Ethanol Dehydrogenation over Copper-Silica Catalysts: From Sub-Nanometer Clusters to 15 nm Large Particles

Abstract

Non-oxidative ethanol dehydrogenation is a renewable source of acetaldehyde and hydrogen. The reaction is often catalyzed by supported copper catalysts with high selectivity. The activity and long-term stability depend on many factors, including particle size, choice of support, doping, etc. Herein, we present four different synthetic pathways to prepare Cu/SiO 2 catalysts (∼2.5 wt % Cu) with varying copper distribution: hydrolytic sol−gel (sub-nanometer clusters), dry impregnation (A̅ = 3.4 nm; σ = 0.9 nm and particles up to 32 nm), strong electrostatic adsorption (A̅ = 3.1 nm; σ = 0.6 nm), and solvothermal hot injection followed by Cu particle deposition (A̅ = 4.0 nm; σ = 0.8 nm). All materials were characterized by ICP-OES, XPS, N 2 physisorption, STEM-EDS, XRD, RFC N 2 O, and H 2 -TPR and tested in ethanol dehydrogenation from 185 to 325 °C. The sample prepared by hydrolytic sol−gel exhibited high Cu dispersion and, accordingly, the highest catalytic activity. Its acetaldehyde productivity (2.79 g g −1 h −1 at 255 °C) outperforms most of the Cu-based catalysts reported in the literature, but it lacks stability and tends to deactivate over time. On the other hand, the sample prepared by simple and cost-effective dry impregnation, despite having Cu particles of various sizes, was still highly active (2.42 g g −1 h −1 acetaldehyde at 255 °C). Importantly, it was the most stable sample out of the studied materials. The characterization of the spent catalyst confirmed its exceptional properties: it showed the lowest extent of both coking and particle sintering.

Copper Phosphinate Complexes as Molecular Precursors for Ethanol Dehydrogenation Catalysts

“…Three Cu­(II) phosphinate complexes 1 – 3 were used as precursors for copper and phosphorus deposition on porous commercial SiO 2 by wet impregnation to prepare the Cu-phosphate/SiO 2 catalysts ( CuP-1 – 3 ). A benchmark sample without phosphorus was prepared by the dry impregnation method similar to a promising Cu/SiO 2 ethanol dehydrogenation catalyst working at 325 °C ( Cu-DI ) . A benchmark catalyst containing phosphorus was prepared using two methods.…”

“…On the contrary, the sample prepared by dry impregnation of copper nitrate ( Cu-DI ) exhibited diffractions corresponding to copper­(II) oxide (PDF: 00–048–1548) . According to the Debye–Scherrer equation, the crystallite size was estimated to be 22 nm …”

“…Crystallite sizes estimated by the Debye–Scherrer equation for Cu-DI after both H 2 treatment and the whole catalytic cycle remained similar (ca. 22 nm) …”

“…Cu 2+ species were reduced entirely in the spent catalyst to Cu 0 /Cu + (peak at 932.8 eV). Unfortunately, Cu 0 and Cu + cannot be distinguished using the Cu 2p peak (Figure D), and the surface copper concentration is too low to observe Cu LMM signal (Auger electrons) …”

“…The weight of the complex and support was calculated to obtain 2.5 wt % Cu loading for CuP-1 – 3 and 25 wt % Cu loading for CuP-3-TEP (masses used in the preparation are summarized in Table S4). The Cu-DI benchmark catalyst was prepared by the previously reported procedure . Cu­(NO 3 ) 2 ·2.5H 2 O (91.5 mg, 0.393 mmol) was dissolved in water (10 cm 3 ) and mixed with silica (Aerosil 300, 1.0 g) to form a paste.…”