Joaquim Henrique Teles

Ti‐MWW is an important member of Ti‐containing zeolites, industrially employed as heterogeneous catalyst for selective oxidations. However, the development of more active variants is often hampered by the lack of characterization methods able to track transformations and interplay between a multitude of species; this is further exacerbated by the use of post‐synthetic modifications. Combining microscopy and various spectroscopic techniques show that the promotional role of Zn is closely linked to its healing of hydrophilic silanol nests, while simultaneously generating small TiO _x clusters. Notably, these changes are accompanied by Ti weight loading dependent modifications of the extra‐framework TiO ₂ and formation of Zn species. Overall, our study not only highlights the interplay between a plethora of species, but enables a more rational development of post‐modification of Ti‐containing zeolites.

Ti‐MWW is an important member of Ti‐containing zeolites, industrially employed as heterogeneous catalyst for selective oxidations. However, the development of more active variants is often hampered by the lack of characterization methods able to track transformations and interplay between a multitude of species; this is further exacerbated by the use of post‐synthetic modifications. Combining microscopy and various spectroscopic techniques show that the promotional role of Zn is closely linked to its healing of hydrophilic silanol nests, while simultaneously generating small TiO _x clusters. Notably, these changes are accompanied by Ti weight loading dependent modifications of the extra‐framework TiO ₂ and formation of Zn species. Overall, our study not only highlights the interplay between a plethora of species, but enables a more rational development of post‐modification of Ti‐containing zeolites.

Titanium silicalite‐1 (TS‐1) is industrially used for selective oxidation processes. Despite being discovered over 40 years ago, TS‐1 is seeing an increase in use in the chemical industry. The catalyst structure, especially the Ti speciation and local environment in the zeolitic framework, as well as the nuclearity of the Ti sites in the pristine zeolite material, are still debated. Here, we address this issue by applying high field (28.2 T) ^47/49 Ti NMR and ¹⁷ O NMR spectroscopy for an array of TS‐1 catalysts. For extra‐framework‐free TS‐1 catalysts, the Ti sites are associated with a distinct NMR signature—δ _iso ( ⁴⁹ Ti) = −900 ppm; C _Q,0 ( ⁴⁹ Ti) = 7.2 MHz—which can be translated into structural parameters and a dominant first coordination environment according to an extended Czjzek model. With the implementation of a ^47/49 Ti NMR crystallography protocol, benchmarked on molecular models, this signature was assigned to the presence of mononuclear Ti framework sites in pristine TS‐1 structures. The observed distribution of the NMR parameters presumably originates from variations of mononuclear Ti framework sites in the first and second coordination sphere, likely due to the occupancy of several T‐sites.

Titanium silicalite‐1 (TS‐1) is industrially used for selective oxidation processes. Despite being discovered over 40 years ago, TS‐1 is seeing an increase in use in the chemical industry. The catalyst structure, especially the Ti speciation and local environment in the zeolitic framework, as well as the nuclearity of the Ti sites in the pristine zeolite material, are still debated. Here, we address this issue by applying high field (28.2 T) ^47/49 Ti NMR and ¹⁷ O NMR spectroscopy for an array of TS‐1 catalysts. For extra‐framework‐free TS‐1 catalysts, the Ti sites are associated with a distinct NMR signature—δ _iso ( ⁴⁹ Ti) = −900 ppm; C _Q,0 ( ⁴⁹ Ti) = 7.2 MHz—which can be translated into structural parameters and a dominant first coordination environment according to an extended Czjzek model. With the implementation of a ^47/49 Ti NMR crystallography protocol, benchmarked on molecular models, this signature was assigned to the presence of mononuclear Ti framework sites in pristine TS‐1 structures. The observed distribution of the NMR parameters presumably originates from variations of mononuclear Ti framework sites in the first and second coordination sphere, likely due to the occupancy of several T‐sites.

Titanium silicalite-1 (TS-1) is industrially used for selective oxidations processes. Despite being discovered over 40 years ago, TS-1 is seeing an increase in use in the chemical industry. The catalyst structure, especially the Ti speciation and local environment in the zeolitic framework as well as the nuclearity of the Ti sites in the pristine zeolite material are still debated. Here, we address this issue by applying high field (28.2 T) 47/49Ti NMR and 17O NMR spectroscopy, for an array of TS-1 catalysts. For extra-framework-free TS-1 catalysts the Ti sites are associated with a distinct NMR signature – δiso (49Ti) = -900 ppm; CQ,0(49Ti) = 7.2 MHz – which can be translated into structural parameters and a dominant first coordination environment according to an extended Czjzek model. With the implementation of a 47/49Ti NMR crystallography protocol, benchmarked on molecular models, this signature was assigned to the presence of mononuclear Ti framework sites in pristine TS-1 structures. The observed distribution of the NMR parameters presumably originates from variations of mononuclear Ti framework sites in the first and second coordination sphere, likely due to the occupancy of several T-sites.

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.

Ti-containing zeotypes, notably titanosilicalite-1 (TS-1), are prominent examples of heterogeneous catalysts that have found applications in selective oxidation processes with hydrogen peroxide. Despite extensive characterization studies including using various probe molecules to interrogate the nature and the local environment of Ti sites, their detailed structure (as well as reactivity) remains elusive. Here, we demonstrate that using low temperature ¹⁵N magic angle spinning (MAS) ssNMR spectroscopy of adsorbed pyridine on TS-1 combined with Ti K-edge XANES on a range of samples (dehydrated, hydrated, contacted with H₂O₂ and pyridine) provides unique information regarding the Ti sites, highlighting their reactivity and dynamic nature. While dehydrated TS-1 shows only Lewis acid sites, the presence of H₂O generates Brønsted acid sites, whose amount correlates with water loading. Moreover, the methodology─based on ¹⁵N ssNMR and Ti K-edge XANES─applied to a library of samples with various Ti-loadings and absence of extraframework TiO₂ also enables quantification of the amount of Lewis acid sites and to establish a structure-activity descriptor (ratio of pyridine adsorbed on silanols vs titanium). Complementary analysis including computational modeling reveals that the reaction of Ti sites with H₂O generates an acidic bridging silanol Ti-(OH)-Si, upon hydrolysis of one Ti-O-Si linkage, where Ti expands its coordination from four to pentacoordinated according to XAS.

Ti-containing zeotypes, notably titanosilicalite-1 (TS-1), are prominent examples of heterogeneous catalysts that have found applications in selective oxidation processes with hydrogen peroxide. Despite extensive characterization studies including using various probe molecules to interrogate the nature and the local environment of Ti sites, their detailed structure (as well as reactivity) remains elusive. Here, we demonstrate that using low temperature 15N magic angle spinning (MAS) ssNMR spectroscopy of adsorbed pyridine on TS-1 combined with Ti K-edge XANES on a range of samples (dehydrated, hydrated, contacted with H2O2 and pyridine) provides unique information regarding the Ti sites, highlighting their reactivity and dynamic nature. While dehydrated TS-1 shows only Lewis acid sites, the presence of H2O generates Brønsted acid sites, whose amount correlate with water loading. Moreover, the methodology – based on 15N ssNMR and Ti K-edge XANES – applied to a library of samples with various Ti-loadings and absence of extraframework TiO2 also enables quantification of the amount of Lewis acid sites and to establish a structure-activity descriptor (ratio of pyridine adsorbed on silanols vs. titanium). Complementary analysis including computational modelling reveals that the reaction of Ti sites with H2O generates an acidic bridging silanol Ti-(OH)-Si, upon hydrolysis of one Ti-O-Si linkage, where Ti expands its coordination from four to pentacoordinated according to XAS.

Propionic acid is, after acetic, formic and 2‐ethylhexanoic acids, the fourth largest industrially produced carboxylic acid in terms of capacity. In the last decades the technology to produce propionic acid has evolved and more efficient, i. e. more sustainable processes, based on the hydroformylation of ethylene to propionaldehyde and subsequent oxidation with O₂ to propionic acid are capturing the growing capacity for this product. For this reason, a perspective is given on developments in sustainable production of propionic acid and derivatives on industrial scale, which is based on a recently published chapter on the same topic in the Ullmann′ Encyclopedia of Industrial Chemistry.