DFT计算结合固体NMR研究富铝SSZ-13的铝分布和Brønsted酸性

Aluminum Distribution and Brønsted Acidity of Al-Rich SSZ-13 Zeolite:A Combined DFT Calculation and Solid-State NMR Study

具有菱沸石(CHA)结构的SSZ-13分子筛在甲醇制烯烃(MTO)及柴油机车尾气氨选择性催化还原(NH3-SCR)反应中具有重要的应用,采用富铝SSZ-13可以调节MTO反应的烯烃选择性和提升NH3-SCR的低温脱硝活性,因此SSZ-13中的铝含量和分布与对应的酸性决定了其催化性能。本文采用密度泛函理论结合固体核磁共振实验研究了富铝和富硅HSSZ-13的Al位置与Brønsted酸强度的内在关系。通过计算取代能发现,对于孤立Al位,质子位于Al周围4个不同O位时能量差异较小,最稳定的B酸位点是O(1)-H。对于富铝SSZ-13,两个Al原子位于同一六元环的对位是Al-Si-Si-Al(NNNN)序列中最稳定的结构,而Al-Si-Al(NNN)序列中能量最优的Al分布是两个铝原子排布于六棱柱上下不同的六元环上。通过计算最稳定构型下的质子亲和势、NH3脱附能和吸附氘代乙腈后的1H NMR化学位移,发现富铝SSZ-13中含有Si(2Al)分布的NNN序列导致了其Brønsted酸强度弱于高硅的分子筛。分峰拟合29Si魔角旋转核磁共振(MAS NMR)谱图表明富铝SSZ-13中Si(2Al)的含量在43%以上,而吸附氘代乙腈后的1H MAS NMR实验显示富铝SSZ-13的桥羟基化学位移向低场移动,进一步证明富铝SSZ-13具有较弱的Brønsted酸强度。

SSZ-13 zeolite with a chabazite(CHA)topology structure has important applications for methanol to olefin(MTO)conversion and selective catalytic reduction of nitrogen oxides(NOx)by ammonia(NH3-SCR)to reduce diesel engine exhaust emissions.It has been reported that the Al-rich SSZ-13 zeolite can be used to tune the selectivity of olefins in the MTO reaction,and significantly enhance NO conversions at lower temperatures in NH3-SCR.Thus,the aluminum content and distribution as well as the corresponding acidity in SSZ-13 zeolite determine the catalytic performance of the zeolite for different catalytic reactions.Herein,quantum chemical computing using density functional theory(DFT)combined with multinuclear solid-state nuclearmagnetic-resonance(NMR)experiments were performed to investigate the correlation of Al location and Brønsted acidity of H-SSZ-13 zeolite with the Si/Al ratio varying from 5.8 to 25.The most favorable acid site in the 1Al model is O(1)-H in which a proton is bonded with the O(1)atom near the isolated Al atom of the zeolite framework.Nevertheless,energy differences were rather small when comparing the substitution energies of an Al atom replacing a Si atom in the zeolite framework with a proton located in different O sites.As the Si/Al ratio decreased,the Al-rich SSZ-13 zeolite contained more Al substitutions in its framework.This system exhibited the lowest substitution energy when two Al atoms were located at the diagonal of the same six-membered ring for the Al-Si-Si-Al(NNNN)sequence in the framework of the Al-rich SSZ-13 zeolite.However,for the Al-Si-Al(NNN)sequence,the most favorable distribution involved two Al atoms located in different six-membered rings of the double six-membered ring units(D6R).The proton affinities(PA),NH3 desorption energies,and 1H NMR chemical shifts after d3-acetonitrile adsorption were calculated in the most stable models to characterize the Brønsted acid strength of the SSZ-13 zeolite with different Si/Al ratios.All computing results suggested that the Al-rich SSZ-13 zeolite exhibited weaker Brønsted acid strength than that of the Si-rich counterpart due to the presence of Si(2Al)groupings with the NNN sequence in the framework.Quantitative 29Si magic-angle spinning(MAS)NMR measurements after deconvolution demonstrated that the content of Si(2Al)groupings in the Al-rich SSZ-13 was˃43%.The 1H MAS NMR experiments after d3-acetonitrile adsorption showed that the chemical shift of the bridging hydroxyls in the Al-rich SSZ-13 moved to the lower field,further confirming that it had a weaker Brønsted acid strength than the Si-rich counterpart.