Metal-free Boron Containing Catalysts For Propane Oxidative Dehydrogenation

Propene is among the most important feedstock in the petrochemical industry.The dehydrogenation of propane to propene is an important technology to solve the diversification of olefin raw materials,which will reduce the dependence on petroleum.Compared to the direct dehydrogenation process,oxidative dehydrogenation(ODH)of propane is an energy efficient way featured by its intrinsic nature of free of equilibrium limitation and coking.At present,metal oxide catalysts have been extensively investigated to benefit from its thermodynamic advantages for propene production.A major obstacle in its industrialization is the deep oxidation of olefin products over metal oxides catalysts,leading to poor yield of propene and serious CO₂ emission.The discovery of boron-based catalysts offered a promising choice to realize the ODH of light alkanes to corresponding olefins with high activity and selectivity while negligible formation of CO₂.This thesis focuses on designing new boron-based catalysts with superior catalytic performance on ODH of propane,and further studies the effect of crystal structure,morphology and pore structure on ODH catalytic performance and explores the chemical environment of boron species,in order to identify the active sites of metal-free boron-based catalysts and understand the mechanism in ODH of propane.(1)Using porous silica as support,silica-supported boron oxide catalysts were prepared showing low temperature activity and high selectivity for ODH of propane.Boron species can be dispersed and immobilized through B-O-Si bonds.The catalyst with 10 wt%B₂O₃ displayed detectable activity at as low as 405?,The olefin selectivity and productivity were 83.1%and 2.47 golefin g_cat^-1 h^-1 respectively,with a propane conversion of 31.5%.The experimental characterizations and the theoretical calculations indicated that four types of boron species exist in silica-supported boron oxide catalysts,and tri-coordinated boroxol B(3a)and hydroxylated linear boron species B(3b)are the active sites for the ODH of propane.The boron sites with more OH groups generating via hydrolysis of B(3a)and B(3b)species were easier to dehydrogenate by dioxygen to form>B-O·radical,which was highly active towards the subsequent dehydrogenation of propane.The>B-O·radical activated propane rapidly to obtain·C₃H₇ radical,which further interacted with oxygen to generate·OOC₃H₇ radical and then produce propene.In addition,the oxophilicity of boron sites enabled its high affinity to bind with alkoxide to suppress its further oxidation and guarantee high selectivity for olefin products.(2)Boron phosphate crystals with a three-dimensionally interconnected ordered macroporous structure were fabricated by using rotary evaporation and hydrothermal-assisted synthesis with polymer templates.Owing to the reinforcement of the boron phosphate framework during pyrolysis process,the macroporous structure was continuous and robust.Benefited from the improved mass diffusion and higher number of exposed active sites in the ordered macroporous structure,the catalyst exhibited a remarkable olefin productivity of?16 golefin g_cat^-1 h^-1 with WHSV of 94 gc3H8 g_cat^-1 h^-1,which is higher than that of ODH catalysts reported to date.The selectivity for olefins was 91.5%(propene:82.5%)at 515?,with a propane conversion of 14.3%.At the same time,the selectivity for the unwanted deep-oxidized CO₂ product remained less than 1.0%.The ordered macroporous boron phosphate framework exhibited a remarkable structural stability,and the tri-coordinated surface boron species were identified as the active catalytic sites for the ODH of propane.(3)By using self-propagating high-temperature synthesis,cubic(BP)and rhombohedral(B12P2)boron phosphides have been prepared by controlling the content of reductant.It is easy to form rhombohedral boron phosphides when the proportion of reductant increased.The rhombohedral boron phosphides exhibited superior catalytic performance and structure stability over its cubic allotrope.The selectivities for propene and olefin of rhombohedral boron phosphides were 79.5%and 90.8%,respectively,with a propane conversion of 20.6%at 480 ?,and the selectivity for the unwanted deep-oxidized CO₂ product remained less than 1.0%.Moreover,the more content of rhombohedral structure in boron phosphides,the higher catalytic activity in the ODH of propane,and boron phosphate is more easily formed from cubic boron phosphide in the ODH reaction.Based on the XPS experiments,the oxidative tri-coordinated boron species were proposed to be the active sites for the ODH of propane.The rhombohedral B12P2 with higher activity was mainly existing tri-coordinated BO₃ and B₂O₃ on the surface,and the lower activity of the cubic BP was attributed to the more abundant distribution of low oxidation state rather than the tri-coordinated boron species on its surface under the ODH conditions.These results revealed the structure-activity relationship of boron phosphides catalysts in the ODH of propane,and their crystal structure determined the types and content of boron species on their surfaces,leading to their different catalytic performance.