Prepartion Of High-performance Raney-Cu/Al₂O₃ Catalyst On Glycerol Hydrolysis To Give 1,2-propanediol

Catalytic hydrogenlysis is an efficient method to convert glycerol,a cheap by-product of biodiesel industry,to high-value1,2-propanediol.Copper-based catalysts are widely used in glycerol hydrogenlysis process for the excellent selectivity of 1,2-propanediol and higher catalytic activity.In this paper,Raney Cu/Al₂O₃catalysts were prepared by the following procedures.Copper-aluminum alloy powder and boehmite were mixed and extruded,then the strips were dried and calcinated.Raney Cu/Al₂O₃was finally acquired by dealumination in alkaline solution.The catalyst demonstrated excellent catalytic performance on glycerol hydrogenlysis and it was suitable for fix-bed reactor.The characterization results revealed that Cu Al₂was the main crystal phase in calcinated strips,the precursors of catalyst.Part of Cu Al₂decomposed and were oxidated toα-Al₂O₃when precursors calcining in air with temperature higher than 850℃.However,the remaining Cu Al₂converted to skeleton copper by the following alkaline treatment.Theα-Al₂O₃improved compressive strength of catalysts and skeleton coppers were the active sites for glycerol hydrogenolysis.Although higher calcining temperature and longer calcinating time improved compressive strength,it decreased the amount of the skeleton copper in catalyst,leading to lower glycerol conversion.As microcrystals in catalyst were unlikely to aggregate with calcinating precursors in argon,it exhibited higher glycerol conversion,though selectivity of 1,2-propanediol declined.According to experimental results,the best Raney Cu/Al₂O₃catalyst was obtained by the following procedures.Copper-aluminum alloy powder and boehmite were mixed with weight ratio of 6:4,then the mixture was extruded and the dried strips were calcinated in air for 1h at the temperature of 850℃.Raney Cu/Al₂O₃catalyst was finally acquired by activating calcinated precursors in 15 wt%Na OH solution.This catalyst was applied in fixed-bed reactor for its high compressive strength of54.75 N?mm^-1.Glycerol conversion and 1,2-propanediol selectivity amounted to 52.47%and 90.39%,respectively when the catalyst was performed at T=215℃,P（H₂）=3 MPa,LHSV=0.8 h^-1,hydrogen-oil valume ratio=100 and w（glycol）=20 wt%.Further results revealed that solvents were critical to catalytic performance on glycol hydrogenlysis.The selectivity of 1,2-propanediol remain still on 90%while glycerol conversion decreased from 52.47%to14.89%over the 100-hour reaction process when the solvents with higher polarity（such as water）serving as solvent.This was partly attributed to polymers from side reactions.Moreover,crystalline copper aggregated by skeletal Cu and its growth in reaction were also contributors to lower catalytic activity.Nevertheless,when ethanol,a solvent with lower surface tension,acted as solvent,catalytic activity of catalyst was obviously improved.This meant glycerol conversion remain stable at100%approximately over the 100-hour reaction process,regardless of that 1,2-propanediol selectivity declined from 83.60%to 70.00%.Meanwhile,it must be noted that acetol selectivity increased from 2.37%to 13.10%.This was attached to aggregation of small Cu crystals in the reaction.The two used catalysts were calcinated for regeneration and the results showed that the one used to be performed in water restored its activity with glycerol conversion and1,2-propanediol selectivity reaching43.87%and 88.03%,respectively.For the one used to be performed in ethanol,the selectivity of 1,2-propanediol dropped by 11.93%,though glycol conversion was 96.78%.