Hydrogenation of some commercial polymers can produce new polymeric materials that are difficult or expensive to synthesize by other means, e.g., by polymerization of the corresponding monomer(s). For example, polystyrene (PS) can be hydrogenated to produce poly(vinyl cyclohexane) (PVCH). The glass transition temperature of PVCH is about 50° C. higher than that of PS (150° C. versus 100° C.), and PVCH may be able to compete with poly(bisphenol A carbonate), a more expensive polymer than PS, in some high-temperature applications.
Despite the potential advantages of hydrogenating inexpensive, commercially-available polymers to create more valuable materials, hydrogenation of polymers is not widely practiced on a commercial scale. A solid catalyst is required to carry out such hydrogenations selectively, at a reasonable rate. The viscosity of a solution of polymer in a solvent is very high, even if the concentration of the polymer in the solution is relatively low. The high viscosity of the polymer solution causes at least two serious problems: First, if the hydrogenation is carried out by suspending small catalyst particles in the polymer solution, these particles are very difficult to separate from the final solution of the hydrogenated polymer when the reaction is complete. Because of the high viscosity of the solution, separation techniques such as filtration, sedimentation, and centrifugation can become impracticably slow. Second, the high viscosity of the solution causes the heat and mass transfer coefficients to be low between the solution and the catalyst particles, and between the solution and the fluid phase that contains the H2. These low coefficients, in turn, lower the rate of hydrogenation and increase the temperature of the catalyst particles. The higher temperature of the catalyst particles can lower the selectivity of the reaction, i.e., undesired hydrogenation reactions such as chain scission can occur.
In a conventional hydrogenation, using a catalyst with a small particle size (0.1 to 100 μm) that is dispersed in the liquid phase, the transport limitation between the gas phase and the liquid (polymer solution) can be reduced or eliminated by employing a high rate of mechanical agitation. This is illustrated in FIG. 1, which shows the results of hydrogenating a solution of polystyrene in decahydronaphthalene in a 50 cc stirred autoclave equipped with a mechanical agitator. The extent to which the phenyl groups of polystyrene have been hydrogenated in a fixed period of time, at a fixed set of conditions, is plotted against the rotational speed of the agitator. Note that the extent of ring hydrogenation increases as the rotational speed of the agitator is increased, up to a speed of about 2000 rpm. Above 2000 rpm, the rate of ring hydrogenation is no longer sensitive to stirring speed. This shows that an agitation rate of greater than 2000 rpm is required to essentially eliminate the resistance to H2 transfer between the gas phase and the liquid phase at the conditions of FIG. 1.
Unfortunately, varying the rotational speed or the design of the agitation system has no little or no effect on the transport resistance between the liquid phase and a small catalyst particle that is suspended in the liquid. Moreover, the agitation rate/agitator design has no effect on the internal (pore diffusion) transport resistance. Consequently, increasing the agitation rate is only a partial solution to the second problem described above, and it does not contribute to solving the first problem. Accordingly, there is a need for new techniques for carrying out hydrogenation reactions on polymers.