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A new catalyst for n-butane isomerization: persulfate-modified Al2O3–ZrO2
Applied Catalysis A: General 185 (1999) 293–300
A new catalyst for n-butane isomerization: persulfate-modified Al2 O3 –ZrO2 Yongde Xia, Weiming Hua, Zi Gao ∗ Department of Chemistry, Fudan University, Shanghai 200433, PR China Received 12 January 1999; received in revised form 22 April 1999; accepted 11 May 1999
Abstract The n-butane isomerization reaction on Al-promoted zirconia catalysts modified by persulfate was studied both at low temperature in the absence of hydrogen and at high temperature in the presence of hydrogen. The addition of Al shows a significant enhancement in the activity and stability of the samples for n-butane isomerization at 250◦ C. After running on stream for 200 h, the n-butane conversion on the catalyst containing 3 mol% Al2 O3 remains steady at 72% of its equilibrium conversion and no observable trend of further deactivation has been observed. The different behavior of the with or without Al-promoted catalysts at low temperature and high temperature is probably associated with a change of reaction mechanism from bimolecular to monomolecular. Microcalorimetric measurements of ammonia adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths effective for the n-butane isomerization reaction. ©1999 Elsevier Science B.V. All rights reserved. Keywords: n-butane isomerization; ZrO2 modified by persulfate; Al-promoted persulfated zirconia; Acid strength distribution
1. Introduction The isomerization reaction of straight-chain paraffins to more highly branched isomers is an important process for the production of clear-burning fuels in the petrochemical refining industry. For example, n-butane, which is undesirable for gasoline, can be converted to isobutane, a valuable precursor to MTBE and other fuel additives. The technology used currently for n-butane isomerization is based on the Pt/Al2 O3 –Cl catalyst which operates at elevated temperatures where the equilibrium product distribution ∗ Corresponding author. Tel.: +86-21-65642792; fax: +86-21-65641740; e-mail: [email protected]
of isobutane is not favorable [1], and the use of superacids as catalysts has been suggested [2]. Owing to more and more strict environmental concerns, researchers have shown more interest in solid superacid catalysts than in liquid superacids. A considerable number of reports dealing with sulfate-promoted zirconia and other metal oxides as catalysts for n-butane isomerization have appeared recently [3–15]. Often being regarded as a typical solid superacid, sulfated zirconia catalyst gives a high activity for n-butane isomerization at low temperature, but a rapid deactivation of the catalyst has often been observed at high temperature. In order to improve the life time of SO2− 4 /ZrO2 catalyst, n-butane isomeriazation reactions in the presence of hydrogen and addition
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 7 6 - 3
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of a small amount of Pt or Ni have been suggested [8,16]. Recently, a new solid superacid, sulfated trinary oxide containing Fe, Mn and Zr has been developed by Hsu et al. [17], and has shown higher superacidity than sulfated zirconia. This new catalyst has been reported to be able to isomerize n-butane at ambient temperature with a rate three orders of magnitude greater than SO2− 4 /ZrO2 . Some researchers [18,19] have confirmed this result but have pointed out that the Fe or Mn-promoted sulfated zirconia catalyst also deactivates quickly at 250◦ C in the presence of hydrogen or at 60◦ C in the presence of nitrogen. Miao et al. [20,21] have reported that the doping of sulfated zirconia with various transition metals, such as Cr or V, could enhance its catalytic activity for n-butane isomerization reaction at low temperature in the absence of hydrogen, which were two to three times more active than sulfated Fe−Mn−Zr for n-butane isomerization, but the doped catalysts are not stable at high temperature in the presence of hydrogen yet. However, in our previous work [22], it has been found that Al promotes the catalytic activity and stability of sulfated zirconia for n-butane isomerization more effectively than doping transition metal catalysts at high temperature in the presence of hydrogen. More recently, we found that zirconia modified by persulfate has shown higher activity for n-butane isomerization than that of sulfated zirconia. Naturally, we want to know whether the promoting effect of Al on sulfated zirconia has a similar effect on persulfated zirconia. In this paper, the n-butane isomerization reaction on a series of persulfated Al-promoted zirconia catalysts was studied both at low and high temperatures. A quantitative measurement of the number and acid strength distribution of the surface acid sites of the catalysts was accomplished by means of a microcalorimetric method. The reasons for the improvement in activity and stability of the catalysts in n-butane isomerization by the addition of Al are discussed based on the experimental results.
2. Experimental The technique used to produce persulfated zirconia in the present work was mainly based on the pro-
cedures described earlier [4,8,11]. Aqueous ammonia was added dropwise to a solution of ZrOCl2 ·8H2 O till pH = 9–10. The hydroxide was washed, filtered and dried at 110◦ C and then immersed in a 0.5 mol l−1 ammonium persulfate ((NH4 )2 S2 O8 ) or 0.5 mol l−1 diluted sulfuric acid solution for 30 min. The persulfated zirconia or sulfated zirconia was then filtered, dried at 110◦ C overnight and calcined at 650◦ C in static air for 3 h. The two catalysts were labeled as ZS2 and ZS, respectively. Al-promoted catalysts were prepared in the same way from a mixed solution of ZrOCl2 ·8H2 O and Al(NO3 )3 ·9H2 O. The Al-promoted catalysts were labeled as ZAS2 . X-ray powder diffraction measurements were carried out on a Rigaku D/MAX-II A instrument with Cu K␣ radiation at 40 kV and 20 mA, scan speed 16◦ min−1 and scan range 5◦ −70◦ . BET surface areas of the samples were acquired on a Micromeritics ASAP 2000 equipment using N2 as the adsorbent. Infrared spectra of the samples were recorded on a Perkin−Elmer 983 G spectrometer. The samples were pressed into thin disks with a density of 3−5 mg cm−2 and placed in a quartz cell with CaF2 windows. Microcalorimetric studies of the adsorption of ammonia were carried out at 150◦ C using a Tian–Calvet type heat-flux calorimeter. The catalysts were evacuated at 250◦ C for 3 h before measurements. The coke deposited on the catalysts was detected on a Carlo−Erba 1106 elemental analysis instrument. Sulfur content in the catalysts was detected by a chemical method. Dehydrated Na2 CO3 and ZnO were used as fusing agents, and the sulfate was turned into BaSO4 and determined by gravimetric method. The isomerization of n-butane was carried out both at low and high temperatures. The isomerization reaction at 35◦ C was performed in a recirculating glass reactor under the closed system pressure of 7 × 104 Pa. The catalyst loading was 0.5 g. The amount of n-butane of 99.9% purity injected for each test was 5 ml of gas (20◦ C, 1 × 105 Pa). The isomerization reaction at 250◦ C was carried out in a flow-type fixed bed reactor under ambient pressure. The catalyst loading was 1.0 g, and a mixture of n-butane (purity above 99.0%) and H2 (1 : 10 molar ratio) was fed at a rate of WHSV 0.3 h−1 . The catalysts were preheated in situ in dry air at 450◦ C for 3 h. The reaction products were analyzed by an on-line gas chromatograph equipped with FID.
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Table 1 Surface area and sulfur content of various samples
Fig. 1. XRD patterns of various catalysts calcined at 650◦ C. (䊏) Tetragonal phase; (䊉) monoclinic phase. (a) ZS; (b) ZS2 ; (c) ZS2 -1; (d) ZS2 -2; (e) ZS2 -3; (f) ZS2 -4; (g) ZS2 -5; (h) ZS2 -6.
3. Results and discussion 3.1. Catalyst characterization XRD patterns of the catalysts after calcining at 650◦ C in air for 3 h were recorded; the results are shown in Fig. 1. The tetragonal phase accompanied with a small portion of the monoclinic phase is observed in ZS2 and ZAS2 -1 samples, which are similar to the XRD result of ZS sample. As regards ZAS2 catalysts with 1.5−10.0 mol% Al2 O3 , the transformation from the metastable tetragonal phase to the thermodynamically favored monoclinic phase is retarded. Only the tetragonal phase appears in all these Al-promoted catalysts except ZAS2 -1. The intensity of the diffractive peak of the tetragonal phase increases slightly with Al2 O3 content up to 3.0–4.5 mol% and then decreases slightly as the Al2 O3 content is further increased. The characteristic peaks of Al2 O3 are not present in any of the Al-promoted samples, indicating that Al2 O3 is rather homogeneously mixed with zirconia. Table 1 presents the data of sulfur content and surface area of the catalysts with different Al2 O3 contents. BET surface areas of the ZAS2 series catalysts
Sample
Al2 O3 content (mol%)
Surface area (m2 g−1 )
SO3 content (wt.%)
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
0 0.5 1.5 3.0 4.5 6.0 10.0 0
58.2 73.8 76.3 80.8 99.4 96.1 85.0 113.0
1.99 2.44 2.74 2.85 3.10 3.24 5.39 3.30
are slightly higher than that of ZS2 , but are a bit lower than that of ZS. The sulfur content of ZS2 is only 60% as high as that of ZS. The lower surface area and sulfur content of ZS2 are probably caused by immersing the amorphous ZrO2 with a solution of aqueous ammonium salt. The sulfate concentration on the catalysts increases significantly with Al2 O3 content. The amount of sulfate on ZS2 corresponds to approximately 60% of a monolayer coverage, assuming that each sulfate group covers 0.25 nm2 [23]. On ZAS2 -6 catalyst, the surface coverage of sulfate groups is close to unity, showing that the incorporation of Al into ZrO2 helps to stabilize the surface sulfate complexes remarkably. After evacuation at 350◦ C for 3 h, infrared spectra of the SO2− 4 promoted superacids display a strong adsorption band at 1380–1390 cm−1 , which represents the characteristic stretching frequency of surface sulfate species with covalent S=O bonds [24]. The characteristic stretching frequency of ZS2 and ZAS2 catalysts appeared at 1398 cm−1 , which is different from that of ZS at 1388 cm−1 ; we postulate that S2 O2− 7 is responsible for such a stretching frequency. These results are coincident with those of M. Bensitel [25] and C. Morterra [26,27]. When one introduces metal cations, such as Al3+ , into the zirconia modified by persulfate sample, due to the interaction of Zr with Al and the formation of Zr–O–Al bond in the binary oxide [28,29], the following complex structures may be formed in some local areas on the surface of the samples:
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Table 2 n-Butane isomerization activities of various samples at 35◦ C Sample
Conversiona (%)
k1 × 103 (h−1 )
k−1 × 103 (h−1 )
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
45.40 45.97 46.32 55.98 68.09 58.68 54.28 37.10
43.56 46.69 61.14 75.11 101.28 81.06 67.45 40.1
13.92 14.88 19.53 24.00 32.36 25.90 21.55 12.81
a Conversion obtained after n-butane isomerization running for 20 h.
According to the principle of electronegativity equalization proposed by Sanderson [30], the electronegativity Sint of the above complex structure and the partial charge δ Zr on Zr4+ can be written as y
x SZr SS SO ]1/2+x+y Sint = [SAl
δZr =
Sint − SZr 1/2
(a) (b)
2.08SZr
where SAl , SZr , SS and SO are the electronegativities of Al, Zr, S and O, respectively, and x, y are the number of Al and O, respectively, in the neighborhood of Zr4+ . Since the electronegativity of Al3+ is larger than that of Zr4+ , the electronegativity of surface complex, Sint , is increased and δ Zr becomes more positive when Al metal cations are introduced into the complex and Zr-O-Al bond is formed, which may help to stabilize the surface sulfur complex and to increase the number of strong acid sites. 3.2. Isomerization reaction In Table 2, the n-butane isomerization activities of the ZS2 , ZS and ZAS2 catalysts running on stream for about 2 days at 35◦ C are presented. Since the isomerization of n-butane on the catalysts at 35◦ C follows the rate law of a first-order reversible reaction [11], the activities are given in terms of the forward and backward rate constants, k1 and k−1 , which are associated with its superacidity, comprising the effect of strength and density of acid. If one compares the activity of ZS catalyst, ZS2 catalyst is more active for n-butane isomerization, implying that the latter has more superacidic sites than the former does. The Al-promoted
catalysts with 0.5–10.0 mol% Al2 O3 are more active than the unpromoted ZS2 catalyst. The isomerization activity of the Al-promoted catalysts increases with the Al2 O3 content up to 3.0–6.0 mol% and then decreases as the Al2 O3 content is further increased. Since the isomerization reaction of n-butane at low temperature can be used as a test reaction for superacidity, in the case of ZAS2 catalysts, the promotion effect in isomerization activity is probably caused by an increase in the number of surface acid sites rather than an increase in acid strength; this is coincident with the IR result. Clearly, the superacidity of the catalysts is out of proportion to the SO3 content of catalysts, which implies that not all the surface sulfate species on the catalysts are superacidic sites. Meanwhile, the conversion of n-butane isomerzation on various catalysts is parallel to its forward and backward rate constants k1 and k−1 , as shown in Table 2. The major reaction product of n-butane isomerization reaction at 250◦ C is isobutane, and the main byproducts are propane and isopentane. The selectivity to isobutane for all the catalysts is above 90%. The variation of the conversion of n-butane at 250◦ C with time on stream for the ZS2 , ZAS2 and ZS catalysts is given in Table 3. Although the initial conversion of ZS2 is lower than that of ZS, it decreases more slowly with time on stream than that of the latter. The steady state conversion of ZS2 after running for 6 h is reduced by about 20%, but SZ catalyst is reduced by about 35%. Under the same reaction conditions, the initial conversions of Al-promoted catalysts for n-butane isomerization increase with Al2 O3 content up to 6.0 mol% and then decrease as Al2 O3 content is further increased. All the initial conversions of Al-promoted catalysts are higher than that of ZS2 catalyst and the initial catalytic activities of Al-promoted samples with an Al2 O3 content of 3.0–6.0 mol% for n-butane isomerization are even two times that of ZS2 . A similar trend is also observed for the steady state conversions of the ZAS2 catalysts after running on stream for 6 h. The effect of varying Al2 O3 content on the steady state activity of the ZAS2 catalysts after running on stream for 6 h is shown in Fig. 2. It is shown that the steady state catalytic activities of Al-promoted samples with an Al2 O3 content of 1.5–6.0 mol% for n-butane isomerization are all above 30% and a maximum steady state activity is observed at a loading of 3 mol% Al2 O3 (ZAS2 -3). When compared with ZS2 , the conversion
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Table 3 n-Butane isomerization activities of various samples at 250◦ C Sample
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
Conversion (%) 2 min
10 min
30 min
60 min
120 min
180 min
240 min
300 min
360 min
25.83 44.61 48.41 51.83 53.35 50.03 43.81 27.70
24.00 35.76 39.83 43.67 42.83 42.97 40.49 25.21
23.90 28.33 35.25 39.47 37.83 39.78 37.24 24.63
22.64 27.49 34.60 38.17 35.08 35.56 33.77 23.50
21.10 23.89 32.73 37.75 34.98 32.99 26.98 21.60
20.91 23.51 32.60 37.68 34.81 32.89 26.62 20.36
20.82 23.49 32.51 37.67 34.06 32.70 26.52 19.34
20.76 23.29 32.41 37.39 33.93 32.67 26.48 18.12
20.43 23.11 32.22 37.33 33.55 32.19 26.46 17.54
Fig. 2. Steady state isomerization activity as a function of Al2 O3 content.
of ZAS2 -3 catalyst becomes 1.8 times more active at steady state. From a practical point of view, the latter catalyst is indeed very promising. Compared with the highest isomerization activity at 35◦ C without hydrogen appearing at the incorporation of 4.5 mol% Al2 O3 (ZAS2 -4), it is found that the highest steady activity for n-butane isomerization at 250◦ C in the presence of hydrogen appeared at a loading of 3 mol% Al2 O3 (ZAS2 -3). At low temperature and in the absence of hydrogen, the n-butane isomerization reaction on ZS2 and ZAS2 catalysts may occur through a complex bimolecular process involving C8 intermediate [31,32], whereas at high temperature and in the presence of hydrogen, a monomolecular mechanism may predominate [33]. In order to investigate the life time of ZAS2 -3 catalyst for longer terms, the reaction has been run at 250◦ C continuously for 200 h. As illustrated in Fig. 3, the initial conversion is 53% and it drops to 37% after 2 h. From then on, it remains steadily at 37% without any further observable trend of deactivation, which means that the isomerization of n-butane has proceeded on ZAS2 -3 at a level of 72% of its equilib-
Fig. 3. Long-term test of ZAS2 -3 sample. Table 4 Coke deposit on various samples Sample
Reaction time (h)
Coke (wt.%)
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
6 6 6 6 200 6 6 6 6
1.14 1.08 1.04 1.00 1.02 1.02 1.04 1.09 1.25
rium conversion in a stable manner. Compared to other Al-promoted persulfated zirconia and sulfated zirconia catalysts, the advantage of ZAS2 -3 is remarkable. In view of its high isomerization activity and stability, especially without containing any precious metals, ZAS2 -3 can be regarded as an excellent candidate for a commercial-scale n-butane isomerization catalyst. The coke deposited on the catalysts after the reaction was analyzed. The results are given in Table 4. The amount of coke deposited on ZS2 catalysts after
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Y. Xia et al. / Applied Catalysis A: General 185 (1999) 293–300
Fig. 4. Deactivation and regeneration of (䊐) ZS2 and (䊉) ZAS2 -3. Regeneration conditions: activated in air for 2 h.
Fig. 5. Differential heat of NH3 adsorption versus adsorbate coverage at 150◦ C for (䊐) ZS2 and (䊏) ZAS2 -3 catalysts.
the latter being on stream for 6 h is lower than that on ZS, but it is slightly higher than that on all the Al-promoted ZAS2 catalysts. Meanwhile, the amount of coke deposited on ZAS2 -3 catalyst remained almost unchanged between 6 and 200 h, showing that the initial drop in activity during 0−2 h was probably caused by catalyst coking and that after that coking and deactivation slowed down. The high steady state activity of ZAS2 -3 catalyst is probably associated with the higher amount of active acid sites left on the catalyst surface after the initial period. In order to confirm the reason for catalyst deactivation, the deactivation and regeneration of two typical catalysts, ZS2 and ZAS2 -3, are illustrated in Fig. 4. Similar to that of ZS2 sample, ZAS2 -3 catalyst can recover its activity completely after being burned in air for 2 h. This result further demonstrated that the deactivation of the catalyst in the initial period of the reaction is mainly caused by the coking deposited on the catalyst surface. 3.3. Microcalorimetric measurement Prior to microcalorimetric measurements, the catalysts were evacuated at 250◦ C. The microcalorimetric results of NH3 adsorption at 150◦ C on ZS2 and ZAS2 -3 catalysts are shown in Fig. 5. The differential heat of adsorption decreases with increasing NH3 coverage, indicating a distribution of acid site strengths on the catalyst surface [34]. The initial differential heats of NH3 adsorption on the strong acid sites of ZS2 and ZAS2 -3 are both about 170 kJ mol−1 . As in the case of NH3 coverage above 25 mol g−1 , under the same coverage, the differential heat of adsorption of ZAS2 -3 is always higher than that of ZS2 sample, implying that a ZAS2 -3 sample contains more acid sites than ZS2 .
Fig. 6. Histograms of acid strength distributions for (a) ZS2 and (b) ZAS2 -3 catalysts.
Histograms and data of the distribution of acid site strengths are shown in Fig. 6 and Table 5. The strengths of the acid sites on ZS2 are more evenly distributed, whereas ZAS2 -3 possesses a greater number of acid sites with differential heats between 125 and 140 kJ mol−1 . Though the number of weak acid sites with differential heats in the range of 80 to 125 kJ mol−1 and strong acid sites with differential heats from 140 to 170 kJ mol−1 for ZAS2 -3 are slightly greater than those for ZS2 , the number of intermediate acid sites with differential heats in the range of 125 to 140 kJ mol−1 for ZAS2 -3 is 4.8 times
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299
Table 5 Microcalorimetric results of the distribution of acid site strengths on samples Sample
ZS2 ZAS2 -3
Acid sites (mol g−1 ) Total
80–125 kJ mol−1
125–140 kJ mol−1
140–170 kJ mol−1
93.0 174.2
61.8 81.3
15.9 75.9
15.3 17.0
greater than that for ZS2 . The total concentration of acid sites with differential adsorption heat of NH3 above 80 kJ mol−1 for ZAS2 -3 is 174.2 mol g−1 , which is nearly two times more numerous than that for ZS2 . Microcalorimetric studies on sulfated zirconia catalysts [35] have already shown that the acid sites with intermediate acid strengths, namely with differential heats of NH3 adsorption between 125 and 140 kJ mol−1 , are active for n-butane isomerization. Hence, the abundance of these acid sites on ZAS2 -3 can explain its extraordinarily high catalytic activity and stability for n-butane isomerization reaction. On the other hand, the strong acid sites on the catalysts with differential heats above 140 kJ mol−1 must also be involved in n-butane isomerization, but they are probably deactivated rapidly in the initial period of the isomerization reaction.
4. Conclusion The results of this study indicate that the promoting effect of aluminum on persulfate modified zirconia is similar to that of aluminum on sulfate modified zirconia. The addition of Al to persulfated zirconia helps to stabilize the surface sulfate complex on the oxide and increases the number of effective acid sites on the catalyst for n-butane isomerization. The remarkable activity and stability of the Al-promoted catalyst under H2 at higher temperatures are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths.
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A new catalyst for n-butane isomerization: persulfate-modified Al2 O3 –ZrO2 Yongde Xia, Weiming Hua, Zi Gao ∗ Department of Chemistry, Fudan University, Shanghai 200433, PR China Received 12 January 1999; received in revised form 22 April 1999; accepted 11 May 1999
Abstract The n-butane isomerization reaction on Al-promoted zirconia catalysts modified by persulfate was studied both at low temperature in the absence of hydrogen and at high temperature in the presence of hydrogen. The addition of Al shows a significant enhancement in the activity and stability of the samples for n-butane isomerization at 250◦ C. After running on stream for 200 h, the n-butane conversion on the catalyst containing 3 mol% Al2 O3 remains steady at 72% of its equilibrium conversion and no observable trend of further deactivation has been observed. The different behavior of the with or without Al-promoted catalysts at low temperature and high temperature is probably associated with a change of reaction mechanism from bimolecular to monomolecular. Microcalorimetric measurements of ammonia adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths effective for the n-butane isomerization reaction. ©1999 Elsevier Science B.V. All rights reserved. Keywords: n-butane isomerization; ZrO2 modified by persulfate; Al-promoted persulfated zirconia; Acid strength distribution
1. Introduction The isomerization reaction of straight-chain paraffins to more highly branched isomers is an important process for the production of clear-burning fuels in the petrochemical refining industry. For example, n-butane, which is undesirable for gasoline, can be converted to isobutane, a valuable precursor to MTBE and other fuel additives. The technology used currently for n-butane isomerization is based on the Pt/Al2 O3 –Cl catalyst which operates at elevated temperatures where the equilibrium product distribution ∗ Corresponding author. Tel.: +86-21-65642792; fax: +86-21-65641740; e-mail: [email protected]
of isobutane is not favorable [1], and the use of superacids as catalysts has been suggested [2]. Owing to more and more strict environmental concerns, researchers have shown more interest in solid superacid catalysts than in liquid superacids. A considerable number of reports dealing with sulfate-promoted zirconia and other metal oxides as catalysts for n-butane isomerization have appeared recently [3–15]. Often being regarded as a typical solid superacid, sulfated zirconia catalyst gives a high activity for n-butane isomerization at low temperature, but a rapid deactivation of the catalyst has often been observed at high temperature. In order to improve the life time of SO2− 4 /ZrO2 catalyst, n-butane isomeriazation reactions in the presence of hydrogen and addition
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 7 6 - 3
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of a small amount of Pt or Ni have been suggested [8,16]. Recently, a new solid superacid, sulfated trinary oxide containing Fe, Mn and Zr has been developed by Hsu et al. [17], and has shown higher superacidity than sulfated zirconia. This new catalyst has been reported to be able to isomerize n-butane at ambient temperature with a rate three orders of magnitude greater than SO2− 4 /ZrO2 . Some researchers [18,19] have confirmed this result but have pointed out that the Fe or Mn-promoted sulfated zirconia catalyst also deactivates quickly at 250◦ C in the presence of hydrogen or at 60◦ C in the presence of nitrogen. Miao et al. [20,21] have reported that the doping of sulfated zirconia with various transition metals, such as Cr or V, could enhance its catalytic activity for n-butane isomerization reaction at low temperature in the absence of hydrogen, which were two to three times more active than sulfated Fe−Mn−Zr for n-butane isomerization, but the doped catalysts are not stable at high temperature in the presence of hydrogen yet. However, in our previous work [22], it has been found that Al promotes the catalytic activity and stability of sulfated zirconia for n-butane isomerization more effectively than doping transition metal catalysts at high temperature in the presence of hydrogen. More recently, we found that zirconia modified by persulfate has shown higher activity for n-butane isomerization than that of sulfated zirconia. Naturally, we want to know whether the promoting effect of Al on sulfated zirconia has a similar effect on persulfated zirconia. In this paper, the n-butane isomerization reaction on a series of persulfated Al-promoted zirconia catalysts was studied both at low and high temperatures. A quantitative measurement of the number and acid strength distribution of the surface acid sites of the catalysts was accomplished by means of a microcalorimetric method. The reasons for the improvement in activity and stability of the catalysts in n-butane isomerization by the addition of Al are discussed based on the experimental results.
2. Experimental The technique used to produce persulfated zirconia in the present work was mainly based on the pro-
cedures described earlier [4,8,11]. Aqueous ammonia was added dropwise to a solution of ZrOCl2 ·8H2 O till pH = 9–10. The hydroxide was washed, filtered and dried at 110◦ C and then immersed in a 0.5 mol l−1 ammonium persulfate ((NH4 )2 S2 O8 ) or 0.5 mol l−1 diluted sulfuric acid solution for 30 min. The persulfated zirconia or sulfated zirconia was then filtered, dried at 110◦ C overnight and calcined at 650◦ C in static air for 3 h. The two catalysts were labeled as ZS2 and ZS, respectively. Al-promoted catalysts were prepared in the same way from a mixed solution of ZrOCl2 ·8H2 O and Al(NO3 )3 ·9H2 O. The Al-promoted catalysts were labeled as ZAS2 . X-ray powder diffraction measurements were carried out on a Rigaku D/MAX-II A instrument with Cu K␣ radiation at 40 kV and 20 mA, scan speed 16◦ min−1 and scan range 5◦ −70◦ . BET surface areas of the samples were acquired on a Micromeritics ASAP 2000 equipment using N2 as the adsorbent. Infrared spectra of the samples were recorded on a Perkin−Elmer 983 G spectrometer. The samples were pressed into thin disks with a density of 3−5 mg cm−2 and placed in a quartz cell with CaF2 windows. Microcalorimetric studies of the adsorption of ammonia were carried out at 150◦ C using a Tian–Calvet type heat-flux calorimeter. The catalysts were evacuated at 250◦ C for 3 h before measurements. The coke deposited on the catalysts was detected on a Carlo−Erba 1106 elemental analysis instrument. Sulfur content in the catalysts was detected by a chemical method. Dehydrated Na2 CO3 and ZnO were used as fusing agents, and the sulfate was turned into BaSO4 and determined by gravimetric method. The isomerization of n-butane was carried out both at low and high temperatures. The isomerization reaction at 35◦ C was performed in a recirculating glass reactor under the closed system pressure of 7 × 104 Pa. The catalyst loading was 0.5 g. The amount of n-butane of 99.9% purity injected for each test was 5 ml of gas (20◦ C, 1 × 105 Pa). The isomerization reaction at 250◦ C was carried out in a flow-type fixed bed reactor under ambient pressure. The catalyst loading was 1.0 g, and a mixture of n-butane (purity above 99.0%) and H2 (1 : 10 molar ratio) was fed at a rate of WHSV 0.3 h−1 . The catalysts were preheated in situ in dry air at 450◦ C for 3 h. The reaction products were analyzed by an on-line gas chromatograph equipped with FID.
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Table 1 Surface area and sulfur content of various samples
Fig. 1. XRD patterns of various catalysts calcined at 650◦ C. (䊏) Tetragonal phase; (䊉) monoclinic phase. (a) ZS; (b) ZS2 ; (c) ZS2 -1; (d) ZS2 -2; (e) ZS2 -3; (f) ZS2 -4; (g) ZS2 -5; (h) ZS2 -6.
3. Results and discussion 3.1. Catalyst characterization XRD patterns of the catalysts after calcining at 650◦ C in air for 3 h were recorded; the results are shown in Fig. 1. The tetragonal phase accompanied with a small portion of the monoclinic phase is observed in ZS2 and ZAS2 -1 samples, which are similar to the XRD result of ZS sample. As regards ZAS2 catalysts with 1.5−10.0 mol% Al2 O3 , the transformation from the metastable tetragonal phase to the thermodynamically favored monoclinic phase is retarded. Only the tetragonal phase appears in all these Al-promoted catalysts except ZAS2 -1. The intensity of the diffractive peak of the tetragonal phase increases slightly with Al2 O3 content up to 3.0–4.5 mol% and then decreases slightly as the Al2 O3 content is further increased. The characteristic peaks of Al2 O3 are not present in any of the Al-promoted samples, indicating that Al2 O3 is rather homogeneously mixed with zirconia. Table 1 presents the data of sulfur content and surface area of the catalysts with different Al2 O3 contents. BET surface areas of the ZAS2 series catalysts
Sample
Al2 O3 content (mol%)
Surface area (m2 g−1 )
SO3 content (wt.%)
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
0 0.5 1.5 3.0 4.5 6.0 10.0 0
58.2 73.8 76.3 80.8 99.4 96.1 85.0 113.0
1.99 2.44 2.74 2.85 3.10 3.24 5.39 3.30
are slightly higher than that of ZS2 , but are a bit lower than that of ZS. The sulfur content of ZS2 is only 60% as high as that of ZS. The lower surface area and sulfur content of ZS2 are probably caused by immersing the amorphous ZrO2 with a solution of aqueous ammonium salt. The sulfate concentration on the catalysts increases significantly with Al2 O3 content. The amount of sulfate on ZS2 corresponds to approximately 60% of a monolayer coverage, assuming that each sulfate group covers 0.25 nm2 [23]. On ZAS2 -6 catalyst, the surface coverage of sulfate groups is close to unity, showing that the incorporation of Al into ZrO2 helps to stabilize the surface sulfate complexes remarkably. After evacuation at 350◦ C for 3 h, infrared spectra of the SO2− 4 promoted superacids display a strong adsorption band at 1380–1390 cm−1 , which represents the characteristic stretching frequency of surface sulfate species with covalent S=O bonds [24]. The characteristic stretching frequency of ZS2 and ZAS2 catalysts appeared at 1398 cm−1 , which is different from that of ZS at 1388 cm−1 ; we postulate that S2 O2− 7 is responsible for such a stretching frequency. These results are coincident with those of M. Bensitel [25] and C. Morterra [26,27]. When one introduces metal cations, such as Al3+ , into the zirconia modified by persulfate sample, due to the interaction of Zr with Al and the formation of Zr–O–Al bond in the binary oxide [28,29], the following complex structures may be formed in some local areas on the surface of the samples:
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Table 2 n-Butane isomerization activities of various samples at 35◦ C Sample
Conversiona (%)
k1 × 103 (h−1 )
k−1 × 103 (h−1 )
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
45.40 45.97 46.32 55.98 68.09 58.68 54.28 37.10
43.56 46.69 61.14 75.11 101.28 81.06 67.45 40.1
13.92 14.88 19.53 24.00 32.36 25.90 21.55 12.81
a Conversion obtained after n-butane isomerization running for 20 h.
According to the principle of electronegativity equalization proposed by Sanderson [30], the electronegativity Sint of the above complex structure and the partial charge δ Zr on Zr4+ can be written as y
x SZr SS SO ]1/2+x+y Sint = [SAl
δZr =
Sint − SZr 1/2
(a) (b)
2.08SZr
where SAl , SZr , SS and SO are the electronegativities of Al, Zr, S and O, respectively, and x, y are the number of Al and O, respectively, in the neighborhood of Zr4+ . Since the electronegativity of Al3+ is larger than that of Zr4+ , the electronegativity of surface complex, Sint , is increased and δ Zr becomes more positive when Al metal cations are introduced into the complex and Zr-O-Al bond is formed, which may help to stabilize the surface sulfur complex and to increase the number of strong acid sites. 3.2. Isomerization reaction In Table 2, the n-butane isomerization activities of the ZS2 , ZS and ZAS2 catalysts running on stream for about 2 days at 35◦ C are presented. Since the isomerization of n-butane on the catalysts at 35◦ C follows the rate law of a first-order reversible reaction [11], the activities are given in terms of the forward and backward rate constants, k1 and k−1 , which are associated with its superacidity, comprising the effect of strength and density of acid. If one compares the activity of ZS catalyst, ZS2 catalyst is more active for n-butane isomerization, implying that the latter has more superacidic sites than the former does. The Al-promoted
catalysts with 0.5–10.0 mol% Al2 O3 are more active than the unpromoted ZS2 catalyst. The isomerization activity of the Al-promoted catalysts increases with the Al2 O3 content up to 3.0–6.0 mol% and then decreases as the Al2 O3 content is further increased. Since the isomerization reaction of n-butane at low temperature can be used as a test reaction for superacidity, in the case of ZAS2 catalysts, the promotion effect in isomerization activity is probably caused by an increase in the number of surface acid sites rather than an increase in acid strength; this is coincident with the IR result. Clearly, the superacidity of the catalysts is out of proportion to the SO3 content of catalysts, which implies that not all the surface sulfate species on the catalysts are superacidic sites. Meanwhile, the conversion of n-butane isomerzation on various catalysts is parallel to its forward and backward rate constants k1 and k−1 , as shown in Table 2. The major reaction product of n-butane isomerization reaction at 250◦ C is isobutane, and the main byproducts are propane and isopentane. The selectivity to isobutane for all the catalysts is above 90%. The variation of the conversion of n-butane at 250◦ C with time on stream for the ZS2 , ZAS2 and ZS catalysts is given in Table 3. Although the initial conversion of ZS2 is lower than that of ZS, it decreases more slowly with time on stream than that of the latter. The steady state conversion of ZS2 after running for 6 h is reduced by about 20%, but SZ catalyst is reduced by about 35%. Under the same reaction conditions, the initial conversions of Al-promoted catalysts for n-butane isomerization increase with Al2 O3 content up to 6.0 mol% and then decrease as Al2 O3 content is further increased. All the initial conversions of Al-promoted catalysts are higher than that of ZS2 catalyst and the initial catalytic activities of Al-promoted samples with an Al2 O3 content of 3.0–6.0 mol% for n-butane isomerization are even two times that of ZS2 . A similar trend is also observed for the steady state conversions of the ZAS2 catalysts after running on stream for 6 h. The effect of varying Al2 O3 content on the steady state activity of the ZAS2 catalysts after running on stream for 6 h is shown in Fig. 2. It is shown that the steady state catalytic activities of Al-promoted samples with an Al2 O3 content of 1.5–6.0 mol% for n-butane isomerization are all above 30% and a maximum steady state activity is observed at a loading of 3 mol% Al2 O3 (ZAS2 -3). When compared with ZS2 , the conversion
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Table 3 n-Butane isomerization activities of various samples at 250◦ C Sample
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
Conversion (%) 2 min
10 min
30 min
60 min
120 min
180 min
240 min
300 min
360 min
25.83 44.61 48.41 51.83 53.35 50.03 43.81 27.70
24.00 35.76 39.83 43.67 42.83 42.97 40.49 25.21
23.90 28.33 35.25 39.47 37.83 39.78 37.24 24.63
22.64 27.49 34.60 38.17 35.08 35.56 33.77 23.50
21.10 23.89 32.73 37.75 34.98 32.99 26.98 21.60
20.91 23.51 32.60 37.68 34.81 32.89 26.62 20.36
20.82 23.49 32.51 37.67 34.06 32.70 26.52 19.34
20.76 23.29 32.41 37.39 33.93 32.67 26.48 18.12
20.43 23.11 32.22 37.33 33.55 32.19 26.46 17.54
Fig. 2. Steady state isomerization activity as a function of Al2 O3 content.
of ZAS2 -3 catalyst becomes 1.8 times more active at steady state. From a practical point of view, the latter catalyst is indeed very promising. Compared with the highest isomerization activity at 35◦ C without hydrogen appearing at the incorporation of 4.5 mol% Al2 O3 (ZAS2 -4), it is found that the highest steady activity for n-butane isomerization at 250◦ C in the presence of hydrogen appeared at a loading of 3 mol% Al2 O3 (ZAS2 -3). At low temperature and in the absence of hydrogen, the n-butane isomerization reaction on ZS2 and ZAS2 catalysts may occur through a complex bimolecular process involving C8 intermediate [31,32], whereas at high temperature and in the presence of hydrogen, a monomolecular mechanism may predominate [33]. In order to investigate the life time of ZAS2 -3 catalyst for longer terms, the reaction has been run at 250◦ C continuously for 200 h. As illustrated in Fig. 3, the initial conversion is 53% and it drops to 37% after 2 h. From then on, it remains steadily at 37% without any further observable trend of deactivation, which means that the isomerization of n-butane has proceeded on ZAS2 -3 at a level of 72% of its equilib-
Fig. 3. Long-term test of ZAS2 -3 sample. Table 4 Coke deposit on various samples Sample
Reaction time (h)
Coke (wt.%)
ZS2 ZAS2 -1 ZAS2 -2 ZAS2 -3 ZAS2 -3 ZAS2 -4 ZAS2 -5 ZAS2 -6 ZS
6 6 6 6 200 6 6 6 6
1.14 1.08 1.04 1.00 1.02 1.02 1.04 1.09 1.25
rium conversion in a stable manner. Compared to other Al-promoted persulfated zirconia and sulfated zirconia catalysts, the advantage of ZAS2 -3 is remarkable. In view of its high isomerization activity and stability, especially without containing any precious metals, ZAS2 -3 can be regarded as an excellent candidate for a commercial-scale n-butane isomerization catalyst. The coke deposited on the catalysts after the reaction was analyzed. The results are given in Table 4. The amount of coke deposited on ZS2 catalysts after
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Fig. 4. Deactivation and regeneration of (䊐) ZS2 and (䊉) ZAS2 -3. Regeneration conditions: activated in air for 2 h.
Fig. 5. Differential heat of NH3 adsorption versus adsorbate coverage at 150◦ C for (䊐) ZS2 and (䊏) ZAS2 -3 catalysts.
the latter being on stream for 6 h is lower than that on ZS, but it is slightly higher than that on all the Al-promoted ZAS2 catalysts. Meanwhile, the amount of coke deposited on ZAS2 -3 catalyst remained almost unchanged between 6 and 200 h, showing that the initial drop in activity during 0−2 h was probably caused by catalyst coking and that after that coking and deactivation slowed down. The high steady state activity of ZAS2 -3 catalyst is probably associated with the higher amount of active acid sites left on the catalyst surface after the initial period. In order to confirm the reason for catalyst deactivation, the deactivation and regeneration of two typical catalysts, ZS2 and ZAS2 -3, are illustrated in Fig. 4. Similar to that of ZS2 sample, ZAS2 -3 catalyst can recover its activity completely after being burned in air for 2 h. This result further demonstrated that the deactivation of the catalyst in the initial period of the reaction is mainly caused by the coking deposited on the catalyst surface. 3.3. Microcalorimetric measurement Prior to microcalorimetric measurements, the catalysts were evacuated at 250◦ C. The microcalorimetric results of NH3 adsorption at 150◦ C on ZS2 and ZAS2 -3 catalysts are shown in Fig. 5. The differential heat of adsorption decreases with increasing NH3 coverage, indicating a distribution of acid site strengths on the catalyst surface [34]. The initial differential heats of NH3 adsorption on the strong acid sites of ZS2 and ZAS2 -3 are both about 170 kJ mol−1 . As in the case of NH3 coverage above 25 mol g−1 , under the same coverage, the differential heat of adsorption of ZAS2 -3 is always higher than that of ZS2 sample, implying that a ZAS2 -3 sample contains more acid sites than ZS2 .
Fig. 6. Histograms of acid strength distributions for (a) ZS2 and (b) ZAS2 -3 catalysts.
Histograms and data of the distribution of acid site strengths are shown in Fig. 6 and Table 5. The strengths of the acid sites on ZS2 are more evenly distributed, whereas ZAS2 -3 possesses a greater number of acid sites with differential heats between 125 and 140 kJ mol−1 . Though the number of weak acid sites with differential heats in the range of 80 to 125 kJ mol−1 and strong acid sites with differential heats from 140 to 170 kJ mol−1 for ZAS2 -3 are slightly greater than those for ZS2 , the number of intermediate acid sites with differential heats in the range of 125 to 140 kJ mol−1 for ZAS2 -3 is 4.8 times
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299
Table 5 Microcalorimetric results of the distribution of acid site strengths on samples Sample
ZS2 ZAS2 -3
Acid sites (mol g−1 ) Total
80–125 kJ mol−1
125–140 kJ mol−1
140–170 kJ mol−1
93.0 174.2
61.8 81.3
15.9 75.9
15.3 17.0
greater than that for ZS2 . The total concentration of acid sites with differential adsorption heat of NH3 above 80 kJ mol−1 for ZAS2 -3 is 174.2 mol g−1 , which is nearly two times more numerous than that for ZS2 . Microcalorimetric studies on sulfated zirconia catalysts [35] have already shown that the acid sites with intermediate acid strengths, namely with differential heats of NH3 adsorption between 125 and 140 kJ mol−1 , are active for n-butane isomerization. Hence, the abundance of these acid sites on ZAS2 -3 can explain its extraordinarily high catalytic activity and stability for n-butane isomerization reaction. On the other hand, the strong acid sites on the catalysts with differential heats above 140 kJ mol−1 must also be involved in n-butane isomerization, but they are probably deactivated rapidly in the initial period of the isomerization reaction.
4. Conclusion The results of this study indicate that the promoting effect of aluminum on persulfate modified zirconia is similar to that of aluminum on sulfate modified zirconia. The addition of Al to persulfated zirconia helps to stabilize the surface sulfate complex on the oxide and increases the number of effective acid sites on the catalyst for n-butane isomerization. The remarkable activity and stability of the Al-promoted catalyst under H2 at higher temperatures are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths.
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