Properties and Characterization of Modified HZSM-5 Zeolites

Transcription

1 Journal of Natural Gas Chemistry 12(2003)56 62 Properties and Characterization of Modified HZSM-5 Zeolites Renqing Lü 1, Hejin Tangbo 1, Qiuying Wang 2, Shouhe Xiang 1 1. Institute of New Catalytic Materials, Department of Material Chemistry, Nankai University, Tianjin, , China 2. Catalytic Factory, Nankai University, Tianjin, , China [Manuscript received July 12, 2002; revised November 4, 2002] Abstract: Physicochemical and catalytic properties of phosphorus and boron modified HZSM-5 zeolites treated with 100% steam at 673 K were investigated. The acidity and distribution of acidic sites were studied by infrared spectroscopy using pyridine as probe molecule and temperature programmed desorption (TPD) of ammonia. The structure of the samples was characterized by XRD, and the textural properties of the catalysts were determined by nitrogen isothermal adsorption-desorption measurements and scanning electron microscopy (SEM). The XRD results show that the modified samples have no novel crystalline phase, indicating a high dispersion of phosphorus and boron species. After treatment, the microporous volume and surface area of the samples markedly decrease, implying the blockage of the channel. The nitrogen adsorption-desorption measurements suggest that the isothermal type of all samples is a combination of isothermal type I and IV, and all hysteresis loops resemble the H4-type in the IUPAC classification. The total acidity of the modified samples, determined by pyridine adsorption IR and TPD of ammonia, decreases in contrast to that of the parent HZSM-5. The conversion of n-heptane over P and B steammodified HZSM-5 is higher than that of P and B-modified HZSM-5 zeolites but lower than that of the parent HZSM-5. Key words: HZSM-5 zeolite, steam treatment, phosphorus, boron, secondary pore, texture, cracking activity 1. Introduction The properties of catalysts are carefully tuned for the desired catalytic process before use. Zeolites are crystalline aluminosilicate, and their acidbase properties depend on the aluminum content in the framework. The adjustment of the acidity may be realized by proper SiO 2 /Al 2 O 3 molar ratio crystallization, other elements replacing framework constituents, or modification of the zeolite. Dealumination, the removal of framework aluminum from the zeolite lattice, is a well known procedure for stabilizing zeolites and creating mesopores, which help overcome diffusional problems in the zeolite micropores [1]. ZSM-5 is a member of the pentasil family of highsilica zeolites and, due to its unusual properties, has found a wide range of applications as a catalytic material. Modification of HZSM-5 zeolites by impregnation with phosphoric or boric acid has been investigated because of its promising catalytic properties in many reactions (e.g. conversion of methanol to hydrocarbons (MTH) [2,7,15,17,22], n-hexane cracking [3,11], disproportionation of toluene [4,6,8,10], alkylation of toluene with methanol [4 6,10,13,15,16,20,21], xylene isomerization [6], alkylation of benzene with ethanol [9,23], alkylation of ethylbenzene [12], conversion of methyl chloride to ethylene and propylene Corresponding author. Tel: (022) ; shxiang@public.tpt.tj.cn. On leave from Chemistry and Chemical Engineering College, University of Petroleum (East China), Dongying, Shandong Province

2 Journal of Natural Gas Chemistry Vol. 12 No [18,19]). To our knowledge, no attempts have been made to combine phosphoric or boric acid modification with steam treatment over HZSM-5 zeolites. 2. Experimental 2.1. Catalyst preparation A template-free synthesized commercial HZSM-5 zeolite (SiO 2 /Al 2 O 3 =50), supplied by the Catalytic Factory of Nankai University, was used as the starting material (denoted parent 50). The zeolite material was impregnated with an aqueous solution of phosphoric acid (H 3 PO 4 ) in order to reach a 1%P content. After being dried in air, the product was heated to 393 K in a muffle furnace for 1 h. Then, the temperature was increased to 823 K and held for 3 h. This product was designated as P501. Another product, B501, with a B content of 1% was prepared in a similar manner except for the replacement of H 3 PO 4 with H 3 BO 3. Some P501 and B501 samples were treated with 100% steam of 673 K for 4 h and designated as P5014 and B5014, respectively Catalyst characterization X-ray diffraction patterns were recorded on D/max-2500 powder diffractometer using nickelfiltered Cu K α radiation (λ= nm) and equipped with a graphite monochromator. The step scans were taken over a 2θ range from 5 to 50 o. The BET specific surface area and porosity texture of each sample were determined by nitrogen adsorption measurements at liquid nitrogen temperature with an automatic Micromeritics ASAP 2400 apparatus. The samples were first degassed at 573 K for approximate 6 h and then studied with a static volumetric technique. IR measurements were carried out using pyridine as the probe molecule, and the vibration spectra of chemisorbed pyridine were recorded between 1,400 and 1,700 cm 1. The samples were pressed into selfsupporting wafers 20 mm in diameter and heated to 673 K in a special IR cell under vacuum (0.04 Pa) for 1 h. After cooling to room temperature, excess pyridine was adsorbed and outgassed at 433 K to eliminate the physisorbed pyridine. The concentrations of Brönsted and Lewis sites able to retain pyridine at 433 K were determined using the extinction coefficients and the adsorbance surface of the corresponding bands at around 1,540 and 1,450 cm 1, respectively. TPD patterns of chemisorbed ammonia were recorded using a DuPont 2000 thermoanalyzer by means of NH 3 adsorption-desorption Catalytic activity measurements The catalytic activity of samples in n-heptane cracking was determined in a pulse microreactor (i.d. 4 mm) connected to a gas chromatograph. The reaction was carried out with 0.2 g catalyst, a 30 cm 3 /min N 2 flow rate, a 2 µl pulse and at 773 K. Before the activity was measured, the catalyst was activated in situ at 793 K for 1 h in dry nitrogen stream. 3. Results and discussion 3.1. Results of catalyst characterization The measurement of physicochemical properties Specific surface area of the catalysts was computed according to the BET method from the nitrogen adsorption isotherms obtained at 77 K, taking a value of nm 2 for the cross-section of the adsorbed N 2 molecule at that temperature. BET areas of the various samples are summarized in Table 1. The BET surface area, microporous area and microporous volume of all modified samples significantly decreased compared to the parent zeolite. Among the modified samples, BET surface area, mesoporous area and mesoporous volume of the steam-treated P5014 and B5014 are higher than those of P501 and B501, respectively, resulting in diffusion benefits. However, Table 1. Physicochemical properties of samples Sample BET surface area Micropore area Mesopore area Micropore volume Mesopore volume (m 2 /g) (m 2 /g) (m 2 /g) (cm 3 /g) (cm 3 /g) Parent P P B B

3 58 Renqing Lü et al./ Journal of Natural Gas Chemistry Vol. 12 No the microporous area and microporous volume of steam-treated P5014 and B5014 are lower than those of P501 and B501. These results suggest that the channel of phosphorus and boron-modified samples is occluded, and steam treatment may result in secondary pore formation X-ray powder analysis The x-ray powder pattern peaks of the five samples, exhibited by all samples in the XRD patterns, are typically of the MFI topology. The patterns indicate that crystallinity was retained after treatment. Also, there is no novel crystalline phase, which indicates that phosphorus and boron species are highly dispersed on the zeolites. The interaction between ZSM-5 zeolites and phosphorus and boron species may result in the peak split around 2θ=23 o IR measurement of adsorbed pyridine Information about the type of acid sites and their distribution in the catalysts could be obtained from the infrared spectra of pyridine adsorbed on the samples in the 1,400 1,700 cm 1 spectral region. The acidity of the five samples is shown in Figure 1, and a qualitative estimation of the band intensity ratio representing pyridine adsorbed at Brönsted acid sites and pyridine absorbed at Lewis acid sites is illustrated in Table 2. As shown in Figure 1 and Table 2, the parent zeolite possesses the largest number of both Brönsted and Lewis acid sites of the five samples. The reason for the difference between the Brönsted and Lewis acid number in P501 and P5014 remains unclear. Steam-treated B5014 and P5014 have a lower acidity compared to B501 and P501, respectively TPD of ammonia TPD profiles of the parent HZSM-5 and modified HZSM-5 are shown in Figure 2. The two-peak pattern is well documented for HZSM-5 [24], indicating the existence of weak and strong acid sites in the parent ZSM-5 zeolite. The profiles of modified samples reveal similar patterns as the parent sample, but the peak intensity of weak and strong acid sites in the modified samples markedly decreased. The significant decrease in the acidity of the single-phosphorus modified P501 and the single-boron modified B501 may be explained by the phosphorus and boron species combining with bridging hydroxyl Al(OH)Si groups [2]. After steam treatment at 673 K, some framework aluminum atoms are partially hydrolysed to form nontetrahedrally symmetric aluminum atoms, which act as a strong electron withdrawal centers for the remaining tetrahedral framework aluminum atoms thus creating stronger Brönsted acids [26]. Table 2 shows the acidity of all samples, strongly suggesting the decrease in acidity of the modified samples in contrast to Parent50. The acidity of the samples determined by the pyridine adsorbed IR method is lower than that of their counterparts determined by TPD of ammonia. This difference may be the results of differences in the molecular size of these bases, i.e. the smaller molecules of ammonia may penetrate through more pores than the larger molecules of pyridine. Figure 1. Total acidity of samples measured by FT- IR Figure 2. Ammonia TPD profiles of samples

4 Journal of Natural Gas Chemistry Vol. 12 No Table 2. Ratio of the B to L acid site intensity (denoted Ratio) and acidity measured by TPD of NH 3 (denoted TPD) Sample Ratio TPD (mmol/g) Parent P P B B Porosity measurement The porous structure of all samples was determined by N 2 adsorption-desorption measurements, and the nitrogen isotherm for the samples is illustrated in Figure 3. According to IUPAC [25], the shape of the adsorption isotherm can be classified into one of six groups. Of these, the most common are type I (Langmuir) isotherms for purely microporous solids, and type IV for mesoporous goods in which capillary condensation takes place at higher pressures of adsorbate as well as a hysteresis loop. As is shown in Figure 3, the adsorption volume at very low relative pressures (p/p 0 <0.1) is high, indicating the presence of microporous adsorption. Increasing the relative pressure causes capillary condensation, which illustrates type IV behavior. All five materials show a hysteresis loops that resembles the H4 type in the IUPAC classification. This can be attributed to the crystalline agglomerates that result in the mesoporous structure formed by the interparticle space and steam treatment that causes the formation of secondary pore. A distinct increase in the adsorbate volume in the low p/p 0 region and the hysteresis loop in the high p/p 0 region indicate the presence of micropores associated with mesopores. Therefore, the isothermal type of all samples is a combination of type I and type IV. Figure 3. N 2 adsorption-desorption isotherms of all samples Scanning electron microscopy A scanning electron micrograph (SEM, Figure 4) of all the samples indicates the morphology of the parent and modified HZSM-5 zeolite crystals. The SEM photographs reveal a change in the morphology of HZSM-5 upon steam treatment. A comparison between P501 and P5014 as well as between B501 and

5 60 Renqing Lu et al./ Journal of Natural Gas Chemistry Vol. 12 No B5014 reveals some cracks and faults that appeared on the surface of steam-treated B5014 and P5014. This shows the formation of secondary pores and this formation is an important explanation for the higher heptane cracking activity because cracking is often limited by diffusion inside the micropores of the zeo- lite [27]. P501 has few cracks or faults, and this may be because of the stronger acidity of H3 PO4 than that of H3 BO3. This clarified that the mesoporous volume of P501 is higher than that of B501 and the heptane cracking activity of P501 is higher than that of B501. Figure 4. SEM pictures of all samples: (a) parent, (b) enlarged parent, (c) P501, (d) P5014, (e) B501, (f ) B5014

6 Journal of Natural Gas Chemistry Vol. 12 No Activity of n-heptane cracking over the catalyst To further study the activity of catalysts, n-heptane cracking was used as a test reaction. The results of an n-heptane cracking conversion over the catalysts are presented in Table 3. It reveals an order of conversion (parent50=b5014>p5014>p501>b501) that shows no correlation to the acidity determined by pyridine adsorption and TPD-NH 3. The activity of steamtreated phosphorus and boron-modified samples is higher than that of single-phosphorus and boronmodified samples. This may be the result of steam enhancement of the BET surface area and mesoporous area as well as proper steam treatment enhancement of acid strength. The selectivity of products is also shown in Table 3. It can be seen that B5014 shows the highest C = 3 selectivity, while B501 shows the highest C = 4 selectivity. According to the acidic results calculated by TPD of ammonia and FT-IR, the acidity of steam-treated P5014 and B5014 is slightly lower than that of P501 and B501, respectively. This suggests a decrease in diffusion constraints brought about by the creation of mesopores in the steam-treated samples (as seen from the enhancement of the mesoporous area and mesoporous volume of the steam-treated samples). In addition, higher acid site strength may contribute to the activity enhancement. Table 3. Product selectivity and conversion of heptane cracking Sample Product selectivity (%) C 1 -C 2 C 3 C = 3 C 4 C = 4 C + 5 Conversion (%) Parent P P B B Conclusions The BET surface area, microporous area and microprous volume of modified samples decreased pronouncedly in contrast to Parent50. Phosphorus and boron species were highly dispersed over the HZSM- 5 (as suggested by XRD and SEM). The acidity of treated samples (measured by FT-IR and TPD of ammonia) pronouncedly decreased. The isothermal type of all samples is a complex of type I and IV, while hysteresis loops belong to the H4 type. The heptane cracking activity of a phosphorus or boron-modified sample is lower than that of the parent zeolite. The activity of steam-treated P5014 is higher than that of only phosphorus-modified P501, while the activity of steam-treated B5014 is remarkably enhanced compared to B501. Acknowledgements Financial support from Catalytic Key Laboratory of China Petroleum and Natural Gas Group Corporation (University of Petroleum) was greatly appreciated. We thank the National Science Foundation Committee for Grant NSFC References [1] Bertea L, Kouwenhoven H W, Prins R. Appl Catal A, 1995, 129(1): 229 [2] Kaeding W W, Butter S A. J Catal, 1980, 61(1): 155 [3] Vinek H, Rumplmayr G, Lercher J A. J Catal, 1989, 115(2): 291 [4] Ashton A G, Batmanian S, Dwyer J et al. J Catal, 1986, 34(1): 73 [5] Nunan J, Cronin J, Cunningham J. J Catal, 1984, 87(1): 77 [6] Young L B, Butter S A, Kaeding W W. J Catal, 1982, 76(2): 418 [7] Vedrine J C, Auroux A, Dejaifve P et al. J Catal, 1982, 73(1): 147 [8] Kaeding W W, Chu C, Ying L B et al. J Catal, 1981, 69(2): 392 [9] Chandawar K H, Kulkarni S B, Ratnasamy P. Appl Catal, 1982, 4: 287 [10] Chen N Y, Kaeding W W, Dwyer F G. J Am Chem Soc, 1979, 101(20 22): 6783 [11] Li D, Tao L, Zhang Y et al. Shiyou Huagong (Petrochem Technol), 1990, 19(3): 449 [12] Kaeding W W. J Catal, 1985, 95(2): 512 [13] Kaeding W W, Chu C, Ying L B et al. J Catal, 1981, 67(1): 159

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