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Development of New Catalysts Formulations for Higher Alcohols Synthesis. Caracterisation, Reactivity, Mechanistic Studies and Predictive Correlations
Guczi, L. et al. (Editors), New Frontiers in Cufalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All righa nscrved
DEVELOPMENT OF NEW CATALYSTSFORMULATIONSFOR HIGHER ALCOHOLS SYNTHESIS. CARACTEMSATION,REACTIVITY, MECHANISTIC STUDIES AND PREDICTIVECORRELATIONS A. Kiennemanna, S.Boujana", C. Diagn8 and P. Chaumetteb
aEHICS, URA-CNRS 469, 1,rue Blaise-Pascal,67008Strasbourg Cedex, France bIFP, 4, avenue de Bois Preau, 92502 Rueil-Malmaison, France
Abstract This work has demonstrated the benefical effect of molybdenum addition to copper-cobalt catalysts. Catalysts were characterised by XRD, STEM and TPR.The comparison between fixed and slurry reactor was performed. The possibility to predict total alcohol selectivity and alcohol chain growth is demonstrated.
INTRODUCTION: c 1 - c 6 alcohols mixtures can easily be obtained from natural gas via synthesis gas, and can be used as octane boosting agent for unleaded gasoline [l].Four types of catalyst formulations have been studied for higher alcohol synthesis: - methanol Synthesis catalysts modified by addition of alkali metals, and/or other metals such as chromium lead to good alcohol selectivities, but the C2+0H fraction is limited (25 to 40 wt%) [21, and they are operated in severe conditions (T=623-693& P=12-16MPa). - catalytic systems based on noble metals, such as rhodium based catalysts, essentially drive to ethanol 131. - modified Fischer-Tropsch catalysts, such as copper-cobalt based catalysts developped by I.F.P.,allow to get a c 1 - C ~alcohol mixture under moderate operating conditions (T=533-593K,P=6-8MPa)and with an alcohol selectivity of 70 to 80 C%. Similar performances have been gained on Cu-Ni catalysts [ll. - catalysts based on molybdenum sulfide promoted by CoS and alkali metals have been developped by Dow and Union Carbide [4,51, whereas Tatsumi et al. claim to get high alcohol productivities with Mo-KCYSi02 catalysts 161.
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Furthermore, the promoting effect of molybdenum towards alcohol synthesis has been evidenced for catalysts which normally lead t o hydrocarbon formation, as examplified with Ru-Mo-Na20/A1203 171, Ir-MoNa2O [81, and Mo(Co)S-K20 [91 catalysts. Yano et al. [lo] have detected a synergetic effect between ruthenium and molybdenum explained by the formation on Ru-O-Mo bonds. These positively charged ruthenium sites would be the one responsible for alcohols formation. Mechanistic studies have been performed by probe molecule addition to syngas: olefins, dichloro o r diiodoalkanes as carbene precursors [6,11-131, by coupling reactions, and by chemical trapping of active intermediates on the surface of catalysts [lll. They show that higher alcohols are formed by insertion of carbon monoxide or a formyl species in a metal-carbene bond. This mechanism implies a priori the presence of two types of sites: the first one responsible for CO dissociation into carbene in the presence of hydrogen, the second one for non dissociative CO chemisorption, or hydrogenation into formyl, Reduced metals sites are known to be active in CO dissociation and carbene formation [13], whereas strong interactions between metals [lo], bimetallic alloys [14], and positively charged metal sites [10,151 have been proposed as non dissociative adsorption sites for CO. Alkali metal promoters also affect the reduction of metal precursors either in the bulk El61 or on the surface. This leads t o a modification of the adsorption and dissociation of CO, and can help stabilize oxygenated intermediates on the surface of the corresponding catalysts (e.g. formyl, acetate, ...1. The present work is devoted t o the study of the effect of molybdenum addition to copper-cobalt-zincformulations, Different Cu-Co-Mo-Zn catalysts have been prepared by coprecipitation, and characterized by X-ray diffraction, S.T.E.M. and thermoprogrammed reduction. The performances of these catalytic systems have been evaluated in gas phase/fixed bed and liquid phase/slurry bed type reactors. In good keeping with the mechanistic scheme already proposed [11,171, the methodology described for other copper-cobalt based formulation [171 has been used t o predict the activity and selectivity of these systems.
EXPERIMENTAL: i>Catalysts Catalysts have been prepared by coprecipitation by ammonia at pH = 7.6 and 353 K of an aqueous solution containing copper, cobalt, zinc nitrates and ammonium heptamolybdate. After agitation for 3 hours the precipitate obtained was washed with water, dried (433K, 6 hrs) and air calcined (623 K, 6 hrs). The oxides obtained have the following theoretical compositions : Cat .I :ColCw.70Moi.55 OZ Cat -11 : C o l C ~ . 7 0 M o l . S S ~ ~ . 2 8 ~ z Cat .I11 : ColCuq,70Mo1.55Zn7.l60~ Cat .Iv: ColCu5.78Mo1.75Zn5.780~
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Cat .V : C O ~ C U ~ O M O ~ Z ~ ~ O O Z . Cat .VI (phase A of cat.V) : Co0.23Mo2.3Cu1.8ZnO~ Cat.VII (phase B of cat.V) : Col.1M00.4Cu0.66ZnOZ Phases A and B have been prepared following - - the same reaction scheme described below for phase A : 0.23 Co(N03)2.6H20+1.8Cu(N03)2.3H20+l Zn(N03)2.6H20+2.3Na2M004.2H20 pH =7 1.44 NaOH (353K)
1 1
Coo~~3Mo~.~Cul.~Zn~OH~,.mH~0 Dried (433K)
Calcined (623K)
~~0.23M02.3Cu1.EZn10z
ii) X-Ray diffraction experiments (XRD) have been performed on a SIEMENS D 500 diffractometer equiped with a DACO-MP microprocessor, using the Ka radiation of cobalt. iii) The characterization by STEM was performed on a high resolution VG HB5 dedicated STEM equiped with a KEVEX energy dispersive X-ray spectrometer, a GATAN 607 electron energy loss spectrometer and a high sensitivity TV camera for the collection of electron microdiffraction patterns. Thin foils of the compounds (t c 100 nm) at each step of their preparation were deposited on the nylon microscopy grids. The foils were obtained by ultramicrotomy with a diamond knife on solid grains previously embedded in an epoxy resin.Quantitative information was obtained from the X-ray emission spectra using a quantitative method which is derived from the CliffLorrimer approach for the use the so called K-factors to correlate the peak intensities with concentrations in the thin film approximation. The use of ultramicrotomy foils made the thin film criterion valid in most analyses. iv) Carbon monoxide hydrogenation : The reduction was performed first from room temperature up to 513K, under diluted hydrogen (6% in N2, 2 1.h-1.gcat-1) with three steps at 433K, 463K and 513K, then from 513K up to 773K (593K in slurry bed reactor) under hydrogen (2 1.h-1.gcat-1;0.2 K.min-l).The reaction was performed at 8.5 MPa with a syngas flow rate of 3 1.h-1.gcat-1and a H2XO ratio of 211. v) Temperature programmed desorption of acetaldehyde Catalysts samples were charged (500 mg) into a tubular glass reactor (1cm O.D.) and reduced under hydrogen as here above. After cooling the reactor down to room temperature, the catalysts was exposed to a helium flow (10 1.h1.g cat-1i4-5 hrs) and then to acetaldehyde vapor (10 mm Hg). After flushing with He to remove all acetaldehyde from the gas phase, the temperature was raised up to 773K (6K.min-1)under a carrier flow of helium (4 1.h-1.gcat-1.1.A Gow-Mac conductivity cell system allowed the detection of desorbed molecules from the catalytic surface. Desorption products were analyzed and identified by two on-line gas chromatograph.
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RESULTS AND DISCUSSION: Figure 1 compares the alcohols selectivity obtained at isoconversion with catalyst V in a fixed bed or a slurry reactor. The same conversion is observed in the slurry reactor after a 10K increase of the operating temperature when compared to the fixed bed test. Figure 1: Evolution of alcohols selectivity versus conversion in slurry and fixed bed reactor. 65’1
2
1
6
10
14
I
Conversion into ROH+HC (%I
The performances obtained with the former type of reactor are summarized in Table I. Table I : Performances of catalysts in slurry bed reactor.
- ZnO addition (cat.11) to the basic formulation (cat.1) does not change much the overall activity: a 10K increase of the reaction temperature is sdlicient to
reach the same conversion as with cat.1. On the opposite, a sharp increase of the alcohol selectivity (55 C% versus 32 C%) and C2+0H selectivity (28 C% versus 10 C%) is noticed. T.P.R. studies (Figure 2) performed on catalysts I allow t o detect two reduction peaks: the first one at 498K corresponding to copper reduction, the second one at 575K which is tentatively attributed to the reduction of the mixed oxide phase CuxCoyMozOt detected by X.R.D.. On the contrary, the thermogram of cat. I1 shows only one broad reduction peak situated at 553K. The absence of a peak at 495K reveals that a lower amount of copper is reduced, which in turn can lead to a lower reduction of other metals. This may explain the slightly lower conversion (less free metallic cobalt present) and the higher alcohol selectivity ( increase of ionic cobalt species and/or of the Cu-Co interaction) observed with catalyst 11. Indeed, the beneficial influence of either the Co2+-Coo or the Cuo-Coo associations on the alcohol selectivity has been stressed in recent publications 118,193.These associations allowing t o increase the cobalt dispersion and reduction, decreasing the hydrogenationhydrogenolysisactivity of metallic cobalt and promoting the formation of alcohols.
lil
Figure 2 :TPR ofcakfdystS1, 11 and III.
""I
Figure3 :TPR ofcatalysta IV and V.
so
2 10 0
oJ
.
.
. - .
. . . I 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0
Temperature K
300
400
500
600
Temperature K
700
800
- The increase of the ZnO content (cat.111) drives to a further decrease of the catalytic activity (comparison of cat.11 and cat.111 in Table 1). Neither the alcohol selectivity, nor the chain growth are modified significantly. The thermoprogrammed reduction (Fig.2) shows a lower degree of reduction of this catalyst, with a second reduction peak situated at 670K. The latter temperature is well over the temperature at which the catalyst was reduced before catalytic evaluation in a slurry reactor (T=595K). - An increase of the Mo/Co ratio from 1.55 to 1.75,and of the Cu/Co+Mo ratio from 1.83to 2.1 (cat IV)causes a decrease of the overall catalytic activity, a 20K increase of the catalyst bed temperature being necessary to reach the same CO conversion as with cat.11. T.P.R. studies of cat.N (figure 3) shows three reduction peaks at 490K, 520K, and 580-590K, the first one corresponding to the partial reduction of copper. Thus, after the reduction procedure followed before the catalytic test (up to 593 or 773K), this catalyst is not totally reduced, which explains the lower performances obtained.
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- On the contrary, a decrease of the Mo/Co ratio t o 1.5 with a Cu/Co+Mo ratio of 2 (cat.V) leads to better performances: a conversion‘of CO into alcohols and hydrocarbons between 6.6 and 14.3 at 530-550K, and an alcohol selectivity 59 C%, together with a chain growth (C2+0N) between 27 and 32 C%. The complete reduction of this catalyst in T.P.R. conditions between 440K and 550K (Figure 3) explains these catalytic test results. Diffraction lines corresponding to copper oxide and zinc oxide are detected on the X-ray diagram of cat.V, as well as other lines which can tentatively be attributed to a mixed CuxCoyMo,Ot phase which might also contain zinc oxide. The characterization of this most performant catalyst has thus been undertaken by scanning transition microscopy (S.T.E.M.), and X-ray emission spectra in order to get some information on the local composition.
Figure 4 and 5 : STEM analysis of CuCoMoZn actives phases. Figure 4 Figure 5 B
B
I
A
1
4
A
I
+B +B
0.bp
t B
0.16
t
B
The S.T.E.M. analysis of this catalyst (global composition: Cu1oCo2Mo3Zn10Oz) indicates that it is composed of four different phases (Figuree 4 and 6): ZnO, CuO, a molybdenum-rich phase (phase A) having the following average composition: Cu1.8C00.23M02.3Zn10z, and a cobalt-rich phase (phase B) of average composition: Cu0.66Col.lMo0.4Zn10~.A careful1 control of the preparation steps and of the composition of the starting solution allowed us to prepare selectively each of these phases. Phaee A has a B.E.T. surface area of 46 mz/g, to be compared t o 151 m2/g for phase B, and 44 mz/g for catalyst V at the same calcination temperature (823K). On the X-ray diagram of phase A (Figure 6), no crystallized CuO, ZnO or cobalt oxide phase is detected, but the presence of lines situated very near those of the mixed copper-molybdenum phase Cu3Mo20g is detected.
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Phase B is amorphous after calcination at 623K,whereas at 823K,poorly defined diffraction lines corresponding to copper and zinc oxides appeart no free cobalt oxide phase being detected.
Figure 6 : XRD of catalyst phase A after calcination at 823K.
t
PHASE A CuCoMoZn
I
Figure 7 allows to compare the alcohol selectivities obtained with cat.V, phase A and B at the same conversion levels. It is clearly seen that phase B is the one responsible for alcohol synthesis in catalyst V. Phase A which has a low cobalt content, presents a low activity (0.7 % conversion to be compared to 5.1% conversion for phase B in slurry reactor at 523K),furthermore, it leads essentially to hydrocarbon formation. Figure 7 :Evolution of alcohole eelectivity vmw c o n d o n . Cataly8t.a V,pham A and B.
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MECHANISTIC STUDIES: We have confirmed on the present catalysts the results of the mechanistic studies (probe molecules, coupling reactions, etc...) previously performed on CuCo catalysts [11,17].In good keeping with the mechanistic scheme already proposed, the tools allowing to predict the alcohol and C2+0H selectivities have been extended to these new CuCoMoZn formulations. As already described {ll] ethylene addition to syngas leads to an increase of propionaldehyde and propanol production. Propane and butane are also detected as by products together with ethane. The orientation towards C3 oxygenates or C3 hydrocarbons depends on the nature of catalyst. The ratio between the increase of C3 oxygenates production and the total increase of C3 products (hydrocarbons + oxygenates) after ethylene addition can thus give us a good approximation of the alcohol orientation of the catalysts as can be seen in figure.8. Figure 8 :Observed alcohole selectivityversus predicted selectivity (ROH %).
0
20
40
60
80
100
Predicted selectivity (ROH %)
Figure 9 :TPD of acetaldehyde on cat.VI.
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In order to obtain a more quantitative prediction, thermoprogrammed desorption after acetaldehyde adsorption can be used. Figure 9 illustrates all the products obtained on cat VII (cat V phase B). Three main desorption domains are roughly observed: at about 373K (acetaldehyde and ethanol), between 420 and 450K (acetone sometimes associated with propene), at T>630K,CO, C02 and CH4 -~~ formation. The formation of-these three groups of products can be rationalized by the following - scheme : ~
CHSCHO, CH3CH20H
CH3CHO-
(Y
surf. c
\
CH3COOads
CH3CHOads
(2)_
r -
__c
(CH,+CO2) or (C02+CH3COCH3) CH3COCH3 (lowtemperature)
The first pathway corresponds t o a desorption without modification or to a hydrogenation, the second one t o an oxidation and to the formation of carboxylate species, the third reaction yields to the formation of acyl species considered as key species in the alcohol chain growth. Acetone formation at low temperature is present on CoCuMoZn and CoCuZnAl catalysts, whereas it is not detected on the support (ZnO,ZnAlaO4) or on a 15%Cu/ZnA1204 catalyst (17). This low temperature acetone desorption cannot arise from acetate decomposition, since the temperature is too low and furthermore, no C02 is evolved at the same time as acetone. Acetone is therefore suggested to arise from the trapping of acyl species by CH3 entities present on the surface, as already indicated for CuCo and Ni based catalysts [19,201. If it is assumed that the three groups of products obtained in acetaldehyde desorption (ethanol+acetaldehyde, acetone, CH4+CO+C02)are representative of the three main reactions occuring on CuCo or CuCoMoZn catalysts, (e.g. hydrogenation, alcohol formation via acyl species, hydrocarbon synthesis via surface carbon and carboxylates species), the ratio between the acetone peak area and the total desorption peaks area give an estimate of the higher alcohol orientation of the catalyst. The C2+OH selectivity thus predicted is compared with the selectivity obtained in catalytic tests under pressure (Figure 10). It can seen that there is a good agreement between those two selectivities even for new CuCoMoZn active phases as already described for the CuCo type catalysts.
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Figure 10 : Observed selectivity (C2+OH) versus predicted selectivity(C2+OH%).
CONCLUSION: This work has demonstrated the beneficial effect of molybdenum addition to copper-cobalt based catalysts for alcohols synthesis from syngas. The quantitative studies performed by S.T.E.M. has lead to the determination of the composition of the mixed phases present in these molybdenum doped formulations, and coprecipitation experiments allowed to prepare the two mixed CuCoMoZn phases detected in a pure state. One of these phases (phase B) having the following composition: C U O . ~ C O ~ . ~ M Ogives ~.~ rise Z to ~ ~a ~ Z , high activity and selectivity towards alcohols synthesis, the other one yielding mainly hydrocarbons. The mechanistic scheme for alcohols synthesis has been found t o be identical to the one already proposed for copper-cobalt based catalysts. The predictive tools developped on the basis of these mechanistic conclusions, and already used in the case of CuCo catalysts, have been extended to the molybdenum doped formulations. This model allows to predict the alcohols selectivity and the chain growth towards higher alcohols, and thus constitutes a very helpful1 tool for the rapid selection of alcohol synthesis catalysts. It can be used for all catalysts yielding alcohols through a mechanism involving the insertion of a C1 oxygenated entity (CO, formyl) into an adsorbed carbenic chain developped through carbene condensation on the surface of the catalyst (i.e. on cobalt, iron and nickel based catalysts), but, it cannot be used in the case of alkalinized methanol synthesis catalysts
.
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1 Ph. Courty, A. Forestihre, N. Kawata, T. Ohno, C. Raimbault and M. Yoshimoto, Industrial Chemicals via C1 Processes ACS Symposium Series 328 (1987)42. 2 A. Paggini and D. Sanfilippo, in "Production of mixed alcohols from synthesis gas", Spring National Meeting, New Orleans, US.,6-10th April 1986. 3 G.van der Lee and V. Ponec, Catal. Rev. Sci. Eng., 29 (1987)183. 4 G.B. Murchison, M.M. Conway, R.R. Stevens and G.J. Quarderer, Proceedings 9th International Congress on Catalysis, vol. 11, Calgary (19881, 626 M.J. Phillips and M. Ternan Editors. 5 A. Hakki, C. Tellis, in "Octane enhancers for motor-fuels", Spring National Meeting, New Orleans, U.S.,6-10th April 1986. 6 T. Tatsumi, A. Muramatsu, K. Yokota and H. Tominaga, J. Catal., 115 (1989)388. 7 M.Inoue, T. Miyake, S. Yonezawa, D. Medhanavyn, Y. Takegami and T. Inui, J. Mol. Catal., 45 (1988)111. 8 M.Inoue, A. Kurusu, H. Wakamatsu, K. Nakajima and T. Inui, Appl. Catal. 29 (1987)36 9 T.Tatsumi, A. Muramatsu, K. Yokota and H. Tominaga, J. Mol. Catal. 41 (1987)385. 10 T. Yano, Y. Ogata, K. Aika and T. Onishi, Chem. Lett. (1986) 303. 11 A. Kiennemann, C. Diagne, J.P. Hindermann, P. Chaumette and Ph. Courty, Appl. Catal., 53 (1989)197. 12 M. Ichikawa, A. Fukuoka and T. Kimura, Proceeding 9th Int. Congr. Catal., vol. I1 Calgary (1988)569. 13 R. Snel and R.L. Espinoza, J. Mol. Catal., 43 (1987)237. 14 R.H. Bailliard-Letournel, A.J. Gomez, C. Mirodatos, M. Primet and J.A. Dalmon, Catal. Lett. 2 (1989)149. 15 A. Razzaghi, J.P. Hindermann and A. Kiennemann, Appl. Catal., 13 (1984)193. 16 A. Muramatsu, T. Tatsumi, H. Tominaga, Bull. Chem. SOC.J a p 60 (1987)3157. 17 A. Kiennemann, S. Boujana, C. Diagne, Ph. Courthy and P. Chaumette, Studies in Surface Science and Catalysis, vol. 61 (1991)243 A. Holmen et a1 Editors. 18 J.E. Baker, R. Burch and N. Yugin, Appl. Catal., 73 (1991)135. 19 J. A. Dalmon, P. Chaumette, C. Mirodatos, Review accepted for publication in Catal. Today. 23 J. Cressely, S. Riegert-Kamel, A. Kiennemann and A. Deluzarche, Bull. SOC. Chim. Fr. I1 (1982)171.
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DISCUSSION Q: M. Ichikawa (Japan) 1) You characterized the catalyst hase by XRD and others, depending on the different formulation. In the multimeta lics system of the catalysts, it is not easy to elucidate the surface composition, it is interesting to know how changes in surface structure of the catalysts under working condition and with time-on-stream during syngas reaction, is reflected by changes in product distribution. Could you show us evidence for the structural trend of the catalysts for higher selectivities towards higher alcohols. 2) In your conclusion, you propose the mechanism of HCHO insertion in the surface carbene to propagate the alcohol-carbon chain. Is this a naive mechanism to be found in the organometallic chemistry to produce higher alcohols ?
f
A. A. Kiennemann 1) To answer the first question concerning the catalyst after the reaction, w e have evidenced that the Co-Cu-Mo-Zn catalyst is composed of three different phases: ZnO, phase A (Mo rich) and phase B (Co rich) and that only one phase (B) is active and selective in alcohol formation. As in many cases in alcohol synthesis (s methanol formation) the most active phase is badly crystallized tmorphous at 400 and the well formed crystal only appear after calcination (at 600 C) of the working catalyst. The only evidence is that some crystallinity appears during the aging of the catalyst with formation of distinct copper or/and cobalt oxide particles. To go deeper into the characterization of the used catalysts, we normally used the association of XRD and STEM as in the case of the calcined catalysts. This as been done and published on other Cu-Co formulations [l]. 2) Concerning the insertion it is not possible to distinguish between insertion of CO and/or formyls (not formaldehyde). Formyl species have been characterized by chemical trapping on the catalytic surface but both CO or formyl can lead to the formation of acyl species. [l] Ph. Courty et al., J. Mol. Car., 17, 241 (1982)and Proceedings 8zh Inz. Cong. Cuzal., Vol. XI., p 81 (1984)
%)
Q: T. Koerts (The Netherlands) In your mechanism for the formation of C2+-oxygenates, you suggest that CO insertion can result in two intermediates: one bonded by oxygen atoms, and one bonded by a carbon atom to the catalyst. What is the evidence for the existence of these species '? Can you speculate on the relative reactivity of these intermediates to produce C2+-oxygenates ? A: A. Kiennemann Some evidences have been obtained during the coupling and chemical trapping experiments and they have been given elsewhere [2].Furthermore, the higher alcohols distribution obtained follows the Anderson-Schulz-Flory distribution and therefore we can conclude that the chain growth is obtained by the insertion of an oxygen containing C, entity into a carbene type hydrocarbon species with one or more carbon atoms to form acyl species. This could not be confirmed by FT-IR because of the presence of zinc in the catalyst (see publications on Cu-Zn-Al catalysts). However, as described [3], a link could be establis ed between the amount of acyl species (evidenced by the low temperature (150-180 C) desorption peak of acetone after acetaldehyde chemisorption) and the alcohol chain growth. This was evidenced on Co-Cubut also on Fe and/or Mo catalysts on which w e believe that alcohols are formed by the same mechanism. [2] Appl. Catal., 53, 197 (1989) [3] Studies in Surf.Sci. and CaZaZ., 61,243(1991)
6
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Q: K. P. de Jong (The Netherlands) The differences in selectivities between slurry and fixed-bed reactors (Figure 1) seem surprising. The more so, because the differences are apparent mainly at higher conversions.. Two questions arise: 1) What are the details of the Sherry reactor (product work up, liquid phase, and extent of backmixing) ? 2) How d o you explain the differences between the Sherry and the fixed-bed reactor ? A: A. Kiennemann 1) The comparison between performances in fixed bed and slurry phase reactor have been performed with some differences in the working up of the catalytic system. These differences are essentially seen in the reduction procedure of the catalyst. In fixed bed reactor the reduction has been performed in the conditions described [3]: first from mom temperature to 240 OC under diluted hydrogen flow (6 % H in N 2 1 h-l gcat‘l) with three steps at 160 OC 190 % and 240 OC then from 240 to 5& % reduction under pure hydrogen (2 I h-{ gcat-l, 0.2 Oc mn-l). In the slurry reactor, the slurry solvent used (Cl0 to C18 n-alkane fraction) is stable in presence of the catalyst only up to 320 OC. As a consequence, the reduction procedure (in situ procedure) has been modified as follows: from mom temperature to 160 % under diluted hydrogen flow (6 % H2 in N2,2 I h-l gcat-l, 0.2O C mn-l) then from 160 OC to 320 OC reduction under pure hydrogen (2 1 h-l gcat-’, 0.2 Oc mn-l) after a step at 160 q. The other conditions are the same: introduction of synthesis gas before reaction, etc. For a Co-Cu-ZnAI2O4-Na catalyst the same phenomenon is observed as can be seen in the figure below.
oc?
80% 15 70
-
Slurry (reduction32OT)
65-
Fixed bed
(reduction sO0”c)
60‘ 55
a
-
50‘
45
-
40,
Conversion
1 ~
(ROH+HC)(%)
When the catalyst is prereduced ”ex-situ” (in the same conditions as for the fixed bed reactor) the same trend is observed. However, the catalyst is more sensitive to the temperature increase. 2) The difference between fixed bed and slurry phase reactor might a priori be explained by the conjunction of three factors. a) The different reduction procedures. It as been shown hereabove that it does not seem to be the most important parameter. b) The difference of the effective H d C O ratio at the contact with the catalytic system in slurry and in fixed bed. In slurry phase, the enhancement of the CO percentage
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in the synthesis gas leads to a decrease in conversion and of the overall alcohol selectivity but in an enhancement of the higher alcohol selectivity. c) A better removal of the reaction heat by the slurry agent. It could be observed, indeed, that the alcohol selectivity is less sensitive to the temperature changes than in the fixed bed reactor. Figure 1 shows in fact that, at low conversion (low temperature), the differences in selectivity are negligible and that the increase with the % of CO conversion (increase of reaction temperature). In a fixed bed reactor, local temperature raised can occur inside the catalyst grains although the temperature of the catalyst bed looks isothermal. The better heat removal in slurry reactor helps to avoid this phenomenon and stabilize the alcohol selectivity.
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All righa nscrved
DEVELOPMENT OF NEW CATALYSTSFORMULATIONSFOR HIGHER ALCOHOLS SYNTHESIS. CARACTEMSATION,REACTIVITY, MECHANISTIC STUDIES AND PREDICTIVECORRELATIONS A. Kiennemanna, S.Boujana", C. Diagn8 and P. Chaumetteb
aEHICS, URA-CNRS 469, 1,rue Blaise-Pascal,67008Strasbourg Cedex, France bIFP, 4, avenue de Bois Preau, 92502 Rueil-Malmaison, France
Abstract This work has demonstrated the benefical effect of molybdenum addition to copper-cobalt catalysts. Catalysts were characterised by XRD, STEM and TPR.The comparison between fixed and slurry reactor was performed. The possibility to predict total alcohol selectivity and alcohol chain growth is demonstrated.
INTRODUCTION: c 1 - c 6 alcohols mixtures can easily be obtained from natural gas via synthesis gas, and can be used as octane boosting agent for unleaded gasoline [l].Four types of catalyst formulations have been studied for higher alcohol synthesis: - methanol Synthesis catalysts modified by addition of alkali metals, and/or other metals such as chromium lead to good alcohol selectivities, but the C2+0H fraction is limited (25 to 40 wt%) [21, and they are operated in severe conditions (T=623-693& P=12-16MPa). - catalytic systems based on noble metals, such as rhodium based catalysts, essentially drive to ethanol 131. - modified Fischer-Tropsch catalysts, such as copper-cobalt based catalysts developped by I.F.P.,allow to get a c 1 - C ~alcohol mixture under moderate operating conditions (T=533-593K,P=6-8MPa)and with an alcohol selectivity of 70 to 80 C%. Similar performances have been gained on Cu-Ni catalysts [ll. - catalysts based on molybdenum sulfide promoted by CoS and alkali metals have been developped by Dow and Union Carbide [4,51, whereas Tatsumi et al. claim to get high alcohol productivities with Mo-KCYSi02 catalysts 161.
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Furthermore, the promoting effect of molybdenum towards alcohol synthesis has been evidenced for catalysts which normally lead t o hydrocarbon formation, as examplified with Ru-Mo-Na20/A1203 171, Ir-MoNa2O [81, and Mo(Co)S-K20 [91 catalysts. Yano et al. [lo] have detected a synergetic effect between ruthenium and molybdenum explained by the formation on Ru-O-Mo bonds. These positively charged ruthenium sites would be the one responsible for alcohols formation. Mechanistic studies have been performed by probe molecule addition to syngas: olefins, dichloro o r diiodoalkanes as carbene precursors [6,11-131, by coupling reactions, and by chemical trapping of active intermediates on the surface of catalysts [lll. They show that higher alcohols are formed by insertion of carbon monoxide or a formyl species in a metal-carbene bond. This mechanism implies a priori the presence of two types of sites: the first one responsible for CO dissociation into carbene in the presence of hydrogen, the second one for non dissociative CO chemisorption, or hydrogenation into formyl, Reduced metals sites are known to be active in CO dissociation and carbene formation [13], whereas strong interactions between metals [lo], bimetallic alloys [14], and positively charged metal sites [10,151 have been proposed as non dissociative adsorption sites for CO. Alkali metal promoters also affect the reduction of metal precursors either in the bulk El61 or on the surface. This leads t o a modification of the adsorption and dissociation of CO, and can help stabilize oxygenated intermediates on the surface of the corresponding catalysts (e.g. formyl, acetate, ...1. The present work is devoted t o the study of the effect of molybdenum addition to copper-cobalt-zincformulations, Different Cu-Co-Mo-Zn catalysts have been prepared by coprecipitation, and characterized by X-ray diffraction, S.T.E.M. and thermoprogrammed reduction. The performances of these catalytic systems have been evaluated in gas phase/fixed bed and liquid phase/slurry bed type reactors. In good keeping with the mechanistic scheme already proposed [11,171, the methodology described for other copper-cobalt based formulation [171 has been used t o predict the activity and selectivity of these systems.
EXPERIMENTAL: i>Catalysts Catalysts have been prepared by coprecipitation by ammonia at pH = 7.6 and 353 K of an aqueous solution containing copper, cobalt, zinc nitrates and ammonium heptamolybdate. After agitation for 3 hours the precipitate obtained was washed with water, dried (433K, 6 hrs) and air calcined (623 K, 6 hrs). The oxides obtained have the following theoretical compositions : Cat .I :ColCw.70Moi.55 OZ Cat -11 : C o l C ~ . 7 0 M o l . S S ~ ~ . 2 8 ~ z Cat .I11 : ColCuq,70Mo1.55Zn7.l60~ Cat .Iv: ColCu5.78Mo1.75Zn5.780~
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Cat .V : C O ~ C U ~ O M O ~ Z ~ ~ O O Z . Cat .VI (phase A of cat.V) : Co0.23Mo2.3Cu1.8ZnO~ Cat.VII (phase B of cat.V) : Col.1M00.4Cu0.66ZnOZ Phases A and B have been prepared following - - the same reaction scheme described below for phase A : 0.23 Co(N03)2.6H20+1.8Cu(N03)2.3H20+l Zn(N03)2.6H20+2.3Na2M004.2H20 pH =7 1.44 NaOH (353K)
1 1
Coo~~3Mo~.~Cul.~Zn~OH~,.mH~0 Dried (433K)
Calcined (623K)
~~0.23M02.3Cu1.EZn10z
ii) X-Ray diffraction experiments (XRD) have been performed on a SIEMENS D 500 diffractometer equiped with a DACO-MP microprocessor, using the Ka radiation of cobalt. iii) The characterization by STEM was performed on a high resolution VG HB5 dedicated STEM equiped with a KEVEX energy dispersive X-ray spectrometer, a GATAN 607 electron energy loss spectrometer and a high sensitivity TV camera for the collection of electron microdiffraction patterns. Thin foils of the compounds (t c 100 nm) at each step of their preparation were deposited on the nylon microscopy grids. The foils were obtained by ultramicrotomy with a diamond knife on solid grains previously embedded in an epoxy resin.Quantitative information was obtained from the X-ray emission spectra using a quantitative method which is derived from the CliffLorrimer approach for the use the so called K-factors to correlate the peak intensities with concentrations in the thin film approximation. The use of ultramicrotomy foils made the thin film criterion valid in most analyses. iv) Carbon monoxide hydrogenation : The reduction was performed first from room temperature up to 513K, under diluted hydrogen (6% in N2, 2 1.h-1.gcat-1) with three steps at 433K, 463K and 513K, then from 513K up to 773K (593K in slurry bed reactor) under hydrogen (2 1.h-1.gcat-1;0.2 K.min-l).The reaction was performed at 8.5 MPa with a syngas flow rate of 3 1.h-1.gcat-1and a H2XO ratio of 211. v) Temperature programmed desorption of acetaldehyde Catalysts samples were charged (500 mg) into a tubular glass reactor (1cm O.D.) and reduced under hydrogen as here above. After cooling the reactor down to room temperature, the catalysts was exposed to a helium flow (10 1.h1.g cat-1i4-5 hrs) and then to acetaldehyde vapor (10 mm Hg). After flushing with He to remove all acetaldehyde from the gas phase, the temperature was raised up to 773K (6K.min-1)under a carrier flow of helium (4 1.h-1.gcat-1.1.A Gow-Mac conductivity cell system allowed the detection of desorbed molecules from the catalytic surface. Desorption products were analyzed and identified by two on-line gas chromatograph.
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RESULTS AND DISCUSSION: Figure 1 compares the alcohols selectivity obtained at isoconversion with catalyst V in a fixed bed or a slurry reactor. The same conversion is observed in the slurry reactor after a 10K increase of the operating temperature when compared to the fixed bed test. Figure 1: Evolution of alcohols selectivity versus conversion in slurry and fixed bed reactor. 65’1
2
1
6
10
14
I
Conversion into ROH+HC (%I
The performances obtained with the former type of reactor are summarized in Table I. Table I : Performances of catalysts in slurry bed reactor.
- ZnO addition (cat.11) to the basic formulation (cat.1) does not change much the overall activity: a 10K increase of the reaction temperature is sdlicient to
reach the same conversion as with cat.1. On the opposite, a sharp increase of the alcohol selectivity (55 C% versus 32 C%) and C2+0H selectivity (28 C% versus 10 C%) is noticed. T.P.R. studies (Figure 2) performed on catalysts I allow t o detect two reduction peaks: the first one at 498K corresponding to copper reduction, the second one at 575K which is tentatively attributed to the reduction of the mixed oxide phase CuxCoyMozOt detected by X.R.D.. On the contrary, the thermogram of cat. I1 shows only one broad reduction peak situated at 553K. The absence of a peak at 495K reveals that a lower amount of copper is reduced, which in turn can lead to a lower reduction of other metals. This may explain the slightly lower conversion (less free metallic cobalt present) and the higher alcohol selectivity ( increase of ionic cobalt species and/or of the Cu-Co interaction) observed with catalyst 11. Indeed, the beneficial influence of either the Co2+-Coo or the Cuo-Coo associations on the alcohol selectivity has been stressed in recent publications 118,193.These associations allowing t o increase the cobalt dispersion and reduction, decreasing the hydrogenationhydrogenolysisactivity of metallic cobalt and promoting the formation of alcohols.
lil
Figure 2 :TPR ofcakfdystS1, 11 and III.
""I
Figure3 :TPR ofcatalysta IV and V.
so
2 10 0
oJ
.
.
. - .
. . . I 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0
Temperature K
300
400
500
600
Temperature K
700
800
- The increase of the ZnO content (cat.111) drives to a further decrease of the catalytic activity (comparison of cat.11 and cat.111 in Table 1). Neither the alcohol selectivity, nor the chain growth are modified significantly. The thermoprogrammed reduction (Fig.2) shows a lower degree of reduction of this catalyst, with a second reduction peak situated at 670K. The latter temperature is well over the temperature at which the catalyst was reduced before catalytic evaluation in a slurry reactor (T=595K). - An increase of the Mo/Co ratio from 1.55 to 1.75,and of the Cu/Co+Mo ratio from 1.83to 2.1 (cat IV)causes a decrease of the overall catalytic activity, a 20K increase of the catalyst bed temperature being necessary to reach the same CO conversion as with cat.11. T.P.R. studies of cat.N (figure 3) shows three reduction peaks at 490K, 520K, and 580-590K, the first one corresponding to the partial reduction of copper. Thus, after the reduction procedure followed before the catalytic test (up to 593 or 773K), this catalyst is not totally reduced, which explains the lower performances obtained.
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- On the contrary, a decrease of the Mo/Co ratio t o 1.5 with a Cu/Co+Mo ratio of 2 (cat.V) leads to better performances: a conversion‘of CO into alcohols and hydrocarbons between 6.6 and 14.3 at 530-550K, and an alcohol selectivity 59 C%, together with a chain growth (C2+0N) between 27 and 32 C%. The complete reduction of this catalyst in T.P.R. conditions between 440K and 550K (Figure 3) explains these catalytic test results. Diffraction lines corresponding to copper oxide and zinc oxide are detected on the X-ray diagram of cat.V, as well as other lines which can tentatively be attributed to a mixed CuxCoyMo,Ot phase which might also contain zinc oxide. The characterization of this most performant catalyst has thus been undertaken by scanning transition microscopy (S.T.E.M.), and X-ray emission spectra in order to get some information on the local composition.
Figure 4 and 5 : STEM analysis of CuCoMoZn actives phases. Figure 4 Figure 5 B
B
I
A
1
4
A
I
+B +B
0.bp
t B
0.16
t
B
The S.T.E.M. analysis of this catalyst (global composition: Cu1oCo2Mo3Zn10Oz) indicates that it is composed of four different phases (Figuree 4 and 6): ZnO, CuO, a molybdenum-rich phase (phase A) having the following average composition: Cu1.8C00.23M02.3Zn10z, and a cobalt-rich phase (phase B) of average composition: Cu0.66Col.lMo0.4Zn10~.A careful1 control of the preparation steps and of the composition of the starting solution allowed us to prepare selectively each of these phases. Phaee A has a B.E.T. surface area of 46 mz/g, to be compared t o 151 m2/g for phase B, and 44 mz/g for catalyst V at the same calcination temperature (823K). On the X-ray diagram of phase A (Figure 6), no crystallized CuO, ZnO or cobalt oxide phase is detected, but the presence of lines situated very near those of the mixed copper-molybdenum phase Cu3Mo20g is detected.
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Phase B is amorphous after calcination at 623K,whereas at 823K,poorly defined diffraction lines corresponding to copper and zinc oxides appeart no free cobalt oxide phase being detected.
Figure 6 : XRD of catalyst phase A after calcination at 823K.
t
PHASE A CuCoMoZn
I
Figure 7 allows to compare the alcohol selectivities obtained with cat.V, phase A and B at the same conversion levels. It is clearly seen that phase B is the one responsible for alcohol synthesis in catalyst V. Phase A which has a low cobalt content, presents a low activity (0.7 % conversion to be compared to 5.1% conversion for phase B in slurry reactor at 523K),furthermore, it leads essentially to hydrocarbon formation. Figure 7 :Evolution of alcohole eelectivity vmw c o n d o n . Cataly8t.a V,pham A and B.
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MECHANISTIC STUDIES: We have confirmed on the present catalysts the results of the mechanistic studies (probe molecules, coupling reactions, etc...) previously performed on CuCo catalysts [11,17].In good keeping with the mechanistic scheme already proposed, the tools allowing to predict the alcohol and C2+0H selectivities have been extended to these new CuCoMoZn formulations. As already described {ll] ethylene addition to syngas leads to an increase of propionaldehyde and propanol production. Propane and butane are also detected as by products together with ethane. The orientation towards C3 oxygenates or C3 hydrocarbons depends on the nature of catalyst. The ratio between the increase of C3 oxygenates production and the total increase of C3 products (hydrocarbons + oxygenates) after ethylene addition can thus give us a good approximation of the alcohol orientation of the catalysts as can be seen in figure.8. Figure 8 :Observed alcohole selectivityversus predicted selectivity (ROH %).
0
20
40
60
80
100
Predicted selectivity (ROH %)
Figure 9 :TPD of acetaldehyde on cat.VI.
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In order to obtain a more quantitative prediction, thermoprogrammed desorption after acetaldehyde adsorption can be used. Figure 9 illustrates all the products obtained on cat VII (cat V phase B). Three main desorption domains are roughly observed: at about 373K (acetaldehyde and ethanol), between 420 and 450K (acetone sometimes associated with propene), at T>630K,CO, C02 and CH4 -~~ formation. The formation of-these three groups of products can be rationalized by the following - scheme : ~
CHSCHO, CH3CH20H
CH3CHO-
(Y
surf. c
\
CH3COOads
CH3CHOads
(2)_
r -
__c
(CH,+CO2) or (C02+CH3COCH3) CH3COCH3 (lowtemperature)
The first pathway corresponds t o a desorption without modification or to a hydrogenation, the second one t o an oxidation and to the formation of carboxylate species, the third reaction yields to the formation of acyl species considered as key species in the alcohol chain growth. Acetone formation at low temperature is present on CoCuMoZn and CoCuZnAl catalysts, whereas it is not detected on the support (ZnO,ZnAlaO4) or on a 15%Cu/ZnA1204 catalyst (17). This low temperature acetone desorption cannot arise from acetate decomposition, since the temperature is too low and furthermore, no C02 is evolved at the same time as acetone. Acetone is therefore suggested to arise from the trapping of acyl species by CH3 entities present on the surface, as already indicated for CuCo and Ni based catalysts [19,201. If it is assumed that the three groups of products obtained in acetaldehyde desorption (ethanol+acetaldehyde, acetone, CH4+CO+C02)are representative of the three main reactions occuring on CuCo or CuCoMoZn catalysts, (e.g. hydrogenation, alcohol formation via acyl species, hydrocarbon synthesis via surface carbon and carboxylates species), the ratio between the acetone peak area and the total desorption peaks area give an estimate of the higher alcohol orientation of the catalyst. The C2+OH selectivity thus predicted is compared with the selectivity obtained in catalytic tests under pressure (Figure 10). It can seen that there is a good agreement between those two selectivities even for new CuCoMoZn active phases as already described for the CuCo type catalysts.
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Figure 10 : Observed selectivity (C2+OH) versus predicted selectivity(C2+OH%).
CONCLUSION: This work has demonstrated the beneficial effect of molybdenum addition to copper-cobalt based catalysts for alcohols synthesis from syngas. The quantitative studies performed by S.T.E.M. has lead to the determination of the composition of the mixed phases present in these molybdenum doped formulations, and coprecipitation experiments allowed to prepare the two mixed CuCoMoZn phases detected in a pure state. One of these phases (phase B) having the following composition: C U O . ~ C O ~ . ~ M Ogives ~.~ rise Z to ~ ~a ~ Z , high activity and selectivity towards alcohols synthesis, the other one yielding mainly hydrocarbons. The mechanistic scheme for alcohols synthesis has been found t o be identical to the one already proposed for copper-cobalt based catalysts. The predictive tools developped on the basis of these mechanistic conclusions, and already used in the case of CuCo catalysts, have been extended to the molybdenum doped formulations. This model allows to predict the alcohols selectivity and the chain growth towards higher alcohols, and thus constitutes a very helpful1 tool for the rapid selection of alcohol synthesis catalysts. It can be used for all catalysts yielding alcohols through a mechanism involving the insertion of a C1 oxygenated entity (CO, formyl) into an adsorbed carbenic chain developped through carbene condensation on the surface of the catalyst (i.e. on cobalt, iron and nickel based catalysts), but, it cannot be used in the case of alkalinized methanol synthesis catalysts
.
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1 Ph. Courty, A. Forestihre, N. Kawata, T. Ohno, C. Raimbault and M. Yoshimoto, Industrial Chemicals via C1 Processes ACS Symposium Series 328 (1987)42. 2 A. Paggini and D. Sanfilippo, in "Production of mixed alcohols from synthesis gas", Spring National Meeting, New Orleans, US.,6-10th April 1986. 3 G.van der Lee and V. Ponec, Catal. Rev. Sci. Eng., 29 (1987)183. 4 G.B. Murchison, M.M. Conway, R.R. Stevens and G.J. Quarderer, Proceedings 9th International Congress on Catalysis, vol. 11, Calgary (19881, 626 M.J. Phillips and M. Ternan Editors. 5 A. Hakki, C. Tellis, in "Octane enhancers for motor-fuels", Spring National Meeting, New Orleans, U.S.,6-10th April 1986. 6 T. Tatsumi, A. Muramatsu, K. Yokota and H. Tominaga, J. Catal., 115 (1989)388. 7 M.Inoue, T. Miyake, S. Yonezawa, D. Medhanavyn, Y. Takegami and T. Inui, J. Mol. Catal., 45 (1988)111. 8 M.Inoue, A. Kurusu, H. Wakamatsu, K. Nakajima and T. Inui, Appl. Catal. 29 (1987)36 9 T.Tatsumi, A. Muramatsu, K. Yokota and H. Tominaga, J. Mol. Catal. 41 (1987)385. 10 T. Yano, Y. Ogata, K. Aika and T. Onishi, Chem. Lett. (1986) 303. 11 A. Kiennemann, C. Diagne, J.P. Hindermann, P. Chaumette and Ph. Courty, Appl. Catal., 53 (1989)197. 12 M. Ichikawa, A. Fukuoka and T. Kimura, Proceeding 9th Int. Congr. Catal., vol. I1 Calgary (1988)569. 13 R. Snel and R.L. Espinoza, J. Mol. Catal., 43 (1987)237. 14 R.H. Bailliard-Letournel, A.J. Gomez, C. Mirodatos, M. Primet and J.A. Dalmon, Catal. Lett. 2 (1989)149. 15 A. Razzaghi, J.P. Hindermann and A. Kiennemann, Appl. Catal., 13 (1984)193. 16 A. Muramatsu, T. Tatsumi, H. Tominaga, Bull. Chem. SOC.J a p 60 (1987)3157. 17 A. Kiennemann, S. Boujana, C. Diagne, Ph. Courthy and P. Chaumette, Studies in Surface Science and Catalysis, vol. 61 (1991)243 A. Holmen et a1 Editors. 18 J.E. Baker, R. Burch and N. Yugin, Appl. Catal., 73 (1991)135. 19 J. A. Dalmon, P. Chaumette, C. Mirodatos, Review accepted for publication in Catal. Today. 23 J. Cressely, S. Riegert-Kamel, A. Kiennemann and A. Deluzarche, Bull. SOC. Chim. Fr. I1 (1982)171.
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DISCUSSION Q: M. Ichikawa (Japan) 1) You characterized the catalyst hase by XRD and others, depending on the different formulation. In the multimeta lics system of the catalysts, it is not easy to elucidate the surface composition, it is interesting to know how changes in surface structure of the catalysts under working condition and with time-on-stream during syngas reaction, is reflected by changes in product distribution. Could you show us evidence for the structural trend of the catalysts for higher selectivities towards higher alcohols. 2) In your conclusion, you propose the mechanism of HCHO insertion in the surface carbene to propagate the alcohol-carbon chain. Is this a naive mechanism to be found in the organometallic chemistry to produce higher alcohols ?
f
A. A. Kiennemann 1) To answer the first question concerning the catalyst after the reaction, w e have evidenced that the Co-Cu-Mo-Zn catalyst is composed of three different phases: ZnO, phase A (Mo rich) and phase B (Co rich) and that only one phase (B) is active and selective in alcohol formation. As in many cases in alcohol synthesis (s methanol formation) the most active phase is badly crystallized tmorphous at 400 and the well formed crystal only appear after calcination (at 600 C) of the working catalyst. The only evidence is that some crystallinity appears during the aging of the catalyst with formation of distinct copper or/and cobalt oxide particles. To go deeper into the characterization of the used catalysts, we normally used the association of XRD and STEM as in the case of the calcined catalysts. This as been done and published on other Cu-Co formulations [l]. 2) Concerning the insertion it is not possible to distinguish between insertion of CO and/or formyls (not formaldehyde). Formyl species have been characterized by chemical trapping on the catalytic surface but both CO or formyl can lead to the formation of acyl species. [l] Ph. Courty et al., J. Mol. Car., 17, 241 (1982)and Proceedings 8zh Inz. Cong. Cuzal., Vol. XI., p 81 (1984)
%)
Q: T. Koerts (The Netherlands) In your mechanism for the formation of C2+-oxygenates, you suggest that CO insertion can result in two intermediates: one bonded by oxygen atoms, and one bonded by a carbon atom to the catalyst. What is the evidence for the existence of these species '? Can you speculate on the relative reactivity of these intermediates to produce C2+-oxygenates ? A: A. Kiennemann Some evidences have been obtained during the coupling and chemical trapping experiments and they have been given elsewhere [2].Furthermore, the higher alcohols distribution obtained follows the Anderson-Schulz-Flory distribution and therefore we can conclude that the chain growth is obtained by the insertion of an oxygen containing C, entity into a carbene type hydrocarbon species with one or more carbon atoms to form acyl species. This could not be confirmed by FT-IR because of the presence of zinc in the catalyst (see publications on Cu-Zn-Al catalysts). However, as described [3], a link could be establis ed between the amount of acyl species (evidenced by the low temperature (150-180 C) desorption peak of acetone after acetaldehyde chemisorption) and the alcohol chain growth. This was evidenced on Co-Cubut also on Fe and/or Mo catalysts on which w e believe that alcohols are formed by the same mechanism. [2] Appl. Catal., 53, 197 (1989) [3] Studies in Surf.Sci. and CaZaZ., 61,243(1991)
6
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Q: K. P. de Jong (The Netherlands) The differences in selectivities between slurry and fixed-bed reactors (Figure 1) seem surprising. The more so, because the differences are apparent mainly at higher conversions.. Two questions arise: 1) What are the details of the Sherry reactor (product work up, liquid phase, and extent of backmixing) ? 2) How d o you explain the differences between the Sherry and the fixed-bed reactor ? A: A. Kiennemann 1) The comparison between performances in fixed bed and slurry phase reactor have been performed with some differences in the working up of the catalytic system. These differences are essentially seen in the reduction procedure of the catalyst. In fixed bed reactor the reduction has been performed in the conditions described [3]: first from mom temperature to 240 OC under diluted hydrogen flow (6 % H in N 2 1 h-l gcat‘l) with three steps at 160 OC 190 % and 240 OC then from 240 to 5& % reduction under pure hydrogen (2 I h-{ gcat-l, 0.2 Oc mn-l). In the slurry reactor, the slurry solvent used (Cl0 to C18 n-alkane fraction) is stable in presence of the catalyst only up to 320 OC. As a consequence, the reduction procedure (in situ procedure) has been modified as follows: from mom temperature to 160 % under diluted hydrogen flow (6 % H2 in N2,2 I h-l gcat-l, 0.2O C mn-l) then from 160 OC to 320 OC reduction under pure hydrogen (2 1 h-l gcat-’, 0.2 Oc mn-l) after a step at 160 q. The other conditions are the same: introduction of synthesis gas before reaction, etc. For a Co-Cu-ZnAI2O4-Na catalyst the same phenomenon is observed as can be seen in the figure below.
oc?
80% 15 70
-
Slurry (reduction32OT)
65-
Fixed bed
(reduction sO0”c)
60‘ 55
a
-
50‘
45
-
40,
Conversion
1 ~
(ROH+HC)(%)
When the catalyst is prereduced ”ex-situ” (in the same conditions as for the fixed bed reactor) the same trend is observed. However, the catalyst is more sensitive to the temperature increase. 2) The difference between fixed bed and slurry phase reactor might a priori be explained by the conjunction of three factors. a) The different reduction procedures. It as been shown hereabove that it does not seem to be the most important parameter. b) The difference of the effective H d C O ratio at the contact with the catalytic system in slurry and in fixed bed. In slurry phase, the enhancement of the CO percentage
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in the synthesis gas leads to a decrease in conversion and of the overall alcohol selectivity but in an enhancement of the higher alcohol selectivity. c) A better removal of the reaction heat by the slurry agent. It could be observed, indeed, that the alcohol selectivity is less sensitive to the temperature changes than in the fixed bed reactor. Figure 1 shows in fact that, at low conversion (low temperature), the differences in selectivity are negligible and that the increase with the % of CO conversion (increase of reaction temperature). In a fixed bed reactor, local temperature raised can occur inside the catalyst grains although the temperature of the catalyst bed looks isothermal. The better heat removal in slurry reactor helps to avoid this phenomenon and stabilize the alcohol selectivity.