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Dehydration of fructose to 5-hydroxymethylfurfural over H-mordenites
~ A PA LE IY DSS CP AT L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 145 (1996) 211-224
Dehydration of fructose to 5-hydroxymethylfurfural over H-mordenites Claude Moreau a,*, Robert Durand a Sylvie Razigade a Jean Duhamet b, Pierre Faugeras b, Patrick Rivalier b Pierre Ros b G6rard Avignon c a Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, URA CNRS 418, Ecole Nationale Sup£rieure de Chimie, 8, Rue de I'Ecole Normale, 34053 Montpellier Cedex 1, France b CEA, D C C / D R D D / SEMP/SGC, BP 171, 30207 Bagnols-sur-Ckze Cedex, France c Le Petit Buscon, 47310 Estillac, France
Received 16 January 1996; revised 4 April t996; accepted 4 April 1996
Abstract
Dehydration of fructose to 5-hydroxymethylfurfural was performed in a batch mode in the presence of a series of dealuminated H-form mordenites as catalysts, at 165°C, and in a solvent mixture consisting of water and methyl isobutyl ketone (1:5 by volume). Under the operating conditions used, the reaction was not controlled by external or internal diffusional limitations. Fructose conversion and selectivity to 5-hydroxymethylfurfural were found to depend on acidic and structural properties of the catalysts used as well as on the micropore vs. mesopore volume distribution of those catalysts. A maximum in the rate of conversion of fructose was observed for the H-mordenite with a Si/A1 ratio of l 1. A maximum in the selectivity to 5-hydroxymethylfurfural was observed only for H-mordenites with a low mesoporous volume. The high selectivity obtained ( > 90%) was correlated with the shape selectivity properties of H-mordenites (bidimensional structure), and particularly with the absence of cavities within the structure allowing further formation of secondary products. The influence of the microporosity vs. mesoporosity on the selectivity to 5-hydroxymethylfurfural was also studied, the formation of mesopores upon dealumination procedures being damaging to obtain a high selectivity. A significant increase in the selectivity (10%) was also obtained by simultaneous extraction of 5-hydroxymethylfurfural with methyl isobutyl ketone circulating in a countercurrent manner in a continuous catalytic heterogeneous pulsed column reactor.
* Tel.: (+ 33-67) 144320; fax: (+ 33-67) 144349; e-mail: [email protected]. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 3 6 - 6
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C. Moreau et al. / Applied Catalysis A." General 145 (1996) 211-224
Finally, taking into account the most recent results reported in the literature and our own results, it is possible to revise the mechanism of the dehydration of fructose. Keywords: Fructose; Dehydration; Zeolites; Shape selectivity; Microporosity
1. Introduction In a previous work [1], we have reported on the renewed interest of biomass transformation into valuable furanic compounds, particularly the preparation of non-petroleum derived polymeric materials such as polyesters, polyamides, polyurethanes, and other industrial intermediates [2-4]. Comparing the methods reported in the academic [5-10] and patented [11-14] literature over the last ten years concerning the preparation of 5-hydroxymethylfurfural through dehydration of fructose, in particular the use of strongly acidic ion exchange resins, zeolites seem to have several advantages: (i) they are more selective than ion exchange resins in water as the solvent; (ii) they can work at high temperatures, thus favoring the formation of 5-hydroxymethylfurfural as compared to its decomposition; (iii) they are capable of adsorbing organic acids partly responsible for the further degradation of 5-hydroxymethylfurfural, and (iv) they are easily regenerated by thermal processes. The shape selectivity properties of those catalytic materials played also an important role, mordenites being more selective than Y-faujasites and beta zeolites. The lower selectivity to 5-hydroxymethylfurfural observed in the presence of ion-exchange resins compared to zeolites in water as the solvent could result from the presence of macropores in the former catalysts and from the formation of hydronium species within these macropores. The aim of this work was therefore to perform dehydration of fructose over a series of new dealuminated H-form mordenites, with different microporous and mesoporous volumes, and particularly with Si/A1 ratios close to those giving high selectivities to 5-hydroxymethylfurfural (Si/A1 from 6.9 to 18).
2. Experimental
2.1. Catalysts Dealuminated H-form mordenites were obtained from IFP, PQ Zeolites, ",Jetikon and Zeocat. For these dealuminated catalysts, total and framework /A1 ratios were given as identical. ~roporous and mesoporous volumes were deduced from the isotherms of q,on of nitrogen at 77 K (T-plot method).
C. Moreau et al./ Applied Catalysis A: General 145 (1996) 211-224
213
2.2. Operating conditions 2.2.1. Process research Experiments were carried out in a 0.3-1 stirred autoclave (Autoclave Engineers Magne-Drive), working in batch mode and equipped with two valves for sampling liquid from aqueous and organic phases. The procedure was as follows: fructose (3.5 g), catalyst (1 g), water (35 ml) and methyl isobutyl ketone (175 ml) were poured into the autoclave. After it had been purged with nitrogen, the temperature was increased under nitrogen and the agitation speed was 700 rpm. Zero time was set when the temperature reached 165°C.
2.2.2. Description and operating principle of the pilot plant The pilot consists of a stainless steel drum column with an inside diameter of 25.5 mm, effective height of 8220 mm, packed with cut stainless steel disks in 25 mm spacing with 25% open free area, maintained under pressure by nitrogen bubbling, with thermoregulation by circulation of heat exchanging fluid in a double envelope [15]. Fructose solution and zeolites were introduced by a membrane dosing pump. Zeolites were maintained in suspension in the aqueous phase by means of a mechanical pulsator in counter current circulation with the extracting solvent of the intermediate component.
2.3. Analyses Analyses of the aqueous phase were performed by HPLC using a Shimadzu LC-6A pump and a refractive index RID-6A detector. The column used was a strong ion-exchange resin in H ÷ form (Biorad HPX 87H). The mobile phase was trifluoroacetic acid (10 -3 M). Analyses of the organic phase were also performed by HPLC using a Shimadzu UV Spectrophotometer SPD-6A detector at 280 nm. A PLRPS column was used (Polymer Laboratories) and the mobile phase was methanol/water (70/30 by volume).
2.4. Kinetic modelling The initial rate constants were deduced from the experimental plots of concentrations vs. time by curve fitting and simulation using the AnaCin software [ 16].
3. Results and discussion
Experimental results obtained for the dehydration of fructose over a series of dealuminated H-mordenites with Si/A1 ratios ranging from 6.9 to 18 are summarized in Table 1.
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Table 1 Initial reaction rates, conversion of fructose and selectivity to 5-hydroxymethylfurfural after 30 and 60 min of reaction at 165°C and in water/methyl isobutyl ketone (1:5 by volume) Catalyst entry
0 1 2 3 4 5 6
Si/AI
Initial rate ( × 106 m o l / s )
1.6 5.7 4.8 7.5 7.6 5.1 5.1
6.9 7.5 10 11 15 18
% HMF selectivity
% Fructose conversion
30 min
60 rain
30 min
60 min
15 35 36 52 54 34 36
(32) (65) (57) (77) (76) (64) (65)
40 97 96 68 92 73 79
(38) (81) (77) (60) (91) (56) (62)
From the results reported in Table 1, it can be seen that, in the absence of catalysts, low fructose conversion and low selectivity to 5-hydroxymethylfurfural are observed after 30 and 60 min of reaction. In the presence of mordenites, both conversion and selectivity are increased, but the selectivity tends to be better at low conversion. A maximum in both activity and selectivity at 30 or 60 min of reaction is observed for the mordenite Si/A1 = 11 (entry 4). This catalyst will then serve as experimental support for the detailed determination of external and internal diffusion parameters.
3.1. External diffusion Under the operating conditions used, it is easily shown that dehydration of fructose in water as the solvent is not limited by external diffusion as illustrated in the plots of the initial reaction rates against agitation speed (Fig. 1), catalyst weight (Fig. 2) and fructose concentration (Fig. 3). Figs. 2 and 3 agree with a classical Langmuir-Hinshelwood mechanism in which the products and the
kobs
40
30
10 0 = 0
, > agitation
200
400
600
800
Fig. I. Plot of observed reaction rate constants (X 105 s - l) against agitation speed (rpm).
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
215
VO 12" 10
~
•
o-.//
8
•
4
2./ 0 0
1
, > catalyst 3
2
Fig. 2. Plot o f initial r e a c t i o n rates ( × 10 6 rnol s - 1) a g a i n s t catalyst w e i g h t (g).
VO 12 ~ 10' 8' 6"
•
4
2 J 0
. 0
, 1
. . . . . . . 2 3 4
, 5
.
, 6
.
, 7
.
, > fructose 8
Fig. 3. Plot o f initial reaction rates ( × 10 6 m o l s - ~) a g a i n s t f r u c t o s e w e i g h t (g).
solvent are not involved in the rate equation, V0 = kAF[F]/1 + AF [F]. As shown in the plot of the initial reaction rates against the catalyst weight (Fig. 2), at low coverage of the catalyst, an apparent kinetic first order is observed. At high coverage, the saturation of the catalyst occurs and a zero order is observed. These kinetic orders are confirmed in the plot of the initial reaction rates against the fructose weight (Fig. 3), and from both figures it is possible to calculate the maximum value of the reaction rate, i.e. about 8 - 1 0 . 10 - 6 tool s -~. Confirmation of the validity of the kinetic model is also obtained by plotting the reciprocal of the initial reaction rates against the reciprocal of the catalyst weight (Fig. 4) and fructose weight (Fig. 5). From both plots and their intercepts with the Y-axis, it is thus possible to calculate the reaction rate constant, 2.7- 10 -5 tool s -1 g-1 for the mordenite Si/A1 = l l . Those calculations of reaction rates and rate constants are also possible from the plots of the concentrations in fructose and 5-hydroxymethylfurfural against time (Fig. 6), providing the reaction strictly obeys a consecutive reaction scheme with a nearly
216
C. Moreau et al. /Applied Catalysis A: General 145 (1996) 211-224
1N0 4
~
3 2 1
0
0
2
4
,
,
6
8
, >
l/catalyst
10
Fig. 4. Plot of reciprocal of initial reaction rates ( X 1O- 5 s tool - ~) against reciprocal of catalyst weight (g- ~).
1N0 4 -/
3-
2
1
0 0.0
0.5
1.0
, > 1.5
1/fructose
Fig. 5. Plot of reciprocal of initial reaction rates ( X 10 -5 s tool- t ) against reciprocal of fructose weight (g- 1).
,M%
~oo2
ructose
..
.--~-'~
HMF
j"
50 z /
Acids
J f
o, . -
..
0 o ........ I
30
_-.--''t
60
I
t, min
120
Fig. 6. Kinetic reaction scheme for dehydration of fructose over H-mordenite (Si/A1 = 11) at 165°C in water/methyl isobutyl ketone (1/5 by volume).
C. Moreau et al.//Applied Catalysis A: General 145 (1996) 211-224
217
CH2OH 0 HO H
H OH OH
H
H÷P-
C HOHz
H÷
~
Levulinic acid I~
CHO
+
Formic acid
5-hydroxymethyl furfural
CH20H Fructose
Scheme 1. Simplified kinetic reaction scheme for dehydration of fructose.
quantitative mass balance (Scheme 1). In this case, pseudo first-order rate constants are obtained. Finally, definitively to rule out the possibility for external diffusion to occur, activation energies for formation and disappearance of the intermediate 5-hydroxymethylfurfural have been measured from the plots of the logarithms of the rate constants against the reciprocal of the temperature (Fig. 7). The values calculated, 141 kJ/mol for the formation of 5-hydroxymethylfurfural and 64 kJ/mol for its disappearance, are higher than the limit values generally accepted for external diffusion limitations [17]. Furthermore, they are of the same order of magnitude than those reported in the homogeneous catalyzed reaction [7], thus confirming the reaction is under chemical regime control. Another feature resulting from the analysis of the Arrhenius plot (Fig. 7) is that the selectivity to 5-hydroxymethylfurfural is higher at high temperatures which cannot be reached unfortunately for ion exchange resins.
3.2. Internal diffusion The importance of internal diffusion can be estimated from the equation proposed by Weisz [18]: d N / d t × 1/C o × R2/Deee < 0.1, in which d N / d t is Ln k
-6
.81
-
mo, 1
0
-
~ EH=64kJ/mol
-12 -14 0.0022
, ) 1/T
0.0023
0.0024
Fig. 7, Arrhenius plot of logarithms of reaction rate constants against reciprocal of temperature (K- 1 ) for the formation of 5-hydroxymethylfurfural ([]) and its disappearance (•).
218
C. Moreau et al. / A p p l i e d Catalysis A: General 145 (1996) 211-224 VO 8
0
Si/AI 6.9
7.5
10
11
15
18
Fig. 8. Effect of dealumination of H-mordenites on the initial reaction rates (X 10 6 mol s i).
the reaction rate (mol/s), C O is the initial reactant concentration (mol/cm3), R is the particle radius (cm) and Deff is the effective diffusivity coefficient (cm2/s). With a maximum reaction rate of 10. 1 0 - 6 m o l / s , an initial concentration of 1.11 • 1 0 - 3 m o l / c m 3 and a mean particle radius of 1 5 . 1 0 -4 cm, there is no limitation by internal diffusion if D~ff is greater than 2 • 10 -7 cm2/s, a reasonable value for the diffusivity coefficient of liquids in microporous systems.
3.3. Influence of the acidic properties of the catalysts The initial rates of dehydration of fructose go through a maximum as the dealumination extent becomes more important (Fig. 8). This maximum lies for the Si/A1 ratio at about 10-11, as is generally observed in other reactions carried out over H-mordenites, and interpreted in terms of a balance between the number and the strength of the acidic sites [19-21]. The catalytic aim is, of course, to increase activity, but, according to the kinetic reaction scheme proposed for the dehydration of fructose (Scheme 1) and from the energies of activation for both H+-catalyzed steps, a relatively low acidity is required for the dehydration step to 5-hydroxymethylfurfural. The selectivity tends to decrease by increasing the Si/A1 ratio, i.e. by increasing the acidic properties of the catalysts, thus allowing secondary reactions to take place, such as the formation of formic and levulinic acids or polymeric materials referred to as humins.
3.4. Influence of the mesoporosity of the catalysts The most important point to be noted in Table 1 is that the selectivity to 5-hydroxymethylfurfural is lower, by about 20%, for some catalysts in the region of the maximum of activity (Table 1, entries 2 and 3). A possibility for such a decrease in the selectivity might result from the dealumination process used for the preparation of the catalysts, leading to differences in the distribution
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
219
Table 2 Microporous and mesoporous volumes for dealuminated H-mordenites Catalyst entry
Si/AI
Micropore volume (cm 3/g)
Mesopore volume (cm ~/g)
1 2 3 4 5 6
6.9 7.5 10 II 15 18
0.204 0.201 0.189 0.192 0.185 0.197
0.040 0.047 0.163 0.056 0.198 0.128
of micropores and mesopores. Microporous and mesoporous volumes measured for the mordenites used are reported in Table 2. For all catalysts, microporous volumes are nearly similar. Depending on the mode of dealumination used by the different catalysts suppliers, relatively important mesoporous volumes are measured. The influence of the mesoporosity of the catalysts on the selectivity to 5-hydroxymethylfurfural is clearly shown in Fig. 9. The catalysts with the lowest mesoporous volumes are the most selective, and, as for the macroporous catalysts in water as the solvent, the formation of hydronium species within the mesopores may be expected, thus increasing local acidity and further degradation of 5-hydroxymethylfurfural. In addition, a low selectivity gives a poor mass balance, and curve fitting is less simple due to a combination of consecutive and parallel pathways.
3.5. Influence of the structural properties of the catalysts In addition to their acidic properties which can be easily modified through dealumination processes, zeolites are also capable of acting as shape selective catalysts due to their structural properties. The bidimensional structure of
0lil
H M F (%) ~
.--=-
1001~
~']
~
~
~
,.,EII
,
60
rain.
60
40 2
• ~ 0.04
0.047
0.163
0.05~
0.198
v o l u m e (cc/g)
0.128
Fig, 9. Effect of rnesoporous volume on the selectivity to 5-hydroxymethylfurfural over dealuminated H-rnordenites.
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C. Moreau et a l . / Applied Catalysis A." General 145 (1996) 211-224
mordenites with only one large channel allow the accessibility of fructose to the catalytic sites and the rapid diffusion of 5-hydroxymethylfurfural once formed, thus avoiding its rearrangement into higher molecular weight compounds. The consequence of the shape selective properties of the mordenites is the high selectivity to 5-hydroxymethylfurfural with a corresponding high mass balance, except when dealumination processes lead to important mesoporous volumes and further rearrangement, in this case too, into higher molecular weight compounds.
3.6. Influence of continuous extraction of 5-hydroxymethylfurfural The influence of the extraction of 5-hydroxymethylfurfural with an organic solvent like methyl isobutyl ketone was already shown to improve the selectivity of the reaction of dehydration of fructose [7]. In the present work, this procedure was carried out in a pilot plant [15] in which 5-hydroxymethylfurfural is extracted continuously in a countercurrent manner with respect to the fructose and catalyst feed. Several advantages have been noted: (i) due to the heterogeneous process itself, the intermediate formed in the aqueous phase is relatively kept away from the protonic sites present on the surface of the catalyst. The pH measured in the vicinity of the catalyst for the aqueous solution is about 3.3, whereas a pH value of about 2 was calculated from kinetic measurements [22], thus concentrating the strong acidity zone close to the surface of the catalyst; (ii) simultaneous extraction by an organic solvent shortens in this way the residence time of the intermediate in the aqueous phase with, as a consequence, an increase in the selectivity; (iii) the organic phase does not contain acid species capable of performing the degradation of the intermediate, contrarily to mineral acids like phosphoric acid which are partly and significantly dissolved in the organic methyl isobutyl ketone phase, (iv) a concentration gradient reactor provides conversion and selectivity performances superior to those obtainable
HMF(%) 1001 80" 60' 40
0
~plabatc h 20
40
~t .
,
60
.
,
80
.
,
),
1O0
conversion
Fig. 10. Plot of selectivity in 5-hydroxymethylfurfural against conversion for dehydration of fructose over H-mordenite (Si/A1 = 10) in batch and continuous processes.
22
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
~H20H
~HO
CH20H Fructose
~H20H
~HOH
H?OH CH20H
HO~H H?OH H?OH CH2OH
COH II COH I H?OH
2t3-enedlel
l
CH20H Msnnose
- H20
l~2-enedlol
- H20
CHO f HCOH I HO~H H~OH H?OH CH20H
~H2OH
~=o
COH Jl H~OH CH2OH
~-
CHO CN H~OH
H?OH
H~OH CH20H
H~OH
Glucose
CH20H
3-deoxy-hexosulose
CHO I
~=o
CH
- OH20
L CH20H
- H20 CHO I C=O ICH H~OH
OH2OH
~
~ H2
.
H20
~H2OH
I
CHO
- H20
CH~OH
- H20
- H20
c~CH2OH II O
Hydroxvscetylturane
I~ ~ C H O Furfural
HOH2C'~CHO Hydroxymethylfurfural
Scheme 2. Revisitedreaction scheme for dehydrationof fructose.
222
C. Moreau et al./Applied Catalysis A: General 145 (1996) 211-224
from a stirred slurry reactor in the case of positive order consecutive reactions, and (v) the outgoing raffinate is brought into contact with entering solvent containing no solute, ensuring a low loss of solute. In this a way, the gain in selectivity to 5-hydroxymethylfurfural when passing from a slurry to a continuous reactor is close to 10%, as illustrated in Fig. 10 with the H-mordenite (Si/A1 = 10). 3.7. Revised reaction scheme
The recent mechanisms proposed for dehydration of fructose generally consider two routes, the acyclic route [23] and the cyclic route through cyclisation of alkenic species in their cis-form [24]. According to the presence of small amounts of glucose and mannose resulting from isomerization of fructose, it seems more convenient now to consider the acyclic route as the more likely for the dehydration of fructose with one key intermediate, 1,2-enediol (Scheme 2). The presence of 1,2-enediol accounts for isomerization of fructose to both mannose and glucose. This intermediate is also in equilibrium with 2,3-enediol which accounts for the formation, in small amounts, of hydroxy-acetylfurane through two further dehydration steps. The main route is, however, the formation of 3-deoxy-hexosulose. Recent results corroborate this mechanism from measurements of reaction rates in heavy water [25]. The primary kinetic isotope effect measured, 12.3, led the authors to propose that the rate determining step followed the first elimination of a water molecule and preceded the second one, i.e. dehydration of 3-deoxy-hexosulose (Scheme 2). Confirmation of this hypothesis was found through the kinetic measurement of the appearance of furfural. The presence of furfural was already reported in the literature as a minor product which does not result from the cleavage of the hydroxymethyl group of 5-hydroxymethylfurfural [26], but from a fast reverse-aldol cleavage often observed in carbohydrate chemistry [27]. In the presence of zeolites as catalysts, it still remains as a minor product (Table 3), but we have shown that furfural and 5-hydroxymethylfurfural
Table 3 Yields in 5-hydroxymethylfurfural (HMF) and furfural (FF) as a function of fructose conversion over H-mordenite (Si/A1 = 11) at 165°C and in water/methyl isobutyl ketone (1:5 by volume) Time (min)
Fructose conversion (%)
HMF yield (%)
FF yield (%)
HMF/FF ratio
0 10 20 30 60 90 120
0 21 37 54 76 87 93
0 19 32 50 69 74 73
0 1 2 4 5 5 5
19 16 13 14 15 15
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
223
are formed through a parallel pathway. The ratio 5-hydroxymethylfurfural/furfural is constant with time with a mean value around 15 over the H-mordenite (Si/A1 = 11), and confirmation is thus obtained that furfural does not result from 5-hydroxymethylfurfural itself. That would also imply that they are both formed from 3-deoxy-hexosulose, in agreement with the primary kinetic isotope effect reported, and that the intermediate resulting from the cleavage step is formed faster than it is dehydrated to yield furfural in order to account for the kinetic parallel appearance of both furfural and 5-hydroxymethylfurfural.
4. Conclusion From the results reported in this work, it appears that dehydration of fructose is easily achieved in the presence of H-form zeolites, in water as the solvent and methyl isobutyl ketone as simultaneous extraction solvent of 5-hydroxymethylfurfural. This constitutes, of course, an important improvement with respect to the reactions using mineral acids or ion-exchange resins for a further development on a pilot scale. These materials can work at higher temperatures, and they can be easily regenerated without loss of both activity and selectivity, and used for several runs. It should also be emphasized than water can be definitively considered as a classical solvent of particular interest for reactions involving carbohydrates or highly hydrophilic compounds. However, one must take into account the hydrophilic/hydrophobic properties of zeolites to avoid external and internal diffusional limitations. Among the different catalysts used, mordenites with a low mesoporous volume, in addition to their shape selective properties, were found to be the most selective catalysts. The selectivity is further increased by means of a continuous liquid-liquid extractor working in a cocurrent or a countercurrent mode. Finally, according to recent results reported in the literature and our own results, it was possible to revise the complex reaction mechanism of dehydration of fructose.
Acknowledgements IFP, PQ Zeolites, Uetikon and Zeocat are gratefully acknowledged for providing us with catalysts samples, and Agrichimie for financial support.
References [1] C. Moreau, R. Durand, C. Pourcheron and S, Razigade, Industrial Crops Products, 3 (1994) 85. [2] M. Kunz, in A. Fuchs (Editor), Inulin and Inulin-containing Crops, Elsevier, Amsterdam, 1993, p. 149.
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C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
[3] A. Gandini, in S.L. Aggarwal and S. Russo (Editors), Comprehensive Polymer Science, 1st Suppl., Pergamon Press, Oxford, 1992, p. 527. [4] A. Gandini, in J.E. Glass and G. Swift (Editors), Agricultural and Synthetic Polymers, Biodegradability and Utilization, ACS Symp. Ser., Vol. 433, 1990, p. 195. [5] S.A. Grin', S.R. Tsimbalaev and S.Yu. Gel'fand, Kinet. Catal., 34 (1993) 177. [6] L. Cottier and G. Descotes, Trends Heterocyclic Chem,, 2 (1991) 233. [7] B.F.M. Kuster, Starch/Stgrke, 42 (1990) 314. [8] T. El Hajj, A. Masroua, J.C. Martin and G. Descotes, Bull. Soc. Chim. Fr., 1987, p. 885. [9] H.E. van Dam, A.P.G. Kieboom and H. van Bekkum, Starch/Starke, 38 (1986) 95. [10] L. Rigal and A. Gaset, Biomass 8 (1985) 267. [I1] G. Braca, A.M. Raspolli Galletti, A. Criscuelo and G. Sbrana, Italian Patent, Appl. A 002363 (1992) assigned to CNR. [12] C. Neyrete, L. Cottier, H. Nigay and G. Descotes, French Patent, FR 2,664,273 (1992) assigned to Beghin-Say. [13] G. Avignon, R. Durand, P. Faugeras, P. Geneste, C. Moreau, P. Rivalier and P. Ros, French Patent, FR 2,670,209 (1992), PCT Int. Appl. WO 92/10486 (1992), assigned to CEA. [14] K. Rapp, German Patent, DE 3,601,209 (1988), assigned to SUdzucker. [15] P. Rivalier, J. Duhamet, C. Moreau and R. Durand, Catal. Today, 24 (1995) 165. [16] J. Joffre, P. Geneste, A. Gnida, G. Szabo and C. Morean, in J.L. Rivail (Editor), Modelling of Molecular Structures and Properties (Studies in Physical and Theoretical Chemistry, Vol. 71 ), Elsevier, Amsterdam, 1990, p. 409. [17] G.C. Bond, Heterogeneous Catalysis: Principles and Applications, 2nd ed., Clarendon Press, Oxford, 1987, p, 57. [18] P.B. Weisz and E.W. Swegler, J. Phys. Chem., 59 (1955) 823. [19] F. Fajula, R. Ibarra, F. Figueras and C. Gueguen, J. Catal., 89 (1984) 60. [20] C. Mirodatos and D. Barthomeuf, J. Chem. Soc. Chem. Commun., (1981) 39. [21] R. Durand, P. Geneste, J. Joffre and C. Moreau, Stud. Surf. Sci. Catal., 78 (1993) 647. [22] R. Durand, P. Geneste, C. Moreau and S. Mseddi, Stud. Surf. Sci. Catal., 20 (1985) 319. [23] M.S. Feather and J.F. Harris, Adv. Carbohydr. Chem. Biochem., 28 (1973) 161. [24] M.J. Antal, W.S.L. Mok and G.N. Richards, Carbohydr. Res., 199 (1990) 91. [25] S.A. Grin', S.R. Tsimbalaev and S.Yu. Gel'fand, Kinet. Catal., 34 (1993) 430. [26] R. Krishna, M.R. Kallury, C. Ambidge, T.T. Tidwell, D.G.B. Boocock, F.A. Agblevor and D.J. Stewart, Carbohydr. Res., 158 (1986) 253. [27] D.A. Nelson, P.M. Molton, J.A. Russell and R.T. Hallon, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 471.
ELSEVIER
Applied Catalysis A: General 145 (1996) 211-224
Dehydration of fructose to 5-hydroxymethylfurfural over H-mordenites Claude Moreau a,*, Robert Durand a Sylvie Razigade a Jean Duhamet b, Pierre Faugeras b, Patrick Rivalier b Pierre Ros b G6rard Avignon c a Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, URA CNRS 418, Ecole Nationale Sup£rieure de Chimie, 8, Rue de I'Ecole Normale, 34053 Montpellier Cedex 1, France b CEA, D C C / D R D D / SEMP/SGC, BP 171, 30207 Bagnols-sur-Ckze Cedex, France c Le Petit Buscon, 47310 Estillac, France
Received 16 January 1996; revised 4 April t996; accepted 4 April 1996
Abstract
Dehydration of fructose to 5-hydroxymethylfurfural was performed in a batch mode in the presence of a series of dealuminated H-form mordenites as catalysts, at 165°C, and in a solvent mixture consisting of water and methyl isobutyl ketone (1:5 by volume). Under the operating conditions used, the reaction was not controlled by external or internal diffusional limitations. Fructose conversion and selectivity to 5-hydroxymethylfurfural were found to depend on acidic and structural properties of the catalysts used as well as on the micropore vs. mesopore volume distribution of those catalysts. A maximum in the rate of conversion of fructose was observed for the H-mordenite with a Si/A1 ratio of l 1. A maximum in the selectivity to 5-hydroxymethylfurfural was observed only for H-mordenites with a low mesoporous volume. The high selectivity obtained ( > 90%) was correlated with the shape selectivity properties of H-mordenites (bidimensional structure), and particularly with the absence of cavities within the structure allowing further formation of secondary products. The influence of the microporosity vs. mesoporosity on the selectivity to 5-hydroxymethylfurfural was also studied, the formation of mesopores upon dealumination procedures being damaging to obtain a high selectivity. A significant increase in the selectivity (10%) was also obtained by simultaneous extraction of 5-hydroxymethylfurfural with methyl isobutyl ketone circulating in a countercurrent manner in a continuous catalytic heterogeneous pulsed column reactor.
* Tel.: (+ 33-67) 144320; fax: (+ 33-67) 144349; e-mail: [email protected]. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 1 3 6 - 6
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Finally, taking into account the most recent results reported in the literature and our own results, it is possible to revise the mechanism of the dehydration of fructose. Keywords: Fructose; Dehydration; Zeolites; Shape selectivity; Microporosity
1. Introduction In a previous work [1], we have reported on the renewed interest of biomass transformation into valuable furanic compounds, particularly the preparation of non-petroleum derived polymeric materials such as polyesters, polyamides, polyurethanes, and other industrial intermediates [2-4]. Comparing the methods reported in the academic [5-10] and patented [11-14] literature over the last ten years concerning the preparation of 5-hydroxymethylfurfural through dehydration of fructose, in particular the use of strongly acidic ion exchange resins, zeolites seem to have several advantages: (i) they are more selective than ion exchange resins in water as the solvent; (ii) they can work at high temperatures, thus favoring the formation of 5-hydroxymethylfurfural as compared to its decomposition; (iii) they are capable of adsorbing organic acids partly responsible for the further degradation of 5-hydroxymethylfurfural, and (iv) they are easily regenerated by thermal processes. The shape selectivity properties of those catalytic materials played also an important role, mordenites being more selective than Y-faujasites and beta zeolites. The lower selectivity to 5-hydroxymethylfurfural observed in the presence of ion-exchange resins compared to zeolites in water as the solvent could result from the presence of macropores in the former catalysts and from the formation of hydronium species within these macropores. The aim of this work was therefore to perform dehydration of fructose over a series of new dealuminated H-form mordenites, with different microporous and mesoporous volumes, and particularly with Si/A1 ratios close to those giving high selectivities to 5-hydroxymethylfurfural (Si/A1 from 6.9 to 18).
2. Experimental
2.1. Catalysts Dealuminated H-form mordenites were obtained from IFP, PQ Zeolites, ",Jetikon and Zeocat. For these dealuminated catalysts, total and framework /A1 ratios were given as identical. ~roporous and mesoporous volumes were deduced from the isotherms of q,on of nitrogen at 77 K (T-plot method).
C. Moreau et al./ Applied Catalysis A: General 145 (1996) 211-224
213
2.2. Operating conditions 2.2.1. Process research Experiments were carried out in a 0.3-1 stirred autoclave (Autoclave Engineers Magne-Drive), working in batch mode and equipped with two valves for sampling liquid from aqueous and organic phases. The procedure was as follows: fructose (3.5 g), catalyst (1 g), water (35 ml) and methyl isobutyl ketone (175 ml) were poured into the autoclave. After it had been purged with nitrogen, the temperature was increased under nitrogen and the agitation speed was 700 rpm. Zero time was set when the temperature reached 165°C.
2.2.2. Description and operating principle of the pilot plant The pilot consists of a stainless steel drum column with an inside diameter of 25.5 mm, effective height of 8220 mm, packed with cut stainless steel disks in 25 mm spacing with 25% open free area, maintained under pressure by nitrogen bubbling, with thermoregulation by circulation of heat exchanging fluid in a double envelope [15]. Fructose solution and zeolites were introduced by a membrane dosing pump. Zeolites were maintained in suspension in the aqueous phase by means of a mechanical pulsator in counter current circulation with the extracting solvent of the intermediate component.
2.3. Analyses Analyses of the aqueous phase were performed by HPLC using a Shimadzu LC-6A pump and a refractive index RID-6A detector. The column used was a strong ion-exchange resin in H ÷ form (Biorad HPX 87H). The mobile phase was trifluoroacetic acid (10 -3 M). Analyses of the organic phase were also performed by HPLC using a Shimadzu UV Spectrophotometer SPD-6A detector at 280 nm. A PLRPS column was used (Polymer Laboratories) and the mobile phase was methanol/water (70/30 by volume).
2.4. Kinetic modelling The initial rate constants were deduced from the experimental plots of concentrations vs. time by curve fitting and simulation using the AnaCin software [ 16].
3. Results and discussion
Experimental results obtained for the dehydration of fructose over a series of dealuminated H-mordenites with Si/A1 ratios ranging from 6.9 to 18 are summarized in Table 1.
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Table 1 Initial reaction rates, conversion of fructose and selectivity to 5-hydroxymethylfurfural after 30 and 60 min of reaction at 165°C and in water/methyl isobutyl ketone (1:5 by volume) Catalyst entry
0 1 2 3 4 5 6
Si/AI
Initial rate ( × 106 m o l / s )
1.6 5.7 4.8 7.5 7.6 5.1 5.1
6.9 7.5 10 11 15 18
% HMF selectivity
% Fructose conversion
30 min
60 rain
30 min
60 min
15 35 36 52 54 34 36
(32) (65) (57) (77) (76) (64) (65)
40 97 96 68 92 73 79
(38) (81) (77) (60) (91) (56) (62)
From the results reported in Table 1, it can be seen that, in the absence of catalysts, low fructose conversion and low selectivity to 5-hydroxymethylfurfural are observed after 30 and 60 min of reaction. In the presence of mordenites, both conversion and selectivity are increased, but the selectivity tends to be better at low conversion. A maximum in both activity and selectivity at 30 or 60 min of reaction is observed for the mordenite Si/A1 = 11 (entry 4). This catalyst will then serve as experimental support for the detailed determination of external and internal diffusion parameters.
3.1. External diffusion Under the operating conditions used, it is easily shown that dehydration of fructose in water as the solvent is not limited by external diffusion as illustrated in the plots of the initial reaction rates against agitation speed (Fig. 1), catalyst weight (Fig. 2) and fructose concentration (Fig. 3). Figs. 2 and 3 agree with a classical Langmuir-Hinshelwood mechanism in which the products and the
kobs
40
30
10 0 = 0
, > agitation
200
400
600
800
Fig. I. Plot of observed reaction rate constants (X 105 s - l) against agitation speed (rpm).
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
215
VO 12" 10
~
•
o-.//
8
•
4
2./ 0 0
1
, > catalyst 3
2
Fig. 2. Plot o f initial r e a c t i o n rates ( × 10 6 rnol s - 1) a g a i n s t catalyst w e i g h t (g).
VO 12 ~ 10' 8' 6"
•
4
2 J 0
. 0
, 1
. . . . . . . 2 3 4
, 5
.
, 6
.
, 7
.
, > fructose 8
Fig. 3. Plot o f initial reaction rates ( × 10 6 m o l s - ~) a g a i n s t f r u c t o s e w e i g h t (g).
solvent are not involved in the rate equation, V0 = kAF[F]/1 + AF [F]. As shown in the plot of the initial reaction rates against the catalyst weight (Fig. 2), at low coverage of the catalyst, an apparent kinetic first order is observed. At high coverage, the saturation of the catalyst occurs and a zero order is observed. These kinetic orders are confirmed in the plot of the initial reaction rates against the fructose weight (Fig. 3), and from both figures it is possible to calculate the maximum value of the reaction rate, i.e. about 8 - 1 0 . 10 - 6 tool s -~. Confirmation of the validity of the kinetic model is also obtained by plotting the reciprocal of the initial reaction rates against the reciprocal of the catalyst weight (Fig. 4) and fructose weight (Fig. 5). From both plots and their intercepts with the Y-axis, it is thus possible to calculate the reaction rate constant, 2.7- 10 -5 tool s -1 g-1 for the mordenite Si/A1 = l l . Those calculations of reaction rates and rate constants are also possible from the plots of the concentrations in fructose and 5-hydroxymethylfurfural against time (Fig. 6), providing the reaction strictly obeys a consecutive reaction scheme with a nearly
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C. Moreau et al. /Applied Catalysis A: General 145 (1996) 211-224
1N0 4
~
3 2 1
0
0
2
4
,
,
6
8
, >
l/catalyst
10
Fig. 4. Plot of reciprocal of initial reaction rates ( X 1O- 5 s tool - ~) against reciprocal of catalyst weight (g- ~).
1N0 4 -/
3-
2
1
0 0.0
0.5
1.0
, > 1.5
1/fructose
Fig. 5. Plot of reciprocal of initial reaction rates ( X 10 -5 s tool- t ) against reciprocal of fructose weight (g- 1).
,M%
~oo2
ructose
..
.--~-'~
HMF
j"
50 z /
Acids
J f
o, . -
..
0 o ........ I
30
_-.--''t
60
I
t, min
120
Fig. 6. Kinetic reaction scheme for dehydration of fructose over H-mordenite (Si/A1 = 11) at 165°C in water/methyl isobutyl ketone (1/5 by volume).
C. Moreau et al.//Applied Catalysis A: General 145 (1996) 211-224
217
CH2OH 0 HO H
H OH OH
H
H÷P-
C HOHz
H÷
~
Levulinic acid I~
CHO
+
Formic acid
5-hydroxymethyl furfural
CH20H Fructose
Scheme 1. Simplified kinetic reaction scheme for dehydration of fructose.
quantitative mass balance (Scheme 1). In this case, pseudo first-order rate constants are obtained. Finally, definitively to rule out the possibility for external diffusion to occur, activation energies for formation and disappearance of the intermediate 5-hydroxymethylfurfural have been measured from the plots of the logarithms of the rate constants against the reciprocal of the temperature (Fig. 7). The values calculated, 141 kJ/mol for the formation of 5-hydroxymethylfurfural and 64 kJ/mol for its disappearance, are higher than the limit values generally accepted for external diffusion limitations [17]. Furthermore, they are of the same order of magnitude than those reported in the homogeneous catalyzed reaction [7], thus confirming the reaction is under chemical regime control. Another feature resulting from the analysis of the Arrhenius plot (Fig. 7) is that the selectivity to 5-hydroxymethylfurfural is higher at high temperatures which cannot be reached unfortunately for ion exchange resins.
3.2. Internal diffusion The importance of internal diffusion can be estimated from the equation proposed by Weisz [18]: d N / d t × 1/C o × R2/Deee < 0.1, in which d N / d t is Ln k
-6
.81
-
mo, 1
0
-
~ EH=64kJ/mol
-12 -14 0.0022
, ) 1/T
0.0023
0.0024
Fig. 7, Arrhenius plot of logarithms of reaction rate constants against reciprocal of temperature (K- 1 ) for the formation of 5-hydroxymethylfurfural ([]) and its disappearance (•).
218
C. Moreau et al. / A p p l i e d Catalysis A: General 145 (1996) 211-224 VO 8
0
Si/AI 6.9
7.5
10
11
15
18
Fig. 8. Effect of dealumination of H-mordenites on the initial reaction rates (X 10 6 mol s i).
the reaction rate (mol/s), C O is the initial reactant concentration (mol/cm3), R is the particle radius (cm) and Deff is the effective diffusivity coefficient (cm2/s). With a maximum reaction rate of 10. 1 0 - 6 m o l / s , an initial concentration of 1.11 • 1 0 - 3 m o l / c m 3 and a mean particle radius of 1 5 . 1 0 -4 cm, there is no limitation by internal diffusion if D~ff is greater than 2 • 10 -7 cm2/s, a reasonable value for the diffusivity coefficient of liquids in microporous systems.
3.3. Influence of the acidic properties of the catalysts The initial rates of dehydration of fructose go through a maximum as the dealumination extent becomes more important (Fig. 8). This maximum lies for the Si/A1 ratio at about 10-11, as is generally observed in other reactions carried out over H-mordenites, and interpreted in terms of a balance between the number and the strength of the acidic sites [19-21]. The catalytic aim is, of course, to increase activity, but, according to the kinetic reaction scheme proposed for the dehydration of fructose (Scheme 1) and from the energies of activation for both H+-catalyzed steps, a relatively low acidity is required for the dehydration step to 5-hydroxymethylfurfural. The selectivity tends to decrease by increasing the Si/A1 ratio, i.e. by increasing the acidic properties of the catalysts, thus allowing secondary reactions to take place, such as the formation of formic and levulinic acids or polymeric materials referred to as humins.
3.4. Influence of the mesoporosity of the catalysts The most important point to be noted in Table 1 is that the selectivity to 5-hydroxymethylfurfural is lower, by about 20%, for some catalysts in the region of the maximum of activity (Table 1, entries 2 and 3). A possibility for such a decrease in the selectivity might result from the dealumination process used for the preparation of the catalysts, leading to differences in the distribution
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
219
Table 2 Microporous and mesoporous volumes for dealuminated H-mordenites Catalyst entry
Si/AI
Micropore volume (cm 3/g)
Mesopore volume (cm ~/g)
1 2 3 4 5 6
6.9 7.5 10 II 15 18
0.204 0.201 0.189 0.192 0.185 0.197
0.040 0.047 0.163 0.056 0.198 0.128
of micropores and mesopores. Microporous and mesoporous volumes measured for the mordenites used are reported in Table 2. For all catalysts, microporous volumes are nearly similar. Depending on the mode of dealumination used by the different catalysts suppliers, relatively important mesoporous volumes are measured. The influence of the mesoporosity of the catalysts on the selectivity to 5-hydroxymethylfurfural is clearly shown in Fig. 9. The catalysts with the lowest mesoporous volumes are the most selective, and, as for the macroporous catalysts in water as the solvent, the formation of hydronium species within the mesopores may be expected, thus increasing local acidity and further degradation of 5-hydroxymethylfurfural. In addition, a low selectivity gives a poor mass balance, and curve fitting is less simple due to a combination of consecutive and parallel pathways.
3.5. Influence of the structural properties of the catalysts In addition to their acidic properties which can be easily modified through dealumination processes, zeolites are also capable of acting as shape selective catalysts due to their structural properties. The bidimensional structure of
0lil
H M F (%) ~
.--=-
1001~
~']
~
~
~
,.,EII
,
60
rain.
60
40 2
• ~ 0.04
0.047
0.163
0.05~
0.198
v o l u m e (cc/g)
0.128
Fig, 9. Effect of rnesoporous volume on the selectivity to 5-hydroxymethylfurfural over dealuminated H-rnordenites.
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C. Moreau et a l . / Applied Catalysis A." General 145 (1996) 211-224
mordenites with only one large channel allow the accessibility of fructose to the catalytic sites and the rapid diffusion of 5-hydroxymethylfurfural once formed, thus avoiding its rearrangement into higher molecular weight compounds. The consequence of the shape selective properties of the mordenites is the high selectivity to 5-hydroxymethylfurfural with a corresponding high mass balance, except when dealumination processes lead to important mesoporous volumes and further rearrangement, in this case too, into higher molecular weight compounds.
3.6. Influence of continuous extraction of 5-hydroxymethylfurfural The influence of the extraction of 5-hydroxymethylfurfural with an organic solvent like methyl isobutyl ketone was already shown to improve the selectivity of the reaction of dehydration of fructose [7]. In the present work, this procedure was carried out in a pilot plant [15] in which 5-hydroxymethylfurfural is extracted continuously in a countercurrent manner with respect to the fructose and catalyst feed. Several advantages have been noted: (i) due to the heterogeneous process itself, the intermediate formed in the aqueous phase is relatively kept away from the protonic sites present on the surface of the catalyst. The pH measured in the vicinity of the catalyst for the aqueous solution is about 3.3, whereas a pH value of about 2 was calculated from kinetic measurements [22], thus concentrating the strong acidity zone close to the surface of the catalyst; (ii) simultaneous extraction by an organic solvent shortens in this way the residence time of the intermediate in the aqueous phase with, as a consequence, an increase in the selectivity; (iii) the organic phase does not contain acid species capable of performing the degradation of the intermediate, contrarily to mineral acids like phosphoric acid which are partly and significantly dissolved in the organic methyl isobutyl ketone phase, (iv) a concentration gradient reactor provides conversion and selectivity performances superior to those obtainable
HMF(%) 1001 80" 60' 40
0
~plabatc h 20
40
~t .
,
60
.
,
80
.
,
),
1O0
conversion
Fig. 10. Plot of selectivity in 5-hydroxymethylfurfural against conversion for dehydration of fructose over H-mordenite (Si/A1 = 10) in batch and continuous processes.
22
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
~H20H
~HO
CH20H Fructose
~H20H
~HOH
H?OH CH20H
HO~H H?OH H?OH CH2OH
COH II COH I H?OH
2t3-enedlel
l
CH20H Msnnose
- H20
l~2-enedlol
- H20
CHO f HCOH I HO~H H~OH H?OH CH20H
~H2OH
~=o
COH Jl H~OH CH2OH
~-
CHO CN H~OH
H?OH
H~OH CH20H
H~OH
Glucose
CH20H
3-deoxy-hexosulose
CHO I
~=o
CH
- OH20
L CH20H
- H20 CHO I C=O ICH H~OH
OH2OH
~
~ H2
.
H20
~H2OH
I
CHO
- H20
CH~OH
- H20
- H20
c~CH2OH II O
Hydroxvscetylturane
I~ ~ C H O Furfural
HOH2C'~CHO Hydroxymethylfurfural
Scheme 2. Revisitedreaction scheme for dehydrationof fructose.
222
C. Moreau et al./Applied Catalysis A: General 145 (1996) 211-224
from a stirred slurry reactor in the case of positive order consecutive reactions, and (v) the outgoing raffinate is brought into contact with entering solvent containing no solute, ensuring a low loss of solute. In this a way, the gain in selectivity to 5-hydroxymethylfurfural when passing from a slurry to a continuous reactor is close to 10%, as illustrated in Fig. 10 with the H-mordenite (Si/A1 = 10). 3.7. Revised reaction scheme
The recent mechanisms proposed for dehydration of fructose generally consider two routes, the acyclic route [23] and the cyclic route through cyclisation of alkenic species in their cis-form [24]. According to the presence of small amounts of glucose and mannose resulting from isomerization of fructose, it seems more convenient now to consider the acyclic route as the more likely for the dehydration of fructose with one key intermediate, 1,2-enediol (Scheme 2). The presence of 1,2-enediol accounts for isomerization of fructose to both mannose and glucose. This intermediate is also in equilibrium with 2,3-enediol which accounts for the formation, in small amounts, of hydroxy-acetylfurane through two further dehydration steps. The main route is, however, the formation of 3-deoxy-hexosulose. Recent results corroborate this mechanism from measurements of reaction rates in heavy water [25]. The primary kinetic isotope effect measured, 12.3, led the authors to propose that the rate determining step followed the first elimination of a water molecule and preceded the second one, i.e. dehydration of 3-deoxy-hexosulose (Scheme 2). Confirmation of this hypothesis was found through the kinetic measurement of the appearance of furfural. The presence of furfural was already reported in the literature as a minor product which does not result from the cleavage of the hydroxymethyl group of 5-hydroxymethylfurfural [26], but from a fast reverse-aldol cleavage often observed in carbohydrate chemistry [27]. In the presence of zeolites as catalysts, it still remains as a minor product (Table 3), but we have shown that furfural and 5-hydroxymethylfurfural
Table 3 Yields in 5-hydroxymethylfurfural (HMF) and furfural (FF) as a function of fructose conversion over H-mordenite (Si/A1 = 11) at 165°C and in water/methyl isobutyl ketone (1:5 by volume) Time (min)
Fructose conversion (%)
HMF yield (%)
FF yield (%)
HMF/FF ratio
0 10 20 30 60 90 120
0 21 37 54 76 87 93
0 19 32 50 69 74 73
0 1 2 4 5 5 5
19 16 13 14 15 15
C. Moreau et al. / Applied Catalysis A: General 145 (1996) 211-224
223
are formed through a parallel pathway. The ratio 5-hydroxymethylfurfural/furfural is constant with time with a mean value around 15 over the H-mordenite (Si/A1 = 11), and confirmation is thus obtained that furfural does not result from 5-hydroxymethylfurfural itself. That would also imply that they are both formed from 3-deoxy-hexosulose, in agreement with the primary kinetic isotope effect reported, and that the intermediate resulting from the cleavage step is formed faster than it is dehydrated to yield furfural in order to account for the kinetic parallel appearance of both furfural and 5-hydroxymethylfurfural.
4. Conclusion From the results reported in this work, it appears that dehydration of fructose is easily achieved in the presence of H-form zeolites, in water as the solvent and methyl isobutyl ketone as simultaneous extraction solvent of 5-hydroxymethylfurfural. This constitutes, of course, an important improvement with respect to the reactions using mineral acids or ion-exchange resins for a further development on a pilot scale. These materials can work at higher temperatures, and they can be easily regenerated without loss of both activity and selectivity, and used for several runs. It should also be emphasized than water can be definitively considered as a classical solvent of particular interest for reactions involving carbohydrates or highly hydrophilic compounds. However, one must take into account the hydrophilic/hydrophobic properties of zeolites to avoid external and internal diffusional limitations. Among the different catalysts used, mordenites with a low mesoporous volume, in addition to their shape selective properties, were found to be the most selective catalysts. The selectivity is further increased by means of a continuous liquid-liquid extractor working in a cocurrent or a countercurrent mode. Finally, according to recent results reported in the literature and our own results, it was possible to revise the complex reaction mechanism of dehydration of fructose.
Acknowledgements IFP, PQ Zeolites, Uetikon and Zeocat are gratefully acknowledged for providing us with catalysts samples, and Agrichimie for financial support.
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[3] A. Gandini, in S.L. Aggarwal and S. Russo (Editors), Comprehensive Polymer Science, 1st Suppl., Pergamon Press, Oxford, 1992, p. 527. [4] A. Gandini, in J.E. Glass and G. Swift (Editors), Agricultural and Synthetic Polymers, Biodegradability and Utilization, ACS Symp. Ser., Vol. 433, 1990, p. 195. [5] S.A. Grin', S.R. Tsimbalaev and S.Yu. Gel'fand, Kinet. Catal., 34 (1993) 177. [6] L. Cottier and G. Descotes, Trends Heterocyclic Chem,, 2 (1991) 233. [7] B.F.M. Kuster, Starch/Stgrke, 42 (1990) 314. [8] T. El Hajj, A. Masroua, J.C. Martin and G. Descotes, Bull. Soc. Chim. Fr., 1987, p. 885. [9] H.E. van Dam, A.P.G. Kieboom and H. van Bekkum, Starch/Starke, 38 (1986) 95. [10] L. Rigal and A. Gaset, Biomass 8 (1985) 267. [I1] G. Braca, A.M. Raspolli Galletti, A. Criscuelo and G. Sbrana, Italian Patent, Appl. A 002363 (1992) assigned to CNR. [12] C. Neyrete, L. Cottier, H. Nigay and G. Descotes, French Patent, FR 2,664,273 (1992) assigned to Beghin-Say. [13] G. Avignon, R. Durand, P. Faugeras, P. Geneste, C. Moreau, P. Rivalier and P. Ros, French Patent, FR 2,670,209 (1992), PCT Int. Appl. WO 92/10486 (1992), assigned to CEA. [14] K. Rapp, German Patent, DE 3,601,209 (1988), assigned to SUdzucker. [15] P. Rivalier, J. Duhamet, C. Moreau and R. Durand, Catal. Today, 24 (1995) 165. [16] J. Joffre, P. Geneste, A. Gnida, G. Szabo and C. Morean, in J.L. Rivail (Editor), Modelling of Molecular Structures and Properties (Studies in Physical and Theoretical Chemistry, Vol. 71 ), Elsevier, Amsterdam, 1990, p. 409. [17] G.C. Bond, Heterogeneous Catalysis: Principles and Applications, 2nd ed., Clarendon Press, Oxford, 1987, p, 57. [18] P.B. Weisz and E.W. Swegler, J. Phys. Chem., 59 (1955) 823. [19] F. Fajula, R. Ibarra, F. Figueras and C. Gueguen, J. Catal., 89 (1984) 60. [20] C. Mirodatos and D. Barthomeuf, J. Chem. Soc. Chem. Commun., (1981) 39. [21] R. Durand, P. Geneste, J. Joffre and C. Moreau, Stud. Surf. Sci. Catal., 78 (1993) 647. [22] R. Durand, P. Geneste, C. Moreau and S. Mseddi, Stud. Surf. Sci. Catal., 20 (1985) 319. [23] M.S. Feather and J.F. Harris, Adv. Carbohydr. Chem. Biochem., 28 (1973) 161. [24] M.J. Antal, W.S.L. Mok and G.N. Richards, Carbohydr. Res., 199 (1990) 91. [25] S.A. Grin', S.R. Tsimbalaev and S.Yu. Gel'fand, Kinet. Catal., 34 (1993) 430. [26] R. Krishna, M.R. Kallury, C. Ambidge, T.T. Tidwell, D.G.B. Boocock, F.A. Agblevor and D.J. Stewart, Carbohydr. Res., 158 (1986) 253. [27] D.A. Nelson, P.M. Molton, J.A. Russell and R.T. Hallon, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 471.