Abstract
The methylation of amines for the synthesis of methylamines and dimethylamines as platform chemicals has been attempted mostly by homogeneous catalysts with acid additives. However, there are scarcely any reports on heterogeneous catalytic methylation reactions except for a routine approach under high temperature and high pressure of CO2 and H2 gases for extended reaction times. Here we report a heterogeneously catalyzed selective methylation of aromatic amines using reactive and nontoxic formic acid as the only source for the construction of methyl groups, under ambient pressure in an additive-free one-pot reaction condition. Equal proportions of Pd and Ag in the PdAg/Fe3O4/N-rGO catalyst deliver highly selective amine methylation without aromatic ring hydrogenation, as the strained Pd in the alloy is combined with the graphene-derived support, preventing nanoparticle agglomeration and the action of magnetite as a promoter. Both N-methylation and N,N-dimethylation of various substituted aromatic amines were performed with complete conversion and excellent 90–97% selectivity by controlling the reaction times in the range of 10–24 h at 140 °C without unwanted aromatic ring hydrogenation. Furthermore, the developed bimetallic catalyst provided high yields (88–91%) of methylation with CO2+H2 gas under high pressure, which are as good as the results of homogenous catalysts with an acid additive. To the best of our knowledge, our use of this environmentally friendly methodology is the first time that this durable heterogeneous catalyst has readily performed highly selective methylation at ambient pressure, which is attractive for industrial applications. A heterogeneous palladium—silver alloy can catalyse selective methylation of aromatic amines under ambient pressure and without additives. Methylation of amines is used to manufacture chemicals used in fertilizers, fungicides, synthetic leathers and polymers. Formic acid, which is produced from biomass, is an environmentally friendly reagent for producing amines. Ajay Singh, Yoon-Ho Hwang and Dong-Pyo Kim at Pohang University of Science and Technology in Korea used a heterogeneous palladium—silver catalyst to selectively methylate aromatic amines at ambient pressure in a one-pot reaction that employs only formic acid as the source. They achieved complete conversion and high selectivities in the range 90–97% for methylation and dimethylation of various substituted aromatic amines. The catalyst realized yields comparable to those of homogenous catalysts with acid additives for methylation with carbon dioxide and hydrogen under high pressure. A heterogeneous palladium–silver alloy can catalyze selective methylation of aromatic amines under ambient pressure and without additives. Methylation of amines is used to manufacture chemicals used in fertilizers, fungicides, synthetic leathers and polymers. Formic acid, which is produced from biomass, is an environmentally friendly reagent for producing amines. They achieved complete conversion and high selectivities in the range 90–97% for methylation and dimethylation of various substituted aromatic amines only by formic acid as the source. The catalyst realized yields comparable to those of homogenous catalysts with acid additives for methylation with carbon dioxide and hydrogen under high pressure.
Highlights
Amine methylation leads to the formation of methylamine,1 dimethylamine2 and formamide,3 which are used as platform chemicals for the synthesis of fertilizers, fungicides, synthetic leathers, and polymers and are directly used as solvents and formulation agents.3 The most common syntheses for the methylation of amines involve the use of methyl iodide,4,5 dimethyl sulfate,6 formaldehyde7 or diazomethane8 as methylating reagents with excellent reactivity.their toxicity and the stoichiometric generation of inorganic salt wastes raise environmental concerns
Equal proportions of Pd and Ag in the PdAg/Fe3O4/N-rGO catalyst deliver highly selective amine methylation without aromatic ring hydrogenation, as the strained Pd in the alloy is combined with the graphene-derived support, preventing nanoparticle agglomeration and the action of magnetite as a promoter. Both N-methylation and N,N-dimethylation of various substituted aromatic amines were performed with complete conversion and excellent 90–97% selectivity by controlling the reaction times in the range of 10–24 h at 140 °C without unwanted aromatic ring hydrogenation
The catalytic performance was evaluated using the model reaction between aniline and formic acid under an additive-free and solvothermal one-pot condition (140 °C, 10 h), in comparison to mono- and tri-metallic catalysts (Table 1 and see ESI† Supplementary Table S4). Both monometallic Pd/N-rGO and commercial Pd/C catalysts produced a mixture of cyclohexanone and dicyclohexylamine via active hydrogenation, as expected, which was consistent with the reported results,21–25 while a metal-free carbocatalyst (GO and N-rGO)28,29 and mono-metals (Ag, Co, Au, Pt, Ni and Sn) loaded on N-rGO showed almost no catalytic effect
Summary
The most common syntheses for the methylation of amines involve the use of methyl iodide, dimethyl sulfate, formaldehyde or diazomethane as methylating reagents with excellent reactivity Their toxicity and the stoichiometric generation of inorganic salt wastes raise environmental concerns. The methylation of amines via environmentally acceptable reagents or routes is highly desirable From this perspective, there have been various attempts at N-methylation by using a C1-building block source (CO2, formic acid) and/or a reductant (H2, silanes and boranes) with organometallic catalysts, including rhodium, ruthenium and zinc complexes.. There have been various attempts at N-methylation by using a C1-building block source (CO2, formic acid) and/or a reductant (H2, silanes and boranes) with organometallic catalysts, including rhodium, ruthenium and zinc complexes.1,3,9–14 These homogenous catalysts require challenging synthesis processes with an elaborately designed ligand complex, which is difficult to separate and recycle from the products.. These homogenous catalysts require challenging synthesis processes with an elaborately designed ligand complex, which is difficult to separate and recycle from the products. additives such as organic acids are needed for good performance, and high pressure and/or temperature are required
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