the assembly of zeolite nanocrystals without any secondary
porogen,[10] and the creation of mesopores by using nano-
structured carbons,[11] polymers,[12] surfactants[13] and inor-
ganic nanoparticles.[14] Recently, Ryoo and co-workers fabri-
cated truly ordered mesoporous mordenite framework in-
verted (MFI) by using linear gemini-type templates consist-
ing of several bridged ammonium cations terminated by
long alkyl chains.[13j] To avoid the phase-separation process
ployed to characterize the obtained products. Its catalytic
activity was verified in reactions involving bulky molecules,
the Claisen–Schmidt condensation of 2-hydroxyacetophe-
none with benzaldehyde and the esterification reaction of
lauric acid with 2-ethylhexanol, compared with those of con-
ventional ZSM-5 and Al-MCM-41.
between the mesoporogen and the crystalline wall,
a
Results and Discussion
number of attempts have been made, including using cation-
ic polymer or surfactant template,[12c,f,13a,b,k] silane-functional-
ized surfactant or polymer,[12b,13e] steam-assisted crystalliza-
tion,[11a,f,12g,13i] and dense mesoporogen/silica composite hy-
drothermal crystallization.[11b,c,12d] However, the above syn-
theses employed the elaborately built “hard” porogens in-
cluding carbon nanotubes, carbon particles and pre-
organized carbon scaffolds, or soft templates, such as spe-
cially designed dual-functional and environmental-unfriend-
ly surfactants, which involved either multi-step procedures
or less cost-effective templates in the synthesis. Therefore,
the simple synthesis of mesoporous zeolites by using more
conventional, environmentally friendly and cost-effective
templates is still a challenge.
Ammonium-modified chitosan, N-(2-hydroxy)propyl-3-tri-
methylammonium chitosan chloride (HTCC) was obtained
by the reaction of glycidyltrimethylammonium chloride
(GTMAC) and chitosan. The FTIR spectra of chitosan and
HTCC are presented in Figure S1 in the Supporting Infor-
mation. In spectrum a, the broad peak at 3400–3000 cmÀ1, as
À
À
well known, can be attributed to O H and N H stretching
vibrations, and the peak at 1600 cmÀ1 corresponds to NH2
deformation. In the spectrum b, a new peak positioned at
1485 cmÀ1 appears, which corresponds to the C H bending
À
of methyl groups of quaternary ammonium, indicating the
introduction of quaternary ammonium group on the HTCC
chains. Moreover, compared with the negligible change in
the characteristic peaks of primary alcohol and secondary
alcohol between 1110 and 1070 cmÀ1, the peak at 3400–
3000 cmÀ1 becomes sharper and the peak at 1600 cmÀ1 disap-
pears in the spectrum b, demonstrating that GTMAC
mainly reacted with NH2 groups in chitosan rather than with
the OH groups.[17] Due to the introduction of cationic am-
monium groups into the chitosan chains, the dissolubility of
HTCC is greatly improved, as shown in Figure S2 in the
Supporting Information, whose solution is transparent with-
out any precipitate.
Chitosan, or poly(b-ACTHNUTRGNEUNG(1-4)-2-amino-2-deoxy-d-glucose), is
the deacetylated natural product of chitin by simple alkali
treatment, which is an abundant, low-cost, highly environ-
mentally friendly and biocompatible biopolymer in the exo-
skeleton of crustaceans. The porogenic capability of chitosan
was verified in literature by using titania or silica as the inor-
ganic component in the composites.[15] Recently, amorphous
mesoporous silica–alumina was synthesized by means of the
formation of inorganic–organic composites with the addition
of chitosan biopolymer, and enhanced activity and accessi-
bility were observed in the chitosan-containing catalyst.[16]
Unfortunately, however, its indissolubility under basic condi-
tion, which is usually required in the crystallization of zeo-
lites, makes it difficult to synthesize mesoporous zeolites
using chitosan biopolymer as mesoporogens. To the best of
our knowledge, there are no reports on the synthesis of mes-
oporous zeolites using chitosan as mesoporogens directly to
date.
The success of using cationic polymers in such a synthesis
inspired us to functionalize the chitosan with ammonium for
improving its base-dissolubility and enhancing the interac-
tion between positively charged ammonium-modified chito-
san and the negatively charged inorganic precursor simulta-
neously. Moreover, the higher thermal stability of ammoni-
um-modified chitosan assures its high dispersion in the zeo-
lite synthesis systems without decomposition.[17]
Figure 1a shows the XRD patterns of mesoporous ZSM-5
(designated as MZ) and conventional ZSM-5. MZ exhibits
the characteristic peaks associated with the MFI zeolite
structure, the same as that of conventional ZSM-5. The
comparable intensity shows that the addition of HTCC did
not affect the crystallization significantly.
N2 adsorption-desorption isotherms and Barrett–Joyner–
Halenda (BJH) pore-size distribution of MZ and conven-
tional ZSM-5 are given in Figure 1b. Conventional ZSM-5
exhibits a representative type I (Langmuir) isotherm accord-
ing to the classification of IUPAC, which is characteristic of
microporous materials. In contrast, the isotherm of MZ
presents a notably mixture of type I and IV, with a larger
adsorption amount and a broad hysteresis loop at the rela-
tive pressure of 0.6–0.9, indicating the presence of meso-
porosity in MZ crystals. Accordingly, the BJH pore-size dis-
tribution of MZ derived from the desorption branch reveals
the presence of mesopores in sample MZ of 5–20 nm in di-
ameter (Figure 1b, inset), again confirming the presence of
mesoporosity. As summarized in Table 1, the specific surface
area (422 m2 gÀ1), external specific surface area (238 m2 gÀ1),
total pore volume (0.42 cm3 gÀ1), and external volume
(0.34 cm3 gÀ1) of MZ are larger than those of conventional
ZSM-5 due to the introduction of mesoporosity. Such in-
In this work, we first synthesized cationic N-(2-hydroxy)-
propyl-3-trimethylammonium chitosan chloride (HTCC) by
the reaction of glycidyltrimethylammonium chloride
(GTMAC) and chitosan, and investigated its potentiality as
the meso-template in the hydrothermal synthesis of mesopo-
rous ZSM-5 material. Various technologies including XRD,
N2 adsorption, SEM, TEM, NMR spectroscopy, TGA, and
temperature-programmed desorption (NH3-TPD) were em-
16550
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16549 – 16555