Characterisation of the microporosity of chromia- and titania-pillared montmorillonites differing in pillar density: II. Adsorption of benzene and water

A saponite from Ballarat (USA) was intercalated with the recently described aluminium polycation [Al13(µ3-OH)6(µ2-OH)12(heidi)6(H2O)6]3+. Different Al:heidi ratios during the preparation of the intercalating solutions and Al:clay ratios during the intercalation process were considered. Solids with a basal spacing of 21.8–25.3 A were obtained in this way, the ordering in their structure depending on the Al:heidi and Al:clay ratios. The surface area of these samples was very low, about 25 g-1, probably because of the large size of the polycations that fill up the internal galleries of the solids. When calcining the intercalated solids at 500°C, pillared solids with basal spacings of about 16–17.5 A were obtained, by elimination of the organic part of the polycations. The surface area of the pillared solids increased to values up to 350 g-1.

Al-pillared saponites Part 4. Pillaring with a new Al13 oligomer containing organic ligands

Argilas pilarizadas formadas pela intercalacao de oxidos de metais de transicao, capazes de prevenir o colapso da estrutura, constituem materiais micro- e mesoporosos com diferentes atividades cataliticas e estabilidades. Argilas pilarizadas com cromo e com ferro foram utilizadas como catalisadores em reacoes de oxidacao do cicloexano em fase gasosa. Os resultados indicam que os catalisadores sao ativos e seletivos na producao de cicloexanona e cicloexanol. Entretanto, especial atencao deve ser dada a acidez superficial dos solidos, responsavel pela formacao de cicloexeno no meio reacional, com consequente reducao dos produtos de interesse.

Oxidação de ciclohexano em fase gasosa catalisada por argilas pilarizadas com ferro e cromo

Hydroxy-chromium beidellite is of interest for use in catalysis and as adsorbents. The micro porosity of the materials differing in pillar density has been explored. Nitrogen adsorption data obtained at very low relative pressures were used. This paper determines the influence of the OH-Cr species on the structure and texture of modified dioctahedral smectites (beidellite). Minerals varying in cation exchange capacity pillared with various Cr pillar precursor ions can provide a family of catalysts that differ in surface area, porous structure and concentration of active sites. These data were analyzed by means of the method as and the BET, Langmuir and Dubinin–Radushkevich approaches. Different types of isotherm equations are used, one to describe adsorption in micro pores while another one for describing adsorption on external surfaces. All methods applied demonstrated that these solids possess micro pores over a broad pore-size range. Chromium pillared clays contain a significant amount of ultramicropores. Interlayer with larger spacing resulted in different specific surface areas and micro pore volumes. Generally, this behaviour could be related to the different structural characteristics of the smectites.

	 

	Key words: Chromium pillared clays, micro porosity analysis, nitrogen adsorption-desorption, textural study.

The textural property of chromium pillared clay

Preparação de novos materiais a base de argila natural para aplicação como adsorventes na remediação ambiental

The porous structure of montmorillonite fixed by chromium oxide was studied by nitrogen, water vapor, and benzene adsorption. The adsorption data obtained were analyzed in detail by the α s -method and the theory of volume filling of micropores. The fixation of acid-treated montmorillonite K10 by hydroxochromium cations resulted in the formation of the pillared clay with considerable mesoporosity. Additional micropores appeared, and new polar adsorption sites were formed. The role of new absorption sites belongs to hydroxochromium cations or chromium oxide clusters localized in the interlayer space of the pillared clay. A microporous material was found to be formed mostly by the fixation of Oglanli bentonite. The calcination of the synthesized pillared clays at 673 K resulted in an increase in the average width of their micropores and mesoporosity. The porous montmorillonite structure fixed by chromium oxide can be purposefully formed using clays of different chemical composition and physical and chemical properties.

E.M. van Oers

Papers

Study of porous structure of pillared clays : 1. Montmorillonite fixed by chromium(III) oxide