Short CommunicationThermodynamics and reaction pathways of furfuryl alcohol oligomer formation
Introduction
The conversion of lignocellulosic biomass into fuels and chemicals has been rigorously investigated as a response to the depletion of petroleum resources, increasing demand for oil, environmental concern over greenhouse gas emissions, and energy security [1], [2], [3]. Among the biomass derived chemicals, furfuryl alcohol (FA) is an attractive intermediate chemical for the production of levulinic acid [4], alkyl levulinate [5], and various useful polymer products [6], [7], [8]. FA can be obtained through the hydrogenation reaction of furfural and further converted into FA oligomers by the acid-catalyzed condensation reaction [9], [10], [11]. FA oligomers have been studied due to their application in fuel blending components [12] (Scheme 1). Buijtenen et al. have shown that after hydrogenation of FA oligomers with Ni/Al2O3, the fuel blend containing 10 vol.% of these C9–C20 carbon coupled FA oligomers fulfills the European diesel specification [12]. The acid-catalyzed condensation of FA is the major source for the FA oligomerization [13], [14]. Depending on the experimental conditions and method of analysis, different oligomer/polymer structures such as linear [9], [15], [16], cross-linked [17], [18], and open ring [19] have been proposed from the results of XRD [20], NMR [15], [21], [22], [23], UV–Vis [16], [21], [24], [25], IR [16], [20], [21], Raman spectroscopy [8], [9], [10], [13], [20], and DFT calculations [9]. We have recently shown that the FA monomer can dimerize in acidic solution and two possible structures can be produced through dehydration or deprotonation reactions (Eqs. (1), (2), the changes in free energy are given in kcal/mol) [9].
From the free energies formation of 2-hydroxymethyl-5(5-furfuryl)furan (HFF) is thermodynamically more favorable than 2,2′-difurfuryl ether (DFE). Even though, the formation of dimer from two FA monomers is thermodynamically favorable, because of the higher carbocation reactivity, dimer formation via the carbocation may be preferred for kinetic reasons. In addition to these two dimers, Khusnutdiv et al. reported that 2,2′-difurfurylmethane (DFM) can be also produced in the presence of the Rh(PPh3)3Cl catalyst using H2O and CCl4 as solvents [26]. The following mechanism for formation of DFM (Eq. (3)) was proposed by Sunjic et al. [27] and Korneeva et al. [28]. DFM and formaldehyde are produced through decomposition of DFE derived from the condensation of two FA monomers.
Using the 13C NMR spectroscopy, Choura et al. proposed a similar reaction mechanism (Eq. (4)) for the conversion of 5-methylfurfuryl alcohol (MFA) to the bis[2-(5-methylfuryl)]methane (BMFM) [15]. 2,2′-dimethylfurfuryl ether is formed first, which then decomposes into formaldehyde and BMFM.
Although the oligomerization of FA appears in a number of literature citations, the detailed understanding of reaction mechanisms and thermodynamics for the oligomerization processes (multiple pathways) is not available. Such an understanding will substantially enhance the utilization of FA as a platform chemical for the formation of longer chain hydrocarbons used in liquid transportation fuels. The aim of the presented study is to understand the reaction mechanism of oligomer formation and provide an accurate analysis of oligomer products. Here we have determined the product distribution of dimers (C9–C10) in detail and employed density functional theory to provide a qualitative confirmation of the experimentally observed dimer distribution trends.
Section snippets
Catalytic reaction
FA (98%) and sulfuric acid (99.999%) were obtained from Sigma-Aldrich and used without further purification. FA was mixed with dilute sulfuric acid (1 M) to catalyze the production of oligomers.
Initial GC/MS studies were undertaken to establish the identity of the major oligomeric species formed during the reaction (see Supporting information for the details of GC/MS experiment). Gas chromatographic analyses were performed using an Agilent 5890 Series II equipped with a flame ionization detector
Acid-catalyzed oligomerization of FA
Oligomerization of FA was performed with an aqueous solution containing 3.00 g FA and 0.03 g 1 M H2SO4 at 25 °C and ambient pressure, as shown in Fig. 1. During the reaction, the conversion values (mass basis) monotonically increase with increasing reaction time, and asymptotically reach ~ 32 wt.% in 24 h (Fig. 1a). Heavy oligomer (> C18) formation monotonically increases, while FA monomer (C5) wt.% decreases as a function of reaction time (Fig. 1b). It was found that up to 4 h, the wt% of > C18 was
Conclusions
In this work, we investigated the products and reaction mechanisms for the liquid-phase oligomerization of FA monomer using gas chromatography (experimental) and density functional methods (theoretical). The selectivity toward high molecular weight oligomers (> hexamer) increases with increasing reaction time. Four different kinds of dimers were observed: methylene bridged (D1), ether bridged (D2), terminal CH2OH (D3), and internal OH (D4) dimers. Theoretical studies were carried out to
Acknowledgments
This work was supported as part of the Institute for Atom-efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Argonne is managed by UChicago Argonne, LLC, for the U.S. Department of Energy under contract DE-AC02-06CH11357. Use of the computational resources from Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic
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Present address: Chemical and Molecular Engineering Department, Stony Brook University, NY 11787, USA.