Short reaction times for hydrogenolysis of polyolefins by overcoming mass transfer limitations

Abstract

The recycling of polyolefins is gaining attention as society transitions toward a more circular economy. Pyrolysis is a promising method; however, its product distribution can be unpredictable. Moreover, the resulting compounds often require additional hydrogenation if they are to be used as feedstock for naphtha crackers. An alternative approach is hydrogenolysis, in which polyolefins are depolymerised into shorter, fully saturated alkanes using a heterogeneous catalyst under a hydrogen atmosphere. Literature indicates that the hydrogenolysis of polyolefins appears to be a slow process, requiring reaction times up to 96 hours to achieve a significant yield of useful products, such as naphtha or fuels. In this work, it is shown that these long reaction times are resolved when physical mass transport limitations are overcome: in 40 minutes, full conversion of low-density polyethylene to gas and liquid products is reached. Introducing a hollow-shaft mechanical stirrer instead of no or limited stirring significantly increases the gas contact area and mass transfer coefficient to the polymer melt, resulting in a decrease in mass transport limitations and thus an increase in overall reactivity. Monitoring the (hydrogen) pressure over time generates more insight into the reaction kinetics, as at a similar hydrogen consumption level, the product distribution changes if the system is stirred instead of kept stagnant. The authors would like to emphasise the importance of these findings regarding the influence of hydrogen mass transfer through the melt, as this could also result in novel catalysts possibly performing even better than currently reported, making hydrogenolysis a more viable option for the chemical recycling of polyolefins.