Design and synthesis of spherical-platelike ternary copper-cobalt-manganese catalysts for direct conversion of syngas to ethanol and higher alcohols
Graphical abstract
Introduction
With the gradually dwindling of oil reserves and soaring crude oil price, the catalytic conversion of syngas (a mixture of H2 and CO) derived from natural gas or shale gas, coal and biomass feedstock to variable liquid fuels and chemicals is currently considered as one of the promising but challenging topics in the field of C1 chemistry [1], [2], [3]. The catalytic synthesis of ethanol and higher alcohols (C2+ longer alcohols, HA) through CO hydrogenation has attracted extensive interest, owing to the relevance of these products as clean liquid fuels (fuel additives), hydrogen carriers for fuel cell and precursors for important chemicals and pharmaceuticals [4], [5], [6].
Generally, ethanol and higher alcohols synthesis (HAS) catalysts can be divided into four types: i.e., noble Rh-based catalysts, modified Mo/MoS2-based catalysts, modified Cu-based or ZnCr-based methanol synthesis (MS) catalysts and modified Co-based or Fe-based Fischer-Tropsch synthesis (FTS) catalysts [7], [8], [9]. Rh-based catalyst, aiming at developing the atomically adjacent Rh0-Rh+ species, is a high-efficiency catalytic system to produce ethanol from syngas, however, it could not be utilized in large scale processes due to the exorbitant price and limited availability [5], [10]. Modified Co-based catalysts have been extensively investigated for ethanol and higher alcohols synthesis (HAS) from syngas since the Institut Francais du Petrole (IFP) firstly reported the CuCo catalyst in 1970s [11], [12], [13], [14]. Researches from academic and industrial circles on Co-based catalysts were mostly concentrated on CuCo system owing to the low feedstock cost and high selectivity of higher alcohols [15], [16], [17]. The CuCo catalyst exhibits an efficient synergetic catalysis between copper and cobalt active sites with distinguished roles, in which metallic cobalt catalyzes CO dissociation to generate surface alkyl group (CHx) while metallic copper enables adsorption and insertion of non-dissociation CO and subsequent step-wise hydrogenation to the ethanol and higher alcohols, as envisaged [18], [19], [20]. Accordingly, the intimate contact between highly dispersed Cu and Co active phases has been recognized as a crucial factor for the satisfactory ethanol or HA selectivity [20], [21]. Prieto et al. have combined DFT calculations and microcosmic kinetics model with accurate preparation routes to design a CuCo/MoOx catalyst with a Cu/(Cu + Co) ratio of 0.3, which presents a higher HA selectivity (ca. 26%) and time yield of total alcohols. The superior nanoscale metal intimacy of CuCo NPs prevents the segregation of Cu phase and greatly enhances catalytic performance [20]. Furthermore, a uniform dispersion of metal Cu and Co elements in the Cu@(CuCo-alloy) structure is responsible for HAS according to Wei et al.’s research [15]. Apart from the necessity of homogeneous uniform of active sites in catalyst particles scale, the group of Kruse developed a CoCuMn catalyst with an atomic-scale mixing in nanoparticles of Cu and Co by the oxalate co-precipitation route, which presented excellent selectivity of long-chain terminal alcohols (C8-C14OH) through CO hydrogenation [22].
Currently, most of the investigations about the CuCo catalysts are aimed at further enhancing catalytic activity and higher alcohols selectivity. Nevertheless, a wide distribution of higher alcohols (C2-C6 terminal alcohols) is generally overlooked, which is uneconomical and inconvenient for the separation of production. Undoubtedly, this unfavorable influence limits the large-scale industrial production of ethanol or other single higher alcohol through CO hydrogenation. Therefore, the CuCo-based catalyst should be designed and optimized to narrow the distribution of alcohols and improve HA selectivity, especially ethanol selectivity in the CO hydrogenation reaction.
Layered double hydroxides (LDHs) with the general formula [M1−x2+Mx3+(OH)2]x+[An−]x/n·mH2O, known as anionic clays or hydrotalcite-like compounds, a unique synthetic two-dimensional (2D) nanostructured materials with metal cations (M2+ and M3+) distributed in a uniform manner in brucite-like layer, are a class of individual catalyst precursors for supported metal catalysts with high density and homogeneous distribution of active sites [23], [24], [25], [26]. Meanwhile, a topological transformation of LDH precursors can offer the benefits of synergistic catalysis between mixed metal oxides, and enhance catalytic activity and stability [25]. Besides, as an efficient promoter, Mn element has been widely employed to improve the selectivity of higher alcohols in alkali-metal-modified Cu-based and Co-based systems in the CO hydrogenation reaction [22], [27], [28], [29].
With this background we developed a CuCoMn catalyst with a “spherical-platelike” nanosized particles structure derived from LDH precursor, which turned out to be an efficient catalyst for the synthesis of ethanol and higher alcohols during CO hydrogenation reaction. The SEM-EDX line spectra, HRTEM and HAADF-STEM results confirmed the unique “spherical-platelike” structure with a CuMn-rich sphere structure and a Co-dominated nanosheet. The resulting CuCoMn catalyst demonstrated a significant enhancement of selectivity to total alcohols of 46.2% and a narrow alcohol distribution of C1-C3 mixed terminal alcohols of 91.4%, in which ethanol accounted for 45.4%. In order to better cognize and design the highly active catalyst for heterogeneous CO hydrogenation to ethanol and HA, the unique catalyst structure and physicochemical properties were discussed in detail and associated with corresponding intrinsic catalytic performance. In addition, a synergistic effect between Cu+ and metallic Co active sites with different functionalities over CuCoMn catalyst was investigated by in situ XPS, NAP-XPS and in situ DRIFT characterizations.
Section snippets
Preparation of the structured CuCoMnxAl catalysts
The CuCoMnxAl precursors with different chemical compositions ((Cu + Co + Mn)/Al = 3, Cu/Co = 2/1, x = 0, 0.5, 1, 1.5, 2, 2.5) were synthesized by co-precipitation (CP) method at constant pH. In a typical synthetic procedure, a mixture of Cu(NO3)2·3H2O, Co(NO3)2·6H2O, Mn(NO3)2·4H2O (M2+ = 0.06 mol) and Al(NO3)3·9H2O (M3+ = 0.02 mol) was dissolved in 300 mL of deionized water. A second aqueous solution was acquired by dissolving 0.13 mol NaOH and 0.04 mol Na2CO3 in 300 mL of deionized water.
Structural and morphological characteristics of catalysts
XRD patterns shown in Fig. 1a revealed the structure of CuMn-LDH, CoMn-LDH, CuCo-LDH and CuCoMnx-LDH (x = 0.5, 1.5, 2.5) hydrotalcite presursors, which were synthesized by co-precipitation (CP) method. Apart from CuCoMn2.5-LDH precursor, typical diffraction peaks were observed in all precursors at 2 theta = 11.7° and 23.7°, which were attributed to (0 0 3) and (0 0 6) basal crystal planes in the hydrotalcite structure with a rhombohedral symmetry (R3), respectively [25], [30]. The lattice
Conclusions
In summary, we have demonstrated here that ternary “CuCoMn” catalysts can be designed to produce ethanol and higher alcohols with high selectivity during CO hydrogenation. Quite different from the classical co-impregnation catalyst (CuCoMn-IM) and the subsequent impregnated as-synthesized CuCo nanosheets catalyst (Mn/CuCo) with a homogeneous distribution of Cu, Co and Mn species on alumina support, the optimized “CuCoMn” catalyst, which was prepared by introducing Cu, Co and Mn elements
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21573269, U1510110 and 91645113) and the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDB-SSW-JSC043). We are grateful to Prof. Yifan Han for providing the corresponding NAP-XPS results. The authors also thank Dr. Pengchao Ren for XPS operation.
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