Heck reaction catalyzed by colloids of delaminated Pd-containing layered double hydroxide

A Hierarchical-Structured Impeller with Engineered Pd Nanoparticles Catalyzing Suzuki Coupling Reactions for High-Purity Biphenyl

Suzuki cross-coupling reactions catalyzed by palladium are authoritative protocols in fine-chemical synthesis. Mass transfer and catalyst activity are both significant factors affecting the reaction efficiency in heterogeneous reactions. Although the holistic catalysts hold great promise in heterogeneous reactions due to the enhanced mass transport and convenient recycling, the unsatisfied catalytic activity has impeded further large-scale applications. In addition, another pronounced barrier is the product separation in the intricate system. Here, the catalytic production and separation of biphenyl (purity of 99.7%) were achieved by integrating the Suzuki cross-coupling reactions and the crystallization separation for the first time. A hierarchical-structured impeller with Pd nanoparticles (NPs) loaded on the Ni(OH)₂ nanosheets was prepared to catalyze the Suzuki reactions for bromobenzene, which exhibits a high turnover frequency (TOF) value of 25,976 h^-1 and a yield of 99.5%. The X-ray absorption fine structure (XAFS) analysis has unveiled that the electron transfer between the Pd NPs and Ni(OH)₂ accounts for the greatly enhanced catalytic activity. The findings inspire new insights toward rational engineering of highly efficient holistic catalysts for Suzuki reaction, and the innovative integrated technology offers an avenue for the separation and collection of products.

Preparation and Reactivation of Heterogeneous Palladium Catalysts and Applications in Sonogashira, Suzuki, and Heck Reactions in Aqueous Media

A new type of heterogeneous palladium catalyst, PdMgAl-LDH, was facilely prepared by the immobilization of Pd2+ species in the layers of a Mg-Al layered double hydroxide (LDH) with co-precipitation, and then fully characterized by using powder XRD, thermogravimetric differential thermal analysis, TEM, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy techniques. These catalysts can efficiently catalyze copper-free Sonogashira, Suzuki and Heck coupling reactions of various aryl iodides, bromides, and chlorides in aqueous media under phosphine-ligand- and organic-base-free conditions. These catalysts feature easy recovery through simple filtration and could be reused at least six times without a marked loss in activity. Notably, they can be facilely reactivated by a combination of nitrolysis with co-precipitation. The basic LDH skeletons could effectively stabilize the Pd0 species created in situ and donate electron density to the Pd0 center to facilitate the oxidative addition of aryl halides, thus the PdMgAl-LDH catalysts are stable during catalysis.

Highly stable layered double hydroxide colloids: a direct aqueous synthesis route from hybrid polyion complex micelles

Aqueous suspensions of highly stable Mg/Al layered double hydroxide (LDH) nanoparticles were obtained via a direct and fully colloidal route using asymmetric poly(acrylic acid)-b-poly(acrylamide) (PAA-b-PAM) double hydrophilic block copolymers (DHBCs) as growth and stabilizing agents. We showed that hybrid polyion complex (HPIC) micelles constituted of almost only Al(3+) were first formed when mixing solutions of Mg(2+) and Al(3+) cations and PAA3000-b-PAM10000 due to the preferential complexation of the trivalent cations. Then mineralization performed by progressive hydroxylation with NaOH transformed the simple DHBC/Al(3+) HPIC micelles into DHBC/aluminum hydroxide colloids, in which Mg(2+) ions were progressively introduced upon further hydroxylation leading to the Mg-Al LDH phase. The whole process of LDH formation occurred then within the confined environment of the aqueous complex colloids. The hydrodynamic diameter of the DHBC/LDH colloids could be controlled: it decreased from 530 nm down to 60 nm when the metal complexing ratio R (R = AA/(Mg + Al)) increased from 0.27 to 1. This was accompanied by a decrease of the average size of individual LDH particles as R increased (for example from 35 nm at R = 0.27 down to 17 nm at R = 0.33), together with a progressive favored intercalation of polyacrylate rather than chloride ions in the interlayer space of the LDH phase. The DHBC/LDH colloids have interesting properties for biomedical applications, that is, high colloidal stability as a function of time, stability in phosphate buffered saline solution, as well as the required size distribution for sterilization by filtration. Therefore, they could be used as colloidal drug delivery systems, especially for hydrosoluble negatively charged drugs.