Steam Reforming of Ethanol on Copper Catalysts Derived from

Article pubs.acs.org/IECR

Steam Reforming of Ethanol on Copper Catalysts Derived from Hydrotalcite-like Materials A. F. Cunha,† Y. J. Wu,† J. C. Santos,† and A. E. Rodrigues*,† †

Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465, Porto, Portugal ABSTRACT: Steam reforming of ethanol (SRE) and sorption enhanced steam reforming of ethanol (SE-SRE) on Cu,Zn,Al materials was studied in the temperature range between 200 and 600 °C for hydrogen production. Precursors with different structures, such as a hydrotalcite-like compound (CuZnAl-HT), zincian malachite derivative (CuZnAl-REF, a reference catalyst precursor for the synthesis of methanol) and amorphous material with embedded copper particles (CuZnAl-ECP), were employed to obtain the active phases. Among the three samples used, the CuZnAl-HT catalyst shows the highest activity (ethanol and water conversion) and selectivity (hydrogen yield). This material also shows a satisfying stability, as the ethanol conversion only decreases around 20% at 400 °C during 12 h of lifetime test. In addition, carbon dioxide uptake during the transient period of SRE was found on the CuZnAl based materials. Especially for the CuZnAl-HT catalyst a SE-SRE phenomena was perfectly observed at 400 °C due to a relative good interaction of carbon dioxide formed in the initial transient period of reaction and the ZnO phase present in the catalyst. Probably, also retained traces of the hydrotalcite-like precursor structure on the CuZnAl-HT catalyst used are responsible for the observed SE-SRE phenomena. Breakthrough tests were performed with the CuZnAl-HT catalyst. This material shows a carbon dioxide adsorption capacity from 0.11 to 0.18 mol/kg, which is a prerequisite for SE-SRE. morphological and surface changes of the copper particles,11 and Cu/Zn alloy formation.12 Other effects like lattice strain13 or planar defects14 have a strong influence on the activity. Binary Cu/ZnO systems are in general, used as a model system. However, these binary systems may additionally contain 5 to 10 mol % Al2O3, which increases and stabilizes the catalytic performance due to its promoter effect.15 SRE over a CuO/ZnO/Al2O3 catalyst was first investigated by Cavallaro and Freni.16 It was found that Cu is very effective for the dehydrogenation of ethanol due to its high ability to maintain the C−C bond below 350 °C. However, in order to cleave the C−C bond, SRE requires higher reforming temperatures than steam reforming of methanol. Due to the aggregation of copper on the surface of support materials at high temperatures, copper-based catalysts always undergo a relative fast deactivation. Mariño et al.17 has found that the copper dispersion is favored with lower copper loadings, indicating that SRE is a structure-sensitivity reaction. It was also reported by Mariño et al.18 that Cu/Ni/K/Al2O3 catalysts are suitable for maximum hydrogen production in SRE at 300 °C. The addition of nickel enhances ethanol gasification, increasing the gas yield and reducing acetaldehyde and acetic acid production. Finally, Mariño et al.19 prepared CuNiAl HTlc precursors with K/γAl2O3 as support. They found that the addition of nickel favors the formation of HTlc structure, with Cu2+, Ni2+, and Al3+ ions in the cationic layers. The calcination of the HTlc precursor

1. INTRODUCTION Nowadays, most of the energy is derived from nonrenewable sources, such as fossil fuels. The gradual depletion of these fossil fuel reserves as well as involved greenhouse gas emissions increased substantially the research efforts in new alternative energy systems.1 Hydrogen is generally accepted to be the energy system of the future, which is clean and efficient in fuel cells.2 Steam reforming is the most important process for hydrogen production.3 For a hydrogen infrastructure based on renewable resources, there is a need for efficient technologies. Recently, some attention has been given to the production of hydrogen from ethanol. Especially bioethanol is an interesting source, because it can be easily derived from biomass.4 Moreover, it is abundant, renewable as an energy source, and free of sulfur, and it has a relatively high H/C ratio for steam reforming: C2H5OH(g) + 3H 2O(g) ↔ 2CO2 (g) + 6H 2(g) 0 (ΔH298K = +173.3 kJ·mol−1)

(1)

Many catalytic systems have been used for steam reforming of ethanol (SRE);5−7 however, the research efforts on SRE over copper-based catalysts are relatively young. Copper-based catalysts are frequently studied systems in the methanol synthesis8 and steam reforming of methanol9 due to their high selectivity and activity. Quite good results are obtained from metallic copper based systems, often in the presence of a ZnO phase and eventually other metal oxides such as Al2O3. The research efforts suggest ZnO as an optimal medium to disperse the active copper metal. It is worth noting, that the relatively good activity of this system may be a result of the strong interaction between CuO and ZnO phases,10 such as synergistic effects in the solid solution (Cu in ZnO), © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13132

June 22, 2012 September 14, 2012 September 14, 2012 September 14, 2012 dx.doi.org/10.1021/ie301645f | Ind. Eng. Chem. Res. 2012, 51, 13132−13143

Industrial & Engineering Chemistry Research

Article

higher reduction temperatures. Additionally, they found that a reduction temperature above 700 °C is required to reduce Ni2+ ions to a metal state due to the strong interaction of nickel oxide with magnesium oxide. Finally, Guil-López et al.28 prepared Cu oxides from hydrotalcite-like precursors and tested them in oxidative SRE. Highly crystalline HTlc-precursors were obtained using the urea hydrolysis method. The particle sizes of the segregated active metal oxide phases could be decreased by the crystallinity increase of the HTlc precursor. Indeed, small particle sizes favor the strong interactions between active metals and the amorphous matrix of Al-modified cations, resulting in an active metal phase stabilization. It is now clear that HTlc are proper materials for metal crystallite stabilization; they show apart from a wide range of applications such as catalysts,29 catalyst support properties,30 also quite good capacities for the selective capture of carbon dioxide.31 It must be emphasized that HTlc are layered double hydroxide (LDH) structures with the general formula [M2+1‑xM3+x(OH)2x]x+ (An‑x/n•mH2O, where M2+ can be Mg (typical), Ni, Zn, or Cu, M3+ is Al (typical), Cr, Mn, Co, and Fe, and finally An‑ is the negative charged anion). Besides, HTlc have a so-called “memory effect” feature,32 which means the LDH structure can be regenerated by exposure to anions after a relatively gentle calcination. Hence, it can be employed as a regenerable carbon dioxide sorbent. As an example, it is known that the hydrotalcite (Mg6Al2(OH)16CO3•4H2O) is an anionic clay with a structure similar to brucite (Mg(OH)2). Part of the Mg2+ cations are replaced with Al3+ in a HTlc, and the resulting positive charge is compensated by anions, typically carbonate, in the interlayer between the brucite-like sheets.33 The activation of HTlc involves a heat treatment wherein the layered structure is destroyed to form a mixed oxide. However, in a next step the material can be rehydrated to restore the original HTlc structure to a large extent and with the exclusion of other anions and CO2, Brønsted base sites (OH−) are incorporated in the interlayer.34−38 It can be found from the literature that a copper-based catalyst system derived from Cu,Zn,Al precursors with a hydrotalcite (HT) structure could be especially interesting for SRE. The calcination of the precursor with a HT structure leads to a mixture of oxides, which can be useful in catalytic applications due to their high surface area, large pores, and the copper dispersion on surface. Nevertheless, after H2 reduction catalysts with highly dispersed metallic Cu can be obtained, avoiding the aggregation of copper. In the present work we report results obtained on Cu,Zn,Al systems derived from precursors with different structures for SRE and the possibility for SE-SRE. A systematic study was performed to investigate the effect of the operating conditions on the activity and selectivity for hydrogen production from SRE with copper as active catalyst phase. Finally, the catalyst derived from the precursor with a HT structure was also analyzed on possible selective carbon dioxide sorption for hydrogen production via sorption-enhanced steam reforming of ethanol. The carbon dioxide adsorption capacity was further examined by carrying out breakthrough tests in a fixed-bed reactor with the feed of helium, carbon dioxide, and steam.

produces a CuO segregated phase and/or a phase of copper referred as a “surface spinel”. Surface spinel phases consist of Cu2+ ions dispersion on the Al2O3 surface. However, the ratio of Cu/Al on surface was not influenced by the nickel content or the temperature of the thermal treatment. Ethanol decomposition was investigated by Segal et al.20 over Cu/Al layered double hydroxide (LDH) derived catalysts. Catalytic dehydrogenation of ethanol into hydrogen and acetaldehyde is the major mechanism. The active catalytic phase is derived from the initial Cu/Al LDH structure during the process. Hydrogen formation occurred at around 200 °C and is related with the formation of metallic Cu species formed during in situ modification of the initial LDH structure. Significant deactivation of the catalyst used occurred above 350 °C. Cu−Ni supported catalysts were prepared by Carrero et al.21 with the incipient wetness impregnation method. The best catalytic performance was achieved at 600 °C. It was found that the product distribution is related with metal particles sizes, ́ et al.22 prepared metal content, and Ni/Cu ratio. Later, Vizcaino Cu−Ni supported catalysts with different support materials. They discovered that the nickel phase is responsible for most of the hydrogen production, while the copper phase decreases the CO production and coke deposition. In another work, a solution combustion synthesis was used by Kumar et al.23 to prepare Ni−Fe−Cu containing catalysts, using a controlled volume combustion method. The Ni1Fe0.5Cu1 catalyst was found to be the most promising system, yielding ∼80% in terms of conversion and ∼42% in terms of hydrogen selectivity at ∼415 °C for the ethanol decomposition reaction. Nevertheless, at lower temperatures Ni is the dominant active and selective phase for hydrogen and methane production, while the Cu phase is selective for acetaldehyde formation and the Fe phase is selective for carbon dioxide and ethane formation. Sau et al.24 performed SRE over Cu/Zn/Al based catalysts at moderate temperatures. Hydrogen, carbon dioxide, methane, ethylene, acetaldehyde, and diethylether were detected as products for all operating conditions, indicating the presence of several parallel reaction pathways. Busca et al.25 prepared Ni/Co/Zn/Al HTlc as precursors to obtain high surface area mixed oxide catalysts for SRE with the urea hydrolysis method. It has been found that the selectivity is linked with acetate ions, assuming that they are the key intermediates during SRE. The selectivity toward hydrogen and carbon dioxide was attributed to a less reduced state of cobalt, while on more or completely reduced surfaces acetates decompose to methane and COx species. The hydrogen yield obtained was close to 90% with a water-to-ethanol feed ratio of 6 at 540 °C. However, the hydrogen yield decreases with increasing temperatures due to the WGS equilibrium.26 In addition, the selectivity toward CO2 decreases simultaneously with an increase of selectivity toward CO. Finally, higher waterto-ethanol ratios (9 to 12) did not improve the selectivity toward hydrogen but tend to decrease the ethanol conversion. Li et al.27 prepared nickel based catalysts derived from thermal decomposition of NiMgAl HTlc precursors with a coprecipitation method. XRD measurements have shown that the main crystal phase was a Ni−Mg−O solid solution, and when the Ni/Mg ratio became lower less Ni0 was reduced. In this specific case, it must be pointed out that inferior catalytic performances are related with insufficient Ni0 metal, so that the catalytic activity and stability was only improved by the use of

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. The Cu,Zn,Al materials used in this work were prepared in an automated laboratory reactor 13133

dx.doi.org/10.1021/ie301645f | Ind. Eng. Chem. Res. 2012, 51, 13132−13143


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Experimental runs were carried out between 200 and 600 °C in 50 °C intervals, and each run was measured for 4 h. During these intervals, the temperature was subsequently increased exclusively in the presence of the carrier gas helium, with a heating rate of 5 °C/min, to ensure total absence of product gases. An HPLC pump (Merck L-2130) was used to introduce the liquid ethanol−water mixture inside the reactor. In all runs, the premixed liquid flow rate of ethanol−water was adjusted to 0.3 mL/min, corresponding to a molar ratio of 1:10. Helium or nitrogen (99.999 mol %) was used as a carrier gas during experimental runs, and the flow rates (V̇ = 50 or 200 Ncm3/ min) were controlled by mass flow controllers. The liquid−gas mixture was vaporized in a spider tube inside the upper part of the reactor (21 cm long). This gas stream passed over the material placed inside the lower part of the reactor. During the reaction, the product vapors exiting the reactor were passed through a condenser (stainless steel heat exchanger consisting on a bundle of tubes) and an ice-cooled trap using a Dewar. The composition of the outgoing off-gas stream was determined with a gas chromatograph (GC 1000, Dani Chromatographs) equipped with an online multiport 16-valve system for sample injection (Valco Instruments Company Inc.), a capillary column (Carboxen 1010 Plot, Supelco) and a thermal conductivity detector. The liquid products, including the unreacted ethanol present, were periodically collected in the ice-cooled trap and separately analyzed with the same aforementioned gas chromatograph, using a capillary column (Poraplot-U) and a flame ionization detector. The water conversion was estimated via the measured amounts of carbon dioxide formed during reaction (indirect method), in agreement with the global SRE equation; 2/3 mol of carbon dioxide formed may consume 1 mol of water. A schematic representation of the experimental steam reforming unit used can be found in a previous work.39 Reaction studies were carried out and the materials used were evaluated. The effects of temperature and space time were investigated. The activity of the materials is assessed by measuring the conversions of ethanol and water. The conversions of ethanol and water were calculated based on the feed stoichiometry, one mol of ethanol and ten mol of water:

(Mettler-Toledo Labmax) by constant pH precipitation from aqueous metal solutions, with Na2CO3 as precipitating agent and a subsequent individual aging time in the mother liquor. The continuously obtained precursor was prepared by a slightly different production method as it was precipitated accordingly from a Cu and Zn nitrate solution, using Na2CO3 solution to which an adequate amount of sodium aluminate was added. This slurry was continuously fed into a spray-dryer (Niro minor mobile, Tinlet = 200 °C, Toutlet = 100 °C). The active catalysts were obtained in situ from these precursors by calcination in air and reduction in pure hydrogen. The materials obtained are as follows: CuZnAl-HT (hydrotalcite sample) with Cu:Zn:Al [mol %] = 50:17:33, precipitation pH = 8.0, and precipitation temperature 25 °C is the sample derived from HTlc structure precursor; CuZnAl-REF (reference sample) with Cu:Zn:Al [mol %] = 60:25:15, precipitation pH = 6.5, and precipitation temperature 65 °C which has already been employed for methanol synthesis was used as a reference; CuZnAl-ECP (embedded Cu-particles sample) with Cu:Zn:Al [mol %] = 61:26:13, precipitation pH = 6.5, and precipitation temperature 65 °C was prepared continuously exhibiting strongly embedded Cu-particles. 2.2. Characterization of the Materials. The materials were characterized before and after SRE runs, applying different physicochemical techniques. The content of copper, magnesia or zincian, and alumina in the materials were determined with inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (model 70 Plus Jobin Yvon, Unterhaching, Germany). The specific surface areas of the fresh materials were calculated by the BET method from the nitrogen equilibrium adsorption isotherms at 77 K, determined with a Quantachrome Autosorb-1 apparatus. The decomposition behavior during calcination was simulated with thermogravimetric analysis (TG) in a Netzsch STA 449C Jupiter analyzer with coupled mass spectrometry (MS) in a Pfeiffer Omnistar or with high pressure differential scanning calorimetry (DSC) in a HP DSC 827e, Mettler-Toledo equipment. X-ray diffraction (XRD) measurements were carried out in a transmission STOE Stadi-P with autosampler, or reflection geometry Bruker D8 Advance diffractometer, using Cu Kα radiation. The crystallite sizes of the metals in the materials were calculated by the Scherrer equation. A high-resolution transmission electron microscopy (HRTEM) study of the materials was carried out with a Philips CM200FEG microscope operated at 200 kV. Temperature programmed reduction (TPR) in hydrogen atmosphere was carried out in a TPDRO 1100, CE instrument. Carbon dioxide sorption capacity was determined from breakthrough curves with the feed of helium, carbon dioxide, and water. Helium and water were employed for sorbent regeneration. The liquid water flow-rate was controlled with an HPLC pump (Merck-Hitachi, Japan) and vaporized before entering into the column. Both carbon dioxide and helium flowrates were controlled by independent mass flow controllers. 2.3. Reaction Studies. Reaction studies were carried out in a 42 cm long tubular stainless steel fixed-bed continuous downflow reactor (i.d. = 3.3 cm). The continuous down-flow reactor was loaded with the precursors and placed inside a heating furnace with a PID temperature controller (Termolab, Fornos Electricos Lda).

XEtOH [%] =

X H2O[%] =

nEtOH ̇ ̇ ,0 − nEtOH nEtOH ̇ ,0 n ̇H2O ,0 − n ̇H2O n ̇H2O ,0

× 100% (2)

× 100% (3)

For convenience, the molar product distribution was calculated (dry basis) from the mole number of one product formed, divided by the total mole number of products Pi[mol%] =

ni̇ × 100% ∑ ni̇

(4)

where ṅi represents the molar flow rate of one product, and ∑ṅi represents the total molar flow rate of products in the dry product stream (water and organic byproduct formed are here excluded). However, in order to calculate the selectivity toward hydrogen, it is required to obtain the yield of hydrogen, which was calculated considering the reactants ethanol and water, respectively: 13134


Industrial & Engineering Chemistry Research YH2[%] =

n ̇H2 − n ̇H2,0

×

nEtOH ̇ ,0

Article

υEtOH × 100% υ H2

Associated with these results is Figure 1 with the nitrogen equilibrium sorption isotherms obtained at 77 K for CuZnAlHT.

(5)

and the hydrogen selectivity, SH2, can then be obtained by

S H2 =

YH2 XEtOH

(6)

Finally, for all the measurements made in this work, an average error of around ±5% should be taken into account.

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The results for the composition of the materials used are collected in Table 1. Table 1. Materials Composition materials

mprec.a [g]

mcat.b [g]

Cu:Zn:Alc [mol %]

Cu:Zn:Ald [mass %]

CuZnAl-HT CuZnAl-ECP CuZnAl-REF

15.0 10.0 7.0

7.0 6.6 4.4

50:17:33 61:26:13 60:25:15

61:21:17 65:29:6 65:28:7

Figure 1. N2 equilibrium adsorption−desorption isotherms at 77 K of the precursor, calcined precursors and after SRE; CuZnAl-HT.

a

Total mass of the precursor. bTotal mass of the catalyst. cMolar percentage. dMass percentage obtained by ICP-AES for the reduced materials.

It might be observed that the shape of all these isotherms is quite similar, which can be interpreted as a type IV isotherm associated with a hysteresis of type H1. The isotherms obtained are typical for mesoporous materials with pores in the range between 2 and 50 nm. The hysteresis of type H1 is normally an indication that the material is composed of rigid agglomerates with spherical shapes and uniform size arrangement. The pore size distribution in the mesoporous region (2−50 nm) can be estimated from the desorption curve following the method of Barrett−Joyner−Halenda (BJH). This method assumes cylindrical pores, and the results are collected in Table 3. It can be found that the CuZnAl-HT material shows a decrease, from 19 to 4 nm. In order to investigate the structural effects on carbon dioxide adsorption, it is important to study dehydration and decarbonation of the materials used. Decomposition products were monitored during the heating of the materials from room temperature to 700 °C in typical thermal gravimeter - mass spectrometer (TG-MS) experiments which were conducted with 21% O2 in argon at a flow rate of V̇ = 100 Ncm3/min. The results are presented in Figure 2. During the calcination of the precursors, hydroxycarbonates are released due to thermal decomposition processes forming corresponding metal oxides. Every step corresponds to a certain process caused by the temperature increase. Nevertheless, it is possible to check whether the decomposition of hydroxyl carbonates into oxides is completed under the applied calcination conditions or if hightemperature carbonates are still present. Decomposition of the precursor CuZnAl-HT is accompanied by a mass loss as a result of water and carbon dioxide release (Figure 2a). Most of the loosely held water (m/z = 18) in the interlayer space of the ex-hydrotalcite structure CuZnAl-HT is lost before the material is heated to 300 °C, with the majority being evolved in the peakings of 135 and 195 °C. On the other hand, the CuZnAl-ECP material (Figure 2b) loses all the water at 400 °C with a peaking at 151 °C. These materials show also initial decarbonation, which occurs in the temperature range of around 100 to 200 °C. At about the same temperature range, evolutions in m/z = 18 were also observed. It could be the result of the disappearance of OH-groups (dehydroxylation) in the hydrotalcite structure. High-temperature carbonates of CuZnAl-

It might be observed that the results obtained from the ICPAES are in good agreement with the nominal mass percentage values. The mass losses between the precursors and the reduced materials are considerable which can be explained by decomposition reactions occur during thermal treatment of these materials. The morphology of a material can influence a catalytic behavior or possible sorption effects (e.g., carbon dioxide adsorption) during a reaction process. The Brunauer− Emmett−Teller surface area (SBET), the pore size distribution, and the shape of the obtained isotherms from the physisorption process further helps to interpret possible phenomena during SRE or SE-SRE. Tables 2 and 3 collect nitrogen physisorption properties of the materials used for SRE. The BET surface areas of Table 2. Characterizations Obtained by BET

a

materials

SBETa [m2/g]

SBETb [m2/g]

SBETc [m2/g]


112 87 84

96 89 62

57 129 82

Precursor. bCalcined precursor. cAfter SRE.

Table 3. Pore Sizes Obtained by BJH

a

materials

d̅porea [nm]

d̅poreb [nm]

d̅porec [nm]


19 3 6

19 5 9

4 4 4

Precursor. bCalcined precursor. cAfter SRE.

precursors, calcined precursors, and materials obtained after SRE are collected in Table 2. The BET results show, in general, relatively high surface areas. It can be found that the surfaces of the Cu,Zn,Al based materials are relatively independent. 13135



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Figure 2. TG-MS of the precursors of CuZnAl-HT (a) and CuZnAl-ECP (b).

effects, observed in SE-SRE runs can only be explained by the fact that ZnO has the ability to adsorb CO2 on its surface.41 The XRD patterns measured for the precursor CuZnAl-ECP (Figure 3b) with the composition of (Cu,Zn,Al)x(OH)y(CO3)z showed the presence of two phases: Cu0.4Zn1.85Al0.75O5 and Zn5(OH)6(CO3)2. Only CuO, ZnO, and Al2O3 were detected after calcination, suggesting that phase segregation occurred during calcination. Not depicted, the in situ hydrogen reduced material showed a total reduction of CuO into metallic Cu in the bulk phase. After SRE, the material used goes through phase transformations: apart from the formerly detected Cu and ZnO, Zn2Al2O4 was detected which most probably corresponds to syn-Gahnite and CuAl2O4 detected as well as corresponds to a syn-Spinel, suggesting a partial oxidation of the material used during the SRE process. XRD data obtained on the CuZnAl-REF precursor (Figure 3c) can be found in an earlier work.8 However, this material shows after calcination and in situ hydrogen reduction (not shown here) similar copper reflection planes toward the reduced CuZnAl-HT. Cu, ZnO, CuO, and Al2ZnO4 (aluminum zinc oxide) are present in the bulk structure as well. From XRD analysis, it is clear that the Cu,Zn,Al materials used do not show the presence of a HTlc structure before SRE, which is a prerequisite for the sorption of carbon dioxide. These results also suggest that during the SRE reaction significant phase transformations occur, as e.g. the oxidation of Cu into CuO. Table 5 collects the crystal sizes of copper for the materials used before and after SRE. The formation of small copper particles is desired in terms of catalytic performance. It is observed that the crystal sizes of copper, obtained after reduction with hydrogen and after SRE, are relative independent of the type of materials used. However, after SRE the crystal sizes are higher, that means copper has sintered, as expected from the severe reaction conditions (T > 300 °C). Scanning electron microscopy (SEM) was done. Figure 4 depicts the SEM micrographs of selected samples. The precursor (Figure 4a) and calcined precursor (Figure 4b) of CuZnAl-HT show platelet-like morphologies, meaning that the structure was kept. In addition, a typical transmission electron microscopy (TEM) picture for CuZnAl-HT was obtained (Figure 5a). Very small Cu particles embedded in the oxidic matrix are observed. Copper particle size distribution was also calculated, and a histogram is presented (Figure 5b). The analyses show

HT (Figure 2a) and CuZnAl-ECP (Figure 2b) occur at higher temperatures: around 600 °C and around 400 °C, respectively. These are the results obtained for the materials, which form solid solutions of ZnO and Al2O3. A schematic model of the phase transformations occurring on the CuZnAl-HT precursor, with the approximated composition of Cu:Zn:Al = 5:2:3 can be found in an earlier report.8 The initial state of the HT precursor (Cu2+, Zn2+, Al3+, CO32‑, OH−, H2O) shows perfect distribution of all species. At 200 °C the loss of interlayer water and hydroxyl groups from the layers occurs, and carbonate remains in the material (Cu2+, Zn2+, Al3+, CO32‑). A starting segregation of Al3+ to fulfill the Zn:Al ratio of 1:2 for spinel formation is the next step (two Al3+ meet at a Zn2+ site, Cu2+ and Zn2+ remain static). At around 300 °C a new microstructure is obtained resulting in Al depleted (Cu rich) and dense Al-rich areas (Cu2+, Zn2+, Al23+, CO32‑); emissions of CO2 from the former areas due to the crystallization of CuO are also observed; finally, the decomposition of residual carbonate and complete segregation into crystalline CuO, ZnAl2O4, and possibly low amounts of ZnO occurs at around 600 °C. X-ray diffraction (XRD) was done to characterize the bulk structure of the materials used, as shown in Figure 3 associated also with Table 4. The phase composition may help to interpret possible sorption effects of carbon dioxide during the first minutes of SRE (SE-SRE), if e.g. the presence of a HTlc structure is detected. The precursor of CuZnAl-HT (Figure 3a) was identified as a typical hydrotalcite-like compound (HTlc). It matches with the patterns reported for Cu3Zn3Al2(OH)16CO3•4H2O.40 No other crystalline phase is observed. The d-spacings of (003) and (006) reflections correspond to the fwhm of brucite-like sheets. After calcination of the CuZnAl-HT, the following phases were identified: CuO, ZnO, and CuAlO2. The HTlc structure was almost destroyed. The calcined CuZnAl-HT was then reduced in situ with hydrogen and subjected to XRD analysis (Table 4). Typical reflection planes of Cu, assigned to a metallic faced centered cubic phase, were identified. This is an indication that most of the copper was reduced. Remember that prior to SRE runs this precursor was calcined in air at 275 °C, followed by activation with pure hydrogen flow. Accordingly, when SRE was started, most probably only not detectable traces of the HTlc structure remain in the bulk structure of the material. XRD patterns were also taken after SRE reaction with the following phases identified: Cu, CuO, Cu2Al4O7, and CuAl2O4 (corresponding to a syn-Spinel). Possible carbon dioxide sorption 13136



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reduction stages are well visible, which are connected to copper oxides reduction (CuO → Cu2O → Cu).42 3.2. Catalysts Performance. Reaction studies were carried out, and the materials used were evaluated to investigate the effects of temperature and space time. The conversions of both ethanol and water in the presence of the Cu,Zn,Al catalysts are shown in Figure 7, with inert-gas flow-rates of V̇ = 50 Ncm3/ min (a) and V̇ = 200 Ncm3/min (b). Among the three catalysts used, the CuZnAl-HT sample shows the best performance for ethanol and water conversion. In agreement with Carrero et al.,21 the catalytic performance can be explained with the metal particle sizes obtained from XRD examinations (section 2.2), and the active metal content in the catalysts (ICP-AES). The CuZnAl-HT catalyst shows the lowest copper content (Table 1), which may explain the best catalytic performance among the three catalysts tested (less probability of sintering). In addition, it must be mentioned that the CuZnAl-ECP catalyst has large amounts of embedded catalyst particles which is the reason for the lowest catalytic performance. The CuZnAl-HT catalyst is an ex-HTlc, meaning that a high crystallinity of the HTlc precursor may be obtained after calcination. As similar phenomena was also reported by Guil-López et al.28 It is also observed that relative high reaction temperatures are required for the activation of water. In addition, the conversion of water is higher with an inert-gas flow of V̇ = 200 Ncm3/min (Figure 7a) as for an inert-gas flow of V̇ = 50 Ncm3/min (Figure 7b). This is possibly related to the decrease of water concentration in the feed, thus the number of active copper sites on catalyst surface limits the adsorption of the reactant. In other words, diffusional limitations may be the reason for the lower water conversion when a lower inert-gas flow-rate regime is applied. As a result, the conversion increases with the dilution of the reactants. Cu,Zn,Al materials show satisfying activities without apparent deactivation phenomena during 4 h measurement in a temperature range from 200 to 600 °C. Certainly, the reaction time used is not sufficient to assert the stability of the catalyst. Therefore, a longer time measurement has been performed, as depicted in Figure 8. However, it must be emphasized that the temperature increase compensates in a certain level the activity loss. At this point it is also necessary to stress that compared with other active phases, copper is more likely to deactivate, especially because of thermal sintering. The predominant sintering mechanism in the bulk is known to be the vacancy diffusion.43 SRE process with copper-based catalysts should be operated at relatively low temperatures, generally lower than 300 °C.44 However, the latest report indicates that copper-based catalysts may also work at 400 °C.45 In order to investigate the stability of the copper-based materials, a lifetime test was carried out with the CuZnAl-HT catalyst at 400 °C for 12 h (Figure 8), as previous mentioned. The CuZnAl-HT catalyst shows deactivation, as expected. Both the conversions of ethanol and water decrease with time, especially the ethanol conversion. However, the activity became stable after the first 10 h, suggesting the effects of the intimate contact between all three components Cu, ZnO, and Al2O3, as well as high dispersion of the active copper phase, in agreement with reports of Guil-López et al.28 In addition, the product distribution shows good hydrogen purity (∼85 mol %) associated with ∼10 mol % of carbon dioxide. The contents of other gases like methane and carbon monoxide only accounted for a very small part, which is no more than 3 mol %.

Figure 3. XRD patterns of the materials used: CuZnAl-HT (a), CuZnAl-ECP (b), and CuZnAl-REF (c).

that the Cu particle of the CuZnAl-HT material has a mean size of 7.7 nm. Finally, a temperature programmed reduction (TPR) profile was recorded for the CuZnAl-HT. This TPR was performed with 5% of H2 in an argon inert gas atmosphere, using a total flow rate of V̇ = 100 Ncm3/min, as shown in Figure 6. It can be seen that the reduction of copper starts at about 125 °C. The reduction curve has a peak at a temperature of 275 °C, ending at around 325 °C. It is also observed that two partially resolved 13137



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Table 4. Phases Identified in the Materials with the Corresponding 2θ Positions material

treatment

phase

2θ positions and reflection planes

PDF-File ICDD

CuZnAla

precursor calcined

Cu3Zn3Al2(OH)16(CO3)•3H2O CuO ZnO CuAlO2 Cu Cu CuO CuAl2O4 (Spinel) Cu2Al4O7 Cu0.4Zn1.85Al0.75O5 Zn5(OH)6(CO3)2 CuO ZnO Al2O3 Cu ZnO ZnAl2O4 CuAl2O4 Cu3Zn3Al2(OH)16(CO3)•3H2O Cu2(OH)2(CO3)2 (syn-Malachite) NaOH•H2O CuO ZnO (Zincite) Al2O3 Cu ZnO CuO Al2ZnO4

11.8° (003), 23.7° (006), 34.7° (012), 39.4° (015), 47.1° (018), 60.2° (110) 35.7° (−111), 38.9° (111), 49.0° (−202) 31.8° (100), 34.4° (002), 36.3° (101), 56.6° (110) 31.6° (004), 37.1° (101), 39.7° (102), 54.9° (105) 43.3° (111), 50.4° (200), 74.1° (220) 43.3° (111), 50.4° (200), 74.1° (220) 35.7° (−111), 38.9° (111), 49.0° (−202) 31.3° (220), 36.9° (311), 59.4° (511), 65.3° (440) 31.6° (004), 37.1° (101), 39.7° (102), 54.9° (105) 12.4° (003), 20.0° (006), 26.7° (101), 34.1° (012), 36.6° (014) 13.1° (200), 24.2° (310), 28.2° (−311), 30.5° (311), 36.2° (221) 35.7° (−111), 38.9° (111), 49.0° (−202) 31.8° (100), 34.4° (002), 36.3° (101), 56.6° (110) 31.2°(400), 31.6°(−401), 32.8°(002), 36.7°(111), 38.9° (310) 43.3° (111), 50.4° (200), 74.1° (220) 31.8° (100), 34.4° (002), 36.3° (101), 56.6° (110) 19.0°, 31.3°, 36.9°, 59.5°, 65.4° 31.3° (220), 36.9° (311), 59.4° (511), 65.3° (440) 11.8° (003), 23.7° (006), 34.7° (012), 39.4° (015), 47.1° (018), 60.2° (110) 14.8° (020), 17.6° (120), 24.1° (220), 31.2° (20−1), 35.6° (240) 15.0° (020), 30.1° (040), 33.2° (201), 35.8° (221), 36.2° (122) 35.7° (−111), 38.9° (111), 49.0° (−202) 31.8° (100), 34.4° (002), 36.3° (101), 56.6° (110) 31.2° (400), 31.6° (−401), 32.8° (002), 36.7° (111), 38.9° (310) 43.3° (111), 50.4° (200), 74.1° (220) 31.8° (100), 34.4° (002), 36.3° (101), 56.6° (110) 35.7° (−111), 38.9° (111), 49.0° (−202) 31.3° (220), 36.8° (311), 55.6° (422), 65.2° (440)

37-0629 65-2309 89-0510 40-1037 04-0836 04-0836 89-5898 78-1605 83-1476 38-0874 99-0062 65-2309 80-0075 86-1410 99-0034 80-0075 73-1961 73-1958 37-0629 41-1390 30-1194 89-2531 99-0111 75-1862 04-0836 89-1397 89-5899 65-3104

reduced after SRE

CuZnAlb

precursor calcined

after SRE

CuZnAlc

precursor

calcined

after SRE

a

HT. bECP. cREF.

Table 5. Copper Crystal Sizes Obtained from the Cu (111) Reflex

a

material

da [nm]

db [nm]


7 5 5

17 19 32

After reduction. bAfter SRE.

Figure 5. TEM image (a) and Cu particle size distribution (b) of CuZnAl-HT.

this work indicate no significant changes for the BET surface areas (Table 2) and N2 equilibrium adsorption−desorption isotherms (Figure 1). Nakamura et al.47 showed that the Cu/ZnO interface plays a crucial role in terms of catalyst performance for the methanol synthesis and reverse WGS. Under reducing conditions, ZnOx species partly migrate onto the copper particles.14 This effect is described as strong metal support interaction.11,48−50 In general, it is widely accepted that ZnO, in the ternary catalytic system, acts both as an electronic and a structural promoter, which requires intense contact between the Cu and the ZnO interface, while Cu is definitively the active species.51,52 Finally, the addition of Al2O3 to Cu/ZnO as well as the manufacturing

Figure 4. SEM micrographs of the precursor of CuZnAl-HT (a) and calcined precursor CuZnAl-HT (b).

An earlier study reports stability measurements obtained on Cu-based materials for standard methanol synthesis conditions.46 The ternary Cu/ZnO/Al2O3 system which has a similar copper loading as the CuZnAl materials used in this work shows the highest initial catalytic activity as well as satisfying stability. It has the highest relative initial catalytic activity (a = 1), with a copper surface area of ∼32 m2/g after reduction, which decreases to a value of ∼20 m2/g after the activity test. The other used binary Cu/MgO or Cu/Al2O3 systems showed much lower initial catalytic activity. The characterizations obtained in 13138



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Figure 6. TPR on calcined materials CuZnAl-HT. Experimental conditions: 5% of H2 in argon with a total flow rate of V̇ = 100 Ncm3/ min.

Figure 8. Ethanol (filled symbol) XEtOH and water (open symbol) XH2O conversion as well as product distribution on dry basis Pi [mol %] as function of reaction time t [min] over CuZnAl-HT. Operating conditions: mcat = 3.4 g, pmass = 60%, T = 400 °C, inert-gas flow-rate of V̇ = 50 Ncm3/min.

procedures used may increase the thermal stability.15,53 Both, the binary and the ternary system show a linear correlation between specific copper surface area and activity.50 However, the copper crystal sizes obtained (Table 4) indicate clearly sintering; the CuZnAl-HT catalyst shows the lowest difference in crystallite size change, explaining once again the better catalytic performance. Finally, the results depicted in Table 3 indicate that after SRE, several mixed oxide phases appear, suggesting catalyst oxidation and/or intimate contact between the phases. The product distributions on dry basis as evolution of the reaction temperature during SRE at steady state conditions in the presence of the CuZnAl materials are depicted in Figure 9. The results show that all CuZnAl materials had similar product distributions. At low temperatures nearly pure hydrogen was obtained from the outlet stream. The increase of the reaction temperature subsequently caused a decrease in hydrogen purity. Apart from the presence of carbon dioxide in the dried gas stream, traces of methane and carbon monoxide were observed. At reaction temperatures higher than 400 °C, the amounts of byproduct increase considerably. Especially the appearance of ethylene was also notable. Ideally, SRE should only generate carbon dioxide and hydrogen. In practice many other reactions take place, leading to the formation of intermediates and byproduct, as mentioned in the Introduction.

It is well-known that two main reaction pathways can occur during SRE:5−7 the dehydration and the dehydrogenation route. The former one consists of the formation of ethylene (produced via dehydration of ethanol C2H5OH ⇌ C2H4 + H2O), which can be further decomposed into carbonaceous species (C2H4 ⇌ CH4 + C or C2H4 ⇌ 2H2 + 2Cx). Carbon deposition may also occur via the Boudouard reaction54 (2CO ⇌ CO2 + C) and methane decomposition55 (CH 4 ⇌ 2H 2 + C x ). This “encapsulating” carbon blocks the active sites of the catalyst results in deactivation. At this point it is important to stress that in this work a water/ethanol ratio of 10 was used in the feed, while in a thermodynamic study it has been found that high water/ethanol ratios can suppress the carbon deposition reaction.26 As a result, the aforementioned carbon deposition was not detected during all the experiments. In accordance with ́ et al.,22 copper phases decrease the CO formation and Vizcaino coke deposition. The products found in the liquid phase mainly consist of condensed water, acetaldehyde, acetone, and acetic acid. The appearance of these products is strongly dependent on the reaction temperature, copper loadings, and nature of the catalysts used. For instance, high amounts of acetaldehyde were detected in the liquid products at low reaction temperatures, while at higher reaction temperatures acetone starts to

Figure 7. Ethanol (filled symbol) XEtOH and water (open symbol) XH2O conversion as a function of reaction temperature T [°C] at steady state conditions, with an inert-gas flow-rate of V̇ = 50 Ncm3/min (a) and V̇ = 200 Ncm3/min (b) for the materials: (■) CuZnAl-REF with mcat = 4.4 g and pmass = 65%; (●) CuZnAl-HT with mcat = 7.0 g and pmass = 60%, and (▲) CuZnAl-ECP with mcat = 6.6 g and pmass = 65%. 13139



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Indeed, methane starts to appear as a result of the acetic acid decomposition reaction that takes place at 400 °C and higher. Product distributions as evolution of the reaction time during SRE in the presence of CuZnAl materials are shown in Figure 10. It could be found that comparing with CuZnAl-REF and CuZnAl-ECP the product distributions for the CuZnAl-HT material taken at 400 °C (Figure 10c and 10d) showed an obvious indication of the breakthrough periods. Dehydrogenation reactions, which could occur in the transient periods and would contribute to an apparently higher hydrogen production, have a low probability of occurring, since at 400 °C or higher the WGS reaction and the SRE reaction are dominant, according to our findings and literature reports.56 Carbon dioxide starts only to appear around 5 min of reaction indicating that this material has the ability to capture carbon dioxide. Carbon dioxide is preferentially retained due to a relative good interaction with ZnO,41 suggesting reaction rates which approach the adsorption rates of the materials used. However, the capacity for carbon dioxide sorption is quickly saturated, as observed in the very short transient periods. It has been found that the HTlc structure of CuZnAl could be partially recovered though a hydrothermal treatment at 180 °C after calcination in air at 400 °C, the so-called memory effect.57 Valente et al.58 also reported that the calcined material can be reversibly rehydrated to the initial layered structure after exposure to water vapor. However, heating or calcination higher than 450 °C may irreversibly destroy the layered structure of HTlc.59 The thermal decomposition causes removal of hydroxyl groups as water and the loss of interlayer carbonate anions as carbon dioxide, leading to the mixed oxides of ZnO and CuO.9,60,61 The TG-MS test of the precursor (Figure 3a) also indicates this phenomenon by a broad CO2 peak in the temperature range of 500−700 °C. As a result, the HTlc structure has not been found from XRD patterns of Cu,Zn,Al materials used after SRE experiments. However, there is a good probability that during the SRE measurement performed at 400 °C some traces of the HTlc structure remained on the CuZnAl-HT catalyst. Overall, the CuZnAl-HT catalyst used shows carbon dioxide capture and may shift the equilibrium during the first minutes of reaction toward the formation of hydrogen. This sets the duration of the feed step in a cyclic pressure swing adsorption reactor process. On the other hand, product distributions of the CuZnAl-REF catalyst as well as CuZnAl-ECP catalyst (Figure 10 a, b, e, f) did not show significant carbon dioxide capture. Additionally, it can be found that the hydrogen contents in the transient periods are lower compared with the CuZnAl-HT catalyst, and they quickly reach steady state. Indeed, these catalysts do not show HTlc structures in their precursor state. Besides, CuZnAl-ECP catalyst (Figure 10 e, f) has a similar product distribution as the CuZnAl-HT catalyst (Figure 10 c, d) at steady state. But the ethanol conversion of the CuZnAl-ECP catalyst is lower than that of the CuZnAl-HT catalyst at the same temperature (Figure 7), which means the former catalyst has a lower hydrogen yield than the CuZnAl-HT material. As a result, the CuZnAl-HT catalyst has the best performance among these materials at steady state. In order to investigate the carbon dioxide adsorption performance of the CuZnAl-HT material, three breakthrough tests with different carbon dioxide flow rates were carried out. The results of the breakthrough experiments, for different molar fractions of CO2, are shown in Figure 11.

Figure 9. Product distribution on dry basis Pi [mol %] as function of reaction temperature T [°C] at steady state conditions, with an inertgas flow-rate of V̇ = 50 Ncm3/min (open symbols) and V̇ = 200 Ncm3/ min (filled symbols) for the materials: (a) CuZnAl-REF with mcat = 4.4 g and pmass = 65%; (b) CuZnAl-HT with mcat = 7.0 g and pmass = 60%; and (c) CuZnAl-ECP with mcat = 6.6 g and pmass = 65%.

condensate. The obtained products in this work are in agreement with the report of Sau et al.24 The formation of acetaldehyde suggests that ethanol is decomposed through the dehydrogenation route (C2H5OH ⇌ CH3CHO + H2), as Segal et al.20 and Kumar et al.23 reported. Besides, acetic acid was also detected in traces at lower temperatures, suggesting the oxidation of acetaldehyde in the presence of steam (CH3CHO + H2O ⇌ CH3COOH + H2). Acetic acid can decompose into methane and carbon dioxide (CH3COOH ⇌ CH4 + CO2). The appearance of acetic acid, followed by the formation of methane and carbon monoxide suggest that the catalysts have been completely reduced (Figure 6), as stated by Busca et al.25 Das et al.44 reported the formation of acetaldehyde could be suppressed when the water/ethanol ratio in the feed increased from 3 to 6, due to the formation of acetic acid through acetaldehyde reacts with water steam. 13140



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Figure 10. Product distributions on dry basis Pi [mol %] as a function of reaction time t [min] on CuZnAl-REF with mcat = 4.4 g and pmass = 65% (ab), CuZnAl-HT with mcat = 7.0 g and pmass = 60% (c-d), and CuZnAl-ECP with mcat = 6.6 g and pmass = 65% (e-f) at T = 400 °C, and inert gas flowrates of V̇ = 50 Ncm3/min (open symbols) and V̇ = 200 Ncm3/min (filled symbols).

sorption capacity of the material. The CO2 sorption capacity of this material changed from 0.11 to 0.18 mol/kg. Table 6 presents selected results obtained from Figure 10 where the comparative performances at transient state (0−10 min) and steady state of the catalysts used are shown. It can be observed that, for the CuZnAl-HT catalyst, during the transient period either the conversions of ethanol and water, yield and selectivity of hydrogen is relative higher than in the steady state, Table 6. Comparative Performances of the Materials Used in SREc transient state

Figure 11. Breakthrough curves with different molar flow rate of CO2 for CuZnAl-HT. Operating conditions: helium flow-rate of V̇ = 50 Ncm3/min in the presence of carbon dioxide at T = 400 °C and p = 100 kPa.

materials CuZnAlHT CuZnAlECP

It could be observed that CuZnAl-HT had shown the ability of carbon dioxide sorption, as expected. It has been reported that Cu1‑xAlx-HT-CO3 with x = 0.43−0.48 has selectivity for carbon dioxide adsorption.62 The breakthrough time, for each molar fraction of carbon dioxide, is directly related with the

CuZnAlREF

steady state

XEtOH [%]

YH2/EtOH [%]

SH2/EtOH []

XEtOH [%]

YH2/EtOH [%]

SH2/EtOH []

68.2a 67.8b 42.5a 39.7b 52.3a 37.8b

16.6 16.2 6.3 8.6 8.7 8.2

24.3 23.9 14.8 21.7 16.6 21.6

55.7 52.8 41.4 37.6 51.9 39.3

12.5 12.2 5.9 8.1 8.8 9.4

22.5 23.1 14.3 21.6 16.9 23.9

V̇ inert = 50 Ncm3/min. bV̇ inert = 200 Ncm3/min. conditions: T = 400 °C, p = 100 kPa. a

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suggesting that the carbon dioxide uptake slightly enhances the reaction process.

4. CONCLUSIONS A series of Cu,Zn,Al based materials were prepared and tested for steam reforming of ethanol in a fixed-bed reactor. It was found that SRE can be performed on Cu,Zn,Al based materials. The effects of the operating conditions and materials nature on the course of ethanol steam reforming products were studied. High purity hydrogen was generated in the temperature range between 200 and 600 °C. The Cu,Zn,Al based materials show relatively good thermal stability, due to the presence of the ZnO and Al2O3 phases. Carbon dioxide uptake during the transient period of SRE was found with the Cu,Zn,Al based material derived from an HTlc precursor due to a relative good interaction of carbon dioxide formed during the initial transition period of SRE with the ZnO phase present in the CuZnAl catalyst and also probably due to the reconstruction of HTlc structure in the presence of steam after calcination for the CuZnAl-HT catalyst. Carbon dioxide uptake during the transient period of SRE is possible since an HTlc structure is kept after the synthesis of the materials used, as it was experimentally proved for the Cu,Mg,Al materials. These materials act as a relatively good carbon dioxide sorbent at 400 °C. A sorption enhanced reaction process concept may be verified for these systems. Higher amounts of hydrogen were yielded during the initial transient periods.

■

■

SE-SRE = sorption-enhanced steam reforming of ethanol SERP = sorption enhanced reaction process SEM = scanning electron microscopy SRE = steam reforming of ethanol TEM = transmission electron microscopy TG-MS = thermal gravimeter - mass spectrometer TPR = temperature programmed reduction XRD = X-ray diffraction

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AUTHOR INFORMATION

Corresponding Author

*Phone: +351-22508-1671. Fax: +351-22508-1674. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are very grateful to Prof. Dr. Robert Schlögl and Dr. Malte Behrens, Fritz-Haber-Institute of the Max-PlanckSociety, Inorganic Chemistry Department, Faradayweg 4-6, 14195 Berlin, Germany, who kindly supplied the CuZnAl precursors used in this work as well as assistance in the samples characterization. A.F.C. is gratefully to Fundaçaõ para a Ciência e Tecnologia (FCT) − Portugal, for providing financial support for this research program and postdoctoral grant (SFRH/BPD/ 62968/2009). Y.J.W. gratefully acknowledges doctoral grant from China Scholarship Council (2010674011). This work is supported by project PEst-C/EQB/LA0020/2011, financed by FEDER through COMPETE - Programa Operacional Factores de Competitividade and by FCT - Fundaçaõ para a Ciência e a Tecnologia.

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ABBREVIATIONS BET = Brunauer−Emmett−Teller BJH = Barrett−Joyner−Halenda ECP = embedded Cu-particles sample HT = hydrotalcite HTlc = hydrotalcite-like compound ICP-AES = inductively coupled plasma atomic emission spectroscopy LDH = layered double hydroxide REF = reference catalyst 13142



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