Ind. Eng. Chem. Res. 2010, 49, 2103–2106
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Methanol to Olefin over Ca-Modified HZSM-5 Zeolites Suhong Zhang,*,†,‡ Bianling Zhang,†,‡ Zhixian Gao,† and Yizhuo Han† Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, P. R. China
A series of HZSM-5 zeolites modified with various Ca loadings were prepared by an incipient impregnation technique and characterized by Fourier transform infrared spectra of adsorbed pyridine and temperatureprogrammed desorption of NH3 and CO2. Methanol to olefin (MTO) over Ca-modified HZSM-5 zeolites was performed in a flow-type fixed bed reactor. The results indicated that Ca modification increased both the catalytic stability and the light olefin selectivity. However, an excess Ca loading resulted in an inferior catalytic performance. Hence, there was an optimal Ca loading for a specific HZSM-5, and the thus prepared Ca/ HZSM-5 had little free Bro¨nsted acid sites and presented the best catalytic performance of MTO. A possible mechanism for MTO involving acid-base centers on Ca-modified HZSM-5 zeolites has been suggested on the basis of the results obtained. 1. Introduction
2. Experimental Section
The methanol to olefin (MTO) process provides an alternative route for the production of light olefins such as ethylene and propylene which are important chemicals for the chemical industry. Therefore, this process has received great attention because of the shortage of crude oil in the foreseeable future. Since this reaction was discovered in early 1970s, the development of the process and the search for more efficient catalysts have become a subject of intense study for decades.1-3 In particular, ZSM-5 zeolite with special pore structure shows excellent resistance to coking and has been proven to be one effective catalyst for the conversion of MTOs under the relatively mild conditions.3,4 There are many reports about the transformation of methanol with modified HZSM-5 zeolites.5-11 The MTO catalytic performance can be increased over modified ZSM-5 with alkaline earth metal, especially Ca. Calcium phosphate-modified ZSM-5 showed that the catalyst life was prolonged and the light olefin selectivity was improved.12 Al-Jarallah et al.9 modified ZSM-5 by impregnation with various metal salts. Especially, Camodified HZSM-5 catalyst exhibited the highest light olefin selectivity (68.1%) at the methanol conversion of 85.9%. They thought that the increase of light olefin selectivity was due to reduction of the apparent pore size of ZSM-5 zeolite by modification. Zhang et al.13 found that the catalytic stability and the light olefin selectivity were significantly increased over Ca-modified HZSM-5. In a word, the catalytic performance was enhanced by Ca modification. However, the influence of Ca loading on the catalyst acidity and the catalytic performance and the role of Ca, have not been studied. In this paper, HZSM-5 zeolites modified with different Ca loading were characterized in detail and used as catalysts for MTO aiming at exploring the above points. The increased catalytic performance was correlated with the formation of active sites on modified HZSM-5 zeolite. A possible mechanism for MTO involving acid-base centers on Ca-modified HZSM-5 zeolites has been suggested on the basis of the results obtained.
2.1. Catalyst Preparation. HZSM-5 zeolites (Si/Al2 ) 48) were impregnated with calcium nitrate solution and kept at room temperature overnight. Subsequently the samples were dried at 120 °C for 4 h and then calcined at 540 °C for 4 h. The obtained samples were hereafter denoted as xCa/HZSM-5, where x was the element loading in weight percent. 2.2. Catalyst Characterization. The nature of acid sites was investigated by Fourier transform infrared spectra of adsorbed pyridine (Py-FTIR) in an infrared quartz cell with CaF2 windows. Self-supporting wafer of the sample was pretreated at 400 °C for 90 min under a high vacuum, and then cooled to room temperature. The background of the sample was recorded. Pyridine pulses were injected until the sample was saturated. The temperature of the sample was increased to 200 °C, and excess pyridine was desorbed at 200 °C for 60 min. Finally, the spectra were recorded on a BRUKER Vector 22 spectrometer. The temperature-programmed desorption of ammonia (NH3TPD) was recorded on a homemade TPD apparatus. A given amount of the sample, 0.2 g, was pretreated in flowing nitrogen at 550 °C for 30 min, cooled to 100 °C, and then saturated by ammonia. The sample was subsequently purged with nitrogen at 100 °C for 30 min to remove physisorbed NH3. The operation was conducted in following nitrogen (40 mL/min) from 100 to 550 °C at a rate of 10 °C/min. CO2-TPD was used to examine the basicity of the modified samples in a procedure similar to NH3-TPD. 2.3. Catalyst Testing. Experiments were carried out at 520 °C in a flow-type fixed bed reactor under atmospheric pressure. The catalyst charge was 3.0 g, the feed was 40 wt % methanol in water and the methanol weight hourly space velocity (WHSV) was 2.1 h-1. The hydrocarbon products were analyzed in a GC with flame-ionization detector (FID). Other products were analyzed in a GC with a thermal conductivity detector (TCD). The methanol conversion was calculated regarding both methanol and DME as reactant.
* To whom correspondence should be addressed. E-mail: zhangsh04@ mails.gucas.ac.cn. † Institute of Coal Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
3. Results 3.1. Catalyst Characterization. Table 1 lists NH3-TPD data of HZSM-5 and Ca/HZSM-5 catalysts. NH3 desorbed at low and high temperature corresponded to the weak and strong acid sites, respectively. As expected, the catalyst acidity decreased after Ca
10.1021/ie901446m 2010 American Chemical Society Published on Web 01/27/2010
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Table 1. Acidity and Strong/Weak Ratios of HZSM-5 and CaModified HZSM-5 Zeolites
1 Ca(NO3)2 f CaO + 2NO2 + O2 2
(2)
CaO + ZOH f ZO-Ca-OH
(3)
ZO-Ca-OH + ZOH f ZO-Ca-OZ + H2O
(4)
2ZO-Ca-OH f ZO-Ca-O-Ca-OZ + H2O
(5)
a
acidity catalyst
total
weak
strong
strong/weak
HZSM-5 0.4Ca/HZSM-5 1Ca/HZSM-5 2Ca/HZSM-5 6Ca/HZSM-5 12Ca/HZSM-5
1.0741 0.7370 0.4902 0.3141 0.1585 0.1542
0.6248 0.4568 0.3336 0.2555 0.1385 0.1342
0.4493 0.2802 0.1566 0.0586 0.0200 0.0200
0.72 0.61 0.47 0.23 0.14 0.15
a As mmolNH3/g; weak, from 100 to 373 °C; strong, from 373 to 550 °C.
Figure 1. FT-IR spectra of pyridine adsorbed on the samples, HZSM-5 (a) and modified HZSM-5 with 0.4 wt % (b), 1 wt % (c), 2 wt % (d), 6 wt % (e), at 200 °C.
modification. For Ca/HZSM-5, the total acidity decreased with the increase of Ca loading. When Ca content changed from 6% to 12%, only slight decrease of the total acidity was obtained. With increasing Ca loading, both weak and strong acid sites decreased. And the strong acid sites decreased much faster than the weak acid sites, so the strong/weak ratio decreased with the increase of Ca loading. Figure 1 shows the IR spectra of pyridine adsorbed on HZSM-5 and Ca/HZSM-5 catalysts. The spectrum of pyridine of HZSM-5 exhibited the characteristic bands at 1548 and 1445 cm-1, which are attributed to pyridinium ions (pyridine chemisorbed on Bro¨nsted acid sites) and coordinatively bound pyridine (pyridine interacting with Lewis acid sites), respectively.14,15 With the addition of Ca to the zeolite, the spectra changed noticeably and depended on the Ca loading. Figure 1 shows that the intensity of the band at 1548 cm-1 decreased and at 1445 cm-1 increased after Ca modification of HZSM-5 as in the case of Mg-modified HZSM-5.16,17 The band at 1445 shifted to 1443 cm-1, which can be assigned to the presence of Ca cation. Therefore, Ca cation substituted the protons of hydroxyl groups and entered into ion exchange positions. With an increase in the Ca loading up to 6%, the band at 1548 cm-1 almost completely disappeared. On the basis of the results of Py-FTIR obtained above, the following equations were therefore suggested: The decrease of Bro¨nsted acid illuminated that Ca interacted with Bro¨nsted acid sites and entered into ion exchange positions to form ZO-Ca-OH (eq 1-3). The formed ZO-Ca-OH may undergo further reaction with free hydroxyl to give ZO-Ca-OZ (eq 4), and two ZO-Ca-OH in close vicinity may react to produce ZO-Ca-O-Ca-OZ (eq 5). Besides, Ca may exist in other species such as calcium oxide after calcinations, especially for samples of excessive loadings (eq 2). 1 Ca(NO3)2 + ZOH f ZO-Ca-OH + 2NO2 + O2 2
(1)
(ZOH:HZSM-5) 3.2. Catalyst Testing. The methanol conversion and product distribution of the MTO reaction over catalysts are quite dependent on reaction conditions such as temperature. In addition to reaction conditions, the amount of Ca loading of the catalyst is also an important factor in determining the conversion and product distribution over modified HZSM-5 catalysts, as shown in Figure 2. As a comparison, the result of HZSM-5 was also given. Figure 2a shows the changes of methanol conversion. At the initial time, methanol conversion was complete with HZSM-5, 0.4Ca/HZSM-5, and 1Ca/HZSM-5 while methanol was not completely transformed with 2Ca/HZSM-5 and 6Ca/HZSM-5 where a small number of DME appeared in products at the beginning and then disappeared with time on stream. As for 12Ca/HZSM-5, the initial methanol conversion was not complete and decreased with time on stream. In addition, the run time for methanol conversion over 98% increased with the increase of Ca loading from 0.4 to 6% but decreased at 12% Ca loading. For each catalyst, the methanol conversion declined remarkably at the time of serious catalyst deactivation. But the beginning time of remarkable decline of methanol conversion was gradually postponed with Ca loading from 0.4 to 6% and then advanced at 12% Ca loading. Figure 2b presents the variations of the light olefin (C22- to 2C4 ) selectivity. For HZSM-5, the selectivity to light olefin was as low as 42% at the initial time, and then rapidly decreased with time on stream. For Ca-modified HZSM-5 catalysts, the light olefin selectivity decreased with time on stream but the decreasing rate decreased with increasing Ca loading. Moreover, the light olefin selectivity increased with increasing Ca loading from 0.4 to 6% but decreased at 12% Ca loading. From the above results obtained, it can be seen that the highest light olefin selectivity was gained with Ca loading of 6%. 4. Discussion MTO is a well-known acid-catalyzed reaction. However, except for light olefins, the reaction of methanol over acid
Figure 2. Methanol conversion and product selectivity over Ca/HZSM-5, (a) conversion, (b) C22- to C42-. Reaction conditions: temperature ) 520 °C, pressure ) 0.1 MPa, feed ) 40% methanol in water; methanol WHSV ) 2.1 h-1.
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Figure 3. CO2-TPD spectra of the modified HZSM-5 with 2% (a), 6% (b), 12% (c) Ca loading.
catalysts can lead to the formation of byproduct such as higher olefin.2,18 The relationship between the catalyst acidity and the catalytic performance of solid acid catalysts for MTO has been studied by many researchers.19-21 They have claimed that low catalyst acidity is responsible for the increase of the light olefin selectivity, and high catalyst acidity, especially involving much strong acid sites, may further convert light olefins to paraffins, aromatic, naphthenes and higher olefins, which makes the formation of light olefin decrease. In our study, a comparison of both NH3-TPD data and the catalytic performance of HZSM-5 before and after Ca modification leads to the conclusion that the decreased secondary reaction of light olefins can be partially attributed to the decrease of the catalyst acidity, especially strong acid sites. Besides, the formation of the catalytic active centers may be the reason of the increased light olefin selectivity due to Ca modification. By correlating the results of catalyst evaluation with the data of Py-FTIR, it can be found there is a relationship between the catalytic performance and the nature of acid sites on Ca-modified HZSM-5. Py-FTIR data indicate that Ca interacts with Bro¨nsted acid sites. Moreover, the addition of Ca for HZSM-5 greatly increased the selectivity to light olefin, suggesting that the interaction between Ca and Bro¨nsted acid sites is favorable for the formation of light olefin. However, HZSM-5 modified with 12% Ca exhibited lower catalytic activity and light olefin selectivity but its acidity was almost identical to that of HZSM-5 modified with 6% Ca. This phenomenon strongly suggests that there must be another significant factor affecting the catalytic performance. To indentify this factor, the basicities of modified HZSM-5 zeolites with different Ca contents were measured by CO2-TPD as shown in Figure 3. From this figure, with 2% Ca loading, it is found that there are three peaks for Ca-modified samples, which are at about 180 °C (peak 1), 460 °C (peak 2), and 710 °C (peak 3), respectively. This modification can provide the basic sites needed for the transformation of methanol as described in the oxonium ylide mechanism.18,22 With increasing Ca loading from 2% to 6%, the intensity of both peak 1 and peak 2 increased, and the intensity of the peak 3 decreased slightly. When Ca loading increased to 12%, the intensity of peak 1 decreased, and the intensity of both peak 2 and peak 3 increased, especially peak 3. In this figure, peak 3 represents stronger basicity than the other two peaks. Therefore, if the basic site is too strong, the molecule adsorbed on the basic site cannot be readily dissociated. Thus, the next step of the reaction cannot efficiently
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proceed. The inferior catalytic performance of the modified sample with 12% Ca was attributed to excessive stronger basic sites. Therefore, the above results of Ca-modified samples clearly demonstrate that there exists an optimal Ca loading for a specific HZSM-5; the best catalytic stability as well as the highest light olefin selectivity was realized with Ca/HZSM-5 that had little free Bro¨nsted acid sites. In addition, the Ca/HZSM-5 catalyst with optimal Ca loading also demonstrated pretty good stability. The used sample was applied more than 10 times through air regeneration, and nearly the same catalytic activity and light olefin selectivity were obtained. The fact that the deactivated catalyst can be regenerated by treatment with air at high temperature indicates that the loss of activity is due to coke formation. Since Ca/HZSM-5 with excessive loading did not show longer run time, the coke was believed to be deposited on acidic sites of the catalysts.23 Thus, it has been postulated that Ca species such as ZO-Ca-OZ and ZO-Ca-O-Ca-OZ may hydrolyze to free Bro¨nsted acid sites in the presence of water, and subsequently the freed Bro¨nsted acid sites and Ca in close vicinity constituted acid-base centers needed for the catalytic transformation of MTO. 5. Conclusions Ca modification of the HZSM-5 zeolites increased both the MTO catalytic stability and the light olefin selectivity. For a specific HZSM-5, there was an optimal Ca loading, and the thus prepared Ca/HZSM-5 showed the best catalytic stability and the highest light olefin selectivity. Py-FTIR data presented that nearly Bro¨nsted acid sites diminished at the optimum Ca loading, and then CO2-TPD characterization of the modified samples demonstrated that basic sites needed for MTO were provided by Ca modification. A possible mechanism for the MTO involving acid-base centers on Camodified HZSM-5 zeolites has been suggested on the basis of the results obtained. Acknowledgment The authors thank for the financial support from Laboratory of Applied Catalysis and Green Chemical Engineering of Institute of Coal Chemistry, Chinese Academy of Sciences. Supporting Information Available: 1#-HZSM-5 (Si/Al2 ) 48), 2#-HZSM-5 (Si/Al2 ) 106); (1) catalyst preparation: Ca/ 2#-HZSM-5, Ca/Al2O3, Ca/HMOR, Na/1#-HZSM-5 and Ca/Na/ 1#-HZSM-5; (2) NH3-TPD data of Ca/2#-HZSM-5 and Na/1#HZSM-5 catalysts, Py-IR spectra of Ca/2#-HZSM-5; (3) catalyst testing: Ca/1#-HZSM-5, Ca/2#-HZSM-5, Al2O3 and 2Ca/Al2O3, Na/1#-HZSM-5 and 2Ca/1Na/1#-HZSM-5, and Ca/HMOR testing. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Chang, C. D. Methaonl Conversion to Light Olefins. Catal. ReV. Sci. Eng. 1984, 26, 323. (2) Sto¨cker, M. Methanol-to-Hydrocarbons: Catalytic Material and Their Behavior. Microporous Mesoporous Mater. 1999, 29, 3. (3) Liu, Z. M.; Sun, C. L.; Wang, G. W.; Wang, Q. X.; Cai, G. Y. New Progress in R&D of Lower Olefin Synthesis. Fuel Process. Technol. 2000, 62, 161. (4) Chen, J. Q.; Bozzano, A.; Glover, B.; Fuglerud, T.; Kvisle, S. Recent Advancements in Ethylene and Propylene Production Using the UOP/Hydro MTO Process. Catal. Today 2005, 106, 103.
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ReceiVed for reView September 15, 2009 ReVised manuscript receiVed January 7, 2010 Accepted January 14, 2010 IE901446M
