Ind. Eng. Chem. Res. 1992,31, 125-130 Lycourghiotis, A.; Vattis, D. Hydrodeeulfurizationof Thiophene over Na-Doped CoMo/Alp03 Catalysts Prepared by Inverse Impregnation. React. Kinet. Catal. Lett. 1982,21, 23-27. Massoth, F. E. Characterization of Molybdena Catalysts. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic: New York, 1978; Vol. 27. Massoth, F. E.; Kibby, C. L. Studies of Molybdena-Alumina Catalysts V. Relation Between Catalyst Sulfided State and Activity for Thiophene Hydrodesulfurization. J. Catal. 1977,47,300-315. Mulligan, D. J.; Berk, D. Reduction of Sulfur Dioxide with Methane Over Selected Transition Metal Sulfides. Znd. Eng. Chem. Res. 1989,28,926-931.
Okamato, Y.; Nakano, H.; Shimokawa, T.; Imanaka, T.; Teranishi, S. Stabilization Effect of Co for Mo Phase in Co-Mo/A1203Hy-
125
drodesulfurization Catalysts Studied with X-Ray Photoelectron Spectroscopy. J. Catal. 1977,50,447-454. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide with Methane over Activated Alumina. Znd. Eng. Chem. Res. 1988,27, 1951-1954. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide by Methane over Transition Metal Oxide Catalysts. Chem. Eng. Commun. 1991, in press. Universal Oil Products Company. Catalyseurs Bimetalliques Utilisable Notamment pour la Reduction de Composes Oxysoufres. Fr. Patent 2223081,1974.
Received for review April 25, 1991 Revised manuscript received September 11, 1991 Accepted September 24, 1991
Methyl tert -Butyl Ether Synthesis over Titanium-Silicalite I Catalysts Kyung-Ho Chang, Geon-Joong Kim, and Wha-Seung Ahn* Department of Chemical Engineering, Znha University, Incheon 401 - 751, Korea
The vapor-phase reaction of methanol with isobutene to form MTBE (methyl tert-butyl ether) was carried out using titanium-modified silicalites at 70-110 "C under atmospheric pressure, and the results were compared with those obtained using HZSM-5. The acid sites responsible for MTBE synthesis were mainly of weak to medium acid strength. MTBE synthesis reaction kinetic data could be fit with the Langmuir-Hinshelwood mechanism, which assumes the reaction between adsorbed methanol molecules with isobutene adsorbed at two different acid sites is the rate-determining step.
Introduction Methyl tert-butyl ether (MTBE) has been regarded as a promising alternative octane booster to toxic lead additives in gasoline (Reynolds et al., 1975). The reaction for obtaining MTBE could also be of interest as a means of quantitative separation of isobutene from 1-butene in C4 petroleum cuts (Fattore et al., 1981). MTBE can be formed by addition of methanol to the double bond of isobutene using acid catalysts. Commercially, MTBE is obtained in the liquid phase below 100 "C at 200 psig using a cation-exchangedresin (Amberlyst) catalyst (Pecci and Floris, 1977). The ion-exchanged resin is,however, not stable above 90 "C; overheating causes the release of acid materials from the catalyst (Takesono and Fujiwara, 1980), and there exists a need for developing a stable alternative catalyst. According to Chu and Kiihl(1987), zeolites ZSM-5 and ZSM-11 produced both high conversion and high selectivity to MTBE. Furthermore, it is expected that isomorphous substitution of Ti into the zeolite framework would produce mild strength acid sites over the catalyst surface, which may be effective for MTBE reaction. The objective of the present study is to prepare a series of isomorphously substituted Tiailicalites and to evaluate their catalytic activities and selectivitiesto MTBE in the vapor-phase reaction of methanol with isobutene. Experimental Section Materials. Fine amorphous silica powder (KoFran Co., 91.8% sio2-8.2% H20), sodium hydroxide (Junsei Co., 95%1, tetrapropylammonium bromide (Fluka Co., TPABr), and Ti(OC3H7I4(Aldrich, 99%) were used for Ti-silicalite preparation. Methanol (James Burrough Ltd., Witham), with a minimum purity of 99% containing less than 0.1% water, and isobutene (Korea Standard Research Institute), with a minimum purity of 99%, were used without further
purification for the MTBE synthesis reaction. Zeolite Synthesis and Characterization. Tiailicalite samples were prepared from the substrate compositions of 0.13Na20Si02-(0.0014-0.014)Ti02-39H20-0.12TPABr. The reaction mixture was stirred for 30 min, and the hydrothermal synthesis reaction was carried out in a stainless steel tube (100-mL capacity) at 140-170 OC without agitation. The solid products were washed, filtered, and dried at 120 "C for 12 h. The crystalline phase obtained was identified using an X-ray diffractometer (Philips, PW-1700, Cu Ka, Ni filter), and the relative crystallinity of the sample was determined by comparing the peak areas at 28 = 20-25O with those of the best crystallized silicalite I. Morphologies were examined with a scanning electron microscopy (Hitachi, X-650), and IR spectra were obtained with a Nicolet lOOMX spectrometer. The acid type and acid strength of the zeolite catalysts used in the MTBE synthesis reaction were determined by means of temperature-programmed desorption (TPD) and IR analysis after pyridine preadsorption at room temperature. Reactant adsorption characteristics on different acid sites have also been examined. MTBE Synthesis Reaction and Analysis. The vapor-phase MTBE synthesis reaction were carried out in a f=ed bed reactor at atmospheric pressure. N2carrier gas was bubbled through a wash bottle filled with methanol, and the methanol-saturated N2 was then mixed with isobutene gas before the reactant mixture was introduced into the Pyrex reactor. Lines suspected of condensation were all electrically heated. The inside/outside temperature gradient of the reactor was measured by means of two chromel-alumel thermocouples. Catalysta were pretreated in a N2environment at 500 OC for 4 h. A sampling valve at the reactor outlet allowed on-line analysis of the reactor effluent using Hitachi 263-30 gas chromatography equipped with a thermal conductivity detector. A 2 m X 0.0032 rn O.D.stainless steel GC column packed with Porapak Q (80/100 mesh) was used for product separation. The
OSSS-5SS5/92/2631-0125$03.Q0~0 0 1992 American Chemical Society
126 Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 Table I. Composition, Lattice Parameter, Surface Area, and N2Sorption Capacity of Various Ti-Silicalites composition of crystal lattice parameter BET surface N, substrate composition Si/Al Si/Ti a b c area/(m2/g) sorbed/img/g) 0. 13Na20-SiO2-O.01TiO2-39H2O-O.12TPABr m 67 19.96 19.84 13.39 460 212 0. 13Naz0-SiO2-O.02TiO2-39H2O-O. 12TPABr m 48 20.02 19.96 13.41 419 161 20.08 20.04 13.44 330 137 27 0.13Na20-Si02-0.033Ti02-39H20-0.12TPABr m
w
0)
5 Y
.r Y
E, +-4L
70
65
60
55
50
45
40
35
30
25
20
15
10
5
1100
800
V
2 8
Figure 1. X-ray diffractograms of Ti-silidte (A),TiOppowder (B), and a physical mixture of silicalite I with TiOz powder (C).
column temperature was maintained at 150 OC,and the helium (Korea Industrial Gases Ltd., 99.99% pure) carrier gas flow rate was 30 cm3/min. At this condition, successful separation of isobutene, methanol, MTBE, diisobutene, and tert-butyl alcohol was accomplished. Kinetic experiments were performed in a ditferential reactor mode, keeping the conversion less than 10%. The catalyst used (1g) was of 100-200-mesh size to avoid possible internal mass-transfer resistance. ‘ m e totalflow rate at the reactor outlet was kept relatively high at 100 cm3/min throughout the experiment, giving W/FT = 4.08 g/(mol/h). The partial pressure ranges of the reactanta were 0.02-0.22 atm for isobutene and 0.02-0.06 atm for methanol, respectively.
Results and Discussion X-ray diffractograms of Ti-modified silicalite, TiOz powder obtained from titanium alkoxide, and silicalite I physically mixed with TiOz powder are shown in Figure 1. No X-ray diffraction peaks corresponding to crystalline titanium oxides were detected for Tiailicalite, and Ti(1V) seems to exist in the structural framework of silicalite. However, the formation of titanium(n3 oxides precipitated as small particles inside pores could not be ruled out completely. IR spectra of Ti-silicalite with different Ti contents are shown in Figure 2. The IR spectra in the midinfrared region were closely examined to verify the isomorphous substitution of Ti4+ions into the silicalite framework. The band around 1100 cm-’ corresponds to asymmetric vibrations of internal tetrahedra, and the symmetric stretching
Wave number (cm-l)
Figure 2. IR spectra of Ti-silicalite and silicalite I. The numbers in parentheaes given after each catalyst denote the Si02/Ti02ratios.
of the T-O bond (T = Al, Si, ...) is found around 820-780 cm-’ (Breck, 1974). As the content of Ti increases, these absorption bands near 800-1100 cm-I shift progressively toward a lower frequency region compared with the absorption band of silicalite I. It can be speculated that the shift to lower frequency on the spectra is due to increases in the unit cell parameters caused by Ti existing in tetrahedral sites. It has been shown (Borade, 1987) that the substitution of Ti for Si in the zeolite framework results in an absorption band shift to a lower frequency region owing to the longer T i 4 bond distance 89 comparded with Si-O bond, while P substitution due to the shorter tetrahedral P-O distance leads to the absorption band shift to a higher frequency region. Table I shows the chemical composition of crystals, unit cell parameters, BET surface area, and N2 amount adsorbed with various Ti-silicalites. As the Si02/Ti02ratio of Ti-silicalite crystals decreased, the unit cell parameters increased. The increase in the unit cell parameters of Ti-silicalite is in agreement with the isomorphous substitution of bigger Ti04tetrahedron to the Si04unit in the zeolite framework. Figure 3 represents the conversion and selectivity to MTBE as a function of time on stream over Ti-silicalite and HZSM-5 catalyst. Both Ti-silicalite and HZSM-5 catalysts gave steady total conversion and MTBE selectivity without any deactivation for 20 h. With HZSM-5, a small amount of DME (dimethyl ether) was produced via methanol dehydration, but with Ti-silicalite, extremely
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 127 20
-T
00
95 90
15
2
-
15
0 : MTBE select. 0 : MTBE select.
.r
L"
. .c
10-
7
V
; 2 LA
80
5.
c
--
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A : Total c o w . v : Total c o w . : DME select.
>
85
I I*
-5"
"
v
E
c 0
t l \
M
-J
2 x L
5
I/
I'
v-
0
4
8
12
16
20
0'
Time-on-stream (hr)
Figure 3. Conversion and selectivity to MTBE over Ti-silicalite (37) (0,V) and HZSMd (20) (m, A, 0 ) : coke free basis; reaction temperature, 90 OC; I-C4H8/CH30Hmole ratio, 6.5; WHSV, 3.6 h-l (FI= 6 X mol/h). The numbers given in parentheses after each catalyst denote the SiOz/TiOz or SiOz/Al2O3ratio.
high selectivity to MTBE (essentially100%) was obtained; no byproducts, such as DME, diisobutene, or TBA (tertbutyl alcohol), were detected in the reactor effluent stream. With HZSM-5, DME was observed for a short while at the beginning of the reaction, and, as time elapsed, MTBE began to appear at the reactor outlet. This phenomenon seems to be due to the differences in diffusion rates of the reactants into the catalyst interior. The pore size of ZSM-5 is approximately 5.4 X 5.6 and 5.1 X 5.5 A,which allows rapid diffusion of methanol (molecular dimension of 3.7 X 4.2 A) into the zeolite, while diffusion of isobutene (3.9 X 5.4 A) is significantly hindered (Chu and Kiihl, 1987; Olson et al., 1981). Thus, an isobutene molecule migrating into the zeolite interior may encounter a large excess of methanol molecules adsorbed in the pores, which may produce DME via dehydration reaction by themselves. The lower conversion at the beginning of the reaction was probably a result of hindrance to isobutene diffusion. In addition, it was observed that over the HZSM-5 catalyst an increasing isobutene/methanol ratio in the inlet stream improved the MTBE selectivity considerably. The effect of reaction temperature on the reaction rate and selectivity to MTBE is shown in Figure 4. The MTBE synthesis reaction is reversible and fairly exothermic = -15.63 Kcal/mol) and very sensitive to reaction temperature. Equilibrium in the MTBE synthesis reaction is more favorable at lower temperatures, as the reverse reaction of the decomposition becomes more noticeable with rising temperature, reducing the yield of MTBE; however, the reaction rate decreases considerably by lowering the temperature. With HZSM-5, the maximum reaction rate was obtained at about 80 "C, whereas the maximum reaction rate was obtained at about 90 O C over Ti-silicalite, and the MTBE yield increased as the Ti content in the Ti-silicalite crystals increased because of the increased number of total acid sites. The selectivity to MTBE over HZSM-5 was decreased due to DME formation, but DME formation was greatly suppressed over Ti-modified zeolite catalysts. The results of temperature-programmed desorption of pyridine for Ti-silicalite, HZSM-5, TiOz, and silicalite I physically mixed with TiOz (10 w t %) are shown in Figure 5. No significant desorption peaks of pyridine in TPD
A : Ti-Silicalite (54)
0 : Ti-Silicalite (37)
v: '
70
1
80
HZSM-5 (20) I
I
I
90
100
110
Reaction temperature
L
120
(OC)
Figure 4. Effect of reaction temperature on the reaction rate and selectivity to MTBE over Ti-silicalite and HZSM-5: i-C4H8/CHSOH mol ratio, 6.5; reaction time, 4 h; WHSV, 3.6 h-' (FI= 6 X mol/h). The numbers in parentheses given after each catalyst denote the Si02/Ti02 or Si02/A1203ratio.
-.-
-
/'i
3
:Ti-Si1 ical i te(96) :HZSM-5 :T i -Si 1 i ca 1 i te ( 3 7 )
I:.'.:
4 1
V W
a 0 y1
0 al
0
100
200
300
Desorption temp.
400
500
(OC)
Figure 5. Temperature-programmeddesorption of pyridine from Ti-sificdite, TiOz,and silicalite I physically mixed with TiOz (10 w t %), and HZSM-5. Pyridine is initially adsorbed at 25 O C .
curves were detectable for pure Ti02,silicalite I, and the silicalite I-Ti02 physical mixture. TPD curves indicate that complete desorption of pyridine from Ti-silicalite occurs at lower desorption temperatures than that from HZSM-5. The highest desorption peak on TPD curves for Ti-silicalite (around 350 "C) was located at a lower temperature than HZSM-5, which indicates that the acid strength of the strong acid site for Ti-silicalite was weaker than HZSM-5 (around 450 "C). This TPD result also reveals that the distribution of acid sites changes when Ti4+ ions are introduced into the zeolite framework, and it was significantly different from that of HZSM-5, TiOz, or silicalite I mixed with titanium oxides. IR analysis of Figure 6 showed that the acidity of the zeolite catalysts tested are mainly of Bronsted acid type; the band around 1550 cm-l is attributed to Bronsted acidity, and the band near 1450 cm-', to Lewis acidity
128 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table 11. Reaction Mechanisms Proposed for MTBE Synthesis isobutene methanol mechanism adsorption adsorption I + 1 1.1 M + 1 M.1 1 I + 1 1.1 M + 1 Mal 2 I + I 1.1 2M + 1 Mz.1 3 I + 21 211p1 M + 1 Mal 4 I + 21 I.12 M + 1 M.1 5
---
--+ +
surface reaction 1.1 + Mal E.1 + 1 1.1 + M.1 + 1 Eel + 21 21.1 + M9.l 2E.1 + 1 21,p.l + M.1 E4 + 21 1-12 + M.1 Ea12 + 1
MTBE desorption E4 E + 1 Eel E 1 Em1 E 1 E.1- E + 1 Em12 E + 21
---
-- + -+ -
~
Ti-Silicalite
-: --- :
1550 1450
' 37 1
CH301I i-C4H8
~~
1700
1600
1500
1400
Wave number ( c m - l )
Figure 6. IR spectra of pyridine-adsorbed zeolites.
(Ward, 1970; Topsae et al., 1981). TPD experiments of reactants were also conducted for Ti-ilicalite and HZSM-5 preadsorbed with isobutene and methanol, respectively, a t room temperature. This TPD result is shown in Figure 7. With both catalysts,isobutene molecules seem to be adsorbed on two different acid sites, and both molecules were found to be weakly adsorbed over Ti-silicalite than over HZSM-5. Figures 8 and 9 show the dependence of reaction rates on the isobutene (PI)or methanol partial pressures (PM), keeping the pressure of the other reactant constant. As can be seen, the reaction rate increases with PIat constant PM, while it decreases with PM at constant PI; the reaction rate is enhanced by isobutene, while methanol strongly inhibits the reaction in the pressure range examined. The data of Figures 8 and 9 could be well fit into a simple power rate law type kinetic expression as follows: = kP
M
D e s o r p t i o n temp.
!OC)
Figure 7. TPD profiles of Ti-silicalite (37) and HZSM-5 (20) adsorbed with CH30H or i-C4Hs. CH30H and i-C,H8 are initially adsorbed at room temperature. The numbers given in parentheses after each catalyst denote the Si02/TiOzor SiOz/Al2O3ratio.
-1.lP0.69
I
In order to evaluate the applicability of LangmuirHinshelwood-Hougen-Watson (LHHW) mechanisms for the MTBE synthesis reaction, five mechanisms and six kinetic models with different rate-determining steps were proposed, as listed in Tables I1 and 111. These kinetic equations assume that up to three active centers can participate in the rate-determining step of the reaction, and isobutene was assumed to be adsorbed on more than one active center. Because dissociative adsorption of reactants seems unlikely at such mild reaction temperatures, the mechanism in which methanol or isobutene is assumed to be dissociatively adsorbed on two active sites was not considered. According to Tejero et al. (1989), models 1,5, and 6 represented the experimental data of MTBE synthesis on the ion-exchange resin catalyst (Amberlyst 15) reasonably well. Among the models proposed in Table 111, kinetic model 6 fit the experimental data well, which is derived from a mechanism whose rate-determining step is the reaction
1
I
1
2
P I x 10 ( a t m )
Figure 8. Reaction rates versus isobutene partial pressures at different partial pressures of methanol at 80 "C. The solid lines are the fits to the data given by mechanism 5 of Table 11.
between methanol adsorbed on one center and isobutene adsorbed on two centers without cleavage. The values of the adsorption equilibrium cnstants KI,KM,and KE were
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 129 Table 111. Kinetic Equations Derived from the Mechanisms in Table I1 (Tejero et al., 1988) rate-determining kinetic mechanism step model kinetic equation 1
surface reaction
1
i-C,Ha adsorption
2
surface reaction
3
MTBE desorption
4
surface reaction
5
surface reaction
6
7
6
-
5
. F" L
E
2 4
-E
0
P
0
4
x 3 L
2
1
0 1
2
3
4
5
6
7
PM x 190 ( a t m )
Figure 9. Reaction rates versus methanol partial pressures at different partial pressures of isobutene at 80 "C. The solid lines are the fits to the data given by mechanism 5 of Table 11.
1.0 atm-l, 5.0 atm-', 6.2 atm-', respectively, for model 6. The equilibrium constant of the MTBE synthesis reaction was determined from the equation reported by Tejero et al. (1988) as follows:
+
In K = 7300/T - 4.75 In T 1.17 X 10-2T4.8 X lo%? + 2.5 X 10-9p + 4.80 The calculated parameters of these models, except model
I
I
100
50
150 Temp.
1
200
I
250
300
(OC)
Figure 10. DTA and TG diagrams of coke-deposited HZSM-5 (20) and Ti-ailicalite (37)with different reaction times: for HZSM-5 (20), (- -) 6 h, (- -) 12 h, and (- -) 24 h; for Ti-silicalite (37), (-) 12 h. The numbers given in parentheses after each catalyst denote the SiOz/Ti02or SiOz/Al2O3ratio.
-
.-
6, exhibited a negative value for one of the parameters, so these models were rejected. Coke deposited on HZSM-5 and Ti-ilicalite was examined by TG and DTA, as shown in Figure 10. Coke formation over HZSM-5 was increased as the reaction time
130
Ind. Eng. Chem. Res. 1992,31, 130-137
elapsed; for HZSM-5, DTA showed that the exothemic peak area at high temperature increased with reaction time. On the other hand, the amount of coke deposited over Ti-modified silicalite was negligible, probably due to the weak surface acidity that resulted from Ti introduced into the zeolite framework.
Conclusions Ti-modified silicalites were hydrothermally synthesized from NazO/SiOz = 0.05-0.2, H20/Na20 = 300-600, SiOz/TiOz = 30-=~,and TPABr/Si02 = 0.12 at 140-170 "C. The results of IR spectra and the differences in the acidity shown by TPD were indicative of the isomorphous substitution of Ti4+ion into the zeolite lattice sites. Compared with HZSM-5 catalyst, Ti-silicalite showed the higher selectivity to MTBE, and catalyst deactivation or coke formation was not observed. The TPD results of pyridine, methanol, and isobutene indicated that the strength of the acid sites and the adsorption strength of Ti-silicalite were weaker than those of HZSM-5. The best fitting Langmuir-Hinshelwood rate equation for the MTBE synthesis reaction was the one derived from a mechanism in which the rate-determining step is assumed to be the reaction between the methanol adsorbed on one site and the isobutene adsorbed on two different active sites. Nomenclature E = methyl tert-butyl ether (MTBE) FT = total molar flow rate FI = isobutene molar flow rate I = isobutene K = equilibrium constant for the reaction, atm-' K i= adsorption equilibrium constant of substance i, atm-I k = reaction rate constant 1 = active center M = methanol pi = partial pressure of substance i, atm
r = reaction rate, mo1.h-l.g-l T = absolute temperature, K
Subscripts E = methyl tert-butyl ether (MTBE) I = isobutene M = methanol Registry No. MTBE, 1634-04-4; Ti, 7440-32-6; i-C,H8, 11511-7; CH,OH, 67-56-1.
Literature Cited Borade, K. B. Synthesis and characterization of ferrisilicate zeolite of pentasil group. Zeolites 1987, 7, 398. Breck, D. W. Zeolites Molecular Sieves; Wiley: New York, 1974; p 415. Chu, P.; Kfihl, G. H. Preparation of Methyl tert-butyl Ether (MTBE) over Zeolite Catalysta. Ind. Eng. Chem. Res. 1987,26, 366. Fattore, V.; Massi Mauri, M.; Oriani, G.; Paret, G. Crack MTBE for Isobutylen. Hydrocarbon Process. 1981, 60, 101-106. Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. Crystal Structure and Structure-Related Properties of ZSM-5. J. Phys. Chem. 1981,85, 2238. Pecci, G.; Floris, T. Ether ups antiknock of gasoline. Hydrocarbon Process. 1977,56, 98. Reynolds, R. W.; Smith, J. S.; Steinmetz, I. Methyl ethers as motor fuel components. Prepr.-Am. Chem. SOC.,Diu.Pet. Chem. 1975, 20,255. Takesono, T.; Fujiwara, Y. U.S.Pat. 4,182,913, 1980. Tejero, J.; Cunill, F.; Izquierdo, J. F. Equilibrium Constant for the Methyl tert-Butyl Ether Vapor-Phase Synthesis. Ind. Eng. Chem. Res. 1988, 27,338-343. Tejero, J.;Cunill, F.; Izquierdo, J. F. Vapor-Phase Addition to Isobutene on Macroporous Resin: A Kinetic Study. Ind. Eng. Chem. Res. 1989,28, 1269-1276. Topsoe, N. Y.; Pedersen, K.; Derouane, E. G. Infrared and Temperature Programmed Desorption Study of the Acidic Properties of ZSM-5 Type Zeolites. J. Catal. 1981, 70,41. Ward, J. W. The Nature of Active Sites on Zeolites. J.Catal. 1970, 17,355.
Receiued for review April 2, 1991 Accepted September 4,1991
Heterogenized Homogeneous Catalyst. 5. The Theory of Solvent Effect and the Effect of Solvent on Adsorption and Diffusivity Tse-Chuan Chou* and Huey-Jiuan Yeh Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C.
70101
The effects of solvent on the adsorption and diffusivity of benzaldehyde on the support of heterogenized homogeneous catalyst were studied. The theory of solvent effect on this reaction system was analyzed. Aprotic and protic solvents such as carbon tetrachloride, ethyl acetate, acetone, acetonitrile, propionic acid, and acetic acid were used. On the basis of both theoretical analysis and experimental results, the equations of solvent effects in terms of main properties of solvent on the adsorption and diffusivity, respectively, of the aldehyde were obtained. The parameters of solvent properties are electrostatic force, af,dispersion force, 6,, and specific solvation, ET. The results indicate that the relationships between parameters of solvent properties and saturated adsorption, qm, and were obtained as follows: effective diffusivity De,of benzaldehyde, respectively, in Dowex 50WX8-400, + 0.0690(6,- 6mcHo)' - 0.07453, (1);In De 19.0705 - 40.0392 CY, 0.12626: In qrn 4.7874 - 3.4784~~ - 0.7409ET (2).
+
Introduction In previous papers (Chou and Lee, 1985; Hwang and Chou, 1987; Kuo and Chou, 1987; Chou et al., 1990), the characteristics of the oxidation of oxygenated hydrocarbons by using heterogenized homogeneous catalyst were reported. In this study, the effects of solvent on the ad-
sorption and diffusivity of aldehyde for this oxidation system are explored. The roles of solvent in a reaction system have been described by several investigators (Reichardt, 1979; Morrison and Boyd, 1983). The effects of solvent on the equilibrium constant of keto-enol tautomerism of ethyl-
0888-5885/92/2631-0130$03.00/0 @ 1992 American Chemical Society
