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Hydrolysis of disaccharides by dealuminated Y-zeolites
190
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All fights reserved.
HYDROLYSIS OF DISACCHARIDES BY DEALUMINATED Y-ZEOLITES Christoph Buttersack and Daniela Laketic Sugar Institute, Technical University, P.O. Box 4636, D-38036 Braunschweig, Germany Hydrolysis of X-O-a-D-glucopyranosyI-D-fructoses w i t h X = 2 (sucrose), X = 5 (leucrose) and X = 6 (isomaltulose) by protonated Y-zeolites w a s s h o w n to be strongly accelerated by dealumination. T w o or less protons per unit cell provide a maximal proton related rate constant of the sucrose hydrolysis. The rate c o n s t a n t is the same compared to diluted homogeneous acid solution. Hydrophobic interactions essentially contribute to the adsorption of the sucrose molecule but not to the energy of the activated transition state. It w a s proven that the zeolite pores are able to include a sucrose molecule plus at least one w a t e r molecule interacting w i t h the glycosidic bond. Because of the limited space inside the zeolite pore a change of c o n f o r m a t i o n compared w i t h the state in solution is required. Introduction The acid catalyzed hydrolysis of sucrose is generally considered to follow an A1 mechanism in which a fast pre-equilibrium protonation is followed by a unimolecular, rate-determining heterolysis of the fructosyl-oxygen bond [1]. The reaction is established on industrial scale using acid ion-exchange resins [2]. In contrast to the polymeric acid system, which bears some problems concerning the microenvironment and swelling [3], acid zeolites are characterized by a fixed geometry. The precisely uniform pore size enables specific interactions, which are the basis for chromatographic separation of carbohydrates on zeolites [4]. To our knowledge nothing has been reported on the utilization of zeolites for the hydrolysis of saccharides. One is tempted to expect that the dimensions of disaccharide molecules are too big to enter even the relative large pores of Y-zeolites [4]. However, it was shown recently that the interaction of sucrose, leucrose and isomaltulose can be significantly enhanced by dealumination of the zeolites [5]. Therefore, we were encouraged to start investigations concerning their corresponding catalytic activities. Experimental section Dealuminated NaY-zeolites (Si/AI ratio: 26, 55, 110) produced by the SiCI4 method [6] and the original Y-zeolite (Si/AI = 2.4) were treated with NH, CI [7] and heated (110 ~ / 16 h plus slowly up to 450 ~ / 16 h). An EDX and AAS analysis revealed a reduction of Na § by more than 60%. Hydrolysis of the disaccharides was carried out in a thermostatted stirred solution of 100 g/L saccharide with 10 g/L zeolite. Results and Discussion A considerable hydrolysis of all three disaccharides was obtained with the highest dealuminated zeolite (Si/AI= 110). As expected, the reaction was found to obey a first order kinetics, and up to a conversion of 50% at 70 ~ no considerable amounts of side products were formed. The rate constant per mass of catalyst observed with a usual Y-zeolite (Si/AI = 2.4) was very low. For sucrose the factor of reduction was about 800. Assuming the ideal case that the process of dealumination by SiCI4 [6] replaces the AI of the original framework in a pure statistical manner, the unit cell of the highest dealuminated zeolite contains only 1.7 Ai-atoms. With reference to the generally accepted assumption that 58% of the quickly exchangeable cation positions are located inside the supercage [7] one can postulate that only one proton per unit cell (8 supercages) exists as an acid site for the catalysis. Table 1 shows the results obtained with Y-zeolites of varying degree of acid sites and different temperature. The rate of hydrolysis was related to the number of acid sites in the supercage nil§ (in mole/g). Thus, the kinetics should obey
.dcs
= knH+ Ccat cs dt where cr and c= are the zeolite respective sucrose concentration in g/L. k represents an effective
191
rate constant composed of the actual value and the concentration of sucrose inside the pores n=. On deactivated zeolites (Na*-form) the adsorption was completed within some minutes, and the analysis of the corresponsing isotherm suggests that n, corresponds to the saturation value (about 170 mg/g) when no influence of pore diffusion occurs. The activation energies are 29.7 and 30.1 kcal mole 1 K~ for Si/AI = 110 and 26 resp. These values are very close to that for the homogeneous solution. For the hydrolysis with HCI a value of 2 5 . 9 + 0 . 7 kcal mole "1 K"~ [8] is published. Therefore, the activated transition complex of the sucrose inside the zeolite is very similar to that in the liquid phase with respect to the activation enthalpy. The concentration of the zeolite particles was always 10 g/L. In case of the zeolite with the ratio Si/AI = 110 the amount of protons (58% of the AI-content) is 0.867 mole/I of the reaction volume. Surprisingly, a corresponding diluted solution of HCI causes about the same rate constants (Table 1). Thus one is tempted to postulate that despite the high concentration of protons and sucrose molecules inside the zeolite volume the separation of the reaction centres by the zeolite lattice has the same effect as a dilution of a concentrated solution. In other words: no difference exists when the sucrose molecules are separated by many water molecules or by thin "walls" mainly consisting of SiO2. Literature 1 2 3 4 5 6 7 8
T.L. Mega and R. van Etten, J. Am. Chem. Soc. 110 (1988) 6372 G. Siegers and F. Martinola, Int. SugarJ. 87 (1985) 23 C. Buttersack, React. Polymers 10 (1989) 143 C. Buttersack, W. Wach and K. Buchholz, Stud. Surf. ScL Catal. (1994), this congress C. Buttersack, W. Wach and K. Buchholz, J. Phys. Chem. 97 (1993) 11861 H.K. Beyer and I. M. Belenykaja, Stud. Surf. Sci. CataL 5 (1980) 203 T.L.M. Maesen, H. van Bekkum, T. G. Verbourg, Z. I. Kolar and H. W. Kouwenhoven, J. Chem. Soc., Faraday Trans. 87 (1991) 787 K. Vukov, Int. Sugar J. 67 (1967) 172
Table 1 Rate constants k of the sucrose hydrolysis in Y-zeolites with different degrees of dealumination (The concentration of protons available for the sucrose molecule was assumed to be 58 % of the bulk aluminum content. Data for hydrolysis in acid solution was taken from literature [8]. The k-value for Si/AI=2.4 at 40"C was extrapolated assuming the same activation energy found for Si/AI=27).
charge
A
C
Si/AI
2.43
k [10"1 L mole-1 min"1]
H +
per unit cell
[mmole /g] 30"C
40 "C
50 "C
60 "C
0.096
32.5
0.171
[0.00042]
27 55 110
4.1 2.0 1.0
0.340 0.171 0.0867
0.0072 0.536 0.566
0.024
110
1.0
0.0867
0.128
0.625
2.46
10.5
0.157
0.622
2.22
7.59
0.57 M HCI
70 "C 0.029 0.505 52.12
23.7 ,..
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All fights reserved.
HYDROLYSIS OF DISACCHARIDES BY DEALUMINATED Y-ZEOLITES Christoph Buttersack and Daniela Laketic Sugar Institute, Technical University, P.O. Box 4636, D-38036 Braunschweig, Germany Hydrolysis of X-O-a-D-glucopyranosyI-D-fructoses w i t h X = 2 (sucrose), X = 5 (leucrose) and X = 6 (isomaltulose) by protonated Y-zeolites w a s s h o w n to be strongly accelerated by dealumination. T w o or less protons per unit cell provide a maximal proton related rate constant of the sucrose hydrolysis. The rate c o n s t a n t is the same compared to diluted homogeneous acid solution. Hydrophobic interactions essentially contribute to the adsorption of the sucrose molecule but not to the energy of the activated transition state. It w a s proven that the zeolite pores are able to include a sucrose molecule plus at least one w a t e r molecule interacting w i t h the glycosidic bond. Because of the limited space inside the zeolite pore a change of c o n f o r m a t i o n compared w i t h the state in solution is required. Introduction The acid catalyzed hydrolysis of sucrose is generally considered to follow an A1 mechanism in which a fast pre-equilibrium protonation is followed by a unimolecular, rate-determining heterolysis of the fructosyl-oxygen bond [1]. The reaction is established on industrial scale using acid ion-exchange resins [2]. In contrast to the polymeric acid system, which bears some problems concerning the microenvironment and swelling [3], acid zeolites are characterized by a fixed geometry. The precisely uniform pore size enables specific interactions, which are the basis for chromatographic separation of carbohydrates on zeolites [4]. To our knowledge nothing has been reported on the utilization of zeolites for the hydrolysis of saccharides. One is tempted to expect that the dimensions of disaccharide molecules are too big to enter even the relative large pores of Y-zeolites [4]. However, it was shown recently that the interaction of sucrose, leucrose and isomaltulose can be significantly enhanced by dealumination of the zeolites [5]. Therefore, we were encouraged to start investigations concerning their corresponding catalytic activities. Experimental section Dealuminated NaY-zeolites (Si/AI ratio: 26, 55, 110) produced by the SiCI4 method [6] and the original Y-zeolite (Si/AI = 2.4) were treated with NH, CI [7] and heated (110 ~ / 16 h plus slowly up to 450 ~ / 16 h). An EDX and AAS analysis revealed a reduction of Na § by more than 60%. Hydrolysis of the disaccharides was carried out in a thermostatted stirred solution of 100 g/L saccharide with 10 g/L zeolite. Results and Discussion A considerable hydrolysis of all three disaccharides was obtained with the highest dealuminated zeolite (Si/AI= 110). As expected, the reaction was found to obey a first order kinetics, and up to a conversion of 50% at 70 ~ no considerable amounts of side products were formed. The rate constant per mass of catalyst observed with a usual Y-zeolite (Si/AI = 2.4) was very low. For sucrose the factor of reduction was about 800. Assuming the ideal case that the process of dealumination by SiCI4 [6] replaces the AI of the original framework in a pure statistical manner, the unit cell of the highest dealuminated zeolite contains only 1.7 Ai-atoms. With reference to the generally accepted assumption that 58% of the quickly exchangeable cation positions are located inside the supercage [7] one can postulate that only one proton per unit cell (8 supercages) exists as an acid site for the catalysis. Table 1 shows the results obtained with Y-zeolites of varying degree of acid sites and different temperature. The rate of hydrolysis was related to the number of acid sites in the supercage nil§ (in mole/g). Thus, the kinetics should obey
.dcs
= knH+ Ccat cs dt where cr and c= are the zeolite respective sucrose concentration in g/L. k represents an effective
191
rate constant composed of the actual value and the concentration of sucrose inside the pores n=. On deactivated zeolites (Na*-form) the adsorption was completed within some minutes, and the analysis of the corresponsing isotherm suggests that n, corresponds to the saturation value (about 170 mg/g) when no influence of pore diffusion occurs. The activation energies are 29.7 and 30.1 kcal mole 1 K~ for Si/AI = 110 and 26 resp. These values are very close to that for the homogeneous solution. For the hydrolysis with HCI a value of 2 5 . 9 + 0 . 7 kcal mole "1 K"~ [8] is published. Therefore, the activated transition complex of the sucrose inside the zeolite is very similar to that in the liquid phase with respect to the activation enthalpy. The concentration of the zeolite particles was always 10 g/L. In case of the zeolite with the ratio Si/AI = 110 the amount of protons (58% of the AI-content) is 0.867 mole/I of the reaction volume. Surprisingly, a corresponding diluted solution of HCI causes about the same rate constants (Table 1). Thus one is tempted to postulate that despite the high concentration of protons and sucrose molecules inside the zeolite volume the separation of the reaction centres by the zeolite lattice has the same effect as a dilution of a concentrated solution. In other words: no difference exists when the sucrose molecules are separated by many water molecules or by thin "walls" mainly consisting of SiO2. Literature 1 2 3 4 5 6 7 8
T.L. Mega and R. van Etten, J. Am. Chem. Soc. 110 (1988) 6372 G. Siegers and F. Martinola, Int. SugarJ. 87 (1985) 23 C. Buttersack, React. Polymers 10 (1989) 143 C. Buttersack, W. Wach and K. Buchholz, Stud. Surf. ScL Catal. (1994), this congress C. Buttersack, W. Wach and K. Buchholz, J. Phys. Chem. 97 (1993) 11861 H.K. Beyer and I. M. Belenykaja, Stud. Surf. Sci. CataL 5 (1980) 203 T.L.M. Maesen, H. van Bekkum, T. G. Verbourg, Z. I. Kolar and H. W. Kouwenhoven, J. Chem. Soc., Faraday Trans. 87 (1991) 787 K. Vukov, Int. Sugar J. 67 (1967) 172
Table 1 Rate constants k of the sucrose hydrolysis in Y-zeolites with different degrees of dealumination (The concentration of protons available for the sucrose molecule was assumed to be 58 % of the bulk aluminum content. Data for hydrolysis in acid solution was taken from literature [8]. The k-value for Si/AI=2.4 at 40"C was extrapolated assuming the same activation energy found for Si/AI=27).
charge
A
C
Si/AI
2.43
k [10"1 L mole-1 min"1]
H +
per unit cell
[mmole /g] 30"C
40 "C
50 "C
60 "C
0.096
32.5
0.171
[0.00042]
27 55 110
4.1 2.0 1.0
0.340 0.171 0.0867
0.0072 0.536 0.566
0.024
110
1.0
0.0867
0.128
0.625
2.46
10.5
0.157
0.622
2.22
7.59
0.57 M HCI
70 "C 0.029 0.505 52.12
23.7 ,..