Copper(II)-Substituted Polyoxotungstates Immobilized on Amine-Functionalized SBA-15: Efficient Heterogeneous Catalysts for Liquid Phase Oxidative Reaction

SBA-15 Anchored Metal Containing Catalysts in the Oxidative Desulfurization Process

Recalcitrant sulfur compounds are common impurities in crude oil. During combustion they produce SOx derivatives that are able to affect the atmospheric ozone layer, increasing the formation of acid rains, and reducing the life of the engine due to corrosion. In the last twenty years, many efforts have been devoted to develop conventional hydrodesulfurization (HDS) procedures, as well as alternative methods, such as selective adsorption, bio-desulfurization, oxidative desulfurization (ODS) under extractive conditions (ECODS), and others. Among them, the oxidative procedures have been usually accomplished by the use of toxic stoichiometric oxidants, namely potassium permanganate, sodium bromate and carboxylic and sulfonic peracids. As an alternative, increasing interest is devoted to selective and economical procedures based upon catalytic methods. Heterogeneous catalysis is of relevance in industrial ODS processes, since it reduces the leaching of active species and favors the recovery and reuse of the catalyst for successive transformations. The heterogenization of different types of high-valent metal transition-based organometallic complexes, able to promote the activation of stoichiometric benign oxidants like peroxides, can be achieved using various solid supports. Many successful cases have been frequently associated with the use of mesoporous silicas that have the advantage of easy surface modification by reaction with organosilanes, facilitating the immobilization of homogeneous catalysts. In this manuscript the application of SBA-15 as efficient support for different active metal species, able to promote the catalytic ODS of either model or real fuels is reviewed, highlighting its beneficial properties such as high surface area, narrow pore size distribution and tunable pore diameter dimensions. Related to this topic, the most relevant advances recently published, will be discussed and critically described.

Catalytic depolymerization of lignin over cesium exchanged and transition-metal substituted heterogeneous polyoxometalates

It is of great importance to develop new pathways for low-cost, high-selectivity conversion of lignin into aromatics. In this work, a series of cesium exchanged and transition-metal substituted heterogeneous polyoxometalates were prepared and applied as efficient catalysts for lignin depolymerization. The catalytic oxidation of lignin has been monitored by GPC, 2D NMR HSQC, and GC/MS technologies, and the reaction pathway was also confirmed by the examination with a dimeric β-O-4 lignin model compound. Under optimal conditions, these catalysts showed high activity toward oxidative cleavage of β-O-4 linkages, as well as β-5 and β-β CC linkages. In particular, the Co substituted polyoxometalates gave high yield of ca. 9.58% monomeric products at 150°C within 3h under an oxygen atmosphere. Results demonstrated the catalyst was easily separated from products and could be repeatedly used at least five cycles without significant loss of activity. Further, the possible reaction mechanism was proposed by a two-step oxygen-based electron transfer and oxygen transfer reaction mechanism. The design and application of the multifunctional POMs based heterogeneous catalytic system make this strategy of great interest in the production of aromatic products from lignin and inspire a new insight to utilize the entire lignocellulosic biomass.

Current advances in the utility of functionalized SBA mesoporous silica for developing encapsulated nanocatalysts: state of the art

The cavities of SBA mesoporous silica materials can be used as nanoreactors for embedding catalytic species such as nanoparticles, complexes and heteropolyacids etc.