Can PDMS membranes separate aldehydes and alkenes at high temperatures?

Abstract

The hydroformylation reaction, the reaction of alkenes and syngas to aldehydes, is one of the most energy-intensive processes in today's chemical industry. In order to intensify hydroformylation, a novel membrane reactor concept was recently introduced, which combines the catalytic reaction with in-situ aldehyde removal through a poly(dimethylsiloxane) (PDMS) membrane. The selective aldehyde removal significantly improves downstream processing during the reaction and prevents unwanted side reactions, which can otherwise lead to catalyst deactivation. PDMS was chosen as the membrane material because of its high hydrocarbon permeabilities at ambient temperatures, as well as high thermal and chemical stability. However, aldehyde permeabilities through PDMS at reaction-relevant temperatures have not been documented, which lacks sufficient information for process simulations and process optimizations for hydroformylation membrane reactors. This work presents permeation properties of the hydrocarbons C2H4, C3H6, C4H8, C3H6O, C4H8O, C5H10O, and N2 as reference gas. Pure- and mixed-gas alkene and N2 permeabilities were measured within the temperature range of 20∘C to 120∘C in a gas-permeation set-up. In addition, a novel vapor-permeation set-up allows to conduct permeation experiments below atmospheric pressures. With this set-up, aldehyde transport properties at temperatures up to 75∘C could be measured. Subsequently, the activation energies of permeation were used to extrapolate the aldehyde permeabilities to higher temperatures with an Arrhenius relation, which enabled the calculation of ideal aldehyde/alkene selectivities for a temperature range up to 120∘C. These selectivities decreased with increasing temperatures, but even at 120∘C, PDMS was aldehyde selective with selectivities ≥ 4.8. Furthermore, PDMS showed no signs of chemical or thermal degradation within alkene or aldehyde experiments.