Meso-macroporous Materials Research Articles

Kinetic investigation of the aldol condensation of n-butanal on mesoporous and macro-mesoporous heterogeneous catalysts

The kinetics of the aldol condensation of n-butanal on modified mesoporous and macro-mesoporous heterogeneous catalysts was studied in the liquid phase. Applying liquid phase 1H NMR the degree of modification was determined quantitatively, in order to calculate the turnover numbers of the n-butanal conversion. The analysis of the kinetic data has shown that the reaction on primary amines follows a 1st order model, whereas on secondary amines the 2nd model gives the best correlation. Tertiary and aromatic amines were not active. Based on the kinetic measurements reaction mechanisms are proposed. The difference in the activation energies suggests that the modification process on mesoporous materials proceeds preferentially on the external surface, while on meso-macroporous materials the modification proceeds in the mesopores.

Kinetic study of distillation and cracking of heavy oil on bimodal meso–macroporous materials of silica and aluminosilicate by thermogravimetry

Bimodal silica and aluminosilicate materials containing mesopores and macropores were synthesized by hydrothermal method using sodium silicate, aluminum nitrate, P123 triblock copolymer, n-decane, hydrochloric acid and water. The uncalcined silica compound was previously characterized by thermogravimetry, and the calcined materials were characterized by X-ray diffraction, infrared absorption spectroscopy, and scanning electron microscopy. The obtained materials have bimodal characteristics with mesopores and macropores in their structures, which pores are ideal for processing bulky molecules, such as hydrocarbons present in heavy oil. In this work, the synthesized silica and aluminosilicate were used as materials for distillation and cracking of a petroleum sample with oAPI = 17.4. The process was evaluated by thermogravimetry, in the temperature range from 25 to 900 °C, in a nitrogen atmosphere. The determination of activation energies (Ea) as a function of the degree of conversion was obtained by using the Ozawa–Flynn–Wall kinetic model and at heating rates of 5, 10 and 20 °C min−1. It was observed that the catalytic cracking of the oil presented Ea of 196 kJ mol−1 for 90% conversion. At the same conversion, for the oil containing 10% of Si-MBB catalyst, the Ea decreased to 170 kJ mol−1 and for AlSi-BMM, the Ea was 155 kJ mol−1, showing the efficiency of bimodal materials containing mesopores and macropores in its structure for processing of heavy oil.

Probing the confinement of β-galactosidase into meso-macro porous silica by Raman spectroscopy

Abstract Immobilization of β–galactosidase (β-gal) into porous materials might afford to supported biocatalysts for the hydrolysis of diary products or to food additives for lactose intolerant people. Activity and stability of the loaded material generally depend on the interactions between the enzyme and the support, as well as on the pore size. Herein, Raman spectroscopy was used to evaluate the specific adsorption the enzyme into meso-macroporous silica materials, containing interconnected mesopores of 9 nm and macropores of 200 nm. Non-porous silica was used as reference material to determine the Raman fingerprint of physisorbed enzyme in the absence of any confinement. While the β-gal physisorbed on the surface of non-porous silica material exhibits the same Raman spectra as the free enzyme, the enzyme physisorbed onto meso-macroporous materials show frequency displacements of characteristic amide groups as a function of initial concentration of the feed enzymatic solution. In fact, at low initial concentration in enzyme, no shifts of the amides were recorded on Raman spectra as compared with free enzyme, indicating a preferential physisorption into macropores. By increasing the enzyme concentration, the frequency of Amide I was shifted to lower values, suggesting thus a confinement into mesopores. Finally, the enzyme concentration effect can be demonstrated by the increment of the amide band intensity in the range of 1700-1500 cm−1 as the amount of adsorbed enzyme increases. Thus, the textural properties of silica materials seem to be the key factor in the enzyme adsorption.

Synthesis of TiO₂ with Hierarchical Porosity for the Photooxidation of Propene.

The elimination of volatile organic compounds (VOCs) at low concentration is a subject of great interest because these compounds are very harmful for the environment and human health. In this work, we have developed a synthesis methodology of TiO2 that allows obtaining meso-macroporous materials with hierarchical porosity and with high thermal stability for their application as photocatalysts in the removal of VOCs, specifically propene. The materials synthesized in this work were characterized by Scanning electron microscope (SEM), Transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Thermogravimetric Analysis (TG), and nitrogen adsorption. It is observed that the samples calcined at 250 °C and 500 °C present a high photoactivity for the photooxidation of propene, which is similar to the benchmark material P25 (commercial TiO2). Moreover, the textural properties are better than those for P25, indicating that the samples are interesting for the preparation of photocatalysts with different conformations, such as in the form of coatings and fillings in different size scales.

Photon transport in cylindrically-shaped disordered meso-macroporous materials

We theoretically and experimentally investigate light diffusion in disordered meso-macroporous materials with a cylindrical shape. High Internal Phase Emulsion (HIPE)-based silica foam samples, exhibiting a polydisperse pore-size distribution centered around 19 μm to resemble certain biological tissues, are realized. To quantify the effect of a finite lateral size on measurable quantities, an analytical model for diffusion in finite cylinders is developed and validated by Monte Carlo random walk simulations. Steady-state and time-resolved transmission experiments are performed and the transport parameters (transport mean free path and material absorption length) are successfully retrieved from fits of the experimental curves with the proposed model. This study reveals that scattering losses on the lateral sides of the samples are responsible for a lowering of the transmission signal and a shortening of the photon lifetime, similar in experimental observables to the effect of material absorption. The recognition of this geometrical effect is essential since its wrong attribution to material absorption could be detrimental in various applications, such as biological tissue diagnosis or conversion efficiency in dye-sensitized solar cells.

Controlled release of ibuprofen by meso–macroporous silica

Structured meso–macroporous silica was successfully synthesized from an O/W emulsion using decane as a dispersed phase. Sodium silicate solution, which acts as a silica source and a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (EO19PO39EO19) denoted as P84 was used in order to stabilize the emulsion and as a mesopore template. The materials obtained were characterized through transmission electron microscopy (TEM), scanning electron microscopy (SEM), small-angle X-ray diffraction scattering (SAXS) and nitrogen adsorption–desorption isotherms. Ibuprofen (IBU) was selected as the model drug and loaded into ordered meso–macroporous materials. The effect of the materials’ properties on IBU drug loading and release was studied. The results showed that the loading of IBU increases as the macropore presence in the material is increased. The IBU adsorption process followed the Langmuir adsorption isotherm. A two-step release process, consisting of an initial fast release and then a slower release was observed. Macropores enhanced the adsorption capacity of the material; this was probably due to the fact that they allowed the drug to access internal pores. When only mesopores were present, ibuprofen was probably adsorbed on the mesopores close to the surface. Moreover, the more macropore present in the material, the slower the release behaviour observed, as the ibuprofen adsorbed in the internal pores had to diffuse along the macropore channels up to the surface of the material. The material obtained from a highly concentrated emulsion was functionalized with amino groups using two methods, the post-grafting mechanism and the co-condensation mechanism. Both routes improve IBU adsorption in the material and show good behaviour as a controlled drug delivery system.

Functional and Disordered Meso-Macroporous <I>γ</I>-Al<SUB>2−<I>x</I></SUB>M<SUB><I>x</I></SUB>O<SUB>3±<I>y</I></SUB> (M = Cu and/or Ce)

Disordered meso-macro porous Cu-Ce-Al2O3 nanocomposite (gamma-Al(2-x)M(x)O3 +/- y, M = Cu and or Ce) with different compositions has been synthesized. In situ templated sol-gel method has been adopted with simple EDTA ethylenediamine tetra aceticacid and ethylenediamine molecules to prepare gamma-Al(2-x)M(x)O3 +/- y, (M = Cu and or Ce). Above meso-macro porous materials were characterized by structural, spectroscopy, microscopy and textural techniques. Detailed characterization indicates that Cu2+ ions are introduced into the ceria and alumina lattice positions. Nano composite nature of the gamma-Al(2-x)M(x)O3 +/- y has been confirmed by detailed microscopy investigations. Catalytic activity of the above nanocomposite materials have been screened for environmentally important CO oxidation reaction. 30% Ce-60% Al and 10% Cu containing material shows the best activity among other meso-macroporous material with (50%) 100% CO oxidation at (107 degrees C) 145 degrees C.

Hierarchically Mesoporous Hematite Microspheres and Their Enhanced Formaldehyde‐Sensing Properties

There is intense interest in the synthesis of hierarchically porous materials (such as micro–mesoporous, bimodalmesoporous, and meso–macroporous materials), possessing multilevel porous structures and high specifi c surface areas, that can provide many novel properties and have important prospects in practical industrial processes, such as catalysis, adsorption, separation, chemical sensing, electrode materials, and biomaterials engineering. [ 1–3 ] Based on the organictemplate route, many hierarchically porous metal oxides have been synthesized, such as TiO 2 , ZrO 2 , WO 3 , Nb 2 O 5 , Ta 2 O 5 , Al 2 O 3 , and CeO 2 , [ 4–9 ] whose walls are almost amorphous or semicrystalline. However, a crystallized wall structure can be expected to provide better thermal and mechanical stability, as well as superior electric and optical properties. [ 10–12 ] However, it still remains a challenge to obtain hierarchically porous materials with crystalline walls as most of these metal oxides often suffer from structural collapse during their crystallization. Recently, the surfactant-directed assembly of preformed crystalline nanoparticles into mesostructures was successfully utilized to synthesize some crystalline mesostructured materials. For example, Corma et al. reported mesoporous CeO 2 assembled from CeO 2 nanocrystals, [ 13,14 ] Niederberger and co-workers prepared mesoporous SnO 2 using SnO 2 nanocrystals, [ 15 ] and Li and co-workers also realized the synthesis of porous 3D spheres using various nanocrystals, such as Ag, Ag 2 S, and Ag 2 Se. [ 16,17 ] However, this approach requires multistep procedures, including the synthesis and collection of nanoparticles with well defi ned shapes and sizes, their redispersion without agglomeration in assembly solution, and the fi nal assembly. Moreover, the assembly in the above-mentioned cases usually occurred on a single-scale level and thus resulted in materials with single-level porous structures. In this Communication, we demonstrate that the dual assembly of nanoparticles can be realized directly at its synthetic solution, without collection and redispersion procedures, and that this fi nally allows us to obtain a hierarchical porous material.

Self-formation phenomenon to hierarchically structured porous materials: design, synthesis, formation mechanism and applications

In this paper, we will thoroughly review a novel and versatile self-formation phenomenon that can be exploited to target porous hierarchies of materials without need of any external templates only on the basis of the chemistry of metal alkoxides and alkylmetals. These hierarchically porous materials have unique structures, which are made of either parallel funnel-like/straight macrochannels or 3D continuous interconnected macroporous foams with micro/mesoporous walls. The self-generated porogen mechanism has been proposed, leading to a series of techniques to tailor porous hierarchy, i.e. the use of different chemical precursors (single metal alkoxides, mixed metal alkoxides, single molecular precursors with two different alkoxide functionalities, alkylmetals, etc., …), the control of their hydrolysis and condensation rates (pH, chelating agents,…) and the addition of alkoxysilanes as co-reactant. Various chemical compositions from single or binary metal oxides, to aluminosilicates, aluminophosphates, silicoaluminophosphates, metallophosphates,… can be prepared, offering a panel of potential applications. Some perspectives have been proposed to transform the synthesized materials with a hierarchy of pore sizes to micro-meso-macroporous crystalline materials with zeolite architectures. The advantages of this self-formation preparation method have been discussed compared to traditional templating methods. The possibility to combine with other strategies, for example soft or hard templating, to target even more sophisticated hierarchically meso-macroporous materials with specific structure and function for various applications has been presented. The "hierarchical catalysis" concept has been re-visited.

Niobia–silica aerogel mixed oxide catalysts: Effects of the niobium content, the calcination temperature and the surface hydrophilicity on the epoxidation of olefins with hydrogen peroxide

Niobia–silica mixed oxides can be synthesized under acidic conditions and evaporated under supercritical conditions to yield meso-macroporous materials. The amount of active metal in the catalyst changes the acidity of the final material and in the epoxidation of allylic alcohols (geraniol) a high amount of niobium improves the production of glycols. Surface modification with methyl groups is possible by addition of methyltriethoxysilane without any change in the morphological properties of the final materials. The extent of methylation does not change significantly the surface area and the porosity but does change the catalytic activity of the materials in the epoxidation of the cyclooctene and geraniol. Methylation improves the activity of the catalysts and the use of the oxidant in the epoxidation of unsubstitued olefins, whereas only small differences were observed in the epoxidation of geraniol.

Highly Ordered Mesoporous and Hierarchically Nanostructured Meso-macroporous Materials for Nanotechnology, Biotechnology, Information Technology and Medical Applications

A great deal of progress has recently been made in the field of ordered porous materials with uniform channel dimensions that can be adjusted over a wide range of length scales. The present paper describes the state of the art and the evolution from highly ordered mesoporous silica, aluminosilicate and pure carbon materials to our very recent success in hierarchically structured meso-macroporous carbon materials, single and binary oxides and aluminosilicates. By means of selected examples, we want to shed some light on the current strategies for the conception of these sophisticated materials. The great potential applications of these new nanomaterials emerge while their real utilisation in some expected and traditional industrial processes such as catalysis, separation, electrode materials for fuel cells and biomaterials still faces some important challenges, for example, the high price, the pelletisation and the stability (thermal, hydrothermal and water resistance). Some new perspectives, which can open the concrete applications of these materials in nanotechnology, biotechnology, information technology and medical purposes, will be prospected.

Insights into hierarchically meso–macroporous structured materials

A great deal of progress has recently been made in the field of ordered porous materials having uniform channel dimensions which can be adjusted over a wide range of length scales. Incorporation of macropores in mesoporous materials combines benefits from both the mesoporous and macroporous structures. Hierarchical materials containing both interconnected macroporous and mesoporous structures have enhanced properties compared with single-sized pore materials due to increased mass transport through the material and maintainance of a specific surface area on the level of fine pore systems. Bimodal mesoporous–macroporous inorganic materials can be prepared by using a self-assembling surfactant or amphiphilic block copolymer species in conjunction with macrotemplates such as colloidal crystals, polymer foams, bio-celluloses, emulsions, inorganic salts and ice crystals, or by macroscopic phase separations. Here, we review the state of the art for hierarchical meso–macroporous inorganic materials and their carbon replicas from the viewpoint of synthesis strategies and emerging applications. Detailed synthetic processes are described, in which the very recently developed spontaneous formation of meso–macroporous (single and binary) metal oxides, metal phosphates and aluminosilicates is specifically addressed. These novel meso–macroporous materials have found a number of applications, including HPLC separation, catalysis, fuel cell electrode materials, biomaterials engineering, controlled drug delivery devices, and membrane reactors, and these are discussed illustratively.