Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity,ACS Catalysis

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Introducing Catalytic Diversity into Single-Site Chabazite Zeolites of Fixed Composition via Synthetic Control of Active Site Proximity
ACS Catalysis ( IF 11.3 ) Pub Date : 2017-09-01 00:00:00 , DOI: 10.1021/acscatal.7b01273
John R. Di Iorio ₁ , Claire T. Nimlos ₁ , Rajamani Gounder ₁

Affiliation

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 Brønsted acid-catalyzed reaction. Specifically, synthetic routes are reported that result in systematic changes in the arrangement of aluminum atoms (Al–O(−Si-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 Brønsted acid sites, respectively, and normalize rates of methanol dehydration to dimethyl 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 ∼10× higher at paired protons. Such behavior reflects faster dissociative (surface methoxy-mediated) 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.

中文翻译：

通过对活性位点的合成控制，将催化多样性引入固定组成的单点菱沸石中

我们报告了一种合成-结构-功能关系，该关系描述了固定成分的单个四面体位点（T位点）沸石的不同结晶途径如何导致框架Al原子的不同排列，进而导致框架外质子活性位点集合的显着影响布朗斯台德酸催化反应的周转率。具体来说，据报道，合成途径导致菱沸石内铝原子（Al-O（-Si-O）_x -Al）的隔离（x > 2）和成对（x = 1，2）构型的系统变化有效固定成分（Si / Al = 14-17）的（CHA）沸石骨架。包含不同结构导向剂和铝源的前体溶液可将CHA沸石与一种有机物结晶每个CHA笼中都吸附有N，N，N-三甲基-1-金刚烷铵阳离子，并且被吸附的Na ⁺阳离子的量随成对的骨架Al含量线性增加（0-44％）。氨水和二价钴离子滴定法分别用于定量总布朗斯台德酸位和成对的布朗斯台德酸位，并标准化甲醇脱水成二甲醚的速率。一阶和零阶甲醇脱水速率常数（每H ⁺（415 K）随CHA沸石中成对质子的比例而系统地增加，而在成对质子中则高出约10倍。这种行为反映了更快的离解（表面甲氧基介导的）途径，在成对的质子上胜过在分离的质子上的较慢的缔合（甲醇二聚体介导的）途径，这与原位红外光谱一致。这些发现表明，即使通过不同的合成途径（影响其原子排列）制备了固定元素组成的沸石，即使当晶体骨架包含一个独特的T位点时，也可以表现出催化多样性。

更新日期：2017-09-04

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