A green approach for enhancing the hydrophobicity of UiO-66(Zr) catalysts for biodiesel production at 298 K

Article abstract of DOI:10.1039/d0ra08217a

Recently, the incorporation of hydrophobicity on the surface of UiO-66(Zr) has received much attention due to the deactivation of hydrophilic active sites of UiO-66(Zr) upon water adsorption. In this work, we report UiO-66(Zr) catalysts with an assortment of surface hydrophobicities fabricated by the solvent-free method to elucidate the impact of the environment framing Lewis acid sites on their catalytic activity in the production of fatty acid methyl ester (biodiesel) via the esterification of fatty acids at room temperature with high selectivity (100%) and good recyclability. A detailed structural analysis of the materials by N2 sorption, FT-IR, SEM, XRD, water contact angle measurement, dynamic liquid scattering (DLS), NMR and TGA revealed the fabrication of stearic acid-grafted UiO-66(Zr) catalysts (10SA/UiO-66) with fine particle size and a highly hydrophobic network. 10SA/UiO-66(Zr) with enhanced hydrophobicity exhibited superior catalytic performance in the esterification of a fatty acid with a long alkyl chain compared with conventional solid acid catalysts and even liquid acid catalysts. Detailed kinetic studies corroborated that the adsorption of lipophilic acids at the Lewis acid sites besides the enhancement of wettability between the reactants was facilitated by the hydrophobic environment, thus significantly motivating the esterification reaction at room temperature. Furthermore, 10SA/UiO-66(Zr) showed good catalytic activity in the esterification of oleic acid in the presence of water (～10% in the light of acid weight).

Full text of DOI:10.1039/d0ra08217a

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A green approach for enhancing the
hydrophobicity of UiO-66(Zr) catalysts for biodiesel
Cite this: RSC Adv., 2020, 10, 41283
production at 298 K†
ab
a
b
*
Ahmed M. El-Nahas
*
Ahmed S. Abou-Elyazed,
and Ahmed M. Yousifbc
Yinyong Sun,
Recently, the incorporation of hydrophobicity on the surface of UiO-66(Zr) has received much attention
due to the deactivation of hydrophilic active sites of UiO-66(Zr) upon water adsorption. In this work, we
report UiO-66(Zr) catalysts with an assortment of surface hydrophobicities fabricated by the solvent-free
method to elucidate the impact of the environment framing Lewis acid sites on their catalytic activity in
the production of fatty acid methyl ester (biodiesel) via the esterification of fatty acids at room
temperature with high selectivity (100%) and good recyclability. A detailed structural analysis of the
materials by N2 sorption, FT-IR, SEM, XRD, water contact angle measurement, dynamic liquid
scattering (DLS), NMR and TGA revealed the fabrication of stearic acid-grafted UiO-66(Zr) catalysts
(10SA/UiO-66) with fine particle size and a highly hydrophobic network. 10SA/UiO-66(Zr) with
enhanced hydrophobicity exhibited superior catalytic performance in the esterification of a fatty acid
with a long alkyl chain compared with conventional solid acid catalysts and even liquid acid catalysts.
Detailed kinetic studies corroborated that the adsorption of lipophilic acids at the Lewis acid sites
besides the enhancement of wettability between the reactants was facilitated by the hydrophobic
environment, thus significantly motivating the esterification reaction at room temperature.
Furthermore, 10SA/UiO-66(Zr) showed good catalytic activity in the esterification of oleic acid in the
presence of water (ꢀ10% in the light of acid weight).
Received 25th September 2020
Accepted 28th October 2020
DOI: 10.1039/d0ra08217a
rsc.li/rsc-advances
One of the most signicant forms of biofuel is biodiesel,
which is produced by the transesterication of vegetable oils/
1. Introduction
animal fats or fatty acids with small-molecular-weight alco-
hols, such as methanol and ethanol; methanol is the most
widely utilized in this type of reaction owing to its strong
nucleophilicity and low price.6 Methanolysis reactions of vege-
table oils or fatty acids occur in the existence of homogenous
catalysts, which can be basic, acidic or an enzyme.7–10 Homog-
enous base and acid catalysts are common in the industrial
production of biodiesel as they have a high ability to accelerate
the reaction under mild conditions and are less energy-
intensive. However, there are some difficulties that limit the
utilization of homogeneous base catalysts, including the diffi-
culty in their separation from the reaction mixture,11 in addition
to the need for a large quantity of water to compensate and
rene the products.12 Economically, they increase biodiesel
production costs.
The dependence of the world on fuel has reached notable levels.
It is necessary to nd alternative sources of renewable energy,
such as solar energy, wind energy, hydroelectric energy and
biofuel energy, in order to increase the energy security of the
world.1 Therefore, renewable fuels are an ideal way to solve this
serious problem. Biofuels obtained from biomass are consid-
ered to be one of the most promising renewable sources of
energy.2 They are characterized by the absence of toxic oxides
(CO2, SOx) and thus limit the process of global warming, air
pollution, and acid rains. In general, they have reduced health
risks compared to fossil fuels.3–5
aMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and
Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin,
150001,
China.
E-mail: yysun@hit.edu.cn;
Tel:
Recently, heterogeneous catalysts have been introduced to
solve the aforementioned problems because of their many
advantages, including easy separation from the reaction
mixture, environmental friendliness and good reusability.13,14
Various types of heterogeneous catalysts, such as ion-exchange
resins,15 alkoxides,16,17 Mg–Al hydrotalcites,18–20 metal oxides
and hydroxides,21–24 are used in biodiesel production by the
+86-45186413708
bChemistry Department, Faculty of Science, Menoua University, Shebin El-Kom,
Egypt. E-mail: amelnahas@hotmail.com; Tel: +20 1064607974
cChemistry Department, College of Science and Arts, Jouf University, Alqurayyat,
Kingdom of Saudi Arabia
† Electronic supplementary information (ESI) available: N2 sorption isotherms of
the prepared samples, textural and crystallinity features of the catalysts and
surface roughness values of the catalysts. See DOI: 10.1039/d0ra08217a
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alcoholysis of vegetable oils, animal fats or various fatty acids. active sites by graing a saturated fatty acid (stearic acid) using
Heterogeneous base catalysts are more efficient in activating the a one-pot and solvent-free method. This promoted the hydro-
reaction than acid catalysts. However, the crude oils contain phobicity of the UiO-66(Zr) network without decreasing the
higher free fatty acids (FFAs) and water, which largely decrease reactivity of the zirconium sites. Thus, this facile and low-cost
the catalytic activity and reusability of the base catalysts as strategy can be widely applied with other types of MOFs and
a result of soap formation and deactivation of active sites.25 In in other elds of heterogeneous catalysts to achieve a highly
contrast, acid catalysts can promote both esterication of free active acid catalyst for biomass conversion.
fatty acids (FFAs) and transesterication of triglycerides with
alcohol, even in the presence of a little amount of water.26 A high
percentage of water that is present in the crude oil or produced
as a byproduct from the reaction may be adsorbed on the
2. Experimental procedures
2.1. Materials
catalysts due to the unique hydrophilic nature of the acidic sites
on their surface and then cause the deactivation of the catalytic
sites or hydrolysis of the frameworks.27–29 In addition, many
metal-based Lewis acids tend to decompose into metal
hydroxides in water-containing environments.30 Therefore, the
development of solid acids with appropriate hydrophobicity can
largely overcome these issues and upgrade the catalytic activity
and reusability.
It is well-known that UiO-66(Zr) is a metal–organic frame-
work (MOFs) that shows exceptional stability, high activity and
excellent reusability in various organic reactions.31,32 These
features allow its use in a wide scope of thermal and chemical
conditions. To the best of our knowledge, the key to their
stability is the high topological connectivity of the
[Zr6O4(OH)4]12+ secondary building unit (SBU), which is
attached through the strong Zr–O bonds to 12 terephthalate
(BDC) linkers. Thus, defects are created in the structure of
UiO-66(Zr) when one or more linkers are replaced or removed
from the crystalline network.33 Experimentally, these defects
are responsible for the enhancement in the catalytic activity of
UiO-66(Zr) in various organic reactions,34,35 since losing one or
more linkers may introduce coordinately unsaturated Zr sites
in the solid, resulting in the enrichment of open Lewis acid
sites.
To date, most of the porous solid acid catalysts are
hydrophilic owing to the hydrophilic nature of their frame-
works and the acidic sites on their surfaces. Although some
organic frameworks, such as carbonaceous materials, are
naturally hydrophobic.36,37 Water, a typical byproduct and the
negative component of acid-catalyzed reactions, coadsorbs on
the surface of hydrophilic solid acids, causing partial deac-
tivation of the acidic sites and hydrolysis of the frameworks in
some cases.38 Recently, numerous investigations have been
done on the fabrication of solid acid catalysts with a hydro-
phobic environment surrounding the active sites in the
catalysts, which facilitate the adsorption of oleophilic mate-
rials and desorption of hydrophilic materials, such as water,
All chemicals were used directly as received: zirconyl chloride
octahydrate (ZrOCl2$8H2O), 1,4-benzenedicarboxylic acid
(BDC), stearic acid (Tianjin Guangfu, 98%), MIL-100(Fe) (Tian-
jin Guangfu), ZMS-5 (Tianjin Guangfu), bentonite (Innochem,
98%), levulinic acid (Innochem, 98%), oleic acid (Tianjin Zhi
Yuan, 98%), acetic acid (Sinopharm, 99.5%), methanol (Sino-
pharm, 99.7%), ethanol (Sinopharm, 99%), ethyl acetate
(Aladdin, 99.5%), toluene (Sinopharm, 99.5%) and potassium
hydroxide (Sinopharm, 99.5%).
2.2. Catalyst preparation
Stearic acid-graed UiO-66(Zr) was synthesized by a green
method; ZrOCl2$8H2O (1.5 mmol) as the metal precursor, 1,4-
benzenedicarboxylic acid (BDC) as the organic linker (1.5
mmol) and 10% of stearic acid (according to the weight of the
BDC linker) were ground together for about 10 min at room
temperature. Then, the powder was moved into an autoclave at
130 ꢁC for 24 h. Aer cooling to room temperature, the obtained
white solid was washed with 70 ꢁC ethanol and dried for 24 h at
150 ꢁC under vacuum, as stated in the previous work.41 The
catalyst was referred to as 10SA/UiO-66(Zr).
Moreover, UiO-66(Zr)-green was synthesized as described in
the literature,42 and stearic acid-graed UiO-66(Zr) was
prepared by a solvothermal method, as described below:
ZrOCl2$8H2O (1.5 mmol) as the metal precursor, BDC as the
organic linker (1.5 mmol) and 10% stearic acid based on the
weight of BDC were dissolved together in 25 mL DMF in
a Teon-lined stainless-steel autoclave at room temperature.
The obtained homogeneous mixture was closed and placed in
a pre-heated oven for 24 h at 130 ꢁC. Aer cooling to room
ꢁ
temperature, the resulting gel was washed with 70 C ethanol
and soaked in CH2Cl2 for 2 days and then centrifuged and dried
for 24 h at 150 ꢁC under vacuum. The catalyst was referred to as
10SA/UiO-66(Zr)-solvent.
2.3. Characterization of the formed catalyst
on their surface. For instance, Chen and coworkers reported All the prepared samples were characterized using different
Pt@UiO-66@GO/rGO as a solid hydrophobic catalyst for the techniques, such as BET, FT-IR, powder-XRD, SEM-EDX, NMR
hydrogenation of nitrobenzene and p-nitrophenol into cor- and TGA.
responding amino derivatives.39 Further, Du and coworkers
Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda
immobilized Aspergillus niger lipase (ANL) on hydrophobic (BJH) methods were used to estimate the surface area and pore
UiO-66-PDMS to form
production.40
a
solid catalyst for biodiesel volume. The nitrogen sorption isotherms were recorded at
ꢂ196 ꢁC on a 3H-2000PS1 Gas Sorption and Porosimetry system
Inspired by the previous efforts, we prepared UiO-66(Zr) with for determining the surface area and pore characteristics. The
defects and enhanced the hydrophobic environment around the samples were regularly arranged for examination aer
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degassing at 150 ꢁC under vacuum until the nal pressure
reached 1 ꢃ 10ꢂ3 torr.
kl
D ¼
(2)
b cos q
The acidity of the UiO-66(Zr) samples was recorded with and
without stearic acid incorporation. All the samples were acti-
vated by degassing for 2 h at 150 ꢁC, then cooled under vacuum
and saturated with liquid pyridine. The samples were again
heated to 150 ꢁC to liberate the physisorbed pyridine. The FTIR
spectra were recorded with the KBr-pellet technique using
a Bruker Equinox 55 Fourier transform infrared spectropho-
tometer, and the diffuse reectance spectra were scanned over
the range of 1000–1800 cmꢂ1 with a resolution (2 cmꢂ1) of 100
scans per measurement.
where D, k, l, b and q are the crystallite size (nm), Scherrer
constant (0.9), wavelength (1.5418 nm), the full width at half-
maximum-height of the peak FWHM (radian) and the peak
position (radian), respectively.
The scanning electron microscopy (SEM) images were
recorded on a SUPRA 55 at an acceleration voltage of 20 kV.
Dynamic light scattering (DLS) was performed with an ALV-
5000/E DLS instrument (Malvern Instruments, U.K.) at a xed
scattering angle of 90ꢁ aer ltering the samples through 0.45
mm Millipore lters.
The XRD patterns were obtained on a Rigaku D/Max-2550
diffractometer furnished with a SolX detector and Cu Ka radi-
The 1H NMR spectra were recorded on a Bruker Advance 400
MHz spectrometer with a digested deuterated mixture as the
solvent at room temperature; the chemical shis (d) are
expressed in ppm.
The thermogravimetric analysis (TGA) was conducted on
a Shimadzu TA-50. The Fourier transform infrared (FT-IR)
spectra were acquired on a NicoLET iS10 spectrometer.
The water contact angles were measured with a CAST V2.6
(Solon Tech. Shanghai Co., Ltd., China). The volume of the
water droplet was xed at 3.0 mL, and the contact angle was
estimated at 30 s aer attachment to the material surface.
ꢀ
ation with l ¼ 1.5418 A. The data were recorded with step
scanning at 2q ¼ 0.02ꢁ per second from 5ꢁ to 50ꢁ. The inuence
of stearic acid on the crystallinity and particle size of UiO-66(Zr)
were investigated by using the crystallinity degree and Scherrer
equations, respectively, as seen below:
area of crystalline peaks
Crystallinity ¼
area of all peaks ðcrystalline þ amorphousÞ
ꢃ 100
(1)
Fig. 1 XRD patterns of the samples: (a)–(c) are the magnifications of the local XRD patterns in different 2q ranges.
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2.4. Calculation of the acid site density
This was performed with 50 mg of each catalyst aer pretreat-
ꢁ
ment at 150 C for 2 h in the air before exposure to the probe
molecule (pyridine). 15–20 mg ofꢁpyridine-covered samples were
subjected to heating up to 150 C for 2 h under vacuum. The
mass loss because of pyridine desorption from the acidic sites
was computed as a function of the total surface acidity at the
sites (gꢂ1). The equation used to compute the acid site density is
as follows:43
Acid site density ¼
moles of pyridine adsorbed ꢃ Avogdro’s number ðsites per molÞ
wt of catalystðgÞ ꢃ BETðm2 gꢂ1Þ
(3)
Fig. 2 The N2 sorption isotherms and pore distribution curves (inset)
of the samples.
2.5. Catalyst activity
The esterication reactions of oleic acid (fatty acid) with
methanol to form methyl oleate (biodiesel) were performed in
a batch reactor at 298 K, where 1 mmol of acid and 39 mmol of
methanol were brought in contact with the catalyst (6% of the
weight of acid), according to previous work.42 Aer esterica-
tion, the catalyst was separated from the reaction medium by
ltration and reused in the next esterication run. The
conversion of oleic acid was determined using 0.1 M alcoholic
KOH as the titrant in the presence of phenolphthalein as the
indicator. The volume of KOH consumed was noted; the
conversion of free fatty acid (FFA) and the product selectivity
were estimated using eqn (4) and (5). The biodiesel was also
quantitatively analyzed by gas chromatography on an Agilent
7890A GC with an FID detector, and the detailed outcomes were
consistent with a standard deviation of ꢄ0.01–0.025 mass%. All
data are from triplicate experiments.
particle sizes. These results match well with those from the SEM
and DLS analyses. Undoubtedly, the addition of stearic acid
inuenced the coordination between the Zr component and the
organic linker.
The N2 sorption results of the UiO-66(Zr)-green and 10SA/
UiO-66(Zr) samples are shown in Fig. 2. The samples UiO-
66(Zr)-green and 10SA/UiO-66(Zr) exhibited type I isotherms at
relatively low pressure, indicating the presence of micropores.
Additionally, the adsorption data listed in (Table S2†) reveal
that 10SA/UiO-66(Zr) possessed a higher BET surface area (1150
m2 gꢂ1) and pore volume (0.92 cm3 gꢂ1) than those of UiO-
66(Zr)-green (701 m2 gꢂ1, 0.68 m3 gꢂ1), suggesting that stearic
acid had graed into the framework of UiO-66(Zr) and did not
accumulate inside the pores of UiO-66(Zr).
The FT-IR spectra of stearic acid (SA), UiO-66(Zr)-green and
10SA/UiO-66(Zr) were used to characterize the functional groups
on the organic linkers in the prepared materials and to eluci-
date the interaction between stearic acid and the zirconium
ions in the UiO-66(Zr) matrix. As shown in Fig. 3, the FT-IR
spectrum of stearic acid (SA) showed peaks centered around
ai ꢂ af
% Conversion ¼
ꢃ 100
(4)
ai
aj
ai ꢂ af
% Selectivity ¼
ꢃ 100
(5)
where ai, af, aj are the initial and nal concentrations of the
sample and the molar concentration of the product,
respectively.
3. Results and discussion
3.1. Structural characterization
The structural integrity of UiO-66(Zr)-green and 10SA/UiO-
66(Zr) was examined by XRD, as shown in Fig. 1. The peaks of
10SA/UiO-66(Zr) were in good agreement with the characteristic
peaks of pristine UiO-66(Zr), which revealed that the crystal
structure of UiO-66(Zr) was well maintained. However, the
differences in the peak intensity in the diffractogram of 10SA/
UiO-66(Zr) compared with that of UiO-66(Zr) suggested
a change in the crystallinity degree and particle sizes due to the
incorporation of stearic acid, as presented in Table S1.†
As seen in Fig. 1, the crystallinity slightly decreased, and
Fig. 3 FT-IR spectra of (a) stearic acid, (b) Zr-stearate, (c) UiO-66(Zr)-
stearic acid assisted the formation of UiO-66(Zr) with smaller green and (d) 10SA/UiO-66(Zr).
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UiO-66(Zr). Therefore, we are condent that the stearic acid
peaks revealed by the NMR analysis aer activation arise from
the molecules graed into the UiO-66 framework.
The SEM images of the UiO-66(Zr)-green and 10SA/UiO-
66(Zr) samples are displayed in Fig. 5. Both samples illus-
trated the same morphology but different crystal sizes. UiO-
66(Zr)-green seemed to have an irregular morphology with
a crystal size of around 200 nm (Fig. 5a), while 10SA/UiO-66(Zr)
exhibited irregular shape and crystal sizes below 150 nm
(Fig. 5b). This indicates the introduction of stearic acid had an
inhibition effect on the crystal growth of UiO-66(Zr), which
could explain the reason why the surface area and pore volume
of UiO-66(Zr) were increased aer the addition of stearic acid.
Further, the dynamic liquid scattering curves proved that
10SA/UiO-66(Zr) possessed a smaller particle size than UiO-
66(Zr)-green (Fig. 6).
Fig. 4 1H NMR spectra of the UiO-66(Zr)-green and 10SA/UiO-66(Zr)
samples.
Additionally, the SEM images were simulated to display the
surface roughness of UiO-66(Zr)-green and 10SA/UiO-66(Zr)
using the Image J soware. Fig. S1† shows the surface rough-
ness plot and particle size distribution of both samples.
Notably, the surface roughness value for 10SA/UiO-66(Zr) was
higher than that for UiO-66(Zr)-green (more than 35%)
(Fig. S2†), suggesting that possibly stearic acid has an effect on
the wettability of the material.
2830 cmꢂ1 and 2920 cmꢂ1, which were assigned to the
stretching vibrations of the CH2-groups (Fig. 3a). Moreover, the
peak located at around 1700 cmꢂ1 belonged to the stretching
vibration of the carbonyl group from stearic acid, which over-
lapped with the carbonyl group of the BDC linker and gave two
new characteristic peaks at 1480 and 1600 cmꢂ1 with high
intensities (Fig. 3d). Moreover, the disappearance of the char-
acteristic peak at 1670 cmꢂ1 corresponding to stearic acid
indicated that chemical interaction between stearic acid and
zirconium ions had occurred.44,45 Notably, the FT-IR spectra of
UiO-66(Zr) and 10SA/UiO-66(Zr) (Fig. 3c and d) exhibited
a strong and broad band centered at 3440 cmꢂ1 due to the
condensation of crystalline and physisorbed water inside the
cavities.46
The results from the thermogravimetric analysis indicated
that 10SA/UiO-66(Zr) possessed high thermal stability similar to
The organic components (e.g., BDC, stearic acid) were iden-
1
tied by H liquid NMR spectroscopy. As seen in Fig. 4, there
were only distinct resonance signals, which could be con-
dently assigned to the BDC-linker, Zr–OH and stearic acid. This
is an early indication that stearic acid (SA) indeed compensates
for the defects in the UiO-66(Zr) framework. However, it is of
extreme signicance to ascertain whether stearic acid (SA) is
actually graed into the UiO-66(Zr) framework or simply trap-
ped in the pores as a free acid. We washed and activated the
samples before the NMR analysis to conrm that all stearic acid
(SA) molecules were successfully removed from the pores of
Fig. 6 The dynamic liquid scattering (DLS) curves of UiO-66(Zr)-green
and 10SA/UiO-66(Zr).
Fig. 5 SEM images of (a) UiO-66(Zr)-green and (b) 10SA/UiO-66(Zr).
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Fig. 7 TGA curves of (a) UiO-66(Zr)-green and (b) 10SA/UiO-66(Zr).
UiO-66(Zr) prepared via green and conventional methods.41 In the dehydroxylation of the zirconium oxo-clusters. The third
Fig. 7, two distinct weight-loss regions can be identied for UiO- weight-loss above 400 ꢁC was attributed to the decomposition of
66(Zr); the rst one before 400 ꢁC could be assigned to the UiO-66 as a result of the burning of organic linkers (BDC and
removal of adsorbed water from different sites. The second one stearic acid) in the framework.
above 400 ꢁC resulted from the decomposition of the frame-
Interestingly, to elucidate the inuence of stearic acid on the
work. In the case of 10SA/UiO-66(Zr), the initial weight-loss wettability, the water contact angles for UiO-66(Zr)-green and
occurring in the temperature range of 25–90 ꢁC was due to 10SA/UiO-66(Zr) were measured (Fig. 8). In the case of 10SA/
the removal of physisorbed water, whereas the second weight- UiO-66(Zr), a contact angle of around 107.5ꢁ was recorded,
loss observed in the temperature range of 90–235 ꢁC was while UiO-66(Zr)-green illustrated a contact angle of around
related to the removal of the aliphatic chain of stearic acid and 81.6ꢁ. Furthermore, the strong affinity of 10SA/UiO-66(Zr)
Fig. 8 Contact angles of static water over (a) UiO-66(Zr)-green and (b) 10SA/UiO-66(Zr).
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Fig. 9 The pyridine FT-IR spectra of UiO-66(Zr)-green and 10SA/UiO-
66(Zr).
Scheme 1 Proposed scheme for the esterification of oleic acid over
the 10% SA/UiO-66(Zr) catalyst.
toward the hydrophobic medium was also conrmed by the
dispersion properties in two different liquid phases (water and
ethyl acetate), as presented in Fig. 8. The catalysts (50 mg) were
dispersed in a water–ethyl acetate (1 : 1 v/v) mixture. Most of the
UiO-66(Zr)-green particles preferentially dispersed between the
two phases to give a suspension (Fig. 8a). However, 10SA/UiO-
66(Zr) was completely distributed in the hydrophobic ethyl
acetate phase (Fig. 8b). The strong affinity for the hydrophobic
phase could be assigned to the enhancement in the hydro-
phobicity of 10SA/UiO-66(Zr) that resulted from the introduc-
tion of stearic acid into the UiO-66(Zr) framework.
Lastly, to conrm the acidity of 10SA/UiO-66(Zr) and UiO-
66(Zr)-green, we recorded the FT-IR spectra of pyridine
adsorption on UiO-66(Zr)-green and 10SA/UiO-66(Zr), as depic-
ted in Fig. 9. Fig. 9 indicates that 10SA/UiO-66(Zr) contained
more acidic sites (Lewis & Bronsted acid sites) than UiO-66(Zr)-
green, which could be attributed to the missing BDC linkers
that connect to the Zr centers during the preparation. Further,
calculations proved that 10SA/UiO-66(Zr) had a higher acid site
density than UiO-66(Zr)-green (Table S1†).
ꢀ94.5%, and the turnover frequency (TOF ¼ 1.45 minꢂ1) was
calculated according to the following eqn (6).42
XVorMc
TOF ¼
(6)
WsMrT
where X, Vo, Mc, Mr, Ws, T are the conversion of the substrate,
the volume (mL) of the substrate, the density (g mLꢂ1) of the
substrate, the molar mass (g molꢂ1) of the catalyst, the molar
mass (g molꢂ1) of the substrate, the weight (g) of the catalyst,
and reaction time (min), respectively.
The pore volume of 10SA/UiO-66(Zr) was enhanced
compared with that of UiO-66(Zr)-green (by about 26%) upon
treatment with stearic acid (Table S2†). The enlargement in pore
volume is explained in terms of the one or more missing BDC
linkers, which lead to the expansion of pore volume and a slight
increase in the acidity of the zirconium sites, as displayed in
Fig. 9. The combination of both acidity and pore volume, beside
All these results demonstrate the advantages that UiO-66(Zr)-
green had gained by the green graing of stearic acid in the
UiO-66(Zr) network. This increases the hydrophobic environ-
ment around the zirconium active sites, enhances the acidic
sites (open Zr-sites) by the absence of one or more BDC linkers
during synthesis and increases the surface area and pore
volume by decreasing the particle size of UiO-66(Zr).
3.2. Catalytic performance evaluation
Upon testing the catalytic performance of 10SA/UiO-66(Zr), UiO-
66(Zr)-green and other prepared samples in the esterication
reaction of oleic acid with methanol to form methyl oleate
(biodiesel), as shown in Scheme 1, it was found that 10SA/UiO-
66(Zr) exhibited the highest catalytic activity among these
catalysts (Fig. 10).
Indeed, the superiority of 10SA/UiO-66(Zr) over UiO-66(Zr)-
green and other catalysts in motivating the esterication reac-
tion between the two different liquid phases (oleic acid and
Fig. 10 Catalytic activities of the samples in the esterification reaction
methanol) at 298 K was evident with a high yield estimated at of oleic acid with methanol.
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produces an excellent solid acid catalyst not only for direct
biodiesel production but also for many organic reactions that
take place in an aqueous medium, such as the hydrolysis of
esters.
3.4. Comparison of catalytic activities and adsorption
capabilities
The catalytic performances of 10SA/UiO-66(Zr) and other re-
ported homogeneous and heterogeneous acid catalysts were
tested in the esterication reaction of oleic acid with methanol
at 298 K, as shown in Fig. 12a. With the exception of H2SO4,
which gave the same yield, the results revealed the superiority of
10SA/UiO-66(Zr) as a solid catalyst over the other reported
catalysts. However, the utilization of sulfuric acid as a homo-
geneous acid was accompanied by high energy consumption for
catalyst separation and disposal, in addition to product puri-
cation. In contrast, 10SA/UiO-66(Zr) as a heterogeneous acid
catalyst could easily be separated from the reaction mixture and
reused. To understand the high catalytic performance of 10SA/
UiO-66(Zr) compared with other reported catalysts, such as MIL-
100(Fe), UiO-66(Zr), ZMS-5, Amberlyst-15 and bentonite, the
adsorption capabilities of the catalysts for oleic acid were
examined, and the results are displayed in Fig. 12b. Each solid
acid (0.1 g) was stirred in a mixture of oleic acid (0.05 g) and
toluene (5 mL) at room temperature. Aer 1 h, oleic acid
concentration in the solution was quantitatively analyzed by
titration and gas chromatography (GC). The results revealed
Fig. 11 Catalytic performance of 10SA/UiO-66(Zr) in the esterification
of oleic acid with methanol in the presence of water at 298 K. The
water content was 2–10 vol% with respect to the amount of acid. Red
and green bars represent the yield after 2 h and 4 h, respectively.
the hydrophobic environment surrounding the acidic sites,
apparently encourages the adsorption of reactants on the solid
acid sites and facilitates the diffusion of reactants and products
from the bulk solution to the surface of the solid catalyst and
vice versa.43,47
3.3. Water tolerance of the 10SAUiO-66(Zr) catalyst
It is known that the acidic/basic sites on conventional solid
acid/base catalysts are largely deactivated by water.48 Therefore,
the water tolerance test is of practical importance because it can
well reect the performance of the catalysts in biodiesel
production from water-containing oil. Consequently, the
impact of water content on the catalytic performance of 10SA/
a
high adsorption capability for 10SA/UiO-66(Zr) toward
hydrophobic oleic acid due to the hydrophobic nature and
microporous structure of the framework.
3.5. Leaching test
UiO-66(Zr) was examined in the esterication reaction of oleic Generally, leaching of the active metal species into the reaction
acid by the addition of various volumes of water to the reaction medium causes contamination of the nal product and also
medium at 298 K. The catalytic results are displayed in Fig. 11. leads to a decrease in catalyst activity. Thus, we tested the
The conversion over 10SA/UiO-66(Zr) reached ꢀ94.5% within catalytic heterogeneity of the reaction by the hot ltration
4 h in the absence of water. But the conversion over 10SA/UiO- method.49 The catalyst was separated from the reaction medium
66(Zr) aer 4 h slightly decreased with an increasing volume of by ltration aer 5 min (with 83% yield of methyl oleate), and
water in the reaction up to 11% when a large volume of H2O the reaction was performed with the ltrate under the same
(10 vol%) was utilized. This result indicates that the hydro- reaction conditions. The results of these experiments (pre-
phobic modication of UiO-66(Zr)-green by graing stearic acid sented in Fig. S5†) showed that the reaction stopped with the
Fig. 12 Comparison of the (a) catalytic activities and (b) adsorption capabilities of 10SA/UiO-66(Zr) and other reported catalysts for oleic acid.
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Fig. 13 (a) Reusability of 10SA/UiO-66(Zr) in the reaction of oleic acid with methanol; (b) SEM image, (c) N2 sorption isotherms and (d) XRD
patterns of the spent catalyst.
removal of the solid catalyst (10SA/UiO-66(Zr)). These results 66(Zr) could almost maintain the structural properties aer
demonstrated that the catalytic system did not behave like reuse, suggesting that 10SA/UiO-66(Zr) was stable. Undoubt-
a homogeneous system.
edly, the adsorption of unreacted oleic acid and products on the
catalyst surface led to a decrease in surface area (Fig. 13c).
Furthermore, the FT-IR results (Fig. S4†) of the spent catalysts
showed two sharp absorption bands at 2855 cmꢂ1 and
2960 cmꢂ1 assigned to the symmetric and asymmetric –CH2
stretching vibrations from oleic acid, respectively, suggesting
that unreacted oleic acid might be adsorbed onto the surface of
the catalysts and could block the active sites.
3.6. Catalyst reusability
Since cost is the major concern in biodiesel production, the
development of recyclable solid catalysts has gained great
attention for ease of separation, low cost and lower environ-
mental problems. The recyclability of 10SA/UiO-66(Zr) was
evaluated in the esterication reaction of oleic acid with
methanol at 298 K. Aer each cycle, the catalyst was separated
and rinsed with methanol three times to remove adsorbed oleic
acid. Then, the product was dried and activated for 6 h at 150 ꢁC
for the next cycle. As displayed in Fig. 13a, the yield aer six
3.7. Kinetic study of the esterication reaction over 10SA/
UiO-66(Zr)
cycles was maintained above 83%. These results indicated that The kinetic behavior of 10SA/UiO-66(Zr) in the esterication
the catalyst can be reused with no signicant loss of activity. To reaction of oleic acid was studied under the optimum condi-
detect the structural stability of 10SA/UiO-66(Zr), the porosity, tions of 6 wt% catalyst loading and oleic acid to methanol molar
crystallinity and surface morphology of the spent catalyst were ratio of 1 : 39 in the temperature range of 298–353 K (Fig. 14).
also examined (Fig. 13). The result indicated that 10SA/UiO- The conversion of oleic acid increased with the reaction
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CA ¼ CA0(1 ꢂ x)
(8)
where CA and x are the initial concentration and conversion of
0
oleic acid, respectively. Thus, the integration of the above
equation for concentration CA with regard to time t gives the
rate constant k of the nth order, as shown in eqn (9).
ln(1 ꢂ x) ¼ kt
(9)
The activation energy was estimated from the plot of (ln k)
against (1/T), as derived from the Arrhenius equation:
k ¼ Ae(ꢂE /RT)
(10)
a
where Ea is the activation energy, A is the frequency factor, R is
the universal gas constant, and T is the absolute temperature.
Fig. 15 illustrates the linear relationship between ꢂln(1 ꢂ x) and
time at different reaction temperatures (Fig. 15a) and the
Arrhenius plot of (ln k) versus (1/T) (Fig. 15b). This plot gives
a straight line with a slope of (Ea/R) and an intercept of (ln A).
Table 1 elucidates an increase in the rate constant with rising
temperature.52
The derived activation energy over 10SA/UiO-66(Zr) was
32.53 kJ molꢂ1, which is lower than the previously reported
values (46.69 and 50.74 kJ molꢂ1).53,54 This result claries the
vital role of the combining effect of hydrophobicity, acidity and
pore volume in enhancing the rate of the methanolysis of oleic
acid by decreasing the energy barrier (Ea). Furthermore, the
activation energy value conrmed that the esterication
Fig. 14 The effect of reaction temperature on the conversion of oleic
acid over the 10SA/UiO-66(Zr) catalyst under optimized reaction
conditions.
temperature.50,51 The overall reaction is depicted in the
following equation:
k1
*
Oleic acid þ methanol ) oleic acid MEðbiodieselÞ þ water
k2
The overall reaction rate (rA) was evaluated based on the
following equations:
n
rA ¼ ꢂ(dCA/dt) ¼ kCA
(7)
Fig. 15 (a) The linear relationship between ꢂln(1 ꢂ x) vs. t at different temperatures; (b) Arrhenius plot of ln k vs. 1/T.
Table 1 Temperature dependency of the rate constant and calculated activation energy
Order of the
reaction (n)
Temp. (K)
Rate constant k (hꢂ1
)
Activation energy (Ea) (kJ molꢂ1 Kꢂ1
)
Frequency factor (A ꢃ 10ꢂ5
)
298
313
333
343
353
0.9945
0.9973
1.0135
1.0000
1.0104
0.0441
0.0235
0.2288
0.0976
0.2753
32.53
6.45
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reaction was kinetically controlled not diffusively controlled.54
This suggests that the esterication reaction over 10SA/UiO-
66(Zr) was chemically controlled and not by diffusion or mass
transfer limitations.
from Jatropha oil using calcined waste animal bones as
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4. Conclusions
In short, graing of stearic acid on UiO-66(Zr) by a facile route
increased the hydrophobicity of the framework. The enhance-
ment in hydrophobicity encourages the adsorption of oleophilic
material (oleic acid) and prevents the ester molecules from
being hydrolyzed back into the reactant by desorption of the
hydrophilic material (water) on the surface. Further, the surface
area and pore volume of UiO-66(Zr) greatly increased on
decreasing the particle size. These merits are important in
catalyzing the esterication reaction of oleic acid at 298 K to
produce biodiesel. Moreover, the catalyst could still maintain
high conversion when the reaction was carried out in the
presence of different quantities of water, which conrms the
role of the hydrophobic character acquired by graing stearic
acid in UiO-66(Zr). Furthermore, 10SA/UiO-66(Zr) exhibited
excellent reusability with negligible changes in catalyst char-
acteristics. This synthetic strategy provides a new and green
route to prepare MOFs with enhanced hydrophobicity for
catalyzing various organic reactions that involve different liquid
phases under mild conditions.
13 F. Guo, Z. Fang, C. C. Xu and R. L. Smith Jr, Solid acid
mediated hydrolysis of biomass for producing biofuels,
Prog. Energy Combust. Sci., 2012, 38, 672–690.
Conflicts of interest
14 W. Thitsartarn and S. Kawi, An active and stable CaO–CeO2
catalyst for transesterication of oil to biodiesel, Green
Chem., 2011, 13, 3423–3430.
There are no conicts to declare.
¨
˘
¨
¨
15 N. Jalilnejad Falizi, T. Gungoren Madenoglu, M. Yuksel and
N. Kabay, Biodiesel production using gel-type cation
exchange resin at different ionic forms, Int. J. Energy Res.,
2019, 43, 2188–2199.
Acknowledgements
The authors acknowledge the nancial support from Key
Laboratory of Functional Inorganic Material Chemistry (Hei-
longjiang University), Ministry of Education, China.
16 V. Kampars, Z. Abelniece and S. Blaua, The Unanticipated
Catalytic Activity of Lithium tert-Butoxide/THF in the
Interesterication of Rapeseed Oil with Methyl Acetate, J.
Chem., 2019, 2019, 1509706.
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