The effect of sodium species on methanol synthesis and water–gas shift Cu/ZnO catalysts: utilising high purity zincian georgeite

Abstract

The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na, using a supercritical CO antisolvent technique, were characterised and tested for the low temperature water-gas shift reaction and also CO hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na concentration, with the 2.1 wt% Na-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na contents (<100-2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100-2500 ppm) Na containing samples, but with a lower Cu surface area, which indicates that Na species block Cu sites. Activity of the catalysts for the water-gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na content, suggesting that Na directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na content resulting in slightly reduced rates of growth under reaction conditions.

Highly active Cu/ZnO–Al catalyst for methanol synthesis: effect of aging on its structure and activity

“…The presence of Na + in this sample could also contribute to the low activity because these species can act as poison by increasing the basicity of the catalyst or modifying the Cu-ZnO interactions. 23 The initial intrinsic activity of the catalysts derived from aged malachite precipitates, Cu/ZnO-100 and Cu/ZnO-160, was 10% higher than the intrinsic activity of the catalyst derived from the amorphous georgeite. It is widely accepted that CH 3 OH synthesis on Cu appears to be sensitive to the Cu surface structure 42,43 and therefore the differences in the intrinsic activity observed in the catalysts were derived from changes in the "quality" of the Cu surfaces as a result of the different structure of the aged precipitates.…”

“…4) characteristic of zincian georgeite whereas the precipitates removed aer the pH drop showed crystalline zincian malachite which slowly transformed with ageing. In the observed evolution of the precipitates with aging it is important to note the presence of 7,23 In this scenario NaNO 3 could be the result of the evolution of the intermediate Znsodium salt generated in the initial crystallization step which transformed to more favourable hydroxycarbonates during washing 8 (Scheme 1). Upon aging the concentration of the intermediate Zn-sodium salts decreased, incorporating its Zn into the zincian malachite by dissolution and accordingly it could be the reason for the absence of NaNO 3 in the precipitates aged aer the crystallization of malachite from georgeite.…”

“…Small crystalline domains of CuO, of about 4 nm, were observed in the Scheme 1 Evolution of precipitates during the initial crystallization and aging of malachite and the subsequent washing. 23 or zincian malachite precipitates 1,5,35 because these precursors retained the high temperature carbonates aer calcination that preserved the segregation and crystallization of CuO. The calcined CuO/ZnO-Al-70 precursor exhibits particles of CuO with a large domain size (7.5 nm) segregated from ZnO.…”

Methanol Synthesis from CO2: A Review of the Latest Developments in Heterogeneous Catalysis

“…The desired properties in the final catalysts are a high Cu surface area (depending on small particle size and meso-porosity), good Cu–ZnO interaction or even formation of surface Cu–Zn partial oxidized mixed oxides, and minimal contamination from alkali metals (Na + ) introduced during the co-precipitation process [65]. The selective preparation of the hydroxycarbonate precursors involves the control of parameters in the precursor formation as elemental formulation pH, temperature, ageing time [63,64], and complete removal of Na + [63,65,66]. There is some controversy in the literature about which of the hydroxycarbonate phases is the best to obtain an optimal catalyst for the synthesis of methanol from CO 2 .…”

“…In this way, different studies have been carried out with catalysts derived from aurichalcite ((Cu,Zn) 5 (CO 3 ) 2 (OH) 6 ) [62,67], zincian malachite ((Cu,Zn) 2 CO 3 (OH) 2 ) [65,68,69], hydrotalcite (Cu 1−x−y ZnyAlx(OH) 2 (CO 3 ) x/2 ) [70,71], and even the amorphous zincian georgeite phase ((Cu,Zn)(CO 3 )(OH) 2 ) [65,69,72]. The best catalytic performance seems to be linked with the presence of the zincian malachite phase in the hydroxycarbonate precursor with a maximum amount of Zn incorporated into the structure of the malachite [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72]. However, recent studies also indicate that there is an additional factor to the presence of zincian malachite that is also key to obtaining a good catalyst for the synthesis of methanol.…”