Structure–activity relationships on metal-oxides: alcohol dehydration

Kostestkyy, Pavlo; Yü, Jen; Gorte, Raymond J.; Mpourmpakis, Giannis

doi:10.1039/c4cy00632a

Cited by 113 publications

(160 citation statements)

References 51 publications

(71 reference statements)

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“…35−41 Kwak et al. 36 supported a two-step mechanism for the alkene formation, but Vlachos et al [37][38][39]41 and Kostestkyy et al 40 rather favored a concerted E2 mechanism ( Figure 2). Very few studies address the formation of ether, the selectivity issue, the potential coexistence of active sites on different facets, and compare experimental and DFT results.…”

Section: Introductionmentioning

confidence: 90%

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

et al. 2015

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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.

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Section: Introductionmentioning

confidence: 90%

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

et al. 2015

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“…Metal oxides are active alcohol dehydration catalysts [3][4][5][6] . In general, removal of oxygen from biomass increases energy density and the overall value of the products, making alcohol dehydration one of the most important reactions in the conversion of biomass to fuels and chemicals.…”

Section: Introductionmentioning

confidence: 99%

Structure-activity relationships in the production of olefins from alcohols and ethers: a first-principles theoretical study

Kostetskyy

Mpourmpakis

2015

Catal. Sci. Technol.

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Olefin formation pathways on Lewis acid (LA) sites of Al 2 O 3 , Ga 2 O 3 and In 2 O 3 and Gallium-and Indiumdoped alumina were investigated using Density Functional Theory (DFT) calculations. We considered two different olefin formation pathways, through alcohol dehydration and ether decomposition via both E1 and E2 type of elimination mechanisms. Both pathways evolve through a concerted E2 type of mechanism. Our results indicate that alumina is most active in alcohol dehydration reactions and the presence of dopants does not improve the catalytic activity. Correlations between the calculated energy barriers and the physicochemical properties of the oxides were identified and a dehydration model was constructed. The model accounts for surface acidity, base-site strength and the alcohol carbenium ion stability (CIS). We demonstrate a strong correlation between the ether decomposition and the alcohol dehydration barriers allowing us to extend the dehydration model predictions to ether decomposition chemistry.

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“…All of these observations are important in the development of structure−activity relationships in biomass conversion 15 …”

Section: ■ Computational Methodsmentioning

confidence: 95%

Understanding the Importance of Carbenium Ions in the Conversion of Biomass-Derived Alcohols with First-Principles Calculations

2015

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Dehydration reactions play a key role in the conversion of biomass derivatives to valuable chemicals, such as alcohols to alkenes. Both Lewis and Brønsted acid-catalyzed dehydration reactions of biomass-derived alcohols involve transition states with carbenium ion characteristics. In this work, we employed high-level ab initio theoretical methods to investigate the effect of molecular structure on the physicochemical properties of a set of alcohols that appear to control dehydration chemistry. Specifically, we calculated the carbenium ion stability (CIS, alkene-binding H + ) and proton affinity (PA, alcohol-binding H + ) of various C2−C8 alcohols to show the effect of alcohol size and degree of primary heteroatom substitution on the properties of the reactive species. Our results show a strong linear correlation between CIS and PA, following the substitution order of the reacting alcohols (i.e., primary < secondary < tertiary). Additionally, the calculated binding free energy (BE) of water on the formed carbenium ions was found to be exothermic and to decrease in magnitude with increasing alcohol substitution level. We demonstrate that the CIS and/or the PA are excellent structural descriptors for the alcohols and, most importantly, they can serve as reactivity descriptors to screen a large number of alcohols in the conversion of biomass-based alcohols involving the formation of carbenium ions. We demonstrate this concept in both Lewis and Brønsted acid-catalyzed dehydration reactions. ■ INTRODUCTIONGlobal primary energy consumption, including commercial renewable energy, increased by 5.6% in 2010, reaching its highest value since 1973. China accounted for 20.3% of the total global energy consumption, followed closely by the United States at 19%. 1 The depleting fossil fuel resources and increasing pollution and global warming concerns require the utilization of alternative and sustainable methods for the production of energy and chemicals. 2 Biomass is an abundant and inexpensive resource with a worldwide production of 560 billion tons of carbon. 3 Biomass-derived energy represents ∼14% of the world's primary energy supply with 25% usage in developed and 75% usage in developing countries. The total sustainable worldwide biomass energy potential is ∼2.47 × 10 13 kJ/m 2 , corresponding to a third of the current total global energy consumption. 4 The abundant quantity of biomass and its sustainable nature make it a plausible alternative source of energy and chemicals production (ethanol, lactic acid, acetone, etc.).Glucose and fructose (major sugars), polyols, and simpler alcohols can be derived from cellulosic biomass processing and further converted into valuable chemicals. For instance, glucose can be reduced to sorbitol, which in turn can be converted to simpler alkanes such as hexane and used as a fuel, through a series of catalyzed dehydration and hydrogenation reactions. 5 Several processes currently exist to convert carbohydrates to liquid fuels. These include (among others) the formation of bio-oils by liq...

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Structure–activity relationships on metal-oxides: alcohol dehydration

Cited by 113 publications

References 51 publications

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

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

Structure-activity relationships in the production of olefins from alcohols and ethers: a first-principles theoretical study

Understanding the Importance of Carbenium Ions in the Conversion of Biomass-Derived Alcohols with First-Principles Calculations

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