545622 Elucidating the Role of CO2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts

Tuesday, June 4, 2019: 5:12 PM
Republic ABC (Grand Hyatt San Antonio)
James H. Carter1, Ewa Nowicka2, Christian Reece3, Sultan Althahban4, Khaled Mohammed5, Simon Kondrat6, David J. Morgan7, Qian He2, David Willock8, Stanislaw Golunski2, Christopher J Kiely9, Graham J. Hutchings1 and Nicholas Dummer2, (1)School of Chemistry, Cardiff Catalysis Institute, Cardiff, United Kingdom, (2)Cardiff University, Cardiff, United Kingdom, (3)Harvard University, Cambridge, MA, United States Minor Outlying Islands, (4)Lehigh University, Bethlehem, PA, (5)UK Catalysis Hub, Research complex at Harwell, Harwell, United Kingdom, (6)Loughborough University, Loughborough, United Kingdom, (7)Carfidff University, Cardiff, United Kingdom, (8)Cardiff University, cardiff, United Kingdom, (9)Materials Science and Engineering, Lehigh University, Bethlehem, PA

Elucidating the Role of CO2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts

Abstract

A mixed oxide support containing Ce, Zr, and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO2 as the oxidant. The activity of the support was compared with that of fully formulated catalysts containing palladium. The Pd/CeZrAlOx material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h), which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH3 and CO2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature-programmed reduction showed that palladium promoted both the reduction and re-oxidation of the support. When the role of CO2 was investigated in the absence of gas-phase oxidant, using a temporal analysis of products (TAP) reactor, it was found that CO2 dissociates over the reduced catalyst, leading to formation of CO and selective oxygen species. It is proposed that CO2 has the dual role of regenerating selective oxygen species and shifting the equilibrium for alkane dehydrogenation by consuming H2 through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive.

Summary of Results and Discussion

5 % Pd/CeZrAlOX was identified as an optimal catalyst after comparing its activity with different Pd loadings and various formulations of Ce:Zr:Al supports (data not presented here). Further investigation into the catalyst, its stability and the reaction mechanism were then carried out. The stability of the catalyst is a major issue in the commercial operation of propane dehydrogenation processes. Most current technologies, which are based on direct (non-oxidative) dehydrogenation, are performed at higher temperatures to achieve cost-effective yields of propene, but this can lead to rapid deactivation of the catalysts through coking. Therefore, in order to maintain a stable yield of propene, the commercial processes (Catofin, STAR and Oleflex) require addition of steam to the gas-feed (to supress coking), frequent regeneration of the catalyst, continuous partial replacement of the catalyst, or a rising profile for the operating temperature. Nonetheless the commercial catalysts generally exhibit relatively short lifetimes, up to a maximum of 3 years.

We have therefore tested our materials for extended periods to observe any changes in catalytic activity. In our investigation, we focused on the selectivity profile since it became quite apparent that the selectivity improves within time-on-stream. The 5% Pd/CeZrAlOx catalyst exhibited high stability over 140 h of reaction at 500 °C, as shown in Figure 1. Selectivity to propene clearly improved during the reaction whereas the C3H8 conversion significantly decreased during the first 1 h of reaction. This is consistent with our interpretation that unselective oxygen is initially consumed from the catalyst, followed by CO2 dissociation on the catalyst surface, resulting in the formation of selective oxygen species which are subsequently consumed in the oxidative dehydrogenation reaction. ADDIN EN.CITE <EndNote><Cite><Author>Nowicka</Author><Year>2018</Year><IDText>Elucidating the Role of CO2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts</IDText><DisplayText><style ISI&gt;://WOS:000430154100092</url></related-urls></urls><isbn>2155-5435</isbn><titles><title>Elucidating the Role of CO2 in the Soft Oxidative Dehydrogenation of Propane over Ceria-Based Catalysts</title><secondary-title>Acs Catalysis</secondary-title></titles><pages>3454-3468</pages><number>4</number><contributors><authors><author>Nowicka, E.</author><author>Reece, C.</author><author>Althahban, S. M.</author><author>Mohammed, K. M. H.</author><author>Kondrat, S. A.</author><author>Morgan, D. J.</author><author>He, Q.</author><author>Willock, D. J.</author><author>Golunski, S.</author><author>Kiely, C. J.</author><author>Hutchings, G. J.</author></authors></contributors><added-date format="utc">1530628972</added-date><ref-type name="Journal Article">17</ref-type><rec-number>760</rec-number><last-updated-date format="utc">1530628972</last-updated-date><accession-num>WOS:000430154100092</accession-num><electronic-resource-num>10.1021/acscata1.7b03805</electronic-resource-num><volume>8</volume></record></Cite></EndNote>1 It is interesting to note that a gradual decrease of activity to the 4% conversion level was observed, and then relatively stable activity was maintained until the end of the 140 h test. This implies that our 5% Pd/CeZrAlOx catalyst underwent changes not only during the first hour of the reaction but also during the remaining period of the test. To further understand this aspect, we performed detailed characterization of the ‘fresh’ and ‘140 h used’ material.

B)

 

A)

 

Figure 1. A) Propane conversion and B) propene selectivity during oxidative dehydrogenation over an extended reaction period using the 5% Pd/CeZrAlOx catalyst. Reaction conditions: total flow 15 cm3 min-1 (37% C3H8, 37% CO2, 26% He), GHSV = 6000 h−1, T = 500 °C.

We used XAFS to probe the local structures of the fresh and used samples of CeZrAlOx and Pd/ CeZrAlOx, concentrating on the Pd-K and Ce-L3 edge. These clearly show that the PdO present in the fresh Pd-CeZrAlOx catalyst is reduced to metallic Pd during reaction. The evidence for PdO is constituted by the clear Pd-O distance at 2.01 Å, which is entirely absent from the samples analysed after 4 h and 140 h on-line. In addition, the Pd-Pd distances shorten from those characteristic of PdOx in the fresh sample to that of fcc Pd metal in the used samples. Also clear from the EXAFS fitting data is an increase in Pd coordination number with time on-line which is associated with a growth in Pd particle size during reaction. The Ce L3 edge XANES is rich in information regarding the Ce oxidation state. After reaction with propane the Ce underwent gradual reduction with the average valance state dropping to 3.3 after 4 h on-line and further decreasing to 3.2 after 140 h on-line.

To confirm the dissociation of CO2 on the catalyst surface, CO2 was pulsed over reduced CeZrAlOx and 5%Pd/CeZrAlOx catalysts using a TAP reactor. The experiment was first performed at room temperature, and it was found that CO2 irreversibly binds to both materials. It was also found that the 5%Pd/CeZrAlOx was able to dissociate CO2 to form CO, whereas the CeZrAlOx was not, indicating the Pd facilitates the formation of CO at lower temperatures. When the experiment was repeated at a higher temperature (400°C) a stronger CO response was detected in both cases, demonstrating that CO2 was being dissociated more readily at the higher temperature. The increased temperature also caused the CO2 adsorption to become reversible, causing the CO2 response to increase. These data were also supported by complementary DRIFTS experiments.

Interestingly the CO2/CO mass balance was consistently 100% ± 2% during ODH over the Pd/CeZrAlOx, which suggests minimal deposition of carbon on the catalyst during the reaction. This was supported by elemental analysis of the used catalyst after 4 hours of reaction, which showed that the amount of retained carbon was equivalent to only 3.7 % of all the converted propane. These results infer that an additional function of the Pd is to suppress carbon deposition on the catalyst by enhancing the mobility of the surface oxygen or by promoting the reverse-Boudouard reaction.

Conclusions

The role of CO2 as a soft oxidant in the dehydrogenation of alkanes over reducible metal oxides has previously been attributed to its ability either (i) to reoxidize the reduced surface as part of an MvK reaction mechanism or (ii) to consume the hydrogen released by direct dehydrogenation through the RWGS reaction. These two pathways have previously been thought to be mutually exclusive. It is worth noting that the RWGS reaction may also proceed by a redox mechanism. Our results indicate that, in ceria-based catalysts such as CeZrAlOx, the MvK and RWGS mechanisms can be linked by a common step. We propose that lattice oxygen ions abstract hydrogen from the reactant alkane molecules to form the product alkene and H2O, while the CO2 replenishes these selective oxygen species, releasing CO into the gas phase (see Figure 2). Although the byproducts (CO and H2O) are also the products of the RWGS, this route to alkenes does not require the stepwise formation of H2 as an intermediate. The high selectivity to the desired alkene can be explained by the exclusive formation of fully reduced oxygen ions when CO2 dissociatively adsorbs at surface oxygen vacancy sites. In contrast, the adsorption of O2 is known to lead to the formation of transient electrophilic oxygen species (such as adatoms, O2−, O−) before O2− ions are formed. Although Figure 13. Representative scheme of the reactions involved in the overall ODH process these electrophilic species are generally short-lived, they account for the formation of CO2 (and any CO) when O2 is used as the oxidant. The ability of Pd to increase the oxidation activity of redox metal oxide catalysts is well-known and correlates with the observed enhanced reducibility and re-oxidation of the metal oxide surface. This rapid cycle of creating surface oxygen vacancies and then refilling them from the gas phase prevents the rate of the catalytic reaction becoming limited by the transfer of lattice oxygen ions. However, when O2 is the oxidant, the presence of Pd promotes the rate of both the selective and nonselective oxidation reactions. By way of contrast, our results demonstrate that, when CO2 is used as the oxidant, the presence of Pd has the critical function of increasing conversion without sacrificing selectivity to propene.

Figure 2. Summary of proposed mechanism of propane dehydrogenation using CO2 over Pd/CeZrAlOx.

References

 ADDIN EN.REFLIST 1.         E. Nowicka, C. Reece, S. M. Althahban, K. M. H. Mohammed, S. A. Kondrat, D. J. Morgan, Q. He, D. J. Willock, S. Golunski, C. J. Kiely and G. J. Hutchings, Acs Catalysis, 2018, 8, 3454.


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