Acidity Adjustment of HZSM-5 Zeolites by Dealumination and

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J. Phys. Chem. B 2006, 110, 15411-15416

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Acidity Adjustment of HZSM-5 Zeolites by Dealumination and Realumination with Steaming and Citric Acid Treatments Yu Fan,†,‡ Xiaojun Bao,*,†,‡ Xiuying Lin,‡ Gang Shi,‡ and Haiyan Liu‡ State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing 102249, P. R. China, and The Key Laboratory of Catalysis, China National Petroleum Corporation, China UniVersity of Petroleum, Beijing 102249, P. R. China ReceiVed: February 5, 2006; In Final Form: May 26, 2006

This article describes a novel method for acidity adjustment of HZSM-5 zeolites with steaming and citric acid treatments and demonstrates the realumination effect of citric acid on HZSM-5 zeolites dealuminated by steaming. A series of modified HZSM-5 zeolites were prepared by streaming and/or acid treatments and characterized by means of X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), 27Al MAS NMR spectroscopy, hydroxyl infrared spectroscopy (OH-IR), pyridine-adsorbed infrared spectroscopy, and N2 adsorption in the present investigation. The results showed that compared with single HCl or citric acid treatment, steaming treatment, and steaming/HCl treatments, citric acid treatment after steaming exclusively increased the amount of framework Al due to reinsertion of extraframework Al into the defective sites of the steamed HZSM-5 framework. This realumination effect of the citric acid treatment on the steamed HZSM-5 zeolite, which is reported here for the first time to the best of our knowledge, could nearly recover the pore structure of the steamed zeolite to that of the parent HZSM-5 zeolite and appropriately tailor the amount and strength of different acid sites, which sheds light on optimizing the physicochemical properties of HZSM-5 zeolites. It was also found that the steaming treatment prior to the citric acid treatment was the precondition of the realumination of HZSM-5 zeolites, suggesting that the lattice defect sites generated during steaming were necessary for citric acid to work.

1. Introduction Since being invented by Mobil Oil Co. in the early 1970s,1 zeolite ZSM-5 has received extensive studies and gained its central importance as a high-efficient catalyst composition in a number of commercially important processes.2-5 The structure of zeolite ZSM-5 consists of two sets of channels: one set of straight channels (elliptical cross section of about 5.4 Å × 5.6 Å) and the other set of sinusoidal channels (circular cross section of about 5.1 Å × 5.4 Å),6 which confer the good shape-selective property on the zeolite for its use in catalytic applications.7,8 However, too many strong acid sites in H-form ZSM-5 zeolites lead to lower hydroisomerization selectivity and poorer product yield because of higher cracking activity.9,10 Moreover, the strong acidity property of HZSM-5 zeolites results in the quick deactivation of their derived catalysts due to the massive formation of coke in acid-catalyzed reactions, such as n-heptane aromatization,11 the conversion of acetone/n-butanol mixtures to aromatics,12 and the hydroisomerization and aromatization of olefins in FCC gasoline.13 Therefore, it is necessary to develop a novel method that can elaborately tune the acidity of HZSM-5 zeolites to improve the activity and stability of their derived catalysts. It has been found that the dealumination of zeolites by steaming and treatments with ammonium hexafluorosilicate (AHFS), hydrochloric acid (HCl), oxalic acid, chelating agent EDTA, and so on are the effective methods for controlling the * Corresponding author. Fax: +86 (0)10 8973 4979. E-mail: baoxj@ cup.edu.cn. † State Key Laboratory of Heavy Oil Processing. ‡ The Key Laboratory of Catalysis.

acidity of zeolitic materials.14-18 Among them the steaming and acid treatments are considered to be the most feasible dealumination techniques from the point of view of environmental protection and industrial application.19 Dealuminating HZSM-5 zeolites by steaming produce many extraframework Al (EFAl) species that have a detrimental effect on their catalytic and transport properties,20,21 so acid leaching is usually followed. However, mineral acids (e. g., HCl) commonly used in leaching treatments can cause an undesirable effect, i.e., the further dealumination of HZSM-5 zeolites after steaming.22 To offset this defect, it is reasonable to consider the moderate realumination of steamed HZSM-5 zeolites. As to the realumination of zeolitic materials, it had been reported that the EFAl species produced in hydrothermal treatment processes could be subsequently reinserted into the framework of Y,23,24 β,25 and ZSM526 by treatments with basic solutions at elevated temperature; the treatment with AlCl3 vapor at higher temperature was also said to be able to incorporate Al atoms into the framework of high-silica ZSM-5 zeolite and thus adjust its acidity and activity.27-29 In addition, Sano et al.30 reported that mineral acid (i.e. HCl) treatment was effective in inducing the reinsertion of nonframework Al into the lattice of dealuminated HZSM-5 zeolites, but this conclusion is contrary to the well-known dealumination effect of mineral acids on Al-rich zeolites. Omegna et al.31 reexamined the results obtained by Sano et al.30 and found no evidence of realumination but the dealumination during the mineral acid treatment of dealuminated HZSM-5 zeolites. Recently, Xie et al. reported the realumination effect of single citric acid treatment on β zeolite,32 but the treatment of HZSM-5 either by citric acid or by streaming followed with

10.1021/jp0607566 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006

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TABLE 1: Treatment Methods and Typical Properties of the Resulting Samples sample

treatment method

SiO2/Al2O3 (mol/mol)a

relative crystallinity (%)b

PHZM HZMA1 HZMA2 SHZM SHZMA1 SHZMA2

parent HCl citric acid steaming steaming/HCl steaming/citric acid

50.6 51.1 50.8 50.6 57.7 54.8

100 102 102 94 96 100

a Calculated from X-ray fluorescence spectroscopy (XRF) data. Obtained by comparing the X-ray diffraction (XRD) peak (051, 2θ ≈ 23.1°) intensities of the modified samples with that of the parent HZSM-5 zeolite that is considered to be 100% crystallinity.

b

the citric acid treatment has not been touched in the open literature, to which the present investigation is addressed. To elucidate whether and how the citric acid treatment influences the acidity and pore structure of HZSM-5 zeolites, HZSM-5 zeolites were treated by citric acid and steaming/citric acid, and the Al state, acidity, and pore structure of the resulting zeolites were compared with those of the conventional steaming and/or HCl treated HZSM-5 zeolites in this study. 2. Experimental Section 2.1. Modifications of HZSM-5 Zeolites. First, the calcined parent H-form ZSM-5 zeolite (sample PHZM, SiO2/Al2O3 (mol/ mol) ) 50.6, Shanghai Huaheng Chemical Plant, Shanghai, P. R. China) was treated with excess 1 M HCl and 1 M citric acid solutions at 363 K for 6 h, respectively, and filtrated and washed with deionized water to obtain two acid-treated HZSM-5 zeolites (samples HZMA1 and HZMA2). Second, the parent HZSM-5 zeolite was hydrothermally treated at 753 K for 6 h in flowing 100% steam with a weight hourly space velocity (WHSV) of 1 h-1 to produce the steamed HZSM-5 zeolite (sample SHZM). Finally, sample SHZM was further treated with 1 M HCl and 1 M citric acid solutions at 363 K for 6 h, respectively, and filtrated and washed with deionized water to obtain the steaming/ acid treated HZSM-5 zeolites (samples SHZMA1 and SHZMA2). All the parent and modified samples were dried at 393 K for 6 h and then stored over a saturated MgCl2 solution to equilibrate with water vapor. All the samples prepared and the corresponding treatment methods applied are listed in Table 1. 2.2. Characterizations. The phase structure of the samples was characterized by XRD analyses conducted on a Shimadzu 6000 diffractometer (Kyoto, Japan) that uses Cu KR radiation and is operated at 40 kV and 30 mA with 2θ scanning speed at 4°/min and diffraction lines of 2θ between 5 and 35°. The relative crystallinity of the zeolites was determined by comparing the (051) peak intensities of the modified samples with that of the parent HZSM-5 zeolite which was considered to be 100% crystallinity. The Si and Al elemental analyses of the samples were performed by XRF (ZSX-100e, Osaka, Japan). The 27Al MAS NMR experiments were conducted on a Varian Infinity plus AS400 spectrometer operated with frequency at 104.26 MHz, pulse width at 0.5 µs, radio frequency field strength at 50 G, pulse delay at 0.5 s, spinning rate at 7 kHz, and 85 000 scans. For a quantitative comparison, all samples were weighed and a 1 M aqueous Al(NO3)3 solution was used as standard reference for 27Al. The infrared (IR) spectra were measured on a MAGNA-IR 560 FTIR (Fourier transform infrared) instrument (Nicolet Co., Madison, WI) with a resolution of 1 cm-1. The samples, each

Figure 1. XRD patterns of the samples.

15 mg, were extruded into the self-supported 12 mm diameter circular wafers. The wafers were evacuated in-situ in IR cells at 623 K for 4 h, and after the temperature was decreased to room temperature, the IR spectra in the range of 3400-4000 cm-1 were recorded to study the nature of hydroxyl groups. Subsequently, the samples were dehydrated at 773 K for 5 h under a vacuum of 1.33 × 10-3 Pa, followed by the adsorption of purified pyridine vapor at room temperature for 20 min. Finally, the system was evacuated at different temperatures and pyridine-adsorbed IR spectra were recorded. Total Lewis acidity and total Bro¨nsted acidity, medium and strong Lewis acidity and medium and strong Bro¨nsted acidity, and strong Lewis acidity and strong Bro¨nsted acidity can be calculated from the IR measurement results of pyridine adsorption at 473, 573, and 673 K, respectively. The surface area and pore volume measurements of the samples were conducted on a volumetric adsorption apparatus (ASAP 2405N, Micromeritics Co., Atlanta, GA) at 78 K using liquid N2. The samples were evacuated at 573 K for 4 h under a vacuum of 1.33 × 10-3 Pa prior to the analysis. 3. Results and Discussion 3.1. Relative Crystallinity and Total SiO2/Al2O3 Ratios. The XRD patterns of the samples are shown in Figure 1. All the samples are highly crystalline, as indicated by the strong intensity of the characteristic peaks and the weak background noises in the XRD patterns, suggesting that the different modification methods used do not obviously degrade the crystalline structure of the HZSM-5 zeolites. Combining Figure 1 and Table 1, we can see that after steaming, the relative crystallinity of the HZSM-5 zeolite decreases from 100% of sample PHZM to 94% of sample SHZM due to the formation of amorphous phases during steaming, and the subsequent acid treatments can recover its crystallinity to different degrees (from 94% of sample SHZM to 96% of sample SHZMA1 after the HCl treatment and to 100% of sample SHZMA2 after the citric acid treatment). Table 1 also shows that the single acid treatment with either HCl or citric acid (sample HZMA1 or HZMA2) only slightly increases the relative crystallinity of the parent HZSM-5 zeolite due to the leaching of intraporous EFAl, in accordance with the results reported in the literature.33 The XRF analyses of the samples can provide the information on the total SiO2 and Al2O3 contents (including both framework and extraframework species) that can be used to calculate bulk SiO2/Al2O3 ratios. As shown in Table 1, the bulk SiO2/Al2O3 molar ratios of the single acid treated samples (HZMA1 and

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J. Phys. Chem. B, Vol. 110, No. 31, 2006 15413

Figure 3. FTIR spectra of the samples in the range of OH stretching vibrations. Figure 2.

27Al

MAS NMR spectra of the samples.

TABLE 2: Concentrations of Different Al Species in the Samples Obtained by the Quantitative Analysis of 27Al MAS NMR Spectra sample

AlIV at 54 ppm (µmol/g)

AlVI at 0 ppm (µmol/g)

PHZM HZMA1 HZMA2 SHZM SHZMA1 SHZMA2

374 367 369 253 204 286

20 14 17 86 35 29

HZMA2) are almost identical with that of the parent sample (PHZM) because of the neglectable effect of the acid treatments on the framework Al (FAl) of the HZSM-5 zeolite, which will be further confirmed in the following discussion. Because the EFAl species formed during the hydrothermal treatment are not leached out of the solid, the steamed sample (SHZM) and the parent sample (PHZM) have the same bulk SiO2/Al2O3 molar ratio, as reflected by the higher bulk SiO2/Al2O3 molar ratios of the steaming/acid treated samples (SHZMA1 and SHZMA2) than that of the steamed sample (SHZM).34 3.2. State of Al in Modified HZSM-5 Zeolites. The properties and the catalytic performance of zeolites depend on the state of Al species in them, and therefore the investigation of the state of Al is needed. For this purpose, 27Al solid-state MAS NMR is usually applied. 3.2.1. 27Al MAS NMR Spectra. The 27Al MAS NMR spectra of the parent HZSM-5 and those modified by the different methods are shown in Figure 2. The quantitative analysis results of the deconvoluted spectra are presented in Table 2. From Figure 2 and Table 2, we can see that the parent HZSM-5 zeolite (sample PHZM) shows a strong peak at 54 ppm due to tetrahedrally coordinated FAl (AlIV) and a very weak peak at 0 ppm due to octahedrally coordinated EFAl (AlVI) that were formed during calcination. The peak intensities and the tetrahedrally coordinated FAl concentrations of the single HCl and citric acid treated samples (HZMA1 and HZMA2) at 54 ppm are almost the same as those of the parent sample (PHZM), suggesting the inability of the single acid treatments to dealuminate HZSM-5 zeolites, as pointed out by Kooyman et al.;35 the peak intensities and the octahedrally coordinated EFAl

concentrations of the single HCl and citric acid treated samples (HZMA1 and HZMA2) at 0 ppm are slightly lower than those of the parent sample (PHZM) due to the removal of EFAl. Compared with the single acid treatments, the steaming treatment (sample SHZM) can remarkably promote the transformation of tetrahedrally coordinated FAl into octahedrally coordinated EFAl, as seen from the comparison of the peak intensities and the Al concentrations of sample SHZM with those of samples HZMA1 and HZMA2 at 54 and 0 ppm, respectively. In other words, the steaming treatment is more effective in dealuminating HZSM-5 zeolites than the acid treatments, consistent with the results reported by Triantafillidis et al.34 As observed by Omegna et al.,31 the HCl treatment after steaming (sample SHZMA1) causes the further framework dealumination of the steamed sample (SHZM) during leaching of EFAl, which further confirms impossibility of realumination during the mineral acid treatment of the dealuminated HZSM-5 zeolites. Contrarily, the citric acid treatment after steaming (sample SHZMA2) increases the peak intensity and concentration of tetrahedrally coordinated FAl at 54 ppm, indicating the realumination effect of the citric acid treatment on the steamed HZSM-5 zeolite. 3.2.2. OH-IR Spectra. OH-IR spectra can also provide the information on the state of Al in HZSM-5 zeolites, because the OH groups on the zeolite surfaces are usually linked to Al atoms in zeolites. Thus, the assignment of OH bands is important for distinguishing the state of Al. The FTIR spectra of the parent and modified samples in the range of OH stretching vibrations are shown in Figure 3. There is a general agreement that the band at 3743 cm-1 corresponds to the OH stretching mode of terminal silanol groups,36 while those at 3610 and 3667 cm-1 are due to bridged Si(OH)Al groups related to the FAl species37 and OH groups attached to EFAl species,38,39 respectively. The quantitative analysis results of the different OH groups are presented in Table 3. From Figure 3 and Table 3, it is observed that the intensities of terminal silanol groups at 3743 cm-1 are in the order of the steaming and HCl treatments (sample SHZMA1) > the steaming treatment (sample SHZM) > the steaming and citric acid treatments (sample SHZMA2) > the single citric acid treatment (sample HZMA2) ≈ the single HCl treatment (sample HZMA1) ≈ the

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TABLE 3: Intensities of IR Bands of OH Groups in the Samples Obtained according to the Band Areas in OH-IR Spectra

sample

bridged Si(OH)Al groups at 3610 cm-1 (arb. unit)

OH groups attached to EFAl species at 3667 cm-1 (arb. unit)

terminal silanol groups at 3743 cm-1 (arb. unit)

PHZM HZMA1 HZMA2 SHZM SHZMA1 SHZMA2

150 141 146 67 54 83

10 7 9 42 19 15

24 23 25 41 49 30

parent sample (PHZM), indicating that the HCl treatment after steaming produces more structural defects and the citric acid treatment after steaming reduces the amount of structural defect sites in the steamed HZSM-5 zeolite due to the direct proportion of the intensities of terminal silanol groups to the amount of structural defect sites.31 The single acid treatments (samples HZMA1and HZMA2) have no obvious influence on the intensity of bridged Si(OH)Al groups at 3610 cm-1, in agreement with the results obtained by 27Al MAS NMR spectra, as shown in Figure 2. After the parent sample (PHZM) is treated by steaming (sample SHZM), the intensity of bridged Si(OH)Al groups at 3610 cm-1 quickly decreases and that of OH groups attached to EFAl species at 3667 cm-1 significantly increases, demonstrating the remarkable effect of steaming on dealumination of HZSM-5 zeolites, in accordance with the results obtained by the 27Al MAS NMR analysis. The two kinds of acid treatments after steaming have the similarity and difference: on one hand, they lead to the leaching of EFAl, as evidenced by the decrease in the intensity of the band at 3667 cm-1; on the other hand, they have distinct effects on FAl, with the steaming/citric acid treatments (sample SHZMA2) having better retention for FAl than the steaming/HCl treatments (sample SHZMA1). The higher FAl retention of the citric acid treatment can be attributed to the reinsertion of EFAl into the framework of the steamed HZSM-5 zeolite (i.e., realumination), as shown by the higher intensity of the band at 3610 cm-1 of sample SHZMA2 than that of sample SHZM. In summary, both the spectra of OH-IR and those of 27Al MAS NMR confirm that the citric acid treatment after steaming brings forth the realumination of the HZSM-5 zeolite, behaving distinctly different from the dealumination of the conventional HCl treatment after steaming. Here, it should be pointed out that the steaming treatment increases not only the realumination sensitivity of HZSM-5 zeolite to the citric acid treatment but also its dealumination sensitivity to the HCl treatment, so acid treatments without prior steaming can hardly affect FAl of HZSM-5 zeolite. Unlike HZSM-5 zeolite, β zeolite can be realuminated by the single citric acid treatment,32 meaning that the realumination behavior of citric acid is closely related to the zeolite structure. The

Figure 4. Concentration of framework Al and intensity of bridged Si(OH)Al groups vs strong Bro¨nsted acidity of the different modified samples.

realumination effect of the citric acid treatment on the steamed HZSM-5 zeolite observed in this study can be taken as an important basis for the fine modification of HZSM-5 zeolites. 3.3. Acidity and Pore Structure. 3.3.1. Acidity. To further understand the effects of the different modification methods on the acidity property of HZSM-5 zeolites, the pyridine-adsorbed FTIR measurements were carried out and the results are presented in Table 4. It can be seen that the single acid treatments (samples HZMA1 and HZMA2) only slightly affect the zeolite Lewis and Bro¨nsted acidities of different strengths, whereas the steaming treatment (sample SHZM) brings about substantial changes in the zeolite acidities: significant decrease in the strong Bro¨nsted acidity and great increase in the strong Lewis acidity. Although both the HCl and citric acid treatments of the steamed sample (samples SHZMA1 and SHZMA2) lead to the decrease in the Lewis acidity (especially in strong Lewis acidity), the latter increases the Bro¨nsted acidity (especially strong Bro¨nsted acidity) and reversely the former decreases the Bro¨nsted acidity (especially strong Bro¨nsted acidity). How are (if they are) the above phenomena related to the dealumination or realumination of the HZSM-5 zeolite framework? To answer this question, the relationship of the zeolite acidity versus the state of Al is pursued in the present investigation. In view of the significant change of the strong Bro¨nsted acidity by the different treatments, the concentration of the framework Al and the intensity of the framework-bridged Si(OH)Al groups that are widely recognized to be associated with the Bro¨nsted acidity40-43 are correlated to the strong Bro¨nsted acidity, respectively, as shown in Figure 4. It can be seen that the resulting two curves are straight lines with their correlation coefficients at 0.994 and 0.971, respectively, regardless of the modification methods used. These results make it clear that the strong Bro¨nsted acid sites are closely related to the concentration of the FAl species in the HZSM-5 zeolite, or more exactly, the

TABLE 4: Acid Type and Strength Distribution of the Samples acidity (µmol/g) weak acid

medium acid

strong acid

sample

Lewis

Bro¨nsted

Lewis

Bro¨nsted

Lewis

Bro¨nsted

tot.

PHZM HZMA1 HZMA2 SHZM SHZMA1 SHZMA2

21.4 16.4 18.6 63.6 34.4 30.1

79.7 71.4 75.6 36.2 20.6 38.9

21.2 19.5 19.8 54.2 28.7 23.4

53.3 51.4 52.1 31.6 21.3 36.9

12.1 10.2 10.9 98.6 23.4 18.7

294.2 287.6 290.7 88.7 61.2 108.5

481.9 456.5 467.7 372.9 189.6 256.5

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TABLE 5: Pore Structure Data of the Samples 2

surface area (m /g)

pore vol (mL/g)

sample

micropore

tot.

micropore

tot.

PHZM HZMA1 HZMA2 SHZM SHZMA1 SHZMA2

284 281 283 260 272 280

366 363 365 344 353 362

0.136 0.133 0.134 0.118 0.126 0.132

0.277 0.274 0.275 0.266 0.269 0.274

decrease in the strong Bro¨nsted acidity originates from the removal of the FAl species. Thus, the minor change in the Bro¨nsted acidity of the parent sample after the single acid treatments can be ascribed to their negligible influence on the FAl content, while the decreases of the different degrees in the Bro¨nsted acidity after the steaming and steaming/acid treatments are due to the disparate reduction of the FAl content by the different modification methods. It should be emphasized that the higher strong Bro¨nsted acidity of the HZSM-5 zeolite after the steaming/citric acid treatments (sample SHZMA2) than that after the steaming treatment (sample SHZM) is due to the increase in the content of FAl caused by the realumination nature of the citric acid treatment. The varying trend of the Lewis acidity of the HZSM-5 zeolite observed in the present investigation follows the rule that Lewis acid sites are associated with EFAl species.43,44 Despite the presence of “NMR-invisible” EFAl species in the steamed HZSM-5 zeolite,45,46 the EFAl peak at 0 ppm for sample SHZM can be clearly observed in the 27Al MAS NMR spectra (Figure 2), suggesting the generation of some “NMR-visible” EFAl species during steaming. It is the very EFAl species that result in the significant increase of the strong Lewis acidity of the parent sample after steaming (Table 4), as reported by Kumar et al.33 The acid treatments after steaming extract the EFAl species from the HZSM-5 zeolites and diminish their amount (referring to Figure 2 and Table 2), correspondingly leading to the decrease in the Lewis acidities of different strengths, especially in the strong Lewis acidity. The negligible changes in the Lewis acidities of the parent HZSM-5 sample after the single acid treatments (Table 4) can be explained by the almost identical amounts of the EFAl species before and after the single acid treatments, as shown in Figure 2 and Table 2. In brief, the causality between the state and amount of Al in the HZSM-5 zeolites and their acidities holds. 3.3.2. Pore Structure. To understand the effects of the different treatments on the shape selectivity of HZSM-5 zeolites, the pore structures of the different samples are studied and the results are shown in Table 5. It can be seen that the single acid treatments (samples HZMA1 and HZMA2) have no obvious effect on the pore structure parameters of the parent HZSM-5 zeolite (sample PHZM), but the steaming treatment (sample SHZM) considerably reduces the surface area and the pore volume of the parent HZSM-5 zeolite due to the blockage of the EFAl species in the pore channels, especially in micropores. Although both the subsequent HCl treatment and the subsequent citric acid treatment (sample SHZMA1 or SHZMA2) can remove a part of the EFAl species generated during steaming (Figure 2 and Table 2) and thus make the pore channels of the zeolites more open, the pore structures of the HZSM-5 zeolites obtained by the two methods are clearly different from each other due to the dealumination effect of HCl and the realumination effect of citric acid on the steamed HZSM-5 sample. 4. Conclusions HZSM-5 zeolites were modified by the different steaming and/or acid treatments, and the realumination effect of the citric

acid treatment on the dealuminated HZSM-5 zeolite was reported for the first time. The characterization results showed that all the modified HZSM-5 samples obtained had good crystallinity, while the modification effects of the different methods on HZSM-5 zeolites were disparate, leading to the different changes in the acidity and pore structure of HZSM-5 zeolites. The single acid treatments (HCl and citric acid) could hardly dealuminate the framework of HZSM-5 zeolites, and their only effect was the leaching of EFAl, so the samples treated by these methods still had the very strong acidity that was inherited from the parent HZSM-5 sample. Compared with the single acid treatments, the steaming treatment was more effective for dealuminating HZSM-5 zeolites, as shown by the greatly promoted transformation of tetrahedrally coordinated FAl into octahedrally coordinated EFAl. This transformation resulted in the narrowed zeolite pore channels because of the blockage of EFAl species, the stronger Lewis acid sites generated by the increase of EFAl, and the weakened strong Bro¨nsted acidity caused by the decrease of FAl in the steamed HZSM-5 sample. The HCl treatment after steaming resulted in the reduction of the Bro¨nsted and Lewis acidity because of the further dealumination of the HZSM-5 zeolite framework and the removal of EFAl compared with the steaming treatment. The citric acid treatment after steaming reinserted EFAl species into the framework of the steamed HZSM-5 sample and, thus, increased its Bro¨nsted acidity (especially strong Bro¨nsted acidity) and restored its pore structure into nearly the same of the parent HZSM-5. The realumination property of the citric acid treatment could elaborately adjust the acidity of HZSM-5 zeolite while keeping its good shape selectivity, imbuing this method with great vitality for further development. Acknowledgment. We acknowledge the financial supports from the Ministry of Science and Technology of China through the National Basic Research Program (Grant No. 2004CB217807) and from China National Petroleum Co. (CNPC) through the CNPC Innovation Foundation (Grant No. 05E7020). References and Notes (1) Argauer, R. J.; Landolt, G. R. U.S. Patent 3702886, 1972. (2) Chen, N. Y.; Garwood, W. E. Catal. ReV.sSci. Eng. 1986, 28, 185. (3) Degnan, T. F.; Chitnis, G. K.; Schipper, P. H. Microporous Mesoporous Mater. 2000, 35/36, 245. (4) Nagamori, Y.; Kawase, M. Microporous Mesoporous Mater. 1998, 21, 439. (5) Salazar, J. A.; Cabrera, L. M.; Palmisano, E.; Garcia, W. J.; Solari, R. B. U.S. Patent 5770047, 1998. (6) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nature 1978, 272, 437. (7) Namba, S.; Kim, J. H.; Komatsu, T.; Yashima, T. Microporous Mater. 1997, 8, 39. (8) Halgeri, A. B.; Das, J. Catal. Today 2002, 73, 65. (9) Lugstein, A.; Jentys, A.; Vinek, H. Appl. Catal., A: Gen. 1999, 176, 119. (10) de Lucas, A.; Valverde, J. L.; Sa´nchez, P.; Dorado, F.; Ramos, M. J. Appl. Catal., A: Gen. 2005, 282, 15. (11) Sahoo, S. K.; Viswanadham, N.; Ray, N.; Gupta, J. K.; Singh, I. D. Appl. Catal., A: Gen. 2001, 205, 1. (12) de Lucas, A.; Can˜izares, P.; Dura´n, A. Appl. Catal., A: Gen. 2001, 206, 87. (13) Fan, Y.; Bao, X.; Shi, G. Catal. Lett. 2005, 105, 67. (14) Lonyi, F.; Lunsford, J. H. J. Catal. 1992, 136, 566. (15) Triantafillidis, C. S.; Evmiridis, N. P. Ind. Eng. Chem. Res. 2000, 39, 3233. (16) Jacobs, P. A.; Tielen, M.; Nagy, J. B.; Debras, G.; Derouane, E. G.; Gabelica, Z. In Proceedings of the 6th International Zeolite Confererence; Olson, D., Bisio, A., Eds.; Butterworth: Guildford, London, 1984; p 783.

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