Gas-phase Methylation Of Phenol Research Articles

Mixed cerium nanooxides as efficient and selective C-alkylation catalysts for gas phase methylation of phenol

Phenol methylation is an acid-base catalysed reaction that produces valuable chemicals that are used in fine chemical synthesis. Methylation of phenol was studied over iron, vanadium and zirconium doped cerium nano oxides under optimized reaction conditions such as catalyst weight 0.5 g, phenol/methanol molar ratio 1/3 and WHSV 1.4 h−1 at 300 °C and 2 h. 80-90% conversion and 90% ortho selectivity are achieved with all prepared mixed oxides. o-cresol and 2,6-xylenol are the major products of the reaction. Catalytic activity depends on the number and strength of moderate and strong acidic sites on the surface which in turn depends on the nature and composition of the catalyst. Structural stability of the catalyst substantiates the heterogeneous nature of the catalyst.

A cascade mechanism for a simple reaction: The gas-phase methylation of phenol with methanol

The gas-phase alkylation of phenol with methanol, a reaction triggered for the production of o-cresol and 2,6-xylenol, is catalysed by MgO-based catalysts. Despite the industrial use of this process, the mechanism of the reaction – which is commonly believed to be based on a classical electrophilic attack of activated methanol onto the aromatic ring – is far from being fully understood. In some previous studies we reported that the reaction intermediate is salicylic alcohol, which is formed by the reaction between the adsorbed phenolate and formaldehyde, the latter being formed in-situ by methanol dehydrogenation. Here we elucidate the following steps of the reaction mechanism, by combining reactivity experiments and DFT calculation, with MgO as a model catalyst. It was found that salicylic alcohol dehydrates into quinone methide, which is then reduced via H-transfer by methanol to o-cresol. Moreover, a dehydrogenation/hydrogenation equilibrium is established between salicylic alcohol and salicylic aldehyde. The methide can also react with methanol to form 2-methoxymethylphenol, which may decompose into o-cresol, thus providing an alternative pathway for the formation of the alkylated compound.

Gas-Phase Methylation of Phenol over Iron–Chromium Catalyst

Phenol methylation to 2,6-dimethylphenol was investigated in gas-phase conditions using an iron–chromium catalytic fixed bed. The experiments were carried out at 603 K, at the atmospheric pressure. The long-term stability of the catalyst and the effects of the reactor packing on the phenol conversion in one- and three-section reactor were studied. It was found that, in both types of the reactor, the phenol conversion exceeded 99%. 2,6-Dimethylphenol, 2-methylphenol, and 2,4,6-trimethylphenol were identified in the outlet liquid, and methane, hydrogen, and carbon oxides were identified in the outlet gas. Selectivity to the main liquid product: 2,6-dimethylphenol of about 85% was obtained. Phenol conversion and selectivity to 2,6-dimethylphenol decreased during the process, and therefore, the regeneration of the catalyst was necessary.

Gas-phase phenol methylation over Mg/Me/O (Me = Al, Cr, Fe) catalysts: mechanistic implications due to different acid–base and dehydrogenating properties

This contribution reports about an in situ FT-IR investigation and the catalytic reactivity of Mg/Me(3+) mixed oxides (Me = Cr, Fe, or Al; Mg/Me = 2, atomic ratio) in the gas-phase methylation of phenol with methanol. It is the second of two papers concerning the mentioned systems, and its purpose is twofold: to confute the classic and not accurate theory concerning the reaction mechanism, and to propose a novel interpretation based on the combined use of catalytic tests and in situ molecular spectroscopy. Results here reported highlight that: (i) the reaction mechanism in phenol methylation, when catalysed by basic systems, is not a classical electrophylic substitution, as generally reported in the literature, but proceeds through the formation of formaldehyde as an intermediate, and (ii) the catalytic behaviour in respect to both methanol and phenol reactants is strictly dependent on catalyst features. Although all investigated systems exhibit a basic-type behaviour with regard to phenol, which dissociates to yield an adsorbed phenolate species, the distribution of phenolic compounds obtained with the Mg/Al/O catalyst was that typically observed with acid catalysts, with prevailing formation of anisole when the reaction was carried out below 350 degrees C and of mono and poly-C-alkylated compounds when the reaction temperature was above 350 degrees C. On the contrary, the reactivity shown by both Mg/Fe/O and Mg/Cr/O systems was that reported in the literature as typical of mixed oxides possessing basic features. The extent of methanol decomposition into light compounds was maximum in the case of Mg/Fe/O catalysts, because of the pronounced redox behaviour typical of Fe(3+) species, whereas neither methanol dehydrogenation nor decomposition were ever observed with Mg/Al/O up to 400 degrees C. Reactivity tests and spectroscopic experiments hinted for methanol dehydrogenation to formaldehyde as the first step in the ring-methylation of phenol with Mg/Cr/O and Mg/Fe/O: in that case, o-cresol and 2,6-xylenol were the only reaction products. But, with Mg/Al/O systems, for which no methanol dehydrogenation occurred, the formation of anisole was due to the synergistic effect of stronger basic features and the presence of Lewis acidic sites, that facilitate the reaction between phenol and methanol after activation over the two different types of catalytic sites.

Mechanistic studies of the role of formaldehyde in the gas-phase methylation of phenol

The gas-phase reaction between phenol and formaldehyde was studied using Brønsted acidic and basic catalysts to gain insight into the components of phenol methylation with methanol. Formaldehyde and methylformate form from methanol during phenol methylation over basic catalysts. In the basic-catalyzed methylation of phenol with formaldehyde, the prevailing phenolic products were o-cresol and 2,6-xylenol; the same products were obtained in the methylation with methanol. When formaldehyde was used with the acidic catalyst, the distribution of products was similar to that obtained over basic catalysts. Comparing these results with those of methylation with methylformate, methyliodide, and methanol shows that with basic catalysts, formaldehyde was the true methylating agent in the reaction between phenol and methanol. This indicates that the transformation of methanol differentiates the reaction pattern in phenol methylation over basic or acidic catalysts. The role of the catalyst is to generate the methylating species; the nature of the latter then determines the type of products obtained in phenol methylation, whereas the catalyst type (either acidic or basic) has little influence on it.

Changes in surface composition of Fe-Si-Cr-K oxide alkylation catalyst: the marked role of potassium ferrites

The influence of gas-phase methylation of phenol on the state of the Fe-Si-Cr-K oxide catalyst surface was investigated by thermodesorption of K atoms and ions. Changes in potassium desorption energies, as determined from Arrhenius-like plots, varied from 3.07 eV to 1.21 eV for the atoms and from 2.59 eV to 2.89 eV for ion desorption from the active and deactivated samples, respectively. The results were discussed in terms of transformation of the catalyst surface and formation of β-ferrite.

Redistribution of Cations Amongst Different Lattice Sites in Cu1-xCoxFe2O4Ferrospinels During Alkylation: Magnetic Study

A series of Cu1−xCoxFe2O4 (x = 0, 0.25, 0.5, 0.75, 1.0) ferrospinels prepared by low temperature coprecipitation method and glycine nitrate combustion method has been studied in gas phase methylation of phenol. Phenol methylation gives mainly o-cresol and 2,6-xylenol as major products and among various compositions, x = 0.50 shows good catalytic performance irrespective of the preparation method. The difference in properties of the fresh and spent catalysts was thoroughly characterized by adopting various physico-chemical characterization techniques with special emphasize on magnetic measurements. Various conclusions derived from magnetic study are in good agreement with our previous study of XRD and Mossbauer on same catalyst system. Redistribution of cations occurred during the reaction is evidenced from the increase of saturation magnetization in the spent. Spent x = 0.0 shows high Tc close to the value of Fe3O4 indicating that the material has ended with a solid solution of Fe3O4 and CuFe2O4 along with other reduced phases.

Gas-phase methylation of phenol and anisole by CH3XCH3 +(X = F, Cl, or Br) ions

Dimethylhalogenonium ions CH3XCH3+(X = F, Cl, or Br), obtained in the dilute gas state from the γ-radiolysis of the corresponding methyl halide CH3X, were allowed to react with representative ambident substrates, such as phenol and anisole, either neat or in competition with benzene, in the pressure range 100–760 Torr, and in the presence of a gaseous base (NH3). The competition between the n- and π-type nucleophilic centres of phenol is kinetically biased in favour of O-methylation, leading to the predominant (CH3FCH3+ 6 : 1; CH3ClCH3+ 5 : 1; CH3BrCH3+ 4 : 1) formation of anisole with respect to cresols under conditions, i.e. high CH3X pressure and in the presence of NH3, which favour kinetic control of the products. For ring methylation, both the selected substrates undergo preferential substitution at the ortho- and para-positions. In general, ortho-attack is favoured at low pressure and in the absence of NH3. The results of competition experiments show that phenol (S) reacts faster than benzene (B) with gaseous CH3XCH3+, the substrate selectivity of the electrophile increasing in the order CH3FCH3+(kS/kB 2.0) < CH3ClCH3+(kS/kB 13.8) < CH3BrCH3+(kS/kB 40.9). The same conclusion cannot be reached for anisole because of the occurrence of extensive O-methylation with formation of dimethylphenyloxonium ion, which turns out to be very stable under the conditions used. The results are consistent with a model involving kinetically predominant attack on the oxygen atom of the ambident substrate, which, in the case of CH3FCH3+ ion, can be preceded by an extensive electrostatic interaction with formation of a ‘chelated’ complex. The same interaction helps also explaining the relatively high reactivity of the ring position ortho to the n-donor substituent, observed at low pressure. The substrate and positional selectivity of the CH3XCH3+ ions toward ambident compounds is discussed and compared with related gas-phase results and with conventional Friedel–Crafts methylation data.