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Role of support on higher alcohol synthesis from syngas
Accepted Manuscript Title: Role of support on higher alcohol synthesis from syngas Author: Dong Ju Moon K. Hariprasad Reddy Jin Hee Lee Jae Sun Jung Eun-Hyeok Yang PII: DOI: Reference:
S0926-860X(14)00267-1 http://dx.doi.org/doi:10.1016/j.apcata.2014.04.026 APCATA 14799
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
22-1-2014 21-3-2014 14-4-2014
Please cite this article as: D.J. Moon, K.H. Reddy, J.H. Lee, J.S. Jung, E.-H. Yang, Role of support on higher alcohol synthesis from syngas, Applied Catalysis A, General (2014), http://dx.doi.org/10.1016/j.apcata.2014.04.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract (for review)
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Graphical abstract
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*Highlights (for review)
Highlights
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Nature of support in playing vital role in catalytic performance. Low coordinated oxygen atoms were observed with zinc supported catalyst. Cu-Co supported on ZnO has showed higher activity of 70% conversion with 70% Higher Alcohols selectivity CO2 along with the feed increased the conversion and selectivity.
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Role of support on higher alcohol synthesis from syngas Jin Hee Lee1,2, K. Hariprasad Reddy1, Jae Sun Jung1,2, Eun-Hyeok Yang1,2, and Dong Ju Moon1, 2*,
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Clean Energy Research Center, KIST, Seoul, South Korea
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Clean Energy & Chemical Engineering, UST, Dae-jeon, South Korea
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Corresponding Author's E-mail:[email protected]
Abstract
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A series of Cu-Co based catalysts were prepared with different supports by co-precipitation method and well characterized by various physico chemical methods such as N2 adsorption, XRD, TPR, TPD NH3, TGDTA, SEM and TEM. CO hydrogenation to higher alcohols were performed and found that zinc supported catalysts showed higher conversion (70%) and selectivity towards higher alcohols (70%). It was found that the addition of CO2 to syngas enhances the conversion and selectivity. It was considered that low coordinated oxygen atoms in case of zinc supported catalyst are responsible for the high activity compared to other catalysts. It was also found that the zinc supported Cu-Co catalyst has potential for the production of higher alcohols from syngas.
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Keywords: Syngas, Cu-Co based catalyst, Higher alcohols, Role of acidity of the support
1. Introduction
The catalytic conversion of syngas to higher alcohols is a potential route to synthetic transportation fuels or fuel additives. The syngas can be generated by reforming of natural gas or by gasification of bio-mass, coal and carbon containing waste fractions [1-3]. Alcohols have high octane numbers and it makes the alcohol interesting as gasoline additives and for replacements [4-8] Ethanol production from syngas is one of the major processes in chemical industry, however low yield and poor selectivity still remain the major hurdles associated with the use of known catalysts. Therefore developing catalysts for synthesizing ethanol efficiently and selectively has been one of the major challenges in catalysis [9-13]. Four types catalysts for ethanol synthesis from syngas are reported, Rh based catalysts [14-16], Mo based catalysts
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[17,18] modified F-T synthesis catalysts and Cu based catalysts [19,20]. Rh based catalysts possess a high selectivity for higher alcohols, but Rh metal is too expensive for commercial applications [15]. Mo based catalysts had a high catalytic activity, but they must be carried out at high temperature and pressure and usually have a long activity induction period during the
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reaction [18]. Modified F-T synthesis catalysts produce large amount of hydrocarbons, which causes a decrease in the yield of alcohol. Among them, much less expensive Cu based catalysts
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have become an attractive option [21-23]. These catalysts are typically alkali promoted cu based catalysts. Overall, Cu based catalysts have been widely used and produced promising results for
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ethanol formation from syngas [24]. Supported Cu-Co based catalysts are employed in a wide range of commercial scale processes such as methanol synthesis, methanol steam reforming and
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other alcohol dehydrogenation reaction systems. The life time and deactivation behavior of these catalysts has drawn great attention with efforts to improve their activity and stability [25, 26].
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The major challenge of carbon monoxide hydrogenation is the efficient control of the reaction selectivity to olefins, long chain paraffins and oxygenated products. Cobalt based catalysts are used for the synthesis of long-chain paraffin's using Fischer -Tropsch reaction,
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while copper -containing catalysts are selective in methanol synthesis from syngas. Both copper
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and cobalt catalysts are currently used in relevant large scale industrial process. It could be
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expected that mixed copper-cobalt catalysts would have higher selectivity to higher alcohols. In the present study the effect of nature of support and interaction of Cu species with different supports on CO hydrogenation to give higher alcohols were investigated. The catalysts prepared with different supports were well characterized by various physico chemical techniques such as XRD, TPR, N2 sorption analysis, TPD ammonia, TG/DTA, SEM and TEM. Catalytic activities of the prepared materials were studied for low pressure selective higher alcohol synthesis with fixed bed reactor at moderate temperature.
2. Experimental 2.1. Catalyst preparation Catalysts were prepared by co-precipitation method using Na2CO3 as a precipitant. In a typical procedure, aqueous solution of Na2CO3 was added to an aqueous solution of mixed
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metallic nitrates of Cu (NO3)2 3H2O, Co (NO3)2 6H2O and Al (NO3)3 9H2O with weight loads 25, 25 and 50 respectively. The pH of the solution was maintained around 8-9 during the precipitation at 70 °C. A precipitate was obtained after filtering the slurry at room temperature and it was thoroughly washed with deionised water for several times. The resultant precipitate
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was then dried at 110 °C for 12 h and finally calcined at 500 °C for 5 h. Same procedure was
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followed to prepare the Cu-Co based catalysts with other supports such as Mg and Zn.
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2.2. Catalyst characterization
The XRD patterns of the prepared catalysts were recorded on a diffractometer (M/S,
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Shimadzu Instruments, Japan)with Ni-filtered Cu Kα (λ = 0.15418 nm). The operating voltage was 40 kV, and the current was 30 mA, with a 2θ scanning rate of 2◦min−1. The BET surface areas and N2adsorption–desorptionmeasurements were performed at 77 K using an automated
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gas sorption system (Moon sorp-I). The Barrett–Joyner–Halanda (BJH) method was used for the calculation of pore size distribution. The TPR experiments were carried out using a temperature
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program analyzer (BELCAT, BEL Japan, Inc.). For TPR studies, 0.1 g of a calcined sample was
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placed between quartz wool in a U-type quartz reactor. The sample was thermally treated under an Ar stream at 400 °C for 2 h to remove physically adsorbed water and other impurities. The
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catalysts were cooled down to the room temperature under pure Ar gas. After the pretreatment, the samples were heated at 10 °C min−1from room temperature up to 800 °C in 5% H2/Ar stream with a flow rate of 30 mL min−1. TEM images of the Cu-Co supported catalysts were characterized by using FEI (Technai F20 G2, The Netherlands) microscopy working in STEM mode. The catalyst sample was dispersed in ethanol solution, and the suspension was carried out using an ultrasonic bath for 30 min. The resulting solution was deposited on copper grid coated with a carbon film, and the alcohol was evaporated. Finally, the sample containing the copper grid was used for the TEM analysis.
2.3. Catalytic performance CO hydrogenation to higher alcohols was carried out in a stainless steel continuous fixedbed flow reactor (10 mm i.d and 200 mm long) with 0.5 g catalyst. Prior to the reaction, the catalyst was reduced under pure H2 with a flow rate of 30 ml min-1 at 300 °C for 3 h under
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atmospheric pressure. Then the reactor was cooled to 493 K and the reaction mixture was admitted by raising the pressure to 35 bar. The reactions were carried out over a temperature range of 220-300 °C. Mole ratio of reactants was maintained at 2 through the studies and the space velocity (GHSV) of 3000 ml/h/g. The products were analyzed by an on-line gas
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chromatograph (HP -7890 A) with porapack-Q and HP-Innowax columns connected to TCD and FID, respectively. The former column was used for the analysis of H2, CO, CH4, methanol and
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CO2, while the latter was used for the analysis of hydrocarbons and alcohols.
3. Results and Discussion
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Prepared catalysts were characterized by various physico chemical techniques and are discussed in the following sub headings.
3.1. Surface area
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To study the effect of surface characteristics such as surface area, pore volume and average pore diameter on the activity and selectivity, BET analysis were measured and the
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results are reported in Table1. The results stated that BET surface area, pore volume and mean pore diameter of the catalysts changed from one support to another support. γ-Al2O3 supported
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Cu-Co catalyst showed higher surface area among all other supported catalysts. In fact, nonreducible metal oxides, with high porous structure supported catalysts show high surface area as compared to reducible metal with less porous oxides supported catalysts. So that the surface area of γ-Al2O3 and MgO supported catalysts showed higher surface area than ZnO supported catalysts. Pore volume and mean pore diameter of the catalysts are also followed the same trend. The activity of the catalyst are independent of the surface area, ZnO supported catalyst having least surface area showed maximum activity. From the Table 1, it is clear that, the spent catalysts have considerably lower surface areas than the fresh catalysts. The particle sizes estimated from XRD are varied for the fresh and spent catalysts, which is in agreement that the sintering as the reason for the diminished surface area. In the particle size from TEM (Fig. 4), there is an element of uncertainty due to the limited number of particles. Another possible reason for the loss of surface area during operation is that
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carbon deposits formed during the CO hydrogenation block the surface and the pores of the catalyst and lower the available surface area. For these observations are consistent with the notion that carbonaceous deposits are formed during the operation. Additionally, it could seem as if the protruding particle in the TEM image of the spent catalyst is largely encircled by an
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amorphous layer, and this could be related to a deposition of carbon on the catalyst. Previously several other authors have reported that, increased carbon content in Cu-Co catalysts used for
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CO hydrogenation. There are strong indications that the Cu-Co catalysts lose surface area as a result of the buildup of carbonaceous deposits during CO hydrogenation.
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In the determination of the surface area of the spent catalysts, an element of uncertainty is however that minor amounts of long-chained products are formed in the reaction. This may
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leads to the formation of waxy deposits in the pores of the catalyst, and if these deposits are not removed in the evacuation procedure prior to the BET measurement, this would lead to an underestimation of the surface area. The surface area of the evaluated Cu-Co-MgO catalyst is
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unchanged after the CO hydrogenation experiment. As will be shown subsequently, this catalyst is essentially inactive in syngas conversion, and the formation of carbonaceous deposits is
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therefore less likely for this catalyst.
3.2 X-ray diffraction patterns of the catalysts
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The powder X-ray diffraction patterns of the samples were recorded and represented in Fig. 1. It can be seen that the precursors γ-Al2O3 and MgO supported catalysts are a typical hydrotalcite-like phase with sharp diffraction peaks at 2θ = 11.8, 23.6 and 34.9 o corresponding to the reflection of crystalline planes of (003), (006), and (009), respectively. The positions of the remaining peaks are in agreement with such an assignment. Many authors [27, 28] reported that it is possible to obtain single stoichiometric hydrotalcite layers (HTL) phases in the range of MIII/ (MIII + MII) = 0.2–0.4. For our case, the ratio of Al3+ / (Al3+ / Cu2+ / Co2+) is 0.5, which is considerably higher than the referred value above. Perez-Bernal et al. [29] found that the excess amount of aluminum in the Co (Zn)–Al HTls samples is segregated forming the additional phases, instead of incorporating into the hydrotalcite type structure or changing the structure. Contrarily, in ZnO supported catalyst showed similar diffractions patterns with low intensity and it can be attributed that the amorphous nature of that phase.
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The XRD patterns of the samples reduced at 300 °C for 3h are shown in Fig. 1. It is wellknown that the Cu–Co and Cu–Co-support spinel phases are difficult to be reduced [30]. Therefore, no obvious change can be seen for the spinel phases after reduction treatment except in ZnO catalyst. Diffraction peaks with high intensity of metallic Cu0 could be observed in the
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reduced γ-Al2O3 and MgO supported Cu-Co catalysts, which indicates that bigger-sized Cu particles are formed along with the stable spinel phase. As far as the sample Cu-CoxOx -ZnO is
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concerned, great changes take place after reduction and metallic Cu is detected with low intensity. However, the diffraction peaks related to copper were hardly identified, implying that
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copper was highly dispersed in Cu-Co-ZnO catalyst. In addition, the appearance of diffraction peaks at 31.8, 34, 36, 42, 56 and 62o can be attributed to the spinel phases of CuCo2O4 and/or
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ZnCo2O4.
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3.3. TG/DTA analysis
The TG-DTA profiles of the supported catalysts were analyzed and represented in Fig. 2 .
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The results revealed that Cu-Co–Al2O3 has two obvious weight loss processes, the first weight
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loss corresponded to the Cu-Co–Al2O3 hydrotalcite loses interbedded water at about 185 °C, second weight loss related to the dehydroxylation, decarbonylation of OH-1 and CO3 -2 anion in
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the layer and between the layers, respectively, at about 250 °C. For MgO, γ-Al2O3 supported catalysts, the weight loss processes are usually included in the above three processes, and generally no further weight loss is recorded above 500 °C. However, in all catalysts the modification of structure with temperature only by changing the support. It is found that weight loss of MgO and ZnO supported catalysts slightly shifted to higher temperature region.
3.4. Scanning electron microscopy (SEM) analysis Fig. 3 reveals the SEM images of the calcined and used catalysts. In the case of alumina supported catalyst, SEM image was not altered before and after the reaction. Whereas in MgO supported catalyst uniform arrangement observed before the reaction, however in spent catalyst morphology was totally different and changed from spherical to flower shape. This might be the reason for the low activity of the catalyst, with decreasing the surface area. In the case of the
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ZnO catalyst exhibit lamellar shape was observed before the reaction and small size granules shape observed in spent catalysts. The support of catalysts is also an effective approach to decrease crystalline size and
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improve surface area of Cu-Co catalysts. Meanwhile, the low intensity diffraction peaks of the Cu-Co-Zn sample mean better dispersion of active components. SEM measurements show a significant difference of the morphology in the Cu-Co-ZnO catalysts relative to the other
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catalysts. The particle of the catalyst is not uniform and the conglomeration phenomena occur for the MgO and γ-Al2O3 samples, indicated that active components are not well dispersed. XRD
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results and morphology observations show that ZnO support is favorable to decrease the
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crystalline size and it showed best activity for the CO hydrogenation to produce higher alcohols.
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3.5. Transmission electron microscopy (TEM) analysis
The morphology of the catalysts before and after the CO hydrogenation was investigated
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by means of transmission electron microscopy. Fig. 4 shows TEM pictures of reduced and spent Cu-Co samples. In the Fig. 4, upper and lower TEM images represented reduced and spent
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different supported Cu-Co catalysts. In reduced catalysts (Fig 4 a and b), the spherical shape
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particles which are uniformly distribution over the support leads to the formation of nano size and average particle size is 10nm. Whereas in Fig. 4(c), shows the needle-shaped crystallites. In the spent catalysts, formations of bulk particles were observed in all catalysts. The average particle size is below 15 nm. During the reaction conditions catalyst particles agglomerate leads to increase in particle size.
3.6. TPR patterns of prepared catalysts Reducibility is an important property of the alcohol synthesis catalyst from syngas, as they are active only in reduced state and have to be pre-reduced prior to the CO hydrogenation reaction studies. Therefore, the reduction characteristics of the catalysts were studied with TPR in hydrogen. TPR profiles shown in Fig. 5 were acquired by continuously following the H2 signal with a quadruple mass spectrometer with linear increase in temperature. A sharp intense and symmetrical reduction peak at 230 °C is detected in each Cu-Co catalyst. This hydrogen
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consumption could be attributed to the reduction of CuO to Cu [31], the small shoulder peak at 350 °C in Alumina supported and 350 °C in Mg supported can be attributed to the reduction of Cu-Co spinal phases respectively. In contrast, quite different profiles could be observed for zinc supported catalyst. From XRD results, it is clear that, CuO, Co3O4 and Co2O4 are the main
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reducible oxides. Thus, the first peak at 230 °C could be due to the reduction of CuO to Cu and the other two overlaps at higher temperatures are attributed to the reduction of Co3O4 to Co
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consisting of two steps [32-34]. And final high temperature reduction peak at 450 °C ascribed to the reduction of (CuZn) Co2O4 spinal phase. All the Cu-Co catalyst showed a peak at 240 °C,
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which is due to the reduction of CuO support in that catalyst, promotes the reducibility of CuO [31]. From the reported results, it was found that highly dispersed CuO gave TPR signals at
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much lower temperature than bulk CuO [35, 36].
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3.7. Temperature programmed desorption of NH3
NH3-TPD patterns of reduced supported Cu-Co catalysts are shown in Fig. 8. From the TPD
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patterns clearly indicate that three different strength of acidic cites presented in each catalyst.
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The acid strength of peaks assigned as weak, medium and strong temperature at 110, 480 and 800 oC, respectively. However, ZnO supported catalyst showed a strong NH3 desorption peak at
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950 oC. The low temperature NH3 desorption peak could be assigned to the desorbed NH3 that adsorbed weakly on the surface of Cu [16]. The medium acidic peaks could be ascribed to the desorbed NH3 that adsorbed strongly onto the acid sites of Co–Al–O, Co–Mg–O, Co–Zn–O supports of Al2O3, MgO and ZnO supported catalysts respectively [17-19]. The strong acidic sites were associated with low-coordination oxygen atoms. These results implied that the acidity of Cu-Co catalysts affected by the support. Low temperature desorption of ammonia is typically assigned to Bronsted bound ammonia, whereas desorption taking place at higher temperatures indicates the presence of Lewis-bound ammonia.
3.8 Catalytic activity studies 3.8.1. Effect of support on CO hydrogenation
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In order to understand the effect of support on CO hydrogenation to produce higher alcohols, three different supports were selected namely, γ-Al2O3, MgO and ZnO. Active metallic species Cu-Co are introduced on to the support by co-precipitation method. The activity of the catalysts were studied and reported in Table 2. From the results it is clear that ZnO supported
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catalyst is superior compared to the other supports. This is due to the fact that, Cu species generating strong and medium acid sites by interacting with Zn support, where as γ-alumina
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support generates medium acid sites and MgO only weak acid sites. Cu dispersed on ZnO showed 18% CO conversion with 83% selectivity towards methanol. Weak acid sites showed
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very less CO conversion, where as high selectivity towards methanol from syngas. Cu species on the support first generates methanol by hydrogenation reaction, the so formed methanol further
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reacting with CO to produce DME. In the second step weak acid role is very important, if the catalyst posses strong acid sites it will leads to dimethylether (DME) instead of higher alcohols. Strong acidity predominates dehydration instead of CO insertion. Al with moderate acidity
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produces more hydrocarbons instead of higher alcohols.
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3.8.2. Effect of temperature on CO hydrogenation
Catalytic activities of Cu-Co supported catalysts were studied for the CO hydrogenation
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reactions. Since temperature is an important parameter in any chemical reaction, the effect of
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temperature was studied in the range of 220-300 °C with Zinc oxide supported catalyst and results are represented in Fig. 6. From Fig 6. it is clear that, CO conversion increases with increase in temperature, whereas methanol selectivity decreases. Lower temperature favors the methanol whereas higher temperature predominates the higher alcohols. 3.7.3. Effect of CO2 as a co-feed
Effect of CO2 as a cofeed in higher alcohol synthesis with copper supported catalysts was studied and the results were represented in table 3. Results revealed that, CO2 enhanced the conversion and selectivity for higher alcohols. CO2 inhibits the formation of methanol and enhances the higher alcohol selectivity by increasing oxygen coverage’s on Cu surfaces and by titrating basic sites required for aldol-type chain growth steps. Inhibition effects are weaker on catalysts with high Cu-site densities [37]. Copper supported on Zinc oxide showed best selectivity towards higher alcohol due to least acidic sites as confirmed by TPD ammonia measurements. With
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copper supported catalysts, the abundance of Cu sites allows reactants to reach methanol synthesis equilibrium and maintain a sufficient number of Cu surface atoms for bi functional
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condensation steps, even in the presence of CO2.
4. Conclusions
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Cu-Co on different support were synthesized by co-precipitation and tested for CO
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hydrogenation reaction at moderate reaction conditions. It was found that active metal species supported on Zinc oxide showed the higher catalytic performance and it was well correlated with XRD, SEM TEM and TPR results. Cu species generates moderately strong acid sites by
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interacting with the zinc support are playing vital role in CO hydrogenation to higher alcohols. It was also found that the low coordinated oxygen atoms in case of zinc supported catalyst and
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strong acidic sites are responsible for the high activity. The zinc supported Cu-Co catalyst has
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Acknowledgements
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potential for the production of higher alcohols from syngas with CO2 as co-feed.
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Authors thanks to the Korea Institute of Science and Technology (KIST) and the Ministry of Trade, Industry & Engineering in Korea.
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Table
Spent 50 43 22
Calcined 28 35 25
Spent 33 40 30
Calcined 22 31 30
Spent 18 28 26
Mean pore diameter (nm) 0.44 0.29 0.29
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Cu-Co-Al2O3 Cu-Co-MgO Cu-Co-ZnO
Calcined 79 47 37
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Catalyst
Pore volume (ml/gm)
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Crystallite size of copper from XRD (nm)
BET surface area (m2/gm)
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Table 1. Surface characteristics of the catalysts before and after the reaction
Catalysts
CO conversion (%)
Methanol selectivity
13 15 18
85 70 83
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Cu-Co-Al2O3 Cu-Co-MgO Cu-Co-ZnO
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Table 2.Effect of support on CO Conversion
Reaction conditions: Temperature: 240 °C, Pressure 35 bar, molar ratio of CO:H2=1:2, GHSV 3000 h-1.
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CO+CO2 as feed
Higher CO
Methanol
alcohol
CO+CO2
conversion
selectivity
selectivity
conversion
30
68
22
20
74
40
33
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33
62
alcohol
selectivity
selectivity (C2-C5)
40
36
60
35
23
75
70
22
70
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Cu-CoAl2O3 Cu-CoMgO Cu-Co-ZnO
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(C2-C5)
Higher
Methanol
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Catalyst
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Only CO as Feed
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Table 3. Effect of CO2 as a co-feed on catalytic performance
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GHSV 3000 h-1.
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Reaction conditions: Temperature: 300 °C, Pressure: 35 bar, molar ratio of CO:CO2:H2 = 0.3:1:2,
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Figure
Fig. 1. (A) XRD patterns of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 1. (B) XRD patterns of reduced Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 2. TG/DTA analysis of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 3. SEM images of supported catalysts: (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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and spent samples: (d) Cu-Co-Al2O3 (e) Cu-Co-MgO (f) Cu-Co-ZnO.
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Fig. 4. TEM images of reduced samples: (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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and spent samples: (d) Cu-Co-Al2O3 (e) Cu-Co-MgO (f) Cu-Co-ZnO.
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Fig. 5. TPR patterns of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 6. TPD of NH3 of Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 7. Effect of Temperature on catalytic performance.
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S0926-860X(14)00267-1 http://dx.doi.org/doi:10.1016/j.apcata.2014.04.026 APCATA 14799
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
22-1-2014 21-3-2014 14-4-2014
Please cite this article as: D.J. Moon, K.H. Reddy, J.H. Lee, J.S. Jung, E.-H. Yang, Role of support on higher alcohol synthesis from syngas, Applied Catalysis A, General (2014), http://dx.doi.org/10.1016/j.apcata.2014.04.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract (for review)
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Graphical abstract
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*Highlights (for review)
Highlights
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Nature of support in playing vital role in catalytic performance. Low coordinated oxygen atoms were observed with zinc supported catalyst. Cu-Co supported on ZnO has showed higher activity of 70% conversion with 70% Higher Alcohols selectivity CO2 along with the feed increased the conversion and selectivity.
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Role of support on higher alcohol synthesis from syngas Jin Hee Lee1,2, K. Hariprasad Reddy1, Jae Sun Jung1,2, Eun-Hyeok Yang1,2, and Dong Ju Moon1, 2*,
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Clean Energy Research Center, KIST, Seoul, South Korea
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Clean Energy & Chemical Engineering, UST, Dae-jeon, South Korea
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Corresponding Author's E-mail:[email protected]
Abstract
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A series of Cu-Co based catalysts were prepared with different supports by co-precipitation method and well characterized by various physico chemical methods such as N2 adsorption, XRD, TPR, TPD NH3, TGDTA, SEM and TEM. CO hydrogenation to higher alcohols were performed and found that zinc supported catalysts showed higher conversion (70%) and selectivity towards higher alcohols (70%). It was found that the addition of CO2 to syngas enhances the conversion and selectivity. It was considered that low coordinated oxygen atoms in case of zinc supported catalyst are responsible for the high activity compared to other catalysts. It was also found that the zinc supported Cu-Co catalyst has potential for the production of higher alcohols from syngas.
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Keywords: Syngas, Cu-Co based catalyst, Higher alcohols, Role of acidity of the support
1. Introduction
The catalytic conversion of syngas to higher alcohols is a potential route to synthetic transportation fuels or fuel additives. The syngas can be generated by reforming of natural gas or by gasification of bio-mass, coal and carbon containing waste fractions [1-3]. Alcohols have high octane numbers and it makes the alcohol interesting as gasoline additives and for replacements [4-8] Ethanol production from syngas is one of the major processes in chemical industry, however low yield and poor selectivity still remain the major hurdles associated with the use of known catalysts. Therefore developing catalysts for synthesizing ethanol efficiently and selectively has been one of the major challenges in catalysis [9-13]. Four types catalysts for ethanol synthesis from syngas are reported, Rh based catalysts [14-16], Mo based catalysts
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[17,18] modified F-T synthesis catalysts and Cu based catalysts [19,20]. Rh based catalysts possess a high selectivity for higher alcohols, but Rh metal is too expensive for commercial applications [15]. Mo based catalysts had a high catalytic activity, but they must be carried out at high temperature and pressure and usually have a long activity induction period during the
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reaction [18]. Modified F-T synthesis catalysts produce large amount of hydrocarbons, which causes a decrease in the yield of alcohol. Among them, much less expensive Cu based catalysts
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have become an attractive option [21-23]. These catalysts are typically alkali promoted cu based catalysts. Overall, Cu based catalysts have been widely used and produced promising results for
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ethanol formation from syngas [24]. Supported Cu-Co based catalysts are employed in a wide range of commercial scale processes such as methanol synthesis, methanol steam reforming and
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other alcohol dehydrogenation reaction systems. The life time and deactivation behavior of these catalysts has drawn great attention with efforts to improve their activity and stability [25, 26].
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The major challenge of carbon monoxide hydrogenation is the efficient control of the reaction selectivity to olefins, long chain paraffins and oxygenated products. Cobalt based catalysts are used for the synthesis of long-chain paraffin's using Fischer -Tropsch reaction,
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while copper -containing catalysts are selective in methanol synthesis from syngas. Both copper
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and cobalt catalysts are currently used in relevant large scale industrial process. It could be
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expected that mixed copper-cobalt catalysts would have higher selectivity to higher alcohols. In the present study the effect of nature of support and interaction of Cu species with different supports on CO hydrogenation to give higher alcohols were investigated. The catalysts prepared with different supports were well characterized by various physico chemical techniques such as XRD, TPR, N2 sorption analysis, TPD ammonia, TG/DTA, SEM and TEM. Catalytic activities of the prepared materials were studied for low pressure selective higher alcohol synthesis with fixed bed reactor at moderate temperature.
2. Experimental 2.1. Catalyst preparation Catalysts were prepared by co-precipitation method using Na2CO3 as a precipitant. In a typical procedure, aqueous solution of Na2CO3 was added to an aqueous solution of mixed
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metallic nitrates of Cu (NO3)2 3H2O, Co (NO3)2 6H2O and Al (NO3)3 9H2O with weight loads 25, 25 and 50 respectively. The pH of the solution was maintained around 8-9 during the precipitation at 70 °C. A precipitate was obtained after filtering the slurry at room temperature and it was thoroughly washed with deionised water for several times. The resultant precipitate
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was then dried at 110 °C for 12 h and finally calcined at 500 °C for 5 h. Same procedure was
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followed to prepare the Cu-Co based catalysts with other supports such as Mg and Zn.
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2.2. Catalyst characterization
The XRD patterns of the prepared catalysts were recorded on a diffractometer (M/S,
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Shimadzu Instruments, Japan)with Ni-filtered Cu Kα (λ = 0.15418 nm). The operating voltage was 40 kV, and the current was 30 mA, with a 2θ scanning rate of 2◦min−1. The BET surface areas and N2adsorption–desorptionmeasurements were performed at 77 K using an automated
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gas sorption system (Moon sorp-I). The Barrett–Joyner–Halanda (BJH) method was used for the calculation of pore size distribution. The TPR experiments were carried out using a temperature
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program analyzer (BELCAT, BEL Japan, Inc.). For TPR studies, 0.1 g of a calcined sample was
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placed between quartz wool in a U-type quartz reactor. The sample was thermally treated under an Ar stream at 400 °C for 2 h to remove physically adsorbed water and other impurities. The
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catalysts were cooled down to the room temperature under pure Ar gas. After the pretreatment, the samples were heated at 10 °C min−1from room temperature up to 800 °C in 5% H2/Ar stream with a flow rate of 30 mL min−1. TEM images of the Cu-Co supported catalysts were characterized by using FEI (Technai F20 G2, The Netherlands) microscopy working in STEM mode. The catalyst sample was dispersed in ethanol solution, and the suspension was carried out using an ultrasonic bath for 30 min. The resulting solution was deposited on copper grid coated with a carbon film, and the alcohol was evaporated. Finally, the sample containing the copper grid was used for the TEM analysis.
2.3. Catalytic performance CO hydrogenation to higher alcohols was carried out in a stainless steel continuous fixedbed flow reactor (10 mm i.d and 200 mm long) with 0.5 g catalyst. Prior to the reaction, the catalyst was reduced under pure H2 with a flow rate of 30 ml min-1 at 300 °C for 3 h under
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atmospheric pressure. Then the reactor was cooled to 493 K and the reaction mixture was admitted by raising the pressure to 35 bar. The reactions were carried out over a temperature range of 220-300 °C. Mole ratio of reactants was maintained at 2 through the studies and the space velocity (GHSV) of 3000 ml/h/g. The products were analyzed by an on-line gas
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chromatograph (HP -7890 A) with porapack-Q and HP-Innowax columns connected to TCD and FID, respectively. The former column was used for the analysis of H2, CO, CH4, methanol and
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CO2, while the latter was used for the analysis of hydrocarbons and alcohols.
3. Results and Discussion
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Prepared catalysts were characterized by various physico chemical techniques and are discussed in the following sub headings.
3.1. Surface area
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To study the effect of surface characteristics such as surface area, pore volume and average pore diameter on the activity and selectivity, BET analysis were measured and the
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results are reported in Table1. The results stated that BET surface area, pore volume and mean pore diameter of the catalysts changed from one support to another support. γ-Al2O3 supported
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Cu-Co catalyst showed higher surface area among all other supported catalysts. In fact, nonreducible metal oxides, with high porous structure supported catalysts show high surface area as compared to reducible metal with less porous oxides supported catalysts. So that the surface area of γ-Al2O3 and MgO supported catalysts showed higher surface area than ZnO supported catalysts. Pore volume and mean pore diameter of the catalysts are also followed the same trend. The activity of the catalyst are independent of the surface area, ZnO supported catalyst having least surface area showed maximum activity. From the Table 1, it is clear that, the spent catalysts have considerably lower surface areas than the fresh catalysts. The particle sizes estimated from XRD are varied for the fresh and spent catalysts, which is in agreement that the sintering as the reason for the diminished surface area. In the particle size from TEM (Fig. 4), there is an element of uncertainty due to the limited number of particles. Another possible reason for the loss of surface area during operation is that
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carbon deposits formed during the CO hydrogenation block the surface and the pores of the catalyst and lower the available surface area. For these observations are consistent with the notion that carbonaceous deposits are formed during the operation. Additionally, it could seem as if the protruding particle in the TEM image of the spent catalyst is largely encircled by an
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amorphous layer, and this could be related to a deposition of carbon on the catalyst. Previously several other authors have reported that, increased carbon content in Cu-Co catalysts used for
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CO hydrogenation. There are strong indications that the Cu-Co catalysts lose surface area as a result of the buildup of carbonaceous deposits during CO hydrogenation.
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In the determination of the surface area of the spent catalysts, an element of uncertainty is however that minor amounts of long-chained products are formed in the reaction. This may
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leads to the formation of waxy deposits in the pores of the catalyst, and if these deposits are not removed in the evacuation procedure prior to the BET measurement, this would lead to an underestimation of the surface area. The surface area of the evaluated Cu-Co-MgO catalyst is
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unchanged after the CO hydrogenation experiment. As will be shown subsequently, this catalyst is essentially inactive in syngas conversion, and the formation of carbonaceous deposits is
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therefore less likely for this catalyst.
3.2 X-ray diffraction patterns of the catalysts
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The powder X-ray diffraction patterns of the samples were recorded and represented in Fig. 1. It can be seen that the precursors γ-Al2O3 and MgO supported catalysts are a typical hydrotalcite-like phase with sharp diffraction peaks at 2θ = 11.8, 23.6 and 34.9 o corresponding to the reflection of crystalline planes of (003), (006), and (009), respectively. The positions of the remaining peaks are in agreement with such an assignment. Many authors [27, 28] reported that it is possible to obtain single stoichiometric hydrotalcite layers (HTL) phases in the range of MIII/ (MIII + MII) = 0.2–0.4. For our case, the ratio of Al3+ / (Al3+ / Cu2+ / Co2+) is 0.5, which is considerably higher than the referred value above. Perez-Bernal et al. [29] found that the excess amount of aluminum in the Co (Zn)–Al HTls samples is segregated forming the additional phases, instead of incorporating into the hydrotalcite type structure or changing the structure. Contrarily, in ZnO supported catalyst showed similar diffractions patterns with low intensity and it can be attributed that the amorphous nature of that phase.
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The XRD patterns of the samples reduced at 300 °C for 3h are shown in Fig. 1. It is wellknown that the Cu–Co and Cu–Co-support spinel phases are difficult to be reduced [30]. Therefore, no obvious change can be seen for the spinel phases after reduction treatment except in ZnO catalyst. Diffraction peaks with high intensity of metallic Cu0 could be observed in the
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reduced γ-Al2O3 and MgO supported Cu-Co catalysts, which indicates that bigger-sized Cu particles are formed along with the stable spinel phase. As far as the sample Cu-CoxOx -ZnO is
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concerned, great changes take place after reduction and metallic Cu is detected with low intensity. However, the diffraction peaks related to copper were hardly identified, implying that
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copper was highly dispersed in Cu-Co-ZnO catalyst. In addition, the appearance of diffraction peaks at 31.8, 34, 36, 42, 56 and 62o can be attributed to the spinel phases of CuCo2O4 and/or
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3.3. TG/DTA analysis
The TG-DTA profiles of the supported catalysts were analyzed and represented in Fig. 2 .
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The results revealed that Cu-Co–Al2O3 has two obvious weight loss processes, the first weight
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loss corresponded to the Cu-Co–Al2O3 hydrotalcite loses interbedded water at about 185 °C, second weight loss related to the dehydroxylation, decarbonylation of OH-1 and CO3 -2 anion in
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the layer and between the layers, respectively, at about 250 °C. For MgO, γ-Al2O3 supported catalysts, the weight loss processes are usually included in the above three processes, and generally no further weight loss is recorded above 500 °C. However, in all catalysts the modification of structure with temperature only by changing the support. It is found that weight loss of MgO and ZnO supported catalysts slightly shifted to higher temperature region.
3.4. Scanning electron microscopy (SEM) analysis Fig. 3 reveals the SEM images of the calcined and used catalysts. In the case of alumina supported catalyst, SEM image was not altered before and after the reaction. Whereas in MgO supported catalyst uniform arrangement observed before the reaction, however in spent catalyst morphology was totally different and changed from spherical to flower shape. This might be the reason for the low activity of the catalyst, with decreasing the surface area. In the case of the
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ZnO catalyst exhibit lamellar shape was observed before the reaction and small size granules shape observed in spent catalysts. The support of catalysts is also an effective approach to decrease crystalline size and
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improve surface area of Cu-Co catalysts. Meanwhile, the low intensity diffraction peaks of the Cu-Co-Zn sample mean better dispersion of active components. SEM measurements show a significant difference of the morphology in the Cu-Co-ZnO catalysts relative to the other
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catalysts. The particle of the catalyst is not uniform and the conglomeration phenomena occur for the MgO and γ-Al2O3 samples, indicated that active components are not well dispersed. XRD
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results and morphology observations show that ZnO support is favorable to decrease the
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crystalline size and it showed best activity for the CO hydrogenation to produce higher alcohols.
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3.5. Transmission electron microscopy (TEM) analysis
The morphology of the catalysts before and after the CO hydrogenation was investigated
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by means of transmission electron microscopy. Fig. 4 shows TEM pictures of reduced and spent Cu-Co samples. In the Fig. 4, upper and lower TEM images represented reduced and spent
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different supported Cu-Co catalysts. In reduced catalysts (Fig 4 a and b), the spherical shape
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particles which are uniformly distribution over the support leads to the formation of nano size and average particle size is 10nm. Whereas in Fig. 4(c), shows the needle-shaped crystallites. In the spent catalysts, formations of bulk particles were observed in all catalysts. The average particle size is below 15 nm. During the reaction conditions catalyst particles agglomerate leads to increase in particle size.
3.6. TPR patterns of prepared catalysts Reducibility is an important property of the alcohol synthesis catalyst from syngas, as they are active only in reduced state and have to be pre-reduced prior to the CO hydrogenation reaction studies. Therefore, the reduction characteristics of the catalysts were studied with TPR in hydrogen. TPR profiles shown in Fig. 5 were acquired by continuously following the H2 signal with a quadruple mass spectrometer with linear increase in temperature. A sharp intense and symmetrical reduction peak at 230 °C is detected in each Cu-Co catalyst. This hydrogen
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consumption could be attributed to the reduction of CuO to Cu [31], the small shoulder peak at 350 °C in Alumina supported and 350 °C in Mg supported can be attributed to the reduction of Cu-Co spinal phases respectively. In contrast, quite different profiles could be observed for zinc supported catalyst. From XRD results, it is clear that, CuO, Co3O4 and Co2O4 are the main
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reducible oxides. Thus, the first peak at 230 °C could be due to the reduction of CuO to Cu and the other two overlaps at higher temperatures are attributed to the reduction of Co3O4 to Co
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consisting of two steps [32-34]. And final high temperature reduction peak at 450 °C ascribed to the reduction of (CuZn) Co2O4 spinal phase. All the Cu-Co catalyst showed a peak at 240 °C,
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which is due to the reduction of CuO support in that catalyst, promotes the reducibility of CuO [31]. From the reported results, it was found that highly dispersed CuO gave TPR signals at
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much lower temperature than bulk CuO [35, 36].
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3.7. Temperature programmed desorption of NH3
NH3-TPD patterns of reduced supported Cu-Co catalysts are shown in Fig. 8. From the TPD
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patterns clearly indicate that three different strength of acidic cites presented in each catalyst.
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The acid strength of peaks assigned as weak, medium and strong temperature at 110, 480 and 800 oC, respectively. However, ZnO supported catalyst showed a strong NH3 desorption peak at
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950 oC. The low temperature NH3 desorption peak could be assigned to the desorbed NH3 that adsorbed weakly on the surface of Cu [16]. The medium acidic peaks could be ascribed to the desorbed NH3 that adsorbed strongly onto the acid sites of Co–Al–O, Co–Mg–O, Co–Zn–O supports of Al2O3, MgO and ZnO supported catalysts respectively [17-19]. The strong acidic sites were associated with low-coordination oxygen atoms. These results implied that the acidity of Cu-Co catalysts affected by the support. Low temperature desorption of ammonia is typically assigned to Bronsted bound ammonia, whereas desorption taking place at higher temperatures indicates the presence of Lewis-bound ammonia.
3.8 Catalytic activity studies 3.8.1. Effect of support on CO hydrogenation
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In order to understand the effect of support on CO hydrogenation to produce higher alcohols, three different supports were selected namely, γ-Al2O3, MgO and ZnO. Active metallic species Cu-Co are introduced on to the support by co-precipitation method. The activity of the catalysts were studied and reported in Table 2. From the results it is clear that ZnO supported
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catalyst is superior compared to the other supports. This is due to the fact that, Cu species generating strong and medium acid sites by interacting with Zn support, where as γ-alumina
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support generates medium acid sites and MgO only weak acid sites. Cu dispersed on ZnO showed 18% CO conversion with 83% selectivity towards methanol. Weak acid sites showed
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very less CO conversion, where as high selectivity towards methanol from syngas. Cu species on the support first generates methanol by hydrogenation reaction, the so formed methanol further
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reacting with CO to produce DME. In the second step weak acid role is very important, if the catalyst posses strong acid sites it will leads to dimethylether (DME) instead of higher alcohols. Strong acidity predominates dehydration instead of CO insertion. Al with moderate acidity
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produces more hydrocarbons instead of higher alcohols.
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3.8.2. Effect of temperature on CO hydrogenation
Catalytic activities of Cu-Co supported catalysts were studied for the CO hydrogenation
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reactions. Since temperature is an important parameter in any chemical reaction, the effect of
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temperature was studied in the range of 220-300 °C with Zinc oxide supported catalyst and results are represented in Fig. 6. From Fig 6. it is clear that, CO conversion increases with increase in temperature, whereas methanol selectivity decreases. Lower temperature favors the methanol whereas higher temperature predominates the higher alcohols. 3.7.3. Effect of CO2 as a co-feed
Effect of CO2 as a cofeed in higher alcohol synthesis with copper supported catalysts was studied and the results were represented in table 3. Results revealed that, CO2 enhanced the conversion and selectivity for higher alcohols. CO2 inhibits the formation of methanol and enhances the higher alcohol selectivity by increasing oxygen coverage’s on Cu surfaces and by titrating basic sites required for aldol-type chain growth steps. Inhibition effects are weaker on catalysts with high Cu-site densities [37]. Copper supported on Zinc oxide showed best selectivity towards higher alcohol due to least acidic sites as confirmed by TPD ammonia measurements. With
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copper supported catalysts, the abundance of Cu sites allows reactants to reach methanol synthesis equilibrium and maintain a sufficient number of Cu surface atoms for bi functional
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condensation steps, even in the presence of CO2.
4. Conclusions
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Cu-Co on different support were synthesized by co-precipitation and tested for CO
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hydrogenation reaction at moderate reaction conditions. It was found that active metal species supported on Zinc oxide showed the higher catalytic performance and it was well correlated with XRD, SEM TEM and TPR results. Cu species generates moderately strong acid sites by
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interacting with the zinc support are playing vital role in CO hydrogenation to higher alcohols. It was also found that the low coordinated oxygen atoms in case of zinc supported catalyst and
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strong acidic sites are responsible for the high activity. The zinc supported Cu-Co catalyst has
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Acknowledgements
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potential for the production of higher alcohols from syngas with CO2 as co-feed.
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Authors thanks to the Korea Institute of Science and Technology (KIST) and the Ministry of Trade, Industry & Engineering in Korea.
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Table
Spent 50 43 22
Calcined 28 35 25
Spent 33 40 30
Calcined 22 31 30
Spent 18 28 26
Mean pore diameter (nm) 0.44 0.29 0.29
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Cu-Co-Al2O3 Cu-Co-MgO Cu-Co-ZnO
Calcined 79 47 37
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Catalyst
Pore volume (ml/gm)
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Crystallite size of copper from XRD (nm)
BET surface area (m2/gm)
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Table 1. Surface characteristics of the catalysts before and after the reaction
Catalysts
CO conversion (%)
Methanol selectivity
13 15 18
85 70 83
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Cu-Co-Al2O3 Cu-Co-MgO Cu-Co-ZnO
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Table 2.Effect of support on CO Conversion
Reaction conditions: Temperature: 240 °C, Pressure 35 bar, molar ratio of CO:H2=1:2, GHSV 3000 h-1.
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CO+CO2 as feed
Higher CO
Methanol
alcohol
CO+CO2
conversion
selectivity
selectivity
conversion
30
68
22
20
74
40
33
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33
62
alcohol
selectivity
selectivity (C2-C5)
40
36
60
35
23
75
70
22
70
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Cu-CoAl2O3 Cu-CoMgO Cu-Co-ZnO
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(C2-C5)
Higher
Methanol
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Catalyst
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Only CO as Feed
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Table 3. Effect of CO2 as a co-feed on catalytic performance
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GHSV 3000 h-1.
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Reaction conditions: Temperature: 300 °C, Pressure: 35 bar, molar ratio of CO:CO2:H2 = 0.3:1:2,
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Figure
Fig. 1. (A) XRD patterns of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 1. (B) XRD patterns of reduced Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 2. TG/DTA analysis of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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Fig. 3. SEM images of supported catalysts: (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
Ac ce p
and spent samples: (d) Cu-Co-Al2O3 (e) Cu-Co-MgO (f) Cu-Co-ZnO.
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d
Fig. 4. TEM images of reduced samples: (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
Ac ce p
te
and spent samples: (d) Cu-Co-Al2O3 (e) Cu-Co-MgO (f) Cu-Co-ZnO.
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ip t cr us an M d te Ac ce p
Fig. 5. TPR patterns of supported Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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ip t cr us an M d te Ac ce p
Fig. 6. TPD of NH3 of Cu-Co catalysts. (a) Cu-Co-Al2O3 (b) Cu-Co-MgO (c) Cu-Co-ZnO
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ip t cr us an M d te Ac ce p
Fig. 7. Effect of Temperature on catalytic performance.
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