N-butenes, alkylation

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

Nivarthy, G.S., Seshan, K., and Lercher, J.A. (1998) The influence of acidity on zeolite H-BEA catalyzed isobutene /N-butene alkylation. Micropor. 

J.A. (1998) Influence of the activation temperature on the physical properties and catalytic activity of La-X zeolites for isobutane/n-butene alkylation. Micropor. Mesopor. Mater., 22, 379-388. 

Besides the rearrangement of carbocations resulting in the formation of isomeric alkylated products, alkylation is accompanied by numerous other side reactions. Often the alkene itself undergoes isomerization prior to participating in alkylation and hence, yields unexpected isomeric alkanes. The similarity of product distributions in the alkylation of isobutane with n-butenes in the presence of either sulfuric acid or hydrogen fluoride is explained by a fast preequilibration of n-butenes. Alkyl esters (or fluorides) may be formed under conditions unfavorable for the hydride transfer between the protonated alkene and the isoalkane. 

Although no specific analysis was made to Identify sec-butyl sulfate in the acid phase, there seems little doubt that this sulfate was the reaction product obtained in the n-butene runs. First, the n-butenes and acid reacted on essentially a 1 1 molar basis only a slight excess of acid was needed In all cases. Of Interest, formation of butyl sulfates has also been reported to occur In conventional alkylation reactors (13). Second, the reaction product was similar 1n many respects as compared to sec-propyl sulfate the product was unstable at higher temperatures and 1t reacted with isobutane in the presence of sulfuric aicd to form alkylate (9). Third, the reaction products from all three n-butenes alkylated in identical manners as will be discussed later (9). Fourth, McCauley (14) has shown that butyl fluorides can be formed and are quite stable at low temperatures such as used here he had contacted HF with n-butenes. 

The activity and decay behaviour of the different porous heteropolycompounds were compared in two reactions requiring strong acid sites the n-butane isomerization and the isobutane/2-butene alkylation. Although these two reactions are important in the petroleum refining industry, n-butane isomerization is often used as a "test reaction" since it is known that this reaction requires very strong acid sites and only a limited number of oxides are active in this reaction, under mild conditions (T = 473 K). 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

With respect to the co-catalytic activity of alkyl halides, BF3 occupies a special position, since these (other than fluorides) cannot form complexes with BF3 for steric reasons. It has indeed been found [31a] that in MeCl solution the n-butenes are not polymerised by BF3. MeCl cannot act as co-catalyst in this system and some other (e.g., S02) was required. The mode of action of S02 is still obscure, but it is possible that H2S03 was the real co-catalyst. 

Feller, A., Zuazo, L, Guzman, A., Barth, J.-O., and Lercher, J.A. (2003) Gommon mechanistic aspects of liquid and solid acid catalyzed alkylation of isobutane with n-butene./. Catal., 216, 313-323. 

Isobutylene is more reactive than n-butene and has several industrial uses. It undergoes dimerization and trimerization reactions when heated in the presence of sulfuric acid. Isobutylene dimer and trimers are use for alkylation. Polymerization of isobutene produces polyisobutenes. Polyisobutenes tend to be soft and tacky, and do not set completely when used. This makes polyisobutenes ideal for caulking, sealing, adhesive, and lubricant applications. Butyl rubber is a co-polymer of isobutylene and isoprene containing 98% isobutene and 2% isoprene. 

A two-step (two-reactor) process292,293,305,306 may be operated at lower operating costs (lower ratios of isobutane to n-butenes, lower levels of agitation and acid consumption). More importantly, it affords alkylates of higher quality (99-101 octane number). In the first step sec-butyl sulfate is produced using a limited amount of acid. This then is used to alkylate isobutane with additional acid added. The two-step alkylation can be carried out in the temperature range of —20 to 0°C with the second reactor usually operated at lower temperature. 

The alkylation of isobutane with n-butenes to give C8 alkylate [Eq. (21)] is a widely used and increasingly important process in petroleum refining. The... 

PFAS can be supported on silica using a dehydrating solvent, obtaining an active catalyst. Spectroscopic studies suggest that the interaction between PFAS and the support is covalent. PFAS-Si02 catalyses the alkylation of isobutane with n-butenes to yield a mixture of products, of which saturated octanes are the major fraction. Trimethylpentanes are the main constituent of this fraction. The best results were obtained using thionyl chloride as the dehydrating solvent. 

Liang, Chin-Huang, Alkylation of isobutane with n-butenes over solid catalysts, (U.M.I., Dissertation N 9411297, 1994). 

Production of light olefins (propylene, n-butenes and isobutene) will be one of the main targets of FCC untis in the near future. These olefins can be fed to alkylation and etherification units to produce additional high octane environmentally acceptable gasoline components, or used as petrochemical feedstock. Johnson and Avidan (85) used higher amounts of ZSM-5 (10-20%) to increase the production of light olefins, mainly propylene. 

The relative location of refinery and acid plant is one of the most important factors in the economic decision between sulfuric acid and anhydrous hydrogen fluoride as a catalyst for alkylation. Besides the distance, other factors such as regeneration of spent acid, energy costs, the nature of the feed and increasingly stringent regulatory constraints play an important role in the selection of alkylation catalyst. Sulfuric acid is selected for alkylation if feed is rich in pentenes or n-butene. HF is selected if the feed is rich in propenes or isobutane. 

Similarly, hydrogen transfer reactions occur when isobutane is alkylated with n-butenes or with amylenes. There is no chain termimtion taking place in hydrogen transfer hydrogen transfer represents either chain initiation or chain transfer. 

The products from butene-1 and n-butenes mixture containing mainly butene-2 were of comparable quality, probably because of Isomerization of the double bonds of the butenes, as In the case of H2SO4 alkylation. This was confirmed by a special experiment where butene-1 diluted with ten-fold n-butane was passed over the catalyst employed under the conditions of alkylation. In the product obtained both butene-1 and butene-2 Isomers were present In thermodynamic quantities. Moreover unexpectedly we have found that n-butane had been alkylated with butene-1 resulting In a liquid product composed mainly of D(W. 72% of the DMH fraction was 3,4 DMH - the product of the direct Interaction of n-butane and butene-1. This reaction In the presence of conventional mineral acids Is not known and Is very Interesting from a theoretical standpoint. 

Use of Catalysts Containing Transition Metal Cations. Ethyl -ene being alkylated over certain zeolite catalysts reacts specifically. Ethylene can not, however, be alkylated with Isobutane In the presence of H2SO., because of the formation of stable ethylsulphates. We examined the Isobutane - ethylene alkylation over crystalline aluminosilicates and found that those catalysts containing RE and/or Ca In combination with transition metal cations were most active. The alkylation has resulted In not hexanes as would be expected, but an alkylate containing octane Isomers as the major product (about 80%). Moreover, the product composition was similar to that obtained from n-butene over CaREY. The TMP-to-DMH ratios were 7.8 and 7.1 respectively. 

The explanation of the experimental results is that the alkylation proceeds In two steps - first ethylene dimerization takes place (7) and then n-butene (or its precursor) formed alkylated with Isobutane as follows ... 

This scheme was confirmed by the presence of n-butenes In the reaction gases, and the close similarity of the products obtained from ethylene and n-butenes over zeolite CaMEY, as Indicated In Table IV. ME represents a transition metal nickel, chromium, and cobalt were all found effective for ethylene alkylations. Nickel was the metal used for experiments reported In Tables IV and V. 

If acid-olefin reactions which occurred in this investigation at -10 C or lower also occur during conventional alkylations at 10 C or higher, some if not all of the significant differences in the type of alkylations with n-butenes and with isobutylene can be explained, as will be discussed later (10). 

As has been Indicated earlier (1,2), C, olefins react to a significant, and maybe even predominant, extent before Isotutane reacts during alkylations using sulfuric acid as a catalyst. In the previous paper (3) of this series the reactions that occur when n-butene and Isobutylene were contacted with relatively small amounts of sulfuric acid In the temperature range of about -30 to -10°C were Investigated. These reactions defined as first-step reactions In the two-step alkylation process were found to be as follows ... 

The butyl sulfate formed from 1-butene or the two z-butenes reacted In all cases In Identical manners. Such a conclusion was, of course, expected since all three n-butenes produce sec-butyl sulfate. Storage of two mixtures of butyl sulfate and sulfuric acid for several days at -20 C had no effect on the subsequent alkylation reactions. Identical results were obtained as conpared with freshly prepared mixtures. 

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