CO Ratio and Temperature for Dimethyl Ether

Article pubs.acs.org/IECR

Optimization of CO2/CO Ratio and Temperature for Dimethyl Ether Synthesis from Syngas over a New Bifunctional Catalyst Pair Containing Heteropolyacid Impregnated Mesoporous Alumina Aysegul Bayat and Timur Dogu* Department of Chemical Engineering, Middle East Technical University, 06800 Ankara, Turkey

ABSTRACT: Dimethyl ether (DME) is a promising nonpetroleum diesel fuel alternate. Synthesis of DME was achieved directly from synthesis gas having different compositions, over bifunctional hybrid catalysts. Silicotungstic acid impregnated mesoporous alumina (STA@MA) was shown to be an excellent catalyst to be used as the dehydration component of the hybrid catalyst combination. Results proved that feed composition, as well as reaction temperature had important influence on product distributions, as well as on DME yield. Results obtained with different CO/CO2 ratios in the feed stream proved the positive effect of CO2 on both overall conversion of CO + CO2 and DME yield. Overall fractional conversion of CO + CO2 and DME yield values were shown to increase with an increase in CO2/CO ratio and also with an increase in reaction temperature, reaching to the values of 0.70 and 0.55, respectively, at 275 °C, with the feed stream composition of H2/CO/CO2 = 50/10/40. However, from the DME selectivity point of view, a DME selectivity value of about 0.9 was obtained with a feed stream composition of H2/ CO/CO2 = 50/40/10, at 275 °C. Further increase of CO2/CO ratio in the feed stream caused some decrease in DME selectivity, due to increased contribution of reverse water gas shift reaction, which caused formation of higher amounts of water. Increase of water concentration in the product stream has a negative effect on the dehydration reaction of methanol.

1. INTRODUCTION

Recent developments have shown that DME could be directly synthesized from synthesis gas, using bifunctional catalyst pairs, containing methanol synthesis and dehydration sites at the same proximity within the reactor. Synthesis gas, which can be produced by gasification of biomass, municipal solid wastes, agricultural residues, and coal; steam reforming of methane and other hydrocarbon mixtures; and dry reforming of biogas, can be converted to this green transportation fuel alternate, namely DME. Most of the studies reported in the recent literature were concentrated on the development of different bifunctional catalyst combinations for direct synthesis of DME.4,6,15−26 Direct synthesis of DME involves methanol synthesis reactions from CO and CO2 (R.2,R.3), as well as methanol dehydration (R.1) and water−gas shift reactions R.4.

Fast depletion of fossil fuels and related environmental issues necessitated the development of alternative nonpetroleum green transportation fuels.1−4 Dimethyl ether (DME), with its high cetane number (55−60) and clean burning properties, has been regarded as one of the most promising nonpetroleum compression-engine fuels.5,6 As compared to conventional diesel fuel, much lower CO, NOx, and particulate matter emissions were reported from DME powered engines. Due to presence of about 34% oxygen in its structure, DME can be considered as an oxygenated fuel, resulting in cleaner combustion. Conventionally, it is produced by dehydration of methanol (R.1) over solid acid catalysts, like γ-alumina, zeolites, etc.7−11 As reported in our recent studies, Nafion and heteropolyacid incorporated silicate structured catalytic materials were also highly promising catalysts for dehydration of alcohols.12−14 2CH3OH ↔ CH3−O−CH3 + H 2O © XXXX American Chemical Society

Received: August 5, 2016 Revised: October 5, 2016 Accepted: October 19, 2016

(R.1) A

DOI: 10.1021/acs.iecr.6b03001 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research CO + 2H 2 ↔ CH3OH

(R.2)

CO2 + 3H 2 ↔ CH3OH + H 2O

(R.3)

CO + H 2O ↔ CO2 + H 2

(R.4)

distribution and DME yield by changing the ratio of methanol synthesis and dehydration catalysts within the reactor.43 In most of the studies reported for direct synthesis of DME, mixtures of CO and H2 were used as the reactor feed stream. In fact, the effect of CO/H2 ratio on product distribution was discussed in some of these studies.4,20,22,24,26 It was generally considered that increase of H2/CO ratio of the feed stream had a positive effect on DME yield. In some other recent studies, it was illustrated that, DME can also be produced through hydrogenation of CO2.31−37 In most of those studies, zircona based Cu catalysts were used as the methanol synthesis component of the hybrid catalyst combinations. However, problems were reported in terms of low CO2 conversion and DME yield values.34 In two of the recent studies, DME yield and CO2 conversion values lower than 10% and 20% were reported, respectively.33−35 However, a DME yield value of 16.8% was reported by Liu et al. with a catalyst containing 1% ZrO2.36 In the work of Stiefel et al. it was shown that, addition of 8% CO2 to the CO + H2 mixture caused some decrease in DME selectivity, at 250 °C.22 However, in another study positive effect of CO2 on DME selectivity was mentioned.15 Stiefel et al. have also reported that, in the case of water feeding, WGSR became the dominating reaction, enhancing CO2 formation and decreasing DME selectivity. In another study, direct synthesis of DME was also achieved by Song et al., at 270 °C, using a feed stream composition of H2/CO/CO2 = 52/24/ 23.6 Their results indicated quite high DME selectivity values, approaching 85%. Contribution of CO2 to DME yield was expected to be strongly related to the catalyst composition, as well as feed composition and reaction temperature. In fact, it was shown in our recent work that, DME selectivity values of about 85% could be achieved at 250 °C, with a syngas containing 10% CO2, over a hybrid catalyst mixture, which was composed of TRC-75(L) type solids acid with a commercial Cu/ZnO based commercial methanol synthesis catalyst.15 Detailed investigation of the effect of CO/CO2 ratio in the feed stream on DME yield is missing in the literature. The main objective of the present study was to investigate the effect of CO/CO2 ratio on the product distribution by keeping the H2/(CO+CO2) ratio constant and to evaluate the best feed ratio of CO/CO2 and temperature for the maximization of DME yield during its synthesis directly from syngas. These investigations were performed using a hybrid catalyst system, which was composed of a commercial methanol synthesis catalyst and a novel silicotungstic acid impregnated mesoporous alumina. During the detailed investigation of the effect of feed composition on the product distribution, reaction temperature was also varied in a range of 180−300 °C. All these results were also compared with the product distributions that would be approached at equilibrium.

The overall stoichiometry of DME synthesis from CO and H2 can then be expressed as follows:15,16 3CO + 3H 2 ↔ CH3−O−CH3 + CO2

(R.5)

2CO + 4H 2 ↔ CH3−O−CH3 + H 2O

(R.6)

Direct synthesis of DME from synthesis gas was reported to have major thermodynamic advantages over the two step process, involving consecutive methanol synthesis and dehydration reactions. Equilibrium conversion of CO and/or CO2 to DME strongly depends upon the reaction temperature and the feed composition. As it was also illustrated in our recent publication, equilibrium conversion values of CO to methanol (R.2) were much less than the equilibrium conversion values estimated from the overall stoichiometry of DME synthesis reactions (R.5, R.6).15 It was also experimentally illustrated in that work that, DME yield values obtained with the mixed catalyst system (mixture of commercial methanol synthesis catalyst and TRC-75(L) type acidic dehydration catalyst), were about an order of magnitude higher than the DME yield values obtained in a system where these catalyst beds were arranged consecutively. Those results were obtained with a feed stream containing 50% CO and 50% H2. In many of the studies, Cu/ZnO/Al2O3 type catalytic materials were used as the methanol synthesis component of the bifunctional catalyst combinations.6,15,22−30 In some recent studies, catalytic performances of zirconia and/or iron based copper catalysts were also investigated.31−37 As for the methanol dehydration component of the bifunctional hybrid catalyst combinations are concerned, acidic characteristics of these materials are quite important. In most of the recent studies, different zeolites or γ-alumina were used for dehydration of the synthesized methanol.6,9,10,18 Due to their very strong surface acidity with very high proton mobility and their good redox properties, heteropolyacids have been considered quite attractive catalytic materials for acid catalyzed reactions.38−40,13−15 Ethanol dehydration reactions performed with different heteropolyacid catalysts showed that activity of silicotungstic (STA) was higher than tungstophosphoric acid (TPA) and molybdophosphoric acid.40 STA was also reported be in completely dehydrated form, at room temperature and more stable than TPA at the reaction temperatures of the present study.40,41 However, low surface area (less than 1 m2/ g) of heteropolyacids limit their widespread use as solid acid catalysts. As it was reported in our earlier publications, catalytic performance of these materials can be significantly improved by impregnation of STA or TPA into high surface area supports.13−15,42 It was also shown in our recent publication that, STA incorporated mesoporous silicate structured TRC-75 type catalyst showed good performance for dehydration of methanol during DME synthesis.15 Bifunctional catalyst combinations used in direct synthesis of DME from syngas were prepared either as physical mixtures of methanol synthesis and dehydration catalysts, or by the synthesis of new catalysts, containing both methanol synthesis and dehydration sites. In the case of using hybrid systems, which were composed of mixtures of methanol synthesis and dehydration catalysts, it is possible to modify the product

2. EXPERIMENTAL SECTION 2.1. Bifunctional Hybrid Catalyst. Direct synthesis of DME was investigated using bifunctional catalyst mixtures, which were composed of a Cu based commercial methanol synthesis catalyst (Alfa Aesar; C18W019) and mesoporous alumina (MA) based solid acid catalysts, which were synthesized in the present study. Initial experiments were performed with pure mesoporous alumina (MA), as the solid acid component of this mixture. However, in most experiments silicotungstic acid impregnated MA was used as the dehydration component of this hybrid catalyst combination. B


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Industrial & Engineering Chemistry Research

CO/CO2 ratios in the feed stream being as 50/50/0, 50/40/10, 50/25/25, and 50/10/40. These tests were repeated at each reaction temperature. Fractional conversions of CO, CO2, and CO + CO2 were evaluated as the ratios of moles CO and/or CO2 converted to the moles of CO and/or CO2 fed to the reactor. 2.3. Characterization of Synthesized Materials. Pore structures and surface area values of the synthesized catalysts were evaluated by nitrogen adsorption/desorption analysis. Nitrogen adsorption/desorption isotherms of MA and STA@ MA were obtained by a Quantachrome Autosorb-6 instrument. Samples were degassed at 120 °C for 6 h, before these tests. TEM images obtained with a Jeol 2100F HRTEM instrument also gave information about the pore structures of the materials. Before TEM analysis, samples were suspended in alcohol by an ultrasonic stirrer for 15 min. Then, a drop of this suspension was placed over HC300-Cu Holey Carbon film grid and dried overnight. XRD patterns of the synthesized materials were obtained by a Rigaku Ultima-IV X-ray Diffractometer for small angle analysis and by a Phillips PW 1840 instrument for wide angle measurements. One of the most important properties of dehydration catalysts is their acidic characteristics. To obtain information about the relative strengths of Lewis and Bronsted acidities of MA and STA@MA, diffuse reflectance FT-IR spectroscopic (DRIFTS) analyses of pyridine adsorbed samples were made by a PerkinElmer Spectrum One instrument. The catalyst samples were dried at 110 °C before the DRIFTS tests, and they were kept at 40 °C after the pyridine adsorption step.

Mesoporous alumina with an ordered pore structure is a recently discovered catalyst support with high thermal stability.39 Its acidic characteristics are also quite suitable for dehydration reaction of methanol. Due to its larger pore sizes than conventional microporous supports, it is less susceptible to catalyst deactivation due to coke formation and also has much lower diffusion resistance of reactants to the active sites. Mesoporous alumina with an ordered pore structure was synthesized by an evaporation-induced self-assembly process, as described in the literature.39,40 In this process, Pluronic 123 and aluminum isopropoxide were used as the structure directing template and the alumina precursor, respectively. A solution of aluminum isopropoxide in nitric acid and ethanol mixture was added to the P-123 solution dropwise and this mixture was continuously stirred for about 18 h. Solvent evaporation and aging (2 days) was achieved in an oven at 60 °C. Solid product was calcined in a tubular furnace, which was heated to 700 °C, with a heating rate of 1 °C/min and kept at that temperature for 6 h. Since initial DME synthesis experiments, which are discussed in the following section, indicated that the acidity of pure MA was not sufficiently high for the dehydration function of the hybrid catalyst mixture, another solid acid catalyst was prepared by wet impregnation of 10% silicotungstic acid (STA) on mesoporous alumina. Silicotungstic acid is a solid acid catalyst with a very high Bronsted acidity and it shows high activity in dehydration reactions.13,33,42 It is also quite stable up to 400 °C. During the impregnation process, MA was mixed with deionized water and a solution of STA was added to this mixture. Resultant mixture was continuously stirred at 30 °C for 47 h. This mixture was then evaporated to dryness at 70 °C, and then calcined at 350 °C for 6 h. This new catalyst was denoted as STA@MA. 2.2. DME Synthesis Experiments. Activity tests of direct synthesis of DME were performed in a fixed bed tubular reactor at 50 bar. A bifunctional hybrid catalyst was prepared by physically mixing equal amounts of Cu-based methanol synthesis catalyst (Alfa Aesar; C18W019) and the MA based solid acid dehydration catalyst. Methanol synthesis catalyst was crushed and sieved to the size of MA based solid acid catalyst, before the preparation of the hybrid catalyst mixture. This catalyst mixture (0.2 g) was placed into the center of the stainless steel tubular reactor and supported by quartz wool, at both ends. Composition of the reactant feed stream was adjusted by using mass flow controllers placed on the flow lines of each reactant. All of the flow lines were heated to 150 °C, to prevent any condensation. Total flow rate of the feed gas was adjusted to 25 mL/min, as evaluated at room temperature. The space time within the reactor was 0.48 s·gcatalyst/mL (at STP). A high pressure metering valve was placed just after the reactor, to decrease the pressure to atmospheric value. An SRI multigas gas chromatograph, which was equipped with a Carbosphere column was connected to the reactor outlet stream for online analysis of the product composition. Activity tests were performed in a temperature range of 180−300 °C, with intervals of 25 °C. Experimental results were found to reach steady state within a period of 60 min. After steady state was reached, successive analysis of the product stream was made and the averages of these successive tests were used for the conversion and selectivity calculations. DME synthesis tests were performed with different feed stream compositions, by varying the CO/CO2 ratio, while keeping the H2/(CO + CO2) ratio constant. Hence, experiments were performed with H2/

3. RESULTS AND DISCUSSIONS 3.1. Characterization of MA-Based Catalysts. Results of nitrogen adsorption desorption tests of the catalysts proved that the alumina material synthesized in this study (MA) had a typical Type IV isotherm in the mesopore range of relative pressures (P/Po) of N2, between 0.5 and 0.8 (Figure 1).

Figure 1. Nitrogen adsorption/desorption isotherms of MA and STA@MA.

Hysteresis loop was steep and parallel, which can be described as Type H1 according to IUPAC classification. These results indicated that the synthesized MA possessed uniform, cylindrical open ended pores with a long-range order. However, in the case of nitrogen adsorption/desorption isotherm of silicotungstic acid impregnated mesoporous alumina (STA@ MA), the shape of hysteresis loop was between Types H1 and H3. Adsorption and desorption branches were broad and were not very sharp, indicating some disturbance of the ordered C


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Figure 2. TEM images of STA@MA.

mesopore structure of MA due to STA impregnation. TEM images of STA@MA also indicated the presence of mesopores with short-range order within this material (Figure 2). Analysis of the nitrogen adsorption/desorption isotherms of the synthesized materials showed some decrease of BET surface area of MA, as a result of STA impregnation (Table 1). BET Table 1. Physical Properties of the Synthesized Dehydration Catalysts BET surface area, m2/g BJH mesopore volume, mL/g BJH pore diameter, nm DR micropore volume, mL/g

MA

STA@MA

323 0.77 5.7 0.16

289 0.47 4.9 0.14

Figure 3. XRD patterns of MA and STA@MA.

Figure 4. The bands observed at about 1445 and 1595 cm−1 correspond to the Lewis acid sites. As shown in Figure 4,

surface area and average pore diameter values of MA decreased from 323 m2 and 5.7 nm to 289 m2 and 4.9 nm, respectively, as a result of STA impregnation. However, there was little difference in the micropore volume values of MA and STA@ MA. These results indicated that, impregnated STA covered mainly the mesopore surfaces and caused some decrease in mesopore volume. Results proved that both MA and STA@MA were mesoporous materials with sufficiently high surface area values for catalytic applications. Small angle XRD results showed a typical peak of mesoporous alumina at about 2θ = 1° (corresponding to d100) in the patterns of both MA and STA@MA. Wide angle XRD patterns of MA showed that it was in amorphous structure (Figure 3). As it was also reported in the literature, mesoporous alumina possesses amorphous pore walls, when calcined at 400 °C, whereas formation of crystalline γ-Al2O3 was expected at higher calcination temperatures.44,45 In the case of STA@MA, formation of some γ-Al2O3 crystals were indicated by the small peaks observed at 2θ values of 66°, 46°, and 34°. Diffuse reflectance FT-IR (DRIFT) analysis of pyridine adsorbed catalytic materials gave valuable information about their acidic characteristics. DRIFT spectra obtained with pyridine adsorbed and nonadsorbed samples are shown in

Figure 4. DRIFT spectra of MA and STA@MA.

intensity of these bands were significantly increased as a result of impregnation of STA into MA. Results proved that acidity of STA@MA was more than three times higher than the acidity of MA. As for the Bronsted acid sites, bands would be expected at 1540 and 1640 cm−1. Results reported in Figure 4 indicated that Bronsted acidities of both MA and STA@MA were quite low. The band observed at 1490 cm−1 in the spectrum of D


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Industrial & Engineering Chemistry Research STA@MA can be due to contributions of both Lewis acid sites and Bronsted acid sites. However, absence of bands at 1640 and 1540 cm−1 indicated that the contribution of Bronsted acid sites to the band at 1490 cm−1 was also small. In order to have some indication of dispersion of STA within the MA matrix EDX mapping of STA@MA was also obtained. As shown in Figure 5, tungsten (green spots), which were due to impregnated silicotungstic acid, was well dispersed within the structure of STA@MA.

Figure 6. Fractional conversion values of CO over MSC+MA and MSC+STA@MA hybrid catalysts (feed stream: 50% CO + 50% H2; 50 bar).

Figure 7. Product distributions obtained with MSC+MA hybrid catalyst (feed stream: 50% CO + 50% H2; 50 bar).

Figure 5. EDX mapping of STA@MA (W: green).

3.2. Initial Activity Tests for DME Synthesis with a Feed Stream of 50% CO + 50% H2. In order to compare the catalytic performances of hybrid catalyst combinations, which were composed of methanol synthesis catalyst (MSC) with MA or STA@MA, a set of initial experiments were performed with a feed stream containing 50% CO and 50% H2. Fractional conversion values of CO were then evaluated using the reactor outlet stream compositions: XCO = (FCO,inlet − FCO,outlet)/FCO,inlet

Here, the inlet flow rate of CO, FCO,inlet (mol/s), was evaluated from the outlet stream compositions, by making a carbon balance. Fractional conversion values of CO were significantly enhanced in the case of using STA@MA as the dehydration component of the hybrid catalyst mixture. As shown in Figure 6, fractional conversion values of CO obtained with the MSC +MA catalyst mixture were 0.13 and 0.35 at 250 and 300 °C, respectively. However, in the case of using MSC+STA@MA as the catalyst mixture, corresponding CO conversion values were 0.28 and 0.50, respectively. These results indicated that acidic properties of MA was not sufficiently high for in situ conversion of produced methanol to DME. Product distributions obtained with MSC+MA and MSC +STA@MA hybrid catalysts strongly supported the conclusion that higher conversion values obtained with STA@MA than MA were mainly due to the higher acidity of STA@MA than MA, which facilitated in situ conversion of produced methanol to dimethyl ether (Figures 7 and 8). Selectivity values reported in these figures were defined as the ratios of moles of CO converted to a specific product to the total converted moles of

Figure 8. Product distributions obtained with MSC+STA@MA hybrid catalyst (feed stream: 50% CO + 50% H2; 50 bar).

CO. Hence, selectivity values of DME and methanol were defined as follows: SDME = 2FDME/(FCO,inlet − FCO,outlet)

SMetOH = FMetOH/(FCO,inlet − FCO,outlet)

As shown in Figure 7, the main product was methanol with the MSC+MA catalyst mixture, at temperatures lower than 250 °C. DME selectivity was quite low at such low temperatures and increased to a value of about 0.5 at 275 °C. On the other hand, much higher DME selectivity values were obtained with the catalyst combination of MSC+STA@MA (Figure 8). DME selectivity value reached to about 0.62 at 275 °C over the MSC E


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Industrial & Engineering Chemistry Research +STA@MA hybrid catalyst. DME yield values were also evaluated using the following definition: YDME = (2FDME)/FCO,inlet)100

Comparison of DME yield values obtained with MSC+MA and MSC+STA@MA catalysts (Table 2) also proved the superior Table 2. Comparison of DME Yield Values Obtained over MSC+MA and MSC+STA@MA with a Feed Stream Composition of 50% CO + 50% H2 temperature, °C

225

250

275

DME yield over MSC+MA, % DME yield over MSC+STA@MA, %

1.6 5.6

4.5 14.6

13.0 26.0

Figure 9. Comparison of fractional conversion values of CO2 at different feed compositions.

dehydration performance of STA@MA catalyst over MA. These results can be explained by the differences of acidic properties of MA and STA@MA. As it was discussed in the previous section, Lewis acidity of STA@MA was much higher than the acidity of MA (Figure 4). In order to test the catalytic performance of the Cu based methanol synthesis catalyst used in this study, another set of experiments were performed using only MSC catalyst in the reactor. In this case, much lower CO conversions were obtained. For instance, fractional conversion of CO was only about 0.1 at 275 °C with the MSC catalyst, while corresponding CO conversions were 0.26 and 0.42, with MSC+MA and MSC +STA@MA, respectively. All these results proved the positive effect of in situ conversion of produced methanol to DME, to achieve higher CO conversions. Based on these initial results, the rest of the direct synthesis experiments of DME were continued with the MSC+STA@MA catalyst mixture, using different feed compositions. 3.3. Effects of CO/CO2 Ratio on DME Selectivity and Yield. The effects of CO/CO2 ratio of the feed stream on the product distribution and DME yield were investigated using the MSC+STA@MA hybrid catalyst system, in a temperature range of 180−300 °C and at a pressure of 50 bar. In this regard, results obtained with different feed compositions of H2/CO/ CO2 being 50/50/0, 50/40/10, 50/25/25, and 50/10/40 were compared. In these experiments the ratio of H2/(CO + CO2) was kept constant as 50/50. Fractional conversions of CO and CO2, as well as the overall conversion of (CO + CO2) were evaluated in each experiment. Results obtained with different CO/CO2 ratios proved that both CO2 and CO acted as resources in the synthesis of DME. Carbon dioxide fractional conversion values reported in Figure 9 clearly showed that very high CO2 conversion values could be achieved, especially at temperatures lower than 250 °C. As it will be discussed in relation to overall conversion values of (CO + CO2) and product distributions, a significant fraction of CO2 was converted to CO at low temperatures through reverse water−gas shift reaction (RWGS, reverse of R.4). In fact, the amount of CO in the product stream did not decreased but increased at such low temperatures. As shown in Figure 9, increase of CO/CO2 ratio of the feed stream caused much sharper decrease of fractional conversion of CO2 with an increase in temperature. In fact, for the feed stream with a composition of H2/CO/CO2 = 50/40/10, negative values of CO2 conversion were obtained at temperatures higher than 250 °C, indicating that the production rate of CO2 through water gas shift reaction was more than its consumption rate for the production of methanol and DME.

In order to have a better understanding of the effects of feed composition, overall conversion values of (CO + CO2) and product distributions were also analyzed. Overall conversion of (CO + CO2) was defined as follows: X(CO + CO2) = [(FCO,inlet + FCO2,inlet) − (FCO,outlet + FCO2,outlet)] /(FCO,inlet + FCO2,inlet)

This definition of overall conversion of (CO + CO2) also corresponds to X(CO + CO2) = (FMetOH + 2FDME + FCH4 + ...) /(FCO,inlet + FCO2,inlet)

As shown in Figure 10, significant increase of fractional conversion of (CO + CO2) to the products, namely methanol,

Figure 10. Overall conversion of CO + CO2.

DME, etc., was achieved with an increase in CO2/CO ratio, up to a value of 0.7 for H2/CO/CO2 = 50/10/40 at 275 °C (Figure 10). As may be expected, increase of reaction temperature from 180 to 275 °C also caused significant increase in fractional conversion of (CO + CO2). As was discussed in relation to Figure 9, some conversion of CO to CO2 took place through WGSR, for the feed composition containing a CO/CO2 ratio of 40/10, especially at temperatures higher than 250 °C. Conversion of CO to CO2 was more significant for the feed stream containing no CO2. In fact, while the total fractional conversion of CO was 0.42 at 275 °C with F


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DME selectivity values were obtained with the feed stream composition of H2/CO/CO2 = 50/40/10. This behavior is more clearly seen in Figure 12. The highest DME selectivity was obtained as 0.9 with this feed stream, at 275 °C, where overall methanol selectivity was only 0.1.

the feed stream composition of H2/CO/CO2 = 50/50/0, fractional conversion of CO to the desired products (DME and methanol) was only about 0.3. This difference was simply due to conversion of some of CO to CO2 through WGSR. Increase of mole fraction of CO2 in the feed stream hindered to conversion of CO to CO2 through WGSR. Product distributions obtained within the temperature range of 180−300 °C proved formation of mainly DME and methanol (Figure 11). Formation of negligibly small amounts

Figure 12. Overall DME selectivity values, based on converted CO + CO2.

Achievement of DME selectivity values approaching 0.9 were highly promising. Addition of some CO2 to the feed stream hindered the occurrence of WGSR. However, further increase of CO2/CO ratio over 10/40 caused some decrease in DME selectivity (Figures 12 and 13). This was explained by the

Figure 13. Fractional conversion values of CO + CO2 and overall DME selectivity values obtained with different feed compositions, at 275 °C.

Figure 11. Product distributions obtained with feed compositions of H2/CO/CO2 being (A) 50/40/10; (B) 50/25/25; (C) 50/10/40.

of side products (like methane) started at temperatures higher than 275 °C. Also, catalyst deactivation problems starts at temperatures higher than 300 °C.15 Overall selectivity values were defined as the ratios of moles of (CO + CO2) converted to a specific product to the total converted moles of CO + CO2.

increase of H2O formation through reverse water gas shift reaction, as a result of the increase in CO2/CO ratio. Increase of H2O in the product stream was expected to have a negative effect on the dehydration step of methanol to produce DME. Although the highest DME selectivity was obtained with the feed stream containing H2/CO/CO2 = 50/40/10, overall conversion of (CO + CO2) increased with an increase in CO2/ CO ratio in the feed stream. As a result of this increase in overall conversion of (CO + CO2), maximum DME yield was obtained with the feed stream composition of H2/CO/CO2 = 50/10/40 (Figure 13). In fact, the highest value DME yield was obtained as about 0.55, at 275 °C, with this feed stream composition. Thermodynamic calculations were also performed to predict the fractional conversion values of CO at equilibrium. In these

SDME,overall = 2FDME/[(FCO,inlet + FCO2,inlet) − (FCO,outlet + FCO2,outlet)]

SMetOH,overall = FMetOH/[(FCO,inlet + FCO2,inlet) − (FCO,outlet + FCO2,outlet)]

As shown in Figure 11, the main product was DME, especially at temperatures over 225 °C. The highest overall G



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calculations simultaneous occurrence of methanol synthesis, dehydration and water gas shift reactions were considered. Results obtained for equilibrium conversions of CO at different feed compositions are shown in Figure 14. Experimental values

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of TUBITAK (Scientific and Technological Research Council of Turkey) through the project 108M571, contributions of Middle East Technical University (METU) Research Fund and METU Central Laboratory are gratefully acknowledged.

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REFERENCES

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Figure 14. Equilibrium and experimental fractional conversion values of CO over MSC+STA@MA, at different temperatures and with different feed compositions.

of fractional conversion of CO are also shown in the same figure. Results indicated some decrease of equilibrium conversion of CO with an increase in CO2/CO ratio. This was considered to be due to the contribution of reverse water gas shift reaction. As shown in Figure 14, experimental values of fractional conversion of CO approached to the corresponding equilibrium conversions, at temperatures higher than 275 °C.

4. CONCLUSIONS Comparison of experimental results obtained over hybrid catalyst combinations MSC+STA@MA and MSC+MA with the results obtained by using only the methanol synthesis catalyst (MSC) in the reactor, proved that in situ conversion of produced methanol to DME caused significant enhancement in CO conversion as well as DME yield. It was concluded that, impregnation of MA with STA resulted in a catalyst with more strong surface acidity, which facilitated in situ dehydration of methanol to DME, in its direct synthesis from synthesis gas. Experimental results obtained with feed streams having different CO2/CO ratios, by keeping the H2/(CO + CO2) ratio the same, proved the positive effect of CO2 on both overall conversion of CO + CO2 and DME yield. Very high DME selectivity values were shown to be obtained with a feed stream containing a CO/CO2 ratio of 4/1, which reached to a value of 0.9 at 275 °C. Further increase of the CO2/CO ratio in the feed stream caused some decrease in DME selectivity, due to the increased contribution of reverse water gas shift reaction, which caused formation of higher amounts of H2O. Increase of H2O concentration has a negative effect on the dehydration step of methanol to DME. DME selectivity values approaching 0.9 and yield values of about 0.55 showed superior performance of the hybrid catalyst combination containing STA@MA as the dehydration component of this mixture. Furthermore, positive contribution of CO2 to DME yield was also considered as a promising result, from the point of view of using one of the most abundant greenhouse gases (CO2) to produce a valuable fuel alternate, namely DME. H


Article


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CO Ratio and Temperature for Dimethyl Ether

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