Higher alcohols synthesis on cobalt based model catalysts

Catalysis Today, 15 (1992) 101-127 Elsevier Science Publishen, B.V., Amsterdam

101

HIGHER ALCOHOLS SYNTHESIS ON COBALT BASED MODEL CATALYSTS J.A. Dalmona, P. Chaumetteb and C. Mirodatosa a) Institut de Recherches sur la Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne Gdex, France b) Institut Francais du P&role, 1 & 4 Avenue de Bois-Pr&u, 92506 Rueil Malmaison Gdex, France

Abstract A review of the main features of higher alcohols synthesis on model cobalt-based catalysts is presented. The specific effects of modifiers such as alumina, copper, zinc and sodium, typical of patented formulations, are analyzed at each step of the catalyst synthesis: preparation, activation and catalytic test. On the basis of these effects and other data obtained from the literature, the nature of the active sites is discussed in relation to reaction mechanisms. 1. INTRODUCTION Despite the current relatively low price of oil, there is still at present a major interest in syngas conversion processes for various reasons. First of all, the relative instability of crude oil prices is a major incentive to look for non-petroleum based processes. Second, natural gas resources have increased and, if expressed in oil energy equivalent, now constitute around 95% of crude oil resources around the world, cheap gas being available in remote areas [l]. A third incentive comes from the decisions to remove lead from motor fuels (European countries) or to add oxygenates to motor fuel for environmental reasons (Clean Air Act in the U.S.). In this context, syngas chemistry is now considered as the main way of natural gas upgrading, and in particular alcohol synthesis from syngas looks attractive. Although at present MTBE has taken the lead on the octane booster market, in the future larger amounts of oxygenates may be necessary for motor fuel blending, and then syngas chemistry may help to provide these products. It is thus not astonishing that alcohols synthesis from syngas has been extensively studied by many companies [1,2]. The catalytic systems developed for this transformation are of different types: alkalized methanol synthesis catalysts [1,3-61, rhodium based catalysts which are selective towards C2 oxygenated molecules [7,8], molybdenum sulphide based catalysts [9-l 11, and copper-nickel or copper-cobalt based catalyst which may be classified as modified Fischer-Tropsch catalysts [ 1,2,12].

1992 Elaevier Science Publishers B.V.

102

It is the aim of this review to focus on copper-cobalt based catalysts since a great deal of work has been performed by the “Institut Fraqais du P&role” (IFP) or by research groups working in collaboration with IFP to understand how alcohols can be selectively formed on these particular formulations [13-151 and since more recently, other laboratories have paid attention to these catalytic systems [2,17,18]. In order to understand the role of each component of the catalyst, we have considered model CuCoZnAlNa formulations, and tried to understand through these formulations as well as through published data the effect of the addition of Cu,Zn,Na additives to a typical cobalt-alumina Fischer-Tropsch catalyst. These effects have been looked at after each step of the catalyst synthesis: preparation, activation, catalytic test, in order to give a more precise idea of the catalytic sites and of the overall alcohol synthesis mechanism. 2. PREPARATION AND ACTIVATION 2.1. Effect of copper addition The effect of copper addition to cobalt aluminate has been studied on a series of catalysts prepared according to the coprecipitation technique described in refs 13, 14, 20. Prior to any characterization, these samples were calcined at 450°C under flowing air for 3h. Keeping the total metal loading almost constant, the Cu/Co atomic ratio was varied from 0 to about 2, a cobalt-free copper aluminate being also prepared for comparison (Table 1). Table 1 Chemical composition of the Al-Co-Cu calcined precursors as reported in ref. 22.

Catalyst

COICU atomic ratio

Al-Co

weight percentage (%) Al

co

cu

30

30

0

Al-Co-Cu

2 1 0.5

30 27 28

19 15 12

9 16 22

Al-CU

0

26

0

31

Cu effect on calcined precursors. A first effect of copper, observed on calcined precursors, is to modify significantly the dispersion of the cobalt aluminate reference sample (Fig. 1). In particular, the XRD pattern of the copper-free Al-Co precursor exhibits

At-Co

Al-Co-Cu (CQKu= 2)

At-Co-Cu

(Co/Cu=1)

At-Co-Cu

(Co/Cu=O,5)

Al- Cu

1

20

*

,

30

.

,

1

*

40

d0

28 (degree1

30

’

i

’

?O

---

Fig.1 : XRD patterns of Al - Co - Cu oxide precursors, from [22].

essentially narrow lines characteristic of a spinel-type Co304 and CoAl204 structure, suggesting the coexistence of two phases, rather badly dispersed, as discussed elsewhere [14,23]. After copper addition, these lines are broadened and weakened, the more the ratio Co/Cu is decreased. Since no CuO phase is detected, at variance with the CuAl2O4 reference sample, it is deduced that copper promotes the dispersion of the cobalt containing mixed oxide phase, without any phase separation. It should be noted that the phase separation observed for the case of CuAl2O4 is consistent with the poor affinity of Cu2+ ions for tetrahedral sites combined with the preferential occupancy of octahedral sites by A13+ ions in an alumina structure [24]. This beneficial effect of copper on phase dispersion has been confirmed by XPS measurements (T.able 2): bulk and surface atomic ratios Cu/Al are found to be very close for the mixed oxides, indicating that copper is inserted into the aluminate matrix, while for the case of Co-free Al-Cu sample the large surface enrichment with copper has to be related to the phase separation and the formation of CuO [21,22]. Table 2 Bulk (chemical analysis) ahd surface (XPS) atomic ratios of calcined precursors [21,22].

Catalysts

COKU bulk

surface

Al-Co Al-Co-Cu

Al-Cu

2.3 1.0 0.6

1.5 0.6 0.5

Co/Al bulk

surface

0.46

0.38

0.28 0.24 0.19

0.18 0.13 0.14

Cu/Al bulk

surface

0.12 0.24 0.33

0.12 0.20 0.28

0.51

0.25

This complex and successful combination of Co and Cu ions in spinel-like structures is considered to be the key step leading to the tight interaction observed subsequently between the two elements in the activated catalysts, which are selective towards alcohol synthesis. In this respect, the studies carried out on supported bi-metallic Co-Cu catalysts, prepared by usual impregnation procedure without an initial formation of mixed oxide precursors [17,25], can hardly be compared with the present results obtained on coprecipitated catalysts. Cu effect on catalyst reduction. A second major effect of copper addition observed during the step of catalyst reduction is to promote the reduction of the cobalt phase, both by increasing its reducibility and by improving the dispersion of the metallic phase.

106

These effects have been clearly demonstrated via temperature programmed reduction experiments (TPR) (Fig.2) and magnetic measurements (Fig.3). As shown on the TPR curve (Pig. 2a), it has been stated first that the reference cobalt aluminate Al-Co was reduced in several steps, in agreement with refs 13b, 14,19,22: i) at T < 6OO”C,the reduction of the oxide phase proceeds via the following steps Co304 + Hz --> 3 Co0 + H20 Ill CoO+H2-> Co”+H20 121 ii) at T > 6OO*C, the stable ions of the ruminate are then prog~~vely reduced as follows c0Al2O4 + H2 --> CO’ f Al203 + H20 131 From the TPR curve (Fig.lza), the respective amounts of Co304 and C0Al204 may be estimated to be 20 and 80% respectively. The reducibility of Co ions is also represented in Fig. 3a which reports the extent of reduction as a function of the temperature of reduction (each point resulting from an isothermal reduction for 15h). It is apparent that some ions, probably those in tetrahedral positions, require a long time at T>600’C to be reduced. This can be confirmed since the largest fraction of the Co ions (aluminate) requires temperatures between 400 and 600°C to be reduced, but a small fraction of the Co ions is difficult to reduce and requires r~uction tern~~tu~s higher than 600°C. Addition of copper to Al-Co greatly modifies the cobalt ~ucib~ity. On the corresponding TPR curves (Fig.2b,c,d), a large proportion of the Co ions in the aluminate is reduced at lower temperature (shift to the left of the high T peak), this effect being more marked for the less stabilized spine1 ions i.e. in octahedral positions. It may be seen also that this shift to lower temperature is independent of the copper content, the effect of copper being only to reduce the amount of cobalt ions in the aluminate phase. The reduction of Co304 is found to be coincident with that of the Cu ions and no precise information may be obtained from the TPR studies. From the reduction-versus-temperature curve obtained by magnetic m~su~rnen~ (Fig.3a) it may be seen that the reduction of the readily reducible Co304 ions is also improved by copper addition. Again it is clear from Figure 3a that this effect of copper on the Co reducibility does not depend on the copper content. This feature has to be considered for the optimization of copper addition in the final formulations. Together with this large effect of copper on reducibility, a striking effect on cobalt dispersion appears in Figure 3d: poorly dispersed when unpromoted, the cobalt is reduced in a well dispersed metallic state after copper promotion (average diameter always < 5nm, even for totally reduced samples). This double effect of copper addition on r~ucibili~ and dispersion has been ascribed to different origins: i) reducibility: copper which is reduced at low temperature (from 200 to 500°C) acts as a catalyst for cobalt reduction by providing activated hydrogen [18], ii) dispersion: the improved metallic dispersion of promoted cobalt originates from the fine dispersion of the oxide precursors, as shown previously by XRD patterns. Let us point out that this effect on Co dispersion clearly depends on the preparation procedure since all attempts to prepare Co-Cu compounds by simple impregnation on

106

Effect of sodium

Effect of copper

Effect of copper and sodium

Al-Co

Al-Co-Ci-Na +l% df’

!M

f’

mm5oom

Temperature

Na

-

-

lkl

of reduction PC)

c AL-Co-Cu-Na Al-CU

Al-Cu-Na 2%

Ll

Na

Temperature

Al-Co-Cu-Na +I%

Na

l!kL (ml

i

mmm

+6% Na

of reduction PC)

Fig.2 : TPR profiles of Al - Co - Cu - Na model catalysts, from [22].

b700

-

Effsct of copper

Effect of sodium

Effect of copper

and sodium

If

”

^.

supports instead of coprecipitation in an alumina matrix led to poorly dispersed metallic phases (see for instance Perrichon et al. with Co-Cu on MgO, Ce02 and Zr02 [25] or Baker et al. with Co-Cu/Si02 who obtained average particle diameter larger than 20 nm

wm Cu effect on bulk and surface composition of activated catalysts. A preliminary statement that can be made is that the bulk and surface composition of Al-Co catalysts strongly depend on the reduction conditions chosen for the catalytic test of alcohol synthesis. Thus, in the study carried out by Blanchard et al. [271, the unpromoted cobalt aluminate precursor was reduced under flowing CO/H2 mixture up to 200°C only. Although CO is expected to be a more efficient reducing agent than hydrogen, it is likely that this low reduction temperature led to a very low content of metallic cobalt, according to our experimental observations (Fig. 3). In a similar manner, Baker et al. [25] reduced their Cu/Co/Zn/Al mixed oxides at 250°C for one hour under flowing H2, which, as stated by the authors, allowed only a small fraction of the cobalt to be in the metallic state (around 10% according to our results). The choice of the reduction temperature seems therefore to be one of the sensitive parameters for this class of catalysts. For the studies carried out in our laboratory on model catalysts, the mixed oxides were reduced at 500°C under flowing hydrogen (3.6 1 h-l) for 15h. According to the relationship between the degree of cobalt reduction and the temperature of reduction (Fig. 3), it is apparent that around 70 % of the cobalt phase is reduced at 500°C without Cu promotion and at least 10% more after Cu promotion. It can therefore be concluded that a much higher content of metal is present in our operating catalysts than for those system previously quoted. It is however also obvious that the parameters that are important for the reaction of CO/H2 will be the surface composition of the activated catalysts and the further changes induced by the reaction itself both during the start-up and subsequently during the stabilized operating conditions. A major question that arises at this point concerns whether a specific surface interaction develops between cobalt and copper, which are the two main components of such catalysts. Bailliard-Letoumel et al. [28] first observed by magnetic measurements that bulk Co-Cu alloy was never formed whatever the relative concentration of the two elements or the temperature of reduction. This observation is in agreement with the poor copper solubility revealed by the phase diagram (limited at around 10% for that case). Second, they observed i) a shift of the CO-Co FTIR band towards a lower frequency after copper addition (Fig. 4A), this shift of around 50 cm-l being independent of the CO coverage (Fig. 4B) and ii) a reverse shift of the CO-Cu band towards higher frequency (from 2105 cm-l for pure copper to 2125 cm-l for copper modified by Co). On the basis of these observations, the authors have concluded that a strong Co-Cu surface interaction was developing on the Co particles, with an electron donation from Cu towards Co. They proposed a type of “cherry model” with a core of pure cobalt surrounded by a shell of Co-Cu alloy. Part of the added copper was shown to remain in a metallic state free from the Co interaction. It may be added that this surface interaction still exists on fully reduced catalysts, after H2 treatement at 700°C [22,28],

wavanumber

2400

2200

2000

1600

1600

100 lempenture

A

cm-’

200 of

dcsorptlon

PC)

0

A : Infrared spectra of CO adsorbed on Al - Co and Al - Co - Cu after Fig.4. adsorption of 4 kPa CO at 25 “C (full line) and after desorption at 25 “C (dotted line). B : Change in position of the J CO band adsorbed on cobalt versus temperature of desorption after CO adsorption at 25 “C on Al - Co and Al - Co - Cu, from

WI. Concerning the influence of the copper content on this Co-Cu interaction, it has been shown that for high Cu content (ColCu = 0.5) the electronic effect upon alIoying revealed by the IR shifts tended rapidly to stabilize, as expected from the limit of solubiity of Cu in the Co phase. Once again an optimization of the copper addition is required since a too high content will i) not improve the Co promotion and ii) result in a larger unalloyed Cu phase, mainly yielding methanol from CO hydrogenation [6], at the expense of the higher alcohols selectivity.

110

On the basis of such a surface configuration, speculation about the nature of the active sites has been developed in connection with the catalytic data, as discussed in a subsequent section. It may be worthwhile to note here that no definitive experiments have shown that this surface alloy is stable during the reaction. “In situ” IR studies (i.e. in CO/H2 reaction conditions) would probably answer this delicate question. Another point to discuss in this section is that the previous observation of a surface alloy on model Al-Co-Cu catalysts has not been reported on other types of Co-Cu catalysts such as Co-Cu supine on MgO, CeO2 and ZrO2 B5]. Though homogen~usly dispersed (as shown by STEM),*these bimetallic phases did not present any IR evidence of alloying. As already noted, only very large particles were obtained on those supports; this probably attests to a large sintering of the oxide phases during thermal treatment, which would lead to some phase separation, thus explaining the absence of alloyed metallic phases. In contrast, Baker et al. [18] have deduced from the observation that propane hydrogenolysis was inhibited after copper addition to cobalt supported on silica that Co and Cu were alloyed, in agreement with TEM results. However, as stressed by the authors, only very large particles were obtained, at variance with the model catalysts prepared by cop~ipi~tion. 2.2. Effect of sodium addition The effect of sodium addition will be first discussed for unpromoted Al-Co catalysts, then after copper addition. In all cases, alkalizing has been carried out by incipient wetness impregnation of calcined oxides with an aqueous solution of Na2C03, followed by a second air calcination at 350°C [22]. It should be noted however that other ways to alkalize the oxides (such as during the initial coprecipitation) may lead to very distinct effects. Table 3 gives the composition of the alkalized oxide precursors. AI-Co

catalysts.

Na effect on ca&d AI-Co precursors. No significant changes in the XRD patterns presented in Fig. 1 have been noted after sodium addition, which indicates that no mixed oxides were formed between the mixed Al-Co and the sodium carbonate, probably just coating the surface, as suggested by XPS studies [22]. Na effect on Co reduction and dispersion. As observed after Cu addition, alkalizing AlCo catalysts globally promotes the reducibility of Co ions. However, from the TPR curves (Fig. 2f,g), it is apparent that this effect concerns only the reduction of the less stable cobalt ions of the aluminate (~~h~r~ site> since i) the most stable ions in the tetrahedral sites remain unaffected and ii} almost a reverse effect is observed for the easily reducible Co203 phase. From the extent of reduction determined by magnetic measurements (Fig. 3b) it may be determined that this effect on reducibility depends on the sodium content, which was not the case for copper.

Table 3 Composition of the alkalized oxide precursors.

Weight percent in the precursor

Catalysts % Al

%Co

%Cu

%Na

Al-Co

30 29 26 22

30 29 22 22

0 0 0 0

0 2 7 12

Al-Co-Cu (Co/Cu = 1)

27 27 25 24

1.5 14 13 13

16 16 14 14

0 1 4 7

Al-CU

26 26

0 0’

31 31

0 2

In addition to the the promoting effect on reducibility, sodium addition tends to exhibit a deleterious effect on the Co dispersion (Fig. 3e), again in a way somewhat proportional to the sodium content, at variance with the positive effect of copper. A chemical explanation may be tentatively proposed for rationalizing these various effects: First, it has been observed by IR spectroscopy that sodium addition tended to neutralize the protonic acidity of the support after reduction at 500°C with the formation of stable carbonate and formate surface groups (IR bands between 1590 and 1380 cm-‘). As already proposed for incompletely reduced systems such as supported iron or nickel [29], the redox equilibrium Co2+ + H2 < == > Coo + 2H+ 141 would be displaced towards the metallic cobalt after Na addition. This would account for the promoting effect on Co reducibility. Second, the change in the surface composition would also decrease the interaction between cobalt particles (oxide and/or metal) and the support, for instance via a surface wetting by a Na2CO3 film; this would favour the sintering of the cobalt phase. From the observation of these two opposite effects of sodium on Co reducibility and dispersion, we may understand how delicate is the right balance of sodium concentration in the final catalyst composition. Na effect on surface composition for activated Al-Co catalysts. IR spectroscopy shows that the effect of Na on the Al-Co model catalyst activated at 500°C strongly depends on the alkali content [22].

112

At low content (around 2 wt. %), a new CO band at a low wavenumber (1790 cm-3 (Fig. 5b) may be ascribed to a direct interaction between CO adsorbed on metallic cobalt and sodium ions, forming the surface complex Co-CO***Na. This type of surface interaction between a strong base such as an alkali ion and a carbonyl group has been also proposed for higher alcohol and oxygenate synthesis over Cu/ZnO/A1203 catalysts promoted with cesium [38]. At high Na content (7 wt. %), the IR spectrum was corn letely modified with the appearance of a new set of bands between 2040 and 1880 cm-P (Fig. 5~). This se.t of bands, also observed for Co catalysts supported on basic oxides like magnesia [25], was ascribed to cobalt tetracarbonyl species [Co(CO),]- possibly formed from the dismutation of Co2(CO)8 species as proposed by Horns et al. on partially hydroxylated magnesia [33]. The ability to form carbonyl species under CO atmosphere will be related later to sintering phenomena occurring in the course of CO hydrogenation. At this point, we will stress the fact that high sodium contents tend to stabilize carbonyl species on the surface. Al-Co-Cu

catalysts.

Na effect on calcin~Al-Co-Cn precursors. As for unpromoted Al-Co solids, the alkalization of copper-promoted mixed oxides did not affect their Ibulk structure (no change in XRD patterns), but modified their surface composition by carbonate coating. Na effect on Co and Cu reduction in AIXo-Cu catalysts. The isolated effect of sodium on copper reduction has been studied by comparing Al-CU and Al-Cu-Na aluminates (Table 3). From TPR curves (Fig. 2h,i), it was shown that the sodium coating tended to hinder the reduction of copper, as observed previously for the readily reducible Co304. The effect of sodium on Al-Co-Cu oxides reduction may be considered as a combination of the single effects of Cu and Na respectively: from the TPR curves (Fig. 2j-m), it is apparent that, as the Na content increases, i) the reduction of the CuO and Co304 (low T peak) is hindered, ii) the reduction of the octahedral ions of the aluminate is promoted, and iii) the reduction of the stable tetrahedral ions remains unaltered. Similarly, it may be observed from the curves demonstrating the extent of Co reduction as a function of the reduction temperature (Fig. 3c) the positive and negative effects of sodium superimposed with the effect of copper, depending on the temperature range. Regarding the effect on Co dispersion, it may be seen that the positive effects of copper addition are still largely prevailing up to 6OO”C, i.e. in the range of the standard activation for catalysis; at higher temperature, the negative effects of Na which promotes the Co sintering (via the surface mobility of metal particles) lead to the poorest dispersions ObSeNed (Fig. 30. Na effects on Al-Co-Cu surface composition. CO adsorption followed by IR spectroscopy revealed once again the influence of the sodium content on the state of the surface for Al-Co-Cu samples activated af 500°C [22]. At low Na content (up to 2 wt. %), both the Co-Cu surface alloy and the direct interaction Co-CO*.*Na were still observed, while at higher Na content (e.g. 7 wt. %), the formation of stabilized carbonyl species appeared to become the main surface process. It may be anticipated that such a

Al- Co

Al-CU

a

.

..’

.I..

_..

5

b

r 23C -

2ooo

1700

1400

Wave number (cm-l)

Fig.5 : Infrared spectra of CO adsorbed on Al - Co, Al - Cu and Al - Co - Cu model catalysts, versus sodium content, after activation at 500 “C. Full lines : after 4 kPa CO adsorption at 25 “C ; dotted lines : after desorption at 25 “C, from [22]. (d) without Na (a) without Na (f) without Na (e) 2 % Na

114

sensitivity of the surface interactions to the Na content will largely control the catalytic behaviour of the alkalized model catalysts. 2.3. Effect of zinc addition As for the other promoting elements, the effect of zinc addition has been studied on a series of Co-based catalysts prepared according to the coprecipitation method already described [20]. Zn has been introduced in different solids containing Al-Co, Al-Co-Cu and Al-Co-Cu-Na in order to investigate the possible interactions between the metals in the formulation [28]. All the solids were air-calcined at 450°C before characterization and activation. Table 4 gives the chemical composition of the prepared solids. Table 4

Composition (wt% metal) of the precursors [28]

Precursor

co

Al-Co Al-Co-Zn

21.1 8.7

cu

Zn

Na

Al

21.7

19.5 16.9 18.5 16.2

Al-Co-Cu Al-Co-Cu-Zn

7.9 3.3

14.5 7.6

19.7

Al-Co-Cu-Na Al-Co-Cu-Zn-Na

7.3 3.0

13.9 7.4

19.0

1.7

17.8 15.2

Effect of Zn addition on calcined precursors. XRD studies of the calcined precursors did

not permit the determination of whether C0Al204 or ZnA1204 structures were present in the Zn containing solids (their patterns are very similar). However it is known that ZnA1204 is thermodynamically favoured by comparison with C0Al204 [24]. Moreover, XPS measurements carried out on similar formulations have shown that A13+ and Zn2+ were effectively combined in a spine1 structure [14]. It is also seen on XRD patterns that the presence of zinc causes the lines of Co304 present in the Zn-free samples to disappear, suggesting that the zinc addition induces a better dispersion for this phase. Effect of Zn addition on catalyst reduction. The activation of the catalysts has been performed under flowing hydrogen at temperatures up to 65O”C, the extent of reduction of cobalt and the average particle size of Co being obtained by means of magnetic measurements. At variance with Cu, addition of Zn did not modify the extent of reduction in a significant way; however the dispersion was clearly improved by Zn addition whatever the composition of the catalyst (Al-Co, Al-Co-Cu or Al-Co-Cu-Na), as indicated in table 5

P81.

115

Table 5 Average particle diameter (in nm) after reduction at 77313 (fresh) and after the catalytic test (used).

Average particle diameter (nm)

Sample AlCo

AlCoZn

AlCoCu

AlCoCuZn

AlCoCuNa

AlCoCuNaZn

3.9

2.8

fresh

(20%)*

5.0

4.2

3.2

Used

(25%)*

7.2

4.7

3.2

2.9

o*:

non superparamagnetic samples. These data give the percent of Coo forming particles larger than ca. 15 nm. This promoting effect of Zn addition on the average particle size has been attributed to the better dispersion of Co304 in the Zn-containing samples, as deduced from KRD experiments. As far as the the extent of cobalt reduction is concerned, a promoting effect of Zn may be expected, Zn2 + ions being favoured with respect to Co2+ in the competition for the occupancy of the tetrahedral sites in the spine1 structure [24], which would limit the amount of non-reducible Co. To account for the observed non-promoting effect of Zn, it has been proposed that, due to the rather low calcination temperature used in the preparation, the samples have probably not completely undergone the solid-solid reaction leading to mixed oxide formation [28]. Effect of Zn addition on surface interaction. Infrared spectroscopy of adsorbed CO did not indicate important modification of the surface of the Al-Co(CuNa) samples after Zn addition, suggesting a limited interaction between Co(CuNa) and Zn. However magnetic measurements performed during H2 adsorption-desorption experiments revealed that zinc could prevent some reoxidation of the metallic cobalt by blocking the redox equilibrium between the Co species and the hydroxyl groups of the support, in a way somewhat similar to the Na effect described before (eq. [4]). 3. CATALYTIC PROPERTIES In this section we present the main catalytic trends observed during CO hydrogenation carried out on the series of model catalysts previously described. These results will be discussed further in the general discussion section in connection with the main physico-chemical features related both to the activated and to the reacted materials. A large amount of research [ 1,201 has permitted the observation of the influence of the “in situ” operating conditions on the catalytic performances of these complex catalysts.

116

Thus, it has been shown that a given Co-Cu system could either lead to the alcohol or to the hydrocarbon synthesis, depending on the start-up procedure [34]. The procedure selected for the present model catalysts is reported in [22]. We have compared the catalytic properties obtained at the same step of the catalytic test: 25O”C, under 50 bar of CO + 2 H2, with a GHSV of lo4 h-l and after 22h on stream. Table 6 reports the catalytic activity, the selectivities towards CO2 hydrocarbons and alcohols, the yields of methanol and higher alcohols, which are the target products of the reaction and the chain growth coefficient, calculated from the Anderson-Schulz-Flory relationship (mainly for hydrocarbon formation, since the series of alcohols was generally not long enough to determine accurately the corresponding coefficient) [22,28]. Fig. 6 presents a schematic overview of the productivities (except for the zinc series, which was not studied under strictly identical conditions). Al-Co reference catalyst (Table 6, line 1; Fig. 6a). This solid, initially the most active of the whole series, deactivated rapidly due to a critical sensitivity to the temperature changes, which rendered it hard to handle in the catalytic test. Only saturated hydrocarbons were observed, with a dominant production of methane. Al-Co-Cu catalysts (Table 6, lines 3,4; Fig. 6d). The addition of copper has been shown i) to decrease the overall activity of the solids and stabilize it, this effect being little dependent on the Cu content and ii) to promote the formation of alkenes and higher alcohols. Let us emphasize that if the pure copper aluminate led, as expected, to a selective formation of methanol, this product was only observed as traces in the reactor effluents on Al-Co-Cu catalysts. It was also observed that the productivity in higher alcohols was maximized for a ratio Co/Cu = 1. Al-Co-Na catalysts (Table 6, lines 5,6; Fig. 6b,c). The alkalizing of the unpromoted AlCo material revealed two main features: i) the activity was significantly decreased and stabilized in proportion to the added sodium, ii) at low content (2%), the alcohol formation was mainly orientated towards methanol, while at higher Na content (7%), higher alcohols were formed at the expense of methanol. A slight amount of acetaldehyde was also detected together with higher alcohols, as recently observed by Matsuzaki et al. on Co&i02 systems modified by alkali [40]. In all cases a tendency to form larger amount of CO2 after alkalizing was noted, in good keeping with the general observation that the water-gas-shift reaction is enhanced by alkali [38]. Al-Co-Cu-Na catalysts (Table 6, lines 7,8; Fig. 6 e,f). The double promotion by copper and sodium gave interesting selectivities towards higher alcohols, optimized productivities being achieved for low sodium content and Co/Cu ratio near one. Again, the stabilizing effect of Na has to be noted. Zn effect (Table 6, lines 9,lO). The main effect of zinc was to stabilize the catalytic activity, limiting the ageing observed on zinc-free samples. Concerning selectivity, no major change was noted on Al-Co samples. The best results concerning both the stabilizing effect and the selective production of the mixture methanol/higher alcohols were obtained, as expected, for the triply promoted samples, like the formulations patented by IFP .

-

(%I

16

218

33

142

88

221

142

205

27

3

24

9

42

14

7

4

6

12

45

90

51

67

35

75

74

78

9

88 5

27 15

7 30

25 22

24 23

17 15

11 67

19 30

18 30

85

0 68

12

0.3

0.1

0.2

10

10

0.1

0.1

84

0

2.0

0.7

0.1

0.3

1.1

29

0.1

0.2

56

0

CH3OH

HC

1.9 0.45

11 0.59

6.2 0.43

25 0.45

11.9 0.41

5

18 0.46

27 0.45

0.3 0.33

0 0.59

C2+OH

Productivity a(chain (mg/gcat/h) growth)

TAHLE 6 : Main catalytic properties of Al-Co-Cu-Zn-Na model catalysts in the CO/H2 reaction at 250 OC after 22 h on stream.

0.16

0.41

lO.Al-Co-Cu-Zn-Na (2%)

0.40

0.91

S.Al-Co-Cu-Na (4%)

-

-

-

-

9.Al-Co-Zn

0.85

-

6.Al-Co-Na (7%)

I.Al-Co-Cu-Na (1%)

-

0.46

4,Al.-Co-Cu

S.Al-Co-Na (2%)

1.1

58

2.Af-Cu

3.Al-Co-Cu

Selectivity

(mgCO/gcat/h) CO2 HC ROH CH4 CH3OH

Total activity

370

atomic ratio

Co/Zn

l.Al-Co

CATALYSTS

Co/Cu

1

118

Al-Co

Al-Co-Cu

370

a 300 % :g %.

1

‘.

2

‘.

3

‘a

4

/,

S

‘f

6

‘.

7

‘.

’ 8nc

b

‘.‘.‘.‘.‘.‘.‘.I --234SblS

i

nc

Na

eo 1

2

3

E

300 250

%

:% o=.

1

‘.

2

-.

‘3 3

4

5

8

4

S

6

7

AL-Co- Cu-Na (4% Na CofCu ,l)

Al-Co-Na

‘tE

~~

Al-Co-Cu-

AL-Co-Na (1% Na)

870

Y

o

7

8n,

0

1

2

3

4

S

6

7

Sn,

Fig.6 : Productivities at 2.50 “C after 22 h on stream in CO/29 conversion for alkanes 888 alcohols fi and alkenes M , versus the carbon number (nc) of the products, from [223.

119

4, CHANGES IN CATALYST SURFACE COMPOSITION AND MORPHOLOGY INDUCED BY CATALYSIS. If an accurate characterization of the activated samples before reaction is required for understanding their catalytic behaviour, the occurrence of aging phenomena and some changes in the catalytic properties observed during the reaction rendered the characterization of the solids after reaction also very informative. It should be noted that the use of actual “in situ” techniques of ch~c~~~tion would be ~viou~y more advisable, but, up to now, few of the usual physical techniques have been adapted to the rather severe conditions of the CO/H2 reaction under pressure. The main features of the changes in surface composition and particle morphology reported here were obtained from i) temperature programmed hydrogenation (IPH) experiments which consist in hydrogenating into methane the carbonaceous species deposited during the reaction in order to evaluate their amount and reactivity (Pig. 7) [39], ii) magnetic measurements performed periodically during the reaction in a special cell working under pressure, the catalytic system being simply “frozen” under He from the reaction temperature to the temperature of analysis [39], and iii) XRD and XPS m~surements carried out on samples passivat~ after the CO/H2 run,

(al

Pig.7 : CH4 yields produced by temperature programmed hydrogenation (TPH) of carbon deposits after CO/H2 test. a/Al-Co;b/Al-Co-Na2%;dAl-Co-Nal2g; d/Al-Co-Cu;elAl-Co-Cu-Na4%;f/Al-Co-Cu-Na7%,from[22].

120

Al-Co reference catalyst. No major structural change was noted by XRD and XPS after CO/H2 reaction on unpromoted Al-Co catalyst. A large amount of carbon deposited on the surface of the Co particles (and not in the bulk as attested by magnetic measurements) can be detected by TPH (Fig. 7,a); a ratio C/Cosurfa~ of around 3 is determined, confirming the idea of surface carbon accumulation, probably in connection with the significant aging phenomena noted previously. Al-Co-Cu catalysts. The main crystallographic change induced by the reaction on copper promoted samples is the observation of a metal sintering attested by narrower XRD peaks and a growth of around 40% of the fe~magneti~ particles. This phenom~on of sintering at low temperature has been related to a cobalt transfer via carbonyi or subcarbonyl species migrating from particle to particle, as studied and modelled for the case of Ni catalysts under similar conditions [30]. It should be noted however that, despite this sintering press, the final cobalt dispersion of copper-promoted samples remains much better than the one observed for the reacted unpromoted Al-Co sample. The positive effect of copper addition was not therefore totally decreased during the reaction. Moreover, the loss of the active surface has been shown to account only for a minor part in the catalyst deactivation. The main part of the deactivation (which, as already emphasized, is much less significant than for the unpromot~ sample) can be assigned to surface poisoning by carbon deposits, as revealed by TPH experiments (Fig, 7,d). It was also proposed that the rather low amounts of these deposits could be due to the dilution effect of Co particles by Cu atoms, which hindered the formation of long hydrocarbon chains, precursors of toxic graphite

[=I* Al-Co-Na catalysts. A major effect of alkalization induced in the reaction course is to promote both a bulk carbidization of the cobalt particles and the stablltion of carbonyl species SCOT]as observedby FTIR. The occurrence of Co2C formation is reflected i) by the disappearance of the ferromagnetic phase after the CO/H2 reaction, for high content of Na and ii) by a large amount of stable carbon detected by TPH (Fig. 7b,c), requiring tern~~~res of up to 600°C to be hyd~gena~. Although not clearly established, a process of carbidization linked to the dismutation of Co2(CO)g species in presence of sodium ions has been proposed [22]. A second type of carbon, still more stable (hydrogenated at 700°C) observed on TPH curves only for high contents of Na (7% or more)(Fig. 7c) has been ascribed to sodium carbonate species, It is therefore obvious that the alkalization of the Co phase induces deep morphological and surface composition changes in the course of the reaction. Al-Co-Cu-Na catalysts. For the case of doubly promoted samples, a combination of the effects of Cu and Na was qualitatively observed: i) sintering of the metallic phase by ~bonylation but at a reduced extent due to the s~bili~tion of carbonyl species by sodium ions, ii) ~bidi~tion of the Co phase but incomplet~y even for high Na content, probably due to the surface dilution of Co by Cu atoms which could hinder the carbonyl dismutation leading to the formation of the bulk carbide phase. Zn effects. The effects of Zn addition on morphological changes induced by the CO/H2 reaction were much less pronounced than the specific effects of Na and Cu. Thus, no significant effect on Co sintering was observed on Al-Co-Zn samples. A trend to decrease

121

the carbon deposits was deduced from TPH experiments [28]. The stabilization of the hydrogenating properties of the Co phase after Zn addition could account for this reduced deposition of toxic carbon. 5. GENERAL DISCUSSION In this section we will first discuss the role of the different elements and ~mbinations on the vie~int of the catalytic ~rforrn~~ (activity, selectivity, safe). The second part of the discussion will be devoted to the nature of the active site(s) and to the possible routes for the higher alcohols synthesis over cobalt-based catalysts. 5.1. Role of the different elements and combinations. Cobalt. As generally observed, Co alone is very selective towards formation of alkanes. This underlines the main role of cobalt which is to dissociate CO and to hydrogenate selectively the resulting surface carbon species into hydrocarbons. The hydrocarbon distribution follows the classical Anderson-Schulz-Flory relationship, indicating that cobalt is able to achieve the C-C bond formation and the chain growth. This high selectivity towards saturated hyd~~bons is observed whatever the extent of reduction of the metallic phase suggesting that the presence of Co”+ species does not induce specific properties. The stability of the unpromoted cobalt catalyst is very poor: owing to the low dispersion of the Co catalyst, small amounts of inactive species can drastically decrease the activity. Copper. As expected for copper-based solids [40], Al-Cu acts mainly as a methanol synthesis catalyst. This high selectivity towards CH30H underlines the non-dissociative activation of CO on copper and the absence of C-C bond formation. Cobalt-Copper. The association of Co and Cu leads in CO hydrogenation to the appearance of higher alcohols which are not observed when using either pure cobalt or pure copper. It appears as if both metals add their own specificities, cobalt forming the hydr~n group and copper adding the alcohol function. According to the ~h~~~~~tion studies, the cobalt and copper association results in the formation of a surface CoCu alloy on the cobalt particles. It has been proposed that this alloy allows the formation of dual sites made of Co-Cu mixed ensembles able both to form carbon chain and to insert CO. This proposal is supported by the ability of alloyed copper to activate CO without CO bond rupture [28] and by the fact that the electrondeticient character of alloyed Cu may promote the CO insertion into the cobalt-hydrocarbon bond [32,33]. This model also agrees with catalytic studies on the same solids involving probe molecules and chemical trapping WI. A striking feature of these CoCu samples is that they produce only very small amounts of methanol: a possible explanation could be that, in relation with the above model, the CO adsorbed on the Cu reacts preferentially with the carbon chains generated on the neighbouring Co thus leading only to C,OH alcohols with n 2 2. However, owing to the copper loading and particle dispersion, free copper is present in the sample, which should produce some methanol. Complementary experiments have shown that these Cobased catalysts were able to convert CH30H, in particular into methane [22]. The absence

122

of CH3OH in the products of CO hydrogenation over the present CoCu solids is therefore not surprising. Besides the changes in selectivity, copper addition induces an important decrease of activity, as also mentionned by Lin et al. [42]. This effect can be assigned to the dilution of the active Co phase upon alloying with Cu, in agreement with the above proposal and previous observations on NiCu alloys [35]. As far as stability is concerned, copper addition has a positive effect which can be related to the lowering of the carbon deposition, a feature often observed in the case of bimetallic catalysts. Cobalt-sodium.Sodium addition completely modifies the nature and distribution of the products. For low Na contents the activity is decreased when compared with pure cobalt and the selectivity is mainly orientated towards Cl products (CH4 and CH3OH). The methanol formation could originate from CO insertion into Co-H bonds: this insertion could here be facilitated by the CO species interacting with neighbouring Na atoms identified by IR spectroscopy experiments. Similar surface complexes have indeed been proposed by Sachtler [36] in connection with this reaction. With regards to methane formation, it may be reasonably proposed that part of the methanol formed on the Co-Na sites is reformed into CH4 on the unpromoted Co sites [22]. For high Na contents, the conversion is markedly decreased, but oletinics and C2+ alcohols appear. The complete transformation of the IR spectra suggests an important change in the nature of the surface associated with new catalytic properties. The presence, after reaction, of high amounts of Co2C and the observation of [Co(CO)4]- agree with an interaction occurring between Cq(CO)g and Na surface species. From the observation that carbonyl species were present in the reacting medium, a side mechanism for higher alcohols synthesis involving mobile Co carbonyl species has been proposed 1211, similar to those proposed for the homogeneous reactions of hydroformylation and alcohol homologation. In keeping with this proposal, the two latter reactions have been shown to be also catalysed by Co when promoted with high Na contents [22]. Although carried out on slightly different systems, the recently published works of Matsuzaki et al. on Co&i02 modified by alkali [40], deserve to be quoted here. The main effects of alkalizing were found to be: i) the overall reaction rate is decreased, ii) the mole fraction of methane in hydrocarbons is decreased, iii) the mole fraction of alkenes is increased, iv) the kind of oxygenated compounds is shifted from alcohols to aldehydes. A general interpretation assumes that the alkali metal cations reduce the hydrogenation ability of cobalt catalysts via some electronic effects; this would account for the enhanced selectivity towards alkenes and aldehydes which are the intermediate products leading to the formation of alkanes and alcohols after hydrogenation. This overall scheme appears globally consistent with our observations on Al-Co-Na systems; however, discrepancies concerning the detailed product distributions are obvious, such as the selective orientation towards methanol observed on weakly alkalized AI-Co compared with the orientation towards higher alcohols and aldehydes observed on similarly alkalized Co/SiO2 (thus, a Na-ColSi02 catalyst (0.6 wt. % Na) exhibits a selectivity of 2

123

% towards methanol (10% for Al-Co-N@ and 41% towards higher alcohols (1 96 for AlCo-Na) under close enough operating conditions [35]). It could be therefore speculated that the so-called electronic effects induced by alkali actually depend on parameters such as the nature of the support and/or the dispersion of the cobalt phase. Thus, the Co-CO***Na complex observed on Al-Co-Na catalyst would not necessarily exist on alkalized Co/SiO2 catalysts. The works of Nunan et al. on alkali doping of cobalt-free catalysts [38] may also be mentions: in p~cul~, after promotion by cesium at low content (0.73 mol%), the production of methanol was increased on Cu-ZnO-A1203 catalysts, while this effect was less marked at higher content of Cs. This is in agreement with the effect of Na content observed on the present Co-based catalysts. Indeed, the effect of alkalizing on methanol synthesis was also widely studied on other metals such as Pd and it is worthwhile to recall here that mechanistic routes via formate intermediates involving OH groups from the support have been considered [39]. Cobalt-Copper-Sodium.The presence of copper and sodium, both combined with cobalt for alcohol formation, makes the analysis of their specific role on selectivity somewhat difficult. For low Na contents, some methanol is produced which reflects the high selectivity of the CoNa association for CH3OH synthesis. These catalysts present a good selectivity towards C2+0H (effect of copper), a limited production of hydrocarbons (effect of sodium) and a good stability (effect of sodium). For high Na contents, the total activity is decreased and the selectivity less orientated towards higher alcohols. This may be related to the partial disappearance of metallic cobalt (formation of Co2C and [Co(CO>4]- species), weakening the CoCu surface interaction. The solid behaves more like a Al-Co-Na sample (the CH3OH generated by the Cu phase can undergo homologation on the CoNa phase [22]). Cobakinc. Concerning catalytic activity, the main effect of Zn is to improve the stability of the catalyst. As indicts above, this promoting role has been attributed to the blocking effect of ‘Znon the oxidation of the metallic active cobalt via a redox equilibrium involving hydroxyl groups. Concerning selectivity, the Al-Co-Zn system compares with the Al-Co one; both are characterized by a high selectivity towards hydrocarbons. This observation agrees with the absence of major modification of the surface properties upon Zn addition, as deduced from IR spectroscopy. It must be however mentioned that for some cases, Al-Co-Zn formulations may lead selectively to methanol, in so far as strict operating procedures are respected f34]; this tends to prove that the zinc addition may induce m~h~istic effects beside the one of actin as a textural promoter. As a matter of fact, one could speculate that for these cases, Zn2$ ions could act in a similar fashion as Na+ ions and form surface complexes favouring the CO insertion into Co-H bonds. Coba&-copper-zinc.In agreement with the morphological data, Cu and Zn simply add their specific promoting effects on the catalytic properties. These catalysts produce alcohols like Co-Cu systems, but are more stable (Zn effect).

124

Cobalt-copper-zinc-sodilrm. This triply promoted model system leads to a good balance concerning alcohols selectivity, as expected from the IFP formulations; moreover, the rather low initial activity of this material related to the combined effect of copper and sodium is compensated for long catalytic run by its excellent stability. 5.2. Nature of the active site(s).

From the above discussion it appears that, beside the side reactions via carbonyl species, at least two types of active surfaces are able to produce alcohols on the present cobalt based catalysts: the Co-Cu surface alloy and the Co-Na (and possibly Co-Zn) association. Before speculating about possible routes and mechanisms on these surfaces, it is of interest to discuss other proposals from the literature concerning the nature of the active sites. Most of these deal with a surface which associates metallic and ionic cobalt species [17,18,27,37]; as a matter of fact all these catalysts have been activated at low temperature, i.e. in the 250-450°C range, which results in a much lower extent of reduction than the present solids, as already discussed in the section “Preparation and Activation”. In this Con+/Coo site, the unreduced species is assumed to adsorb CO undissociatively which would permit CO insertion to occur [27]. Concerning the present model catalysts, it has been shown that the extent of reduction did not change the selectivity in a significant way and that no evidence suggested that the presence of surface Co”+ led to oxygenates: partially (50%) reduced Al-Co samples lead to hydrocarbons with 99% selectivity, while fully reduced Al-Co-Cu samples display a good selectivity towards alcohols, higher than the one observed on partially (70%) reduced Al-Co-Cu [22,28]. Takeuchi et al. have also shown that well dispersed and fully reduced Co/SiO2 catalysts can lead to the formation of alcohols [43]. On the other hand promotion by Na (and possibly by Zn) which leads to alcohols synthesis tends to improve the reduction extent, which also is not in favour, for the present solids, of an active Con+/Coo site. As the modes of preparation, the activation conditions and the start-up procedures are different in those various works, one can easily understand that the active sites may differ. According to a generally agreed scheme [2], the higher alcohol synthesis requires a surface capable of two distinct functions: i) undissociative CO activation (site A) and ii) formation of surface C, (n 2 1) species (site B). Coo is known to form these C, species and therefore any other surface site able to first activate CO associatively and secondly insert CO, will lead to higher alcohols, provided that both types of site are in a close vicinity. Con+ Cu and the CoNa association have been proposed as able to activate CO without rupture’and all these species could then act as site A for metallic cobalt (site B) in alcohols synthesis. Depending on the respective amounts of the different A species in the vicinity of Coo different sites are possible: - For catalysts reduced at low temperature, small amounts of well dispersed Coo are formed in the surface of a Co”+ matrix and interactions between both Coo and Co”+ species are favoured; for these conditions, the CoCu alloy is actually not yet formed.

125

- For catalysts reduced at high temperatures, the opposite picture holds: The con+ and Coo phases are now separated and Cu and Co can form the active alloy in a large extent. In the light of the above discussion, it appears that the conclusion by Baker et al. [18] stressing that “bimetallic Cu/Co particles are not the active centres for higher alcohols synthesis” is excessive. Moreover, data given in [15] indicate that the selectivity towards C2+OH is generally higher in the presence of Cu for a given temperature of reduction. We would rather suggest that, depending on the nature of the catalyst and on its activation conditions, different active centres are created, which all associate a site able to activate undissociatively CO and a site where the hydrocarbon chain may be formed. 6. CONCLUSION The experiments carried out on model cobalt-based catalysts prepared by coprecipitation have led to a better understanding of the specific role of copper, zinc and sodium in complex AlCoCuZnNa active phases for Cl-C6 alcohols synthesis. These modifiers of the cobalt phase help to reduce cobalt oxides to metallic cobalt (copper, sodium), to increase or stabilize the metallic particle dispersion (copper), to promote the formation of active sites for alcohol synthesis (copper, sodium) and to decrease the carbon deposits by avoiding long-chain hydrocarbon formation or stabilizing the hydrogenating activities (copper, zinc). Though it is discussed that various types of sites can lead to alcohol synthesis depending on the catalyst composition and the operating conditions, the cobalt-copper association appears as the heart of the AlCoCuZnNa formulation, round which the present review is centred. Provided that they are well dispersed into the oxide matrix, cobalt and copper can form an alloy after reduction which is proposed to be the site which activates CO both in a dissociative and non-dissociative way, a dual function required for alcohols formation. ACKNOWLEDMENTS: The authors thank Drs Ph. Courty, G.A. Martin, V. Perrichon, M. Primet and Prof. A.J. Gomez Cobo for helpful discussions. 7. REFERENCES 1 2 3 4 5

Ph. Courty , P. Chaumette, C. Raimbault and Ph. Travers, Revue de 1’I.F.P. 45, n”4, (1990) 561. Xu Xiaoding, E.B.M.Doesburg and J.J.F. Scholten, Catalysis Today, 2 (1987) 125. E. Supp, AICHE Spring National Meeting, New Orleans, 6th-10th April 1986. K.J. Smith and R.B. Anderson, J.Catal., 85 (1984) 428. G.A.Vedage, G.W. Himmelfarb, G.W. Simmons and K. Klier, in “Solid State Chemistry in Catalysis”, R.K. Grasselli and J.F. Brazdil Eds., ACS Symposium Series 279, Am. Chem. Sot., Washington D.C., 295 (1985).

126

6 7

8

9

10 11 12 13 14 15

16 17 18 19 20 21

22

23 24 25

26

A. Kiennemann, H. Idriss, R. Kieffer, P. Chaumette and D. Durand, Ind. Eng. Chem. Prod. Res. Dev. 30 (1991) 1130. M. Ichikawa, T. Fukushima, K. Shikakura and J. Kanegawa, Proc. of the 8th Int. Cong. on Catal., Berlin, Springer Verlag, 2 (1984) 69 and T. Fukushima, M. Ichikawa, S. Matsushita, T. Tanaka and T. Saito, J. Chem. Sot., Chem. Commun., 1209 (1985). S. Arimutu, K. Tanaka and T. Saito, in “Progress in Cl Chemistry in Japan”, Industrial Chemistry Library Vol. 1, The Research Association for Cl Chemistry Ed., Kodansha Ltd and Elsevier Pub., (1989) 143. G.J. Quarderer, AICHE Spring National Meeting, New Orleans, sth-lO* April 1986. M. Inoue, A. Kurusu, H. Wakamatsu, K. Nakajima and T. Inui, Appl. Catal., 29 (1987) 361. T. Tatsumi, A. Muramatsu and H. Tominaga, J.Catal., 101 (1986) 553 and T. Tatsumi, A. Muramatsu, K. Yokota and H. Tominaga, J.Catal., 115 (1989) 388. S. Uchiyama, N. Kawata, Sekiyu Gakkaishi, 33 (1990) 1. P. Grandvallet, Ph. Courty and E. Freund, Proc. of the 8th Int. Cong. on Catal., Berlin, Springer Verlag, 2(1984) 81. P. Grandvallet, PhD Thesis, Technip Ed., 1985. Ph. Courty, A. Forestiere, N. Kawata, T. Ohno, C. Raimbault and M. Yoshimoto, in “Industrial Chemicals via Cl Processes”, D.R. Fahey Ed., ACS Symposium Series 328, Am. Chem. Sot., Washington D.C., 42 (1987). A.Kiennemann, S. Boujana, C. Diagne, Ph. Courty and P. Chaumette, Stud. Surf. Sci. Catal. 61 (1991) 243. J.E. Baker, R. Burch and S.E. Golunski, Appl. Catal., 53 (1989) 279. J.E.Baker, R. Burch, S.J. Hibble and P.K. Loader, Appl. Catal., 65 (1990) 281. J.E. Baker, R. Burch and Niu Yuquin, Appl. Catal., 73 (1991) 135. Ph Courty and C. Marcilly, Revue de I’IFP, 33 (1978) 83; ibid., Pr0c.“3’~ Int. Symp. on Preparation of Catalysts”, Elsevier, Amsterdam, 1983, p.485. A.J. Gomez Cobo, N. Mouaddib, J.A. Dalmon, C. Mirodatos, V. Perrichon, M. Primet, P. Chaumette and Ph. Courty, “Natural Gas Conversion”, A. Holmen, K.J. Jens and S. Kolboe Eds., Elsevier,61 (1991) 257. A.J. Gomez Cobo, P. Chaumette, J.A. Dalmon, M. Primet and C. Mirodatos, Proc. 12th Ibero-american Symposium on Catalysis, I.B.P. Eds., 2 (1990) 377; ibid, J. Catal., submitted for publication.; A.J. Gomez-Cobo, Thesis, Lyon UCB 94-90, 1990. Ph. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catal. 17 (1982) 241. A. Navrotsky and O.J. Kleppa, J. Inorg. Nucl. Chem., 29 (1967) 2701; J. Inorg. Nucl. Chem., 30 (1968) 479. N. Mouaddib and V. Perrichon, Proc. gth Inter. Cong. on Catal., M.J. Phillips and M Ternan Eds., 2 (1988) 521; N. Mouaddib, V. Perrichon and M. Primet, J. Chem. Sot., Faraday Trans. I, 85 (1989) 3413. H.F.J. van’t Blick and R. Prins, J. Catal. 97 (1986) 188.

127

27 28 29

30 31 32 33 34

35 36 37 38 39 40 41 42 43 44

M. Blanchard, H. Derule and P. Canesson, Catal. Let. 2 (1989) 319. R.M. Bail&rd-Letoumel, A.J. Gomez Cobo, C Mirodatos, M. Primet and J.A. Dalmon, Catal. Let., 2 (1989) 149; ibid, J. Catal., submitted for publication. J.A. Dalmon, C. Mirodatos, P. Turlier and G.A. Martin, “Spillover of adsorbed species” G. Pajonk et al. Eds., Elsevier, 17 (1983) 169. M. Agnelli, M. Kolb, C. Nicot and C. Mirodatos, “Catalyst Deactivation 1991”, C.H. Bartholomew and J.B. Butt Eds., Elsevier, 68 (1991) 605. J.M. Driessen, E.K. Poels, J.P. Hindermamm and V. Ponec, J. Catal., 82 (1983) 26. N.A. Anikin, A.A. Bagatur’yants, G.M. Zhidomirov and V.B. Kazanskii, Russ. J. Phys. Chem., 57 (1983) 393. N. Horns, A. Chaplin, P.R. de la Piscina, L. Huang, E. Garbowski, R. SanchezDelgado, A. Theoher and J.M. Basset, Inorg. Chem., 27 (1988) 4030. P. Chaumette, Ph. Courty, A. Kiennemann, R. Kieffer, S. Boujana, G.A. Martin, J.A. Dalmon, P. Meriaudeau, C. Mirodatos, B. HBlhein, D. Mausbeck, J. Hubert and A. Noels, Proceedings of the “Oil and Gas Conference”, Palerme, 9-2/10/1990, to appear in Int. J. Energy Research (1992). J.A. Dalmon and G.A. Martin, Proc. 7th Int. Cong. Catal., Tokyo 1980, Kodanska Ltd, (1981) 402. W.M.H. Sachtler, D.F. Schriver and M. Ichikawa, J.Catal., 99 (1986) 513. G.R. Sheffer, R.A. Jacobson and T.S. King, J. CataI., 116 (1989) 95. J.G. Nunan, R.G. Herman and K. Klier, J. Catal, 116 (1989) 222. Y. Kikuzono, S. Kagami, S. Naito, T. Onishi and K. Tamaru, Chem. Letters, (1981) 1249; Faraday Discuss. Chem. Sot., 72 (1981) 183. T. Matsuzaki, T. Hanaoka, K. Takeuchi, Y. Sugi and M. Rein&amen, Catal. Lett. 10 (1991) 193. K. Klier, Adv. Catal., 11 (1980) 243. F.N. Lin, F. Penella, “Catalytic conversion of syngas and alcohols to chemicals”, Herman G. Edt., Plenum Press, (1984) 53. K. Takeushi, T. Matsuzaki, T. Hanaoka, H. Arakawa, Y. Sugi and K. Wei, J. Mol. Catal. 55 (1989) 361. C. Mirodatos, H. Praliaud and M. Primet, J. Catal., 107 (1987) 275.