Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR.

Dong Xiao,Xiuwen Han,Bao-Lian Su,Xinhe Bao,Shutao Xu,Subhradip Paul,Frédéric Blanc,Zhongmin Liu,Li-Hua Chen,Nick J Brownbill,Fabien Aussenac,Shane Pawsey

doi:10.1039/c8sc03848a

Abstract

Acidic zeolites are porous aluminosilicates used in a wide range of industrial processes such as adsorption and catalysis. The formation of carbocation intermediates plays a key role in reactivity, selectivity and deactivation in heterogeneous catalytic processes. However, the observation and determination of carbocations remain a significant challenge in heterogeneous catalysis due to the lack of selective techniques of sufficient sensitivity to detect their low concentrations. Here, we combine 13C isotopic enrichment and efficient dynamic nuclear polarization magic angle spinning nuclear magnetic resonance spectroscopy to detect carbocations in zeolites. We use two dimensional 13C-13C through-bond correlations to establish their structures and 29Si-13C through-space experiments to quantitatively probe the interaction between multiple surface sites of the zeolites and the confined hydrocarbon pool species. We show that a range of various membered ring carbocations are intermediates in the methanol to hydrocarbons reaction catalysed by different microstructural β-zeolites and highlight that different reaction routes for the formation of both targeted hydrocarbon products and coke exist. These species have strong van der Waals interaction with the zeolite framework demonstrating that their accumulation in the channels of the zeolites leads to deactivation. These results enable understanding of deactivation pathways and open up opportunities for the design of catalysts with improved performances.

Highlights

Carbocations are important intermediates in many homogeneous[1,2] and heterogeneous reactions,[3,4,5,6] especially those catalysed by solid acids,[7,8,9] and are formed from the corresponding hydrocarbons through protonation by the acidic protons of the Brønsted acid sites
We show that a range of various membered ring carbocations are intermediates in the methanol to hydrocarbons reaction catalysed by different microstructural b-zeolites and highlight that different reaction routes for the formation of both targeted hydrocarbon products and coke exist
A more representative parameter to evaluate the dynamic nuclear polarization (DNP) efficiency as compared with standard NMR at room temperature is the overall DNP gain S†. This takes into account the increase of sensitivity from thermal Boltzmann distribution going from 298 K to 110 K, signal attenuation due to the paramagnetic relaxation effects from the exogenous stable biradicals and cross-effect induced depolarization under magic

Summary

Introduction

Carbocations are important intermediates in many homogeneous[1,2] and heterogeneous reactions,[3,4,5,6] especially those catalysed by solid acids (e.g. acidic zeolites),[7,8,9] and are formed from the corresponding hydrocarbons through protonation by the acidic protons of the Brønsted acid sites. The DNP-enhanced 13C CP MAS NMR spectra of the activated zeolites are shown in green in Fig. 3 and reveal 13C signal enhancements 3C CP of 10 and 40 at 9.4 T for M-b and MMM-b, respectively.

Results

Conclusion