Current Catalysis - Volume 6, Issue 3, 2017

Background: Isomerization of glucose to fructose is recognized as one of the key reactions for the transformation of cellulosic biomass, a potent substitution for the depleting petroleum resources. Zeolites are among the most useful catalysts for the isomerization process and substantial achievements have been achieved thus far. A state-of-the-art review is given within this content, and catalytic mechanisms and competitive reactions are discussed as well. Methods: We performed a comprehensive search of literatures and the qualities of retrieved papers were appraised using standard tools. Literatures that represent the substantial findings are highlighted reflecting the progresses made with regard to the isomerization of glucose to fructose catalyzed by zeolites. Results: Substantial achievements have been made with regard to the zeolite-catalyzed isomerization of glucose to fructose. The joint experimental-computational studies provide insightful clues to reaction mechanisms; e.g., the isomerization reaction proceeds in sequence of pyranose ring opening, hydride shift from C2 to C1 (rate-decisive) and furanose ring closure while the epimerization reaction is characterized by the intramolecular 1,2-carbon shift; Both of the defects SnOH and proximate silanol promote significantly the isomerization reaction while proximate silanol disfavors the epimerization reaction. Conclusion: On basis of the fruitful catalytic results obtained thus far, two outlets are put forward that should be given priority with regard to the utilization of cellulosic biomass, as the combination of the isomerization reaction with other catalytic processes and the exploitation of the direct transformation of cellulosic biomass to platform chemicals.

Background: Azlactones, also known as oxazolones, are a class of versatile intermediates and are commonly used for the synthesis of various heterocyclic scaffolds and unnatural α-amino acids. In particular, because of their inherent multiple sites of reactivity, azlactones have recently attracted a great deal of research efforts from the synthetic community. By exploring different sites of reactivity, a variety of cycloaddition reactions have been developed to construct diverse synthetically and biologically important hetereocycles as well as α-amino acid derivatives bearing quaternary carbons. With the development of asymmetric catalysis, a range of catalytic asymmetric variants have also been developed by metal and organocatalysis to access various valuable enantioenriched molecules. However, to the best of our knowledge, there is still no comprehensive review devoted to this emerging topic to date. Methods: We performed a systemic survey of azlactone-mediated cycloaddition reactions by SciFinder search. To calibrate the reasonable scope of this survey, we highlight the recent advances in the field of cycloaddition reactions of azlactones, and their applications to the construction of heterocycles according to the different types of reaction modes. Thus, thirty four leading papers were included in this review and accordingly organized by different sites of reactivity, including [2+3], [2+4], [2+8], and [3+2] cycloaddition reactions. Conclusion: The results shown in this review confirmed the significance of such type of versatile intermediates. The discovery of new suitable reactive species and application of other catalytic modes can provide a fruitful platform for the development of new cycloadditions of azlactones.

Background: The enzymes, oxidoreductases catalyze various vital biological oxidation/ reduction reactions at the cellular level and thus a wide spectrum research in the area of enzyme technology has focused on their applications in different disciplines such as bioanalysis, biodegradation, biosynthesis, biotechnology, therapeutics and industry etc. Methods: Oxidoreductases have successfully immobilized on the surface of simple and modified magnetic nanoparticles or entrapped within the network of polymeric magnetic nanomatrices. Prior to their use for the immobilization of enzymes, these supports have been functionalized by using different kinds of activating agents. Results: The immobilized oxidoreductases were found significantly more stable against the inactivation mediated by pH, heat, organic solvents, detergents and other kinds of denaturants and were found quite resistant to inhibition mediated by their specific inhibitors. The bound enzymes have proved their potential in synthetic reactions due to high stability in the presence of organic solvents. The obtained immobilized oxidoreductases have demonstrated remarkably high operational stability and reusability as a biosensor, in batch as well as in continuous bioreactors during synthesis and transformation of useful compounds. Conclusion: The novel properties of magnetic nanoparticles make them an excellent choice as a support for efficient high yield immobilization and stabilization, and easy separation of enzymes. The immobilized oxidoreductases exhibited very high activity, stability against different kinds of denaturants and on storage and reusability. These results were dependent on the type of nanomatrices and method employed for immobilization enzyme. The findings in this manuscript have a good technological potential.

Background: The generation of hydrogen by electrochemical reduction of water, represents an attractive approach for storing the electrical energy transiently produced by renewable energy sources, and catalysts based on transition metal compounds are used to increase the reaction rate. Despite much progress in water reduction catalysis, structural complexity, insolubility in aqueous media and low pH conditions severely limit their practical utility. To meet this goal, we design an electrocatalyst based on Co(TCNQ)2 (TCNQ: 7,7,8,8-tetracyanoquinodimethane). This compound is water soluble and can catalyze proton or water reduction. Methods: The reaction of LiTCNQ with CoCl2.6H2O yields a cobalt-TCNQ compound, Co(TCNQ)2 1. This compound has been characterized by spectroscopic, physics-chemical and electro-chemical methods. Results: Electrochemical studies show that the electrocatalytic system formed by compound 1 achieves a turnover frequency (TOF) of 767 moles h-1 H2 from a neutral buffer solution (pH 7.0) at an overpotential (OP) of 0.638 V. Conlusions: The result suggests that the addition of TCNQ is the key feature for highly catalytic activity of 1. This can be attributed to the introduction of TCNQ- ion to cobalt center, stabilizing the low oxidation state of cobalt.

Background: 5-Hydroxymethylfurfural (HMF) and its derivatives are versatile materials having tremendous applications in a variety of industries. One of the interesting derivatives of HMF, 5, 5' (oxy-bis (methylene)) bis-2-furfural (OBMF), has potential uses in polymer and pharmaceuticals. The synthesis of OBMF is mainly reported by using homogeneous catalyst and only a few reports are published by using heterogeneous catalyst. Herein, we synthesized OBMF efficiently from HMF using supported heteropolyacid catalyst 20% (w/w) Cs2.5H0.5PW12O40/K-10 (CDK). Methods: A wet impregnation method was employed for the preparation of CDK. Initially a dry K-10 was impregnated by aqueous solution of CsCl with vigorous stirring. Then it was dried at 150°C for 3 h and preformed material was again impregnated by a solution of DTP in methanol. Finally, it was well dried and calcined at 300°C for 3 h. All the catalysts were characterized by different techniques such as XRD, FTIR, ASAP, TPD and TGA. Effects of various kinetic parameters were studied to fix the reaction conditions. Reaction mechanism was predicted; and kinetic model was developed. Results: Among all the catalysts, CDK was found active, selective, and reusable catalyst for OBMF synthesis. The yield of OBMF was increased as water formed in reaction was removed by using Dean Stark apparatus. OBMF was efficiently obtained with 98% selectivity at HMF conversion of 95% over CDK. A second order kinetics was found to fit the data well. Conclusion: The strong acidic nature of CDK makes it an effective heterogeneous catalyst among sulfated zirconia and amberlyst- 15 for OBMF synthesis. It shows good reusability up to four cycles. Temperature, catalyst loading and mole of HMF were influencing the reaction rate. Maximum yield of OBMF can be obtained at following reaction conditions: temperature; 100ºC, amount of catalyst; 0.015 g mL-1, mole of HMF; 1.5 mmol. The kinetic model was successfully developed for the reaction and follows LHHW mechanism. The calculated activation energy was 10.7 kcal/mol.

Background: Supporting of choline chloride (CC):2urea as a deep eutectic solvent (DES) on nonomagnetite gives the core-shell of nanoFe3O4@CC:2urea which in DES coating inhibits the agglomeration of nanoFe3O4 and nanoFe3O4 facilitates reusability of DES. The ratio of nanoFe3O4 to DES in nanoFe3O4@CC:2urea was determined as 8:1. These synergic effects led to a high catalytic performance for nanoFe3O4@CC:2urea versus individual nanoFe3O4 or CC:2urea in synthesis of pyrazole derivatives by various synthetic strategies. Faster reactions with higher yields at room temperature occur with low loading of this recyclable hybrid catalyst. Method: NanoFe3O4 and DESs were prepared by co-precipitation of FeCl3.6H2O and FeCl2.4H2O with NH3 and heating the mixture of CC and hydrogen donors to a homogeneous liquid. Then, nanoFe3O4@CC:2urea was prepared by sonication of nanoFe3O4 and dissolved CC:2urea in methanol. After characterization by FT-IR, SEM, TEM, TGA, and VSM, the efficacy of nanoFe3O4@CC:2urea was examined in one-pot synthesis of pyrazoles and pyranopyrazoles. Results: NanoFe3O4@CC:2urea is a hybrid organic/inorganic catalyst which in DES coating inhibits the agglomeration of Fe3O4 nanoparticles and nanoFe3O4 improves the reusability of DES. These synergic effects deliver a high catalytic performance for nanoFe3O4@CC:2urea in synthesis of pyrazoles and pyranopyrazoles. Faster reactions with higher yields at room temperature occur with low loading of this magnetically separable and reusable catalyst. Conclusion: In summary, synergic effects of CC:2urea and nanomagnetite in nanoFe3O4@CC:2urea lead to a reusable catalyst with superior catalytic activity in room temperature three- or four- component synthesis of pyrazoles and pyranopyrazoles. This magnetically separable DES provides higher yields of products, faster reactions, and milder conditions compared to previous reported methods.

Background: A variety of catalysts have been developed for HMF production from glucose. However, there is less information available concerning the mechanism of HMF dehydration from glucose arisen from heterogeneous catalyst in aqueous medium. It is of great significance to figure out the mechanism of HMF formation from glucose catalyzed by molecular sieve. Methods: A combined experimental and theoretical research on dehydration of glucose to HMF catalyzed by Sn-KIT-6 in aqueous media has been performed. We conducted Density functional theory (DFT) to investigate the effects of catalyst on the reaction mechanism. Results: In the first part, Sn-KIT-6 catalyst can greatly reduce the enthalpy of 1, 2-enediol, the key intermediate, from 70.88kcal/mol in non-catalyst system to 46.57kcal/mol. The second part undergoes sequential removal of three water molecules from fructose. The elimination of the second water molecule in the presence of Sn-KIT-6 is the rate-limiting step in the second part with enthalpy of 29.47kcal/mol, which is less than that of non-catalyst condition with enthalpy of 55.69kcal/mol. Conclusion: The reaction mechanism was developed so that the isomerization of glucose into fructose together with the subsequent dehydration is generally considered as two main parts of the whole reaction. The change of the reaction enthalpy calculated from DFT in the rate-controlling step illustrates that Sn-KIT-6 catalyst greatly promotes HMF yield from dehydration of glucose.

Background: Residue hydroprocessing is the most important processes in the refinery to obtain transportation fuels from the low quality petroleum resources. But the upgradation of residues is a difficult task, due to the presence of high contaminants such as sulfur, metals (Ni and V), asphaltenes and residue carbon, which cause rapid deactivation of the catalysts. The most effective approach to control catalyst deactivation is the use of the mesoporous alumina as support. Mesoporous γ-aluminas through a sol-gel method was synthesized without using any surfactant or templates. The synthesized highly porous aluminas are used to prepare NiMo catalysts for the hydroprocessing of vacuum residues. Methods: In the present work, an attempt has been made to synthesize large pore mesoporous alumna with better thermal stability in a very simple way by using different alcoholic solvents. NiMo catalysts were further prepared using synthesized aluminas using sequential impregnation method. The catalytic activity has been evaluated using vacuum residue as feed having 21.98wt% asphaltene in a batch reactor. Results: The mesoporous alumina synthesized using isopropanol as solvent had a wide porosity with appropriate surface area. The number of total acid sites is more for the mesoporous alumina synthesized using mixture of solvents (isopropanol and butanol). The ratio of octahedral/tetrahedral species in catalysts prepared by isopropanol is quite high indicating that the presence of weakly bonded octahedral species in this catalyst is higher than that of catalysts prepared by mixture of solvents. Hydrotreating activity suggests that due to the presence of high concentration of octahedral species in NiMo/Al2O3 catalysts prepared by isopropanol has high hydrogenation activity and hence shows higher HDM, HDS and HDAs conversions. Conclusion: A simple, convenient and effective way is used to synthesize mesoporous alumina as support for the NiMo hydrotreating catalysts. Residue hydrotreating activity of the catalysts suggests that the coke formation by NiMo catalysts prepared by isopropanol is very low due to the presence of large porous structure and high hydrogenation activity of the catalyst. It is also found that the catalyst having higher total acidity facilitates coke formation.