Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates

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Abstract

The synthesis of many industrial bulk and fine chemicals frequently involves electrophilic aromatic substitution (SEAr) reactions. The most widely practiced example of the SEAr mechanism is the zeolite-catalysed ethylation of benzene, using ethylene as an alkylating agent. However, the current production route towards ethylbenzene is completely dependent on fossil resources, making the recent commercial successes in the zeolite-catalysed benzene ethylation process using bioethanol (instead of ethylene) very encouraging and noteworthy. Unfortunately, there is no information available on the reaction mechanism of this alternative synthesis route. Here, by employing a combination of advanced solid-state NMR spectroscopy and operando UV-Vis diffuse reflectance spectroscopy with on-line mass spectrometry, we have obtained detailed mechanistic insights into the bioethanol-mediated benzene ethylation process through the identification of active surface ethoxy species, surface-adsorbed zeolite–aromatic π-complexes, as well as the more controversial Wheland-type σ-complex. Moreover, we distinguish between rigid and mobile zeolite-trapped organic species, providing further evidence for distinctive host–guest chemistry during catalysis.

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Fig. 1: Schematic illustration of the zeolite-catalysed EB reaction.
Fig. 2: Operando UV-Vis DRS measurements during the EB reaction.
Fig. 3: The electronic effect on the formation of a Wheland-type reaction intermediate during the zeolite-catalysed ethylation of aromatics.
Fig. 4: 1D 13C solid-state NMR spectra of zeolite trapped products after EB reaction.
Fig. 5: 13C mobile molecules in J-coupling-based MAS solid-state NMR spectra.
Fig. 6: MAS solid-state NMR correlations of rigid zeolite trapped molecules.
Fig. 7: Proposed catalytic cycle of the zeolite catalysed EB reaction.

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Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement (no. 704544 to A.D.C.), a European Research Council (ERC) Advanced Grant (no. 321140 to B.M.W.) as well as a Veni grant (no. 722.015.003 to G.T.W.) and a Middelgroot programme (no. 700.58.102 to M.B.) from the Netherlands Organization of Scientific Research (NWO). K.H. is supported by uNMR-NL, an NWO-funded National Roadmap Large-Scale Facility of the Netherlands (grant no. 184.032.207).

Author information

A.D.C. and B.M.W. conceived the research ideas and directed the overall project. A.D.C. designed and performed the experiments and analysed the data, in particular the bulk characterization, including the operando UV-vis diffuse reflectance spectroscopy measurements. K.H. collected and processed the NMR data. K.H. and M.B. analysed NMR spectroscopy data. G.T.W. prepared all extrudate materials. G.T.W. and S.C. characterized extrudate materials. A.D.C., K.H. and B.M.W. co-wrote the paper. All authors discussed the results and commented on the different versions of the manuscript.

Correspondence to Bert M. Weckhuysen.

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Supplementary Methods, Supplementary Figs. 1–14, Supplementary Tables 1–2, Supplementary References

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Chowdhury, A.D., Houben, K., Whiting, G.T. et al. Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates. Nat Catal 1, 23–31 (2018) doi:10.1038/s41929-017-0002-4

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