Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(1), 57-62, January (2013) Res.J.Chem. Sci. International Science Congress Association 57 Synthesis of Some Baria-Modified -Al for Methanol Dehydration to Dimethyl EtherSafaeeHoda, Sohrabi Morteza andFalamaki CavusChemical Engineering Department, Amirkabir University of Technology, Hafez Avenue, Tehran 15914, IRAN Available online at: www.isca.in Received 26th September 2012, revised 10th October 2012, accepted 22nd November 2012Abstract In the present study a kinetic model for the catalytic dehydration of methanol to dimethyl ether, using some baria-modified gamma alumina as the reaction catalyst have been presented. Three catalyst samples were prepared consisting of Ba2+ impregnated on -Al, using barium nitrate solution. It was noted that the Ba2+ content of the catalysts had a high impact on the activity of the latter. The operating temperature range was 260-290 șC and the pressure was 1 bar. Catalysts' activity and kinetic measurements were carried out using a catalytic fixed bed micro- reactor. Keywords: Modified -Al, methanol dehydration, dimethyl ether.Introduction Owning to the increasing air pollution during the last decade, and imposing of tougher environmental regulations, dimethyl ether (DME) has received global attention due to its potential use as a clean alternative fuel for diesel engines. In addition DME is an intermediate in the preparation of a number of industrial chemicals1-3. DME can be produced from either of the following two methods: i. a single step process, that is the direct formation of DME from synthesis gas over hybrid catalysts and ii. dehydration of methanol over solid acid catalysts4-14 according the following reaction: 2CHOH CHOCH + HO H = -23.4 kJ/mol (1) Among the solid acid catalysts used for methanol dehydration, -Al has been studied intensively for both academic and commercial purposes15-18. However, this compound is normally sintered during a relatively long period of application. As sintering is the most serious cause of gamma alumina deactivation; the main purpose of the present study, was to prepare some catalysts with longer life times and lower operating temperature. Three catalyst samples were prepared, characterized and tested. The sample with the highest activity was considered as a suitable catalyst. Material and Methods The chemicals used in the present study were all analytical grades and supplied by Merck, BASF and Condea Plural, Germany. These were barium nitrate [Ba(NO], Methanol, Al and Pseudo boehmite. -Al: Gamma aluminawas prepared by thermal decomposition of the pseudo boehmite precursor at 600 șC for 20 min, applying a heating rate of 20 șC/min. -Al3 modification: Introduction of BaO to -Al should be performed up to an optimum value. It is well known that BaO largely inhibits the phase transition19. In addition, excess BaO reduces the catalyst’s surface area leading to the reduction of catalytic activity. As the purpose of this work containing various amounts of barium oxide (0.1, 1, 5 wt%) were prepared. Barium oxide was impregnated to -Al3 by first dissolving the barium nitrate, in deionized water (100 ml) and mixing the latter with an aqueous suspension of -Al3 (10 g) in deionized water (200 ml). The slurry was then stirred for 3h, and gradually dried in a rotary evaporator at 70 șC for 3h, to avoid segregation of impregnated nitrate. Heating of the sample was continued over night, in an oven at 110 șC, before being calcined at 535 șC for 1h, to remove hydroxyl water and nitrate. The nitrates of divalent cations generally decompose to corresponding oxides at relatively low temperature, around 500 șC 20-21. 2 Ba(NO + heat 2 BaO + 4 NO +O (2) The ratio of barium oxide to -Al3 was fixed at 0.1, 1 and 5 wt%. These samples and the one containingno barium oxide were analyzed by STA test (TGA+DTA) to determine and compare the transformation temperature. Figure 1a and b shows the DTA and TG curves of the raw and modified -Al3 samples containing various amounts of BaO. The exothermic transition peaks were observed at 1209 and 1216 șC for the raw sample and the one modified with 0.1 wt% BaO, respectively, whereas no exothermic transition peak was detected in case of other samples. These measurements were conducted using STA equipment, model PLSTA. In case of TG measurements, a heating rate of 10șC.min-1 under continuous flow of air was applied. The sample's weight was 10 mg. In TG curve for the sample containing 0.1 wt% barium nitrate an endothermic peak was observed around 200șC. This may be related to the loss of hydroxyl water and nitrate 20. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(1), 57-62, January (2013) Res. J. Chem. Sci. International Science Congress Association 58 Figure-1a DTA curves of different sample Figure-1b TG curves of different samples As the rest of the samples had been dried before the DTA test, such a peak was not appeared in TG curves of the latter. In case of the sample containing 5wt% of barium nitrate, a peak was observed around 500șC. This could be due to the barium nitrate decomposition to BaO at such a temperature19. Such a peak was not significant for all other samples as they had been dried at high temperature before the STA test. STA measurements demonstrated that a content of 0.1 wt% BaO retarded the to phase transition about 7șC, while larger contents shifted the transition temperature to higher than 1400 șC. The results are given in table 1. Based on the catalytic performance of the samples, it was deduced that addition of 1wt% BaO increases the life time of catalyst while maintaining a reasonable catalyst activity. The results are shown in table 1. Catalytic Activity: Catalytic activity of the selected samples was studied under steady state conditions in a fixed bed reactor (16 mm i.d. and 70 cm length) equipped with an on-line GC apparatus. Experimental runs were all performed at atmospheric pressure, and a temperature of 260șC. As previous studies on Al3 activity have shown that with particles smaller than 0.17 mm in size the intra-particle resistances are negligible22, prior to catalytic testing, the samples were pressed, crushed and then sieved to 0.11 mm pellets. In each experiment, 1 g of the catalyst was loaded to the reactor and fixed between two quartz packing (0.01 mm in size) at both ends of the catalyst bed. Nitrogen saturated with pure methanol (0.35 bar partial pressure of methanol in nitrogen) was used as the feed, with a flow rate of 665.57 cm3 min-1. The catalyst was first activated at atmospheric pressure and temperature of 410șC for 1h, applying a flow of nitrogen gas with the rate of 100 cm3 min-1. The nitrogen flow rate was then increased to 665.57 cm3 min-1 with simultaneous decrease in temperature to 260șC. When a stable temperature was established, methanol was injected to the nitrogen stream with a flow rate of 0.6 cm3 min-1. After steady state conditions were prevailed within the system (about 45 min), analysis of the outlet gas from the reactor was performed using a GC online apparatus. The results are shown in table 2. In order to examine the stability and activity of the catalyst containing 1wt% BaO during a long period of application, an experimental run was performed using such a catalyst for 120 h time on stream. It was found that neither structure nor activity of the sample was affected during this period. Based on such an observation, it was deduced that addition of 1wt% BaO increases the life time of catalyst while maintaining a reasonable catalytic activity. Table-1 Phase transformation temperatures of the catalyst samples Samples Raw. Al 2 O 3 0.1 wt%1 wt%5 wt% Phase transformation Temperature șC 1206 1216 �1400 �1400 Table-2 Methanol conversion on different catalyst samples Raw. Al 2 O 3 Commercial -Al0.1wt % 1 wt%5 wt% Methanol conversion 28.34 28.42 25.56 21.31 7.3 Results and DiscussionCatalyst Characterization: The raw -Al catalyst and the modified sample with 1wt% BaO were characterized applying BET, FTIR, XRD and SEM analyses. The BET specific surface area of the 1wt% modified sample and raw -Al are given in table 3. Table-3 BET specific surface area 0 wt% 1 wt% Catalyst sample 200 143.2 BET result (m 2 /g) As it is apparent from the table, the BET of the modified sample is less than that of the raw Al. This may be due to the presence of large BaO molecules in the modified sample. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(1), 57-62, January (2013) Res. J. Chem. Sci. International Science Congress Association 59 The FTIR spectra of the raw and modified catalysts are shown in figure 3a and b. The raw catalyst shows an adsorption band in a region close to 3455.14 cm-1. The bands around this region are usually attributed to bridging hydroxyls (free bridging or Bronsted acid sites). On the other hand, the Lewis acid sites appeared in 1465 cm-123. It is evident that the number of both acid types in modified catalyst is higher than that of the raw sample. This analysis was carried out using a Nicolet FTIR equipment, model NEXUS 70 EP. Figure-3a FTIR spectrum of the raw sample Figure-3b FTIR spectrum of the modified sample Figure 4a and b demonstrates the XRD patterns of the raw and modified -Al24. The peaks appeared at 2 close to 26, 34 and 45 are related to BaO. The X-ray analysis have been carried out applying a Philips X-ray diffractometer, model XPERT.  Figure-4a X-Ray diffractometery of raw sample, the peaks appeared at = 13.85, 36.89, 39.77, 46.49 and 66.65 are related to Al3 24 The microstructure of raw -Al and modified catalyst containing 1 wt% BaO was revealed by scanning electron microscopy analysis (Philips SEM, model XL30). As it may be observed from figure 5a and b, presence of about 1 wt% of BaO in the samples retards the sintering of -Al particles. Prior to the SEM analysis, the samples were prepared as small tablets and heated in a furnace for 1h at 1400șC. Figure-4b The X-Ray diffractometery of the modified sample Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(1), 57-62, January (2013) Res. J. Chem. Sci. International Science Congress Association 60 Kinetic investigation: The process conditions applied in the kinetic investigations and the conversions of methanol are presented in table 4. A plug flow was assumed for the gas phase within the reactor. Figure-5a SEM of the raw sample Figure-5b SEM of the modified sample A number of studies on methanol dehydration to DME have been carried out, including those of Bercic and Levec (1992, 1993) Royaee et al (2008). Both Eley-Ridel and Langmuir-Hinshelwood type mechanisms have been proposed for the catalytic dehydration reaction of methanol25-29. A kinetic model based on Langmuir-Hinshelwood surface controlled reaction with dissociative adsorption of methanol, neglecting adsorption term for DME, was found to be well correlated with the experimental results. In this model the term describing the reversible nature of the reaction was disregarded. This was due to the identical results obtained with and without considering the latter term in the kinetic model. The reaction mechanism proposed is as follows, + 2 M.S K C C = 2 MS C (2) M.S + M.S + W.S + 3 S R = 2 2 MS (3) W.S + S KW W WS (4) Leading to the following model, 22 0.54 [12()] MMMMWWkKCKCKC++ (5) Where, M, W and D stand for methanol, water and DME, respectively; S, is the active site; KM and K, are the adsorption coefficients of methanol and water, respectively; C, W and C are concentrations of methanol, water and free active sites, respectively; , is the rate of methanol dehydration; , is the rate coefficient; 2 MS C and WS, are the concentrations of methanol and water adsorbed on the active sites. Table 5 shows the optimized parameters for the reaction scheme presented. A comparison has been made between the experimental data for DME concentrations and those predicted from the model as shown in figure 6. The mean absolute deviation is within the range of 6-10%. Table-4 The process parameters and measured conversion of methanol MeOH (mol%) MeOH flow rate (cmmin-1) Nitrogen flow rate (cmmin-1) Conversion at 260șC Conversion at 280șC Conversion at 290șC 20 0.30 716.76 27.73 39.73 46.33 27.5 0.45 691.165 24.38 34.49 40.02 35 0.60 665.57 21.31 29.90 36.85 42.5 0.75 601.705 17.55 27.85 34.51 50 0.90 537.84 15.44 25.23 31.70 Table-5 Optimal calculated parameters for reaction w (m 3 /kmol) K M (m 3 /kmol) K (kmol/hr) Temperature 739.68 21.13 0.66 260 șC 560.11 11.5 2.76 280 șC 384.2 9 5.5 290 șC Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(1), 57-62, January (2013) Res. J. Chem. Sci. International Science Congress Association 61 Figure-6 Calculated data versus experimental data of DME outlet concentrations Conclusion Analysis of the synthesized catalysts revealed that modification of -Al by 1 wt% barium oxide imposed a high resistance to phase transition and sintering of the catalyst 17. A comparison was made between the performance capability of the raw and modified -Al in dehydration of methanol under identical operating conditions (T=260 șC, P= 1 bar, methanol partial pressure= 0.35 bar). As it is apparent from the present results, with both raw and modified -Al at low temperatures and pressure, reasonable methanol conversions are obtained. Lower temperature and pressure mean lower energy consumption and thus, less expenses. Decrease in conversion of methanol with application of -Al impregnated with 1wt% barium oxide in comparison with raw -Al may be completely compensated by increase in life time of modified -Al. 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