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10.1002/cctc.201601269
ChemCatChem
FULL PAPER
Acetic acid ketonization over Fe3O4/SiO2 for pyrolysis bio-oil
upgrading
James A. Bennett,[a] Christopher M.A. Parlett,[a] Mark A. Isaacs,[a] Lee J. Durndell,[a] Luca Olivi,[b] Adam
F. Lee*[a] and Karen Wilson*[a]
Abstract: A family of silica supported, magnetite nanoparticle
catalysts was synthesized and investigated for continuous flow
acetic acid ketonization as a model pyrolysis bio-oil upgrading
reaction. Physicochemical properties of Fe3O4/SiO2 catalysts were
characterized by HRTEM, XAS, XPS, DRIFTS, TGA and
porosimetry. Acid site densities were inversely proportional to Fe3O4
particle size, although acid strength and Lewis character were size
invariant, and correlated with the specific activity for vapor phase
acetic ketonization to acetone. A constant activation energy (~110
kJ.mol-1), turnover frequency (~13 h-1) and selectivity to acetone of
60 % were observed for ketonization across the catalyst series,
implicating Fe3O4 as the principal active component of Red Mud
waste.
growth and improves oil stability by removing reactive oxygenate
components, but does not neutralise the intrinsic acidity which
indeed induces catalyst deactivation. Hydrodeoxygenation is an
effective means to obtain cyclic and aliphatic alkanes as drop-in
transportation bio-fuels, however this requires a sustainable
source of molecular hydrogen, while the metal component of
HDO catalysts is susceptible to leaching in acidic bio-oils and
hence their neutralisation should help minimise precious metal
usage. Ketonization, through the condensation of two carboxylic
acid molecules to form a heavier ketone while eliminating CO2
and water (Scheme 1), affords a facile means to simultaneously
reduce the acidity and oxygen content of pyrolysis vapor
(through close-coupling to a pyrolysis unit) or associated bio-oil
condensate. For a monocarboxylic acid (RCOOH) such as
acetic acid, ketonization lowers the oxygen content by 75 % and
increases the chain length by (R-1) carbon atoms.
Introduction
Metal oxides have been widely demonstrated as active
catalysts for ketonization,[8] including iron oxides[9] which are a
major component of Red Mud. Red Mud is an industrial waste
material from bauxite mining for aluminium production,[10] and
comprises a toxic and caustic mixture of transition, alkali and
alkali earth metal oxides. Such waste is generally sent to landfill,
and hence in conjunction with the scale (120 million tons per
annum) of this hazardous material production, additional
opportunities are sought to add value to Red Mud waste
streams.[11] Consequently, there are several literature reports of
potential processes addressing the valorisation of Red Mud,
including its use in construction,[12] wastewater treatment,[13]
preparation of geopolymers[14] and magnetic materials,[15] energy
storage[16] and catalysis for diverse transformations such as
biodiesel production[17], biomass pyrolysis,[18] oxidation[19] and
hydrogen production.[20] and the upgrading of fast pyrolysis bio-
Bio-oil is a renewable (and potentially sustainable) liquid fuel
prepared by pyrolysis of biomass feedstocks such as agricultural
or forestry waste, energy crops, or microalgae solid residues
and sewage sludge.[1] Direct use of unprocessed fast pyrolysis
bio-oils is hindered by undesirable physicochemical properties,
including a low heating value due its high oxygen content, high
viscosity, and high acidity which renders it corrosive and
(thermo)chemically unstable.[2] The latter arises from the
presence of significant concentrations of carboxylic acids formed
during the thermal decomposition of cellulose and hemicellulose
biomass components, with acetic acid at levels between 1-10 %.
Heterogeneous catalysis affords several routes to the upgrading
of
pyrolysis
bio-oils,
including
esterification,[3]
(HDO),[5]
aldol
and
condensation,[4]
hydrodeoxygenation
ketonization,[6] each offering advantages and drawbacks.
Esterification of bio-oil condensates over solid Brönsted acids
can afford low temperature liquid phase upgrading of the
aqueous bio-oil fraction,[7] but requires a sustainable alcohol
source (although self-esterification with phenolic bio-oil
components is possible) and only slightly lowers the oxygen
content. Aldol condensation over solid bases enables chain
oils.[21] Hematite, α-Fe2O3, is
a major catalytically active
component of Red Mud, constituting typically 30-50 wt%,[22] and
has been investigated for the ketonization of formic and acetic
acid mixtures as model reactions for upgrading of pyrolysis bio-
oils. Hematite present in Red Mud is reported to reduce to
ferromagnetic Fe3O4 during reaction >350 oC.[21] This reduced
mixture is itself catalytically active, but exhibits superior
selectivity to the parent Red Mud with 10-20 % higher ketone
selectivity.[21-22] Acetic acid ketonization over bulk hematite is
also reported to induce in situ catalyst reduction to Fe3O4, which
is proposed to exhibit superior activity to Fe2O3.[23] Indeed,
Taimoor et al report that Fe2O3 ketonization activity in enhanced
upon the addition of 50 vol% H2 to the feedstream,[9] although
direct evidence for Fe3O4 formation was not provided.
Nevertheless, the literature consensus is that magnetite is
probably the stable, and catalytically active, iron oxide phase
present during ketonization.
[a]
Dr. J.A. Bennett, Dr. C.M.A. Parlett, Dr. M.A. Isaacs, Dr. L.J.
Durndell, Prof. A.F. Lee, Prof. K. Wilson
European Bioenergy Research Institute
Aston University
Birmingham, B4 7ET (UK)
E-mail: k.wilson@aston.ac.uk
Dr L. Olivi
[b]
Sincrotrone TriesteTrieste
34012 Basovizza (Italy)
Supporting information for this article is given via a link at the end of
the document.
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