(432g) Study of Transmethylation As Primary Step on the Decomposition of Methoxy-Rich Lignin Model Compound over Zeolite Catalyst

Authors: 
Beatriz, F., Cranfield University, UK
In the context of the bio-based economy, lignin is a major source of aromatic compounds. Fast pyrolysis of lignin followed by bio-oil upgrading is a promising route for retrieving these aromatic chemical platforms and making them available for subsequent processing. Fast pyrolysis of lignin and upgrading of the derived bio-oil has been previously studied. However, the conversion pathway followed by each compound (or functional group) in the complex liquid product still needs to be accurately understood in order to tailor the process towards desired products.

Significant number of compounds containing methoxy functional group, such as anisole, guaiacol, syringol, and their derivatives, are found in primary liquids obtained from the thermochemical conversion of lignin. These compounds are further converted into phenolic and aromatic compounds via secondary pyrolysis and upgrading reactions. Anisole is normally used as model compound of methoxy-rich compounds present in the liquids from fast pyrolysis of lignocellulosic biomass because the only functionality in the molecule is the methoxy group. Research on anisole decomposition has been mainly focused on the deoxygenation process of the phenyl-oxygen bond. Study of the mechanism of transmethylation as reaction occurring prior to deoxygenation is less extensive, despite it is essential to understand the entire process of anisole decomposition.

In this work the transmethylation as the primary step on the catalytic decomposition of anisole is investigated. The decomposition of anisole was carried out in a fluidised bed reactor, and HZSM-5 zeolite was used as catalyst. The yield and product distribution in the liquid fraction, with particular focus on the phenolic compounds, was evaluated by gas chromatography. Non-catalytic anisole decomposition experiments were also conducted for comparison purposes.

Maximum yield of liquid products was observed at the minimum temperature at which complete conversion of anisole is achieved (650°C for non-catalytic decomposition and 400°C for catalytic decomposition). Phenolic compounds were the primary products at these key temperatures and below. In the case of catalytic decomposition, phenol, and ortho-cresol and para-cresol were the main products. In addition, multi-methylphenols (xylenols and trimethyl-phenols) were abundantly produced over HZMS-5. Yields of aromatic compounds were larger than those of phenols at higher temperatures.

Experimental results suggest that transmethylation occurs as primary reaction when anisole conversion is not complete at low temperatures. A plausible mechanism for low-to-moderate temperatures is that, in the presence of the zeolite catalyst, the anisole is converted into phenol followed by the relocation of the methyl radical to form o-cresol and p-cresol. The addition of another methyl radical to the cresol molecule gives rise to the formation of xylenols. The rearrangement to trimethyl phenols is observed to a lesser extent. In all cases the major orientations for transmethylation are the ortho- and para-positions, and are favoured by the increase in temperature. As the temperature increases, anisole conversion is maintained but phenolic compounds yields decrease which points out to phenolics as precursors of aromatic compounds. Formation of aromatics occurs by means of deoxygenation as secondary step during anisole decomposition.