Binary caesium–lanthanum oxide supported on MCM-41: A new stable heterogeneous basic catalyst

Abstract

Heterogeneous mesoporous stable basic catalysts have been prepared by wet or solid-state impregnation of MCM-41 with caesium acetate and lanthanum nitrate followed by thermal decomposition. ¹³³Cs MAS NMR data of CsLa/MCM-41 show an increase in the Cs–O bond length in the CsLa mixed oxides compared with that in Cs oxides supported on MCM-41. A small difference in chemical shift between the hydrated and dehydrated materials is observed, indicating a weak interaction of water with Cs⁺ cations. CO₂ temperature-programmed desorption (TPD) suggests that these materials possess small oxide clusters of mild basicity. The thermal stability of the materials, which is independent of the framework aluminium content, is reflected by an unaffected CO₂ desorption and BET surface area on successive thermal cycles. Alkali metal–lanthanum binary oxides of the Na, K and Rb-type appear to be thermally less stable, in the sense of a significantly smaller CO₂ desorption indicative of particle clustering. ²³Na MAS NMR shows several resonances for the NaLaO_x materials. The presence of the single Rb-oxide component in the RbLaO_x materials is confirmed by ⁸⁷Rb MAS NMR. The mild base strength of the CsLaO_x/MCM-41 is demonstrated by its ability to remove a proton from enolates having a pK_a⩽10.2. The presence of lanthanum and the mesoporous framework govern the activity and product selectivity in the liquid-phase Michael addition of ethyl cyanoacetate to ethyl acrylate. The Knoevenagel addition of enolates to benzaldehyde in aqueous media is also catalysed by the CsLa-oxide/MCM-41 system. The catalyst is re-usable after regeneration at high temperature without loss of activity.

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