(48f) Generation of Novel Diesel Additives Via Furan Condensation and Partial Hydrogenation of Biomass-Derived Molecules

Generation of Novel
Diesel Additives via Furan Condensation and Partial Hydrogenation of Biomass-Derived
Molecules

Eric R. Sacia,
Madhesan Balakrishnan, and Alexis
T. Bell

Department of
Chemical and Biomolecular Engineering,

University of
California-Berkeley, Berkeley, California 94720

     Production of alternative fuels from
biomass sources remains one of the greatest contemporary challenges for
catalysis. Due to its many advantages for growth and production, lignocellulosic biomass has been identified as a beneficial
source of  chemical
energy for biofuels.¹ However, in order to synthesize fuels of the
desired size and properties for blending into the diesel fuel supply, catalytic
C-C bond formation is required. Towards this aim, 5-hydroxymethylfurfural (HMF)
and furfural, dehydration products from the glucose and xylose
sugars generated by cellulose and hemicellulose
hydrolysis, have been broadly identified as useful platform molecules.^2-3

     While several pathways exist to produce
diesel-range fuel additives through C-C bond formation, including aldol condensation and acid-catalyzed alkene
oligomerization, the condensation of 2-methylfuran
with biomass-derived aldehydes provides an
innovative, high-yield pathway to fuel precursors.^4-5   The products of furan condensation have thus
far been subjected to extensive hydrodeoxygenation to
form branched 6-alkyl undecanes. In order to improve the
feasibility of these reactions, we have elucidated methods for solvent-free
acid-catalyzed furanylation at low temperatures
followed by partial hydrogenation.  We
have demonstrated that this scheme generates products that display attractive
fuel characteristics while requiring 38% less H₂ input than full
hydrogenation to alkanes due to the beneficial effect
of ethers on cetane number.⁶

     The overall scheme for furfural
condensation with 2-methylfuran is provided in Figure 1.  By performing the furan condensation with
furfural and a 10% stoichiometric excess of
2-methylfuran, 93% yields to the targeted product, 1, could be achieved within 2 hours at 65°C with homogeneous
catalysts under solvent-free conditions. In order to better facilitate acid
catalyst recovery, heterogeneous Brønsted and Lewis
acids were also screened. Brønsted acidic ionic
liquids (IL) of tunable hydrophilicity were
synthesized in order to create a liquid/liquid bi-phasic
catalyst system in which the hydrophobic reactants and products would separate
from the hydrophilic catalyst phase. 
Through modification of the hydrophilicity of
the IL's anion, the acidic IL phase could catalyze the furan condensation with activity
equal to that of homogeneous sulfonic acids.

Figure 1.  Scheme for
furan condensation followed by partial hydrogenation to targeted products

     Heterogeneous catalysts, such as sulfated zirconia and Amberlyst-15, were also tested for activity in
furan condensation reactions but were demonstrated to display lower activity
than the tested homogeneous counterparts. 
In order to more efficiently catalyze the furan condensation, several
alkyl ?linkers? were used to tether sulfonic acid
groups to high surface area silica.  The
resulting catalysts achieved complete conversion within 30 minutes while
maintaining high selectivity and recyclability. Furan condensation reactions were
also tested under these optimized conditions with 5-methylfurfural (97% yield),
HMF (78% yield), and 5-methylfurfuryl alcohol (89% yield).

     It was determined that condensation
product 1had a cetane
number of 25, significantly below the target of 40, and also had too high of a
viscosity to readily blend into diesel by itself.  Therefore, we sought to exploit the high cetane number of ethers compared to aromatics to generate
previously unreported diesel fuel candidates, 1c and 2c. Palladium
supported on traditional supports proved unselective for the partial
hydrogenation of 1 and 2 to their tetrahydrofuranyl
analogs, often producing ring-opened products before complete ring saturation.  However, we found that Pd supported on a
surface-tethered ionic liquid on SiO₂ gave high selectivity to 1c and 2c (92 and 85% respectively), even in solvent-free conditions.  Further, the products of partial
hydrogenation were tested and found to have significantly improved cetane numbers and viscosities compared to the starting
materials. The unique selectivity of these catalysts and their mechanism of
production of high-value additives will be further discussed. The role of
surface hydrophobicity and dispersion on the
selectivity of this hydrogenation will also be explored. 

References

1.  Somerville, C.; Youngs, H.; Taylor, C.; Davis, S. C.; Long, S. P., Feedstocks for Lignocellulosic Biofuels. Science 2010, 329 (5993), 790-792.

2.  Huber, G. W.; Iborra, S.; Corma, A., Synthesis
of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem Rev 2006, 106 (9), 4044-4098.

3.  Alonso, D. M.; Bond, J. Q.; Dumesic, J. A., Catalytic conversion of biomass to biofuels. Green Chemistry 2010, 12 (9), 1493-1513.

4.  Corma,
A.; de la Torre, O.; Renz, M.; Villandier,
N., Production of High-Quality Diesel from Biomass Waste Products. Angewandte Chemie International
Edition 2011, 50 (10), 2375-2378.

5.  Corma,
A.; de la Torre, O.; Renz, M., High-Quality Diesel
from Hexose- and Pentose-Derived Biomass Platform
Molecules. ChemSusChem 2011, 4 (11), 1574-1577.

6.  Murphy, M. J.; Taylor, J. E.;
McCormick, R. L., Compendium of Experimental Cetane
Number Data. NREL, Golden, Colo., 2004. ADDIN EN.REFLIST