Most studies involving the catalytic activity of basic catalysts make us of
either alkali-exchanged zeolites or zeolites impregnated with sodium metal clusters
or alkali oxides (Dartt and Davis, 1994). Ordered mesoporous
silicas such as MCM-41 (Kresge et al., 1992;
Qi et al., 2011), SBA-15 (Zhao
et al., 1998; Thielemann et al., 2011)
and zeolite-MCM-41 composite (Li et al., 2011)
are attractive materials due to their high surface area and ordered arrays of
uniform channels. SBA-15 has ordered straight mesopores as well as disordered
interchannel micropores in the mesopore wall (Zhang et
al., 2006; Yang et al., 2003). Recent
research focus is directed towards the design of basic mesoporous materials
for various reactions (Rymsa et al., 1999). In
this respect, SBA-15 is one of the most widely investigated for use as catalysts
in many reactions.
Alkaline metal oxides are often used to improve the basic strength of a solid
catalyst due to their strong basic behavior. Mesoporous strong solid bases can
be prepared through impregnation of mesoporous material with alkali metals or
alkaline earth metals. For example, lithium can be used as the guest component
to generate strong basicity in various porous hosts such as MCM-41 and zeolites
(Clacens et al., 2002). Kloetstra
and van Bekkum (1997) successfully demonstrated the preparation of strong
basic mesoporous solid, especially MCM-41 using cesium acetate solution. The
prepared basic materials showed rather poor stability because cesium oxide could
react with the silica host and damaged the mesoporous frame works (Kloetstra
and van Bekkum, 1997; Zhao et al., 1998).
With relatively thicker pore walls , SBA-15 is reported to be more stable than
MCM-41. However, limited result on its stability to strong base is available
in the literature.
There are two main factors which are considered as the obstacles to the successful
generation of strong basicity in mesoporous silicas. Firstly, weak host-guest
interaction between silica and the base precursors can lead to the difficulty
in the decomposition of the base precursors to their basic forms. It was reported
that only a small amount of alkali salt could be decomposed on silica, even
when the sample was activated at the high temperature of 600°C (Sun
et al., 2008a). The second factor is the poor resistance of mesoporous
silicas against alkali. It is due to the reaction between alkali hydroxides
with silica hydroxylsilicates. This can result in the collapse of the mesoporous
structure after the formation of this strongly basic species (Sun
et al., 2008b).
The objective of this study is to demonstrate the effects of strong metal hydroxide on the porous structures of SBA-15. Detail study on this topic is rarely reported and understanding on the surface and structural phenomena is yet to be established. Thus, through several surface analysis method, specific changes that occur upon exposure to strong metal hydroxide can be elucidated.
MATERIALS AND METHODS
Material preparation: Mesoporous silica (SBA-15) was synthesized according
to areported method by Zhao et al. (1998). Briefly,
4 g of triblock copolymer P123 (EO20PO70EO20,
M = 5800, Aldrich) as the templating agent was dissolved in 90 mL of water and
60 mL of 4 M HCl aqueous solution with stirring at 40°C for 2 h. Next, 8.5
g of tetraethyl orthosilicate (TEOS) was then added to the homogeneous solution
and stirred at this temperature for 22 h. Finally, the temperature was heated
to 100°C and held at this temperature for 24 h under static condition. The
prepared sample was recovered by Filtration washed with water and air-dried
at room temperature. The removal of the template was carried out at 550°C
in air for 6 h. Alkali metal LiOH was introduced through wet impregnation process.
An identical amounts of LiOH (namely 5, 10 and 20 wt. %) were used for all samples.
The required amount of LiOH was dissolved in deionized water followed by the
addition of the host (SBA-15). After stirring at room temperature for 24 h,
the mixture was evaporated at 80°C and subsequently dried at 100°C for
4 h. The obtained solid was then calcined at 550°C for 5 h in air.
Material characterization: The materials obtained were characterized using XRD, surface analyzer, SEM and TEM. The N2 adsorption-desorption isotherms were measured using a Belsorp II system at-196°C. Those samples were degassed at 300°C for 4 h prior to analysis. The Brunauer-Emmett-Teller (BET) surface area was calculated using adsorption data at relative pressures ranging from 0.04 to 0.20. The total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of about 0.99. The pore diameter was calculated from the adsorption branch by using the Barrett-Joyner-Halenda (BJH) method. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) analyses were performed using a Zeiss Supra 35VP equipment operated at 3.00 kV. The TEM images of the prepared samples were analyzed using a Phillips CM 12 transmission electron microscope equipped with an image analyzer and operated at 120 kV. The basic strengths of materials (H-) were determined using Hammett indicators method. About 25 mg of the sample was shaken with 5.0 mL of a solution of Hammett indicators diluted with methanol and left to equilibrate for 2 h. After the equilibration, the color on the catalyst was observed. The following Hammett indicators were used: methyl red (H_ = 4.8), neutral red (H_ = 6.8), bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.3), 2, 4-dinitroaniline (H_ = 15.0) and 4-nitroaniline (H_ = 18.4).
RESULTS AND DISCUSSION
Effects on surface characteristics: The basic strength and surface characteristics of the catalysts used in this work are shown in Table 1. It is noted that the basic strength of the prepared samples increased after loading with LiOH from 5 to 20 wt. %. The basic strength of SBA-15 was found at H_<4.8. However, after loading 20 wt% LiOH, it was noted that basic strength was significantly increased to 9.3<H_<15. This observation suggested that alkali metal loading directly involved in the basicity of the materials, regardless of their surface or structural properties.
It could be deduced from the sharp decrease in the area of alkali metal loaded samples that structure of SBA-15 might collapse due to the reaction with alkali metal (LiOH). This was also proven by changes in the mesoporous area of these samples as shown in the same table. It was found that the mesoporous area and pore volume also sharply decreased in agreement with the decreasing surface area of these samples. Obviously, it was an indication of major loss of mesoporous structure that presented in these samples.
Data in the same table proved that the surface area of the prepared SBA-15 samples sharply decreased after loading with alkali metal. These results suggested that major destruction must have occurred with the corresponding loss of porosity and surface area. Clearly, the mesoporous channels in the SBA-15 should have collapsed to form non-porous mass of silica due to the reaction with the alkali metals. The solid evidently consisted of small particles that were readily dispersible in water or aqueous solution during basicity test as the basicity was found to increase with metal loading.
Nitrogen adsorption-desorption isotherms of SBA-15 and SBA-15 with different
LiOH loadings are given in Fig. 1. Parent SBA-15 showed the
isotherm and hysteresis loop which are generally associated with mesoporous
materials (Zhao et al., 1998). However, for LiOH/SBA-15
sample (5) with small loading of LiOH, the hysteresis loop sharply decreases
along with a decrease in surface area and pore volume. The shapes of the curves
for sample LiOH/SBA-15 (10) and LiOH/SBA-15 (20) generally agree with the Type
I isotherm according to the IUPAC nomenclature which is a characteristic of
microporous materials. This result was in agreement with data in Table
1 that show loss of mesoporousity.
In addition, according to further classified shapes of the hysteresis loops,
these three metal loaded SBA-15 samples exhibited the type H3 hysteresis
loops. This type of hysteresis is usually shown by aggregates or agglomerates
of particle forming slit shaped pores (plates or edged particles like cubes).
Thus, in agreement with previous study on titania (Sun et
al., 2008a), the incorporation of high loading of alkali metals led
to the collapse of some mesostructures leading to a severe drop in porosity.
|| Characteristics of LiOH/SBA-15 samples prepared with different
||Nitrogen adsorption-desorption isotherms for LiOH/SBA-15 samples
with different LiOH loadings
As such, the synthesis of alkali metal loaded porous silica materials through
simple wet impregnation was indeed a challenging task.
The collapse of porous structure of SBA-15 was deemed to be directly caused by the alkali-silica reaction between the loaded metal and the silica matrix in aqueous condition. This reaction led to the formation of lithium silicate hydrates. This reaction caused the expansion of the altered aggregate by the formation of a swelling gel of these hydrates. This gel increased in volume with water and exerts an expansive pressure inside the SBA-15 material, causing spalling and loss of its mesoporous structure.
Another strong possibility which might be involved in the destruction the porous
silica structure of SBA-15 is the change in media pH. According to Zhao
et al. (1998), the mesoporous SBA-15 structure is formed and stable
under a strong acidic media (pH<3). Therefore, when trying to load a strong
alkali LiOH, its media changed from strong acidic to week acidic (pH>3).
As silica structure could not be stable under this media, it might collapse
badly leading to the major loss of porosity.
Effects on particle structure: Based on the XRD pattern (result not
shown), SBA-15 exhibited regular XRD patterns, with an intense (100) diffraction
peak and two or more well-resolved peaks (100, 110, 200) which were indexed
as 2D hexagonal symmetry (Yang et al., 2003).
||TEM images of, (a) pure SBA-15 and (b) SBA-15 loaded with
20 wt % of LiOH
Compared to the SBA-15 sample, the intensity of the peaks for LiOH loaded SBA-15
samples were very weak and their positions were not clear. This observation
suggested that the original mesoporous structure of SBA-15 was not sustained.
Similar characteristics of the prepared samples were also observed in TEM images.
TEM images of sample SBA-15 and LiOH/SBA-15 prepared by impregnation method
with 20 wt. % of LiOH loadings are shown in Fig. 2a and b,
respectively. SBA-15 showed well-ordered hexagonal pores with clear channel
tubing inside the structure. However, with 20 wt. % of LiOH loading, the ordering
of the SBA-15 support was observed to disappear.
Figure 3 shows SEM images of the prepared SBA-15, LiOH/SBA-15
(5 wt. %) and LiOH/SBA-15 (20 wt. %).
|| SEM images at a magnification of 5000 X, (a) SBA-15, (b)
5% LiOH/SBA-15 and (c) 20% LiOH/SBA-15
In Fig. 3a, the microscopic morphology of prepared SBA-15
is similar to generally published mesoporous structure of SBA-15 (Rymsa
et al., 1999; Li et al., 2011). As
observed in Fig. 3b, 5 wt. % LiOH loading over SBA-15 consisted
of a large amount of small spherical particles with spiky structures on the
surface, evidently contributed by the loaded metal. When the LiOH loading was
increased to 20%, the shape of SBA-15 structure had entirely different looks
with a fluffy type and sharp edges. No specific SBA-5 morphology that can be
seen in Fig. 3b and c, they were generally
irregular in shape. This was an evidence of mesoporous structure collapse.
As reported by different researchers (Sun et al.,
2008b; Thielemann et al., 2011), mesoporous
SBA-15 could have weak resistance to strong alkali. In this work, its structure
could be destroyed in the presence of alkali metals. The morphology of 20 wt.
% LiOH loaded over SBA-15 materials (Fig. 3c) was seen to
be made up of more fluffy fibers with large circular ball shape. Thus, the analysis
of TEM and SEM images of prepared LiOH loaded samples showed that LiOH well-dispersed
over the surface of SBA-15. It could be concluded that the morphology of SBA-15
structure underwent significant changes after loading with this strong alkali
metal. The newly prepared structures of these samples seemed to be fluffier
and denser than the parent SBA-15. This type of material is unlikely to act
as good basic catalyst due to its non-porous characteristics.
Increasing severity of mesoporous structure destruction was detected when SBA-15 was impregnated with LiOH with increasing LiOH loading. This destruction was ascribed to the chemical reaction between strong alkali metal and mesoporous matrices to form metal hydrosilicates. The new structure of alkali-loaded SBA-15 was found to be fluffier and lighter than the parent material. The new structure was neither microporous nor mesoporous and was entirely different from parent structure of SBA-15. However, the basic strength of the prepared material was found to increase with increasing lithium loading. Thus, despite having thicker pore walls compared to other mesoporous materials, SBA-15 still shows poor resistance to strong basic metals.