Aliphatic alkylation, zeolites

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

The alkylation of the naphthenic cation causes formation of complex aliphatic carbonium ions. Transformation of such intermediates according to Poustma [30] gives the molecules of light saturated hydrocarbons and aromatics. It is generally accepted that the formation of condensed aromatic rings being a coke precursors is difficult in the pores of ZSM-5 zeolite. The fact that the products of the toluene transformation reaction in all cases contained 1 - and 2-methylnaphthalene seems to prove their formation from the olefinic or naphthenic carbocations. Transformation of the naphthenic carbocations occuring in zeolite pores and on the external zeolite surface is the most probable source of methyinaphthalene isomers [23]. 

Alcohol Dehydration using New Shape Selective Zeolites. - The shape selectivity of zeolites has been referred to and exploited in numerous ways. Nevertheless, within the last ten years a new class of zeolite (Type ZSM-5) has been reported, which possesses pore openings, intermediate in size between the large and small pore zeolites its sieving properties in respect of alkyl aromatics and various aliphatic hydrocarbons have been reported. 

H NMR. - NMR was also used to study HY zeolites coked with n-heptane at 450°C The study discriminated among aliphatic or alicyclic protons, alkyl aromatic protons, aromatic protons, or protons linked to carbon near oxygen atoms, before and after oxidation treatment at 400°C. It was found that almost all the alkyl aromatic and aromatic protons are eliminated by the oxidation treatment, while the protons linked to saturated carbons are little affected, and signals characteristic of protons situated near oxygen atoms could be observed. 

In two papers by Walsh and Rollman [14-C]labelled hydrocarbons were used to study the origin of carbonaceous deposits on zeolites. With feeds composed of an aliphatic + an aromatic hydrocarbon, the initial reaction involved in the formation of coke was the alkylation of aromatics by the olefmic fragments of alkane cracking. Since ZSM-5 and mordenite have the same framework A1 content, it was possible to compare directly the coke yields of these zeolites. Under the same experimental conditions it was found that C deposition on mordenite was almost two orders of magnitude greater than on ZSM-5. The differences were explained in terms of pore size. In the smaller ZSM-5 pore, the alkylaromatics, once formed were prevented from reacting further to produce coke, because of the spacial constraints. 

---