Ti-MWW分子筛吸附H_2O和NH_3的结构和振动光谱的密度泛函理论计算（英文）  

Structures and vibrational spectra of Ti-MWW zeolite upon adsorption of H_2O and NH_3: A density functional theory study

作　　者：乔溢铭 范志琳 蒋艳娇[1] 李娜[1] 董浩[1] 贺宁[2] 周丹红[1] 

机构地区：[1]辽宁师范大学化学化工学院,辽宁大连116029 [2]大连理工大学精细化工国家重点实验室,辽宁大连116024

出　　处：《催化学报》2015年第10期1733-1741,共9页

基　　金：supported by the National Natural Science Foundation of China(21343010)~~

摘　　要：Ti-MWW分子筛具有10元环(10MR)孔道体系和12MR超笼以及外表面杯状空穴,在以H2O2水溶液为氧化剂的催化氧化反应中表现出不同于其他钛硅分子筛的特殊溶剂效应和立体选择性.已有的实验和密度泛函理论(DFT)计算研究表明,骨架Ti(IV)可能分布在10MR孔道和12MR超笼中.最近,我们采用DFT计算研究了Ti-MWW分子筛中骨架钛落位,通过比较Ti/Si替代能和红外振动光谱,提出Ti(IV)最可能落位在T1和T3位,并以[Ti(OSi)4]形态存在,显示960 cm–1钛特征振动峰.[Ti(OSi)4]物种水解时Ti–O键发生选择性断裂,生成具有翻转Ti–OH的[Ti(OSi)3OH]物种.由于Ti中心具有Lewis酸性,与配体分子络合后使Ti(IV)的配位状态改变.Ti-MWW分子筛中不同的骨架Ti(IV)落位和形态可能呈现不同的催化选择性.本文应用DFT研究了Ti-MWW分子筛中T1和T3位上不同钛物种与H2O和NH3的吸附作用,考察了其几何结构、吸附能以及红外振动光谱性质,为深入理解骨架Ti(IV)的微观结构及实验红外光谱表征提供参考数据.计算采用36T簇模型,从MWW分子筛晶体结构中分别以T1和T3为中心截取七层骨架原子,末端设为Si–H键并固定为1.46?.结构优化时松弛内部四层骨架原子并固定最外三层骨架原子.所有计算在B3LYP/6-31G(d,p)理论水平完成,计算的吸附能都经过BSSE校正,计算的频率以约化因子0.961校正.所有计算在Gaussian 09软件包完成.计算结果表明,四配位的[Ti(OSi)4]和[Ti(OSi)3OH]物种都能与H2O或NH3分子作用生成三角双锥的五配位络合物.H2O或NH3分子有选择性地进攻Ti–O键的Ti端,形成近乎直线的L–Ti–O键,L–Ti距离可达2.2–2.4?.T1位钛物种的Lewis酸性比T3位的略高.对于[Ti(OSi)3OH]物种,Ti–OH的存在使得Ti(IV)的酸性大大增强,表现出很强的吸附作用.此外,[Ti(OSi)3OH]物种也能通过Ti–OH基团与H2O和NH3形成氢键络合物,但是其吸附能比形成配位络合物的能量更小,说明配体分The structures and vibrational spectroscopic features of framework Ti（IV） species in Ti-MWW zeo-lite upon adsorption of H2O and NH3 were investigated by density functional theory. The calcula-tions were carried out on cluster models up to 36 tetrahedra at the B3LYP/6-31G（d,p） level of the-ory. The calculated results indicate that both Ti（OSi）4 and Ti（OSi）3OH species can interact with H2O or NH3 molecules to form five-coordinated complexes. The Ti（OSi）3OH species has higher Lewis acidity and adsorbs the ligands more easily than Ti（OSi）4. The Ti-specific band is attributed to the collective vibration of the antisymmetric stretching of Ti–O–Si bonds. The vibrational frequencies of coordinated Ti species can be divided into two regions： the Ti-specific vibration region and the hydroxyl group vibration region. After adsorption of H2O, the Ti-specific band of the Ti（OSi）4 species shifted from 960 to 970 cm？1, and the Ti-specific bands of the Ti（OSi）3OH species shifted from 990 cm？1 （T1 site） and 970 cm？1 （T3 site） to 980 cm？1. The frequencies of the corresponding NH3 adducts were about 5 cm？1 higher. The Ti（OSi）3OH species can also form hydrogen bonded complexes with H2O and NH3 through Ti–OH, resulting in the hydroxyl stretching band of Ti–OH red shifting by 500–1100 cm？1 and appearing in the 2700–3200 cm？1 region.

关 键 词：密度泛函理论 钛硅分子筛 络合 振动频率 红外光谱 

分 类 号：O647.31[理学—物理化学]