E.F. Vansant

Silylation of micro-, meso- and non-porous oxides: a review

Changing the surface characteristics of silicon or mixed Si-Al oxides has created new prospects in both catalysis and separation technology. This review presents the surface modification of these materials by silylation over a wide range of pore sizes. The influence on the porosity, the adsorption characteristics and thermodynamic background of these modifications are summarised. The reaction mechanisms are reviewed for a wide range of silylation reagents.

Plugged hexagonal templated silica: a unique micro- and mesoporous composite material with internal silica nanocapsulesElectronic supplementary information (ESI) available: Fig. S1: X-ray diffractogram of a PHTS material. Fig. S2: TEM images of SBA-15 and PHTS-2. Fig. S3: hydrothermal stabilities. See http://www.rsc.org/suppdata/cc/b2/b201424f/

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro- and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures.

The relation between the synthesis of pillared clays and their resulting porosity

Abstract Clays can be converted to stable microporous materials by the incorporation of pillars. The type of pillars determine to a large extent the porosity features of the pillared clays. Different pillaring species (Al, Ti, Zr, Fe) were investigated and discussed in terms of pore volume, pore size and pore size distribution. The porosity induced by pillaring can be further modified by small modifications during the synthesis. More meso- and macroporosity can be achieved by choosing clays with smaller clay layer sizes (laponite compared to montmorillonite) or by the way of drying (air or freeze drying). Incorporation of Zr and Cr in the Fe-pillars creates mixed oxide pillared clays and pillared clays with new properties were obtained. The adsorption strength strongly increased resulting in higher adsorption capacities as demonstrated for gases (O2, N2 , CO2, CO and CH4) and chlorinated hydrocarbons. Pre-adsorption of amines effectively reduces the pillar density (Fe-PILL) and distributes the pillars more homogeneously between the layers (Al- and Ti-PILC) creating higher pore volumes and higher adsorption strength. Amines were also useful for the orientation of laponite clay layers to more regularly ordered face-to-face stacked pillared laponite clays which exhibit significant increased pore volumes.

Synthesis and characterization of a controlled-micropore-size carbonaceous adsorbent produced from walnut shell

Abstract A new and simple method is presented for the production of carbonaceous adsorbents with a controlled pore size from walnut shells. The carbonized walnut shells (char) were mixed with a solution of potassium hydroxide. The mixture was then activated thermally in the absence of air. The products formed were washed and dried in order to obtain a carbonaceous adsorbent. A systematic study of the effect of the activation temperature, activation time and KOH char ratio on the microporosity of the final product was carried out. The prepared samples were subjected to adsorption with benzene at room temperature for characterization in accordance with the Dubinin-Radushkevich, BET and Langmuir equations and with respect to the micropore volume, pore width, adsorption energy and surface area. Probe molecules were also used in order to define the microporous structure. A higher temperature, a longer activation time and a higher KOH content increased the adsorption capacity and resulted in a widening of the pores. A milder activation favored the production of adsorbents with small micropores, accentuating the molecular sieving properties. By altering the synthesis conditions, the size of the pores in the adsorbent can be controlled in the ultramicropore (pore diameter

Porous Ceramic Membranes: Preparation, Transport Properties and Applications

The preparation and characterization of porous ceramic membranes is presented. These membranes consist of a macroporous support system, with or without a mesoporous intermediate layer, and a microporous top layer. For the macroporous support membranes two manufacturing routes are described: a conventional and a RBAO (Reaction Bonded Aluminium Oxide) route. The mesoporous γ-Al2O3 layer is obtained by means of a sol-gel dipcoating technique. Three microporous top layers are considered: SiO2, Al2O3-pillared montmorillonite and Laponite. These top layers have different pore structures which results in different gas transport properties. A SiO2 membrane can be used for H2 removal from a gas mixture. Al2O3-pillared montmorillonite and Laponite membranes do not show specific gas separation properties. Dehydration of water/2-propanol mixtures by means of pervaporation also proved a different behavior for these microporous membranes.

Acidic porous clay heterostructures : study of their cation exchange capacity

Abstract The cation exchange capacities (CECs) of two porous clay heterostructures (PCHs), derived from natural montmorillonite (PMH) and synthetic saponite (PSH), have been studied. Both materials are highly porous, with surface areas of 997 m 2 /g (PMH) and 1118 m 2 /g (PSH). In order to obtain exchangeable ammonium cations in the pore structures of the PCHs, different modifications are performed on calcined and extracted PMH and PSH. Three methods for the formation of NH 4 + -exchanged PCH forms are described and evaluated: (1) adsorption of ammonia under a gas flow on calcined and extracted PCHs in acidified methanol; (2) direct exchange in NH 4 Cl solution; (3) solvent extraction with NH 4 Ac/EtOH/H 2 O. The obtained ammonium containing materials are subsequently exchanged for K + cations in aqueous solution in order to determine the CEC of the PCH solids. With Kj-N analysis and infrared-DRIFT technique, evidence is found for the presence of ammonium cations on the surface of PCHs. The stability (crystallinity and porosity) of the porous solids under the different treatments has been investigated using X-ray diffraction and N 2 -adsorption isotherms. The adsorption under NH 3 gas flow proves to be the best method for maintaining the porosity of the studied PCHs. Maximal K + loadings of 0.55 mmol/g (PMH) and 0.37 mmol/g (PSH) are obtained for ammonia-loaded and extracted PCH samples in MeOH/HCl.

Influence of polymer matrix and adsorption onto silica materials on the migration of α-tocopherol into 95% ethanol from active packaging

In this study, the effect of polymer materials with different polarity, namely low density polyethylene (LDPE) and ethylene vinyl acetate (EVA), on the migration behaviour of α-tocopherol from active packaging was investigated. The antioxidant was also adsorbed onto silica materials, namely SBA-15 (Santa Barbara-15) and Syloblock, in order to protect the antioxidant during extrusion and to ensure a controlled and sufficient release during the shelf-life of the food product. Migration experiments were performed at 7.0 ± 0.5°C and 95% ethanol was used as fatty food simulant. All films contained a high concentration of α-tocopherol, ∼ 2000 mg kg−1, to obtain an active packaging. Polymer matrix had a small influence on the migration profile. The migration of 80% of total migrated amount of antioxidant was retarded for 2.4 days by using LDPE instead of EVA. When α-tocopherol was adsorbed onto both silica materials, the migration of 80% of total migrated amount of antioxidant was retarded for 3.4 days in comparison to pure α-tocopherol. No difference was seen between the migration profiles of α-tocopherol adsorbed onto both silica materials. In the case of pure α-tocopherol, 82% of the initial amount of α-tocopherol in the film migrated into the food simulant at a rather fast migration rate. In the case of adsorption on silica materials, a total migration was observed. These antioxidative films can have positive food applications.

Preparation and characterization of zirconium pillared laponite and hectorite

The Zr-pillaring of natural hectorite and synthetic laponite clay was investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to characterize the different substrates. The difference in particle size of both clays is responsible for their differences in crystallinity, surface area (SA) and micropore volume (μPV) and consequently for their pillared forms too. Pre-adsorption of ethylenediamine in the interlayer space of laponite is performed in order to reduce the important contribution of edge-to-face and edge-to-edge stacking of the clay layers and creating a more homogeneous substrate for pillaring. As is proven by XRD, TGA and FTIR, ethylenediamine is exchanged completely for the Zr-pillaring precursors during the pillaring reaction. Surface areas and micropore volumes of, respectively, 482 m2/g and 0.34 cm3/g for Zr-pillared laponite and 171 m2/g and 0.064 cm3/g for Zr-pillared hectorite after calcination are obtained. Through pillaring, especially small pores (<0.71 nm), additional secondary micropores are formed with laponite while for hectorite, pores over a broad size range are observed. The prepared pillared interlayered clays (PILCs) were tested for their gas adsorption behaviour. Gas adsorption measurements on Zr-laponite reveal high adsorption capacities for N2 and O2 with a low N2/O2 selectivity at 0°C.

Preadsorption of organic compounds on iron oxide-pillared clays

Abstract Iron(III) oxide-pillared clays were prepared by a reaction of Na + -montmorillonite with base-hydrolyzed solutions of Fe 3+ salts and a subsequent thermal conversion of the intercalated polycations. Those pillared clays have low micropore volumes (0.04 cm 3 /g) and surface areas (95 m 2 /g). The low porosity is due to a high density of Fe pillars between the clay sheets. This disadvantage can be overcome by a preadsorption of amines between the clay sheets prior to pillaring with the iron precursor. As a result, the pillar density decreases because a part of the interlayer space is occupied by the amine. During calcination, the pillaring precursor is converted into rigid iron oxide pillars. and the organic compound is removed. This results in a micropore volume and surface area 2.5 times higher reflecting an important increase of the adsorption capacity.

Study of Fe2O3-pillared clays synthesized using the trinuclear Fe(III)-acetato complex as pillaring precursor

Abstract Pillaring montomorillonite with the trinuclear Fe(III)-acetato complex results in an Fe 2 O 3 -pillared clay. The chemical nature of these intercalates, however, is still a point of discussion. With the aid of Fourier transform infrared (FTIR) techniques (mid-IR: diffuse reflectance, circular internal reflection; far-IR: transmission) it was possible to elucidate the nature of the pillaring precursor before, during and after the pilaring process. Changes in the acetato configuration during this process could easily be monitored in the mid-IR, while changes in the triangular M 3 O unit were followed in the far-IR. More information on the pillar composition was obtained by thermogravimetric analysis (TGA) (precentage acetyl) and electron probe micro analysis (EPMA) (percentage Fe). A further characterization of the resulting pillared clays was carried out using nitrogen adsorption to determine the specific surface area and the porosity. More detailed information on the pore size was obtained using the logarithmic adsorption isotherm and the derivative of this curve. It became clear that the orginal Fe complex changed seriously during hydrolysis; large Fe-oxyhydroxide polymers were formed. Intercalation of these species resulted in a mainly mesoporous structure.