Stereoselective hydrogenation of phenyl alkyl acetylenes on pillared clays supported palladium catalysts

Highly Active Bimetallic Single-Atom Alloy PdAg Catalysts on Cerium-Containing Supports in the Hydrogenation of Alkynes to Alkenes

Abstract

A study of a series of single-atom-alloy catalysts Pd1Ag3/Al2O3, Pd1Ag3/CeO2–Al2O3, and Pd1Ag3/CeO2–ZrO2 in the selective hydrogenation of diphenylacetylene (DPA) showed a significant (five-fold) increase in activity for the PdAg3/CeO2–ZrO2 sample in comparison with that of Pd1Ag3/Al2O3. It was especially noted that the increase in activity was not accompanied by a decrease in the selectivity for the target product. This catalytic behavior can be explained by two factors: (1) a more than twofold increase in the dispersity of the PdAg3/CeO2–ZrO2 catalyst and (2) a change in the electronic state of the nanoparticles, as determined from the results of an IR-spectroscopic study of adsorbed CO. The retention of the high selectivity of the synthesized catalysts indicated the stability of the structure of Pd1 monoatomic sites in the catalysts prepared by deposition on Ce-containing supports, which was also confirmed by the IR spectroscopy of adsorbed CO. The experimental results indicate that Ce-containing supports are promising for the synthesis of catalysts for the selective hydrogenation of substituted alkynes.

Liquid-Phase Hydrogenation of 1-Phenyl-1-propyne on the Pd1Ag3/Al2O3 Single-Atom Alloy Catalyst: Kinetic Modeling and the Reaction Mechanism

This research was focused on studying the performance of the Pd₁Ag₃/Al₂O₃ single-atom alloy (SAA) in the liquid-phase hydrogenation of di-substituted alkyne (1-phenyl-1-propyne), and development of a kinetic model adequately describing the reaction kinetic being also consistent with the reaction mechanism suggested for alkyne hydrogenation on SAA catalysts. Formation of the SAA structure on the surface of PdAg₃ nanoparticles was confirmed by DRIFTS-CO, revealing the presence of single-atom Pd₁ sites surrounded by Ag atoms (characteristic symmetrical band at 2046 cm⁻¹) and almost complete absence of multiatomic Pd_n surface sites (<0.2%). The catalyst demonstrated excellent selectivity in alkyne formation (95–97%), which is essentially independent of P(H₂) and alkyne concentration. It is remarkable that selectivity remains almost constant upon variation of 1-phenyl-1-propyne (1-Ph-1-Pr) conversion from 5 to 95–98%, which indicates that a direct alkyne to alkane hydrogenation is negligible over Pd₁Ag₃ catalyst. The kinetics of 1-phenyl-1-propyne hydrogenation on Pd₁Ag₃/Al₂O₃ was adequately described by the Langmuir-Hinshelwood type of model developed on the basis of the reaction mechanism, which suggests competitive H₂ and alkyne/alkene adsorption on single atom Pd₁ centers surrounded by inactive Ag atoms. The model is capable to describe kinetic characteristics of 1-phenyl-1-propyne hydrogenation on SAA Pd₁Ag₃/Al₂O₃ catalyst with the excellent explanation degree (98.9%).

Highly-Ordered PdIn Intermetallic Nanostructures Obtained from Heterobimetallic Acetate Complex: Formation and Catalytic Properties in Diphenylacetylene Hydrogenation

Formation of PdIn intermetallic nanoparticles supported on α-Al₂O₃ was investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and hydrogen temperature-programmed desorption (H₂-TPD) methods. The metals were loaded as heterobimetallic Pd(μ-O₂CMe)₄In(O₂CMe) complex to ensure intimate contact between Pd and In. Reduction in H₂ at 200 °C resulted in Pd-rich PdIn alloy as evidenced by XRD and the disappearance of Pd hydride. A minor amount of Pd₁In₁ intermetallic phase appeared after reduction at 200 °C and its formation was accomplished at 400 °C. Neither monometallic Pd or in nor other intermetallic structures were found after reduction at 400⁻600 °C. Catalytic performance of Pd₁In₁/α-Al₂O₃ was studied in the selective liquid-phase diphenylacetylene (DPA) hydrogenation. It was found that the reaction rate of undesired alkene hydrogenation is strongly reduced on Pd₁In₁ nanoparticles enabling effective kinetic control of the hydrogenation, and the catalyst demonstrated excellent selectivity to alkene.

Photodeposited Pd Nanoparticles with Disordered Structure for Phenylacetylene Semihydrogenation

Developing effective heterogeneous metal catalysts with high selectivity and satisfactory activity for chemoselective hydrogenation of alkyne to alkene is of great importance in the chemical industry. Herein, we report our efforts to fabricate TiO₂-supported Pd catalysts by a photodeposition method at room temperature for phenylacetylene semihydrogenation to styrene. The resulting Pd/TiO₂ catalyst, possessing smaller Pd ensembles with ambiguous lattice fringes and more low coordination Pd sites, exhibits higher styrene selectivity compared to two contrastive Pd/TiO₂ samples with larger ensembles and well-organized crystal structure fabricated by deposition-precipitation or photodeposition with subsequent thermal treatment at 300 °C. The sample derived from photodeposition exhibits greatly slow styrene hydrogenation in kinetic evaluation because the disordered structure of Pd particles in photodeposited Pd/TiO₂ may prevent the formation of β-hydride phases and probably produce more surface H atoms, which may favor high styrene selectivity.