Organic functionalization of mesopore walls in hierarchically porous zeolites

Article abstract of DOI:10.1039/b815540b

Mesopore walls of hierarchically meso-/microporous zeolites (MFI, BEA and LTA) are covered with silanol groups, so that the zeolites can be functionalized with various organic groups via silylation; the organic-functionalized hierarchical zeolites exhibit

Full text of DOI:10.1039/b815540b

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Organic functionalization of mesopore walls in hierarchically porous
zeolitesw
Dong-Hwan Lee, Minkee Choi, Byung-Woo Yu and Ryong Ryoo*
Received (in Cambridge, UK) 5th September 2008, Accepted 20th October 2008
First published as an Advance Article on the web 13th November 2008
DOI: 10.1039/b815540b
Mesopore walls of hierarchically meso-/microporous zeolites
(MFI, BEA and LTA) are covered with silanol groups, so that
the zeolites can be functionalized with various organic groups via
silylation; the organic-functionalized hierarchical zeolites exhibit
hydrothermal stability and reusability in catalytic applications, as
compared with organic-functionalized mesoporous silica.
can be functionalized with various organic groups, to a high con-
centration that is comparable to functionalization of mesoporous
silica SBA-15. Furthermore, the crystalline zeolite frameworks
offer the advantage of improved chemical stabilities as compared
with mesoporous silicas having amorphous frameworks.
Hierarchical zeolites with three types of framework (MFI, BEA
and LTA) were synthesized following the reported procedure.11,12
The zeolite samples are denoted by the 3-letter structural codes18
following ‘MP-’, where MP means ‘mesoporous’. Nitrogen
adsorption–desorption isotherms and corresponding mesopore
size distributions are presented in Fig. 1. As the isotherms show,
there is a sharp increase in adsorption in 0.2 o P/P0 o 0.8, which
corresponds to the capillary condensation in mesopores. The pore
size analysis by BJH algorithm gave a distribution curve centered
at 3.4 nm for MP-MFI, 2.7 nm MP-BEA, and 9.9 nm MP-LTA.
The mesoporous samples exhibited X-ray powder diffraction
patterns similar to the corresponding conventional zeolites (ESIw).
Due to the lack of structural order in mesoscale, the materials did
not show XRD peaks in the low-angle regime.
Organic functionalization of inorganic materials (e.g., mesoporous
silica, silica gel and zeolite) has been extensively studied in
heterogeneous catalysis for green chemistry during the last
decade.1–9 Organic-functionalized inorganic matrices can provide
synergistic properties of organic and inorganic components, such
as high functionalizability, accessible surface area and structural
stability. Among the various inorganic supports, mesoporous
(2 o pore diameter o 50 nm) silicas have attracted much
attention in recent years because the large pore diameter allows
grafting of bulky organic moieties.1–6 However, one drawback of
mesoporous silica is its low hydrothermal and chemical stabilities
due to amorphous nature in the framework.6,10 Crystalline zeolites
are more stable, but in ordinary zeolites the solely microporous
structure (pore diameter o2 nm) and lack of silanol (RSi–OH)
groups make the incorporation of bulky organic moieties difficult.
Nanocrystalline zeolites8 and delaminated zeolites9 with enlarged
surface area were used for functionalization.
Reactions between zeolite silanol groups and alkoxysilanes
were carried out by refluxing in toluene solution containing
trimethoxy or triethoxy silane, which are similar to the
conventional functionalization of mesoporous silicas reported
previously.1–6 To prevent moisture contamination, zeolites
were immediately used after calcination in air at 450 1C and
anhydrous toluene (Aldrich, 99.9%) was used as a solvent.
The organic content after functionalization was analyzed
by thermogravimetric analysis (TGA), and confirmed by
elemental analysis (EA). The organic content in hierarchical
zeolites was compared with those of conventional zeolites and
SBA-1519 (Table 1).
Here we show that hierarchically meso-/microporous zeolites
(‘hierarchical zeolites’ hereafter for brevity) can serve as a suitable
inorganic support for organic functionalization. Hierarchical
zeolites with various structures are currently available according
to various synthesis routes developed recently, such as directly
hydrothermal synthesis using organosilane surfactants as a meso-
pore generator,11–13 desilication of pre-synthesized zeolites,14,15
and synthesis using presynthesized solid template.16,17 In the
present work, we have chosen the synthesis route which adds
organosilane surfactants into conventional zeolite synthesis
compositions.11–13 The hierarchical zeolites thus obtained exhibit
uniform and tailorable mesopores. The mesopore walls are
composed of microporous crystalline zeolite frameworks, which
are terminated with silanol groups at the wall surface. Due to the
large mesopore surface area, the amount of silanol groups is very
high as compared with that on the external surface of conven-
tional zeolite crystal. The silanol groups can readily be reacted
with various alkoxysilanes (Scheme 1). Thus, the mesopore walls
As the result with 3-aminopropylsilane shows, the grafting
reaction was almost completed within 3 h. Ordinary MFI
National Honor Scientistist Progaram in Korea, Center for Functional
Nanomaterials and Department of Chemistry, Korea Advanced
Institute of Science and Technology, Daejeon 305-701 Korea.
E-mail: rryoo@kaist.ac.kr; Fax: 82-42-869-8130; Tel: 82-42-869-8131
w Electronic supplementary information (ESI) available: General
experimental procedures, synthesis and characterization details. See
DOI: 10.1039/b815540b
Scheme 1 Organic functionalization of hierarchical zeolites.
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Scheme 2 Grafting of dichloro-b-oxoiminato-Pd on mesopore walls
of hierarchical zeolites, and their catalytic application in Sonogashira
reaction.
Si–O–Si network) showed continuous decreases in weight even
in the region of 300–800 1C. It was therefore reasonable that
many of the SBA-15 silanol groups could be lost during the
calcination at 450 1C.
To explore the catalytic applicability of organic-functionalized
hierarchical zeolites, we have investigated catalytic activities in
the Sonogashira coupling reaction, after grafting dichloro-b-
oxoiminato-palladium (Scheme 2, and ESIw for details). The
Pd-catalyzed Sonogashira cross-coupling reaction of terminal
alkynes is one of the most powerful methods for synthesizing
arylated alkynes.20 A major challenge in Sonogashira reaction
has been the recycling of expensive Pd catalyst. The reaction is
carried out in the presence of Na2CO3, as a base to prevent Pd
leaching by the product HCl. Nevertheless, low hydrothermal
stability of the support is still a major problem for recycling,
particularly in the case of silica-supported catalyst. As our results
show in Table 2, the Pd complexes grafted on hierarchical
zeolites had similar catalytic activities to those on SBA-15. It
should be noted that the MP-MFI-based catalyst exhibited only
a slight decrease in catalytic activity (from 96 to 91% in yield)
during recycling for five times. In comparison, SBA-15 exhibited
the decrease in catalytic conversion from 84 to 38%. Thus, the
hierarchical zeolites exhibited much higher catalyst durability in
the basic medium than SBA-15 did. The high recyclability of the
hierarchical zeolite may be attributed to the enhanced hydro-
thermal stability, due to the crystalline nature of the zeolite
framework. Pore size analysis of the used catalyst by N2 isotherm
showed that the functionalized MP-MFI maintained the original
mesoporosity up to 24 h under the basic reaction condition at
90 1C, whereas the functionalized SBA-15 completely lost the
mesoporosity in 12 h. MP-LTA indicated even higher stability
than MP-MFI. The Al-rich LTA zeolite maintained their
Fig. 1 (a) N2 adsorption–desorption isotherms, and (b) BJH mesopore
size distributions for hierarchical zeolites. The isotherm for MP-LTA
was vertically offset by 300 ml gÀ1
.
zeolites having solely microporous structure showed negligible
organic content due to the small area of the external surface.
In contrast, the functionalization of hierarchical zeolites led
to a similar or even higher amount of organic groups than
SBA-15 mesoporous silica, as compared with the same weight
of samples. For example, in the case of 3-aminopropyl
functionalization, the MP-MFI zeolite contained 25% more
organic groups than SBA-15. Note that the SBA-15 sample
has approximately 2 times larger mesopore surface area
(683 m2 gÀ1) than the MP-MFI zeolite (350 m2 gÀ1), according
to the crude estimation using t-plot analysis. Such a t-plot
analysis can underestimate mesopore surface area in the case
of the aluminosilicate zeolite. It is thus reasonable that the
surface functionalization density would be at least 2 times
higher at the mesopore walls of MFI than SBA-15. The high
surface functionalization indicates that more silanol groups
per surface area were available in the case of hierarchical
zeolite. This seemed to be due to the presence of silanol
(RSi–OH) groups on the external surface (i.e., mesopore
surface) of the zeolite framework, where the fully crosslinked
Si–O–Si linkage could not allow further condensation. TGA
analysis showed that the hierarchical zeolite lost physisorbed
water during 200–300 1C, but no further weight losses indicat-
ing silanol condensation were detected thereafter. In contrast,
SBA-15 (atomically disordered, and partially condensed
Table 2 Sonogashira coupling of chlorobenzene with terminal
alkynesa
Table 1 Functionalization of hierarchical zeolites with oganosilanes,
RC3H6Si(OCH3)3 or RC3H6Si(OC2H5)3
Catalyst
R1
Temp./1C
Time/h
Yield (%)b
Material
R
Organic content/mmol gÀ1
MP-MFI
MP-BEA
MP-LTA
SBA-15
MP-MFI
MP-MFI
MP-MFI
MP-BEA
MP-LTA
SBA-15
Ph
Ph
Ph
Ph
CH2OH
n-C4H9
n-C8H17
n-C8H17
n-C8H17
n-C8H17
70
70
70
70
90
90
90
90
90
90
6
6
6
6
6
8
8
8
8
8
96(93)
89(84)
91
MP-MFI
MP-MFI
MP-BEA
MP-LTA
SBA-15
NH2
NH2
NH2
NH2
NH2
NH2
Cl
C6H4NH2
1.13a
1.18
1.10
0.61
0.94
0.14
1.20
1.31
1.29
1.28
1.13
76
92(89)
86
48
29
88(85)
67
MFI
MP-MFI
MP-MFI
MP-MFI
MP-MFI
MP-MFI
NH(CH2)2NH2
NHC(CH3)CHCOCH3
NHC(C6H5)CHCOC6H5
a
Reaction conditions: chlorobenzene (1.0 mmol), alkynes (1.2 mmol),
CuI (1 mol%), Na2CO3 (2.0 mmol), solvent (0.7:0.7 ml = H2O:DMA)
a
b
and Pd catalyst (1 mol%). GC yield was determined using n-dode-
Reaction was carried out for 3 h using 1 g material, 5.0 mmol
organosilane, and 10 ml toluene. Other samples were reacted for 12 h.
cane as an internal standard. Isolated yield is given in parenthesis.
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mesoporosity and catalytic activity after the hydrothermal treat-
ment for 4 d. The hierarchical zeolites, functionalized with
similar Pd-complexes, were also effective for other C–C coupling
reactions such as Suzuki–Miyaura, Stille and Heck of aryl
chlorides. Further studies of these coupling reactions are
currently in progress.
Notes and references
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In conclusion, hierarchically meso-/microporous zeolites
(MFI, BEA and LTA) were functionalized by grafting organo-
silanes onto the silanol groups located at the surface of mesopore
walls. Whereas the solely microporous zeolites could not be
functionalized due to the lack of free silanol groups inside
micropores, the hierarchical zeolites could be functionalized at
the mesopore surface by using high density of silanol groups.
The organic-functionalized hierarchical zeolites exhibited signi-
ficantly enhanced hydrothermal stability and catalytic reusability
as compared with mesoporous silica SBA-15 having amorphous
framework. It was also remarkable that the surface functionali-
zation density was higher than that of SBA-15, although the
total amounts of functional groups were similar if based on
weight. We only demonstrated catalytic application, but we
believe that organic-functionalized hierarchical zeolites might
provide new opportunities in the design of catalyst, sensor,
adsorbent and ion-exchange materials. In particular, it would
be interesting to design bifunctional materials that can combine
the acid and ion-exchange properties of zeolite with various
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This work was supported by the National Honor Scientist
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