Engineering of Surface Environment of Pd Nanoparticle Catalysts on Carbon Support with Pyrene–Thiol Ligands for Semihydrogenation of Alkynes

Abstract

A new type of pyrene–thiol derivative-modified Pd nanoparticle (NP) catalyst on a carbon black support for the efficient semihydrogenation of alkynes to alkenes is reported herein. Colloidal Pd NPs surrounded by pyrene–thiol modifiers were prepared using the two-phase Brust method followed by impregnation of carbon black materials. Based on the structural characterization of the prepared catalyst (PyC12S-Pd/VC) by NMR, UV–vis, FT-IR, TEM, HAADF-STEM, Pd K-edge XAFS, XRD, N2 adsorption, and XPS, we show that highly dispersed Pd NPs are immobilized on the catalysts via π–π interaction between pyrene groups bound to the Pd NPs and carbon black supports. PyC12S-Pd/VC efficiently catalyzes the alkyne semihydrogenation reaction while maintaining high alkene selectivity; an alkene selectivity of 94% is attained at 98% conversion after 5 h of reaction, and the selectivity was retained around 80% in 10 h of reaction. This performance is superior to that of a catalyst without pyrene groups and that of a commercial Lindlar catalyst. The steric hindrance of pyrene groups restricts access of the substrates to Pd NP surfaces, suppressing the unfavorable overhydrogenation of alkenes to alkanes, which is revealed by the solvent and substrate dependency on the catalytic performance and a DFT calculation study. Furthermore, the high selectivity and stability of PyC12S-Pd/VC are caused by the strong interaction between pyrene groups and carbon supports, which prevents the separation of pyrene modifiers and the leaching or sintering of Pd NPs during the catalytic reaction. It is demonstrated that the combination of Pd NPs, pyrene–thiol modifiers, and carbon supports offers high activity, alkene selectivity, and stability in the semihydrogenation reaction.

Pyrene‐Thiol‐modified Pd Nanoparticles on Carbon Support: Kinetic Control by Steric Hinderance and Improved Stability by the Catalyst‐Support Interaction

“…[18][19][20][21] For example, amine-or thiol-modified Pd NP catalysts show high alkene selectivity in semi-hydrogenation reactions of alkynes, in which organic modifiers control the adsorption of alkenes to the Pd surface, inhibiting the overhydrogenation of alkenes to alkanes. [22][23][24][25] Furthermore, Pd NPs stabilized by thiol or phosphine compounds have been reported to catalyze CÀ C coupling reactions with high turnover frequency. [26][27][28] However, there are few applications of surfacemodified Pd NP catalysts to oxidation reactions, due to the lower stability of organic ligands on Pd NP surfaces under O 2 atmosphere and high temperature conditions.…”

“…Our recent work revealed that pyrene-thiol ligands-modified Pd NPs on carbon supports suppressed the separation and leaching of thiol ligands with the aid of π-π interactions between pyrene groups and the carbon surface, enhancing the catalyst stability in the semi-hydrogenation reaction. [22] This result inspired us to apply Pd-based nanohybrid materials with pyrene-thiol ligands on carbon supports for highly stable oxidation reactions.…”

“…PyC 12 SH and BnC 12 SH were synthesized based on our reported method via Williamson ether synthesis, and each molecular structure was confirmed by 1 H NMR analysis (for details, see Scheme S1). [22,29] Subsequently, Pd NP colloids were formed by the two-phase Brust method. [30] Each thiol was dissolved in toluene, and mixed with Na 2 PdCl 4 aqueous solution and a phase transfer catalyst of tetraoctylammonium bromide (TOAB), followed by reduction with NaBH 4 aqueous solution to yield dark-brown colored colloidal NPs.…”

“…The absorption edge energies (E 0 ) of the catalysts prepared by the colloidal method (PyC 12 S-Pd/ VC, C 12 S-Pd/VC, and C 12 N-Pd/VC) are higher than that of Pd/VC, suggesting that surface Pd species coordinated with the thiol or amine ligands were partially oxidized, as consistent with the previous study. [22,23] The catalytic performance of prepared catalysts was examined in the dehydrogenation of dibenzylamine with 1 atm of O 2 at 110°C. [15,16,38] Figure 2A summarizes the conversion of dibenzylamine after 4 h of reaction using each prepared catalyst.…”

“…It is noted that C 12 S-Pd NPs was employed instead of PyC 12 S-Pd NPs in the in situ FT-IR test, because it is difficult to distinguish the absorption change derived from the imine product and that from the pyrene ligands. [22] Figure 3 shows the in situ FT-IR spectra measured using the C 12 N-Pd NPs and C 12 S-Pd NPs. In both spectra, two distinct absorption bands at around 1690 and 1510 cm À 1 were observed at 50°C, which can be assigned to the C=N and aromatic CÀ C stretching vibrations, respectively, judging from the experimental and DFT-simulated FT-IR spectra ( Figure S9).…”