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Reaction of Glucose Catalyzed by Framework and Extraframework Tin Sites in Zeolite Beta.

机译：β型沸石骨架和骨架外锡位点催化的葡萄糖反应。

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摘要

The isomerization of glucose into fructose is a large-scale reaction for the production of high-fructose corn syrup, and is now being considered as an intermediate step in the possible route of biomass conversion into fuels and chemicals. Recently, it has been shown that a hydrophobic, large pore, silica molecular sieve having the zeolite beta structure and containing framework Sn4+ (Sn-Beta) is able to isomerize glucose into fructose in aqueous media. Here, I have investigated how this catalyst converts glucose to fructose and show that it is analogous to that achieved with metalloenzymes. Specifically, glucose partitions into the molecular sieve in the pyranose form, ring opens to the acyclic form in the presence of the Lewis acid center (framework Sn4+), isomerizes into the acyclic form of fructose and finally ring closes to yield the furanose product. Akin to the metalloenzyme, the isomerization step proceeds by intramolecular hydride transfer from C2 to C1. Extraframework tin oxides located within hydrophobic channels of the molecular sieve that exclude liquid water can also isomerize glucose to fructose in aqueous media, but do so through a base-catalyzed proton abstraction mechanism. Extraframework tin oxide particles located at the external surface of the molecular sieve crystals or on amorphous silica supports are not active in aqueous media but are able to perform the isomerization in methanol by a base-catalyzed proton abstraction mechanism. Post-synthetic exchange of Na+ with Sn-Beta alters the glucose reaction pathway from the 1,2 intramolecular hydrogen shift (isomerization) to produce fructose towards the 1,2 intramolecular carbon shift (epimerization) that forms mannose. Na+ remains exchanged onto silanol groups during reaction in methanol solvent, leading to a near complete shift in selectivity towards glucose epimerization to mannose. In contrast, decationation occurs during reaction in aqueous solutions and gradually increases the reaction selectivity to isomerization at the expense of epimerization. Decationation and concomitant changes in selectivity can be eliminated by addition of NaCl to the aqueous reaction solution. Thus, framework tin sites with a proximal silanol group are the active sites for the 1, 2 intramolecular hydride shift in the isomerization of glucose to fructose, while these sites with Na-exchanged silanol group are the active sites for the 1, 2 intramolecular carbon shift in epimerization of glucose to mannose.
机译：葡萄糖异构化为果糖是生产高果糖玉米糖浆的大规模反应，目前被认为是生物质转化为燃料和化学品的可能途径的中间步骤。最近，已经显示出具有沸石β结构并且含有骨架Sn4 +（Sn-β）的疏水性大孔二氧化硅分子筛能够在水性介质中将葡萄糖异构化为果糖。在这里，我研究了这种催化剂如何将葡萄糖转化为果糖，并表明它与金属酶的作用相似。具体而言，葡萄糖以吡喃糖形式分配到分子筛中，在路易斯酸中心（骨架Sn4 +）存在下，环打开为无环形式，异构化为果糖的无环形式，最后环闭合以生成呋喃糖产品。类似于金属酶，异构化步骤通过从C2到C1的分子内氢化物转移进行。位于分子筛疏水通道内的排除液态水的骨架外氧化锡也可将葡萄糖在水性介质中异构化为果糖，但通过碱催化的质子提取机理实现。位于分子筛晶体外表面或无定形二氧化硅载体上的骨架外氧化锡颗粒在水性介质中不具有活性，但能够通过碱催化的质子提取机理在甲醇中进行异构化。 Na +与Sn-Beta的合成后交换将葡萄糖反应路径从1,2分子内氢转移（异构化）改变为生成果糖，向1,2分子内碳转移（异构化）形成甘露糖。在甲醇溶剂中的反应过程中，Na +仍然交换到硅烷醇基团上，从而导致葡萄糖向差向甘露糖的异构化几乎完全转变。相反，在水溶液中反应期间发生阳离子化，并以差向异构化为代价逐渐提高了对异构化反应的选择性。通过在反应水溶液中添加氯化钠，可以消除选择性的降低和随之而来的变化。因此，具有近端硅烷醇基团的骨架锡位点是葡萄糖异构化为果糖时1、2分子内氢化物转移的活性位点，而这些具有Na交换硅烷醇基团的位点是1、2分子内碳原子的活性位点葡萄糖差向异构化转变为甘露糖。

著录项

  • 作者

    Bermejo-Deval, Ricardo.;

  • 作者单位

    California Institute of Technology.;

  • 授予单位 California Institute of Technology.;
  • 学科 Chemical engineering.;Inorganic chemistry.;Organic chemistry.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 218 p.
  • 总页数 218
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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