Conversion of Methanol to Hydrocarbons: spectroscopic characterization of carbonaceous species formed over H-ZSM-5

Transcription

1 Conversion of Methanol to Hydrocarbons: spectroscopic characterization of carbonaceous species formed over H-ZSM-5 Francesca Bonino 1, Luisa Palumbo 1, Morten Bjørgen 2, Pablo Beato 2, Stian Svelle 3, Silvia Bordiga 1, Adriano Zecchina 1 1 Dipartimento di Chimica IFM -NIS Centre of Excellence, Università di Torino, Via P. Giuria 7, Torino and INSTM unità di Torino Centro di Riferimento 2 Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark 3 Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway luisa.palumbo@unito.it In this study, the mechanism of the methanol-to-hydrocarbons (MTH) reaction over H-ZSM- 5, which is the archetype MTH catalyst, has been pursued. From isotopic labeling experiments, it is demonstrated that the reaction scheme for alkene formation from methanol over H-ZSM-5 is essentially different from those previously drawn for H-beta and H-SAPO- 34. Besides to effluent compounds analysis, a study of the organic material confined within the H-ZSM-5 pores during the reaction was performed by GC-MS technique. In order to perform a complete screening on the sample, a spectroscopic characterization of carbonaceous species formed over H-ZSM-5 after methanol conversion was done. Methanol can be made from carbonaceous feedstocks via synthesis gas and further converted into gasoline range hydrocarbons or alternatively light alkenes. The zeolite catalysed conversion of methanol to hydrocarbons is commonly denoted the MTH reaction, but, as process conditions and catalyst choice alter the product selectivities, the abbreviations MTO (methanol to olefins) and MTG (methanol to gasoline) are frequently used. Throughout the years, the MTH reaction has challenged the catalysis community by its complex mechanism 1. The long-standing question has been the mechanism behind the step where C-C bonds are formed from oxygenates (methanol/dimethylether). There are presently strong evidences disfavoring direct mechanisms where methanol/dimethylether molecules are combined to form e.g. ethene during steady state conversion 1,2. On the other hand, an all-dominating indirect route known as the hydrocarbon pool mechanism has been gradually accepted over the last years 1-4. The hydrocarbon pool is now described as a catalytic scaffold, constituted by large organic molecules adsorbed in the zeolite, to which methanol/dimethylether is added and from which alkenes and water are formed in a closed catalytic cycle. The most extensive fundamental mechanistic understanding has until now been attained for the H-SAPO-34 5 and

2 H-beta 6 catalysts, but, as will be shown, many aspects may vary with the zeolite topology. This study is devoted to H-ZSM-5, which is the archetype MTG catalyst. Therefore, catalytic tests were performed in order to understand the identity and the operation of the mechanism on H-ZSM-5. It has been demonstrated, from isotopic labeling experiments, that the reaction scheme for alkene formation from methanol over H-ZSM-5 is essentially different from those previously drawn for H-SAPO-34 and H-beta. Besides effluent compounds analysis, a study of the organic material confined within the zeolite pores during the reaction was performed by GC-MS technique. This approach has been successfully used to characterize species entrapped inside the pores and assign them their role in the reaction mechanism. One of the major issues for a commercial application of the MTH process, is the relatively fast catalyst deactivation due to the formation of carbonaceous species. Unfortunately the characterization of these species is very difficult and at the moment no clear data are available to establish their formation mechanism. In this work we present a spectroscopic study on the carbonaceous species formed on the catalyst upon methanol treatment by a combination of DRUV-Vis, Luminescence and Raman Spectroscopy. Commercially available, as well as home made H-ZSM-5 samples have been used. Spectroscopic studies have been performed in situ on samples previously actived in high vacuo, and then contacted with methanol partial pressure at 370 C for increasing contact times. A second set of data have been obtained ex situ measuring the samples treated in a fixed bed reactor (He stream up to 370 C in order to remove most of adsorbed water and then methanol flux). H-ZSM-5 behaves very differently from H-SAPO-34, not only with respect to product selectivity but also with respect to reaction and deactivation mechanism. The longer life-time of ZSM-5 has been attributed to size constraints within the pore structure, preventing the formation of larger condensed aromatic ring systems 7. Applying a combination of spectroscopies we have characterized the carbonaceous molecules formed along the reaction with methanol. After methanol conversion, the H-ZSM-5 samples were studied by means of vibrational (FTIR and Raman) and electronic (DRUV-Vis and Fluorescence) spectroscopies. For sake of brevity, only FTIR and DRUV-Vis results will be here reported. In figure 1 FTIR spectrum of the sample outgassed and oxidized at 500 C for 2 h (dashed line) is reported. It is characterised by two well-defined signals at 3615 and 3745 cm -1,

3 assigned to the acidic bridged OH of Si(OH)Al (Brønsted sites) and to the terminal SiOH (Isolated Silanols), respectively 8. In the same figure it is also reported methanol dosage on the zeolite (solid line). Upon methanol interaction the band associated to Brønsted site becomes completely eroded and gives rise to a rather broad band with maximum around 3270 cm -1 ; while the Isolated Silanols are partially consumed and shifted to 3645 cm -1. In the spectra region around 3000 cm -1 it is possible to observe the CH stretching mode of the methanol interaction, while in the spectra region around 1500 cm -1 are present CH bending mode. Figure 1: IR spectra of outgassed sample (dashed line) and after methanol dosage (solid line) After methanol dosage, the sample was treated at the methanol conversion temperature (370 C) for increasing times, as reported in figure 2 that summarized two subsequent experiments. Part a) reports IR spectra of H-ZSM-5 sample contacted by methanol (70 torr) and heated at 370 C for 5h (solid line). The spectrum presents two signals at about 1470 cm -1 and 1380 cm - 1 that correspond to CH 3 bending mode of methanol interacting the zeolite. Another peak at about 1510 cm -1 apperars after 5h of methanol conversion reaction time. In order to understand the character of this new component, NH 3 is dosed on the zeolite (dashed line): the peak at 1510 cm -1 desappears, therefore it is possible to conclude that it is due to the presence of carbocationic species. Part b) shows IR spectra of H-ZSM-5 sample contacted by methanol (70 torr) and heated at 370 C at differents reaction time in a range between 1-100h (solid line). The CH bending modes are obviously persistent, but it is easy to observe that the peak at about 1510cm -1, by

4 increasing the reaction time, becomes more structured and it is possible to distingiush two different peaks at about 1518 cm -1 and 1495 cm -1. Also in this case, to verify the character of the species, NH 3 is dosed, but the peaks remain in the same position. Therefore, from these results, it is possible to conclude that during the MTO process we observe carbocationic species formation just during the fisrt steps of the reaction. Figure 2: IR spectra of methanol convertion on the H-ZSM-5: part a) zeolite contacted by methanol at 370 C for 5h (solid line) and after NH 3 dosage (dashed line); part b). zeolite contacted by methanol at 370 C for reaction times in a range between 1-100h (solid lines) and after NH 3 dosage (dashed line) DRUV-Vis results will be here reported in figure 3 that summarizes three different experiments. Part a) reports DRUV-Vis spectra of H-ZSM-5 sample contacted with methanol (70 torr) under static conditions and heated at different temperatures, in the C range. Along the series we observe the growth of components at about 400, 500 and 600 nm. During the treatment in methanol atmosphere, the sample colour passes from white to pale grey. The last spectrum of this series is very close to the first spectrum reported in part b). Part b) shows the case in which the reaction is performed in a bed-fixed reactor. The sample has been treated in He stream up to 370 C and then contacted at the same temperature with a methanol flux for increasing reaction times (1-110 h). In this case the spectra were performed on the reacted samples in air. The colour of the more reacted sample is dark grey: in fact a multi-component band is present in the nm range. It seems that the component at 400 nm is present in all the spectra with a similar intensity, while the bands at lower wavenumber become completely mixed in the broad absorption extending till 800 nm. If this is the case,

5 the species associated to the band at 400 nm could be the intermediate state in which the reactive species are transformed before to become coke. Part c) reports the effect of NH 3 dosage on the H-ZSM-5 sample previously treated in CH 3 OH. It is clearly observed that some of the components (bands at 400 nm and at 500 nm) decrease in intensity and shift upwards. The component at about 600 nm seems less perturbed. This behaviour is assigned to the presence of carbocationic species. The result is confirmed by the fact that a prolonged outgassing reveals a partial reversibility of the phenomenon. These preliminary results point out that the H-ZSM-5 sample, after methanol conversion, gives rise to species absorbing at 400 nm, which have a carbocationic character and that could be the coke precursors as they are present not only after a treatment in static conditions, but also upon a treatment in flux. Figure 3: DRUV-Vis spectra: part a) H-ZSM-5 contacted by methanol (70 torr) and heated at different temperatures in the C range in static condiction; part b) H-ZSM-5 treated at 370 C with a methanol flux, for increasing reaction times (1-110 h) by using a bed-fixed reactor; part c) the effect of NH 3 dosage on H-ZSM-5 previously contacted and heated in methanol atmosphere. 1 M. Stöcker, Microporous Mesoporous Mater 1999,. 29, W. Song, D. M. Marcus, H. Fu, J. O. Ehresmann, J. F. Haw, J. Am. Chem. Soc. 2002, 124, D. M. Marcus, K. A. McLachlan, M. A. Wildman, J. O. Ehresmann, P. W. Kletnieks, J. F. Haw, Angew. Chem., Int. Ed. 2006, 45, U. Olsbye,M. Bjørgen, S. Svelle, K. P. Lillerud, S. Kolboe, Catal. Today, 2005, 106 (1-4), B. Arstad, S. Kolboe, J. Am. Chem. Soc., 2001, 123, M. Bjørgen, U. Olsbye, D. Petersen, S. Kolboe, J. Catal. 2004, 221 (1), G.F. Froment, W.J.H. Dehertog, A.J.Marchi, A rewiew of the literature, Catalysist , 1 8 A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Spanò, F. Geobaldo, J. Chem. Soc. Faraday Trans. 1996, 92 (23),