Zeolite nanofiber assemblies as acid catalysts with high activity for the acetalization of carbonyl compounds with alcohols

Article abstract of DOI:10.1039/c3ra47952h

Zeolite nanofiber assemblies (HNB-MOR) as efficient heterogeneous catalysts for the formation of a range of acetals in good yields. The mesoporosity of HNB-MOR benefits mass transfer, and the strong acidic sites on HNB-MOR facilitate acetalization activity. The catalyst can be reused 10 times without loss of activity.

Full text of DOI:10.1039/c3ra47952h

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Zeolite nanofiber assemblies as acid catalysts with
high activity for the acetalization of carbonyl
compounds with alcohols
Cite this: RSC Adv., 2014, 4, 18217
*
*
Taotao Liu, Wenqian Fu, Xiang Zheng, Jun Jiang, Maolin Hu and Tiandi Tang
Received 24th December 2013
Accepted 28th February 2014
Zeolite nanofiber assemblies (HNB-MOR) as efficient heterogeneous catalysts for the formation of a range
of acetals in good yields. The mesoporosity of HNB-MOR benefits mass transfer, and the strong acidic sites
on HNB-MOR facilitate acetalization activity. The catalyst can be reused 10 times without loss of activity.
DOI: 10.1039/c3ra47952h
www.rsc.org/advances
Recently, we reported a facile method for the synthesis of
nanober assemblies of mordenite (NB-MOR).17 This material
Introduction
Acetals are an important class of ne chemicals.1 In multi-stage
synthesis, acetalization is also an important protection strategy
for carbonyl groups.2 Generally, acetalization can be catalyzed
efficiently by conventional mineral acids with high activity.
However, these acid catalysts are toxic, corrosive and difficult to
remove from the reaction medium, which strongly limits their
practical applications in industry. One solution to these prob-
lems is to replace mineral acids with highly active solid acid
catalysts, which are advantageous as isolation and purication
of the intermediates is not required.
Different kinds of solid catalysts such as heteropoly acids,3
imidazolium salts,4 sulfated zirconia,5 montmorillonite,6
envirocat EPZG,7 alumina/KSF8 and natural kaolinitic clay9 have
been employed for the acetalization of carbonyl compounds.
However, these solid acid catalysts have lower activity due to
their relatively small surface area. Other solid acids such as
cationic exchange resins10 and metal organic frameworks11
show better activity toward acetalization reactions, but the
activity of such catalysts cannot be regenerated by calcination
aer catalyst deactivation due to their relatively lower
thermostability.
has parallel-mesopore channels in the nanober assemblies of
the microporous mordenite. Aer ion-exchange of NH4 and
+
calcination, the H-form of NB-MOR (HNB-MOR) was obtained.
The HNB-MOR catalyst is strongly acidic and shows high
activity in the acetalization of various carbonyl compounds with
a series of alcohols involving bulky organic substrates under
mild reaction temperatures (50 ꢀC), compared with HAlMCM-41
and mesopore-free HMOR catalysts.
Experimental
Material synthesis
Mordenite nanober assemblies (NB-MOR) as well as meso-
pore-free mordenite (MOR) were synthesized in a similar way to
previous work.16 Mesoporous aluminosilicate (Al-MCM-41) was
synthesized according to the literature.15 The H-form of the
samples was obtained by ion-exchanging twice with NH4NO3
solution (1 M) at 80 ꢀC for 4 h, followed by calcination at 550 ꢀC
for 4 h.
Catalyst characterization
Great attention has been dedicated to the uses of acidic
aluminosilicate zeolites to achieve the acetalization reaction.12
However, one disadvantage of conventional micropore zeolites
is that the pore sizes are too small to allow bulky molecules
access. Ordered mesopore molecular sieves such as Al-MCM-
4113 and Al-SBA-1514 could overcome this pore size limitation,
but higher activity was not achieved by these catalysts for acetal
formation. This is attributed to the lower acidity of ordered
mesoporous materials due to the amorphous nature of meso-
porous walls.15,16 Therefore, the development of an economic,
green and sustainable catalytic system for this fundamental
transformation is highly desirable.
Nitrogen adsorption–desorption isotherms were obtained using
a Micromeritics ASAP 2020M apparatus at liquid nitrogen
temperature (À196 ꢀC). Specic surface areas were calculated
from the adsorption data using the Brunauer–Emmett–Teller
(BET) equation. The mesopore size distributions were calcu-
lated using the Barrett–Joyner–Halenda (BJH) model. The
acidity of the materials was determined by stepwise tempera-
ture-programmed desorption of ammonia (NH3-STPD) on a
Micromeritics ASAP 2920 instrument. The detailed operating
procedure and conditions are described in a previous work.18
Reaction and analysis
In a typical experimental procedure for an acetalization reaction
in the presence of a catalyst (30 mg), the carbonyl compound
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
325035, P. R. China. E-mail: maolin@wzu.edu.cn; tangtiandi@wzu.edu.cn
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(0.2 mmol) and alcohol (2.0 mL) were placed in a 10 mL glass reaction was carried out in the absence of a catalyst. Table 2
vessel. The reaction was then allowed to proceed at 50 ꢀC for 12 shows the reaction activity of the acetal formation of various
h. Aer the reaction nished, the catalyst was separated by aldehydes with methanol over HNB-MOR, HMOR and
centrifugation and the liquid phase was passed through lter HAlMCM-41 catalysts. Clearly, the HNB-MOR, HMOR and
paper. The liquid was analyzed with an Agilent 7890A GC HAlMCM-41 catalysts gave high yields for aldehydes not only
equipped with a FID detector and mass spectrometer. The with electron-withdrawing groups as the halogen (entries 1–3)
products were obtained by ash chromatography (hex- but also with electron-donating groups as –CH3 and –CH2CH3
ane : EtOAc). 1H NMR (500 MHz) and 13C NMR (125 MHz) were (entries 4–6). However, when the aldehyde with an electron-
performed by spectrometers at 20 ꢀC using CDCl3 as the solvent. withdrawing nitro substituent was used as the substrate, the
Chemical shis are given in parts per million relative to TMS as product yields over HNB-MOR reached 97–100%, and the yields
the internal standard at room temperature.
over HMOR and HAlMCM-41 catalysts were only 48 and 14%
(entries 7 and 8). Meanwhile, the activity of zeolite HNB-MOR
and HMOR catalysts was much higher than HAlMCM-41 in the
reaction entries 5–8.
Results and discussion
Table 3 shows the reaction activity of the acetal formation
of p-nitrobenzaldehyde with various alcohols over HNB-MOR,
HMOR and HAlMCM-41 catalysts. The reactions with ethanol,
ethanediol, n-propanol, n-butanol and benzyl alcohol over the
HNB-MOR catalyst proceeded readily and gave the corre-
sponding acetals in very high yields. In contrast, the product
yields over HMOR and HAlMCM-41 catalysts are very low
except for the reaction using ethanol as a substrate (entry 1).
In addition, the product yields over the HMOR catalyst were
reduced with increasing molecule dimensions of the alcohols.
Particularly when aromatic alcohol was used as a substrate,
the yields were very low over the HMOR and HAlMCM-41
catalysts. The results from Tables 2 and 3 indicate that the
HNB-MOR catalyst shows excellent activity in the acetalization
with aldehydes and alcohols including the bulky organic
substrates.
It is worth mentioning that not only aldehydes, but also
ketones are good substrates for the HNB-MOR-catalyzed ace-
talization reaction relative to HMOR and HAlMCM-41 cata-
lysts. Table 4 clearly shows that the HNB-MOR catalyst gives
the highest conversions in the acetalization of ketone with a
series of alcohols. These results indicate that the HNB-MOR
catalyst shows good activity in the acetalization of ketone with
alcohols. The reusability of HNB-MOR catalysts was also
surveyed. Aer the reaction, the catalyst was simply separated
by ltration and washed with ethanol, dried at 50 ꢀC, and
reused 10 times without loss of activity (Table 5). These results
indicate that the HNB-MOR catalyst has a good catalyst life,
which is one of the key features of catalysts for industrial
applications.
Fig. 1 shows the nitrogen adsorption isotherm, pore size
distribution and NH3-STPD proles of the samples. The
nitrogen sorption isotherm of HNB-MOR exhibits a hysteresis
loop at a relative pressure of 0.85–0.95, which is typically
assigned to the presence of a mesoporous structure (Fig. 1a).
The mesopore-size distributions of HNB-MOR and HAlMCM-41
are mainly centered at 33 and 2.5 nm (Fig. 1b and c), respec-
tively. Sample textural parameters are presented in Table 1. The
NH3-STPD proles of the samples are shown in Fig. 1d. For the
characterization of acidity by NH3-STPD, the acidic strength can
be differentiated as weak, middle and strong according to the
desorption temperature.19 Clearly, the concentrations of rela-
tively strong (250–350 C) and strong (>350 C) acidic sites of
HNB-MOR and HMOR catalysts are much higher than those of
HAlMCM-41.
The activity and scope of the acetalization of various alde-
hydes with a series of alcohols over HNB-MOR, HMOR and
HAlMCM-41 catalysts are shown in Tables 2–5. The blank
experiment showed that no product was obtained when the
ꢀ
ꢀ
Generally, the catalytic performance of zeolite in the ace-
talization reaction can be co-inuenced by acidity and pore
structure. For the acetalization of aldehydes with small
dimensions of alcohols, the reactant as well as the product can
diffuse into the micropores in HMOR so that HNB-MOR and
HMOR have a comparable catalytic activity (entries 1–6, Table
2). However, when using a substrate alcohol with large
molecular dimensions, the conversion of aldehyde over HMOR
is much lower than the conversion over HNB-MOR (entries 2–5,
Table 3). This is due to the difference in mesoporosity of both
catalysts. HNB-MOR has a mesopore surface area of 158 m2 gÀ1
(Table 1) which could favour mass-transfer, while HMOR has
an external surface area of only 12 m2 gÀ1. It is reasonable that
Fig. 1 (a) N2 adsorption isotherm, (b) pore size distribution of HNB-
MOR, (c) pore size distribution of HAlMCM-41, (d) NH3-STPD curves of
the samples.
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Table 1 Textural parameters of the catalyst samples
SBETa (m2 gÀ1
)
Sexter.b (m2 gÀ1
)
Vmeso (cm3 gÀ1
)
Vmicrod (cm3 gÀ1
)
c
Samples
HMOR
HNB-MOR
HAlMCM-41
346
474
876
12
158
824
0.002
0.50
0.67
0.13
0.13
0.01
a
c
BET surface area. b External surface area including the mesoporous surface area. Mesoporous volume. d Microporous volume.
Table 2 Acetal formation of aldehydes with methanol over HNB-
MOR, HMOR and HAlMCM-41 catalystsa
Table 3 Acetal formation of p-nitrobenzaldehyde with various alco-
hols over HNB-MOR, HMOR and HAlMCM-41 catalystsa
Yields [%]
a/b/cb
Entry
1
Product
Yieldsb,c,d [%]
100(94)/100/100
100(91)/100/99
Entry
1
Product
Time (h)
12
a/b/cb
2
3
100(93)/98/84
100(93)/100/100
2
3
12
18
100(91)/83/72
100(89)/77/65
4
5
6
100(92)/100/100
100(89)/95/90
100(87)/97/94
4c
12
12
87(81)/37/11
99(90)/70/53
7
100(95)/93/58
97(91)/48/14
5d
a
Reaction conditions: p-nitrobenzaldehyde (0.2 mmol), HNB-MOR (30
mg), alcohol (2 mL), 50 ꢀC, 12 h. The yields were obtained by GC
analysis (isolated yields in parentheses). a) HNB-MOR as the
8
b
catalyst; b) HMOR as the catalyst; c) HAlMCM-41 as the catalyst.
c
d
Benzyl alcohol (1.2 mmol), CH3CN (1.5 mL). Ethanediol (0.6
mmol), CH3CN (1.5 mL).
a
Reaction conditions: aldehyde (0.2 mmol), methanol (2 mL), catalyst
(30 mg), 50 ꢀC, 12 h. The yields were obtained by GC analysis (isolated
yields in parentheses). a) HNB-MOR as the catalyst; b) HMOR as the
b
catalyst; c) HAlMCM-41 as the catalyst.
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Table 4 HNB-MOR-catalyzed acetal formation of ketone with series Table 5 Recyclability experiments of the HNB-MOR systema
alcoholsa
Entry
Recycle
Yield [%]
Yield [%]
a/b/cb
1
2
3
4
5
6
7
8
9
10
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7
Run 8
Run 9
Run 10
100
100
100
>99
100
100
100
100
100
>99
Entry
1c
Product
91/53/38
2
3
4
98/97/94
82/75/64
69/65/25
a
Reaction conditions: p-nitrobenzaldehyde (0.2 mmol), HNB-MOR (30
mg), ethanol (2 mL), 50 ꢀC, 12 h. The yields were obtained by GC
analysis.
acidic sites on the catalyst play an important role in acetal
formation reactions.
Conclusions
In summary, we have developed an example of a zeolite material
(HNB-MOR) as an outstanding catalyst for the synthesis of
acetals with excellent yields under mild conditions. The meso-
porosity of HNB-MOR benets mass transfer and the strong
acidic sites on HNB-MOR facilitate its acetalization activity.
5d
42/27/5
a
Reaction conditions: cyclohexanone (0.2 mmol), HNB-MOR (30 mg),
alcohol (2 mL), 50 ꢀC, 12 h. The yields were obtained by GC analysis
b
and the products were detected by GC-MS. a) HNB-MOR as the
catalyst; b) HMOR as the catalyst; c) HAlMCM-41 as the catalyst.
Ethanediol (0.6 mmol), CH3CN (1.5 mL). Benzyl alcohol (1.2
mmol), CH3CN (1.5 mL).
c
d
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (U1162115, 21076163 and 21371137) and
the Science and Technology Program of Zhejiang Province
(2010C31096).
the HNB-MOR catalyst gives higher conversion compared with
mesopore-free HMOR. Compared with HNB-MOR, although
HAlMCM-41 has a relatively high mesopore volume (0.67 m3
gÀ1) and mesopore surface area (876 m2 gÀ1), it still showed a
lower acetalization capability with the bulky substrate, which
may be related to the difference in acidity between the HNB-
MOR and HAl-MCM-41 catalysts. It has been reported that
acidic sites on catalysts facilitate acetalization reactions.20 In
this work, the HNB-MOR zeolite with strong and abundant
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