Mechanistic Investigation of Isopropanol Conversion on Alumina Catalysts: Location of Active Sites for Alkene/Ether Production

Abstract

Alcohol dehydration is of prominent relevance in the context of biomass conversion. This reaction can be efficiently catalyzed by alumina surfaces, but the nature of active sites, the mechanisms involved, and the key parameters to tune both the activity and the alkene/ether selectivity remain a matter of debate. In the present paper, isopropanol dehydration to propene and diisopropylether over γ-alumina, δ-alumina, and sodium-poisoned γ-alumina was investigated through a combined experimental and theoretical study. The experimental kinetic study shows that dehydration occurs following the same reaction mechanism on all materials, although γ-alumina activated above 450°C exhibits the highest density of active sites and the highest global activity. Results suggest that all the reaction pathways involved in dehydration require the same set of adjacent active sites located on the (100) facets of γ-alumina. DFT transition-state calculations of the formation of propene and diisopropylether on the main terminations of alumina, (110) and (100), were also performed. The less activated pathways for both the formation of the olefin (E2 mechanism) and the formation of the ether (S N 2 mechanism) were found on a Al V Lewis acidic site of the (100) termination, with calculated activation enthalpies (125 and 112 kJ·mol −1 for propene and diisopropylether formation, respectively) in good agreement with the experimental values (128 and 118 kJ·mol −1 , respectively). The higher or lesser selectivity toward propene or ether appears to originate from significantly different activation entropies. The effect of coadsorbed sodium on the reaction is linked to the poisoning of Al sites by neighboring, Na-stabilized OH groups, but no influence of sodium on distant sites is evidenced. Reaction temperature is identified as the main key parameter to tune alkene/ ether selectivity rather than morphology effects, which in turn affect drastically the number of available active sites, and thus catalytic activity.

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

“…The ASA supercell contains about 220 atoms and required a 1×1×2 k‐points grid. Transition states were calculated using CI‐NEB, and thermodynamic corrections were performed from vibrational calculations …”

Hydrothermal Stability of Ru/SiO2–C: A Promising Catalyst for Biomass Processing through Liquid-Phase Reactions

“…Furthermore, it can be seen that the addition of ruthenium Both supports present very low isopropanol conversion (X IPA ), evidencing the low surface acidity of these supports. It can be seen that the propylene and acetone products are in similar proportions, indicating the presence of weak Lewis acid sites and strong Lewis basics sites [26]. Table 4 shows the result of characterization of ruthenium catalysts (fresh and after reaction).…”

Kinetics and mechanisms of alcohol dehydration pathways on alumina materials

“…The nearly five-fold variation (0.9-4.7 × 10 −4 mol g −1 ) in the amount of penta-coordinated Al 3+ ions on thermally treated γ-Al 2 O 3 samples at temperatures from 773 to 1373 K from the report by Hu et al 60 mentioned above supports this conclusion. Larmier et al 56 also suggested that the observed lower 2-propanol conversion rate on δ-Al 2 O 3 compared to γ-Al 2 O 3 at 473 K is a consequence of a different ratio of (100)/(110) surface facets. The results presented in Tables 1-3 show that α-Al 2 O 3 has a lower ethene formation rate constant normalized per surface area compared to γ-and η-Al 2 O 3 demonstrating a lower surface density of ethene formation sites on α-Al 2 O 3 compared to high-surface area alumina polymorphs tested in this report.…”

“…Knözinger and Scheglila 29 proposed an E2-type mechanism for mono-alcohol dehydration on γ-Al 2 O 3 involving concerted cleavage of the C β -H and C-OH bond and recent DFT calculations have shown lower barriers for an E2-type mechanism for Lewis-acid catalyzed mono-alcohol dehydration. 37,38,[40][41][42][43]54,56 Based on these reports, we consider an E2-type mechanism for ethene formation; however, we note that independent of the detailed mechanistic pathways, the rate expression shown in eqn (1) rigorously describes the measured kinetics of mono-ethanol dehydration at 573 K on α-, γ-, and η-Al 2 O 3 .…”

“…This observation led the authors to propose that penta-coordinated Al 3+ ions on the surface of alumina catalysts are the primary active sites for mono-ethanol dehydration. Larmier et al 56 had previously proposed penta-coordinated aluminum on the (100) surface facet of γ-Al 2 O 3 as the active site for 2-propanol dehydration by showing lower activation barriers for dehydration compared to those on tetra-coordinated aluminum on the (110) surface facet using computational chemistry. Based on an assessment of the temperature variation of the initial rate of propene synthesis and diisopropyl ether synthesis on γ-and δ-Al 2 O 3 at 433-483 K, the authors also reported activation barriers to be the same on the two alumina materials within experimental error and inferred that similar active sites exist on the two alumina materials.…”