Catalysis via Sol–Gel Acid Silicas: An Important Chemical Technology for 2nd Generation Biorefineries

摘要: First described by Kato and co-workers in Japan 1990 further developed two years later Kresge’s team the United States, mesoporous silicas with ordered amorphous pore walls large specific surface areas are now a well-established class of functional materials numerous chemical applications. Since inception so named MCM-41 materials, synthesized via self-assembled liquid crystal mechanism involving silicon alkoxides sol–gel precursors ionic surfactants (which form hexagonally packed rod-shaped micelle structure), other members M41S family have been introduced including cubic, MCM-48, lamellar, MCM-50, forms. These molecular sieves generally obtained using long-chain alkyltrimethylammonium halides as structure-directing agents (SDA). The surfactant is eventually removed extraction (or calcination) to afford sizes from about 15 100 . In 1998, Stucky SBA-15 larger (50–300 ) made nonionic (triblock star diblock) copolymers templates. Almost concomitantly, Ozin reported discovery socalled periodic organosilicas (PMO), starting bridged organosilanes. Functionalization inner porosity these solids metal nanoparticles, organic molecules, base or acid groups, polymers, affords that can be used catalysts, sensors, controlled release well make innovative optical electronic devices. 1998 Jacobs preparation sulfonic acid-containing hexagonal silica (HMS), their use catalysts for condensation esterification reactions. Acid catalysis such significant area progress field was rapid Wilson Clark (in 2000), Davis 2002), were already able summarize achievements related silicasupported acids catalytic esterification, dehydration, acetalization, cyclization, etherification, aldol Sol–gel true sponges adsorbing concentrating reactants at surface, whose mesoporosity (pore diameter <20–100 favors diffusion broad variety molecules entrapped moieties resulting effective solid-acid liquid-phase. 2002 referred heterogeneous over an emerging green chemistry synthetic methodology applicative potential. For example, Kanabe Hclderich 1999 identified 127 petrochemical industrial processes (such alkylation, isomerization, amination, cracking, etc.) some 180 different solid (zeolites, oxides, complex phosphates, ion-exchange resins, clays, etc.). Applied synthesis liquid-phase would play role similar played zeolites gas phase petrochemistry. Yet, zeolite limited small (<15 often require high temperatures, which makes them poorly suitable reactions most substrates interest chemistry. Today number tunable strength commercially available scale, supplied either xerogel irregular microparticles spherically shaped microparticles, typically 45–70 mm few diameters 60–150 range. Canada’s SiliCycle commercializes silica-supported propyl silica-entrapped tosic acid, strong both under batch flow conditions; while Heterogeneously catalyzed selectively many valued products biomass components. versatile nanochemistry process allows shape catalytically attractive morphologies, functionalize active species affording nanocomposites consecutive one-pot. Using selected examples recent research, we identify approach will likely produce enhanced performance, practical application.