Synthesis of Sulfated and CaO-Modified UiO-66 Bifunctional
Catalysts from Zircon Sand for Conversion of
Waste Cooking Oil
Article
Marvin Horale Pasaribu
, Retno Agnestisia
, Remi Ayu Pratika
, Erwin Prasetya
Toepak
, Karelius Karelius
*
Department of Chemistry, Faculty of Mathematics and Science, Universitas Palangka Raya,
Palangka Raya (Indonesia)
A
bstract
Zircon-based UiO-66 metal–organic frameworks (MOFs) were synthesized using zirconium oxychloride
extracted from natural zircon sand from Central Kalimantan, Indonesia, and subsequently modified to
produce two distinct heterogeneous catalysts for the stepwise conversion of waste cooking oil (WCO)
into biodiesel. Sulfated UiO-66 (UiO-66/SO4) was developed as a solid acid catalyst for the esterification
step, while CaO-modified UiO-66 (UiO-66/CaO) was employed as a solid base catalyst for the subsequent
transesterification reaction. Structural characterization by XRD and FTIR confirmed the preservation of
the
UiO-66
framework
after
functionalization,
with
successful
incorporation
of
sulfate
and
calcium
species. During esterification process, UiO-66/SO4 exhibited the highest activity at a catalyst loading of
0.5 wt%, reducing the free fatty acid (FFA) content of WCO by 51.7%. Following this pretreatment, the
transesterification
step
catalyzed
by
UiO-66/CaO
at
the
same
loading
yielded
the
best
performance,
producing 61.82% biodiesel. The formation of fatty acid methyl esters (FAME) was confirmed by FTIR
analysis,
showing
characteristic
ester
bands
at
1744,
1165,
1031,
and
1456
cm-1.
These
results
demonstrate that zircon sand–derived UiO-66 can be effectively tailored into complementary acid and
base catalysts, each optimized for a specific reaction step, providing a sustainable and efficient route for
biodiesel production from high-FFA waste cooking oil.
Keyword
s
:
B
iodiesel,
heterogeneous catalyst,
UiO
-
66,
waste cooking oil
,
zircon sand
*
Corresponding author
Email address:
karelius@chem.upr.ac.id
DOI:
https://doi.org/10.22437/chp.v10i1.49575
Received
October 31
st
2025;
Accepted
February 22
nd
2026;
Available online
May 18
th
2026
Copyright © 2026 by Authors, Published by Chempublish Journal. This is an open access article under the CC BY License
(
https://creativecommons.org/licenses/by/4.0
)
63
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
Graphical Abstract
Introduction
The last two decades have been marked by a
tightening global energy landscape, and the
pressure to shift away from fossil fuels has
never
felt
more
immediate
[1].
Among
the
many alternatives being explored, biodiesel
often
stands
out
because
it
behaves much
like
regular
diesel
while
carrying
the
environmental
benefits
of
being
biodegradable
and
producing
lower
greenhouse
gas
emissions
[2].
Still,
producing
biodiesel
at
a
competitive
price
remains
a
challenge,
especially
when
the
feedstock depends on refined vegetable oils.
This is one reason waste cooking oil (WCO)
keeps attracting attention: it is inexpensive,
abundant, and turns what would ordinarily
be
a
disposal
problem
into
a
usable
fuel
source that fits neatly into circular-economy
thinking [3].
Processing
WCO,
however,
comes
with
its
own
set
of
complications.
The
oil
often
contains
a
significant
amount
of
free
fatty
acids
(FFAs),
and
these
FFAs
have
an
unfortunate
habit
of
forming
soap
during
alkaline-catalyzed
transesterification.
Once
that
happens,
the
reaction
becomes
messy—emulsions
form,
yields
drop,
and
purification
becomes
bothersome
[4].
A
practical
workaround
is
a
two-step
route:
first, an acid-catalyzed esterification to push
the
FFA
level
down
to
safer
territory,
generally below 2.5%; then a base-catalyzed
transesterification
to
convert
the
triglycerides
into
methyl
esters
[5].
This
strategy
effectively
reduces
FFA
content
prior
to
transesterification,
improving
biodiesel
yield
and
product
quality.
Consequently,
the
development
of
heterogeneous
acid–base
bifunctional
catalysts has become a key research focus in
biodiesel
production
from
low-quality
feedstocks [6].
Zirconia
(ZrO
2
)
frequently
appears
in
these
discussions. Its stability—both thermal and
mechanical—paired
with
its
intrinsic
acid–
base
character,
makes
it
a
versatile
option
for
catalysis
[7,8].
Efforts
to
enhance
its
surface area and porosity have led to the use
of
metal–organic
frameworks
(MOFs)
as
structural
templates,
particularly
the
well-
known UiO-66. This zirconium-based MOF is
prized for its durability and adjustable pore
properties,
and
when
it
is
calcined,
it
can
transform
into
a
porous
form
of
ZrO
₂
that
retains
much
of
parent
framework’s
architectural
advantages
[9].
Previous
64
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
studies
have
shown
that
UiO-66
can
be
effectively functionalized to introduce acidic
or basic sites for biodiesel-related reactions.
For example, sulfonic acid–modified UiO-66
catalysts
have
achieved
fatty
acid
esterification conversions above 80% under
optimized conditions [10].
CaO-based
catalysts
are
widely
recognized
for their strong basicity and high activity in
transesterification reactions; however, their
broader application is often constrained by
limited stability, catalyst leaching, and poor
reusability
[11].
To
mitigate
these
shortcomings,
recent
efforts
have
focused
on
immobilizing
CaO
on
zirconium-
containing supports. For instance, CaO/ZrO
₂
catalysts derived from UiO-66(Zr) have been
reported
to
achieve
biodiesel
yields
of
approximately
90%
under
optimized
laboratory
conditions,
while
exhibiting
improved
structural
stability
compared
to
bulk CaO [12]. Despite these advances, most
UiO-66-based catalytic systems reported to
date
still
rely
on
commercially
sourced
zirconium
precursors,
which
increases
production
costs
and
poses
challenges
for
large-scale
implementation.
Moreover,
the
direct
utilization
of
locally
available
zircon-
rich
minerals
as
precursors
for
MOF
synthesis remains largely unexplored, even
in
regions
with
abundant
zircon
resources,
such as Indonesia [8,13].
In this study, zirconium is sourced not from
an
imported
chemical
reagent,
but
from
natural
zircon
sand
found
in
Central
Kalimantan, Indonesia. Although the region
is
rich
in
zircon
minerals,
much
of
this
resource
remains
underutilized
despite
its
promise as a local precursor for zirconium-
based
materials
[13].
By
extracting
zirconium oxychloride (ZrOCl
2
·8H
2
O) directly
from
the
sand,
the
present
work
offers
a
route toward producing UiO-66 that is both
economically
and
geographically
advantageous.
The
synthesized
MOF
is
subsequently
modified
in
two
directions:
sulfation
to
form
UiO-66/SO
4
for
the
esterification step, and incorporation of CaO
to
generate
UiO-66/CaO
for
the
transesterification
stage
[12,14].
Together,
these
materials
create
a
bifunctional
catalytic
system
capable
of
handling
high-
FFA
WCO
efficiently,
while
also
demonstrating
the
broader
potential
of
turning
local
mineral
resources
into
high-
value catalytic materials.
Materials and Methods
Materials and Instrumentations
Natural
zircon
sand
was
collected
from
Kereng
Pangi,
Pulang
Pisau,
Central
Kalimantan,
Indonesia
during
Juni
2019.
Analytical
grade
chemicals
were
employed,
namely hydrochloric acid (HCl, 37%), sodium
hydroxide (NaOH), terephthalic acid (H
2
BDC),
N,N' Dimethylformamide (DMF), chloroform,
methanol,
ammonium
sulfate
((NH
4
)
2
SO
4
),
calcium
oxide
(CaO),
were
purchased
from
Merck Reagent, Germany. Characterization
instruments
utilized
included
X-ray
diffraction (XRD) (Philips with X’Pert MPD, at
room
temparature),
Scanning
Electron
Microscopy (SEM) (ThermoFisher Phenom XL
G2,
magnifications
of
350,000x
to
over
1,000,000x), and Fourier Transform Infrared
Spectroscopy
(FTIR)
(Shimadzu
FTIR-8400S
spectrometer,
signals
to
noise
>55.000:1/min).
Extraction
of
Zirconium
Precursor
from
Zircon Sand.
The zirconium precursor was extracted from
locally
sourced
zircon
sand
using
an
acid
leaching technique [15]. The raw zircon sand
was
first
washed,
oven-dried,
and
magnetically
separated
to
eliminate
ferromagnetic
impurities.
The
purified
material
was
sieved
through
a
100-mesh
screen
to
ensure
uniform
particle
size.
Subsequently, 6 g of the
sieved zircon was
65
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
leached in 60 mL of concentrated HCl under
continuous stirring at 80 °C for 90 min. The
mixture
was
then
filtered,
and
the
clear
filtrate
obtained
was
subjected
to
recrystallization
to
yield
the
zirconium
precursor (ZrOCl
2
·8H
2
O).
Synthesis
of
UiO-66
from
Zircon-Derived
ZrOCl
2
.8H
2
O
UiO-66
was
synthesized
using
a
solvothermal
approach
[16].
A
0.0045
mol
solution
of
ZrOCl
₂
.8H
₂
O
in
35
mL
of
DMF
(Solution
A)
and
a
0.0045
mol
solution
of
H
₂
BDC
in
35
mL
of
DMF
(Solution
B)
were
prepared separately and stirred for 15 min.
Both
solutions
were
then
combined
and
mixed
vigorously
for
30
min
before
being
transferred
into
a
Teflon-lined
autoclave.
The sealed reactor was heated at 125 °C for
24
h.
The
resulting
white
precipitate
was
collected
and
sequentially
washed
with
30
mL each of DMF, chloroform, and methanol
for 12 h per solvent. The purified UiO-66 was
dried at 120 °C for 24 h to obtain a fine white
powder.
Synthesis of Sulfate-Modified Acid Catalyst
(UiO-66/SO4)
The
acid
catalyst
(UiO-66/SO
₄
)
was
synthesized via wet impregnation of sulfate
ions onto the UiO-66 framework. In a typical
procedure, 3 g of UiO-66 was dispersed in 30
mL
of
1.5
M
ammonium
sulfate
solution
(1:10 w/v) and stirred continuously at room
temperature for 24 h. The impregnated solid
was then separated, oven-dried at 100 °C for
2 h.
Synthesis
of
Calcium
Oxide-Modified
Base
Catalyst (UiO-66/CaO)
The
The
base
catalyst
(UiO-66/CaO)
was
prepared
through
a
hydrothermal
impregnation method [17]. UiO-66 (30 wt%
relative
to
CaO)
was
dispersed
in
a
suspension of 5 g CaO in 50 mL of distilled
water
and
stirred
thoroughly.
The
mixture
was transferred into a Teflon-lined autoclave
and treated hydrothermally at 100 °C for 24
h. The obtained solid was then filtered, oven-
dried at 105 °C to constant weight.
Esterification
of
WCO
with
UiO-66/SO
₄
Catalyst
The esterification process was performed to
reduce
the
free
fatty
acid
(FFA)
content
of
pre-treated waste cooking oil (WCO). The oil
was
mixed
with
methanol
at
a
1:15
molar
ratio and refluxed at 45 °C for 10 min [17].
Subsequently,
the
UiO-66/SO
₄
acid
catalyst
was introduced at loadings of 0.25, 0.5, and
0.75 wt% relative to the total reactants. The
reaction proceeded under reflux at 65 °C for
30
min.
After
completion,
the
catalyst
was
separated,
and
the
esterified
oil
was
collected
for
FFA
determination
and
subsequent transesterification.
Transesterification of WCO with UiO-66/CaO
Catalyst
The
transesterification
process
was
performed
using
the
esterified
oil
and
methanol at a 1:18 molar ratio. The mixture
was preheated under reflux at 45 oC for 10
min [17]. followed by the addition of the UiO-
66/CaO base catalyst at loadings of 0.25, 0.5,
and 0.75 wt%. The reaction proceeded under
reflux
at
70
oC
for
60
min.
Afterward,
the
catalyst
was
separated,
and
the
upper
biodiesel layer was collected and purified for
subsequent analysis.
Analytical Methods
The free fatty acid (FFA) content of both the
oil
and
the
produced
biodiesel
was
determined
through
an
acid–base
titration
method. In this procedure, 1 g of the sample
was dissolved in 10 mL of methanol, heated
to
45
°C,
and
titrated
with
a
standardized
KOH. The FFA percentage was subsequently
calculated based on the molecular weight of
66
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
oleic
acid
(288.45
g/mol)
by
applying
the
formula in equation 1[17].
FFA content (%)
=
V
KOH
x N
KOH
x MW
KOH
weight of sampel (g)
x 100%
(1)
The
biodiesel
yield
was
determined
gravimetrically after catalyst separation and
phase purification. The presence of fatty acid
methyl
esters
(FAME)
was
qualitatively
confirmed
by
FTIR
spectroscopy
through
characteristic
ester
absorption
bands.
No
chromatographic
quantification
using
external FAME standards was performed in
this study.
The
functional
groups
present
in
both
the
synthesized
catalysts
and
the
produced
biodiesel
were
examined
using
FTIR
spectroscopy
(Shimadzu).
The
crystalline
characteristics of the catalytic materials were
identified through X-ray diffraction analysis
(Bruker D8), and their surface features were
further observed using a scanning electron
microscope (Thermo Fisher Scientific)
Result and Discussion
Extraction
of
Zirconium
Oxychloride
from
Zircon Sand
Zirconium
oxychloride
was
extracted
from
natural zircon sand (ZrSiO
₄
) through an acid-
leaching
route.
The
powdered
sand
was
digested
in
concentrated
HCl
at
elevated
temperature,
during
which
the
zircon
gradually
broke
apart
and
released
zirconium
species
into
solution.
Once
the
dissolution
reached
a
steady
state,
the
mixture was filtered to separate remaining
silica-rich residues, yielding a zirconium-rich
filtrate containing ZrCl
₄
species. Subsequent
recrystallization
of
this
solution
produced
zirconium
oxychloride
octahydrate.
The
overall transformations occurring during the
leaching
and
precipitation
processes
are
summarized in equation (2) and (3) [8,18].
ZrSiO
4
+
4 HCl
(aq,hot)
→ ZrCl
4 (aq)
+ SiO
2 (s)
+ 2 H
2
O
(2)
ZrCl
4 (aq)
+
9 H
2
O
→ ZrOCl
2
.
8 H
2
O
(aq)
+ 2 HCl
(aq)
(
3
)
6Zr
4+
+
4O
2−
→ 4OH
−
+ 4BDC
2−
+
Zr
6
O
4
(OH)
4
(BDC)
6
(4)
Synthesis of UiO-66
UiO-66
was
prepared
through
a
solvothermal
reaction
using
zircon-derived
ZrOCl
2
.8H
2
O
and terephthalic acid (H
2
BDC).
The metal-to-ligand ratio was kept close to
1:1 to help the framework assemble evenly
and to reduce coordination defects such as
missing
linkers
[19,20].
When
both
components
were
dispersed
in
DMF,
the
solvent
partially
deprotonated
H
2
BDC,
allowing
the
BDC
2-
ligand
to
bind
to
hydrolyzed
Zr
4+
species.
Through
gradual
hydrolysis
and
condensation,
these
zirconium units formed Zr
6
O
4
(OH)
4
clusters
that
connected
with
BDC
to
generate
the
UiO-66
framework,
as
outlined
in
equation
(4) [21].
The
solvothermal
treatment
produced
a
crystalline
3D
porous network
in which
Zr
6
nodes
are
linked
by
aromatic
BDC
ligands
[21].
After
synthesis,
the
solid
was
washed
with DMF, chloroform, and methanol to clear
unreacted species, followed by a methanol
exchange
that
opened
the
pores
further
[19].
he
final
product
appeared
as
a
fine
white crystalline powder (Figure 1a).
67
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
a
b
c
d
e
Figure 1.
UiO-66 (a); (b) SO
4
-Modified UiO-66 (b); (c) CaO-Modified UiO-66; (d) Waste Cooking
Oil (WCO); (e) Transesterified Biodiesel
Synthesis
of
Acidic
Sulfate-Modified
and
Basic CaO-Modified UiO-66 Catalysts
Sulfation was performed to strengthen the
acidity of UiO-66 by introducing sulfate ions
(SO
4
2-
)
into
its
Zr-based
clusters.
Sulfate
species from ammonium sulfate interacted
with Zr–OH groups and Zr–O–Zr linkages on
the
Zr
6
O
4
(OH)
4
nodes,
forming
Zr–O–SO
₃
H
units
that
contribute
both
Brønsted
and
Lewis
acidity
[9,19].
This
interaction,
summarized in equation 5 [22], increases the
material’s
acid
strength
while
keeping
the
original
UiO-66
framework
intact
[23].
The
modified
UiO-66–SO
4
appeared
as
a
fine
white
powder
(Figure
1b),
indicating
successful
incorporation
of
sulfate
groups
and
the
creation
of
an
efficient
solid
acid
catalyst for esterification.
For
the
basic
catalyst,
UiO-66/CaO
was
produced
through
a
hydrothermal
process
designed to merge the structural stability of
UiO-66
with
the
strong
basicity
of
CaO.
During synthesis, CaO partially hydrated into
Ca(OH)
2
,
which
then
formed
Ca-O-UiO
linkages by interacting with oxygen atoms in
the UiO-66 framework [24]. The underlying
mechanism is illustrated in Eq. (6) [25]. Some
of the CaO dissolved and redeposited within
the
MOF’s
pore
network
under
hydrothermal conditions, creating a porous
and
thermally
stable
composite
while
maintaining active CaO sites on the surface
[26-28].
(
UiO
−
66)
−
Zr
−
OH
+
SO
4
2−
→
(UiO
−
66)
−
Zr
−
O
−
SO
3
H (acidic site)
(5)
Zr
6
O
4
(OH)
4
(BDC)
6
+
CaO
+
H
2
O
→ Zr
6
O
4
(OH)
4
(BDC)
6
−
Ca(OH)
2
(
6
)
Characteristics
of
UiO-66,
SO
₄
-Modified,
and CaO-Modified UiO-66
The FTIR spectra presented in Figure 2 and
Table
1
highlight
the
key
vibrational
signatures
of
UiO-66
and
its
sulfate-
and
calcium-modified
forms.
In
the
unmodified
MOF,
a
broad
absorption
at
3365
cm
-1
reflects
O–H
stretching
from
hydroxyl
groups
within
the
Zr
6
O
4
(OH)
4
cluster
units
[26].
The
bands
at
1573
and
1396
cm
-1
correspond
to
the
aromatic
C=C
and
asymmetric O-C-O stretching modes of the
terephthalate
linker,
confirming
coordination
between
BDC
and
the
Zr-O
nodes
[19],
[29].
Additional
absorptions
at
744 and 661 cm
-1
assigned to Zr-O bending
and
stretching
signal
the
presence
of
well-
formed Zr
6
clusters in the framework [10,30].
68
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
Figure 2.
FTIR spectra of UiO-66 MOF, UiO-66/SO
4
, and UiO-66/CaO composites
Once
sulfate
groups
were
introduced,
the
spectrum
of
UiO-66/SO
4
showed
a
new
absorption around 1114 cm
-1
of asymmetric
S–O
stretching.
This
feature
indicates
that
sulfate
species
successfully
bonded
to
the
zirconium
centers,
creating
stronger
Brønsted
and
Lewis
acidic
sites
[9].
Minor
shifts
in
the
carboxylate
and
aromatic
regions
also
suggest
subtle
electrostatic
interactions,
although
the
integrity
of
the
organic
linker
remained
intact.
The
UiO-
66/CaO
sample
displayed
two
additional
bands typical of basic calcium species: an –
OH
stretching
signal
at
3664
cm
-1
from
Ca(OH)
₂
and a peak near 874 cm
-1
associated
with
O–C–O
bending
from
CaCO
3
.
These
bands point to the presence of hydrated and
carbonated
CaO,
confirming
that
calcium
species were successfully incorporated onto
the UiO-66 architecture [31].
Table 1
. Comparative Analysis of FTIR Absorption Bands for Various Materials
FTIR Absorption bands
Wavenumbers (cm
-1
)
UiO-66
UiO-66
/SO
4
UiO-66
/CaO
References
OH
group of Ca(OH)
2
-
-
3664
3640-4642 [31]
O-H
stretching
3365
3122
3410
3200-3600 [26]
C=C
stretching (Aromatic Ring)
1573
1576
1598
1576 [29]
O–C–O a
symmetric
stretching
1396
1402
1417
1430 [19]
S-O a
symmetric
Stretching
-
1114
-
1126 [9]
O–C–O
bending
of Ca(CO)
3
874
-
-
872-876 [31]
Zr-O
bending
744
745
748
771 [32]
Zr-O
Stretching
661
618
662
663 [30]
69
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
Figure
3
shows
the
XRD
profiles
of
UiO-66
and its modified forms, confirming that both
sulfation and CaO incorporation altered the
framework
as
intended.
The
unmodified
UiO-66
displays
clear
peaks
at
7.4
o
,
8.5
o
,
25.8
o
,
and
30.8
o
,
matching
the
(111),
(200),
(600),
and
(711)
planes
of
its
cubic
lattice
assembled from Zr
6
O
4
(OH)
4
nodes and BDC
linkers which associated with UiO-66 pattern
(CCDC
number
733458)
[10,33].
After
sulfation, these peaks remain at nearly the
same
positions
but
become
broader
and
slightly
weaker.
This
soft
decrease
in
crystallinity
suggests
that
SO
4
2-
groups
created
local
defects
and
mild
distortions
without
disrupting
the
underlying
UiO-66
topology [9,23].
Figure 3.
XRD diffractograms of UiO-66 MOF, UiO-66/SO
4
, and UiO-66/CaO composites
The
diffraction
pattern
of
the
UiO-66/CaO
composite
exhibits
additional
reflections
attributed
to
calcium-containing
phases,
with
peaks
at
33.81
o
and
54.02
o
corresponding
to
cubic
CaO
(ICCD
00-037-
1484),
reflections
at
17.74
o
,
46.80
o
,
and
50.47
o
assigned
to
Ca(OH)
2
(ICCD
00-044-
1481); and a peak at 28.37
o
associated with
rhombohedral
CaCO
3
(ICCD
00-003-1123)
[17,34].
The
presence
of
these
phases
partially dampens the characteristic UiO-66
reflections at 7.4
o
and 8.5
o
, indicating their
contribution
to
the
overall
diffractogram.
These results indicate that CaO modification
leads
to
the
formation
of
a
UiO-66/CaO
composite
in
which
the
UiO-66
framework
remains
crystallographically
intact,
while
CaO
is
present
as
a
physically
associated
surface
phase
rather
than
inducing
bulk
structural
modification.
The
minor
Ca(OH)
2
and CaCO
3
phases are likely formed during
cooling,
as
CaO
encounters
trace
moisture
or
CO
2
[26,35].
These
phases
are
predominantly
associated
with
the
surface
of
the
UiO-66
framework
and
do
not
significantly affect the bulk crystal structure.
Subtle
shifts
in
several
peaks
hint
at
interactions between Ca
2+
ions and the UiO-
66
carboxylate
groups,
forming
Zr–O–Ca
linkages
that slightly
distort
the
lattice
and
strengthen
the
interface
within
the
composite [36].
The SEM images in Figure 4 show how the
morphology
of
UiO-66
changes
after
sulfation and CaO loading. The unmodified
UiO-66
produced
from
zircon
sand
forms
submicron
polyhedral
particles
that
pack
into compact aggregates with smooth faces
70
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
and
clean
edges—a
good
sign
that
the
Zr-
BDC
framework
crystallized
properly.
The
relatively
even
particle
sizes
and
the
small
voids
between
them
suggest
controlled
nucleation
during
solvothermal
growth,
aligning
well
with
earlier
descriptions
of
highly
crystalline
UiO-66
exhibiting
mild
aggregation
and
good
microporosity
[20],
[37], This confirms that zircon sand serves as
a reliable precursor for generating a stable
and porous UiO-66 phase.
(a)
(b)
(c)
Figure 4.
Surface morphology (SEM) of (a) UiO-66 MOF; (b) UiO-66/SO4, and UiO-66/CaO (c)
composite
Once
sulfated,
the
surface
of
UiO-66/SO
4
becomes
noticeably
rougher.
Wrinkled
textures
and
amorphous
patches
appear
between
the
crystalline
grains,
indicating
that sulfate groups have interacted with the
Zr-O nodes. These local distortions generate
additional Brønsted acid sites and open up
more accessible surface area, features that
tend to enhance molecular diffusion during
catalysis [9,23]. In contrast, brighter particles
observed
in
the
UiO-66/CaO
suggest
the
presence
of
Ca-containing
species
associated with the external surface
of the
MOF
crystallites,
although
elemental
mapping
was
not
performed.
Their
even
dispersion
suggests
strong
interfacial
contact
with
the
UiO-66
matrix,
creating
a
composite with both structural stability and
pronounced
basicity—traits
that
are
advantageous
for
transesterification
in
biodiesel
synthesis.
This
morphology
mirrors
previous
reports
[12],
further
indicating
that
CaO
was
successfully
introduced without undermining the MOF’s
crystalline backbone.
Esterification
of
WCO
Using
UiO-66/SO
4
Catalys
Figure
5(a)
shows
a
clear
drop
in
the
FFA
content of waste cooking oil from 1.2547%
down
to
1.0885%,
0.6057%,
and
0.6490%
after
esterification
using
UiO-66/SO
4
at
loadings of 0.25%, 0.5%, and 0.75%. The 0.5%
dosage stands out, achieving a conversion of
51.7%,
which
seems
to
strike
the
right
balance
between
having
enough
acid
sites
and keeping the surface open for reactants
to
reach
them.
At
higher
loading,
the
particles
tend
to
clump
together,
reducing
the amount of accessible active area [38].
The
esterification
itself
proceeds
through
activation of the fatty acid carbonyl, where -
SO
3
H Brønsted sites and Zr
4+
Lewis centers
work together to increase its electrophilicity,
making it easier for methanol to attack and
form methyl esters and water [9] as shown
in figure
6a. The improved performance
at
0.5%
likely
comes
from
efficient
diffusion
through
the
pores
of
UiO-66,
which
allows
methanol
and
fatty
acids
to
interact
more
freely [17]. Once the loading reaches 0.75%,
viscosity
and
interfacial
resistance
rise,
71
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
slowing mass transfer, while excess sulfate
can
interfere
with
Zr-O
coordination
and
narrow the pores [14]. Therefore, 0.5% UiO-
66/SO
4
offers
the
best
compromise
of
acidity, stability, and diffusivity, giving more
than 50% FFA conversion and preparing the
oil for transesterification with minimal risk of
soap formation [17].
a
b
Figure
5.
%
FFA
reduction
of
WCO
versus
catalyst
weight
for
the
(a)
esterification
and
(b)
transesterification reactions.
Figure 6.
Proposed catalytic mechanism for the esterification and transesterification of waste
cooking oil over UiO-66/SO
₄
and UiO-66/CaO catalysts.
Transesterification
of
WCO
Using
UiO-
66/CaO Catalyst
After the transesterification step using UiO-
66/CaO, the FFA content in the esterified oil
dropped
substantially—from
1.2547%
to
0.5392%, 0.5115%, and 0.5070% for catalyst
loadings
of
0.25%,
0.5%,
and
0.75%,
respectively. These values correspond to FFA
reductions of roughly 57–60%, as shown in
Figure
5b
and
Table
2.
In
contrast,
the
uncatalyzed reaction produced no biodiesel
under
moderate
temperatures,
consistent
with
reports
that
transesterification
barely
proceeds
at
moderate
conditions
[11].
The
absence of product is tied to the high energy
barrier of methanolysis: without a catalyst or
supercritical
methanol,
methoxide
ions
simply cannot form in sufficient quantity to
attack triglyceride carbonyls [39,40]. Among
the tested loadings, the 0.5% catalyst yielded
the
highest
biodiesel
content
at
61.82%.
Increasing the loading to 0.75% reduced the
72
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
yield
to
53.64%,
likely
due
to
increased
viscosity and poorer methanol diffusion [17],
[29]. For comparison, bulk CaO catalysts can
reach
biodiesel
yields
of
about
80%
under
optimized
conditions,
as
reported
by
[41]
snail
shell–derived
CaO
applied
to
waste
cooking oil. The lower yield obtained in this
study
likely
reflects
differences
in
catalyst
structure,
feedstock
quality,
and
reaction
conditions. The basic -O
2-
and -OH sites on
CaO,
supported
by
the
UiO-66
framework,
facilitate
methanol
deprotonation
to
methoxide
an
essential
nucleophile
that
converts triglycerides into methyl esters and
glycerol [12] as shown in figure 6b.
The strong performance at 0.5% suggests a
good balance between available basic sites
and efficient reactant transport through the
porous
UiO-66 structure.
The
uniform
CaO
dispersion
improves
surface
contact
and
adsorption,
while Zr-O-Ca
interactions
help
maintain
structural
robustness
during
reaction
[14],
[17].
At
higher
loadings,
particle
aggregation
and
partial
pore
blockage
reduce
surface
accessibility
and
hamper
conversion
[19].
Overall,
UiO-
66/CaO
at
0.5%
offers
an
effective
combination
of
basicity,
stability,
and
diffusion pathways for biodiesel production,
as illustrated in Figure 1e.
Table
2
.
Biodiesel
Yield
with
Variation
of
Basic Catalyst Mass
Catalyst
Yield (%)
Uncatalyzed
UiO
-
66/CaO
0.25
%
wt
-
[11]
56.91
UiO
-
66/CaO
0.50
%
wt
61.82
UiO
-
66/CaO
0.75
%
wt
53.64
Figure 7.
FTIR spectra of waste cooking oil and the produced biodiesel.
The FTIR spectra in Figure 7 clearly indicate
that the waste
cooking oil was successfully
converted into biodiesel. In the original WCO
spectrum, the strong carbonyl band at 1743
cm
-1
typical
of
triglycerides
appears
alongside
methylene
C-H stretches at 2921
and
2853
cm
-1
[42].
After
transesterification,
new
absorptions emerge at 1165 and 1031 cm
-1
,
which correspond to C-O stretching in -OCH
3
and
-OCH
2
-C
groups,
confirming
the
formation
of
methyl
esters
[31].
The
carbonyl
peak
near
1744
cm
-1
remains
visible,
signaling
the
presence
of
ester
73
M.H. Pasaribu et al.,
Chempublish Journal, 10(1) 2026, 63-77
functionalities,
while
the
disappearance
of
the
1157
cm
-1
glycerol
band
reflects
the
breakdown of triglycerides [43].
A distinct band at 1456 cm
-1
, associated with
CH
3
bending
of
-CO-O-CH
3
,
provides
additional evidence of FAME formation [17].
Meanwhile, the persistent peak at 721 cm
-1
linked to long chain -CH
2
- bending indicates
that the fatty acid chains remain structurally
intact
[44].
Together,
the
features
at
1165,
1031, and 1456 cm
-1
act as reliable diagnostic
fingerprints
for
successful
biodiesel
production [31].
Conclusions
This
work
shows
that
zircon
sand
can
be
turned
into
a
reliable
precursor
for
producing
UiO-66
and
its
bifunctional
variants, which perform well as catalysts for
converting waste cooking oil into biodiesel.
The
solvothermal
synthesis
produced
a
stable
UiO-66
framework,
and
its
subsequent
modification
with
sulfate
and
CaO
successfully
generated
acid
and
base
catalytic
sites.
XRD
patterns
retained
the
hallmark
UiO-66
peaks
at
2θ
=
7.4
o
,
8.5
o
,
25.8
o
, and 30.8
o
, while additional reflections
at 33.8
o
and 54.0
o
confirmed CaO formation.
FTIR signals at 1114 cm
-1
(S-O), 3664 cm
-1
(O-
H
from
Ca(OH)
2
),
and
874
cm
-1
(O-C-O
of
CaCO
2
)
further
verified
the
surface
functionalization.
Catalytic
tests
revealed
that
0.5
wt%
UiO-66/SO
4
was
the
most
efficient for esterification, lowering FFA from
1.2547% to 0.6057% (51.7% conversion). The
subsequent
transesterification
step
using
0.5 wt% UiO-66/CaO reduced the FFA level to
0.5115%
and
delivered
a
biodiesel
yield
of
61.82%.
FTIR
analysis
of
the
final
product
showed clear ester signatures at 1744, 1165,
1031,
and
1456
cm
-1
,
confirming
the
formation
of
fatty
acid
methyl
esters.
Overall,
the
bifunctional
UiO-66/SO
4
and
UiO-66/CaO catalysts offer a promising route
for
upgrading
low-quality
feedstocks
into
biodiesel with good conversion efficiency.
Acknowledgement
The
author
expresses
gratitude
to
the
Faculty
of
Mathematics
and
Natural
Sciences, Universitas Palangka Raya for the
research
grant
that
was
provided
in
2025
under
contract
number
187/UN.24.10/AL/2025.
Author Contributions
Conceptualization,
Marvin
Horale
Pasaribu
and Karelius; Methodology, Retno Agnestisia
and
Remi
Ayu
Pratika;
Software,
Erwin
Prasetya
Toepak;
Validation,
Marvin
Horale
Pasaribu,
Karelius
and
Remi
Ayu
Pratika.;
Formal
Analysis,
Retno
Agnestisia;
Investigation, -; Resources, -; Data Curation, -
; Writing – Original Draft Preparation, Marvin
Horale Pasaribu; Writing – Review & Editing,
Marvin Horale Pasaribu; Visualization, Retno
Agnestisia;
Supervision,
Karelius;
Project
Administration,
Remi
Ayu
Pratika;
Funding
Acquisition.
Conflict of Interest
The authors declare that there is no conflict
of interest regarding the publication of this
paper.
Ethical Standards
This
article
does
not
contain
any
studies
involving human or animal subjects.
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