Metal-free Heterogeneous Catalyst Research Articles

Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride.

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10–5–4 × 10–3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (BN), vacancies (VB and VN), and Stone–Wales defects. SSNMR and binding-energy calculations show that VN are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.

Degradation of Bisphenol A using peroxymonosulfate activated by one-step prepared sulfur-doped carbon nitride as a metal-free heterogeneous catalyst

While sulfate radical-generating Advanced Oxidation Processes (AOPs) are promising techniques to degrade Bisphenol A (BPA), transition metals are typically required as catalysts to activate peroxymonosulfate (PMS) to generate sulfate radicals for BPA degradation. To reduce environmental impact and associated costs from metallic catalysts, metal-free catalysts for activating PMS are more attractive and should be further developed. Here, we propose to employ a one-step prepared sulfur-doped carbon nitride (CNS) as a non-metal and easy-to-prepare catalyst to activate PMS for BPA degradation. The as-prepared CNS exhibited a significantly higher surface area and catalytic activity for activating PMS than undoped carbon nitride (CN). Under visible light irradiation, BPA degradation extent and kinetics by CNS-activated PMS were also much higher than CN-activated PMS. The more effectiveness of CNS-activated PMS can be attributed to the synergy of sulfur and nitrogen co-doping, which might enable CNS to exhibit higher catalytic and photo-catalytic activities. Factors influencing BPA degradation were also examined, including temperature, pH, and co-existing ions. CNS-activated PMS was much more favorable to degrade BPA at elevated temperatures and under neutral conditions. The effect of concentrated NaCl did not significantly inhibit PMS activation by CNS for BPA degradation. The mechanism and pathway of BPA degradation by CNS-activated PMS were also proposed by evaluating effects of radical scavengers and degradation intermediates. CNS was also reusable to activate PMS for BPA degradation over multiple cycles without significant efficiency loss. These features demonstrate that CNS is a conveniently-prepared and promisingly effective non-metal catalyst to activate PMS for BPA degradation.

Efficient conversion of acids and esters to amides and transamidation of primary amides using OSU-6

OSU-6, an MCM-41 type hexagonal mesoporous silica with strong Bronsted acid properties, has been used to promote the high-yield conversion of carboxylic acids and esters to carboxamides as well as transamidations of primary amides in a one-pot solventless approach. A metal-free heterogeneous catalyst that promotes all of these processes has not been previously reported. OSU-6 enables these transformations to proceed in shorter times and at lower temperatures for a broad range of substrates. An added benefit is that the catalyst can be recycled and reused multiple times without significant loss of activity.

One-pot synthesized multi-functional graphene oxide as a water-tolerant and efficient metal-free heterogeneous catalyst for cycloaddition reaction

A series of functional graphene oxide (F-GO) materials were prepared by silylanization of graphene oxide (GO) with chlorine-terminal silanes and subsequent nucleophilic substitution reaction with tertiary amine in a one-pot approach. The quaternary ammonium salt was generated and immobilized simultaneously in situ, and the primary amine is beneficial to improve the number of surface functionalities on GO. This is the first time that a multi-functional GO material was designed, prepared and used for the synthesis of cyclic carbonates through the cycloaddition of carbon dioxide (CO2) to epoxides. The F-GO catalyst can be easily separated and reused for at least five times without significant loss of activity (TOF=46.4h−1). The excellent performance is attributed to the synergetic effect of silanol group and halide anion for ring opening of epoxide as well as the role of amine for CO2 adsorption and activation. A possible mechanism is proposed for the cycloaddition reaction over F-GO.

Sulfonic acid functionalized mesoporous SBA-15 as catalyst for styrene carbonate synthesis from CO2 and styrene oxide at moderate reaction conditions

A sulfonic acid tethered mesoporous silica material (SBA-15-SO3H) was successfully synthesized and characterized using SAXS, N2 physisorption studies, TEM, EA and XPS. This metal-free heterogeneous catalyst with terabutyl ammonium bromide (TBAB) co-catalyst was found to be an efficient catalyst system for styrene carbonate synthesis from CO2 and styrene oxide. The synergistic mechanistic pathways of SBA-15-SO3H with TBAB have been explained by hydrogen bond interactions and nucleophilic effects. A turn over number of 920 was obtained at mild reaction temperature of 80°C. The catalyst was thermally stable and reused for five times without loss of any significant activity.

Efficient fixation of CO2into cyclic carbonates catalysed by silicon-based main chain poly-imidazolium salts

A new method for the synthesis of main chain poly-imidazolium salts was developed from bisimidazole and silicon tetrachloride. These types of silicon-based poly-imidazolium salts are found to be among the most efficient metal-free heterogeneous catalysts (TOF = 90 h−1) for the fixation of CO2 with epoxides into cyclic carbonates under metal- and solvent-free conditions.

Metal-free oxidation of sulfides by carbon nitride with visible light illumination at room temperature

Mesoporous graphitic carbon nitride (mpg-C3N4) has been developed as a non-metal, heterogeneous photocatalyst for the selective oxidation of sulfides to sulfoxides with O2 at room temperature. Especially, the combination of mpg-C3N4 and aldehydes was a highly active system under visible-light irradiation. For example, mpg-C3N4/isobutyraldehyde catalytic oxidation of methyl phenyl sulfide afforded 97% conversion with 98% selectivity for the methyl phenyl sulfoxide in 4 h. Moreover, the mpg-C3N4 can be easily recovered by filtration and then reused at least four times without losing activity. By exploring the electron spin resonance and some comparative experiments, a catalytic mechanism of this oxidation was provided. Finally, the system also works well in the oxidation of a number of sulfides, including sulfides bearing various groups, and phenyl disulfide. The use of a metal-free heterogeneous catalyst and visible light energy, along with the mild reaction conditions makes this oxidation reaction an environmentally benign and energy-saving chemical process.

Supported sulfonic acids: Metal-free catalysts for the oxidation of hydroquinones to benzoquinones with hydrogen peroxide

Some silica- and polystyrene-supported sulfonic acids, prepared by tethering or sol–gel techniques, were employed as metal-free heterogeneous catalysts in the oxidation of hydroquinones to the corresponding 1,4-benzoquinones, carried out with 30% aqueous hydrogen peroxide. The activity of these catalysts was tested in the model oxidation of methylhydroquinone under batch conditions: high yield and selectivity at room temperature and under environmentally friendly conditions can be achieved in the presence of SiO 2–(CH 2) 3–SO 3H. The heterogeneous catalyst, that can be easily recovered by simply filtration, can be used for at least three cycles affording the same good results (∼80% yield, ∼91% selectivity).