Alexander Yakimov

Last Name

Yakimov

First Name

Alexander

ORCID

0000-0002-8624-1002

Organisational unit

03872 - Copéret, Christophe / Copéret, Christophe

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Publications 1 - 10 of 20

Structure of Metiram, a Crystalline Zinc Coordination Complex with Amorphous Side Phase
Dinnebier, Robert E.; Seidel, Karsten; Terban, Maxwell W.; et al. (2024)
Crystal Growth & Design
Ethylenebis(dithiocarbamates) (EBDTCs) have been extensively used as fungicides in agriculture for nearly 80 years. Modern fungicides based on EBDTCs contain metal ions, such as zinc in Zineb and Metiram, manganese in Maneb, or combinations of both, such as in Mancozeb. Despite being commercially available since the 1940s, the molecular structure of the metal complexes of EBDTCs was not described in detail until the crystal structure of Zineb was published in 2020. Zineb (C₄H₆N₂S₄Zn) is a single-phase crystalline material. In this study, we present a comprehensive multimethod structural characterization of Metiram (C₁₂H₂₇N₉S₁₂Zn₃), which is the active ingredient of Polyram WG, a highly effective and plant-compatible organic contact fungicide. Our findings reveal that Metiram comprises two distinct phases. The primary phase, phase I, of Metiram is ethylenebis(dithiocarbamate) zinc(II) ammine, which constitutes 81 wt % of the material, or three-quarters of EBDTC. It is a zinc-coordinating crystalline phase. The crystal structure of this phase was determined by powder X-ray diffraction, revealing 1D S-shaped chains of EBDTC connected by strongly distorted Zn[NH₃][S₄] tetragonal pyramids. These pyramids share their sulfur atoms, and ammonia molecules occupy the apex of the pyramids, pointing alternately up and down. The secondary phase, phase II, constitutes 19 wt % of the material or one-quarter of EBDTC, and is amorphous. Using a combination of different techniques, including microscopy, diffraction, and spectroscopy, we have concluded that phase II consists of Zn-free EBDTC. We infer that the primary structure of this secondary constituent aligns with previous assumptions, notably the absence of a significant amount of Zn and the presence of disulfide bonds.
Grafting of Group-10 Organometallic Complexes on Silicas: Differences and Similarities, Surprises and Rationale
Gioffrè, Domenico; Rochlitz, Lukas; Payard, Pierre-Adrien; et al. (2022)
Helvetica Chimica Acta
Surface organometallic chemistry (SOMC) represents a unique synthetic platform for the preparation of model heterogeneous catalysts resembling those broadly applied in industry. SOMC techniques usually rely on the grafting of tailored molecular precursors onto the surface OH groups of oxide supports. The development of such precursors and the understanding of their reactivity with the supports are therefore crucial for the development of well-defined surface species. While a large number of organometallic precursors of early transition metals are known, only few examples of group-10 metal complexes are reported, in spite of the great interest for heterogeneous catalysts based on the Pt-group elements. Herein, we report the reactivity of a family of group-10 (Ni, Pd and Pt) alkyl complexes, towards partially dehydroxylated SiO2 yielding well-defined supported species. We studied the effect of the metal, ligand, and support on the grafting mechanism of such precursors through a combined experimental and computational approach. Ultimately, we showed that at least two grafting pathways are possible for these compounds, namely the protonolysis of the M-alkyl bond by surface OH groups and the opening of strained siloxane bridges: the proportion of the two depending on the nature of the metal and its ancillary ligand.
Solid-state NMR spectra of protons and quadrupolar nuclei at 28.2 T: Resolving signatures of surface sites with fast magic angle spinning
Berkson, Zachariah J.; Björgvinsdóttir, Snædís; Yakimov, Alexander; et al. (2022)
JACS Au
Advances in solid-state nuclear magnetic resonance (NMR) methods and hardware offer expanding opportunities for analysis of materials, interfaces, and surfaces. Here, we demonstrate the application of a very high magnetic field strength of 28.2 T and fast magic-angle-spinning rates (MAS, >40 kHz) to surface species relevant to catalysis. Specifically, we present as case studies the 1D and 2D solid-state NMR spectra of important catalyst and support materials, ranging from a well-defined silica-supported organometallic catalyst to dehydroxylated γ-alumina and zeolite solid acids. The high field and fast-MAS measurement conditions substantially improve spectral resolution and narrow NMR signals, which is particularly beneficial for solid-state 1D and 2D NMR analysis of 1H and quadrupolar nuclei such as 27Al at surfaces.
Probing the Nature of Surface Hydrides by Deuterium Quadrupolar Parameters: A Case Study on Silica-Supported Zirconium Hydrides
Docherty, Scott R.; Schärz, Philipp; Gioffrè, Domenico; et al. (2024)
Helvetica Chimica Acta
Supported metal hydrides are key reactive intermediates in various catalytic processes, such as hydrogenation and dehydrogenation, but are often challenging to characterize spectroscopically. Here, deuterium solid state nuclear magnetic resonance spectroscopy is used to understand the structure of the corresponding silica-supported zirconium hydrides after H/D exchange as an illustrative example of supported metal hydrides, which have been shown to display notable reactivity towards small molecules (e. g., CO₂ and N₂O) and to activate both C−H and C−C bonds, hence their use in to the conversion of hydrocarbons (alkanes, polyolefins etc.)
Direct Identification of Zeolite Beta Polymorphs by Solid-State ²⁹Si Nuclear Magnetic Resonance Spectroscopy
Kolyagin, Yury G.; Yakimov, Alexander; Zasukhin, Dmitry S.; et al. (2022)
The Journal of Physical Chemistry Letters
Stacking disorder and polymorphism in zeolite and zeolite-like materials hinder their structural characterization. In this work, we propose an advanced approach based on applying “pure shift” solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy for the structural investigation of zeolitic materials containing intergrown polymorphs. The approach developed in the case study of zeolite beta allows for the resolution of 21 29Si signals, attributing them to non-equivalent T sites in polymorphs A, B, and C, reconstruction of individual 29Si magic angle spinning NMR spectra for each polymorph, and determination of the polymorph composition with higher accuracy than X-ray diffraction. The results reveal that two widely used synthetic routes for zeolite beta, alkaline and fluoride synthesis, lead to different polymorph compositions. These findings indicate that “pure shift” solid-state 29Si NMR can serve as a superior tool for the elucidation of polymorphism in zeolites.
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.
Developing Versatile Contactors for Direct Air Capture of CO2 through Amine Grafting onto Alumina Pellets and Alumina Wash-Coated Monoliths
Grossmann, Quirin; Stampi-Bombelli, Valentina; Yakimov, Alexander; et al. (2023)
Industrial & Engineering Chemistry Research
The optimization of the air-solid contactor is critical to improve the efficiency of the direct air capture (DAC) process. To enable comparison of contactors and therefore a step toward optimization, two contactors are prepared in the form of pellets and wash-coated honeycomb monoliths. The desired amine functionalities are successfully incorporated onto these industrially relevant pellets by means of a procedure developed for powders, providing materials with a CO2 uptake not influenced by the morphology and the structure of the materials according to the sorption measurements. Furthermore, the amine functionalities are incorporated onto alumina wash-coated monoliths that provide a similar CO2 uptake compared to the pellets. Using breakthrough measurements, dry CO2 uptakes of 0.44 and 0.4 mmol gsorbent-1 are measured for pellets and for a monolith, respectively. NMR and IR studies of CO2 uptake show that the CO2 adsorbs mainly in the form of ammonium carbamate. Both contactors are characterized by estimated Toth isotherm parameters and linear driving force (LDF) coefficients to enable an initial comparison and provide information for further studies of the two contactors. LDF coefficients of 1.5 x 10(-4) and of 1.2 x 10(-3) s(-1) are estimated for the pellets and for a monolith, respectively. In comparison to the pellets, the monolith therefore exhibits particularly promising results in terms of adsorption kinetics due to its hierarchical pore structure. This is reflected in the productivity of the adsorption step of 6.48 mol m-3 h(-1) for the pellets compared to 7.56 mol m-3 h(-1) for the monolith at a pressure drop approximately 1 order of magnitude lower, making the monoliths prime candidates to enhance the efficiency of DAC processes.
Structure of Na Species in Promoted CaO-Based Sorbents and Their Effect on the Rate and Extent of the CO2 Uptake
Krödel, Maximilian; Abduly, Lorenz; Nadjafi, Manouchehr; et al. (2023)
Advanced Functional Materials
To advance CaO-based CO₂ sorbents it is crucial to understand how their structural parameters control the cyclic CO₂ uptake. Here, CaO-based sorbents with varying ratios of Na₂CO₃:CaCO₃ are synthesized via mechanochemical activation of a mixture of Na₂CO₃ and CaCO₃ to investigate the effect of sodium species on the structure, morphology, carbonation rate and cyclic CO₂ uptake of the CO₂ sorbents. The addition of Na₂CO₃ in the range of 0.1–0.2 mol% improves the CO₂ uptake by up to 80% after 10 cycles when compared to ball-milled bare CaCO₃, while for Na₂CO₃ loadings >0.3 mol% the cyclic CO₂ uptake decreases by more than 40%. Energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy, X-ray absorption spectroscopy (XAS), and ²³Na MAS NMR, reveal that in sorbents with Na₂CO₃ contents <0.3 mol% Na exists in highly distributed, noncrystalline [Na₂Ca(CO₃)₂] units. These species stabilize the surface area of the sorbent in pores of diameters >100 nm, and enhance the diffusion of CO₂ through CaCO₃. For Na₂CO₃ contents >0.3 mol%, the accelerated deactivation of the sorbents via sintering is related to the formation of crystalline Na₂Ca(CO₃)₂ and the high mobility of Na.
Surface Sites and Ligation in Amine-capped CdSe Nanocrystals
Cao, Weicheng; Yakimov, Alexander; Qian, Xudong; et al. (2023)
Angewandte Chemie. International Edition
Converting colloidal nanocrystals (NCs) into devices for various applications is facilitated by designing and controlling their surface properties. One key strategy for tailoring surface properties is thus to choose tailored surface ligands. In that context, amines have been universally used, with the goal to improve NCs synthesis, processing and performances. However, understanding the nature of surface sites in amine-capped NCs remains challenging, due to the complex surface compositions as well as surface ligands dynamic. Here, we investigate both surface sites and amine ligation in CdSe NCs by combining advanced NMR spectroscopy and computational modelling. Notably, dynamic nuclear polarization (DNP) enhanced 113Cd and 77Se 1D NMR helps to identify both bulk and surface sites of NCs, while 113Cd 2D NMR spectroscopy enables to resolve amines terminated sites on both Se-rich and nonpolar surfaces. In addition to directly bonding to surface sites, amines are shown to also interact through hydrogen-bonding with absorbed water as revealed by 15N NMR, augmented with computations. The characterization methodology developed for this work provides unique molecular-level insight into the surface sites of a range of amine-capped CdSe NCs, and paves the way to identify structure-function relationships and rational approaches towards colloidal NCs with tailored properties.

Publications 1 - 10 of 20

Alexander Yakimov

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