Ind. Eng. Chem. Res. 2010, 49, 12383–12389
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Ethanol Steam Reforming over Rh(1%)MgAl2O4/Al2O3: A Kinetic Study Cecilia Graschinsky,† Miguel Laborde,† Norma Amadeo,*,† Anthony Le Valant,‡ Nicolas Bion,‡ Florence Epron,‡ and Daniel Duprez‡ Laboratorio de Procesos Catalı´ticos, Departamento de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, UniVersidad de Buenos Aires, Ciudad UniVersitaria, Buenos Aires 1428, Argentina, and Laboratoire de Catalyse en Chimie Organique, UMR6503 CNRS-UniVersite´ de Poitiers, AV. Recteur Pineau 40, Poitiers 86022, France
A kinetic study of ethanol steam-reforming reaction on Rh catalyst supported over a spinel structure (MgAl2O4/ Al2O3) was carried out. From the analysis of products distribution, the following four reactions were proposed as the reaction scheme: C2H5OH f CO + CH4 + H2 (ED), C2H5OH + H2O f CO2 + CH4 + 2H2 (ER), water-gas shift reaction, and methane steam-reforming reaction. When the initial rate method was applied, it was demonstrated that in the rate-determining step (RDS) two active sites of the same type are involved. With use of our experimental results and data obtained from the literature, a sequence of 14 elementary steps were proposed, in agreement with the reaction scheme. Taking into account both the reaction mechanism and the occurrence of two active sites in the RDS, four different kinetic expressions can be formulated in which the RDS is (1) dissociative adsorption of ethanol, (2) dehydrogenation of ethoxide, (3) C-C bond scission, or (4) reaction between two adsorbed species. Finally, when discrimination models were applied, it was verified that the mechanism that fits experimental data is that in which the RDS is the surface reaction. Introduction Availability of energy has always been essential to humanity. In the search for new forms of energy to replace fossil fuels, hydrogen has been identified as an energy carrier.1 Fuel cells are electrochemical systems that are not limited by Carnot cycle, capable of converting chemical energy contained in H2 directly into electricity, with high efficiency. They have virtually no atmosphere emissions other than water. However, depending on the raw material employed, H2 production may lead to significant formation of carbon dioxide. The use of renewable resources, like ethanol, allows recycling of CO2 and a net decrease of emissions of this gas into the atmosphere. Besides being a renewable resource, ethanol presents other advantages as a raw material for H2 production. It is not toxic and has a high content of H2 and a high availability, and the storage and transportation are safe. Bioethanol can be obtained from a wide variety of raw materials: crops that are widespread all over Latin America as corn and sugar cane. Furthermore, the possibility to obtain the ethanol from lignocellulosic resources such as wood and wastes from agriculture and forest is very attractive. Ethanol steam reforming (ESR) is a strong endothermic reaction involving a complex system of reactions. Besides H2 and CO2, nondesired products appear during the process such as CH4 and CO; even more, at low temperatures and low contact times ethanol dehydration and dehydrogenation can lead to ethylene and acetaldehyde, respectively.2-4 Different metals have been studied as catalysts for the ESR: Co, Ni, Cu, and noble metal such as Rh, Ru, Pd, and Pt.2,3,5-10 Ni and Rh showed higher activity and selectivity to H2.11-13 The support has a fundamental role in the formation of coke on the catalyst since acidic sites promote ethylene production, a precursor of coke formation.14 Alumina possesses relatively strong Lewis acid sites and weak Bro¨nsted acid sites.15 When it is modified * To whom correspondence should be addressed. Fax: +54-11-45763211. E-mail:
[email protected]. † Universidad de Buenos Aires, Ciudad Universitaria. ‡ UMR6503 CNRS-Universite´ de Poitiers.
with Mg, alumina acquires basic properties that give the catalyst better stability.7,13 Few articles were published concerning the kinetics and mechanism of this reaction. Several authors12,16-20 working with different metals (Ni, Co, Rh) agree that the path followed by the adsorbed ethanol is its dehydrogenation to acetaldehyde. Few authors have made the mechanistic adjustment of kinetic parameters. Akande et al.21 and Akpan et al.,22 working with 15% Ni/Al2O3 and commercial Ni catalysts, respectively, found that the rate-determining step (RDS) is the ethanol adsorption. They proposed a different path to that of acetaldehyde, where the adsorbed ethanol interacts with an adjacent empty site with the scission of the C-C bond. Sahoo et al.18 and Mas et al.12 found in their adjustment that surface reactions are the RDSs. In this paper a kinetic study of the ESR reaction is presented. Rhodium supported over a spinel structure (MgAl) was used as catalyst (Rh(1%)MgAl2O4/Al2O3). The products distribution was determined with the objective to identify most probable reaction schemes. Mechanistic kinetic models were postulated and fitting software was implemented, which allowed a discrimination between them. Experimental Section Catalyst. Catalytic experiments were carried out using a supported catalyst, RhMgAl2O4/Al2O3. Spheres of γ-alumina (Axens) were employed as support (200 m2/g, 1-2 mm diameter). The support was prepared by impregnation of γ-alumina with magnesium acetate (Mg(CH3COO)2 · 4H2O) to obtain 5% in weight of magnesium in the support. The spinel was formed via a solid-solid reaction at high temperature between the magnesium oxide and γ-alumina spheres. Acetate was chosen to obtain better control over acid-base properties of the support. The catalyst with 1% in weight of Rh was prepared by wet impregnation with rhodium chloride (RhCl3) for 4 h at room temperature. This precursor was chosen because it favors good rhodium dispersion on the support surface without producing a significant increase of the solid acidity. The impregnated support was evaporated under stirring at 45 °C for 24 h and dried at 120 °C for 15 h. The catalyst was finally
10.1021/ie101284k 2010 American Chemical Society Published on Web 11/16/2010
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calcined in air flow (30 cm3/min) at 973 K for 4 h (2 K/min). The complete experimental procedure and catalyst characteristics were described in detail in ref 7. The nomenclature of the catalyst is Rh(1%)MgAl2O4/Al2O3. Catalytic Runs. The kinetic measurements were carried out in conventional flow laboratory equipment. A quartz reactor, 4 mm internal diameter, where the catalytic bed was placed, was located in an electrical oven. Reaction temperature was controlled by a thermocouple placed inside the catalytic bed. The reactants, an ethanol (Merck) and water mixture, were fed as liquid with a HPLC pump. The mixture was evaporated in an electrical oven at 623 K, before entering the reactor, and diluted with an inert gas stream. The catalyst was ground to a diameter between 44 and 88 µm with the aim of avoiding internal diffusion effects and diluted with inert material of the same diameter to avoid temperature gradients inside the catalytic bed.23 The liquid flow was varied between 3.6 and 5.5 mL min-1, the reaction temperature between 773 and 873 K and mass of catalyst was varied between 2 and 10 mg. Before catalytic evaluation, experiments were carried out to verify the contribution of homogeneous phase reaction and absence of external and internal diffusion limitations. It was found that catalyst particles diameter smaller than 88 µm and a total volumetric flow bigger than 390 mL min-1 guaranteed chemical control. Prior to reaction, the catalyst was reduced in situ under H2 flow of 50 mL min-1 at 773 K for 1 h. Temperature was programmed from ambient temperature to 773 K at 10 K min-1, and after 1 h on stream, H2 flow was changed to N2 flow of 100 mL min-1 at reaction temperature for 30 min. The analysis of the feed and the products stream were carried out online by gas chromatography in an Agilent Chromatograph, Model GC 6820, with two columns (Innowax and Carbonplot) and a flame ionization detector and a thermal conductivity detector. The condition of steady state was reached approximately after 1 h of reaction. The reproducibility of experimental results was checked and the experimental error was less than 2%. Carbon balance was close to 100% in all experiments, which corroborates the following two observations: no carbon formation was detected and good stability of the catalyst during a run time (8 h).
Figure 1. H2 yield as a function of space time. yet0 ) 0.008; yH0 2O ) 0.09; T ) 873 K.
Figure 2. Product distribution. 9, YCO; 2, YCO2; ×, YCH4, as a function of space time.
The catalytic results are discussed in terms of Ethanol conversion: Xet )
Fet0 - Fet Fet0
CO, CO2, and CH4 yields: Yi(%) )
Fi 2Fet0
× 100
H2 yield: YH2 )
FH2 Fet0
Space time: W Fet0 CO and CO2 selectivities: Si )
Fi 2(Fet0
- Fet)
Results and Discussion Distribution of Products. The reaction schemes were proposed from the analysis of products distribution (Figures 1 and 2). Reaction experiments were carried out varying the space time, keeping constant the feed composition (y0et ) 0.008; yH0 2O ) 0.09) and the reaction temperature at 873 K. Values of conversion and yields correspond to steady state. Ethylene and acetaldehyde were observed in the order of traces during reaction experiments. In Figure 1, H2 yield as a function of space time is shown. The yield increases with the space time and reaches a maximum of about 5; other product yieldssCO, CO2, and CH4sare shown in Figure 2. It can be seen that CH4 and CO yields present a maximum, which demonstrate their intermediate character due to the presence of water-gas shift (WGS) and CH4 steamreforming (SRM) reactions: CO + H2O T CO2 + H2
(WGS)
CH4 + H2O T CO + 3H2
(SRM)
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Figure 4. Ethanol conversion as a function of initial ethanol molar fraction. T ) 873 K; W/FV0 ) 0.015 mgcat · min/mL. Figure 3. Initial rates of reaction as a function of initial ethanol molar fraction. yH0 2O ) 0.09; T ) 873 K; W ) 2-10 mg.
In a previous paper7 it was proposed that there is a direct route for CO and CH4 formation. This independent route could involve the ethanol cracking represented by ED: C2H5OH f CO + CH4 + H2
(ED)
From the thermodynamic analysis4 and previous works2,12 ethanol steam reforming reaction (ESR) can be also proposed. C2H5OH + H2O f CO2 + CH4 + 2H2
(ER)
In conclusion, two reaction schemes could agree with the product distribution obtained: Scheme A: ED, WGS, and SRM; Scheme B: ED, ER, WGS, and SRM. The regression analysis will be used to decide which model fits the experimental data better. Initial Rates Method. Catalytic tests were carried out with different ethanol molar fractions, varying the space time for each feed composition. From curves of conversion of ethanol as a function of space time for different ethanol molar fractions, initial rates were calculated for each initial composition, estimating it from the design equation of a plug flow reactor when the space time tends to zero.24 dXet d(W/F0et)
|
) r0
0 )f0 (W/Fet
In order to solve this equation the experimental data were fitted to a second order polynomial. Xet ) a
( ) ( ) W F0et
2
+b
W +c F0et
In Figure 3 the initial rate as a function of the ethanol molar fraction fed to the reactor is presented. The maximum in the initial rate curve versus ethanol molar fraction indicates that two active sites of the same type must be involved in the rate-determining step (RDS). It must be remarked that each point in Figure 3 represents, on average, six experimental measurements obtained at a determined feed composition and variation of the space time. Furthermore, the maximum obtained in the conversion as a function of ethanol feed molar fraction (Figure 4) confirms this conclusion.12 Reaction Mechanism. Although the information published about the mechanism of the ethanol steam-reforming reaction is scarce, most authors agree that the first elementary step is the molecular adsorption of ethanol on the active site.20-22,25-27 Particularly, Akpan et al.22 found that the adsorption of
ethanol over Ni site is the RDS, whereas Akande et al.21 assumed the dissociative adsorption of ethanol on active sites as the RDS. Few others proposed that the ethanol simply decomposes on the active site without molecular adsorption.16,18,19,28 The subsequent reactions have been discussed in previous papers.6,29-31 Over metals such as Ni, Pt, Pd, and Rh ethanol is dissociated to ethoxide and adsorbed hydrogen. Ethoxide species sequentially are dehydrogenated to acetaldehyde and upon further heating the acetaldehyde will go through a C-C bond scission, producing adsorbed CO, H2, and CHx species. Some authors17,19 found that ethanol over Rh in the absence of water as reactant follows a different mechanism. Ethoxide gives an oxametallacycle intermediate ((a)CH2CH2O(a)) instead of acetaldehyde. This later decomposes to CO. However, this does not happen in our system because of the excess of water in the feed. Basic catalysts promote the dehydrogenation to acetaldehyde and acid catalysts favor the dehydration to ethylene.32 Fatsikostas and Verykios6 demonstrated that γ-Al2O3 was very selective for ethylene. With modification of the alumina with Mg, it acquires basic features that allow greater mobility of OH groups, favoring ethanol steam-reforming reaction.33 Compared with catalysts supported on alumina, those catalysts supported on spinel exhibit a slightly higher basicity, whereas the surface acidity is strongly reduced. In summary, the acidic and basic properties of supports are essential parameters that directly affect the primary selectivity to acetaldehyde or ethylene. Ethylene would be produced only on the support, with an essential role of the acidic sites in olefin formation. The dehydrogenation, however, mainly occurs over metallic sites.7 Moreover, the ethylene produced over the catalyst is rapidly decomposed to hydrocarbon fragments (coke), whereas the acetaldehyde is further transformed into reaction products.18 Water activation can follow different routes. Many authors propose the dissociative adsorption to OH and H.18,20,26 Other authors have proposed that water in the gas phase reacts with adsorbed species following an Eley-Rideal mechanism.20,22 Nevertheless, this kind of mechanism has a very poor probability of occurrence.34 The spinel support promotes the water activation and possesses very mobile OH groups favoring the reaction with the CHxOy species adsorbed on the metal particles.7 For all these reasons the following reaction mechanism is proposed: (1) C2H5OH + (a) f C2H5OH(a)
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(2) C2H5OH(a) + (a) f CH3CH2O(a) + H(a)
(III) RDS: C-C Bond scission: (4) CH3CHO(a) + (a) f CH3(a) + CHO(a)
(3) CH3CH2O(a) + (a) f CH3CHO(a) + H(a)
r)
(4) CH3CHO(a) + (a) f CH3(a) + CHO(a)
kK1K2yet 1 2 √K12yH den 2
(5) CHO(a) + (a) f CO(a) + H(a) (6) CH3(a) + H(a) f CH4(a) + (a)
den ) 1 + Ayet + ByCOyH1/22 + C
(7) CHO(a) + OH(a) f CO2(a) + H2 + (a) F
(8) H2O + 2(a) f OH(a) + H(a)
yH2O yH1/22
yCH4 yH1/22
+ DyCH4yCO +
+ GyCH4 + HyCO + IyCO2 + JyH1/22 + L
yet yH1/22
(9) CH4(a) f CH4 + (a) (IV) RDS: Surface reaction, Scheme A: (5) CHO(a) + (a) f CO(a) + H(a)
(10) CO(a) f CO + (a) (11) CO2(a) f CO2 + (a)
(13) CH3(a) + OH(a) f CO(a) + 2H2 + (a)
(12) 2H(a) f H2 + 2(a)
(14) CO(a) + OH(a) f CO2(a) + H(a)
(13) CH3(a) + OH(a) f CO(a) + 2H2 + (a) (14) CO(a) + OH(a) f CO2(a) + H(a)
rED ) k5
Assuming different steps as RDS, kinetic expressions were obtained based on Langmuir-Hinshelwood-Hougen-Watson theory (LHHW). Only those steps that involve two active sites were considered as we concluded using the concept of initial rate (Figures 3 and 4). Possible RDS and its corresponding rate expressions are detailed below: (I) RDS: Dissociative adsorption of ethanol: (2) C2H5OH(a) + (a) f CH3CH2O(a) + H(a)
rER ) k7
(
rSRM ) k13
(
K9K1/2 12
K1/2 12
yH1/22
EyCH4yCOyH1/22 + F
yH2O yH1/22
yCH4 yH1/22
1
yCH4yH1/22 den2
)
1/2 - k-13K11K1/2 12 yCO2yH2
)
+ DyCH4yCO +
yCH4 yH1/22
+F
yH2O yH1/22
+ GyCH4 +
HyCO + IyCO2 + JyH1/22 + L
+ GyCH4 + HyCO + IyCO2 + JyH1/22
(II) RDS: Dehydrogenation of ethoxide: (3) CH3CH2O(a) + (a) f CH3CHO(a) + H(a)
1 den2
K8K9 yCH4yH2O 1 - k-14K10yCOyH2 2 K6K12 yH2 den2
den ) 1 + Ayet + ByCOyH1/22 + C den ) 1 + Ayet + ByCOyH1/22 + C
yet
K1K2K3K4K6K8 yetyH2O 1 K9K12 yCH4yH2 den2
K8K10 yCOyH2O
rWGS ) k14 1 r ) kK1yet den2
K1K2K3K4K6
yet yH1/22
(V) RDS: Surface reaction, Scheme B: (5) CHO(a) + (a) f CO(a) + H(a) (7) CHO(a) + OH(a) f CO2(a) + H2 + (a) (13) CH3(a) + OH(a) f CO(a) + 2H2 + (a)
kK1K2yet 1 r) 2 √K12yH den
(14) CO(a) + OH(a) f CO2(a) + H(a)
2
den ) 1 + Ayet + F
yH2O yH1/22
ByCOyH1/22
+C
yCH4 yH1/22
rED ) k5
+ DyCH4yCO +
+ GyCH4 + HyCO + IyCO2 + JyH1/22 + L
yet yH1/22
rER ) k7
K1K2K3K4K6 K9K1/2 12
yet
1
yCH4yH1/22 den2
K1K2K3K4K6K8 yetyH2O 1 K9K12 yCH4yH2 den2
+M
yet yH2
Ind. Eng. Chem. Res., Vol. 49, No. 24, 2010 Table 1. Terms of Denominators of Rate Expressions and the Adsorbed Species Corresponding to Each One terms of denominator
empty sites CH3CH2OH
B)
K10K1/2 12 K5
CHO
C)
K10K1/2 12 K5
CH3
D)
K9K10 K4K5K6
CH3CHO
E)
K9K10K1/2 12 K4K5K6
CH3CH2O
F)
K8
CH4
H ) K10
CO
I ) K11
CO2
J ) K1/2 12
H
K1K2
rWGS
K8K10 yCOyH2O K1/2 12
7.6 × 10-2 6.5 × 10-2 6.4 × 10-2 1.8 × 10-2 1.7 × 10-2
rER ) kER
(
rSRM ) kSRM
(
yH1/22
-
)
)
yCH4 yH1/22
+F
yH2O yH1/22
yH1/22
)
- k-SRMyCO2yH1/22
yCH4yH2O yH2
- k-WGSyCOyH2 2
yet yH1/22
+M
yH2O yH1/22
( (
+ GyCH4 +
HyCO + IyCO2 + JyH1/22 + L
yetyH2O 1 yCH4yH2 den2
ki ) k0,i exp -
1 den2
K8K9 yCH4yH2O 1 ) k14 - k-14K10yCOyH2 2 K6K12 yH2 den2
den ) 1 + Ayet + ByCOyH1/22 + C
1
yCH4yH1/22 den2
den ) 1 + Ayet + F
1/2 k-13K11K1/2 12 yCO2yH2
(
yet
yCOyH2O
rWGS ) kWGS
CHO
K9K1/2 12
(
I II III IV V
rED ) kED
CH3CHO
K1K2K3K4K6
rSRM ) k13
sum of squared residues
CH3CH2O
K1/2 12
K1K2K3 M) K12 N)
mechanism
reactions, fit better the experimental data. To discriminate between them, CO yield along space time for both models was calculated assuming the plug flow model in the laboratory reactor. In Figure 6 these values and the experimental results are shown. It can be seen that values predicted by model V adjust the experimental values much better than the values predicted by model IV. Finally, the kinetic expressions corresponding to model V are the following:
OH
K1/2 12
G ) K9
L)
Table 2. Sum of Squared Residues for Each Mechanism
adsorbed species
1 A ) K1
12387
yet yH2
Each term of the denominators corresponds to a fraction of sites occupied by an adsorbed species. In Table 1, each term and the corresponding adsorbed species are detailed. It must be pointed out that, in mechanisms I, II, and III, SRM (Step 13) and WGS (Step 14) were considered at equilibrium. To discriminate between the proposed mechanisms, the software Athena Visual Workbench was used. The number of data points used for the regression analysis was 100 and the corresponding variable levels were inlet composition, temperature, and space time. Figure 5 shows H2 molar fraction values predicted by the model as a function of values observed experimentally for each mechanism. On Table 2 the sum of squared residues for each proposed kinetic model are informed. It can be seen that the results shown in Figure 5 enable us to discard mechanisms I, II, and III. Mechanisms IV and V, in which RDS are surface
)
1 den2 1 den2
+ IyCO2
Eai 1 1 j R T T
))
It must be pointed out that, for each level of temperature analyzed, the parameters that fitted significant values in the inhibition term of the rate expressions were those related with ethanol, OH and CO2 adsorbed. Table 3 shows activation energies calculated for model V and values reported by other authors. Morgensen et al.10 and Mas et al.12 studied ESR employing catalysts based on Ni and adjusted a kinetic expression type potential low. For the reactions involving ethanol as reactant (reactions ED and ER), they found similar values in order of magnitude, but lightly lower than those obtained in this work. Sahoo et al.18 used Co as catalyst and proposed a reaction scheme formed by ethanol decomposition, ethanol reforming and WGS reaction. In the three reactions the activation energy reported by this author are lower than the corresponding values presented in this work. Finally Hou et al.35 and Xu and Froment36 carried out a kinetic study of SRM and WGS reactions. They informed values of activation energy lightly higher compared with those reported in this work, attributed to these authors employing Ni-based catalysts. Conclusions From the distribution of products observed during the ethanol steam reforming over RhMgAl2O4/Al2O3, and the regression
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Figure 5. Predicted H2 molar fraction for each model as a function of observed H2 molar fraction.
C2H5OH f CO + CH4 + H2 C2H5OH + H2O f CO2 + CH4 + 2H2 CO + H2O T CO2 + H2 CH4 + H2O T CO + 3H2
Figure 6. yCO as a function of space time. yet0 ) 0.008; T ) 873 K; mcat ) 2-10 mg. [, Experimental; s, mechanism IV; ---, mechanism V.
analysis performed, the scheme which represents the reaction system is
The analysis of the reaction initial rate behavior suggested that two active sites of the same type participate in the RDS. In the late stage of the kinetic study the sequence of elementary steps that represents the reaction system was determined. Considering the results obtained with the initial rate method, four possible reaction mechanisms were proposed. The fitting of experimental data allowed verification of the validity of the mechanism in which RDS are the following surface reactions:
Table 3. Activation Energy (J/mol) model V EaED EaER EaSRM EaWGS
8.59 × 104 4.18 × 105 1.51 × 105 1.07 × 105
Mas et al.12 1.44 × 105
Morgenstern et al.35
Sahoo et al.18
1.49 × 105
7.13 × 104 8.27 × 104 4.36 × 104
Hou et al.36
Xu and Froment37
1.54 × 104 2.09 × 105
1.08 × 105 2.40 × 105
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(5) CHO(a) + (a) f CO(a) + H(a) (7) CHO(a) + OH(a) f CO2(a) + H2 + (a) (13) CH3(a) + OH(a) f CO(a) + 2H2 + (a) (14) CO(a) + OH(a) f CO2(a) + H(a) The activation energy values obtained for each reaction have the same order of magnitude as the values reported by other authors using other catalysts. Nomenclature Fi F0i FV0 W yi y0i r0 rj Kj j T
molar flow of ith compound molar flow of ith compound fed to the reactor total volume flow weight of the catalyst molar fraction of ith compound molar fraction of ith compound fed to the reactor reaction initial rate reaction rate of the jth elementary step equilibrium constant of the jth elementary step average temperature
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ReceiVed for reView June 14, 2010 ReVised manuscript receiVed October 3, 2010 Accepted October 12, 2010 IE101284K
