Clays and Clay Minerals, VoL 44~ No. 2. ~70-180. 199@ INTERACTION OF TRIALKYL PHOSPHITES WITH MONTMORILLONITES G. Dlos CANCELA, E. ROMERO TABOADA, E J. HUERTAS, A. HERNANDEZ LAGUNA, AND F. S,~NCHEZ RASERO Estacidn Experimental del Zaidfn, Consejo Superior de Investigaciones Cientfficas Profesor Albareda, I, 18008 Granada, Spain Abstract--Complexes formed between montmorillonite, saturated in Li +, Na +, Mg 2+, Ca 2+, Co 2+, Fe 3+, Cu 2+ and Zn 2§ and trimethyl phosphites (TMP) and triethyl phosphites (TEP) were studied. In all of the cases, ph~3sphites penetrate into the interlayer space of the montmorillonite and produce solvates whose basal spacing varies depending on the characteristics of the exchangeable cation. All the complexes with low basal spacing (Li +, Na +, Mg z+, Co 2~ and Zn 2+) are stable in vacuum, whereas those with high basal spacing, formed by the Ca 2+ sample with TMP, and Ca 2" and Fe ~* samples with TEP are transformed into low basal spacing complexes in vacuum. The complexes with high basal spacing (Cu ~+ sample with TMP and TEP) are stable in vacuum. The TMP and TEP complexes stable in vacuum with low spacing are thermally destroyed in one or two stages with two loss maxima, as a result of partial burning of phosphite molecules. Those with high spacing (Cu 2+) are destroyed in two stages; the first is probably the result of the transformation process from high to low spacing, as a consequence of the structural reorganization of the molecules which remain in the interlayer space, and the second, could be associated with the destruction of low spacing complexes. The IR spectra show that the molecule and the cation are linked by the P of the phosphite, which produces a reinforcement of lhe other bonds in the molecule, caused by an inductive effect_ The phosphite intercalation is accompanied by a partial isomerization of phosphite to phosphonate. The heat of adsorption of phosphites shows that the molecule-cation bond is ion-dipole. In the Cu sample with trimethyl phosphite, this bond seems to be reinforced by retrodonation of electrons from copper to ligand. Finally, the possible disposition of phosphite molecules in the interlayer space is con- sidered. For this purpose, ab initio calculations have been performed on the different conformers of the TMP molecule at 6-3lG* and 6-31+G* basis sets. Key Words---Ab initio Calculations, Complexes, Formation Mechanism, Montmorillonite, Stability, Tri- alkyl Phosphites. INTRODUCTION One characteristic of the clay minerals is their ca- pacity to form interlayered complexes with organic compounds, which have distinctive basal spacings. The preparation and study of such complexes is of interest, not only for their applications in distinguish- ing and diagnosing minerals in mixtures, but also for their practical applications for industry and agricul- ture. The study of interlayer complexes of saturated smectites with cations with semifilled d orbitals and polar molecules, such as phosphines and phosphites, has become very important. These solvates have great relevance to the hydrogenation and isomerization pro- cesses, because they minimize many of the technical and economic barriers which are associated with the use of such catalyzers in homogeneous dissolution (Farzaneh and Pinnavaia 1983; Pinnavaia and Welty 1975; Pinnavaia et al. 1976, 1979; Quayle and Pin- navaia 1979; Raythatha and Pinnavaia 1981, 1983). The aim of this present work is to investigate the complexes formed between trialkyl phosphites (Tri- methyl, TME and Triethyl, TEP) and montmorillonite saturated with cations with noble gas stnmtnres (al- Copyright 9 1996, The Clay Minerals Society kalines and alkaline earths) and transition element cat- ions, in order to discover their capacity for retention of organic liquids, their thermal stability and their sta- bility under vacuum conditions, as well as the mech- anism for the formation of the complexes as revealed by infrared spectrometry and calorimetry. This re- search can provide data of interest on: the influence of the exchangeable cation and the molecular dimension and basicity of the ligand with respect to the stability and the stoichiometry of the complexes, the magnitude of the swelling, the disposition of the ligand mole- cules, the type of cation coordination involved, and the determination of the group or groups which participate in the rnetal-ligand bonding. All of these questions are of great interest because they can be applied in cata- lyrical processes. MATERIALS AND METHODS Samples Wyoming montmorillonite with the following char- acteristics, SWy-1, <2 txm fraction, 752 m2/g of sur- face area, 112 meq/100 g of cation exchange capacity, when dried at 120~ and 4.07 esu/cm 2 of charge den- sity, was used in this study. 170