Confinement and surface sites control methanol adsorbate stability on MFI zeolites, SBA-15, and a silica-supported heteropoly acid

Abstract

We herein investigate methanol adsorbates on a variety of heterogeneous catalysts. We quantitatively desorb methanol from saturated MFI zeolites, SBA-15 materials and silicotungstic acid (STA) supported on silica, all in the respective siliceous, Na- and H-forms. Surface species are identified by ¹H and ¹³C MAS NMR and DRIFTS. On saturated surfaces, we find liquid-like methanol in weak surface interaction. For siliceous materials, adsorption on silanol Si(OH) groups is dominant, especially on materials with amorphous pore walls like SBA-15. Weak methanol binding on microporous silicalite is caused by a repulsive effect due to micropore confinement. For Na-form materials, methanol complexes at Na⁺ counter ions dominate the adsorption of methanol in Na-ZSM-5 micropores. The strong confinement leads to stronger methanol adsorption compared to less confined systems. Without confinement, no complex at Na⁺ is observed and Si(OH) groups dominate adsorption. On H-form materials, methanol complexes at acid sites form in a higher quantity under confinement in H-ZSM-5. After treatment at 423 K, the formation of dimethyl ether (DME) was evidenced by IR and ¹³C MAS NMR spectroscopy and the acid site proton peak found at δ_¹H = 14.4 ppm. Si(OH) groups bind methanol stronger than counter ions and acid sites (Na⁺ and H⁺). This explains why defects and the Si(OH) density influence heterogeneous reactions. Our findings show that confinement in micropores is crucial for the stabilization of methanol complexes at counter ions Na⁺ and acid sites H⁺.