Abstract: Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the Ce^IV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the Ce^IV content. This i s believed to be due to the water formed during the oxidation of Ce^III, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce³⁺ was found more benefit for the hydride transfer compared with La³⁺, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

Key words: CeX zeolite, Calcination atmosphere, Isobutane alkylation, Brønsted acid, Hydride transfer

摘要： Lanthanum-containing (LaX) and cerium-containing X zeolites (CeX) were prepared by a double-exchange, double-calcination method. By changing the calcination atmospheres between nitrogen and air, the Ce^IV contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established. The crystallinity of CeX zeolite was found to be negatively correlated with the Ce^IV content. This i s believed to be due to the water formed during the oxidation of Ce^III, which facilitates the framework dealumination. As a consequence, calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection, this phenomenon was mostly hindered and the sample's acidity was preserved. When tested in a continuously flowed slurry reactor, the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration. In addition, Ce³⁺ was found more benefit for the hydride transfer compared with La³⁺, which is ascribed to the stronger polarization effect on the CH bond of isobutane. Moreover, the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules. Based on the product distribution, a new catalytic cycle of dimethylhexanes (DMHs) involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

关键词: CeX zeolite, Calcination atmosphere, Isobutane alkylation, Brønsted acid, Hydride transfer