Catalytic features of cesium hydrogen salts and triammonium salt of heteropoly acid H₃PW₁₂O₄₀ in the alkylation of ammonia with methanol to methylamines

Catalytic features of Cs_xH₃-_xPW₁₂O₄₀ (CS_xPW; x=0-3) and (NH₄)₃PW₁₂O₄₀ (NPW) in the methylamine synthesis by sequential alkylations of NH₃ with CH₃OH were investigated by conducting the reaction (NH₃/CH₃OH/ He = 2/1/7) at 0.1 MPa by flow method. All the H⁺- or NH₄⁺-containing heteropoly compounds were effective for the methylamine synthesis at ca. 573-723 K. In contrast with SiO₂ - Al₂O₃, all the heteropoly catalysts exhibited a long induction period in the initial stage of the reaction, during which significant sorption of CH3OH by the catalysts was observed. For the H⁺-containing heteropoly catalysts, the uptake of NH₃ was also observed in its early stage. The uptakes of CH₃OH by Cs_2.5PW and Cs_2.95PW were in the range of 2-3 moles per mol-polyanion, while those of NH₃ as NH₄⁺, estimated by TPD analysis, were 1.0 per H⁺ in each fresh samples. These results suggest that both CH₃OH and NH₃ can diffuse into their rigid crystal bulk. After such an induction period, the catalysts showed steady activity. The activity of Cs_xPW and the distribution of the amines during the steady state greatly varied with Cs-content x. Among the high surface area salts, Cs2.5PW, Cs2.95PW, NPW, etc. that showed high catalytic activity, Cs_2.95PW gave almost 100% selectivity to the smaller amines, MMA (monomethylamine: 0.4 nm) and DMA (dimethylamine: 0.49 nm), with a negligible yield of TMA (trimethylamine: 0.61 nm) even at high CH₃OH conversions. On the other hand, NPW and Cs_xPW with xca. 2.9 as well as SiO₂·Al₂O₃ behaved as a non-shape selective catalyst yielding TMA with the smaller amines even at low conversions. It was found, however, that Cs_2.95PW has almost the same pore distribution (Type IV) as Cs_2.5PW: both salts have not only small pores (< 1 nm) but also larger pores (> 2 nm) in a significant proportion. This fact implies that the high shape-selective salt, Cs_2.95PW, has the active sites (H⁺) mostly in its small micropores (probably, < 0.6 nm) where no TMA can be formed or diffuse, while non-shape-selective one, Cs_2.5PW, has H⁺ throughout its micro- to macropores. In fact, TPD studies suggested that the both salts easily chemisorb MMA and DMA as well as NH₃ (as ammonium ions) even into their bulk or very small pores, but Cs_2.95PW can hardly chemisorb TMA, while Cs_2.5PW chemisorbs TMA only onto the surface of its relatively large pores in reasonable amounts.