Tuning Catalytic Selectivity in Cascade Reactions by Light Irradiation

Selectivity of cascade redox reactions: the reduction of nitrobenzene to azoxybenzene and then to azobenzene and the oxidation of benzyl alcohol to benzaldehyde and then to benzoic acid, is discovered to be tuneable via light irradiation over plasmonic gold photocatalysts. The representative photocatalyst of Au/CeO2 was characterized by TEM, EDX, UV–Vis and XPS to determine its morphology, elemental composition, photo absorptivity and oxidation state of gold. The catalytic test results demonstrate that the net contribution of light irradiation correlates with the ability of incident light to excite electrons and light absorption of catalysts. These findings may inspire peer researchers in developing new photocatalytic processes or in designing new photocatalysts for clean chemicals synthesis.


Introduction
Controlling selectivity in clean chemicals synthesis is crucial to obtaining target products with high purity [1,2], and it is more challenging in cascade reactions [3,4]. Apart from optimizing reaction kinetics and mass transfer kinetics to achieve better catalytic efficiency [5], developing new heterogeneous catalysts requires controlling active sites (e.g. composition, orientation, and distribution) [6,7], and structures (e.g. pores, sizes, and shapes) [1,8]. Electrons (e.g. density, interaction, and transfer) also play an indispensable role in manipulating catalytic performances, particularly in photocatalysis which involves light-excited electrons [9,10]. In parallel, strategies associated with energy input are developed to enhance product selectivity, such as the use of microwave, ultrasound, an additional electric or electromagnetic field, and light irradiation [11][12][13].
Photocatalysis offers a promising opportunity to obtain a high product selectivity due to its capacity to influence catalytic reactions at ambient temperatures. More significantly, the unique property of incident light interacting with reactants or photocatalysts is able to initiate or accelerate reactions with low energy barriers and to give a certain product [14][15][16][17]. Recent studies demonstrate that plasmonic photocatalysts based on the localized surface plasmon resonance (LSPR) effect can actually achieve these goals [18][19][20]. The LSPR effect features a collective and coherent oscillation of the free conduction electrons with the incident photons on the surface of metal nanoparticles (NPs) [21][22][23]. These strong oscillating electrons can drive the target reaction locally (around active sites) and affect the product selectivity fundamentally [24][25][26]. However, the functional mechanism of light irradiation to tune reactive procedure remains ambiguous.
Herein, a tuneable laser source and a solar simulator are used to explore the processing mechanism involving incident light, light absorption, electron excitation, electron transfer and activating the reactants to obtain targeted products. In clean chemicals synthesis, the catalytic hydrogenation of nitro compounds is an ideal process to produce amines, mainly azobenzene (other products of azoxybenzene and aniline can also be produced via this route) [14], and the catalytic oxidation of benzyl alcohol plays a significant role to produce aromatic aldehydes [27]. Conventional catalytic processes of these two consecutive reactions always necessitate complicated methodologies to minimize the production of side-products [28][29][30][31]. This study highlights that light irradiation over supported plasmonic gold catalysts [14,32] can effectively tune the catalytic selectivity of the aforementioned two cascade reactions, with the net contribution depending upon light wavelength and intensity at a certain temperature.

Catalyst Preparation
In a glass beaker with a magnetic stirring bar, 2.5 g of support powders, such as CeO 2 , were dispersed into the 100 ml of 3.8 × 10 −3 mol/L HAuCl 4 aqueous solution. Subsequently, 20 ml of 0.53 mol/L lysine was added dropwise to the mixture under stirring, and after that the stirring was prolonged for 30 min. To this suspension, 10 ml of 0.35 mol/L NaBH 4 solution was added dropwise, followed by adding 10 ml of 0.3 mol/L hydrochloric acid (to achieve a pH 9.5). The mixture was stirred for 1 h and aged for 24 h. Then the solid was separated, washed firstly with 600 ml deionized water for three times in a 250-ml centrifuge bottle to remove the chlorine ions, and the residual solution was tested by 0.1 mol/L AgNO 3 solution and no AgCl precipitate was observed. The solids were collected, dried at 60 °C for 16 h, and used directly as the photocatalyst.

Photocatalytic Tests
The reaction was conducted in a 50 ml round-bottomed Pyrex glass flask with a sealed spigot and a magnetic stirrer. The reaction temperature was controlled by an air-conditioner within a sealed wooden box. The light intensity in the reaction position was set at 0.1 W/cm 2 and could be adjusted by changing the distance between the reactor and the light source. The wavelength range was tuned by using various glass filters to cut off the irradiation below a certain value of wavelength. A tuneable laser source (OPOTECK VIBRANT, 350 II) and a solar simulator (NEWPORT, 150 W) were used as light source, respectively.
Catalytic reduction of nitrobenzene was conducted under the argon (Ar) atmosphere. Typically, 3 mmol of nitrobenzene, 30 ml of isopropanol as solvent, and 3 ml of 0.1 mol/L KOH solution in isopropanol were mixed in the reactor, followed by adding 100 mg of the catalyst and purging with argon gas, and then stirred during reaction and illuminated with the light source.
Catalytic oxidation of benzyl alcohol was conducted under the oxygen (O 2 ) atmosphere. Typically, 3 mmol of benzyl alcohol was dissolved into 30 ml of isopropanol, and 3 ml of 0.1 mol/L KOH solution in isopropanol and 100 mg of the catalyst were added into the mixture. The mixture was stirred during reaction and illuminated with the light source.
During these reactions, 0.5 ml aliquots were collected at given irradiation time intervals and filtered through a Millipore filter (pore size 0.45 µm) to remove the catalyst particulates. Then flask was purged with Ar or O 2 again for more than 3 min to remove air and then sealed. The filtrates were analyzed by an Agilent 6890 gas chromatograph with the 1 3 HP-5 column. Quantities of the products and reactants were calculated from the peak areas of the compounds using calibration curves. An Agilent HP5973 mass spectrometer was used to determine and analyze the product compositions.

Characterization
TEM images were taken with a JEOL JEM-2100 transmission electron microscope employing an accelerating voltage of 200 kV. The specimens were sample powders deposited onto a copper microgrid coated with a holey carbon film. The element composition of some samples was determined by energy-dispersive X-ray spectroscopy attached on an FEI Quanta 200 scanning electron microscopy (The average gold loading was 2.8 wt%). The diffuse reflectance UV-Vis spectra of the samples were recorded on a Cary 5000 spectrometer. The XPS data were recorded on an ESCALAB 250 spectrometer and Al Ka radiation was used as the X-ray source. The C1s peak at 284.5 eV was used as a reference for the calibration of the binding energy scale.

Results and Discussion
The catalytic performances of the hydrogenation of nitrobenzene are presented in Table 1. The effect of light irradiation on tuning the product selectivity is evident. In particular, at 40 °C and with the light off, the selectivity of azoxybenzene can be up to 91%; whereas with the light on, azoxybenzene disappears and the yield of azobenzene can be up to > 99%.
At a higher temperature (80 °C), the influence of light is negligible and both light reaction and thermal reaction can achieve a complete conversion. Generally, the light irradiation can increase the reaction conversion compared with the reaction with light off at lower reaction temperatures and have significant influence on product selectivity, thereby playing a threshold/switch role in tuning the catalytic reaction procedure.
To explain this phenomenon, the physiochemical properties of catalysts are characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-Vis spectroscopy. In Fig. 1a, the typical TEM images of the Au-NPs on the CeO 2 support show that Au NPs are well dispersed and have narrow size distributions within 2-6 nm. The XPS spectra (Fig. 1b) provides the spectra of Au 4f and confirms that the binding energies of Au4f 7/2 and Au4f 5/2 electrons are 84.0 and 87.7 eV, respectively, corresponding to the oxidation state of gold metal [33]. The UV-Vis spectra of Au/CeO 2 shows the absorption band centres at about 539 nm in wavelength owing to the LSPR effect; on the contrary, the CeO 2 support itself shows no absorption in the visible light region, which substantially demonstrates that the contribution of light irradiation to affecting the catalytic activity and selectivity via the LSPR effect of Au NPs only.
It would be exciting if the underlying mechanistic reasons for the light irradiation which affects catalytic selectivity are unravelled. At the current stage of this study, we propose that the distinct differences of reaction pathway at a certain reaction temperature can be ascribed to "the reduction barriers of intermediate products" and "the light-induced electrons interaction" between reactant molecules and Au NPs, and these proposed associations have been initially explored in our previous studies [24,25,34].
To illustrate, the reduction of nitrobenzene can be induced by both heat and the light irradiation over supported gold catalysts [14,28], undergoing different reaction pathways and thus giving different product selectivity. In the lightinduced process, the solvent of isopropanol also serves as a hydrogen donor to form the intermediate species of Au-H bonds [14], and these species can break the N-O bonds to reduce nitrobenzene. Meanwhile, the resonating electrons of Au NPs strongly interact with the electrophilic nitro groups of nitrobenzene to facilitate the cleavage of the N-O bonds by Au-H species. For these reasons, even at a lower temperature of 40 °C, the reduction of nitrobenzene still occurs efficiently; however, at a higher temperature of 80 °C, both heat and light irradiation take effect, and it is difficult to identify the individual contribution of heat or light, thereby showing identical conversions and similar selectivity. Hence, the similar experimental results at 80 °C with light on and off convince the switch role of light irradiation at lower reaction temperatures of 60 °C and 40 °C ( Table 1). As the light irradiation essentially affects the catalytic reaction procedure, then it is sensible to deduce that the catalytic performances should differ with the light source, light absorption and energy transfer. It is shown in Fig. 2a that the For instance, with the same light absorption, at 505 and 582 nm, the conversions are roughly the same, but the reaction proceeds to different stages, namely, the selectivity is different. In the case of 460 and 623 nm, the latter only proceeds at the initial stage, having a > 99% of selectivity towards azoxybenzene. Besides light wavelength, the light intensity also has influence on the catalytic selectivity as shown in Fig. 2b. Increasing light intensities at the fixed wavelength (at 539 nm) boost more energetic electrons, thus resulting in different product selectivity; the conversions also increase and undergo latter stages, that is, much higher yields of azobenzene and aniline are obtained.
Further to test the generality of this finding that light irradiation can tune the product selectivity, the selective oxidation of benzyl alcohol to benzaldehyde and then to benzoic acid was also investigated, and the catalytic results are shown in Table 2. At 40 °C, the oxidation of benzyl alcohol mainly produces benzaldehyde, but the conversion is much higher under visible light irradiation. On the other hand, the heat effect is significant at 80 °C, and benzyl alcohol is oxidized into benzaldehyde with light off, but it further transforms to benzoic acid if the light irradiation applies; these results show that the light irradiation acts as a "switch" to control the production of benzaldehyde or benzoic acid at 80 °C. This observation is inspiring because it not only proves the "switch role" of light irradiation for these two selected representative cascade reactions but also demonstrates that this phenomenon applies to both reductive and oxidative cascade reactions.

Conclusion
This study highlights the contribution of light irradiation to tuning the selectivity of redox cascade reactions by means of supported plasmonic gold photocatalysts. The reduction of nitrobenzene and the oxidation of benzyl alcohol exhibit different selectivity with light on and off at a certain temperature. The net contribution correlates to light wavelength and intensity. To further identify or even quantify the contribution of light irradiation and to clarify their underlying mechanisms, more investigation will be performed on such as photocatalysts with different gold loadings or sizes and other redox cascade reactions. Overall, this study presents a promising approach to refining the product selectivity in clean chemicals synthesis, and could inspire the design of new catalytic materials and processes.