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.