Aloe-vera flower shaped rutile TiO₂ for selective hydrogenation of nitroaromatics under direct sunlight irradiation

2.1 Chemicals and reagents

Titanium tetrachloride (TiCl₄), tetra-butyl titanate (TBT), ethanol, toluene, iso-propyl alcohol (IPA), nitrobenzene, m-dinitrobenzene, m-nitroaniline and m-phenylenediamine and 2,2-dinitrobiphenyl, benzo[c]cinnoline were purchased from Loba Chemicals Ltd. Ultra-filtered de ionized water was obtained from Millipore- Milli-Q (Conductivity 35 mho cm⁻¹ at 25 °C. Commercial P25-TiO₂ was procured from Degussa Corporation, Germany. All the chemicals were used without further purification.

2.2 Synthesis of Aloe-vera shaped rutile TiO₂ nanostructures (F-TiO₂) and P25 rutile TiO₂ (R-TiO₂)

The Aloe-vera flower shaped rutile TiO₂ was synthesized by surfactant free hydrothermal procedure (Xiang et al., 2012). In a typical procedure the TiCl₄ aqueous solution (50 wt%, solution A) was prepared by dropwise addition of TiCl₄ to the ice-cooled de ionized water under vigorous stirring. Solution B was prepared by drop wise addition of TBT (4 mL) to ice-cooled 30 mL toluene. After 1 h of stirring, 4 mL of solution A was added to it, and further stirred for another 1 h. The contents were shifted to a Teflon lined stainless steel autoclave (50 mL) and kept at 150 °C for 24 h. The powdered product was filtered, washed several times using ethanol and dried at 80 °C. The R-TiO₂ was prepared (Kaur and Pal, 2014) by thermally treated P25 at 800 °C for 4 h in a high temperature muffle furnace. The obtained powder was cooled at the room temperature for further analysis and application.

2.3 Characterization techniques

The optical absorption spectrum of as-synthesized photocatalysts dispersed in ethanol was recorded on a UV–Visible spectrophotometer (Analytic-Jena Specod 205). The band gap energies were calculated with the help of ‘Tauc relation’ (αhν = A (hν − Eg)ⁿ) where α is absorption coefficient, A is a constant, E_g is bandgap energy, hν is photon energy and exponent n is the transition type (n = 2 for indirect and ½ for direct band gap). The crystallographic studies were performed on X-ray diffractometer (PANalytical X’ Pert PRO using Cu-K_α radiation (operated at 45 kV, λ = 1.54 Å) with 2θ set in the range of 20–80°. Time-resolved decay profile was measured by time correlated single photon counting (TCSPC) Edinburgh FL920 model with excitation diode lasers. The sample (suspended in ethanol) was exposed with a diode laser at 390 nm wavelength. Photoluminescence (PL) was monitored at 370 nm laser wavelength. Surface area (S_BET) studies were performed by pre treating 50 mg of the catalyst at 150 °C in vacuum for 2 h and then analyzed on BET (Brunauer Emmett Teller) surface area analyzer (BEL Sorp-max) at cryogenic temperature. The scanning electron microscope (SEM) images were taken on a 2 Å resolution Hitachi 7500 instrument, operating at 120 kV and carbon coated copper grid was used for the sample preparations.

2.4 Photocatalytic activity study

As synthesized F-TiO₂, R-TiO₂ and P25 catalysts (20 mg) were analyzed for the photoreduction study of nitroaromatic compounds (NB, m-DNB and DNBP; Scheme 1). The nitroaromatic solution (5 mM) was prepared as an aqueous mixture (1:1) in iso-propanol and deionized water by sonication (30 min). In a typical process 5 mL solution of nitroaromatic compound (25 µmol) and 20 mg of the catalyst were charged in a test tube and purged with argon (15 min) to remove dissolved oxygen. The test tube was covered with gas tight rubber septum and kept under UV (125 W Hg arc, 10.4 mW/cm²; λ > 300 nm), the sunlight based experiment was performed during the month of September 2016 (15–20 and 23–27 September, 11.00 h. to 16.00 h. IST) with average solar flux ∼40 mW/cm² at ∼36.4 °C temperature. The reaction mixture was continuously stirred at 700 rpm. The samples for the analysis were withdrawn at different time intervals and the contents were centrifuged, filtered through cellulose micro filter (0.22 µm) and 20 µL of the sample was injected into the HPLC column using methanol: water (7:3) solvent system. The quantification of the samples were done on a C-18 column (250 mm × 4.6 mm, 5 µm, at room temperature with flow rate of 1 mL/min) installed on Agilent 1120 Compact LC HPLC, against the standard samples of the products. Furthermore, the identification of the reaction products was done on a gas chromatography mass spectrometer (GC–MS, Shimadzu, GC–MS-QP 2010 plus and GC-2010 with RTX-5 Sil-MS capillary column (30 mm × 0.25 mm × 0.25 µm) using Helium as carrier gas with the flow rate of 1 mL/min. The injector temperature was maintained at 240 °C and the oven was programmed from 50 °C to 260 °C with the ramp of 10 °C per minute. The quantification of acetone was done with the help of standard acetone (99.9%) as external standard keeping the injection volume constant at 2 μL. The hydrogen (H₂) produced was quantified by manually injecting 1 mL of gas evolved during the reaction to gas chromatography (GC, Nucon Ltd, India) with thermal conductivity detector (TCD) and molecular sieve column (5XA, 1.5 m). The temperatures of the injector, column oven and detector were maintained at 35 °C. The amount of H₂ produced was measured by comparing it with standard 180 PPM H₂ (Sigma Gasses Ltd, India).

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Scheme 1 Photoreduction of various nitroaromatics to their corresponding aromatic amines byF-TiO₂, R-TiO₂ and P25 under direct sunlight and UV light exposure.

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3.1 Photocatalytic studies

The photocatalytic reduction of every nitroaromatic compounds NB, m-DNB and DNBP (25 µmol in 50% isopropanol solution) to their respective amino derivatives (AN, m-NA and m-PDA, and BC) was carried out using 20 mg TiO₂ dosage and studied both under UV and sunlight irradiation as per Scheme 1. The HPLC chromatographs of DNBP reduced to BC by different TiO₂ catalysts under both UV and Sunlight irradiation for 7 h are comparatively displayed in Fig. ESI-4. The peak intensity of DNBP (t_R = 5.6 min) is reduced while as intensity of peak at t_R = 5.2 min increased corresponding to the BC formation. It was disclosed that maximum amount DNBP reduction to BC formation took place by F-TiO₂ under sunlight and P25 under UV light irradiation for 7 h and comparatively R-TiO₂ showed no or least catalytic activity under similar conditions. Similarly the GC analysis (using authentic samples, Fig. 6) further confirm the highest amount of BC formation (as evident from the difference in GC peak intensity at t_R = 12.9 min) from DNBP reduction (GC peak t_R = 15 min) by F-TiO₂ under 7 h sunlight relative to lower activity of P25 under UV light exposure and the characteristic mass spectra of photo produced BC is also complying with the authentic commercial sample. Similarly, the reduction of NB and m-DNB to AN, m-NA and m-PDA respectively, under both UV and sunlight irradiation was also quantitatively calculated by HPLC and GC–MS analysis in comparison to respective authentic samples (Figs. ESI-5–ESI-7).

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Fig. 6 GC chromatogram and mass spectra of authentic DNBP and BC and their respective reduction products under 7 h UV and sunlight illumination.

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During photoreduction the concentration of all nitroaromatics decreased linearly with increasing amount of amino derivatives as a function of light irradiation time, moreover the reaction kinetics study showed that the reaction obeys 1st order (C = C₀e^−kt, k = rate constants) kinetics as shown in the time course graphs (Fig. 7), and it was also observed that the maximum yield of amino compounds is achieved after 7 h light irradiation. However, m-NA formation is gradually increased up to 3–4 h irradiation and thereafter starts decreasing due to reduction of second —NO₂ group to —NH₂ group resulting into maximum amount of m-PDA formation. Similar trend is also seen in case of NB reduction to AN as shown in Fig. ESI-8. It was also revealed that the photoreduction rate (Fig. 7b) of m-DNB and DNBP by F-TiO₂ is drastically enhanced (4–5 h) relative to P25 and R-TiO₂under (Fig. 7c-d) direct sunlight irradiation as compared to their photoactivity (Fig. 7a) under UV radiation.

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Fig. 7 Time course graphs for photoreduction of m-DNB to m-NA and m-PDA, and DNBP to BC under (a and b) UV light and (c and d) direct sunlight exposure for 7 h.

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The higher rate of reduction for DNBP by F-TiO₂ (k = 4.7 × 10⁻¹ min⁻¹) was observed as compared to P25 (k = 3.9 × 10⁻² min⁻¹) under direct sunlight and UV light exposure (F-TiO₂ (k = 2.2 × 10⁻¹ min⁻¹), P25 (k = 1.2 × 10⁻¹ min⁻¹)) suggested the overall excellent photoreduction efficiency of F-TiO₂ (Fig. 7c-d) under sunlight irradiation. The comparative and quantitative photoactivity (yield and % conversion) of the different photocatalysts for the photoreduction of nitro organics to respective aromatic amines under 7 h continuous UV and sunlight exposure is presented in Fig. 8 and ESI-Table-1. The results disclose that compared to R-TiO₂ and F-TiO₂ the percentage yield gradually increases in case of F-TiO₂ for production of AN (20–72%), m-NA (15–42%), m-PDA (30–100%) and BC (10–100%) that was better (Fig. 8a) in comparison to P25 (m-PDA, 100% and BC, 65%) under UV light irradiation. Despite having rutile phase F-TiO₂ microstructure imparted the best photoreduction activity (10.5 µmol/42% AN, 22 µmol/88%m-PDA and 23.5 µmol/94% BC) under sunlight in contrast to P25 (22–60% yield) and R-TiO₂ giving 10–30% yield under UV light. As seen in Fig. 8b. one key feature of this anisotropic F-TiO₂ rutile phase reveals that it could simultaneously be capable of exhibiting the highest photoactivity both upon UV and sunlight irradiation which is hardly found in the reported photocatalysts.

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Fig. 8 Histogram showing the yield (%) of aromatic amines formed under 7 h (a) UV light and (b) direct sunlight exposure.

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Along with the surface structural properties of rutile, the selection of hole scavenger also plays an important role in the photoreduction processes. Several solvents like ethanol, methanol and isopropanol have been used as hole scavenger for various nitroaromatic reduction reactions. It was observed that the iso-propanol is a best medium for the photocatalytic reactions this is also supported by earlier findings (Kaur and Pal, 2014; Zhu et al., 2010). Due to smaller bandgap (2.8 eV = 420 nm) and upon excitation of F-TiO₂, electron-hole (e⁻-h⁺) pairs are photogenerated in the conduction and valance band, respectively, that eventually diffuses to the surface for redox reactions of the adsorbed substrates as shown in Scheme 2. It is reported (Tamaki et al., 2006) that the photoexcited energetic holes in the valence band causes the oxidation of an hole scavenger alcohol to corresponding carbonyl compound and hydrogen as bi products, and the reduction of a nitrocompound occurs by the excited electron and the in process use of hydrogen produced during oxidation of the solvent. As the average lifetime (120 µs) of the photo-formed charge species over F-TiO₂ is significantly much higher than R-TiO₂ (34 µs) and P25 (45 µs), the photocatalytic redox reaction occurs in faster rate for reduction of all nitroaromatics (e.g., DNBP to BC) and oxidation of isopropanol to acetone and H₂ molecules. Moreover, larger specific surface area and more surface exposed reduction sites (1 1 0 and 1 0 1 crystal planes) present over F-TiO₂ are also responsible for better adsorption and photoreductive activity than R-TiO₂ and P25 catalysts, previously it has been reported that (1 1 0) and (1 0 1) crystal planes of rutile TiO₂ possess large number of reductive sites, Ohno et al. (2002) have reported the selective reduction of metal nanoparticles on (1 1 0) plane, similarly Farneth et al. (1999) have also reported that the selective reduction of Ag¹⁺ takes place on 1 1 0 plane followed its deposition. Theoretically, one molecule of DNBP reduction requires 8e⁻ for a molecule of BC formation and simultaneously, 8h⁺ in the valance band initiates the oxidation of IPA to 4 molecules of acetone and 4 molecules of H₂ (requires 2h⁺ for each IPA oxidation) molecules. Similarly, 12 e^- are needed for hydrogenation of two —NO₂ group (6e⁻ per nitro group) of m-DNB to m-PDA and simultaneously 12h⁺ will oxidize IPA into 6 acetone and 6 H₂ molecules. In a typical a stoichiometric calculation (BC/m-PDA:H₂ = 1:2 ratio) it found that for 23.5 µmol BC formation required 47 µmol H₂ obtained during IPA oxidation to 47 µmol acetone, and for 22 µmol m-PDA formation needs 44 µmol H₂ during IPA oxidation to 44 µmol acetone after DNBP and m-DNB reduction. Quantitative analysis by GC revealed that amount of acetone (chromatogram Fig. ESI-10) and H₂ (chromatogram Figs. ESI-11 and 12) are gradually increasing with increased amount of BC and m-PDA production with instantaneous reduction of substrates (DNBP and m-DNB) concentration with sunlight irradiation time as in Fig. 9. After 4–6 h irradiation complete reduction of both the substrates occurs and the highest amount of amino compounds (22–24 µmol) and acetone (40–45 µmol) formation (Fig. 9a) is observed after 7 h photoreaction. It could be seen in Fig. 9b that H₂ production increases to a maximum of 156–180 µmol after 5 h irradiation followed by a decrease to a minimum value (36–60 µmol) that remained after complete reduction of nitro to amino derivatives (highest yield) after 7 h irradiation. This decrease (156–180 to 36–60 µmol) in H₂ concentration with irradiation time (5–7 h) strongly evidencing its utilization for hydrogenation of —NO₂ moiety of nitro-organics (Zhu et al., 2010) to-NH₂ groups during reduction process. It also evident that the amount of H₂ formed thus measured is relatively higher than required stoichiometric amount 40–47 µmol needed for nitroaromatic reduction which could be attributed to the possibility of over oxidation of IPA and water oxidation. Moreover, the amount (40–45 µmol) of acetone thus experimentally determined after 7 h reaction is in good accordance with stoichiometric amount (44–47 µmol) needed for BC and m-PDA production as discussed above.

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Scheme 2 Schematic illustration of reaction set-up and photoreduction process of DNBP under direct sunlight exposure.

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Fig. 9 Time course graph of (a) acetone formation and (b) hydrogen production during DNBP and m-DNB photoreduction up to 7 h sunlight exposure.

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Numerous reports (Chen et al., 2011; Kaur and Pal, 2014) have been published for the selective reduction of various nitroaromatic compounds using conventional rutile and P25-TiO₂ catalysts. However, selective reduction of m-dinitrobenzene to m-phenylenediamine, azoxybenzene and 2,2-dinitrobiphenyl to benzo[c]cinnoline using as prepared anisotropic rutile TiO₂ sensitive to both UV and direct sunlight irradiation is rarely found in the literature. Hence, this finding of F-TiO₂ catalyst consisting of rutile phase prepared at low temperature demonstrating the drastic improvement in the photoreductive activity than P25 and rutile TiO₂ under sun light illumination is quite significant for nitroaromatics photoreduction. This photocatalyst appreciably enhanced the hydrogenation activity of DNBP to which undergoes intra-molecular reductive cyclization for BC formation because of the close spatial proximity of the interacting NO₂ groups in two different benzene rings. Generally R-TiO₂ prepared by high temperature sintering of P25 titania leads to bigger aggregated particles size, hence loss the nanoscale properties and catalytic activity [40]. As F-TiO₂ possess exposed surface facets, appropriate band energetics (absorb visible light; λ = 420 nm) and beneficial surface structural morphology in the nanoscale dimensions could be ascribed for superior photoactivity under sun light exposure. The reason for higher selectivity of this material is attributed to higher surface area that provides large number of active sites and easy diffusion of ionic species at the surface of F-TiO₂ as compared to R-TiO₂ and P25 which directly affected the photocatalytic efficiency. Surface facets (1 1 0) and (1 0 1) with dominant reductive species as reported previously provide diffusion path for surface —OH groups and helped in attaching of reactant species (Huo et al., 2012) with the surface for catalytic reaction by Aloe-vera shaped rutile phase under direct sunlight exposure. Moreover, the electronic energy levels of the (1 1 0) and (1 0 1) facet helps in the quick separation of photoexcited electrons and holes for the rutile [30, 31] than anatase particles as supported by PL and time resolved spectroscopy. This variation in the surface energy of the conduction and valence band of different crystal faces and their atomic arrangements thus affect the TiO₂ photoreactivity; hence facilitating the nitro to amine conversion by the surface trapped electrons, enabling superior photoreduction of nitroaromatics. In F-TiO₂, reactive sites are the Ti³⁺ species which are positioned at the oxygen vacancies on the surface behaving as the adsorption sites for —NO₂ group reduction and trapping sites for the electrons in conduction band (Abazović et al., 2009; Shiraishi et al., 2013). Furthermore, these tuned morphological rutile TiO₂ nanostructures allow the multidirectional flow of photo generated charge carriers, leading to improved sunlight absorption. Typically, reduction products of nitroaromatic compounds have been used for the synthesis of polymers, herbicides, food additives, pharmaceutical and agrochemicals etc., which are commonly synthesized at high pressure and temperature using toxic solvents and strong reducing agents [42, 43] e.g., NaBH₄ and metal catalysts. Hence the development of visible light sensitive TiO₂ catalysts would be beneficial for the hydrogenation of nitro aromatic compounds as they could utilize natural sunlight, inexpensive, nontoxic, and chemically stable. Therefore, such redox combined photocatalytic reduction of nitroaromatics by different anisotropic shape of nanoscale rutile TiO₂under direct solar irradiation could be extended to produce many industrially important amino derivatives in more eco-friendly approach.