1,721,238 research outputs found
Advances in Catalysis for More Sustainable Synthesis of Phenolics
No full textIn this chapter, we examine the synthesis of phenolic compounds via catalytic reactions and processes, with a special focus on sustainability issues.
In recent years, considerable steps forward have been made with the aim of
developing greener routes for the functionalisation of phenol and diphenols.
Examples of these include: (a) the use of methanol instead of methylchloride
or dimethylsulphate for the synthesis of ethers, such as anisole, guaiacol and
veratrol, which are key intermediates for the synthesis of a plethora of fine
chemicals and specialties; (b) the use of alkylcarbonates for the synthesis of
alcohol-ethers (e.g. phenoxyethanol), cresols, and ethers; and (c) the use of
aldehydes instead of halogenated alkanes for the hydroxyalkylation of phenolics to alcohols, such as piperonyl alcohol. Indeed, many of these reactions
were inspired by the successful industrial application of methanol as an electrophile for the synthesis of o-cresol and 2,6-xylenol. The latter reaction may
be considered the very first ‘green’ process for the functionalisation of phenol; surprisingly, despite its industrial use for several decades, only in recent
years has the mechanism of this reaction been elucidated. Some emblematic
examples of the more sustainable synthesis of phenolic compounds, briefly
discussed here, are 2,6-xylenol, guaiacol, vanillin, methylendioxobenzene,
phenoxyethanol, hydroxytyrosol and piperonal
The characterization and the catalytic activity of modified Wells-Dawson-type polyoxometalates in the oxidehydrogenation of isobutane to isobutene
No full texthermal stabilities of Wells-Dawson polyoxometalates of overall skeleton composition P2W18O626- were examined and their catalytic properties in isobutane oxidehydrogenation were tested as a function of temperature, isobutane and oxygen content and presence of steam. It is shown that the lacunary K10P2W17O61 and the iron-substituted K7P2W17FeO61 are stable under oxidizing conditions up to 500degreesC. At higher temperature structural modifications occur with breaking of the polyanion framework and formation of K/W/O and K/P/O-containing phases. These rearrangements are thermally induced (they occur under both air and inert atmosphere) and do not involve the formation of the Keggin-type phases, as previously observed with K6P2W18O62. The Wells-Dawson compounds were active and selective in isobutane oxidehydrogenation to isobutene. However, a high activity was evident only under conditions which favoured the polyoxometalate reduction, and the possible generation of radical species which propagated into the gas phase, thus yielding a reactivity typical of heterogeneously-initiated, homogeneous reactions. These conditions were found at temperature higher than 450 degreesC, high isobutane concentration and high oxygen concentration in feed. A specific role of water in the reaction has been found; specifically, in the presence of steam the contribution of homogeneous reactions was enhanced
Key aspects of catalyst design for the selective oxidation of paraffins
No full textThis review examines some aspects in the development of heterogeneous catalysts for the oxyfunctionalization of light paraffins. Particular attention is devoted to the reaction of paraffin oxydehydrogenation to olefins and of -butane oxidation to maleic anhydride. Most catalyst compositions are based on vanadium oxide as the main component, and the peculiar properties of this element with respect to the catalytic performance are discussed. These properties are also examined in light of the stability of the product of partial oxidation towards consecutive unselective oxidation reactions, and with respect to the mechanism of paraffin activation
Valorization of ethanol by anaerobic transformation over vanadium containing catalyst
No full textA systematic study of the catalytic behaviour of a FeVO4sample in the anaerobic ethanol transformation was performed and compared with reference oxides V2O5and Fe2O3. It was found that the alcohol is able to reduce the samples at 300°C. The FeVO4and V2O5samples have almost identical catalytic behaviour even if they form completely different structures during reduction. It was concluded that in the first case, Fe plays a role as a "stabilizer" of the reduced phase thanks to the formation of a spinel phase where the V is mainly present as V3+whereas in the case of V2O5the reduction under the same conditions leads mainly to a VO2phase and the presence of this V4+vanadium species catalyse side reactions
Oxidation of 1-Butene and Butadiene to Maleic Anhydride. 2. Kinetics and Mechanism
No full textThe kinetics of the oxidation of 1-butene and butadiene over a vanadium-phosphorus mixed oxide catalyst prepared
by reduction of V,O, in an organic medium were investigated. Increased 1-butene concentration in the feed strongly
inhibited the rate of maleic anhydride formation but did not affect the rate of formation of the other products (methyl
vinyl ketone, crotonaldehyde, acetaldehyde, and carbon oxides). The inhibition was interpreted by a Langmuir-
Hinshelwood type model where the ratedetermining step is the reaction between adsorbed oxygen and the
intermediate butadiene adsorbed on two adjacent sites. The rates of formation of the other byproducts are
interpreted by expressions which correspond to the reaction between 1-butene or butadiene adsorbed on one site
and adsorbed or gaseous oxygen. Satisfactory fit of the derived rate expression with the experimental data was
obtaine
Vanadium/phosphorus mixed oxide from the precursor to the active phase:Catalyst for the oxidation of n-butane to maleic anhydride
No full textThis review examines the recent scientific and patent literature dealing with V/P/O-basedcatalysts for the synthesis of maleic anhydride by n-butane oxidation. Attention is focused on the different methods of preparation claimed by each company, as well as on the main parameters in precursor preparation and thermal treatment affecting the final catalytic performance. The role of the various promoters reported in the literature is also discussed. © 1995 Elsevier B.V
CHAPTER 11: Advances in Catalysis for More Sustainable Synthesis of Phenolics
No full textIn this chapter, we examine the synthesis of phenolic compounds via catalytic reactions and processes, with a special focus on sustainability issues. In recent years, considerable steps forward have been made with the aim of developing greener routes for the functionalisation of phenol and diphenols. Examples of these include: (a) the use of methanol instead of methylchloride or dimethylsulphate for the synthesis of ethers, such as anisole, guaiacol and veratrol, which are key intermediates for the synthesis of a plethora of fine chemicals and specialties; (b) the use of alkylcarbonates for the synthesis of alcohol-ethers (e.g. phenoxyethanol), cresols, and ethers; and (c) the use of aldehydes instead of halogenated alkanes for the hydroxyalkylation of phenolics to alcohols, such as piperonyl alcohol. Indeed, many of these reactions were inspired by the successful industrial application of methanol as an electrophile for the synthesis of o-cresol and 2,6-xylenol. The latter reaction may be considered the very first 'green' process for the functionalisation of phenol; surprisingly, despite its industrial use for several decades, only in recent years has the mechanism of this reaction been elucidated. Some emblematic examples of the more sustainable synthesis of phenolic compounds, briefly discussed here, are 2,6-xylenol, guaiacol, vanillin, methylendioxobenzene, phenoxyethanol, hydroxytyrosol and piperonal
Editorial for the Special Issue on “Multidisciplinary Insights on Bone Healing”
Full text linkAnimal and human bone damage can be considered differently according to a macro- or micro-level analysis [...
Three-Punch Alveolar Ridge Reconstruction Technique: A Novel Flapless Approach in Eight Consecutive Cases
No full textThis clinical and histologic case series aims to evaluate a novel flapless approach to alveolar ridge reconstruction (ARR) of compromised extraction sockets by means of collagenated xenograft sealed with three resorbable layers of hole punched membrane. Eight postextraction sockets without buccal and/or palatal bone walls and with adjacent natural teeth from eight consecutive patients were included. Pretreatment CBCT scanning was performed. After debridement of the selected sites, a flapless grafting procedure was carried out, and the three membrane protection was applied. After 6 to 17 months, at implant placement, a posttreatment alveolar ridge CBCT was taken, and a bone core biopsy sample was harvested for histologic and morphometric analyses. Clinical outcomes showed predictable horizontal bone regeneration in all postextraction sockets with good preservation of soft tissue architecture. Pretreatment ridge CBCT measurements showed limited bone width (2.6 +/- 1.08 mm). Posttreatment measurements revealed adequate bone width (9.05 +/- 1.29 mm) with a mean bone gain of 6.4 +/- 1.34 mm. Histologic and morphometric analyses revealed the absence of inflammatory cells and the presence of 25.4% +/- 8.7% of new bone and 31.8% +/- 8.3% of graft particles inside the biopsy samples. Many graft particles were surrounded and interconnected by new bone, thus demonstrating the formation of a bone-graft network. Rare osteoclasts were found. This novel technique seems to be effective in treating alveolar sockets prior to implant placement, preventing inflammation and bone resorption and promoting bone regeneration. Int J Periodontics Restorative Dent 2021;41:875-884. doi: 10.11607/prd.491
- …
