Tungsten Polyoxide Catalysts for Oxidative Condensation of Methane
Tolkyn S. Baizhumanova1,2, Svetlana A. Tungatarova1,2*, Zauresh T. Zheksenbaeva1,2, Kaysar Kassymkan1, Gulnar N. Kaumenova1,2, Rabiga O. Sarsenova1
1 – D.V. Sokolsky Institute of Fuel, Catalysis and Electrochemistry, 142 Kunaev str., Almaty, 050010, Kazakhstan
2 - al-Farabi Kazakh National University, 71 al-Farabi ave., Almaty, 050040, Kazakhstan
* corresponding tungatarova58@mail.ru
Introduction
The increasing need for industry in ethylene and the largest reserves of natural gas make the process of oxidative conversion of hydrocarbons especially attractive. Additional opportunities are opened at embedding the process in the technological chain of production of valuable products from natural gas (motor fuels and their components, products and semi-products of organic and petrochemical synthesis) without intermediate deep cleaning of ethylene. The most active and selective catalysts provide 20% yield of C2 hydrocarbons. Kazakhstan has also conducted research on the ODM. The obtained results are perspective and allow carrying out an economically and environmentally beneficial recycling of natural gas.
Materials and Methods
Experiments were carried out in flow type system with remove of products from cooling zone. Tubular quartz reactor with fixed bed of catalysts was used for investigation. The catalysts were prepared by impregnation of granulated carriers. Catalytic activity of supported catalysts prepared with using of Mo and W polyoxide compounds was investigated in OCM process. Tests were realized in reaction mixture, containing CH4, O2, inert gas with or without water vapor at the temperature of reaction 873-1173 K, contact time ≤ 0.5s, and atmospheric pressure. The catalysts were prepared by impregnation of different carriers (aluminosilicate, alumina, cordierite, active carbon, zeolites, clinoptilolite, etc.) by HPC solutions. The concentration of active component is varied from 1 to 20%. The size of catalytic granules, contact time and relationship of components are varied widely. IRS, ESDR, XRD, TPR, DTA, and isotopic methods were used for physical-chemical investigation.
Results and Discussion
The reaction of oxidative condensation of methane (OCM) to C2-hydrocarbons (C2-HC) is the most rapidly developing among the various pathways of methane oxidation. Publications on this problem are much higher than the publication of other possible pathways of partial oxidation. The mechanisms of this process are proposed, but the processes that are embedded in the production do not exist yet. The maximum yield and selectivity of C2-HC formation from CH4 (YC2-HC = 9.6% and 10.0%, SC2-HC = 80.0% and 57.6 %, SC2H4 = 51.5% and 39.2%, respectively) were achieved on the 5% and 0.5% H4SiW12O40/AlSi catalysts. C2H4/C2H6 ratio in the products increases sharply toward the formation of C2H4 for each of the investigated catalysts with increasing of reaction temperature from 600 to 900oC and the transition from high-percentage to low-percentage catalysts. In some cases it was possible to obtain only C2H4 from CH4 (at T = 800-900oC) and at T = 600oC - only C2H6.
The most active catalysts show the highest selectivity for C2-hydrocarbons: 5% H4SiW12O40/AlSi (SÑ2H4 = 51.5%, SC2-HC = 80.0%) and 15% H3PW12O40/AlSi (SC2H4 = 39.2%, SC2-HC = 60.2%). Catalysts based on W-HPC with P as central atom in all cases are similar in activity, but less selective in OCM reaction to ΣC2-hydrocarbons compared to HPC with Si as central atom. The yield of C2-hydrocarbons on Mo-containing catalysts are lower than on the W-containing catalysts (2.3% sample – 7.8%, 5.0% sample – 9.3%, 10.0% sample – 9.7%, T = 750-900oC). Carriers form series with respect to increasing influence on YC2H4 and SC2H4 at 800oC: SÑ2Í4, %: SiO2 KSK-2,5 (23,3) < zeocare-2 (37,1) < zeolite NaA (37,2) < zeolite CaX (38,9) < SiO2 KSM-5 (43,4) < AlSi (49,0) < periclase MgO (57,9) < pentasile CaAZ (58,5). Elements of the I and II groups of the Periodic System (Na, Mg, Cd, Cs) are optimal substituents for the proton in HPC of synthesis of ethylene from methane.
The influence of reaction mixture on the formation of C2-HC was studied. The yield and selectivity of formation of C2H4 increases with the ratio of CH4 : O2 from 1 : 1 to (1.5-30) : 1. The composition of mixture (mol: CH4 – 0.0018-0.0031, O2 – 0.0013, inert gas – 0.008-0.004) is optimal for synthesis of C2H4 from CH4. Introduction of water vapor (CH4 : O2 = 1 : 0,21, mol) in the reaction mixture has a positive effect on reaction. The results show that the OCM process can be optimized through a rigorous selection of process parameters of reaction and improvement the composition of supported catalysts. The optimal reaction conditions by varying the composition of HPC, the nature of carrier, the ratio of components of reaction mixture and reaction temperature were established. [PW12]-HPA at replacement of proton to the Na, Cs as well as [SiW12]-HPA at replacement of proton to the Na, Mg, Cd possess the highest activity at 750-850oC and when using the Si-containing oxides as carrier, such as pentasile, SiO2, or AlSi at supporting of 0,5-15% HPC on carrier and carrying out the experiment in the reaction mixtures: CH4 – 0.0018-0.0031, O2 – 0.001, mol, CH4 : H2O = 1 : 0,21, Ar is residual, t = 0,38-0,46 s. Implementation of OCM process on polyoxide systems in installations with one or two reactors (Table 1), as well as on polyoxide catalysts of the complex composition allowed to increase the conversion of CH4 to 46.0%, the yield of C2H4 - up to 18.5% at ΣYC2-HC to 22,0%.
Preliminary mathematical analysis of the kinetic results of reaction was conducted with a view to recommending the optimal design of an industrial reactor for the developed OCM process and optimal conditions for the realization of the industrial process on catalysts. It was offered the presumed optimal model of the industrial reactor. Analysis of results showed that the use of industrial-scale multi-shelf (5-7 layers) adiabatic reactor is the best. Productivity of the process according to preliminary estimates may be up to 3.2-3.4 tons of ethylene per day from 1 ton of catalyst.
Thus, the catalysts have been developed from research and the optimal technological parameters of the oxidative conversion of natural and associated gas into ethylene on molybdenum polyoxometalates have been determined.
Thermal stability of supported catalysts based on the 12th series W-HPC in a high temperature gas phase OCM process was examined by XRD and TPR methods under influence of the medium (O2 + H2O + Ar + CH4) at T = 20–800°C. It is caused by preservation of fragmented formations of HPC at temperatures up to 800–850°C on a carrier, light renewability under the action of reaction medium containing water vapor, or vapor-air treatment of reactive oxygen-containing fragments of HPA.
The process of OCM into ethylene can be intensified by creating a technology that is close to non-waste technology by additional oxidative dehydrogenation of the product of OCM - ethane to ethylene, and by adding additives to CH4 of other alkanes in order to increase the conversion of CH4 to C2H4 and C2H6.
On the proposed supported catalysts based on 12th series W-HPC (their compositions are close to those used in the OCM) for the concomitant process of oxidative dehydrogenation of C2H6 (ODE), the C2H4 yield in the presence of water vapor is 41.6-51.3% or 527-641 g C2H4/m3 C2H6, selectivity for C2H4 reaches 86–94%.
This publication has been made within the project Grant No AP05133881, which is funded by the Ministry of Education and Science of the Republic of Kazakhstan.
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