Probing the Titanium Sites in Ti−MCM41 by Diffuse Reflectance and Photoluminescence UV−Vis Spectroscopies

Abstract

Ti−MCM41 catalysts prepared by anchoring titanocene onto the inner walls of MCM-41 exhibit high catalytic performance in the epoxidation of cyclohexene and in oxidation of bulky cyclic and aromatic compounds because of the presence, as revealed by means of XANES and EXAFS spectroscopy, of a high concentration of accessible, well-spaced, and structurally well-defined Ti tetrahedral active sites (see Maschmeyer et al. Nature, 1995, 378, 159). These results have prompted us to study the nature and the coordinative environment of Ti active sites in MCM-41 also by means of diffuse reflectance UV−visible and luminescence spectroscopy. We revealed, particularly by means of photoluminescence spectroscopy, the presence of more than one kind of tetrahedral titanium site.

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

“…As the Mg 2+ and Zn 2+ ions are of similar ionic size, it would be reasonable to assume similar levels of incorporation of these ions, thereby translating to a 15 similar fraction of Brønsted acid sites. The experimental observations, however, accentuate the degree of complexity involved in the kinetics of crystallisation, and show that while Mg 2+ ions are readily incorporated into the framework; facilitating the formation of Brønsted acid sites, Zn 2+ ions prefer 20 to exist as extra-framework species. This observation is confirmed when comparing the CO adsorbed FTIR spectra of MgAlPO-5, ZnAlPO-5 and AlPO-5 ( Figure S3).…”

“…In all cases the maximum derivation from the undoped crystalline parameters was less than 1%, in agreement with the low quantities of heteroatoms doped into the framework (Table S2). This variance To gain further insights on the location of the zinc ions, specific surface area measurements were employed to compare the bimetallic and monometallic catalysts to that of the undoped 20 AlPO-5 framework (295 m 2 /g, Table 1). The greatest variation was seen in the monometallic ZnAlPO-5 system, that revealed a significant decrease in surface area (165 m 2 /g), which is indicative of pore obstruction by extra-framework Zn species.…”

“…[32] The direct correlation between the contrasting catalytic behaviour of the SiAlPO-5 and MgAlPO-5, coupled with the FTIR findings underpinning the nature of the active sites, emphasises the influence of acidity for the complementary catalytic reactions employed in this study (Scheme 1). 20 By comparing the overall activity of the monometallic catalysts in both processes, it is readily apparent that the ZnAlPO-5 catalyst is far inferior to the Mg-or Si-AlPO-5 analogues ( Figure 6A, 6B, S5 & S6). In accordance with the findings from the XRD and spectroscopic data, the zinc ions are 25 reluctant to undergo isomorphous framework substitution into the AlPO architecture to form Brønsted acid sites.…”

“…The continued demand for the development of sustainable chemical processes has prompted the discovery and design of novel catalytic materials that operate in an environmentally 20 benign fashion, in order to meet the stringent requirements of clean technology and energy efficiency. [1][2][3] Understanding the nature of the active site at the molecular level is therefore a fundamental prerequisite, for facilitating structure-property correlations, that enable the design of catalytically active entities 25 for enhancing rates and selectivity.…”

“…In this work, we demonstrate how Zn 2+ ions, introduced into a microporous molecular sieve, are able to adroitly tailor the nature of both Mg 2+ and Si 4+ sites, that reside in the immediate vicinity of the framework. It is widely known that Zn acts as a modifier in 20 certain chemical transformations, [30] and, given the comparable ionic size and valency to Mg, it was hoped that Zn would be a good candidate for prompting synergistic interactions. In this work, we compare the behaviour of Zn-containing and Zn-free catalysts for two acid-catalysed transformations: the vapour- 25 phase Beckmann rearrangement of cyclohexanone oxime and the isopropylation of benzene (Scheme 1), with a view to understanding the nature of the acid sites and the influence of zinc.…”