Heteropoly acid-encapsulated metal-organic framework as a stable and highly efficient nanocatalyst for esterification reaction

Article abstract of DOI:10.1039/c9ra03209f

Metal-organic frameworks (MOFs) have been shown to be promising candidates in the recent decades for the immobilization of guest active species due to their diversified structures, porosity, and high surface area. In this study, silicotungstic acid (HSiW) encapsulated UiO-66 has been successfully synthesized by one-pot synthesis strategy. The synthesized composite (HSiW-UiO-66) was characterized by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FT-IR), nitrogen adsorption-desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric (TG) techniques, and the HSiW-UiO-66 was also employed as a solid acidic catalyst for the synthesis of biodiesel by esterification of lauric acid with methanol. The results revealed that the HSiW-UiO-66 composite had better textural properties (such as large BET surface area (758.3 m² g^-1), large pore volume (0.438 cm³ g^-1), and smaller particle size (about 50-200 nm)), good thermal stability, and high catalytic activity. In addition, the effect of the esterification process parameters on biodiesel production was analyzed using single factor experimental method combined with response surface methodology (RSM); a high conversion of 80.5% with single factor optimization and 92.8% with RSM was obtained. More importantly, the conversion of lauric acid decreased from 80.5% to just 70.2% when the catalyst had been reused six times, which exhibited that the catalyst had excellent reusability. The high activity of HSiW-UiO-66 benefited from low activation energy of 27.5 kJ mol^-1. This work thus offers an underlying application for green biodiesel obtained from free fatty acids esterification or pre-esterification of low-cost oils with high acid value.

Full text of DOI:10.1039/c9ra03209f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Heteropoly acid-encapsulated metal–organic
framework as a stable and highly efficient
nanocatalyst for esterification reaction†
ac
Cite this: RSC Adv., 2019, 9, 16357
Tingting Yang,a Xiaofang Liu,b Caiyan Yue,a Liufang Ao,a
*
Qiuyun Zhang,
a
ac
*
Taoli Deng and Yutao Zhang
Metal–organic frameworks (MOFs) have been shown to be promising candidates in the recent decades for
the immobilization of guest active species due to their diversified structures, porosity, and high surface area.
In this study, silicotungstic acid (HSiW) encapsulated UiO-66 has been successfully synthesized by one-pot
synthesis strategy. The synthesized composite (HSiW-UiO-66) was characterized by X-ray diffraction (XRD),
Fourier transform infrared spectrometry (FT-IR), nitrogen adsorption–desorption isotherms, scanning
electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric (TG)
techniques, and the HSiW-UiO-66 was also employed as a solid acidic catalyst for the synthesis of
biodiesel by esterification of lauric acid with methanol. The results revealed that the HSiW-UiO-66
composite had better textural properties (such as large BET surface area (758.3 m2
g
ꢀ1), large pore
volume (0.438 cm3 ꢀ1), and smaller particle size (about 50–200 nm)), good thermal stability, and high
g
catalytic activity. In addition, the effect of the esterification process parameters on biodiesel production
was analyzed using single factor experimental method combined with response surface methodology
(RSM); a high conversion of 80.5% with single factor optimization and 92.8% with RSM was obtained.
More importantly, the conversion of lauric acid decreased from 80.5% to just 70.2% when the catalyst
had been reused six times, which exhibited that the catalyst had excellent reusability. The high activity of
HSiW-UiO-66 benefited from low activation energy of 27.5 kJ molꢀ1. This work thus offers an underlying
application for green biodiesel obtained from free fatty acids esterification or pre-esterification of low-
cost oils with high acid value.
Received 29th April 2019
Accepted 13th May 2019
DOI: 10.1039/c9ra03209f
rsc.li/rsc-advances
applied in any compression-ignition engine without
modication.4
1. Introduction
Biodiesel can be prepared by transesterication of oils or
esterication of FFAs with short chain alcohols catalyzed by an
acid or base catalyst. Nevertheless, the cost of the raw materials
is the major contributor to biodiesel production.5 In this regard,
low-cost oils (such as non-edible oils and restaurant waste oils)
are economical raw materials for biodiesel production.
However, low-cost oils usually contain abundant FFAs and
cannot be used with an alkaline catalyst, which causes signi-
cant saponication problems.6 Thus, as they cannot be trans-
lated to biodiesel by a one-step conversion process, most
researchers focuses on the development of a two-step technique
including acid-catalyzed esterication of the FFAs and the
following alkali-catalyzed transesterication of triglycerides to
biodiesel.7 On the other hand, the two-step methods will
increase the cost due to complex operation. Therefore, acidic
catalysts are more suitable for the conversion of FFAs and low-
cost oils into biodiesel.
The increase in industrialization and urbanization energy
demands have led to the depletion of the world's oil reserves
and intensied environmental degradation. Thus, it is a great
challenge for investigators to develop alternative renewable
energy to replace petroleum-based fuels.1,2 At present, the
conversion of rened edible oils, waste oil, and free fatty acids
(FFAs) into biodiesel is receiving more and more attention3 due
to the merits of sustainability, lower pollution and greenhouse
gas emissions, biodegradability, non-toxicity, and better
lubricity in comparison with fossil diesel fuels and it can be
aSchool of Chemistry and Chemical Engineering, Anshun University, Anshun 561000,
Guizhou, China. E-mail: sci_qyzhang@126.com; zyt0516@126.com
bFood and Pharmaceutical Engineering Institute, Guiyang University, Guiyang 550005,
China
cEngineering Technology Center of Control and Remediation of Soil Contamination of
Provincial Science
Guizhou, China
& Technology Bureau, Anshun University, Anshun, 561000,
Although homogeneous acid catalysts have high catalytic
capability for the esterication of FFAs, it has several limita-
tions such as non-reusability, high equipment corrosion, the
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03209f
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 16357–16365 | 16357

View Article Online
RSC Advances
Paper
requirement of separation and purication steps, and envi- stainless steel autoclave for hydrothermal treatment at 393 K
ronmental problems.8 Other than homogenous acid catalysts, for 6 h, followed by cooling to the ambient temperature. Then,
heterogeneous solid acid catalysts afford high activity and the formed precipitate was obtained by centrifugation, washing
stability, ease of separation, low corrosive and eco-friendly several times with DMF and ethanol to remove the un-reacted
process.9 In this paper, various solid acid catalysts have been moieties, and drying at 80 C for 24 h (denoted as HSiW-UiO-
ꢁ
extensively investigated for the esterication of FFAs with 66). For comparison, the blank sample without silicotungstic
methanol such as metal oxides,10 zeolites,11 and solid super- acid was also synthesized under the same conditions and is
acids.12 Recently, heteropolyacids were widely studied as solid denoted as UiO-66. The obtained UiO-66 and HSiW-UiO-66
ꢁ
acid catalysts with good to excellent activity for esterica- catalyst were dried at 120 C before use in the catalytic esteri-
tion.13–16 However, bulk heteropolyacids have low specic cation reaction.
surface area and are soluble in polar organic media that make
X-ray diffraction (XRD) spectra were obtained using a D8
˚
their recycling difficult. In order to avert these drawbacks, ADVANCE spectrometer (Germany) using CuK˛I (1.5406 A)
multiple approaches were attempted to modify the hetero- radiation. The FT-IR spectra were obtained on a PerkinElmer
polyacids, for instance, embedding them into supports,17 partial Spectrum 100 using the KBr disc technique (4000–400 cmꢀ1).
or full substitution of the heteropolyacids' H+ with large Nitrogen physisorption measurements were performed using
monovalent ions,18 and graing on functionalized surfaces.19 a Quantachrome Quadrasorb EVO apparatus (Quantachrome
Nevertheless, the preparation of stable encapsulated hetero- Instruments, Boynton Beach, USA) and the specic surface area
polyacids that do not leach in polar organic media is urgently was calculated by the Brunauer–Emmett–Teller (BET) algo-
required. In particular, the reported studies have shown that rithm, whereas the pore size and pore volume distributions
metal–organic frameworks (MOFs) are able to interact and were calculated using the Barrett–Joyner–Halenda (BJH)
stabilize the dispersed active species due to controllable method. Thermogravimetric analysis (TGA) was performed on
composition, a large number of uniformly sized micropores, a NETZSCH/STA 409 PC Luxx simultaneous thermal analyzer
and large surface area.20–22 Among these MOF materials, the with a heating rate of 5ꢁC minꢀ1 under an air ow rate of 20
zirconium-based MOF UiO-66 has emerged as an attractive mL minꢀ1. Scanning electron microscopic (SEM) images were
catalyst support for the production of efficient and stable solid obtained using a Hitachi S4800 scanning electron microscope.
catalysts because of the high robustness and chemical The microscopic features were obtained by TEM on a FEI Tecnai
stability.23
In this work, silicotungstic acid (HSiW) encapsulated on
G2 20 with an accelerating voltage of 200 kV.
UiO-66 was prepared, characterized, and investigated for the
esterication of lauric acid with methanol. The effects of reac-
tion parameters on the esterication conversion were also
investigated by using one variable statistical method and
response surface methodology (RSM). Subsequently, the cata-
lyst reusability and kinetic parameters of the reaction were
studied.
2.3. The esterication test
The catalytic activity of the HSiW-UiO-66 catalyst was evaluated
through the catalytic esterication of lauric acid with methanol
for the preparation of biodiesel (Scheme 1). In a typical run,
lauric acid, methanol, and the catalyst (w/w of lauric acid) were
loaded into a stainless-steel high-pressure autoclave reactor and
ꢁ
reacted at 160 C under autogenous pressure. When the desig-
2. Experimental
2.1. Materials
nated time had elapsed, the reactor was cooled to room
temperature, and the catalyst was separated from the reaction
mixture by centrifugation. Then, the substrate was puried
under reduced pressure distillation to remove water and excess
methanol. The acid values (AV, mg KOH gꢀ1) of lauric acid and
the products were determined according to the ISO 660-2009
standard and the conversion of lauric acid could be determined
based on the following formula:
The following chemicals were procured from Shanghai Aladdin
Industrial Inc: zirconium(IV) chloride, terephthalic acid, silico-
tungstic acid (HSiW, H4SiW12O40$nH2O), lauric acid (98%),
oleic acid, methanol (>99%), stearic acid (98%), myristic acid
(98%), and palmitic acid (98%). All chemicals were of analytical
grade and used without further purication, unless otherwise
noted.
AV1 ꢀ AV2
Conversionð%Þ ¼
ꢂ 100%
AV1
where AV1 is the acid value before the reaction and AV2 is the
2.2. Materials' preparation and characterization
acid value aer the reaction.
Silicotungstic acid encapsulated metal–organic framework
(UiO-66) was prepared by a previously reported one-pot hydro-
thermal method with slight modications.24 Zirconium(IV)
chloride (0.51 g) and silicotungstic acid (0.5 g) were initially
mixed into 18 mL N,N-dimethylformamide (DMF) at room
temperature with stirring, followed by the addition of tereph-
thalic acid (0.3275 g). Aer stirring at room temperature for 3 h,
Scheme 1 Esterification of lauric acid with methanol over nano-solid
the substrate mixture was transferred into a Teon-lined acid.
16358 | RSC Adv., 2019, 9, 16357–16365
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
W]O, Si–O, W–Oc–W, and W–Oe–W, respectively,27 and it can
be detected for the Keggin structures. For UiO-66, the peak at
1660 cmꢀ1 is attributed to the C]O stretching vibrations of
BDC carboxylic acid, and the peaks at 1583 and 1397 cmꢀ1 are
representing the O–C–O asymmetric and symmetric stretching
of the BDC ligand, respectively. Meanwhile, the peaks at 745
and 667 cmꢀ1 belong to the O–H and C–H vibrations in the BDC
ligand, whereas the band at around 553 cmꢀ1 is assigned to the
Zr-(OC) asymmetric stretch, which are in good agreement with
the previous results.28 Compared to the pristine HSiW and UiO-
66, the FT-IR spectra of the HSiW-UiO-66 composite contains
typical infrared peaks belonging to HSiW with the Keggin
structures and the FT-IR spectra of the HSiW-UiO-66 composite
is similar with the spectra of UiO-66, suggesting that HSiW
incorporated the pores in the UiO-66 material. The aforemen-
tioned analysis is in accordance with the results drawn from the
XRD experiments.
3.1.3 N2 adsorption–desorption analysis of the catalysts. In
order to evaluate the porosity of the catalysts, N2 adsorption
isotherms were plotted (see Fig. 3), which conrmed that the
UiO-66 and HSiW-UiO-66 structures were type I isotherms,
indicating the typical predominant microporous feature of the
samples; the porosity of UiO-66 was retained aer encapsula-
tion of pristine HSiW. Additionally, the specic surface area
(BET) of UiO-66 was 667.2 m2 gꢀ1 with 0.431 cm3 gꢀ1 pore
volume and the average pore size was mainly distributed at
2.58 nm. Aer the incorporation of HSiW, the BET surface area
was 758.3 m2 gꢀ1 with a pore volume of 0.438 cm3 gꢀ1 and
average pore size of 2.3 nm. It is interesting that HSiW-UiO-66
had a higher BET surface area than UiO-66, which is bene-
cial for good catalytic activity. However, the average pore sizes of
HSiW-UiO-66 became smaller than UiO-66 since the pore
channel was partly occupied by the incorporated HSiW, which
further conrms that HSiW was encapsulated in the porous
cages of UiO-66.
3. Results and discussion
3.1. Catalysts' characterization
3.1.1 The XRD spectra of the catalysts. X-ray diffraction
(XRD) patterns of pristine HSiW, UiO-66, and HSiW-UiO-66 are
shown in Fig. 1. UiO-66 shows peaks at 2q ¼ 7.2ꢁ, 8.5ꢁ, 12.1ꢁ,
14.7ꢁ, 17.2ꢁ, 22.3ꢁ, 25.7ꢁ, 30.8ꢁ, and 33.3ꢁ correspond to the
(111), (002), (022), (113), (004), (115), (224), (046), and (137)
planes and its XRD pattern UiO-66 is in accordance with the
published literature.25 XRD patterns of UiO-66 and HSiW-UiO-
66 do not show any differences except that the intensity of
peaks is weakened with the entry of pristine HSiW inside the
cage. The crystallinity reduction of HSiW-UiO-66 indicates
proper encapsulation of HSiW into the UiO-66 material, which
is in good agreement with the previous results.26 Moreover, the
XRD pattern of HSiW cannot be obtained from the XRD pattern
of the HSiW-UiO-66 sample, implying the uniform distribution
of HSiW. Based on this analysis, these XRD results suggest that
the HSiW clusters are mainly incorporated into the UiO-66
cavities.
3.1.2 The FT-IR spectra of the catalysts. Fig. 2 shows the FT-
IR spectra of pristine HSiW, UiO-66, and HSiW-UiO-66. For
pristine HSiW, the peaks at 980, 927, 884, and 804 cmꢀ1
represent the particular antisymmetric stretching vibrations of
3.1.4 SEM analysis of the catalysts. Fig. 4 shows several
representative SEM images of UiO-66 and HSiW-UiO-66. As is
obvious, the morphology of UiO-66 (Fig. 4a) displays near-
spherical shapes for the aggregates of irregular inter-grown
crystallites, the particle size of which was nearly 200 nm. As
Fig. 1 XRD spectra of HSiW, UiO-66, and HSiW-UiO-66.
Fig. 3 N2 adsorption–desorption isotherms of UiO-66 and HSiW-
Fig. 2 FT-IR spectra of HSiW, UiO-66, and HSiW-UiO-66.
UiO-66.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 16357–16365 | 16359

View Article Online
RSC Advances
Paper
shown in Fig. 6. At the very beginning, weight loss (about 25%)
was observed up to 300 ꢁC due to the loss of physically adsorbed
water, guest water and solvent molecules. The weight loss (about
26%) at 300–650 ꢁC is mainly attributed to the release of a portion
of the organic ligand, the decomposition of the Keggin structure
of HSiW and the framework structure of UiO-66 into simple
oxides.29 Accordingly, this gure demonstrates that the HSiW-
UiO-66 composite has better thermal stability and thus enables
it to be used as a solid acid catalyst for biodiesel production.
Fig. 4 SEM images of UiO-66 (a) and HSiW-UiO-66 (b).
can be seen from Fig. 4b, the incorporation of HSiW gave rise to
an octahedral morphology, where the crystals are uniformly
distributed and the sizes are about 50–200 nm. Such results
suggested the interaction between the guest HSiW molecules
and matrix UiO-66, which prevented the aggregation of the UiO-
66 crystallites to a certain extent and increased the dispersion.
Meanwhile, the incorporation of HSiW into UiO-66 did not
affect the morphology of HSiW-UiO-66, which displayed its
strong stability.
3.1.5 TEM analysis of the catalysts. Fig. 5 shows the TEM
images of UiO-66 and HSiW-UiO-66. From comparison of the
two samples, we can clearly observe that a signicant change in
the morphological features do not emerge, revealing that UiO-
66 has a highly stable structure and the guest HSiW mole-
cules encapsulated in UiO-66 do not alter the structure and
morphology of UiO-66. These results are in accordance with the
results obtained from XRD and SEM analysis.
3.2. Effect of reaction parameters on the catalytic activity of
HSiW-UiO-66 nanocatalyst
The conversion to biodiesel could be affected by the parameters
of esterication of lauric acid with methanol. Therefore, the
inuences of different esterication reaction parameters (i.e.,
catalyst amount, reaction time, reaction temperature, and
molar ratio) on the catalytic performance of the HSiW-UiO-66
nanocatalyst towards the esterication of lauric acid and
methanol were explored.
3.2.1 Effect of reaction temperature. The effect of reaction
temperature in the esterication of lauric acid is multifaceted.
In general, the higher temperature would facilitate the collision
probability of molecules and catalysts, and give rise to acceler-
ated reaction rate. Fig. 7a presents the effect of temperature on
esterication conversion. From Fig. 7a, it can be clearly seen
that ester conversion improves from 51.6% to 80.5% with the
ꢁ
ꢁ
rise in temperature from 110 C up to 160 C. At the reaction
temperatures beyond 160 ꢁC, the lauric acid conversion
remained almost constant. As a result, the optimum reaction
3.1.6 Thermogravimetric (TG) analysis of HSiW-UiO-66. To
measure the thermal stability of the catalyst, the HSiW-UiO-66
composite was investigated from 40 ꢁC to 800 ꢁC (Fig. 6), as
ꢁ
temperature of 160 C was chosen for further analysis.
3.2.2 Effect of catalyst amount. In general, the higher the
catalyst amount employed, the more are the catalytic active sites
available.30 The effect of catalyst amount ranging from 0 wt% to
11 wt% was investigated on lauric acid conversion by the HSiW-
UiO-66 nanocatalyst with other operating conditions xed, as
depicted in Fig. 7b. Without the catalyst, the lauric acid
conversion was lower (17.6%). The conversion increased as the
catalyst amount increased, as could be seen that the lauric acid
conversion was found to increase from 47.4% to 80.5% when
the catalyst amount increased from 1 wt% to 7 wt%. However,
further increase in the catalyst amount led to the reduction of
the esterication conversion due to the increasing viscosity of
the reaction mixture, which led to poor diffusion of the reac-
tants and decrease in the biodiesel conversion.31 Thus, the
appropriate catalyst amount was 7 wt% for the reaction.
3.2.3 Effect of reaction time. The reaction time needed for
the completion of esterication relies on the extent of proper
contact among the reactant molecules.32 Therefore, the effect of
reaction time on the conversion of lauric acid is shown in
Fig. 7c. From Fig. 7c, it can be found that the conversion of
methyl laurate at the beginning was low and then increased
steadily in the time range between 1 h and 4 h. The conversion
of 81.0% is obtained aer 5 h and then, as the reaction time is
prolonged, the esterication reaction is basically constant.
Therefore, 4 h was selected as the appropriate reaction time for
the process.
Fig. 5 TEM images of UiO-66 (a) and HSiW-UiO-66 (b and c).
Fig. 6 TG curve of HSiW-UiO-66.
16360 | RSC Adv., 2019, 9, 16357–16365
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
3.2.4 Effect of lauric acid to methanol molar ratio. Lauric
acid to methanol molar ratio is one of the important parameters
that has a considerable effect on the conversion as well as on the
production cost. In general, the molar ratio of methanol and
lauric acid is 1 : 1 and the esterication reaction is a reversible
process. In order to keep the equilibrium on the right side of the
reaction, a slightly more amount of methanol is required.
Fig. 7d graphically shows the relationship between lauric acid to
methanol in a certain range (1 : 5, 1 : 10, 1 : 15, 1 : 20, 1 : 25,
1 : 30) and the esterication conversion. The best methyl lau-
rate conversion of 80.5% was obtained with the optimal lauric
acid to methanol ratio of 1 : 20, whereas the conversion
decreased beyond the lauric acid to methanol ratio of 1 : 20
probably because of lower interactions between the reactants
and the catalyst, and the dilution of lauric acid and the catalyst
in the mixture, an observation that was similarly reported by
Nongbe et al.33 Thus, the lauric acid to methanol ratio of 1 : 20
was the optimal ratio for esterication.
3.3. Response surface methodology for optimal conversion
According to the preliminary single factor experiments given
above, it is indicated that the reaction temperature, reaction
time, and catalyst amount are signicant variables for the
esterication of lauric acid with methanol. Therefore, a 3-level-
3-factor Box–Behnken design of experiments was used to
discover the inuence of operational factors on the conversion
achieved with lauric acid to methanol ratio of 1 : 20. The levels
of the variables studied in this work are shown in Table 1S† and
the experimental results and the predicted responses based on
Box–Behnken design are summarized in Table 2S.† From the
RSM results and analysis, a second-order quadratic model
relationship between conversion (C) and the various variables in
coded units are given in eqn (1) as
Cconversion ¼ 79.94 + 6.73X1 + 4.30X2 + 3.52X3 ꢀ 2.45X1X2 +
2
2
2
1.60X1X3 + 3.15X2X3 ꢀ 2.57X1 ꢀ 0.32X2 ꢀ 1.62 X3
(1)
where Cconversion is the lauric acid conversion, X1, X2, and X3 are
the coded variables in the esterication process.
The model was statistically analyzed using the soware in
order to assess the ANOVA and to study the model t, and the
results are presented in Table 1 in terms of sum of squares,
degrees of freedom, mean square, as well as F- and p-values. As
can be seen in Table 1, the model F-value and p-values of 34.81
and <0.0001, respectively, imply that the model is signicant.
Apart from that, the ‘Lack of Fit P-value’ of 0.3756 is higher than
0.05, which indicates that Lack of Fit is not signicant relative
to pure error. Meanwhile, the coefficient of determination (R2)
of 0.9781 implies that the model can explain 97.8% of the
variability. The R2-predicted of 0.8068 is in good agreement with
the R2-adjusted of 0.9500. Under the circumstances, X1, X2, X3,
Fig.
7 Methyl laurate conversion affected by various reaction
2
parameters: (a) reaction temperature (molar ratio of 1 : 20, 7 wt%
catalyst weight, and 4 h), (b) catalyst weight (molar ratio of 1 : 20,
160 ꢁC, and 4 h), (c) reaction time (molar ratio of 1 : 20, 7 wt% catalyst
weight, and 160 ꢁC), and (d) lauric acid to methanol molar ratio (160 ꢁC,
7 wt% catalyst weight, and 4 h).
X1X2, X2X3, and X1 are signicant model terms, and insigni-
2
cant terms such as X1X3, X22, and X3 have negative effects on
the esterication conversion.
The three-dimensional response surfaces of lauric acid
conversion towards the independent variables are presented in
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 16357–16365 | 16361

View Article Online
RSC Advances
Paper
Table 1 ANOVA of the proposed second-order modela
Probability
Source
Sum of squares
df
Mean square
F value
(P) > F
Model
X1
X2
725.18
361.81
147.92
99.40
24.01
10.24
39.69
27.81
0.43
11.05
16.20
8.17
8.03
741.38
9
1
1
1
1
1
1
1
1
1
7
3
4
80.58
361.81
147.92
99.40
24.01
10.24
39.69
27.81
0.43
34.81
156.32
63.91
42.95
10.37
4.42
17.15
12.02
0.19
<0.0001
<0.0001
<0.0001
0.0003
0.0146
0.0735
0.0043
0.0105
0.6790
0.0652
Signicant
X3
X1X2
X1X3
X2X3
2
X1
X2
X3
2
2
11.05
2.31
2.72
4.77
Residual
Lack of t
Pure error
Cor total
1.36
0.3756
Not signicant
2.01
16
a
R2 pred ¼ 0.8086, R2 adj ¼ 0.9500, R2 ¼ 0.9781.
Fig. 1S.† The experimental results were optimized and found to be concluded that the quadratic model was fair and the
ꢁ
be as follows: reaction time 5 h, 170 C reaction temperature, regression equation effectively reected the effect of various
and 11 wt% of catalyst amount. The maximum conversion of variables on esterication conversion.
lauric acid achieved under these optimum conditions was
92.28%. Under these optimum conditions, the experiments
3.4. Lauric acid esterication reaction kinetics over HSiW-
UiO-66 nanocatalyst
were conducted in parallel triplicate and the average conversion
of lauric acid that could be achieved was 92.8%, which was in
accordance with the predicted value. From these results, it can
For the kinetic studies, independent experiments were carried out
under the optimized conditions for lauric acid conversion: the
lauric acid/methanol molar ratio of 1 : 20, 7 wt% catalyst amount,
and the reaction temperature was varied in each experiment (160,
150, and 140 ꢁC). During each experiment, the results were
collected at different reaction times (from 0.5 to 5 h), as given in
Fig. 2S.† The obtained maximum variation in the conversion of
methyl laurate was different at different temperatures and thus,
the kinetic studies were performed for different temperatures. In
each of the experiments, due to methanol being taken in large
excess, the reaction rate was considered to follow rst-order
kinetics and is consistent with previous literature reports.34–36
The reaction rate (r) according to the following equation is:
Fig. 8 (a) Linear relationship between –ln(1 ꢀ X) and time at different
reaction temperatures for esterification in the presence of the HSiW-
UiO-66 nanocatalyst (b) the linear Arrhenius plot for ln k versus 1/T for Fig. 9 Reusability study of HSiW-UiO-66 and UiO-66 catalysts in
esterification in the presence of the HSiW-UiO-66 nanocatalyst.
esterification.
16362 | RSC Adv., 2019, 9, 16357–16365
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 2 Comparison of performance of several catalysts with the present work for esterification
Reaction conditions
FFAs : MeOH
Catalysts
Temperature (ꢁC)
molar ratio
Amount of catalyst
Time (h)
Activity
Ref.
H3PW12O40/K10
WO3/USY
Niobic acid
HPW@MIL-100
[BMIm][CH3SO3]
HSiW-UiO-66
HSiW-UiO-66
165
200
249
111
140
160
170
1 : 8
1 : 6
5 wt%
10 wt%
0.7 g
15 wt%
4 wt%
7 wt%
11 wt%
5
2
—
5
5
4
5
z100%
74%
90%
40%
93%
34
41
42
43
1 : 10.83
1 : 11
1 : 2
1 : 20
1 : 20
44
80.5%
92.8%
This work
This work
molar ratio of lauric acid to methanol 1 : 20, reaction temper-
ature 160 ꢁC, and reaction time 4 h). Aer each run, the catalyst
was separated by centrifugation, washed with methanol, and
directly used for the next run. The experimental results are
shown in Fig. 9. Aer six successive runs, the lauric acid
conversion decreased from 80.5% to 70.2% with the HSiW-UiO-
66 nanocatalyst and the conversion decreased from 75.0% to
38.6% with the blank UiO-66, suggesting that the HSiW-UiO-66
nanocatalyst was efficient and reusable for biodiesel produc-
tion. This result may be due to the HSiW-UiO-66 nanocatalyst
having a relatively strong interaction between the HSiW and
UiO-66 material aer the encapsulation of HSiW, resulting in
a synergistic effect, similar to the previous literature results.39,40
It could be also concluded that the incorporation of HSiW into
the UiO-66 cavities improved the conversion of lauric acid with
methanol and the stability of the HSiW-UiO-66 nanocatalyst.
However, the small loss in the reusability of the HSiW-UiO-66
nanocatalyst is probably due to the leaching of the active sites
of the catalyst in the reaction mixture. The FT-IR spectrum of
the recovered HSiW-UiO-66 (Fig. 3S†) was similar to that of the
fresh catalyst, suggesting the structure and composition of
HSiW encapsulated UiO-66 was mostly maintained. Based on
the above analysis, the as-obtained HSiW-UiO-66 nanocatalyst
exhibits great stability during the esterication reaction.
dCA
ꢀ
¼ kCA
(2)
dt
where k represents the modied rate constant and CA represents
the concentration of lauric acid. CA is dened as:
CA ¼ CA0 (1 ꢀ X)
(3)
where X and CA0 are the conversion of used lauric acid and the
initial concentration of lauric acid, respectively. Thus, eqn (3)
can be changed to eqn (4).
ꢀln(1 ꢀ X) ¼ kt
(4)
Fig. 8a shows a linear relationship of the rst-order for ꢀln (1
ꢀ X) versus t. The reaction rate constants (k) at different reaction
temperatures were obtained by linear regression and the k is
found to be increasing with increasing reaction temperature
with a good straight-line t. It can be clearly seen that elevating
the temperature promotes the esterication process due to
enhanced mass transfer effect. Moreover, the activation energy
for the lauric acid esterication reaction was evaluated graphi-
cally from the plot of ln k versus 1/T (see Fig. 8b) and using the
Arrhenius equation given as follows.
ln k ¼ ꢀEa/RT + ln A
(5)
3.6. Comparison of different catalysts for esterication
From Fig. 8b, the observed activation energy (Ea) was
27.5 kJ molꢀ1, and the Ea in our work is much lower. Meanwhile,
it has been reported that the activation energy for diffusion-
limited reactions is relatively low (10–15 kJ molꢀ1) and reac-
The catalytic activities of various solid acid catalysts in esteri-
cation were compared and the observed results are shown in
Table 2. From Table 2, it can be seen that HSiW-UiO-66 exhibits
higher catalytic activity with 92.7% conversion under mild
reaction conditions, whereas the other catalysts show low
conversion (40%) or require higher temperature ($200 ꢁC) to
obtain similar conversion. According to the explanation given
above, the HSiW-UiO-66 nanocatalyst has appropriate catalytic
properties, attributed to the high surface areas, strong acidity,
and better thermal stability. Therefore, it can be used as
a promising solid acid catalyst for biodiesel production.
tions controlled by a truly chemical step usually show Ea higher
than 25 kJ molꢀ1
.
Hence, in the present study, the esteri-
37,38
cation reaction is kinetically controlled by a chemical step
owing to the high activation energy.
3.5. Reusability of catalyst
The catalyst reusability test is of great importance as it indicates
the advantage of using a catalyst to reduce the preparation cost
for its application in industrial production. Therefore, the
4. Conclusions
reusability of blank UiO-66 and HSiW-UiO-66 nanocatalyst was In conclusion, silicotungstic acid was successfully incorporated
investigated in the esterication of lauric acid with methanol into UiO-66 through a one-pot hydrothermal synthesis. The
under optimum reaction conditions (catalyst amount 7 wt%, HSiW-UiO-66 nanocatalyst exhibited excellent textural
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 16357–16365 | 16363

View Article Online
RSC Advances
Paper
properties and good thermal stability. Meanwhile, the optimum 12 C. C. Huang, C. J. Yang, P. J. Gao, N. C. Wang, C. L. Chen and
catalytic performance in the conversion of lauric acid with J. S. Chang, Green Chem., 2015, 17, 3609–3620.
methanol to produce biodiesel was achieved by the HSiW-UiO- 13 Y. Chen, X. L. Zhang, M. M. Dong, Y. H. Wu, G. P. Zheng,
66 nanocatalyst, using which the conversion can reach up to
80.5% with single factor optimization and 92.8% with RSM.
J. Huang, X. X. Guan and X. C. Zheng, J. Taiwan Inst.
Chem. Eng., 2016, 61, 147–155.
Moreover, there existed no remarkable reduction in the catalytic 14 Q. Y. Zhang, F. F. Wei, Q. Li, J. S. Huang, Y. M. Feng and
performance even aer six runs. The reaction mechanism fol- Y. T. Zhang, RSC Adv., 2017, 7, 51090–51095.
lowed pseudo-rst-order kinetic model and the result show that 15 N. A. S. Ramli, D. Sivasubramaniam and N. A. S. Amin,
the activation energy was 27.5 kJ molꢀ1. Thus, the HSiW-UiO-66
BioEnergy Res., 2017, 10, 1105–1116.
nanocatalyst presented here has great potential to be a stable 16 Q. Y. Zhang, C. Y. Yue, Q. L. Pu, T. T. Yang, Z. F. Wu and
and highly efficient solid acid catalyst for biodiesel production Y. T. Zhang, ACS Omega, 2019, 4, 9041–9048.
and may have broader applications in other acid-catalyzed 17 A. B. Gawade, M. S. Tiwari and G. D. Yadav, ACS Sustainable
reaction systems.
Chem. Eng., 2016, 4, 4113–4123.
18 J. Cai, Q. Y. Zhang, F. F. Wei, J. S. Huang, Y. M. Feng,
H. T. Ma and Y. T. Zhang, J. Oleo Sci., 2018, 67, 427–432.
19 Y. Hanada, M. Kamada, K. Umemoto, H. Kominami and
Y. Kera, Catal. Lett., 1996, 37, 229–233.
Conflicts of interest
There are no conicts to declare.
20 K. Fujie, T. Yamada, R. Ikeda and H. Kitagawa, Angew.
Chem., Int. Ed., 2015, 126, 11484–11487.
21 C. S. Diercks, Y. Liu, K. E. Cordova and O. M. Yaghi, Nat.
Mater., 2018, 17, 301–307.
Acknowledgements
This work was nancially supported by the Technical Talent
Support Program of Guizhou Education Department (No. KY 22 A. Jrad, B. J. Abu Tarboush, M. Hmadeh and M. Ahmad, Appl.
[2018]069), the Joint Science and Technology Funds of Guizhou Catal., A, 2019, 570, 31–41.
S&T Department, Anshun City People's Government and 23 Y. Cao, Y. X. Zhao, Z. J. Lv, F. J. Song and Q. Zhong, J. Ind.
Anshun university (No. LH [2016]7278), the Creative Research Eng. Chem., 2015, 27, 102–107.
Groups support program of Guizhou Education Department 24 L. H. Wee, S. R. Bajpe, N. Janssens, I. Hermans,
(No. KY [2017]049), Construction Projects of Innovation Plat-
form from Science and Technology Bureau in Anshun (No.
K. Houthoofd, C. E. A. Kirschhock and J. A. Martens,
Chem. Commun., 2009, 46, 8186–8188.
[2016]-4), Guizhou Science & Technology Foundation (No. [2019] 25 M. Kandiah, M. H. Nilsen, S. Usseglio, S. Jakobsen,
1009), the Youth Growth Science & Technology Personnel
Foundation of Guizhou Education Department (No. KY [2018]
292).
U. Olsbye, M. Tilset, C. Larabi, E. A. Quadrelli, F. Bonino
and K. P. Lillerud, Chem. Mater., 2010, 22, 6632–6640.
26 G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange,
J. Dutour, S. Surble and I. Margiolaki, Science, 2005, 309,
2040–2042.
27 K. M. Parida and S. Mallick, J. Mol. Catal. A: Chem., 2007,
275, 77–83.
28 J. Tang, W. J. Dong, G. Wang, Y. Z. Yao, L. M. Cai, Y. Liu,
X. Zhao, J. Q. Xu and L. Tan, RSC Adv., 2014, 4, 42977–42982.
29 X. L. Yang, L. M. Qiao and W. L. Dai, Microporous Mesoporous
Mater., 2015, 211, 73–81.
Notes and references
1 A. F. Lee, J. A. Bennett, J. C. Manayil and K. Wilson, Chem.
Soc. Rev., 2014, 43, 7887–7916.
2 S. X. Tan, S. Lim, H. C. Ong and Y. L. Pang, Fuel, 2019, 235,
886–907.
¨¨
3 I. Ambat, V. Srivastava and M. Sillanpaa, Renewable
Sustainable Energy Rev., 2018, 90, 356–369.
4 F. Muhammad, R. Anita and S. Duvvuri, J. Clean. Prod., 2013,
59, 131–140.
5 Q. Y. Zhang, X. F. Liu, T. T. Yang, C. Y. Yue, Q. L. Pu and
Y. T. Zhang, RSC Adv., 2019, 9, 8113–8120.
6 P. Jaggernauth-Ali, E. John and P. Bridgemohan, Fuel, 2015,
158, 372–378.
30 F. H. Alhassan, U. Rashid, R. Yunus, K. Sirat, I. M. Lokman
and Y. H. Tauq-Yap, Int. J. Green Energy, 2015, 12, 987–994.
31 S. L. Lee, Y. C. Wong, Y. P. Tan and S. Y. Yew, Energy Convers.
Manage., 2015, 93, 282–288.
32 S. Sahani, S. Banerjee and Y. C. Sharma, J. Taiwan Inst. Chem.
Eng., 2018, 86, 42–56.
33 M. C. Nongbe, T. Ekou, L. Ekou, K. B. Yao, E. L. Grognec and
F. X. Felpin, Renewable Energy, 2017, 106, 135–141.
7 P. Mazumdar, S. R. Dasari, V. B. Borugadda, G. Srivasatava,
L. Sahoo and V. V. Goud, Biomass Convers. Bioren., 2013, 34 K. Y. Nandiwale and V. V. Bokade, Ind. Eng. Chem. Res., 2014,
3, 361–369.
53, 18690–18698.
8 Q. Y. Zhang, F. F. Wei, P. H. Ma, Y. T. Zhang, F. H. Wei and 35 N. Kaur and A. Ali, Renewable Energy, 2015, 81, 421–431.
H. L. Chen, Waste Biomass Valorization, 2018, 9, 911–918.
9 C. Pirez, A. F. Lee, J. C. Manayil and C. M. A. Parlett, Green
Chem., 2014, 16, 4506–4509.
36 S. Shalini and S. Y. Chandra, Energy Convers. Manage., 2018,
171, 969–983.
37 V. Brahmkhatri and A. Patel, Appl. Catal., A, 2011, 403, 161–
10 Q. Y. Zhang, H. Li and S. Yang, J. Oleo Sci., 2018, 67, 579–588.
172.
11 K. H. Chung and B. G. Park, J. Ind. Eng. Chem., 2009, 15, 388– 38 A. Patel and V. Brahmkhatri, Fuel Process. Technol., 2013,
392.
113, 141–149.
16364 | RSC Adv., 2019, 9, 16357–16365
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
´
39 D. D. Yu, W. X. Gao, S. Y. Xing, L. L. Lian, H. Zhang, 42 L. L. Rade, L. Letıcia, C. O. T. Lemos, M. A. S. Barrozo,
X. Y. Wang and D. W. Lou, RSC Adv., 2019, 9, 4884–4891.
40 M. Zhang, Q. G. Shang, Y. Q. Wan, Q. R. Cheng, G. Y. Liao
and Z. Q. Pan, Appl. Catal., B, 2019, 241, 149–158.
R. M. Ribas, R. S. Monteiro and C. E. Hori, Renewable
Energy, 2018, 115, 208–216.
43 F. J. Liu, W. Li, Q. Sun, L. F. Zhu, X. J. Meng, Y. H. Guo and
F. S. Xiao, ChemSusChem, 2011, 4, 1059–1062.
41 A. A. Costa, P. R. S. Braga, J. L. de Macedo, J. A. Dias and
S. C. L. Dias, Microporous Mesoporous Mater., 2012, 147, 44 F. Guo, Z. Fang, X. F. Tian, Y. D. Long and L. Q. Jiang,
142–148.
Bioresour. Technol., 2013, 140, 447–450.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 16357–16365 | 16365