中孔Beta沸石的孔结构、酸性及其萘的苄基化催化性能研究

发布时间：2018-06-15 20:21

本文选题：中孔Beta沸石催化剂 + 可接近性　；参考：《太原理工大学》2017年硕士论文

【摘要】：Beta沸石是一类具有三维孔道结构的沸石分子筛,较大的孔径和较强的酸性使其在芳烃苄基化反应中表现出较高的催化活性。但是在涉及更大分子的催化反应中,0.7nm的孔径仍然不能满足大分子反应的需求。大量的研究表明,多级孔沸石催化剂在保持微孔沸石固有的酸性的基础上,多级孔沸石中二级孔的引入促进了反应物分子对酸性位的接近性,扩大了沸石催化剂的反应空间,缩短了分子在孔道内的扩散路径,提高了催化剂的利用率,并且产物分子能够及时扩散出孔道,避免了二次反应的发生,从而有效提高了催化剂的催化性能。多级孔Beta沸石分子筛同样是一类同时包含有相互交联的微孔和中孔的沸石催化材料,其固有的强酸性为催化反应提供活性位,而发达的中孔体系有助于酸性位的接近和分子在其内的扩散,从而有助于催化反应特别是涉及大分子的反应的进行。本论文采用中孔Beta沸石作为研究对象,通过X射线衍射、N2吸附脱附、扫描电镜对其孔径及形貌进行表征。研究结果表明,中孔Beta沸石样品的结晶度下降,孔径主要分布在3-10nm,并且晶体颗粒较小。然而,中孔的引入对于沸石分子筛的孔结构和表面酸性有很大的影响。尤其是表面酸的类型、酸的强度、酸的浓度以及酸位点的分布。本论文通过NH3-TPD技术对其总酸量和酸强度进行表征,研究结果表明,随着介孔量的增大,酸中心均向低温区移动,并且强酸量明显下降。其中MHB4样品的ATH/ATL值下降到了1.76,是微孔样品HB的1/2。此外采用吡啶原位红外技术对中孔Beta沸石表面不同酸类型的酸强度和酸浓度进行表征,采用不同的大尺寸探针分子2,6-二叔丁基吡啶和三甲基乙腈有机碱对其酸性位的分布进行表征。研究结果表明,随着外表面积的增大,其酸强度依次减弱,Br?nsted酸和Lewis酸的可接近性依次增大。为了探究沸石酸性和孔结构的改变对其芳烃苄基化反应的催化活性的影响,以大分子萘和苄基氯的苄基化反应对其催化活性进行评价。研究结果显示,微孔沸石催化剂HB虽然具有较高的初始转化率,但是随着反应的进行逐渐失活。但是中孔沸石样品MHBn和GHBn都表现出了较高的催化活性。并且随着外表面积的增大,催化活性逐渐增大。另外,对于不同的中孔沸石样品,较弱的酸性有利于反应的进行。在Br?nsted酸和Lewis酸的可接近性都较高的催化剂中,表现出较好的催化活性。说明Br?nsted酸和Lewis酸协同促进芳烃苄基化的进行。
[Abstract]:Beta zeolite is a kind of zeolite molecular sieve with three dimensional pore structure. Because of its large pore size and strong acidity, Beta zeolite exhibits high catalytic activity in benzylation of aromatics. However, in the catalytic reactions involving larger molecules, the pore size of 0.7 nm still can not meet the demand of macromolecular reactions. A large number of studies have shown that on the basis of keeping the inherent acidity of microporous zeolite, the introduction of secondary pore in multi-porous zeolite promotes the proximity of reactant molecules to acidic sites and expands the reaction space of zeolite catalyst. The diffusion path of the molecule in the pore channel is shortened, the utilization rate of the catalyst is improved, and the product molecule can diffuse out the pore channel in time, thus avoiding the occurrence of the secondary reaction, thus effectively improving the catalytic performance of the catalyst. Multiporous Beta zeolite zeolites are also a kind of zeolites with cross-linking micropores and mesoporous materials. Their intrinsic strong acidity provides active sites for catalytic reactions. The developed mesoporous system is conducive to the proximity of acidic sites and the diffusion of molecules in it, thus facilitating the catalytic reactions, especially those involving macromolecules. In this paper, mesoporous Beta zeolite was used as the object of study. The pore size and morphology of mesoporous Beta zeolite were characterized by scanning electron microscope (SEM). The results show that the crystallinity of mesoporous Beta zeolite samples decreases, the pore size is mainly distributed from 3 to 10 nm, and the crystal particles are smaller. However, the introduction of mesopore has a great influence on the pore structure and surface acidity of zeolites. In particular, the types of surface acids, the strength of acids, the concentration of acids, and the distribution of acid sites. In this paper, the total acid content and acid strength were characterized by NH _ 3-TPD technique. The results show that with the increase of mesoporous volume, the acid center moves to the low temperature region, and the strong acid content decreases obviously. The ATH / ATL value of the MHB4 sample decreased to 1.76, which is 1 / 2 of the HB of the microporous sample. In addition, the acid strength and acid concentration of different acid types on mesoporous Beta zeolite surface were characterized by pyridine in situ infrared spectroscopy. The distribution of acid sites was characterized by using different large size probe molecules 2H 6 di tert Ding Ji pyridine and trimethyl acetonitrile organic base. The results show that with the increase of the surface area, the acid strength decreases in turn, and the accessibility of Brnsted acid and Lewis acid increase in turn. In order to investigate the effect of acidity and pore structure of zeolite on the catalytic activity of benzylation of aromatic hydrocarbons, the catalytic activity of benzylation of macromolecular naphthalene and benzyl chloride was evaluated. The results showed that the initial conversion of the catalyst HB was higher, but gradually deactivated with the reaction. However, both MHBn and GHBn of mesoporous zeolite samples showed high catalytic activity. And with the increase of the surface area, the catalytic activity increases gradually. In addition, for different mesoporous zeolite samples, the weak acidity is favorable for the reaction. The catalyst with higher accessibility to Brnsted acid and Lewis acid showed better catalytic activity. The results showed that Brnsted acid and Lewis acid promoted the benzylation of aromatics.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.36

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