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催化学报  2015 

Ti-MWW分子筛吸附H2O和NH3的结构和振动光谱的密度泛函理论计算

DOI: 10.1016/S1872-2067(15)60900-7, PP. 1733-1741

Keywords: 密度泛函理论,钛硅分子筛,络合,振动频率,红外光谱

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Abstract:

?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960cm-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Gaussian09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,说明配体分子更趋向于吸附在Ti中心形成配位络合物.自然键轨道分析表明,Ti(IV)中心的Lewis酸性归因于Ti的空4p轨道接受配体提供的孤对电子,并且属于LUMO+3.所有吸附络合物的特征振动频率分布在两个区域,即钛特征振动区域和羟基振动区域.T1和T3位的[Ti(OSi)4]物种的钛特征振动频率都在960cm-1,与H2O形成五配位的吸附络合物之后,钛特征振动频率位移到970cm-1.[Ti(OSi)3OH]物种的钛特征振动频率分别为990cm-1(T1位)和970cm-1(T3位),吸附H2O分子后都位移到980cm-1.相应的NH3吸附络合物的钛特征振动峰频率都高出5cm-1.分析表明,钛特征振动模式归属于Ti-O-Si键的不对称伸缩振动的协同振动.在羟基伸缩振动区域,气相H2O、末端Si-OH基团以及Ti-OH基团的羟基伸缩振动在3600-3760cm-1.吸附H2O后,羟基伸缩振动移到3460-3150cm-1区域.[Ti(OSi)3OH]物种与NH3和H2O形成氢键络合物后,钛羟基的伸缩振动频率分别红移500和1100cm-1,出现在2700和3200cm-1区域.吸附分子的O-H和N-H的伸缩振动频率略微蓝移,这反映了Ti物种具有Lewis酸性.

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