(327e) A Paradigm Shift in Catalyst Stability for Toluene Alkylation with Methanol

One of the commercially most important aromatic compounds is p-xylene, which is used for the production of various fine chemicals. Due to its cheap feedstocks, toluene alkylation with methanol is one of the most commercially attractive routes to produce p-xylene; however, challenges such as low catalyst stability and single pass p-xylene yield need to be addressed. Here we present a study of the catalytic properties of MWW-type zeolite catalyst, for toluene alkylation with methanol under high pressure conditions (600 psia).

To understand the catalytic behavior of MWW, we deconvoluted structure-function relationships for different topological features (supercages, sinusoidal channels, and external surface pockets) having profound impact on catalyst performance. Deconvolution was achieved by selectively passivating the active sites in specific pore systems, and then comparing their activity with the parent MWW catalyst.

Under high pressure operating conditions, the catalyst life increases multi-fold at high toluene conversion with side reactions such as methanol-to-hydrocarbon reactions being suppressed in the absence of water or hydrogen co-feed. Interestingly, the selectivity of xylene isomers changes significantly during the course of reaction, from thermodynamic equilibrium mixture to 55% p-xylene selectivity.

The combination of structure-deconvoluting experiments and DFT calculations reveals that the active sites in surface pockets are unselective and fast isomerization on external surface leads the reaction towards the thermodynamically determined xylene product distribution, while microporous sinusoidal channels and/or supercages preferentially form p-xylene. Overall, this study introduces a commercially viable route for p-xylene production as well as new insight into the reaction mechanism over MWW zeolite catalysts to aid in the development of optimized processes for various alkylation reactions.