Carboxylic acids to butyl esters over dealuminated-realuminated beta zeolites for removing organic acids from bio-oils

Article abstract of DOI:10.1039/c7ra05298g

This article describes a novel method to dealuminate and realuminate H-beta zeolites as catalysts for removing organic acids from bio-oils via their esterification reactions with alcohols. Modified H-beta zeolites were prepared by leaching with solutions of oxalic acid, dl-malic acid, and dl-tartaric acid that have different numbers of hydroxyl groups. The results showed that, while all three organic acids can dealuminate the parent H-beta zeolite, with Al(vi)_a atoms and Al(iv)_c ones being preferentially removed, they show quite different realumination abilities, with tartaric acid with two hydroxyl groups having the highest realumination ability. The concomitance of dealumination and realumination and their dependence on the hydroxyl group numbers of the organic acids provide the possibility of finely tuning the Al and acidity distributions of the resulting zeolites. Among the three acid treated H-beta zeolites, the one obtained from malic acid leaching exhibited the best performance in catalyzing the esterification reaction between acetic acid and sec-butyl alcohol, attributed to its suitable quantity and density of medium and strong Br?nsted acid sites and enhanced aluminum gradient. The catalytic results obtained in a fixed-bed microreactor revealed that the malic acid leached H-beta exhibited dramatically enhanced catalytic performance compared to the commercial ion-exchange resin Amberlyst 15, demonstrating great potential for industrial application.

Full text of DOI:10.1039/c7ra05298g

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Carboxylic acids to butyl esters over
dealuminated–realuminated beta zeolites for
removing organic acids from bio-oils†
Cite this: RSC Adv., 2017, 7, 33714
Jianhua Li,‡ Haiyan Liu,‡* Tingting An, Yuanyuan Yue and Xiaojun Bao *b
a
a
a
b
This article describes a novel method to dealuminate and realuminate H-beta zeolites as catalysts for
removing organic acids from bio-oils via their esterification reactions with alcohols. Modified H-beta
zeolites were prepared by leaching with solutions of oxalic acid, DL-malic acid, and DL-tartaric acid that
have different numbers of hydroxyl groups. The results showed that, while all three organic acids can
a c
dealuminate the parent H-beta zeolite, with Al(VI) atoms and Al(IV) ones being preferentially removed,
they show quite different realumination abilities, with tartaric acid with two hydroxyl groups having the
highest realumination ability. The concomitance of dealumination and realumination and their
dependence on the hydroxyl group numbers of the organic acids provide the possibility of finely tuning
the Al and acidity distributions of the resulting zeolites. Among the three acid treated H-beta zeolites,
the one obtained from malic acid leaching exhibited the best performance in catalyzing the esterification
reaction between acetic acid and sec-butyl alcohol, attributed to its suitable quantity and density of
medium and strong Br o¨ nsted acid sites and enhanced aluminum gradient. The catalytic results obtained
in a fixed-bed microreactor revealed that the malic acid leached H-beta exhibited dramatically enhanced
catalytic performance compared to the commercial ion-exchange resin Amberlyst® 15, demonstrating
great potential for industrial application.
Received 10th May 2017
Accepted 26th June 2017
DOI: 10.1039/c7ra05298g
rsc.li/rsc-advances
reactions between acids and alcohols are considered to be
promising. Esterication reactions are usually catalyzed by
1
. Introduction
4
Due to the fast depletion of fossil fuel reserves and the acidic catalysts, so development of high-performance acidic
increasing awareness of the environmental impacts of fossil catalysts becomes key for the efficient conversion of acids in
fuel combustion, renewable energy is now becoming key in bio-oils.
1
moving the world economy towards a more sustainable future.
Nowadays liquid mineral acids are the most heavily used
In this respect, biomass is presently the only sustainable source catalysts for esterication reactions, but they suffer from several
2
of organic carbon, which necessitates the development of drawbacks such as serious corrosion to process devices,
various processes to convert biomass to bio-oils as alternative unavoidable formation of unwanted by-products, and difficult
3
fuels. Production of liquid bio-oils from biomass through separation from the reaction system. To overcome the above
thermal processes under fast pyrolysis conditions has great drawbacks, various solid acid catalysts such as ion-exchange
5
,6
7
8
9
potential since the resulting bio-oils can be more conveniently resins, heteropoly acids, zeolites, bres and solid super-

10
11
tted into the existing transportation fuel facilities than many acids like sulphated zirconia and niobium acid have been
other biofuels. However, a substantial amount of organic acids extensively explored. Particularly, zeolite-based catalysts are
existing in thermal-pyrolysis bio-oils can lead to serious corro- now receiving increasing attention because of their high
sion of process devices and lower storage stability. Therefore, it hydrothermal stability which can endow catalysts with longer
is necessary to decrease the content of organic acids in thermal life as compared with ion-exchange resins and their tunable
pyrolysis bio-oils. Among various routes available, esterication acidity which can avoid the occurrence of side reactions as
12
encountered for heteropoly acids and solid superacids.
It has been reported that, compared with ultra-stable zeolite
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing Y (USY) with faujasite structure and zeolites with MFI structure,
a
1
02249, P. R. China. E-mail: baoxj@fzu.edu.cn; Fax: +86 591 22865220
zeolite beta with BEA structure exhibits much better perfor-
b
National Engineering Research Center of Chemical Fertilizer, College of Chemical
Engineering, Fuzhou University, Fuzhou 350116, P. R. China
13,14
mance for esterication reactions.
However, excessive strong
acid sites and improper Al and acidity distributions in H-form
†
Electronic supplementary information (ESI) available: Characterization
beta (H-beta) zeolites lead to poorer esterication selectivity
methods and data. See DOI: 10.1039/c7ra05298g
15,16
and product yield.
Therefore, it is necessary to develop
‡
These authors contributed equally to this work.
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a novel method to nely tune the acidity of H-beta zeolites. It beta and T-beta, respectively. All of the above samples were
has been found that dealumination of beta zeolites by steam- stored in a drier over a saturated CaCl solution to equilibrate
2
1
7
18
ing, leaching with mineral acids such as HCl, and treatments with water vapor.
19
with chemicals such as ammonium hexauorosilicate, and
organic acids
20,21
are effective for this purpose. Among these
2.2 Catalytic tests
approaches, acid leaching is considered to be the most feasible
route from the point of view of environmental protection and
industrial application, but the negative effects of dealumination
by acid leaching are structural changes in zeolite crystals and
uncontrollable loss of framework aluminum atoms that
decreases active sites. Therefore, it is desirable to reinsert
aluminum atoms into the framework of the partially deal-
uminated framework of zeolite beta. Several routes have been
proposed for the post-synthetic incorporation of aluminum into
Catalysts were made by crushing the zeolite samples into
particles of size 20–40 mesh. For comparison purpose, a resin
catalyst Amberlyst® 15 (A15, dry, exchange capacity $ 4.70 eq.
kg , Rome & Hass) that is widely applied as esterication
catalyst in industry was used as a reference catalyst.
The esterication reaction of acetic acid ($99.5 wt%,
Aladdin) with sec-butyl alcohol ($99 wt%, Aladdin), which was
used as a model reaction representing the stabilization of bio-
oils, was performed in a continuously owing tubular xed-
bed microreactor of internal diameter 14.0 mm and length 650
mm made from a 316L stainless steel pipe. During catalytic test,
ꢀ
1
3 3
zeolites. The treatment with AlCl or AlBr vapor at higher
temperature was found to be able to incorporate aluminum
atoms into the framework of high-silica zeolite ZSM-5 and thus
3.0 g of a catalyst sample or A15 were packed in the middle of the
22
adjust its acidity and activity. It was also reported that extra-
framework Al species produced in hydrothermal treatment
reactor and xed between a layer of quart spheres with a diam-
eter of 2 mm in the bottom and a layer of quartz sands in the top.
A mixture of acetic acid and sec-butyl alcohol in a molar ratio
of 1 : 2 was pumped into the reactor by a mass owmeter at
2
3
could be subsequently reinserted into the frameworks of Y,
ZSM-5
solution at elevated temperature. In addition, Omegna et al.
24
25
and beta by treatments with an aqueous alkaline
26
ꢀ1
a weight hourly space velocity (WHSV) of 0.8 h and heated to
93 K. The reaction pressure was controlled at 2.5 MPa by
reported that zeolite beta could be dealuminated by treatment
with hydrochloric acid and realuminated by treating the deal-
3
passing nitrogen through the reactor. The esterication prod-
ucts were analyzed with a SP-3420A gas chromatograph (GC)
installed with a PEG-20M column (30 m ꢁ 0.25 mm ꢁ 0.25 mm)
and a ame ionization detector. The analysis results were
calculated by using acetone (AR, Beijing Chemical Reagent Co.,
27
uminated zeolite beta with aluminum isopropoxide. Fan et al.
observed the realumination effect of citric acid on HZSM-5
zeolites dealuminated by steaming, and found that the steam-
ing treatment prior to the citric acid leaching was the precon-
28
dition of realuminating HZSM-5 zeolites. Xie et al. also
reported the realumination effect of single citric acid treatment
on beta zeolite. While the aforementioned researches demon-
strated the feasibility of realuminating dealuminated zeolites, it
seems that there is no report on how to nely control the real-
umination process of beta zeolites by acid treatment, to which
the present investigation is addressed.
29
PR China) as the internal standard. The fractional conversion
(X
AA) of acetic acid, the selectivity (SSBAC) to sec-butyl acetate
(SBAC) and the yield (YSBAC) of SBAC were calculated as follows:
n
acetic acid in feed ꢀ nacetic acid in product
X
AA ð%Þ ¼
ꢁ 100
n
acetic acid in feed
Herein we present a thorough investigation on the modi-
cation of H-beta zeolite using the solutions of three organic
acids (oxalic acid, DL-malic acid, and DL-tartaric acid) that all
contain two carboxyl groups but different numbers of hydroxyl
groups, with the aim of understanding whether and how the
number of hydroxyl groups in these organic acids inuences the
Al state and acidity of the resulting beta zeolites and laying
a fundamental basis for developing a zeolite beta based catalyst
for removing organic acids from bio-oils via their esterication
reactions with alcohols.
n
SBAC
S
SBAC ð%Þ ¼ n
ꢁ 100
SBAC þ nsec-butyl ether þ nmethyl ethyl ketone
Y
SBAC (%) ¼ XAA ꢁ SSBAC ꢁ 100
where nacetic acid in feed and nacetic acid in product are the moles of
acetic acid in the feed and product, respectively; nSBAC, nsec-butyl
and nmethyl ethyl ketone are the moles of SBAC, sec-butyl ether
and methyl ethyl ketone in the product, respectively.
ether
2
. Experimental
2
.3 Characterizations
Detailed information on characterizations can be found in the
2 3
/Al O
(mol ESI.†
2.1 Modications of beta zeolites
A parent zeolite beta in protonic form (H-beta, SiO
2
ꢀ
1
mol ) ¼ 21, Catalyst Plant of Nankai University, PR China) was
treated with a 0.1 M solution of oxalic acid ($98 wt%, Beijing
Chemworks, PR China), DL-malic acid ($99.0 wt%, Aladdin) and
DL-tartaric acid (AR, Aladdin), respectively, at 333 K for 4 h,
repeatedly ltrated and washed with deionized water until the
3
3
. Results and discussion
.1 Bulk and surface chemical compositions: FTIR, XRF and
XPS characterizations
effluent water became neutral, and nally dried at 393 K for 6 h The skeletal vibration FTIR spectra of the beta zeolites are
ꢀ1
to obtain three modied H-beta zeolites denoted by O-beta, M- shown in Fig. 1. The absorption bands at 525 and 575 cm are
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signies that the bulk molar Si/Al ratios of the acid-leached beta
zeolites increase with the increasing acidity of the organic acids
used. The FTIR results show that the framework molar Si/Al
ratio of T-beta is identical to that of M-beta, while the XRF
analysis results reveal that the bulk molar Si/Al ratio of T-beta is
higher than that of M-beta, suggesting that more extra-
framework Al species are removed from T-beta than from
M-beta.
One most important feature responsible for the enhanced
catalytic performance of beta zeolite is the presence of
32
aluminum gradient across the zeolite crystals. For this reason,
XPS measurements were performed to study the aluminum
gradients in the different samples and the results are presented
in Table 1. In Table 1, the (Si/Al)surf/(Si/Al)bulk values indicating
Al excess or deciency on crystal surface compared with in the
bulk phase are also given. It can be seen that: the value of (Si/
Al)surf/(Si/Al)bulk for H-beta is about 1, indicating that aluminum
atoms are uniformly distributed in the zeolite; however, the
values of (Si/Al)surf/(Si/Al)bulk for O-beta and M-beta decrease to
Fig. 1 FTIR spectra of H-beta, O-beta, M-beta and T-beta in the
region of framework vibrations.
characteristic of zeolite beta due to the presence of ve- and six-
30
membered rings in the structure. The band at about 1092
0.56 and 0.92, respectively, demonstrating the preferential
ꢀ
1
cm represents the O–T–O asymmetric stretching vibration
a(OTO)) which is sensitive to the content of framework
dealumination from the zeolite surface. This can be rational-
ized by the preferential attack of the zeolite outer surface by
hydrogen ions in the acidic media. Interestingly, the value of (Si/
Al)surf/(Si/Al)bulk for T-beta remained at about 1, in contrast to
the intuition that the stronger acidity of tartaric acid than malic
acid should have given a lower value of (Si/Al)surf/(Si/Al)bulk than
that of M-beta. It is speculated that this is because of the rein-
sertion of partial aluminum atoms in the solution that were
removed from the zeolite structure into the zeolite framework
on the external surface, which will be further discussed below.
(n
aluminum; more exactly, this wavenumber increases with the
decreasing aluminum content in the zeolite structure, therefore
the shi of this wavenumber indicates the change in the
31
framework molar Si/Al ratio. As seen in Fig. 1 and Table 1,
ꢀ
1
n
a(OTO) appearing at 1092 cm
for H-beta shis to higher
ꢀ
1
wavenumbers at 1098, 1094 and 1094 cm for O-beta, M-beta
and T-beta, respectively, indicating the decreased content of
framework aluminum. This should be ascribed to the leaching
of framework aluminum species by the organic acids that
results in the increased framework molar Si/Al ratios. Because
the acidity of oxalic acid is the strongest among the three
3.2 States of Al in beta zeolites
organic acid, the most decreased framework aluminum content The properties and catalytic performance of zeolites depend on
of O-beta should be related to the strongest dealumination the state of Al species in them, therefore the study of the Al
3
3
27
ability of oxalic acid. Interestingly, although the acidity of tar- coordination state is needed.
taric acid is stronger than that of malic acid, na(OTO) for T-beta is copy is known as an efficient tool to determine the coordination
Al (MQ) MAS NMR spectros-
34
identical to that for M-beta, so there may be other phenomenon and local structure of aluminum atoms in zeolitic materials.
2
7
accompanied with dealumination.
The one-dimensional (1D) Al MAS NMR spectra of H-beta
XRF analysis of zeolites can provide the information on the and its derived samples are shown in Fig. 2. In the spectrum
bulk SiO and Al O contents. Table 1 shows that the bulk molar of H-beta, there are an intense and sharp peak at about 53 ppm
2
2 3
Si/Al ratios of the beta zeolites increase aer the organic acid ascribed to aluminum atoms in tetrahedral coordination and
treatments and are in the order of O-beta > T-beta > M-beta, a signal at about 0 ppm assigned to octahedrally coordinated
which exactly matches with the acidity order of the three aluminum atoms. Fig. 3(a) shows the corresponding two-
2
7
organic acids (oxalic acid (0 hydroxyl group) > tartaric acid (two dimensional (2D) Al MQ MAS NMR spectrum of H-beta in
2
7
hydroxyl groups) > malic acid (one hydroxyl group)). This the form of contour plot, with the 1D Al MAS NMR spectrum
Table 1 Chemical compositions of H-beta, O-beta, M-beta and T-beta
b
c
Bulk composition
Surface composition
a
ꢀ1
Entry
Sample
n
a(OTO) (cm
)
SiO (wt%) Al
2
2
O
3
(wt%)
(Si/Al)bulk
Si 2p
Al 2p
(Si/Al)surf
(Si/Al)surf/(Si/Al)bulk
1
2
3
4
H-Beta
O-Beta
M-Beta
T-Beta
1092
1098
1094
1094
92.02
98.87
95.09
96.00
7.42
1.08
4.75
3.81
10.5
77.5
17.0
21.4
29.18
30.48
29.71
29.93
2.58
0.70
1.89
1.37
11.31
43.54
15.72
21.85
1.08
0.56
0.92
1.02
a
b
c
Evaluated from Fig. 1 according to na(OTO)
.
Calculated from XRF data. Calculated from XPS data.
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aluminum in a crystalline silicoaluminate framework. Accord-
36
ing to the literature, they can be assigned to aluminum atoms
occupying different T-sites: the peak Al(IV)b is assigned to
aluminum atoms on positions T1 and T2 in the framework of
zeolite beta, and the peak Al(IV)
positioned in T3–T9 sites. An indication of a third tetrahedral
species, Al(IV) , is visible in the isotropic dimension, which is
a
corresponds to aluminum
c
characterized by a strong quadrupolar interaction. This species
may be locally distorted aluminum atoms associated with
defective sites in the beta framework, or extra-framework
amorphous aluminum-oxide species which were formed
during the removal of template at high temperature or the
26
process converting the zeolite into protonic form. Two reso-
nances are seen in the octahedral region. The sharp octahedral
27
Fig. 2
Al NMR MAS spectra of H-beta, O-beta, M-beta and T-beta.
a
peak represented by Al(VI) has been ascribed to framework
connected octahedral aluminums formed by partial hydro-
37,38
lysis.
These octahedral Al atoms show a weak quadrupolar
being put on the top. The projection of the 2D spectrum along
F1 axis represents the pure isotropic spectrum, free from
anisotropic quadrupolar broadening. The tetrahedral region
between 53 and 65 ppm consists of at least two components,
interaction and a small distribution in the isotropic chemical
shis (Table 2). However, the resonance Al(VI) which is a weak
b
35
signal shows strong quadrupolar interaction and is associated
with highly distorted aluminum atoms. Table 2(a) and (b) gives
2
7
Al(IV)
a b
and Al(IV) , which are well-resolved in the isotropic
the deconvolution results of the Al (MQ) MAS NMR spectra to
understand the change of Al sites in the beta zeolites. From the
table, it is also noted whether these species are of framework
dimension. They resonate close to the diagonal, indicating
a small quadrupolar induced shi (Table 2) and being typical of
27
27
Fig. 3
Al MQ MAS NMR spectra of (a) H-beta, (b) O-beta, (c) M-beta, and (d) T-beta, in which the corresponding Al MAS NMR spectra are
given on the top of each figure. The F1 projection represents the pure isotropic spectrum.
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Table 2 NMR parameters and relative Al contents determined by Al (MQ) MAS NMR
a) Peak parameters determined from the spectra
Paper
27
(
Isotropic chemical
shi (ppm)
Quadrupolar coupling
constant (MHz)
Entry
Peak
1
2
3
4
5
Al(IV)
Al(IV)
Al(IV)
Al(VI)
Al(VI)
a
b
c
58.7
54.7
59.1
ꢀ0.1
2.3
2.2
1.9
5.5
0.25
5
a
b
(b) Relative peak intensities of the spectra of beta zeolites
Peak Al(IV)
FW
a
Peak Al(IV)
FW
b
Peak Al(IV)
N-FW
c
Peak Al(VI)
FW
a
Peak Al(VI)
N-FW
b
Relative total
intensity
a
Ratio Al(IV) /
Al(IV)
b
a
b
Entry
Sample
1
2
3
4
H-Beta
O-Beta
M-Beta
T-Beta
21.51
10.77
23.86
26.47
62.74
89.23
63.64
63.53
7.17
0.00
5.68
4.12
2.43
0.00
2.84
5.88
6.15
0.00
3.98
0.00
1.00
0.03
0.59
0.77
0.34
0.12
0.38
0.42
a
b
Framework. Extra-framework.
(FW) or non-framework (N-FW) in nature. A typical t illustrated beta are similar to that of H-beta (Fig. 3), although they have
in Fig. S1† shows excellent agreement between the data and t. changed relative intensity of the peaks (Table 2). The malic acid
Fig. 2 shows that the overall intensity of the spectrum of O- and tartaric acid treatments result in the decreases of extra-
beta obtained by leaching with the solution of oxalic acid framework tetrahedral species Al(IV)
signicantly diminishes. This indicates that a plenty of drally coordinated species Al(VI) , whereas traces of octahedrally
aluminum atoms are removed from O-beta, consistent with the coordinated framework species Al(VI) are still present. As re-
higher molar Si/Al ratio (77.5) of O-beta obtained by the XRF ported in literature, the decreases in the contents of Al(IV) and
characterization. Through the oxalic acid leaching, framework Al(VI) indicated that the treatment with the solution of organic
c
and the distorted octahe-
b
a
39
c
a
octahedral species Al(VI) , extra-framework octahedral species acid removed aluminum atoms preferentially from tetrahe-
a
Al(VI) and tetrahedral species Al(IV) are completely removed, drally coordinated extra-framework species represented by
b
c
and a considerable quantity of the framework tetrahedral Al(IV)
c
and distorted octahedrally coordinated framework
are extracted, as revealed by the Al species characterized by Al(VI) . It was also reported that the
MQ MAS NMR characterization results (Fig. 3 and Table 2). Al(IV) species changed the coordination to octahedral upon
Moreover, the obvious decrease in the amount of Al(IV) species partial hydrolysis while maintaining the connection with the
2
7
species Al(IV)
a
and Al(IV)
b
a
a
a
indicates that the oxalic acid treatment preferentially removes framework. The malic acid (with one hydroxyl group) and tar-
aluminum atoms from crystallographic positions represented taric acid (with two hydroxyl groups) treatments increase the
by Al(IV) . This is consistent with the previous results reporting relative content of the framework tetrahedral species Al(IV) of
a
a
that Al(IV) species were partially dealuminated by hydrothermal the resulting samples, indicating their realumination effect on
a
36
treatment at high temperature and that Al(IV)
a
species were H-beta zeolite in addition to dealumination, in accordance with
preferentially leached by washing beta zeolite with a 1 M the result that the citric acid (with one hydroxyl group) treat-
26
hydrochloric acid solution. The Al(IV)
to aluminum atoms positioned at T3–T9 sites are assigned to tion functions.
aluminum atoms partially connected to the framework repre- The above results reveal that the leaching of H-beta with the
a
species corresponding ment of zeolite beta have both dealumination and realumina-
28
senting lattice defective sites in zeolite H-beta. It was reported solutions of the three organic acids does not result in the
that the Al(IV) species corresponding to aluminum atoms in T1 formation of any new aluminum species. The dominant process
b
and T2 sites appeared to be more resistant to nitric acid treat- during the organic acid leaching is the partial removal of
ment than those in T3–T9 sites, and it seems that the latter sites framework tetrahedral aluminums represented by peaks Al(IV)
are present in four-membered rings, which exhibit the smallest and Al(IV) . Table 2(b) compares the ratios of the two resonances
strain with the lowest T–O–T angle allowing the easier removal corresponding to framework tetrahedral aluminums Al(IV) and
as determined from the isotropic dimension spectra of
Fig. 2 also shows that the overall intensity of the spectra of the zeolite samples (Fig. 3). The ratio of Al(IV) /Al(IV) from 0.34
a
b
a
35
of Al atoms from these sites.
Al(IV)
b
a
b
M-beta and T-beta obtained by leaching with the solutions of for H-beta decreases to 0.12 for O-beta, but rises back to 0.38 for
malic acid and tartaric acid, respectively, does not obviously M-beta and 0.42 for T-beta. The higher ratios of Al(IV) /Al(IV) of
a
b
2
7
decrease, and the Al MQ MAS NMR spectra of M-beta and T- M-beta and T-beta than H-beta conrm that aluminum atoms
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were incorporated into the defect sites of the framework and matrix and the removal of partial aluminum atoms from the
transformed into Al(IV) species. It is worth noting that tartaric zeolite framework. Therefore, the framework molar Si/Al ratio is
a
acid with two hydroxyl groups has stronger acidity than malic in the order of O-beta > M-beta ¼ T-beta > H-beta, consistent
acid with one hydroxyl group, but they resulted in M-beta and T- with the FTIR results. It is noticed that, although the acidity of
beta with the identical framework molar Si/Al ratio, with the tartaric acid is stronger than that of malic acid, they have the
framework Al content of T-beta being higher than that of M-beta identical leaching effect on the zeolite structure, as reected by
and therefore concluding the stronger realumination ability of the identical 2q and d values of M-beta and T-beta for the (302)
tartaric acid than malic acid.
reection. This further conrms the stronger realumination
ability of tartaric acid than malic acid.
3
.3.2 Pore structure characterization. The nitrogen
3
.3 Phase structure, textural property and acidity
characterizations
.3.1 XRD characterization. Fig. 4 gives the XRD patterns of
adsorption–desorption isotherms of H-beta and the organic
acid leached beta zeolites are shown in Fig. S2.† They all exhibit
a type IV isotherm with a hysteresis loop. The BET specic
surface areas and pore volumes of the different beta zeolites are
given in Table 3. It can be seen that the three organic acids
leached samples have obvious increases in BET surface area and
micropore volume due to the removal and/or reinsertion of
extra-framework Al species that endow the resulting zeolites
with opener pore channels and nearly intact crystallinity.
3
the different beta zeolites. It can be seen that, similar to the
parent H-beta zeolite, all of the organic acid leached samples
have the XRD patterns displaying representative Bragg reec-
tions corresponding to the typical BEA zeolite topology,
all have well-maintained crystalline structure, in agreement
40,41
and
39
with the literature results for acid leached beta zeolites. It is
ꢂ
well known that the distinct signal at 2q ¼ 6.5–8.5 ascribed to
3
.3.3 Acidity characterizations. To understand the effects
of the different organic acid treatments on the acidity of the
resulting beta zeolites, NH -TPD and Py-IR measurements were
(101) reection is an indication of a highly distorted structure
due to the presence of different isomorphs in the zeolite
structure.
3
42,43
The relatively wider full widths at half-maxima of
carried out and the results are given in Fig. 5 and S3† and
Table 4.
ꢂ
ꢂ
the diffraction peaks at 2q ¼ 7.72 and 22.52 signify that the
44
zeolite beta employed in this work has a smaller crystal size.
3
Fig. 5 shows the NH -TPD proles of H-beta and the organic
ꢂ
The narrow main diffraction peak near 22–23 ascribed to
302) reection is generally taken as an evidence of lattice
acid leached beta zeolites. There exist two kinds of acid sites in
H-beta: weak acid sites represented by a typical desorption peak
at around 513 K and strong acid ones signied by a typical
desorption peak at around 683 K. Similar results were obtained
for all of the modied samples. For the oxalic acid leached
sample O-beta, the amount of weak acid and strong acid
dramatically decreased, indicating the removal of aluminum
atoms in a large amount; for the malic acid leached sample M-
beta and the tartaric acid leached sample T-beta, the amount of
weak acid obviously decreased, but that of strong acid remained
almost unchanged, possibly ascribed to the predominant
removal of extra-framework Al species from the parent H-beta
zeolite by the treatments with organic acids with hydroxyl
groups.
Fig. S3† shows the Py-IR spectra of H-beta and the organic
acids leached samples in the region of 1575–1400 cm . It can
be seen that there are three sharp bands due to C–C stretching
vibrations of pyridine, with the strong band at 1490 cm
assigned to pyridine molecules adsorbed on both Br ¨o nsted (B)
and Lewis (L) acid sites, and those at 1540 and 1450 cm to
protonated pyridine molecules by B acid sites and pyridine
(
45
contraction/expansion of the beta structure. As seen in Fig. 4
and Table S1,† for the parent H-beta zeolite, the narrow main
diffraction peak is at 22.52 and the corresponding d302 spacing
is 3.945 A; for the oxalic acid leached sample O-beta, this peak
ꢂ
˚
ꢂ
shis to 22.60 and the corresponding d302 spacing decreases to
˚
3.932 A, indicating some relaxation of the BEA matrix and
suggesting the removal of aluminum atoms from zeolite
framework in a great degree; and for the malic acid leached
sample M-beta and tartaric acid leached sample T-beta, this
peak shis to 22.54 and the corresponding d302 spacing
decreases to 3.941 A, indicating the slight relaxation of the BEA
46
ꢂ
˚
21
ꢀ1
ꢀ
1
ꢀ
1
47
molecules adsorbed on L acid sites, respectively. For organic
acid leached beta zeolites, the intensity of all the bands at 1490
ꢀ
1
cm is considerably weaker than that for H-beta, suggesting
that the decreased total acid sites.
Table 4 presents the quantitative results of the Py-IR
47
spectra. It can be seen that O-beta obtained by leaching H-
beta with the solution of oxalic acid has the largely decreased
quantities of both B and L acids due to the massive removal of
aluminum atoms from the zeolite. These results are in good
4
8
49
agreement with those of Maache et al. and Apelian et al. who
Fig. 4 XRD patterns of H-beta, O-beta, M-beta and T-beta zeolites.
The asterisks refer to the reflections listed in Table S1.†
reported the decreased quantities of B and L acids in H-beta
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a
Table 3 Textural properties of H-beta, O-beta, M-beta and T-beta
a
2
ꢀ1
b
3
ꢀ1
c
3
ꢀ1
Entry
Sample
Specic surface area (m g
)
Micropore volume (cm g
)
Total pore volume (cm g )
1
2
3
4
H-Beta
O-Beta
M-Beta
T-Beta
528
591
553
563
0.19
0.22
0.21
0.20
0.36
0.40
0.39
0.39
a
b
c
Calculated with the BET method. Calculated with the t-plot method. Calculated from nitrogen single point adsorption at P/P
0
¼ 0.99.
28
with the results of Xie et al. It was reported that the quantity of
strong B acid of beta zeolite was increased and the distribution
of aluminum atoms was changed, ascribed to the concurrence
of dealumination and realumination during the citric acid
treatment. T-Beta has a higher quantity of medium and strong B
acid than M-beta, indicating that more aluminum atoms were
reinserted into the framework of the former. In addition,
51
52
Guisnet et al. and Sulikowski et al. reported that, for zeolites
mazzite and Y, the framework Si(2Al) site corresponding to
(
SiO) (AlO)Si–OH–Al(OSi) is less acidic than the framework
2 3
Si(1Al) site corresponding to (SiO) Si–OH–Al(OSi) . From this
3
3
point of view, we can infer that the framework aluminum atoms
of M-beta and T-beta at Si(2Al) sites may be partially removed
from the zeolite framework, and some aluminum species in the
solutions of malic acid and tartaric acid were retransformed to
framework Si(1Al) species with the help of hydroxyl groups of
the organic acids, therefore the quantities of medium and
strong B acid of M-beta and T-beta are higher than that of H-
beta. Similarly, more framework aluminum atoms of H-beta
at Si(2Al) sites could be removed from its framework by tarta-
ric acid that has stronger acidity than malic acid, and more
aluminum species in the tartaric acid solution can be trans-
formed into framework Si(1Al) species with the help of more
hydroxyl groups in the tartaric acid solution than in the solution
malic acid, leading to the increased medium and strong B
acidity of T-beta than M-beta.
3
Fig. 5 NH -TPD profiles of H-beta, O-beta, M-beta and T-beta.
Table 4 Acid types and strength distributions of H-beta, O-beta, M-
beta and T-beta
ꢀ1
Acidity (mmol g
)
Medium
and strong
acid
Weak acid
Total
L
Entry Sample
L
B
L
B
B
L + B
1
2
3
4
H-Beta
O-Beta
40.0 98.8 203.5 68.3 243.5 167.1 410.6a
0.0 47.4 25.4 31.0 12.7 78.4 103.8
In addition, in view of the signicant change of the medium
and strong B acidity caused by the organic acid treatments, the
distribution of tetrahedrally coordinated framework Al species
M-Beta 19.4 90.1 125.3 69.7 144.7 159.8 304.5
T-Beta
6.6 75.3
91.6 78.6
98.2 153.9 252.1
a
a b
(Al(IV) /Al(IV) ) is correlated to the medium and strong B acidity
Total(L+B) ¼ 2total
L
+ total
B
.
zeolites treated with inorganic acids such as HCl or organic
acids such as dicarboxylic acid. Unexpectedly, the quantity of
medium and strong L acid of O-beta is higher than that of
total L acid, indicating the elimination of water that results in
the conversion of two B acid sites into one L acid site at high
50
temperature, as reported by Ward et al. The interconversion
between L and B acid sites of aluminum atoms connected to the
zeolite framework seems to be a characteristic of zeolite because
18
of its rich structure defects.
It can be also seen that M-beta and T-beta obtained by
leaching H-beta with the solution of malic acid and the solution
of tartaric acid, respectively, have the increased quantity of
medium and strong B acid, indicating the reinsertion of partial
aluminum atoms into the framework and being in agreement
Fig. 6 Al(IV)
a
/Al(IV)
b
vs. medium and strong B acidity.
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28
and the results are shown in Fig. 6. It can be seen that the
As reported by Xie et al., the realumination mechanism of
medium and strong B acidity is closely related to the distribu- beta zeolite is isomorphous substitution when it is treated with
tion of tetrahedrally coordinated framework species. citric acid (with one hydroxyl group), as encountered in the
2
7
By combining these results and the Al (MQ) MAS NMR treatments with malic acid (with one hydroxyl group) and tar-
results, it can be seen that, partial framework aluminum atoms taric acid (with two hydroxyl groups). While malic acid and
were removed from the H-beta zeolite during the organic acid tartaric acid can also function as chelating agents to complex
+
treatments. Because extra-framework Al species in zeolites are Al(OH)
2
in the malic acid solution and tartaric acid solution
21
responsible for weak acid sites, the decreased quantities of formed in the dealumination step (see step 2(b) and (c)), their
weak L and B acids of the beta zeolites aer organic acid different numbers of –OH groups can reduce the chelating
leaching (Table 4) indicate the extraction of most extra- extent of the resulting Al–malic acid complex and Al–tartaric
framework Al species from the zeolite pore channels. The acid complex. Therefore, more Al(OH)2 cations in these
oxalic acid leaching not only removes framework aluminum solutions are hydrolyzed to produce Al(OH) anions (see step
28
+
ꢀ
4
ꢀ
atoms from the zeolite framework in large amount but also 3), then Al(OH)
4
anions can be reinserted into the framework
brings them out of the zeolite pore channels leading to opener of beta zeolite at the defect sites (see step 4). On the one hand,
pore channels, i.e., the oxalic acid leaching has dual roles: the more framework tetrahedral Al species were removed by tartaric

rst is to hydrolyze Al–O–Si linkages (see step 1), and the second acid leaching than by malic acid leaching, resulting in more
+
is as a chelating agent to complex the tetrahedral Al species and Al(OH)
2
cations in the tartaric acid solution at the rst step,
then to move the resulting species out of the pore channels (see ascribing that the stronger acidity of tartaric acid; on the other
ꢀ
step 2(a)). Similar observations were reported for the deal- hand, more Al(OH)4 anions were formed and reinserted into
49
umination of beta zeolite using dicarboxylic acid and toluene- the framework of T-beta at the defect sites with the help of two
53
4
-sulfonic acid. On the whole, there is only dramatic deal- hydroxyl groups. Here, we must emphasize that the malic acid
umination during the oxalic acid (with no hydroxyl group) and tartaric acid leaching plays a third role, i.e., providing more
ꢀ
leaching.
Step 1:
Al(OH)
4
in the solutions for reinsertion into the zeolite
framework, as illustrated in Scheme 1.
Step 2:
3.4 Catalytic performance
+
2ꢀ
n1ꢀ2n
The ability of strong B acid sites in catalyzing esterication
Al(OH)
Al(OH)
Al(OH)
2
2
2
+ n[C
2
O
4
]
# Al(OH)
2
[C
2
O
4
]
(n ¼ 0, 1, 2, 3.)
reaction has been demonstrated by the well-known macro-
(a)
12
porous cation exchange resin A15 with strong acidity, so A15
was used as the reference catalyst in the present investigation.
Fig. 7 shows the values of XAA, SSBAC and YSBAC obtained over A15
and the different beta zeolites. The values of XAA and YSBAC for
A15 are 42.8% and 32.6%, respectively. The sole strong B acid
+
+
2ꢀ
+ n[H
3
C
4
O
4
(OH)]
#
1
ꢀ2n
Al(OH)
2
[H
3
C
4
O
4
(OH)]
n
(n ¼ 0, 1, 2, 3.)
(b)
2ꢀ
#
+ n[H
2
C
4
O
4
(OH)
2
]
1
ꢀ2n
Al(OH) [H C O (OH) ]
(n ¼ 0, 1, 2, 3.) (c) sites of A15 can also serve as active centers for etherication,
which makes the selectivity to the target product SBAC lower
than 80%, as shown in the GC-MS results in Fig. S4.†
Compared with A15, all of the zeolites including the parent
2 2
4ꢀ
+ 2H O # Al(OH)
2
2
4
4
2 n
Step 3:
Step 4:
+
Al(OH)
H-beta gave dramatically increased XAA and signicantly
enhanced SSBAC, with T-beta giving the highest conversion and
M-beta giving the highest selectivity. It is rational that the
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Scheme 1 Schematic representation of the dealumination and realumination mechanisms of beta zeolite via organic acid leaching.
enhanced selectivity obtained over the different zeolites can be Table 4 was correlated with XAA and the results are shown in
ascribed to the conning effect of the pore channels (6.6 ꢁ 6.7 Fig. 8(a). Because the medium and strong B acidity is positively
2
2
˚
˚
A along a- and b-axis and 5.6 ꢁ 5.6 A along c-axis) of the beta correlated to the distribution (Al(IV) /Al(IV) ) of tetrahedrally
a
b
54
zeolites. With the increasing numbers of hydroxy groups of the coordinated framework Al species, the effect of Al(IV) /Al(IV) on
a
b
organic acids used, YSBAC obtained over the resulting zeolites
XAA is also presented in Fig. 8(a). Unexpectedly, XAA does not

rst increased and then dropped, with M-beta giving the high- monotonically increase with the increasing quantity of medium
est YSBAC. This can be attributed to the compromised deal- and strong B acid, suggesting that the medium and strong B
umination and realumination abilities of malic acid leaching acid sites are not the sole factor affecting XAA. The medium and
a b
that provide the resultant M-beta with ideal Al and acidity strong B acidity and Al(IV) /Al(IV) of O-beta are lower than those
distributions. of H-beta, but XAA obtained over O-beta is higher than that over
It is surprising to see that all the beta zeolites especially H-beta. This suggests that the Al(IV) atoms at T1 and T2 sites
b
those obtained by acid leaching gave much higher conversion should be the main active sites for the esterication of sec-butyl
rates despite their much weaker acidity than A15, which may be alcohol with acetic acid.
55
due to the synergistic effect of B and L acid sites in the zeolites.
As aforementioned, the presence of aluminum gradient
In view of the fact that A15 only has B acidity, it is conceived across beta zeolite crystals is also an important factor
that, for the beta zeolites concerned here, B acidity should be accounting for the catalytic performance. To conrm this, the
mainly responsible for esterication activity. To validate this, (Si/Al)surf/(Si/Al)bulk values of the different beta zeolites in Table
the medium and strong B acidity of the different beta zeolites in 1 were correlated with XAA and the results are shown in Fig. 8(b).
It is seen that XAA rst increases and then decreases with the
increasing (Si/Al)surf/(Si/Al)bulk ratio. This can be interpreted as
follows: on the one hand, the chemical attack of hydrogen ions
in the leaching solutions on the H-beta zeolite occurs prefer-
entially at the outer surface of the H-beta zeolite that leads to
the preferential extraction of extra-framework Al species on the
outer surface of the zeolite particles and thereby the exposure of
more framework Al species; on the other hand, the presence of
hydroxyl groups in the malic acid and tartaric acid solutions
promotes the reinsertion of aluminum species formed by
leaching onto the outer surface of the zeolite framework, also
leading to the increased aluminum gradient. Therefore, the
values of XAA obtained over M-beta and T-beta were higher than
that of H-beta and O-beta, with T-beta with more medium and
strong B acid sites having higher XAA than M-beta. As a result of
it, the values of XAA obtained over the different zeolites are in the
order of T-beta > M-beta > O-beta > H-beta.
Fig. 7
XAA, SSBAC and YSBAC of A15, H-beta, O-beta, M-beta and T-
beta.
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Fig. 8 Effects of medium and strong B acidity and (Si/Al)surf/(Si/Al)bulk of beta zeolites on XAA
.
Interestingly, we found that SSBAC depends upon the sulphated zirconia showed an optimum calcination tempera-
density of medium and strong B acid sites over the beta ture for the esterication of acetic acid with butanol even both
zeolites, dened as medium and strong B acidity divided by the amount and acidity of the acid sites increased mono-
total surface area. As seen in Fig. 9, SSBAC presents a maximum tonically with the increasing calcination temperature. The
with the increasing density of medium and strong B acid sites, stability of M-beta in the esterication of acetic acid and sec-
indicating that a suitable density of medium and strong B butanol was tested and the results are shown in Fig. S5.† It can
acid sites is necessary for achieving an optimum SSBAC. It can be seen that M-beta exhibited excellent stability aer reaction
also be seen that T-beta has the lowest SSBAC because of its for 104 h.
highest density of medium and strong B acid sites and larger
total pore volume that benet the formation of the by-product
sec-butyl ether. Among the three acids used, tartaric acid with
4. Conclusions
two hydroxyl groups has the strongest dealumination and H-Beta zeolites were modied by leaching with the organic
realumination abilities, resulting in the excessive quantity acids with different numbers of hydroxyl groups, and the
and overhigh density of medium and strong B acid sites of T- effects of hydroxyl group numbers in the different organic
beta and thus negatively impacting SSBAC of the resulting T- acids on the dealumination and realumination behavior of
beta; whereas oxalic acid with no hydroxyl group has only beta zeolite were comprehensively investigated. The charac-
dealumination ability without realumination ability, leading terization results showed that: the oxalic acid (with no
to the inadequate density of medium and strong B acid sites of hydroxyl group) leaching could only dealuminate the Al atoms
O-beta, unfavorable for the formation of SBAC; distinctly, at Al(IV)
c
, Al(VI)
a
, and Al(VI)
b
sites and massive framework
malic acid with one hydroxyl group owns compromised deal- tetrahedral Al atoms at Al(IV)
a
and Al(IV) sites, resulting in the
b
umination and realumination abilities, yielding a zeolite decreased quantities of Lewis and Br ¨o nsted acids to a great
M-beta with appropriate quantity and density of medium degree, while the malic acid and tartaric acid leaching could
and strong B acid sites and thereby the highest SSBAC
,
not only partially dealuminate the Al atoms at the outer
+
1
2
consistent with the results of Peters who reported that the surface preferentially but also reinsert the Al(OH)
2
species in
the leaching solutions into the framework of beta zeolites at
T3–T9 sites, with tartaric acid with two hydroxyl groups having
higher realumination ability than malic acid with only one
hydroxyl group. Among the three organic acid treated H-beta
zeolites, the one obtained by malic acid leaching exhibited
the best performance in catalyzing the esterication, attrib-
uted to its suitable quantity and density of medium and strong
Br ¨o nsted acid sites and enhanced aluminum gradient. The
malic acid treatment could be taken as an effective way to
optimize the distribution of Al species in H-beta zeolite and
thus improve the catalytic performance of the resulting H-beta
zeolite.
Acknowledgements
We gratefully acknowledge the National Natural Science Foun-
dation of China for providing nancial support through grants
U1462203 and 91434206.
Fig. 9 Density of medium and strong acid B sites of the beta zeolites
vs. SSBAC
.
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2
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