Synthesis and catalytic testing of Lewis acidic nano-MFI zeolites for the epoxide ring opening reaction with alcohol
Synthesis and catalytic testing of Lewis acidic nano-MFI zeolites for the epoxide ring opening reaction with alcohol
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DOI：
10.1016/j.apcata.2018.08.018
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
Aamena Parulkar;Rutuja Joshi;N. Deshpande;N. Brunelli
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文献类型：
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作者：
Aamena Parulkar;Rutuja Joshi;N. Deshpande;N. Brunelli
沸石具有明确的孔，可以赋予形状选择性，但也可以导致扩散限制，降低催化性能。虽然已经研究了几种策略来克服布朗斯台德酸性沸石的扩散限制，但是有限的研究已经研究了改性刘易斯酸性沸石的材料性质。在这项工作中，通过低温合成含有刘易斯酸性锡位点的纳米沸石，克服了催化反应的扩散限制。使用动态光散射监测锡取代的沸石的生长，表明在沸石合成凝胶中以200：1（Si：Sn）包含锡作为杂原子不影响沸石生长速率。虽然对于纯二氧化硅和锡取代的MFI观察到类似的生长速率，但低温合成方法导致低的锡掺入，这可以通过向合成凝胶中添加更多的锡前体来克服，同时仍然产生纳米颗粒。纳米尺寸的沸石被证明可以克服常规Sn取代的沸石MFI在与醇的环氧开环反应中遇到的表观扩散限制。事实上，常规Sn-MFI材料可以催化1，2-环氧己烷与甲醇的开环，并且在24小时后仅实现55%的转化率;纳米Sn-MFI沸石在相同的时间内实现>95%的转化率。催化剂重复使用实验表明，纳米沸石不会浸出锡，但有机物在孔隙中的积累导致表观催化活性的损失。可以通过煅烧除去有机物，从而能够恢复催化活性。总的来说，具有减小的晶体尺寸的沸石的工程化提供了克服大体积分子的扩散限制的有效途径。
Zeolites have well-defined pores that can impart shape selectivity, but can also cause diffusion limitations, reducing catalytic performance. While several strategies have been investigated to overcome diffusion limitations for Brønsted acidic zeolites, limited research has investigated modifying material properties for Lewis acidic zeolites. In this work, diffusion limitations for catalytic reactions are overcome through the low temperature synthesis of nanozeolites containing Lewis acidic tin sites. The growth of the tin-substituted zeolites is monitored using dynamic light scattering, showing that including tin as a heteroatom at a 200:1 (Si:Sn) in the zeolite synthesis gel does not impact the zeolite growth rate. While similar growth rates are observed for pure silica and tin-substituted MFI, the low temperature synthesis methods result in low tin incorporation that can be overcome through adding more tin precursor to the synthesis gel while still producing nanoparticles. The nano-sized zeolites are demonstrated to overcome apparent diffusion limitations that conventional Sn-substituted zeolite MFI encounter for the epoxide ring opening reaction with alcohol. Indeed, conventional Sn-MFI materials can catalyze the ring opening of 1,2-epoxyhexane with methanol and achieve only 55% conversion after 24 h; the nano Sn-MFI zeolites achieve >95% conversion in the same amount of time. Catalyst reuse experiments demonstrate that the nano zeolite does not leach Sn, but that organic accumulation in the pores causes loss in apparent catalytic activity. The organic can be removed through calcination, enabling recovery of the catalytic activity. Overall, engineering of zeolites with reduced crystal size provides an efficient route to overcome diffusion limitations of bulky molecules.