1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Sustainable Chemistry Efficient and Scalable Production of Alkyl Levulinates from Cellulose-Derived Levulinic Acid Using Heteropolyacid Catalysts Sharath Bandibairanahalli Onkarappa, Manjunath Javoor, Sib Sankar Mal,* and Saikat Dutta* [a] This work reports a straightforward and scalable synthesis of a series of alkyl levulinates from cellulose-derived levulinic acid and alkyl alcohols using commercially available heteropolyacid catalysts under homegenous conditions. The reaction was optimized on parameters such as temperature, molar ratio of reagents, type and loading of catalyst. The solvent-free reactions afforded alkyl levulinates in high isolated yields (> 85%) using only slight excess of alcohols and 10 wt% of catalyst at 120 °C in 6 h. Further, the catalysts were successfully recycled for three consecutive cycles without significant loss in activity. Introduction Cellulosic biomass has been considered by many as a renew- able source of carbon in producing transportation fuels and chemicals which are otherwise derived from non-renewable fossilized sources of carbon. [1] In a biorefinery concept, in analogy to a petrorefinery, a handful of chemical building blocks are produced initially which are then synthetically upgraded into fine chemicals and materials. [2] Levulinic acid (LA), produced by acid-catalyzed degradation of biomass- derived sugars and carbohydrates, has received significant attention over the past two decades as a biomass-derived chemical building block that can be synthetically upgraded into products of commercial value. [3] LA was chosen as one of the top-ten biomass-derived chemical building block of commercial potential by the United States Department of Energy in 2004 and then again in 2010 by Bozell et al. [4] Having two reactive functionality i. e. ketone and carboxylic acid, LA can be used as a renewable chemical intermediate to access various important classes of products such as fuels and fuel oxygenates, solvents, polymers, plasticizers, and agrochemicals. [5] Alkyl levulinates (AL) is an important class of product synthesized from LA with potential applications as renewable solvent and fuel oxygenate. [6] They can also be used as a chemical intermediate for the synthesis of other valuable products such as γ-valerolactone and 2-metyltetrahydrofuran. [7] AL can be produced by Fischer esterification between LA and alkyl alcohols in presence of a suitable acid catalyst, via acid- catalyzed dehydration of sugars in alcohols, or by alcoholysis of furfuryl alcohol. [8] A vast range of acid catalysts ranging from mineral acids, Lewis acid, sulfonated resins, to zeolites have been examined for the Fischer esterification of LA. [9] However, there is a constant search for more efficient catalysts that will allow the reaction to proceed under mild condition, afford product in high yield, non-toxic in nature, and can be recycled conveniently. In this regard, heteropolyacids (HPA) are well- structured metal-oxygen clusters with strong Brönsted acidity, high thermal stability, and tunable solubility. [10] HPAs have been used as efficient catalyst for various organic trans- formations and also in renewable chemistry. [11] Supported HPAs have been used for the synthesis of butyl levulinate. [12,13] Heterogeneous catalysts have certain advantages over homo- geneous catalysts such as convenient recovery of catalysts from the reaction media. On the other hand, homogeneous catalysts work under milder conditions but faces challenges in product purification and catalyst recycling. [14] In this regard, HPAs have decent solubility in alcohols especially at higher temperatures and in presence of a small amount of water. However, they are completely immiscible in non-polar organic solvents. We envisioned that the HPAs will behave as a homogeneous catalyst in polar solvents like alcohol under heating and in presence of water byproduct but can be isolated conveniently after reaction by using a non-polar organic solvent such as diethylether. In this work, we report the use of commercially available Keggin-type solid HPAs, namely, phosphotungstic acid (H 3 PW 12 O 40 ), phosphomolybdic acid (H 3 PMo 12 O 40 ), and silicotungstic acid (H 4 SiW 12 O 40 ) as catalysts for the esterification of LA with monohydric alkyl alcohols (C 1 - C 6 ) under homogeneous condition (Scheme 1). Results and Discussion In order to find the best reaction conditions, the conversion of Levulinic acid into alkyl levulinates was investigated using different heteropolyacids, levulinic acid to alcohol ratios, [a] S. B. Onkarappa, M. Javoor, Dr. S. S. Mal, Dr. S. Dutta Department of Chemistry, National Institute of Technology Karnataka (NITK), Surathkal, Mangalore-575025, Karnataka, India E-mail: sdutta@nitk.edu.in malss@nitk.edu.in Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201803641 Communications DOI: 10.1002/slct.201803641 2501 ChemistrySelect 2019, 4, 2501–2504 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim