Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

Abstract

a b s t r a c tAmorphous silica-aluminas (ASA) are prominent solids for their acidic properties. They are of first interest in catalysis, in particular for cracking reactions. Their relative acidity as compared to zeolites is a long-lasting general issue, driven by the limited knowledge of the surface structure of ASA, due to their amorphous nature. In the present contribution, thanks to a first principles approach based on our original ASA surface model, and on the study of a model mordenite zeolite (MOR), we propose a rational explanation for this feature. We compare by density functional theory calculations the ability of ASA and MOR to generate carbenium species from isobutene, versus p-complexes and alkoxide species. On ASA, carbenium species can be formed on pseudo-bridging silanols (PBS) only and are much less stable than in MOR. Then, we investigate the cracking pathway of a model alkene, 2,4,4-trimethyl-2-pentene (DIB), by quantifying the stability of relevant intermediates of carbocationic and alkoxide natures. The carbocationic pathway is favored in MOR, whereas on ASA, this pathway is strongly activated. By contrast, the PBS on ASA initiate preferentially a tertiary alkoxide route, or possibly a combined carbocationic/alkoxy route. Finally, the i-butene desorption step is also limiting in MOR due to the confinement effect induced by the zeolite pores. As a result, the higher cracking reactivity of zeolites as compared to ASA is mainly attributed to the favored nature of the carbenium route, stabilized by higher electrostatic confinement effect.

Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta

“…Within the periodic DFT+D2 formalism, we aim here to compare the energy levels of the three species adsorbed on sites located in the bulk versus sites on the outer surface of the BEA zeolite. To provide a more general analysis, we extend the comparison to Brønsted sites located on amorphous silica‐alumina surfaces, in particular the pseudobridging silanol (PBS‐Al) site and a surface Al−(H 2 O) species, as performed in our previous work . According to our previous theoretical findings, PBS are silanol groups in which the O atom is close to an Al or Si atom, in such a way that they can form a new O−Al or O−Si bond once the proton is removed .…”

“… DFT+D2 energy profiles for the protonation of isobutene on ASA (PBS‐Al and Al‐(H 2 O), values taken from Ref. ) and Beta (bulk and surface sites). Δ r U is referenced to gas‐phase isobutene.…”

Isopropanol Dehydration on Amorphous Silica–Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo‐Bridging Silanols

“…Thel atter mechanism was ruled out as the activation energy was very high (220 kJ mol À1 ), and the carbenium intermediate was found to be highly unstable and to evolve into an alkoxide by binding to an eighboring oxygen atom from the surface.T he stability of carbocations on aluminosilicates is indeed strongly dependent on both the type of carbocation (stability:primary < secondary < tertiary) and the local electrostatic field that is found to be higher in the pores of zeolites than on ASA. [31,32] Thec arbocation generated here is secondary,and the planar ASA surface does not generate astrong enough electrostatic field (even weaker than on alumina, which already does not stabilize carbenium ions;T able S1) to stabilize it as alocal energy minimum. Note that the E1 mechanism was also found to be unfavorable in the case of isopropanol dehydration in HZSM-5.…”

Reaction path analysis for 1-butanol dehydration in H-ZSM-5 zeolite: Ab initio and microkinetic modeling

“…Dispersive corrections for the van der Waals interactions were included by adding a pairwise interaction term to the Kohn-Sham energy using the DFT-D2 approach proposed by Grimme [38] and extended by Kerber et al [39] for periodic PBE calculations. Although systematic deviations may be observed due to the overestimation of the dispersion interaction [40][41][42][43], DFT-D2 has been widely applied for the theoretical investigation of adsorption [44][45][46][47] and reaction in zeolites [47,48] and is known to provide reasonably accurate results [41,49]. The electronic charge on atoms and fragments was calculated using Bader analysis [50] as implemented by Henkelman et al [51] Transition state search was performed using Nudged Elastic Band (NEB) [52] and dimer [53,54] calculations.…”