Methanol formation in the pyrolysis of formaldehyde

Huang, Ted Tong-Shan (1968) Methanol formation in the pyrolysis of formaldehyde. Masters thesis, Memorial University of Newfoundland.

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Abstract

The pyrolysis of formaldehyde has been studied at 420°C and 452°C in the pressure range 20-130 mm. The products are mostly carbon monoxide, methanol and hydrogen as previously reported. The ratio (CH₃OH)/(H₂) is approximately four at 420°C and one at 452°C. When formaldehyde was completely pyrolyzed a good carbon balance was obtained, indicating that no condensation reaction occurred, at least at the end of the reaction. Reports of a condensation reaction have appeared in the literature. -- Conditioning of the reactor by pumping over night with the oil diffusion pump had no effect upon the rate of methanol formation. The rate of hydrogen formation was sometimes erratic. As the reactor aged the rate of hydrogen formation decreased slightly but no effect on methanol formation could be detected. -- The addition of foreign gas (CO₂) increased the rate of hydrogen formation, but had no effect upon the rate of methanol formation. -- The reaction is approximately second order for methanol with a suggestion of temperature dependence of order. Hydrogen formation is also approximately second order but the results are erratic. For H₂ the observations are consistent with the results of previous work at ≈500°C and at ≈1500°C and for CH₃OH they are consistent with some preliminary results which have been reported. Activation energies estimated for methanol formation and hydrogen formation were 31 and 39 kcal mol⁻¹ respectively. -- A radical chain mechanism for methanol formation is proposed which, when combined with a mechanism for hydrogen formation, can account for all of the observations in the present work: the reaction orders, the dependence of the rate of formation of H₂ on foreign gas pressure and the independence of the rate of formation of CH₃OH on foreign gas pressure, the sometimes erratic rate of H₂ formation and the reproducible rate of CH₃OH formation. The activation energy estimated from this mechanism is close to the experimental value for CH₃OH. The mechanism, however, cannot explain the large increase in the rate of hydrogen formation found by Klein in the unbaked vessel and by Ke in the unconditioned vessel. Heterogeneous reactions occurring in the unbaked or unconditioned vessels are therefore proposed.

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