Hydrogenation of CO and CO2 with K and Mn promoted iron catalysts

Hydrogenation of CO and CO2 with K and Mn promoted iron catalysts

M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholz and M.S. Scurrell (Editors) Natural Gas Conversion IV 231 Studies in Surface Science and C...

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M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholz and M.S. Scurrell (Editors) Natural Gas Conversion IV

231

Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.

Hydrogenation o f C O and CO2 with K and M n promoted iron catalysts M.L. Cubeiro, G. Valderrama, M.R. Goldwasser, F. Gon~ez-Jim6nez, M.C. Da Silva and M.J. P6rez-Zurita. Centro de Cat~ilisis Petr61eo y Petroquimica, Escuela de Quimica, Facultad de Ciencias, Universidad Central de Venezuela, A.P. 47102, Caracas, Venezuela.

Abstract

Two iron catalysts promoted with potassium and manganese: FeKMn/AI203 and KMn/Laterite were tested in the H2+CO and H2+CO2 reactions at 563 K, 1.3 MPa and various space velocities. Bulk iron phases were analysed by M6ssbauer spectroscopy before and after reaction. After a reduction and carburation pre-treatment a higher proportion of carbided iron was attained for the laterite based solid compared to the FeKMn/AI203, which showed a mixture of magnetite and iron carbide. The higher activity in the H2+CO reaction of the KMn/Laterite is attributed to its higher carbide proportion, as well as to its susceptibility to be changed in situ by syngas. However, in the H2+CO2 reaction similar activity was attained with both solids, which is related to the fact that magnetite can catalyse the primary reverse water gas shift reaction.

1. INTRODUCTION Iron based solids, because of its performance, have been traditionally used as catalysts for the Fischer-Tropsch Synthesis (FTS); potassium [1-3] and manganese [4-6] being promoters known to enhance alkenes selectivity and to depress methane formation. A characteristic of iron catalysts is that their behaviour strongly depends not only upon composition and preparation conditions, but also on pre-treatment and reaction conditions, which determine the iron phases composition of the working catalyst [7-8]. Besides hydrocarbons, iron catalysts produce CO2 mainly by the water gas shift reaction (WGS) which also produces hydrogen, an advantage in working with low H2/CO ratios. However, CO2 is an important pollutant responsible for the green-house effect and for this reason attention is being paid to minimise its formation, as well as on possibilities for its recycling, such as CO2 hydrogenation. There is agreement in the literature [9-10] that CO2 hydrogenation to hydrocarbons proceeds via CO formation, so in principle a good catalyst should have a double function, to catalyse both the reverse WGS reaction and the FTS, iron catalysts being potential candidates. In the present work the catalytic behaviour in CO and CO2 hydrogenation of two iron based catalysts was studied. An attempt to correlate the bulk iron phases with its behaviour was also made.

232 2. EXPERIMENTAL

The two catalysts KMn/Lat. and FeKMn/Ai203, in particle size ranging from 0.18 to 0.25 mm were prepared as follows: the previously washed and sieved laterite was successively impregnated by incipient wetness with Mn(NO3)2.4H20 and K2CO3, dried at 413 K and calcined in air flow at 723 K for 3 h. The supported catalyst was prepared by co-impregnating Kentjen CK-300 ~t-A1203 with Fe(NO3)a.gH20 and the promoter salts followed by the same procedure used for the laterite solid. The iron and promoters content, determined by ICP argon plasma and flame emission, was Fe:K:Mn 23:5:4 for the laterite based solid and 16:6:4 wt.% for the alumina supported one. The specific surface areas of the two solids (Micromeritics Flowsorb II 2300, one point method) were 115 m2/g and 128 m2/g for the laterite based and the alumina supported solids respectively. Before reaction the catalysts were reduced in flowing H2 at 723 K for 16 h and carburated afterwards with either CO or syngas (H2/CO=2, 5% N2) for 5 h at 423 K followed by 16 h at 573 K (pre-treatment S). For the alumina supported solid, a milder pre-treatment with H2 at 623 K for 3 h was also carried out (pre-treatment M). The catalytic tests were performed using 2-7 g of the pre-treated solids in a fixed bed reactor at 563 K, 1.3 MPa and various space velocities to attain different conversions. In each test the initial and final space velocities were kept the same in order to check the stability of the solids during times on stream up to 12 days. Reaction gases with H2/CO=I.1 and H2/CO2=2.2, both with 5% N2 (internal standard), were used. Feed and effluent gases were analysed by gas chromatography with thermal conductivity and flame ionisation detectors, methane was used to correlate the chromatograms. The bulk iron phases of the solids, before and after reaction, were analysed by 57Fe M6ssbauer spectroscopy, using a source of 57Co in a Pd matrix. Spectra were recorded at room temperature and fitted by means of a least-squares regression program.

3. RESULTS AND DISCUSSION 3.1. Mfssbauer spectroscopy Although the iron and promoter contents as well as the specific surface area of both solids were similar, they are different in nature: while the laterite is a mixture of oxihydroxides and oxides of iron, alumim'um, silicon and titanium [11] and the catalyst is prepared by impregnating the promoters on it, the FeKMn/Ai203 catalyst is prepared by co-impregnating metal and promoters on alumina. M6ssbauer spectra indicated that the use of either CO or syngas as carburating agent lead to the same bulk iron phases composition on each catalyst. A higher degree of transformation is attained by the reduction and carburation pre-treatment on the promoted laterite (almost 70% of Hiigg carbide, table 1) compared to the FeKMn/AI203 catalyst. With the last one a significant proportion of magnetic oxides (41%) appears, which indicates a less extended iron reduction on this solid. This component of the spectrum shows broadened lines and lower hyperfme fields than bulk magnetite suggesting either the presence of a highly dispersed Fe304 phase or aluminium [ 12] and manganese ions in the spinel lattice. The iron phase composition of the promoted laterite catalyst was similar before and atter 12 days on stream for CO hydrogenation with a slight increase of the Hiigg carbide proportion (table 1). During CO2 hydrogenation the magnetite phase tends to increase and the Hiigg carbide to reoxidize, as the results for the alumina supported catalyst showed. According to the

233 stoichiometry for hydrocarbon formation, the amount of water produced in CO2 hydrogenation is twofold compared to that for CO hydrogenation. As was intended, pre-treatment M over the FeKMn/A1203 solid produced magnetite as major iron phase. Table 1 M6ssbauer determined bulk iron phases of the catalysts after pre-treatmem and reaction a-Fe ~-FesC2 Fe304 ot-Fe203 Fe 3+ Fe 2+ KMn/Lat. after pre-treatment S 67 21 12 after H2+CO reaction a 74 14 12 FeKMn/AI203 29 41 15 15 after pre-treatment S 20 56 24 after H2+CO2 reaction a 2 60 6 20 12 after pre-treatment M 3 3 70 11 13 after H2+CO2 reaction b

a12 days on stream, b2 days on stream 3.2.

Activity

3.2.1 CO hydrogenation

Between both solids the KMn/Lat. catalyst showed higher activity during the H2+CO reaction (table 2). A tendency to activate on the first two days on stream (fig. 1) was also observed, which suggests a modification of the solid under syngas. A change in metal and promoters surface composition could have occurred, as has been reported for similar systems [13-14]. A continuation of the reduction-carburation of iron is also possible, together with particle fragmentation by effect of carburation [15-16]. Activation under syngas exposition has also been detected for the single potassium and manganese promoted laterite [ 17]. For the manganese promoted laterite a more gradual activity increase occurred, together with an increase in CO2 and a decrease in methane selectivities during 7 days on stream. Activation on stream of the double promoted laterite was then followed by deactivation, carbon and high molecular weight hydrocarbons deposition as well as sinterization of the solid being possible causes. 3.2.2 CO2 hydrogenation

For the H2+CO2reaction no great differences in activity were observed with both catalysts, being slightly higher for the alumina supported one (table 2). For this reaction, the activity of both solids remains constant after 12 days on stream, as is shown in fig. 2 for the alumina supported catalyst, contrary to what was observed for the CO hydrogenation, where a faster deactivation occurred. Since activity of the solids in the H2+CO2 reaction was constant with time on stream, the results obtained allow us to get information about the dependency of conversion with space velocity. It was observed that a great decrease on space velocity produces a low increase in conversion. Mass and heat transfer effects can not be discarded, however these results can also be related to an inhibiting effect from the water evolved.

234 The FeKMn/A1203 catalyst submitted to pre-treatment M, with a higher proportion of magnetite and absence of iron carbides (table 1) gave rise in the H2+CO2 reaction to 7% CO2 conversion, compared to 21% attained at the same reaction conditions with a mixture of Hiigg carbide and magnetite on the solid. These results indicate that to achieve higher CO2 hydrogenation activity the presence of iron carbides is necessary. Table 2 Catal~ic results for CO and CO2 h~drogenation reactions a H2+CO H2+CO2 KMn/Lat. FeKMn/AI203 KMn/Lat. FeKMn/AI203 SV (l/gc~t/h) 1.2 0.7 0.6 0.6 COx conv. (%) 69 15 15 21 Selectivity (%)b COy 48 50 54 20 Selectivity COy free (%)c CH4 6.9 7.2 34.1 20.7 C2-C4 29.8 40.2 47.8 43.1 C5+ 63.3 52.6 18.1 36.2 C2-C4 alkenes 25.7 36.6 29.8 32.0 apre-treatment S, 3 days on stream, bc converted to a given product/C total converted c %Selectivity to a fraction/(100-%Selectivity to COy) x=l ,y=2 for the H2+CO reaction, x-2, y= 1 for the H2+CO2reaction

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time on stream (h) Figure 2. CO2 conversion as function of time on stream for the FeKMn/AI203 catalyst. Space velocities E! 0.6, • 0.15, + 1.2 l/(gcat.h)

The differem catalytic behaviour in the carbon oxide hydrogenation reactions of both solids could be explained by two effects: differences in the iron phases composition and in situ modification of the catalyst by syngas. Concerning the first effect, iron carbides are active for both CO and CO2 hydrogenation to hydroearbom. However, the roll of magnetite appears different for both reactions. Magnetite catalyses the WGS reaction, which is a secondary one in

235 the H2+CO reaction, while in the hydrogenation of CO2 it catalyses the primary reverse of the WGS reaction. Starting from H2+CO2, magnetite can form CO, which then reacts over carbided iron to produce hydrocarbons. That could explain why the alumina supported catalyst, with a higher magnetite and lower iron carbide proportion, showed lower activity in CO hydrogenation than the laterite based solid but similar activity in CO2 hydrogenation. On the other hand, the H2+CO mixture performs changes on the laterite based catalyst, which increases its activity, not observed for the H2+CO2 reaction environment. 3.3.

Selectivity

For the H2+CO reaction both catalysts showed high selectivity to CO2 (table 2). Potassium promoted iron catalysts produce high selectivity to the WGS reaction [2-3], which is also catalysed by magnetite. The alumina supported catalyst, with a high proportion of magnetite phase showed 50% CO2 selectivity even at low conversion. Fig. 3 demonstrates the character of CO as primary product from which hydrocarbons are formed in the H2+CO2 reaction. A strong dependence of CO and hydrocarbon selectivities with conversion was observed for CO2 conversions lower than 20%. As conversion increases it is possible to obtain an almost complete transformation to hydrocarbons, which is an advantage compared to CO hydrogenation. 100

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COx conversion (%) Figure 4. Olefins percentage in the C 2 - C 4 fraction vs. COx conversion. E2 KMn/Lat., A FeKMn/AI203. Open symbols: H2+CO, closed symbols: H2+CO2.

A significant amount of C2+ hydrocarbons was attained in the H2+CO2reaction, mainly with the FeKMn/AI203 catalyst (table 2). The formation of lighter hydrocarbons and more methane in the H2+CO2 reaction compared to the CO hydrogenation earl be related to the fact that for both reactions hydrocarbons are formed through CO. Under the reaction conditions used, the selectivity to alkenes was lower for the H2+CO2 reaction (fig. 4, C2-C4 fraction). Alkenes appear as primary products which can suffer secondary hydrogenation. As was observed for CO hydrogenation, changes on the laterite based solid occurred during the H2+CO2 reaction which modified its catalytic behaviour. In this case, an increase in alkenes selectivity with time on stream was observed, the alkenes percentage in the C2-C4 fraction was 63% at 3 days and 70% at 12 days on stream at isoconversion (15%).

236 4. CONCLUSIONS Differences in the nature of the potassium and manganese promoted iron catalysts produce different degrees of iron transformation by pre-treatment and also different activity behaviour in both CO and CO2 hydrogenation. Iron, as magnetite, showed lower activity for the H2+CO2 reaction than a mixture of magnetite and carbide, with differences observed for magnetite concerning its roll in the hydrogenation of CO and CO2 reactions. CO is a primary product in the CO2 hydrogenation, from which hydrocarbons are formed. The CO selectivity in the H2+CO2 reaction strongly depends on conversion and it seems feasible to obtain an almost complete transformation to hydrocarbons at the higher conversions. Due to the fact that hydrocarbons formation goes through CO, lighter hydrocarbons were obtained with H2+CO2 as reactants.

Acknowledgements The authors want to thank to the Scientific and Humanistic Council of the Central University of Venezuela C.D.C.H.-U.C.V. for the financial support.

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