磷酸与分子筛HZSM-5之间的密度泛函理论研究

对HZSM-5分子筛与H3PO4之间的相互作用进行了研究。结果表明,焙烧或蒸汽处理之前,磷酸的主要存在形式是化学吸附,吸附能分别是-49.31,-45.96kJ/mol,两者都是负值,表明吸附是一个放热过程,原因是氢键作用的存在。化学吸附包括H3PO4及质子化的H4PO4＋,H5P2O7＋在分子筛表面上的吸附。煅烧或蒸汽之后,磷酸与分子筛发生脱水反应,形成两种可能的作用模式,即磷化合物通过一个或两个氧桥同分子筛相连。因为由一个氧桥到两个氧桥模式的反应能垒是125.73kJ/mol,反之是47.59kJ/mol,所以有一个氧桥的模式较稳定,是主要存在模式,且较低的能垒使两个氧桥结构易于转化成一个氧桥稳定结构。

The Study of H3PO4 and HZSM-5 by the Density Functional Theory

The interaction of orthophosphoric acid and zeolite HZSM-5 was investigated.It is found that chemisorption of orthophosphoric acid and cationic species including H5P2O+7 and H4PO+4 are possible structures of P/HZSM-5 before calcination or steaming,adsorption energy with-49.31,-45.96 kJ/mol are negative,it shows that adsorption is an exothermic process,the reason is the hydrogen bonding is the major form of interaction in between.After calcination or steaming,dehydration reaction occurs,two different models are proposed,that’s to say,P species are combined with the framework of zeolite by one or two bridged oxygen atoms.It is show that the energy barrier,for the reaction: a oxygen bridge to two oxygen bridges,is 125.73 kJ/mol for forward reaction;while 47.59 kJ/mol for reversed one,so the model with only one bridged oxygen atoms is the energetically stable one,which is regarded as the major model after dehydration,the lower energy barrier makes two oxygen bridges easy into an oxygen bridge structure.