Dual-Role Membrane as NH3 Permselective Reactor and Azeotrope Separator in Urea Alcoholysis.

Abstract

Urea methanolysis is a green alternative to synthesize dimethyl carbonate (UM-to-DMC). However, it is strongly challenged by the generated NH3 induced thermodynamic equilibrium limitation and the azeotropic products’ separation. Herein, these predicaments are well-relieved by introducing membranes in both reaction and product separation. An NH3 permselective membrane reactor (MR) based on modified SAPO-34 membrane is successfully realized for UM-to-DMC. The permselectivity and acidity of the SAPO-34 membrane are significantly adjusted to cater the strict molecular sieving of NH3/methanol and chemical inertness upon the reaction. The MR exhibits excellent reactant conversion and DMC selectivity, resulting in >139% higher DMC yield than that of the nonmembrane reactor, due to in situ removal of NH3 by the membrane. The MR also demonstrates reliable chemical, thermal, and mechanical stability during >2000 h. Moreover, the regular SAPO-34 membrane with controlled thickness presents remarkable separation performance for the methanol–DMC azeotrope, in which the methanol–DMC separation factors and the methanol permeance are 1–2 orders of magnitude higher than those of the polymeric membranes. This study suggests the great potential that integration of such membranes offers for process intensification, energy savings, and efficiency improvement in a series of urea alcoholysis and even other NH3 releasing reactions.