Wolfgang Kleist

Tuning functional sites and thermal stability of mixed-linker MOFs based on MIL-53(Al)

The MIXMOF concept has been applied to the metal-organic framework compound MIL-53(Al). The random incorporation of two different linker molecules (benzene-1,4-dicarboxylate and 2-aminobenzene-1,4-dicarboxylate) in the framework structure and its influence on thermal stability were proven using several complementary techniques (XRD, ATR-IR, MAS-NMR and TG).

Aerobic Epoxidation of Olefins Catalyzed by the Cobalt‐Based Metal–Organic Framework STA‐12(Co)

A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90 %. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism.

Post-synthetic immobilization of palladium complexes on metal–organic frameworks – a new concept for the design of heterogeneous catalysts for Heck reactions

MIL-53-NH2(Al) was modified in a two-step post-synthetic reaction using maleic anhydride and palladium acetate. The resulting Pd-containing frameworks were used as catalysts in Heck-type CC coupling reactions where they exhibited high conversion and selectivity. The results of leaching tests indicate that the reaction is partly catalyzed heterogeneously.

Dynamic transformation of small Ni particles during methanation of CO2 under fluctuating reaction conditions monitored by operando X-ray absorption spectroscopy

A 10 wt.-% Ni/Al2O3 catalyst with Ni particles of about 4 nm was prepared and applied in the methanation of CO2 under dynamic reaction conditions. Fast phase transformations between metallic Ni, NiO and NiCO3 were observed under changing reaction atmospheres using operando X-ray absorption spectroscopy (XAS). Removing H2 from the feed gas and, thus, simulating a H2 dropout during the methanation reaction led to oxidation of the active sites. The initial reduced state of the Ni particles could not be recovered under methanation atmosphere (H2/CO2 = 4); this was only possible with an effective reactivation step applying H2 at increased temperatures. Furthermore, the cycling of the gas atmospheres resulted in a steady deactivation of the catalyst. Operando XAS is a powerful tool to monitor these changes and the behavior of the catalyst under working conditions to improve the understanding of the catalytic processes and deactivation phenomena.

Supported gold- and silver-based catalysts for the selective aerobic oxidation of 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid and 5-hydroxymethyl-2-furancarboxylic acid

The sustainable synthesis of two important intermediates relevant for the production of bio-based polymers, 2,5-furandicarboxylic acid (FDCA) and 5-hydroxymethyl-2-furancarboxylic acid (HFCA), via oxidation of 5-(hydroxymethyl)furfural (HMF) was investigated using supported gold- and silver-based catalysts in water with air as the oxidant. High yields and selectivities for the production of FDCA (89%) and HFCA (≥98%) were achieved under the optimized reaction conditions with Au/ZrO2 and Ag/ZrO2 catalysts, respectively. While FDCA was mainly formed in the presence of gold catalysts at a maximum productivity of 67 molFDCA h−1 molAu−1, silver catalysts showed a remarkably high activity in aldehyde oxidation producing HFCA in almost quantitative yields with a maximum productivity of 400 molHFCA h−1 molAg−1. By variation of the reaction parameters, the Au/ZrO2 catalyst could be tuned to produce also HFCA, whereas the Ag/ZrO2 catalyst exclusively produced HFCA in a wide range of reaction parameters. The observed differences in catalyst selectivities can be taken as a starting point for further mechanistic investigation on the oxidation of HMF, contributing to a fundamental understanding of this reaction which is particularly important for establishing the production of bio-based polymers.

Synthesis strategies and structural arrangements of isoreticular mixed-component metal-organic frameworks

In recent years, the synthesis of mixed-metal and mixed-linker metal-organic frameworks with multiple metals and/or linker molecules combined in one framework has become a growing field of interest. These mixed-component or multivariate metal-organic framework materials provide the possibility to introduce multiple functionalities inside one framework. The interaction of guest molecules with different functionalities in the same material is a promising approach in the fields of gas storage, separation, catalysis and drug delivery. Furthermore, the combination of different components may lead to synergistic effects that cannot be achieved otherwise. These mixed-component approaches open up new pathways to an even larger range of possible customizations in the field of metal-organic frameworks.

CHAPTER 6:Hydrodeoxygenation of Lignocellulose-Derived Platform Molecules

This chapter addresses the hydrodeoxygenation of 5-hydroxymethylfurfural and levulinic acid for the production of chemicals, synthetic fuels and fuel additives. The reaction pathways for the conversion of these platform chemicals are discussed through the perspective of the effective H/C ratio concept. Even for these rather simple platform molecules, the reaction network is very complex, and apart from higher reaction rates especially the selectivity towards the desired molecules, such as 2,5-dimethylfuran, 2,5-bis(hydroxymethyl)furan, γ-valerolactone, is an important aspect. A fundamental understanding of the mechanisms accounting for the product distributions is identified as one of the future challenges. In situ spectroscopic methods, such as operando EXAFS and ATR-IR, are presented as powerful tools for the clarification of these mechanisms.