Boron Nitride Based Catalyst For Methane Conversion

Methane is the main component of natural gas,its activation and conversion are the key to produce liquid fuels and basic chemicals from natural gas instead of petroleum.As the most stable low carbon alkane molecule,the activation of methane is very difficult.Furthermore,the target products are usually more active than methane molecules and deep oxidation easily occurred under reaction conditions.Therefore,the development of high-efficiency catalysts is the key to achieve high methane conversion and target product selectivity.Boron nitride(BN)is a synthetic ceramic material with excellent oxidation resistance,thermal conductivity,and high hardness.Due to its low specific surface area(20-30 m2·g-1),and few surface functional groups,BN is generally regarded as an inert material in the field of traditional catalysis.In recent years,the superior activity of BN in oxidative dehydrogenation of propane was demonstrated.It is proved that BN can active low carbon alkane(C2-C4)molecules and inhibit the deep oxidation.Based on the above considerations,in this thesis,methane activation and conversion was studied based on the BN material.First,methane activation and conversion was studied over BN catalyst and the active sites and reaction mechanism were discussed;then,a monolithic boron nitride catalyst with honeycomb structure was designed to optimize the mass transfer,and regulate the selectivity of ethane and ethylene;at last,a boron nitride-hydrotalcite composite catalyst was designed and applied to the methane dry reforming reaction,the effect of boron nitride on catalyst activity and anti-coke ability was investigated.The specific research results are as follows:(1)The oxidative conversion of methane over BN catalyst.BN can effectively activate the C-H bond of methane,the conversion reaches 20.5%at 690?,with the products distribution of CO(76.4%),H2(13.7%),C2H4(9.6%),C2H6(9.7%)and a small amount of CO2(4.3%).Combined with in-situ IR,isotopic experiments and theoretical studies,the reaction mechanism and path were speculated,and a novel H2O-assisted O2 and CH4 synergetic mutual activation mechanism was proposed.Specifically,a small amount of B-O-B and B-O-H groups exist at the edge of BN,the B-O-H groups facilitate the insertion of O2 on B-N bond to form transient intermediate B-O-O-N,subsequently,the gaseous CH4 are activated on B-O-O-N site to generate methoxy and methyl radicals,resulting in the formation of partial oxidation and coupling products by a series of gaseous radical or surface reactions.In the reaction process,boron nitride was partially oxidized to produce B-O-B site,which then reacted with in-situ formed H2O in the reaction process to form B-O-H site,and B-O-H site was further involved in the reaction cycle as the active site.(2)Catalytic oxidation of methane over BN/Cordierite monolithic catalysts.Based on the mechanism of methane oxidative conversion over BN,a BN wash-coated cordierite monolithic catalyst was designed.The well-developed channel of the catalyst provides space for the gas reaction and guaranteed the reaction operation at gas hourly space velocity(GHSV),which facilitated the gas phase coupling of CH3· and reduced its collision with BN to generate CHO·.Thus,the selectivity of ethane and ethylene was improved.At the GHSV of 210 L·gBN-1·h-1,the methane conversion was 19.0%at 770? with the ethane and ethylene selectivity of 19.3%and 32.4%.For the oxidative dehydrogenation of propane,BN/Cordierite achieved a propane conversion rate of 16.8%,propylene selectivity of 82.1%,ethylene selectivity of 14.2%,and CO selectivity of 3.7%under high space velocity(576 L·gBN-1·h-1)reaction conditions,with a high propylene yield of 18.6 gc3H6·gBN-1·h-1.(3)The NiMgAl@BN composite catalyst catalyzes the methane dry reforming.A novel NiMgAl@BN catalyst is prepared through electrostatic self-assembly of exfoliated NiMgAl LDHs nanoplates and ball-milled BN nanosheets.The catalyst exhibits excellent activity and anti-coke ability in the methane dry reforming reaction,due to the modulating of "geometric structure" and "electronic structure" of Ni by boron nitride.At 750?,the CH4 conversion is 81%and the CO2 conversion is 88%,with the H2/CO ratio of 0.9.After 350 h of reaction,the catalyst activity remains stable,and no carbon deposits are detected.Even under severe coking conditions at 600?,the conversion of CH4 and CO2 over the NiMgAl@BN catalyst are 36%and 47%,respectively,with no carbon formation detected after 20 hours of reaction.The BN nanosheet spatially separates and stabilizes the original layered structure of NiMgAl LDH and prevent the aggregation of Ni nanoparticles(?5.8nm);the surface of Ni particle is partially covered by BN nanosheet,and the exposed surface area of facets and step edges are reduced;the electron cloud density of Ni decreases due to the electron transfer from Ni to BN.As a result,the CO disproportionation and CH4 decomposition are suppressed over NiMgAl@BN catalyst,the small amount of carbon produced is quickly removed by CO2,resulting in the inhibition of coke deposition.