The curious effects of integrating bimetallic active centres within nanoporous architectures for acid-catalysed transformations

Abstract

ARTICLEThis journal is © The Royal Society of Chemistry 20xxJ. Name., 2013, 00, 1-3 | 1 Please do not adjust marginsPlease do not adjust margins The resourceful combination of distinct Mg, Zn and Si active-sites within a single aluminophosphate framework, via simultaneous isomorphous substitution, has afforded unique bimetallic nanoporous heterogeneous catalysts. Unique sitespecific interactions have been engineered, at the molecular level, to facilitate catalytic modifications and optimize product yield. By the dextrous incorporation of individual transition-metal active centres, we are able to intricately control the precise nature of the Brønsted acid sites, thereby influencing their catalytic behaviour for the industrially relevant acidcatalysed Beckmann rearrangement of cyclohexanone oxime and isopropylation of benzene. IntroductionIn the modern era of ever-growing demands and restrictions imposed on the chemical industry, the requirement for cleaner and more sustainable production of bulk chemicals is becoming progressively more urgent. Advances in catalysis are attempting to meet these challenges by pursuing a range of possibilities; some by employing more sustainable feedstocks, while others seek to improve existing chemical routes through the use of more effective catalytic materials. Despite the diverse expansion of modern catalysis, it is clear that any significant advances in sustainability will require the development of single-site heterogeneous catalysts that are both versatile and robust for industrial manipulation. This subsequently places significant importance on the design and creation of increasingly intricate active sites at the molecular level, with a view to rationalising structure-activity relationships in our quest for maximising atom-efficiencies and optimising product yields. [1][2][3][4][5] In the last decade the need to adroitly construct welldefined active centres has prompted a surge in the development of bifunctional and multimetallic heterogeneous catalysts. [6][7][8][9] A growing number of examples in the literature describe how the proficient cooperation of different metals may influence the structure-property correlations between the individual active sites and the surrounding support (framework) infrastructure, which may in turn influence the generation of the desired transition-state for facilitating enhanced catalytic turnovers and selectivity. [10][11][12] Such notions are already wellunderstood in the field of nanoparticle chemistry [11][12][13] , whereby precise control of active-site location and crystal morphology is the targeted goal. While several synthetic protocols are already well-established to modulate such properties, the deft combination of individual monometallic species at the molecular level is not trivial. However, the rational introduction of a second metal offers a plethora of new synthetic possibilities towards modulating and optimising catalytic performance. [6,10] While such systems are still under development, current synthetic methods to engineer s...

Investigating the role of framework topology and accessible active sites in silicoaluminophosphates for modulating acid-catalysis

“…[9][10][11][12][13] Similarly a range of species have been synthesized with the aim of increasing the lactam yield (Table 1). [14][15][16][17][18][19][20][21][22][23][24] The Beckmann rearrangement of cyclohexanone oxime has more recently been performed at low temperature, liquidphase conditions, [15,25,26] achieving near-quantitative yield of εcaprolactam (> 98 % under optimized conditions) using a SAPO-37 catalyst. [14,15] Zeolite Y, which possesses the same framework topology (FAU) as SAPO-37, on the other hand, leads to much inferior yields of ε-caprolactam (29 mol%, Table 1), under identical conditions.…”

The Molecular Design of Active Sites in Nanoporous Materials for Sustainable Catalysis

“…In contrast, the Si 4+ -doped analogue (SAPO-5) contained significantly weaker acid sites that proved well-suited for catalysing the transformation ( Figure 4 ) [ 48 ]. Significantly, by combining the strong acid sites from Mg 2+ -doping, with the weaker acid sites from Si 4+ -doping, it was possible to create a bimetallic system of intermediate acidity that showed improved caprolactam yield in the vapour-phase [ 20 , 49 ]. The aluminium-doped, mesoporous silica, Al-MCM-41, has also proven to be of great interest as a Beckmann rearrangement catalyst, since improved mass-transport behaviour has been linked to enhanced caprolactam selectivity and catalytic lifetime.…”

“…In fact, even those atoms eligible for framework substitution can be made to occupy extra-framework sites, provided an appropriate synthesis procedure is adopted. For example, in an effort to dope Zn 2+ ions into AlPO-5, extra-framework species have been created [ 20 , 48 ]. Whilst, these extra-framework sites did not provide additional catalytic capabilities, their presence was found to modify the acidity of Mg- and Si-doped AlPO-5 systems antagonistically.…”