Combining Theoretical and Experimental Methods to Probe Confinement within Microporous Solid Acid Catalysts for Alcohol Dehydration

Abstract

Catalytic transformations play a vital role in the implementation of chemical technologies, particularly as society shifts from fossil-fuel-based feedstocks to more renewable bio-based systems. The dehydration of short-chain alcohols using solid acid catalysts is of great interest for the fuel, polymer, and pharmaceutical industries. Microporous frameworks, such as aluminophosphates, are well-suited to such processes, as their framework channels and pores are a similar size to the small alcohols considered, with many different topologies to consider. However, the framework and acid site strength are typically linked, making it challenging to study just one of these factors. In this work, we compare two different silicon-doped aluminophosphates, SAPO-34 and SAPO-5, for alcohol dehydration with the aim of decoupling the influence of acid site strength and the influence of confinement, both of which are key factors in nanoporous catalysis. By varying the alcohol size from ethanol, 1-propanol, and 2-propanol, the acid sites are constant, while the confinement is altered. The experimental catalytic dehydration results reveal that the small-pore SAPO-34 behaves differently to the larger-pore SAPO-5. The former primarily forms alkenes, while the latter favors ether formation. Combining our catalytic findings with density functional theory investigations suggests that the formation of surface alkoxy species plays a pivotal role in the reaction pathway, but the exact energy barriers are strongly influenced by pore structure. To provide a holistic view of the reaction, our work is complemented with molecular dynamics simulations to explore how the diffusion of different species plays a key role in product selectivity, specifically focusing on the role of ether mobility in influencing the reaction mechanism. We conclude that confinement plays a significant role in molecular diffusion and the reaction mechanism translating to notable catalytic differences between the molecules, providing valuable information for future catalyst design.

2-Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

“…Our results fall short of resolving the influence of intraparticle mass transport through the pores of UiO-66 influencing the rate and pathways of the catalytic reaction. In a recent report, Potter et al . interrogated pore size and confinement effects in the dehydration of a series of alcohols of varying sizes on aluminophosphates (AIPOs) with varying pore sizes.…”

Disordered medium-pore zeolite PST-24 as an efficient low-temperature isobutanol dehydration catalyst

“…A wide variety of solid acids, including amorphous silica-alumina, mixed-metal oxides and hydroxyapatites, have been investigated for alcohol dehydration so far. 5–9 Owing to its cost-effective availability, for example, γ-alumina has been extensively studied for isobutanol dehydration. 10–12 However, a significant decrease in dehydration activity by water is the serious drawback of this catalyst, despite its high selectivity toward isobutene: 12 at 285 °C, isobutanol conversion decreased from 53–61 to 28–42% upon co-feeding of 15 wt% water.…”

In-Depth Understanding of Highly Active Silicotungstic Acid Catalysts for Ethanol Dehydration to Ethylene under Industrially Favorable Conditions

“…Various acid catalysts, including oxides, − zeolites, − and heteropolyacid (HPA), − are active for the ETE process. Among these catalysts, the alumina-based catalysts are predominantly used in industrial ethanol dehydration to ethylene, operated at 300–500 °C under atmospheric pressure.…”