Efterpi S. Vasiliadou

One-step propylene formation from bio-glycerol over molybdena-based catalysts

This work presents a novel, one-step catalytic process, enabling highly selective propylene formation via glycerol hydro-deoxygenation (HDO) reactions. Fe–Mo catalysts, supported on black and activated carbons, are selective towards C–O bond cleavage, thus converting glycerol to propylene with high yields. BET, XRD, TPD-NH3 and TPD-He methods have been employed for the characterization of the samples. Molybdenum oxide, at its reduced state, is essential for driving selectively the reaction towards complete deoxygenation. The only product of glycerol HDO is propene, in the gas phase, while 2-propenol, propanols and propylene glycol have been detected, among others, in the liquid phase. Under the standard reaction conditions (300 °C temperature, 8.0 MPa hydrogen pressure), glycerol conversion exceeds 88% and selectivity to propene reaches 76% after 6 hours of reaction. This study includes the investigation of the operating conditions effect (i.e. reaction time, reaction temperature, catalyst loading and H2 pressure) regarding glycerol HDO towards propene formation.

Glycerol hydro-deoxygenation aided by in situ H2 generation via methanol aqueous phase reforming over a Cu–ZnO–Al2O3 catalyst

A tandem catalytic cycle of methanol aqueous phase reforming–glycerol hydro-deoxygenation targeted to 1,2-propanediol formation under inert conditions is investigated. The H2 needed for glycerol hydro-deoxygenation is provided in situ via methanol reforming. The effects of reaction time, temperature, methanol concentration and system pressure were investigated over a Cu : Zn : Al bulk catalyst. The catalytic results showed that 1,2-propanediol selectivity and yield depend on reaction temperature and reaction time combination. Higher methanol concentrations favor glycerol hydro-deoxygenation towards the desired pathway, resulting in a significant increase in 1,2-propanediol selectivity. Under optimum reaction conditions (t = 1 h, T = 250 °C, 36 v/v% CMeOH + 9 v/v% CGLY, 1.0 < PN2 < 3.5 MPa), glycerol was almost fully converted (95.9%), with 79.4% selectivity (76.2% maximum yield) to 1,2-propanediol. Upon catalyst reuse, the Cu : Zn : Al catalyst showed satisfactory stability. An initial loss of activity (35.8%) was observed, which was ascribed to Cu agglomeration; however, catalyst performance was improved and stabilized after the third run, possibly due to Cu re-dispersion. It is proposed that metallic Cu0 efficiently catalyzes glycerol hydro-deoxygenation, while methanol reforming is mainly catalyzed by metallic Cu0 and facilitated by the interaction of Cu0 with ZnO–Al2O3 structures.

Exploring the reaction pathways of bio-glycerol hydro-deoxygenation to propene over Molybdena-based catalysts

The one-step reaction of glycerol with hydrogen to form propene selectively is a particularly challenging catalytic pathway that has not yet been explored thoroughly. Molybdena-based catalysts are active and selective to C-O bond scission; propene is the only product in the gas phase under the standard reaction conditions, and further hydrogenation to propane is impeded. Within this context, this work focuses on the exploration of the reaction pathways and the investigation of various parameters that affect the catalytic performance, such as the role of hydrogen on the product distribution and the effect of the catalyst pretreatment step. Under a hydrogen atmosphere, propene is produced primarily via 2-propenol, whereas under an inert atmosphere propanal and glycerol dissociation products are formed mainly. The reaction most likely proceeds through a reverse Mars-van Krevelen mechanism as partially reduced Mo species drive the reaction to the formation of the desired product.

Zeolite‐Catalyzed Formaldehyde–Propylene Prins Condensation

Prins condensation of formaldehyde with propylene to form 3‐buten‐1‐ol is investigated using microporous solid acid catalysts. Zn/H‐beta shows high conversion but leads to a broad product distribution composed primarily of pyrans. Mechanistic studies revealed that 3‐buten‐1‐ol reacts via Prins cyclization or dehydrate to 1,3‐butadiene that further reacts with formaldehyde via a hetero‐Diels–Alder reaction. These secondary reactions are suppressed over ZSM‐5 catalysts: 3‐buten‐1‐ol is the predominant product over H‐ZSM‐5 zeolite under all conditions investigated. 3‐Buten‐1‐ol selectivity of up to 75 % is achieved. In a second step 3‐buten‐1‐ol dehydrates at temperatures as low as 423 K, forming 1,3‐butadiene. Although Brønsted acid sites are the primary catalytic sites, ion exchange of ZnII increases the overall rate and 3‐buten‐1‐ol selectivity. H‐ZSM‐5 showed significant differences in reactivity and selectivity as a function of the Si/Al ratio; optimal catalytic properties were observed within Si/Al=40–140.

Formaldehyde-isobutene Prins condensation over MFI-type zeolites

The liquid phase Prins condensation of formaldehyde with butenes on H-ZSM-5 (MFI) zeolite catalysts was investigated showing that reaction rates follow the order isobutene > 1-butene > cis-2-butene. The catalytic rates for medium-pore zeolite H-ZSM-5 were not only a larger than for H-beta, but also selective for 3-methyl-3-buten-1-ol during formaldehyde condensation with isobutene. Isoprene forms, under the optimal reaction conditions, either in a second dehydration step of the unsaturated alcohol, or in a single step over H-ZSM-5 with optimal Si/Al ratio (Si/Al = 40). Mechanistic studies revealed that sequential Prins cyclization and hetero-Diels–Alder reactions of the desired products, forming six carbon species, occur only at slow rates over H-ZSM-5. Using DFT methods it was determined that the reaction follows a two-step mechanism: protonation of formaldehyde and electrophilic attack to the alkene, and deprotonation of the resulting intermediate carbocation. For isobutene and 1-butene, the electrophilic addition is coordinated with the protonation of the formyl group, but for cis-2-butene, it is coordinated with the proton back-donation step. The rate-limiting step switches from electrophilic addition for isobutene to proton elimination for the other two C4 isomers.

Selective hydrodeoxygenation of tartaric acid to succinic acid

A novel one-step process for catalytic production of succinic acid from tartaric acid, which is largely available in the waste streams of wine making, is developed. A liquid-phase system comprised of a molybdenum oxide catalyst supported on carbon black (MoOx/BC) and hydrobromic acid in acetic acid under a H2 atmosphere is effective for selective cleavage of the C–O bonds of tartaric acid and selective hydrogenation of the resulting unsaturated carbons. Temperature, hydrogen pressure, and catalyst pre-treatment are optimized to understand their impact on catalytic activity and product distribution, leading to an 87% yield of succinic acid at 170 °C. Importantly, reduction of the carboxyl groups and C–C bond cleavage are retarded. Pre-reduction and characterization studies (TPR, XRD, XPS, and XAS) reveal that the high catalyst activity of MoOx/BC is correlated with the lower Mo oxidation states (+4 to 0) formed during pre-reduction that promote cleavage of the C–O bonds of tartaric acid and hydrogenation of the CC bond of the intermediate fumaric acid. Recyclability studies and structural characterization of the recovered catalyst indicate that MoOx/BC remains active upon reuse.