【作者】 田玲；

【导师】 李建伟；

【作者基本信息】 北京化工大学 ， 化学工程， 2008， 硕士

【摘要】 采用浸渍法制备了一系列用于苯与1-十二烯烷基化的磷改性MCM-22分子筛催化剂,以XRD、BET、NH₃-TPD、TG和FT-IR等表征技术和催化剂性能评价相结合的方式,较为系统地研究了磷改性对MCM-22分子筛催化剂的物理化学特性及其性能的影响。结果表明,用磷改性MCM-22分子筛,虽然不会影响分子筛的晶体结构,但会降低分子筛的结晶度;适当量磷的引入可增加分子筛孔径,起到扩孔的作用,但过大的磷含量可能导致多聚态磷物种的增加,从而使分子筛孔容和平均孔径迅速减小,但比表面积变化不大。此外,采用磷改性的方法可以调节分子筛酸性,使得强酸减少,弱酸增多,酸强度降低,总酸量和B酸中心数随着磷含量的增加呈现先增后减的规律,L酸减少。适当的磷含量有利于提高MCM-22分子筛对苯与1-十二烯烷基化反应的催化选择性和稳定性,以用途最广的2-十二烷基苯（2-LAB）产物收率最大为目标函数确定的最佳改性磷含量为0.5%左右。较低的浸渍温度以及充足的浸渍时间,有利于磷元素与分子筛骨架原子的相互作用,提高分子筛催化性能。高温焙烧脱铝虽然可以调节分子筛酸性,但是达不到磷改性处理的效果。对失活分子筛上残留物质分析发现,来不及扩散出去的反应产物、副产物以及原料液中的短链多烷基苯及多环芳烃等覆盖在活性中心或沉积在分子筛孔道,造成分子筛失活。关于磷改性机理的研究,IR谱表明磷原子与分子筛骨架原子存在着强的相互作用,使T-O-T对称性伸缩振动频率增大;²⁷Al MAS NMR表征结果说明,磷的加入会破坏分子筛上Si-O-Al键,使部分骨架四配位铝转化为六配位铝,并可能产生扭曲的四配位或五配位铝,且均有可能与磷发生键合;³¹P MAS NMR可以检测到聚合态磷酸盐物质,并有磷酸铝物种产生。当磷含量较小时,低聚态磷酸盐物质居多;随着磷含量的增加,多聚态磷酸盐明显增加。磷改性分子筛有两种可能的连接方式,其主要区别是磷上的桥氧与铝配位还是端基氧与铝配位,采用H原子饱和的方法建立两种可能连接方式的模型簇,并基于Gaussian98程序和采用PM3半经验量子化学计算方法对模型簇进行优化计算发现,端基氧与铝的配位在能量上更有利,是最有可能的磷改性键合方式。更多还原

【Abstract】 A series of MCM-22 zeolite catalysts modified with phosphor were prepared by means of incipient wetness impregnation.The effects of phosphor on the physical chemistry characteristics and catalytic performance of the modified MCM-22 zeolite catalyst were investigated based on the combined characterizations of XRD,BET,NH₃-TPD,TG and FT-IR,with experimental evaluation.The results showed that the crystal structure was not destroyed after modified,but the relative crystallinity of zeolite samples decreased. Impregnating zeolite with proper amount of phosphor could enlarge the pore size of the zeolite,the phosphor played a role of expanding agent.However, the specific pore volume and average size of zeolite minished rapidly when phosphor was too much,that because polyphosphate chains were increased. But the specific surface areas of the zeolite samples changed a little.Moreover, the acidity of MCM-22 zeolite was changed when phosphor was introduced, with the increase of the phosphor content,the quantity of the weak acid sites increased gradually,the quantity of the strong acid sites decreased,the quantities of total acid sites and the Brφnsted acid sites increased firstly and then decreased,and the quantity of Lewis acid sites decreased.Proper phosphor content could improve the selectivity to 2-phenyldodecane and the catalytic stability of MCM-22 catalyst for the alkylation of benzene with 1-dodecene.The maximum yield of 2-phenyldodecane（2-LAB）was obtained in case that the phosphor content was 0.5%.Long time impregnation at lower temperature was benefic to the interactions between P species and framework atoms of zeolite,therefore,the catalytic performance of modified zeolites was improved.High temperature calcination couldn’t reach the purpose of modification with P.The characterization of compounds deposited in deactive zeolites showed that the deactivation of catalysts was caused by a jam of bulkier molecules, such as linear alkylbenzenes,branch alkylbenzenes and aromatic compounds in raw material.About modifying mechanism,IR spectras of modified zeolites showed that symmetric stretch vibrations of T-O-T tended to low frequency,that probably because the strong interactions between the P species and the framework atoms of zeolite.²⁷Al MAS NMR demonstrated that partial Al-O-Si bonds were broken,and some tetrahedral aluminum was converted to octahedral aluminum.³¹p MAS NMR found the formation of aluminum phosphate species and polyphosphate chains.At low phosphor content,short chain polyphosphate species were more,as phosphor content increasing, highly condensed branching polyphosphates increased obviously. The zeolite modified with phosphorus has two possible structures,their main difference was the bridge oxygen of phosphorus coordination with aluminum or terminal oxygen coordination.The two forms of model cluster were established by atomic H saturation approach.Gaussian98 program and PM3 semi-empirical quantum chemical calculation were employed to study the stablility of two possible phosphorus modified structures,the modelling results showed that the terminal oxygen coordination was more possible according to the heat of formation.更多还原