Active sites in the acylation of anisole with acetic anhydride over zeolite Beta

Paine, G.S., 2002. Active sites in the acylation of anisole with acetic anhydride over zeolite Beta. PhD, Nottingham Trent University.

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Acylation is a key step in the manufacture of aromatic ketones used as intermediates in the fine chemical and pharmaceutical industry. The conventional method, and at the present time, the most widely applied method of preparation of these ketones is the homogeneous Freidel Crafts acylation of aromatics with carboxylic acid derivatives using Lewis acid anhydrous metal halides as catalysts, most notably AICI3. However, recently, the first industrial process has been introduced for the acylation of anisole and veratrole using zeolite Beta and zeolite Y respectively. Such a process overcomes the environmental problems associated with using metal halides, the latter are required in more than stoichiometric amounts and lead to a significant amount of inorganic by-product. The effect of hydrothermal, thermal, chemical modification, ion exchange, and oxidative regeneration on the acidic properties of zeolite Beta is probed using a combination of FTIR, MAS NMR, and XRD studies. The acidic properties of ZSM-5, zeolite Y, mordenite, and mesoporous MCM-41 are also considered briefly. It was found that the catalytic activity of the entire range of zeolite Beta samples is related to the concentration of framework tetrahedral atoms, and to their partial positive charge. On active zeolite Beta samples, the reversible transformation of framework tetrahedral aluminium to framework octahedral aluminium was observed upon adsorption of the acylating agent, acetic anhydride. Only limited transformation was observed on the inactive Na-Beta. A mechanism is proposed for the acylation of anisole with acetic anhydride over zeolite Beta. It is a modified Eley-Rideal type mechanism whereby acetic anhydride interacts with framework tetrahedral aluminium (which acts as a Lewis acceptor), forming an octahedral complex. This polarises the anhydride molecule, forming the active electrophilic cation intermediate. A molecule of aromatic substrate (anisole) impinges from the liquid phase, reacting with the acylium cation and forming a Wheland intermediate. The decomposition of the Wheland intermediate to p-methoxyacetophenone (p-MAP) and acetic acid completes the process. The desired product, p-MAP, is rapidly and selectively formed on all zeolite Beta samples. The exclusive products of the reaction, p-MAP and acetic acid were found to be the main contributors of catalyst deactivation. The catalytic activity of ZSM-5, zeolite Y, mordenite, and mesoporous MCM-41 was significantly lower than on zeolite Beta samples. Catalytic data would suggest that pore size and acidic strength play no major role in the activity of the different materials tested. The much higher catalytic activity observed over zeolite Beta samples may be related to the unusual flexibility of framework tetrahedral aluminium exhibited by this material, allowing it to adopt higher coordination environments.

Item Type: Thesis
Creators: Paine, G.S.
Date: 2002
ISBN: 9781369313888
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 18 Sep 2020 07:39
Last Modified: 19 Jul 2023 09:33

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