Differences in the reactivity of ammonium salts with methylamine
The heterogeneous uptake of methylamine (MA) onto (NH 4) 2SO 4, NH 4HSO 4, NH 4NO 3 and NH 4Cl was investigated using a Knudsen cell reactor coupled to a quadrupole mass spectrometer, in situ Raman spectrometer and theoretical calculations. Exchange reactions were observed between MA and NH 4NO 3, (NH 4) 2SO 4, and NH 4Cl were observed at 298 K. Simple acid-base reaction for MA was found taking place on NH 4HSO 4. CH 3NH 3NO 3 and CH 3NH 3Cl are not stable at low pressure and have higher dissociation vapor pressure than methylammonium sulfate. The observed uptake coefficients of MA on (NH 4) 2SO 4, NH 4HSO 4, NH 4NO 3 and NH 4Cl at 298 K were measured to be 6.30±1.03×10 −3, 1.78±0.36×10 −2, 8.79±1.99×10 −3 and 2.29±0.28×10 −3, respectively. A linear free energy relationship was found for the heterogeneous reactions between MA and NH 4Cl, (NH 4) 2SO 4 and NH 4NO 3. Namely, the natural logarithm of uptake coefficients of MA on these ammonium salts is linearly related to the electrostatic potential of the H atom in the NH 4+ group.