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The sorption of trimethylamine by zeolite rho was studied to provide a rational basis of catalyst selectivity for the methylamines synthesis reaction from ammonia and methanol. Sorption kinetics, obtained for the range 150–250°C, yield an activation energy of 9 kcal/mole. Mass spec data indicate that trimethylamine is both physisorbed and chemisorbed with relative amounts of 10 and 90%, respectively. The physisorbed entity desorbs intactly while, above 300°C, the chemisorbed trimethylamine pyrolyzes forming mono- and dimethylamines plus some residue. For a given zeolite rho sample, the yield of trimethylamine in the methylamines synthesis reaction was found to correlate with the quantity of physisorbed trimethylamine. For zeolite rho catalysts, an activation energy of 42 kcal/mole was determined for the appearance of trimethylamine in the product stream.

The relative contributions of external and intracrystalline acidic sites of small pore H-RHO zeolite for the selective synthesis of methylamines from methanol and ammonia have been studied. Nonselective surface reactions which produce predominantly trimethylamine can be eliminated by “capping” the external acidic sites with trimethylphosphite (TMP) and other reagents, thus improving the selectivity toward the formation of dimethylamine. For small pore zeolites, neither the zeolite pore size nor the internal acidic sites is significantly affected by this treatment. In situ infrared and MAS-NMR studies show that TMP reacts irreversibly with the zeolite acidic sites via a modified Arbusov rearrangement to form surface-bound dimethylmethylphosphonate.

HZK-5 is a highly selective catalyst for dimethylamine (DMA) synthesis in the reaction of methanol and ammonia. Deep-bed (DB) calcination of NH4ZK-5 containing 4 Cs uc results in DMA selectivities of 60–75% but with 15–30% dimethyl ether (DME) yields. Deep-bed or shallow-bed steam calcination of NH4ZK-5 with only 1 Cs uc gives similar high DMA selectivities but lower DME yields of 5–10%. Shallow-bed calcination of NH4ZK-5 results in DMA selectivities of ~45%. The activity of HZK-5 in the methylamine synthesis reaction is lower than that of zeolite HRHO.

Typical RHO preparations can contain impurities such as unreacted gel, chabazite, P c , and pollucite. These impurities were synthesized in pure form to investigate their effects on the behavior of RHO as a dimethylamine (DMA)-selective catalyst in the methanol-ammonia reaction. Gel and p c , after exchange and calcination, were amorphous and inactive at typical reaction temperatures of 300–325 °C. Chabazite, always present in RHO preparations at 5–10% levels, is active and DMA selective after deep-bed calcination. After shallow-bed calcination, it loses much of its crystallinity and becomes inactive. Pollucite, appearing in the later stages of RHO crystallization, loses activity at 325 °C after shallow-bed calcination. However, after deep-bed calcination, it is an active and nonselective methylamine catalyst. Thus, the presence of pollucite can be a primary cause of lower DMA selectivity of deep-bed calcined RHO catalysts. Steaming, one of the most effective methods of reducing the activities of impurities, destroys pollucite crystallinity and/or nonselective Surface sites.

Al2O3 and SiO2 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. SiO2 is more effective than Al2O3 for improving DMA selectivity. SiO2 coatings from monosilicic acid (MSA) reduce dimethyl ether (DME) yields over shallow-bed nitrogen (SBN)-calcined HRHO, whereas Al2O3 coatings and SiO2 coatings [from tetraethylorthosilicate (TEOS)]do not. Correlations between thickness of SiO2 (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 Al2O3 or SiO2 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. SiO2 coatings on HZK-5 increased DMA selectivity and decreased DME yields, but reduced activity.

Sorption measurements of alcohols were used to rationalize the performance of zeolites used as catalysts for the synthesis of methylamines via the sequential reaction of methanol and ammonia. Low methanol absorption corresponds to low catalytic activity while high isopropanol absorption corresponds to zeolites producing an equilibrium distribution of methylamines. Generally, zeolites, with sorption values for methanol (or ethanol) of 10–30 w o and little or no isopropanol sorption, selectively produce mono- and dimethylamines versus trimethylamine. Mineral chabazites, while having similar activities, surprisingly provide a wide range of product selectivities. The Geometric Selectivity Index, GSI, denned as the ratio of methanol sorption to the sorption of n -propanol, was found to correspond to the observed catalytic selectivity of the mineral chabazites.