The selective formation of hydrocarbons from CO hydrogenation over oxide catalysts

Colloids and Surfaces, 38 (1989) 93-101 Elsevier Science Publishers B.V., Amsterdam -

93 Printed in The Netherlands

The Selective Formation of Hydrocarbons from CO Hydrogenation over Oxide Catalysts TAKAHARU ONISHI, KEN-ICHI MARUYA, KAZUNARI DOMEN, TAKESHI FUJISAWA and TORU ARAI Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227 (Japan) (Received 10 August 1988; accepted 21 November 1988)

ABSTRACT The hydrogenation of carbon monoxide was carried out over difficult-to-reduce metal oxide catalysts to form hydrocarbons under mild conditions. Over ZrO,, the selective formation of isobutene (88% in carbon base) from CO hydrogenation proceeds at 623-673 K and 67 kPa initial pressure. During the course of the reaction, the ZrO, surface was covered with formate and methoxide species, which were observed by FT-IR. The effect of addition of some organic compounds on the CO hydrogenation reaction was examined. The addition of dimethyl ether shows a marked enhancement effect on the hydrocarbon formation and retains the high selectivity for isobutene. Over In,OB (10% )-CeO,, ethylene is the main product (45% ) among hydrocarbons produced from CO hydrogenation at 773 K and 67 kPa (Hz/CO = 3 ). The effect of the addition of In,O, to the CeO, catalyst is discussed on the basis of the results obtained by XPS and XRD.

INTRODUCTION

There have been a number of studies on the hydrogenation of CO to form mainly linear-chain hydrocarbons over various transition metal catalysts. The distributions of hydrocarbons produced are generally described by the SchulzFlory rule. Therefore, there have been only few studies on the selective formation of particular hydrocarbons over metal catalysts, which were reported by Fraenkel and Gates [ 11, Nazar et al. [ 21, Iwasawa and Ito [ 31, Tatsumi et al. [ 41 and Venter et al. [ 51. These metal catalysts gradually lose activity and selectivity for lower olefin formation during the reactions. On the other hand, difficult-to-reduce metal oxides, such as ThOz studied by Pichler and Ziesecke [ 61, and LazO, and Dy,O, by Kieffer et al. [ 71, have been used as catalysts for the CO-H, reaction to produce branched-chain, saturated aliphatic hydrocarbons such as isobutane under severe conditions. Maehashi et al. [8] reported that ZrOa selectively forms isobutene under mild conditions at 623 K and 67 kPa. In this paper, we report the selective formation

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94

of isobutene over ZrO, and also ethylene over In203-Ce02 from CO hydrogenation under mild conditions. EXPERIMENTAL

Catalystpreparation

Hydroxides of Y, La, Ce and Th were prepared with NH,OH from aqueous solution of the nitrates. Hydroxides of Zr and Hf were prepared with NH,OH from aqueous solution of the oxynitrate and the oxychloride, respectively. Hydrated niobium pentoxide was prepared with NH,OH from aqueous solution of potassium niobate. These hydroxides were calcined at 773 K for 3 h. Mixed oxide catalysts were prepared by co-precipitation from mixed aqueous solution of each metal nitrate by the same procedure as that of the oxide. The atomic ratios of metal (Al, Ga and In) to Ce were 1: 10. Procedure

The CO hydrogenation reactions were carried out in a glass vacuum reaction system with a gas-circulating pump. A mixture of H, and CO (3 : 1) was introduced onto catalysts (1.5 g) which were pre-evacuated at 973 K for 3 h. The initial pressure was 67 kPa and the gas flow rate was N 35 ml (STP) min-‘. The reaction was also carried out in a flow system with a quartz reactor. The catalyst was heated at 873 K for 3 h under a nitrogen stream, and H2 and CO were introduced at a given reaction temperature. Analyses

Products were analyzed by gas chromatography equipped with Porapak Q (for COz, Hz0 and hydrocarbons), VZ-7 and VZ-10 (for isomers of C, hydrocarbons) and molecular sieve 5A (for CO) columns. The crystal structure of the oxide catalyst was determined by XRD, and surface analyses of the catalysts were performed by ESCA (Shimadzu ESCA 750). The binding energies were corrected by Au ( 4f7,2) (83.8 eV). Infrared spectra during the course of the reaction were recorded on a JEOL JIR 100 FT-IR spectrometer with a mercury cadmium telluride detector. Typically, 256 scans (total scan time: 2 min ) were accumulated at 4 cm-’ resolution. RESULTS AND DISCUSSION

CO hydrogenation

over metal oxides

The CO-H, reactions at 673 K and 67 kPa of total pressure proceed over metal oxides such as Y203, La203, CeO,, ThOz, ZrO,, HfO, and Nb,O,. The

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carbon-containing products are hydrocarbons, CO, and small amounts of methanol and/or dimethyl ether (DME), as shown in Table 1.CeO, shows the highest activity for the formation of hydrocarbons. The hydrocarbon product distributions with metal oxides are shown in Fig. 1. ZrO,, HfOz and ThOa catalysts are remarkably selective for the formation of C, hydrocarbons, and CeO, catalyzes the formation of C, hydrocarbons. A Schulz-Flory distribution is not followed for the formation of large amounts of C, hydrocarbons over metal oxide catalysts. The catalytic activities of forTABLE 1 Hz-CO reaction over some metal oxide catalysts” Catalyst

Surface area (m* g-‘1

57 14 29 49 57 37 28

y203

LasO, CeOz ThO, ZrOz HfO, Nb,Os

Rateb Hydrocarbon

MeOH + Me,0

CO,

44 20 223 84 40 41 1

1.0 5.6 0.0 1.4 0.7 0.0 0.0

390 490 1000 370 170 190 140

“Reactions were carried out over metal oxide catalysts (1.5 g) at 673 K and 67 kPa (HJCO = 3) in a gas-circulating glass system. bThe values are average rates for the initial 24 h: pmol mm2h-l.

cl

cZ

c3

c4

c5

c6

Carbon number

Fig. 1. Hydrogenation product distributions with some metal oxide catalysts: (a) Y,O,; (b) La,O,; (c) CeO,; (d) ThO,; (e) ZrO,; (f) HfO,; (g) Nbz05.

96

mation of hydrocarbons over metal oxide catalysts are generally one order of magnitude lower than those over supported metal catalysts [ 91. CO hydrogenation

over ZrOz

The yield of products formed from the Hz-CO reaction over ZrO, in the temperature range 473-723 K for the initial 24 h are shown in Fig. 2. Yields of methanol and DME reach maximum values at around 523 and 573 K, respectively, and that of hydrocarbons increases rapidly with an increase in the reaction temperature. Table 2 shows the distributions of hydrocarbons produced in the temperature range 523-723 K. The apparent activation energies for C, and C3 hydrocarbons are 79 and 84 kJ mol-l, respectively. The activation energy for linear C, hydrocarbons is 96 kJ mol-‘, while that evaluated in the range 523-623 K for branched-chain C4 hydrocarbons is N 210 kJ mol-l. The isomer distributions in Table 3 show that at 523 K 1-butene is mainly formed; however, above 573 K the main product is isobutene. Activity and high selectivity for isobutene formation over ZrO, after 240 h were not changed. These results indicate that branched-chain hydrocarbons are probably formed by a different path from linear hydrocarbons.

rTl 5 9 J 200 E m % m D 100 _n 9 >r c ? k

0

473

523

573 623

673

723

T/K

Fig. 2. CO-H, reaction over ZrO, at various reaction temperatures. TABLE

2

Selectivity

in hydrocarbons

(carbon base %) over ZrO,

T(K)

CH,

C,H,

C,H,

C3Hs

C,H,

C,H,

C,H,,

523 573

20 3

30 20

1 +

22 13

_

27 64

-

623

1

4

+

4

673 723

4 18

3 8

1 6

5 10

1 3

C,,

88

1

2

76 39

3 10

7 6

91

TABLE 3 Product distribution in C, hydrocarbons from CO hydrogenation over ZrO,

T(K)

523 573 623 673 723

Selectivity (% )

n-GHlo

i-&HI0

l-&H,

trans-2-&H,

cis-2-&H,

i-&H8

3.0 0.3 0.0 0.3 10

0.4 0.0 1.1 4.2 11

63 0.5 0.2 1.7 3.0

3.3 7.1 1.0 3.4 8.0

1.3 0.5 0.7 2.4 11

29 91 97 88 57

Effect of metal oxide additives over ZrO, The effects of addition of metal oxides such as NaOH (13% ), CaO (10% ), A&O, (10% ) and SiO, (10% ) to ZrOz on the activity and selectivity for hydrocarbon formation at 673 K were studied as reported by Maruya et al. [lo]. Among the mixed oxide catalysts, the SiOz-ZrO, catalyst is not C,-selective but rather methane-selective. The presence of NaOH on ZrO, causes an increase in isobutene selectivity (85% C-base) compared with that (76% ) over the unmodified ZrO, catalyst at 673 K, while NaOH-ZrOz shows much lower activity than ZrO, alone. The addition of CaO and A1203to ZrO, decreases the activity and selectivity for Cd-hydrocarbon formation. IR spectra of adsorbed species over ZrO, When a mixture of Hz and CO was introduced over ZrO:, at room temperature, we observed FT-IR spectra of adsorbed species soon after its introduction, as shown in Fig. 3 (a). Bands at 3760 (OH), 2200 (adsorbed CO), 1560, 1371 and 1116 cm-’ (three bands for Zr-H species) were observed, as reported by Onishi et al. [ 111, and additional bands due to the formation of a new species in the region 1100-900 cm-’ appeared. Figure 3 (b) shows the IR spectrum of adsorbed species at room temperature after reaction for 1 h. It was found that in the spectra, surface formate ions (bands at 2872,1558,1389 and 1365 cm-l) appear, even at room temperature. The three bands at 1135,1116 and 931 cm-l are characteristic of paraformaldehyde-type adsorbed species, as studied by Onishi et al. [ 121. It is clear that at room temperature some paraformaldehyde-type adsorbed species are formed on the surface from the reaction of H,-CO. At 373 K, after reaction for 30 min, weak bands at 1144 and 1028 cm-’ due to methoxy species appear in the spectra. At 523 K, where the main product is methanol, the surface of ZrOz was fully covered only with methoxide species and formate ions. Under the reaction condition at 673 K, where the selective formation of iso-

98

I, 4000

3603

3200

2800

24w

MOO

16W

1200

800

Wavenumber/cm-l

Fig. 3. (a) Infrared spectrum of adsorbed species formed from Hz-CO reaction over ZrO, at room temperature soon after introduction of the reaction gases; (b) after reaction for 1 h.

LA 000

3EOO

3200

2803

2400

2OCO

1600

1200

800

Wavenumber,‘cm-’

Fig. 4. Infrared spectrum

of surface species on ZrO, during CO-H, reaction at 673 K.

butene proceeds, an IR spectrum of adsorbed species over ZrO, was observed, as shown in Fig. 4. The spectrum was essentially similar to that at 523 K, and the surface was covered only with methoxide and formate ions. Adsorbed methoxide would be an important intermediate for the formation of methanol and hydrocarbons. Adsorbed formate ions decompose for form CO and hydrogen, and are not converted into methoxide on the basis of carbon isotope experimental results which will be reported elsewhere by Onishi et al. [ 131. Effect of additives on the CO-H,

reaction

over ZrO,

The activity and selectivity for hydrocarbon formation from the CO-H, reaction in the presence of some organic compounds were studied, as shown in Table 4. When DME alone was introduced over Zr02 at 643 K, small amounts

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TABLE 4 Effect of additives to CO-H, reaction over ZrO, (5 g) on hydrocarbon distribution at 643 K Additive”

None DME CH,=CH, CH,CHO CH,CH(OCH,), CH,=CHCH, CH,CH,CHO CH,COCH,

Product (C-base ,umol min-‘)

Selectivity i-C,H,/C., (% )

C,

C,

C,

C,

C 5-t

0.03 21 0.3 0.2 0.2 0.2 0.2 0.3

0.2 34

0.2 15 1.2 1.0 0.2 2.7 2.2

2.8 36 6.0 9.2 4.4 2.4 0.4 0.4

0.3 10 0.2 0.7 0.2 0.1 5.0 0.2

1.2 0.4 0.3 0.4 0.3

“The feed rate for the mixed gas (CO/H,/N, were 10-l pm01 min-‘.

97 97 84 97 97 35 24

= 2 : 2 : 2 : 1) was 100 ml min-’ and those for additives

of hydrocarbons, which consist mainly of ethylene, were obtained. It was found by FT-IR that DME adsorbs over ZrO, to form methoxide species. The addition of small amounts of DME to a mixture of Hz-CO gases markedly enhanced the formation of lower molecular hydrocarbons without changing the selectivity for isobutene among C, hydrocarbons. This means that isobutene is not formed from methoxide species alone, and the addition of DME to the mixture of CO-H, gases results in an increase of C, hydrocarbons which consist mainly of isobutene. Maruya et al. [lo]reported a second-order dependence of isobutene formation on the CO pressure. This result suggests the slow formation of C, intermediate from the reaction between CO and a C, intermediate which can be formed from methoxide species. The increase of isobutene and isopentenes by the addition of acetaldehyde and propionaldehyde, respectively, suggests the fast addition of the oxygenates adsorbed to the C, intermediate to form C4 and C, olefins. Selective ethylene formation over CeO, catalyst Among oxide catalysts, CeO, shows the highest activity for hydrocarbon formation and the highest selectivity of C, hydrocarbons from CO hydrogenation, as shown in Table 1 and Fig. 1. In order to improve the selectivity for C, hydrocarbons, the effect of addition of some oxides, such as A1203, Ga,O,, and In,O,, to the Ce02 catalyst was examined at 673 K and 67 kPa, as shown in Table 5. It was found that the addition of 10% In20, (or Ga,O,) to CeO, showed a marked increase in the activity and selectivity for C2 hydrocarbons. In Table 6, the activity and selectivity changes in the temperature range 623-723 K over CeO, and In,03-Ce02 catalysts are shown. The In,O,-CeO, catalyst exhibits

100 TABLE 5 CO hydrogenation Catalyst area)

over CeO, and mixed CeOP catalysts

(surface

Rate H.C.”

CeO, (21) AI,O:,-Ce02 (64) GapO,-CeO% (59) In,O,-CeO, (28)

Selectivity

2.0 2.8 2.7 5.7

“Rate of hydrocarbon

formation;

(1: 10) at 673 K and 67 kPa (HJCO

= 3 : 1)

(C-base %)

C1

C,

C3

C,

CS

C 6+

25 18 25 24

29 30 43 43

9 13 14 10

21 24 15 13

8 13 3 7

8 2 +3

C-base prnol rn-’ h-l.

TABLE 6 Activity and selectivity changes with the reaction temperature Catalyst

T(K)

Rate H.C.”

over CeOP and In,O,-Ce02 catalysts

(C-base % )

Selectivity C,

C,

C,

Cd

C,

C,+

CeO,

623 673 723

0.6 2.0 8.1

10 25 34

6(86)b 29(96) 35(96)

4(63)b 9(84) 12(81)

32 21 15

31 8 3

18 8 1

In,O,-CeO,

623 673 723

2.5 5.7 8.6

23 24 30

39(99) 43(99) 45(98)

9(91) lO(94) 12(94)

15 13 10

11 7 2

3 3 +

“Rate of hydrocarbon formation: C-base pmol mm2 h-l. hParentheses show the selectivity for alkenes. TABLE 7 Characterization Catalyst

of CeO, and In,O,-CeO, XRD

(1: 10) Binding energy of

InW4,,2) (eV) CeO,

In,O,S-CeOp In& In metal

fluorite fluorite In,O,-type

444.4 444.4 443.3

Surface ratio (76) of In to Ce

11 _

the highest selectivity of 45% for C, hydrocarbons which consist of 98% ethylene. During the reaction for 48 h, In20,-CeO, is stable with a good mass balance, and the activity and selectivity for C2 hydrocarbons were not changed. The state of the In,O,-CeO, mixed oxides was examined by XRD and XPS, as shown in Table 7. For In,O,--CeO, only diffraction patterns of CeO, (fluor-

101

ite type) were observed, although the catalyst contains 10% In,O,. This means that indium is highly dispersed and no In,O, phase was observed in the mixed oxides. The binding energy of In (3d,,,) for the mixed oxides was observed at 444.4 eV, the value of which is higher than that at 443.3 eV for metallic indium. This result indicates that indium in the mixed oxides is not reduced to the metallic state and retains the higher oxidation state under the reaction conditions. The surface composition of the In,O,-CeO, catalyst was estimated from XPS data and found that the surface ratio of In/Cc is close to the bulk ratio, which supports the fact that the added indium is uniformly dispersed in CeO,. At higher temperatures, CeOz exhibits high activity and selectivity for ethylene formation (Table 6). These results suggest that the active sites for hydrocarbon formation may be Ce3+ on CeO, in the In,O,-CeO, catalyst. Since the difference of activity for hydrocarbon formation between CeO, and InpOaCeO, is quite large at lower temperatures, the role of indium may be to produce and stabilize the state of Ce3 +, especially at low temperatures.

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