Rationalising the role of solid-acid sites in the design of versatile single-site heterogeneous catalysts for targeted acid-catalysed transformations

Abstract

Probing multifunctional acid centres in nanoporous architectures through in situ spectroscopy affords a strategy for predictive design of novel catalysts.

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.…”

“…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. In light of this, we resolved and quantified the FTIR signals observed using the 2,6-Dimethylpyridine (2,6-DMP) as a molecular probe (see Figure 8B), and this not only highlighted the subtle differences in acid-site strength that existed between our catalysts; but more importantly emphasised the differences in the quantity of accessible active (acid) sites.…”

“…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.…”

“…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. It was concluded from these studies 71,75 that through the inclusion of zinc in a microporous framework architecture, it becomes possible to subtly modify the (acid) strength of different sites. Furthermore, it was revealed 75 that zinc is capable of provoking different catalytic responses from different acid sites; this therefore provides a new tool for tuning acidity and targeting catalytic outcomes by predictively designing active centres.…”