Different Ti-Si catalysts, viz. TiO₂ supported on amorphous SiO₂ or Si-MCM-41, TiO₂-SiO₂ xerogels, and Ti zeolites (TS-1 and Ti-beta), were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88–92% at conversions below 50%. From DR UV–vis spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO₄ sites is higher than those of polymerized TiO₅ or the octahedral TiO₆ counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO₄ sites may be attributed to an increased number of vacant sites (e.g., two for TiO₄, zero for TiO₆). Lactide productivity thus highly benefits from an increased dispersion of Ti sites on the catalyst surface and could be increased by a factor of 2.5 (up to 10 g_LD g_cat^–1 h^–1) when TiO₂ is dispersed on a Si-MCM-41 support, with higher surface areas in comparison to amorphous SiO₂ gels.

中文翻译：

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从可再生的烷基乳酸酯生产丙交酯的二氧化钛-二氧化硅催化剂：结构-活性关系

不同的Ti-Si催化剂，即。的TiO ₂支撑在无定形的SiO ₂或Si-MCM-41，二氧化钛₂ -SiO _2个干凝胶，和Ti沸石（TS-1和Ti-β），在乳酸甲酯于直接转化活性和选择性方面进行了比较丙交酯在气相中。除Ti-β外，所有催化剂在低于50％的转化率下均表现出88-92％的高丙交酯选择性。从DR紫外可见光谱法可以看出，四面体TiO ₄位点的催化活性高于聚合的TiO ₅或八面体TiO _{6的位点。}不论催化剂的结构如何，ToF-SIMS测量均支持这种分析。动力学分析表明，催化活性与催化剂表面上空位的数量成正比。因此，对于四面体TiO ₄位点观察到的活性增加可以归因于空位的数目增加（例如，对于TiO ₄为两个，对于TiO ₆为零）。因此，丙交酯的生产率得益于催化剂表面上Ti位置增加的分散性，当TiO ₂时，其生产率可以提高2.5倍（最多10 g _LD g _cat ^–1 h ^–1）。将其分散在Si-MCM-41载体上，与无定形SiO ₂凝胶相比表面积更大。