Detection of Carbocationic Species in Zeolites: Large Crystals Pave the Way

Large zeolite crystals have been used as model systems for the investigation of diffusion and catalytic reactivity phenomena in microporous host materials for at least two decades. However, their potential in assisting the detection of elusive reactive intermediates appears to have been underestimated. Herein, we show that a complementary use of vibrational and optical spectroscopy in combination with theoretical calculations allows for the unambiguous identification of transient carbocationic species generated upon the acid-catalyzed oligomerization of styrene derivatives within zeolite H-ZSM-5. Thanks to the mediated diffusion of the reactant in the large H-ZSM-5 crystals and minimal external surface the reaction intermediates can be accumulated within zeolite micropores in sufficient concentrations to allow their detection by synchrotron-based IR microspectroscopy. The UV/Vis and IR spectra display strong polarization dependence of on the molecular alignment of the dimeric styrene carbocations imposed by the zeolite channels and cages that can be rationalized in terms of the electronic and vibrational transitions of the intrazeolite carbocations. Based on these findings, a molecular-level picture of the macroscopic arrangement of the reaction intermediates confined within microporous zeolite matrices can be devised.