IR Spectral Investigation of the Multistage Single Catalytic Process

Energy & Fuels 2001, 15, 835-840

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IR Spectral Investigation of the Multistage Single Catalytic Process of CH4 Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element Moein B. Sayed* Chemistry Department, Al-Azhar University, Cairo, Egypt Received October 17, 2000. Revised Manuscript Received March 8, 2001

The acidic zeolite HZSM-5 is modified by partial isomorphous substitution of the framework aluminum (Al) with an oxidative element during the zeolite synthesis. This is to develop an oxidative-acidic mediated catalyst suitable for the direct conversion of methane (or natural gas) into higher hydrocarbons in a multistage single catalytic process. The pure and modified ZSM-5 materials are highly crystalline of high morphology, which is shown by XRD and SEM. Pure ZSM-5 shows a single decomposition TGA band for the TPA+ cations associated with the anionic framework Al. Besides, modified ZSM-5 shows a lower temperature band revealing weaker interaction of TPA+ cations with the modifier element. The infrared spectrum of modified ZSM-5 shows a high frequency 3685 cm-1 band assigned to Bronsted sites weakly interacting with the modifier in addition and compared to the well-established more acidic Al-associated Bronsted sites absorbing at 3610 cm-1. Thus, XRD, TGA, and IR show evidence of incorporating the modifier element in the framework. The modifier site catalyzes the CH4 oxidation into H2CdO using molecular O2 near 1 bar and 373 K. This is explicit in the spectral appearance of υCdO at 1711 cm-1 and δC-H at 1420 and 1375 cm-1. This stage of the catalytic conversion is so peculiar to modified ZSM-5 that can never be catalyzed at any circumstance over ordinary ZSM-5. Bronsted sites associated with Al and modifier element interact strongly with H2CdO yielding unsaturated species defined by the weak υCdC at 1675 cm-1. Evidence of such interaction is shown as a downward shift of 23 cm-1 in the υCdO of the gaseous H2CdO at 1734 cm-1 to 1711 cm-1 on the surface and absence of the Bronsted sites absorbing at 3610 and 3685 cm-1. In effect, the latter Bronsted bands recover at the end of the catalytic process. The unsaturated species are higher aldehydes equilibrated in a keto-enol tautomeric structure. They eventually decompose in the temperature 473-573 K range into a mixture of hydrocarbons identified by υC-H at 2960, 2935, 2913, and 2868 cm-1 and δC-H at 1469 and 1385 cm-1 (aliphatics) and by υring at 1600 and 1510 cm-1 (aromatics). The IR spectral profile of the products is similar to that of Mobil methanolto-gasoline MTG process. The mechanistic implications of this process are proposed on the basis of the IR spectral findings observed in the reaction gas phase and on the surface that shows no evidence for the MTG route, however.

Introduction Methane conversion to liquid hydrocarbons has been a subject of much interest at the academic and industrial levels. This is due to the fast consumption of the petroleum oil and growing exploration of natural gas late of the 20th century. Natural gas has currently been made ready for export either as ordinary gas in pipelines to neighboring countries or as liquefied gas shipped by sea to rather far remote countries. Methane represents over 90% of the natural gas content. Sincere efforts have been paid for economically converting methane (or natural gas) into higher liquid hydrocarbons. Two major catalytic processes are now in progress. In the first process, methane is partially oxidized over oxidative catalysts1-6 to CH3OH. In Mobil’s plant, CH3OH is dehydrated7 into CH3OCH3 that propagates8 to higher ethers, viz., CH3OC2H5, CH3OC3H7, and CH3OC4H9 over * E-mail: [email protected].

HZSM-5. The higher ethers then decompose to the corresponding primary olefin intermediates C2H4, C3H6, and C4H8, respectively, which are eventually alkylated by methanol regenerated in the previous step,8 oligomerized into higher paraffins and olefins, and cyclized into aromatics in the gasoline, C5-C10 range. Because of the peculiar activity and selectivity of HZSM-5, (1) Topp-Jorgensen, J. In Methane Conversion; Bibby, M., Chang, C. D., Howe, R. F., Yurchak, S., Eds.; Elsevier: Amsterdam, 1988; p 293. (2) Onsager, T.; Soraker, P.; Lodeng, R. Reprints Symp. on CH4 Activation, Conversion and Utilization: 1989, 4th Int. Chem. Eng., Pacific Basin Soc.; Honolulu, Hawaii, p 113. (3) Guzci, L. New Trends in CO Activation; Elsevier: Amsterdam, 1991. (4) Smith, K. J.; Young, C. W.; Herman, R. G.; Klier, K. Ind. Chem. Eng. Res. 1991, 30, 61. (5) Hu, X. D.; Foley, H. C.; Stiles, A. B. Ind. Chem. Eng. Res. 1991, 30, 1419. (6) Sie, S. T. Ind. Chem. Eng. Res. 1992, 31, 1881. (7) Sayed, M. B. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1149. (8) Sayed, M. B. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1771.

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gasoline forms with high efficiency and octane number.9,10 The second process involves steam reforming10 of methane into synthesis gas, CO and H2. In FischerTropsch’s plant, synthesis gas reacts in varying stoichiometries (the CO/H2 ratio can be modified using the water-gas shift catalysis) on hydrogenation catalysts, e.g., Ru to heavy hydrocarbons,11 which are eventually hydrocracked into poor octane gasoline, kerosene, diesel, and lighter and heavier fractions. Neither Mobil nor Fischer-Tropsch has yet been regarded as an inexpensive synthesis of the future liquid hydrocarbon fuels based on natural gas.10 Combining the oxidative catalysis of methane and acid catalysis of the formed oxygenates in a single process over an oxidative-acid mediated catalyst would successfully bring synthesis of the future liquid fuels to the commercial scale. This could be anticipated by modifying HZSM-5 with an oxidative element by impregnation or ion exchange.12-14 Since it is difficult to control such kinds of modification, the modified zeolite had always been lacking strong Bronsted acidity and had, therefore, been exclusively oxidative.15 The cut-off product of CH4 conversion over such surfaces is CO and CO2 with only limited fractions of hydrocarbons. Isomorphous substitution of the zeolite Al by oxidative elements may be performed during the zeolite synthesis. The modifier can serve as an acid site retaining its oxidative feature. HZSM-5 modified by the latter method has proven powerful oxidative-acid mediated catalyst so that it successfully performs a 100% conversion of methane into gasoline in a single catalytic process at low temperatures and pressures. The present IR spectral study, albeit performed under a static catalysis, affords a direct measurement for the reaction performed on the surface, which is unreadable in dynamic catalyses. Despite the static nature of the measurements, the spectral data shed light on a possible reaction pathway; its mechanistic implications are derived from the spectral findings of the intermediates and products detected in the reaction gas phase and on the solid surface throughout the catalytic process. Experimental Section Materials. The highly siliceous ZSM-5 was modified by inclusion of a small percentage of a suitable oxidative transition element in the form of an oxide as a substitute for alumina in the synthesis medium.16 The composition was 8TPABr: 8Al2O3:100SiO2:1100H2O. The silica source (Fluka) was of an analytical grade sodium silicate. The alumina (Merck) was 60G neutral Al2O3. TPABr (Aldrich) was used as a template. NH4F (Merck) was used for crystal growth. The modifier transition element was added in 10% of the alumina composition. NH4F was added in 25% of the template composition. The zeolite synthesis was performed in a Teflon vessel- contained stainless steel Deutch & Neumann High-Pressure Autoclave 100-350-2 (9) Sayed, M. B.; Vedrine, J. C. J. Catal. 1986, 101, 43; see also, Weisz, P. B. Pure Appl. Chem. 1980, 52, 209. (10) Mills, G. A. Fuel 1994, 73 (8), 1243. (11) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: New York, 1984. (12) Brown, D. M.; Bhatt, B. L.; Hsiung, T. H.; Leonard, J. J.; Waller, F. J. Catal. Today 1991, 8, 279. (13) Han, S.; Martenak, D. J.; Palermo, R. E.; Pearson, T. A.; Walsh, D. E. J. Catal. 1994, 148, 134. (14) Kucherov, A. V.; Slinkin, A. A. J. Mol. Catal. 1994, 90, 323. (15) Sayed, M. B. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1751. (16) Sayed, M. B.; Auroux, A.; Vedrine, J. C. J. Catal. 1989, 116, 1.

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Figure 1. XRD of pure ZSM-5. The inset shows the SEM of its single crystal of 6 µm diameter.

Figure 2. XRD of modified ZSM-5. The inset shows the SEM of its single crystal of 6 µm diameter. for two weeks with continuous stirring at 448 K. XRD of the oxidant-modified acidic ZSM-5 showed (Figure 2) a typical pattern of pure ZSM-5 (Figure 1), with only intensity change. SEM of modified ZSM-5 showed (Figure 2-inset) a cubic crystal of 6 µm diameter, compared to the spherical crystal (Figure 1-inset) of pure ZSM-5. Full characterization of the status of the modifier element is the subject of a separate article. However, a number of concrete evidences have teamed up to reveal its existence, nature, and role. NH4Cl (BDH) was used for ZSM-5 extraction into the acid form after a mild thermal decomposition for the TPA+ and NH4+ cations. The oxidative source in the CH4 reactions was a Medical grade O2. CH4 was a high purity grade from Merck. Methods and Equipment. TPA+ in pure and modified ZSM-5 was thermally characterized17 under a N2 flow of 30 cm3 min-1 and a heating rate of 5 K min-1, using a PerkinElmer TGA with a P. E. Nelson 1022 Controller. The zeolite powder was pressed into a self-supported thin wafer suitable for the IR measurements. It was pretreated in a specially designed IR reactor of two sections: silica for the heat treatment and Pyrex glass for the ir spectral measurements, which were jointly connected by a graded seal. The glass segment was fitted with vacuum tight and easily removable IR-transparent KBr windows. The IR cell and the vacuum line were greaseless, capable of maintaining good vacuum of 10-8 bar. TPA+ decomposition into Bronsted sites was followed by (17) Kassem, M. E.; Sayed, M. B.; Arafa, W. M.; El-Samman, H. M.; Al-Emadi, I. M. Thermochim. Acta 1992, 197, 265.

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Figure 3. TGA thermogram of TPA+ decomposition for pure and modified TPA+ZSM-5. ir spectroscopy during the zeolite pretreatment at 10-8 bar and 673 K in order to ensure complete transfer of ZSM-5 into the acid form. The methane conversion was safely performed below the atmospheric pressure because of the glass nature of the ir reactor. CH4 (600 mbar) and O2 (200 mbar) were successively admitted onto the pretreated zeolite at 293 K. Successive spectral measurements were performed under such static conditions at rising temperature for both the reaction gas phase and the catalytic surface, using the Ft-IR spectral facilities of a Nicolet 510P Ft-IR spectrometer.

Results and Discussion Correlation Thermal TGA Analysis of Pure and Modified TPA-ZSM-5. TPA+ decomposes respectively at 550, 653, 738, and 753 K for pure TPABr,17 physically occluded in silicalite,18 associated with Ga in GaZSM5,18 and associated with Al in pure ZSM-5.17 Gradual rise in TPA+ decomposition temperature explains stronger interaction with more anionic framework.19 TPA+ decomposes respectively at 768 and at 708 and 768 K (Figure 3) for pure and modified ZSM-5. Appearance of more than a decomposition band indicates inclusion of TPA in more than a phase. The 768 K-band is of higher temperature than that (753 K) previously reported for pure ZSM-5,17 which assigns stronger interaction to the more anionic framework Al of the present zeolite.20 The lower temperature 708 K-band assigns yet weaker interaction of TPA+ with the modifier element than that (738 K) shown for Ga in GaZSM-5.18 Involvement of the 708 K-band along with the 768 K-band in modified ZSM5, does not only show evidence for incorporation of the modifier element in the framework but also predicts lower Lewis affinity to the modifier than to Al or Ga. Reduction in Lewis affinity to the modifier goes as rise to Al that explains the stronger interaction. The protons associated with the modifier sites exhibit low acidity as confirmed by IR spectroscopy. Nevertheless, they interact as strongly as for those associated with the zeolite Al, see IR analysis. It is important to add that no evidence of physically occluded TPA+ is shown in the (18) Kosslick, H.; Tuan, V. A.; Fricke, R.; Jedamzik, J.; Lanch, H. D. J. Thermal Anal. 1991, 37, 631. (19) Parker, L. M.; Bibby, D. H.; Patterson, J. E. Zeolites 1984, 4, 168. (20) Minchev, Ch.; Weyda, H.; Minkov, V.; Penchev, V.; Lecher, H. J. Thermal Anal. 1991, 37, 573.

Figure 4. Surface infrared spectra of (CH4 + O2)/modified HZSM-5 catalytic conversion: (a) Modified ZSM-5 pretreated at 10-8 bar and 573 K containing residual TPA+ cations. (b) Modified HZSM-5 pretreated at 10-8 bar and 673 K. (c) CH4 sorbed at 293 K and reacted with O2 at 373 K over the pretreated zeolite of step b. (d) As in (c), after reaction at 473 K. (e) As in (c), after reaction at 573 K.

present ZSM-5. Occluded TPA+ would decompose at a lower temperature of 653 K as shown in silicalite.18 FT-IR Spectral Analysis of CH4 Conversion over Modified HZSM-5. i. Infrared Spectral Correlation during Pretreatment of the Zeolite Surface. Infrared spectrum of ZSM-5 pretreated at 10-8 bar and 573 K shows a surface background dominated by spectral features of residual TPA+ (Figure 4a) at 2982, 2945, and 2882 cm-1 for its υC-H mode and at 1476, 1459, and 1382 cm-1 for its δC-H mode. Weakly interacting terminal silanols absorb IR radiation at 3750 and 1625 cm-1 for υO-H and δO-H, respectively. Spectral features near 2000 cm-1 are of little importance because of their weak interference with sorption and catalysis. Pretreatment at a higher temperature of 673 K effects complete decomposition for TPA+ and NH4+ into Bronsted sites (Figure 4b) absorbing at lower frequencies compared to the weakly acidic terminal silanols absorbing at 3750 cm-1. HZSM-5 shows the related υO-H at 3610 cm-1 for the Bronsted site associated with the framework Al. Typical absorption is shown for the pure and modified HZSM-5. In addition, a mid frequency band is shown (Figure 4b) at 3685 cm-1 for the modified

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HZSM-5 and is assigned to Bronsted sites associated with the modifier element, being tetrahedrally coordinated. The higher frequency explains association with lower Lewis affinity to the modifier compared to Al. However, the modifier- associated Bronsted sites interact as strongly (Figure 4c) as those associated with the zeolite Al of the higher Lewis acidity. ii. Infrared Spectra of the (CH4 + O2)/Modified HZSM-5 Catalytic Conversion. CH4 was sorbed at 293 K and the ir spectrum was measured. O2 was sorbed at the specified stoichiometry (Experimental) and the mixture was then warmed to 373 K below 1 bar for 15 min. Infrared spectrum of the surface (Figure 4c) reveals an intense υCdO band at 1711 cm-1 and weaker υCdC shoulder bands at 1772 and 1675 cm-1 of intermediates. δC-H of the intermediates appears at 1420 and 1375 cm-1. Such findings at early conversion favor a reaction pathway based on H2CdO than CH3OH intermediate. Methanol would chemisorb strongly to form a methoxy species (2942 cm-1) showing a downward shift of 30 cm-1 from the dominant υC-H of gaseous methanol at 2972 cm-1.8 Neither the gaseous methanol nor the surface methoxy species is evident in the present study. In effect, gaseous methanol shows other fundamental modes of υO-H (3675 cm-1), υC-H (2842 cm-1), δC-H (1458 cm-1), δO-H (1350 cm-1), and the very strongly absorbing υC-O (1033 cm-1),8 which are not apparent in the present spectra. Also, the primary intermediate of CH3OH reactions over HZSM-5 surfaces is (CH3)2O that absorbs very strongly at 1178 cm-1 for the υC-O, which is not shown either. The υCdO band at 1711 cm-1 and δC-H bands at 1420 and 1375 cm-1 are for typical H2CdO modes. Formaldehyde interacts strongly with the surface Bronsted sites and to a lesser extent with the surface terminal silanols of weak acidity, which is explicit in disappearance of the Bronsted bands at 3685 and 3610 cm-1 and in only downward shift of 25 cm-1 in the silanol band at 3750 cm-1. The latter spectral shift is associated with the appearance of a broad band centered at 3480 cm-1. The weak bands at 1772 and 1675 cm-1 favor assignment to unsaturated aldehyde than to olefin intermediates at such an early conversion because of conjugation with the aldehydic CdO bond. Raising the measurement temperature to 473 K allows these aldehydes to react into hydrocarbons identified (Figure 4d) by υC-H at 2960, 2935, 2913, and 2868 cm-1 and δC-H at 1469 and 1385 cm-1 for the aliphatic hydrocarbons and by υring at 1600 and 1510 cm-1 for the aromatic hydrocarbons. The simultaneous absence of the Bronsted bands at 3610 and 3685 cm-1 and aldehyde bands at 1772, 1711, and 1675 cm-1 (Figure 4b,c) can be regarded as evidence for the active participation of the associated species in the catalytic reaction. In effect, the Bronsted 3610 and 3685 cm-1 bands recover (Figure 4d) when the spectral features of reactant methane and the aldehyde intermediates vanish (Figure 4e) at end of the catalytic process. iii. Infrared Spectra of the Catalytic Conversion Gas Phase. The gas-phase spectrum (Figure 5) reveals complementary data of useful importance for predicting a possible reaction pathway for methane conversion. Figure 5a illustrates a typical CH4 spectrum that is dominated by υC-H {PQR; Q at 3017 cm-1}, δC-H {PQR; Q at 1306 cm-1} and γC-H {PQR; Q at 585 cm-1}.

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Figure 5. Gas-phase infrared spectra of CH4 reaction with O2 over modified HZSM-5: (a) CH4 at 600 mbar and 293 K. (b) As in (a), after reaction with O2 (200 mbar) over modified HZSM-5 at 373 K. (c) As in (b), after reaction at 473 K. (d) As in (b), after reaction at 573 K.

Reaction of CH4 and O2 at 373 K (Figure 5b) shows a complex IR pattern revealing key intermediates. Gaseous H2CdO is characterized by υCdO mode at 1734 cm-1. The downward shift of 23 cm-1 shown in this mode on the surface (1711 cm-1) corresponds to the strong interaction of H2CdO with the zeolite Bronsted sites that concurrently disappear. In the high-frequency range of the spectrum, evidence of strong intramolecular hydrogen bonding is explicit in a broad band centered at 3365 cm-1. Chain CH2 bands form at the expense of dropping CH4, which is observed in the spectral region of the C-H stretching at 2925 and 2853 cm-1. The CH2 species could be part of higher hydrocarbons or more likely of higher aldehydes at such early stages of the catalytic conversion. Beside the residual CH4 deformation bands, the spectrum reveals a weak υCdC band at 1624 cm-1, weak δC-H bands at 1464 and 1405 cm-1, a strong γC-H band at 802 cm-1, weak νC-C bands at 1165 ( 20 cm-1, and strong υC-O bands at 1090, 1075, and 1024 cm-1. The higher υC-C and lower υCdC band frequencies, with respect to that expected, assign a resonance structure for the aldehyde intermediates as in the keto-enol tautomeric structure. Reaction at 473 K raises the CH2 band intensity (Figure 5c) at the expense of dropping CH4 bands.

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Scheme 1. Aldehyde-Based Pathway for Oxidative-Acid Mediated Catalysis of CH4 Conversion to Higher Hydrocarbons in a Single Catalytic Process

a The modifier element catalyzes CH oxidation. Higher olefins form on decarbonylation of Aldol’s products. Olefins then oligomerize 4 and cyclize into higher aliphatics and aromatics, respectively.

Raising the temperature to 573 K effects a 100% CH4 conversion. It is interesting to denote that H2CdO reforms during the aldehyde propagation and decomposition, which is shown (Figure 5d) by a sudden rise in its υCdO band intensity at 1734 cm-1. A similar observation of CH3OH reforming in Mobil process was reported.8 Also, CH3 forms at late stages of the conversion, which is shown as late appearance of its υC-H mode at 2960 and 2873 cm-1 and its δC-H mode at 1420 and 1385 cm-1. The late formation of CH3 is an essential mechanistic step for terminating the hydrocarbon chain. To propose a reaction mechanism supporting this

multistage complex reaction, key observations derived from the gas and solid-phase spectra should now be reconciled and further emphasized. a. The initial intermediate of partial CH4 oxidation is definitely H2CdO (not CH3OH) that explains a fair oxidation. A mild oxidative element might result in CH4 oxidation to CH3OH, where Mobil MTG process could be the reaction pathway. b. H2CO propagates to higher aldehydes that are equilibrated in a keto-enol tautomeric structure, which is evident by the appearance of further υCdO, υCdC, υO-H along with new υC-H and δC-H modes.

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c. Since it is a gas-state feature, the broad υO-H band at 3365 cm-1 favors intra- to intermolecular hydrogen bonding association. This conforms to the proposed keto-enol tautomerism, which equilibrates the higher aldehydes with the counterpart glycols. d. The lower CdC and higher C-C band frequencies, than expected, are in favor of the keto-enol tautomeric structure of the propagating aldehydes. e. H2CdO reforms at late stages of the conversion as CH3OH reforming in Mobil MTG process. f. CH2 forms at earlier stages of the conversion as part of propagating aldehydes, whereas CH3 forms at later stages for terminating the chains of lately appearing hydrocarbons. g. The few short chain paraffins contained in natural gas accelerate its conversion, compared to methane of the higher molecular symmetry and hence much lower reactivity. This would conclude a more fruitful application for the present oxidative-acid mediated zeolitic catalyst to the natural gas conversion into higher hydrocarbons suitable for the future fuel and petrochemical industries. Finally, the author would like to ask for the reader’s kind patience and permission for keeping the identity

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of the modifying element closed with a promise of disclosing it shortly in a next article. However, various spectral evidences are provided that reveal clearly its existence, nature, and role in the difficult task of methane conversion into higher hydrocarbons, using diverse techniques. Working out the present gas- and surface-phase infrared spectral findings, the mechanistic implications of the multistage catalytic conversion of methane may now be postulated. Acknowledgment. I would like to dedicate the fruit of this work to the Soul of my Mother, Amina Al-SebÆei and to the Human, Prof. Ralph P. Cooney, Vice Chancellor and Dean, Faculty of Science, Auckland University, New Zealand. They reinforced Patience and Enthusiasm at my Heart. The author sincerely acknowledges the great support made by the University of Qatar during his affiliation with Qatar. He is highly indebted to Prof. Ibrahim S. Al-Naimi, the Director. He would ever remember that Peaceful Country and would like to dedicate this work to its generous people, with a great hope of applying this project to their benefit soon. EF0002288