title: Selective nickel-catalyzed conversion of model and lignin-derived phenolic compounds to cyclohexanone-based polymer building blocks.
abstract: Valorization of lignin is essential for the economics of future lignocellulosic biorefineries. Lignin is converted into novel polymer building blocks through four steps: catalytic hydroprocessing of softwood to form 4-alkylguaiacols, their conversion into 4-alkylcyclohexanols, followed by dehydrogenation to form cyclohexanones, and Baeyer-Villiger oxidation to give caprolactones. The formation of alkylated cyclohexanols is one of the most difficult steps in the series. A liquid-phase process in the presence of nickel on CeO2 or ZrO2 catalysts is demonstrated herein to give the highest cyclohexanol yields. The catalytic reaction with 4-alkylguaiacols follows two parallel pathways with comparable rates: 1) ring hydrogenation with the formation of the corresponding alkylated 2-methoxycyclohexanol, and 2) demethoxylation to form 4-alkylphenol. Although subsequent phenol to cyclohexanol conversion is fast, the rate is limited for the removal of the methoxy group from 2-methoxycyclohexanol. Overall, this last reaction is the rate-limiting step and requires a sufficient temperature (>250 °C) to overcome the energy barrier. Substrate reactivity (with respect to the type of alkyl chain) and details of the catalyst properties (nickel loading and nickel particle size) on the reaction rates are reported in detail for the Ni/CeO2 catalyst. The best Ni/CeO2 catalyst reaches 4-alkylcyclohexanol yields over 80 %, is even able to convert real softwood-derived guaiacol mixtures and can be reused in subsequent experiments. A proof of principle of the projected cascade conversion of lignocellulose feedstock entirely into caprolactone is demonstrated by using Cu/ZrO2 for the dehydrogenation step to produce the resultant cyclohexanones (≈80 %) and tin-containing beta zeolite to form 4-alkyl-ε-caprolactones in high yields, according to a Baeyer-Villiger-type oxidation with H2 O2 .
authors.affiliation: Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee (Belgium), Fax: (+32) 16-321998.
authors.forename: Wouter
authors.initials: W
authors.lastname: Schutyser
authors.affiliation: Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee (Belgium), Fax: (+32) 16-321998.
authors.forename: Sander
authors.initials: S
authors.lastname: Van den Bosch
authors.affiliation: Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee (Belgium), Fax: (+32) 16-321998.
authors.forename: Jan
authors.initials: J
authors.lastname: Dijkmans
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp (Belgium).
authors.forename: Stuart
authors.initials: S
authors.lastname: Turner
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp (Belgium).
authors.forename: Maria
authors.initials: M
authors.lastname: Meledina
authors.affiliation: Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp (Belgium).
authors.forename: Gustaaf
authors.initials: G
authors.lastname: Van Tendeloo
authors.affiliation: Institute of Condensed Matter and Nanoscience, Molecules, Solids and Reactivity (IMCN/MOST), Université catholique de Louvain, Croix du Sud 2 box L7.05.17, 1348 Louvain-La-Neuve (Belgium).
authors.forename: Damien P
authors.initials: DP
authors.lastname: Debecker
authors.affiliation: Centre for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, 3001 Heverlee (Belgium), Fax: (+32) 16-321998. bert.sels@biw.kuleuven.be.
authors.forename: Bert F
authors.initials: BF
authors.lastname: Sels
mesh_terms: D002384:Catalysis; D002563:Cerium / Q000737:chemistry*; D003300:Copper / Q000737:chemistry; D003512:Cyclohexanones / Q000737:chemistry; D006139:Guaiacol / Q000737:chemistry*; D008031:Lignin / Q000737:chemistry; D009532:Nickel / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry; D011108:Polymers / Q000737:chemistry; D014934:Wood; D015040:Zirconium / Q000737:chemistry
chemical_list: D003512:Cyclohexanones; D010636:Phenols; D011108:Polymers; D002563:Cerium; C036468:cyclohexanone; C030583:ceric oxide; D006139:Guaiacol; D003300:Copper; D009532:Nickel; D008031:Lignin; D015040:Zirconium; C028541:zirconium oxide
keywords: biomass; heterogeneous catalysis; lignin; nickel; synthesis design