George C. Sonnichsen

Methylamines synthesis: A review☆

Monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA) are major industrial chemical intermediates. Generally, these compounds are prepared by reaction of methanol and ammonia over a dehydration catalyst such as silica alumina. While thermodynamics favors TMA formation, market demand is for DMA. This has led to the development of highly DMA selective zeolite-based catalysts. In this article, the history of, the reported routes to, the design of new catalysts for, and the mechanism of the synthesis of methylamines are discussed.

Selective synthesis of dimethylamine over small pore zeolites

Abstract Zeolite H-RHO is highly selective for the synthesis of dimethylamine (DMA) from methanol and ammonia. Shallow-bed dry calcination of NH4-RHO at temperatures from 400 to 700 °C results in progressive deammoniation, dehydroxylation, and dealumination and in dimethylamine (DMA) and trimethylamine (TMA) selectivities that progressively change from 50 to 67% and 25 to 5%, respectively. Concurrent changes in dealumination and internal and external acid sites suggest a process whereby DMA selectivity arises from destruction of nonselective sites on impurity phases such as pollucite and/or from hindered TMA diffusivity by either nonframework Al species (NFA) or NFA/methylamine adsorption complexes. Shallow-bed steam calcination at temperatures from 400 to 700 °C results in higher degrees of dehydroxylation and dealumination than shallow-bed dry calcination and in DMA and TMA selectivities that change from 50 to 71% and 20 to 2%, respectively. The higher DMA selectivities result from the higher degree of dealumination and/or the more effective deactivation of impurity phases. Increased activity of shallow-bed steamed H-RHO is correlated with destruction of most of the highly acidic 3610 cm−1 Bronsted sites and to their replacement with weakly acidic 3640 cm−1 OH groups. High dimethyl ether yields observed in samples calcined above 650 °C are associated with an OH band at 3680 cm−1 and attributed to the condensation of nonframework Al species.

Selective synthesis of dimethylamine over small-pore zeolites. IV: Effects of SiO2 and Al2O3 coatings

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.

Selective synthesis of dimethylamine over small-pore zeolites: III. HZK-5

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 Csuc 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 Csuc 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.

Selective synthesis of dimethylamine over small-pore zeolites: catalytic selectivity and sorption behavior

Abstract 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.

Selective synthesis of dimethylamine over small-pore zeolites: II. Effects of impurities on catalytic properties of H-RHO

Abstract 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.