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Effect of the binder on the properties of a mordenite catalyst for the selective conversion of methanol into light olefins
J. Weitkamp, H.G. Karge, H. Pfeifcr and W. Hdldcrich (Eds.) Zeolites and Relaled Microporous Malerials: Siaie of lhe Arr 1994 Studies in Surface Science and Caulysis, Vol. 84 0 1994 Elsevier Science B.V. All rights rcscrvcd.
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Effect of the binder on the properties of a mordenite catalyst for the selective conversion of methanol into light olefins. J. M. Fougerita, N.S. Gnepa, M. Guisneta, P Amiguesb, J.L. Duplanb and F. Huguesb
a - Universite de Poitiers - URA CNRS 350 - 40,Avenue du Recteur Pineau - 86022 Poitiers Cedex - France. b - lnstitut FranGais du Petrole, CEDI, BP 3,69390 Vernaison, France.
ABSTRACT
The association of an inactive binder with a dealuminated mordenite in a catalyst developed for the selective conversion of methanol into light olefins causes a decrease in the activity of the zeolite and an improvement of its stability. Ammonia thermodesorption and model reactions - n-heptane cracking at 803 K and metaxylene isomerization at 623 K - show that the activity decrease is due to a neutralization of the strongest protonic acid sites by exchange with the alkaline or alkaline-earth cations of the binder during the preparation of the catalyst. This neutralization is also responsible to part of the stability improvement. However this improvement is mainly due to the trapping by the binder of coke precursors with consequently a significant decrease in the amount of coke deposited on the zeolite, as shown by experiments with mechanical mixtures.
1. INTRODUCTION
Zeolite catalysts are generally manufactured by embedding the zeolite crystallites in a matrix. This matrix (or binder) can be constituted of natural materials (clays : kaolinite, halloysite etc ...) used as such or chemically or thermally modified orland of synthetic materials (silica, alumina, silica alumina etc ...). Obviously most of the information concerning the manufacture of catalysts is considered as confidential by the producers. However the role of matrix in the catalysts used in the most important process of refining i.e. FCC is quite well - known (1, 2).The primary role of matrix is to provide the size and the shape of the catalyst particles and to improve the mechanical strength. However the matrix can also play a chemical role. Thus in the cracking of heavy feeds the matrix must be active for preconverting the bulky molecules which cannot diffuse in the zeolite pores (1-4). Another function of the matrix is to improve the resistance of the zeolite to poisoning or to structural
1724
degradation by trapping nitrogen-containing compounds and metals such as vanadium (2). Modifications of the zeolite properties linked to the presence of the binder can also occur during the catalyst preparation. Thus the activity for acid catalyzed reactions of a high silica ZSM5 zeolite was enhanced when the zeolite was associated with alumina. The formation of new acid sites in the zeolite was demonstrated and attributed to the migration of soluble Al species from the binder into the zeolite framework and to their insertion in tetrahedral coordination (5). During steaming at high temperature of a USY zeolite intimately mixed with a silica matrix, an active silica alumina phase would be created at the zeolite matrix interface by reaction of extra framework aluminium species with silica. This active silica alumina caused an increase in gas oil cracking activity and an improvement of selectivities to gasoline and diesel oil (6). The matrix can also improve the zeolite stability. Thus the resistance to steam deactivation of Y and offretite zeolites was increased by incorporating them into an amorphous silica alumina matrix (7,8). This stability increase can be explained by a substitution of silicon atoms of the matrix for aluminium atoms expelled from the framework by steaming as demonstrated thanks to an isotopic 29Si labelling of the matrix (8). The matrix can also act as a sink for sodium ions of zeolites, which increases their hydrothermal stability (2). Deactivation of zeolites by coking can also depend on the binder and on the distribution of the zeolite crystallites in the catalyst (9). A catalyst selective for the conversion of methanol into light alkenes and particularly into propene has been recently developed by the French Institute of Petroleum (10, 11). This catalyst is constituted of a dealuminated mordenite associated to a kaolinite binder. We show here that this binder affects both the acid and catalytic properties of the mordenite.
2. EXPERIMENTAL
The industrial catalyst, the corresponding dealuminated mordenite and the binder were supplied by the French Institute of Petroleum (IFP, Vernaison). The industrial catalyst was constituted of a mixture of mordenite and of binder. The size of the particles was chosen equal to 60 pm except for the physical mixture used for coke measurements. Dimethylether transformation, m-xylene isomerization and n-heptane cracking were carried out in a flow reactor. The operating conditions used for measuring the activity of the samples were : - Dimethylether transformation : 803 K, p dimethylether = 0.2 bar, p N2 = 0.8 bar, flow rate of dimethylether = 74 mmo1e.h-1, - m-Xylene transformation : 623 K,p m-xylene = 0.2 bar, p N2 = 0.8 bar, flow rate of m-xylene = 30 mmo1e.h-1, - n-Heptane transformation : 803 K, p n-heptane = 0.1 bar, p N2 = 0.9 bar, flow rate of n- heptane = 14.2 mmo1e.h-1. The amounts of industrial catalyst (IC), mordenite (MOR),binder (Bf) and physical mixture of MOR and BI are indicated in the section results and discussion.
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Reaction products from all the transformations were analyzed on-line by gas chromatography. The GC apparatus was equipped with a flame ionization detector and with capillary columns : CP-SiI 5 CB (length : 30 m) and PLOT Alumina (length : 50 m) supplied by Chrompack for dimethylether and n-heptane transformations respectively and DB Wax (length : 30 m ) supplied by J & W Scientific for m-xylene transformation. The acidity was characterized by NH3 TPD, the amount of desorbed ammonia being measured with a thermal conductivity detector calibrated through volumetric titration with HCI solution. The ammonia adsorption was carried out at 323 K and the desorption from 323 K to 773 K step by step, each step lasting 1 hour. Coke contents were determined by the "Service Central d'Analyse - CNRS Vernaison- France".
3. RESULTS AND DISCUSSION 3-1 Comparative study of dimethylether transformation on the industrial catalyst (IC) and on its components mordenite (MOR) and binder (BI)
The same products : methanol and C1 - C12 hydrocarbons (mainly propene, butenes and pentenes) are formed on MOR and IC while BI catalyzes mainly the formation of methanol and of methane. Figure 1 compares the conversion of dimethylether into hydrocarbons for different time-on-stream on IC (50 mg) and on MOR (amount equal to that contained in 50 mg of IC). XO the initial conversion is slightly lower on IC than on MOR. BI is practically inactive (& c 1 % on the amount of BI equal to that contained in 50 mg of IC). The deactivation of IC is slower than
0
50
150
250
Time-on-stream (mn)
Figure 1. Conversion of dimethylether into hydrocarbons (XHC) on 50mg of industrial catalyst (IC) and on mordenite (MOR) as a function of time - on - stream.
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that of MOR. Furthermore the product distribution depends on the catalyst. Thus the percentages of propene, butenes and methylbenzenes are greater with MOR than with IC, those of C5 - C7 are lower. However the percentages of methane and ethylene are identical.
A decrease in acidity from MOR to IC could explain the decrease in the initial activity. This acidity decrease could also explain the decrease in selectivity to methylbenzenes, propene and butenes. Indeed the formation of these products requires various successive steps : - cyclization of > cf3 alkenes and hydrogen transfer from naphthenes to alkenes for the formation of methylbenzenes - alkylation of light alkenes with dimethylether then cracking of the resulting > C5 alkenes for the formation of part of the propene and the butenes (12). Most of the steps are bimolecular and therefore sensitive to acid site density (13-15). Furthermore the stabilizing effect of the binder can be attributed to a decrease in coking. Indeed the deactivation of zeolites during the reactions of organic compounds is mainly due to the formation and blockage inside the pores or on the outer surface of the crystallites of heavy secondary products generally called coke (16). Kinetics of coking and characterization of acidity through N H 3 thermodesorption and through model reactions were carried out to confirm the origin of the activity decrease and of the stability increase. 3-2 Characterization of acidity
NH3 t her modesorption The preparation of IC has no effect on the total number of acid sites estimated from the amount of NH3 desorbed above 323 K. However it causes a decrease in
‘P
2.5
400
500
600
700
800
Temperature (K)
Figure 2. NH3 TPD. Comparison of the number of ammonia molecules desorbed from the industrial catalyst (IC) and from an equivalent mordenite-binder mixture (MOR + BI).
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the acid strength. This is shown in Figure 2 which compares the number of ammonia molecules desorbed from IC to the number of ammonia molecules desorbed from an identical mixture of MOR and BI estimated from separate experiments with MOR and BI.
Model reactions While ammonia thermodesorption can give information on the distribution in strength of the acid sites (Lewis + Bronsted) model reactions allow to characterize specifically the active acid sites, generally the protonic ones (17, 18). Two model reactions : n-heptane cracking at 803 K and metaxylene isomerization at 623 K were used to specify the effect of the catalyst preparation on the mordenite acidity. Both reactions were carried out on 167 mg of IC or of an equivalent intimate physical mixture of MOR and BI (MOR + BI) and on the amounts of MOR or of BI contained in 167 mg of IC.
n-heptane transformation Whatever the catalyst Ci-Cg alkenes, alkanes (mainly C3-C4) and a small amount of C7 isomers are formed. Figure 3 shows that XO the initial conversion obtained on IC is lower than XO obtained on MOR or on the MOR-BI physical mixture. The activity of BI is very low compared to that of the other samples. The deactivation is more rapid with MOR than with IC, the physical mixture having an intermediate behaviour.
6
I*-*-*L*
I
O O
50
100
150
Time-on-stream (mn 1
Figure 3. n-Heptane cracking. Conversion of n-heptane (X) on 167mg of industrial catalyst (IC), on mordenite (MOR), on binder (BI) and on 167mg of a mordenite binder physical mixture (MOR+BI) as a function of time - on - stream.
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The product distribution was determined at various contact times. At a given conversion the C i - C t j distribution does not depend on the catalyst. On the other hand the olefinlalkane ratio is slightly greater with IC than with MOR and with the mixture (1.85 against 1.75 at 10 % conversion). The high value of this ratio shows that protolysis is mainly responsible for n-heptane activation. This is not surprising, since both for high temperatures (19) and for low acid site densities (20) this mechanism is favoured compared to the bimolecular hydride transfer mechanism. This model reaction confirms that the preparation of IC causes a decrease in the acidity of the mordenite component. The decrease in activity can be related to a decrease in the number and/or the strength of active acid sites, the increase in the olefin/alkane ratio to a decrease in the density of active sites. The identical behaviour of the physical mixture and of MOR means that the change in acidity occurs probably during the preparation of IC and not during the thermal treatment . On the other hand the presence of BI improves the stability of the mordenite component in the physical mixture and particularly in IC. m-xy lene transformation
With all the catalysts, m-xylene isomerizes into ortho and para-xylenes and leads through disproportionation to toluene and trimethylbenzenes. Benzene and tetramethylbenzenes are also observed at high conversion. Again BI is practically inactive and the preparation of IC causes a decrease in the initial activity of MOR. Indeed the initial conversions XO obtained with MOR or with the physical mixture (MOR-BI) are about 1.7 times greater than XO obtained with IC. No differences are observed in the stability of the catalysts probably because of the high rate of deactivation. From ammonia thermodesorption and from model reactions it can be concluded that the lower activity of the industrial catalyst (compared to pure mordenite) for dimethylether transformation into hydrocarbons is linked to a decrease in the number of mordenite protonic sites strong enough to catalyze this transformation. This neutralization of acid sites which occurs during the preparation of IC is probably due to a partial exchange of mordenite acid sites by the alkaline and alkaline-earth cations of the binder. Indeed the binder contains about 0.05 wt% of Na and K, 0.4 wt% of Ca and 0.1 % of Mg. This exchange can occur during the mixing of MOR and 81 in water orland during the drying. The effect of MOR exchange by Na and Ba ions on the rate of dimethylether conversion into hydrocarbons was previously determined (21). From the results obtained it can be concluded that the decrease of activity from MOR to IC can be due to the exchange of 5-10 % of the protonic sites of MOR by alkaline or alkaline-earth cations. It must be underlined that this partial exchange of MOR improved slightly the stability, the effect however being less pronounced than the one observed in this study. This stabilizing effect was attributed to the neutralization of the strongest acid sites (21) which are known as the most active for coke formation.
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3-3 Influence of the binder on the coke formation
The formation of coke during the conversion of dimethylether into hydrocarbons was investigated on 167 mg of IC or of an equivalent physical mixture of MOR and BI and on the amounts of MOR and BI contained in 167 mg of IC.To allow the analysis of coke after reaction on the components of the mixture different particle sizes were chosen : about 60 pm for MOR and 200 pm for BI. Under the operating conditions the initial conversion into hydrocarbons was equal to about 95 % with MOR, IC and the physical mixture and only 2 YOwith 81. After 4 1/2 hours reaction the conversion was equal to 0.3 % with BI and between 5 and 10 % with the other catalysts. Again the stability of IC was better than that of the physical mixture and much better than that of MOR. Figure 4 shows that whatever the time-on-stream the amount of coke formed on IC is about twice greater than that formed on MOR. The amount of coke formed on BI which yet is practically inactive for the formation of hydrocarbons is close to that formed on MOR (Fig 4). Therefore it could be concluded that coke is formed independently and in practically equal amounts on the MOR and BI components of IC or of the physical mixture. However the analysis of the amount of coke on the components of the physical mixture shows that the amount of coke formed on the MOR component is lower than on pure MOR (4.2 mg after 4.1/2 hours reaction against 6.4 mg) while the reverse is found on the BI component (8.5 mg against 7 mg on pure 61).This decrease in the amount of coke on the MOR component allows to explain that the stability of IC and of the physical mixture is better than that of pure MOR.
16 714
-E" 12 I
s
8
10
-0 8
E a
6
a
2
0 0 0
2
4
6
Time-on-stream (h 1
Figure 4. Amount of coke (mg) formed during dimethylether transformation on 167mg of industrial catalyst (IC), on mordenite (MOR), on binder (81)and on 167mg of a mordenite - binder physical mixture (MOR+BI) as a function of time - on - stream.
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The increase in the amount of coke formed on the BI component and the decrease on the MOR component can be related to a trapping by the binder of coke precursors. 4-CONCLUSION
The association of a binder with a dealuminated mordenite in a catalyst developed for the selective conversion of methanol into light olefins causes a decrease in the acidity on the zeolite with consequently a decrease in the activity. This acidity decrease can be related to a partial exchange of the protonic sites by alkaline and alkaline-earth cations of the binder during the catalyst preparation. A pronounced increase in stability is also observed which is mainly due to the trapping of coke precursors by the binder hence to a decrease in the deposit of coke on the zeolite.
REFERENCES
J. Biswas and I.E. Maxwell, Appl. Catal., 63 (1990) 197. J. Scherzer, Octane-Enhancing Zeolite FCC Catalysts, Chemical Industries, n042, H. Heinemann (Ed.), Marcel Dekker, New York, 1990. A. Humphries and J.R. Wilcox, Oil and Gas Journal, (1989) 45. 3. C.M.T. Hayward and W.S. Winkler, Hydrocarbon Processing, (1990) 55. 4. D.S. Shihabi, W.E. Garwood, P. Chu, J.N. Miale, R.M. Lago, C.T.W. Chu and 5. C.D. Chang, J. Catal., 93 (1985) 471. A. Corma, M. Grande and V. Fornes, Appl. Catal., 66 (1990) 247. 6. 7. P. Gelin and C. Gueguen, Appl. Catal., 38 (1988) 225. P. Gelin and T. Des Courieres, Appl. Catal., 72 (1991) 179. 8. J.W. Dean and D.B. Dadyburjor, Ind. Eng. Chem. Res., 28 (1989) 271. 9. 10. B. Juguin, F. Hugues and C. Hamon, ACS Symposium, The Production of Ethylene and Other Olefins by Non-Conventional Means, Boston, 1990. 11. J.P. Burzynski, F. Hugues, J.L. Duplan, D. Vuillemot, Communication EUROGAS, Oslo, 1992. 12. J.M. Fougerit, Ph.D. Thesis, No. 620, University of Poitiers, 1993. 13. L.A. Pine, P.J. Maher and W.A. Watcher, J. Catal., 85 (1984) 466. 14. M.L. Martin de Armando, N.S. Gnep and M. Guisnet, J. Chem. Res., Synop., (1981) 8 ; J. Chem. Res., Miniprint, (1981) 243. 15. M. Guisnet, F. Avendano, C. Bearez and F. Chevalier, J. Chern. SOC.,Chem. Commun. (1985) 336. 16. M. Guisnet and P. Magnoux, Appl. Catal., 54 (1989) 1. 17. H.A. Benesi and B.H.C. Winquist, Adv. Catal., 27 (1978) 97. 18. M.R. Guisnet, Acc. Chem. Res., 23 (1990) 392. 19. W.O. Haag and R.M. Dessau, Proc. 8th Intern. Congress, Catal., Dechema, Verlag Chemie, Weinheim, Vol. II (1984) 305. 20. G. Giannetto, S. Sansare and M. Guisnet, J. Chem. SOC.,Chem. Commun., (1985)336. 21. P. Roger, Ph. D. Thesis, No. 262, University of Poitiers, 1989. 1. 2.
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Effect of the binder on the properties of a mordenite catalyst for the selective conversion of methanol into light olefins. J. M. Fougerita, N.S. Gnepa, M. Guisneta, P Amiguesb, J.L. Duplanb and F. Huguesb
a - Universite de Poitiers - URA CNRS 350 - 40,Avenue du Recteur Pineau - 86022 Poitiers Cedex - France. b - lnstitut FranGais du Petrole, CEDI, BP 3,69390 Vernaison, France.
ABSTRACT
The association of an inactive binder with a dealuminated mordenite in a catalyst developed for the selective conversion of methanol into light olefins causes a decrease in the activity of the zeolite and an improvement of its stability. Ammonia thermodesorption and model reactions - n-heptane cracking at 803 K and metaxylene isomerization at 623 K - show that the activity decrease is due to a neutralization of the strongest protonic acid sites by exchange with the alkaline or alkaline-earth cations of the binder during the preparation of the catalyst. This neutralization is also responsible to part of the stability improvement. However this improvement is mainly due to the trapping by the binder of coke precursors with consequently a significant decrease in the amount of coke deposited on the zeolite, as shown by experiments with mechanical mixtures.
1. INTRODUCTION
Zeolite catalysts are generally manufactured by embedding the zeolite crystallites in a matrix. This matrix (or binder) can be constituted of natural materials (clays : kaolinite, halloysite etc ...) used as such or chemically or thermally modified orland of synthetic materials (silica, alumina, silica alumina etc ...). Obviously most of the information concerning the manufacture of catalysts is considered as confidential by the producers. However the role of matrix in the catalysts used in the most important process of refining i.e. FCC is quite well - known (1, 2).The primary role of matrix is to provide the size and the shape of the catalyst particles and to improve the mechanical strength. However the matrix can also play a chemical role. Thus in the cracking of heavy feeds the matrix must be active for preconverting the bulky molecules which cannot diffuse in the zeolite pores (1-4). Another function of the matrix is to improve the resistance of the zeolite to poisoning or to structural
1724
degradation by trapping nitrogen-containing compounds and metals such as vanadium (2). Modifications of the zeolite properties linked to the presence of the binder can also occur during the catalyst preparation. Thus the activity for acid catalyzed reactions of a high silica ZSM5 zeolite was enhanced when the zeolite was associated with alumina. The formation of new acid sites in the zeolite was demonstrated and attributed to the migration of soluble Al species from the binder into the zeolite framework and to their insertion in tetrahedral coordination (5). During steaming at high temperature of a USY zeolite intimately mixed with a silica matrix, an active silica alumina phase would be created at the zeolite matrix interface by reaction of extra framework aluminium species with silica. This active silica alumina caused an increase in gas oil cracking activity and an improvement of selectivities to gasoline and diesel oil (6). The matrix can also improve the zeolite stability. Thus the resistance to steam deactivation of Y and offretite zeolites was increased by incorporating them into an amorphous silica alumina matrix (7,8). This stability increase can be explained by a substitution of silicon atoms of the matrix for aluminium atoms expelled from the framework by steaming as demonstrated thanks to an isotopic 29Si labelling of the matrix (8). The matrix can also act as a sink for sodium ions of zeolites, which increases their hydrothermal stability (2). Deactivation of zeolites by coking can also depend on the binder and on the distribution of the zeolite crystallites in the catalyst (9). A catalyst selective for the conversion of methanol into light alkenes and particularly into propene has been recently developed by the French Institute of Petroleum (10, 11). This catalyst is constituted of a dealuminated mordenite associated to a kaolinite binder. We show here that this binder affects both the acid and catalytic properties of the mordenite.
2. EXPERIMENTAL
The industrial catalyst, the corresponding dealuminated mordenite and the binder were supplied by the French Institute of Petroleum (IFP, Vernaison). The industrial catalyst was constituted of a mixture of mordenite and of binder. The size of the particles was chosen equal to 60 pm except for the physical mixture used for coke measurements. Dimethylether transformation, m-xylene isomerization and n-heptane cracking were carried out in a flow reactor. The operating conditions used for measuring the activity of the samples were : - Dimethylether transformation : 803 K, p dimethylether = 0.2 bar, p N2 = 0.8 bar, flow rate of dimethylether = 74 mmo1e.h-1, - m-Xylene transformation : 623 K,p m-xylene = 0.2 bar, p N2 = 0.8 bar, flow rate of m-xylene = 30 mmo1e.h-1, - n-Heptane transformation : 803 K, p n-heptane = 0.1 bar, p N2 = 0.9 bar, flow rate of n- heptane = 14.2 mmo1e.h-1. The amounts of industrial catalyst (IC), mordenite (MOR),binder (Bf) and physical mixture of MOR and BI are indicated in the section results and discussion.
1725
Reaction products from all the transformations were analyzed on-line by gas chromatography. The GC apparatus was equipped with a flame ionization detector and with capillary columns : CP-SiI 5 CB (length : 30 m) and PLOT Alumina (length : 50 m) supplied by Chrompack for dimethylether and n-heptane transformations respectively and DB Wax (length : 30 m ) supplied by J & W Scientific for m-xylene transformation. The acidity was characterized by NH3 TPD, the amount of desorbed ammonia being measured with a thermal conductivity detector calibrated through volumetric titration with HCI solution. The ammonia adsorption was carried out at 323 K and the desorption from 323 K to 773 K step by step, each step lasting 1 hour. Coke contents were determined by the "Service Central d'Analyse - CNRS Vernaison- France".
3. RESULTS AND DISCUSSION 3-1 Comparative study of dimethylether transformation on the industrial catalyst (IC) and on its components mordenite (MOR) and binder (BI)
The same products : methanol and C1 - C12 hydrocarbons (mainly propene, butenes and pentenes) are formed on MOR and IC while BI catalyzes mainly the formation of methanol and of methane. Figure 1 compares the conversion of dimethylether into hydrocarbons for different time-on-stream on IC (50 mg) and on MOR (amount equal to that contained in 50 mg of IC). XO the initial conversion is slightly lower on IC than on MOR. BI is practically inactive (& c 1 % on the amount of BI equal to that contained in 50 mg of IC). The deactivation of IC is slower than
0
50
150
250
Time-on-stream (mn)
Figure 1. Conversion of dimethylether into hydrocarbons (XHC) on 50mg of industrial catalyst (IC) and on mordenite (MOR) as a function of time - on - stream.
1726
that of MOR. Furthermore the product distribution depends on the catalyst. Thus the percentages of propene, butenes and methylbenzenes are greater with MOR than with IC, those of C5 - C7 are lower. However the percentages of methane and ethylene are identical.
A decrease in acidity from MOR to IC could explain the decrease in the initial activity. This acidity decrease could also explain the decrease in selectivity to methylbenzenes, propene and butenes. Indeed the formation of these products requires various successive steps : - cyclization of > cf3 alkenes and hydrogen transfer from naphthenes to alkenes for the formation of methylbenzenes - alkylation of light alkenes with dimethylether then cracking of the resulting > C5 alkenes for the formation of part of the propene and the butenes (12). Most of the steps are bimolecular and therefore sensitive to acid site density (13-15). Furthermore the stabilizing effect of the binder can be attributed to a decrease in coking. Indeed the deactivation of zeolites during the reactions of organic compounds is mainly due to the formation and blockage inside the pores or on the outer surface of the crystallites of heavy secondary products generally called coke (16). Kinetics of coking and characterization of acidity through N H 3 thermodesorption and through model reactions were carried out to confirm the origin of the activity decrease and of the stability increase. 3-2 Characterization of acidity
NH3 t her modesorption The preparation of IC has no effect on the total number of acid sites estimated from the amount of NH3 desorbed above 323 K. However it causes a decrease in
‘P
2.5
400
500
600
700
800
Temperature (K)
Figure 2. NH3 TPD. Comparison of the number of ammonia molecules desorbed from the industrial catalyst (IC) and from an equivalent mordenite-binder mixture (MOR + BI).
1121
the acid strength. This is shown in Figure 2 which compares the number of ammonia molecules desorbed from IC to the number of ammonia molecules desorbed from an identical mixture of MOR and BI estimated from separate experiments with MOR and BI.
Model reactions While ammonia thermodesorption can give information on the distribution in strength of the acid sites (Lewis + Bronsted) model reactions allow to characterize specifically the active acid sites, generally the protonic ones (17, 18). Two model reactions : n-heptane cracking at 803 K and metaxylene isomerization at 623 K were used to specify the effect of the catalyst preparation on the mordenite acidity. Both reactions were carried out on 167 mg of IC or of an equivalent intimate physical mixture of MOR and BI (MOR + BI) and on the amounts of MOR or of BI contained in 167 mg of IC.
n-heptane transformation Whatever the catalyst Ci-Cg alkenes, alkanes (mainly C3-C4) and a small amount of C7 isomers are formed. Figure 3 shows that XO the initial conversion obtained on IC is lower than XO obtained on MOR or on the MOR-BI physical mixture. The activity of BI is very low compared to that of the other samples. The deactivation is more rapid with MOR than with IC, the physical mixture having an intermediate behaviour.
6
I*-*-*L*
I
O O
50
100
150
Time-on-stream (mn 1
Figure 3. n-Heptane cracking. Conversion of n-heptane (X) on 167mg of industrial catalyst (IC), on mordenite (MOR), on binder (BI) and on 167mg of a mordenite binder physical mixture (MOR+BI) as a function of time - on - stream.
1728
The product distribution was determined at various contact times. At a given conversion the C i - C t j distribution does not depend on the catalyst. On the other hand the olefinlalkane ratio is slightly greater with IC than with MOR and with the mixture (1.85 against 1.75 at 10 % conversion). The high value of this ratio shows that protolysis is mainly responsible for n-heptane activation. This is not surprising, since both for high temperatures (19) and for low acid site densities (20) this mechanism is favoured compared to the bimolecular hydride transfer mechanism. This model reaction confirms that the preparation of IC causes a decrease in the acidity of the mordenite component. The decrease in activity can be related to a decrease in the number and/or the strength of active acid sites, the increase in the olefin/alkane ratio to a decrease in the density of active sites. The identical behaviour of the physical mixture and of MOR means that the change in acidity occurs probably during the preparation of IC and not during the thermal treatment . On the other hand the presence of BI improves the stability of the mordenite component in the physical mixture and particularly in IC. m-xy lene transformation
With all the catalysts, m-xylene isomerizes into ortho and para-xylenes and leads through disproportionation to toluene and trimethylbenzenes. Benzene and tetramethylbenzenes are also observed at high conversion. Again BI is practically inactive and the preparation of IC causes a decrease in the initial activity of MOR. Indeed the initial conversions XO obtained with MOR or with the physical mixture (MOR-BI) are about 1.7 times greater than XO obtained with IC. No differences are observed in the stability of the catalysts probably because of the high rate of deactivation. From ammonia thermodesorption and from model reactions it can be concluded that the lower activity of the industrial catalyst (compared to pure mordenite) for dimethylether transformation into hydrocarbons is linked to a decrease in the number of mordenite protonic sites strong enough to catalyze this transformation. This neutralization of acid sites which occurs during the preparation of IC is probably due to a partial exchange of mordenite acid sites by the alkaline and alkaline-earth cations of the binder. Indeed the binder contains about 0.05 wt% of Na and K, 0.4 wt% of Ca and 0.1 % of Mg. This exchange can occur during the mixing of MOR and 81 in water orland during the drying. The effect of MOR exchange by Na and Ba ions on the rate of dimethylether conversion into hydrocarbons was previously determined (21). From the results obtained it can be concluded that the decrease of activity from MOR to IC can be due to the exchange of 5-10 % of the protonic sites of MOR by alkaline or alkaline-earth cations. It must be underlined that this partial exchange of MOR improved slightly the stability, the effect however being less pronounced than the one observed in this study. This stabilizing effect was attributed to the neutralization of the strongest acid sites (21) which are known as the most active for coke formation.
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3-3 Influence of the binder on the coke formation
The formation of coke during the conversion of dimethylether into hydrocarbons was investigated on 167 mg of IC or of an equivalent physical mixture of MOR and BI and on the amounts of MOR and BI contained in 167 mg of IC.To allow the analysis of coke after reaction on the components of the mixture different particle sizes were chosen : about 60 pm for MOR and 200 pm for BI. Under the operating conditions the initial conversion into hydrocarbons was equal to about 95 % with MOR, IC and the physical mixture and only 2 YOwith 81. After 4 1/2 hours reaction the conversion was equal to 0.3 % with BI and between 5 and 10 % with the other catalysts. Again the stability of IC was better than that of the physical mixture and much better than that of MOR. Figure 4 shows that whatever the time-on-stream the amount of coke formed on IC is about twice greater than that formed on MOR. The amount of coke formed on BI which yet is practically inactive for the formation of hydrocarbons is close to that formed on MOR (Fig 4). Therefore it could be concluded that coke is formed independently and in practically equal amounts on the MOR and BI components of IC or of the physical mixture. However the analysis of the amount of coke on the components of the physical mixture shows that the amount of coke formed on the MOR component is lower than on pure MOR (4.2 mg after 4.1/2 hours reaction against 6.4 mg) while the reverse is found on the BI component (8.5 mg against 7 mg on pure 61).This decrease in the amount of coke on the MOR component allows to explain that the stability of IC and of the physical mixture is better than that of pure MOR.
16 714
-E" 12 I
s
8
10
-0 8
E a
6
a
2
0 0 0
2
4
6
Time-on-stream (h 1
Figure 4. Amount of coke (mg) formed during dimethylether transformation on 167mg of industrial catalyst (IC), on mordenite (MOR), on binder (81)and on 167mg of a mordenite - binder physical mixture (MOR+BI) as a function of time - on - stream.
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The increase in the amount of coke formed on the BI component and the decrease on the MOR component can be related to a trapping by the binder of coke precursors. 4-CONCLUSION
The association of a binder with a dealuminated mordenite in a catalyst developed for the selective conversion of methanol into light olefins causes a decrease in the acidity on the zeolite with consequently a decrease in the activity. This acidity decrease can be related to a partial exchange of the protonic sites by alkaline and alkaline-earth cations of the binder during the catalyst preparation. A pronounced increase in stability is also observed which is mainly due to the trapping of coke precursors by the binder hence to a decrease in the deposit of coke on the zeolite.
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