Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity
Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity
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DOI：
10.1021/acscatal.7b01273
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发表时间：
2017-10-01
期刊：
影响因子：
12.9
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
--
作者：
Di Iorio, John R.;Nimlos, Claire T.;Gounder, Rajamani
我们报告了一个合成结构函数关系描述如何不同的路线结晶单四面体网站（T-网站）沸石的固定组合物导致不同的安排框架铝原子，反过来，框架外质子活性位点合奏显着影响营业额率的布朗斯台德酸催化反应。具体地，报道了导致铝原子（Al-O（SiO）O）（x）-Al）在有效固定组成（Si/Al = 14-17）的菱沸石（CHA）沸石骨架内以孤立（x > 2）和成对（x = 1，2）构型排列的系统变化的合成路线。含有不同结构导向剂和铝源的前体溶液使CHA沸石结晶，其中每个CHA笼封闭一个有机N，N，N-三甲基-1-金刚烷基铵阳离子，并且封闭的Na+阳离子的量随成对骨架Al含量（0-44%）线性增加。氨和二价钴离子滴定分别用于量化总的和成对的布朗斯台德酸位点，并将甲醇脱水至二甲基醚的速率归一化。一级和零级甲醇脱水速率常数（每H+，415 K）随着CHA沸石中成对质子的分数而系统地增加，并且在成对质子处类似于高10倍。这种行为反映了更快的解离（表面甲氧基介导的）途径，在成对的质子比较慢的关联（甲醇二聚体介导的）途径在孤立的质子，在原位红外光谱一致。这些发现表明，固定元素组成的沸石，即使当结晶框架含有一个独特的T-位点，可以表现出催化多样性时，通过不同的合成路线，影响其原子排列制备。
We report a synthesis structure function relation describing how different routes to crystallize single tetrahedral-site (T-site) zeolites of fixed composition lead to different arrangements of framework Al atoms and, in turn, of extraframework proton active site ensembles that markedly influence turnover rates of a Bronsted acid-catalyzed reaction. Specifically, synthetic routes are reported that result in systematic changes in the arrangement of aluminum atoms (Al-O(SiO) O)(x)-Al) in isolated (x > 2) and paired (x = 1, 2) configurations within chabazite (CHA) zeolite frameworks of effectively fixed composition (Si/Al = 14-17). Precursor solutions containing different structure-directing agents and aluminum sources crystallize CHA zeolites with one organic N,N,N-trimethyl-1-adamantylammonium cation occluded per CHA cage, and with amounts of occluded Na+ cations that increase linearly with paired framework Al content (0-44%). Ammonia and divalent cobalt ion titrations are used to quantify total and paired Bronsted acid sites, respectively, and normalize rates of methanol dehydration to dirnethyl ether. First order and zero-order methanol dehydration rate constants (per H+, 415 K) systematically increase with the fraction of paired protons in CHA zeolites and are similar to 10x higher at paired protons. Such behavior reflects faster dissociative (surface methoxymediated) pathways that prevail at paired protons over slower associative (methanol dimer-mediated) pathways at isolated protons, consistent with in situ infrared spectra. These findings demonstrate that zeolites of fixed elemental composition, even when crystalline frameworks contain one unique T-site, can exhibit catalytic diversity when prepared via different synthetic routes that influence their atomic arrangements.