- Email: [email protected]
Evaluation of MY zeolites (M=Pt, Pd, Ni) in the transalkylation of trimethylbenzene with benzene
Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 Elsevier B.V. All rights reserved
809
Evaluation of MY Zeolites (M= Pt, Pd, Ni) in the Transalkylation of Trimethylbenzene with Benzene M. S. Ramos a, S. T. Grecco a, L. P. Gomes a, A. C. Oliveira a, P. Reyes b, M. Oportus b and M. C. Rangel a aGECCAT Grupo de Estudos em Cin6tica e Catfilise, Instituto de Quimica, Universidade Federal da Bahia. Campus Universitfirio de Ondina, Federagao. 40 170-280. Salvador, Bahia, Brazil. E-mail: [email protected] bFacultad de Ciencias Quimicas, Universidad de Concepci6n, Casilla 3-C Concepci6n, Chile The trimethlybenzene transalkylation with benzene has attracted increasing attention since valuable aromatic petrochemicals such as xylenes can be obtained from low value aromatics. Mordenite is the most used catalyst but it easily deactivates with time. In order to get more efficient catalysts, the effect of nickel, palladium and platinum on catalytic properties of Y zeolite was studied. It was found that nickel and platinum improve the conversion of trimethylbenzene but palladium is ineffective. Nickel produces mainly ethylbenzene and platinum produces large amount of xylenes, being the best metal to improve the catalyst. Nickel and platinum are also able to prevent coke deposition on Y zeolite.
I. INTRODUCTION Zeolite catalysts have received increasing attention and importance in petroleum refining and in chemical industry because of their unique properties. They are the most used catalysts in refining and petrochemical processes [1,2] as well as in the synthesis of fine and specialty chemicals [3] and in the depollution of industrial and automobile exhausts [4]. These applications are closely related to their high specific surface area, adsorption capacity, acid sites, size of the channels and intricate channel structure [5]. All these properties can be tailored for a particular application and are ultimately dependent on the thermal and on the hydrothermal stability of these materials [6]. Despite these catalytically desirable properties, zeolites often deactivate during industrial processes mainly by coke deposition [7, 8]. In most cases, the cost of catalyst deactivation is high enough to become the mastering of catalyst stability at least as important as the control of activity and selectivity [9]. In the transalkylation of aromatic hydrocarbons, for instance, the catalyst stability is one of the main issues, because coking can occur rapidly as a consequence of the high content of aromatics in the feed [10]. In these processes, valuable aromatic petrochemicals such as benzene and xylenes can be obtained from C9 aromatics and toluene [ 11 ]. Benzene and xylenes are important starting materials for several processes such as the production of synthetic fibers, plasticizers and resins. They are commercially produced by naphtha reforming and as a by-product of naphtha cracking for ethylene production (pyrolysis gasoline), in which substantial amounts of Cv and C9 aromatics are also produced [ 12, 13]. Xylene production is then optimized by upgrading these
810 low value streams through aromatic transalkylation reactions. These reactions also offer the possibility of developing environmental friendly processes at low operating costs [ 12]. The transalkylation of toluene with trimethylbenzenes takes place on solid acid catalysts and it is part of a complex network of reversible reactions. The transalkylation reaction is mainly in equilibrium with the disproportionation reaction and the isomerization of polymethylbenzenes also occurs [ 13, 14]. The distribution of the several isomers is governed by kinetics factors like the reactivity of the aromatic hydrocarbon, the nature of the catalytic sites, the texture and the morphology of the catalysts [13]. It is one of the most important reactions of conversion of methylaromatics aiming at the xylene production. However, with the recent market reduction of benzene as a consequence of environmental restrictions, benzene transalkylation with C9+ aromatics emerges as a potentially important reaction for commercial applications and its investigation has attracted increasing attention. Zeolite based catalysts are useful for a better control of the complex network of reversible reactions that take place during the transalkylation processes, showing activity and selectivity higher than the amorphous silica-alumina [11, 13]. Among these solids, mordenite was shown to be the most active, but it easily deactivates because of its unidimensional structure of channels [ 15]. Therefore, it is important to develop new catalysts for this reaction. With this goal in mind, this work deals with the effect of nickel, palladium and platinum on the catalytic properties of Y zeolite. Because of large pores of Y zeolite and the hydrogenation capacity of the metals, it is expected that the catalysts can be more resistant against coke deposition than mordenite.
2. E X P E R I M E N T A L
The catalysts were prepared using a commercial NaY zeolite (CENPES/Petrobr/ls) with a silicon to aluminum molar ratio of 2.6. The metal (1%) was incorporated in the zeolite pores through ion exchange, by dispersing the solid (10g) in 40 mL of an ammonium nitrate solution (1 mol.L -~) and keeping the system at 80 ~ under stirring, for 1 h. The suspension was centrifuged and the process was repeated six times. The solid (NH4Y) was then dispersed in a solution of the metal precursor and kept at 80 ~ under stirring, for 12 h. After centrifugation, the solid was dried (100 ~ C) and heated (1 ~ -l) up to 550~ and kept in this temperature for 5 h. Nickel nitrate,tetraminpalladium chloride and hexachloroplatinic acid and were used as precursors to get catalysts with nickel, platinum and palladium respectively. The metal contents in solids were determined by inductively coupled plasma atomic emission spectroscopy (ICP/AES) using an Arl 3410 model equipment and samples (0.1g) dissolved in concentrated hydrofluoric acid. The X-ray diffraction experiments were performed at room temperature in a Shimadzu model XD3A instrument using CuK~t radiation generated at 30 kV and 20 mA and a nickel filter. The specific surface area and the porosity were measured in a model AZAP 2010 Micromeritics equipment on samples (0.1 g) previously heated at 300 ~ for 1 h, under nitrogen flow. The temperature programmed reduction (TPR) was performed in a model TPD/TPO 2900 Micromeritics equipment, using a 5% H2/N2 mixture. The acidity of the solids was measured by ammonia desorption using a Micromeritics model TPD/TPR 2900. The sample (0.7 g) was heated (10~ t) under argon (45 mL/min-1) for 30 min to take the humidity out. Ammonia was then injected in the loop by pulses until saturation. The sample was cooled to room temperature and then heated (10~ -1) to promote the ammonia desorption.
811 The transalkylation reaction was Carried out in a tubular reactor at 470 ~ and 1 atm, using a reaction mixture with 98% benzene and 2% 1,2,4-trimethylbenzene, H2/hydrocarbon (molar)= 4 and a WHSV- 1.0 h -1. Prior to the tests, the catalyst (0.3 g) was heated at 500 ~ under hydrogen for 2h. The gaseous effluent was analyzed by on line gas chromatography, using a model Trace GC Thermofinnigan instrument with thermal conductivity and ionization detectors. A commercial mordenite was also evaluated as a reference. The carbon amount in solids was measured in an EA112 Carlo Erba model equipment using 0.15 of sample which was introduced in a quartz tube and heated up to 900~ under oxygen flow. The combustion product was separated in a Porapak column using helium as a carrier and analyzed with a conductivity detector at 60~
3. RESULTS AND DISCUSSION
The amount of metal incorporated in zeolites was around 0.95% as shown in Table 1. The Xray diffractograms showed the typical pattern of Y zeolite regardless the presence of the metals. In all cases, the zeolite structure was not affected during the reaction. The textural properties of the catalysts are also displayed in Table 1. The samples showed different specific surface areas, depending on the presence and on the kind of the metal. The palladium-based sample presented the lowest value while the nickel-containing zeolite showed the highest one. Table 1. Chemical analysis results and textural properties of the flesh catalysts. Sample Metal content Sg, BET Pore volume t-plot micropore Average pore (%) (m2.g-l) BJH (cm3.g-1) volume (cm3.g-1) diameter (nm) Y -527 0.03337 0.2243 2.00 NiY 0.95 612 0.03491 0.2999 6.44 PdY 0.94 377 0.08935 0.1472 4.21 PtY 0.96 438 0.04084 O.1998 2.18 All solids showed I type isotherms, which are typical of microporous solid with limited mesoporosity, as illustrated in Figure 1, in accordance with previous work [16]. The curve rises almost vertically up to a nearly horizontal region and then rises again, as saturation is approached and bulk condensation begins to occur. The initial rise is due to adsorption related to micropore filling which takes place progressively in order of increasing size, although under a driving force of quite low relative pressure. After the micropores have filled, very little sorption takes place thereafter for there is essentially no place remaining on which adsorption can occur [17]. It can also see a hysteresis loop near saturation, in all curves, showing the presence of mesopores. The palladium-based zeolite showed a wider hysteresis loop indicating that it has a large amount of mesopores. This explains why it has the lowest specific surface area. In agreement with this conclusion, this sample also has the highest value of pore volume BHJ. As a whole, the addition of the metals decreased the specific surface area and the micropore volume. On the other hand, the pore volume and the average pore diameter were increased by the metals. Figure 2(a) displayed the TPR profiles of the samples. As expected, Y zeolite has no reduction peak. The curve of the nickel-based sample showed three broad peaks above 300 ~ The low temperature peak is related to the reduction of the metal in the large cages, while
812
_~- 160-
"v 160
i
Or)
155
150-
~" 150 ~O ~ 145
~ 140~
.
.
.
.
mm:mm-- __,m:.O-~-'= ~
=
-
~ 130-
0
~" 140
120-
~ 135
o 110- .i <
0
~ 130 < 0.(
0'.2 0'.4 016 0'.8 Relative pressure (p/po)
(a)
1.0
0.0
0.2 0.4 0.6 0.8 Relative pressure (p/po)
1.0
(b)
Figure 1. (a) Typical nitrogen isotherm of the Y, Ni and PtY and zeolite. (b) Nitrogen isothem of PdY zeolite:_ AdsortiowDesorption. the high temperature peaks can be assigned to the reduction of nickel in sodalite and hexagonal prism cages I18]. The palladium-based zeolite showed a reduction curve with two broad peaks typical of the reduction of palladium from Pd 2+ to Pd ~ in sodalite cages [19]. They are very small, as compared with previous work [20], suggesting that this metal may be not completely reduced in the zeolite structure. The TPR curve of the platinum-based sample showed three well-defined peaks around 190, 400 and 610 ~ assigned to the metal reduction in the large cages and in sodalite and hexagonal prism, respectively [ 18]. By comparing the different curve, one can see that the easiness of reducing decreases in the order Pt>Pd>Ni. The acidity of he samples are shown in Table 2. One can note that the nickel-based sample is the most acidic one followed by platinum and palladium-based zeolites. The acidity curves of the samples, Figure 2 (b), showed two broad desorption peaks in the 150-350 ~ and 350-800 ~ range. The low temperature peak, which seems to include two components, can be ascribed to weak and medium strength acid sites, while the other one is typical of strong acidity [21]. In all cases the low temperature peak is less intense that the high temperature one, indicating that most of the acid sites has weak and medium strength. These observations are in accordance with other results previously reported [22]. There is no simple relationship among these results and the amount of coke produced on the catalysts, as shown in Table 2. As coke formation is catalyzed by acid sites [23] it could be expected that the highest amount of coke could be produced on the most acidic solid. However, it was found that nickel-based catalysts formed the lowest amount of coke although it is the most acidic solid. On the other hand, it is known that coke is also produced on metallic sites [23] which in turn can hydrogenate and then eliminate it. Therefore, the results can be explained by considering that most of coke is deposited on the metal which hydrogenates and eliminates a part of it. From Table 2, we can conclude that nickel is the most efficient metal in hydrogenating coke followed by platinum and palladium. All samples were active in trimethylbenzene transalkylation with benzene (Table 2). In all cases, the metals improved the conversion in the order Ni>Pt>Pd. The selectivity changed depending on the kind of the metal. The Y zeolite produced almost exclusively toluene and showed low conversion. The introduction of palladium increased the conversion and led to the production of xylenes but it was not significant. However, the presence of nickel and platinum largely increased the conversion and the selectivity to xylenes. Nickel increased around ten
813
"7 el) O9
.Y NiY
0
~ . ~ - - ~ ~
O9
PdY o o r
< i
0
2()
46 66 8() Temperature (~
100
0
i
200
4()0 600 Temperature (~
(a)
I
800
(b)
Figure 2. (a) TPR curves of the samples. (b) Ammonia molecules adsorbed per gram of catalyst as a function of time (TPD). Table 2. Acidity (desorbed ammonia in mols per gram of catalyst), amount of coke on the catalysts (carbon) and conversion of trimethylbenzene (C) and selectivity (S) to aromatics and pentane.
Sample Y NiY PdY PtY Mor
NH3
C o k e (%)
(mol. g'~) -2.44 x 104 1.21 x 104 1.86 x 104 . . . .
15.68 2.57 12.11 8.24
C (%)
5.:4 50.2 7.0 29.2 10.6
Sxylenes Sethylbenzene
(%) 0.51 10.8 2.8 84.2 19.6
(%) 2.6 42.4 2.1 0.0 0.0
Stolueno
Spentane
(%) 95.5 37.2 86.9 0.0 80.3
(%) 1.4 0.0 4.9 0.0 0.0
times the conversion and produced xylenes, toluene and ethylbenzene. Platinum improved the catalyst even more and showed a conversion even higher, producing the highest amount of xylenes. By comparing these results with nitrogen isotherms, we can conclude that there is no effect of accessibility on the catalytic activity since the palladium-based catalyst would be expected to be the most active. Figure 3 displays the conversion of trimethylbenzene on the catalysts as a function of time. Y zeolite showed very low conversions during all the reaction time. The palladium-based catalyst showed a decrease in conversion after 4 h, probably due to coke deposition. The same behavior was shown by mordenite. In the case of nickel and platinum-based zeolite, there was a decrease in the first 4 h of reaction followed by an increase, which can be assigned to the hydrogenation activity of these metals. In both cases, the activity was completely recovered.
4. CONCLUSIONS From the results, one can conclude that nickel and platinum increase the conversion of trimethylbenzene with benzene, but palladium is ineffective9 Nickel produces large amounts of ethylbenzene, a high value chemical used in the production of styrene, and low amounts of xylenes. On the other hand, platinum produces large amount of xylenes and then is the best metal to improve the properties of Y zeolite to get theses compounds. Nickel and platinum are also able to prevent coke deposition in Y zeolite.
814
~ 40 = o
30
.,..~
20 o
~10 =
i
=
2
-
3
-
4
i
i
5'
d
=-
m
9
7' 8
Time (h) Figure 4. Conversion of trimethylbenzene in the transalkylation with benzene as a function of time. = Y zeolite;-o- NiY; ---0- P d Y ; + P t Y ~ - - mordenite. ACKNOWLEDGEMENTS
MSR, STG, LPG, DCS and ACO acknowledge PIBIC/CNPq, CAPES and CNPQ for their scholarship. This work was supported by CAPES, CNPq and FINEP. REFERENCES
[ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [ 11 ] [12] [ 13] [14] [15] [16] [ 17] [ 18] [ 19] [20] [21] [22] [23]
R.H. Harding, A. W. Peters and J. R. D. Nee, Appl. Catal. A: Gen., 221 (2001) 389. A. Corma, J. Catal., 216 (2003) 298. T.F. Jr. Degnan, J. Catal. 216 (2003) 32. R.Q. Long and R. T. Yang, J. Catal., 194 (2000) 80. A. Corma, Chem. Rev., 95 (1995) 559. A.Corma, Chem. Rev., 97 (1997) 2273. M. Guisnet and P. Magnoux, Stud. Surf. Sci.Catal., 88 (1994) 53. H. Kodama and S. Kazaki, J. Catal., 132 (1991) 512. G.F. Froment, in J. L. Figueredo (Ed.), Progress in Catalyst Deactivation (1982) 103. M. Guisnet, P. Magnoux, Catal. Today, 36 (1997) 477. J. Wu and L. Leu, Appl. Catal., 7 (1983) 283. L. Forni, G. Cremona, F. Missineo, G. Bellusi, C. Perego and G. Pazzuconi, Appl. Catal. A-Gen., 121 (1995) 261. T. Tsai, S. Liu and I. Wang Ikai, Appl. Catal. A-Gen., 181 (1999) 355. E.Dumitriu,V.Hulea, S.Kaliaguine and M. M. Huang, Appl. Catal. A-Gen., 135 (1996) 57. C.J. Kuei and L. J. Eu, Zeolites, 9 (1989) 193. J.C.Groen,L.A.A. Peffer and J.P6rez-Ramirez, Microp. Mesop. Mater., 60 (2003) 1. P. A. Webb and C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instruments Coorporation, Norcross, 1997, p. 58. M. Suki, K. Tsutsumi, H. Takahashi and Y. Saito, Zeolites, 9 (19) 98. O. Feeley and. W. M. H. Sahtler, Appl. Catal., 67 (1990) 141. O. Feeley and. W. M. H. Sahtler, Appl. Catal., 75 (1991) 93. S. Narayanan, A. Sultana, Q. T. Le and A. Auroux, Appl. Catal. A, 168 (1998) 373. S.Scir6, S.Minic6,C.Crisafulli,G.Burgio and V.Giuffrida, Stud. Surf. Sci. Catal. (2002) 1023. J.M. Parera and J. N. Beltramini, J. Catal., 112 (1988) 357.
809
Evaluation of MY Zeolites (M= Pt, Pd, Ni) in the Transalkylation of Trimethylbenzene with Benzene M. S. Ramos a, S. T. Grecco a, L. P. Gomes a, A. C. Oliveira a, P. Reyes b, M. Oportus b and M. C. Rangel a aGECCAT Grupo de Estudos em Cin6tica e Catfilise, Instituto de Quimica, Universidade Federal da Bahia. Campus Universitfirio de Ondina, Federagao. 40 170-280. Salvador, Bahia, Brazil. E-mail: [email protected] bFacultad de Ciencias Quimicas, Universidad de Concepci6n, Casilla 3-C Concepci6n, Chile The trimethlybenzene transalkylation with benzene has attracted increasing attention since valuable aromatic petrochemicals such as xylenes can be obtained from low value aromatics. Mordenite is the most used catalyst but it easily deactivates with time. In order to get more efficient catalysts, the effect of nickel, palladium and platinum on catalytic properties of Y zeolite was studied. It was found that nickel and platinum improve the conversion of trimethylbenzene but palladium is ineffective. Nickel produces mainly ethylbenzene and platinum produces large amount of xylenes, being the best metal to improve the catalyst. Nickel and platinum are also able to prevent coke deposition on Y zeolite.
I. INTRODUCTION Zeolite catalysts have received increasing attention and importance in petroleum refining and in chemical industry because of their unique properties. They are the most used catalysts in refining and petrochemical processes [1,2] as well as in the synthesis of fine and specialty chemicals [3] and in the depollution of industrial and automobile exhausts [4]. These applications are closely related to their high specific surface area, adsorption capacity, acid sites, size of the channels and intricate channel structure [5]. All these properties can be tailored for a particular application and are ultimately dependent on the thermal and on the hydrothermal stability of these materials [6]. Despite these catalytically desirable properties, zeolites often deactivate during industrial processes mainly by coke deposition [7, 8]. In most cases, the cost of catalyst deactivation is high enough to become the mastering of catalyst stability at least as important as the control of activity and selectivity [9]. In the transalkylation of aromatic hydrocarbons, for instance, the catalyst stability is one of the main issues, because coking can occur rapidly as a consequence of the high content of aromatics in the feed [10]. In these processes, valuable aromatic petrochemicals such as benzene and xylenes can be obtained from C9 aromatics and toluene [ 11 ]. Benzene and xylenes are important starting materials for several processes such as the production of synthetic fibers, plasticizers and resins. They are commercially produced by naphtha reforming and as a by-product of naphtha cracking for ethylene production (pyrolysis gasoline), in which substantial amounts of Cv and C9 aromatics are also produced [ 12, 13]. Xylene production is then optimized by upgrading these
810 low value streams through aromatic transalkylation reactions. These reactions also offer the possibility of developing environmental friendly processes at low operating costs [ 12]. The transalkylation of toluene with trimethylbenzenes takes place on solid acid catalysts and it is part of a complex network of reversible reactions. The transalkylation reaction is mainly in equilibrium with the disproportionation reaction and the isomerization of polymethylbenzenes also occurs [ 13, 14]. The distribution of the several isomers is governed by kinetics factors like the reactivity of the aromatic hydrocarbon, the nature of the catalytic sites, the texture and the morphology of the catalysts [13]. It is one of the most important reactions of conversion of methylaromatics aiming at the xylene production. However, with the recent market reduction of benzene as a consequence of environmental restrictions, benzene transalkylation with C9+ aromatics emerges as a potentially important reaction for commercial applications and its investigation has attracted increasing attention. Zeolite based catalysts are useful for a better control of the complex network of reversible reactions that take place during the transalkylation processes, showing activity and selectivity higher than the amorphous silica-alumina [11, 13]. Among these solids, mordenite was shown to be the most active, but it easily deactivates because of its unidimensional structure of channels [ 15]. Therefore, it is important to develop new catalysts for this reaction. With this goal in mind, this work deals with the effect of nickel, palladium and platinum on the catalytic properties of Y zeolite. Because of large pores of Y zeolite and the hydrogenation capacity of the metals, it is expected that the catalysts can be more resistant against coke deposition than mordenite.
2. E X P E R I M E N T A L
The catalysts were prepared using a commercial NaY zeolite (CENPES/Petrobr/ls) with a silicon to aluminum molar ratio of 2.6. The metal (1%) was incorporated in the zeolite pores through ion exchange, by dispersing the solid (10g) in 40 mL of an ammonium nitrate solution (1 mol.L -~) and keeping the system at 80 ~ under stirring, for 1 h. The suspension was centrifuged and the process was repeated six times. The solid (NH4Y) was then dispersed in a solution of the metal precursor and kept at 80 ~ under stirring, for 12 h. After centrifugation, the solid was dried (100 ~ C) and heated (1 ~ -l) up to 550~ and kept in this temperature for 5 h. Nickel nitrate,tetraminpalladium chloride and hexachloroplatinic acid and were used as precursors to get catalysts with nickel, platinum and palladium respectively. The metal contents in solids were determined by inductively coupled plasma atomic emission spectroscopy (ICP/AES) using an Arl 3410 model equipment and samples (0.1g) dissolved in concentrated hydrofluoric acid. The X-ray diffraction experiments were performed at room temperature in a Shimadzu model XD3A instrument using CuK~t radiation generated at 30 kV and 20 mA and a nickel filter. The specific surface area and the porosity were measured in a model AZAP 2010 Micromeritics equipment on samples (0.1 g) previously heated at 300 ~ for 1 h, under nitrogen flow. The temperature programmed reduction (TPR) was performed in a model TPD/TPO 2900 Micromeritics equipment, using a 5% H2/N2 mixture. The acidity of the solids was measured by ammonia desorption using a Micromeritics model TPD/TPR 2900. The sample (0.7 g) was heated (10~ t) under argon (45 mL/min-1) for 30 min to take the humidity out. Ammonia was then injected in the loop by pulses until saturation. The sample was cooled to room temperature and then heated (10~ -1) to promote the ammonia desorption.
811 The transalkylation reaction was Carried out in a tubular reactor at 470 ~ and 1 atm, using a reaction mixture with 98% benzene and 2% 1,2,4-trimethylbenzene, H2/hydrocarbon (molar)= 4 and a WHSV- 1.0 h -1. Prior to the tests, the catalyst (0.3 g) was heated at 500 ~ under hydrogen for 2h. The gaseous effluent was analyzed by on line gas chromatography, using a model Trace GC Thermofinnigan instrument with thermal conductivity and ionization detectors. A commercial mordenite was also evaluated as a reference. The carbon amount in solids was measured in an EA112 Carlo Erba model equipment using 0.15 of sample which was introduced in a quartz tube and heated up to 900~ under oxygen flow. The combustion product was separated in a Porapak column using helium as a carrier and analyzed with a conductivity detector at 60~
3. RESULTS AND DISCUSSION
The amount of metal incorporated in zeolites was around 0.95% as shown in Table 1. The Xray diffractograms showed the typical pattern of Y zeolite regardless the presence of the metals. In all cases, the zeolite structure was not affected during the reaction. The textural properties of the catalysts are also displayed in Table 1. The samples showed different specific surface areas, depending on the presence and on the kind of the metal. The palladium-based sample presented the lowest value while the nickel-containing zeolite showed the highest one. Table 1. Chemical analysis results and textural properties of the flesh catalysts. Sample Metal content Sg, BET Pore volume t-plot micropore Average pore (%) (m2.g-l) BJH (cm3.g-1) volume (cm3.g-1) diameter (nm) Y -527 0.03337 0.2243 2.00 NiY 0.95 612 0.03491 0.2999 6.44 PdY 0.94 377 0.08935 0.1472 4.21 PtY 0.96 438 0.04084 O.1998 2.18 All solids showed I type isotherms, which are typical of microporous solid with limited mesoporosity, as illustrated in Figure 1, in accordance with previous work [16]. The curve rises almost vertically up to a nearly horizontal region and then rises again, as saturation is approached and bulk condensation begins to occur. The initial rise is due to adsorption related to micropore filling which takes place progressively in order of increasing size, although under a driving force of quite low relative pressure. After the micropores have filled, very little sorption takes place thereafter for there is essentially no place remaining on which adsorption can occur [17]. It can also see a hysteresis loop near saturation, in all curves, showing the presence of mesopores. The palladium-based zeolite showed a wider hysteresis loop indicating that it has a large amount of mesopores. This explains why it has the lowest specific surface area. In agreement with this conclusion, this sample also has the highest value of pore volume BHJ. As a whole, the addition of the metals decreased the specific surface area and the micropore volume. On the other hand, the pore volume and the average pore diameter were increased by the metals. Figure 2(a) displayed the TPR profiles of the samples. As expected, Y zeolite has no reduction peak. The curve of the nickel-based sample showed three broad peaks above 300 ~ The low temperature peak is related to the reduction of the metal in the large cages, while
812
_~- 160-
"v 160
i
Or)
155
150-
~" 150 ~O ~ 145
~ 140~
.
.
.
.
mm:mm-- __,m:.O-~-'= ~
=
-
~ 130-
0
~" 140
120-
~ 135
o 110- .i <
0
~ 130 < 0.(
0'.2 0'.4 016 0'.8 Relative pressure (p/po)
(a)
1.0
0.0
0.2 0.4 0.6 0.8 Relative pressure (p/po)
1.0
(b)
Figure 1. (a) Typical nitrogen isotherm of the Y, Ni and PtY and zeolite. (b) Nitrogen isothem of PdY zeolite:_ AdsortiowDesorption. the high temperature peaks can be assigned to the reduction of nickel in sodalite and hexagonal prism cages I18]. The palladium-based zeolite showed a reduction curve with two broad peaks typical of the reduction of palladium from Pd 2+ to Pd ~ in sodalite cages [19]. They are very small, as compared with previous work [20], suggesting that this metal may be not completely reduced in the zeolite structure. The TPR curve of the platinum-based sample showed three well-defined peaks around 190, 400 and 610 ~ assigned to the metal reduction in the large cages and in sodalite and hexagonal prism, respectively [ 18]. By comparing the different curve, one can see that the easiness of reducing decreases in the order Pt>Pd>Ni. The acidity of he samples are shown in Table 2. One can note that the nickel-based sample is the most acidic one followed by platinum and palladium-based zeolites. The acidity curves of the samples, Figure 2 (b), showed two broad desorption peaks in the 150-350 ~ and 350-800 ~ range. The low temperature peak, which seems to include two components, can be ascribed to weak and medium strength acid sites, while the other one is typical of strong acidity [21]. In all cases the low temperature peak is less intense that the high temperature one, indicating that most of the acid sites has weak and medium strength. These observations are in accordance with other results previously reported [22]. There is no simple relationship among these results and the amount of coke produced on the catalysts, as shown in Table 2. As coke formation is catalyzed by acid sites [23] it could be expected that the highest amount of coke could be produced on the most acidic solid. However, it was found that nickel-based catalysts formed the lowest amount of coke although it is the most acidic solid. On the other hand, it is known that coke is also produced on metallic sites [23] which in turn can hydrogenate and then eliminate it. Therefore, the results can be explained by considering that most of coke is deposited on the metal which hydrogenates and eliminates a part of it. From Table 2, we can conclude that nickel is the most efficient metal in hydrogenating coke followed by platinum and palladium. All samples were active in trimethylbenzene transalkylation with benzene (Table 2). In all cases, the metals improved the conversion in the order Ni>Pt>Pd. The selectivity changed depending on the kind of the metal. The Y zeolite produced almost exclusively toluene and showed low conversion. The introduction of palladium increased the conversion and led to the production of xylenes but it was not significant. However, the presence of nickel and platinum largely increased the conversion and the selectivity to xylenes. Nickel increased around ten
813
"7 el) O9
.Y NiY
0
~ . ~ - - ~ ~
O9
PdY o o r
< i
0
2()
46 66 8() Temperature (~
100
0
i
200
4()0 600 Temperature (~
(a)
I
800
(b)
Figure 2. (a) TPR curves of the samples. (b) Ammonia molecules adsorbed per gram of catalyst as a function of time (TPD). Table 2. Acidity (desorbed ammonia in mols per gram of catalyst), amount of coke on the catalysts (carbon) and conversion of trimethylbenzene (C) and selectivity (S) to aromatics and pentane.
Sample Y NiY PdY PtY Mor
NH3
C o k e (%)
(mol. g'~) -2.44 x 104 1.21 x 104 1.86 x 104 . . . .
15.68 2.57 12.11 8.24
C (%)
5.:4 50.2 7.0 29.2 10.6
Sxylenes Sethylbenzene
(%) 0.51 10.8 2.8 84.2 19.6
(%) 2.6 42.4 2.1 0.0 0.0
Stolueno
Spentane
(%) 95.5 37.2 86.9 0.0 80.3
(%) 1.4 0.0 4.9 0.0 0.0
times the conversion and produced xylenes, toluene and ethylbenzene. Platinum improved the catalyst even more and showed a conversion even higher, producing the highest amount of xylenes. By comparing these results with nitrogen isotherms, we can conclude that there is no effect of accessibility on the catalytic activity since the palladium-based catalyst would be expected to be the most active. Figure 3 displays the conversion of trimethylbenzene on the catalysts as a function of time. Y zeolite showed very low conversions during all the reaction time. The palladium-based catalyst showed a decrease in conversion after 4 h, probably due to coke deposition. The same behavior was shown by mordenite. In the case of nickel and platinum-based zeolite, there was a decrease in the first 4 h of reaction followed by an increase, which can be assigned to the hydrogenation activity of these metals. In both cases, the activity was completely recovered.
4. CONCLUSIONS From the results, one can conclude that nickel and platinum increase the conversion of trimethylbenzene with benzene, but palladium is ineffective9 Nickel produces large amounts of ethylbenzene, a high value chemical used in the production of styrene, and low amounts of xylenes. On the other hand, platinum produces large amount of xylenes and then is the best metal to improve the properties of Y zeolite to get theses compounds. Nickel and platinum are also able to prevent coke deposition in Y zeolite.
814
~ 40 = o
30
.,..~
20 o
~10 =
i
=
2
-
3
-
4
i
i
5'
d
=-
m
9
7' 8
Time (h) Figure 4. Conversion of trimethylbenzene in the transalkylation with benzene as a function of time. = Y zeolite;-o- NiY; ---0- P d Y ; + P t Y ~ - - mordenite. ACKNOWLEDGEMENTS
MSR, STG, LPG, DCS and ACO acknowledge PIBIC/CNPq, CAPES and CNPQ for their scholarship. This work was supported by CAPES, CNPq and FINEP. REFERENCES
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