Catalysis Today最新文献

Glycerol carbonate is a versatile molecule used as a fuel additive and a chemical building block. It is a prominent route for glycerol valorization, improving the biodiesel production chain. This base-catalyzed reaction can be carried out using sodium-type zeolites if accessibility to the basic sites is ensured. Herein, we report a facile strategy to fine-tune the number and access to basic sites of Na-LTA zeolite precursors via monitoring the crystallization kinetics of the microporous material. It is demonstrated that an optimal glycerol to glycerol carbonate conversion is obtained when using the solid obtained at the beginning of the crystallization (90 min at 100 °C of hydrothermal step). Textural and spectroscopy characterization reveals that semicrystalline material presents many basic aluminate anions in a very open structure, allowing facile access to glycerol and propylene carbonate. Moreover, it is shown that this catalyst is stable in four successive reaction cycles and active in different reactions conditions

The escalating increase in global temperatures has highlighted hydrogen (H₂) as a promising alternative clean energy carrier. Hydrogen can be derived from biofuels such as bioethanol, offering an efficient liquid carrier that addresses availability, storage, and distribution issues hindering a widespread use of H₂. This study reports on the synthesis of La_0.4Sr_0.4Ti_0.8Ni_0.2O_3-δ catalysts followed by the exsolution process to enhance catalytic activity for H₂ production via ethanol steam reforming (ESR) reaction. Single-phase compounds with exsolved metallic nanoparticles were successfully tested for ESR, revealing an ethanol conversion rate of 40 % and over 40 % H₂ production for the catalyst reduced at 1000 °C for 12 hours. Stability tests demonstrated the catalyst's capacity for regeneration in both water vapor and N₂. The experimental data demonstrate that exsolving metallic nanoparticles is a viable strategy for producing a stable catalyst for the ESR reaction.

New formulations of unsupported tetrametallic catalysts derived from layered double hydroxides (LDH) compounds were synthesized by a constant pH coprecipitation method, characterized, and evaluated in the selective hydrodesulfurization (HDS) of thiophene in the presence of cyclohexene. The layered double hydroxides were prepared using terephthalate (C₈H₈O₂)^2- as the interlayer anion with nominal aluminum molar fraction of 0.5, and nominal cobalt to M^II (M = Zn or Mn) ratios of 0.2, 0.4 and 0.6. Molybdenum introduction in the LDH precursor was accomplished by an ion exchange process using the heptamolybdate anion. The calcined Mn samples showed a higher surface area than those obtained from Zn. The sulfidation was carried out in situ with a heating ramp up to 673 K and the catalysts were evaluated in a tubular flow reactor. The catalytic evaluation of Zn and Mn-based materials for simultaneous thiophene HDS and cyclohexene hydrogenation (HYD) was conducted at 573 K and 20 bar. The results show that the Zn-based unsupported catalysts synthesized here exhibited higher selectivity for the HDS reaction than an industrial one.

Dinuclear chromium(III) complexes [Cr(Z)-2-(HCN)-2-OCH₃C₆H₄)(THF)Cl₂]₂ (Z= C₄H₃N, L1-Cr₂) or [Cr(Z)-2-(HCN)-2-OCH₃C₆H₄)(THF)Cl₃]₂ (Z = C₄H₃S, L2-Cr₂; Z = 5-methyl-C₄H₂S, L3-Cr₂; Z = 5-phenyl-C₄H₂S, L4-Cr₂) were synthesized by reaction of the phenyl-bridged bifunctional imine-pyrrole/thiophene ligands with [CrCl₃(THF)₃]. DFT calculations for L2-Cr₂ revealed that both isomers (anti and syn) are stable presumably due to the nonrigid skeleton of this class of ligands. On activation with MAO, all chromium complexes showed catalytic activity in ethylene oligomerization, providing a non-selective distribution of α-olefins (C₄-C₁₂⁺) together with varying amounts of polymer (3.6 – 45.0 wt%). The nature of the pendant heterocyclic moieties plays a substantial influence on the catalytic performances of these binuclear Cr catalysts. L1-Cr₂ bearing pyrrolide unit yielded a significant quantity of polymer (45.0 wt%) while the binuclear chromium complexes based on thiophene unit showed higher productivities towards production of oligomers (up to 94.4 wt%). These latter Cr complexes showed higher productivities in comparison to their mononuclear analogues. DFT calculations, assuming the formation of cationic chromium (III) species after treatment with MAO in ethylene atmosphere, suggested the coordination of the thiophene group to the chromium metal center, and thus influencing the catalytic performance of L2Cr₂ – L4-Cr₂. By adjusting the reaction conditions, L3-Cr₂ showed TOF of 131,100 mol ethylene·mol Cr⁻¹·h⁻¹ producing predominantly oligomers (89.3 wt% of total products) with high selectivity for α-olefins (>91.5 wt%).

In refineries, one of the most important production units is Fluid Catalytic Cracking (FCC). In them, the incorporation of additives to the catalyst is a very interesting option to reduce the sulfur content of gasoline, and thus obtain commercial fuel specifications. In this study, the transformation of thiophene into a cyclohexane (HC) stream for its in-situ desulfurization has been investigated, using catalysts based on niobium or zinc (6 wt%) incorporated into three types of zeolites: HUSY, HBeta and HZSM-5. Catalytic tests were carried out at 773 K in an automatically controlled fixed-bed tubular reactor. The reaction products were analyzed online by gas chromatography, with FID detector for hydrocarbons and SCD for sulfur compounds, and the catalysts were characterized by XRD, FTIR, N₂ adsorption isotherms, HRTEM, XRF, ²⁹Si and ²⁷Al solid state NMR, NH₃-TPD and FTIR-pyridine techniques. The results indicate that the addition of metals did not significantly alter the textural and structural properties of the zeolites, and that the Nb/HB and Nb/HY catalysts were the most effective in the conversion of thiophene, and selective toward paraffins and H₂S mainly. The performance of these materials was attributed to the existence of an optimal balance between the Brønsted and Lewis acid sites, which increased the hydrogen transfer coefficient (HTC) during the cyclohexane transformation, resulting in higher desulfurization activity. The addition of zinc drastically increased the density of the Lewis acid sites and decreased the HTC, favoring the production of aromatics and thiophene condensation reactions, thus transferring sulfur to the gasoline streams. In general, the catalysts investigated are of significant interest as additives for sulfur reduction in FCC gasoline streams, either by producing easily removable H₂S or by transferring sulfur to heavier fractions.

The fragrance industry continually searches for compounds that introduce new scent notes or improve existing ones. Herein, we report a methodology for obtaining two novel aldehydes, potentially interesting for application in fragrance formulations, via rhodium-catalyzed hydroformylation of (1R,5R)-sabinene, a biorenewable monoterpene available in many essential oils. One of the aldehydes, which retained the original sabinene skeleton, was derived from the hydroformylation of the exocyclic C-C double bond through the accepted hydroformylation mechanism. The other aldehyde arose from the interaction of rhodium with the cyclopropane ring through an unusual mechanism, which resulted in the cyclopropane ring cleavage, forming a product that shares structural similarities with established fragrant molecules. The selectivity was controlled by adjusting the reaction conditions (temperature, pressure, P-ligand, and P/Rh ratio). Each aldehyde was obtained in a 70–80 % yield, depending on the reaction conditions. Combining our experimental findings with DFT calculations allowed us to propose an unusual hydroformylation reaction mechanism involving cyclopropane moiety in vinylcyclopropanes.

Ethanol has captured a lot of interest from the industry in recent years due to its sustainable appeal, being obtained through the process of biomass fermentation. Not only its application in the energy sector as a potent fuel, but its conversion to higher added value products has been a commercial highlight. In this work, we sought to synthesize a new mixed catalyst Mg_xAlO_y-SiO₂ to investigate its performance in the upgrading of ethanol to a product of significant relevance, particularly for the plastics and polymers industry, namely1,3-butadiene. The molar ratio of Mg:Al:Si in the catalyst was tuned, revealing a remarkable impact on the selectivity of the reaction products. For the production of 1,3-butadiene, the optimal catalyst composition was determined to be 3:1:1, exhibiting a selectivity of 23 % at 723 K. This composition offered a favorable amount of acid and basic active sites, indicated by the acid to basic sites density ratio of 0.109, with medium basic sites predominantly represented. This arrangement is believed to have facilitated dehydrogenation, condensation, and dehydration reactions, each playing a crucial role in the overall process.

This work aimed at the green synthesis of MWW zeolites using rice husk silica as an alternative raw material. It was also performed a post-synthesis desilication procedure with NaOH and CTABr aiming to obtain MWW zeolites with improved accessibility. The set of characterization techniques (XRD, ²⁷Al MAS NMR, N₂ physisorption, SEM, TEM, and Pyridine-FTIR) revealed the phase purity and combined structure of micro-mesopores after desilication. The zeolites were evaluated as catalysts for polystyrene pyrolysis, producing benzene, toluene and ethylbenzene, besides styrene monomers, and dimers. Without any catalyst, polystyrene produces only styrene monomers, dimers and trimers. The desilication increases the amount of Brønsted and Lewis sites and the external area, catalyzing the production of polyaromatics and naphthalene derivatives. The activation energy decreased for the catalyzed reactions, reflecting other reaction pathways.

Understanding the dynamics of zeolite formation is key to synthesising high-quality zeolitic materials with controllable properties, in order to develop more efficient and performant materials. X-ray photon correlation spectroscopy (XPCS) using coherent X-rays offers new possibilities for in situ observation of nano to micron-scale fluctuation dynamics during crystal growth. An in situ cell, which is capable of collecting time-resolved coherent X-ray scattering data under hydrothermal conditions has been developed and used to study, by in situ XPCS combined with small and wide angle X-ray scattering, zeolite formation and dynamics. Analysis of the results using two-time correlations enables to accurately identify the successive growth and crystallisation steps, revealing the dissolution process of the LTA topology during the SOD growth. This approach opens a powerful new avenue for studying the dynamics of nanomaterials formation, phase transitions and growth processes under in situ conditions that will enable profound insights into the nanoscale synthesis mechanisms.

The possibility of SnO₂ incorporation and immobilization as films forming composites opens new perspectives for TiO₂ to profit visible light and to facilitate the photocatalytic process, respectively. In this study, 0 %, 1 % and 10 % wt. of SnO₂ was incorporated into TiO₂ (SnO₂-TiO₂) by coprecipitation in a sol-gel method by ammonia addition, followed for calcination at 500 °C. The photocatalysts were characterized by N₂ adsorption-desorption, FTIR spectroscopy, XRD Rietveld refinement, TG-DTG, SEM-EDS, DRS and elemental analysis. The performance of all solids was evaluated in the photocatalytic degradation of the cationic dye methylene blue in aqueous phase, under visible and UV irradiation, at 25 °C for 2 h. The results showed that the incorporation of Sn into TiO₂ improved the textural properties and decreased the bandgap. All solids presented only TiO₂ typical diffractograms, with anatase as the main phase, but catalyst with 10 % SnO₂ presented also brookite phase, inferring that Sn atoms were incorporated into TiO₂ structure, corroborated by MEV results. All tin-based photocatalysts show high activity under UV and visible light, with the 10 % SnO₂ material reaching 83 % and 88 % degradation after 2 h under UV and visible radiation, respectively. This material was tested as an immobilized film, achieving 14 % of decolorization, and the reuse was also evaluated. Our investigation demonstrates that SnO₂-TiO₂ catalysts could be used to decompose a dye under UV and Visible light as powder in a batch reactor and immobilized as a composite film with chitosan, that opens new perspectives to facilitate the application using solar light.