Zeolite ZnY catalysts prepared by solid-state ion exchange

Zeolites ZnY with various overall zinc contents were prepared from mixtures of zeolite NH₄Y and crystalline zinc chloride by solid-state ion exchange. The obtained materials were investigated with carbon monoxide, xenon, and nitrogen adsorption as well as with ¹²⁹Xe NMR and XRF spectroscopy. From the results of these measurements, the zinc cation distributions between the different types of cages of the faujasite framework as well as between the crystallographic positions SIII and SII within the large voids (supercages) were quantitatively determined. The concentrations of zinc cations in the supercages of the presently prepared zeolites are considerably higher than in materials obtained from NaY by conventional wet ion exchange using aqueous zinc salt solutions. Experimental evidence is provided for salt inclusion under certain conditions of preparation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Number of metallic clusters in Y zeolites obtained from129Xe NMR

We present a simple model which permits to obtain the number of metallic clusters entrapped in Y zeolites from the curves of the chemical shifts of¹²⁹Xe NMR against the pressure of Xe. From the metal loading the average number of atoms per cluster can also be calculated. We apply the model to Pt/NaY and the bimetallic Pt-Cu/NaY.     

Search for confinement effects in mesoporous supports: hydrogenation of o-xylene on Pt°/MCM-41

The occurrence in mesoporous supports of a confinement effect related to the adsorption strength of a reaction intermediate on a metallic phase dispersed in the mesopores is examined, as well as its effect on the activity and selectivity. A model is proposed in order to determine the catalyst requirements (size of the pores and of the metal particles), which would allow to maximise the confinement. Alkyltrimethylammonium surfactants with increasing alkyl chain length were used to prepare MCM-41 supports, with controlled pore size between 22 and 39 Å and Pt° particles of controlled size (from 11 to 18 Å) were formed in the porosity of the MCM-41 supports. The reaction of hydrogenation of o-xylene was chosen as model reaction, because (i) previous studies have highlighted the influence of the adsorption strength of 1,2-dimethylcyclohexene (DMCHE, reaction intermediate formed by cis-addition of four hydrogen atoms on o-xylene) on the metallic surface on the selectivity toward trans-dimethylcyclohexane (trans-DMCH) and (ii) the kinetic diameter of DMCHE (6.4 Å) is close to the distance between the metallic particle surface and the silica wall of the series of Pt°/MCM-41 catalysts. Slight decrease in the turn-over frequency (TOF) and increase in the selectivity toward trans-DMCH are observed for the smaller pore sizes, and the assignment of this effect to a confinement of the reaction intermediate is discussed.     

新型固体超强酸SO_4~(2-)/ZrO_2/MCM-41催化合成二芳基乙烷的研究

用水热合成法合成了一种新型的复合型固体超强酸SO42-/ZrO2/MCM-41催化剂,并通过IR和XRD方法进行了表征。在固定床反应器中,以苯乙烯和二甲苯为原料,对该催化剂催化合成二芳基乙烷的反应进行了研究,确定最佳反应条件:苯乙烯与二甲苯混合进料物质的量比为1∶4,反应温度75℃,空速1 h-1。在此条件下,苯乙烯单程转化率可达93.5%,二芳基乙烷的选择性为78.9%。表明该催化剂是替代液体酸合成二芳基乙烷的理想固体酸催化剂。     

One-pot synthesis of fluorescent mesoporous materials for detection of the presence of Be<Superscript>2+</Superscript> ion

A fluorescent mesoporous silica solid (F-MCM-41) has been successfully prepared by one-pot hydrothermal synthesis from TEOS and 2-((3-(triethoxysily)propylimino)methyl)phenol in the presence of CTMABr surfactant, followed by removal of CTMABr in F-MCM-41 from extraction of ethanol solution at 50 °C for 5 h. The fluorescent sample was characterized with X-ray diffraction, nitrogen isotherms, nuclear magnetic resonance (NMR), and fluorescence spectroscopy, and the obtained results show that the sample has ordered hexagonal mesoporous symmetry, uniform pore size, high BET surface area, large pore volume as well as good fluorescent properties. When this sample is used to detect Be²⁺ ions in water, the degree of fluorescence quenching is dependent on the concentration of Be²⁺ solutions ranged from 1 to 20 μg/L.     

Hydrodesulfurization of Dibenzothiophene Over Ni-Mo Sulfides Supported by Proton-Exchanged Siliceous MCM-41

Strong acid sites on the surface of mesoporous MCM-41 were generated by ion-exchanging siliceous MCM-41 with dilute HNO₃ solution (0.5 M). The XRF determination indicates that most of the sodium cations contained in MCM-41 can be removed by the proton exchange, and dealuminization was observed during the proton exchange. The acidity of the mesoporous materials was characterized by means of NH₃-TPD and the Hammett indicators. It is revealed that new strong acid sites (-5.6 > H₀ > -8.2) were generated after the first 2 h of ion exchange and that the following ion exchanges had little effect on the acidic properties. XRD patterns of the mesoporous materials indicate that the structure of siliceous MCM-41 was improved by HNO₃ ion exchange. When Ni-Mo sulfides were supported on the prepared solid acid (H⁺-MCM-41), high performance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was observed. However, the HDS activity was decreased while the selectivity of biphenyl (BP) was increased, when H⁺-Si-MCM-41 was ion exchanged with Na₂CO₃ aqueous solution. TPR profiles of the supported Ni-Mo oxides reveal that the acidic properties of the supports greatly influence the hydrogenation activities of the bimetallic oxides. The high performance of H⁺-MCM-41-supported Ni-Mo catalysts may be attributed to the enhanced hydrogenation activity. The introduction of Na cation into the support led to the decrease of the HDS activity due to the poor hydrogenation ability of the supported bimetallic oxides. The HDS activity is well correlated with the low H₂ consumption temperature in the TPR profiles.     

Chemisorption of H2 on supported Pt clusters probed by129Xe NMR

The surface of Pt clusters with average size between 1 and 8 nm supported on SiO₂, -Al₂O₃ or Y-zeolite was probed by¹²⁹Xe NMR as the Pt surface coverage with hydrogen, , was increased. A distinct change in the structure of the hydrogen overlayer at 0.3 was inferred from the NMR spectra. This change is believed to take place when the chemisorbed hydrogen fills all the next nearest neighbor metal sites and the nearest neighbors start to be occupied. These new observations clarify previously reported determinations of the average number of Pt atoms in supported clusters by means of Xe NMR and other techniques. It also appears that interfacial metal-support interactions may be probed by Xe NMR.     

Oligomerization of propene on an alumoxane-grafted MCM-41 host with bis(cyclopentadienyl)zirconium dimethyl (Cp2Zr(CH3)2)

The anchoring of alumoxane, synthesized by the in situ hydrolysis of trimethylaluminum, on the internal pore walls of a mesoporous MCM-41 support generates a highly active and selective host for bis(cyclopentadienyl)zirconium dimethyl (Cp₂Zr(CH₃)₂) in the oligomerization of propene. The regioselective preference of the immobilized metallocene is preserved and a typical Flory–Schulz distribution for the propene oligomers is obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Immobilization of new Mn(salen) complex over MCM-41 and its activity in asymmetric epoxidation of styrene

New tetradentate chelates of bis-Schiff bases were synthesized and then these chiral salen ligands were immobilized over mesoporous MCM-41 by using the ion-exchange method. The efficiency of the chiral catalyst was examined in the asymmetric epoxidation of styrene. Chiral Mn(salen) complexes immobilized onto mesoporous MCM-41 were stable during the reaction without any leaching and exhibited relatively high enantioselectivity for epoxidation as compared with homogeneous complexes. This revised version was published online in August 2006 with corrections to the Cover Date.     

A new solid acid catalyst grafting BF3·Et2O on mesoporous molecular sieve MCM-41

A new Solid acid catalyst has been prepared by grafted BF₃·Et₂O on mesoporous molecular sieve MCM-41. The solid catalyst would effectively promote the opening reaction of epichlovohydrin with isobutanol to form 1-isobutoxy-3-chloropropanol, and showed a nice activity and selectivity.     

Adsorption behavior and kinetics of exchange of Co2+ and Eu3+ ions on polymer composite ion exchanger

Equilibrium and kinetics of exchange of ⁶⁰Co and ^152+154Eu radionuclides on hydrous titanium oxide incorporated in polyacrylonitrile, TiO-OH-PAN, were studied. The capacities of TiO-OH-PAN for ⁶⁰Co and ^152+154Eu were found to be 2.35 and 2.69 meq/g respectively. Freundlich and Langmiur isotherms were investigated. The validity of the Langmuir isotherm over the specified concentration range indicates monolayer coverage of the surface of TiO-OH-PAN by cobalt and europium. Sorption data was treated with the Dubinin-Radushkevich equation. The results indicated that hydrous titanium oxide incorporated in polyacrylonitrile can be considered as a promising material for removal of cobalt and europium ions from aqueous solution.     

Ship-in-Bottle synthesis of Pt9-Pt15 carbonyl clusters inside NaY and NaX zeolites,in-situ FTIR and EXAFS characterization and the catalytic behaviors in13CO exchange reaction and NO reduction by CO

[Pt₉(CO)₁₈]^2–/NaY (orange-brown, 2056 and 1798 cm^–1), [Pt₁₂(CO)₂₄]^2–/NaY (dark-green, 2080 and 1824 cm^–1 and [Pt₁₅(CO)₃₀]^2–/NaX (yellow-green, 2100 and 1865 cm^–1) were stoichiometrically synthesized by the reductive carbonylation of [Pt(NH₃)₄]²⁺/NaY, Pt²⁺/NaY and Pt²⁺/NaX, respectively. The IR bands characteristic of their linear carbonyls shift to higher frequencies whereas the bridging CO bands to lower frequencies, compared with those on the external zeolites and in solution. In-situ FTIR studies suggested that the subcarbonyl species such as PtO(CO) and Pt₃(CO)₃(₂ –CO)₃ are formed as the proposed intermediates towards [Pt₁₂(CO)₂₄]^2–/NaY in the reductive carbonylation of Pt²⁺/NaY.¹³CO exchange reaction preceded with the different intrazeolite Pt carbonyl species in the following order of activity at 298–343 K: Pt₃(CO)₃(₂ –CO)₃/NaY PtO(CO)/NaY>[Pt₉(CO)₁₈]^2–/NaY >[Pt₁₂(CO)₂₄]^2–/NaY. Pt-L₃-edge EXAFS measurment for these synthesized samples demonstrated that they are consistent with the Pt carbonyl clusters having trigonal prismatic Pt₉ and Pt₁₂ frameworks infered to a series of the Chini complexes such as [NEt₄]₂[Pt₃(CO)₆] _n ( _n = 3–5). The intrazeolite Pt₉ and Pt₁₂ carbonyl clusters exhibited higher cataytic activity in NO reduction by CO towards N₂ and N₂O at 473 K, compared with those on the conventional Pt/Al₂O₃ catalysts. The mechanism of intrazeolite Pt₉-Pt₁₅ carbonyl cluster formation are discussed in terms of the intrazeolite basicity and acidity.On leave from National Laboratory for Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 129 Street, China.     

Preparation of MCM-41 Supported Heterogenized Chiral Salen Mn (III) Complex and the Catalytic Activity in the Asymmetric Epoxidation

A chiral Manganese (III) salen complex was immobilized on the walls of MCM-41 (mobile crystalline material) through the multi-grafting method. The immobilized complex was characterized by XRD, FTIR, UV-Vis, ICP and Nitrogen sorption, and was applied to the asymmetric epoxidation of unfuctionalized alkenes including 1,2-dihydronaphthalene, α-methylstyrene, cis-β-methylstyrene, styrene using NaClO and m-chloroperbenzoic acid (m-CPBA) as oxidants respectively. The immobilized complex showed good activity and enantioselectivity in the epoxidation of 1,2-dihydronaphthalene by using NaClO as oxidant. It could also be run for 4 times in the epoxidation of α-methylstyrene without obvious loss of activity or enantiomeric excess.     

129Xe NMR to probe microscopic mixing state of composite catalysts

¹²⁹Xe NMR spectra obtained from microporous solid mixtures such as NaY-Pt/NaY and NaY-Pt/Al₂O₃ have indicated that the NMR peak pattern can depend greatly on the mixing method.¹²⁹Xe NMR can thus be a very simple and useful technique to probe the physical mixing state of many composite catalysts.     

Comparison of (non)-sulfided NiNaY zeolite catalysts prepared by ion-exchange and impregnation by xenon adsorption and129Xe NMR

Ni²⁺ exchanged and NiCl₂ impregnated non-sulfided and sulfided NaY zeolites were characterized by xenon adsorption isotherms,¹²⁹ Xe NMR and thiophene hydrodesulfurization. The nickel species are located mainly inside the micropores of the zeolites in all samples. Ion-exchange results in a more homogeneous distribution of these species than impregnation. This explains their lower hydrodesulfurization activity compared to the ion-exchanged samples.On leave of absence from the Institute of Isotopes, Budapest, Hungary.     

FTIR study of NO, C3H6 and O2 adsorption and interaction on gold modified MCM-41 materials

This paper is devoted to the detailed FTIR study of the adsorption, co-adsorption, and interaction of all the reagents used in NO HC-SCR process addressed to lean-burn engines with the surface of new gold catalysts based on ordered mesoporous materials. Gold was introduced into silicate and niobiosilicate matrices by the impregnation (Au/MCM-41 and Au/NbMCM-41, respectively) and via co-precipitation with siliceous and niobium sources (AuNbMCM-41). The in situ FTIR study allowed the estimation of the possible chemisorption of the reagents and their interaction towards intermediates, depending on the chemical composition of the catalyst and the way of gold introduction. It has been found that propene is chemisorbed, but not, NO, on gold species at room temperature. Chemisorbed C₃H₆ interacts with NO only in the presence of oxygen excess. Oxygen oxidizes NO to NO₂, the latter interacts with chemisorbed propene towards carboxylates (1570 cm⁻¹) and NO₂ is reduced to N₂O. At higher temperatures carboxylates interact with gaseous NO to carbonate, N₂O, CO and CO₂. The presence of niobium in the NbMCM-41 matrix enhances the oxidative properties of the catalysts and as a consequence the interaction between intermediates in NO reduction with propene in the oxygen excess. The co-precipitated AuNbMCM-41 exhibits higher NOx storage properties than the impregnated one.     

Selectivity Changes Due to Restructuring of the Pt(533) Surface in the NH3 + O2 Reaction

Ammonia oxidation with oxygen over Pt(533) in the pressure range 10^-7-10^-3 mbar has been studied using LEED and scanning tunneling microscopy (STM) to detect reaction-induced structural changes. Below 620 K, strong hysteresis effects in the rate occur upon cyclic variation of one of the p, T parameters. The rate changes are associated with structural modifications of the substrate. The Pt(533) surface exhibiting single atomic steps after preparation undergoes a doubling in step height and terrace width connected with a shift in selectivity from preferential N₂ production to strong NO formation. Restoring the single atomic steps in vacuum requires heating above 1100 K, but under reaction conditions the substrate changes are reversible at least down to 600 K.     

Preparation and luminescent properties of Tb3+ doped Y2O2SO4 microflakes

Abstract

Tb³⁺ doped Y₂O₂SO₄ (Y₂O₂SO₄:Tb³⁺) microflakes were prepared by a combination method of electrospinning and calcination. The two-dimensional microflakes had smooth surface and high radial/axial ratio. Crystal structures of the Y₂O₂SO₄:Tb³⁺ microflakes resulted in layer by layer growth in axial direction. A possible formation mechanism was proposed on the basis of experimental results, which indicated that poly(vinyl pyrrolidone) played the role of the nanostructure directing template and revealed the growth priority in radial direction. The microflakes showed a favourable fluorescent property symbolised by the characteristic green emission (541 nm) resulting from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions under 229 nm ultraviolet excitation. The maximum intensity of Tb³⁺ emission of the Y₂O₂SO₄:Tb³⁺ microflakes was 2·3 times stronger than that of the Y₂O₂SO₄:Tb³⁺ bulk powders with the same doping concentration.     

Initial activity/selectivity of H-gallosilicate (MFI) in propane aromatization: influence of H+ exchange and thermal/hydrothermal pretreatments

Initial activity/selectivity of H-gallosilicate (MFI) zeolite with different degrees of H⁺ exchange and pretreated under different thermal and hydrothermal conditions in propane aromatization (at 500C) has been determined using a pulse microreactor connected to GC. It is found to be strongly influenced by the degree of H⁺ exchange, calcination temperature and hydrothermal treatment at different temperatures and concentrations of steam. There exists a close relationship between the acidity (measured in terms of pyridine chemisorbed at 400 C) of the gallosilicate and its initial propane conversion and aromatization activity. Presence of strong acidic sites (attributed to FW Ga) at high concentration is essential for the well dispersed non-FW Ga oxide species to be active for dehydrogenation in the propane aromatization over the zeolite.     

离子交换法去除选矿循环水中SO_4~(2-)的研究

采用离子交换法脱除选矿循环水中SO42-,考察了SO42-初始浓度、运行流量、树脂层高度及水温对模拟废水中SO42-去除效果的影响,并在最佳工艺条件下对实际选矿循环水进行了试验验证。结果表明:各运行条件对废水中SO42-的去除效果影响均较小,在不同试验条件下,SO42-去除率均保持在90%以上。在流量为45 L/h、树脂层高度为78 cm的条件下处理实际选矿循环水,出水中SO42-浓度、pH值、浊度和色度均达到GB/T 19923—2005《城市污水再生利用工业用水水质》的要求。