Al matrix as catalysts for Fischer-Tropsch synthesis

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.

337

Hydrogenated Zr-Fe alloys encapsulated in Al203/Al matrix as catalysts for Fischer-Tropsch synthesis S.F. Tikhov* a, V.I. Kurkinb, V.A. Sadykov a, E.V. Slivinsky b, Yu.N. Dyatlova ", A.E. Kuz'minb, E.I. Bogolepova b, S.V. Tsybulya ", A.V. Kalinkin ", V.B. Fenelonov ", V.P. Mordovin aBoreskov Institute of Catalysis SB RAS, 5 Lavrentieva St., 630090, Novosibirsk bTopchiev Institute of Petrochemical Synthesis RAS, 19 Leninskii Av., 117912, Moscow, RUSSIA

ABSTRACT The genesis of the ZrFe intermetallides with different atomic ratios during preparation of ZryFe~Hy/AlzO3/A1 catalysts and their performance in FischerTropsch synthesis has been studied. The effect of structural, surface and textural properties on their activity had have been discussed. 1. INTRODUCTION The polymetallic catalysts derived from intermetallic hydrides based upon transition metals of YIII group and the elements of the 4 th period of the Periodic Table are known to be active for the hydrogenation reactions, hydrogenated ZrFe bulk catalysts demonstrating the highest output of C5+ hydrocarbons [1,2]. However, the use of such catalysts as pure active component (AC) in the Fischer-Tropsch synthesis (FTS) is hampered due to fragility of hydride particles and a low stability towards coking [3]. A possible way of improvement of the properties of hydrogenated alloys is their encapsulation into the AlzO3/A1 matrix via mild oxidative procedure [4]. In the work presented, the model catalysts based upon hydrogenated, pure and encapsulated Zr-Fe alloys with a varied Zr/Fe atomic ratio have been studied. Structural, textural, surface and catalytic properties of the catalysts have been elucidated in details.

2. EXPERIMENTAL The powdered catalysts with different Zr/Fe atomic ratio ~2" 1, 1"1 and 1:2 were prepared by alloying Zr and Fe at 2200-2400~ under Ar followed by

338 hydrogenation of those alloys at a room temperature and hydrogen pressure of 5 MPa. Approximate stoichiometry of hydrides measured gravimetrically corresponds to H1.sZrz6Fe, H0.sZrFe, HoZrFe2. Hydrogenated AC (fraction 0.50.25 mm) were mixed with the aluminum powder and subjected to hydrothermal treatment at 100~ in a special die followed by drying and calcination at 540~ This procedure results in self-pressing of blends into compact mechanically strong metallo-ceramic monoliths [4,5]. Two types of aluminum powder (PA-4 and PA-HP) were used, the latter possessing a higher reactivity thus permitting to encapsulate more AC. The catalyst performance in FTS was evaluated for hydrogenated AC and composite catalysts (fraction 2-3 mm) in a flow type reactor at 300-310~ 3 MPa, H2:CO ~2"1, GHSV ~6500-7400 h -~. The phase composition was analyzed with URD-63 diffractometer using CuK~ radiation. The total pore volume was estimated from the values of true and apparent densities of granulated cermets [4]. The true densities of samples were estimated by using a helium Autopycnometer 1320 Micromeritics instrument. The details of microtexture were characterized by using the nitrogen adsorption isotherms measured at 77 K on an ASAP-2400 Micromeritics instrument. The crushing strength was measured using a PK-2-1 machine. The XPS spectra were recorded with an ESCA-3 spectrometer using A1K~ radiation. 3. RESULTS

As is seen in the Table 1, the following sequence of activity for pure hydrogenated alloys has been found: Zrz.6Fe < ZrFe2 < ZrFe. As for the composite catalysts ZryFe~Hx/AlzO3/A1, a large enhancement of CHx productivity has been observed with a different sequence: ZrFe2 < Zrz.6Fe < ZrFe, 1" 1 sample being again the most effective. The same order of the C5+ yield for pure and encapsulated alloys has been found. The increase of the AC content Table 1 Catalytic properties of hydrogenated Zr-Fe intermetallides in Fischer-Tropsch synthesis. Active component Conversionof Hydrocarbonproductivity, g/kg.h SelecCO, % tivity Type Content, Total To C 0 2 Total C5+ to CH4, wt.% Per kgcat Per kgAc Per kgcat Per kgAc % 78 78 27 27 27,7 Zr2,6FeHx (2:1) 100 20.9 2.3 189 473 39 98 37.3 + PA-HP 40.0 11.0 0.8 267 1041 45 174 18.4 + PA-4 25.6 16.6 2.5 ZrFeHx (1:1) 100 59.1 15.0 231 231 57 57 35.2 +pA-Hp 40.0 24.4 2,6 381 953 99 248 34.9 +PA-4 25.5 21.1 1.5 300 1172 82 320 34.9 ZrFe2Hx(1:2) 100 36.6 5.1 106 106 28 28 30.2 +PA-4 25.4 21.5 0.6 189(188) 746 63(34) 247 27.0(46) In brackets the data for non-hydrogenated AC are presented.

339 up to 40% permits to enhance the activity of 1:1 composite catalysts, but effectiveness of the AC (productivity per weight of AC) drops (Tabl. 1). The activity of non-hydrogenated AC is the same but selectivity towards CH4 is much higher then than that for hydrogenated ones. The hydrocarbon products distribution demonstrates a non-ideal character due to enhanced CH4 formation; the average parameter ~x of the Anderson-Schulz-Flory distribution being in the 0.6-0.7 range for all the catalysts studied. Comparing these data with the particle size for pure (2.5 mm) and encapsulated (0.375 mm) AC permits to conclude that, in the first approximation, the specific output (per unit wt. of ZryFe~) is proportional to the AC active surface. Hence, one of the advantages of AC encapsulation into the A1203/A1 matrix is the use of more dispersed and much more productive AC particles, which is not possible for the conventional fixed bed catalyst design [3]. According to XRD (Fig.l), in pure intermetallides tetragonal Zr2Fe and ZrFe2 (hexagonal for 2:1, cubic for 1:1 and 1:2 samples) phases dominate. The amount of the first phase increases, while that of the second decreases with the Fe content. For 1:2 sample a cubic [3-Zr phase is also observed. After hydrogenation, tetragonal ~-Zr2H-type phase with varied lattice parameters appears to be formed from Zr2Fe, but in 1:2 sample this phase is absent at all. ZrFe2 phase becomes cubic for all samples, the lattice parameter increasing from 2:1 to 1:2 samples. In addition, another hydride phase ZrxFeyHy with varied lattice parameters is observed only for 1:1 hydrogenated sample. This phase appears to be formed on the boundary between ZrFe2 and ~-Zr2H phases. After encapsulation into the A1203/A1 matrix for all samples A1~ 7-A1203 c~-Fe203 and zirconia phases are observed. For the latter one, the ratio of the amount of monoclinic to tetragonal phases increases with the increasing Fe content. In 2" 1 and 2:1 samples, a cubic ZrFe2 phase is identified. For 1:2 sample [3-Zr phase is not observed. As for hydride ~;-ZrH1.95 phase, its content is decreased in 2:1 and is almost zero in 1"1 sample. The analysis of the texture of hydrogenated intermetallides (fraction 0.50.25 mm) revealed that for all samples but 1:2 one, their specific surface area is much higher than the geometric one, thus demonstrating a high roughness of their particles due to cracks and pores formation. All types of AC have mesopores ( d - 390-460 A) with a small pore volume. The hydrothermal and calcination treatments have only a weak effect on the textural characteristics of AC. As for encapsulated catalysts, the total micropore volume of the catalysts as well as the specific surface area is higher but the average micropore diameter is significantly lower as compared to hydrogenated AC. All these characteristics are close to that of a pure A1203/A1 matrix and are proportional to the alumina content being higher for PA-HP aluminum powder (Tabl.2). Nevertheless, the total pore volume of the composite catalysts is much higher than that of

340

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Figure 1. X- ray diffraction patterns of intermetallides on the different stages of synthesis: 1 - initial, 2 - after hydrogenation, 3 - after capsulation in AlzO3/A1 matrix. Designation of phases: Y - ZrFe2 (cubic); Yh - ZrFeh (hexag.); o - ZrH1.95(tetrag.); A - ZrO2; * - Zr2Fe; + - a - F e 2 0 3 ; 9 Zr(Fe)H•

micropores, and it increases with the AC content in the composite catalysts thus demonstrating a developed macropore (d > 1 ~tm) structure [4]. The crushing strength of the granulated AlzO3/A1 matrix is higher for PA-HP derived composite due to a higher alumina content and a self-pressing in the press-form in the course of hydrothermal oxidation [6]. According to XPS, even after hydrogenation, the surface of AC is in the oxidized state: Fe z+, Fe 3+ with BE(Fe2p 3/2)~711 eV; Zr 4+ with BE(Zr3d 5/2) ~182 eV, due to a contact with the air. The surface layer is enriched by Fe, segregation being increased after hydrogenation. The surface Fe content which is characterized by the Fe/Zr ratio is increased as compared with the bulk Fe content. Surprisingly, after encapsulation, a very low surface AC content characterized by the Fe/A1 ratio < 0.013 is observed, possibly due to a shielding effect of porous A1203 partially covering the surface of AC in the course of encapsulation procedure. 4. D I S C U S S I O N As is seen from comparison of the catalytic, the bulk phase and the surface composition data, hydrogenation results in the enrichment and more random distribution of iron on the surface of AC. Evidently, the surface layer of AC

341 Table 2 Textural and mechanical properties of the catalysts ZryFezHx,ZryFezHx/A1203/A1,prepared from different aluminum powders (PA-HP, PA-4) and compared with A1203/A1matrix. Adsorption-desorption isotherm data Crushing Active component V total, SSA, V micro, d, under strength, ml/g m 2/g ml/g 2000 A MPa Type + Content, wt.% aluminum powder Zr2,6FeHx 0 0,17 0,010 390 + PA-HP 40.0 0,74 15,02 0,019 50,5 1,27 +PA-4 25.6 0,21 10,61 0,017 51 0,32 ZrFeHx 100 0,22 0,012 440 +pA-np 40.0 0,64 42,14 0,051 49,1 2,08 + PA-4 25.5 0,31 11,25 0,019 53 0,23 ZrFe2H,, 100 0,03 <0,001 460 + PA-4 25.4 0,24 9,09 0,015 50,8 0,58 A1203/A1, PA-HP 0 0,39 34,52 0,040 40,1 3,53 A1203/A1, PA-4 0 0,37 17,30 0,032 51,0 0,78 exists as FeCx in the reaction media [7], but preliminary hydrogenation possibly facilitates this process positively influencing the selectivity of FeZr system. The order of activity related per the Fe surface content (g/"SFe"'h) estimated from SSA and Fe/Zr ratio was found to be passing through maximum to bulk Fe content: Zrz6Fe (~2010) < ZrFe (~11550) < ZrFe2 (~5480) also demonstrating a higher effectiveness of the surface active centers for 1:1 active component. However, the "real" (per mass of AC) effectiveness of catalysts also depends upon their SSA which is at maximum for 1"1 sample. Possibly, this SSA is provided by the optimum multiphase composition with hydrogenated zirconium and catalytically active iron, characterized by enhanced surface roughness and existence of microcracks. For aluminum powder-based catalysts the porosity and the average pore diameter are closely related to the average particle size of initial A1 powders [6]. Supposing a negligible blocking effect of alumina on the surface concentration of active sites due to permeation of gaseous substances through the porous layer, we found that, as compared with the known catalysts [8,9], a general level of the structural parameter X; [8] is very low (x1016 m-I) 9 ~1.9; ~1.3 and ~0.1 for 2"1; 1"1 and 1:2 PA-4-capsulated catalysts, respectively, which is explained by a very high amount of ultramacropores in the composite catalyst [5,6]. Trends in the selectivity variation differ to methane (Tabl. 1) and C5+ products (16.7; 27.2; 33.1). These deviations from regularities observed earlier [8,9] can be explained by the unusual pore size distribution of the composite catalysts. Also, AC is not distributed randomly on the surface of alumina, while porous alumina decorates the surface of AC by a thick (up to few gm) layer, which FTS products need to penetrate before reaching the ultramacropores.

342 However, all catalytic composites demonstrate a high activity which remarkably exceeds the highest level achieved by earlier described intermetallic catalysts. Thus, according to [2], on ZrFe intermetallide at 350 ~ C and 20 MPa a C5+ productivity of 200 g/lcat'h (~60-70 g/kgcat'h) was obtained. In terms of the simplest FTS kinetics on Fe-contained catalysts (nearly l~t order to H2 and 0 ~0,2 order to CO), one can estimate that at 3 MPa and 350~ such a catalyst would produced ~10-20 g/kgcat'h C5+. For composite catalysts, this value is in the range of 50-100 g/kgcat'h. 4. C O N C L U S I O N The encapsulation of hydrogenated Zr-Fe intermetallides into the A1203/A1 matrix permits to preserve their main properties as well as to increase their specific activity. The multiphase composition of easy hydrogenated and FTS active component provides a non-monotonous variation of their performance with Zr:Fe ratio. The macroporous A1203/A1 matrix prevents the loss of AC and permits to use highly dispersed AC while ensuring the efficient transport of reagents and products due to developed macro/microporous structure, but the selectivity towards C5+ needs to be improved. The composite catalysts prepared as thick coating layers on narrow tubes can ensure intensification of the heat transfer in fixed tube reactors at high linear velocities [ 10]. ACKNOWLEDGEMENTS This work was financially supported by the Russian Foundation for Basic Research (Grant 02-03-32277). REFERENCES [1] V.V. Lunin, A.Z. Khan, J. Molec. Catal., 25 (1984) 317. [2] A.Ya. Rozovsky, Kinet. Catal., 40 (1999) 358. [3] L.A. Vytnova, V.P. Mordovin, G.A. Kliger, E.I. Bogolepova, V.I. Kurkin, A.N. Shuykin, E.V. Marchevskaya, E.V. Slivinsky, Russ. J. Petrochemistry, 42 (2002) 111. [4] S.F. Tikhov, V.A. Sadykov, Yu.V. Potapova, A.N. Salanov, S.V. Tsybulya, G.N. Kustova, G.S. Litvak, V.I. Zaikovskii, S.N. Pavlova, A.S. Ivanova, A.Ya. Rozovskii, G.I. Lin, V.V. Lunin, V.N. Ananyin, V.V. Belyaev, Stud. Surf. Sci. Catal., No 118 (1998) 797. [5] S.F. Tikhov, Yu.V. Potapova, V.A. Sadykov, A.N. Salanov, S.V. Tsybulya, G.S. Litvak, L.F. Melgunova, React. Kinet. Catal. Lett., 77 (2002) 267. [6] S.F. Tikhov, V.B. Fenelonov, V.A. Sadykov, Yu.V. Potapova, A.N. Salanov. Kinet. Catal., 41 (2000) 907. [7] S. Li, G.D. Meitzner, E. Iglesia, Stud. Surf. Sci. Catal., No 136 (2001) 387. [8] E. Iglesia, S.C. Reyes, R.J. Madon, S.L. Soled, Adv. Catal., No 39 (1992) 221. [9] G.W. Huber, C.H. Bartolomew, Stud. Surf. Sci. Catal., No 136 (2001) 283. [10] M.E. Dry, Catal. Today, 71(2002) 227.