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Al ratios on propane aromatization over highly active H-GaAlMFI
Catalysis Communications 7 (2006) 166–169 www.elsevier.com/locate/catcom
Influence of Si/Ga and Si/Al ratios on propane aromatization over highly active H-GaAlMFI T.V. Choudhary a
a,*
, A. Kinage b, S. Banerjee b, V.R. Choudhary
b
ConocoPhillips Company, Bartlesville Technology Center, Bartlesville, OK 74004, USA b Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India
Received 2 September 2005; received in revised form 21 October 2005; accepted 21 October 2005 Available online 22 December 2005
Abstract Conversion of propane to aromatics has been investigated at 550 C and space velocity range of 3000–12 000 cc/g/h over galloaluminosilicate zeolites (GaAlMFI) containing similar bulk Si/(Al + Ga) ratio but markedly different individual Si/Al and Si/Ga ratios. While Al showed a propensity to be distributed predominantly in the zeolite framework, a large fraction of total Ga was present as non-FW Ga species; due to this the strong acidity was higher for the GAlMFI zeolite with the larger bulk Al content. The zeolite with the larger bulk Al content (GAL1) showed superior propane aromatization (initial and time-on-stream) activity compared to the zeolite with the larger Ga content (GAL2). The aromatic distribution was also significantly influenced by the bulk Si/Al and Si/Ga ratios of the zeolites. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1. 2005 Elsevier B.V. All rights reserved. Keywords: Aromatization; Si/Ga ratio; Si/Al ratio; Propane; Ga-based zeolites; Zeolitic factors
1. Introduction The commercially important propane aromatization reaction has been investigated over a variety of Ga-based ZSM-5 type zeolites, viz., physically mixed Ga2O3 and HZSM-5, Ga-exchanged or Ga-impregnated H-ZSM-5 (Ga/H-ZSM-5) [1–3], H-Gallosilicates (GaMFI) [4–10], and H-Galloaluminosilicate (H-GaAlMFI) [11–14]. Amongst the various Ga-modified zeolites, GaAlMFI shows the highest activity/selectivity for the propane aromatization process [11]. The superior performance of these zeolites has been attributed to high zeolitic acidity in combination with well dispersed extra framework Ga species [11,14–16]. The bulk Si/Ga and Si/Al ratios are known to strongly influence the strong zeolitic acid sites and nonframework Ga species and thereby the propane aromatization activity for Ga-based zeolites [2,8]. This important *
Corresponding author. E-mail address: [email protected] Choudhary).
(T.V.
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.006
effect has not yet been investigated in case of the highly active GaAlMFI type zeolites. In this study, two GaAlMFI zeolites with similar bulk Si/Al + Ga ratio but considerably different individual Si/Al and Si/Ga ratios were synthesized and investigated for the propane aromatization reaction. The objectives of the study were to understand the effect of bulk Si/Al and Si/Ga ratios on: (a) the distribution of Al and Ga in the zeolite framework and non framework sites, (b) strong acidity of the zeolites and (c) activity (initial and time-on-stream) and product selectivity for the propane aromatization reaction. 2. Experimental 2.1. Catalyst synthesis H-GaAlMFI zeolites was synthesized by hydrothermal crystallization from a gel consisting of Na-trisilicate (Fluka), gallium nitrate (Aldrich), aluminium nitrate (BDH), TPA-Br (Aldrich), sulfuric acid and deionized water in a stainless steel autoclave at 180 C for 96 h.
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
167
Table 1 Bulk and surface properties of the H-GaAlMFI zeolites Zeolite
Bulk composition
Framework (FW) composition
Si/Al
Si/Ga
Si/(Al + Ga)
Si/Al
Si/Ga
Si/(Al + Ga)
GAL1 GAL2
17.7 49.6
46.6 15.3
12.8 11.7
25.0 54.1
88.6 44.8
19.5 24.5
Crystal size (lm)
Crystal morphology
Non-FW Ga (mmol/g)
Strong acidity (mmol/g)
3.5 ± 0.5 3.7 ± 1
Spherical hexagonal Spherical hexagonal
0.16 0.65
0.54 0.41
The zeolite crystals were washed thoroughly with deionized water and dried at 120 C for 10 h. After calcination at 550 C for 15 h under static air, it was converted into its NH4 form by repeated exchanging with 1 M ammonium nitrate solution at 80 C. The zeolite samples were obtained by deammoniation of their NH4 form by calcination at 600 C for 1 h in a flow of moisture free air (1800 cc/g/h). The bulk and framework composition for the two zeolites is shown in Table 1.
550 C and space velocity of 3000 cc/g/h using a propane–N2 mixture (33.3 mol% propane) as feed. The dehydrogenation/cracking (D/C) ratio was determined from the product selectivities as follows:
2.2. Catalyst characterization
Table 1 shows the bulk and surface properties of the HGaAlMFI zeolites. SEM studies revealed a similar crystal morphology and crystal size for the two zeolites. The zeolites also have a similar bulk Si/Al + Ga ratio but markedly different individual Si/Al and Si/Ga ratios; GAL1 has a higher bulk Al content (Si/Al = 17.7 and Si/Ga = 46.6) while GAL2 has a higher bulk Ga content (Si/Al = 49.6 and Si/Ga = 15.3). It is noteworthy that Al has a propensity to be distributed predominantly in the zeolite framework. In contrast a large fraction of total Ga is present as non-FW Ga species, which are formed in the zeolite channels by degalliation of FW Ga during calcination. The tendency of the Ga species to be present as non-FW Ga species appears to increase with increasing bulk Ga content (Table 1); this may be observed from the large difference in the bulk and FW Si/Ga ratios. Since the ionic ˚ ) is larger than that of Al3+ radius of Ga3+ (0.62 A ˚ (0.51 A), the isomorphous substitution of Al by Ga is expected to result in a less stable MFI structure. The lower ability of the Ga species to be accommodated in the zeolite FW may be attributed to its tendency to destabilize the MFI structure. The strong acidity of the GaALMFI zeolite depends on the FW (tetrahedral) Ga and Al content. Although the bulk Si/Al + Ga ratios are the same for the two zeolites, they have considerably different FW Si/ Al + Ga ratios. The lower acidity of GAL2 can therefore be attributed to the lower FW Al and Ga content. The total propane conversion and aromatic yields at 550 C for the zeolites are shown in Fig. 1(a) and (b) at space velocities of 3000 and 6000 cc/g/h, respectively. GAL1 showed higher propane conversion and aromatic yield than GAL2 at all the investigated space velocities (only data at 3000 and 6000 cc/g/h are shown here). Our previous studies have shown that high zeolitic acidity and optimum content/dispersion of non-FW Ga species are responsible for superior propane aromatization activity of GaAlMFI type zeolites [11]. The better performance of the GAL1 zeolite as compared to GAL2 may be attributed to its higher strong zeolitic acidity (higher number of
The strong acidity of the zeolite samples was measured in terms of pyridine chemisorbed at 400 C. Details of the procedure to determine acidity by this method have been described earlier [8]. The FW Si/(Ga + Al) ratios were determined from 29Si MAS NMR and the individual FW Si/Al and Si/Ga ratios were estimated from the FW Si/ (Ga + Al), octahedral Al (determined from 27Al MAS NMR) and bulk concentration of Al in the zeolite. The determination of the FW Si/Al and Si/Ga ratio from the 29 Si MAS NMR is not very accurate as the T sites in ZSM-5-type zeolites are not crystallographically identical [17]. Nevertheless this information can be appropriately used, when large changes in the FW ratios are observed. The MFI structure of the zeolites was confirmed by XRD. The zeolites were characterized for their crystal morphology by SEM, using JOEL scanning electron microscope. 2.3. Activity tests The propane aromatization reaction was carried out in an atmospheric pressure continuous flow quartz reactor equipped with a Chromel–Alumel thermocouple in the center of the catalyst bed (1 g catalyst and 1:1 propane–N2 mix as feed). The conversion and selectivity data at different space velocities (3000–12 000 cc/g/h) were obtained at 550 C in the absence of catalyst deactivation (for initial activity and selectivity). This was accomplished by employing the square pulse technique by passing the reaction mixture over a fresh catalyst for a short period (5 min) under steady state and then replacing the reaction mixture with pure N2 during the period of product analysis. The reaction products were analyzed by an on-line GC with FID, using Poropak-Q (3 mm · 3 m) and Benton-34 (5%) and dinonylthalate (5%) on Chromosorb-W (3 mm · 5 m) columns. For time on stream studies, the propane aromatization reaction was investigated as a function of time at
D=C ¼ ½100 ðselectivity for C1 ; C2¼ and C2 Þ =ðselectivity for C1 ; C2¼ and C2 Þ. 3. Results and discussion
168
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
Aromatic Yield Propane Conversion
Zeolite
GAL2
GAL1
50.0
55.0
60.0
65.0
70.0
75.0
80.0
Conversion/Aromatic Yield (%)
a
Fig. 2. Product selectivity for GAL1 and GAL2 in the propane aromatization reaction; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
Aromatic Yield Propane Conversion GAL2
Zeolite
Table 2 D/C ratio for GAL1 and GAL2 at different space velocities Space velocity (cc/g/h)
D/C ratio
GAL1 GAL2
GAL1
3000
6000
12 000
4.5 5.2
4.2 4.5
8.6 9.1
Reaction temperature = 550 C.
b
45.0
50.0
55.0
60.0
65.0
70.0
75.0
Conversion/Aromatic Yield (%)
Fig. 1. Propane conversion and aromatic yields for GAL1 and GAL2 at: (a) 3000 cc/g/h; (b) 6000 cc/g/h; Reaction temperature = 550 C.
strong acid sites); this is in good agreement with previous studies wherein, we have observed a strong dependence of aromatic yields on the strong zeolitic acidity [11,14]. The higher non-FW Ga content in case of GAL2 is apparently not sufficient to compensate for the activity contribution from the higher strong acidity of GAL1. Fig. 2 shows the product selectivity for the zeolites at 3000 cc/g/h; a similar trend in selectivity is observed at other investigated space velocities. While the methane selectivity was higher for GAL1 the propylene selectivity was higher for GAL2. Higher methane selectivity for GAL1 may be attributed to its higher strong zeolitic acidity (higher cracking activity), while higher propylene selectivity for GAL2 may be related to its higher non-FW Ga content (higher dehydrogenation activity). No significant difference in selectivity was observed for the other products. The dehydrogenation/cracking (D/C) ratio for the zeolites is shown in Table 2. At all the investigated space velocities GAL2 has a higher D/C ratio. Our previous studies have shown that dehydrogenation activity increases with increasing nonFW Ga content [8]; thus the higher D/C ratio for the GAL2 zeolite may be attributed to its higher non-FW Ga content.
Fig. 3 shows the aromatic distribution for the zeolites at 3000 cc/g/h. Although there was no significant difference in the selectivity for total aromatics in case of the two zeolites, distinct differences could be observed in the aromatic distribution at all investigated space velocities. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1. No significant differences in the selectivity of C9+ aromatic compounds were observed. 100.0 90.0 80.0
Selectivity (%)
40.0
Benzene Toluene C8 Aromatics C9 Aromatics
70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
GAL1
GAL2
Zeolite
Fig. 3. Aromatic distribution for GAL1 and GAL2 in the propane aromatization reaction; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
(c) Due to the higher content of non-FW Ga, GAL2 showed a larger dehydrogenation/cracking ratio for the propane aromatization reaction. (d) GAL1 showed a higher selectivity for methane, while GAL2 was more selective for propylene formation. (e) Although the zeolites showed no difference in the selectivity for total aromatics, distinct differences could be observed in the distribution of aromatic compounds. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1.
85 GAL1 Aromatic Yield GAL2 Aromatic Yield GAL1 TotalConversion GAL2 TotalConversion
80
Conversion/Yield (%)
75
169
70 65 60 55 50 45
References
40 0
50
100
150
200
250
300
350
400
Time on Stream (mins.)
Fig. 4. Time-on-stream propane aromatization data for GAL1 and GAL2; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
Time on stream propane aromatization studies were undertaken at 550 C and space velocity of 3000 cc/g/h. A decrease in the total propane conversion and aromatic yield is observed for GAL1and GAL2 as a function of reaction time (see Fig. 4). The deactivation in propane aromatization activity may be attributed to the poisoning of the strong zeolitic acid sites by coke [8]. GAL1 showed higher propane aromatization than GAL2 throughout the time on stream reaction. 4. Conclusions The salient features of the study are summarized below: (a) GAL1, the GaMFI type zeolite with larger bulk Al content and lower Ga content, showed superior propane aromatization (initial and time-on-stream) activity as compared to GAL2. (b) A considerable fraction of total Ga was present as non-FW Ga species. In contrast, Al was predominantly present in the zeolite framework.
[1] M. Guisnet, N.S. Gnep, F. Alario, Appl. Catal. 89 (1992) 1. [2] V.R. Choudhary, K. Mantri, C. Sivadinarayana, Micropor. Mesopor. Mater. 37 (2000) 1. [3] G. Giannetto, R. Monque, R. Galliasso, Catal. Rev. Sci. Eng. 36 (1994) 271. [4] C.R. Bayense, A.J.H.P. van der Pol, J.H.C. van Hoof, Appl. Catal. 72 (1991) 81. [5] C.R. Bayense, J.H.C. van Hoof, Appl. Catal. A 79 (1991) 127. [6] G. Giannnetto, A. Montes, N.S. Gnep, A. Florentino, P. Cartraud, M. Guisnet, J. Catal. 145 (1993) 86. [7] V.R. Choudhary, A.K. Kinage, C. Sivadinarayana, S.D. Sansare, M. Guisnet, Catal. Lett. 33 (1995) 401. [8] V.R. Choudhary, A.K. Kinage, C. Sivadinarayana, P. Devdas, S.D. Sansare, M. Guisnet, J. Catal. 158 (1996) 34. [9] V.R. Choudhary, P. Devdas, A.K. Kinage, C. Sivadinarayana, M. Guisnet, J. Catal. 158 (1996) 537. [10] V.R. Choudhary, C. Sivadinarayana, A.K. Kinage, P. Devdas, M. Guisnet, Appl. Catal. A 136 (1996) 125. [11] V.R. Choudhary, A.K. Kinage, C. Sivadinarayana, M. Guisnet, J. Catal. 158 (1996) 23. [12] V.R. Choudhary, A.K. Kinage, T.V. Choudhary, Appl. Catal. A 162 (1997) 239. [13] T.V. Choudhary, A.K. Kinage, S. Banerjee, V.R. Choudhary, Micropor. Mesopor. Mater. 70 (2004) 37. [14] T.V. Choudhary, A.K. Kinage, S. Banerjee, V.R. Choudhary, Micropor. Mesopor. Mater., in press. [15] V.R. Choudhary, K.C. Mondal, S.A.R. Mulla, Angew Chem. Int. Ed. 44 (2005) 4381. [16] T.V. Choudhary, A.K. Kinage, S, Banerjee, V.R. Choudhary, submitted for publication. [17] J.M. Thomas, X.S. Lin, J. Phys. Chem. 90 (1986) 4843.
Influence of Si/Ga and Si/Al ratios on propane aromatization over highly active H-GaAlMFI T.V. Choudhary a
a,*
, A. Kinage b, S. Banerjee b, V.R. Choudhary
b
ConocoPhillips Company, Bartlesville Technology Center, Bartlesville, OK 74004, USA b Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India
Received 2 September 2005; received in revised form 21 October 2005; accepted 21 October 2005 Available online 22 December 2005
Abstract Conversion of propane to aromatics has been investigated at 550 C and space velocity range of 3000–12 000 cc/g/h over galloaluminosilicate zeolites (GaAlMFI) containing similar bulk Si/(Al + Ga) ratio but markedly different individual Si/Al and Si/Ga ratios. While Al showed a propensity to be distributed predominantly in the zeolite framework, a large fraction of total Ga was present as non-FW Ga species; due to this the strong acidity was higher for the GAlMFI zeolite with the larger bulk Al content. The zeolite with the larger bulk Al content (GAL1) showed superior propane aromatization (initial and time-on-stream) activity compared to the zeolite with the larger Ga content (GAL2). The aromatic distribution was also significantly influenced by the bulk Si/Al and Si/Ga ratios of the zeolites. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1. 2005 Elsevier B.V. All rights reserved. Keywords: Aromatization; Si/Ga ratio; Si/Al ratio; Propane; Ga-based zeolites; Zeolitic factors
1. Introduction The commercially important propane aromatization reaction has been investigated over a variety of Ga-based ZSM-5 type zeolites, viz., physically mixed Ga2O3 and HZSM-5, Ga-exchanged or Ga-impregnated H-ZSM-5 (Ga/H-ZSM-5) [1–3], H-Gallosilicates (GaMFI) [4–10], and H-Galloaluminosilicate (H-GaAlMFI) [11–14]. Amongst the various Ga-modified zeolites, GaAlMFI shows the highest activity/selectivity for the propane aromatization process [11]. The superior performance of these zeolites has been attributed to high zeolitic acidity in combination with well dispersed extra framework Ga species [11,14–16]. The bulk Si/Ga and Si/Al ratios are known to strongly influence the strong zeolitic acid sites and nonframework Ga species and thereby the propane aromatization activity for Ga-based zeolites [2,8]. This important *
Corresponding author. E-mail address: [email protected] Choudhary).
(T.V.
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.10.006
effect has not yet been investigated in case of the highly active GaAlMFI type zeolites. In this study, two GaAlMFI zeolites with similar bulk Si/Al + Ga ratio but considerably different individual Si/Al and Si/Ga ratios were synthesized and investigated for the propane aromatization reaction. The objectives of the study were to understand the effect of bulk Si/Al and Si/Ga ratios on: (a) the distribution of Al and Ga in the zeolite framework and non framework sites, (b) strong acidity of the zeolites and (c) activity (initial and time-on-stream) and product selectivity for the propane aromatization reaction. 2. Experimental 2.1. Catalyst synthesis H-GaAlMFI zeolites was synthesized by hydrothermal crystallization from a gel consisting of Na-trisilicate (Fluka), gallium nitrate (Aldrich), aluminium nitrate (BDH), TPA-Br (Aldrich), sulfuric acid and deionized water in a stainless steel autoclave at 180 C for 96 h.
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
167
Table 1 Bulk and surface properties of the H-GaAlMFI zeolites Zeolite
Bulk composition
Framework (FW) composition
Si/Al
Si/Ga
Si/(Al + Ga)
Si/Al
Si/Ga
Si/(Al + Ga)
GAL1 GAL2
17.7 49.6
46.6 15.3
12.8 11.7
25.0 54.1
88.6 44.8
19.5 24.5
Crystal size (lm)
Crystal morphology
Non-FW Ga (mmol/g)
Strong acidity (mmol/g)
3.5 ± 0.5 3.7 ± 1
Spherical hexagonal Spherical hexagonal
0.16 0.65
0.54 0.41
The zeolite crystals were washed thoroughly with deionized water and dried at 120 C for 10 h. After calcination at 550 C for 15 h under static air, it was converted into its NH4 form by repeated exchanging with 1 M ammonium nitrate solution at 80 C. The zeolite samples were obtained by deammoniation of their NH4 form by calcination at 600 C for 1 h in a flow of moisture free air (1800 cc/g/h). The bulk and framework composition for the two zeolites is shown in Table 1.
550 C and space velocity of 3000 cc/g/h using a propane–N2 mixture (33.3 mol% propane) as feed. The dehydrogenation/cracking (D/C) ratio was determined from the product selectivities as follows:
2.2. Catalyst characterization
Table 1 shows the bulk and surface properties of the HGaAlMFI zeolites. SEM studies revealed a similar crystal morphology and crystal size for the two zeolites. The zeolites also have a similar bulk Si/Al + Ga ratio but markedly different individual Si/Al and Si/Ga ratios; GAL1 has a higher bulk Al content (Si/Al = 17.7 and Si/Ga = 46.6) while GAL2 has a higher bulk Ga content (Si/Al = 49.6 and Si/Ga = 15.3). It is noteworthy that Al has a propensity to be distributed predominantly in the zeolite framework. In contrast a large fraction of total Ga is present as non-FW Ga species, which are formed in the zeolite channels by degalliation of FW Ga during calcination. The tendency of the Ga species to be present as non-FW Ga species appears to increase with increasing bulk Ga content (Table 1); this may be observed from the large difference in the bulk and FW Si/Ga ratios. Since the ionic ˚ ) is larger than that of Al3+ radius of Ga3+ (0.62 A ˚ (0.51 A), the isomorphous substitution of Al by Ga is expected to result in a less stable MFI structure. The lower ability of the Ga species to be accommodated in the zeolite FW may be attributed to its tendency to destabilize the MFI structure. The strong acidity of the GaALMFI zeolite depends on the FW (tetrahedral) Ga and Al content. Although the bulk Si/Al + Ga ratios are the same for the two zeolites, they have considerably different FW Si/ Al + Ga ratios. The lower acidity of GAL2 can therefore be attributed to the lower FW Al and Ga content. The total propane conversion and aromatic yields at 550 C for the zeolites are shown in Fig. 1(a) and (b) at space velocities of 3000 and 6000 cc/g/h, respectively. GAL1 showed higher propane conversion and aromatic yield than GAL2 at all the investigated space velocities (only data at 3000 and 6000 cc/g/h are shown here). Our previous studies have shown that high zeolitic acidity and optimum content/dispersion of non-FW Ga species are responsible for superior propane aromatization activity of GaAlMFI type zeolites [11]. The better performance of the GAL1 zeolite as compared to GAL2 may be attributed to its higher strong zeolitic acidity (higher number of
The strong acidity of the zeolite samples was measured in terms of pyridine chemisorbed at 400 C. Details of the procedure to determine acidity by this method have been described earlier [8]. The FW Si/(Ga + Al) ratios were determined from 29Si MAS NMR and the individual FW Si/Al and Si/Ga ratios were estimated from the FW Si/ (Ga + Al), octahedral Al (determined from 27Al MAS NMR) and bulk concentration of Al in the zeolite. The determination of the FW Si/Al and Si/Ga ratio from the 29 Si MAS NMR is not very accurate as the T sites in ZSM-5-type zeolites are not crystallographically identical [17]. Nevertheless this information can be appropriately used, when large changes in the FW ratios are observed. The MFI structure of the zeolites was confirmed by XRD. The zeolites were characterized for their crystal morphology by SEM, using JOEL scanning electron microscope. 2.3. Activity tests The propane aromatization reaction was carried out in an atmospheric pressure continuous flow quartz reactor equipped with a Chromel–Alumel thermocouple in the center of the catalyst bed (1 g catalyst and 1:1 propane–N2 mix as feed). The conversion and selectivity data at different space velocities (3000–12 000 cc/g/h) were obtained at 550 C in the absence of catalyst deactivation (for initial activity and selectivity). This was accomplished by employing the square pulse technique by passing the reaction mixture over a fresh catalyst for a short period (5 min) under steady state and then replacing the reaction mixture with pure N2 during the period of product analysis. The reaction products were analyzed by an on-line GC with FID, using Poropak-Q (3 mm · 3 m) and Benton-34 (5%) and dinonylthalate (5%) on Chromosorb-W (3 mm · 5 m) columns. For time on stream studies, the propane aromatization reaction was investigated as a function of time at
D=C ¼ ½100 ðselectivity for C1 ; C2¼ and C2 Þ =ðselectivity for C1 ; C2¼ and C2 Þ. 3. Results and discussion
168
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
Aromatic Yield Propane Conversion
Zeolite
GAL2
GAL1
50.0
55.0
60.0
65.0
70.0
75.0
80.0
Conversion/Aromatic Yield (%)
a
Fig. 2. Product selectivity for GAL1 and GAL2 in the propane aromatization reaction; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
Aromatic Yield Propane Conversion GAL2
Zeolite
Table 2 D/C ratio for GAL1 and GAL2 at different space velocities Space velocity (cc/g/h)
D/C ratio
GAL1 GAL2
GAL1
3000
6000
12 000
4.5 5.2
4.2 4.5
8.6 9.1
Reaction temperature = 550 C.
b
45.0
50.0
55.0
60.0
65.0
70.0
75.0
Conversion/Aromatic Yield (%)
Fig. 1. Propane conversion and aromatic yields for GAL1 and GAL2 at: (a) 3000 cc/g/h; (b) 6000 cc/g/h; Reaction temperature = 550 C.
strong acid sites); this is in good agreement with previous studies wherein, we have observed a strong dependence of aromatic yields on the strong zeolitic acidity [11,14]. The higher non-FW Ga content in case of GAL2 is apparently not sufficient to compensate for the activity contribution from the higher strong acidity of GAL1. Fig. 2 shows the product selectivity for the zeolites at 3000 cc/g/h; a similar trend in selectivity is observed at other investigated space velocities. While the methane selectivity was higher for GAL1 the propylene selectivity was higher for GAL2. Higher methane selectivity for GAL1 may be attributed to its higher strong zeolitic acidity (higher cracking activity), while higher propylene selectivity for GAL2 may be related to its higher non-FW Ga content (higher dehydrogenation activity). No significant difference in selectivity was observed for the other products. The dehydrogenation/cracking (D/C) ratio for the zeolites is shown in Table 2. At all the investigated space velocities GAL2 has a higher D/C ratio. Our previous studies have shown that dehydrogenation activity increases with increasing nonFW Ga content [8]; thus the higher D/C ratio for the GAL2 zeolite may be attributed to its higher non-FW Ga content.
Fig. 3 shows the aromatic distribution for the zeolites at 3000 cc/g/h. Although there was no significant difference in the selectivity for total aromatics in case of the two zeolites, distinct differences could be observed in the aromatic distribution at all investigated space velocities. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1. No significant differences in the selectivity of C9+ aromatic compounds were observed. 100.0 90.0 80.0
Selectivity (%)
40.0
Benzene Toluene C8 Aromatics C9 Aromatics
70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
GAL1
GAL2
Zeolite
Fig. 3. Aromatic distribution for GAL1 and GAL2 in the propane aromatization reaction; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
T.V. Choudhary et al. / Catalysis Communications 7 (2006) 166–169
(c) Due to the higher content of non-FW Ga, GAL2 showed a larger dehydrogenation/cracking ratio for the propane aromatization reaction. (d) GAL1 showed a higher selectivity for methane, while GAL2 was more selective for propylene formation. (e) Although the zeolites showed no difference in the selectivity for total aromatics, distinct differences could be observed in the distribution of aromatic compounds. While the benzene and C8 aromatics selectivity was higher for GAL2, the selectivity for toluene was higher for GAL1.
85 GAL1 Aromatic Yield GAL2 Aromatic Yield GAL1 TotalConversion GAL2 TotalConversion
80
Conversion/Yield (%)
75
169
70 65 60 55 50 45
References
40 0
50
100
150
200
250
300
350
400
Time on Stream (mins.)
Fig. 4. Time-on-stream propane aromatization data for GAL1 and GAL2; Reaction temperature = 550 C and space velocity = 3000 cc/g/h.
Time on stream propane aromatization studies were undertaken at 550 C and space velocity of 3000 cc/g/h. A decrease in the total propane conversion and aromatic yield is observed for GAL1and GAL2 as a function of reaction time (see Fig. 4). The deactivation in propane aromatization activity may be attributed to the poisoning of the strong zeolitic acid sites by coke [8]. GAL1 showed higher propane aromatization than GAL2 throughout the time on stream reaction. 4. Conclusions The salient features of the study are summarized below: (a) GAL1, the GaMFI type zeolite with larger bulk Al content and lower Ga content, showed superior propane aromatization (initial and time-on-stream) activity as compared to GAL2. (b) A considerable fraction of total Ga was present as non-FW Ga species. In contrast, Al was predominantly present in the zeolite framework.
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