Masataka
Yasui
,
Kiyofumi
Katagiri
,
Shoji
Yamanaka
and
Kei
Inumaru
*
Department of Applied Chemistry, Institute of Engineering, Hiroshima University, 1-4-1, Higashi-Hiroshima, Hiroshima 739-8527, Japan. E-mail: inumaru@hiroshima-u.ac.jp
First published on 17th September 2012
Molecular selective photocatalytic decomposition of alkylanilines in water was investigated by using crystalline TiO2 particles (P-25) and their nanocomposites with mesoporous silica. In the nanocomposites, pre-formed TiO2 particles were embedded into surfactant-templated mesoporous silica with tuned pore sizes (1.4–5.3 nm) as described in our previous reports (Chem. Commun. 2005, 2131–2133; J. Mater. Chem. 2011, 21, 12117–12125). Contrary to the generally accepted knowledge of TiO2 photocatalysts, pristine TiO2 particles showed molecule selective photocatalysis: decomposition of 4-n-heptylaniline (NHA) was much faster than that of 4-n-butylaniline (NBA) or p-toluidine (4-methylaniline, PT) in a mixed aqueous solution. After NHA had almost disappeared, decomposition of NBA accelerated. When the TiO2 particles were surrounded by mesoporous silica, the molecular selectivity was strongly modified: the nanocomposites showed high molecular selectivity for NBA decomposition and the reaction of PT was considerably suppressed. The origin of molecular selectivity can basically be ascribed to preferential molecular adsorption, and the difference in selectivity between alkylanilines and alkylphenols is discussed in terms of the interaction of the molecules with the nanocomposite surfaces.
Surfactant-templated mesoporous silica32–41 has been a subject of great interest because of its potential applications as host structures, adsorbents, and catalysts for reactions involving large molecules.42–47 To develop new functions such as catalysis, many studies have been carried out by combining transition metal oxides with mesoporous structures.48–58 The most frequently used method is to disperse transition metal oxide particles onto the pore-walls of mesoporous silica. To improve the activity of the photocatalysts, many researchers have attempted to combine TiO2 with mesoporous silica by simple methods such as impregnation, sol–gel methods, and surface grafting. However, it seems that materials of this type generally have poorer photocatalytic performance than well-crystallized pure TiO2 particles such as commercially available “P-25”. This is probably because of the low crystallinity of the highly-dispersed TiO2 particles on the mesoporous supports, and the limited TiO2 loading level (ca. 10–25 wt%). To overcome these limitations, some papers have reported crystalline mesoporous TiO2 materials and their applications in photocatalysis.59,60
We reported an approach to combine preformed crystalline TiO2 and surfactant-templated mesoporous silica61–63 to achieve molecule selective photocatalysis for the decomposition of organic contaminants in water. We employed preformed well-crystallized TiO2 particles (P-25) directly as the source material for nanocomposite synthesis. These nanocomposites have a unique structure in which TiO2 crystalline particles are surrounded by mesoporous silica.62,63 After our reports, a trial of selective organic transformation was also made using similar TiO2–mesoporous silica nanocomposites, which indicates the potential applications of this composite material.64 In our previous study, we investigated the decomposition of alkylphenols.62,63 The molecular selectivity of the photocatalytic decomposition was ascribed to the molecular selectivity of preferential adsorption into the nanospaces of the mesoporous silica. Since phenolic hydroxyl groups are known to show acidity, examining the behaviour of molecules such as anilines, containing basic groups, is the next stage.
Here we report molecule selective photocatalytic decomposition of alkylanilines by TiO2 and TiO2–mesoporous silica nanocomposites. The differences between the behaviour of alkylanilines and that of alkylphenols were revealed and discussed in terms of molecule–photocatalyst surface interactions.
Mesoporous silica (MPS(2.7)) and the composite catalyst (NC(2.7)) were prepared by the following procedure: 4.4 g hexadecyltrimethylammonium bromide was dissolved in ca. 230 g hot water (333 K). Then 17 g of 35% ammonia solution was added. A weighed amount of TiO2 (Degussa P-25, 7.3 g) or C18–TiO2 was ultrasonically dispersed in the solution. After adjusting the pH to 11.8 by adding ammonia solution, 17 g tetraethoxysilane (TEOS) was quickly added to the mixture, under vigorous stirring. After aging for 1 h, the precipitate was filtered, washed with water, and dried at 353 K for one day. The materials were calcined at 813 K for 6 h to obtain the mesoporous composite materials. The nanocomposites prepared from unmodified TiO2 and C18–TiO2 are denoted as NC(2.7) and C18-NC(2.7), respectively. “2.7” represents the pore diameter (nm), as determined by nitrogen adsorption at 77 K. All nanocomposites contain 60 wt% TiO2. MPS(2.7) was prepared according to the same procedure, without the addition of TiO2.
To investigate the effect of pore size on the molecular selectivity of the photocatalysts, we prepared a series of samples using surfactants with different alkyl-chain lengths. The preparation procedures were the same as those used to prepare NC(2.7), except that C8H17N(CH3)3Br (3.2 g) or C10H21N(CH3)3Br (4.4 g) was used as the template. To expand the pores, 3.68 g tridecane was added to a solution of C16H33N(CH3)3Br (3.7 g dissolved in 34 g water),65 and then the nanocomposite was prepared in a similar way to that used for NC(2.7). In this case, 6.36 g TiO2 and 14.7 g triethoxysilane were used.
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Fig. 1 X-ray diffraction patterns of mesoporous silica and nanocomposites: (a), TiO2 (P-25); (b) mesoporous silica (MPS(2.7)); (c) NC(1.4); (d) NC(1.9); (e) NC(2.7); (f) NC(5.3); (g) C18-NC(2.7). See ref. 63. |
Sample | Templateb | S BET c(m2 g−1) | V p d(cm3 g−1) | Pore sizee (nm) | |
---|---|---|---|---|---|
d K | d C | ||||
a See ref. 63. b Cn represents CnH2n+1(CH3)3NBr. c BET surface area. d Total pore volume. e Diameters. dK was determined using Kelvin's equation and multilayer adsorption thickness. dC = 4Vp/SBET. | |||||
MPS(2.7) | C16 | 1016 | 0.87 | 2.7 | 3.4 |
NC(5.3) | C16 + tridecane | 482 | 0.60 | 5.3 | 5.0 |
NC(2.7) | C16 | 405 | 0.36 | 2.7 | 3.5 |
NC(1.9) | C10 | 452 | 0.24 | (1.9) | 2.1 |
NC(1.4) | C8 | 310 | 0.15 | (1.4) | 1.9 |
C18-NC(2.7) | C16 | 444 | 0.37 | 2.7 | 3.3 |
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Fig. 2 Time courses of concentrations of alkylanilines in aqueous solutions under irradiation. (a) Blank test, (b) mesoporous silica (MPS(2.7)) only, without TiO2 addition. |
Fig. 3 shows the time courses during the photocatalytic reactions for pristine TiO2 and for some nanocomposite photocatalysts. It should be noted here, as shown in Fig. 3a, that pristine TiO2 showed considerable molecular selectivity for decomposition of NHA; when TiO2 was added to a mixed solution of alkylanilines, the concentration of NHA decreased significantly because of adsorption onto TiO2, while the adsorption of NBA was quite small, and the adsorption of PT was much smaller again. After the irradiation started, NHA decomposed much faster than NBA or PT. The first-order plot for this reaction (Fig. 3b) clearly shows the difference in the decomposition rates between these molecules. In Fig. 3a and 3b, it is clearly observed that decomposition of NBA accelerated just after almost all NHA disappeared. A slight acceleration of PT decomposition was also observed once NBA decomposition had largely proceeded. These trends indicate that pristine TiO2 preferentially decomposed the alkylanilines in the order NHA ≫ NBA > PT. The molecule selective photocatalysis of pristine TiO2 will be discussed in a later section.
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Fig. 3 Time courses of concentrations of alkylanilines in aqueous solutions under irradiation. (a), (b) Pristine TiO2 (P25); (c), (d) NC(2.7); (e), (f) C18-NC. (b), (d), and (f) are the corresponding first-order plots. |
In the case of NC(2.7) (Fig. 3c and 3d), there was a much clearer difference in the molecular selectivity of photocatalytic decomposition between NBA and PT. Their initial reaction rate constants were very different, as can be seen from the initial stages of the reaction time courses. This point is clearly indicated in the first order plots of this reaction (Fig. 3d), in contrast to the case of pristine TiO2 (Fig. 3b). The reaction rates were in the order NHA > NBA ≫ PT, based on the initial reaction stage.
Fig. 4 compares the rate constants derived from the first order plots. The rate constant k for NHA was clearly larger in the presence of the nanocomposite photocatalyst NC(2.7) than in the presence of pristine TiO2 (P25). This means that the photocatalytic reaction was accelerated by incorporating the TiO2 into a composite with mesoporous silica. This acceleration may be ascribed to concentration of the NHA molecules into the nanospaces of the mesoporous silica surrounding the TiO2 particles; the effective concentration around the TiO2 surfaces could be increased by this preferential adsorption of NHA. On the other hand, the decomposition rate of PT was significantly lower in the presence of NC(2.7), compared with that in the presence of pristine TiO2, while the rate of decomposition of NBA was similar with both catalysts.
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Fig. 4 First-order rate constants for photocatalytic decomposition of alkylanilines. The constants were determined from the initial 30 min of the first-order plots. |
Fig. 4 also contains results obtained with pore-size controlled nanocomposites. As the pore size increased to 5.3 nm (NC(5.3)) the decomposition selectivity decreased slightly. Although the results of NC(1.9) were similar to those of NC(2.7); for NC(1.4) the reaction rates of all compounds were significantly smaller than those obtained with the other nanocomposites. This may be ascribed to slow diffusion rates of the molecules in the narrow pores of NC(1.4) and/or access to the TiO2 surfaces being partly hindered by silica overlayers.
Fig. 3e and f represent the time courses of the reactions and the first order plots for C18-NC(2.7), which was prepared using surface organo-modified TiO2. In this case, the molecular selectivity for NBA was significantly higher than for the other nanocomposites: the kinetic constant as well as the decomposition rate of NBA exceeded those of NHA under these conditions.
In contrast, for the nanocomposites such as NC(2.7) and C18-NC, in which TiO2 particles were surrounded by mesoporous silica, selectivity of NBA against PT was greatly increased compared to the selectivity found for pristine TiO2. For NC(2.7), the reaction rate of PT was much smaller than that of NBA, while for pristine TiO2, the reaction rates of NBA and PT are comparable. For C18-NC(2.7), the enhancement of selectivity for the decomposition of NBA was significant: the order of the molecular selectivity was reversed, although the amount of NHA adsorbed was much larger than that of NBA (Fig. 3e). That is, it appeared that the NBA molecules concentrated into the nanospaces of the nanocomposites such as C18-NC(2.7) were more efficiently decomposed than the NHA molecules. Such a phenomenon was not observed for alkylphenols; the order of decomposition rates of alkylphenols can be simply explained by the order of the amounts adsorbed onto the composite photocatalysts.61–63 It should be noted here again that, in the case of pristine TiO2, the order of reaction rates of alkylphenols could also be simply explained by the amounts adsorbed. At present the reason for these differences between alkylanilines and alkylphenols are not clearly proven. It is likely, however, that molecular interactions between silica surfaces and the adsorbed molecules play an important role. As was shown in our previous study,4 amino groups of alkylanilines strongly interact with SiOH groups on the walls of mesoporous silica via hydrogen bonding and/or weak acid–base interactions. In the case of the TiO2–mesoporous silica nanocomposites, a large amount of NHA was adsorbed onto the silica walls; Fig. 2b shows this very high affinity of NHA molecules for mesoporous silica surfaces. A strong interaction between NHA molecules and Si–OH might retard the mobility of NHA molecules in the mesoporous silica channels. Of course, increasing the effective concentration of NHA molecules around TiO2 must contribute to the reaction rate of NHA molecules. The strong adsorption of a molecule onto the silica walls has trade-off effects on the reaction rates: the concentration of the molecules around the TiO2 increases, but the mobility of the molecules is decreased by the strong adsorption. In contrast, NBA molecules showed moderate adsorption onto the silica walls. This is likely to both increase the concentration of NBA molecules around TiO2 and maintain high mobility of the molecules in the mesoporous channels.
Here, the diffusion of active species such as hydroxyl radicals from the TiO2 surfaces should be considered. The lifetime and diffusion distance of hydroxyl radicals depend greatly on the conditions. It was reported that, in ambient air, hydroxyl radicals diffuse as far as 70–125 μm (diffusion coefficient D = 0.22 × 10−4 m2 s−1, and their lifetime was estimated to be 170 μs).67 In water, the diffusion is much slower (D = 2.8 × 10−9 m2 s−1)68,69 and the diffusion distance is significantly shorter as a result of the presence of reactant molecules or radical scavengers; for example, the diffusion distance can be as short as 6 nm in cells.70 In the case of our composite photocatalysts, organic reactants were concentrated into the nanospaces of the mesoporous silica around the TiO2 particles, and it is likely that the reaction of hydroxyl radicals and the reactants occurs inside the pores. Under such conditions, even if the hydroxyl radicals diffuse faster than the reactant molecules, the molecular selectivity is expected to be governed by the adsorption selectivity of the reactant molecules.
The significant difference between NC(2.7) and C18-NC(2.7) (Fig. 4) is interesting, and the reasons for the difference are not yet clear but are probably concerned with the structure of the composites. A possibility is that although C18-NC(2.7) contains a very high concentration of TiO2 particles embedded in the mesoporous silica, the surface organo-modifications on the TiO2 may produce some gaps or voids between the TiO2 surfaces and the walls of the silica mesopores, resulting in slow diffusion of the molecules strongly adsorbed onto the silica surfaces to the TiO2 surfaces. This point will be an interesting subject for future studies.
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