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Hydrogen Transfer Reactions Catalyzed by Raney-Nickel: Stability, Surface Analysis and Mechanistic Properties


Kennema,  Marco Oskar
Research Group Rinaldi, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Kennema, M. O. (2016). Hydrogen Transfer Reactions Catalyzed by Raney<sup<®</sup>-Nickel: Stability, Surface Analysis and Mechanistic Properties. PhD Thesis, Ruhr-Universität Bochum, Bochum.

Cite as: https://hdl.handle.net/21.11116/0000-000D-5C57-B
The majority of industrial fine chemical production employs the aromatic fraction of crude oil as a building block.1-4 Crude oil consumption can typically be classified into two separate categories: transportation fuels (high volume, low value) and aromatics (low volume, high value).5 Several methods are already commercially viable for the use of the carbohydrate fraction of lignocellulosic biomass. However, full utilization of lignin remains to be achieved.5 Additionally, lignin conversion belongs to a niche category of biomass conversion, whereby the potential products of the process are of far higher value than would be achieved by simply burning the lignin.4
Two significant challenges related to the production of chemicals and fuels from lignin are the high intrinsic oxygen content and the complex mixture of molecules and functional groups in the extracted product fractions.3, 4 The use of catalysts for hydrogenation and hydrodeoxygenation (HDO) of bio-derived streams constitutes a rapidly growing field of chemistry. In this context, gaining an in-depth understanding of how molecules interact with the catalyst at the solid-liquid interface is typically challenging.
This thesis is centered on two processes: the Early-stage Catalytic Conversion of Lignin (ECCL)6 and the indirect upgrading of lignin oil.7 Both processes share similarities in that they use isopropanol (i-PrOH) as a hydrogen source, via a hydrogen transfer reaction, and Raney® Ni as a catalyst.6-9 To better understand the reactivity of Raney® Ni during the production and upgrading of highly aromatic oil obtained during the processing of lignocellulosic biomass, the interaction of a variety of small molecules with the surface of Raney® Ni is analyzed using an array of analytical techniques. Developing a better comprehension of the mechanism by which molecules react on the surface of Raney® Ni could lead to changes in the design and chemical structure of the catalyst, in order to promote desired reactions or inhibit undesired formation of side products.
Several challenges must be overcome when attempting to use lignocellulosic biomass as a starting material for the production of fine chemicals. These challenges will be presented in Chapter 2. State of the Art, followed by a description of the two processes on which this thesis is focused. Chapter 2 further characterizes Raney® Ni and challenges associated with the study of its surface, as well as giving a description of the surface adsorption of a number of different alcohols, acids and aromatic species on transition metals (where possible, focusing on Ni, Al and alloys of NixAlx). The chapter concludes with a brief explanation of the two specialized reaction installations used for analysis in the subsequent chapters.
The breakdown of hemicellulose typically leads to the formation of polyols and acids in the oil obtained from lignocellulosic biomass; Chapter 3. Inhibition Experiments describes a detailed investigation into the effect of a variety of simple alcohols and acids on the stability of Raney® Ni in the hydrogen transfer from i-PrOH to phenol (to yield acetone and cyclohexanol), performed in a Continuous Stirred Tank Reactor (CSTR). The chapter critically compares trends in the inhibition behavior of different classes of molecules. The inhibition strength of primary vs. secondary alcohols are discussed and compared to adsorption data for the surface of Raney® Ni collected using Attenuated Total Reflection Infrared (ATR-IR) spectroscopy. The chapter investigates an array of parameters relating to the competitive inhibition of molecules on the surface, including temperature, acetone production, cyclohexanone production and gas formation/analysis.
Building upon the insight obtained in Chapter 3, Chapter 4. Competitive Adsorption vs. Inhibition analyses the competitive adsorption of methanol (MeOH) against i-PrOH and phenol. Initially, the interaction of each molecule is investigated individually using ATR-IR, followed by the competitive adsorption of these molecules. This information is compared to the data presented in Chapter 3 to provide a more complete picture of the inhibition process of alcohols on the surface of Raney® Ni in H-transfer from i-PrOH to phenol.
Chapter 5. Surface Adsorption of Methoxyphenol Isomers investigates the differences in the interaction of the three structural isomers of methoxyphenol (ortho, meta and para) and compares them against phenol using ATR-IR. Each isomer is compared in terms of dissolved-phase and adsorbed-phase IR spectra. The precise adsorption behavior is characterized according to surface selection rules, shifts in the absorption bands upon adsorption, and the appearance or disappearance of specific absorbance bands. This information is combined with information in Chapter 6 to propose mechanisms for reaction of phenol and structural isomers of methoxyphenol.
Experiments employing deuterated Raney® Ni are described in Chapter 6. Mechanistic Study of H-transfer Reactions between i-PrOH and Phenols Catalyzed by Raney® Ni. The reactivity of each methoxyphenol isomer was analyzed over the course of 2.5 h and the isotopomeric weight distribution of each parent ion, separated by GC-MS, was characterized. The change in selectivity of the product distribution between experiments performed under deuterated conditions and without is discussed in detail. The information collected was used to decipher potential mechanisms for the conversion of each isomer of methoxyphenol by comparison to the ATR-IR data presented in Chapter 5 and mechanisms previously proposed by other research groups. Furthermore, 13C NMR studies confirm the preferred position of the exchange of deuterium atoms from the surface with hydrogen atoms on the aromatic ring for Phenol.
The final chapter, Chapter 8. Conclusions and Future Directions provides an overall summary of all of the primary insights evolving from the results chapters and identifies key areas for further research.