Photochemistry with microwaves: Catalysts and environmental applications

Microwave radiation has recently become an active source of thermal energy in numerous chemical reactions. As such, the microwave energy is not ordinarily and is not likely to be used to drive photochemical reactions. Accordingly, is the role of microwaves then relegated solely to be a source of heat? They do not have to be since photochemical reactions can be activated indirectly by microwaves using the UV light emitted from certain gas-fills excited by microwave radiation. This article examines the microwave radiation not only as a dielectric heat source but also a source of vacuum-UV radiation and UV light through microwave discharge electrodeless lamp devices, which in some cases (depending on design) can also serve as photoreactors.     

Microwave-assisted pyrolysis of biomass: Catalysts to improve product selectivity

This study was intended to evaluate the effects of catalysts on product selectivity of microwave-assisted pyrolysis of corn stover and aspen wood. Metal oxides, salts, and acids including K₂Cr₂O₇, Al₂O₃, KAc, H₃BO₃, Na₂HPO₄, MgCl₂, AlCl₃, CoCl₂, and ZnCl₂ were pre-mixed with corn stover or aspen wood pellets prior to pyrolysis using microwave heating. The thermal process produced three product fractions, namely bio-oil, gas, and charcoal. The effects of the catalysts on the fractional yields were studied. KAc, Al₂O₃, MgCl₂, H₃BO₃, and Na₂HPO₄ were found to increase the bio-oil yield by either suppressing charcoal yield or gas yield or both. These catalysts may function as a microwave absorbent to speed up heating or participate in so-called “in situ upgrading” of pyrolytic vapors during the microwave-assisted pyrolysis of biomass. GC–MS analysis of the bio-oils found that chloride salts promoted a few reactions while suppressing most of the other reactions observed for the control samples. At 8 g MgCl₂/100 biomass level, the GC–MS total ion chromatograms of the bio-oils from the treated corn stover or aspen show only one major furfural peak accounting for about 80% of the area under the spectrum. We conclude that some catalysts improve bio-oil yields, and chloride salts in particular simplify the chemical compositions of the resultant bio-oils and therefore improve the product selectivity of the pyrolysis process.     

H, V and N nuclear magnetic resonance studies of structure and properties of vanadia supported on TiOx/SiO2

¹H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of TiO_x/SiO₂ catalysts suggest the interaction of surface TiO_x. species with Si-OH groups of the silica. Simultaneously, Ti-OH groups from surface titania species appear. The distribution of TiO_x species over SiO₂ is non-uniform, since a considerable part of surface OH groups remains unreacted with supported titania. Supported vanadia species interact both with Si-OH and Ti-OH groups. ⁵¹V NMR spectra suggest the interaction of vanadia with supported titania species and show the non-uniform distribution of titania over the SiO₂ surface. Deposition of titania as well as vanadia produces strong electron-accepting (Lewis) sites which interact with the terminal N atom of adsorbed N₂O molecules, resulting in a downfield shift of the resonance in ¹⁵N NMR spectra. The acid strength of electron-accepting sites is similar in both cases. Only about 10% of the total amount of supported titania and vanadia create Lewis sites.     

Polymerization behavior of propene with [t-BuNSiMe2Flu]TiMe2: effects of solvents and cocatalysts

The titanum complex [η¹:η³tert-butyldimethylfluorenylsilyl]-amido)dimethyltitanum ([t-BuNSiMe₂Flu]TiMe₂, Cat.A) was synthesized by an alternatively quicker one pot procedure. Polymerization of propene in the presence of [t-BuNSiMe₂Flu]TiMe₂/dried pure methylaluminoxane (d-PMAO) system at 0 °C was investigated in toluene and heptane. A strong enhancement of productivity was observed in toluene compared to heptane. Polymerization of propene was also investigated with Cat.A in heptane, with different cocatalysts, d-PMAO and dried pure modified methylaluminoxane (d-MMAO), which was prepared from the mixture of trimethylaluminum (TMA) and triisobutylaluminum (TIBA). A 10-fold higher number average molecular weight (M_n) and broad molecular weight distribution (MWD) was obtained with d-PMAO in heptane, even though kinetic features and other parameters signified the living nature of the polymerization process. However, the broadening of MWD was attributed to the poor insolubility of d-PMAO in heptane.     

Metal-supported ultrathin oxide film systems as designable catalysts and catalyst supports

Thin oxide films lend themselves as model supports for studies in heterogeneous catalysis, for example, to study the growth and reaction of metal deposits (atoms, clusters and nanoparticles). If the thickness of the film is chosen appropriately these thin films are reasonable models to mimic the situation of bulk materials. If thin films below a critical thickness are studied these materials exhibit properties in their own right. Their structural properties may be tuned to control their functional characteristics. Possible implications for heterogeneous catalysis are discussed.     

Modular ligands in asymmetric synthesis. Copper-mediated cyclopropanation

The chiral imidazoline/copper catalyst system efficiently mediates asymmetric intermolecular cyclopropanations. Complexes derived from (R,R)- or (S,S)-1,1-diphenylethylenediamine, cyclic ketones, and Cu(I) or Cu(II) triflates were compared. The reaction between (−)-menthyl diazoacetate and 1,1-diphenylethylene affords cyclopropane carboxylates in up to 80% yield and with up to 78% de.     

Thermal degradation of polyethylene and polystyrene from the packaging industry over different catalysts into fuel-like feed stocks

Thermal degradation of waste polymers was carried out as a suitable technique for converting plastic polymers into liquid hydrocarbons, which could be used as feed stock materials. The catalytic degradation of waste plastics (polyethylene and polystyrene) was investigated in a batch reactor over different catalysts (FCC, ZSM-5 and clinoptillolite). The effects of catalysts and their average grain size on the properties of main degradation products (gases, gasoline, diesel oil) are discussed. The temperature range of 410-450 °C was used in the process. Both equilibrium FCC catalyst and natural clinoptilolite zeolite catalyst had good catalytic activity to produce light hydrocarbon liquids, and ZSM-5 catalyst produced the highest amount of gaseous products. Gases and liquids formed in cracking reactions were analyzed by gas chromatography. The liquid products consisted of a wide spectrum of hydrocarbons distributed within the C₅-C₂₈ carbon number range depending on the cracking parameters. The composition of hydrocarbons had linear non-branched structure in case of polyethylene, while from polystyrene more aromatics (ethyl-benzene, styrene, toluene, and benzene) were produced. The yields of volatile products increased with increasing degradation temperature. The olefin content of liquids was measured with an infrared technique and an olefin concentration of 50-60% was observed. The concentration of unsaturated compounds increased with decreasing temperature, and in the presence of catalysts. The activation energies were calculated on the basis of the composition of volatile products. The apparent activation energies were decreased by catalysts and catalyst caused both carbon-chain and double bond isomerisation.     

Oxovanadium(IV) and copper(II) complexes of 1,2-diaminocyclohexane based ligand encapsulated in zeolite-Y for the catalytic oxidation of styrene, cyclohexene and cyclohexane

N,N′-Bis(salicylidene)cyclohexane-1,2-diamine (H₂sal-dach) reacts with oxovanadium(IV) and copper(II) exchanged zeolite-Y in refluxing methanol to yield the corresponding zeolite-Y encapsulated metal complexes, abbreviated herein as [VO(sal-dach)]-Y and [Cu(sal-dach)]-Y. Spectroscopic studies (IR, electronic and ¹H NMR), thermal analysis, scanning electron micrographs (SEM) and X-ray diffraction patterns have been used to characterise these complexes. These encapsulated complexes catalyse the oxidation, by H₂O₂, of styrene, cyclohexene and cyclohexane efficiently in good yield. Under the optimized conditions, the oxidation of styrene catalysed by [VO(sal-dach)]-Y and [Cu(sal-dach)]-Y gave 94.6 and 21.7% conversion, respectively, where styreneoxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde being the major products. Oxidation of cyclohexene catalysed by these complexes gave cyclohexeneoxide, 2-cyclohexene-1-ol, cyclohexane-1,2-diol and 2-cyclohexene-1-one as major products. Conversion of cyclohexene achieved was 86.6% with [VO(sal-dach)]-Y and 18.1% with [Cu(sal-dach)]-Y. A maximum of 78.1% conversion of cyclohexane catalysed by [Cu(sal-dach)]-Y and only 21.0% conversion by [VO(sal-dach)]-Y with major reaction products of cyclohexanone, cyclohexanol and cyclohexane-1,2-diol have been obtained.     

Europium-Containing Organic Gels and Organic and Carbon Aerogels. Preparation and Initial Applications in Catalysis

Summary. We prepared organic gels and organic and carbon aerogels doped with europium through sol–gel processes. Eu-gels were prepared by sol–gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde followed by ion-exchange with Eu(OTf)₃. Eu–organic aerogels were obtained after CO₂ supercritical drying of the gels and Eu–carbon aerogels were obtained by pyrolysing the organic aerogels. The Eu-gels containing 12% europium proved to be efficient as recoverable catalyst in Michael additions. Deceased on February 20, 2006     

Recovery of polyols from flexible polyurethane foam by “split-phase” glycolysis with new catalysts

Polyurethanes (PU) represent one of the most important groups of plastics, so the increasing quantity of wastes makes their recycling an urgent task. The general purpose of polyurethane chemical recycling is to recover constituent polyol, a valuable raw material. Among the suitable processes, glycolysis, specially in two phases, allows better quality products. In this study glycolysis reactions of flexible polyurethane foams were conducted in “split-phase” with different catalysts, in order to study their activity. Diethanolamine, titanium n-butoxide as well as octoate salts, which are novel compounds for this application, showed suitable catalytic activity. Reaction kinetics and glycolysis products were investigated. Times to reach complete conversion, chemical properties of the polyol phase and its purity depend on the catalyst employed. The novel catalysts developed have been probed to be a worthy and economic alternative to traditional catalysts.