Polymers, conjunct

With both liquid acid catalysts, but presumably to a higher degree with sulfuric acid, hydrides are not transferred exclusively to the carbenium ions from isobutane, but also from the conjunct polymers 44,46,71). Sulfuric acid containing 4-6 wt% of conjunct polymers produces a much higher quality alkylate than acids without ASOs (45). Cyclic and unsaturated compounds, which are both present in conjunct polymers, are known to be hydride donors (72). As was mentioned in Section II.B, these species can abstract a hydride from isobutane to form the -butyl cation, and they can give a hydride to a carbenium ion, producing the corresponding alkane, for example the TMPs, as shown in reactions (7) and (8). 

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

Only scant information is available about the influence of coke formation on the alkylation mechanism. It has been proposed that, similar to the conjunct polymers in liquid acids, heavy unsaturated molecules participate in hydride transfer reactions. However, no direct evidence was given for this proposition (69). In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced from an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

Fig. 8. Concerted alkylation mechanism including alkylation, self-alkylation , cracking, dimerization, and hydride transfer via isobutane and via conjunct polymers.

Propene undergoes little polymerization when treated with 96% sulfuric acid, the chief product being isopropyl hydrogen sulfate which yields isopropyl alcohol on hydrolysis. When 98% sulfuric acid is used, propylene is converted to conjunct polymer. Ethylene cannot be polymerized by sulfuric acid because the stable ethyl hydrogen sulfate and ethyl sulfate are formed attempts to obtain the polymerization by increasing the reaction temperature are unsuccessful because oxidation occurs. 

Phosphoric acid may be used for the polymerization of all the gaseous olefins. Ethylene is converted to ethyl phosphoric acid at temperatures below 250°. At higher temperatures, the ester decomposes to yield conjunct polymer including isobutane. Propylene Undergoes either conjunct or true polymerization depending on whether the reaction temperature is above or below 300°. The butylenes undergo true polymerization chiefly. 

During the last 40 years, an enormous effort was put into searching for a solid catalyst [4, 5]. The main obstacle still to be overcome is the formation of acid-soluble oils (ASO, also known as conjunct polymers or red oil) which accompanies the alkylation process. This material contains highly unsaturated cyclic hydrocarbons, which rapidly passivate the catalyst. When liquid catalysts are used, they can be easily withdrawn from the process and replaced, without interrupting the alkylation operation. UOP has developed the Alkylene technology, which uses the proprietary HAL 100 catalyst in a process that is claimed to be commercially competitive [6]. 

A problem that is characteristic of sulfuric acid-catalyzed alkylation is its capabihty to oxidize hydrocarbons. H2SO4 decomposes in the presence of isoalkanes to form water, SO2, and alkenes. This is a slow process, and so it occurs predominantly when the acid is in contact with hydrocarbons for a longer period. Higher temperatures favor the formation of SO2 (10). Some irreversible reactions between acid and hydrocarbons also take place during alkylation. Sulfone, sulfonic acid, and hydroxy groups have been detected in conjunct polymers produced with H2SO4 as the catalyst (8,96). Kramer (97) reported that... 

Doshl and Albright, and earlier Hoffmann, Schrleshelm and this author (13) have recognized that alkylation performance Is related to the presence of oil soluble hydrocarbons, commonly called red oil or conjunct polymers. These species are usually considered to be saturated and unsaturated cations which can function as Intermediates In the transfer of hydride Ions from Isobutane to other alkyl cations. Assuming that hydride transfer Is a limiting factor, the discovery of means to augment the rate should result In Improved alkylation. This report deals with research which has led to the successful application of cationic surfactants for this purpose In commercial plants. 

Conjunct Polymers. Conjunct polymers (frequently called acid-soluble oils in HF alkylation, red oils in sulfuric acid alkylation) are an exceedingly complex mixture of highly unsaturated, cyclic hydrocarbons. These polymers are by-products of tertiary butyl carbonium ions, and their formation undoubtedly Involves a complexity of reactions. Miron and Lee (1963) found the bulk of an HF conjunct polymer to be mode up of molecules containing 2-4 rings with an average ring size of 5-6 carbon atoms. They estimated the number of double bonds per molecule of polymer at about 2.5 to 3. Thus, these polymers are hydrogen-deficient. 

When isobutane-olefin mixtures are contacted with sulfuric acid at alkylation conditions of coimierclal Interest, the olefins often. If not always, disappear from the hydrocarbon phase at a faster rate than the Isobutane (1,2). Subsequently, the Isobutane reacts to produce alkylate tdilch Is often predominantly trimethyl pentanes (TMP s). Other Isoparaffins formed to a large extent during the Initial stages of alkylation Include dimethyl-hexanes (DMH s), C5-C7 Isoparaffins referred to as light ends (LE s), Cg and heavier Isoparaffins often called heavy ends (HE s), and acid-soluble hydrocarbons sometimes referred to as conjunct polymer or red oil. The relatively rapid disappearance of the olefins from the hydrocarbon phase Is undoubtedly caused In part by the relatively high solubility of the olefins In the acid phases. 

Reactions between Isobutylene and sulfuric acid were very different than those between n-butene and sulfuric acid. At least three types of reactions occurred. These were dimerization to form Cb olefins, formation of heavy ends Including Cg to Ci4 hydrocarbons (many of which are olefins), and acid-soluble hydrocarbons. These latter hydrocarbons probably Included conjunct polymers and perhaps t-butyl sulfate. As will be reported later... 

Second-order dependency relative to the Hammett acidity Is based on the results for the degradation of TMP s In the presence of sulfuric acid (11). The acidity is, of course, decreased because of dissolved acid-soluble hydrocarbons Including conjunct polymers and butyl sulfates and because of dissolved water. The term, -Hj. - m(BS) - 0.065(CP), was employed to predict the effective acidity of the acids used. It Is an expansion of the expression used earlier (11) for acids containing dissolved conjunct polymers. 

C.-Cg cations and olefins. The latter olefins also quickly pro-tonateo forming cations. Hydride transfer from isobutane or acid-soluble hydrocarbons resulted in the production of C to Cg isoparaffins. Heavy ends and conjunct polymers were, of course, produced to some extent during the second-step reactions, but most of these compounds were probably produced during the first-step reactions. 

Isobutylene was also found to react In the presence of sulfuric acid to form acid-soluble hydrocarbons that reacted with Isobutane to form alkylate (5). Although the exact nature of these acid-soluble hydrocarbons is not known. It Is thought that they are in part at least t-butyl sulfates (see Reaction 1-2) or that they complex (or react) with the conjunct polymer cations (R" "), as shown in Reaction J. In both cases, isobutylene would be liberated by reverse reactions, and the isobutylene would then alkylate Isobutane. 

Based on the above reasoning, rather significant differences likely occur when HF Is used for two-st p alkylations as compared to two-step processes using sulfuric acid. For HF alkylations, butyl fluorides have been reported to be Intermediates (23). Since Isobutane Is much more soluble In HF as compared to sulfuric acid, there is a strong possibility that less heavy ends or conjunct polymers would be produced In two-step alkylations using HF. 

One of the most important facets of the mixer con arison was the effect of the mixer type on alkylate quality. As mentioned previously, since acid composition does have a profound effect on quality it is necessary to compare mixing effects at constant acid composition. For this study, the acid con ionent which seemed to play the key role in this composition/quality effect is the acid soluble oil. This component, which is characteristic of strong acid processes, is a complex mixture in which cyclic conjunct polymers predominate ( ). Since alkylate quality does vary markedly with acid soluble oil content, many theories on its role in quality enhancement have been proposed such as a hydride transfer agent, surfactant, or agent to increase isobutane solubility. 

Since the hydrocarbons or conjunct polymers in the used sulfuric acid catalyst are unsaturated, attempts have been made to hydrogenate the catalyst in an effort to saturate the polymers, and thus make them insoluble in the acid. Attempts have also been made to conduct the alkylation under hydrogen pressure. 

Various solvents have been used to try to extract the soluble or chemically bound conjunct polymers from the used catalyst. 

Attempts have been made to separate the acid from the conjunct polymers by crystallization of the acid, and washing of the polymers from the acid crystals. This work was promising and a large pilot unit was operated. A paper on the process by Mr. S. Robert Stiles is scheduled for this Symposium (2). 

One line of attack, which is the subject of this paper, is to convert the sulfuric acid in the used catalyst to dialkyl sulfates, which are soluble in hydrocarbons, and then extract the dialkyl sulfates from the conjunct polymers and water. Considerable background information was available prior to the late 1950 s when a study was started with the specific objective of developing such a recovery process. When it is considered that sulfuric acid had a net cost of not over about one cent per pound, or 20.00 per ton, and a net cost in alkylation of about... 

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