A computational study of the role of chlorine in the partial oxidation of methane by MgO and Li/MgO

We investigate the surface defect chemistry of MgO and Li/MgO paying special attention to the effects of added chloride ions on the active sites in the materials when used as partial oxidation catalysts. We calculate that chloride ions will segregate to the surface of these materials and will be preferentially sited at low coordinate sites. The [LiCl] defect cluster is also calculated to be bound, both of which effects will have a major influence on catalytic properties. From our calculations we also comment on the siting and selectivities of the active sites in MgO and Li/MgO.     

C2 selectivity enhancement in oxidative coupling of methane over microwave-irradiated catalysts

The oxidative coupling of methane over Li/MgO and BaBiO_{3 - x} catalysts irradiated by microwaves and classically heated is reported. Enhanced selectivities in C₂⁺ products are observed at lower temperatures under microwave conditions, especially with the Li/MgO catalyst.

The complex permittivity measurements of BaBiO_{3 - x} show that the regeneration of the active oxygen species on the surface is lower under microwave irradiation than classical heating. X-ray diffraction analyses of the catalyst before and after catalytic reaction, when it is classically heated and when it is heated by microwave irradiation, corroborate these results. Therefore, the CH₃⁻ carbanions are less oxidated at the catalyst surface under microwave irradiation.

On the other hand, the quenching of the output gas probably decreases the oxidation of CH°₃ radicals in the gas phase when the Li/MgO catalyst is irradiated by microwaves. The quenching of the output gas is a unique consequence of microwave irradiation which heats the catalyst without heating the wall of the reactor.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

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     

MgO-SBA-15 Supported Pd-Pb Catalysts for Oxidative Esterification of Methacrolein with Methanol to Methyl Methacrylate☆

Novel MgO–SBA-15 supported catalysts were prepared for oxidative esterification of methacrolein (MAL) with methanol to methyl methacrylate (MMA). The MgO–SBA-15 supports were synthesized with different...     

Carbon dioxide reforming of methane over nickel catalysts supported on mesoporous MgO

In this paper, ordered mesoporous MgO nanocrystals [MgO(M)] were synthesized, and the nickel catalysts supported on MgO(M) were facilely prepared by impregnation method. The obtained Ni/MgO(M) catalysts with advantageous textural properties were investigated as the catalysts for the carbon dioxide reforming of methane reaction. It was found that compared with the Ni/MgO(C) catalyst [MgO(C): commercial MgO], the mesoporous pore structure of MgO(M) could effectively limit the growth of the activity metal, and the Ni/MgO(M) catalysts showed high catalytic activities as well as long catalytic stabilities toward this reaction. The results showed that the conversions of CH₄ and CO₂ were only decreased <5 % after 100 h of reaction at 650 °C. The improved catalytic performance was suggested to be closely associated with both the advantageous structural properties, such as large specific surface area, uniform pore size, and the “confinement effect” of the mesoporous matrixes contributed to stabilize the Ni active sites during the reaction. The carbon species deposited on the spent Ni/MgO(M) catalyst were analysized by TG and Raman, and the results exhibited that the carbon species after 100 h of reaction were mainly active carbon species.     

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

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.     

Influence of support on surface basicity and catalytic activity in oxidative coupling of methane of Li–MgO deposited on different commercial catalyst carriers

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     

Comparison of behaviour of rare earth containing catalysts in the oxidative dehydrogenation of ethane

Catalyst promotion by addition of either La and Sm to MgO or Na aluminate to Sm₂O₃ and La₂O₃ has been investigated for the oxidative dehydrogenation of ethane in the temperature range 550–700°C. With all unpromoted and promoted catalysts, the selectivity to ethylene is strongly enhanced by the temperature, the highest values being obtained at 700°C. Sm₂O₃ is the most active among the bulk oxides, while samarium addition to MgO results in higher surface area, but does not enhance the catalytic activity. Ethylene productivity on La₂O₃ promoted MgO samples is higher than with pure La₂O₃, Sm₂O₃ and MgO, not only due to the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk oxides and rare earth promoted MgO, the selectivity to ethylene strongly increases by decreasing the O₂/C₂H₆ feed ratio, while it is quite unaffected by ethane conversion and catalyst composition, in agreement with the hypothesis that the main role of catalyst in the experimental conditions investigated is to produce ethyl radicals which are converted in the gas phase to CO and C₂H₄. When La₂O₃ is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, likely due to a different, mostly heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover, only CO₂ is produced as by-product, the formation of CO being quite negligible.     

The selective oxidation of methane to ethane and ethylene over doped and un-doped rare earth oxides

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.     

Oxidative dehydrogenation of propane over vanadium oxide based catalysts Effect of support and alkali promoter

The oxidative dehydrogenation of propane was investigated using vanadia type catalysts supported on Al₂O₃, TiO₂, ZrO₂ and MgO. The promotion of V₂O₅/Al₂O₃ catalyst with alkali metals (Li, Na, K) was also attempted. Evaluation of temperature programmed reduction patterns showed that the reducibility of V species is affected by the support acid–base character. The catalytic activity is favored by the V reducibility of the catalyst as it was confirmed from runs conducted at 450–550°C. V₂O₅/TiO₂ catalyst exhibits the highest activity in oxydehydrogenation of propane. The support’s nature also affects the selectivity to propene; V₂O₅ supported on Al₂O₃ catalyst exhibits the highest selectivity. Reaction studies showed that addition of alkali metals decreases the catalytic activity in the order non-doped>Li>Na>K. Propene selectivity significantly increases in the presence of doped catalysts.     

Lithium-doped sulfated-zirconia catalysts for oxidative coupling of methane to give ethylene and ethane

Li-doped sulfated-zirconia catalysts were found to be effective for oxidative coupling of methane (OCM). The catalyst performances depend on the sulfate content and calcination temperature. A maximum C₂ yield is attained over the catalysts, which contain 6 wt.% sulfate and calcined at 923–973 K, being closely related to the preparation conditions of sulfated-ZrO₂ as solid super-acids. When the performances of the Li-doped sulfated-ZrO₂ (Li/SZ) catalysts were tested at 1023 K as a function of reaction time, both the C₂ and CO_x selectivities remained constant over the range of 8 h, but the CH₄ conversion decreased from 17.5% to 11.9%. The nature of Li/SZ catalysts for the OCM was investigated by X-ray diffraction, XPS, and NH₃ and CO₂ TPD measurements. It could be postulated that the sulfated-ZrO₂ surface could play an important role in the formation of a catalytically active structure by Li-doping.     

Oxidative coupling of methane over doped Li/MgO catalysts

A series of zirconia doped Li/MgO catalysts with a fixed amount of zirconia and varying concentrations of lithium was used for the oxidative coupling of methane. It was found that an increase in lithium concentration resulted in a decrease in initial activity, while the selectivity was not affected. The life-time of Zr doped Li/MgO catalysts with a fixed concentration of ZrO₂ is a function of the lithium concentration. Previous results have shown that Li₂Mg₃ZrO₆ is active and selective but it is now shown to be instable under reaction conditions.     

Catalytic properties of various MgO catalysts for oxidative coupling of methane

A series of MgO catalysts for the oxidative coupling of methane prepared by different methods have been investigated. Specific surface area, XRD and XPS measurement results reveal that at lower temperatures catalysts with larger specific surface area, larger lattice distortion, smaller crystal dimension, and higher amount of unsaturated coordinated surface oxygen give higher catalytic activity. However, if we compare the catalytic properties of the samples in terms of unit surface area, the dependence of catalytic properties of the samples will be different.     

MgO/APT催化剂酸碱性质对1,4-丁二醇单醋酸酯转化反应的影响

以Mg(NO3)2为镁源,采用等体积浸渍法制备系列镁改性凹凸棒土催化剂Mg O/APT,利用XRD、BET、FT-IR和TPD对催化剂进行表征。结果表明,Mg O/APT催化剂具有与凹凸棒土相同的晶相结构;随着Mg O负载量的增加,Mg O/APT的碱强度和碱含量显著增加,较强酸量明显减少,弱酸酸位增加。在固定床反应装置上,考察催化剂气相催化转化1,4-丁二醇单醋酸酯反应性能。结果表明,催化剂表面的酸碱性对1,4-丁二醇单醋酸酯气相转化反应的产物分布有显著影响。酸性为主的催化剂APT上有利于1,4-丁二醇单醋酸酯发生分子内酯交换环化反应生成四氢呋喃;而适量酸-碱活性位有助于实现1,4-丁二醇单醋酸酯发生脱水-水解反应,生成3-丁烯-1-醇及3-丁烯-1-醇醋酸酯,过多碱性位有助于1,4-丁二醇单醋酸酯发生酯水解反应生成1,4-丁二醇。     

Catalyst development for oxidative methane coupling

The heterogeneously catalyzed oxidative coupling of methane has been investigated for several catalysts. A Ce/Li/MgO system was found to be the most attractive catalyst. The regeneration of the active sites seems to play a major role in the radical generating mechanism. The influence of the operating conditions on the conversion/selectivity has been studied, in order to find conditions for an optimal yield of C₂₊ - hydrocarbons. It is demonstrated that it is unrewarding to compare different types of catalysts only under selected standard conditions.     

MgO含量对高温甲烷化催化剂性能的影响

采用共沉淀法制备以Ni为活性组分的Ni-Mg-Al-O高温甲烷化催化剂,考察MgO质量分数为8%、10%、12%和14%时对催化剂性能的影响,并采用N2低温吸附、XRD、H2-TPR和H2-脉冲化学吸附对样品进行表征。结果表明,MgO质量分数超过10%时,会增加活性组分与载体间的相互作用,提高Ni O的还原温度,降低活性组分的分散度;MgO质量分数为10%时有助于提高催化剂的低温催化活性和高温稳定性。     

MgO/ZrO_2的制备表征及催化合成碳酸二异辛酯

分别采用物理研磨、浸渍和共沉淀3种方法制备了负载型固体碱MgO/ZrO2催化剂,并将其用于催化碳酸二甲酯与异辛醇的酯交换反应合成碳酸二异辛酯。结果表明,催化剂的活性在焙烧温度为973 K时达到最佳,合成产物的收率分别为67.00%,51.00%,59.93%。催化剂的X射线衍射、CO2程序升温脱附、FTIR表征表明,催化剂的活性及稳定性取决于MgO/ZrO2的结构。四方氧化锆晶相对催化活性有积极的影响,当单斜氧化锆晶相明显时,催化活性显著下降。用物理研磨法制备的催化剂,由于MgO均匀分散于载体表面而使其显示高催化活性。用共沉淀法制备的催化剂,由于Mg2+进入Zr4+的晶格形成MgO/ZrO2固溶体结构,使其显示了高的稳定性和活性。     

The effect of Nb2O5 and ZrO2 additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane

Incorporation of Nb₂O₅ or ZrO₂ into both Li/MgO and Li/Na/MgO systems produced ternary and quaternary catalysts, respectively, capable of attaining optimal C₂ yields and selectivities at lower temperatures relative to the unpromoted materials. The degree of enhancement effected by these metal oxide additives was compared to that produced by Li/MgO and Li/Na/MgO catalysts promoted with SnO₂ or Co₃O₄. At reaction temperatures < 700°C, the Li/Co/MgO ternary system showed marked differences in behaviour compared to the other ternary catalysts tested. This was particularly evident in the variation in C₂ selectivity with time on stream during ageing studies of (i) untreated materials, (ii) materials pretreated in CO₂, and (iii) materials dosed periodically with CHCI₃.     

MgO/NaY催化甲醇与碳酸乙烯酯酯交换合成碳酸二甲酯

以硝酸镁为前体,通过等体积浸渍法制备不同负载量的MgO/NaY催化剂,用CO2-TPD和TEM对催化剂进行了表征,考察MgO负载量、反应温度、反应时间等条件对甲醇与碳酸乙烯酯（EC）酯交换反应合成对碳酸二甲酯（DMC）的影响。实验结果表明：MgO的负载量对催化剂表面的碱量和MgO分散程度有着重要影响。高分散的MgO物种越多,其催化剂碱量越高。采用12%MgO/NaY为催化剂、反应温度70 ℃、n(甲醇)∶n(EC)= 8∶1、反应时间3 h时,EC的转化率和DMC的选择性最佳,DMC收率高达89%。