SILCA—Supported ionic liquid catalysts for fine chemicals

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Abstract

Various ionic liquids (ILs), together with Pd-metal species were immobilized on a high-surface area, structural active carbon cloth. The resulting SILCA (supported ionic liquid catalyst) catalysts were studied in the production of fine chemicals, i.e. in the hydrogenation of unsaturated aldehydes, citral and cinnamaldehyde. These model molecules are challenging ones, due to the possibility of several parallel and consecutive reactions that can occur, depending on the experimental conditions and the nature of the catalyst. In this paper we illustrate the feasibility of altering the selectivity profiles, not only by means of altered reaction conditions but also by variation of the nature of the IL present in the catalytic layer. The catalysts were characterized by means of, e.g. nitrogen physisorption, XPS as well as FESEM and EFTEM. The results revealed that Pd derived from a Pd(acac)2-precursor, initially dissolved into the ionic liquid, undergoes a change of oxidation state from Pd2+ to Pd4+, regardless of the ionic liquid in question. However, upon decomposition of the precursor, at 373 K, under H2-flow, a transition to either Pd+ or Pd0 occurred, indicating the formation of catalytically active Pd complexes or nano particles, respectively.

Graphical abstract

Various ionic liquids (ILs), together with Pd-metal species were immobilized on a high-surface area, structural active carbon cloth. The resulting SILCA (supported ionic liquid catalyst) catalysts were studied in the production of fine chemicals, i.e. in the hydrogenation of unsaturated aldehydes, citral and cinnamaldehyde. In this paper we illustrate the feasibility of altering the selectivity profiles, not only by means of altered reaction conditions but also by variation of the nature of the IL present in the catalytic layer.

Introduction

Ionic liquids (ILs) have emerged as a novel class of materials or neoteric solvents that are applied in many fields, from solvents for electrochemistry [1] and organic synthesis [2], as materials for recovery of metals from aqueous solution [3], synthesis of nano-structured materials [4] and sequestration of carbon dioxide [5], to entrapment and activation of enzymatic and metal species for catalytic applications [6], [7]. The vast number of anticipated possibilities to form various ionic liquids, at least a million or even 1018 [8], alone gives the possibilities almost beyond our imagination, enabling task-specific configurations for different technology disciplines. Moreover, the choice of liquid-ionic compounds, or materials with similar performance, is likely to be even greater when taking into account zwitterionic species (molecules with internal charge distribution) [9] and deep eutectic melts (such as choline chloride–zinc or tin chloride) [10].

Room temperature ionic liquids (RTILs) posses unique characteristics such as an extremely wide liquidus range (up to hundreds of degrees); they display unusual dissolution properties (materials like carbohydrates [11] and cellulose [12] can be dissolved); RTILs are often associated with very low vapor pressures and non-flammability (although not exclusively: an aerosol of certain ionic liquids will burn [13]); RTILs have a large electrochemical potential window [14]. However, as emphasized in a recent polemic, Deetlefs and Seddon [15] importantly map the facts and fiction associated with ionic liquid literature: generic statements are considered specific whereas specific assumed as generic.

Various authors (e.g. [16]) have studied the concept of immobilized metal nanoparticles in ionic liquids and, for instance, the pioneering work of prof. Dupont has shown that many kinds of organometallic species can be immobilized in ionic liquids and applied successfully in various catalytic transformations (e.g. Refs. [17], [18], [19], [20], [21]).

The quest for greener chemistry, speculations about the global climate change as well as the legislative pressures such as REACH (Registration, Evaluation and Authorization of Chemicals) [22] set by the European Union to meet the criteria for sustainable chemical products has increased the research efforts and inspired the researchers to come up with alternative, more atom-efficient technologies and methods. Ionic liquids have already proven their potential in various cases and there is certainly more to come!

One of the key challenges in the application research of ionic liquids has been the availability and cost issues. Although a multitude of recipes has been published to carry out the synthesis of ionic liquids, not all laboratories focused on applied research have the expertise, work practices and equipment necessary to carry out synthesis work. Also, to prepare pure, dried RTILs or to execute post synthesis purification steps is often challenging. On the other hand, commercial availability of some ionic liquids is restricted to small volumes or the cost of many liquids is still unacceptable for applied engineering research. Therefore, the concept of immobilized ionic liquids entrapped, for instance, on the surface and pores of various porous solid materials offers an attractive alternative to study the performance of ILs cost-effectively. Also, due to the higher relative viscosity of many RTILs, compared to that of classical molecular solvents, the established thin ionic liquid layer is an advantage, facilitating more rapid diffusion and mass transfer.

The question remains, how to ensure that the ionic liquid is immobilized on the carrier. There are a few possibilities, such as to anchor the IL covalently, either via the anion or cation onto, e.g. the hydroxyl groups of the surface [23], [24], [25]. This is conveniently carried out, for instance, via a reaction with the appended alkyl–silyl side branch tethered into the anion or cation, respectively. This method, however, principally allows only anchoring of relatively isolated cation–anion molecule pairs and is, thus, strictly speaking not a true liquid layer. Moreover, the physico-chemical properties of the ionic liquid are influenced by the structural changes induced by appended side branches.

In this work we chose a different preparation method relying elegantly on the solubility limitations of ionic liquids. In fact, the situation resembles that of a biphasic operations, only that the RTIL remains attached to the surface of a porous solid material where it was appended. The ionic liquids applied were butyl–methyl imidazolium hexafluorophosphate [bmim][PF6], butyl–methyl imidazolium tetrafluoroborate [bmim][BF4], Aliquat 336® hexafluorophosphate [A336][PF6] and n-butyl-4-methylpyridinium tetrafluoroborate [NB4MPy][BF4]. The synthesis of the Aliquat-based ionic liquid is described in earlier publications [26], [27].

The sample reactions, hydrogenation of citral and cinnamaldehyde, respectively, are interesting ones since both are multi-unsaturated compounds containing both carbon–carbon double bonds as well as a carbonyl group. Upon hydrogenation, numerous parallel and consecutive reactions, can take place. The reaction schemes for the hydrogenation of α,β unsaturated aldehydes, citral as well as cinnamaldehyde, are presented in Scheme 1, Scheme 2, respectively. A few of the interesting hydrogenation products of citral are citronellal, citronellol and dihydrocitronellal that are utilized, e.g. in the perfumery industry. A more thorough list of products and their sample applications can be found in our earlier publication [26].

Section snippets

Experimental

The supported ionic liquid catalysts were prepared as follows: the ionic liquid, [bmim][PF6] (Merck, 98%), [bmim][BF4] (Merck, 98%), self-made [A336][PF6] [27] or [NB4MPy][BF4] (Merck, 98%) and the transition metal salt (palladium acetylacetonate, Aldrich 99%) were both dissolved in dry acetone (Acros Organics, 50 ppm H2O), mixed and, thereafter, a pre-dried (378 K, 1 h) structural solid, high surface area Kynol® Active Carbon Cloth/matt (ACC, Fig. 1) was wetted (‘incipient wettness’) with the

Catalyst characterization

The amount of ACC-support (ACC, active carbon cloth) was approximately 1.1–1.4 g (dry), corresponding to a macroscopic surface area of 4 cm × 10 cm (see Fig. 1). According to our own nitrogen physisorption measurements (Carlo Erba Sorptomatic 1900 Sorptometer), the specific surface area of unloaded ACC-cloth is 1680 m2/g (Dubinin; 1120 m2/g B.E.T.) and the micropore volume 0.60 cm3/g. The active carbon cloth had a fibre diameter of Ø = 9.2 μm [28].

SEM images revealed the morphology of the active carbon

Conclusions

Supported ionic liquid catalysts (SILCA) were characterized by means of various techniques, such as (FE)SEM, EFTEM and XPS and their performance evaluated in batch hydrogenations of citral and cinnamaldehyde, respectively. The analyses confirmed the formation of Pd nanoparticles, these being the catalytically active species. Although the amount of the active metal, palladium, entrapped into the thin ionic liquid layer was very small, the catalysts were very active and durable against

Acknowledgments

The financial support from the Academy of Finland is gratefully acknowledged (Decisions nos. 209391 and 211463). This work is part of the activities at the Åbo Akademi Process Chemistry Centre (PCC) within the Finnish Centre of Excellence Programmes (2000–2005 and 2006–2011) by the Academy of Finland.

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