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21.
The catalytic behavior of the novel MCM-22 zeolite for the continuous alkylation of isobutane with 2-butene has been investigated at a temperature of 50°C, 2.5 MPa total pressure, and a variety of olefin space velocities. At high olefin conversions the MCM-22 zeolite showed a very high initial cracking activity attributable to strong Brønsted acid sites, as well as to the existence of strong diffusional restrictions of the TMP's (formed inside the zeolite) to exit through the channels. At short times on stream (TOS), TMP's account for ca. 40% of the C8 fraction. The olefin conversion and the cracking activity rapidly decline with TOS, while the alkylate product became richer in dimethylhexenes, indicating a predominance of 2-butene dimerization and a loss of hydrogen transfer activity as the catalyst aged. Moreover, MCM-22 gives less TMP's than large-pore zeolites (USY, beta, mordenite), but more than the mediumpore ZSM-5 at similar 2-butene conversion. The latter catalyst was much more selective for olefin dimerization than for isobutane alkylation, presumably because formation of the bulkier TMP's was strongly impeded in its smaller pores. 相似文献
22.
Siyang Jiang Yan Kong Jun Wang Xiaoqian Ren Qijie Yan 《Journal of Porous Materials》2006,13(3):341-346
A series of Sn-Zn modified-MCM41 has been synthesized by direct hydrothermal method and characterized using ICP, XRD, TG/DTA,
FT-IR, HRTEM and N2-adsorption techniques. Catalytic performances of the obtained materials were evaluated in the hydroxylation of phenol with H2O2. Results indicated that all the samples exhibited typical hexagonal arrangement of mesoporous structure with high surface
area and the heteroatoms were probably incorporated into the framework of MCM41. Catalytic tests revealed that the bimetallic
incorporated materials were effective catalysts in the hydroxylation of phenol. The conversion of phenol could be reach to
56.8% for the catalysts with Sn: Zn: Si = 2.69: 4.57: 100 (molar ratio) under the optimized reaction conditions. Moreover,
the materials containing Sn and Zn exhibited higher catalytic activity than monometallic Sn and Zn modified MCM41. 相似文献
23.
Hydrogen evolution during formaldehyde oxidation in alkaline solution has been monitored by Differential Electrochemical Mass Spectrometry on Au(111) and polycrystalline gold. The current efficiency for hydrogen evolution increases with higher concentration and is in the same range on both, polycrystalline Au and Au(111) electrode. The onset potentials and half-wave potentials are higher on Au(111). Reaction orders for the faradaic current on the bare gold electrodes have been determined as 0.21 for higher and 0.76 for lower concentrations. Reaction orders for hydrogen evolution during formaldehyde oxidation are 1.4 times higher in each case. Tafel slopes in the range of 140-160 mV are found. This signifies that the first reaction step involving the formation of adsorbed hydrogen is largely determining the overall reaction rate. In the presence of thallium adlayers hydrogen evolution from formaldehyde oxidation is largely suppressed. On the thallium modified polycrystalline Au, formaldehyde oxidation is shifted for 100 mV to higher potentials where Tl is partially desorbed and hydroxide is coadsorbed on the modified surface. On thallium modified Au(111), a similar process takes place, but in the same potential region as the onset of formaldehyde oxidation on the bare surface and therefore the formaldehyde oxidation is only slightly shifted. Tafel slopes are decreased to 80 mV/dec in the presence of thallium. In the presence of adsorbed thallium, the first reaction step is in equilibrium, the coverage with adsorbed hydrogen is smaller and its recombination to H2 is largely suppressed. 相似文献
24.
K. Juodkazis J. Juodkazyt? T. Juodien? V. Šukien? I. Savickaja 《Electrochimica acta》2006,51(27):6159-6164
The idea of underpotential oxidation of water, taking place on the surface of noble metals in the range of potentials preceding molecular oxygen evolution, is more than 40-years old. Chemisorbed oxygen atom—Ochem is considered to be the main intermediate in the underpotential oxidation of H2O, leading eventually to chemical oxidation of noble metals and formation of anhydrous surface oxides of MeO type. This concept is still used for the evaluation of the real surface area of the noble metal electrodes and also for the interpretation of new experimental results.The existence of reversible metal-oxide electrodes demonstrated experimentally for Pt, Pd, Rh, Ir and Au electrodes, oxidized anodically in the range of potentials preceding O2 evolution, shows that noble metals do have electrochemistry of their own. Nanometric layers of amorphous, slightly soluble hydroxides, hydrous oxides or oxides can form electrochemically on the electrode surface during the anodic process and be reduced during the cathodic one. Such alternative concept admits the reversibility of anodic and cathodic processes and changes essentially the understanding and interpretation of the phenomena of anodic oxidation of noble metals. 相似文献
25.
MCM-41 silicates prepared in the presence of octyltrimethylammonium bromide either by a conventional method or by post-synthesis
hydrothermal treatment were characterized by nitrogen adsorption in a wide range of relative pressure from 10-6 to 1. Hydrothermally restructured samples were found to have lower BET surface areas, lower external surface areas and thicker
silica walls than the non-treated sample. More importantly, in addition to their characteristic mesopores (ca. 3 nm), they
were shown to have considerable amounts of micropores. The relative amount of micropores and mesopores was shown to be dependent
on the treatment conditions. Thus, it is demonstrated that the postsynthesis hydrothermal restructuring is a convenient synthesis
route to MCM-41 silicates with bimodal pore size distribution involving controllable amounts of microporosity.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
26.
Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO20.1 Al2O31 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH4)2O1.5 Na2O300 H2O. The MCM-41 sample was calcined in O2 flow at 813 K and subsequently ion exchanged with Ca2+. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH3)
4
2+
. The Pt(NH3)
4
2+
ion supported on MCM-41 has been activated in O2 flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in129Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve. 相似文献
27.
28.
Masashi Shou Hiroyuki Takekawa Dong-Ying Ju Tokio Hagiwara Da-ling Lu Ken-ichi Tanaka 《Catalysis Letters》2006,108(3-4):119-124
Catalytic activity of a 1 wt% Au/TiO2 catalyst is markedly improved by loading a large amount of FeOx, on which the oxidation of CO in excess H2 is selectively promoted at temperature lower than 60 °C. Oxidation of CO with O2 on the FeOx/Au/TiO2 catalyst is markedly enhanced by H2, and H2O moisture also enhances the oxidation of CO but its effect is not so large as the promotion by H2. We deduced that activation of Au/TiO2 catalyst by loading FeOx is not caused by the size effect of Au particles but a new reaction path via hydroxyl carbonyl intermediate is responsible
for the superior activity of the FeOx/Au/TiO2 catalyst. 相似文献
29.
Propylene epoxidation over Ti/MCM-41 catalysts prepared by chemical vapor deposition 总被引:3,自引:0,他引:3
Ti-containing mesoporous catalysts were prepared by chemical vapor deposition (CVD) of TiCl4 on silica MCM-41 in the 700–900 °C temperature range. These samples were characterized (with XRD, ICP, nitrogen adsorption, FT-IR, ESCA, and TEM) and evaluated for the epoxidation of propylene with two alkyl hydroperoxides. The increase of CVD temperature resulted in the decrease of titanium content, catalyst hydroxyl population, crystallinity, and surface area. Catalyst selectivity to the desired product – propylene oxide – was highly sensitive to the deposition temperature. The best Ti/MCM-41 catalyst was prepared at the temperature of 800 °C, which had the maximum propylene oxide yield of 94.3%. 相似文献
30.
Abdullahi Nuhu Jorge Soares Monica Gonzalez-Herrera Andrew Watts Ghulam Hussein Michael Bowker 《Topics in Catalysis》2007,44(1-2):293-297
We have investigated the adsorption and reaction of methanol with Au/TiO2 catalysts using a pulsed flow reactor, DRIFTS and TPD. The TiO2 (P25) surface adsorbed a full monolayer of methanol, much of it in a dissociative manner, forming methoxy groups associated
with the cationic sites, and hydroxyl groups at the anions. The methoxy is relatively stable until 250 °C, at which point
decomposition occurs, producing mainly dimethyl ether by a bimolecular surface reaction. As the concentration of methoxy on
the surface diminishes, so the mechanism reverts to a de-oxygenation pathway, producing mainly methane and water (at ~330 °C
in TPD), but also with some coincident CO and hydrogen. Au catalysts were prepared by the deposition-precipitation method
to give Au loadings between 0.5–3 wt %. The effect of low levels of Au on the reactivity is marked. The pathway which gives
methane, which is characteristic of titania, remains, but a new feature of the reaction is the evolution of CO2 and H2 at lower temperature (a peak is seen in TPD at 220 °C), and the elimination of the DME-producing state. Clearly this is associated
with the presence of Au and appears to be due to the production of a formate species on the surface of the Au component. This
formate species is mainly involved in the reaction of methanol with the Au/TiO2 catalysts which results in a combustion pathway being followed, with complete conversion occurring by ~130 °C. 相似文献