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

Abstract

The ability to adroitly tailor acid-strength using specificallyengineered bimetallic nanoporous materials has been investigated with a view to exploiting their potential in solidacid catalysed transformations. Further, it has been demonstrated that through site-specific interactions, extra-10 framework zinc ions can suitably modify the acidity of Bronsted acid sites, to stimulate diverse catalytic responses, when combined with isomorphously-substituted framework metal cations within porous architectures, for the Beckmann rearrangement of cyclohexanone oxime and in the 15 isopropylation of benzene.

Predictive design of engineered multifunctional solid catalysts

“…Figure 8: Detailing the use of molecular probes (CO and 2,6-DMP) for quantifying nature and type of solid-acid centres. 71,75 The TPD measurements on our solid acid catalysts 71 were in good agreement with the FTIR findings on the position of the ν 8a (CC) bands ( Figure 8C), but it was difficult to decipher subtle differences between the monometallic and bimetallic solid acid catalysts used in this study 71 purely on the basis of the NH 3 -TPD data. It is to be noted that the NH 3 probe that was used in the TPD measurements bears little resemblance to the substrates (see Page 9 for catalytic data) used in our study 71 and it was more pertinent to employ a probe-molecule that bears a greater resemblance to the reagents, so that only the catalytically relevant species may be observed.…”

“…It is often beneficial to combine the information garnered from FT-IR with other spectroscopic methods, such as temperature-programmed desorption (TPD), to complement and categorise the respective fractions of 'strong', 'weak' or 'intermediate' acid sites present on a specific species. In our studies 71,75 that were particularly aimed at establishing structure-property correlations in the industrially significant Beckmann rearrangement of cyclohexanone oxime and in the isopropylation of benzene to cumene, it was important to establish selection rules for the classification and quantification of the Brønsted acid centres present in our catalysts. Figure 8: Detailing the use of molecular probes (CO and 2,6-DMP) for quantifying nature and type of solid-acid centres.…”

“…80 Figure 9: The role of Zn in modifying the acid strength (A, B); and facilitating catalytic synergy in bimetallic Mg(II)Si(IV)AlPO-5 for acid catalysis (C, D). 71,75 Despite the inactivity of zinc in the framework, a bimetallic MgZnAlPO-5 and ZnSiAlPO-5 species were synthesised. This required the development of a one-step hydrothermal synthesis protocol to influence and optimise the catalytic profiles of both reactions and to further contrast the findings with the monometallic catalysts.…”

“…With a view to fully rationalising these findings, a comprehensive spectroscopic study was undertaken 75 to understand the remarkably contrasting effects of adding zinc to the monometallic catalysts. Again, through the use of the 2,6-DMP as a molecular probe, it was found that the addition of Zn had weakened the acid site strength of the magnesium sites; whilst it had modified and somewhat strengthened the acid strength associated with the silicon sites, in accordance with the catalytic findings.…”

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts

“…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.…”