Breaking good: In situ ATR-IR spectroscopy tracked the Ni catalyzed deconstruction and hydrodeoxygenation of organosolv lignin in n-hexadecane, towards the production of naphthenes.
Abstract The mechanism of the catalytic reduction of palmitic acid to n ‐pentadecane at 260 °C in the presence of hydrogen over catalysts combining multiple functions has been explored. The reaction involves rate‐determining reduction of the carboxylic group of palmitic acid to give hexadecanal, which is catalyzed either solely by Ni or synergistically by Ni and the ZrO 2 support. The latter route involves adsorption of the carboxylic acid group at an oxygen vacancy of ZrO 2 and abstraction of the α‐H with elimination of O to produce the ketene, which is in turn hydrogenated to the aldehyde over Ni sites. The aldehyde is subsequently decarbonylated to n ‐pentadecane on Ni. The rate of deoxygenation of palmitic acid is higher on Ni/ZrO 2 than that on Ni/SiO 2 or Ni/Al 2 O 3 , but is slower than that on H‐zeolite‐supported Ni. As the partial pressure of H 2 is decreased, the overall deoxygenation rate decreases. In the absence of H 2 , ketonization catalyzed by ZrO 2 is the dominant reaction. Pd/C favors direct decarboxylation (−CO 2 ), while Pt/C and Raney Ni catalyze the direct decarbonylation pathway (−CO). The rate of deoxygenation of palmitic acid (in units of mmol mol total metal −1 h −1 ) decreases in the sequence r (Pt black) ≈ r (Pd black) > r (Raney Ni) in the absence of H 2 . In situ IR spectroscopy unequivocally shows the presence of adsorbed ketene (CCO) on the surface of ZrO 2 during the reaction with palmitic acid at 260 °C in the presence or absence of H 2 .
Ni nanoparticles supported on sulfonated carbon have been developed for phenol hydrodeoxygenation in liquid phase. Through combination of kinetic and spectroscopic studies the dehydration was elucidated to be the rate determining reaction. While the hydrodeoxygenation was determined by hydrogenation and elimination reactions, the successful deconstruction of lignin was mainly driven by hydrogenolysis of ether bonds in lignin polymer.
Abstract Sulfonated carbons were explored as functionalized supports for Ni nanoparticles to hydrodeoxygenate (HDO) phenol. Both hexadecane and water were used as solvents. The dual‐functional Ni catalysts supported on sulfonated carbon (Ni/C‐SO 3 H) showed high rates for phenol hydrodeoxygenation in liquid hexadecane, but not in water. Glucose and cellulose were precursors to the carbon supports. Changes in the carbons resulting from sulfonation of the carbons resulted in variations of carbon sheet structures, morphologies and the surface concentrations of acid sites. While the C‐SO 3 H supports were active for cyclohexanol dehydration in hexadecane and water, Ni/C‐SO 3 H only catalysed the reduction of phenol to cyclohexanol in water. The state of 3–5 nm grafted Ni particles was analysed by in situ X‐ray absorption spectroscopy. The results show that the metallic Ni was rapidly formed in situ without detectable leaching to the aqueous phase, suggesting that just the acid functions on Ni/C‐SO 3 H are inhibited in the presence of water. Using in situ IR spectroscopy, it was shown that even in hexadecane, phenol HDO is limited by the dehydration step. Thus, phenol HDO catalysis was further improved by physically admixing C‐SO 3 H with the Ni/C‐SO 3 H catalyst to balance the two catalytic functions. The minimum addition of 7 wt % C‐SO 3 H to the most active of the Ni/C‐SO 3 H catalysts enabled nearly quantitative conversion of phenol and the highest selectivity (90 %) towards cyclohexane in 6 h, at temperatures as low as 473 K, suggesting that the proximity to Ni limits the acid properties of the support.
The state of Ni supported on HZSM-5 zeolite, silica, and sulfonated carbon was studied during aqueous-phase catalysis of phenol hydrodeoxygenation using in situ extended X-ray absorption fine structure spectroscopy. On sulfonated carbon and HZSM-5 supports, NiO and Ni(OH)2 were readily reduced to Ni(0) under reaction conditions (≈35 bar H2 in aqueous phenol solutions containing up to 0.5 wt. % phosphoric acid at 473 K). In contrast, Ni supported on SiO2 was not stable in a fully reduced Ni(0) state. Water enables the formation of Ni(II) phyllosilicate, which is more stable, that is, difficult to reduce, than either α-Ni(OH)2 or NiO. Leaching of Ni from the supports was not observed over a broad range of reaction conditions. Ni(0) particles on HZSM-5 were stable even in presence of 15 wt. % acetic acid at 473 K and 35 bar H2 .
Microalgae to green diesel oils The mechanism of the catalytic reduction of palmitic acid to n-pentadecane at 260 °C in the presence of hydrogen over catalysts combining multiple functions has been explored by C. Zhao, J. A. Lercher, et al. in their Full Paper on page 4732 ff. A new redundancy catalysis route has been established on the surface of Ni/ZrO2 in the cascade hydrodeoxygenation reaction.
