Juan Carlos Colmenares

Nanostructured Photocatalysts and Their Applications in the Photocatalytic Transformation of Lignocellulosic Biomass: An Overview

Heterogeneous photocatalysis offer many possibilities for finding appropiate environmentally friendly solutions for many of the the problems affecting our society (i.e., energy issues). Researchers are still looking for novel routes to prepare solid photocatalysts able to transform solar into chemical energy more efficiently. In many developing countries, biomass is a major energy source, but currently such countries lack of the technology to sustainably obtain chemicals and/or fuels from it. The Roadmap for Biomass Technologies, authored by 26 leading experts from academia, industry, and government agencies, has predicted a gradual shift back to a carbohydrate-based economy. Biomass and biofuels appear to hold the key to satisfy the basic needs of our societies for the sustainable production of liquid fuels and high value-added chemicals without compromising the scenario of future generations. In this review, we aim to discuss various design routes for nanostructured photocatalytic solid materials in view of their applications in the selective transformation of lignocellulosic biomass to high value-added chemicals.

A sustainable approach for lignin valorization by heterogeneous photocatalysis

The depletion of the Earths fossil fuel reserves and the rapid increase in the emission of greenhouse gases and other environmental pollutants are driving the development of renewable energy technologies. Lignin is one of the three main subcomponents of lignocellulosic biomass in terrestrial ecosystems and makes up nearly 30% of the organic carbon sequestered in the biosphere. As a result of its rich content of aromatic carbon, lignin has the potential to be decomposed to yield valuable chemicals and alternatives to fossil fuels. However, the complex and stable chemical bonds of lignin make the depolymerization of lignin a difficult challenge with regard to its valorization. In this review, we highlight recent advances in the selective decomposition of lignin-based compounds via photocatalysis into other value-added chemicals and the treatment of waste water containing lignin. The photocatalytic transformation of lignin under mild conditions is particularly promising.

High-value chemicals obtained from selective photo-oxidation of glucose in the presence of nanostructured titanium photocatalysts.

Glucose was oxidized in the presence of powdered TiO(2) photocatalysts synthesized by an ultrasound-promoted sol-gel method. The catalysts were more selective towards glucaric acid, gluconic acid and arabitol (total selectivity approx. 70%) than the most popular photocatalyst, Degussa P-25. The photocatalytic systems worked at mild reaction conditions: 30°C, atmospheric pressure and very short reaction time (e.g. 5 min). Such relatively good selectivity towards high-valued molecules are attributed to the physico-chemical properties (e.g. high specific surface area, nanostructured anatase phase, and visible light absorption) of novel TiO(2) materials and the reaction conditions. The TiO(2) photocatalysts have potential for water purification and energy production and for use in the pharmaceutical, food, perfume and fuel industries.

Efficient and simple reactive milling preparation of photocatalytically active porous ZnO nanostructures using biomass derived polysaccharides

ZnO nanocrystals of different shapes and sizes have been synthesized using an innovative, simple and efficient dry reactive milling methodology using Zn(NO3)2 and various polysaccharides as sacrificial templates. Optimum results were achieved using extracted agar from the red seaweed Gracilaria gracilis. Upon template removal after calcination at 600 °C, the protocol gave rise to a range of porous metal oxide nanomaterials of different shapes and nanoparticle sizes which were found to have excellent photocatalytic properties in aqueous phenol degradation as compared to commercial P25 Evonik titania.

Sonophotodeposition of bimetallic photocatalysts Pd-Au/TiO2 : application to selective oxidation of methanol to methyl formate.

The aim of this work is to develop bimetallic Pd-Au/TiO2 P90 systems, which are highly active and selective for the photocatalytic oxidation of methanol to form methyl formate. Modification of commercial TiO2 P90 with Pd-Au nanoparticles was successfully achieved for the first time by means of a sonophotodeposition (SPD) method. The prepared materials were characterized by TEM, UV/Vis spectroscopy, X-ray photoelectron spectroscopy, and powder XRD. The Pd-Au bimetallic nanoparticles supported on titania exhibited remarkably enhanced catalytic activity in selective methanol oxidation to form methyl formate due to the synergism of Au and Pd particles, as well as the strong interaction between TiO2 and Pd-Au. SPD is a green methodology that can be used to prepare well-defined bimetallic surfaces on semiconductor supports with great promise for catalytic applications, in which selectivity can be tuned through adjustment of the surface composition.

Sustainable hybrid photocatalysts: titania immobilized on carbon materials derived from renewable and biodegradable resources

This review comprises the preparation, properties and heterogeneous photocatalytic applications of TiO2 immobilized on carbon materials derived from earth-abundant, renewable and biodegradable agricultural residues and sea food waste resources. The overview provides key scientific insights into widely used TiO2 supported on carbonaceous materials emanating from biopolymeric materials such as lignin, cellulose, cellulose acetate, bacterial cellulose, bamboo, wood, starch, chitosan and agricultural residues (biochar, charcoal, activated carbon and their magnetic forms, coal fly ash) or seafood wastes namely eggshell, clamshell and fish scales; materials that serve as a support/template for TiO2. Heightened awareness and future inspirational developments for the valorisation of various forms of carbonaceous functional materials is the main objective. This appraisal abridges various strategies available to upgrade renewable carbon-based feedstock via the generation of sustainable TiO2/carbon functional materials and provides remarks on their future prospects. Hopefully, this will stimulate the development of efficient and novel composite photocatalysts and engender the necessary knowledge base for further advancements in greener photocatalytic technologies.

Sonication‐Assisted Low‐Temperature Routes for the Synthesis of Supported Fe–TiO2 Econanomaterials: Partial Photooxidation of Glucose and Phenol Aqueous Degradation

Fe‐doped TiO2‐supported photocatalytic materials prepared by the use of a mild ultrasound‐assisted protocol have been found to possess excellent selectivities in glucose oxidation and total phenol mineralization. These materials were characterized by a number of techniques such as XRD, diffuse reflectance UV/Vis spectroscopy, N2 physisorption, X‐ray photoelectron spectroscopy, SEM with X‐ray microanalysis, HRTEM, and flame atomic absorption spectroscopy. The proposed synthesis method was found to have a significant effect on textural properties, morphology, and visible‐light responsiveness of nanophotocatalysts. The dopant agent was found to be in the form of Fe3+ ions. The effects of the textural properties, together with the Fe presence and the adsorption ability of the material (acidic properties), seem to be involved in the optimum photocatalytic activities found for Fe–TiO2 supported on zeolite. In phenol photodegradation, although Fe doping demonstrated the detrimental effect on the conversion rate, an improvement in the direct mineralization of phenol to CO2 and water, particularly for the SiO2‐supported photocatalyst, with a negligible content of toxic byproducts in water has been observed in comparison with commercially available Evonik P25.

A novel biomass-based support (Starbon) for TiO2 hybrid photocatalysts: a versatile green tool for water purification

The average size of the TiO2 nanoparticles was controlled at around 30 nm on Starbon 800 without using any surfactant, which is attributed to the hydrolysis and condensation (it is also proposed to be a plausible condensation with the carboxylate groups on the carbon surface) of the dissolved titanium(IV) isopropoxide into TiO2 by ultrasonic waves. As the Starbon in the composites has very good contact with the TiO2 nanoparticles (there is no Ti leaching after 240 min of photocatalysis, XRF analysis) it enhances the photo-electron conversion (better than Norit and graphene oxide carbons) of TiO2 by reducing the recombination of photo-generated electron–hole pairs. We have prepared an excellent new hybrid photocatalyst for aqueous phase phenol total mineralization.

Carbonaceous residues from biomass gasification as catalysts for biodiesel production

Abstract Tars and alkali ashes from biomass gasification processes currently constitute one of the major problems in biomass valorisation, generating clogging of filters and issues related with the purity of syngas production. To date, these waste residues find no useful applications and they are generally disposed upon generation in the gasification process. A detailed analysis of these residues pointed out the presence of high quantities of Ca (>30 wt%). TG experiments indicated that a treatment under air at moderate temperatures (400–800°C) decomposed the majority of carbon species, while XRD indicated the presence of a crystalline CaO phase. CaO enriched valorized materials turned out to be good heterogeneous catalysts for biodiesel production from vegetable oils, providing moderate to good activities (50%–70% after 12 h) to fatty acid methyl esters in the transesterification of sunflower oil with methanol.

Mechanochemical Synthesis of TiO2 Nanocomposites as Photocatalysts for Benzyl Alcohol Photo-Oxidation

TiO2 (anatase phase) has excellent photocatalytic performance and different methods have been reported to overcome its main limitation of high band gap energy. In this work, TiO2-magnetically-separable nanocomposites (MAGSNC) photocatalysts with different TiO2 loading were synthesized using a simple one-pot mechanochemical method. Photocatalysts were characterized by a number of techniques and their photocatalytic activity was tested in the selective oxidation of benzyl alcohol to benzaldehyde. Extension of light absorption into the visible region was achieved upon titania incorporation. Results indicated that the photocatalytic activity increased with TiO2 loading on the catalysts, with moderate conversion (20%) at high benzaldehyde selectivity (84%) achieved for 5% TiO2-MAGSNC. These findings pointed out a potential strategy for the valorization of lignocellulosic-based biomass under visible light irradiation using designer photocatalytic nanomaterials.