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The Catalyst Deactivation in the Alkylation of Xylenes with Methanol using Type Y Zeolite
J. Weitkamp, H.G. Kargc, H. Pfcifcr and W. Holdcrich (Eds.) Zeoliies and Related Microporous Materials: State of ihe Art 1994 Studies in Surface Scicnce and Calrilysis. Vol. 84 0 1994 Elsevicr Scicnce B.V. All rights rcscrvcd.
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The Catalyst Deactivation in the Alkylation of Xylenes with Methanol using Type Y Zeolite P i h e n , M.E.,Krause, A.O.I. Helsinki University of Technology, Department of Chemical Engineering Kemistintie 1, FIN-02150 ESPOO, FINLAND and ,)time on stream (TOS) on the The influence of the partial pressure of methanol ha, alkylation of xylenes was studied using a Y zeolite as the catalyst. The partial pressure of methanol had a major influence on the catalyst stability, selectivity and rate of coke-forming. The comparison of the activities on a constant coke level was difficult due to changes of coke quality with TOS. The rate of coke-forming was independent of the xylene-isomer fed. 1. INTRODUCTION
The alkylation of benzene, toluene and xylenes with methanol is an important step in the preparation of various chemicals. Several studies have been published on the alkylation of the benzene ring with methanol [l-181. ZSM-5 type zeolites have been most frequently used as catalysts. Y-type zeolites can also be used, but it is much more difficult to control the selectivities with these wide pore zeolites. They also deactivate more rapidly. Therefore, it is very important to know what causes the deactivation of the catalysts. The characterization methods, coke types, mechanisms for the coke formation and location of coke on zeolites have been studied widely in the literature [19-251. In acid-catalyzed hydrocarbons the main reason for the catalyst deactivation is coking. At least two kinds of coke have been described: a so-called "white-coke'' and "black-coke". Higher reaction temperatures and extended time on stream favor the formation of more bulky polyaromatic coke-componentswith low H E ratio (black-coke) [19, 201. The amount of methanol has been found to influence the rate of coking on USHY [21]. Coke is more aromatic for large pore zeolites than for small or intermediate size pores [20]. In this study we wanted to gain more insight into the role of methanol in the deactivation process. The time on stream (TOS), the coke content of the catalyst (C%) and the partial pressure of methanol (pMco,,) can all influence the performance of the catalyst. 2. EXPERIMENTAL
The experiments were carried out in a tubular downflow reactor. The catalyst, commercial Y zeolite LZY-82 (Union Carbide), was crushed and sieved into a particle size of 0.5-1.0 mm and packed with silicon carbide. The followingprocess conditions were used, temperature 300 "C and WHSV 20 h-'. Nitrogen was used as an inert diluent. The partial pressure of xylene was 0.2 bar and that of methanol 0, 0.05, 0.1, 0.3 or 0.6 bar in the rn- and p-xylene experiments and 0 and 0.1 in the o-xylene experiments. In order to determine the rate of coke
formation at each partial pressure of methanol, the coke content of the catalyst was measured at TOS (time on stream) 1, 3, 5, 10, 15, 30, 60, 120, 240 and 360 min. The coke profile of the catalyst was also measured without xylene at TOS 60 min at bH ranging from 0.05-1 bar. The runs with metu-xylene were also repeated at 275"C, in order to determine the influence of temperature on the coking of the catalyst. The weight loss of the coked catalysts was analyzed by means of TGA in air at a CP-MAS temperature range of 200-700°C. The types of coke were also analyzed with Nh4R using a Jeol 400 MHz instrument.
3. RESULTS AND DISCUSSION 3.1 Activity Profiles
The activity profiles can be divided into two periods. In the first period, the conversions decreased very rapidly. In the second period, the decline of the conversions were slower (Figure 1). 80 70
2 60
f
5
30 20 10 0
0
50
100
150
200
250
300
350
400
TOS (min)
Figure 1 . Reactions of metu-xylene; 300"C, WHSV = 20 hl, pMaH = 0.1 bar, isomerization, + disproportionation, 0 alkylation.
total
0
In the first period, methanol was converted into light hydrocarbons, and xylene, alkylated by methanol, also isomerized and disproportionated. The conversions, especially alkylation, were high (total conversion 50-8056, TOS < 10 min), but decreased rapidly. When the haH was low (0.05 bar) the alkylation conversion of meru-xylene decreased from 18.3 (at TOS 10 min) to 7.8 % (at TOS 60 min). In the case of para-xylene the decrease was from 11.8 (at TOS 10 min) to 4.4 % (at TOS 60 min). The conversions at TOS 10 min are shown in Table 1 . One should remember, though, that the activities are not necessarily comparable at constant TOS with different reaction conditions because of the possible variations in the coke content of the catalyst. The length of this unstable first period depended on the P M ~ H . When xylene was present and the P M ~ Hwas low (0.05 bar), the first period was longer than at high partial pressures, and in addition to alkylation isomerization and disproportionation reactions of xylene also occurred. When the pMcOH was high (0.6 bar), the isomerization and disproportionation conversions decreased more rapidly close to zero. However, the precise definition of the length of the first period is somewhat difficult since the conversions continued to decrease throughout the whole run.
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Table 1 Conversions of mefa- and para-xylenes at TOS 10 min.
p-X y lene
m-Xylene PMmH
0 0.05 0.1
0.3 0.6
alk
isom
dis
tot
alk
isom
dis
tot
18.3 31.2 33.0 32.0
17.5 13.8 10.7 3 .O 1.5
28.8 19.3 11.4 2.0 0.8
51.1 53.4 58.9 40.7 36.1
11.8 18.4 27.7 21.1
18.6 16.6 13.5 4.0 1.5
40.4 28.5 21.8 4.4 1.8
63.4 64.5 54.6 34.8 26.1
pWoH= partial pressure of methanol, bar alk = alkylation conversion, % isom = isomerization conversion, % dis = disproportionation conversion, %
In the second period, the main reaction was alkylation (Table 2). The rak of decrease of the conversions depended again on the pMmH.At high methanol concentrations more polymethylated benzenes were produced. The rate of deactivation was strongly influenced by the hmH. The second state was reached faster, when the P M ~ Hwas high, and conversions was low. declined more slowly in the second period when the pMmH Table 2 Conversions of mefa- and para-xylenes at TOS 360 min. m-X ylene
p-X ylene
PM&H
alk
isom
dis
tot
alk
isom
dis
tot
0 0.05 0.1 0.3 0.6
4.3 6.1 5.2 2.5
2.0 0.5 0.7 0.5 0.5
2.1 0.6 0.7 0.3 0.2
4.2 5.7 8.0 6.0 3.2
3.1 4.0 5.1 2.2
1.7 0.4 0.4 0.3 0.2
5.4 1.6 1.4 0.8 0.2
7.5 5.4 6.0 6.4 2.3
pMcOH = partial pressure of methanol, bar alk = alkylation conversion, % isom = isomerization conversion, % dis = disproportionation conversion, 96
3.2 Product Distributions In addition to alkylation isomerizationand disproportionationreactions also occurred in the first period. This could be seen as product xylene-isomers and toluene. When methanol was not present, the disproportionation reaction was the most favored in the case of para-xylene and isomerization in the case of onho-xylene. mefa-Xylene isomerized and disproportionated equally. The main isomerization product obtained from para- and orfho-xylenes was mefa-xylene.
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mefu-Xylene produced both para- and orfho-isomers equally. This suggests the 1,2methylshift mechanism in the isomerization. Ethylbenzene was also produced from puruxylene. As the P M ~ H increased, the amounts of tri- (TMB), tetra-, penta- and hexamethylbenzenes were increased. The main TMB produced was the 1,2,4-isomer. When methanol was present, the P M ~ Hhad no clear influence on the isomer distributions. However, the amount of heavy methylbenzenes was increased, and the amounts of toluene and product xylenes decreased with increasing haH.
3.3 Coking Profiles As previously discussed, the catalyst deactivated very rapidly during the reactions of xylenes and methanol. In Figure 2 the coke and total conversion profiles are shown as a function of TOS. The coke profiles and total conversion profiles were mirror images: as the coke content increased, the total conversions decreased with increasing TOS at each haH. In the deactivation process, coking seemed to have a very important role.
._ f
9 60
7 E
0
2
20
1 0
0
0
50
100
150
200
250
300
350
0 400
TOS (mid
Figure 2. Total conversion and coke content of the catalyst during the alkylation of mefuxylene with methanol; pMaH 0.1 bar, T = 30O0C, WHSV = 20 h-', coking, 0 total conversion. When the coke vs. TOS profiles were determined at different pMaH, methanol was observed to increase the rate of coking (Figure 3). When the haH was increased, the amount of polymethylated benzenes, which can form coke precursors, was also increased.
1
100
10
lo00
TOS (mid
Figure 3. Coke content during the run, mefu-xylene; 30O0C, W H S V = 20 h-', pMaH = rn 0, 0 0.05, + 0.1, 0 0.3, A 0.6 bar (partial pressure of xylene, 0.2 bar).
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Pure methanol also coked the catalyst very rapidly. In order to clarify the influence of methanol, the coke contents at TOS 60 min and at various P M ~ Hwere compared in the presence @Xyl 0.2 bar) and in the absence of xylene (Figure 4). The coke contents were higher when xylene was present, which indicates that the presence of xylene increases the formation of coke precursors. 8.5
8
q 7.5
3
7
6.5 O
B
6
5 u
0 5.5
4.5
0
0.2
0.4
0.6
0.8
1
p MeOH (bw)
Figure 4. Influence of haH to the coke content of the catalyst at TOS 60 min, 30O0C, WHSV = 20 h-', mefa-xylene (0.2 bar), 0 para-xylene (0.2 bar) and + without xylene. Because it is known that the carbon content influences the behavior of the catalyst, the conversions and selectivities should not be cornpared at constant TOS but at the constant coke level of the catalyst (C%). The initial activities of rn-xylene on ultrastable Y have been studied by extrapolation the activity vs. TOS curve to zero [26, 271. In this study, however, the error from such extrapolation appeared to be too high due to the shape of the conversion vs. TOS curve and the high initial conversions. For this reason the activities were compared on a partially deactivated catalyst, at a constant coke level. This coke content was chosen to be 7.8 w-%. The chosen coke level was achieved more rapidly at high P M ~ H . For example, the same coke content, 7.8 w-%, was achieved at TOS 315 min at PM~H,0.05 bar and at 10 min at P M ~ H , 0.6 bar. When methanol was not present, the catalyst coke content 7.8 w-% was not achieved at all during the 360-min run (Figure 3). This made the comparison of the catalyst at a constant C% difficult. It was difficult to find a precise TOS at which the constant coke level was achieved because of the shape of the coke profile. However, both the coke content and the conversions no longer changed so rapidly in the second period, which made the comparisons easier. The conversions of mefa-xylene at coke level 7.8 w-% are shown in the Figure 5 . The conversions at the catalyst coke content 7.8 w-% were higher at high P M ~ Hcompared conversions at the same coke level. When there was more methanol, more with the low htoH alkylation took place, as expected. In addition to alkylation more isomerization and disproportionation reactions also occurred at higher pMdH at this constant coke level. However, at the highest PM~H,0.6 bar, the isomerization and disproportionation conversions were lower than at 0.3 bar. With this exception methanol Seems to increase all reactions when the catalysts are compared at the same coke level. The selectivity for alkylation increased with increasing pMcOH, diminishing the isomerization and disproportionation selectivities.
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40
1
5
0
'2 20 0
2
1.6 .. 1.4 .. 1.2 -.
15
t 0.8 .. c
5
0.4 .. 0.2 .-
0
0
0.2 0.4 p MeOH (bar)
0.6
0
0.2
0.4
0.6
p MeOH (bar)
Figure 5 . Influence of the partial pressure of methanol on the reactions of mefu-xylene at catalyst coke level 7.8 w-%; 30O0C, WHSV = 20 h-', total conversion, 0 alkylation, + isomerization, 0 disproportionation. The increase of the alkylation conversions with increasing P M ~ His an indication of the positive reaction order of methanol in the alkylation reaction. At first assumption, one would expect that methanol would not influence the isomerizationand disproportionationconversions when the coke content of the catalyst is constant. However, the change of the isomerization indicates that the surface of the and disproportionation conversions with changing haH catalyst varies with the P M ~ H even when the coke content remains the same. Thermogravimetric analysis showed that the coke formed at TOS 360 min contained more components burning at higher temperatures than the coke formed at TOS 1 min (Figure 6). Also NMR analysis confirmed that the coke in the beginning of the experiment was different compared to the coke formed later. There were some indications of a shift towards more condensed polyaromatics during the TOS. The influence of the hcoH at a given coke content on the selectivities could be explained by the different nature of the coke formed at different TOS.
360 min
c
200-350
350-450
450-550
550-700
Temperature Ranger (OC)
Figure 6. The amount of coke burned at various temperatures. Catalysts used in the conversion of mefu-xylene @Xyl 0.2 bar) and methanol @ M a H 0 . 3 bar); 30O0C, W H S V = 20 h-', TOS 1 min and 360 min.
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Although there were clear differences in the selectivities of the three xylene-isomers, their coking rate was almost similar (Figure 7). when methanol was not present, the catalyst showed slightly faster coking with para-xylene compared with the other isomers. This could be caused by the increased disproportionation,which produces heavy trimethylbenzenes.
0 l 1 10
100
t 1000
TOS (minl
Figure 7. Coking of the three xylene-isomers at partial pressures of methanol: 0 bar: 0 p, o and 0.1 bar: 0 m, A p, A o; 300"C,W H S V = 20 h-'.
*
m,
Temperature at the studied range (275-300°C) had a surprisingly low influence on the coking profile of mefa-xylene. The influence at a larger range would probably be more pronounced because of the changes in conversions and selectivities. 4. CONCLUSIONS
Results obtained indicate that deactivation of Y-type zeolite in the xylene alkylation reaction is caused by coke. The partial pressure of methanol increases the formation of polymethylated benzenes; which can produce coke precursors. The rate of coke-forming is highest at the beginning of the run. The rate of coke formation is strongly dependent on the partial pressure of methanol and less dependent on of the xylene-isomer fed or temperature (at the studied range). The presence of xylene increases the rate of coking compared with pure methanol. The influence of the pMaH at a given coke content on the selectivitiescould be explained by the different nature of the coke formed at different TOS. The comparison of the activities on a constant coke level is difficult due to changes of coke quality with TOS.
ACKNOWLEDGEMENT We wish to thank Dr. Sigmund Csicsery for helpful discussions during the course of this work and Dr. Andrew Root from Neste Oy, Scientific Services, Analytical Research, for NMR-analysis. This work was financed by the Neste Oy Foundation.
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REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27
Rakwzy, J. and Romotowski, T.,Zeolifes, 13 (1993) April/May 256-260. Huang, C.S. and KO,A.-N., Catal.Lett. 19 (1993) 319-326. Coughlan, B., Carroll, W.M. and Nunan, J., J.Chem.Sw.Faraday Trans 79 (1983) 297-309. Raj, A., Reddy, S. and Kumar, R., J.Catal. 138 (1992) 518-524. Giordiano, N., Pino, L., Cavallaro, S., Vitarelli, P. and Rao, B.S., Zeolites, 7 (1987) 131-134. Kodama, H.and O W , S., J.Catal., 132 (1991) 512-523. Vinek, H.,Derewinski, M., Mirth, G. and Lercher, J.A., AppLCatal., 68 (1991) 277-284. Vayssilov, G., Yankov, M. and Hamid, A., Appl.Catal. A: General 94 (1993) 117130. Mantha, R., Bhatia, S. and Rao, M.S., Ind.Eng.Chem.Res., 30 (1991) 281-286. Kaeding, W.W., Chu, C., Young, L.B., Weinstein, B. and Butter, S.A., J.Catal. 67 (1981) 159-174. Young, L.B., Butter, S.A. and Kaeding, W.W., J.Catal. 76 (1982) 418-432. Parker, D.G., Appl.Catal., 9 (1984) 53-61. Rakwzy, J., React.Kinet.Catal.Lett.,48 (1992) 401-409. Beschmann, K. and Riekert, L., J.Catal. 141 (1993) 548-565. Wei, J., J.Catal., 76 (1982) 433-439. Rakoczy, J. and Sulikowski, B., React.Kinet.Catal.Lett.,36 (1988) 241-246. Blanco, A., Campelo, J.M., Garcia, A., Luna, D., Marinas, J.M. and Romero, A.A., J.Catal. 137 (1992) 51-68. Bhat, Y.S., Halgeri, A.B. and Prasada Rao, T.S.R., 1nd.Eng.Chem.Res. 28 (1989) 890-894. Karge, H.G., in Introduction to Zeolite Science and Practice, van kkkum, H., Flanigen, E.M. and Jansen, J.C. (eds.), Stud.Surf.Sci.Catal. 58, Elsevier, Amsterdam (1991) 531-570. Guisnet, M. and Magnoux, P., Appl.Catal., 54 (1989) 1-27. Rozwadowski, M., Wloch, J., Erdmann, K. and Kornatowski, M., Collect.Czech.Chem.Commun.,57 (1992) 959-968. Dimon, B., Cartraud, P. and Guisnet, M., Appl.Catal. A:General, 101 (1993) 351369. Magnoux, P., Canaff, C., Machado, F. and Guisnet, M., J.Catal., 134 (1992) 286298. Liu, Z. and Dadyburjor, D.B., J.Catal., 134 (1992) 583-593. Pradhan, A.R. and Rao, B.S., J.Catal., 132 (1991) 79-84. Corma, A., FornBs, V., Perez-Pariente, J., Sastre, E., Martens, J.A. and Jacobs, P.A., ACS Symp.Ser., 368 (1988) 555-568. Corma, A., Llopis, F. and Monton, J.B., J.Catal., 140 (1993) 384-394.
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The Catalyst Deactivation in the Alkylation of Xylenes with Methanol using Type Y Zeolite P i h e n , M.E.,Krause, A.O.I. Helsinki University of Technology, Department of Chemical Engineering Kemistintie 1, FIN-02150 ESPOO, FINLAND and ,)time on stream (TOS) on the The influence of the partial pressure of methanol ha, alkylation of xylenes was studied using a Y zeolite as the catalyst. The partial pressure of methanol had a major influence on the catalyst stability, selectivity and rate of coke-forming. The comparison of the activities on a constant coke level was difficult due to changes of coke quality with TOS. The rate of coke-forming was independent of the xylene-isomer fed. 1. INTRODUCTION
The alkylation of benzene, toluene and xylenes with methanol is an important step in the preparation of various chemicals. Several studies have been published on the alkylation of the benzene ring with methanol [l-181. ZSM-5 type zeolites have been most frequently used as catalysts. Y-type zeolites can also be used, but it is much more difficult to control the selectivities with these wide pore zeolites. They also deactivate more rapidly. Therefore, it is very important to know what causes the deactivation of the catalysts. The characterization methods, coke types, mechanisms for the coke formation and location of coke on zeolites have been studied widely in the literature [19-251. In acid-catalyzed hydrocarbons the main reason for the catalyst deactivation is coking. At least two kinds of coke have been described: a so-called "white-coke'' and "black-coke". Higher reaction temperatures and extended time on stream favor the formation of more bulky polyaromatic coke-componentswith low H E ratio (black-coke) [19, 201. The amount of methanol has been found to influence the rate of coking on USHY [21]. Coke is more aromatic for large pore zeolites than for small or intermediate size pores [20]. In this study we wanted to gain more insight into the role of methanol in the deactivation process. The time on stream (TOS), the coke content of the catalyst (C%) and the partial pressure of methanol (pMco,,) can all influence the performance of the catalyst. 2. EXPERIMENTAL
The experiments were carried out in a tubular downflow reactor. The catalyst, commercial Y zeolite LZY-82 (Union Carbide), was crushed and sieved into a particle size of 0.5-1.0 mm and packed with silicon carbide. The followingprocess conditions were used, temperature 300 "C and WHSV 20 h-'. Nitrogen was used as an inert diluent. The partial pressure of xylene was 0.2 bar and that of methanol 0, 0.05, 0.1, 0.3 or 0.6 bar in the rn- and p-xylene experiments and 0 and 0.1 in the o-xylene experiments. In order to determine the rate of coke
formation at each partial pressure of methanol, the coke content of the catalyst was measured at TOS (time on stream) 1, 3, 5, 10, 15, 30, 60, 120, 240 and 360 min. The coke profile of the catalyst was also measured without xylene at TOS 60 min at bH ranging from 0.05-1 bar. The runs with metu-xylene were also repeated at 275"C, in order to determine the influence of temperature on the coking of the catalyst. The weight loss of the coked catalysts was analyzed by means of TGA in air at a CP-MAS temperature range of 200-700°C. The types of coke were also analyzed with Nh4R using a Jeol 400 MHz instrument.
3. RESULTS AND DISCUSSION 3.1 Activity Profiles
The activity profiles can be divided into two periods. In the first period, the conversions decreased very rapidly. In the second period, the decline of the conversions were slower (Figure 1). 80 70
2 60
f
5
30 20 10 0
0
50
100
150
200
250
300
350
400
TOS (min)
Figure 1 . Reactions of metu-xylene; 300"C, WHSV = 20 hl, pMaH = 0.1 bar, isomerization, + disproportionation, 0 alkylation.
total
0
In the first period, methanol was converted into light hydrocarbons, and xylene, alkylated by methanol, also isomerized and disproportionated. The conversions, especially alkylation, were high (total conversion 50-8056, TOS < 10 min), but decreased rapidly. When the haH was low (0.05 bar) the alkylation conversion of meru-xylene decreased from 18.3 (at TOS 10 min) to 7.8 % (at TOS 60 min). In the case of para-xylene the decrease was from 11.8 (at TOS 10 min) to 4.4 % (at TOS 60 min). The conversions at TOS 10 min are shown in Table 1 . One should remember, though, that the activities are not necessarily comparable at constant TOS with different reaction conditions because of the possible variations in the coke content of the catalyst. The length of this unstable first period depended on the P M ~ H . When xylene was present and the P M ~ Hwas low (0.05 bar), the first period was longer than at high partial pressures, and in addition to alkylation isomerization and disproportionation reactions of xylene also occurred. When the pMcOH was high (0.6 bar), the isomerization and disproportionation conversions decreased more rapidly close to zero. However, the precise definition of the length of the first period is somewhat difficult since the conversions continued to decrease throughout the whole run.
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Table 1 Conversions of mefa- and para-xylenes at TOS 10 min.
p-X y lene
m-Xylene PMmH
0 0.05 0.1
0.3 0.6
alk
isom
dis
tot
alk
isom
dis
tot
18.3 31.2 33.0 32.0
17.5 13.8 10.7 3 .O 1.5
28.8 19.3 11.4 2.0 0.8
51.1 53.4 58.9 40.7 36.1
11.8 18.4 27.7 21.1
18.6 16.6 13.5 4.0 1.5
40.4 28.5 21.8 4.4 1.8
63.4 64.5 54.6 34.8 26.1
pWoH= partial pressure of methanol, bar alk = alkylation conversion, % isom = isomerization conversion, % dis = disproportionation conversion, %
In the second period, the main reaction was alkylation (Table 2). The rak of decrease of the conversions depended again on the pMmH.At high methanol concentrations more polymethylated benzenes were produced. The rate of deactivation was strongly influenced by the hmH. The second state was reached faster, when the P M ~ Hwas high, and conversions was low. declined more slowly in the second period when the pMmH Table 2 Conversions of mefa- and para-xylenes at TOS 360 min. m-X ylene
p-X ylene
PM&H
alk
isom
dis
tot
alk
isom
dis
tot
0 0.05 0.1 0.3 0.6
4.3 6.1 5.2 2.5
2.0 0.5 0.7 0.5 0.5
2.1 0.6 0.7 0.3 0.2
4.2 5.7 8.0 6.0 3.2
3.1 4.0 5.1 2.2
1.7 0.4 0.4 0.3 0.2
5.4 1.6 1.4 0.8 0.2
7.5 5.4 6.0 6.4 2.3
pMcOH = partial pressure of methanol, bar alk = alkylation conversion, % isom = isomerization conversion, % dis = disproportionation conversion, 96
3.2 Product Distributions In addition to alkylation isomerizationand disproportionationreactions also occurred in the first period. This could be seen as product xylene-isomers and toluene. When methanol was not present, the disproportionation reaction was the most favored in the case of para-xylene and isomerization in the case of onho-xylene. mefa-Xylene isomerized and disproportionated equally. The main isomerization product obtained from para- and orfho-xylenes was mefa-xylene.
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mefu-Xylene produced both para- and orfho-isomers equally. This suggests the 1,2methylshift mechanism in the isomerization. Ethylbenzene was also produced from puruxylene. As the P M ~ H increased, the amounts of tri- (TMB), tetra-, penta- and hexamethylbenzenes were increased. The main TMB produced was the 1,2,4-isomer. When methanol was present, the P M ~ Hhad no clear influence on the isomer distributions. However, the amount of heavy methylbenzenes was increased, and the amounts of toluene and product xylenes decreased with increasing haH.
3.3 Coking Profiles As previously discussed, the catalyst deactivated very rapidly during the reactions of xylenes and methanol. In Figure 2 the coke and total conversion profiles are shown as a function of TOS. The coke profiles and total conversion profiles were mirror images: as the coke content increased, the total conversions decreased with increasing TOS at each haH. In the deactivation process, coking seemed to have a very important role.
._ f
9 60
7 E
0
2
20
1 0
0
0
50
100
150
200
250
300
350
0 400
TOS (mid
Figure 2. Total conversion and coke content of the catalyst during the alkylation of mefuxylene with methanol; pMaH 0.1 bar, T = 30O0C, WHSV = 20 h-', coking, 0 total conversion. When the coke vs. TOS profiles were determined at different pMaH, methanol was observed to increase the rate of coking (Figure 3). When the haH was increased, the amount of polymethylated benzenes, which can form coke precursors, was also increased.
1
100
10
lo00
TOS (mid
Figure 3. Coke content during the run, mefu-xylene; 30O0C, W H S V = 20 h-', pMaH = rn 0, 0 0.05, + 0.1, 0 0.3, A 0.6 bar (partial pressure of xylene, 0.2 bar).
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Pure methanol also coked the catalyst very rapidly. In order to clarify the influence of methanol, the coke contents at TOS 60 min and at various P M ~ Hwere compared in the presence @Xyl 0.2 bar) and in the absence of xylene (Figure 4). The coke contents were higher when xylene was present, which indicates that the presence of xylene increases the formation of coke precursors. 8.5
8
q 7.5
3
7
6.5 O
B
6
5 u
0 5.5
4.5
0
0.2
0.4
0.6
0.8
1
p MeOH (bw)
Figure 4. Influence of haH to the coke content of the catalyst at TOS 60 min, 30O0C, WHSV = 20 h-', mefa-xylene (0.2 bar), 0 para-xylene (0.2 bar) and + without xylene. Because it is known that the carbon content influences the behavior of the catalyst, the conversions and selectivities should not be cornpared at constant TOS but at the constant coke level of the catalyst (C%). The initial activities of rn-xylene on ultrastable Y have been studied by extrapolation the activity vs. TOS curve to zero [26, 271. In this study, however, the error from such extrapolation appeared to be too high due to the shape of the conversion vs. TOS curve and the high initial conversions. For this reason the activities were compared on a partially deactivated catalyst, at a constant coke level. This coke content was chosen to be 7.8 w-%. The chosen coke level was achieved more rapidly at high P M ~ H . For example, the same coke content, 7.8 w-%, was achieved at TOS 315 min at PM~H,0.05 bar and at 10 min at P M ~ H , 0.6 bar. When methanol was not present, the catalyst coke content 7.8 w-% was not achieved at all during the 360-min run (Figure 3). This made the comparison of the catalyst at a constant C% difficult. It was difficult to find a precise TOS at which the constant coke level was achieved because of the shape of the coke profile. However, both the coke content and the conversions no longer changed so rapidly in the second period, which made the comparisons easier. The conversions of mefa-xylene at coke level 7.8 w-% are shown in the Figure 5 . The conversions at the catalyst coke content 7.8 w-% were higher at high P M ~ Hcompared conversions at the same coke level. When there was more methanol, more with the low htoH alkylation took place, as expected. In addition to alkylation more isomerization and disproportionation reactions also occurred at higher pMdH at this constant coke level. However, at the highest PM~H,0.6 bar, the isomerization and disproportionation conversions were lower than at 0.3 bar. With this exception methanol Seems to increase all reactions when the catalysts are compared at the same coke level. The selectivity for alkylation increased with increasing pMcOH, diminishing the isomerization and disproportionation selectivities.
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40
1
5
0
'2 20 0
2
1.6 .. 1.4 .. 1.2 -.
15
t 0.8 .. c
5
0.4 .. 0.2 .-
0
0
0.2 0.4 p MeOH (bar)
0.6
0
0.2
0.4
0.6
p MeOH (bar)
Figure 5 . Influence of the partial pressure of methanol on the reactions of mefu-xylene at catalyst coke level 7.8 w-%; 30O0C, WHSV = 20 h-', total conversion, 0 alkylation, + isomerization, 0 disproportionation. The increase of the alkylation conversions with increasing P M ~ His an indication of the positive reaction order of methanol in the alkylation reaction. At first assumption, one would expect that methanol would not influence the isomerizationand disproportionationconversions when the coke content of the catalyst is constant. However, the change of the isomerization indicates that the surface of the and disproportionation conversions with changing haH catalyst varies with the P M ~ H even when the coke content remains the same. Thermogravimetric analysis showed that the coke formed at TOS 360 min contained more components burning at higher temperatures than the coke formed at TOS 1 min (Figure 6). Also NMR analysis confirmed that the coke in the beginning of the experiment was different compared to the coke formed later. There were some indications of a shift towards more condensed polyaromatics during the TOS. The influence of the hcoH at a given coke content on the selectivities could be explained by the different nature of the coke formed at different TOS.
360 min
c
200-350
350-450
450-550
550-700
Temperature Ranger (OC)
Figure 6. The amount of coke burned at various temperatures. Catalysts used in the conversion of mefu-xylene @Xyl 0.2 bar) and methanol @ M a H 0 . 3 bar); 30O0C, W H S V = 20 h-', TOS 1 min and 360 min.
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Although there were clear differences in the selectivities of the three xylene-isomers, their coking rate was almost similar (Figure 7). when methanol was not present, the catalyst showed slightly faster coking with para-xylene compared with the other isomers. This could be caused by the increased disproportionation,which produces heavy trimethylbenzenes.
0 l 1 10
100
t 1000
TOS (minl
Figure 7. Coking of the three xylene-isomers at partial pressures of methanol: 0 bar: 0 p, o and 0.1 bar: 0 m, A p, A o; 300"C,W H S V = 20 h-'.
*
m,
Temperature at the studied range (275-300°C) had a surprisingly low influence on the coking profile of mefa-xylene. The influence at a larger range would probably be more pronounced because of the changes in conversions and selectivities. 4. CONCLUSIONS
Results obtained indicate that deactivation of Y-type zeolite in the xylene alkylation reaction is caused by coke. The partial pressure of methanol increases the formation of polymethylated benzenes; which can produce coke precursors. The rate of coke-forming is highest at the beginning of the run. The rate of coke formation is strongly dependent on the partial pressure of methanol and less dependent on of the xylene-isomer fed or temperature (at the studied range). The presence of xylene increases the rate of coking compared with pure methanol. The influence of the pMaH at a given coke content on the selectivitiescould be explained by the different nature of the coke formed at different TOS. The comparison of the activities on a constant coke level is difficult due to changes of coke quality with TOS.
ACKNOWLEDGEMENT We wish to thank Dr. Sigmund Csicsery for helpful discussions during the course of this work and Dr. Andrew Root from Neste Oy, Scientific Services, Analytical Research, for NMR-analysis. This work was financed by the Neste Oy Foundation.
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