Tarabanko V.E., Smirnova M.A., Chernyak M. Yu. and Morozov A.A.

Institute of Chemistry and Chemical Technology, SB RAS,

Akademgorodok, 50-24, Krasnoyarsk, 660036, Russia, veta@icct.ru

ON THE REASONS OF SELECTIVITY DECREASE OF THE CARBOHYDRATE ACID-CATALYZED CONVERSION

One of the most common way to process hexose carbohydrates is their acid-catalyzed conversion in water medium to levulinic acid (4-ketopentanoic acid, LA) and 5-hydroxymetylfurfural (5-hydroxymethyl-2-furaldehyde, 5-HMF). These compounds are used in different fields including biofuel production [1 - 2].

The most important problem of the carbohydrate acid-catalyzed conversion to LA and 5-HMF is to maintain the high selectivity with increasing the substrate concentration until 0.8 – 1 M. High 5-HMF yields (80 – 95 mol. %) are obtained only in solutions of 0.1 – 0.5 M carbohydrate concentrations, and they decrease dramatically at higher concentrations (Fig. 1) [1-4]. This problem is extremely important from the industrial viewpoint and require to be interpreted in terms of the reaction mechanisms.

Fig. 1 [4]. The carbohydrate concentration influence on the yields of LA, 5-HMF and their derivatives in different media (1 – fructose - water NaHSO₄– butanol, 2 – fructose - water – HCl, 3 – sucrose - water – HCl, 4 – fructose – butanol - H₂SO₄, 5 – sucrose – water NaHSO₄– butanol, 6 – fructose - water NaHSO₄.

The aim of this paper is to study the influence of glucose and LA progressive additions on the selectivity of the fructose acid-catalyzed conversion and to propose a mechanism for the described above and discussed below data on the selectivity decrease.

Experimental

Fructose and glucose of food quality and levulinic acid of ACROS-ORGANICS (USA) were used in the experiments. The experiments were carried out in a 250 ml thermostated magnetically stirred glass flask. The byproducts and intermediates were analyzed by GC-MS with Hewlett – Packard GCD Plus spectrometer.

Results and discussion

The effect of LA and glucose addition on fructose conversion. The LA addition at the beginning of the fructose conversion greatly decreases the maximum yield of LA obtained in the process, favoring the increase of the humin mass (Figures 2 and 3). There is a line dependence between masses of added levulinic acid and humic substances formed (Figure 3), and tangent of these straight lines is 1.5±0.1. In

Fig. 2. Influence of LA additions on the LA yield in fructose conversion (0.4 M, 108^0С, 2M NaHSO4, 1.7 M H2SO4. The initial added LA concentration: 1 : without LA addition, 2 : 0.02 M, 3 : 0.07 M, 4:0.09 M.	Fig. 3. Influence of added LA concentration on the yield of humic substances in fructose conversion (108^0C, 2 M NaHSO4, 1.7 M H2SO4. The initial fructose concentration: 1 : 0.4 M, 2 : 0.27 M, 3 : 0.14 M).

all the experiments, mass of humic substances increases with increasing fructose concentration. These facts indicate that levulinic acid reacts with fructose producing humic substances.

The glucose addition similarly affects the fructose conversion (Fig. 4). The influence of the glucose concentration on the humic substance yield shows linear character (Fig. 5) and tangent of straight line is 1.3±0.1. This results show that glucose reacts with levulinic acid giving rise to humic substances.

The rate ratio of fructose to glucose conversion is approximately 20 – 30 [3,4]. Therefore glucose additives into the solution of converted fructose can not detectably increase the overall LA yield, but purely demonstrate the formation of humic substances from carbohydrates.

Comparing the concentration data of Figures 2 and 4 shows that levulinic acid is approximately 50 times more active in depressing the LA formation compared to glucose. This means that high final concentration of levulinic acid, but not high initial carbohydrate concentration in the process, principally limited selectivity of acid catalyzed carbohydrate conversion.

Fig. 4. Influence of glucose additions on the LA yield in fructose conversion (0.4 M, 108^0С, 2 M NaHSO4, 1.7 M H2SO4.Glucose concentration: 1 : without glucose addition, 2 : 0.12 M, 3 : 0.24 M, 4 : 0.4 M.	Fig. 5. Influence of added glucose concentration on the humic substance mass in fructose conversion (0.4 M, 108^0C, 2M NaHSO4, 1.7 M H2SO4.)

On the mechanism of humin formation. In order to explain the obtained results and the just known data we suggest a novel hypothesis on the mechanism of target products and byproducts formation involving common carbocation species.

The first stage of LA and carbohydrate conversion is proton accepting and formation of the corresponding carbocations, immediately or after the removal of a water molecule:

R₂CHOH + H⁺ = R₂CHO⁺H₂ = R₂C⁺H + H₂O

CH₃C(O)CH₂CH₂COOH + H⁺ = CH₃C⁺(OH)CH₂CH₂COOH

These carbocation species can react then with the different molecules present in solution. These interactions can be divided in two groups and therefore the two possible pathways of carbocations conversion can be discussed:

(A) – interaction with water molecule (or solvent molecule);

(B) – interaction with substrate molecule or molecular products of its conversion (5-HMF or LA etc.)

(A) pathway of interaction with water molecules results finally in levulinic acid formation [5]:

(B) pathway of carbocation interaction with reagent or target product molecules results in increasing the product molecular mass, oligomerization and formation of humic substances. For example, the interaction of the carbocation A1 formed from fructose with enolic form of LA can give rise to ether derivatives of higher molecular weight with respect to the starting reagents:

Thus, selectivity of the acid-catalyzed conversion depends on the relative contributions of (A) and (B) pathways. The relationship between these pathways depends on the ratio of water to substrate activity, and, hence, levulinic acid yield has to decrease with increasing the carbohydrate concentration in the reaction mixture.

The suggested mechanism allows to assume that in order to increase the selectivity of concentrated carbohydrates acid-catalyzed conversion one have to extract levulinic acid from the acid carbohydrate solution. This approach was successfully realized by us adopting two-phase water-butanol system (Fig. 1) [4].

Acknowledgements. Financial support from Russian Foundation for Basic Research (Grant No. 13-03-00754) is gratefully appreciated.

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