Kinetic Parameters of Polymer Degradation by SAPO-37

High density poly(ethylene) has been submitted to thermal degradation alone, and in the presence of silicoaluminophosphate SAPO-37. The processes were carried out in a reactor connected on line to a gas chromatograph/mass spectrometer in order to analyze the evolved products. Polymer degradation was also evaluated by thermogravimetry, from room temperature until 800°C, under nitrogen dynamic atmosphere, with multiple heating rates. From TG curves, the activation energy related to degradation process was calculated using the Flynn and Wall multiple heating rate kinetic model for pure polymer (PE) and for polymer in the presence of catalyst (PE/S37). SAPO-37 showed good selectivity for low molecular mass hydrocarbons in PE catalytic degradation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study on Adsorption Properties of Ammonium Exchanged Chabazite for CO2

Ammonium type chabazite (NH₄CHA) and magnesium type chabazite (MgCHA) was prepared by hydrothermal synthesis and hydrothermal ion exchange. The structure and morphology of ion‐exchanged chabazite was characterized with various experimental techniques such as XRD, SEM, and ICP. The gas adsorption amount of chabazite decreases gradually with increase in temperature or decrease in pressure. The adsorption capacity of NH₄CHA to CO₂ was 3.33 mmol · g^–1 at 273 K, and the saturated adsorption capacity of NH₄CHA reached 3.88 mmol · g^–1 under extreme pressure. The CO₂ molecule was more likely to be adsorbed by all chabazite samples than the N₂ molecule due to its linear molecular structure and greater molecular polarity. NH₄CHA exhibited larger CO₂/N₂ selectivity, which first increased and then declined with the temperature decreasing, and reached the maximum value between 400 K to 450 K.     

Synthesis of Discrete CHA Zeolite Nanocrystals without Organic Templates for Selective CO2 Capture

Small-pore zeolites such as chabazite (CHA) are excellent candidates for the selective separation of CO₂; however, the current synthesis involves several steps and the use of organic structure-directing agent (OSDA), increasing their cost and energy requirements. We report the synthesis of small-pore zeolite crystals (aluminosilicate) with CHA-type framework structure by direct synthesis in a colloidal suspension containing a mixture of inorganic cations only (Na⁺, K⁺, and Cs⁺). The location of CO₂ molecules in the host structure was revealed by 3D electron diffraction (3D ED). The high sorption capacity for CO₂ (3.8 mmol g⁻¹ at 121 kPa), structural stability and regenerability of the discreate CHA zeolite nanocrystals is maintained for 10 consecutive cycles without any visible degradation. The CHA zeolite (Si:Al=2) reaches an almost perfect CO₂ storage capacity (8 CO₂ per unit cell) and high selectivity (no CH₄ was adsorbed).     

Measurements of Heat of Zeolite Dehydration by Scanning Heating

A procedure for measurement of the heat of zeolite dehydration by scanning heating has been designed. Simultaneous data on heat flow (DSC) and mass loss (TG) are required for evaluation. The heating rate depends on the experimental conditions (point-spread function, sample mass, crucible design, and calorimetric reproducibility). Dehydration measurements have three advantages as compared with the sorption procedure: i) one can investigate samples with irreversible dehydration; ii) no approximation model is needed for calculation of the partial molar heat of dehydration; and iii) the procedure is not labor-consuming.The procedure was tested on the natural zeolites heulandite, chabazite and mordenite. The results are close to those measured by the sorption procedure. The partial molar heat of dehydration was found to depend on the water content. It increases from 50 to 87 J mol^–1 K^–1 for heulandite, from 53 to 81 J mol^–1 K^–1 for chabazite, and from 51 to 71 J mol^–1 K^–1 for mordenite.The approximation of the heat of sorption by linear regression was found to be wrong. Detection of a phase transitioN after this approximation has no meaning.This revised version was published online in November 2005 with corrections to the Cover Date.     

The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime prediction reliability

The thermooxidative degradation of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA), was carried out in a thermogravimetric (TG) analyser, at various temperatures (in the 473–533 K range), in isothermal heating conditions. The resulting set of experimental TG data was used to determine the apparent activation energy (E_a) of degradation through two isothermal literature methods, as well as through a very simple method we set up, based on the direct regression of the experimental mass loss data, in order to verify the general applicability of our method to various polymers. The results from different methods were in good agreement. Degradation experiments in dynamic heating conditions, which were also performed, gave E_a values in good agreement with those in isothermal heating conditions for PS, PC and PMMA, while for PE a large discrepancy was observed, which was discussed and interpreted. The results suggested the general applicability of our method to all polymers, independently on their structure and degradation mechanism. A long-term (about 13 months) isothermal degradation experiment was also carried out with the same polymers at relatively low temperature (423 K). Only PE and PS evidenced appreciable mass loss in the investigated period, but the experimental data were not in agreement with those from the short-term degradations at higher temperatures, thus suggesting different degradation kinetics, and a low reliability of the lifetime predictions for polymers in service based on experiments at higher temperatures.     

Dealumination and characterization of chabazite for catalytic application

Dealumination of the small-pore zeolite chabazite (CHA) was performed with ammonium hexafluorosilicate under solid and liquid-state conditions to increase the Si-to-Al ratio from 2.0 to 6.0. In the solid state, the mesopore fraction increased with repeating hydrothermal synthesis at 423 K, which was confirmed by nitrogen adsorption?Cdesorption. In the liquid state, the formation of mesopores decreased substantially and the resulting CHA had an Si-to-Al ratio of ca. 5. The result of desorption of NO?CNO₂ from the Cu ion-exchanged dealuminated CHA in the solid state indicated that the presence of mesopores reduced NO?CNO₂ adsorption and desorption of NO₂ occurred at 383 and 683 K whereas for the high-silica analog SSZ-13 desorption of NO₂ occurred mostly at 473 and at 673 K.     

异相晶种诱导快速合成低硅菱沸石分子筛

在不含有机模板剂体系(OSDAs)中,利用异相晶种(T型分子筛)诱导快速合成出纯相的低硅菱沸石分子筛。采用XRD、SEM、TEM、27Al MAS NMR和紫外拉曼等手段表征分子筛的结构属性和形貌特点。详细研究了菱沸石分子筛的晶化过程以及晶种添加量、nAl2O3/nSi O2、nH2O/nSiO2和碱度对菱沸石分子筛晶化的影响,并探讨T型分子筛晶种诱导合成菱沸石分子筛的晶化机理。原位合成体系中仅形成L型分子筛晶相,而一定量T型分子筛异相晶种的加入诱导溶胶快速制备出纯相的菱沸石分子筛。T型分子筛晶体在一定的水热条件下不断溶解而释放的六元环(6R)和四元环(4R)迅速形成菱沸石分子筛特征笼(CHA笼),抑制了L型分子筛特征单元和特征笼(不含四元环的CAN笼)的形成。     

理性设计核-壳Rh@沸石催化材料用于二烯烃选择加氢反应

Selective hydrogenation of dienes and alkynes to monoenes is an important topic of research in the fields of pharmacology and organic synthesis. Catalyst design plays a key role in this process, where a general principle involves controlling the steric diene adsorption by modifying the surface of the metal nanoparticles. For example, upon introducing Bi species into Rh nanoparticles, the resulting RhBi/SiO₂ showed 90% selectivity to 2-hexene, with 95% conversion of 1, 4-hexadiene under ambient conditions, because of the suppressed adsorption of the internal C＝C bond. However, the catalyst activity decreased remarkably; that is, the activity of the unmodified Rh/SiO₂ was about 27 times higher than that of RhBi/SiO₂. Controlled steric adsorption of the diene molecules could also be achieved by the constructing porous channels around the metal nanoparticles. For example, metal-organic framework (ZIF-8) or mesoporous silica (MCM-41) encapsulated noble metals showed high selectivity for the hydrogenation of terminal C＝C bonds. However, these catalysts had poor durability under the thermal/hydrothermal reaction/regeneration conditions. In contrast, zeolites have superior durability under harsh reaction conditions, but they are rarely used in semi-hydrogenation reactions. We recently found that metal nanoparticles fixed within zeolite crystals (e.g., ZSM-5 and Beta) efficiently catalyze the selective hydrogenation of molecules bearing multiple reducible groups. Thus inspired, we developed a catalyst by fixing Rh nanoparticles within zeolite crystals via an inter-zeolite transformation method. The Rh@CHA catalyst was synthesized by introducing Rh species into the parent Y zeolite (Rh@Y) and transformation of the Y zeolite to chabazite (CHA zeolite) under hydrothermal conditions. X-ray diffraction patterns, N₂ sorption isotherms, scanning/transmission electron microscopy images, and model reactions (hydrogenation of probe molecules) confirmed the successful fixation of the Rh nanoparticles inside the CHA zeolite crystals. As expected, the Rh@CHA catalyst was highly selective for the hydrogenation of dienes. For example, Rh@CHA showed a 2-hexene selectivity of 86.7%, with 91.2% conversion of 1, 4-hexadiene. In contrast, the generally supported Rh nanoparticle catalyst (Rh/CHA) showed a low 2-hexene selectivity of 37.2% under identical reaction conditions. Considering that Rh@CHA and Rh/CHA comprise the same CHA zeolite crystals and have similar Rh nanoparticle sizes, the remarkably high selectivity of Rh@CHA is assigned to the steric adsorption of dienes on the Rh surface controlled by the micropores of the CHA zeolite. This work demonstrates that a zeolite-fixed metal core-shell structure is a powerful tool for developing efficient catalysts to be used in diene hydrogenation.     

Modification of Potassium Chabazites Derived from Fly Ash by Dosing Extra Cations: Promoted CO2 Adsorption Capacities and Fine‐Tuned Frameworks

Potassium chabazite (K‐CHA), a typical microporous zeolite with excellent CO₂ separating properties, was synthesized with waste fly ash and modified via cation dosing treatments using cesium and zinc cations, respectively. The resulting CHAs were analyzed by XRF, XRD, FT‐IR, SEM, and N₂ physisorption, whose CO₂ adsorption properties were then tested on the reorganized TGA apparatus. It showed from XRF data that cesium and zinc cations were successfully imported in the original K‐CHA by cation dosing, but the CHA microstructures and morphologies of K‐CHA were perfectly retained as confirmed by XRD, FT‐IR, SEM and N₂ physisorption. Since there were still over 9 potassium cations per unit cell in cation dosed Cs‐CHA and Zn‐CHA, they both maintained the favored properties of K‐CHA as “molecular trapdoors”. In the following adsorption experiments, the CO₂ uptakes of Cs‐CHA and Zn‐CHA at 333 K and 1 bar, compared with K‐CHA, elevated from 1.70 mmol · g^–1 to 2.34 and 2.03 mmol · g^–1, and the import of zinc cation also presented a positive effect on the adsorption kinetics. Detailed comparisons suggested modifications with cesium and zinc cations fine‐tune the CHA complying with different mechanisms, and CHAs modified via cation perform more approvingly than fully ion‐exchanged ones, providing us important insights into CHA modifications and applications in practice.     

Temperature-induced transformations in CoAPO-34 molecular sieve: a combined in situ X-ray diffraction and FTIR study

Thermally induced processes of CoAPO-34, an aluminophosphate molecular sieve with chabasite-type structure, synthesized in the presence of morpholine as a structure-directing agent and HF as a mineralizing agent, have been studied by in situ X-ray synchrotron powder diffraction augmented with Fourier transform (FT) IR analysis. A time-resolved experiment was performed using a translating imaging plate system. At room temperature, the structure refinement by full-profile Rietveld analysis showed P-1 symmetry and the presence of one Al site with sixfold coordination. At around 400 degrees C, both fluorine and morpholine are lost, and the four-connected chabazite (CHA)-type topology is restored. Notwithstanding the metrically rhombohedral values of the cell parameters, the symmetry remains triclinic P-1. Inhomogeneous dealumination of the framework begins at 725 degrees C, accompanied by a strong triclinization of the unit cell and followed by the collapse of the structure above 775 degrees C. The insertion of cobalt ions within the CHA framework was monitored by FTIR spectroscopy, which showed that bridged Co(2+)-O(H)-P hydroxyls are present after morpholine removal.     

Effects of synthetic and natural zeolites on morphology and thermal degradation of poly(lactic acid) composites

Poly(lactic acid) (PLA) composites containing 5 wt% synthetic (type 4A) and natural (chabazite) zeolites were prepared using extrusion/injection molding. Morphological, structural, and thermal properties of composites were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC results revealed that the glass transition and melting temperatures were not significantly changed; however, the incorporation of both type 4A and chabazite zeolites enhanced the nucleation of PLA crystallites as well as increased the percent crystallinity. Thermal degradation properties of PLA and PLA/zeolite composites were studied by non-isothermal thermogravimetric analysis (TGA) in nitrogen atmosphere. TGA results showed that at temperatures above 300 °C, PLA/type 4A synthetic zeolite composites were thermally decomposed more easily than the PLA and PLA/chabazite natural zeolite composites. The apparent activation energies of thermal degradation of PLA and PLA/zeolites composites estimated using both the Flynn-Wall-Ozawa and Kissinger methods followed the same order: PLA/type 4A < PLA/chabazite < PLA.     

Surface oxidation and degradation of polyethylene in a mixed argon-oxygen plasma

Changes in surface chemical composition and the rates of formation of gaseous degradation products under the action of a mixed argon-oxygen plasma on polyethylene (PE) were studied using multiple attentuated total internal reflection (MATIR) IR spectroscopy and mass spectrometry. It was found that the dilution of a gas mixture with argon decreased the rate of degradation of the material at argon contents higher than 30%. It was demonstrated that absorbance at the absorption bands of the main oxygen-containing groups in a PE film remained unchanged over an argon concentration range from 0 to 90%. Conceivable mechanisms of processes occurring in gas and solid phases, which allowed us to describe the phenomena observed, are discussed.     

TiO2光催化降解聚乙烯薄膜

TiO2光催化降解聚乙烯薄膜;纳米TiO2;固相光催化;聚乙烯塑料;降解     

Microfluidics as A Tool to Understand the Build‐Up Mechanism of Exponential‐Like Growing Films

Microfluidics is used here for the first time to efficiently tune the growth conditions for understanding the build‐up mechanism of exponentially growing polyelectrolyte (PE) films. The velocity of PE supply and time of interaction can be successfully altered during the layer‐by‐layer assembly. Another advantage of this method is that the deposition of poly‐L ‐lysine/hyaluronic acid (PLL/HA) films in microchannels can be monitored online by fluorescence microscopy. The study demonstrates that PE mass transport to the film surface and diffusion in the film are key parameters affecting PLL/HA film build‐up. Increase of PE supply rate results in a change in the “transition” (exponential‐to‐linear growth) towards higher number of deposition steps, thus indicating a mass transport‐mediated growth mechanism.     

High-resolution TG for the Characterization of Diesel Fuel Additives

High-resolution thermogravimetric analysis (Hi-Res^TM TG) in its variable heating rate mode was applied to commercially available diesel fuel additives under a nitrogen atmosphere. A TGA 2950 from TA Instruments was used to run the experiments. For the selected diesel fuel additives, the use of variable heating rate resulted in sharper transitions and clearer onset of degradation than in traditional constant heating rate experiments. Moreover, the time required for experiments giving reasonable resolution was remarkably reduced compared to constant heating rate experiments. The technique proved to be very useful in the study of multi-component systems, such as diesel fuel additives, since it provides more detailed information regarding mass changes in the sample as a result of heat treatment when total decomposition involves more than one stage i.e. mass changes stem from consecutive reactions. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Kinetic Study of the Thermal and Oxidative Degradations of a New Poly(arylene)ether Copolymer

The degradation of a new thermoplastic poly(arylene)ether copolymer was carried out in both dynamic and isothermal heating conditions, under nitrogen flow and in a static air atmosphere. The measurements showed that the copolymer degraded through two stages in both investigated environments with the formation of a stable residue in N₂ and complete mass loss in air. The apparent activation energy values associated with the degradation processes were evaluated. The obtained results suggested different degradation mechanisms between N₂and air. Results were discussed and compared with those obtained for several polymers previously investigated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calytic Degradation of Mixed Plastics Using Natural Clinoptilolite Catalyst

Catalytic degradation of the mixture of polyethylene (PE), polypropylene (PP), and polystyrene (PS) was investigated in a semi-batch reactor. The main degradation product was liquid oil with gasoline range carbon number distributions. Clinoptilolite a natural zeolite (HNZ) showed as good a catalytic performance as silica-alumina (SA). The protonated catalysts (HNZ, HZSM-5) showed an increased production of ethylbenzene, but a decrease of styrene production. Increase of degradation temperature resulted in a decrease of ethylbenzene and propylbenzene, but an increase of styrene. Thermogravimetric analysis revealed that the increased degradation rate in the mixed plastics came from the intermolecular carbenium ion transfer between the degraded fragments of PE, PP, and PS.This revised version was published online in December 2005 with corrections to the Cover Date.     

Investigation of hexabromocyclododecane thermal degradation pathways by gas chromatography/mass spectrometry

The decomposition products of hexabromocyclododecane (HBCD), a widely used brominated flame retardant, were investigated by gas chromatography/mass spectrometry (GC/MS). HBCD thermal degradation was conducted under a moderate heating rate (10 degrees C/min) in a batch reactor using both inert and oxidizing atmospheres. GC/MS analysis allowed the identification of substances derived from the primary pyrolysis process at the moderate heating rates used. The presence of oxygen seems to have a negligible influence on the degradation products obtained in HBCD decomposition, at least at moderate heating rates. Based on the identified products, the main pathways of HBCD thermal degradation were assessed and a mechanism for HBCD decomposition was proposed. The results obtained indicate that hexa-, penta- and tetrabrominated polyaromatic structures seem not to be primary products of HBCD decomposition, and may only be obtained by secondary bromination reactions.     

Degradation of a model naphthenic acid, cyclohexanoic acid, by vacuum UV (172 nm) and UV (254 nm)/H2O2

The mechanism of hydroxyl radical initiated degradation of a typical oil sands process water (OSPW) alicyclic carboxylic acid was studied using cyclohexanoic acid (CHA) as a model compound. By use of vacuum ultraviolet irradiation (VUV, 172 nm) and ultraviolet irradiation in the presence of hydrogen peroxide UV(254 nm)/H(2)O(2), it was established that CHA undergoes degradation through a peroxyl radical. In both processes the decay of the peroxyl radical leads predominantly to the formation of 4-oxo-CHA, and minor amounts of hydroxy-CHA (detected only in UV/H(2)O(2)). In UV/H(2)O(2), additional 4-oxo-CHA may also have been formed by direct reaction of the oxyl radical with H(2)O(2). The oxyl radical can be formed during decay of the peroxyl-CHA radical or reaction of hydroxy-CHA with hydroxyl radical. Oxo- and hydroxy-CHA further degraded to various dihydroxy-CHAs. Scission of the cyclohexane ring was also observed, on the basis of the observation of acyclic byproducts including heptadioic acid and various short-chain carboxylic acids. Overall, the hydroxyl radical induced degradation of CHA proceeded through several steps, involving more than one hydroxyl radical reaction, thus efficiency of the UV/H(2)O(2) reaction will depend on the rate of generation of hydroxyl radical throughout the process. In real applications to OSPW, concentrations of H(2)O(2) will need to be carefully optimized and the environmental fate and effects of the various degradation products of naphthenic acids considered.     

Determination of the Thermal Degradation Kinetic Parameters of Carbon Fibre Reinforced Epoxy Using TG

In this work, a kinetic study on the thermal degradation of carbon fibre reinforced epoxy is presented. The degradation is investigated by means of dynamic thermogravimetric analysis (TG) in air and inert atmosphere at heating rates from 0.5 to 20°C min⁻¹ . Curves obtained by TG in air are quite different from those obtained in nitrogen. A three-step loss is observed during dynamic TG in air while mass loss proceeded as a two step process in nitrogen at fast heating rate. To elucidate this difference, a kinetic analysis is carried on. A kinetic model described by the Kissinger method or by the Ozawa method gives the kinetic parameters of the composite decomposition. Apparent activation energy calculated by Kissinger method in oxidative atmosphere for each step is between 40–50 kJ mol⁻¹ upper than E _a calculated in inert atmosphere. The thermo-oxidative degradation illustrated by Ozawa method shows a stable apparent activation energy (E _a ≈130 kJ mol⁻¹ ) even though the thermal degradation in nitrogen flow presents a maximum E _a for 15% mass loss (E _a ≈60 kJ mol⁻¹ ). This revised version was published online in July 2006 with corrections to the Cover Date.