Side-chain alkylation of 4-picoline with formaldehyde over alkali-modified zeolites Original Research Article

Catalytic side-chain alkylation of 4-picoline (4P) with formaldehyde (F) (37/1 (w/v) aqueous solution) was studied over alkali and alkaline earth cation-modified zeolites in vapor phase conditions at atmospheric pressure, and at a reaction temperature of 300 °C. Mixtures of vinylpyridine and ethylpyridine were formed in the alkylation of the corresponding picoline over Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba cation-modified zeolites. Catalytic activity of side-chain alkylation of picoline was studied over various alkali-modified zeolite molecular sieves like ZSM-5 (30), X, Y, Mordenite and MCM-41. Alkali-modified ZSM-5 (30) catalyst was found more active in side-chain alkylation of picoline when compared to other zeolites. Among all these catalysts studied, K-modified ZSM-5 (30) and K-Cs-ZSM-5 (30) gave the best conversion of picoline and selectivity to vinylpyridine. Cs-ZSM-5 (30) and K-ZSM-5 (30) were employed to study the reaction parameters like reaction temperature, weight hourly space velocity, molar ratio, and time on stream for 4-picoline. The effects of alkali metal ion content (K, Cs) and precursors of potassium ion on catalytic activity of side-chain alkylation were studied. The catalytic activities of side-chain alkylation of picoline using metal nitrate-modified catalysts were compared to potassium tert-butoxide (KOtBu)-, KF-, and KOH-modified ZSM-5 (30) catalysts. The catalysts were characterized by XRD, IR, irreversible sorption of CO2, surface area, temperature-programmed desorption of NH3 (NH3-TPD), and CO2-TPD techniques. An attempt has been made to correlate between the basicity and the activity of side-chain alkylation. A bifunctional catalyst is required containing medium or weak acidic centers and basic centers in the side-chain alkylation, this is understood through the proposed reaction mechanism. The selectivities of 4-vinylpyridine (4-VP) were 95.6, 94.5 and 89.1% at 80.4, 90.2 and 87.2% conversions over K-ZSM-5 (30), Rb-ZSM-5 (30) and Cs-K-ZSM-5 (30), respectively.