【作者】 王阳；

【导师】 陆安慧；

【作者基本信息】 大连理工大学 ， 工业催化， 2021， 博士

【摘要】 甲烷是天然气的主要组分,其活化和转化是以天然气替代石油生产液体燃料和基础化学品的关键。甲烷分子热力学性质稳定,直接活化非常困难,而且目标产物通常比甲烷分子更为活泼,易发生过度氧化,因此发展高效催化剂是实现甲烷活化和定向转化的有效途径。氮化硼(BN)是一种人工合成陶瓷材料,具有抗氧化性好、导热性好、硬度高等优点,但由于比表面积小(20-30 m2·g-1)、表面官能团少,氮化硼在传统催化领域通常被视为惰性载体材料,最近研究证实,氮化硼具有优异的低碳烷烃(C2～C4)氧化脱氢活性,能高效活化低碳烷烃分子并抑制过度氧化。基于此,本论文以氮化硼材料为基础,围绕甲烷活化转化进行研究。探究了氮化硼催化甲烷临氧转化,揭示了活性位点和反应机理;设计了具有蜂窝陶瓷结构的涂层式氮化硼催化剂,研究了催化剂结构对甲烷临氧转化反应中产物乙烷、乙烯选择性的调控;设计了氮化硼-水滑石复合催化剂并应用于甲烷-二氧化碳重整反应,探索了氮化硼对催化剂活性及抗积碳性能的调控。具体研究内容如下:(1)氮化硼催化甲烷临氧转化。证实了氮化硼可以有效活化甲烷C-H键,690℃时甲烷转化率可以达到20.5%,产物包括一氧化碳(76.4%)、氢气(13.7%)、乙烯(9.6%)、乙烷(9.7%)和二氧化碳(4.3%)。结合原位红外表征、同位素示踪实验和密度泛函理论计算等方法推测了反应机理和路径,提出了水辅助的甲烷-氧气协同活化机理,即氮化硼边缘含有少量的B-O-B和B-O-H位点,B-O-H位点可以弱化相邻B-N键,进而促进O2的化学吸附,形成B-O-O-N位点,随后气相CH4在+B-O-O-N位点发生活化,打断O-O键,生成甲氧基和甲基自由基,进而发生一系列气相和表面自由基反应,生成氧化和偶联产物。在反应过程中,氮化硼被部分氧化产生B-O-B位点,进而与反应过程中生成的H2O作用形成B-O-H位点,B-O-H位点作为活性位进一步参与反应循环。(2)涂层式氮化硼/堇青石催化剂催化甲烷临氧转化。基于对氮化硼催化甲烷临氧转化的反应机理分析,设计了具有蜂窝结构的氮化硼/堇青石催化剂,催化剂发达的直通孔道为气相反应提供了空间,而且保证反应在高空速下顺利运行,有利于促进CH3·气相偶联,减少其与氮化硼碰撞生成CHO·,从而提高了乙烷、乙烯选择性。当反应气体空速为210 L·gBN-1·h-1,反应温度为770℃时,甲烷转化率为1 9.0%,乙烷和乙烯选择性可达到1 9.3%和32.4%。将氮化硼/堇青石拓展应用于丙烷氧化脱氢反应,催化剂在高空速下显示出优异的活性和选择性,在空速576 L·gBN-1·h-1时,丙烷转化率为16.8%,丙烯和乙烯选择性可达82.1%和14.2%,CO选择性仅为3.7%,没有检测到CO2产物,丙烯产率可达 18.6 gC3H6 gBN-1 h-1。(3)氮化硼-水滑石复合催化剂催化甲烷-二氧化碳重整制合成气。通过静电驱动组装的方法制备了氮化硼-镍镁铝水滑石复合催化剂(NiMgAl@BN-r)。通过氮化硼对Ni的“几何结构”和“电子结构”进行了调控,使催化剂在甲烷干气重整反应中表现出优异的活性和抗积碳性能。750℃时甲烷转化率达到81%,二氧化碳转化率达到88%,H2/CO比例为0.9,反应350h后催化剂活性保持稳定,且没有检测到积碳形成。在积碳严重的反应温度(600℃)下催化剂活性保持稳定,甲烷和二氧化碳转化率分别为36%和47%,反应20 h后没有检测到积碳生成。通过氮化硼纳米片对镍镁铝水滑石的分隔和锚定,抑制了焙烧过程中水滑石纳米片的团聚,进而抑制了 Ni颗粒的烧结;Ni颗粒表面被氮化硼覆盖,减少了暴露的缺陷位点和大面积平台位点;Ni与氮化硼之间存在电子相互作用,使Ni颗粒电子云降低。因此,甲烷深度裂解和一氧化碳歧化反应被有效抑制,生成的微量碳迅速被二氧化碳氧化消除,从而抑制了反应过程中的积碳。更多还原

【Abstract】 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.更多还原