Effect of CO2 in the reaction of oxidative dehydrogenation of propane over Cr/ZrO2 catalysts João F. S. de Oliveira1, Diogo P. Volanti2, José M. C. Bueno1, Adriana P. Ferreira1 1 DEQ - UFSCar, São Carlos-SP, 13.565-905, Brazil IQ - UNESP, São José do Rio Preto-SP, Brazil *Corresponding author: [email protected]) 2 Keywords: Chromium-Zirconium oxides; Oxidative dehydrogenation of Propane; Basic sites; CO2; microwave 1. Introduction Propene is a major raw material for the petrochemical industry mainly used in polymer and rubber industry and its growing demand requires new production processes, as the transformation of alkanes in their respective alkenes. Since a recent past, an alternative route largely explored has been the catalytic non-oxidative dehydrogenation of propane (CDP) [1], which operates at high temperatures (above 800 K) to overcome thermodynamics restrains, with consequent catalyst deactivation by propane/propene cracking with coke deposition [2] and a loss in the propene selectivity and yield. The oxidative dehydrogenation of propane (ODP) becomes attractive due to the fact the reaction is exothermic and can operate at lower temperatures, suppressing coke formation [3]. However this reaction favors complete oxidation of alkanes and olefins and the reactants mixture hydrocarbonoxygen can be explosive, affecting drastically the olefins yield due to low concentrations of propane. A technological advancement can be achieved increasing propene yield via higher propane concentrations in process and/or coke gasification by CO2 [4]. Furthermore CO2 must shift the equilibrium to the product side and/or promote the dehydrogenation through reaction coupling between a simple dehydrogenation of propane and the reverse water–gas shift reaction [4]. Herein, we analyze the catalytic measurements in presence and absence of CO2 using Cr/ZrO2 catalysts obtained by conventional and microwave assisted hydrothermal treatments to address three major issues: (i) what is the effect of CO2 on performance Cr/ZrO2 catalyst in the dehydrogenation of propane; (ii) what is the effect of structural properties of Cr/ZrO2, obtained via different methods and Cr contents, on activity and selectivity; (iii) the CO2 is effective in the reactivation of Cr/ZrO2 catalyst used in ODP reaction. 2. Experimental Part Cr/ZrO2 samples were prepared by hydrothermal synthesis with two methods: conventional (autoclaves with solutions were heated a stove with static atmosphere at 180oC during 24h) and assisted by microwave (reactors with solutions were coupled into the adapted microwave and heated at 150oC during 2.5h). Aqueous solution of zirconium nitrate (Aldrich) and specific amounts of chromium nitrate (Aldrich) were used to prepare samples containing 2.5, 5, 10 and 15 % (wt.) of Cr. Precipitates were washed, dried and calcined in air at 600oC, during 4h. These catalysts were named y-xCZ, where "y” means c-Conventional or m– Microwave assisted; “x” is the chromium load in the catalysts. The catalysts were characterized by nitrogen physisorption, X-ray diffraction, Raman spectroscopy, temperature programmed reduction and desorption of CO2 (TPD-CO2). The catalytic performance tests were recorded in temperatureprogrammed on stream reactions (TPSR) during 300 minutes, flowing 20 mL/min of feed gas in ODP (2.5 % (v/v) propane, 6.5 % (v/v) CO2 and balance with He) or CDP (2.5 % (v/v) propane and He balance). For used c-5CZ catalyst was flowing 20 mL/min of air synthetic for 5 minutes or pure CO2 for 30 minutes, at 600oC. At each catalytic cycle of 45 minutes on stream in CDP reaction conditions, a regeneration procedure was applied and, then, a new cycle was performed. 3. Results and discussion The Cr/ZrO2 catalysts containing various chromium contents between 2.5 and 15 wt. % of Cr were prepared by hydrothermal conventional and microwave assisted methods. The tetragonal ZrO2 9 t-ZrO2) is formed decreasing the crystallites sizes with increases of Cr contents, the amorphous phase is achieved for samples containing 10 and 15 wt% of Cr. Smaller t-ZrO2 crystallites sizes are obtained by microwave assisted method. TPR-H2 profiles indicate CO2 was not efficient to promote re-oxidation of Cr+3 into Cr+6 species in reduced samples. Figure 2. Propene yield of the fresh (A) conventional and (B) microwave assisted y-xCZ (y= c- or m-; x = 2.5 to 15 % wt. of Cr) catalysts under CDP and ODPCO2 reaction conditions. Figure 1. Mass spectra of CO2 (M.M.= 44 g.mol-1) registered during TPD-CO2 from the y-xCZ (x = 0 to 15 % wt. of Cr) catalysts (left) and from the c-10CZ catalyst fresh and previously reduced (right). Factors between parenthesis indicate that curve signal is multiplied by it. Figure 1 presents TPD-CO2 results from y-xCZ catalysts. Cr addition on ZrO2 changes drastically its basicity (Fig. 1b). The strong basic sites are formed and a maximum of density this sites is observed in samples containing about 10 wt % of chromium. A fraction of CO2 is desorbed at temperatures higher than 500oC and characteristic of strong basic site at interface like Cr6+-ZrO2. The presence and density of these sites depends on Cr contents and preparation methods. Figure 2 presents propene yield of the fresh yxCZ catalysts. In CDP the selectivity to propene decreases with decreasing of Cr contents and crystallization of t-ZrO2. The effects of t-ZrO2 on selectivity decrease with increasing Cr contents, which reflect in formation of ZrO2 amorphous phase. The selectivity to propene decreases with presence of CO2 and becomes less sensitive to chromium contents than in absences of CO2 in CDP reaction. The decreasing of initial activity depends on presence of strong basic sites, which desorbs CO2 at high temperature. The strong basic property is assigned to interfacial sites like Cr6+-O-Zr. The maximum of strong is observed in catalysts with about 10 wt. % of chromium. On basis of catalytic measurements in presence of CO2, the selectivity to propene decrease with presence of CO2 and indicates that a relationship between stability of activity with time on stream and propane conversion is strongly dependent of presence of strong basic sites. The catalysts that exhibit only weak basic sites, the activity is insensitive to CO2 and the deactivation occur by reduction of Cr+6 species and by deposition of carbonaceous species. The catalysts that exhibit strong basic sites the activity decreases by presence of CO2, caused through of strong interaction of CO2 site with activity sites like Cr6+-O-Zr. The CO2 is more effective to maintain the activity with time on stream for catalyst that contains strong basic sites. The reactivation with CO2 indicates that the mechanism is not self-sustained trough modulating propane and CO2 in feed, the activity decrease with number of cycles due a deposition of carbonaceous species, which is effectively reactivated in presence of O2. 4. Conclusions The Cr/ZrO2 catalysts containing various chromium contents between 2.5 and 15 wt % of Cr were prepared by hydrothermal conventional and microwave assisted methods. The tetragonal ZrO2 is formed, decreasing the crystallites sizes with increases of Cr contents and smaller crystallites sizes are obtained in microwave assisted method. The TPD-CO2 results show that a fraction of CO2 is desorbed at temperatures high than 500oC and characteristic of strong basic site at interface like Cr6+-ZrO2. The presence and density of these sites depends on Cr contents and preparation methods. On basis of catalytic measurements in presence of CO2, the selectivity to propene decrease with presence of CO2. The stability with time on stream depends on presence strong basic sites. The catalyst that shows only weak basic sites the activity decreases with time on stream by reduction of Cr6+ species and accumulation of carbonaceous species. The catalyst with strong basic site the activity slight decreases with time on stream in presence of CO2. The initial activity decreases with number of cycles upper than 2. The thermal treatment of used catalysts in presence of O2 reestablishes the activity. Acknowledgments We acknowledge the financial support from CNPq and CAPES. References (Time new roman, 9 pts) [1] M. S. Kumar, J.C. Walmsley, A. Holmen, Catal. Commun, 2008, 9, 747. [2] E. Daniel, E. Resasco, G.L. Haller, in: J.J. Spivey, S.K. Agarwal (Eds.), Catalysis, vol. 11, Royal Society of Chemistry, Cambridge, 1994, p. 379. [3] R. Grabowski, Catal. Rev., 2006, 48, 199–268. [4] C. Boucetta, M. Kacimi, A. Ensuqu, J.Y, Piquemal, F. BozonVerduraz, M. Ziyad. Applied Catalysis A: Gen, 2009, 356, 201– 210.
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