Low-Pressure Hydrogenolysis of Vapor-Phase Glycerol Over Heterogeneous Catalysts

The recent increase in biodiesel production has resulted in a global glut of glycerol, the major byproduct of biodiesel transesterification, which has decreased the value of crude glycerol and rendered the product detrimental to biodiesel economics. This study presents a feasible process for the production of value-added products from glycerol via hydrogenolysis of vapor-phase glycerol over various heterogeneous catalysts at atmospheric pressure. Dynamic system behavior was observed via on-line gas chromatography. In order to determine the effect of H₂ concentration on catalyst activity and product selectivity, reactions were typically conducted under H₂ followed by 10% H₂ (balance He) flow. Under all conditions tested, catalyst activity was greater under H₂ than 10% H₂ flow. Cu catalysts were found to be selective for hydrogenation products, such as 1,2-propanediol (1,2-PDO), under H₂ and dehydration products, such as acetol and acrolein, under 10% H₂ flow. Acrolein, a known thermal reaction product of glycerol, was produced most selectively when acetol was injected over blank quartz wool and glycerol was dehydrated over blank SiO₂. A network of reaction pathways for glycerol hydrogenolysis over Cu catalysts was proposed based on the observed temporal product selectivity behavior. Supported Ni catalysts were found to be highly active for C-C bond hydrogenolysis, generally producing C1 products, such as methanol, more selectively under 10% H₂ and C2 products, such as ethylene glycol, more selectively under H₂ flow. When reactions were performed over Ni/Al₂O₃ under 5% H₂ flow, methanol was produced with highest selectivity, which is significant because methanol can be reused as a reactant in biodiesel transesterification.