Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Abstract

Structure-property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO(2+) ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems.

Predictive design of engineered multifunctional solid catalysts

“…70 Figure 7: Establishing structure-property relationships through the use of in situ spectroscopic and structural characterisation methods. 3,4,70,72 In situ diffuse reflectance (DR) UV-Vis spectroscopy is another useful tool for ascertaining the local environment of the metal centres; it is of particular use when examining the redox capabilities of the transition metals and their coordination environments. The presence of certain spectroscopic features can ascertain the extent to which transition metals are incorporated into a framework.…”

“…72 EPR is also a potent technique to probe the local structural environment of paramagnetic transition-metal ions. 73 In partnership with our collaborators, 3,74 we have employed continuous wave (CW) and electron spin echo measurements to elucidate the oxidation states and coordination geometries of our CoTiAlPO-5 and VTiAlPO-5 catalysts, that were particularly active in selective oxidation reactions. In order to quantitatively probe the local environment of the individual metal ions, with a view to studying the nature of the transitionmetal substitution mechanism (see Scheme 2), an array of hyperfine techniques ( Figure 7D) associated with EPR can be utilised; namely Electron Spin Echo Envelop Modulation (ESEEM), Electron Nuclear Double Resonance (ENDOR), Hyperfine Sublevel Correlation (HYSCORE).…”

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts

“…To quantify such interactions at the molecular level requires a detailed understanding of the nature of the active sites, and it is necessary to employ a range of physico-chemical, operando and spectroscopic characterisation techniques, that are best complemented when integrated with atomic level modelling studies. [9][10][11][12][13][14][15] In our recent work 16 we extended the family of transition-metal doped aluminophosphate (AlPO) frameworks, 10,17,18 to obtain isomorphous incorporation of bimetallic active centres, that display superior catalytic activity in oxidation reactions ( Figure S1 and Table S1). 16 Through a rational selection of appropriate metal combinations and synthetic strategy, it is possible to engineer and exploit synergic interactions between individual metal sites, deliberately placed within sufficiently close proximity such that their local geometry and electronic structure is modified to facilitate catalytic improvements.…”

“…It is possible to engineer this phenomenon not only between different dopants, but also for different industrially relevant catalytic transformations. 9,16,[19][20][21] In this paper we discuss the synergic effects obtained by isomorphously substituting cobalt and titanium ions simultaneously into the same AlPO-5 framework, to yield a bimetallic CoTiAlPO-5 system. We have previously shown that individually these two metal dopants (as monometallic entities) are capable of catalysing a range of oxidation reactions.…”

Investigating site-specific interactions and probing their role in modifying the acid-strength in framework architectures

“…[4,5] It is well-known that a diverse array of synthetic approaches [6][7][8][9][10] can be adopted for heterogenizing a wide-variety of molecular precursors on high-area supports for generating multimetallic metal nanoparticle catalysts that lead to enhanced catalytic 30 turnovers. [11][12][13] Recently, [4,14,15] the simultaneous isomorphous substitution of multimetallic transition-metal ions into the framework of porous architectures has resulted in considerable enhancements in catalytic synergy in selective oxidation reactions. The implementation and execution of such a design 35 strategy often requires precise dexterity in the synthesis procedure, manipulation and control of the pore dimensions of the framework architecture, choice of dopant metal precursors, coupled with a detailed elucidation of the nature of the active site by employing advanced spectroscopic and structural 40 characterisation methods.…”