Christoph J. Kaul

Heterogeneous olefin metathesis catalysts composed of highly dispersed Mo sites on support carriers are used to produce key olefin building blocks for higher value-added products, e.g. the Shell Higher Olefin Process (SHOP). However, understanding the nature of Mo sites in catalysts, i.e. molybdenum oxides supported on amorphous supports like silica, MoOx/SiO2, is hampered by a low fraction of surface metal sites (low loading), the possible distribution of surface species and the lack of very sensitive characterization methods to distinguish subtle changes in the coordination environment of Mo sites on amorphous supports. Here, we show how 95Mo NMR spectroscopy enables identifying various Mo sites in olefin metathesis catalysts, pointing out the presence of isolated tetrahedral Mo-dioxo sites along with MoOx clusters, whose quantities depend on the preparation methods and Mo-loadings. Most notable is the use of NMR signatures as a structural refinement tool. Following a validation with molecular mono- and di-oxo Mo(VI) complexes characterized by single-crystal XRD measurements, we are able to determine the exact coordination environment, geometry and structural parameters of the dominant Mo sites, a highly dispersed dioxo Mo bis-grafted species in a tetrahedral environment with d(Mo=O) = 1.71 ± 0.07 Å, d(Mo-OSi) = 1.91 ± 0.15 Å, ∠(O-Mo-O) = 107 ± 7° and ∠(Mo-O-Si) = 140 ± 40°. This structure closely resembles this of the [MoO2(OSi(OtBu)3)2] molecular analogue. Additionally, this signature indicates a narrow distribution of Mo sites, highlighting the well-defined nature of these sites. Noteworthily, while well-defined, the large promotional effect observed upon co-feeding 2,3-dimethyl-1-butene further supports the role of nearby protons to initiate metathesis, rather than the sole nature of the isolated Mo sites themselves, hence the importance of promoters and the small fraction of active sites in their absence.

Imido ligands strongly influence high‐valent molybdenum olefin‐metathesis catalysts, yet their electronic impact remains difficult to quantify. Here, a series of Mo(VI)(NAr)Cl ₂ (OtBuF ₉ ) ₂ pre‐catalysts is investigated by ^{9 5} Mo NMR spectroscopy, catalytic testing, and DFT calculations. Within the aryl‐imido family, electron‐withdrawing substituents give more shielded ^{9 5} Mo isotropic shifts and higher activities in 1‐nonene self‐metathesis after reduction with 1‐methyl‐3,6‐bis(trimethylsilyl)‐1,4‐cyclohexadiene, whereas electron‐donating substituents show the opposite trend. Shielding‐tensor, natural chemical shielding, and molecular‐orbital analyses show that this correlation is mainly encoded in σ ₁₁ through stabilization of the occupied π(Mo═NAr) HOMO, which reduces paramagnetic deshielding. Comparison with oxo and tert ‐butyl‐imido analogues reveals that similar ^{9 5} Mo shifts can arise from distinct occupied‐orbital manifolds, making tensor, and orbital analyses essential for interpreting ^{9 5} Mo NMR descriptors.

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.

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 TiOx clusters. Notably, these changes are accompanied by Ti weight loading dependent modifications of the extra-framework TiO2 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 employed as heterogeneous catalyst in the ammoximation reaction. Despite the industrial prevalence of Ti-MWW catalysts, 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 challenge is exacerbated by the introduction of secondary metals, such as Zn, which allows to significantly enhance catalytic performance. Hence, we set out to employ an array of characterization methods, featuring microscopy and various spectroscopic techniques, enabling a more accurate handle on these changes. Thereby we find that the promotional role of Zn is closely linked to its healing of hydrophilic silanol nests, while simultaneously generating small TiOx clusters. These changes are accompanied by Ti weight loading dependent modifications of the extra-framework TiO2 and formation of Zn species. Overall, our study highlights the interplay between a plethora of species, enabling the assessment of different species, and 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.