Aluminum Fluoride Modified HZSM-5 Zeolite with Superior

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Aluminum Fluoride Modified HZSM-5 Zeolite with Superior Performance in Synthesis of Dimethyl Ether from Methanol Qi Yang, Meng Kong, Zheyong Fan, Xiangju Meng, Jinhua Fei,* and Feng-Shou Xiao Institute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou 310028, China S Supporting Information *

ABSTRACT: A series of HZSM-5 catalysts modified with various loadings of aluminum fluoride (AlF3) were prepared from a mechanical mixture route. Combined characterizations of X-ray diffraction, Fourier transform infrared (FT-IR), 27Al, 29Si, 19F MAS NMR, N2 sorption, and NH3-tempeature-programmed desorption (NH3-TPD) techniques show that the structure, texture, and acidity of HZSM-5 catalysts can be adjusted with the loading of AlF3. A suitable amount of AlF3 modification (2 wt %) could increase the framework aluminum content and the surface area of HZSM-5. However, when the loading of AlF3 came to 3 wt % or more, the contrary results were obtained, which could be ascribed to the dealumination of the zeolitic framework. The catalytic activities for dehydration of methanol to dimethyl ether (DME) show that suitable amount of AlF3-modified HZSM-5 exhibited much higher activity and better stability than parent HZSM-5. The combination of “tunable” synthesis and “superior” properties is very much valuable in the academic and industry.

1. INTRODUCTION Dimethyl ether (DME) as an environmentally friendly chemical has received much attention due to its potential use as a substitute of diesel and liquefied petroleum gas.1,2 The superior features of DME are the low NOx emission, nearly no black smoke and SOx, and little engine noise, compared with traditional diesel fuels.3 In addition, DME is also an important intermediate for production of chemicals such as dimethyl sulfate, methyl acetate, and light olefins.4−6 Therefore, a large amount of DME is needed in the market. Generally, DME is synthesized from a methanol-to-dimethyl ether (MTD) process over solid-acid catalysts such as γ-Al2O3, HZSM-5, HY, SAPO, and SiO2−Al2O3 [eq 1].7−11 Of course, it can also be synthesized from a syngas-to-dimethyl ether (STD) process [eq 2] over a bifunctional catalyst consisting of methanol synthesis component and solid-acid component, which should be more attractive than MTD route in consideration of the equilibrium limitation.12,13 2CH3OH → CH3OCH3 + H 2O (1) CO + H 2 → CH3OH → CH3OCH3 + H 2O

Notably, zeolitic acidity is mainly modified with alkali metals, alkaline earth metals, transition metals, and rare earth metals.4,15,16 By these modifications, the partially strong acid sites of zeolites can be eliminated, leading to the reduction of undesirable byproduct for DME synthesis. It is well-known that the acidity of HZSM-5 zeolite is strongly dependent on the amount of framework Al.17 Normally, in order to achieve the preferred Si/Al ratio, hydrothermal synthesis in the presence of organic structuredirecting agent (OSDA) and post-treatment synthesis by alumination/dealumination process have been adopted. In the past decades, dealumination of zeolites with steaming,18−20 acid,21,22 and fluoride reagents such as (NH4)2SiF6, HF, and NH4F has been employed.23−29 Except for the case of steam, the high pressure, high treatment temperature, long time, and complicated procedure limit its wide application. Obviously, although the fluoride reagents can easily cause dealumination of zeolites, the uses of these reagents have the same problem of environmental pollution. In contrast, a little attention is paid on alumination of zeolites. Dessau and Kerr reported new shapeselective HZSM-5 formed by the treatment of high-silica HZSM-5 with gas phase aluminum chloride, which is a good example for alumination of zeolites from external aluminum source.30 To the best of our knowledge, aqueous aluminum fluoride solutions can be observed to substitute A13+ into highly siliceous ZSM-5 zeolitic frameworks by replacement of Si4+.31 Therefore, as an insoluble solid-phase reagent containing Al and F, AlF3 might provide a feasible way. Herein, the facile modification of HZSM-5 by solid AlF3 for synthesis of DME from methanol was reported. In the present work, we prepared AlF3-modified HZSM-5 zeolites by a facile mechanical mixing

(2)

Among these solid-acid catalysts, γ-Al2O3 is the most widely used material due to its high DME selectivity, long life, and low cost. However, a disadvantage of the γ-Al2O3 is its low hydrothermal activity and stability. Accordingly, HZSM-5 zeolite is usually used as dehydration catalyst for DME synthesis, because its hydrothermal stability is higher than that of the γ-Al2O3, and it operates at a lower temperature.14 However, undesirable byproducts in DME synthesis such as hydrocarbons and even coke formed on the HZSM-5 zeolite catalyst due to the presence of strong acid sites.4 Therefore, a great effort has been made to modify the acidity of HZSM-5 zeolite to inhibit the deactivation, thereby enhancing catalytic stability. © 2012 American Chemical Society

Received: April 16, 2012 Revised: June 19, 2012 Published: June 21, 2012 4475

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3. RESULTS AND DISCUSSION 3.1. Structural Properties of the Catalysts. Figure 1 shows XRD patterns of various catalysts. Clearly, no crystalline

route. Catalytic tests in MTD process show that the modified HZSM-5 catalysts exhibit not only better stability but also higher activity than unmodified HZSM-5 catalysts.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The AlF3-modified HZSM-5 zeolites (H-type with the Si/Al ratio of 50, Nankai University, China) were prepared from mechanical mixtures of AlF3·3H2O with HZSM-5 zeolite at room temperature, followed by the treatments at 110 and 500 °C. The weight composition of the starting mixtures was (100 − x)% for HZSM-5 and x% for AlF3, where x was 0, 1, 2, 3, and 5, respectively. As a typical run for preparing the catalyst with 3 wt % AlF3, 4.85 g of HZSM-5 zeolite (Si/Al = 50) was mechanically mixed with 0.25 g of AlF3·3H2O by grinding for 1 h at room temperature, followed by drying at 110 °C for 24 h and calcining at 500 °C for 3 h. These AlF3-modified samples were designated as HZ(x), where x was the mass percentage ratio of AlF3·3H2O. The commercial AlF3·3H2O, γ-Al2O3, and initial HZSM-5 used as catalysts were pretreated by the same condition and designated as AlF3, γ-Al2O3, and HZ(0), respectively. 2.2. Characterization of Zeolite Catalysts. X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX 2550/PC diffractometer with Ni-filtered Cu Kα radiation operated at 40 kV and 100 mA. Fourier transform infrared (FT-IR) spectra (400−1500 cm−1) were investigated on Nicolet 6700 instrument using the KBrpellet technique. 27Al and 29Si MAS NMR experiments were carried out on a Varian Infinity plus-400 spectrometer (fitting the sample in a 7 mm ZrO2 rotor, spinning at 4 kHz with the number of scans being 308, and a collection time of 25.6 min). 19F MAS NMR spectra were collected by Bruker AdvanceIII 600, spinning rate 30 kHz, 1 pulse program, pi/4 pulse width 2.2 us. The chemical shifts are referred to CFCl3. The surface area and pore volume measurements of the catalysts were conducted on a volumetric adsorption apparatus (ASAP 2020M, Micromeritics) at −196 °C using liquid N2. The samples were evacuated at 300 °C for 4 h under a vacuum of 1.33 × 10−3 Pa prior to adsorption. The acidity of the catalysts were measured by the temperatureprogrammed desorption of ammonia (NH3-TPD). The catalyst (0.2 g, 40−60 mesh) was pretreated at 500 °C in a N2 flow for 60 min, followed by the adsorption of NH3 at 100 °C for 30 min. After saturation, the catalyst was purged by N2 flow for 30 min to remove the physically adsorbed ammonia on the sample. Then, desorption of NH3 was carried out from 100 to 600 °C with a heating rate of 10 °C/ min. The amount of NH3 desorbed from the sample was obtained by comparing the area under the curve with a sample containing a known amount of NH3 using a thermal conductivity detector. The amounts of coke deposited on the used catalysts were carried out by temperature programmed oxidation (TPO) technique. The used catalysts (0.1 g) were treated at 500 °C in a N2 stream for 30 min, cooled to 50 °C, and then heated from 50 to 700 °C with a slope of 10 °C/min in 20% O2/N2 stream (50 mL/min). The amount of CO2 released from the reactor in O2-TPO process was continuously detected using a mass spectrometer (Ametek, LC-200). 2.3. Catalytic Performance. The catalytic activities over HZ(0), AlF3, γ-Al2O3, and HZ(x) for the MTD process were carried out in a quartz fixed-bed reactor with an inner diameter of 8 mm. The catalysts (40−60 mesh) were placed, pretreated in a stream of N2 at 300 °C for 2 h and then cooled to 180 °C. The feed of methanol controlled by a high pressure liquid chromatography (HPLC) pump (ASI 501) with the liquid hourly space velocity (LHSV) of 40 h−1 and N2 regulated by mass flow controller with the gas hourly space velocity (GHSV) of 6000 h−1 was introduced continuously through an evaporator maintained at 180 °C into the reactor. The products in the effluent were analyzed by an online gas chromatography (HP5890) equipped with a Porapak-Q column and a thermal conductivity detector.

Figure 1. XRD patterns of (a) HZ(0), (b) HZ(1), (c) HZ(2), (d) HZ(3), (e) HZ(5) catalysts, and (f) AlF3 sample.

diffraction peak of AlF3 was detected in the modified zeolite, implying a high dispersion of AlF3 in the zeolites. In addition, all the AlF3-modified zeolites show similar XRD patterns to that of parent HZSM-5, suggesting that the basic MFI-type structure is still remained after the modification. Interestingly, HZ(0), HZ(1), and HZ(2) have singlet at 24.4° and 29.2°, while HZ(3) and HZ(5) give split peaks. The appearance of doublets in place of singlet at about 24.4° and 29.2° have has been considered as the crystallographic symmetry changes from the typical orthorhombic system to the monoclinic system for HZSM-5 zeolite.32,33 In addition, the relative crystallinity of catalysts is influenced by the modification of AlF3 amount. Compared with HZ(0) (100%), the relative crystallinity of HZ(1), HZ(2), HZ(3), and HZ(5) is estimated at 97.4, 96.2, 93.9, and 89.9%, respectively. This phenomenon might be related to the effect of defects created during alumination/ dealumination. FT-IR spectra of the catalysts (Figure S1, Supporting Information) reveal that the asymmetric stretching vibration frequencies of the Si−O−T linkages at about 1098 and 1223 cm−1 for HZ(0) slightly shifted to lower wavenumber for HZ(2) (1095, 1221, respectively), while it shifted to higher wavenumber for HZ(3) (1105, 1232, respectively). It is wellknown that the frequencies of the vibrations were straightforwardly related with the content of zeolitic framework Al.34,35 The results exhibit that the framework Si/Al ratio decreased slightly for HZ(2) and increased for HZ(3). Furthermore, according to the equation for Si/Al ratio determination from XRD by an empirical formula of %Al2O3 = 16.5−30.8δ (δ, angle unit, the distance of the two peaks between 45.0 and 45.5°),36 the Si/Al ratios in the framework of HZ(0), HZ(1), HZ(2), HZ(3), and HZ(5) were 50.3, 37.6, 37.0, 160.8, and 298.0, respectively (Table 1). Figure 2 shows 27Al and 29Si MAS NMR spectra of HZ(0), HZ(2), and HZ(3). As observed in the 27Al spectra (Figure 2A), HZ(0) displays a strong and narrow peak at 56 ppm associated with tetrahedrally coordinated framework aluminum (FAl) and a weak and broad peak at 0 ppm attributed to octahedrally coordinated extra-framework aluminum (EFAl).37 For HZ(2) catalyst, the signal at 0 ppm is very weak, indicating 4476

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Table 1. Textural and Structural Parameters of Various Catalysts Brunauer−Emmett−Teller (BET) surface area (m2/g)

pore vol, (cm3/g)

catalysts

total

Smic

Smes

total

Vmic

Vmes

Si/Ala

relative crystallinity (%)

HZ(0) HZ(1) HZ(2) HZ(3) HZ(5) AlF3

329.5 350.3 352.4 314.8 316.4 20.3

323.2 337.3 342.1 189.7 99.8 8.7

6.3 13.0 10.3 125.1 216.7 11.6

0.189 0.196 0.203 0.212 0.167 0.099

0.151 0.150 0.159 0.091 0.047 0.004

0.038 0.046 0.044 0.121 0.120 0.095

50.3 37.6 37.0 160.8 298.0

100 97.4 96.2 93.9 89.9

a

Analyzed by the empirical formula: % Al2O3 = 16.5−30.8δ (δ, angle unit, the distance of the two peaks between 45.0 and 45.5°) using a scan speed of 0.2°/min and a step of 0.002° from 44.5 to 46°.

Figure 3. 19F MAS NMR spectra of (a) HZ(2) and (b) HZ(3) catalysts. * denotes spinning sideband, chemical shift referred to CFCl3.

are three peaks at −117, −148, and −173 ppm for HZ(3). Peaks at −155 ppm and −148 ppm are in a typical region for bridging F-sites in octahedral AlFxO6−x-species.40,41 According to the literature, octahedral AlFxO6−x-species are formed in the dealumination process,29,42 and HZ(2) shows an obvious peak at −155 ppm, but the results of XRD (Table 1) show that the framework Si/Al ratio of HZ(2) is lower than that of HZ(0). Therefore, it is concluded that both alumination and dealumination occur in the modified process. For HZ(2), alumination plays the dominating role, while the appearance of a wide peak at −117 ppm observed for HZ(3) is due to the presence of highly dispersed F− counterion of H+ occurring in the channels of zeolites, and a sharp peak at −173 ppm is assigned to partially hydrated AlF3.41 For HZ(3), the partially hydrated AlF3 might be the result of excessive loading of AlF3·3H2O. Compared to HZ(2), dealumination plays the dominating role for HZ(3). 3.2. Textural Properties of the Catalysts. Figure 4 shows that the textural properties of these samples were determined by N2 isotherms. All samples show a significant uptake at P/P0 0.4 than HZ(0). The hysteresis loop at relative pressures, which is higher than 0.4, could be ascribed to the presence of mesopores in zeolites.34,43 The step occurring at P/P0 = 0.2 is due to a structural rearrangement of the adsorbed nitrogen molecules experiencing a fluid-to-crystal-like phase transition in the zeolite

Figure 2. (A) 27Al and (B) 29Si MAS NMR spectra of (a) HZ(0), (b) HZ(2), and (c) HZ(3) catalysts.

that the EFAl species are a little. Considering the fact (Table 1, calculation from XRD technique) that the Si/Al ratio of HZ(2) is lower than that of HZ(0), it is suggested that the aluminum species of AlF3 might be entered into the framework of HZ(2) zeolite. However, when the content of AlF3 reaches to 3%, the peak intensity for FAl on HZ(3) decreases obviously, and the peak intensity for EFAl increases significantly, compared with HZ(0). It indicates that AlF3 can also play a similar role with the other liquid or gaseous fluorine agents in dealumination process. For 29Si MAS NMR spectra (Figure 2B), HZ(0) and HZ(2) show obvious peaks at about −101, −106, −113, and −116 ppm, which are assigned to Si(2Al), Si(1Al), Si(0AlT1), and Si(0AlT2) species, respectively,38 where T1 and T2 are used to distinguish the two peaks assigned to the same Si(nAl).39 For the HZ(3) catalyst, peaks appear at −107.6, −109.8, −113.5, and −116.6 ppm, which are assigned to Si(1AlT1), Si(1AlT2), Si(0AlT1), and Si(0AlT2) species, respectively. Compared with HZ(0), the relative peak intensity of Si(1Al) species on HZ(3) decreases apparently, while relative peak intensity for Si(0 Al) species increases. It is further confirmed fluorine aluminum aroused dealumination of zeolite as other liquid and gaseous fluorine agents did. Figure 3 shows 19F MAS NMR spectra of HZ(2) and HZ(3). There is only one peak at −155 ppm for HZ(2), however, there 4477

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Table 2. Estimation of Acid Sites and Activity in MTD Process sample

weak acid sitesa [I] (mmol/g)

strong acid sitesa [II] (mmol/g)

total acid sites [I+II] (mmol/g)

I/II ratio

methanol conversionb (%)

HZ(0) HZ(1) HZ(2) HZ(3) HZ(5) AlF3

0.499 0.577 0.643 0.308 0.275 0.062

0.399 0.421 0.425 0.253 0.226 0.033

0.898 0.998 1.068 0.561 0.501 0.095

1.251 1.371 1.513 1.217 1.217 1.879

77.69 86.34 87.66 20.12 17.29 1.22

a Estimation by NH3-TPD curves: weak and strong acid sites are ranged in the region of 100−300 and 300−600 °C, respectively. b Reaction conditions: T = 240 °C, LHSV = 40 h−1.

exhibits relatively low acidic concentration. The order of total acidity and the ratios of I and II acid sites for various catalysts is HZ(2) > HZ(>1) > HZ(0) > HZ(3) > HZ(5). However, it is clearly observed that AlF3 shows very weak acid property on its surface. It is well-known that incorporation of some fluorine atoms into the H-forms of high-silica zeolites can result in a significant increase in their acidity,26 furthermore, this order is well consistent with the content of tetrahedrally coordinated Al in the framework of zeolites.45 3.4. Effect of AlF3. All of the results indicate that appropriate content of AlF3 (2 wt %) mainly played an alumination role in modifying HZSM-5 zeolite, while the excessive content of AlF3 (3 wt %) induced severe dealumination of HZSM-5 zeolite. A probable role of AlF3 in interaction with the zeolite framework was proposed based on the results obtained. Since AlF3·3H2O is not stable enough when it is heated,46 fluoride species can be obtained and inserted into the zeolitic framework at high temperature and cause dealumination of zeolite as other liquid and gaseous fluorine agents do. Meanwhile, aluminum fluoride as an external aluminum source can be introduced into the framework of HZSM-5; consequently, the alumination of zeolitic framework occurs. Sánchez et al. proved the same trend for aluminum ions insertion into the lattice when HY zeolite was modified with F2.29 Since the framework Si/Al ratio can be affected by different amounts of AlF3, the acidity of catalysts are tunable by the modification. 3.5. Catalytic Performance of the Catalysts. Figure 6 shows catalytic activities in MTD reaction over various catalysts. In the operated conditions, all the catalysts have very high selectivity for DME (above 99.9%), and byproducts such as CO and CH4 are trace ( HZ(1) > HZ(0) > γ-Al2O3 > HZ(3) > HZ(5) > AlF3. This order is well consistent with the total acidity and ratio of I and II acid sites on the samples (Table 2), indicating that catalytic activity for MTD strongly depends on the total acidity and I/II ratio of catalyst. In contrast, AlF3 is nearly inactive. It is clear that catalyst activity is highest when the total acidity and I/II ratio of the sample have maximum values. In contrast, when the total acidity and ratios of I and II acid sites are relatively smaller, catalyst activity is lower. Obviously, suitable content of AlF3 for modification of HZSM-5 is favorable for enhancing the catalytic activity.

Figure 4. N2 adsorption/desorption curves of (a) AlF3, (b) HZ(0), (c) HZ(1), (d) HZ(2), (e) HZ(3), and (f) HZ(5) samples.

micropores. 44 The textural parameters analyzed by N 2 isotherms are presented in Table 1. Compared with HZ(0), the micropore volume and surface area increase slightly for HZ(1) and HZ(2). In contrast, the micropore volume and micropore surface area for HZ(3) and HZ(5) reduces and gives very high mesopore volume and mesopore surface area, indicating that in the dealumination process fragments blocked some micropores and new mesopores were generated. 3.3. Acidic Properties of the Catalysts. Figure 5 shows the NH3-TPD profiles of various samples. In HZ(0), two peaks

Figure 5. NH3-TPD profiles of (a) HZ(0), (b) HZ(1), (c) HZ(2), (d) HZ(3), (e) HZ(5), and (f) AlF3 samples.

can be observed at around 205 °C (T1) and 430 °C (T2), corresponding to NH3 eluted from the weak and strong acid sites, respectively, which are consistent with the reported results.19 The desorption temperature basically does not change on HZ(1) and HZ(2). However, it obviously shifts to lower temperature on HZ(3) and HZ(5). Table 2 presents the estimation results of acid sites on the various samples. Notably, compared with HZ(0), the concentration of the weak and strong acid sites on HZ(2) increases significantly, while HZ(3) 4478

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Figure 6. Dependences of catalytic activities in MTD reaction over (a) HZ(0), (b) HZ(1), (c) HZ(2), (d) HZ(3), (e) HZ(5) catalysts, (f) AlF3, and (g) γ-Al2O3 samples. (Reaction conditions: LHSV = 40 h−1).

Figure 8. Temperature-programmed oxidation of (a) HZ(0) and (b) HZ(2) spent catalysts.

Figure 7 shows dependences of methanol conversion on reaction time in MTD process over HZ(0) and HZ(2)

pore system, which is good for more resistant to coke. In addition, the increase of the concentration of acid sites is mostly contributed by weak acid sites (85%) which are less sensitive to deactivation than strong ones. Meantime, the increased weak acid sites on HZ(2) also have good activity in MTD process, which might somewhat inhibit the coke deposited on strong acid sites. It is also be explained by the rate of activity decay from carbon deposition expressed by decay constant here. The roughly calculated constants of HZ(0) and HZ(2) were 1.29 and 1.07, respectively. The results show that the decay constant is negatively related to I/II ratios (Table. 2), indicating that the value of I/II ratios is related to the coke formation. Therefore, HZ(2) exhibits not only high activity but also stability, compared with HZ(0).

4. CONCLUSION The structure, texture, and acidity of HZSM-5 zeolite could be tunable by physically mixing with different amounts of AlF3. When modified HZSM-5 with no more than 2 wt % of AlF3, not only the framework aluminum content and acidity were improved but also surface area and pore volumes increased; especially, mesopore volume increased. When the loading of AlF3 was over 2 wt %, it obtained opposite results except for mesopore volume. For methanol dehydration, the order of activity was determined to be HZ(2) > HZ(1) > HZ(0)> HZ(3) > HZ(5). It is worth noting that methanol conversion was shown to be greater for catalysts with higher ratios of I to II acid sites and total acidity. In addition, the decay constant is negatively related to I/II ratios, indicating that the value of I/II ratios is related to the coke formation. These results suggest that the modified zeolite would be potentially important as an alternative catalyst for production of DME. Compared with conventional fluoride modification route, the novel, mild, and facile method is of great importance for industrial application due to the solvent-free and effective use of fluoride reagents.

Figure 7. Conversion of methanol on stream on (a) HZ(0) and (b) HZ(2) catalysts. (Reaction conditions: LHSV = 40 h−1).

catalysts. The temperature of the oven remained at 235 °C. For HZ(2) catalyst, the conversion of methanol slightly decreased from 86 to 75% during the reaction time of 120 h. In contrast, the conversion of methanol significantly decreased from 73 to 28% over HZ(0) catalyst. This phenomenon is reasonably attributed to the differences in the coke deposited on the catalysts in MTD process.47 In order to better understand the discrepancy of the two catalysts, the characterization of spent catalysts were performed by TPO technique. Figure 8 shows the amount of CO2 released after long-time runs (120 h). The amount of CO2 formed by HZ(0) is about 2.33 times to that of HZ(2), indicating that the coke deposited on HZ(0) is much heavier than that on HZ(2). The results properly explain that the proper modified HZSM-5 zeolite exhibits less sensitive to deactivation than initial HZSM-5. In the previous study,14 HZSM-5 zeolite via a fluoride-mediated route also exhibited extremely more resistant to coke formation in the MTD process. The lower content of coke deposited on the surface of HZ(2) may be attributed to enhanced accessibility and weak acid sites. After modification by 2% of AlF3, the increase of total pore volume from 0.189 to 0.203 cm3/g and surface area from 329.5 to 352.4 m2/g make the active sites more accessible to reactants and diffuse out from

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra of (a) HZ(0), (b) HZ(2), and (c) HZ(3) catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. 4479

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AUTHOR INFORMATION

Corresponding Author

*Fax: 86-571-88273283. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the Hi-Tech Research and Development Program of China (2007 AA 05Z415) and the National Basic Research Program of China (2007CB210207).

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dx.doi.org/10.1021/ef3006383 | Energy Fuels 2012, 26, 4475−4480