Trimethylsilylation of Framework Brønsted Acid Sites in Microporous Zeolites and Silico-Aluminophosphates

Abstract

Framework-bound alkoxy groups are well-studied intermediates in zeolite chemistry, but their low stability complicates their spectroscopic study in high-temperature reactions such as alkylation or dealkylation. Taking advantage of the much higher bond strength of Si-O versus C-O, we synthesized trimethylsilylated zeolites by reacting them with phenyltrimethylsilane in a catalytic flow reactor at 648 K. In favorable cases, the reaction accurately titrated the acid sites, and 29Si and 13C MAS NMR spectra of the derivatized catalysts measured at room temperature confirmed the proposed reaction.

Al(ORF)3 (RF = C(CF3)3) activated silica: a well-defined weakly coordinating surface anion

“… 12–15 The 29 Si NMR chemical shift of trimethylsilyl functionalized zeolites appear at 17 ppm. 63 These results are inconsistent with a silylium character in these materials. To the best of our knowledge, the only [R 3 Si][oxide] type species is i Pr 3 Si + supported on sulfated zirconia ( δ 29 Si = 53 ppm).…”

“…The available 29 Si chemical shi values for R 3 Si-supported on silica and silica-alumina suggested that alkylsilanes do not form R 3 Si + sites. [12][13][14][15]63 This is a result of formation of^SiO-SiR 3 sites on these material surfaces.…”

Well-defined (supra)molecular structures in zeolite methanol-to-olefin catalysis

“…This result can be rationalized by the similarity of C-C and C-O bond energies. Recently, we used the reaction in figure 12(c) to synthesize a thermally stable TMS-derivatized HBeta (*BEA topology) zeolite at 648 K in a flow reactor [33]. In this case, formation of the stronger Si-O bond with clean elimination of benzene resulted in material with reduced acid site density and modified pore characteristics.…”

Properties and applications of Zeolites

“…Post-synthetic modification 120,121 is a method that is employed to reduce the effective channel dimensions of a zeolite, to reduce the number of framework acid sites or to introduce some desired new reactive functionality. Haw and his coworkers recently functionalised the large-pore zeolite H-beta by reaction with trimethylphenylsilane at 648 K, which was found to convert some of the acid sites to their trimethylsilyl derivatives 122 . This work has been extended to a two-step post-synthetic sequence in which HZSM-5 is functionalised with a high selectivity at ambient temperatures.…”