Isobutylene selectivity

These processes are aH characterized by low isobutane conversion to achieve high isobutylene selectivity. The catalytic processes operate at conversions of 45—55% for isobutane. The Coastal process also operates at 45—55% isobutane conversion to minimize the production of light ends. This results in significant raw material recycle rates and imposing product separation sections. 

Similarly, the polymerization process will pull the isobutylene selectively out of the C4 stream. Polyisobutylene is used mainly as a viscosity index improver in lubricating oils and in caulking and sealing compounds. Some of the low molecular weight polyisobutylenes are particularly suited for use in the construction field because it doesn t solidify. They remain a tacky fluid and when properly formulated with clay fillers, etc., take on the properties of a sticky, putty-like substance. 

The existence of a catalytic site different from a proton donor site, e.g., a Lewis acid site, has not been considered. It is known that Lewis acid sites could play a role in such reactions (14) however, among the catalysts that are selective for this isomerization, most have little or no Lewis acidity. Moreover, often when the catalysts arc nonmicroporous materials, such as halogenated aluminas (15) and SiOa—ALO3, the highest isobutylene selectivities are obtained in the presence of a mixture of olefin and water, with the water partial pressure being close to that of the olefin. The existence of Lewis acid sites under these conditions is very unlikely. 

From the previous discussion, it is expected that if the two reaction mechanisms operate simultaneously, a change in temperature changes the isobutylene selectivity the dimerization reaction is favored by low temperatures and consequently the isobutylene selectivity is low at low tempera-... 

The isobutylene selectivity at given conversion increases with TOS in such operations. The catalytic behavior is illustrated in Figs. 9 and 10. 

Additional indirect experimental evidence was reported by the same authors 19, 49). They observed that when n-butenes reacted at different pressures in the presence of a catalyst exhibiting moderate isobutylene selectivity (50-70%), the highest isobutylene selectivity was observed at the lowest butene pressure this result was interpreted by considering that a decrease in butene pressure causes a greater decrease in the bimolecular reaction than in the monomolecular reaction. Similar results and a similar... 

Similar behavior was observed for ZSM-23 (38), MeAPO-11 (14), and ZSM-22 (35). For these unidimensional pore systems (MTT, AEL, and TON), the coking rate was less than that for FER catalysts furthermore, the magnitude of the differences in activity and selectivity between fresh and coked samples was less than that for catalysts with bidimcnsional pore systems. For example, when ZSM-23 was used at 693 K and a W.HSV of 171 h , the micropore volume decreased from 58.4 to 11 ptl/g after 20 h on stream. Also, the number of acidic sites estimated by butene TPD decreased from 0.45 to 0.06 mol per unit cell, whereas the butene conversion decreased only from 41 to 31% the isobutylene selectivity increased from 72 to 92% (38). Evidently, the catalyst pore geometry significantly affects the coke deposition and thus the selectivity. The relevant literature is discussed in the following section. 

Ion exchange (%) Normalized remaining acidity Butene conversion (%) Isobutylene selectivity (%)... 

These results arc important because they clearly demonstrate the role of shape selectivity in the selective skeletal isomerization of n-butene. Similar results and interpretation were obtained independently by Kwak et al (JJ), who modified the ferrierite catalyst by replacing part of the protons with Mg +. They observed a smaU decrease in microporosity which was related to an increase in the isobutylene selectivity. Again, it was demonstrated that the micropore size is a key factor governing the selectivity for isobutylene. 

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. 

Again, this difference in isobutylene selectivity between ZSM-23 and PER, when both were treated with oxalic acid and had few external acid sites, was attributed to the presence of cavities in PER. These cavities, which are not accessible to oxalic acid, are too large to restrict the formation of Cs olefinic intermediates. 

A family of catalysts with undimensional pores was compared for n-butene conversion. Data were obtained at short TOS so that coking was negligible and the catalysts could be compared on the basis of their intrinsic pore geometries 63). The results, summarized in Pig. 15, show that the isobutylene selectivity increases with decreasing pore dimensions as follows ... 

The authors concluded that the major factor governing the isobutylene selectivity is the free space around the active site, as was proposed for PER (62). 

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). 

In an investigation of PER synthesized without a template (66), it was shown that an acid treatment causes the removal of non-shape-selective acidic sites located in pores larger than the zeolitic pores or on the external surfaces of the ferrierite grains (66). Consequently, isobutylene selectivity... 

For such treated samples it is not easy to discriminate between two possible effects of dealumination, namely, the removal of some acid sites and the decrease in microporosity due to the deposition of aluminum-containing debris in the pores. Thus, hydrothermally dealuminated FER, hydro thermally dealuminated acid-washed FER. acid-washed FER, and CsFER were compared under the same experimental conditions (62). The results indicate the following order of isobutylene selectivities untreated FER < acid-treated FER < hydrothermally treated FER < hydrothermally acid-treated FER < CsFER 61). These results, obtained with noncoked catalysts, reinforce the interpretation in terms of shape selectivity. The hydrothermally acid-treated sample has acid sites located only in the micropores, and the aluminium debris in the micropores creates an additional constraint playing a role identical to that of Cs" in FER. 

The main factor governing the isobutylene selectivity is restricted transition-state shape selectivity. The space available around the acid site governs the isobutylene selectivity by allowing the reaction to proceed mainly through the monomolecular and not the bimolecular mechanism. 

The acid strength does not have an important influence on isobutylene selectivity, and the acid site density seems to play only a minor role. 

The isobutane pyrolysis studies by Egloff, Thomas, and Linn (IS) established gaseous product selectivity relationships for operation at 14.2 and 99.5 psia. Unfortunately, liquid products were not reported, and their propylene and isobutylene selectivity data are probably high. Nevertheless, their data are useful for establishing shapes of curves and trends. 

Figure 3. Pyrolysis of isobutane. Isobutylene selectivity decreases with conversion and pressure.

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