Effect of Operating Conditions on Dimethyl Ether Steam Reforming

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Effect of operating conditions on dimethyl ether steam reforming over a CuFeO/#-AlO bifunctional catalyst 2

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Lide Oar-Arteta, Aingeru Remiro, Andres Tomas Aguayo, Javier Bilbao, and Ana Guadalupe Gayubo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02368 • Publication Date (Web): 27 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015

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Industrial & Engineering Chemistry Research

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Effect of operating conditions on dimethyl ether steam reforming over a CuFe2O4/γ-Al2O3 bifunctional catalyst Lide Oar-Arteta, Aingeru Remiro*, Andrés T. Aguayo, Javier Bilbao, Ana G. Gayubo Chemical Engineering Department, University of the Basque Country, P. O. Box 644, 48080, Bilbao, Spain. Phone: +34 946 015361. Fax: +34 946 013 500. *Email: [email protected] Abstract A study is conducted on the effect the operating variables (temperature, space time, steam/DME molar ratio, DME partial pressure) have on DME steam reforming over CuFe2O4/γ-Al2O3 bifunctional catalyst in order to obtain high H2 yield with minimum deactivation by coke. The experiments were carried out in a fluidized bed reactor under the following operating conditions: 250-400 ºC; space-time, 0.029-1.16 gcatalysth/gDME; steam/DME molar ratio, 3-8; DME partial pressure, 0.05-0.2 atm. A good balance is required between the reaction rates for both steps, DME hydrolysis (on the acid function) and the steam reforming of methanol (on the metallic function), in order to minimize methanol concentration in the reaction medium because it promotes deactivation of the metallic function. A high steam/DME molar ratio minimizes deactivation. Steady H2 yield of 82 % along 10 h is obtained at 350 ºC, S/DME of 4 and space-time of 0.58 gcatalysth/gDME.

Keywords: DME steam reforming, Hydrogen, Copper ferrite spinel, Boehmite, Operating conditions, Deactivation

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1. Introduction In response to the global growing energetic demand, H2 emerges as a clean raw material and energy vector, which may be produced at large scale from renewable sources.1 Moreover, it can be used directly in transport and stationary power generation or by means of a hydrogenated intermediate (H2 carrier) that can be transformed in situ into H2 by reforming for use in hydrogen fuel cells.2 Although methanol is a suitable H2 carrier because it is easily and selectively converted into H2-rich gas at low temperatures (150-300 ºC) by steam reforming,3 its toxicity and corrosiveness boost the use of alternative H2 vectors. Amongst these, dimethyl ether (DME) has clear advantages, since it is relatively inert, non-corrosive, non-carcinogenic, it can be stored and handled as LPG, and consequently it is more readily used in fuel cells.4 Moreover, single-step DME synthesis has aroused great interest since it is considered a key factor for the viability of lignocellulosic biomass gasification5,6 and for the large scale upgrading of CO2 by co-feeding it with the syngas.7-9 The steam reforming of DME (SRD) proceeds over bifunctional catalysts by the following steps: DME hydrolysis (over the acid function)

(CH 3 ) 2 O + H 2 O ⇔ 2CH 3 OH

MeOH steam reforming (over the metallic function) CH 3 OH + H 2 O ⇔ 3H 2 + CO 2

(1) (2)

Consequently, the stoichiometry of the reaction is: ( CH 3 ) 2 O + 3 H 2 O ⇔ 6 H 2 + 2 CO 2

(3)

Appropriate catalysts and operating conditions must be employed in order to promote H2 formation and minimize secondary reactions, such as methanol decomposition, reverse WGS and methanation reactions. The most commonly used acid function for the preparation of the bifunctional catalyst is γ-Al2O3, which requires temperatures in the 300-400 ºC range to promote the DME hydrolysis step. However, DME decomposition and reverse WGS reaction are also promoted at these temperatures, and consequently CH4 and CO formation increases considerably.10,11 HZSM-5 zeolite allows reforming at lower temperature, since its high acidity enhances the hydrolysis reaction.12,13 However, it may favor the formation of hydrocarbons, via MTH (methanol to hydrocarbons) reactions, which drastically diminishes H2 production14-16 and also contributes to increasing deactivation by coke deposition.17 Consequently, in order to avoid both MTH reactions and catalyst deactivation, the acidity of the HZSM-5 zeolite must be attenuated.16,18 CuO-ZnO-Al2O3 (CZA) is the most used metallic function in the preparation of the bifunctional catalyst for SRD due to its high activity and H2 selectivity in the methanol reforming step (eq 2).10,13,16,17,19-22 Whereas ZnO promotes Cu dispersion,23,24 Al2O3 provides


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specific surface area and improves Cu dispersion,25 which contributes to attenuating coke deactivation.22 Nevertheless, CZA catalyst undergoes Cu sintering above 300 ºC, which is a disadvantage for the regeneration of the bifunctional catalysts by coke combustion, since the coke deposited on the acid function burns only above 350 ºC.16,26 Copper-spinel type metallic functions have been proposed as a solution to the regenerability problem associated with the CZA metallic function. Thus, they are thermally stable because they do not undergo Cu sintering even at very high temperature (up to 900 ºC). Although most research has been conducted with Cu-Fe spinel, CuFe2O4, other Cu spinels have also been studied, such as CuMxO4 (M = Al, Co, Cr, Ga, Mn, Ni).15,26-34 Oar-Arteta et al.26 proved the complete regenerability of this metallic function by studying the catalyst in successive reaction-regeneration cycles. A previous study on the performance in the SRD reaction of several catalysts prepared by wet extrusion of the Cu-Fe spinel metallic function together with different acid functions highlighted the interest in using boehmite (AlO(OH)) as a precursor of the acid function.35 The advantages of using boehmite as precursor of the acid function instead of commercial γAl2O3 and HZSM-5 zeolite are the following: i) boehmite acts as a binder of the metallic function, thus conferring to the catalyst high mechanical strength; ii) during the activation of the catalyst boehmite is transformed into γ-Al2O3,36 which has higher acidity than the commercial one, and consequently shows higher activity in the DME hydrolysis step. The acidity of γ-Al2O3 obtained from boehmite is slightly lower than that of the alkali treated HZSM-5, thus decreasing deactivation by coke and avoiding the formation of hydrocarbons from methanol.35 In addition to these advantages, boehmite is cheaper than HZSM-5 zeolite. The objective of this work is to delimit the optimal range of operating conditions (temperature, space time, steam/DME ratio and DME partial pressure) for maximizing H2 yield with minimum CO formation and catalyst deactivation in the DME steam reforming over a bifunctional catalyst made of Cu-Fe spinel as the metallic function (CuFe2O4) and γAl2O3 obtained from boehmite (by calcination at 550 ºC) as the acid function. For that purpose, a detailed analysis of the effect operation conditions have on the conversion of DME and yield of products has been carried out. As previously remarked, this catalyst was proved to be a promising catalyst for SRD process since, it is regenerable, stable, and provides high activity for both steps in SRD (DME hydrolysis and MeOH steam reforming), with high hydrogen selectivity.35



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2. Experimental 2.1 Catalyst preparation The copper ferrite spinel (CuFe2O4) was prepared by the sol-gel method from a citrate complex precursor, according to Shimoda et al.15 An aqueous solution of Cu and Fe nitrates was stirred for 2 h at 60 °C, and the citric acid was then added and kept stirring for one hour more. After this, it was heated to 90 °C in order to eliminate the remaining water. Subsequently, the solution was maintained into a muffle furnace until the oxide powder was formed, firstly at 150 °C for 30 min and then at 300 °C for 5 h in order to decompose the citric acid. Finally, the resulting spinel was calcined at 900 °C for 10 h. The binder boehmite (AlO(OH) form) was provided by Sasol, commercially denoted as DISPERAL®. The bifunctional catalyst was prepared by wet physical mixing of CuFe2O4 spinel and commercial boehmite, with a mass ratio of 1/1 between the solids, which is the suitable mass ratio for attaining the maximum hydrogen yield with high stability.35 Subsequently, the resulting mixture was treated by wet extrusion and dried overnight. Finally, the catalyst was ground and sieved (90-250 µm) and calcined at 550 ºC for 2 h in order to transform the boehmite into γ-Al2O3 acid function. The CuFe2O4/γ-Al2O3 bifunctional catalyst was subjected to an equilibration treatment in order to attain high activity and reproducible behavior in the SRD reaction. The equilibration treatment involved a reaction-regeneration cycle under the following conditions: steam reforming under high coke deposition conditions (10 h time on stream at 400 ºC and steam/DME molar ratio of 3), and subsequent regeneration by coke combustion with air at 500 ºC.35 The physicochemical properties of the fresh catalyst, after equilibration, are as follows: SBET of 85 m2/g and Vpore of 0.25 cm3/g, calculated by N2 adsorption-desorption in a Micromeritics ASAP 2010; metal surface area of 14.4 m2/gCu and metal crystallite size of 38 nm, determined by CO pulse chemisorption at 50 ºC in He atmosphere in a Micromeritics AutoChem 2920 coupled to a Balzers Instruments Omnistar mass spectrometer. 2.2 Experimental device and product analysis The kinetic runs were carried out in an automated reaction equipment (Microactivity Reference from PID Eng & Tech) provided with an stainless steel isothermal fluidized-bed reactor (22 mm of internal diameter and total length of 460 mm). The fluidized bed reactor ensures bed isothermicity during the steam reforming of DME (a highly endothermic reaction), which is necessary to properly assess the role of temperature in the reaction. The catalytic bed consisted of the catalyst (90-250 µm) mixed with an inert solid (carborundum, SiC, 30-50 µm particle size), at 8/1 inert/catalyst mass ratio in order to attain suitable hydrodynamic regime. Preliminary studies show that this range of particle size and the


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inert/catalyst mass ratio are suitable for achieving appropriate hydrodynamic properties in the catalytic bed.37 A gas chromatograph (Agilent Micro GC 3000) was used for continuous product quantification. The device was provided with four modules for the analysis of the following: (1) permanent gases (O2, H2, CO, and CH4) with 5A molecular sieve capillary column; (2) light oxygenates (C2-), CO2 and water, with Plot Q capillary column; (3) C2-C4 hydrocarbons, with alumina capillary column; (4) oxygenated compounds (C2+) with Stabilwax type column. The reactor was connected on-line to the gas chromatograph through a thermostated line (heated at 140 ºC) in order to avoid water condensation, so that all products at the reactor outlet (water included) are analyzed in the gas chromatograph. The operating conditions employed for the parametric study were the following: temperature range from 250 to 400 ºC; space time from 0.029 to 1.16 gcatalysth/gDME; total pressure, 1.3 atm; steam/DME molar ratio from 3 to 8; PDME, from 0.05 to 0.2 atm. Total pressure was kept constant and the DME partial pressure for each S/DME ratio was modified using the corresponding ratio for the inert gas, He/DME. Long duration runs (up to 33 h) were conducted in order to study the effect of operating conditions on the stability of the catalyst. Prior to the catalytic runs, the catalyst was reduced in situ using 10 % H2 in He at 350 ºC for 2 h, with a total flow rate of 100 ml/min. 2.3 Reaction indices The catalytic performance was quantified based on the following reaction indices: DME conversion

X=

Yields of products

Yi =

FDME,0 − FDME FDME,0

Fi FDME,0 ⋅ υi

(4)

(5)

where FDME,0 and FDME are the molar flow rates of DME at the reactor inlet and outlet, respectively, Fi is the molar flow rate of each i product (H2, CO2, CO and MeOH) at the reactor outlet, which is evaluated based on both its molar fraction (determined from chromatographic results) and the total molar flow rate (determined by atomic balances for H, C and O), and υi is the stoichiometric coefficient for the formation of component i from methanol, which is υi = 2 for CO2 and CO and υi = 6 for H2. Moreover, an additional reaction index is defined, corresponding to the methanol effective conversion in the second step of the SRD reaction: X MeOH =

FMeOH,0 − FMeOH FMeOH,0


(6)


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where FMeOH,0 is the methanol molar flow rate calculated from the DME conversion, namely, twice the molar flow of DME converted, according to the DME hydrolysis stoichiometry. FMeOH is the methanol molar flow rate at the reactor outlet, determined experimentally by chromatography. It is important to highlight that the calculation of methanol conversion allows evaluating the activity of the metallic function in the methanol steam reforming step (eq 2).22 Under conditions with negligible deactivation (those corresponding to high values of space time, above 0.5 gcatalysth/gDME) the reaction indices corresponding to zero time on stream were determined by means of dynamic runs conducted under increasing temperature steps of 25 ºC and 2 h time on stream at each temperature, from 250 to 400 ºC. Stable behavior of the catalyst along time on stream was proven for each temperature in this dynamic runs. Under operating conditions with significant deactivation, the reaction indices at zero time on stream were obtained by extrapolating to zero time the values of evolution of reaction indices with time on stream determined in long duration runs at constant temperature.

3. Results and discussion This study analyzes the effect of temperature, space time, steam/DME molar ratio in the feed and DME partial pressure on the reaction indices at zero time on stream and on their evolution with time on stream for the CuFe2O4/γ-Al2O3 (1/1, w/w) bifunctional catalyst. The effect of total pressure is not addressed since hydrogen production is not thermodynamically favored at high pressure. The total pressure was around 1.3 atm (small overpressure due to the continuous sampling device).

3.1. Temperature Figure 1 shows the effect of temperature on the values of DME and methanol conversions and on H2, CO2 and CO yields at zero time on stream under high space-time (1.16 gcatalysth/gDME) and high steam/DME molar ratio in the feed (S/DME = 8). The values of product yields corresponding to the thermodynamic equilibrium (dashed lines, which have been calculated by minimizing the Gibb’s free energy with Pro II-Simsci® 8.3 software) are also shown for comparison. The conversions of DME and MeOH in the thermodynamic equilibrium are full in the whole range of temperatures studied. Full methanol conversion is reached in the whole temperature range studied, whereas DME conversion increases sharply with temperature in the 300-350 ºC temperature range, which involves an almost parallel increase in H2 and CO2 yields. CO yield, which is very low below 350 ºC, increases considerably by increasing the temperature in the 350-400 ºC range, since reverse WGS reaction (CO2 +H2 ⇔ CO + H2O) is favored. The promotion of reverse WGS reaction at high temperature explains that CO2 yield remains almost constant above 375 ºC, whereas DME conversion continues increasing to full


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conversion at 400 ºC. Furthermore, Figure 1 shows that the yield of CO obtained is higher than that corresponding to the thermodynamic equilibrium for temperatures above 360 ºC, and also that H2/CO2 molar ratio (which is 3 until 350 ºC) increases slightly as temperature is increased above 350 ºC. These results are attributed to methanol decomposition (CH3OH CO + 2H2) at high temperatures, but not to DME decomposition (CH3OCH3 CO + CH4+ H2) because no increase in CH4 yield is observed. Figure 1 It must be remarked that no significant CH4 formation took place by methanation (CO + 3H2 ⇔ CH4 + H2O) nor by DME decomposition (CH3OCH3 CO+CH4+H2) in the whole temperature range studied for DME steam reforming on the CuFe2O4/γ-Al2O3 catalyst. The effect of temperature on catalyst stability was analyzed by means of long duration runs under low space time conditions, given that for the high space time values in Figure 1 extremely long runs are required in order to observe the decrease in reaction indices with time on stream. Therefore, Figure 2 shows the evolution with time on stream of DME conversion, methanol conversion (a) and H2 and CO yields (b) at different temperatures and for 0.073 gcatalysth/gDME and S/DME = 3. Figure 2 First, it must be remarked that, for this low value of space time, the methanol conversion is not full at zero time on stream and it notably decreases with temperature in the range studied. This result may be explained by the different effect temperature has on the reaction rate of both reaction steps, DME hydrolysis and methanol steam reforming. Thus, at low temperature and low space time, DME conversion is low and the low quantity of methanol formed is almost fully reformed over the spinel metallic function. Nonetheless, when temperature is increased, the reaction rate for DME hydrolysis increases faster than that for methanol steam reforming, and consequently the calculated methanol conversion decreases. According to the opposite evolution with temperature of DME and methanol conversions at zero time on stream for 0.073 gcatalysth/gDME space time, the initial H2 yield remains almost constant in the 325-375 ºC range. The effect of temperature on the methanol conversion for low space time value evidences the difficulty in selecting the suitable metallic function/acid function mass ratio. Indeed, as determined by Faungnawakij et al.,14 for each temperature there would be a different optimum ratio of both functions, given that temperature affects in a different way the reaction rates over each function of the catalyst. Regarding catalyst stability, Figure 2 shows that deactivation increases with reaction temperature. Thus, DME and methanol conversion and H2 yield undergo a slow decrease with time on stream at 325 ºC, whereas at 375 ºC a considerable decrease takes place in all reaction



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indices, which is due to deactivation by coke deposition, since no sintering of the CuFe2O4 metallic function occurs under these conditions.26 At high temperature, when the deactivation gains significance, the effective methanol conversion decreases with time on stream faster than DME conversion. Thus, at 375 ºC methanol conversion decreases from 73 % to 25 % (Figure 2a) and DME conversion from 56 % to 35 % in the same period of time on stream. This different evolution of both reaction indices seems to indicate that steam reforming of methanol is more affected than DME hydrolysis by coke deactivation. A similar result was obtained by Vicente et al.38 with a bifunctional catalyst made up of CZA and alkali treated HZMS-5 zeolite. For this catalyst, coke deposition mainly takes place over the metallic function, thereby selectively decreasing the catalytic activity for the methanol steam reforming step. These results support the theory by several authors, who associate coke deposition with the presence of methoxy ions, which are intermediates in the methanol steam reforming reaction, and they propose that these intermediates also act as precursors in the coke formation mechanism.38-42 The detection of methoxy ions in the coke deposited on Cu/ZnO/Al2O3 catalyst supports this hypothesis.40 According to this hypothesis, deactivation by coke on the metallic CuFe2O4 sites is favored under conditions of high methanol concentration in the reaction medium, and therefore with high concentration of methoxy species, which act as coke precursors. This is the case in Figure 2, which shows that the decrease in the reaction indices is more accentuated at 375 ºC than at 325 ºC. Although the study involving the coke evolution mechanism is beyond the scope of this paper, the metallic and acid sites are expected to contribute to this evolution by activating the coke evolution steps, i.e., oligomerization-cyclization-dehydrogenation reactions.43,44.

3.2. Space time The effect of space time has been analyzed in different kinetic runs by varying the catalyst mass in the reaction bed for equal values of the other operating conditions. Figure 3 shows the evolution with space time of the initial values (zero time on stream) of DME and methanol conversions (graph a) and yields of H2, CO2 and CO (graph b), for two temperatures (300 and 400 ºC). The products yields corresponding to the thermodynamic equilibrium at 300 ºC (solid lines) and at 400 ºC (dashed lines) are also plotted in Figure 3b. Methanol conversion (Figure 3a) increases very rapidly with space time, especially at low temperature, and almost reaches full conversion at both temperatures for space times close to 0.6 gcatalysth/gDME. The increase in DME conversion with space time is very low at low temperature, so that at 300 ºC increases almost linearly, indicating the low reaction rate of


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DME hydrolysis at 300 ºC. However, DME conversion increases sharply with space time at 400 ºC, reaching 90 % conversion for 0.29 gcatalysth/gDME. Figure 3 As previously observed in Figure 2, Figure 3a also shows that methanol conversion is higher for a lower temperature, with this effect being more accentuated for low values of space time. This is due to the reason mentioned above that the small amount of methanol formed by DME hydrolysis at low temperature (especially for low space time) is easily reformed by the metallic function available. However, in order to reform all the methanol formed in DME hydrolysis at high temperature (for which the DME conversion is high even at very low space time) a higher amount of metallic function would be needed, thus obtaining higher methanol conversion at 300 ºC than at 400 ºC, with this effect being more pronounced at a low space time. Figure 3b shows that H2 and CO2 yields are low and both increase almost linearly with space time at 300 ºC, in parallel to the increase in DME conversion observed in Figure 3a, and at this low temperature the CO formation is negligible. At 400 ºC the increase in H2 and CO2 yields with space time is fast for low values of this variable, and attenuates at higher space times. A comparison of the results at 300 and 400 ºC confirms the important effect of temperature on H2 and CO2 yields mentioned in the previous section. Thus, for 0.58 gcatalysth/gDME H2 yield is lower than 20 % at 300 ºC, whereas it increases to 80 % at 400 ºC. The high CO yield (near 40 %) obtained at 400 ºC and 0.58 gcatalysth/gDME is a consequence of reverse WGS reaction being greatly favored at high temperature, and explains the value of H2 yield obtained (80 %), despite both DME and methanol conversions being almost full under these conditions (Figure 3a). The contribution of methanol decomposition reaction at 400 ºC should also be taken into account, given that it explains the slightly higher value than that corresponding to the thermodynamic equilibrium obtained for CO yield at a space time of 0.58 gcatalysth/gDME, as mentioned in the previous section. As a consequence of the aforementioned results, H2 selectivity, which is almost constant with space time, decreases when temperature is increased (from 75 % at 300 ºC to 70 % at 400 ºC), whereas CO selectivity, which is negligible at 300 ºC, increases with space time to 15 % at 400 ºC. Figure 4 shows the effect of space time on catalyst stability. This figure depicts the evolution with time on stream of DME and methanol conversions (graph a) and H2 and CO yields (graph b) at 350 ºC, S/DME molar ratio of 4 and for three different values of space time: 0.073 gcatalysth/gDME (GHSV = 12707 h-1), 0.145 gcatalysth/gDME (GHSV = 6379 h-1) and 0.58 gcatalysth/gDME (GHSV = 1588 h-1). As observed, for a high space time, 0.58 gcatalysth/gDME, reaction indices remain constant along 10 h time on stream. Nonetheless, a notable catalyst deactivation takes place in 10 h time on stream for the lowest space time, which involves a



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noticeable decrease in all the reaction indices, and mainly affects methanol steam reforming reaction, since methanol conversion decreases more sharply (from 92 % to 52 % in 10 h) than DME conversion (from 30 % to 20 % in 10 h). These results corroborate that the metallic function undergoes coke deactivation to a greater extent than the acid function. The faster deactivation observed for low space time values is explained because the amount of metallic sites available under those conditions is not enough to promote the methanol reforming reaction over them, and consequently the methoxy species adsorbed are more likely to evolve towards coke formation, thereby decreasing faster the reaction indices. Figure 4

3.3. Steam/DME molar ratio In order to study the effect of steam to DME molar ratio on the DME steam reforming reaction, different kinetic runs were conducted at different temperatures and high space time (1.16 gcatalysth/gDME) for three different steam/DME molar ratios (3, 4 and 8). At each temperature, the same values of catalyst mass and DME flow rate in the feed stream were used, whereas the water flow rate in the feed was varied, as well as the inert flow rate (so that the total molar flow rate at the reactor inlet remains constant in order to attain good hydrodynamic conditions in all the kinetic runs). Figure 5 shows the evolution with temperature of the values at zero time on stream of DME conversion (graph a) and H2 and CO yields (graph b) for the three values of steam/DME molar ratio and high space time (1.16 gcatalysth/gDME). It must be remarked that the effective methanol conversion (not shown) is full in the whole temperature range studied. It is also noteworthy the important role the steam/DME molar ratio plays in the effect of temperature on the H2 yield (Figure 5b). Thus, for low S/DME (≤ 4), H2 yield goes through a maximum with temperature because, although DME conversion increases, high temperatures favor reverse WGS reaction, thus decreasing H2 selectivity and also H2 yield. Nevertheless, high S/DME hinders reverse WGS reaction, so that H2 yield increases almost in parallel to the conversion increase. Figure 5 Concerning the effect of S/DME molar ratio on the reaction indices, it is observed that an increase in this variable involves a notable decrease in DME conversion at zero time on stream for low temperature values (Figure 5a), whereas this effect attenuates at higher temperature. This result indicates that a high water content in the reaction medium attenuates the reaction rate for DME hydrolysis, which is the limiting step in the SRD reaction under these operating conditions (which corresponds to full methanol conversion). The decrease in


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DME conversion with steam/DME molar ratio is expected to be more accentuated for lower space time values than that corresponding to Figure 5, namely, when operating nearer to the kinetic control regime than the thermodynamic control regime of the reaction. Probably, methanol steam reforming reaction will be also negatively affected by an increase in steam/DME ratio under these operating conditions. Presumably, the cause is the saturation of acid or metallic sites by excess steam in the reaction medium. Furthermore, in the whole temperature range studied, an increase in steam/DME molar ratio involves a significant decrease in CO yield (Figure 5b) since the high water content inhibits reverse WGS reaction, thereby displacing the reaction equilibrium towards H2 formation. Consequently, H2 selectivity increases with steam/DME molar ratio. Due to these two aforementioned opposite effects of steam/DME ratio on DME conversion and on H2 selectivity, the effect this variable has on the H2 yield is complex and depends on the temperature range used for the reaction. Thus, at low temperatures, H2 yield decreases with the increase in steam/DME molar ratio because the negative effect of water on DME conversion (by attenuating the reaction rate of DME hydrolysis) prevails over the positive effect on H2 selectivity (by inhibiting reverse WGS reaction). Nonetheless, at high temperature the opposite effect takes place, and accordingly H2 yield increases for higher steam/DME molar ratios. Figure 6 shows the effect of the steam/DME molar ratio on the catalyst stability, showing the evolution with time on stream of the reaction indices at 375 ºC, 0.1 atm DME partial pressure and for two different steam/DME ratios (4 and 8). These results correspond to a lower space time value (0.29 gcatalysth/gDME) than that used in previous experiments (Figure 5) in order to better appreciate the catalyst deactivation. As observed, an increase in steam/DME molar ratio has a positive effect on catalyst stability, since it attenuates deactivation. Thus, a high water content attenuates the decrease in both DME and methanol conversion with time on stream, and therefore also attenuates significantly the decrease in H2 yield with time on stream. This result is explained by the attenuation in coke deposition when the water content in the reaction medium is increased, as previously observed for bifunctional catalysts with CZA metallic function and HZSM-5 zeolites.38 The attenuation of coke deposition is probably due to the fact that an increase in water content in the reaction medium hinders the formation of adsorbed methoxy species that degrade to form coke. Figure 6 As a conclusion of the results in Figures 5 and 6, it must be remarked that a high steam/DME molar ratio attenuates the reaction rate of DME hydrolysis (and probably also the steam reforming reaction rate, although this effect has not been observed at the high values of space time used in Figure 5), and specially contributes to inhibiting the reverse WGS reaction. Therefore, under operating conditions in which reverse WGS reaction has an important



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contribution to the global reaction mechanism (namely, at high temperatures), the presence of high water content increases both the yield and selectivity of H2 and decreases the yield and selectivity of CO. Moreover, a high steam/DME molar ratio contributes to attenuating catalyst deactivation by coke deposition (Figure 6). Nevertheless, due to the high cost requirement that involves heating a great amount of water, an optimization is needed for the selection of the most suitable steam/DME molar ratio in the SRD reaction. In view of the aforementioned results, steam/DME molar ratios in the 4-8 range seem to be the more appropriate for the reaction.

3.4. DME partial pressure The effect of DME partial pressure has been studied in the 0.05-0.2 atm range at 350 ºC, a S/DME ratio of 4 and space time of 0.145 gcatalysth/gDME. In order to keep the hydrodynamic conditions, the experiments were conducted with different He and DME molar flow rates, so that the total molar flow rate at the reactor inlet remained the same. The amount of catalyst was also varied in order to maintain the same value of space time in all experiments. Figure 7a shows the DME partial pressure effect on the values at zero time on stream of DME and methanol conversions, whereas Figure 7b depicts the effect of this variable on the H2, CO2 and CO yields. Figure 7 Figure 7 shows that an increase in DME partial pressure to 0.1 atm involves an increase in both DME and methanol conversions (Figure 7a) and in H2, CO2 and CO yields (Figure 7b) Nevertheless, a further increase in DME partial pressure to 0.2 atm does not involve further increase in these reaction indices. Methanol conversion is more affected than DME conversion, thereby indicating that an increase in the partial pressure of the limiting reactant (DME) accelerates the reaction rate for methanol steam reforming to a greater extent than for the DME hydrolysis. Furthermore, Figure 7b shows that higher concentrations of H2 and CO2 in the reaction medium enhance slightly reverse WGS reaction, thus favoring CO formation at high DME partial pressures. Figure 8 analyzes the effect of DME partial pressure on the evolution with time on stream of DME and methanol conversion (Figure 8a) and of H2 and CO yields (Figure 8b). These results correspond to the same temperature, space time and steam/DME molar ratio values used in Figure 7, and for two values of DME partial pressure. As observed, an increase in DME partial pressure in the feed attenuates deactivation. Thus, methanol conversion decreases rapidly and progressively with time on stream for low DME partial pressures, from 60 to 30 % in 10 h time on stream (Figure 8a). Nevertheless, for high DME partial pressures methanol conversion decreases only about 3 % in 10 h time on stream. A similar effect of DME partial pressure on deactivation rate has been also observed previously for a


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CZA/HZSM-5 zeolite catalyst.39 The effect of DME partial pressure on deactivation is in line with the aforementioned hypothesis about the role of methoxy species in the deactivation by coke deposition. An increase in DME partial pressure favors selectively the conversion of methoxy species into reforming products, in a more pronounced way than their evolution to coke precursors. DME conversion decreases following the same trend for both DME partial pressures studied, and stabilizes after 3 h time on stream. These results indicate that DME partial pressure has negligible impact on the catalytic activity for the acid function responsible for DME hydrolysis. Consequently, the results are consistent with the theory that deactivation by coke selectively affects metallic sites. Figure 8 All the aforementioned results allow concluding that in order to attain high DME conversion and H2 yield on the CuFe2O4/γ-Al2O3 catalyst, with minimum catalyst deactivation and CO formation, the most suitable operating conditions are as follows: temperature from 350 to 375 ºC, space time higher than 0.60 gcatalysth/gDME, DME partial pressure above 0.1 atm and steam/DME molar ratios in the 4-8 range. At temperatures in the 350-375 ºC range, steam/DME of 8 and space time of 1.16 gcatalysth/gDME the H2 yield obtained varies from 78 to 93 % and the CO yield from 3 to 8 %, both yields remaining constant along 10 h time on stream. Nevertheless, as aforementioned, operating with high water content involves an increase in the energy requirement for heating the feed, and the subsequent recovery of H2 from a highly diluted stream, which would also increase the cost of the product recovery equipment. Therefore, an energetic optimization is required in order to determine the optimum water content for the process.

3.5. Assessment of catalyst performance The aforementioned results remark the complexity of the SRD reaction, which attains similar H2 yields for different operating conditions. These properties are of interest for further work on the selection of the optimum conditions for different requirement. However, they hinder the comparison of the results obtained in this study with those from literature. An additional difficulty lies in the definition of the reaction indices, which is different according to the authors. H2 production rate, defined as the molar flow rate of H2 per gram of catalyst, is a suitable reaction index for comparing the results of activity and H2 selectivity in SRD reaction obtained in this work with others reported in the literature on different catalysts.



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Figure 9 shows the evolution with time on stream of H2 production rate values obtained in this study at different temperatures for low (Figure 9a) and high (Figure 9b) space time values. For 0.073 gcatalysth/gDME space time (Figure 9a), at 350 ºC the initial H2 production is about 550 mmolH2/(gcatalysth), which decreases to 260 mmolH2/(gcatalysth) in the first 5 h time on stream and only to 200 mmolH2/(gcatalysth) in the following 5 h time on stream. At 400 ºC the initial H2 production rate is rather high, about 750 mmolH2/(gcatalysth), but decreases very rapidly, thus reaching values only slightly higher than those obtained at 350 ºC after 10 h time on stream. Figure 9 For high space times (Figure 9b) in the temperature range studied, the catalyst is stable and the H2 production rate remains constant along 10 h time on stream. At 300 ºC the production is about 60 mmolH2/(gcatalysth) (similar value to that obtained at zero time on stream at this temperature and for the low space time in Figure 9a). An increase of 25 ºC in the reaction temperature from 300 to 325 ºC involves a substantial increase in the H2 production rate to 166 mmolH2/(gcatalysth) at 325 ºC. However, a further increase of 25 ºC in temperature to reach 350 ºC, only raises slightly the H2 production rate to 185 mmolH2/(gcatalysth). This value is very similar to that obtained by Vicente et al.38 at a lower temperature (300 ºC), but under similar values for the remaining conditions, using a catalyst composed of CZA and modified HZSM5 zeolite as metallic and acid functions, respectively.38 Therefore, the catalyst studied in this work is slightly less active, but has the advantage of being completely regenerable. The complete regeneration of the CZA metallic function is not possible,26 which restricts its use for SRD in reaction-regeneration cycles. Semelsberger et al.45 analyzed the kinetic behavior of bifunctional catalysts based on Cu-ZnO (CZ) metallic function and different acid functions (γ-Al2O3 and ZSM-5 zeolite). These authors improved their results by using a bifunctional catalyst based on a commercial CZA catalyst as the metallic function for methanol steam reforming and a HZSM-5 zeolite as the acid function, which produced 92 mmolH2/(gcatalysth) at 275 ºC (with a 94 % H2 yield).13 Faungnawakij and co-workers are pioneers in the use of bifunctional catalysts based on CuFe2O4 spinel.15,28-34,46 The results obtained by these authors using a γ-Al2O3 acid function46 were improved by using a ZSM-5 zeolite as acid function and subjecting the catalyst to a prior heating at 900 ºC for 10 h in order to activate the spinel.15 The catalyst used in this study performed well due to the use of CuFe2O4 spinel as the metallic function together with boehmite as the precursor of the γ-Al2O3 acid function. The resulting CuFe2O4/γ-Al2O3 catalyst here reported has higher activity than that prepared by ourselves with CuFe2O4 spinel and a commercial γ-Al2O3, and similar activity to that obtained with an alkali treated zeolite, but with the advantage of higher stability.35 This γ-Al2O3 obtained from boehmite performs better in the SRD reaction than other types of γ-Al2O3 used


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as the acid function for the bifunctional catalyst. Moreover, the use of boehmite confers the catalyst the mechanical resistance required for use in the fluidized bed reactor, thus avoiding solid loss by attrition. 5. Discussion Figure 10 shows a scheme of the reaction steps for the SRD reaction (DME hydrolysis, methanol reforming, reverse WGS reaction and methanol decomposition), which have been considered for the interpretation of the aforementioned results. Consequently, these steps should be considered for establishing a kinetic model that quantifies the effect reaction conditions have on the yields and product distribution. Figure 10 The results show a good kinetic performance of the catalyst used in this study. This good performance of the catalyst stems from a suitable balance between the reaction rates of the DME hydrolysis step and methanol steam reforming step. This balance is required to avoid catalyst deactivation by coke, which is severe when DME hydrolysis reaction rate is higher than the steam reforming rate because this involves an increase in the methoxy species adsorbed on the catalyst and the subsequent coke formation by degradation of these species. However, to strike this balance is difficult because the operating variables (temperature, space time and S/DME) have different effect on these two reaction steps (DME hydrolysis and methanol steam reforming), as well as on the reverse WGS reaction and methanol decomposition. Besides, the use of CuFe2O4 spinel instead of CZA function guarantees the regenerability of the catalyst.26 Furthermore, the acidity of the γ-Al2O3 obtained by calcining the boehmite at 550 ºC is slightly lower than that of the zeolite HZSM-5 modified by alkaline treatment. Therefore, the acid function used in this work is highly active in the DME hydrolysis step and avoids the formation of hydrocarbons, which contribute to the zeolite deactivation by coke. Moreover, the use of boehmite as binder provides high mechanical strength to the catalyst (suitable to avoid attrition in the fluidized bed). Difficulties arise for establishing the optimum reaction conditions due to the relationship between the steps in Figure 10 and especially for the effect of operating conditions on catalyst deactivation. An analysis of the results on the effect of temperature in the 300-400 ºC range (section 3.1) shows that an adequate value of space-time is required to strike a balance between the reaction rates. Thus, a significant decrease in H2 yield at zero time on stream and a notable increase in catalyst deactivation takes place below 0.29 gcatalysth/gDME (section 3.2), which is attributable to the accumulation of methoxy ions promoting coke formation and its deposition on the spinel. It is also difficult to determine the most appropriate value of S/DME molar ratio (section 3.3), since this variable has a complex effect on the reaction rates of the two reaction steps (DME hydrolysis and methanol steam reforming), on the reverse WGS reaction and on deactivation.



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Thus, at low temperature the S/DME molar ratio has a negative effect on H2 yield at zero time on stream because the attenuating effect of water on the reaction rate of DME hydrolysis prevails. However, it has a positive effect at high temperature because under those conditions the inhibition of reverse WGS reaction is more significant that the attenuation of DME hydrolysis reaction rate. Therefore, H2 yield increases and that of CO decreases for high S/DME molar ratios. Moreover, under all the operating conditions used, a high S/DME molar ratio favors process stability, thereby attenuating the catalyst deactivation by coke deposition. The kinetic results are consistent with the hypothesis that methoxy species formed by methanol adsorption are intermediate species in coke formation, and that coke deposition has a greater impact on the activity of the metallic sites of the catalyst. The role of S/DME molar ratio is to control the formation of these methoxy species, whose concentration should not exceed that liable to the reformed over the metallic function.


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6. Conclusions The bifunctional catalyst with CuFe2O4 spinel metallic function and γ-Al2O3 acid function obtained by calcination of boehmite at 550 ºC, and prepared using the same mass ratio of metallic and acid functions, shows an appropriate behavior for the steam reforming of DME operating in the 350-375 ºC range, with S/DME molar ratio in the 4-8 range, space-time higher than 0.58 gcatalysth/gDME and DME partial pressure higher than 0.1 atm. Besides, the use of CuFe2O4 spinel instead of CZA function guarantees the regenerability of the catalyst. Moreover, the use of boehmite as binder provides the catalyst high mechanical strength (suitable to avoid attrition in the fluidized bed). The acidity of the γ-Al2O3 obtained by calcination at 550 ºC of boehmite is slightly lower than that of the zeolite HZSM-5 modified by alkaline treatment. Therefore, the acid function used in this work is highly active in the DME hydrolysis step and avoids the undesirable hydrocarbon formation. It has been proven that a moderate temperature is needed for a suitable balance of the reaction rates in the DME hydrolysis and methanol reforming steps. Thus, at 350 ºC, a space-time of 0.58 gcatalysth/gDME, S/DME of 4 and a partial pressure of DME in the feed of 0.1 atm, a steady H2 yield higher than 82 % is obtained. The temperature increase above 350 ºC allows obtaining higher H2 yield at zero time on stream, but at the expense of a faster deactivation of the catalyst.

Acknowledgments This work was carried out with the financial support of the Basque Government (Project IT748-13), of the University of the Basque Country (UFI 11/39 and post doctoral grant of A. Remiro), of the Ministry of Economy and Competitiveness of the Spanish Government and ERDF funds (Project CTQ2012-35263) and the Ministry of Science and Innovation (PhD grant BES-2010-033241 of L. Oar-Arteta).



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Notation Fi

molar flow rate of i component at the reactor outlet, mol min-1

FDME, FDME,0

molar flow rate of DME at the reactor outlet and inlet, respectively, mol min-1

FMeOH, FMeOH,0

molar flow rate of methanol at the reactor outlet and that formed by the DME hydrolysis reaction, respectively, mol min-1

GHSV

gas hourly space velocity, h-1

P, PDME

total pressure and DME partial pressure in the feed, respectively, bar

S/DME

steam to DME molar ratio

XDME

DME conversion

XMeOH

methanol effective conversion

Yi

yield of i component

υi

stoichiometric coefficient for the formation of i component from DME


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Figure captions Figure 1.

Effect of temperature on the values at zero time on stream of DME conversion, methanol effective conversion, and H2, CO and CO2 yields. Operating conditions: PDME, 0.1 atm; S/DME, 8; space time, 1.16 gcatalysth/gDME (GHSV from 667 (at 250 ºC) to 858 h-1 (at 400 ºC)).

Figure 2.

Effect of temperature on the evolution with time on stream of DME conversion and methanol effective conversion (a), and H2 and CO yields (b). Operating conditions: PDME, 0.2 atm; S/DME, 3; space time, 0.073 gcatalysth/gDME (GHSV from 12197 (at 325 ºC) to 13217 h-1 (at 375 ºC)).

Figure 3.

Effect of space time on the values at zero time on stream of DME conversion and effective methanol conversion (a), and H2, CO and CO2 yields (b) at different temperature. Operating conditions: PDME , 0.2 atm; S/DME, 3.

Figure 4.

Effect of space time on the evolution with time on stream of DME conversion and methanol effective conversion (a), and H2 and CO yields (b). Operating conditions: 350 ºC; PDME, 0.2 atm; S/DME, 4.

Figure 5.

Evolution with temperature of the values at zero time on stream of DME conversion (a), and H2 and CO yields (b) at different steam/DME molar ratios (S/DME = 3, 4 and 8). Operating conditions: PDME, 0.1 atm; space time, 1.16 gcatalysth/gDME (GHSV from 730 (at 300 ºC) to 858 h-1 (at 400 ºC)).

Figure 6.

Effect of the steam/DME molar ratio on the evolution with time on stream of DME conversion and methanol conversion (a), and H2 and CO yields (b). Operating conditions: 375 ºC; PDME, 0.1 atm; space time, 0.29 gcatalysth/gDME (GHSV= 3304 h-1).

Figure 7.

Effect of DME partial pressure on the values at zero time on stream of DME conversion and methanol conversion (a), and H2, CO and CO2 yields (b). Operating conditions: 350 ºC; S/DME, 4; space time, 0.145 gcatalysth/gDME.

Figure 8.

Effect of DME partial pressure on the evolution with time on stream of DME conversion and methanol conversion (a), and H2 and CO yields (b). Operating conditions: 350 ºC; S/DME, 4; space time, 0.145 gcatalysth/gDME.

Figure 9.

Effect of temperature on evolution with time on stream of H2 production rate at 0.073 gcatalysth/gDME (a) and 0.58 gcatalysth/gDME (b). Operating conditions: PDME, 0.2 atm; S/DME, 4 (graph a) and 3 (graph b).

Figure 10. Kinetic scheme for the SRD process on a CuFe2O4/γ-Al2O3 bifunctional catalyst.



Page 24 of 34

24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

“For Table of Contents Only”


Page 25 of 34

1 XDME XMeOH YH2 YCO YCO2 YH2eq YCOeq YCO2eq

0.8

Xi, Yi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0 250

300

350

Temperature, ºC

Figure 1


400


1

a

XDME, XMeOH

0.8 XDME XMeOH 325 ºC

0.6

350 ºC 375 ºC 0.4

0.2

0 0

500

1000

1500

2000

time on stream, min 0.5

b YH2 YCO

0.4

325 ºC

YH2, YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

350 ºC

0.3

375 ºC 0.2

0.1

0 0

500

1000

1500

time on stream, min

Figure 2


2000

Page 27 of 34

1

a

XDME, XMeOH

0.8 DME MeOH 0.6

300 ºC 400 ºC

0.4

0.2

0 0

0.1

0.2

0.3

0.4

0.5

0.6

W/F, gcatalysth/gDME 1

300 ºC

H2 CO CO2 300 ºC 400 ºC

0.8

YH2,eq 400 ºC

YCO2,eq

b

YH2, YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 ºC 400 ºC

0.6

0.4 400 ºC

YCO,eq

300 ºC

0.2

0 0

0.1

0.2

0.3

0.4

W/F, gcatalysth/gDME Figure 3


0.5

0.6


1

a

XDME, XMeOH

0.8

0.6 W/F, gcatalysth/gDME 0.073 0.145 0.58

0.4

DME MeOH

0.2

0 0

100

200

300

400

500

600

time on stream, min 1

b 0.8 W/F, gcatalysth/gDME

YH2, YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

0.073 0.145 0.58

0.6

H2 CO

0.4

0.2

0 0

100

200

300

400

time on stream, min Figure 4


500

600

Page 29 of 34

1

a 0.8

XDME

0.6

S/DME 8 4

0.4

3

0.2

0 300

400

350

Temperature, ºC 1

b 0.8 H22 CO

YH2,YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

S/DME 8 4 3

0.4

0.2

0 300

350

Temperature, ºC Figure 5


400


1

a

XDME, XMeOH

0.8

0.6

0.4 DME MeOH S/DME 8

0.2

4

0 0

100

200

300

400

500

600


b 0.8

YH2, YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

0.6

0.4

H2

CO S/DME 8 4

0.2

0 0

100

200

300

400



500

600

Page 31 of 34

1

a

XDME, XMeOH

0.8

0.6

0.4 DME MeOH

0.2

0 0

0.05

0.1

0.2

0.15

0.25

PDME, atm 1

b 0.8

0.6

Yi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2

0.4

CO

CO2 Experimental Equilibrium

0.2

0 0

0.05

0.1

0.15

PDME, atm Figure 7


0.2

0.25


1

a

DME MeOH PDME, atm 0.2

XDME, XMeOH

0.8

0.05 0.6

0.4

0.2

0 0

100

200

300

400

500

600

time on stream, min 1 H2 CO PDME, atm

b

0.2

0.8

0.05

YH2, YCO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

0.6

0.4

0.2

0 0

100

200

300

400



500

600

Page 33 of 34

H2 production rate, mmol/gcatalysth

800

a

300 ºC 325 ºC 350 ºC 400 ºC

600

400

200

0 0

100

200

300

400

500

600


H2 production rate, mmol/gcatalysth

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 ºC 325 ºC 350 ºC 400 ºC

b

200

100

0 0

100

200

300

400



500

600


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

CO + H 2 Decomposition

(CH3)2O + H2O

Hydrolysis

CH3OH

Steam reforming

H2 + CO2

Metallic function

Acid function Coke

+H2O

Coke

Figure 10


r-WGS Metallic function

H2 O + CO

Effect of Operating Conditions on Dimethyl Ether Steam Reforming

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