Catalytic hydrogenation of butadiene copolymers

Abstract

Selective hydrogenation of the olefin residing within nitrile butadiene rubber (MBR) yields an elastomer that possesses remarkable thermal and oxidative stability. A new generation of homogenous catalysts have been identified for the reaction, having the general form OsHCl(CO)(L)(PCy3)2(1a, L=vacant; 2a, L=O2). In the present work, the merits of this new technology have been assessed in comparison to the Rh(I) phosphine catalysts that are currently in use. Comprehensive kinetic and selectivity data have been acquired under reaction conditions that are industrially relevant. These measurements, coupled with fundamental studies of the structure and reactivity of 1a and 2a, have improved our understanding of the novel catalytic chemistry of this class of complexes.

The rates of hydrogenation supported by 2a are superior to those produced by RhCl(PPh3)3 over the entire range of conditions studied. Unique to the 2a system is an apparent second order dependence of the reaction rate on the concentration of H2. At pressures exceeding 60 bar, this reaction order diminishes until the rate is virtually independent of the hydrogen pressure. In contrast, the hydrogenation of substrates lacking nitrile functionality is indifferent to [H2] at all system pressures. An unconventional catalytic mechanism in which two molecules of H2 are required to bring about the rate determining step is supported by the kinetic data.

A spectroscopic analysis of HNBR produced using 2a revealed no evidence for the reduction of the copolymer's nitrile unsaturation to amine. However, the olefin hydrogenation was accompanied by an undesirable crosslinking reaction that was not observed for the rhodium catalysts. Detailed studies of the effect have indicated that elevated pressures and minimized catalyst concentrations suppress, but do not eliminate, the crosslinking process. By monitoring the evolution of the side-reaction product with time, it has been connected to the presence of residual olefin. Various mechanisms by which the cross-linking process could occur have been explored.

An alternative to batch HNBR production has been explored in the form of a continuous, plug-flow reactor configuration. The design considerations underlying the new production strategy have been detailed along with criteria used for its evaluation. To demonstrate the operating principles of the new approach, a bench-sale prototype has been constructed and assessed according to these standards. The breadth and form of the residence time distribution afforded by the unit have been measured and related to its hydrogenation performance. The data suggests that HNBR may be synthesized efficiently by this method.