Engineering the porosity and acidity of H-Beta zeolite by dealumination for the production of 2-ethylanthraquinone via 2-(4′-ethylbenzoyl)benzoic acid dehydration

Article abstract of DOI:10.1039/c7ra13576a

Environmentally-friendly zeolites have been used commercially to replace concentrated sulfuric acid and oleum in the alkylation reactions and dehydration of alcohols. However, moderate activity, associated with access and diffusion limitations, low intramolecular dehydration selectivity, associated with unsatisfactory acidity, and unknown reusability have hampered their industrial implementation in the dehydration of bulky 2-(4′-ethylbenzoyl)benzoic acid (E-BBA) to 2-ethylanthraquinone (2-EAQ). Herein, we have discovered that after being treated with mild HNO₃, nano-sized H-Beta zeolite showed outstanding catalytic activity, selectivity and reusability, compared with a commercial oleum catalyst. A number of techniques, such as XRD, XPS, XRF, ²⁹Si MAS NMR, ²⁷Al MQ MAS NMR, FTIR, NH₃-TPD, argon physisorption and HR-TEM, have been employed to decouple the interdependence between acidity, porosity and catalytic performance. It was found that mild HNO₃ treatment could clean out the extra-framework aluminium deposits and selectively extract the aluminium species on the outer surface of Beta zeolites, which strengthened the acidity of the Br?nsted acid sites (Si(OH)Al) inside the H-Beta micropores, thus increasing the possibility of intramolecular dehydration of E-BBA. Moreover, this mild HNO₃ treatment also dredged the network of intercrystalline mesopores, alleviating the diffusion constraints. Therefore, through the dual adjustment of acidity and porosity, dealuminated H-Beta zeolite has a promising future in the green synthesis of 2-EAQ.

Full text of DOI:10.1039/c7ra13576a

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Engineering the porosity and acidity of H-Beta
zeolite by dealumination for the production of 2-
ethylanthraquinone via 2-(40-ethylbenzoyl)benzoic
acid dehydration†
Cite this: RSC Adv., 2018, 8, 9731
a
a
a
a
b
a
*
N. He, C. Y. Liu, G. R. Wang, Q. Xin and H. C. Guo
J. X. Liu,
Environmentally-friendly zeolites have been used commercially to replace concentrated sulfuric acid and
oleum in the alkylation reactions and dehydration of alcohols. However, moderate activity, associated
with access and diffusion limitations, low intramolecular dehydration selectivity, associated with
unsatisfactory acidity, and unknown reusability have hampered their industrial implementation in the
dehydration of bulky 2-(40-ethylbenzoyl)benzoic acid (E-BBA) to 2-ethylanthraquinone (2-EAQ). Herein,
we have discovered that after being treated with mild HNO3, nano-sized H-Beta zeolite showed
outstanding catalytic activity, selectivity and reusability, compared with a commercial oleum catalyst. A
number of techniques, such as XRD, XPS, XRF, 29Si MAS NMR, 27Al MQ MAS NMR, FTIR, NH3-TPD, argon
physisorption and HR-TEM, have been employed to decouple the interdependence between acidity,
porosity and catalytic performance. It was found that mild HNO3 treatment could clean out the extra-
framework aluminium deposits and selectively extract the aluminium species on the outer surface of
Beta zeolites, which strengthened the acidity of the Brønsted acid sites (Si(OH)Al) inside the H-Beta
micropores, thus increasing the possibility of intramolecular dehydration of E-BBA. Moreover, this mild
HNO3 treatment also dredged the network of intercrystalline mesopores, alleviating the diffusion
constraints. Therefore, through the dual adjustment of acidity and porosity, dealuminated H-Beta zeolite
has a promising future in the green synthesis of 2-EAQ.
Received 22nd December 2017
Accepted 19th February 2018
DOI: 10.1039/c7ra13576a
rsc.li/rsc-advances
The demand for 2-EAQ worldwide in 2016 was more than 20 000
tons and is expected to increase at a rate of 8%.
Introduction
Three industrialized synthetic routes for producing 2-EAQ
have been developed: the phthalic anhydride (PHA) method, the
anthracene oxidation method, and the naphthoquinone
method.2 From the viewpoint of the low production cost, the
abundance of the raw materials, and the facile operation
process, the PHA method is widely used in industry. The
commercially synthetic process includes two steps; in the rst
step, AlCl3 is used as the catalyst for the acylation of ethyl-
benzene with PHA to produce 2-(40-ethylbenzoyl)benzoic acid
(E-BBA); in the second step, concentrated sulfuric acid or oleum
is used as the catalyst for the dehydration of E-BBA to 2-EAQ.4–7
Generally, each ton of 2-EAQ product would consume 6.5
tons of oleum, which could result in more than 16 tons of waste
acid (about 40 wt% concentration). The waste acid is very
difficult to use again and is extremely difficult to be disposed of
because it also contains a lot of organic compounds. In addition
to such environmental problems, the oleum catalysed E-BBA
dehydration also suffers from the formation of a large amount
of sticky tar by-products due to heavily intermolecular oligo-
merization. Consequently the selectivity of 2-EAQ is only 80%.
Therefore, under the dual pressure of a fast-growing demand
With the issue of environmental protection becoming more and
more critical, some pollution technologies in the chemical
industries urgently need to be improved or replaced. With 2-
ethylanthraquinone (2-EAQ), an important raw material, 90% of
it is used in the synthesis of hydrogen peroxide (H2O2),1,2 and
the other 10% is used for the synthesis of dye intermediates and
photosensitizers.3 Hydrogen peroxide (H2O2) is a highly efficient
and green oxidant because it has the highest content of active
oxygen (47.1% w/w) and only H2O as the by-product. In recent
years, commercial H2O2 has been widely applied in the epoxi-
dation of propylene to produce propylene oxide and the oxida-
tion of cyclohexanone amine to produce cyclohexanone oxime.
Nowadays, H2O2 is manufactured almost exclusively through
the anthraquinone process using 2-EAQ for hydrogenation.3
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian, 116024, China. E-mail: hongchenguo@dlut.edu.
cn; Fax: +86-411-84986120
bState Key Laboratory for Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, Liaoning, China
† Electronic supplementary information (ESI) available: Additional catalytic data.
See DOI: 10.1039/c7ra13576a
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for 2-EAQ and the need for environmental protection, nowadays
2-EAQ producers eagerly desire new environmentally benign
and more selective E-BBA dehydration catalysts to replace the
concentrated sulfuric acid and oleum catalysts.
Characterization
HR-TEM images of the nano-sized H-Beta zeolites were recor-
ded on a JEOL JEM-2100 (200 kV) microscope.
X-ray diffraction (XRD) patterns were obtained with a Rigaku
D/max-2004 diffractometer with Cu Ka radiation (40 kV, 100
mA) and a 0.02ꢀ minꢁ1 (2q) scanning speed.
X-ray uorescence (XRF) measurements were performed
with a Bruker SRS3400 spectrometer to determine the bulk
silicon to aluminum ratio.
X-ray photoelectron spectroscopy (XPS) analyses were con-
ducted with a VG ESCALAB MK2 instrument using Al Ka radi-
ation (1486.6 eV) in order to estimate the surface silicon to
aluminum ratio. The voltage and power used for the measure-
ments were 12.5 kV and 250 W, respectively. The vacuum in the
test chamber during spectrum collection was maintained at 2 ꢂ
10ꢁ10 mbar. The binding energies were calibrated for the
surface charge by referencing to the C1S peak of the contami-
nant carbon at 284.6 eV.
Zeolites are well-known solid acid catalysts, which have
been used in many industrial processes. Zeolites have many
merits such as being highly eco-friendly and having high
thermal stability, adjustable acidity and porosity and, more
importantly, unique shape-selectivity. Zeolites have already
been used to replace liquid acid catalysts such as AlCl3$HCl,
HF, concentrated sulfuric acid and oleum in ethylbenzene
synthesis, cumene synthesis, alkylation, and alcohol dehy-
dration.8,9 H-Beta zeolite catalysed E-BBA dehydration has
been reported by Xu et al.10 The study showed that HNO3
treatment (concentration: 0.5–5 M, 80 ꢀC, time: 2 h twice,
liquid/solid ratio: 10) could enhance E-BBA conversion and
the 2-EAQ selectivity of the H-Beta zeolite catalyst. However,
little is known about the catalytic chemistry from the limited
publications.11
Solid-state MAS NMR measurements were performed with
an Agilent DD2-500 MHz spectrometer. 27Al MAS NMR spectra
were acquired at 130.2 MHz using a 4 mm MAS NMR probe with
14 kHz spinning speed. The chemical shis of aluminum were
referenced to (NH4)Al(SO4)2$12H2O at ꢃ0.4 ppm as a secondary
reference. The spectra were accumulated for 200 scans with a p/
12 ip angle and 2 s pulse delay. 27Al MQ MAS NMR spectra
were collected using a three-pulse sequence incorporating a z-
lter. A two-dimensional Fourier transformation followed by
a shearing transformation gave a pure absorption mode 2D
contour plot. The second-order quadrupolar effect (PQ) and
isotropic chemical shi (diso) values were calculated according
to the procedure introduced in the reference.15 29Si MAS NMR
spectra were collected at 99.3 MHz using a 6 mm MAS probe
with 4 kHz spinning speed, for 400 scans and with a 4 s pulse
delay. 4,4-Dimethyl-4-silapentane sulfonate sodium (DSS) was
used as the chemical shi reference for the 29Si MAS NMR
spectroscopy.
In recent years, we have carried out systematic studies on H-
Beta zeolite catalysed E-BBA dehydration. In this study, a nano-
sized H-Beta zeolite and three kinds of HNO3 leached nano-
sized H-Beta zeolite were used as catalysts for the dehydration
of E-BBA to 2-EAQ. A number of techniques, such as XRD, XPS,
XRF, 29Si MAS NMR, 27Al MQ MAS NMR, FTIR, NH3-TPD, argon
physisorption and HR-TEM, have been employed to illuminate
the relationship between the catalytic properties and the
performance. Moreover, density functional theory (DFT) studies
were also performed to gain insight into the advantages of H-
Beta zeolite. It was found that the mild HNO3 leaching treat-
ment is helpful for enhancing both E-BBA conversion and 2-
EAQ selectivity. At full conversion of E-BBA, the selectivity of 2-
EAQ over mildly treated nano-sized H-Beta was 89%, which is
higher than that of a commercial oleum catalyst (80%). More-
over, the zeolite catalyst could be recycled aer regeneration at
least seven times without a notable degradation in catalytic
performance.
NH3-TPD measurements were employed to investigate the
overall acidity of the catalysts. Proles were obtained with
a
Quantachrome ChemBet 3000 chemisorb instrument.
Experimental
Materials
Samples (150 mg, 380–830 mm sieve fraction) were pretreated in
He at 600 ꢀC for 1 h and then cooled down to 100 ꢀC for
ammonia adsorption. The ammonia adsorption was carried out
at 100 ꢀC for 30 minutes with a mixture of 5% NH3 in He. Aer
the adsorption the cell was purged in 50 mL minꢁ1 He ow for
30 min to remove all non-chemically adsorbed NH3. Then the
NH3-TPD proles were recorded in a 50 mL minꢁ1 He ow by
Nano-sized NH4-Beta zeolites (20–50 nm, Fig. S1†) with a Si/Al
ratio of 12 were supplied by Dalian Ligong Qiwangda Chem-
ical Technology (DQ-TECH). H-Beta zeolite was obtained from
the calcination of NH4-Beta zeolite under a dry air ow of up to
ꢀ
540 C for 6 hours.
ꢀ
ramping the temperature from 100 to 600 C at a rate of about
Dilute nitric acid leaching was carried out at 85 ꢀC under
stirring the zeolite with 0.3 M, 0.6 M and 2.0 M concentrations
of HNO3. The acid solution to zeolite ratio was 30 mL gꢁ1 and
the leaching time was 5 h. Aer leaching, the zeolitic powders
were recovered by ltration and washed repeatedly until the pH
value was equal to that of deionized water. Then, the lter cakes
were dried at 110 ꢀC for 24 h and calcined at 540 ꢀC for 3 h. The
calcined samples were labeled as H-Beta(0.3), H-Beta(0.6), and
H-Beta(2.0), corresponding to 0.3 M, 0.6 M, and 2.0 M HNO3
solutions, respectively.
ꢁ1
ꢀ
16 C min
.
Fourier transform infrared spectroscopy (FTIR) was also
used to characterize the acidity of the H-Beta zeolite samples.
The spectra for surface hydroxyl (OH) vibrations and pyridine
adsorption were obtained with a Nicolet is10 FT-IR spectrom-
eter. The zeolitic samples were pressed into a self-supporting
thin wafer (approximatelyꢁ315 mg) and decontaminated at
400 ꢀC under vacuum (10 Pa) for 4 h in a quartz IR cell
equipped with CaF2 windows. Aer the pretreatment, the cell
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was cooled down to RT for sample measurement. Spectra were
recorded from 4000 to 400 cmꢁ1 with an optical resolution of
4 cmꢁ1. The hydroxyl vibration spectra were obtained by sub-
tracting the background spectrum (recorded with an empty IR
cell in the absence of sample) from the measured sample
spectra. The spectra for pyridine were obtained as follows: rst,
pyridine adsorption was carried out at 150 ꢀC for 10 minutes in
pyridine vapor (approximately 2.2 mbar). Then, evacuation
treatment (10ꢁ3 Pa) was conducted for 30 minutes at 150 ꢀC.
The spectra were obtained by subtracting the background
spectrum (obtained with decontaminated wafers before pyri-
dine adsorption) from the measured sample spectra.
Fig. 1 The 72T ONIOM cluster (the atoms treated with uB97XD and
AM1 are shown in ball and stick modelling and the wireframe,
respectively).
Argon physisorption was conducted on a Micromeritics ASAP
2020 instrument at 87 K to obtain textural information. Prior to
measurement, the samples (380–830 mm sieve fraction) were
degassed at 623 K for 6 h. The surface area was calculated using
the Brunauer–Emmett–Teller (BET) method using the adsorp-
tion branch in the p/p0 range from 0.10 to 0.15: the pore
volumes were estimated at p/p0 of 0.99, while micro- and mes-
oporosity was discriminated using the t-plot method.
catalytic reactions were treated at high-level with the functional
uB97XD along with the 6-31+G(d,p) basis set for accuracy,
which is a long-range corrected functional.12 The regions away
from the active center were treated at lower-level with the semi-
empirical calculation method (AM1) for efficiency. During all
calculations, the positions of terminal Si–H groups in the
cluster model were held xed in their crystallographic positions
to retain the zeolite structure, whereas the positions of the
remaining atoms and reactant molecule were optimized. The
transition state structures were characterized by means of
frequency calculations with only one imaginary frequency. The
intrinsic reaction coordinate (IRC) method was used when
necessary to identify the two minima connected by a transition
state. In addition, the acidity of the E-BBA molecule and H-Beta
zeolites was calculated using NH3 as the probe molecule.
Catalytic tests
Dehydration of 2-(40-ethylbenzoyl)-benzoic acid (E-BBA) was
performed under atmospheric pressure in a three-necked ask.
The temperature of the reaction was maintained using an
electric heater. In a typical experiment, 2.0 g E-BBA was loaded
into the ask and heated to melt with the solvent, to which 2.0 g
H-Beta zeolite powder catalyst was added whilst stirring, then
the mixture was heated to reaction temperature and the reac-
tion carried out at a given temperature for 3.5 h with vigorous
stirring. Aer the reaction, the mixture in the ask was cooled
down to room temperature and dissolved in 1,4-dioxane. The
zeolite catalyst was removed by ltration, washed with 1,4-
Results and discussion
Chemistry of E-BBA dehydration to 2-EAQ
ꢀ
dioxane, dried in an oven at 90 C, and further regenerated by
As shown in Scheme 1, the dehydration of E-BBA over concen-
trated sulphuric acid and oleum as catalysts has two possible
reaction paths: (1) intramolecular dehydration to 2-EAQ; (2)
intermolecular dehydration to oligomer by-products. Owing to
the lack of shape-selectivity of oleum as a catalyst, the selectivity
of 2-EAQ is not ideal, because of the large amount of sticky tar
by-products produced through the heavily intermolecular
oligomerization.
calcining it in air at 500 ꢀC for 3 h if necessary. The ltrate was
analyzed using a liquid-chromatograph (Waters 1525) equipped
with a SunFire C18 column with a particle size of 5 mm and
dimensions 150 ꢂ 4.6 mm. The mobile phase was a mixture of
H2O, CH3OH and tetrahydrofuran (THF), owing at a rate of 0.5
mL minꢁ1, with HPLC spectra collected at a UV wavelength of
274 nm. The calibration curves were linear for E-BBA (r ¼
0.9995) and 2-EAQ (r ¼ 0.9997). The weights of E-BBA and 2-EAQ
were calculated based on their peak areas and the correction
factors. The liquid was analyzed by atmospheric pressure ioni-
zation electrospray mass spectrometry (API-ES-MS) with
a HP1100LC/MSD mass spectrometer.
Beta zeolite possesses a three-dimensional 12-ring pore
structure constituted of perpendicular straight channels (6.6 ꢂ
9
˚
˚
6.7 A) and a sinusoidal channel (5.6 ꢂ 5.6 A). Compared with
the molecular size of E-BBA (shown in Fig. 2), these dimensions
DFT calculations
A 72T cluster model with 247 atoms and the Al atoms placed in
the T6 lattice positions was obtained by cutting the periodic
structure, as shown in Fig. 1. The dangling silicon atoms are
terminated by substituting the lattice O atoms with H atoms,
˚
giving Si–H bond lengths of 1.46 A. To improve the energetic
properties and take into account the effect of the entire zeolite
framework on the reaction mechanism, a two-layer scheme
Scheme 1 Reaction paths of E-BBA dehydration over oleum and
ONIOM was employed. Therefore, the regions related to the concentrated H2SO4 catalysts.
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Fig. 2 Molecular length of the E-BBA molecule measured from
different directions (A˚) (the white balls are H atoms, the red balls are O
atoms and the grey balls are C atoms).
Fig. 3 The potential energy surface of the active site (Brønsted acid
site) of H-Beta zeolites. The detailed structures of the zeolite and
molecules are shown in Fig. S2.†
51.92 kcal molꢁ1). This indicates that the acidity of the catalyst
is critical for this reaction. Therefore, the acid strength of the E-
BBA molecule and the different hydroxyl groups over H-Beta
zeolite were determined by calculating the adsorption ener-
gies of NH3 as a probe molecule on different hydroxyl groups. As
listed in Table 1, the framework aluminium species at different
T-positions of H-Beta zeolite have similar acid strengths. The
acid strength of the E-BBA molecule and different hydroxyl
groups follows the sequence Si(OH)Al > E-BBA > SiOH. This
indicates that the framework aluminium species with strong
Brønsted acidity could be the effective active sites in E-BBA
intramolecular dehydration.
mean that the E-BBA molecule can only enter the channel of
Beta zeolite in a parallel position (0.58 nm), which is favourable
for intramolecular dehydration. However, owing to spatial
connement, intermolecular dehydration between two E-BBA
molecules does not happen easily inside the zeolite channel.
For H-Beta zeolite, most of the active sites are located inside the
microporous channels of the zeolite. Therefore, the selectivity of
E-BBA intramolecular dehydration over H-Beta zeolite aer
precise modication is expected to be higher than that over
oleum and concentrated H2SO4, because of the shape-selective
catalysis.
In order to gain insight into the active site of H-Beta zeolite
for E-BBA intramolecular dehydration to 2-EAQ, DFT calcula-
tions were performed to obtain the potential energy surface for
E-BBA intramolecular dehydration over the Si(OH)Al acid sites.
The reaction path is shown in Scheme 2; this path started with
physical adsorption, underwent a C–H bond dissociation step,
and ended with the desorption of 2-EAQ and H2O as products.
As seen from Fig. 3, the formation of the O–H bond takes place
between the C]O group of the E-BBA molecule and H+ on H-
Beta zeolite. Then the adsorbed molecule has to go through
a transition state (TS_1) by means of H transfer from the C–H
bond of ethylbenzene to the O atom of C]O, and at the same
time there is formation of a C–C bond. Obviously, the dissoci-
ation of a C–H bond and formation of a C–C bond in molecular
E-BBA is the rate determining step of the whole reaction due to
the high intrinsic activation energy (DE ¼ 23.81 ꢁ (ꢁ28.11) ¼
MOH + NH3 / MOH/NH3
(1)
(2)
DEads ¼ E(MOH/NH ) ꢁ (EMOH + ENH
)
3
3
The effects of dealumination on the composition of H-Beta
zeolites
To the best of our knowledge, two major challenges existed for
the large, organic molecule E-BBA’s dehydration to 2-EAQ over
H-Beta zeolites: (1) strong transportation limitations and (2) low
intramolecular dehydration selectivity. In order to overcome
these challenges, commercial nano-sized H-Beta zeolite was
chosen as the parent catalyst and HNO3 leaching treatment was
applied to adjust the distance between two neighboring
Brønsted acid sites (Si(OH)Al) to suppress undesired intermo-
lecular oligomerization.
As shown in Fig. S3,† the XRD patterns of all samples were
obtained. Compared with those of the parent H-Beta zeolite, the
XRD patterns of all dealuminated H-Beta zeolites have no
Table 1 Adsorption energies of NH3 as a probe molecule on different
hydroxyl groups of H-Beta zeolites and the E-BBA molecule
Si(OH)Al (Al
Si(OH)Al (Al
at T1–T2)
at T3–T9)
SiOH
E-BBA
Scheme 2 The mechanism of intramolecular dehydration of E-BBA to
2-EAQ over the Brønsted acid sites of H-Beta zeolites.
DE (kcal molꢁ1
)
ꢁ30.59
ꢁ32.71
ꢁ3.47
ꢁ15.21
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obvious change, indicating that there was no signicant struc-
ture damage aer acid leaching. The effects of HNO3 treatment
on the composition of nano-sized H-Beta zeolite were investi-
gated through XRF and 29Si MAS NMR spectroscopy (Fig. S4†) to
obtain the total and framework Si/Al ratios, respectively. And
XPS analysis was applied to determine the Si/Al ratio on the
outer surface of the prepared samples.
As shown in Table 2, an obvious increase in Si/Al ratios can
be observed aer HNO3 treatment. The Si/Al ratios of deal-
uminated H-Beta zeolites increased with increasing HNO3
concentration. For the parent H-Beta zeolite, the total and
framework Si/Al ratios were 12 and 22, respectively, indicating
the existence of extra-framework aluminum (EFAL) species.
Aer 0.3 M HNO3 treatment, the total and framework Si/Al
ratios were 30 and 32, respectively, indicating that most of the
aluminium species located in the framework of the zeolite and
most of the EFAL species had been cleaned out by this mild acid
leaching. At the same time, the surface Si/Al ratio obviously
increased to 112, suggesting that the aluminium species in the
HB0.3 catalyst are framework aluminium, located inside the
zeolite channel. However, during the acid treatment with higher
concentration, severe extraction of framework aluminium
species occurred as the framework Si/Al ratio increased faster
than the total Si/Al ratio.
To obtain deep insight into the effects of acid leaching on Al
distribution, 27Al MQ MAS NMR spectra of H-Beta and deal-
uminated H-Beta zeolites were obtained, and the 27Al MAS NMR
spectra are shown on top of the MQ MAS spectra in Fig. 4. For H-
Beta zeolite, two kinds of aluminium species were visible. The
resonances corresponding to framework aluminium species in
the region 50–60 ppm were assigned to aluminium species at
different T-sites in the zeolite called Al(IV)a and Al(IV)b. The
resonances corresponding to EFAL species were visible in the
region ꢁ20 to 20 ppm.15–20 For HB0.3, the main aluminium
species observed were framework aluminium and most of the
EFAL species had been removed. The intensity and resonances
of Al(IV)a and Al(IV)b were clearly discriminated at 54 and
57 ppm, respectively, suggesting that the acid leaching had
removed part of one kind of framework aluminium species so
that the two signals become optimal in terms of resolution.20 In
line with the XRF, 29Si MAS NMR and XPS results, it was
Fig. 4 27Al MQ MAS NMR spectra of H-Beta and dealuminated H-Beta
zeolites. The corresponding 27Al MAS NMR spectrum is given on top of
the MQ MAS plot. The F1 projection shows a purely isotropic dimen-
sion, showing a clear resolution of the resonances corresponding to
tetrahedral aluminum at different T-positions.
As the HNO3 concentration increased from 0.6 M to 2.0 M,
the decrease of the resonance intensity of Al(IV) was accompa-
nied by the resonance intensity of Al(VI) increasing and a new
resonance at 30 ppm attributed to penta-coordinated
aluminium Al(V) appeared.19 As deduced from the XPS, 29Si
MAS NMR and XRF results, the high HNO3 concentration led to
the extraction of framework aluminium species to form EFAL
species (EFAL). During this process, the penta-coordinated
aluminium existed as the transition species for the extraction
of framework aluminium.
The effect of dealumination on the acidic properties of H-Beta
zeolites
deduced that the mild HNO3 treatment had selectively removed The effect of acid leaching on the acidic properties of H-Beta
the EFAL species and aluminium species on the external surface was studied using NH3-TPD and OH-IR and pyridine-IR spec-
of the zeolite.
troscopy. Based on the NH3-TPD proles shown in Fig. 5(a),
0.3 M HNO3 treatment did not change the total number of acid
sites very much, and just shied the desorption peak at high
temperature to a higher temperature. However, for HB0.6 and
HB2.0, both the number and strength of the acid sites obviously
decreased.
Table 2 Textural properties of H-Beta and dealuminated H-Beta
zeolite
For H-Beta zeolite, four bands can be clearly distinguished in
the spectral range of hydroxyl group stretching modes
(Fig. 5(b)); the most intense bands at 3736 cmꢁ1 and 3745 cmꢁ1
are asymmetric and correspond to isolated internal SiO–H
groups and isolated external SiO–H groups, respectively.21–25
The band at 3610 cmꢁ1 is commonly assigned to the bridging
hydroxyl groups (Si–(OH)–Al), which cause strong Brønsted
acidity. The band at 3782 cmꢁ1 is commonly assigned to the
Cat
Si/Ala
Si/Alb
Si/Alc
HB
12
30
63
163
22
32
83
185
59
HB0.3
HB0.6
HB2.0
112
136
242
a
Molar Si/Al ratio determined by XRF. b Molar Si/Al ratio determined by
29Si MAS NMR spectroscopy.13 Molar Si/Al ratio determined by XPS.14
c
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a substantial amount of framework aluminium species had
been extracted.
The same phenomenon was also observed in the pyridine-IR
results (Fig. 5(c)). With 0.3 M HNO3 treatment of H-Beta zeolite,
the number of Lewis acid sites (the bands at 1454 and
1622 cmꢁ1), most of which are attributed to EFAL, decreased. In
contrast, the number of Brønsted acid sites (the band at
1547 cmꢁ1), which are attributed to framework Al, was well
preserved. As the concentration of acid increased to 0.6 and
2.0 M, the number of Brønsted and Lewis acid sites simulta-
neously decreased signicantly. In line with the above results,
acid treatment using different concentrations obviously has
different effects on the acidity of different catalysts. For HB0.3,
the acid strength of the Brønsted acid sites (Si(OH)Al) located
inside the zeolite channels was strengthened by cleaning out
the EFAL species and aluminium species on the external surface
of the zeolite. Meanwhile for HB0.6 and HB2.0, the cleanout of
the aluminium species was more extensive than this, resulting
in a decreased number and strength of the acid sites.
The effect of dealumination on the porosity of H-Beta zeolites
The theoretically calculated molecular sizes of both intra- and
inter-molecular dehydration products of E-BBA (Scheme S1†)
were compared with the crystallographic micropore diameters
of H-Beta zeolite (Table S1†), and it is easy to see that the shape-
selective function of the porous channel of H-Beta zeolite plays
a crucial role in crippling the inter-molecular dehydration of E-
BBA. The direct comparison of these sizes indicates that the
micropores of H-Beta zeolite can exclude most of the formation
of inter-molecular dehydration products inside the zeolite
channel. Thus, the microporosity of catalysts is also an impor-
tant factor determining the catalytic performance in this reac-
tion. High-resolution Ar physisorption, an effective method to
study the microporosity of zeolites, was employed here. From
Fig. 6 it can be seen that all four samples showed a sorption
isotherm of type IV with similar hysteresis loops. Compared
with that for the parent sample HB, the amount of adsorption
increased for all treated samples, but the increase in the degree
of adsorption was greater for lower concentrations of acid, viz.,
Fig. 5 Acidic properties of H-Beta and dealuminated H-Beta zeolites:
(a) NH3-TPD profiles, (b) OH-IR profiles and (c) Py-IR profiles.
AlO–H groups in which Al atoms belong to the extra framework
aluminum (EFAL) species. Acid treatment with 0.3 M HNO3
resulted in the disappearance of the band at 3782 cmꢁ1, indi-
cating that the EFAL species are very sensitive to acid treatment.
Furthermore, the obvious increase in the intensity of the bands
assigned to bridging hydroxyl groups (3610 cmꢁ1) and the
silanol bands (3745 cmꢁ1 and 3736 cmꢁ1), accompanied by the
appearance of the band between 3400 and 3600 cmꢁ1 (attrib-
uted to hydrogen-bonded hydroxyl groups), both indicate the
partial extraction of framework aluminium species.25 As the
HNO3 concentration increased to 0.6 and 2.0 M, the intensity of
the band between 3400 and 3600 cmꢁ1 increased more obvi-
ously, at the expense of that at 3610 cmꢁ1, suggesting that
Fig.
6
High-resolution Ar adsorption/desorption isotherms at
ꢁ186 ꢀC.
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HB0.3, HB0.6, and HB2.0, which indicates that mild acid
treatment may dredge the channel of zeolites. To get some more
details about the pore structure change, the pore size distribu-
tions estimated from the adsorption branch of the isotherms
using an NLDFT method were investigated in the following.
For the parent sample HB in Fig. 7, there are two main peaks
centered at 0.56 nm and 0.65 nm. Also, a small peak centered at
0.48 nm was observed. The existence of micropores with
a diameter of 0.48 nm could possibly originate from the
blocking of the microporous channels of Beta zeolites by EFAL
species. These small micropores disappeared aer 0.3 M HNO3
treatment. In correlation with XRF and NMR results, the EFAL
deposits blocking the micropores were cleared away by this
mild HNO3 treatment, thus resulting in the increased Ar
adsorption amount. On further increasing the HNO3 concen-
tration to 0.6 M and 2.0 M, the Ar adsorption volume of the
HB0.6 and HB2.0 catalysts began to decrease. According to
previous results, the removal of a fraction of framework Al
species leads to the formation of EFAL species and the
condensation of these species blocked the micropores of zeolite
again. Moreover, as shown in Fig. 8, the mesopores, with
diameters between 2 and 5 nm, were generated aer HNO3
leaching. Due to the small single-crystal size of the nano-sized
H-Beta parent (20–50 nm), most of these mesopores could
possibly be identied as intercrystalline mesopores, which were
blocked by EFAL species before. However, the possibility of the
existence of intracrystalline mesopores cannot be excluded.
Fig. 8 Selected region of the mesopore size distribution of H-Beta
and dealuminated H-Beta zeolites.
and 2.0 M, even though the acid treatment can also clear the
channels of the zeolite, some framework Al species were
transformed into EFAL deposits, which blocked the micropo-
rous channels of the zeolite to some extent, leading to decreased
Ar adsorption compared to that of the HB0.3 catalyst.
The effect of dealumination on nano-sized H-Beta zeolites in
the dehydration of E-BBA
E-BBA dehydration over H-Beta and dealuminated H-Beta
High-resolution transmission electron microscopy (TEM) zeolites was studied and their performance was compared
provides insight into the local structure of crystalline phases. with that of a commercial oleum catalyst. Nano-sized H-Beta
Consistent with previous results, TEM spectra gave a direct view zeolite could catalyze the dehydration of E-BBA but the
(Fig. 9) of 0.3 M HNO3 treatment having cleared away the EFAL activity and 2-EAQ selectivity were unsatisfactory (Fig. 11). Aer
species. Moreover, the HR-TEM spectra (Fig. 10) have also 0.3 M HNO3 treatment of the H-Beta zeolites, the catalytic
shown that this mild HNO3 treatment did not inuence the performance improved greatly. The E-BBA conversion and 2-
porous structure of H-Beta zeolite. Therefore, the crucial role of EAQ selectivity increased to 99% and 89%, respectively. Further
0.3 M HNO3 treatment is that of washing the EFAL deposits and raising the HNO3 solution concentration to 0.6 M and 2.0 M had
dredging the second network of pores between zeolite nano- little inuence on E-BBA conversion but brought down the 2-
crystals, leading to better accessibility of micropores. EAQ selectivity to 84% and 76%, respectively. In order to obtain
However, if further increasing the concentration of HNO3 to 0.6 insight into this reaction, detailed product analysis was per-
formed. Taking the products over the HB0.3 catalyst as exam-
ples, the main product 2-EAQ was rst collected through
recrystallization and its purity was higher than 98.5%, deter-
mined by comparison with a standard reagent. Then the le-
over product was further analyzed through API-ES mass
Fig. 7 Pore size distributions of H-Beta and dealuminated H-Beta
zeolites obtained by application of the NL-DFT method to the Ar
adsorption isotherm at ꢁ186 ꢀC in the range 0.4–2 nm.
Fig. 9 TEM images of H-Beta and dealuminated H-Beta zeolites.
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Fig. 10 HRTEM images of H-Beta and dealuminated H-Beta zeolites.
Fig. 13 Comparison of HB0.3 with an oleum catalyst for the dehy-
dration of E-BBA. Reaction conditions for HB0.3 catalysed reaction:
temperature 210 ꢀC, pressure 0.1 MPa, catalyst loading 2.0 g, E-BBA
dosage 2.0 g and reaction time 3.5 h; reaction conditions for oleum
catalysed reaction: temperature 98 ꢀC, pressure 0.1 MPa, catalyst
loading 9.6 g, E-BBA dosage 2.0 g and reaction time 2 h.
Besides the catalytic activity, the reusability of catalysts is
also an important issue for industrial application. So, the used
HB0.3 catalyst was recovered by facile regeneration in owing
ꢀ
air at 540 C for 3 hours. The loss of some catalysts is usually
Fig. 11 The effect of HNO3 treatment on nano-sized H-Beta zeolite in
E-BBA dehydration. Reaction conditions: m(E-BBA)/m(Cat) ¼ 1.0,
210 ꢀC, 3.5 h and 0.1 MPa.
inevitable during the regeneration process. Thus, in order to
make sure of comparability between the fresh and regenerated
catalysts, the mass ratio of E-BBA to HB0.3 catalyst was always
maintained at 1 for each test. As seen in Fig. 14, the E-BBA
activity and 2-EAQ selectivity remain constant even aer seven
cycles of regeneration. HB0.3 catalyst showed another signi-
cant advantage in the E-BBA dehydration reaction when
compared with non-recyclable oleum. The treated Beta zeolites
have a promising future in the green synthesis of 2-EAQ via E-
BBA dehydration.
spectrometry analysis (shown in Fig. 12). It was found that there
were small amounts of unreacted E-BBA and side-products of
dimer and trimer produced by the intermolecular dehydration
of E-BBA. The selectivity for total oligomers is only around 3%
and no formation of larger molecules was found, indicating that
the HB0.3 catalyst is more effective in the suppression of heavily
intermolecular dehydration (Fig. 13), compared with the high
selectivity for sticky tar, 10%, with oleum as catalyst. Thus, the
HB0.3 catalyst exhibited better performance than a commercial
oleum catalyst in suppressing the formation of large molecular
oligomers.
Therefore, in line with previous characterization results and
reaction data, it was found that the low E-BBA conversion over
the parent Beta zeolite could be attributed to the blockage of
Fig. 14 Reusability of the HB0.3 catalyst after regeneration for E-BBA
Fig. 12 Product analysis for E-BBA dehydration over HB0.3 catalyst. 2- dehydration. Reaction conditions: temperature 210 ꢀC, pressure
EAQ products obtained by NaOH recrystallization (left) and the API-ES 0.1 MPa, m(E-BBA)/m(HB0.3 catalyst loading) ¼ 1.0, and reaction time
mass spectrum of the washing liquor (right).
3.5 h.
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a substantial amount of micropores and intercrystalline meso- not only inhibited intermolecular dehydration but also
pores by EFAL species, resulting in difficult accessibility of the strengthened the acidity of the Brønsted acid sites (Si(OH)Al),
E-BBA reactant to Brønsted acid sites located inside zeolite thus increasing the possibility of intramolecular dehydration of
channels. And owing to a lack of space restriction, aluminium E-BBA. Moreover, this mild HNO3 treatment also dredged the
species on the outer surface of the parent Beta zeolite are network of intercrystalline mesopores, alleviating the diffusion
responsible for the intermolecular dehydration of E-BBA to constraints. The DFT calculation results showed that the 2-EAQ
dimer and trimer by-products. Dilute HNO3 treatment (0.3 M) product formed by intramolecular dehydration is free to diffuse
could clean out the extra-framework aluminium deposits and out through both straight and sinusoidal channels, however the
selectively extracted the aluminium species on the outer surface diffusion of the dimer by-products formed by intermolecular
of H-Beta zeolite, which not only suppressed the intermolecular dehydration is restricted. Herein, we rstly propose that the
dehydration but also strengthened the acidity of the Brønsted shape-selective catalysis of mild HNO3 dealuminated H-Beta
acid sites (Si(OH)Al). Moreover, this mild HNO3 treatment also zeolite determines its superior performance in E-BBA dehydra-
dredged the network of intercrystalline mesopores, alleviating tion. Therefore, through the dual adjustment of acidity and
the diffusion constraints. Thus, the HB0.3 catalyst exhibited porosity, dealuminated H-Beta has a promising future in the
better performance in both E-BBA conversion and 2-EAQ green synthesis of 2-EAQ.
selectivity than the HB catalyst. However, when further
increasing the HNO3 concentration to 0.6 and 2.0 M, the E-BBA
conversion was not inuenced but the 2-EAQ selectivity
decreased to some extent. As deduced from NMR and Ar phys-
isorption results (Table 2 and Fig. 4 and 8), some of the
Conflicts of interest
There are no conicts to declare.
framework aluminium species were extracted from the zeolite
framework lattice generating a large amount of mesopores with
diameters between 2 and 5 nm. It is reasonable to deduce that
References
most of these newly formed EFAL species were located in these
mesopores, which is different from the location of EFAL species
in the parent H-Beta zeolite. The latter is mainly located in
micropores and intercrystalline mesopores. The diffusion of E-
BBA and 2-EAQ products in the HB0.6 and HB2.0 catalysts is
faster than that in H-Beta zeolite. And the accessibility of the
acid sites inside the channels of these two catalysts is sufficient
to maintain high E-BBA conversion, even though the strength
and number of Brønsted acid sites in HB0.6 and HB2.0 appar-
ently decreased (Fig. 5(b) and (c)). These remaining acid sites
are sufficient for E-BBA conversion, but the existence of
framework vacancies provides a larger space for the intermo-
lecular dehydration of E-BBA, leading to decreased 2-EAQ
selectivity.
1 G. Blanco-Brieva, J. M. Campos-Martin and J. L. G. Fierro,
Angew. Chem., Int. Ed., 2006, 45, 6962.
2 G. Blanco-Brieva, M. P. de Frutos, J. M. Campos-Martin and
J. L. G. Fierro, Green Chem., 2010, 12, 1163.
3 C. G. Bi, Z. S. Li, W. H. Qiao, G. R. Wang and J. X. Yu, Fine
Spec. Chem., 2007, 15, 20.
4 I. Kenji, S. Hiroyuki and I. Iwasaky, Jap. Pat., 7118198, 1995.
5 E. Williaml, US Pat., US 4404140, 1983.
6 D. Michel, WO Patent 96/28410, 1996.
´
´
7 A. Corma, A. Martınez and C. Martınez, J. Catal., 1994, 146,
185.
´
8 W. J. Song, Y. S. Liu, E. Barath, L. L. Wang, C. Zhao, D. H. Mei
and J. A. Lercher, ACS Catal., 2016, 6, 878.
9 J. B. Higgins, R. B. Lapierre, J. L. Schlenker, A. C. Rohrman,
J. D. Wood, G. T. Kerr and W. J. Rphrbaugh, Zeolites, 1988, 8,
446.
In summary, the framework aluminium species with strong
Brønsted acidity located inside the microporous channels were
the main active sites for intramolecular dehydration of E-BBA 10 R. Xu, X. Guo, G. Wang, J. Liu, Z. Zhang and H. Liu, Catal.
over H-Beta zeolites to 2-EAQ. Meanwhile the intermolecular Lett., 2006, 107, 149.
dehydration of E-BBA to oligomer by-products is catalysed by 11 Q. J. Hou, B. M. Zheng, C. G. Bi, J. M. Luan, Z. K. Zhao,
the aluminium species on the outer and mesoporous surface of
H. C. Guo, G. R. Wang and Z. S. Li, J. Catal., 2009, 268, 376.
H-Beta zeolites. The diffusion of reactant and product in the 12 J.-D. Chai and M. Head-Gordon, Phys. Chem. Chem. Phys.,
zeolite channels is another factor in determining the activity of
E-BBA transformation over H-Beta zeolites.
2008, 10, 6615.
13 M. A. Camblor, A. Corma and S. Valencia, Microporous
Mesoporous Mater., 1998, 25, 59.
14 T. F. Chaves, H. O. Pastone, P. Hammer and D. Cardoso,
Microporous Mesoporous Mater., 2015, 202, 198.
Conclusions
The catalytic performance of H-Beta and dealuminated H-Beta 15 J. A. van Bokhoven, D. C. Koningsberger, P. Kunkeler, H. van
zeolites in E-BBA dehydration to 2-EAQ has been systemati-
Bekkum and A. P. M. Kentgens, J. Am. Chem. Soc., 2000, 122,
cally studied. It was found that aer being treated by 0.3 M
12842.
HNO3, nano-sized H-Beta showed outstanding catalytic activity, 16 P. J. Kunkeler, B. J. Zuurdeeg, J. C. van der Waal, J. A. van
selectivity and reusability, compared with a commercial oleum
catalyst. The mild HNO3 treatment could clean out the extra-
Bokhoven, D. C. Koningsberger and H. van Bekkum, J.
Catal., 1998, 180, 234.
framework aluminium deposits and selectively extract the 17 R. Baran, Y. Millot, T. Onfroy, J. M. Kra and S. Dzwigaj,
aluminium species on the outer surface of H-Beta zeolite, which
Microporous Mesoporous Mater., 2012, 163, 122.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 9731–9740 | 9739

View Article Online
RSC Advances
Paper
18 Y. J. Zhu, B. B. Chen, R. R. Zhao, Q. Zhao, H. Gies, F. S. Xiao, 22 J. P. Marques, I. Gener, P. Ayrault, J. C. Bordado, J. M. Lopes,
D. D. Vos, T. Yokoi, X. H. Bao, U. Kolb, M. Feyen, S. Maurer, F. R. Ribeiro and M. Guisnet, C. R. Chim., 2005, 8, 399.
A. Moini, U. Muller, C. Shi and W. P. Zhang, Catal. Sci. 23 R. Otomo, T. Yokoi, J. N. Kondo and T. Tatsumi, Appl. Catal.,
Technol., 2016, 6, 6581. A, 2014, 470, 318.
19 M. Muller, G. Harvey and R. Prins, Microporous Mesoporous 24 J. P. Marques, I. Gener, P. Ayrault, J. C. Bordado, J. M. Lopes,
Mater., 2000, 34, 135.
20 D. M. Roberge, H. Hausmann and W. F. Holderich, Phys.
Chem. Chem. Phys., 2002, 4, 3128.
F. R. Ribeiro and M. Guisnet, Microporous Mesoporous
Mater., 2003, 60, 251.
25 A. Omegna, M. Vasic, J. A. van Bokhoven, G. Pirngruber and
R. Prins, Phys. Chem. Chem. Phys., 2004, 6, 447.
21 M. Maache, A. Janin and J. C. Lavalley, Zeolites, 1993, 13, 419.
9740 | RSC Adv., 2018, 8, 9731–9740
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