A spectroscopic investigation of the structure and interactions of hierarchical zeotype catalysts for the Beckmann Rearrangement

Abstract

The acid-catalysed Beckmann rearrangement (BR) of cyclohexanone oxime is a key step in the production of ε-caprolactam, a high-value bulk chemical and the precursor to nylon-6. Conventionally, the BR is effected by a mineral acid catalyst, but the hazards and inefficiencies of the homogeneous process have motivated research into heterogeneous alternatives. Whilst zeotype materials have proven effective as solid-acid catalysts for the BR, their performance is subject to the steric constraints of the micropores. In this regard, the development of hierarchically-porous (HP) zeotypes has provided a means to combine the catalytic properties of a microporous (MP) framework with the mass transport properties of a secondary, mesoporous network.
Recently, organosilane-templated HP silicoaluminophosphates (SAPOs) were investigated as catalysts for the vapour-phase BR, with the result of enhanced activity and lifetime relative to their MP analogues. In this case, the improved performance of the HP SAPOs was not solely attributed to mass transport effects, since the organosilane mesoporogen also created weakly acidic silanol sites in the mesopores. The silanol sites were implicated in the catalytic activity of the HP SAPOs, yet the mode and degree to which they might have facilitated the BR was unknown. To this end, an integrated empirical and theoretical study has been undertaken to determine whether the siliceous mesopores play a catalytic role in the BR, or simply facilitate mass transport to the Brønsted acid sites in the micropores. In this undertaking, neutron spectroscopies have provided a unique insight into the substrate-framework interactions in HP SAPOs, allowing reactive species to be studied near independent of the catalyst matrix. Inelastic neutron spectroscopy (with complementary FTIR, NMR, and computational studies) has evidenced substrate-activating interactions in the mesopores, which facilitate the protonation of cyclohexanone oxime in a physical mixture at ambient temperature. Moreover, quasi-elastic neutron spectroscopy has revealed that these substrate-silanol interactions lead to an interesting interplay with the molecular dynamics in HP SAPO. Significantly, these studies suggest that the HP structure allows the siliceous mesopores to act in concert with the Brønsted sites in the micropores to catalyse the BR.
These investigations also provided a rationale for the application of the organosilane mesoporogen in the synthesis of a new HP SAPO-37 material. By this synthetic route, HP SAPO-37 was prepared the crystalline FAU-type structure, notwithstanding the mesoporositydetected by low-angle XRD, gas adsorption, positron annihilation lifetime spectroscopy, and TEM imaging. Probe-based characterisation (including TPD, FTIR, and NMR) revealed that the acidity of SAPO-37 was moderated by soft-templating, yet this proved effective for the vapour-phase BR (300 °C), in which HP SAPO-37 maintained > 99 % conversion of cyclohexanone oxime (whilst the activity of MP SAPO-37 declined to < 35 %) over 8 hours on-stream. The sustained performance of HP SAPO-37 also elaborated the role of the siliceous mesopores, which were identified to increase resistance to deactivation through coking.
These investigations demonstrate how a multifaceted spectroscopic and computational analysis can support the design of targeted, solid acid-catalysts.

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ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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