密度泛函理论结合固体核磁共振谱学研究SSZ-39分子筛骨架铝分布与Brönsted酸性关系

A Combination of DFT and Solid-state NMR Study on the Relationship between Framework Al Distribution and Brönsted Acidity in SSZ-39 Zeolite

含铜的SSZ-39分子筛(AEI拓扑结构)在机动车尾气氨气选择性催化还原(NH_(3)-SCR)反应中性能优异,其中SSZ-39分子筛的骨架铝分布与对应的Brönsted酸性质对反应性能影响至关重要.我们通过密度泛函理论计算同时结合固体核磁共振谱学实验探究了高硅和富铝SSZ-39分子筛骨架Al位置以及与相应Brönsted酸强度之间的关系.通过比较骨架铝在不同位置的取代能发现,高硅H-SSZ-39分子筛的骨架铝主要以孤立Al形式存在,同晶取代后落位在T3位上,其相应的Brönsted酸质子与O7结合时最稳定.而富铝SSZ-39分子筛的骨架铝主要以NNNN与NNN序列的2Al形式存在,当两个骨架铝原子分别位于六元环和四元环对位的T3位上时体系能量最低,此时两个Brönsted酸质子指向分子筛的超笼和八元环孔道.在最优构型下计算质子亲核势、 NH_(3)吸附态微观结构与脱附能以及吸附氘代乙腈后^(1)H NMR化学位移来表征Brönsted酸性,发现随着SSZ-39分子筛铝含量增加相应的Brönsted酸含量增加,而Brönsted酸强度趋于减弱.这些理论计算结果与NH_(3)-TPD及吸附氘代乙腈的^(1)H MASNMR实验结果一致,为调控SSZ-39分子筛酸性以及合理设计高效催化剂提供了依据.

Cu-SSZ-39 zeolite with AEI framework structure shows excellent activities for the selective catalytic reduction of nitrogen oxides by ammonia(NH_(3)-SCR) to reduce diesel engine exhaust emissions. It is proposed that the Al distribution and corresponding Brönsted acidity of SSZ-39 play a vital role in reaction performances. In this article, Al distribution and its relationship to the corresponding Brönsted acidity of Si-rich and Al-rich SSZ-39 zeolites are investigated via density functional theory(DFT) and solid-state NMR spectroscopy. By comparing the substitution energies of framework Al in Si-rich SSZ-39, it is found that T3 site is the most readily to be replaced by isolated Al and the Brönsted acid prefers to be formed at O7 site near the isolated Al atom. In Al-rich SSZ-39 zeolite, the framework Al tends to form 2 Al pairings in Al-Si-Al(NNN) and Al-Si-Si-Al(NNNN) sequences. The most favorable distribution for NNN sequence involves in two Al atoms located in four-membered ringunit. While for NNNN sequence two Al atoms occupying the diagonal of the same six-membered ring exhibit the lowest substitution energy. The Brönsted acid strength of the SSZ-39 zeolite with different Si/Al ratios was characterized by the proton affinities(PA), NH_(3) desorption energies and 1 H NMR chemical shifts after d3-acetonitrile adsorption. It is found that the acid strength tends to decrease with the increase of Al content. These computing results agree with the experimental results of NH_(3)-TPD and ^(1)H MAS NMR with d3-acetonitrile as probe molecule. This study on the Al location at the specific T-site and the Brönsted acid strength could provide insight into finely tuning acidity of SSZ-39 and rational design of zeolite catalysts.