Molecular selective photocatalytic decomposition of alkylanilines by crystalline TiO₂ particles and their nanocomposites with mesoporous silica

Abstract

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.

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Introduction

Molecular selectivity is becoming increasingly important in environmental protection, because it provides an effective way to remove dilute pollutants. One strategy is molecular selective adsorption using nanoscale-designed organic–inorganic materials.^1–4 Another important strategy is to design molecule selective photocatalysts. TiO₂ is a well known, versatile photocatalyst.^5–19 Environmental purification is a promising and extensively studied fields for the application of TiO₂ photocatalysts. However, TiO₂ photocatalysts are known not to be molecule selective. A promising way towards molecule selective photocatalysis is to combine molecule selective adsorption and photocatalysis.^20–22 Although many researchers have studied the enhancement of photocatalytic activity by dispersing TiO₂ onto various adsorbents,^23–31 few studies have reported molecule selective photocatalysis.

Surfactant-templated mesoporous silica^32–41 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 silica^61–63 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.

Experimental methods

Sample preparation

Detailed sample preparation procedures were described in our previous reports.^62,63n-Octadecyl-grafted TiO₂ (C18-TiO₂) particles were prepared by the following procedure: 2 g TiO₂ powder (Degussa P-25) was dehydrated in vacuo at 473 K for 1 h in a glass high vacuum system, and then refluxed with 5 g n-octadecyltriethoxysilane in freshly-distilled toluene for 48 h. After refluxing, the reaction mixture was filtered and the residue was washed with distilled toluene and anhydrous methanol. Using a CHN analyser (PerkinElmer 2400II), the number of grafted alkyl groups on the TiO₂ particles was determined to be 2.5 × 10⁻⁴ mol g⁻¹, corresponding to a surface density of 2.9 molecules nm⁻².

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.

Characterization of nanocomposites

Nitrogen adsorption isotherms were measured at 77 K using a Belsorp-mini (BEL Japan Inc., Osaka, Japan). TEM and SEM images were taken with a JEM-2010 transmission electron microscope and a JSM-6320FS scanning electron microscope, respectively (JEOL, Tokyo, Japan). X-ray powder diffraction patterns were measured with X-ray diffractometers M18XHF (MacScience, Japan; low angle regions) and D8 Advance (Bruker AXS, Germany; high angle regions) using Cu-Kα radiation.

Photocatalytic test

Photocatalytic tests were carried out as follows: typically 30 mg of the composite photocatalyst was added to 300 g of a mixed aqueous solution of 4-n-heptylaniline (NHA), 4-n-butylaniline (NBA), and p-toluidine (4-methylaniline, PT). Their initial concentrations were 2.4–2.6 × 10⁻⁵ mol dm⁻³. After adsorption of the phenols onto the catalysts had reached equilibrium, the mixture was irradiated with a 500 W Xe lamp, under stirring. The solution was analysed using an HPLC equipped with a UV detector (Shimadzu LC-10).

Results and discussion

Structure of the nanocomposites

The nanocomposites used in this study are the same as those reported in our previous study. The X-ray powder diffraction patterns of selected samples⁶³ are shown in Fig. 1. Mesoporous silica (MPS(2.7)), without TiO₂ incorporation, showed intense diffraction peaks in the low angle region, attributed to hexagonally ordered mesopores. The nanocomposites gave much weaker diffraction peaks than mesoporous silica, and also showed peaks from crystalline TiO₂ (mixture of anatase and rutile types) at higher angles. The TiO₂ diffraction peaks for the nanocomposites were identical to the TiO₂ peaks in the original TiO₂ (P-25), indicating that there was no phase change in the TiO₂ crystals. The nitrogen adsorption isotherms of the nanocomposites showed a step from nitrogen capillary condensation, indicating the presence of mesopores templated by the surfactants. Table 1 summarizes the properties derived from the nitrogen adsorption isotherms.⁶³ According to TEM and SEM images, which were presented in our previous paper,⁶³ a large fraction of the TiO₂ particles was embedded into the mesoporous silica. In the case of C18-NC(2.7), almost all the TiO₂ particles were embedded into mesoporous silica.

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.

Table 1 Properties of nanocomposites determined by N₂ adsorption^a

Sample	Template^b	S BET ^c(m^2 g^−1)	V p ^d(cm^3 g^−1)	Pore size^e (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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Molecular selective photocatalysis of pristine TiO₂ and the nanocomposite photocatalysts

First we checked the behaviour of the mixed molecules under irradiation without photocatalysts (blank test). Fig. 2a shows the time courses for the blank test. After the photo-irradiation started, the concentrations of the three compounds decreased, indicating that decomposition of these molecules occurred without photocatalysts, but the decomposition rates were slow. When MPS(2.7) was added to the solution, a large decrease in NHA concentration was observed because of preferential adsorption of the NHA molecules to MPS(2.7) prior to the start of irradiation (Fig. 2b). A decrease in the concentration of NBA was also observed, but it was much smaller than that observed for NHA. The adsorption of PT on MPS was small, similar to that of NBA.

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.

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.

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.

The factors governing molecular selectivity

It has been widely considered that TiO₂ photocatalysts show no molecular selectivity. In our previous study,^62,63 we demonstrated that a TiO₂ particle–mesoporous silica nanocomposite showed molecule selective decomposition of nonylphenol in a mixed aqueous solution of alkylphenols. In that case, nonylphenol was preferentially adsorbed into the nanospaces of the mesoporous silica and then was decomposed selectively. In the present decomposition experiment with mixed alkylanilines, however, the pristine TiO₂ (P-25) showed molecular selectivity: NHA decomposed much faster than NBA or PT in the mixed aqueous solution. Since it was obvious that NHA was preferentially adsorbed onto TiO₂ (see Fig. 3a), the molecule selective decomposition of NHA can reasonably be ascribed to selective adsorption on the TiO₂ surface. As shown in Fig. 3a and b, decomposition of NBA accelerated after NHA was almost completely decomposed, and decomposition of PT accelerated after the concentration of NBA had decreased. This order may be the same as the order of preferential adsorption of molecules from aqueous solution onto TiO₂ surfaces. It is notable that the order (NHA > NBA > PT) coincides with the order of hydrophobicity. There are some reports that preferential molecular adsorption onto TiO₂ leads to molecule selective photocatalysis.^20,66 Recently, Liu et al. reported that anatase particles with exposed {001} facets showed molecule selective photocatalysis towards the decomposition of dyes.⁶⁶

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.

Conclusions

Molecule selective photocatalytic decomposition of alkylanilines in water was investigated using crystalline TiO₂ particles (P-25) and their nanocomposites with mesoporous silica. 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. When the TiO₂ particles were surrounded by mesoporous silica, the molecular selectivity was strongly modified. The nanocomposites with mesoporous silica showed much higher molecular selectivity for NBA decomposition. The reason for the molecular selectivity is likely to be the effect of concentration and mobility of the molecules in the mesoporous channels; it is likely that a crucial factor for the molecular selectivity is chemical interactions (such as hydrogen bonding and weak acid–base interaction) between the organic molecules and silica surfaces.

Acknowledgements

This study was partly supported by a Grant-in-Aid on Priority Areas (Area No. 417, Grant No. 17029043) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), by Grants-in-Aid for Scientific Research (B) (Nos. 20350095 and 17360389), and for Creative Scientific Research (No. 17GS0206), and the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program from Japan Society for the Promotion of Science (JSPS).

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