Abstract:Metal-substituted beta-zeolites have proven to be effective catalysts for various important reactions involving the transformation of biomass-derived molecules. In this study, a combination of quantum mechanical calculations and integrated quantum mechanics−molecular mechanics along with a polarizable continuum model were used to determine the preferred substitution site of Sn, Ti, and Ge metals in zeolite beta (BEA) and in the polymorphism C of zeolite beta (BEC), as well as the Lewis acidity and the hydrothe… Show more
“…In order to generate the Lewis acid active site at M‐Beta zeolites, one silicon atom at a T9 site was replaced by one of the tetravalent metal atoms (Sn, Ge, Hf or Zr). Upon hydrolyzing, a (M−OH) metal site and a silanol (Si−OH) group are formed . The metal can act as Lewis acid, and the hydroxide (OH) group as Lewis base.…”
We investigate the tautomerization of phenol catalyzed by acid‐base active pair sites in Lewis acidic Beta zeolites by means of density functional calculations using the M06‐L functional. An analysis of the catalytic mechanism shows that hafnium on the Beta zeolite causes the strongest absorption of phenol compared to zirconium, tin, and germanium. This can be rationalized by the highest delocalization of electron density between the Lewis site and the oxygen of phenol which can in turn be quantified by the perturbative E(2) stabilization energy. The reaction is assumed to proceed in two steps, the phenol O−H bond dissociation and the protonation of the intermediate to form the cyclohexa‐2,4‐dien‐1‐one product. The rate determining step is the first one with a free activation energy of 26.3, 25.0, 22.1 and 22.7 kcal mol−1 for Ge‐Beta, Sn‐Beta, Zr‐Beta, and Hf‐Beta zeolites, respectively. The turnover frequencies follow these reaction barriers. Hence, the intrinsic catalytic activity of the Lewis acidic Beta zeolites studied here is in the order of Hf‐Beta≈Zr‐Beta>Sn‐Beta> Ge‐Beta for the tautomerization of phenol.
“…In order to generate the Lewis acid active site at M‐Beta zeolites, one silicon atom at a T9 site was replaced by one of the tetravalent metal atoms (Sn, Ge, Hf or Zr). Upon hydrolyzing, a (M−OH) metal site and a silanol (Si−OH) group are formed . The metal can act as Lewis acid, and the hydroxide (OH) group as Lewis base.…”
We investigate the tautomerization of phenol catalyzed by acid‐base active pair sites in Lewis acidic Beta zeolites by means of density functional calculations using the M06‐L functional. An analysis of the catalytic mechanism shows that hafnium on the Beta zeolite causes the strongest absorption of phenol compared to zirconium, tin, and germanium. This can be rationalized by the highest delocalization of electron density between the Lewis site and the oxygen of phenol which can in turn be quantified by the perturbative E(2) stabilization energy. The reaction is assumed to proceed in two steps, the phenol O−H bond dissociation and the protonation of the intermediate to form the cyclohexa‐2,4‐dien‐1‐one product. The rate determining step is the first one with a free activation energy of 26.3, 25.0, 22.1 and 22.7 kcal mol−1 for Ge‐Beta, Sn‐Beta, Zr‐Beta, and Hf‐Beta zeolites, respectively. The turnover frequencies follow these reaction barriers. Hence, the intrinsic catalytic activity of the Lewis acidic Beta zeolites studied here is in the order of Hf‐Beta≈Zr‐Beta>Sn‐Beta> Ge‐Beta for the tautomerization of phenol.
“…2 , wherein M(IV)-FER and M(IV)-CHA consist of two and four unit cells along the c (1 × 1 × 2) and a × b (1 × 2 × 2) lattice vectors, respectively. Different from CHA and FER zeolites where all T sites are indistinguishable (referred to as T1), BEA possesses nine crystallographically distinct T sites, and as recommended elsewhere 28 – 30 , 33 , 35 , 38 , 39 , 53 – 61 , T2 site that is the most energetically favorable and T9 and T6 sites that are situated at the intersection of two channels were investigated. Figure 2 Periodic models of ( A ) BEA, ( B ) CHA and ( C ) FER zeolites labeling the T sites presently investigated.…”
M(IV)-incorporated zeolites have recently aroused wide interest due to outstanding catalytic effects while their active sites remain largely elusive. Here periodic density functional theory calculations are conducted finding that active sites are determined jointly by identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites. All M2(IV) active sites in BEA zeolites are penta-coordinated with chemisorption of one water while subsequent water molecules that form only H-bonds promote chemisorption of the first water, especially the second water possessing comparable or even higher adsorption strengths as the first water; Ti(IV) and Ge(IV) active sites at the intersection remain penta-coordinated and Sn(IV) and Zr(IV) active sites prefer to hexa-coordination although potentially expanded to hepta-coordination. Different from other zeolites, Ti(IV) active sites in FER zeolites are hexa-coordinated as Sn(IV) active sites, due to the promoting effect of the first water. Lewis acidic defects expand Ti(IV) active sites to hexa-coordination while inhibit the formation of hepta-coordinated Sn(IV) species. Two forms of Brϕnsted acidic defects exist for Sn(IV) sites instead of only one for Ti(IV) sites, and all M(IV) Brϕnsted acidic defects, regardless of different acidic forms and M(IV) ions, can chemisorb only one water.
“…[26] Supercells consisting of two and four unit cells along the c (1 x 1 x 2) and a x b (1 x 2 x 2) lattice vectors were constructed for FER and CHA, and framework-M(IV) ions occupy respectively the T 2 , T 1 and T 1 sites in BEA, FER and CHA, see Figure 1. [27][28][29][30] For comparison, p-DFT calculations were performed in parallel for FER with one unit cell (referred to as mFER).…”
Albeit the isomerization mechanism of glucose to fructose catalyzed by M(IV)‐incorporated zeolites is widely studied, scant attention has been given to the adsorption of related sugars that is critical to catalysis. Here p‐DFT calculations are conducted to have a comprehensive understanding within this context, considering the effects of adsorption modes, identity of framework‐M(IV) ions, pore topology and conformational states of glucose. Monodentate rather than bidentate adsorption structures of glucose are the most energetically favorable within all investigated zeolites except Sn‐CHA. Adsorption performances of different M(IV)‐incorporated BEA zeolites decline as Zr > Sn > Ti > Ge, where Ti‐ and Ge‐BEA, especially the latter, is obviously inferior for sugar adsorption and catalysis. Pore topology of zeolites plays an even more pronounced effect during glucose adsorption. Non‐covalent interactions contribute significantly to the adsorption processes. Dispersion effects of different framework‐M(IV) ions, although close to each other, show a clear opposite trend as adsorption energies. FER rather than other zeolites shows surprisingly high dispersion effects (e. g.; −218 kJ/mol for Sn‐FER vs. −123 kJ/mol for Sn‐BEA). It also shows that dispersion effects for the various conformational states of glucose are closely related with structural flexibilities.
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