CaO Catalysts

CaO Catalysts

Journal of Natural Gas Chemistry 16(2007)200–203 Article Catalytic Oxidation of Dimethyl Ether to Hydrocarbons over SnO2/MgO and SnO2/CaO Catalysts ...

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Journal of Natural Gas Chemistry 16(2007)200–203

Article

Catalytic Oxidation of Dimethyl Ether to Hydrocarbons over SnO2/MgO and SnO2/CaO Catalysts Lin Yu∗ ,

Jieyu Xu,

Ming Sun,

Xuetao Wang

Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, Guangdong, China [ Manuscript received December 25, 2006; revised February 1, 2007 ]

Abstract: A novel reverse microemulsion method was used to prepare SnO2 /MgO and SnO2 /CaO catalysts. It was found that both the catalysts were active for the reaction of catalytic oxidation of dimethyl ether (DME) in the temperature range of 275 to 300 ℃. SnO2 /CaO catalyst exhibits much higher activity than SnO2 /MgO. On SnO2 /CaO catalyst, DME conversion of 21.8% was obtained at 300 ℃, while selectivities to methyl formate (MF) and dimethoxyethane (DMET) of 19.1% and 59.0% respectively were obtained at 275 ℃. Key words: dimethyl ether; catalytic oxidation; methyl formate; dimethoxyethane

1. Introduction Dimethyl ether is expected to be a potential alternative fuel for diesel engines due to its low NOx emission, near-zero smoke, and less engine noise compared with traditional diesel fuels [1,2]. It is also a useful chemical intermediate for the preparation of many important chemicals [3]. Much attention has been given to the mechanism of gas phase oxidation [4,5] and electrocatalytic oxidation of DME [6,7]. But only a few reports have addressed to the catalytic oxidation of DME to hydrocarbons. Liu et al. [8−10] studied the selective oxidation of dimethyl ether to formaldehyde on MoOx species supported on MgO, Al2 O3 , ZrO2 , and SnO2 . But the selective oxidation of DME to larger molecules containing C—C bond has scarcely been reported. Yagita et al. [11] studied the selective oxidation of dimethyl ether to DMET on SnO2 /MgO catalyst. Liu et al. [12] studied the onestep selective synthesis of dimethoxymethane (DMM) ∗

achieved by oxidation of dimethyl ether (DME) on unsupported and SiO2 -supported heteropolyacids with Keggin structures. However, the conversions of DME they obtained were just 10.8% and 2.0% respectively. In this study, we focused on how to obtain higher carbonic hydrocarbons (i.e. MF, DMET, and C2 H5 OH) from DME through the catalytic oxidation of DME over the novel SnO2 /CaO and SnO2 /MgO catalysts. 2. Experimental 2.1. Catalyst preparation SnO2 /MgO and SnO2 /CaO catalysts were prepared with reverse microemulsion method in a system consisting of n-heptane, emulsifier OP-10, 1-octanol and water. And the volume ratio of n-heptane, emulsifier OP-10, 1-octanol and water is 11:3:3:3. Mixed Mg(NO3 )2 or (Ca(NO3 )2 ) and SnCl4 solution at a mass ratio of 15:1 was added into the reverse mi-

Corresponding author. Tel: +86-20-39322201; Fax: +86-20-39322237; E-mail: amy [email protected] This work was supported by the Natural Science Foundation of Guangdong Province (4205301, 06021468), Project of Science and Technology of Guangdong Province (2004B33401003, 2005B10201053), Project of Science and Technology of Guangzhou (2006 J1-C0501) and National Natural Science Foundation of China (20203012).

Journal of Natural Gas Chemistry Vol. 16 No. 2 2007

croemulsion drop by drop at 60 ℃ with vigorous stirring. Then, NaOH solution was slowly added as a precipitating agent into the reverse microemulsion. The formed precipitates were filtered and washed by water and ethanol to remove residual chloride ions present in the system. The precipitates were then dried at 100 ℃ to obtain the precursor. SnO2 /MgO and SnO2 /CaO catalysts were prepared by calcining the precursor powder in dry air at 700 ℃ for 4 h.

201

DME conversion reached a maximum value of 21.8% at 300 ℃. However, on SnO2 /MgO catalyst, the maximum value of DME conversion is just 18.2% under the same reaction condition. On both SnO2 /MgO and SnO2 /CaO catalysts, the DME conversion initially increased with increasing temperature and reached the maximum values at 300 ℃, then slowly declined with the increase of the reaction temperature.

2.2. Activity measurements Catalytic activities were measured in a continuous flow reactor made of a quartz tube (12 mm i.d.). Catalyst pellets of 40−60 mesh (0.3 g) were loaded, and a feed gas mixture of 38% DME, 8% oxygen and 54% He was introduced into the reactor at a space velocity of 3480 ml/(g·h). The reaction was performed from 150 ℃ to 350 ℃ under 0.1 MPa, and the effluent gas was analyzed by gas chromatographs equipped with TCD and FID detectors. The conversion of DME and the selectivities to various products were calculated by carbon balance method. 2.3. Structural characterization The BET surface areas of the catalysts were measured by N2 adsorption using a Micromeritics (Model ASAP 2380) apparatus. Particle sizes of the catalysts were examined by transmission electron microscopy (TEM). XRD patterns were recorded on a Shimadzu XD-3A instrument operating at a voltage of 40 kV and a current of 25 mA with Cu-Kα radiation. The full XRD profile was obtained with 2θ ranging from 10o to 90o . 3. Results and discussion Figure 1 shows the temperature dependence of the DME conversion on SnO2 /MgO and SnO2 /CaO catalysts. As shown in Figure 1, the reaction proceeded at temperature above 250 ℃, and SnO2 supported on CaO led to a significantly higher DME conversion than that supported on MgO. On SnO2 /CaO catalyst,

Figure 1. Temperature dependence of DME conversion over SnO2 /MgO (1) and SnO2 /CaO (2) catalysts Reaction conditions: 38% DME, 8% oxygen and 54% He as feed gas; space velocity 3480 ml/(g·h); reaction temperature 150–350 ℃; 0.1 MPa

Table 1 shows the temperature dependence of the selectivities to the desired-products of MF and DMET on SnO2 /MgO and SnO2 /CaO catalysts. As shown in Table 1, on both the catalysts, the most optimal range of reaction temperature for catalytic oxidation of DME was from 275 to 300 ℃. SnO2 /CaO catalyst seems to be more selective to the desired catalytic oxidation products of MF (21.2% vs. 13.7%) and DMET (59.0% vs. 33.6%). It is obvious that SnO2 /CaO is more active for the reaction. This can be reasonably speculated that both supports of MgO and CaO are alkaline earth oxides, and the basicity of CaO is stronger than that of MgO, consequently, enhancement of the basicity of the supported catalysts may be responsible for the improvement of the activity of catalysts.

Table 1. Selectivities for MF and DMET over SnO2 /MgO and SnO2 /CaO catalysts Temperature (℃)

Selectivity for MF (%) SnO2 /MgO

Selectivity for DMET (%)

SnO2 /CaO

SnO2 /MgO

SnO2 /CaO

275

6.6

19.1

19.7

59.0

300

11.7

21.2

33.6

42.0

325

13.7

18.7

10.7



350

12.9

12.9





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Surface areas and particle sizes of the SnO2 /MgO and the SnO2 /CaO catalysts are presented in Table 2. Usually, the catalyst with higher surface area has smaller particle size. The surface area of the SnO2 /CaO catalyst is far smaller than that of the SnO2 /MgO catalyst. Associated with the above results of activity evaluation, it was found that the catalyst with smaller surface area showed higher selectivities to the desired-products. In the reaction of oxidative coupling of methane [13], surface area is a negative exponential function of selectivity to C2 formation. Therefore, it is rationally reasoned that relatively small surface area can inhibit the deep oxidation of DME and improve the selectivities of desiredhydrocarbons. Table 2. Surface areas and particle sizes of SnO2 /MgO and SnO2 /CaO catalysts Catalyst

Surface area (m2 /g)

SnO2 /MgO

30.9

19

SnO2 /CaO

16.3

36

Particle size (nm)

XRD patterns for the SnO2 /MgO and the SnO2 /CaO catalysts are shown in Figure 2. It can be seen that, the characteristic diffraction peaks of SnO2 /MgO catalyst are attributed to the cubic phase of MgO and SnO2 at (2θ) 37.98o , 42.66o, 62.04o, 37.81o, 51.16o and 58.63o , while the characteristic diffraction peaks of SnO2 /CaO catalyst are attributed to the cubic phase of CaO and orthorhombic phase of SnO2 at (2θ) 32o , 37.2o , 53.7o , 22.3o and 32.7o respectively. However, the intensity of the characteristic diffraction peaks of SnO2 is very low. Thus,

Figure 2. XRD Patterns of SnO2 /MgO (1) and SnO2 /CaO (2) catalysts Experimental conditions: 40 kV, 25 mA with Cu-Kα radiation, 2θ=10o to 90o .

it indicates that the active component of SnO2 dispersed well on the surface of the catalysts, but a little amount of SnO2 entered into the bulk phase of the catalysts. Both the catalysts comprise two different types of oxides (acidic and basic), and the acidic tin oxide spreads out on the surface of the basic support. It is considered that the irreducible oxides (MgO and CaO) can form an oxygen vacancy, and the oxygen vacancy can activate the gas-phase oxygen to produce absorbed unsaturated active oxygen species [14−16]. As a result, we suggest that the basic supports are in favor of the conversion of gas-phase oxygen to absorbed active oxygen species. Besides, SnO2 is a reducible oxide, the lattice oxygen can be released via redox cycles of Sn4+ /Sn2+ [17]. In the reaction of oxidative coupling of methane, it was found that the enhancement of catalyst basicity leads to the increase in methane conversion and selectivities to C2 formation [15]. For the basicity of CaO is stronger than that of MgO, we may conclude that the higher catalyst performance of SnO2 /CaO catalyst should be attributed to its stronger basic support. The stronger basicity of CaO leads to the stronger interaction between the catalyst active component and the support of SnO2 /CaO catalyst, which can promote the conversion of gas-phase oxygen to absorbed active oxygen species and enhance the redox cycles of Sn4+ /Sn2+ to release more lattice oxygen. The oxidation of DME to the desiredhydrocarbons proceeds in two steps: the decomposition of DME to the adsorbed species of CH3 OCH2 , CH3 and CH3 O [3,5,18−23]; and the oxidation of decomposition products to the desired-hydrocarbons. The primary pathway for the decomposition of DME involves the cleavage of C—H bond and C—O bond, which may be activated by the active oxygen species and lattice oxygen. As speculated, the process may be consisted of the following steps: CH3 OCH3 (a) −→ CH3 OCH2 (a) + H(a)

(1)

CH3 OCH3 (a) −→ CH3 O(a) + CH3 (a)

(2)

Where a in the parenthesis (a) means the adsorbed species. It is considered that DME is easily decomposed to adsorbed CH3 OCH2 [5,20,21,23], CH3 and CH3 O [3,18,19,21−23] species on the surface of catalysts; therefore, the following reaction steps are considered for the partial oxidation of DME to desired-hydrocarbons:

Journal of Natural Gas Chemistry Vol. 16 No. 2 2007

CH3 OCH2 (a) + CH3 OCH2 (a) −→ CH3 OCH2 CH2 OCH3

(3)

CH3 O(a) + CH3 (a) −→ CHO(a) + CH4 + H(a) CHO(a) + CH3 O(a) −→ HCOOCH3

(4) (5)

Francisco [20,23] found that the hydrogen abstraction by molecular oxygen was a dominating process compared with the C—O fragmentation process in initiating DME combustion, and the CH3 OCH2 radical can be formed at low temperature. Zhang [18] found that CH3 and CH3 O radicals can be formed at high temperature through the Equation (2). From literature, the bond energies for the C—H bond and the C—O bond are 200.3 kJ/mol and 340.6 kJ/mol, respectively. Associated with the above results of activity evaluation, it was found that the selectivity for DMET is much higher than that of MF in the temperature range of 275 to 300 ℃. So we suggest that the C—H bond fragmentation pathway in catalytic oxidation of DME is energetically favored and then followed by Equation (3) at relatively low temperature (275−300 ℃), while the C—O bond fragmentation pathway is energetically favored at relatively high temperature (>300 ℃). This conclusion is in good agreement with Francisco’s and Zhang’s study. 4. Conclusions Briefly, both the SnO2 /MgO and the SnO2 /CaO catalysts were active for the reaction of catalytic oxidation of DME, and the activity of the SnO2 /CaO catalyst was much higher. The oxidation of DME to the desiredhydrocarbons comprises the decomposition of DME and the oxidation of decomposition products to the desired-hydrocarbons. Associated with the activity results, it may be concluded that the C—H bond fragmentation pathway in DME decomposition is energetically favored at relatively low temperature, while the C—O bond fragmentation pathway in DME decomposition is energetically favored at relatively high temperature.

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References [1] Wang S Z, Ishihara T, Takita Y. Appl Catal A, 2002, 228(1/2): 167 [2] Xu M T, Goodman D W, Bhattacharyya A. Appl Catal A, 1997, 149(2): 303 [3] Solymosi F, Cser´enyi J, Ov´ ari L. J Catal, 1997, 171(2): 476 [4] Lang J, Zhang K. Neiranji Gongcheng (Chin Int Comb Eng Eng), 2005, 26(6): 30 [5] Hiroyuki Y, Kotaro S, Hideki S, Namil C, Atsumu T. Comb Flam, 2005, 140(1/2): 24 [6] Wang S Z, Ishihara T. Cuihua Xuebao (Chin J Catal), 2003, 24(9): 695 [7] Zhao J, Yin G P, Shao Y Y. Gaoxiao Huaxue Gongcheng Xuebao (J Chem Eng Chin Univ), 2005, 19(4): 493 [8] Liu H, Iglesia E. J Catal, 2002, 208(1): 1 [9] Liu H, Cheung P, Iglesia E. J Catal, 2003, 217(1): 222 [10] Liu H, Cheung P, Iglesia E. J Phy Chem B, 2003, 107: 4118 [11] Yagita H, Asami K, Muramatsu A. Appl Catal, 1989, 53(1): L5 [12] Liu H, Iglesia E. J Phy Chem B, 2003, 107: 10840 [13] Yang X G, Bi Y L, Zhen K J, Wu Y. Yingyong Huaxue (Chin J Appl Chem), 1995, 12(6): 1 [14] Karasuda T, Aika K. J Catal, 1997, 171(2): 439 [15] Yu L, Xu Y D, Cai Sh, Li X Sh, Huang J Sh, Guo X X. Cuihua Xuebao (Chin J Catal), 1996, 17(1): 22 [16] Xu Y D, Yu L, Guo X X. J Natural Gas Chem, 1999, 8(1): 18 [17] Nagaoka K, Karasuda T, Aika K. J Catal, 1999, 181(1): 160 [18] Zhang Q, Li X H, Fujimoto K, Asami K. Appl Catal A, 2005, 288(1/2): 169 [19] Solymosi F, Klivenyi G. Sur Sci, 1998, 409(2): 241 [20] Good A D, Francisco S J. Chem Phy Lett, 1997, 266(5/6): 512 [21] Daly A C, Simmie M J, Wrmel J, Djeba¨Ili N, Paillard C. Comb Flam, 2001, 125(4): 1329 [22] Bugyi L, Solymosi F. Sur Sci, 1997, 385(2/3): 365 [23] Francisco S J. Comb Flam, 1999, 118(1/2): 312