Diesel consumption is expected to grow between 46 and 200 % compared to its 2010 level during the next four decades, according to the World Energy Council.[1] Meanwhile, air pollution has reached alarming levels in large cities forcing governments to adopt more stringent emission standards. In this context, polyoxymethylene dimethyl ethers (OME) have recently gained attention as a new type of diesel additive or substitute due to their appealing properties.[2] Their large-scale implementation requires stable and acidic catalysts. However, the role of Lewis and Brønsted acid sites, and their potential synergy remains ambiguous for the synthesis of OME. Therefore, the goals of this study were twofold. First, we sought to acquire a deeper understanding of the type of acidity which is involved in the different OME synthesis steps. Second, we aimed at investigating the nature of the synergy between Lewis and Brønsted acid sites.

These objectives were pursued by synthetizing a series of BEA zeolites with various amounts of Brønsted and Lewis acid sites applied to several reactions for OME synthesis. Lewis acidity was introduced in the zeolite framework via Sn incorporation by grafting in dichloromethane, as it was shown to lead to high Sn content with little extra-framework Sn. Various characterization techniques such as N₂ physisorption, solid-state NMR or infrared spectroscopy of adsorbed pyridine were used to study the catalysts structure and acidity. The synthesized catalysts were used to produce OME using various reactions involving different reactants. Additionally, attenuated total transmittance Fourier-transform infrared spectroscopy was used to study the adsorption modes of OME₁and trioxane (TRI) on the synthesized catalysts.

The characterization results confirmed that Sn grafting resulted in the incorporation of Sn in a tetrahedral coordination within the dealuminated BEA framework. Then, our reaction results demonstrated that Brønsted sites were active in all steps of OME synthesis while Lewis acid sites were only active in OME growth, paraformaldehyde (PF) decomposition and hemiacetal acetalization. Notably, TRI dissociation and OME₁activation did not occur on Lewis acid sites. The presence of both sites resulted in a synergy when OME₁was used with PF or TRI. Partially dealuminated, Sn-modified Beta zeolites exhibited a significant increase in turnover frequency and reduction in byproduct generation compared to the parent H-Beta zeolite. This synergistic effect is explained by a more efficient formaldehyde (FA) insertion into OME on Lewis acid sites, while generation of FA units by TRI or PF decomposition occurred on Brønsted acid sites. The interaction between tetrahedral Sn and the carbonyl group of FA resulted in an activated FA, likely to be inserted into OME.

[1] World Energy Council, Global Transport Scenarios 2050, 2011, DOI: 10.1016/j.enpol.2011.05.049.
[2] C. J. Baranowski, A. M. Bahmanpour, O. Kröcher, Appl. Catal. B Environ, 2017, 407, DOI: 10.1016/j.apcatb.2017.06.007.