【作者】 楼银川；

【导师】 王野；

【作者基本信息】 厦门大学 ， 工业催化， 2008， 博士

【摘要】 本论文主要研究了介孔分子筛SBA-15负载氧化钼催化剂(MoO_x/SBA-15)上的乙烷和乙烯的选择氧化反应。论文详细考察了催化反应性能,表征了催化剂结构,阐述了结构和性能之间的关联,建立了构效关系;通过动力学和原位红外光谱等研究提出了合理的乙烷和乙烯选择氧化反应途径和反应机理。研究表明,较高钼含量(≥15.4 wt%)的MoO_x/SBA-15上的乙烷转化率以及单个Mo原子上的乙烷转化速率均高于较低钼含量(≤6.6 wt%)的催化剂。钼含量较低时含氧有机产物以乙醛为主(可大于40%),而钼含量较高时则以甲醛为主(可大于30%)。在9.6 wt%MoO_x/SBA-15上得到最高乙醛单程收率为0.95%,在20.1wt%MoO_x/SBA-15上得到最高甲醛单程收率为4.2%以及总的醛收率为5.0%。MoO_x/SBA-15上的甲烷选择氧化中的转化率和转化速率随钼含量变化的趋势与乙烷选择氧化中类似。对MoO_x/SBA-15进行了详细的表征研究,结果表明钼含量≤4.9 wt%的催化剂上Mo物种主要以单核分散状态存在;而钼含量在6.6～9.5 wt%时,出现低聚的氧化钼小簇;当Mo为15.4～20.1 wt%时,则以～10-20 nm的纳米颗粒及少量的氧化钼微晶状态存在。关联催化剂的结构和催化性能,我们推断多聚的Mo物种比孤立的Mo物种更有利于乙烷的活化,并且孤立的Mo物种主要倾向于生成乙醛而多聚的Mo物种主要倾向于生成甲醛。20.1 wt%MoO_x/SBA-15上的乙烷选择氧化反应动力学研究表明,乙烯和乙醛是反应的初级产物而甲醛是次级产物。进一步针对乙烯和乙醛的反应动力学研究表明乙烯是生成甲醛的主要反应中间体,其贡献是乙醛贡献的十倍左右。在MoO_x/SBA-15催化剂上详细研究了乙烯选择氧化反应,发现较高钼含量(～15.4-20.1 wt%)的催化剂有利于乙烯向甲醛的选择氧化反应。在此基础上,我们设计了一种以乙烷氧化脱氢催化剂为上游催化剂,以20.1 wt%MoO_x/SBA-15为下游催化剂的乙烷选择氧化双床层催化体系,获得了超过14%的甲醛单程收率。论文对4.9 wt%和20.1 wt%MoO_x/SBA-15催化剂上构效关联的本质和催化反应机理进行了初步的研究。研究结果表明在MoO_x纳米粒子上的可利用晶格氧比单核分散的MoO_x物种上更多、更容易被还原,而且具有更好的反应性以及可能具有更好的流动性能。我们推测这是导致纳米粒子物种活性更优的重要原因之一。另外,含纳米粒子的催化剂的表面酸性要略强于单核分散的MoO_x物种,结合文献报道,我们推测这可能是纳米粒子上乙烯生成甲醛的原因之一。而机理研究表明在MOO_x纳米粒子上,乙烯吸附之后主要经历了乙二氧基中间体而在孤立Mo物种上则可能主要经历了乙氧基中间体。这可能是两种催化剂在产物选择性上表现出差异的重要原因。更多还原

【Abstract】 This dissertation focuses on the studies of selective oxidation of C₂H₆ and C₂H₄ over SBA-15-supported molybdenum oxide catalysts by O₂.The catalysts have been characterized in detail and the structure-reactivity relationships have been investigated. The reaction pathways and the mechanisms for C₂H₆ and C₂H₄ selective oxidation by O₂ have also been studied.The conversion of C₂H₆ and the conversion rate of C₂H₆ per Mo atom over MoO_x/SBA-15 with higher Mo contents（≥15.4 wt%） are much higher than those over the catalysts with lower Mo contents（≤6.6 wt%）.The catalysts with lower Mo contents favor the formation of CH₃CHO（Its selectivity can be higher than 40%）, while those with higher Mo contents can give HCHO with higher selectivity（HCHO selectivity can be higher than 30%）.The highest single-pass yield of CH₃CHO（0.95%） can be obtained over the 9.6 wt%MoO_x/SBA-15,while the highest single-pass yield of HCHO（4.2%） and of total aldehyde（CH₃CHO+HCHO,5.0%） can be obtained over the 20.1 wt%MoO_x/SBA-15.The trend in the change of conversion rates of CH₄ per Mo atom over the MoO_x/SBA-15 with changing Mo content is similar to that observed in C₂H₆ selective oxidation.Our characterizations suggest that monomeric MoO_x species exist mainly in the catalysts with Mo content≤4.9 wt%,and that an increase in Mo content from 4.9 to 9.5 wt%causes the formation of MoO_x oligomers or small polymeric MoO_x clusters in SBA-15 mesoporous channels.For the samples with Mo contents between 15.4 and 23.5 wt%,the MoO_x nano- particles with -10-20 nm are the dominant molybdenum species as well as small amount of crystalline MoO₃.The correlation of catalytic performances and catalyst structures reveals that the polymeric MoO_x species are more active than the monomeric ones.The catalysts with highly dispersed MoO_x species favor the formation of CH₃CHO,whereas those containing polymeric MoO_x species show higher HCHO formation rates.The kinetic studies over the 20.1 wt%MoO_x/SBA-15 suggest that HCHO in C₂H₆ oxidation is mainly produced through the consecutive oxidation of C₂H₄.This is a new reaction to our best knowledge.Detail investigations on the selective oxidation of C2H4 show that the catalysts with higher Mo contents（～15.4-20.1 wt%） favor the selective oxidation of C₂H₄ to HCHO.Based on these results,we have designed a double-layer catalytic reaction system for C₂H₆ selective oxidation,which comprises Dy₂O₃-Li⁺-MgO-Cl^-（an C₂H₆ ODH catalyst） as up-stream catalyst and 20.1 wt% MoO_x/SBA-15 as down-stream catalyst.A HCHO single-pass yield of 14.2%can be achieved using this system.The dissertation have preliminarily studied the natures of structure-reactivity relationships and reaction mechanism over 4.9 wt%and 20.1 wt%MoO_x/SBA-15 catalysts.The results suggest that the available lattice oxygen in 20.1 wt%MoO_x/ SBA-15 is much more in amounts,easier to be reduced,better reactivities,and perhaps easier to transfer than that in 4.9 wt%MoO_x/SBA-15.On the other hand,the surface acidity of MoO_x nanoparticals are stronger than that of isolated MoO_x species.In situ FTIR spectroscopic studies suggest that ethylene dialkoxide（-OCH₂CH₂O-） is an intermediate over polymeric Mo species while ethoxide（-OC₂H₅） might be an intermediate over the monomeric Mo species.更多还原