545082 The Role of Activated Carbon Supported CeOx and VOx Phases during Oxidative Dehydrogenation of Propane to Propylene Using CO2 As a Soft Oxidant

Monday, June 3, 2019
Texas Ballroom Prefunction Area (Grand Hyatt San Antonio)
Petar Djinovic1, Janez Zavasnik2,3, Ivo Jerman4 and Gregor Zerjav1, (1)Department for Environmental Sciences and Engineering, National Institute of Chemistry, Ljubljana, Slovenia, (2)Department of Structure and Nano-/ Micromechanics of Materials, Max-Planck-Institut für Eisenforschung, Dusseldorf, Germany, (3)Centre for Electron Microscopy and Microanalysis, Jožej Stefan Institute, Ljubljana, Slovenia, (4)Department of Materials Chemistry, National Institute of Chemistry, Ljubljana, Slovenia

The role of activated carbon supported CeOx and VOx phases during oxidative dehydrogenation of propane using CO2 as a soft oxidant

Petar Djinovica*, Janez Zavašnikb,c, Ivo Jermand, Ervin Šestd and Gregor Žerjava

aDepartment for Environmental Sciences and Engineering, National Institute of Chemistry,
Hajdrihova 19, SI-1001 Ljubljana, Slovenia

bCentre for Electron Microscopy and Microanalysis, Jožef Stefan Institute, Jamova cesta 39,
SI-1000 Ljubljana, Slovenia

cMax-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, DE-40237 Düsseldorf, Germany

dDepartment of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19,
SI-1001 Ljubljana, Slovenia

*petar.djinovic@ki.si

Keywords: CeO2, VOx, oxidative dehydrogenation, propylene, CO2 activation, lattice oxygen

1.        Introduction

Propylene is a commodity chemical used for synthesis of acetone, acrylic acid, acrylonitrile, polypropylene plastic, etc. A growing gap between the demand and production of propylene in the last years requires additional propylene production pathways. Catalytic oxidative dehydrogenation of propane using O2 (O2-ODH) or CO2 (CO2-ODH) emerge as possible alternatives. The use of a soft oxidant (such as CO2, reaction 1) could offer the benefits of higher propylene selectivity, less stringent active site architecture (compared to O2-ODH) and an additional chemical reaction for CO2 conversion.1,2

C3H8 + CO2 → C3H6 + CO + H2O                          H298K=167 kJ/mol                                                                                 (1)

The propane CO2-ODH reaction proceeds with involvement of O species from both CeOx and VOx redox phases via the Mars van Krevelen mechanism.3 The distribution of reaction products is strongly influenced by the parallel RWGS and non-oxidative dehydrogenation reactions.4 This work focuses on synthesis, characterization and catalytic testing of bulk and supported CeOx, VOx and CeVOx phases over activated carbon. We aim at identifying the morphological, chemical and redox properties that regulate propane and CO2 activation, as well as their consequences on activity and selectivity in the propane CO2-ODH reaction.

2.        Experimental/methods

Pure CeO2 was synthesized by dissolving 4.9 g of Ce(NO3)3 in 84 ml of MQ water; 140 ml of 0.1 M aqueous NaOH was added, mixed for 30 min and transferred to Teflon® lined autoclaves, where it was kept for 24 h at 180°C. Bulk V2O5 was synthesized by calcination of vanadium acetylacetonate in air at 600°C for 4 h. Bulk CeVO4 was synthesized by dissolving 92 mg of NH4VO3 in water, adding 1 drop of concentrated HNO3 and 340 mg of Ce(NO3)3 dissolved in 10 ml of water. The formed suspension was stirred at 80°C for additional 2 h, filtered, dried at 70°C and calcined for 4 h at 600°C in air. Activated carbon (Norit RX3 Extra from Cabot) supported CeOx and VOx catalysts were synthesized exactly as CeVO4, with the exception of powdered AC being added together with HNO3 and final calcination in argon instead of air. Catalysts were characterized by N2 physisorption, XRD, Raman, H2-TPR-MS, C3H8-TPR-MS, CO2-TPO-MS, in-situ DRIFT and HRTEM techniques. Catalytic tests were performed in a continuous-flow quartz tubular reactor at 550°C using 300 mg of a catalyst and a flow of C3H8, CO2 and He (10 ml/min each, WHSV= 4 L/gcat*h).

3.        Results and discussion

The XRD and Raman analyses on CeOx/AC catalysts showed CeO2 is formed with the average crystal size of 4 nm, regardless of loading (7-17wt. %). With increasing VOx loading from 2 to 12 wt. %, a series of VOx/AC catalysts containing only oligomeric VOx species, or VOx in coexistence with crystalline V2O5 were synthesized. In the CeVOx/AC catalysts, crystalline CeVO4 phase was formed in coexistence with small fractions of CeO2 and V2O5. Reactivity of lattice oxygen was probed with H2-TPR and C3H8-TPR-MS analyses. Different reactivity and quantity of reactive lattice oxygen was identified in the tested bulk materials: CeO2 started to reduce at 200 °C and CeVO4 much later at 380°C. No reduction was observed for V2O5 in H2 atmosphere up to 550 °C. The reduction of CeVOx/AC samples was initiated at 330°C.

Distinctly different catalytic performance of the tested materials was observed in the propane CO2-ODH reaction (Fig. 1A). Activity of CeO2 is low, stable, with low (15 %) propylene selectivity. Bulk CeVO4 shows no CO2 conversion; propylene is formed exclusively due to non-oxidative dehydrogenation. Propene selectivity drops to zero within 2h over bulk V2O5. VOx/AC catalysts exibited a propene selectivity decrease from 86 to 80 %, and a propane conversion increase from 3 to 5% when VOx loading was increased from 2 to 12%, suggesting oligomeric VOx sites mainly responsible for the propane CO2-ODH reaction. When CeOx content was increased from 7 to 17 % in CeOx/AC samples, higher catalytic activity was accompanied by a drop of propene selectivity from 84 to 61%.

By increasing CeVOx content from 10 to 40 wt. % in CeVOx/AC catalysts, both propane and CO2 conversions rose twofold (Fig. 1A), along with propene selectivity which reached 78%. Deactivation of all catalysts was observed with time on stream with concentrations of C3H6 and CO in the reactor discharge decreasing continuously (Fig. 1B). Contrary, concentrations of H2, CH4 and trace amounts of C2H4 were stable, indicating two parallel and independent reaction pathways: (i) propane CO2-ODH and (ii) non catalytic (thermal) propane cracking.

In-situ DRIFT analysis revealed that activation of propane is associated with V2O5 and CeO2 lattice oxygen abstraction and catalyst reduction. Bulk CeVO4 does not react with C3H8. The CeO2 to a lesser extent, but CeVO4/AC and especially V2O5 suffer from very limited ability to activate CO2, therefore lattice O2- vacancy cannot be re-oxidized to close the catalytic cycle. Subsequently, this results in a progressive decline of their CO2-ODH activity.

         

Fig. 1. A) propane and CO2 conversion, as well as C3H6 selectivity for tested catalysts after 4 h of propane CO2-ODH reaction, and B) temporal distribution of reaction products over 15Ce8V/AC catalyst (inset shows C3H8 and CO2 conversion along with C3H6 selectivity).

4.        Conclusions

Two parallel and independent reaction pathways occur: propane CO2-ODH which is related to participation of lattice oxygen, and propane cracking to methane, H2 and carbon. Ethylene is formed only in trace amounts suggesting fast C=C bond cleavage. Activation of propane is associated with lattice oxygen abstraction and catalyst reduction. Based on the C3H8-TPR and CO2-TPO results we can postulate that that bulk CeVO4 formation contains negligible activity for propane and CO2 activation. The ability of bulk crystalline V2O5 catalyst to activate CO2 is very limited leading to a progressive decline of its CO2-ODH activity. Bulk CeO2 acts favorably towards catalyst re-oxidation with CO2 already at 450°C. Presence of vanadium species strongly changes the redox and catalytic performance of CeO2, hindering its CO2 activation ability. Finely dispersed, non-crystalline VOx species appear as the ones mainly responsible for propane activation, leading to propylene formation.

Acknowledgement

The financial support of the Slovenian research agency (ARRS) through research program P2-0150 and project J7-7294 is kindly acknowledged. We thank the Cabot Company for providing the AC sample.

References

(1)         Carrero, C. A.; Schloegl, R.; Wachs, I. E.; Schomaecker, R. Critical Literature Review of the Kinetics for the Oxidative Dehydrogenation of Propane over Well-Defined Supported Vanadium Oxide Catalysts. ACS Catal. 2014, 4 (10), 3357–3380.

(2)         Cavani, F.; Ballarini, N.; Cericola, A. Oxidative Dehydrogenation of Ethane and Propane: How far from Commercial Implementation? Catal. Today 2007, 127 (1–4), 113–131.

(3)         Martínez-Huerta, M. V; Deo, G.; Fierro, J. L. G.; Bañares, M. A. Operando Raman-GC Study on the Structure - Activity Relationships in V5+/CeO2 Catalyst for Ethane Oxidative Dehydrogenation: The Formation of CeVO4. J. Phys. Chem. C 2008, 112 (30), 11441–11447.

(4)         Nowicka, E.; Reece, C.; Althahban, S. M.; Mohammed, K. M. H.; Kondrat, S. A.; Morgan, D. J.; He, Q.; Willock, D. J.; Golunski, S.; Kiely, C. J.; et al. Elucidating the Role of CO 2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts. ACS Catal. 2018, 8 (4), 3454–3468.

 


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