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Shape selectivity of hydrothermally treated H-ZSM-5 in toluene disproportionation and xylene isomerization
Applied Catalysis, 57 (1990) 167-177 Elsevier Science Publishers B.V., Amsterdam -
167 Printed in The Netherlands
Shape Selectivity of Hydrothermally Treated H-ZSM-5 in Toluene Disproportionation and Xylene Isomerization U. KURSCHNER, H.-G. JERSCHKEWITZ, Zentralinstitutfiir
Physikalische
(German Democratic
E. SCHREIER and J. VOLTER
Chemie, Akademie
der Wissenschaften
der DDR, Berlin,
1199
Republic)
(Received 22 March 1989, revised manuscript received 10 August 1989)
ABSTRACT Controlled hydrothermal treatment of H-ZSM-5 at 500°C causes a transformation of framework into non-framework aluminium, as indicated by means of cation exchange and IR spectroscopy. After treatment with nitric acid, partial extraction of non-framework aluminium was observed. The non-framework aluminium-containing samples showed an enhanced p-xylene selectivity in the catalytic disproportionation of toluene and in the isomerization of m-xylene. The subsequently extracted samples had lost this property. A model of the decisive role of nonframework aluminium in shape selectivity is discussed.
INTRODUCTION
Selectivity is a major challenge in the catalytic conversion of hydrocarbons. H-ZSM-5, a pentasil-type zeolite, has provided unique possibilities of shape selectivity. This has led to the development and commercial use of several novel processes in the petrochemical industry [ 11, including the disproportionation of toluene and the isomerization of xylene. These are acid-catalysed reactions and the activity of H-ZSM-5 is due to its strong acid sites. Direct correlations of these sites with the activity in the disproportionation [2-51 and in the isomerizations [ 61 have been established. Still more interesting is the shape selectivity. p-Xylene is a valuable intermediate in the production of polyester fibres and its selective formation is of both practical and theoretical interest. Usually mixtures of isomers are obtained, containing only 24% ofp-xylene, according to the thermodynamic equilibrium. The dimensions of the channels in H-ZSM-5 show diameters very close to those of the benzene molecule and the selectivity feature can be readily understood from an interplay of catalytic reaction with mass transfer, severly discriminating against large molecules [ 11. Larger crystals of H-ZSM-5 with their long diffusion path induce increased
0166.9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
168
selectivity for p-xylene [ 1,7]. A variety of different reagents have been successfully used as modifiers of H-ZSM-5 in order to induce para-selectivity, including compounds of phosphorus [ 11, magnesium [ 11, silicon [ 81 and antimony [ 11. Recently we observed that thermal treatment of H-ZSM-5 enhances the shape selectivity and it was suggested that thermally formed non-framework aluminium species could be the reason [ 5,9]. This study was aimed at elucidating the relationship between shape selectivity and non-framework aluminium. Samples containing well determined amounts of framework and nonframework aluminium were prepared by hydrothermal treatment and expected consequences with regard to shape selectivity were studied for the disproportionation of toluene and for the isomerization of m-xylene. EXPERIMENTAL
The parent ZSM-&type zeolite prepared without any template was obtained from VEB Chemiekombinat Bitterfeld (G.D.R.). The silicon-to-aluminium atomic ratio of the starting material was 15. It was converted into the ammonium form by four successive exchanges using 0.5 it4 ammonium nitrate solution at 80” C. The dealumination of the zeolite framework was achieved by treating the sample in flow of water vapour at 500°C. In order to vary the degree of dealumination the exposure was varied from 0.25 to 24 h. Refluxing of dealuminated samples with 1 M nitric acid resulted in partial removal of the non-framework aluminium. Applying higher concentrations of acid and longer extraction times did not cause more complete removal. In order to determine framework aluminium and consequently the degree of dealumination, the samples were exchanged for ammonium ions again. The amount of ammonia back-exchanged with 0.2 A4 ammonium acetate solution was measured by the Kjeldahl method. The comparison of these results with the temperature-programmed desorption of ammonia showed good quantitative agreement [lo]. For IR spectroscopic studies, the zeolites were pressed into thin wafers (48 mg/cm2 ) . The wafers were fixed into the IR cell and preheated under vacuum at 400 oC for 1 h. After cooling to room temperature, pyridine was adsorbed at 10 Torr (1 Torr= 133.322 Pa) and then thermally desorbed at temperatures from 100 to 400°C. The intensity of the 1550 cm-l pyridinium ion band after desorption at 200°C related to the weight of the wafer is a measure of the concentration of Brsnsted sites [ 11-131. The IR spectra were recorded on a UR 20 spectrometer. The catalytic reactions, the disproportionation of toluene and the isomerization of m-xylene were performed in a fixed-bed glass apparatus with a continuous flow at atmospheric pressure. The reaction products were analysed with an on-line gas chromatograph equipped with a flame ionization detector.
169
A 2-m Bentone 34 column was found to be adequate for performing the necessary analytical separations. Prior to reaction, the catalysts were calcined first in an air flow and then in a nitrogen flow at 500°C. The feed for catalytic reactions was prepared by saturation of a nitrogen stream with the appropriate hydrocarbon at 17 oC. Toluene disproportionation was carried out using 0.5 g of catalyst at 350°C and a flow-rate of 1 l/h. The reaction temperature for the m-xylene isomerization was 250’ C in order to avoid the formation of by-products. The amount of catalyst was varied between 0.02 and 0.5 g and the flow-rate between 0.5 and 2.5 l/h in order to achieve different conversions in the desired range. RESULTS
Dealumination H-ZSM-5 samples were dealuminated by steaming at 500” C. The remaining framework aluminium was determined by measuring the NH,+ ion-exchange capacity. This capacity indicates the number of Bronsted sites [ 14-161. As previously suggested, Bronsted sites in zeolites are assigned to bridging OH groups and these are directly related to framework aluminium [ 14,17-201. Therefore, the ion-exchange capacity was taken as a direct measure of the framework aluminium content. Knowing the total aluminium content from chemical analysis, the non-framework aluminium content was calculated as the difference of total and framework aluminium. The dependence of the concentrations of framework and non-framework aluminium on the time of hydrothermal treatment is shown in Fig. 1. All samples are derived from the same parent H-ZSM-5 and differ only in the duration of hydrothermal treatment at 500” C. Results for the hydrothermally treated H-ZSM-5 and for the samples subr
TIME
OF HYDROTHERMAL
Fig. 1. Aluminium
TREATMENT
distribution
Ih
in H-ZSM-5
as a function
of hydrothermal
treatment
at 500 ‘C.
170 TABLE 1 Framework, non-framework and extracted aluminium after hydrothermal treatment and after subsequent extraction with 1 M nitric acid Time of Al after hydrothermal Al after subsequent HNO, treatment hydrothermal treatment treatment Framework Non-framework Framework Non-framework Extracted Extracted (h) AI/u.c. AI/u.c. AI/u.c. AI/u.c. Al AI/u.c. (%o) 0.6 2.9 3.5 3.7 4.2 4.7 4.9 5.1
5.1 2.8 2.2 2.0 1.5 1.0 0.8 0.6
0
0.25 0.5 1.5 3.0 6.0 12.0 24.0
r
,
,
1
2
FRAMEWORK
3
,
,
4
5
ALUMINIUM
/
_ 2.0 1.5 1.0 0.8 0.6
2.7 2.3 1.8 1.6 1.5
1.0 1.9 2.9 3.3 3.0
27 45 62 67 59
U. C.
Fig. 2. Correlation between extinction of the IR band of pyridine at 1550 cm-i and framework aluminium per unit cell ( U.C.), determined by cation exchange.
sequently extracted with nitric acid are listed in Table 1. The greatest decrease in framework aluminium content was observed after only a short time. The degree of extraction of non-framework aluminium was found to increase with increasing time of hydrothermal treatment. Further investigations of framework dealumination were carried out by chemisorption of pyridine followed by recording of the IR spectra. In these zeolites the interaction of pyridine with the Brranstedsites of the ZSM-5 causes the disappearence of the IR band at 3610 cm-’ and the appearance of a band at 1550 cm-‘, which is assigned to pyridine adsorbed on Bronsted sites. As the results in Fig. 2 illustrate, the intensity of the IR band related to the weight of sample corresponds to the amount of framework aluminium measured by the ion-exchange capacity. All the results show good agreement between the hy-
171
drothermal dealumination and the acidity determined by two independent methods. Shape selectivity Studies of the catalytic disproportionation of toluene indicate a considerable influence of the non-framework aluminium content on the xylene distribution in the product. As shown in Fig. 3 (and also Fig. 5) the p-xylene selectivity increases with increasing non-framework aluminium content of ZSM-5. The high non-framework aluminium values are attributed to samples having been exposed longer to hydrothermal treatment (see Table 1) . The open circles in Fig. 3 represent values of samples extracted with nitric acid after hydrothermal treatment. Results of chemical analysis show that all the extracted zeolites have a low non-framework aluminium content. The p-xylene selectivities of these samples approach the equilibrium composition of p-xylene. Considering the corresponding pairs of samples with the same hydrothermal treatment but with and without extraction, it is obvious that the extraction causes a decrease in the non-framework aluminium content and also a decrease in the p-xylene selectivity. In Fig. 4, the activity andp-xylene selectivity of catalysts in toluene disproportionation are shown as a function of the framework aluminium content. All catalysts were used after extraction of the non-framework Al with nitric acid. The results indicate that a higher activity, expressed as the total amount of xylene formed, corresponds to larger amounts of framework aluminium. This direct correlation between acidity and conversion is in accordance with previous results [ 2-61. Although the framework aluminium content and as a consequence the catalytic activity differed by nearly one order of magnitude, the p-xylene selectivity was constant, remaining close to the equilibrium. It is thus demonstrated that under these conditions of low conversions there is no correlation between activity and shape selectivity.
3 1
2
3 NON
4 FRAMEWOAK
5 ALUMINIUM
/UC.
Fig. 3. Para-selectivity in disproportionation of toluene on dealuminated H-ZSM-5 before and after extraction of non-framework aluminium. The time of hydrothermal treatment is indicated in hours.
172
PARA
SELECTIVITY
I
I
2
1
FRAMEWORK
6
5
.4
3
ALUMINIUM
/
U.C.
Fig. 4. Conversion and selectivity for disproportionation of toluene on H-ZSM-5 after dealumination and extraction.
660 NOT
L
b
EXTRACTED EXTRACTED
LO
t
I
I
0.5
1.0 TOTAL
1.5 XYLENE
2.0
2.5 YIELD
I
%
Fig. 5. Para-selectivity in disproportionation of toluene as a function of total xylene yield on hydrothermal treated H-ZSM-5 samples before and after extraction of non-framework aluminium.
In order to prove that the shape selectivity is independent of catalytic activity under the chosen conditions, the p-xylene selectivities of extracted and non-extracted hydrothermally treated H-ZSM-5 were compared with the total xylene yield. In Fig. 5, the para-selectivity is plotted against the total xylene yield, which is taken as a measure of activity. The high shape selectivity of the non-extracted samples with a low xylene formation is caused by the high nonframework aluminium content in these samples, containing only small amounts of framework aluminium. All the extracted zeolites, which means those containing almost no non-framework aluminium, display no shape selectivity. Equilibrium distribution is observed, although the total xylene yield varies between 0.1 and 2.3%. The comparison of the two curves reveals that only samples with non-framework aluminium exhibitparu-selectivity, whereas for samples without non-framework aluminium the selectivity is independent of conversion. Therefore, the observed puru-selectivity cannot be caused by decreased conversion. Further, the selectivity and the conversion of toluene disproportionation were measured as a function of flow-rate in order to study the influence of the residence time. The H-ZSM-5 sample used had been pretreated hydrothermally for 6 h at 500’ C. The results are shown in Fig. 6. The puru-selectivity
173
1
2
L
3 SPACE
TIME
/~4O-~h
Fig. 6. Total xylene formation and para-selectivity as a function of space
I
v&city.
I
5
10
15 CONVERSION
20 /
%
Fig. 7. Ratio of p- to o-xylene in m-xylene isomerization on hydrothermally treated and subsequently extracted H-ZSM-5 samples.
clearly decreases with increasing space velocity whereas the hourly produced total amount of p-xylene remains nearly constant. In addition, the isomerization of m-xylene was used as a test reaction for the study of the shape selectivity of ZSM-5 catalysts. The ratio of the p- and o-xylene reaction products was taken as a measure of the shape selectivity. At the equilibrium this ratio is 1.09 : 1. The p-to-o-xylene ratio was studied as a function of the conversion, using a dealuminated sample (6 h, 500°C) before and after extraction. The results, shown in Fig. 7, demonstrate an increasing selectivity with decreasing conversion for both samples. Nevertheless, there is a marked difference between the extracted and the non-extracted samples. The extraction of non-framework aluminium lowers the para-selectivity, but this is observable only at low conversions. This indicates that the p- to o-xylene ratio is not only a function of conversion but also depends on the extraction of the non-framework aluminium.
174 DISCUSSION
Dealumination Steaming at 500’ C was successfully used for controlled dealumination. After only 30 min more than 50% of the lattice aluminium is dislodged. The amount of remaining framework aluminium was determined by applying two different methods, measurement of the ion-exchange capacity and IR spectroscopy of adsorbed pyridine. These methods rely on the strong acidity of the bridged OH groups. The number of acid sites and the number of framework aluminium atoms were regarded as being equal. Direct proportionality between the results of the two methods was found, which shows that both methods are suitable for the determination of the number of acid sites in ZSM-5 and therefore for the determination of framework aluminium content. Previous ESCA experiments [lo] have given hints concerning the location of non-framework aluminium in these samples. They indicated that (i) after hydrothermal treatment aluminium is considerably enriched on the outer surface and (ii) after subsequent treatment with nitric acid this aluminium on the outer surface is nearly exclusively dissolved. Obviously the hydrothermal treatment causes not only dealumination but also transportation of non-framework aluminium towards the outer surface. Para-selectivity The dealumination should cause a decrease in catalytic activity [21]. As already mentioned, the dealumination eliminates framework aluminium and hence acid sites. Acid sites are known to be active sites in acid-catalysed reaction and both the disproportionation of toluene and the isomerization of m-xylene are acid catalysed [2-61. Indeed, a decrease in activity with a decreasing content of framework aluminium was observed (Fig. 4). More interesting is the influence of the dealumination on the shape selectivity. The products of the disproportionation of toluene are, apart from benzene, the three isomers of xylene in proportions determined by the thermodynamics, viz, 24% p-xylene, 22% o-xylene and 54% m-xylene [ 1,221. Surprisingly, dealuminated samples display considerable shape selectivity, the content of p-xylene being increased at the expense of o- and m-xylene. A correlation between the para-selectivity and the content of non-framework aluminium could be found (Fig. 3). After reaching a threshold value, increasing amounts of nonframework aluminium substantially increase the para-selectivity. This conclusion is supported by the observed significant differences in the para-selectivities of samples with low and high contents of non-framework aluminium at the same conversion (Fig. 5). Therefore, the kinetic effects can be ruled out as the reason for the observed shift in the selectivity. Overall the
175
position of the aluminium in the zeolite crystal controls the catalytic behaviour. Framework aluminium is responsible for the active sites and the activity, but non-framework aluminium is responsible for the shape selectivity. The generation of paru-selectivity by means of hydrothermal treatment is always accompanied by a decreased catalytic activity and therefore a diminished conversion. It is well known that the product selectivities can be influenced by the conversion. In order to exclude this kinetic effect, ZSM-5 catalysts with and without non-framework aluminium were compared at the same degree of conversion. It could be shown for toluene disproportionation that in the applied range of conversion the selectivity is still independent of the conversion (Figs. 4 and 5), using samples after extraction of the non-framework aluminium. This indicates that the observed shift in selectivity is indeed due to an effect of the non-framework aluminium and not to a kinetic effect of the conversion. The results for m-xylene isomerization demonstrate a considerable influence of conversion on the shape selectivity (Fig. 7). Only in the region near the equilibrium, which means at maximum conversion of m-xylene (46% ), does the ratio of p- to o-xylene become equilibrated. At low conversions of m-xylene this ratio is enhanced for all the ZSM-5 samples used. However, a distinct difference in thep- to o-xylene ratio can be observed between extracted and non-extracted samples. Obviously the non-framework aluminium has a substantial influence on shape selectivity in m-xylene isomerization, although some workers have come to the opposite conclusion [lo]. The results of m-xylene isomerization confirm the enhancement of paraselectivity by non-framework aluminium already observed in toluene disproportionation. The question remains of why non-framework aluminium can cause paraselectivity. According to published papers [ 1,171, the disproportionation proceeds in two steps. The first is the reaction of two molecules of toluene to give a bulky diphenyl intermediate and the decomposition of the latter. Owing to a transition-state selectivity in the narrow channels, the only product, apart from benzene, is p-xylene. The second step is the isomerization of p-xylene to the equilibrium composition of the xylene isomers. We suggest that this isomerization takes place mainly outside the pores. Inside the pores p-xylene diffuses much faster than the bulkier molecules of m- and o-xylene. The kinetic diameter of the latter two molecules is slightly larger than the diameter of the pores. Consequently, mainlyp-xylene reaches the outer surface. The number of acid sites on the outer surface is more than one order of magnitude lower than the number of acid sites inside the pores. On the other hand, the rate of isomerization is more than three orders of magnitude higher than the rate of disproportionation [ 171. This means that all the p-xylene formed in the slow disproportionation on the sites within the channels can be
176
isomerized on the outer surface owing to the high rate of the isomerization, overcompensating the drawback of the small number of outer surface sites. This model with two reaction steps taking place at inner and outer sites, provides a basis for the explanation of our results. The hydrothermal dealumination forms non-framework aluminium, which, as already mentioned is located predominantly at the outer surface [ 10,231. Therefore, we assume that non-framework aluminium species block active acid sites on the outer surface. This would inhibit the reaction, predominantly taking place here, of the isomerization of p-xylene to o- and m-xylene. This model is in accordance with all our results. H-ZSM-5 as synthesized without non-framework aluminium is non-selective in toluene disproportionation. Pura-selectivity is only observable after the formation of non-framework aluminium (Fig. 3, closed circles). Moreover, it is in accordance with the observed decrease in the paru-selectivity with increasing space velocity (Fig. 6). Owing to the increase in space time, more p-xylene is isomerized on the outer surface. The first step, the conversion of toluene, is mainly controlled by the product diffusion inside the pores and not by the conditions on the external surface of the crystals. Therefore, the total amount of xylenes formed is nearly constant. An analogous model of an increased isomerization in toluene disproportionation on the outer surface has been suggested previously in order to explain the observed decrease in paru-selectivity with increasing temperature
[51.
The results for the m-xylene isomerization also display a considerable shape selectivity of the extracted samples. Therefore, we conclude that the reaction proceeds to a certain extent on shape-selective sites, i.e., on internal sites, whereas another part of the reaction proceeds on non-selective sites, i.e., on external or surface sites. However, samples containing non-framework aluminium display a distinctly higher shape selectivity. Assuming here also a predominant blocking of the external sites by non-framework aluminium, the ratio of external to internal sites has been shifted towards the internal sites. Consequently, the puru-selectivity should be increased. This effect has been observed. Blocking of the pore mouth can be excluded becauses it would inhibit the access of reactant to the internal sites. As a consequence, the reaction would proceed mainly on the external sites, giving a product of equilibrium distribution. It can thus be ruled out that the changed shape selectivity is due to blocking of the pore mouth. Overall, an increase in shape selectivity by dealumination of H-ZSM-5 could be observed in the disproportionation of toluene in addition to the isomerization of m-xylene. This has been explained by the same model. In both reactions the isomerization of the xylene isomers should proceed at least partly on the outer surface of the zeolite. The non-framework aluminium, formed by dealu-
mination, should predominantly block the outer, non-selective sites, which promotes the efficiency of the internal, selective sites.
REFERENCES D.H. Olson and W.O. Haag, in Catalytic Materials, ACS Symposium Series, No. 248, American Chemical Society, Washington, DC, 1984, p. 275. 2 N.R. Meshram, S.G. Hegde, S.B. Kulkarni and P. Ratnasamy, Appl. Catal., 8 (1983) 359. 3 N.R. Meshram, S.G. Hegde and S.B. Kulkarni, Zeolites, 6 (1986) 433. 4 VS. Nayak and L. Riekert, Appl. Catal., 23 (1986) 403. 5 U. Kiirschner, B. Parlitz, E. Schreier, G. ohlmann and J. Viilter, Appl. Catal., 30 (1987) 159. 6 G.P. Babu, M. Santra, V.P. Shiralkar and P. Ratnasamy, J. Catal., 100 (1986) 458. 7 P. Ratnasamy, G.P. Babu, A.J. Chandwadkow and S.B. Kulkarni, Zeolites, 6 (1986) 98. 8 M. Niwa, M. Kalo, T. Tattori and Y. Mewakami, J. Phys. Chem., 90 (1986) 6233. 9 J. Volter, G. Lietz, U. Ktirschner, E. Liiffler and J. Caro, Catal. Today, 3 (1988) 407. 10 G. iihlmann, H.-G. Jerschkewitz, G. Lischke, B. Parlitz, M. Richter and R. Eckelt, Z. Chem., 28 (1988) 161. 11 J. Datka, J. Catal., 102 (1986) 43. 12 P.A. Jacobs, J. Phys. Chem., 86 (1982 ) 3050. 13 N.-Y. Topsoe, J. Catal., 70 (1981) 41. 14 W.O. Haag, R.M. Lago and P.B. Weiz, Nature (London), 309 (1984) 589. 15 J.R. Anderson and K.C. Pratt, Introduction to Characterization and Testing of Catalysis, Academic Press, New York, 1985, p. 223. 16 N.Y. Chen and W.E. Carwood, Catal. Rev. Sci. Eng., 28 (1986) 185. 17 W.O. Haag, in B.L. Shapiro, Heterogeneous Catalysis, Vol. 2, Texas A & M University Press, TX, 1984, p. 95. 18 G.B. Babu, S.G.Hedge, S.B. Kulkarni and P. Ratnasamy, J. Catal., 81 (1983) 471. 19 G.W. Skeels and W.H. Flank, in ACS Symposium Series, No. 218, American Chemical Society, Washington, DC, 1983, p. 369. 20 P. Weisz, Ind. Eng. Chem., Fundam., 25 (1986) 53. 21 V.S. Nayak and V.R. Choudhary, Appl. Catal., 10 (1984) 137. 22 W.W. Kaeding, C. Chu., L.B. Young, B. Winterstein and S.A. Butter, J. Catal., 67 (1981) 159. 23 G. Derbas, A. Courgue, J.B. Nagy and G.C. Clippeleir, Zeolites, 6 (1986) 241. 1
167 Printed in The Netherlands
Shape Selectivity of Hydrothermally Treated H-ZSM-5 in Toluene Disproportionation and Xylene Isomerization U. KURSCHNER, H.-G. JERSCHKEWITZ, Zentralinstitutfiir
Physikalische
(German Democratic
E. SCHREIER and J. VOLTER
Chemie, Akademie
der Wissenschaften
der DDR, Berlin,
1199
Republic)
(Received 22 March 1989, revised manuscript received 10 August 1989)
ABSTRACT Controlled hydrothermal treatment of H-ZSM-5 at 500°C causes a transformation of framework into non-framework aluminium, as indicated by means of cation exchange and IR spectroscopy. After treatment with nitric acid, partial extraction of non-framework aluminium was observed. The non-framework aluminium-containing samples showed an enhanced p-xylene selectivity in the catalytic disproportionation of toluene and in the isomerization of m-xylene. The subsequently extracted samples had lost this property. A model of the decisive role of nonframework aluminium in shape selectivity is discussed.
INTRODUCTION
Selectivity is a major challenge in the catalytic conversion of hydrocarbons. H-ZSM-5, a pentasil-type zeolite, has provided unique possibilities of shape selectivity. This has led to the development and commercial use of several novel processes in the petrochemical industry [ 11, including the disproportionation of toluene and the isomerization of xylene. These are acid-catalysed reactions and the activity of H-ZSM-5 is due to its strong acid sites. Direct correlations of these sites with the activity in the disproportionation [2-51 and in the isomerizations [ 61 have been established. Still more interesting is the shape selectivity. p-Xylene is a valuable intermediate in the production of polyester fibres and its selective formation is of both practical and theoretical interest. Usually mixtures of isomers are obtained, containing only 24% ofp-xylene, according to the thermodynamic equilibrium. The dimensions of the channels in H-ZSM-5 show diameters very close to those of the benzene molecule and the selectivity feature can be readily understood from an interplay of catalytic reaction with mass transfer, severly discriminating against large molecules [ 11. Larger crystals of H-ZSM-5 with their long diffusion path induce increased
0166.9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
168
selectivity for p-xylene [ 1,7]. A variety of different reagents have been successfully used as modifiers of H-ZSM-5 in order to induce para-selectivity, including compounds of phosphorus [ 11, magnesium [ 11, silicon [ 81 and antimony [ 11. Recently we observed that thermal treatment of H-ZSM-5 enhances the shape selectivity and it was suggested that thermally formed non-framework aluminium species could be the reason [ 5,9]. This study was aimed at elucidating the relationship between shape selectivity and non-framework aluminium. Samples containing well determined amounts of framework and nonframework aluminium were prepared by hydrothermal treatment and expected consequences with regard to shape selectivity were studied for the disproportionation of toluene and for the isomerization of m-xylene. EXPERIMENTAL
The parent ZSM-&type zeolite prepared without any template was obtained from VEB Chemiekombinat Bitterfeld (G.D.R.). The silicon-to-aluminium atomic ratio of the starting material was 15. It was converted into the ammonium form by four successive exchanges using 0.5 it4 ammonium nitrate solution at 80” C. The dealumination of the zeolite framework was achieved by treating the sample in flow of water vapour at 500°C. In order to vary the degree of dealumination the exposure was varied from 0.25 to 24 h. Refluxing of dealuminated samples with 1 M nitric acid resulted in partial removal of the non-framework aluminium. Applying higher concentrations of acid and longer extraction times did not cause more complete removal. In order to determine framework aluminium and consequently the degree of dealumination, the samples were exchanged for ammonium ions again. The amount of ammonia back-exchanged with 0.2 A4 ammonium acetate solution was measured by the Kjeldahl method. The comparison of these results with the temperature-programmed desorption of ammonia showed good quantitative agreement [lo]. For IR spectroscopic studies, the zeolites were pressed into thin wafers (48 mg/cm2 ) . The wafers were fixed into the IR cell and preheated under vacuum at 400 oC for 1 h. After cooling to room temperature, pyridine was adsorbed at 10 Torr (1 Torr= 133.322 Pa) and then thermally desorbed at temperatures from 100 to 400°C. The intensity of the 1550 cm-l pyridinium ion band after desorption at 200°C related to the weight of the wafer is a measure of the concentration of Brsnsted sites [ 11-131. The IR spectra were recorded on a UR 20 spectrometer. The catalytic reactions, the disproportionation of toluene and the isomerization of m-xylene were performed in a fixed-bed glass apparatus with a continuous flow at atmospheric pressure. The reaction products were analysed with an on-line gas chromatograph equipped with a flame ionization detector.
169
A 2-m Bentone 34 column was found to be adequate for performing the necessary analytical separations. Prior to reaction, the catalysts were calcined first in an air flow and then in a nitrogen flow at 500°C. The feed for catalytic reactions was prepared by saturation of a nitrogen stream with the appropriate hydrocarbon at 17 oC. Toluene disproportionation was carried out using 0.5 g of catalyst at 350°C and a flow-rate of 1 l/h. The reaction temperature for the m-xylene isomerization was 250’ C in order to avoid the formation of by-products. The amount of catalyst was varied between 0.02 and 0.5 g and the flow-rate between 0.5 and 2.5 l/h in order to achieve different conversions in the desired range. RESULTS
Dealumination H-ZSM-5 samples were dealuminated by steaming at 500” C. The remaining framework aluminium was determined by measuring the NH,+ ion-exchange capacity. This capacity indicates the number of Bronsted sites [ 14-161. As previously suggested, Bronsted sites in zeolites are assigned to bridging OH groups and these are directly related to framework aluminium [ 14,17-201. Therefore, the ion-exchange capacity was taken as a direct measure of the framework aluminium content. Knowing the total aluminium content from chemical analysis, the non-framework aluminium content was calculated as the difference of total and framework aluminium. The dependence of the concentrations of framework and non-framework aluminium on the time of hydrothermal treatment is shown in Fig. 1. All samples are derived from the same parent H-ZSM-5 and differ only in the duration of hydrothermal treatment at 500” C. Results for the hydrothermally treated H-ZSM-5 and for the samples subr
TIME
OF HYDROTHERMAL
Fig. 1. Aluminium
TREATMENT
distribution
Ih
in H-ZSM-5
as a function
of hydrothermal
treatment
at 500 ‘C.
170 TABLE 1 Framework, non-framework and extracted aluminium after hydrothermal treatment and after subsequent extraction with 1 M nitric acid Time of Al after hydrothermal Al after subsequent HNO, treatment hydrothermal treatment treatment Framework Non-framework Framework Non-framework Extracted Extracted (h) AI/u.c. AI/u.c. AI/u.c. AI/u.c. Al AI/u.c. (%o) 0.6 2.9 3.5 3.7 4.2 4.7 4.9 5.1
5.1 2.8 2.2 2.0 1.5 1.0 0.8 0.6
0
0.25 0.5 1.5 3.0 6.0 12.0 24.0
r
,
,
1
2
FRAMEWORK
3
,
,
4
5
ALUMINIUM
/
_ 2.0 1.5 1.0 0.8 0.6
2.7 2.3 1.8 1.6 1.5
1.0 1.9 2.9 3.3 3.0
27 45 62 67 59
U. C.
Fig. 2. Correlation between extinction of the IR band of pyridine at 1550 cm-i and framework aluminium per unit cell ( U.C.), determined by cation exchange.
sequently extracted with nitric acid are listed in Table 1. The greatest decrease in framework aluminium content was observed after only a short time. The degree of extraction of non-framework aluminium was found to increase with increasing time of hydrothermal treatment. Further investigations of framework dealumination were carried out by chemisorption of pyridine followed by recording of the IR spectra. In these zeolites the interaction of pyridine with the Brranstedsites of the ZSM-5 causes the disappearence of the IR band at 3610 cm-’ and the appearance of a band at 1550 cm-‘, which is assigned to pyridine adsorbed on Bronsted sites. As the results in Fig. 2 illustrate, the intensity of the IR band related to the weight of sample corresponds to the amount of framework aluminium measured by the ion-exchange capacity. All the results show good agreement between the hy-
171
drothermal dealumination and the acidity determined by two independent methods. Shape selectivity Studies of the catalytic disproportionation of toluene indicate a considerable influence of the non-framework aluminium content on the xylene distribution in the product. As shown in Fig. 3 (and also Fig. 5) the p-xylene selectivity increases with increasing non-framework aluminium content of ZSM-5. The high non-framework aluminium values are attributed to samples having been exposed longer to hydrothermal treatment (see Table 1) . The open circles in Fig. 3 represent values of samples extracted with nitric acid after hydrothermal treatment. Results of chemical analysis show that all the extracted zeolites have a low non-framework aluminium content. The p-xylene selectivities of these samples approach the equilibrium composition of p-xylene. Considering the corresponding pairs of samples with the same hydrothermal treatment but with and without extraction, it is obvious that the extraction causes a decrease in the non-framework aluminium content and also a decrease in the p-xylene selectivity. In Fig. 4, the activity andp-xylene selectivity of catalysts in toluene disproportionation are shown as a function of the framework aluminium content. All catalysts were used after extraction of the non-framework Al with nitric acid. The results indicate that a higher activity, expressed as the total amount of xylene formed, corresponds to larger amounts of framework aluminium. This direct correlation between acidity and conversion is in accordance with previous results [ 2-61. Although the framework aluminium content and as a consequence the catalytic activity differed by nearly one order of magnitude, the p-xylene selectivity was constant, remaining close to the equilibrium. It is thus demonstrated that under these conditions of low conversions there is no correlation between activity and shape selectivity.
3 1
2
3 NON
4 FRAMEWOAK
5 ALUMINIUM
/UC.
Fig. 3. Para-selectivity in disproportionation of toluene on dealuminated H-ZSM-5 before and after extraction of non-framework aluminium. The time of hydrothermal treatment is indicated in hours.
172
PARA
SELECTIVITY
I
I
2
1
FRAMEWORK
6
5
.4
3
ALUMINIUM
/
U.C.
Fig. 4. Conversion and selectivity for disproportionation of toluene on H-ZSM-5 after dealumination and extraction.
660 NOT
L
b
EXTRACTED EXTRACTED
LO
t
I
I
0.5
1.0 TOTAL
1.5 XYLENE
2.0
2.5 YIELD
I
%
Fig. 5. Para-selectivity in disproportionation of toluene as a function of total xylene yield on hydrothermal treated H-ZSM-5 samples before and after extraction of non-framework aluminium.
In order to prove that the shape selectivity is independent of catalytic activity under the chosen conditions, the p-xylene selectivities of extracted and non-extracted hydrothermally treated H-ZSM-5 were compared with the total xylene yield. In Fig. 5, the para-selectivity is plotted against the total xylene yield, which is taken as a measure of activity. The high shape selectivity of the non-extracted samples with a low xylene formation is caused by the high nonframework aluminium content in these samples, containing only small amounts of framework aluminium. All the extracted zeolites, which means those containing almost no non-framework aluminium, display no shape selectivity. Equilibrium distribution is observed, although the total xylene yield varies between 0.1 and 2.3%. The comparison of the two curves reveals that only samples with non-framework aluminium exhibitparu-selectivity, whereas for samples without non-framework aluminium the selectivity is independent of conversion. Therefore, the observed puru-selectivity cannot be caused by decreased conversion. Further, the selectivity and the conversion of toluene disproportionation were measured as a function of flow-rate in order to study the influence of the residence time. The H-ZSM-5 sample used had been pretreated hydrothermally for 6 h at 500’ C. The results are shown in Fig. 6. The puru-selectivity
173
1
2
L
3 SPACE
TIME
/~4O-~h
Fig. 6. Total xylene formation and para-selectivity as a function of space
I
v&city.
I
5
10
15 CONVERSION
20 /
%
Fig. 7. Ratio of p- to o-xylene in m-xylene isomerization on hydrothermally treated and subsequently extracted H-ZSM-5 samples.
clearly decreases with increasing space velocity whereas the hourly produced total amount of p-xylene remains nearly constant. In addition, the isomerization of m-xylene was used as a test reaction for the study of the shape selectivity of ZSM-5 catalysts. The ratio of the p- and o-xylene reaction products was taken as a measure of the shape selectivity. At the equilibrium this ratio is 1.09 : 1. The p-to-o-xylene ratio was studied as a function of the conversion, using a dealuminated sample (6 h, 500°C) before and after extraction. The results, shown in Fig. 7, demonstrate an increasing selectivity with decreasing conversion for both samples. Nevertheless, there is a marked difference between the extracted and the non-extracted samples. The extraction of non-framework aluminium lowers the para-selectivity, but this is observable only at low conversions. This indicates that the p- to o-xylene ratio is not only a function of conversion but also depends on the extraction of the non-framework aluminium.
174 DISCUSSION
Dealumination Steaming at 500’ C was successfully used for controlled dealumination. After only 30 min more than 50% of the lattice aluminium is dislodged. The amount of remaining framework aluminium was determined by applying two different methods, measurement of the ion-exchange capacity and IR spectroscopy of adsorbed pyridine. These methods rely on the strong acidity of the bridged OH groups. The number of acid sites and the number of framework aluminium atoms were regarded as being equal. Direct proportionality between the results of the two methods was found, which shows that both methods are suitable for the determination of the number of acid sites in ZSM-5 and therefore for the determination of framework aluminium content. Previous ESCA experiments [lo] have given hints concerning the location of non-framework aluminium in these samples. They indicated that (i) after hydrothermal treatment aluminium is considerably enriched on the outer surface and (ii) after subsequent treatment with nitric acid this aluminium on the outer surface is nearly exclusively dissolved. Obviously the hydrothermal treatment causes not only dealumination but also transportation of non-framework aluminium towards the outer surface. Para-selectivity The dealumination should cause a decrease in catalytic activity [21]. As already mentioned, the dealumination eliminates framework aluminium and hence acid sites. Acid sites are known to be active sites in acid-catalysed reaction and both the disproportionation of toluene and the isomerization of m-xylene are acid catalysed [2-61. Indeed, a decrease in activity with a decreasing content of framework aluminium was observed (Fig. 4). More interesting is the influence of the dealumination on the shape selectivity. The products of the disproportionation of toluene are, apart from benzene, the three isomers of xylene in proportions determined by the thermodynamics, viz, 24% p-xylene, 22% o-xylene and 54% m-xylene [ 1,221. Surprisingly, dealuminated samples display considerable shape selectivity, the content of p-xylene being increased at the expense of o- and m-xylene. A correlation between the para-selectivity and the content of non-framework aluminium could be found (Fig. 3). After reaching a threshold value, increasing amounts of nonframework aluminium substantially increase the para-selectivity. This conclusion is supported by the observed significant differences in the para-selectivities of samples with low and high contents of non-framework aluminium at the same conversion (Fig. 5). Therefore, the kinetic effects can be ruled out as the reason for the observed shift in the selectivity. Overall the
175
position of the aluminium in the zeolite crystal controls the catalytic behaviour. Framework aluminium is responsible for the active sites and the activity, but non-framework aluminium is responsible for the shape selectivity. The generation of paru-selectivity by means of hydrothermal treatment is always accompanied by a decreased catalytic activity and therefore a diminished conversion. It is well known that the product selectivities can be influenced by the conversion. In order to exclude this kinetic effect, ZSM-5 catalysts with and without non-framework aluminium were compared at the same degree of conversion. It could be shown for toluene disproportionation that in the applied range of conversion the selectivity is still independent of the conversion (Figs. 4 and 5), using samples after extraction of the non-framework aluminium. This indicates that the observed shift in selectivity is indeed due to an effect of the non-framework aluminium and not to a kinetic effect of the conversion. The results for m-xylene isomerization demonstrate a considerable influence of conversion on the shape selectivity (Fig. 7). Only in the region near the equilibrium, which means at maximum conversion of m-xylene (46% ), does the ratio of p- to o-xylene become equilibrated. At low conversions of m-xylene this ratio is enhanced for all the ZSM-5 samples used. However, a distinct difference in thep- to o-xylene ratio can be observed between extracted and non-extracted samples. Obviously the non-framework aluminium has a substantial influence on shape selectivity in m-xylene isomerization, although some workers have come to the opposite conclusion [lo]. The results of m-xylene isomerization confirm the enhancement of paraselectivity by non-framework aluminium already observed in toluene disproportionation. The question remains of why non-framework aluminium can cause paraselectivity. According to published papers [ 1,171, the disproportionation proceeds in two steps. The first is the reaction of two molecules of toluene to give a bulky diphenyl intermediate and the decomposition of the latter. Owing to a transition-state selectivity in the narrow channels, the only product, apart from benzene, is p-xylene. The second step is the isomerization of p-xylene to the equilibrium composition of the xylene isomers. We suggest that this isomerization takes place mainly outside the pores. Inside the pores p-xylene diffuses much faster than the bulkier molecules of m- and o-xylene. The kinetic diameter of the latter two molecules is slightly larger than the diameter of the pores. Consequently, mainlyp-xylene reaches the outer surface. The number of acid sites on the outer surface is more than one order of magnitude lower than the number of acid sites inside the pores. On the other hand, the rate of isomerization is more than three orders of magnitude higher than the rate of disproportionation [ 171. This means that all the p-xylene formed in the slow disproportionation on the sites within the channels can be
176
isomerized on the outer surface owing to the high rate of the isomerization, overcompensating the drawback of the small number of outer surface sites. This model with two reaction steps taking place at inner and outer sites, provides a basis for the explanation of our results. The hydrothermal dealumination forms non-framework aluminium, which, as already mentioned is located predominantly at the outer surface [ 10,231. Therefore, we assume that non-framework aluminium species block active acid sites on the outer surface. This would inhibit the reaction, predominantly taking place here, of the isomerization of p-xylene to o- and m-xylene. This model is in accordance with all our results. H-ZSM-5 as synthesized without non-framework aluminium is non-selective in toluene disproportionation. Pura-selectivity is only observable after the formation of non-framework aluminium (Fig. 3, closed circles). Moreover, it is in accordance with the observed decrease in the paru-selectivity with increasing space velocity (Fig. 6). Owing to the increase in space time, more p-xylene is isomerized on the outer surface. The first step, the conversion of toluene, is mainly controlled by the product diffusion inside the pores and not by the conditions on the external surface of the crystals. Therefore, the total amount of xylenes formed is nearly constant. An analogous model of an increased isomerization in toluene disproportionation on the outer surface has been suggested previously in order to explain the observed decrease in paru-selectivity with increasing temperature
[51.
The results for the m-xylene isomerization also display a considerable shape selectivity of the extracted samples. Therefore, we conclude that the reaction proceeds to a certain extent on shape-selective sites, i.e., on internal sites, whereas another part of the reaction proceeds on non-selective sites, i.e., on external or surface sites. However, samples containing non-framework aluminium display a distinctly higher shape selectivity. Assuming here also a predominant blocking of the external sites by non-framework aluminium, the ratio of external to internal sites has been shifted towards the internal sites. Consequently, the puru-selectivity should be increased. This effect has been observed. Blocking of the pore mouth can be excluded becauses it would inhibit the access of reactant to the internal sites. As a consequence, the reaction would proceed mainly on the external sites, giving a product of equilibrium distribution. It can thus be ruled out that the changed shape selectivity is due to blocking of the pore mouth. Overall, an increase in shape selectivity by dealumination of H-ZSM-5 could be observed in the disproportionation of toluene in addition to the isomerization of m-xylene. This has been explained by the same model. In both reactions the isomerization of the xylene isomers should proceed at least partly on the outer surface of the zeolite. The non-framework aluminium, formed by dealu-
mination, should predominantly block the outer, non-selective sites, which promotes the efficiency of the internal, selective sites.
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