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Aromatization of n-hexane over Ga, Mo and Zn modified H-ZSM-5 zeolite catalysts
Catalysis Communications 72 (2015) 49–52
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Aromatization of n-hexane over Ga, Mo and Zn modified H-ZSM-5 zeolite catalysts Themba E. Tshabalala a,b, Michael S. Scurrell b,⁎ a b
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa Department of Civil and Chemical Engineering, University of South Africa, P O Box 392, UNISA 0003, South Africa
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 28 June 2015 Accepted 29 June 2015 Available online 12 September 2015 Keywords: Aromatization n-Hexane H-ZSM-5 BTX Mo Ga and Zn
a b s t r a c t n-Hexane aromatization was investigated at 500 °C on parent and metal (i.e. Ga, Mo and Zn) modified H-ZSM-5 zeolite catalysts. Conversion reached 88% over H-ZSM-5 and was stable. Addition of metal resulted in lower conversion (b 80%). Formation of aromatic compounds was favored on Ga/H-ZSM-5 (N 35%) and Zn/H-ZSM-5 (N 40%) while H-ZSM-5 and Mo/H-ZSM-5 showed higher cracking activity. Gallium and zinc favored aromatization. At 600 °C a decrease in activity with increasing TOS was observed. A decrease in aromatics selectivity was also observed. The aromatics selectivity with increase in TOS of Ga/H-ZSM-5 (43–27%) and Mo/H-ZSM-5 (35–27%) catalysts was higher than for Zn/H-ZSM-5 (46–7%). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Conversion of light alkanes into aromatic compounds is of great industrial interest [1–3]. Alkanes are converted into aromatics in a medium-pore catalyst containing a metal as a promoter [4,5]. These catalysts however can display low aromatics selectivity and deactivation [6]. The ZSM-5 system enhances the selectivity of the catalysts in methanol to gasoline [7], cracking [8], alkylation [9] and xylene isomerization [8]. This zeolite was found to be the most suitable for alkane aromatization [10–12]. Incorporation of metals into H-ZSM-5 assists in aromatization, involving a bifunctional process where metal species provide dehydrogenation sites leading to olefins which convert further to BTX products [13–15]. The metal can also reduce cracking selectivities. Dihydrogen is a useful co-product on metal-modified catalysts. Mo/H-ZSM-5 was seen to convert methane into benzene with 80– 100% selectivity at a conversion of 10–12% [16–19]. It was suggested that molybdenum carbides were involved in activating the methane. Subsequently, the aromatization of n-heptane and n-octane over Mo2C catalysts supported on different supports, H-ZSM-5, SiO2 and Al2O3 was studied [20,21]. The Mo2C/H-ZSM-5 catalyst showed a particularly high performance. However, the conversion of higher alkanes on molybdenum-based systems has been relatively neglected compared with the studies on these catalysts in methane and propane ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.S. Scurrell).
http://dx.doi.org/10.1016/j.catcom.2015.06.022 1566-7367/© 2015 Elsevier B.V. All rights reserved.
aromatization. The present paper aims to address this by focusing on a comparative study of n-hexane aromatization over molybdenum-, zinc- and gallium-modified H-ZSM-5 catalysts in what we believe is the first such comparative study of this reaction. 2. Experimental H-ZSM-5 was synthesized hydrothermally [22]. Mo, Ga or Zn were introduced using incipient wetness impregnation. Further details and catalyst characterization work are found in the SI. Aromatization was carried out in a fixed-bed microreactor with 0.5 g catalyst at 500 °C and a space velocity of 1200 cm3 g−1 h−1. 3. Results and discussion The impregnation of 2 wt% metal in the parent H-ZSM-5 zeolite catalyst does not greatly affect the BET surface areas and pore volumes (Table S1). The TPD profiles of the metal impregnated H-ZSM-5 catalysts displayed a shoulder at 268 °C, signifying the formation of medium acid sites (Fig. S2). The conversion of n-hexane over H-ZSM-5 catalyst was ca. 85% (Fig. 1(a)). The addition of metal resulted in a decrease in conversion and slight catalyst deactivation with increase in time-on-stream (tos). Decreasing activity is attributed to the decreased number of strong acid sites. Mo/H-ZSM-5 showed better stability over Ga/H-ZSM-5 and Zn/H-ZSM-5. The conversion of n-hexane over Mo/H-ZSM-5 and
50
T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52
Fig. 1. The catalytic conversion of n-hexane over metal promoted H-ZSM-5 catalysts of 2 wt% loading as the function of time-on-stream at 500 °C.
Zn/H-ZSM-5 catalysts was ca. 75%, and that of Ga/H-ZSM-5 was ca. 60%. The Ga/H-ZSM-5 catalyst suffered a rapid decrease with tos than molybdenum and zinc catalysts. Catalysts with low conversions exhibited good aromatic selectivities (Fig. 1(b)). Aromatic selectivities for Ga/HZSM-5 and Zn/H-ZSM-5 catalysts were around 38 and 41% respectively; those for H-ZSM-5 and Mo/H-ZSM-5 were b 30%. The high aromatic selectivity associated with gallium and zinc catalysts is attributed to the dehydrogenation activity of the metals [23,24]. Low aromatic selectivity of H-ZSM-5 and Mo/H-ZSM-5 is due to the cracking activity that characterizes these catalysts. The low aromatic selectivity of Mo/H-ZSM-5 is attributed to the presence of molybdate species anchored on Brønsted acid sites with limited chain growth, cyclization of hydrocarbons [25] and high cracking activity (Fig. 2). In summary, addition of gallium and zinc to the H-ZSM-5 catalyst increased the formation of aromatics and decreased cracking. This is due to the dehydrogenation activity of these metals which is effective in = the conversion of small cracked products especially C= 3 and C4 olefins that are mostly converted into aromatic compounds by secondary reactions. The introduction of molybdenum on H-ZSM-5 gave a material that did not show any activity in transforming the cracked products into aromatics. The yields of the main C3 and C4 compounds were almost similar to those produced by metal-free H-ZSM-5. This might be
Fig. 2. The cracking and aromatic activity of the metal modified H-ZSM-5 zeolite catalysts.
due to the absence of dehydrogenation activity and the dominance of cracking. The A/C ratio of Ga/H-ZSM-5 and Zn/H-ZSM-5 was higher than that of Mo/H-ZSM-5 and H-ZSM-5. (See detailed product distributions, Table 1.) High yields of olefins and aromatics are obtained over gallium and zinc catalysts. The addition of molybdenum did not have a major effect on the product distribution. The yields of major olefins, propene and the butenes, remained constant after the addition of molybdenum. Addition of gallium and zinc enhanced the yield of aromatics. The cracking of nhexane is compromised by the presence of gallium and zinc. This is evident from the yield of C5 which is ca. 1% for gallium and zinc catalysts and for H-ZSM-5 and molybdenum catalysts is 2.4 and 3% respectively. The TGA profiles of the parent H-ZSM-5 and metal modified H-ZSM5 zeolite catalysts after reaction at 500 °C for 12 h are shown in Fig. S3. Minimal deposition of coke was found (b 30 mg/gcat) consistent with the selectivity results presented in Table 1 which show low coke yields (b10%).
Table 1 The product distribution of the aromatic compounds of n-hexane over metal modified H-ZSM-5 zeolite catalysts taken at 77% n-hexane conversion. (Reaction temperature of 500 °C, space velocity of 1200 cm3 g−1 h−1). Catalysts
H-ZSM-5
Ga/H-ZSM-5
Zn/H-ZSM-5
Mo/H-ZSM-5
%Conversion
85.3
76.4
77.3
77.5
Products
Percentage Yield
Methane Ethylene Ethane Propylene Propane C= 4s / C4s C5s C6s Benzene Toluene m,p-Xylene Et-Benzene o-Xylene C9s ∑Aromatics Coke Coke (mg/gcat)#
2.3 4.0 6.3 21.9 5.7 8.9 2.0 2.4 0.3 4.0 8.7 5.8 2.1 2.9 1.5 23.5 6.5 27.4
2.4 0.93 3.8 9.8 1.9 4.3 1.4 1.1 1.04 10.2 10.0 7.9 2.3 2.1 9.7 32.4 8.4 17.0
1.8 3.9 6.5 20.5 4.3 9.5 2.7 3.0 1.4 3.0 7.2 4.1 2.5 1.0 1.0 19.9 5 20.7
#TGA quantified.
2.1 4.3 2.6 12.1 3.5 5.0 1.6 1.3 0.4 6.4 12.1 8.9 3.3 2.1 3.9 32.9 6.8 24.3
T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52 Table 2 Effect of temperature n-hexane conversion on Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM5 catalysts containing 2 wt% metal at 1, 5 and 10 h on-stream. Metal
Temperature °C
n-Hexane conversion (%)
Aromatic selectivity (%)
1h
5h
10 h
1h
5h
10 h
Ga
500 550 600 500 550 600 500 550 600
87.4 99.8 98.7 75.5 99.1 99.9 84.4 67.1 73.0
70.0 99.4 81.3 77.6 90.4 79.9 77.4 68.1 63.3
62.2 97.1 59.1 67.2 74.9 53.9 72.4 59.5 47.7
32.2 54.5 43.2 44.9 56.9 46.5 25.2 28.7 34.8
41.7 50.8 39.6 44.5 34.8 24.1 24.9 34.9 33.6
36.2 49.4 27.5 43.5 18.7 7.0 23.6 30.0 27.2
Zn
Mo
The influence of temperature on the conversion of n-hexane over Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 catalysts was studied at temperatures between 500 and 600 °C. Tables 2 and 3 present the results of the effect of temperature on the aromatization of n-hexane taken after 1, 5 and 10 h on-stream. For reaction at 550 °C (Table 2) high stability of gallium catalysts was seen at conversions of 96–99% with aromatics selectivity N 50%. At 600 °C a decrease in aromatic selectivity 43 to 27% was noted with increasing tos. For zinc catalysts, the conversion of n-hexane reached 100% at 600 °C followed by a rapid decrease as the time increased at temperatures of 550 or 600 °C, attributed to catalyst deactivation due to coke formation and zinc volatilization due to high reaction temperatures [26]. This also led to a decrease in the aromatics selectivity from 56 to 18% at 550 °C and from 46 to 7% at 600 °C. The conversion of n-hexane was initially 61 and 73% with molybdenum catalysts with an increase in aromatics selectivity from 25 to 35% with increase in temperature. Higher conversion observed at 600 °C after 1 h on-stream can be attributed to the fact that the molybdenum species could be in the form of the carbide at this temperature [27]. It was also shown that molybdenum carbide species could only be formed at temperatures above 580 °C, when butane was reacted over Mo/H-ZSM-5 catalysts [28]. n-Hexane conversion decreased with increase in tos, especially at 600 °C, where the conversion decreased from 73 to almost 48%. This decrease is due to catalyst deactivation, attributed to coking. A decrease in aromatics selectivity with tos at 500 and 600 °C was observed but at 550 °C a stable aromatic selectivity was maintained. The decrease in the aromatic selectivity can be attributed to the decrease in conversion of n-hexane and one contributing factor is the change in the geometry of the zeolite pores due to coke blockage [29]. One other reason is that presence of molybdenum species favor cracking reaction over aromatization (Fig. 2). It is worth noting that gallium and zinc catalysts showed better catalytic conversion of hexane at 550 °C with increase in tos. However, the difference in the aromatic selectivity with zinc catalyst with increase in due to the decrease in dehydrogenolysis activity of the catalysts which is attributed to decrease in zinc content in the catalysts cause
51
by volatilization. This suggests that the conversion of hexane is dependent on the reaction temperature and aromatic selectivity and on the availability of dehydrogenolysis sites in the catalyst. The effect of reaction temperature is most noticeable on benzene and toluene yields. At 500 °C, toluene dominated, but at higher temperature, the benzene yield increased, reaching maximum values of 18 and 12% for the gallium and zinc catalysts respectively at 550 °C. This increase in benzene was compensated for a decreased C8 aromatics yield (ethyl-benzene and xylenes). An increase in temperature inhibited alkylation and favored dealkylation. The low yields of o-xylene relative to m,p-xylenes are due to the shape selective character of the zeolite. The p-xylene contributes to increasing the yields of m,p-xylenes [30]. Higher benzene and slightly lower toluene yield are attributed to the demethylation of toluene and C8s. The molybdenum catalyst showed a similar effect to that seen with gallium and zinc catalysts but the yields of for each aromatic compound were b 9%. The yield of benzene increased from 2 to 8% while toluene remained constant with an increase in reaction temperature. In the case of C8s, a decrease in the yield was observed from 500 to 550 °C and remained constant at 600 °C. The m,p-xylenes were also dominant in the C8 aromatics. 4. Conclusions High aromatization activity associated with gallium and zinc metal is due to dehydrogenation activity. Gallium and zinc provides an alternative pathway reaction for the aromatization of n-hexane which gives a high conversion and a high selectivity to aromatic compounds. On the other hand, addition of molybdenum to H-ZSM-5 gave different results. Aromatic products from Ga/H-ZSM-5 and Zn/H-ZSM-5 dominated over those formed by cracking. This showed that the dehydrogenation activity contributes to aromatization and hence these catalysts are more selective to the formation of aromatics. Molybdenum-containing catalysts were more selective towards cracked products. At 550 °C gallium and zinc catalysts showed good activity giving 99% conversions after 1 h on-stream and aromatic selectivities of 55 and 57% were attained, respectively. The gallium catalyst showed good activity and stability with increasing tos. Zinc catalysts show poor stability because of zinc volatilization. The gallium-based catalyst is particularly worthy of further investigation. Acknowledgments We thank the University of the Witwatersrand, University of South Africa, National Research Foundation (NRF) and SASOL for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.06.022.
Table 3 The effect of reaction temperature on the aromatic product distribution (mainly BTX) of aromatization of n-hexane over Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 zeolite catalysts at 500–600 °C. Metal
Ga
Zn
Mo
Temperature °C
500 550 600 500 550 600 500 550 600
%Conversion
87.4 96.4 90.4 75.5 92.2 92.8 61.2 58.0 58.4
%Yield Benzene
Toluene
Et-Benzene
o-Xylene
m,p-Xylene
6.6 18.3 16.4 6.9 12.0 13.7 2.0 6.8 7.9
11.3 14.4 12.0 7.7 10.1 8.4 5.7 6.2 6.1
2.0 2.7 1.7 7.1 4.9 4.6 1.6 0.9 0.9
2.5 1.8 1.5 3.5 2.8 2.3 1.4 0.6 0.9
5.9 6.0 5.3 5.8 4.8 1.9 4.7 2.9 2.9
52
T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Aromatization of n-hexane over Ga, Mo and Zn modified H-ZSM-5 zeolite catalysts Themba E. Tshabalala a,b, Michael S. Scurrell b,⁎ a b
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa Department of Civil and Chemical Engineering, University of South Africa, P O Box 392, UNISA 0003, South Africa
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 28 June 2015 Accepted 29 June 2015 Available online 12 September 2015 Keywords: Aromatization n-Hexane H-ZSM-5 BTX Mo Ga and Zn
a b s t r a c t n-Hexane aromatization was investigated at 500 °C on parent and metal (i.e. Ga, Mo and Zn) modified H-ZSM-5 zeolite catalysts. Conversion reached 88% over H-ZSM-5 and was stable. Addition of metal resulted in lower conversion (b 80%). Formation of aromatic compounds was favored on Ga/H-ZSM-5 (N 35%) and Zn/H-ZSM-5 (N 40%) while H-ZSM-5 and Mo/H-ZSM-5 showed higher cracking activity. Gallium and zinc favored aromatization. At 600 °C a decrease in activity with increasing TOS was observed. A decrease in aromatics selectivity was also observed. The aromatics selectivity with increase in TOS of Ga/H-ZSM-5 (43–27%) and Mo/H-ZSM-5 (35–27%) catalysts was higher than for Zn/H-ZSM-5 (46–7%). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Conversion of light alkanes into aromatic compounds is of great industrial interest [1–3]. Alkanes are converted into aromatics in a medium-pore catalyst containing a metal as a promoter [4,5]. These catalysts however can display low aromatics selectivity and deactivation [6]. The ZSM-5 system enhances the selectivity of the catalysts in methanol to gasoline [7], cracking [8], alkylation [9] and xylene isomerization [8]. This zeolite was found to be the most suitable for alkane aromatization [10–12]. Incorporation of metals into H-ZSM-5 assists in aromatization, involving a bifunctional process where metal species provide dehydrogenation sites leading to olefins which convert further to BTX products [13–15]. The metal can also reduce cracking selectivities. Dihydrogen is a useful co-product on metal-modified catalysts. Mo/H-ZSM-5 was seen to convert methane into benzene with 80– 100% selectivity at a conversion of 10–12% [16–19]. It was suggested that molybdenum carbides were involved in activating the methane. Subsequently, the aromatization of n-heptane and n-octane over Mo2C catalysts supported on different supports, H-ZSM-5, SiO2 and Al2O3 was studied [20,21]. The Mo2C/H-ZSM-5 catalyst showed a particularly high performance. However, the conversion of higher alkanes on molybdenum-based systems has been relatively neglected compared with the studies on these catalysts in methane and propane ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.S. Scurrell).
http://dx.doi.org/10.1016/j.catcom.2015.06.022 1566-7367/© 2015 Elsevier B.V. All rights reserved.
aromatization. The present paper aims to address this by focusing on a comparative study of n-hexane aromatization over molybdenum-, zinc- and gallium-modified H-ZSM-5 catalysts in what we believe is the first such comparative study of this reaction. 2. Experimental H-ZSM-5 was synthesized hydrothermally [22]. Mo, Ga or Zn were introduced using incipient wetness impregnation. Further details and catalyst characterization work are found in the SI. Aromatization was carried out in a fixed-bed microreactor with 0.5 g catalyst at 500 °C and a space velocity of 1200 cm3 g−1 h−1. 3. Results and discussion The impregnation of 2 wt% metal in the parent H-ZSM-5 zeolite catalyst does not greatly affect the BET surface areas and pore volumes (Table S1). The TPD profiles of the metal impregnated H-ZSM-5 catalysts displayed a shoulder at 268 °C, signifying the formation of medium acid sites (Fig. S2). The conversion of n-hexane over H-ZSM-5 catalyst was ca. 85% (Fig. 1(a)). The addition of metal resulted in a decrease in conversion and slight catalyst deactivation with increase in time-on-stream (tos). Decreasing activity is attributed to the decreased number of strong acid sites. Mo/H-ZSM-5 showed better stability over Ga/H-ZSM-5 and Zn/H-ZSM-5. The conversion of n-hexane over Mo/H-ZSM-5 and
50
T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52
Fig. 1. The catalytic conversion of n-hexane over metal promoted H-ZSM-5 catalysts of 2 wt% loading as the function of time-on-stream at 500 °C.
Zn/H-ZSM-5 catalysts was ca. 75%, and that of Ga/H-ZSM-5 was ca. 60%. The Ga/H-ZSM-5 catalyst suffered a rapid decrease with tos than molybdenum and zinc catalysts. Catalysts with low conversions exhibited good aromatic selectivities (Fig. 1(b)). Aromatic selectivities for Ga/HZSM-5 and Zn/H-ZSM-5 catalysts were around 38 and 41% respectively; those for H-ZSM-5 and Mo/H-ZSM-5 were b 30%. The high aromatic selectivity associated with gallium and zinc catalysts is attributed to the dehydrogenation activity of the metals [23,24]. Low aromatic selectivity of H-ZSM-5 and Mo/H-ZSM-5 is due to the cracking activity that characterizes these catalysts. The low aromatic selectivity of Mo/H-ZSM-5 is attributed to the presence of molybdate species anchored on Brønsted acid sites with limited chain growth, cyclization of hydrocarbons [25] and high cracking activity (Fig. 2). In summary, addition of gallium and zinc to the H-ZSM-5 catalyst increased the formation of aromatics and decreased cracking. This is due to the dehydrogenation activity of these metals which is effective in = the conversion of small cracked products especially C= 3 and C4 olefins that are mostly converted into aromatic compounds by secondary reactions. The introduction of molybdenum on H-ZSM-5 gave a material that did not show any activity in transforming the cracked products into aromatics. The yields of the main C3 and C4 compounds were almost similar to those produced by metal-free H-ZSM-5. This might be
Fig. 2. The cracking and aromatic activity of the metal modified H-ZSM-5 zeolite catalysts.
due to the absence of dehydrogenation activity and the dominance of cracking. The A/C ratio of Ga/H-ZSM-5 and Zn/H-ZSM-5 was higher than that of Mo/H-ZSM-5 and H-ZSM-5. (See detailed product distributions, Table 1.) High yields of olefins and aromatics are obtained over gallium and zinc catalysts. The addition of molybdenum did not have a major effect on the product distribution. The yields of major olefins, propene and the butenes, remained constant after the addition of molybdenum. Addition of gallium and zinc enhanced the yield of aromatics. The cracking of nhexane is compromised by the presence of gallium and zinc. This is evident from the yield of C5 which is ca. 1% for gallium and zinc catalysts and for H-ZSM-5 and molybdenum catalysts is 2.4 and 3% respectively. The TGA profiles of the parent H-ZSM-5 and metal modified H-ZSM5 zeolite catalysts after reaction at 500 °C for 12 h are shown in Fig. S3. Minimal deposition of coke was found (b 30 mg/gcat) consistent with the selectivity results presented in Table 1 which show low coke yields (b10%).
Table 1 The product distribution of the aromatic compounds of n-hexane over metal modified H-ZSM-5 zeolite catalysts taken at 77% n-hexane conversion. (Reaction temperature of 500 °C, space velocity of 1200 cm3 g−1 h−1). Catalysts
H-ZSM-5
Ga/H-ZSM-5
Zn/H-ZSM-5
Mo/H-ZSM-5
%Conversion
85.3
76.4
77.3
77.5
Products
Percentage Yield
Methane Ethylene Ethane Propylene Propane C= 4s / C4s C5s C6s Benzene Toluene m,p-Xylene Et-Benzene o-Xylene C9s ∑Aromatics Coke Coke (mg/gcat)#
2.3 4.0 6.3 21.9 5.7 8.9 2.0 2.4 0.3 4.0 8.7 5.8 2.1 2.9 1.5 23.5 6.5 27.4
2.4 0.93 3.8 9.8 1.9 4.3 1.4 1.1 1.04 10.2 10.0 7.9 2.3 2.1 9.7 32.4 8.4 17.0
1.8 3.9 6.5 20.5 4.3 9.5 2.7 3.0 1.4 3.0 7.2 4.1 2.5 1.0 1.0 19.9 5 20.7
#TGA quantified.
2.1 4.3 2.6 12.1 3.5 5.0 1.6 1.3 0.4 6.4 12.1 8.9 3.3 2.1 3.9 32.9 6.8 24.3
T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52 Table 2 Effect of temperature n-hexane conversion on Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM5 catalysts containing 2 wt% metal at 1, 5 and 10 h on-stream. Metal
Temperature °C
n-Hexane conversion (%)
Aromatic selectivity (%)
1h
5h
10 h
1h
5h
10 h
Ga
500 550 600 500 550 600 500 550 600
87.4 99.8 98.7 75.5 99.1 99.9 84.4 67.1 73.0
70.0 99.4 81.3 77.6 90.4 79.9 77.4 68.1 63.3
62.2 97.1 59.1 67.2 74.9 53.9 72.4 59.5 47.7
32.2 54.5 43.2 44.9 56.9 46.5 25.2 28.7 34.8
41.7 50.8 39.6 44.5 34.8 24.1 24.9 34.9 33.6
36.2 49.4 27.5 43.5 18.7 7.0 23.6 30.0 27.2
Zn
Mo
The influence of temperature on the conversion of n-hexane over Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 catalysts was studied at temperatures between 500 and 600 °C. Tables 2 and 3 present the results of the effect of temperature on the aromatization of n-hexane taken after 1, 5 and 10 h on-stream. For reaction at 550 °C (Table 2) high stability of gallium catalysts was seen at conversions of 96–99% with aromatics selectivity N 50%. At 600 °C a decrease in aromatic selectivity 43 to 27% was noted with increasing tos. For zinc catalysts, the conversion of n-hexane reached 100% at 600 °C followed by a rapid decrease as the time increased at temperatures of 550 or 600 °C, attributed to catalyst deactivation due to coke formation and zinc volatilization due to high reaction temperatures [26]. This also led to a decrease in the aromatics selectivity from 56 to 18% at 550 °C and from 46 to 7% at 600 °C. The conversion of n-hexane was initially 61 and 73% with molybdenum catalysts with an increase in aromatics selectivity from 25 to 35% with increase in temperature. Higher conversion observed at 600 °C after 1 h on-stream can be attributed to the fact that the molybdenum species could be in the form of the carbide at this temperature [27]. It was also shown that molybdenum carbide species could only be formed at temperatures above 580 °C, when butane was reacted over Mo/H-ZSM-5 catalysts [28]. n-Hexane conversion decreased with increase in tos, especially at 600 °C, where the conversion decreased from 73 to almost 48%. This decrease is due to catalyst deactivation, attributed to coking. A decrease in aromatics selectivity with tos at 500 and 600 °C was observed but at 550 °C a stable aromatic selectivity was maintained. The decrease in the aromatic selectivity can be attributed to the decrease in conversion of n-hexane and one contributing factor is the change in the geometry of the zeolite pores due to coke blockage [29]. One other reason is that presence of molybdenum species favor cracking reaction over aromatization (Fig. 2). It is worth noting that gallium and zinc catalysts showed better catalytic conversion of hexane at 550 °C with increase in tos. However, the difference in the aromatic selectivity with zinc catalyst with increase in due to the decrease in dehydrogenolysis activity of the catalysts which is attributed to decrease in zinc content in the catalysts cause
51
by volatilization. This suggests that the conversion of hexane is dependent on the reaction temperature and aromatic selectivity and on the availability of dehydrogenolysis sites in the catalyst. The effect of reaction temperature is most noticeable on benzene and toluene yields. At 500 °C, toluene dominated, but at higher temperature, the benzene yield increased, reaching maximum values of 18 and 12% for the gallium and zinc catalysts respectively at 550 °C. This increase in benzene was compensated for a decreased C8 aromatics yield (ethyl-benzene and xylenes). An increase in temperature inhibited alkylation and favored dealkylation. The low yields of o-xylene relative to m,p-xylenes are due to the shape selective character of the zeolite. The p-xylene contributes to increasing the yields of m,p-xylenes [30]. Higher benzene and slightly lower toluene yield are attributed to the demethylation of toluene and C8s. The molybdenum catalyst showed a similar effect to that seen with gallium and zinc catalysts but the yields of for each aromatic compound were b 9%. The yield of benzene increased from 2 to 8% while toluene remained constant with an increase in reaction temperature. In the case of C8s, a decrease in the yield was observed from 500 to 550 °C and remained constant at 600 °C. The m,p-xylenes were also dominant in the C8 aromatics. 4. Conclusions High aromatization activity associated with gallium and zinc metal is due to dehydrogenation activity. Gallium and zinc provides an alternative pathway reaction for the aromatization of n-hexane which gives a high conversion and a high selectivity to aromatic compounds. On the other hand, addition of molybdenum to H-ZSM-5 gave different results. Aromatic products from Ga/H-ZSM-5 and Zn/H-ZSM-5 dominated over those formed by cracking. This showed that the dehydrogenation activity contributes to aromatization and hence these catalysts are more selective to the formation of aromatics. Molybdenum-containing catalysts were more selective towards cracked products. At 550 °C gallium and zinc catalysts showed good activity giving 99% conversions after 1 h on-stream and aromatic selectivities of 55 and 57% were attained, respectively. The gallium catalyst showed good activity and stability with increasing tos. Zinc catalysts show poor stability because of zinc volatilization. The gallium-based catalyst is particularly worthy of further investigation. Acknowledgments We thank the University of the Witwatersrand, University of South Africa, National Research Foundation (NRF) and SASOL for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.06.022.
Table 3 The effect of reaction temperature on the aromatic product distribution (mainly BTX) of aromatization of n-hexane over Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 zeolite catalysts at 500–600 °C. Metal
Ga
Zn
Mo
Temperature °C
500 550 600 500 550 600 500 550 600
%Conversion
87.4 96.4 90.4 75.5 92.2 92.8 61.2 58.0 58.4
%Yield Benzene
Toluene
Et-Benzene
o-Xylene
m,p-Xylene
6.6 18.3 16.4 6.9 12.0 13.7 2.0 6.8 7.9
11.3 14.4 12.0 7.7 10.1 8.4 5.7 6.2 6.1
2.0 2.7 1.7 7.1 4.9 4.6 1.6 0.9 0.9
2.5 1.8 1.5 3.5 2.8 2.3 1.4 0.6 0.9
5.9 6.0 5.3 5.8 4.8 1.9 4.7 2.9 2.9
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T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52
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