materials Article Solvent-Free Esterification of Carboxylic Acids Using Supported Iron Oxide Nanoparticles as an Efficient and Recoverable Catalyst Fatemeh Rajabi 1,*, Mohammad Abdollahi 1 and Rafael Luque 2 1 Department of Science, Payame Noor University, P.O. Box 19395-4697, Tehran 19569, Iran; [email protected] 2 Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, km 396, Cordoba 14014, Spain; [email protected] * Correspondence: [email protected]; Tel.: +98-281-333-6366; Fax: +98-281-334-4081 Academic Editor: Eduardo J. García-Suárez Received: 9 May 2016; Accepted: 23 June 2016; Published: 12 July 2016 Abstract: Supported iron oxide nanoparticles on mesoporous materials (FeNP@SBA-15) have been successfully utilized in the esterification of a variety carboxylic acids including aromatic, aliphatic, and long-chain carboxylic acids under convenient reaction conditions. The supported catalyst could be easily recovered after reaction completion and reused several times without any loss in activity after up to 10 runs. Keywords: supported iron oxide nanoparticles; esterification; carboxylic acid 1. Introduction Esters play a significant role in daily living and the chemical industry. The reaction of carboxylic acids with alcohols to form esters is among the mildest and most efficient of organic transformations, largely a consequence of the high accessibility and stability of reactants. High ester usage in the synthesis of drugs, fine chemicals, pharmaceuticals, solvents and plasticizers as intermediates makes these substrates one of the most important types of compounds in organic chemistry [1]. In this context, some protocols for the synthesis of esters are well-known, including Fisher esterifications [2] and methylation reactions [3] of carboxylic acids. Due to the wide synthetic and biological applications of esters, a number of reagents such as ortho esters [4], N,N-dimethylformamide dialkyl acetals [5], triazene derivatives [6] and O-dialkyl isoureas [7] have been reported for the esterification of various aromatic/aliphatic carboxylic acids. The reaction of carboxylic acids and alcohols in the absence of catalysts is very slow and requires a long time for the reaction to reach equilibrium. To accelerate reaction rates, a number of catalysts have been reported inthe literature. These include classical solid acids such as ion exchange resins [8–10], zeolites [11,12], super acids [13,14], heteropolyacids [15–18] and supported chlorides [19]. Metal oxides such as CaO and MgO [20], metal-layered hydroxides [21,22], and efficient enzymatic catalysts [23,24] have also been employed in esterification reactions. However, many of these methods suffer from inherent drawbacks such as the need for expensive or harmful materials as reagents and catalysts, the formation of undesired side products, highly acidic conditions, the use of hazardous and toxic solvents, high reaction temperatures, low yield of products and prolonged reaction times. A need to develop an improved catalytic system for the synthesis of esters in terms of operational simplicity and economic viability is of utmost importance. Herein, a nanomaterial based on supported iron oxide nanoparticles on SBA-15 (FeNP@SBA-15) has been utilized as an efficient catalyst for a mild esterification of various carboxylic acids to their corresponding esters (Scheme1). The combination of iron nanoparticles and the mesoporous structure of the material showed excellent synergistic effects on the enhancement of Materials 2016, 9, 557; doi:10.3390/ma9070557 www.mdpi.com/journal/materials Materials 2016, 9, 557 2 of 9 Materials 2016, 9, 557 2 of 9 synergistic effects on the enhancement of activity and stability of the catalyst. Apart from a high Materials 2016, 9, 557 2 of 9 activityactivity, and stability the successful of the recycling catalyst. of Apart this catalytic from a system high activity, allows athe more successful economicrecycling and environmentally of this catalytic Materials 2016, 9, 557 2 of 9 systemfriendly allows process a more which economic is of and special environmentally advantage for friendly large-scale process preparations which is of and special industrial advantage for large-scalesynergistic preparationseffects on the andenhancement industrial of applications. activity and stability of the catalyst. Apart from a high activity,applications.synergistic the effectssuccessful on therecycling enhancement of this catalytic of activity system and allows stability a more of the economic catalyst. and Apart environmentally from a high friendlyactivity, theprocess successful which recycling is of ofspecial this catalytic advantage system for allows large-scale a more economicpreparations and environmentallyand industrial COOH COOMe applications.friendly process which is of specialFeNP@SBA-15 advantage for large-scale preparations and industrial applications. MeOH, Reflux COOH FeNP@SBA-15 COOMe COOH COOMe Scheme 1. Esterification ofFeNP@SBA-15 benzoic acid with FeNP supported on SBA-15. Scheme 1. Esterification ofMeOH, benzoic acidReflux with FeNP supported on SBA-15. MeOH, Reflux 2. Results and Discussion 2. Results and DiscussionScheme 1. Esterification of benzoic acid with FeNP supported on SBA-15. The catalyticScheme performance 1. 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(SEM), Transmission sizes were in electron the 5–7 nm (Figure 1). Fe species in the synthesized materials as measured by ICP/MS were found to be around range,microscopy withTEM an images excellent(TEM) of and the homogeneous X-ray catalyst photoe indicatedlectron dispersion that spectroscopy the iron of theoxide (XPS) iron nanoparticle [27]. oxide nanoparticles sizes were in onthe the5–7 supportnm 0.5–0.6 wt. %, with average iron oxide nanoparticle sizes in the 5–8 nm range. XRD of the materials (Figurerange,1).TEM Fe with species images an excellent inof thethe synthesizedcatalysthomogeneous indicated materials dispersion that the as ironof measured the oxide iron nanoparticle oxide by ICP/MS nanoparticles sizes were were found on