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Realumination of dealuminated HZSM-5 zeolites by acid treatment and their catalytic properties
Microporous and Mesoporous Materials 31 (1999) 89–95
Realumination of dealuminated HZSM-5 zeolites by acid treatment and their catalytic properties T. Sano *, Y. Uno, Z.B. Wang, C.-H. Ahn, K. Soga School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan Received 21 December 1998; accepted for publication 15 February 1999
Abstract Effects of acid treatment variables on the realumination efficiency for realumination of dealuminated HZSM-5 zeolite (reinsertion of non-framework aluminums into the zeolite framework) were investigated using 27Al MAS NMR and nitrogen adsorption. Among various mineral acids employed, HCl solution was the most effective for the realumination. The realumination efficiency was also studied, evaluating the change in the cracking activity of cumene. The cumene cracking activity of the realuminated HZSM-5 zeolite was comparable to that of the parent zeolite, whose framework SiO /Al O ratio was the same as that of the realuminated zeolite. From the results obtained, it 2 2 3 was found that no structural degradation of HZSM-5 zeolite crystals takes place during the acid treatment and that some of the non-framework aluminums in dealuminated HZSM-5 zeolites are reinserted into the framework. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Acid treatment; HZSM-5; Non-framework aluminum; Reinsertion
1. Introduction In general, the acidic and catalytic properties of zeolites are known to be effectively related to the content of tetrahedrally coordinated framework aluminums. An irreversible loss of catalysis efficiency observed after a number of reaction– regeneration cycles is attributable to the removal of framework aluminums from the zeolite lattice [1]. Therefore, realumination of dealuminated zeolites has received considerable attention from a standpoint of regeneration of zeolite catalyst as well as control of physicochemical properties of zeolites such as thermal stability and ion-exchange * Corresponding author. Tel.: +81-761-511611; Fax: +81-761-511625. E-mail address: [email protected] ( T. Sano)
abilities. There are a large number of papers concerning the realumination of dealuminated zeolites by treatment with alkali solutions, suggesting the reinsertion of non-framework aluminums into the framework [2–14]. However, dissolution of a part of zeolite framework has been pointed out. Kooyman et al. recently reported that dealumination of HZSM-5 zeolite is not straightforward in the acid solution [15]. Very recently, we found that the reinsertion of non-framework aluminums in dealuminated HZSM-5 zeolites takes place by treatment with HCl solution at 100°C [16,17]. As it is recognized that leaching with mineral acid is one of the dealumination procedures for dealumination of Al-rich zeolites such as Y and mordenite zeolites, realumination by the acid treatment may be a very puzzling phenomenon. In this paper, in order to develop a more systematic understanding
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of the realumination of dealuminated HZSM-5 zeolites by treatment with acid solutions, the effects of acid treatment variables on the realumination efficiency for reinsertion of non-framework aluminums into the zeolite framework and the catalytic performance of realuminated zeolite were investigated.
2. Experimental 2.1. Zeolite samples and treatment HZSM-5 zeolite with 69 of a bulk SiO /Al O 2 2 3 ratio was prepared following the procedure described previously [18]. Dealumination of the HZSM-5 zeolite was carried out by thermal treatment at 600°C for 48 h using a muffle furnace. Acid treatment was carried out as follows: 1 g of the dealuminated HZSM-5 zeolite was treated with 100 ml of acidic solution at 100°C under stirring for a given period of time. The product was filtered off, washed thoroughly with deionized water, dried at 100°C and calcined at 380°C for 8 h. Acetylacetone was adsorbed onto the parent and treated HZSM-5 zeolites to try to complex non-framework aluminums including the NMR invisible species [19,20]. The samples were dried in an oven at 380°C to remove adsorbed water, and transferred to a desiccator, containing acetylacetone, and then left to equilibrate over 5 days at room temperature. 2.2. Characterization The identification of zeolites obtained was achieved by X-ray diffraction (Rigaku RINT 2000). The chemical composition was measured by X-ray fluorescence ( XRF, Philips PW2400). Textural properties were determined by nitrogen adsorption (Bel Japan Belsorp 28SA). Before adsorption measurements at −196°C, the powdered zeolites (c. 0.1 g) were evacuated at 400°C for 12 h. 27Al MAS NMR spectra were recorded on a Varian VXP-400 spectrometer at 104.3 MHz with a c. 4.5-kHz spinning speed and 1.73-ms pulses for 4000 scans. The chemical shift reference was hydrated Al3+ in solid aluminum nitrate enneahy-
drate. The surface composition was measured by X-ray photoelectron spectroscopy ( XPS, UlvacPhi 5600). The content of coke was measured using a Perkin-Elmer TGA 7 thermal analyzer (air flow: 40 ml min−1; heating rate: 10°C min−1). 2.3. Catalytic evaluation Cracking of cumene was carried out in an atmospheric pressure flow system. The zeolite catalyst placed into a quartz tube reactor of a 10-mm inner diameter was dehydrated at 400°C for 1 h in a nitrogen stream. The temperature was then brought into a reaction temperature (250°C ). The reactant was fed into the catalyst bed with a microfeeder. Nitrogen was used as a carrier gas (40 ml min−1). The contact time ( W/F ) was varied from 0.4 to 1.9 g h mol−1 by changing the weight of catalyst, and the partial pressure of cumene was 7.9 kPa. An on-line product analysis was performed on a Shimadzu GC-17A gas chromatograph (FID) with a GL-Science TC-1 capillary column (30 m).
3. Results and discussion 3.1. Effect of acid treatment variables The key acid treatment variables studied included a kind of acid, acid concentration, treatment time and treatment temperature. The contents of framework aluminums of HZSM-5 zeolites before and after the acid treatment were determined from the spectra of 27Al MAS NMR using a calibration curve, which was produced by measuring the 27Al MAS NMR spectra of parent HZSM-5 zeolites with various SiO /Al O ratios 2 2 3 [18]. The dealuminated HZSM-5 zeolites with 120–140 of framework SiO /Al O ratios were 2 2 3 prepared by thermal treatment at 600°C for 48 h of the HZSM-5 zeolite with 69 of the framework SiO /Al O ratio and used as a sample for the acid 2 2 3 treatment. At first, the effect of a kind of mineral acid solution on the realumination efficiency was investigated using the dealuminated HZSM-5 zeolite with 121 of the framework SiO /Al O ratio. 2 2 3 Table 1 (Nos 1–4) shows the degree of remaining
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T. Sano et al. / Microporous and Mesoporous Materials 31 (1999) 89–95 Table 1 Realumination of dealuminated HZSM-5 zeolite with various acidic solutionsa Run no.
Treatment solution
Concentration (M )
Degree of reamaining framework aluminums (%)
Micropore volume W (N )b [cm3( liquid ) g−1] 0 2
1 2 3 4 5 6 7 8 9c
HCl H SO 2 4 HNO 3 CH COOH 3 AlCl 3 Al(NO ) 33 Al (SO ) 2 43 NH NO 4 3 –
2 1 2 2 2 2 1 2 –
81.0 71.2 71.2 69.3 62.2 66.9 67.1 66.2 58.2
0.183 0.189 0.185 – – 0.187 0.187 – 0.192
a Treatment conditions: Temperature=100°C, Time=120 h. b Determined by the Dubinin–Radushkevich (D–R) equation. c Before acid treatment (dealumianted HZSM-5).
framework aluminums after the acid treatment, which is defined as [Al/(Si+Al )] / treated zeolite [Al/(Si+Al )] . The degree of remaining frameparent work aluminums is dependent on the type of mineral acid. Among the mineral acids employed, the treatment with HCl solution exhibited the highest realumination efficiency. The exact reason for the high realumination efficiency of HCl solution is not clear at present due to limited data. The results of realumination of the dealuminated HZSM-5 zeolite with various aqueous solutions of Al-containing compounds and ammonium nitrate are also summarized in Table 1 (Nos 5–8). A slight increase in the degree of remaining framework aluminums was observed. To check the structural degradation of zeolite crystals during the realumination treatment, nitrogen adsorption isotherms of realuminated zeolites were measured, and the micropore volumes were calculated. As shown in Table 1, no differences in micropore volume [ W (N )] determined by the 0 2 Dubinin–Radushkevich (D–R) equation were observed between dealuminated and realuminated HZSM-5 zeolites, indicating no structural degradation. From these results, HCl solution was selected in the following acid-treatment experiments. The variations in the degree of remaining framework aluminums and the micropore volume [ W (N )] with HCl concentration are illustrated 0 2 in Fig. 1. The dealuminated HZSM-5 zeolite with 139 of the framework SiO /Al O ratio was used 2 2 3
Fig. 1. Effect of HCl concentration on realumination efficiency. Treatment conditions: Temperature=100°C; Time=120 h.
as a sample. The degree of the remaining framework aluminums increased markedly with an increase in HCl concentration and reached a constant value at more than 2 M HCl. The micropore volumes [ W (N )] hardly changed, even after 4 M 0 2 HCl treatment. The effect of treatment time on the realumination efficiency was also studied using the dealuminated HZSM-5 zeolite with 139 of the framework SiO /Al O ratio, and the result is 2 2 3 illustrated in Fig. 2. The degree of remaining framework aluminums increased with treatment time and became constant after more than 120 h. The reduction in micropore volume due to structural degradation of zeolite crystals hardly
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that the removal of framework silicon atoms hardly takes place during the acid treatment, indicating the high stability of HZSM-5 zeolite in the acid solution. Therefore, it can be concluded that the realumination of dealuminated zeolite occurs during the acid treatment, although the degree of reinsertion of non-framework aluminums into the zeolite framework is not so high. 3.2. Catalytic performance
Fig. 2. Effect of HCl treatment time on realumination efficiency. Treatment conditions: Temperature=100°C, HCl concentration: 2 M.
occurred, even after a long period of treatment of more than 300 h. As it is possible that the increase in the framework SiO /Al O ratios of the realuminated 2 2 3 HZSM-5 zeolites may be attributable to the removal of the framework silicon atoms, the influence of treatment conditions on dissolution of zeolite framework was investigated. The silicon and aluminum concentrations in the liquid phase of HCl treatment were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP, Seiko SPS7700), and the degrees of elution of silicon and aluminum atoms from the dealuminated zeolite are listed in Table 2. It became clear
Next, from a standpoint of regeneration of dealuminated zeolite catalyst, the catalytic properties of the realuminated HZSM-5 zeolites with the HCl treatment were evaluated by the cumene cracking reaction. The conversion profiles of cumene on various HZSM-5 zeolites at 250°C are illustrated in Fig. 3. There are significant differences between these zeolites with respect to initial activity and deactivation, showing that the HCl treatment undoubtedly has an influence on the catalytic property of zeolite. The initial activity for cumene cracking of the dealuminated zeolite was improved by the HCl treatment. Of course, the activity was not completely recovered. For parent HZSM-5 zeolite, it has been found that a linear correlation between activity and concentration of Brønsted acidic site [bridged hydroxyl of
Table 2 Degrees of elustion of silicon and aluminum atoms from dealuminated HZSM-5 zeolites during acid treatment Treatment conditions
Degree of elutiona
Solution
Time (h)
Si (at.%)
Al (at.%)
0.5 M HCl 1 M HCl 2 M HCl 2 M HCl 4 M HCl 1 M H SO 2 4 2 M HNO 3 2 M CH COOH 3 2 M NH NO 4 3
120 120 120 360 120 120 120 120 120
1.80 1.48 1.56 1.27 0.78 1.36 1.07 1.23 1.03
6.57 7.34 7.98 11.95 9.23 5.09 4.36 3.56 2.36
a Determined by ICP.
Fig. 3. Conversion profiles of cumene on various HZSM-5 zeolites. Reaction conditions: Temperature=250°C; W/F=1.88 g h mol−1; partial pressure of cumene=7.9 kPa. #: Parent HZSM-5; $: Dealuminated HZSM-5 (600°C, 48 h); %: Realuminated HZSM-5 (2 M HCl, 100°C, 120 h).
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Si(OH )Al ] is observed in several hydrocarbon conversions such as cumene and hexane cracking reactions [21,22]. Therefore, the increase in activity seems to indicate the partial regeneration of dealuminated HZSM-5 zeolite catalyst. It is well recognized that an enhancement of catalytic activity is observed for zeolites dealuminated under mild conditions due to a synergistic interaction between Brønsted acidic sites in the zeolite and any type of Lewis acidic sites from non-framework aluminum species, resulting in an increase in acidic strength of bridged hydroxyls (e.g. [23–26 ]). Thus, in order to clarify the reason for the increase in the initial conversion for cumene cracking by the acid treatment, the rate constants for cumene cracking on realuminated HZSM-5 zeolites were compared with those on parent HZSM-5 zeolites. It is difficult to determine the initial conversion by extrapolating the curve of cumene conversion vs. time on stream to zero time due to the rapid change in conversion with time on stream in the initial reaction stage. Therefore, the conversion of the first pulse sampling was regarded as the initial conversion, X . Assuming 0 first-order kinetics, −ln(1−X ) was plotted 0 against the contact time ( W/F ), to obtain a linear plot, the slope of which was a first-order rate constant. Fig. 4 shows the relationship between
the framework aluminum (Brønsted acidic site) content and the first-order rate constant of cumene cracking. It is evident that a good linear correlation exists between the rate constant and the framework aluminum content in parent HZSM-5 zeolites, indicating that all the acidic sites have an equal activity. A considerable decrease in the rate constant for cumene cracking was observed for dealuminated HZSM-5, whereas an increase in the rate constant for the realuminated zeolite was observed. Data for the dealuminated and realuminated HZSM-5 zeolites are fitted by a common straight line of parent HZSM-5, suggesting that new acidic sites with a higher acidity are not generated by the HCl treatment. As shown in Fig. 3, the steady-state activity of realuminated HZSM-5 zeolite after 45 min on stream was higher than that of the parent or the dealuminated zeolite. The amounts of coke on the parent, dealuminated and realuminated zeolites after 2 h on stream measured by thermogravimetric analysis were 4.8, 3.1 and 3.8 wt%, respectively. To clarify the difference in deactivation behavior, the external surface SiO /Al O ratio of zeolite 2 2 3 was measured by XPS ( Table 3). No difference in either external surface or bulk SiO /Al O ratios 2 2 3 was observed between the parent and dealuminated zeolites. A large difference in external surface SiO /Al O ratio was observed between the 2 2 3 parent and realuminated zeolites, whereas a slight difference was observed in bulk SiO /Al O ratio. 2 2 3 Taking into account that the coke deposition on external surfaces of zeolite crystal is mainly related to the deactivation, the difference in the steadystate activity between the parent and the realuminated zeolites may be attributable to the difference Table 3 Surface and bulk SiO /Al O ratios of various HZSM-5 zeolites 2 2 3 Zeolite
Fig. 4. Relationship between framework aluminum content and first-order rate constant for cumene cracking at 250°C. #: Parent HZSM-5; $: Dealuminated HZSM-5 (600°C, 48 h); %: Realuminated HZSM-5 (2 M HCl, 100°C, 120 h)
Parent HZSM-5 Dealuminated HZSM-5 Realuminated HZSM-5 a Determined by XPS. b Determined by XRF.
SiO /Al O ratio 2 2 3 Surface a
Bulkb
58 58 86
69 69 74
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in surface aluminum content, probably surface framework aluminum content. A similar phenomenon was also reported by Namba et al. [27]. These results provide strong evidence of the partial reinsertion of non-framework aluminums in the dealuminated HZSM-5 into the zeolite framework by the acid treatment, although the selective removal of non-framework aluminums on external surfaces of zeolite crystals proceeds. 3.3. Plausible mechanism of realumination The realumination treatment was conducted using HCl solution at 100°C, followed by thermal treatment at 380°C. In order to clarify the realumination process, the framework aluminum concentrations of realuminated HZSM-5 zeolites before and after the thermal treatment were measured using 27Al MAS NMR. No difference was observed between the framework aluminum concentrations of zeolites before and after the thermal treatment, indicating that the realumination takes place during HCl treatment at 100°C. It is well known that the non-framework aluminum is not uniform and exists in different states [28,29]. Therefore, to obtain further information concerning the realumination process, the chemical state of non-framework aluminum species in the dealuminated HZSM-5 zeolites before and after HCl treatment was investigated using the 27Al MAS NMR. Acetylacetone was used to visualize
NMR invisible aluminum species in the dealuminated zeolite. In the 27Al MAS NMR spectrum of dealuminated HZSM-5 zeolite, as shown in Fig. 5, a very weak peak at c. 30 ppm and a sharp peak at c. 53 ppm assigned to framework aluminums were observed. It has been considered that the 30-ppm peak is assigned to non-framework aluminum species where the aluminum atom is connected to the zeolite framework only by one or two remaining chemical bonds, probably either aluminum species in a highly distorted tetrahedral environment or a penta-coordinated aluminum species [30,31]. The peak intensity at 30 ppm decreased as a result of HCl treatment, as shown in Fig. 5. The reinsertion of framework aluminums hardly takes place on HZSM-5 zeolite dealuminated under severe hydrothermal treatment condition [17], in which the complete extraction of aluminum atoms from the framework seems to proceed. Therefore, this result seems to suggest that realumination with acid treatment proceeds mainly through the reinsertion of non-framework aluminum species, where the aluminum atom is connected to the zeolite framework only by one or two remaining chemical bonds. Of course, some of the non-framework aluminums also dissolves into the solution during the acid treatment, as described above. From the above results, as well as the fact that the alumination of silicalite with an aqueous solution of AlCl did not take place, we speculate that 3
Fig. 5. 27Al MAS NMR spectra of dealuminated HZSM-5 zeolites before and after HCl treatment. (A) Parent HZSM-5; (B) Dealuminated HZSM-5 (600°C, 48 h); (C ) Realuminated HZSM-5 (2 M HCl, 100°C, 120 h).
T. Sano et al. / Microporous and Mesoporous Materials 31 (1999) 89–95
Fig. 6. Plausible mechanism of realumination of dealuminated HZSM-5 zeolite by acid treatment.
the realumination mechanism by acid treatment proceeds through the reinsertion not of non-framework aluminums completely removed from the zeolite lattice but of those connected to the lattice, as shown in Fig. 6.
4. Conclusions Although the degree of reinsertion of framework aluminums into the zeolite framework is not so high, it was found that some of the nonframework aluminums in dealuminated HZSM-5 zeolite are reinserted into the framework by acid treatment; in particular, HCl solution is effective. The reinsertion of non-framework aluminums was also confirmed from cumene cracking.
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[8] H. Ishida, Y. Fukuoka, Nippon Kagaku Kaishi (1994) 690. [9] J. Datka, B. Sulikowski, B. Gil, J. Phys. Chem. 100 (1996) 11242. [10] V. Calsavara, E.. Falabella Sousa-Aguiar, N.R.C. Fernandes Machado, Zeolites 17 (1996) 340. [11] D. Lin, S. Bao, Q. Xu, Zeolites 18 (1997) 162. [12] C. Yang, Q. Xu, J. Chem. Soc., Faraday Trans. 93 (1997) 1675. [13] G.L. Woolery, G.H. Kuehl, H.C. Timken, A.W. Chester, J.C. Vartuli, Zeolites 19 (1997) 288. [14] W. Lutz, W. Gessner, R. Bertram, I. Pitsch, R. Fricke, Microporous Mater. 12 (1997) 131. [15] P.J. Kooyman, P. van der Waal, H. van Bekkum, Zeolites 18 (1997) 50. [16 ] T. Sano, R. Tadenuma, Z.B. Wang, K. Soga, J. Chem. Soc., Chem. Commun. (1997) 1945. [17] T. Sano, R. Tadenuma, Y. Uno, Z.B. Wang, K. Soga, in: M.M.J. Treacy, B. Marcus, J.B. Higgins, M.E. Bisher (Eds.), Proc. 12th Int. Zeolite Conf., Material Research Society (1999) 2743. [18] Y. Iwami, T. Sano, Y. Kawakami, Nippon Kagaku Kaishi (1996) 303. [19] S.M. Alexander, D.M. Bibby, R.F. Howe, R.H. Meinhold, Zeolites 13 (1993) 441. [20] L.D. Fernandes, P.E. Bartl, J.L.F. Monteiro, J.G. Silva, S.C. Menezes, M.J.B. Cardoso, Zeolites 14 (1994) 533. [21] K. Yamagishi, S. Namba, T. Yashima, J. Catal. 121 (1990) 47. [22] W.O. Haag, Stud. Surf. Sci. Catal. 84 (1994) 1375. [23] R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt, G.T. Kerr, in: Y. Murakami, A. Iijima, J.W. Ward ( Eds.), Proc. 7th Int. Zeolite Conf., Kodansha (1986) 677. [24] N.Y. Topsoe, F. Joensen, E.G. Derouane, J. Catal. 110 (1988) 404. [25] Y. Sendoda, Y. Ono, Zeolites 8 (1988) 101. [26 ] D.B. Lukyanov, Zeolites 11 (1991) 325. [27] S. Namba, A. Inake, T. Yashima, Zeolites 6 (1986) 107. [28] E. Loeffler, U. Lohse, Ch. Peuker, G. Oehlmann, L.M. Kustov, V.L. Zholobenko, V.B. Kazansky, Zeolites 10 (1990) 266. [29] Y. Iwami, Y. Nakajima, T. Sano, Y. Kawakami, Nippon Kagaku Kaishi (1996) 617. [30] J. Sanz, V. Fornes, A. Corma, J. Chem. Soc., Faraday Trans. 184 (1988) 3113. [31] J.-P. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie, M.P. Shatlock, J. Chem. Soc., Chem. Commun. (1987) 91.
Realumination of dealuminated HZSM-5 zeolites by acid treatment and their catalytic properties T. Sano *, Y. Uno, Z.B. Wang, C.-H. Ahn, K. Soga School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan Received 21 December 1998; accepted for publication 15 February 1999
Abstract Effects of acid treatment variables on the realumination efficiency for realumination of dealuminated HZSM-5 zeolite (reinsertion of non-framework aluminums into the zeolite framework) were investigated using 27Al MAS NMR and nitrogen adsorption. Among various mineral acids employed, HCl solution was the most effective for the realumination. The realumination efficiency was also studied, evaluating the change in the cracking activity of cumene. The cumene cracking activity of the realuminated HZSM-5 zeolite was comparable to that of the parent zeolite, whose framework SiO /Al O ratio was the same as that of the realuminated zeolite. From the results obtained, it 2 2 3 was found that no structural degradation of HZSM-5 zeolite crystals takes place during the acid treatment and that some of the non-framework aluminums in dealuminated HZSM-5 zeolites are reinserted into the framework. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Acid treatment; HZSM-5; Non-framework aluminum; Reinsertion
1. Introduction In general, the acidic and catalytic properties of zeolites are known to be effectively related to the content of tetrahedrally coordinated framework aluminums. An irreversible loss of catalysis efficiency observed after a number of reaction– regeneration cycles is attributable to the removal of framework aluminums from the zeolite lattice [1]. Therefore, realumination of dealuminated zeolites has received considerable attention from a standpoint of regeneration of zeolite catalyst as well as control of physicochemical properties of zeolites such as thermal stability and ion-exchange * Corresponding author. Tel.: +81-761-511611; Fax: +81-761-511625. E-mail address: [email protected] ( T. Sano)
abilities. There are a large number of papers concerning the realumination of dealuminated zeolites by treatment with alkali solutions, suggesting the reinsertion of non-framework aluminums into the framework [2–14]. However, dissolution of a part of zeolite framework has been pointed out. Kooyman et al. recently reported that dealumination of HZSM-5 zeolite is not straightforward in the acid solution [15]. Very recently, we found that the reinsertion of non-framework aluminums in dealuminated HZSM-5 zeolites takes place by treatment with HCl solution at 100°C [16,17]. As it is recognized that leaching with mineral acid is one of the dealumination procedures for dealumination of Al-rich zeolites such as Y and mordenite zeolites, realumination by the acid treatment may be a very puzzling phenomenon. In this paper, in order to develop a more systematic understanding
1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 05 9 - 1
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T. Sano et al. / Microporous and Mesoporous Materials 31 (1999) 89–95
of the realumination of dealuminated HZSM-5 zeolites by treatment with acid solutions, the effects of acid treatment variables on the realumination efficiency for reinsertion of non-framework aluminums into the zeolite framework and the catalytic performance of realuminated zeolite were investigated.
2. Experimental 2.1. Zeolite samples and treatment HZSM-5 zeolite with 69 of a bulk SiO /Al O 2 2 3 ratio was prepared following the procedure described previously [18]. Dealumination of the HZSM-5 zeolite was carried out by thermal treatment at 600°C for 48 h using a muffle furnace. Acid treatment was carried out as follows: 1 g of the dealuminated HZSM-5 zeolite was treated with 100 ml of acidic solution at 100°C under stirring for a given period of time. The product was filtered off, washed thoroughly with deionized water, dried at 100°C and calcined at 380°C for 8 h. Acetylacetone was adsorbed onto the parent and treated HZSM-5 zeolites to try to complex non-framework aluminums including the NMR invisible species [19,20]. The samples were dried in an oven at 380°C to remove adsorbed water, and transferred to a desiccator, containing acetylacetone, and then left to equilibrate over 5 days at room temperature. 2.2. Characterization The identification of zeolites obtained was achieved by X-ray diffraction (Rigaku RINT 2000). The chemical composition was measured by X-ray fluorescence ( XRF, Philips PW2400). Textural properties were determined by nitrogen adsorption (Bel Japan Belsorp 28SA). Before adsorption measurements at −196°C, the powdered zeolites (c. 0.1 g) were evacuated at 400°C for 12 h. 27Al MAS NMR spectra were recorded on a Varian VXP-400 spectrometer at 104.3 MHz with a c. 4.5-kHz spinning speed and 1.73-ms pulses for 4000 scans. The chemical shift reference was hydrated Al3+ in solid aluminum nitrate enneahy-
drate. The surface composition was measured by X-ray photoelectron spectroscopy ( XPS, UlvacPhi 5600). The content of coke was measured using a Perkin-Elmer TGA 7 thermal analyzer (air flow: 40 ml min−1; heating rate: 10°C min−1). 2.3. Catalytic evaluation Cracking of cumene was carried out in an atmospheric pressure flow system. The zeolite catalyst placed into a quartz tube reactor of a 10-mm inner diameter was dehydrated at 400°C for 1 h in a nitrogen stream. The temperature was then brought into a reaction temperature (250°C ). The reactant was fed into the catalyst bed with a microfeeder. Nitrogen was used as a carrier gas (40 ml min−1). The contact time ( W/F ) was varied from 0.4 to 1.9 g h mol−1 by changing the weight of catalyst, and the partial pressure of cumene was 7.9 kPa. An on-line product analysis was performed on a Shimadzu GC-17A gas chromatograph (FID) with a GL-Science TC-1 capillary column (30 m).
3. Results and discussion 3.1. Effect of acid treatment variables The key acid treatment variables studied included a kind of acid, acid concentration, treatment time and treatment temperature. The contents of framework aluminums of HZSM-5 zeolites before and after the acid treatment were determined from the spectra of 27Al MAS NMR using a calibration curve, which was produced by measuring the 27Al MAS NMR spectra of parent HZSM-5 zeolites with various SiO /Al O ratios 2 2 3 [18]. The dealuminated HZSM-5 zeolites with 120–140 of framework SiO /Al O ratios were 2 2 3 prepared by thermal treatment at 600°C for 48 h of the HZSM-5 zeolite with 69 of the framework SiO /Al O ratio and used as a sample for the acid 2 2 3 treatment. At first, the effect of a kind of mineral acid solution on the realumination efficiency was investigated using the dealuminated HZSM-5 zeolite with 121 of the framework SiO /Al O ratio. 2 2 3 Table 1 (Nos 1–4) shows the degree of remaining
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T. Sano et al. / Microporous and Mesoporous Materials 31 (1999) 89–95 Table 1 Realumination of dealuminated HZSM-5 zeolite with various acidic solutionsa Run no.
Treatment solution
Concentration (M )
Degree of reamaining framework aluminums (%)
Micropore volume W (N )b [cm3( liquid ) g−1] 0 2
1 2 3 4 5 6 7 8 9c
HCl H SO 2 4 HNO 3 CH COOH 3 AlCl 3 Al(NO ) 33 Al (SO ) 2 43 NH NO 4 3 –
2 1 2 2 2 2 1 2 –
81.0 71.2 71.2 69.3 62.2 66.9 67.1 66.2 58.2
0.183 0.189 0.185 – – 0.187 0.187 – 0.192
a Treatment conditions: Temperature=100°C, Time=120 h. b Determined by the Dubinin–Radushkevich (D–R) equation. c Before acid treatment (dealumianted HZSM-5).
framework aluminums after the acid treatment, which is defined as [Al/(Si+Al )] / treated zeolite [Al/(Si+Al )] . The degree of remaining frameparent work aluminums is dependent on the type of mineral acid. Among the mineral acids employed, the treatment with HCl solution exhibited the highest realumination efficiency. The exact reason for the high realumination efficiency of HCl solution is not clear at present due to limited data. The results of realumination of the dealuminated HZSM-5 zeolite with various aqueous solutions of Al-containing compounds and ammonium nitrate are also summarized in Table 1 (Nos 5–8). A slight increase in the degree of remaining framework aluminums was observed. To check the structural degradation of zeolite crystals during the realumination treatment, nitrogen adsorption isotherms of realuminated zeolites were measured, and the micropore volumes were calculated. As shown in Table 1, no differences in micropore volume [ W (N )] determined by the 0 2 Dubinin–Radushkevich (D–R) equation were observed between dealuminated and realuminated HZSM-5 zeolites, indicating no structural degradation. From these results, HCl solution was selected in the following acid-treatment experiments. The variations in the degree of remaining framework aluminums and the micropore volume [ W (N )] with HCl concentration are illustrated 0 2 in Fig. 1. The dealuminated HZSM-5 zeolite with 139 of the framework SiO /Al O ratio was used 2 2 3
Fig. 1. Effect of HCl concentration on realumination efficiency. Treatment conditions: Temperature=100°C; Time=120 h.
as a sample. The degree of the remaining framework aluminums increased markedly with an increase in HCl concentration and reached a constant value at more than 2 M HCl. The micropore volumes [ W (N )] hardly changed, even after 4 M 0 2 HCl treatment. The effect of treatment time on the realumination efficiency was also studied using the dealuminated HZSM-5 zeolite with 139 of the framework SiO /Al O ratio, and the result is 2 2 3 illustrated in Fig. 2. The degree of remaining framework aluminums increased with treatment time and became constant after more than 120 h. The reduction in micropore volume due to structural degradation of zeolite crystals hardly
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that the removal of framework silicon atoms hardly takes place during the acid treatment, indicating the high stability of HZSM-5 zeolite in the acid solution. Therefore, it can be concluded that the realumination of dealuminated zeolite occurs during the acid treatment, although the degree of reinsertion of non-framework aluminums into the zeolite framework is not so high. 3.2. Catalytic performance
Fig. 2. Effect of HCl treatment time on realumination efficiency. Treatment conditions: Temperature=100°C, HCl concentration: 2 M.
occurred, even after a long period of treatment of more than 300 h. As it is possible that the increase in the framework SiO /Al O ratios of the realuminated 2 2 3 HZSM-5 zeolites may be attributable to the removal of the framework silicon atoms, the influence of treatment conditions on dissolution of zeolite framework was investigated. The silicon and aluminum concentrations in the liquid phase of HCl treatment were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP, Seiko SPS7700), and the degrees of elution of silicon and aluminum atoms from the dealuminated zeolite are listed in Table 2. It became clear
Next, from a standpoint of regeneration of dealuminated zeolite catalyst, the catalytic properties of the realuminated HZSM-5 zeolites with the HCl treatment were evaluated by the cumene cracking reaction. The conversion profiles of cumene on various HZSM-5 zeolites at 250°C are illustrated in Fig. 3. There are significant differences between these zeolites with respect to initial activity and deactivation, showing that the HCl treatment undoubtedly has an influence on the catalytic property of zeolite. The initial activity for cumene cracking of the dealuminated zeolite was improved by the HCl treatment. Of course, the activity was not completely recovered. For parent HZSM-5 zeolite, it has been found that a linear correlation between activity and concentration of Brønsted acidic site [bridged hydroxyl of
Table 2 Degrees of elustion of silicon and aluminum atoms from dealuminated HZSM-5 zeolites during acid treatment Treatment conditions
Degree of elutiona
Solution
Time (h)
Si (at.%)
Al (at.%)
0.5 M HCl 1 M HCl 2 M HCl 2 M HCl 4 M HCl 1 M H SO 2 4 2 M HNO 3 2 M CH COOH 3 2 M NH NO 4 3
120 120 120 360 120 120 120 120 120
1.80 1.48 1.56 1.27 0.78 1.36 1.07 1.23 1.03
6.57 7.34 7.98 11.95 9.23 5.09 4.36 3.56 2.36
a Determined by ICP.
Fig. 3. Conversion profiles of cumene on various HZSM-5 zeolites. Reaction conditions: Temperature=250°C; W/F=1.88 g h mol−1; partial pressure of cumene=7.9 kPa. #: Parent HZSM-5; $: Dealuminated HZSM-5 (600°C, 48 h); %: Realuminated HZSM-5 (2 M HCl, 100°C, 120 h).
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Si(OH )Al ] is observed in several hydrocarbon conversions such as cumene and hexane cracking reactions [21,22]. Therefore, the increase in activity seems to indicate the partial regeneration of dealuminated HZSM-5 zeolite catalyst. It is well recognized that an enhancement of catalytic activity is observed for zeolites dealuminated under mild conditions due to a synergistic interaction between Brønsted acidic sites in the zeolite and any type of Lewis acidic sites from non-framework aluminum species, resulting in an increase in acidic strength of bridged hydroxyls (e.g. [23–26 ]). Thus, in order to clarify the reason for the increase in the initial conversion for cumene cracking by the acid treatment, the rate constants for cumene cracking on realuminated HZSM-5 zeolites were compared with those on parent HZSM-5 zeolites. It is difficult to determine the initial conversion by extrapolating the curve of cumene conversion vs. time on stream to zero time due to the rapid change in conversion with time on stream in the initial reaction stage. Therefore, the conversion of the first pulse sampling was regarded as the initial conversion, X . Assuming 0 first-order kinetics, −ln(1−X ) was plotted 0 against the contact time ( W/F ), to obtain a linear plot, the slope of which was a first-order rate constant. Fig. 4 shows the relationship between
the framework aluminum (Brønsted acidic site) content and the first-order rate constant of cumene cracking. It is evident that a good linear correlation exists between the rate constant and the framework aluminum content in parent HZSM-5 zeolites, indicating that all the acidic sites have an equal activity. A considerable decrease in the rate constant for cumene cracking was observed for dealuminated HZSM-5, whereas an increase in the rate constant for the realuminated zeolite was observed. Data for the dealuminated and realuminated HZSM-5 zeolites are fitted by a common straight line of parent HZSM-5, suggesting that new acidic sites with a higher acidity are not generated by the HCl treatment. As shown in Fig. 3, the steady-state activity of realuminated HZSM-5 zeolite after 45 min on stream was higher than that of the parent or the dealuminated zeolite. The amounts of coke on the parent, dealuminated and realuminated zeolites after 2 h on stream measured by thermogravimetric analysis were 4.8, 3.1 and 3.8 wt%, respectively. To clarify the difference in deactivation behavior, the external surface SiO /Al O ratio of zeolite 2 2 3 was measured by XPS ( Table 3). No difference in either external surface or bulk SiO /Al O ratios 2 2 3 was observed between the parent and dealuminated zeolites. A large difference in external surface SiO /Al O ratio was observed between the 2 2 3 parent and realuminated zeolites, whereas a slight difference was observed in bulk SiO /Al O ratio. 2 2 3 Taking into account that the coke deposition on external surfaces of zeolite crystal is mainly related to the deactivation, the difference in the steadystate activity between the parent and the realuminated zeolites may be attributable to the difference Table 3 Surface and bulk SiO /Al O ratios of various HZSM-5 zeolites 2 2 3 Zeolite
Fig. 4. Relationship between framework aluminum content and first-order rate constant for cumene cracking at 250°C. #: Parent HZSM-5; $: Dealuminated HZSM-5 (600°C, 48 h); %: Realuminated HZSM-5 (2 M HCl, 100°C, 120 h)
Parent HZSM-5 Dealuminated HZSM-5 Realuminated HZSM-5 a Determined by XPS. b Determined by XRF.
SiO /Al O ratio 2 2 3 Surface a
Bulkb
58 58 86
69 69 74
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in surface aluminum content, probably surface framework aluminum content. A similar phenomenon was also reported by Namba et al. [27]. These results provide strong evidence of the partial reinsertion of non-framework aluminums in the dealuminated HZSM-5 into the zeolite framework by the acid treatment, although the selective removal of non-framework aluminums on external surfaces of zeolite crystals proceeds. 3.3. Plausible mechanism of realumination The realumination treatment was conducted using HCl solution at 100°C, followed by thermal treatment at 380°C. In order to clarify the realumination process, the framework aluminum concentrations of realuminated HZSM-5 zeolites before and after the thermal treatment were measured using 27Al MAS NMR. No difference was observed between the framework aluminum concentrations of zeolites before and after the thermal treatment, indicating that the realumination takes place during HCl treatment at 100°C. It is well known that the non-framework aluminum is not uniform and exists in different states [28,29]. Therefore, to obtain further information concerning the realumination process, the chemical state of non-framework aluminum species in the dealuminated HZSM-5 zeolites before and after HCl treatment was investigated using the 27Al MAS NMR. Acetylacetone was used to visualize
NMR invisible aluminum species in the dealuminated zeolite. In the 27Al MAS NMR spectrum of dealuminated HZSM-5 zeolite, as shown in Fig. 5, a very weak peak at c. 30 ppm and a sharp peak at c. 53 ppm assigned to framework aluminums were observed. It has been considered that the 30-ppm peak is assigned to non-framework aluminum species where the aluminum atom is connected to the zeolite framework only by one or two remaining chemical bonds, probably either aluminum species in a highly distorted tetrahedral environment or a penta-coordinated aluminum species [30,31]. The peak intensity at 30 ppm decreased as a result of HCl treatment, as shown in Fig. 5. The reinsertion of framework aluminums hardly takes place on HZSM-5 zeolite dealuminated under severe hydrothermal treatment condition [17], in which the complete extraction of aluminum atoms from the framework seems to proceed. Therefore, this result seems to suggest that realumination with acid treatment proceeds mainly through the reinsertion of non-framework aluminum species, where the aluminum atom is connected to the zeolite framework only by one or two remaining chemical bonds. Of course, some of the non-framework aluminums also dissolves into the solution during the acid treatment, as described above. From the above results, as well as the fact that the alumination of silicalite with an aqueous solution of AlCl did not take place, we speculate that 3
Fig. 5. 27Al MAS NMR spectra of dealuminated HZSM-5 zeolites before and after HCl treatment. (A) Parent HZSM-5; (B) Dealuminated HZSM-5 (600°C, 48 h); (C ) Realuminated HZSM-5 (2 M HCl, 100°C, 120 h).
T. Sano et al. / Microporous and Mesoporous Materials 31 (1999) 89–95
Fig. 6. Plausible mechanism of realumination of dealuminated HZSM-5 zeolite by acid treatment.
the realumination mechanism by acid treatment proceeds through the reinsertion not of non-framework aluminums completely removed from the zeolite lattice but of those connected to the lattice, as shown in Fig. 6.
4. Conclusions Although the degree of reinsertion of framework aluminums into the zeolite framework is not so high, it was found that some of the nonframework aluminums in dealuminated HZSM-5 zeolite are reinserted into the framework by acid treatment; in particular, HCl solution is effective. The reinsertion of non-framework aluminums was also confirmed from cumene cracking.
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