Pedro K. Kiyohara

High performance magnetic separation of gold nanoparticles for catalytic oxidation of alcohols

We present the magnetic separation approach to facilitate the recovery of gold nanoparticle (AuNP) catalysts. The use of magnetically recoverable supports for the immobilization of AuNPs instead of traditional oxides, polymers or carbon based solids guarantees facile, clean, fast and efficient separation of the catalyst at the end of the reaction cycle. Magnetic separation can be considered an environmentally benign separation approach, since it minimizes the use of auxiliary substances and energy for achieving catalyst recovery. The catalyst preparation is based on the immobilization of Au3+ on the surface of core–shell silica-coated magnetite nanoparticles, followed by metal reduction using two different methods. AuNPs were prepared by thermal reduction in air and by hydrogen reduction at mild temperature. Interestingly, the mean particle size of the supported AuNPs was similar (ca. 5.9 nm), but the polydispersity of the samples is quite different. The catalytic activity of both catalysts in the aerobic oxidation of alcohols was investigated and a distinct selectivity for benzyl alcohol oxidation was observed.

Superparamagnetic nanoparticle-supported palladium: a highly stable magnetically recoverable and reusable catalyst for hydrogenation reactions

Here we present a magnetically recoverable palladium catalyst prepared by immobilization of palladium over silica-coated magnetite nanoparticles. The catalyst reduced by molecular hydrogen contains palladium nanoparticles well distributed and stabilized in the magnetizable support surfaces and converts cyclohexene to cyclohexane under mild reaction conditions (75 °C and 6 atm) with TOF of 11 500 h−1. The catalyst was easily recovered with a permanent magnet in the reactor wall and reused for up to 20 recycles of 2500 TON each without any significant loss in catalytic activity, demonstrating an efficient recycling process for hydrogenation reactions.

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (ALCOA 123) and aqueous solution of acetic acid at 96 ◦ C±1 ◦ C. The white powder of Al(OH)(CH3COO)2 is constituted by agglomerates of crystalline plates, having size about 10 µm. The crystals were fired from 200 ◦ C to 1550 ◦ C, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300 ◦ C); delta (750 ◦ C); alpha (1050 ◦ C). The aluminas maintain the original morphology of the Al(OH)Ac2 crystal agglomerates, up to 1050 ◦ C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550 ◦ C. High surface area aluminas are formed in the temperature range of 700 ◦ C to 1100 ◦ C; the maximum value of 198m 2 /g is obtained at 900 ◦ C, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al2O3 which acts as precursor for the sequence gamma- to alpha-Al2O3.

Gold nanoparticles supported on magnesium ferrite and magnesium oxide for the selective oxidation of benzyl alcohol

Au nanoparticles (Au NPs) have gained significant attention as catalysts for the selective oxidation of alcohols; however, the catalytic activity is highly dependent on the presence of a base. Alternatives to the use of strong bases, such as NaOH, are still needed. Here, we explored the basicity of magnesium ferrite/oxide supports to study the catalytic behaviour of supported Au NPs for the oxidation of benzyl alcohol. The presence of Mg2+ ions in the ferrite structure improved the catalytic activity of supported Au NPs to ca. 35% conversion in the absence of an additional base. After modifying the support with MgO, the catalytic activity of supported Au NPs was further improved to ca. 50% conversion, but the catalyst deactivated in successive recycling tests. When the catalysts were tested in the presence of a sub-stoichiometric amount of K2CO3, they became more active and remained stable upon recycling with no loss of activity and selectivity for the preferential production of benzoic acid.

Thermal transformation of synthetic euhedral and fibrillar crystals of boehmite into aluminas

Abstract Fibrillar pseudoboehmite and euhedral lath-like boehmite aqueous sols were prepared from the reaction of aluminium secondary butoxide and acetic acid in order to have a solid content of 70g/1 of AlOOH. The dried solids were fired from 200 °C to 1500 °C and examined by TEM, SEM, SAED, and XRD to compare formation of aluminas and their crystal morphological changes at the same temperatures. It was observed that the sequence of transformations of fibrillar pseudoboehmite is closer to that for euhedral boehmite than to the “gelatinous boehmite” described in the literature.

Pseudomorphic transformations of euhedral crystals of γ-AlOOH into aluminas

Abstract The thermal transformation of synthetic well-shaped hexagonal laths of boehmite-AlOOH was studied between 200°C and 1500°C by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD), to characterize the transition aluminas that appear before the formation of α-Al 2 O 3 . It was observed in the single crystals that dehydroxylation produces, between 400°C and 500°C, pores regularly organized and parallel to the larger diameter cf the hexagons which have the same direction (100) as the original boehmite crystal; this gives a striated appearance to the pseudomorphs, which is maintained until after heating at 900°C, when the transformation from the γ phase coexist; the pores have lost their orderly disposition and larger pores or openings are now irregularly distributed in the mixed single crystals, whose crystalline structure can be observed in the lattice image by the use of high-resolution electron microscopy. The α-alumina pseudomorphs exists as platy elongated single crystals at 1100°C and coalesce at 1200°C; from 1300°C to 1500°C the pseudomorphs coalesce into round polygonal particles of α-alumina, whose crystalline structure could be demonstrated by SAED, XRD or lattice images.

Copper nanoparticles synthesized by thermal decomposition in liquid phase: the influence of capping ligands on the synthesis and bactericidal activity

We explored here the synthesis of copper nanoparticles (CuNPs) by thermal decomposition of copper(II) acetate in diphenyl ether in the presence of different capping ligands. To look for any specific role in thermal decomposition, we performed reactions in the presence of oleic acid, oleylamine, and 1,2-octanediol, or in the presence of different combinations of these capping ligands, or in the absence of them. The CuNPs obtained in the presence of oleic acid and oleylamine (in the presence or absence of 1,2-octanediol) were stabilized as Cu(0) NPs, and the “naked” NPs prepared in solvent only easily oxidized to CuO. Therefore, both oleic acid and oleylamine can act as capping ligands to prepare air-stable Cu(0) NPs. The 1,2-alkyldiol is not necessary for metal reduction during the synthesis, but its presence improves size and morphology control. The presence of capping ligands significantly reduced the bactericidal activity exhibited by the Cu NPs against the gram-negative bacteria Escherichia coli.

PLGA microspheres containing bee venom proteins for preventive immunotherapy

Bee venom (BV) allergy is potentially dangerous for allergic individuals because a single bee sting may induce an anaphylactic reaction, eventually leading to death. Currently, venom immunotherapy (VIT) is the only treatment with long-lasting effect for this kind of allergy and its efficiency has been recognized worldwide. This therapy consists of subcutaneous injections of gradually increasing doses of the allergen. This causes patient lack of compliance due to a long time of treatment with a total of 30-80 injections administered over years. In this article we deal with the characterization of different MS-PLGA formulations containing BV proteins for VIT. The PLGA microspheres containing BV represent a strategy to replace the multiple injections, because they can control the solute release. Physical and biochemical methods were used to analyze and characterize their preparation. Microspheres with encapsulation efficiencies of 49-75% were obtained with a BV triphasic release profile. Among them, the MS-PLGA 34kDa-COOH showed to be best for VIT because they presented a low initial burst (20%) and a slow BV release during lag phase. Furthermore, few conformational changes were observed in the released BV. Above all, the BV remained immunologically recognizable, which means that they could continuously stimulate the immune system. Those microspheres containing BV could replace sequential injections of traditional VIT with the remarkable advantage of reduced number of injections.

Comparative study between yeasts immobilized on alumina beads and on membranes prepared by two routes

Alumina channeled beads and rough surface membranes prepared from aqueous sols of fibrillar pseudoboehmite are able to immobilize yeasts for ethanol fermentation of sugar solutions. This paper describes comparative results of assays carried out with yeasts immobilized onto alpha-alumina beads and membranes prepared under two different conditions of processing and firing. The fermentation tests evaluated by the decrease of fermentable sugars, referred as Brix degrees per hour, indicated that the yeasts immobilized on beads had similar performance, probably because their surfaces, even being morphologically different, presented the same value of open porosity. One type of membrane (asymmetrical; precursor: pseudoboehmite; firing temperature 1,150oC; crystal structure; alpha-alumina) had better performance than the other type (asymmetrical; precursor: fibrillar pseudoboehmite plus aluminum hydroxiacetate mixture; 1,150oC; alpha-alumina) because the yeast cells entered into their porous interior through the surface slits, were immobilized and their growth was easier than on the external surface.

Aluminas by Thermal Activation of Fibrillar Pseudoboehmite Powders

The aluminum monohydroxide pseudoboehmite is a common precursor for catalysts and ad/absorbents. Fibrillar pseudoboehmite powders were synthesized by the same preparation method from aqueous sols but aged in different times: 24, 48 and 168 hours. The sol particles were from fibrillar pseudoboehmite; the average length of the microfibrils changed sample to sample, the sizes increasing with the thermal aging time. The sols were dried by different methods: at room temperatures, forming a solid membrane; spray dried; dried at 60°C and 110°C from a gel from the sol aged 168 hours; all were fired between 200°C and 1000°C in oxidative atmosphere to produce activated transition aluminas. The phases formed was identified by XRD. The aim of the present paper is to present data showing that method of drying the sol has an effect on the temperature of formation of alpha-alumina due to the anisodiametric shape of the pseudoboehmite crystals.