Deciphering Active Sites in Titanium Silicalite-1 via Solid-State NMR and X-ray Spectroscopic Signatures

ConspectusPropylene oxide manufacturing has experienced one of the fastest growth rates of commodity chemicals in recent years (ca. 21% from 2020 to 2023) and is expected to reach a market size of 36 billion US $ by 2031. Newly installed production facilities frequently use Ti-zeotype catalysts, most notably, titanium silicalite-1 (TS-1). Owing to their industrial relevance, these catalysts have been studied intensively. However, to date, many aspects of this catalytic process remain unclear, in particular, regarding the nature of the active sites. Most commonly, the active sites have been described as framework-incorporated isolated metal sites. Yet, an increasing number of reports has highlighted the role of defect sites and/or the presence of di- (or multi) nuclear sites. Many of the assignments have, however, remained tentative due to limited structural resolution or the lack of suitable molecular references, where so far UV/vis, IR, Raman, and K-edge XAS spectroscopy have predominately been utilized. In this Account, we show how the combination of advanced solid-state nuclear magnetic resonance spectroscopy (ssNMR) and/or X-ray absorption spectroscopy (XAS) augmented by computational modeling and classical characterization approaches can yield molecular-level understanding of active sites in titanosilicate zeotype catalysts. Specifically, we focus on understanding their structure and dynamics, with the ultimate goal of extracting guideline principles to develop optimal catalysts. We also highlight how developing new methods for low-γ, quadrupolar, and metal-centered NMR spectroscopy has allowed us to gain unprecedented insights into their electronic structure and how related detailed information can be obtained from X-ray absorption-based methods. This account discusses the following challenges and associated learning opportunities: (i) how key peroxo intermediates can be identified based on ¹⁷O ssNMR, how their stability can be quantified, and how it relates to the presence of TiO₂ domains and the overall catalyst performance; (ii) how novel approaches based on direct metal characterization, in particular ^47/49Ti ssNMR, yield information on Ti-site symmetry and help to assign T-site (distribution); (iii) how soft X-rays (Ti L_2,3-edge NEXAFS) can help detecting octahedral Ti sites and can be used to measure, track the conversion, and distinguish mono- and dinuclear Ti-peroxo species; and (iv) how ambient conditions affect the active-site structure and how water-induced structural rearrangement gives rise to Brønsted acidity in TS-1. Finally, we provide an outlook on ongoing developments that are needed to further expand the scope of the methodology discussed herein, in particular, with a focus on characterizing reactive intermediates and translating the methodology to other Ti-based catalysts.