The synthesis of fine chemicals over solid catalysts often involves the use of solvents that may strongly influence the catalyst performance. Thus , the choice of suitable solvents is frequently critical to obtain high catalytic activity and selectivity. However , the optimal solvent selection requires a detailed knowledge on the relationship between the chemical nature of the solvents and the interactions taking place in the gas-liquid-solid catalytic systems. One of the most common types of catalytic reactions carried out in the presence of solvents is the hydrogenation of organic compounds. In particular , the selective hydrogenation of aromatic ketones into the corresponding alcohols on metal-based catalysts in the presence of different solvents has been widely studied. Here , the solvent effect on catalyst activity and selectivity for the liquid-phase hydrogenation of acetophenone to 1-phenylethanol was thoroughly investigated over Ni/SiO2. Solvents of different properties and polarities were used: i) protic solvents: methanol , ethanol , 1-propanol and 2-propanol; ii) aprotic polar solvents: acetonitrile , g-butyrolactone and tetrahydrofuran; iii) aprotic apolar solvents: cyclohexane , toluene and benzene. The relative interactions solvent–catalyst , solvent–reactant and reactant–catalyst and their influence on the activity pattern were considered in the analysis. Concerning the influence of the solvent-reactant interaction , classical polarity parameters (e.g. dipole moment m and dielectric constant ε) and other solvatochromic scales (e.g. hydrogen-bond donor (a) and hydrogen-bond acceptor (b) parameters , p* polarity/polarizability index , Kosower’s Z and ET(30) scales) were taken into account. The influence of H2 solubility in the solvents was also considered. For the solvent-catalyst and reactant-catalyst interactions , the corresponding molar adsorption enthalpies were measured calorimetrically and compared.
Synthesis of Fine Chemicals by Heterogeneous Catalysis: A Critical Approach of the Role of Solvent for Hydrogenation Reactions
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