Hydrotalcite-Derived Mixed Oxides for the Synthesis of a Key Vitamin A Intermediate Reducing Waste

The synthesis of hydroxenin monoacetate, a key intermediate in the manufacture of vitamin A, relies on the undesirable use of stoichiometric amounts of organic bases such as pyridine. Although the final product (vitamin A acetate) can be produced from hydroxenin diacetate, using the monoacetylated intermediate improves the overall process yield. Aiming to identify more efficient, environmentally benign alternatives, this work first studies the homogeneous acetylation reaction using pyridine. The addition of the base is found to enhance the rate of hydroxenin monoacetate formation, confirming its catalytic role, but also yields non-negligible amounts of hydroxenin diacetate. On the basis of these insights, Mg- and Al-containing hydrotalcites are explored because of their broad scope as base catalysts and the ability to finely tune their properties. The reaction kinetics are greatly enhanced via controlled thermal activation, forming high surface area mixed metal oxides displaying Lewis basic sites. In contrast, a Brønsted basic material synthesized by the reconstruction of a mixed oxide performs similarly to the as-synthesized hydrotalcite. Variation of the Mg/Al ratio from 1 to 3 has no significant impact, but activity losses are observed at higher values because of a reduced number of basic sites. After optimizing the reaction conditions, hydroxenin monoacetate yields >60% are obtained in five consecutive cycles without the need for any intermediate treatment. The findings confirm the potential of hydrotalcite-derived materials as highly selective catalysts for the production of vitamins with reduced levels of organic waste

Protection Strategies for the Conversion of Biobased Furanics to Chemical Building Blocks

In recent years, the urgency to replace fossil-based resources by renewablebiomass for obtaining chemical building blocks has only increased. Carbohydrate-derivedfuranic compounds are regarded as promising platforms for a renewable value chain. Thehigh reactivity of such biobased intermediates requires the development of novel catalyticchemistry to enhance product yield. The protection of reactive functional groupsprovides a way to improve the product selectivity. Such protection strategies are commonpractice in the synthesis offine chemicals and pharmaceuticals but are not fully exploredfor the conversion of furanic compounds. In this perspective, several examples ofprotection strategies focusing on the selective passivation of 5-HMF are discussed.Formation and removal of these protection groups are highlighted as well as theapplication of the neutralized 5-HMF in further processing. A guide for selecting theappropriate protection strategy depending on the targeted chemistry and operatingconditions is provided

Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM-5-Catalyzed Methane Dehydroaromatization

Non-oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H2/CO ratio close to two is the main side-product of coke combustion. Using 13C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one-fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.ChemE/Catalysis Engineerin

Methane Dehydroaromatization by Mo/HZSM-5: Mono- or Bifunctional Catalysis?

The active site requirements for methane dehydroaromatization by Mo/HZSM-5 were investigated by employing as catalysts physical mixtures of Mo-bearing supports (HZSM-5, SiO₂, γ-Al₂O₃, and activated carbon) and HZSM-5. Separation of the two catalyst components after activation or reaction was possible by using two different sieve fractions. Our comparison demonstrates that migration of volatile Mo oxides into the micropores of HZSM-5 is at the origin of the observed catalytic synergy in methane dehydroaromatization for physical mixtures. The propensity of Mo migration depends on the activation method and the Mo–support interaction. Migration is most pronounced for Mo/SiO₂. Prolonged exposure of HZSM-5 zeolite to Mo oxide vapors results in partial destruction of the zeolite framework. Mo carbide dispersed on nonzeolitic supports afforded predominantly coke with only very small amounts of benzene. The main function of the zeolite is to provide a shape-selective environment for the conversion of methane to benzene. A comparison of Mo/HZSM-5 and Mo/Silicalite-1 demonstrates that aromatization of methane is an intrinsic ability of molybdenum carbides dispersed in the 10-membered-ring micropores of MFI zeolite. Thus, one important role of the Brønsted acid sites is to promote the dispersion of the Mo oxide precursor and, accordingly, the active Mo carbide phase in the micropores of HZSM-5