Top Catal (2011) 54:888–896 DOI 10.1007/s11244-011-9708-8
ORIGINAL PAPER
Coke Formation on Pt–Sn/Al2O3 Catalyst in Propane Dehydrogenation: Coke Characterization and Kinetic Study Qing Li • Zhijun Sui • Xinggui Zhou • Yian Zhu • Jinghong Zhou • De Chen
Published online: 28 July 2011 Ó Springer Science+Business Media, LLC 2011
Abstract The influences of gas compositions on the rates of coke formation over a Pt–Sn/Al2O3 catalyst are studied. The coke formed on the catalyst is characterized by thermal gravimetric analysis, IR spectroscopy, Raman spectroscopy and elemental analysis. Two kinds of coke are identified from the TPO profiles and assigned to the coke on the metal and the coke on the support, respectively. The coke formed on the metal is softer (containing more hydrogen) than that formed on the support. The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen pressures but increasing with the propane pressure, while the rate of coke formation on the support is increasing with the propane and propylene pressures and decreasing with the hydrogen pressure. Based on the kinetic analysis, a mechanism for the coke formation on the Pt–Sn/Al2O3 catalyst is proposed, and the dimerization of adsorbed C3H6 is identified to be the kinetic relevant step for coke formation on the metal. Keywords Coking mechanism Kinetics Propane dehydrogenation Pt–Sn/Al2O3 catalyst
Q. Li Z. Sui X. Zhou (&) Y. Zhu J. Zhou State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China e-mail:
[email protected] D. Chen Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491, Trondheim, Norway
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1 Introduction Recently the rapidly rising needs for propylene [1–3] have driven searching for new propylene production techniques. Propane dehydrogenation, developed commercially in 1980s, is an on-purpose technique for propylene production and is now making big contributions to the world propylene supply in recent years [4]. Pt based catalyst is the most popular catalyst for propane dehydrogenation and is used in commercial Oleflex process. This catalyst, as well as other catalysts for alkane dehydrogenation, is quickly deactivated, mainly by coke formation, and the mechanism of coke formation on the Pt catalyst is still not clear. Understanding the mechanism of coke formation is of great significance. On one hand, based on the mechanism of coke formation, one can optimize the reaction conditions to reduce the frequency of dehydrogenation and regeneration cycle by controlling the rate of coke formation so as to increase the propylene productivity. On the other hand, it can help the rational design of the catalyst to reduce the rate of coke formation while maintaining the activity and selectivity in dehydrogenation. Coke formation on Pt or modified Pt catalysts during propane dehydrogenation has been studied by a number of researchers. Most of the studies were targeted to reduce the coking rate by modifying Pt, for example, with Sn or alkali metals, and/or using different catalyst supports. Praserthdam et al. [5] showed that alkali metals (such as Li, Na, and K) would help to reduce the coking rate by providing excess mobile electrons to the Pt catalyst. Another straightforward method to reduce the amount of coke formed is to use supports with weak acidity, e.g. SBA-15 [6], which is effected by weakening propylene adsorption and suppressing propylene dehydrogenation or polymerization.
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The rate of coke formation depends highly on the operating conditions. Larsson et al. [7] studied the coke formation on Pt/Al2O3 and Pt–Sn/Al2O3 catalysts, and suggested that only a small part of the formed coke was responsible for catalyst deactivation, and that a major part of the coke was formed regardless of the gas composition but dependent on the temperature. They also concluded that hydrogen could reduce the rate of coke formation and correspondingly the rate of catalyst deactivation by suppressing coke precursor formation, but hydrogen could not remove the coke that had already been formed on the catalyst. Rebo et al. [8] studied the coke formation on Pt– Sn/Al2O3 catalyst using an oscillating microbalance reactor and concluded that coke formation was a structure sensitive reaction and hydrogen could decrease the rate of coke formation as well as the deactivating effect of the coke formed. Coke could be formed either on the support or on the metal, and the coke deposited on different sites is supposed to have different effects on catalyst deactivation. Studies on the influences of reaction conditions on the rate of coke formation and the nature of the coke deposited on different sites are essential for the understanding of the mechanism of coke formation as well as the mechanism of catalyst deactivation. However, studies on the influences of reaction conditions, especially gas compositions, during propane dehydrogenation are seldom reported. In this work, we study the influences of gas compositions on coke formation on the metal and support of a Pt– Sn/Al2O3 catalyst during propane dehydrogenation. Based on these results, the kinetic relevant step in coke formation on the metal is identified and a mechanism of coke formation is then proposed.
2 Experimental
889 Table 1 Catalyst characteristics Pt–Sn/Al2O3
Pt/MgO
Shape
Pellet
Pellet
Diameter (mm)
1.0
0.13
56.6
47.7 0.24
Surface area (m2/g) 3
Pore volume (m /g)
0.25
Average pore diameter (nm)
18.5
19.2
Pt (%)
0.50
0.75
Sn (%)
1.50
–
Besides, alumina (Pural 200), which was the support for the Pt–Sn catalyst, was also used as a blank catalyst for comparison. 2.2 Coking Experiments The catalysts were evaluated in a tubular stainless steel reactor with an inner diameter of 6 mm, the temperature of which was maintained by an electrical furnace jacketing outside of the reactor. Inserted inside the catalyst packing (100 mg) was a thermocouple to indicate the real temperature of reaction. Silica particles as inert packing were used for uniform flow distribution. An online gas chromatography (Agilent 4890D) was used to measure the outlet gas concentrations. The catalyst (or the alumina), which had particle sizes between 0.1 and 0.15 mm, was firstly reduced at 500 °C in flowing hydrogen (10 ml/min) for 100 min and the temperature was then ramped to 575 °C in argon (40 ml/min). The feed gas was then introduced into the reactor for coking experiment. In all the experiments, argon was used as a balance gas and the total flow rate was maintained at 80 ml/min. After 80 min of reaction, the reactor was switched to pure argon flow and then cooled down to room temperature. Table 2 shows the feed compositions and averaged gas compositions in the reactor during the experiments.
2.1 Catalyst Preparation and Characterization 2.3 Coke Characterization The Pt–Sn/Al2O3 catalyst used for this study was prepared by incipient impregnation of alumina (Pural 200) with H2PtCl6 and SnCl4 solutions, followed by calcination at 530 °C and treatment in steam to remove chlorine. A Pt/ MgO catalyst was also prepared by impregnation method. The catalysts were characterized by N2 physisorption on ASAP 2010 (Micromeritics, USA) at -196 °C after outgassing the samples for at least 5 h at 190 °C and 1 mm Hg vacuum. The specific surface areas were calculated with BET equation, and the pore volumes and pore size distributions were determined from the N2 desorption isotherms by using the BJH method. Table 1 summarizes the characteristics of the catalysts.
The amounts of coke formed on the spent catalysts (or coked alumina) were determined by TG (thermal gravimetric) analysis (SDT Q600, TA Company, USA) in air. During the TG analysis, the temperature was increased from ambient temperature to 600 °C (sometimes higher) at a rate of 10 °C/min, during which the temperature programmed oxidation (TPO) profiles and heat flow curves were recorded. Elemental analysis was carried out on a Vario EL III Elementar analyzer. The coked samples were firstly dissolved in a hydrofluoric acid solution (40%) at room temperature to liberate the coke from the support. Then the coke was dried at 50 °C and collected for testing.
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890
Samples
C3H6
S1a
34.3
0.0
S2a
35.6
S3a
37.0
a
S4
35.0
S5a
36.0
S6a S7a a
S8
C3H8
C3H6
8.3
30.0
2.9
3.0
6.6
31.2
5.6
10.5
9.5
9.1
34.5
11.1
10.1
7.8
0.0
32.4
9.4
1.9
7.3
12.0
34.0
8.5
13.6
20.0
0.0
0.0
15.9
4.0
5.3
34.9
0.0
0.0
30.0
3.3
4.0
H2
42.7
4.3
11.9
0.0
0.0
10.0
0.0
0.0
9.9
0.0
35.3
0.0
0.0
35.0
0.2
0.2
S9
0.0
H2
49.1
b
S10c
Averaged gas compositionse (%)
Feed compositionsd (%) C3H8
a
0.020
Summary of the experimental conditions (temperature,
Al2O3
c
Pt/MgO
d
Gas compositions at the inlet of the reactor
S1 S2 S3
0.015
0.010
0.005
0.000 100
200
e
Average compositions of gas between the inlet and outlet of the reactor
FTIR analysis was carried out on a Bruker Equinox-55 with a resolution of 4.0 cm-1 and the scanned wave number was ranged from 4000 to 400 cm-1. Raman analysis was performed at room temperature under ambient conditions on a Renishaw inVia ? Reflex Raman spectrometer with a 514.5 nm Ar-ion laser beam.
3 Results 3.1 Thermal Gravimetric Analysis
300
400
500
600
o
Temperature ( C)
4.5
Pt–Sn/Al2O3
b
-(Deriv. Weight) (%)
Table 2 575 °C)
Top Catal (2011) 54:888–896
Fig. 1 TPO profiles of Pt–Sn/Al2O3 catalyst coked under different partial pressures of propylene (575 °C; S1: C3H8, 30.0%; C3H6, 2.9%; H2, 11.9%; S2: C3H8, 31.2%; C3H6, 5.6%; H2, 10.5%; S3: C3H8, 34.5%; C3H6, 11.1%; H2, 10.1%; Ar, balance)
Table 3 Characterization results of coked samples Samples
AI
AII
Total
AI/AII
QI/QII
ID/IG
H/C
S1
1.10
0.33
1.43
3.33
–
–
1.89
S2
0.96
0.49
1.45
1.96
3.02
–
1.83
S3
1.16
1.20
2.36
0.97
2.48
–
1.78
S4
0.82
2.98
3.80
0.28
–
–
1.18
S5
0.78
0.66
1.44
1.18
2.45
–
2.00
S6
0.31
0.39
0.70
0.79
–
0.66
–
S7
0.70
0.54
1.24
1.30
–
0.68
1.81
S8
1.18
1.19
2.37
0.99
–
0.60
1.79
S9
n.d.
3.21
3.21
/
–
0.64
1.17
S10
1.21
n.d.
1.21
/
–
0.80
2.66
AI and AII: peak areas of TPO Peak I and Peak II in Figs. 1, 2 and 3 AI/AII: ratio of area of Peak I to area of Peak II
Figures 1–3 show the TPO profiles of the spent Pt–Sn/ Al2O3 catalysts coked with different gas compositions at 575 °C for 80 min. Two peaks, locating in the interval of 150–280 °C (Peak I) and in the interval of 380–430 °C (Peak II), respectively, are present in each profile. However, for coked alumina (Sample S9) and Pt/MgO catalyst (Sample S10), only one peak, at around 470 and 310 °C, respectively, is present in the TPO profiles (not shown here). To determine the amount of coke losses, the TPO profiles are deconvoluted by multiple Gaussian functions using a non-linear least-squared optimization procedure based on Levenberg–Marquardt algorithm. This method fits the TPO profiles quite well and the results are summarized in Table 3. Figure 1 shows that the area of Peak II increases while that of Peak I is almost unchanged when the partial pressure of propylene is increased (Samples S1–S3).
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QI/QII: ratio of released heats during the oxidation of coke represented by Peak I and Peak II ID/IG: intensity ratio of D band to G band in the Raman spectrum H/C: ratio of hydrogen atoms to carbon atoms of the coke n.d. not detected, – not characterized, / not calculated
Consequently the total amount of coke increases with the partial pressure of propylene. Figure 2 indicates that the addition of hydrogen in the feed has greatly suppressed coke formation: if there is only 1.9% hydrogen (Sample S4) in the gas, the total amount of coke is as high as 3.80 wt%, while if the hydrogen content in the gas is increased to 13.6 wt% (Sample S5), the total coke amount is decreased to 1.44 wt%. It is also noted that for sample S4 or S5, the decrease in the total amount of coke, denoted by the areas of Peak I and Peak II, is mainly due to the decrease of the area of Peak II, as seen from Table 3.
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3.2 Elemental Analysis
0.040
-(Deriv. Weight) (%)
0.035
S4 S3 S5
0.030 0.025 0.020 0.015 0.010 0.005 0.000 100
200
300
400
500
600
Temperature (o C) Fig. 2 TPO profiles of Pt–Sn/Al2O3 catalyst coked under different partial pressures of hydrogen (575 °C; S4: C3H8, 32.4%; C3H6, 9.4%; H2, 1.9%; S3: C3H8, 34.5%; C3H6, 11.1%; H2, 10.1%; S5: C3H8, 34.0%; C3H6, 8.5%; H2, 13.6%; Ar, balance)
-(Deriv. Weight) (%)
0.015 S6 S7 S8
0.010
0.005
0.000 100
200
300
400
500
600
o
Temperature ( C) Fig. 3 TPO profiles of Pt–Sn/Al2O3 catalyst coked under different partial pressures of propane (575 °C; S6: C3H8, 15.9%; C3H6, 4.0%; H2, 5.3%; S7: C3H8, 30.0%; C3H6, 3.3%; H2, 4.0%; S8: C3H8, 42.7%; C3H6, 4.3%; H2, 4.5%; Ar, balance)
Figure 3 shows that the areas of both Peak I and Peak II increase with the partial pressure of propane. Moreover, Peak II shifts to higher temperature, from 385.3 to 389.6 and then to 398.0 °C, as the partial pressure of propylene increases (Fig. 1) and shifts to lower temperature as the partial pressure of hydrogen increases (Fig. 2). Peak I also shifts to higher temperature as the partial pressure of propane increases (Fig. 3). The released heats by oxidation of the cokes on Samples S2, S3, and S5 at the two peak temperatures are calculated from the integration of the heat flow curve and have already been listed in Table 3 in terms of the heat ratios. It is observed that the heat ratio is always larger than the area ratio.
The results of elemental analysis of the coke are listed in Table 3 in terms of H/C ratio. For most samples (except for Samples S4, S9, and S10), the H/C ratios are in the range of 1.7–2.0, which is an indication of the relatively high hydrogen content of the coke formed on Pt–Sn/Al2O3 catalysts. However, the H/C ratio of S4 is much lower than those of other coked Pt–Sn samples, which is because of the very low hydrogen concentration in the feed flow. The coked alumina (S9) has the lowest while the coked Pt/MgO (S10) has the highest H/C ratio, which indicates that the coke formed on the alumina is relatively deficient in hydrogen while the coke formed on the Pt surfaces is rich in hydrogen.
3.3 IR Characterization Figures 4 and 5 show the IR spectra of the Pt–Sn/Al2O3 catalysts coked with different gas compositions. Several absorption bands appear between 600 and 3000 cm-1, from which different characteristics of the cokes can be identified. Generally, the bands at 1350–1470 cm-1 reflect the bending vibrations of C–H in CH2 and CH3 groups [9], while the stronger absorption at 2850–2960 cm-1 reflects the stretching vibrations of C–H in CH, CH2 and CH3 groups [9–12]. The vibration bands of C–H in alkenes (between 675 and 1000 cm-1) [13] overlap the deformation vibration bands of C–H in aromatics (between 680 and 880 cm-1) [14], while the bands between 1640 and 1680 cm-1 (representing the stretching vibrations of C=C in alkenes [13]) overlap the bands 1660–2000 cm-1 (representing the external-plane-bending vibration of C–H in aromatic rings [15]). The bands between 1450 and 1600 cm-1 are originated from the skeleton vibration of C=C in aromatic rings [12, 16]. The absorption at 3000–3020 cm-1 is the characteristic of C=C bonds in aliphatic hydrocarbon, while the absorption at 3020–3200 cm-1 is the characteristic of C=C bonds in aromatics [9, 17]. Based on this information, the strongest absorption at about 2920 cm-1 is chosen to represent the aliphatic nature of the coke and the absorption at around 3060 cm-1 is chosen as the sign of the aromatic nature of the coke. The involved bands are shown in Figs. 4 and 5, in which, the absorption intensities at 2920 cm-1 are scaled to the same magnitude for convenience of comparison. As shown in Figs. 4 and 5, the intensity at 3060 cm-1 increases with the partial pressure of propylene but decreases with the partial pressure of hydrogen. Propylene promotes while hydrogen inhibits the formation of aromatic coke. These results are in good agreement with the shift of Peak II in Figs. 1 and 2.
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Intensity (a.u.)
1336
1602
S6
Intensity (a.u.)
0
2920 -CH2 (a)
2962 -CH3 (a) S1 S2 S3
2750 2800 2850 2900 2950
3020 3040 3060 3080 3100 -1
Wavenumbers (cm )
Intensity (a.u.)
Fig. 4 FT-IR spectra of catalyst coked under different partial pressures of propylene (575 °C; S1: C3H8, 30.0%; C3H6, 2.9%; H2, 11.9%; S2: C3H8, 31.2%; C3H6, 5.6%; H2, 10.5%; S3: C3H8, 34.5%; C3H6, 11.1%; H2, 10.1%; Ar, balance)
2920 -CH2 (a) 2962 -CH3 (a)
S4 S3 S5
2750 2800 2850 2900 2950
3020 3040 3060 3080 3100
Wavenumbers (cm -1) Fig. 5 FT-IR spectra of catalyst coked under different partial pressures of hydrogen (575 °C; S4: C3H8, 32.4%; C3H6, 9.4%; H2, 1.9%; S3: C3H8, 34.5%; C3H6, 11.1%; H2, 10.1%; S5: C3H8, 34.0%; C3H6, 8.5%; H2, 13.6%; Ar, balance)
3.4 Raman Spectral Analysis Figure 6 shows the Raman spectra of the coked catalysts (Samples S6, S7, S8, S9, and S10). Five peaks are identified at 1336, 1602, 2700, 2920, and 3200 cm-1, respectively. The bands at 1336 and 1602 cm-1 are generally considered as the D band and G band [18, 19] and attributed to the ring stretching in polyaromatic compounds, which are considered to be the graphite-like carbon species [20, 21]. The region of 2700–3000 cm-1 is supposed to be the C–H stretching in alkyl hydrocarbon [19, 21, 22] while the band at 3200 cm-1 is regarded as the C–H stretching in
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1000
1500
2000
2500
3000
3500
4000
S7 S8
1000 1000 1000
1500 1500 1500
2000 2000 2000
2500 2500 2500
3000 3000 3000
3500 3500 3500
S9
4000 4000 4000
S10
1000
1500
2000
2500
3000
3500
4000
Raman shift (cm -1) Fig. 6 Raman spectra of coked samples (S6–S8 are coked Pt–Sn/ Al2O3 catalysts, S9 is coked alumina, S10 is coked Pt/MgO catalyst;575 °C; S6: C3H8, 15.9%; C3H6, 4.0%; H2, 5.3%; S7: C3H8, 30.0%; C3H6, 3.3%; H2, 4.0%; S8: C3H8, 42.7%; C3H6, 4.3%; H2, 4.5%; S9: C3H8, 0.0%; C3H6, 9.9%; H2, 0.0%; S10: C3H8, 35.0%; C3H6, 0.2%; H2, 0.2%; Ar, balance)
aromatics [21, 22]. The intensity ratio of D band to G band is always used as an important measure of the degree of graphitization of carbon materials [23, 24]. A high ratio indicates a low degree of graphitization. Table 3 lists the ID/IG ratios of the coked catalysts. As shown in Table 3, the ID/IG ratios of Samples S6–S9 are very close to each other. However, the ID/IG ratio of the coked Pt/MgO catalyst (Sample S10) is large indicating the coke on the Pt/MgO catalyst has a low degree of graphitization. This is consistent with the results of elemental analysis and indicates that the coke on the alumina and Pt– Sn catalysts has a H/C ratio lower than that on the Pt/MgO catalyst. Furthermore, as shown in Fig. 6, the lower intensity of C–H vibration in the aromatics (3200 cm-1) on coked Pt/MgO catalyst were observed comparing to other samples. It indicates that the degree of graphitization of the coke on the Pt surfaces is lower, and coke molecules are more like olefin/paraffin. 3.5 Reaction Orders to Propane, Propylene and Hydrogen Kinetic experiments were carried out under different gas compositions as summarized in Table 2, where the averaged compositions of gas between inlet and outlet of the reactor are also listed. During the course of reaction for 80 min, we observed that the amounts of the two types of coke were both increasing approximately linearly with time on stream (not shown here). The linear build-up of coke with time was also reported by Kumar et al. [25]. Therefore, the average coking rates were estimated in the present
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-6.0 r1, coking rate on the metal r2, coking rate on the support
-6.5 -7.0 -7.5 -8.0 -8.5
k=0.0
S1
S3
S2
-9.0 S2
-9.5 k=1.0
-10.0 -10.5
S1
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
ln(PC3H6)(PC3H6, 101Pa) Fig. 7 The relationship between coke formation rates and partial pressures of propylene (575 °C, open square Samples S1–S3; filled circle Samples S1–S3)
ln(r) (r, mg_coke/(10 2 mg_cat.s))
-6.0 r1, coking rate on the metal
-6.5
r2, coking rate on the support
S3 S3
-7.0
S2
-7.5 -8.0
k=-0.7 S4 S1 k=0.0
-8.5 -9.0
S5
S7
S4
S5 S7
-9.5 -10.0
k=-0.7
5.2
5.6
6.0
6.4
S1
6.8
7.2
ln(PH2 ) (PH2, 101Pa) Fig. 8 The relationship between coke formation rates and partial pressures of hydrogen (575 °C, open square Samples S1–S5 and S7; filled circle Samples S1 and S7, Samples S3–S5)
pressure on r1. Figure 9 shows that r1 is dependent on the partial pressure of propane with an apparent reaction order of 1.7. Summarizing Figs. 7, 8 and 9 we see that r2 is dependent on the partial pressures of propane, propylene and hydrogen and the reaction orders are 1.4, 1.0 and -0.7, respectively.
4 Discussions 4.1 Assignment of Peaks in the TPO Profiles TG is a very useful method to identify and quantify the coke on the catalysts [26–28]. The peaks in the TPO profiles can be attributed to the loss of carbonaceous deposits with different compositions and structures and/or the carbonaceous deposits located at different sites on the catalyst. The two peaks identified in the TPO profiles of coked catalysts shown above indicate that two types of coke are formed. The two types of coke are usually considered to be formed on the metal and support, respectively [29, 30]. To confirm the locations of the coke, comparative experiments were carried out on pure alumina, which was used as the support for the Pt–Sn catalyst, and the Pt/MgO catalyst, which had no acid sites on the support (Table 1). For coked alumina, a single peak emerges at around 470 °C in the TPO profile, while for coked Pt/MgO catalyst the peak appears at around 310 °C. This information confirms that Peak I and Peak II in the TPO profiles of Samples S1–S8 are corresponding to the coke formed on the metal and support, respectively. As a result, in this study, we assign Peak I to the coke deposited on the metal and Peak II to the coke deposited on the support.
ln(r) (r, mg_coke/(10 2 mg_cat.s))
ln(r) (r, mg_coke/(10 2 mg_cat.s))
work based on the measured coke contents in 80 min of time on stream by assuming a constant coking rate. The coking rates r1 and r2 were determined based on Peak I and II from the TPO spectra. The logarithms of the rates were then plotted against the logarithms of the partial pressures of propane, propylene and hydrogen, respectively, as shown in Figs. 7, 8 and 9. Figure 7 shows that r1 is independent of the propylene pressure. As a result, Samples S1–S5 and Sample S7 are all used to find the influences of the hydrogen pressure on r1, because these samples were obtained under similar partial pressures of propane. Figure 8 shows that r1 is also independent of the partial pressure of hydrogen. Samples S1– S8 are then used to find the influences of the propane
-6.0 r1, coking rate on the metal
-6.5
r2, coking rate on the support
S2
-7.0
S4 S7
-7.5 -8.0
S1
-8.5 -9.0
S3 S8
k=1.7
S5 k=1.4
S6
S7
-9.5 -10.0 -10.5
S6
7.4
7.6
7.8
8.0
8.2
8.4
ln(PC3H8) (PC3H8, 101Pa) Fig. 9 The relationship between coke formation rates and partial pressures of propane (575 °C, open square Samples S1–S8; filled circle Samples S6–S8)
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4.2 The Nature of Coke It is interesting to note from Table 3 that the ratios of the combustion heats of the two types of coke are much higher than the ratios of the masses determined by TPO. This fact clearly indicates that the coke on the metal has a higher H/C ratio than that on the support, which is consistent with the results of elemental analysis (Samples S9 and S10). Based on the results of TPO, TG, and DTA of the Pt/Al2O3 and Pt–Sn/Al2O3 catalysts coked in n-butane dehydrogenation, Zhang et al. [31] also suggested that the coke formed on the metal had a higher hydrogen content. The same conclusion was obtained by Srihiranpullop et al. [32] when studying the coke formation on Pt, Pt–Sn, and Pt–Sn–K catalysts during n-hexane dehydrogenation. In addition, we note that Peak I shifts to higher temperature when the propane pressure is increased. This is because the coke covers the metal surface and reduces the rate of coke combustion by increasing the resistance of oxygen diffusion [29]. The Raman spectral analysis of Samples S9 and S10 shows that the ID/IG ratio of Sample S9 is much lower than that of Sample S10, which indicates that the coke formed on the support is more graphitized. The very weak intensity of the band at 3200 cm-1 in the Raman spectrum of coked Pt/MgO catalyst also confirms that the coke on the metal is less dehydrogenated. Based on this analysis, the catalyst samples containing more coke on the support should have lower ID/IG ratios. Hence the ID/IG ratios of Samples S6, S7, S8, S9, and S10 should have been in the decreasing order of S10 [ S7 [ S8 [ S6 [ S9. However, the ID/IG ratio of Sample S8 is the lowest, and the coke is hard to burn, as indicated by the rightward shift of the position of peak II of Sample S8. This is because the high propane concentration greatly increases the concentration of coke precursor on the support, which leads to an increased degree of polymerization and aromatization of the coke. Increasing the propylene concentration will increase the degree of polymerization of the polymers [33], which will be further dehydrogenated to aromatics. The degree of the aromatization of the coke is enhanced as a result of the increased propylene concentration and weakened as a result of the increased hydrogen concentration, as hydrogen will reduce the molecular weight of the polymer during propylene polymerization [34]. This fact also explains the shift of Peak II in Figs. 1 and 2. In general, the H/C ratios of the cokes are very high (Table 1), especially on the metal surfaces, indicating that the coke contains mainly aliphatic hydrocarbons. The high H/C ratios on metal surfaces are consistent with the relatively large peak of 2920 cm-1 in IR spectra and
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the relatively large peak of 2700–3000 cm-1 in Raman spectra. 4.3 Dependence of Coking Rate on Gas Concentration The rate of coke formation on the metal is found dependent on the propane pressure (Fig. 9). The apparent reaction order is about 1.7 with respect to propane and zero reaction order with respect to hydrogen and propylene. Figures 7 and 9 also show that the rate of coke formation on the support increases with the propylene or propane pressures, while Fig. 8 shows that it decreases with the hydrogen pressure. The apparent reaction orders of coking on the support are about first order with respect to propylene (Fig. 7), -0.7 order with respect to hydrogen (Fig. 8) and 1.4 order with respect to propane (Fig. 9). It suggests that coke can be formed both from propane and propylene, but with different reaction mechanisms. An apparent reaction order of 1.4 with respect to propane can not explain that propylene is the main precursor for coke formation, since the reaction order to propane is about one for propylene formation, and the reaction order to propylene is about one for coke formation. 4.4 Coke Formation Mechanism 4.4.1 Coke Formation on the Metal Many reaction mechanisms have been tested, and only the following mechanism of coke formation on the metal can describe the experimental observation: C3 H8 ðg) þ 2 ) C3 H7 þH
ð1Þ
C3 H7 þ ) C3 H6 þH
ð2Þ
C3 H6 ) C3 H6 ðg) þ
ð3Þ
2C3 H6 ) C6 H12 þ
ð4Þ
H þH , H2 þ 2
ð5Þ
The apparent reaction order of propane of about 1.7 indicates that two C3 intermediates are involved in the carbon formation. Furthermore, the H/C ratios of the coked Pt–Sn catalysts of 1.7–2.0 indicate a high hydrogen content of the coke. Therefore, the formation of C6H12* from two C3H6* intermediates is proposed as the kinetic relevant step for coke formation (Step 4). It is assumed that the dehydrogenation steps (Steps 1 and 2) are both irreversible and far from equilibrium [35–37], and the desorption of propylene (Step 3) is also considered irreversible. Step 4 is assumed to be the kinetic relevant step and the rate of Step 4 is considered to be much slower than those of Steps 2 and 3.
Top Catal (2011) 54:888–896
895
dhC3H7 ¼ k1 PC3H8 h2 k2 hC3H7 h ¼ 0 dt dhC3 H6 ¼ k2 hC3 H7 h k3 hC3 H6 ¼ 0 dt
ð6Þ ð7Þ
Then the site coverages of C3H7* and C3H6* are determined by, h C3 H 7 ¼ h C3 H 6 ¼
k1 PC3 H8 h k2
ð8Þ
Coking rate on the metal (Model)
-7.0 -7.5
Coking rate on the metal (Experiment)
-8.0 -8.5 -9.0 -9.5 -10.0 S1
ð9Þ
k3
The coke formation rate on the metal is given by Eq. (10) by assuming C3H7* as the most abundant surface specie: kc P2C3 H8 ð1 þ KI PC3 H8 Þ
4
ð10Þ
where, 2 k1 kc ¼k4 k3
ð11Þ
k1 k2
ð12Þ
KI ¼
-6.5
-10.5
k1 PC3 H8 h2
rc ¼ k4 h2C3 H6 ¼
-6.0
ln(r) (r, mg_coke/(10 2 mg_cat.s))
By assuming the steady-state of the intermediates, C3H7* and C3H6*, we obtain,
Increasing propane concentration in the gas phase concurrently increases the concentration of adsorbed propylene (C3H6*) and hence the amount of coke on the metal. A relatively low value of KI in Eq. (10) can lead to the apparent order of 1.7 with respect to propane. The irreversible desorption of propylene subsequently leads to the zero order to propylene. The irreversible reaction of steps in propane dehydrogenation and relatively weak adsorption of hydrogen comparing to the C3 surface intermediate result in the weak effect of hydrogen. By fitting the rates of coking on the metal at different gas compositions, kc and KI are estimated to be 4.15 9 10-5 mg coke/(mg cat.s) and 7.18 9 10-1, respectively. Moreover, one can see that the model fits the experimental data quite well (Fig. 10), which implies the proposed kinetic model for coking on the metal is reasonable. It is noted here that although the hydrogen pressure has little effect on the rate of coking on the metal, it changes significantly the hydrogen content in the coke. At higher hydrogen partial pressure, the coke is less dense and compact. In addition, the coke formed on the metal can be further softened through hydrogenation [38].
S2
S3
S4
S5
S6
S7
S8
Samples Fig. 10 Comparison between model prediction and experiments of coking rates on metal
4.4.2 Coke Formation on the Support Coke formation on the support mainly involves polymerization/oligomerization, condensation, cyclization, hydride transfer, etc. Referring to the mechanism proposed by Caeiro et al. [39], coke formation on the support can be divided into two stages, i.e., the conversion of propane to olefins (propylene and ethylene) by protolysis and transformation of these olefins to aromatic hydrocarbons. The second stage involved a number of reactions, such as oligomerization–cracking reactions (producing C4–C10 olefins), hydrogen transfer (producing dienes), cyclization (generating cycloalkenes), and further hydrogen transfer (producing cyclic diolefins and finally aromatics). Based on this mechanism, increasing the propylene partial pressures will certainly increase the amount of coke on the support. The coke precursor formed on the metal may migrate to the support [32] and then undergoes subsequent polymerization/oligomerization, condensation and so on. Thus, increasing the partial pressure of propane would increase the rate of coke formation on the support. Sn in the Pt catalyst will weaken the binding of hydrocarbon to the metal [30, 40], and promote the migration of the coke precursor from the metal to the support. The presence of hydrogen will weaken the acidity of the support by converting Brønsted acid sites to Lewis acid sites and thus reduce the coke formation rate. Based on the above discussions, a coke formation mechanism is proposed as in Fig. 11. Propane is firstly dissociated on the metal and the coke precursor is formed through dehydrogenation; then the ‘‘soft coke’’ is generated on the metal from the coke precursor. The coke precursor generated on the metal will also migrate to the acid sites. On these acid sites, the coke precursor and the adsorbed propylene undergo polymerization/oligomerization, condensation, cyclization and
123
896
Top Catal (2011) 54:888–896 Propylene
Propane
Acid site
Pt
Migration
Al2O3
Coke on metal Coke precursor
Coke on support
Fig. 11 Coke formation mechanism
hydride transfer, etc., resulting in the formation of ‘‘hard coke’’.
5 Conclusion Two types of coke are identified on coked Pt–Sn/Al2O3 catalyst and assigned to coke on the metal and the support, respectively. Coke formed on the metal has aliphatic hydrocarbon characteristic, containing more hydrogen than that formed on the support. Coke formed on the support has an aromatic characteristic. The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen pressures but increases concurrently with the propane pressure while the rate of coke formation on the support increases concurrently with the propane and propylene pressures and decreases with the hydrogen pressure. A mechanism for coke formation has been proposed based on the kinetic analysis. The reaction between the two strong adsorbed C3H6*, which was formed by dehydrogenation of propane, was identified as the kinetic relevant step for the coke formation on the Pt surfaces. A portion of the precursor migrates to the acid site and is involved in the coke formation on the support. In addition, the propylene in the gas phase can also adsorb on the support and form coke through reactions such as polymerization/oligomerization, condensation and so on. Acknowledgment This work is supported by Natural Science Foundation of China (No. 20736011)
References 1. Galvita V, Siddiqi G, Sun P, Bell AT (2010) J Catal 271:209 2. Liu H, Zhang L, Li X, Huang S, Liu S, Xin W, Xie S, Xu L (2009) J Nat Gas Chem 18:331
123
3. Sun K (2004) Petrochem Des 21:25 4. Cosyns J, Chodorge J, Commereuc D, Torck B (1998) Hydrocarb Process 77:61 5. Praserthdam P, Mongkhonsi T, Kunatippapong S, Jaikaew B, Lim N (1997) Stud Surf Sci Catal 111:153 6. Santhosh Kumar M, Holmen A, Chen D (2009) Microporous Mesoporous Mater 126:152 7. Larsson M, Hulte´n M, Blekkan EA, Andersson B (1996) J Catal 164:44 8. Rebo HP, Chen D, Blekkan EA, Holmen A (1998) Stud Surf Sci Catal 119:617 9. Yang JL, Stansberry PG, Zondlo JW, Stiller AH (2002) Fuel Process Technol 79:207 10. Eisenbach D, Gallei E (1979) J Catal 56:377 11. Van Doorn J, Moulijn JA (1990) Fuel Process Technol 26:39 12. Cerqueira HS, Sievers C, Joly G, Magnoux P, Lercher JA (2005) Ind Eng Chem Res 44:2069 13. Geach A (1996) Wear Check Afr Tech Bull 2:1 14. Wolthuis E, Bossenbroek B, DeWall G, Geels E, Leegwater A (1963) J Org Chem 28:148 15. Sato K, Ikeda S, Iida M, Oshima A, Tabata Y, Washio M (2003) Nucl Instrum Methods Phys Res Sect B 208:424 16. Matsushita K, Hauser A, Marafi A, Koide R, Stanislaus A (2004) Fuel 83:1031 17. Jong SJ, Pradhan AR, Wu JF, Tsai TC, Liu SB (1998) J Catal 174:210 18. Guichard B, Roy-Auberger M, Devers E, Rebours B, Quoineaud AA, Digne M (2009) Appl Catal A 367:1 19. Korhonen ST, Airaksinen SMK, Ban˜ares MA, Krause AOI (2007) Appl Catal A 333:30 20. Zeng Z, Natesan K (2003) Chem Mater 15:872 21. Airaksinen SMK, Ban˜ares MA, Krause AOI (2005) J Catal 230:507 22. Chua YT, Stair PC (2003) J Catal 213:39 23. Dumont M, Chollon G, Dourges M, Pailler R, Bourrat X, Naslain R, Bruneel JL, Couzi M (2002) Carbon 40:1475 24. Li J, Naga K, Ohzawa Y, Nakajima T, Shames AP, Panich AI (2005) J Fluor Chem 126:265 25. Santhosh Kumar M, Chen D, Holmen A, Walmsley JC (2009) Catal Today 142:17 26. Yang Z, Zhang Y, Wang X, Zhang Y, Lu X, Ding W (2010) Energy Fuels 24:785 27. Sun L, Guo X, Liu M, Wang X (2010) Ind Eng Chem Res 49:506 28. Li X, Zhang W, Li X, Liu S, Huang H, Han X, Xu L, Bao X (2009) J Phys Chem C 113:8228 29. Duprez D, Hadj-Aissa M, Barbier J (1989) Appl Catal 49:67 30. Liwu L, Tao Z, Jingling Z, Zhusheng X (1990) Appl Catal 67:11 31. Tao Z, Jingling Z, Liwu L (1991) Stud Surf Sci Catal 68:143 32. Srihiranpullop S, Praserthdam P, Mongkhonsi T (2000) Korean J Chem Eng 17:548 33. Novokshonova LA, Tsvetkova VI, Chirkov NM (1963) Russ Chem Bull 12:1077 34. Soga K, Siono T (1982) Polym Bull 8:261 35. Li Q, Sui Z, Zhou X, Chen D (2011) Appl Catal A 398:18 36. Azzam KG, Jacobs G, Shafer WD, Davis BH (2010) Appl Catal A 390:264 37. Biloen P, Dautzenberg FM, Sachtler WMH (1977) J Catal 50:77 38. Liu K, Fung SC, Ho TC, Rumschitzki TC (2003) Ind Eng Chem Res 42:1543 39. Caeiro G, Carvalho RH, Wang X, Lemos MAND, Guisnet M, Ramoˆa Ribeiro F (2006) J Mol Catal A 255:131 40. Lieske H, Sa´rka´ny A, Vo¨lter J (1987) Appl Catal 30:69
