A Recyclable Acidic Ionic Liquid Gel Catalyst for Dehydration: Comparison with an Analogous SILP Catalyst

Abstract

An acid-functionalized ionic liquid was entrapped within a silica gel to yield a recyclable liquid phase catalyst for the dehydration of rac-1-phenyl ethanol. Hot filtration tests showed that the activity was within the gel. Comparison with an analogous SILP system revealed fundamental differences in the properties and behavior of the materials.The discovery of facile ionic-liquid silica gel formation, 1,2 enabled the introduction of the first ionic liquid mediated sol-gel methods for the preparation of solid catalysts. [3][4][5] There followed a period of relatively few publications, during which ionic liquid gels were seldom investigated as catalysts. Currently, in parallel with the development of new classes of ionic liquid, and an increase in interest in softmatter, the area of ionic liquid gel catalysts has come back into focus, and new methods are emerging. This catalyst preparation methodology involves the entrapment of an ionic liquid within an inorganic oxide matrix by a gel method. At the same time a catalyst is co-entrapped, 3,[5][6][7] or, alternatively the catalytic center can be associated with the ionic liquid. 4 The worked-up material is then used as a heterogeneous catalyst. Materials consisting of an ionic liquid within a porous oxide are often referred to as ionogels. 8 These catalysts are a sub-set of the sol-gel prepared entrapped catalysts pioneered by Blum and Avnir. 9,10 In contrast to the gel-based catalysts, the related SILP catalysts, in which a heterogeneous catalyst is prepared by layering an ionic liquid on a pre-existing oxide matrix, have elicited an explosion of interest in the last ten years. Several reviews and concept articles have been published covering the method. [11][12][13][14][15][16][17] Exemplars have been demonstrated in the gaseous phase, 18 and using supercritical fluids, [19][20][21] and the application of SILP technology is at an advanced stage. The simplicity of the system enables a wide variety of matrices, ionic liquids and dopants to be screened easily. In liquid phase reactions the ease of addition and removal of the ionic liquid is potentially detrimental, and leaching can be facile.The development of supported nanoparticles catalysts by ionic-liquid assisted gel methods 3 has been recently developed further by Han and co-workers. 22 Catalysts with hierarchical pores were achievable by employing a metal salt (e.g. CaCl 2 ) initiated gelation. Doping with gold afforded a catalyst for esterification, and ruthenium catalyzed hydrogenation.The introduction of functionalized ionic liquids that are 'task specific' 23-26 opens up many new possibilities for the application of ionic liquids to liquid phase reactions, not least the incorporation of Brønsted-Lowry Acidic/Basic sites. Acid functionalized ionic liquids have been demonstrated as robust catalysts for continuous esterification in a miniplant. 27 In order to improve separation and prevent corrosion caused by liquid phase acids, it would be fortuitous to develop a heterogenised system. The combinat...

Taming the Base Catalyzed Sol–Gel Reaction: Basic Ionic Liquid Gels of SiO2 and TiO2

“…3,4,5 More recently the autocatalytic synthesis of ionic liquid gels was introduced using an acidic ionic liquid. 6 Occasionally ionic liquid gels have been synthesised using neutral ionic liquids, with an additional base as the catalyst, 7 but these materials are reported to have low mechanical strength. Potential advantages of developing basic routes to ionic liquid gels are demonstrated by the application of basic ionic liquid SILPs (Supported Ionic Liquid Phases).…”

“…10 Building on the understanding gained from our work on autocatalytic systems for acidic ionic liquid gels, 6 our hypothesis was that the combination of a viscous ionic liquid with the right level of Brønsted basicity could slow down the sol-gel process, and allow a more controlled sol-gel reaction. We found that, by careful selection of Brønsted basic ionic liquid, we were able to tame the sol-gel reaction to form silica and titania ionic liquid gels on a slower time frame.…”

Low molecular weight gelators (LMWGs) for ionic liquids: the role of hydrogen bonding and sterics in the formation of stable low molecular weight ionic liquid gels

“…27 Of the gels presented here several have been successfully formulated from functional ionic liquids which have shown promise in diverse applications. 28 : acidic catalytic membranes. In some of these applications the ionic liquid could be made more easily applied in the gel state.…”

“…%. An interesting observation is seen with [TEAPS][NTf 2 ], an acidic ionic liquid,28 where 1 did not gel at 4 wt. % but 2 did.…”

A Brønsted‐Acidic Ionic Liquid Gel as an Efficient and Recyclable Heterogeneous Catalyst for the Synthesis of Bis(indolyl)methanes under Solvent‐Free Sonication

“…Recently, Marr and co‐workers have developed a recyclable acidic ionic liquid gel catalyst for the dehydration of rac ‐1‐phenyl ethanol. It was reported that this ionic liquid gel demonstrated significantly improved activity as well as negligible leaching . In this work, we described the application of an analogue of this Brønsted acidic ionic liquid gel for the preparation bis(indolyl)methanes from aldehydes and indoles via Friedel–Crafts alkylation assisted by sonication under solvent‐free condition.…”