Catalytic Methanation Of CO₂ Over Ni Supported On Modified Bentonite

The major clay minerals in bentonite is montmorillonite. Montmorillonite is a 2:1 layer clay mineral made up by a succession of silicon oxide, aluminum hydroxide, and silicon oxide sheets. Bentonite's unique structure ensures its many excellent features, such as swelling, adsorptive, ion-exchangeable and so on. This last property makes possible the modification of bentonite.After modification of bentonite by acidation, alkalization and Al-pillared, the clay has larger specific surface area, better thermal stability, and its pore distribution is unique, which makes it a better support. In the paper, Bentonite acidified by HCl, alkalized by KOH and pillared by Al are used as the supports for Ni catalysts, which are prepared by aqueous impregnation method. Methanation of carbon dioxide is used as a probe reaction to characterize the catalytic performance. The samples are characterized by XRD, BET, H2-TPR, CO2 (H2)-TPD, IR and TG. The main contents and results of this paper are included as following:1. HCl and KOH are used to modify bentonite(BEN). The Ni catalysts supported on acidified and alkalified bentonite have been prepared by aqueous impregnation. We have investigated the catalytic performances of the catalysts under different reaction temperatures. In this work, it is found that the catalytic activity of Ni/alkali-BEN for CO2 methanation is higher than Ni/acid-BEN. On both catalysts, the highest activity can be achieved at 350℃. After alkalization by KOH, the interaction between NiO and support becomes weaker, which facilitates the formation of NiO crystallites with lower reduction temperature. On the other hand, the acidity of the support decreases, leading to more CO2 adsorption. In addition, the amount of H2 adsorption for alkalized bentonite also increases. Indeed, these could be the main reasons leading to the higher catalytic activity of Ni/ alkali-BEN.2. NaOH,Na2CO3 and NH3·H2O are used to modify acid-BEN. The Ni catalysts supported on modified bentonite have been prepared by aqueous impregnation. We have investigated the catalytic performances of the catalysts under different reaction temperatures. In this work, it is found that Ni/NaOH-BEN has the best catalytic activity for CO2 methanation, who shows the best activity at 400℃with a CO2 conversion of 57.9% and a CH4 selectivity of 98.4%. The possible reason is that NiO is easier reduced to metal nickel because of the weak interaction between NiO and NaOH-BEN.3. The Al-pillared bentonites have been synthesized by microwave under the different operating conditions using the sodium benionite, which is purified and modified from bentonite. The Ni catalysts supported on modified bentonite have been prepared by aqueous impregnation. We have investigated the catalytic performances of the catalysts under different reaction temperatures. At the same time, the properties of Al-pillared by microwave radiation are compared with these by tradition heating. The catalytic activity of catalysts is also compared. Methanation of carbon dioxide is used as a probe reaction to determine the optimal preparation condition.The best preparation condition for Al-pillared by microwave is as follows:microwave radiation power is 350 watts; Radiating time is 10 minutes; Na-bentonite suspensions at 2%,30% and dry clay,any one of them is ok. Compared with Na-bentonite, both the Al-pillared bentonites have better thermal stability, larger specific surface area and d001-spacing. Comparing with the traditional merhod, the microwave radiation method has apotentiallypractical value as it can be produced quickly at a high Na-bentonite suspension concentration.We have investigated the catalytic performances of the catalysts under different reaction temperatures. As a comparison, all of the three catalysts reach the highest activity at 400℃. The catalyst Ni/MIAl-PILC shows the best activity at 400℃with a CO2 conversion of 60.3% and a CH4 selectivity of 99.1%. The possible reason is that the larger specific surface area of MIA1-PILC facilitates the dispersion of active metal, which prevents the agglomeration process of plenty of NiO crystallites.