Selective synthesis of dimethylamine over small-pore zeolites: IV. Effects of SiO 2 and Al 2O 3 coatings

Abstract

Al 2O 3 and SiO 2 coatings are effective in increasing dimethylamine (DMA) and decreasing trimethylamine (TMA) selectivities of small-pore zeolites HRHO and HZK-5 used as methylamine catalysts. The HRHO catalysts typically contain chabazite or chabazite and pollucite impurities. SiO 2 is more effective than Al 2O 3 for improving DMA selectivity. SiO 2 coatings from monosilicic acid (MSA) reduce dimethyl ether (DME) yields over shallow-bed nitrogen (SBN)-calcined HRHO, whereas Al 2O 3 coatings and SiO 2 coatings [from tetraethylorthosilicate (TEOS)]do not. Correlations between thickness of SiO 2 (TEOS or MSA) coatings and DMA selectivity as well as a decrease in the n-hexane rate of sorption suggest a physical hindrance to egress of TMA from RHO channels and cages to the product stream. Coating deep-bed calcined H-pollucite with either Al 2O 3 or SiO 2 from TEOS reduces activity and increases DMA selectivity through deactivation of nonselective surface sites. Improvement of DMA selectivity by coating DB-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) deactivation of external acid sites of HRHO and H-pollucite and secondarily from deactivation of H-chabazite. Improvement of DMA selectivity by coating SBN-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) further deactivation of external acid sites of HRHO and amorphous H-chabazite and secondarily from the deactivation of H-pollucite. SiO 2 coatings on HZK-5 increased DMA selectivity and decreased DME yields, but reduced activity.