(288c) Selective Conversion of Guaiacol to Substituted Alkylphenols in Supercritical Ethanol over MoO3

Recently, transition metal phosphides, nitrides, carbides and oxides have been intensively employed in biomass conversion{Ma, 2015 #41}.^[1-4] The earth-abundant MoO₃ is regarded as a representative of promising alternative catalysts.^[2] However, the bottleneck for the development of HDO technique over MoO₃ lies in the consumption of hydrogen and high temperature. In this work, the conversion of guaiacol to various substituted alkylphenols is investigated using MoO₃ in supercritical ethanol at a low temperature of 280 ^oC in inert nitrogen. Furthermore, the preliminary reaction pathways are proposed and the molybdenum active state is explored.

The conversion of guaiacol was carried out in a batch reactor at a temperature, in 260-300 °C, for a desired reaction time, 1-5 h over a MoO₃ catalyst. The product sample was injected neat into a GC-MS and a GC-FID instrument for analysis. Several possible reaction intermediates (phenol, catechol, anisole, 4-ethylphenol and 2-isopropylphenol) were employed as reactants to understand the transformation path of guaiacol. XRD, XPS and Raman spectroscopy were used to characterize the catalyst samples.

Fig. 1 exhibits that substituted phenols such as ethylphenols, iso-propylphenols, butylphenols (tert-, iso-) and tert-amylphenol were obtained as the dominating products with a high combined selectivity of 86% and at a conversion of 99% over MoO₃ under 280 ^oC for 4 h in supercritical ethanol.

To explore the reaction pathways of guaiacol conversion, control experiments with phenol, anisole and catechol as the starting materials under the same reaction condition were carried out. Catechol gave a complete conversion while phenol and anisole did not. Besides, a similar product distribution compared to guaiacol as the reactant is achieved with catechol. Considering phenol was never found in the guaiacol conversion process, we propose that catechol is formed as the intermediate via demethylation of guaiacol and followed by direct conversion to alkylphenols without the formation of phenol. Subsequently, we employed 4-ethylphenol and 2-isopropylphenol as the reactants and obtained higher alkylphenols as the products. There were no isopropyl, tertiary (iso) butyl or tertiary pentyl derived aliphatic compounds or n-propyl, n-butyl, n-pentyl substituted phenols detected in the liquid and gaseous products of guaiacol conversion. Thus, we speculate that the higher alkylphenols are from consecutive substitution of lower alkylphenols with methyl and ethyl groups produced by ethanol medium, instead of direct substitution with corresponding substituent groups or isomerization with interrelated alkylphenols (Fig. 2).

The Mo (3d) energy region of used MoO₃ is shown in Fig. 3. As can be seen, it contains large amounts of Mo⁵⁺ (56.4%) with the 3d_5/2 and 3d_3/2 bands located at 230.9 eV and 234.1 eV, respectively. Moreover, molybdenum trioxide (Mo⁶⁺ state) and molybdenum dioxide (Mo⁴⁺ state) have been proved to be inactive in control experiments. Therefore, Mo⁵⁺ was considered to be responsible for the excellent performance of guaiacol conversion.