Fe原位改性小晶粒Silicalite-1分子筛催化醛氨缩合性能研究

Study on the catalytic performance of Fe in-situ modified small crystallite Silicalite-1 zeolite in Chichibabin condensation reaction

  • 摘要: 吡啶及其衍生物统称为吡啶碱,其广泛应用于农药、医药等领域。Chichibabin醛氨缩合反应是目前工业制取吡啶碱最广泛的路线。目前,使用最广泛的ZSM-5分子筛受制于硅铝骨架结构的不稳定性,高活性反应周期较短(5 h),针对这一问题,本研究选用热稳定性、水热稳定性优异的Silicalite-1分子筛,使用聚乙烯吡咯烷酮(PVP)作为胶体分散剂,在水热合成分子筛的过程中向骨架中引入Fe,结合XRD、SEM、TG、BET、NH3-TPD、Py-FTIR等表征方法探究了晶化条件对Silicalite-1分子筛结晶度、孔结构和酸性质的影响。实验结果表明,在晶种投入量15%、PVP添加量3.75%时产品相对结晶度达到最高(103%),粒径约为200 nm。改性后的Silicalite-1具有更丰富的酸位点,醛氨缩合反应的初始活性由66%增加至85%,在反应进行15 h后,原料转化率和吡啶碱收率分别保持在66%和40%以上。研究提出的原位改性Silicalite-1分子筛策略极大扩宽了纯硅沸石在酸催化领域的应用,具有显著的科研价值和工业化潜力。

     

    Abstract: Pyridine and its derivatives, collectively referred to as pyridine bases, are widely used in industries such as pesticides and pharmaceuticals, serving as crucial intermediates in the chemical industry. In recent years, with the development of the pesticide and pharmaceutical industries, the demand for pyridine bases has rapidly increased. The Chichibabin condensation reaction is the most commonly route for industrial production of pyridine bases. Currently, the most used ZSM-5 zeolite catalyst is limited by the instability of its silicon-aluminum framework structure, resulting in a short active reaction cycle (5 h). To address this limitation, this study selected the thermally stable and hydrothermally stable Silicalite-1 zeolite. Polyvinylpyrrolidone (PVP) was employed as a colloidal dispersant and Fe was introduced into the MFI framework through in situ modification during the hydrothermal synthesis of zeolite. The influence of PVP dosage, template agent dosage, and other crystallization conditions on the crystallinity, pore structure, and acidity of Silicalite-1 zeolite products was investigated using XRD, SEM, TG, and N2 adsorption-desorption measurement. The acidity of Fe-modified Silicalite-1 zeolites was characterized using NH3-TPD, Py-IR, FT-IR, and XPS. These results indicated that the introduction of seed crystals effectively reduced the particle size of the zeolite to about 200 nm. Fe-modified Silicalite-1 displayed a disk-like morphology with excellent crystal dispersion. The highest relative crystallinity of the zeolite reached 103% with 15% seed crystal input and 3.75% PVP addition. The Fe-modified Silicalite-1 possessed a significantly enhanced abundance of both Lewis (L) and Brønsted (B) acid sites, resulting in an increase in the initial activity from 66% to 85% for the pyridine bases synthesis through the Chichibabin condensation. Compared to ZSM-5, Fe-modified Silicalite-1 exhibited superior catalytic stability, maintaining the total carbon conversion and pyridine bases yield above 66% and 40%, respectively, over a 15 h reaction period. Furthermore, the strategy proposed in this study, employing polyvinylpyrrolidone as a colloidal stabilizer to modify Silicalite-1 zeolite, could significantly broadened the application prospects of weakly acidic pure silica zeolites in the field of acid catalysis. This approach has demonstrated significant scientific value and industrial potential.

     

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