Catalyst, hydrogenation cooperative active sites

A remarkable example of the cooperation of different active sites in a polyfunctional catalyst is the one-step synthesis of 2-ethylhexanol, including a combined hydroformylation, aldol condensation, and hydrogenation process [17]. The catalyst in this case is a carbonyl-phosphine-rhodium complex immobilized on to polystyrene carrying amino groups close to the metal center. Another multistep catalytic process is the cyclooligomerization of butadiene combined with a subsequent hydroformylation or hydrogenation step [24, 25] using a styrene polymer on to which a rhodium-phosphine and a nickel-phosphine complex are anchored (cf Section 3.1.5). 

The reduction of unsaturated C-C bonds constitutes one of the largest applications of heterogeneous catalysis [48]. On the surface of a metal particle, hydrogen is either cleaved homolytically to give M-H units or is adsorbed in the intact form. The metal sites that are responsible for either activation path of Hj on the catalyst surface as well as the chemical-physical characteristics of the adsorbed H2 are still a matter of debate. In this respect, molecular systems mimicking the activation of Hj on a single metal site without the cooperation of a solvent are expected to provide valuable information on the mechanism of Hj adsorption/release over heterogeneous metal catalysts. 

Systems such as iron on an acid-base-type support follow Mechanism 2 or 3 (both are two-step pathways). Mechanism 2 proposes the occurrence of RWGS reaction coupled with dehydrogenation of EB in the presence of CO2. In this mechanistic pathway, the acid-base bifunctionality of the catalyst plays an important role in C02-mediated oxidative dehydrogenation reactions [49]. The acid properties of the catalyst are important in the activation of EB, whereas the basic sites are for CO2 adsorption to form carbonates for hydrogen blockage. The cooperative action of acid sites and basic sites is fundamental [49]. 

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