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Schlagwörter:
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Zusammenfassung:
Catalysis plays a critical role in the production of commodity chemicals from crude
oil.1 Heterogeneous catalysis is key to approximately 90% of such processes in the
petroleum refinery.2 Biomass is expected to be a sustainable carbon source that can
replace oil within the current refinery scheme.3‐5 Several similarities can be drawn
between the petrochemical processes and biofuel production.6 However, the great
differences in composition between crude oil and biomass make utilization of
biomass challenging.7 The high oxygen content leads to poor quality fuels and for
this reason (hydro)‐deoxygenation processes have to be applied. Deoxygenation of
biomass can be achieved via hydrogenation and hydrodeoxygenation (HDO). In both
circumstances, a hydrogenation catalyst is required in order to activate hydrogen,
which is then added to the substrate. Raney Ni is an inexpensive skeletal metal
catalyst. By using concentrate sodium hydroxide solutions, this heterogeneous
catalyst is produced via the partial leaching of aluminum from an Ni‐Al (1:1) alloy.
Skeletal Ni catalysts have been employed for several types of reaction, including
hydrogenation, transfer hydrogenation,8 hydrogenolysis,9,10 and dehydrogenation.11
In this thesis, Raney Ni is investigated as an H‐transfer catalyst for hydrogenolysis
and hydrodeoxygenation of lignin, which is one of the major components of lignocellulosic biomass. The goal is selective deconstruction of lignin into
intermediates for the production of value‐added chemicals.
In Chapter Two, an introduction of biomass composition and structure is
provided. The primary existing fractionation processes of lignocellulosic biomasses
are succinctly reviewed. Furthermore, the state‐of‐the‐art utilization of biomass‐
derived commodities and catalytic processes recently developed for the production
of chemicals from lignin are introduced.
In Chapter Three, hydrogenolysis reactions of lignin are described. Raney Ni is
selected as a hydrogenation, catalyst and molecular hydrogen (H2) is added to the
lignin structure in order to depolymerize lignin into corresponding monomers.
Together with hydrogenolysis, which leads to the production of ortho‐
methoxyphenols, hydrogenation of the ring occurs and cyclohexanols are obtained.
Lignin feedstocks were obtained from an organosolv process and from the
mechanocatalytic depolymerization of wood.12 A comparison of processes performed
on these raw materials shows that the conversion is higher for the organosolv lignin
compared to lignin obtained via the mechanical catalytic depolymerization of wood
(MCP‐L). The difference in reactivity may be related to the poor solubility of MCP‐L
in organic solvents (e.g. acetone). However, other factors may also influence the
conversion of lignin (e.g. carbohydrate content and condensation).
In Chapter Four, the procedure for mechanical catalytic depolymerization of
wood is improved. The saccharification step is performed in the presence of 2‐methyl
tetrahydrofuran (2‐MeTHF) water mixture. Lignin is mostly extracted into the
organic layer (2‐MeTHF), whilst carbohydrates (e.g. cellulose and hemicellulose) are
depolymerized into monomers and recovered in aqueous phase. The obtained lignin
(E‐L) exhibits high solubility in organic solvents and, therefore, higher reactivity than
MCP‐L. Moreover, hydrogenolysis reactions, performed in the presence of Raney Ni
and H2, leads selectively to ortho‐methoxyphenols without saturation of the ring. In Chapter Five, a low‐temperature deoxygenation route,13 performed in the
presence of Raney Ni and a solid acid catalyst, is applied to a variety of phenol in
order to convert them into arenes. 2‐Propanol (2‐PrOH) is employed as the hydrogen
source for the hydrogenation of phenol into cyclohexanol, which is dehydrated by
the solid acid catalyst into cyclohexene. Raney Ni catalyzes the dehydrogenation of
cyclohexene into benzene as a tandem process. The reaction conditions are optimized
and applied to a lignin‐derived bio‐oil.14
Finally, in Chapter Six, a new procedure for the selective conversion of phenol
into arenes is described. Raney Ni alone catalyzes the selective conversion of phenol
derivatives into arenes in the presence of alkanes, which can be also employed as a
hydrogen source. The presence of a solvent with low basicity is strictly necessary for
this reaction. Indeed, a solvent with high basicity (e.g. 2‐PrOH, 2‐MeTHF) could
inhibit the Lewis acidic sites of Raney Ni and lead to hydrogenation of the ring. The
stability of the catalyst is explored and enhanced by using stoichiometric quantities
of alcohols. Furthermore, the reaction is successfully applied to the reaction of a
lignin‐derived bio‐oil.
