Catalytic Conversion of Biomass to Monofunctional Hydrocarbons and Targeted Liquid-Fuel Classes

It is imperative to develop more efficient processes for conversion of biomass to liquid fuels, such that the cost of these fuels would be competitive with the cost of fuels derived from petroleum. We report a catalytic approach for the conversion of carbohydrates to specific classes of hydrocarbons for use as liquid transportation fuels, based on the integration of several flow reactors operated in a cascade mode, where the effluent from the one reactor is simply fed to the next reactor. This approach can be tuned for production of branched hydrocarbons and aromatic compounds in gasoline, or longer-chain, less highly branched hydrocarbons in diesel and jet fuels. The liquid organic effluent from the first flow reactor contains monofunctional compounds, such as alcohols, ketones, carboxylic acids, and heterocycles, that can also be used to provide reactive intermediates for fine chemicals and polymers markets

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 COx 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 C8C12 compounds which can undergo subsequent hydrodeoxygenation to produce C8C12 alkanes. The aldol-condensation step can be carried out at 573 K in the presence of H2 on a bi-functional CuMg10Al7Ox catalyst, where the Mg10Al7Ox 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 CuMg10Al7Ox catalyst. This treated organic liquid was passed over a CuMg10Al7Ox 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 CeZrOx 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/CeZrOx 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

Catalytic Production and Upgrading of Biomass Derived Monofunctional Hydrocarbons

We have studied the deoxygenation/ reforming of biomass derived carbohydrates to yield monofunctional hydrocarbons that can be utilized as fuels, commodity chemicals and solvents. Additionally, we have developed C-C coupling processes to upgrade these monofunctional species into fuel grade components that can be utilized in the current transportation infrastructure. We have demonstrated the catalytic conversion of biomass-derived carbohydrates, including monosaccharides and sugar alcohols, into hydrophobic mixtures of monofunctional C4-C6 hydrocarbon species containing alcohols, ketones, heterocyclic compounds and carboxylic acids. The conversion occurs over carbon supported Pt-Re catalysts and removes more than 80% of the initial oxygen content of the sugars and polyols, yielding a spontaneously-separating organic phase. This process operates at moderate pressures (20-30 bar) and temperatures (283-523 K) and utilizes highly concentrated aqueous feeds (40-60%) of sorbitol or glucose. At 503 K and 18 bar, Pt-Re/C showed excellent stability for longer than one month time-on-stream and yielded an organic stream containing ~ 50% of the carbon found in the 60 wt% sorbitol feed. A yield of 70% of the maximum possible conversion of the carbon in sorbitol to monofunctional species was obtained, corresponding to the production of 1 kg of organic for every 4 kg of sorbitol. Furthermore, we have studied catalytic C-C coupling processes to convert functional species (carboxylic acids, alcohols and ketones ) derived from carbohydrate conversion into C7-C12 ketones, that can be converted into diesel grade alkanes via deoxygenation over solid acid supported metal catalysts such as Pt/NbOPO4. These processes include ketonization in which two carboxylic acid molecules combine to form a linear ketone, CO2 and water, and aldol condensation/hydrogenation in which two ketone or secondary alcohol molecules combine to form a singly-branched ketone. We have studied ketonization of the aforementioned carbohydrate derived organic species over a CeZrOx catalyst at 648-673K and found near 100% conversion of carboxylic acids into C7-C11 linear ketones. The aldol condensation/hydrogenation process occurs on bi-functional catalysts that contain acid/basic functionality as well as metal sites to dissociate hydrogen. Aldol condensation/hydrogenation was studied over a low loading (0.25 wt%) Pd/CeZrOx catalyst at 623K, and was found to convert ~60% of the condensable ketones and alcohols found in the aforementioned carbohydrate-derived mixtures into C8-C12 branched ketones. Further investigation of the aldol condensation/hydrogenation reaction was performed by examining the reactivity of a representative ketone - 2-hexanone over Pd/CeZrOx and CeZrOx catalysts at temperatures between 573 and 673 K, and pressures of 5 to 26 bar. Reaction kinetics studies show that in addition to the expected C12 condensation product (7-methyl-5-undecaone), the CeZrOxbased catalysts produce C18 and C9 secondary species, along with light alkanes (<C7). Low loadings of Pd (e.g., 0.25 wt. %) lead to optimal activity and selectivity for the production of C12 species. The high activation energy of C9 formation (140 kJ/mol) compared to the formation of C12 and C18 species (15 and 28 kJ/mol, respectively) indicate that these species may be formed as a result of the decomposition of heavier condensation products. The self-coupling of 2-hexanone was found to be positive order in both 2-hexanone and hydrogen. The addition of primary alcohols and carboxylic acids as well as water and CO2 to the feed was found to reversibly inhibit the self-coupling activity of 2-hexanone

Dynamic Surface Processes of Nanostructured Pd₂Ga Catalysts Derived from Hydrotalcite-Like Precursors

The stability of the surface termination of intermetallic Pd₂Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd₂Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd₂Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd₂Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga₂O₃, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd₂Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd₂Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd₂Ga particles