Design and control of Lewis acid sites in Sn-substituted microporous architectures

Abstract

Monometallic and bimetallic tin-containing framework architectures have been prepared by hydrothermal methods. Structural and spectroscopic techniques were used to probe the nature of the solid-acid sites, at the molecular level, using a combination of XRD, DR UV-Vis, solid state MAS NMR ( 119 Sn, 27 Al and 31 P) and XAFS. The nature and strength of the solidacid sites were experimentaly probed by FT-IR spectroscopy using CD3CN as a probe molecule. To elucidate further the localstructure, the structural characteristics of the Sn sites were probed using DFT calculations, with a view to rationalising the experimental findings. These detailed structural and spectroscopic studies revealed the presence of multiple Sn environments, with the monometallic SnAlPO-5 catalyst displaying a greater number of tetrahedral Sn(IV) active centres. These framework Sn(IV) centres generated stong Lewis acid sites, when compared with their bimetallic Co-Sn analogue, thereby affording attractive possibilities for modulating catalytic reactivity. IntroductionTetrahedral (Td) sites in microporous solids are widely reported as catalytic active centres in selective oxidation and acidcatalysed transformations, 1-4 of which Ti in TS-1, 5 Fe in ZSM-5 6 and Co in AlPO-5 7 are significant examples. Controlling the nature of these active sites allows one to tune the Lewis 8 and Brønsted acidity, 9 and regulate redox 10 centres for oxidation catalysis. The addition of a second heteroatom allows for further optimisation of these properties, as well as influencing the coordination geometry of the initial heteroatom. [2][3][4] The incorporation of Sn into zeolitic 11 and non-zeolitic 1,4 architectures has received significant attention for its promising performance in Baeyer-Villiger oxidations. 11 The activity of these systems was correlated with the presence of Sn in Td sites, with well isolated Lewis acidic centres, which can be used for a wide range of catalytic reactions. [11][12][13] The synthetic findings were supported by a large number of characterization techniques, including DR UV-Vis, 119 Sn MAS NMR and in situ IR spectroscopy. 11,14 Although such materials could be expected to show significant catalytic functionality, the incorporation of Sn into AlPO-5 frameworks in Td sites is, however, very limited and poorly understood.This work focuses on the isomorphous substitution of P(V) with Sn (IV) within AlPO-5 using a modified hydrothermal preparation method. 4 Cobalt (Co 2+ /Co 3+ ) ions can be simultaneously introduced into the AlPO-5 architecture 2,3,15 by the substitution of (Al 3+ ) sites. Recently, a combined DFT and EXAFS study demonstrated the potential of effecting catalytic synergy in Co−Ti bimetallic nanoporous frameworks. 16 However, the prospects of introducing a relatively heavier element like Sn, alongside a redox transition-metal such as Co, could pave the way for probing bifunctional catalytic transformations. Nevertheless, the structural and spectroscopic features of such a system needs to be probed in meticulous detail...

Revealing Interplay of Defects in SnO2 Quantum Dots for Blue Luminescence and Selective Trace Ammonia Detection at Room Temperature

“…14 However, the above studies prove that QDs possess a very high surface-to-volume ratio along with a high abundance of Sn 2+ and Sn 4+ defects, which act as a Lewis acid to turn into an adsorption site for a Lewis base. 25 However, the confirmation of the adsorption sites after the postadsorption effect on the optical sensor is yet to be drawn and remains as a gap area in the available literature. Hence, with the above findings, the PL-based sensor study is devised to carry forward with NH 3 , a strong Lewis base, due to the presence of a lone pair of electrons.…”

“…The decisive role of acidic sites in the SnO 2 surfaces for interactions with Lewis base analyte NH 3 is thus discussed as a possible mechanism for PL sensor responses. 16,51 In metal oxides, coordinately unsaturated metal cations (Sn 4+ ) act as Lewis acid sites 16,24,25,51 and the oxygen anions as Lewis base sites. 22,51 These acid sites play a vital role in the interaction with basic molecules.…”

“…6 Interestingly, these states become Lewis acid sites and promote attraction for the Lewis base. 24,25 Such base−acid interaction has also been exploited in various catalytic reactions, while the acidic site becomes a critical point for the oxidation process of the basic analyte like toluene. 24,26 However, such an interaction is rarely depicted to describe any optical modulation.…”

Structure‐Activity Relationships in Highly Active Platinum‐Tin MFI‐type Zeolite Catalysts for Propane Dehydrogenation

“…[37] However, when compared to bulk SnO 2 , which contains octahedrally coordinated Sn(IV), the lower intensity observed for both the SnÀ O contribution in the FT and the main edge in the XANES indicates differences in the Sn coordination environment, due to the presence of at least some Sn(IV) species in tetrahedral coordination. [38] The presence of higher R-space features between 3-4 Å in the EXAFS FT (Figure S9b) confirms that some octahedral Sn (IV) is still present in the form of SnO 2 clusters. We propose tetrahedral Sn (IV) sites to occupy framework positions due to the incorporation of Sn into the framework vacancies.…”

Butane Isomerization as a Diagnostic Tool in the Rational Design of Solid Acid Catalysts

“…Ultimately this would lead to artificially enhanced reaction rates which are not representative of acidic strength. Such functionalities are known to exist for MAlPO materials, such as the redox behavior of CoAlPOs, or the Lewis acidity of SnAlPOs [84][85][86].…”