Nature of Catalytically Active Sites over Solid Acids. I. Selective Poisoning of Lewis Acid Sites on Silica–Alumina with Pyridine and Its Application to Olefin Polymerization

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Abstract
When pyridine chemisorbed on silica–alumina was evacuated at 350°C, the 1460 cm−1 band due to Lewis-bound pyridine (LPY) remained while the 1540 cm−1 band due to Brønsted-bound pyridine (BPY) completely disappeared. This disappearance of the BPY band was revealed to accompany a loss of the Brønsted acid sites themselves besides their liberation. However, the lost Brønsted acid sites were regenerated by allowing the sample to stand overnight at 110°C. Thus, strong Lewis acid sites of silica–alumina were able to be selectively poisoned by pyridine without any damage to the Brønsted acid sites. By poisoning catalysts in situ by controlled amounts in this way, the nature of the active site was investigated for polymerization of propylene or cis-2-butene at a low temperature of 30 °C. The initial activity declined sharply as the extent of pyridine poisoning of the Lewis acid sites increased. Therefore, it was concluded that strong Lewis acid sites (25–30% of the total Lewis acid content) were active in polymerization over silica–alumina.