Silica-supported sulfonic acids as recyclable catalyst for esterification of levulinic acid with stoichiometric amounts of alcohols

Silica-supported sulfonic acids as recyclable catalyst for esterification of levulinic acid with stoichiometric amounts of alcohols Raimondo Maggi*1, N. Raveendran Shiju*2, Veronica Santacroce1,2, Giovanni Maestri1, Franca Bigi1,3 and Gadi Rothenberg2 Full Research Paper Address: 1Clean Synthetic Methodology Group, Dipartimento di Chimica, Università di Parma, Parco Area delle Scienze 17A, I-43124 Parma, Italy, 2Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands. Tel: +31-20-5256515 and 3Istituto IMEM-CNR, Parco Area delle Scienze 37/A, I-43124 Parma, Italy Open Access Beilstein J. Org. Chem. 2016, 12, 2173–2180. doi:10.3762/bjoc.12.207 Received: 27 July 2016 Accepted: 22 September 2016 Published: 12 October 2016 This article is part of the Thematic Series "Green chemistry". Email: Raimondo Maggi* - ; N. Raveendran Shiju* * Corresponding author Guest Editor: L. Vaccaro © 2016 Maggi et al.; licensee Beilstein-Institut. License and terms: see end of document. Keywords: esterification; heterogeneous catalysis; renewable feedstocks; supported organic catalysts; sustainable chemistry Abstract Converting biomass into value-added chemicals holds the key to sustainable long-term carbon resource management. In this context, levulinic acid, which is easily obtained from cellulose, is valuable since it can be transformed into a variety of industrially relevant fine chemicals. Here we present a simple protocol for the selective esterification of levulinic acid using solid acid catalysts. Silica supported sulfonic acid catalysts operate under mild conditions and give good conversion and selectivity with stoichiometric amounts of alcohols. The sulfonic acid groups are tethered to the support using organic tethers. These tethers may help in preventing the deactivation of the active sites in the presence of water. Introduction Vegetal biomass is mankind’s only source of renewable carbon on a human timescale. It is abundantly available, with the potential of replacing fossil-based carbon on a scale sufficient for covering the worldwide demand for non-fuel chemicals [1-4]. Currently, the main research thrust is directed at lignocellulose, the most abundant fraction of biomass. The mass com- 2173 Beilstein J. Org. Chem. 2016, 12, 2173–2180. position of lignocellulose could be roughly represented by a 5/3/2 ratio of cellulose, hemicellulose and lignin, respectively. All of these polymers are the subject of many studies [5-11]. Levulinic acid (LA) is one of the most important platform chemicals as it is a versatile building block for a variety of value-added agrochemicals, fine chemicals and pharmaceutical intermediates [12,13] (Scheme 1, bottom). Moreover, it can be obtained from cellulose with relative ease and high selectivity (see Scheme 1, top) [14]. Scheme 1: Synthesis of levulinic acid from ligno-cellulosic feedstocks and its principal uses to access fine chemicals. Levulinic acid esters are of particular interest for the chemical industry [12,13]. Their main current market is represented by the formulation of flavours and fragrances [15], although the scale of these preparations did not boosted demand yet. However, the seek to develop more eco-compatible solvents might grant to levulinates a novel route of application. By tailoring their physicochemical properties they could become complementary to common esters and other solvents, which might be more harmful for both humans and the environment [16]. It should be also noted that ethyl levulinate could shrink the emission of nitrogen oxides from exhausts of diesel engines when used as additive [17,18]. Due to their importance, new strategies have been developed for the production of levulinic esters [19-22]. Homogeneous Brønsted acids could catalyse the esterification of levulinic acid in the presence of alcohols and reports on this reactivity date back to the nineties [23]. Although this route could ensure high chemical yields, it still presents a series of drawbacks. In particular, issues with catalyst recycling and product separation limits the environmental viability of this strategy. As a result, it remains of high interest to develop alternatives to trigger this reaction, which are more sustainable, for instance through the design of suitable and recyclable solid acid catalysts. In the literature, methods that use solid heteropolyacids, such as ammonium or mixed ammonium and silver-doped phosphotungstic acid, sulfated metal oxides (such as sulfated titania, sulfated zirconia), zeolites and hydrotalcites have been reported [24-30]. These solid catalysts share several advantages, including high activity and an easy recovery, which might provide a real basis for future application in commercial processes. Nevertheless, they require high temperatures (usually above 100 °C) and long reaction times [24-30]. Furthermore, they often share another common pitfall, namely the use of large molar excess of alcohol, either for practical convenience [31] or to minimise ester hydrolysis. As meaningful examples, it has been recently reported that acid ZSM-5 zeolites, with encapsulated maghemite particles to allow magnetic catalyst recover, could be used to directly convert furfuryl alchol into an alkyl levulinate upon warming at 130 °C for 8 hours in the presence of a large excess of alchol as solvent/reagent (100 equiv) [32]. Although the behaviour of many metal oxides has been investigated, reports featuring the activity of supported organic Brønsted acids are very few. In particular, Tejero reported that sulfonic acid supported on polymeric resins could catalyse the esterification of LA, providing conversions up to 94% upon warming at 80 °C for 8 hours in the presence of 3 equiv of n-butanol [33]. Melero described the synthesis of mesostructured silica frameworks featuring pending organosulfonic arms. The best catalyst provided quantitative conversion of LA upon warming of the reaction mixture at 130 °C for 2 hours in the presence of a fivefold molar excess of ethanol, used as solvent/reagent [34]. Here we present an alternative strategy in which a heterogeneous catalyst triggers the selective esterification of levulinic acid with a stoichiometric amount of alcohol. 2174 Beilstein J. Org. Chem. 2016, 12, 2173–2180. In the last years, many methods have been developed for the transformation of homogeneous catalysts into recyclable heterogeneous ones. To prevent leaching, a common strategy is tethering the active species with the support via covalent bonds [35]. This approach increases the stability of the catalyst itself compared to impregnation (Figure 1). Furthermore, the activity of the catalyst can be tuned through adoption of a suitable linker. Figure 1: Anchoring methodologies: a) impregnation; b) covalent binding. Results and Discussion As part of our interest in acid catalysis [36-38], we prepared a set of solid materials for the esterific (...truncated)