(193d) Catalytic Conversion of Biomass-Derived Carbohydrates to Functional Molecules On Carbon-Supported Pt-Re Catalysts

New processes
for the conversion of biomass to liquid fuels in a limited number of processing
steps are essential to make transportation fuels produced from ligno-cellulosic
biomass cost-competitive with those produced from petroleum.  Recent work has
lead to the development of a novel catalytic approach which converts
carbohydrates (sorbitol and glucose) derived from cellulose to the same monofunctional
chemical intermediates currently derived exclusively from fossil fuels.  These
molecules can, in turn, be converted to higher molecular weight alkanes (e.g.,
C₅-C₁₂ for gasoline, C₉-C₁₆ for jet
fuel, and C₁₀-C₂₀ for diesel applications).  Sorbitol and
glucose are converted to monofunctional hydrocarbon intermediates such as
alcohols, ketones, carboxylic acids, and heterocyclic compounds with 4-6 carbon
atoms on Pt-Re/C catalysts at moderate temperatures and pressures (483-523 K,
18-27 bar).  Subsequent upgrading of these molecules via aromatization,
isomerization, aldol-condensation, and/or ketonization processes leads to
alkanes suitable for use as fuel components.  This approach represents an
advance toward the economic conversion of biomass to liquid alkane fuels in
that a limited number of catalytic reactors or beds (e.g., 2) are employed, and
in that the liquid alkane products can be both processed and distributed by
existing petrochemical technologies and infrastructure with immediate use in
existing transportation vehicles.  An additional benefit of this approach is
that the mono-functional compounds produced as intermediates have use in
chemical applications, forming a platform for the production of liquid fuels
for the high-volume transportation market, and/or the production of
intermediates for the lower-volume, but higher value, chemicals and polymers
markets.

The initial step
of the process presented herein involves partial deoxygenation of the
carbohydrate/polyol feed.  The H₂ for these deoxygenation reactions
is supplied from reforming a portion of the feed on Pt-Re, in which adsorption
and dehydrogenation of the feed molecule with subsequent C-C cleavage leads to
adsorbed CO species which react with water to form H₂ and CO₂. 
Thus, the formation of CO₂ is necessary, and balancing these
reforming reactions that produce H₂ with deoxygenation reactions
requires that a minimum amount of the carbon in the feed be converted to CO₂. 
Alternatively, these adsorbed polyol species can undergo successive C-O bond
scissions leading to surface intermediates that either desorb as monofunctional
hydrocarbons or alkanes.  These reaction pathways on Pt-Re/C involving C-C and
C-O bond scission lead to the formation of CO, CO₂, and H₂
when C-C cleavage rates are high, whereas alkanes and mono-oxygenated species
are produced when rates of C-O cleavage are high.  The conversion of sorbitol
leads to the production of high molecular weight oxygenates with between 4-6
carbon atoms and 0-1 monofunctional oxygen groups.  These organic molecules
spontaneously separate from an aqueous effluent (which contains more highly
oxygenated species) into a hydrophobic phase.  The gaseous effluent contains CO_x
species and light alkanes.  Increasing pressure results in a shift of the
effluent carbon distribution from aqueous phase species to organic phase
species at 483 K and from aqueous phase species to gaseous species at 503 K.  The
production of alkanes increases at the expense of oxygenated species as
pressure increases from 18 bar to 27 bar at constant temperature.  Increasing
temperature at constant pressure leads to an increase in the production of
alkanes and a decrease in high molecular weight oxygenates.  Most of the CO₂
(70-80%) produced during sorbitol conversion is associated with the stoichiometric
CO₂ discussed previously while the remainder results from excess
water-gas shift reaction.

The second step
in this approach involves reactions that form C-C bonds amongst the
monofunctional intermediates from carbohydrate conversion, and the removal of
the remaining oxygen to give high molecular weight alkanes suitable for
transportation applications.  The C₄-C₆ ketones and
secondary alcohols in the organic liquid derived from the conversion of
sorbitol on Pt-Re/C can undergo C-C coupling by aldol-condensation on basic
catalysts to produce C₈?C₁₂ compounds which can undergo
subsequent hydrodeoxygenation to produce C₈?C₁₂ alkanes. 
The aldol-condensation step can be carried out at 573 K in the presence of H₂
on a bi-functional CuMg₁₀Al₇O_x catalyst, where
the Mg₁₀Al₇O_x component provides basic sites
for aldol-condensation, and Cu sites provide for both hydrogenation of C=C
double bonds in dehydrated aldol-adducts and dehydrogenation of secondary
alcohols to ketones.   The small amounts of organic acids and esters in the
organic liquid derived from sorbitol were removed prior to aldol condensation
(via hydrolysis/neutralization in a 20 wt% NaOH solution) because these
compounds cause deactivation of the CuMg₁₀Al₇O_x
catalyst.  This treated organic liquid was passed over a CuMg₁₀Al₇O_x
catalyst at 573 K and 5 bar pressure with a H₂ co-feed.  At these
reaction conditions, 2-ketones undergo self aldol condensation or crossed aldol
condensation with 3-ketones, whereas self-aldol condensation of 3-ketones is
less likely due to steric and electronic effects.  The primary alcohols present
in the liquid organic phase undergo crossed aldol condensation with ketones
(taking place via the intermediate formation of aldehydes).  Light species
containing between 4 and 6 carbon atoms and 0 and 1 oxygen atoms comprise 55%
of the carbon in the products.  These light species contain C₄
alcohols (3% of total carbon) and heterocyclic hydrocarbon compounds
(substituted tetrahydrofurans and tetrahydropyrans; 9% of total carbon) which
will form C₄-C₆ alkanes upon hydrodeoxygenation.  C₅-C₆
ketones and secondary-alcohols contribute 32% of the carbon in the products
while hexane and pentane contribute 10% of the carbon.  The remaining carbon
(45%) is associated with condensation products containing between 8 and 12
carbon atoms and 0 and 1 oxygen atoms.  The condensation products can be
converted by hydrodeoxygenation to the corresponding alkane products.  Alternatively,
the C₈-C₁₂ fraction can be separated from the C₄-C₆
fraction and converted to heavy alkane products, while the C₄-C₆
fraction (consisting primarily of 3-hexanone, 3-pentanone, tetrahydrofurans,
and tetrahydropyrans) can be used as fuel additives, solvents or chemical
intermediates.

Liquid fuel
components can also be produced by reacting oxygenated hydrocarbons over
H-ZSM-5 to produced aromatics, olefins and paraffins.  Accordingly, the
hydrophobic phase from sorbitol conversion can be converted to liquid fuel
components by first hydrogenating the ketones to alcohols (at 433 K and 55 bar
H2 pressure over 5 wt% Ru/C), followed by dehydration/alkylation at 673 K and
atmospheric pressure over H-ZSM-5.  This processing step converts 25% and 29%
of the carbon in the sorbitol-derived organic phase to paraffins and olefins containing
3 and 4 carbon atoms, respectively, and 38% of the carbon to aromatic species. 
Within this aromatic fraction, 12% (5% of total) and 37% (14% of the total) are
benzene and toluene, respectively, while 51% (19% of the total) is more highly
substituted benzenes.

An additional
process to form C-C bonds involves ketonization reactions between two
carboxylic acid molecules to form a ketone, CO₂, and H₂O. 
This reaction can be performed instead of the hydrolysis step, eliminating the
use of non-renewable agents such as NaOH, and is effective for feeds with high
concentrations of organic acids such as those produced from glucose conversion
over Pt-Re/C.  The ketonization on CeZrO_x at 573 K of the
hydrophobic molecules from glucose conversion yielded 85% conversion of the
monofunctional oxygenates to a liquid organic product stream and achieved
greater than 98% conversion of the carboxylic acids in the feed to C₇-C₁₁
ketones.  This ketonization step can be combined with aldol-condensation on
Pd/CeZrO_x at 623 K leading to a product stream in which 57% of the
carbon is in the form of C₇₊ ketones with 34% of the ketones resulting
from ketonization and 23% of the ketones resulting from aldol-condensation. 
Products with carbon-chain length greater than C₁₂ were also
observed, likely resulting from aldol condensation of methyl ketones with the C₇₊
ketones formed during ketonization.  The combined ketonization and
aldol-condensation process completely converted the carboxylic acids into C₇₊
ketones.

The removal of
oxygen atoms in tandem with C-C bond formation to produce chemical
intermediates with the desirable functionality for chemical applications or
conversion to the same molecules which comprise existing liquid fuels is an
attractive option for the processing of ligno-cellulosic biomass.  The
catalytic approach shown herein represents an advance in the conversion of biomass
to fuels and chemicals because it employs a limited number of flow reactors,
thus achieving low capital costs but retaining sufficient flexibility such that
it can be employed to produce a variety of liquid-fuel components.