Mapping the Methanol-to-Gasoline Process Over Zeolite Beta.

Decarbonizing the transportation sector is among the biggest challenges in the fight against climate change. CO2 -neutral fuels, such as those obtained from renewable methanol, have the potential to account for a large share of the solution, since these could be directly compatible with existing power trains. Although discovered in 1977, the zeolite-catalyzed methanol-to-gasoline (MTG) process has hardly reached industrial maturity, among other reasons, because maximizing the production of gasoline range hydrocarbons from methanol has proved complicated. In this work, we apply multimodal operando UV/Vis diffuse reflectance spectroscopy coupled with an online mass spectrometer and "mobility-dependent" solid-state NMR spectroscopy to better understand the reaction mechanism over zeolites H-Beta and Zn-Beta. Significantly, the influential co-catalytic role of oxymethylene species is linked to gasoline formation, which impacts the MTG process more than carbonylated species.

Modifying the Hydrogenation Activity of Zeolite Beta for Enhancing the Yield and Selectivity for Fuel-Range Alkanes from Carbon Dioxide.

In order to empower a circular carbon economy for addressing global CO2 emissions, the production of carbon-neutral fuels is especially desired, since addressing the global fuel demand via this route has the potential to significantly mitigate carbon emissions. In this study, we report a multifunctional catalyst combination consisting of a potassium promoted iron catalyst (Fe-K) and platinum containing zeolite beta (Pt-beta) which produces an almost entirely paraffinic mixture (up to C10 hydrocarbons) via CO2 hydrogenation in one step. Here, the Fe catalyst is responsible for modified Fischer-Tropsch synthesis from CO2 while Pt-beta is instrumental in tuning the product distribution almost entirely towards paraffins (both linear and branched) presumably via a combination of cracking and hydrogenation. The optimal temperature of operation was estimated to be 325 °C for the production of higher paraffins (C5 -C10 ) with a selectivity of ca. 28 % at a CO2 conversion of ca. 31 %.

Designing a Multifunctional Catalyst for the Direct Production of Gasoline-Range Isoparaffins from CO2.

The production of carbon-neutral fuels from CO2 presents an avenue for causing an appreciable effect in terms of volume toward the mitigation of global carbon emissions. To that end, the production of isoparaffin-rich fuels is highly desirable. Here, we demonstrate the potential of a multifunctional catalyst combination, consisting of a methanol producer (InCo) and a Zn-modified zeolite beta, which produces a mostly isoparaffinic hydrocarbon mixture from CO2 (up to ∼85% isoparaffin selectivity among hydrocarbons) at a CO2 conversion of >15%. The catalyst combination was thoroughly characterized via an extensive complement of techniques. Specifically, operando X-ray absorption spectroscopy (XAS) reveals that Zn (which plays a crucial role of providing a hydrogenating function, improving the stability of the overall catalyst combination and isomerization performance) is likely present in the form of Zn6O6 clusters within the zeolite component, in contrast to previously reported estimations.

A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion.

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(µ-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.