Electrodeposited Ni-Rich Ni-Pt Mesoporous Nanowires for Selective and Efficient Formic Acid-Assisted Hydrogenation of Levulinic Acid to γ-Valerolactone

Abstract

In pursuit of friendlier conditions for the preparation of high-value biochemicals, we developed catalytic synthesis of γ-valerolactone by levulinic acid hydrogenation with formic acid as the hydrogen source. Both levulinic and formic acid are intermediate products in the biomass transformation processes. The objective of the work is twofold: the development of a novel approach for milder synthesis conditions to produce γ-valerolactone and the reduction of the economic cost of the catalyst. Ni-rich Ni-Pt mesoporous nanowires were synthesized in an aqueous medium using a combined hard-soft-template-assisted electrodeposition method, in which porous polycarbonate membranes controlled the shape and the Pluronic P-123 copolymer served as the porogen agent. The electrodeposition conditions selected favored nickel deposition and generated nanowires with nickel percentages above 75 atom %. The increase in deposition potential favored nickel deposition. However, it was detrimental for the porous diameter because the mesoporous structure is promoted by the presence of the platinum-rich micelles near the substrate, which is not favored at more negative potentials. The prepared catalysts promoted the complete transformation to γ-valerolactone in a yield of around 99% and proceeded with the absence of byproducts. The coupling temperature and reaction time were optimized considering the energy cost. The threshold operational temperature was established at 140 °C, at which, 120 min was sufficient for attaining the complete transformation. Working temperatures below 140 °C rendered the reaction completion difficult. The Ni₇₈Pt₂₂ nanowires exhibited excellent reusability, with minimal nickel leaching into the reaction mixture, whereas those with higher nickel contents showed corrosion.

Scheme 1. Conversion of LA and FA to γ-Valerolactone over Various Heterogeneous Catalysts under mild conditions

Figure 1

(a) CVs of 3 mM Na₂PtCl₆ + 200 mM NiCl₂ + 200 mM H₃BO₃, 25 mM NH₄Cl + 10 g L^–1 P-123 solution (black line) and blank solution without metallic precursors (blue line) on Si/Ti/Au at 50 mV s^–1 and 30.0 °C. (b) Reduction region of the CVs of the Ni–Pt bath on Si/Ti/Au (black line) and the polycarbonate membrane coated with an Au layer (green line) at 50 mV s^–1 and 30.0 °C.

Figure 2

FE-SEM micrographs, at various magnifications, of mesoporous Ni–Pt films at (a) −0.7 V, (b) −0.8 V, (c) −0.9 V, and (d) −1.0 V vs Ag|AgCl|Cl^– after circulating at 0.45 C cm^–2. Scale bar: 50 nm. (e) Schematic representation of the electrodeposition of mesoporous Ni–Pt films via block copolymer template electrodeposition. Adapted with permission from ref (54). (f) Ni content as a function of the electrodeposition potential used to electrosynthesize the mesoporous films.

Figure 3

TEM micrographs, at various magnifications, of mesoporous NWs prepared at (a) −0.35 V, (b) −0.80 V, and (c) −1.0 V vs Ag|AgCl|Cl^– after circulating at 0.45 C cm^–2. Scale bar: 20 nm.

Figure 4

(a) XRD patterns, (b) nitrogen adsorption–desorption isotherms, and (c) Pt 4f and (d) Ni 2p XPS spectra of mesoporous NWs prepared at (i) −0.35 V, (ii) −0.80 V, and (ii) −1.0 V vs Ag|AgCl|Cl^– after circulating at 0.45 C cm^–2.

Figure 5

(a) Reusability experiments of the conversion of LA and FA into GVL over Pt and Ni-rich Ni–Pt mesoporous NWs (reaction conditions: 1 g of LA, catalyst loading = 5 mg, 180 °C, 3 h). (b) Nickel leaching of Ni₇₈Pt₂₂ and Ni₉₄Pt₆ catalysts after each reusability experiment (reaction conditions: 1 g of LA, catalyst loading = 5 mg, 180 °C, 3 h). (c) LA conversion and GVL yield over Ni₇₈Pt₂₂ mesoporous NWs (reaction conditions: 1 g of LA, catalyst loading = 5 mg).

Figure 6

¹H NMR spectra recorded in acetone-d₆ of three reaction samples at (a) t = 0, (b) t = 90, and (c) t = 120 min of hydrogenation. Temperature = 140.0 °C. Dense gray circles correspond to the solvent signal.