Additive‐free Semihydrogenation of an Alkynyl Group to an Alkenyl Group over Pd−TiO₂ Photocatalyst Utilizing Temporary In‐situ Deactivation

Abstract

Lindlar's catalyst, i. e., calcium carbonate‐supported palladium (Pd) modified with lead, has been used for semihydrogenation of an alkynyl group in the presence of hydrogen gas (H2). We examined hydrogenation of an alkynyl group in organosilane and hydrocarbon in methanolic suspensions of a Pd‐loaded titanium(IV) oxide (Pd−TiO2) photocatalyst without the use of additives and H2. In the photocatalytic reaction, Pd particles worked as co‐catalysts for hydrogenation and alkyne hydrogenation had priority to alkene hydrogenation. Since the Pd co‐catalyst was temporarily deactivated during the reaction owing to accumulation of the oxidized product(s) of methanol, the capacity of hydrogenation of the unsaturated C−C bond was limited. By optimizing the capacity and amount of alkynes, almost complete semihydrogenation of alkynes was achieved under a poison‐free condition. Pd−TiO2 can be regenerated by very simple treatments, i. e., washing and drying at room temperature.

Photo-thermo semi-hydrogenation of acetylene on Pd1/TiO2 single-atom catalyst

“…Despite of these great progress, whether photo-thermocatalysis is possible to boost semi-hydrogenation of acetylene still remains inconclusive. Few studies related to photocatalysis of selective hydrogenation for nitrobenzene 53 , benzaldehyde 54 , and alkynyl group [55][56][57][58][59] were reported, but the related works for photo-thermocatalytic acetylene semi-hydrogenation are limited. Swearer et al firstly used Pd NPs and aluminum nanocrystals (AlNC) to construct a heterometallic antenna-reactor complexes photocatalyst for semi-hydrogenation of acetylene but with a low product yield 60 .…”

Control of selectivity in organic synthesis via heterogeneous photocatalysis under visible light

“…However, this selective hydrogenation is quite challenging since the semihydrogenation of triple bonds (C≡C) to double bonds (C=C) is difficult to proceed without overreduction to single bonds [132][133][134]. The photocatalytic hydrogen transfer strategy provides one efficient technique to realize the selective semihydrogenation of alkynes [135][136][137][138][139][140][141][142][143], which also avoids the employment of dangerous high-pressure H2 and an external heat source. The basic idea of photocatalysis is to generate active hydrogen atoms on the surface of photocatalysts via the activation of hydrogen donors such as alcohols or water by holes and electrons [144,145].…”

Solvothermal Treatment of Visible‐Light‐Responsive TiO2 toward Enhanced Photocatalytic Degradation Activity of Tetracycline

“…[12][13][14] However, the wide band gap of TiO 2 makes it only excited by ultraviolet light, which greatly reduces the efficiency of solar energy utilization and seriously limits its practical application in photocatalysis. [15,16] Wang et al successfully synthesized free-standing two-dimensional TiO 2 sheets with narrow band gap by a one-step solvothermal method. [17] The obtained TiO 2 sheets possessed intensive absorption in the visible-light region and showed the enhanced photocatalytic activity.…”

“…Based on these properties, TiO 2 can be used for environmental pollution control, such as water and air purification, stain prevention and sterilization [12–14] . However, the wide band gap of TiO 2 makes it only excited by ultraviolet light, which greatly reduces the efficiency of solar energy utilization and seriously limits its practical application in photocatalysis [15,16] . Wang et al.…”