Selective Oxofunctionalization of Cyclohexane and Benzyl Alcohol over BiOI/TiO₂ Heterojunction

Abstract

Heterogeneous photocatalysis under visible light irradiation allows performing of selective oxofunctionalization of hydrocarbons at ambient temperature and pressure, using molecular oxygen as a sacrificial reagent and potential use of sunlight as a sustainable and low-cost energy source. In the present work, a photocatalytic material based on heterojunction of titanium dioxide and bismuth oxyiodide was used as photocatalyst on selective oxofunctionalization of cyclohexane and benzyl alcohol. The selective oxidation reactions were performed in a homemade photoreactor equipped with a metal halide lamp and injected air as a source of molecular oxygen. The identified oxidized products obtained from oxofunctionalization of cyclohexane were cyclohexanol and cyclohexanone. On the other hand, the product obtained from oxofunctionalization of benzyl alcohol was benzaldehyde. The yield obtained with BiOI/TiO₂ photocatalysts was higher than that obtained with pure bismuth oxyiodide. The higher performance of this material with respect to pure BiOI was attributed to its higher specific area.

1. Introduction

Heterogeneous photocatalysis is an advanced oxidation process (AOP) that allows the use of light as the driving energy to perform chemical transformations. The photocatalytic activity of semiconductor material is based on their ability to absorb photons with energy equal to or greater than the forbidden energy band of the semiconductor material. The energy of photon irradiated over semiconductor material promotes a photoelectron from the valence band to the conduction band, generating a photohole–photoelectron pair [1]. The generated photohole and photoelectron have oxidizing and reducing properties, respectively. These species in the presence of water and oxygen can generate radical hydroxyl, hydrogen peroxide and other reactive oxygen species.

Many semiconductor materials have been used as photocatalysts. Among the semiconductor materials with photocatalytic activity, titanium dioxide is one of the most widely used due to its stability, photoreactivity, low cost and non-toxicity [1]. However, due to the broad forbidden energy band of titanium dioxide (3.2 eV), this material can only be activated by electromagnetic radiation (λ < 387 nm), which limits their use in photocatalytic processes under visible light. Since less than 5% of the solar radiation incident on the earth’s surface corresponds to the ultraviolet region, many efforts have been focused on the development of new semiconductor materials for photocatalytic applications for an efficient application of solar radiation as an energy source in photocatalytic processes [2].

Some metal ions such as Fe, Cr, V, Mo and Co have been used to modulate the electronic structure and improve the photocatalytic activity of TiO₂ [3,4].

Bismuth oxyhalide compounds are representative novel photocatalysts. These materials exhibit a tetragonal matlokite structure where the central bismuth atom is surrounded by four oxygen atoms and four halogen atoms. Bismuth oxyhalides present a lamellar structure with [Bi₂O₂]²⁺ sheets alternately interspersed with negatively charged halide sheets, generating an internal static electric field perpendicular to each sheet. These electrical properties confer to these materials an effective separation of the generated photoelectron–photohole pairs [5]. The interest in these materials is due to their particular layered crystal structure, suitable bandgap, and high photocatalytic activity under both ultraviolet and visible light irradiation. [6] Bismuth oxyiodide (BiOI) exhibits the narrowest bandgap within the bismuth oxyhalides with an energy bandgap of 1.85 eV in the visible region electromagnetic spectra (λ < 670 nm) [7].

Traditionally, photocatalysis has been employed in wastewater treatment [8], destruction of microorganisms [9], and degradation of toxins [10], dyes [11], surfactants [12], fatty acids [13], antibiotics [14,15] and organic pollutants [16,17]. Due to their high reactivity, photocatalytic systems based on semiconductors are non-specific and have been rarely applied in the selective conversion of organic compounds [18]. Nevertheless, the rational use of appropriate photoactive semiconductor materials and fine control of the chemical reaction may lead to highly selective organic transformations by heterogeneous photocatalysis [19].

Selective aerobic oxidation of hydrocarbons to generate oxygenated products such as alcohols, ketones and carboxylic acids is a commercially important process [20]. Nevertheless, the processes performed to obtain these oxygenated products are frequently severe, require corrosive mixtures of chemical reagents, and are non-selective [21]. In this context, semiconductor photocatalysts have attracted much attention because they make possible this type of transformation under mild reaction conditions and the potential use of sunlight as a sustainable and low-cost energy source [22].

In previous works, high selectivities were obtained in photocatalytic oxofunctionalization reactions of cyclohexane under visible light irradiation using bismuth oxyiodide-based photocatalysts. However, despite the high selectivity obtained, low yields were achieved in the photocatalytic oxofunctionalization reactions of cyclohexane. Since the photocatalytic oxofunctionalization reactions occur on the surface of the semiconducting material, the low surface area of bismuth oxyiodide may influence the low yields achieved using this material as a photocatalyst.

This work focuses on the synthesis and evaluation of photocatalytic activity of a BiOI/TiO₂ hybrid material with the aim of obtaining a semiconductor material with photocatalytic performance similar to pure bismuth oxyiodide, but with higher specific area and its testing in the photocatalytic oxofunctionalization of the model substrates cyclohexane and benzyl alcohol.

2. Results

2.1. Photocatalyst Characterization

The structural and electronic properties of the synthesized semiconductor photocatalysts are presented in

Table 1. Structural and electronic properties of the synthesized semiconductor photocatalysts.

Photocatalyst	Crystallite Size a [nm]	BET Surface Area [m2 g−1]	Pore Size [nm]	Pore Volume [cm3 g−1]	Bandgap b [eV]
TiO2	15.74	19.07	7.66	0.0365	3.09
BiOI	22.58	11.97	6.73	0.0201	1.89
20% BiOI/TiO2	13.84	37.62	6.95	0.0654	1.91

^a Calculated from XRD measurements using the Debye–Scherrer equation. ^b Calculated from diffuse reflectance UV-Vis spectra using Kubelka–Munk function.

. The Xray diffraction data of the titanium dioxide photocatalyst sample (Figure 1A) agree with Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 4-477, corresponding to the anatase phase of titanium dioxide. The diffraction peaks of the bismuth oxyiodide (BiOI) photocatalyst sample (Figure 1B) can be indexed to the tetragonal phases of BiOI (JCPDS Card No. 73-2062). On the other hand, Xray diffraction data of the BiOI/TiO₂ heterostructure sample show peaks corresponding to both the titanium dioxide anatase phase (JCPDS Card No. 4-477) and the tetragonal phase of BiOI (JCPDS Card No. 73-2062).

It has been reported that the exposure ratio of (1 1 0)/(0 0 1) planes is important in the photocatalytic activity of bismuth oxyiodide because of the ability of (1 1 0) planes to generate hydroxyl radicals under visible light irradiation [23]. It can be observed from Figure 1B that the (1 1 0)/(0 0 1) ratio is 5.3. This value is higher than the optimal ratio found in previous work for hydroxyl radical generation [24].

Figure 2 shows the DRS-UV–Vis spectra of the synthesized photocatalysts. Figure 2A shows that titanium dioxide samples do not have the ability to efficiently absorb photons from the visible region of the electromagnetic spectrum. Unlike titanium dioxide, BiOI samples (Figure 2B) exhibit an efficient ability to absorb photons from the visible region (Figure 2B). In Figure 2C, it can be observed that the BiOI/TiO₂ heterostructure has the ability to absorb photons of higher wavelengths in comparison to pure titanium dioxide. Figure 2D shows the transformed Kubelka–Munk function versus the light energy absorbed by the synthesized materials. From the latter plots, the forbidden energy band of the photocatalysts was determined. The forbidden energy bands of the photocatalysts were 3.09, 1.89 and 1.91 eV for the TiO₂, BiOI and BiOI/TiO₂ samples, respectively.

The adsorption desorption isothermal curves of the TiO₂ samples (Figure 3A) were classical type IV and contained hysteresis, suggesting a mesoporous presence [25]. The BiOI and BiOI/TiO₂ samples (Figure 3B,C, respectively) presented type II adsorption and desorption isotherms, suggesting a non–porous presence or possibly macroporous materials [26].

The TiO₂ sample presented clusters of TiO₂ nanoparticles (Figure 4A). By contrast, the sample of BiOI (Figure 4B) presented hierarchically assembled microspheres of approximately 2 µm consisting of nanosheets. The BiOI/TiO₂ sample (Figure 4C) presented a close physical contact between the TiO₂ nanoparticle clusters and the BiOI nanosheets. The above is corroborated by the mapping of Bi and Ti elements in Figure 4D, which shows that both BiOI and TiO₂ photocatalyst are uniformly distributed on the surface of the hybrid material.

2.2. Cyclohexane and Benzyl Alcohol Oxofunctionalization

Photocatalytic oxofunctionalization of cyclohexane and benzyl alcohol reactions was performed in a homemade photoreactor equipped with a 400 W metal halide lamp (Osram, Powerstar HQI-E 400 W/D Pro Daylight, Munich, Germany) [27]. The spectral distribution of the metal halide lamp, reported in the product datasheet, is shown in Figure 5, and the band gap energies of the photocatalysts assayed are represented by vertical lines over the spectrum. From the emission spectrum of the lamp used, it is concluded that the emission source used is suitable for the use of the synthesized semiconductor materials as photocatalysts, due to the fact that the spectral range of emission of the lamp allows the promotion of an electron from the valence band to the conduction band in the synthesized materials.

2.2.1. Photocatalytic Oxofunctionalization of Cyclohexane

A total of 25 µL of deionized water (1.375 mmol) was placed in a 25 mL volumetric flask and made up to capacity with cyclohexane (231.2 mmol). The mixture was taken into an ultrasonic bath until the formation of an inverse emulsion. The inverse emulsion of water in cyclohexane was transferred to a 50 mL two-necked round bottomed flask equipped with a reflux condenser in the presence of 25 mg of the respective photocatalyst (1 mg/mL). In each case, during the reaction time, the reaction system was magnetically stirred and saturated by air bubbling (1 atm) as a source of molecular oxygen. According to the solubility of O₂ in cyclohexane [28], its concentration was about 2.6 mmol L⁻¹. To avoid an increase in pressure inside the reaction system, the reflux condensers were capped with a rubber stopper pierced through by a hollow needle. The temperatures inside the photocatalytic reactor reached 37 ± 2 °C. As a control experiment, a cyclohexane oxofunctionalization test was carried out in the absence of photocatalyst under the same conditions as those performed in the presence of photocatalyst.

2.2.2. Photocatalytic Oxofunctionalization of Benzyl Alcohol

A total of 25 mL of benzyl alcohol solution 5 mmol L⁻¹ was transferred to a 50-mL two-necked round-bottomed flask equipped with a reflux condenser in the presence of 25 mg of the respective photocatalyst (1 mg/mL). The oxofunctionalization reaction was carried out under the same conditions used in the oxofunctionalization of cyclohexane. As a control experiment, a benzyl alcohol oxofunctionalization test was carried out in the absence of photocatalyst under the same conditions as those performed in the presence of photocatalyst.

2.3. Identification and Quantification of the Oxofunctionalized Products

2.3.1. Oxofunctionalized Products from Cyclohexane

To identify the compounds obtained from the photocatalytic oxidation of cyclohexane, a 1.0 µL aliquot was removed from the reaction mixture and injected into a GC–MS system. The analytes were separated in a crosslinked 5% diphenyl–95% dimethylsiloxane column (Hewlett Packard Corporation, Palo Alto, CA, USA). After a reaction time of 180 min, the products cyclohexanol and cyclohexanone were identified in all photocatalytic systems of oxofunctionalization of cyclohexane (Figure 6). The cyclohexanol and cyclohexanone concentrations of the reaction mixture aliquot were calculated by interpolating the area of the peaks of cyclohexanol and cyclohexanone in a standard curve. In order to evaluate the selectivity of photocatalytic systems cyclohexanol/cyclohexanone, the (CHol/CHone) ratios were calculated. In a control experiment, the generation of cyclohexane oxidation products was not evidenced.

For further comparison, typical results for photocatalytic oxofunctionalization of cyclohexane reported in the literature are summarized in

Table 2. Selective photocatalytic oxidation of cyclohexane over synthesized photocatalysts and others reported photocatalysts.

Refs.	Catalyst	BET Specific Area [m2 g−1]	Illumination Source	Experimental Conditions	Time [h]	CHol Yield [µmol]	CHone Yield [µmol]	CHol/CHone Ratio
	TiO2 25 mg	19.07	400 W metal halides-lamp	cyclohexane (24.975 mL), water (0.025 mL), air (1 bar) a	3	1.01	11.18	0.09
	BiOI 25 mg	11.97	400 W metal halides-lamp	cyclohexane (24.975 mL), water (0.025 mL), air (1 bar) a	3	2.61	0.64	4.08
	20% BiOI/TiO2 25 mg	37.62	400 W metal halides-lamp	cyclohexane (24.975 mL), water (0.025 mL), air (1 bar) a	3	5.46	1.54	3.92
[29]	Nanosized TiO2 30 mg	410	250 W high pressure mercury-lamp	cyclohexane (10 mL), acetonitrile (10mL),	3	75.5	13.3	5.68
[30]	TiO2 P25 30 mg	-	solar simulator (1000 W m−2)	cyclohexane (2 mL), O2 saturated acetonitrile (18 mL)	6	3.2	34.0	0.09
[30]	FeO0.13-TLO		solar simulator (1000 W m−2)	cyclohexane (2 mL), O2 saturated acetonitrile (18 mL)	6	21.2	18.8	1.13
[31]	NH2-M125/P25-4 50 mg	571.2	300 W xenon-lamp (λ ≥ 420 nm)	cyclohexane (10 mL), acetonitrile (10mL), O2 (0.1 MPa), temperature (25 °C)	5	246.58	431.52	0.57
[32]	FeIII(5-chloro-8-hydroxyquinoline)3 0.01 mmol		35 W tungsten-bromine lamp	cyclohexane (1.0 mmol), acetonitrile (5.0 mL), H2O2 (4 mmol), temperature (18–20 °C)	5	55.20	60.80	0.91

^a Reaction conditions: 1 bar air; 24.975 mL cyclohexane (0.2312 mol); 25 µL of nanopure water (55.5 mmol L⁻¹); 25 mg photocatalyst; magnetic stirring.

. The results show that the synthesized 20% BiOI/TiO₂ hybrid material shows higher photocatalytic activity under visible light in the selective oxofunctionalization of cyclohexane to cyclohexanol with respect to the synthesized BiOI and TiO₂ photocatalysts. Although the obtained yields are low with respect to others reported in literature (Table 2), it should be considered that in the present work the photocatalytic oxofunctionalization reactions were carried out in the absence of organic solvents or metallic salts, conditions that could influence the reaction yields.

2.3.2. Oxofunctionalized Products from Benzyl Alcohol

After a reaction time of 180 min, the photocatalysts were separated from the suspension by filtration through a 0.22 µm polyethersulfone membrane filter. An aliquot of 10 µL of the solution was analysed by high performance liquid chromatography (HPLC). The analytes were separated in a reversed phase column (C18 column). The product benzaldehyde was identified in all assayed systems (Figure 7). The benzaldehyde concentrations of the reaction mixture aliquot were calculated by interpolating the area of the peaks of benzaldehyde in a standard curve. In a control experiment, the generation of benzyl alcohol oxidation products was not evidenced.

For further comparison, typical results reported in the literature are summarized in

Table 3. Selective photocatalytic oxidation of benzyl alcohol over synthesized photocatalysts and others reported photocatalysts.

Refs.	Catalyst	BET Specific Area [m2 g−1]	Illumination Source	Experimental Conditions	Time [h]	[Benzaldehyde]/[Benzyl Alcohol]0
	TiO2 25 mg	19.07	400 W metal halides-lamp	25 mL aqueous solution of benzyl alcohol 5 mmol L−1 (0.125 mmol), air (1 bar) a	3	0.103
	BiOI 25 mg	11.97	400 W metal halides-lamp	25 mL aqueous solution of benzyl alcohol 5 mmol L−1 (0.125 mmol), air (1 bar) a	3	0.002
	20% BiOI/TiO2 25 mg	37.62	400 W metal halides-lamp	25 mL aqueous solution of benzyl alcohol 5 mmol L−1 (0.125 mmol), air (1 bar) a	3	0.005
[33]	BiOBr 50 mg	-	500 W Xe lamp (λ ≥ 420 nm)	10 mL benzyl alcohol 0.5 mmol L−1 in acetonitrile (0.005 mmol)	8	0.214
[33]	Bi3O4Br 50 mg	-	500 W Xe lamp (λ ≥ 420 nm)	10 mL benzyl alcohol 0.5 mmol L−1 in acetonitrile (0.005 mmol)	8	0.356
[34]	30-W18O49/ HU-CNS 20 mg	-	Xe lamp (500 mW cm−2)	100 mL aqueous solution of benzyl alcohol 1 mmol L−1 (0.100 mmol)	1	0.397
[35]	CUO-0.1 32 mg	-	300 W Xe lamp (λ ≥ 400 nm)	benzyl alcohol (0.200 mmol), air flow rate (2 mL·min−1), acetonitrile (3 mL)	3	0.463
[32]	C3N4BF0.5 0.01 mmol	-	35 W tungsten-bromine lamp	benzyl alcohol (1 mmol), H2O2 (4 mmol), acetonitrile (5 mL)	12	0.085

^a Reaction conditions: 1 bar air; 25 mL benzyl alcohol 5 mmol L⁻¹ (540.7 ppm); 25 mg photocatalyst; magnetic stirring. [Benzyl Alcohol]₀ represents the initial benzyl alcohol content (time = 0).

. The results suggest that titanium dioxide is the photocatalyst that exhibits the best photocatalytic activity under visible light under the conditions tested for the conversion of benzyl alcohol into benzaldehyde among the synthesized materials. This could be related to the fact that titanium dioxide shows higher polarity than the BiOI. Therefore, the adsorption of benzyl alcohol on the surface of TiO₂ would be more efficient than on the surface of BiOI.

To identify the radicals generated during the oxofunctionalization of benzyl alcohol, spin trap EPR experiments were performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO), α-(4-pyridyl 1-oxide)-N-tert-butylnitrone (POBN) and N-tertbutyl-α-phenylnitrone (PBN). At 10 min after initiation of the photocatalytic oxofunctionalization reaction of benzyl alcohol conducted by titanium dioxide, the spectra shown in Figure 8A–C were recorded. The Figure 8A shows DMPO- OH adduct EPR signal (aN = 14.9 G; aH = 14.9 G). The Figure 8B shows POBN- OH adduct EPR signal (aN = 15.0 G; aH = 1.7 G). These results confirm the generation of hydroxyl radicals during the photocatalytic oxidation process of benzyl alcohol conducted by titanium dioxide. It was not possible to identify the generation of radical species during the photocatalytic oxidation processes of benzyl alcohol conducted by bismuth oxyiodide and the BiOI/TiO₂ heterojunction. This may be due to the fact that the generation of these species is under the detection limits of the instrumental technique. For this reason, the generation of radical species during the oxidative processes conducted by bismuth oxyiodide and the BiOI/TiO₂ heterojunction cannot be completely discarded.

To identify the radicals generated during the oxofunctionalization of cyclohexane, spin trap EPR experiments were performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-tertbutyl-α-phenylnitrone (PBN). At 10 min after initiation of the photocatalytic oxofunctionalization reaction of cyclohexane conducted by titanium dioxide, the spectra shown in Figure 9A,B were recorded. The Figure 9A corresponds to a triplet of doublet signals (aN = 13.0 G; aH = 6.0 G), whereas Figure 9B corresponds to a broad triplet signal (aN = 13.6 G). A comparison of the hyperfine coupling constants with literature values for oxygen-centered radicals permit us to conclude that the EPR spectra correspond to the adduct formed between the respective spin trap and the C₆H₁₁O• radical [36]. In Figure 9A, no hydroxyl radical generation is observed in the aliquots obtained from the photocatalytic oxofunctionalization reaction of cyclohexane conducted by titanium dioxide. This may be due to the polar nature of the DMPO-OH• adduct and the non-polar nature of the aliquot. Figure 9C shows an EPR signal similar to that observed in Figure 9A (aN = 13.0 G; aH = 6.0 G), showing that in the BiOI-driven cyclohexane oxofunctionalization reaction C₆H₁₁O• radicals are generated corresponding to a broad triplet signal (aN = 13.6 G). Figure 9D shows two EPR signals. The signals were identified as two partially overlapping triplets. The first signal was assigned to C₆H₁₁O• (aN = 13.6 G) and the second assigned to PBNox (PhCON(O•)Bu^t (aN = 8.1 G) and its formation was evidence of the •OH radical [37]. Figure 9E shows a weak signal associated with C₆H₁₁O• (aN = 13.6 G). Figure 9F shows an EPR signal (aN = 8.1 G) assigned to PBNox, evidencing that in the oxofunctionalization of cyclohexane conducted by BiOI/TiO₂ heterojunction under visible light irradiation as in the BiOI-driven system, there is generation of hydroxyl radicals. Interestingly, in the oxidation of cyclohexane hydroxyl, radical generation is only evident in the photocatalytic systems driven by the photocatalysts containing bismuth oxyiodide. Moreover, the systems in which hydroxyl radical generation is evidenced are more selective for cyclohexanone. These results are in agreement with what was observed by Henriquez et al. 2017 [27].

3. Discussion

The titanium dioxide photocatalyst allows an oxidation with a high selectivity to cyclohexanone while the bismuth oxyiodide-based photocatalyst allows a photocatalytic oxidation of cyclohexane with higher selectivity to cyclohexanol than to cyclo-hexanone. These findings are consistent with what has been observed in previous work [27]. Comparing the yields of the products in the benzyl alcohol and cyclohexane photocatalytic oxidation experiments, it was observed that the titanium dioxide photo-catalyst exhibits higher photocatalytic activity than the bismuth oxyiodide photocatalyst. This behavior may be due to the higher production of hydroxyl radicals in titanium dioxide-driven photocatalytic systems. This is suggested since only in titanium dioxide-driven systems, the generation of hydroxyl radicals is evidenced by in situ electron paramagnetic resonance experiments (Figure 8A and Figure 9B). The generation of a hybrid BiOI/TiO₂ material allows obtaining similar selectivities to those obtained with pure bismuth oxyiodide in the oxofunctionalization reactions of cyclohexane and benzyl alcohol under visible light irradiation. Nevertheless, BiOI/TiO₂ heterostructures allow obtaining higher yields than those obtained using pure BiOI as a photocatalyst (Table 2 and Table 3). This is due to the dispersion of bismuth oxyiodide through the formation of BiOI/TiO₂ heterostructures allowing us to obtain a material with higher specific area than pure bismuth oxyiodide (Table 1).

4. Materials and Methods

4.1. Synthesis of Photocatalysts

4.1.1. Titanium Dioxide Photocatalysts

Titanium dioxide samples were prepared according to the synthesis method described by Qamar et al. 2014 [38]. A total of 0.5 g of Pluronic 123 was dissolved in 20 mL absolute ethanol and solution acidity was adjusted to pH 2 by adding nitric acid. The resultant solution was stirred for 6 h in Erlenmeyer flasks under continuous magnetically agitation. A total of 4.915 mmol of tetraisopropoxide of titanium IV (TTIP) was added drop by drop into the solution and was kept under agitation for 2 additional h, followed by the addition of 0.5 mL of deionized water to perform a controlled hydrolysis of TTIP. The solution was kept under agitation until the formation of colloidal particles. Solvents were evaporated over a hot plate and obtained dried gel was pulverized and dried at 80 °C overnight followed by calcination at 450 °C.

4.1.2. BiOI/TiO₂ Photocatalysts

Bismuth oxyiodide sample was prepared according to a modified synthetic procedure described by Li et al. 2018 [39]. A total of 4.915 mmol as-prepared TiO₂ and 0.983 mmol of Bi(NO₃)₃ · 5H₂O were dissolved in 20 mL of absolute ethanol (Bi/Ti molar ratio 0.2). The resultant solution was magnetically stirred and sonicated to form solution A. On the other hand, 0.983 mmol of KI was dissolved in 20 mL of deionized water to form solution B. Solution B was slowly added to solution A dropwise to form a colored precipitate. The pH of the suspension was adjusted to 10, and the resulting solution was stirred for 1 h at room temperature. Then, the suspension was filtered, washed with deionized water, and fully dried at 70 °C. Pure BiOI sample was sample was prepared by the same synthetic procedure in the absence of TiO₂.

4.2. Photocatalyst Characterization

The Xray diffraction patterns of the synthesized photocatalysts was used to investigate the phase structure of the materials. On the other hand, the crystallite sizes of the photocatalysts were estimated using the Debye–Scherrer formula Equation (1), where D is the average crystallite size, K is a dimensionless shape factor (in this case, with a value of 0.94), λ is the X-ray radiation wavelength (Cu Kα = 0.154056 nm), β is the band broadening at half the maximum intensity (FWHM), and θ is the diffraction angle.

$$D=\frac{K \lambda }{\beta \cos \theta }$$

(1)

The bandgap of the semiconductor photocatalyst was determined from a Tauc plot obtained from the UV–vis diffuse reflectance spectra Equation (2) [40].

$$\alpha \left(h\nu \right)=A{\left(h\nu -E_g\right)}^{n/2}$$

(2)

h is Planck’s constant, ν is the vibration frequency, α is the absorption coefficient, E_g is the bandgap, and A is a proportional constant. The value of the exponent n denotes the nature of the simple transition. For an indirect transition, n = 4 [41]. The α term in the Tauc equation was substituted with the Kubelka–Munk function, F(R_∞). The crystal morphology of the products was observed by scanning electron microscopy (NanoSEM 200, FEI-Nova, Japan). Nitrogen adsorption-desorption isotherms at −196 °C will be obtained using a surface area and porosity analyzer (TriStar II 3020, Micromeritics Instrument Corporation, Norcross, GA, USA).

4.3. Hydrocarbons Oxofunctionalization

4.3.1. Cyclohexane Oxofunctionalization

The oxofunctionalization of 24.975 mL of cyclohexane was performed in a 50 mL 2-neck round-bottom flask fitted with a reflux condenser in the presence of 25 mg of catalyst (1 mg/mL) and 25 μL of deionized water. For each reaction, the system was saturated with of air (1 atm) for the duration of the reaction time (3 h). Photocatalytic oxidation reactions were carried out under visible radiation generated by a 400 W metal halide lamp (Osram Powerstar HQI–E 400 W/D Pro Daylight). The temperature inside the photocatalytic reactor reached 37 ± 2 °C. To identify the compounds obtained from the photocatalytic oxidation of cyclohexane, a 1 µL aliquot was removed from the reaction mixture and injected into a GC–MS system. GC–MS analysis was performed on a 7890 gas chromatograph (Agilent Technologies, Inc., Wilmington, DE, USA) inter-faced with a 5975C Series Mass Selective Detector (Agilent Technologies, Inc., Wilmington, DE, USA). The analytes were separated in a crosslinked 5% diphenyl–95% dimethylsiloxane column (Hewlett Packard Corporation, Palo Alto, CA, USA). The temperature programme was an isotherm at 60 °C for 15 min. Helium was used as the carrier gas at a constant flow of 1 mL/min, and data acquisition was performed in electron impact ionization (EI) mode. The temperature inlet was set to 250 °C, and the source was set to 230 °C. Data acquisition and peak integration of the oxofunctionalized products from cyclohexane were performed using Agilent MSD ChemStation Software (Agilent Technologies, Inc., Wilmington, DE, USA, Rev. E.02.01.1177). To identify the compounds obtained from the photocatalytic oxidation of cyclohexane, a 1.0 µL aliquot was removed from the reaction mixture and injected into a GC–MS system. GC–MS analysis was performed on a 7890 gas chromatograph (Agilent Technologies, Inc., Wilmington, DE, USA) inter-faced with a 5975C Series Mass Selective Detector (Agilent Technologies, Inc., Wilmington, DE, USA).

4.3.2. Benzyl Alcohol Oxofunctionalization

The oxofunctionalization of 25 mL of 5 mmol L⁻¹ benzyl alcohol aqueous solution was performed in a 50-mL 2-neck round-bottom flask fitted with a reflux condenser in the presence of 25 mg of catalyst (1 mg/mL). For each reaction, the system was saturated with of air (1 atm) for the duration of the reaction time (3 h). Photocatalytic oxidation reactions were carried out under visible radiation generated by a 400 W metal halide lamp (Osram Powerstar HQI–E 400 W/D Pro Daylight). The temperature inside the photocatalytic reactor reached 37 ± 2 °C.

To identify the compounds obtained from the photocatalytic oxidation of benzyl alcohol, a 10 µL aliquot was removed from the reaction mixture and injected into an HPLC system. HPLC analysis was performed on a FLEXAR™ Liquid Chromatography System (PerkinElmer, Inc., Waltham, MA, USA) inter-faced with a FLEXAR™ UV-Vis Detector (PerkinElmer, Inc., Waltham, MA, USA). The analytes were separated in an Inertsil ODS-3 (particle size: 5µm; length: 250 mm; inner diameter: 3 mm) analytical column (GL Sciences Inc., Torrance, CA, USA). The detection wavelength was set at 254 nm, the flow rate was set at 1 mL/min. The mobile phase used was composed of acetic acid 0.1% v/v (solution A) and acetonitrile (solution B). From 0 to 5 min a linear gradient of mobile phase was used, 10% to 30% A in (A + B). From 5 to 10 min an isocratic gradient of mobile phase was used, 30% A in (A + B). From 10 to 15 min a linear gradient of mobile phase was used, 30% to 10% A in (A + B). Data acquisition and peak integration of the oxofunctionalized products from cyclohexane were performed using Chromera^® Software (PerkinElmer, Inc., Waltham, MA, USA, Version 3.4.0.5712).

In Situ Electron Paramagnetic Resonance Experiments

To detect the radical species formed during the cyclohexane and benzyl alcohol oxofunctionalization over the photoactivated surface of synthesized photocatalysts, in situ electron paramagnetic resonance (EPR) measurements were performed with an EMX micro 6/1 Bruker ESR spectrometer working in the X-band and equipped with a Bruker Super High QE cavity resonator (Billerica, MA, USA). 5,5-dimethyl-1-pyrroline N-oxide (DMPO) [42,43,44], α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) and N-Tertbutyl-α-phenylnitrone (PBN) [24] and were used as spin traps.

In situ detection of radicals in oxofunctionalization of cyclohexane was performed dispersing 1 mg of photocatalyst in 1 mL of cyclohexane containing 10 mmol L⁻¹ spin trap and 0.1% (v/v) nanopure water previously saturated with air, as a source of molecular oxygen. Then, the reaction was initiated by turning on the irradiation source. The reactions were carried out in an Wilmad WG-1228-7 NMR sample tube (Wilmad-LabGlass™ SP Scienceware™, Vineland, NJ, United States) inside the EPR cavity, irradiated by the 400 W metal halide lamp described above.

In situ detection of radicals in oxofunctionalization of benzyl alcohol was carried out under the same conditions used for detection of radicals in oxofunctionalization of cyclohexane. A total of 1 mg of photocatalyst was dispersed in 1 mL of 5 mmol L⁻¹ benzyl alcohol aqueous solution containing 10 mmol L⁻¹ spin trap, previously saturated with air. The photocatalytic reaction was initiated by turning on the irradiation source. The reactions were carried out in an Wilmad WG-1228-7 NMR sample tube (Wilmad-LabGlass™ SP Scienceware™, Vineland, NJ, USA) inside the EPR cavity, irradiated by the 400 W metal halide lamp. In situ measurements were performed at room temperature. Typical instrumental conditions were as follows: Center field, 3514 G; sweep width, 100 G; microwave power, 20 dB; modulation frequency, 100 kHz; time constant, 0.01 ms; sweep time, 30 s; modulation amplitude, 1.00 G; and receiver gain, 30 dB.

5. Conclusions

Three materials, titanium dioxide, bismuth oxyiodide and a 20% BiOI/TiO₂ hybrid material were synthesized and characterized. Titanium dioxide exhibited the highest photocatalytic activity in the oxofunctionalization of cyclohexane and benzyl alcohol under visible light. This behavior was attributed to its ability to generate hydroxyl radicals in situ under visible light irradiation. Bismuth oxyiodide, on the other hand, exhibited high selectivity for cyclohexanol in the cyclohexane oxofunctionalization reaction under visible light irradiation. Finally, the BiOI/TiO₂ material exhibits similar selectivity to that obtained with pure bismuth oxyiodide. However, with BiOI/TiO₂ a better photocatalytic performance than that obtained with pure BiOI was obtained. These results allowed us to evaluate the potential use of these photocatalysts for the oxofunctionalization of cyclohexene and benzyl alcohol, which opens possibilities of applying these photocatalytic systems for the oxofunctionalization, for example, of lignin model compounds, taking advantage of solar energy, to establish a more sustainable process.