Liquid-phase esterification of levulinic acid with ethanol
catalyzed by sulfonated carbon catalysts: effects of additional
surface functional groups
Yukei Suzuki, Isao Ogino,* Shin R. Mukai
Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
*Corresponding author: [email protected]
Keywords: esterification, levulinic acid, acetic acid, sulfonated carbon catalyst, liquid-phase reaction
1. Introduction
Additional functional groups present near acid sites
in solid acid catalysts often have a significant
influence on the overall catalyst performance
because they affect adsorption equilibrium of
reacting molecules and sometimes modify acid
strength of active sites. In this work, we report such
effects for sulfonated carbon catalysts on their
catalytic
performance
in
the
liquid-phase
esterification reaction of levulinic acid with ethanol.
Levulinic acid has been identified as the most
important value-added chemicals derived from
biomass [1] and its esters have been used as
ingredients for flavour and fragrance.[2] To
disentangle effects caused by potentially modified
acid strength of active sites, modified adsorption
properties of reacting molecules, and highly swelling
ability of some sulfonated carbon catalysts, a series
of sulfonated carbon catalysts was synthesized with
different concentrations of additional functional
groups, swelling ability, and porous properties. Then,
they were tested in the liquid-phase esterification
reaction of levulinic acid with ethanol as well as in
the esterification reaction of acetic acid with ethanol
because the latter test reaction can probe primarily
the performance of active sites.
2. Experimental
Sulfonated carbon catalysts were synthesizing via
sulfonation of various carbon materials using
sulfonic acid at 423 K. In addition to commercially
available activated carbon and multi-walled carbon
nanotubes (MWCNT), four different micromesoporous carbon materials were synthesized from
resorcinol-formaldehyde (RF) resins through
pyrolyzation and three different carbon materials
were synthesized from cellulose or glucose through
pyrolyzation or hydrothermal treatment. The
synthesized catalysts were characterized by N2
adsorption, water vapor adsorption, Boehm titration,
IR and Raman spectroscopies, and X-ray
photoelectron spectroscopy (XPS). Esterification
reactions were conducted in a batch reactor operated
at 333 K with reactants consisting of equimolar
acetic acid and ethanol or 1:3 molar ratio of levulinic
acid and ethanol. Products were analyzed using a gas
chromatography. In some experiments, to identify
and quantify active sites, a set of catalysts was
prepared by neutralizing a fraction of –SO3H sites by
Na+ to different degrees and tested their activity in
the esterification reaction of acetic acid with ethanol.
3. Results and discussion
Synthesized catalysts (Table 1) bear strong acid sites
(–SO3H groups, <1.0 mmol g–1) as characterized by
elemental analysis, base titration, and XPS. All
catalysts contain some additional oxygen-containing
functional groups such as –COOH and – OH groups
as characterized by IR and XP spectroscopies. In
particular, catalysts derived from carbohydrates and
graphite oxides are rich in –COOH groups (≈1.4
mmol g–1). RF resins-derived SCG catalysts contain
moderate concentrations of –COOH and –OH
groups (0.2–0.9 mmol g–1, each).
The synthesized catalysts exhibit significantly
different porous properties as well as swelling ability
in the presence of water vapor as characterized by
N2 and water vapor adsorption experiments. SAC
and SCNT retain high internal and external surface
areas
after
sulfonation,
respectively.
The
carbohydrate-derived catalysts are nonporous in dry
form but swell significantly in the presence of water
vapor as reported in various literature. SGO also
exhibits similar swelling ability although it exhibits
a low surface area in dry form. SCG catalysts are
micro-mesoporous with similar surface areas but
different mesopore volumes.
Although the synthesized catalysts exhibit vastly
different porous properties and concentrations of
surface functional groups, their apparent turnover
frequencies (TOFs) expressed with respect to the
concentration of –SO3H group were approximately
0.02 mol-ester/(mol-(–SO3H)·s) in the liquid-phase
esterificaiton reaction of acetic acid with ethanol.
Our previous work with SCG catalyst [3] show that
reaction experiments conducted using a set of SCG1 catalyst with different degrees of poisoning of acid
sites with Na+ show a linear decrease in TOF as the
degree of poisoning was increased with the line
extrapolated to zero activity at 100% poisoning.
These data indicate that –SO3H groups function as a
single site because of their low concentration, all
reactants can access them without mass transfer
limitations, and additional functional groups made
negligible contributions to the catalytic performance
for this esterification reaction.
swelling ability show relatively high performance.
These results suggest that such variations in activity
were caused either by favorable interaction of
reacting molecules with the additional surface
functional groups, presumably through the γ-keto
group of levulinic acid, or by potentially different
reaction mechanism between the two reactions.
Table 1. Sulfonated carbon catalysts tested in this work
Catalyst
name
SAC
Source of carbon
material
activated carbon
Remarks
SCNT
multi-walled carbon
SBET = 251 m2 g–1
SGO
graphite oxide
SBET = 77 m2 g–1, highly
swellable in water vapor
SGlu
glucose pyrolyzed at
673 K
nonporous,
highly
swellable in water vapor
SCel
cellulose pyrolyzed at
673 K
nonporous,
highly
swellable in water vapor
SHTGlu
glucose treated under
hydrothermal
condition
RF resin pyrolyzed at
673 K
nonporous,
highly
swellable in water vapor
SCG-2
RF resin pyrolyzed at
673 K
SBET = 658 m2 g–1,
Vmeso = 0.46 cm3 g–1
SCG-3
RF resin pyrolyzed at
1273 K
SBET = 596 m2 g–1,
Vmeso = 0.31 cm3 g–1
SCG-4
RF resin pyrolyzed at
1273 K
SBET = 570 m2 g–1,
Vmeso = 1.5 cm3 g–1
SCG-1
microporous,
SBET = 947 m2 g–1
micro-mesoporous,
SBET = 618 m2 g–1,
Vmeso = 0.20 cm3 g–1
Although the reaction of levulinic acid with
ethanol is considered to proceed via the same
mechanism as that for the reaction of acetic acid
with ethanol, it shows a variation of activity among
tested catalysts. The reaction tends to proceed faster
with higher concentrations of additional functional
groups (–COOH and –OH) as shown in a cross-plot
in Figure 1. Despite the high mesopore volume,
SCG-4, it shows a lower activity than SCG-1. In
addition, the catalysts with low porosity but high
Figure 1. Liquid-phase esterification reaction of levulinic
acid with ethanol catalyzed by the following catalysts at
333 K in a batch reactor: ●, SAC; ●, SCNT; ▲,SGO; ▲,
SGlu; ▲, SCel; ▲, SHTGlu; ◼, SCG-1; ◼, SCG-2; ◼,
SCG-3; ◼, SCG-4.
4. Conclusions
Presence of additional functional groups such as –
COOH and –OH groups led to promotional effects
on the catalytic performance of the sulfonated
carbon catalysts for the liquid-phase esterification of
levulinic acid with ethanol. Such promotional effects
are absent in the reaction of acetic acid with ethanol.
Thus, the promotional effects observed in the former
reaction suggest favorable interaction of reacting
molecules with the catalyst surface or slightly
different reaction mechanisms in the two
esterificaiton reactions. More precise control of the
distribution of these additional groups around –
SO3H groups will allow for further investigations on
such promotional effects and thereby enable design
of high performance catalysts.
Acknowledgments
A part of this work was supported by JSPS KAKENHI Grant
Number 26420774.
Reference
[1] J. J. Bozell, G. R. Petersen, Green Chem. 2010, 12, 539.
[2] A. Scott, One Company’s Big Plans for Levulinic Acid.
Chem. Eng. News, 2016, 94, 19 .
[3] I. Ogino, Y. Suzuki, S. R. Mukai, ACS Catal. 2015, 5, 4951.