Kinetics of glucose dehydration catalyzed by homogeneous Lewis acidic metal salts in water
Graphical abstract
Homogeneous Lewis acid salt relative activity for glucose conversion.
Introduction
Carbohydrates represent a promising carbon source, since they account for 75% of the renewable and abundant biomass feedstock. As such, the development of efficient and economical processes that can convert carbohydrates to useful chemical intermediates is very desirable. United States Department of Energy (US DOE) has identified 5-hydroxymethyl furfural (HMF) as one of the top 12 platform chemicals, having the potential to be a building block in synthesizing furanic-based polyamides, polyesters, and polyurethanes analogous to petroleum-based terephthalic acid [1], [2], [3], [4], [5], [6], [7].
HMF can be produced from C6 carbohydrates (glucose and fructose) using acid catalysts. In the past decade, extensive research efforts have studied the production of HMF from fructose. However, the more abundant and economical monosaccharide found in nature, glucose, is preferred as the feedstock. Compared to fructose, the lesser success in converting glucose to HMF is due to the stability of the pyranose ring structure [8]. As such, harsher reaction conditions such as higher reaction temperature would be required for glucose dehydration, which more likely leads to low HMF yields because of HMF decomposition. Therefore, several research groups have developed a sequential reaction pathway where glucose is first isomerized to produce fructose, which is subsequently dehydrated to HMF.
In order to implement the tandem isomerization–dehydration strategy to get high yields of HMF, researchers have focused on utilizing enzymes, solid bases or zeolites as isomerization catalysts with Brønsted acid catalysts for the dehydration reaction. Huang et al. [9] reported a two-step reaction system where glucose is first converted to fructose by glucose isomerase in aqueous media, and the produced fructose is dehydrated to HMF with HCl as the catalyst in a biphasic reactor, giving HMF yields of 63%. Using a similar approach, Takagaki et al. [10] reported another process in which Mg–Al hydrotalcite was used as a solid base to isomerize glucose to fructose and Amberlyst-15 as a solid acid to dehydrate fructose, giving HMF yields of 42% at 73% conversion in a reaction media of N,N-dimethylformamide. Solid Lewis acid zeolites, such as Sn-beta, have been used as metalloenzyme-like catalysts to isomerize glucose to fructose with high activity [11], [12], [13]. Using Sn-beta zeolites and HCl as catalysts in an aqueous/THF biphasic reaction system, HMF yields of 57% were obtained from glucose in a report by Nikolla et al. [14]. In addition to carrying out the reaction in aqueous or organic solvents, considerable efforts have focused on using ionic liquids as the solvent [15], [16]. For example, Zhao and co-workers [15] reported HMF yields from 68% to 70% using 1-ethyl-3-methyl-imidazolium chloride as a solvent with CrCl2 as the catalyst. The authors attributed the high catalytic performance obtained for the system to the facile ring opening of glucose and the stabilization of the transition state by Cr as a Lewis acid center.
Recently, various Lewis acidic metal salts, including Al3+, Ga3+, In3+, Sn4+, La3+, Dy3+, Yb3+, Zn2+, Ge4+, Cr3+, Cr2+, Cu2+, have been reported as active catalysts for converting C5 or C6 aldose sugars (glucose or xylose) to furanics in an aqueous media [17], [18], [19], [20], [21], [22], [23], [24], [25], organic solvents [26], or ionic liquids [15], [27]. Most of these reports have similarly focused on integrating the Lewis acid properties of the metal as the coordination center for glucose isomerization, and the protons resulting from the metal hydrolysis as a Brønsted acid to catalyze the dehydration reaction in order to achieve high yields of the furanic compounds. For example, a HMF yield of over 60% was achieved by using a combination of AlCl3 and HCl in a one-pot biphasic system as reported by two research groups [17], [23]. However, to the best of our knowledge, there is no study in the literature to systematically tune the reaction conditions such as Lewis acidic metal type, pH values of the reaction media, temperature, etc. As a result, a comprehensive examination of intrinsic kinetic properties including activation energies (Ea) and reaction rates so as to understand the Lewis acidic metal catalysts would provide further insight into how their Lewis acidic characteristics affect the glucose dehydration results.
It is generally known that when metal chlorides are contacted with water, depending on their hydrolysis constant values, they can be classified as either “water-compatible” or “water-sensitive” categories. For example, “water-compatible” Lewis acid metal cations refer to those associated with small hydrolysis constant values, resulting in a limited extent of hydrolysis. Kobayashi and Manabe [28] extensively studied “water-compatible” Lewis acids represented by lanthanide trifluoromethanesulfonates, Ln(OTf)3 for facilitating various organic transformations such as CC and CO bond formation. The Lewis activity of Ln(OTf)3 was ascribed to the high pKh values of the Ln cations of between 7.6 and 8.5 leading to their existence as coordinately unsaturated Lewis acidic aqua ions, which were believed to be the catalytically active species. Also, exchange rate constants for the substitution of inner-sphere water ligands or the water exchange rate constant (WERC) have been suggested to play a role in controlling the Lewis acidity of metal ions in water. It is commonly thought that Lewis acidic metal salts, such as AlCl3, are inactive in aqueous solutions, due to their favorable formation of mononuclear and polynuclear species in the presence of water molecules. Studies have shown that the speciation of these metal cations in water is highly dependent on the concentration of the metal cation and the solution pH [29], [30]. In this regard, metal salts such as AlCl3 can work as efficacious, water-compatible Lewis acids as well as metal salts if used at a suitable pH.
We have recently reviewed the catalytic dehydration of C6 sugars to HMF and concluded that strategies combining Lewis acid-catalyzed isomerization and Brønsted acid-catalyzed dehydration steps to convert glucose to HMF in either a biphasic system or in ionic liquid “one-pot” reactor configuration are quite promising [31]. Although many reports have focused on the efficacy of such systems on the overall conversion and selectivity, there have been limited studies performed to examine the kinetic profiles for the isomerization/dehydration steps. Better kinetic information can provide insights into the optimal ratio for the two catalytic functions as well as process and reactor conditions to optimize HMF production. Additionally, since many of the Lewis acids readily hydrolyze in water, which can either be the solvent or be produced during reaction, studies that can distinguish Brønsted and Lewis acidity and their effect on the catalytic process are necessary. In the current study, the effect of the nature of the Lewis acidic metal salts and different initial solution pH values ranging from 2.5 to 5.5 on the conversion kinetics of glucose were explored by examining Al, Ga, and In salts, which are hydrolysable Lewis acids, and La, Dy, and Yb salts, which are water-compatible Lewis acids. By using the same initial pH conditions, the intrinsic effect of Lewis acid strength on the glucose dehydration kinetic profile could be determined. In addition, by calculating the activation energies between 150 and 170 °C, a relationship between observed activities and reaction mechanism hypothesis can be correlated. Mechanistic studies have been performed by Davis and co-workers on the Sn-beta type solid Lewis acid for glucose conversion in water and they concluded glucose/fructose isomerization proceeded through a C2 position hydride shift mechanism [13]. While Choudhary et al. performed kinetic isotope effects of labeled glucose catalyzed by CrCl3, and AlCl3 under aqueous conditions [32], we also examined the kinetic isotope effect with glucose molecules labeled at the 2-H position under biphasic conditions relevant to glucose dehydration.
Section snippets
Preparation of reaction solution
Two types of Lewis acid salt solutions were prepared for reaction testing: water-compatible lanthanide metal salts and water-sensitive (hydrolysable) salts (e.g. AlCl3). For the lanthanide metal salts, 25 mM LaCl3, DyCl3, or YbCl3 (Aldrich) aqueous solutions were made by adding the appropriate amount of LnCl3 into nanopure water (18 MΩ cm) while adjusting the pH to the targeted value through the addition of HCl. Similarly, 5 mM AlCl3 or 25 mM InCl3 or GaCl3 (Aldrich) aqueous solutions were prepared
Effect of pH on the Lewis acid salt activities
In the aqueous media, the reactant, 0.3 M glucose, together with the catalyst, 5 mM AlCl3, was used under different solution pH values ranging from 2.5 to 5.5. Saturated levels of NaCl (∼33% by weight) were employed to facilitate the partitioning of HMF produced to the organic phase, which was 2-sec-butyl phenol (SBP). It has been shown in previous reports that SBP is very effective in extracting the furans made from carbohydrate dehydration in the reactive aqueous phase without concomitant
Conclusions
In summary, the use of homogeneous Lewis acid metal salts to catalyze glucose conversion particularly in aqueous media is of importance for developing a basic scientific understanding of the reaction as well as potentially aiding the practical deployment for the reaction. To examine this dehydration reaction in a more systematic manner, different Lewis acids at various pH conditions were studied. It was found that, depending on the hydrolysis properties of each Lewis acid, the H+ concentration
Acknowledgement
This material is based upon work supported by the National Science Foundation under Award No. EEC-0813570.
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