Novel synthesis of -caprolactam from cyclohexanone-oxime via Beckmann rearrangement over mesoporous molecular sieves MCM-48

Vapor-phase synthesis of -caprolactam (-C) from cyclohexanone-oxime (CHO) has been studied at 1 atm and 300–400 °C using SiMCM-48 and AlMCM-48(X) with Si/Al molar ratios X in a fixed-bed, continuous flow reactor. The catalysts were characterized with ICP-AES, XRD, TEM, FT-IR, N₂-adsorption, ²⁷Al and ²⁹Si MAS NMR and TPD of ammonia. An increase of X value in AlMCM-48(X) enhances both the BET surface area and the unit cell parameter but diminishes the acid amount. In the reaction of CHO, benzene, toluene, ethanol and 1-hexanol were utilized as solvents. The CHO conversion increases with the reaction temperature, whereas the -C selectivity exhibits the opposite trend due to side reactions. The catalyst stability is greatly enhanced by using ethanol and 1-hexanol as the solvents due to their production of water vapor via dehydration. Excellent catalytic performance of AlMCM-48(10) is attained at 1 atm, 350 °C and W/Fc 74.6 g h/mol by using 1-hexanol in the feed; the CHO conversion and the -C selectivity exhibit higher than 99% and 90%, respectively, during at least 130 h process time.     

Numerical studies of a tube-in-tube helically coiled heat exchanger

In the present study a tube-in-tube helically coiled (TTHC) heat exchanger has been numerically modeled for fluid flow and heat transfer characteristics for different fluid flow rates in the inner as well as outer tube. The three-dimensional governing equations for mass, momentum and heat transfer have been solved using a control volume finite difference method (CVFDM). The renormalization group (RNG) k– model is used to model the turbulent flow and heat transfer in the TTHC heat exchanger. The fluid considered in the inner tube is compressed air at higher pressure and cooling water in the outer tube at ambient conditions. The inner tube pressure is varied from 10 to 30 bars. The Reynolds numbers for the inner tube ranged from 20,000 to 70,000. The mass flow rate in the outer tube is varied from 200 to 600 kg/h. The outer tube is fitted with semicircular plates to support the inner tube and also to provide high turbulence in the annulus region. The overall heat transfer coefficients are calculated for both parallel and counter flow configurations. The Nusselt number and friction factor values in the inner and outer tubes are compared with the experimental data reported in the literature. New empirical correlations are developed for hydrodynamic and heat-transfer predictions in the outer tube of the TTHC.     

Cs exchanged phosphotungstic acid as an efficient catalyst for liquid-phase Beckmann rearrangement of oximes

Cs exchanged phosphotungstic acid is a highly efficient and environmentally benign solid acid catalyst for the liquid-phase Beckmann rearrangement of ketoximes to the corresponding amides. The catalysts Cs_xH_3−xPW₁₂O₄₀ (x = 1.5, 2, 2.5 and 3) were prepared by a titration method. The characterization results indicated that the primary Keggin structure remained intact after exchanging the protons with Cs ions. Moreover, the Cs exchanged catalysts were insoluble and exhibited larger BET surface area than the parent acid. The catalysts exhibited high reactivity and selectivity for the formation of -caprolactam, the precursor of Nylon 6, from cyclohexanone oxime. The catalyst can be recovered after reaction without any structural transformation.     

Development of mesoporous Al,B-MCM-41 materials: Effect of reaction temperature on the catalytic performance of Al,B-MCM-41 materials for the cyclohexanone oxime rearrangement

A series of aluminum–boron–silicate MCM-41 mesoporous materials and their counterparts treated with NH₄F aqueous solution were synthesized and characterized by using XRD, MAS NMR, nitrogen physisorption, DRIFT, TG-DTA, TP/MS and pyridine adsorption. All of the samples showed typical MCM-41 structural and textural properties. ²⁷Al MAS NMR showed that the aluminum environment was mainly four-coordinated and six-coordinated aluminum for non-fluorinated samples and fluorinated ones, respectively. Boron was in the trigonal framework environment at ca. catalytic reaction temperatures and the NH₄F treatment did not affect the boron environment in our Al,B-MCM-41 materials. All of the Al,B-MCM-41 materials studied contained both Brønsted and Lewis acid sites. However, the strong acid Brønsted/Lewis ratios decreased in the fluorinated catalysts. Moreover, the influence of temperature was studied on the cyclohexanone oxime conversion and the product selectivity in the 623–798 K range. Results indicated that temperatures lower than 748 K favored Beckmann rearrangement to -caprolactam, whereas, at higher temperatures the main reaction was cyclohexanone oxime hydrolysis to cyclohexanone. The aluminum–boron–silicate MCM-41 mesoporous materials treated with NH₄F improved both the selectivity to -caprolactam (related mainly to boron content) and their life span (related to their lower ratios of strong Brønsted/Lewis acid sites).     

Synthesis of tadpole-shaped copolyesters based on living macrocyclic poly(-caprolactone)

Synthesis of an asymmetric tadpole-shaped aliphatic copolyester consisting of a poly(-caprolactone) ring and two poly(l-lactide) tails was reported for the first time. First, a high molecular weight cyclic PCL macroinitiator (M_n = 31,000) was prepared by intramolecular photocross-linking of “living” chains. Polymerization of l-lactide was resumed by the tin dialkoxide containing macrocycles, thus making the targeted tadpole-shaped copolyester available. A preliminary investigation of the crystallization of these copolyesters was carried out by differential scanning calorimetry and polarized optical microscopy.     

Preparation of poly(-caprolactone) grafted titanate nanotubes

This work reported for the first time the surface functionalization of titanate nanotubes (TNTs) with biodegradable poly(-caprolactone) (PCL). A “grafting from” approach based on in situ ring-opening polymerization of -caprolactone from TNTs with a special surface modification was adopted to prepare the PCL-g-TNTs. The thickness of the grafted PCL shell can be controlled by increasing reaction time. After grafted with PCL, both the dissolubility and flexibility of the tubes were greatly improved. The obtained PCL-g-TNTs can easily disperse in several organic solvents, and the dispersal stability depends on solvent polarity and PCL shell thickness. Furthermore, the PCL immobilized on the surface of TNTs still possessed a good biodegradable capacity and could be completely decomposed in the presence of Pseudomonas (PS) lipase. The PCL-g-TNTs reported here are promising in biotechnology applications due to good dissolubility, flexibility, biocompatibility and the tubular nano-structure.     

A novel route to α,ω-telechelic poly(-caprolactone) diols, precursors of biodegradable polyurethanes, using catalysis by decamolybdate anion

A new convenient route for the synthesis of poly(-caprolactone) (PCL) with α,ω-telechelic diols' end-groups is presented. Synthesis of α,ω-telechelic PCL diols (HOPCLOH) was achieved by ring-opening polymerization (ROP) of -caprolactone (CL) catalyzed with ammonium decamolybdate (NH₄)₈[Mo₁₀O₃₄] and using diethylene glycol (DEG) as initiator. Obtained HOPCLOH was characterized by ¹H and ¹³C NMR, FT-IR, GPC and MALDI-TOF. Comparative studies demonstrate that ammonium decamolybdate (NH₄)₈[Mo₁₀O₃₄] is better catalyst than Sn-octanoate (SnOct₂) toward CL polymerization in presence of DEG, under the conditions tested. A biodegradable poly(ester-urethane-urea) derivative was efficiently prepared from synthesized HOPCLOH. Obtained polymer shows minor differences with respect to the properties recorded for a poly(ester-urethane-urea) obtained from commercial HOPCLOH.     

Structure and magnetic properties of Fe1−xCox nanowires in self-assembled arrays

Fe_1−xCo_x nanowires in self-assembled arrays with varying compositions were produced by the template-assisted pulsed electrochemical deposition method. The structural and magnetic properties of the arrays were investigated using several experimental techniques. TEM analyses indicated that the nanowires were regular, uniform, 8 μm in length and 50 nm in diameter. The results of X-ray diffraction indicated that the body-centered-cubic (bcc) (α), face-centered-cubic (fcc) (γ), and hexagonal-close-packed (hcp) () Fe–Co phases appeared in different compositions. Magnetic measurements showed that the coercivity and squareness of the hysteresis loops of the Fe_1−xCo_x changed with their compositions, which may be attributable to shape anisotropy. The room temperature ⁵⁷Fe Mössbauer spectra of the arrays of the Fe_1−xCo_x nanowires revealed strong shape anisotropy.     

Novel synthetic strategy toward shape memory polyurethanes with a well-defined switching temperature

Shape memory polyurethanes (SMPUs) have been synthesised via a novel synthetic methodology, resulting in an improvement of the phase separation in the multi-block structure of the polyurethane and in its shape memory properties. ABA block copolymers based on semi-crystalline poly(-caprolactone) and amorphous poly(propylene oxide) (PPO) were used as precursor for the SMPUs. For their synthesis, poly(-caprolactone) diols have been converted into isocyanate end-capped prepolymers by using a mixture of 3(4) isocyanato-1-methyl-cyclohexylisocyanate isomers, after which a coupling with low-T_g poly(propylene oxide) oligomers is done. The shape memory polymers are obtained by reaction of the ABA block copolymers with hexamethylenediisocyanate and 1,4-butanediol as chain extender. Using this new strategy, a flexible segment (PPO) was introduced between the hard and the switching segments of the SMPU. For comparison, SMPUs without flexible segment have also been prepared with the conventional synthetic route. DSC, isostrain experiments and cyclic shape memory tests revealed narrower switching temperatures for the SMPUs including a flexible segment.     

Effect of carbonization time on the structure and electromagnetic parameters of porous-hollow carbon fibres

A series of polyacrylonitrile-based porous-hollow carbon fibres (PAN-PHCFs) were prepared by carbonizing PAN porous-hollow cured fibres at 1073 K for different times in nitrogen. The effects of carbonization time on the structure, electrical volume conductivity and electromagnetic parameters were investigated. Results indicate that the degree of graphitization increases as carbonization time increases. The electrical volume conductivity increases as the degree of graphitization and carbonization time increase. The real and imaginary parts of the complex permittivity (′ and ″) increase with carbonization time increasing. The values of ′ and ″ of composites of PAN-PHCFs and paraffin are 13.76 and 10.09 when the carbonization time is 240 min, and the electrical volume conductivity of PAN-PHCFs is 190.47 Ω⁻¹ m⁻¹.     

The effect of solids on the dense phase gas fraction and gas–liquid mass transfer at conditions close to the heterogeneous regime in a mechanically agitated vessel

In the heterogeneous regime, there is a strong literature evidence (discussed herein) that solids can supplant small bubbles in the dense phase and reduce the gas hold-up. This work examines the effects of the addition of 205 μm glass ballotini on the gas hold-up and k_La in a 0.286 m diameter stirred tank operated under intense conditions (P/V ≥ 5 kW m⁻³) close to the heterogeneous regime and above the agitator speed corresponding to the just suspended point, N_JS. The tests were carried out on two systems: air–water (coalescing) and air–0.2 M sodium sulphate (salt) solution (which resists coalescence in the bubble regime). For the air–water system it was observed that the overall gas plus solids hold-up remains approximately constant until all the small gas bubbles are supplanted and then increased in direct relation to the solids volume. The k_La mirrored the gas hold-up trend and decreased with a fall in gas hold-up. In the salt solution k_La decreased in direct relationship to the solids concentration, to 40% of the no solids value at around 19% solids by volume of dispersion. Dynamic engagement and disengagement experiments established that the salt solution behaves differently than water with the small bubbles initially growing in size rather than being coalesced directly into the large bubble population.     

Dielectric properties and microstructures of CaSiO3 ceramics with B2O3 addition

The effects of B₂O₃ additives on the sintering behavior, microstructure and dielectric properties of CaSiO₃ ceramics have been investigated. The B₂O₃ addition resulted in the emergence of CaO–B₂O₃–SiO₂ glass phase, which was advantageous to lower the synthesis temperature of CaSiO₃ crystal phase, and could effectively lower the densification temperature of CaSiO₃ ceramic to as low as 1100 °C. The 6 wt% B₂O₃-doped CaSiO₃ ceramic sintered at 1100 °C possessed good dielectric properties: _r = 6.84 and tan δ = 6.9 × 10⁻⁴ (1 MHz).     

Aerobic oxidation of alcohols over novel crystalline MoVO oxide

Novel crystalline MoVO oxide was employed as the catalyst in the aerobic oxidation of alcohols to the corresponding carbonyl compounds. Reactions were mainly conducted at 353 K in pure oxygen or air (1 atm). The selectivities for benzaldehydes were more than 95% in all cases. The conversions of benzyl alcohols varied from 10% to 99% depending on the substituent. A Hammett plot gave a moderate ρ-value of −0.249 (r² = 0.98), suggesting that the reaction processes may involve hydride abstraction. The oxidation of primary alkanols afforded aldehydes, and secondary alcohols were mainly dehydrated to olefins. It was found that the conversion of linear alkanols decreased with the length of alkanols. Kinetic analysis showed that catalytic reaction rate was first-order dependent on the concentrations of substrate and of catalyst. The apparent activation energy was estimated to be 45.7 kJ mol⁻¹. Catalytic reactions took place on the 6- or 7-member rings on the a–b basal plane, where highly dense unsaturated metal cation centers and oxygen anion might serve as catalytic active sites.     

Influence of liquid properties on flow regime and backmixing in a special bubble column

An experiment aimed to link the extent of axial mixing in a special configuration bubble column reactor with different liquid properties (water, 10% K₂CO₃ solution, 20% K₂CO₃ solution, paraffine). The experimental results proved that, increase of liquid viscosity will delay the mean residence time and weaken gas axial backmixing. Increased surface tension leads to lower flow regime transition point and higher overall gas holdup. Surface tension is the dominant factor to influence of gas axial backmixing degree. A simple RTD model for homogeneous–heterogeneous regime is developed in the column of 0.1 m diameter and the corresponding correlation of gas axial dispersion coefficients is . The model is verified by experiments with air/water/paraffine system. Good agreement is found. As a byproduct, a non-empirical formula for gas holdup results, _g/(1−_g)⁴ = 0.579 (u_gμ/σ)^0.918 (μ⁴g/ρ_Lσ³)^−0.252. But both correlations cannot be available for K₂CO₃ solution with addition of small quantities of surface tension in pure liquid.     

Modelling of heat transfer and hydrodynamic with two kinetics approaches during supercritical water oxidation process

Supercritical water oxidation is an innovative and very efficient process to treat hazardous organic waste. In order to better understand the complex physic phenomena involved in this process, and to design more efficient reactors or to insure future efficient scale-up, a simulation with the Computational Fluid Dynamics software FLUENT was carried out for a simple tubular reactor.The turbulent non-reactive flow is well-represented using the k– model. Nevertheless, the k–ω model gives better results when a source term is added to take into account the chemical reaction.Two approaches are used to model the reaction rate: an Arrhenius law and the Eddy Dissipation Concept (EDC) generally used to describe combustion reactions.The results of this simulation using Arrhenius law, are in good agreement with experimental data although a simple thermohydraulic model was used. Moreover, the sensitiveness to the inlet temperature has been demonstrated. It influences the reaction start-up and the shape of the measured wall temperature peak. Equally, the simulated temperature profiles using Eddy Dissipation Concept model are in good agreement with experimental ones. Hence, the two approaches give similar results. Nevertheless, the EDC model predicts more precisely the thermal peak location at the reactor wall.     

Electrochemical behaviour of N-acetyl-l-cysteine on gold electrode—A tentative reaction mechanism

The electrochemical behaviour of N-acetyl-l-cysteine (NAC) has been investigated by linear and cyclic voltammetry on gold electrode at room temperature. The results showed two oxidation peaks under acid and neutral conditions and only one in basic medium. For each oxidation, as many electron was exchanged as proton. The influence of both the concentration and the potential scan rate on the peak currents highlighted a diffusion-controlled phenomenon for the first peak and an adsorption-limited reaction rate for the second one. The diffusion coefficient of NAC in solution and the surface concentration of the adsorbed species at pH 3 and 7 were close to 2 × 10⁻⁴ to 2 × 10⁻⁵ cm² s⁻¹ and 6 × 10⁻⁹ to 6 × 10⁻¹⁰ mol cm⁻², respectively. Film transfer experiments resulted in an irreversible adsorption of NAC on gold electrode, and the formation of a self-assembled monolayer (SAM).     

Acid- and base-catalyzed hydrolyses of aliphatic polycarbonates and polyesters

Poly(propylene carbonate) (PPC) was synthesized by the zinc glutarate catalyzed copolymerization of carbon dioxide and propylene oxide (PO). Hydrolytic degradability of the PPC polymer was examined in tetrahydrofuran solutions containing 10 wt.% acidic or basic aqueous solutions of varying pH using viscometry and GPC analysis. Further, the hydrolysis behaviors of all PPC solutions were compared with those of poly(-caprolactone) (PCL) and poly(d,l-lactic acid) (PLA). All polymers studied show higher degradability in strong basic conditions than in strong acidic conditions, but very low degradability in moderate acidic, basic and neutral conditions. Moreover, PPC is degraded less in strong acidic conditions than the polyesters, while in strong basic conditions, the polycarbonate is more easily degraded. The difference in degradabilities of these polymers in acidic conditions is associated with the different nucleophilicities of their carbonyl oxygen atoms, while in basic conditions the differences are associated with the different electrophilicities of the corresponding carbonyl carbon atoms. With regard to the hydrolysis results and the structural and chemical nature of the polymer backbones, degradation mechanisms are proposed for the acid- and base-catalyzed hydrolyses of PPC, PCL and PLA.     

Preferential oxidation of CO in excess hydrogen over a nanostructured CuO–CeO2 catalyst with high surface areas

CuO–CeO₂ is prepared by coprecipitation and ethanol washing and characterized using BET, HR-TEM, XRD and TPR techniques. The results show that CuO–CeO₂ is nanosized (r_TEM = 6.5 nm) and possesses high surface area (S_BET = 138 m² g⁻¹). Furthermore, some lattice defects in the surface of CuO–CeO₂ are found, which are beneficial to enhance catalytic performance of CuO–CeO₂ in preferential oxidation of CO in excess hydrogen (PROX). Consequently, the nanostructured CuO–CeO₂ exhibits perfect catalytic performance in PROX. Namely, CO content can be lowered to less than 100 ppm at 150 °C with 100% selectivity of O₂ in the presence of 8% CO₂ and 20% H₂O at .     

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O₂ ^– and O₂ ^2– adspecies), most probably O₃ ^2– formed from reversible redox coupling of an O₂ adspecies at an anionic vacancy with a neighboring O^2– in the surface lattice. This active precursor may initiate H abstraction from CH₄ and be itself converted to OH^–+O₂ ^–, or it may abstract an electron from the oxide lattice and be converted to O₂ ^2–+O^–. The prospect of developing this type of OCM catalysts is discussed.     

Sorption of boron by invasive marine seaweed: Caulerpa racemosa var. cylindracea

The sorption of boron from aqueous solution onto Caulerpa racemosa var. cylindracea (CRC), collected from Seferihisar/Izmir region in Turkey, was investigated as a function of pH, temperature, initial boron concentration, adsorbent dosage, contact time and ionic strength. Optimum conditions for the sorption of boron were obtained at pH 7.5, 318 K, 8 mg L⁻¹ initial boron concentration, 0.2 g of CRC, 2.5 h contact time and greater ionic strength (10⁻¹ M NaCl). As the temperature was increased the boron removal took place with higher percentages. In experiments conducted at optimum conditions, maximum boron sorption was determined to be about 63%. The experimental data were analyzed by Freundlich, Langmuir and Dubinin–Radusckevich (DR) equations. Freundlich and DR models provide best conformity with the experimental data. In order to describe kinetics of boron sorption onto CRC, first-order Lagergren equation, pseudo-second-order kinetic model and intraparticle diffusion model were used. It was seen that the first order Lagergren equation was better described than the pseudo-second-order kinetic model. Thermodynamic parameters of sorption process were also calculated. It was obtained that sorption process was not spontaneous. The characterization of CRC was carried out by Fourier transform infrared spectroscopy (FTIR) analysis.