La交换NaY分子筛中的离子定位和迁移规律

以NaY分子筛为原料,采用二次交换二次焙烧工艺,制得了LaNH₄Y-1,LaY-1,LaNH₄Y-2和LaY-2四种分子筛样品.采用粉末X射线衍射结合Rietveld结构精修确定了La离子在分子筛中的分布.结果表明,在离子交换过程中,La离子首先定位于分子筛超笼里;经焙烧脱水后,La离子迁移到分子筛方钠石笼并定位在SI′位置.T-O-T键角的变化表明,LaNH₄Y-1表现出最大的骨架扭曲性和结构不稳定性.La离子定位在SI′位置与O3配位,增加了T-O3的键长,表明稀土离子不仅通过控制骨架脱铝抑制晶胞收缩,还可能通过影响T-O3键长使晶胞增大.在REY分子筛中,稀土的主要作用是稳定分子筛结构,从而调节其酸性和催化活性.采用红外光谱和NH₃程序升温脱附考察了La离子和NH₄⁺对分子筛酸性的影响,并总结了稀土离子稳定Y型分子筛的机理.

Cation location and migration in lanthanum-exchanged NaY zeolite

LaNH₄Y and LaY zeolites were prepared using a double-exchange double-calcination method by La exchange of NaY zeolite. The distribution of the La cations was determined by powder X-ray diffraction with Rietveld refinement. The results indicate that the La cations are initially located in supercages, and then their hydration shells are stripped off and they migrate to small sodalite structures located at SI′ during heating and dehydration. The changes in the T-O-T angles show significant distortion of the flexible framework and instability in LaNH₄Y obtained using a one-exchange one-calcination process. For La cations located at SI′ and coordinated with O3, the T-O3 bond distance increased, which indicates that the rare-earth cations not only restrain framework dealumination, suppressing condensation of the unit cell, but also have an effect on the T-O3 bond distance, increasing the unit cell volume. The role of the rare-earth species is to ensure the hydrothermal stability of the zeolite in order to control the acid site density and catalytic activity. The effects of La cations and NH₄⁺ cations on the zeolite acidity were studied using infrared spectroscopy and NH₃ temperature-programmed desorption, and the mechanism of rare-earth stabilization of the Y zeolite is described.