Catalytic features of CsxH3-xPW12O40 (CSxPW; 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 Cs2.5PW and Cs2.95PW 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 CsxPW 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, Cs2.95PW 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 CsxPW 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 Cs2.95PW has almost the same pore distribution (Type IV) as Cs2.5PW: 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, Cs2.95PW, 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, Cs2.5PW, 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 Cs2.95PW can hardly chemisorb TMA, while Cs2.5PW chemisorbs TMA only onto the surface of its relatively large pores in reasonable amounts.
|Number of pages||13|
|Journal||Sekiyu Gakkaishi (Journal of the Japan Petroleum Institute)|
|Publication status||Published - 1998|