(80e) The Demonstration of Lewis Acid Transforms to Brønsted Acid in a Lewis Acid Modified Solid Acid Catalyst

The demonstration of Lewis acid transforms to Brønsted acid in a Lewis acid modified solid acid catalyst

Naiwang Liu, Xin Pu, Li Shi*

The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China

A Lewis acid modified solid acid catalyst is prepared: AlCl₃-montmorillonite. Pyridine-FTIR Spectra of the sample show Lewis acid of the catalyst is almost 7 times higher after modification, the content of Lewis acid increased from 57.3% to 83.3%. Friedel-Crafts alkylation reaction (paraxylene and 1-octene) was used to investigate the catalytic properties of this catalyst. The conversion of 1-octene in the reaction catalyzed by pure Lewis acid (AlCl₃, AlCl₃-activated carbon) and Brønsted acid (H⁺-montmorillonite) is 1%, 93% separately, which proves that under this reaction condition (1.0 Mpa, 448K)  Brønsted acid sites are the active position, Lewis acid cannot catalyze this reaction. However, the conversion of 1-octene in the reaction catalyzed by AlCl₃-montmorillonite is 91%. This phenomenon reveals Lewis acid modified catalyst can catalyze the reaction while pure Lewis acid cannot. Stemmed from this fact, a hypothesis is made: the process of alkylation reaction in this catalyst is separated into two parts. i) The active position of this catalyst is Brønsted acid sites. ii) With the reaction, Brønsted acid sites are covered by coke. Lewis acid, interacted with the water in montmorillonite interlayer, transforms to Brønsted acid, these Brønsted acid sites act as the catalyze position, rather than Lewis acid sites catalyze the reaction directly.

In order to support this hypothesis, in-situ pyridine-FTIR characterization of the catalyst is conducted. In the early stage of the reaction, the content of Brønsted acid decreased 77.1%, the content of Lewis acid decreased 12.8%; In the middle of the reaction, the content of Brønsted acid decreased 10.6%, the content of Lewis acid decreased 76.3%; In the later reaction, the content of Brønsted acid and Lewis acid decreased 5.2%, 3.3% separately. This result does consist with the hypothesis.

Based on the experimental results, a conclusion could be draw that in the early stage the active position of this catalyst is Brønsted acid sites, after Brønsted acid sites covered by coke, Lewis acid, interacted with the water in montmorillonite interlayer, transforms to Brønsted acid then catalyze the reaction continually.