A novel approach to the scientific design of oxide catalysts for the partial oxidation of methane to methanol

M. de P o n t e s , R.L. E s p i n o z a , C.P. N i c o l a i d e s , J.H. S c h o l z and M.S. Scurrell (Editors)

Natural Gas Conversion IV

41

S t u d i e s in S u r f a c e S c i e n c e and Catalysis, Vol. 107 9 1997 E l s e v i e r S c i e n c e B.V. All rights r e s e r v e d .

A NOVEL A P P R O A C H TO THE SCIENTIFIC DESIGN OF O X I D E C A T A L Y S T S FOR THE PARTIAL O X I D A T I O N OF M E T H A N E TO M E T H A N O L

Graham J. Hutchings 1, Justin S.J. Hargreaves 2, Richard W. Joyner 2 and Stuart H. Taylor 1 1 Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX. 2 Catalysis Research Centre, Nottingham Trent University, Clifton Lane, Nottingham, NG11 8NS.

ABSTRACT The approach adopted for the scientific design of methane partial oxidation catalysts has identified oxides that are capable of activating methane and oxygen, but do not destroy methanol, the desired product. From the perspective of methanol stability Sb O 3 showed the best performance, destroying . . . only 3% of the methanol feed at 500~ ~ e majority of oxides totally combusted methanol below 400~ The oxides M o O. 3, . . Nb 2O 5, Ta 2 O 5 and WO all produced high methanol conversion, however, high selecUvllaes towards f6rmaldehyde and dimethylether were obtained with only low levels of carbon oxides throughout the range of conversions. The products formaldehyde and dimethylether were desirable by-products from a methane partial oxidation process hence these oxides are suitable catalyst components. The activation of methane has been probed by the exchange reaction with deuterium under non-oxidative conditions. The most active catalyst was Ga 2 O 3 which exhibited normalised . . exchange rates several orders of magnitude greater than the other catalysts. A relauonship between the rate of methane activation and the predicted basic strength of the rare earth sesquioxides was established. This relationship indicates that methane activation took place via the abstraction of H § to form a surface methyl carbanion species. On the basis of these results we have designed and tested a limited range of two component oxide catalysts for methane partial oxidation. The best catalyst was GazO3/MoO. prepared by physically mixing the component oxides. This catalyst showed an increased' methanol yield compared with the homogeneous gas phase reaction in the reactor tube packed with quartz chips. The increased methanol yield has been attributed to the development of a co-operative effect between the Ga O and MoO 3 oxide phases. The degree of success from this approach indicates the validit~ 0]~ this novel study. 1. INTRODUCTION The direct partial oxidation of methane to methanol would offer considerable ~onomic advantages over the current two stage process, by-passing the costly steam reforming step. Such a process would also facilitate the utilisation of the vast natural gas reserves which are often located in remote areas. The partial oxidation of methane to methanol has received considerable research attention and although appearing a relatively simple transformation the development of efficient catalysts has met with little success [ 1]. One of the major challenges encountered with methane oxidation in many heterogeneous systems is the high temperatures required to activate the methane molecule, the use of such temperatures often leads to the production of deep oxidation products which are more thermodynamically stable products compared to methanol. In the present study a novel approach for the scientific design of new methane partial oxidation catalysts has been adopted. This approach has studied the activation of methane and oxygen on single metal oxides and the stability of the methanol over the same oxides.

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By studying these interactions the suitability of single oxides for inclusion as components in methane oxidation catalysts can be assessed. 2. EXPERIMENTAL Experiments to determine methanol stability were carried out in a conventional liquid feed microreactor with a fused silica reactor tube. Reaction conditions used a methanol[pxygen/helium feed in the ratio 1/4/12, with a gas hourly space velocity ca. 12,000 h . Methanol stability was determined in the temperature range from 100~ to 500~ Product analysis was performed on-line using a Varian GC-MS system. A low dead volume isotope pulsing reactor was designed for methane/deuterium exchange studies. Under normal operation methane and hydrogen (1/1) were passed over the catalyst held in a fused silica reactor tube. The hydrogen supply was then switched to deuterium and the exchange reaction monitored using a Varian GC and a Hiden quadrup_qle mass spectrometer. Studies were carried out using a gas hourly space velocity of 300 h . The exchange of O / O isotopes in the presence of oxides has been widely studied by Winter [2] and Boreskov [3]. We have therefore uulised these results, concentrating on the work of Winter [2], although general agreement between the two research groups is good. Catalytic activity for methane partial oxidation was studied in a silica lined stainless steel microreactor at 15 bar. The reactant feed consisted of methane/oxygen/helium in the ratio 23/2/5 .atnd experiments were conducted at a constant gas hourly space velocity of ca. 5,000 h . 18

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3. RESULTS A N D DISCUSSION A ranking order for the stability of methanol over candidate oxides was based on the temperature at which 30% of the methanol was converted to carbon oxides (figure 1). ~11o OrlOll OvO

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200 220

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140 280

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8ElO

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400 Cllo

420

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440 40o 4co NblOI --'-'-- 500

T9| O S M o O I , WOe. 11tl0:1

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Figure 1. Ranking for methanol stability based on the temperature at which 30% methanol feed was converted to carbon oxides.

43 The majority of oxides tested totally combusted methanol below 400~ Over Sb_O 3 methanol showed exceptional stability as only 3% methanol conversion was exhibited at 500~ M o O 3, Nb O 5, Ta 2 .O and WO 3 were . all at high temperatures in the methanol stability ranking, q~laese oxides all showed high methanol conversion however, major reaction products were formaldehyde and dimethylether which are desirable by-products from a methane partial oxidation process. Comparison of the methanol stability results with the surface oxygen exchange activity of the oxides showed that a correlation was evident [4]. This correlation, albeit weak, can be expected to be important for the ultimate design of methane partial oxidation catalysts. The pattern of activity for the activation of methane at 500~ normalised for surface area effects is shown in figure 2.

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Gs203

43 42 41

ZnO Cr203 Ls203 Nd203

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39

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In

Gd203, Pr6Oll CaO AI203 8m203, MgO V203 Yb208 MnO ZrO2

Fe304 Ce02 TI02

Rate

-37

36

36

34

33

Figure 2. Methane/deuterium surface area normalised exchange rates at 500~ The most active catalyst by a considerable margin was Ga 2 O 3 . which showed an exchange rate several orders of magnitude greater than any other orade. ZnO also showed high activity for the exchange reaction. It is interesting to note that these two oxides are recognised for their high activity in other processes involving hydrocarbon activation [5]. For all the oxides included in figure 2 the exchange process took place in a step wise manner. CH D was the primary product' whilst traces of CH 2 D 2 were detected at higher 9 . . . rates of exchange. The oxides MoO 3 and WO 3 were reduce2t under reacuon condmons and these oxides along with Nb205 and Ta205 were inactive or showed very low activity for the exchange reaction. A correlation has been established between the exchange activity and the ionic radii of the lanthanide sesquioxides. It has been established that the basicity of the lanthanide oxides decreased in a gradual manner as the 3+ metal ionic radius decreased [6]. Thus the relationship observed indicates that the exchange rates were related to the strength of basic sites on the oxide surface and is consistent with methane activation by the abstraction of H § to form a surface methyl anion.

44 The results discussed above were used as a guide for the selection of suitable methane oxidation catalyst components. The best of a very limited set of catalysts tested so far is a physical mixture of Ga O and MoO Ga O was selected for the ability to activate methane whilst MoO 3 was selected i3or two reasons. Ftrstly, the major product from methanol oxidation was formaldehyde, produced with 97% selectivity at 500~ Secondly, the mechanism of oxygen exchange identified by Winter [2] was of R 3 type. This type of mechanism involved exchange with the whole of the lattice oxygen, not merely the surface layer indicating that the mobility of oxygen throughout the oxide lattice was a facile process. The lability of lattice oxygen is an important concept in selective oxidation reactions, particularly with respect to the Mars van Krevelan mechanism, in which lattice oxygen is the active oxygen insertion species. The maximum methanol per pass yields based on methane conversion for the methane partial oxidation process are shown below for Ga203/MoO3, Ga203, MoO 3 and the reactor tube packed with quartz chips (figure 3). 9

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Maximum C H 3 0 H per p a n yield/%

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Figure 3. Maximummethanol per pass yields for MoO3, Ga203, Ga203/MoO3and quartz chips. The Ga203/MoO_ catalyst produced the highest yield of methanol which was approxirriately 2 0 ~ greater than that over the quartz chips. Comparison with the activity of the empty reactor tube showed that methanol yields were higher in the absence of any packing. However, in the present study we consider that the catalytic activity should be considered against the homogeneous methane oxidation activity of the quartz chips bed. This comparison provides a more valid reference point as residence time in the heated zone of the reactor and the heat transfer throughout the catalyst bed will be similar. The methanol yields over MoO and Ga O were in contrast very low Comparison of the methane oxidation results for ~ o O and Ga O showed that MoO was more selectwe for methanol production whilst GazO 3 was more acuve showing a higher methane conversion. These observations were consistent with the high activity for methane activation over Ga O and the oxygen exchange mechanism over MoO The p~ayslcal mixture of Ga O and MoO. combined die beneficml aspects of both oxides for methane partial oxadauon. These were the high acttvlty of GagO3 and the selective oxidation function of MoO 3. The increased yield of methanol over [he homogeneous gas 2

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45 phase reaction in the quartz chips reactor suggests that there was the development of a cooperative effect between the two phases. Such an effect can be envisaged by considering the bifunctional nature of the catalyst. The surface of Ga O could provide the methane activation function to produce a surface methyl activated species. Such a species would be surface mobile and able to migrate to sites on the MoO. phase responsible for oxygen insertion leading to methanol formation. The mttmate mLx of the two component oxides provides many phase boundaries at which methanol may be formed. M=O species on MoO. surfaces have been proposed as active sites for methane partial ox~datton to formaldehyde at atmospheric pressure [7]. The same type of M=O sites have also been proposed as important for other selective oxidation reactions [8]. The migration of the surface methyl species to this site could lead to the insertion of oxygen forming a surface methoxy species. The desorption of this species to the gas phase may then be a relatively facile process as the diffusion of oxygen throughout the MoO_ lattice to regenerate the surface M=O site would be rapid [2]. The regeneration of such surface 3 . sRes by bulk lattice oxygen has been previously proposed for selective methane oxidation on MoO 3 .[7]. The production of methanol from a methoxy precursor has also been proposed as an Important mechanistic step by other research groups [9, 10]. The production of methanol from the methoxy species may take place either via the release of the methoxy radical to the gas phase followed by hydrogen abstraction from another source, usually methane. Methanol may also be formed from a surface reaction of the methoxy group with a supply of surface hydrogen, possible from surface hydroxyl groups, followed by desorption of the product. Co-operative effects in catalysis have often been attributed to the formation of a new phase which is more active or selective than the individual phases [ 11 ]. Characterisation of the Ga203]lVloO 3 catalyst by powder X-ray diffraction and X-ray photoelectron spectroscopy before and after use did not reveal any evidence for the formation of a new mixed oxide phase. 9

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4. CONCLUSIONS The Ga O / M o O catalysts which has been developed from the design approach has shown an increased 3held of methanol when compared to the homogeneous gas phase oxidation of methane in a quartz chips packed reactor. This increased yield has been attributed to a co-operative effect which can be explained from results of the individual activation steps. The results presented in the present study emphasise the validity of the approach that has been adopted. 2

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5. ACKNOWLEDGMENT We would like to acknowledge the Gas Research Institute, Chicago, for their fmancial support of this project. REFERENCES [1] T.J. Hall, J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner, S.H Taylor, Fuel Processing Technology, 42, 1995, 151. [2] E.R.S. Winter, J. Phys. Chem. (A), 1968, 2889. [3] G.K. Boreskov, Disc. Faraday Soc., 41, 1966, 263. [4] S.H. Taylor, J.S.J. Hargreaves, G.J. Hutchings, R.W. Joyner, Appl. Catal., 126, 1995, 287. [5] M. Guisnet, N.S. Gnep, F. Alario, Appl. Catal., 89, 1992, 1. [6] T. Moeller, H.E. Kremers, Chem. Rev., 37, 1945, 97. [7] M.R. Smith, U.S. Ozkan, J. Catal., 141, 1993, 124. [8] H.H. Kung, Stud. in Surf. Sci and Catal., 45, 1989, 169-199. [9] H.F. Liu, R.S. Liu, K.Y. Liew, R.E. Johnson, J.H. Lunsford, J. Am. Chem. Soc., 106, 1984, 41179

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[10] N.D. Spencer, C.J. Pereira, R.K. Grasselli, J. Catal., 126, 1990, 546. [11] L.T. Weng, B. Delmon, Appl. Catal., 81(A), 1992, 141.