NO removal by CH4 on Co-NaX-CO and Ag-NaX catalysts in a dual-bed system

NO removal using CH₄ as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co²⁺-, Ag⁺-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co₃O₄, which promotes NO oxidation to NO₂ by excess O₂ at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N₂ by CH₄ in the presence of excess O₂ and H₂O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N₂ in a deNO bed at 523 K and a deNO₂ bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H₂O 10% gas mixture after 160 min.     

Influence of monovalent cations and CuO nanoparticles on X-nanozeolite in uranium anionic species separation from contaminated drinking water

The main purpose of this work was the modification of NaX nanozeolite using copper oxide nanoparticles and various monovalent cations such as K~+, Cs~+, and Ag~+in order to make the negatively charged zeolite surface accessible for anionic forms of uranium which are the most dominant species of uranium in the contaminated radioactive waters at natural p H. Various methods such as the X-ray fluorescence(XRF), X-ray powder diffraction(XRD), field emission scanning electron microscopy(FE-SEM), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy(FT-IR), and atomic absorption spectroscopy(AAS) were used to characterize the final synthesized absorbents. Batch technique was used to study the adsorption behavior of uranium ions from polluted drinking water by Na X nanozeolite and its modified forms. In order to better understand the performance of them, the results were compared with those that were obtained for synthesizing bulk NaX zeolite and Na-form of clinoptilolite natural zeolite. Preliminary results indicated that uranium sorption increased as the loading level of CuO nanoparticles on NaX nanozeolite increased from 2.1 wt% to 11.2 wt%. In addition,from the obtained data, an increase in uranium removal efficiency resulted as charge/ionic radius ratio of exchanged cation decreased. Also, the effect of contact time, solid–liquid ratio, initial concentration and temperature on the adsorption process was studied. It is worth mentioning that, in this study, the sorption of uranium was performed under natural conditions of pH and the presence of competing cations and anions which are available in drinking waters.     

Use of NaX porous materials in the recovery of iron ions

Nowadays, the porous materials have an economic impact in certain industrial fields and particularly that of the environment. NaX materials economically prove very interesting; small quantities of these last are enough to treat great toxic volumes of effluent. This work aims at the use of NaX solid materials in the recovery of the ions iron poisons and vermin to the environment. We used a treatment process based on ions exchange reactions. Several parameters related to these operations of treatment were studied and the used materials are types of NaX faujasites. The obtained results show that NaX faujasite and its hydrogenated form NaHX can be used like ion exchangers in the recovery of the ions iron Fe³⁺. NaX and NaHX materials preserve their crystalline structures during the treatment process until 0.2 mol/l Fe³⁺ concentrations.     

Kinetics of alkylation of phenol with methanol over Ce-exchanged NaX zeolite

The alkylation of phenol with methanol was investigated in vapor phase over a series of cerium-exchanged NaX zeolite with Ce loadings ranging from 0 to 10.43 wt%. The catalyst MX-4 with 8.86 wt% cerium was found to be the best one with total cresols selectivity of 72% at a temperature of 573 K and MeOH to phenol mole ratio of 4:1. The catalyst was also found to be quite stable in the operating range investigated. It was established that the stronger acid sties are required for C-alkylation compared to O-alkylation. From the study of the effects of various parameters, the optimum operating condition for highest cresols selectivity were determined as: MeOH to phenol mole ratio, 4:1; temperature, 623 K; space-time, 10.2 kg h/kgmole under atmospheric pressure. From the kinetic analysis of the experimental data, the apparent activation energy for the reaction was determined as 57.2 kJ/mole.     

Multidimensional NMR diffusion studies in microporous materials

The diffusional behavior of selected hydrocarbons adsorbed in NaX zeolites were investigated with pulsed magnetic field gradient (PFG) NMR methods in two different ways. First, the exchange time τ_exch of n-pentane between the interior and the exterior of the zeolite crystals was determined by two-dimensional diffusion exchange spectroscopy. The results were compared to the mean life time τ_intra as obtained by fast NMR tracer desorption method. Second, the diffusion of a propylene–propane mixture adsorbed in the zeolite was studied by Fourier transform PFG NMR, thus employing the chemical shift of the individual organic constituents as a second dimension for a selective evaluation of the PFG NMR signal decays and the subsequent extraction of corresponding diffusion coefficients. Modifications of NMR pulse sequences necessary for the application of ultra-high pulsed magnetic field gradients of up to in microporous materials are briefly discussed for both kind of experiments.     

微波合成纳米NaX分子筛的研究与表征

在常压回流微波加热条件下合成了NaX分子筛,并对其进行了XRD,FTIR,TEM,NMR表征,考察了合成配比对NaX分子筛的晶粒尺寸、形貌、硅铝比及相对结晶度的影响。表征结果显示,合成配比对NaX分子筛的晶粒尺寸、形貌、硅铝比及相对结晶度具有较为显著的影响。XRD和TEM表征结果显示,合成的NaX分子筛的晶粒尺寸为30¹00 nm。FTIR表征结果显示,当焙烧温度高于600℃时,纳米NaX分子筛的骨架开始崩塌,表明纳米NaX分子筛的热稳定性低于常规NaX分子筛。NMR表征结果显示,合成的纳米NaX分子筛具有较低的硅铝比,且硅物种配位峰的化学位移向高场偏移。微波合成的NaX分子筛的晶粒度明显小于常规合成的NaX分子筛,且晶化时间明显缩短,充分证明了微波加热快速均匀且能得到较小晶粒度的分子筛。     

Hexairidium carbonyl clusters in the micropores of faujasite zeolites: evidence from transmission electron microscopy

[Ir₆(CO)₁₆] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)₂(acac)] followed by treatment in CO + H₂. [Ir₆(CO)₁₅]²⁻ in zeolite NaX was prepared similarly. Each sample was characterized by high‐resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost no scattering centers indicating iridium outside these pores. The supported [Ir₆(CO)₁₆] and [Ir₆(CO)₁₅]²⁻, which have previously been characterized by infrared and extended X‐ray absorption fine structure spectroscopies, are among the most uniform and structurally best defined supported metal clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

NaX分子筛填充PDMS膜对丙烯／丙烷分离性能的研究

采用了对丙烯／丙烷有吸附选择性的NaX型分子筛填充PDMS,通过真空涂敷法涂敷至PPESK／PEI中空纤维基膜上,制备了NaX／PDMS中空纤维复合膜。研究了固化温度、固化时间、涂敷时间（抽真空时间）、NaX填充含量对丙烯／丙烷分离性能的影响。实验结果表明,室温下,在固化温度80℃,固化时间80min,涂敷时间6min,NaX填充含量30％时,复合膜对丙烯,丙烷的分离性能最佳,分离系数达到2．68。为丙烯／丙烷的分离提供了一种新的可能性,开辟了一条研制高性能丙烯／丙烷分离有机膜的新途径。     

The study on the selective oxidation of H<Subscript>2</Subscript>S over the mixture zeolite NaX-WO<Subscript>3</Subscript> catalysts

At temperatures lower than 250 °C the deactivation of zeolite NaX catalyst occurred in the presence of water vapor. The gradual accumulation of water vapor on the surface of catalyst could cause deactivation of catalyst. The zeolite NaX-WO₃ catalysts were prepared to study a method preventing deactivation of catalysts from the adsorption of water vapor. The zeolite NaX-WO₃ (9 : 1) with a low content of WO₃ showed the highest conversion of H₂S. It is believed that the addition of WO₃ caused either a decrease of the strong adsorption of water vapor on the zeolite NaX or an increase of the reducibility of WO₃ by some interactions between zeolite NaX and WO₃. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.