Effects of Calcination Temperature on the Acidity and Catalytic

Journal of Natural Gas Chemistry 14(2005)213–220

Effects of Calcination Temperature on the Acidity and Catalytic Performances of HZSM-5 Zeolite Catalysts for the Catalytic Cracking of n-Butane Jiangyin Lu,

Zhen Zhao∗,

Chunming Xu,

Aijun Duan,

Pu Zhang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China [Manuscript received September 30, 2005; revised November 11, 2005]

Abstract: The acidic modulations of a series of HZSM-5 catalysts were successfully made by calcination at different treatment temperatures, i.e. 500, 600, 650, 700 and 800 ℃, respectively. The results indicated that the total acid amounts, their density and the amount of B-type acid of HZSM-5 catalysts rapidly decreased, while the amounts of L-type acid had almost no change and thus the ratio of L/B was obviously enhanced with the increase of calcination temperature (excluding 800 ℃). The catalytic performances of modified HZSM-5 catalysts for the cracking of n-butane were also investigated. The main properties of these catalysts were characterized by means of XRD, N2 adsorption at low temperature, NH3 -TPD, FTIR of pyridine adsorption and BET surface area measurements. The results showed that HZSM-5 zeolite pretreated at 800 ℃ had very low catalytic activity for n-butane cracking. In the calcination temperature range of 500–700 ℃, the total selectivity to olefins, propylene and butene were increased with the increase of calcination temperature, while, the selectivity for arene decreased with the calcination temperature. The HZSM-5 zeolite calcined at 700 ℃ produced light olefins with high yield, at the reaction temperature of 650 ℃ the yields of total olefins and ethylene were 52.8% and 29.4%, respectively. Besides, the more important role is that high calcination temperature treatment improved the duration stability of HZSM-5 zeolites. The effect of calcination temperature on the physico-chemical properties and catalytic performance of HZSM-5 for cracking of n-butane was explored. It was found that the calcination temperature had large effects on the surface area, crystallinity and acid properties of HZSM-5 catalyst, which further affected the catalytic performance for n-butane cracking. Key words: HZSM-5 zeolite catalyst, acidic modification, calcination temperature, n-butane, catalytic cracking, olefin

1. Introduction C4 fraction will be another choice of the valuable petrochemical materials that can produce important chemicals like ethylene and propylene. C4 fraction is mainly produced from catalytic cracking and steam pyrolysis processes. The C4 fraction in high demand in the chemical industry was C4 olefins, while the C4 alkanes were primarily used as fuel. The current supply of ethylene and propylene, which are among the most important basic organic ∗

chemicals, can not satisfy the growing demand of high quality petrochemical materials. To develop new technology and methods for increasing the production of propylene including the catalytic cracking of C4 olefins [1,2] or C4 hydrocarbon feedstocks [3] are indispensable and important for petrochemical industry. In recent years, zeolites catalysts were paid more and more attention and widely applied in refinery, the petrochemical industry and environmental catalysis [4–8]. ZSM-5 zeolite has been extensively investi-

Corresponding author. Tel: (010)89731586; E-mail: [email protected].

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gated as the active catalyst for a variety of reactions owing to its activity and special pore structure [9–11]. Wakui et al. [12,13] investigated cracking of n-butane over rare earth-loaded HZSM-5 and they found that the formation of aromatic and heavier products was remarkably suppressed by the loading of rare earths. Besides, they reported a dehydrogenation cracking double-stage reaction of n-butane [14]. Ji et al. [3] used ZSM-23 to crack C4 alkanes, and obtained high yields of olefins. The acid property of the catalysts plays an important role in the cracking of hydrocarbons. Therefore, the study on the modification of the acidity of ZSM-5 catalysts is very significant for understanding the relation between the acid property and their catalytic cracking performances. In this work, the acidic modulations, including acidic amount, acid strength and acid types of a series of HZSM-5 catalysts were made by calcination at different treatment temperatures. A systematic investigation of their catalytic performances for cracking of n-butane was carried out and the nature of calcination temperature effects was discussed. 2. Experimental 2.1. Catalyst preparation NaZSM-5 zeolite (obtained from Qi Lu Petrochemical Corporation Catalysts Factory) with a Si:Al ratio of 32 was chosen as the catalyst precursor. Then, the NaZSM-5 zeolite was ion-exchanged twice with 1M NH4 NO3 solution at 90 ℃ for 1 h. After drying at 120 ℃ for 4 h, the zeolite was calcined at 500 ℃ in air for 4 h. After that, HZSM-5 zeolite samples were calcined at 500, 600, 650, 700 and 800 ℃, respectively, for 4 h and marked as 1#, 2#, 3#, 4#, and 5#, correspondingly. After being crushed and sieved, the 0.2–0.45 mm fraction was chosen for activity measurement. 2.2. Catalyst characterization Crystal structure and relative crystallinity of the zeolite were determined by X-ray diffraction (XRD). Using Cu Kα radiation operating at 40 kV and 30 mA (XRD-6000, Shimadzu, Japan), diffraction peaks were recorded between 5o and 50o , and scanning rate was 4o /min. The height of strongest peak at 2θ=23.02o was used to calculate the crystallization. The BET specific surface area of the catalyst was measured with linear parts of BET plot of N2 ad-

sorption isotherms, using a Micromeritics ASAP 2010 analyzer. Acidic amounts of the zeolite were measured by NH3 -TPD (Temperature-programmed desorption of ammonia) method. 0.2 g samples with 40–80 mesh were pretreated at 500 ℃ for 2 h, cooled to 120 ℃ and adsorbed NH3 for 30 min. Then temperature-programmed desorption started at a rate of 15 ℃/min from 120 ℃ to 800 ℃, and the signal was monitored with a TCD. In order to obtain the total acid amount, the desorbed ammonia was absorbed by HCl solution (0.01 mol/L) and then titrated by NaOH solution (0.01 mol/L), finally the acidic amount and its density on the zeolite were calculated. According to the temperature of the desorption peak the relative intensity of acid sites of the samples was analyzed, and the acid amount of the acid sites with different intensities by peak areas were calculated. Acid types and their relative strengths of HZSM5 catalysts calcined at different temperatures were measured by FT-IR of pyridine adsorption. First the samples need to be outgassed at elevated temperature under high vacuum conditions (1×10−3 Pa). And then the background spectra are recorded. After equilibration of pyridine, the sample is treated at elevated temperatures to remove physisorbed pyridine, and finally the spectra in the range of 1700– 1400 cm−1 of adsorbed pyridine are recorded. Optionally, in order to obtain information of B and L acid with different strengths, the pyridine-adsorbed samples were degassed at 200 and 350 ℃, respectively. The L and B acid amount desorbed at 200 and 350 ℃ are attributed to all acid with different strengths, strong and medium strong acids. Peaks at 1540 and 1450 cm−1 characterize B acid and L acid, respectively. 2.3. Activity measurement The pyrolysis and catalytic cracking reactions over HZSM-5 catalysts were carried out in a fixed bed reactor with a diameter of 6 mm (i.d.) by passing a gaseous n-butane (2 ml/min, 99.9%) in a N2 flow at a total flow rate of 40 ml/min over 200 mg catalyst (total pressure: 0.1 MPa). Products were analyzed by gas chromatography (Beijing Analytical Apparatus Factory, Model. SP-3420) with a 50 m PONA capillary column and a flame ionization detector (FID). The test conditions are: column temperature of 34 ℃, detector oven temperature of 180 ℃, the flow rate of carrier gas at 150 ml/min.

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3. Results and discussion 3.1. Ef fect of calcination temperature on the catalytic performance of HZSM-5 catalysts for cracking of n-butane The catalytic performances of HZSM-5 samples calcined at different temperatures for the catalytic cracking of n-butane are shown in Table 1. From Table 1, it can be seen that the conversion of n-butane slowly decreases with the increase of calcina-

tion temperatures when the calcination temperature was below 700 ℃. But HZSM-5 zeolite sample had little activity as the calcination temperature reached 800 ℃. At the same reaction temperature, as the temperature of thermal pretreatment of HZSM-5 zeolite samples became lower, the cracking activity of the zeolite samples became higher. Apart from the HZSM5 zeolite sample calcined at 800 ℃, all of the other HZSM-5 zeolite samples had good cracking activities at 650 ℃.

Table 1. The catalytic performances of HZSM-5 samples calcined at dif ferent temperatures for the cracking of n-butane Catalyst 1#

2#

3#

4#

5#

a b c

Temperature ( ℃) 600 625 650 600 625 650 600 625 650 600 625 650 600 625 650

Conversion (%) 89.4 96.8 99.4 86.6 95.0 98.9 80.0 92.6 98.4 63.8 82.3 93.5 8.1 13.9 21.9

Methane 10.8 11.5 12.1 10.6 11.5 11.9 10.2 11.1 11.6 10.3 10.2 10.7 9.2 9.1 10.0

Ethane 13.7 12.8 11.6 12.5 11.9 10.8 10.5 9.6 8.4 14.0 11.9 10.6 12.5 10.6 9.6

Propane 3.8 2.7 1.7 4.0 2.9 2.0 3.3 2.7 2.2 3.0 2.4 2.1 0.8 0.7 0.9

Selectivity (%) Ethene Propene Butenea 27.8 21.3 4.8 29.4 17.4 2.9 30.9 13.9 1.5 25.6 20.5 5.0 27.9 17.9 3.3 29.3 14.3 1.7 25.0 22.2 6.7 27.6 19.2 4.0 29.0 15.2 1.9 27.8 28.4 11.6 28.9 24.2 7.4 31.4 20.8 4.3 16.5 25.5 35.2 17.3 24.8 29.2 20.3 25.9 24.6

Arceneb 15.9 19.9 23.7 15.1 19.7 25.2 8.9 14.3 19.4 6.7 10.4 17.4 0.0 0.0 0.0

Total olefinsc 54.0 49.7 46.3 51.1 49.1 45.3 53.9 50.8 46.2 67.9 60.5 56.5 77.2 71.3 70.8

The butene was mainly 1-butene, 2-butene(including trans, 2-butene and cis, 2-butene). The acene included benzene and toluene. The total selectivity for olefins included ethylene, propylene and butene but not dibutene.

The calcination temperature could influence the product selectivity over the HZSM-5 zeolite samples for catalytic cracking of n-butane. Generally, the selectivity for alkanes could not be changed significantly by the calcination temperature and the reaction temperature, but the selectivity for propane of 5# catalyst was obviously lower than that of other catalysts. In the reaction temperature range of 600–650 ℃, the selectivity for methane increased with the rising of reaction temperature, while the selectivities for ethane and propane showed the opposite trends. This could be explained by that methane had higher thermal stability and high reaction temperature would suppress the hydrogen transfer reaction, i.e., high temperature is favorable for the further cracking of C2 and C3 hydrocarbons into methane. Apart from the 5#, the selectivity for aromatic hydrocarbon increased remarkably with the rising of reaction temperature, and

calcination temperature. The general trends of alkene selectivities were similar for all samples except for 5# sample, the total selectivities for alkene, propylene and butene decreased with the increase of reaction temperature. But they increased with the rising of calcination temperature. Similarly, the selectivity for ethylene dramatically increased with the rising of reaction temperature, while it almost kept constant at about 30% selectivity to ethylene when the calcination temperature was below 700 ℃ and it seriously decreased with further rising of calcination temperature to 800 ℃. Consequently, the total selectivity to olefins in n-butane catalytic cracking reaction reached a maximum for the HZSM-5 zeolite sample calcined at 700 ℃. When n-butane was cracked over HZSM-5 zeolite samples, the main products of butene were mostly 1-butene, and secondly 2-butene (including trans-2butene and cis-2-butene).

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In order to study the nature of catalytic cracking of n-butane over HZSM-5 zeolite samples and to exclude the effect of pyrolysis of n-butane, a comparative study of n-butane pyrolysis was also made. The product yield of n-butane pyrolysis as a function of reaction temperature is shown in Figure 1.

n-butane could be excluded. Therefore, below 600 ℃, the dominant reaction occurred at HZSM-5 zeolite sample should be catalytic cracking reaction. But when the reaction temperature is higher than 600 ℃, catalytic cracking reaction often coexists with pyrolysis reaction. The higher the reaction temperature, the more obvious is the pyrolysis reaction. It can also be observed that the selectivities for methane and ethylene increased with the rising of reaction temperature, but the selectivities for propane, propylene and butene decreased. This might be explained by that the pyrolysis reaction played some role in the whole reaction process. The catalytic cracking activity would increase with the rising of the reaction temperature. But, if the temperature was too high, the proton acid (B acid) on the catalyst surface would lose by dehydration and some of them changed into L acid site, so the total number of the acid sites decreased and resulted in the loss of catalytic activity. 3.2. Ef fects of calcination temperature on the catalytic stability of HZSM-5 for the cracking of n-butane.

Figure 1. Product yield of n-butane pyrolysis as a function of reaction temperature

From Table 1 and Figure 1, it is seen that since no cracking products of n-butane was detected without catalyst below 600 ℃, the pyrolysis reaction of

The HZSM-5 samples calcined at 500 and 700 ℃ were used to test their stability on n-butane catalytic cracking activity under atmospheric pressure and reaction temperature at 650 ℃. The whole experiment lasted 30 h, and the results are shown in Figure 2.

Figure 2. Duration test of HZSM-5 calcined at dif ferent temperatures for n-C4 cracking (a) Conversion of n-C4 , (b) Yield of total olefins, (c) Yield of ethylene, (d) Yield of propylene (1) Calcined at 500 ℃, (2) Calcined at 700 ℃ (Reaction temperature at 650 ℃)


It can be found from Figure 2 that in the initial period of the experiment, high n-butane conversion could be obtained over HZSM-5 sample calcined at 500 ℃, which may be due to the presence of large numbers of acid sites on the zeolite surface under the lower calcination temperature. With the prolonging of reaction time, the conversion of n-butane decreased a lot, i.e., the catalyst deactivated rapidly. After 10 h reaction, the conversion of n-butane over HZSM-5 zeolite sample calcined at 500 ℃ was almost the same as that of the sample calcined at 700 ℃. Then it deactivated more quickly after 10 h. The conversion of n-butane over HZSM-5 zeolite calcined at 700 ℃ decreased relatively slowly after 10 h, and the conversion was higher than the former, and still kept 63.3% at 29 h and then decreased quickly. The n-butane conversions of two samples were almost the same at 30 h and they kept 53.9% and 54.7%, respectively. The yield of total olefins for the cracking of n-butane over HZSM-5 sample zeolite calcined at 500 ℃ was obviously lower than that of the sample calcined at 700 ℃. The initial yield of total olefins was 46.0% when using HZSM-5 zeolite calcined at 500 ℃, however, with the prolongation of reaction time, the yield of total olefins decreased significantly. For example, at 30 h, it was only 22.9%. For the HZSM5 zeolite calcined at 700 ℃, the initial yield of total olefins was 52.8%, with the extension of reaction time, the yield of total olefins decreased slowly, and at 30 h, it was still as high as 45.9%.

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version of the former sample decreased slowly with reaction time. At 29 h, it was still kept at 63.3%. The total yield of olefins and the yields of ethylene and propylene were also kept at high levels. After 30 h, they were 45.9%, 18.4% and 16.5%, respectively. All of them were higher than the results of the HZSM-5 sample calcined at 500 ℃ which were 22.9%, 8.0% and 8.4%, respectively. This maybe be because that at high calcination temperature, the total acid amount and acid density of the HZSM-5 zeolite samples decreased somewhat, which benefited the yield of ethylene and propylene, but also depressed the reaction rates of hydrogen transfer and carbon deposition. It could also be used to explain why there was a maximum value for the yield of propylene over HZSM-5 catalyst calcined at 500 ℃ in n-butane cracking. 3.3. Ef fects of calcination temperature on HZSM-5 structure stability XRD patterns of the HZSM-5 zeolite samples calcined at different temperatures are shown in Figure 3. The height of the strongest peak at 2θ=23.02o, was used to calculate the crystallization. 0# sample is used as the standard, 100%, and the relative value of other samples are obtained by the strongest height of other samples divided by that of 0# sample. The results of relative crystallinity of the HZSM-5 catalysts are listed in Table 2. The results indicate that the calcined HZSM-5 catalysts show good thermal stabil-

The yield of ethylene decreased with the increase of reaction time. In the beginning period of experiment, these two samples had little difference, but with the increase of reaction time, the yield of ethylene of the former sample decreased from 30.7% to 8.0% which is more rapidly than that of the latter one, which is decreased only from 19.3% to 18.4%. The yield of propylene also decreased with the prolongation of reaction time. The propylene yield decreased steadily from 19.5% to 16.5% (30 h) with HZSM-5 sample calcined at 700 ℃. For HZSM-5 zeolite calcined at 500 ℃, the propylene was first increased from 13.8% to 16.2% (13 h) and then decreased rapidly to 8.4% at 30 h. In addition, during reaction time of 8 h to 19 h, the propylene yield reached the maximal value of about 16.0%. The above results indicated that in the n-butane cracking reaction, the stability of HZSM-5 zeolite sample calcined at 700 ℃ was much better than that of the sample calcined at 500 ℃. The n-butane con-

Figure 3. XRD patterns of HZSM-5 zeolite samples (1) 0#, (2) 1#, (3) 2#, (4) 3#, (5) 4#, (6) 5#, (7) 6#

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ities of the framework. Except for 5#, XRD patterns of all samples show the strongest peak at 2θ=23.02o. Compared with the fresh HZSM-5 catalyst, the one calcined at 700 ℃ merely lost 6.0% of the crystallinity. Wu et al. [15] pointed out that the phenomenon in which singlet peaks between 24.25o–24.50o, 29.12o–29.35o and 48.42o–48.86o split into doublet peaks was due to the transformation of HZSM zeolite

crystal structure from orthorhombic to monoclinic. The relative crystallinity of spent samples of HZSM-5 calcined at 700 ℃ slightly decreased after they were used for the n-butane crackings for 30 h at the reaction temperature of 650 ℃. Therefore, the deactivation of this catalyst after reaction is mainly due to carbon deposition rather than the change in their structures.

Table 2. Crystallinity of HZSM-5 zeolite samples calcined at dif ferent calcination temperatures 0#a

Catalyst

1#

2#

3#

4#

5#

6#b

Crystallinity(%)

100

99.3

93.0

92.5

94.0

87.0

88.3

24.4o peak shape

single

double

double

double

double

double

double

a b

Fresh HZSM-5 zeolite sample. Spent sample of HZSM-5 calcined at 700 ℃, and used for the reaction of the cracking of n-butane for 30 h at 650 ℃

The BET surface areas of HZSM-5 zeolite samples calcined at different temperatures are listed in Table 3. The results indicated that the surface area decreases with increasing calcination temperature. But the BET areas of HZSM-5 zeolite samples calcined at 700 and 800 ℃ did not decrease. According to lit-

erature [16], the frame structure of the zeolite would begin to break down above 800 ℃, indicating that HZSM-5 zeolite sample bear good thermal stability. Taken all the facts into consideration, 700 ℃ was a proper calcination temperature for the thermal treatment of HZSM-5 zeolite catalysts.

Table 3. NH3 -TPD results of HZSM-5 samples calcined at dif ferent temperatures Catalyst

ABET

Peak 1

Peak 2

m2 /g

Temperature (℃) Weak acid amount

Temperature (℃) Strong acid amount

Acid amount Acid density (mmol/g)

(µmol/m2 )

1#

394.8

272.9

0.2561

518.1

0.5359

0.7920

2.006

2#

362.2

273.0

0.2340

518.4

0.5080

0.7420

2.046

3#

352.6

274.9

0.1611

518.9

0.4069

0.5680

1.611

4#

318.0

277.8

0.1127

519.7

0.3618

0.4745

1.492

5#

323.5

288.9

0.0838

615.8

0.1897

0.2735

0.845

3.4. Acid amount and types From Table 3, NH3 -TPD results indicated that acid site density and acidic amount of HZSM-5 catalyst rapidly decreased with the increase of calcination temperature. Inversely, the shifting of NH3 desorption peak temperatures at both low and high temperatures to high values indicates that the strengths of both weak acid strength and strong acid strength increase with the increase of calcination temperature. The change of acid amount was resulted from the dehydroxylation of HZSM-5 zeolite catalysts, and some amounts of the B acid sites began to convert to L acid sites, when the temperature was higher than 600 ℃. The dehydroxylation of HZSM-5 zeolite catalysts was accelerated and could result in the decrease of catalyst activity. At the same time, some alu-

minums in the frame structure of zeolite would be lost. The elimination of aluminum was initiated from 400 ℃ and it was intensified above 500 ℃ [17,18], then the L acid site would be less than before. The HZSM-5 samples for pyridine adsorption were degassed at 200 and 350 ℃, respectively. The corresponding analysis results are listed in Table 4. The numbers of medium and strong acid sites of B and L acid sites were determined by FT-IR of pyridine adsorption at 200 and 350 ℃, respectively. It could be seen from Table 4 the amount of B acid sites was far more than that of L acid sites. The number of B acid sites rapidly decreased with the increase of calcination temperature, while the number of L acid sites had little change as the calcination temperature increased. When the calcination temperature was 800 ℃, the amount of L acid decreased drastically.

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Table 4. The results of FT-IR of pyridine adsorption Acid amount (µmol/g) Catalyst

Mass (mg)

Total (200 ℃)

Medium and strong (350 ℃)

B

L

B+L

L/B

B

L

B+L

L/B

1#

10.5

283.1

73.8

356.9

0.26

239.0

45.2

284.2

0.19

2#

10.5

279.7

77.4

357.1

0.28

230.5

56.0

286.5

0.24

3#

10.4

150.8

72.9

223.7

0.48

108.5

50.2

158.7

0.46

4#

10.4

111.9

70.2

182.1

0.63

71.2

51.2

122.4

0.72

5#

10.5

39.0

22.6

61.6

0.58

23.7

11.9

35.6

0.50

3.5. The nature of calcination temperature ef fect on the catalytic performances of HZSM5 for n-butane cracking With the change of calcination temperature the changes in total acid amounts and acid site density of HZSM-5 determined by NH3 -TPD are found to be very consistent with those of total amounts of B-type and L-type acid, which were determined by FT-IR of pyridine adsorption. A very good correlation between total acid amounts of HZSM-5 samples and their catalytic activities for the n-butane cracking can be obtained (See Figure 4). Therefore, the catalytic ac-

tivity of HZSM-5 catalysts for n-butane cracking was determined by their total amounts of acid. For the HZSM-5 catalysts calcined at low temperatures (e.g., 500 and 600 ℃) their total amounts of B-type acid are much greater than that of L-type acid and, at the same time, their n-butane conversions are much higher than those of the samples calcined at high temperatures (e.g., 700 and 800 ℃). These results indicate that B-type acid mainly control the catalytic activity for n-butane cracking although Ltype acid also plays some role in the n-butane cracking process. This consideration is consistent with that reported in literature [19].

Figure 4. Ef fect of calcaination temperature on the total acid amount, B+L acid amount of HZSM-5 samples and their conversion of n-butane ( Reaction temperature at 600 ℃)

It can be seen from Table 4, that the amount of B-type acid especially the medium and strong Btype acid decreased with the increase of calcination temperature, while the amount of L-type acid almost did not change, except for 5# sample (calcined at 800 ℃). And the ratio of L/B increased with the rising of calcination temperature. Except for 5# sample

(calcined at the highest temperature of 800 ℃), the similar trend of total selectivity to olefins, i.e., the total selectivity to olefins increased with the rising of calcination temperature, was observed indicating that the ratio of L/B may be related to the total selectivity to olefins. The proper L/B ratio is favorable for getting high total selectivity to olefins.

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As mentioned in the literature [19], strong B-type acid sites associate with high rate of cracking, high hydrogen transfer and coke deposition. Therefore, the decrease in the number of strong B-type acid sites may result in high selectivity to olefins.

support from the National Basic Research Program of China (grant No. 2004CB 217806), the National Natural Science Foundation of China (Grant No. 20373043), and the Scientific Research Key Foundation for the Returned Overseas Chinese Scholars of State Education Ministry.

4. Conclusions References (1) The acid property of HZSM-5 catalysts were modified by calcination treatment. The total acid amount and B-type acid amount of HZSM-5 catalysts rapidly decreased, while there was almost no change for the amount of L-type acid, thus the ratio of L/B was obviously enhanced with the increase of calcination temperature. (2) 700 ℃ is the best calcination temperature for thermal treatment, The HZSM-5 catalyst calcined at 700 ℃ gave very high yield of low olefins. At the reaction temperature of 650 ℃, the total olefin yield reached 52.8% and the yield of ethylene and propylene were as high as 29.4% and 19.4%. (3) High calcination temperature could improve the duration stability of HZSM-5 zeolites. Compared to HZSM-5 zeolite sample calcined at 500 ℃, the sample calcined at 700 ℃ has a good duration stability, and also achieve the higher conversion of n-butane as well as the yields of total olefins, ethylene and propylene. (4) The catalytic cracking activity of HZSM-5 catalysts is closely related to their total amounts of acid sites. The larger the number of B and L acid sites, the higher activity of HZSM-5 zeolite sample. The ratio of L/B may be related to the total selectivity for olefins. The proper L/B ratio is favorable for getting high total selectivities for olefins. (5) The calcination temperature has large effect on the surface area, crystallinity and acidic characteristics of HZSM-5 catalysts, thus further affect their catalytic performances for the cracking of n-butane. Acknowledgements The authors would like to thank the financial

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