Simultaneous Photocatalytic Production of H2 and Acetal from Ethanol with Quantum Efficiency over 73% by Protonated Poly(heptazine imide) under Visible Light

In this work, protonated poly(heptazine imide) (H-PHI) was obtained by adding acid to the suspension of potassium PHI (K-PHI) in ethanol. It was established that the obtained H-PHI demonstrates very high photocatalytic activity in the reaction of hydrogen formation from ethanol in the presence of Pt nanoparticles under visible light irradiation in comparison with K-PHI. This enhancement can be attributed to improved efficiency of photogenerated charge transfer to the photocatalyst's surface, where redox processes occur. Various factors influencing the system's activity were evaluated. Notably, it was discovered that the conditions of acid introduction into the system can significantly affect the size of Pt (cocatalyst metal) deposition on the H-PHI surface, thereby enhancing the photocatalytic system's stability in producing molecular hydrogen. It was established that the system can operate efficiently in the presence of air without additional components on the photocatalyst surface to block air access. Under optimal conditions, the apparent quantum yield of molecular hydrogen production at 410 nm is around 73%, the highest reported value for carbon nitride materials to date. The addition of acid not only increases the activity of the reduction part of the system but also leads to the formation of a value-added product from ethanol-1,1-diethoxyethane (acetal) with high selectivity.

Selective direct oxidation of 1-butanol into acetal using hydrogen peroxide and Cs5MPW11(H2O)O39 (M=Fe, Co, Cu) catalysts

Direct catalytic oxidation of alcohols to acetals in one step process is very attractive, because the two steps process leads firstly to aldehydes, which are unstable species serving as reactive intermediates to generate several by-products reducing process efficiency and selectivity. In this work, a new selective catalytic acetalization of 1-butanol into 1-1-dibutoxybutane acetal using H2O2 in one step process is investigated using Keggin-type polyoxometalates catalysts. The materials developed consisted of new lacunary phophotungstate salts Cs5MPW11(H2O)O39 (M=Fe, Co and Cu) which were prepared by inorganic solution condensation method and characterized using XRD, IR, SEM and EDX analysis to verify their structure, surface morphology and chemical composition. Cs5CuPW11(H2O)O39 catalyst allowed the highest performance for the oxidation of 1-butanol at 60 °C using H2O2 excess with a yield approaching 92 % and a turnover number of 784. Such activity is related to a bifunctional behavior of the catalyst as superacid and redox operating system and the synergistic effect created between the [PW11O39]7- Keggin framework, Cs+ and Cu2+.

Selective Photocatalytic Activation of Ethanol C-H and O-H Bonds over Multi-Au@SiO2/TiO2: Role of Catalyst Surface Structure and Reaction Kinetics

The chemical bond diversity and flexible reactivity of biomass-derived ethanol make it a vital feedstock for the production of value-added chemicals but result in low conversion selectivity. Herein, composite catalysts comprising SiO₂-coated single- or multiparticle Au cores hybridized with TiO₂ nanoparticles (mono- or multi-Au@SiO₂/TiO₂, respectively) were fabricated via electrostatic self-assembly. The C-H and O-H bonds of ethanol were selectively activated (by SiO₂ and TiO₂, respectively) under irradiation to form CH₃CH^•(OH) or CH₃CH₂O^• radicals, respectively. The formation and depletion kinetics of these radicals was analyzed by electron spin resonance to reveal marked differences between mono- and multi-Au@SiO₂/TiO₂. Consequently, the selectivity of these catalysts for 1,1-diethoxyethane after 6 h irradiation was determined as 81 and 99%, respectively, which was attributed to the more pronounced effect of localized surface plasmon resonance for multi-Au@SiO₂/TiO₂. Notably, only acetaldehyde was formed on a Au/TiO₂ catalyst without a SiO₂ shell. Fourier transform infrared (FTIR) spectroscopy indicated that the C-H adsorption of ethanol was enhanced in the case of multi-Au@SiO₂/TiO₂, while NH₃ temperature-programmed desorption and pyridine adsorption FTIR spectroscopy revealed that multi-Au@SiO₂/TiO₂ exhibited enhanced surface acidity. Collectively, the results of experimental and theoretical analyses indicated that the adsorption of acetaldehyde on multi-Au@SiO₂/TiO₂ was stronger than that on Au/TiO₂, which resulted in the oxidative coupling of ethanol to afford 1,1-diethoxyethane on the former and the dehydrogenation of ethanol to acetaldehyde on the latter.

Upgrading of Ethanol to 1,1-Diethoxyethane by Proton-Exchange Membrane Electrolysis

The direct acetalization of ethanol is a significant challenge for upgrading bioethanol to value-added chemicals. In this study, 1,1-diethoxyethane (DEE) is selectively synthesized by the electrolysis of ethanol using a proton-exchange membrane (PEM) reactor. In the PEM reactor, a Pt/C catalyst promoted the electro-oxidation of ethanol to acetaldehyde. The Nafion membrane used as the PEM served as a solid acid catalyst for the acetalization of ethanol and electrochemically formed acetaldehyde. DEE was obtained at high faradaic efficiency (78 %) through sequential electrochemical and nonelectrochemical reactions. The DEE formation rate through PEM electrolysis was higher than that of reported systems. At the cathode, protons extracted from ethanol were reduced to H₂ . The electrochemical approach can be utilized as a sustainable process for upgrading bioethanol to chemicals because it can use renewable electricity and does not require chemical reagents (e. g., oxidants and electrolytes).

Selective Chloride-Mediated Neat Ethanol Oxidation to 1,1-Diethoxyethane via an Electrochemically Generated Ethyl Hypochlorite Intermediate

Selective primary alcohol oxidation to form aldehydes products without overoxidation to carboxylic acids remains a key chemistry challenge. Using simple alkylammonium chloride as the electrolyte with a glassy carbon working electrode in neat ethanol solvent, 1,1-diethoxyethane (DEE) was prepared with >95% faradaic efficiency (FE). DEE serves as a storage platform protecting acetaldehyde from overoxidation and volatilization. UV-vis spectroscopy shows that the reaction proceeds through an ethyl hypochlorite intermediate as the sole chloride oxidation product, and that this intermediate decomposes unimolecularly (rate constant k = (6.896 ± 0.516) × 10^-4 s^-1) to form HCl catalyst and acetaldehyde, which undergoes rapid nucleophilic attack by ethanol solvent to form the DEE product. This indirect oxidation mechanism enables ethanol oxidation at much less positive potentials due to the fast kinetics for chloride anion oxidation.