Stability of Pt/γ-Al₂O₃ Catalysts in Lignin and Lignin Model Compound Solutions under Liquid Phase Reforming Reaction Conditions

Abstract

The stability of a 1 wt % Pt/γ-Al 2 O 3 catalyst was tested in an ethanol/water mixture at 225°C and autogenic pressure, conditions at which it is possible to dissolve and depolymerize various kinds of lignin, and structural changes to the catalysts were studied by means of X-ray diffraction (XRD), 27 Al MAS NMR, N 2 physisorption, transmission electron microscopy (TEM), H 2 chemisorption, elemental analysis, thermogravimetric analysis-mass spectrometry (TGA-MS), and IR. In the absence of reactants the alumina support is found to transform into boehmite within 4 h, leading to a reduction in support surface area, sintering of the supported Pt nanoparticles, and a reduction of active metal surface area. Addition of aromatic oxygenates to mimic the compounds typically obtained by lignin depolymerization leads to a slower transformation of the support oxide. These compounds, however, were not able to slow down the decrease in dispersion of the Pt nanoparticles. Vanillin and guaiacol stabilize the aluminum oxide more than phenol, anisole, and benzaldehyde because of the larger number of oxygen functionalities that can interact with the alumina. Interestingly, catalyst samples treated in the presence of lignin showed almost no formation of boehmite, no reduction in support or active metal surface area, and no Pt nanoparticle sintering. Furthermore, in the absence of lignin-derived aromatic oxygenates, ethanol forms a coke-like layer on the catalyst, while oxygenates prevent this by adsorption on the support by coordination via the oxygen functionalities.

Deactivation of Ni-MoS2 by bio-oil impurities during hydrodeoxygenation of phenol and octanol

“…Ni-MoS2/ZrO2 was chosen as catalyst, as promotion by nickel previously has been shown to be slightly superior to cobalt promotion for HDO purposes [9,25] and ZrO2 was previously found as a suitable support for HDO [31][32][33][34][35] and does apparently not suffer from the instability in high concentration of water, as reported for the conventionally used -Al2O3 support for hydrotreating catalysts [9,36,37].…”

Silica deposition as an approach for improving the hydrothermal stability of an alumina support during glycerol aqueous phase reforming

“…Similarly, studies have shown some difficulty in controlling Pt dispersion on a shows the typical resonances at 8 ppm and 65 ppm, attributed to octahedrally and tetrahedrally coordinated aluminum species, respectively [23]. Expectedly, surface overcoating followed by precursor decomposition did not lead to obvious differences in the 27 Al MAS NMR spectrum. No peaks corresponding to pentahedrally coordinated aluminum species were observed, indicating that silica deposition leads to a surface modification only, rather than the formation of any detectable (bulk) silica-alumina phases [42].…”

“…Solid-state 27 Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were taken using a Bruker AV3 400 solid spectrometer. A high MAS rate of 12 kHz and a high magnetic field of 9.4 T helped to ensure that the spectra were quantitatively reliable.…”

“…The spent catalysts were characterized by XRD, 27 Al NMR, thermal gravimetric analysis (TGA), STEM and N 2 physisorption to better understand the impact of surface silica deposition on the support layer stability of the catalyst during the APR reaction.…”

“…Previous studies suggested this transformation to start with hydrolytic attack of the Lewis acid sites of the alumina support [24,25]. Indeed, supported metal particles as well as biomass-derived oxygenates in the reaction solution were reported to have a beneficial effect [26,27] on catalyst stability by "capping" specific, coordinatively unsaturated surface Al atoms. For example, Copeland et al found that biomass-derived polyols can block Lewis acidic Al sites of γ-Al 2 O 3 by formation of multidentate alkoxy surface species that protect the alumina from hydrolytic attack [25].…”