水热处理USY二次孔形成规律研究

Study on the Formation of Mesopore during Hydrothermal Dealumination of Y Zeolite

研究了氧化钠含量不同Y型分子筛水热处理形成二次孔的规律.结果表明,NH4NaY升温至650℃,分子筛处于介稳态,其比表面积、孔容等均显著低于NaY.红外研究未发现脱铝及其他物种生成,通入水蒸气后,有脱铝过程发生,但其比表面积、孔容等大幅增加,总孔容也大于NaY;进一步处理上述参数又有降低.这种变化被归结为升温形成的具有强电子亲和势的质子在高温下和骨架氧发生强相互作用,造成骨架扭曲、松弛或断裂,而有Na+存在的方钠石笼则保持相对稳定.在高温水热合成条件下,可能以稳定的方钠石笼为单位进行结构重排,骨架脱铝形成的非骨架铝和无定形氧化硅物种的存在,抑制了分子筛按照原始NaY的结构愈合,只能形成具有破损方钠石笼(缺陷)的八面沸石结构,这可能是二次孔形成的原因.

Mesopore formation of Y zeolite in hydrothermal dealuminated process was studied over NH4NaY with different Na2O content. A 'pseudo-steady state' (PSS) was observed by merely raising temperature to 650 degreesC, IR characteristic absorbency maintained and few dealumination occurred in this stage, therefore BET surface area, pore volume leveled down substantially. However, they could increase by introducing steam, even could be larger than that of the original NaY in 1 h, and could decrease by further treatment. It was pronounced that protons with high electron affinity formed in temperature-raising stage interacted strongly with frame oxygen, and brought about twist, relaxation and split of zeolite frame. However, sodalite cages with a certain amount of Na+ kept relatively stable. Then structure could be rearranged based on the stabilized sodalites under the high temperature, hydrothermal condition, but it couldn't heal over due to the existance of extraframe alumina and amorphous silica coming from framework dealumination, only sharp faujasite with damage (defects) in sodalite could be formed. This probably is the mechanism of mesopore formation.