Renewable hydrogen production by aqueous phase reforming of
oxygenated biomass over hydrotalcite derived Ni/Mg/Al catalysts

Abstract

The conversion, selectivity for hydrogen, and stability of different Ni based hydrotalcite derived catalysts were studied in aqueous phase reforming of ethylene glycol and phenol and were benchmarked against Ni/γ-Al2O3 and commercial 5 wt.% Pt/C and 5 wt.% Pd/C. Hydrotalcite catalysts were given a nomenclature as aNib, were ‘a’ is M2+/M3+ ratio and ‘b’ is Ni loading. The highest conversion obtained was for 3Ni20 catalyst giving a mass activity of 24.4 mmol of substrate per g of catalyst (38% conversion), which was similar to the mass activity of commercial 5 wt.% Pt/C and 5 wt.% Pd/C catalyst. However, selectivity for hydrogen of commercial catalysts was at least 1.5 times more than 3Ni20. The highest selectivity for hydrogen for hydrotalcite derived catalyst was obtained with 8Ni15 catalyst, which was similar to the selectivity obtained by Pt/C, however, mass activity of 8Ni15 was only 11.5 mmol of substrate per g of catalyst. With an increase in Ni loading in Ni-hydrotalcite derived catalysts (keeping the M2+/M3+ ratio constant) conversion of ethylene glycol increased (3Ni5 < 3Ni10 < 3Ni15 < 3Ni20) but the selectivity towards hydrogen decreased (3Ni5 > 3Ni10 > 3Ni15 > 3Ni20). With an increase in M2+/3+ ratio (keeping Ni loading constant) conversion of ethylene glycol decreased (2Ni15 > 3Ni15 > 6Ni15 > 8Ni15) but the selectivity to hydrogen increased (2Ni15 < 3Ni15 < 6Ni15 < 8Ni15). Monometallic Ni/γ-Al2O3 demonstrated poorer selectivity compared to all hydrotalcite derived catalysts. Post reaction pXrd patterns showed that the hydrotalcite structure was changed to a spinel structure (in case of hydrotalcite derived catalysts) and boehmite in case of Ni/γ-Al2O3. Ni leaching was lower in hydrotalcite derived catalysts as compared to monometallic Ni/γ-Al2O3, except for sample 8Ni15, indicating that higher Mg content can negatively affect catalyst stability. Spent 2Ni15, 3Ni20 and Pd/C were re-tested in APR of ethylene glycol. For hydrotalcite derived catalysts, conversion dropped by 50% as compared to fresh catalyst while conversion remained almost same in Pd/C. Selectivity for hydrogen of spent and fresh catalysts were almost same. The major reason for deactivation was leaching of Ni. With the increase in reduction temperature, conversion increased but the selectivity dropped. In case of aqueous phase reforming of phenol, no conversion of phenol was observed with any catalyst and consequently, no hydrogen was produced, in contrast to literature reports.