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1.
ChemSusChem ; 2020 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-32281748

RESUMO

Organosolv pulping releases reactive monomers from both lignin and hemicellulose by the cleavage of weak C-O bonds. These monomers recombine to form undesired polymers through the formation of recalcitrant C-C bonds. Different strategies have been developed to prevent this process by stabilizing the reactive monomers (i.e., lignin-first approaches). To date, all reported approaches rely on the addition of capping agents or metal-catalyzed stabilization reactions, which usually require high pressures of hydrogen gas. Herein, a metal- and additive-free approach is reported that uses zeolites as acid catalysts to convert the reactive monomers into more stable derivatives under organosolv pulping conditions. Experiments with model lignin compounds showed that the recondensation of aldehydes and allylic alcohols produced by the cleavage of ß-O-4' bonds was efficiently inhibited by the use of protonic ß zeolite. By applying a zeolite with a preferred pore size, the bimolecular reactions of reactive monomers were effectively inhibited, resulting in stable and valuable monophenolics. The developed methodology was further extended to birch wood to yield monophenolic compounds and value-added products from carbohydrates.

2.
Materials (Basel) ; 12(12)2019 Jun 17.
Artigo em Inglês | MEDLINE | ID: mdl-31213003

RESUMO

Different metalorganic lamellar hybrid materials based on associated nanoribbons were synthesized by the use of alkyl-benzyl monocarboxylate spacers, containing alkyl tails with variable lengths, which acted like structural growing inhibitors. These molecular agents were perpendicularly located and coordinated to aluminium nodes in the interlayer space, controlling the separation between individual structure sub-units. The hybrid materials were studied by X-ray diffraction (XRD), chemical and thermogravimetrical analysis (TGA), nuclear magnetic resonance (NMR) and infrared spectroscopy (IR), and field emission scanning electron microscopy (FESEM)/transmission electron microscopy (TEM), showing their physicochemical properties. The specific capacity of the metalorganic materials to be exfoliated through post-synthesis treatments, using several solvents due to the presence of 1D structure sub-units and a marked hydrophobic nature, was also evidenced.

3.
Chem Sci ; 10(7): 2053-2066, 2019 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-30842863

RESUMO

Novel MOF-type materials with different morphologies based on assembled 1D organic-inorganic sub-domains were prepared using specific monodentate benzylcarboxylate spacers with functional substituents in the para-position as structure modulating agents. The combination of electron-withdrawing or electron-donating functions in the organic spacers with suitable solvothermal synthesis conditions allowed modulating the structuration level (2D or 3D), vacancies, physico-chemical properties and Lewis acidity strength of the metal-organic structures. Furthermore, bimetallic (Al/Fe) MOF-type materials were synthesized by a one-pot direct process without modification of the structural framework. The activity of these hybrid materials as Lewis acid catalysts was evaluated to prepare cyanohydrins as precursors for the synthesis of biologically active compounds, and for aerobic oxidation of thiols to disulfides. The catalytic results showed that the derived MOFs exhibited modulatable Lewis acid capacities which are a function of the morphology, functionality of monodentate substituents present in the networks and a cooperative effect between metallic nodes of different nature.

4.
Dalton Trans ; 47(15): 5492-5502, 2018 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-29611585

RESUMO

Novel aluminium MOF-type materials structured by 1D subdomains, such as organic-inorganic nanoribbons, were synthesized by modifying the conditions of solvothermal synthesis and the nature of the solvents in the presence of aryl monocarboxylate linkers with long alkyl chains, which acted as growth-modulating agents. Specifically, three different families of materials were prepared with various morphological characteristics: (i) isoreticular MIL-53(Al)-type materials, (ii) mesoscopic metalorganic structures and (iii) lamellar aluminium MOFs. The length of the alkyl chain in the aryl linker and the hydrophobic/hydrophilic nature of the solvothermal synthesis media determined the structuration level that was achieved. The derived Al-MOFs are active and stable catalysts for the synthesis of fine chemicals. This was illustrated by the efficient synthesis of 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine.

5.
Catal Sci Technol ; 8(22): 5835-5847, 2018 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-30713656

RESUMO

A new chiral mesoporous hybrid material was synthesized based on pyrrolidine units included in a siliceous framework, HybPyr, and integrated into the organic-inorganic structure, from a specific bis-silylated precursor. A fluoride sol-gel methodology under soft synthesis conditions and in the absence of sophisticated structural directing agents allowed the generation of a mesoporous architecture with a homogeneous distribution of active chiral moieties along the network. The hybrid material was studied by means of different characterization techniques (TGA, NMR and IR spectroscopy, chemical and elemental analyses, TEM, and textural measurements), verifying the stability and integrity of the asymmetric active sites in the solid. The hybrid material, HybPyr, is an excellent asymmetric heterogeneous and recyclable catalyst for enantioselective Michael addition of linear aldehydes to ß-nitrostyrene derivatives with high stereocontrol of the reaction products.

6.
ChemSusChem ; 6(7): 1224-34, 2013 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-23754795

RESUMO

Glycerol carbonate esters (GCEs), which are valuable biomass-derivative compounds, have been prepared through the direct esterification of glycerol carbonate and long organic acids with different chain lengths, in the absence of solvent, and with heterogeneous catalysts, including acidic-organic resins, zeolites, and hybrid organic-inorganic acids. The best results, in terms of activity and selectivity towards GCEs, were obtained using a Nafion-silica composite. A full reaction scheme has been established, and it has been demonstrated that an undesired competing reaction results in the generation of glycerol and esters derived from a secondary hydrolysis of the endocyclic ester group, which is attributed to water formed during the esterification reaction. The influence of temperature, substrate ratio, catalyst-to-substrate ratio, and the use of solvent has been studied and, under optimized reaction conditions and with the adequate catalyst, it was possible to achieve 95% selectivity for the desired product at 98% conversion. It was demonstrated that the reaction rate decreased as the number of carbon atoms in the linear alkyl chain of the carboxylic acid increased for both p-toluenesulfonic acid and Nafion-silica nanocomposite (Nafion SAC-13) catalysts. After fitting the experimental data to a mechanistically based kinetic model, the reaction kinetic parameters for Nafion SAC-13 catalysis were determined and compared for reactions involving different carboxylic acids. A kinetic study showed that the reduced reactivity of carboxylic acids with increasing chain lengths could be explained by inductive as well as steric effects.


Assuntos
Carbonatos/química , Polímeros de Fluorcarboneto/química , Glicerol/química , Dióxido de Silício/química , Ácidos Carboxílicos/química , Catálise , Esterificação , Ésteres , Cinética , Água/química
7.
Chemistry ; 18(28): 8659-72, 2012 Jul 09.
Artigo em Inglês | MEDLINE | ID: mdl-22678926

RESUMO

A family of hybrid mesoporous materials with high temperature stability was obtained by the suitable covalent combination of two types of siloxane precursors. Specifically, cubic T(8) polyhedral oligomeric (POSS) and aryl bridged silsesquioxane monomers (1,4-bis(triethoxysilyl)benzene, BTEB) play the role of nanobuilders. An optimal molar ratio of the two precursors (5-25 mol% of total silicon content from the BTEB disilane) generated a homogenous, highly accessible, and well-defined mesoporous material with hexagonal symmetry and narrow pore-size distribution. Physicochemical, textural, and spectroscopic analysis corroborated the effective integration and preservation of the two different nanoprecursors, thereby confirming the framework of the mesoporous hybrid materials. A post-synthesis amination treatment allowed the effective incorporation of amino groups onto the aryl linkers, thereby obtaining a stable and recyclable basic catalyst for use in C-C bond-formation processes.

8.
J Am Chem Soc ; 132(42): 15011-21, 2010 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-20879788

RESUMO

Novel layered zeolitic organic-inorganic materials (MWW-BTEB) have been synthesized by intercalation and stabilization of arylic silsesquioxane molecules between inorganic zeolitic MWW layers. The organic linkers are conformed by two condensed silyl-arylic groups from disilane molecules, such as 1,4-bis(triethoxysilyl)benzene (BTEB), which react with the external silanol groups of the zeolitic layers. The hybrids contain micropores within the inorganic layers and a well-defined mesoporous system in between the organic linkers. An amination post-treatment introduces basic groups in the organic linkers close to the acid sites present in the structural inorganic counterpart. Through this methodology it has been possible to prepare bifunctional acid-base catalysts where the acid sites are of zeolitic nature located in the inorganic building blocks and the basic sites are part of the organic structure. The resultant materials can act as bifunctional catalysts for performing a two-step cascade reaction that involves the catalytic conversion of benzaldehyde dimethylacetal into benzylidene malononitrile.

9.
ChemSusChem ; 1(1-2): 85-90, 2008.
Artigo em Inglês | MEDLINE | ID: mdl-18605669

RESUMO

Sorbitol fatty esters have been prepared through a two-step catalytic process which involves the protection of the polyalcohol by ketalization followed by esterification with the fatty acid. Inorganic molecular sieves with different pore topologies as well as heteropolyacids (HPA) were used as acid catalysts to perform both the ketalization and the esterification step. These catalysts are found to hydrolyze the ketal function and promote the esterification between the free hydroxy groups and oleic acid in a cascade process. The results show a positive shape-selectivity effect when the process is carried out using tridirectional, and especially monodirectional (mordenite), zeolites, such that the ratio of mono- to higher esters and the hydroxy number of the final product are increased as compared to those obtained for the homogeneous-catalyzed process.


Assuntos
Biomassa , Ésteres/síntese química , Ácidos Graxos/química , Sorbitol/química , Tensoativos/síntese química , Esterificação , Ésteres/química , Ácido Oleico/química , Tensoativos/química , Zeolitas/química
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