Preparation of nanocrystalline γ-Al2O3 catalyst using different procedures for methanol dehydration to dimethyl ether

Journal of Natural Gas Chemistry 20(2011)334–338

Preparation of nanocrystalline γ -Al2O3 catalyst using different procedures for methanol dehydration to dimethyl ether Ahmad Reza Keshavarz1 ,

Mehran Rezaei1,2∗ , Fereydoon Yaripour3

1. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran; 2. Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran; 3. Catalyst Research Group, Petrochemical Research & Technology Company, National Petrochemical Company (NPC), P. O. Box 149650115, Tehran, Iran [ Manuscript received September 27, 2010; revised November 23, 2010 ]

Abstract A series of nanocrystalline γ-alumina are synthesized by different procedures, namely, thermal decomposition method (sample A), precipitation method (sample B) and sol-gel method using sucrose and hexadecyltrimethyl ammonium bromide (CTAB) as templates (samples C and D, respectively). Textural and acidic properties of γ-alumina samples are characterized by XRD, N2 adsorption-desorption and NH3 -TPD techniques. Vapor-phase dehydration of methanol into dimethyl ether is carried out over these samples. Among them, sample C shows the highest catalytic activity. NH3 -TPD analysis reveals that the sample with smaller crystallite size possesses higher concentration of medium acidic sites and consequently higher catalytic activity. Thermal decomposition method leads to decrease in both surface area and moderate acidity, therefore it is the cause of lower catalytic activity. Key words methanol dehydration; dimethyl ether; gama alumina; sol-gel

1. Introduction Dimethyl ether (DME) has been found to be an alternative diesel fuel because it has low NOx emission, near-zero smoke amounts and less engine noise compared with traditional diesel fuels [1,2]. It can also be used to replace chlorofluorocarbons (CFCs) which destroy ozone layer of the atmosphere and used as an intermediate for producing many valuable chemicals such as lower olefins, methyl acetate, dimethyl sulfate and liquified petroleum gas (LPG) alternative. It is also used in power generation and as an aerosol propellant, such as in hair spray and shaving cream, due to its liquefaction property [3−7]. Hence, there is a growing demand to produce a large amount of DME to meet the global need. Dimethyl ether can be produced by methanol dehydration over a solid-acid catalyst or direct synthesis from syngas by employing a hybrid catalyst, comprising a methanol synthesis component and a solid-acid catalyst [8]. Methanol dehydration to dimethyl ether is a potentially important process and more favorable in the views of thermodynamics and economy [9]. Commercially, γ-Al2 O3 is used as the cat∗

alyst for this reaction. It has high surface area, excellent thermal stability, high mechanical resistance and catalytic activity for DME formation due to its surface acidity. Recently, many methods have been applied to synthesize alumina with a higher specific surface area and activity for DME synthesis [10]. In the present work, the catalytic dehydration of methanol to DME has been studied over nanocrystalline γ-alumina prepared by four different methods. The effects of preparation method on the textural, acidic properties and catalytic activity of γ-alumina samples have been investigated. 2. Experimental 2.1. Catalyst preparation 2.1.1. Thermal decomposition method γ-Al2 O3 was obtained by the thermal decomposition of the boehmite (γ-AlOOH) precursor at 600 ◦ C for 6 h at a heating rate of 2 ◦ C·min−1 and named as sample A.

Corresponding author. Tel: +98-361-5912469; E-mail: [email protected] This work was supported by the Petrochemical Research & Technology Company of National Petrochemical Company in Iran.

Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60157-0

Journal of Natural Gas Chemistry Vol. 20 No. 3 2011

2.1.2. Precipitation method Initially, aluminium nitrate nanohydrate (ANN, 98.5 wt%) was dissolved in water under continuous stirring. The molar ratio of water to ANN was 1. Then precipitation was carried out by adding aqueous ammonia to the above stirred solution. The pH of solution was adjusted to about 7.5. After that, the precipitate was filtered and dried at 110 ◦ C overnight. The final solid was then calcined in a flow of air from room temperature to 600 ◦ C with a heating rate of 2 ◦ C·min−1 , and kept at this temperature for 6 h. The obtained catalyst is named as sample B. 2.1.3. Sol-gel method via sucrose as template Firstly, aluminium isopropoxide as an aluminium precursor (AIP, 99 wt%) and sucrose as template were dissolved separately in water (molar ratios of AIP : H2 O and sucrose : H2 O were 1 : 2). The above solutions were mixed together. After that, the mixture was peptized using nitric acid (10 wt%) under vigorous stirring by carefully adjusting the pH value to 5.5. The resulting mixture was aged at 80 ◦ C for 5 h. The solid product was dried at 110 ◦ C for 12 h in static air and calcined in air at 600 ◦ C for 6 h. The obtained catalyst is referred herein as sample C. 2.1.4. Sol-gel method via CTAB as template In a typical preparation, firstly aluminum isopropoxide (AIP, 99 wt%) and hexadecyltrimethyl ammonium bromide as template (CTAB, 99 wt%) were dissolved in water. The molar ratios of water to AIP and CTAB to AIP were chosen to be 90 and 0.8, respectively. After that, the mixture was peptized using nitric acid (10 wt%) under vigorous stirring by carefully adjusting the pH value to 5.5. The mixture was aged at 80 ◦ C for 5 h. The solid product was dried at 110 ◦ C for 15 h and finally calcined at 600 ◦ C for 6 h. The obtained catalyst is referred herein as sample D.

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ventional flow apparatus, which included an on-line thermal conductivity detector (TCD). In a typical analysis, 0.25 g of the sample is degassed at 500 ◦ C under helium flow (30 mL·min−1) for 1 h. After that, the sample is cooled to 150 ◦ C and then saturated with a mixture of helium and 2% ammonia for 1.5 h. The sample is then purged with a helium flow for 0.5 h to remove weakly and physically adsorbed NH3 on the surface of the catalyst. After this operation, the sample is cooled to room temperature and then is heated at a rate of 10 ◦ C·min−1 under a flow of helium carrier gas (40 mL·min−1) from 25 ◦ C to 700 ◦ C and the amount of ammonia in the effluent is measured using TCD and recorded as a function of temperature. 2.3. Catalytic performance Vapor-phase dehydration of methanol was conducted in the Chemical Data Systems (CDS) unit. Figure 1 shows a simplified flow diagram of the CDS unit. Nitrogen, as the internal standard, was fed through a set of mass flow controllers (Bronkhorst HI-TECH, EL-FLOW), and methanol was pumped from a feed tank through a set of metering pumps (ILSHIN Autoclave Co., Ltd.). Methanol and N2 were subsequently introduced into a preheater that was set at a temperature of 250 ◦ C. The temperature of the down stream effluent was constantly maintained at temperatures above 150 ◦ C to avoid the possible condensation of water, methanol, or DME. Catalytic activity was studied under steady state conditions in a fixed-bed reactor (stainlessness steel with an internal diameter of 2.7 mm and a length of 305 mm). The reactor was heated by three individual furnaces located at top, middle and bottom sections.

2.2. Characterization techniques The BET surface area, the total pore volume and the mean pore diameter were measured using a N2 adsorptiondesorption isotherm at liquid nitrogen temperature (−196 ◦ C), using a NOVA 2200 instrument (Quantachrome). Prior to the adsorption-desorption measurements, all the samples were degassed at 200 ◦ C in a N2 flow for 16 h to remove the moisture and other adsorbates. The X-ray diffraction (XRD) patterns of all the calcined samples were recorded on a Philips X’Pert (40 kV, 30 mA) X-ray diffractometer, using a Cu Kα radiation source ˚ and a nickel filter in the 2θ range of 10o –76o. (λ = 1.542 A) The acidity of the samples was measured via temperatureprogrammed desorption of ammonia (NH3 -TPD), using a TPR/TPD 2900 instrument (Micromeritics, USA) with a con-

Figure 1. A simplified flow diagram of the chemical data systems (CDS) unit

Prior to the catalytic activity measurements, the samples were crushed, sieved to 60−120 mesh size, and then treated in situ at a heating rate of 5 ◦ C·min−1 under N2 flow (50 mL·min−1) from room temperature to 220 ◦ C and kept at this temperature for 2 h under atmospheric pressure. In a typical experiment, 1 g of catalyst (size 60−120 mesh) was loaded in the middle section of the reactor and methanol was pumped with different weight hourly space velocities (WHSV = 1.75 and 11.6 h−1 ). Activity tests were conducted at 300 ◦ C under

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atmospheric pressure. The reaction products were analyzed by an on-line gas chromatograph (GC) of Varian CP-3800, equipped with a PoraPlot-Q-HT column to separate the reaction products. 3. Results and discussion The XRD patterns of the samples (Figure 2) clearly indicate that the catalysts exhibit the typical γ-phase. Table 1 presents the crystallite sizes of various samples calculated by Deby-Scherer formula. Crystallite sizes of various catalysts

Figure 2. XRD patterns of γ-Al2 O3 samples prepared by (1) thermal decomposition, (2) precipitation, (3) sol-gel via sucrose as template and (4) sol-gel via CTAB as template

decreased as follows: sample A > sample B > sample C > sample D. The γ-Al2 O3 prepared by the addition of CTAB as template exhibited the smallest crystallite size among the samples. Therefore it can be concluded that the templates strongly affected the textural properties. Textural characteristics of catalysts were listed in Table 1. As described in Table 1, the surface area of samples decreased as follows: sample D > sample C > sample B > sample A. It can be observed that both γ-alumina samples D and C prepared in the presence of the surfactant showed higher surface area than the other two catalysts samples A and B. This shows that the using of template increases the surface area. It means that template prevents more intimate contact and aggregation among alumina particles during preparation. Moreover, sucrose and CTAB acted as chelating agent. During calcining, the chelated complex decomposed, and the produced gases prevent agglomeration and helped to form pores and fine particles with high surface area in the final products [11,12]. It was also observed that sample D has higher surface area than sample B. On the other hand, the influence of CTAB on the surface area is more dominant compared to the effect of sucrose, which is possibly due to the stronger ability of CTAB to facilitate the process of template formation. Probably, the cationic charge of CTAB micelles favors their adsorptions on aluminium hydroxides species during sol-gel preparation. Therefore CTAB causes a lower degree of solid aggregation. It is also possible that CTAB decomposes into more and larger molecules than sucrose during combustion process.

Table 1. Physicochemical properties of the prepared γ -Al2 O3 samples Sample A B C D a

Surface area (m2 ·g−1 ) 192 241 292 375

Pore volume (cm3 ·g−1 ) 0.44 0.80 0.54 0.61

Pore diameter (nm) 8.9 13.2 7.4 16.2

Particle size (nm)a 8.5 6.7 5.6 4.3

Crystallite size (nm)b 6.3 4.5 4.2 3.9

ψc 2.42 3.35 2.32 1.36

Acidity (mmolNH3 ·g−1 Cat ) weak & moderate strong 0.030 0.072 0.035 0.052 0.039 0.040 0.047 0.580

T d /◦ C peak 1 peak 2 232 424 204 418 201 406 197 389

Determined by BET area; b Determined by XRD results; c Partial sintering factor; T d : Temperature of desorption maxima

The theoretical particle sizes are also calculated from surface area, assuming spherical particles, by the following equation: 6000 DBET = (1) ρ×S where, DBET is the equivalent particle diameter (nm), ρ is the density of the material (g·cm−3), and S is the specific surface area (m2 ·g−1 ) [13]. It can be observed that the equivalent particle diameter decreased with the addition of surfactant. This observation confirmed the positive effect of surfactant addition in decreasing the particle size. As a result, it can be seen that the particle sizes of samples C and D are smaller than those of samples A and B. Based on aforementioned discussion, template prevents intimate contact and aggregation among alumina particles during preparation. Hence, particle size decreases in this case. The ψ as a factor is used to reflect the partial sintering extent of the primary crystallites and it is calculated by the following equation:

 ψ=

DBET DXRD

3 (2)

The experimental data showed that ψ increased for the samples A and B prepared without surfactant. This is due to severe sintering of their primary crystals. Although the specific surface area is one of the most important parameters, it must be taken into account that, sometimes, there is no direct relationship between catalyst activity and the physical properties of the catalyst such as surface area. Such predictions can be validated by the aid of chemisorption measurements. Here the number of catalytically active surface atoms is determined by temperature-programmed desorption of ammonia (NH3 -TPD). NH3 -TPD measurements are performed to determine the acid strength and the amounts of acid sites on catalyst surface. Desorption peaks in the range of 180−250 ◦ C, 260−330 ◦ C, and 340−500 ◦ C in the NH3 TPD profiles are commonly attributed to NH3 that has been

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chemisorbed on weak, moderate and strong acid sites, respectively [14]. NH3 -TPD of samples is shown in Figure 3. Temperatures of desorption maxima peaks (Td ) and the acidity content of the catalysts are summarized in Table1. The amounts of weak and moderate acid sites decreased as follows: sample D > sample C > sample B > sample A. It is clearly observed that the total amounts of weak and moderate acid sites of samples D and C are more than those on the other samples, and the thermal decomposition of the boehmite precursor produced a catalyst (sample A) with the lowest moderate acid sites. Also among these samples, sample D has the highest proportion of weak and moderate acidic sites.

Figure 4. Methanol conversion to DME over γ-Al2 O3 samples prepared by thermal decomposition, precipitation, sol-gel via sucrose as template and solgel via CTAB as template in two levels of WHSV

4. Conclusions

Figure 3. NH3 -TPD profiles of γ-Al2 O3 samples prepared by (1) thermal decomposition, (2) precipitation, (3) sol-gel via sucrose as template and (4) sol-gel via CTAB as template

Figure 4 shows the catalytic performance for methanol dehydration over γ-Al2 O3 catalysts at 300 ◦ C and WHSV of 1.75 and 11.66 h−1 under steady-state conditions. As indicated in Figure 4, sample A prepared by thermal decomposition method shows the lowest catalytic activity among all the samples. It can also be observed that samples B, C and D show approximately equivalent catalytic activity. The experimental results clearly showed that a low WHSV of 1.75 h−1 is not suitable for catalyst screening. Therefore, the catalytic activities are investigated under a higher space velocity (WHSV = 11.66 h−1 ). It is noted that the selectivity to DME over these catalysts is almost 100% under these reaction conditions. Comparison between activities under different WHSV shows that the activities of all samples decreased with increasing WHSV. Besides, the catalytic activity decreased as follows: sample D > sample C > sample B > sample A. Both samples D and C prepared in the presence of surfactant showed higher activity than samples A and B. In particular, the sol-gel method with the addition of CTAB produces a more active catalyst (sample D) than the others. Comparison of catalytic activity, crystallite size and moderate acidity of samples confirmed that the catalyst with smaller crystallite size gives more favorable active sites for methanol dehydration to dimethyl ether.

This work shows that the textural and acidic properties of γ-alumina are greatly influenced by the preparation method. From the catalytic activity and acidity results, it can be concluded that the γ-Al2 O3 with the addition of cationic surfactant presented a better catalytic performance for the dehydration of methanol to dimethyl ether (DME) in terms of activity and stability in a wide range of WHSV from 1.75 to 11.66 h−1 . Furthermore, catalyst with more moderate acidic sites showed higher activity for this reaction. As a final result, the addition of surfactant decreased the crystallite size and increased the surface area. Therefore the accessibility of medium acidic sites increased the amount of favorable active site for the dehydration of methanol to DME. Acknowledgements The authors wish to acknowledge Petrochemical Research & Technology Company of National Petrochemical Company in Iran for their financial support of this study.

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