Engineering of Active Sites in Metal–Organic Frameworks for Biodiesel Production

Abstract

Despite the high biodiesel yields, the use of such harsh conditions requires a pure source of triglycerides, since both the presence of water or FFAs decreases the yields. This is due, on the one hand to the dilution or neutralization of the base in the aqueous phase, and on the other hand, to the formation of soaps resulting from the saponification of the FFAs. The generation of these byproducts/emulsions consumes the Brönsted base and complicates the isolation of the desired fatty acid methyl esters (FAMEs).When the feedstock is rich in FFA, which can acidify the final biodiesel product, the direct esterification of such compounds with light alcohols is preferred (Scheme 1b). This is the case of food and/or biomass process plants, where it is necessary to perform the esterification in addition to the transesterification of the oil. [7] For this particular process, mineral or organic acids serve as homogeneous catalysts (e.g., HCl, p-toluenesulfonic acid, etc.), but suffer from manipulation problems and contribute to the corrosion of reactors, as well as needing further neutralization steps. An environmentally benign alternative for this process, as well as for the transesterification of triglycerides is to use solid catalysts. Solid heterogeneous catalysts are sustainable alternatives to soluble catalysts, either as mineral, organic (chemo-catalysts), or biologic (enzymatic catalysts). In fact, a solid material does not require additional neutralization and complicated isolation steps, since a simple filtration allows the recovery and recycling of the heterogeneous catalyst. [8] The use of synthetic heterogeneous catalysts offers better stability under broader operational conditions and often lower cost with respect to many homogeneous or enzymatic catalysts, despite the high efficiency of biocatalysis methods. [9,10] Among the traditional solid catalysts, porous solids with windows and cavities wide enough (i.e., at the nanoscale), and/or with high external surface area for the facilitated diffusion of the fats, oil feedstock and biodiesel product are alternatives to amorphous low surface area carbon or polymers. For example, clays, [11] zeolites, [12,13] and mesoporous silicas [12,[14][15][16] have been employed as highly robust solids with well-defined active sites, e.g., Lewis and/or Brönsted acid species, for the production of biodiesel. [17] During the last years, metal-organic frameworks (MOFs) appeared as interesting class of ordered porous materials that combine inorganic-organic functionalities, as well as large and often hierarchical void spaces. [18,19] These porous frameworks are ready to introduce fine-tuned active sites for the valorization of complex molecules from biomass feedstock. [20][21][22][23] The production of biodiesel using solid catalysts is a sustainable methodology to renewable fuels from the valorization of biomass derived oils and fats. Here, the use of tailorable multifunctional metal-organic frameworks (MOFs) as heterogeneous catalysts for esterification/transesterification reac...

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

“…In addition to the introduced strategies for possible (photo)catalytic sites in MOFs, the microenvironments surrounding the catalytic sites and their performance can be tuned by defectengineering, 367 engineering the pore diameters, and changing the pore diameter, morphology and particle size of MOFs. 75,368 Engineering catalytic sites via the creation of defects in crystalline MOFs to obtain defective MOFs, also known as defect engineering, is a strategy to increase the catalytically active sites, porosity, mass transport pathways, and selectivity of MOF materials. Indeed, defect engineering offers an alternative procedure to post-synthetically modify MOFs with various organic/inorganic species and tune their photo(catalytic) activity, 83,329,369,370 besides the above-mentioned linker and node functionalization.…”

Advances in Upgrading Biomass to Biofuels and Oxygenated Fuel Additives Using Metal Oxide Catalysts

“…The synergism of the Brønsted–Lewis acid sites and porous structure of the MOFs facilitates the participation of active sites via carbonyl group activation. The structure–activity relationship and properties of MOFs have raised interest among researchers for biodiesel production …”

Texture Regulation of Metal–Organic Frameworks, Microwave Absorption Mechanism‐Oriented Structural Optimization and Design Perspectives

“…[ 11 ] Generally, the metal ions and ligands determine the chemical composition of the final product. For example, the common zeolitic imidazolate framework (ZIF) materials typically take Zn or Co as the central ions, [ 12 , 13 , 14 , 15 , 16 , 17 ] while the Universitetet i Oslo (UiO), [ 18 , 19 ] Prussian blue (PB) [ 20 , 21 ] and Material Institute Lavoisier (MIL) [ 22 , 23 ] MOFs generally contain Zr, [ 24 ] Fe, [ 25 ] and Cr, [ 26 ] respectively. The topology of MOFs (1D, [ 27 , 28 ] 2D, [ 29 , 30 , 31 ] and 3D, [ 32 , 33 , 34 ] ) can not only be affected by the type of MOF, but also chemical or physical factors, such as solvent, [ 35 ] surfactant, [ 36 ] and temperature.…”