3.1. Hydroisomerization of n-Butane over Various Catalysts
Figure 1 shows the SEM picture of the mixture of Pt/Al
2O
3 with Cs2.5 (mass ratio = 1:1) after grinding in a mortar for 30 min.
Figure 1.
SEM picture of Pt/Al2O3+Cs2.5 (mass ratio = 1:1) after grinding for 30 min.
Figure 1.
SEM picture of Pt/Al2O3+Cs2.5 (mass ratio = 1:1) after grinding for 30 min.
The size of Pt/Al2O3 particles was about 0.5 μm. The Cs2.5 became small powder particles adhered to the Pt/Al2O3 particles due to the low mechanical strength of Cs2.5. The Pt/Al2O3 particles and the Cs2.5 particles contacted closely with each other. Moreover, the mechanical strength was greatly strengthened after pressing the mixture into a disk at 40 kg cm–2 for use as a catalyst.
Table 1 lists conversion and selectivity for the hydroisomerization of
n-butane at 573 K over various catalysts. The data at 5 min could be regarded as the initial catalytic performance and the data at 5 h could be regarded as the stable catalytic performance over each catalyst.
Table 1.
Catalytic hydroisomerization of n-butane over various catalysts at 573 K.
Table 1.
Catalytic hydroisomerization of n-butane over various catalysts at 573 K.
Catalyst | Time on stream | Conv. (%) | Selectivity (%) |
---|
C1 | C2 | C3 | i-C4 | C4= | C5+ |
---|
Pt/Al2O3 | 5 min | 4.2 | 9.7 | 12.6 | 9.3 | 66.2 | 0 | 2.2 |
| 5 h | 2.8 | 8.6 | 9.2 | 8.1 | 71.3 | 0 | 2.8 |
Cs2.5 | 5 min | 23.5 | 2.6 | 6.8 | 9.6 | 76.5 | 0.8 | 3.7 |
| 5 h | 9.2 | 1.8 | 5.1 | 7.3 | 82.3 | 0.7 | 2.8 |
Pt/Cs2.5 | 5 min | 66.1 | 1.9 | 3.2 | 4.7 | 88.4 | 0.7 | 1.0 |
| 5 h | 42.2 | 1.6 | 2.5 | 4.1 | 90.3 | 0.6 | 0.9 |
Pt/Al2O3+Cs2.5 | 5 min | 70.3 | 1.2 | 2.4 | 3.9 | 91.2 | 0.6 | 0.6 |
| 5 h | 64.8 | 0.8 | 2.0 | 3.3 | 92.5 | 0.5 | 0.8 |
Pt/Al
2O
3 showed a low conversion (2.8%) after 5 h on stream, which indicates that catalyst acidity is indispensable for the hydroisomerization of
n-butane. Cs2.5 showed an initial conversion of 23.5% and an initial selectivity to isobutane of 76.5%. The solid acid Cs2.5 could catalyze the hydroisomerization of
n-butane even without Pt [
44]. However, the conversion after 5 h on stream over Cs2.5 was low (9.2%), due to the severe deactivation. The initially white Cs2.5 catalyst became black after reaction at 573 K for 5 h, indicating that carbonaceous deposits were formed on the catalyst surface, covering the acid sites of Cs2.5. This is the reason for the deactivation of the Cs2.5 catalyst. On the other hand, Pt/Cs2.5 showed an initial conversion of 66.1% and a stable conversion of 42.2%. The synergy between Pt and Cs2.5 was great, because both the initial conversion and the stable conversion were remarkably improved by introducing Pt in the solid catalyst Cs2.5. Moreover, Pt/Al
2O
3+Cs2.5 exhibited a higher stable conversion (64.8%) and a higher stable selectivity to isobutane (92.5%) than those over Pt/Cs2.5 for the hydroisomerization of
n-butane after 5 h on stream. On the other hand, the mixture of Pt/Cs2.5 and Al
2O
3 just showed a similar performance (not shown in
Table 1) to that of Pt/Cs2.5 for the hydroisomerization of
n-butane. Thus the method for combining Pt with Cs2.5 is important for increasing the catalytic activity of bifunctional catalysts in the hydroisomerization of
n-butane.
Scheme 1 shows the mechanism of
n-butane hydroisomerization over a Brönsted acid catalyst. Since a heteropolyacid is a kind of typical Brönsted acid [
1], it is very probable that the hydroisomerization of
n-butane over Cs2.5 also takes place
via this mechanism [
44]. At first, a
sec-butyl carbenium cation was formed by a step of proton addition, followed by a step of H
2 elimination. Then, the
sec-butyl carbenium cation was transformed to a
tert-carbenium cation by a shift of the methyl group. Finally, the
tert-carbenium cation captured a H
2 molecule and eliminated a proton to form an isobutane molecule. All of these steps were carried out on the Brønsted acid sites.
Scheme 1.
Mechanism of n-butane hydroisomerization over a Brønsted acid catalyst.
Scheme 1.
Mechanism of n-butane hydroisomerization over a Brønsted acid catalyst.
Scheme 2 shows the mechanism of
n-butane hydroisomerization over a bifunctional catalyst containing metal and heteropolyacid [
18,
19]. At first, the
n-butane molecule eliminated a H
2 molecule to form a
n-butene molecule on the Pt sites. Then, the formed
n-butene molecule moved to the solid acid sites to form a
sec-butyl carbenium cation by obtaining a proton. Then, the
sec-butyl carbenium cation was transformed to a
tert-carbenium cation by a shift of the methyl group on the acid sites. Then, the
tert-carbenium cation eliminated a proton to form an isobutene molecule. Finally, the isobutene molecule moved to the Pt sites to form an isobutane molecule by a process of H
2 addition.
Scheme 2.
Mechanism of n-butane hydroisomerization over a bifunctional catalyst.
Scheme 2.
Mechanism of n-butane hydroisomerization over a bifunctional catalyst.
As a result, although the carbenium cation is a key intermediate in the hydroisomerization of
n-butane over either a heteropolyacid catalyst or a bifunctional catalyst containing Pt and heteropolyacid, the path for forming the carbenium intermediate over a bifunctional catalyst is different from that over a heteropolyacid catalyst. Because the rate of formation of a carbenium species by adding proton to
n-butene is much faster than that by adding proton to
n-butane, in the hydroisomerization of
n-butane the bifunctional catalysts (Pt/Cs2.5 and Pt/Al
2O
3+Cs2.5) showed much higher conversions than those observed over the monofunctional heteropolyacid catalyst Cs2.5 (
Table 1).
Figure 2 shows H
2 uptake by Pt/Al
2O
3+Cs2.5 and Pt/Cs2.5 at 298 K. The H
2 uptake was used as for calculating the Pt surface area, Pt dispersion degree, and Pt particle size for the samples. The total H
2 uptake contains the reversible H
2 uptake (physical absorption) and the irreversible H
2 uptake (chemical absorption). The irreversible H
2 uptake at 0 torr could be obtained from the total H
2 uptake at 0 torr and the reversible H
2 uptake at 0 torr. The obtained irreversible H
2 uptake at 0 torr was used for calculating Pt surface area and Pt dispersion degree of each sample. As shown in
Figure 2, the Pt surface area and Pt dispersion degree over Pt/Al
2O
3+Cs2.5 were much larger than those over Pt/Cs2.5. Thus Al
2O
3 is a good support for Pt as comparison with Cs2.5. The high Pt dispersion degree gave Pt/Al
2O
3+Cs2.5 a higher selectivity for isobutane than that over Pt/Cs2.5 in the hydroisomerization of
n-butane (
Table 1).
Figure 2.
(a) H2 uptake by Pt/Al2O3+Cs2.5 at 298 K. (b) H2 uptake by Pt/Cs2.5 at 298 K.
Figure 2.
(a) H2 uptake by Pt/Al2O3+Cs2.5 at 298 K. (b) H2 uptake by Pt/Cs2.5 at 298 K.
Although the catalysts prepared by directly supporting Pt on solid acids (using the impregnation method) are usually used for the hydroisomerization of
n-alkanes, the mechanical mixtures of solid acids with Pt/SiO
2 or Pt/Al
2O
3 have also been applied for the hydroisomerization of
n-alkanes for several purposes. Firstly, a mechanical mixture of Pt/SiO
2 with aluminum silicates had been used for investigating the mechanism of
n-alkanes hydroisomerization over Pt-promoted acid catalysts [
40]. Because Pt sites and solid acid sites achieved their functions independently, the mechanical mixed catalyst showed a catalytic activity similar to that seen over the directly impregnated catalysts [
40]. Secondly, because the Pt-supported zeolites have low mechanical strength and poor molding properties, the mechanical mixtures of acidic zeolites with Pt/SiO
2 or Pt/Al
2O
3 have been used to increase the mechanical strength and the molding ability of the bifunctional catalysts [
33,
35,
45]. Thirdly, in the cause of solid superacid SO
42--ZrO
2, the directly supported Pt/SO
42--ZrO
2 catalyst is not a bifunctional catalyst due to the interaction of Pt with the sulfur on the SO
42--ZrO
2 surface, while the mechanical mixed catalyst Pt/Al
2O
3+SO
42--ZrO
2 is a bifunctional catalyst [
27]. Therefore, the mechanical mixed catalysts Pt/SiO
2+SO
42--ZrO
2 and Pt/Al
2O
3+SO
42--ZrO
2 showed high catalytic performances for the hydroisomerization of
n-butane [
27,
28,
46]. Fourthly, in the case of the heteropolyacid Cs
2.5H
0.5PW
12O
40, the mechanical mixed catalyst Pt/Al
2O
3+Cs2.5 showed a higher activity than that of the impregnated catalyst Pt/Cs2.5 for the hydroisomerization of
n-pentane and
n-hexane [
5,
6,
7]. In the present study, we found that Pt/Al
2O
3+Cs2.5 showed a higher catalytic performance for the hydroisomerization of
n-butane in comparison with Pt/Cs2.5. The directly supported catalyst Pt/Cs2.5 had a low Pt surface area and a low Pt dispersion degree, probably because Pt
2+ interacted with PW
12O
403– during the impregnation process.
3.2. Deactivation of Various Catalysts in the Hydroisomerization of n-Butane
Figure 3 shows the time courses of
n-butane hydroisomerization over various catalysts at 573 K. Pt/Al
2O
3 showed a very low conversion due to the lack of strong acid sites. Thus Pt/Al
2O
3 can barely be used as an independent catalyst for the hydroisomerization of
n-butane. The presence of strong acid sites in the catalyst is indispensable for the hydroisomerization of
n-butane. Cs2.5 showed an initial conversion (after 5 min on stream) of 23.5%, but the conversion decreased to 9.2% after 5 h on stream. The carbonaceous deposits which formed by the polymerization of alkene intermediates and by-products (such as
n-butene,
iso-butene, and so on) covered the solid acid sites and caused a serious deactivation of the Cs2.5 catalyst. The carbonaceous deposits are hydrocarbons with large molecular weights and high ratios of C to H. When Pt was introduced into Cs2.5 (
i.e., Pt/Cs2.5 and Pt/Al
2O
3+Cs2.5), the deactivation was greatly repressed in the hydroisomerization of
n-butane. Pt catalyzes the hydrogenation of the carbonaceous deposits (covered acid sites) by supplying hydrogen. In concrete, the remarkable effect of Pt in suppressing the catalyst deactivation was brought about by the activated hydrogen, which were formed on Pt, transferred to Cs2.5, and utilized to remove the carbonaceous deposits [
5,
6]. Pt/Al
2O
3+Cs2.5 showed a higher catalytic stability than that of Pt/Cs2.5 for the hydroisomerization of
n-butane because the Pt dispersion degree in Pt/Al
2O
3+Cs2.5 was much higher than that in Pt/Cs2.5 (
Figure 2).
Figure 3.
Time courses of the hydroisomerization of n-butane over various catalysts at 573 K.
Figure 3.
Time courses of the hydroisomerization of n-butane over various catalysts at 573 K.
The amount of the carbonaceous deposits on the used catalyst could be calculated by a temperature-programmed oxidation (TPO) method [
47,
48,
49]. After the reaction was carried out over each catalyst for 5 h at 573 K, the reactor was cooled to room temperature in flowing N
2 gas. The catalyst was then treated in air flow (1.5 L h
−1) by increasing the temperature at 2.5 K min
−1 to change the carbonaceous deposits to CO
2. The formed CO
2 could be detected by a TCD GC.
Figure 4 shows the dependence of the rate of CO
2 (formed from carbonaceous deposits) on the calcination temperature over various catalysts after 5 h on stream at 573 K.
Figure 4.
Dependence of the rate of CO2 (formed from carbonaceous deposits) on the calcination temperature over various catalysts after 5 h on stream at 573 K.
Figure 4.
Dependence of the rate of CO2 (formed from carbonaceous deposits) on the calcination temperature over various catalysts after 5 h on stream at 573 K.
An integration of the rate of CO
2 formation gave the amount of total CO
2, from which the amount of the carbonaceous deposits (after 5 h on stream at 573 K) were calculated as 4.5, 0.6, and 0.4 wt% on Cs 2.5, Pt/Cs2.5, and Pt/Al
2O
3+Cs2.5, respectively. Moreover, according to the amount of CO
2 and H
2O formed in the TPO measurement for various catalysts after 5 h on stream at 573 K, the H/C ratios were calculated as about 0.3, 0.6, and 0.7 for the carbonaceous deposits on Cs 2.5, Pt/Cs2.5, and Pt/Al
2O
3+Cs2.5, respectively. Two peaks with the highest rate at around 473 and 598 K were observed from the plot of Cs2.5. In general, the peak at lower temperature could be regarded as “soft coke” and the peak at the higher temperature could be regarded as “hard coke” [
50,
51]. Pt/Cs2.5 deposited both “soft coke” and “hard coke” on the surface, but the amount deposited on the Pt/Cs2.5 surface was much lower than that on the Cs2.5 surface. Thus Pt/Cs2.5 showed a much higher stability than that over Cs2.5 at 573 K (
Figure 3). Further, the amount of carbonaceous deposit on Pt/Al
2O
3+Cs2.5 (0.4 wt.%) was lower than that on Pt/Cs2.5 (0.6 wt.%) after 5 h on stream. In the TPO profile of Pt/Al
2O
3+Cs2.5, the peak of “hard coke” at 573–598 K was very small and just became a shoulder peak for the peak of “soft coke” at low temperature. Therefore, Pt/Al
2O
3+Cs2.5 showed a very high stability for the hydroisomerization of
n-butane (
Figure 3).
3.3. Hydroisomerization of n-Butane over the Pt/Al2O3+Cs2.5 Catalyst
Figure 5 shows the effect of Pt amount in the Pt/Al
2O
3+Cs2.5 catalyst for the hydroisomerization of
n-butane at 573 K. Either the pressure of
n-butane or the pressure of H
2 was 0.1 atm. The initial conversion greatly increased when a small amount of 2 wt.% Pt/Al
2O
3 was added to Cs2.5, but it almost remained at a constant value when the amount of 2 wt.% Pt/Al
2O
3 was more than 0.1 g. On the other hand, 0.5 g of 2 wt.% Pt/Al
2O
3 was necessary for suppressing the deactivation to make the stationary conversion close to the initial conversion. Thus Pt has two effects in the hydroisomerization of
n-butane. Firstly, Pt achieves a hydrogenation-dehydrogenation function, which greatly improves the initial conversion over Pt/Al
2O
3+Cs2.5. It seems that 0.1 g of 2 wt.% Pt/Al
2O
3 is enough for increasing the initial conversion. Therefore, the hydrogenation-dehydrogenation on Pt sites is a limiting step when the amount of Pt/Al
2O
3 is less than 0.1 g in Pt/Al
2O
3+Cs2.5, and the isomerization on Cs2.5 sites is a limiting step when the amount of Pt/Al
2O
3 is more than 0.1 g in Pt/Al
2O
3+Cs2.5. Therefore, the speed of hydrogenation-dehydrogenation on Pt is very fast and a small amount of Pt can achieve the hydrogenation-dehydrogenation function. Secondly, Pt plays an important role for suppressing the catalyst deactivation and for maintaining the catalytic stability for the hydroisomerization of
n-butane. The deactivation can not be completely suppressed with a small amount of Pt/Al
2O
3 (
Figure 5). It needs a relatively large amount of 2 wt.% Pt/Al
2O
3 (about 0.5 g) to eliminate the catalyst deactivation over Pt/Al
2O
3+Cs2.5 for the hydroisomerization of
n-butane.
Figure 5.
Effect of Pt amount in the Pt/Al2O3+Cs2.5 catalyst for the hydroisomerization of n-butane at 573 K. (■) Initial conversion. (●) Stationary conversion. Cs2.5: 0.5 g. Reaction conditions: n-butane: 0.1 atm; H2: 0.1 atm; N2: 0.8 atm; total flow rate: 20 mL min–1.
Figure 5.
Effect of Pt amount in the Pt/Al2O3+Cs2.5 catalyst for the hydroisomerization of n-butane at 573 K. (■) Initial conversion. (●) Stationary conversion. Cs2.5: 0.5 g. Reaction conditions: n-butane: 0.1 atm; H2: 0.1 atm; N2: 0.8 atm; total flow rate: 20 mL min–1.
Figure 6 shows the time courses of the hydroisomerization of
n-butane over Pt/Al
2O
3+Cs2.5 at 573 K under various H
2 pressures. The partial pressure of
n-butane was 0.1 atm and the partial pressure of H
2 was changed from 0 to 0.5 atm. Under a N
2 atmosphere (P
H2 = 0), the initial conversion was very high, but the deactivation was serious. With increasing H
2 pressure, the deactivation was gradually suppressed, but the initial conversion decreased. Thus H
2 has two effects in the hydroisomerization of
n-butane: decreasing the initial conversion and suppressing the catalyst deactivation. Actually, H
2 plays two roles in the hydroisomerization of
n-butane over bufunctional catalysts. One role of H
2 is the hydrogenation of isobutene to isobutane on Pt sites (the last step in
Scheme 2). According to
Scheme 2, the first step is the dehydrogenation of
n-butane to
n-butene for the hydroisomerization of
n-butane over a bifunctional catalyst. Under a high H
2 partial pressure, the equilibrium of the dehydrogenation step shifts to
n-butane, which causes the decrease of the concentration of
n-butene intermediates in the reaction system. As a result, because the presence of H
2 is not favorable for the dehydrogenation step in the hydroisomerization of
n-butane (the first step in
Scheme 2), the initial conversion decreases with increasing H
2 pressure over Pt/Al
2O
3+Cs2.5. Another role of H
2 in the hydroisomerization of
n-butane over bifunctional catalysts is the suppression of catalyst deactivation. As discussed above, the carbonaceous deposits that formed on the catalyst surface cause the catalyst deactivation. H
2 molecules can form active H atoms on Pt sites during the reaction. The formed active H atoms shift (spillover) to the acid sites and hydrogenate the carbonaceous deposits on the acid sites of Cs2.5. Therefore, the catalyst deactivation can be suppressed by increasing H
2 pressure in the hydroisomerization of
n-butane over Pt/Al
2O
3+Cs2.5.
Figure 6.
Time courses of the hydroisomerization of n-butane over Pt/Al2O3+Cs2.5 at 573 K under various H2 pressures. N-butane: 0.1 atm; N2: balance; total flow rate: 20 mL min–1.
Figure 6.
Time courses of the hydroisomerization of n-butane over Pt/Al2O3+Cs2.5 at 573 K under various H2 pressures. N-butane: 0.1 atm; N2: balance; total flow rate: 20 mL min–1.
Figure 7 shows the time courses of the
n-butane hydroisomerization over Pt/Al
2O
3+Cs2.5 at 573 K under various
n-butane pressures. The partial pressure of H
2 was 0.1 atm and the partial pressure of
n-butane was changed from 0.1 to 0.7 atm.
The initial conversions over Pt/Al
2O
3+Cs2.5 showed almost the same values under various
n-butane pressures. This implies that the initial rate is proportional to the
n-butane pressure (
i.e., first order in
n-butane). Thus the “bimolecular mechanism” (by alkylation-cracking a C
8 intermediate) is not important for the hydroisomerization of
n-butane over Pt/Al
2O
3+Cs2.5 [
6,
43]. On the other hand, the conversion after 5 h on stream decreased with increasing the
n-butane pressure. The increase of
n-butane pressure means the decrease in the ratio of H
2 to
n-butane in the feed gas, which caused the catalyst deactivation during the reaction. Therefore, it needs a high H
2 pressure to maintain the catalyst stability for the
n-butane hydroisomerization over Pt/Al
2O
3+Cs2.5 under a high partial pressure of
n-butane.
Figure 7.
Time courses of the n-butane hydroisomerization over Pt/Al2O3+Cs2.5 at 573 K under various n-butane pressures. H2: 0.1 atm; N2 balance; total flow rate: 20 mL min–1.
Figure 7.
Time courses of the n-butane hydroisomerization over Pt/Al2O3+Cs2.5 at 573 K under various n-butane pressures. H2: 0.1 atm; N2 balance; total flow rate: 20 mL min–1.
3.4. Comparison of Various Bifunctional Catalysts for the Hydroisomerization of n-Butane
Table 2 lists the results obtained in the hydroisomerization of
n-butane with various bifunctional catalysts. We compared the Pt/Al
2O
3+Cs2.5 catalyst with the catalysts of Pt/Al
2O
3+SZ and Pt/Al
2O
3+HZ in this study because both Pt-promoted solid super acid SZ (SO
42--ZrO
2) and Pt-promoted acidic zeolite HZ (H-ZSM-5) have been reported as effective catalysts for the hydroisomerization of
n-butane [
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35].
Table 2.
Comparison of various bifunctional catalysts for the hydroisomerization of n-butane.
Table 2.
Comparison of various bifunctional catalysts for the hydroisomerization of n-butane.
Catalyst | Time on stream | Conv. (%) | Selectivity (%) |
---|
C1 | C2 | C3 | i-C4 | C4= | C5+ |
---|
Pt/Al2O3+Cs2.5 | 5 min | 70.3 | 1.2 | 2.4 | 3.9 | 91.2 | 0.6 | 0.6 |
| 5 h | 64.8 | 0.8 | 2.0 | 3.3 | 92.5 | 0.5 | 0.8 |
Pt/Al2O3+SZ | 5 min | 77.6 | 7.7 | 10.6 | 14.3 | 61.2 | 1.3 | 4.8 |
| 5 h | 20.5 | 4.6 | 7.2 | 10.1 | 71.7 | 1.0 | 5.2 |
Pt/Al2O3+HZ | 5 min | 39.7 | 2.7 | 3.1 | 5.7 | 87.1 | 0.9 | 0.5 |
| 5 h | 32.2 | 1.8 | 3.3 | 4.6 | 88.7 | 1.1 | 0.4 |
Pt/Al
2O
3+Cs2.5, Pt/Al
2O
3+SZ and Pt/Al
2O
3+HZ had the same Pt dispersion because the Pt metal was supported on Al
2O
3 for all three catalysts. Therefore, the difference of solid acids (Cs2.5, SZ, and HZ) determined the difference of catalytic performances for the hydroisomerization of
n-butane over various catalysts. As shown in
Table 2, Pt/Al
2O
3+Cs2.5 showed the highest conversion (64.8%) and the highest selectivity for isobutane (92.5%) among the various catalysts after 5 h on stream. Pt/Al
2O
3+SZ showed the highest initial conversion (77.6%), but the conversion rapidly decreased to 20.5% after 5 h on stream. Moreover, the selectivity for isobutane over Pt/Al
2O
3+SZ was low. Pt/Al
2O
3+HZ showed lower conversion and selectivity than those over Pt/Al
2O
3+Cs2.5. As a result, Pt/Al
2O
3+Cs2.5 is the best catalyst for the hydroisomerization of
n-butane at a low H
2 pressure (0.1 atm). Brønsted acid sites in Cs2.5 contributed to the acid function of Pt/Al
2O
3+Cs2.5 for the hydroisomerization of
n-butane since heteropolyacids are pure Brønsted acids without Lewis acid sites [
1].
Figure 8 shows the NH
3-TPD profiles of various catalysts. The NH
3-TPD is a powerful tool for measuring the acidic strength of a solid acid. The NH
3 molecules desorbed from the weak acid sites at low temperatures and desorbed from the strong acid sites at high temperatures.
Figure 8.
NH3-TPD profiles of various catalysts. (A): Pt/Al2O3; (B): Pt/Cs2.5; (C): Pt/Al2O3+Cs2.5; (D): Pt/Al2O3+HZ; (E): Pt/Al2O3+SZ.
Figure 8.
NH3-TPD profiles of various catalysts. (A): Pt/Al2O3; (B): Pt/Cs2.5; (C): Pt/Al2O3+Cs2.5; (D): Pt/Al2O3+HZ; (E): Pt/Al2O3+SZ.
Pt/Al
2O
3 showed a weak and broad peak at about 473 K in the NH
3-TPD profile, indicating that the acid sites in Pt/Al
2O
3 are very weak. Although Pt/Cs2.5 and Pt/Al
2O
3+Cs2.5 showed the peaks with similar shape in the NH
3-TPD profiles, the maximum temperature of NH
3 desorption from Pt/Cs2.5 was slightly lower than that from Pt/Al
2O
3+Cs2.5. Thus the acidic strength of Pt/Cs2.5 was slightly weaker than that of Pt/Al
2O
3+Cs2.5. This is the reason that Pt/Al
2O
3+Cs2.5 showed a higher initial conversion than that over Pt/Cs2.5 (
Figure 3). The decrease of the acid strength of Pt/Cs2.5 probably because the ion exchange of Pt
2+ with H
+ in Cs
2.5H
0.5PW
12O
40 occurred in the impregnation stage of the synthesis of Pt/Cs2.5. For a solid acid, the strongest acid sites provide its main character for the acid-catalyzed reactions. The peak at the maximum temperature in the NH
3-TPD profile corresponds to the strongest acid sites and thus it determines the acidic strength of a solid acid. According to the peak position at the maximum temperature in the NH
3-TPD profile of various samples (
Figure 8), the acidic strength of various catalysts was in an order of Pt/Al
2O
3+SZ > Pt/Al
2O
3+Cs2.5 > Pt/Cs2.5 > Pt/Al
2O
3+HZ > Pt/Al
2O
3. Pt/Al
2O
3+HZ showed two peaks at around 473 K and 673 K in the NH
3-TPD profile, while Pt/Al
2O
3+SZ showed two peaks at around 473 K and 873 K in the NH
3-TPD profile. These results indicate that both Pt/Al
2O
3+HZ and Pt/Al
2O
3+SZ have two types of acid sites: weak acid sites and strong acid sites. The hydroisomerization of
n-butane occurs through carbenium cation intermediates which are easy to be formed on the strong acid sites. Thus the strong acid sites provide the main effect for the catalytic performances over Pt/Al
2O
3+HZ and Pt/Al
2O
3+SZ. However, the weak acid sites also catalyze the reaction although the reaction rate is low. Because the products formed from the strong acid sites and the weak acid sites are different, the broadly distributed acid sites caused the decrease of the selectivity for isobutane over Pt/Al
2O
3+HZ and Pt/Al
2O
3+SZ (
Table 2). On the other hand, either Pt/Cs2.5 or Pt/Al
2O
3+Cs2.5 showed only one peak in the NH
3-TPD profile, implying that the strength of the acid sites were distributed uniformly on the surfaces of Pt/Cs2.5 and Pt/Al
2O
3+Cs2.5. The uniform acid strength gave Pt/Cs2.5 and Pt/Al
2O
3+Cs2.5 high selectivity for isobutane in the hydroisomerization of
n-butane (
Table 1).
Figure 9 shows the time courses of various bifunctional catalysts for the hydroisomerization of
n-butane at 573 K. Either the pressure of
n-butane or the pressure of H
2 was 0.1 atm.
Figure 9.
Time courses various bifunctional catalysts for the hydroisomerization of n-butane at 573 K. (●) Pt/Al2O3+Cs2.5, (■) Pt/Al2O3+SZ, (▲) Pt/Al2O3+HZ.
Figure 9.
Time courses various bifunctional catalysts for the hydroisomerization of n-butane at 573 K. (●) Pt/Al2O3+Cs2.5, (■) Pt/Al2O3+SZ, (▲) Pt/Al2O3+HZ.
For a bifunctional catalyst in the hydroisomerization of
n-alkanes, the balance between acid and metal is very important for obtaining the optimum performance [
52,
53]. The acid strength is an important factor for controlling the activity and the selectivity in the hydroisomerization of
n-alkanes. Strong acids usually exhibit high activity but also usually show low selectivity and serious deactivation.
As shown in
Figure 9, the initial conversion was in the order Pt/Al
2O
3+SZ > Pt/Al
2O
3+Cs2.5 > Pt/Al
2O
3+HZ. This order coincided with the order of the acid strength of the various catalysts. Moreover, Pt/Al
2O
3+SZ showed a serious deactivation under a low H
2 pressure (0.1 atm) and the activity after 5 h on stream was low. A high H
2 pressure is need for the hydroisomerization of
n-butane over the Pt-promoted SZ (SO
42--ZrO
2) catalysts [
25,
26,
27,
28,
29,
30,
31]. Actually, the deactivation of Pt/Al
2O
3+SZ could be suppressed under a high H
2 pressure of 0.7 atm. Moreover, because the conversion over Pt/Al
2O
3+Cs2.5 decreased with increasing H
2 pressure (
Figure 6), Pt/Al
2O
3+SZ showed a higher stationary conversion than that over Pt/Al
2O
3+Cs2.5 under a high H
2 pressure of 0.7 atm. On the contrary, Pt/Al
2O
3+Cs2.5 showed a higher stationary conversion than that over Pt/Al
2O
3+SZ under a low H
2 pressure of 0.1 atm (
Figure 9). Thus Pt/Al
2O
3+Cs2.5 is a good catalyst under low H
2 pressures and Pt/Al
2O
3+SZ is a good catalyst under high H
2 pressures for the hydroisomerization of
n-butane. In comparison with Pt/Al
2O
3+SZ, using Pt/Al
2O
3+Cs2.5 as a catalyst for the hydroisomerization of
n-butane decreases the cost of the process (because H
2 is much more expensive than N
2) and improves the safety of the operation (because a gas stream containing a large amount of H
2 is very dangerous).