Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

Abstract

Metal substitution of molecular sieve systems is a major driving force in developing novel catalytic processes to meet current demands of green chemistry concepts and to achieve sustainability in the chemical industry and in other aspects of our everyday life. The advantages of metal-substituted molecular sieves include high surface areas, molecular sieving effects, confinement effects, and active site and morphology variability and stability. The present review aims to comprehensively and critically assess recent advances in the area of tetra- (Ti, Sn, Zr, Hf) and pentavalent (V, Nb, Ta) metal-substituted molecular sieves, which are mainly characterized for their Lewis acidic active sites. Metal oxide molecular sieve materials with properties similar to those of zeolites and siliceous molecular sieve systems are also discussed, in addition to relevant studies on metal–organic frameworks (MOFs) and some composite MOF systems. In particular, this review focuses on (i) synthesis aspects determining active site accessibility and local environment; (ii) advances in active site characterization and, importantly, quantification; (iii) selective redox and isomerization reaction applications; and (iv) photoelectrocatalytic applications.

Experimental and Theoretical Evidence for Distorted Tetrahedral ≡Ti–OH Sites Supported on Amorphous Silica and Their Effect on the Adsorption of Polar Molecules

“…We note that extinction coefficients for pyridine adsorbed to Lewis acid metal-supported silicates fall in the range of 0.7−1.5 cm/μmol. 51 We also note that our experimental setup, i.e., the brand of spectrometer, detector type, pellet size, and mass were similar to those used for the experiment reported in ref 72. This further supports the applicability of the extinction coefficients reported by the authors of this work to our experiments.…”

Electron Transfer Reduction by Hydrogen Creates Porosity in Tantalate Crystals and Produce Multifunctionality

“…Among these materials, WO 3 stands out as a unique MS owing to its empty d orbitals and absence of template cations within its cavities, facilitating direct utilization of its framework structure for photocatalysis (water splitting or half-reactions). This is in contrast to d 0 metal ion-incorporated MS . However, WO 3 has a conduction band minimum that makes it thermodynamically challenging to reduce protons .…”

“…Among these materials, WO 3 7 stands out as a unique MS owing to its empty d orbitals and absence of template cations within its cavities, facilitating direct utilization of its framework structure for photocatalysis (water splitting or half-reactions). This is in contrast to d 0 metal ion-incorporated MS. 8 However, WO 3 has a conduction band minimum that makes it thermodynamically challenging to reduce protons. 9 Therefore, there is a need for alternative elements such as Ta and Nb, which exhibit proton reduction capabilities, as framework atoms in MS materials to advance material development in the field of photocatalysis.…”

Toward a Better Understanding of Intrinsic Activity Trends in Zr-Based Heterogeneous Catalysts for Meerwein–Pondorf–Verley Reduction of Levulinate Esters

“…Dumesic and co-workers were the first to report the successful implementation of MPV chemistry for both batch and continuous production of GVL from levulinates, where ZrO 2 was identified as the best catalyst among numerous metal oxide catalysts . This pioneering work has stimulated a decade of intensive research on the design and synthesis of “novel” heterogeneous catalysts for the MPV reduction of LA and its esters under mild conditions, among which numerous Zr­(Hf)-based materials have emerged as efficient catalysts for this important class of biomass transformations. − The Zr­(Hf)-based heterogeneous catalysts hitherto reported for the MPV reductions of bioderived carbonyl compounds can be broadly classified into purely inorganic (hydroxides, oxides, and zeolites ,,− ) and crystalline/noncrystalline inorganic–organic hybrid catalysts (e.g., metal–organic-frameworks (MOFs) and Zr-phytates, among others ,,,,− ). Despite the wide spectrum of MPV catalysts developed over the past decade, a systematic comparison and in-depth understanding of their catalytic performances is generally lacking, stymying more focused efforts to analyze, design, and optimize truly promising systems for the target reactions.…”