Mechanism and kinetic parameters of glucose and fructose dehydration to 5-hydroxymethylfurfural over solid phosphate catalysts in water

Catalysis Today xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Mechanism and kinetic parameters of glucose and fructose dehydration to 5hydroxymethylfurfural over solid phosphate catalysts in water Nicolás I. Villanueva, Teresita G. Marzialetti

⁎

Department of Chemical Engineering, Faculty of Engineering, University of Concepcion, Concepcion, Chile

A R T I C L E I N F O

A B S T R A C T

Keywords: 5-Hydroxymethylfurfural Zirconium hydrogen phosphate Simulation Acid sites Glucose.

The reaction of dehydration of glucose and fructose was studied over solid acid catalysts (zirconium hydrogen phosphates, and sulfonated and carboxylated activated carbons) in water at 125, 135 and 145 °C. Zirconium hydrogen phosphate enhanced their activity and selectivity toward 5-HMF from glucose or fructose after being calcined at 700 °C (ZrPO-700). The activation energy of the first step of the dehydration of fructose (formation of an intermediary) was higher than the one of the second step (formation of 5-HMF from intermediary), 209 and 25 kJ/mol, respectively. Additionally, the activation energy of glucose isomerization to fructose was 97 kJ/mol. Overall; the formation of intermediary between fructose and 5-HMF was the limiting step in the formation of 5-HMF from glucose or fructose. Fitting simulations of experimental data estimated maximum yield and selectivity toward 5-HMF from glucose using ZrPO-700 at 125, 135 and 145 °C. These predictions help investigators to estimate conversions and selectivity when pilot plants need to be simulated.

1. Background As new carbon-neutral energy continues to be searched for, the attention for the chemistry of carbohydrates rises for they are the most abundant natural source of carbon [28]. One way to exploit carbohydrates and produce high-value substances is the reaction of dehydration, which produces, mainly, furanic compounds, levulinic acid and formic acid. 5-Hydroxymethylfurfural is a furanic compound obtained by catalytic dehydration of hexoses, such as glucose and fructose. In the later times, several studies have reported that it can be used in biofuels production (especially dimethylfuran) [22], monomers for commercial polymers [5] and others interesting substances [12]. Although good results for dehydration of glucose and fructose have been achieved in organic solvents [8], biphasic systems [21] and ionic liquids [26], there is not enough information regarding kinetic parameters and mechanism of this reaction in aqueous phase. Carnili et al. [6] reported the dehydration of hexoses over solid catalysts in water, but their approach was not extended enough. This information must be extended and completed to be usefully applied to an industrially scalable process. Amongst solid catalysts [28], phosphate salts have shown interesting results regarding selectivity and yield to 5-HMF. Asghari et al. [1] and Ordomsky et al. [24] studied the dehydration of fructose and

⁎

glucose, respectively, to 5-HMF over zirconium hydrogen phosphate catalysts in water discussing the role of different type of acid sites in the reaction. Asghari et al. tested calcined and non-calcined zirconium phosphate salts, sulfonated and carbonated activated carbons at 125, 135 and 145 °C achieving selectivity toward 5-HMF of 61.3% from fructose and 39.0% from glucose. Meanwhile, Ordomsky et al. examined aluminum, niobium, zirconium and titanium phosphate at 135 °C getting a maximum selectivity to 5-HMF of approximately 30% on ZrPO from glucose. Although they gathered much information they did not show kinetics or thermodynamics parameters, neither they proposed a reaction mechanism. Their conclusions about the role of the different acid sites are contradictory leaving open a discussion about the mechanism and kinetic parameters for this reaction. Consequently, the purpose of this study was to comprehend the mechanism of the reaction, the role of the acid sites, and obtain kinetic parameters such as kinetic constants and activation energies to be used on the design of a pilot or industrial scale plant to produce 5-HMF from forest biomass.

Corresponding author. E-mail address: [email protected] (T.G. Marzialetti).

http://dx.doi.org/10.1016/j.cattod.2017.04.049 Received 29 November 2016; Received in revised form 24 March 2017; Accepted 22 April 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Villanueva, N.I., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.04.049

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

2. Experimental

Ammonia TPDs. Fifty milligram of catalyst were added to a certain amount of distilled water at 25 °C, under vigorous stirring. An electronic pHmeter (Denver Instrument UB-10) measured pH values of liquid mixture at all times. Then 0.1 M NaOH were added drop by drop to the mixtures until pH value remained constant (the point of zero charge), which is around pH = 3.6 for zirconium phosphates [36] and rages from 2.2 [37] to 6.70 [38] for acid-functionalized AC. pH of 2.7 was reached in this study for all activated catalysts. Accordingly, the amount of acid sites in the catalyst could be estimated by knowing moles of NaOH added.

2.1. Materials Zirconium (IV) hydrogen phosphate was purchased from SigmaAldrich (Sigma-Aldrich, CAS number: 13772-29-7) and either used directly from package or after proceedings depending on the case. Anhydrous glucose was purchased from Merck Company (Germany, CAS number: CAS 50-99-7) and used directly from package. Fructose and 5-hydroxymehtylfurfural (for reaction or analysis) were obtained from Sigma-Aldrich Co., ultra-pure grade. Activated carbon was obtained from Pittsburgh Activated Carbon Co. Sulfuric acid (97%, Merck Millipore, Germany) and formic acid (98%, Sigma-Aldrich, Germany) were used directly from package.

2.3.4. Catalytic tests In a typical reaction, 1.00–7.00 g of catalyst were added to 90 mL of distilled water heating up to the reaction temperature in a stirred batch SS316 Parr reactor. Reactions were carried out at ascending stirring velocities until there were no differences in the reaction rates (∼300 rpm). One gram of carbohydrate (glucose or fructose) or 5HMF dissolved in 20 mL of distilled water was manually injected into the reactor when the reactor had reached the reaction temperature setpoint, assigning that time as t = 0. Internal mass transfer limitation was discarded accordingly to Weisz–Prater criterion (see supplementary information). Conversion (X) of species A was defined as shown in Eq. 1:

2.2. Catalyst preparation 2.2.1. Activated carbons Sulfonated (AC-SA) and carboxylated (AC-FA) activated carbons were prepared by acid impregnation as Onda et al. described [23]. In a typical impregnation, 5 g of activated carbon were added to 100 mL of concentrated sulfuric or formic acid in a SS316 reactor coated with Teflon. Under constant stirring, the mixture was heated up to 150 °C for 16 h. Afterwards, the activated carbon was washed using 3 L of distilled water at 80 °C until no ions were found in the washing water proved by testing the conductivity of the washing water with an electronic conductivity meter (Denver Instrument UB-10). The catalysts were then mixed with distilled water at 140 °C for 2 h under stirring, simulating the dehydration reactions, observing that there was no leaching of ions.

XA =

nA0 − nA , nA0

(1)

where nA0 is the initial amount of moles of A and nA is the amount of moles of species A at the time of measure. In the same way, yield (Y) of product P was defined as shown in Eq. 2:

YP =

2.2.2. Zirconium phosphates Different phases of Zirconium hydrogen phosphate (Zr(HPO4)2) were obtained after the calcination of original zirconium (IV) hydrogen phosphate (Sigma-Aldrich, CAS number: 13772-29-7). This material changes phases after exposed them to calcination at 620–650 °C and 1100 °C, according to reported in the literature [7,31]. Consequently, calcinations in this study were carried out in nitrogen up to 700 °C for 2 h, with a heating rate of 10 °C/min.

nP − nP 0 , νP nA0

(2)

where nP0 is the initial amount of moles of P, np is the amount of moles of species P at sampling time and νP is the relationship between P:A stoichiometric coefficients. Finally, selectivity (S) toward a product P is defined as follows (Eq. 3):

SP =

nP − nP 0 Y = P. νP (nA0 − nA) XP

(3)

A HPLC (model YL9100) determined concentration of carbohydrates, acids and furfurals by using 5 mM of sulfuric acid as a mobile phase and a HiPlex-8 μm chromatographic column.

2.3. Catalysts characterization 2.3.1. Surface area and porosity Surface area and pore size distribution of the catalysts were determined by nitrogen physisorption. The experiments were carried out in a Micromeritics Gemini VII apparatus at 77 K and 1 atm; surface areas were calculated using the adsorption isotherms applying BET adsorption equations.

3. Results and discussion 3.1. Catalysts characterization 3.1.1. Surface area and porosity Formic acid treatment increases in about 20% the surface area of activated carbon, similar to the effect produced by phosphoric acid [18]. On the other hand, AC-SA had similar surface area than untreated activated carbon, just as Onda et al. observed [23]. Zirconium salts had decreasing their surface area after calcination; alike it was reported in the literature [31]. Calcination up to 700 degrees turned into a loss of 30% of area, probably due to pore plugging, as Ciesla et al. described [33]. Table 1 summarized BET surface area and C coefficients. AC catalysts exhibited similar pore size distribution, between 25 and 45 Armstrong according to shown in Fig. 1 (left). Although AC-FA had a bigger surface area than AC-SA, formic acid treatment may open more pores in the carbon. In the case of zirconium phosphates, it is also seen that the distribution of the pore sizes was similar, but the height of the peak around 35 Armstrong displayed in Fig. 2 was small for ZrPO700, indicating less numbers of pores of that size.

2.3.2. Ammonia TPD The distribution and strength of acid sites in each catalyst were obtained from Temperature Programmed Desorption of ammonia. This technique was carried out in a Chembet 5652 apparatus. Prior to TPDs, samples (50 mg) were degassed by heating them up to 300 °C at 10 °C/ min under vacuum, cooled down to 50 °C and exposed to ammonia gas (loops of 50 μL) until the surface was saturated (ammonia titrations). Afterwards, a flow of pure helium at 50 °C for 2 h was passed through the samples eliminating any rest of physisorbed ammonia. Finally, all samples were heated up to 800 °C at 5 °C/min to evacuate surface ammonia molecules. At all time, relative concentration of ammonia in the effluent gas was determined by a Thermal Conduction Detector to get desorption profiles. 2.3.3. NaOH mass titration Titration with NaOH in water can also be used to estimate total acid sites. NaOH mass titration values supplemented the data obtained from

3.1.2. NaOH mass titrations Activated carbon showed significant improving in the acid site 2

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

According to Figs. 2 and 3 and Table 2 strength and number of acid sites are different on all catalysts; however, ZrPO-700 showed a wider site strength distribution and, consequently, undefined type of sites. As we know, ZrPO catalysts have Lewis and Brønsted acid sites [30] and AC catalysts have almost exclusively Brønsted acid sites [20], which are active for aldose-ketose isomerization [25]. Additionally, conversion rates should increase with increasing both number and strength of acid sites. Nonetheless, selectivity toward desired product depends on whether they are weak or strong acid sites. The goal of this study was to associate these four catalysts having different numbers and strength of sites to the selective formation of 5-HMF.

Table 1 Surface areas of the tested catalysts obtained through BET equation and C coefficient calculated from BET. Catalyst

BET surface area (m2/g)

C coefficient

AC AC-FA AC-SA ZrPO ZrPO-700

877 1036 898 277 193

55 241 133 101 101

concentration after being treated with sulfuric and formic acid, as shown in Table 2. Despite commercial catalysts may have higher number of acid site per gram of catalyst (Amberlyst-15 holds 4.7 meq/g, as listed by manufacturers), modified AC reached an important number of acid sites. On the other hand, zirconium catalysts showed less number of acid sites per gram of catalyst after being calcined. The amount of acid sites made a significant impact on the rate of conversion of carbohydrates. The concentration of acid sites in AC-FA and AC-SA are comparable when similar impregnation procedure and conditions (time and temperature) were applied, suggesting that the type of acid does not played a significant role on rising acid site numbers. It is also remarkable that the surface area of ZrPO catalysts was reduced up to ∼30%. Similarly, this catalyst has lost ∼36% in the number of acid sites implying that acid sites were homogeneously distributed around the surface of the zirconium salts.

3.2. Catalysts activity and reaction rates 3.2.1. Fructose dehydration The selectivity toward 5-HMF from fructose achieved with ZrPO700 catalyst at 135 °C shown in Fig. 4 has a turning point at 34% of conversion (or at 90 min of reaction), after that point the slope of the selectivity of 5-HMF decreased. The lessening on 5-HMF selectivity could be due to a change in the nature of the active sites during the reaction or the reaction occurred throughout an intermediary, in which reaction rates are similar in order of magnitude. Stability tests of this catalyst showed that it slowly loses activity as well suggesting that the reaction occurred in two steps. The reaction with ZrPO catalyst exhibits the highest rate of conversion of fructose of the two zirconium catalysts but lower selectivity. While the explanation of the differences in fructose conversion could be the larger amount of acid sites in ZrPO catalyst (Table 2), the answer to the better selectivity achieved by ZrPO-700 catalyst might be the difference on the strengths of acid sites of each catalyst. Ammonia TPD analyses suggest that ZrPO catalyst has the higher number of strong acid sites between the two. Benvenuti et al. [2] reported that strongly acidic sites promote rehydration reactions of 5HMF to levulinic and formic acid, or polymerization reactions of 5-HMF and fructose. On the other hand, Ordomsky et al. [24] reported that the Lewis acid sites are responsible for these polymerization reactions. According to Ammonia TPDs, AC catalysts had more strong acid sites than ZrPO catalysts suggesting that rehydration and polymerization reactions of 5-HMF occurred decreasing 5-HMF yields. A yield of 7% was reached at studied reaction conditions over AC-FA but only 4% over AC-SA. Fig. 5 represents selectivity of 5-HMF against conversion of fructose at 135 °C over AC-FA and AC-SA, respectively. As expected, better selectivity achieved AC-FA catalyst because it could have more weak acid sites than AC-S A (Table 3), although less amount of total acid sites (Table 2). However, AC-SA and AC-FA showed similar evolution during reaction time. Regarding the effect of the temperature for ZrPO catalysts, decreasing reaction temperature increases the selectivity toward 5-HMF on calcined Zr(HPO4)2 as shown in Fig. 6. This effect was bigger at higher

3.1.3. Ammonia TPD TPD profiles for ammonia desorption displayed in Fig. 2 (a) and (b) show that ZrPO-700 catalyst had three adsorption sites at 120, 200 and 400 °C, while ZrPO had only one adsorption site at 400 °C. These profiles indicate that some strong sites turned into weak sites after calcination, similarly to what Benvenuti et al. observed [2]. We suggest that the water molecules placed in the structure of Zr(HPO4)2 were removed during calcination altering its structure to change to ZrP2O7. This phenomenon has been described previously in the literature [7] and it could be similar in zeolites [29]. The calcination process diminished the acid sites of zirconium catalysts by 32%, estimated by integrating ammonia TPD curves. This observation is consistent with the calculations made by NaOH titration tests for these catalysts (Table 2). Ammonia desorption profiles of AC catalysts show on Fig. 3 suggest that AC-FA (left) had apparently more weak acid sites than AC-SA (right). Additionally, AC-SA shows a new peak at 500 °C that can be related to sulfonic groups [16]. Although AC-FA had apparently fewer amount of total acid sites than AC-SA, estimated by integrating NH3 TPD curves, which is also consistent with the information provided by NaOH titrations (Table 2).

Fig. 1. Pore size distribution of activated carbons (left) and zirconium salts (right), obtained by N2 desorption.

3

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

Fig. 2. Ammonia TPDs of ZrPO-700 (a) and ZrPO (b). Heating rate of 5 °C/min. Table 2 Acid site concentration and distribution of the catalysts tested, measured by NaOH titration to point of zero charge and by NH3 TPD. Catalyst

Acid-site concentration by NaOH titration (meq/ g)

Peak in NH3 TPD (°C)

Area of NH3 TPD × 10−6 (a.u.)

Total sites on NH3 TPD (%)

Acid-site concentration by NH3 TPD (meq/g)

AC AC-FA

0.091 2.56

2.68

ZrPO

0.85

ZrPO-700

0.54

– 3.8 4.5 24.1 6.3 17.8 13.8 1.5 3.0 7.5 1.6 3.4 3.2

– 11.5 13.6 74.9 16.5 47.0 36.5 10.5 25.3 64.2 19.4 41.3 39.3

– 2.69

AC-SA

– 100 180 380 100 375 520 200 300 450 120 200 400

2.91 Fig. 4. Selectivity vs conversion of fructose over Zirconium catalysts at 135 °C. Solvent: Distilled water 110 mL. Mass of catalyst: 1.00 g. Mass of carbohydrate: 2.00 g.

0.92

temperatures (145–135 °C). It is also interesting to observe that the reaction at 135 °C dramatically changes the slope of the rate of the production of 5-HMF reaching similar values than the one at 145 °C. The rate of formation of 5-HMF at 125 °C was higher than the rate of conversion of fructose (Fig. 6) suggesting that if this reaction occurs in two steps as we propose, the second one was slower (conversion of the intermediate to 5-HMF).

0.58

Fig. 3. Ammonia TPD profiles of AC-FA (left) and AC-SA (right). Heating rate of 5 °C/min. (TPDs tests were performed at 8x attenuation to avoid saturation).

4

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

Assuming all reactions are first-order, the model is mathematically represented by Eq. 4–6:

dCF = −k3 CF − kB2 CF , dt

(4)

dCI1 = K3 CF − k 4 CI1, dt

(5)

dCHMF = K4 CI1, dt

(6)

where kF = kB2 + k3 Initial conditions are declared in Eq. 7–9:

Fig. 5. Dehydration of fructose at 135 °C over AC-SA and AC-FA.Solvent: Distilled water 110 mL. Mass of catalyst: 1.00 g. Mass of carbohydrate: 2.00 g.

T (°C)

kF [min

125 135 145 Eapp [kJ/ mol]

1.2 × 10−3 4.8 × 10−3 1.8 × 10−2 186

]

k3 [min

−1

]

8.0 × 10−4 3.9 × 10−3 1.7 × 10−2 209

k4 [min

−1

]

1.2 × 10−2 1.3 × 10−2 1.6 × 10−2 24

kB2 [min

−1

]

4.3 × 10−4 8.9 × 10−4 1.6 × 10−3 91

(7)

CI0 (t = 0) = 0,

(8)

CHMF (t = 0) = 0.

(9)

Concentrations as function of time can be obtained as shown Eq. 10–12.

CF = CF 0 e−kF t ,

Table 3 Value of the pseudo first-order kinetic constants and activation energies for the fructose dehydration to 5-HMF over ZrPO-700. −1

CF (t = 0) = CF0,

CI =

Smax = k3/kF 0.65 0.81 0.91

(10)

CF 0 k3 e−k 4 t (1 − e−(kF − k 4 ) t ) , kF − k 4

CHMF =

CF 0 k3 [kF (1 − e−k 4 t ) − k 4 (1 − e−kF t )] , kF (kF − k 4 )

(11)

(12)

where kF = kB2 + k3. Least squares fitted the model to conversion and yield data as shown in Fig. 7 for the reaction tested over ZrPO-700 catalyst at 135 °C. Accordingly, estimated parameters kF, k3 and k4 are 4.87 × 10−3, 3.9 × 10−3 and 1.3 × 10−2, respectively. The model has physical meaning since kF was higher than k3, and k4 was only one order of magnitude bigger than k3 suggesting that there is no rate-determining step. The ratio between k3 and kF define the maximum selectivity, as shown in Eq. 13.

Smax =

dCI dt dCI dCBP * dt dt

=

k3 CF k 3.9 × 10−3 = 3 = = 0.81. (k3 + kB2 ) CF kF 4.8 × 10−3

(13)

It is interesting to note that fructose conversion follows a normal first-order type conversion indicating that our hypothesis is plausible. Same data treatment was made for the reactions at 125 and 145°. The constants and estimated apparent activation energy are summarized in Table 3. The trend in the activation energies against the inverse of the absolute temperature is almost linear, which means that the amount of substrate adsorbed do not significantly change with tem-

Fig. 6. Effect of reaction temperature on the dehydration of fructose over ZrPO-700 catalyst. Evolution of selectivity of 5-HMF against conversion. Solvent: Distilled water 110 mL. Mass of catalyst: 1.00 g. Mass of carbohydrate: 2.00 g.

3.2.2. Proposed mechanism for the dehydration of fructose. ZrPO-700 was chosen to adjust proposed model since it was the catalyst that showed the best selectivity toward 5-HMF. Based on our experimental results and literature reports, a two-step pathways mechanism of the reaction should fit experimental results. The mechanism we proposed includes the degradation of fructose to polymerized undesired products. Accordingly, we agreed to propose a model where all reactions suit a first-order reaction, as shown in Scheme 1.

Fig. 7. Pseudo first-order model fitted (line) to the experimental data (points) of fructose dehydration over ZrPO-700 at 135 °C. Fructose: 2.00 g. Catalyst: 1.00 g. Solvent: 210 mL distilled water.

Scheme 1. Reaction pathway proposed for the decomposition of fructose over ZrPO-700. F: fructose, I1: intermediary, BP: by-products.

5

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

perature, and therefore the heat of adsorption should have been lower than the activation energy. Table 3 also includes the value of kB2 calculated by subtracting k3 from kF. The activation energies estimated in this study are much higher than the ones obtained by Carniti et al. [6] over niobic phosphate in water within 90 and 110 °C (Ea = 65 ± 8), although they are in the same order of magnitude. Even though they worked with different catalysts, they also observed an increasing selectivity trend, but they did not propose a mechanism. The theoretical maximum selectivity (defined in Eq. 13) rises with temperature, while Fig. 6 shows that increasing the temperature decreased selectivity toward 5-HMF. This apparent contradiction occurs because the activation energy on the first step (k3) was much bigger than on the second step (k4) according to Table 3. At 125 °C k3 was two orders of magnitude bigger than k4, implying that the intermediary rapidly turned into 5-HMF. On the other side, at 145 °C k3 and k4 were similar between each other, but kB2 was one order of magnitude smaller than k3 and k4. Additionally, increasing the temperature increased kB2, which reduced 5-HMF selectivity. The maximum theoretical selectivity estimated at this point did not consider further degradation reactions of 5-HMF.

Fig. 9. Conversion of glucose and yield to 5-HMF and fructose for glucose dehydration at 125 °C over AC-FA. Solvent: 110 mL Distilled water. Catalyst: 1.00 g. Carbohydrate: 2.00 g.

much slower than the one of fructose, and so was the rate of formation of 5-HMF. Also, selectivity toward 5-HMF from fructose was low because glucose underwent other reactions besides isomerization to fructose.

3.2.3. Glucose dehydration The mechanism of glucose dehydration whether it goes through fructose or not is still under discussion. Although experimental data on different catalysts and reaction conditions from many investigations has shown that fructose dehydration to 5-HMF is much faster than glucose dehydration to 5-HMF, some authors still argue that isomerization to fructose is not a required pathway. To contribute somehow to this discussion, Fig. 8 shows glucose conversion, fructose and 5-HMF yields for the dehydration of glucose over ZrPO-700 at 125 °C. The initial rate of 5-HMF formation was smaller than the initial rate of fructose formation suggesting that the isomerization of glucose to fructose has to happen. Isomerization reaction occurred in this catalyst because it has Lewis acid sites, which are required for aldose-ketose isomerization according to the literature [3,4,25]. In consequence, Lorenzelli et al. reported that zirconium hydrogen phosphate catalysts have Lewis and Brønsted acid sites based on FTIR spectra of adsorption of pyridine, acetone and acetonitrile [17]. In addition, Ordomsky et al. [24] reported dehydration reactions of glucose in water using a zirconium phosphate having much fewer Brønsted acid sites than Lewis acid sites. Only one peak of ammonia was observed around 250 °C according to their ammonia TPD spectra. Consequently, they achieved low maximum selectivity and yield of 5-HMF of 25% and 15%, respectively, at 135 °C. On the other hand, AC catalysts had negligible production of fructose and 5-HMF, but a high conversion of glucose (Fig. 9). This observation reinforces that the isomerization of glucose to fructose is a needed step in the mechanism of this reaction since AC has only Brønsted acid sites [4]. As expected, the conversion rate of glucose was

3.2.4. Proposed mechanism of reaction for glucose dehydration The reaction pathway proposed for the production of 5-HMF from glucose over ZrPO-700 involved a first reversible step of isomerization of glucose to fructose based on our experimental data, followed by the mechanism proposed above. This mechanism also included the formation of one undesired product (BP1, kB1). Scheme 2 pictures this mechanism: Equilibrium constants of the isomerization of glucose at 125, 135 and 145 °C estimated by using Van’t Hoff expression and data at 25 °C obtained from literature [34] such as Keq = 0.87, ΔH° = 2.8 kJ/mol and ΔCp = 76J/mol K are summarized in Table 4. According to the literature [35] standard enthalpy does not significantly change with temperature within this range of temperature. Differential equations that describe concentrations of glucose, fructose, intermediary and 5-HMF are represented by Eq. 14–17.

⎛ k ⎞ dCG = −kG CG + ⎜ 1 ⎟ CF , dt ⎝ Keq ⎠

(14)

dCF = k1 CG − kF CF , dt

(15)

dCI = k3 CF − k 4 CI , dt

(16)

dCHMF = k 4 CI . dt

(17)

Subsequently, estimated values of kinetic constants and selectivity toward fructose (isomerization reaction) are summarized in Table 4. Selectivity toward fructose was estimated using Eq. 18.

SF =

k1 . kG

(18)

The activation energy of glucose consumption (kG, Ea = 124 kJ/

Fig. 8. Conversion of glucose and yield to 5-HMF and fructose for glucose dehydration at 125 °C over ZrPO-700. Solvent: 110 mL Distilled water. Catalyst: 1.00 g. Carbohydrate: 2.00 g.

Scheme 2. proposed mechanism for the production of 5-HMF from glucose, where kG = k1 + kB1.

6

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

Table 4 Estimated values of the equilibrium and kinetic constants for isomerization of glucose toward fructose and their selectivity. T (°C)

Keq

kG [min−1]

k1 [min−1]

kB1 [min−1]

SF

125 135 145

2.25 2.92 3.45

1.4 × 10−3 3.4 × 10−3 8.2 × 10−3

1.1 × 10−3 2.5 × 10−3 5.2 × 10−3

1.26 × 10−2 3.18 × 10−2 7.71 × 10−2

0.82 0.72 0.63

mol) was bigger than the one estimated for the fructose formation (k1, Ea = 106 kJ/mol) suggesting that the isomerization reaction of glucose to fructose become less selective at high temperature favoring byproducts production (BP1). Regarding the activation energy of the formation of fructose, values reported in the literature were consistent with ours like the 104 kJ/mol estimated by Moreau et al. using zeolites [19] or the 126 kJ/mol reported by Lacomte et al. over modified hydrotalcites [15]. Although our proposed mechanism did not involve the decomposition reactions of 5-HMF, several authors stated that 5-HMF decomposes and rehydrates under acidic conditions [10,13–15]. Consequently, 5HMF conversion was briefly analyzed against temperature over ZrPO700 represented in Fig. 10. A pseudo first-order reaction suited experimental data achieving kinetic constants of 3.61 × 10−4, 7.40 × 10−4 and 1.47 × 10−3 min−1 at 125, 135 and 145 °C, respectively. Note that these kinetic constants were lower than those estimated for fructose and glucose conversions (Tables 3 and 4, respectively). The activation energy estimate for this reaction was 97 kJ/mol, which was slightly lower than the one calculated by Girisuta et al. [10], who reported activation energy of 111 kJ/mol for the homogeneous decomposition of 5-HMF catalyzed by sulfuric acid. To validate that the strong acid sites promoted further reactions of 5-5-HMF to humins or other compounds, we measured the rate of formation of levulinic acid (LA) from 5-HMF at 135 °C over the catalysts tested in this study. As we expected, Fig. 11 shows that the rate of formation of levulinic acid linearly increased increasing the number of strong acid sites (TPD peaks over 350 °C). The kinetic constant for the formation of levulinic acid from 5-HMF was smaller than the one for 5-HMF decomposition suggesting that 5HMF underwent other reactions at tested reaction conditions.

Fig. 11. Pseudo first-order constant for the formation of levulinic acid from 5-HMF against the number of strong acid sites estimated by integrating NH3 TPD peaks over 350 °C. Catalyst: 1.00 gram. 5-HMF: 1.00 g. Solvent: 110 mL distilled water. Reaction temperature: 135 °C.

Fig. 12. Simulation of the conversion of glucose against selectivity toward 5-HMF over ZrPO-700 catalyst at 125, 135 and 145 °C.

selectivity toward 5-HMF from glucose was 60, 53 and 46% at 125, 135 and 145 °C, respectively. In order to reach a maximum selectivity, simulations were kept up to 2500, 900 and 400 min for the reactions at 125, 135 and 145 °C, respectively. A comparative plot of experimental data against the model prediction displayed in Fig. 13 showed that the proposed mechanism for the dehydration of furfural fitted very well our experimental data at 125 and 145 °C. The turning point of the slope of selectivity toward 5-HMF from fructose at 135 °C highlighted in a previous section is evidenced in this plot (circle marked in Fig. 13).

3.2.5. Simulation and validation of proposed mechanism Kinetic parameters estimated allowed above predict glucose conversion as well as yields of fructose and 5-HMF at periods of time longer than reaction times tested in this study. Simulated profiles displayed in Fig. 12 shows that the selectivity toward 5-HMF reaches a maximum within 50 and 60%, depending on the reaction temperature, before the reaction gets 90% of the conversion of glucose. Specifically, maximum

Fig. 10. Concentration profiles for the 5-HMF consumption at 125 °C (green triangles), 135 °C (blue diamonds) and sky blue stars (145 °C). 110 mL of distilled water, 1.00 g of ZrPO-700 and 1.02 g of initial 5-HMF.

Fig. 13. Comparing the model prediction to experimental data of selectivity toward 5HMF from fructose.

7

Catalysis Today xxx (xxxx) xxx–xxx

N.I. Villanueva, T.G. Marzialetti

niobium phosphate as highly acidic viable catalysts in aqueous medium: Fructose dehydration reaction, Catal. Today 118 (3) (2006) 373–378. [7] A. Clearfield, J.A. Stynes, The preparation of crystalline zirconium phosphate and some observa- tions on its ion exchange behaviour, J. Inorg. Nucl. Chem. 26 (1) (1964) 117–129. [8] Solenne Despax, Carole Maurer, Boris Estrine, Jean Le Bras, Norbert Hoffmann, Sinisa Marinkovic, Jacques Muzart, Fast and efficient dmso-mediated dehydration of carbohydrates into 5-hydroxymethylfurfural, Catal. Commun. 51 (2014) 5–9. [10] B. Girisuta, L.P.B.M. Janssen, H.J. Heeres, A kinetic study on the decomposition of 5- hydroxymethylfurfural into levulinic acid, Green Chem. 8 (8) (2006) 701–709. [12] Navneet Kumar Gupta, Shun Nishimura, Atsushi Takagaki, Kohki Ebitani, Hydrotalcite- supported gold-nanoparticle-catalyzed highly efficient base-free aqueous oxidation of 5- hydroxymethylfurfural into 2, 5-furandicarboxylic acid under atmospheric oxygen pressure, Green Chem. 13 (4) (2011) 824–827. [13] Jinder Jow, Gregory L. Rorrer, Martin C. Hawley, Derek T.A. Lamport, Dehydration of d- fructose to levulinic acid over lzy zeolite catalyst, Biomass 14 (3) (1987) 185–194. [14] Jacob S. Kruger, Vinit Choudhary, Vladimiros Nikolakis, Dionisios G. Vlachos, Elucidating the roles of zeolite h-bea in aqueous-phase fructose dehydration and 5HMF rehydration, ACS Catal. 3 (6) (2013) 1279–1291. [15] Jérôme Lecomte, Annie Finiels, Claude Moreau, Kinetic study of the isomerization of glucose into fructose in the presence of anion-modified hydrotalcites, StarchStärke 54 (2) (2002) 75–79. [16] Xuezheng Liang, Minfeng Zeng, Chenze Qi, One-step synthesis of carbon functionalized with sulfonic acid groups using hydrothermal carbonization, Carbon 48 (6) (2010) 1844–1848. [17] Vincenzo Lorenzelli, Paola Galli, Aldo La Ginestra, Pasquale Patrono, et al., A fourier-transform infrared and catalytic study of the evolution of the surface acidity of zirconium phosphate following heat treatment, J. Chem. Soc. Faraday Trans. 1 83 (3) (1987) 853–864. [18] M. Molina-Sabio, F. Rodriguez-Reinoso, F. Caturla, M.J. Selles, Porosity in granular carbons activated with phosphoric acid, Carbon 33 (8) (1995) 1105–1113. [19] Claude Moreau, Robert Durand, Alain Roux, Didier Tichit, Isomerization of glucose into fructose in the presence of cation-exchanged zeolites and hydrotalcites, Appl. Catal. A: Gen. 193 (1) (2000) 257–264. [20] Kiyotaka Nakajima, Michikazu Hara, Amorphous carbon with so3 h groups as a solid brønsted acid catalyst, ACS Catal. 2 (7) (2012) 1296–1304. [21] Eranda Nikolla, Yuriy Román-Leshkov, Manuel Moliner, Mark E. Davis, “One-pot” synthesis of 5-(hydroxymethyl) furfural from carbohydrates using tin-beta zeolite, Acs Catal. 1 (4) (2011) 408–410. [22] Shun Nishimura, Naoya Ikeda, Kohki Ebitani, Selective hydrogenation of biomassderived 5-hydroxymethylfurfural (5-HMF) to 2, 5-dimethylfuran (dmf) under atmospheric hydrogen pressure over carbon supported pdau bimetallic catalyst, Catal.Today 232 (2014) 89–98. [23] Ayumu Onda, Takafumi Ochi, Kazumichi Yanagisawa, Selective hydrolysis of cellulose into glucose over solid acid catalysts, Green Chem. 10 (10) (2008) 1033–1037. [24] V.V. Ordomsky, V.L. Sushkevich, J.C. Schouten, J. van der Schaaf, T.A. Nijhuis, Glucose de- hydration to 5-hydroxymethylfurfural over phosphate catalysts, J. Catal. 300 (2013) 37–46. [25] Yuriy Román-Leshkov, Christopher J. Barrett, Zhen Y. Liu, James A. Dumesic, Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates, Nature 447 (7147) (2007) 982–985. [26] Carsten Sievers, Ildar Musin, Teresita Marzialetti, B. Mariefel, Valenzuela Olarte, Pradeep K. Agrawal, Christopher W. Jones, Acid-catalyzed conversion of sugars and furfurals in an ionic-liquid phase, Chem. Sus. Chem. 2 (7) (2009) 665–671. [28] Robert-Jan van Putten, Jan C. van der Waal, E.D. De Jong, Carolus B. Rasrendra, Hero J. Heeres, Johannes G. de Vries, Hydroxymethylfurfural, a versatile platform chemical made from re- newable resources, Chem. Rev. 113 (3) (2013) 1499–1597. [29] Jacques C. Védrine, Aline Auroux, Vera Bolis, Pierre Dejaifve, Claude Naccache, Piotr Wierz- chowski, Eric G. Derouane, Janos B. Nagy, Jean-Pierre Gilson, Jan H.C. van Hooff, Jan P. van den Berg, Jillus Wolthuizen, Infrared, microcalorimetric, and electron spin resonance investiga- tions of the acidic properties of the h-zsm-5 zeolite, J. Catal. 59 (2) (1979) 248–262. [30] Ronen Weingarten, Geoffrey A. Tompsett, Wm Curtis Conner Jr., George W. Huber, Design of solid acid catalysts for aqueous-phase dehydration of carbohydrates: The role of lewis and brønsted acid sites, J. Catal. 279 (1) (2011) 174–182. [31] Shoji Yamanaka, Masami Tanaka, Formation region and structural model of γzirconium phosphate, J. Inorg. Nucl. Chem. 41 (1) (1979) 45–48. [33] U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schüth, Formation of a porous zirconium oxo phosphate with a high surface area by a surfactant-assisted synthesis, Angew. Chem. Int. Ed. Eng. 35 (5) (1996) 541–543.

4. Conclusions A two-steps mechanism thought an intermediary I described the degradation of fructose to 5-HMF over calcined Zr(PO4)2 catalyst (ZrPO-700). The rate constants of both steps were similar in magnitude suggesting that there is no rate-determining step in this reaction. However, kinetic parameters estimated for glucose dehydration to 5HMF probed that the isomerization of glucose to fructose was the critical step in formation of 5-HMF contributing with fundamental data to this controversial reaction. The activation energy of the first step of dehydration of fructose (fructose to intermediary) was found to be bigger than the one estimated for the second step (transformation of intermediary to 5HMF). Apparent activation energy was estimated in 209 and 25 kJ/mol for the first and second step, respectively. Consequently, the ratedetermining step for the dehydration of fructose was the second step of dehydration at high temperatures; however, at lower temperatures it was the first one. Fitting simulations to experimental data estimated the maximum selectivity and yield toward 5-HMF from glucose over ZrPO-700 achieving selectivity values up to 60% at long reaction times. Accordingly, a reaction at 125 °C should take up to 42 h to react 54% of selectivity, while a reaction at 145 °C could reach 60% of selectivity in 11 h. Finally, a comparative plot of simulated data against experimental data showed that proposed mechanism fitted experimental data when dehydration of fructose occurred at 125 or 145 °C. Acknowledgments This work was mainly financially supported by Fondef-ConicytChile CA13I10266. The authors also acknowledge financial support from Basal PFB-27 of UDT-UdeC. The authors want to acknowledge Ms. Verónica Sierra for her unlimited collaboration. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.04.049. References [1] Feridoun Salak Asghari, Hiroyuki Yoshida, Dehydration of fructose to 5-hydroxymethylfurfural in sub-critical water over heterogeneous zirconium phosphate catalysts, Carbohydr. Res. 341 (14) (2006) 2379–2387. [2] Federica Benvenuti, Carlo Carlini, Pasquale Patrono, Anna Maria Raspolli Galletti, Glauco Sbrana, Maria Antonietta Massucci, Paola Galli, Heterogeneous zirconium and titanium catalysts for the selective synthesis of 5-hydroxymethyl-2-furaldehyde from carbohydrates, Appl. Catal. A: Gen. 193 (1) (2000) 147–153. [3] Ricardo Bermejo-Deval, Rajeev S. Assary, Eranda Nikolla, Manuel Moliner, Yuriy Román-Leshkov, Son-Jong Hwang, Arna Palsdottir, Dorothy Silverman, Raul F. Lobo, Larry A. Curtiss, et al., Metalloenzyme-like catalyzed isomerizations of sugars by lewis acid zeolites, Proc. Natl. Aca. Sci. 109 (25) (2012) 9727–9732. [4] Prasenjit Bhaumik, Paresh Laxmikant Dhepe, Chapter 1 conversion of biomass into sugars, In biomass sugars for non-fuel applications, The Royal Society of Chemistry, 2016, pp. 1–53. [5] Teddy Buntara, Sebastien Noel, Pim Huat Phua, Ignacio Melián-Cabrera, Johannes G. de Vries, Hero J. Heeres, Caprolactam from renewable resources: Catalytic conversion of 5- hydroxymethylfurfural into caprolactone, Angew. Chem. Int. Ed. 50 (31) (2011) 7083–7087. [6] Paolo Carniti, Antonella Gervasini, Serena Biella, Aline Auroux, Niobic acid and

8