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HZSM-5 Catalyst: On the Nature of the Active Sites for the Dehydrogenation Reaction
Guai, L a nl (Editom), New Frontiers in Catalysk procctdinga of the 1Chh Inttmationnl Qngrcac on Catalysis, 19-24 July. 1992, Budapest, Hungary 0 1993 Elsevier Science Publishem B.V. All rights nscrved
TRANSFORMATION OF PROPANE OVER G-ZSM-5 CATALYST: ON THE NATURE OF THE ACIlVE SITES FOR THE DEHYDROGENATION REACTION P. Meriau&au and C. Naccache Institut de Recherches sur la Catalyse, 2 av. Albert Einstein, 69626 Villeurbanne =ex,
France
Summary Ga2O3 - HZSM-5 are characterized by several techni ues and used for propane cyclisation. A tentative model is given for the irst reaction step, propane dehydrogenation.
1
Introduction Ga2O3 HZSM-5 or allosilicate catalysts are active and selective for pro ane aromatization (1). f'hese catalysts have been extensively studied a n 8 the bifunctional character of these catal sts is now well acce ted. It has been shown that the catalytic properties o these solids were molified as they were reacted at hi h temperature (773 K range) with C3Hg 2) H2 and 02 (3). The mechanism y which the solid properties were c anged is still under discussion. The aim of this work is to describe the possible modifications of the active sites occuring during 0 2 - H2 treatments.
%
fy
6
Experimental ZSM-5 (SUAI = 15) sample was synthetised according to the literature. Gallium oxide (.2 to 5 wt %) was deposited by incipient wetness impregnation technique and samples were activated over night under 0 2 at 773 K ; subsequent H2 and 0 2 treatments were performed before propane reaction. The propane reaction was studied at low conversion in order to focus our attention on the first reaction steps namely C3 cracking into C and C2H4 and C3 dehydro enation into C3Hg. The localization of the gal ium phase was determined y using T.E.M. and S.T.E. M. techniques. Infrared spectroscopy was used to monitor the chan e in the concentration of Briinsted acid sites as a function of the nature of the tierma1 treatment and of the gallium loading. IR spectra of adsorbed pyridine was used to measure the ratio between Bronsted and Lewis acid sites.
%
I
Results and discussion On the table 1 are reported the change of the propane conversion for different treatments.
2432
Table 1
I
I %Ga IHZSM exchan
I %Ga HZSM5 :xchanled
Sample
HZSM
0.25%Ga HZSM5
I %Ga HZSM5
2.5%Ga HZSM5
Treatment
4
A B C
A B C
A B C
A B C
A
A
Propane conversion
3.9
2.6
4.4
5
4.4 4.3 3.3
5.6
0.3
%
B
0.9
Adsorbanc 3.8 3.8 [3610jcm-l x 10-
3.4 3.4
3.2
5.9
6
6.2 6.2
?k;R5
2. I
4.10-3 4.10.
Adsorbanc 0.026 [ I 5401cm- 1
0.0:
Treatments : A : 0 2 overnight 773 K B : A + [H2 lhr, O lhr] twice 773 K C : B + H2 l h r 573 K On this table are listed the concentration of pyridinium ions ( = 1540 c m - l ) and pyridine adsorbed on Lewis acid sites ( = 450 cm-1) together with the concentration of Bronsted acid sites ( = 3610 cm- ). Electron micrographs of 5 wt% Ga2O3 - HZSM-5 submitted to 0 2 treatment and to 0 2 - H2 treatments are shown on fig. 1. Table 2 gives the relative amount of allium (as measured by S.T.E.M. E.D.X.) present in the zeolite grain at di erent places.
1
f
2433
Table 2 S.T.E.M. (EDX) analysis o f : 5 % Ga2O3 / HZSM-5 (after 0 2 treatment 773 K) 5 % Ga2Og / HZSM-5 (after 0 2 and H2 treatments 773 K), The percentage of Ga203 / HZSM-5 as a function of the zone examined. 0 2 and H2 treatments
0 2 treatment Zone number
I
zone number
%Ga
1 2 3
0.5 0.2 59
4
1 7 6
I
%IGa
12,l 5,3 7,7 7,8
1’ 2’ 3’
4’
0 2 and H2 treatments
0 2 treatment
The results clearly indicate that Ga 0 is mainly on the external surface of the zeolite grain for 5 wt% Ga2O3 - H&d-5 calcined under 0 2 . After subsequent 02-H treatment gallium species apparently migrated within the zeolite crysta ; these results (T.E.M. and S.T.E.M. analysis) are in good agreement with the XPS data we reported earlier (4) which indicated that the Ga/Si atomic ratio decreased from . I to .04 when Ga 0 3 - ZSM- 5 activated in oxygen was further H treated at 773 K. The ecrease of the surface Ga concentration was attri uted to a migration of Ga inside the zeolite pores. The infrared results summarized in table 1 showed that the number of acidic OH roups present on the HZSM-5 sample decreased by increasin the gallium foading. In addition the OH groups decreased in number ollowing the reducing treatment and the reducing oxidizing treatments. It was concluded that the gallium species at high temperature (773 K) and H2 atmosphere rem laced H+ in cationic ositions thus decreasing the overall Bronsted aci ity. This hypothesis was urther reinforced throu h the analysis of the I. R. spectra of adsorbed pyridine : table 1 shows that t e number of pyridinium ions decreased after the H? treatment ; along with the decrease in Bronste acidity, the number of Lewis acid sites revealed by I.R. vibration at 1450 cmincreases. It is not possible with the results relative to pyridine adsorption (pyridine on Lewis acid sites) to distinguish between Ga+x in ionic exchange position and pyridine adsorbed on Ga2O oxide located on / in the zeolite channels since it has been shown that the a sorption of p ridine over Ga2O3 (5) reveals the presence of Lewis sites ( = 1450 crn- ), From the above
T
CT
i
P
a
F
fl
a
P
r
2434
discussion one concludes that after activation treatment in 0 2 - H2. gallium is distributed within the zeolite channels, part of G a being in ionic exchange ositions, part of it being in the form ofwell dispersed oxide. f h e conversion of propane studied on these solids clearly indicate the beneficial effect of both the addition of G a and the activation (H2 - 0 ) treatment. The maximum of the activity is obtained for a gallium loading of 2.5 wt%. Table 1 indicates that the 1 % GaHZSM-5 is much more active than 1 % GaNaZSM-5, both samples being prepared by ionic exchange ; this result suggests that the acidity of the su port is an important factor affecting the roperties of Ga-ZSM-!. Two different ex lanations could be proposed or such a phenomenon : as proposed by us (67it could be assumed that the dehydrogenation of propane is a reaction requiring a double site [Ga+x H+ or that the dehydrogenation of propane occurs on reduced gallium ions (Ga+ ), this reduction being favored with the acidity of the sup ort (7). Since it is observed (table 1) that after having reached the maximum o activity (treatment B) the conversion does not change dramatically if a additionnal H2 treatment is erformed (except for the 5 % Ga HZSM-5 sample for which a decrease is o served) it appears that the explanation (6) is more convincing than (7). The decrease to the activity of 5 % Ga HZSM-5 is in agreement with (6) since for this sample, the number of remaining Bronsted acid sites after reduction treatment is too low to balance the amount of Ga+x ions. In conclusion it is observed that the H2 and 0 2 treatments performed over GaHZSM-5 catalysts cause the migration of the gallium oxide from the external surface to the interior of the zeolite grain ; the active site for the dehydrogenation reaction of propane (first step in the cyclisation of propane) is a dual site, Ga+x in the vincinity of a proton.
f
f! i
f!
t
References (I)
E.E. Davies andA.J. Kolombos, Australian Patent G.B. 530121 (1979) to BP
(2)
L. Petit, J.P. Buriionville and F.Raatz, Zeolites : "facts, figure. future". P.A. Jacobs and R.A. Van Santen (Eds) Elsevier Amsterdam, 1989 p I163
(3)
E.P. Kieffer, Australian Patent 565365 (1985)
(4)
P. Meriaudeau and C. Naccache. Catalyst deactivation 1991 C.H. Bartholomew and J.B. Butt (Eds) Elsevier, 1991 p 767
(5)
P. Meriaudeau unpublished results
(6)
P. Meriaudeau and C. Naccache, J. Mol. Catal., 59 (1990) L31
(7)
G.L. Price and V. Kanazirev J. Mol. Catal., 66 (1991) 115
TRANSFORMATION OF PROPANE OVER G-ZSM-5 CATALYST: ON THE NATURE OF THE ACIlVE SITES FOR THE DEHYDROGENATION REACTION P. Meriau&au and C. Naccache Institut de Recherches sur la Catalyse, 2 av. Albert Einstein, 69626 Villeurbanne =ex,
France
Summary Ga2O3 - HZSM-5 are characterized by several techni ues and used for propane cyclisation. A tentative model is given for the irst reaction step, propane dehydrogenation.
1
Introduction Ga2O3 HZSM-5 or allosilicate catalysts are active and selective for pro ane aromatization (1). f'hese catalysts have been extensively studied a n 8 the bifunctional character of these catal sts is now well acce ted. It has been shown that the catalytic properties o these solids were molified as they were reacted at hi h temperature (773 K range) with C3Hg 2) H2 and 02 (3). The mechanism y which the solid properties were c anged is still under discussion. The aim of this work is to describe the possible modifications of the active sites occuring during 0 2 - H2 treatments.
%
fy
6
Experimental ZSM-5 (SUAI = 15) sample was synthetised according to the literature. Gallium oxide (.2 to 5 wt %) was deposited by incipient wetness impregnation technique and samples were activated over night under 0 2 at 773 K ; subsequent H2 and 0 2 treatments were performed before propane reaction. The propane reaction was studied at low conversion in order to focus our attention on the first reaction steps namely C3 cracking into C and C2H4 and C3 dehydro enation into C3Hg. The localization of the gal ium phase was determined y using T.E.M. and S.T.E. M. techniques. Infrared spectroscopy was used to monitor the chan e in the concentration of Briinsted acid sites as a function of the nature of the tierma1 treatment and of the gallium loading. IR spectra of adsorbed pyridine was used to measure the ratio between Bronsted and Lewis acid sites.
%
I
Results and discussion On the table 1 are reported the change of the propane conversion for different treatments.
2432
Table 1
I
I %Ga IHZSM exchan
I %Ga HZSM5 :xchanled
Sample
HZSM
0.25%Ga HZSM5
I %Ga HZSM5
2.5%Ga HZSM5
Treatment
4
A B C
A B C
A B C
A B C
A
A
Propane conversion
3.9
2.6
4.4
5
4.4 4.3 3.3
5.6
0.3
%
B
0.9
Adsorbanc 3.8 3.8 [3610jcm-l x 10-
3.4 3.4
3.2
5.9
6
6.2 6.2
?k;R5
2. I
4.10-3 4.10.
Adsorbanc 0.026 [ I 5401cm- 1
0.0:
Treatments : A : 0 2 overnight 773 K B : A + [H2 lhr, O lhr] twice 773 K C : B + H2 l h r 573 K On this table are listed the concentration of pyridinium ions ( = 1540 c m - l ) and pyridine adsorbed on Lewis acid sites ( = 450 cm-1) together with the concentration of Bronsted acid sites ( = 3610 cm- ). Electron micrographs of 5 wt% Ga2O3 - HZSM-5 submitted to 0 2 treatment and to 0 2 - H2 treatments are shown on fig. 1. Table 2 gives the relative amount of allium (as measured by S.T.E.M. E.D.X.) present in the zeolite grain at di erent places.
1
f
2433
Table 2 S.T.E.M. (EDX) analysis o f : 5 % Ga2O3 / HZSM-5 (after 0 2 treatment 773 K) 5 % Ga2Og / HZSM-5 (after 0 2 and H2 treatments 773 K), The percentage of Ga203 / HZSM-5 as a function of the zone examined. 0 2 and H2 treatments
0 2 treatment Zone number
I
zone number
%Ga
1 2 3
0.5 0.2 59
4
1 7 6
I
%IGa
12,l 5,3 7,7 7,8
1’ 2’ 3’
4’
0 2 and H2 treatments
0 2 treatment
The results clearly indicate that Ga 0 is mainly on the external surface of the zeolite grain for 5 wt% Ga2O3 - H&d-5 calcined under 0 2 . After subsequent 02-H treatment gallium species apparently migrated within the zeolite crysta ; these results (T.E.M. and S.T.E.M. analysis) are in good agreement with the XPS data we reported earlier (4) which indicated that the Ga/Si atomic ratio decreased from . I to .04 when Ga 0 3 - ZSM- 5 activated in oxygen was further H treated at 773 K. The ecrease of the surface Ga concentration was attri uted to a migration of Ga inside the zeolite pores. The infrared results summarized in table 1 showed that the number of acidic OH roups present on the HZSM-5 sample decreased by increasin the gallium foading. In addition the OH groups decreased in number ollowing the reducing treatment and the reducing oxidizing treatments. It was concluded that the gallium species at high temperature (773 K) and H2 atmosphere rem laced H+ in cationic ositions thus decreasing the overall Bronsted aci ity. This hypothesis was urther reinforced throu h the analysis of the I. R. spectra of adsorbed pyridine : table 1 shows that t e number of pyridinium ions decreased after the H? treatment ; along with the decrease in Bronste acidity, the number of Lewis acid sites revealed by I.R. vibration at 1450 cmincreases. It is not possible with the results relative to pyridine adsorption (pyridine on Lewis acid sites) to distinguish between Ga+x in ionic exchange position and pyridine adsorbed on Ga2O oxide located on / in the zeolite channels since it has been shown that the a sorption of p ridine over Ga2O3 (5) reveals the presence of Lewis sites ( = 1450 crn- ), From the above
T
CT
i
P
a
F
fl
a
P
r
2434
discussion one concludes that after activation treatment in 0 2 - H2. gallium is distributed within the zeolite channels, part of G a being in ionic exchange ositions, part of it being in the form ofwell dispersed oxide. f h e conversion of propane studied on these solids clearly indicate the beneficial effect of both the addition of G a and the activation (H2 - 0 ) treatment. The maximum of the activity is obtained for a gallium loading of 2.5 wt%. Table 1 indicates that the 1 % GaHZSM-5 is much more active than 1 % GaNaZSM-5, both samples being prepared by ionic exchange ; this result suggests that the acidity of the su port is an important factor affecting the roperties of Ga-ZSM-!. Two different ex lanations could be proposed or such a phenomenon : as proposed by us (67it could be assumed that the dehydrogenation of propane is a reaction requiring a double site [Ga+x H+ or that the dehydrogenation of propane occurs on reduced gallium ions (Ga+ ), this reduction being favored with the acidity of the sup ort (7). Since it is observed (table 1) that after having reached the maximum o activity (treatment B) the conversion does not change dramatically if a additionnal H2 treatment is erformed (except for the 5 % Ga HZSM-5 sample for which a decrease is o served) it appears that the explanation (6) is more convincing than (7). The decrease to the activity of 5 % Ga HZSM-5 is in agreement with (6) since for this sample, the number of remaining Bronsted acid sites after reduction treatment is too low to balance the amount of Ga+x ions. In conclusion it is observed that the H2 and 0 2 treatments performed over GaHZSM-5 catalysts cause the migration of the gallium oxide from the external surface to the interior of the zeolite grain ; the active site for the dehydrogenation reaction of propane (first step in the cyclisation of propane) is a dual site, Ga+x in the vincinity of a proton.
f
f! i
f!
t
References (I)
E.E. Davies andA.J. Kolombos, Australian Patent G.B. 530121 (1979) to BP
(2)
L. Petit, J.P. Buriionville and F.Raatz, Zeolites : "facts, figure. future". P.A. Jacobs and R.A. Van Santen (Eds) Elsevier Amsterdam, 1989 p I163
(3)
E.P. Kieffer, Australian Patent 565365 (1985)
(4)
P. Meriaudeau and C. Naccache. Catalyst deactivation 1991 C.H. Bartholomew and J.B. Butt (Eds) Elsevier, 1991 p 767
(5)
P. Meriaudeau unpublished results
(6)
P. Meriaudeau and C. Naccache, J. Mol. Catal., 59 (1990) L31
(7)
G.L. Price and V. Kanazirev J. Mol. Catal., 66 (1991) 115