Kathleen A. Carrado

Synthetic organo- and polymer–clays: preparation, characterization, and materials applications

Abstract We have over the years developed and patented a general technique for the hydrothermal synthesis of clay minerals in the presence of organic, organometallic, and polymeric intercalants. This review will summarize the details for crystallization of modified hectorites along with their characterization and materials applications. Among the several potential uses of these synthetic materials, there are two important applications concerning catalysis and composites. The fate of the template dictates which of these applications is pertinent. First, if the organic molecule or polymer is used with the intention of acting as templates of pore structure, then the organic template is removed after the modified clay has been crystallized. Upon template removal, the now porous materials are examined for their use as potential catalysts and catalyst supports. We have recently proven a correlation between catalyst pore size in the mesoporous range and the size and concentration of a polymeric template that is used. Preliminary hydrodesulfurization catalytic results have been obtained using these materials. If, on the other hand, intercalants are allowed to remain as a part of the structure, then a distinctive class of organic–inorganic composites becomes possible. When polymeric intercalants are used, especially at high concentrations, the materials have relevance to nanocomposite applications. Work in this area has focused on incorporating polymers at higher than 85 wt.% of the nanocomposite.

Polyvinyl alcohol-clay complexes formed by direct synthesis

Synthetic hectorite clay minerals were hydrothermally crystallized with direct incorporation of a series of five water-soluble polyvinyl alcohols (PVA) of molecular weights from 9000-146,000. The molecular weight of PVA had little effect on the success of hydrothermal hectorite synthesis, d-spacing or the amount of polymer incorporated. The basal spacings range from 19.5 Å to 20.8 Å and the amount of polymer incorporated ranges from 20 wt.% to 23 wt.%. Incorporation of PVA within the clay inter-layers, along with Li(I) ions to compensate the lattice charge, is indicated. Thermal gravimetric analysis and small angle neutron scattering were used to further examine the polymer-clay systems. Small PVA-clay crystallites that are coated with excess PVA are indicated. Removal of the polymer does not alter the extended synthetic clay network, and the nitrogen BET surface area increases from <5 m2/g to >200 m2/g.

New carbon electrodes for secondary lithium batteries

The authors have synthesized and electrochemically tested carbon samples that are suitable as anodes for lithium secondary batteries. The unique synthesis is based on the use of the inorganic templates and organic precursors. Pyrolysis of these carbons was performed at 500 and 700 C. High reversible capacity (up to 825 mAh/g) was found on samples heated to 700 C. The cells were tested continuously for more than 20 cycles with a drop in capacity of only 10%. X-ray powder diffraction showed that the samples heated to 700 C have a highly disordered structures.

Low temperature steam reforming of methanol over layered double hydroxide-derived catalysts.

The catalytic production of hydrogen by steam reforming of methanol (SRM) has been carried out over Mg/Al, Cu/Al, Co/Al, and Ni/Al layered double hydroxides (LDHs). The catalytic reactions were performed at temperatures of 150-400 C and atmospheric pressure. The most efficient catalyst was the Cu/Al LDH, which became active at {approx}230 C, with concomitant H{sub 2} production. The Ni/Al and Co/Al LDHs were also active in SRM, however, the activation temperature was significantly higher (315-320 C). No catalytic activity was observed for the Mg/Al LDH. Significant LDH decomposition occurred during the catalytic reactions. The reducibility of the divalent cations present in the LDH was a crucial parameter in determining the steam reforming activity of the catalysts. Pre-activation of the Cu/Al LDH by calcination in air (400 C), followed by reduction in dilute H2, did not significantly change the catalytic activity. The onset of H2 production was slightly lower for the pre-activated versus as-prepared Cu/Al LDH ({approx}218 C), also the CH{sub 3}OH conversion was 5-10% lower.

A study of organo-hectorite clay crystallization

Abstract A method has been developed to synthesize organo-hectorite clays directly from a Mg-silicate gel containing organic or organometallic molecules that are expected to be incorporated within the interlayer space. Complete crystallization occurs upon aqueous reflux for 48 h. The progress of clay layer formation was monitored by X-ray powder diffraction (XRD), differential thermal gravimetry (DTG), and infrared (IR) spectroscopy. Evidence of clay XRD peaks occurs after just 4 h of hydrothermal treatment, and Mg(OH)2 is no longer observable after 14 h. Observable changes in DTG and IR occur at about this time as well. Warren line-shape analysis of the 110 reflection indicates that when growth is complete the clay lamellae are on average ~50% and 25% of the size of natural hectorites and montmorillonites, respectively. The N2 BET surface areas for all materials are also compared. Small angle neutron scattering shows that addition of tetraethyl ammonium (TEA) ions does not alter the structural integrity over that of the purely inorganic form of Li-hectorite, but that use of a cationic polymer does significantly alter the microstructure. The effect of temperature is critical, for at room temperature only the layered Mg hydroxide mineral brucite crystallizes unless very long time scales are used. The crystallizations carried out at room temperature show that clay will form after about 3 months, but that the presence of organics (at least TEA) acts to hinder this process greatly. The role of the organic molecules on silicate clay layer formation is compared with the role of organics in zeolite synthesis.

Crystallization and textural porosity of synthetic clay minerals

The crystallization of synthetic layered magnesium silicate hectorite clays from both silica sol and organosilane sources is compared. For the silica sol-derived clays, a templating method is employed wherein organic or polymeric molecules are included during clay crystallization that are then removed from the composites via calcination. The mechanism of silane-derived hectorite formation is followed by XRD, TGA, 29Si MAS NMR, and small angle X-ray scattering (SAXS), and results are compared to those obtained for the sol-derived hectorite. The mechanism appears to be similar but the rate is approximately doubled when the silane is used rather than silica sol. Analytical transmission electron microscopy (TEM) is exploited to glean structural morphology information towards resolving the nature of the resulting pore network structures. Results are compared with nitrogen adsorption–desorption isotherm behavior; dominant hysteresis loops are present in the type IV isotherms. Pore size distributions based on both the adsorption and desorption isotherms are compared. Small angle neutron scattering (SANS) experiments reveal that the average particle size increases as synthetic laponite < sol-derived hectorite < silane-derived hectorite < natural hectorite. Contrast matching SANS studies in aqueous and organic solvents are carried out to extract information about pore accessibility.

Carbons for lithium battery applications prepared using sepiolite as an inorganic template

Carbon anodes for Li-ion cells were prepared by the in situ polymerization of olefins such as propylene and ethylene in the channels of a sepiolite clay mineral. Upon dissolution of the inorganic framework, a disordered carbon was obtained. The carbon was tested as an anode in coin cells, yielding an average reversible capacity of 633 mAh/g discounting the first cycle, which is 1.70 times higher than the capacity delivered by graphitic carbon assuming 100% efficiency. The coulombic efficiency was higher than 90%. Morphologies of the clay, carbon/clay composite, and final carbon were examined by TEM. The structure of the carbon and its electrochemical performance were monitored in situ by small angle x-ray scattering.

REACTIVITY OF ANISOLES ON CLAY AND PILLARED CLAY SURFACES

Pillared bentonites were found to be efficient catalysts for the O-methyl bond cleavage of anisoles (e.g., m-methylanisole, guaiacol, and creosol) under very mild, static conditions (150°C, a few hours, inert atmosphere). The O-methyl bond cleavage led to phenolic products. Gas chromatographymass spectrometry and solid-state 13C nuclear magnetic resonance (NMR) techniques used to probe 13C-labeled anisoles revealed that dealkylation and transalkylation reactions occurred to a large extent, and that conversion was efficient at >95% after two days. Ortho- and para-isomers were observed exclusively, without any evidence of meta-substitution. Volatile products were determined by mass spectrometry to be 13CH3OH and (13CH3)2O. Magic-angle spinning 13C NMR experiments showed that the molecules were fairly mobile in the clay micropores prior to catalysis. After catalysis, cross-polarization NMR showed that molecular motion had decreased markedly. Ultraviolet-visible spectroscopy of the colored complexes suggested some quinone formation. The trend of clay reactivity was found to be: pillared bentonite ≫ acid-washed montmorillonite > untreated bentonite > pillared fluorhectorite ≃ untreated fluorhectorite.

Materials derived from synthetic organo-clay complexes as novel hydrodesulfurization catalyst supports.

A series of mesoporous synthetic organo-clay complexes has been prepared by hydrothermal crystallization of gels containing silica, magnesium hydroxide, lithium fluoride, and an organic of choice, followed by calcination to remove the organics. The organic serves to impart structural order to the inorganic network that does not disappear upon its removal. The choice of organic modifier can be used to control the pore structure of the resulting mesoporous materials. Pore size distributions appear in some cases to be related to the type of polymer packing upon clay formation in situ. These materials are being explored as Co Mo hydrodesulfurization (HDS) catalyst supports. Preliminary HDS results show performance commensurate with commercial catalysis for the mesoporous materials when a model heavy oil feed is used (1 wt% S as dibenzothiophene in hexadecane). Temperature programmed reduction experiments of used catalysts suggest a relationship between HDS activity and ease of reduction of the CoMo/clay catalysts. Reactivity of the CoMo clay also correlates with the percentage of mesopore volume remaining after reaction. Losses in mesopore volume are largely recouped by recalcination, suggesting that reversible coke is formed inside the pore structure of clays faster than inside conventional alumina.

Novel forms of carbon as potential anodes for lithium batteries

The objective of this study is to design and synthesize novel carbons as potential electrode materials for lithium rechargeable batteries. A synthetic approach which utilizes inorganic templates is described and initial characterization results are discussed. The templates also act as a catalyst enabling carbon formation at low temperatures. This synthetic approach should make it easier to control the surface and bulk characteristics of these carbons.