Christoph Kaul

Last Name

Kaul

First Name

Christoph

ORCID

0000-0003-0919-8538

Organisational unit

03872 - Copéret, Christophe / Copéret, Christophe

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Publications 1 - 6 of 6

Well-Defined Ti Surface Sites in Ziegler–Natta Pre-Catalysts from ⁴⁷/⁴⁹Ti Solid-State Nuclear Magnetic Resonance Spectroscopy
Yakimov, Alexander; Kaul, Christoph; Kakiuchi, Yuya; et al. (2024)
The Journal of Physical Chemistry Letters
Treatment of Ziegler–Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how ⁴⁷/⁴⁹Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of ⁴⁷/⁴⁹Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit ⁴⁹Ti NMR signatures (δ_iso, exp = −170 ppm; C_Q, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl_2 support, formed upon post-treatment with BCl_3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.
Nature of Reactive Sites in TS-1 from ¹⁵N Solid-State NMR and Ti K-Edge X-Ray Absorption Spectroscopic Signatures Upon Pyridine Adsorption
Lätsch, Lukas; Kaul, Christoph; Yakimov, Alexander; et al. (2024)
Journal of the American Chemical Society
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.
Designer Phospholipid Capping Ligands for Soft Metal Halide Nanocrystals
Morad, Viktoriia; Stelmakh, Andriy; Svyrydenko, Mariia; et al. (2024)
Nature
The success of colloidal semiconductor nanocrystals (NCs) in science and optoelectronics is inextricable from their surfaces. The functionalization of lead halide perovskite (LHP) NCs1–5 poses a formidable challenge due to their structural lability, unlike the well-established covalent ligand-capping of conventional semiconductor NCs6,7. We posited that the vast and facile molecular engineering of phospholipids as zwitterionic surfactants can deliver highly customized surface chemistries for metal halide NCs. Molecular dynamics simulations inferred that ligand-NC surface affinity is primarily governed by the structure of the zwitterionic headgroup, particularly by the geometric fitness of the anionic and cationic moieties into the surface lattice sites, as corroborated by the NMR and FTIR data. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic-inorganic LHPs (FAPbBr3 and MAPbBr3, FA-formamidinium; MA-methylammonium) and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to acetone and alcohols. These NCs exhibit photoluminescence quantum yield (PL QY) above 96% in solution and solids and minimal PL intermittency at the single particle level with an average ON fraction as high as 94%, as well as bright and high-purity (ca. 95%) single-photon emission.
Identifying Active Centers of a Ring-Shaped, Industrial Vanadyl Pyrophosphate Catalyst for Maleic Anhydride Production
Mestl, Gerhard; Böcklein, Sebastian; Wolf, Florian K.; et al. (2025)
ChemCatChem
We have investigated how the phosphorus selectivity-activity moderator interacts with active sites on industrial vanadyl pyrophosphate catalysts used for n-butane oxidation to give maleic anhydride. Combining phosphoric acid doping experiments with catalytic performance tests and several spectroscopic techniques, active sites could be identified. It is shown that the sites consist of coordinatively unsaturated Lewis acid V$^{4+}$ centers and Brønsted acid P-OH groups that are connected by V-O-P bridging bonds. Phosphorus blocks these sites but does not alter their chemical nature. Catalytic performance tests correlate with spectroscopic results, demonstrating a direct relationship between the proposed active site structure and the catalytic behavior. It is suggested that the active sites exist in the previously identified amorphous surface layer on the crystalline vanadyl pyrophosphate probably formed by VOPO$_4$ species as proposed in the literature. The data show that only about 11% of the catalyst surface is catalytically active, making earlier active site suggestions based on crystallographic considerations of vanadyl pyrophosphate unlikely. At high phosphorus loadings, the β-VO(PO$_3$)$_2$ phase forms which has a known increased intrinsic activity but lower selectivity to maleic anhydride. The findings contribute to a more comprehensive understanding of the maleic anhydride catalyst functionality.
NMR Signatures of Transition-Metal Nuclei: From Local Environments and Electronic Structures to Reactivity Descriptors in Molecular and Heterogeneous Catalysis
Berkson, Zachariah J.; Cao, Weicheng; Gioffrè, Domenico; et al. (2025)
JACS Au
This Perspective summarizes the current state of the art in understanding the local environments of metal sites across homogeneous and heterogeneous catalysts by means of solid-state nuclear magnetic resonance (NMR), augmented with first-principles density functional theory (DFT) calculations, focusing on transition-metal nuclei and emphasizing the potential of this approach for understanding reactivity. We illustrate in particular how NMR parameters of transition-metal nuclei provide unique insights into the electronic structures and coordination environments of metal sites, complementary to information that can be obtained from ¹³C, ¹⁵N, or ¹⁷O NMR parameters of metal-bound ligands. Using the examples of solid-state NMR analyses of supported and molecular systems containing NMR-active transition-metal nuclei (⁹⁵Mo, ¹⁹⁵Pt, ¹⁰⁹Ag, ¹⁸³W, ⁵¹V, and 47/49Ti), we show how NMR parameters can be determined and related to structural and electronic features of molecular and surface metal sites. Moreover, analyzing the origins of the chemical shift tensors of these metal nuclei through DFT computations helps to connect NMR signatures to specific local coordination environments and electronic structures (frontier molecular orbitals) and the corresponding reactivity of specific metal sites, thereby opening the possibility of establishing structure–activity relationships across catalytic systems, including industrially relevant heterogeneous catalysts.
Deciphering Active Sites in Titanium Silicalite-1 via Solid-State NMR and X-ray Spectroscopic Signatures
Lätsch, Lukas; Kaul, Christoph; Yakimov, Alexander; et al. (2025)
Accounts of Chemical Research
Propylene 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 L2,3-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.

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Christoph Kaul

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