Understanding the role of diffusion in catalysis is essential in the design of highly active, selective, and stable industrial heterogeneous catalysts. By using a combination of advanced in situ spectroscopic characterization tools, particularly quasi-elastic and inelastic neutron scattering, we outline the crucial differences in diffusion modes and molecular interactions of active sites within solid-acid catalysts. This, coupled with 2D solid-state NMR and probe-based FTIR spectroscopy, reveals the nature of the active site in our SAPO-37 catalyst and affords detailed information on the evolution of solid-acid catalysts that can operate at temperatures as low as 130 °C, for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam (precursor for Nylon-6). The versatility of this approach leads to structure−property correlations that contrast the dynamics of the diffusion process in the different materials studied. Our results illustrate the power of these techniques in unravelling the interplay between active site and molecular diffusion in single-site heterogeneous catalysts, which can play a vital role in designing low-temperature, sustainable catalytic processes.

Using a distinctive bottom-up approach, hierarchical SAPO-34 has been synthesized using CTAB encapsulated within ordered mesoporous silica (MCM-41) that serves as both the silicon source and mesoporogen. The structural and textural properties of the hierarchical SAPO-34 were contrasted against its microporous analogue, and the nature, strength and accessibility of the Brønsted acid sites were studied using a range of physicochemical characterization tools; notably probe-based FTIR and solid-state (SS) MAS NMR. Whilst CO was used to study the acid properties of hierarchical SAPO-34, bulkier molecular probes (including pyridine, 2,4,6-trimethylpyridine and 2,6-di-tert-butylpyridine) allowed particular insight into the enhanced accessibility of the acid sites. The activity of the hierarchical SAPO-34 catalyst was evaluated in the industrially-relevant, acid-catalysed Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, under vapor-phase conditions. These catalytic investigations revealed a significant enhancement in the yield of ε-caprolactam using our hierarchical SAPO-34 catalyst compared to SAPO-34, MCM-41, or a mechanical mixture of these two phases. The results highlight the merits of our design strategy for facilitating enhanced mass transfer, whilst retaining favorable acid site characteristics.

Modern society is placing increasing demands on commodity chemicals, driven by the ever‐growing global population and the desire for improved standards of living. As the polymer industry grows, a sustainable route to ϵ‐caprolactam, the precursor to the recyclable nylon‐6 polymer, is becoming increasingly important. To this end, we have designed and characterized a recyclable SAPO catalyst using a range of characterization techniques, to achieve near quantitative yields of ϵ‐caprolactam from cyclohexanone oxime. The catalytic process operates under significantly less energetically demanding conditions than other widely practiced industrial processes.

Porosity and acidity are influential properties in the rational design of solid-acid catalysts.P robing the physicochemical characteristics of an acidic zeotype framework at the molecular level can provide valuable insights in understanding intrinsic reaction pathways,f or affording structure-activity relationships.H erein, we employ av ariety of probe-based techniques (including positron annihilation lifetime spectroscopy(PALS), FTIR and solid-state NMR spectroscopy) to demonstrate howahierarchical design strategy for afaujasitic (FAU)z eotype (synthesized for the first time,v ia as ofttemplating approach,w ith high phase-purity) can be used to simultaneously modify the porosity and modulate the acidity for an industrially significant catalytic process (Beckmann rearrangement). Detailed characterization of hierarchically porous (HP) SAPO-37 reveals enhanced mass-transport characteristics and moderated acidity,w hichl eads to superior catalytic performance and increased resistance to deactivation by coking,c ompared to its microporous counterpart, further vindicating the interplay between porosity and moderated acidity.

At the forefront of global development, the chemical industry is being confronted by a growing demand for products and services, but also the need to provide these in a manner that is sustainable in the long-term. In facing this challenge, the industry is being revolutionised by advances in catalysis that allow chemical transformations to be performed in a more efficient and economical manner. To this end, molecular design, facilitated by detailed theoretical and empirical studies, has played a pivotal role in creating highly-active and selective heterogeneous catalysts. In this review, the industrially-relevant Beckmann rearrangement is presented as an exemplar of how judicious characterisation and ab initio experiments can be used to understand and optimise nanoporous materials for sustainable catalysis.