ABSTRACT
Citraconic anhydride (CA, methylmaleic anhydride) is an important intermediate for the preparation of synthetic resins, and is also employed for the synthesis of pharmaceuticals. Due to different functional groups which are present in this relatively small molecule, several synthetic routes can be envisaged, starting from different raw materials (1-4). CA can be synthesized either i) starting from condensation of acetone to yield mesityl oxide, which is then oxidized, or ii) from maleic anhydride, through methylation with formaldehyde or methanol, iii) via pyrolysis of citric acid, iv) from succinic anhydride, via condensation with formaldehyde, or v) by selective oxidation of iso-Cs hydrocarbons, such as isoamylene or isoprene. The direct oxidation of iso-Cs hydrocarbons is particularly interesting, since the Cs fraction is one of the main component of pyrolytic gasoline from naphtha steam cracking. Isoprene in this fraction can be either separated or transformed to ter-amylmethylether; the residual Cs raffmate mainly contains n-and isopentane, pentenes and cyclopentane. It is of industrial interest to find chemical uses for these hydrocarbons, as feedstocks for the synthesis of valuable intermediates for the petrochemical and polymer industry. In a previous paper, the reactivity of isopentane over various vanadiwn oxidebased systems was examined (5). Low selectivities to CA were in generally
found, due to the preferential formation of maleic anhydride and of carbon oxides. More promising results were obtained starting from isoprene. The present study involved a thorough investigation of the main parameters which affect isoprene conversion and selectivity to CA over titania-supported vanadium oxide as the heterogeneous catalyst, in order to fmd conditions which maximize the yield to the desired product. In addition, this reaction represents an example of how the catalytic performance in the selective oxidation of hydrocarbons with vanadium oxide-based catalysts can be affected through control of the reaction parameters.
