Al2O3 Catalysts - Energy & Fuels

Energy & Fuels 2000, 14, 1195-1199

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Ethanol Steam Reforming on Rh/Al2O3 Catalysts S. Cavallaro* Dipartimento di Chimica Industriale ed Ingegneria dei Materiali dell’Universita` , P.O. Box 29, 98166 Sant’Agata di Messina, Italy Received April 17, 2000

Ethanol steam reforming on Rh/Al2O3 catalysts has been the object of our research project. The mixture used for the research testing was prepared with a high water content (H2O/C2H5OH ) 8.4 mol/mol) in order to simulate the composition of the ecological fuel product from vegetable biomass fermentation. The experimental tests were carried out in a fixed bed reactor at a programmed temperature between 323 and 923 K. The maximum temperature (T ) 923 K) is the standard working temperature of a molten carbonate fuel cell able to make direct use of the hydrogen produced for ethanol steam reforming. The reaction mechanism starts with the initial dehydrogenation and/or dehydration of the ethanol, followed by rapid conversion of the products into methane, carbon monoxide and carbon dioxide. The acid support (Al2O3) assists the dehydration of the alcohol, while all the other reactions are catalyzed by the Rh, although in different measures. For this reason, with increase in Rh content, there is also a progressive increase in the C1 (CH4, CO, and CO2), while the C2 (ethylene, ethanol, and acetaldehyde) disappear gradually from the outlet gaseous stream. The acetaldehyde, that is produced by dehydrogenation, is formed during an intermediate stage, but rapidly decomposes to CH4 and CO when working at high temperatures. Traces of acetaldehyde and hydrogen in a ratio of 1:1 are also present in the reaction products when pure Al2O3 is used. However in this case the main product of the reaction is the ethylene obtained by dehydration on the acid sites of the Al2O3. Obviously the presence of the ethylene assists the formation of carbon whiskers very strongly, which after only a few hours can be easily seen on the depleted catalyst. Vice versa, in the presence of Rh at high temperatures (T ) 923 K), coking does not occur and the catalyst maintains its activity for several hours transforming all the ethanol into C1 and H2.

Introduction Even though hydrogen production using ethanol steam reforming has very little interest from an economical point of view, soon, certain industries could obtain great advantages from the refining of similar technology. In fact, research is already underway aimed at obtaining ethanol from vegetable biomass1,2 for use in internal combustion engines. However, because of the extreme dilution of the raw product (H2O/C2H5OH molar ratio = 8.4 mol/mol) and the possible coproduction of not easily separable toxic products, direct use of ethanol-water mixtures is not advisable at the present moment. As an alternative to concentration through distillation and/or the use of additives, the preliminary transformation into hydrogen3 could, however, permit the creation of a gaseous stream in situ, and, because it is easily transportable and not highly toxic, ethanol will always be advantageous from an ecological point of view. During a recent research study4 the possibility * E-mail: [email protected]. (1) Ogier, J. C.; Ballerini, D.; Leygue, J. P.; Rigal, L.; Pourquie, J. Ethanol-Production from Lignocellulosic Biomass. Oil Gas Sci. Technol. (formerly Rev. Inst. Fr. Pet.) 1999, 54, 67-94. (2) Teixeira, L. C.; Linden, J. C.; Schroeder, H. A. Alkaline and Peracetic-Acid Pretreatments of Biomass for Ethanol Production. Appl. Biochem. Biotechnol. 1999, 77-79, 19-34. (3) Haga, F.; Nakajima, T.; Miya, H.; Mishima S. Catalytic Properties of Supported Cobalt Catalysts for Steam Reforming of Ethanol. Catal. Lett. 1997, 48, 223-227.

of converting ethanol in two stages was taken into consideration, to minimize coke production and to increase the value of the ethanol by using it, at least partially, for the coproduction of acetaldehyde (a product with an increased value). After preliminary selective dehydrogenation at low temperature, this process transforms the fuel into a mixture of CO, CO2, H2O, CH4, and H2 that is able to feed an internal combustion engine, or a molten carbonate fuel cell (MCFC). However, if the final aim is a product mainly for use in MCFC, the use of a two-stage process would be expensive and the resulting system would not be sufficiently compact. So the aim of this research is to evaluate the possibility of transforming the ethanol-water mixture into hydrogen in a single stage and, selectively, at the standard working temperature of a MCFC (Tr ) 923 K). To achieve this result, a Rh-supported catalyst was studied to evaluate the complex reaction mechanism, at least at the preliminary stage. Experimental Section Catalysts with different Rh contents were prepared by impregnating RhCl2 on γ-Al2O3 (AKZO-NOBEL 001-3P) using the incipient wetness technique. The Rh content was determined by atomic absorption analysis. Steady-state reactions of ethanol steam reforming were carried out under atmospheric pressure in a conventional flow reactor. The continuous flow microreactor consists of a quartz tube (φint ) 4 mm) packed

10.1021/ef0000779 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/16/2000

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Cavallaro

with a catalytic bed ranging from 2 to 3 cm. The bed is made of ≈0.016 g of the active catalyst pellets (40-70 mesh) diluted with a large amount of inert material (10 times in volume SiC) to minimize thermal spot effects. Before each test, catalyst was pretreated at T ) 923 K for 16 h in air and then reduced in situ at T ) 575 K for 1 h under hydrogen flow FH2 ) 100 cm3 min-1. Endurance tests were carried out at constant temperature (Tr ) 923 K), while catalyst fast screening performance was evaluated using the temperature-programmed reaction mode (TPR). TPR was carried out by increasing Tr from Tr ) 323 K to Tr ) 923 K at the heating rate of 1.5 K min-1. The same reaction mixture of composition 9.6 vol % C2H5OH + 80.4 vol % H2O + 10 vol % N2 was used in all catalytic experiments. The ethanol/water reaction mixture, with an inlet steam/ carbon molar ratio S/C ) 4.2 mol/mol (H2O/C2H5OH ) 8.4 mol/ mol), was very quickly vaporized in a thermo-regulated (Tv g 473 K) vaporizer packed with SiC, where it was also simultaneously mixed with the nitrogen (FN2 ) 20 cm3 min-1). By high precision control of the reagent inlet flow with an isocratic HP 1100 pump, the gas hourly space velocity (GHSV) was carefully calculated as the “gas flow”/“overall catalyst volume” ratio. Hydrogen (before the test) and nitrogen flow rates were carefully controlled using a mass flow regulator system ASM 2500. Both reactants and products were analyzed on-line by a Hewlett-Packard 6890 Plus gaschromatograph using three columns (Molecular sieve 5A, Porapack Q, and Hysep) and thermo-conductivity detector (TCD). A complex timers and electro-valves system provided to sample (0.5 cm3) the gas stream and to switch the sampled gas in order to realize the best separation. The helium (FHe ) 70 cm3/min) was used as carrier gas, while nitrogen (10 mol % in the inlet mixture) represents the internal standard. The size of the Rh crystallites (80-190 Å) was determined using transmission electron microscopy (TEM) analysis carried out with a Philips CM-12 instrument.

Results and Discussion When a mixture of ethanol and water is used to supply a heated coil reactor, the reagents are transformed according to the reaction behavior pattern provided by the chemical nature of the catalyst. In the case where a dual function acid-dehydrogenant catalyst is used, it is reasonable to think that the main reactions will be those described in the scheme shown in Figure 1. A reaction pattern of this kind assumes that the ethanol is converted during the first stage into ethylene or into acetaldehyde by dehydration or dehydrogenation, respectively, depending upon the prevailing catalytic function:

C2H5OH ) CH3CHO + H2

(1)

C2H5OH ) C2H4 + H2O

(2)

The acetaldehyde can undergo decarbonylation to form methane and carbon monoxide:

CH3CHO ) CO + CH4

(3)

while the methane, like the other hydrocarbons present (ethylene and acetaldehyde), can undergo steam reforming, followed by the shift reaction of the CO (eq 5).

CH4 + H2O ) CO + 3 H2

(4)

C2H4 + 2 H2O ) 2 CO + 4 H2

(4′ )

Figure 1. Pathways for the steam reforming of ethanol over Rh/Al2O3 catalysts.

CH3CHO + H2O ) 2 CO + 3 H2

(4′′)

CO + H2O ) CO2 + H2

(5)

In addition, according to information already widely discussed in various articles5,6 for T > 800 K, the ethanol, acetaldehyde, and ethylene are almost completely converted into C1, so that, in the outlet mixture, apart from a very small percentage of methane and the strong excess of water already present in the original mixture, only H2, CO, and CO2 can be found. Table 1 shows the equilibrium constants calculated7 for 573 K < T < 923 K, while Table 2 shows the relative progress of a TPR scan on a catalyst Rh(5 wt %)/Al2O3. Given that it is possible to find acetaldehyde in reaction products only at 850 K < T < 923 K, it was possible to calculate the mass action ratio (MAR) relative to eqs 1 and 3 in this temperature range only. No traces of ethylene were ever formed, therefore the MAR relative to eq 2 is not determined. By maintaining the temperature constant (Tr ) 923 K) for a few minutes at the end of the experiment, even the residual traces of ethanol were fully converted into C1. For this reason, once the system is stabilized it is possible to calculate the log MAR from eqs 4 and 5 only. After comparison with the Kp values in brackets (interpolated between those in Table 1), the presence of an excess of CO and acetaldehyde (where present) seems obvious. This observation leads us to the conclusion that although the catalyst is active for all the reactions reported Figure (4) Freni, S.; Mondello, N.; Cavallaro, S.; Cacciola, G.; Parmon V. N.; Sobyanin, V. A. Hydrogen Production by Steam Reforming of Ethanol: Two Step Process. Catal. Lett., submitted. (5) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Steam Reforming of Ethanol for Hydrogen Production: Thermodynamic Analysis. Int. J. Hydrogen Energy 1996, 21, 13-18. (6) Garcia, E. Y.; Laborde, M. A. Hydrogen Production by Steam Reforming of Ethanol: Thermodynamic Analysis. Int. J. Hydrogen Energy 1991, 16, 307-312. (7) Stull, D. R.; Westrum, E. R., Jr.; Sinke G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley & Sons: New York, 1969.

Ethanol Steam Reforming on Rh/Al2O3 Catalysts

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Table 1. Decimal Logarithms of Equilibrium Constants Were Calculated According to Stull et al.7 temp (K)

log K1

log K2

log K3

log K4

log K5

298 300 400 500 600 700 800 900 1000

-6.1300 -6.0570 -3.0420 -1.1970 0.0520 0.9540 1.6350 2.1690 2.5980

-1.3700 -1.3220 0.6710 1.8850 2.7010 3.2840 3.7190 4.0550 4.3220

9.6000 9.5800 8.7680 8.2930 7.9760 7.7470 7.5710 7.4280 7.3120

-24.9020 -24.6800 -15.6150 -10.0650 -6.3020 -3.5760 -1.5110 0.1110 1.4170

4.9920 4.9470 3.1660 2.1140 1.4290 0.9520 0.6030 0.3410 0.1360

Table 2. Logarithms of Mass Action Ratios (MARs) Were Calculated from TPR Test over Rh(5 wt %)/Al2O3 Catalyst (log10(Kp) data are reported in brackets for comparison) temp (K) 665 688 734 757 780 849 886 923 923

log MAR1

-0.46 (1.83) -0.02 (2.01) 0.12 (2.19)

log MAR3

0.87 (7.52) 0.90 (7.47) 0.75 (7.42)

log MAR4

log MAR5

-1.61 (-4.18) -1.18 (-3.67) -0.87 (-2.73) -0.23 (-2.29) 0.25 (-1.93) 0.89 (-0.90) 1.89 (-0.34) 2.55 (0.20) 2.52 (0.20)

-1.78 (0.73) -1.46 (0.67) -1.00 (0.51) -0.44 (0.42) -0.23 (0.33) -0.21 (0.33)

1, it does not react as efficiently for eq 5. Moreover, although it may not be desirable in the case where it is necessary to supply an MCFC with H2, a high CO/H2, ratio could represent an advantage if it was decided to use the mixture for other purposes (i.e., methanol synthesis). The presence of acetaldehyde for conversion from ethanol less than 100% shows that dehydrogenation (eq 1) represents an intermediate stage of the entire reaction cycle, even if, because of rapid decarbonylation,8 CH3CHO is completely converted into CH4 and CO at Tr ) 923 K. Finally, the absence of ethylene, an important precursor of coke whiskers, could be explained because of two opposed mechanistic theories: (a) dehydration (eq 2) does not occur; (b) the steam reforming of the ethylene is faster than its formation (stationary state) and/or eventual coking. To clarify this point TPR tests were run on catalysts with different Rh content (Figures 2-5). Figure 2 shows the result of a TPR test on the support of γ-Al2O3 calcined at T ) 923 K. Far from being nonactive, this support shows a strong capability to dehydrate and dehydrogenate ethanol even at temperatures only a little over 600 K. Naturally, due to its acid nature, the dehydration reaction (eq 2) is prevalent compared to its dehydrogenation and it forms some hydrogen only at high temperature. On the other hand, notwithstanding the fact that the conversion of ethanol reaches 80% already at T ) 800 K, we did not find any products with a single atom of carbon (CH4, CO, or CO2) in the outlet stream, even though at the end of the test, the catalyst was covered with a considerable amount of coke. Therefore, when Rh is not present, coking is faster than both steam reforming and every other reaction shown in Figure 1. The presence of marked amounts of ethylene (as much as 80 vol %) precedes the formation of undesirable coke whiskers9 to the disadvantage of the other conversion reactions to H2, of the products obtained during the first stage. (8) Houtman, C. J.; Barteau, M. A. Divergent Pathways of Acetaldehyde and Ethanol Decarbonylation on the Rh(111) Surface. J. Catal. 1991, 130, 528-546.

Figure 2. Temperature programmed reforming (TPR) test over pure Al2O3. (dTr/dt ) 1.5 K min-1; S/C ) 4.2 mol/mol; GHSV ) 75000 h-1; unreported steam and nitrogen yields).

Figure 3. TPR test over Rh(1 wt %)/Al2O3 catalyst (see Figure 2 caption for details).

Figures 3-5, for catalysts doped with Rh 1, 2, and 5 wt %, show how the ethylene gradually disappears, by increasing the Rh content and/or the temperature. The depleted catalysts do not show noteworthy quantities of coke, and the CO and CO2 (end products of reactions (9) Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In Catalysis, Science and Technology; Anderson, J. R., Boudart M., Eds.; SpringerVerlag: Berlin, 1984; Vol. 5, Chapter1.

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Cavallaro Table 3. Endurance Test over Rh(5 wt %) Doped Catalyst. Ethanol Is Fully Converted at T ) 923 K; GHSV ) 75000 h-1 time on stream (h)

log MAR4 (K4 ) 0.20)

log MAR5 (K5 ) 0.33)

0.38 0.76 1.15 1.53 1.91 2.30 2.68 3.06 3.45 3.83 4.21 4.60 4.98

2.74 2.82 2.85 2.86 2.85 2.84 2.80 2.78 2.79 2.77 2.80 2.84 2.86

0.0165 0.0002 0.0031 0.0003 -0.0087 -0.0144 -0.0387 -0.0523 -0.0504 -0.0558 -0.0462 -0.0333 -0.0203

Figure 4. TPR test over Rh(2 wt %)/Al2O3 catalyst (see Figure 2 caption for details).

Figure 6. Product distribution as a function of GHSV, for endurance ethanol steam reforming tests over Rh(5 wt %)/ Al2O3 (Tr ) 923 K; S/C ) 4.2 mol/mol).

Figure 5. TPR test over Rh(5 wt %)/Al2O3 catalyst (see Figure 2 caption for details).

reported in Figure 1) gradually increase among the reaction products. In catalysts with a low Rh content (1-2 wt %) the acetaldehyde is present even at Tr ) 600 K, while C1 (CH4, CO, and CO2) can be found in the two catalysts with higher Rh percentages (2-5 wt %). Moreover, the catalyst with 5 wt % Rh, produces traces of acetaldehyde only at 800 K < T < 900 K and its selectivity is totally for C1. Even though in the TPR test sets shown in Figures 3-5, complete conversion of

ethanol has never been shown, after a stabilizing period of a few minutes at T ) 923 K, both the catalyst at 2 wt % and that at 5 wt % show 100% conversion rates producing exclusively C1. As shown in Table 3, at T ) 923 K and GHSV ) 75000 h-1, no noticeable decay of the catalyst that produces C1 only (without acetaldehyde or ethylene) at 100% conversion, has been recorded. Moreover, during this test as well, as has already been noted, the CO content is higher than the amount that is thermodynamically predictable and therefore eq 5 is slower than the others. To evaluate the influence of reaction kinetics better, Figure 6 shows the influence of the GHSV on the selectivity. Each test was carried out at Tr ) 923 K and the analytical result was obtained after 4 h on stream at low contact times (τ ) 1/GHSV ) (1.2-5) × 10-2 s). Only at GHSV of 75000 h-1 (τ ) 0.05 s) was it possible to convert all the ethanol to C1, obtaining ratios CO/ CH4 and CO2/CH4 that decrease slightly with the increasing GHSV (CO/CH4 ) 2.43-2.68 mol/mol and CO2/CH4 ) 4.45-5.15 mol/mol, respectively). In this test set too, an excess of CO was noted compared to the equilibrium amounts. This can be explained because of the slower time needed to reach eq 5. The absence of

Ethanol Steam Reforming on Rh/Al2O3 Catalysts

any traces of C2 even at τ ) 0.012 s, however, seems to indicate that the acetaldehyde decarbonylation (eq 3) and the ethylene steam reforming proceed with far higher speed than those of their formation (eqs 1 and 2); therefore at Tr ) 923 K only C1 and traces of non reacted ethanol can be present. No coke has been found in the discharged catalysts. Conclusions Ethanol steam reforming on Rh/Al2O3 is capable of producing H2, CO, and CO2 selectively with higher CO yields and lower CH4 yields compared to what would be expected according to thermodynamics. The strong excess of water (S/C ) 4.2 mol/mol) is able to guarantee the absence of coking even after several hours working and does not compromise the following use of hydrogen, since the water can be easily removed by simple

Energy & Fuels, Vol. 14, No. 6, 2000 1199

condensation if necessary. The acetaldehyde, formed by dehydrogenation of the ethanol, is easily decarbonylated by the Rh to form CH4 and CO, while ethylene, produced by the dehydration, is converted to C1 by steam reforming. Both of these reactions proceed at a speed higher than that required for the formation of the first intermediate (acetaldehyde or ethylene). Thanks to this situation, coke formation is prevented also on an acid support as well (Al2O3), as long as the process is carried out at high temperatures with a sufficiently large amount of Rh (g5 wt %). Finally, although these tests were carried out with extremely low contact times (τ ) 1-5 × 10-2 s) and after a time on stream of several hours (t > 4 h) the 5 wt % Rh catalyst has always led to the selective forming of C1 and no coke formation has ever been recorded. EF0000779