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1.
A new environmentally friendly method for the production of 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) based on the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H 2O 2 over various Ti-containing mesoporous silicate materials is reported. Both well-organized Ti-containing mesoporous mesophase silicate (Ti-MMM), having hexagonal arrangement of uniform mesopores, and amorphous TiO 2–SiO 2 mixed oxides (aerogels and xerogels) produced TMBQ with good to high yield. All the materials studied have been proved to operate as truly heterogeneous catalysts. No titanium leaching occurred from the solid matrixes during the oxidation process. Titanium dispersion and its accessibility were found to be crucial factors determining the catalytic properties. For samples with similar titanium loading, both the catalytic activity and TMBQ yield appeared to fall in the order TiO 2–SiO 2 aerogel>Ti-MMM>TiO 2–SiO 2 xerogel and correlate with average mesopore diameter and mesopore volume. The best results (96–98% selectivity to TMBQ at 99–100% TMP conversion) were obtained with TiO 2–SiO 2 aerogels, containing 1.7–6.5 wt.% Ti. 相似文献
2.
Mesostructured SiO 2–TiO 2 mixed oxides have been prepared by a soft-templating sol–gel route, using a non-ionic triblock copolymer as structure-directing agent. Tetraethylorthosilicate (TEOS) and titanium tetraisopropoxide (TTIP) have been employed as Si and Ti sources, respectively. Using a prehydrolysis TEOS step allows mixed oxides to be produced with a homogeneous porosity and with no phase segregation, in a wide range of Si/Ti compositions. Both the hydrolysis molar ratio and the silicon content have been found to be important factors determining the final properties of these materials. For instance, mixed oxides containing low silicon concentrations exhibit N 2 physisorption isotherms typical of mesoporous materials, although with an important contribution of microporosity. On the other hand, increasing the hydrolysis molar ratio makes more difficult to reach a total dispersion of SiO 2 through the TiO 2 matrix. Even with low SiO 2 loadings, the thermal stability is effectively enhanced, when compared to the equivalent pure TiO 2 materials, as a consequence of a delay in the titania crystallization to anatase. Thus, after calcination at 300 °C for 3 h, mixed oxides containing low Si/Ti ratios (20/80) show BET surface area in the range 290–346 m 2/g, while pure TiO 2 materials largely collapse under the same treatment and their BET surface area drop strongly to values around 125 m 2/g. This synthesis route, therefore, provides mesoporous TiO 2-rich materials with enhanced stability and textural properties, which is of high interest for applications as catalysts and supports. 相似文献
3.
TiO 2–SiO 2 mixed oxides were prepared by sol–gel processes with one-stage (mix up fully hydrolyzed titania- and silica-sol), two-stage (with pre-hydrolysis) and modified two-stage synthesis routes. The photoresponse and AC impedance characterization of the derived catalysts are studied and correlated for the first time with the photocatalytic activities in water decomposition under UV illumination. Synergistic effects in terms of photocatalytic activity and electronic properties including band-gap energy, flat band potential and doping density were observed on atomically mixing TiO 2 and SiO 2 by the two-stage synthesis route. Meanwhile, the decline of photocurrent density were found on TiO 2–SiO 2 relative to bare TiO 2, which could be attributed to low quality crystalline structure of the former compared to that of the latter. The superior photocatalytic performance of TiO 2–SiO 2 is ascribed to the higher flat band potential, band-gap energy, and doping density than those of bare TiO 2. 相似文献
4.
Supported nickel phosphides were prepared by treating an amorphous Ni–B alloy on silica–alumina support with phosphine (15 vol.% PH 3/H 2) at relatively low temperature. The amorphous Ni–B/SiO 2–Al 2O 3 precursors were synthesized by silver-induced electroless plating. The amorphous precursors and catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, BET surface area and inductively coupled plasma measurements. The transmission electron micrographs of the Ni 2P/SiO 2–Al 2O 3 particles with their size ranging from 60 to 80 nm showed that they were homogeneously dispersed over the SiO 2–Al 2O 3 support. The as-prepared catalysts exhibited an excellent catalytic activity in the hydrodesulfurization (HDS) of dibenzothiophene. 相似文献
5.
The hydrogenation of CO over an Rh vanadate (RhVO 4) catalyst supported on SiO 2 (RhVO 4/SiO 2) has been investigated after H 2 reduction at 500°C, and the results are compared with those of vanadia-promoted (V 2O 5–Rh/SiO 2) and unpromoted Rh/SiO 2 catalysts. The mean size of Rh particles, which were dispersed by the decomposition of RhVO 4 after the H 2 reduction, was smaller (41 Å) than those (91–101 Å) of V 2O 5–Rh/SiO 2 and Rh/SiO 2 catalysts. The RhVO 4/SiO 2 catalyst showed higher activity and selectivity to C 2 oxygenates than the unpromoted Rh/SiO 2 catalyst after the H 2 pretreatment. The CO conversion of the RhVO 4/SiO 2 catalyst was much higher than that of V 2O 5–Rh/SiO 2 catalyst, and the yield of C 2 oxygenates increased. We also found that the RhVO 4/SiO 2 catalyst can be regenerated by calcination or O 2 treatment at high temperature after the reaction. 相似文献
6.
A series of sulphated ZrO 2–TiO 2 mixed oxide with different nominal sulphate loadings in the range of 2–15 wt.% was prepared and characterized for their structural properties, surface acidity and benzene isopropylation activity. The catalyst with 10 wt.% nominal sulphate loading showed highest surface area and uniform pore size distributions. Surface acidity, measured by NH 3–TPD method, showed increase in acidity with sulphate loading and the 10 wt.% sulphate loaded catalyst showed highest acidity. The activities of these catalysts were tested for isopropylation of benzene to cumene using 2-propanol as the alkylating agent. The 10 wt.% sulphate-loaded catalyst also showed highest activity for this reaction with 97% cumene selectivity. The higher activity of this catalyst was attributed to its higher acidity. 相似文献
7.
A series of V 2O 5–TiO 2 aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO 2. The aerogel catalysts showed much higher surface areas and total pore volumes than V 2O 5–TiO 2 xerogel and impregnated V 2O 5–TiO 2 catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H 2S without harmful emission of SO 2. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO x species and high reducibility. 相似文献
8.
TiO 2-SiO 2 with various compositions prepared by the coprecipitation method and vanadia loaded on TiO 2-SiO 2 were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH 3 and in the undesired oxidation of SO 2 to SO 3, using BET, XRD, XPS, NH 3-TPD, acidity measurement by the titration method and activity test. TiO 2-SiO 2, compared with pure TiO 2, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V 2O 5. The SCR activity and N 2 selectivity are found to be more excellent over vanadia loaded on TiO 2-SiO 2 with 10–20 mol% of SiO 2 than over that on pure TiO 2, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH 3 adsorbed on the catalysts. With increasing SiO 2 content, the remarkable activity decrease in the oxidation of SO 2 to SO 3, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V 4+ on TiO 2-SiO 2, while V 5+ exists on TiO 2, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO 2-SiO 2 with low SiO 2 content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO x activity but also the low SO 2 oxidation performance. 相似文献
9.
The present paper gives a detailed review of the different studies under investigation in our laboratory concerning the use of TiO 2 and TiO 2–Al 2O 3 composites prepared by chemical vapor deposition (CVD) as support for sulfide catalysts in the HDS of dibenzothiophene (DBT) derivatives. The supports investigated here are: TiO 2 (from Degussa, 50 m 2/g), Al 2O 3 (Nikki, 186 m 2/g) and TiO 2–Al 2O 3 supports prepared by CVD of TiCl 4 on alumina. Using several characterization techniques, we have demonstrated that the support composite presents a high dispersion of TiO 2 over γ-Al 2O 3 without forming precipitates up to ca. 11 wt.% loading. Moreover, the textural properties of the support composite are comparable to those of alumina. XPS investigations of Mo and NiMo catalysts supported on the different carriers show that Mo-oxide species exhibit a higher degree of sulfidation on the surface of TiO 2 and TiO 2–Al 2O 3 than on alumina. The HDS tests of 4,6-DMDBT under mild operating conditions (573 K, 3 MPa) show that sulfide catalysts supported on the composite support (ca. 11 wt.%) are more active than those supported on to TiO 2 or Al 2O 3. This higher HDS catalytic activity is attributed to the promotion of the hydrodesulfurization pathway, whereby the pre-hydrogenation of one of the aromatic rings adjacent to the thiophenic one may reduce the steric hindrance caused by the two methyl groups adjacent to the sulfur atom during the C–S bond cleavage. 相似文献
10.
Phosphorus containing ET(P)S-4 and ET(P)S-10 were synthesized from gels of composition xNa 2O–0.6KF–0.2TiO 2–(1.28 − 4 y) xHCl– yP 2O 5–1.49SiO 2–39.5H 2O at 190 °C for 3 days. The XRD patterns are almost identical to the corresponding ETS samples. The presence of phosphorus in the initial gel influences the kinetic parameters of the crystallization process, the morphology and the size of the crystals. The 31P-NMR of the samples show that part of the phosphorus is attached at the siliceous surface as dihydrogenophosphate SiOPO(OH) 2 groups and cannot be introduced in a tetrahedral or octahedral framework position. 相似文献
11.
ZrO 2–TiO 2 mixed oxide (30–70 mol/mol) was prepared by low-temperature sol–gel followed by solvo-thermal treatment (1 day) at various temperatures (40, 80, 120, 160 and 200 °C). Selected samples of the corresponding single oxides were also prepared. Materials characterization was carried out by N 2 physisorption, XRD, thermal analysis (TG-DTA) and UV–vis DRS, infra-red and Laser-Raman spectroscopies. Binary solids of enhanced pore volume and pore size diameter were obtained by increasing the post-treatment severity. Anatase TiO 2 micro-segregation was evidenced by Raman spectroscopy for the mixed oxide solvo-treated at the highest temperature. This solid also showed the highest crystallization temperature to ZrTiO 4 (702 °C). Mo impregnated (2.8 atom nm −2) on various mixed oxides was sulfided under H 2S/H 2 (400 °C, 1 h), the catalysts being tested in the dibenzothiophene hydrodesulfurization (HDS, T = 320 °C, P = 5.59 MPa). By increasing the severity of the solvo-treatment improved supports for MoS 2 phase were obtained. The HDS activity of the catalyst with carrier post-treated at 200 °C was 40% higher (in per total mass basis) than that of sulfided Mo supported on the binary oxide solvo-treated at 80 °C. The ZrO 2–TiO 2-supported catalysts showed higher selectivity to products from the hydrogenation route than their counterparts supported on either single oxide. 相似文献
12.
TiO 2 nanocrystalline particles dispersed in SiO 2 have been prepared by the sol-gel method using titanium- and silicon-alkoxides as precursors. Nano-composite thin films were formed on the glass substrates by dip-coating technique and heat treated at temperatures up to 500 °C for 1 h. The size of the TiO 2 nanocrystalline particles in the TiO 2–SiO 2 solution ranged from 5 to 8 nm. The crystalline structure of TiO 2 powders was identified as the anatase phase. As the content of SiO 2 increased, the anatase phase tended to be stabilized to higher temperature. TEM results revealed the presence of spherical TiO 2 particles dispersed in a disk-shaped glassy matrix. Photocatalytic activity of the TiO 2–SiO 2 (1:1) thin films showed decomposition of 95% of methylene blue solution in 2 h and a contact angle of 10°. The photocatalytic decomposition of methylene blue increased and the contact angle decreased with the content of TiO 2 phase. TiO 2–SiO 2 with the molar ratio of 1:1 showed a reasonable combination of adhesion, film strength, and the photocatalytic activity. 相似文献
13.
Co–Nb 2O 5–SiO 2 catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl 5 and SiCl 4 as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl 4. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min −1) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb 2O 5) were obtained. The characterization of the catalysts was performed using various techniques: N 2 adsorption and desorption curves at 77 K, NH 3- and H 2-chemisorption, TPO, XPS, XRD, and solid state 1H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support. 相似文献
14.
The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al 2O 3, TiO 2 and SiO 2 have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al 2O 3 supported catalyst, while the activity of the TiO 2 and SiO 2 supported catalysts increased. However, at high concentrations of water both the SiO 2 and TiO 2 supported catalyst deactivated. Common for all catalysts was an increase in C 5+ selectivity and a decrease in the CH 4 selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H 2 chemisorption and X-ray diffraction (XRD). 相似文献
15.
The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on SiO 2, SiO 2–Al 2O 3, activated carbon, mordenite type zeolites, MgO, TiO 2 and Al 2O 3. Supported chromium oxide catalysts were more active than any other metal oxide catalysts including noble metal examined in the present study. PCE removal activity of chromium oxide catalysts mainly depended on the type of supports and the content of metal loaded on the catalyst surface. TiO 2 and Al 2O 3 containing high surface areas were effective for the high performance of PCE removal, since the formation of well dispersed Cr(VI) active reaction sites for the present reaction system, was enhanced even for the high Cr loading on the catalyst surface. CrO x catalysts supported on TiO 2 and Al 2O 3 also exhibited stable PCE removal activity at a low feed concentration of PCE of 30 ppm up to 100 h at 350°C. However, significant catalyst deactivation was observed at high PCE concentration of 10 000 ppm. CrO x/TiO 2 revealed stronger water tolerance than CrO x/Al 2O 3 due to the surface hydrophobicity. 相似文献
16.
Various partial oxidation products were identified on the surface of TiO 2 and an 8% SiO 2–TiO 2 binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid. The SiO2–TiO2 photocatalyst is more active and appears to deactivate slower than TiO2. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO2. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO2 is cofumed with TiO2, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it. 相似文献
17.
采用浸渍法制备了不同CoCr_2O_4负载量x CoCr_2O_4/SiO_2催化剂(x=5%、10%、20%和30%),考察其对二氯甲烷催化燃烧性能的影响。结果表明,催化剂的整体活性顺序为:30CoCr_2O_4/SiO_220CoCr_2O_4/SiO_210CoCr_2O_4/SiO_25CoCr_2O_4/SiO_2,但按照活性组分CoCr_2O_4质量归一化后本征活性顺序为:10CoCr_2O_4/SiO_2≈5CoCr_2O_4/SiO_220CoCr_2O_4/SiO_230CoCr_2O_4/SiO_2。表征结果发现催化剂本征活性与可还原性能和表面酸性存在密切关系。10CoCr_2O_4/SiO_2和5CoCr_2O_4/SiO_2具有较高的表面酸性和耗氢量,因此具有较高的本征活性。 相似文献
18.
Nanosized pure TiO 2 particles were prepared by hydrolysis of TTIP in the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. TiO 2/SiO 2 nanoparticles were also prepared from TEOS as a silicon source and TTIP as a titanium source. These particles were characterized by TEM, XRD, FT-IR, BET, TGA and DTA. From thermal analysis and XRD analysis, the anatase structure of pure titania appeared in the 300–600 °C calcination temperature range and the rutile structure was showed above 700 °C. However, no rutile phase was observed for the TiO 2/SiO 2 particles up to 800 °C. The crystallite size decreased and the surface area of TiO 2/SiO 2 particles monotonically increased with an increase of the silica content. From FT-IR analysis, the band for Ti–O–Si vibration was observed and the band intensity for Si–O–Si vibration increased with an increase of the silica content. The micrographs of TEM showed that the TiO 2/SiO 2 nanoparticles had a spherical and a narrow size distribution. In addition, TiO 2/SiO 2 particles showed higher photocatalytic activity than pure TiO 2 and the TiO 2/SiO 2 (90/10) particles showed the highest activity on the photocatalytic decomposition of p-nitrophenol. 相似文献
19.
The local structure and the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides (SiO 2 content was fixed as 30 at.% with respect to TiO 2) was investigated by using XRD, FT-IR, BET, UV-vis spectra, and electron paramagnetic resonance (EPR) measurement. In FT-IR analysis, boron was incorporated into the framework of titania matrix with replacing Ti---O---Si with Si---O---B or Ti---O---B bonds. Also, paramagnetic species such as O − and Ti 3+ defects were formed by the boron incorporation. In SiO 2/TiO 2 mixed oxides, a blue shift in the light absorption band was observed due to the quantization of band structure. All B 2O 3–SiO 2/TiO 2 samples had pure anatase phase and no rutile phase was formed even though the calcination temperature was over 900 °C. Incorporating boron oxides of more than 10% enlarges the grain size of anatase phase and causes a red shift of the light absorption spectrum. The surface area was monotonically decreased with increasing the content of boron content. As a result, the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides was greatly influenced by the content of boron oxide. The highest photoactivity (g moles/min l) was obtained when the boron content was 5% and seven times higher than that of silica/titania binary mixed oxide. In addition, the specific photoactivity (g moles/m 2 l) was maximum still at 5%. It was concluded that the large reduction of surface area, the change of band structure, and more formation of bulk Ti 3+ sites are responsible for the deterioration in the photoactivity of B 2O 3–SiO 2/TiO 2 ternary mixed oxides when the content of boron is over 10%, although their crystallinity was enhanced by increasing the calcination temperature with keeping anatase phase. 相似文献
20.
The WO 3–TiO 2 catalysts with different WO 3 loadings prepared by the coprecipitation method were investigated in comparison with those prepared by the conventional impregnation method for the activity and durability in the high temperature SCR of NO by NH 3 and the structural and physico-chemical properties which were characterized by BET and XRD measurements, IR, Raman and XPS spectroscopies. The catalyst prepared by coprecipitation, as compared with that prepared by impregnation, was found to exhibit a higher SCR activity at high temperatures and also to possess a larger surface area, higher Brønsted acidity and larger monolayer capacity of the support with WO 3. Increasing the WO 3 loading of the catalysts enhances the SCR activity and simultaneously increases the Brønsted acidity. The observed improvement of SCR activity for the catalyst prepared by coprecipitation is mainly attributed to the higher Brønsted acidity and the presence of the more highly dispersed WO 3 species which is suggested by the larger monolayer capacity of ca. 13 μmol(W)/m 2 and no crystalline WO 3 on TiO 2 detected with XRD at the high WO 3 loading up to 40 wt.%. The catalyst with 20 wt.% WO 3, as compared with that prepared by impregnation, was found to exhibit a better thermal durability at high temperatures from 550 to 600 °C. The better durability is attributed to that the reduction of the surface area and the formation and subsequent growth of crystalline WO 3 upon aging are more remarkably inhibited. 相似文献
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