Synthesis of hydrocarbons from methanol on highly siliceous zeolites of various chemical composition

Journal

of Molecular

SYNTHESIS SILICEOUS

31 (1985)

251

251

- 267

OF HYDROCARBONS FROM METHANOL ON HIGHLY ZEOLITES OF VARIOUS CHEMICAL COMPOSITION

V. N. ROMANNIKOV Institute

Catalysis,

of Catalysis,

and K. G. IONE Novosibirsk

630090

(U&S.

R.)

(Received May 20, 1984; accepted November 10, 1984)

Summary Effective group rate constants for the consumption of methanol (Ku) and for the formation of aromatic hydrocarbons (KAr) have been obtained for the synthesis of hydrocarbons from methanol over a series of ZSM-5 zeolites by assuming that the reaction is inhibited by the products formed. It was observed that the value of Ku increases and that of K, decreases as the aluminium content of the zeolites decreases. An acid centre model for catalysis is presented to explain these results.

Introduction It is known [l, 21 that Brijnstead acid centres (BAC), i.e. bridging hydroxyl groups bound to aluminium atoms in isomorphous Si4+ positions of the zeolite framework, act as catalytically active centres in zeolites. On the basis of NMR data [2], on the results obtained from studies of adsorption properties [ 31 and on the existence of a proportionality between exchanged cesium cations and the aluminium content [4], it has been suggested that the aluminium atoms in ZSM-5 zeolites are always in a tetrahedral coordination. If this is correct, the nature of BAC should be independent of the degree of substitution, i.e. (Si04)4- * (A104)5-, and the concentration of these centres, and hence, the catalytic activity of highly siliceous zeolites, should increase linearly with increasing aluminium content. Olsen et al. [4] have shown that for highly siliceous ZSM-5 type zeolites of varying SiOJA120s mole ratio within the range of 89 - 8660, the relative first-order rate constant for the cracking of n-hexane (o-test value, as examined by Chen and Reagan [5]) increases in direct proportion to the aluminium content in the zeolite. A linear correlation between the o-test value and the rate constant has also been observed [5, 61 for the conversion of methanol to olefins over ZSM-5 zeolites. This type of regular relationship suggests that the proportionality must arise from the dependence of the reaction rate on the chemical composition of such highly siliceous zeolites. However, as mentioned by Haag [2], such conclusions 0304-5102/85/$3.30

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252

will only be correct if all the aluminium atoms are situated in tetrahedral positions in .the zeolite framework and if the catalytic activity of the acid centres is not diminished by poisoning arising from residual sodium cations, present either as impurities in the initial reagents or by coke deposition during hydrocarbon conversion. Recently, some evidence for the heterogenous state of aluminium and silicon atoms in zeolite active centres has been described [7]. The use of *‘Al NMR spectroscopy [8] has shown that decationation of zeolites leads to the development of a portion of the framework aluminium atoms in cationic extralattice positions. This results in the formation of Lewis acid centres (LAC) [9, 10 J and centres of multipoint interactions [ 71. In this respect, highly siliceous ZSM-5 type zeolites are no exception [ll]. The probability of a such irreversible hydrolytic scission of Si-O-Al bonds is assumed [7,12] to increase as the number of aluminium atoms in the second coordination sphere of the silicon atom increases, i.e. with increasing values of IZ in the structure (SiO)4_ ,Si(OAl),. From 29Si NMR data it has been shown [lo, 131 that an increase in the overall content of aluminium in zeolites results in an increase in the mean value of n in the above structure as a result of a statistical irregularity in the distribution of aluminium over the zeolite framework. Thus, on the basis of experimental observations [7, 8,10, 12, 131 it may be expected that a variation in the chemical composition for zeolites with the same structure will result in a non-proportional change, not only of the concentration, but also of the type of acid-base centres in these catalysts. On the base of literature data [ 14 - 171, the sequences involved in methanol conversion to hydrocarbons may be described by the following scheme: (autocatalytic pCH,OH, [ -HzO~‘“‘:~O]

=

chain propagation)

--pH,O

C2<

>

Hzo,

+ 2) -

(oligomerization) hydrogen redistribution reactions

+

paraffins

+ aromatic

hydrocarbons

As seen from this scheme, synthesis of hydrocarbons from methanol includes parallel and successive stages of a quite different nature: dehydration, alkylation, cracking. Indeed, reported data [ 14 - 161 allow the first step of methanol conversion to olefins to be considered as a sequence of similar reactions involving the formation of ether and its dehydration. However in contrast, the second step, i.e. the conversion of olefins to paraffins and aromatic only includes hydrocarbon conversions, although the hydrocarbons, hydrogen redistribution reactions involved have also been assumed [2, 171 to consist of a successive sequence of similar stages: oligomerization and cracking with the elimination of the saturated paraffin fragment.

253

If the reaction is considered as a whole, and if account is taken of the distribution and ionic state of the aluminium atoms in the zeolite framework as described in the literature, it is hardly surprising that a simple proportional dependence does not exist between the rate constants for every route and the chemical composition of the zeolite. In the work described in this paper, the dependence of the rate constants involved in the consumption of methanol and in the formation of hydrocarbons during methanol conversion on the SiOz/A1203 mole ratio has been investigated under gradientless conditions for a series of highly siliceous ZSM-5 type zeolites. The data obtained have been compared with the rate constants for ethylene aromatization performed over the same series of catalysts. On the basis of the principles established, an acid centre model for the behaviour of such zeolites is discussed.

Experimental The catalytic properties of the samples were examined as the aluminium content in the zeolite crystals varied over the range 0.28 to 4.9 wt.% A1203, which is equivalent to decreasing the SiOJAl,Os mole ratio from 540 to 24 (Table 1). The synthesis and physicochemical characterization of these zeolites has already been described [ 111. TABLE 1 Chemical composition of the H-forms of hydrated ZSM-5 zeolite? Sample No.

Na content (wt.%)

SiOz content (wt.%)

Al203 content (wt.%)

SiOz /A1203 mole ratio

1 2 3 4 5 6 7

0.27 0.43 0.30 0.32 0.29 0.36 0.24

90.3 88.2 88.1 87.7 86.5 82.5 70.7

0.285 0.525 0.695 1.18 2.03 3.59 4.95

540 285 216 127 72 39 24

f f f f + f f

50 8 10 7 1 0 1

aThe remainder of the analysis in each case corresponds to water.

Thus the zeolites were synthesized by hydrothermal crystallization of the system Na@--(TPA),O-Al@-SiO,-H,C at a temperature of 175 + 1 “C for a period of 108 h. Constant initial molar ratios of (TPA-OH + NaOH)/SiOz = 0.35, TPA-OH/Si02 = 0.14 and H20/Si02 = 35.8 were maintained for all the starting reaction mixtures, while the initial molar ratio of SiOz/A1203 was changed over the range 33 - 655. To prepare the initial mixture of reactants, weighed quantities of sodium hydroxide and sodium aluminate were dissolved in a given quantity of water, and the corresponding weighed quantity of a solution of tetrapropyl ammonium hydroxide was

254

then added. A corresponding quantity of silica sol was weighed separately and placed in a receptacle. The reaction mixture was prepared via the gradual addition of the silica sol to the intimately mixed aluminium-containing solution. Each freshly prepared reaction mixture was then placed in a Teflon container which was installed in an airtight steel autoclave and heated to the crystallization temperature in an air thermostat provided with a system of automatic temperature controls and capable of intensive internal air ventilation at corresponding intervals. On completion of crystallization the autoclave was rapidly cooled, the Teflon container removed, and the sample filtered, rinsed with cool distilled water and then dried for 10 - 15 h at room temperature. The following reagents were used for the preparation of the reaction mixtures: TPA-OH (20 wt.% water solution, pure, Fluka), sodium aluminate (pure, 40.4 wt % NazO and 50.9 wt.% AlzOJ, sodium hydroxide (pure, 97.5 wt.% NaOH), silica sol (28.5 - 29.0 wt.% SiOZ). The synthesized zeolites were analyzed after thermal treatment of the samples in an oxygen flow for 10 - 12 h at a temperature of 530 - 550 “C followed by ion exchange for 4 - 6 h at room temperature in a 1 M ammoniacal buffer solution. After ion exchange was completed, each sample was filtered, rinsed with distilled water and dried at room temperature. The H-forms of the samples were prepared by thermal treatment of the NH4-forms in an oxygen flow for 10 - 12 h at a temperature of 530 - 550 “C. The silica content was determined by gravimetry. Thus a melt with sodium carbonate was first prepared, then dissolved in water and the silicic acid precipitated after the solution had been neutralized at a pH of 6.5 - 7.0. The aluminium and sodium contents were determined by atomic absorption spectrophotometry using a SPEKTR type instrument. In this case the weighed sample was dissolved in a mixture of sulphuric and hydrofluoric acids, the resulting solution evaporated and that obtained after addition of water to the solid residue analyzed to establish the sodium and aluminium content. The H-form powders were employed as granules of 0.25 - 0.5 mm diameter without the use of a binding agent. The conversion of methanol and ethylene was studied through the use of a glass gradientless flow circulating system at 370 f 3 “C and atmospheric pressure, and with contact times within the range 0.01 - 4.7 s. The contact time was calculated from the ratio of the catalyst bed volume (in cm3) to the flow rate of the initial gas mixture (in cm3 s-l), the latter consisting of from 1 to SO vol.% of reagent contained in helium. The methanol conversion observed was generally in the range 40 - 95%, while the ethylene conversion was in the range 10 75%. In some experiments water vapour was added to the initial gas mixture. The composition of the reaction mixture was analyzed at the inlet and outlet of the circulating system using a gas chromatograph with columns filled with y-alumina (surface area CU. 500 m2 g-‘) and polyphenyl ether on a C22 support. Water could not be estimated with this separation arrangement.

255

The parameters (K, a, b) of the reaction rate equation

W=KC,"CBb

(1)

where W is. the rate of the steady-state reaction, and CA and Ca are the steady-state concentrations of the reactants have been determined by plotting the dependencies W = KCAa (at CB = 1) and W = KCBb (at CA = 1) using log~thmic coordinates. The parameter K has been determined from such dependencies using the condition CA = CB = 1. To obtain the dependencies, the data were calculated via the linear interpolation method using the equations:

(log wcB= 1 =

(log C&

=1 =

t1ogWC, =1 =

(1%CB)CA where

(Iok!

w) Itlog

CB)2

-

(log

w)2(l”g

(log

&3)2

-

(log

cB)

(log

CAhflog (log

(log

w)I(l”g (log

(log

CBhtlog

1

CB)2

-

flog

cA)2(10g

cB)2

-

(log

CB) 1

CA)2

-

(log

CA)2

-

(log

CB)l

wh.(log

cB)l

CAh

CAlI

CA)2

-

(log

cB)2(10g

CA)2

-

(lo&f CA)I

CA)1

= t =

(log

(log

WI,

(log

CA)I,

(lokit CBh

and

(log

W2,

(log

cA)2,

(log

CB)2

me

arbitrary combinations of experimental reaction rate values together with the related values of the concentrations CA and Ca. To reduce the error in the dete~ination of the parameters, various pairs of experimental combinations (experimental points) were used. For each zeolite studied the number of experimental points was not less than 10. Consequently, the number of pair combinations (i.e. points from which the particular dependences were calculated) was not less than 45. The reaction selectivity towards the ith hydrocarbon (Si) was calculated by means of the equation: Sf = z

reag

X 100 (mol%)

where Wi is the rate of accumulation of the ith hydrocarbon, W,,, is the rate of reagent consumption (methanol or ethylene) (in mmol s-’ (g catalyst)-‘), nj is the number of carbon atoms in the ith hydrocarbon molecule and EZis the number of carbon atoms in the reagent molecule.

256

Results The rate of methanol

consumption

This rate depends on the sum of the steady-state concentration of methanol and twice the steady-state concentration of dimethyl.ether in the reaction mixture (CM), since the general routes for hydrocarbon synthesis from methanol and dimethyl ether coincide [14, 151. As seen from the typical dependencies depicted in Fig. 1 for a zeolite with an SiOz/A120s mole ratio of 39 (sample 6, Table l), an increase in the steady-state concentration of methanol leads to an increase in. the rate of methanol consumption, while an increase in the contact time leads to a decrease in the rate. The latter may be attributed to the reversible poisoning of the catalyst by the aromatic reaction products, whose selectivity increases with increasing contact time. The data depicted in Fig. 2 shoyv that wide-ranging variations in the conditions for methanol conversion do not influence the average number NC

84

I

__________________o‘_

;r; n_~.__~__~_._~_H.__H

i.0 0.0 logCM (relative units)

log

ISi02/AI~O~l,o~

Fig. 1. Dependence of the rate of methanol consumption during conversion to hydrocarbons at 370 f 3 “C on the steady-state concentrations of methanol and dimethyl ether in the reaction mixture, and on the contact time with zeolite of SiOz/AlsOs mole ratio equal to 39. The contact times were as follows: curve 1, 0.02 s; curve 2, 0.1 s; curve 3, 0.53 s; and curve 4, 1.0 s. Fig. 2. Average number of carbon atoms (No) in molecules of (1) aromatic and (2) paraffin hydrocarbons obtained by the conversion of methanol or ethylene on highly siliceous zeolites of various chemical composition. Reaction temperature, 370 f 3 “C; initial reagent concentration, 1 -80 vol.%;contact time, 0.01- 4.7 s; atmospheric pressure,

of carbon atoms in the final aromatic hydrocarbon molecules to any great extent, the latter being within the range 8.2 - 8.4. This means that the effect of each aromatic hydrocarbon on the rate of methanol consumption cannot be considered separately, but that a group value, i.e. a concentration sum of the aromatic hydrocarbons, must be employed. To accommodate

257

this fact, the equation for the rate of methanol at 370 “C must be written as:

conversion

to hydrocarbons

WM = KM(M)CMaCArb (mm01 s-l (g catalyst)-‘)

(3)

where WM is the rate of methanol consumption during conversion to hydrocarbons, CM is the steady-state concentration of methanol and dimethyl concentration of the sum of the aromatic ether, CAr is the steady-state hydrocarbons, and KM(M) is the effective rate constant which depends on the chemical composition of the zeolite, i.e. on the SiOZ/A120s mole ratio (M). The parameters determined by means of eqn. (3) are listed in Table 2. As seen from this table, the kinetic orders linking the rate of methanol TABLE 2 Parameters deduced from eqn. (3) for the rate of methanol consumption sion to hydrocarbons on ZSM-5 zeolites of various chemical compositiona Sample No.~

log KM

a

b

1 2 3 4 5 6 7

-3.12 -2.90 -2.84 -2.80 -2.70 -2.53 -2.49

0.97 1.06 1.10 1.11 1.13 1.14 1.07

-0.77 -0.77 -0.96 -1.00 -1.40 -1.35 -0.93

during conver-

aReaction temperature, 370 “C. bCorresponding to those listed in Table 1.

consumption to the methanol and aromatic hydrocarbon concentrations are only slightly affected by variations in the chemical composition of the zeolites over the range studied. From this we may conclude that to a high degree of accuracy the rate of methanol consumption on the zeolites studied is proportional to the methanol concentration and inversely proportional to the concentration of aromatic hydrocarbons. The rate constant increases as the aluminium content in the zeolites increases, but not in proportion. Thus, when the aluminium content is increased by a factor of 17 in the samples studied (Table l), the effective rate constant only increases by a factor of four. The rate of formation

of the. sum of aromatic

hydrocarbons

from methanol

In this work this rate was found to depend on the steady-state concentration of olefins in the reaction mixture. As shown previously [14], olefins are intermediates in the conversion of methanol to aromatic and paraffin hydrocarbons, with ethylene having been suggested [18] as the most probable primary olefin hydrocarbon. The regular logarithmic dependencies found for a zeolite with an SiOz/A1,03 mole ratio of 39 are

258

shown in Fig. 3. These dependencies show that the rate of formation of aromatic hydrocarbons increases with increasing steady-state concentrations of ethylene and decreases with an increase in the contact time. If it is assumed that the latter is a result of the inhibiting effect of the conversion products, i.e. aromatic hydrocarbons, the equation for the rate of formation of the sum of aromatic hydrocarbons from methanol at a reaction temperature of 370 “C!must be written as:

W Al-/M

= KArIM(hf)CC2H4P

CArq

(mmd

s-’

(g Catdyst)-‘)

(4)

where WArIMis the rate of formation of aromatic hydrocarbons from methaconcentrations of ethylene and of nol, CC2H4 and CAr are the steady-state the sum of aromatic hydrocarbons respectively, and KArjM(M) is the effective rate constant which depends on the Si02/A1203 mole ratio (M). The parameters calculated from an application of eqn. (4) are listed in Table 3.

Fig. 3. Dependence of the rate of formation of C+,- C 10 aromatic hydrocarbons from methanol over a zeolite of SiOz/Alz03 mole ratio equal to 39 at 370 + 3 “C on the steadystate concentration of ethylene in the reaction mixture and on the contact time. The contact times were as follows: curve 1, 0.02 s; cutie 2, 0.1 s; curve 3, 0.53 s; and curve 4, 1.0 s.

TABLE 3 Parameters deduced from eqn. (4) for the rate of formation of the sum of aromatic hydrocarbons from methanol over ZSM-5 zeolites of various chemical compositiona Sample No.~

log

1

-4.38 -4.04 -3.48 -3.03 -2.46 -2.05

2 4 5 6 7

KArIM

aReaction temperature, 370 “C. bCorresponding to those listed in Table 1.

P

Q

1.80 2.35 2.18 2.19 2.54 2.47

-0.94 -1.01 -1.11 -1.22 -1.02 -1.41

259

The rate orders p and q in eqn. (4) change very little with a variation in the chemical composition of the zeolites over the range studied (Table 3). Hence, we may conclude that the formation rate of aromatic hydrocarbons from methanol is proportional to the square of the ethylene concentration and inversely proportional to the concentration of aromatic hydrocarbons. A non-linear dependence is observed between the reaction rate constant KArIM in eqn. (4) and the Si02/A120s mole ratio (Table 3); this constant increased more than 200 times when the aluminium content of the samples studied was increased 17 times (Table 1). Of interest is the fact that the values of q in eqn. (4) and b in eqn. (3) are close. This suggests that the mechanism of the inhibiting effect of aromatic hydrocarbons in both reactions (i.e. those expressed by eqns. (3) and (4)) is the same. In an attempt to identify the reaction stage which may be inhibited by aromatics, the conversion of the intermediate product (ethylene) has also been investigated with the same series of catalysts (Table 1) at 370 “C and with a wide variation in the contact time. The rate of formation

of the sum of aromatic

hydrocarbons

from ethylene

The results obtained from such studies are depicted in Fig. 4. From dependencies given in the figure, an increase of the steady-state concentration of ethylene results in an increase in reaction rate. However, in contrast to previous results for reactions involving the participation of methanol (Figs. 1 and 3), no variations in the rate of formation of the sum of aromatic hydrocarbons were observed when the contact time was varied. For this reason, the possible inhibition of the reaction by products of ethylene

I

lo

i.0 0.0 log CCzH.(relativeunits)

1.0

Fig. 4. Dependence of the rate of formation at 370 f 3 “C of C6 - Cl0 aromatic hydrocarbons from ethylene over highly siliceous zeolites of various chemical composition on the steady-state concentration of ethylene in the reaction mixture. The numbers on the curves correspond to the sample number listed in Table 1. The points depicted as black correspond to experiments performed in the presence of water vapour at 55 torr. The contact times were as follows: o, 4.1 - 5.1 s; A, A, 1.8 - 2.1 s; l, q, 1.0 - 1.3 s; and x, 0.6 - 0.8 s.

260

aromatization can be discounted. Hence, the linear sections depicted in Fig. 4 may be described by the following equation: WArlC2n4=K

A~/CZH@)~C~H~”

(mm01

S--l (g

catalyst)--‘)

of the curves

(5)

where WAr/C 2H 4 is the rate of formation of aromatics from ethylene, CC2nq is the steady-state concentration of ethylene and KAr,C2n4(M) is the effective rate constant, which depends on the SiO,/Al,O, mole ratio (M). The parameters calculated from this equation are presented in Table 4. As seen from data in the table, the order p in eqn. (5) changes very little as the chemical composition of the zeolites is varied. For this reason, the reaction rate may be considered as proportional to the square of the steady-state concentration of ethylene. At the same time the effective rate constant for this reaction decreases significantly as the ~uminium content of the zeolites decreases: thus, a decrease in the aluminium content of the zeolites of 17-times results in a decreasing in KAr,C2H4 of more than three orders of magnitude. TABLE 4 Parameters deduced from eqn. (5) for the rate of formation of the sum of aromatic hydrocarbons from ethylene over ZSM-5 zeoiites of various chemical compositiona Sample No.~

1”g KA~/c~H~,

P

1 2 3 4 5 6 7

-6.56 -4.80 -4.45 -3.98 -3.12 -2.56 -2.12

2.22 2.08 2.05 2.02 2.16 2.02 2.00

“Reaction temperature, 370 “C. bCorresponding to those listed in Table 1.

This disproportional decrease in the catalytic activity of zeolites towards aromatics formation with decreasing aluminium content cannot be explained by diffusional restrictions. Thus, a comparison of the constants in eqn. (5) (Table 4) over the linear sections of the curves in Fig. 4 indicated that no changes in the magnitude of p occurred. Secondly, although curvatures do occur in all the plots depicted in Fig. 4, which are likely to be due to diffusional limitations, these sections of the curves were excluded from such an analysis. It is interesting that an increase in the aluminium content in the series of ZSM-5 zeolites studied results, on the one hand, in a considerable increase in the magnitude of the reaction rate at which curvature commences and, on the other hand, in a significant decrease in the average size of the zeolite crystals (Table 5, samples 1 - 5). A further increase in the aluminium content (Table 5, samples 5 - 7) has no effect either on the reaction rate value or on the size of the zeolite crystals present in these samples.

261 TABLE 5 Comparison of the values of the reaction rates during ethylene aromatization at the point at which initial curvature in the slope of kinetic dependencies (Fig. 4) is observed with the average size of the zeolite crystals employed as catalysts Sample No.~

Average size of zeolite crystals (data taken from ref. 11) (Pm)

Reaction rate value at which initial curvature in the kinetic plot is observed (mmol s-r (g catalyst)-‘) x 104

1 2 4 5 6 7

13 - 14 7-9

0.6 1.8 4.0 5.6 5.6 5.6

b

0.5 - 1.5 0.3 - 1.0 0.1 - 0.5

Vorresponding to those listed in Table 1. bNo data available.

The marked difference between the dependence of the catalytic activity of zeolites in ethylene aromatization and in methanol conversion on the aluminium content in the zeolite crystals could also possibly be related to the different concentration of water vapour present in the reaction mixtures. To check this suggestion, ethylene aromatization was performed using a model helium/ethylene reaction mixture containing water vapour at a partial pressure of 55 torr. The data obtained (Fig. 4) show that such addition of water has no effect on the rate of ethylene aromatization over ZSM-5 zeolites at 370 “CL Discussion In the present work the effective rate- constants for hydrocarbon synthesis from methanol over a series of highly siliceous ZSM-5 type zeolites of different Si02/A120, mole ratios have been determined. Employing a general scheme for methanol conversion, three reaction routes have been considered. The first is methanol consumption relative to the sum of hydrocarbon products formed, the rate for which depends on the methanol (and dimethyl ether) concentration. The corresponding rate constant provides a quantitative characterization principally of the formation of the sum of olefins, including the various stages of olefin alkylation by methanol to ethers and dehydration of the latter to higher olefins [14 - 161. The second route is the formation of ‘the sum of aromatic hydrocarbons from ethylene, which includes oligomerization and cracking as the main stages [Z, 171. The rate over this route is regarded as being dependent upon the ethylene concentration, the corresponding rate constant providing a quantitative characterization of the aromatization reaction, The rates of formation of paraffins have not been considered in the present work, since syn-

262

thesis of these hydrocarbons proceeds strictly in accordance with the known stoichiometry [17,X3]. Finally, the third route is the formation of the sum of aromatic hydrocarbons from methanol, whose rate depends upon the ethylene concentration in the reaction mixture. As indicated by the general scheme, a definitive interpretation cannot be provided by the corresponding rate constant, PC,,,, for this route since the latter includes steps from both the first and the second routes. As expected, our results show that an increase in the aluminium content of the zeolites leads to an increase in the effective rate constants for all the routes under consideration. However, the precise nature of the changes observed for the various constants differs as the chemical composition of the zeolites changes, with non-linear dependences being apparent for all the cases studied (Tables 2 - 4). Suggestions made previously in the literature lead to the conclusion that bridging OH groups generated in zeolites at sites of SiOt- * A1045substitution can act as catalytically active centres in hydrocarbon synthesis from methanol. If this is so, a linear relationship should exist between the concentration of such catalytically active centers and the aluminium content of the zeolites employed. For this reason we have related each of the three effective rate constants determined in this work (Tables 2 - 4) to 1 g atom of aluminium in each of the zeolites studied, taking into account the chemical composition of the individual samples (Table 1). The resulting values of the standardized effective rate constants (K*) in relation to the chemical composition of the zeolites employed are plotted in Fig. 5. An increase in the Si02/A1203 mole ratio in the zeolites results, on the one hand, in an increase of approximately four times in the st~dardized rate constant for methanol consumption in

I ,

1.0

2.0 log

3.0

LSi01/A120J,d

Fig. 5. Dependence of the standardized rate constants (K*) for (1) methanol consumption during conversion to hydrocarbons, (2) for the formation of C6 - Cre aromatic hydrocarbons from methanol and (3) for the formation of C6 - Cte aromatic hydrocarbons from ethylene C6 - Cre on the chemical composition of highly siliceous zeolites at areaction temperature of 370 + 3 “C.

263

the conversion to hydrocarbons (KG, curve 1), and, on the other hand, in a decrease of more than two orders in magnitude in the standardized rate constant for ethylene aromatization (K&lC2Hq, curve 3). As the SiO,/ A1203 ratio increases, the standardized rate constant for the formation of aromatic hydrocarbons from methanol (Ki,,,) decreases by more than one order of magnitude (curve 2). Finally, a more complicated change in the character of KT\,,, in comparison with K& and K&,C2H4 occws as the Si0,/A1203 ratio increases; at SiOz/Al,O, ratios less than 100 the values of C&M and Klfir~~,n,_,are similar but they increasingly differ as the SiOZ/A120s ratio exceeds 100. These experimental observations indicate that two different types of catalytically active zeolite centres ‘participate in methanol transformation. In the first route, the concentration (or strength) of these centres increases noticeably with increasing Si02/A120s ratio in the zeolite crystals (Fig. 5, curve l), while that of centres participating in the second and the third routes rapidly decreases (Fig. 5, curves 2,3) with an increase in the same ratio. To obtain a better insight into the nature of the changes in catalytic activity of highly siliceous zeolites as their composition is changed, let us consider some aspects in the description of acid centres in these catalysts. Following the views of Haag [ 21, an acid centre may be approximated by 0 O\ or [ O&C-&-A10,]H6 +. a bridging OH group depicted as 0-Si--&-A& ‘0 [ 0’ However, because of the statistical irregularity in the distribution of aluminium over the zeolite framework, an alternative representation of the centre model may be [(SiO),_ nSi(OAl),]“6-(H*+)n (0 < n < 4). Quantitative estimations of this centre model [ 191 have shown that increasing values of n lead to a decrease in the degree of protonization (6+) of the H atom with the simultaneous growth in the energy of proton abstraction, which corresponds to a decrease in the acidity of the centre. As established previously [13, 201, the average statistical value of n for a given centre decreases as the SiOz/A120s molar ratio in the zeolite crystals increases. Hence, it is not unreasonable to expect an increase in the average acid strength of a given centre with increasing Si02/A.120, molar ratio, because of the presence in zeolites of several types of bridging OH groups whose acidity is dependent on the value of n. Does this allow the possibility of accounting for the non-linear change in the catalytic properties of zeolites when their chem;cal composition is varied? To answer this question, let us assume that the first route for methanol conversion occurs, for example, mainly through centres of the type [(SiO)~Si(OAl)]~-(Hi’) (n = l), ie. on the strongest centres, whose proportion increases as the Si0,/A120, molar ratio increases (Fig. 5, curve 1). The second route for the conversion (i.e. the formation of aromatics and paraffins) will take place mainly on centres of the type [(SiO)Si(OA1)s]3s-(H”), (n = 3), ie. on the weakest centres, whose proportion rapidly de-

1

264

creases ‘with increasing SiOZ/AIZO, ratio (Fig. 5, curve 3). But these two types of acid eentre are basically similar in nature. Hence, the characteristics of reactions which proceed via their p~icipation and the types of equations necessary to describe the corresponding reaction rates must be the same for both centres. However, aromatic hydrocarbons affect the rate of the first route (Table 2), but do not influence that of the second (Table 4). If bridging OH groups are the centres for the consumptibn of methanol during conversion to olefins, the inhibiting effect of aromatics on the rate of this route may be explained by the decrease in the proportion of these centres available for adsorption and further conversion of methanol and ethers, as a result of the strong competitive adsorption of aromatic hydrocarbons. However, it is also possible that such bridging OH groups should not be considered as centres for ethylene aromatization, since if this were so the inhibiting effect of aromatics found for methanol conversion would also have been observed for aromatization reactions involving the less basic ethylene. Hence, from the above it is possible to conclude that centres of a different nature, whose concentration (or strength) decreases rapidly with increasing SiOz/A1,03 ratio, participate in ethylene aromatization rather than OH groups. The appearance of centres with an alternative nature in zeolites is assumed [7,8, lo] to result from a partial loss of aluminium from the zeolite framework during thermochemical treatment. As found from measurements of the heat of adsorption of carbon monoxide [lo], such centres are strongly aprotic, their appearance probability depending on the composition of the second coordination sphere of the silicon atom in the zeolite framework (Le. on the value of IZ in the above centre model) [ 121. Hence, the portion of aluminium participating in the formation of aprotic centres in zeolites can be anticipated as decreasing rapidly with increasing Si0,/A1,03 ratio. This has been observed in the series of zeolites investigated in the present work through the use of “Al NMR techniques at high resolution [ 111. Containing this line of reasoning, it follows that it is more correct to represent the structure of the acid centre in zeolites as [(SiO)+ .Si(O(0 G m < n Q 4), where an increase in the Al) ]‘+(H’+) _ (A136+) SiO”,/A120s mtlemratio in”ge zeolite framework corresponds to a decrease in the average values of IZ and m. On this basis, A13+ cations emerging from the zeolite framework should also compensate for the excess negative charge in the structure. Hence, an increase in the Si01/A120j ratio in zeolites may not only reinforce the protic eentres (due to a decrease in the average value of n), but also increase their concentration per g atom of ~uminium due to a rapid decrease in the proportion of aluminium in cationic extralattice positions. Both these factors act in the same direction to account for the increase of K& with increasing SiOz/Al,03 ratio (Fig. 5, curve 1). At the same time, a rapid decrease in K&ZHJ with increasing SiOz/A120s ratio also occurs as a result of the rapid decrease in the concentration of aprotic centres in the system (Fig. 5, curve 3).

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Provided that the above suggestions regarding the acid centre model are valid, it follows that there should be little difference between the magnitudes of the standardized effective rate constants for the formation of aromatic hydrocarbons from ethylene and from methanol (i.e. K&,c2n4 and Ki,,,). However, such similar values are only observed for zeolites with rather low SiOJAl,O, ratios (Fig.’ 5, curves 2 and 3). As this ratio increases, greater differences appear between the ma~itudes of the aromatization constants mentioned. These differences, however, are probably more apparent than real. In fact, it is in zeolites with a rather low SiOz/A120s ratio that one would expect the greatest proportion of extralattice aluminium. Hence, such zeolites will be characterized by a reduced proton acidity and, simultaneously, by a relatively high concentration of aprotic centres participating in olefin aromatization. Accordingly, over such a range of SiOz/A1,Os ratios, the difference between the rate of olefin formation from methanol and in the rate of consumption of the latter should be least. Actually (Fig. 6, curve l), an increasing content of ~uminium in the zeolites

Fig. 6. Dependence of the selectivity of methanol conversion to C2 - C5 olefins (curve 1) and of the composition of the Cz - C4 olefin fraction (curve 2, ethylene; curve 3, propylene; curve 4, butylenes) on the chemical composition of highly siliceous zeolites. Reaction temperature, 370 “C; contact time, 1.0 5; initial methanol concentration, 20 vol.%; degree of methanol conversion, 85 - 95%.

leads to a rapid decrease in the selectivity towards the sum of olefins. Yet, the participation of CZ - C4 olefins in secondary aromatization reactions depends strongly on their reactivity. As reported previously [21 J, the reactivity of olefins increases rapidly in the sequence C,H, < CaH, < C&Is. Hence, in this case, an increase in the overall aluminium content of the zeolites will result not only in an increase in the rate of aromatization but also in a redistribution of the olefin fraction. An analysis of the compo-

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sition of the Cz - C4 olefin fraction (Fig. 6, curves 2 - 4), obtained through an examination of the series of zeolites under the same experimental conditions, reveals that an increase in the aluminium content of these zeolites leads to an increase in the ethylene fraction of the olefin product and, simultaneously, to a decrease in the fractions of both propylene and butylenes in the product. In accordance with this observation, a comparison of the rate of formation of the sum of aromatic hydrocarbons from methanol with the steady-state concentration of ethylene alone (as was undertaken in the present work (Table 3)) is insufficient to characterize the aromatization constant Isli,,, and will lead to values of this constant which are too high (Fig. 5, curve 2). A more correct equation for the rate of formation of the sum of aromatic hydroc~bons from methanol appears to be one that includes a function expressing the composition of all the olefin fractions found in the reaction mixture. Since for zeolites with a reasonably high content of aluminium this fraction consists mainly of ethylene (Fig. 6), such a description of the rate of methanol aromatization on these zeolites is more accurate, and the values of KRIC2u4 and Ri,,, resulting from its application are found to be quite close.

Conclusions Investigations of the rate and selectivity of hydrocarbon synthesis from methanol on ZSM-5 type zeolites with SiOzfAIzOs mole ratios in the range 24 - 540 lead to the conclusion that at least two different types of centres with an acid-base character exist in such catalysts. By assuming that methanol conversion on zeolites is inhibited by the products, i.e. by aromatic hydrocarbons, it is possible to analyze the group rate constants for the different routes involved in the process on the basis of the chemical composition of the catalysts. These dependencies exhibit non-linear characteristics. Fu~he~ore, on a g atom aluminium basis the rate constant for the consumption of methanol increases considerably, while that for the formation of the sum of aromatic hydrocarbons decreases rapidly with increasing Si02/A1,03 molar ratio in the zeolite crystals. From this it is concluded that the description of acid-base centres in zeolites as bridging OH groups is only a first approximation, which is quite insufficient for understanding the overall data for the catalytic properties of these systems. In this work we have considered the catalytic action of zeolites in terms of an active centre model, which includes the [(SiO),_ RSi(OA1),]“S-(H6+) _ (A136+)m,3 group of atoms, where the parameters II and m are functronr of the total aluminium content in the zeolites. These functions are of a non-linear nature as a result of the statistical irregularity of aluminium d~tribution over the crystal, and they determine the activity and selectivity of zeolite catalyst action. Thus, with increasing total alum~i~ content in the zeolites a noticeable decrease occurs in the standardized

267

constant for the first step for hydrocarbon synthesis from methanol, i.e. the constant relating to the methanol consumption during conversion to olefins expressed on a g atom aluminium basis. This decrease appears to arise from a decrease in both the strength and the concentration of bridging OH groups (the latter also being referred to on a g atom aluminium basis). Simultaneously, an increase in the aluminium content results in a rapid increase in the magnitude of the standardized constant for the second step of the process in question, i.e. that for olefin aromatization, which occurs because of the considerable increase of the aluminium concentration in cation extralattice positions. Hence, the acid-base catalytic action of zeolites of varying chemical composition is determined not only by the concentration, but also by the distribution, of aluminium over the zeolite crystal, a fact that may be predicted using the active centre model considered in this work.

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