Propan-2-ol dehydration on H-ZSM-5 and H-Y zeolite: a DFT study

Abstract

The catalytic dehydration of propan-2-ol over H-Y and H-ZMS-5 aluminated\ud zeolite models, mimicking both internal cavities and external surfaces, was\ud studied by DFT calculations to investigate the reaction mechanism. After the adsorption of propan-2-ol on the zeolite, the dehydration mechanism starts with alcohol protonation, occurring by one acidic –OH group of the zeolite fragment, followed by a concerted b-elimination to give propene. The catalytic activity is\ud affected by the size of the zeolite cavity, which is larger in the H-Y than in the H-ZMS-5 zeolite. The adsorption energy of the reagent, as an example, decreases in the order: H-Y cavity = H-ZMS-5 > surface H-ZMS-5 cavity, pointing that the adsorption process should preferentially occur either on open surface or inside larger cavity. More interestingly, confinement effects play a twofold role in driving the reaction pathway, resulting in two different effects on the reaction outcomes. The thermodynamic stability, evaluated by the standard free energy difference of\ud the products (water and propene) with respect to the reactant (propan-2-ol), would indeed suggest that the reaction more smoothly could occur for the systems: H-ZMS-5 surface > non-catalyzed > H-Y cavity > H-ZMS-5 cavity. The activation standard free energy of the process conversely decreases in the order: non-catalyzed > H-ZMS-5 surface > H-ZMS-5 cavity > H-Y cavity, suggesting that the reaction is faster inside zeolite cavities. Experimental and computational results are in agreement, giving confidence on the atomistic-level insights provided

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

“…Some structures of purely silicic surfaces were proposed for BEC, LTL, and LTA [22] (with Si/Al=∞ or 1, Na or Ca form, for the latter), for the sake of comparison by using high‐resolution electron microscopy. Cluster models of the external surface of ZSM‐5 were used recently for reactivity purposes (butene isomerization or propanol dehydration) but again without any systematic investigation of the location of protons . Al−OH and Si−OH were invoked at the surface, the latter of which had a higher reactivity for butene isomerization .…”

“…Al−OH and Si−OH were invoked at the surface, the latter of which had a higher reactivity for butene isomerization . For propanol dehydration, only surface bridging OH groups were considered, which had enhanced adsorption properties but hindered reactivity compared to that of the bulk.…”

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

“…Hence, one cannot disregard the possibility of an alternative reaction mechanism for the latter case. A recent DFT study by Prestianni et al [25] indicated a concerted E2 b-elimination mechanism for the dehydration of 2-propanol on H-ZSM-5 and H-Y zeolite.…”

On the Acid Sites of Phosphorous Modified Zeosils

“…Alcohol dehydration has been extensively used as a probe reaction to characterize acidity in zeotype materials, 6,[35][36][37][38] and recent computational as well as experimental studies have led to detailed understanding of underlying reaction mechanisms and pathways of light (C1-C4) alcohol dehydration on solid acid catalysts. 37,[39][40][41][42][43][44][45][46][47] The ubiquitous themes tying these studies together are: (i) the evidence for the coupling of unimolecular and bimolecular dehydration pathways, where adsorbed dimers that produce di-alkyl ethers (bimolecular dehydration product) can also contribute to olefin formation (unimolecular dehydration product), 43,44 (ii) inhibition of olefin formation rates at high alcohol partial pressures (> 50 torr) due to the higher stability of adsorbed alcohol dimers relative to monomers, 37,39 and (iii) an increased preference to unimolecular dehydration with increasing temperatures. 37,39,43,44 The favorability of E1/E2 elimination pathways during alcohol dehydration on zeolites depends on the stability of carbenium formed during the prominence of E2 type pathways as the RDS for isopropanol (a secondary alcohol) dehydration in aluminosilicates MFI, 41,48 and FAU, 41 while primary alcohol dehydration primarily occurs through E1-type elimination pathways.…”