P.A. Cox - Inorganic chemistry (793955), страница 62
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The former reaction (see Topic F5) is exothermic and the equilibrium constant therefore decreases withtemperature. In the Haber process a potassium-promoted iron catalyst is used, this relatively reactive metal beingnecessary to adsorb and dissociate the very stable N2 molecule at moderate temperatures (400°C). The yield of NH3 isimproved by working at high pressure.
Large-scale catalytic hydrogenation of alkenes is normally carried out with aNi-SiO2 catalyst. More selective hydrogenation for specialized purposes (e.g. pharmaceuticals where specific isomersJ5—INDUSTRIAL CHEMISTRY: CATALYSTS271are required) is possible with homogeneous catalysts; for example, Wilkinson’s catalyst (2), which undergoes oxidativeaddition of H2 under mild conditions (see Topic H9).Oxidation reactionsCatalytic oxidation steps are involved in the manufacture of the major industrial chemicals sulfuric and nitric acid.
HNO3manufacture starts with ammonia, which is oxidized to NO (see Topic F5). A very active Pt-Rh catalyst is used, thecontact time being minimized to avoid forming the thermodynamically more favorable products N2 and N2O. To makeH2SO4 it is necessary to oxidize SO2 to SO3 (see Topic F8). The catalyst is vanadium pentoxide, V2O5.
Oxides oftransition and post-transition metals are also used for selective oxidation of hydrocarbons; for example, fromalkenes to carbonyl compounds and acid anhydrides, which are used for polymer manufacture. The action of thesecatalysts appears to depend on the ability of the metallic element to change oxidation state and coordination number.Oxygen is transferred to the adsorbed reactant molecules from the catalyst surface, which is then reoxidized in anotherstep.Alkene polymerizationDomestic and industrial plastics are mostly formed by polymerizing alkenes:The reaction is exothermic and can be initiated by free radicals (often from peroxo compounds), but organometalliccatalysts give more controllable results.
Most widely used are Ziegler-Natta catalysts made by mixing Al2Et6 (whereEt is the ethyl group) with TiCl4. Solid TiCl3 is formed and catalysis occurs at surface Ti-Et groups, to which alkenemolecules coordinate and undergo insertion into the Ti-R bond (see Topic H10). An advantage of these catalysts is thatthey may form stereoregular polymers where all the R groups in -C(R)H-CH2- have the same stereochemicalconfiguration. This gives stronger materials with higher melting points than the random stereochemistry resulting fromradical polymerization.A new generation of catalysts is based on cyclopentadienyl compounds of early transition metals such as 3, which inthe presence of aluminoxane (MeAlO)n forms the active species [(η5−C5H5)2ZrCH3]+.272SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTSGasoline and automobile catalystsNatural petroleum contains organic sulfur compounds, which must be removed before further processing, as they blockactive sites in some catalysts and so act as poisons.
When burnt they also give the environmental pollutant SO2 (seeTopic J6). Hydrodesulfurization is the reaction in which organic sulfur is converted to H2S, which is easily removed.Catalysts based on mixed Co-Mo sulfides are used. Subsequent processing of petroleum involves catalytic crackingand re-forming in which long-chain hydrocarbons are reduced to shorter ones, together with isomerization processesgiving a more desirable mixture of compounds. Bifunctional catalysts for these reactions contain metals such as Ptthat are active for hydrogenation, and zeolites (see Topic D5) as acid catalysts providing H+ to give carbocations thatreadily isomerize.Catalysts for automobile exhaust systems are designed to remove environmental pollutants such as unburnedhydrocarbons, CO formed from incomplete combustion and oxides of nitrogen. Three-way catalysts are based on Ptand Rh together with various additives that together perform a complex series of reactions, including removal ofhydrocarbons by oxidation and steam re-forming (see above), andTheir operation depends on the absence of poisons such as lead compounds, and on a fuel injection system that providesan almost perfect stoichiometric ratio of fuel and oxygen to the engine: this is achieved by a feedback system using asensor that monitors the O2 content of the exhaust gases, based on an electrochemical cell using the ionic conductorZrO2 as a solid electrolyte (see Topic D7).Section J—Environmental, biological and industrial aspectsJ6ENVIRONMENTAL CYCLING AND POLLUTIONKey NotesIntroductionThe carbon cycleOther nonmetallicelementsHeavy metalsRelated topicsThe cycling of elements is driven by energy fluxes that producecirculation of the crust, oceans and atmosphere, and that allowphotosynthetic and photochemical transformations.
The presence ofliquid water and of life contribute to the complexity of theseprocesses.Carbon is cycled by both inorganic processes (involving CO2,and carbonates) and by photosynthesis and respiration. The slowburial of fossil fuels has been accompanied by the production of O2,but the current burning of fossil fuels is increasing CO2 in theatmosphere and leading to global warming.S and N are cycled by life and by atmospheric photochemistrythrough many oxidation states. Natural Si and P compounds areinvolatile and less mobile in the environment. Environmentalproblems include acid rain, and pollution by soluble phosphates andorganochlorine compounds.Compounds of Cd, Hg and Pb are potentially serious pollutants.Their use (especially that of Pb, which has been widespread) isdeclining.Geochemistry (J2)Bioinorganic chemistry (J3)IntroductionThe cycling of substances through the environment is driven by energy fluxes within the Earth and at its surface.
Theradioactive decay of elements in the mantle and core drives tectonic processes that lead to crust formation, volcanicactivity, and hydrothermal processes in aqueous solutions deep within the crust (see Topic J2). Absorption of solarenergy drives the physical circulation of winds and ocean currents. It also fuels the physicochemical hydrologicalcycle, which entails the evaporation of water from oceans and lakes, and subsequent rainfall giving rivers that flow intothe sea. Solar energy has in addition some direct chemical consequences, through photosynthesis by green plants, andatmospheric photochemistry, which depends on reactive species produced by absorption of UV radiation. Humanactivity contributes to these cycles through the burning of fossil fuels and the extraction and use of elements intechnology.The existence of liquid water and the presence of life are two features that make the chemistry of the Earth’s surfaceuniquely complex among the known planets.
Biological processes cycle some elements (especially C, N, O and S)through different oxidation states, and photosynthesis has given us both a strongly oxidizing atmosphere and buried274SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTSfossil fuels. Hydrological cycling entrains many other substances, through the chemical breakdown of rocks and byevaporation from the oceans.Elements respond to these driving forces in ways that depend on their chemical characteristics. Volatile moleculesformed by nonmetallic elements enter the atmosphere from volcanic emissions, as ‘waste products’ of life, and fromhuman energy use and industry.
Some volatile compounds are rapidly oxidized by photochemical processes, and someare quickly washed out by dissolving in rainfall. Elements (especially metallic ones) that do not form volatile compoundsunder normal conditions are confined to the solid and liquid parts of the environment. Soluble ions (e.g. Na+, Cl−) areremoved from rocks in weathering processes and end up in sea water. Other elements (e.g. Al, Ti) that form veryinsoluble oxides or silicates are by comparison highly immobile.Some pollutants from human activity are natural substances (e.g.
CO2) produced in excessive amounts that unbalancethe natural cycles. Others are synthetic (e.g. organochlorine compounds) and are harmful either because they are toxicto life, or because they interfere with natural chemical processes (e.g. in the ozone layer).The carbon cycleThe environmental cycling of carbon compounds involves a flux of over 2×1014 kg C per year, much larger than for anyother substance except water (about 5×1017 kg per year in the hydrological cycle). Understanding the carbon cycle hasbecome especially urgent as the atmospheric CO2 content is currently increasing, producing global warming throughthe trapping of IR radiation in the atmosphere.
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