Investigation of the carbon dioxide adsorption behavior and the heterogeneous catalytic efficiency of a novel Ni-MOF with nitrogen-rich channels

Article abstract of DOI:10.1039/d0ra05233g

MOFs have attracted remarkable attention as solid sorbents in CO2 capture processes for their low-energy post-combustion. In this paper, a new Ni-based MOF, Ni4(TATB)1.5(EtO)3.5(NEt3)4, was synthesized and characterized using various physicochemical techn

Full text of DOI:10.1039/d0ra05233g

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Investigation of the carbon dioxide adsorption
behavior and the heterogeneous catalytic
efficiency of a novel Ni-MOF with nitrogen-rich
channels†
Cite this: RSC Adv., 2020, 10, 29772
Sima Aryanejad
and Naser Valipour Motlagh*
MOFs have attracted remarkable attention as solid sorbents in CO
post-combustion. In this paper, a new Ni-based MOF, Ni (TATB)1.5(EtO)3.5(NEt
characterized using various physicochemical techniques. The efficiency of the as-prepared Ni-MOF as
a solid sorbent for CO capture was investigated, and acceptable adsorption was exhibited. Furthermore,
2
capture processes for their low-energy
4
3 4
)
, was synthesized and
2
this Ni-MMOF was used as a catalyst in toluene selective oxidation, for eliminating a volatile organic
compound, with tert-butyl hydroperoxide as an oxidant in the absence of organic solvents. The obtained
results indicated that Ni-MOF has good catalytic activity and could be reused three times without
considerable loss of its catalytic activity. This study highlights the great potential of developing MOFs to
achieve green chemistry goals with the removal of hazardous liquid and gas compounds such as toluene
Received 14th June 2020
Accepted 20th July 2020
DOI: 10.1039/d0ra05233g
rsc.li/rsc-advances
2
and CO .
their many applications, CO
challenges that the eld of MOFs is well-prepared to address.
2
capture stands out as specic
Introduction
The explosive growth of energy consumption due to anthropo- Particularly, the reticular chemistry of MOFs has been grown to
genic activities has been associated with increasing CO2 the point that a chemist can systematically and rationally
concentration. CO2 is the primary contributor to the green- modulate the interplay between the structure of MOF and the
12
house gas effect and the most important perpetrator of global desired properties. The previous reports exhibited that the
1,2
climate change. As a subject of public concern, there is an incorporation of some functional groups, e.g. unsaturated metal
urgent requirement to reduce the amount of CO
for sustain- sites, heteroatoms and non-metallic functional groups of SBUs,
able development and environmental protection. Carbon enhance the affinity of the framework towards selective CO
2
2
13
capture and storage (CCS) is one of the most hopeful capture. Of these, heteroatoms incorporation within the
3
approaches to diminish this issue. Three main technologies, backbone has indicated excellent promise for providing strong
14,15
including cryogenic distillation, absorption, and adsorption, interactions with CO
have been employed for CO capture. Among them, adsorption
With these concepts for structural design in hand, a novel
is a promising technique because it is energetically efficient and Ni-MOF was designed and synthesized using a rigid trigonal
2
.
2
4
–6
7
0
00
economically competitive. Porous materials such as zeolites,
planar ligand, 4,4 ,4 -s-triazine-2,4,6-triyl-tribenzoic acid
(H TATB), which contains two different functional groups, the
capture; however, they had carboxylate groups will form neutral MOFs, and the imine
8
9
10
carbons, porous aromatic frameworks, and calcium oxide
were previously examined for CO
3
2
the disadvantages of being insufficiently selective or difficult groups can act as secondary functional groups in the pores.
regeneration. Thus, there is a continuous quest for better Aer full characterization of the synthesized Ni-MOF (named as
adsorbents with improved efficiency.
UoB-20), its adsorption behavior for CO was investigated that
2
Emerging metal–organic frameworks (MOFs) for the last the results revealed the acceptable activity.
decades, as a new class of porous materials, have been receiving
The removal of volatile organic compounds (VOCs) has been
considerable attention because of their tunable porosities and the main trend because of hazardous effects on human health
11
structures and a wide range of potential applications. Among and the environment. Whereas, the potential of them to trans-
form into valuable and harmless chemicals is usually
16,17
ignored.
Toluene is one of the toxic and carcinogen VOCs
Department of Chemistry, Faculty of Science, University of Birjand, Birjand, Iran.
E-mail: n.valipour.m@birjand.ac.ir; nasservalipoormotlagh@gmail.com; Fax: +98- that its oxidation to useful chemical products i.e., benzyl
5
†
1
632202515; Tel: +98-9153636841
alcohol, benzaldehyde, and benzoic acid is extremely attractive.
Unfortunately, due to the high activation of C–H bonds, toluene
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra05233g
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oxidation reaction usually performs in harsh conditions such as coupled plasma atomic emission spectroscopy (ICP-AES) anal-
high oxygen pressure, high temperature, or the presence of ysis was conducted on OPTIMA 7300DV to determine the metal
18,19
additives.
oxidation processes is the uncontrollable over-oxidation of IsoPrime100 Elemental Analyser. Infrared (IR) samples were
products and convert to CO , which is the main problem in prepared as KBr pellets, and spectra were recorded in the range
them. Therefore, there is the main challenge to control the of 400–4000 cm
Also, another problem associated with the toluene content. Elemental analyses for C, H, and N were obtained
2
ꢁ
1
using a NICOLET system. Transmission
toluene oxidation reaction to produce desirable products Electron Microscope (TEM) analysis was performed on a TEM
without additive under mild conditions. To overcome this issue, Philips EM 208S. Scanning electron microscopy (SEM) was ob-
it is necessary to introduce a heterogeneous catalyst without the tained SEM FEI Quanta 200. Gas sorption measurements were
above disadvantages. MOFs can be used as efficient heteroge- performed a MicroActive for TriStar II Plus. Before the
ꢀ
neous catalysts, due to having polymeric nature and organic– measurement, UoB-20 was degassed under vacuum at 140 C for
inorganic hybrid composition, which make them good recy- 10 h. The surface areas and pore size distributions were deter-
clable and reusable catalysts.
mined by the BET and BJH method, respectively. NMR experi-
With the objective of the catalytic activity of Ni-MOF for ments were performed on a Bruker UltraShield™ spectrometer
producing ne chemicals from the transformation of hazardous operating at 300 and 50 MHz for proton and carbon, respec-
compounds, its efficiency in the toluene oxidation to benzal- tively. The chemical shis are expressed in ppm relative to tet-
dehyde was investigated. However, to the best of our knowledge, ramethylsilane as the internal reference.
there has rarely been reported on oxidation reactions breaking
chemically inert C–H bonds using MOFs as catalyst under mild Catalytic activity test
conditions.
For liquid oxidation of toluene, toluene (1 mmol, 9.42 mL) and
catalyst (2.5 mol%) were performed into two necks round
Experimental
Materials
bottom ask under a mechanical stirrer. The reaction mixture
was heated at 75 C in the oil bath, and then tert-hydrogen
ꢀ
peroxide (3 eq.) was added dropwise. The mixture remained at
the set temperature for 3 h. The reaction progress was moni-
tored by Thin-Layer Chromatography (TLC). The recyclability of
catalyst was performed as the same reaction conditions. At the
end of the reaction, catalyst was isolated from the cooled
mixture via simple centrifugation, thoroughly washed with
All the chemicals and solvents in the experiments were analytical
grade and used as received without further purication. Cyanuric
chloride (C N Cl ), nickel acetate (Ni(OAc) $4H O), acetic anhy-
3
3
3
2
2
dride ((CH
3
CO)
2
O), and toluene were provided Sigma-Aldrich
), acetic acid
) were purchased from Merck
4,4 ,4 -(1,3,5-Triazine-2,4,6-triyl)tribenzoic acid
Company. Anhydrous aluminum chloride (AlCl
3
(HOAc), and chrome oxide (CrO
3
ꢀ
ethanol several times, dried at 100 C for 10 h, and then reused
in the next cycle.
0
00
Company.
3
(H TATB) linker was synthesized according to previously pub-
20
lished methods (for further information see ESI†).
Result and discussion
Ni-MOF synthesis and characterization
Ni-MOF synthesis
The incorporation of chemically available Lewis basic sites into
the channels of MOFs is a great challenge. So, it is very signif-
icant to develop novel functional porous materials for various
H
(
3
TATB (1 mmol, 0.452 g) linker was dissolved in a DMF
21 mL) by adding triethylamine (4 mL) to form a clear solution.
Then, at ambient temperature. Meanwhile, Ni(OAc) $4H
3 mmol, 0.746 g) was added to deionized water (90 mL) and
ethanol (12 mL) for the same time. Subsequently, the
Ni(OAc) $4H O solution was gradually added into the H TATB
2
2
O
applications, for example, selective CO
heterogeneous catalysis.
2
adsorption and
Several new approaches were re-
ported for generation available Lewis basic sites inside MOF
(
21–24
2
2
3
25
channels. Most of the reported methods rely on employing
multi-topic bridging ligands containing Lewis basic sites, which
solution and stirred at room temperature for 30 minutes. Then,
the mixed solution was kept in the same condition for over-
night. Aer that, the solid product was ltered off and washed
with water and ethanol three times. Finally, the synthesized
framework, namely as UoB-20 (University of Birjand), was dried
ꢀ
at 100 C in a vacuum oven for 10 hours.
Characterization
The X-ray diffraction (XRD) pattern was recorded using Philips
PW 1730/10 instrument with Cu Ka radiation in the 2q range of
ꢀ
5
–50 . X-ray photoelectron spectra (XPS) measurements were
carried out by a Thermo Scientic with monochromatic Al Ka
(
(
1486.6 eV) at 15 kV and 10 mA. Thermogravimetric analysis
ꢀ
TGA) was obtained from SDT-Q600 between 30 and 710 C with
Scheme 1 Schematic illustration of synthesis and chemical structure
ꢀ
ꢁ1
a heating rate of 10 C min under argon gas. Inductively of UoB-20.
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stretching vibrations of carboxylic acid groups, respectively.
ꢁ1
Two distinct absorption peaks at 1490 and 1405 cm were
attributed to the aromatic ring of N-containing and the side
ꢁ
1
rings. By contrast, the peak of C]O at 1716 cm disappears,
which detects the carboxyl group coordinated to Ni. The bands
ꢁ1
at 1623 and 1361 cm were assigned to the asymmetric and
symmetric stretching modes of the coordinated carboxylate
ꢁ1
group, respectively. The broad band centered at 3413 cm from
O implies the coordinated water and hydrogen-bonding in
H
2
UoB-20. Furthermore, the coordinate modes of carboxylate
group to metal ion were determined by the wavenumber
difference (Dy) between yas and y . The difference between
s
ꢁ
asymmetric and symmetric stretching of COO anion is
51 cm , which is greater than 262 cm in UoB-20, indicating
ꢁ
1
ꢁ1
6
3
Fig. 1 FTIR spectra of (a) H TATB; (b) UoB-20.
ꢁ
2+
that the COO of H
mode.
3
TATB coordinated to Ni in a bidentate
XPS analytical technique was performed to gain further
detailed insight into the chemical composition and the chem-
ical valence states of nickel in the UoB-20. The main peaks
related to Ni 2p, C 1s, N 1s and O 1s in the full spectrum of UoB-
20 proved the existence of Ni, O, and C elements (Fig. 2a). XPS
spectrum of Ni 2p exhibited two peaks at 856.3 eV for Ni 2p3/2
and 873.8 eV for Ni 2p1/2 with a spin-energy separation of
17.5 eV (Fig. 2b). The results well conrmed the presence form
of nickel ions is Ni(II) in the UoB-20, which are in good agree-
ment with the XPS data known for Ni(II).
Fig. 2 (a) XPS spectra of UoB-20; (b) Ni 2p spectra of UoB-20.
20,21
To clarify the composition of UoB-20, the as-prepared
product was further studied by element analysis (EA). EA
cannot easily be coordinated to metal ions during the synthesis (%) result found as follows: C, 68.17; N, 10.37; Ni, 10.02.
ꢀ
of MOF. Also, the produce of intact 3 amine-based Lewis basic According to electrical neutrality supposition and the EA
sites is rather challenging due to their high basicity. In order to results, the probable molecular formula was supposed to be
achieve this aim, we have focused on the synthesis of the Ni (TATB)1.5(EtO)3.5(NEt ) . The calculated EA% (C, 68.13; N,
4
3 4
ꢀ
functional MOF by H
sites, through the simple procedure, which described in the well with the determined value.
Experimental section (Scheme 1).
TG analysis of UoB-20 was performed under an argon
3
TATB, as a 3 amine-based Lewis basic 10.08; Ni, 9.49) based on the proposed formula matched very
2
+
ꢁ1
ꢀ
The coordination mode of H TATB to Ni for the MOF was atmosphere at a heating rate of 10 C min to evaluate its
3
revealed by the FT-IR spectrum of the as-prepared product. The thermal stability (Fig. 3a). It is to note that the losing weight
ꢀ
H TATB, as a strong coordinated ligand, has various coordinate (13.57%) was observed below 330 C, indicating the removal of
3
modes, such as unidentate, chelating bidentate, and bridging coordinated ethoxy groups in the UoB-20. This amount
bidentate. For easy comparison, the FT-IR spectra of the compatible with well the calculated value based on the
H TATB and Ni-based MOF were presented in Fig. 1. The FT-IR proposed formula (13.3%). The TG curve shows a considerable
3
ꢀ
ꢀ
spectrum of aromatic carboxylic acid has been investigated loss weight between 370 C and 570 C for the decomposition of
considerably, and all absorption peaks were assigned to the UoB-20 to NiO.
26
corresponding vibrations. As shown in Fig. 1a, the peaks at
The PXRD pattern of UoB-20 is presented in Fig. 3b, and its
716 and 1065 cm were attributed to the n(C]O) and d(O–H) pattern shows intense and clear peaks in 16.12, 21.31, and 26.66
ꢁ
1
1
Fig. 3 (a) TGA curve; (b) XRD pattern of UoB-20, (c) XRD pattern of Ni-BTC.
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Fig. 4 (a) SEM; (b) TEM images of UoB-20.
2q. The presence of the sharp peaks conrmed the synthesized
MOF has high crystallinity. According to the size of the crystals
was the smaller than the adequate size for single-crystal deter-
mination, the PXRD of UoB-20 was compared to the reported
XRD of the previous studies. The overall XRD pattern of the
UoB-20 is in good agreement with the Ni-BTC pattern (Fig. 3c),
22
as reported in the literature, indicating that both of them
(UoB-20 & Ni-BTC) are isostructural.
The morphology and microstructure of the as-prepared
product were investigated by SEM and TEM, respectively. The
panoramic view in Fig. 4 exhibits that UoB-20 consists of nano-
rods in high quantity. The sizes of UoB-20 rods are relatively
uniform with the scale of nanometers. Based on the SEM
images, the size of UoB-20 nano-rods were about 16 nm without
the uniform length. TEM image (Fig. 4b) was obtained to gain
further insight into the architecture of UoB-20, which displayed
the as-prepared ultrathin nano-rods with nano-scale size. It
should be noted that this result is consistent with the result of Fig. 5 (a) N
2
adsorption–desorption isotherms; (b) pore width of UoB-
0; (c) CO adsorption at 273 K in UoB-20.
2
2
the SEM image. Furthermore, the TEM image revealed no
amorphous component in the synthesized nano-size.
To evaluate the porous nature of UoB-20, Brunauer–Emmett–
Teller (BET) gas-sorption measurements were carried out. The
N adsorption–desorption isotherms at 77 K and the corre-
2
sponding Barrett–Joyner–Halenda (BJH) pore size distribution
plot of as-prepared UoB-20 are displayed in Fig. 5. The results
adsorption of CO2 was studied at 273 K and the related
isotherm was plotted in Fig. 5c. The result shows a typical type-
I isotherm, which a steep rise at the very low-pressure region.
3
ꢁ1
The adsorbed CO
(
2
amount for UoB-20 was 33 cm
1.7 mmol g ). It should be noted that most MOFs exhibit
dissatisfactory CO adsorption capacities at low pressures,
which are the realistic pressure of CO emission. Based on
g
ꢁ
1
2
exhibited a type IV isotherm with an H hysteresis loop, based
on the IUPAC denition, which revealed the mesoporous
structure of UoB-20 (Fig. 5a). The distribution curve of the pore
diameter, calculated by the BJH method, exhibited that the
dominant mesopores are centered approximately in the range
of 3–11 nm with a peak maximum of 6 nm (Fig. 5b). Also, the
BET surface area and the pore volume was calculated to be 181
2
2
previous research, the incorporation of heteroatoms, espe-
cially high polarity of them or with a nucleophilic nature,
within the framework, have exhibited a strong interactions
1
3
with CO2. The CO capture properties of organic linker with
2
2
ꢁ1
3
ꢁ1
a range of aromatic, primary, secondary and tertiary amines
cm g and 0.41 cm g , respectively.
2
3
was conrmed in the literature. Therefore, it can be
concluded that the relatively desirable CO adsorption for
2
CO adsorption
2
UoB-20 is due to the incorporation of the high concentration
ꢀ
MOFs have exhibited the potential to address the world's of 3 amine-based Lewis basic sites in the framework.
energy and environmental problems, especially as solid
adsorption for CO . In this regard, the ability of UoB-20 was result was compared to the previously reported literature
investigated in the CO adsorption eld. The low-pressure
To further conrm the efficiency of UoB-20, the obtained
2
2
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Table 1 CO
2
adsorption capacities of some adsorbents at 1 bar and 273 K
Langmuir surface
Adsorption capacity
2
ꢁ1
ꢁ1
Adsorbent
area (m g
)
(mmol g
)
Ref.
UiO-66-CF
UiO-66-(COOH)
UiO-66-Br
NH -MIL-53
3
799
217
339
1100
780
390
612
398
1131
181
1.53
1.70
1.40
1.50
1.44
1.31
1.25
1.44
1.45
1.44
24
24
24
25
26
27
28
28
2
2
2
a
n
{[Zn2.66
O
0.66(L)
2
]$2H
2
O}
[
Zn
2
(DMF)(py-CF
3
)
2
]
n
VPI-100 (Ni)
VPI-100 (Cu)
TKL-104 (Ni-tpt)
UoB-20
b
29
This work
a
0
0
b
L: 2,2 -bis-triuoromethyl-biphenyl-4,4 -dicarboxylic acid. tpt: 2,4,6-tri(4-pyridinyl)-1,3,5-triazine.
(Table 1). The result of the comparison table revealed that UoB- catalytic amounts of UoB-20 in different solvents and at diverse
2
0 has an acceptable CO adsorption capacity, despite the lower temperatures to provide benzaldehyde.
2
30–32
surface area compared to the listed MOFs in the table. It can
also be due to the presence of 3 amine-based Lewis basic sites e.g., H O, EtOH, and CH CN were selected as the reaction
in UoB-20. Finding the positive effect of 3 amine-based Lewis solvents at room temperature, but no product was achieved. In
basic sites in the framework for CO adsorption capacity can be the next step, it was decided not to use any solvent that can be
At rst, according to reported articles
the polar solvents,
ꢀ
2
3
ꢀ
2
useful for future research in developing excellent CO
absorbing materials.
2
-
used at a higher temperature without the requirement of
a reux condenser. However, no product was achieved again
from performing the reaction in solvent-free conditions at room
temperature. To improve the reaction conditions, the temper-
The catalytic activity of UoB-20 towards toluene selective
oxidation
ꢀ
ature was set at 75 C in a solvent-free medium. At this time, the
desirable product was gained with a good yield. Next, other
solvents were used instead of the solvent-free condition and the
Several reports have been presented for the toluene oxidation
reaction until now. Although many of these reports are suitable
for laboratory synthesis, they are not effective for industrial-
scale fabrication due to required difficult conditions of reac-
tion. For instance, most of the reported reactions were per-
ꢀ
reaction repeated in the presence of UoB-20 (2 mol%) at 75 C
for 90 minutes (Fig. 6). The toluene conversion was increased in
a weak polar solvent (such as DMF), while selectivity was drastic.
Whereas, benzaldehyde as the main oxidation product was
obtained in strong polar solvent like water, but with the little
conversion. Actually, the signicant conversion and selectivity
were achieved in a solvent-free medium.
ꢀ
formed at high reaction temperatures above 100 C. Therefore,
supplying more simple procedures with available and usual
equipment at lower temperatures are required to develop
toluene oxidation. Accordingly, the main aim of this study was
placed in simplifying the toluene selective oxidation reusable
and efficient catalyst by a novel MOF with economical and
environmentally friendly conditions at lower temperature
To further study the effect of temperature, the toluene
oxidation was investigated at various temperatures (55, 75, 95,
ꢀ
and 110 C). It is obvious that the conversion of toluene
(Scheme 2).
To select the best conditions for oxidation of toluene, model
research was performed with toluene in the presence of various
Fig. 6 Effect of solvent on toluene oxidation. Reaction condition
Scheme 2 Toluene oxidation reaction with UoB-20 as a heteroge- toluene: 1 mmol; UoB-20: 2 mol%; toluene/TBHP mole ratio: 1/3;
ꢀ
neous catalyst.
time: 90 min, temperature: 75 C.
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Fig. 7 Effect of temperature on toluene oxidation. Reaction condition
toluene: 1 mmol; UoB-20: 2 mol%; toluene/TBHP mole ratio: 1/3;
time: 90 min.
Fig. 9 Effect of the molar ratio effect of toluene/TBHP on toluene
oxidation. Reaction condition toluene: 1 mmol; UoB-20: 2 mol%; time:
ꢀ
9
0 min, temperature: 75 C.
increased from 72% to 88% while the reaction temperature
of oxygen on the decomposition of TBHP, which is responsible
for the toluene oxidation. While an excessive amount of
released oxygen is the favour for over-oxidation and the
formation of benzoic acid.
ꢀ
ꢀ
increased from 75 C to 110 C, while the benzaldehyde selec-
tivity decreased from 87% to 55%. The good conversion of
ꢀ
toluene at 75 C can be due to the thermal effect of the kineti-
cally controlled reaction. So, the conversion of toluene
increased as the reaction temperature rose. It should be
considered that the reaction selectivity was decreased and more
by-products produce with increasing temperature. It could be
due to over oxidation of benzaldehyde or the decomposition of
TBHP in higher temperature (Fig. 7). Another important factor,
which inuences the oxidation of toluene, is the catalyst
amount (Fig. 8). Toluene conversion increased from 50% to
It is necessary to mention that H
oxidant in the toluene oxidation, and TBHP exhibited a higher
activity than H . In the oxidation of toluene with H as an
oxidant, carboxylic acid was produced as a by-product, but no
by-product was gained in the presence of TBHP.
2 2
O was also used as an
2
O
2
2 2
O
UoB-20 was also examined for reusability in the oxidation of
toluene at the optimized reaction conditions (Fig. 10). Aer
completion of each reaction, the catalyst was separated by
7
2
2%, with an increase in the catalyst amount from 1 mol% to
mol%. Whereas, no remarkable change was observed in the
ꢀ
centrifuge, washed with ethanol, dried at 80 C for overnight,
and employed in the next run. The toluene conversion
decreased from 72% to 64% and selectivity of benzaldehyde
decreased from 88% to 78% aer the 3rd cycle. The decreasing
the catalytic activity in the subsequent run may be due to the
partial lling porous during the reaction.
A possible mechanism for toluene oxidation by TBHP in the
present of UoB-20 was proposed and is depicted in Scheme 3. It
should initially involve the redox reactions of the oxidant TBHP
by Ni(II)/Ni(I) metal centers and the formation of oxygen-based
radicals of t-BuOOc/t-BuOc nickel(II) rstly turned into nickel(I)
with donating an electron to TBHP and forming t-BuOOc. Then,
toluene conversion by taking 3 mol% of UoB-20. Although, the
selectivity was decreased due to occurring over-oxidation along
with increasing catalyst amount. The effect of toluene/TBHP
molar ratio with respect to time on the oxidation of toluene
was also investigated under the condition which other param-
eters were retained constant (Fig. 9). With increasing the molar
ratio of toluene to TBHP from 1 : 2 to 1 : 3, the toluene oxidation
was increased. It may be due to the release of the high amount
Fig. 8 Effect of catalyst amount on toluene oxidation. Reaction
condition toluene: 1 mmol; toluene/TBHP mole ratio: 1/3; time: Fig. 10 Recyclability tests of UoB-20 for the oxidation reaction of
ꢀ
9
0 min, temperature: 75 C.
toluene to benzaldehyde.
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Acknowledgements
The authors would like to acknowledge the nancial support of
University of Birjand for this research under contract number
1396/D/26005.
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00
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taining imine groups, and conrmed by various characteriza-
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2
Conflicts of interest
There are no conicts to declare.
MOF CAU-1, Energy Environ. Sci., 2011, 4(11), 4522–4527.
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