在无有机导向剂条件下采用乙醇为助剂合成高硅沸石

高硅沸石具有优异的热稳定性、水热稳定性、大的微孔体积、高表面积和均匀的微孔孔道,因而广泛应用于催化领域.然而,高硅沸石的合成往往需要使用有机结构导向剂,不但增加了沸石合成成本,而且还产生了大量的三废排放.为了解决这个问题,我们发展了在无有机导向剂存在条件下采用沸石晶种诱导合成沸石的方法,但是该方法合成的沸石产物骨架富铝,不能合成高硅沸石,实现绿色方法合成工业上大量使用的高硅沸石问题仍然没有解决.最近,我们又报道了采用沸石晶种导向和醇填充相结合的方法合成纯硅沸石,但仍不能合成高硅沸石.本文首次在无有机结构导向剂存在条件下采用乙醇为助剂合成高硅沸石,并成功地合成了硅铝比(Si/Al)为38?240的ZSM-5沸石.此方法成功的关键是使用具有四配位铝物种的硅铝酸盐前驱体作为起始原料.因为乙醇的沸点低,溶液中的乙醇可以循环使用,大幅度地降低了传统高硅沸石合成有机模板的三废排放成本.通过固体核磁证明,在沸石的晶化过程中,硅物种发生重排与聚合,而铝物种一直保持着四配位的状态不变,这表明Si-O-Al类的连接在晶化过程中基本上保持稳定.另外,采用该方法合成了高硅TON,MTT,*MRE沸石,表明了此方法的普适性.此外,还可以将Fe和B等杂原子引入到沸石骨架中,成功地合成了铁硅和硼硅沸石.X射线粉末衍射测试与扫描电子显微镜表征结果表明,所合成的高硅ZSM-5沸石具有高纯度和结晶度,样品呈现出典型的块状ZSM-5晶体形貌.样品的BET表面积和微孔体积分别为382 m²/g和0.16 cm³/g,这与传统方法以TPAOH为有机结构导向剂合成的ZSM-5沸石的BET表面积和微孔体积是几乎一致的.更重要的是,所制备的ZSM-5沸石在甲醇制丙烯反应中,具有良好的催化性能,并与使用有机结构导向剂合成的ZSM-5沸石具有相似的活性、选择性和催化剂寿命.综上所述,在沸石晶种和乙醇存在的条件下,以硅铝酸盐前驱体为起始原料,在无有机结构导向剂存在条件下,成功地合成出了高硅MFI,TON,MTT和*MRE沸石.同时,也可利用硼硅酸盐和铁硅酸盐为起始原料来合成B-ZSM-5和Fe-ZSM-5沸石.与传统的合成高硅沸石路线相比,此方法避免了昂贵和有毒的有机结构导向剂的使用、减少了废水的排放、简化了合成过程和提高了产品收率;同时所合成沸石具有优异的催化性能,为高硅沸石的产业化生产和催化应用奠定基础.

Alcohol-assisted synthesis of high-silica zeolites in the absence of organic structure-directing agents

In this work, we show for the first time that high-silica zeolites (MFI, TON, MTT, and *MRE) could be synthesized from a combined strategy of both zeolite seeding and alcohol filling in the absence of organic structure-directing agents (OSDAs). High-silica ZSM-5 zeolites with Si/Al ratios ranging from 38 to 240 (TF-Al-ZSM-5) could be synthesized via this route. The key to the success of this technique was the employment of an aluminosilicate precursor with a fully 4-coordinated alumi-num species as the initial source, wherein the rearrangement and condensation of the silicate spe-cies, rather than the aluminate species, occurred during zeolite crystallization. In addition, heteroa-toms, such as Fe and B, could be incorporated into the zeolite frameworks. Catalytic tests for the methanol-to-propylene (MTP) reaction exhibited good catalytic performance for TF-Al-ZSM-5, which was comparable to that of the aluminosilicate ZSM-5 zeolite synthesized with OSDAs. Hence, this method offers viable opportunities for the industrial production and catalytic application of high-silica zeolites in the future.