Catalysis and petrochemistry

Vapor-phase oxidation of propylene glycol-methanol mixture to methyl lactate on CeO₂/Al₂O₃ catalyst

M.E. Sharanda¹, A.M. Mylin¹, O.Yu. Zinchenko², V.V. Brei¹

¹Institute for Sorption and Problems of Endoecology of NAS of Ukraine, 13 General Naumov Str., Kyiv, 03164, Ukraine
²Manufacturing Group Techinservice Limited, Makiivskyy provulok, 1, Kyiv, 04114, Ukraine
Е-mail: brei@ukr.net, net@techinservice.com.ua

ABSTRACT

The vapor-phase oxidation of mixtures of propylene glycol with methanol and ethanol to methyl and ethyl lactate, respectively, on supported CeO₂/Al₂O₃ catalyst with 10 wt.% CeO₂ content was studied. The steel flow reactor with a fixed catalyst bed (4 cm3) was used. 20 wt.% solution of propylene glycol in alcohol was fed to the reactor inlet by Waters 950 pump at LHSV= 0.5-0.8 h-1. Reaction temperature and pressure were varied in the interval of 190-250 0C and 1.3-1.8 bars respectively. Compressed air was given to the reactor inlet at the molar ratio of propylene glycol/O2 = 1. The reaction products were analyzed using gas chromatography (Agilent 7820A) and 3C NMR (Bruker Avance 400) methods.
Studied oxidation of propylene glycol in the presence of methanol describes by total reaction
CH₃CHOHCH₂OH +O₂ + СН₃OH = CH₃CHOHCOOСН₃ +2H₂O
At first, hydroxyacetone is formed that is further oxidized to pyruvic aldehyde, which attaches alcohol to form hemiacetal. Then, hemiacetal of methyl glyoxal rearranges into methyl lactate by Cannizzaro. At 220 ⁰C and load on a catalyst of < 2 mmol PG/gcat/h, the selectivity towards methyl lactate reaches 70 wt.% at 100 % propylene glycol conversion. The main by-products are formed as the result of acetaldehyde transformation. Acetaldehyde could be formed at hydroxyacetone aldol decondensation.
In the presence of ethanol, the formation of a significant amount of acetaldehyde and its aldol condensation products as well as the formation of diethoxyethane are observed. Therefore, ethyl lactate selectivity at 100 % propylene glycol conversion does not exceed 45 %.
Supported CeO₂/SiO₂ contact was tested in this oxidation reaction also. However, CeO₂/SiO₂ provides the low, up to 25%, selectivity towards methyl lactate at full propylene glycol conversion.
It was shown that at the same conditions methyl lactate is formed with higher selectivity then ethyl lactate. The high methyl lactate yield up to 70 wt.% could be obtained via vapor-phase oxidation of 20% mixture of propylene glycol with methanol by air oxygen on supported CeO₂/Al₂O₃ catalyst at 210 - 220°С and at time contact of 3-4 seconds.

KEYWORDS

methyl lactate, lactide, propylene glycol, cerium dioxide, supported catalyst.

REFERENCES