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Mechanism of 1-butene hydration over acidic zeolite and ion-exchange resin catalysts
Applied Catalysis A: General 12 1 ( 1995) 45-56
Mechanism of l-butene hydration over acidic zeolite and ion-exchange resin catalysts D&es Ka116 *, R. Magdolna MihAyi Central Research
Institutefor Chemistry, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary
Received 17 December
1993; revised 23 August 1994; accepted 3 1 August I994
Abstract Vapour-phase hydration of 1-butene at atmospheric pressure above 373 K shows similar features on both H-clinoptilolite and sulphonic acid ion-exchange resin: the initial reaction rate linearly increases with the partial pressure of 1-butene and passes through a maximum as a function of water pressure. Kinetics was evaluated accordingly. A decrease in the 1-butene isomerization rate indicated that under hydration conditions more than 99.9% of the acidic sites are hydrated. For the hydration, however, hydroxonium ions are the active centres where an adsorbed water molecule reacts with gasphase 1-butene in the rate determining step. The equilibrated desorption of secondary butanol ends the reaction. The kinetic inhibition by butanol can be suppressed with water. Above ca. 0.2 bar water pressure inactive polyhydrated hydroxonium ions are formed. Keywords: Butene hydration; Clinoptilolite; Ion exchange; Resin; Solid acids
1. Introduction Contrary to dehydration of alcohols on acidic catalysts the mechanism of the reverse reaction, i.e. that of alkene hydration has scarcely been investigated [ 11. Tanabe and Nitta [ 21 studied the vapour-phase hydration of ethylene over various cationic forms of molecular sieve A. The formation of an adsorbed ethyl carbenium ion from ethylene and the proton of the acid sites was found to be the rate controlling step of the hydration. No hydrogen-deuterium exchange was observed during the hydration of ethylene with D20. The gas-phase hydration of ethylene on the H-forms of different zeolites with various Si/Al ratio as MFI, MOR, FER and FAU, Y was not influenced by crystal * Corresponding author. 0926-860X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X(94)00194-4
46
D. Kalld. R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
structure, but the activity showed a volcano-shaped change with the aluminium content [3]. Similar behaviour was found for the aqueous-phase hydration of butenes [ 451. Hydrophobic zeolites (pentasil- and ferrierite-types) were the active H-zeolite catalysts in the high-pressure hydration of ethylene, propene and I-butene to alcohols [ 61. It was found that the hydration activity of zeolites decreased with increasing amount of adsorbed water per aluminium content of the catalysts. These findings suggest that catalyst acidity has a decisive role in alkene hydration. Neier and Woellner [ 71 proposed that the hydration of propene to isopropanol on ion-exchange resins takes place via formation of carbenium ions; the same mechanism was considered by others for the reverse reaction, i.e., for the dehydration [ 8,9]. This includes the formation of alcoxonium ions by protonation of alcohol, which step is followed by the loss of water resulting in carbenium ions bound to the surface. The catalytic cycle is closed by the desorption of the alkene molecules leaving back protons on the surface. It was shown that data obtained for alkene hydration on many different catalysts formally satisfy the Langmuir-Hinshelwood kinetics. Accordingly, the rate determining surface reaction proceeds between adsorbed water and alkene [ 10,l I]. Mechanisms outlined above may involve different interactions between the reactants or products and the acid catalysts. During the dehydration of tert-butanol on sulphonic acid ion-exchange resin the product water inhibits the reaction by hydrating the protons. Hydrated protons are of significantly lower activity than protons of the -S03H groups [ 121. Alcohols in higher concentration, i.e., at partial pressures of a few tenths of a bar also inhibit the reaction by solvating the protons [ 13 ] . In alcoholic media the acidity of solvated protons increase in the order of MeOH < EtOH < n-PrOH < n-BuOH, which corresponds to the decreasing dielectric constant of the alcohols [ 141. When alcohols are added to iso-butene and alkyl tert-butyl ethers are formed on sulphonic acid resin, the inhibiting effect of the alcohols decreases in the same sequence [ 151, i.e., in the sequence of decreasing dielectric constants. Alkenes can only be protonated by solvated protons when tertiary carbenium ions can form, while secondary carbenium ions are not formed at all under such conditions [ 141. As a consequence of microscopic reversibility the dehydration of alcohols and the hydration of the corresponding alkenes should proceed via the same route. The dehydration, however, results in equimolar formation of water and alkene while during the hydration the water-to-alkene ratio can be varied over a large range, which enables one to study the interactions between the active sites and the reaction components separately. Owing to different conditions in dehydration and hydration experiments different mechanisms may prevail. 1-Butene was used as reactant, since its isomerization is a probe for its interaction with the acidic sites. An H-zeolite and a cation-exchange resin in H-form were the two catalysts investigated. They represent acidic contacts of different structures. This may allow us to come to some general conclusions on how acidic sites operate.
D. KalM, R.M. Mihdlyi / Applied Catalysis A: General 121 (1995) 45-56
41
It was reported earlier [ 161 that the H-clinoptilolite (H-CLI) and the sulphonated styrene-divinyl benzene ion-exchange resin (Vat-ion KSM, Hungary) were appropriate catalysts. On both catalysts the acidic sites are the source of activity. Reaction conditions of gas-phase hydration, where no by-products are formed and catalyst deactivation is not observable, are similar for the two catalysts.
2. Experimental
2. I. Materials 1-Butene (B) produced by Linde/Germany was of 99.0% purity. As diluent high purity nitrogen was used with traces of argon. A rhyolite Hungarian tuff [ 171 contained 60% clinoptilolite besides catalytically inactive quartz, cristobalite and volcanic glass. It was ion-exchanged to 78% for ammonium by treatment with 2 M (NH,)NOj solution for 8 h at 350 K, then deammoniated at 623 K in nitrogen stream for 2 h ‘in situ’. The H-form of embedded clinoptilolite produced in this way contained 0.86 meq H+/g sample (H-CLI). The crystal size of the clinoptilolite was around 0.1 pm, the porosity of the rock was 0.2 ml/g in the diameters ranged from 10 to 1000 nm. Cation-exchange resin Varion KSM (Nitrokemia/Hungary ) was a sulphonated product of styrene-divinyl benzene copolymer containing 10% divinyl benzene. A pure acidic form was produced by treatment with 3 M HCl and dried at 478 K. Ionexchange capacity was 4.94 meq/g, specific surface area (with nitrogen adsorption) 37.4 m*/g, pore volume 0.49 ml/g, pore diameter 14-17 nm, density (dead volume filled with helium after drying) 1.32 g/ml. 2.2. Methods A conventional flow reactor with 10 g H-CL1 or 1 g Vat-ion KSM was used for I-butene hydration. Experiments were started with feeding water in a nitrogen stream; as contacting the fresh acidic catalysts with 1-butene will otherwise result in oligomerization and formation of carbonaceous deposits. After exothermic adsorption of water the reactor was cooled to reaction temperature and the 1-butene feed was started. The feed rates of 1-butene (B) and water ( W) were changed and the desired partial pressures established. A total pressure of 1 bar was attained by admixing nitrogen. At room temperature the effluent was separated into two phases. The liquid and the gas obtained were analyzed separately with gas chromatography (GC) . In the liquid phase was 2-butanol (A) the only product; in the gas phase only n-butene isomers were detected. Under reaction conditions (383-4 13 K; atmospheric pressure, partial pressures: PB = 0.07-0.43 bar, pw = 0.1-0.8 bar; space time up to 1.2. lo5 g,, mol; ’ s) the
48
D. Ka116, R.M. Mihdlyi/Applied
Catalysis A: General 121 (1995) 45-56
conversions were between 0.1 and 1%. At higher conversions the reaction dramatically slows down (see Section 3). Conversions were determined with an error of less than f 4 relative percent. Initial reaction rates were calculated from the initial slopes of the conversion curves. No catalyst deactivation was observed for some ten hours, hence experiments could be carried out in series. 1-Butene isomerization was investigated in the absence of water using 10 mg catalyst in a closed recirculation apparatus of 89.1 ml volume. Catalyst were pretreated ‘in situ’ by evacuation at 10e6 bar. The pretreatment temperature was 623 K for H-CL1 and reaction temperature for Varion KSM. Conversions were determined with GC. In spite of deactivation initial isomerization rates could be determined by working with fresh catalyst in each experiment. In the presence of water vapour the isomerization was studied in the flow reactor used also for hydration. Conversion curves and initial isomerization rates were determined for both catalysts while the same reaction conditions ( T, pe, pw) were maintained as during hydration. 7
3.0 d : *” F ; 2.0 5 L %
1.0
“0 z OL 0.25 pe
0.2 PW
0.5
, (bar1
0.4
0.6
0.8
, (bar1
Fig. 1. The initial rate of hydration on H-CL1 as a function of (a) the partial pressure of I-C,Hs at constant Hz0 pressure of 0.57 bar; (b) the partial pressure of Hz0 at a constant l-C4H, pressure of 0.20 bar.
D. Kall6, R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
49
3. Results The initial rate of 2-butanol formation on both catalysts linearly increases with the partial pressure of 1-butene at a constant water pressure (Figs. la and 2a). If the partial pressure of 1-butene is constant, the initial rate as a function of pw first increases, passes through a maximum, and then decreases on H-CL1 (Fig. lb) as well as on Vat-ion KSM (Fig. 2b). Conversion vs. space time (T) curves are shown in Fig. 3 for H-CL1 and in Fig. 4 for ion-exchange resin. For the sake of clarity only the fitted curves are plotted and the experimentally determined data are not. It is important to remark that reaction slows down when space time is increased although conversions are far below the equilibrium values (see Table 1). Steadystate conditions, however, persist in the whole range of conversions which is proved as follows: 2-Butanol was admixed to I-butene plus water feed and the concentration expressed in terms of conversion was fitted to the experimental conversion curve (points 0 in Figs. 3,4). This feed composition corresponds to a product compo7 I: :
i
3.0 t
393 K
*” 2
2.0
d 1 .E
Ia)
1.0
k/ 0
0
383
0.25
ps
7 i J
i
K
0.5
, Ibarl
1.5
*” *”
1.0
d L s
0.5
0 0.4
0.8
PW , (bar1 Fig. 2. The initial rate of hydration on Varion KSM as a function of (a) the partial pressure of 1C.,Hs at constant Hz0 pressure of 0.57 bar; (b) the partial pressure of H,O at a constant l-&H, pressure of 0.20 bar.
50
D. Ka116,R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56 0.6
-
s
4,
pw =O.bbar pw =0.4bar
.
4.0 lo-‘+T
PW =0.2bar
6.0
12.0
(g,,,*mol-‘I_c,H,*s)
Fig. 3. The formation of 2-butanol as a function of space time at 403 K, 0.40 bar I-butene pressure and at different water pressures, on H-CL1 (full line); A ,O,U indicate the conversion in the presence of 2-butanol in the feed (initial 2-butanol concentrations were expressed as percent conversion and are indicated by symbol 0). 0.6
Pw =0.6bar
I
5 E ‘P t z x
0.4
8.0
4.0 lo-‘*T
12.0
Ig,,,.*mol~‘1_c,4*sI
Fig. 4. The formation of 2-butanol as a function of space time at 393 K. 0.20 bar I-butene pressure and at different water pressures on Varion KSM (full line) ; A, 0, n indicate the conversion in the presence of alcohol in the feed (initial 2-butanol concentrations were expressed as percent conversion and are indicated by symbol 0). Table 1 Equilibrium Temp. (K)
403 403 403 393 393 393
conversion
of I-butene hydration
under conditions
specified at Figs. 3 and 4
Pressure (bar) of
Equilibrium
1-C,H,
Hz0
0.4 0.4 0.4 0.2 0.2 0.2
0.2 0.3 0.6 0.2 0.4 0.6
conv. (%)
4.89 7.23 13.73 7.21 13.57 19.21
sition at a hypothetical space time for I-butene which can be obtained as the abscissa value of point 0. Under steady-state reaction conditions the 2-butanol content of the outlet expressed as percent conversion fits the conversion curve well at the space time calculated by adding the above hypothetical and actual space time for I-butene in the feed containing the admixed 2-butanol (cf. the pairs of 0 and full points denoted with the same numbers in Figs. 3,4). It means that the effect of the formed and the admixed 2-butanol on the conversion is the same, resulting in a declining conversion curve. Therefore, the observed space time dependence of the
D. Ka116, R.M. Mihdlyi/Applied
Table 2 Water coverages Catalyst
Temp. (K) Part. press. (bar)
393
403
Varion KSM
51
at different partial pressures of water and the initial rates of I-butene isomerization
l-C,H, H-CL1
Catalysis A: General 121 (1995) 45-56
383
393
H,O
0.5 0.4
_
0.4 0.4 0.4 0.4
_
Isom. rate. 10’ ’ (mol, _ cJHIIs - ’ g,,’ ) r”
OH,0
(%) b
PHrn
126
0.2--0.6
0.056
99.9924
0.261 0.093 0.056
99.9773 99.9921 99.9953
1.04
99.830
2.08
99.895
1177
0.2 0.4 0.6 988
0.4 0.4
0.2--0.57
0.4 0.42
0.57
1219
a The initial rate of I-butene isomerization in the absence (P) and in the presence ( dto) b The fraction of active centres covered with water determined as ( 1 -&o/P) 100.
of water.
conversion is not due to accumulation of an unknown inhibitor on the catalyst. 2-Butanol is not irreversibly bound since the increase of water pressure (pw) suppresses the inhibiting effect of 2-butanol (cf. conversion curves for different pw values in Figs. 3, 4). Flat conversion curves reflect kinetic product inhibition under steady-state reaction conditions. Both in the absence and presence of water I-butene isomerization rates were determined from the initial slopes of the corresponding conversion curves (Table 2). It is well known that the isomerization proceeds on these catalysts through a 2butyl carbenium ion [ 18,191, the formation of which is inhibited by adsorption of water on the acid sites, as the proton acceptor strength of water is higher than that of 1-butene [ 121. From the decrease of isomerization rates the fraction of acid sites covered with water could be calculated (last column in Table 2). The rate of isomerization i.e. the adsorption of B decreases by nearly four orders of magnitude on H-CL1 and two orders of magnitude on Varion KSM in the presence of water.
4. Discussion Since essential features of I-butene hydration, e.g., the dependence of the reaction rate on the partial pressures and temperature are very similar for the two acidic catalysts of different composition and structure, it is conceivable that Bransted acid sites are the active centres, and the main features of the reaction can be discussed similarly for both catalysts.
52
D. Kall6, R.M. Mitilyi/Applied
Catalysis A: General 121 (1995) 45-56
In order to avoid uncertainties encountered by formal kinetic evaluation of experimental data applying mathematically derived rate equations [ 201, the evaluation of results is based rather on kinetic evidence. If the adsorption of water (W) were the rate determining step, the initial reaction rate (r’) should be independent of the partial pressure of 1-butene (B) :
If the adsorption of B were rate determining,
then
r” #_f(Pw) If the desorption of the product secondary butanol (A) were rate determining, then (3) Since Eqs. ( 1) , (2) and (3) contradict experimental findings (Figs. 1, 2)) the surface reaction, either between adsorbed W and gas-phase B or between adsorbed B and gas-phase W, must be the rate determining step. A rate determining surface reaction between B and W both adsorbed on similar active sites can be disregarded since the rate equation derived for this case
r”=
KBPBKW PW
k
(1 +KBPB+KWPW)*
(4)
(k: rate constant; K: adsorption equilibrium constant) does not express the correlation between r and PB, pw found experimentally (Figs. 1, 2). Extreme forms of Eq. (4) when active sites are practically covered with W
+&@_ KWPW
(W
or with B ro =
kKwpw KBPB
(4b)
do not hold either. Some chemical considerations reveal this conclusion too. In terms of Langmuir-Hinshelwood kinetics a reaction between surface species adsorbed on similar active sites is required. It means that W and B both adsorbed on a Bronsted acid site have to react, i.e., a reaction between positively charged H30+ and set - C,’ HP should proceed which is rather improbable. The reaction between adsorbed W and gas-phase B seems more likely than between adsorbed B and gas-phase W, because ( i) 9 increases linearly with PB over the whole range of PB (Figs. 1,2) indicating a negligible adsorption of B;
D. Kalld, R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
53
(ii)
in the low pw range (initial sections in Figs. lb, 2b) when the minute adsorption of B is further suppressed by W (Table 2)) the rate of hydration increases with pw; consequently adsorbed B may not be involved in the hydration reaction; (iii) the hydration conversion can be increased by increasing pw when A exerts kinetic product inhibition (vide infra and in Figs. 3, 4); kinetic product inhibition can be suppressed by adsorbing reactant, in our case by W. Most probably the hydration of B takes place through rate determining surface reaction between adsorbed W and gas-phase B. For the related reaction of methanol addition to isobutene on sulphonic acid resin, tert-butyl carbenium ions perfectly block the acid sites at low methanol pressures [ 151. The formation of secondary alkyl carbenium ions can, however, be suppressed by the alcohol [ 141. Water has a similar effect in our catalytic system indicating that reaction K- set-C,+H,+H,O+K-
H30+ +n-C,H,
(5)
is shifted to right (K- represents deprotonated site) [ 121. These observations seem to support the above outlined interactions. In the investigated pw range more than 99 percent of acid sites are blocked by W (Table 2) in the form of H30+, therefore a linear increase of r” with pw can not be a consequence of the increase of H,O+ concentration but an increase of the concentration of hydrated hydroxonium ions: H,O;,
+ nH,O + H,O + - ~zH*O~~
(6)
The existence of such ensembles under conditions established in the present work was substantiated previously by others [ 2 1] : on increasing pw first mono-, then bi-, tri- etc. hydrates are formed. Presumably the formation of hydroxonium monohydrate in low concentration results first in a proportional increase of r with pw, i.e. first order kinetics in W (Figs. lb, 2b). When pw is further increased r passes through a maximum and thereafter decreases monotonously, indicating that hydroxonium polyhydrates may inhibit the reaction. Based on these observations the initial reaction rate can be expressed in form of Es. (7). P’k’(lFKEp$)
(7)
where k’ is a phenomenological rate constant and K,, is the equilibrium constant of the reaction (6) when n > 1. Parameters in Eq. (7) were determined (Table 3) by fitting calculated curves to the experimental points (as shown in Figs. 1,2). Experimental data do not permit to obtain more detailed information, e.g., the separate determination of the equilibrium constants for the formation of bi-, triand tetrahydrated hydroxonium ions. Water in polyhydrated hydroxonium ion complexes is very probably not ‘activated’ as in hydroxonium monohydrate. In the
54
D. Kalld, R.M. Mihdlyi / Applied Catalysis A: General I2 I (I 995) 45-56
Table 3 Constants in initial rate equation of I-butene hydration, Catalyst
Temp. (K)
H-CL1
393 403 413
Varion KSM
383 393
r,,= k’p, p&l
+ K,,,p”)
k’~10’“(mol,_,,,s~‘g~~~~ 1.41 1.12 I .30 29.0 25.1
K w,, h (harm”)
II c
62.5 12.7 1.6
3 3 3
62.5 13.0
3 4
’ Phenomenological rate constant. h Equilibrium constant of hydration of hydroxonium ion with n water molecules. ’ Average number of water molecules hydrating hydroxonium ions
polyhydrated H+ each water molecule is equivalent [21] and, therefore, their interaction with H+ is too weak, the H-OH bond should be less loosened than in monohydrate. The concentration of the reaction intermediate monohydrate is negligibly small and is substantially decreased by further hydration. The trihydrated form is conceivably the abundant species under reaction conditions [ 2 11. It should be noted, however, that the phenomenological rate constant, k’, does not vary with temperature suggesting that the phenomenological activation energy must be zero. This is reflected by the initial, linearly increasing sections of r =f(pw) curves which are perfectly overlapping for both catalysts at different temperatures (Figs 1b and 2b). If k’ is supposed to be the product of the real rate constant k and the equilibrium constant of formation of the monohydrated hydroxonium ion Kw ,, the phenomenological activation energy can be given by Eq. (8)
(8) In other words, the coverage of H,O’ sites by water decreases with increasing temperature in the same rate within experimental errors as the addition reaction accelerates. At higher pw values, however, r” increases with temperature (Figs. 1a, 2a, at pw = 0.57 bar; Figs. lb, 2b, right to the maxima) which can be attributed to the decrease of K,,, (Table 3). The above observations can be summarised as follows. Water adsorbed on Bronsted acid sites is involved in hydration. It may not be present in the form of hydroxonium ions since all the protons are hydrated even at the lowest pw values where experiments were carried out. The only reasonable explanation for rate increase of hydration with pw is that hydrated, most probably monohydrated, hydroxonium ions react with gas-phase B. At higher pw reaction rate decreases because the conditions favour the formation of polyhydrated hydroxonium ions and formation of monohydrated hydroxonium ions is suppressed [ 2 1 ] . One could argue that the reaction is inhibited by water since water fills up all the pores. This kind of inhibiting effect, however, must be very different for a zeolite with a rigid structure and for a resin that is able to swell. Such structural differences
D. Kal16, R.M. Mihdlyi / Applied Catalysis A: General 121 (I 995) 45-56
55
may explain differences in the catalytic activity but can not result in differences in the mode of catalytic action. Being far from equilibrium (cf. conversion at high space time in Figs. 3,4 and data in Table 1)) the fast decline of the conversion curve, i.e., the slowdown of the hydration with increasing T is attributed to the inhibition of product A. Hydroxonium ions are substituted by secondary butyl alcoxonium ions. In the case of completely solvated protons the reaction rate can be given as KWPW r=rOCKw~w+K~~n)
(9)
where K, and KA are the equilibrium constants for H,O+ and secondary butyl alcoxonium ion formation, respectively. From the slope of conversion curves reaction rates and from reaction rates using Eq. (9) ratios of KA to Kw can be estimated. Obtained K,IK, is very high, more than 100. Due to the limited experimental accuracy a more detailed evaluation does not seem justified. The large ratio is due to the highly different dielectric constants which are 78.3 for W and 15.8 for A at 298 K since the adsorption on Bronsted acid sites increases when the dielectric constant of adsorption decreases [ 22-241. Because of strong adsorption of A on Bronsted acid sites a high concentration of W is needed for substituting A by W. It seems likely that the reaction route of hydration differs from alcohol dehydration because in the latter reaction initially no water is present. In absence of water H30+ ions which are the active sites for hydration can not be formed. This infers that assuming microscopic reversibility of the hydration-dehydration would be unjustified for the initial period of the reaction.
5. Conclusion It seems likely that hydrogen bonded water in monohydrated hydroxonium ion reacts with gas-phase 1-butene in a rate determining surface reaction. Rearrangement of the bonds between the four atoms to a position required by the Markownikoff rule may be anticipated: H I K-H+-a-O-m-H-O 1 H
i
; i CH2=CH-CH2-CH3
The secondary butanol product substitutes most of the water molecules in the hydroxonium ions and inactive alcoxonium ions may form, exerting kinetic inhibition.
56
D. Kalld, R.M. Mihdlyi / Applied Catalysis A: General 121 (1995) 45-56
Acknowledgements The authors are grateful to the National Scientific Research Foundation (OTKA) for financial support by contract T 014532.
References [ I] J.M. Witterbottom, in C. Kemball and D.A. Dowden (Editors) Catalysis, Roy. Sot. Chem., London, 1981, Vol4. p. 141. [2] K. Tanabe and M. Nitta, Bull. Jpn. Petrol. Inst., 14 ( 1972) 47. [3] M. Iwamoto, M. Tajima and S. Kagawa, J. Catal., 101 (1986) 195. [4] F. Fajula, R. Ibbara, F. Figueras and C.J. Gueguen, J. Catal., 89 ( 1984) 60. [5] B. Coq, R. Durand, F. Fajula, C. Moreau, A. Finiels, B. Chiche, F. Figueras and P. Geneste, Stud. Surf. Sci. CataL. (1988) 241. [6] K. Equichi, T. Tokiai and H. Arai, Appl. Catal., 34 ( 1987) 275. [7] W. Neier and J. Woellner, Hydrocarb. Process., (Nov. 1972) 113. [ 81 S.J. Gentry and R. Rudham, J. Chem. Sot. Faraday Trans. 1.70 ( 1974) 1685. [9] P.A. Jacobs, M. Tielen and J.B. Uytterhoeven, J. Catal., 50 ( 1977) 98. [ IO] L. Beranek and M. Kraus, in C.H. Bramford and C.F.H. Tipper (Editors), Comprehensive Chemical Kinetics, Elsevier, Amsterdam, 1978, p. 323. [ Ill H. Momose, K. Kusomoto, Y. Izumi and Y. Mizutani, J. Catal., 77 ( 1982) 23,554. [ 121 B.C. Gates and W. Rodriguez, J. Catal., 31 ( 1973) 27. [ 131 R. Thornton and B.C. Gates, J. Catal., 34 ( 1974) 275. [ 141 F. Ancilotti, M. Massi Massuri and E. Pescarollo, J. Catal., 46 ( 1977) 49. [ 151 F. Ancilotti, M. Massi Massuri and E. Pescarollo, J. Mol. Catal., 4 ( 1978) 37. [ 161 R.M. Mihalyi and D. Kalld, Acta Chim. Hung., 118 (1985) 331. [ 171 D. Kallo, J. Papp and J. Valyon, Zeolites, 2 ( 1982) 13. [ 181 D. Kallo and I. Preszler, J. Catal., 12 ( 1968) 1. [ 191 D. Kallo, E. Detrekoy and G. Schay, Acta Chim. Hung., 94 ( 1977) 19. [201F. Nagy,in 2. Szabo, D. Kallo (Editors), Kinetics of Heterogeneous Catalytic Reactions, In Contact Catalysis, Elsevier, Amsterdam, 1976, p. 437. ]211 P. Kebarle, S.K. Searles, A. Zolla, J. Scarborough and M. Arshadi, J. Am. Chem. Sot., 89 ( 1967) 6393. [22] D.J. Pietrzyk, Talanta, 13 ( 1966) 209. [23] D.J. Pietrzyk, Talanta, 13 (1966) 225. [ 241 T.C. Gilmer and D.J. Pietrzyk, Anal. Chem., 43 ( 197 I ) 1585.
Mechanism of l-butene hydration over acidic zeolite and ion-exchange resin catalysts D&es Ka116 *, R. Magdolna MihAyi Central Research
Institutefor Chemistry, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary
Received 17 December
1993; revised 23 August 1994; accepted 3 1 August I994
Abstract Vapour-phase hydration of 1-butene at atmospheric pressure above 373 K shows similar features on both H-clinoptilolite and sulphonic acid ion-exchange resin: the initial reaction rate linearly increases with the partial pressure of 1-butene and passes through a maximum as a function of water pressure. Kinetics was evaluated accordingly. A decrease in the 1-butene isomerization rate indicated that under hydration conditions more than 99.9% of the acidic sites are hydrated. For the hydration, however, hydroxonium ions are the active centres where an adsorbed water molecule reacts with gasphase 1-butene in the rate determining step. The equilibrated desorption of secondary butanol ends the reaction. The kinetic inhibition by butanol can be suppressed with water. Above ca. 0.2 bar water pressure inactive polyhydrated hydroxonium ions are formed. Keywords: Butene hydration; Clinoptilolite; Ion exchange; Resin; Solid acids
1. Introduction Contrary to dehydration of alcohols on acidic catalysts the mechanism of the reverse reaction, i.e. that of alkene hydration has scarcely been investigated [ 11. Tanabe and Nitta [ 21 studied the vapour-phase hydration of ethylene over various cationic forms of molecular sieve A. The formation of an adsorbed ethyl carbenium ion from ethylene and the proton of the acid sites was found to be the rate controlling step of the hydration. No hydrogen-deuterium exchange was observed during the hydration of ethylene with D20. The gas-phase hydration of ethylene on the H-forms of different zeolites with various Si/Al ratio as MFI, MOR, FER and FAU, Y was not influenced by crystal * Corresponding author. 0926-860X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X(94)00194-4
46
D. Kalld. R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
structure, but the activity showed a volcano-shaped change with the aluminium content [3]. Similar behaviour was found for the aqueous-phase hydration of butenes [ 451. Hydrophobic zeolites (pentasil- and ferrierite-types) were the active H-zeolite catalysts in the high-pressure hydration of ethylene, propene and I-butene to alcohols [ 61. It was found that the hydration activity of zeolites decreased with increasing amount of adsorbed water per aluminium content of the catalysts. These findings suggest that catalyst acidity has a decisive role in alkene hydration. Neier and Woellner [ 71 proposed that the hydration of propene to isopropanol on ion-exchange resins takes place via formation of carbenium ions; the same mechanism was considered by others for the reverse reaction, i.e., for the dehydration [ 8,9]. This includes the formation of alcoxonium ions by protonation of alcohol, which step is followed by the loss of water resulting in carbenium ions bound to the surface. The catalytic cycle is closed by the desorption of the alkene molecules leaving back protons on the surface. It was shown that data obtained for alkene hydration on many different catalysts formally satisfy the Langmuir-Hinshelwood kinetics. Accordingly, the rate determining surface reaction proceeds between adsorbed water and alkene [ 10,l I]. Mechanisms outlined above may involve different interactions between the reactants or products and the acid catalysts. During the dehydration of tert-butanol on sulphonic acid ion-exchange resin the product water inhibits the reaction by hydrating the protons. Hydrated protons are of significantly lower activity than protons of the -S03H groups [ 121. Alcohols in higher concentration, i.e., at partial pressures of a few tenths of a bar also inhibit the reaction by solvating the protons [ 13 ] . In alcoholic media the acidity of solvated protons increase in the order of MeOH < EtOH < n-PrOH < n-BuOH, which corresponds to the decreasing dielectric constant of the alcohols [ 141. When alcohols are added to iso-butene and alkyl tert-butyl ethers are formed on sulphonic acid resin, the inhibiting effect of the alcohols decreases in the same sequence [ 151, i.e., in the sequence of decreasing dielectric constants. Alkenes can only be protonated by solvated protons when tertiary carbenium ions can form, while secondary carbenium ions are not formed at all under such conditions [ 141. As a consequence of microscopic reversibility the dehydration of alcohols and the hydration of the corresponding alkenes should proceed via the same route. The dehydration, however, results in equimolar formation of water and alkene while during the hydration the water-to-alkene ratio can be varied over a large range, which enables one to study the interactions between the active sites and the reaction components separately. Owing to different conditions in dehydration and hydration experiments different mechanisms may prevail. 1-Butene was used as reactant, since its isomerization is a probe for its interaction with the acidic sites. An H-zeolite and a cation-exchange resin in H-form were the two catalysts investigated. They represent acidic contacts of different structures. This may allow us to come to some general conclusions on how acidic sites operate.
D. KalM, R.M. Mihdlyi / Applied Catalysis A: General 121 (1995) 45-56
41
It was reported earlier [ 161 that the H-clinoptilolite (H-CLI) and the sulphonated styrene-divinyl benzene ion-exchange resin (Vat-ion KSM, Hungary) were appropriate catalysts. On both catalysts the acidic sites are the source of activity. Reaction conditions of gas-phase hydration, where no by-products are formed and catalyst deactivation is not observable, are similar for the two catalysts.
2. Experimental
2. I. Materials 1-Butene (B) produced by Linde/Germany was of 99.0% purity. As diluent high purity nitrogen was used with traces of argon. A rhyolite Hungarian tuff [ 171 contained 60% clinoptilolite besides catalytically inactive quartz, cristobalite and volcanic glass. It was ion-exchanged to 78% for ammonium by treatment with 2 M (NH,)NOj solution for 8 h at 350 K, then deammoniated at 623 K in nitrogen stream for 2 h ‘in situ’. The H-form of embedded clinoptilolite produced in this way contained 0.86 meq H+/g sample (H-CLI). The crystal size of the clinoptilolite was around 0.1 pm, the porosity of the rock was 0.2 ml/g in the diameters ranged from 10 to 1000 nm. Cation-exchange resin Varion KSM (Nitrokemia/Hungary ) was a sulphonated product of styrene-divinyl benzene copolymer containing 10% divinyl benzene. A pure acidic form was produced by treatment with 3 M HCl and dried at 478 K. Ionexchange capacity was 4.94 meq/g, specific surface area (with nitrogen adsorption) 37.4 m*/g, pore volume 0.49 ml/g, pore diameter 14-17 nm, density (dead volume filled with helium after drying) 1.32 g/ml. 2.2. Methods A conventional flow reactor with 10 g H-CL1 or 1 g Vat-ion KSM was used for I-butene hydration. Experiments were started with feeding water in a nitrogen stream; as contacting the fresh acidic catalysts with 1-butene will otherwise result in oligomerization and formation of carbonaceous deposits. After exothermic adsorption of water the reactor was cooled to reaction temperature and the 1-butene feed was started. The feed rates of 1-butene (B) and water ( W) were changed and the desired partial pressures established. A total pressure of 1 bar was attained by admixing nitrogen. At room temperature the effluent was separated into two phases. The liquid and the gas obtained were analyzed separately with gas chromatography (GC) . In the liquid phase was 2-butanol (A) the only product; in the gas phase only n-butene isomers were detected. Under reaction conditions (383-4 13 K; atmospheric pressure, partial pressures: PB = 0.07-0.43 bar, pw = 0.1-0.8 bar; space time up to 1.2. lo5 g,, mol; ’ s) the
48
D. Ka116, R.M. Mihdlyi/Applied
Catalysis A: General 121 (1995) 45-56
conversions were between 0.1 and 1%. At higher conversions the reaction dramatically slows down (see Section 3). Conversions were determined with an error of less than f 4 relative percent. Initial reaction rates were calculated from the initial slopes of the conversion curves. No catalyst deactivation was observed for some ten hours, hence experiments could be carried out in series. 1-Butene isomerization was investigated in the absence of water using 10 mg catalyst in a closed recirculation apparatus of 89.1 ml volume. Catalyst were pretreated ‘in situ’ by evacuation at 10e6 bar. The pretreatment temperature was 623 K for H-CL1 and reaction temperature for Varion KSM. Conversions were determined with GC. In spite of deactivation initial isomerization rates could be determined by working with fresh catalyst in each experiment. In the presence of water vapour the isomerization was studied in the flow reactor used also for hydration. Conversion curves and initial isomerization rates were determined for both catalysts while the same reaction conditions ( T, pe, pw) were maintained as during hydration. 7
3.0 d : *” F ; 2.0 5 L %
1.0
“0 z OL 0.25 pe
0.2 PW
0.5
, (bar1
0.4
0.6
0.8
, (bar1
Fig. 1. The initial rate of hydration on H-CL1 as a function of (a) the partial pressure of I-C,Hs at constant Hz0 pressure of 0.57 bar; (b) the partial pressure of Hz0 at a constant l-C4H, pressure of 0.20 bar.
D. Kall6, R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
49
3. Results The initial rate of 2-butanol formation on both catalysts linearly increases with the partial pressure of 1-butene at a constant water pressure (Figs. la and 2a). If the partial pressure of 1-butene is constant, the initial rate as a function of pw first increases, passes through a maximum, and then decreases on H-CL1 (Fig. lb) as well as on Vat-ion KSM (Fig. 2b). Conversion vs. space time (T) curves are shown in Fig. 3 for H-CL1 and in Fig. 4 for ion-exchange resin. For the sake of clarity only the fitted curves are plotted and the experimentally determined data are not. It is important to remark that reaction slows down when space time is increased although conversions are far below the equilibrium values (see Table 1). Steadystate conditions, however, persist in the whole range of conversions which is proved as follows: 2-Butanol was admixed to I-butene plus water feed and the concentration expressed in terms of conversion was fitted to the experimental conversion curve (points 0 in Figs. 3,4). This feed composition corresponds to a product compo7 I: :
i
3.0 t
393 K
*” 2
2.0
d 1 .E
Ia)
1.0
k/ 0
0
383
0.25
ps
7 i J
i
K
0.5
, Ibarl
1.5
*” *”
1.0
d L s
0.5
0 0.4
0.8
PW , (bar1 Fig. 2. The initial rate of hydration on Varion KSM as a function of (a) the partial pressure of 1C.,Hs at constant Hz0 pressure of 0.57 bar; (b) the partial pressure of H,O at a constant l-&H, pressure of 0.20 bar.
50
D. Ka116,R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56 0.6
-
s
4,
pw =O.bbar pw =0.4bar
.
4.0 lo-‘+T
PW =0.2bar
6.0
12.0
(g,,,*mol-‘I_c,H,*s)
Fig. 3. The formation of 2-butanol as a function of space time at 403 K, 0.40 bar I-butene pressure and at different water pressures, on H-CL1 (full line); A ,O,U indicate the conversion in the presence of 2-butanol in the feed (initial 2-butanol concentrations were expressed as percent conversion and are indicated by symbol 0). 0.6
Pw =0.6bar
I
5 E ‘P t z x
0.4
8.0
4.0 lo-‘*T
12.0
Ig,,,.*mol~‘1_c,4*sI
Fig. 4. The formation of 2-butanol as a function of space time at 393 K. 0.20 bar I-butene pressure and at different water pressures on Varion KSM (full line) ; A, 0, n indicate the conversion in the presence of alcohol in the feed (initial 2-butanol concentrations were expressed as percent conversion and are indicated by symbol 0). Table 1 Equilibrium Temp. (K)
403 403 403 393 393 393
conversion
of I-butene hydration
under conditions
specified at Figs. 3 and 4
Pressure (bar) of
Equilibrium
1-C,H,
Hz0
0.4 0.4 0.4 0.2 0.2 0.2
0.2 0.3 0.6 0.2 0.4 0.6
conv. (%)
4.89 7.23 13.73 7.21 13.57 19.21
sition at a hypothetical space time for I-butene which can be obtained as the abscissa value of point 0. Under steady-state reaction conditions the 2-butanol content of the outlet expressed as percent conversion fits the conversion curve well at the space time calculated by adding the above hypothetical and actual space time for I-butene in the feed containing the admixed 2-butanol (cf. the pairs of 0 and full points denoted with the same numbers in Figs. 3,4). It means that the effect of the formed and the admixed 2-butanol on the conversion is the same, resulting in a declining conversion curve. Therefore, the observed space time dependence of the
D. Ka116, R.M. Mihdlyi/Applied
Table 2 Water coverages Catalyst
Temp. (K) Part. press. (bar)
393
403
Varion KSM
51
at different partial pressures of water and the initial rates of I-butene isomerization
l-C,H, H-CL1
Catalysis A: General 121 (1995) 45-56
383
393
H,O
0.5 0.4
_
0.4 0.4 0.4 0.4
_
Isom. rate. 10’ ’ (mol, _ cJHIIs - ’ g,,’ ) r”
OH,0
(%) b
PHrn
126
0.2--0.6
0.056
99.9924
0.261 0.093 0.056
99.9773 99.9921 99.9953
1.04
99.830
2.08
99.895
1177
0.2 0.4 0.6 988
0.4 0.4
0.2--0.57
0.4 0.42
0.57
1219
a The initial rate of I-butene isomerization in the absence (P) and in the presence ( dto) b The fraction of active centres covered with water determined as ( 1 -&o/P) 100.
of water.
conversion is not due to accumulation of an unknown inhibitor on the catalyst. 2-Butanol is not irreversibly bound since the increase of water pressure (pw) suppresses the inhibiting effect of 2-butanol (cf. conversion curves for different pw values in Figs. 3, 4). Flat conversion curves reflect kinetic product inhibition under steady-state reaction conditions. Both in the absence and presence of water I-butene isomerization rates were determined from the initial slopes of the corresponding conversion curves (Table 2). It is well known that the isomerization proceeds on these catalysts through a 2butyl carbenium ion [ 18,191, the formation of which is inhibited by adsorption of water on the acid sites, as the proton acceptor strength of water is higher than that of 1-butene [ 121. From the decrease of isomerization rates the fraction of acid sites covered with water could be calculated (last column in Table 2). The rate of isomerization i.e. the adsorption of B decreases by nearly four orders of magnitude on H-CL1 and two orders of magnitude on Varion KSM in the presence of water.
4. Discussion Since essential features of I-butene hydration, e.g., the dependence of the reaction rate on the partial pressures and temperature are very similar for the two acidic catalysts of different composition and structure, it is conceivable that Bransted acid sites are the active centres, and the main features of the reaction can be discussed similarly for both catalysts.
52
D. Kall6, R.M. Mitilyi/Applied
Catalysis A: General 121 (1995) 45-56
In order to avoid uncertainties encountered by formal kinetic evaluation of experimental data applying mathematically derived rate equations [ 201, the evaluation of results is based rather on kinetic evidence. If the adsorption of water (W) were the rate determining step, the initial reaction rate (r’) should be independent of the partial pressure of 1-butene (B) :
If the adsorption of B were rate determining,
then
r” #_f(Pw) If the desorption of the product secondary butanol (A) were rate determining, then (3) Since Eqs. ( 1) , (2) and (3) contradict experimental findings (Figs. 1, 2)) the surface reaction, either between adsorbed W and gas-phase B or between adsorbed B and gas-phase W, must be the rate determining step. A rate determining surface reaction between B and W both adsorbed on similar active sites can be disregarded since the rate equation derived for this case
r”=
KBPBKW PW
k
(1 +KBPB+KWPW)*
(4)
(k: rate constant; K: adsorption equilibrium constant) does not express the correlation between r and PB, pw found experimentally (Figs. 1, 2). Extreme forms of Eq. (4) when active sites are practically covered with W
+&@_ KWPW
(W
or with B ro =
kKwpw KBPB
(4b)
do not hold either. Some chemical considerations reveal this conclusion too. In terms of Langmuir-Hinshelwood kinetics a reaction between surface species adsorbed on similar active sites is required. It means that W and B both adsorbed on a Bronsted acid site have to react, i.e., a reaction between positively charged H30+ and set - C,’ HP should proceed which is rather improbable. The reaction between adsorbed W and gas-phase B seems more likely than between adsorbed B and gas-phase W, because ( i) 9 increases linearly with PB over the whole range of PB (Figs. 1,2) indicating a negligible adsorption of B;
D. Kalld, R.M. Mihdlyi/Applied Catalysis A: General 121 (1995) 45-56
53
(ii)
in the low pw range (initial sections in Figs. lb, 2b) when the minute adsorption of B is further suppressed by W (Table 2)) the rate of hydration increases with pw; consequently adsorbed B may not be involved in the hydration reaction; (iii) the hydration conversion can be increased by increasing pw when A exerts kinetic product inhibition (vide infra and in Figs. 3, 4); kinetic product inhibition can be suppressed by adsorbing reactant, in our case by W. Most probably the hydration of B takes place through rate determining surface reaction between adsorbed W and gas-phase B. For the related reaction of methanol addition to isobutene on sulphonic acid resin, tert-butyl carbenium ions perfectly block the acid sites at low methanol pressures [ 151. The formation of secondary alkyl carbenium ions can, however, be suppressed by the alcohol [ 141. Water has a similar effect in our catalytic system indicating that reaction K- set-C,+H,+H,O+K-
H30+ +n-C,H,
(5)
is shifted to right (K- represents deprotonated site) [ 121. These observations seem to support the above outlined interactions. In the investigated pw range more than 99 percent of acid sites are blocked by W (Table 2) in the form of H30+, therefore a linear increase of r” with pw can not be a consequence of the increase of H,O+ concentration but an increase of the concentration of hydrated hydroxonium ions: H,O;,
+ nH,O + H,O + - ~zH*O~~
(6)
The existence of such ensembles under conditions established in the present work was substantiated previously by others [ 2 1] : on increasing pw first mono-, then bi-, tri- etc. hydrates are formed. Presumably the formation of hydroxonium monohydrate in low concentration results first in a proportional increase of r with pw, i.e. first order kinetics in W (Figs. lb, 2b). When pw is further increased r passes through a maximum and thereafter decreases monotonously, indicating that hydroxonium polyhydrates may inhibit the reaction. Based on these observations the initial reaction rate can be expressed in form of Es. (7). P’k’(lFKEp$)
(7)
where k’ is a phenomenological rate constant and K,, is the equilibrium constant of the reaction (6) when n > 1. Parameters in Eq. (7) were determined (Table 3) by fitting calculated curves to the experimental points (as shown in Figs. 1,2). Experimental data do not permit to obtain more detailed information, e.g., the separate determination of the equilibrium constants for the formation of bi-, triand tetrahydrated hydroxonium ions. Water in polyhydrated hydroxonium ion complexes is very probably not ‘activated’ as in hydroxonium monohydrate. In the
54
D. Kalld, R.M. Mihdlyi / Applied Catalysis A: General I2 I (I 995) 45-56
Table 3 Constants in initial rate equation of I-butene hydration, Catalyst
Temp. (K)
H-CL1
393 403 413
Varion KSM
383 393
r,,= k’p, p&l
+ K,,,p”)
k’~10’“(mol,_,,,s~‘g~~~~ 1.41 1.12 I .30 29.0 25.1
K w,, h (harm”)
II c
62.5 12.7 1.6
3 3 3
62.5 13.0
3 4
’ Phenomenological rate constant. h Equilibrium constant of hydration of hydroxonium ion with n water molecules. ’ Average number of water molecules hydrating hydroxonium ions
polyhydrated H+ each water molecule is equivalent [21] and, therefore, their interaction with H+ is too weak, the H-OH bond should be less loosened than in monohydrate. The concentration of the reaction intermediate monohydrate is negligibly small and is substantially decreased by further hydration. The trihydrated form is conceivably the abundant species under reaction conditions [ 2 11. It should be noted, however, that the phenomenological rate constant, k’, does not vary with temperature suggesting that the phenomenological activation energy must be zero. This is reflected by the initial, linearly increasing sections of r =f(pw) curves which are perfectly overlapping for both catalysts at different temperatures (Figs 1b and 2b). If k’ is supposed to be the product of the real rate constant k and the equilibrium constant of formation of the monohydrated hydroxonium ion Kw ,, the phenomenological activation energy can be given by Eq. (8)
(8) In other words, the coverage of H,O’ sites by water decreases with increasing temperature in the same rate within experimental errors as the addition reaction accelerates. At higher pw values, however, r” increases with temperature (Figs. 1a, 2a, at pw = 0.57 bar; Figs. lb, 2b, right to the maxima) which can be attributed to the decrease of K,,, (Table 3). The above observations can be summarised as follows. Water adsorbed on Bronsted acid sites is involved in hydration. It may not be present in the form of hydroxonium ions since all the protons are hydrated even at the lowest pw values where experiments were carried out. The only reasonable explanation for rate increase of hydration with pw is that hydrated, most probably monohydrated, hydroxonium ions react with gas-phase B. At higher pw reaction rate decreases because the conditions favour the formation of polyhydrated hydroxonium ions and formation of monohydrated hydroxonium ions is suppressed [ 2 1 ] . One could argue that the reaction is inhibited by water since water fills up all the pores. This kind of inhibiting effect, however, must be very different for a zeolite with a rigid structure and for a resin that is able to swell. Such structural differences
D. Kal16, R.M. Mihdlyi / Applied Catalysis A: General 121 (I 995) 45-56
55
may explain differences in the catalytic activity but can not result in differences in the mode of catalytic action. Being far from equilibrium (cf. conversion at high space time in Figs. 3,4 and data in Table 1)) the fast decline of the conversion curve, i.e., the slowdown of the hydration with increasing T is attributed to the inhibition of product A. Hydroxonium ions are substituted by secondary butyl alcoxonium ions. In the case of completely solvated protons the reaction rate can be given as KWPW r=rOCKw~w+K~~n)
(9)
where K, and KA are the equilibrium constants for H,O+ and secondary butyl alcoxonium ion formation, respectively. From the slope of conversion curves reaction rates and from reaction rates using Eq. (9) ratios of KA to Kw can be estimated. Obtained K,IK, is very high, more than 100. Due to the limited experimental accuracy a more detailed evaluation does not seem justified. The large ratio is due to the highly different dielectric constants which are 78.3 for W and 15.8 for A at 298 K since the adsorption on Bronsted acid sites increases when the dielectric constant of adsorption decreases [ 22-241. Because of strong adsorption of A on Bronsted acid sites a high concentration of W is needed for substituting A by W. It seems likely that the reaction route of hydration differs from alcohol dehydration because in the latter reaction initially no water is present. In absence of water H30+ ions which are the active sites for hydration can not be formed. This infers that assuming microscopic reversibility of the hydration-dehydration would be unjustified for the initial period of the reaction.
5. Conclusion It seems likely that hydrogen bonded water in monohydrated hydroxonium ion reacts with gas-phase 1-butene in a rate determining surface reaction. Rearrangement of the bonds between the four atoms to a position required by the Markownikoff rule may be anticipated: H I K-H+-a-O-m-H-O 1 H
i
; i CH2=CH-CH2-CH3
The secondary butanol product substitutes most of the water molecules in the hydroxonium ions and inactive alcoxonium ions may form, exerting kinetic inhibition.
56
D. Kalld, R.M. Mihdlyi / Applied Catalysis A: General 121 (1995) 45-56
Acknowledgements The authors are grateful to the National Scientific Research Foundation (OTKA) for financial support by contract T 014532.
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