Robust Ruthenium Catalysts Supported on Mesoporous Cyclodextrin-Templated TiO2-SiO2 Mixed Oxides for the Hydrogenation of Levulinic Acid to γ-Valerolactone.

In this paper, we present a versatile template-directed colloidal self-assembly method for the fabrication in aqueous phase of composition-tuned mesoporous RuO2@TiO2-SiO2 catalysts. Randomly methylated ß-cyclodextrin/Pluronic F127 supramolecular assemblies were used as soft templates, TiO2 colloids as building blocks, and tetraethyl orthosilicate as a silica source. Catalysts were characterized at different stages of their synthesis using dynamic light scattering, N2-adsorption analysis, powder X-ray diffraction, temperature programmed reduction, high-resolution transmission electron microscopy, high-angle annular bright-field and dark-field scanning transmission electron microscopy, together with EDS elemental mapping. Results revealed that both the supramolecular template and the silica loading had a strong impact on the pore characteristics and crystalline structure of the mixed oxides, as well as on the morphology of the RuO2 nanocrystals. Their catalytic performance was then evaluated in the aqueous phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) under mild conditions (50 °C, 50 bar H2). Results showed that the cyclodextrin-derived catalyst displayed almost quantitative LA conversion and 99% GVL yield in less than one hour. Moreover, this catalyst could be reused at least five times without loss of activity. This work offers an effective approach to the utilization of cyclodextrins for engineering the surface morphology of Ru nanocrystals and pore characteristics of TiO2-based materials for catalytic applications in hydrogenation reactions.

Understanding the role of cyclodextrins in the self-assembly, crystallinity, and porosity of titania nanostructures.

A series of mesoporous titania photocatalysts with tailorable structural and textural characteristics was prepared in aqueous phase via a colloidal self-assembly approach using various cyclodextrins (CDs) as structure-directing agents. The photocatalysts and the structure-directing agents were characterized at different stages of the synthesis by combining X-ray diffraction, N2-adsorption, field emission scanning electron microscopy, transmission electron microscopy, UV-visible spectroscopy, dynamic light scattering, and surface tension measurements. The results demonstrate that the cyclic macromolecules efficiently direct the self-assembly of titania colloids, resulting in a fine-tuning of the crystal phase composition, crystallite size, surface area, particle morphology, pore volume, and pore size. Depending on the chemical nature of the substituents in the cyclodextrin ring, synergistic or competitive effects arising from the adsorption capacity of these cyclic oligosaccharides onto titania surface, surface-active properties, and ability to aggregate in water by intermolecular interactions were found to substantially impact the characteristics of the final material. We propose that, in contrast to the native cyclodextrins, which tend to favor the local agglomeration of titania nanoparticles due to the strong intermolecular interactions, the substitution of hydroxyl groups by a relatively large number of methoxyl or 2-hydropropoxyl ones in the ß-CD derivatives allows for creating smoother interfaces, thus facilitating the self-assembly of the colloids in a more homogeneous network. The photocatalytic activity of those titania materials was evaluated in the photodegradation of a toxic herbicide, phenoxyacetic acid, and was correlated to the structural and textural characteristics of the photocatalysts.

Effect of Functional Group on the Catalytic Activity of Lipase B from Candida antarctica Immobilized in a Silica-Reinforced Pluronic F127/α-Cyclodextrin Hydrogel.

Surface modification plays a key role in the fabrication of highly active and stable enzymatic nanoreactors. In this study, we report for the first time the effect of various functional groups (epoxy, amine, trimethyl, and hexadecyl) on the catalytic performance of lipase B from Candida antarctica (CALB) incorporated within a monolithic supramolecular hydrogel with multiscale pore architecture. The supramolecular hydrogel formed by host-guest interactions between α-cyclodextrin (α-CD) and Pluronic F127 was first silicified to provide a hierarchically porous material whose surface was further modified with different organosilanes permitting both covalent anchoring and interfacial activation of CALB. The catalytic activity of nanoreactors was evaluated in the liquid phase cascade oxidation of 2,5-diformylfuran (DFF) to 2,5-furandicarboxylic acid (FDCA) under mild conditions. Results showed that high FDCA yields and high efficiency conversion of DFF could be correlated with the ability of epoxy and amine moieties to keep CALB attached to the carrier, while the trimethyl and hexadecyl groups could provide a suitable hydrophobic-hydrophilic interface for the interfacial activation of lipase. Cationic cross-linked ß-CD was also evaluated as an enzyme-stabilizing agent and was found to provide beneficial effects in the operational stability of the biocatalyst. These supramolecular silicified hydrogel monoliths with hierarchical porosity may be used as promising nanoreactors to provide easier enzyme recovery in other biocatalytic continuous flow processes.

Oxidation of 2,5-diformfylfuran to 2,5-furandicarboxylic acid catalyzed by Candida antarctica Lipase B immobilized in a cyclodextrin-templated mesoporous silica. The critical role of pore characteristics on the catalytic performance.

HYPOTHESIS: Porous silica has been extensively used as suitable carrier for the immobilization of various enzymes. Randomly Methylated ß-Cyclodextrin (RaMeßCD) has surface active properties and very high solubility in water and could therefore be used as template in the fabrication of silica particles with tunable pore size. EXPERIMENTS: Silica particles were prepared by sol-gel process in alkaline medium with and without use of RaMeßCD. Lipase Bfrom Candida antarctica (CALB) was either incorporated within the pores of RaMeßCD-derived support or covalently attached on the surface of CD-free silica particles and its catalytic performance was assayed in the oxidation of 2,5-diformylfuran (DFF) to 2,5-furandicarboxylic acid (FDCA). Enzymatic reactors were characterized by N2-adsorption analysis, small angle XRD, TG/DSC experiments, ATR-FTIR spectroscopy, HR-TEM and LSCM, while reaction products were determined based on 1H NMR spectroscopy combined with HPLC. FINDINGS: Results showed that the use of RaMeßCD as structure directing agent led to mesoporous silica composed of uniform 8 nm-sized particles with 11 nm-sized mesopores compatible with the dimensions of CALB (3.0 nm × 4.0 nm × 5.0 nm). Incorporation of CALB within the pores of RaMeßCD-derived silica caused almost a two-fold increase in specific activity after 7 h at 40 °C when compared to lipase immobilized on the surface of CD-free silica particles (33.2 µmol g-1 min-1vs. 14.4 µmol g-1 min-1). Moreover, the RaMeßCD-derived biocatalyst demonstrated enhanced operational stability during the recycling experiments, retaining more than 90% of its initial activity after five 24 h-reaction cycles. These findings open up new avenues for future research on the use of cyclodextrins in the development of enzyme-based nanoreactors.

Confinement of Candida Antarctica Lipase B in a Multifunctional Cyclodextrin-Derived Silicified Hydrogel and Its Application as Enzymatic Nanoreactor.

Supramolecular hydrogels with a three-dimensional cross-linked macromolecular network have attracted growing scientific interest in recent years because of their ability to incorporate high loadings of bioactive molecules such as drugs, proteins, antibodies, peptides, and genes. Herein, we report a versatile approach for the confinement of Candida antarctica lipase B (CALB) within a silica-strengthened cyclodextrin-derived supramolecular hydrogel and demonstrate its potential application in the selective oxidation of 2,5-diformylfuran (DFF) to 2,5-furandicarboxylic acid (FDCA) under mild conditions. The enzymatic nanoreactor was deeply characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, N2-adsorption, dynamic light scattering, UV-visible spectroscopy, transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy, while the reaction products were established on the basis of 1H nuclear magnetic resonance spectroscopy combined with high-performance liquid chromatography. Our results revealed that while CALB immobilized in conventional sol-gel silica yielded exclusively 5-formylfuran-2-carboxylic acid (FFCA), confinement of the enzyme in the silicified hydrogel imparted a 5-fold increase in DFF conversion and afforded 67% FDCA yield in 7 h and almost quantitative yields in less than 24 h. The hierarchically interconnected pore structure of the host matrix was found to provide a readily accessible diffusion path for reactants and products, while its flexible hydrophilic-hydrophobic interface was extremely beneficial for the interfacial activation of the immobilized lipase.

Robust Mesoporous CoMo/γ-Al2O3 Catalysts from Cyclodextrin-Based Supramolecular Assemblies for Hydrothermal Processing of Microalgae: Effect of the Preparation Method.

Hydrothermal liquefaction (HTL) is a promising technology for the production of biocrude oil from microalgae. Although this catalyst-free technology is efficient under high-temperature and high-pressure conditions, the biocrude yield and quality can be further improved by using heterogeneous catalysts. The design of robust catalysts that preserve their performance under hydrothermal conditions will be therefore very important in the development of biorefinery technologies. In this work, we describe two different synthetic routes (i.e., impregnation and cyclodextrin-assisted one-pot colloidal approach), for the preparation in aqueous phase of six high surface area CoMo/γ-Al2O3 catalysts. Catalytic tests performed on the HTL of Nannochloropsis gaditana microalga indicate that solids prepared by the one-pot colloidal approach show higher hydrothermal stability and enhanced biocrude yield with respect to the catalyst-free test. The positive effect of the substitution of the block copolymer Tetronic T90R4 for Pluronic F127 in the preparation procedure was evidenced by diffuse reflectance UV-visible spectroscopy, X-ray diffraction, N2-adsorption-desorption, and H2-temperature-programmed reduction measurements and confirmed by the higher quality of the obtained biocrude, which exhibited lower oxygen content and higher-energy recovery equal to 62.5% of the initial biomass.

Photocatalysis of Volatile Organic Compounds in water: Towards a deeper understanding of the role of cyclodextrins in the photodegradation of toluene over titanium dioxide.

HYPOTHESIS: Cyclodextrin-assisted photodegradation of toluene was investigated in water in the presence of a photo-irradiated commercial titanium dioxide photocatalyst. It was expected that cyclodextrins could form water-soluble supramolecular host/guest complexes with the toluene and thus promote the approach of the pollutant on the TiO2 surface and enhance the phototocatalytic oxidation efficiency. EXPERIMENTS: Photodegradation kinetics of toluene were investigated under UV-C and near-visible light radiation in aqueous suspensions of TiO2. Impact of cyclodextrin (CD) on the photocatalytic efficiency of TiO2 was evaluated with different cyclodextrins: α-CD, ß-CD, γ-CD and RAME-ß-CD. Host-guest association constants were determined by static headspace gas chromatography and affinity of cyclodextrins for the TiO2 surface by isothermal adsorption studies. Issue of the cyclodextrin stability during the degradation process was examined using Total Organic Carbon, NMR and MALDI-TOF analyses. FINDINGS: Toluene could be fully mineralized by TiO2 in water within hours, even if the presence of cyclodextrin caused a delay in the photodegradation process. The chemical nature of cyclodextrins was found to exert a significant influence on the extent of inhibitory effect, which was discussed in terms of balance between solubilization efficiency, substrate protection and coverage of active sites of TiO2 by competitive adsorption. The cyclodextrin degradation was also studied and discussed.

An efficient route to aqueous phase synthesis of nanocrystalline γ-Al2O3 with high porosity: from stable boehmite colloids to large pore mesoporous alumina.

In this paper we emphasise the important role of Pluronic F127 on the porosity of mesoporous alumina prepared from boehmite colloids. By focusing on the F127/boehmite interactions we show how the concepts of interface science may help to predict and improve the textural characteristics of mesoporous alumina. By varying the synthetic parameters, in particular the copolymer content, we show that the porosity of γ-Al(2)O(3) can be enhanced by 400% and the average pore diameter can be expanded from 5 to 14 nm. These results are discussed in terms of interactions between the Pluronic F127 and boehmite colloids, and are correlated to the critical micelle concentration (CMC) of the copolymer. The textural characteristics of the mesoporous alumina can be further improved either by introducing hydrocarbons in the preformed boehmite/copolymer sols or by concentrating the sols. In comparison with as-synthesised alumina, those prepared with F127 showed improved thermal stability. Furthermore, boehmite/copolymer sols were stable for all surfactant concentrations investigated and can give high quality coatings suitable for catalytic applications.

Rheological behaviour and spectroscopic investigations of cerium-modified AlO(OH) colloidal suspensions.

The rheological behaviour of aqueous suspensions of boehmite (AlO(OH)) modified with different Ce-salts (Ce(NO(3))(3), CeCl(3), Ce(CH(3)COO)(3) and Ce(2)(SO(4))(3)) was investigated at a fixed Ce/Al molar ratio (0.05). Freshly prepared boehmite suspensions were near-Newtonian and time-independent. A shear-sensitive thixotropic network developed when Ce-salts with monovalent anions were introduced in the nanoparticle sols. The extent of particle aggregation dramatically increased with ageing for Ce(NO(3))(3) and CeCl(3) whereas an equilibrium value was reached with Ce(CH(3)COO)(3). The addition of Ce(2)(SO(4))(3) with divalent anions involved no thixotropy but rather a sudden phase separation. The combined data set of IRTF and DRIFT spectra indicated that free NO(3)(-) anions of peptized boehmite adsorb on the nanoparticle surface by H-bond. The introduction of Ce-salts in the boehmite sol led to the coordination between Ce(3+) ions and NO(3)(-) anions adsorbed on boehmite i.e. to [Ce(NO(3))(4)(H(2)O)(x)](-) complex. Such coordination led to a thixotropic behaviour which was lower with Ce(NO(3))(3) compared to CeCl(3) and Ce(CH(3)COO)(3). In contrast, Ce(2)(SO(4))(3) formed insoluble complexes with dissolved aluminium species. The formation of H-bonded surface nitrate complexes was found to play a decisive role on the particle-particle interactions and consequently on the rheological behaviour of the sols.

Effect of PEG on rheology and stability of nanocrystalline titania hydrosols.

Very stable titania hydrosols were prepared by fast hydrolysis of titanium isopropoxide in a large excess of water. XRD patterns show that these sols contain nanocrystals (5-6 nm) of anatase (70%) and brookite (30%). TEM images indicate that these primary particles form aggregates whose mean hydrodynamic diameter, determined by photon correlation spectroscopy, is in the range of 80-90 nm. The flow curves of these colloids, recorded for several volume fractions of nanoparticles, can be perfectly fitted, in the range 0-100 s(-1), with a power-law model. In this range the behavior is Newtonian but for larger shear rates a shear thinning is observed. The viscosity dependence on particle concentration can be predicted by a Batchelor-type model were the volume fraction of particles is replaced by an effective volume fraction of aggregates, taking into account their fractal dimension. Addition of polyethylene glycol (PEG 2000) induced a marked decrease (more than 50%) of the sol viscosity down to a minimum. This is explained by assuming that PEG adsorbs on the surface of TiO(2) particles producing stabilization by steric effects and leading to formation of more compact aggregates. Without PEG the sol viscosity strongly decreases on aging. This effect is not caused by the growth of primary particles. It is rather interpreted as a progressive reorganization of the aggregates toward a more compact packing.