H2 on new SiO2-supported PtW and PtCr bimetallic catalysts

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APPLIED CATALYSIS A: GENERAL

ELSEVIER

Applied Catalysis A: General 128 (1995) L1-L6

Letter

Selective methanol synthesis from C O 2 / H 2 on new SiO2-supported PtW and PtCr bimetallic catalysts Changpin Shao a, Li

F a n b, K a o r u F u j i m o t o

b,*,

Yasuhiro Iwasawa c

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 161 Zhongshan Road, Dalian, China u Department of Applied Chemistry, Facul~" of Engineering, The Universi~ ofTo~'o, Hongo, Bun~'o-ku, Tokyo 113, Japan c Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 20 March 1995; accepted 28 April 1995

Abstract

New SiO2-supported PtW and PtCr bimetallic catalysts prepared by using bimetal carbonyl hydride complexes as precursors exhibited remarkably higher activity and methanol selectivity (typically above 90%) in carbon dioxide hydrogenation than conventionally prepared monometallic or bimetallic catalysts. At 473 K and 30 bar, the carbon dioxide conversion was 2.6% while methanol selectivity reached 92.2% for the complex-derived PtW/SiO2 catalyst even if the reactant gas ratio w a s CO2/H 2 = 1/3.

Keywords." Carbon dioxide; Hydrogenation; Methanol synthesis: Bimetallic catalyst; Catalyst preparation

1. Introduction Efficient conversion and utilization of carbon dioxide has received considerable attention, from a chemical viewpoint as well as from the viewpoint of environmental protection, i.e., green-house effects. Most research on methanol synthesis from carbon dioxide hydrogenation has been concentrated on Cu-Zn oxide catalysts [ 15 ]. A high reaction temperature is not favorable for this reaction as it is exothermal. However, at low reaction temperature, Cu-based catalysts may interact with coexistent carbon dioxide and water to form CuCO3 and Cuz(OH)2CO3, resulting in catalyst deactivation. For carbon dioxide hydrogenation, supported transition metal catalysts such as Ni [6,7], Rh [8], or Ru [8,9] have been shown to be effective * Corresponding author. Tel./fax. ( + 8l-3) 56890469. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X(95) 001 09-3

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C. Shao et al. /Applied Catalysis A: General 128 (1995) L1-L6

for methane formation. Furthermore, Fe [ 10] and Co [ 11 ] catalysts are effective for C2+ hydrocarbon formation. Supported noble metal catalysts for methanol synthesis from hydrogenation of carbon dioxide were not intensively studied until now. Ramaroson et al. [ 12,13 ] conducted this reaction under very severe conditions ( 120 bar, 623 K) using supported palladium catalysts. When a basic oxide support like La203 was employed, high methanol selectivity was feasible. They also prepared a Pd-Li/SiO 2 catalyst by co-impregnation and claimed a methanol selectivity of 83.9% was obtained under the same reaction conditions as mentioned above. Unfortunately, they did not take the amount of carbon monoxide formation in consideration in their calculation of methanol selectivity, as pointed out in another report [ 14]. Solymosi and co-workers [14,15] studied palladium catalysts supported on A1203, SiO2, TiO2 and MgO but methanol selectivity was not high. The comparatively lower reaction pressure (9.5 bar) in their experiment seems to be the reason for this. Some of the present authors [ 16,17] found that high-temperature-reduced Pd/CeO2 catalyst exhibited strong metal-support interaction behavior and could give high methanol selectivity. Although platinum is active for hydrogenation reactions, carbon dioxide hardly adsorbs on platinum under the usual hydrogenation reaction conditions [ 18], leading to less ability for carbon dioxide hydrogenation. With the aim of utilizing the hydrogenation ability of platinum, we tried to introduce metal additives on which carbon dioxide can effectively adsorb, to prepare Pt-based bimetallic catalysts for this reaction. In this study we report that new SiO 2supported PtW and PtCr bimetal catalysts, prepared by using bimetallic carbonyl hydride complexes as precursors, exhibited very high selectivity and lifetime for methanol synthesis from a mixture of carbon dioxide and hydrogen. In comparison, the conventionally impregnated monometallic and bimetallic catalysts were almost inert or produced mainly carbon monoxide.

2. Experimental Heteronuclear-metal carbonyl hydride complexes (I) of Pt-Cr and Pt-W were synthesized by reaction of trans-Pt(H)Cl(PPh3)2 with Cr(CO)6 or W(CO)6 in THF under argon atmosphere. Trans-Pt(H)Cl(PPh3)2 was synthesized from cisPtClz(PPh3)2 which was obtained from K2PtC14 + PPh3 with a method similar to that described in the literature [19-21]. The reaction of trans-Pt(H)Cl(PPh3)2 with Cr(CO) 6 or W ( C O ) 6 in THF was conducted in the presence of Na-Hg at 313 K in a way similar to that for the synthesis of Pt-M (M:Cr, Mo and W) bimetallic complexes from Pt ( CzH4) (PPh3) 2 and M (CO) 5PPh2H [ 22 ]. The details of the synthesis and characterization of (I) will be reported elsewhere, where (I) is proposed to be (PPh3) (H) Pt (/z-PPh2) (/x-CO) M (CO) 4 ( M: Cr, Mo and W) [ 23 ]. The bimetallic complex (I) was purified on a SiO2-column chromatograph, giving

C Shao et al./Applied Catalysis A: General 128 (1995) L1-L6

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violet-red elution, from which crystallized (I), with a violet-olive color, was obtained with 30-40% yield. SiO2-supported [PtCr] and [PtW] catalysts were prepared by an impregnation method using a THF solution of (I) under argon (99.9999%) atmosphere. The solvent was removed by evacuation at room temperature. The obtained catalysts were transferred to a flow-type high pressure reactor within l0 min in air, and used for carbon dioxide hydrogenation without any pretreatment. For comparison, monometallic and bimetallic catalysts were also prepared by an impregnation (or coimpregnation) method using HzPtC16, CrC13, and WC16, followed by drying at 393 K for 2 h and reduction in flowing hydrogen at 673 K for 2 h. Carbon dioxide hydrogenation was carried out in a pressurized fixed-bed flow-type reactor; typical reaction conditions are catalyst weight = 0.20 g, P(total) -- 30 bar, H2/CO2 = 3, and W/F = 10 gcat h/mol. Effluent products were analyzed by a gas chromatograph with flame ionization detector (FID) using a Porapak R column for hydrocarbons, methanol, and C2-oxygenates, and by a gas chromatograph with thermal conductivity detector (TCD) using an active carbon column for carbon dioxide and carbon monoxide.

3. Results and discussion

The performance of the complex-derived bimetallic catalysts (denoted as [PtCr]/SiO2 and [PtW]/SiO2), the conventionally prepared bimetallic catalysts (Pt-Cr/SiO 2 and Pt-W / SiO2), and monometallic catalysts (Pt / SiO2, Cr/SiO2 and W/SiO2) for carbon dioxide hydrogenation with hydrogen is summarized in Table 1. The Cr/SiO2 and W/SiO2 catalysts were almost inactive at 473-573 K. The activity of the Pt/SiO2 catalyst at 473 K was also very low and the main product was carbon monoxide. Carbon monoxide selectivity was above 97%, accompanied by traces of hydrocarbons and methanol. Increased reaction temperatures also gave virtually nothing but carbon monoxide, even if carbon dioxide conversion was enhanced. On these catalysts methanol production was not significant. Addition of chromium to the Pt/SiO2 system by conventional impregnation enhanced the catalytic activity and the yield of methanol, but the methanol selectivity of the Pt-Cr/SiO2 catalyst was as low as 15.5% at 473 K and 12.8% at 573 K. Addition of tungsten to the Pt/SiO2 catalyst in a conventional way also enhanced the activity, but the methanol selectivity (1.6%) was not improved, as shown in Table 1. These results indicate that chromium and tungsten in the conventional PtCr/SiO2 and Pt-W/SiO2 catalysts promoted mainly the reverse water-gas shift reaction (H2 + CO2 ~ H20 + CO). In contrast, complex-derived [ PtCr]/SiO2 and [PtW] /SiO2 catalysts exhibited activities that were higher by 5.5-6.5 times at 473 K than the Pt/SiO2 catalyst. More interesting, the methanol selectivity was dramatically improved for the [PtCr]/SiO2 and [PtW]/SiO2 catalysts. The promotion effect of the added chro-

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Table 1 Activities and selectivities of CO2 hydrogenation on SiO2-supported monometal and bimetal catalysts Catalyst

Reaction temp./K

Conversion/%

Selectivity/%

(CO2)

CH4

C2 -

CHsOH

CO

[ PtCr]/SiO 2 (2.4-0.6 wt.-%) [PtW]/SiO2 (2.0-1.9 wt.-%)

473

2.2

0

0

51.1

48.9

473 513

2.6 4.0

0 0

0 0

92.2 86.6

7.8 13.4

Pt/SiO2 (2.4 wt.-%) Cr/SiO_, ( 1.3 wt.-%) W/SiOz (2.3 wt.-%)

473 573 473 573 473 573

0.4 3.1 0 0.3 0 0

0.2 0.04 0 0 0 0

0.4 0.03 0 0 0 0

1.9 0.3 0 0 0 0

97.5 99.6 0 100 0 0

Pt-Cr/SiO2 ( 2.4~).7 wt.-% ) Pt-W/SiQ (2.4-1.1 wt.-%)

473 573 473

2.5 5.3 1.9

0.4 0.2 0.1

0. l 0.1 0.1

15.5 l 2.8 1.6

84.0 86.9 98.2

Catalyst weight: 0.20 g; total pressure: 30 bar; H2/CO2 = 3; W / F = I0 g~,, h/mol.

mium and tungsten on the activity was similar, but the effect on methanol selectivity was more obvious for the W-promoted catalyst than for the Cr-promoted one. As a result, the methanol yield increased 148 times by chromium addition and 315 times by tungsten addition. The temperature dependence of the carbon dioxide conversion and selectivity of the [PtW]/SiO 2 is shown in Fig. 1. Carbon dioxide conversion of the catalyst increased with increasing reaction temperature, while the methanol selectivity remained at a high level up to 513 K, typically above 90%. At 533 K, methanol selectivity dropped to 75% which is still much higher than the selectivity of 1.9% for the Pt/SiO2 catalyst at 473 K. For the catalyst lifetime, both carbon dioxide conversion and methanol selectivity of [PtCr]/SiO2 and [PtW]/SiO2 were very stable throughout a period of 50 h, and these catalysts were used repeatedly. The superiority of the complex-derived [PtCr]/SiO2 and [PtW]/SiO2 bimetallic catalysts may be ascribed to the interaction of two metals, resulting in advantages. At first, platinum metal sintering may be suppressed by chromium and tungsten additives. In fact, the mean platinum particle sizes were determined to be 4.4 nm (Pt/SiO2) > 3.5 nm (Pt-W/SiO2) > 3.4 nm (Pt-Cr/SiO2) > 2.5 n m ( [PtW] / SiO2) > 1.0 n m ( [PtCr]/SiO2), by transmission electronic microscopy (TEM) (JOEL, JEM-3010, operated at 300 kV). Secondly, active bimetallic ensemble sites might form on the catalysts prepared by using the bimetallic complexes. The molar ratio of chromium to platinum in a metal particle on a SiO2 of [ PtCr] /SiO2 catalyst was estimated by analytical TEM (resolution: 6.5 nm) to be about twice higher

C. Shao et al. / A p p l i e d Catalysis A: General 128 (1995) L I - L 6 10

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Reaction temperature / K Fig. I. Conversion and selectivity in CO2 hydrogenation on the [ P t W ] / S i O 2 catalyst as a function of reaction temperature. P ( t o t a l ) = 30 bar, H2/CO2 = 3, W~ F = 10 gc~, h / m o l .

than that of Pt-Cr/SiO2 catalyst, suggesting that the two metals in the complexderived catalyst were well mixed, while a relatively larger part of the two metals in the conventional catalyst was separately supported on SiO2. The particle size of the supported metal is crucial in determining the product formation route in carbon monoxide or carbon dioxide hydrogenation. It is well known that in the case of carbon monoxide hydrogenation, small particles of palladium of a Pd/SiO2 catalyst gave methane selectively whereas large palladium particles favored methanol formation on this catalyst [ 24,25 ]. For carbon dioxide hydrogenation on supported palladium catalysts, it is also observed that highly dispersed palladium produced mainly methane while poorly dispersed palladium gave methanol [14]. Different from the palladium catalysts mentioned above, supported PtW and PtCr bimetallic catalysts here exhibited the reverse trend: small platinum particles favored methanol and large ones gave carbon monoxide. It is considered that added tungsten or chromium changed not only the distribution state remarkably, but also the electronic state of platinum. Surely, the addition effect of tungsten or chromium is largely controlled by the structure of the bimetallic ensemble site, where the latter is dominated by the preparation method of these bimetallic catalysts.

4. C o n c l u s i o n

The preparation method and reaction performance of the new bimetallic catalysts, obtained by a controllable procedure from bimetallic complex precursors, are entirely different from those of conventionally prepared catalysts. High activity and methanol selectivity of the complex-derived PtW/SiO2 and PtCr/SiO2 catalyst should be attributed to its well-dispersed platinum particles, and uniform mixture

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state between platinum and the added metal. Further studies on the active site structure of these catalysts, test of other additive metals, and enhancement of carbon dioxide conversion are now in progress.

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