Some technological aspects of methane aromatization (direct and via oxidative coupling)

Fuel Processing Technology 87 (2006) 511 – 521 www.elsevier.com/locate/fuproc

Some technological aspects of methane aromatization (direct and via oxidative coupling) Krzysztof Skutil, Marian Taniewski ⁎ Silesian Technical University, Chair of Chemical Organic Technology and Petrochemistry, ul. Krzywoustego 4, 44-101 Gliwice, Poland Received 2 November 2005; received in revised form 16 November 2005; accepted 12 December 2005

Abstract The investigations on transformation of methane to benzene and naphthalene have been carried out in aim to verify and supplement earlier reported data and on this basis to estimate real industrial perspectives of the CH4 aromatization concept, the main challenges and barriers. Methane aromatization (direct and via oxidative coupling) has been studied over Mo/HZSM-5 catalyst used both for direct methane dehydroaromatization and for aromatization of methane oxidative coupling (OCM) products. The effects of Mo content in the catalyst, temperature, space velocity, the presence of CO2, CO, H2O, C2H4, C2H6 and their mixtures in the feed have been studied. The effectiveness of the catalyst regeneration in the air was also examined. All results were confronted with the literature data and analyzed from technological point of view. It was confirmed that direct CH4 aromatization process was characterized by a low CH4 single-pass conversion, low single-pass yields of the main products (benzene, hydrogen and naphthalene) and a low catalyst stability (rapid catalyst deactivation). Various possible technological schemes were analyzed. It was concluded that real industrial chances of direct methane aromatization or aromatization via OCM would depend largely on the advancement in the cost-effective separation techniques. The methane aromatization concept was also confronted with other methane conversion processes. © 2006 Elsevier B.V. All rights reserved. Keywords: Methane; Methane dehydroaromatization; Methane aromatization via oxidative coupling; Mo/HZSM-5 catalysts

1. Introduction It is generally accepted that in the present 21st century, natural gas (methane) will be increasingly important as a source of energy (gaseous fuel), transportable liquid fuels and chemicals (petrochemicals) [1,2]. Numerous advantages of natural gas over other energy sources (abundant resources, also in remote areas and as hydrates, ease of purification, lower emissions of pollutants, higher efficiency of electricity, heat and power production, etc.) cause that the steady growth of natural gas share in total primary energy is widely predicted. The advancement of C1 chemistry and technology will lead to a wider chemical utilization of natural gas (methane) both as a raw material for liquid fuels and as alternative feedstock for the petrochemical industry, gradually replacing more rapidly exhausting resources of crude oil. Among already developed productions of liquid fuels from natural gas (GTL, Gas-to-Liquids) are: gasoline via CH4 reforming to syngas and Fischer–Tropsch synthesis, gasoline, via CH4 reforming to ⁎ Corresponding author. Tel.: +48 32 2372014; fax: +48 32 2371032. E-mail address: [email protected] (M. Taniewski). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2005.12.001

syngas, CH3OH synthesis and MTG (methanol-to-gasoline) process, diesel fuel, via syngas, Fischer–Tropsch synthesis and mild hydrocracking of wax, derivatives of methanol (MTBE— methyl-tert-butyl ether, DME—dimethyl ether, FAME—fatty acids methyl esters). The importance of natural gas (methane) for contemporary petrochemical industry is connected mainly with manufacturing of syngas and hydrogen and to much less extent also of acetylene, chloromethanes, etc. Extensive studies have been carried out in recent years on the catalytic oxidative coupling of CH4 (OCM) to ethylene (C2+ hydrocarbons). The single-pass C2 yield obtained so far, approaching 25% and the selectivity to C2 approaching 80%, were found to be not sufficient for immediate industrial application of the OCM process. No doubt, that in the case of successful development in the future of the costeffective technology of direct conversion of methane into ethylene, such process will be commercialized and, in consequence, the structure of feedstocks applied by petrochemical industry will be thoroughly changed. Other attempts, lasting already for many years, were concerned with the search for an effective method of direct selective oxidation of CH4 to methanol or/and formaldehyde. Unfortunately, these attempts were not

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successful up to now and the obtained single-pass formaldehyde yield did not exceed 5–6%. Although there are still some hopes for improvement, the development of such process appears to be one of the toughest goals in C1 catalysis and technology. The advanced research is also carried out on catalytic decomposition of CH4 to pure hydrogen for fuel cells and to nanocarbons. In such circumstances were undertaken in 1990s the investigations on direct conversion of CH4 to benzene. Aromatization (conversion to aromatics) of methane is a part of a wider problem of aromatization of alkanes and this, in turn, of a still larger field of aromatization of non-aromatic hydrocarbons. Aromatization of non-aromatics has a double aim. It is a route to high-octane liquid fuels and to individual aromatic hydrocarbons for petrochemical syntheses. The processes of transformation of certain aromatics (alkylaromatics, etc.) into other aromatic individuals are obviously not included into this term. Short characteristics of hydrocarbon aromatization processes and, on this basis, the indication of unique aspects of CH4 aromatization, are given below. Aromatization of C6+ alkanes, cycloalkanes and alkenes contained in low-octane naphtha fractions is the main transformation process taking place during the multipath catalytic reforming (apart of isomerization, hydrocracking, etc.). The aromatization proceeds on bifunctional reforming catalysts by dehydrocyclodehydrogenation of alkanes, dehydrogenation of naphthenes, etc. Highly aromatized reformate is applied as a high-octane component of gasoline or as a source of toluene, xylenes and benzene for petrochemical syntheses. Aromatization of C2+ alkanes, cycloalkanes and alkenes contained in gaseous and liquid hydrocarbon feedstocks (from ethane through naphtha, up to hydrocracking residue) takes place during their noncatalytic, high-temperature steam cracking (pyrolysis), in manufacturing of ethylene and co-products (propylene, butenes, butadiene and aromatics), being the major building blocks for petrochemicals. The aromatization, which accompanies the main processes of thermal decomposition, proceeds as a multistep thermal transformation of non-aromatic components. Aromatic-rich pyrolysis gasoline (pyrobenzine) is produced with high yields, especially by such steam crackers which apply liquid hydrocarbon fractions as a feedstock. Pyrobenzine is a rich source of benzene, but also of toluene, xylenes, ethylbenzene and (potentially) styrene for petrochemical syntheses. Aromatization of C2–C4 alkanes contained in LPG was developed in 1980s and commercialized in the late 1990s (Cyclar and related processes). Aromatization of lower alkanes proceeds on bifunctional catalysts by dehydrooligocyclodehydrogenation (dehydrogenation of alkanes to alkenes followed by oligomerization of lower alkenes to higher alkenes, their cyclization to naphthenes and dehydrogenation to aromatics). Aromatization of methane could be accomplished either by indirect multistep methods via synthesis gas (syngas) and Fischer–Tropsch Synthol synthesis (developed), via syngas, methanol and MTG synthesis (developed), and via oxidative coupling (OCM) followed by aromatization (both steps not developed) or by direct single-step aromatization (not developed). Aromatization processes via Fischer–Tropsch synthesis

or via methanol synthesis (and perhaps also, in the future, via OCM) are treated first of all as routes from gas to high-octane liquid fuels (GTL). The direct aromatization of methane could open the way to a CH4-based manufacturing of benzene for synthesis. If carried out under nonoxidative conditions, it would lead also to a co-product H2, what might be regarded as an additional advantage of this process. The direct aromatization (dehydrocondensation, dehydroaromatization) of methane still remains in its early basic research stage. The transformation of C1 molecule into aromatics is obviously much more complex than aromatization of C3–C4, not mentioning C6+ hydrocarbons. Methane is the most stable among alkanes. The structure of CH4 molecule with its four C–H covalent bonds of extra-high bond energies (D(CH3–H) ≅ D(H–H) = 435 kJ mol− 1), zero dipole moment, no multiple bonds or functional groups and no asymmetry, is responsible for an extremely low CH4 reactivity. Thermodynamically, methane is unstable only above 803 K, becoming more unstable than benzene above 1303 K [3]. The conversion of methane to benzene (6CH4 = C6H6 + 9H2) has a rather high ΔH value (523.018 kJ mol− 1). The methane thermodynamic equilibrium conversion at 1 atm is low at moderate temperatures (about 5% at 873 K, 11.4% at 973 K and 16.2% at 1023 K) [4]. The direct aromatization of methane to benzene (like CH4 transformation to ethylene or oxygen-containing compounds) requires an initial activation of CH4 molecule. The activation of C–H bond in alkanes, and particularly in the most stable CH4, under mild conditions necessary for selective synthesis of derivatives, became one of the most intriguing and challenging problems in contemporary hydrocarbon chemistry and catalysis. The structure of CH4 molecule is responsible for exceptional difficulties in controlled activation, particularly of one or two of its four identical C–H bonds (see Reactions (1)–(3), below). O2

ð1Þ

CH4 Y CH3 OH; CH2 O; etc:

O2

ð2Þ

CH4 Y C6 H6 ; C10 H8 ; H2 ; etc:

ð3Þ

CH4 Y C2 H4 ; C2 H6 ; etc:

The breakage of all four C–H bonds in the CH4 molecule is relatively somewhat less constraint and that is why only indirect conversion of CH4 via syngas into higher hydrocarbons or chemicals was already commercialized (reaction (4)). For the same reason, there are some chances to develop in the future the process of catalytic decomposition of CH4 to carbon (desirably in the form of nanostructures) and pure hydrogen (reaction (5)). H2 O;O2 ;CO2

P CO þ H2 Y CH4 PPPPPP Y

ð4Þ

CH4 Y nano  C þ H2

ð5Þ

The activation and direct transformation of CH4 into hydrocarbons is thermodynamically more favorable with the

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assistance of oxidants than under nonoxidative conditions. In some cases and under certain conditions nonoxidative transformation is thermodynamically totally unfavorable. That is why the conversion of CH4 to ethylene is studied, most often, as the catalytic oxidative coupling in the presence of O2 or other oxidants. However, the usage of O2 as an activator usually leads to a poor selectivity caused by simultaneous formation of carbon oxides, most favorable thermodynamically. This is, certainly, the main reason for observed limited selectivity of the OCM process under conditions when sufficiently high CH4 conversion is achieved, and as a consequence, of the limitations in C2 yields. Similarly to OCM, also the methane aromatization in the presence of oxygen is thermodynamically favorable but proceeds with a limited selectivity. No wonder, that investigations on selective transformation of CH4 to aromatics are usually carried out in the absence of oxygen, taking advantage of the fact that thermodynamically, the transformation of CH4 under nonoxidative conditions is somewhat more favorable to aromatics than to olefins. Under such conditions, valuable H2 is also formed as a co-product. The investigations are carried out, as a rule, in the presence of catalysts, most often transition metals supported zeolites, in aim to enhance kinetics and to reach the results, as close as possible to those predicted by thermodynamics. Since the pioneering work of Wang et al. [5], who demonstrated the possibility of direct, though only limited, conversion of CH4 to benzene at about 973 K under non-oxidizing condition in a flow reactor mode on Mo/HZSM-5 and related catalysts, numerous research reports, have been published. The investigations were carried out by research groups in the USA, Hungary, Japan, etc., and in the recent years mostly in China. Several extensive reviews summarizing the results of research on direct aromatization of CH4 under nonoxidative conditions have been already published [6–10]. The results of the research on aromatization of methane via OCM have been mentioned only occasionally [11–13]. The most widely applied and thoroughly studied catalyst for oxygen-free aromatization of methane was a bifunctional Mo/ HZSM-5 catalyst (and its modifications), with dehydrogenation and acid sites. Many other Mo-zeolites (Mo/MCM-22, etc.), promoter (Ga, etc.)-Mo-zeolites and other metal (Re, etc.)zeolites catalysts were also examined. It is generally believed that molybdenum carbide Mo2C or oxycarbide species, formed initially by reduction of MoOx with CH4, are responsible for the mild activation and conversion of CH4 to C2H4 (or C2H2, as suggested by some authors). Intermediate C2H4 oligomerizes and cyclizes to aromatics (benzene, naphthalene, etc.) over acid sites within the channels of the zeolite. Apart of aromatics, also H2 and C2+ hydrocarbons are formed (in minor amounts also CO, H2O, etc.). Various alternative schemes of the reaction mechanism have been postulated [9]. The investigations on direct nonoxidative aromatization of CH4 carried out in the last decade, were focused almost exclusively on fundamental issues. The remarkable advancement was achieved in establishing the active forms of Mo, the activation and reaction mechanisms, the effects of catalyst composition, modificators and promoters, preparation and

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pretreatment methods, the effect of applied zeolites, the character of interactions between the metallic sites and the zeolite supports, the effects of the Brønsted acidity and of the channel structure, the role of operational conditions, the induction period and deactivation phenomena, the nature of carbonaceous deposits, the effects of CO, CO2, H2, O2 and C2 admixtures in CH4, etc. The results of fundamental research on CH4 aromatization became a source of important information advancing our knowledge concerning this intriguing subject. However, it should be also noted that some reported detailed observations and data appeared to be controversial and uncertain. Technological aspects of the possible future aromatization process were almost not touched in the literature. This was, perhaps, a consequence of the fact that the results obtained so far were not encouraging from technological point of view. Indeed, the highest obtained single-pass conversion of methane was usually very low (about 8–10% at 973 K), in a full accordance with thermodynamics. Reported by some authors as somewhat higher conversions, exceeding equilibrium values, seem to be disputable [14]. Although the maximum selectivity to benzene reached 70–80%, the benzene yield did not exceed 6–8%. The selectivity to naphthalene was always lower and varied incomprehensibly from case to case. The applied catalysts had a low stability and were gradually (often rapidly) deactivated due to a coke deposition. The efforts to suppress coke formation, by adding to the feed small amounts of CO, CO2, etc. or by catalyst modifications, gave only limited effect. No effective regeneration methods have been found, though the addition of minor amounts of NO or hydrogasification of the coke in the stream of H2 were proposed. Nevertheless, it seems to be already a proper time to initiate discussion on technological as well as economic aspects of now considered concept, to establish main barriers and chances (including those connected with the changing market situation of benzene) and on this basis to estimate the real industrial perspectives of this project. Such approach and discussion are particularly needed in view of some overoptimistic opinions concerning the industrial prospects of this process, expressed sometimes in the papers dealing with fundamental issues. Such preliminary discussion concerning only some technological aspects, was undertaken in this study. In view of some discrepancies concerning certain experimental observations and results reported in the literature, and a lack of some information, it became necessary to carry out several series of reliable and reproducible own experiments, in aim to verify published results and to obtain complementary data. The experiments on direct CH4 aromatization and on aromatization via oxidative coupling were carried out in the presence of the most commonly applied Mo/HZSM-5 catalyst. 2. Experimental 2.1. Materials and catalyst preparation Main gases used in the experiments were of high purity: – methane, 99.995%, supplied by Linde AG; – argon, 99.995%, supplied by Linde Gas Poland;

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– – – –

hydrogen, N99.9%, supplied by Linde Gas Poland; ethylene, polymer grade, supplied by Orlen, Plock; ethane, 99.95%, supplied by Air Products carbon monoxide, 99.5%, supplied by Messer Greisheim GmbH; – nitrogen, carbon dioxide, air, technical grade, supplied by Linde Gas Poland.

diameter, 2 m in length) was employed for separation and analysis of the products. A GC 505 (INCO) Chromatograph with a FID detector equipped with deactivated Al2O3 column (3 mm in diameter, 2.5 m in length) was applied for separation of nonaromatic hydrocarbons.

Mo/HZSM-5 catalysts containing 0.5–6 wt.% Mo were prepared by impregnation of HZSM-5 (SH-27 type, ALSIPENTA GmbH) with a desired amount of acidified to pH = 6 (HNO3) aqueous solution of (NH4)6MoO24·4H2O. The resulting materials were dried at 383 K, and calcined in air for 5 h at 773 K. The calcined samples were pressed, crushed and sieved to 0.3–0.6 mm for catalytic evaluation. In all series of experiments, except of those concerned with the effect of Mo concentration, was used the catalyst denoted as 4.5%Mo/HZSM-5 containing 4.27 wt.% Mo (fresh), 4.45 wt.% Mo (calcined) and 4.13 wt.% Mo (spent, after 4 h of work), as determined by AAS (Solaar 4). The corresponding BET surface areas were 276, 263 and 194 m2 g− 1, micropore areas 248, 233 and 178 m2 g− 1, micropore volumes 0.12, 0.11 and 0.085 m3 g− 1, average pore diameters 1.7, 1.7 and 1.8 nm (ASAP 2000, Micromeritics).

The calculation of CH4 conversion was based on the determined compositions and flow rates of the incoming and outgoing reaction mixtures. The yields of individual products were calculated from the ratio of the number of CH4 moles converted to the particular product to the number of incoming CH4 moles. For calculation of the H2 yield, the value 2 was taken as the stoichiometric factor of CH4 to H2 transformation. The yield of carbonaceous deposit was calculated from the balance of C element contained in the substrates and the products.

2.2. Catalytic evaluation The experiments were carried out at atmospheric pressure in a conventional continuous fixed-bed quartz microreactor (3 cm3 in volume), placed in the electric furnace and equipped with all necessary facilities for regulation, measurements, control, sampling, etc. Reactor was packed with about 2 g of catalyst. After pretreatment of a catalyst in air flow (40 ml min− 1) for 5 h at 773 K, a N2 stream and next a feed gas mixtures (CH4–N2, CH4–N2admixtures, N2-admixtures, etc.) were introduced. The experiments on methane aromatization were carried out, as a rule, at the temperature 998 K and space velocity 1500 cm3 g− 1 h− 1. The effluent products were analyzed chromatographically with the use of 2 integrated gas chromatographs equipped with FID and TCD detectors. A SRI 8610 Chromatograph with a FID detector equipped with TCPE column (Chromosorb Paw-100/120 mesh, 2 mm in diameter, 2.5 m in length) and with a TCD detector equipped with active carbon column (30–40 mesh, 3 mm in

2.3. Calculations

3. Results and discussion 3.1. The reproducibility of CH4 aromatization tests The conversion of CH4 and the yields of benzene, naphthalene and H2 obtained in the presence of four loadings of the same 4.5%Mo/HZSM-5 catalyst at 998 K are shown in Fig. 1, as a function of the time on stream. As shown, the reproducibility of the products yields obtained in the presence of four catalyst samples was very good. Somewhat less satisfactory was the reproducibility of CH4 conversion, evidently because of higher relative error in ΔCH4 determinations under conditions of a low CH4 conversion. The satisfactory reproducibility of the basic results enabled us to draw reliable conclusions from the results of all series of our experiments. It was confirmed that the aromatization of CH4 to benzene showed indeed an induction period in the early stages of the reaction when CH4 conversion was significant and the major products were H2 and coke with only small amounts of benzene and other aromatics formed. According to the widespread view, the induction period might correspond to the reduction of MoOx with CH4 to Mo2C and/or MoOxCy species, being subsequently the active species for aromatization. After an induction period, the catalyst reached maximum aromatization activity, then a

14

10 Hydrogen

8 10

Benzene

Yield [%]

CH4 conversion [%]

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6 4

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Naphthalene

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Fig. 1. The reproducibility test of CH4 conversion and product's yield (4 identical loadings of 4.5%Mo/HZSM-5 catalyst: x, ◊, □, Δ, T = 998 K, GHSV= 1500 cm3 g− 1 h− 1).

K. Skutil, M. Taniewski / Fuel Processing Technology 87 (2006) 511–521 8

14

fresh

12

Benzene yield [%]

CH4 conversion [%]

515

10 fresh

8 6

after 2nd reg.

4

after 1st reg.

after 3rd reg.

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after 1st reg.

4 after 2nd reg.

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after 3rd reg.

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Fig. 2. The catalytic performance of 4.5%Mo/HZSM-5 catalyst, fresh and regenerated successively 1, 2 and 3 times in air (T = 998 K, GHSV = 1500 cm3 g− 1 h− 1).

short period of quasi steady-state activity was observed and soon its activity began gradually decline (Fig. 1). The maximum CH4 conversion was found to be about 12% and the yields of benzene, naphthalene and H2 were 7%, 1% and 10%, respectively. Our observations confirmed that heavy carbonaceous deposits were formed during the reaction that led to the severe catalyst deactivation. 3.2. Regeneration of 4.5%Mo/HZSM-5 catalyst in the air The attempts to restore the aromatization activity of the spent catalyst by its regeneration in the air were performed for 2 h at 723–833 K. The results of the comparative tests (each lasting 4 h) on the catalytic performance at 998 K of the fresh catalyst and the same catalyst after successive 3 regenerations are shown in Fig. 2. As shown, the benzene yield declined after each successive regeneration (cf. [15]). Also CH4 conversion changes, in spite of some dispersion of the points, confirmed low effectiveness of regeneration. The observed results could be connected, as suggested by some researchers, with migration and sublimation of Mo species and the changes in structure of the Mo active sites occurring at higher temperatures. 3.3. The role of Mo concentration The catalytic performance of several Mo/HZSM-5 samples containing 0.5–6.0 wt.% of Mo at 1023 K and 1500 cm3 g− 1 h− 1 is shown in Fig. 3.

As shown, the highest benzene yield (and CH4 conversion after induction period) was obtained in the presence of catalyst containing about 4.5 wt.% Mo. 3.4. The role of temperature and space velocity The effects of temperature (973–1073 K ) and of space velocity (1000–1900 cm3 g− 1 h− 1) on aromatization activity of 4.5%Mo/HZSM-5 catalyst are shown in Figs. 4 and 5, respectively. As demonstrated, the changes of both parameters affect the CH4 conversion, aromatic yields and induction period, in accordance with expectations. The results indicated that the temperature 1073 K was evidently too high and led to the rapid deactivation of the catalyst. 3.5. The role of CO2 and CO Some authors noticed (see e.g. [10]) that the addition of a few percent of CO and CO2 to the CH4 feed promoted benzene production and enhanced catalyst stability. On the other hand, the addition of large amounts of carbon oxides totally suppressed the activity of Mo/HZSM-5 catalyst. In our own experiments, after 90 min (in some cases after 120 min) of aromatization (998 K, 1500 cm3 g− 1 h− 1), CO2 or CO was added in various amounts. The results are shown in Figs. 6 and 7. As shown, we did not observe the effect of CO2 addition, up to about 2.5%, on the benzene yield and CH4 conversion during the whole time on stream, whereas the addition of higher

12 3 wt%

Benzene yield [%]

CH4 conversion [%]

8 10

4.5 wt%

8 6

6 wt%

4 2 wt% 1 wt%

2

6 4.5 wt% 3 wt%

4

6 wt% 2 wt%

2

1 wt%

0.5 wt%

0.5 wt%

0

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0

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Fig. 3. The catalytic performance of Mo/HZSM-5 catalysts with different content (0.5–6 wt.%) of Mo (T = 998 K, GHSV = 1500 cm3 g− 1 h− 1).

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1073 K 1073 K

16

Benzene yield [%]

CH4 conversion [%]

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1023 K

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8 973 K

8 6 4

4

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998 K 973 K

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Time on stream [min]

Fig. 4. The effect of temperature (973–1073 K) on catalytic performance of 4.5%Mo/HZSM-5 (GHSV = 1500 cm3 g− 1 h− 1).

amounts of CO2 (5% and especially 11%) caused a rapid deactivation. The addition of 8.5% of CO led to the increase in activation and to the enhancement in stability of the catalyst. 3.6. The role of H2O A promotional effect of small amounts of H2O and a sudden drop in catalytic activity after some time on stream were observed in one work [16]. The results of our own experiments (Fig. 8) indicated that the addition of minor amounts of H2O (2%) did not influence the benzene yield and CH4 conversion, whereas the addition of larger amounts of H2O (9.5%) caused a total deactivation of the catalyst. 3.7. The role of C2H4 and C2H6 As shown in Fig. 9A, the presence of minor amounts of C2H4, C2H6 or both C2s in the CH4 + N2 mixture significantly increased the benzene yield. This expected observation was fully compatible with the results of aromatization of C2H4 or C2H6 (in their mixtures with N2) which led to very high benzene yields and relatively high naphthalene yield (Fig. 9B). 3.8. The aromatization of OCM products The combination of the OCM process with the process of aromatization of its products (C2 components) is one of the

obvious technological options. However, very few researchers have studied such combined process experimentally [11–13]. One team of investigators applied a complex integrated recycle system involving OCM reactor, C2H6 dehydrogenation reactor, C2H2 hydrogenation reactor, CO2 and H2O removal, aromatization reactor, product condensation vessel and CH4 recycling pump [13]. Other authors applied only one reactor without a recycle [12]. According to their reports, rather promising results were obtained. Our own results on the aromatization of a CH4 + N2 + C2H4 + C2H6 + CO2 mixture, simulating the composition of OCM products (except of the lack of H2O), confirmed once again the deteriorating effect of CO2. This clearly followed from the confrontation of the obtained benzene yield curve with the curves corresponding to the aromatization of CH4 + N2, CH4 + N2 + C2H4 and CH4 + N2 + C2H4 + C2H6 mixtures (Fig. 10A). In the next step, we carried out the direct experiment on the combination of the OCM and the aromatization of its products in one reactor filled with two consecutive layers of catalysts (Na/CaO for OCM and 4.5%Mo/HZSM-5 for aromatization). Before the main experiment, it was confirmed that both steps carried out independently (OCM: 998 K, 1.125 g Na/CaO, 1200 cm3 g− 1 h− 1, O2 : CH4 = 0.52; methane direct aromatization: 998 K, 0.9 g 4.5%Mo/HZSM-5, 1500 cm3 g− 1 h− 1) gave typical results. In the OCM process, CH4 conversion—36.7%, O2 conversion—98%; C2H4 yield—8.7%; C2H6 yield—6.4% were observed. As shown in Fig. 10B, the combined process

8

6

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1000 cm3g-1h-1

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Fig. 5. The effect of space velocity (1000–1900 cm3 g− 1 h− 1) on catalytic performance of 4.5%Mo/HZSM-5 (T = 998 K).

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Fig. 6. The effect of different amounts of CO2 (2.5–11 vol.%) to the feed on catalytic performance of 4.5%Mo/HZSM-5 catalyst (T = 998 K, GHSV= 1500 cm3 g− 1 h− 1).

failed to produce benzene, contrary to the above-mentioned reports. One can suppose that the presence of relatively large amounts of CO2 and H2O in the OCM products was mainly responsible for the deactivation of aromatization catalyst. All our experimental results (Figs. 1–10), if confronted with the literature observations and data, led to the following detailed conclusions: – the appearance of an induction period in the early stage of reaction was confirmed; – it was confirmed that after an induction period the yield of benzene attained a maximum and then gradually decreased, evidently due to formation of carbonaceous deposits (deactivation); – it was found that the naphthalene yield was much lower than that of benzene and was practically constant under experimental conditions at least during 4 h; – it was demonstrated that the regeneration in air of spent catalyst could not restore its original activity and that this activity felt down with each subsequent regeneration; – it was shown that the optimum initial content of Mo in Mo/ HZSM-5 catalyst was about 4.5 wt.%; – at the early stages of the process carried out under chosen conditions (temperature 998 K, space velocity 1500 cm3

–

– – –

– –

g− 1 h− 1 ), CH4 conversion—12%, benzene yield—7%, naphthalene yield—1%, hydrogen yield—10% were obtained; it was shown that the yields of products were unaffected by the presence of small amounts of CO2 (about 2.5%), whereas its larger amounts caused a rapid decrease in the yields; the enhancing effect of small amounts of CO2 on the product yields or catalyst stability, was not observed as suggested sometimes [10]; a weak enhancing effect exerted by small amounts of CO on CH4 conversion and the yields of benzene and hydrogen were confirmed; the effect of small amounts of CO on the yield of naphthalene was not detected; it was shown that small amounts of H2O (about 2%) did not influence the benzene yield and CH4 conversion (cf. [16]), whereas its larger amounts caused a rapid catalyst deactivation; it was confirmed that the addition of C2H6 or/and C2H4 significantly increased the benzene yield; it was demonstrated that the replacement of a CH4–N2 mixture with a CH4–C2H4–C2H6–CO2 mixture, of the composition modeling the composition of the OCM products (except of the lack of H2O), led to the marked decrease in the benzene yield, evidently due to deteriorating effect of CO2; Added CO

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Fig. 7. The effect of addition of CO (8.5 vol.%) to the feed on catalytic performance of 4.5%Mo/HZSM-5 catalyst (T = 998 K, GHSV = 1500 cm3 g− 1 h− 1).

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Fig. 8. The effect of addition of different amounts of H2O (2–9.5 vol.%) to the feed on catalytic performance of 4.5%Mo/HZSM-5 catalyst (T = 998 K, GHSV = 1500 cm3 g− 1 h− 1).

– it was demonstrated that, unlike some earlier reported results [12,13], the combination of the OCM and aromatization processes led to the significant decrease in benzene yield, evidently due to deteriorating effects of both CO2 and H2O. 3.9. Basic alternative technological schemes of methane aromatization The basic alternative technological schemes of the possible process of CH4 aromatization (direct and via OCM) in the absence of oxygen are shown in Fig. 11. Scheme 1 illustrates the basic concept of the direct aromatization process with the product's separation and possible recycling of unreacted CH4. Scheme 2 shows the direct process leading to higher CH4 conversions due to continuous removal of the formed H2 (e.g. in the catalytic membrane reactor). Schemes 3-7 illustrate the CH4 aromatization via OCM, involving formation of the mixtures containing C2 hydrocarbons, CO2, CO, H2O, H2, etc. in OCM and aromatization of C2 hydrocarbons contained in this mixture in the second step.

A

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Schemes 3 and 4 show a two-bed single-reactor system without or with interzonal separation of the OCM products, with separation of aromatics-contained products and possible recycling of unreacted CH4. By separation of OCM products, undesirable CO2 and H2O components could be removed. Scheme 5 shows a modification of schemes 3 and 4 indicating the possibility of simultaneous production of olefins-contained products and aromatics-contained products in one reactor. Schemes 6 and 7, being analogous to schemes 3 and 4, show a two-bed double-reactor system without or with interzonal separation of the OCM products, separation of aromatics-contained products and possible recycling of unreacted CH4. All schemes involve expensive operations of separation of the reaction products. All schemes, with the exception of scheme 2, require also costly recycling of unreacted CH4 caused by a low conversion of a feedstock. Scheme 2 providing higher conversions does not necessarily require CH4 recycling, but there are strong indications that the increase in CH4 conversion in the aromatization process would lead to the rapid decrease in the catalyst stability due to the rise in the rate of coke formation. Besides, the analysis of the state-of-the-art in construction of

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Fig. 9. The effect of C2H4 or/and C2H6 presence in the CH4–N2 mixture on catalytic performance (A). The results of transformation of C2H4 or C2H6 in their mixtures with N2 (B). Conditions: 4.5%Mo/HZSM-5 catalyst, T = 998 K, GHSV = 1500 cm3 g− 1 h− 1.

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Fig. 10. The effect of C2H4, C2 H6 and CO2 presence in the CH4–N2 mixture on catalytic performance (4.5%Mo/HZSM-5 catalyst, T = 998 K, GHSV = 1500 cm3 g− 1 h− 1) (A). The benzene yield obtained in the process combining direct methane aromatization (MDA) and oxidative coupling of MDA products (OCM) (B); conditions—see text.

catalytic membrane reactors indicates that such systems could be ready for implementation for large-scale productions only in the more distant future. The analysis of possible technological schemes from the point of view of industrial chances of the methane aromatization concept, clearly shows its decisive dependence on the future advancement of the separation techniques, their effectiveness and economy. 4. Conclusions 1. The direct nonoxidative CH4 aromatization is characterized by a low CH4 single-pass conversion and low single-pass yields of the main products (benzene, naphthalene and hydrogen), being intrinsic features of this reaction, as well as by a low stability of applied catalysts (a rapid catalyst deactivation). 2. A low single-pass conversion of CH4 to benzene and H2 at applied temperatures and atmospheric pressure is determined by thermodynamics (equilibrium-limitation). The chances to increase the single-pass conversion (and single-pass yields) could lie, in principle, in shifting the equilibrium of dehydroaromatization by continuous removal of the products, best of all of H2. However, the removal of H2 will cause, along with the rise in CH4 conversion, also the rise in the rate of coke formation by the products carbonization and the fall in the rate of coke removal by its hydrogasification. As a result, the decrease in the catalyst stability (the increase in the rate of catalyst deactivation) should be expected. Indeed, such undesirable effect has been already observed during the attempts to remove H2 by using catalytic permselective membrane reactor [17–19]. The alternative and more realistic way leading to the high overall CH4 conversion, would require the costly separation of diluted products from the unreacted CH4 and multiple recycling of CH4. The analysis of possible technological schemes confirms that the chances to upgrade the aromatization process depend on the advancement in the effectiveness and economy of separation techniques.

3. A low stability and gradual deactivation of the applied sort of catalyst is a consequence of simultaneous carbonization of CH4, C2H4, C2H6, etc., proceeding probably both on the acid sites and on the Mo sites. The desirable increase in CH4 conversion may even lead to the intensification of coke formation and the enhancement of catalyst deactivation (see above). The deactivation can be only slightly slowed down by reducing the acidity of HZSM-5, admixing minute amounts of H2, CO, CO2, O2, etc. The achieved effects are, however, far from being satisfactory from technological point of view. The repeated regenerations of Mo/HZSM-5 catalyst with air had shown the gradual decrease in achieved maximal activity, what might be connected with migration and sublimation of Mo species and the changes in structure of the Mo active sites. Apart of a low CH4 conversion, the low stability of the catalyst due to its deactivation would be another serious barrier in the development of CH4 aromatization technology. Thus, the further improvement of the catalytic performance and the development of the effective regeneration methods are desirable. 4. The above-mentioned features of direct aromatization process characterize also the two-step CH4 aromatization process combining methane catalytic oxidative coupling with subsequent aromatization of C2s contained in the OCM products and carried out in one reactor or in two separate reactors. Our experimental results were rather disappointing, unlike those reported in literature [11–13]. As some components of OCM products (large amounts of CO2 and H2O) are undoubtedly detrimental to the aromatization step carried out on such catalysts as Mo/HZSM-5, it would be necessary to eliminate them from the OCM effluents, before directing products to the aromatization section. This costly and technologically complex separation could be implemented in somewhat easier way into two-reactor system. There could be some hope that by changing the catalyst, the detrimental influence of some OCM products might be reduced. 5. The direct conversion of methane (natural gas) to fuels and petrochemicals, omitting costly production of syngas, and

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Fig. 11. The alternative technological schemes of the process of CH4 aromatization, direct oxygen-free or via OCM (A—aromatization, S—separation, a-cp— aromatics-contained products, o-cp—olefins-contained products). Detailed description—see text.

leading to ethylene, oxygen-containing compounds, aromatics, hydrogen, etc., should be certainly intensively studied, in view of its remarkable fundamental and technological significance. However, until now, the processes of direct CH4 conversion (including aromatization) still remain technologically not fully developed and economically unattractive, unlike multistep and costly, but mature indirect processes based on CH4 conversion to syngas followed by its transformation to desirable products. The utilization of abundant natural gas reserves will be concentrated for many years ahead, rather on more advanced and economically justified routes, such as reforming technologies, Fischer– Tropsch synthesis, methanol synthesis, etc. (apart of the transport of CNG or LNG).

Acknowledgements A participation of undergraduates Mr. Emanuel Tabacarz and Mr. Grzegorz Nowak in the experiments is acknowledged with gratitude. A financial support from the Polish Scientific Research Committee (Research Project 3 T09B 026 26) is gratefully acknowledged. References [1] J.H. Lunsford, Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century, Catal. Today 63 (2000) 165–174.

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