Synthesis and characterization of poly-L-leucine initialized and immobilized by rehydrated hydrotalcite: understanding stability and the nature of interaction.

PLLs were synthesized by the ring-opening polycondensation (ROP) method using α-L-leucine N-carboxyanhydride (NCA) and initialized by triethylamine (Et3N), water or rehydrated hydrotalcite (HTrus). The role of temperature, different initiators and water in ROP was further investigated. In general, the initiators used in the polymerization reaction lead to PLL alpha-helical chains containing 5-40 monomers with NCA endgroups via a monomer-activated mechanism. However, the water has a twofold effect on ROP, as both a nucleophile and a base, which involves competition between two different types of initiating mechanisms (nucleophilic attack or deprotonation of the NCA monomer) in the polymerization reaction. This competition provides as a main product NCA endgroups with an alpha-helical structure and leads to the formation of the PLL cyclic-chains and beta-sheet structures which reduce the polymer Mw and the PD of the polypeptide. Furthermore, the water can hydrolyze the NCA endgroups resulting in PLL alpha-helical chains that contain living groups as the main product. On the other hand, the HTrus presents a double role: as both an initiator and a support. The polymers synthesized in the presence of HTrus presented a HT-carboxylate endgroup. The PLLs immobilized in HTrus through an anion-exchange method performed for just 30 minutes presented the PLL immobilized in the interlayer space of the HTrus. The PLL chains of the immobilized counterpart are stabilized by H-bonding with the M-OH of the HT structure. All the polypeptides and biohybrid materials synthesized have been characterized using different techniques (EA, ICP, XRD, Raman, MALDI-TOF, ESI-TOF, FT-IR at increasing temperatures, TG/DT analyses and TEM).

Biohydrogen production by dark fermentation of glycerol using Enterobacter and Citrobacter Sp.

Glycerol is an attractive substrate for biohydrogen production because, in theory, it can produce 3 mol of hydrogen per mol of glycerol. Moreover, glycerol is produced in substantial amounts as a byproduct of producing biodiesel, the demand for which has increased in recent years. Therefore, hydrogen production from glycerol was studied by dark fermentation using three strains of bacteria: namely, Enterobacter spH1, Enterobacter spH2, and Citrobacter freundii H3 and a mixture thereof (1:1:1). It was found that, when an initial concentration of 20 g/L of glycerol was used, all three strains and their mixture produced substantial amounts of hydrogen ranging from 2400 to 3500 mL/L, being highest for C. freundii H3 (3547 mL/L) and Enterobacter spH1 (3506 mL/L). The main nongaseous fermentation products were ethanol and acetate, albeit in different ratios. For Enterobacter spH1, Enterobacter spH2, C. freundii H3, and the mixture (1:1:1), the ethanol yields (in mol EtOH/mol glycerol consumed) were 0.96, 0.67, 0.31, and 0.66, respectively. Compared to the individual strains, the mixture (1:1:1) did not show a significantly higher hydrogen level, indicating that there was no synergistic effect. Enterobacter spH1 was selected for further investigation because of its higher yield of hydrogen and ethanol.

Hydrogen substitutes for the in situ generation of H2O2: an application in the Fenton reaction.

This study investigates the ability of formic acid, hydrazine and hydroxylamine to act as H(2) substitutes in conducting phenol degradation by Fenton reaction using in situ generated hydrogen peroxide. The processes were performed with semi-heterogeneous (Pd/Al(2)O(3)+soluble Fe(2+)) and fully heterogeneous (FePd/Al(2)O(3)) catalytic systems under ambient conditions. In contrast to bulk H(2)O(2) production conditions, hydrazine is able to produce H(2)O(2)in situ followed by phenol degradation using Pd/Al(2)O(3)+Fe(2+) at pH 3 without the need for halide ions. However, a degree of mineralization exceeding 37% could not be achieved. The significant production of in situ H(2)O(2) at the inherent acidic pH of hydroxylammonium sulfate in the presence of Pd/Al(2)O(3)+Fe(2+) was also found to differ from the bulk production of H(2)O(2), in which no H(2)O(2) was detected. A remarkable degree of mineralization (ca. 65%) as well as fast phenol degradation during the reaction started at pH 7 over FePd/Al(2)O(3) may be an advantage of using hydroxylamine. On the other hand, using formic acid, H(2)O(2) was produced at a moderate rate, thereby achieving higher efficiency in the mineralization of phenol. Most importantly, the catalysts were more stable in the presence of formic acid than hydrazine or hydroxylamine.

Simultaneous in situ generation of hydrogen peroxide and Fenton reaction over Pd-Fe catalysts.

High mineralization degree of organic compounds can be achieved by a novel environmentally-friendly full heterogeneous Pd-Fe catalytic system, which involves in situ generation of hydrogen peroxide from formic acid and oxygen, and oxidation of organic compounds by Fenton process in a one-pot reaction.

Aldol condensations over reconstructed Mg-Al hydrotalcites: structure-activity relationships related to the rehydration method.

Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.

Supported choline hydroxide (ionic liquid) as heterogeneous catalyst for aldol condensation reactions.

Choline hydroxide was used as a basic catalyst for aldol condensation reactions to produce new C-C bonds between several ketones and aldehydes. Choline supported on MgO exhibits higher TOF values than other well known basic catalysts in these reactions.