Catalytic farming: reaction rotation extends catalyst performance

Abstract

The use of heterogeneous catalysis has key advantages compared to its homogeneous counterpart, such as easy catalyst separation and reusability. However, one of the main challenges is to ensure good performance after the first catalytic cycles. Active catalytic species can be inactivated during the catalytic process leading to reduced catalytic efficiency, and with that loss of the advantages of heterogeneous catalysis. Here we present an innovative approach in order to extend the catalyst lifetime based on the crop rotation system used in agriculture. The catalyst of choice to illustrate this strategy, Pd@TiO₂, is used in alternating different catalytic reactions, which reactivate the catalyst surface, thus extending the reusability of the material, and preserving its selectivity and efficiency. As a proof of concept, different organic reactions were selected and catalyzed by the same catalytic material during target molecule rotation.

Scheme 1. Reactions used to demonstrate the catalytic farming concept. (A) Sonogashira coupling is catalyzed by supported PdNP upon visible light irradiation in methanol (MeOH) and Ar atmosphere in the presence of base (K₂CO₃). (B) Ullmann homo-coupling of methyl 4-iodobenzoate (Ar′-I) proceeds under UV-vis light irradiation in the presence of catalyst (Pd@TiO₂) and base (Cs₂CO₃) utilizing tetrahydrofuran (THF) as solvent and Ar atmosphere. (C) Alkene isomerization of estragole can be carried out upon blue light irradiation of a methanolic suspension of Pd@TiO₂ under argon atmosphere. (D) Alkene hydrogenation of estragole can be performed under the isomerization conditions by switching the light to UV light.

Fig. 1. Conversions (dark bars) and yields (light bars) obtained after several catalytic cycles of reactions (A) and (B) in Scheme 1. While reaction (A) experiences a dramatic efficiency drop, reaction (B) can be catalyzed with excellent conversions and yields for several catalytic cycles. Reaction conditions: (A) Sonogashira coupling upon 450 nm irradiation at 2.7 W cm^–2, (B) Ullmann homo-coupling upon 368 nm and 465 nm irradiation at 0.3 and 1.6 W cm^–2, respectively.

Scheme 2. Suggested mechanisms under UVA (A) or visible (B) irradiation. (A) Upon UVA excitation an electron is pumped from the valence band (VB) into the conduction band (CB) of the semiconductor (TiO₂). The electron can be trapped by the Pd nanoparticle attached to the surface slowing down the electron–hole recombination kinetics. Therefore, electron acceptor reagents (EA) can react more easily on the catalyst surface whereas a sacrificial electron donor (SED), frequently the solvent, quenches the hole. (B) Under visible light excitation, the generation of hot electrons on the Pd surface can photocatalyze reactions through (1) local heat generation or (2) hot electron transfer (eT), the latter being the accepted mechanism for this type of non-plasmonic nanoparticles.

Fig. 2. Pd 3d HR-XPS spectra for Pd@TiO₂ catalyst. (A) Fresh catalyst: Pd 3d core-level spectrum deconvoluted by using two spin–orbit split Pd 3d_5/2 and Pd 3d_3/2 components centred at 336.6 eV and 342.0 eV and separated by ∼5.4 eV; attributed to PdO. Small contribution of more reduced palladium species are also found on the material (components at 335.1 eV and 340.2 eV). (B) Catalyst after Sonogashira reaction: high contribution of more reduced species (spin–orbit components at 335.0 (336.0) eV and 340.4 (341.5) eV). (C) Catalyst after Ullmann reaction: similar contribution of both oxidized and less oxidized species. (D) Catalyst after Sonogashira reaction and post-treatment with THF: oxidation state of Pd restored to almost the same as in the fresh material (336.1 and 341.5 eV).

Fig. 3. Conversions (dark bars) and yields (light bars) for catalytic farming of Pd@TiO₂ by rotation between Sonogashira coupling (black) upon 450 nm irradiation at 2.7 W cm^–2 for 30 min and Ullmann homo-coupling (red) upon 368 nm and 465 nm irradiation at 0.3 and 1.6 W cm^–2 for 1 h. Compare to Fig. 1A and B. The spiral at the top-right corner helps us visualize the sequence of reactions with the number representing the reaction sequence, and the color the type of reaction. Similar spirals are included in other figures.

Fig. 4. Conversions (dark bars) and yields (light bars) obtained after several catalytic cycles of (C) alkene isomerization upon 450 nm irradiation at 2.7 W cm^–2 and (D) alkene hydrogenation upon 368 nm irradiation at 0.3 W cm^–2. Notice that each catalytic cycle implies catalyst separation – cleaning cycles – before reusability test.

Fig. 5. Conversions (dark bars) and yields (light bars) for catalytic farming of Pd@TiO₂ by rotation of three different reactions: alkene isomerization (blue), Ullmann homo-coupling (red) and Sonogashira coupling (black). Alternating reactions rotations show different performance for each reaction. See ESI Table S3.

Fig. 6. Conversions (black) and yields (grey) obtained after 3 catalytic cycles for Sonogashira coupling. Cycle 3 shows the recovery of catalytic activity after THF treatment (†). Compare with cycle 3 in Fig. 1A and ESI Table S10.