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I. Ogino et al.
CatalysisTodayxxx(xxxx)xxx–xxx
exhibit activity comparable to the sulfonated carbon catalysts. This
analysis has raised questions about why some sulfonated carbon cata-
lysts seem to perform better than others, and about whether there is a
specific carbon catalyst performs better than others. In this work, we
chose to investigate carbon-based catalysts for this reaction, aimed to
address the second question, and thereby obtain a design principle to
improve catalyst performance for this reaction further. Carbon-based
catalysts potentially provide advantages because their porous structure
and surface properties can be tailored by choosing the type of carbon
materials and synthesis conditions of carbon precursors. In our previous
work with phenolic-resin-derived carbon catalysts [24], we demon-
strated that surface hydrophobicity and mesoporosity of catalysts can
be tailored by synthesis conditions of precursor resins and their pyr-
olyzation temperature. In addition, we showed how these factors in-
fluence the performance of catalysts in three different acid-catalyzed
reactions. In the current work, the type of carbon materials has been
expanded to traditional carbon catalysts based on activated carbon and
carbon nanotubes as well as graphite oxides and carbohydrate-based
catalysts, which have become more common recently [13,20,25].
Challenges to achieve this goal include complex interplays of effects
of porous structures, surface hydrophobicity/hydrophilicity, and swel-
ling ability of some catalysts [25] that is also common in ion-exchange
resin catalysts but challenging to evaluate its effect on overall perfor-
mance [26]. Thus, we sought a way to resolve these effects by syn-
thesizing a set of sulfonated carbon catalysts from various carbon
sources, and conducting a test esterification reaction of acetic acid
(AcA) with EtOH in addition to that of LA with EtOH (Scheme 1). We
chose the reaction of AcA with EtOH because it can be considered as
prototypical for esterification reaction, and the reactants are small en-
ough to minimize effects of mass transfer limitations (kinetic diameter
of AcA is reported to be 0.436 nm [27]). In addition, by comparing the
data for the reaction of LA with that of AcA, we anticipated that we can
investigate the effect of γ-keto group of LA on catalysis. We compare
catalyst performance by apparent initial turnover frequency (TOF) ra-
ther than rate per catalyst mass to compare active-site performance and
to minimize effects of product water on –SO3H groups [28,29]. We
report our findings that additional surface functional groups such as
–COOH and –OH groups in some carbon catalysts (carbonaceous cata-
lysts) facilitate the reaction of LA with EtOH.
2.2. Syntheses of catalysts
Catalysts were synthesized by sulfonating carbon materials using
sulfuric acids under N2 atmosphere. Graphite oxides (GO) were syn-
thesized by oxidizing natural graphite powder (z-5F) by Hummers
method [30]. Pyrolyzation of glucose and cellulose was conducted by
heating 1.0 g of D-glucose and cellulose powder, respectively, in
100 mL min−1 N2 to 673 K at a ramp rate of 5 K min−1 and holding the
temperature at 673 K for 15 h. The resultant materials are designated as
Glu and Cel, respectively. Hydrothermal treatment of glucose was
conducted by heating a D-glucose solution (1 g/mL) in a sealed 23-mL
Teflon-lined autoclave at 453 K for 24 h under static conditions. The
resultant material is designated as HTGlu. Four different carbon gels
(CG) were synthesized by pyrolyzing two different resorcinol-for-
maldehyde resins at different temperatures [24,31]. The resins were
synthesized at a molar ratio of resorcinol (R) and sodium carbonate (C)
of 50 and 200, and pyrolyzed at 673 or 1273 K. The resultant materials
are designated as CG-x-y where x and y represent a R/C ratio and
pyrolyzation temperature, respectively. CGs were sulfonated as re-
ported previously [24]. Activated carbon (AC), multi-walled carbon
nanotubes (CNT), GO, Glu, Cel, and HTGlu were sulfonated by heating
them in concentrated H2SO4 at 353 K for 15 h, followed by deep
washing with distilled water. Sulfonated materials are designated by
adding S at the beginning of each name. For example, sulfonated AC
and CNT are designated as SAC and SCNT, respectively.
2.3. Characterization of catalysts
Surface functional groups of the carbon catalysts were characterized
by IR spectroscopy. Measurements were performed in a transmission
mode under dynamic vacuum using a JASCO FT/IR-6100 Fourier
transform spectrometer with a spectral resolution of 4 cm−1. Acidic
functional groups of the carbon catalysts were quantified by Boehm
titration [32]. The standardization of NaOH solutions was performed
using potassium hydrogen phthalate as the primary standard and phe-
nolphthalein as the indicator. The carbon and oxygen concentrations in
the catalysts were determined through CHN analysis (CHN: MICRO
CORDER JM10, J-SCIENCE LAB Co.) while their sulfur concentrations
were determined by ion chromatography analysis (Dionex ICS1600,
Thermo Fisher Scientific Inc.) both at the Global Facility Center of the
Creative Research Institution at Hokkaido University. The carbon and
oxygen concentrations obtained from the elemental analysis were cor-
rected after the moisture content of carbon catalysts was determined by
thermogravimetric analysis (TGA), which assumed that the weight loss
below 373 K arises from desorption of physisorbed water. Powder X-ray
diffraction of carbon materials were measured on a Rigaku RINT Ultima
IV with Cu Kα radiation (λ = 1.5418 Å) and a D/teX Ultra detector.
Data were collected in a continuous mode over 5 ≤ 2θ ≤ 60° in 0.05°
step width with a scan speed of 0.16° s−1. TG measurements were
conducted on a Shimadzu TGA-50 thermogravimetric analyzer by
heating approximately 10 mg of a catalyst in a platinum crucible to
1073 K at 10 K min−1 in a 20 cm3 min−1 N2 flow. The textural property
of catalysts was characterized through nitrogen adsorption measure-
ments. The adsorption isotherms were collected at 77 K on an adsorp-
tion apparatus BELSORP-mini II (MicrotracBEL Co.). Prior to analysis,
samples were heated at 523 K in a 30-mL min−1 N2 flow for 4 h. The
surface areas of the catalysts were calculated by the Brunauer-Emmett-
Teller (BET) method [33] in a relative pressure range of 0.05–0.3. The
micropore volume (Vmicro) of catalysts was calculated from N2 uptake at
P/P0 = 0.15. The mesopore volume (Vmeso) of catalysts was calculated
by subtracting Vmicro from the total volume calculated from N2 uptake
at P/P0 = 0.98. Mesopore size distributions were determined by ap-
plying the Dollimore-Heal method [34] to the adsorption branch of
isotherms.
2. Experimental
2.1. Materials
Levulinic acid (97%), acetic acid (99.7%), ethanol (99.5%), D-glu-
cose, cellulose, 0.01 N NaOH aq., and 0.01 N HCl aq. were purchased
from Wako Pure Chemical Industries Ltd. Resorcinol (99.0%), for-
maldehyde (36.0 wt% aq.), sodium carbonate (99.8%), 1 N HCl aq.,
tert-butyl alcohol (99.0%), and sulfuric acid (98%) were purchased
from Tokyo Chemical Industry Co. Ltd. Activated carbon (Norit GAC
1240W), Amberlyst-15 (hydrogen form), and multi-walled carbon na-
notubes (SMW210, Sigma-Aldrich, BET surface area = 350 m2 g−1
)
were purchased from Sigma-Aldrich Japan. Natural graphite powder (z-
5F) was received from Ito Graphite Industry Co. Ltd.
To evaluate hydrophilicity of catalyst surface, water vapor adsorp-
tion isotherms were measured at 298 K on an adsorption apparatus
Scheme 1. Esterification reactions tested in this work.
2