Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level

Abstract

Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host–guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.

Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted–Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process

“…25,71,72 The relatively broad line-width of the $51 ppm peak (belonging to the side-on mode of zeolite-bound methanol) suggests heterogeneity in its molecular environment. 23 In addition to the identication of acetal species (-O-CH 2 -O-) on both post-reacted zeolites (cf. Fig.…”

“…This is a distinctive signature of a mobile species, which typically displays much narrower features under cross-polarization-based dipolar/ through-space magnetization transfer schemes. 15,22,23 To produce acetal, methanol rst undergoes dehydrogenation to form in situ carbon monoxide. Then, a surface-formate species was formed as a result of Koch-carbonylation of zeolite (Fig.…”

“…1,[15][16][17][18][19][20] Solid-state NMR spectroscopy is a quantitative and nondestructive/invasive approach to elucidate molecular structures along with their physiognomies accurately. 15,16,[21][22][23] However, this technique has not (yet) entirely overcome all its challenges. The natural abundance of the element of interest is one of the primary criteria (or concerns) behind its success.…”

“…Therefore, the utilization of 13 C-labelled species/reactant(s) is a popular strategy in multiple research elds, including heterogeneous catalysis, to deal with the low sensitivity issue. 2,3,15,16,[21][22][23][24][25][26][27][28] However, absolute isotope-enrichment of an element with low natural abundance is not always a practical solution, particularly in the eld of heterogeneous catalysis and surface/materials science. To overcome the low-sensitivity concerns, dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) has recently gained a lot of interest from the scientic community.…”

“…40,[45][46][47][48] To understand the reaction mechanism involving micro-porous zeolite catalysts, fully 13 C-enriched reactants have typically been used to deal with the natural abundance issue of the hetero-nuclei, including in MTH catalysis. 2,3,15,[21][22][23][24][26][27][28][49][50][51][52][53] Although very informative, this approach primarily involves the use of non-ideal reactors (e.g., mostly in situ reaction cells with a very high 'dead volume'), as well as shorter reaction times (cf. <60 min), which do not necessarily reect the real reaction conditions.…”