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cobalt catalysts
Catalysis, 53 (1989) 279-297 Elsevier Science Publishers B.V., Amsterdam -
Applied
279
Printed in The Netherlands
Synthesis of Higher Alcohols over Copper/Cobalt Catalysts Influence of Preparative Procedures on the Activity and Selectivity of Cu/Co/Zn/Al Mixed Oxide Catalysts J.E. BAKER, R. BURCH” and S.E. GOLUNSKI Catalysis Research Whiteknights,
Group, Chemistry
Department,
University
of Reading, P.O. Box 224,
Reading RG6 2AD (U.K.)
(Received 18 February 1989, revised manuscript received 27 April 1989)
ABSTRACT The effects of selected preparative variables on the performance of Cu/Co/Al/Zn catalysts during the conversion of synthesis gas to higher alcohols have been assessed. Using the PlackettBurman statistical technique, the following order of importance has been observed for the preparation of dry (co-precipitated) precursors: ageing of precipitate > precipitation temperature > pH > total metal ion concentration > drying temperature. Samples prepared accordingly are comprised mainly of a crystalline hydrotalcite-type phase, but some amorphous malachite-type phase is also present. However, the degrees of crystallinity and homogeneity do not greatly influence the activity or selectivity of the calcined and reduced catalysts. It has become apparent that the thermal activation of the dry precursors is the single most important variable in the preparation of these catalysts. The importance of the degree of reduction of the various cobalt phases in determining the catalytic selectivity is discussed.
INTRODUCTION
The conversion of synthesis gas (CO/CO,/H,) to higher alcohols can be achieved using a variety of catalytic processes, which are based, for example, on modified methanol synthesis catalysts, rhodium containing catalysts, alkalised MO&, or Cu/Co oxides [ 11. Of the catalysts currently available, those developed by Institut Francais du P&role (IFP) [2-111 are among the most interesting, because of their stability, their high activity at relatively low pressures, and their selectivity towards alcohols rather than hydrocarbons. Several patents have been published which describe these catalysts [ 2-81, which are based on a combination of copper and
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0 1989 Elsevier Science Publishers B.V.
cobalt. Typically they contain another trivalent metal (Al orCr), Zn, an alkali metal, and possibly a number of minor components. It is claimed [ 61 that the preferred catalyst contains: Cu: l&45%; Co: 9-20%; Al: 7-25%; Zn: 1550%; with alkali/Al: o-0.003; Zn/Al: 0.4-2.0; Co/Al: 0.2-0.75; Cu/Al: 0.1-3.0. The patents [ 3-61 emphasize the importance of maintaining the homogeneity of the precursors during each step in the preparation and pretreatment of the catalysts. However, although detailed procedures are given in the patents concerning the choice of conditions (precipitation temperature, pH, ageing time and temperature, drying and calcination methods, etc.), this information alone does not provide an insight into the way in which the preparation of these materials affects their catalytic properties. Furthermore, there have as yet been few publications on these, or closely related, catalysts in the open literature [9,10,12-171 and as far as we are aware no systematic study of the preparation of these multicomponent Cu/Co/Zn/Al catalysts has been published. Complementary studies on single crystals and on simple binary oxides are to be published [ 18,191. EXPERIMENTAL
Catalystpreparation We have used the Plackett-Burman statistical technique {as reported by Stowe and Mayers [ 201 for catalyst preparation by co-precipitation) to examine the effect of selected parameters on the catalytic properties of Cu/Co/ Zn/Al catalysts. The technique allows several variables to be screened using a small number of preparations. Each variable is given a high ( + ) or low ( - ) value and then, by using an appropriate matrix, a set of preparations is derived which allows all the variables to be graded in importance. The variables and the values used in our experiments are shown in-Table 1, together with the matrix summarizing the catalyst preparations. As an example of the use of the matrix, the precursor of catalyst A was prepared from solutions with a total metal ion concentration of 0.5 mol dmp3 by precipitation at pH 9, at 330-340 K; the suspension was aged for 1 h at 350 K; the precipitate was dried at 360 K for 16 h and then at 390 K for 3 h. Preparation of the precursors involved co-precipitation from two solutions, one of which contained the metal nitrates and the other sodium carbonate (see Table 1) . All reagents were of AnalaR grade. The overall concentrations of Cu, Co, Zn, and Al were chosen to match those reported by IFP for the preparation of one of their best catalysts (catalyst A in ref. 6). The two solutions were mixed in a glass reaction vessel (volume, 100 cm3) initially containing deionised water which was at the required precipitation temperature. The solutions were injected below the surface of the rapidly stirred water, and the flow-rates were adjusted to maintain the pH at the selected
281
TABLE 1 Variables used in the statistical preparation procedure (a) Variables
and levels
Variable
(i) (ii) (iii) (iv) (v) (vi) (vii)
Level
pH during precipitation Precipitation temp. (K) Ageing of suspension Drying temp. (K)/time (h) Blank Total metal cont. (mol dm-“) Blank
Low (-)
High (+)
7 293 none 360/16 & 390/3
9 330-340 350 K/l h 390/19
0.5
1.5
(b) Matrix
Preparation
Variable
(i)
(ii)
(iii)
I (IFP method) (c) Solutions
Solution 1 Low(-)
High (+)
0.1 M 0.15 M 0.066 M 0.2 M
0.3 M 0.45 M 0.198 M
0.6 M
Cu nitrate Zn nitrate Co nitrate Al nitrate
Solution 2 Low (-)
High ( + )
0.577 M
1.731 M
Na carbonate
(iv)
(v)
(vi)
(vii)
282
value. Usually, the total flow-rate was about 10 cm3 min-‘, giving a residence t.ime of about 10 min. The suspension drained out through a sidearm into a large collecting vessel. After discarding the first 300 cm’, the suspension was collected and aged if required. The catalyst precursor was separated from the mother liquor by centrifugation, washed with 30 dm3 distilled water to remove Na+ ions, and dried as required. Separate portions of each dried sample were calcined either (a) in a muffle furnace or (b) in flowing oxygen. In the case of (a) the samples were heated at 723 K for 4 h, followed by heating in the catalytic reactor in nitrogen for 0.5 h at 773 K to remove adsorbed contaminants: in the case of (b), the samples were heated at 10 K min-’ to 723 K and held at this temperature for 4 h. The catalyst samples (100 mg) were reduced in situ in the catalytic reactor (see later) by heating in pure hydrogen (at 1 atm, 101.3 kPa) at 10 K min-l to 523 K and holding at this temperature for 1 h. Characterization
of precursors
X-ray diffraction (XRD) was used to determine the structure of the crystalline component of the dry precursors using a General Electrics diffractometer with Cu Ka radiation. The surface composition was examined by scanning electron microscopy (SEM) and semi-quantitative X-ray fluorescence (SQXRF) using a Cambridge Scanning Casmscan 3130 electron microscope combined with a Link Analyser System 290. Thermal analysis was carried out on samples (100 mg) which were heated at 10 K min-’ to 775 K. During differential thermal analysis (DTA) using a Stanton Redcroft Standata 6-25 instrument, heat treated cr-A&O, was used as the inert reference material, and each sample was heated under a flowing atmosphere of oxygen-free nitrogen or high purity oxygen (both from B.O.C.). Simultaneous thermogravimetry (TG) and derivative thermogravimetry (DTG) were performed using a Sartorius vacuum microbalance linked to a BBC microcomputer. Evolved gas analysis (EGA) was achieved by heating a sample under flowing helium (B.O.C.) and monitoring selected peaks in the mass spectrum of the effluent gas, using a Spectramass Dataquad 100-V3.15 quadrupole mass spectrometer; the data were collected and processed on an Amstrad 1512 computer. Characterization
of catalysts
Surface areas of the calcined samples were determined by nitrogen adsorption at 77 K (BET method) in a glass volumetric apparatus. Chemical analysis for sodium was carried out using a Perkin Elmer 1100B atomic absorption spectrometer. Analysis for Cu, Co, Zn, and Al was performed using a Philips PW 1410 X-ray fluorescence spectrophotometer. XRD
283
and SQXRF were used to examine the bulk structure and surface composition, respectively. The reducibility of the samples was determined by temperature-programmed reduction (TPR) with a heating rate of 15 K min-l under a flowing atmosphere of 5% H,/95% Ar. Catalyst testing
The activity and selectivity of the various samples for the synthesis of alcohols from synthesis gas mixtures were determined using a glass-lined stainless steel microreactor (I.D. 4 mm) capable of operating at pressures up to 50 atm and temperatures of 773 K. The samples (100 mg) were reduced in pure hydrogen at 1 atm by heating at 10 K min-’ to 523 K and holding at this temperature for 1 h. The microreactor was pressurized to the reaction pressure (40 atm). Premixed gases (B.O.C. Special gases, lo%, CO/lo% CO,/80% H, and 25% CO/75% HZ) were admitted to the reactor through Krohne mass flow controllers (15 cm3 mine1 ). Th e activity and selectivity were determined at 523,533 and 543 or at 533,543 and 553 K (5 h at each temperature) using an automated gas analysis system. This comprised a Perkin Elmer 8500 series dual channel FID gas chromatograph fitted with a heated gas sampling valve. Products were separated into oxygenates and hydrocarbons using a 2-m precolumn filled with Reoplex 400 on Chromosorb GAW and then the two product streams were further separated using a pair of 3-m Porapak QS columns in parallel. The outputs from the two detectors were fed simultaneously to a Daisy two channel integrator for data acquisition and storage. Response factors for the various products were taken from the literature [ 211. RESULTS
Chemical analysis
Table 2 summarizes the analytical results on the catalysts. Rather surprisingly the wt.-% of each of the metals in the catalysts does not vary a lot between the different preparations, indicating that the method of preparation has little influence on the amounts of the metals precipitated. Samples C,D and E have high sodium contents. C and E were prepared using concentrated solutions at pH 9 without ageing; D was prepared using concentrated solutions and a precipitation temperature of 330-340 K. It is possible that under these conditions some sodium becomes incorporated into the brucite layers making it very difficult to remove by washing. Samples which were precipitated using dilute solutions and/or at 333-343 K, and/or aged, were very low in sodium content. Comparison with the nominal composition reported for the IFP catalyst [ 61 used as a reference for our preparations (see Table 2) shows that our catalysts
284 TABLE 2 Analytical data on the catalysts Weight percentage
Catalyst
A B C D E F G H I IFP catalyst”
cu
co
Zn
Al
Na
24.1 23.1 18.5 21.4 22.7 22.7 23.2 24.7 23.7 19.7
13.0 12.6 10.6 10.8 11.9 11.8 12.9 10.0 13.2 11.2
24.1 23.8 24.5 20.2 22.3 21.9 23.4 23.5 24.1 30.9
11.4 12.6 11.8 13.0 10.6 14.9 12.7 13.7 11.6 11.2
<0.2 < 0.2 5.7 5.1 4.4 <0.2 to.2 to.2 (0.2 to.2
“Nominal values reported in ref. 6, based on composition of precursors.
contain similar amounts of Co and Al, rather more Cu and significantly less Zn. Surface area measurements The surface areas of the calcined samples ranged from 41 to 230 m2 g-l as shown in Table 3. Samples calcined in a muffle furnace generally have lower surface areas than those heated in flowing oxygen although there are excepTABLE 3 Surface areas of calcined and heat-treated samples Catalyst
Surface area
(m’ g-’ I
Air”
Oxygenb
182 191 56 148 42 195 111 169 135
200 173 56 160 41 230 224 133 208
“Samples calcined in air. “Samples heated in flowing oxygen.
tions. Samples C and E have particularly low surface areas (56 and 41 m2 g-i); both were precipitated at pH 9 using high metal concentrations. High surface area samples were obtained when at least two of the following conditions were met: pH 7; low ion concentration; samples aged, precipitation temperature, 330-340 K. X-ray diffraction
measurements
The X-ray diffractograms for the various dried precursors were all very similar. A typical diffractogram is shown in Fig. 1. This indicates the presence of the hydrotalcite phase previously reported [9,10,12]. The intensities of the lines on the various diffractograms differed quite a lot, indicating the presence of varying amounts of the crystalline phase. Table 4 summarizes the intensity data for the (003 ) peak, which corresponds to the d-spacing between the brucite layers. The peak widths at half maximum also varied as shown in Table 4, indicating differences in the degree of order of the crystalline phase. Samples D, F, G, H and I gave sharp peaks indicating that samples precipitated at pH 7 contained the most ordered crystalline phase. Of these samples, F, H and I were prepared from dilute solutions. However, if precipitation was performed at 330-340 K, or if the samples were aged, the diffractogram was still well defined, irrespective of the concentration of the metal ions in solution. Clearly there are competing effects during the preparation of such complex mixed oxides. However, it appears that to produce a highly crystalline, well
a
-
Fig. 1. X-ray diffractogram of dried precursor. (e) 015; (f) 018; (g) 1010.
Miller indices:
(a) 003; (b) 006; (c) 012; (d) 104;
286 TABLE 4 X-ray diffraction data on the various dried catalyst precursors Catalyst
(003) peak height
Peak width
11.5 3.8 4.5 15.0 11.0 11.5 12.0 5.5 10.0
72 77 57 50 60 41 44 44 41
“peak width at half maximum, units are nm.
ordered material, precipitation should be done at pH 7, 330-340 K, with low concentrations of metal ions, followed by ageing of the suspension. XRD of the calcined catalysts revealed that the only crystalline phase present is a CoAl,O, spinel. This is consistent with the findings of Courty et al.
[91. Temperature-programmed
reduction
experiments
The TPR traces for the catalyst exhibit two features as shown in Fig. 2. The first feature is a peak occurring between 490 and 540 K (samples A, B, D, F, G, H, I), or, in the case of samples C and E, a doublet occurring between 540 and 610 K. This feature probably corresponds to the reduction of CuO. The second feature is a flat, broad doublet extending from 570 to 770 K, probably due to the stepwise reduction of Co,O, and/or CoAl,O, [1,9,11]. Samples C and E were prepared by precipitation from concentrated solutions at pH 9. These conditions seem to produce a form of CuO which is more difficult to reduce. Samples heated in flowing oxygen rather than in a muffle furnace are slightly more difficult to reduce as shown in Fig. 2. Typically the reduction peak is moved upwards by 20 K. It is noticeable, however, that the shapes of the TPR traces are unaffected by the method of preheating used. In our catalytic experiments the samples are reduced for 1 h at 523 K. The TPR results show that the copper should be fully reduced after this time but indicate that only a small fraction of the cobalt will be in the metallic state. Thermal analysis
During DTA (using a flowing atmosphere of either nitrogen or oxygen), each precursor produced an endotherm with an extrapolated onset temperature (T,)
287
(a) (ij
T/K Fig. 2. Temperature-programmed
reduction
of (a) sample I and (b) sample C, after calcination
(i) in muffle furnace and (ii) under flowing oxygen.
I
I 500
300
700
T/K s-
z 100.
e
k
P
F 6 ;
0
8
rdr
rb)
‘p 0
4--
‘\
80--
a
__ 300
4 300 Fig. 3. Thermal
I 500
700
analysis of sample G. (a) DTA;
500
700
T/K (b) TG; (c) DTG;
(d) EGA.
of 423 t- 5 K. In most cases (Fig. 3a shows a typical example) this peak was preceded by a gradual drift of the baseline (between 315 and 385 K), which was accompanied by the desorption of water (Fig. 3~). A further, but much more abrupt, change in baseline was usually observed at ca. 550 K. The corresponding TG and DTG traces (Fig. 3b and c) show that both the peak and the sudden shift of baseline are associated with substantial losses of mass (ca.
288
17 and ll%, respectively), which EGA revealed to be due to the concurrent evolution of carbon dioxide and water (Fig. 3d). Thermal analysis of the precursor of catalyst C shows it to be an exception (Fig. 4). In particular, the DTA trace (Fig. 4a) contains a distinct peak (T, = 533 K) in place of the baseline shift; this is associated with evolution of carbon dioxide and water, amounting to a loss in mass of 18% (Fig. 4b). Furthermore, samples C, D and E, which have the highest Nat content (Table 2) and are very hygroscopic, yielded an additional peak in both the DTA and DTG traces (e.g., see Fig. 4a and c), corresponding to the release of water between 315 and 450 K. We attribute the common DTA peak (T, = 423 K) to the decomposition of the hydrotalcite phase. The subsequent baseline shift (or peak in the case of sample C) and the associated DTG peak occur at a temperature which is typical of a malachite-type phase containing zinc [22], and so we suggest that they are due to the dissociation of an amorphous Cu-Zn hydroxycarbonate. We consider, therefore, that the relative sizes of the two major DTG peaks reflect the proportions of the crystalline (hydrotalcite-type) and the amorphous (malachite-type) phases. Finally, the absence of peaks at ca. 460-470 K and at ca. 525 K implies that there is little, if any, free Cu-Co hydroxycarbonate [ 131, or of the amorphous Al-Zn-Cu phase (“roderite” ) reported by Hijppener and co-workers [ 231.
T/K
300
so0
760
300
500
700
T/K
Fig. 4. Thermal analysis of sample C. (a) DTA; (b) TG; (c) DTG; (d) EGA.
289 2 (b)
P3 0
1
8
8
L-
parthcles
Fig. 5. SQXRF analysis of surface composition of (a) sample E and (b) sample G. Particles pl, p2, p6, p7, p8 refer to dry precursors; p3, p4, p5, p9, ~10, pll refer to calcined catalysts. (o ) Cu/ Al ratio; (0 ) Zn/Al ratio, (A ) Co/Al ratio.
SEM and SQXRF measurements The scanning electron micrographs showed that the materials had a flocculent appearance. The SQXRF facility was used to obtain analytical information on a number of dried precursors and calcined samples. Several particles of each material were selected and analysis performed at points about 10 pm apart. Typical plots are shown in Fig. 5 for samples E and G. These show that sample E is very heterogeneous both between particles and also within a single particle. The heterogeneity mainly reflects a variation in the amount of Al relative to Cu, Co or Zn, since the results show that the Cu/Al, Co/Al and Zn/ Al ratios are all clustered together at each point on a sample. Sample E was also found by XRF to have the lowest aluminium content of all the samples prepared. Clearly the conditions used to prepare sample E are not ideal. In contrast to sample E, the results for sample G show that this is quite homogeneous both in the dried and calcined states. As we shall see later this material turns out to be the most active catalyst in the synthesis gas reaction. Catalytic measurements
The catalysts calcined in a muffle furnace behaved in many respects quite differently to those heated in flowing oxygen so we shall consider the two treatments separately. Note, however, that the calcined samples were further heated in flowing nitrogen at 773 K before reduction so it is possible that the differ-
290
ences between “calcined” samples and those heated in flowing oxygen are due t,o the higher temperature used in the final treatment of the calcined samples rather than to the choice of the gaseous environment. Calcined samples. It was found to be exceedingly difficult to remove all the sodium from some of the catalyst precipitates. This unfortunately complicates the analysis of the results in terms of preparation variables. Nevertheless, some general conclusions can be drawn, and some clear differences between the catalysts can be observed from the results shown in Table 5. All the catalysts have rather low activities for the CO/H2 reaction. There is no obvious pattern to the activity data in terms of total surface area, sodium content, etc. The selectivity data show that catalyst C, which is heavily contaminated by sodium, is selective for the formation of higher oxygenates, producing about 57% C,, oxygenates. The presence of sodium is undoubtedly important in this particular case. However, comparison with catalysts D and E shows that the mere presence of sodium is not sufficient to provide selectivity to higher alcohols. Presumably the distribution of sodium with respect to the active phase is important. Leaving aside the alkali-containing catalysts we still observe significant differences between the catalysts. The selectivity to oxygenates rather than hydrocarbons varies from about 49% (catalyst F) to over 80% (catalyst H) at comparable conversions. The selectivity to higher oxygenates rather than methanol varies from about 10% (catalyst H) to over 30% (catalyst I). Table 6 summarizes the product distributions for the various catalysts. These TABLE 5 Activity and selectivity of various calcined catalysts for the reaction of a 25% CO/75% H2 mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
0.76 0.50 0.54 0.59 0.21 0.89 1.09 1.07 0.75
Activity”
12.8 8.3 9.0 10.0 3.4 14.8 19.6 17.5 12.1
Selectivity/%
CH,
C,+b
CH,OH
C,H,OH
C,+OH’
8.8 11.2 12.2 15.1 40.2 22.6 16.6 8.5 8.5
13.2 13.5 20.7 16.4 19.4 28.6 22.4 11.2 12.2
54.7 52.0 10.2 50.8 11.1 27.7 36.8 70.1 49.3
15.1 14.9 9.3 11.4 11.2 14.2 15.3 6.8 19.1
8.2 8.4 47.6 6.3 18.1 6.9 8.9 3.4 10.9
“Units are pmol CO, converted/g catalyst/min. ‘All hydrocarbon products containing two or more carbon atoms. ‘All oxygen-containing products excluding methanol and ethanol.
291 TABLE 6
Product distributions obtained from various calcined catalyst for the reaction of 25% CO/‘75% H2 at 40 atm pressure and 543 K Product”
Catalyst A
B
c
D
E
F
G
C,+h
11.2 6.3
14.2 6.8
25.0 14.0
18.9 7.9
58.4 9.8
32.6 14.1
23.1 11.2
9.7 4.8
11.3 6.0
CH,lOH
CH,
H
I
69.6
66.1
21.0
63.6
16.0
40.0
51.3
80.3
65.6
C&OH
9.6
9.5
9.5
7.1
8.2
10.2
10.7
3.9
12.7
C:,H,OH C,H,OH
2.7 0.6
2.8 0.6 _
9.0 6.2
4.4 3.3 _
2.5 0.6
2.9 0.9
1.1 0.2
1.8
2.0 0.5 _
3.2 1.2 _
5.1 2.4 1.9
-
-
C,H, ,OH CH ,CHO C,H,CHO C ,H,CHO CH,,COOCH,,
_
_
4.1
“Units are mol-% of product. ‘All hydrocarbon products containing two or more carbon atoms.
show that for catalyst C a wide range of oxygen-containing species are produced, including aldehydes and esters. In all cases a range of alcohols were produced but methanol was by far the most important oxygen-containing product. When compared with the results expected for a pure copper catalyst (i.e. a methanol producing catalyst) or a pure cobalt catalyst (i.e. a hydrocarbon producing catalyst) it is observed that our mixed catalysts are intermediate in character. All produced methanol as a major product and all produce a range of hydrocarbons. However, they also produce some higher alcohols indicating that there is a synergistic interaction between the copper and cobalt. From the statistical tests applied to the preparation procedure we can derive the order of importance for the experimental variables investigated. In terms of the activity for the formation of higher alcohols the order of importance (and the level, see Table 1) of the variables is as follows: ageing ( + ) > precip. temp. ( + ) > pH ( - ) > ion cont. ( + ) > drying temp. ( + ). On this basis we would predict that catalyst G should be the most active for the formation of higher alcohols. This is indeed what is observed. If the activity for the formation of ethanol is taken as the appropriate statistical response, then the order (and level) is: ageing ( + ) >pH ( - ) > precip. temp. ( + ) > ion cont. ( - ) > drying temp. ( + ). This predicts that catalyst I should be the most active. In fact, this catalyst is the second most active for the formation of ethanol. However, this is not a serious discrepancy since the only difference between
292
the preparations of G and I is the concentration of the metal ions in solution which is a variable of relatively low importance (see the order above). Samples heated in flowing oxygen. Table 7 shows the activity and selectivity results obtained with catalysts heated in flowing oxygen. Comparison with Table 5 shows that these catalysts differ in many important respects as compared with the samples calcined in air. In general, these catalysts are more active and the order of activity is quite different. Furthermore, the selectivity is very different with a much larger amount of methane being formed in almost every case. Selectivity to methanol is suppressed, but in general the selectivity to ethanol is somewhat higher. The detailed product distributions are given in Table 8. The overall pattern is similar to that shown in Table 6 for calcined samples with catalyst C being the only one which produces detectable amounts of aldehydes and esters. It is clear from these results that the thermal treatment of these catalysts is important. At this stage it is not known whether the crucial difference between the two heat treatments is the maximum temperature used or the use of flowing nitrogen rather than oxygen at the highest temperature. Further detailed studies on the influence of pretreatment conditions on catalytic properties are currently underway. The analysis of the activity results using the statistical tests for the preparation showed that the same catalysts should be most active for the formation of higher alcohols (catalyst G) and for the formation of ethanol (catalyst I) as was predicted from the data for the calcined catalysts. As before, it is found TABLE 7 Activity and selectivity of various oxygen-treated catalysts for the reaction of a 25% CO/75% H, mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
1.35 1.50 0.78 0.85 0.48 0.94 2.33 0.76 1.77
Activity”
19.1 25.1 13.2 14.0 7.9 15.7 38.7 12.7 29.3
Selectivity/%
CH,
c 2+ b
CH,OH
C,H,OH
C,+OH”
25.8 28.3 25.3 19.6 36.6 24.1 25.2 18.7 27.0
26.4 30.2 23.1 25.0 18.1 28.3 32.8 23.3 29.4
27.8 18.6 7.7 34.8 14.7 23.0 17.3 29.2 14.8
13.3 14.8 11.3 13.1 14.1 15.9 15.1 16.2 15.9
6.8 8.1 32.7 7.6 16.6 8.8 9.7 9.9 11.3
“Units are pmol CO, converted/g cataIyst/min. ‘All hydrocarbon products containing two or more C atoms. ‘All oxygen-containing products excluding methanol and ethanol.
293
TABLE 8 Product distributions obtained from various oxygen-treated catalysts for the reaction of 25% CO/ 75% H.Jat 40 atm pressure and 543 K Product”
Catalyst A B
C
D
E
F
G
H
I
CH,
36.4
42.5
45.5
27.5
52.6
35.6
39.9
27.8
42.9
C,+h
12.1 39.2
14.7 28.0
13.0 13.9
11.1 48.9
9.0 21.1
14.6 34.1
16.1 21.4
12.1 43.4
15.6 23.4
9.4
11.1
10.2
9.2
10.6
11.7
12.0
12.1
12.6
2.4 0.6 _
2.7 1.1
6.0 4.9 1.7
4.4 2.7
3.0 1.0 _
3.2 1.4
3.7 0.9
3.2 1.3
_
2.0 1.1
2.5 0.8 _ -
0.2 1.5
-
CH:,OH C,H,OH C:,H,OH C,H,OH
C,H,,OH
CH:,CHO C,H,CHO C,,H,CHO CH,,COOCH., C,H,COOH
_
_ _
_
_
_ 1.1
“Units are mol-% of product. ‘All hydrocarbon products containing two or more carbon atoms.
that catalyst G is most active for the formation of higher alcohols but that catalyst I is about 20% less active than catalyst G for the formation of ethanol. Nevertheless, the overall predictive value of the statistical method has been demonstrated for the CO/H2 reaction.
Table 9 summarizes the activity and selectivity results obtained for the CO/ CO,/H, reaction over the catalysts heated in flowing oxygen. Comparison with Table 7 shows that all the catalysts are much more active when there is carbon dioxide in the reaction mixture. The rate of formation of methanol is about the same as in the absence of carbon dioxide; the rate of formation of ethanol and higher alcohols is about 4 times larger; and the rate of formation of hydrocarbons is about 5-10 times larger. The net effect of these changes is that the selectivity for the formation of alcohols is lower in the presence of carbon dioxide although the activity is much greater. Table 10 shows the product distribution observed for this reaction. In this case all the catalysts tend to produce the same wide range of products, which contrasts with the observed product distribution in the absence of carbon dioxide. When the statistical test is applied to the preparation procedure it is found that the best catalyst for the synthesis of ethanol in the presence of carbon dioxide should be catalyst I (as before). This is indeed what is observed. For the formation of higher alcohols the statistical results are distorted by the re-
294
TABLE 9 Activity and selectivity of various oxygen-treated catalysts for the reaction of a 10% CO/lo% CO,/80% He mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
4.58 5.06 1.58 4.56 4.47 7.59 5.91 4.51 9.49
Activity”
Selectivity/%
62.4 65.6 20.3 60.9 57.9 100.2 77.9 59.5 127.4
CH,
c 2+ b
CH,OH
GH,OH
C,+OH’
34.7 41.9 27.8 31.1 21.6 43.7 39.2 42.0 41.5
23.4 25.1 25.2 31.3 31.6 33.9 32.2 29.4 33.4
16.5 8.1 4.5 7.3 4.4 2.6 6.0 6.1 4.4
13.2 13.4 11.9 13.8 12.8 11.6 12.2 11.7 11.2
10.2 9.5 27.6 14.5 27.6 9.2 11.4 8.8 10.5
“Units are pmol CO, converted/g catalyst/min. hAll hydrocarbon products containing two or more carbon atoms. ‘All oxygen-containing products excluding methanol and ethanol. TABLE 10 Product distributions obtained from various oxygen-treated catalysts for the reaction of 10% CO/ 10% CO,/80% H> at 40 atm pressure and 543 K Product”
CH,
C,+h CH,,OH C,H,OH C,,H,OH C,H,OH C,H,,OH CH,,CHO CH:,COOCH,, C,H,COOH C:,H,COOH CH,,CHOHCH,,
Catalyst A
B
C
D
E
F
G
H
I
50.7 11.0 24.0 9.6 1.9 1.1 0.2
61.8 12.3 11.9 9.9 1.8 1.0 0.2
51.1 15.2 8.4 11.0 5.5 5.3 1.4 0.6 0.6
52.7 15.9 12.3 11.6 2.5 1.4 0.5 -
43.3 19.5 8.9 12.8 4.5 4.7 1.8 0.4 0.6
67.2 16.5 4.1 9.0 1.6 0.7 0.2 -
60.3 16.0 9.2 9.4 1.9 1.1 0.4
63.4 14.6 9.2 8.9 1.7 0.9 0.2 _ _
63.6 16.5 6.7 8.5 1.7 1.1 0.3
0.9 0.2 0.1
1.0 -
2.1 0.6 0.1
2.7 0.7 0.1
1.1 0.4 0.1
1.3 0.4
0.9 0.2 0.1
1.1 0.4 0.1
_ 0.8 0.4 0.1
“Units are mol-% of product. “All hydrocarbon products containing two or more carbon atoms.
sults for samples D and E (both containing sodium) which show large increases in activity in the presence of carbon dioxide. The result is that none of the catalysts is expected to be significantly more active than the others. In fact
295
catalyst I is again the most active for this reaction. DISCUSSION
In a previous study, Marchi and co-workers [ 131 observed how the preparative conditions influence the crystallinity and homogeneity of the precursors of Cu/Co/Al catalysts, We have assessed similar variables in the present study of the Cu/Co/Al/Zn system, but we have related them to the catalytic performance of the activated samples. In common with the work on Cu/Co/Al [ 131 we have found that the homogeneity and the degree of crystallinity of our dry precursors reflect the conditions used during precipitation. In fact, in our experience, it is very difficult to prepare the hydrotalcite phase exclusively, because it is usually associated with an amorphous malachite-type phase. However, even though the statistical screening of a group of catalysts has allowed us to optimize the procedure for preparing dry precursors, the variation in catalytic performance is relatively small. In brief, the conditions during precipitation and drying have a marked effect on the composition and structure of the precursors, and yet these are not the most important factors in relation to either the activity or selectivity of the catalysts during the conversion of syngas to higher alcohols. As Tables 5 and 7 show, thermal activation has a much more pronounced effect on the catalytic properties than does the method of preparation. In particular, heating in nitrogen at 770 K generally produces much less active catalysts (which are more selective for the formation of methanol) than is found when the same materials are heated in oxygen at 720 K. Furthermore, changes in the subsequent treatment (reduction conditions; duration of catalyst testing) do not greatly influence the performance of the catalysts. Unlike the precursors of the Cu/Co/Cr catalysts [ 161, our samples do not show any signs of bulk oxidation during thermal activation (even DTA of samples heated under flowing oxygen did not reveal any exothermic peaks), suggesting that the changes are limited to the surface. It seems that by initially choosing a calcination temperature which was high enough to ensure the removal of adsorbed contaminants [24] we were effectively destroying potentially active sites. Thus, thermal activation emerges as a critical step in the formation of an effective catalyst. One important outstanding question concerning these multicomponent catalysts is the role of cobalt. The initial work by Courty et al. [9] encourages belief in the importance of forming small bimetallic Cu/Co clusters. Certainly it is true that separate copper and cobalt metallic phases will be unlikely to produce higher alcohol with a high selectivity. Instead, it would be expected that copper would produce methanol and cobalt would produce hydrocarbons. It is well established, and confirmed in the present work, that in co-precipitated catalysts the reduction of cobalt is rather difficult and, depending on the
support, may not occur to any significant extent under normal catalyst activation conditions [ 141. In our work, we have used a very mild reduction treatment on catalysts previously exposed to a rather vigorous calcination. We would anticipate that only a small amount of the cobalt would be in the fully reduced state at the commencement of the activity testing. Therefore, it is interesting that in the early stages of the catalyst testing we observe the formation of significant amounts of higher alcohols and that the selectivity does not change very much with time-on-stream. This suggests either that metallic cobalt is not required for the formation of higher alcohols or that only a very small amount of cobalt is necessary to modify a copper particle sufficiently to generate higher alcohols in preference to methanol. It is noticeable in the work of Courty et al. [ 91 that the selectivity to higher alcohols slowly declines with time-on-stream, while, presumably, the degree of reduction of the cobalt will increase. One plausible explanation for these various effects can be derived from the work of Mouaddib and Perrichon [ 141. These workers have studied the influence of the support on the synthesis of alcohols over Cu/Co catalysts. They observe that on MgO the selectivity to methanol is 75% and to higher alcohols it is lo%, whereas on ZrOz the selectivity to methanol is 39% and to higher alcohols it is 25%. On TiOa the selectivity to methanol is only 12% and to higher alcohols it is only 13%. The explanation offered is that the ease of reduction of cobalt increases in the order: MgO < Zr02 < TiOa. The MgO-supported Cu/Co catalyst then contains copper particles modified by a small amount of metallic cobalt, and so is essentially a methanol synthesis catalyst. The TiO*-supported catalyst contains Cu/Co particles rich in cobalt and is essentially a hydrocarbon synthesis catalyst. The Zr02 catalyst is intermediate in behaviour. Providing these results can be transferred to our Cu/Co/Zn/Al catalysts then the formation of higher alcohols would be due to the presence of a small amount of metallic cobalt intimately mixed with the copper. More complete reduction of cobalt would be disadvantageous. However, one has to be careful about making assumptions about the composition of the surface of Cu/Co bimetallic clusters. Surface science studies on single crystal and polycrystalline materials [ 181 provide evidence of a significant amount of surface enrichment by copper in such samples. Further work is in progress to try to ascertain the relationship between selectivity for the formation of higher alcohols and the degree of reduction of the various cobalt phases present in these multicomponent catalysts. ACKNOWLEDGEMENTS
We are grateful to the S.E.R.C. for supporting this research and for the award of a studentship to JEB and a fellowship to SEG.
297 REFERENCES
7 8 9 10
11 12
13 14 15 16 17 18 19 20 21 22 23 24
X. Xiaoding, E.B.M. Doesburg and J.J.F. Scholten, Catal. Today, 2 (1987) 125. M. Stoel, ECN Process Supplement, Nov. (1985). A. Sugier and E. Freund, U.S. Patent 4 122 110 (1987). A. Sugier and E. Freund, U.S. Patent 4 291 126 (1979). P. Courty, D. Durand, A. Sugier and E. Freund, G.B. Patent 2 118 061 (1983). P. Chaumette, P. Courty, D. Durand, P. Grandvallet and C. Travers, G.B. Patent 2 158 730 (1985). A. Sugier and E. Freund, G.B. Patent 1547 687 (1977). A. Sugier and E. Freund, G.B. Patent 2 037 179 (1979). P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catol., 17 (1982) 241. R. Szymanski, C. Travers, P. Chaumette, P. Courty and D. Durand in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 738. P. Grandvallet, P. Courty and E. Freund, Proc. 8th Int. Congr. Catal. Berlin, 1984, Vol. II, Verlag Chemie, Weinheim, 1984, p. 81. S. Gusi, F. Pizzoli, F. Trifiro‘, A. Vaccari and G. de1 Piero, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 753. A.J. Marchi, A.G. Sedran and C.R. Apesteguia, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, paper H7. Mouaddib, N., and Perrichon, V., 9th International Congress on Catalysis, Calgary, 1988, Vol. 2, The Chemical Institute of Canada, Ottawa, 1988, p. 521. A.J. Marchi, J.I. Di Cosimo and C.R. Apesteguia, 9th International Conress on Catalysis, Calgary, 1988, Vol. 2 The Chemical Institute of Canada, Ottawa, 1988, p. 529. G.R. Sheffer and T.S. King, Appl. Catal., 44 (1988) 153. W.X. Pan, R. Cao and G.L. Griffin, J. Catal., 114 (1988) 447. D. Chadwick, J. Pritchard, and A. Taylor, in preparation. P.A. Sermon, in preparation. R. Stowe and R. Mayer, Ind. Eng. Chem., 58 (1966) 236. W.A. Dietz, J. Gas Chromatogr., (1967) 68. P. Porta, S. De Rossi, G. Ferraris, M. Lo Jacono, G. Minelli and G. Moretti, J. Catal., 109 (1988) 367. R.H. HSppener, E.B.M. Doesburg and J.J.F. Scholten, Appl. Catal., 25 (1986) 109. J.E. Baker, R. Burch and SE. Golunski, Thermochimica Acta, 142 (1989) 329.
Applied
279
Printed in The Netherlands
Synthesis of Higher Alcohols over Copper/Cobalt Catalysts Influence of Preparative Procedures on the Activity and Selectivity of Cu/Co/Zn/Al Mixed Oxide Catalysts J.E. BAKER, R. BURCH” and S.E. GOLUNSKI Catalysis Research Whiteknights,
Group, Chemistry
Department,
University
of Reading, P.O. Box 224,
Reading RG6 2AD (U.K.)
(Received 18 February 1989, revised manuscript received 27 April 1989)
ABSTRACT The effects of selected preparative variables on the performance of Cu/Co/Al/Zn catalysts during the conversion of synthesis gas to higher alcohols have been assessed. Using the PlackettBurman statistical technique, the following order of importance has been observed for the preparation of dry (co-precipitated) precursors: ageing of precipitate > precipitation temperature > pH > total metal ion concentration > drying temperature. Samples prepared accordingly are comprised mainly of a crystalline hydrotalcite-type phase, but some amorphous malachite-type phase is also present. However, the degrees of crystallinity and homogeneity do not greatly influence the activity or selectivity of the calcined and reduced catalysts. It has become apparent that the thermal activation of the dry precursors is the single most important variable in the preparation of these catalysts. The importance of the degree of reduction of the various cobalt phases in determining the catalytic selectivity is discussed.
INTRODUCTION
The conversion of synthesis gas (CO/CO,/H,) to higher alcohols can be achieved using a variety of catalytic processes, which are based, for example, on modified methanol synthesis catalysts, rhodium containing catalysts, alkalised MO&, or Cu/Co oxides [ 11. Of the catalysts currently available, those developed by Institut Francais du P&role (IFP) [2-111 are among the most interesting, because of their stability, their high activity at relatively low pressures, and their selectivity towards alcohols rather than hydrocarbons. Several patents have been published which describe these catalysts [ 2-81, which are based on a combination of copper and
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
cobalt. Typically they contain another trivalent metal (Al orCr), Zn, an alkali metal, and possibly a number of minor components. It is claimed [ 61 that the preferred catalyst contains: Cu: l&45%; Co: 9-20%; Al: 7-25%; Zn: 1550%; with alkali/Al: o-0.003; Zn/Al: 0.4-2.0; Co/Al: 0.2-0.75; Cu/Al: 0.1-3.0. The patents [ 3-61 emphasize the importance of maintaining the homogeneity of the precursors during each step in the preparation and pretreatment of the catalysts. However, although detailed procedures are given in the patents concerning the choice of conditions (precipitation temperature, pH, ageing time and temperature, drying and calcination methods, etc.), this information alone does not provide an insight into the way in which the preparation of these materials affects their catalytic properties. Furthermore, there have as yet been few publications on these, or closely related, catalysts in the open literature [9,10,12-171 and as far as we are aware no systematic study of the preparation of these multicomponent Cu/Co/Zn/Al catalysts has been published. Complementary studies on single crystals and on simple binary oxides are to be published [ 18,191. EXPERIMENTAL
Catalystpreparation We have used the Plackett-Burman statistical technique {as reported by Stowe and Mayers [ 201 for catalyst preparation by co-precipitation) to examine the effect of selected parameters on the catalytic properties of Cu/Co/ Zn/Al catalysts. The technique allows several variables to be screened using a small number of preparations. Each variable is given a high ( + ) or low ( - ) value and then, by using an appropriate matrix, a set of preparations is derived which allows all the variables to be graded in importance. The variables and the values used in our experiments are shown in-Table 1, together with the matrix summarizing the catalyst preparations. As an example of the use of the matrix, the precursor of catalyst A was prepared from solutions with a total metal ion concentration of 0.5 mol dmp3 by precipitation at pH 9, at 330-340 K; the suspension was aged for 1 h at 350 K; the precipitate was dried at 360 K for 16 h and then at 390 K for 3 h. Preparation of the precursors involved co-precipitation from two solutions, one of which contained the metal nitrates and the other sodium carbonate (see Table 1) . All reagents were of AnalaR grade. The overall concentrations of Cu, Co, Zn, and Al were chosen to match those reported by IFP for the preparation of one of their best catalysts (catalyst A in ref. 6). The two solutions were mixed in a glass reaction vessel (volume, 100 cm3) initially containing deionised water which was at the required precipitation temperature. The solutions were injected below the surface of the rapidly stirred water, and the flow-rates were adjusted to maintain the pH at the selected
281
TABLE 1 Variables used in the statistical preparation procedure (a) Variables
and levels
Variable
(i) (ii) (iii) (iv) (v) (vi) (vii)
Level
pH during precipitation Precipitation temp. (K) Ageing of suspension Drying temp. (K)/time (h) Blank Total metal cont. (mol dm-“) Blank
Low (-)
High (+)
7 293 none 360/16 & 390/3
9 330-340 350 K/l h 390/19
0.5
1.5
(b) Matrix
Preparation
Variable
(i)
(ii)
(iii)
I (IFP method) (c) Solutions
Solution 1 Low(-)
High (+)
0.1 M 0.15 M 0.066 M 0.2 M
0.3 M 0.45 M 0.198 M
0.6 M
Cu nitrate Zn nitrate Co nitrate Al nitrate
Solution 2 Low (-)
High ( + )
0.577 M
1.731 M
Na carbonate
(iv)
(v)
(vi)
(vii)
282
value. Usually, the total flow-rate was about 10 cm3 min-‘, giving a residence t.ime of about 10 min. The suspension drained out through a sidearm into a large collecting vessel. After discarding the first 300 cm’, the suspension was collected and aged if required. The catalyst precursor was separated from the mother liquor by centrifugation, washed with 30 dm3 distilled water to remove Na+ ions, and dried as required. Separate portions of each dried sample were calcined either (a) in a muffle furnace or (b) in flowing oxygen. In the case of (a) the samples were heated at 723 K for 4 h, followed by heating in the catalytic reactor in nitrogen for 0.5 h at 773 K to remove adsorbed contaminants: in the case of (b), the samples were heated at 10 K min-’ to 723 K and held at this temperature for 4 h. The catalyst samples (100 mg) were reduced in situ in the catalytic reactor (see later) by heating in pure hydrogen (at 1 atm, 101.3 kPa) at 10 K min-l to 523 K and holding at this temperature for 1 h. Characterization
of precursors
X-ray diffraction (XRD) was used to determine the structure of the crystalline component of the dry precursors using a General Electrics diffractometer with Cu Ka radiation. The surface composition was examined by scanning electron microscopy (SEM) and semi-quantitative X-ray fluorescence (SQXRF) using a Cambridge Scanning Casmscan 3130 electron microscope combined with a Link Analyser System 290. Thermal analysis was carried out on samples (100 mg) which were heated at 10 K min-’ to 775 K. During differential thermal analysis (DTA) using a Stanton Redcroft Standata 6-25 instrument, heat treated cr-A&O, was used as the inert reference material, and each sample was heated under a flowing atmosphere of oxygen-free nitrogen or high purity oxygen (both from B.O.C.). Simultaneous thermogravimetry (TG) and derivative thermogravimetry (DTG) were performed using a Sartorius vacuum microbalance linked to a BBC microcomputer. Evolved gas analysis (EGA) was achieved by heating a sample under flowing helium (B.O.C.) and monitoring selected peaks in the mass spectrum of the effluent gas, using a Spectramass Dataquad 100-V3.15 quadrupole mass spectrometer; the data were collected and processed on an Amstrad 1512 computer. Characterization
of catalysts
Surface areas of the calcined samples were determined by nitrogen adsorption at 77 K (BET method) in a glass volumetric apparatus. Chemical analysis for sodium was carried out using a Perkin Elmer 1100B atomic absorption spectrometer. Analysis for Cu, Co, Zn, and Al was performed using a Philips PW 1410 X-ray fluorescence spectrophotometer. XRD
283
and SQXRF were used to examine the bulk structure and surface composition, respectively. The reducibility of the samples was determined by temperature-programmed reduction (TPR) with a heating rate of 15 K min-l under a flowing atmosphere of 5% H,/95% Ar. Catalyst testing
The activity and selectivity of the various samples for the synthesis of alcohols from synthesis gas mixtures were determined using a glass-lined stainless steel microreactor (I.D. 4 mm) capable of operating at pressures up to 50 atm and temperatures of 773 K. The samples (100 mg) were reduced in pure hydrogen at 1 atm by heating at 10 K min-’ to 523 K and holding at this temperature for 1 h. The microreactor was pressurized to the reaction pressure (40 atm). Premixed gases (B.O.C. Special gases, lo%, CO/lo% CO,/80% H, and 25% CO/75% HZ) were admitted to the reactor through Krohne mass flow controllers (15 cm3 mine1 ). Th e activity and selectivity were determined at 523,533 and 543 or at 533,543 and 553 K (5 h at each temperature) using an automated gas analysis system. This comprised a Perkin Elmer 8500 series dual channel FID gas chromatograph fitted with a heated gas sampling valve. Products were separated into oxygenates and hydrocarbons using a 2-m precolumn filled with Reoplex 400 on Chromosorb GAW and then the two product streams were further separated using a pair of 3-m Porapak QS columns in parallel. The outputs from the two detectors were fed simultaneously to a Daisy two channel integrator for data acquisition and storage. Response factors for the various products were taken from the literature [ 211. RESULTS
Chemical analysis
Table 2 summarizes the analytical results on the catalysts. Rather surprisingly the wt.-% of each of the metals in the catalysts does not vary a lot between the different preparations, indicating that the method of preparation has little influence on the amounts of the metals precipitated. Samples C,D and E have high sodium contents. C and E were prepared using concentrated solutions at pH 9 without ageing; D was prepared using concentrated solutions and a precipitation temperature of 330-340 K. It is possible that under these conditions some sodium becomes incorporated into the brucite layers making it very difficult to remove by washing. Samples which were precipitated using dilute solutions and/or at 333-343 K, and/or aged, were very low in sodium content. Comparison with the nominal composition reported for the IFP catalyst [ 61 used as a reference for our preparations (see Table 2) shows that our catalysts
284 TABLE 2 Analytical data on the catalysts Weight percentage
Catalyst
A B C D E F G H I IFP catalyst”
cu
co
Zn
Al
Na
24.1 23.1 18.5 21.4 22.7 22.7 23.2 24.7 23.7 19.7
13.0 12.6 10.6 10.8 11.9 11.8 12.9 10.0 13.2 11.2
24.1 23.8 24.5 20.2 22.3 21.9 23.4 23.5 24.1 30.9
11.4 12.6 11.8 13.0 10.6 14.9 12.7 13.7 11.6 11.2
<0.2 < 0.2 5.7 5.1 4.4 <0.2 to.2 to.2 (0.2 to.2
“Nominal values reported in ref. 6, based on composition of precursors.
contain similar amounts of Co and Al, rather more Cu and significantly less Zn. Surface area measurements The surface areas of the calcined samples ranged from 41 to 230 m2 g-l as shown in Table 3. Samples calcined in a muffle furnace generally have lower surface areas than those heated in flowing oxygen although there are excepTABLE 3 Surface areas of calcined and heat-treated samples Catalyst
Surface area
(m’ g-’ I
Air”
Oxygenb
182 191 56 148 42 195 111 169 135
200 173 56 160 41 230 224 133 208
“Samples calcined in air. “Samples heated in flowing oxygen.
tions. Samples C and E have particularly low surface areas (56 and 41 m2 g-i); both were precipitated at pH 9 using high metal concentrations. High surface area samples were obtained when at least two of the following conditions were met: pH 7; low ion concentration; samples aged, precipitation temperature, 330-340 K. X-ray diffraction
measurements
The X-ray diffractograms for the various dried precursors were all very similar. A typical diffractogram is shown in Fig. 1. This indicates the presence of the hydrotalcite phase previously reported [9,10,12]. The intensities of the lines on the various diffractograms differed quite a lot, indicating the presence of varying amounts of the crystalline phase. Table 4 summarizes the intensity data for the (003 ) peak, which corresponds to the d-spacing between the brucite layers. The peak widths at half maximum also varied as shown in Table 4, indicating differences in the degree of order of the crystalline phase. Samples D, F, G, H and I gave sharp peaks indicating that samples precipitated at pH 7 contained the most ordered crystalline phase. Of these samples, F, H and I were prepared from dilute solutions. However, if precipitation was performed at 330-340 K, or if the samples were aged, the diffractogram was still well defined, irrespective of the concentration of the metal ions in solution. Clearly there are competing effects during the preparation of such complex mixed oxides. However, it appears that to produce a highly crystalline, well
a
-
Fig. 1. X-ray diffractogram of dried precursor. (e) 015; (f) 018; (g) 1010.
Miller indices:
(a) 003; (b) 006; (c) 012; (d) 104;
286 TABLE 4 X-ray diffraction data on the various dried catalyst precursors Catalyst
(003) peak height
Peak width
11.5 3.8 4.5 15.0 11.0 11.5 12.0 5.5 10.0
72 77 57 50 60 41 44 44 41
“peak width at half maximum, units are nm.
ordered material, precipitation should be done at pH 7, 330-340 K, with low concentrations of metal ions, followed by ageing of the suspension. XRD of the calcined catalysts revealed that the only crystalline phase present is a CoAl,O, spinel. This is consistent with the findings of Courty et al.
[91. Temperature-programmed
reduction
experiments
The TPR traces for the catalyst exhibit two features as shown in Fig. 2. The first feature is a peak occurring between 490 and 540 K (samples A, B, D, F, G, H, I), or, in the case of samples C and E, a doublet occurring between 540 and 610 K. This feature probably corresponds to the reduction of CuO. The second feature is a flat, broad doublet extending from 570 to 770 K, probably due to the stepwise reduction of Co,O, and/or CoAl,O, [1,9,11]. Samples C and E were prepared by precipitation from concentrated solutions at pH 9. These conditions seem to produce a form of CuO which is more difficult to reduce. Samples heated in flowing oxygen rather than in a muffle furnace are slightly more difficult to reduce as shown in Fig. 2. Typically the reduction peak is moved upwards by 20 K. It is noticeable, however, that the shapes of the TPR traces are unaffected by the method of preheating used. In our catalytic experiments the samples are reduced for 1 h at 523 K. The TPR results show that the copper should be fully reduced after this time but indicate that only a small fraction of the cobalt will be in the metallic state. Thermal analysis
During DTA (using a flowing atmosphere of either nitrogen or oxygen), each precursor produced an endotherm with an extrapolated onset temperature (T,)
287
(a) (ij
T/K Fig. 2. Temperature-programmed
reduction
of (a) sample I and (b) sample C, after calcination
(i) in muffle furnace and (ii) under flowing oxygen.
I
I 500
300
700
T/K s-
z 100.
e
k
P
F 6 ;
0
8
rdr
rb)
‘p 0
4--
‘\
80--
a
__ 300
4 300 Fig. 3. Thermal
I 500
700
analysis of sample G. (a) DTA;
500
700
T/K (b) TG; (c) DTG;
(d) EGA.
of 423 t- 5 K. In most cases (Fig. 3a shows a typical example) this peak was preceded by a gradual drift of the baseline (between 315 and 385 K), which was accompanied by the desorption of water (Fig. 3~). A further, but much more abrupt, change in baseline was usually observed at ca. 550 K. The corresponding TG and DTG traces (Fig. 3b and c) show that both the peak and the sudden shift of baseline are associated with substantial losses of mass (ca.
288
17 and ll%, respectively), which EGA revealed to be due to the concurrent evolution of carbon dioxide and water (Fig. 3d). Thermal analysis of the precursor of catalyst C shows it to be an exception (Fig. 4). In particular, the DTA trace (Fig. 4a) contains a distinct peak (T, = 533 K) in place of the baseline shift; this is associated with evolution of carbon dioxide and water, amounting to a loss in mass of 18% (Fig. 4b). Furthermore, samples C, D and E, which have the highest Nat content (Table 2) and are very hygroscopic, yielded an additional peak in both the DTA and DTG traces (e.g., see Fig. 4a and c), corresponding to the release of water between 315 and 450 K. We attribute the common DTA peak (T, = 423 K) to the decomposition of the hydrotalcite phase. The subsequent baseline shift (or peak in the case of sample C) and the associated DTG peak occur at a temperature which is typical of a malachite-type phase containing zinc [22], and so we suggest that they are due to the dissociation of an amorphous Cu-Zn hydroxycarbonate. We consider, therefore, that the relative sizes of the two major DTG peaks reflect the proportions of the crystalline (hydrotalcite-type) and the amorphous (malachite-type) phases. Finally, the absence of peaks at ca. 460-470 K and at ca. 525 K implies that there is little, if any, free Cu-Co hydroxycarbonate [ 131, or of the amorphous Al-Zn-Cu phase (“roderite” ) reported by Hijppener and co-workers [ 231.
T/K
300
so0
760
300
500
700
T/K
Fig. 4. Thermal analysis of sample C. (a) DTA; (b) TG; (c) DTG; (d) EGA.
289 2 (b)
P3 0
1
8
8
L-
parthcles
Fig. 5. SQXRF analysis of surface composition of (a) sample E and (b) sample G. Particles pl, p2, p6, p7, p8 refer to dry precursors; p3, p4, p5, p9, ~10, pll refer to calcined catalysts. (o ) Cu/ Al ratio; (0 ) Zn/Al ratio, (A ) Co/Al ratio.
SEM and SQXRF measurements The scanning electron micrographs showed that the materials had a flocculent appearance. The SQXRF facility was used to obtain analytical information on a number of dried precursors and calcined samples. Several particles of each material were selected and analysis performed at points about 10 pm apart. Typical plots are shown in Fig. 5 for samples E and G. These show that sample E is very heterogeneous both between particles and also within a single particle. The heterogeneity mainly reflects a variation in the amount of Al relative to Cu, Co or Zn, since the results show that the Cu/Al, Co/Al and Zn/ Al ratios are all clustered together at each point on a sample. Sample E was also found by XRF to have the lowest aluminium content of all the samples prepared. Clearly the conditions used to prepare sample E are not ideal. In contrast to sample E, the results for sample G show that this is quite homogeneous both in the dried and calcined states. As we shall see later this material turns out to be the most active catalyst in the synthesis gas reaction. Catalytic measurements
The catalysts calcined in a muffle furnace behaved in many respects quite differently to those heated in flowing oxygen so we shall consider the two treatments separately. Note, however, that the calcined samples were further heated in flowing nitrogen at 773 K before reduction so it is possible that the differ-
290
ences between “calcined” samples and those heated in flowing oxygen are due t,o the higher temperature used in the final treatment of the calcined samples rather than to the choice of the gaseous environment. Calcined samples. It was found to be exceedingly difficult to remove all the sodium from some of the catalyst precipitates. This unfortunately complicates the analysis of the results in terms of preparation variables. Nevertheless, some general conclusions can be drawn, and some clear differences between the catalysts can be observed from the results shown in Table 5. All the catalysts have rather low activities for the CO/H2 reaction. There is no obvious pattern to the activity data in terms of total surface area, sodium content, etc. The selectivity data show that catalyst C, which is heavily contaminated by sodium, is selective for the formation of higher oxygenates, producing about 57% C,, oxygenates. The presence of sodium is undoubtedly important in this particular case. However, comparison with catalysts D and E shows that the mere presence of sodium is not sufficient to provide selectivity to higher alcohols. Presumably the distribution of sodium with respect to the active phase is important. Leaving aside the alkali-containing catalysts we still observe significant differences between the catalysts. The selectivity to oxygenates rather than hydrocarbons varies from about 49% (catalyst F) to over 80% (catalyst H) at comparable conversions. The selectivity to higher oxygenates rather than methanol varies from about 10% (catalyst H) to over 30% (catalyst I). Table 6 summarizes the product distributions for the various catalysts. These TABLE 5 Activity and selectivity of various calcined catalysts for the reaction of a 25% CO/75% H2 mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
0.76 0.50 0.54 0.59 0.21 0.89 1.09 1.07 0.75
Activity”
12.8 8.3 9.0 10.0 3.4 14.8 19.6 17.5 12.1
Selectivity/%
CH,
C,+b
CH,OH
C,H,OH
C,+OH’
8.8 11.2 12.2 15.1 40.2 22.6 16.6 8.5 8.5
13.2 13.5 20.7 16.4 19.4 28.6 22.4 11.2 12.2
54.7 52.0 10.2 50.8 11.1 27.7 36.8 70.1 49.3
15.1 14.9 9.3 11.4 11.2 14.2 15.3 6.8 19.1
8.2 8.4 47.6 6.3 18.1 6.9 8.9 3.4 10.9
“Units are pmol CO, converted/g catalyst/min. ‘All hydrocarbon products containing two or more carbon atoms. ‘All oxygen-containing products excluding methanol and ethanol.
291 TABLE 6
Product distributions obtained from various calcined catalyst for the reaction of 25% CO/‘75% H2 at 40 atm pressure and 543 K Product”
Catalyst A
B
c
D
E
F
G
C,+h
11.2 6.3
14.2 6.8
25.0 14.0
18.9 7.9
58.4 9.8
32.6 14.1
23.1 11.2
9.7 4.8
11.3 6.0
CH,lOH
CH,
H
I
69.6
66.1
21.0
63.6
16.0
40.0
51.3
80.3
65.6
C&OH
9.6
9.5
9.5
7.1
8.2
10.2
10.7
3.9
12.7
C:,H,OH C,H,OH
2.7 0.6
2.8 0.6 _
9.0 6.2
4.4 3.3 _
2.5 0.6
2.9 0.9
1.1 0.2
1.8
2.0 0.5 _
3.2 1.2 _
5.1 2.4 1.9
-
-
C,H, ,OH CH ,CHO C,H,CHO C ,H,CHO CH,,COOCH,,
_
_
4.1
“Units are mol-% of product. ‘All hydrocarbon products containing two or more carbon atoms.
show that for catalyst C a wide range of oxygen-containing species are produced, including aldehydes and esters. In all cases a range of alcohols were produced but methanol was by far the most important oxygen-containing product. When compared with the results expected for a pure copper catalyst (i.e. a methanol producing catalyst) or a pure cobalt catalyst (i.e. a hydrocarbon producing catalyst) it is observed that our mixed catalysts are intermediate in character. All produced methanol as a major product and all produce a range of hydrocarbons. However, they also produce some higher alcohols indicating that there is a synergistic interaction between the copper and cobalt. From the statistical tests applied to the preparation procedure we can derive the order of importance for the experimental variables investigated. In terms of the activity for the formation of higher alcohols the order of importance (and the level, see Table 1) of the variables is as follows: ageing ( + ) > precip. temp. ( + ) > pH ( - ) > ion cont. ( + ) > drying temp. ( + ). On this basis we would predict that catalyst G should be the most active for the formation of higher alcohols. This is indeed what is observed. If the activity for the formation of ethanol is taken as the appropriate statistical response, then the order (and level) is: ageing ( + ) >pH ( - ) > precip. temp. ( + ) > ion cont. ( - ) > drying temp. ( + ). This predicts that catalyst I should be the most active. In fact, this catalyst is the second most active for the formation of ethanol. However, this is not a serious discrepancy since the only difference between
292
the preparations of G and I is the concentration of the metal ions in solution which is a variable of relatively low importance (see the order above). Samples heated in flowing oxygen. Table 7 shows the activity and selectivity results obtained with catalysts heated in flowing oxygen. Comparison with Table 5 shows that these catalysts differ in many important respects as compared with the samples calcined in air. In general, these catalysts are more active and the order of activity is quite different. Furthermore, the selectivity is very different with a much larger amount of methane being formed in almost every case. Selectivity to methanol is suppressed, but in general the selectivity to ethanol is somewhat higher. The detailed product distributions are given in Table 8. The overall pattern is similar to that shown in Table 6 for calcined samples with catalyst C being the only one which produces detectable amounts of aldehydes and esters. It is clear from these results that the thermal treatment of these catalysts is important. At this stage it is not known whether the crucial difference between the two heat treatments is the maximum temperature used or the use of flowing nitrogen rather than oxygen at the highest temperature. Further detailed studies on the influence of pretreatment conditions on catalytic properties are currently underway. The analysis of the activity results using the statistical tests for the preparation showed that the same catalysts should be most active for the formation of higher alcohols (catalyst G) and for the formation of ethanol (catalyst I) as was predicted from the data for the calcined catalysts. As before, it is found TABLE 7 Activity and selectivity of various oxygen-treated catalysts for the reaction of a 25% CO/75% H, mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
1.35 1.50 0.78 0.85 0.48 0.94 2.33 0.76 1.77
Activity”
19.1 25.1 13.2 14.0 7.9 15.7 38.7 12.7 29.3
Selectivity/%
CH,
c 2+ b
CH,OH
C,H,OH
C,+OH”
25.8 28.3 25.3 19.6 36.6 24.1 25.2 18.7 27.0
26.4 30.2 23.1 25.0 18.1 28.3 32.8 23.3 29.4
27.8 18.6 7.7 34.8 14.7 23.0 17.3 29.2 14.8
13.3 14.8 11.3 13.1 14.1 15.9 15.1 16.2 15.9
6.8 8.1 32.7 7.6 16.6 8.8 9.7 9.9 11.3
“Units are pmol CO, converted/g cataIyst/min. ‘All hydrocarbon products containing two or more C atoms. ‘All oxygen-containing products excluding methanol and ethanol.
293
TABLE 8 Product distributions obtained from various oxygen-treated catalysts for the reaction of 25% CO/ 75% H.Jat 40 atm pressure and 543 K Product”
Catalyst A B
C
D
E
F
G
H
I
CH,
36.4
42.5
45.5
27.5
52.6
35.6
39.9
27.8
42.9
C,+h
12.1 39.2
14.7 28.0
13.0 13.9
11.1 48.9
9.0 21.1
14.6 34.1
16.1 21.4
12.1 43.4
15.6 23.4
9.4
11.1
10.2
9.2
10.6
11.7
12.0
12.1
12.6
2.4 0.6 _
2.7 1.1
6.0 4.9 1.7
4.4 2.7
3.0 1.0 _
3.2 1.4
3.7 0.9
3.2 1.3
_
2.0 1.1
2.5 0.8 _ -
0.2 1.5
-
CH:,OH C,H,OH C:,H,OH C,H,OH
C,H,,OH
CH:,CHO C,H,CHO C,,H,CHO CH,,COOCH., C,H,COOH
_
_ _
_
_
_ 1.1
“Units are mol-% of product. ‘All hydrocarbon products containing two or more carbon atoms.
that catalyst G is most active for the formation of higher alcohols but that catalyst I is about 20% less active than catalyst G for the formation of ethanol. Nevertheless, the overall predictive value of the statistical method has been demonstrated for the CO/H2 reaction.
Table 9 summarizes the activity and selectivity results obtained for the CO/ CO,/H, reaction over the catalysts heated in flowing oxygen. Comparison with Table 7 shows that all the catalysts are much more active when there is carbon dioxide in the reaction mixture. The rate of formation of methanol is about the same as in the absence of carbon dioxide; the rate of formation of ethanol and higher alcohols is about 4 times larger; and the rate of formation of hydrocarbons is about 5-10 times larger. The net effect of these changes is that the selectivity for the formation of alcohols is lower in the presence of carbon dioxide although the activity is much greater. Table 10 shows the product distribution observed for this reaction. In this case all the catalysts tend to produce the same wide range of products, which contrasts with the observed product distribution in the absence of carbon dioxide. When the statistical test is applied to the preparation procedure it is found that the best catalyst for the synthesis of ethanol in the presence of carbon dioxide should be catalyst I (as before). This is indeed what is observed. For the formation of higher alcohols the statistical results are distorted by the re-
294
TABLE 9 Activity and selectivity of various oxygen-treated catalysts for the reaction of a 10% CO/lo% CO,/80% He mixture at 40 atm pressure and 543 K Catalyst
Conversion/%
4.58 5.06 1.58 4.56 4.47 7.59 5.91 4.51 9.49
Activity”
Selectivity/%
62.4 65.6 20.3 60.9 57.9 100.2 77.9 59.5 127.4
CH,
c 2+ b
CH,OH
GH,OH
C,+OH’
34.7 41.9 27.8 31.1 21.6 43.7 39.2 42.0 41.5
23.4 25.1 25.2 31.3 31.6 33.9 32.2 29.4 33.4
16.5 8.1 4.5 7.3 4.4 2.6 6.0 6.1 4.4
13.2 13.4 11.9 13.8 12.8 11.6 12.2 11.7 11.2
10.2 9.5 27.6 14.5 27.6 9.2 11.4 8.8 10.5
“Units are pmol CO, converted/g catalyst/min. hAll hydrocarbon products containing two or more carbon atoms. ‘All oxygen-containing products excluding methanol and ethanol. TABLE 10 Product distributions obtained from various oxygen-treated catalysts for the reaction of 10% CO/ 10% CO,/80% H> at 40 atm pressure and 543 K Product”
CH,
C,+h CH,,OH C,H,OH C,,H,OH C,H,OH C,H,,OH CH,,CHO CH:,COOCH,, C,H,COOH C:,H,COOH CH,,CHOHCH,,
Catalyst A
B
C
D
E
F
G
H
I
50.7 11.0 24.0 9.6 1.9 1.1 0.2
61.8 12.3 11.9 9.9 1.8 1.0 0.2
51.1 15.2 8.4 11.0 5.5 5.3 1.4 0.6 0.6
52.7 15.9 12.3 11.6 2.5 1.4 0.5 -
43.3 19.5 8.9 12.8 4.5 4.7 1.8 0.4 0.6
67.2 16.5 4.1 9.0 1.6 0.7 0.2 -
60.3 16.0 9.2 9.4 1.9 1.1 0.4
63.4 14.6 9.2 8.9 1.7 0.9 0.2 _ _
63.6 16.5 6.7 8.5 1.7 1.1 0.3
0.9 0.2 0.1
1.0 -
2.1 0.6 0.1
2.7 0.7 0.1
1.1 0.4 0.1
1.3 0.4
0.9 0.2 0.1
1.1 0.4 0.1
_ 0.8 0.4 0.1
“Units are mol-% of product. “All hydrocarbon products containing two or more carbon atoms.
sults for samples D and E (both containing sodium) which show large increases in activity in the presence of carbon dioxide. The result is that none of the catalysts is expected to be significantly more active than the others. In fact
295
catalyst I is again the most active for this reaction. DISCUSSION
In a previous study, Marchi and co-workers [ 131 observed how the preparative conditions influence the crystallinity and homogeneity of the precursors of Cu/Co/Al catalysts, We have assessed similar variables in the present study of the Cu/Co/Al/Zn system, but we have related them to the catalytic performance of the activated samples. In common with the work on Cu/Co/Al [ 131 we have found that the homogeneity and the degree of crystallinity of our dry precursors reflect the conditions used during precipitation. In fact, in our experience, it is very difficult to prepare the hydrotalcite phase exclusively, because it is usually associated with an amorphous malachite-type phase. However, even though the statistical screening of a group of catalysts has allowed us to optimize the procedure for preparing dry precursors, the variation in catalytic performance is relatively small. In brief, the conditions during precipitation and drying have a marked effect on the composition and structure of the precursors, and yet these are not the most important factors in relation to either the activity or selectivity of the catalysts during the conversion of syngas to higher alcohols. As Tables 5 and 7 show, thermal activation has a much more pronounced effect on the catalytic properties than does the method of preparation. In particular, heating in nitrogen at 770 K generally produces much less active catalysts (which are more selective for the formation of methanol) than is found when the same materials are heated in oxygen at 720 K. Furthermore, changes in the subsequent treatment (reduction conditions; duration of catalyst testing) do not greatly influence the performance of the catalysts. Unlike the precursors of the Cu/Co/Cr catalysts [ 161, our samples do not show any signs of bulk oxidation during thermal activation (even DTA of samples heated under flowing oxygen did not reveal any exothermic peaks), suggesting that the changes are limited to the surface. It seems that by initially choosing a calcination temperature which was high enough to ensure the removal of adsorbed contaminants [24] we were effectively destroying potentially active sites. Thus, thermal activation emerges as a critical step in the formation of an effective catalyst. One important outstanding question concerning these multicomponent catalysts is the role of cobalt. The initial work by Courty et al. [9] encourages belief in the importance of forming small bimetallic Cu/Co clusters. Certainly it is true that separate copper and cobalt metallic phases will be unlikely to produce higher alcohol with a high selectivity. Instead, it would be expected that copper would produce methanol and cobalt would produce hydrocarbons. It is well established, and confirmed in the present work, that in co-precipitated catalysts the reduction of cobalt is rather difficult and, depending on the
support, may not occur to any significant extent under normal catalyst activation conditions [ 141. In our work, we have used a very mild reduction treatment on catalysts previously exposed to a rather vigorous calcination. We would anticipate that only a small amount of the cobalt would be in the fully reduced state at the commencement of the activity testing. Therefore, it is interesting that in the early stages of the catalyst testing we observe the formation of significant amounts of higher alcohols and that the selectivity does not change very much with time-on-stream. This suggests either that metallic cobalt is not required for the formation of higher alcohols or that only a very small amount of cobalt is necessary to modify a copper particle sufficiently to generate higher alcohols in preference to methanol. It is noticeable in the work of Courty et al. [ 91 that the selectivity to higher alcohols slowly declines with time-on-stream, while, presumably, the degree of reduction of the cobalt will increase. One plausible explanation for these various effects can be derived from the work of Mouaddib and Perrichon [ 141. These workers have studied the influence of the support on the synthesis of alcohols over Cu/Co catalysts. They observe that on MgO the selectivity to methanol is 75% and to higher alcohols it is lo%, whereas on ZrOz the selectivity to methanol is 39% and to higher alcohols it is 25%. On TiOa the selectivity to methanol is only 12% and to higher alcohols it is only 13%. The explanation offered is that the ease of reduction of cobalt increases in the order: MgO < Zr02 < TiOa. The MgO-supported Cu/Co catalyst then contains copper particles modified by a small amount of metallic cobalt, and so is essentially a methanol synthesis catalyst. The TiO*-supported catalyst contains Cu/Co particles rich in cobalt and is essentially a hydrocarbon synthesis catalyst. The Zr02 catalyst is intermediate in behaviour. Providing these results can be transferred to our Cu/Co/Zn/Al catalysts then the formation of higher alcohols would be due to the presence of a small amount of metallic cobalt intimately mixed with the copper. More complete reduction of cobalt would be disadvantageous. However, one has to be careful about making assumptions about the composition of the surface of Cu/Co bimetallic clusters. Surface science studies on single crystal and polycrystalline materials [ 181 provide evidence of a significant amount of surface enrichment by copper in such samples. Further work is in progress to try to ascertain the relationship between selectivity for the formation of higher alcohols and the degree of reduction of the various cobalt phases present in these multicomponent catalysts. ACKNOWLEDGEMENTS
We are grateful to the S.E.R.C. for supporting this research and for the award of a studentship to JEB and a fellowship to SEG.
297 REFERENCES
7 8 9 10
11 12
13 14 15 16 17 18 19 20 21 22 23 24
X. Xiaoding, E.B.M. Doesburg and J.J.F. Scholten, Catal. Today, 2 (1987) 125. M. Stoel, ECN Process Supplement, Nov. (1985). A. Sugier and E. Freund, U.S. Patent 4 122 110 (1987). A. Sugier and E. Freund, U.S. Patent 4 291 126 (1979). P. Courty, D. Durand, A. Sugier and E. Freund, G.B. Patent 2 118 061 (1983). P. Chaumette, P. Courty, D. Durand, P. Grandvallet and C. Travers, G.B. Patent 2 158 730 (1985). A. Sugier and E. Freund, G.B. Patent 1547 687 (1977). A. Sugier and E. Freund, G.B. Patent 2 037 179 (1979). P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catol., 17 (1982) 241. R. Szymanski, C. Travers, P. Chaumette, P. Courty and D. Durand in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 738. P. Grandvallet, P. Courty and E. Freund, Proc. 8th Int. Congr. Catal. Berlin, 1984, Vol. II, Verlag Chemie, Weinheim, 1984, p. 81. S. Gusi, F. Pizzoli, F. Trifiro‘, A. Vaccari and G. de1 Piero, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 753. A.J. Marchi, A.G. Sedran and C.R. Apesteguia, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, paper H7. Mouaddib, N., and Perrichon, V., 9th International Congress on Catalysis, Calgary, 1988, Vol. 2, The Chemical Institute of Canada, Ottawa, 1988, p. 521. A.J. Marchi, J.I. Di Cosimo and C.R. Apesteguia, 9th International Conress on Catalysis, Calgary, 1988, Vol. 2 The Chemical Institute of Canada, Ottawa, 1988, p. 529. G.R. Sheffer and T.S. King, Appl. Catal., 44 (1988) 153. W.X. Pan, R. Cao and G.L. Griffin, J. Catal., 114 (1988) 447. D. Chadwick, J. Pritchard, and A. Taylor, in preparation. P.A. Sermon, in preparation. R. Stowe and R. Mayer, Ind. Eng. Chem., 58 (1966) 236. W.A. Dietz, J. Gas Chromatogr., (1967) 68. P. Porta, S. De Rossi, G. Ferraris, M. Lo Jacono, G. Minelli and G. Moretti, J. Catal., 109 (1988) 367. R.H. HSppener, E.B.M. Doesburg and J.J.F. Scholten, Appl. Catal., 25 (1986) 109. J.E. Baker, R. Burch and SE. Golunski, Thermochimica Acta, 142 (1989) 329.