Importance to catalysts

Surface Area. This property is of paramount importance to catalyst performance because in general catalyst activity increases as the surface area of the catalyst increases. However because some reaction rates are strongly dependent on the nature of the stmcture of the catalytic surface, a linear correlation of catalyst activity with surface area should not be expected. As the catalyst surface area increases, for many reactions the selectivity of the catalyst is found to decrease. If the support material is completely inert to the reactants and products, this effect may be diminished somewhat. 

Compounds and their conformational models were imported to Catalyst and subjected to the HypoGen routine to build chemical feature-based pharmacophore models using hydrogen bond acceptor lipid (HBA), hydrogen bond donor (HBD), positive ionizable (PI), ring aromatic (RA) and hydrophobic ... 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

Cobalt has an odd number of electrons, and does not form a simple carbonyl in oxidation state 0. However, carbonyls of formulae Co2(CO)g, Co4(CO)i2 and CoJCO),6 are known reduction of these by an alkali metal dissolved in liquid ammonia (p. 126) gives the ion [Co(CO)4] ". Both Co2(CO)g and [Co(CO)4]" are important as catalysts for organic syntheses. In the so-called oxo reaction, where an alkene reacts with carbon monoxide and hydrogen, under pressure, to give an aldehyde, dicobalt octacarbonyl is used as catalyst ... 

Apart from using an environmentally friendly solvent, it is also important to clean up the chemical reactions themselves by reducing the number and amount of side-products formed. For this purpose catalysts are a versatile tool. Catalysts have been used for thousands of years in processes such as fermentation and their importance has grown ever since. In synthetic oiganic chemistry, catalysts have found wide applications. In the majority of these catalytic processes, organic solvents are used, but also here the use of water is becoming increasingly popular . 

There is a growing interest in modeling transition metals because of its applicability to catalysts, bioinorganics, materials science, and traditional inorganic chemistry. Unfortunately, transition metals tend to be extremely difficult to model. This is so because of a number of effects that are important to correctly describing these compounds. The problem is compounded by the fact that the majority of computational methods have been created, tested, and optimized for organic molecules. Some of the techniques that work well for organics perform poorly for more technically difficult transition metal systems. 

The carboaylatioa of methanol to give formic acid is carried out ia the Hquid phase with the aid of a basic catalyst such as sodium methoxide. It is important to minimi2e the presence of water and carbon dioxide ia the startiag materials, as these cause deactivatioa of the catalyst. The reactioa is an equHibrium, and elevated pressures are necessary to give good conversions. Typical reaction conditions appear to be 80°C, 4.5 MPa (44 atm) pressure and 2.5% w/w of catalyst. Under these conditions the methanol conversion is around 30% (25). 

Nickel also is an important iadustrial catalyst. The most extensive use of nickel as a catalyst is ia the food iadustry ia connection with the hydrogenation or dehydrogenation of organic compounds to produce edible fats and oils (see Fats and FATTY oils). 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

Improved feedstock pretreatment is important to minimize catalyst consumption and reduce subsequent spent-catalyst handling requirements. Selective hydrogenation of dienes can be used to reduce acid consumption, both in HF and H2SO4 alkylation (29). More effective adsorptive treating systems have been appHed to remove oxygen-containing contaminants that are frequently introduced in upstream processing steps. 

Many agents have been proposed and patented including copper sulfate (34), zinc chloride (35), ferric chloride (36), aluminum chloride (36), and phosphoms pentoxide (37) ferric chloride, zinc chloride, and phosphoms pentoxide have been most widely used. The addition of these agents may vary from 0.1 to 3%, depending upon the feedstock and the desired characteristics of the product (Table 5) and all asphalt feedstocks do not respond to catalysts in the same way. Differences in feedstock composition are important qualifiers in determining the properties of the asphalt product. The important softening point-penetration relationship, which describes the temperature susceptibiUty of an asphalt, also varies with the source of the feedstock. Straight-reduced, air-blown, and air-blown catalytic asphalts from the same cmde feedstock also vary considerably. 

Supports are often prepared first and the catalyst and promoter components added later (60). Metal oxide supports are usually prepared by precipitation from aqueous solutions. Nitrates are commonly used anions alkaUes and ammonium are commonly used cations. Metal oxide supports, eg, sihca and alumina, are prepared in the form of hydrogels. Mixed oxides such as siUca—alumina are made by cogelation. Carefiil control of conditions such as pH is important to give uniform products. 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

Catalyst Cation. The logarithms of extraction constants for symmetrical tetra- -alkylammonium salts (log rise by ca 0.54 per added C atom. Although absolute numerical values for extraction coefficients are vastly different in various solvents and for various anions, this relation holds as a first approximation for most solvent—water combinations tested and for many anions. It is important to note, however, that the lipophilicity of phenyl and benzyl groups carrying ammonium salts is much lower than the number of C atoms might suggest. Benzyl is extracted between / -propyl and -butyl. The extraction constants of tetra- -butylammonium salts are about 140 times larger than the constants for tetra- -propylammonium salts of the same anion in the same solvent—water system. 

Composition. The results of elemental analyses are almost always included among the specifications for a commercial catalyst. Depending on the accuracy desired and whether or not the catalyst can be rendered soluble without great difficulty, elemental analysis may be performed by x-ray methods, by one of the procedures based on atomic absorption, or by traditional wet-chemical methods. Erequentiy it is important to determine and report trace element components that may have an effect on catalyst performance. 

There is an enormous volume of Hterature available on the appHcations for supported catalysts. Examples are compiled here based on important synthesis methods and industrial uses. The organization is according to specific reactions and appHcations rather than according to catalyst type. 

Type AD-G is used in an entirely different sort of formulation. The polymer is designed for graft polymerisation with methyl methacrylate. Typically, equal amounts of AD-G and methyl methacrylate are dissolved together in toluene, and the reaction driven to completion with a free-radical catalyst, such as bensoyl peroxide. The graft polymer is usually mixed with an isocyanate just prior to use. It is not normally compounded with resin. The resulting adhesive has very good adhesion to plasticised vinyl, EVA sponge, thermoplastic mbber, and other difficult to bond substrates, and is of particular importance to the shoe industry (42,43). 

Chromium is conventionally deposited from chromic acid solutions containing at least one anionic catalyst, which is usually the sulfate ion. The weight ratio of chromic acid to catalyst is important and, for sulfate-cataly2ed solutions, is maintained about 100 1. Formulations and conditions for operating hard chromium plating solutions are shown in Table 5. 

For commercial appHcation, catalyst activity is only one of the factors to be considered. Equally important is catalyst life, but Htde has been pubHshed on this aspect. Partly because of entrainment losses and partly through loss of acid as volatile triethyl phosphate, the catalyst loses activity unless compensating steps are taken. This decline in activity can be counteracted by the periodic or continuous addition of phosphoric acid to the catalyst during use, a fact that seems to have been disclosed as early as 1940 (94). A catalyst subjected periodically to acid addition could remain in service indefinitely, according to a report by Shell (91). A later Shell patent (85) states that complete reimpregnation with acid is required every 200 mn-days. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

The powders of zeolites of various trademarks are used to produce petroleum-refining catalysts. In this connection, it is very important to have complete information concerning not only chemical composition and distribution of impurity elements, but also shape, surface, stmcture and sizes of particles. It allows a more detailed analysis of the physical-chemical characteristics of catalysts, affecting their activity at different stages of technological process. One prospective for solving these tasks is X-ray microanalysis with an electron probe (EPMA). 

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