(93d) The Effects of Supercritical Carbon Dioxide on the Zeolite-Based Heterogeneous Friedel-Crafts Acylation of Anisole

Acylation of aromatics is of considerable industrial importance for making intermediates, which are used in the production of pharmaceuticals, insecticides, perfumes, agrochemicals and other fine chemicals. Pharmaceutical drug sales alone were an estimated 124.5 billion dollars in 1998. For example, the acylation of isobutylbenzene with hydrogen fluoride is currently used to manufacture 4-isobutylacetophenone, a key intermediate for the bulk active pharmaceutical ibuprofen. However, the use of large amount of hazardous catalysts (e.g., Lewis acid catalyst such as AlCl3) and solvents (e.g., nitrobenzene) in order to achieve high selectivity make most traditional Friedel-Crafts acylation processes inherently dirty and highly polluting. The development of an efficient and environmentally friendly catalytic acylation of aromatics via the Friedel-Crafts reaction remains a major challenge and a high priority for clean technology. As alternatives to the Lewis acid catalysts, zeolites have been used as solid acid catalysts in heterogeneous reactions for synthesis of organic compounds for over two decades, due to their high activity, stability and selectivity. However, there is a problem with maintaining the activity of the zeolite catalysts for long periods of time due to catalyst deactivation by deposition of heavy reaction products within the pores or the outer surface of the crystallites.The acylation of anisole with acetic anhydride using beta zeolite catalysts with high framework SiO2/Al2O3 ratio in the absence and presence of supercritical CO2 as a solvent has been investigated in a high-pressure 25 ml batch stainless steel (SS 316) reactor. The effects of reaction conditions (e.g., temperature, catalyst concentration, reaction time and pressure, etc.) on anisole conversion and product yield are also evaluated using 0.2-0.8g of catalyst at 60-150oC and pressures of about 1200 - 3000 psi. In the absence of supercritical CO2 at catalyst loading of 0.4g conversions of anisole as high as 80% at 90oC was attainable but yields of 4-methyl acetophenone (4-MAP), the desired product were low (4-5%), and the catalyst deactivated rapidly at temperatures above 90oC after a reaction time of just 4h. However, in the presence of scCO2 at 90oC conversion values over 99% were observed especially at higher pressures (= 2000 psi or higher) and yields increased ten-fold (up to 47%). Also, the activity of the catalyst was maintained for longer reaction time periods (> 20h) in the presence of scCO2. The presence of scCO2 significantly enhanced both the conversion of anisole and yields of 4-MAP, and the enhancements were more pronounced at higher pressures. This suggests that supercritical CO2 strips out possible coke and acylation products or facilitates faster desorption of the products, prolonging and improving the activity of the catalyst. The results demonstrate that the acylation of anisole with acetic anhydride to mainly 4-methyl acetophenone (the desirable intermediate ketone) as products could be accomplished very effectively in an environmentally friendly manner using a beta type zeolite catalyst in supercritical CO2.The zeolite and carbon dioxide based approach shows promise as a greener alternative to the inherently dirty and highly polluting traditional Friedel-Crafts acylation processes, which uses typically nonregenerable Lewis acid like aluminum chloride (AlCl3) as catalyst and nitrobenzene as solvent.

References:

Y.G. Adewuyi, Zeolite-Based Catalysis in Supercritical CO2 for Green Chemical Processing. In: Environmental Catalysis; V. Grassian, editor; CRC Press; A Division of Taylor and Francis, Inc.2005, 609-626.