Shape‐Selective Conversion of Methanol to Hydrocarbons Over 10‐Ring Unidirectional‐Channel Acidic H‐ZSM‐22

Abstract

With the forecasted depletion in global oil reserves, new routes to petrochemical products from natural gas, coal, or biomass are becoming increasingly important. The methanolto-hydrocarbons (MTH) reaction constitutes the final step in one such route. The MTH reaction proceeds over Brønstedacidic zeolite or zeotype catalysts, and near-commercial processes exist for the methanol-to-gasoline (MTG) reaction over ZSM-5, as well as the methanol-to-olefin (MTO) reaction over A breakthrough in the mechanistic understanding of the MTH reaction was the formulation of the "hydrocarbon pool mechanism" by Dahl and Kolboe, [2,3] which postulates that methanol is continuously added to aromatic reaction centers, from which light alkenes are split off in later reaction steps. [4] Recently, the importance of methylation and cracking of alkenes over ZSM-5 was highlighted. [5,6] ZSM-22 (TON) is less well studied [7-10] as a MTH catalyst. It incorporates one-dimensional non-interacting 10-ring channels with diameters of 0.46 0.57 nm. [11] Song and co-workers [7][8][9] reported the failure of ZSM-22 to convert methanol into olefins. Their studies of ZSM-22 showed a low production of olefins during the first pulses of methanol, however the amount of olefin quickly decreased to essentially zero. This failure as an MTH catalyst was ascribed to the narrow pores, which were assumed to be too small to accommodate the complete catalytic cycle of the hydrocarbon pool mechanism. Flow experiments (at 250-400 8C) afforded relatively constant yields of trace amounts of ethene and propene. The low reactivity was believed to be the result of traces of ZSM-11, impurities in the methanol (acetone) and/or external acid sites. Herein, we report studies of the MTH reaction over ZSM-22 at a wider range of reaction conditions and demonstrate that the previous conclusions [7][8][9][10] are not universally valid. Under suitable conditions, ZSM-22 has a conversion capacity comparable to that of SAPO-34, reaction intermediates reside within the pores, and the product spectrum is intermediate to those found for reactions for the MTO and MTG processes.Several batches of ZSM-22 with different Si/Al ratios were synthesized and all were found to be active catalysts for the MTH reaction. The crystallinity and purity of the product were confirmed by X-ray diffraction. Scanning electron microscopy (SEM) revealed needle shaped crystals of 2-3 mm length. 27 Al NMR spectroscopy indicated that, for the samples discussed here, Al was located exclusively in the framework, both for as-made and calcined/ion-exchanged samples. BET surface areas were in the range 160-207 m 2 g À1 . Two different ZSM-22 catalysts (Si/Al = 30 by inductively coupled plasma atomic emission spectroscopy) with BET surface areas of 173 and 207 m 2 g (weight hourly space velocity (WHSV) = 2.05 h À1 ) was 100 % and appreciable conversion took place for several hours (Figure 1 a) . However, at 350 8C, deactivation was very rapid. The feed rate in this case was lower than that used in previous studies...

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

“…In the one-dimensional pores of H-ZSM-22 only alkenes were found to be active HP species resulting in the production of branched C 5+ alkenes, [72][73][74][75][76][77][78] whereas aromatics were found to be dominating HP species in H-SAPO-34 and large pore zeolites H-beta and H-SSZ-24. 22,24,[79][80][81][82] A high activity of aromatics is mostly coupled with a high selectivity towards ethene and propene.…”

“…Several experimental studies clearly pointed out that methanol conversion in H-ZSM-22 occurs through the alkene cycle and that aromatic HP species are not active. [73][74][75]77,78,125 Cui et al…”

Conversion of methanol over H-ZSM-22: The reaction mechanism and deactivation

“…Nearly complete conversion of methanol could be obtained over ZSM-22 at 450 • C (WHSV = 10 h −1 ) [21]. In a very recent report from the group of Olsbye [22], they also found that under suitable conditions ZSM-22 has a conversion capacity comparable to that of SAPO-34. These results have been somewhat contradictive with the report of Cui et al [18].…”

Highly Selective Sorption of Small Polar Molecules by a Nonporous Ionic Crystal of a Lacunary Keggin-type Heteropoly Anion and Alkali Metal Cations

“…[3][4][5] Zeolites have well-defined molecular-sized pores, in which shape-selective adsorption and catalysis can occur. [3][4][5][6][7][8][9][10][11] Recently, there is great interest in organic zeolites constructed with organic and organometallic building blocks because they can have flexible channels when appropriate organic functional groups are used, leading to unique selective-inclusion properties. 1,2,12 Ionic crystals of polyoxometalates are nanostructured materials that are capable of selectively absorbing molecules.…”

“…[13][14][15][16][17][18][19][20] The absorption properties are finely controlled by adjusting the charge of the polyoxometalates and the organic moiety of the macrocations. For example, K 3 [Cr 3 O(OOCH) 6 solution with vigorous stirring. The resulting solution was allowed to stand for 1 day at room temperature, during which time a white solid precipitated.…”