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Kinetics of methanol dehydration on dealuminated H-mordenite: Model with acid and basic active centres
Applied
Catalysis, 69 (1991)
Elsevier
Science
Publishers
139-148 B.V..
139
Amsterdam
Kinetics of methanol dehydration on dealuminated H-mordenite: Model with acid and basic active centres Jean Bandiera* and Claude Naccache Institut de Recherches sur la Catalyse, Laboratoire Propre du C.N.R.S., Conuentionnd k l’Uniuersit6 Claude Bernard, Lyon I, 2 Avenue Albert Einstein, 69626 Villeurbanne C&den (France),
tel. (+33)
72445341, fax. (+33)
(Received
19 July 1990, revised manuscript
72445389. received 5 September
1990)
Abstract The kinetics of methanol dehydration catalysed by dealuminated H-mordenite were studied using a packed bed flow reactor at atmospheric pressure in the 473-573 K temperature range. The data were interpreted in terms of Langmuir-Hinshelwood rate equations and they suggested that two different sites are operative during the dehydration, probably an acid site and its adjacent basic site on which methanol forms respectively [CH,.OH,]+ and [CH,O] _ species which, upon condensation, give dimethyl ether and water. Dimethyl ether and/or water compete with the methanol adsorption particularly at lower reaction temperatures and the surface coverages by activated complexes are always large, so that one observes a kinetic order with respect to methanol of less than one. Keywords: methanol dehydration, ether )
zeolites, acid sites, basic sites, kinetics, zeolites, selectivity
(dimethyl
INTRODUCTION
The acid zeolite-catalysed conversion of methanol into hydrocarbons involves several reaction steps which have been well documented in the literature
111.
Among the various mechanisms proposed, the oxonium-ylide mechanism and/or oxygen-methylation of the dimethyl ether molecule requires both acid sites and basic sites. It was also clearly shown that the conversion of methanol into hydrocarbons starts by a rapid and reversible formation of dimethyl ether and water. The purpose of this study is to investigate the kinetics of the dehydration of methanol on dealuminated H-mordenite, which was found to be active, selective and stable for the conversion of methanol into olefins [4]. It was sought to determine whether or not the dehydration of methanol involves acid sites and basic sites. [ 2,3] involving oxygen-protonation
140 EXPERIMENTAL
The catalyst was prepared from a Na-mordenite starting material provided by Societe Chimique de la Grande Paroisse-France. The sample was exchanged with dilute hydrochloric acid solution, washed, dried at 373 K and then exposed to a hydrothermal treatment at about 923 K. The solid was then leached at 353 K by a 6 M hydrochloric acid solution; this acid leaching allowed the removal of extra-lattice aluminium. By repeating the hydrothermal treatment followed by the hydrochloric acid leaching several times, a dealuminated H-mordenite with a Si/Al ratio greater than 80 was prepared. This highly dealuminated H-mordenite was found to be very selective and resistant towards deactivation in the conversion of methanol to olefins [ 41. Dehydration of methanol to dimethyl ether (DME) and water was carried out in a thermostatically controlled flow reactor. The gaseous mixture (methanol/nitrogen) was passed through the catalyst bed at atmospheric pressure; the reaction temperature was varied between 473 and 573 K. Differential conversions were determined by gas chromatography; a flame ionization detector was used with a column packed with Porapak Q of 80-100 mesh. The air-dried dealuminated H-mordenite (20 mg) was loaded into the flow reactor and pretreated in situ at 773 K under a stream of oxygen. Although the activity of the catalyst was found to be remarkably constant in continuous flow operating conditions, the kinetic results were established under experimental conditions (the catalyst bed was flushed with a pure nitrogen stream between two measurements of the reaction rate) such that the activity of the catalyst remained constant throughout the kinetic study and such as resulted in perfectly reproducible values of the rate under the same reaction conditions. RESULTS
AND DISCUSSION
Under the reaction conditions indicated, DME and water are the only products formed. The thermodynamic constants for the equilibrium 2 CH30H=CH30CH3+H20 have been calculated for temperatures in the range 473-573 K. The derived values of AGT , the equilibrium constant K,, and the thermodynamic yield Yth are given in Table 1. In order to establish the true chemical kinetic expression, the domain of flow rate, F, where mass transport effects were practically absent, was first determined. Fig. 1. shows the influence of the flow-rate on the conversion of methanol. The rate of the reaction is not dependent on the conversion for values of flow rate above 10 1 h-’ in the temperature range 473-573 K. At the highest temperature (573 K) and methanol pressure (80 Torr, 1 Torr= 133.3 Pa) and for a flow-rate of 10 1 h-l over 20 mg of catalyst, the conversion was about
141
TABLE 1 Thermodynamic data for dehydration of methanol Temperature
(K)
A Cl (J mol-‘1
4
YLh (fro)
473 573
- 17727 - 18735
92.6 52.0
95.1 93.5
Y
-
(%)
20
0.1
I 0.2
F-l
(h
1-l)
Fig. 1. Variation of the conversion level as a function of the contact time for a methanol partial pressure of 80 Torr over 0.2 g of catalyst at (A ) 473 K, (m) 523 K and (e ) 573 K.
16%,a value well below the thermodynamic yield (Table 1). Hence, the kinetic data were established for those reaction conditions where the rate of the reaction was limited neither by transport processes nor by thermodynamics.
142
Effects of methanol partial pressure, P,, on the rate of methanol dehydration, r Figs. 2 and 3 show the dependence of r on POfor three different temperatures. The results are expressed in Fig. 2 as log,, r versus log,, P,, and in Fig.3 in the form PO/rversus POfollowing a Langmuir-Hinshelwood expression. It is clear that a Langmuir-Hinshelwood model best fits all the results for reaction temperatures of 523 and 573 K. However, for the lowest temperature (473 K), deviation from the linear correlation between PJr and P, occurs at high P,, values. In Fig. 4, the data obtained at 473 K are plotted in the form PO/rversus PDME,the dimethyl ether partial pressure. The linear correlation then ob-
log1o 1.5
p0
2
Fig. 2. Logarithmic plots of the methanol dehydration rate versus the methanol partial pressure at (A ) 473 K, (m) 523 K and (0 ) 573 K (r in mmol converted methanol per h and per g of catalyst, P, in Torr ).
143
PO/
r
(tot-r
h gcat
mmol-I) 6
I
I
I
0
40
80
PO (torr)
Fig. 3. Linear transforms of the methanol dehydration rate as a function of the methanol partial pressure (Langmuir-Hinselwood model) at (0 ) 473 K, ( n ) 523 K and (A ) 573 K.
served indicates that at 473 K the Langmuir-Hinshelwood model fits the results only if one considers that the reaction products, DME and/or water, remain adsorbed on the active centres. The inhibiting effect of water on the dehydration of methanol at 473 K was further demonstrated by the data obtained for conversion of methanol/water mixtures. Indeed, at 473 K and P, = 40 Torr, the rate of dehydration decreased from 12.3 to 6.8 mmol h-l g&l when
144
Po/
r
(tow
h g
I
cat
mm01 -I.,
!
I
0.2 Fig. 4. Linear transform
,
0. 4 of the inhibiting
effect of reaction products
I
0.6
PDME
on methanol
(torr1
dehydration
at
473 K.
the partial pressure of water increased from zero to 5 Torr. In contrast, at 573 K, almost no decrease of the reaction rate occurred on adding water to the methanol feed. The kinetic data for the dehydration of pure methanol at two different partial pressures (P, = 10 or 40 Torr ) are collected in an Arrhenius plot in Fig. 5. This figure shows that the apparent activation energy increased with P,, as may be expected for a Langmuir-Hinshelwood mechanism. The results shown in Figs. 1, 2 and 3 indicate that, when neither diffusion nor thermodynamics limit the reaction rate, the order relative to methanol, n, remains between 1 and 0.5. Moreover, n increases with the temperature and decreases with the methanol partial pressure leading to the belief that the rate
145
\ log10
t \
0 \ 0 \
a \
0 \
e \
8,
l
\
l
\
\
l
\ \
l-7
1.9
lo3 K-l
0
x 2.1
Fig. 5. Arrhenius plots of dehydration of methanol over dealuminated H-mordenite for a flow-rate of 20 1 h-’ and a methanol partial pressure of 10 (open symbols) or 40 Torr (filled symbols) (r in mmol converted methanol per h and per g of catalyst).
law obeys an apparent Langmuir-Hinshelwood model. But if one considers that: (i) the rate limiting step for the reaction involves the combination of two
146
superficial species formed by adsorption of two methanol molecules, one on a proton HA+ and one on its adjacent basic site 02CH,-OH+H+=[CH,.OH,]+ and CH,O-H+O’-=[CH30]-+
[OH]-
(ii) dimethyl ether (or/and water) competes with methanol for the protonic site (CH,),O+H+=
[ (CH,),.OH)]+
then the rate expression for methanol dehydration should be r= rZ0,8being the coverage of dual sites by two methanol molecules. Moreover 8, and & the surface coverages by [ CH3 .OH,] + and [ CH30] - respectively, are given by: f31=
a 3,
and
l+b PDME+a,P,
02=
GPO 1+a,p,
From a statistical analysis, it may be shown that f3=& 0,. Hence, the rate equation for the methanol dehydration should be of the form: r=k
ala2PZ l+bPDME+
[a,+
(1+bPDME)a2]Po+a,a,P~
A mechanism whereby methanol molecules are covalently bonded to H + and O*- would lead to an identical kinetic equation. When dimethyl ether (and/or water) is not adsorbed on protons (7’~ 523 K), the value of b is zero, and r=k
ala2E 1+ (a, +a,)P,+a,azP:
It is obvious that the data presented in Figs. 2 and 3 fit this expression only if we assume that (al + a*) PO + a, a2 Pz is much greater than one, that is if the surface coverages by [ CH,.OH,] + and/or [ CHBO] - are important. With such an assumption, the rate expression becomes: r=k
APo l+AP,,
where
A =-
ala2 a1
+a2
Experimental results have indicated that, at 473 K, the adsorption of DME (or/and water) should be considered. If we assume that protons are more covered by DME than by methanol, as may be expected from the proton affinities of DME and methanol, which are equal to 804 and 761 kJ mol-’ respectively
147 TABLE2 Kinetic parameters
for dehydration
Temperature
k
(K)
(mmol h-’
523
147
573
739
of methanol. E
g&i)
(kJ mol-‘)
80.3
one may write bPDME >>a,Po; we can then demonstrate that [a,+ (1+bPDME)a2]Po>>a,a,P~. Under these conditions, the rate equation would be:
[SIT
al a2 PO
r=k(a,
+as)+a,bPDME
APo =kl+Abp DME al
A good fit was observed by plotting P,,/r versus PDME (Fig. 4). Such an analysis of the kinetic data allows are to determine the values of the rate constant, Fz,at 523 and 573 K from the slope of the straight lines of Fig. 3; they are listed in Table 2 with the corresponding value of the true activation energy, E=80 kJ mol-‘. From the latter value, one can calculate the value k= 20.8 mmol h-’ g&t for the rate constant at 473 K. Furthermore, the slope of the straight line of Fig. 4 measures the ratio b/a,K; the value obtained is 6.81 h gcatmmol-l. Hence, b=142 a,, in fair agreement with the assumption that &ME 3 alp,. CONCLUSION
Analysis of the kinetic results has shown that the dehydration of methanol to dimethyl ether proceeds through the combination of two adjacent different activated complexes, formed by adsorption of methanol on two different active centres. At 473 K, dimethyl ether and/or water inhibit the reaction by competing with methanol for protons. At temperatures between 473 and 573 K, the coverages of the active centres by methanol are large. Thus, a dual site mechanism may be proposed for highly dealuminated Hmordenite, as has been suggested for methanol dehydration on sulphonated polystyrene catalysts [ 61. In the case of zeolites, the dual active centre would be one acid site H+ and its adjacent basic site 02-. On acid sites, methanol would be protonated to form [ CH3*OHP] + which then rapidly generates [ CH,] + and H20, while on basic sites methanol would react to form [ CH,O] - and [OH] -. Dimethyl ether
148
may then be formed by combination of two adjacent activated species, the dual active centre being regenerated through the reactions: H,O+
[OH]-=[H,O]++O’-
and [H,O]+=H,O+H+
REFERENCES 1 C.D. Chang, Catal. Rev. Sci. Eng., 25 (1983) 1. 2 J.P. van den Berg, J.P. Wolthuizen and J.H.C. van Hooff, in L.V.C. Rees (Editor), Proc. 5th Conf. on Zeolites, 1980, p. 649. 3 T. Mole and J.A. Whiteside, J. Catal., 75 (1982) 284. 4 J. Bandiera, C. Hamon and C. Naccache, in D. Olson and A. Bisio (Editors), Proc. 6th Conf. on Zeolites, Butterworths, London 1984, p. 337. 5 S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 695. 6 B.C. Gates and L.N. Johanson, J. Catal., 14 (1969) 69.
Catalysis, 69 (1991)
Elsevier
Science
Publishers
139-148 B.V..
139
Amsterdam
Kinetics of methanol dehydration on dealuminated H-mordenite: Model with acid and basic active centres Jean Bandiera* and Claude Naccache Institut de Recherches sur la Catalyse, Laboratoire Propre du C.N.R.S., Conuentionnd k l’Uniuersit6 Claude Bernard, Lyon I, 2 Avenue Albert Einstein, 69626 Villeurbanne C&den (France),
tel. (+33)
72445341, fax. (+33)
(Received
19 July 1990, revised manuscript
72445389. received 5 September
1990)
Abstract The kinetics of methanol dehydration catalysed by dealuminated H-mordenite were studied using a packed bed flow reactor at atmospheric pressure in the 473-573 K temperature range. The data were interpreted in terms of Langmuir-Hinshelwood rate equations and they suggested that two different sites are operative during the dehydration, probably an acid site and its adjacent basic site on which methanol forms respectively [CH,.OH,]+ and [CH,O] _ species which, upon condensation, give dimethyl ether and water. Dimethyl ether and/or water compete with the methanol adsorption particularly at lower reaction temperatures and the surface coverages by activated complexes are always large, so that one observes a kinetic order with respect to methanol of less than one. Keywords: methanol dehydration, ether )
zeolites, acid sites, basic sites, kinetics, zeolites, selectivity
(dimethyl
INTRODUCTION
The acid zeolite-catalysed conversion of methanol into hydrocarbons involves several reaction steps which have been well documented in the literature
111.
Among the various mechanisms proposed, the oxonium-ylide mechanism and/or oxygen-methylation of the dimethyl ether molecule requires both acid sites and basic sites. It was also clearly shown that the conversion of methanol into hydrocarbons starts by a rapid and reversible formation of dimethyl ether and water. The purpose of this study is to investigate the kinetics of the dehydration of methanol on dealuminated H-mordenite, which was found to be active, selective and stable for the conversion of methanol into olefins [4]. It was sought to determine whether or not the dehydration of methanol involves acid sites and basic sites. [ 2,3] involving oxygen-protonation
140 EXPERIMENTAL
The catalyst was prepared from a Na-mordenite starting material provided by Societe Chimique de la Grande Paroisse-France. The sample was exchanged with dilute hydrochloric acid solution, washed, dried at 373 K and then exposed to a hydrothermal treatment at about 923 K. The solid was then leached at 353 K by a 6 M hydrochloric acid solution; this acid leaching allowed the removal of extra-lattice aluminium. By repeating the hydrothermal treatment followed by the hydrochloric acid leaching several times, a dealuminated H-mordenite with a Si/Al ratio greater than 80 was prepared. This highly dealuminated H-mordenite was found to be very selective and resistant towards deactivation in the conversion of methanol to olefins [ 41. Dehydration of methanol to dimethyl ether (DME) and water was carried out in a thermostatically controlled flow reactor. The gaseous mixture (methanol/nitrogen) was passed through the catalyst bed at atmospheric pressure; the reaction temperature was varied between 473 and 573 K. Differential conversions were determined by gas chromatography; a flame ionization detector was used with a column packed with Porapak Q of 80-100 mesh. The air-dried dealuminated H-mordenite (20 mg) was loaded into the flow reactor and pretreated in situ at 773 K under a stream of oxygen. Although the activity of the catalyst was found to be remarkably constant in continuous flow operating conditions, the kinetic results were established under experimental conditions (the catalyst bed was flushed with a pure nitrogen stream between two measurements of the reaction rate) such that the activity of the catalyst remained constant throughout the kinetic study and such as resulted in perfectly reproducible values of the rate under the same reaction conditions. RESULTS
AND DISCUSSION
Under the reaction conditions indicated, DME and water are the only products formed. The thermodynamic constants for the equilibrium 2 CH30H=CH30CH3+H20 have been calculated for temperatures in the range 473-573 K. The derived values of AGT , the equilibrium constant K,, and the thermodynamic yield Yth are given in Table 1. In order to establish the true chemical kinetic expression, the domain of flow rate, F, where mass transport effects were practically absent, was first determined. Fig. 1. shows the influence of the flow-rate on the conversion of methanol. The rate of the reaction is not dependent on the conversion for values of flow rate above 10 1 h-’ in the temperature range 473-573 K. At the highest temperature (573 K) and methanol pressure (80 Torr, 1 Torr= 133.3 Pa) and for a flow-rate of 10 1 h-l over 20 mg of catalyst, the conversion was about
141
TABLE 1 Thermodynamic data for dehydration of methanol Temperature
(K)
A Cl (J mol-‘1
4
YLh (fro)
473 573
- 17727 - 18735
92.6 52.0
95.1 93.5
Y
-
(%)
20
0.1
I 0.2
F-l
(h
1-l)
Fig. 1. Variation of the conversion level as a function of the contact time for a methanol partial pressure of 80 Torr over 0.2 g of catalyst at (A ) 473 K, (m) 523 K and (e ) 573 K.
16%,a value well below the thermodynamic yield (Table 1). Hence, the kinetic data were established for those reaction conditions where the rate of the reaction was limited neither by transport processes nor by thermodynamics.
142
Effects of methanol partial pressure, P,, on the rate of methanol dehydration, r Figs. 2 and 3 show the dependence of r on POfor three different temperatures. The results are expressed in Fig. 2 as log,, r versus log,, P,, and in Fig.3 in the form PO/rversus POfollowing a Langmuir-Hinshelwood expression. It is clear that a Langmuir-Hinshelwood model best fits all the results for reaction temperatures of 523 and 573 K. However, for the lowest temperature (473 K), deviation from the linear correlation between PJr and P, occurs at high P,, values. In Fig. 4, the data obtained at 473 K are plotted in the form PO/rversus PDME,the dimethyl ether partial pressure. The linear correlation then ob-
log1o 1.5
p0
2
Fig. 2. Logarithmic plots of the methanol dehydration rate versus the methanol partial pressure at (A ) 473 K, (m) 523 K and (0 ) 573 K (r in mmol converted methanol per h and per g of catalyst, P, in Torr ).
143
PO/
r
(tot-r
h gcat
mmol-I) 6
I
I
I
0
40
80
PO (torr)
Fig. 3. Linear transforms of the methanol dehydration rate as a function of the methanol partial pressure (Langmuir-Hinselwood model) at (0 ) 473 K, ( n ) 523 K and (A ) 573 K.
served indicates that at 473 K the Langmuir-Hinshelwood model fits the results only if one considers that the reaction products, DME and/or water, remain adsorbed on the active centres. The inhibiting effect of water on the dehydration of methanol at 473 K was further demonstrated by the data obtained for conversion of methanol/water mixtures. Indeed, at 473 K and P, = 40 Torr, the rate of dehydration decreased from 12.3 to 6.8 mmol h-l g&l when
144
Po/
r
(tow
h g
I
cat
mm01 -I.,
!
I
0.2 Fig. 4. Linear transform
,
0. 4 of the inhibiting
effect of reaction products
I
0.6
PDME
on methanol
(torr1
dehydration
at
473 K.
the partial pressure of water increased from zero to 5 Torr. In contrast, at 573 K, almost no decrease of the reaction rate occurred on adding water to the methanol feed. The kinetic data for the dehydration of pure methanol at two different partial pressures (P, = 10 or 40 Torr ) are collected in an Arrhenius plot in Fig. 5. This figure shows that the apparent activation energy increased with P,, as may be expected for a Langmuir-Hinshelwood mechanism. The results shown in Figs. 1, 2 and 3 indicate that, when neither diffusion nor thermodynamics limit the reaction rate, the order relative to methanol, n, remains between 1 and 0.5. Moreover, n increases with the temperature and decreases with the methanol partial pressure leading to the belief that the rate
145
\ log10
t \
0 \ 0 \
a \
0 \
e \
8,
l
\
l
\
\
l
\ \
l-7
1.9
lo3 K-l
0
x 2.1
Fig. 5. Arrhenius plots of dehydration of methanol over dealuminated H-mordenite for a flow-rate of 20 1 h-’ and a methanol partial pressure of 10 (open symbols) or 40 Torr (filled symbols) (r in mmol converted methanol per h and per g of catalyst).
law obeys an apparent Langmuir-Hinshelwood model. But if one considers that: (i) the rate limiting step for the reaction involves the combination of two
146
superficial species formed by adsorption of two methanol molecules, one on a proton HA+ and one on its adjacent basic site 02CH,-OH+H+=[CH,.OH,]+ and CH,O-H+O’-=[CH30]-+
[OH]-
(ii) dimethyl ether (or/and water) competes with methanol for the protonic site (CH,),O+H+=
[ (CH,),.OH)]+
then the rate expression for methanol dehydration should be r= rZ0,8being the coverage of dual sites by two methanol molecules. Moreover 8, and & the surface coverages by [ CH3 .OH,] + and [ CH30] - respectively, are given by: f31=
a 3,
and
l+b PDME+a,P,
02=
GPO 1+a,p,
From a statistical analysis, it may be shown that f3=& 0,. Hence, the rate equation for the methanol dehydration should be of the form: r=k
ala2PZ l+bPDME+
[a,+
(1+bPDME)a2]Po+a,a,P~
A mechanism whereby methanol molecules are covalently bonded to H + and O*- would lead to an identical kinetic equation. When dimethyl ether (and/or water) is not adsorbed on protons (7’~ 523 K), the value of b is zero, and r=k
ala2E 1+ (a, +a,)P,+a,azP:
It is obvious that the data presented in Figs. 2 and 3 fit this expression only if we assume that (al + a*) PO + a, a2 Pz is much greater than one, that is if the surface coverages by [ CH,.OH,] + and/or [ CHBO] - are important. With such an assumption, the rate expression becomes: r=k
APo l+AP,,
where
A =-
ala2 a1
+a2
Experimental results have indicated that, at 473 K, the adsorption of DME (or/and water) should be considered. If we assume that protons are more covered by DME than by methanol, as may be expected from the proton affinities of DME and methanol, which are equal to 804 and 761 kJ mol-’ respectively
147 TABLE2 Kinetic parameters
for dehydration
Temperature
k
(K)
(mmol h-’
523
147
573
739
of methanol. E
g&i)
(kJ mol-‘)
80.3
one may write bPDME >>a,Po; we can then demonstrate that [a,+ (1+bPDME)a2]Po>>a,a,P~. Under these conditions, the rate equation would be:
[SIT
al a2 PO
r=k(a,
+as)+a,bPDME
APo =kl+Abp DME al
A good fit was observed by plotting P,,/r versus PDME (Fig. 4). Such an analysis of the kinetic data allows are to determine the values of the rate constant, Fz,at 523 and 573 K from the slope of the straight lines of Fig. 3; they are listed in Table 2 with the corresponding value of the true activation energy, E=80 kJ mol-‘. From the latter value, one can calculate the value k= 20.8 mmol h-’ g&t for the rate constant at 473 K. Furthermore, the slope of the straight line of Fig. 4 measures the ratio b/a,K; the value obtained is 6.81 h gcatmmol-l. Hence, b=142 a,, in fair agreement with the assumption that &ME 3 alp,. CONCLUSION
Analysis of the kinetic results has shown that the dehydration of methanol to dimethyl ether proceeds through the combination of two adjacent different activated complexes, formed by adsorption of methanol on two different active centres. At 473 K, dimethyl ether and/or water inhibit the reaction by competing with methanol for protons. At temperatures between 473 and 573 K, the coverages of the active centres by methanol are large. Thus, a dual site mechanism may be proposed for highly dealuminated Hmordenite, as has been suggested for methanol dehydration on sulphonated polystyrene catalysts [ 61. In the case of zeolites, the dual active centre would be one acid site H+ and its adjacent basic site 02-. On acid sites, methanol would be protonated to form [ CH3*OHP] + which then rapidly generates [ CH,] + and H20, while on basic sites methanol would react to form [ CH,O] - and [OH] -. Dimethyl ether
148
may then be formed by combination of two adjacent activated species, the dual active centre being regenerated through the reactions: H,O+
[OH]-=[H,O]++O’-
and [H,O]+=H,O+H+
REFERENCES 1 C.D. Chang, Catal. Rev. Sci. Eng., 25 (1983) 1. 2 J.P. van den Berg, J.P. Wolthuizen and J.H.C. van Hooff, in L.V.C. Rees (Editor), Proc. 5th Conf. on Zeolites, 1980, p. 649. 3 T. Mole and J.A. Whiteside, J. Catal., 75 (1982) 284. 4 J. Bandiera, C. Hamon and C. Naccache, in D. Olson and A. Bisio (Editors), Proc. 6th Conf. on Zeolites, Butterworths, London 1984, p. 337. 5 S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 695. 6 B.C. Gates and L.N. Johanson, J. Catal., 14 (1969) 69.