Bronsted-Bronsted Synergies between Framework and Noncrystalline Protons in Zeolite H-ZSM-5
Bronsted-Bronsted Synergies between Framework and Noncrystalline Protons in Zeolite H-ZSM-5
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DOI：
10.1021/acscatal.9b01583
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发表时间：
2019-07-01
期刊：
影响因子：
12.9
通讯作者：
White, Jeffery L.
中科院分区：
文献类型：
--
作者：
Chen, Kuizhi;Abdolrahmani, Maryam;White, Jeffery L.
沸石催化剂是固体布朗斯台德酸，其反应性通常与结晶骨架桥连酸位点（BAS）处的质子数相关。合成后的催化剂改性，滴定与不同大小的一价和二价阳离子，定量自旋计数光谱上的所有质子之前和之后的阳离子交换，胺滴定，和室温原位反应与两种不同的探针分子揭示，沸石HZSM-5的反应性强烈对应于存在的酸性质子从骨架外和/或非晶网站。值得注意的是，室温氢氘（H/D）之间的催化剂和有机探针分子的交换反应表明，反应速率强烈依赖于从框架外和非结晶质子网站的酸性质子的总浓度。在室温探针反应中最活跃的催化剂含有来自BAS和非结晶物质的质子，包括可以通过溶剂处理除去的反应性骨架外铝醇物质。为了证明成对的骨架/骨架外或非结晶布朗斯台德位点对整体催化剂活性的重要性，在用具有不同半径的一价和二价阳离子（Na+、Ca 2+、Cu 2+、Ba 2+）滴定、用六氟硅酸铵（AHFS）化学洗涤以及Si/Al等于15和40的HZSM-5催化剂的不同蒸汽处理程序后，对不同质子的形态进行定量。在Si/Al = 15催化剂中反应性布朗斯台德物种的详细操作使得能够在异丁烷的次甲基和甲基位置处直接实验观察H/D交换，这是迄今为止没有报道的，澄清了围绕该机制的不确定性。反应数据表明，孤立的框架BAS的整体催化剂的反应性比接近的框架/框架外或非结晶布朗斯台德网站的重要性小得多，DFT计算支持接近质子网站的重要性。潜在的布朗斯台德-布朗斯台德协同作用相对于先前提出的布朗斯台德/刘易斯协同作用是独特的，但不排除后者对增加反应性的贡献。
Zeolite catalysts are solid Bronsted acids whose reactivity is typically associated with the number of protons at crystalline framework bridging acid sites (BAS's). Postsynthetic catalyst modification, titrations with monovalent and divalent cations of varying size, quantitative spin-counting spectroscopy on all protons before and after cation exchange, amine titration, and room-temperature in situ reactions with two different probe molecules reveal that zeolite HZSM-5 reactivity strongly corresponds with the presence of acidic protons from extraframework and/or noncrystalline sites. Significantly, room-temperature hydrogen deuterium (H/D) exchange reactions between the catalyst and the organic probe molecules reveal that reaction rates are strongly dependent on the total concentration of acidic protons from extraframework and noncrystalline proton sites. The most active catalysts in room-temperature probe reactions contain protons from both BAS's and from noncrystalline species, including reactive extraframework aluminol species that can be removed by solvent treatments. In order to demonstrate the significance of paired framework/extraframework or noncrystalline Bronsted sites to overall catalyst activity, speciation of different protons were quantified after titration with mono- and divalent cations of varying radius (Na+, Ca2+, Cu2+, Ba2+), chemical washing with ammonium hexafluorosilicate (AHFS), and different steaming procedures for HZSM-5 catalysts with Si/Al equal to 15 and 40. Detailed manipulation of reactive Bronsted species in the Si/Al = 15 catalyst enabled direct experimental observation of H/D exchange at both the methine and methyl positions of isobutane, heretofore not reported, clarifying uncertainties surrounding that mechanism. Reaction data indicates that isolated framework BAS's are much less important to overall catalyst reactivity than proximate framework/extraframework or noncrystalline Bronsted sites, and DFT calculations support the importance of proximate proton sites. Potential Bronsted Bronsted synergies are unique relative to previously proposed Bronsted/Lewis synergies but do not preclude the latter's contribution to increased reactivity.