CO Hydrogenation over Metal Clusters in Zeolites

A. Holmen et al. (Editors), Natural Gas Conuersion 0 1991 Elsevier Science PublishersB.V., Amsterdam

305

CO HYDROGENATION OVER METAL CLUSTERS IN ZEOLITES

M.JIANG, Y.LIU, S.T.WONG and R.F.HOWE Chemistry Department, University of Auckland, Auckland, New Zealand ABSTRACT CO hydrogenation has been carried out over zeolite Y and AEQ-5 loaded with Fe, Mo or Co clusters generated from the correspondingcarbonyl complexes. The activities for hydrocarbon formation and product distributions depend on the exchangeable cations present in the zeolite. INTRODUCTION The catalytic properties of transition metal clusters dispersed in zeolite supports have attracted considerable interest (refs. 1-3). The zeolite allows in principle the possibility of stabilizing high metal dispersions, producing novel particle size effects, and of imposing shape selectivity on reaction products. In the case of CO hydrogenation. studies have been reported of Ru (refs.4.5). Fe (refs. 6-12), Co (refs.13-17), 0 s (ref.18) and Mo (refs.19-21) loaded zeolites. Hydrogenation of CO and CO, over metal loaded AP04-5 sieves has also been described (refs.22.23). The exchangeable cations in zeolites can have an important influence on the properties of metal loaded zeolite catalysts. The role of cations in inhibiting the sintering of noble metal clusters in zeolites has been explored by Sachtler et al. (refs.1.24-26). Zeolite cations can also affect the reducibility of metals in zeolites (ref.27) and participate directly in bimetallic cluster

formation (refs.5,28-31). Bimetallic Fischer-Tropsch hydrocarbon synthesis catalysts on oxide or carbon supports have been investigated, e.g. Fe-Cu (ref.32), Fe-Ru (ref.33), Fe-Co (ref.34),

Co-Mo, Ru-Mo, Rh-Mo, Co-W and Rh-W (ref.33, and Fe-Mn (refs.36,37), as have bimetallic

zeolite catalysts for alcohol synthesis (e.g. ref.38).Hydrocarbon synthesis over zeolites containing more than one transition metal component has not so far been widely studied. We have found that the exchangeable cations in zeolites strongly influence the chemistry of metal cluster formation from transition metal carbonyl complexes (refs.22,39-42). In this paper we report the CO hydrogenation perfomance of the zeolite supported clusters. EXPERIMENTAL Zeolite Nay (Union Carbide) was ion exchanged with Ca2’,Ni’+,Co2*or Cu” from aqueous solution. A1PO,-5

306 was prepared according to the method of Wilson et aL(ref.43). using trielhylamine as the template. The metal carbonyl complexes were outgassed by freezepumping prior to use. CO hydrogenation experiments w e n performed in a pyrex continuous flow micnxeactor. Catalyst samples (typically 0.lg) were prepared in-situ in the reactor. The zeolite support was dehydrated by outgassing in vacuo to 300'C, then exposed at room temperature to the saturated vapour pressure of the metal carbony1 for 24 hours. The catalyst was then outgwed, initially at mom temperatm then stepwise to the final activation temperature (usually 250'C). A 1:1 COH2 mixture (purified by passage over molecular sieve and oxy-traps) was flowed over the catalyst at 1 atmosphere pressure,typically at 20 mL pa minute. Reactor eftluent was directed to a Hewlett Packard HF5890 gas chromatograph (FID, 5m Dumpak column) f u hydrocarbon analysis, or to a Carle GC8700 (TCD,2m zeolite 13X and 2m Pompak Q columns) for CO and CO2 analysis.

RESULTS Table 1 summarizes the compositions of the catalysts studied. Gravimetric adsorption measurements (refs.39,41,42) have shown that exposun of dehydrated zeolite Y to Mo(CO),, Fe(CO), or Co(CO),NO vapour at room temperature causes complete filling of the zeolite supercages (ca. 2 molecules per supercage for Mo(CO), and ca. 3 for Fe(CO), and Co(CO),NO). In A1P04-5complete filling of the available pore volume was found only for Fe(CO),; maximum

uptakes achieved for Mo(CO), and Co(CO),NO were 25% and 50% respectively of the theoretical maximum. As discussed in detail elsewhere (ref.42), the extent of metal loss on subsequent activation by heating in vacuo depends on the particular carbonyl complex and on the zeolite cation. The final metal loadings of the activated catalysts can therefore vary widely. Selected data for CO conversion to hydrocarbons versus time on stream are shown in Figure 1. For the purposes of comparing catalysts, conversions and product selectivities at ca. 12 hours

on stream were chosen (Table 1, Figure 2). The activities of the iron loaded zeolites decreased markedly during the first few hours on stream. Reaction times for FeNaY and FeCoY were extended to 28 hours; FeCoY catalysts remained stable whereas FeNaY showed a further decline in activity over this time scale. Catalyst activities were reproducible to within 25% from one catalyst preparation to another. Regeneration of used catalysts by heating in flowing hydrogen at 350'C gave activities comparable to those of the fresh catalysts (Table 1). FeNaY and FeCoY catalysts were also studied at a GHSV of 800 hi'; the conversions were correspondingly higher (e.g. FeCoY, 5% conversion at 12 hours on stream). Cobalt and molybdenum loaded zeolites showed less decline in activity with time on stream (Figure 1). The zeolite cations also influenced the activity of these catalysts. The largest cation effect was found with molybdenum loaded zeolites; MoNiY catalysts gave conversions 10 times those of MoNaY. The metal loaded A1P04-5 catalysts gave lower conversions than the corresponding zeolites. The lower metal content of these catalysts means however that formal turnover numbers (CO converted per metal atom) are comparable to those of the zeolite catalystshn and cobalt loaded CuY zeolites gave a low initial activity for CO hydrogenation which declined even further with time on stream, whereas the corresponding Cay zeolites gave

307

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REACTION TIME (hours)

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10

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15

20

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REACTION TIME (hours)

AlPO,-5 Zeolites, GHSV=6000h-' A Yo-COY

8 Yo-NiY 0 Yo-NaY

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0 A1P04-5

:: 0.3 a e I

0 Mo-AIP04-5 0

Pe-AIP04-5

..88.

REACTION TINE (hours)

REACTION TIME (houlr)

HGURE 1. CO Conversion to Hydrocarbons Versus Time on Stream

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30

308

100

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80

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HC C1 C 2 C 3 C3= C 4 C4= C 5 C 5 = C>S HYDROCARBON PRODUCT

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=Yo-NtY mYo-coY mY0-N.Y

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Zeolites, CHSV=6000h-' ca. 12 HOURS

80-

60-

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HYDROCARBON PRODUCT

Mo Zeolites, 25OoC, CHSV=3000h-' AT ca. 12 HOURS r

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100

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FIGURE 2. Hydrocarbon Product Distributions (Mole % CO basis)

=

05 2 3 K .

Fe-AIPO -5 5 7 3 K . Yo-AIPd,-S 5 7 3 K . AIP04-5

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TABLE 1 Summary of Catalyst CO Hydrogenation Performance (at ca.12 hours on seeam). Conversion’

Formal M N b l(r s-’

totalHC

0.2

1.o 15 2.0 10 4.0 8.0 10 7.0 0.4 110 6.0 7.0 9.0 4.0 0.2 16

30

%

MoNaY MoNiY MoCoY Mom5 CoNaY CoNiY CoCOY CoCaY CoCUY Corn5 FeNaY FeNiY FeCoY FeCaY FeCuY FeAP5

-

19 19 19 0.1

20

19 19 19 19 0.2 14 18 11 12 20 0.3

56 23 17 56 23 17 20 24 56 23 17 20 24

3.0 0.4

0.We)

OM(0.61) 1.8(1.7) 2.5 131.0) 0.08 8.0(f) 0.8(1.0) lA(1.7) 1.O(OA) O.S(O.25) 0.05 0.2@)

15

56 $5 33 70

Selectivity’ HC>CH, alkene./Wed

62 21 57 54 77 48 48 59 53 52 63 30

1.7 4.8 2.0

80 78

2.3 0.75 4.0

50 84

1.3

0.5

0.2 1.0 6.8 1.2 3A 2.0

.o

1

1.7

a) conversion to hydrocarbons, 250’C, GHSV=3000 (data in parentheses for regenerated catalysts) b) moles of CO converted to hydrocarbons per mole of M, c) mole 96 CO d) for C2 to C5 hydrocarbons r) at 3WC, GHSV=854 e) at 3WC, GHSV=6000 g) at 250’C. GHSV=6000

conversions comparable with Nay. Product distribution data are presented in Figure 2 and Table 1. The effects of zeolite cations

on hydrocarbon product distributions were similar for all 3 metals. Ni2” enhanced the methane selectivity compared with Nay, while both Ca” and Co” decreased the yields of higher hydrocarbons and lowered the alkene to alkane ratios (relative to Nay). In the case of metal loaded AlP04-5, the

carbon number product distributions were similar to those of the

corresponding metals i n zeolite Nay, although the alkene:alkane ratios were significantly lower. DISCUSSION The possible effects of zeolite cations on CO hydrogenation performance of metal loaded zeolites can be classified as electronic promotion (which may be indirect alteration of metal cluster reactivity through long range electrostatic effects within the zeolite cavities, or direct modification through bimetallic cluster formation), or structural promotion (inhibition of migration of metal to the external surface of the zeolite). Relative to Nay, Ni2+enhances total CO hydrogenation activity, but gives a high selectivity

to methane. Preliminary EXAFS data(ref.44) show no evidence in either Mo or Ni EXAFS for Mo-Ni or Ni-Ni bonding in MoNiY catalysts following activation in vacuo at 2 W C or 40O.C. The activity and selectivity patterns of the metal loaded NiY catalysts suggests however that zerovalent Ni is produced under CO hydrogenation conditions. The added transition metal may

310

catalyze the reduction of Ni2+. NiY alone is not appreciably reduced in CO or hydrogen at 250’C, and gave negligible yields of hydmarbons in the absence of Mo, Fe or Co at this temperature. Previous examples of such catalyzed reduction include PtCuY zeolite (ref.28) and silica supported FeCu catalysts (ref.32). Enhancement of methanation in a bimetallic zeolite has also been reported by Jacobs (ref.45) for RuNiY. Cu2+cations do not have a beneficial effect on CO hydrogenation performance of Fe and Co loaded zeolite Y. EPR studies (ref.42) have shown that the characteristic signal of Cu” in CuY is immediately removed on exposure to metal carbonyl vapour at room temperature, and can only be restored by subsequent heating in oxygen, suggesting that reduction of Cuz+ and corresponding oxidation of the added transition metal during catalyst preparation is responsible for the resulting low activity for CO hydrogenation. The influence of Co2+cations is less dramatic than that of Ni2’ or Cu2+.Inthe case of Fe loaded zeolites, X P S experiments show that ion exchange with Co” inhibits the migration of Fe to the external surface of the zeolite occurring with FeNaY (ref.40). Jacobs et al. (ref.10) have shown that such migration of Fe modifies the CO hydrogenation activity of FeNaY zeolites; during CO hydrogenation most of the Fe in FeNaY was shown to be transformed to an external carbide phase. The FeNaY zeolites subjected to prolonged CO hydrogenation tests by Zwart and Vink (ref.12) presumably contained external Fe. The enhanced activity and greater stability of FeCoY catalysts (Figure 1) may therefore be due to structural promotion i.e.inhibition of sintering by Co”. Bimetallic FeCo clusters on alumina show enhanced activity for CO hydrogenation compared with Fe alone (ref.34), but X P S measurements show no evidence for reduction of Co in FeCoY (ref.40); bimetallic cluster formation seems unlikely in this case. Migration of metal to the external surface does not occur during activation of Mo and Co loaded zeolites. The observed promotion effects of Co” in MoCoY and CoCoY must therefore be attributed to electronic interactions. EXAFS measurements have so far produced no evidence for Co-Mo bond formation in MoCoY (ref.44), and the observed suppression of alkene production in MoCoY and CoCoY is contrary to the high alkene yields reported for bimetallic MoCo-alumina catalysts prepared by sequential adsorption of Mo(CO), and Co,(CO), (ref.35). Decomposition of Co(CO),NO in NaY produces a mixture of zerovalent Co and Co2+(refs.40,42).Thehigher ratio of oxidized to zerovalent Co in CoCoY appears to enhance activity but suppress alkene formation; we attribute this and the corresponding promotion of MoCoY to an indirect electronic influence of Co2+.Ca” cations modify the CO hydrogenation performance of Fe and Co loaded zeolites only slightly; this may be due to removal of Na’ cations which promote alkene formation(ref.46). Differences between product distributions in MoNaY, FeNaY and CoNaY and the corresponding metal loaded A1PO,-5 supports which contain no alkali metal cation are also consistent with this explanation.

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We note finally that the structural and electronic promotion effects observed in these metal loaded zeolite catalysts prepared by unconventional methods may be found also in conventionally prepared catalysts, and warrant further investigation. REFERENCES 1 W.M.H.Sachtler, "Chemisq and Physics of Solid Surfaces VZII" (R.F.Howe and R.Vanselow,eds),SpringerVerlag,Heidelberg.l990, p 69. 2 P.A.Jacobs, "Metal Clusters in Catalysis'' (B.C.Gates,L.Guczi and H.Knozinger,eds),Elsevier Studies in Surface Science and Catalysis 29 (1986), 357. 3 G.A.Ozin, ChemRev. 89 (1989). 1749. 4 H.H.Nijs and P.A.Jacobs, J.Catal. 66 (1980). 401. 5 D.J.Elliot and J.H.Lunsford, J.Catal. 57 (1979). 11. 6 D.Ba1livet-Tkatchenko and G.Coudurier, 1norg.Chem. 18 (1979). 558. 7 D.BaIlivet-Tkatchenko et al., ACS Symp.Ser. 152 (1981). 187.

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