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

Chemical Science View Article Online Open Access Article. Published on 02 October 2018. Downloaded on 11/26/2018 1:33:49 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2018, 9, 8184 All publication charges for this article have been paid for by the Royal Society of Chemistry View Journal | View Issue Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR† Dong Xiao, ‡abc Shutao Xu, ‡d Nick J. Brownbill,c Subhradip Paul, e Li-Hua Chen, f Shane Pawsey,g Fabien Aussenac,h Bao-Lian Su, fi Xiuwen Han,a Xinhe Bao, a Zhongmin Liuad and Frédéric Blanc *cj 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 13 C isotopic enrichment and efficient dynamic nuclear polarization magic angle spinning nuclear magnetic resonance spectroscopy to detect carbocations in zeolites. We use two dimensional correlations to establish their structures and 29 13 C–13C through-bond 13 Si– C 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 b-zeolites and highlight that different Received 28th August 2018 Accepted 2nd October 2018 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 DOI: 10.1039/c8sc03848a in the channels of the zeolites leads to deactivation. These results enable understanding of deactivation rsc.li/chemical-science pathways and open up opportunities for the design of catalysts with improved performances. Introduction Carbocations are important intermediates in many homogeneous1,2 and heterogeneous reactions,3–6 especially those a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b University of Chinese Academy of Sciences, Beijing 100049, China c Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK. E-mail: d National Engineering Laboratory for Methanol to Olens, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China e DNP MAS NMR Facility, Sir Peter Manseld Imaging Centre, University of Nottingham, Nottingham NG7 2RD, UK f State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, 430070, Wuhan, China g Bruker BioSpin Corporation, 15 Fortune Drive, Billerica, Massachusetts 01821, USA h Bruker BioSpin, 34 rue de I'Industrie BP 10002, 67166 Wissembourg Cedex, France i CMI (Laboratory of Inorganic Materials Chemistry), University of Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium j Stephenson Institute for Renewable Energy, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK † Electronic supplementary 10.1039/c8sc03848a information (ESI) ‡ These authors contributed equally to this work. 8184 | Chem. Sci., 2018, 9, 8184–8193 available. See DOI: catalysed by solid acids (e.g. acidic zeolites),7–9 and are formed from the corresponding hydrocarbons through protonation by the acidic protons of the Brønsted acid sites. They take part in a range of industrial processes such as cracking, isomerization, alkylation, etc., which account for the conversion of hydrocarbons to a range of products.8 For example, cyclic carbocations are proposed as important intermediates involved in the hydrocarbon pool mechanism for the conversion of methanol to hydrocarbons (MTH).9–17 Despite the signicant roles of carbocations in heterogeneous reactions, their identications in solid catalysts are not straightforward as they are reactive, transient, difficult to capture and exist in generally low concentrations,3,15,18–20 and therefore their spectroscopic characterization is very challenging.15,19–21 Solid-state NMR is useful in detecting reactive carbocations on solid catalysts as shown in some limited cases on zeolites where their capture is achieved by quenching the reaction with liquid N2 (ref. 15 and 22) or stabilizing the intermediates with a base (e.g. ammonia).20 However, further development in extending the use of solid-state NMR to the study of carbocations is currently hindered by both the challenge associated with capturing enough highly reactive carbocations formed on solids (vide supra) and the intrinsically low sensitivity of NMR, especially when low natural abundance nuclei (e.g. 1.1% for 13C) This journal is © The Royal Society of Chemistry 2018 View Article Online Open Access Article. Published on 02 October 2018. Downloaded on 11/26/2018 1:33:49 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Edge Article are targeted. Although 13C isotopically enriched reagents are generally used to overcome this inherently poor sensitivity,15,19,22 the small amount of carbocations that can be captured in successful cases (typically 0.01 mmol g
1 in the MTH activated b-zeolite19) usually only permits the acquisition of one dimensional (1D) NMR signals, limiting the application of more informative multidimensional NMR experiments to obtain both the structures of these carbocations and their interaction with the solid catalysts. The structures of adsorbed carbocations are typically derived from such 1D 13C NMR spectra combined with gas chromatography-mass spectrometry (GC-MS) and density functional theory (DFT) calculations15,16 and therefore prior assumption of the existing structures is required. We recently identied the carbocations formed in 13C enriched MTH activated ZSM-5 and investigated their host– guest interaction by obtaining limited structural constraints.23 However, the experimental times needed to acquire the multidimensional and multinuclear NMR data were prohibitively long (>5 days), even in this favourable case where the carbocation concentration is relatively high (>0.02 mmol g
1). This signicantly hinders the systematic use of these powerful approaches on a wider range of solid acids with a lower amount of carbocation intermediates and addressing this challenge necessitates further dramatic boost in NMR sensitivity beyond 13 (...truncated)