Improvements in the Hydrogenation of Nitrile Rubber Using Wilkinson's Catalyst

Abstract

Abstract RhCl(PPh3)3 is an efficient catalyst precursor for the selective hydrogenation of C=C in acrylonitrile butadiene rubber(“nitrile rubber”, NBR). The established technology for the process using RhCl(PPh3)3 is to carry out reaction in the presence of a large excess of triphenylphosphine (PPh3), with monochlorobenzene (MCB) as solvent. In parallel with the hydrogenation of unsaturation in the rubber there is a side reaction involving the MCB, which produces benzene. This likely occurs via oxidative addition of the C-Cl bond in the monochlorobenzene to a Rh intermediate in the catalytic cycle for hydrogenation, followed by reductive elimination of benzene in conjunction with H2 addition to the Rh centre. This leads to formation of less active Rh intermediates which lead to rapid deterioration of catalytic activity in the absence of excess PPh3. It was postulated that some of the PPh3 in solution acts as a base that “mops up” excess HCl formed as a by product of the catalytic cycle. Supporting evidence comes from a novel improvement of the hydrogenation process, where the deactivation of catalyst, can be offset by the presence of bases, such as amines and metal oxides (as an alternative to adding a large excess of PPh3). This modification can improve catalyst activity with respect to levels of Rh used, or could be used to minimize the level of added co-catalysts needed to maintain useful rates of hydrogenation.