Synthesis, Hydrogenation, and Hydrodeoxygenation of Biomass-Derived Furans for Diesel Fuel

Hydrogenation and hydrodeoxygenation (HDO) processes are critical for the chemical conversion of lignocellulosic biomass into liquid transportation fuels such as gasoline, jet, and diesel. Hydro-treatment processes can remove undesired functionalities and reduce the oxygen content in biomass-derived oxygenates to produce candidates suitable for fuel blends. In this study, the liquid phase hydrogenation and hydrogenolysis of several model substrates including, 2,5-dimethylfuran (DMF), 5-methylfurfural (5-MF), bis(5-methylfuran-2-yl)methane (BMFM), and 5-(2-(5-methylfuran-2-yl)vinyl)furan-2-aldehyde (MFVFAL), were investigated over noble metal catalysts (Pd, Pt, Ru) at moderate conditions (353 – 473 K, 0.55 – 5.1 MPa). Understanding the mechanism and kinetics by which undesired functionalities found in these substrates, such as aromatic furans, unsaturated C=C bonds, and C=O bonds, are removed during hydrogenation will allow for the design of efficient catalysts and processes to improve the hydroupgrading reactions of biomass-derived fuels. Unlike DMF and 5-MF, complex substrates such as BMFM and MFVFAL are not available commercially. To this end, we investigated synthetic strategies to produce these C11 – C12 oxygenates. BMFM was synthesized by electrophilic aromatic substitution of 2-MF with formalin catalyzed by acidic protons at 333K. The use of 5-methylfurfuryl alcohol in place of formalin, and heterogeneous SO3H-SiO2 improved the yield of BMFM from 70% (using homogeneous HCl) to 91%. MFVFAL was synthesized via base catalyzed nucleophilic self-condensation of 5-methylfurfural. We identified heterogeneous base K3PO4 that catalyzed the reaction 10 times faster than homogeneous hydroxide and ethoxide bases at 338K in ethanol solution. Through solvent and isotopic exchange studies, we identified dissociated ethoxide species on the surface of K3PO4 to be responsible for the initial proton abstraction, forming the reactive carbanion species involved in condensation reactions. Our investigation in liquid phase hydrogenation of aromatic furans begins with a simple C6 substituted furan, 2,5-dimethylfuran (DMF). The activity and selectivity of carbon-supported Pt, Pd, Rh, and Ru were compared under identical conditions (353 K, 0.55 MPa). Pt/C exhibited the highest activity and selectivity for C-O bond hydrogenolysis, producing 2-hexanone as the major product, whereas all other catalysts exhibited a high selectivity for ring hydrogenation to form 2,5-dimethyltetrahydrofuran. The reaction kinetics were measured in detail over Pt/C, under conditions that excluded the effects of internal or external mass transfer (333K – 363K, pH2 = 0.41 – 0.69MPa). A reaction mechanism is proposed to explain the parallel formation of 2,5-dimethyltetrahydrofuran, 2-hexanone, and 2-hexanol. The mechanism involves C-O hydrogenolysis of the aromatic ring to form an enol intermediate responsible for 2-hexanone and 2-hexanol products. 2,5-dimethyltetrahydrofuran was not an intermediate to ring-opened products under studied reaction conditions. The kinetics of DMF hydrogenation over Pt metals were described by a Langmuir-Hinshelwood model that assumed non-competitive adsorption of organic species and hydrogen atoms. The step involving C-O bond cleavage of the aromatic ring had the highest activation energy, 48.3 kJ/mol, and is favored over the steps involving C=C bond hydrogenation of the furan ring at elevated temperatures and low pH2. We applied the findings from our model system, DMF, to understand the hydrogenation behavior of more complex furanyl substrates, such as BMFM over noble metal catalysts. BMFM contains two adjacent aromatic rings – a structure commonly found in products of hydroalkylation reactions using 2-methylfuran. Kinetics of liquid phase hydrogenation of BMFM over Pt/C and Pd/C at moderate conditions (430 K – 473 K, 2 MPa H2) were investigated. As found in our DMF system, ring opening and ring saturation of the aromatic rings of BMFM occurred in parallel, with the former preferred over Pt/C catalysts and at elevated temperatures. Contrary to the DMF system, hydrogenation of BMFM occurred an order of magnitude more rapidly on Pd/C than on Pt/C, with significant activity toward C-O hydrogenolysis (15-30% selectivity to single ring-opened species). Over Pt/C, products from C-O cleavage of one or multiple rings accounted for 80-90% selectivity of the products. Again, the formation of an enol intermediate following ring opening is evidenced by the parallel production of ketone and alcohol products. Alcohols were predominantly formed over ketones when Pd/C was employed due to the nature of Pd surfaces to readily saturate the C=C bonds of enol intermediates. Additional differences in the activity and selectivity between Pt and Pd catalysts were discussed within the context of the differences in BMFM adsorption energy and bonding configuration over the metal surfaces. The reaction kinetics revealed that BMFM could be selectively hydrogenated to cyclic ethers only over Pd/C at low temperatures conditions (373 K, 2.0 MPa H2). Finally, we address the selective hydrodeoxygenation of 5-MF to DMF and MFVFAL into1,2-bis(5-methyltetrahydrofuran-2-yl)ethane (BMTHFE), a saturated cyclic ether product, in ethanol over noble metals (Pt, Pd, Ru) and Pd bimetallic catalysts. During hydrogenation (453 K, 5.2 MPa H2) over such catalysts, the C=O aldehyde group of these substrates is first hydrogenated to C-OH and hydrodeoxygenated, forming a terminal methyl group and releasing water. Group VIII metals such as Pt, Pd, and Ni demonstrated activity for decarbonylation and ring hydrogenation/hydrogenolysis due to strong interactions between the C=O group and the furan ring, respectively, and the metal surface. These reactions, along with etherification reactions, reduce selectivity to the desired HDO product. The incorporation of 1wt% Cu to 1wt% Pd/C resulted in 15-30% higher selectivity to HDO products for 5-MF and MFVFAL than 1wt% Pd/C. Monometlallic Cu catalysts were inactive toward hydrodeoxygenation and deactivated during reaction. Although PdCu/C catalysts had high activity in converting the C=O aldehyde into –CH3, weak interactions between the aromatic ring and metal surface prevented hydrogenation of the ring into cyclic ethers, which have been determined to be high cetane value diesel fuel candidates. After selective HDO over PdCu/C, a secondary hydrogenation step using 1wt% Pd/C at 373 K, 2.1 MPa H2 resulted in a 78% yield of BMTHFE