Alkylations over zeolites

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Isoparaffin-Olefin Alkylation over Zeolite Catalysts... 

Minachev, K.M., E.S. Mortinov, S.M. Zen kovski, N.V Mostovoy andN.F. Kononov, 1977, Isoparaffin-olefin alkylation over zeolite catalysts, in Industrial and Laboratory Alkylations, Vol. 55 of ACS Symposium Series (American Chemical Society, Washington, DC) pp. 89-95. 

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

In agreement with this, the analysis of the products forming during the alkylation over MCM-22 samples at different reaction time showed that about 80-90 wt. % of the recovered material is adsorbed on the external surface (phenol, o-TBP and p-TBP), whereas only about 10-20 wt.% is formed inside the zeolite ( coke constituted by p-TBP, naphthalenes, 9,10-dihydro-phenanthrene and acenaphthylene). Coke molecules could be formed in both supercages and sinusoidal channels. However, in the large supercages they would be converted into bulkier compounds, which is not the case. Therefore they are most likely located in the sinusoidal channels. 

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

Nevertheless, the discussion whether the intermediates involved in the reactions of hydrocarbons over zeolite surface is the alkyl-aluminumsilyl oxonium ion or the carbocation could not be answered with these previous studies. 

Alkylation over the MWW Zeolite. The MWW (or MCM-22) zeolite developed by Mobil as catalyst for ethylbenzene and cumene production deserves particular attention. Indeed, this zeolite presents unique structural features (Figure 12.5). Its structure is constituted of three independent pore systems " large supercages (inner diameter of 7.1 A dehned by a 12-member-ring [12-MR], height 18.2 A) each connected to six others through 10-MR apertures... 

N. (2008) Continuous supercritical iC4/ C4= alkylation over H-Beta and H-USY Influence of the zeolite strudure. Appl. Catal A, 336, 51-71. 

On selectivity aspects of alkylation of toluene with methanol over zeolites, DGMK tagungsbericht, 9903. Proceedings of DGMK-Confer-ence The Future Role of Aromatics in Refining and Petrochemistry, 1999, pp. 279-286. 

Tsai, T.-C., liu, S.-B., and Wnag, I. (1999) Disproportionation and trans-alkylation of alkylbenzenes over zeolite catalysts. Appl Catal A, 181, 355-398. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

There is increasing interest in studying the alkylation of methylbenzenes in the side chain with methyl alcohol over zeolite catalysts.270 These reactions may lead to new nontraditional technologies in the synthesis of styrene from toluene and that of p-methy I styrene from p-xylene. A one-step route from toluene or p-xylene to the corresponding styrenes would be of great practical importance compared with the presently practiced two-step syntheses (alkylation followed by dehydrogenation). 

Tphe rate-limiting processes in catalytic reaction over zeolites remain A largely undefined, mainly because of the lack of information on counterdiffusion rates at reaction conditions. Thomas and Barmby (7), Chen et al. (2, 3), and Nace (4) speculate on possible diffusional limitations in catalytic cracking over zeolites, and Katzer (5) has shown that intracrystalline diffusional limitations do not exist in liquid-phase benzene alkylation with propene. Tan and Fuller (6) propose internal mass transfer limitations and rapid fouling in benzene alkylation with cyclohexene over Y zeolite, based on the occurrence of a maximum in the reaction rate at about 100 min in flow reaction studies. Venuto et al (7, 8, 9) report similar rate maxima for vapor- and liquid-phase alkylation of benzene and dehydro-... 

Benzene alkylation over Y zeolites has been studied as a function of olefin, olefin aromatic ratio, temperature, and zeolite cation form. The rate has been modeled, and the rate-limiting process has been quantified as product desorption. 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

Finally, highly regioselective dialkylation of naphthalene has been achieved over zeolite HM using ter/-butanol as alkylating agent. By optimisation of the reaction parameters, 2,6-di-ter/-butylnaphthalene has been obtained in a yield of 60 % with a 2,6/2,7 ratio of over 50. This is the highest yield of a 2,6-dialkylnaphthalene and easily the highest 2,6/2,7 ratio yet reported from a direct dialkylation of naphthalene. 

Catalyst deactivation due to coke formation is relatively speedy for a reactions such as alkylation of benzene to ethylbenzene over zeolite, particularly when the benzene to ethylene ratio is low. Another problem of this reaction is the formation of xylenes, the major byproducts. Though their total amount produced in the process is very limited, they are harmful to the process because of the difficulty to remove them from the desired product ethylbenzene. Therefore, investigating the mechanism of catalyst coking is of practical significance for finding the potential ways for prolonging the reactor runtime and decreasing the xylenes selectivity. 

Description The alkylator (1) and benzene stripper (2) operate together as a distillation column. Alkylation and distillation occur in the alkylator (1) in the presence of a zeolite catalyst packaged in patented structured packing. Unreacted ethylene and benzene vapor from the alkylator top are condensed and fed to the finishing reactor (3) where the remaining ethylene reacts over zeolite catalyst pellets. The benzene stripper bottoms is fractionated (5 6) into EB prod-... 

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. 

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. 

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