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Preparation of encaged heteropoly acid catalyst by synthesizing 12-molybdophosphoric acid in the supercages of Y-type zeolite
/
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 165 (1997) 219-226
Preparation of encaged heteropoly acid catalyst by synthesizing 12-molybdophosphoric acid in the supercages of Y-type zeolite Shin R. Mukai*, Takao Masuda, Isao Ogino, Kenji Hashimoto Division of Chemical Engineering, Graduate School of Engineering, Kyoto University Kyoto 606-01, Japan Received 18 February 1997; received in revised form 26 May 1997; accepted 31 May 1997
Abstract
12-molybdophosphoric acid encaged in the supercages of Y-type zeolite was synthesized from molybdenum oxide and phosphoric acid, in a slurry mixture of Y-type zeolite crystals and deionized water. After thorough washing in hot water, the 12-molybdophosphoric acid was found to remain in the Y-type zeolite, at an amount estimated to be 0.09 g (g-support) -1. Catalysts thus obtained were found to show activity for the esterification of acetic acid with ethanol, indicating that this catalyst could be used as a solid acid catalyst in various liquid phase reactions which involve water. ~: 1997 Elsevier Science B.V.
Keywords: Heteropoly acid; 12-molybdophosphoric acid; Y-type zeolite; Solid acid catalyst; Esterification
1. I n t r o d u c t i o n Acids such as sulfuric acid, hydrochloric acid, aluminum chloride and boron trifluoride show high catalytic activities in liquid phase reactions which involve water, such as esterification and hydrolysis. However, it is difficult to separate and recover these catalysts from the products, as the catalysts are highly soluble in water. The separation process consumes a large amount of energy. Moreover, as these acids are highly corrosive, expensive durable reactors are required for their usage, The development of alternative solid acid catalysts active in aqueous solutions is one way to solve these problems. Presently in commercial processes, ionexchange resin is the only solid acid available for this *Corresponding author, 0926-860X/97/$17.00 (t5 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 2 0 3 - 2
purpose. However, such resins lack mechanical strength and thermal stability, and problems such as deactivation due to swelling occur. There have been other attempts to use solids such as high-silica zeolite [ 1] and layer compounds of zirconium phosphonate [2] for this purpose. Another approach is to immobilize an active acidic species, such as a heteropoly acid (HPA), and then practically use it as a solid acid catalyst in the liquid phase. Izumi et al. have entrapped HPAs in activated carbon and showed that the obtained catalyst could be used for the liquid phase etherification of alcohols [3]. However, even though a significant amount of HPA could be entrapped in the carbon, the amount of HPA leakage could not be neglected. Recently, HPAs or HPA-salts have been included in a silica matrix by means of a sol-gel technique, which has drastically reduced the amount of leakage [4,5]. Okuhara et al.
220
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
supercage window
0.74 nm
HY crystal Fig. 1. Schematic illustration of the encagement of HPA molecules in HY crystal.
have applied water insoluble cesium salts of HPAs to liquid phase reactions [6]. This catalyst was found to work as an efficient acid catalyst in Friedel-Crafts alkylation, acylation [7], esterification and hydrolysis reactions in the liquid phase [8]. If HPAs could be formed like a 'ship in a bottle' in a 'cage' whose size is slightly bigger than their anions, the formed HPA anions would not be able to diffuse out of the cage. The catalyst would be expected to act as a solid acid in the liquid phase. The anion of an HPA with a Keggin structure is almost a sphere with a diameter of about 1 nm. The supercages of Y-type zeolites have a diameter of 1.3 nm and are interconnected by windows 0.74 nm in diameter, as is schematically shown in Fig. 1. Therefore, this zeolite is thought to be an ideal material for the encagement of HPA molecules having a Keggin structure,
Recently, Sulikowski et al. [9] reported that Keggin structured 12-tungstophosphoric acid (PW12) could be encaged in the supercages of Y-type zeolite. They also showed that this catalyst was active in isomerization and disproportionation reactions of m-xylene in the gas phase. However, these reactions also proceed over Y-type zeolites not encaging HPAs. Furthermore, when compared with the usage in the gas phase, usage of this catalyst in liquid phase reactions demands a higher catalyst stability. In other words, it must be confirmed that a significant amount of HPA is encaged within the zeolite, and that the encaged HPA does not decompose or leak during usage of this catalyst in the liquid phase. In this work, 12-molybdophosphoric acid (PMol2) was synthesized in the supercages of Y-type zeolite from molybdenum oxide and phosphoric acid. This
S.R. Mukai et al. /Applied Catalysis A: General 165 (1997) 219-226
HPA was selected in order to examine the usefulness of the proposed method, as the water solubility of its materials is rather high when compared with those of PW12. This allows us to easily remove the HPAs formed on the outer surface of the Y-type zeolite as well as the unreacted material by simply washing the catalyst in hot water and filtering. In the case of PWl 2, a significant amount of its materials remain on the outer surface of the Y-type zeolite, even after thorough washing, due to their low water solubility. These materials will eventually leak into the reaction solution when the encaged catalyst is used in liquid phase reactions, The obtained catalyst was thoroughly washed with hot water, and the amount of PMol2 leakage was measured. Finally, the catalytic activity of the obtained solid acid was verified using the esterification of acetic acid with ethanol as a model reaction.
2. Catalyst preparation and characterization Keggin structured PMol2 was used in this work. The Y-type zeolite used was supplied by the Catalyst Society of Japan (JRC-HY4.8 (HY)). However, the frame~vork of this zeolite contains aluminum atoms. As aluminum atoms show basicity, they may accelerate the decomposition of HPAs, or disturb the formation of HPA anions. Therefore, Y-type zeolite dealuminated in a steam atmosphere for 10 h was also prepared. All of the zeolites were ion-exchanged with a 10 wt% NHaCi solution before usage, The method reported by Tsignidos is widely used for the synthesis of PMol2 [10]. We modified this method to form PMoI2 molecules in the supercages of Y-type zeolite. 2.0 g of HY and 7.2 g of molybdenum(VI) oxide (99% purity, Wako Pure Chemicals) were mixed in 70 g of deionized water. This mixture was stirred for 24 h at room temperature. 0.48 g of phosphoric acid (85% purity, Wako Pure Chemicals) was added, and the obtained mixture was stirred for 3 h, at a temperature of 368 K. The synthesized sample (PMo 12 encaged HY) was filtrated and dried at 383 K. For comparison, HY supporting PMol2 was also prepared by the usual impregnation technique, HY crystals were immersed in a 12.5% aqueous
221
solution of PMol2. This mixture (PMol2 supported HY) was stirred for 24 h followed by drying at 383 K. Both of the samples were individually immersed into hot water (353 K). After an hour of agitation, the samples were removed from the hot water by filtration, and the amount of PMol2 dissolved in the hot water was determined by inductively coupled plasma analysis (ICP; Shimadzu, ICPS 1000IV). The amount of PMol2 remaining in the HY was also measured by dissolving a small amount of the washed samples in hydrofluoric acid, and analyzing the obtained solution using ICP analysis. The washing sequence was repeated five times. IR spectra of the washed samples were recorded using a Fourier transform infrared spectrometer (Shimadzu, FTIR8200PC) to determine if the peaks assigned to PMol2 could be detected in the samples. In order to confirm the location of PMol2 formation within HY crystals, adsorption isotherms of iso-butane on PMo 12 encaged HY were measured at 298 K using an adsorption apparatus (Bel Japan, Belsorp 28). Using the isotherms, the limiting micro-pore volume corresponding to the minimum dimension of the adsorbed molecule was determined using the Dubinin-Astakhov equation [11].
3. Reaction The esterification of acetic acid with ethanol was performed using PMo 12 encaged HY as the catalyst to determine if PMo 12 encaged HY has catalytic activity. Reactions using dissolved PMol2 and PMol2 supported HY as the catalyst and reactions using no catalyst were also conducted for comparison. The amounts of PMol2 encaged HY and dissolved PMol2 were adjusted to give a proton concentration of 2.7 mol m 3. The amount of PMo 12 supported HY was made equal to that of PMol2 encaged HY. Reactions were conducted at 333 K in a 100 cm 3 flask using a mixture of acetic acid (0.5 mol) and ethanol (0.5 mol) as the starting material. A small amount of the reactant solution was sampled at a certain interval using a syringe equipped with a filter. The composition of the solution was measured by gas chromatography using a Porapak Q column (3 mm i.d., 2.6 m), and the transient change in the conversion of the reaction was obtained.
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
222
4. Results
"7 "-- 0.20
and discussion
I
I
I
J
I
0 ¢X
When non-dealuminated HY was used for catalyst synthesis, no P M o l 2 could be detected in the obtained catalyst by ICP and FTIR analysis. This is assumed to be due to the basicity of aluminum contained in the framework of HY. Therefore, the HY was steam treated to lower its aluminum content. Fig. 2 shows the XRD patterns of the original and steam treated HY. The intensity of the main peaks of the zeolite is only slightly reduced, indicating that HY still maintained its framework structure after dealumination by steam treatment. This dealuminated HY was employed in further experiments. Both P M o l 2 encaged HY and P M o l 2 supported HY samples synthesized using dealuminated HY had a bright yellow color, which is characteristic of PMo 12. However, the color of P M o l 2 supported HY turned white after only a single washing operation, suggesting that there was no P M o l 2 left in the P M o l 2 I
I
o. ~' 0.15 o~
._= 0.10 ¢-~ .=~ 0,05 < n -i"5 E 0 o-~ E
©
/
I
J
I
I
I
1
2
3
4
5
times of washing operation
Fig. 3. Transient change in the amount of HPA remaining in HY after consecutive washing operations. I
I
I
fresh
c
®
steam
.¢::
treated
5
10
15
20
25
30
2O Fig. 2. XRD patterns of fresh and steam treated HY.
35
223
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
supported HY. This was also confirmed by ICP analysis. On the other hand, PMo 12 encaged HY maintained its color after 5 consecutive washing operations, and almost no phosphorous and molybdenum was detected in the hot water used for the 5th washing by ICP analysis. Fig. 3 shows the transient change in the amount of PMol2 remaining in the HY after each washing sequence, which was determined from ICP analysis. Note that after 5 consecutive washing operations, approximately 0.09 g (g-support) -1 of PMol2 remained within the HY.
L M,,o=Ot
I Mo-Oc-Mo i
Fig. 4 shows the FTIR spectrum o f P M o l 2 encaged HY. The spectra of PMo 12, HY and PMo 12 supported HY are also shown in the figure for comparison. One peak, corresponding to P-O (1062,-~1068 cm 1), among the 4 fingerprint peaks of PMol2 [12] could not be seen for PMol2 encaged HY, because it overlaps with the peak of HY. The remaining 3 peaks corresponding to Mo=Ot (954,-~975 cm-l), Mo-OcMo (869~880 cm -]) and Mo-Oe-Mo (785~810 cm -1) can be clearly observed for PMol 2 encaged HY. These peaks cannot be observed
I
I
I PMo12
__
O E
E E
P M o 1 2 e n c ~ g e d HY
1000
I
f
I
i
900
800
700
600
wave number
/ c m -1
Fig. 4. FTIR spectra of samples.
500
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
224
in the spectrum of PMo 12 supported HY. These facts indicate that PMol2 exists in PMol2 encaged HY, whereas for PMo 12 supported HY prepared by simple impregnation, all of the PMol2 was washed away in the washing process. It is interesting that the difference in the peak positions of PMol2 and PMol2 encaged HY was negligibly small, which suggests that even if the PMol2 molecules are encaged within the supercages of HY, they are allowed to move rather freely within the cages, and are expected to show the same acidity as free PMol2 molecules, The minimum dimension of iso-butane is approximately 0.50 nm, so it can pass through the windows of HY and gain access to the supercages of HY, but it cannot penetrate into the sodalite cages of the zeolite, Therefore, the limiting pore volume obtained using iso-butane as the probe molecule gives a rough value of the effective volume within HY, in which PMol2
80
I
60
1
-
molecules can be formed. Fig. 5 shows the adsorption isotherms of iso-butane on HY and PMol2 encaged HY measured at 323 K, where Va is the volume of isobutane adsorbed at a relative pressure of P/Ps (Ps -- 3.5 x 105 Pa at 323 K). The amount of isobutane adsorbed decreases after the formation of PMol2, which is evidently due to the formation of PMol2 within the supercages of HY. Both isotherms were analyzed by the Dubinin-Astakhov equation with n = 3 to obtain the limiting micro-pore volume corresponding to the minimum dimension of the adsorbed molecules. The obtained limiting micro-pore volumes of HY and PMol2 encaged HY were, 0.247 and 0.156 cm 3 g - l , respectively. This indicates that about one third of the supercages are filled by PMo 12 molecules. In order to confirm the probability of applying the prepared catalysts to reactions in solutions containing
I
I
I
]
1
HY
I---
,40
°
F
0
PMo12 encaged HY
20
0
F 0
I
I
I
0.1
I
I
0.2
PIPs Fig. 5. Adsorption isotherms of iso-butane on HY and PMol2 encaged HY.
0.3
225
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
1.0
.8
i
i
i
m
q
PMo12 o.6
-
"___--o-o-o-
-
0.4
-
/"/~-PMo12
no
ca
-
O~
-
/"~ --~'jZE'-Zt~-
r~
_
0.2
encaged HY
,ys
-
-
I
0
I
1000 t /min
I
2000
Fig. 6. Catalytic activity of the obtained catalysts.
water, the esterification of acetic acid with ethanol was conducted at 333 K. Fig. 6 shows the conversion (x) of the esterification of acetic acid with ethanol as a function of the reaction time (t), using the P M o l 2 encaged HY catalyst. The results of dissolved PMo 12, PMo 12 supported HY catalyst, and no catalyst addition are also shown for comparison. It is obvious that the reaction proceeds to a certain extent without catalyst addition. This is due to the existence of acetic acid as one of the reactants. The data for P M o l 2 supported HY and for the reaction conducted without catalyst addition lay on the same curve, indicating that PMo 12 supported HY does not show catalytic activity. PMo 12 encaged HY evidently has catalytic activity toward this reaction. This implies that this new solid acid catalyst can be used as an alternative to mineral acids, such as sulfuric acid and hydrochloric acid, in various liquid phase processes. However, its activity is lower than that of dissolved PMo 12. One explanation
for this difference is that only the PMol 2 molecules in the supercages which face the surface layer of HY contribute to the reaction. The difference in the size of a supercage and a HPA anion is small enough to limit the diffusion of reactant molecules through the space between them. Therefore, if a HPA anion encaged in a supercage is surrounded by supercages which also encage HPAs, it cannot contribute to the reaction.
5. Conclusion A new solid acid was developed by the encagement of P M o l 2 in the supercages of HY. The amount of leakage of P M o l 2 from the catalyst was found to be negligibly small. This solid acid showed catalytic activity in the esterification of acetic acid with ethanol, implying that it can be used as an alternative to mineral acids in various liquid processes.
226
S.R. Mukai et al./Applied Catalysis A: General 165 (1997)219-226
Acknowledgements T h e authors want to thank Dr. Yusuke Izumi, Professor o f the D e p a r t m e n t o f A p p l i e d Chemistry, S c h o o l o f Engineering, N a g o y a University, for his kind advice. His suggestion that the existence o f a l u m i n u m atoms in the f r a m e w o r k o f zeolite accelerates the d e c o m p o s i t i o n o f h e t e r o p o l y acids led the authors to the successful preparation o f this n e w solid catalyst.
References [1] S. Namba, N. Hosonuma, T. Yashima, J. Catal. 72 (1981) 16. [2] K. Segawa, N. Kihara, H. Yamamoto, J. Mol. Catal. 74 (1992) 213.
[3] Y. Izumi, K. Urabe, Chem. Lett., (1981) 663. [4] Y. Izumi, M. Ono, M. Ogawa, K. Urabe, Chem. Lett., (1993) 825. [5] Y. Izumi, M. Ono, M. Kitagawa, M. Yoshida, K. Urabe, Microporous Materials 5 (1995) 255. [6] T. Okuhara, T. Nishimura, K. Ohashi, M. Misono, Chem. Lett., (1990)1201. [7] Y. Izumi, M. Ogawa, W. Nohara, K. Urabe, Chem. Lett., (1992) 1987. [8] Y. Izumi, M. Ono, M. Ogawa, K. Urabe, Chem. Lett., (1993) 825. [9] B. Sulikowski, J. Haber, A. Kubacka, K. Pamin, Z. Olejniczak, J. Ptaszynski, Catal. Lett. 39 (1996) 27. [10] G.A. Tsigdinos, Ind. Eng. Chem. Prod. Res. Dev. 13 (1974) 4. [11] V.A. Astakhov, M.V. Dubinin, P.G. Romankov, Theor. Osn. Khim. Tekhn. 3 (1969) 292. [12] M. Misono, Catal. Rev. Sci. Eng. 29 (1987) 269.
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 165 (1997) 219-226
Preparation of encaged heteropoly acid catalyst by synthesizing 12-molybdophosphoric acid in the supercages of Y-type zeolite Shin R. Mukai*, Takao Masuda, Isao Ogino, Kenji Hashimoto Division of Chemical Engineering, Graduate School of Engineering, Kyoto University Kyoto 606-01, Japan Received 18 February 1997; received in revised form 26 May 1997; accepted 31 May 1997
Abstract
12-molybdophosphoric acid encaged in the supercages of Y-type zeolite was synthesized from molybdenum oxide and phosphoric acid, in a slurry mixture of Y-type zeolite crystals and deionized water. After thorough washing in hot water, the 12-molybdophosphoric acid was found to remain in the Y-type zeolite, at an amount estimated to be 0.09 g (g-support) -1. Catalysts thus obtained were found to show activity for the esterification of acetic acid with ethanol, indicating that this catalyst could be used as a solid acid catalyst in various liquid phase reactions which involve water. ~: 1997 Elsevier Science B.V.
Keywords: Heteropoly acid; 12-molybdophosphoric acid; Y-type zeolite; Solid acid catalyst; Esterification
1. I n t r o d u c t i o n Acids such as sulfuric acid, hydrochloric acid, aluminum chloride and boron trifluoride show high catalytic activities in liquid phase reactions which involve water, such as esterification and hydrolysis. However, it is difficult to separate and recover these catalysts from the products, as the catalysts are highly soluble in water. The separation process consumes a large amount of energy. Moreover, as these acids are highly corrosive, expensive durable reactors are required for their usage, The development of alternative solid acid catalysts active in aqueous solutions is one way to solve these problems. Presently in commercial processes, ionexchange resin is the only solid acid available for this *Corresponding author, 0926-860X/97/$17.00 (t5 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 2 0 3 - 2
purpose. However, such resins lack mechanical strength and thermal stability, and problems such as deactivation due to swelling occur. There have been other attempts to use solids such as high-silica zeolite [ 1] and layer compounds of zirconium phosphonate [2] for this purpose. Another approach is to immobilize an active acidic species, such as a heteropoly acid (HPA), and then practically use it as a solid acid catalyst in the liquid phase. Izumi et al. have entrapped HPAs in activated carbon and showed that the obtained catalyst could be used for the liquid phase etherification of alcohols [3]. However, even though a significant amount of HPA could be entrapped in the carbon, the amount of HPA leakage could not be neglected. Recently, HPAs or HPA-salts have been included in a silica matrix by means of a sol-gel technique, which has drastically reduced the amount of leakage [4,5]. Okuhara et al.
220
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
supercage window
0.74 nm
HY crystal Fig. 1. Schematic illustration of the encagement of HPA molecules in HY crystal.
have applied water insoluble cesium salts of HPAs to liquid phase reactions [6]. This catalyst was found to work as an efficient acid catalyst in Friedel-Crafts alkylation, acylation [7], esterification and hydrolysis reactions in the liquid phase [8]. If HPAs could be formed like a 'ship in a bottle' in a 'cage' whose size is slightly bigger than their anions, the formed HPA anions would not be able to diffuse out of the cage. The catalyst would be expected to act as a solid acid in the liquid phase. The anion of an HPA with a Keggin structure is almost a sphere with a diameter of about 1 nm. The supercages of Y-type zeolites have a diameter of 1.3 nm and are interconnected by windows 0.74 nm in diameter, as is schematically shown in Fig. 1. Therefore, this zeolite is thought to be an ideal material for the encagement of HPA molecules having a Keggin structure,
Recently, Sulikowski et al. [9] reported that Keggin structured 12-tungstophosphoric acid (PW12) could be encaged in the supercages of Y-type zeolite. They also showed that this catalyst was active in isomerization and disproportionation reactions of m-xylene in the gas phase. However, these reactions also proceed over Y-type zeolites not encaging HPAs. Furthermore, when compared with the usage in the gas phase, usage of this catalyst in liquid phase reactions demands a higher catalyst stability. In other words, it must be confirmed that a significant amount of HPA is encaged within the zeolite, and that the encaged HPA does not decompose or leak during usage of this catalyst in the liquid phase. In this work, 12-molybdophosphoric acid (PMol2) was synthesized in the supercages of Y-type zeolite from molybdenum oxide and phosphoric acid. This
S.R. Mukai et al. /Applied Catalysis A: General 165 (1997) 219-226
HPA was selected in order to examine the usefulness of the proposed method, as the water solubility of its materials is rather high when compared with those of PW12. This allows us to easily remove the HPAs formed on the outer surface of the Y-type zeolite as well as the unreacted material by simply washing the catalyst in hot water and filtering. In the case of PWl 2, a significant amount of its materials remain on the outer surface of the Y-type zeolite, even after thorough washing, due to their low water solubility. These materials will eventually leak into the reaction solution when the encaged catalyst is used in liquid phase reactions, The obtained catalyst was thoroughly washed with hot water, and the amount of PMol2 leakage was measured. Finally, the catalytic activity of the obtained solid acid was verified using the esterification of acetic acid with ethanol as a model reaction.
2. Catalyst preparation and characterization Keggin structured PMol2 was used in this work. The Y-type zeolite used was supplied by the Catalyst Society of Japan (JRC-HY4.8 (HY)). However, the frame~vork of this zeolite contains aluminum atoms. As aluminum atoms show basicity, they may accelerate the decomposition of HPAs, or disturb the formation of HPA anions. Therefore, Y-type zeolite dealuminated in a steam atmosphere for 10 h was also prepared. All of the zeolites were ion-exchanged with a 10 wt% NHaCi solution before usage, The method reported by Tsignidos is widely used for the synthesis of PMol2 [10]. We modified this method to form PMoI2 molecules in the supercages of Y-type zeolite. 2.0 g of HY and 7.2 g of molybdenum(VI) oxide (99% purity, Wako Pure Chemicals) were mixed in 70 g of deionized water. This mixture was stirred for 24 h at room temperature. 0.48 g of phosphoric acid (85% purity, Wako Pure Chemicals) was added, and the obtained mixture was stirred for 3 h, at a temperature of 368 K. The synthesized sample (PMo 12 encaged HY) was filtrated and dried at 383 K. For comparison, HY supporting PMol2 was also prepared by the usual impregnation technique, HY crystals were immersed in a 12.5% aqueous
221
solution of PMol2. This mixture (PMol2 supported HY) was stirred for 24 h followed by drying at 383 K. Both of the samples were individually immersed into hot water (353 K). After an hour of agitation, the samples were removed from the hot water by filtration, and the amount of PMol2 dissolved in the hot water was determined by inductively coupled plasma analysis (ICP; Shimadzu, ICPS 1000IV). The amount of PMol2 remaining in the HY was also measured by dissolving a small amount of the washed samples in hydrofluoric acid, and analyzing the obtained solution using ICP analysis. The washing sequence was repeated five times. IR spectra of the washed samples were recorded using a Fourier transform infrared spectrometer (Shimadzu, FTIR8200PC) to determine if the peaks assigned to PMol2 could be detected in the samples. In order to confirm the location of PMol2 formation within HY crystals, adsorption isotherms of iso-butane on PMo 12 encaged HY were measured at 298 K using an adsorption apparatus (Bel Japan, Belsorp 28). Using the isotherms, the limiting micro-pore volume corresponding to the minimum dimension of the adsorbed molecule was determined using the Dubinin-Astakhov equation [11].
3. Reaction The esterification of acetic acid with ethanol was performed using PMo 12 encaged HY as the catalyst to determine if PMo 12 encaged HY has catalytic activity. Reactions using dissolved PMol2 and PMol2 supported HY as the catalyst and reactions using no catalyst were also conducted for comparison. The amounts of PMol2 encaged HY and dissolved PMol2 were adjusted to give a proton concentration of 2.7 mol m 3. The amount of PMo 12 supported HY was made equal to that of PMol2 encaged HY. Reactions were conducted at 333 K in a 100 cm 3 flask using a mixture of acetic acid (0.5 mol) and ethanol (0.5 mol) as the starting material. A small amount of the reactant solution was sampled at a certain interval using a syringe equipped with a filter. The composition of the solution was measured by gas chromatography using a Porapak Q column (3 mm i.d., 2.6 m), and the transient change in the conversion of the reaction was obtained.
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
222
4. Results
"7 "-- 0.20
and discussion
I
I
I
J
I
0 ¢X
When non-dealuminated HY was used for catalyst synthesis, no P M o l 2 could be detected in the obtained catalyst by ICP and FTIR analysis. This is assumed to be due to the basicity of aluminum contained in the framework of HY. Therefore, the HY was steam treated to lower its aluminum content. Fig. 2 shows the XRD patterns of the original and steam treated HY. The intensity of the main peaks of the zeolite is only slightly reduced, indicating that HY still maintained its framework structure after dealumination by steam treatment. This dealuminated HY was employed in further experiments. Both P M o l 2 encaged HY and P M o l 2 supported HY samples synthesized using dealuminated HY had a bright yellow color, which is characteristic of PMo 12. However, the color of P M o l 2 supported HY turned white after only a single washing operation, suggesting that there was no P M o l 2 left in the P M o l 2 I
I
o. ~' 0.15 o~
._= 0.10 ¢-~ .=~ 0,05 < n -i"5 E 0 o-~ E
©
/
I
J
I
I
I
1
2
3
4
5
times of washing operation
Fig. 3. Transient change in the amount of HPA remaining in HY after consecutive washing operations. I
I
I
fresh
c
®
steam
.¢::
treated
5
10
15
20
25
30
2O Fig. 2. XRD patterns of fresh and steam treated HY.
35
223
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
supported HY. This was also confirmed by ICP analysis. On the other hand, PMo 12 encaged HY maintained its color after 5 consecutive washing operations, and almost no phosphorous and molybdenum was detected in the hot water used for the 5th washing by ICP analysis. Fig. 3 shows the transient change in the amount of PMol2 remaining in the HY after each washing sequence, which was determined from ICP analysis. Note that after 5 consecutive washing operations, approximately 0.09 g (g-support) -1 of PMol2 remained within the HY.
L M,,o=Ot
I Mo-Oc-Mo i
Fig. 4 shows the FTIR spectrum o f P M o l 2 encaged HY. The spectra of PMo 12, HY and PMo 12 supported HY are also shown in the figure for comparison. One peak, corresponding to P-O (1062,-~1068 cm 1), among the 4 fingerprint peaks of PMol2 [12] could not be seen for PMol2 encaged HY, because it overlaps with the peak of HY. The remaining 3 peaks corresponding to Mo=Ot (954,-~975 cm-l), Mo-OcMo (869~880 cm -]) and Mo-Oe-Mo (785~810 cm -1) can be clearly observed for PMol 2 encaged HY. These peaks cannot be observed
I
I
I PMo12
__
O E
E E
P M o 1 2 e n c ~ g e d HY
1000
I
f
I
i
900
800
700
600
wave number
/ c m -1
Fig. 4. FTIR spectra of samples.
500
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
224
in the spectrum of PMo 12 supported HY. These facts indicate that PMol2 exists in PMol2 encaged HY, whereas for PMo 12 supported HY prepared by simple impregnation, all of the PMol2 was washed away in the washing process. It is interesting that the difference in the peak positions of PMol2 and PMol2 encaged HY was negligibly small, which suggests that even if the PMol2 molecules are encaged within the supercages of HY, they are allowed to move rather freely within the cages, and are expected to show the same acidity as free PMol2 molecules, The minimum dimension of iso-butane is approximately 0.50 nm, so it can pass through the windows of HY and gain access to the supercages of HY, but it cannot penetrate into the sodalite cages of the zeolite, Therefore, the limiting pore volume obtained using iso-butane as the probe molecule gives a rough value of the effective volume within HY, in which PMol2
80
I
60
1
-
molecules can be formed. Fig. 5 shows the adsorption isotherms of iso-butane on HY and PMol2 encaged HY measured at 323 K, where Va is the volume of isobutane adsorbed at a relative pressure of P/Ps (Ps -- 3.5 x 105 Pa at 323 K). The amount of isobutane adsorbed decreases after the formation of PMol2, which is evidently due to the formation of PMol2 within the supercages of HY. Both isotherms were analyzed by the Dubinin-Astakhov equation with n = 3 to obtain the limiting micro-pore volume corresponding to the minimum dimension of the adsorbed molecules. The obtained limiting micro-pore volumes of HY and PMol2 encaged HY were, 0.247 and 0.156 cm 3 g - l , respectively. This indicates that about one third of the supercages are filled by PMo 12 molecules. In order to confirm the probability of applying the prepared catalysts to reactions in solutions containing
I
I
I
]
1
HY
I---
,40
°
F
0
PMo12 encaged HY
20
0
F 0
I
I
I
0.1
I
I
0.2
PIPs Fig. 5. Adsorption isotherms of iso-butane on HY and PMol2 encaged HY.
0.3
225
S.R. Mukai et al./Applied Catalysis A: General 165 (1997) 219-226
1.0
.8
i
i
i
m
q
PMo12 o.6
-
"___--o-o-o-
-
0.4
-
/"/~-PMo12
no
ca
-
O~
-
/"~ --~'jZE'-Zt~-
r~
_
0.2
encaged HY
,ys
-
-
I
0
I
1000 t /min
I
2000
Fig. 6. Catalytic activity of the obtained catalysts.
water, the esterification of acetic acid with ethanol was conducted at 333 K. Fig. 6 shows the conversion (x) of the esterification of acetic acid with ethanol as a function of the reaction time (t), using the P M o l 2 encaged HY catalyst. The results of dissolved PMo 12, PMo 12 supported HY catalyst, and no catalyst addition are also shown for comparison. It is obvious that the reaction proceeds to a certain extent without catalyst addition. This is due to the existence of acetic acid as one of the reactants. The data for P M o l 2 supported HY and for the reaction conducted without catalyst addition lay on the same curve, indicating that PMo 12 supported HY does not show catalytic activity. PMo 12 encaged HY evidently has catalytic activity toward this reaction. This implies that this new solid acid catalyst can be used as an alternative to mineral acids, such as sulfuric acid and hydrochloric acid, in various liquid phase processes. However, its activity is lower than that of dissolved PMo 12. One explanation
for this difference is that only the PMol 2 molecules in the supercages which face the surface layer of HY contribute to the reaction. The difference in the size of a supercage and a HPA anion is small enough to limit the diffusion of reactant molecules through the space between them. Therefore, if a HPA anion encaged in a supercage is surrounded by supercages which also encage HPAs, it cannot contribute to the reaction.
5. Conclusion A new solid acid was developed by the encagement of P M o l 2 in the supercages of HY. The amount of leakage of P M o l 2 from the catalyst was found to be negligibly small. This solid acid showed catalytic activity in the esterification of acetic acid with ethanol, implying that it can be used as an alternative to mineral acids in various liquid processes.
226
S.R. Mukai et al./Applied Catalysis A: General 165 (1997)219-226
Acknowledgements T h e authors want to thank Dr. Yusuke Izumi, Professor o f the D e p a r t m e n t o f A p p l i e d Chemistry, S c h o o l o f Engineering, N a g o y a University, for his kind advice. His suggestion that the existence o f a l u m i n u m atoms in the f r a m e w o r k o f zeolite accelerates the d e c o m p o s i t i o n o f h e t e r o p o l y acids led the authors to the successful preparation o f this n e w solid catalyst.
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[3] Y. Izumi, K. Urabe, Chem. Lett., (1981) 663. [4] Y. Izumi, M. Ono, M. Ogawa, K. Urabe, Chem. Lett., (1993) 825. [5] Y. Izumi, M. Ono, M. Kitagawa, M. Yoshida, K. Urabe, Microporous Materials 5 (1995) 255. [6] T. Okuhara, T. Nishimura, K. Ohashi, M. Misono, Chem. Lett., (1990)1201. [7] Y. Izumi, M. Ogawa, W. Nohara, K. Urabe, Chem. Lett., (1992) 1987. [8] Y. Izumi, M. Ono, M. Ogawa, K. Urabe, Chem. Lett., (1993) 825. [9] B. Sulikowski, J. Haber, A. Kubacka, K. Pamin, Z. Olejniczak, J. Ptaszynski, Catal. Lett. 39 (1996) 27. [10] G.A. Tsigdinos, Ind. Eng. Chem. Prod. Res. Dev. 13 (1974) 4. [11] V.A. Astakhov, M.V. Dubinin, P.G. Romankov, Theor. Osn. Khim. Tekhn. 3 (1969) 292. [12] M. Misono, Catal. Rev. Sci. Eng. 29 (1987) 269.