Oxidation of 4-methoxytoluene over copper-containing BEA-type zeolite catalyst

Title : Oxidation of 4-methoxytoluene over copper-containing BEA-type zeolite catalyst

Abstract:

The solvent-free oxidation of 4-methoxytoluene (4-MT or 1-methoxy-4-methylbenzene) in the liquid phase was studied using air as an oxygen source and a partially dealuminated Cu-HBEA catalyst and under the influence of ultrasound (US, 300 W, 25 kHz, 1.5 hours). The HBEA zeolite used to prepare the catalyst was obtained from commercial NH?BEA with a Si/Al₂ molar ratio of 38 (Zeolyst International, USA); the copper form was obtained by ion exchange between copper acetate solution and HBEA zeolite with a Si/Al molar ratio of 27.6; the Cu-HBEA zeolite contained 0.96% copper. Ultrasound-assisted reactions were carried out in a hybrid reactor (UMR-300B, Shinka, Japan), providing ultrasonic irradiation of the reaction mixture directly through the sonotrode.
According to the results of the reaction product analysis carried out using a gas chromatograph-mass spectrometer (GC/MS, Agilent Technologies, 5890B/5977A, USA), several reactions occur during the thermal catalytic oxidation of 4-MT using air oxygen on a Cu-HBEA catalyst at relatively low temperatures (323–362 K), including alkylation, oxidation, isomerization, and cyclization. The main product of these reactions is trans-anethole (1-methoxy-4-[(1E)-prop-1-en-1-yl]benzene, C??H??O), β-linalool ((3R)-3,7-dimethylocta-1,6-dien-3-ol, C??H??O), estragole (4-allylanisole, C??H??O), para-anisaldehyde (4-methoxybenzaldehyde, C?H?O?), formic acid 2-isopropyl ester (C??H??O?), 1-(4-methoxyphenyl)-2-propanone (C??H??O?)and 1-(3-methyl-2- butenoxy-4-(1-propenyl)benzene (C₁₄H₁₈O) are also formed, and in small quantities d-Limonene ((4R)-1-methyl-4-prop-1-en-2-ylcyclohexene, C₁₀H₁₆), 2,4-dimethylanisole (1-methoxy-2,4-dimethylbenzene, C₉H₁₂O), trans-α-bergamotene ((1S,5S,6R)-2,6-dimethyl-6-(4-methylpent-3-enyl)bicyclo[3.1.1]hept-2-ene,C₁₅H₂₄) and 1-(4-methoxyphenyl)propane-1,2-diol (C₁₀H₁₄O₃). The formation of precipitation products on the catalyst – its blackening and coating with resinous products – was observed.
The formation of trans-anethole is unusual both during thermal catalytic oxidation and during the action of ultrasound. The yield of trans-anethole in thermal catalytic oxidation is high, 72%, at 362K. In the oxidation of 4-methoxytoluene  (C₈H??O) in the presence of a Cu-HBEA catalyst, the formation of trans-anethole (C??H??O) and other (C₉-C₁₂) molecules with an elongated side carbon chain in comparison with the methyl group of 4-MT does not agree with the classical ideas of anethole formation from 4-MT. For this, radical halogenation and condensation are used.
In the ultrasound-assisted catalytic oxidation of 4-MT, the selectivity for the formation of the target product, 4-MBA, is approximately six times higher than in thermal catalytic oxidation, and amounts to 41.7% upon complete conversion of 4-MT.

Biography:

Vladimer Tsitsishvili (1947), graduated from the Physics Department of Tbilisi State University (experimental physics, 1970), Candidate of Physical and Mathematical Sciences (Physics of Magnetic Phenomena, 1975), Doctor of Chemical Sciences (Physical Chemistry, 1988), Corresponding Member (2001) and Full Member (2019) of the National Academy of Sciences of Georgia, worked at the Institutes of Cybernetics (1970-78), Pharmacochemistry (1978-90) and Physical and Organic Chemistry (1990-2006) of the Georgian Academy of Sciences, Director (2006-18) and Chairman of the Scientific Council (since 2018) of the Petre Melikishvili Institute of Physical and Organic Chemistry at the Tbilisi State University.