Catalysts for Glycerol and CO2 Catalytic Conversion:Preparation and Performance Study

Author:Song Xiang Hai

Supervisor:xiao guo min


Degree Year:2018





The concentration of carbon dioxide(CO2),an important greenhouse gas,is increasing rapidly in the atmosphere because of the extensive combustion of fossil fuels worldwide.This increase is believed to cause originate global warming,which is associated to a series of problems.In addition,the use of fossil fuels may also cause some environmental problems.Biodiesel,a green and sustainable energy,is one of the main alternatives to fossil fuels.However,the expansion in biodiesel production has generated huge amounts of glycerol as a by-product,which has seriously hampered the development of the biodiesel industry.The conversion of CO2 and glycerol to high value-added products can not only relieve the problems caused by these two by-products,but also make huge profits,in line with the concept of green and sustainable development of current society.Cyclic carbonates,products of both CO2 and glycerol,are important industrial products with extensive applications.In this work,a series of catalysts were designed,prepared and used for the catalytic conversion of CO2 and glycerol to value-added products.Meanwhile,the relationship between the structure of catalysts and the catalytic performance was also studied and possible reaction mechanisms have been proposed based on experimental results and characterization.The main contents and results are as follows:(1)Three kinds of carbon nitride(u-C3N4,m-C3N4 and s-C3N4)were prepared and tested for the catalytic conversion of CO2 with epoxides to cyclic carbonates.The activity test demonstrated that u-C3N4 showed the best catalytic activity which was attributed to the huge amounts of edge defects in its structure and the high surface area.The superior reactivity of epichlorohydrin(ECH)compared with other epoxides was attributed to the Cl atom in ECH structure which can activate epoxide group in ECH.In addition,the reaction between ECH and u-C3N4 can introduce quaternary ammonium groups into the structure of u-C3N4.During the reaction,the Cl atom in ECH,edge defects of u-C3N4 and introduced quaternary ammonium groups synergistically catalyzed the cycloaddition of CO2 with ECH.Moreover,the catalyst was highly recyclable.(2)Melem oligomers were prepared by the polymerization of melamine at moderate temperature.Melem based catalysts with various functional groups were developed by modifying melem oligomers and tested for the catalytic conversion of CO2 into cyclic carbonates.Activity test demonstrated that melem oligomers grafted with both hydroxyl and quaternary ammonium groups(MOSB)showed the best catalytic performance.During the reaction,the edge defects,hydroxyl groups and quaternary ammonium groups in the structure of MOSB synergistically catalyzed the cycloaddition of CO2 with high efficiency.Moreover,MOSB can be applied to a number of epoxides.A possible reaction mechanism was proposed for the cycloaddition of CO2 with epoxides to form cyclic carbonates.(3)Two dual-linker metal-organic frameworks(Co(tp)(bpy)and MOF-508a)were prepared with metal salts,terephthalic acid and 4,4′-bipyridine as precursors,and subsequently used for preparing cyclic carbonates from CO2 and epoxides.The two catalysts prepared showed superior catalytic performance under solventless and co-catalyst-free conditions.In this sense,Co(tp)(bpy)achieved ECH conversions as high as 95.75% and CPC yields of 94.18% under optimal reaction conditions.The high activity of Co(tp)(bpy)was attributed to the co-existence of Lewis acidic and basic active sites on the catalyst derived from incompletely coordinated metal cations and uncoordinated pyridine groups,respectively.In addition,Co(tp)(bpy)maintained this high catalytic performance after five consecutive reaction cycles.(4)A series of efficient and low-cost Li/ZnO catalysts were prepared by a simple impregnation method and investigated for the synthesis of glycerol carbonate(GC)from the transesterification of glycerol with dimethyl carbonate(DMC).It was found that the basicity of the catalysts highly depended on the Li loading and calcination temperature.The weak and moderate basic sites on the catalyst surfaces originated from the ZnO and Li+interaction.The strong basic sites were attributed to the substitution of Zn2+by Li+in the ZnO lattice,which led to the formation of[Li+O-]species.The basic sites of the catalysts will be destroyed beyond specific temperature.The catalytic perfprmance of strong basic sites are superior to weak and moderate basic sites.The best catalytic activity was observed over ZnO loaded with 1 wt% LiNO3 and calcined at 500 ℃.(5)A series of new solid base catalysts were prepared by loading KF on the porous La-Zr-600 support and used for the transesterification of glycerol to afford GC.A large number of various basic sites were generated upon loading KF onto the La-Zr-600 support.The weak basic sites were assigned to the surface hydroxyl groups produced during the formation of LaOF,while the strong and super basic sites were related to the Lewis base produced due to the interaction between KF and the La-Zr-600 support.The excellent activity of these catalysts was attributed to the presence of the strong and super basic sites which favor the transesterification of glycerol with dimethyl carbonate.The production of glycidol(GD)from GC decarbonylation was unfeasible at moderate temperature using the newly developed catalysts,but GD can be formed at high temperature and in the presence of KF.(6)A series of polymers were prepared and used for the“one-pot”synthesis of GC from glycerol and CO2 with propylene oxide(PO)as a coupling reagent.These polymers can not only catalyze the coupling of CO2 with PO to give propylene carbonate(PC),but also catalyze the transesterification of PC with glycerol to afford GC.The experimental results showed that P-DVB-(vIm-BuBr)displayed the best catalytic performance among all the polymers prepared.The high catalytic activity of P-DVB-(vIm-BuBr)was attributed to the incorporation of ionic liquids into the structure of the polymer.In addition,the catalytic performance of the polymer was highly depended on the preparation methods.During the reaction,glycerol served not only as reactants,but also as a co-catalyst to facilitate the coupling of PO with CO2.Moreover,the catalyst was highly recyclable.