Alkylation Of Phenol With Methanol Over Various Catalysts

Alkylation of phenol with methanol, including O-alkylation and C-alkylation(mainly Ortho-alkylation), has gained considerable attention for many years as theproducts are widely used in variety of applications. For example, anisole (productof O-alkylation) is used as an intermediate in the synthesis anethole, dyes;O-cresol and 2,6-xylenol (poducts of C-alkylation), are extensively used aschemical intermediates in the synthesis of pharmaceuticals,agrochemicals(herbicides), resins, antioxidants, special grade paints,polyphenylene oxide (PPO) and various other fine chemicals.Since the first report of alkylation of phenol with methanol in the 1940s,many kinds of materials have been investigated as catalysts for the reaction,including various zeiolites and metal oxides (hydrotalcites, pure and mixed metaloxides, phosphates and spinel oxides so on). Among them, some solid bases,mixed metal oxides and spinel oxides show relatively good catalytic performance.However, the catalytic properties, especially the selectivity and stability, are stillunsatisfied for practical applications. Furthermore, some basic problems such asthe nature of active center, reaction mechanism are still unclear.In this work, various catalyst systems, including alkali supported silicacatalysts, Mn-Si metal oxide catalysts and spinel-type cobalt chromites, wereprepared by different methods, and the catalytic performance was investigated foralkylation of phenol with methanol. The catalysts were characterized by means ofXRD, BET, TPD, TPR, TEM, XPS and FTIR of pyridine adsorption etc. in orderto build a relationship between the structure, acid-base properties, redoxproperties of the catalysts and the catalytic performance. Meanwhile, the nature ofactive center,the reaction mechanism and the reason for deactivation were alsodiscussed. The main experimental results and conclusions are as follows:1. Vapor phase O-alkylation of phenol with methanol over alkalisupported silica catalystsVarious alkali and alkaline earth supported silica catalysts were prepared byan impregnation procedure and the catalytic performance of vapor phaseO-alkylation of phenol with methanol was investigated. Among the tested samples,Cs/SiO2 exhibits the highest catalytic activity. For the Cs/SiO2 catalysts,increasing calcination temperature could result in the decrease of phenolconversion, which might be due to the decrease of basic amount of the sample athigh calcination temperature. Meanwhile, the reaction conditions haveconsiderable effect on the catalytic performance. The relatively low reactiontemperature is in favor of the anisole selectivity, and the introduction ofcoexistence gas (N2) can improve the catalyst stability. For the K/SiO2 catalysts,the potassium source has considerable effect on the catalytic performance. WhenKNO3 is used as the potassium source, the catalyst exhibits the best catalyticperformance. The relatively high BET surface area and big pore volume ofsupports is benefit to the improvement of catalytic performance. Additionally,combined with some related references and various characterization results, areaction mechanism can be proposed, which is: the methanol is adsorbed on thecation, and the phenol is adsorbed on basic sites, then the phenol oxygen atomundergoes a nucleophilic attack on the carbon atom of methanol, leading to theformation of the anisole.2. Vapor phase Ortho-selective alkylation of phenol with methanolover manganese silicon metal oxide catalystsVarious Mn-Si metal oxide catalysts were prepared by coprecipitation method,and the catalytic performance of the vapor phase ortho-selective alkylation ofphenol with methanol was investigated. The Mn/Si ratio, the calcinationtemperature and the reaction temperature have considerable effect on the catalyticperformance. The samples calcined at relatively low temperatures exhibit goodcatalytic performance. Meanwhile, increasing reaction temperature favors theformation of 2,6-xylenol. Additionally, manganese can be partly reduced duringthe reaction, which might be the main reason for the decrease in the catalyticperformance of Mn-Si metal oxide catalyst. Additionally, combined with somerelated references and the results of pyridine adsorption FTIR spectra, it can beproposed that a synergic interaction between relatively strong Lewis acid sites andadjacent base sites should be responsible for the high ortho-selectivity of thecatalyst.3. Vapor phase Ortho-selective alkylation of phenol with methanolover spinel-type chromites catalystsThe gas phase ortho-selective alkylation of phenol with methanol wasinvestigated over a series of chromites prepared by co-precipitation method. Thetypes of chromites have considerable effect on the catalytic performance. Thespinel-type cobalt chromites show relatively good catalytic properties. Amongthem, the spinel-type cobalt chromite with a ratio of Co/Cr = 0.8 exhibits the bestcatalytic performance. Combined with some references and XRD, FTIR and XPScharacterization results, it is proposed that the rich cobalt samples can beconsidered as Co2+ Co3+xCr3+2-xO4 solid solution. Both the acidic amount andstrength of cobalt chromites increase gradually when the ratio of Co/Cr increasesfrom 0.5 to 0.8, which could be due to a part of Co3+ ions occupied the positionof octahedral (Oh) sites. With further increase the ratio of Co/Cr, the acidity ofcobalt chromites samples decrease gradually, which could be due to the fact thatthe presence of excessive Co3+ species in Oh sites. Therefore, it can be concludethat the change of composition and structure of the catalysts could result in thechange of the acidity, thus considerably affect the catalytic performance of thecatalysts.For the Co/Cr = 0.8 sample, the reaction conditions have considerable effecton the catalytic performance. Increasing the reaction temperature and decreasingthe weight hourly space velocity could result in an increase in the phenolconversion and the 2,6-xylenol selectivity. Combined with the related referencesand various characterization results, it is proposed that the existence of relativelyweak acid sites are responsible for the formation of o-cresol, whereas therelatively strong acid sites are in favor of 2,6-xylenol.Additionally, combined with the related references XRD,TPR,XPS andTG-DTA characterization results, it is proposed that a slight decrease in thecatalytic activity of cobalt chromites catalysts could be not due to the formation ofthe carbon deposition during the reaction, but mainly ascribed to the decrease inthe strength of Lewis acid which originated from the part reduction of Co3+ intoCo2+ during the reaction.The gas phase ortho-selective alkylation of phenol with methanol wasinvestigated over a series of potassium supported on the cobalt chromites preparedby an impregnation procedure. The results show that the weaker acid sites favorsthe formation of o-cresol and anisole, whereas the stronger acid sites favors the2,6-xylenol. Meanwhile, the coexistence of the stronger acid sites and therelatively weak (or medium) basic sites of the cobalt chromites catalysts is benefitto the increase in the catalytic activities of the ortho-selective alkylation of phenolwith methanol。The gas phase ortho-selective alkylation of phenol with methanol wasinvestigated over a series of sulfuric acid supported on the cobalt chromitesprepared by an impregnation procedure. The results show the introduction ofsuitable amount of sulfuric acid (S/Cr = 0.001) could improve the selectivity of2,6-xylenol.We attempted to introduce some transition metals to the cobalt chromites. Theaddition of different kinds of transition metals could considerably influence thecatalytic performance. Addition of a certain amount of zinc could enhance thecatalytic performance of the cobalt chromites. Among them, the sample ofZn0.5Co0.5Cr2O4 exhibits the best catalytic performance. Increasing the reactiontemperature and decreasing the weight hourly space velocity could result in anincrease in the phenol conversion and 2,6-xylenol selectivity.