water from the zeolite pores’’. ‘‘Carbidic coke’’ is carbon associate with Mo carbide, the active form of the catalyst, which is oxidized to molybdenum oxide during the TGA procedure. ‘‘Non-carbidic coke’’ is carbon deposited on the catalyst during the run. Run conditions: 700 C at a space velocity of 90 gCH4/ gMo h1; or with the addition of a CO2 stream at a space velocity of 10 gCO2/gMo h1 (CH4/CO2 molar ratio of 25).
A number of studies on other various zeolites such as Mo/ITQ-2,39 Mo/ITQ-13,40 Pt–Ga silicates,41 MFI-supported molybdenum,42 MFI-supported rhenium,43 Mo/HZRP-1,44 metal (Cr, Mo, W)-doped carbon aerogels,45 Ni–Re/Al2O3,46 and various forms of unsupported Mo carbide9,47 show no particular advantage over the ZSM-5,MCM-22, or H–GaAl MFI catalysts discussed above.
4.0 Reactor designs A number of different reactor designs have been proposed, all of which have the goal of maximizing benzene yield by minimizing
Coupled reactor
One reactor design integrates an oxidative coupling of methane (OCM) reactor (which converts methane to ethylene) with a second reactor using Ga/ZSM-548 or 6Mo/HMCM-4949 to convert ethylene to aromatics (Fig. 17). The formation of H2O and CO2 in the OCM reactor then limits carbon formation in the MA reactor downstream.49 4.2
Membrane reactors
These reactors have the goal of removing hydrogen to drive the MA reaction toward the formation of benzene by removing the hydrogen continuously through a selective membrane. In its simplest configuration, it consists of a packed tube containing a solid catalyst, a radially centered tube coated with a selective membrane, through which hydrogen permeates (Fig. 18).50 One study on such a reactor showed a positive effect of hydrogen removal (using a Pd–Ag membrane),50 as shown in Table 5. A closely related study using a similar membrane reactor showed a corresponding increase in benzene yield resulting from hydrogen removal, but also observed more rapid deactivation, presumably due to the lower level of hydrogen in the gas, which can limit carbon formation.52 A theoretical modeling study by the Iglesia group showed the potential for these systems
Table 4 Results for the aromatization of methane over H–GaAl MFI zeolite in the presence of different additives (alkenes or higher alkanes) in the feed (a mixture of CH4, N2, and additive; concentration of methane in feed, 33.3 mol%)7
Conversion (%)
Additive in feed
Additive/CH4 molar ratio
Space velocity (cm3 g1 h1)
Temp. (8C)
CH4
Additive
Selectivity for aromatics (%)
None None n-Butene i-Butene Propene Ethene Propane n-Hexane
0.0 0.0 0.5 0.5 0.8 1.0 0.7 0.8
3100 6200 3100 6200 6200 6200 3100 3100
600 600 600 500 500 600 600 600
0.0 0.0 45.0 44.2 34.7 36.3 12.0 24.1
— — 100 100 99.7 98.8 91.1 100
— — 92.0 93.8 91.8 93.8 84.2 93.8
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SV (mL h1 g1)
H2 separation
Conversion (%)
120
No Yes No Yes No Yes No Yes No Yes No Yes
2.5 7.5 2.4 5.9 2.6 4.9 2.6 4.2 2.4 3.5 2.4 3.3
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180 240 360 720 1440
Selectivity (%) C2
C6H6
C7H8
C10H8
9.6 3.2 5.4 4.1 6.2 5.5 6.9 6.4 8.8 8.3 11.3 8.2
56.0 53.3 70.8 64.4 69.2 65.3 69.2 66.7 70.8 71.4 62.5 63.6
2.4 3.5 2.9 4.4 3.1 4.3 3.8 4.8 4.6 5.4 4.2 5.5
32.0 40.0 20.8 27.1 23.1 24.5 19.2 21.4 16.7 14.6 22.1 22.4
Pressure = 100 kPa; the permeability of H2 through the membrane is ca. 75%.
Fig. 17 Schematic diagram of integrated recycle system for conversion of methane to aromatics. C = mass flow controller; F = flowmeter; P = gas sampling port; R = pressure regulator.48
Fig. 18 Membrane reactor in which hydrogen is continuously removed in the tube side of an annular reactor containing solid catalyst.50
includes almost complete CH4 conversion at B1000 K at practical reactor residence times (o100 s).53 4.3
Two-zone reactor
In this reactor, methane is fed to the middle of the reacting zone, creating two zones in the bed with different environments: an oxidizing atmosphere in the lower part and a reducing atmosphere in the upper part, Fig. 19.51 As the catalyst deactivates by coke formation, this denser catalyst falls to the lower part of the reactor, where it is
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Fig. 19 Two zone fluidized bed reactor.51
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regenerated and rises to the upper (reaction) level. Performance data given in this study using Mo/HZSM-5 show up to 90% benzene + hydrocarbon selectivity, with methane conversion dropping to about 8% after 400 min.88
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5.0 Conclusions Methane aromatization (MA) is of great potential interest as a means of producing both a high-value aromatic compounds as well as pure hydrogen in one reaction. However, the reaction inevitably produces carbon deposits on the catalyst. Carbon formation is thermodynamically favored at all conditions of practical interest, but can be kinetically limited by the addition of hydrogen or oxidants such as CO2 or steam. Thermodynamic calculations suggest that the addition of C2–C4 alkanes/alkenes to the methane feed can improve the selectivity to benzene. However, experimental studies to test this approach are limited, suggesting an area for further study. There are a number of reactor designs that have been evaluated for this reaction, all with the goal of continuous regeneration of the catalyst while maintaining reaction conditions that favor MA in the reaction zone. Although a number of catalysts have been investigated for MA, by far the most well-studied are those based on Mo/HZSM-5 and Mo/MCM-22. These catalysts appear to function by activation of methane on a molybdenum carbide site, formed in the initial stages of the reaction, and oligomerization on the acidic sites of the zeolite, which provides a shapeselective function to maximize benzene selectivity. One area for further study is the application of computational catalysis to this reaction. There are few studies in which DFT methods have been used to follow the reaction mechanism and carbon deposition.54,55 Given the recent advances in these methods, there are likely opportunities to improve the selectivity and kinetically limit carbon formation using the tools of computational catalysis.
Acknowledgements This work was supported by Rubicon LLC (A joint venture of Huntsman and Chemtura).
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