分子筛中限制效应对其酸性表征及催化性能的影响

Influence of zeolite confinement effect on its acidic characterization and catalytic performance

分子筛作为酸催化反应的重要载体,具有酸性可调控、热稳定性强及形状选择性的优点,但特殊刚性孔道结构及内部电荷分布使其具有限制效应会对分子筛的酸性表征及催化反应产生影响。由于分子筛中起催化作用的主要是Brønsted酸位点,因此,本文介绍了Brønsted酸位点及限制效应的形成机理,简述了限制效应对Brønsted酸位点酸强度及酸密度表征的影响,分析了限制效应中的空间约束及局部电场对催化反应的影响。指出在酸性表征中,空间约束限制探针分子对酸位点的可接近性,进一步会影响酸密度的测量。局部电场由于会影响探针分子的吸附与解吸,进而直接影响到酸强度。所以,在分子筛的酸性表征时应该选择与反应物尺寸相近及结构相似的探针分子,才能测量可接近Brønsted酸位点的酸密度与酸强度。在催化主导的热化学反应中,空间约束使分子筛具有形状选择性,通过控制分子筛孔径大小能对热化学反应的反应过程、中间产物过渡态及最终产物分布进行选择。同时,由于局部电场影响表观酸强度,分子筛催化性能与表观酸强度有关。分子筛孔径越小,反应分子所受范德华相互作用越大,通过影响反应过渡态的形成进而改变反应活化能,从而影响催化热化学反应效果。综合分析已有工作表明只有制备一个酸强度适宜、可接近孔径尺寸与反应物分子相近的分子筛才是催化反应的理想酸性载体。

As an important carrier of acid-catalyzed thermal reaction,zeolite has the advantages of controllable acidity,strong thermal stability and shape selectivity,but its special rigid pore structure and internal charge distribution make it have a limiting effect,which will affect the zeolite’s acidic characterization and catalytic reaction performance.Since the catalytic role of zeolite is mainly the Brønsted acid site,the formation mechanism of the Brønsted acid site and the confinement effect is introduced,the influence of the restriction effect on the acid strength and acid density characterization of the Brønsted acid site is briefly described,and the influence of spatial constraint and local electric field on the catalytic reaction performance in the restriction effect is reviewed.It is pointed out that in the acidic characterization,spatial constraints limit the accessibility of probe molecules to acid sites,which further affects the measurement of acid density.The local electric field affects the adsorption and desorption of probe molecules,which in turn directly affects the acid intensity.Therefore,in the acidic characterization of zeolites,probe molecules with similar size and structure to reactants should be selected to measure the acid density and acid strength that can be acquired as the actual results close to the Brønsted acid site.In catalytically dominated thermochemical reactions,spatial constraints make zeolites shape-selective,and the reaction process,intermediate-transition-state product and final product distribution of thermochemical reactions can be selected by controlling the pore size of zeolite.At the same time,since the local electric field affects the apparent acid intensity,the catalytic performance of zeolite is related to the apparent acid intensity.The smaller the pore size of the zeolite,the greater the van der Waals interaction of the reaction molecules,which affects the formation of the transition state of the reaction and then changes the activation energy of the reaction,thereby affecting the catalytic thermochemical reaction efficacy.Comprehensive analysis shows that only a zeolite with appropriate acid strength and an accessible pore size similar to that of the reactant molecule is an ideal acid carrier for catalyzing the thermal reaction.