One-step Preparation of Carbon-based Solid Acid Catalyst from Water Hyacinth Leaves for Esterification of Oleic Acid and Dehydration of Xylose

Article abstract of DOI:10.1002/asia.201701369

Carbon-based solid acid catalysts were successfully obtained via one-step hydrothermal carbonization (HTC) of water hyacinth (WH) in the presence of p-toluenesulfonic acid (PTSA). Increasing the HTC temperature from 180 to 240 °C resulted in carbonaceous materials with increased sulfur content and less adsorbed water. The material obtained at 220 °C (WH-PTSA-220) contains the highest amount of acid sites and promotes the highest initial rate of two transformations, that is, methanolysis of oleic acid and dehydration of xylose to furfural. While all PSTA-treated WH catalysts gave comparable fatty acid conversions (≈97 %) and furfural yields (≈60 %) after prolonged reaction times, the WH-PTSA-240 system bearing a relatively low acid density maintains the most favorable reusability profile. Higher HTC temperatures (220–240 °C) improved the catalyst reusability profiles due to graphitization and hydrophobicity of the carbon surface. The catalyst systems derived herein from biomass may have potential applications in biorefining platforms, utilizing the conversion of waste biomass to chemicals.

Full text of DOI:10.1002/asia.201701369

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: One-step preparation of carbon-based solid acid catalyst
from water hyacinth leaves for esterification of oleic acid and
dehydration of xylose
Authors: Jutitorn Laohapornchaiphan, Christopher Smith, and
Siwaporn Meejoo Smith
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One-step preparation of carbon-based solid acid catalyst from
water hyacinth leaves for esterification of oleic acid and
dehydration of xylose
Jutitorn Laohapornchaiphan,[a] Christopher B. Smith,*[b] and Siwaporn Meejoo Smith*[c]
Abstract: Carbon-based solid acid catalysts were successfully
obtained via one step hydrothermal carbonization (HTC) of water
hyacinth (WH) in the presence of p-toluenesulfonic acid (PTSA).
Increasing HTC temperatures from 180 to 240°C resulted in
carbonaceous materials with increased sulfur content, and less
adsorbed water. Material obtained at 220°C (WH-PTSA-220)
contains the highest amount of acid sites, and promotes the highest
initial rate of two transformations, methanolysis of oleic acid, and
dehydration of xylose to furfural. While all PSTA treated WH
catalysts gave comparable fatty acid conversions (97%) and
furfural yields (60%) after prolonged reaction times, the WH-PTSA-
240 system bearing a relatively low acid density maintains the most
favorable reusability profile. Higher HTC temperatures (220-240°C)
improved catalyst reusability profiles due to graphitization and
hydrophobicity of the carbon surface. The catalyst systems derived
herein from biomass may have potential applications in biorefining
platforms, utilizing the conversion of waste biomass to chemicals.
alcohols, or catalytically converted into various carbon-based
fuels and chemicals through intermediates (e.g. furfural, 5-
hydroxymethylfurfural (HMF), and levulinic acid) by acid-
catalyzed dehydration.[1] Acid catalyst systems are also
important for the production of oleochemicals from free fatty
acids (FFAs), including the esterification of FFA in low-quality
triglyceride feedstocks (e.g. waste cooking oils) for biodiesel
production prior to transesterification.[2]
Although liquid mineral acids (e.g. HCl, H2SO4, and H3PO4)
are cheap and often effective for catalyzing the above processes,
storage, use and transportation of these reagents is a concern
due to their corrosive nature. Recovery, and re-use from
homogeneous systems is difficult, and safe disposal is also a
major concern impacting the environmental footprint of chemical
industries. These factors give credence to the use of solid, easily
separable and re-usable heterogeneous acids for the
sustainable production of biofuels and chemicals.[3]
Carbon is an attractive support material for solid acid
catalyst systems due to its stability, availability, and ease of
functionalization.[4] Its derivation from biomass such as forest
residues, weeds, and agricultural waste, allows for the catalyst
preparation step to be integrated into biorefinery systems. As an
example, carbon bearing strong Brønsted acid (‒SO3H) sites
can be prepared in two steps through substrate (sugars, starch,
cellulose, or lignocellulosic biomass) carbonization followed by
sulfonation.[5] Solid acid catalysts prepared by this method
exhibit high catalytic performance and good stability in cellulose
hydrolysis,[6] dehydration of pentoses or hexoses,[7] and
esterification of FFAs.[8] However, the high-temperature
(≥400°C) thermal treatment required is energy intensive, with
sulfonation requiring very corrosive reagents and generating a
noxious waste stream.
Introduction
The shift from petroleum (e.g. crude oil and natural gas) to
sustainably produced liquid fuels and fine chemicals from
renewable carbon resources (i.e. biomass) requires new
conversion technologies. In the case of biorefining, these
technologies should be environmentally benign, and if possible
utilize feedstock components to enhance sustainability metrics.
Plant derived biomass typically requires depolymerization of
cellulosic/lignocellulosic components into sugar monomers (e.g.
glucose and xylose) which can be fermented to produce bio-
Carbon-based solid acid materials have been prepared by
one step hydrothermal carbonization (HTC) of aqueous furfural
(2-furaldehyde) with hydroxyethylsulfonic acid[9] or p-
toluenesulfonic acid (PTSA),[10] using polyvinyl alcohol as a
carbon substrate,[11] or autoclaving a mixture of glucose and
PTSA.[12] The former show comparable catalytic activity in
esterification reactions to those of sulfuric acid, Amberlyst-15
resin and H-Y zeolite, with the glucose/PTSA system being more
hydrophobic and promoting chemoselectivity towards dimethyl
succinate formation from succinic acid. Moreover, this carbon-
based solid acid catalyst also exhibits good performance and
reusability in fructose dehydration to the platform chemical 5-
hydroxymethylfurfural (5-HMF).
[a]
[b]
J. Laohapornchaiphan
Chemistry Graduate Program, Department of Chemistry
Faculty of Science
Mahidol University
Rama VI Rd, Rajathevi, Bangkok, 10400, Thailand
Dr C. B. Smith
Faculty of Science
Mahidol University
999 Phuttamonthon Sai 4 Rd, Salaya, Nakhon Pathom 73170,
Thailand
E-mail: christopher.smi@mahidol.ac.th
Prof. Dr. S. M. Smith
[c]
Center of Sustainable Energy and Green Materials and Department
of Chemistry
Faculty of Science
Mahidol University
While the above systems allow for facile and environmentally
benign preparations of SO3H-functionalized carbonaceous
materials, there are only limited reports of one-step acid catalyst
preparations from cellulose or lignocellulosic biomass. Several
999 Phuttamonthon Sai 4 Rd, Salaya, Nakhon Pathom 73170,
Thailand
E-mail: siwaporn.smi@mahidol.edu
Supporting information for this article is available on the WWW
For iunntdeerrnhtatpl://udxs.deo,i.oprgl/e10a.s10e02d/aosian.2o0t1xdxxexlxex.te. Submitted_Manuscript
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previous studies have obtained bio-based catalysts from multi-
step processes, involving acid or base treatment of cellulosic
raw materials followed by sulfonation, although the use of
multiple steps is not advantageous from an environmental
standpoint.[8g, 13] In this study, water hyacinth (WH, Eichhornia
crassipes), an invasive rapidly proliferating weed,[14] was chosen
as a biomass carbon substrate due to its abundance. Biomass-
derived solid acids were obtained in one step, using HTC at
different temperatures (180-240°C) with PTSA. The influence of
HTC temperature on physical/chemical properties, reusability
profiles, and catalytic activity of these materials towards two
conversions: oleic acid esterification, and dehydration of xylose
to furfural (Figure 1), was subsequently examined.
Figure 1. Acid-catalyzed reactions used to investigate the catalytic activities of the prepared carbon-based solid acids: (A) esterification of oleic acid and (B)
dehydration of xylose into furfural.
Figure 2. (A) Powder XRD patterns, (B) TGA curves, (C) NH3-TPD spectra, and (D) FT-IR spectra of carbon-based solid acids prepared at different HTC
temperatures.
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(Supporting
Information),
as
determined
by
N2
desorption/adsorption isotherms.
Results and Discussion
Solid acid materials prepared at various HTC temperatures
show variations in elemental composition, water content, and
acid properties. As shown in Table 1, sulfur content is greatly
enhanced at higher preparation temperatures (220°C, 240°C), at
the expense of carbon, hydrogen and oxygen. Decreases in H/C
molar ratio as the temperature is increased may be attributed to
the formation of cross-links between polycyclic aromatic carbon
sheets.[19] Both H/C and O/C ratios for WH-PTSA-240 are
significantly lower than those of materials prepared at lower
temperatures; these data are consistent with XRD results
indicating greater levels of carbonization at this temperature.
Thermogravimetric analysis (Figure 2B) indicates that higher
preparation temperatures results in material having lower overall
water content. Weight loss due to removal of water (between 30-
200°C, Table 1)[8a] is most pronounced in WH-PTSA-180 and
WH-PTSA-200 (approximately 6 wt. %), while markedly lower for
higher preparation temperatures (WH-PTSA-240, 1 wt.%). The
TGA results indicating low water content in WH-PTSA materials
derived at high HTC temperatures is consistent with XRD results
indicating carbon framework graphitization, resulting in a more
hydrophobic character. In these systems, further assessments of
the hydrophobicity is not possible through the use of contact
angle measurements, as these materials are utilized in powder
form to ensure efficient sorption and chemical processes at the
catalyst surface. Pelletization, as is required for contact angle
measurements, may give erroneous results as the measured
values may not be representative of the powdered material.
Indeed, recent reports[20] provide strong evidence that contact
angle values are highly dependent on compression pressure.
Properties of carbonaceous materials
Chemical and thermal properties. PXRD data indicate that
WH-PTSA materials consist of amorphous carbon comprised of
disordered graphitic carbon sheets, as indicated by the broad
and weak diffraction peaks at 2θ10-30° and 40-50° (Figure 2A),
which correspond to the diffraction of C(002), C(100), and
C(101) planes in graphitic carbon.[15] Increases in HTC
temperature from 180°C to 220°C result in marginal shifting of
the broad C(002) peak center to a higher angle, a consequence
of greater degree of carbonization and structural rigidity.[8g]
Further temperature increases (240°C) result in C(002) peak
sharpening and additional shifting to higher angle, although
some broadening related to an amorphous or turbostratic carbon
framework is still evident.[16] Diffraction peaks due to C(101) and
C(004) are more noticeable at this temperature, consistent with
graphitization of the carbon framework.[15b, 17]
Carbon-based materials prepared by one-step HTC of WH in
the presence of PTSA possess different morphologies,
depending on preparation temperature (see Supporting
Information). Materials prepared at 180 and 200°C (WH-PTSA-
180 and WH-PTSA-200, respectively) appear as microscale
(200 µm) structures having flat surfaces interspersed with
cracks and large cavities. Higher preparation temperatures (220
and 240°C) result in smaller particles, with WH-PTSA-220
exhibiting a range of size distributions (5-50 µm) and shapes.
Samples prepared at 240°C (WH-PTSA-240) exist as tightly
packed aggregates of small irregular shard-like (1-40 µm)
particles, although the formation of carbon spheres (0.2-10 µm
diameter) is now clearly noticeable, a feature seen previously in
products derived from corncob HTC.[8g]
.
The formation
Acid properties of WH-PTSA materials. Increases in sulfur
content with HTC temperature for WH-PTSA materials do not
correlate with total acid density, as shown by titration results in
Table 1. WH-PTSA-220, while having markedly higher S content
than material prepared at lower temperatures, shows negligible
difference in acid density in comparison to these materials. In
mechanism of WH-carbon based catalysts can be explained
based on that proposed for conversion of starch to acidic carbon
based materials.[9, 18] During hydrothermal treatment, cellulose
degradation takes place through hydrolysis followed by
carbonization (aromatization) processes, producing hydrochar.
Including PTSA in the treatment creates defect sites on the
treated biomass resulting in surface functionalized hydrochar
(acidic carbon based material). At higher HTC temperatures,
cellulose breakdown occurs more effectively, resulting in the
formation of tightly packed aggregates of small irregular shard-
like particles, and carbon spheres. All samples exhibit similar,
low BET surface areas and micro/mesoporous structure
contrast, WH-PTSA-240, with highest
S
content, has
significantly lower numbers of acid sites suggesting that some
sulfur containing groups are not responsible for acid behavior. It
is likely in WH-PTSA-220 that titrable oxygen-containing
functionalities (e.g. carboxylic, phenolic groups), abundant in
biomass, also contribute to the total acidity of these materials.[21]
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Table 1. Elemental compositions (carbon, hydrogen, sulfur, and oxygen), water content, and total acid density of carbonaceous materials prepared at different
temperatures.
[e]
[e]
Sample
C[a]
(wt.%)
H[a]
(wt.%)
S[a]
(wt.%)
O[b]
(wt.%)
H/C
molar ratio
O/C
molar ratio
Water[c]
(wt.%)
Total acid
density[d]
(mmol g-1)
O1s
S2p
(atomic%)
(atomic%)
WH-240
60.02
60.52
58.81
52.51
51.21
5.45
4.65
4.01
3.25
2.38
0.74
29.87
28.40
30.55
26.57
18.05
1.09
0.92
0.82
0.74
0.56
0.37
0.35
0.39
0.38
0.26
0.95
6.26
6.97
3.83
1.20
1.5±0.1
3.1±0.5
3.1±0.5
3.2±0.6
2.1±0.5
n.d.
n.d.
WH-PTSA-180
WH-PTSA-200
WH-PTSA-220
WH-PTSA-240
4.70
15.86
14.31
14.48
9.67
1.16
1.91
5.19
7.44
4.52
16.07
27.49
[a] C, H, and S contents were determined by CHNS analysis. [b] O content was calculated from CHNS results; %O = 100−(%C+%H+%N+%S). [c] As obtained
from TGA analysis. [d] Total acid density was determined by acid-base back titration. [e] From XPS measurements. n.d. = not determined.
Temperature-programmed desorption of ammonia (NH3-
TPD) provides more details on acid behavior, with acid strength
in solid acids being related to the desorption temperature.
Higher NH3-desorption temperatures infer stronger acidity, with
relative quantities of acid sites being ascertained through
integration of NH3-TPD curve peak area.[13b] As seen in Figure
2C, prepared solid acids contain several types of moderate to
strong Brønsted acid sites (e.g. ‒SO3H, ‒SO4H), as indicated by
NH3 desorption above 300°C, and weak acid sites (e.g. ‒COOH,
X-ray photoelectron spectroscopy (XPS) allows further
probing of the nature of WH-PTSA acid sites. XPS survey
spectra (Figure 3A) confirm that the surface of solid acids is
comprised primarily of carbon, oxygen, sulfur, and nitrogen.
Oxygen to carbon ratio (O/C) trends are consistent with those
obtained from elemental analysis, indicating greater
carbonization levels with temperature. Increases in surface S
content with HTC temperature are consistent with CHNS
analysis (Table 1). The high resolution spectrum of C 1s region
in WH-PTSA (Supporting Information) indicates surface carbon
to be mainly in a non-oxidized, or aromatic state (CHx, C‒C,
C=C, 285 eV), although higher HTC temperatures result in
more contribution from sulfur and oxygen-bound carbon atoms
(e.g. C‒S, C‒O, C=O, O‒C=O),[24] having higher binding
energies. From XPS C1s measurements, the peak
corresponding to C-S is suggestive of sulfonic/sulfonate group
grafting to the WH support. This finding is consistent with results
obtained by Liu et. al. [25], who report SO3H group binding to a
biomass carbon substrate. WH-PTSA-220 exhibits the highest
portion of carbon bound to sulfur or oxygen atoms. Furthermore,
S 2p spectra (Figure 3B) suggest that the surface sulfur atoms in
samples prepared at 180-200°C are predominantly in oxidized
phenol) as indicated by lower (40-250°C) temperature
13b, 22]
desorption.[12b,
Through comparison of peak areas
obtained from equal weights of each material, results indicate
that WH-PTSA-220 contains the highest number of moderate to
strong acid sites (7.4% greater peak area above 300°C) than for
WH-PTSA-200, followed by other materials in the order WH-
PTSA-180 > WH-PTSA-240. A further desorption peak above
750°C in WH-PTSA-220 may indicate the presence of even
higher strength acid sites.
FT-IR spectra of raw WH leaves before hydrothermal
treatment (WH), material prepared from HTC of WH at 240°C
without PTSA (WH-240), and WH-PTSA solid acids are shown
in Figure 2D. All spectra contain OH stretching vibrations (3000-
3700 cm-1) indicative of water content, or phenolic/carboxylic
functional groups, however from comparison of WH and WH-
PTSA the intensity of this vibration decreases slightly on
hydrothermal treatment. Vibrations due to C-O stretching in WH
(950-1350 cm-1) are still observable after HTC (WH-240),
although these are of very low intensity in WH-PTSA. This would
indicate that such weakly acidic residues are retained in WH-
PTSA materials, consistent with titration and NH3-TPD results.
Addition of PTSA followed by HTC results in shifting of the C=O
stretch in WH (1650 cm-1) to higher frequency (1700 cm-1) in
conjunction with the appearance of a new C=C vibration at 1620
cm-1; both of these result from dehydration, and aromatization,
during HTC.[18] Most notably, new resonances at 1034 cm-1 and
1160 cm-1 in WH-PTSA spectra are evident arising from the
presence of sulfonic acid/sulfonate groups.[8g, 12a, 23]
forms, as indicated by the peaks in region B.[8c, 24a, 26] On the
[24a, 26]
contrary, region C belonging to the C–S–C structure,
or
thiol (‒SH) groups,[22, 27] becomes the main contributor to the S
2p peak at higher (220-240°C) HTC temperatures. This indicates
that the acidity of WH-PTSA-220 is likely due to several factors:
a large number of acid sites, the presence of oxy-acid groups
(e.g. carboxylic), some oxidized sulfur groups contributing from
region C, as well as additional but unknown species at higher
binding energy (region A), as noted at high NH3 desorption
temperatures (Figure 2C). The low acidity of WH-PTSA-240 is
thus likely to be a consequence of S being predominately in non-
acidic (e.g. C-S-C) forms, and may likely be involved in cross-
linking of graphitic carbon sheets.[19]
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(approx. 70% after 5 hours), such that a 7 wt.% catalyst loading
and 1:20 oleic acid to methanol ratio provides optimal
performance for esterification using WH-PTSA-240.
Figure 4. Conversion of oleic acid to methyl oleate using (A) carbon-based
solid acids prepared at different HTC temperatures and (B) a comparison of
conversions using commercial acid catalysts and WH solid acids. Reaction
temperature was 65°C, oleic acid/methanol molar ratio was 1:20, catalyst
loading 7 wt.% with respect to oleic acid.
Figure 3. (A) XPS survey spectra and (B) S 2p spectra of carbon-based solid
acids prepared at different HTC temperatures.
Catalytic activity in esterification of oleic acid
Catalytic activity of other WH-PTSA solid acids. As
illustrated in Figure 4, and utilizing the parameters derived from
screening above, solid acids prepared at lower HTC
temperatures show higher activity in esterification of oleic acid
than WH-PTSA-240. This is most apparent after 3-4 h reaction
time, with conversions correlating with the amount and strength
of acid sites as given by NH3-desorption results (Figure 2C).
WH-PTSA-220, having the highest amount of moderate to
strong acid sites, is most active, with WH-PTSA-240 being least
active to esterification. The more graphitized, cross-linked
structure of the WH-PTSA-240 catalyst might also contribute to
its lower activity, potentially by limiting accessibility of the
reactants to the active sites.
The esterification of oleic acid using WH-PTSA-220 versus
commercially available sulfonated ion-exchange resins and the
control (WH-240) highlights its superior catalytic activity (Figure
4B). WH-240 bearing weakly acidic ‒OH and ‒COOH groups is
catalytically inactive, conversion rates being comparable to
those in the absence of catalyst. The activity of the WH-PTSA-
220 system emphasises the importance of the active sulfur acid
Reaction parameter screening. As WH-PTSA-240 was
expected to be least catalytically active of the series due to its
lower acidity it was selected for reaction parameter screening for
the esterification of oleic acid. The results of these experiments
appear in the Supporting Information. For a catalyst loading of 7
wt.% and 1:20 oleic acid to methanol molar ratio, esterification
proved sluggish at 35°C, reaching only 40% conversion after 10
h. Higher reaction temperatures (65-80°C) greatly enhanced
conversion, as could be expected through increased reaction
rates.[28] At 65°C, 96% conversion of oleic acid was achieved in
5 h, with near quantitative conversion after 3 h at 80°C. For
practical purposes, being the boiling point of methanol, 65°C
was selected as the basis for other parameter screening tests.
Increasing the amounts of methanol or catalyst loading resulted
in higher conversions, however, at a methanol molar ratio of
1:20, only slight increases in the conversion resulted from higher
(10 wt.%) catalyst loadings. Halving the amount of methanol
(1:10 molar ratio) dramatically reduces conversion levels
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sites in the catalytic process, although the hydrophobicity of
carbon-based catalysts may contribute to their superior
performance, facilitating adsorption of long-chained fatty acids
and thus preventing adsorption of water produced in the
hydrogen-bonding in WH-PTSA-220, resulting in enhanced
catalytic performance.[6a, 12b]
esterification process leading to potential active site poisoning.[8e,
29]
Catalytic activity in dehydration of xylose
Effects of reaction temperature and catalyst loading on xylose
dehydration to furfural were studied in GVL using WH-PTSA-240,
the lowest acidity system. Increases in reaction temperature or
catalyst loading resulted in higher furfural yields, although
optimal catalyst loadings and reaction temperatures were 2 wt.%
with respect to GVL and 170°C after
3 h, respectively
(Supporting Information). Further increases in reaction
temperature, catalyst loading, or reaction time led to lower yields
of furfural, possibly due to degradation or condensation of the
product to form insoluble humins[7b] as suggested by the
formation of dark solid products on the wall of the reaction
vessel.
Catalytic activities of all WH solid acids in xylose dehydration
using the parameters derived above are shown in Figure 5A. As
in esterification, higher acidity solid acids (e.g. WH-PTSA-220)
provide greater product yields at short reaction times, with WH-
PTSA-240 being least active. However, furfural yields of
approximately 60% can be obtained after 2 h, regardless of the
difference in acid density or acid strength among the prepared
solid acid catalysts. WH-240 alone proved inactive to promoting
dehydration (Figure 5B). Notably, only furfural was observed by
HPLC under conditions of UV detection. The HPLC
chromatogram (277 nm) showed a single peak attributed to
furfural (see Supporting Information). Nevertheless, with a 70%
furfural yield and >95% conversion of xylose, there is the
possibility of other concurrent reactions occurring, perhaps
generating UV inactive by-products. Further investigation to
identify possible by-products using other techniques (such as
high resolution HPLC-MS) is required.
The most active WH solid acid (WH-PTSA-220) compares
favourably in terms of catalyst activity with other heterogeneous
acid catalyst systems (Figure 5B). Commercially available solid
catalysts with strong acid sites (Amberlyst-15, Nafion NR50) are
most active after 30 min; rapid decreases in furfural yields after
this time may result from the heat-sensitivity of Amberlyst-15
resin,[30] or potential poisoning of surface active sites in Nafion
NR50, a material which contains the majority of its active sites
within the polymer interior.[31] In contrast, WH-PTSA-220 exhibits
more consistent performance, with furfural yields in excess of
50% over 4 h. In addition to acidity the presence of ‒OH and
‒COOH groups may facilitate xylose adsorption through
Figure 5. Furfural yields obtained in GVL over (A) carbon-based solid acids
prepared at different HTC temperatures and (B) using different heterogeneous
acid catalysts in comparison with WH materials. (Reaction temperature 170°C,
catalyst loading 2 wt.% with respect to GVL).
The catalytic activity in terms of furfural yield obtained using
WH-PTSA-220 is comparable to that of magnetic porous carbon-
based solid acids prepared from sawdust (61% within 6 h in
DMSO at 150°C), but lower than sulfonated carbon prepared by
a two-step procedure from D-glucose (70% within 20 min in
GVL
at
175°C)[7b]
and
calcined
co-polymerized
PTSA/paraformaldehyde (80% within 10 min in GVL at
170°C).[23] However, the catalysts reported here are prepared in
a single step from an abundant biomass substrate, with the
benefit that the use of corrosives, and high energy consumption
processes (pyrolysis, calcination), are avoided.
Solid acid catalyst reusability profiles
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Catalyst stability with prolonged use is a necessary requirement
in practical applications. As shown in Figure 6, solid acid
prepared at 240°C (WH-PTSA-240) exhibits the most favorable
reusability profile in both esterification and dehydration
processes, with 86% oleic acid conversion and 57% furfural
yields obtained after eight consecutive reaction cycles. Although
higher HTC temperatures result in lower acidity material the
more graphitic, hydrophobic nature of WH-PTSA-240 relative to
other WH solid acids may alleviate active site poisoning through
water adsorption over time. SEM images of fresh and reused
WH-PTSA-240 reveal little change in morphology.
Since the obtained carbon-based catalysts are nonporous
and have very low BET surface areas (< 7 m2/g) compared with
other catalyst systems used for esterification of oleic acid and
dehydration of xylose[8i]
, surface area and pore volume
contributions to catalytic activity in these materials should be low.
FT-IR spectra of the WH solid acids after five consecutive
esterification/dehydration cycles (Supporting Information) show
several differences on comparison with those of freshly prepared
material. Covering of the surface by reaction products, as
indicated by greater intensity C-H stretching vibrations in spent
catalysts, is expected to lead to further reduced surface area,
pore diameter, and pore volume, as indicated through prior work
by D'Agostino and co-workers.[32]
The deactivation or reduced catalytic performance of WH-
PTSA catalysts on prolonged use can be explained based on
FTIR data, as the result of active site contamination and some
leaching of acidic sulfonate/sulfonic species. Decreases in
−SO3H vibration intensities (1034 cm-1 and 1160 cm-1) relative to
fresh WH solid acids (Figure 2D) suggest leaching of, or gradual
poisoning of, these sites by water, with hydration of the catalyst
surface being evident through enhanced broadening of the O‒H
stretching vibration in reused catalyst materials. Furthermore,
NH3-TPD analysis (Supporting Information) confirmed reduced
amounts of weak- and moderate-to-strong acid sites in spent
catalyst material, in good agreement with IR data. Regeneration
of spent catalysts can be carried out by re-sulfonation, although
calcination is not appropriate as high-temperature treatment can
result in decomposition of sulfonate/sulfonic groups, as
addressed previously.[8a, 33]
Figure 6. Reusability of carbon-based solid acid catalysts in (A) esterification
of oleic acid with methanol (reaction temperature 65°C, time 7 h/cycle, catalyst
7 wt.%, oleic acid/methanol molar ratio 1:20), and (B) dehydration of xylose
(reaction temperature 170°C, time 3 h/cycle, catalyst 2 wt.% based on GVL).
these materials to other acid-catalyzed transformations, with
potential for integration of such materials in biorefining.
Experimental Section
Materials
p-Toluenesulfonic acid (AR, Carlo Erba), D-xylose (AR, Loba Chemie), -
valerolactone (≥98%, Sigma Aldrich), oleic acid (Panreac Applichem),
Conclusions
methanol (AR, Burdick
& Jackson), ethanol (AR, RCI Labscan),
Amberlyst-15 H+ form (Sigma Aldrich), Nafion NR50 (Sigma Aldrich),
sodium hydroxide pellets (AR, Merck), barium chloride dihydrate (AR,
Merck), and hydrochloric acid (AR, Carlo Erba) were used as received
without further purification.
Fresh WH was collected from Maha Sawat canal in Nakhon
Pathom, Thailand. After collection, the biomass was washed with large
quantity of tap water to remove dirt and insects. After sun-drying for 1-2
days, leaf material was oven-dried at 105°C for 12 h. The dried leaves
were powdered using a stainless steel high-speed grinder, and then
sieved to pass through a household sifter (40 mesh).
Carbon-based solid acid catalysts for esterification of oleic acid,
and dehydration of xylose to furfural were prepared in a single
step from HTC of readily obtainable water hyacinth biomass.
The temperature of the hydrothermal treatment has a profound
influence on the morphology, chemical composition, acid
property, catalytic activity, stability and reusability of the
obtained materials. The solid acid with highest acid density (WH-
PTSA-220) exhibits highest catalytic activity, although material
prepared at higher temperature (WH-PTSA-240) has a more
favorable reusability profile due to enhanced graphitization and
hydrophobicity. The robustness, high performance and facile
preparation of these materials from an under-utilized biomass
resource augur well for further studies expanding the scope of
One-step preparation of carbon-based solid acid catalysts
A modified methodology to that utilized for furfural-derived materials[9]
was employed for the one step HTC of ground dried WH biomass and
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PTSA. For this, 6.0 g of PTSA was dissolved in 20 mL of deionized water,
and the solution transferred to a 70 mL Teflon-lined stainless steel
autoclave containing 3.0 g of ground dried WH leaves. The autoclave
reactor was then heated in an oven at either 180, 200, 220, or 240°C for
10 h. After being allowed to cool to room temperature, the reactor was
opened and the resulting brown-to-black solid product was collected by
vacuum filtration and washed with hot deionized water until no sulfate
was detectable in the filtrate (using barium chloride solution). After drying
at 120°C overnight, the material was ground to a powder using a mortar
and pestle. A series of materials (WH-PTSA-T) were obtained, where T
denotes the hydrothermal temperature used. Carbonaceous material
from biomass alone without addition of PTSA was prepared at 240°C
(denoted as WH-240) as a control.
determined from the Brunauer-Emmett-Teller (BET) plot with pore
volume and pore size distribution calculated using Barrett-Joyner-
Halenda (BJH) analysis.
Esterification of oleic acid
Esterifications were carried out in a 100 mL glass (Ace) pressure tube
using 3.0 g of oleic acid, solid acid catalyst (5-10 wt.% with respect to
oleic acid) and methanol (10-20:1 molar ratio with respect to oleic acid).
After closing the tube reactions were conducted at a specific temperature
(35°C, 65°C, or 80°C) for a fixed time period (3-10 h). Once the reaction
was deemed complete the tube was cooled to room temperature, opened,
and the mixture transferred to a conical centrifuge tube for centrifugation
at 3000 rpm for 20 min to separate, and collect, the used catalyst. For
reusability tests, used catalyst sedimented in the centrifuge tube was
washed with 20 mL of methanol, transferred to a clean Ace tube, and
dried at 105°C overnight prior to exposure to fresh oleic acid and
methanol under the same reaction conditions.
Characterization of carbonaceous materials
Morphological investigations of prepared carbonaceous materials utilized
SEM imaging (JEOL JSM-6480LV). Samples were coated with gold prior
to imaging, and examined using 1000x magnification and with 15 kV
accelerating voltage.
Conversion of oleic acid to methyl oleate (representative of ester yield)
was determined from changes in acid value through titration,[30] and was
calculated as follows:
Powder X-ray diffraction (XRD) analysis was carried out on a Bruker D8
Advance X-ray diffractometer using Cu Kα radiation ( 1.54 Å) operating
at 40 kV and 35 mA, 2θ range 10°-60°, with a step size of 0.02° and at 2
s/step.
% Conversion = [(AVO–AVS)/AVO]100 (1)
where AVO and AVS are the acid values of oleic acid (before
esterification) and sample (after esterification), respectively.
Weight percentages of carbon, hydrogen, nitrogen, and sulfur (elemental
composition) for all samples were determined using a CHNS analyzer
(LECO CHN628 equipped with Sulfur Add-On Module).
Dehydration of xylose to furfural
Total acidity of carbon materials was determined by acid-base back
titration[12b]. For this, 0.05 g of sample was added to 20 mL of 0.01 M
sodium hydroxide solution. After shaking at 25°C for 24 h, the
supernatant solution was titrated with 0.01 M hydrochloric acid to end-
point using phenolphthalein as indicator.
Dehydration experiments were carried out in a 10 mL Pyrex glass tube
with screw cap kept at constant temperature (160°C, 170°C, or 180°C)
with magnetic stirring. Typically, 40 mg of xylose, 2.0 g of solvent (-
valerolactone, GVL), and catalyst (1-3 wt.% with respect to GVL) were
placed in the tube, the tube was closed, and the reaction mixture was
heated for a specific reaction time (30 min-4 h). After cooling to room
temperature the used catalyst was separated by centrifugation, and
washed with 20 mL of deionized water. For reusability testing the catalyst
was dried at 120°C overnight before exposure to fresh reagents under
the same reaction conditions.
Quantification of acid sites in material samples utilized NH3 temperature-
programmed desorption (NH3-TPD) performed on
a chemisorption
analyzer (Microtrac BELCAT-B) equipped with a thermal conductivity
detector (TCD). Prior to analysis, samples were heated at 150°C under
helium gas flow (50 mL·min-1) for 60 min. The temperature was
subsequently decreased to 50°C and NH3 was introduced under helium
flow (30 mL·min-1) for 30 min. Following this, physically adsorbed NH3
was removed by rapid helium gas flow (50 mL·min-1) for 15 min. The
NH3-TPD desorption was then performed over 50-800°C (heating rate
10°C·min-1). Identical sample weights were used for each measurement
to allow comparisons of intensity differences in acid site regions.
The amount of furfural in the liquid reaction mixture was quantified by
HPLC (Waters 2695 system, Nova-Pak C18 column, Waters 2487 UV
detector) with detection at 277 nm, using 0.1% acetic acid (v/v) in
acetonitrile as mobile phase. Furfural yields were calculated using
equation (2).
Furfural yield (%) = (furfural content (g)/initial wt xylose (g))100 (2)
Surface functionalization of materials was probed using Fourier transform
infrared (FT-IR), with spectra recorded as KBr discs over the range 400-
4000 cm-1 using a PerkinElmer Spectrum GX spectrophotometer.
Acknowledgements
X-ray photoelectron spectroscopy (XPS) was performed using a Kratos
Axis Ultra DLD instrument equipped with a 150W monochromatic Al Kα
source.
The authors are grateful to the financial support from the
Development and Promotion of Science and Technology Talents
project (DPST, Thailand) for a scholarship (J. L.) and partial
support from the Thailand Research Fund (Grant no.
IRG5980007). The authors also would like to thank Suranaree
University of Technology, Chiang Mai University, and
Thammasat University for instrumental support. J. L. would like
to give a special acknowledgement to Asst. Prof. Surajit
Tekasakul (Prince of Songkla University) for meaningful
Thermogravimetric analysis (TGA) measurements were performed on a
TA Instruments 2960 SDT V3.0F analyzer under N2 flow, with heating
rate of 10°C·min-1 from 30-700°C.
Surface area and porosity of carbon-based solid acids were studied by
N2 adsorption/desorption isotherms carried out on a Quantachrome
Autosorb iQ gas sorption analyzer at -196°C. Specific surface areas were
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Entry for the Table of Contents (Please choose one layout)
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Carbon-based solid acid catalysts for
esterification of oleic acid, and
dehydration of xylose to furfural were
prepared in a single step from
Jutitorn Laohapornchaiphan,
Christopher B. Smith,* Siwaporn
Meejoo Smith*
Page No. – Page No.
hydrothermal carbonization of
abundant water hyacinth biomass.
One-step preparation of carbon-
based solid acid catalyst from water
hyacinth leaves for esterification of
oleic acid and dehydration of xylose
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