FTIR研究不同硅铝比HZSM-5沸石的酸性质

A FTIR Spectroscopy Study on Acidic Properties of HZSM-5 Zeolites with Different SiO_2/Al_2O_3 Ratios

用IR和NH_3-TPD测定了硅铝比为31.4～137.1的HZSM-5沸石样品的酸中心数;由Lang-muir吸附模型推导出B、L酸的吸附热公式,并由此计算了B酸的吸附热。发现在Al/u.c.为3.64附近,酸中心数与Al/u.c的关系发生了转折,并发现B酸吸附符合Langmuir模型,L酸强度不均匀而不符合该模型,测定了HZSM-5分子筛的B、L酸消光系数8_B=7.52×10~5cm·mol^(-1)、8_L=1.09×10~6cm·mol^(-1)。

Numbers of acid sites of a series of HZSM-5 zeolites with SiO2/Al2O3 from 31. 4 to 137. 1 were determined using IR and NH3-TPD spectra. To calculate respectively the adsorption heats of Bronsted and Lewis acid sites, an equation, ln0/[( 1 - 0)P] = Q/RT+C, was established with the help of Langmuir model, and the adsorption heats of Bronsted acid sites were calculated with this equation. It was found that there is a transition of the relationship between Al/u. c. and acidic concentration at Al/u. c. =3. 64. The calculated results show that the adsorption on Bronsted acid sites accords with Langmuir model and the adsorption heats of samples with different SiO2/Al2O3 are almost the same, but the strength of Lewis acid sites is heterogeneous and adsorption on Lewis acid sites does not accord with Langmuir model. The extinction coefficients of Bronsted and Lewis sites of HZSM-5 zeolites were also determined, εB=7. 52× 10 cm · mol-1 and εL=1. 09×10 cm · mol-1.