Catalysts containing mercury

Some natural gases have also been found to contain mercury, which is a reformer catalyst poison when present in sufftciendy large amounts. Activated carbon beds impregnated with sulfur have been found to be effective in removing this metal. 

Application Upgrade natural gas condensate and other contaminated streams to higher-value ethylene plant feedstocks. Mercury, arsenic and lead contamination in potential ethylene plant feedstocks precludes their use, despite attractive yield patterns. The contaminants poison catalysts, cause corrosion in equipment and have undesirable environmental implications. For example, mercury compounds poison hydrotreating catalysts and, if present in the steam-cracker feed, are distributed in the C2-C5+ cuts. A condensate containing mercury may have negative added-value as a gas field product. 

Many cathode catalyst materials have been used. For noble metal catalysts, platinum was mainly used in fuel cells for space applications. For terrestrial use, one has to use less expensive materials, and non-noble metal catalysts are therefore mainly employed. Bacon used lithium-doped nickel oxide as a cathode catalyst for high-temperature AFCs. Lithium-doped nickel oxide has a sufficient electrical conductivity at temperatures above 150 °C. Currendy, mainly Raney silver and pure silver catalysts are favored. Developments of silver-supported materials containing PTFE are sometimes successful. Silver catalysts are usually prepared from silver oxide, Raney silver, and supported silver. Typically, the catalysts on the cathode are supported by PTFE because it is highly stable under basic and acidic conditions. In contrast, carbon is oxidized at the cathode in contact with oxygen, when carbon is used as an inexpensive support material. In the past, the silver catalysts frequentiy contained mercury as part of an amalgam to increase the stability and the lifetime of the cathode. Because mercury is partially dissolved during the activation procedure (see below) and during the fuel-cell operation, some electrolyte contamination can be observed. Because of the environmental hazard of mercury, this metal is currently not used in silver catalysts. 

Today, acetaldehyde (CH3CHO) is produced from ethylene (C H ), a petroleum product. In earlier days, though, it was produced by adding water to acetylene (C H ) (the reaction is -1- H O CH3CHO). This reaction requires a catalyst, mercury sulfate (HgSO ). The waste liquid containing mercury compounds was released into the Minamata bay. 

Transfer the contents of a digestion flask with two or three 5 ml distilled water washings together with 0.2 ml 25 % cupric sulfate solution via the fuimel on the steam stripper to the interior sample vessel and close the stopcock D. Open the outlet E to waste and turn stopcock T1 to reconnect B and C. Pass steam until it exits at E. Meanwhile pour an excess of 40% sodium hydroxide into the (closed) fuimel at D. Use 10 ml, 20 ml or 40 ml sodium hydroxide (depending on whether 2 ml, 5 ml or 10 ml of concentrated sulfuric acid was used for the acid digestion.) If the digestion catalyst used contains mercury or mercuric oxide use instead, the same volumes of sodium hydroxide-sodium thiosulfate solution (Note 6). Use the same amount of alkali in both sample and blank determinations. 

From Acetylene. Although acetaldehyde has been produced commercially by the hydration of acetylene since 1916, this procedure has been almost completely replaced by the direct oxidation of ethylene. In the hydration process, high purity acetylene under a pressure of 103.4 kPa (15 psi) is passed into a vertical reactor containing a mercury catalyst dissolved in 18—25% sulfuric acid at 70—90°C (see Acetylene-DERIVED chemicals). 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LET (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution-, this method increases the furnace volume and hence the investment and operation costs. Concentrations between 2% and 20% of LET are optimal, The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.(see Table 1.3.111... 

Modified electrodes containing cyclam derivatives have been prepared. The approach utilizing cyclam incorporated in Nafion film on a carbon electrode shows that the catalytic efficiency of the system is much lower than observed when the catalyst is adsorbed on the mercury. With electrodes prepared following the Langmuir Blodgett technique, only the electrode materials that allow the orientation of the monolayer so that the tail points to the substrate were found to be electrocatalytically active.165... 

Active catalysts for dinitrogen activation have been prepared by reduction at a mercury pool of M0CI5 in basic methanolic solutions containing MgCl2, phospholipids, and various tertiary phosphines.319 The turnover number reached several hundreds per Mo center. Both ammonia and hydrazine were formed in a ratio of about 1 10. 

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. 

Certain alkaloids are able to effect asymmetric induction during a reduction process at a mercury cathode even when present in low concentration in an aqueous alcohol acetate buffer. Asymmetric induction under these conditions was first observed [39] during the conversion of 4-methylcoumarin to 4-methyl-3,4-dihydro-coumariit (sec page 60). Induction results because a layer of alkaloid is strongly adsorbed on the electrode surface thus permitting transfer of a proton to a carban-ion intermediate m an asymmetric environment. Up to 16% asymmetric induction has been achieved in 1-phenylethanol recovered from reduction of acetophenone in a buffer of pH 4.8 containing a low concentration of quinidine. lire pinacol formed simultaneously shows no optical activity. However quinidine is itself reduced at the potential employed so that the actual catalyst for the asymmetric process is not defined [34,40],... 

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