Catalysts containing heavy metals

Some catalysts are ha2ardous materials, or they react to form ha2ardous substances. For example, catalysts used for hydrogenation of carbon monoxide form volatile metal carbonyl compounds such as nickel carbonyl, which are highly toxic. Many catalysts contain heavy metals and other ha2ardous components, and environmentally safe disposal has become an increasing concern and expense. 

Specifically, catalysts are typically in the form of a ceramic support carrying small amounts of metals such as chromium, nickel, or platinum. Alumina and silica are commonly used in the construction of the ceramic support. Die catalysts lose their activity progressively via various deactivation mechanisms (Pavel and Elvin, 1994). Tliermal regeneration is often employed for regaining catalytic activity, if applicable, but some of the particles break during this process. Once the catalyst particles become too small to be useful, they constitute a waste disposal problem, since catalysts may contain heavy metals that are considered hazardous, or other harmful components. 

Recently, there have been many new catalysts developed for the hydration of alkenes, although most are more suited for industrial purposes. These catalysts include zeolites such as pentasil,283 mordenite286 and ferrierite,183 as well as those containing heavy metals,287 heteropolyacids288 and sulfonic acid exchange resins.289 It is reported that some of these new catalysts perform as high as 99.6% conversions and 99.4% selectivity, as illustrated in the hydration of propene into 2-propanol (equation 192).290... 

In the late 1960s, new antipollution initiatives were enacted to reduce nitrogen oxides, carbon monoxide, and lead pollutants from automotive exhaust. Nitrogen oxides were responsible for the brown haze that hung over cities that can still be seen today. The advent of the catalytic converter, a small canister that contained heavy metal catalysts embedded on a ceramic support, helped oxidize carbon monoxide and reverse the reaction that produced nitrogen oxides. However, lead in the exhaust stream deactivated the catalysts in the catalytic converter. The only solution was to remove tetraethyllead from the gasoline. 

In recent years, the EPA has recognized spent catalysts as very dangerous industrial wastes [2], since they may contain heavy metals and carcinogenic compounds present in coke. Also [3-7], these materials tend to be corrosive, may release toxic gases and are subjected to spontaneous burning. As a conclusion, spent catalysts are a powerful source of contamination of ground, water, animal and vegetable life. 

Remnants of the polymerisation process (e.g., monomers, oligomers, catalysts and so on) are possible impurities after the polymerisation is completed. Their concentrations are not known and are usually very low. However, since most of the catalyst systems contain heavy metal ions that are poisonous, as will be outlined shortly, and since some of the monomers can also pose serious health hazards, these additives should be considered as another problematic group. 

The old alkyd paints contained heavy metal salts (lead) acting as curing catalysts which rendered the paint film microbicidal. It should not be under-emphasized that customer perception changed over the years as well and appearance of a paint film to be acceptable is different today from ancient times. 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Other modifications have special but more limited apphcations. A centrifugal bowl may contain, instead of disks, several aimular baffles that take the hquid through a labyrinth path before discharge. The multiple cylinders increase cake capacity to as much as 70 L for easily sedimented sohds. This centrifuge is used for clarification of food symps and antibiotics (qv), and for recovery of heavy metallic salts and catalysts (see Fig. 14c). 

The usual feed is a virgin gas oil that is, the part of crude oil boiling between about 60 °F. and 1050°F. Sometimes material below 600°F will be included into the cat feed but more often, it is put into diesel fuel or home heating oil. The heavy material above 1050°F is not normally used as cat feed because it often contains metallic compounds that contaminate the catalyst. Even if metals are not present, there are sometimes tarry materials that end up on the catalyst. This deposit increases the load on the regenerator, and, hence, the 1050 °F+ material is less desirable than lower boiling feeds. 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

Salts of transition metals are widely used in technological processes for the preparation of various oxygen-containing compounds from hydrocarbon raw materials. The principal mechanism of acceleration of RH oxidation by dioxygen in the presence of salts of heavy metals was discovered by Bawn [46 19] for benzaldehyde oxidation (see Chapter 1). Benzaldehyde was oxidized with dioxygen in a solution of acetic acid, with cobalt acetate as the catalyst. The oxidation rate was found to be [50] ... 

Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes. Mercury is used when sulphonating anthraquinones and copper when reacting arylamines with bromoanthraquinones. Much effort has been devoted to minimising the trace metal content of such colorants and in effluents from dyemaking plants. Metal salts are used as reactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. These structures are described in detail in Chapter 5, where the implications in terms of environmental problems are also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. The environmental impact of the heavy metal salts used in dye application processes is dealt with in Volume 2. 

In order to increase the selectivity in diene hydrogenation, low-temperature basic additives and the use of less polar solvents may help. In special cases, treatment of the catalysts with the salts of heavy metals (Zn, Cd, Pb) can be the method used to modify the activity and selectivity53. Rh and Ir catalysts could be selectively poisoned with CO-containing hydrogen, in order to saturate 1,3-butadiene to 1-butene without isomerization54. 

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