Fluoride-free synthesis of high-silica RHO zeolite for the highly selective synthesis of methylamine

Abstract

Low-silica RHO zeolites have been recognized as efficient catalysts for synthesizing industrial intermediates monomethylamine (MMA) and dimethylamine (DMA) through the reaction of methanol (MeOH) with ammonia. However, they typically suffer from rapid deactivation. Herein, we report the synthesis of high-silica RHO zeolite with small crystal sizes (1–2 μm) via interzeolite conversion of SSZ-13, utilizing a reduced amount of organic structure-directing agent (OSDA) and without the use of fluoride. It was demonstrated that the double eight-membered rings of RHO zeolite can be formed using the single eight-membered ring of SSZ-13, resulting in a reduction of OSDA dosage by approximately 30% and Cs⁺ by about 38%. The resulting high-silica RHO zeolites exhibit an excellent methanol conversion of ∼93%, a high MMA and DMA yield of ∼90%, and good reaction durability under conditions more severe than those of industrial manufacture conditions, at 350 °C with a liquid hourly space velocity at 11.7 h⁻¹. Compared to directly synthesized high-silica RHO zeolites with larger crystal sizes (3–8 μm), interzeolite-converted high-silica RHO zeolites with smaller crystal sizes show a 10–16% higher yield of MMA and DMA. These findings highlight the significant potential of high-silica RHO zeolites for the industrial manufacturing of MMA and DMA.