Luca Maselli

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.

How basic properties of MgO-based mixed oxides affect the catalytic performance in gas-phase and liquid-phase methylation of m-cresol

Abstract The reactivity of MgO-based mixed oxides in the gas-phase alkylation of m-cresol with methanol was studied, with the aim of finding relationships between basic surface features and catalytic performance. Conversion and products distribution were compared with those obtained in liquid-phase methylation. Catalysts investigated included: MgO, Mg/Al/O, Mg/Fe/O, Mg/Al/Zr/O, Mg/Al/Ce/O and H-Y zeolite, the latter used as reference for Bransted-type activity. It was found that the regio-selectivity of the reaction is greatly affected by the presence of (i) medium-strength and strong basic sites, the latter playing a relevant role only in gas-phase reaction, and (ii) metal cations having acid, Lewis-type, coordination properties.

Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme

The reaction scheme for the hydroxyalkylation of 2-methoxyphenol (guaiacol) with aqueous solutions of formaldehyde (formalin) aimed at the synthesis of 3-methoxy-4-hydroxybenzyl alcohol (p-vanillol), catalyzed by an H-mordenite zeolite was analyzed. Specific attention was focused on the nature of the by-products obtained in the presence and in the absence of methanol-a component present in formalin. In the presence of methanol the main products were the vanillol isomers, and the prevailing by-product was 3-methoxy-4-hydroxybenzylmethylether, obtained by reaction between p-vanillol and methanol. In the absence of methanol the prevailing by-products were diarylmethanes. Even though both by-products can be considered secondary ones from a chemical point of view, i.e., formed starting from vanillols, they were found to form even at low guaiacol conversion. Therefore vanillols, once formed in the zeolitic pores, generate therein the corresponding benzyl carbocations and undergo nucleophilic attack by either methanol, guaiacol or another vanillol. The reactivity of vanillic alcohol isomers was also directly checked, both in the presence and in the absence of formaldehyde and of methanol. The results show the different tendencies of the isomers to give consecutive transformations.

A comparison of the reaction mechanism in acid-and in basic-catalysed gas-phase methylation of phenol

This paper compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It was found that with the basic catalyst Mg/Fe/O, the true alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacted with phenol to yield salicylic alcohol, which rapidly dehydrogenated to salicylic aldehyde. The latter was isolated in tests made by variation of the residence time, and in tests made by feeding a formalin/phenol aqueous solution. Salicylic aldehyde then transformed to o-cresol, the main product of the basic-catalyzed methylation of phenol, by means of an intermolecular H-transfer. With an H-mordenite catalyst, instead, the activated methanol reacted with phenol to generate anisole, cresols and polyalkylated phenols.

The control of regioselectivity in the liquid-phase hydroxymethylation of 2-methoxyphenol with aqueous formaldehyde. A comparison between zeolites and basic catalysts

The heterogeneous hydroxyalkylation of 2-methoxyphenol (guaiacol) with an aqueous solution of formaldehyde was carried out using either zeolites or solid bases as catalysts. The reaction is aimed at the production of vanillic alcohols (methoxyhydroxybenzyl alcohols), which are intermediates for the synthesis of the corresponding aldehydes. In the case of H-Mordenites, the catalytic performance was governed by the hydrophobic properties of the zeolite (which were in turn affected by the Si/Al ratio), rather than by the number of acid sites. With H-β zeolites, instead, hydrophobic properties had a smaller influence on the conversion and selectivity. It was found that the regioselectivity of the hydroxymethylation is greatly affected by the catalyst characteristics; while zeolites gave mainly the product of methylation in the para position with respect to the hydroxy group in guaiacol, oxides having basic surface features gave predominantly o-vanillol, the product of ortho-C-hydroxymethylation.