J. Poyato

Alumina-pillared montmorillonite: effect of thermal and hydrothermal treatment on the accessible micropore volume

In the search for materials having larger pores than zeolites, the concept of pillared clays gradually emerged. The substitution of zeolite-based commercial catalysts by pillared clays requires the improvement of the thermal and hydrothermal stability of the latter. Characterization of pillared clays has essentially consisted of nitrogen BET specific surface area and X-ray diffraction powder measurements. The study of the texture of pillared smectites really requires the assessment of their microporosity. However, little detailed information is available on the evolution of microporosity with thermal and hydrothermal treatments of these materials. The aim of this work was to supply such information on an alumina-pillared montmorillonite. The effect of La(III) addition was also studied. Up to now the thermal stability of the pillars has been over-estimated from measurement of the basal spacing. However, it has been found that the accessible volume decreases with increasing calcination temperature at a much greater rate than does the basal spacing. Therefore, the generally accepted conclusion that alumina pillars are stable at temperatures even higher than 500°C may give a misleading picture of pore structure evolution with calcination temperature. Lanthanum appears to exert a small positive effect on the thermal and hydrothermal stability of the pillared montmorillonite.

Montmorillonites intercalated with Al(III), La(III) and alumina pillars: structural aspects and reactivity

Abstract The location, environment and availability of the interlamellar species in modified layered silicates have been investigated using two montmorillonites selected with different charge deficit distributions. A structural study has revealed release of protons and further migration to the lattice as thermal treatments are performed. The proton availability has been examined through a catalytic reaction, all the modified samples showing similar activities in this test. A possible explanation for this levelling effect has been suggested.

Effect Of La(III) on the Thermal Stability of Al-Pillared Montmorillonite

Abstract The promotion effect of La(III) addition upon the thermal stability of an alumina-pillared montmorillonite has been studied. The accessibility of molecular water to the interlamellar spacing has been measured as a test for the adsorbance capacity of the active solids. Nitrogen preheating at 300°C exert a negative influence on the samples containing lanthanum. Thermal treatments at temperatures higher than 300°C lead to a reduction in the uptake of H 2 O, which is not accompanied by a parallel change in the specific surface area or in the basal spacing of the smectite.

Retention of tripositive lanthanides (Gd, Ho, Yb, Lu) by montmorillonite

Abstract A Trancos montmorillonite from Gador (Spain), with a calculated charge deficit per unit cell of 0.87 units, has been saturated with Gd (III), Ho (III), Yb (III) and Lu (III). FT-IR spectra and b-dimension measurements show no migration of the 4f ions from their interlamellar sites to octahedral ones with air-heating treatments at 300°C. The fixation of the lanthanide ions by the montmorillonite with heating should be ascribed exclusively to the hydrolysis of the hydrated ions. The FT-IR spectroscopy results show a large decrease in the extension of the lanthanide montmorillonite hydration, which has been ascribed to a decrease of the more firmly held water, direct coordinated to the cations in a first sphere. The heated lanthanide montmorillonites are capable of swelling, the ones with the heavier 4f ions showing some inertia to it.

Effect of pre-heating on the thermal dehydration of lanthanide montmorillonites

Abstract Simultaneous DTA-TG, TPD-GC, BET specific surface area and FT-IR measurements have been carried out on Ln-montmorillonites (Ln  La, Nd, Gd, Ho or Yb) in order to study the effect of air-heating at 300°C on Ln III fixation and subsequent thermal dehydration. In the unheated samples, the hydration shell of the exchangeable 4f ions contains an average number of H 2 O molecules which ranges from 20 (La) to 12 (Yb). This decrease can be ascribed to partial hydrolysis of interlamellar cations, which increases for all the samples on heating at 300°C. Deprotonation of the montmorillonites by reaction of their structural OH groups with the interlamellar 4f hydroxides is suggested.

Swelling of lanthanide (La, Nd, Gd) montmorillonites heated in air and under vacuum

A sodium-montmorilion ite from Almeria (Spain), with the structural formula (Si7.644+ Al0.363+) (Al3.093+ Fe0.283+ Mg0.6092+) O20 (OH)4 has been used as starting material for the preparation of samples saturated with lanthanum, neodymium and gadolinium. By application of the Greene-Kelly Li+- test the inability of the sample to re-expand with glycerol, has been observed, in agreement with the existence of a predominant charge in the octahedral sheet. The effect of heating at 300° C, in air and under vacuum, on the swelling capacity of the lanthanide montmorillonites has been studied. Prolonged re-expansion in water after thermal treatment in air is necessary to reproduce the initial basal spacing of the samples. From Fourier transform infrared spectra it is concluded that the interlamellar hydrated lanthanide cation concentration decreases on heating, which agrees with the decrease in the cation exchange capacity claimed in the literature. The mechanism of exchangeable ion migration into octahedral vacant sites with thermal treatments, advocated in the case of lithium-saturated montmorillonite, does not seem to occur in the case of lanthanide-saturated samples. The partial hydrolysis of the interlamellar hydrated cations to polyhydroxycations is proposed as the only mechanism for explaining the loss or exchange able 4f ions.