The promotion effect of π-π interactions in Pd NPs catalysed selective hydrogenation.

The utilization of weak interactions to improve the catalytic performance of supported metal catalysts is an important strategy for catalysts design, but still remains a big challenge. In this work, the weak interactions nearby the Pd nanoparticles (NPs) are finely tuned by using a series of imine-linked covalent organic frameworks (COFs) with different conjugation skeletons. The Pd NPs embedded in pyrene-COF are ca. 3 to 10-fold more active than those in COFs without pyrene in the hydrogenation of aromatic ketones/aldehydes, quinolines and nitrobenzene, though Pd have similar size and surface structure. With acetophenone (AP) hydrogenation as a model reaction, systematic studies imply that the π-π interaction of AP and pyrene rings in the vicinity of Pd NPs could significantly reduce the activation barrier in the rate-determining step. This work highlights the important role of non-covalent interactions beyond the active sites in modulating the catalytic performance of supported metal NPs.

Synthesis of Bifunctional Porphyrin Polymers for Catalytic Conversion of Dilute CO2 to Cyclic Carbonates.

Development of efficient solid catalysts for catalytic conversion of dilute CO2 is of extreme importance for carbon capture and utilization. We report the synthesis of bifunctional polymers co-incorporated with porphyrin-Zn as Lewis acid sites and Br- as nucleophiles for the cycloaddition of dilute CO2 with epoxides in this work. It was found that the Br-/Zn ratio has a volcano relation with the activity of bifunctional polymers in a cycloaddition reaction, indicating the synergy effect between Lewis acid sites and nucleophiles. The turnover frequency (TOF) of the bifunctional polymer is more than four-fold that of the physical mixture of tetrabutylammonium bromide and porphyrin-Zn-incorporated polymer, implying the enhanced cooperation between Br- and porphyrin-Zn in the polymer network. The bifunctional polymer with optimized Br-/Zn afforded 99% conversion, 99% selectivity, and a TOF as high as 12,000 h-1 for the cycloaddition of CO2 and propylene oxide, which is among the most active solid catalysts ever reported. Furthermore, the bifunctional polymer could efficiently catalyze the cycloaddition of epichlorohydrin with dilute CO2 (7.5% CO2 balanced by N2) even under ambient conditions, demonstrating its potential application in industrial-scale production.

Cationic Zn-Porphyrin Polymer Coated onto CNTs as a Cooperative Catalyst for the Synthesis of Cyclic Carbonates.

The development of solid catalysts containing multiple active sites that work cooperatively is very attractive for biomimetic catalysis. Herein, we report the synthesis of bifunctional catalysts by supporting cationic porphyrin-based polymers on carbon nanotubes (CNTs) using the direct reaction of 5,10,15,20-tetrakis(4-pyridyl)porphyrin zinc(II), di(1H-imidazol-1-yl)methane, and 1,4-bis(bromomethyl)benzene in the presence of CNTs. The bifunctional catalysts could efficiently catalyze the cycloaddition reaction of epoxides and CO2 under solvent-free conditions with porphyrin zinc(II) as the Lewis acid site and a bromine anion as a nucleophilic agent working in a cooperative way. Furthermore, a relative amount of porphyrin zinc(II) and quaternary ammonium bromide could be facilely adjusted for facilitating cooperative behavior. The bifunctional catalyst with a TOF up to 2602 h-1 is much more active than the corresponding homogeneous counterpart and is one of the most active heterogeneous catalysts ever reported under cocatalyst-free conditions. The high activity is mainly attributed to the enhanced cooperation effect of the bifunctional catalyst. With a wide substrate scope, the bifunctional catalyst could be stably recycled. This work demonstrates a new approach for the generation of a cooperative activation effect for solid catalysts.

Ultrasmall Platinum Stabilized on Triphenylphosphine-Modified Silica for Chemoselective Hydrogenation.

Chemoselective hydrogenation of substrates with more than one functional group that could be hydrogenated is quite challenging and is of fundamental importance. Here, the enhanced chemoselectivity of ultrasmall (<1 nm) platinum nanoparticles (NPs) stabilized on silica modified with triphenylphosphine (PSiO2 ) in hydrogenation reactions is reported. Platinum NPs on PSiO2 exhibit much higher selectivity than those on SiO2 in the hydrogenation of acetophenone to 1-phenylethanol (99.9 versus 36 %) and hydrogenation of phenylacetylene to styrene (85 versus 52 %). The results of NMR spectroscopy, X-ray photoelectron spectroscopy, and in situ CO adsorption FTIR spectroscopy indicate the existence of strong interactions between triphenylphosphine and Pt NPs. Consequently, Pt NPs on PSiO2 have smaller particle sizes and more positive Pt 4 f binding energy than those of Pt/SiO2 ; these are the main contributors to the superior chemoselectivity of Pt NPs. The organically modified silica could act as an efficient solid ligand for tuning the catalytic performance of metal NPs.

Cocatalyst-Free Hybrid Ionic Liquid (IL)-Based Porous Materials for Efficient Synthesis of Cyclic Carbonates through a Cooperative Activation Pathway.

Cocatalyst-free ionic liquid (IL)-based porous polymers (Px -Vy -OHz R) functionalized with an intermolecular hydroxyl group were prepared by means of radical copolymerization of 1-butyl-3-vinylimidazolium bromide, (4-vinylphenyl)methanol (VBzOH), and divinylbenzene (DVB) under solvothermal conditions. As the ratio of 4-vinylphenylmethanol in the initial mixture increased, the content of the hydroxyl groups in the polymer increased from 3.35 to 5.35 mmol g-1 and the Brunauer-Emmett-Teller (BET) surface area of the polymer decreased sharply from 365 to 2.5 m2 g-1 . In the carbonation of CO2 and epoxides, the turnover frequency (TOF) of Px -Vy -OHz R increased gradually from 25 to 67 h-1 as the OH ratio increased irrespective of the sharp decrease in BET surface area, which suggests the existence of a cooperative activation effect between OH and ILs. To obtain a high OH content while still maintaining a high BET surface area, hybrid porous materials (SBA-[Vx OHy ]R-n) were prepared by means of copolymerization of 1-ethyl-3-vinylimidazolium bromide and 4-vinylphenylmethanol in the mesopores of SBA-15. SBA-[Vx OHy ]R-n was more active than its polymer counterpart (TOF: 188 versus 71 h-1 ) in the cycloaddition of CO2 with propyl oxide owing to the combined effect of the high BET surface area and the high OH content. The hybridization of mesoporous materials with polymers represents an efficient strategy for the preparation of high-performance solid catalysts for chemical transformations.