Alkylation process conditions

Optimization of a process or catalyst by experimental design such as two-factorial design can lead to significant reduction in time required to achieve the goal. In their excellent work, Mylroie et al. (3) reduced the time required for the optimization of the reductive alkylation process conditions by a factor of 10. Here, we turn our attention to the optimization of the catalyst rather than the process. 

The alkylation process possesses the advantages that (a) a wide range of cheap haloalkanes are available, and (b) the substitution reactions generally occur smoothly at reasonable temperatures. Furthermore, the halide salts formed can easily be converted into salts with other anions. Although this section will concentrate on the reactions between simple haloalkanes and the amine, more complex side chains may be added, as discussed later in this chapter. The quaternization of amines and phosphines with haloalkanes has been loiown for many years, but the development of ionic liquids has resulted in several recent developments in the experimental techniques used for the reaction. In general, the reaction may be carried out with chloroalkanes, bromoalkanes, and iodoalkanes, with the reaction conditions required becoming steadily more gentle in the order Cl Br I, as expected for nucleophilic substitution reactions. Fluoride salts cannot be formed in this manner. 

The various olefinsulfonates used in this study differed in their alkyl chain lengths and in the position of the ionic head group along the hydrophobe moiety. Furthermore, the composition of some IOS compounds was modulated by using different process conditions. 

Although, as has already been mentioned, under matrix conditions between 10 and 77 K, there is no oxidative addition of a chloroolefin to nickel or palladium atoms (141), it is evident that this is simply a function of reaction and processing conditions, as it has been shown (68) that oxidative addition to C-C or C-H bonds by nickel atoms leads to pseudocomplexes having Ni C H ratios of 2-5 1 2. Klabunde and co-workers investigated the oxidative addition-reactions of palladium atoms with alkyl halides (73) and benzyl chlorides (74). 

For carbon-carbon bond-formation purposes, S 2 nucleophilic substitutions are frequently used. Simple S 2 nucleophilic substitution reactions are generally slower in aqueous conditions than in aprotic organic solvents. This has been attributed to the solvation of nucleophiles in water. As previously mentioned in Section 5.2, Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions (Scheme 6.1). In alkylations of phenoxide ions by benzylic chlorides, S/y2 substitutions can occur both at the phenoxide oxygen and at the ortho and para positions of the ring. In fact, carbon alkylation occurs in water but not in nonpolar organic solvents and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para). The effects of phenol substituents and of cosolvents on the rates of the competing alkylation processes... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

A comparison of the feed and product compositions achievable by this approach is shown in Figure 16.8, which shows the depletion of multi-ring aromatics from the feed in favor of a variety of single ring aromatics with short alkyl chains. A more challenging approach that leads to a higher-value product involves optimization of the catalyst and process conditions to maximize xylene and toluene production for aromatic complex feeds [60]. 

Having established reliable values for all of the important rate constants as a function of alkyl substitution on dibenzothiophenes, it is now possible to examine critically how these rate constants (and associated changes in product selectivity) are affected by other components of commercial gas oils and by the H2S that is produced during the HDS process. It is also possible to evaluate how these various rate constants are affected by changes in catalyst composition and by process conditions. Knowledge of the details of these effects can lead to novel catalyst modifications and process configurations that may be able to reach the new stricter standards of 0.05% S. These topics are discussed in later sections. However, for perspective, we will first summarize what is known about present-day catalyst compositions and catalytic mechanisms that bring about the transformations observed in HDS processes. 

Concentrated sulfuric acid and hydrogen fluoride are still mainly used in commercial isoalkane-alkene alkylation processes.353 Because of the difficulties associated with these liquid acid catalysts (see Section 5.1.1), considerable research efforts are still devoted to find suitable solid acid catalysts for replacement.354-356 Various large-pore zeolites, mainly X and Y, and more recently zeolite Beta were studied in this reaction. Considering the reaction scheme [(Eqs (5.3)—(5.5) and Scheme 5.1)] it is obvious that the large-pore zeolitic structure is a prerequisite, since many of the reaction steps involve bimolecular bulky intermediates. In addition, the fast and easy desorption of highly branched bulky products, such as trimethylpentanes, also requires sufficient and adequate pore size. Experiments showed that even with large-pore zeolite Y, alkylation is severely diffusion limited under liquid-phase conditions.357... 

The ranges of operating conditions normally used in commercial H2S04 and HF alkylation processes are shown in Table I. Both processes operate in the liquid phase at relatively low temperatures. For the best... 

Cumene is industrially produced by propylating benzene over supported acidic catalysts such as phosphoric acid. On the other hand, the largest-scale single industrial alkylation process—that is, ethylation of benzene with ethylene—is still carried out to a significant degree in the liquid phase using acid catalysts since ethylene is less polar than propylene, it requires more forcing conditions in the protolytic initiation step [Eq. (5.71)]. 

Since the stereochemistry of the newly created quaternary carbon center was apparently determined in the second alkylation process, the core of this method should be applicable to the asymmetric alkylation of aldimine Schiffbase 42 derived from the corresponding a-amino adds. Indeed, di-alanine-, phenylalanine- and leucine-derived imines 42 (R1 = Me, CH2Ph, i-Bu) can be alkylated smoothly under similar conditions, affording the desired non-coded amino acid esters 43 with excellent asymmetric induction, as exemplified in Table 5.7 [19]. 

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