Baixe zeolite tissue through wood cell templating e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity!
partially filled. Figure 5 shows the relation between the hole-
depth and the excursion temperature. A hole with a depth of
~30 % was clearly observed after excursion to RT. In Figure 5,
the depth of the holes, burned and measured at 100, 200, and
295 K, is also shown, indicating that stable and deep holes
were observed in the X-ray irradiated glasses.
0
20
40
60
0 100 200 300 Temperature (K)
Hole depth
(%)
Fig. 5. Relation between the hole depth and burning temperature (closed cir- cles) and the excursion temperature (open circles) of an X-ray irradiated Sm 2+^ - doped Al 2 O 3 ±SiO 2 (1:9) glass.
In conclusion, we have demonstrated the fast formation of
PSHB in an Sm
2+
-doped Al 2 O 3 ±SiO 2 glass irradiated with
X-rays. The hole burning rate in this glass was thirty times
faster than that of the Sm
2+
in a H 2 gas treated glass. This fast
formation of PSHB was contributed to by the formation of
Sm
2+
and hole centers by nearby oxygen ions. These fast and
highly efficient hole-burning glasses should make possible the
realization of high density memory.
Received: March 7, 2002 Final version: April 1, 2002
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Zeolitic Tissue Through Wood Cell Templating**
By Angang Dong, Yajun Wang, Yi Tang,* Nan Ren,
Yahong Zhang, Yinghong Yue, and Zi Gao
Crystalline microporous materials, such as zeolites, have
been widely used in the fields of catalysis, separation, and ad-
sorption.
[1]
However, the diffusion rate of the guest species in
practical applications is usually limited by the existence of the
micropores, especially in reactions involving large molecules.
To overcome this problem, and to fill the growing need for
many novel applications, intensive research has been under-
taken toward the construction of hierarchically organized po-
rous zeolites containing tailored meso- or macropores.
[2±7]
Such porous zeolites are expected to be of great technological
importance, since they combine the advantages of each pore-
size regime.
[3]
Although various strategies have been pro-
posed to design their porous structure, template-directed syn-
thesis is currently the most widely used method on account of
its versatility.
[3±7]
In a typical procedure, zeolite nanoparticles
or their precursors are assembled around the skeleton of a
template to form the composite, and the porous zeolite is pro-
duced upon the removal of the template. A variety of tem-
plates, including latex particles,
[3]
polyurethane foams,
[4]
ion-
exchange resins,
[5]
and carbon fibers
[6]
have been employed to
date, resulting in a number of hierarchical porous materials
involving zeolitic structures, including monoliths,
[3]
foams,
[4]
beads,
[5]
hollow fibers,
[6]
and spheres.
[7]
In addition to artificial templates, much current research in
the chemistry of materials uses natural complex templates, as
exemplified by biological tissue, to construct hierarchical po-
rous inorganic materials.
[8±17]
Compared to artificial tem-
plates, biological tissue is not only inherently complex and
hierarchical but also different from species to species even
within the same genus, making it possible to synthesize mate-
rials with unique multilevel structures and morphologies
(namely, biomimetic materials
[8]
). In addition, biological tem-
plates are generally inexpensive, abundant, renewable, and
environmentally benign. Up to now, several kinds of biologi-
cal templates, such as shells,
[9]
skeletons,
[10]
living cells,
[11]
diat-
oms,
[12]
and bacterial superstructures
[13]
have been utilized to
synthesize inorganic materials.
Wood is a natural composite material mainly composed of
cellulose, hemicellulose, and lignin. [14]^ Microscopically, the
tracheidal cells or vessels, which extend from micrometers to
several meters in length, are distributed over the cross section
of the wood to form a uniaxial pore channel system that
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______________________
[] Prof. Y. Tang, A. Dong, Y. Wang, N. Ren, Dr. Y. Zhang, Dr. Y. Yue, Prof. Z. Gao Laboratory of Molecular Catalysis and Innovative Materials Department of Chemistry, Fudan University Shanghai 200433 (China) E-mail: [email protected] [*] This work is supported by the Major State Basic Research Development Program (grant no. 2000077500), the NSFC (grant no. 29873011), the Foundation for University Key Teacher by the Ministry of Education, and the Doctoral Fund of the Ministry of Education.
makes wood a perfect candidate as a template for generating
inorganic materials with both structural and morphological
complexity.
[14±17]
Some progress has been made toward trans-
forming wood's cellular structures into inorganic ceramics
such as titania by repeated infiltration of titanium tetra(iso-
propoxide) into the wood tissue and subsequent hydrolysis.
[16]
Very recently, Shin et al.
[17]
used wood tissue as a template to
prepare hierarchically ordered amorphous silica ceramics.
However, to the best of our knowledge, wood tissue has not
been previously utilized for the formation of hierarchical po-
rous materials involving a zeolitic framework.
In this communication, we report a general and facile meth-
od employing a seeded growth strategy to fabricate self-stand-
ing zeolitic tissue that faithfully inherits the initial cellular
structure of wood at various hierarchical levels. We chose to
work with silicalite-1, a pure silica zeolite, rather than amor-
phous silica, not only because such materials may extend the
application range due to their unique microporous framework
but also because the formation process of zeolitic tissue may
mimic the natural petrification of wood more vividly, since a
crystallization process often occurs in wood silicification.
[18]
Two kinds of tissue, cedar with relatively uniform sized pores
(10±20 lm) and bamboo with non-uniformly sized pores
(2±20 lm), were adopted as templates. To faithfully replicate
both the macro- and microstructure of wood tissue, it is neces-
sary to generate very thin, uniform, and continuous zeolite
membranes on the cell walls. However, the direct hydrother-
mal synthesis seems unable to achieve this aim because the ob-
tained crystals, at the micrometer scale,
[3a,4]
are close in size to
the wood cells, making it difficult to finely replicate the initial
structure of wood tissue. To avoid this problem, a seeded
growth strategy, which has been used to prepare thin and uni-
form zeolite membranes on planar substrates,
[19]
was adopted
in this work as shown by Figure 1. The preformed, negatively
charged silicalite-1 nanocrystals (about 80 nm)
[7c]
were first de-
posited as seed crystals on the wood's cellular walls, which
were premodified by the cationic polyelectrolyte. Afterwards,
the seed-coated wood slices were washed to remove the un-
bound zeolite seeds and then subjected to secondary growth in
a clear synthesis solution at 110 C under reflux for one day to
form the zeolite/wood composite. The zeolitic structure was
formed after the removal of the wood by calcination in air.
We first investigated the zeolitic tissue synthesized from ce-
dar, a coniferous wood whose main cell type is longitudinal
tracheids.
[15]
The nucleation seeds could be dispersed com-
pactly and homogeneously on the cell walls, after their poly-
electrolytic treatment, due to their small size and negative
charge,
[7]
which is of great importance for the subsequent sec-
ondary growth. After reflux in the synthesis solution, the color
of the solution became pale yellow, probably due to the par-
tial leaching of the organic compounds from the wood. How-
ever, the entire cellular structure was well preserved, suggest-
ing that the secondary growth process did not destroy the
skeleton of the original wood tissue (scanning electron
microscopy (SEM) image in Fig. 2a). Since adjacent tracheids
are linked together by middle lamellae,
[15]
zeolite crystals pre-
dominantly grew on the inner cell walls under the induction
of the seeds, resulting in continuous and homogenous zeolite
films approximately 500 nm thick (arrows in Fig. 2a). Upon
the removal of the wood tissue by calcination, the sample
roughly preserved its initial shape and size with little contrac-
tion. This should be attributed to complete penetration of the
seeds in the wood during the dip process and the formation of
continuous and relatively dense zeolite films on the cell walls
during the hydrothermal treatment. These films sustained the
architecture of the whole template-free sample. Thermogravi-
metric analysis (TGA) of the zeolite/wood composite showed
that most of the weight loss occurred between 300 and 550 C,
corresponding to the removal of organic components in the
composite. The obtained residue after heating above 800 C
was pure white and weighed only 6±10 % of the initial com-
posite, indicating the high pore volume of the zeolitic tissue.
SEM images demonstrated that the self-standing zeolitic
tissue was composed of bundles of hollow fibers with diame-
ters ranging from 10 to 20 lm (Figs. 2b,c) and lengths to hun-
dreds of micrometers. The size and shape of the hollow fibers
Adv. Mater. 2002 , 14 , No. 12, June 18 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1206-0927 $ 17.50+.50/0 927
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Fig. 1. Schematic illustration of the fabrication of hierarchically organized zeo- lite materials from wood tissue. One cell, with a curved inner cellular wall, is used to illustrate the fabrication procedure.
Fig. 2. a) SEM image showing cedar/zeolite composite after secondary growth (the arrows indicate newly formed zeolite films on cell inner walls). Inset: The original cedar cells. b) Cross section of the resulting self-standing zeolitic tissue. c) Side view of the zeolitic tissue. d) SEM image of zeolitic tissue composed of solid fibers. Inset: The zeolite crystals that form the rods.
even have many smaller pinholes on the surface (Fig. 5c).
Similar results were also obtained when the template was
bamboo, a grass with woody stems. The non-uniform porous
zeolitic tissue resembles the initial wood (Fig. 6a) and it also
demonstrates the various cell morphologies of bamboo. For
instance, some zeolite fibers show ringlike strips (Fig. 6b),
while some exhibit a remarkable spinelike structure (Fig. 6c).
In conclusion, we have demonstrated a general and facile
route involving a seeded growth strategy for the conversion of
wood tissue into hierarchical porous zeolitic materials. On the
one hand, this strategy offers a great potential for designing
novel zeolitic materials with anisotropic structures. On the
other hand, the various vivid morphologies exhibited by zeo-
litic tissue reveal the complexity of the natural wood cells,
which would provide a novel and important perspective on,
for example, biomineralization. Although only cedar and
bamboo woods were researched for this paper, we believe
that the seeded growth strategy could be extended to other
wood species or even tissue of other organisms with hierarchi-
cal structure. In addition, construction of multilevel porous
zeolites of other types should also be feasible in principle.
Experimental
Materials: Cedar (cedrus) and bamboo (bambusa) wood were imported from Shanghai and cut into 1.00.50.5 cm 3 slices before use. Poly(diallyldimethyl ammonium chloride) (PDDA, Mw < 200 000), tetraethylorthosilicate (TEOS, 98 %), and tetrapropylammonium hydroxide (TPAOH, 1 M) were purchased (Aldrich) and used without further purification. Preparation of Silicalite-1 Nanocrystals: Silicalite-1 nanocrystals were pre- pared as described before [23]. The products were purified by repeated centrifu- gation and washing, then dispersed in distilled water to form a stable suspension with a concentration of approximately 1.0 wt.-% at pH 9.5 (adjusted with NH 4 OH). The zeta potential of this suspension was about ±50 mV. The mean size of the nanocrystals was 80 nm (SEM). Fabrication of Zeolitic Tissue: The original wood slices (2 g) were first soaked in a solution of PDDA (0.5 %) for 2 h. During this period, the samples were shaken in an ultrasonic bath for 5 min to release the air bubbles emanat-
ing from the wood and allow the polyelectrolyte to enter the tracheids more easily. After rinsing with deionized water, the wood slices were dipped into a suspension of zeolite nanocrystals at pH 9.5 for 12 h. For larger wood slices, the dip time was lengthened (2±3 d) or the concentration of the seed suspension in- creased (5.0 wt-%). After washing with 0.01 M NH 4 OH to remove the excess seeds, the seeded wood slices were treated in a clear synthesis mixture (10 mL) with a molar composition of 3 TPAOH/25 SiO 2 /1500 H 2 O/100 EtOH at 110 C under reflux for 1 day. The starting materials of the synthesis solution were TEOS, TPAOH, and distilled water. The wood/zeolite composite was then tak- en out and rinsed again with 0.01 M NH 4 OH. After drying at ambient tempera- ture, the sample was heated to 873 K for 6 h at a heating rate of 2 K min ±1^ in air to remove the organic components (wood tissue, polyelectrolyte, TPAOH). Characterization: SEM studies were performed on a Philips XL 30 instru- ment. XRD patterns were taken on a Rigaku D/MAX-IIA diffractometer using Cu Ka radiation. The N 2 adsorption±desorption isotherms were measured at 77 K using a Micrometrics ASAP 2000 system. The TGA measurements were performed on a Rigaku TG analyzer. FT-IR spectra were recorded on a Magna 550 spectrophotometer using KBr pellets.
Received: February 4, 2002 Final version: April 4, 2002
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Adv. Mater. 2002 , 14 , No. 12, June 18 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1206-0929 $ 17.50+.50/0 929
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Fig. 6. SEM images of zeolitic tissue obtained through bamboo templating: a) cross-sectional view of the hierarchical porous structure, b) ringlike mor- phology, and c) spinelike morphology. Inset: The cross-sectional view of the spinelike morphology.
