P.A. Cox - Inorganic chemistry (793955), страница 61
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Substances made in comparable amounts that are not listed include fuels and organic chemicals produced frompetroleum, and construction metals such as iron. In many cases the basic chemical reactions used to produce thecompounds in Table 1 are simple, although catalysts are often required (see Topic J5). The design of processes to makethe most economical use of energy and raw materials, and to minimize polluting wastes, is, however, notstraightforward. The raw materials needed include air (for N2 and O2), sulfur (mined as native S or obtained fromprocessing sulfide minerals), natural gas and oil (a source of energy and H2), NaCl, and calcium carbonate andphosphate.
It is interesting to consider some details of the chlor-alkali industry, one of the oldest sectors of thechemical industry, which links the production of Cl2, NaOH and Na2CO3. The source material, NaCl, is used in greateramounts than any other raw material in the entire inorganic chemical industry.Cl2 is produced by electrolysis of NaCl (see Topic B4). A molten NaCl-CaCl2 mixture gives metallic Na and Ca atthe other electrode; these metals are used industrially; for example, as reducing agents for production of otherelectropositive metals such as Ti. Much greater quantities of NaCl are electrolyzed in aqueous solution to give NaOH,with H2 as a byproduct. Cells with a mercury electrode give Na-Hg amalgam as the initial product, which is thenreacted with water; modern diaphragm cells giving aqueous NaOH directly are cheaper to run and avoid the use oftoxic mercury.For economic reasons the amount of NaOH produced depends on the demand for Cl2.
However, NaOH andNa2CO3 are interchangeable in many uses (e.g. glass and paper manufacture), and so any short-fall in NaOH productioncan be made up by the carbonate. Some Na2CO3 is obtained from natural deposits but it is also made synthetically bythe ammonia-soda or Solvay process. The overall reaction,266J4—INDUSTRIAL CHEMISTRY: BULK INORGANIC CHEMICALSTable 1.
Production and uses of some major inorganic chemicalsis thermodynamically unfavorable but can be achieved in several steps:(i)(ii)(iii)(iv)NH3 and CO2 are recycled. The equilibrium is shifted in favor of the products by providing heat in the endothermicstages (ii) and (iii). The overall energy required comes from burning coke, mixed with CaCO3 in stage (iii), and someheat is also recycled from the exothermic steps (i) and (iv).SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTS267UsesMany of the substances listed in Table 1 are used primarily for further processes in the inorganic or organic chemicalindustry.
Some important areas of application are summarized below.PetrochemicalsPetrochemicals processing to give polymers and other organic compounds uses large amounts of many inorganicchemicals, including acids and alkalis (mostly H2SO4 and NaOH), Cl2 and HCl for production of chlorinatedcompounds, NH3 and HNO3 for organic N compounds, and H2 for reduction and cracking. Smaller but significantquantities of inorganic compounds are also required as catalysts (see Topic J5) and as additives for polymers.Metallurgical production and processingMetallurgical industry uses the gases O2 (e.g.
for welding, and for oxidation of impurities such as P and S in steelproduction) and N2 and Ar (as an inert blanket to prevent oxidation), refractory materials such as MgO and CaO forfurnace linings, and acids such as HCl and H3PO4 for ‘pickling’ or removing oxide films.AgricultureIntensive agriculture requires a supply of plant nutrients containing K, P and N. K is derived from natural sources(mostly KCl). P is provided in the form of phosphates, a major use of H2SO4 being in the treatment of natural calciumphosphate minerals to obtain more soluble forms. The major sources of N in fertilizers are ammonium nitrate, urea (anonexplosive alternative) and ammonium phosphate, which provides P simultaneously.
Agricultural products consumeover 80% of all industrially produced NH3, itself the highest volume synthetic chemical in terms of molar quantity.Many pesticides used in agriculture are based on organophosphorus compounds, PCl3 and POCl3 being importantintermediates in their production.Glass makingSilicate glass is based on a random network of corner-sharing SiO4 units interspersed with ions such as Na+ and Ca2+(see Topic D5). Its manufacture, by heating together SiO2, CaCO3 and Na2CO3, is a major user of Na2CO3. Borates areadded when increased thermal resistance is required, for example, for ‘Pyrex’ cooking utensils.Paper makingThe digestion of plant cellulose fibers to make paper is facilitated by a variety of inorganic compounds including NaOH,Na2SO4 and SO2.
Bleaching agents (which act by oxidizing colored organic material) are required for white paper andinclude Cl2 and chlorine compounds such as Ca(OCl)2 and ClO2, as well as hydrogen peroxide and O2.Soaps, detergents and bleachesTraditional soap is the sodium salt of long-chain carboxylic acids, made from NaOH and animal fats. Replacing Na by Kreduces the melting point and is used for liquid soaps. Synthetic detergents are based on sulfonaterather thancarboxylatesalts and often contain many additives.
‘Builders’, used to complex or remove Ca2+ from hardwater, include condensed phosphates such as sodium tripolyphosphate Na5P3O10, although these are now regarded as268J4—INDUSTRIAL CHEMISTRY: BULK INORGANIC CHEMICALSenvironmental pollutants and are being replaced by zeolites, which act as ion exchangers (see Topic D5). For similarenvironmental reasons, traditional bleaches based on chlorine compounds such as NaOCl are being increasingly replacedby peroxides, often in the form of the peroxoacid salts such as perborates or percarbonates, which liberate H2O2 onheating (see Topic F7).Section J—Environmental, biological and industrial aspectsJ5INDUSTRIAL CHEMISTRY: CATALYSTSKey NotesIntroductionDihydrogen chemistryOxidation reactionsAlkene polymerizationGasoline andautomobile catalystsRelated topicsCatalysts speed up reactions, and provide selectivity for the desiredproduct.
Homogeneous catalysts (often organometallic compounds)act in the same phase as the reactions, heterogeneous catalysts (oftentransition metals on an oxide support) in another phase.Heterogeneous catalysts are used in the production of synthesis gas(H2-CO) and from it methanol, for ammonia synthesis and for largescale hydrogenation. Organometallic catalysts provide moreselectivity for specialized hydrogenation reactions.Heterogeneous oxidation catalysts are used in the production ofH2SO4 and HNO3 and for the selective oxidation of hydrocarbons.Alkene polymerization is performed by Ti compounds (Ziegler-Nattacatalysts) and by organometallic compounds of early transitionmetals.Petroleum products undergo catalytic hydrodesulfurization and reforming processes. Catalysts in automobile exhaust systems convertpollutants (hydrocarbons, CO, NO) to CO2 and N2.Stability and reactivity (B3)Organometallic compoundsInorganic reactions and(H10)synthesis (B6)Industrial chemistry: bulkinorganic chemicals (J4)IntroductionA catalyst is a substance that speeds up a chemical reaction but is not itself consumed.
Catalysts do not alter thethermodynamics of a reaction or the position of equilibrium, but act by providing an alternative pathway of lower activationenergy. A high proportion of industrial chemical processes, inorganic and organic, use catalysts. They allow manyreactions to be performed at lower temperatures than without a catalyst, and also provide selectivity in producing aspecific product in reactions where several products are feasible thermodynamically.
Enzymes (which often containmetallic elements; see Topic J3) are uniquely selective biological catalysts.A catalyst present in the same phase as the reactants (generally liquid) is called homogeneous, one in a differentphase is heterogeneous. Most heterogeneous catalysts are solids, and act by adsorption of gaseous or liquidreactants on a surface. Homogeneous catalysts are specific molecules, often organometallic compounds, that can betailored in a more specific way to give a required product than is possible with heterogeneous catalysts.
On the otherhand, it is generally harder to separate the products from homogeneous catalysts. Another difference is that270SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTSheterogeneous catalysis is usually performed at higher temperatures than homogeneous catalysis. Metallic elements usedas heterogeneous catalysts are often in the form of small particles on a support such as Al2O3 or SiO2. This provides ahigh surface area for the active catalyst, and reduces the tendency for sintering (i.e. coalescence into larger particles)at the operating temperature.A catalytically active substance must be able to bind the reactant molecules and release the products.
For example,hydrogenation with a metallic Ni catalyst proceeds via adsorbed hydrogen atoms formed by dissociation of H2 on themetal surface (1). Transition metals and their compounds are effective catalysts because of their ability to coordinatemolecules and to change oxidation state (see Topic H1). An appropriate degree of reactivity is often provided byelements from groups 9 and 10, either in metallic form or as organometallic compounds.
More reactive elements (e.g.early transition metals) are more likely to bind reactants irreversibly, less reactive ones (e.g. Au) not to bind them atall.Dihydrogen chemistryH2 is mostly produced by steam re-forming of hydrocarbons. The simplest reaction isIn this endothermic reaction, the equilibrium is driven to the right by using elevated temperatures (900°C). The usualcatalyst is Ni supported on Al2O3. The H2-CO mixture is known as synthesis gas or syngas, and may be used directlyfor further reactions, the most important being methanol synthesis:A Cu-ZnO-Al2O3 catalyst is used, which is less active than ones involving metals in earlier groups, so that the C-O bondis not broken. Methanol is an important stage in further synthesis; see, for example, the Monsanto acetic acid processcatalyzed by Rh compounds (Topic H10).Another important reaction for synthesizing useful organic compounds is hydroformylation:Homogeneous catalysts based on cobalt carbonyl compounds were previously used, but have been largely replaced bythe Union-Carbide process with [(Ph3P)2Rh(CO)Cl] as the catalyst.Two major uses of H2 are for ammonia synthesis and the hydrogenation of unsaturated vegetable oils to makemargarine.
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