(508d) Polymer Hydrogenation by Reactive Extrusion – Pulsed and Continuous Flow Systems

We have modified a twin screw extruder to serve as a novel reactor for the catalyzed hydrogenation or dehydrogenation of polymers. A test reaction poly(styrene) (PS) to poly(vinylcyclohexane) is currently under investigation. The Pd-based catalysts are coated on cordierite monoliths housed in the die. For example, a high dispersion 0.5% Pd/γ-Al₂O₃ was slurry coated on a 100 cpsi cordierite monolith to a coating thickness of ~100 µm to give a catalyst similar in activity and selectivity to those used in previous studies.

The custom-made die housing the monoliths was attached to a counter-rotating twin screw extruder. The polymer solution is pumped from an autoclave at high pressure where hydrogen is pre-dissolved.

Initial reactor studies were performed to determine the stable two-phase flow range of gas to liquid (G/L, liquid = 2-10 wt% PS in 10 vol% THF/cyclohexane) volumetric ratios. These ranged from G/L = 7 at 2.0 MPa to G/L = 20 at 3.5 MPa. At these conditions, observed rate constants were determined by modeling the system in plug flow with first order dependences in aromatic group and hydrogen concentrations. While intrinsic reaction rates (from batch autoclave studies) are in agreement with previous work, observed reaction rates in steady-state reactive extrusion suggest highly mass-transfer limiting conditions. For example, increasing both flow rates simulataneously increases the observed rate constants in qualitative agreement with correlations for gas-limited mass transfer in slug flow monolith systems.

Reaction studies are underway with the reactive extruder operating in pulsed mode at frequencies <10 Hz. In this manner the system alternates between gas- and liquid-rich conditions within the monoliths, oscillating the liquid film thickness to allow for rapid hydrogen mass transfer during part of a cycle. This also allows the reactor to be run under higher G/L in steady-state operation. We are also studying the same reacdtion in a related reactor where oscillation can be controlled by a piston-cam arrangement. These systems are applicable to a broad range of macromolecular reactions, including fuels production from bio-based oils and edible oil hydrogenation. Catalyst selectivity in such serial pathway reactions (low MW hydrogenated oligomers are ultimate products) may also be improved, because pulsing in the correct frequency range can flush intermediate product from the catalyst.