By-product co-feeding reveals insights into the role of zinc on methanol synthesis catalysts

Abstract

Co-feeding of two relevant by-products of methanol synthesis, viz. dimethyl ether and methyl formate, in CO + H₂ and CO + CO₂ + H₂ streams was used as a strategy to gain understanding of the reaction over ternary Cu/ZnO/Al₂O₃ and binary Cu/ZnO, Cu/Al₂O₃, and ZnO/Al₂O₃ catalysts. While dimethyl ether does not participate in the process, the addition of methyl formate remarkably increases the MeOH production by its oxygen-assisted split into active intermediate species (CH₃O^⁎ and HCOO^⁎). Specifically, a breakthrough role was assigned to ZnO as catalytic partner for methyl formate conversion to methanol, since even the Cu-free ZnO/Al₂O₃ sample shows a considerable MeOH yield on MeF co-feeding. Alternatively, in the Zn-free Cu/Al₂O₃ sample, the presence of carbon dioxide in the inlet mixture was crucial to activate the methyl formate conversion path, likely because CO₂ provides readily available oxygen to the Cu surface.

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 Cu^δ+ 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.