A Comparative Study of Ethylene/α-Olefin Copolymerization with Silane-Modified Silica-Supported MAO Using Zirconocene Catalysts

Activities of ethylene/α-olefin copolymerization were found to increase with silane-modified silica-supported MAO using ansa-zirconocene catalyst. The increase in activities was less pronounced when higher α-olefins were used. However, silane modification resulted in the narrower molecular weight distribution of polymers. ¹³C NMR revealed that ethylene incorporation in all systems gave polymers with the similar triad distribution.     

Observation on Different Turnover Number in Two-phase Acid-catalyzed Esterification of Dilute Acetic Acid and 1-Heptanol

This paper reports on the two-phase acid-catalyzed esterification of dilute acetic acid and 1-heptanol. It reveals larger turnover number of the Amberlyst 15 than sulfuric acid. It indicates that adsorbed water on protonated sites is replaced by acetic acid, then moving into the organic phase to promote the reaction.     

Impact of quenching process on the surface defect of titanium dioxide for hydrogen production from photocatalytic decomposition of water

Nanocrystalline titania was prepared by solvothermal reaction of titanium butoxide in toluene at 300 °C for 2 h. Thus obtained-powder was calcined at 300 °C in box furnace for 1 h and then quenched in various media at different temperature. The physiochemical properties of samples were investigated by using X-ray diffraction (XRD), nitrogen adsorption, CO₂-Temperature Programmed Desorption (CO₂-TPD), UV–visible scanning spectrophotometer, Transmission electron microscopy (TEM) and electron spin resonance spectroscopy (ESR) techniques. All physical properties such as phase, BET surface area and crystal size were not changed after quenching processes. While the CO₂-TPD and ESR results indicate the changing of Ti³⁺ contents on the surface of TiO₂ after quenching process. The amounts of Ti³⁺ increased as the quenching temperature decreased. Photocatalytic decomposition of water was carried out to evaluate the catalytic activity of quenched TiO₂. The activity of quenched-powder increased corresponding to the increasing of Ti³⁺ contents increased by following order: air at 77 K > air at RT > air at 373 K > 30 wt% H₂O₂ at RT = 30 wt% H₂O₂ at 373 K > H₂O at RT > H₂O at 373 K.     

Dielectric properties and solubility of multilayer hyperbranched polyimide/polyhedral oligomeric silsesquioxane nanocomposites

Multilayer hyperbranched polyimide/polyhedral oligomeric silsesquioxane (POSS) nanocomposites were synthesized by the reaction of a bromide‐hyperbranched polyether/POSS and a main‐chain polyimide containing hydroxyl‐functional groups. The first layer was formed through the direct reactions of the main‐chain hydroxyl groups with monochloroisobutyl polyhedral oligomeric silsesquioxane (POSS–Cl). The second and third layers were prepared by the repeated reactions of bromine ether branches that incorporated POSS–Cl with 3,5‐dihydroxybenzyl alcohol. Regardless of the fixed amount of POSS, the higher layers yielded lower dielectric constants. Even when the amount of the POSS loading was reduced 4‐fold, the third layer still had the lowest dielectric constants. The lowest dielectric constant of 2.54 was found in the third layer of the hyperbranched polyimide/POSS nanocomposite because of the large free volume and loose polyimide structures. The densities of the hyperbranched polyimide/POSS nanocomposite corresponded to the dielectric constants. The lower the density was, the higher the free volume was and the lower the dielectric constant was. The experimental results indicated that the hyperbranched polyimide/POSS nanocomposite exhibited increased solubility in comparison with pure polyimide. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of Particle Size on the Hydrothermal Stability and Catalytic Activity of Polycrystalline Beta Zeolite

The effect of particle size in the range of 0.2–0.9 μ m on the hydrothermal stability of polycrystalline beta zeolites was investigated in terms of changes in BET surface areas, percent crystallinity, and framework aluminum atoms. It was found that the hydrothermal stability of beta zeolite increased with increasing particle size while the catalytic activities decreased. However, the XRD results have revealed that percent crystallinity of the hydrothermally treated beta zeolite remained relatively high (> 95%) although dealumination occurred in most cases. This high stability is due probably to small amount of Al atoms present in the unit cell of this zeolite.     

New synthesis methods for polypropylene‐co‐ethylene‐propylene rubber

In this research, the reinforcement of polypropylene (PP) was studied using a new method that is more practical for synthesizing polypropylene‐block‐poly(ethylene‐propylene) copolymer (PP‐co‐EP), which can be used as a rubber toughening agent. This copolymer (PP‐co‐EP) could be synthesized by varying the feed condition and changing the feed gas in the batch reactor system using Ziegler–Natta catalysts system at a copolymerization temperature of 10°C. The ¹³C‐NMR tested by a 21.61‐ppm resonance peak indicated the incorporation of ethylene to propylene chains that could build up the microstructure of the block copolymer chain. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA) results also confirmed these conclusions. Under these conditions, the morphology of copolymer trapped in PP matrix could be observed and the copolymer T_g would decrease when the amount of PP‐co‐EP was increased. DMA study also showed that PP‐co‐EP is good for the polypropylene reinforcement at low temperature. Moreover, the PP‐co‐EP content has an effect on the crystallinity and morphology of polymer blend, i.e., the crystallinity of polymer decreased when the PP‐co‐EP content increased, but tougher mechanical properties at low temperature were observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3609–3616, 2007     

Effect of Surface Tungstate W<Superscript>5+</Superscript> Species on the Metathesis Activity of W-Doped Spherical Silica Catalysts

The high surface area W-doped spherical silica (SSP) catalysts were prepared with different sequences of W and Si addition (W–Si_(Alt), Si₁–W_2, and W₁–Si₂) by the sol–gel method with CTAB as a structure directing agent and compared with the impregnated one (W/SSP). All the catalysts exhibited high specific surface area (～?1100 m² g^?1) with a closely perfect spherical shape. The presence of surface/sub-surface tungstate W⁵⁺ species, crystalline bulk WO₃, and tetrahedral tungsten oxide species on the prepared catalysts was investigated by means of X-ray photoelectron spectroscopy depth profile analysis, X-ray diffraction, and Raman spectroscopy. Without in situ reduction by the reactants/products, tungstate W⁵⁺ species was found on the top surface of the as-prepared W–Si_(Alt) whereas for the Si₁–W₂, W/SSP, and W₁–Si₂, the W⁵⁺ appeared only on the sub-surface of the catalysts after 5 and 15 s Ar⁺ etching. The abundance of surface W⁵⁺ species is suggested to facilitate the establishment of the active tungsten carbenes and was correlated well to the catalytic activity in propene metathesis. The surface W⁵⁺-activity relationship of the WO₃-based metathesis catalysts is useful especially when the catalyst activity did not depend solely on the amount of active tetrahedral coordinated tungsten oxides.     

Comparative Study of Lewis Acid Transformation on Non-reducible and Reducible Oxides Under Hydrogen Atmosphere by In Situ DRIFTS of Adsorbed NH<Subscript>3</Subscript>

The Lewis acid transformation to Bronsted acid was investigated over the Pt/γ-Al₂O₃ hybrid catalysts in the presence of hydrogen atmosphere by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed NH₃. The changes of FTIR spectra were monitored during the introduction of hydrogen at 40 °C and atmospheric pressure for 130 min. The degrees of Lewis acid transformation were varied by addition of non-reducible (SiO₂ and Al₂O₃) and reducible (ZrO₂, TiO₂ and CeO₂) oxides to the Pt/γ-Al₂O₃ catalysts as the hybrid catalysts. According to the in situ DRIFTS, the hydrogen temperature programmed reduction (H₂-TPR), and the hydrogen temperature programmed desorption (H₂-TPD) results, the introduction of hydrogen resulted in a decrease in the amount of ammonia adsorbed on Lewis acid sites, and an increase in the amount of ammonium ions on Bronsted acid sites with time on stream. It is proposed that ammonia migration from Lewis acid sites to Bronsted acid sites occurred during the introduction of hydrogen in the presence of Pt particles when compared to the observation of only observed catalysts (without Pt particles). The addition of reducible oxides led to the high rate of Lewis acid transformation, which was higher than those of the non-reducible oxides. Weaker Lewis acid sites and higher amount of hydrogen spillover over the observed catalysts enhanced the rate of Lewis acid transformation in this study. However, the amount of Lewis acid sites at the initial stage did not play an important role in these transformations.     

The influence of Si-modified TiO2 on the activity of Ag/TiO2 in CO oxidation

Nanocrystalline TiO₂ and Si-modified TiO₂ with Si/Ti ratios 0.01, 0.05, 0.1, and 0.3 were prepared by the solvothermal method and employed as the supports for Ag/TiO₂ catalysts for CO oxidation reaction. The incorporation of Si into the TiO₂ lattice in the form of Ti–O–Si as revealed by FT-IR results could inhibit the agglomeration of TiO₂ crystallites, resulting in an increase of both surface area and metal dispersion. However, there existed an optimum content of Si/Ti at ca. 0.05–0.1 which resulted in an improved catalytic activity of Ag/TiO₂ in CO oxidation. Based on the O₂-temperature program desorption (O₂-TPD) results, the catalysts with appropriate amounts of Si/Ti exhibited higher amount of O₂ adsorption and much lower desorption temperature. It is suggested that the presence of Ti–O–Si promoted the formation of active oxygen species and increased the mobility of lattice oxygen so that the catalytic activity was enhanced. There was no improvement in CO oxidation activity of the Ag/TiO₂ catalyst when the Si/Ti was further increased to 0.3 due probably to the formation of amorphous SiO₂ instead of the Ti–O–Si bond.