Quang Thang Trinh

Size-Dependent Catalytic Activity of Palladium Nanoparticles Fabricated in Porous Organic Polymers for Alkene Hydrogenation at Room Temperature

Ultrafine palladium nanoparticles (Pd NPs) with 8 and 3 nm sizes were effectively fabricated in triazine functionalized porous organic polymer (POP) TRIA that was developed by nonaqueous polymerization of 2,4,6-triallyoxy-1,3,5-triazine. The Pd NPs encapsulated POP (Pd-POP) was fully characterized using several techniques. Further studies revealed an excellent capability of Pd-POP for catalytic transfer hydrogenation of alkenes at room temperature with superior catalytic performance and high selectivity of desired products. Highly flammable H2 gas balloon at high pressure and temperature used in conventional hydrogenation reactions was not needed in the present synthetic system. Catalytic activity is strongly dependent on the size of encapsulated Pd NPs in the POP. The Pd-POP catalyst with Pd NPs of 8 nm in diameter exhibited higher catalytic activity for alkene hydrogenation as compared with the Pd-POP catalyst encapsulating 3 nm Pd NPs. Computational studies were undertaken to gain insights into different catalytic activities of these two Pd-POP catalysts. High reusability and stability as well as no Pd leaching of these Pd-POP catalysts make them highly applicable for hydrogenation reactions at room temperature.

Biomass Oxidation: Formyl CH Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves†

An integrated experimental and computational investigation reveals that surface lattice oxygen of copper oxide (CuO) nanoleaves activates the formyl C-H bond in glucose and incorporates itself into the glucose molecule to oxidize it to gluconic acid. The reduced CuO catalyst regains its structure, morphology, and activity upon reoxidation. The activity of lattice oxygen is shown to be superior to that of the chemisorbed oxygen on the metal surface and the hydrogen abstraction ability of the catalyst is correlated with the adsorption energy. Based on the present investigation, it is suggested that surface lattice oxygen is critical for the oxidation of glucose to gluconic acid, without further breaking down the glucose molecule into smaller fragments, because of C-C cleavage. Using CuO nanoleaves as catalyst, an excellent yield of gluconic acid is also obtained for the direct oxidation of cellobiose and polymeric cellulose, as biomass substrates.

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

The activation of methane by transition metal/metal oxide catalysts is pertinent for developing/optimizing processes which help to convert this abundantly available resource to value-added chemicals. First principles calculations reveal that the under-coordinated lattice Cu–O pair on different CuO surfaces synergistically activates methane with barriers as low as 60.5 kJ mol−1 on the high-energy CuO(010) surface and 76.6 kJ mol−1 on the most stable CuO(111) surface. The significantly low activation barrier is due to (1) the stabilization of the transition state (TS) and the reduced strain on the dissociating methane molecule and (2) the stabilization of the co-adsorbed products of dissociation, resulting in favorable thermodynamics. The mechanism, which is also applicable to the chemisorbed oxygen-containing Cu(111) surface, involves simultaneous copper addition and hydrogen abstraction by the chemisorbed/lattice oxygen via a 4-centered (CH3-(Cu)–H-(O)) TS, stabilized by the Cu–CH3 and O–H dipole–dipole interaction. The activation barriers for the subsequent dissociation of surface CH3 moieties and coupling of CH3 with CH2 on the CuO(111) surface are both much higher than the barrier of the first C–H bond dissociation in methane. The mechanistic insights elucidated in this article could be applicable to methane activation by other metal–oxygen (M–O) site-pairs and thus can serve to screen potential oxide surfaces for the purpose.

Selective and Catalyst-free Oxidation of D-Glucose to D-Glucuronic acid induced by High-Frequency Ultrasound

This systematic experimental investigation reveals that high-frequency ultrasound irradiation (550 kHz) induced oxidation of D-glucose to glucuronic acid in excellent yield without assistance of any (bio)catalyst. Oxidation is induced thanks to the in situ production of radical species in water. Experiments show that the dissolved gases play an important role in governing the nature of generated radical species and thus the selectivity for glucuronic acid. Importantly, this process yields glucuronic acid instead of glucuronate salt typically obtained via conventional (bio)catalyst routes, which is of huge interest in respect of downstream processing. Investigations using disaccharides revealed that radicals generated by high frequency ultrasound were also capable of promoting tandem hydrolysis/oxidation reactions.

Mechanistic insights into the catalytic elimination of tar and the promotional effect of boron on it: first-principles study using toluene as a model compound

Tar is the undesired viscous black liquid produced during the gasification of biomass. Catalytic elimination of tar is feasible and is economically promising. In the present investigation, we performed first-principles calculations (i) to elucidate the decomposition mechanism of tar on the popular nickel (Ni) catalyst and (ii) to reveal the promotional effect of boron (B) in improving the activity and stability of the Ni catalyst for tar decomposition. Being the most abundant component of tar, toluene was chosen as a model compound. On the Ni(111) surface, toluene adsorbs strongly in a bridge configuration and the activation barrier for methyl C–H dissociation is 72 kJ mol−1. Toluene can further decompose on the Ni(111) surface via stepwise dehydrogenation of the methyl group. The aromatic C–H bond at the ortho position could only be activated after the complete dehydrogenation of the methyl group, which is followed by subsequent ring opening (activation barriers are 112 and 84 kJ mol−1, respectively) and C–C cleavages, to generate smaller hydrocarbons. The incorporation of subsurface B into the Ni catalyst (B–Ni) results in a corrugated Ni top surface and toluene adsorbs more strongly by 11 kJ mol−1 on the B–Ni catalyst than on pure Ni. Although the mechanism of toluene decomposition remains unchanged after doping with boron, the decomposition of toluene is significantly promoted on B–Ni. The activation barrier for the first methyl C–H dissociation on B–Ni is reduced to 51 kJ mol−1. Subsequent methyl C–H activations are also promoted on the B–Ni catalyst, but to a smaller extent. The aromatic C–H activation is also strongly promoted (89 kJ mol−1vs. 112 kJ mol−1 on pure Ni). Additional calculations on stepped surface models of Ni show that the activation barriers of toluene decomposition on the B–Ni surface are very close to those on B5 and F4 step sites of pure Ni, thus suggesting that the promotional effect of B on the catalytic activity of Ni could be mainly attributed to the creation of step-like corrugations on the Ni surface. These corrugations could also inhibit the formation of graphene-like structures on B–Ni.

Integrated Experimental and Theoretical Study of Shape-Controlled Catalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

We have synthesized CuO nanostructures with flake, dandelion-microsphere, and short-ribbon shapes using solution-phase methods and have evaluated their structure–performance relationship in the heterogeneous catalysis of liquid-phase oxidative coupling reactions. The formation of nanostructures and the morphological evolution were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis, and Fourier transform infrared spectroscopy. CuO nanostructures with different morphologies were tested for the catalytic oxidative coupling of aromatic amines to imines under solvent-free conditions. We found that the flake-shaped CuO nanostructures exhibited superior catalytic efficiency compared to that of the dandelion- and short-ribbon-shaped CuO nanostructures. We also performed extensive density functional theory (DFT) calculations to gain atomic-level insight into the intriguing reactivity trends observed for the different CuO nanostructures. Our DFT calculations provided for the first time a detailed and comprehensive view of the oxidative coupling reaction of benzylamine over CuO, which yields N-benzylidene-1-phenylmethanamine as the major product. CuO(111) is identified as the reactive surface; the specific arrangement of coordinatively unsaturated Cu and O sites on the most stable CuO(111) surface allows N–H and C–H bond-activation reactions to proceed with low-energy barriers. The high catalytic activity of the flake-shaped CuO nanostructure can be attributed to the greatest exposure of the active CuO(111) facets. Our finding sheds light on the prospective utility of inexpensive CuO nanostructured catalysts with different morphologies in performing solvent-free oxidative coupling of aromatic amines to obtain biologically and pharmaceutically important imine derivatives with high selectivity.

Unraveling the mechanism of the oxidation of glycerol to dicarboxylic acids over a sonochemically synthesized copper oxide catalyst

The utilization of low frequency ultrasound (US) offers a straightforward and powerful tool for the production of nanostructured materials, in particular for structurally stable, highly crystalline, and shape-controlled catalytic materials. Herein, we report an unconventional strategy for the synthesis of CuO nanoleaves within 5 min of US irradiation. The as-obtained CuO nanoleaves were found to be selective in the base-free aqueous oxidation of glycerol to dicarboxylic acids (78% yield of tartronic and oxalic acids), in the presence of hydrogen peroxide (H2O2) and under mild reaction conditions. Density Functional Theory (DFT) investigations revealed a synergy between the CuO catalyst and H2O2 in maintaining the structural integrity of the catalyst during the reaction, creating alternative efficient pathways for the selective formation of dicarboxylic acids. Isotope labeling experiments using H218O2 further confirmed that oxygen from hydrogen peroxide, not from CuO, was preferentially incorporated into the dicarboxylic acid, significantly preserving the monoclinic structure of the CuO catalyst.