Isobutylene, formation

Poisoning experiments with varying amounts of preadsorbed pyridine have recently been carried out by KnOzinger and Stolz (47). Pyridine is solely held by Lewis acid sites under the experimental conditions as shown by infrared spectroscopy. The rate of isobutylene formation from f-butanol was essentially independent of the degree of poisoning, and the true activation energy of the reaction remained constant at 25 kcal/mole, when the number of preadsorbed pyridine molecules varied between 3 and 9X 10n/m2. It thus, appears that Lewis sites which retain pyridine at temperatures between 550° and 150°C, respectively, do not interfere in this reaction. 

An idea that was tested experimentally is that if the bimolecular mechanism prevails 19), the products formed from n-butene reactants, on the one hand, and from any of the possible isomers formed by dimerization of n-butenes (such as 3,4-dimethylhex-l-ene), on the other hand, should be similar. Thus, the transformations of 2,2,4-trimethylpent-2-ene, 3,4-dimethylhex-2-ene, and methylheptenes were investigated with micro-porous catalysts such as MnAPO-11 and SAPO-11. The results are summarized in Fig. 11, in which the ratio (propene pentene) /n-butenes is plotted for different reactants. The data show that with a selective isomerization catalyst, this ratio is quite low (<0.1) for n-butene reactants in contrast, it is quite high (approximately 0.8) when 3.4-dimethylhex-2-ene or methylheptenes are the reactants, indicating that these compounds are not intermediates in the selective isomerization of n-butenes. Consequently, the isobutylene formed on selective catalysts results from a monomolecular process. Th ese results are considered to be good indirect evidence that the bimolecular reaction is not selective for isobutylene formation. 

The proposed pathway will be more favorable kinetically than that suggested for the true monomolecular process, whereby a primary carbenium ion is formed. To further test the idea that carbonaceous residues are the active and selective sites for the skeletal isomerization of n-butenes, the authors reported results showing that the rate of isobutylene formation catalyzed by ferrierite passed through a maximum as the conversion continuously decreased (Fig. 12) (51). 

Treatment with oxalic acid has been described as a method for selective removal of the external acid sites of medium-pore zeolites 61). PER and ZSM-23 zeolites were treated with a 1-M solution of oxalic acid at 353 K overnight 39, 62). The characterization of the acid sites showed that the treated materials had a low number of external acid sites compared with the untreated materials and, when used in n-butene isomerization, they exhibited an improved isobutylene selectivity. It was also observed that acid-treated PER does not have a high selectivity for isobutylene formation. It was inferred (62) that the cavities in ferrierite at the intersections of 8- and 10-ring channels are large enough to accommodate butene dimer intermediates, thus favoring the unselective bimolecular path. In contrast, when the external acid sites are removed from a zeolite with a unidimensional pore system (e.g., ZSM-23), the initial isobutylene selectivity is higher (nearly 80%) than that of the untreated sample. 

Hydrothermally dealuminated PER and sample that were subsequently acid treated exhibited better selectivities for isobutylene formation than an untreated PER catalyst (27). Furthermore, hydrothermally dealuminated PER exhibited a lower activity than untreated PER but higher selectivity for isobutylene 30,62,66). A subsequent acid treatment (with 5% HCl solution) further decreased the conversion and increased the isobutylene selectivity. The hydrothermal treatment created mesoporosity by A1 extraction. The A1 extraframework species were located in the mesopores and/or in the micropores. The HCl treatment removed part of the extraframework Al, leaving part in the micropores. The elimination of extraframework A1 from the mesopores was evidently beneficial for isobutylene selectivity. Evidently, the active sites associated with extraframework Al located in large voids are nonselective in contrast, extraframework Al located in the micropores (and not removed by acid treatment) does not contribute to catalytic activity. The steamed and acid-washed ferrierite exhibits excellent isobutylene selectivity and catalytic stability 30). 

The surface acidic sites These sites, which are not selective for isobutylene formation, will also be active sites for coke formation. It has been reported (28) that the removal of these sites by oxalic acid treatment improves the catalyst lifetime. 

A systematic series of synthetic, characterization and butene isomerization catalysis studies of ferrierite and ferrierite-like materials such as ZSM-22,59 ZSM-23,60 and ZSM-35,6 was undertaken to study optimization of isobutylene product. Coke deposits in the pores of these materials play a key role in isobutylene formation as does the overall acidity and structure of the pore system. Such shape selective effects have been probed with TPD methods. 

Reaction pressure was maintained with a dome-loaded back-pressure regulator (Circle Seal Controls). All heated zones were controlled and monitored with a Camile 2500 data acquisition system (Camile Products). Products were analyzed online by gas chromatography with an HP 5890 II GC, equipped with an FID, and a DB-Petro 100 m column (J W Scientific), operated at 35° C for 30 min, ramped at 1.5°/min to 100° C, 5°/min to 250° C for 15 min. An alkylate reference standard (Supelco) allowed identification of the trimethylpentanes (TMP) and dimethylhexanes (DMH). The combined mass of TMP and DMH is referred to hereafter as the alkylate product . As discussed elsewhere [19], propane, an impurity in the isobutane feed, was used as an internal standard for butene conversion calculations. Since isomerization from 1-butene to 2-butene isomers is rapid over acidic catalysts, reported conversion is for all butene isomers to C5 and higher products. Isobutylene formation was not observed under any conditions. 

The reaction mechamism for isobutylene formation is derived from the general mechanism of Grassie and co-workers [42]. Polyisobutyl acrylate degradation produces more simple saturated esters than the other homologous polyermeric esters. 

The degradation products of poly tertiary- mty acrylate are shown in Table 3.4 and Table 3.5 and the formation of carbon dioxide, monomer and dimers are consistent with the mechanisms for poly-w-butyl acrylate. Isobutylene formation is in agreement with the work of Schaefgen and Sarasohn [37] while the presence of carbon dioxide, monomer, and dimers extends their work. 

The autoaeeelerated character of P-i-BuMA degradation was linked to the ester group deeomposition, with isobutylene formation, which gives free radicals in the reaction with NO2 and thus promotes the degradation proeess. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Substituted Phenols. Phenol itself is used in the largest volume, but substituted phenols are used for specialty resins (Table 2). Substituted phenols are typically alkylated phenols made from phenol and a corresponding a-olefin with acid catalysts (13). Acidic catalysis is frequendy in the form of an ion-exchange resin (lER) and the reaction proceeds preferentially in the para position. For example, in the production of /-butylphenol using isobutylene, the product is >95% para-substituted. The incorporation of alkyl phenols into the resin reduces reactivity, hardness, cross-link density, and color formation, but increases solubiHty in nonpolar solvents, dexibiHty, and compatibiHty with natural oils. 

Sulfuric acid is about one thousand times more reactive with isobutylene than with the 1- and 2-butenes, and is thereby very useful in separating isobutylene as tert-huty alcohol from the other butenes. The reaction is simply carried out by bubbling or stirring the butylenes into 45—60% H2SO4. This results in the formation of tert-huty hydrogen sulfate. Dilution with water followed by heat hydrolyzes the sulfate to form tert-huty alcohol and sulfuric acid. The Markovnikov addition implies that isobutyl alcohol is not formed. The hydration of butylenes is most important for isobutylene, either directiy or via the butyl hydrogen sulfate. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

A second route based on olefin disproportionation was developed by Phillips Petroleum (131). Here isobutylene reacts with propylene to form isoamylenes, which are dehydrogenated to isoprene. 2-Butene can be used in place of propylene since it also yields isoamylene and the coproduct propylene can be recycled. Use of mixed butylenes causes the formation of pentenes, giving piperjlene, which contaminates isoprene. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Examples are given of common operations such as absorption of ammonia to make fertihzers and of carbon dioxide to make soda ash. Also of recoveiy of phosphine from offgases of phosphorous plants recoveiy of HE oxidation, halogenation, and hydrogenation of various organics hydration of olefins to alcohols oxo reaction for higher aldehydes and alcohols ozonolysis of oleic acid absorption of carbon monoxide to make sodium formate alkylation of acetic acid with isobutylene to make teti-h ty acetate, absorption of olefins to make various products HCl and HBr plus higher alcohols to make alkyl hahdes and so on. 

According to the above reaction scheme the carbonylation reaction has to be carried out in two steps In the absence of water the olefin is first converted at 20-80°C and 20-100 bar by the aid of mineralic acid and carbon monoxide into an acyliumion. In a second step the acyliumion reacts with water to the carboxylic acid. The mineral acid catalyst is recovered and can be recycled. The formation of tertiary carboxylic acids (carboxylic acids of the pivalic acid type) is enhanced by rising temperature and decreasing CO pressure in the first step of the reaction. Only tertiary carboxylic acids are formed from olefins that have at the same C atom a branching and a double bond (isobutylene-type olefins). 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

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