Cadmium-based catalysts

Cadmium, which possesses substantial catalytic activity as recognised in tbe literature, is considered to possess limited availability and there is a potential future risk to provide an adequate supply of this metal. This compels the development of a new generation of cadmium-based catalysts. Tbe discussion in this chapter focuses on literature-reports of the catalytic activities of diverse cadmium-based materials, in an attempt to present a concise overview of all the cadmium catalysts activities, leading to a prospective growth on this field of cadmium-based sustainable catalysis. 

In recent years, catalytic processes involving cadmium metal have witnessed a remarkable upsurge, particularly with the advent of metal-organic framework (MOF) materials, and semiconducting sulfides. Recent decades have come up with so many catalysts from these two domains that they call for special mention in the regime of cadmium-based catalyst materials. 

Figure 17.1 Classification scheme for cadmium-based catalysts.

It may be noted that all of the homosteric catalysts were chiral zinc systems, the composition of which was expressed as [RZnOR ]x[R,OZnOR,]J,(v/y < 1). Antisteric catalysts were chiral zinc- or cadmium-based systems of a common compositional feature expressed as [RZnOR ]x[R OZnOR ]>, or [RCdOR ]x [R OCdOR ]) (x/y > 2). The nature of homosteric and antisteric stereoelection has not yet been elucidated fully at the molecular level because the structure of the operating species and the polymerisation mechanism with these catalysts are not clearly established [52]. 

Toluene is oxidized in die liquid phase in the presence of a soluble cohalt-based catalyst The neutralization of benzoic add by potash yields insoluble potassium benzoate, which is separated by centrifuging. The disproportionation of potassium benzoate takes place between 400 and 430 C, under C02 pressure ranging from 13 to 3.106 Pa absolute, in the presence of a catalyst consisting of cadmium or zinc oxides. The reaction takes place in the solid phase. Tercphthalic add is released from its salt by the action of sulfuric add. 

M. F. Ilker, H. Skalf, T. Emrick and E.B. Coughlin, Metathesis and Polyolefin Growth on Cadmium Selenide Surfaces Using Ruthenium-Based Catalysts , p. 263... 

Liquid- and vapor-phase processes have been described the latter appear to be advantageous. Supported cadmium, zinc, or mercury salts are used as catalysts. In 1963 it was estimated that 85% of U.S. vinyl acetate capacity was based on acetylene, but it has been completely replaced since about 1982 by newer technology using oxidative addition of acetic acid to ethylene (2) (see Vinyl polymers). In western Europe production of vinyl acetate from acetylene stiU remains a significant commercial route. 

Vinyl acetate (ethenyl acetate) is produced in the vapor-phase reaction at 180—200°C of acetylene and acetic acid over a cadmium, 2inc, or mercury acetate catalyst. However, the palladium-cataly2ed reaction of ethylene and acetic acid has displaced most of the commercial acetylene-based units (see Acetylene-DERIVED chemicals Vinyl polymers). Current production is dependent on the use of low cost by-product acetylene from ethylene plants or from low cost hydrocarbon feeds. 

Henkel Rearrangement of Benzoic Acid and Phthalic Anhydride. Henkel technology is based on the conversion of benzenecarboxyhc acids to their potassium salts. The salts are rearranged in the presence of carbon dioxide and a catalyst such as cadmium or zinc oxide to form dipotassium terephthalate, which is converted to terephthahc acid (59—61). Henkel technology is obsolete and is no longer practiced, but it was once commercialized by Teijin Hercules Chemical Co. and Kawasaki Kasei Chemicals Ltd. Both processes foUowed a route starting with oxidation of napthalene to phthahc anhydride. In the Teijin process, the phthaHc anhydride was converted sequentially to monopotassium and then dipotassium o-phthalate by aqueous recycle of monopotassium and dipotassium terephthalate (62). The dipotassium o-phthalate was recovered and isomerized in carbon dioxide at a pressure of 1000—5000 kPa ( 10 50 atm) and at 350—450°C. The product dipotassium terephthalate was dissolved in water and recycled as noted above. Production of monopotassium o-phthalate released terephthahc acid, which was filtered, dried, and stored (63,64). 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

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