Conversion of syngas to higher alcohols over nanosized LaCo0.7Cu0.3O3 perovskite precursors

Applied Catalysis A: General 326 (2007) 152–163 www.elsevier.com/locate/apcata

Conversion of syngas to higher alcohols over nanosized LaCo0.7Cu0.3O3 perovskite precursors Nguyen Tien-Thao a, M. Hassan Zahedi-Niaki a, Houshang Alamdari b, Serge Kaliaguine a,* a

Department of Chemical Engineering, Laval University, Quebec, Canada G1K 7P4 b Nanox Inc., 4975 Rue Rideau, Quebec, Canada G2E 5H5

Received 23 January 2007; received in revised form 27 March 2007; accepted 1 April 2007 Available online 19 April 2007

Abstract Two LaCo0.7Cu0.3O3 perovskite catalysts synthesized by reactive grinding and by the citrate complex method have been characterized by X-ray diffraction (XRD), BET, SEM, H2-temperature-programmed reduction (TPR) and tested for the synthesis of higher alcohols and hydrocarbons from syngas. The ground sample shows a rather high surface area and nanometric particles. The coexistence of copper and cobalt in the perovskite lattice provides a highly dispersed bimetallic phase after pretreatment under hydrogen. Both samples were reduced in situ prior to being tested for the synthesis of alcohols and hydrocarbons. The catalytic activity and product distribution depend strongly on the process variables, alkali promoter, preparation method, and catalyst morphology. While the ground perovskite is rather selective for the synthesis of higher alcohols, the citrate-derived precursor produces mainly methane and light hydrocarbons in addition to 10–15 wt.% of alcohols. The optimum parameters for catalyst preparation and for alcohol synthesis were determined. Under the optimal reaction conditions, the alcohol productivity is in a broad range of 70–140 mg/gcat/h and the selectivity towards alcohols is about 40–45 wt.%. Both alcohols and hydrocarbons produced obey a classical Anderson–Schulz–Flory (ASF) plot carbon number distribution. The nanocrystalline perovskite precursor shows a better catalytic performance compared to the citrate-derived sample in terms of both alcohol selectivity and productivity. The catalytic stability of the ground perovskite is dependent not only on the crystal domain size (or the size of nanoparticles) and the amount of remnant sodium ions, but also strongly on the compactness of nanoparticles. The existence of slit-shaped spaces between primary nanoparticles and/or grain boundaries hinders the formation of long carbon chains, which are precursors for the formation of coke on the catalyst surface. # 2007 Published by Elsevier B.V. Keywords: La-Co-Cu; Higher-alcohols; Co-Cu metal; Syngas; Stability; Perovskite

1. Introduction The research into the synthesis of methanol and higher alcohols from syngas (H2/CO) has been thoroughly developed during the 1930–1945s. Mixed alcohols are an octane booster for clean gasoline. Indeed, higher alcohols could be added to gasoline in all proportions and make them miscible under a broad range of operating temperature. Since both tetraethyl lead and short ether compounds (MTBE, ETBE, etc.) were banned as gasoline octane improvers in North America, mixed alcohols have been used as additives for reformulated gasoline and their demand has recently undergone in an impressive growth [1,2].

* Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810. E-mail address: [email protected] (S. Kaliaguine). 0926-860X/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apcata.2007.04.009

Up to now, the synthesis of higher alcohols from syngas is an interesting subject for both industrial application and fundamental research. This synthesis usually requires a complex composition catalyst working in severe conditions and produces a mixture of either branched or linear primary alcohols ranging from methanol to hexanol [3,4]. Based on the product distribution, two main types of higher alcohol synthesis catalysts have been reported [5,6]. The first type was established primarily from either high-temperature–pressure Zn-Cr-based catalysts or low-temperature–pressure (Cu-Zn-Al/Cr) methanol synthesis catalysts. The addition of alkali promoters into such systems significantly enhances the selectivity towards higher branchedalcohols [7–12]. Thus, the formation of higher alcohols in this case is generally assumed to be a combination of hydrogenation and carbon–carbon bond formation via aldol condensation of lower alcoholic intermediates and/or oxygenated compounds

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[13–17]. In this case, among C2+-alcohols, isobutanol (2-methyl1-propanol) is always formed as a major branched product because it lacks the two a-hydrogens required for the successive aldol-condensation-chain growth steps [8,18,19]. As a result, this category mainly produces methanol and branched-alcohol products which therefore do not follow a classical Anderson– Schulz–Flory (ASF) product distribution. In contrast, the second type developed from Fischer–Tropsch (FT) and the VI–VIII metal-based catalysts yields a series of linear primary alcohols and gaseous hydrocarbons both with ASF carbon number distribution [4–6,20–26]. Table 1 summarized a series of typical performances established experimentally for selected representative catalysts. This selection of literature data was made for experiments conducted in conditions close to the ones in this work in order to allow some comparisons with our proposed catalyst. As seen from Table 1, modified Fischer–Tropsch catalysts (for example, catalysts # 5–7 in Table 1) produce a mixture of straight alcohols in addition to hydrocarbons and carbon dioxide [20–23,25]. Molybdenum sulfide catalyst (MoS2) promoted with alkali (for example, # 8) also yields a homolog of linear primary alcohols, hydrocarbons [26,30] while Rh-based catalysts (Rh-Mn/SiO2, LaRhO3, # 9) tend to be more selective for C2-oxygenates (alcohols and aldehydes), methane, CO2 [31,32]. Like some modified methanol synthesis catalysts, the modified FT catalysts (# 5–7) show a better selectivity to higher alcohols if a proper amount of alkali promoter is introduced. Both mentioned types of higher alcohol synthesis catalysts are rather active for the water–gas-shift (WGS) reaction and for methanation. A high selectivity to linear primary alcohols is achievable if the catalyst surface contains CO dissociating sites in the vicinity of non-dissociating sites [6,21–23]. Therefore, it is suggested that for all alcohol synthesis catalysts, the transition metals distributed in the crystal structure (e.g., spinel, perovskite) of oxide precursors may be very highly dispersed after being reduced under hydrogen atmosphere [6,21,31,33–34]. Among transition metals, cobalt is known to be very active for the Fischer–Tropsch synthesis while copper tends to form methanol and other alcohols [20,21,34,35]. This leads to the assumption that a Co-Cu mixed oxide with a definite structure such as perovskite or spinel could be a promising catalytic material for conversion of syngas (H2/CO) into alcohols and hydrocarbons

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[25,31,33,36–38]. Accordingly, one might expect that a close contact between cobalt and copper phases may yield an active component for higher alcohol synthesis. By this way, a variety of perovskite systems prepared by conventional methods have been applied in CO hydrogenation in an effort to produce higher alcohols from syngas [31,33,39]. However, such materials have some limitations for this synthesis because their specific surface area is always rather low [33]. In previous studies [40,41], we have reported the reduced Co-Cu metals from nanocrystalline perovskites prepared by mechano-synthesis, acting as catalysts for the synthesis of higher alcohols and hydrocarbons from syngas. A small amount of residual alkali introduced during catalyst preparation has significant effects on alcohol productivity and selectivity [41]. This article is to further optimize the catalyst composition of Co-Cu-based perovskites, preparation, and reaction conditions for the synthesis of higher alcohols from syngas. 2. Experimental 2.1. Perovskite precursor preparation LaCo0.7Cu0.3O3 d perovskite-type mixed oxide (GM) was synthesized by the reactive grinding method also designated as mechano-synthesis in literature [40–43]. The stoichiometric proportions of commercial lanthanum, copper, and cobalt oxides (99%, Aldrich) were mixed together with three hardened steel balls (diameter = 11 mm) in a hardened steel crucible (50 ml). A SPEX high-energy ball mill working at 1000 rpm was used for mechano-synthesis. Milling was carried out for 8 h prior to a second milling step with an alkali additive. Then, the resulting powder was mixed to 50% sodium chloride (99.9%) and further milled under the same conditions for 12 h before washing the additives with distilled water. A sample was added into a beaker containing 1200 ml water and stirred by magnetic stirring for 90 min prior to being sedimented for 3–5 h. After the clean water is removed, the slurry was dried in oven at 60– 80 8C before calcination at 250 8C for 150 min. A reference sample without any alkali promoter, LaCo0.7Cu0.3O3 d designated as CIT, was prepared by thermal decomposition of the corresponding amorphous citrate

Table 1 Alcohol synthesis over various catalysts No.

1 2 3 4 5 6 7 8 9

Catalysts

Cs/Zn/Cr Pd/Cu/Me/Ce Cs/Cu/Zn/Al Cs/Cu/Zn/Cr K/Cu/Co/Zn/Al Cu/Co/Zn/Al Na/Cu/Co/Zn/Al Cs/Co/Mo/Clay LaRhO3 a

Conditions

CO conversion (%)

T (K)

P (MPa)

H2/CO

GHSV

678 583 583 583 563 573 573 573 573

6.9 4.5 7.6 7.6 6.0 6.0 6.0 13.8 0.6

1 1 0.45 0.45 0.5–2 1 1 1.1 1

Not found 6000 l/kgcat/h 5330 l/kgcat/h 5530 l/kgcat/h 4000 h 1 3000 h 1 3000 h 1 4000 h 1 Not found

Calculated on a CO2 free basis.

9.0 10.6 13.8 21.1 21–24 Not found Not found 4.7 Not found

Selectivitya (carbon atom, %)

Productivity (g/kgcat/h)

MeOH

C2+OH

MeOH

72.2 74.4 85.1 48.7 20–27 44.0 37.0 61.8 12.2

0.8 11.6 5.9 29.8 30–50 30.0 33.0 26.11 5.6

116 1 23.6 7.8 436.5 16.9 202.0 51.9 60–100 85.0 85.0 60–64 35–40 16 15

References

C2+OH [27] [13] [28] [28] [21] [29] [29] [30] [31]

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precursor [33,43]: a stoichiometric amount of La(NO3)3 was added into distilled water while slowly heating the mixture on a magnetic stirrer. After a clear transparent solution was obtained, stoichiometric amounts of cobalt and copper nitrates were added. Then 1 mole of citric acid per mole of metal atom was added to the dark pink/red translucent solution. The resulting solution was heated slowly to dryness and then dehydrated at 100–120 8C overnight in a vacuum oven, yielding an amorphous solid precursor. The sample was calcined at 800 8C for 6 h with a ramp of 3 8C/min under air. 2.2. Characterization The chemical analysis (Co, Cu, Na) of the perovskites and the residual impurities was performed by atomic absorption spectroscopy using a Perkin-Elmer 1100B spectrometer. Prior to each analysis, a weighed amount of sample was digested in a 10% HCl solution at 60–70 8C overnight. The specific surface area of all obtained samples was determined from nitrogen adsorption equilibrium isotherms at 196 8C measured using an automated gas sorption system (NOVA 2000; Quantachrome). Three hundred to five hundred milligram of each sample was degassed at 200 8C for 6–7 h in order to remove the humidity prior to N2 adsorption measurements. Phase analysis and particle size determination were performed by powder X-ray diffraction (XRD) using a SIEMENS D5000 diffractometer with Cu Ka radiation (l = 1.54059 nm). Bragg’s angles between 208 and 608 were collected at a rate of 18 min 1. Average crystal domain sizes (D) were evaluated by means of the Debye-Scherrer equation D = Kl/b cos u after Warren’s correction for instrumental broadening. SEM images were recorded using a JEOL JSM-840 electron microscope with magnification of 25,000. The acceleration voltage was 10 kV. The samples were dispersed on an aluminum stub and coated with Au//Pt film. Elemental analysis was performed using an X-ray fluorescence spectrometer (Bruker-AXS) at 15 kV beam voltage. Temperature-programmed reduction (TPR) experiments were carried out using a multifunctional catalyst testing and characterization apparatus (RXM-100 from Advanced Scientific Designs, Inc.). The reactor was connected to a thermal conductivity detector (TCD). Prior to each TPR analysis, a 50 mg sample was calcined at 500 8C for 90 min under flowing 20% O2/He (20 ml/min, ramp 5 8C/min). The sample was then cooled down to room temperature under flowing pure He (20 ml/min). TPR of the catalyst was then carried out by Table 2 Physical properties of LaCo0.7Cu0.3O3 Sample

GM CIT a b

Recipe

Reactive grinding Citrate complex

d

ramping under 4.65 vol.% of H2/Ar (20 ml/min) from room temperature up to 800 8C (5 8C/min). The hydrogen consumption was determined using a TCD with a reference gas of same composition as the reducing gas (H2/Ar). The effluent gas was passed through a cold trap (dry ice/ethanol) in order to remove water prior to detection. 2.3. Catalytic test The catalytic tests were carried out in a stainless-steel continuous flow fixed-bed micro-reactor (BTRS-Jr PC, Autoclave Engineers). The reaction pressure was controlled using a back-pressure regulator. The syngas mixture (H2/CO = 2/1, Praxair) was supplied from a pressurized manifold via individual mass flow controllers. The catalyst pellet size was 40–60 mesh. Catalysts were pretreated in situ under flowing 5 vol.% of H2/Ar (20 ml/min) prior to each reaction test. The temperature was kept at 250 8C (3 h) and 500 8C (3 h) with a ramp of 2 8C/min. Then, the reactor was cooled down to the reaction temperature while pressure was increased to 1000 psi by feeding inert gas before switching to the reaction mixture. The GHSV was set at 5000 h 1 unless otherwise stated. Butane/helium (4.98 vol.%) was used as an internal calibration standard. The products were analyzed using a gas chromatograph equipped with two capillary columns and an automated online gas-sampling valve maintained at 170 8C. The temperature of transfer line between the reactor and the valves was kept at 220 8C in order to avoid any product condensation. Carbon monoxide and carbon dioxide were separated using a capillary column (CarboxenTM 1006 PLOT, 30 m  0.53 mm) connected to the TCD. Quantitative analysis of all organic products was carried out using the second capillary column (Wcot fused silica, 60 m  0.53 mm, Coating Cp-Sil 5CB, DF = 5.00 mm) connected to a FID detector (Varian CP3800) and mass spectrometer (Varian Saturn 2200 GC/MS/MS). The selectivity to a given product is defined as its weight percent with respect to all products excluding CO2 and water. Productivity is defined here as a weight (mg) product per gram of catalyst per hour. 3. Results 3.1. Characterization Table 2 summarizes some physical properties of the LaCo0.7Cu0.3O3 samples prepared by the two different methods. As compared with the conventional citrate method [33], mechano-synthesis always produces nanocrystalline

mixed oxides SBET (m2/g)

21.5 4.7

Pore diameter (nm)a

18.0 16–20

Determined from N2 adsorption/desorption isotherms. Calculated by the Scherrer equation from X-ray line broadening.

Crystallite size (nm)b

10.3 >35

Composition (wt.%) Co

Cu

Na+

16.54 16.68

5.86 6.03

0.38 –

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Fig. 1. XRD spectra of patterns: (a) GM before any washing; (b) GM washed; (c) CIT ((*): NaCl; (+): CuO; P: perovskite).

perovskites having higher specific surface area, in agreement with several results reported earlier [40,41]. XRD patterns are presented in Fig. 1. The ground perovskite sample without any washing (Fig. 1a) shows some strong reflections of NaCl additives in addition to those of perovskite. After leaching, no clear lines corresponding to NaCl are observed in spite of a small amount of additive being detected from AAS (Table 2). The other samples have essentially perovskite structure as compared with JCPDS data (Card Nos. 48-0123 and 81-2124). For sample CIT, two minor peaks at 35.78 and 38.98 ascribed to CuO crystals (Card No. 45-0937) appear, indicating the presence of a small amount of copper oxides.

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Fig. 2A and B shows the very significant difference in topography between the two studied samples (GM and CIT). The SEM photograph of sample CIT shows a foam-like phase with a nonporous structure while that of sample GM presents a micro-porous structure of primary particles with pore diameter of 18 nm (Table 2), in agreement with previous observations [40,43]. The crystal domain sizes calculated by the Scherrer equation from XRD line broadening are about 9–10 nm for the ground perovskite sample. As reported earlier in Refs. [40–42], the introduction of a grinding additive during the second milling step resulted in the partial separation of the crystalline domains, leading to an increased surface area and a decreased volume of grain boundaries. Therefore, this process of catalyst preparation has affected the specific surface area by changing the internal porosity of agglomerates. In this case, the porous system of ground perovskite (GM) is presumably constituted of slit-shaped spaces between primary nanoparticles. These differences in the catalyst morphology account for the difference in physical properties and catalytic activity between the two samples, GM and CIT. 3.2. H2-TPR In order to shed light on the reducibility of Co-Cu-based perovskites, temperature-programmed reduction was carried out from room temperature to 800 8C. Fig. 3 shows such H2TPR curves for both samples. Two main peaks are observed in the temperature range of 180–750 8C. For sample CIT, the small shoulder at 220 8C which is observed at a higher temperature for sample GM, is attributed to the reduction of a

Fig. 2. Photographs of samples: GM (A); CIT (B); GM reduced at 500 8C (C); GM reacted for 14 days (D).

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Fig. 4. XRD spectra of samples CIT, GM reduced at 350, 400 8C, respectively.

Fig. 3. H2-TPR profiles of LaCo0.7Cu0.3O3

d

perovskites.

tiny amount of extra-perovskite framework copper as indicated by X-ray diffraction (Fig. 1). According to the calculation of H2 balance, the first sharp peak with maxima at 275, 357 8C for samples CIT, GM, respectively, is ascribed to the two successive reductions of intra-perovskite lattice Cu2+ and Co3+ to Cu0 and Co2+, respectively, though a minor amount of Co0 formed in the case of sample (GM) under these conditions is not ruled out [40,41,43]. In order to verify this stage of reduction, both samples were collected at the end of the first reduction peak (350 and 400 8C for CIT, GM samples followed by sintering at 325 and 375 8C, respectively, in He for 90 min) and their XRD spectra were recorded. The sintering was made here in order to enhance the metal diffraction peaks that are very low with highly dispersed metals [40]. Fig. 4 shows these spectra and the two typical reflections of copper at 2u = 43.328 and 50.348 (JCPDS card No. 04-0836) for both samples studied. Under these reduction conditions, the perovskite framework is still preserved although a strong modification in structure is observed. Accordingly, the reduced metallic copper along with Co2+ ions are assumed to be atomically dispersed in the perovskite at the end of the first reduction process (Cu2+/Cu0 and Co3+/Co2+) [40,43]. The further reduction of Co2+ to Co0 requires an essentially higher temperature [40,41,44]. Therefore, the second peak is definitely assigned to the reduction of the remaining cobalt ions. In the case of ground perovskite, this broad peak from 400 to 650 8C is resulting from the reduction of cobalt ions at different locations and coordination numbers [41,44]. Moreover, H2-TPR results also indicate that cobalt ions in La-Co-Cu perovskite lattice are more reducible than those in Co-based perovskites (not shown here) [40,41,44]. The reduced copper promotes the reduction of cobalt, apparently by providing hydrogen dissociation sites [40,45]. H2-TPR studies also indicate that the presence of sodium additive inhibits the

reduction of Cu2+ and Co3+ during H2 pretreatment (Fig. 3) [41]. This phenomenon is explained by the inhibition of H2 activation and by the blockage of copper/cobalt sites by the residual sodium ions [41,44,45]. Under hydrogen pretreatment, the perovskite was strongly depleted in Cu and Co, yielding finely divided Co-Cu clusters as observed from the SEM photograph (Fig. 2C). The average particle size in the pretreated sample GM (Fig. 2C) is significantly smaller than that in the parent one (Fig. 2A). This is in good agreement with the XRD studies of the reduction of LaCoCuO3 reported previously [40]. 3.3. Synthesis of alcohols 3.3.1. Effect of reaction temperature The influence of the reaction temperature on the rate of formation of products has been examined in the range of 250– 350 8C under 1000 psi, GHSV = 5000 h 1 and H2/CO = 2 over the two samples GM and CIT. The products obtained under these conditions include C1–C7 linear primary alcohols, C3–C4 secondary alcohols, and C1–C10 hydrocarbons. The formation of oxygenates and hydrocarbons is always accompanied by the production of water, most of which was converted to CO2 through the water–gas-shift reaction [6,21,41]. In this study, %WGS is defined as the percentage of CO converted to CO2. Fig. 5 reports an increased catalytic activity in carbon monoxide hydrogenation with increasing reaction temperatures for both samples. Clearly, the conventional perovskite catalyst (CIT) shows a higher conversion level than the ground sample (GM) does. Meanwhile, alcohol productivity on both these samples passes through a maximum at temperature 300 8C (Fig. 6). At higher temperatures, the water–gas-shift reaction and methanation become fairly predominant [31,45]. Indeed, Fig. 7 shows an increased methane yield with increasing reaction temperature. This can be explained from the mechanistic consideration that hydrogenation of CHx* inter-

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Fig. 5. Effect of reaction temperature on catalytic activity under 1000 psi, 5000 h 1 and H2/CO = 2.

157

Fig. 7. Effect of reaction temperature on methane productivity under 1000 psi, 5000 h 1 and H2/CO = 2.

mediates into methane predominates at higher temperatures [21,31,46]. Comparing the overall activity and productivity presented in Figs. 5–7 indicates that sample CIT is rather favorable for the production of methane and light hydrocarbons while the ground perovskite (GM) shows a fairly high selectivity to higher alcohols in the temperature ranging from 250 to 325 8C. At a higher temperature, the formation of a large amount of CO2 leads to a small change in H2/CO ratio and may also affect the oxidation state of metal sites on the catalyst surface, resulting in a significant change in selectivity of chain growth termination [3,6,21,31,47]. For this reason, both alcohol selectivity and productivity rapidly decrease at high temperatures (>325 8C) [6,21,41]. 3.3.2. Effect of space velocity The effect of velocity (GHSV) of reactants on alcohol synthesis was investigated over the broad range of 2500– 20,000 h 1 with a H2/CO/He = 8/4/3 at 300 8C under 1000 psi for the GM catalyst. The results are reported in Figs. 8 and 9. These figures show a similar variation trend between CO

Fig. 6. Effect of reaction temperature on alcohol productivity under 1000 psi, 5000 h 1 and H2/CO = 2.

Fig. 8. Effect of velocity on CO conversion and WGS over sample GM (300 8C, 1000 psi, H2/CO/He = 8/4/3).

Fig. 9. Effect of velocity on product yield on sample GM at 300 8C (1000 psi, H2/CO/He = 8/4/3).

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Fig. 11. Effect of feed partial pressure on productivity over sample GM at 300 8C (5000 h 1, H2/CO/He = 8/4/3). Fig. 10. Effect of feed partial pressure on CO conversion and water–gas-shift reaction on sample GM at 300 8C (5000 h 1, H2/CO/He = 8/4/3).

conversion and water–gas-shift reaction. The increased rate of reactant feed flow rate (space velocity) results in a decreased contact time, which leads to a gradual decline in CO conversion. Fig. 9 also shows a decreased hydrocarbon productivity with increasing space velocity; whereas alcohol productivity reaches a maximum at GHSV = 12,000–15,000 h 1. At a low space velocity, alcohol compounds strongly adsorbed on the catalyst surface take part in some side reactions, which convert such intermediates into olefins via dehydration reaction on the basic La2O3, for example [7,19,25,34,41]. At a higher rate of gas feeding (>15,000 h 1), a declined CO conversion corresponds to a decreased productivity of all products. Moreover, the formation of higher alcohol is not favored at a very high velocity because the synthesis of alcohols occurs through several intermediate steps [6,20,21,23,34]. 3.3.3. Effect of syngas partial pressure The influence of syngas partial pressure on activity and productivity is presented in Figs. 10 and 11. With increasing Table 3 Effect of H2/CO ratio on alcohol synthesis at 300 8C, 1000 psi, GSVH = 15000 h

syngas partial pressure the activity tends to decrease after passing through a maximum at 500 psi, while CO2 selectivity passes through a climax at 750 psi. Both hydrocarbon and alcohol productivities reach maxima around 800 psi. Fig. 11 also shows that productivity of hydrocarbons is always higher than that of alcohols except at high syngas partial pressure. This seems to indicate that in this pressure range the effect of temperature on alcohol productivity is much more predominant than that of syngas partial pressure at 300 8C. 3.3.4. Effect of H2/CO ratio Table 3 summarizes the effect of the H2/CO ratio in the range of 0.5–2.0 on the overall catalytic activity of sample GM at 300 8C, 1000 psi and space velocity = 15,000 h 1. These conditions have been selected due to the rather high productivity as well as the selectivity to alcohols obtained (Fig. 9). With decreasing H2/CO ratio, CO conversion and water–gas-shift reaction decrease, yielding a small decline in productivity. Among alcohols, methanol selectivity decreases

1

H2/CO ratio

CO conv. (%) WGS (%)

2.0

1.5

1.0

0.5

16.03 2.10

12.25 1.28

9.48 1.08

6.95 0.82

Products

Select

Prod

Select

Prod

Select

Prod

Select

Prod

Methanol Ethanol Propanol Butanol C5+-OH Total alcohols Methane C2+-Hyda

17.6 14.1 4.1 1.5 0.8 38.1 22.9 38.9

74.6 49.8 15.4 5.4 3.4 148.6 87.9 142.9

16.7 14.2 5.5 1.6 0.9 38.9 18.2 42.8

42.9 36.9 11.7 4.1 1.9 97.5 41.8 118.6

15.5 14.2 5.5 1.6 1.2 38.0 16.8 45.1

41.3 37.5 11.5 4.1 2.1 97.7 39.2 121.8

13.8 13.5 6.0 2.0 1.4 36.7 14.5 48.7

31.2 30.5 14.3 4.0 2.5 77.3 30.2 124.2

Select: Selectivity (wt.%); Prod: productivity (mg/gcat/h). a C2+-hydrocarbons.

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with decreasing hydrogen partial pressure due to a deviation from stoichiometry of H2/CO = 2, which is the optimal ratio for the formation of methanol [6,21]. Ethanol selectivity is essentially unchanged while selectivity to higher alcohols (C3+OH) increases modestly. As a result, the average carbon atom number or carbon chain growth probability factor slightly increases [10,31]. As indicated in Table 3, an increase in the H2/ CO ratio promotes the overall chain growth process by favoring CO hydrogenation but an excess of hydrogen tends to slow down the formation of higher products because of a preferential conversion of the C1 surface species to methanol. Methane and hydrocarbon selectivities show therefore two opposite trends. The selectivity to methane gradually decreases while that of C2+-hydrocarbons increases with a small decrease in H2/CO ratio [21,31]. Mechanistically, a higher CO partial pressure favors CO insertion into alkyl-metal surface groups to propagate the carbon chain and limits the hydrogenation of (CHx)* for methanation [20,21,23,46]. Consequently, a higher partial pressure of carbon monoxide results in a slightly increased selectivity to heavier products [48]. The alcohol productivities at 0.5 H2/CO are relatively low due to the lower overall activity. The H2/CO ratio of 2 is most favorable for alcohol production [21,49]. 3.3.5. Effect of residual sodium LaCo0.7Cu0.3O3 perovskite catalyst precursor was tested as a function of the residual sodium content at 300 8C, 1000 psi, H2/ CO/He = 8/4/3, 5000 h 1 for optimizing the concentration of alkali promoter. Fig. 12 shows the variation in alcohol productivity with the amount of remnant sodium ion. C2+alcohol productivity increases monotonically with increasing amount of Na+ from 0 to 0.20 wt.% and then reaches a maximum at the sodium content of 0.2–0.30 wt.%. At higher sodium ion content, the overall activity decreases, leading to a suppressed productivity of higher alcohols. This may be related to the deactivation of a part of metallic cobalts covered by an

Fig. 12. Dependence of alcohol productivity on sodium ion content over LaCo0.7Cu0.3O3 pretreated with H2/Ar (Reaction conditions: 300 8C, 1000 psi, 5000 h 1; H2/CO/He = 8/4/3; Na1-Na3 data were taken from Ref. [41].

159

Fig. 13. Variation in CO conversion with the reaction time (300 8C, 1000 psi, 5000 h 1, H2/CO/He = 8/4/3): (a) sample GM pretreated at 500 8C for 180 min under 20 ml/min of 5 vol.%H2/Ar (3 8C/min); (b) sample GM pretreated at 500 8C for 180 min under 20 ml/min of 20 vol.% CO/Ar (3 8C/min); (c) CIT pretreated at 500 8C for 180 min under 20 ml/min of 5 vol.% H2/Ar (3 8C/min).

excess of surface Na+ and a small loss of specific surface area [41,45]. Moreover, the presence of remnant sodium results in a significantly decreased production of methane and a slightly increased yield of higher alcohols and olefins. Thus, the role of remnant sodium ions is plausibly to reduce the hydrogenation ability of cobalt metals by an electronic interaction between Na+ and metallic cobalt, and thus propagate the carbon chain growth of hydrocarbons and alcohol products [6,21,41]. This explains an enhanced selectivity to olefins and higher alcohols, even some traces of aldehydes, which all are intermediates in the hydrogenation reaction [41,50–52]. 3.3.6. Catalyst stability The extended time-on-stream activity of the two samples (GM and CIT) was examined at 300 8C, 1000 psi, H2/CO/ He = 8/4/3, 5000 h 1. In addition, sample GM was pretreated under different reducing gases (5 vol.% H2/Ar and 25 vol.% CO/Ar) prior to catalytic test. The results are reported in Fig. 13. This figure displays the most constant activity of sample GM pretreated at different reducing atmosphere, whereas the sample CIT shows a gradual decline in the overall activity with time-on-stream. A similar trend was observed in water–gas-shift reaction conversion. The measurable difference in activity between the ground perovskite sample (GM) and the citrate complex catalyst (CIT) may result from a significant difference in catalyst morphology as well as the presence/ absence of a small amount of alkali additive (Table 2). As reported previously in Ref. [43], the slit-shaped spaces between elementary nanometric particles or grain boundaries of a perovskite prepared by reactive grinding play an important role in resistance to sulfur poisoning. Under the present operating conditions, the catalyst surface is partially covered by (CHx)* species which are either hydrogenated to paraffins or kept as adsorbed hydrocarbons [31,34]. The presence of carbonaceous material on the surface is indicated by SEM photograph (Fig. 2D) and X-ray fluorescence microanalysis (Fig. 14). As seen in Fig. 2D, the spent catalyst is still composed of nanometric particles, but its surface seems smoothened likely by a carbonaceous deposit after a long-term performance test.

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Fig. 14. X-ray fluorescence microanalysis spectra: (1) fresh GM; (2) GM pretreated at 500 8C under 20 ml/min of 5 vol.% H2/Ar; (3) sample GM reacted for 2 weeks (300 8C, 1000 psi, H2/CO/He = 8/4/3, 5000 h 1).

X-ray fluorescence microanalysis spectra measured ex situ of sample GM before and after reaction test are shown in Fig. 14. As seen from this figure, line intensities reflecting the surface concentration of cobalt and copper increase from parent to pretreated sample, but decrease for the used sample. Signals of lanthanum, carbon and oxygen appear very strong. By contrast, the signals of copper and cobalt are rather weak amounting to 0.75 and 1.5%, respectively (after subtraction of the gold signal). This result suggests that only a small fraction of these two element are exposed at the external surface of the particles and therefore that only a small number of surface Co/Cu metal sites are sufficient to yield a significant activity in alcohol synthesis from syngas [3,20,21,34,50,53,54]. Indeed, Fig. 15

Fig. 15. Alcohol productivity at 300 8C (1000 psi, 5000 h 1, H2/CO/He = 8/4/ 3): (a) GM pretreated in 5 vol.% H2/Ar; (b) GM pretreated in 20 vol.% CO/Ar; (c) CIT pretreated in 5 vol.% H2/Ar (all sample were pretreated at 500 8C for 180 min, 3 8C/min).

Fig. 16. Hydrocarbon productivity at 300 8C (1000 psi, 5000 h 1, H2/CO/ He = 8/4/3): (a) GM pretreated in 5 vol.% H2/Ar; (b) GM pretreated in 20 vol.% CO/Ar; (c) CIT pretreated in 5 vol.% H2/Ar (all samples were pretreated at 500 8C for 180 min, 3 8C/min).

displays a minor change in alcohol productivity after a rather long course of reaction for sample GM. This figure also reveals a strong variation of alcohol productivity with the pretreatment conditions and catalyst preparation. Comparing sample GM pretreated at two different reducing conditions (5 vol.% H2/Ar and 20 vol.% CO/Ar at 500 8C for 180 min) indicates that the fresh catalyst reduced under H2/Ar is much more active for alcohol synthesis than that pretreated with CO/Ar while the productivity of C2+-hydrocarbons shows the opposite trend (Fig. 16). The lower yield of higher alcohols over sample GM activated in CO/Ar is probably due to the formation of cobalt carbide and/or carbonaceous material deposited during the CO pretreatment. It is known that carbides are intermediates for the production of hydrocarbons in Fischer–Tropsch synthesis [55– 57]. Thus, the presence of these compounds tends to increase the selectivity to paraffins instead of higher alcohols. Indeed, the synthesis of higher alcohols usually requires a dual site of cobalt and copper [21,51,57] so that the separation of copper and cobalt by the formation of carbide (Co2C) and/or the accumulation of carbonaceous species on the surface leads to a lowering of selectivity and activity in higher alcohol synthesis [21,28,32,51,54,57]. In all cases, the productivity of methane and gaseous hydrocarbons are still rather high, particularly in the case of sample CIT. The high methane selectivity for the citrate complex sample may be due to a lower abundance of sites acting for propagation of carbon chain, leading to more individual (CHx)* species on the surface that would be fully hydrogenated to CH4 [25]. In addition, the presence of larger metallic copper crystallites may also be a contributing factor in enhancing selectivity to gaseous hydrocarbons; in contrast to the synthesis of higher alcohols that decreases as the copper dispersion is decreased. Moreover, both alcohol and hydrocarbon yields decreased monotonically with reaction time for the citrate complex sample, demonstrating the lower catalytic stability of this sample (CIT) compared with ground pretreated perovskite (Figs. 13 and 15). This can be explained by the

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Fig. 17. Alcohol selectivity over sample GM (300 8C, 1000 psi, 5000 h 1, H2/ CO/He = 8/4/3).

possibility that the metal surface is either sintered or gradually blocked by the high level of carbon deposited [25,51]. On the contrary, sample GM displays a better stability under the same conditions. The nanocrystalline perovskite precursor, besides having grain boundaries and a large surface-to-volume ratio, always retains a small amount of sodium additive highly dispersed on the catalyst surface (Table 2). This can prevent the agglomeration of small metallic Co-Cu crystallites [34,50,55]. Moreover, the appearance of alkali ions is to promote the formation of alcohols as well as heavier molecular weight hydrocarbons and to decrease the production of methane (Figs. 7 and 8) [41]. Therefore, Co-Cu-Na, triply promoted system shows a good balance concerning alcohol selectivity and productivity [6,21,41,50]. Fig. 17 reports the variations in alcohol selectivity as functions of reaction time. Selectivity to ethanol and higher alcohols (C3+-OH) is almost unchanged during the long-term alcohol synthesis while that to methanol slightly increases, resulting in a modest increase in total alcohol selectivity [29,57]. The origin of the change in MeOH/C2+-OH ratios is likely due to a slow alteration in the nature of the catalyst surface with reaction time (Fig. 17). A similar variation in alcohol selectivity of sample GM pretreated with CO/Ar is observed (not shown in Fig. 17). However, total alcohol selectivity in the latter case (30–32 wt.%) is lower than that of the former (35–48 wt.%, shown in Fig. 15), but still much higher than the one of sample CIT (10–16 wt.%). The low alcohol selectivity of sample CIT must be related to the poor interaction between copper and cobalt and the absence of alkali promoters [33,41]. In all cases, the portion of ethanol in the alcohol fraction is about 50–70 wt.% [57]. 4. Discussion The nanocrystalline perovskite prepared by mechano-synthesis has a rather high surface area and a particular morphology. Addition of sodium chloride into a ground mixed Co-Cu-type perovskite oxide during secondary milling leads to a substan-

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tially increased surface area and creates a cluster of elementary nanometric particles [42,43]. The large surface-to-volume ratio of nanocrystals substantially changes the role played by surface atoms in determining their physical–chemical properties. The reduced coordination number of the surface cobalt atoms dramatically modifies their redox characteristics [40,43,58]. The successful combination of copper and cobalt in the definite perovskite structure results furthermore in the mutual interaction between these two elements in the reduced form. This is a prerequisite for a higher alcohol synthesis catalyst so that any modification of such a characteristic has a detrimental effect on the synthesis of higher alcohols [6,21,40]. Moreover, cobalt and copper located in the grain boundaries of a ground perovskite in the present study is believed to be very active sites for alcohol synthesis [38,39]. All perovskite precursors are prereduced in situ, producing both copper and cobalt metals homogeneously distributed over La2O3 support [40,41,43,58]. The reduction of Cu2+ in Co-Cu-based perovskites to Cu0 was found to be a onestep process whereas that of Co3+ to Co0 occurred in two stages (Co3+/Co2+ and Co2+/Co0) [40,43,44,59]. Simultaneously, the presence of the reduced Cu0 promotes the reducibility of cobalt ions, resulting to a higher reduction extent of cobalt ions and a higher dispersion of metallic cobalt at a lower temperature. For sample GM, a small amount of alkali ions retained on the catalyst surface has remarkable effects on both the reducibility of copper and cobalt ions and the synthesis of higher alcohols [41,49,60]. The conversion of syngas into higher alcohols as well as hydrocarbons is strongly dependent on the process variables. Reaction temperature is a crucial parameter to control both alcohol selectivity and productivity as shown in Fig. 6. A maximal productivity of alcohols is achieved at reaction temperatures close to 300 8C. At temperatures beyond the range of 275–325 8C, the reduced concentration of surface CHx* species is likely to become a limiting factor for the formation of higher alcohols [21,31,41]. The fact that all alcohol productivities decrease simultaneously with temperature is an indication that a thermodynamic limitation due to the proximity of the chemical equilibrium of exothermal alcohol synthesis is not the main factor here. The significantly decreased activity in alcohol synthesis at a higher temperature is likely explained by either a faster rate of desorption or a higher energy of activation for hydrogenation of the intermediates. Consequently, a large amount of methane is formed at temperature higher than 325 8C (Fig. 7) [61–63]. Increasing velocity of reactants gave rise to the suppression of methane as well as light hydrocarbons, but the overall activity also gradually decreased (Fig. 8). Both the synthesis rates of alcohols and hydrocarbon are dependent on the syngas pressure. Therefore, increasing syngas partial pressure results in an increased productivity of all products with the exception of CO2 which is not very sensitive to the change in syngas pressure [21]. The stability of CHx* intermediates and concentration of CO* species adsorbed on the catalyst surface are strongly associated with the amount of alkali promoter and carbon monoxide partial pressure [6,21,41,64,65]. The presence of remnant sodium ions enhances the propagation of carbon chain and accelerates the rate of CO insertion into an alkyl group adsorbed on the surface.

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Simultaneously, the selectivity to methane is significantly suppressed. In the present study, a stoichiometric H2/CO ratio of 2 seems to be an appropriate syngas composition for the synthesis of alcohols (Table 3) though decreasing H2/CO makes a small increase in the selectivity to higher alcohols [6,49,64,65]. Nevertheless, the overall activity is very low at a lower H2/CO ratio so that the synthesis rates of all products are relatively low (Table 3). In the presence of excess CO the decreased activity is due to the limited H2 availability and/or to the catalyst surface being covered by a layer of carbon from Boudouard reaction [6,31,36,37,66]. Two LaCo0.7Co0.3O3 d perovskite precursors synthesized by different preparations show a significant difference in both catalytic activity and stability. Nanocrystalline perovskite precursors have shown better stability for the synthesis of higher alcohols from syngas. The GM catalyst shows indeed a higher activity in alcohol synthesis than does the CIT sample (Fig. 6). Comparing alcohol productivity of the GM catalyst with the data reported in Table 1 indicates that the modified methanol synthesis catalysts (# 1–4) which have higher total alcohol productivity yield also very high selectivity to methanol [9,14,15,28]. By comparison the modified FT catalysts (# 5–7) with lower alcohol productivities are much more selective for higher alcohols. The GM catalyst shows a maximum productivity comparable to the best modified FT catalyst (# 5–8) productivities and comparable C2+OH selectivities [20,21,29,50]. It is noted here that the higher alcohol synthesis reaction is always composed of several elementary steps including CO dissociation, insertion, and hydrogenation so that the catalyst surface must comprise a variety of active sites [6,21–23,66]. Under synthesis conditions, cobalt atoms in the grain boundaries are thought to provide alkyl groups while copper atoms would be responsible for chain termination by addition of CO into an alkyl group, followed by hydrogenation to a straight-chain alcohol [20,23,53]. Therefore, a Co metal should be proximate to a Cu site to facilitate the surface migration of an adsorbed CO* to a metal-alkyl group. The copper dispersion can be stabilized by the presence of sodium promoter [20,21,23,50]. The abundance of ethanol in the reacting system indeed supports this conclusion. Thus, any modifications in the metal surface of Cu-Co metals such as coking, metal agglomeration, and separation of copper from neighboring cobalts would strongly affect the activity in alcohol synthesis [34,66]. The alcohols obtained are mainly linear-products. As shown in Fig. 18, their carbon number distribution is consistent with an Anderson–Schulz–Flory plot, indicating a chain growth mechanism by successive insertion of a C1 species into alkyl-metal on the catalyst surface for the chain growth [40,66]. A modest increase in methanol selectivity with increasing timeon-stream leads to a minor change in the carbon chain growth factors of alcohols [29]. The propagation constant of alcohols (a) estimated from methanol to hexanol decreases from 0.32 to 0.27 after periods of testing times. However, this factor would be constant (0.30) if methanol is eliminated from an ASF plot in the entire range studied. This is explained by the possibility that methanol is formed on a different site and independently of

Fig. 18. ASF distribution of alcohol products at 300 8C (1000 psi, 5000 h 1, H2/CO/He = 8/4/3) after 10 days on stream.

the CO insertion reactions that produce all of the other higher alcohols [3,6,16,21]. 5. Conclusion Two LaCo0.7Cu0.3O3 perovskites synthesized by different preparative recipes have been thoroughly characterized and tested for the synthesis of higher alcohols and hydrocarbons from syngas. The perovskite catalyst prepared by mechanosynthesis always shows a higher specific surface area and a smaller nanophase than that synthesized by the citrate complex method. Both copper and cobalt located in the well-defined perovskite structure results in the mutual interaction between these two elements, providing a highly dispersed bimetallic phase after pretreatment under hydrogen. The reduced forms of LaCo0.7Cu0.3O3 perovskites were very active for the conversion of syngas to higher alcohols and hydrocarbons. The catalytic activity and product distribution depend strongly on the reaction conditions and catalyst morphology. The ground LaCo-Cu perovskite always shows higher alcohol synthesis rate than does the citrate-derived sample. A mixture of products including linear primary alcohols, hydrocarbons, carbon dioxide, and water is identified. The selectivity and productivity of higher alcohols reflect the presence of highly dispersed Co interacting with Cu sites on surface. In addition, the distributions of products as well as the yields of higher alcohols are strongly dependent on the process variables. Reaction temperature is one of the key parameter affecting the alcohol selectivity and productivity. The optimum parameters for catalyst preparation and for alcohol synthesis in this study were determined. Under typical reaction conditions, the alcohol productivity is in a broad range of 70–140 mg/gcat/h and the selectivity towards alcohols is about 40–45 wt.%. The distribution of all products is consistent with a typical ASF plot. The catalytic stability is dependent not only on the size of crystal domain, the size of nanoparticles, the amount of remnant sodium ions, but also strongly on slit-shaped spaces

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between nanoparticles. The overall activity and alcohol productivity of ground perovskite precursors were rather stable after the long-term alcohol synthesis whereas those of the citrate complex sample substantially decreased. The spent catalyst surface is composed of small amount of adsorbed heavy hydrocarbons. The existence of slit-shaped spaces between primary nanoparticles and/or grain boundaries of perovskites plays an important role in resistance to the formation of a long hydrocarbon chain, which is a precursor for the formation of coke. Our findings indicate that nanocrystalline Co-Cu-based perovskite precursors are rather active and stable and may be promising catalysts for the synthesis of higher alcohols from syngas.

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Acknowledgments

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The authors gratefully acknowledge Nanox Inc. (Que´bec, Canada) and the Natural Sciences and Engineering Research Council of Canada for financial support. They also thank Nanox Inc. for preparing the nanocrystalline ground perovskites. References

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