Catalyst supports smectites

Over the past 15-20 years, there has been a renewed and growing interest in the use of clay minerals as catalysts or catalyst supports. Most of this interest has focused on the pillaring of smectite clays, such as montmorillonite, with various types of cations, such as hydrated metal cations, alkylammonium cations and polycations, and polynuclear hydroxy metal cations (1-17). By changing the size of the cation used to separate the anionic sheets in the clay structure, molecular sieve-like materials can be made with pore sizes much larger than those of conventional zeolites. 

The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite... 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

In mineralogy, the term clay is used for a variety of polycrystaUine materials that are well described in clay science, mineralogy properties, and characterization textbooks [2-5]. Clays can be present in fibrous, tubular, lath shaped, and planar geometries. In this chapter, however, our focus will be mainly on the planar clay varieties called smectites that include montmorillonites, the most commonly used clay for the produchon of polyolefin-clay nanocomposites. In this section, we wiU focus on clay characteristics that are relevant to catalyst supporting and particle break-up during polymerization clay chemistry, crystalline structure, and geometry. 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

These conceptions, explaining the fimctions of the chelating agents, were mainly developed for the silica-supported HDS catalysts [12-15]. However, the data presented here indicate their validity for the stevensite-supported HDS catalysts. Nevertheless, taking into account differences in the crystal structure, sur ce chemistry of the smectites, and the corresponding characteristics of silica, additional study is necessary to elucidate the case. 

This method relies on the size of the metal complex rather than on a specific adsorptive interaction. There are two different preparation strategies One, often called the ship-in-a-bottle approach, is based on building up catalysts in well defined cages of porous supports. Recently, enantioselective Mn epoxidation catalysts with different salen ligands have been assembled in zeolites. In zeoHte EMT [35] ees up to 88% and in zeolite Y [36] ees up to 58% were obtained with czs-P-methylstyrene. However, both entrapped catalysts were much less active than their homogeneous counterparts. Rh diphosphine complex were entrapped in the interlayers of Smectite [37]. The resulting catalyst was active for the enantioselective hydrogenation of N-acetamidoacrylic acid (ee 75%). 

Iwasa, N., Yamane, T., Takei, M., Ozaki, J.-I., Arai, M. (2010). Hydrogen production by steam reforming of acetic acid comparison of conventional supported metal catalysts and metal-incorporated mesoporous smectite-like catalysts. International Journal of Hydrogen Energy, 35, 110—117. 

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