Types of Catalysts

The observed rate law depends on the type of catalyst used with promoted iron catalysts a rather complex dependence on nitrogen, hydrogen, and ammonia pressures is observed, and it has been difficult to obtain any definitive form from experimental data (although note Eq. XVIII-20). A useful alternative approach... 

Although acrylonitrile manufacture from propylene and ammonia was first patented in 1949 (30), it was not until 1959, when Sohio developed a catalyst capable of producing acrylonitrile with high selectivity, that commercial manufacture from propylene became economically viable (1). Production improvements over the past 30 years have stemmed largely from development of several generations of increasingly more efficient catalysts. These catalysts are multicomponent mixed metal oxides mostly based on bismuth—molybdenum oxide. Other types of catalysts that have been used commercially are based on iron—antimony oxide, uranium—antimony oxide, and tellurium-molybdenum oxide. 

As with nearly all other polymers, HDPE resin is a collection of polymer chains of different lengths, varying from short, with molecular weights of 500—1000, to very long, with molecular weights of over 10 million. Relative contents of chains with different lengths (ie, the shape and width of MWD) depend mostly on production technology and on the type of catalyst used for polymerization. The MWD width of HDPE resins can be tailored to specific apphcations. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Depending on the type of catalyst used, polymerization of cycloolefins proceeds through either ring opening or by opening of the double bond with the preservation of the ring. 

Typically, reactors require some type of catalyst. Reactors with catalyst can be of the fixed-bed style for fiuid-bed types. Fixed-bed reactors are the most common. The feed often enters the reactor at an elevated temperature and pressure. The reaction mixtures are often corrosive to carbon steel and require some type of stainless steel alloy or an alloy liner for protection. If the vessel wall is less than 6 mm, the vessel is constmcted of all alloy if alloy is provided. Thicker reactor walls can be fabricated with a stainless overlay over a carbon steel or other lower alloy base steel at less cost than an all-alloy wall constmction. 

Phosgene addition is continued until all the phenoHc groups are converted to carbonate functionahties. Some hydrolysis of phosgene to sodium carbonate occurs incidentally. When the reaction is complete, the methylene chloride solution of polymer is washed first with acid to remove residual base and amine, then with water. To complete the process, the aqueous sodium chloride stream can be reclaimed in a chlor-alkah plant, ultimately regenerating phosgene. Many variations of this polycarbonate process have been patented, including use of many different types of catalysts, continuous or semicontinuous processes, methods which rely on formation of bischloroformate oligomers followed by polycondensation, etc. 

Friedel-Crafts reaction of ahyl alcohol with benzene or alkylbenzene yields many kinds of products, in which the reaction species and the product ratio depend on the type of catalyst. Zinc chloride is the most effective catalyst for producing ahyl compounds by this reaction (32). 

Dry reduced nickel catalyst protected by fat is the most common catalyst for the hydrogenation of fatty acids. The composition of this type of catalyst is about 25% nickel, 25% inert carrier, and 50% soHd fat. Manufacturers of this catalyst include Calsicat (Mallinckrodt), Harshaw (Engelhard), United Catalysts (Sud Chemie), and Unichema. Other catalysts that stiH have some place in fatty acid hydrogenation are so-called wet reduced nickel catalysts (formate catalysts), Raney nickel catalysts, and precious metal catalysts, primarily palladium on carbon. The spent nickel catalysts are usually sent to a broker who seUs them for recovery of nickel value. Spent palladium catalysts are usually returned to the catalyst suppHer for credit of palladium value. 

Selectivity, Steering of reaction directions by the type of catalyst cation, eg, O- vs C-alkylation (7), substitution vs dibalocarbene addition (8), as weU as enantioselective alkylations by optical active catalysts (9) have been achieved in some systems. Extensive development is necessary, however, to generate satisfactorily large effects. 

The amount of catalytic coke that is formed depends on the type of catalyst used ia the FCCU, the coking tendency of the feed, the degree of conversion of the feed, and the length of time the catalyst is exposed to the feed (eq. 2) (11). 

At high metals levels, the coking characteristics of a cracking catalyst can be greatly increased that is, the ratio of contaminant coke to catalytic coke can be quite high. The effect of the contaminant metals on the coke response is affected not only by the level of metals but also by the type of catalyst and the use of a metals passivator. Catalysts, which contain effective metals traps to inhibit the contaminant effects, do produce much less contaminant coke than catalyst without metal traps. 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

Polydimethyl siloxane is of moderately high molecular weight. The siHcone is a viscous Hquid. CoUoidal siHca or micronized metal oxides, 5—10 p.m particle size, are added to prepare a paste that is mixed with a tetraalkyl siHcate containing 50% ethoxy groups, eg, commonly tetraethyl orthosiHcate, and 1—2% of an organic tin activator. This type of catalyst has a limited shelf life because of oxidation. 

Many catalysts for the hydration of olefins in general, and of ethylene in particular, are described in the patent Hterature. Practically all of them are acidic. There has been a patent Hterature review through 1937 of the types of catalysts used (47,48) and a general review of olefin hydration (88). 

Front-end hydrogenation is also possible. This approach uses a different type of catalyst, and the reactor is located upstream of the demethanizer. For this design, a deethanizer or depropanizer tower is located upstream of the demethanizer to remove heavy fractions. This approach has been utilized by some Hcensors with some success. 

Poisons ndInhibitors. Catalyst poisons and inhibitors can come from the fuel, the lube oil, from engine wear and corrosion products, and from air ingestion. There are two types of catalyst poisons one poisons active sites, the other is a masking agent. 

Operating Temperature. The operating temperature needed to achieve a particular VOC destmction efficiency depends primarily on the species of pollutants contained in the waste stream, the concentration of the pollutants, and the catalyst type (14). One of the most important factors is the hydrocarbon species. Each has a catalytic initiation temperature which is also dependent on the type of catalyst used (14). 

The microactivity test uses small quantities of catalyst, only 4 grams, and a feed of 1.33 g in 75 seconds, so it is a very fast test, but the test s empirical usefulness is strictly limited to one well-known technology, for an endothermic reaction and one very limited type of catalyst. 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Seebach et al., who first developed the TADDOL ligands [53, 67], have also developed a number of polymer- and dendrimer-bound TiCl2-TADDOLate catalysts derived from the monomeric TADDOLs [68]. Application of 10mol% of this type of catalysts, derived from polymers and dendrimers of 27 and 28, respectively, in the... 

The degree of acylation of PS essentially depends on the type of catalyst used and the molecular weight of initial virgin PS. As it is seen from the data in Table 3, virgin PS with higher molecular weight is acylated to a smaller degree in the equal conditions of modification. 

The MWD is the ratio of Mw Mn and is called the polydispersity index. This largely varies from one grade of polymer to the other, depending on the polymerization conditions and the type of catalyst used. Figure 14 shows different types of MWD for the polymers. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

From these simple intermediates, many important chemicals and polymers are derived through different conversion reactions. The ohjec-tive of this hook is not merely to present the reactions involved in such conversions, hut also to relate them to the different process variables and to the type of catalysts used to get a desired product. When plausi-hle, discussions pertinent to mechanisms of important reactions are included. The hook, however, is an attempt to offer a simplified treatise for diversified subjects dealing with chemistry, process technology, polymers, and catalysis. 

The HVCH ratio is an indicator of dehydrogenation reactions. However, the ratio is sensitive to the reactor temperature and the type of catalyst. A better indicator of nickel activity is the volume of... 

Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

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