Comparison of Lewis acids of different hardness supported on silica gel as catalysts of Diels–Alder reactions of (E)-2-cyanocinnamates

The behaviour of several Lewis acids, derived from zinc, aluminium and titanium, supported on silica gel, as catalysts of the Diels-Alder reactions of cyclopentadiene with methyl, (1R,2S,5R)-menthyl, and (R)-pantolactone (E)-2-cyanocinnamates has been compared. The best catalytic activities were observed with the zinc catalysts. The extent and even the direction of the asymmetric induction changes for the same chiral auxiliary depending on the catalyst used. Ab initio theoretical calculations, carried out on model dienophile-catalyst intermediates, show that the coordination of the softer zinc derivatives at the nitrogen atom is thermodynamically favoured, whereas coordination to the carbonyl oxygen atom is preferred for the harder aluminium catalysts. However, in both cases the most reactive intermediate comes from the coordination of the Lewis acid to the carbonyl group of the dienophile in the s-trans conformation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hetero-Diels–Alder reactions of hetaryl thiochalcones with acetylenic dienophiles

Hetaryl-substituted thiochalcones react with acetylenic mono- and diesters in the THF solution in the presence of LiClO₄ at 65°C to give, after 24?h, 4H-thiopyran carboxylates and dicarboxylates, respectively, in moderate to good yields. The same reactions were performed also in the THF solution without a catalyst under microwave irradiation. In that case, the reaction time was reduced to three minutes and, in most cases, an improvement in the yield of the [4+2]-cycloadduct was observed. The reactions with methyl propiolate occurred regioselectively and the 3-carboxylates were formed exclusively.     

Recyclable tetraarylphosphonium supported chiral imidazolidin-4-one organocatalyst for Diels–Alder reactions

The tetraarylphosphonium supported chiral imidazolidin-4-ones were synthesized and used to catalyze the Diels–Alder reactions of cyclopentadiene and α,β-unsaturated aldehydes. High enantiomeric excesses and good yields were obtained, and catalyst recycling (up to five cycles) was accompanied by almost intact enantioselectivity and some loss of the chemical efficiency.     

PMMA thermoreversible networks by Diels–Alder reaction

Reversibly crosslinked polymethylmethacrylate were prepared by Diels–Alder (DA) reaction using multi-furan and multi-imide precursors. Furan functionalized PMMA were obtained by reactive extrusion (transesterification) between a commercial PMMA and furfuryl alcohol using tin(II)2-ethylhexanoate (Sn(oct)₂) or 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) catalysts. Maleimide based coupling agents were prepared by amine–anhydride reaction. Thermomechanical properties of the PMMA precursors and issued networks were analyzed. Two G′ and G″ cross-over temperatures were obtained during the heating and cooling cycles. The first one at 110 °C can be assimilated to physical gel formation and the second one at 160 °C to its breaking. When the network was formed, an increasing of these temperatures is observed. Also, the G′ between these temperatures increased with the network density. The obtained network had a shape memory behavior.     

Recyclable biobased materials based on Diels–Alder cycloaddition

Poly(ethylene glycol) diglycidyl ether-furfurylamine (PGFA) containing pendant furan was synthesized, and a series of crosslinked materials with thermally reversible capacity were synthesized through a furan/maleimide Diels–Alder (DA) reaction between PGFA and bismaleimide (BMI). The kinetics of the PGFA/BMI DA reaction were studied by Fourier transform infrared spectroscopy (FTIR). The reaction conversion rate, the reaction rate constant, and the energy of the DA reaction at different temperatures were calculated. In addition, the retro Diels–Alder (rDA) reaction was studied via ¹H-NMR, differential scanning calorimetry, and in situ FTIR. The occurrence of the retro DA reaction has been characterized clearly. Finally, the mechanical properties of the materials were obtained by dynamic mechanical and tensile tests. The storage modulus decreased obviously when the temperature reached over 90 °C, which proved that the materials were thermally reversible at high temperature. By changing the proportion of the crosslinking agent BMI, the best-performing materials were obtained, and the properties of the materials were basically unchanged after recycling. Thus we have obtained an excellent reusable material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47352.     

Production of renewable p-xylene from 2,5-dimethylfuran via Diels–Alder cycloaddition and dehydrative aromatization reactions over silica−alumina aerogel catalysts

We report the selective conversion of biomass-derived 2,5-dimethylfuran (DMF) to p-xylene (~ 70% selectivity) through Diels–Alder cycloaddition and subsequent dehydration with silica−alumina aerogel (SAA) catalysts. The high activity of SAA can be attributed to its high surface area, large mesoporous volume, and high acid site concentrations. The conversion of DMF and the yield of p-xylene were strongly dependent on the silica alumina ratio of SAA. A higher aluminum content in SAA led to a progressive increase in the concentration of Brønsted acid sites and a corresponding increase in the p-xylene production rate. The effect of solvent on the production of p-xylene was examined, and it was found that the p-xylene production rate increases significantly in polar aprotic solvents (i.e. 1,4-dioxane).     

Synthesis of silica aerogel microspheres by a two-step acid–base sol–gel reaction with emulsification technique

Journal of Porous Materials - Silica aerogel microspheres were synthesized by a two-step acid–base sol–gel reaction in water-in-oil emulsion systems, in which tetraethoxysilane was used...     

Ultrahydrophobic silica films by sol–gel process

Wettability of solid surfaces is a crucial concern in our daily life as well as in engineering and science. The present research work describes the room temperature (27 °C) synthesis of adherent and water repellent silica films on glass substrates using vinyltrimethoxysilane (VTMS) as a hydrophobic reagent by a single step sol–gel process. The silica sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), water (H₂O) constant at 1:14.69:5, respectively, with 0.01 M NH₄F throughout the experiments and the VTMS/TEOS molar ratio (M) was varied from 0 to 0.97. The effects of M on the surface structure and hydrophobicity have been researched. The static water contact angle as high as 144° and water sliding angle as low as 14° was obtained for silica film prepared from M = 0.97. The hydrophobic silica films retained their hydrophobicity up to a temperature of 255 °C and above this temperature the films became superhydrophilic. The prepared silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity test and static and dynamic water contact angle measurements.     

Zeolite catalytic synthesis of high-performance jet-fuel-range spiro-fuel by one-pot Mannich–Diels–Alder reaction

A one-pot Mannich–Diels–Alder reaction was developed for synthesis of high-performance jet-fuel-range spirocycloalkane with an overall yield of 71.6% from biomass and petroleum derived cyclopentanone, formaldehyde, and cyclopentadiene. HZSM-5 zeolite with Si/Al molar ratio of 130 exhibits much higher catalytic activity, along with good recycling ability. Over it, a high selectivity (81.6%) and yield (80.9%) of target adduct is achieved with almost complete conversion of cyclopentanone, attributed to the synergy effect of appropriate ratio of Lewis to Brønsted acid sites and mass transfer effect of porous zeolite. More importantly, after hydrodeoxygenation, the resultant spirocycloalkane has much higher density and volumetric neat heat of combustion (0.952 g·ml⁻¹, 40.18 MJ L⁻¹) than the wildly used JP-10 fuel (0.936 g ml⁻¹, 39.41 MJ L⁻¹). This work provides a promising route to produce high-performance jet-fuel-range hydrocarbons through co-conversion of biomass and petroleum derivatives.     

Porous water repellent silica coatings on glass by sol–gel method

Development of the solid surfaces with water-repellent and self-cleaning ability has attracted much research interest in recent years. In the present research work, we have prepared water repellent silica coatings on glass at room temperature (~27 °C) by sol gel process and surface silylation technique. Coating sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS), methanol (MeOH) and water (H₂O) constant at 1:12.36:4.25, respectively, with 0.01 M NH₄F. The dip coated silica films were surface silylated using two different silylating agents namely hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ). The HMDSO and HMDZ in hexane solvent were varied from 0 to 1 vol.% and silylation period was varied from 1 to 3 h. The HMDSO and HMDZ modified films showed dense and porous surface morphology, respectively. The HMDSO modified silica films showed static water contact angle of 122° whereas HMDZ modified films showed 165°. The HMDZ modified films displayed the extreme water repellency comparing with that of lotus leaves. The silica films were characterized by surface profilometer, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared microscopy, thermal and chemical aging tests, optical transmission and static water contact angle measurements.     

Relationship on structure and properties of polyurethane modified Diels–Alder addition polymer

A series of linear and crosslinked polyurethane modified materials were prepared by Diels–Alder cycloaddition reaction. Based on raw materials, some novel equations were designed and effects of equation parameters on the mechanical properties of modified material were explored. It was demonstrated that tensile strength of modified materials and elongation at break were changed from 3.68 to 18.71 MPa, 200 to 866%, respectively. In addition, the optimal reaction temperature of retro-Diels–Alder (r-DA) reaction was determined by differential scanning calorimeter (DSC). Gel permeation chromatography (GPC) and tensile experiments were used to characterize the properties of repolymerized material after degradation. The results indicated the optimal temperature of r-DA for linear and crosslinked polyurethane modified materials is 127 and 150 °C. Moreover, after decomposed, the product was slowly repolymerized at 60 °C, and can more effectively restore material strength under the action of the solvent to achieve self-healing effect. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47355.     

Synthesis and characterization of poly(ester-urethane-imide)s by Diels–Alder polyaddition

Poly(ester-urethane-imide)s were prepared by Diels–Alder polyaddition of 1,6-hexamethylene-bis(2-furanylmethylcarbamate) with various bismaleimides containing ester groups in the backbone. The Diels–Alder reaction was carried out in m-cresol, at 110°C, followed by thermal and chemical aromatization of tetrahydrophthalimide intermediates. The monomers and polymers were characterized by IR, ¹H-NMR spectroscopy and elemental analysis. Thermal properties of the polymers were investigated by differential scanning calorimetry and dynamic thermogravimetric analysis.     

Photocatalytic degradation of methanol on titania and titania–silica aerogels prepared by non-alkoxide sol–gel route

Titania and titania–silica aerogels were prepared by alkoxide or non-alkoxide sol–gel route and subsequent supercritical drying with carbon dioxide at low temperature. The resulting aerogels having high surface area and mesoporosity were used as photocatalysts for gas phase methanol degradation reaction. Photocatalytic degradation reactions were carried out on titania and titania–silica aerogels, and commercial Degussa P-25 titania. The photocatalytic activities of titania and titania–silica aerogels were higher than that of the P-25. While the conversion of methanol degradation over the P-25 catalyst was only 50–60%, that for the titania aerogel was observed to be above 98% due to the higher specific surface area and the well developed mesoporous structure. In spite of lower titania contents, much higher surface area and high dispersion of titania of titania–silica aerogel gave rise to the high photocatalytic activity in comparison to those of titania aerogels. Moreover, titania–silica aerogel was also used for the photodegradation and adsorption hybrid system. It was observed that the high removal efficiency for methanol was caused by the combination of higher catalytic activity and adsorption capacity.     

Sol–gel synthesis and characterization of silica supported nickel ferrite catalysts for dry reforming of methane

Silica-supported NiFe₂O₄ spinel was prepared by sol–gel method using tetramethyl orthosilicate as a precursor of silica. B.E.T., XRD, MEB–EDS, TEM, XPS and Raman scattering techniques were used for its characterization. The reducibility by hydrogen was investigated by TPR and HT-XRD. These properties are compared to those of unsupported NiFe₂O₄. Both acidic and redox sites were found by studying the decomposition of isopropanol. First experiments in the dry reforming of methane by CO₂ showed that owing to more acidic properties supporting NiFe₂O₄ on silica provides a more active and selective catalyst that seems less prone to coking.     

Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

Hollow mesoporous silica spheres supported Ag and Ag–Au catalyzed reduction of 4-nitrobenzo-15-crown

Hollow mesoporous silica (HMS) spheres of size within the range 120–220 nm have been prepared using propanol–water solvent as template and cetyltrimethylammonium bromide (CTAB) as stabilizer. HMS supported silver and silver–gold catalysts were prepared by impregnating metal nanoparticles on HMS and were characterized by ultraviolet–visible spectroscopy (UV–vis), dynamic light scattering (DLS), optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductive coupled plasma optical emission spectroscopy (ICP-OES) and N₂ adsorption–desorption. The reduction of 4-nitrobenzo-15-crown (4-NB-15-C) was compared using HMS supported silver and silver–gold nanocatalysts varying experimental parameters. Bimetallic Ag–Au/HMS nanocatalysts was found to be more active than monometallic Ag/HMS nanocatalyst.     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Suzuki–Miyaura reaction catalyzed by graphene oxide supported palladium nanoparticles

Pd supported on polyamine modified graphene oxide (GO-NH₂-Pd^2 +) was fabricated for the first time. The prepared catalyst was characterized by transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy. The catalytic activity of the prepared catalyst was investigated by employing Suzuki–Miyaura coupling reaction as a model reaction. A series of biphenyl compounds were synthesized through the Suzuki–Miyaura reaction using GO-NH₂-Pd^2 + as catalyst. The yields of the products were in the range from 71% to 95%. The catalyst can be readily recovered and reused at least 10 consecutive cycles without significant loss its catalytic activity.     

Thermo-adjusted self-healing epoxy resins based on Diels–Alder dynamic chemical reaction

Micro-damage in materials could be repaired by endowing materials with self-healing performance. Herein, an epoxy resin with excellent self-healing performance grounded on thermo-reversible Diels–Alder dynamic chemical reaction was developed. Results showed that the bending strength and adhesive behavior of epoxy resin were influenced dramatically upon treatment with various temperatures. More importantly, damages created in epoxy resin could be repaired completely after suitable heat treatments. What is more, the healed epoxy resin exhibited much higher bending strength and adhesive performance than the pristine one did. The materials could be damaged and then repaired repeatedly. Meanwhile, the as-prepared self-healing epoxy resin exhibited excellent thermal reversibility and controllable adhesion. The thermo-adjusted self-healing performance endowed epoxy resin with recyclable and reusable performance. Therefore, the research made it possible of recycling waste epoxy resins.