applied catalysis volume 69 issue 1 1991 [doi 10.1016%2fs0166-9834%2800%2983297-2] jean bandiera;...

8/10/2019 Applied Catalysis Volume 69 Issue 1 1991 [Doi 10.1016%2Fs0166-9834%2800%2983297-2] Jean Bandiera; Claude Naccache -- Kinetics of Methanol Deh

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Applied Catalysis, 69 (1991) 139-148

Elsevier Science Publishers B.V.. Amsterdam

139

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

lUniuersit6 Claude Bernard, Lyon I, 2 Avenue Albert Einstein, 69626 Villeurbanne C den

(France), tel. (+33) 72445341, fax. (+33) 72445389.

Received 19 July 1990, revised manuscript 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 di-

methyl ether and water. Dimethyl ether and/or water compete with the methanol adsorption particu-

larly 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, zeolites, acid sites, basic sites, kinetics, zeolites, selectivity dimethyl

ether )

INTRODUCTION

The acid zeolite-catalysed conversion of methanol into hydrocarbons in-

volves several reaction steps which have been well documented in the literature

111

Among the various mechanisms proposed, the oxonium-ylide mechanism

[

2,3] involving oxygen-protonation and/or oxygen-methylation of the di-

methyl 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, selec-

tive 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.


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141

TABLE 1

Thermodynamic data for dehydration of methanol

Temperature

A Cl

4

YLh

(K)

(J mol-1

(fro)

473

-

17727

92.6

95.1

573

-

18735

52.0

93.5

Y

(%)

- 20

I

0.1

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 , value well below the thermodynamic yield (Table 1). Hence, the kinetic

data were established for those reaction conditions where the rate of the re-

action was limited neither by transport processes nor by thermodynamics.


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142

Effects of methanol partial pressure, P,, on the rate of methanol dehydration, r

Figs. 2 and 3 show the dependence of

r

on

PO

or 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 tem-

peratures 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-

1.5

log1o p

0

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 orr ).


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143

PO

r (tot-r h gcat mmol-I)

6

I

I

I

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, ( 523 K and (A

)

573 K.

served indicates that at 473 K the Langmuir-Hinshelwood model fits the re-

sults only if one considers that the reaction products, DME and/or water, re-

main adsorbed on the active centres. The inhibiting effect of water on the

dehydration of methanol at 473 K was further demonstrated by the data ob-

tained 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


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144

Po/ r

(tow h g mm01

,

I.,

cat

I

I

I

0.2

0.

4

0.6

PDME (torr1

Fig. 4. Linear transform of the inhibiting effect of reaction products on methanol 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 par-

tial 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


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145

\

t

\

0

\

log1

0

\

a

\

0

\

8,

e

\

\

\ \

\

\

0

l-7

1.9

lo3 K-l

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


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146

superficial species formed by adsorption of two methanol molecules, one on a

proton HA+ and one on its adjacent basic site 02-

CH,-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 02=

GPO

l+b PDME+a,P,

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

ala2

l+AP,,

where A =-

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 cov-

ered 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


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147

TABLE2

Kinetic parameters for dehydration of methanol.

Temperature

k E

(K)

(mmol h- g i) (kJ mol-)

523

147

80 3

573

739

[SIT

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:

al

a2

PO APo

r=k a, +as)+a,bPDME =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, t 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 com-

peting 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 H-

mordenite, 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


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applied catalysis volume 69 issue 1 1991 [doi 10.1016%2fs0166-9834%2800%2983297-2] jean bandiera;...

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