ABSTRACT
The use of metalloenzyme-like zeolites as Lewis acid catalysts for C-C bond formation reactions has received increasing attention over the past few years. In particular, the observation of direct aldol condensation reactions enabled by hydrophobic zeolites with isolated framework metal sites has encouraged the development of catalytic approaches for producing chemicals from biomass-derived compounds. The discovery of new Diels-Alder cycloaddition/dehydration routes and experimental and computational studies of Lewis acid catalyzed carbonyl-ene reactions have given a further boost to this rapidly evolving field.
ABSTRACT
Hf-, Sn-, and Zr-Beta zeolites catalyze the cross-aldol condensation of aromatic aldehydes with acetone under mild reaction conditions with near quantitative yields. NMR studies with isotopically labeled molecules confirm that acid-base pairs in the Si-O-M framework ensemble promote soft enolization through α-proton abstraction. The Lewis acidic zeolites maintain activity in the presence of water and, unlike traditional base catalysts, in acidic solutions.
Subject(s)
Acetone/chemistry , Aldehydes/chemistry , Ketones/chemical synthesis , Lewis Acids/chemistry , Zeolites/chemistry , Acid-Base Equilibrium , Catalysis , Ketones/chemistry , Molecular StructureABSTRACT
Hf-, Zr- and Sn-Beta zeolites effectively catalyze the coupled transfer hydrogenation and etherification of 5-(hydroxymethyl)furfural with primary and secondary alcohols into 2,5-bis(alkoxymethyl)furans, thus making it possible to generate renewable fuel additives without the use of external hydrogen sources or precious metals. Continuous flow experiments reveal nonuniform changes in the relative deactivation rates of the transfer hydrogenation and etherification reactions, which impact the observed product distribution over time. We found that the catalysts undergo a drastic deactivation for the etherification step while maintaining catalytic activity for the transfer hydrogenation step. (119) Sn and (29) Si magic angle spinning (MAS) NMR studies show that this deactivation can be attributed to changes in the local environment of the metal sites. Additional insights were gained by studying effects of various alcohols and water concentration on the catalytic reactivity.
Subject(s)
Ethers/chemistry , Furaldehyde/analogs & derivatives , Lewis Acids/chemistry , Zeolites/chemistry , Butanols/chemistry , Catalysis , Furaldehyde/chemistry , Hydrogenation , Water/chemistryABSTRACT
The catalytic valorization of cellulose is currently subject of intense research. Isosorbide is among the most interesting products that can be formed from cellulose as it is a potential platform molecule and can be used for the synthesis of a wide range of pharmaceuticals, chemicals, and polymers. A promising direct route from cellulose to isosorbide is presented in this work. The strategy relies on a one-pot bifunctional catalytic concept, combining heteropoly acids, viz. H(4)SiW(12)O(40), and redox catalysts, viz. commercial Ru on carbon, under H(2) pressure. Starting from pure microcrystalline cellulose, a rapid conversion was observed, resulting in over 50% isosorbide yield. The robustness of the developed system is evidenced by the conversion of a range of impure cellulose pulps obtained by organosolv fractionation, with isosorbide yields up to 63%. Results were compared with other (ligno)cellulose feedstocks, highlighting the importance of fractionation and purification to increase reactivity and convertibility of the cellulose feedstock.
Subject(s)
Acids, Noncarboxylic/chemistry , Carbon/chemistry , Cellulose/chemistry , Isosorbide/chemistry , Lignin/chemistry , Ruthenium/chemistry , Sorbitol/chemistryABSTRACT
Carbon nanofibers (CNFs) are a class of graphitic support materials with considerable potential for catalytic conversion of biomass. Earlier, we demonstrated the hydrolytic hydrogenation of cellulose over reshaped nickel particles attached at the tip of CNFs. The aim of this follow-up study was to find a relationship between the acid/metal balance of the Ni/CNFs and their performance in the catalytic conversion of cellulose. After oxidation and incipient wetness impregnation with Ni, the Ni/CNFs were characterized by various analytical methods. To prepare a selective Ni/CNF catalyst, the influences of the nature of oxidation agent, Ni activation, and Ni loading were investigated. Under the applied reaction conditions, the best result, that is, 76 % yield in hexitols with 69 % sorbitol selectivity at 93 % conversion of cellulose, was obtained on a 7.5 wt % Ni/CNF catalyst prepared by chemical vapor deposition of CH(4) on a Ni/γ-Al(2)O(3) catalyst, followed by oxidation in HNO(3) (twice for 1 h at 383 K), incipient wetness impregnation, and reduction at 773 K under H(2). This preparation method leads to a properly balanced Ni/CNF catalyst in terms of Ni dispersion and hydrogenation capacity on the one hand, and the number of acidic surface-oxygen groups responsible for the acid-catalyzed hydrolysis on the other.
Subject(s)
Carbon/chemistry , Cellulose/chemistry , Nanofibers/chemistry , Nickel/chemistry , Catalysis , Hydrogen-Ion Concentration , Hydrogenation , Hydrolysis , Oxidation-Reduction , Temperature , VolatilizationABSTRACT
Acid-catalyzed condensation of levulinic acid and phenol into high yields of diphenolic acid (>50%) is possible with a combination of sulfonated hyperbranched polymers and thiol promotors, either added as a physical mixture or bound to the polymer by ion-pairing.
Subject(s)
Hydroxybenzoates/chemical synthesis , Indoles/chemistry , Sulfhydryl Compounds/chemistry , Catalysis , Levulinic Acids/chemistry , Phenol/chemistryABSTRACT
The hydrolytic hydrogenation of cellulose in the presence of Ru-loaded zeolites and trace amounts of mineral acid shows excellent yields (>90%) for hexitols.
ABSTRACT
A combination of heteropolyacids and Ru on carbon catalyzes the conversion of concentrated cellulose feeds into hexitols under H(2) pressure. Quantitative conversion of ball-milled cellulose was observed with remarkable hexitol volume productivity.
Subject(s)
Acids/chemistry , Carbon/chemistry , Cellulose/chemistry , Ruthenium/chemistry , Sugar Alcohols/chemical synthesis , Catalysis , Hydrogen/chemistry , Sugar Alcohols/chemistryABSTRACT
Conversion of glycerol into high yields of 1,2-propanediol in absence of added hydrogen is possible with Pt impregnated NaY zeolite characterized by extra-zeolitic metal particles combined with zeolite Brønsted acidity.
