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1.
《Applied Catalysis A: General》2002,230(1-2):263-270
Influence of gas atmosphere used in the calcination of MgO (flowing oxygen, air, helium, nitrogen) on its basicity has been investigated. Pure MgO of relatively low basicity and MgO of higher basicity, containing Ca and Na compounds as impurities, were examined. The surface basicity/base strength distribution was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO2. Both methods revealed that the calcination of MgO samples in the stream of oxygen or air gave MgO of lower basicity than obtained by calcination in the stream of helium or nitrogen. This effect may indicate a different character of interactions between various gas-phase molecules and the surface. It was also shown that MgO calcined in oxygen or air when used in the catalytic oxidative coupling of methane, gave lower conversions of methane and oxygen than that calcined in the inert gas atmosphere. The choice of a gas used in the calcination of MgO, may therefore, influence the basicity and catalytic properties of calcined MgO. 相似文献
2.
《Fuel Processing Technology》2002,76(1):41-49
The properties and the role of MgO in oxidative coupling of methane to C2+ hydrocarbons (OCM) have been studied. The transformation of 2-butanol over several unpromoted MgO samples was applied as a test reaction for the determination of the surface basicity/acidity. The specific rates of alcohol transformation and of product formation were determined in a flow silica reactor at 608±1 K. All the investigated MgO samples exhibited significant dehydrogenation activity and extremely weak dehydration activity, producing 2-butanone as the main product and only minor or even negligible amounts of butenes. The samples of MgO containing Ca compounds as impurities showed enhanced basicity, as concluded from the higher rates of ketone formation and from the presence of solely 1-butene among butenes. These samples also showed enhanced catalytic performance in the OCM process. The surface basicity of MgO, affected by the content or the absence of some impurities (Ca, Na, etc. oxides), was recognized as the decisive factor responsible for observed diversity in the catalytic performance of MgO in OCM, more significant than other surface properties or precursors. 相似文献
3.
Addition of promoters, such as Li2O, Na2O, PbO, La2O3, MgCl2 and CaCl2, to MgO causes a large increase in its surface basicity (particularly strong basic sites) and catalytic activity/selectivity in oxidative coupling of methane, but the correlation between the basicity and C2-yield is poor, indicating that factors other than basicity are also important in deciding catalytic performance. 相似文献
4.
Vasant R. Choudhary Shafeek A. R. Mulla Balu S. Uphade 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1998,72(2):99-104
Deposition of Li–MgO catalyst on commonly used supports (containing SiO2, Al2O3, SiC, ZrO2, HfO2, etc.) causes a drastic reduction in the catalytic activity/selectivity for the oxidative methane coupling reaction and also in both the total and strong surface basicity. The decrease in the catalytic activity/selectivity and basicity is attributed to strong chemical interactions between the catalyst and support which occur during the high temperature (750°C) calcination/pretreatment of the catalyst. The chemical interactions result in catalytically less active binary and ternary metal oxides containing Li and/or Mg, thus deactivating the Li–MgO catalyst by consuming its active components. © 1998 SCI 相似文献
5.
S?awomir Ku? 《Fuel》2003,82(11):1331-1338
The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La2O3, Nd2O3, ZrO2 and Nb2O5 has been investigated under various conditions. The results confirmed that the activity of La2O3 and Nd2O3 was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La2O3, Nd2O3, ZrO2 and Nb2O5 was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO2. Both methods showed that La2O3 and Nd2O3 had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO2 had only negligible amount of weak basic sites and Nb2O5 was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO2 it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed. 相似文献
6.
The XBaSrTiO3 (X = Li, Na, Mg) with perovskite structure was prepared by impregnation of LiCl, NaCl or MgCl2 solution on BaSrTiO3 (BST) surface. The products were characterized with XRD, SEM, UV, FT-IR and Raman spectroscopy. Determination of band gap, conduction and basicity of the prepared catalysts revealed that NaBST exhibits the lowest band gap and the most conduction and basicity. The catalytic performances of XBSTs for the oxidative coupling of methane (OCM) in a tubular continuous flow reactor were evaluated. The NaBST showed the maximum catalytic effect on methane conversion (47%), C2+ selectivity (51%) and ethylene yield (24%) at the temperature of 800 °C. 相似文献
7.
Vasant R. Choudhary Vilas H. Rane 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1997,69(1):63-69
CaO–La2O3–MgO and BaO–La2O3–MgO catalysts with different compositions have been studied for their bulk and surface properties (viz. crystal phases, surface area, acidity/acid strength distribution, basicity/base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different processing conditions (reaction temperature, 700–850°C; CH4/O2 ratio in feed, 3·0, 4·0 and 8·0 and GHSV, 102000 and 204000 cm3 g−1 h−1). The surface acidity and strong basicity of La2O3–MgO are found to be increased due to the addition of a third component (CaO or BaO), depending upon its concentration in the catalyst. The addition of CaO or BaO to La2O3–MgO OCM catalyst causes a significant improvement in its performance. Both the CaO- and BaO-containing catalysts show a high activity and selectivity at 800°C, whereas, the activity and selectivity of BaO-containing catalysts at 700°C is lower than that of CaO-containing catalysts. © 1997 SCI. 相似文献
8.
V
R Choudhary S
A
R Mulla M
Y Pandit S
T Chaudhari V
H Rane 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2000,75(9):828-834
The influence of the catalyst precursors (for Li2O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO2 content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH4/O2 feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm3 g−1 h−1) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li2O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO2 contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li2O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry 相似文献
9.
Vasant R. Choudhary Vilas H. Rane Meenakshi Y. Pandit 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1997,68(2):177-186
Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C2-hydrocarbons at different temperatures (700–750°C), CH4/O2 ratios (4·0 and 8·0) in feed, and space velocities (10320 cm3 g−1 h−1). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C2-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI. 相似文献
10.
J. A. Roos S. J. Korf R. H. J. Veehof J. G. van Ommen J. R. H. Ross 《Catalysis Today》1989,4(3-4):441-452
In order to examine the importance of the further oxidation of the desired C2 products in the oxidative coupling of methane, ethylene and ethane have been added to the feed (containing methane and oxygen) to a Li/MgO or Ca/Sm2O3 catalyst. The results of these measurements show that neither of these C2 molecules is stable under these conditions with either of the catalysts. Additionally, the rates of the oxidation of ethane and of ethylene alone have been measured using a gradientless reactor for both catalysts as well as for a quartz bed. It was found that the Ca/Sm2O3 material had higher activities for the oxidation of C2H6 and C2H4 (and also of CH4) than had the Li/MgO material. These higher activities result in a lower optimal reaction temperature for the oxidative coupling of methane and are (at least partially) responsible for the lower selectivity to C2 products observed with the Ca/Sm2O3 catalyst compared to that with the Li/MgO catalyst. 相似文献
11.
The effects of MgO promoter on the physicochemical properties and catalytic performance of Ni/Al2O3 catalysts for the partial oxidation of methane to syngas were studied by means of BET, XRD, H2-TPR, TEM and performance evaluation. It was found that the MgO promoter benefited from the uniformity of nickel species in
the catalysts, inhibited the formation of NiAl2O4 spinel and improved the interaction between nickel species and support. These results were related to the formation of NiO-MgO
solid solution and MgAl2O4 spinel. Moreover, for the catalysts with a proper amount of MgO promoter, the nickel dispersiveness was enhanced, therefore
making their catalytic performance in methane partial oxidation improved. However, the excessive MgO promoter exerted a negative
effect on the catalytic performance. Meanwhile, the basicity of MgO promoted the reversed water-gas shift reaction, which
led to an increase in CO selectivity and a decrease in H2 selectivity. The suitable content of MgO promoter in Ni/Al2O3 catalyst was ∼7 wt-%.
Translated from Journal of Fuel Chemistry and Technology, 2006, 34(4): 450–455 [译自: 燃料化学学报] 相似文献
12.
Shigeru Sugiyama Kohji Satomi Naoya Shigemoto Hiromu Hayashi John B. Moffat 《Catalysis Letters》1994,25(3-4):201-208
The oxidative coupling of methane on magnesium oxide (MgO) has been studied in the presence of carbon tetrachloride (TCM) as a gas-phase additive. Addition of a small amount of TCM to the reactant stream improves the selectivity to C2H4, while the conversion of methane is not influenced by the additive. X-ray photoelectron spectra of the used MgO reveal the formation of chlorine species on the catalyst surface in quantities up to 0.20 of Cl/Mg (atomic ratio), although X-ray diffraction spectra of the catalyst show MgO only and the content of the chlorine species in the bulk phase estimated by X-ray fluorescence analysis is very low. It is concluded that the enhancement of the selectivity to C2H4 primarily results from the presence of surface chlorine species. The chlorinated species on the catalyst has been identified as MgCl2. 相似文献
13.
S. J. Korf J. A. Roos J. M. Diphoorn R. H. J. Veehof J. G. van Ommen J. R. H. Ross 《Catalysis Today》1989,4(3-4):279-292
A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb4O7 (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm2O3 was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm2O3 gives a higher optimum C2 yield than that of unpromoted Sm2O3. Sm2O3 and Ca/Sm2O3 catalysts are more stable than Li/MgO, Li/Sm2O3 or Na/Sm2O3. The addition of Li or Na to Sm2O3 causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm2O3 catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm2O3. 相似文献
14.
Kaushal K. Tiwari Tarak N. Roy Somnath Banerjee Subrato Ganguly Deba P. Bhattacharyya 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1995,63(2):190-194
The oxidative coupling of methane to ethylene and ethane was studied over lithium–cerium-promoted MgO and MgO–CaO catalysts in the presence of molecular oxygen at 730°C and at atmospheric pressure in a continuous flow, fixed bed quartz reactor. The catalysts were prepared by an impregnation method and finally calcined at 900°C. The surface area, pore size distribution and pore volume of the catalysts were determined. The feed consisted of only methane and oxygen in the molar ratio of 2:1. The results obtained over the catalyst systems, viz. (i) lithium–cerium-promoted MgO and (ii) lithium–cerium-promoted MgO–CaO, have been compared. A relatively high C2-selectivity has been obtained with Li–Ce-promoted MgO–CaO catalysts. The optimum yield and selectivity for C2-hydrocarbons were found to be 21·5% and 76·8% respectively at a methane conversion of 28% over Li (7 wt%)–Ce (2 wt%)-doped MgO–CaO (3:1 wt ratio) catalyst. The various factors governing the activity and the selectivity of the catalyst systems have been discussed. 相似文献
15.
Some essential conditions necessary to reach an autothermal regime in methane oxidative coupling on La2O3/MgO catalysts were investigated. The following three ways can be suggested to transfer the process into the autothermal regime: (1) higher initial concentrations of reagents; (2) larger reactor diameter; (3) optimization of the flow rate and the preheating temperature. It was found that the optimal temperature of the autothermal regime of methane oxidative coupling is governed by the nature of the catalyst. 相似文献
16.
MgO/Al2O3 and NiO/MgO/Al2O3 solid bases were prepared by mixing method. The samples were characterized by X-ray diffraction (XRD), CO2 temperature-programmed desorption (CO2-TPD) and surface area measurements. After supported sulfonated cobalt phthalocyanine (CoPcS) the catalytic performance of these catalysts was evaluated in the mercaptan oxidation reaction. The effect of Mg/Al mole ratios on activity, crystal structure, basicity and stability in air was discussed. And the mechanism of the effect of NiO was identified. Results show that the base amount of MgO/Al2O3 increases with increasing Mg/Al mole ratio and catalyst with high Mg/Al mole ratio has a higher initial activity. NiO/MgO/Al2O3–CoPcS shows a higher initial activity and a much longer lifetime than MgO/Al2O3–CoPcS. When nickel oxide is doped into the MgO/Al2O3 support more crystal defects are generated, which increases the amount and types of basic sites. 相似文献
17.
The oxidative coupling of methane (OCM) has been found to be structure sensitive on La2O3 catalysts exhibiting different crystallite morphologies. Thin plates obtained by thermal decomposition of lanthanum nitrate at 650 °C are more selective on OCM reaction performed at 750 °C than the particles obtained by decomposition of the nitrate at 800 °C. It is assumed that the oxycarbonate observed is formed from the methane deep oxidation on the catalyst surface. This compound appears to act as an intermediate in the production of CO2 and is thus important hi the resulting selectivity. 相似文献
18.
Upon promoting MgO (prepared via a sol-gel process) with any binary mixture of the alkali metal chlorides, catalytic systems are obtained which are more active, selective and much more stable with time-on-stream than the respective monoalkali promoted MgO in the oxidative coupling of methane (OCM) to C2 hydrocarbons. The best catalytic performance is obtained over (5 mol% NaCl+5 mol% CsCl)/MgO, which exhibits a C2 yield of 19.7% compared to 5.9 and 4.1% over 10 mol% NaCl/MgO and 10 mol% CsCl/MgO, respectively, at atmospheric pressure, a temperature of 750 °C, a space velocity of 15000 cm3 g–1 h–1,
= 608 Torr and CH4/O2 = 4. A series of different combinations among the five alkali chlorides were made and the afore-mentioned synergistic effect was always observed. The basicity and base strength distribution of the bialkali chloride systems (measured by the gaseous acid adsorption or benzoic acid titration methods) are significantly higher than those of the respective monoalkali halide systems. The relationship between the catalytic performance and basicity/base strength distribution is explored. 相似文献
19.
AtT 650 °C carbon dioxide either formed during reaction or added to the system increases the selectivity for the desired hydrocarbon products during the oxidative coupling of methane and the oxidative dehydrogenation of ethane reaction over Li+/MgO catalysts. Similarly, CO2 inhibits secondary reactions of CH3-radicals with the surface of the Li+/MgO. The improved selectivities are attributed to the poisoning effect that CO2 has on the secondary reactions of alkyl radicals with the surface. 相似文献
20.
A novel plasma-treated Ni/MgO catalyst was prepared by treating coprecipitated NiCO3–MgCO3 with dielectric-barrier discharge plasma. The results by XRD, TEM and N2 adsorption analyses showed that the plasma-prepared Ni/MgO catalyst possessed smaller particle size, enhanced nickel dispersion, and higher specific surface area than a conventionally reduced Ni/MgO catalyst. The plasma-prepared Ni/MgO catalyst also exhibited better catalytic activity for carbon dioxide reforming of methane. More than 20% higher conversions of methane and carbon dioxide were obtained than those over the conventional Ni/MgO catalyst at 700 °C and a space velocity of 96,000 mL/(h?gcat). 相似文献