Oxidative Dehydrogenation Of Light Alkanes(C2-C4) On Boron-based Catalysts

With the shift of energy sources from oil to shale gas and the influence of international geopolitics,more and more attention has been paid to the development of catalytic conversions of light alkanes(C2-C4)to olefins.Compared with direct dehydrogenation(DH),oxidative dehydrogenation(ODH)of light alkanes has the potential advantages including no carbon deposition on the catalysts and no thermodynamic equilibrium limitation.On conventional metal oxide-based catalysts,it is diffiuclt to avoid the over-oxidation of olefin products.On the other hand,non-metallic boron-based catalysts,either the supported B2O3 or h-BN catalysts,have recently been shown to effectively inhibit the over-oxidation of olefins,achieving high olefin selectivities.However,questions such as the active sites and reaction mechanisms involved in ODH of light alkanes remain unanswered.ODH is also highly exothermic.The development of boron-based catalysts in a fixed bed reactor requires the fundamental understanding of mass and heat transfers invovled.To address these issues,this dissertation focuses on the following three aspects in order to provide insight on the catalyst sites,reaction mechanisms involved,and essential information related to the catalytic process to enable the design of more efficient catalysts and development of the ODH reators.(1)Understanding the active site and reaction mechanism of boron-based catalysts.Supported B2O3 catalysts exhibit similar catalytic performance as the h-BN catalysts in ODH of light alkanes,which implies that these boron-based catalysts,including h-BN and B2O3 materials,have similar active sites.Combined with the characterizations of catalysts and the evaluation of catalytic performance,boron(BO3)with three coordination is the active site for light alkane ODH.Through 18O2 isotope exchange experiment,kinetic studies and in-situ infrared spectroscopy,it was found that O2 molecule bounds on the electron deficient B center of two adjacent BO3 sites forming the B-O-O-B sites.This site can be used as a nucleophilic mild oxidation site to activate propane molecules but prevent the attacking of electron-rich olefin molecules,thus inhibiting the further oxidation of alkane and olefin to form COx.This work provides new insights into the active sites and reaction mechanism of boron catalysts in light alkanes ODH,offering further guidance for the design of more efficient boron-based catalysts.(2)Product distribution of the oxidative dehydrogenation of propane(ODHP)over boron-based catalysts.Boron-based catalysts show high olefin selectivities in ODHP reaction.Besides the main product of propylene,a significant amount of ethylene are also produced,and the overall selectivities of C2 products are much higher than that of C1 producst.In this chapter,we took h-BN as an example of boron-based catalysts to understand the influence of different reaction paths on product distributions in ODHP reaction over boron-based and vanadium-based catalysts.Four reaction paths are identified for secondary reactions of methyl radical formed from the cleavage of C-C bond in ODHP reaction over h-BN catalyst:over-oxidation to CO,CO2(COx);hydrogenation to CH4,selective oxidation to oxygen-containing compound(CHxOy)and oxidative coupling to C2 products.Among these reaction pathways,oxidative coupling of methyl radical to C2 product is mainly responsible for the high C2/C1 ratio(>2)observed in the ODHP over boron-based catalysts,and ethylene is formed not only from the cleavage of propane C-C bond,but also from the oxidative coupling of methyl radicals.This study not only provide deep understanding of the reaction mechanism of ODHP over boron-based catalysts,but also guide the design of boron-based catalysts for selective oxidation of methane to ethylene.(3)Hexagonal boron nitride catalyst in a fixed-bed reactor for exothermic propane oxidation dehydrogenation.h-BN is a promising catalyst for potential practice of ODHP.This chapter summarizes the studies of mass and heat transfers in in a fixed bed reactor.Kinetic studies and computational fluid dynamics(CFD)simulations indicate that the hot spot formed in the h-BN catalyst bed is less than 1?,while for vanadium-based catalyst,the hot spot of bed is more than 8?.Even when the diameter of the reactor is further increased to 60 mm,which is generally used in the industrial processes,h-BN catalyst bed with high thermal conductivity can still maintain uniform temperature distribution.However,under the same conditions,a temeperature gradient as high as 47? can be present in the VOx/?-Al2O3 catalyst bed,which negatively affects the catalytic performances.