Photocatalyzed Transformation of Free Carbohydrates

Abstract

In the growing context of sustainable chemistry, one of the challenges of organic chemists is to develop efficient and environmentally friendly methods for the synthesis of high-added-value products. Heterogeneous photocatalytic transformations have brought revolution in this regard, as they take advantage of an unlimited source of energy (solar light) or artificial UV light to onset organic chemical modifications. The abundance of free carbohydrates as chemical platform feedstock offers a great opportunity to obtain a variety of industrial interest compounds from biomass. Due to their chirality and polyfunctionality, the conversion of sugars generally requires multi-step protocols with protection/deprotection steps and hazardous chemical needs. In this context, several selective and eco-friendly methodologies are currently under development. This review presents a state of art of the recent accomplishments concerning the use of photocatalysts for the transformation and valorization of free carbohydrates. It discusses the approaches leading to the selective oxidation of free sugars, their degradation into organic chemicals, or their use for hydrogen production.

Photocatalytic Cellulose Reforming for H2 and Formate Production by Using Graphene Oxide-Dot Catalysts

“…d -Glucose units of cellulose are peeled piece-by-piece off the cellulose chain in alkaline solutions and the peeled units subsequently transform into deprotonated isosaccharinic acid (C 6 ) for further downgrading. The downgrading of a C x H y O z – molecule (C x ) to C x –1 H y –2 O z –1 – (C x –1 ) consists of two steps: first, the C x is deprived of a formate (HCOO – , C 1 ) molecule to form a reduced-state C x –1 H y –1 O z –2 (r-C x –1 ) through hydrolysis, − and then the r-C x –1 is photocatalytically oxidized using photogenerated holes to form C x –1 . ,, H 2 production through water reduction by the photogenerated electrons is accompanied and triggered by the photocatalytic oxidation of r-C x –1 . Without illumination (i.e., in darkness), dissolved O 2 in the solution may oxidize r-C x –1 to form C x –1 .…”

“…The efficiency of the photocatalytic reforming of cellulose is governed by the kinetics of the charge transfer at the catalyst–solution interface . In an alkaline solution, such reforming requires a photocatalyst capable of generating holes to oxidize cellulose and electrons to reduce H 2 O or cellulose derivatives for H 2 evolution under photoexcitation.…”

Gold-Catalyzed Oxidative Transformation of Free Sugars into Biobased Platform Molecules

“…Oxygen was bubbled for 10 min, and then the medium was stirred at room temperature under an O 2 atmosphere for 2 h. The catalyst was then filtered through a nylon membrane (0.25 μm) and washed with MeOH. This fraction was evaporated and dried overnight under vacuum and then weighed and characterized by 1 H and 13 C NMR in DMSO-d 6 . Because the proton spectrum was unfortunately not exploitable (except for 2 experiments performed with galactose and arabinose), all calculations were based on the spectra obtained via a quantitative 13 C NMR sequence.…”

Photocatalytic Hydrogen Production from Aqueous Solutions of Glucose and Xylose over Layered Perovskite-like Oxides HCa2Nb3O10, H2La2Ti3O10 and Their Inorganic-Organic Derivatives

“…Nevertheless, direct combustion of the plant biomass or its processing products does not appear to be sufficiently expedient because of their relatively low calorific value [ 1 ] and the release of carbon dioxide. With that said, there is particular interest in the biomass reforming to obtain more energy-intensive and environmentally friendly fuels such as hydrogen [ 2 , 3 ].…”