Abstract
Porous silicates containing dilute amounts of tri-, tetra- and penta-valent metal sites, such as TS-1, Sn-β and Fe-ZSM-5, have recently emerged as state of the art catalysts for a variety of sustainable chemical transformations. In contrast with their aluminosilicate cousins, which are widely employed throughout the refinery industry for gas-phase catalytic transformations, such metallosilicates have exhibited unprecedented levels of performance for a variety of liquid-phase catalytic processes, including the conversion of biomass to chemicals, and sustainable oxidation technologies with H2O2. However, despite their unique levels of performance for these new types of chemical transformations, increased utilization of these promising materials is complicated by several factors. For example, their utilization in a liquid, and often polar, medium hinders process intensification (scale-up, catalyst deactivation). Moreover, such materials do not generally exhibit the active-site homogeneity of conventional aluminosilicates, and they typically possess a wide variety of active-site ensembles, only some of which may be directly involved in the catalytic chemistry of interest. Consequently, mechanistic understanding of these catalysts remains relatively low, and competitive reactions are commonly observed. Accordingly, unified approaches towards developing more active, selective and stable porous metallosilicates have not yet been achieved. Drawing on some of the most recent literature in the field, the purpose of this mini review is both to highlight the breakthroughs made with regard to the use of porous metallosilicates as heterogeneous catalysts for liquid-phase processing, and to highlight the pertaining challenges that we, and others, aim to overcome during the forthcoming years.
Highlights
Porous silicates containing dilute amounts of tri, tetraand penta-valent metal sites, such as titanium silicalite-1 (TS-1), Sn-β and FeZSM-5, have recently emerged as state of the art catalysts for a variety of sustainable chemical transformations
We have demonstrated that the materials produced by solid-state incorporation (SSI) are as active per mole of active site as their conventionally prepared counterparts for the isomerization of glucose to fructose, Baeyer–Villiger oxidation (BVO) and catalytic transfer hydrogenation (CTH), among others
Therein, we demonstrated that post-synthetic treatment of TS-1 with NH4HF2 and H2O2, following studies of Balducci et al [92] resulted in the formation of a catalyst that was as active for catalytic epoxidation, but dramatically more efficient with respect to the oxidant, H2O2
Summary
Metallosilicates, more commonly known as zeolites, are microporous, crystalline silicate materials composed of corner-sharing TO4 tetrahedra. When the cation is a proton (H+), the material displays high levels of Brønsted acidity, with acid strength comparable to 100% H2SO4 (figure 2) [4]. In addition to Brønsted acidity, conventional aluminosilicates can display Lewis acidity through ‘dealumination’ of the framework In this instance, high-temperature treatments, occasionally in steam or other non-inert chemical atmospheres, dislodge Al3+ from the framework, leading to the formation of ‘extra-framework’ Lewis acid sites, within the pores of the zeolite. It should be noted that the generation of Lewis acid sites through dealumination corresponds to the loss of the cation-exchange sites of the zeolite, and diminishes its Brønsted acidity It typically results in a material possessing both Lewis and Brønsted acidity, because total dislodgement of Al3+ from the framework is highly challenging, and some residual framework Al3+ typically remain. Such materials are rarely uniform, and often display disparate levels of activity, selectivity and/or stability during catalysis
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