Unveiling the organocatalytic pathway to glycerol carbonate from glycerol and CO2: a comprehensive study using phase behavior, operando high-pressure FTIR, and DFT insights

Abstract

The direct synthesis of glycerol carbonate from glycerol and CO₂, using acetonitrile as a dehydration agent and DBU as a catalyst, was investigated to identify the mechanism involved during this reaction. DFT modeling revealed that the most efficient pathway involves forming a C–O bond between CO₂ and glycerol's secondary alcohol. The catalyst reduces energy barriers across steps, particularly for the rate-limiting intramolecular ring closure, making the carbonyl substitution route more favorable than hydroxyl dehydration. However, acetonitrile's hydrolysis proved less effective as a dehydrating agent due to its higher energy barrier compared to direct carbonylation. While reaction parameters like CO₂ pressure and acetonitrile volume had minimal impact on yield, they influenced glycerol conversion. Higher pressure and larger acetonitrile volumes reduced glycerol conversion without changing the yield of glycerol carbonate. Elevated temperatures and prolonged reaction times promoted the formation of side products, primarily monoacetin. The reaction's complex phase behavior, driven by pressure and temperature, revealed that glycerol and acetonitrile become fully miscible only under specific conditions (155 °C, 45 bar). In situ FTIR and HPLC analyses identified kinetic profiles for glycerol carbonate, acetins, and acetamide, with ammonia and urea detected in the gas phase. Complementary DFT calculations confirmed that monoacetin formation predominantly arises from the reaction of glycerol with acetamide, a pathway characterized by the lowest energy barriers, aligning with experimental findings.