Pinacol Boronate Ester-Containing Polysilsesquioxane Membranes for CO₂ Separation

Poly(silsesquioxanes) (PSQs) are organic–inorganic hybrid materials synthesized through the sol–gel reaction of trialkoxysilanes. Carbon dioxide (CO2) separation membranes made of PSQs offer high durability, and the organic groups attached to the silicon atoms can be tailored to control gas separation performance. Conversely, boronate esters, commonly used in palladium-catalyzed cross-coupling reactions, possess both Lewis acidity on the boron atom and basicity on the oxygen atom. Despite the diverse organic groups that can be bound to the silicon atoms in PSQs, there are no reported applications of boronate ester-containing PSQ materials as separation membranes. This study addresses this gap by synthesizing a PSQ material incorporating a pinacol boronate ester unit and evaluating its potential as a CO2 separation membrane. The PSQ membranes incorporating boronate ester units exhibited excellent CO2 separation performance. They achieved a maximum gas permeance of 2.10 × 10–6 mol m–2 s–1 Pa–1 with a CO2/N2 permeance ratio of 11. Density functional theory (DFT) calculations suggest that this superior CO2 separation likely arises from the affinity of the pinacol unit for CO2 molecules.