J. M. Trillo

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.

Structure-directing effect of phyllosilicates on the synthesis of y-Y2Si2O7. Phase transitions in Y2Si2O7

This paper describes the low temperature hydrothermal synthesis of y-Y2Si2O7 from a layered silicate (2 ∶ 1 clay), a precursor which acts as a structure-templating agent. Both the transformations undergone by the clay during the hydrothermal process and the new products formed at different reaction stages have been characterised at both long and short range orders. Secondly, the influence of the aluminium content of the clay on the stability of y-Y2Si2O7 has been investigated. For this purpose, another layered clay, containing nominally no Al, has been submitted to the same hydrothermal treatment as the previous one. The results indicate that the amount of aluminium present in the starting material assists the stabilisation of y-Y2Si2O7. The behaviour of y-Y2Si2O7 with increasing temperature has been studied and the sequence of polymorphs and transition temperatures are reported and compared with those found in the literature.

Lanthanide oxides: Thermochemical approach to hydration

The behaviour toward hydration of a series of Ln2O3 has been studied as a function of water pressure and temperature by means of temperature programmed decomposition measurements. Thermochemical calculations have pointed out the structural change associated in going from ytterbium to lutetium in Ln(OH)3 to be responsible for the apparent anomalous behaviour of Lu2O3. On this basis generalizations about the expected behaviour along the 4f series have been considered taking into account the simple ionic model for solid lanthanide compounds.

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.

Study of the reversibility on the local La3+ environment after thermal and drying treatments in lanthanum-exchanged smectites

Abstract Changes produced by thermal treatments (300°C, 500°C, and 700°C) and after an “in situ” drying treatment on the hydration sphere of the lanthanum ions located in the interlamellar space of montmorillonites have been investigated. X-ray absorption spectroscopy (XAS) spectra for these materials have been recorded with this purpose at room and liquid nitrogen temperature by using a special cell where the temperature and the atmosphere could be controlled. The dry atmosphere employed during the drying treatment produced either drastic or no significant effects on the La L III -edge EXAFS spectra, depending upon the previous thermal treatment given to the sample. These results have allowed us to postulate a dehydration reaction mechanism for these samples and to show how XAS measurement is a very useful tool to be employed in the study of the local environments adopted by the exchangeable cations in clays.

Structure of Lu3+ and La3+ ions intercalated within layered clays as determined by EXAFS

Abstract La 3+ and Lu 3+ ions are intercalated within the interlamellar space of montmorillonite as aquocomplexes. Heating at 700°C for 6 h induces a structural change around the lanthanide ions resulting in the formation of oxide-type environments. The change is complete only for the lutetium sample, which is consistent with the lower value of the dehydration enthalpy of this cation.

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.

Formation at 300°C of a High-Temperature Disilicate from Hydrated Lutetium in a Layered Aluminosilicate

Abstract The interaction between interlamellar Lu(III) cations and the layered silicate structure has been studied by means of MAS-NMR, XRD and EDX. Irreversible fixation of exchangeable Lu(III) cations upon hydrothermal treatment at 300℃ of Lu-saturated montmorillonite is reported, through the formation of the crystalline phase Lu2Si2O7. It is shown that the lowest temperature previously described for this reaction, 400℃ does not indicate a thermodynamic boundary. This basic solid-state finding is relevant for some clay applications such as those related to the safety of nuclear waste repositories with engineering barriers.

EXAFS study of the interaction of lanthanide cations with layered clays upon hydrothermal treatments

Abstract The interaction of ions intercalated in layered clays with the host lattice is of interest concerning the preparation and applications of new clay-based materials. Intercalation reactions are usually reversible, and the structural integrity of the host lattice is formally conserved in the course of the forward and reverse processes. Many studies of this type involving rare earth cations have been carried out because these ions simulate radionuclides in the process of retention of nuclear wastes by clays. In relation with this process, thermal and hydrothermal treatments are known to prevent the reversibility of the exchange reaction with diverse cations, resulting in their “irreversible” fixation. This is of importance for the protection of the biosphere. With the aid of EXAFS, we have studied the reaction of lutetium ions intercalated within montmorillonite layers. We have followed the structural metamorphosis of the initial Lu(III) aquocomplex to the oxide-type environment, or disilicate phase. This process is of fundamental importance in evaluating the safety of the future nuclear waste repositories. Up to now, the hydrothermal treatments have been applied in our laboratory at temperatures higher than the real expected ones. The aim of this paper is to report the first results from a sample submitted to hydrothermal treatment at 200°C and for a long period of time (two months). These results are compatible with the coexistence of Lu(H 2 O) 8 3+ and Lu(OH) 3 , what implies that significant structural changes have been induced by the hydrothermal treatment at 200°C. Since these conditions are very similar to those expected in the projected nuclear waste repositories of the bentonite backfill, the structural effects, when extrapolated to geologic timescales, support the safe fixation of radionuclides.