effect of operating conditions on dimethyl ether steam reforming in a fluidized bed reactor with a...

Effect of Operating Conditions on Dimethyl Ether Steam Reformingin a Fluidized Bed Reactor with a CuO−ZnO−Al2O3 and DesilicatedZSM‑5 Zeolite Bifunctional CatalystJorge Vicente, Javier Erena,* Lide Oar-Arteta, Martin Olazar, Javier Bilbao, and Ana G. Gayubo

Departamento de Ingeniería Química, Universidad del País Vasco UPV/EHU, Apartado 644, 48080 Bilbao, Spain

ABSTRACT: This paper studies the effect of operating conditions in dimethyl ether (DME) steam reforming on a bifunctionalcatalyst synthesized with CuO−ZnO−Al2O3 metallic function and a HZSM-5 zeolite treated with NaOH to moderate acidity.The experimentation has been carried out in a fluidized bed reactor in the 225−325 °C range, with space time between 0.1 and2.2 gcatalyst h/gDME, steam/DME molar ratio between 3 and 6, and DME partial pressure between 0.08 and 0.25 bar. The 275−300°C range is suitable for obtaining high values of DME conversion and H2 yield with minimum CO formation and deactivation bycoke and avoiding hydrocarbon formation. Stable values of DME conversion (0.85), H2 yield (0.81), and H2 production rate(180 mmolH2

/(gcatalyst h)) are obtained during 48 h at 300 °C with a steam/DME ratio of 4 and space time of 0.60 gcatalyst h/gDME.The main cause of deactivation is coke deposition on the metallic function.

1. INTRODUCTIONThe steam reforming of dimethyl ether (SRD) is a promisingway of producing hydrogen-rich gas for fuel cell systems, as itcan take place at low temperatures only slightly higher thanthose required for the steam reforming of methanol (SRM).1

The use of dimethyl ether (DME) as a raw material forproducing H2 by steam reforming has several advantagescompared to methanol, such as high hydrogen content (13 wt %vs 12.5 wt % of methanol), no toxicity or hazard factor, gaslikeproperty, liquid-storage density, and available handling infra-structure (similar to liquefied petroleum gas).2 Furthermore, thesingle-step synthesis of DME on a bifunctional catalyst isconsidered a suitable process for the large-scale valorization ofCO2, given it is thermodynamically more favorable than thesynthesis of methanol, which allows cofeeding CO2 togetherwith the syngas.3−6

The SRD reaction proceeds over bifunctional catalysts via thehydrolysis of DME over the acid function, followed by steamreforming of methanol (MeOH) over the metallic function. Theindividual reactions are

+ ↔ Δ ° =H(CH ) O H O 2CH OH 24 kJ/mol3 2 2 3(1)

+ ↔ + Δ ° =HCH OH H O 3H CO 49 kJ/mol3 2 2 2(2)

The SRD and the reverse water gas shift reaction (r-WGSR)generally take place over a metallic function, and methane andhydrocarbons are also generated via DME decomposition whena strong acidic function or high temperatures are used:7

+ ↔ + Δ ° =HCO H H O CO 41 kJ/mol2 2 2 (3)

→ + + Δ ° = −H(CH ) O CH H CO 1 kJ/mol3 2 4 2(4)

Consequently, suitable metallic and acid functions arerequired in the bifunctional catalysts for attaining high DME

conversion and high H2 selectivity by minimizing the formationof CO (a poison for the anode catalyst in proton exchangemembrane (PEM) fuel cells) and CH4. The more widelystudied metallic functions for SRD are those commonly used inthe methanol reforming process (no bifunctional catalyst isrequired), which may be grouped into two types:8,9 (i) thosecontaining Cu and (ii) those containing metals from groups 8−10. The most widely studied Cu-based metallic function is CZA(CuO−ZnO−Al2O3) with numerous commercial and labo-ratory-synthesized modifications,1,10−16 although other copper-spinel type functions have recently been proposed for SRD,with the aim being a higher resistance to sintering.17−21

Although the latter have their advantages compared to Cu-based catalysts, such as their higher thermal and long-termstability, most studies report their lower activity and selectivityfor methanol reforming, given that they predominantly catalyzemethanol decomposition,8,9 thus producing CO and H2. Awater gas shift (WGS) reaction takes place in the presence ofwater, partially converting CO to CO2, but a significant amountof CO is produced via decomposition. Nevertheless, concerningCu-based catalysts, there is some agreement in the literature onthe existence of a pathway through a methyl formateintermediate, which directly releases CO2 and H2. Conse-quently, the CO produced during methanol steam reformingover Cu-based catalysts is formed by the reverse WGS reaction.Other catalysts, such as Pd/ZnO alloys used in methanolreforming (not assayed for SRD), provide similar results tothose obtained with Cu-based ones.22 Furthermore, Pd metallicfunctions in SRD lead to higher yields of byproducts (especiallyCO),23 so subsequent WGS steps are required to purify the H2

stream.

Received: August 1, 2013Revised: January 27, 2014Accepted: February 11, 2014Published: February 11, 2014

Article

pubs.acs.org/IECR

© 2014 American Chemical Society 3462 dx.doi.org/10.1021/ie402509c | Ind. Eng. Chem. Res. 2014, 53, 3462−3471

pubs.acs.org/IECR

Concerning the acid function, γ-Al2O3 is the more commonlyused one, but due to its low acidity, a high temperature (usuallyin the 300−400 °C range) is required for DME hydrolysis,which is the rate-limiting step in the overall reaction under theseconditions.10,17−19,24 Consequently, the use of γ-Al2O3 as theacid function promotes DME decomposition and the reverse-WGS reaction, which considerably increase CH4 and COformation, as well as the sintering of Cu in the CuO−ZnO−Al2O3 metallic function.HZSM-5 zeolite is an attractive acid function because it

allows reforming at a lower temperature (approximately 100 °Clower) than using γ-Al2O3 does. This is due to its higheracidity,12,25 enhancing the hydrolysis reaction, which even inthis case is slower than methanol reforming. However, it mayfavor the formation of hydrocarbons, via MTH (methanol tohydrocarbons) reactions,26−30 which drastically diminish H2production,7,31−33 and also may contribute to increasingdeactivation by coke deposition.13 The use of HZSM-5 zeoliteswith a high Si/Al ratio (>90), i.e., of moderate acidity,attenuates the formation of hydrocarbons and increases catalyststability.12,25 It has been previously reported that an alkalinetreatment of the HZSM-5 zeolite with a 0.4 M NaOH solutionfor 300 min is suitable for the use of this zeolite as the acidfunction in DME steam reforming,34 as this alkaline treatmentmoderates HZSM-5 acidity and contributes to attenuating theformation of coke.35,36

Consequently, the bifunctional catalysts based on CuO−ZnO−Al2O3 as the metallic function and an alkali treatedHZSM-5 zeolite as the acid function are considered promisingcatalysts for SRD. Given the catalyst’s kinetic performance is aconsequence of the synergism between the activities of themetallic and acid functions, previous studies have paid specialattention to the preparation and composition of the CuO−ZnO−Al2O3 metallic function,37 as well as to the mass ratiobetween this function prepared under the best conditions (Cu/Zn/Al atomic ratio = 4.5:4.5:1.0) and the treated HZSM-5zeolite,34 with the optimum zeolite/metallic function mass ratiobeing 1:1.38 This paper addresses the effects that operatingconditions (temperature, space time, DME partial pressure,steam/DME molar ratio) have on the performance (DMEconversion, yield and selectivity of H2, and stability) of thecatalyst prepared under the conditions optimized in theprevious studies mentioned. The results of this paper allowestablishing suitable conditions for the process in order tomaximize DME conversion and H2 yield, and minimize both theformation of byproducts and catalyst deactivation. An overallstructured study of all these operating variables, paying specialattention to catalyst stability, is of great interest, given that it is akey factor for process viability. The use of a fluidized bed reactoris also of interest for ensuring bed isothermicity, and therebyincreasing catalyst stability.

2. EXPERIMENTAL SECTION2.1. Catalyst. The metallic function CuO−ZnO−Al2O3

(CZA), with an atomic ratio of Cu/Zn/Al = 4.5:4.5:1.0, hasbeen prepared by coprecipitating the corresponding nitrateswith Na2CO3 at pH 7.0 and 70 °C, and then calcining it at 325°C for 3 h.37−39 The parent HZSM-5 zeolite (SiO2/Al2O3 = 30)has been supplied by Zeolyst International in ammonium form.This zeolite has been subjected to an alkaline treatment with a0.4 M NaOH solution for 300 min at 80 °C, followed by fastcooling and ion exchange with ammonium nitrate of theresulting NaZSM-5 zeolite.34 The solid is then washed (distilled

water) and dried at 110 °C, and subsequently calcined at 550°C for 3 h in order to obtain its acid form. The treatment of theHZSM-5 with NaOH efficiently decreases the acid strength andgenerates new mesopores by breaking Si−O−Al bonds(desilication) and maintaining Si−OH−Al bonds.40,41 Thistreatment under moderate conditions (300 °C and 0.4 MNaOH solution) is suitable for increasing the stability of DMEreforming catalysts by minimizing the formation of hydro-carbons and coke.34

The catalyst has been prepared by wet physical mixing of thepreviously calcined metallic and acid functions, with a mass ratioof 1:1, which is suitable for attaining a synergy between thesteps of DME hydrolysis (eq 1) and methanol reforming (eq2).38 The resulting mixture is dried at 110 °C and then pressed,ground, and sieved in the 150−250 μm range.The following properties have been determined for the

catalyst: composition, by inductively coupled plasma atomicelectron spectroscopy (ICP-AES) in an ARL Model 3410; BETsurface area (SBET) and porous structure, by N2 adsorption−desorption in a Micromeritics ASAP 2010; metal surface areaand metal crystal size, by N2O chemisorption in a MicromeriticsAutoChem 2920 connected online to a Balzers InstrumentsOmnistar mass spectrometer; and total acidity and acidstrength, by differential adsorption of ammonia at 150 °C in aTA Instruments SDT 2960 connected online to a BalzersInstruments Thermostar mass spectrometer. Table 1 sets out

the properties of the catalysts prepared based both on theHZSM-5 zeolite treated with alkali (A0.4-300), which has beenused in this paper, and on the parent (untreated) zeolite(HZ30). The physical properties correspond to a combinationof the metallic and acid functions. Microporosity is provided bythe zeolite acid function and mesoporosity by the metallic andacid functions. Treating the zeolite with alkali considerablyincreases the mesopore volume and BET surface area andslightly decreases the micropore volume, although zeolitedesilication does not cause significant changes in the specificmetal surface area or in the Cu particle average size.34

The capacity of the CZA metallic function for NH3adsorption is negligible compared to that of the zeolite, andaccordingly, the total acidity of the catalyst is almost half thatcorresponding to its acid function (accounts for 50 wt %). Theeffect of attenuating the total acidity and average acid strengthof the catalyst by treating the zeolite with NaOH is a direct

Table 1. Properties of the Bifunctional Catalysts, with theSame CZA Metallic Function (Cu/Zn/Al = 4.5:4.5:1.0,Nominal) and with HZ30 (Parent Zeolite with SiO2/Al2O3 =30) and A0.4-300 (Alkali Treated Zeolite) Acid Functions

acid function

HZ30 A0.4-300

wt % metallic function (nominal) 50 50wt % metallic function (real) 51.5 48.6SBET, m

2/gcatalyst 230 207Vmicropore, cm

3/g 0.057 0.041Vmesopore, cm

3/g 0.181 0.330dmesopore, Å 54 82Smetallic, m

2Cu/gCu 56 56

dCu, nm 12.0 11.9total acidity, mmolNH3

/gcatalyst 0.42 0.34

mean acid strength, kJ/molNH3125 103

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consequence of the effect observed in the acid function, i.e., asignificant decrease in total acidity and a slight decrease in theacid strength with this treatment.34

The content of the coke deposited on the deactivated catalysthas been determined by temperature programmed oxidation(TPO) with air in a TA Instruments SDT 2960 thermobalanceconnected online to a Balzers Instruments Thermostar massspectrometer. Prior to combustion, the deactivated catalyst issubjected to sweeping with N2 for 30 min in the reactor itself atthe reaction temperature, in order to desorb the reactionproducts and homogenize the coke.42 The combustionprocedure in the thermobalance is as follows: the sample isstabilized for 30 min with air at 75 °C and the temperature issubsequently increased (5 °C/min) to 550 or 600 °C andmaintained at this value for 30 min to ensure complete cokecombustion. Given that coke combustion is complete, themeasurement of coke content has been carried out based on theCO2 signal in the spectrometer.2.2. Reaction Equipment and Product Analysis. The

kinetic runs have been carried out in automated reactionequipment provided with an isothermal fluidized bed reactorwith a 22 mm internal diameter connected online to a MicroGCAgilent 3000 for product analysis, provided with four modulesfor the analysis of the following: (1) permanent gases (O2, N2,H2, CO, CO2, CH4); (2) oxygenates (MeOH, DME), lightolefins (C2−C3), and water; (3) C2−C6 hydrocarbons; (4) C6−C12 hydrocarbons and oxygenate compounds. The hydro-dynamic properties of the bed have been improved by mixingthe catalyst (particle size between 150 and 250 μm) with aninert solid (CSi, with particle size between 60 and 90 μm) at acatalyst/inert ratio of 1/4. Prior to the catalytic runs, the catalystwas reduced in a stream of 5% H2 in He at 300 °C for 2 h with atotal flow rate of 60 mL/min.The operating conditions are as follows: temperature

between 225 and 325 °C; total pressure, 1.2 bar; partialpressure of DME between 0.08 and 0.25 bar; steam/DMEmolar ratio between 3 and 6; space time up to 2.2 gcatalyst h/gDME(with a catalyst mass between 0.6 and 4 g and DME mass flowrate between 2.06 and 6.72 gDME/h); He flow rate between 16and 200 cm3(NC)/min; water flow rate between 0.047 and0.135 mL/min (2.5−5 times the minimum fluidizationvelocity); time on stream, up to 24 h (48 h runs when spacetime is high and so deactivation is slow). The range of operatingconditions has been established for attaining vigorous fluid-ization, while avoiding excessive bubbling and bed segregation.Furthermore, the conditions established correspond to severedeactivation, with the aim being to clearly determine the effectvariables have on deactivation.2.3. Reaction Indices. DME conversion has been calculated

based on the flow rates at the reactor inlet (FDME,0) and outlet(FDME):

=−

XF F

FDMEDME,0 DME

DME,0 (5)

The yields of H2 and CO have been calculated from the flowrates of these compounds (Fi) at the reactor outlet:

υ=

−Y

FFi

i i1

DME,0 (6)

where υi is the stoichiometric coefficient of i componentproduced in SRD (υ = 6 for H2 and υ = 2 for CO and CO2).

In order to quantify the activity of the metallic function in thebifunctional catalyst, the effective conversion of methanol,XMeOH, has been determined in the second step of the SRDprocess (i.e., methanol steam reforming, eq 2) as

=−

XF F

FMeOHMeOH,0 MeOH

MeOH,0 (7)

where FMeOH,0 is the methanol molar flow rate corresponding toDME conversion, which is calculated as twice the number ofDME moles converted, according to the stoichiometry of DMEhydrolysis (eq 1).The hydrogen production rate, rp,H2

, has been defined as themoles of hydrogen formed by time and catalyst mass unit:

=rF

Wp,HH

2

2

(8)

The selectivity of each j component in the C1 lump (CO,CO2, and CH4) has been defined as the molar fraction of eachcomponent in this lump:

=+ +

SF

F F Fjj

CO CO CH2 4 (9)

3. RESULTS3.1. Kinetic Behavior at Zero Time on Stream.

3.1.1. Effect of Temperature. Figure 1 shows the results at

zero time on stream of DME conversion, MeOH effectiveconversion, and product (H2, CO2, and CO) molar fractions atdifferent temperatures. These results have been obtained byextrapolating the values of composition versus time obtained inlong runs to zero time on stream.The DME hydrolysis reaction rate at 225 °C is very low, and

DME conversion (0.22) is only slightly higher than thatcorresponding to thermodynamic equilibrium. As temperatureis increased, the DME hydrolysis equilibrium shifts to the rightdue to methanol reforming on the metallic function.Consequently, DME conversion increases sharply in the 225−275 °C range, and moderately above 275 °C, to a value of 0.87at 325 °C. The effect increasing temperature has on MeOHeffective conversion is largely insignificant due to the highMeOH reforming reaction rate at 225 °C. The H2 molar

Figure 1. Effect of temperature on the values at zero time on stream ofDME conversion, MeOH effective conversion, and product (H2, CO2,and CO) mole fractions. Conditions: W/FDME,0 = 0.60 gcatalyst h/gDME,steam/DME = 3, and PDME = 0.25 bar.



fraction is low below 250 °C, but it increases sharply as thetemperature is increased, recording a value of 0.66 at 325 °C.Furthermore, as the temperature is increased, the CO

concentration in the outlet stream increases. The molar fractionof CO at 225 °C is insignificant (0.001), and at 325 °C it is0.033 (0.044 on a water-free basis). CO is produced through thereverse WGS reaction (eq 3),8,9 which takes place in series withmethanol reforming (eq 2). On the basis of the aforementionedresults, this reaction should be contemplated in the overall SRDreaction scheme, especially above 275 °C, and when H2 andCO2 concentrations in the reaction environment are high.3.1.2. Effect of DME Partial Pressure. Figure 2 shows the

effect of DME partial pressure on the results at zero time on

stream of DME conversion, MeOH effective conversion, H2yield, and CO2 selectivity based on C1 compounds. The studyhas been carried out with different flow rates of inert gas (He),corresponding to a range of gas relative velocities (u/umf) in thefluidized bed between 2.5 and 5 and the same value for theremaining operating conditions. As observed, an increase inDME partial pressure slightly increases DME conversion andH2 yield. MeOH conversion does not change and, therefore, anincrease in DME partial pressure increases the hydrolysisreaction rate, but it does not affect the methanol reformingreaction rate. Nevertheless, CO2 selectivity based on C1compounds decreases slightly as the DME partial pressure isincreased, given that higher concentrations of H2 and CO2 shiftthe WGS reaction to the left, i.e., it favors CO formation.Consequently, the molar fraction of CO increases from 0.0036to 0.0055 on a water-free basis as the DME partial pressure isincreased from 0.09 to 0.21 atm, respectively.3.1.3. Effect of Steam/DME Molar Ratio in the Feed. The

effect of the steam/DME molar ratio has been studied in therange between 3 (minimum according to stoichiometry) and 6by carrying out runs using different values of water and inert gas(He) flow rates, with the remaining operating conditions beingfixed. Figure 3 shows the results at zero time on stream of DMEconversion, MeOH effective conversion, H2 yield and CO2selectivity based on C1 compounds.An increase in the steam/DME ratio slightly increases DME

conversion and H2 yield by shifting the DME hydrolysisequilibrium (eq 1). This increase is less significant for a steam/DME ratio higher than 4. Furthermore, higher water contenthinders the reverse WGS reaction by shifting its equilibrium tothe left, which explains why the CO concentration decreases

considerably from 0.0089 to 0.0039 on a water-free basis as thesteam/DME ratio is increased from 3 to 6. Nevertheless, theMeOH effective conversion hardly changes because themethanol reforming reaction (eq 2) is almost full under theseconditions.Although an increase in the steam/DME ratio increases DME

conversion and H2 yield, these advantages should be consideredtogether with the disadvantage of a higher cost incurred bysteam generation and product separation, given that productmolar fractions decrease due to dilution (Figure 4).

3.1.4. Effect of Space Time. The effect of space time hasbeen determined by runs using different catalyst mass, with theremaining conditions being fixed. Figure 5 shows the results at300 °C and zero time of DME conversion, MeOH effectiveconversion, H2 yield, and CO2 selectivity based on C1compounds. DME and MeOH conversions and H2 yieldincrease sharply as space time is increased. Above W/FDME,0 =2.2 gcatalyst h/gDME, DME and MeOH conversions and H2 yieldasymptotically approach the values of 0.95, 1.0, and 0.94,respectively.Furthermore, an increase in space time also enhances the

reverse WGS reaction (eq 3), thus increasing the CO yield.Thus, the CO concentration increases from 0.003 to 0.033% ona water-free basis for an increase in space time from 0.1 to 2.2gcatalyst h/gDME. Therefore, CO2 selectivity decreases. A factorcontributing to this increase in CO concentration is the increase

Figure 2. Effect of DME partial pressure on the values at zero time onstream of DME conversion, MeOH effective conversion, H2 yield, andCO2 selectivity based on C1 products. Conditions: 300 °C,W/FDME,0 =0.30 gcatalyst h/gDME, and steam/DME = 3.

Figure 3. Effect of steam/DME molar ratio on the values at zero timeon stream of DME conversion, MeOH effective conversion, H2 yield,and CO2 selectivity based on C1 products. Conditions: 300 °C, W/FDME,0 = 0.20 gcatalyst h/gDME, and PDME = 0.16 bar.

Figure 4. Effect of steam/DME molar ratio on the values at zero timeon stream of product molar fractions. Conditions: 300 °C,W/FDME,0 =0.20 gcatalyst h/gDME, and PDME = 0.16 bar.



in CO2 concentration in the reaction medium due to a higherconversion.16

The aforementioned results and those in the literatureevidence the good performance of catalysts made up of CuO−ZnO−Al2O3 and HZSM-5 zeolite for SRD when they areprepared using a suitable method and composition.14,34,37,38

The reaction indices are better than those reported byMatsumoto et al. using Cu/CeO2−H-mordenite catalysts,given that these authors obtained 86% DME conversion anda hydrogen production rate of 71 mmolH2

/(gcatalyst h) at atemperature of 250 °C and using a very high value of space time(0.35 gcatalyst and a flow rate of DME/H2O/N2 = 2.3:7.7:16.0mL/min (at 25 °C)).23

Studies in the literature deal with the improvement of thecatalyst by modifying the CuO−ZnO−Al2O3 metallic function.Thus, Feng et al. incorporate ZrO2 and obtain full DMEconversion and a H2 yield of more than 90% with a CO2selectivity of around 95% in a fixed bed at 270 °C and using aspace velocity lower than 2461 mL/(gcatalyst h).15,16 Theseresults are open to improvement based on future innovationsand show the potential interest of DME reforming forproducing hydrogen to be used in fuel cells and as a rawmaterial.3.2. Catalyst Stability. The evolution of reaction indices

with time on stream (up to 24 h) has been studied withinoperating condition ranges established for significant deactiva-tion in order to clearly analyze the effect of these conditions.3.2.1. Effect of Temperature. The formation of hydro-

carbons from oxygenates (DME and MeOH) gives way to asignificant decrease in H2 yield.32−34 Furthermore, theformation of hydrocarbons takes place presumably throughthe formation of methoxy ions, which are also intermediates inthe formation of coke in DME synthesis,43 and in thetransformation of oxygenates on the HZSM-5 zeolite throughwell-known mechanisms activated by the zeolite acid sites and,particularly, by the sites of higher acid strength.44−46 In theliterature, the presence of methoxy ions as intermediates inDME reforming has been determined by spectroscopy and theyhave been attributed a coke precursor role.47,48

The effect of temperature on catalyst deactivation has beenstudied based on runs 1−3 in Table 2. It should be noted thatthe moderate acidity of the catalyst allows operating at up to325 °C by avoiding hydrocarbon formation.34 The runs havebeen carried out with a low value of space time (far from

thermodynamic equilibrium) in order to clearly observe thedecrease in catalyst activity with time on stream.Figure 6a shows the decrease in DME and methanol

conversions with time on stream for the three temperaturesstudied, with this decrease being more pronounced at 325 °C.The decrease in DME and methanol conversions with time onstream evidences the deactivation of both the acid and themetallic functions, although the deactivation of the metallicfunction is more significant as the temperature is increased,which gives way to a considerable reduction in the conversion ofmethanol with time on stream at 325 °C.The decrease in the production of H2 (Figure 6b) is also

more severe at 325 °C, given that it is reduced by half afterapproximately 24 h. Therefore, although hydrogen productionat zero time on stream is highest at 325 °C, the averagecumulative production for 24 h at this temperature is 395mmolH2

/(gcatalyst h), which is very similar to the averageproduction corresponding to 300 °C and higher than thatcorresponding to 275 °C (256 mmolH2

/(gcatalyst h)).The selectivity of CO2 based on C1 compounds (Figure 6c)

at zero time on stream decreases as the temperature isincreased, due to the enhancement of the reverse WGSreaction, as mentioned above for the results in Figure 1. Thisselectivity of CO2 is almost the same for 24 h at 275 °C, whichevidences that catalyst deactivation at this low temperature has asimilar effect on the reforming and reverse WGS reactions.Nevertheless, the selectivity of CO2 at 300 and 325 °C increaseswith time on stream (in a more pronounced way at 325 °C)because deactivation affects the reverse WGS reaction to agreater extent than the reforming reaction.The deactivation results in Figure 6 are consistent with the

significant increase in coke deposition as the temperature isincreased, with coke content values being 1.0, 1.8, and 4.8 mg/gcatalyst, for 275, 300, and 325 °C, respectively (Table 2).The cause of deactivation has been studied from the TPO

curves of the coke for determining the nature and location ofthe coke and by N2O chemisorption by pulses for quantifyingpossible Cu sintering. The TPO curves for the catalystdeactivated at different temperatures (Figure 7) show twomean peaks: (i) one at low temperature (∼260 °C),corresponding to the coke deposited on the metallic function,whose combustion is activated by the metal, and (ii) the otherone at high temperature (410−450 °C range), corresponding tothe coke deposited on the acid function.23,49,50 A third peak isalso observed at an intermediate temperature (around 350 °C),

Figure 5. Effect of space time on the values at zero time on stream ofDME conversion, MeOH effective conversion, H2 yield and CO2selectivity based on C1 products. Conditions: 300 °C, steam/DME = 4,and PDME = 0.16 bar.

Table 2. Operating Conditions and Coke Content (Cc) in theCatalyst (For Time on Stream = 24 h) in the Runs forDetermining Catalyst Stability

no. T, °Csteam/DMEmolar ratio

PDME,bar

W/FDME,0,gcatalyst h/gDME

Cc, mg/gcatalyst

1 275 3 0.16 0.13 1.02 300 3 0.16 0.13 1.83 325 3 0.16 0.13 4.84 325 6 0.13 0.13 1.85 300 4 0.08 0.08 1.86 300 4 0.18 0.08 2.17 300 4 0.25 0.08 2.28 300 4 0.25 0.20 1.39 300 4 0.25 0.60 0.210 300 4 0.25 0.60 0.3a

aFor 48 h time on stream.



which should be attributed to the coke deposited at theinterface between the metallic and acid functions, or on the

Al2O3 support of the metallic function, as has been establishedin the study of the coke deposited on CuO−ZnO−Al2O3/γ-Al2O3 catalysts used in the synthesis of DME.50

An increase in the reaction temperature enhances total cokedeposition (Table 2), but mainly the coke deposited on themetallic sites (peak at 260 °C in Figure 7). This result explainsthe effect of temperature by decreasing methanol conversionwith time on stream (Figure 6a).Furthermore, the mechanism of coke formation occurs

presumably through methoxy ion intermediates, as proposedby Agarwal et al. for methanol steam reforming.51 In addition todirectly enhancing the evolution of methoxy ions to form coke,the temperature increase also contributes to increasing theconcentration of these ions by favoring methanol formation byDME hydrolysis. Therefore, methanol reforming is almost inequilibrium at 325 °C and methoxy ions are in excess for thisreaction, thereby favoring coke formation.As a consequence of deactivation, the metal surface area of

the catalyst deactivated at 325 °C (24 h) is 30 m2/gCu. Thislower metal surface area compared to that of the fresh catalyst,56 m2/gCu, is due to the fact coke deposition hinders N2Oadsorption on the metal, which has been proven because themetal surface area corresponding to the fresh catalyst isrecovered through catalyst regeneration by coke combustion at300 °C with diluted O2. This result dismisses Cu sintering as acause of deactivation under the operating conditions studied(with 325 °C as the ceiling reaction temperature, as it is thetemperature used in catalyst calcination).The aforementioned results allow concluding that under the

conditions studied coke deposition on the metal surface area isthe main cause of catalyst deactivation, which attenuates itsactivity for reforming and WGS reactions, with Cu sinteringbeing insignificant. The coke is formed presumably bydegrading methoxy ions, which are intermediates in thetransformation of oxygenates into hydrocarbons, and catalyststability lasts longer below 300 °C, as methoxy ion formation isattenuated.

3.2.2. Effect of Steam/DME Molar Ratio. In addition toincreasing the reforming reaction rate, the presence of water inthe reaction medium contributes to attenuating coke deposi-tion. Sierra et al. have studied this effect in the synthesis of DMEon a CuO−ZnO−Al2O3/γ-Al2O3 catalyst and attributed it tothe inhibition in the formation of methoxy ions, which areintermediates in the formation of hydrocarbons and coke.43

Steam is also important for attenuating coke deposition in thetransformation of methanol and ethanol into hydrocarbons onHZSM-5 zeolite and SAPO catalysts.26,27,52−55

Nevertheless, excess steam in the SRD reaction medium maypartially oxidize the metal active sites.8 Consequently, the effectof the steam/DME ratio on catalyst stability should be studiedin order to delimit the conditions that minimize coke depositionwithout deteriorating the metallic function.Figure 8 shows the evolution with time on stream of DME

and methanol conversions (Figure 8a) and H2 production(Figure 8b) for steam/DME ratio values of 3 and 6. Thedecrease in the conversions with time on stream is lesspronounced for the steam/DME ratio of 6, which is consistentwith the hypothesis whereby coke is formed through methoxyions as intermediates and an increase in the amount of steam inthe reaction medium contributes to inhibiting the formation ofthese ions. Furthermore, an increase in the formation of H2contributes to attenuating coke deposition, given that coke

Figure 6. Effect of temperature on the evolution with time on streamof (a) DME conversion and MeOH effective conversion, (b) H2production rate, and (c) CO2 selectivity based on C1 compounds.Conditions: W/FDME,0 = 0.13 gcatalyst h/gDME, steam/DME = 3, andPDME = 0.16 bar.

Figure 7. TPO profiles of coke combustion for catalyst deactivated atdifferent temperatures. Conditions: W/FDME,0 = 0.13 gcatalyst h/gDME,steam/DME = 3, PDME = 0.16 bar, and time on stream, 24 h.



formation takes place following mechanisms based on aromaticdehydrogenation and condensation.45,56

The higher DME and MeOH conversions obtained as thesteam/DME ratio is increased (Figure 8a) evidence that catalystdeactivation is attenuated, although this attenuation ofdeactivation has presumably different effects on each step(DME hydrolysis and methanol reforming) of the global SRDprocess. The assessment of the effect that this attenuation ofdeactivation has on each step is a complex task, given that thereis a synergy between the two steps; i.e., the decrease in DMEconversion observed with time on stream is due to deactivationaffecting not only the DME hydrolysis step (first), but also themethanol reforming step, given that the latter shifts the DMEhydrolysis equilibrium. In spite of this synergy, the results inFigure 8a allow a quantitative comparison to be made of theeffect that the attenuation of deactivation caused by the increasein the steam/DME ratio has on each step. Thus, for a steam/DME ratio of 3, DME conversion decreases by approximately40% over 24 h (from 0.58 to 0.35), whereas for a steam/DMEratio of 6 it decreases by 27% (from 0.63 to 0.46).Consequently, as the steam/DME ratio is doubled from 3 to6, the decrease in the DME conversion of the overall SRDprocess attenuates by 13%. Furthermore, for a steam/DMEratio of 3, methanol conversion decreases by approximately 21%over 24 h (from 0.97 to 0.77), whereas for a steam/DME ratioof 6 it decreases by 11% (from 0.97 to 0.86), which means thatthere is a difference of 10% in the attenuation of deactivationwhen the steam/DME ratio is doubled from 3 to 6. The 3%difference between these two figures for attenuation (13 and10%) may be attributed to the smaller influence of deactivationon DME hydrolysis. It is therefore concluded that the effect the

steam/DME ratio has on attenuating deactivation is moresevere for methanol reforming than for DME hydrolysis.Furthermore, in order to prove there is no partial oxidation of

Cu during reforming with a steam/DME ratio of 6, thedeactivated catalyst has been treated with H2 and no water hasbeen observed in the chromatographic analysis of the productstream. As Cu crystals are very stable and Cu does not undergosintering, as proven above, as well as the fact the catalyst fullyrecovers its activity subsequent to combustion, evidence thatthe sole cause of catalyst deactivation is coke deposition.Consistent with the results in Figure 8, the coke content in

the catalyst (determined by TPO) (Table 2) is higher for asteam/DME ratio of 3 (4.8 mg/gcatalyst) than for a ratio of 6 (1.8mg/gcatalyst). A comparison of the TPO profiles correspondingto the catalysts deactivated in runs with different steam/DMEratios in the feed (Figure 9) reveals that an increase in the

content of steam in the reaction medium attenuates thedeposition of coke on the metallic sites to a greater extent(peaks at 260 °C), which is consistent with the more severeattenuation of the methanol reforming reaction than DMEhydrolysis, as observed in Figure 8a.On the basis of these results, it is concluded that an increase

in the steam/DME ratio favors catalyst stability in the SRDreaction. In light of the results obtained here on the effect of thesteam/DME molar ratio on the values at zero time on stream ofH2 yield (Figure 3), product concentration (Figure 4), andattenuation of deactivation by coke deposition (Figure 8), asteam/DME ratio in the 4−5 range is established as suitable,given that above this value the energy cost is likely to beexcessive.It is noteworthy that a fluidized bed reactor contributes to

attenuating coke formation due to uniform catalyst activity, sothe effect of attenuating coke formation by steam is thereforemore efficient than in a fixed bed, as has been proven in thetransformation of methanol into hydrocarbons.52

3.2.3. Effect of DME Partial Pressure. Figure 10 shows theevolution of DME conversion, MeOH effective conversion(Figure 10a), and H2 production (Figure 10b) with time onstream for three values of DME partial pressure. An increase inthe DME partial pressure increases DME conversion and H2production at zero time on stream, whereas MeOH effectiveconversion is maintained unaltered. These results are consistentwith those in Figure 2, for a higher value of space time (0.30gcatalyst h/gDME).

Figure 8. Effect of steam/DME ratio on the evolution with time onstream of (a) DME conversion and MeOH effective conversion and(b) H2 production rate. Conditions: 325 °C, W/FDME,0 = 0.13 gcatalysth/gDME, and PDME = 0.16 bar.

Figure 9. TPO profiles for the catalyst deactivated under differentvalues of steam/DME molar ratio. Reaction conditions: 325 °C, W/FDME,0 = 0.13 gcatalyst h/gDME, and PDME = 0.16 bar.



Furthermore, Figure 10 shows there is no significant effect ofDME partial pressure on catalyst stability and the decrease inthe reaction indices with time on stream is similar for the threevalues of DME partial pressure.Moreover, the coke contents for the catalysts deactivated

under the different DME partial pressures are similar, around 2mg/gcatalyst (Table 2), although there is a very slight increase inthe content as the DME partial pressure is increased.The fact that the DME partial pressure does not have a

significant effect on coke deposition is due to two opposingeffects of this variable. On the one hand, as the DME partialpressure is increased the hydrolysis reaction rate increasessignificantly, without affecting the reforming reaction. Con-sequently, the methanol concentration in the medium increases,and therefore the concentration of coke precursor methoxy ionsalso increases. On the other hand, H2 production increases,which inhibits the dehydrogenation reactions involved in cokeformation.It should be noted that the difference in the values of DME

partial pressure studied is small, given that this variable isrestricted in order to maintain steam/DME constant and due tothe need of operating with a gas flow rate that ensures a suitablehydrodynamic performance in the catalytic bed (u/umf relativevelocity between 2.5 and 5). Consequently, the difference inmethoxy ion concentration in the catalyst is limited under theconditions studied.3.2.4. Effect of Space Time. An increase in space time

significantly attenuates the decrease with time on stream ofDME and MeOH conversions, with both conversions (Figure11a) and H2 yield (Figure 11b) being almost constant for 24 hfor a value of 0.60 gcatalyst h/gDME. Furthermore, CO2 selectivity(Figure 11c) remains constant for low values of space time(0.075 gcatalyst h/gDME) and increases with time on stream for

higher values of this variable. This result evidences that thedeactivation by coke deposition has a greater impact on thereverse WGS reaction than the reforming reaction, which isconsistent with the results in section 3.2.1 corresponding to thestudy of the effect of temperature on catalyst stability. Althoughthe results are not shown, the catalyst is stable for long durationexperiments (48 h) with a space time of 0.60 gcatalyst h/gDME.Clearly, as space time is increased deactivation becomes patentfor higher values of time on stream.The aforementioned results concerning the effect of space

time on deactivation are consistent with the coke content in thecatalyst, which sharply decreases as space time is increased(Table 2). This result is in agreement with the hypothesis thatthe coke precursors are the methoxy ions formed from theoxygenates (DME and MeOH). Thus, the higher concen-trations of DME and MeOH in the reaction medium for a lowvalue of space time favors the formation of coke precursormethoxy ions.

Figure 10. Effect of DME partial pressure on the evolution with timeon stream of (a) DME conversion and MeOH effective conversion and(b) H2 production rate. Conditions: 300 °C, W/FDME,0 = 0.08 gcatalysth/gDME, and steam/DME = 4.

Figure 11. Effect of space time on the evolution with time on stream of(a) DME conversion and MeOH effective conversion, (b) H2 yield,and (c) CO2 selectivity based on C1 compounds. Conditions: 300 °C,steam/DME = 4, and PDME = 0.25 bar.



4. CONCLUSIONSAn increase in temperature (between 275 and 325 °C), spacetime (between 0.075 and 2.2 gcatalyst h/gDME), steam/DME ratioin the feed (between 3 and 6), and, to a lesser extent, DMEpartial pressure in the feed (between 0.08 and 0.25 bar) givesway to a significant increase in the conversion and in the yield ofH2 in the reforming of DME on the catalysts studied, althoughCO yield also increases by enhancing the reverse WGS reaction.The problems related to the stability of the catalyst are due to

coke deposition, given there is no Cu sintering under theconditions studied (<325 °C). The main deactivation cause iscoke deposition on the metallic function (a combustion peak inthe TPO curves at 250−260 °C) and, to a lesser extent, thecoke deposited on the interface (peak at 350 °C) and on theacid function itself (peak at 410−450 °C). The coke origin ispresumably the degradation of methoxy ion intermediatesformed from the oxygenates in the reaction medium, andtherefore, coke deposition decreases under the conditions thatgive way to a decrease in the concentration of oxygenates in thereaction medium (high space time) or the formation ofmethoxy ions (high steam/DME ratio). A higher concentrationof H2 in the reaction medium also contributes to attenuatingdehydrogenation reactions that give way to coke deposition.Furthermore, operation should be carried out below 300 °C inorder to minimize the degradation of methoxy ions to coke andits formation from hydrocarbons, whose production issignificant for higher temperatures.Based on the effect operating variables have on deactivation,

the time on stream when deactivation is patent is higher as thetemperature decreases and both space time and the steam/DME molar ratio are increased. At 300 °C with a space time of0.60 gcatalyst h/gDME, with a steam/DME molar ratio of 4, andunder a DME partial pressure in the feed of 0.25 bar, thecatalyst is stable for 48 h, with a DME conversion of 0.85 and aH2 yield of 0.81, with H2 production being 180 mmolH2

/(gcatalysth).Furthermore, encouraging results are obtained in a fluidized

bed reactor, which in turn is a suitable technology for scaling upby maintaining bed isothermicity.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +34 94 6015363. Fax: +34 94 6013500. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been carried out with the financial support of theDepartment of Education, Universities and Research of theBasque Government (Project IT748-13), the University of theBasque Country (UFI 11/39 UPV/EHU), and the Ministry ofScience and Innovation of the Spanish Government (ProjectsCTQ2009-13428 and CTQ2012-35263).

■ NOTATIONCc = coke content in the catalyst, mg/gcatalystdCu = average particle size of Cu, nmdmesopore = average mesopore diameter, ÅFi, Fi,0 = molar flow rates of i component at the reactor outletand in the feed, respectivelyPDME = DME partial pressure, bar

rp,H2= H2 production rate, mmolH2

/(gcatalyst h)SBET = BET surface area, m2/gSj = selectivity of each product in the lump C1Smetallic = specific metal surface area (m2

Cu/gCu)Smicropore = micropore surface area, m2/gVmesopore, Vmicropore = mesopore and micropore volumes,respectively, cm3/gW = catalyst weight, gXDME, XMeOH = DME conversion and methanol effectiveconversion, respectivelyxi = molar fraction of i componentYi = yield of i component

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