Coke formation, location, nature and regeneration on dealuminated HZSM-5 type zeolites

Controlled coking of different catalysts has been carried out during the conversion of acetone/n-butanol mixtures (1:2 (w/w)) to hydrocarbons. A parent HZSM-5 and two additional zeolites obtained after dealumination with steam and steam-HCl combined treatment were used. Infrared spectroscopy, thermogravimetric analysis, specific surface (BET), C/H atomic ratio and adsorption measurement techniques were employed to characterize the different coked samples. Dealuminated samples showed a more aromatic-olefinic type of coke, whereas coke formed in the unmodified HZSM-5 had polyolefinic character. Coke is initially found to be formed in the channel system in parent and steam-HCl treated zeolites. Beyond 5% of coke deposited, it begins to be formed in the outer surface. In the steamed catalyst, the number of external acid sites increases leading to partial blocking of channels even at low coke contents. New active sites formed during dealumination were found to be responsible for the different behavior of steam-treated samples, improving the aromatization stability. Regeneration of coke shows that the rate of oxidation reaction is dependent on the sites of coke formation. Thus, the regeneration of the parent catalyst and that dealuminated by means of steam-HCl combined treatment fitted well to the intracrystalline burning model, whereas steamed catalysts fitted to the crystal surface burning model, both described in literature.