Sodium salts such as Na2CO3 and Na molybdate are active and selective MeOH dehydrogenation catalysts for the prodn. of water-free HCHO. Profound chem. and structural changes are experienced by the molybdate when employed under normal reaction conditions (500-700 Deg). The material reacts with the fluid phase and undergoes sepn. into Na2CO3 and Mo2C. The transition occurs relatively slowly and correlates well with start-up behavior of a catalytic fixed-bed reactor showing that an induction period is required before max. HCHO selectivity is achieved. This induction period reflects the transformation to Na2CO3, which is more selective for HCHO than Na molybdate under the conditions employed. TEM shows that the phase sepn. results in the formation of Na2CO3 filaments extending from Mo-rich centers. Hydrogen reduces the molybdate and reaction with C-contg. gases (including CO and CO2) completes the transformation. In addn. to TEM, electron microanal., X-ray powder diffraction, and diffuse reflectance Fourier-transform IR spectroscopy are used for post-reactor catalyst analyses. In-situ transmission IR spectroscopy conforms that the transformation to carbonate takes place during reaction with MeOH >=500 Deg. [on SciFinder (R)]