A facile approach for the preparation of tunable acid nano-catalysts with a hierarchically mesoporous structure

Article abstract of DOI:10.1039/c4cc01881h

A facile and efficient approach to prepare hierarchically and radially mesoporous nano-catalysts with tunable acidic properties has been successfully developed. The nanospheres show excellent catalytic performance for the acid catalysed reactions, i.e. cr

Full text of DOI:10.1039/c4cc01881h

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A facile approach for the preparation of tunable
acid nano-catalysts with a hierarchically
mesoporous structure†
Cite this: Chem. Commun., 2014,
0, 7652
5
Received 13th March 2014,
Accepted 27th May 2014
Youngbo Choi,‡ Yang Sik Yun,‡ Hongseok Park, Dae Sung Park, Danim Yun and
Jongheop Yi*
DOI: 10.1039/c4cc01881h
www.rsc.org/chemcomm
A facile and efficient approach to prepare hierarchically and radially pH adjusted solution containing preformed hierarchically and
mesoporous nano-catalysts with tunable acidic properties has been radially mesoporous (HRM) nanostructures via a hydrothermal
2
successfully developed. The nanospheres show excellent catalytic treatment. A second hydrothermal treatment was then used to
performance for the acid catalysed reactions, i.e. cracking of introduce heteroatoms into the silica framework, resulting in the
1,3,5-triisopropylbenzene and hydrolysis of sucrose.
maintenance of the unique structure with acidic properties. In
contrast, when a small amount of an Al precursor was included
Hierarchically porous nanomaterials have attracted a great deal of in the initial solution, as in the case of the direct synthesis, HRM
interest in recent years because of their superior physicochemical nanostructures were not obtained (Fig. S1, ESI†).
properties, such as large surface area, high porosity, and low
In the developed procedure, it is noteworthy that the reaction
1,2
density. However, many hierarchically porous nano-materials, time required to preform the HRM nanostructure and the pH shift
such as silica, are chemically and/or catalytically inactive, making prior to the addition of the Al precursor played a crucial role in
it difficult to meet the needs of these materials for practical introducing the functionality while maintaining its nanostructures
applications including catalysis, adsorption, separation, and drug (Fig. S1, ESI†). Interestingly, by simply changing the pH adjusting
3
delivery and sensing.
agent in the procedure to prepare the aluminosilicate nanospheres
The incorporation of heteroatoms into the silica framework (denoted as ASN-X where ‘‘X’’ indicates the Si/Al molar ratio), we
by a direct synthesis or post-grafting method has been widely used to can induce incorporation of P as well as Al into silica framework,
4
create active sites. These conventional methods, however, suffer leading to the formation of aluminosilicophosphate nanospheres
from some critical limitations, such as the collapse of their structure (denoted as ASPN-X where ‘‘X’’ indicates the Si/Al molar ratio).
5
when a large amount of heteroatoms are introduced, complex Furthermore, the Si/Al ratio of both ASN and ASPN samples can be
3
,6
and time-consuming processes, and difficulty in controlling easily controlled by simply changing the concentrations in the
7
the amount of grafted heteroatoms. Therefore, the development initial preparation solution because its ratios in the preparation
of simple and efficient strategies for preparing the hierarchically solution were preserved in the final samples while the content of P
porous nanomaterials with adjustable catalytic functionalities in the ASPN samples is about half that of Al (Table S1, ESI†).
continues to be a great challenge.
The morphology and structure of the ASN and ASPN samples
Herein, we report on a convenient and effective route for the with various Si/Al ratios prepared under optimum conditions
preparation of tunable acid nano-catalysts with a hierarchically were investigated using electron microscopy. As shown in the
and radially mesoporous structure. In this method, referred to scanning electron microscopy (SEM) images, both ASN and
as ‘‘pH-assisted delay addition’’, an Al precursor was added to a ASPN samples are spherical in shape with a uniform particle
size (450–600 nm) (Fig. 1a and b and Fig. S2, ESI†). Magnified
SEM images demonstrate that the wrinkled sheets are arranged
in three dimensions to form a spherical shape with a pore
World Class University Program of Chemical Convergence for Energy & Environment,
Institute of Chemical Processes, School of Chemical and Biological Engineering,
mouth size of 15–50 nm. The large diameters of the pores can
permit guest molecules to easily access the active sites inside
the pores. The transmission electron microscopy (TEM) images
indicate that the pores are radially oriented, and their size
gradually increases from the center to the outer surface (Fig. 1c
and Fig. S2, ESI†). Elemental mapping images demonstrate a
highly homogeneous distribution of elemental Si and Al in the
College of Engineering, Seoul National University, Seoul 151-742, Republic of Korea.
E-mail: jyi@snu.ac.kr; Tel: +82-2-880-7438
†
Electronic supplementary information (ESI) available: Experimental details,
characterization (EPMA, SEM and TEM images, XRD results, NH -TPD, in situ FTIR
adsorption–desorption
3
27
spectra of adsorbed NH
3
,
Al MAS NMR spectra N
2
isotherms, and TG analysis), catalytic activities for the cracking of 1,3,5-TIPB and
hydrolysis of sucrose, and hydrothermal stability test. See DOI: 10.1039/c4cc01881h
‡
These authors contributed equally.
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À1
very similar surface areas (511 Æ 32 m g ) (Fig. 2 and Table S2,
ESI†). This textural uniformity of ASN and ASPN particles across
various compositions clearly demonstrates that the pH-assisted
delayed addition method can overcome the limitations of con-
ventional functionalization methods, in which the incorporation
of a functionality generally leads to a decrease in the textural and
3
structural properties.
The acid and ion exchange sites of aluminosilicate materials
are mainly associated with a tetrahedrally coordinated Al in the
9,10 27
framework.
Al MAS NMR analysis revealed the coordination
environment of Al (Fig. S3, ESI†). All ASN samples showed a strong
peak (51–56 ppm) corresponding to a tetrahedrally coordinated Al
in the framework and a weak peak (0 ppm) attributed to an
10,11
octahedrally coordinated Al in the extra framework,
suggesting
that most of the Al species are incorporated into the framework.
Indeed, the proportion of tetrahedral Al, estimated from the
10,11
relative intensities of the NMR peaks,
exceeded 70% for all
ASN samples (Table S2, ESI†). Moreover, the proportion of tetra-
hedral Al in ASN-40 was about 1.5 times higher than the corres-
ponding values for a commercial mesoporous aluminosilicate
Fig. 1 (a and b) SEM, (c) TEM, and (d) EDS mapping images of (A) ASN-40
and (B) ASPN-40 samples.
ASN, and elemental P as well as Si and Al in the ASPN samples AlMCM-41 with the same Si/Al ratio of 40 (Table S2, ESI†). In the
Fig. 1d), suggesting the effective formation of acid sites.
case of ASPN samples, the spectra were similar to those previously
The porous structure of the ASN and ASPN samples was reported for mesoporous aluminosilicophosphates, indicating the
further evaluated by N adsorption–desorption measurements. presence of tetrahedral and octahedral atoms which are bonded to
(
2
1
2
All ASN samples exhibited a type IV isotherm with a H3 type P atoms via oxygen bridges. These findings demonstrate that
hysteresis loop in the relative pressure range of 0.4–1.0, implying the developed method has a high efficiency for incorporating
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the presence of various sized, slit-shaped mesopores (Fig. 2a).
heteroatoms and creating acid sites.
The BJH adsorption pore volume curves further confirm these
The amount and strength of the acid sites on the ASN and
multimodal pore size distributions, which consist of a relatively ASPN samples were evaluated by the temperature-programmed
sharp peak (3 nm), a feeble shoulder peak (10 nm) and a broad desorption (TPD) of NH
peak (37 nm) (Fig. 2b). Interestingly, although ASN and ASPN strength, the TPD curves were fitted by Gaussian deconvolution
samples were prepared using a wide range of Si/Al ratios and
3
. To reveal the distribution of acid
13
different Al/P ratios, they all had a uniform pore structure and
Fig. 3 (a) NH
3
–TPD curves and (b) in situ FTIR spectra for adsorbed NH
3
of
Fig. 2 (a) N
2
adsorption–desorption isotherms and (b) BJH adsorption
ASN and ASPN samples with different Si/Al molar ratios. Peak assignments in
À1
À1
pore size distribution curves of ASN and ASPN samples with different Si/Al
molar ratios.
FTIR spectra; 1640 cm (Lewis acid sites: LS), 1470 and 1705 cm (Brønsted
acid sites: BS).
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(
Fig. 3a and Table S3, ESI†). The total amount of acid sites progres- with the same Si/Al ratio of 40 were also tested. In the cracking of
À1
sively increased with increasing Al content, from 0.132 mmol g for 1,3,5-TIPB, HZSM-5 showed the lowest activity, since 1,3,5-TIPB
ASN-60 to 0.609 mmol g for ASN-15. The peak maximum of the is too large to enter the pores of the HZSM-5 and the reaction
À1
1
7
TPD curves shifted to higher temperatures with increasing Al content, occurs only on the external surface. AlMCM-41 showed a high
indicating an increase in acid strength. The acid strength distribution initial activity, but was deactivated rapidly. Interestingly, ASN-40
also showed a positive relation between Al content and densities for and ASPN-40 not only exhibited a higher activity, but also
both medium and strong acidity. As compared with the ASN samples retained much longer lifetime than the reference catalysts due
with the same Si/Al ratio, the total acid amount on the ASPN samples to high coke resistance originated from their unique pore
18
was significantly increased. In order to distinguish between Brønsted structure and suitable acidity (Fig. S6, ESI†). The deactivation rate
1
9
and Lewis acid sites on the samples, in situ FTIR spectra for NH
3
constant calculated from the first order deactivation model was in
À1
À1
adsorbed on the samples were also recorded (Fig. 3b and Table S3, the order of ASPN-40 (0.01 h ) o ASN-40 (0.02 h ) o HZSM-5
À1
À1
À1
ESI†). In the spectra, two bands at ca. 1470 and 1705 cm are (0.11 h ) { AlMCM-41 (0.75 h ). Furthermore, ASN-40 was
assigned to NH adsorbed on Brønsted acid sites, and a band at revealed to be more stable in terms of hydrothermal stability than
3
À1
14
1
640 cm is attributed to NH
3
adsorbed on Lewis acid sites.
AlMCM-41, exhibiting a relatively high activity and maintaining
The ratio of Brønsted to Lewis acid sites (BS/LS ratio) on the ASN its structure (Fig. S7 and Table S5, ESI†). In the hydrolysis of
samples was gradually enhanced with increasing Al content, from sucrose, ASN-40 and ASPN-40 showed a higher performance than
0.6 for ASN-60 to 5.5 for ASN-15. In the case of the ASPN samples, the reference catalysts (Fig. 4b). In particular, the conversion of
the BS/LS ratio was noticeably increased compared to the ASN sucrose over ASPN-40 was more than 4 times that of AlMCM-41
samples with the same Si/Al ratio. The larger acid amount and and HZSM-5, mainly due to the high content and easy accessibility
20
higher BS/LS ratio of the ASPN samples can be attributed to the of the Brønsted acid sites in ASPN-40.
OH groups associated with the P atoms which provide additional
weak Brønsted acid sites.
In summary, we report on an attractive route for introducing
tunable acidic properties into the hierarchical mesoporous
9,15
Compared with AlMCM-41, the total amount of acid and the nanospheres. These procedures permit the amount, types,
ratio of Brønsted to Lewis acid sites (BS/LS ratio) in ASN-40 were and strength of acid sites to be adjusted precisely. These are
higher by 1.8 and 1.7, respectively (Fig. S4 and Table S3, ESI†). important properties for solid acids, and textural uniformity
In amorphous aluminosilicates such as ASN and AlMCM-41 can be maintained across various compositions. The excellent
(Fig. S5, ESI†), the Brønsted acidity originates from silanol catalytic properties of the resulting nanospheres suggest that
groups, which are strongly influenced by neighboring Al they have great potential for use as solid acids in the chemical
1
6
atoms. As shown in Fig. 1d, our method results in the highly industry and in expanding the scope of hierarchically structured
homogeneous distribution of Si and Al atoms, which facilitates nanomaterials.
the creation of silanol groups having neighboring Al, which
serves to enhance the acidity.
This research is supported by Korea Ministry of Environment
as ‘‘Converging technology project (202-091-001)’’.
The catalytic performance of the ASN and ASPN samples was
evaluated in two acid-catalysed reactions (Fig. 4). The cracking
of 1,3,5-triisopropylbenzene (1,3,5-TIPB) and the hydrolysis of
sucrose were chosen as model reactions for the transformation
of a hydrocarbon and biomass, respectively. By different product
distributions of 1,3,5-TIPB cracking and conversions of sucrose
over the samples, the tunable acidic properties of ASN and ASPN
samples were confirmed (Table S4, ESI†). To demonstrate the
versatility of the ASN and ASPN catalysts, AlMCM-41 and HZSM-5
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