Abstract

Three nanosized ZSM-5 zeolites were successfully prepared from reactive gels with the same Si/Al ratios by different synthetic procedures that included the use of tetrapropylammonium hydroxide or n-butylamine as a template and a seeding method that did not use an organic additive. The effect of the synthetic method on the physicochemical properties of the prepared samples was investigated by XRD, XRF, XPS, N₂ physisorption, SEM, TEM, ²⁷Al MAS NMR, NH₃-TPD, and Py-FTIR. The catalytic performance of the nanosized ZSM-5 zeolites in the alkylation of naphthalene with methanol was compared. The prepared samples were phase-pure, highly crystalline ZSM-5 zeolites, but they had different bulk and surface Si/Al ratios as well as textural and acidic properties. The study of the prepared catalysts in naphthalene methylation revealed that both the acid characteristics of the ZSM-5 nanosized zeolites and their textural properties were responsible for their activity in the reaction. A difference in the composition of monomethylnaphthalenes and dimethylnaphthalenes was attributed to the ability of the catalyst to isomerize the primary reaction products on acid sites located on the external surface of the zeolite crystals. 2,7-DMN was found to be the preferred reaction product over 2,6-DMN when formed at pore entrances to ZSM-5 channels due to the differences in their dimensions. In contrast, 2,6-dimethylnaphthalene could be produced on weaker external Brønsted acid sites, which are hydroxyls attached to octahedral Al atoms. The presented results show that the method used to synthesize nanoscale ZSM-5 zeolites is a critical factor that determines the physicochemical properties and catalytic performance of the resulting crystals.

中文翻译：

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不同方法合成纳米ZSM-5分子筛及其在萘烷基化反应中的催化性能

摘要

通过不同的合成程序，由具有相同Si / Al比的反应性凝胶成功制备了三种纳米级ZSM-5沸石，包括使用氢氧化四丙基铵或正丁胺作为模板以及不使用有机添加剂的接种方法。通过XRD，XRF，XPS，N ₂物理吸附，SEM，TEM，²⁷ Al MAS NMR，NH ₃ -TPD和Py研究了合成方法对所制备样品的理化性质的影响。-FTIR。比较了纳米ZSM-5沸石在萘与甲醇的烷基化反应中的催化性能。制备的样品是纯相，高度结晶的ZSM-5沸石，但它们具有不同的体积和表面Si / Al比以及质构和酸性。对所制备的萘甲基化催化剂的研究表明，ZSM-5纳米沸石的酸特性和其织构性质均决定了它们在反应中的活性。单甲基萘和二甲基萘组成的差异归因于催化剂使位于沸石晶体外表面的酸性位点上的主要反应产物异构化的能力。发现2,7-DMN是比2更优选的反应产物 由于ZDM-5通道的尺寸不同，在其孔口处形成6-DMN。相反，可以在较弱的外部布朗斯台德酸位上生成2，6-二甲基萘，这些酸是与八面体Al原子相连的羟基。提出的结果表明，用于合成纳米级ZSM-5沸石的方法是决定所得晶体的物理化学性质和催化性能的关键因素。