全天然硅铝矿物合成Y和ZSM-5分子筛材料的研究

Zeolites Y and ZSM-5 are widly used in oil refining and petrochemical industries. At present, most of them are prepared from chemical materials such as soluble sodium silicate (waterglass)and aluminium sulfate. But manufacturing of these chemical materials causes environmental hazard and high energy consumption. Therefore, natural aluminosilicate minerals (NAMs) such as kaolin, rectorite and diatomite minerals which having suitable chemical composition and enough chemical reactivity after thermal activation, were reported to be used as the alternative starting materials for zeolites fabrication. However, preparation of zeolites from NAMs is now confronted with several challenges: (1) NAMs with high crystallinity such as kaolin mineral can only be activated at high temperature of above 600 oC; (2) it is inevitable to use chemical materials such as sodium silicate as a supplementary silica source especially for synthesizing those zeolites which having medium or high molar SiO2/Al2O3 ratio and therefore tuning of the SiO2/Al2O3 ratio of the zeolites is still mostly depending on the addition of pure chemical materials; (3) impurites such as iron in NAMs will unfavor the synthesis process and the properties of the NAMs-derived zeolites. In this project, a series of Y and ZSM-5 with tunable molar SiO2/Al2O3 ratio will be directly prepared from natural kaolin and diatomite minerals, without any supplementary silica/alumina sources. Based on the sub-molton system,the possibility of activation of those NAMs with high crystallinity at a relatively lower temperature (300 oC) will be investigated. The dissociation,aggregation and transformation behavior of silica, alumina sources and iron impurities during NAMs activation and zeolites synthesis processes will be investigated too, and then a method for tuning the molar SiO2/Al2O3 ratio of NAms-derived zeolite can be developed.

目前，Y及ZSM-5等分子筛大多是以水玻璃、硫酸铝等化学试剂为原料合成的，而化学试剂的生产本身存在着高能耗、高污染的缺点，为实现上述分子筛低成本的绿色合成，利用天然硅铝矿物作为硅铝源替代化学试剂是重点研究方向之一。但随之遇到的难题是：(1)结晶态天然矿物(如高岭土)需要在较高的温度下活化；(2)分子筛的合成不能完全避免使用化学试剂,分子筛硅铝比的调变往往依赖于外加化学试剂的量；(3)天然矿物中的杂质铁可能影响分子筛的合成及后续应用。为解决上述难题，本项目拟采用天然高岭土与天然硅藻土为原料，在无外加化学试剂的条件下合成出硅铝比可调的Y和ZSM-5分子筛。基于亚熔盐体系发展天然硅铝矿物的低温活化方法，研究分子筛合成过程中硅、铝以及杂质铁等组分的解离-聚合反应机理以及铁的去向，获得全天然硅铝矿物合成分子筛材料硅铝比的调变方法，形成具有工业应用价值的基于天然黏土矿物的分子筛绿色合成新技术路线。

分子筛是现代化学工业的基石。本项目针对目前Y和ZSM-5等以化学试剂为原料合成所存在的化学试剂制备过程复杂、环境污染严重等问题，开发以天然硅铝矿物为原料，直接合成Y和ZSM-5分子筛的绿色合成新技术路线。主要结果为：(1)分别采用热活化、碱熔活化、亚熔盐活化对天然矿物进行处理。结果表明，经600-900 oC热活化后，累托土或高岭土等天然矿物只能变为偏土，AlO6八面体中键能较小的Al−O键遭到破坏，而SiO4四面体中键能较大的Si−O键大多转化为高聚合态结构；经500 oC以上碱熔活化后，结晶态的SiO4四面体可以解聚为二聚体硅酸盐(Na2SiO3/Na2Si2O7) 和原硅酸盐(Na4SiO4)，AlO6/AlO4结构解聚为NaAlO2/Na2Al2O4；在远低于热活化和碱熔活化的温度 (约250 oC) 下，亚熔盐(SMS)活化可以完全破坏矿物的Si-O键和Al-O键，将其中六配位的铝转化为高活性的四配位铝，将高聚合态硅转化为原硅酸钠Na4SiO4；(2) 分别以累托土原土以及上述三种活化方法所得活化累托土为原料合成ZSM-5分子筛。结果表明，以原土和热活化土为原料合成的ZSM-5分子筛表面光滑，晶粒尺寸较大，为2~4 μm，产物中含有大量的累托土残骸；以碱熔或亚熔盐活化土为原料合成的ZSM-5是由小晶粒堆积而成的椭球状晶体，表面粗糙，晶粒大小均一，约为1~2 μm，且具有一定的介孔; (3) 以热活化累托土和水玻璃为原料，合成出富含晶间介孔的ZSM-5分子筛，其晶粒尺寸3~4 μm，晶体由棒状纳米晶组装而成，纳米晶间的缝隙形成了分子筛的介孔结构；以亚熔盐活化累托土和热活化硅藻土为原料，采用纳米晶解聚-自组装技术，合成出ZSM-5，其相对结晶度为92%。该材料也是由片层晶体堆积而成的具有梯级孔的分子筛，同时存在晶间介孔和晶内介孔，介孔孔径集中在6.5 nm处；(4) 以全天然硅铝矿物为原料合成Y分子筛，进行了10 L/批次和30 L/批次的放大制备。结果表明，所制备的Y型分子筛的相对结晶度平均达到87%以上，晶粒规整，粒径大小集中在400 ~500 nm，SiO2/Al2O3摩尔比达到5.2；(5)经亚熔盐活化后，累托土、高岭土等天然硅铝矿物中的杂质Fe和Ti均可被完全活化，在后续的分子筛合成中有可能取代Si或Al而进入分子筛骨架。