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Modification of HZSM-5 by Diazomethane
165
Modification of HZSM-5 by Diazomethane
Gen-min Lu, Song-Ying Chen and Shao-yi Peng Institute of Coal Chemistry, Academia Sinica, Taiwan, Shanxi, 03OOO1, People's Republic of China
ABSTRACT
The zeolite HZSM-5 was modified by the methylation of the surface protonic hydroxvl groups with diazonethane. After d i fication, i.r. peaks of surface methoxyl groups appeared at around 2970-2860 cm-', which corresponded to two decomposition peaks (295' and 535°C) in Ar and vere assigned to two states of surface methoxyl groups. The amount of irreversibly adsorbed pyridine and surface area dropped to one third of the original values after modification, while the change in catalytic activity for cumene cracking was not parallel to that of the acidity due to the decomposition of surface methoxyl groups under the reaction conditions. The surface Si :A1 ratio of nodif ied catalyst decreased to about one half its original value, which may be due t o the Migration of amorphous aluminum oxide in the pores t o the surface of the zeolite during the modification.
INTRODUCTION
In recent Years, Plodification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Pen8 (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (A1,0,,SiOJ and zeolite catalysts changed significantly by diazomethane modification. In the present paper. the results of modification of HZSM-5 by diazomethane and the influence of modification on its properties of surface acidity, porosity and catalytic activity are reported.
EPER IMENTAL
HZSM-5 vas prepared through ammonium exchange of the sodiur form (AF-5, Si :A1
=W).Four successive exchanges were carried out for lh with a 1M NH,CI solution at 96°C. After washing and drying, the NH,ZSM-5 was calcinated for 4h at 540°C in air.
166 G:m.
Lu, S.-y. Chen and S.-y. Peng
Diazomethane was synthesized from CH,NH,HCl in ethyl ether and the solution was used to react with HZSH-5. The modification reaction was carried out at 0-C until no further color change of the diazomethane solution was observed. The modified catalyst was dried first at aabient temperature in air and then at 120°C in Ar for 2h. The surface methoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabi 1 ities were studied by TemperatureProgrammed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed a m n i a was desorbed in Ar from R.T. to 550°C by 'PD at 1BoC/min. The same procedure was used for the TPDE of surface species on modified catalyst. The pretreated catalysts were contacted with pyridine gas at anbient temperature for one week, then the physically adsorbed pyridine was evacuated. The samples with and without pyridine adsorption were used for FSCA. The bonding energy of O,,, N,,, Si,,, Al,, and C,, and their relative amounts were measured. The specific surface area was measured by nitrogen adsorption at -195OC. The cumene cracking reaction was conducted by pulse technique under the following conditions: 0.10 catalyst, H, flow rate 75 mlhin, Pulse volume 1 ul.
RESULTS AND DISCUSSION
..
PorlaationddecolaDosltlonnfsurfacemethoxvlnrouDs Figure 1 shows the i.r. spectra of surface species on diazoaethane-modified HZSH-5. Three peaks appeared at around 2970-2860 CI-', indicating that surface methoxyl groups were formed during the modification reaction:
where Z-OH represented the protonic acidic site on the zeolite surface. Morrow (3) observed similar i.r. spectra for methanol adsorbed on SiO,, and the peaks were assigned to two states of the surface methoxvl groups. The 'PDE result in Figure 2 indicates two decomposition peaks at 295- and 535OC for the surface species, which were consistent with the above i.r. results. We could therefore assign the i.r. peaks at around 2970-2860 cm-' of modified HZSM-5 to two types of surface methoxyl groups which exibited different thermal stability. The TPDE product before 295'C was methane while the Products consisted of C,, C, and other compounds at higher temperatures, implying that at lower temperatures,
Modification of HZSM-5 by Diazomethane
167
the surface was demthylated via 2 (Z-O-CH,)
--------+
Z(Z-OH)
+
C
+
CH,
(2)
and the complex surface reactions took place at higher temperatures.
3200
3000
2800
cm-1
Figure 1. IR spectra of methoxyl groups on modified HZSW-5.
0
200
Figure 2. Temperature-Programmed Decomposition of surface methoxyl groups in Ar. Influenced' M the P r o wt h n f H Z S M - 5 As shown in Table 1, the aaount of irreversibly adsorbed pyridine dropped to one third its original value after aodification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of BPY peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. Ihe results in Table 1 aslo show that the drop in acidity paralleled that
168 G.-m. Lu, S.-y. Chen and S.-y. Peng
of the specific surface area. The XRD results indicate that the modification had little influence on the structure of HZSM-5. The drop in specific surface area after modification may be considered as the result of the formation of methoxyl groups in the pores of zeolite.
Original Modified Demethylated
0.530 0.170 0.718
444.7 117.7 420.5
100
70 >loo
a rmaol/g, 120°C; b 3OOOC; c 450"C, Ar, 2h.
Figure 3. TPD of irreversibly adsorbed ammonia on untreated (-1 and modified (-.-I HZSM-5 in Ar, The influence of the modification on the surface atomic ratios of Si :A1 as measured by ESCA is shown in Table 2. The enrichment of alumina was observed for modified HZSW-5. The surface Si:Al ratio was decreased t o about one half its original value. It seeled impossible for diazomethane modification to remove the framework alumina of the zeolite. We assumed that the enrichment was caused by the migration of the amorphous aluminum oxide to the surface of the zeolite. The mechanism should be studied further in detail.
Modification of HZSM-5 by Diazornethane 169
It is worth noting that the change in catalytic activity for cumene cracking after modification did not parallel the change of acidity in Table 1. This may be due to the difference in temperatures for adsorption and reaction or the partial decomposition of the surface methoxyl groups under reaction conditions (Figure 2 and equation (2)). It was also observed (Table 1) that the acidity and the catalytic reactivity for cuaene cracking increased when the aodif ied zeolite was treated at higher temperatures (450"C), indicating that the acidic property of the surface hydroxyl groups was enhanced by denethylation (2,4). Based on the above results, we conclude that the diazomethane modification of zeolites is an effective aethod to change selectively the anount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature (<3OO0C) catalytic reactions.
ACKNOWLEDGEWENTS
We wish to thank WS. Ju-aing Hong of Beijing University for assistance of ESCA. This research was supported by the Chinese National Nature Scientific Foundation and partially by the Zhong Guancun Foundation of Instrumental Measurements. REFERENCES 1. Wen-juan Yin and Shao-yi Peng, in Y. Murakaai, A. Iijima and J. W. Ward ( E d s . ) , New Developaents in Zeolite Science and Technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22, 1986), Kodansha/Elsevier,Tokuo/ksterdam, 1986. 2. Wen-Juan Yin and Shao-yi Peng, J. Fuel Chem. Techn. (Chinese), 1509871, 205. 3. B. A. Horrow, J. Chem. SOC. Faraday Trans I, 70(1974), 1527. 4. E. Borello, A. Zecchina and C. Morterra, J. Phys. Cher., 71(1967), 2938.
Modification of HZSM-5 by Diazomethane
Gen-min Lu, Song-Ying Chen and Shao-yi Peng Institute of Coal Chemistry, Academia Sinica, Taiwan, Shanxi, 03OOO1, People's Republic of China
ABSTRACT
The zeolite HZSM-5 was modified by the methylation of the surface protonic hydroxvl groups with diazonethane. After d i fication, i.r. peaks of surface methoxyl groups appeared at around 2970-2860 cm-', which corresponded to two decomposition peaks (295' and 535°C) in Ar and vere assigned to two states of surface methoxyl groups. The amount of irreversibly adsorbed pyridine and surface area dropped to one third of the original values after modification, while the change in catalytic activity for cumene cracking was not parallel to that of the acidity due to the decomposition of surface methoxyl groups under the reaction conditions. The surface Si :A1 ratio of nodif ied catalyst decreased to about one half its original value, which may be due t o the Migration of amorphous aluminum oxide in the pores t o the surface of the zeolite during the modification.
INTRODUCTION
In recent Years, Plodification of zeolites, such as HZSM-5, by phosphoric compounds or metal oxides has been extensively studied, but little information is available on the modification of zeolites by diazomethane, which is an excellent methylating agent for protonic acidic sites. It is capable of entering into the small pores of zeolites because of its small molecular size and linear molecular structure. Yin and Pen8 (1,2) reported that the acidity and specific surface area of the inorganic oxide supports (A1,0,,SiOJ and zeolite catalysts changed significantly by diazomethane modification. In the present paper. the results of modification of HZSM-5 by diazomethane and the influence of modification on its properties of surface acidity, porosity and catalytic activity are reported.
EPER IMENTAL
HZSM-5 vas prepared through ammonium exchange of the sodiur form (AF-5, Si :A1
=W).Four successive exchanges were carried out for lh with a 1M NH,CI solution at 96°C. After washing and drying, the NH,ZSM-5 was calcinated for 4h at 540°C in air.
166 G:m.
Lu, S.-y. Chen and S.-y. Peng
Diazomethane was synthesized from CH,NH,HCl in ethyl ether and the solution was used to react with HZSH-5. The modification reaction was carried out at 0-C until no further color change of the diazomethane solution was observed. The modified catalyst was dried first at aabient temperature in air and then at 120°C in Ar for 2h. The surface methoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabi 1 ities were studied by TemperatureProgrammed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed a m n i a was desorbed in Ar from R.T. to 550°C by 'PD at 1BoC/min. The same procedure was used for the TPDE of surface species on modified catalyst. The pretreated catalysts were contacted with pyridine gas at anbient temperature for one week, then the physically adsorbed pyridine was evacuated. The samples with and without pyridine adsorption were used for FSCA. The bonding energy of O,,, N,,, Si,,, Al,, and C,, and their relative amounts were measured. The specific surface area was measured by nitrogen adsorption at -195OC. The cumene cracking reaction was conducted by pulse technique under the following conditions: 0.10 catalyst, H, flow rate 75 mlhin, Pulse volume 1 ul.
RESULTS AND DISCUSSION
..
PorlaationddecolaDosltlonnfsurfacemethoxvlnrouDs Figure 1 shows the i.r. spectra of surface species on diazoaethane-modified HZSH-5. Three peaks appeared at around 2970-2860 CI-', indicating that surface methoxyl groups were formed during the modification reaction:
where Z-OH represented the protonic acidic site on the zeolite surface. Morrow (3) observed similar i.r. spectra for methanol adsorbed on SiO,, and the peaks were assigned to two states of the surface methoxvl groups. The 'PDE result in Figure 2 indicates two decomposition peaks at 295- and 535OC for the surface species, which were consistent with the above i.r. results. We could therefore assign the i.r. peaks at around 2970-2860 cm-' of modified HZSM-5 to two types of surface methoxyl groups which exibited different thermal stability. The TPDE product before 295'C was methane while the Products consisted of C,, C, and other compounds at higher temperatures, implying that at lower temperatures,
Modification of HZSM-5 by Diazomethane
167
the surface was demthylated via 2 (Z-O-CH,)
--------+
Z(Z-OH)
+
C
+
CH,
(2)
and the complex surface reactions took place at higher temperatures.
3200
3000
2800
cm-1
Figure 1. IR spectra of methoxyl groups on modified HZSW-5.
0
200
Figure 2. Temperature-Programmed Decomposition of surface methoxyl groups in Ar. Influenced' M the P r o wt h n f H Z S M - 5 As shown in Table 1, the aaount of irreversibly adsorbed pyridine dropped to one third its original value after aodification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of BPY peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. Ihe results in Table 1 aslo show that the drop in acidity paralleled that
168 G.-m. Lu, S.-y. Chen and S.-y. Peng
of the specific surface area. The XRD results indicate that the modification had little influence on the structure of HZSM-5. The drop in specific surface area after modification may be considered as the result of the formation of methoxyl groups in the pores of zeolite.
Original Modified Demethylated
0.530 0.170 0.718
444.7 117.7 420.5
100
70 >loo
a rmaol/g, 120°C; b 3OOOC; c 450"C, Ar, 2h.
Figure 3. TPD of irreversibly adsorbed ammonia on untreated (-1 and modified (-.-I HZSM-5 in Ar, The influence of the modification on the surface atomic ratios of Si :A1 as measured by ESCA is shown in Table 2. The enrichment of alumina was observed for modified HZSW-5. The surface Si:Al ratio was decreased t o about one half its original value. It seeled impossible for diazomethane modification to remove the framework alumina of the zeolite. We assumed that the enrichment was caused by the migration of the amorphous aluminum oxide to the surface of the zeolite. The mechanism should be studied further in detail.
Modification of HZSM-5 by Diazornethane 169
It is worth noting that the change in catalytic activity for cumene cracking after modification did not parallel the change of acidity in Table 1. This may be due to the difference in temperatures for adsorption and reaction or the partial decomposition of the surface methoxyl groups under reaction conditions (Figure 2 and equation (2)). It was also observed (Table 1) that the acidity and the catalytic reactivity for cuaene cracking increased when the aodif ied zeolite was treated at higher temperatures (450"C), indicating that the acidic property of the surface hydroxyl groups was enhanced by denethylation (2,4). Based on the above results, we conclude that the diazomethane modification of zeolites is an effective aethod to change selectively the anount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature (<3OO0C) catalytic reactions.
ACKNOWLEDGEWENTS
We wish to thank WS. Ju-aing Hong of Beijing University for assistance of ESCA. This research was supported by the Chinese National Nature Scientific Foundation and partially by the Zhong Guancun Foundation of Instrumental Measurements. REFERENCES 1. Wen-juan Yin and Shao-yi Peng, in Y. Murakaai, A. Iijima and J. W. Ward ( E d s . ) , New Developaents in Zeolite Science and Technology (Proc. 7th Int. Zeolite Conf., Tokyo, August 17-22, 1986), Kodansha/Elsevier,Tokuo/ksterdam, 1986. 2. Wen-Juan Yin and Shao-yi Peng, J. Fuel Chem. Techn. (Chinese), 1509871, 205. 3. B. A. Horrow, J. Chem. SOC. Faraday Trans I, 70(1974), 1527. 4. E. Borello, A. Zecchina and C. Morterra, J. Phys. Cher., 71(1967), 2938.