(476g) Synthesis of Biodiesel Via Etherification of Biomass-Sourced Furanyl Alcohols

Synthesis of Biodiesel via Etherification
of Biomass-Sourced Furanyl Alcohols

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

Energy
Biosciences Institute and Department of Chemical Engineering, University of California, Berkeley, CA 94720

Email: alexbell@uclink.berkeley.edu

The combustion of
petroleum-sourced transportation fuels is one of the leading sources of
anthropogenic CO₂ emissions, and, therefore, is cited as a
significant source of rising atmospheric CO₂ concentrations.¹  For this reason, significant
emphasis has been placed in both the European Union² and the United States³ to replace approximately 10-15% of
the fuel supply with renewable sources in the next ten years.  In order to meet
these mandated targets, innovative schemes of fuel generation must be determined
using the chemical functionality present in biomass derived molecules to replace
not only gasoline blendstocks, but also diesel range molecules since diesel
consumption far outpaces that of gasoline on a global level.⁴

In order to replace the C₁₁-C₂₂
alkanes and aromatics in current generation diesel fuels, molecules possessing
similar properties with a similar cetane number must be synthesized.  While
transesterification of palm, rapeseed, or soybean oil to generate fatty acid
methyl esters can produce fuels that meet cetane number requirements, they
exhibit notable problems with pour point, oxidation stability, and raw material
cost in addition to using a food crop as a fuel raw material.  Diesel fuels can
also be generated from 5-(hydroxymethyl)furfural (HMF) and furfural, the
products of dehydrating glucose and xylose, sugars that can be sourced from
lignocellulosic feedstocks.⁵ While furan condensation⁶ and aldol condensation⁷ derivatives provide intriguing
diesel additives due to their formation of branched and linear alkanes, the
high hydrogen input necessary to make these fuels is problematic.  For this
reason, we have investigated the generation of furanyl ethers as a source of
biorenewable diesel fuel that has shown promise in diesel blends up to and
exceeding 17%.⁸

Our work has focused on
three primary pathways for forming furanyl ethers.  First, we have evaluated
the direct etherification of HMF to 5-(alkoxymethyl)furfurals using a variety
of linear alkyl alcohols  from methanol to 1-butanol as solvents.  We
discovered that acetal formation preceded ether formation for HMF etherification
due to the mesomeric effect causing the aldehyde of HMF to be significantly
electron withdrawing from the ring.  High temperatures and longer reaction
times led to decomposition of 5-(ethoxymethyl)furfural into ethyl levulinate.

In the second pathway, we
began with furanyl alcohols that are formed from input of 1-2 moles of hydrogen
into HMF and furfural.  Hydrogenation of HMF generates bis-(hydroxymethyl) furan (BHMF) and
methylfurfuryl alcohol (MFA).  These molecules have already been shown to be
byproducts in the formation of dimethylfuran (DMF), a candidate gasoline
additive.⁹ The aldehyde linkage of furfural can
readily be reduced to furfuryl alcohol, providing a pathway from hemi-cellulose
to furanyl ethers as well.  By performing etherification reactions on the
entire spectrum of furanyl alcohols, we were able to gain valuable insight into
trends in reactivity and key factors in selectivity of these reactions.  By
optimizing conditions, yields of furanyl ethers in excess of 98% were observed.

The third pathway we
investigated was the direct conversion of fructose into furanyl ethers in
ethanol and butanol as solvents.  At temperatures of 110°C, fructose readily
dehydrated to 5-(ethoxymethyl)furfural (EMF) (71%), the diethylacetal of EMF
(10%) and ethyl levulinate (16%) for a combined yield of biofuel candidate molecules
of 97% using Amberlyst-15, demonstrating an important pathway of forming attractive
fuel candidates from biomass derived sugars.

Using the mechanism and
rate law that were determined using heterogeneous sulfonic acid resin
catalysts, specifically Amberlyst-15, we elucidated that furanyl alcohol
etherification occurs via an S_N1 reaction where the loss of water is
the rate limiting step.  This mechanism causes the kinetics to be first order
in the furanyl alcohol and the acid catalyst and zero order in water and the alkyl
alcohol.  This mechanism further explains trends observed whereby decreasing
alcohol solvent polarity caused an increase in reaction rate.  The ring
substituents of the furanyl alcohol were found to have a profound effect on the
reactivity towards etherification.  Electron donation to the ring greatly
stabilizes the transition state in which the oxonium ion intermediate is
formed.  This causes MFA to have a higher reactivity than BHMF, which in turn
is more active than HMF.  It was also shown using ¹³C NMR spectroscopy
that furanyl alcohol partition into the surface phase of the sulfonic acid
resin plays an important role in the selectivity toward desired product
formation. The results of the present study illustrate how compounds suitable
as diesel can be produced in high yield from HMF and related furanyl alcohols.

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