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

Abstract

a b s t r a c tThis 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.

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…[27][28][29][30][31][32][33][34][35][36][37] Only a limited number of studies have applied techniques that go beyond the static approach to describe chemical transformation in zeolites. [19,20,[38][39][40][41][42][43][44] The strength of a dynamical approach was earlier demonstrated by some of the present authors.We reported that the influence of the presence of multiple protic molecules on the methylation of benzene [44] and methoxide formation [42] in H-ZSM-5 has to be studied with molecular dynamics. These studies demonstrated that formation of protonated clusters of protic molecules has to be taken into account when modeling reactions under conditions of high reactant loadings, in particular in case of protic reactants that can play a solvent-like role.…”

“…[13,43] It was demonstrated that in both catalysts, aromatic hydrocarbon pool compounds play a major role during olefin formation. [13,43,47,48] Figure 2. Schematic representation of the CHA structured H-SAPO-34 and AFI-structured H-SSZ-24 catalysts, with indication of the acid site, the size of the pore opening (taken from the IZA database [49] ) and the dominant hydrocarbon pool species during methanol conversion.…”

“…[13,45,46] H-SAPO-34 is an industrially relevant MTO catalyst, [25] whereas H-SSZ-24 has been studied as a model system as it allows co-feeding studies with bulky aromatic reaction intermediates. [13,43] It was demonstrated that in both catalysts, aromatic hydrocarbon pool compounds play a major role during olefin formation. [13,43,47,48] Figure 2.…”

“…Secondly, it was shown recently that these systems typically exhibit complex potential energy surfaces which cannot be fully captured by static geometry optimizations. [43] Indeed, due to the large available space inside the channels, guest molecules may adopt many configurations, making it difficult to describe reactions with one pre-reactive complex and transition state.…”

“…Therefore, preliminary static calculations may reveal the necessity of molecular dynamics based techniques as was done for methanol and benzene in H-SSZ-24 in reference [43] .…”