Selectivity Enhancement in Methylamine Synthesis via Postsynthesis Modification of Brønsted Acidic Mordenite

Abstract

Methylamine synthesis from methanol and ammonia over parent and modified Brønsted acidic mordenites is studied by in situ infrared spectroscopy and kinetic analysis to elucidate the role of elementary steps for activity and selectivity. In situ infrared spectroscopy reveals that all methylammonium ions are formed in the micropores of these catalysts. The formation of the chemisorbed methylamines, however, is not rate determining. Transient response experiments indicate that the desorption of these methylamines aided by adsorbing ammonia and/or the scavenging of methyl groups with ammonia constitutes the rate determining step. For a given catalyst, the selectivity strongly depends on the methanol conversion and the ammonia to methanol ratio of the feed. Upon modification of mordenite with tetraethoxysilane, the selectivity to the lower methylated amines is strongly enhanced. The transport limitations of the bulkier products, formed in high concentration in the pores, are concluded to cause the enhanced selectivity toward mono-and dimethylamine over the modified catalyst. Since a decrease in activity compared to the parent sample was not observed, it seems that the methyl-scavenging mechanism plays an important role over these catalysts.

Highly Selective Conversion of Carbon Dioxide to Aromatics over Tandem Catalysts

“…The peaks of the NH 2 groups centered at 1,550 and 3,381 cm À1 can be obviously observed in IR. 50,51 It is interesting that the intensity of NH 2 species of ZnZrO/NH 2 -SBA catalyst become weak and the new species appear, centered at 1,460 (inset in Figure 4C), 2,852, 2,954, and 2,983 cm À1 , which can be attributed to the formation of N-CH 3 species on NH 2 -SBA through CO 2 hydrogenation under reaction conditions. [50][51][52] When NH 2 -SBA was treated using methanol under reaction conditions, the N-CH 3 species were also identified over NH 2 -SBA.…”

“…50,51 It is interesting that the intensity of NH 2 species of ZnZrO/NH 2 -SBA catalyst become weak and the new species appear, centered at 1,460 (inset in Figure 4C), 2,852, 2,954, and 2,983 cm À1 , which can be attributed to the formation of N-CH 3 species on NH 2 -SBA through CO 2 hydrogenation under reaction conditions. [50][51][52] When NH 2 -SBA was treated using methanol under reaction conditions, the N-CH 3 species were also identified over NH 2 -SBA. These results proposed the transfer of CH x O species (mainly surface CH 3 O*, CHO* species, and gasphase CH 3 OH) from the surface of ZnZrO onto NH 2 -SBA, because N-CH 3 species was produced through the reaction of CH 3 O*, CHO*, and methanol species with the -NH 2 group.…”

A Theoretical Study on the Mechanism of Methylation of N‐methylaniline with CO2 and Silyl Hydrides

“…The reverse equilibria within the reaction will lead to the complex product mixtures. For example, the selectivity for the three methylamines (MMA, DMA, and TMA) depends on the feed ratio of NH 3 /CH 3 OH . Alternatively, because methylamines feature a C1 group, their synthesis by using CO 2 as a synthon would be an attractive strategy (Scheme , reaction 2).…”

Methylamines

“…For example, use of an alkali metal-modified H þ -mordenite catalyst can reduce TMA selectivity from >50% by weight over an equilibrium catalyst at 2:1 ammonia:methanol (molar) and 320 C to >20% by weight (40). Mechanistic studies of mordenite and chabazite have shown that low TMA selectivity results from diffusion limitations of TMA to the bulk and from suppression of TMA formation within the zeolite pores (41)(42)(43). Silylation of the external surface of the mordenite with SiCl 4 reduces TMA selectivity to <5 wt% by further narrowing the pore size (44).…”