How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

• DOI: 10.1016/j.jcat.2015.01.013
• Publication Date: 2015-06-05T18:59:04Z
• Authors: Marius Westgård E...; Kristof De Wispel...; Karen Hemelsoet; Samuel L.C. Moors; Thomas Deconinck; Michel Waroquier; Stian Svelle; Veronique Van Spe...; Unni Olsbye
• Keywords: Methylation reactions; REACTION-MECHANISM; Ab initio molecular dynamics; TO-HYDROCARBONS; DIMETHYL ETHER; 02 engineering and technology; MOLECULAR-SIEVES; Methanol to olefins; MICROPOROUS MATERIALS; Chemistry; CO-REACTION; H-MFI; Methanol to hydrocarbons; SOLID ACIDS; 0210 nano-technology; Acid strength; CATALYZED METHYLATION REACTIONS; PRODUCT SELECTIVITY; Co-adsorption

Abstract This work encompasses a combined experimental and theoretical assessment of how zeolitic acid strength and composition affects acid-catalysed methylation reactions. Overall, higher methylation rates were observed over the material with higher acid strength. Co-reactions of methanol with benzene at 250 °C over the two isostructural AFI materials H-SSZ-24 and H-SAPO-5 revealed large differences in selectivity. While the stronger acidic H-SSZ-24 mainly produced toluene and polymethylbenzenes, high yields of C 4+ aliphatics were observed over H-SAPO-5. These results strongly suggest that alkene methylation was preferred over H-SAPO-5 even at very low conversion during methanol/benzene co-reactions. Furthermore, a comparison of benzene and propene methylation at 350–400 °C revealed a significantly faster rate of benzene than propene methylation in H-SSZ-24, whereas the rates of benzene and propene methylation were similar in H-SAPO-5. The observed difference in reactivity of the two hydrocarbons in both catalysts could be understood by careful analysis of various molecular dynamics simulations of the co-adsorbed complexes. The probability to form protonated methanol was, as expected, higher in the more acidic material. However, in H-SSZ-24, the probability for methanol protonation was higher when co-adsorbed with benzene than when co-adsorbed with propene, while the same was not observed in H-SAPO-5. Furthermore, it was found that benzene and methanol are more likely to form a reactive co-adsorbed complex in H-SSZ-24 compared to propene and methanol, while the opposite was observed for H-SAPO-5. This work shows that molecular dynamics simulations provide insights into the adsorption behaviour of guest molecules in large pore AFI materials. The obtained insights correlate with the experimentally observed reactivities.