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Abstract--Ce/Al-and La/Al-pillared smectites were prepared by cation exchange of bentonite, saponite and laponite with hydrothermally treated (130-160 ~ for 16-136 h) solutions containing mixtures of aluminumchlorohydrate (ACH) and Ce3+-/and La3+-salts. After calcination at 500 ~ the pillared products are characterized by basal spacings between 24.8 and 25.7 ,~ and surface areas of approximately 430 m 2 g 1. The products are hydrothermally stable at 500 ~ after 2 h in steam. The large basal spacings are due to the formation of a large Ce/La-bearing Al-polyoxocation, whose formation is favored by initially high A1 concentrations ~3.7 M and an OH/A1 molar ratio of approximately 2.5. The Ce/A1 or La/A1 molar ratios can be as low as 1/30. 27A1 nuclear magnetic resonance (NMR) spectroscopy has shown that the polyoxocation has a higher Altetr~earal/A1 ~ ratio than the Keggin structure Al13, which may partly explain the higher stability compared to normal Al-pillared clays. Hydroconversion of n-heptane indicated that the activity of the Pt-loaded pillared products is higher than that of a conventional Pt-loaded amorphous silica-alumina catalyst. Selectivity is strongly dependent on the type of starting clay and its acidity. In industrial hydrocracking of normal feedstock, a Ni/W-loaded Ce/Al-pillared bentonite catalyst showed rapid deactivation due to coke-formation reducing the surface area and the pore volume. Additionally, coke-formation is facilitated by the relatively high iron content of the pillared bentonite (3.43 wt% Fe203).

Valverde et al. (2000) recently reported on the preparation and characterization of Al-pillared smectites modified with Ce and La. Pillaring of clays with A1 polyoxocations (Keggin type Al~3 ) and a large variety of other complexing cations has been reported many times since the seventies. McCauley (1988) described in a patent the preparation of thermally stable pillared clays with large basal spacings of -28 A, followed by publications by Sterte (1991aSterte ( , 1991b and Booij and coworkers (Booij et al., 1996a(Booij et al., , 1996b. These authors used hydrothermally treated or refluxed solutions containing the A1 polyoxocation and the rare earth elements in the form of their chloride salts. The presence of cerium or lanthanum seems to promote polymerization of the A1 polyoxocation. Until now however, nobody has been able to assess the structure of the newly formed pillaring molecules. Although the interlayer spacing is about twice that of the normal A1-pillared clay, chemical analyses indicate smaller amounts of Ce and La than expected from polymerization of the A1 polymer alone. Several other studies on the combination of A1 pillars with Ce or La resulted in the formation of pillared clays with basal spacings characteristic of Al-pillared clays. However, catalytic activity is enhanced owing to Ce or La (e.g., Gonz~ilez et al., 1992;Mendioroz et aL, 1993;Trillo et al., 1993).Unfortunately, Valverde and coworkers (Valverde et aL, 2000) missed most of the important publications on the thermally stable large-pore pillared clays based on AI pillars modified by Ce or La. The basal spacings reported by Valverde et al. (2000) are in a similar range to those reported by others for Al-pillared smectites (Kloprogge, 1998 and references therein), but -6--7 A smaller than those reported for the A1 and rare-earth element (A1/REE) large-pore pillared smectites (McCauley, 1988; Sterte, 1991a Sterte, , 1991bBooij et al., 1996aBooij et al., , 1996b. This indicates that rare earth elements do not form an integral part of the pillars within the interlayer. Although the surface areas are in the order of what is expected for a pillared clay, the micropore volume is significantly lower as, for example, observed for Al-pillared montmorillonite SWy-1 (Kloprogge et al., 1994). However, their micropore volume is much higher than that of large-pore A1/REE pillared clays in which no micropores are present at all because the average pore diameter is --60-70 A (Booij et aL, 1996b). The relatively low micropore volume can probably be explained by the incorporation of a relatively large amount of Ce or La in the interlayer between the pillars during the synthesis of the pillared clays where initial REE/A1 ratios of --<1.8 were maintained. Although the thermal stability of these pillared clays is enhanced, the access to the catalytically active sites in the pillared clays may be severely hindered. However, Valverde et al. (2000) did not report any catalytic testing on their materials.Valverde et al. (2000) also reported on the infrared s...

Two semantic paradoxes, the Liar and Curry’s paradox, are analysed using a newly developed conception of procedural semantics (semantics according to which the truth of propositions is determined algorithmically), whose main characteristic is its departure from methodological realism. Rather than determining pre-existing facts, procedures are constitutive of them. Of this semantics, two versions are considered: closed (where the halting of procedures is presumed) and open (without this presumption). To this end, a procedural approach to deductive reasoning is developed, based on the idea of simulation. As is shown, closed semantics supports classical logic, but cannot in any straightforward way accommodate the concept of truth. In open semantics, where paradoxical propositions naturally ‘belong’, they cease to be paradoxical; yet, it is concluded that the natural choice—for logicians and common people alike—is to stick to closed semantics, pragmatically circumventing problematic utterances.