Short CommunicationBy-product co-feeding reveals insights into the role of zinc on methanol synthesis catalysts
Graphical abstract
Co-feeding of methyl formate in CO + H2 and CO + CO2 + H2 mixtures reveals that ZnO acts as catalytically active species for MeOH production.
Highlights
► Influence of by-product co-feeding on methanol synthesis catalysts is investigated. ► Dependence of feed and catalyst composition on methanol synthesis is rationalized. ► ZnO is identified as catalytically active species in the presence of methyl formate. ► CO2 is required to activate methyl formate split over the zinc-free catalyst.
Introduction
Methanol (MeOH) is a top chemical worldwide, which can be used as a potential substitute for traditional oil-based fuels for automobiles, as an intermediate to produce a wide range of chemicals, or converted into more complex hydrocarbons (methanol upgrading) [1], [2], [3]. MeOH is conventionally obtained from syngas (CO + H2), which can originate from non-renewable (fossil fuels) or renewable (biomass) feedstocks. Besides, the use of CO2 as reactant is beneficial and particularly attractive among the viable strategies to valorize anthropogenic carbon dioxide emissions [3], [4], [5], [6]. Although Cu/ZnO/Al2O3 has been the industrial MeOH synthesis catalyst for decades, the influence of multiple catalyst preparation variables [7], [8], the nature of the active sites [9], and more specifically the role played by each component [10] are still debated. There is general agreement that (i) copper phase is needed to adsorb and to activate both CO and CO2 [11], (ii) Al2O3 increases thermal stability and surface exposure to reaction [9], and (iii) ZnO minimizes the sintering of Cu during ageing/calcination step by formation of aurichalcite [12] or zincian malachite [13] in the co-precipitated precursor. In fact, several investigations have claimed that the stabilization of Cuδ+ can be attributed to its incorporation into the ZnO lattice [14] and/or to the formation of strong metal-support interaction, which is beneficial for CO2 hydrogenation [15], [16]. Moreover, a spill-over mechanism between Cu and ZnO involving migration of H/OH [17] and formate species have been also reported [18].
The mechanism of MeOH synthesis is another conflictive point. A vast number of experimental and theoretical investigations have been conducted to derive insights into the reaction pathways to form MeOH. Although isotopic experiments pointed out to CO2 as the main carbon source over the CuZn system [2], and its hydrogenation proceeds via formate (HCOO⁎) species [19], [20], CO can be also hydrogenated to MeOH via formyl (CHO⁎) species. As shown in Fig. 1, both pathways end up in a common route involving formaldehyde (CH2O⁎) and methoxy (CH3O⁎) hydrogenations. In parallel to CO/CO2 hydrogenations, some side reactions are activated during the process. Among them, the water-gas shift (WGS) equilibrium is the most studied, since it provides an interchange route between CO and CO2 that can be critical for the global reaction [21]. Besides, the reverse WGS produces large amounts of H2O, which can alter the catalyst properties [22]. Another side reaction is the dehydration of MeOH over the acid sites of the Al2O3 support, yielding dimethyl ether (DME, CH3OCH3) and H2O. Finally, methyl formate (MeF, HCOOCH3) can be also formed during CO2 hydrogenation from the most abundant surface intermediate, i.e. HCOO⁎ [19].
In contrast to the multiple studies focusing on the WGS and the effect of H2O [23], [24], [25], the impact of DME and MeF co-feeding on methanol synthesis catalysts Cu/ZnO/Al2O3 remains unexplored. The formation of DME and MeF directly involve MeOH and CO2, respectively, and the hydrogenolysis of MeF has been found energetically advantageous in MeOH synthesis on alkali alkoxides-promoted Cu-based catalysts [26], [27]. Herein, we report on the effect of dimethyl ether and methyl formate as co-reactants in CO + H2 and CO + CO2 + H2 mixtures over ternary (Cu/ZnO/Al2O3) and binary (Cu/ZnO, Cu/Al2O3, and ZnO/Al2O3) catalysts.
Section snippets
Experimental
Ternary Cu/ZnO/Al2O3 (CuZnAl) and binary Cu/ZnO (CuZn), Cu/Al2O3 (CuAl), and ZnO/Al2O3 (ZnAl) catalysts with the same copper content (weight basis) and a molar metal ratio of 60/30/10, 65/35, 60/40, and 75/25, respectively, were synthesized by continuous co-precipitation followed by calcination in static air at 573 K for 2 h, using the method reported elsewhere [28]. The catalytic performance was measured using a stainless steel fixed-bed reactor with an internal diameter of 3 mm and using 0.4 g of
Results and discussion
Table 1 collects basic physico-chemical properties of the samples. Details on the techniques and procedures are described in the Supporting Information. The ternary CuZnAl catalyst displays a much higher exposed copper surface area than the binary counterparts. This relates well with the smaller average crystallite size of copper determined from the XRD patterns of the reduced samples. As expected, the reducibility of the copper is slightly favored in the ternary system, while the
Conclusions
We have described the effect of co-feeding DME and MeF using CO + H2 and CO + CO2 + H2 mixtures to form MeOH over a series of binary and ternary catalysts. While DME plays no role on the reaction under our conditions, MeF revealed a positive effect on the catalytic performance. The MeF-induced enhancement strongly depends on the catalyst formulation and the composition of the inlet mixture. These results confirm that the latter is not simply a by-product formed during CO2 hydrogenation, but also it
Acknowledgments
Total Gas & Power is acknowledged for sponsoring this research and for permission to publish these results. We thank Mr. Luis A. Robles Macías for valuable discussions.
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