Title : Selective catalytic of CO hydrogenation over functionalized Algerian diatomite
Abstract:
Air pollution by various gaseous emissions is becoming a crucial problem, to which it is imperative to find solutions in order to reduce these harmful emissions for our direct environment. Various techniques exist to minimize or eliminate the presence of these pollutants in air, among them adsorption and/or catalysis process are the large useful method due to its efficiency and lower cost. The choose of materials, used as catalyst or adsorbent is considered important in the elimination or abatement process of gas pollutants. The objective of our work is to highlight the role of Algerian diatomite for the conversion of carbon monoxide by hydrogenation process which is an advantageous process for obtaining clean fuels and chemical products. Algerian diatomite contains 79% of silica (SiO2) and it is rich in different types of clays (palygorskite, zeolite ...).
Materials and Methods: A saturated Na+-Clays materials powder were mixed with 1 mol/L NH4NO3 solution according to the solid-liquid mass ratio of 1:20, stirred in a water bath at 80 °C for 2 h30, washed, dried at 120 °C. NH4+-clay materials were converted to H-clay by and calcined at 400 °C. Metals supported-Hclays catalysts were prepared by the deposition-precipitation (DP) method, using urea as a precipitator. Typically, 1.0 g of clay was suspended in an aqueous solution containing hydrated PdCl2 and PtCl2 at different ratios and urea as the precipitating agent. The solution was heated to 80 ºC for 8 h with vigorous stirring, filtered, washed in deionized water and dried under a vacuum at 25 ºC for 12 h. The resulting samples were designated as Metals/H-clays catalysts. We studied the reaction in which carbon monoxide is converted to hydrocarbons, and investigated the behaviors and the combination system of catalysts exchanged with platinum and paladium ions. Experiments were conducted at 1.2 MPa, 523K, and CO/H2=1 ratio. The structure and the texture of the catalysts, assessed by XRD, BET/BJH, and SEM, exhibited a microporosity, which implies a direct effect on the catalytic properties of these materials. The conversions obtained were 49%, 36%, and 28% for Ptn+/Pdn+-catalysts, Ptn+-catalysts, and Pdn+-catalysts respectively. Such conversions could be attributed to the good acidity resulting from the simultaneous presence of Ptn+/Pdn+ at different oxidation states of platinum and paladium, which was revealed by XANES PtLIII/PdLIIIanalysis, and their uniform dispersion within the inner surface and its grain size average conducted by the titration of adsorbed H2-O2. FTIR analysis showed a better distribution of acid sites for bi-exchanged catalysts over mono-exchanged ones, which resulted in a good catalytic activity. These results suggest a strong correlation between the high selectivity of light hydrocarbon products, the ions, and the catalyst types. These differences depended mainly on the facility of forming different products, such as n/iso-alkanes and alkenes. Skeletal isomerization was the main transformation observed on exchanged catalysts, particularly those with Ptn+/Pdn+ ions. A deactivation process of catalysts, versus time-on-stream, begins after 1hour, especially for combined exchanged materials. Finally, the regeneration study of diatomite was performed. The regeneration cycles are 4 cycles in the case of CO hydrogenation. In conclusion, natural and modified Algerian diatomite can be used as an appreciable catalyst in the field of air treatment containing pollutants.
Results and Discussion: XRD patterns show that the structure of clays materials are globally conserved and the structure/texture are not collapsed despite the insertion of metals within the internal surface. The XPS and STEM-EDX characterizations indicated that Pt and Pd nanoparticles with sizes of 2–3 nm were highly dispersed over the clays support. Significantly, we observed strong interactions between Pt and Pd in the bimetallic catalysts, which enhanced the good catalytic performance of the Pt-Pd/H-ZSM-5 catalyst. The deposition-precipitation method was used to prepare a series of catalysts containing monometallic Pt/clay, Pd/clay, and bimetallic Pt-Pd/clay. Subsequently, the catalytic selectivity performances of these agents in the SCR of CO by C3H6 were investigated. The exchange of H-clays catalysts with a 0.6/0.4 Wt% ratio of Pt/Pd facilitated propylene activation and thus reduced the temperature at 450°C window and enhanced 63% of De-CO conversion. It’s observed that in the Pd/clay catalyst, the formation of PdO suggested the apparition of 28% NO2 and secondary products.
