@article{mishra_dudek_gaffney_ding_li_2023, title={Spinel oxides as coke-resistant supports for NiO-based oxygen carriers in chemical looping combustion of methane}, volume={424}, ISSN={["1873-4308"]}, url={https://doi.org/10.1016/j.cattod.2019.09.010}, DOI={10.1016/j.cattod.2019.09.010}, abstractNote={Due to their high activity for methane conversion under a cyclic redox scheme, supported nickel oxides are among the most extensively investigated oxygen carrier materials for chemical looping combustion (CLC) and reforming (CLR) of methane. However, coke formation remains as a key challenge for Ni-containing oxygen carriers. The current study investigates the effect of reducible, spinel-structured supports to enhance coke resistance of NiO-based oxygen carriers. It was hypothesized that reducible supports capable of continued yet slow lattice oxygen donation in the presence of methane can actively retard coke formation on the surface of the oxygen carriers. To evaluate such effects, NiFe2O4, MgFe2O4, and BaFe2O4 are investigated as coke-resistant, reducible supports for NiO using mass spectrometry (MS) and thermogravimetric analysis (TGA) during chemical looping cycles. All three reducible supports were capable of continuous oxygen donation over an extended period of time (>40 min) without signs of coke formation. When used as supports for NiO, the resulting oxygen carriers showed no sign of carbon deposition under typical methane CLC environments. In comparison, NiO supported on inert MgAl2O4 exhibited significant coke formation after only 2.5 min. Moreover, NiO supported on NiFe2O4 and BaFe2O4 exhibited faster redox activity and higher oxygen carrying capacity when compared to the inert MgAl2O4-supported NiO. Detailed investigation of the reduction behavior of NiFe2O4-supported NiO revealed extensive solid-state reactions and Ni/Fe exchanges among the support, NiO, and newly formed phases. Specifically, initial weight loss in NiFe2O4-supported NiO was associated with reduction of the oxygen carrier to metallic Ni and Fe3O4 phases. Subsequent coke inhibition was attributed to the slow reduction of Fe3O4 and FeO phases. Multi-cyclic redox studies indicated that NiFe2O4-supported NiO gradually lost its redox activity. In comparison, both MgFe2O4- and BaFe2O4-supported NiO exhibited satisfactory redox stability, activity, and coke resistance.}, journal={CATALYSIS TODAY}, publisher={Elsevier BV}, author={Mishra, Amit and Dudek, Ryan and Gaffney, Anne and Ding, Dong and Li, Fanxing}, year={2023}, month={Dec} }
 @article{dudek_li_2021, title={Selective hydrogen combustion as an effective approach for intensified chemical production via the chemical looping strategy}, volume={218}, ISSN={["1873-7188"]}, DOI={10.1016/j.fuproc.2021.106827}, abstractNote={Demand continues to grow rapidly for commodity chemicals, such as light olefins, at a time when the chemicals sector must strive to reduce its energy consumption and greenhouse gas emissions. Process intensification provides a framework for producing the same chemical products with higher efficiency and lower emissions. In recent years, chemical looping has received increasing attention as a strategy for intensified chemical production. For example, the chemical looping oxidative dehydrogenation (CL-ODH) of ethane to ethylene offers the potential for near order-of-magnitude reductions in process energy usage and CO2 emissions in comparison to conventional ethane steam cracking, while chemical looping dehydroaromatization (CL-DHA) of methane offers a pathway for aromatics production at higher yields. In these examples, the CL processes rely on selective hydrogen combustion (SHC) to remove the co-produced H2 gas as water, providing several benefits, including yield increase, autothermal operation, and simplified downstream separation. As a yield-enhancing strategy, SHC is not new. However, the design of redox catalysts for SHC in a chemical looping context has only recently begun to be explored. In this perspective, we summarize previous research on SHC in chemical production schemes, and we attempt to outline priority areas of research in the years to come.}, journal={FUEL PROCESSING TECHNOLOGY}, author={Dudek, Ryan B. and Li, Fanxing}, year={2021}, month={Jul} }
 @article{hao_gao_liu_dudek_neal_wang_liu_li_2021, title={Zeolite-assisted core-shell redox catalysts for efficient light olefin production via cyclohexane redox oxidative cracking}, volume={409}, ISSN={["1873-3212"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85098692807&partnerID=MN8TOARS}, DOI={10.1016/j.cej.2020.128192}, abstractNote={This study reports a highly effective redox catalyst platform, i.e. composites of metal-exchanged ZSM5 and CaMn0.75Fe0.25O3@Na2WO4, for “low temperature” (<650 °C) redox oxidative cracking (ROC) of naphtha using a cyclohexane model compound. TEM-EDX showed that Na2WO4 shell covers the CaMn0.75Fe0.25O3 bulk and the zeolite + CaMn0.75Fe0.25O3@Na2WO4 composite can achieve autothermal conversion of cyclohexane under a cyclic redox scheme by selective oxidation of by-product H2 to H2O. Meanwhile, NH3-TPD and pyridine FTIR experiments confirmed that the Brönsted acidity of ZSM5 was primarily responsible for cyclohexane activation. Compared to each catalyst component alone, the synergistic effect of the composite redox catalysts resulted in substantially higher olefin yield (up to 75%), lower alkane yield (~5%), lower aromatic yield (~10%), and higher lattice oxygen utilization (up to 4 wt.cat%). Based on TGA-DSC experiments and ASPEN Plus analysis, the multi-functional redox catalysts facilitate autothermal conversion of cyclohexane with high lattice oxygen utilization from the redox catalysts. Due to the highly effective redox catalysts performance and the ease for heat integration, the novel ROC process has the potential for more energy-efficient light olefins production with significantly reduced CO2 emissions when compared to naphtha steam cracking.}, journal={CHEMICAL ENGINEERING JOURNAL}, author={Hao, Fang and Gao, Yunfei and Liu, Junchen and Dudek, Ryan and Neal, Luke and Wang, Shuang and Liu, Pingle and Li, Fanxing}, year={2021}, month={Apr} }
 @article{tian_dudek_westmoreland_li_2020, title={Effect of Sodium Tungstate Promoter on the Reduction Kinetics of CaMn0.9Fe0.1O3 for Chemical Looping - Oxidative Dehydrogenation of Ethane}, volume={398}, ISSN={["1873-3212"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85085392858&partnerID=MN8TOARS}, DOI={10.1016/j.cej.2020.125583}, abstractNote={Reduction kinetics of unpromoted and Na2WO4-promoted CaMn0.9Fe0.1O3 redox catalysts are measured in the context of chemical looping-oxidative dehydrogenation (CL-ODH). CL-ODH is a promising alternative for ethylene production compared to steam cracking, as it reduces the energy demand and increases the single-pass ethane conversion. The ability of a redox catalyst for selective hydrogen combustion (SHC), i.e. selectively oxidizing hydrogen co-product from ethane dehydrogenation, represents an effective strategy for CL-ODH because it can shift the reaction equilibrium and facilitate exothermic overall reaction. In this work, kinetic models and parameters of unpromoted and Na2WO4-promoted, Fe-doped CaMnO3 (CaMn0.9Fe0.1O3) under H2 and C2H4 were investigated. The reduction of unpromoted CaMn0.9Fe0.1O3 follows reaction-order models. C2H4 reduction has a higher energy barrier and a greater dependence on active lattice oxygen concentration, resulting in an order-of-magnitude decrease in the reduction rate. In comparison, the reduction of Na2WO4-promoted CaMn0.9Fe0.1O3 follows an Avrami-Erofe’ev nucleation and nuclei growth model. The addition of Na2WO4 more significantly suppressed C2H4 combustion relative to H2 oxidation. As a result, the reduction rate of Na2WO4-promoted CaMn0.9Fe0.1O3 under H2 was three orders of magnitude greater than that under C2H4, demonstrating its excellent SHC properties. The resulting redox catalyst was shown to be effective for ethane CL-ODH with measured 90.5% ethylene selectivity and 41.6% ethylene yield at 750 °C. The kinetics models and parameters provide useful information for CL-ODH reactor design and further development of the redox catalyst.}, journal={CHEMICAL ENGINEERING JOURNAL}, author={Tian, Yuan and Dudek, Ryan B. and Westmoreland, Phillip R. and Li, Fanxing}, year={2020}, month={Oct} }
 @article{dudek_tian_jin_blivin_li_2020, title={Reduction Kinetics of Perovskite Oxides for Selective Hydrogen Combustion in the Context of Olefin Production}, volume={8}, ISSN={["2194-4296"]}, url={https://doi.org/10.1002/ente.201900738}, DOI={10.1002/ente.201900738}, abstractNote={Chemical looping represents a novel approach for generating light olefins in which thermal cracking or catalytic dehydrogenation is coupled with selective hydrogen combustion (SHC) by a metal oxide redox catalyst, which enables autothermal operation, increased per‐pass conversion, and greater‐than‐equilibrium yields. Recent studies indicate that Na 2 WO 4 ‐promoted perovskite oxides are effective redox catalysts with high olefin selectivity. Herein, kinetic parameters, rates, and reaction models for the reduction of unpromoted and Na 2 WO 4 ‐promoted CaMnO 3 redox catalysts by H 2 , C 2 H 4 , and C 2 H 6 , is reported. Reduction rates of CaMnO 3 under ethylene and ethane are significantly lower than under H 2 . Model fitting of reduction kinetics show good agreement with reaction order–controlled models for CaMnO 3 reduction and predict greater oxygen site dependence and higher activation energy for CaMnO 3 reduction by C 2 H 4 as compared with H 2 . Avrami–Erofe'ev nucleation and growth models provide the best fit to the reduction of Na 2 WO 4 /CaMnO 3 in H 2 and in C 2 H 4 . After Na 2 WO 4 promotion, the reduction rate of CaMnO 3 is three orders of magnitude lower in ethylene in comparison to hydrogen, consistent with its superior selectivity to hydrogen combustion. The models developed can be applied toward reactor design and optimization in the context of enhanced olefin production via SHC under a cyclic redox scheme.}, number={8}, journal={ENERGY TECHNOLOGY}, publisher={Wiley}, author={Dudek, Ryan B. and Tian, Yuan and Jin, Gaochen and Blivin, Millicent and Li, Fanxing}, year={2020}, month={Aug} }
 @article{hao_gao_neal_dudek_li_chung_guan_liu_liu_li_2020, title={Sodium tungstate-promoted CaMnO3 as an effective, phase-transition redox catalyst for redox oxidative cracking of cyclohexane}, volume={385}, ISSN={["1090-2694"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85082646635&partnerID=MN8TOARS}, DOI={10.1016/j.jcat.2020.03.022}, abstractNote={Oxidative cracking, which combines catalytic oxidation and cracking reactions, represents a promising approach to reduce the energy and carbon intensities for light olefin production from naphtha. The need to co-feed gaseous oxygen with hydrocarbons, however, leads to significant COx formation and safety concerns. The cost and energy consumption associated with air separation also affects its economic attractiveness. In this study, we investigated a redox oxidative cracking (ROC) scheme and evaluated perovskites (La0.8Sr0.2FeO3 and CaMnO3) and Na2WO4-promoted perovskite (La0.8Sr0.2FeO3@Na2WO4 and CaMnO3@Na2WO4) as the redox catalysts for ROC. CaMnO3@Na2WO4 redox catalyst shows high activity, selectivity, and stability for light olefin production from cyclohexane. Operated under a redox oxidative cracking (ROC) scheme, CaMnO3@Na2WO4 enhances the catalytic cracking of cyclohexane, while showing high selectivity towards hydrogen combustion with its built-in, active lattice oxygen. Over three-fold increase in olefin yield compared to thermal cracking and 35% yield increase compared to conventional O2-cofeed oxidative cracking were achieved. Low energy ion scattering (LEIS), X-ray photoelectric spectroscopy (XPS), and differential scanning calorimetry (DSC) indicated a core-shell structure, where a molten Na2WO4 layer covers the CaMnO3 core. Na2WO4 modifies the oxygen donation behavior of CaMnO3 and provides a catalytically active surface for cyclohexane activation. In-situ XRD revealed that CaMnO3@Na2WO4 exhibited excellent structural stability and regenerability. The transformation of Mn4+ ↔ Mn3+ ↔ Mn2+ in CaMnO3, facilitated by reversible phase transition to (Ca/Mn)O solid solution, is responsible for the lattice oxygen donation and uptake during redox cycles. Electrochemical impedance spectroscopy (EIS) measurements further confirmed that the oxygen species were transported through the molten Na2WO4 layer to participate in ROC. These findings offer mechanistic insights to design effective redox catalysts for hydrocarbon valorization using the chemical looping strategy.}, journal={JOURNAL OF CATALYSIS}, author={Hao, Fang and Gao, Yunfei and Neal, Luke and Dudek, Ryan B. and Li, Wenyuan and Chung, Chingchang and Guan, Bo and Liu, Pingle and Liu, Xingbo and Li, Fanxing}, year={2020}, month={May}, pages={213–223} }
 @article{haribal_wang_dudek_paulus_turk_gupta_li_2019, title={Modified Ceria for "Low-Temperature" CO2 Utilization: A Chemical Looping Route to Exploit Industrial Waste Heat}, volume={9}, ISSN={["1614-6840"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074008887&partnerID=MN8TOARS}, DOI={10.1002/aenm.201901963}, abstractNote={Efficient CO2 utilization is key to limit global climate change. Carbon monoxide, which is a crucial feedstock for chemical synthesis, can be produced by splitting CO2. However, existing thermochemical routes are energy intensive requiring high operating temperatures. A hybrid redox process (HRP) involving CO2‐to‐CO conversion using a lattice oxygen‐deprived redox catalyst at relatively low temperatures (<700 °C) is reported. The lattice oxygen of the redox catalyst, restored during CO2‐splitting, is subsequently used to convert methane to syngas. Operated at temperatures significantly lower than a number of industrial waste heat sources, this cyclic redox process allows for efficient waste heat‐utilization to convert CO2. To enable the low temperature operation, lanthanum modified ceria (1:1 Ce:La) promoted by rhodium (0.5 wt%) is reported as an effective redox catalyst. Near‐complete CO2 conversion with a syngas yield of up to 83% at low temperatures is achieved using Rh‐promoted LaCeO4−x. While La improves low‐temperature bulk redox properties of ceria, Rh considerably enhances the surface catalytic properties for methane activation. Density functional theory calculations further illustrate the underlying functions of La‐substitution. The highly effective redox catalyst and HRP scheme provide a potentially attractive route for chemical production using CO2, industrial waste heat, and methane, with appreciably lowered CO2 emissions.}, number={41}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Haribal, Vasudev Pralhad and Wang, Xijun and Dudek, Ryan and Paulus, Courtney and Turk, Brian and Gupta, Raghubir and Li, Fanxing}, year={2019}, month={Nov} }
 @article{dudek_tian_blivin_neal_zhao_li_2019, title={Perovskite oxides for redox oxidative cracking of n-hexane under a cyclic redox scheme}, volume={246}, ISSN={["1873-3883"]}, url={https://doi.org/10.1016/j.apcatb.2019.01.048}, DOI={10.1016/j.apcatb.2019.01.048}, abstractNote={Steam cracking of naphtha is a commercially proven technology for light olefin production and the primary source of ethylene in the Europe and Asia-Pacific markets. However, its significant energy consumption and high CO2 intensity (up to 2 tons CO2/ton C2H4), stemming from endothermic cracking reactions and complex product separations, make this state-of-the-art process increasingly undesirable from an environmental standpoint. We propose a redox oxidative cracking (ROC) approach as an alternative pathway for naphtha conversion. Enabled by perovskite oxide-based redox catalysts, the ROC process converts naphtha (represented by n-hexane) in an auto-thermal, cyclic redox mode. The addition of 20 wt.% Na2WO4 to SrMnO3 and CaMnO3 created highly selective redox catalysts capable of achieving enhanced olefin yields from n-hexane oxy-cracking. This was largely attributed to the redox catalysts’ high activity, selectivity, and stability towards selective hydrogen combustion (SHC) under a redox mode. Na2WO4/CaMnO3 demonstrated significantly higher olefin yield (55–58%) when compared to that from thermal cracking (34%) at 725 °C and 4500 h−1. COx yield as low as 1.7% was achieved along with complete combustion of H2 over 25 cycles. Similarly, Na2WO4/SrMnO3 achieved 41% olefin yield, 0.4% COx yield, and 73% H2 combustion at this condition. Oxygen-temperature-programmed desorption (O2-TPD) indicated that Na2WO4 hindered gaseous oxygen release from CaMnO3. Low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) revealed an outermost perovskite surface layer covered by Na2WO4, which suppressed near-surface Mn and alkaline earth metal cations. The formation of non-selective surface oxygen species was also inhibited. XPS analysis further confirmed that promotion of SrMnO3 with Na2WO4 suppressed surface Sr species by 90%, with a similar effect also observed on CaMnO3. These findings point to the promoting effect of Na2WO4 and the potential of promoted SrMnO3 and CaMnO3 as selective redox catalysts for efficient production of light olefins from naphtha via the ROC process.}, journal={APPLIED CATALYSIS B-ENVIRONMENTAL}, publisher={Elsevier BV}, author={Dudek, Ryan B. and Tian, Xin and Blivin, Millicent and Neal, Luke M. and Zhao, Haibo and Li, Fanxing}, year={2019}, month={Jun}, pages={30–40} }
 @article{tian_dudek_gao_zhao_li_2019, title={Redox oxidative cracking of n-hexane with Fe-substituted barium hexaaluminates as redox catalysts}, volume={9}, ISSN={["2044-4761"]}, url={https://doi.org/10.1039/C8CY02530D}, DOI={10.1039/c8cy02530d}, abstractNote={Promoted hexaaluminate redox catalysts achieved excellent olefin yield while allowing autothermal redox oxidative cracking of naphtha with low COx formation.}, number={9}, journal={CATALYSIS SCIENCE & TECHNOLOGY}, publisher={Royal Society of Chemistry (RSC)}, author={Tian, Xin and Dudek, Ryan B. and Gao, Yunfei and Zhao, Haibo and Li, Fanxing}, year={2019}, month={May}, pages={2211–2220} }