Cascade transformations of (±)-citronellal to menthol over extruded Ru-MCM-41 catalysts in a continuous reactor

Cascade transformations of (±)-citronellal in a continuous mode were investigated over a bifunctional shaped ruthenium catalyst bearing metal clusters of the size 7–13 nm. Four types of Ru/H-MCM-41 extrudates (1.5 × 10 mm) containing 30% of Bindzil-50/80 colloidal silica binder were prepared varying in metal location and metal-to-acid ratio, while the concentration of Brønsted and Lewis acid sites and textural properties of the final extrudates were comparable. Catalytic tests were performed in the trickle-bed reactor under 70 °C, 10 bar of H2, and the initial reactant concentration in cyclohexane 0.086 mol L−1 for the liquid residence time of 12.5 min. As a reactant, isopulegol, citronellol or (±)-citronellal was used. Metal location in extrudates has a significant effect on the catalytic activity and selectivity especially in terms of isopulegol isomers which content correlated with the metal-to-acid site ratio. Stereoselectivity to the desired (±)-menthol was 68–70%. The highest amount of the desired menthol, 32% yield, was obtained over extrudates where Ru was deposited on the catalytic support, i.e. with the shortest distance between the acid and metal sites, the lowest Brønsted acidity, the lowest Brønsted–Lewis acid sites ratio, the highest specific surface area and the narrowest range of the Ru particle size distribution.</p

Supported silver nanoparticles as catalysts for liquid-phase betulin oxidation

The research is funded from the Russian Science Foundation project No. 18-73-00019, Tomsk Polytechnic University Competitiveness Enhancement Program, project VIU-ISHBMT-197/2020 and Tomsk Polytechnic University State Task "Science", project FSWW-2020-0011 (Russia). This work was partially supported by Fundacao para a Ciencia e a Tecnologia, Portugal, through project UIDB/00100/2020 of the Centro de Quimica Estrutural, by national funds though FCT, under the Scientific Employment Stimulus-Institutional Call (CEECINST/00102/2018) and the Associate Laboratory for Green Chemistry-LAQV financed by national funds from FCT/MCTES (UIDB/50006/2020 and UIDP/50006/2020).Herein, it has been shown that betulin can be transformed into its biologically active oxo-derivatives (betulone, betulinic and betulonic aldehydes) by liquid-phase oxidation over supported silver catalysts under mild conditions. In order to identify the main factors determining the catalytic behavior of nanosilver catalysts in betulin oxidation, silver was deposited on various alumina supports (γ-alumina and boehmite) using deposition–precipitation with NaOH and incipient wetness impregnation methods, followed by treatment in H2 or O2. Silver catalysts and the corresponding supports were characterized by X-ray diffraction, nitrogen physisorption, inductively coupled plasma optical emission spectroscopy, photoelectron spectroscopy and transmission electron microscopy. It was found that the support nature, preparation and treatment methods predetermine not only the average Ag nanoparticles size and their distribution, but also the selectivity of betulin oxidation, and thereby, the catalytic behavior of Ag catalysts. In fact, the support nature had the most considerable effect. Betulin conversion, depending on the support, increased in the following order: Ag/boehmite < Ag/boehmite (calcined) < Ag/γ-alumina. However, in the same order, the share of side reactions catalyzed by strong Lewis acid centers of the support also increased. Poisoning of the latter by NaOH during catalysts preparation can reduce side reactions. Additionally, it was revealed that the betulin oxidation catalyzed by nanosilver catalysts is a structure-sensitive reaction.publishersversionpublishe

Transformations of citral over bifunctional Ru-H-Y-80 extrudates in a continuous reactor

One-pot transformations of citral were investigated over Ru-catalysts in a batch and a continuous mode over a powder catalyst and extrudates, respectively. Ru/H-Y-80 catalysts were prepared by different impregnation methods to obtain different particle sizes of Ru.The highest yield of isopulegols was 15% and yield of menthols was 3% with stereoselectivity to the desired (±)-menthol isomer of 42% after 5 h over the Ru/H-Y-80 powder catalyst with the smallest metal particles prepared by the incipient wetness impregnation method with six impregnation steps using Ru(NO)(NO3)3 as a precursor. This catalyst was synthesized with the Bindzil binder and shaped by extrusion.For comparison, also a catalyst with Ru deposited on the binder was prepared. Addition of 30 wt% Bindzil binder to Ru/H-Y-80 catalyst caused significant catalyst deactivation. Z/E-citral ratio decreased with increasing Ru particle size. Citral conversion and the yield of acyclic hydrogenation products were higher over the powder catalysts containing a mixture of zeolite and binder with a larger distance between the metal and the acid sites, i.e. with Ru deposited on the Bindzil binder. Long-term experiments in the continuous mode revealed comparable catalytic behaviour of both Ru-extrudates with the controlled metal location due to the presence of diffusion regime. The maximum yield of menthols, 6%, with stereoselectivity to the desired (±)-menthol of 73% was obtained during the transient state, below 2 h of time-on-stream. The reuse of Ru/H-Y-80 extrudates was successfully demonstrated.</p

Diffusion measurements of hydrocarbons in H-MCM-41 extrudates with pulsed-field gradient nuclear magnetic resonance spectroscopy

Mesoporous materials are promising catalysts for production of biofuels. Herein, H-MCM-41 catalysts with different concentrations of the silica Bindzil binder (10-50 wt%) were prepared and characterized using pulsed-field gradient (PFG) NMR in the powder form and as extrudates. Effective diffusion coefficients (D-e) are measured in all cases. Diffusivities of n-hexadecane were found smaller for extrudates as compared to the powder catalysts. The estimates of diffusive tortuosity were also determined. PFG NMR data showed one major component that reveals diffusion in interconnected meso- and micropores and one other minor component (1-2%) that may correspond to more isolated pores or may represent complex effects of restricted diffusion. Therefore, several approaches including initial slope analysis of spin-echo attenuation curves, two-component fitting and Laplace inversion were used to discuss different aspects of diffusional transport in the studied H-MCM-41 materials. Correlations between D-e and the amount of Bindzil, the specific surface area, the micropore volume, the particle size, the total acid sites and the Lewis acid sites are discussed

Continuous synthesis of menthol from citronellal and citral over Ni-beta-zeolite-sepiolite composite catalyst

One-pot continuous synthesis of menthols both from citronellal and citral was investigated over 5 wt% Ni supported on H-Beta-38-sepiolite composite catalyst at 60–70 °C under 10–29 bar hydrogen pressure. A relatively high menthols yield of 53% and 49% and stereoselectivity to menthol of 71–76% and 72–74% were obtained from citronellal and citral respectively at the contact time 4.2 min, 70 °C and 20 bar. Citral conversion noticeably decreased with time-on-stream under 10 and 15 bar of hydrogen pressure accompanied by accumulation of citronellal, the primary hydrogenation product of citral, practically not affecting selectivity to menthol. A substantial amount of defuctionalization products observed during citral conversion, especially at the beginning of the reaction (ca. 1 h), indicated that all intermediates could contribute to formation of menthanes. Ni/H-Beta-38-sepiolite composite material prepared by extrusion was characterized by TEM, SEM, XPS, XRD, ICP-OES, N2 physisorption and FTIR techniques to</p

Catalytic conversion of glucose to methyl levulinate over metal-modified Beta zeolites

Methyl levulinate was selectively formed from glucose and methanol over a copper modified Beta zeolite bifunctional catalyst at 180 degrees C under argon atmosphere. The selectivity to methyl levulinate substantially exceeded previously reported in the open literature results. The copper modification was done through an ion-exchange method using a solution of copper nitrate, followed by drying and calcination of the catalyst. Copper modification changed the distribution of acid sites namely, less Bronsted and more Lewis sites were observed with FTIR using pyridine adsorption. Application of the proton form H-Beta-25 gave the methyl levulinate yield of ca. 89%, which could be elevated with the addition of copper, as the apparent selectivity exceeds 99%, assuming that methyl glucosides are eventually transformed to methyl levulinate. The non-acidic Cu/SiO2 catalyst was completely inactive in methyl levulinate formation. Metal modification of Beta zeolite with Sn and Zn did not perform as well as Cu in the formation of methyl levulinate during glucose transformation

One-Pot Synthesis of Menthol Starting from Citral over Ni Supported on Attapulgite-H-Beta-38 Extrudates in a Continuous Flow: Effect of Metal Location

The one-pot synthesis of menthol from citral in a continuous mode was investigated over three different types of extrudates, namely Ni supported on H-Beta-38 and extruded with attapulgite, Ni supported on attapulgite and extruded with H-Beta 38, and the extrudates, which were shaped after deposition of Ni on both H-Beta-38 and attapulgite in the powder form. These composite catalysts were characterized by X-ray diffraction, transmission and scanning electron microscopy, temperature-programmed reduction of H-2, mechanical strength, and pyridine adsorption desorption. These three types of extrudates exhibited different metal particle sizes and acidity. The most promising catalyst in the one-pot menthol synthesis was Ni supported first on H-Beta-38 and then extruded with attapulgite giving a 45% yield of menthols in a trickle bed reactor. This catalyst exhibited 10 nm nickel particles and a rather high amount of strong Bronsted acid sites

Citral-to-Menthol Transformations in a Continuous Reactor over Ni/Mesoporous Aluminosilicate Extrudates Containing a Sepiolite Clay Binder

One-pot continuous synthesis of menthols from citral was performed over 5 wt % Ni supported on a mesoporous aluminosilicate catalyst with sepiolite as a binder at 70 degrees C with a selectivity of 75% to menthols. Catalyst deactivation with time-onstream resulted in a decrease of the conversion and selectivity to menthols at the expense of higher selectivity to isopulegols. Stereoselectivity to isopulegols and menthols only slightly changed with conversion and TOS. A kinetic model capable of describing experimental data for transformations of citral to menthol in a continuous mode was developed. It was based on a detailed reaction network and also comprised deactivation on both metal and acid sites. Numerical data fitting confirmed a good correspondence between the experimental data and calculations