白金担持Y型ゼオライト触媒によるn-ヘキサンの異性化反応の選択性に対する固体酸性の影響

Acidity

The motor gasoline is commercially produced from n-paraffin and naphtene by catalytic reforming reaction, and is composed mainly of hydrocarbon molecules of high octane rating such as aromatics and side-chained hydrocarbons. A part of the gasoline is incompletely burned and is exhausted, resulting in air pollution. Hence, in order to completely burn the gasoline, it is desired to increase the fraction of side-chained hydrocarbons, which is completely burned, without reducing its octane rating. These molecules are produced from n-paraffin by isomerization reaction over metal catalysts supported on solid acid catalysts. The main objective of this paper is to investigate the relationship between selectivity of the side-chained hydrocarbon molecules and acidic properties of catalysts by performing isomerization of n-hexane over platinum catalysts supported on Y type zeolites at 603 K. On the basis of results obtained, catalyst of high selectivity of isomerization was prepared. Furthermore, the changes in its catalytic activity caused by thiophene poisoning the continual cycle of reaction and regeneration were examined. Firstly, a series of Y type zeolites with different acidic properties were prepared, by varying sodium content in Y type zeolite by the technique of ion exchange applied to sodium ion. Secondly, platinum was supported, about 0.5-0.6 wt%, on the prepared Y type zeolites, yielding platinum catalysts supported on Y type zeolites. By measuring amounts of chemisorbed hydrogen on the catalysts and contents of platinum in these catalysts, it was confirmed that platinum was supported on Y type zeolites in the same manner (Fig. 1). The acid strength distributions of the catalysts were measured by the amine titration method. Isomerization of n-hexane was performed over the catalysts at 603 K. From the results of reaction and measurement of acidic properties, the selectivity of isomerization was found to be closely related to the acid strength of acid sites distributed on the catalyst, as follows: 1) acid sites having Hammett acidity function (H0) smaller than -8.2 (stronger acid sites) accelerate the cracking reaction of n-hexane as well as the isomerization. 2) acid sites having acid strength H0 in the range from -2 to -6 enhance only isomerization. 3) acid sites with weak acid strength (H0>2) on the catalyst are inactive in isomerization and cracking reactions. The platinum catalyst supported on Y type zeolite, which was prepared by exchanging 45% of proton with sodium ion, (Na(45)Y), had a number of acid sites of acid strengths H0 in the range from -2 to -6 (Fig. 3). Hence, this catalyst accelerated only the isomerization of n-hexane, and its selectivity was 90% (Fig. 2). The deactivated Na(45)Y catalyst was found to be completely regenerated by burning reaction. Furthermore, the reaction rate of the Na(45)Y catalyst was reduced only 16%, when the catalyst was used for 2 h with thiophene of 60 ppm in the reactant gas, while the platinum catalyst supported on activated alumina was completely deactivated under the same conditions.