Persistent methylbenzenium ions in protonated zeolites: The required proton affinity of the guest hydrocarbon

Protonated zeolites are microporous solid Brönsted acids with widespread applications as catalysts in hydrocarbon conversion reactions. The beta zeolite is a wide-pore zeolite, which is particularly promising as catalyst in aromatic-based processes, such as alkylations and transalkylations. Carbocations are likely intermediates in numerous zeolite-mediated hydrocarbon reactions, and the capability of acidic zeolites to generate and stabilize such species is thus an extensively debated aspect of heterogeneous catalysis. Ten to fifteen years ago, zeolites were still believed to possess super acidity, and it was assumed that simple cationic species, such as isopropyl- or tert-butyl cations, were persistent in their cavities. Yet, all attempts to verify the long-lived existence of such simple alkyl cations in zeolites have failed. As a result of an extensive research effort, mainly based on NMR, zeolites were reclassified from super acids to acids that are slightly weaker than 100 % sulfuric acid. Carbenium ions that are persistent in zeolites are still rarely reported, and less than ten species have been identified. It is now a high-priority issue to accurately establish a basicity limit to predict which hydrocarbons may exist as carbenium ions within protonated zeolites. Recently, we reported the formation of the hexamethylbenzenium ion (H-hexaMB+) from hexamethylbenzene (hexaMB) 1 (Scheme 1) in the H-beta zeolite. Until now, hexaMB has the lowest proton affinity (PA) value (860.6 kJ mol-1, 859 kJ mol-1 at the G3(MP2) level) observed for a base that is stable, in its cationic form, in a zeolite. Ultraviolet diffuse reflectance (DRUV)/Vis and Fourier transform infrared (FTIR) spectroscopy proved to be well-suited techniques for verifying the existence of H-hexaMB+ in H-beta. It is known that the protonation of benzene requires an acidity, which should be orders of magnitude stronger than that of 100 % sulfuric acid. The PA of species that may be protonated by zeolites must accordingly be between the values of benzene and hexaMB. Compared to hexaMB, pentaMB 2 (PA=850.7 kJ mol-1, 845 kJ mol-1 in position 6 at the G3(MP2) level) possesses a lower molecular symmetry, and the vibrational ring modes of the neutral species give IR absorptions in the region around 1600 cm-1, which are diagnostic for the corresponding carbenium ion. Thus, FTIR could not unambiguously verify the existence of protonated pentaMB, (H-pentaMB+). DRUV/Vis experiments did, however, suggest that H-pentaMB+ is stable in the cavities of H-beta zeolite. Contrary to pentaMB, the protonation of 1,2,4,5-tetraMB (durene) 3 (G3(MP2) PA=818 kJ mol-1 in position 3), a representative for the next lower polymethylbenzene homologue, will be accompanied by symmetry changes that lead to new distinct vibrational modes. Herein, we present clear results - not only from FTIR but also from DRUV/Vis - which show that protonated 1,2,4,5-tetraMB (H-tetraMB+) is formed in the micropores of the H-beta zeolite. Moreover, we show that the less basic triMB homologue, 1,2,3-triMB 4 (G3(MP2) PA=813 kJ mol-1 in position 4) does not show appreciable protonation in H-beta. Based on the present results, we can therefore estimate a proton affinity region for hydrocarbons (methylbenzenes) able to form stable carbenium ions in zeolites with acidic properties similar to that of the H-beta zeolite. It should, however, be noticed that such a limit may be restricted to species that experience similar steric constraints within the hosting zeolite.