Preparation And Performance Of The Catalysts Applied To Oxidative Dehydrogenation Of Low Alkanes With Carbon Dioxide

Ethylene and propylene are the important basic chemical feedstocks and play a decisive role in petrochemical industry. At the present time, the ethene and propylene are mainly come from the steam cracking and direct dehydrogenation of hydrocarbons. But the way is limited in the industrial production becaused of high reaction temperature, large energy consumption and deactivation of catalyst. Comparing with the steam cracking and direct dehydrogenation of hydrocarbons, the catalytic oxidative dehydrogenation (ODH) of ethane and propane with carbon dioxide is thermodynamically favorable and has been recognized as an attractive alternative. In addition, using CO₂ as a mild oxidant, could not only improve the alkene selectivity, but also reduce the emission of greenhouse gas and be considered as a source of carbon. The SBA-15 mesoporous material possesses high surface areas, hexagonal array of uniform tubular channels, thick walls, and higher thermal stability than many other mesoporous silica materials. So it is a promising new type of porous support for fabrication the catalyst for ODH of ethane and propane with carbon dioxide. The monolithic catalysts have many advantages such as low-pressure drop, favorable heat and mass transfer properties and minimum axial dispersion stem from the uniquely structured multi-channel configuration of monoliths. Therefore, monolithic catalysts have been extensively applied to the control of emissions. In this paper, the supported and metal based monolithic catalysts were prepared and their catalytic performances for the ODH of ethane or propane with carbon dioxde were tested. The structure of the catalysts was characterized by XRD, TEM, N₂ adsorption-desorption, XPS, UV-vis and H₂-TPR. The main research works and results are listed as follows:The Cr/SBA-15, Cr-Co/SBA-15, Cr-Ce/SBA-15 supported catalysts and Cr/SBA-15/Al₂O₃/FeCrAl,Ce/SBA-15/Al₂O₃/FeCrAl,Cr-Co/SBA-15/Al₂O₃ /FeCrAl,Cr-Ce/SBA-15/Al₂O₃/FeCrAl monolithic catalysts were prepared and their catalytic activity of ODH of ethane with CO₂ were tested. The results indicated that the Cr/SBA-15 catalyst showed excellent activity of ethane dehydrogenation. For the Cr-based catalyst, the addition of Co species promoted the dispersion of Cr species and also changed the redox properties and catalytic activity of the catalysts. The 5Cr-1Co/SBA-15 catalyst with 5%Cr and 1%Co exhibited the best catalytic activity in which 53.9% ethane conversion and 94.6% ethylene selectivity could be achieved under GHSV=3600 mL·g^-1 ? h^-1, V_CO2/V_C2H6=3 and 700℃reaction conditions. The Cr-based catalysts modified with Ce also exhibited excellent catalytic activity for the ODH of ethane with CO₂. Adding the Ce species to the 5Cr/SBA-15 catalyst could remarkably increase the relative content of Cr⁶⁺ species. The Cr⁶⁺ species is the main active species in the ODH of ethane. The 55.0% ethane conversion and 96.0% ethylene selectivity could be obtained over 5Cr-10Ce/SBA-15 catalyst at 700℃. The deactivation and regeneration reaction results showed that the addtion of Ce species to the Cr/SBA-15 catalyst could enhance the stability of the catalyst.For the Cr-based monolithic catalysts, the high thermal conductivity of FeCrAl support could avoid the sinter of activity phase (Cr species), which is expected for the oxidative dehydrogenation of ethane occurring at high temperature. In addition, the mesoporous structure of SBA-15 could depress Cr species sintering, and enhance the catalytic activity and stability. During the 1130 h of stability reaction, the conversion of C₂H₆ dropped from 66.5% to 42.3%, and the selectivity for C₂H₄ decreased from 95.5% to 90.1% on 5Cr/SBA-15/Al₂O₃/FeCrAl catalyst. The activity and the stability of the catalysts could be improved when 5Cr/SBA-15/Al₂O₃/FeCrAl was doped by Co and Ce. After modified by Co species, the redox ability and catalytic activity changed and the relative content of the Cr⁶⁺ species increased. The 5Cr-1Co/SBA-Al₂O₃/FeCrAl catalyst exhibited excellent activity with 36.8% ethane conversion and 99.4% ethylene selectivity under GHSV=6000mL·g^-1 ? h^-1, V_CO2/V_C2H6=4 and 650℃reaction conditions. All the catalysts modified by Ce species showed better activity than that of Cr-based monolithic catalysts. The ODH performance could reach maximum at 5Cr-10Ce/SBA-15/Al₂O₃/FeCrAl monolithic catalysts. In the 1300 h stability reaction, the ethane conversion decreases from 51.1 % to 45.2 % and the C₂H₄ selectivity is above the 96.4 %. For the Ce/SBA-15/Al₂O₃/FeCrAl catalysts, the high oxidation state Ce⁴⁺ species had higher catalytic activity than the Ce³⁺ species in the monolithic catalysts. The Ce⁴⁺ species was reduced to Ce³⁺ species in the ethane dehydrogenation process, and the reduced Ce species was reoxidized to the Ce⁴⁺ species by treatment with CO₂. The Ce redox cycle played an important role in the catalyst's high activity.The V/SBA-15, V-Cr/SBA-15 supported catalysts and the V/SBA-15 /Al₂O₃/FeCrAl and Cr modified V-based monolithic catalysts were prepared and the catalytic activity of ODH of propane with CO₂ was tested. The results indicated that, in V-based catalysts, the vanadium exhibited multiple valences, including V⁵⁺ species which is main activit component for ODH of propane. The addition of Cr species in the V/SBA-15 catalyst could enhance the dispersion of the V species and increase the content of the high valence V speicies, especially the relative amount of the V⁵⁺ species. As a result, the catalytic activity of propane dehydrogenation had been enhanced.For the V/SBA-15/Al₂O₃/FeCrAl and Cr modified V-based monolithic catalysts, 10V-10Cr/SBA-15/Al₂O₃/FeCrAl catalyst had the best catalytic activity in which 61.3% propane conversion and 50.7% propylene yiled could be achieved under GHSV=14400 mL·g^-1 ? h^-1, V_CO2/V_C3H8=3 and 700℃reaction conditions. According to the results of characterization, the good catalytic performance of the V-based monolithic catalyst could be related to the higher surface concentration of dispersed tetrahedral V⁵⁺ species and the synergistic action of V and Cr species.