Mössbauer spectroscopic and thermogravimetric studies of tin-clay complexes

Abstract

Complexes of SnCIJmontmorillonite have been prepared from solutions which contained 10, 20, 40 and 200 times the cation exchange capacity of the clay, resulting in complexes which contained 1.7, 1.6, 2.8 and 4-6 wt% of tin. Detailed thermogravimetric and variable temperature llgsn M6ssbauer spectroscopic studies of these complexes, both as prepared and following back exchange with Ca2+-ions, have shown that (i) the complexes are quite acidic in nature, and (ii) the majority of the Sn forms an oxy/hydroxy coating over the montmorillonite surface in which the vibrational freedom of Sn depends upon both the Sn content and the state of hydration of the coating. Adams (1987) has shown that layered silicates with small, highly charged cations residing on the exchange sites can impart considerable catalytic activity to the host structure. The small, highly charged cations polarize the water molecules held in the primary coordination sphere resulting in an acidic proton. Consequently, the clay acts as a solid source of protons and can efficiently catalyse a number of novel and industrially significant reactions such as the hydration of ethene (Atkins et al., 1983), the esterification of organic acids by alkenes (Ballantine et al., 1984) and the low temperature synthesis of tert-methylbutylether from 2-methylpropene and methanol (Adams et al., 1982). More recently, Ni 2+, Co 2+ and Cu e+-exchanged clays have proven to be efficient Diels-Atder cycloaddition catalysts (Adams et al., 1987).Recent investigations into the acidity and catalytic activity of a variety of cationexchanged clays (Breen et al., 1987; Breen, 1991a,b) have shown that when H +, A13+ and Fe 3 § reside on the exchange sites the acidity is predominantly Bronsted in nature, whereas Ni z+ and Co z+ ions result in predominantly Lewis acid centres. Only when the exchange sites are occupied by Cr 3+ is there a mixture of both Lewis and Bronsted acid sites. Clearly, the type of interlayer cation can have a profound influence on the nature of the acid sites and the resulting catalytic activity, insofar as Ni 2+ and Co 2+ are only effective acid catalysts at temperatures above 200~ whereas the other exchange forms referred to are active at temperatures below 100~To our knowledge, no previous attempts have been made to exchange Sn n+ (n = 2,4) cations between the layers of montmorillonite, although several groups have studied tin complexes on clays. Abdo et al. (1980) investigated a Sn-porphyrin complex sorbed on hectorite, Molloy etal. (1986) produced a high spacing intercalate using N-methyl-(3-triphenylstannyl)pyridinium iodide, and Petridis and co-workers (Simopoulos et al., 1988;Petridis et al., 1989) have studied the behaviour of cationic dimethyltin(IV) species 9 1992 The Mineralogical Society

Tin-Clay Complexes: A Mössbauer Study

“…Similar behavior has been reported for Fe(II)-exchanged montmorillonite (Diamant et al 1982) and montmorillonite intercalated with dimethyltin(IV) complexes (Simopoulos et al 1988). It was also discussed by Breen et al (1992) in their study of the Sn(II)-clay interactions. The lack of, or diminished, absorption in the room-temperature MOssbauer spectra from cations residing on clay surfaces has been attributed to water molecules associated with the clay surfaces.…”

“…Recently, the interaction of Sn 2 § with montmorillonite was reported by Breen et al (1992). Their results indicate that, during intercalation, the majority of Sn 2+ (>95%) was oxidized to some Sn 4--oxo/hydroxo form that precipitated on the montmorillonite surfaces.…”

Organometallic cation-exchanged phyllosilicates: exchange with cations derived from (CH3)2SnCl2

“…Indeed small amounts of any unidentified phase present could result in high 22 and Fvalues. However, the high-velocity peak near 4.00 mm s ~, has also been observed in the Me2SnC12/montmorillonite complex prepared at pH 5.5 by Petridis et al (1989) and by the authors in an inorganic tin-exchanged clay (Breen et al, 1992). This component is undoubtedly indicative of the presence of Sn(II) which normally gives rise to doublet spectra due to the asymmetric distribution of electron density from a stereochemically active line pair.…”

Sonochemical synthesis of methyl-4-(hetero)arylmethylene isoxazole-5(4H)-ones using SnII-montmorillonite

“…Montmorillonite is one of the most intensively explored as a heterogeneous support for organic reactions due to its physical and chemical properties (20). To enhance the acidity property and high catalytic activities, SnCl 2 can be used with montmorillonite to be widely used as a powerful catalyst for various organic transformations under mild conditions (21,22).…”