Dalton Transactions PAPER Cite this: DOI: 10.1039/c7dt00035a Received 4th January 2017, Accepted 14th February 2017 DOI: 10.1039/c7dt00035a rsc.li/dalton Lewis acidity quantification and catalytic activity of Ti, Zr and Al-supported mesoporous silica† Maria V. Zakharova, a Freddy Kleitz* b and Frédéric-Georges Fontaine* a Water-tolerant supported Lewis acids are actively sought after, in particular to address the challenging direct amidation reaction. To this aim, a versatile and easysynthesis of large pore silica materials with sup- ported Ti-, Al-, Zr-Lewis acids, using acetyl acetonate as a metal-stabilizing agent, was accomplished. The formation of bulk metal oxides was not observed, even at high concentrations of metal species. The Lewis acidity was demonstrated using quantitative and qualitative titration techniques using a series of Hammett indicators, such as butter yellow, phenylazodiphenylphosphine and dicinnamalacetone. The optimal concentration of metals corresponding to the highest Lewis acidity of solids was found to be 4% for Al-SBA-15, 12–15% for Ti-SBA-15 and 7% for Zr-SBA-15 materials. The water-tolerance of the sup- ported metal centers was explored by a pyridine adsorption-FTIR study before and after water addition. The metalated materials were used as water-tolerant heterogeneous catalysts for the amidation of elec- tron-poor and bulky amines, such as substituted anilines and morpholine, obtaining 59–99% yield of the corresponding amides. Introduction Lewis acid catalysts play a primary role in many industrial chemical processes. Although not inclusive, the list of organic transformations and rearrangements catalyzed by Lewis acid catalysts includes notably the Mukaiyama-aldol, 1 the Diels–Alder 2 and the Friedel–Crafts 3 reactions, which commonly result in the formation of new C–C, C–N and C–O bonds. The typical Lewis acids used for these applications include soluble complexes of Ti 4+ , Al 3+ , Sn 4+ , and Zr 4+ . However, the removal of homogeneous catalysts in large scale processes can be costly and troublesome and heterogeneous catalysis presents itself as an alternate choice. Although many Lewis acidic materials have been reported, most of these compounds exhibit significant moisture sensitivity. 4 Moreover, conventional Lewis acids require pretreatment in order to break any Lewis adducts occurring between the active sites and moisture, which can lead to catalyst degra- dation. In addition, the increasing interest in green solvents for catalysis makes the synthesis of water-tolerant solid Lewis acids a target of great interest. 5 Zeolites are among the most important examples of heterogeneous catalysts comprising both Brønsted and Lewis acidic sites, originating from the aluminum atoms residing in the framework. 6 In the early 1980s, titanium silicalite-1 (TS-1) materials were shown to exhibit isolated Lewis acid sites in crystalline microporous materials without the presence of Brønsted acid sites. 7 However, these Lewis acid sites were hardly accessible for chemical transformations, such as for oxidation reactions, because of the microporosity of the materials. In order to catalyze reactions with larger substrates, Ti 4+ atoms were also introduced into the framework of 12-membered ring zeolites (Ti-Beta) and into ordered mesoporous silica matrices (e.g., Ti-MCM-41). 8 However, in some cases, a lower reactivity with respect to TS-1 was observed suggesting that the location of the Ti sites and the environment surrounding these sites were influencing the activity. 9 Metal–organic frameworks (MOFs) also exhibit catalytically relevant features similar to zeolites, including large internal surface areas and uniform pore and cavity sizes. 10 Some MOFs having Al, 11 Cd, 12 Cu, 13 In, 14 Mn, 15 Sc 16,17 or Cr 18 nanoclusters have been shown to be very active Lewis acid catalysts. However, the low stability of these materials toward moisture, temperature, and some reactants, owing to the presence of organic linkers, especially in comparison to the stability of the covalent Si–O bonds in zeolites, has significantly limited their industrial use. 19 Nevertheless, in the past few years, some new synthetic strategies have been developed in order to overcome the aforementioned drawbacks. 20 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c7dt00035a a Département de Chimie, Centre de Catalyse et Chimie Verte (C3 V), Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V 0A6, Canada. E-mail: frederic.fontaine@chm.ulaval.ca b Département de Chimie, Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V 0A6, Canada. E-mail: freddy.kleitz@chm.ulaval.ca This journal is © The Royal Society of Chemistry 2017 Dalton Trans. Published on 14 February 2017. Downloaded by Université Laval on 10/03/2017 19:49:07. View Article Online View Journal