Photocatalytic hydrogenation of acetophenone on a titanium dioxide cellulose film

A previously developed sustainable immobilization concept for photocatalysts based on cellulose as a renewable support material was applied for the photocatalytic hydrogenation of acetophenone (ACP) to 1-phenyl ethanol (PE). Four different TiO2 modifications (P25, P90, PC105, and PC500) were screened for the reaction showing good performance for PC25 and PC500. PC500 was selected for a detailed kinetic study to find the optimal operating conditions, and to obtain a better understanding of the photocatalytic pathway in relation to conventional and transfer hydrogenation. The kinetic data were analyzed using the pseudo-first-order reaction rate law. A complete conversion was obtained for ACP concentrations below 1 mM using a 360 nm filter and argon as the purge gas within 2–3 hours. High oxygen concentrations slow down or prevent the reaction, and wavelengths below 300 nm lead to side-products. By investigating the temperature dependency, an activation energy of 22 kJ mol−1 was determined which is lower than the activation energies for conventional and transfer hydrogenation, because the light activation of the photocatalyst turns the endothermic to an exothermic reaction. PC500 was immobilized onto the cellulose film showing a 37% lower activity that remains almost constant after multiple use.


Dissolved oxygen concentration
The approximate oxygen concentration dissolved in ethanol from the air in the gas phase is calculated with the 2 Henry constant using Eqn. S1-S3, where x is the mole fraction, p is the pressure of the gas phase, the , 2 density of ethanol, and the molar mass.
(S1)  Figure S1 shows the HPLC chromatogram for different wavelengths applied in the photocatalytic hydrogenation of ACP. Figure S1: Chromatograms of the reaction solution before an experiment containing ACP only and afterward containing the product 1-phenylethanol. When no filter or filter that blocks wavelengths ≤ 300 nm is used, another signal appears which is a side-product.

Side-irradiation reactor (SIR)
For kinetic investigations, the SIR ( Figure S2) was used. The SIR has a total volume of 35 mL and consists of a planar quartz glass window.

Top-irradiation reactor (TIR)
The TIR was used for the experiments with the film ( Figure S3). The TIR consists of borosilicate glass with a total volume of 250 mL and has a removable quartz glass window (Ø 4 cm) on top. The irradiated circular surface of the PCF fixed on the Teflon substrate holder had a diameter of 3 cm. The substrate holder has a total diameter of 3.8 cm, and the lower part, on which the PCF lies, has a height of 0.5 cm. The height of the liquid phase is 11 cm. The total distance between lamp and PCF is 12 cm. The PCF containing TiO2 (PC500) turns blue during the reaction and white when oxygen reaches the PCF, which was already reported in the literature 4,5 Figure S3: Photocatalytic hydrogenation of acetophenone in the TIR (a), PCF before irradiation (b), PCF after irradiation (c), and the substrate with PCF in the liquid phase (d).
Further results from reaction kinetics 4 The reaction was performed at different temperatures (Table S1) to determine the activation energy. In addition to the 1 st order kinetics, the Langmuir-Hinshelwood equation was used to fit the experimental data. Here k is the rate constant, K LH is the Langmuir-Hinshelwood constant, and c ACP is the concentration of acetophenone: When c ACP is low, the LH equation can be simplified into the pseudo 1 st order rate law. The fitting of the concentration-time profiles with the 1 st order rate law and the Langmuir-Hinshelwood equation in Figure S4 have minor differences, which are also shown by the rate constants in Table 2.

Energy profiles for different types of hydrogenation
To compare the different types of hydrogenation, their energy profiles were constructed using Eqn. S5-S6. The standard values for the reactants (Table S3) were taken from the literature and the calculated enthalpies are summarized in Table S4.

Recycling experiment
The film was recycled several times, and the individual concentration profiles were fitted to the pseudo 1st order rate law.
The rate constants are summarized in Table S5. The optical behavior of the films after each recycling is shown in Figure S5.  8

Cellulose characterization
The structure of the modified cellulose, from which the film was prepared, is shown in Figure S6. Figure S6: TEM image cellulose before preparation of the PCF. 9

Catalyst/Cellulose separation
For the recovery of the TiO 2 photocatalyst, the film can be treated with concentrated sulphuric acid, which will lead to complete the dissolution of cellulose. From this dispersion, TiO 2 can be recovered, e.g., by centrifugation. The individual steps are shown in Figure S7.