Sepiolite Accessibility

SEM studies shown in Fig. la revealed that the Pt/sepiolite ICP sample possessed a long range ordered structure at macroscopic level, with ink bottle pores of about 12 pm left by removal of the AC, accessed through an interconnected network with pore entrances of about 2 pm. When the support was a mixture sepiolite-alumina or sepiolite-titania, the bundles of sepiolite fibres act as a matrix in which the particles of alumina or titania are distributed as shown in Fig lb for Pt/sep.Ti02 ICP. 

In this work the catalytic activity of a series of copper oxide catalysts supported on monolithic honeycomb supports in the reduction of nitrogen oxide with propylene in an oxidising atmosphere was studied. The monoliths were produced from acid washed sepiolite, sepiolite or a mixture of sepiolite and alumina in order to study the effect of the support on the activities and selectivities of the catalysts. Tlie introduction of nickel oxide as a second active species on the overall activity was also detennined. Finally tlie application of an alumina washcoat impregnated with the copper and nickel salts to increase the accessibility of tlie gases to be treated to the active phase was studied. 

Catalysts supported on monoliths based on a mixture of sepiolite and alumina had much better selectivities to NO conversion than the corresponding catalysts supported on monoliths prepared from sepiolite. Tliis was thought to be due to the different textural properties of these mixed composition supports, where the presence of wide macropores and the higher smface areas improved the accessibility of the reaction gases to the internal pore network. 

One of the questions that can be answered with the help of adsorption measurements concerns the microtexture of natural clay minerals. Several idealized models for the texture of soil clays (see [5]) have been considered, but rather than assuming one model a priori, one should try to gain useful information from experimental relationships between the size of clay particles and apparent density or surface area and internal porosity, as described in Sections 6.1 and 6.2.1. Experiments aiming at the evaluation of the microtextures of clay minerals were carried out by Ben Ohoud and van Damme [95], who studied kaolinite, sepiolite, palygorskite and 20 monoionic montmorillonite samples. The accessible surface area S of consecutive fractions of size r was measured by N2 adsorption using the classical BET method, whereas the open porosity P was measured from the amounts of adsorbed N2 at a relative vapor... 

Ruiz-Hitzky E (2001) Molecular access to intracrystalline tunnels of sepiolite. J Mater Chem 11 86-91... 

The accessibility of tunnels to the nonpolar gas molecules used in surface area determinations, although questioned in some earlier works (68), has now been clearly demonstrated for sepiolite by Ruiz-Hitzky (69). This was accomplished by using the Howarth-Kawazoe analysis (70) of argon adsorption isotherms. For samples previously degassed at 120°C, the analysis shows the predominance of pores with a diameter of 0.6-0.7 nm. These pores disappear when sepiolite is heated at 350°C under dynamic vacuum due to loss of coordinated water and folding of the structure (71). 

Other organic molecules such as pyridine also have access to the intracrystalline tunnels of sepiolite, as demonstrated clearly by Inagaki (72a), Kuang (72b), and Ruiz-Hitzky (69). The latter has recently reviewed the subject of accessibility and concluded that, for sepiolite, larger molecules (Fig. 13), such as methylene... 

Pore distribution in sepiolite remains almost unaltered through sulphurization, in spite of the variations shown in Fig. 2b and Table 2. Only a decrease in the 10-100 pm range in favour of that of 0.025-0.25 is appreciated. This distribution can be related to a gradual filling of all pore sizes, resulting in a large decrease in surface area and pore volume that leaves nearly constant the values of mean, median and mode pore radius. The rate of sulphur deposition should decrease in line with the fall in accessible sulphur (Fig. 1). 

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