Catalytic and acid properties of pentasil zeolites: Isomerization of o-xylene and alkylation of toluene

Catalytic and acid properties of pentasil zeolites: Isomerization of o-xylene and alkylation of toluene

Catalytic and acid properties of pentasil zeolites: Isomerization of o-xylene and alkylation of toluene J. Datka and Z. Piwowarska Faculty of Chemist...

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Catalytic and acid properties of pentasil zeolites: Isomerization of o-xylene and alkylation of toluene J. Datka and Z. Piwowarska

Faculty of Chemistry, Jagiellonian University, Karasia 3, 30-060, Krakow, Poland and

J. Rakoczy and B. Sulikowski

Institute of Organic Chemistry and Technology, Technical University, Warszawska 24, 31-155 Krakow, Poland (Received 15 June 1987) Acid and catalytic properties of H,NaZSM-5 (Si/AI = 47) zeolites of various exchange degrees (0-98%) were studied. H,NaZSM-5 zeolites contain Lewis acid sites and two kinds of Br6nsted sites (strong sites: 3609 cm -~ OH groups, and weak sites: not being those OH groups). The concentration of all these sites w a s determined. W e a k Br6nsted sites are inactive in all the reactions studied. In the zeolites of exchange degrees higher than 11%, the conversion increases linearly with the number of 3609 cm -~ OH groups. Keywords: ZSM-5 zeolite; acidity; i.r. spectroscopy; toluene alkylation; o-xylene isomerization

INTRODUCTION Zeolite ZSM-5, a well-known member of ~he pentasil zeolite family, exhibits unique catalytic properties, which have been studied extensively during the past years 1-8 in several reactions involving low alcohol conversion, alkylation of toluene, isomerization of xylenes, and cumene cracking. ZSM-5 modified by, for example, phosphorus, magnesium, or boron compounds, shows a marked shape-selectivity in xylene isomerization, toluene disproportionation, or alkylation. ZSM-5 zeolite is used commercially as the isomerization catalyst in many p'ara-xylene plants worldwide. 9 Hence, the relationship between acid and catalytic behavior of this particular zeolite is of great interest. One problem that may arise is whether catalytic activity of ZSM-5 zeolites of different proton exchange degrees strictly follows the concentration of the Br6nsted acid sites. A linear relationship between activity and the number of acid centers in ZSM-5 zeolite has been reported, ~°-xs which suggests that the efficiency of all of the acid sites is the same. However, the variation of the strength of acid sites in H,NaZSM-5 zeolites with the exchange degree and also the heterogeneous acid sites distribution have been observed by other authors. H-18 In this paper we present a systematic study of acid and catalytic properties of ZSM-5 zeolite (Si/AI = 47) with a wide range of proton concentrations. Isomerization of 0-xylene and alkylation of toluene with methanol

© 1988 Butterworth Publishers

were chosen as the test reactions, both of them known as proceeding on the Br6nsted acid sites. 1'1'~ The strength of the acid centers and their concentration were studied by i.r. spectroscopy.

EXPERIMENTAL ZSM-5 zeolite was prepared at the Institute of Industrial Chemistry (Warsaw). The zeolite sample, with a Si/A1 ratio of 47 and Na/AI ratio of 0.81, was highly crystalline to X-rays. Ammonium nitrate solutions of various concentrations were used in order to obtain the NH.I,NaZSM-5 samples listed ill Table I. For catalytic purposes, zeolite powder was pressed without binder to avoid possible solid-state reactions between zeolite and a carrier. 2° The pellets were crushed and sieved. The measurements were carried out in a pulse microreactor working on-line with a GCHF 18.3 gas chromatograph equipped with a flame ionization detector and a CI-100 cnmputing integrator. The reaction products were analyzed using a 3 m column packed with Bentone-34, didecyl phtalate, and silicon oil A on Chromosorb W; further details have been described elsewhere. '-'l T h e stainless-steel electrically heated microreactor was packed with 0.05~ of 0.2-0.3-mm catalyst grains diluted with 0.5 cm" of 0.2-0.3-mm quartz. Additional amounts of quartz were loaded below and above the catalyst charge, the total volume being ca. 3 cm :~. Before the catalytic runs, all the zeolites were preheated in a dry helium stream at 773 K for 2 h.

ZEOLITES, 1988, Vol 8, May 199

Catalytic and acid properties of pentasil zeolites: J. Datka et al. Table 1

Characteristics of ZSM-5 zeolites (Si/AI = 47)

Zeolite NaZSM-5 0.11 NH4,NaZSM-5 0.34 NH4,NaZSM-5 0.39 NHa, NaZSM-5 0.68 NH4,NaZSM-5 0.98 NH4,NaZSM-5

NH~ exchange degree (%)

Idealized u.c. composition

B + 2L/u.c. a

AI - Na/u.c. b

0 11 34 39 68 98

N a 1.gHS.2(AIO2)z dSiO2)s3.s Nal.THo.2(NH4)o.2(AIO2)2.~(SiO2)93.9 Nal.3Ho.2(N H4)o,s(AIO~)zl (SiO2)sz.s N a~.~Ho.~(N H4)o,7(AIO2)z~ (SiO~)93.9 Nao.eHo.2(N H4)1,3(AIOz)z~(SiO~)93.9 Nao.ozHo.=(N H4h.a7(AIO~)z.dSiO2)es.9

0.33 0.39 0.71 0.73 1.33 1.82

0.2 0.4 0.8 0.9 1.5 2.1

a Concentration of Br6nsted acid sites + 2 x concentration of Lewis acid sites per u.c. b Sodium deficit (concentration of AI - concentration of Na per u.c.) c Protons, which were formed as a result of the organic template decompositio.n

Reagents were o f analytical grade and were used without f u r t h e r purification. In o r d e r to nlinimize the effect o f coking o f catalysts t h e , , r o u n d - t r i p p r o c e d u r e was applied for catalytic tests.-For the i.r. studies, the zeolites were pressed into thin wafers• T h e wafers were outgassed in situ in the i.r. cell at 773 K for 1 h. T h e i.r. spectra were r e c o r d e d using a Specord 75IR s p e c t r o m e t e r working on-line with a KSR 4100 minicomputer. T h e acid properties o f zeolites were studied by pyridine a d s o r p t i o n - d e s o r p t i o n experiments. Portions o f pyridine were adsorbed at 443 K up to constant intensities o f the 1545-cm -I (PyH +) and 1455-cm-l (PyL) bands. T h e concentrations o f Br6nsted and Lewis acid sites were calculated from the naaximum intensities o f the 1545- and 1455-cm -I bands. T h e c o r r e s p o n d i n g values o f their extinction coefficients were d e t e r m i n e d previously. 23-'-':' It has been shown earlier "5''-'~ that two kinds o f Br6nsted acid sites exist in ZSM-5 zeolites: (i) strong sites d u e to O H groups absorbing at 3609 cm - t , and (ii) weaker protonic sites not being these O H groups. In o r d e r to study the concentration o f both types o f Br6nsted acid centers, the following e x p e r i m e n t a l p r o c e d u r e has been applied. First, small portions o f pyridine were adsorbed up to constant intensity o f the pyridine ions band. Second, the cell was connected to a liquid nitrogen trap and heated to higher t e m p e r a t u r e s (in the range o f 5 0 0 - 8 8 0 K). After each desorption step, the cell was cooled down to 443 K and the i.r. spectrum was recorded. T h e pyridine molecules neutralizing weak Br6nsted acid sites are r e m o v e d at relatively low t e m p e r a t u r e s (the 1545 cm - l PyH + band decreases, while the 3609 cm - l O H band is still not visible). After reaching a certain "limit t e m p e r a t u r e , " the pyridine molecules are r e m o v e d fi'om strong Br6nsted sites (the 3609 cm-J O H band reappears). T h e a m o u n t o f PyH + ions r e m o v e d by desorption befi)re the 3609 c m - l O H band r e a p p e a r s was taken as the a m o u n t o f weak Br6nsted acid sites, and the r e m a i n d e r , as the a m o u n t o f strong sites. T h e e x p e r i m e n t s described above also provided information on the strength o f the 3609 cm -1 O H groups (i.e., strong sites). T h e fi)llowing p r o c e d u r e was applied• T h e intensities o f PyH + band were m e a s u r e d after the desorption at two temperatures: at the t e m p e r a t u r e at which the 3609 c m - t O H band began to r e a p p e a r (A.) and at 850 K ( A ~ ) , respec-

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ZEOLITES, 1988, Vol 8, M a y

tively. T h e values o f integrated extinction coefficients o f the 3609 c m - l O H band were calculated from the intensities o f the O H bands and the c o r r e s p o n d i n g concentrations o f these O H groups (strong Br6nsted acid centres).

RESULTS AND DISCUSSION T h e characteristics o f NH4,NaZSM-5 samples prepared are given in Table 1. All the zeolites differ with the NH,I- exchange degree. T h e p a r e n t material (Si/A1 = 47) was labeled NaZSM-5, despite the fact that a small n u m b e r o f Br6nsted acid sites (0.20 sites per unit cell) were detected by i.r. spectroscopy

(FigTtre 1 ). Acid properties of zeolites T h e concentrations o f Br6nsted and Lewis acid sites d e t e r m i n e d by pyridine adsorption are presented in Figure I, lines a and d. T h e concentration o f Br6nsted acid centers (Fi~lre 1, lines b and c) increases with the exchange degree, that is, with the

o

• 1.5 a

_= b 1.0

.=1

c :0

m 0.5 "

C

c 0 0

0

20

40 60 80 Exchange degree Igl

100

Figure I Concentrations of acid sites in H,NaZSM-5 zeolites as a function of exchange degree; (a) sum of all Br6nsted acid sites; (b) strong Br6nsted acid sites (3609 cm 1 OH groups); (c) weak Br6nsted acid sites; (d) Lewis acid sites

Catalytic and acid properties of pentasil zeolites: J. Datka et al.

amount of protons introduced into the zeolite. The concentration of Lewis acid sites is relatively low in all the samples, indicating, therefore, that the degree of dehydroxylation is low at the activation conditions. The concentration of Lewis sites increases slightly with the exchange degree (i.e., with the concentration of OH groups). This observation supports the earlier conclusion 24'27 that the zeolites of higher hydroxyl content are more dehydroxylated than are the zeolites with lower hydroxyl content. The concentration of acid sites determined by i.r. spectroscopy will now be compared with those calculated from the chemical analysis. If we assume that Lewis sites are f o r m e d by d e h y d r o x y l a t i o n stoichiometrically (i.e., two Br6nsted acid sites form one Lewis site), then the sum (B + 2L) (where B and L denote the concentrations of Br6nsted and Lewis sites, respectively) represents the concentration of protonic sites after the decomposition of ammonium ions but before the dehydroxylation. The calculated values (B + 2L) are given in Table 1. These data are comparable to the theoretical content of protons, calculated as the difference between aluminium and sodium content in zeolites. The acid strength of 3609 cm -l OH groups was studied by pyridine thermodesorption and by the comparison of the values of integrated extinction coefficients (IEC). The corresponding data visualized in Figure 2 suggest that the acid strength of the 3609 c m - ' OH groups increases with the proton exchange degree. The ratio A850/A0 represents the fraction of 3609 cm-l OH groups that still hold pyridine afier

0.~ o

\

8 0.4 I

20

,

q

t

40 60 80 Exchange degree [g)

I

100

2.5

o 2.0 I,LI

"1.5

I

20

[

I

I

40 60 80 Exchange degree [~}

I

100

2 Acid strength of 3609 cm -~ OH groups in H,NaZSM-5 zeolites; (a) Asso/Ao; (b) integrated extinction coefficient (IEC) as a function of e x c h a n g e degree Figure

the desorption at 850 K. This ratio is therefl>re a measure of the strength of the 3609 cm- l OH groups and is depicted in Figure 2A. Similar effects have also been observed in other series of pentasil zeolites and have been discussed in the terms of a collective model of zeolites acidity.'-'8 According to this model, the variation of the acid strength of OH groups upon the proton exchange degree should be, m practice, negligible because of a minor change in the average electronegativity in the highly siliceous H,NaZSM-5 zeolite. It seems that increase of the acid strength of 3609 cm-I OH groups, higher than expected from the collective model, can be interpreted as the consequence of nonhomogenous AI distribution in ZSM-5 zeolite crystals and the existence of domains of lower and higher AI content (cf. Ref. 28). Inside the domains of higher AI content, the zeolite does not behave as "highly siliceous"; it cannot be excluded that the distance between AI atoms is short enough and that the Na+-H + exchange gives rise to the polarization of neighboring OH groups to the extent that is experimentally observed (Figure 2). Another possible explanation assumes the heterogeneity of OH groups in H,NaZSM-5 zeolites. Thus, the protons first introduced into the zeolite react with more basic oxygen atoms and form less acidic OH groups than the protons introduced later (at higher exchange degrees).

Catalytic properties of zeolites The catalytic activity of the H,NaZSM-5 zeolites was tested in two reactions: alkylation of toluene with methanol and isomerization of 0-xylene. Under the conditions used for studying the toluene methylation, methanol undergoes two reactions: alkylation to xylenes and dehydration to dimethyl ether and olefines. Figure 3 shows the conversion of methanol in both reactions as a function of tbe exchange degree of zeolite. The parent NaZSM-5 and 0.11 H,NaZSM5 zeolites are inactive in dehydration and alkylation at the reaction temperature (593 K). According to the results of acidity studies (Figures 1 and 2), these samples contain weak Br6nsted acid sites and some amount of 3609-cm -I OH groups of low acid strength. It can be concluded that these sites are too weak to catalyse alkylation and dehydratation at this temperature. The increase of the exchange degree of H,NaZSM-5 zeolites results in a sharp rise in their catalytic activity. In all the zeolites, the conversion in methanol dehydration is much higher than in toluene alkylation. The strongest difference between those two methanol conversion pathways is observed in the case of zeolite 0.39 H,NaZSM-5, which is characterized by a relatively low strength of the 3609 cm -1 OH groups. Hence, it is clear that methanol dehydration requires weaker acid sites than does toluene alkylation. This conclusion is in agreement with previous findings.'-' Figures 4 and 5 show the resuhs of catalytic tests in toluene alkylation with methanol and o-xylene isomerization. The fractional conversion is presented

ZEOLITES, 1988, Vol 8, May 201

Catalytic and acid properties of pentasil zeolites: J. Datka et al.

t.o b

and toluene alkylation. Weak Br6nsted acid sites were also found to be inactive; the conversion in both reactions was therefore compared with the concentration of 3609 cm -~ OH groups (Figure 6). Linear correlations were obtained. The lines, however, do not cross the origin of coordinates. The 3609 cm - I O H g r o u p s in N a Z S M - 5 a n d 0.11 H,NaZSM-5 zeolites are inactive in both reactions. They were also inactive in methanol dehydration (Figure 3). The resuhs of the experiments with pyridine thermodesorption and the comparison of the extinction coefficients of 3609 cm -1 OH band (Figure 2) show that the acid strength of these OH groups increases with the exchange degree of H,NaZSM-5 zeolites. In 0.11 H,NaZSM-5, the acid strength of 3609 cm- l OH groups is the lowest: they are too weak to catalyse all the reactions investigated in the present study. In the case of zeolites of exchange degree higher than 1 I%, the conversion increases linearly with the amount of 3609 cm-~ OH groups despite that their acid strength increases (Figure 2). The linear relationships between the catalytic activity and the number of acid sites in ZSM-5 zeolites were also observed by other authors. ~°-z3 It seems that all these results can be readily interpreted assuming (according to Post and Van H o o f ~) the existence of a threshold level for the acid strength of the OH groups: above this level, the acid strength does not anymore influence the catalytic activity of zeolites. The results obtained in the present study suggest that this threshold level is "localized" between the zeolite exchange degree of 11 and 39%.

A



I.

_ Q8

>=

as o

~- o.4

02

I

0

20

I

I

I

40 60 Exchange degree

I

I

l

I

80

100

It]

Figure3 Fractional conversion of methanol and toluene on H,NaZSM-5 zeolites at 593 K as a function of exchange degree; (0), total conversion of methanol; (O), dehydratation of methanol; (L~), conversion of toluene in alkylation

as a function of the reaction temperature. The parent NaZSM-5 zeolite is practically inactive in both reactions. The increase of exchange degree results in the rise of catalytic activity, the highest activity is observed for 0.98 H,NaZSM-5. H,NaZSM-5 zeolites contain Br6nsted acid sites (3609 cm -I OH groups and weak sites) and Lewis acid sites as well. There is no evidence that Lewis acid sites play any catalytic role in xylene isomerization

CONCLUSIONS Lewis acid sites and two kinds of Br6nsted acid sites exist in H,NaZSM-5 zeolites: 3609 cm -l OH groups and weaker acid sites--not being those OH groups. Their concentrations were determined by i.r. spectroscopy.

0.3

A s ~.,~'0~#" 8 0.1 0

f

,

I/"

......

.

,x

.....

573 Temperature

, 673 (K]

"

. ' D ...... D . . . . a - - . - D 773

Figure4 Fractional conversion of toluene to xylenes on H,NaZSM-5 zeolites as a function of temperature; (n), NaZSM-5; (X), 0.11 H,NaZSM-5; (0), 0.39 H,NaZSM-5; (©), 0.68 H,NaZSM-5; (&), 0.98 H,NaZSM-5

202

ZEOLITES, 1988, Vol 8, May

Catalytic and acid properties of pentasil zeolites: J. Datka et al.

0.4

0.3

O

0.2

i

X

0.1

I

II~'-

I

,

573

673 Temperature

773

[K }

Figure 5 Fractional conversion of o-xylene on H,NaZSM-5 zeolites as a function of temperature; graphic representations of zeolites as in Figure 4

In all the ZSM-5 studied, the majority of Br6nsted sites are strong sites (3609 cm -l OH groups). The contribution of weak Br6nsted sites and of Lewis acid sites is relatively low. The acid strength of 3609 cm-1 OH groups increases with the exchange degree of ZSM-5 zeolite. Weak Br6nsted acid sites and Lewis sites are inactive in methanol dehydration, alkylation of

toluene with methanol, and 0-xylene isomerization; the active sites are the 3609 c m - " OH groups. In the parent NaZSM-5 and 0. I 1 H,NaZSM-5 zeolites, their acid strength, however, is insufficient to catalyse all the reactions studied at the conditions used. The existence of a threshold level for the acid strength of the 3609 cm -] OH groups "localized" between the ammonium exchange degree of 11 and 39% is suggested. Above this level, the increase in acid strength of the 3609 cm- 1 OH groups does not affect the catalytic efficiency and the catalytic activity of ZSM-5 zeolites is determined only by the concentration of the acid centers.

ACKNOWLEDGEMENT

O -Q t-

J. R. and B. S. acknowledge with gratitude the support of the Committee of Chemical Sciences, Polish Academy of Sciences, Warsaw.

O

o.1o 0

"

=

REFERENCES

//-t-

.o = o

ao6

I

I

I

J

I

0,4 0.8 1.2 Concentration of strong Br6nsted sites/u.c. Figure6

Relationship between the fractional conversions of

(0) o-xylene or (+) toluene and

concentration

of

strong

Br6nsted acid sites (3609 cm -1 OH groups) per u.c. in H,NaZSM5. zeolites at 593 K

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203

Catalytic and acid properties of pentasil zeolites: J. Datka et al.

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20 Fyte, C. A. et al. J. Am. Chem. Soc. 1986, 108, 522 21 Sulikowski, B. Czasopismo Techn. 1976, 195, 33 22 Matsumoto, H., Take, J. I., and Yoneda, Y. J. Catal. 1968, 11, 211 23 Datka, J. J. Chem. Soc., Faraday Trans. / 1980, 76, 705 24 Datka, J. J. Chem. Soc., Faraday Trans. I 1981, 77, 2877 25 Datka, J. and Tu~nik, E. J. Catal. 1986, 102, 43 26 Datka, J. and Piwowarska, Z., submitted for publication 27 Hoffman, J., Hunger, B., and Schliebs, R. Zeolites 1984, 4, 235 28 Datka, J. and Tu~nik, E., in Proceedings of the/nternationa/ Symposium on Zeolites Cata/ysis, Sibfok, Hungary, May 13-16, 1985, p. 173