Catalytic Processes for Natural Gas Valorization via Bromine Chemistry

Catalytic alkane functionalization into bromoalkanes and their subsequent conversion into liquid fuels and chemicals presents an attractive technology for valorizing abundant and low cost natural gas feedstocks. The industrial implementation of this route is contingent on the full recovery of hydrogen bromide (HBr), which is the end form of the halogen after the activation and upgrading steps. Catalytic oxidation of HBr into bromine (Br2) and catalytic oxybromination of methane (CH4) into bromomethane (CH3Br) and dibromomethane (CH2Br2) are two processes indicating great potential to close the halogen cycle. However, the practical application of these pathways has been impeded by (i) insufficient activity and/or fast deactivation of HBr oxidation catalysts under realistic process conditions and (ii) limited CH4 oxybromination selectivity to CH3Br and CH2Br2. This thesis contributes to the fundamental understanding of CH4 oxybromination, provides guidelines for designing catalysts for this complex reaction scheme, and discovers new families of catalytic materials enabling the efficient bromine looping via both CH4 oxybromination and HBr oxidation. The first insights into the chemistry of CH4 oxybromination were gathered by investigating the activities and product distributions of different bulk oxide and phosphate catalysts and comparing those with the activities of these solids in the oxidation of HBr, CH4, CH3Br, and CH2Br2, the reaction steps that were suggested as participants in the overall mechanism. It was shown that the highest selectivity and yields to bromomethanes were attained over those structurally-stable catalysts that feature both a high activity in HBr oxidation and low propensity to combust CH4 and brominated products. Along with the steady-state kinetics assessment, which implied substantial similarities between CH4 oxybrominarion and non-catalytic CH4 bromination, this observation suggests that that the surface-catalyzed production of Br2 and gas phase bromination of CH4 lead to C-Br bond formation. To address the impact of the hydrogen halide type on the oxyhalogenation, the same series of oxide and phosphate catalysts was further evaluated in CH4 oxychlorination performance. The kinetic fingerprints have revealed that CH4 activation by hydrogen chloride (HCl) and O2 also involves the cooperation between the surface catalyzed and gas phase reactions. However, in contrast to oxybromination in which halogenation of CH4 is proposed to control the activity, chlorine evolution appears to be a rate determining step in oxychlorination. This induces stark activity and selectivity differences between the two halides. In particular, while the oxybromination over mildly oxidizing catalysts favors the production of bromomethanes, the oxychlorination over these solids results in a highly selective production of CO, thereby providing a novel route for upgrading natural gas into this well established platform chemical. In view of halomethanes production, the use of HBr as the halogenating agent enables broader possibilities to enhance the selectivity. To this end, three strategies are devised in this thesis. The first is based on using catalytic materials with redox active metal centers embedded in a matrix featuring a mild combustion propensity and a high structural stability in oxybromination. This led to the development of vanadium phosphate ((VO)2P2O7, VPO), lanthanum vanadate (LaV0.5O2.75), and particularly europium oxybromide (EuOBr) catalysts, that have demonstrated both an outstanding selectivity to bromomethanes (up to 96%) and stable behavior. In addition to its exceptional performance in oxybromination, EuOBr has been shown as a unique catalytic material for HBr oxidation, enabling a high activity and outstanding stability using a stoichiometric feed composition. The specific activity was further enhanced and robustness preserved upon preparing a supported EuOBr/Al2O3 catalyst with 9 wt.% europium loading, demonstrating its realistic practical potential. The second strategy to enhance the selectivity and yield of CH3Br and CH2Br2 exploits the oxybromination under an increased partial pressure of HBr, resulting in a reduced combustion propensity of the catalyst surface due to an increase in the coverage of bromine. This approach was found particularly effective in the case of catalysts with strongly oxidizing action such as ceria (CeO2). In the third strategy, the redox properties of CeO2 are tuned by supporting its nanocrystals over suitable carriers using low oxide loadings (0.5 2 wt.%). In particular, basic MgO support promotes the bromination in the surface region of the catalyst. This minimizes the production of COx under the stoichiometric feed, and eventually results in significantly higher selectivity to bromomethanes compared to the bulk CeO2, which offers attractive opportunities in view of potential catalyst scale-up into technical material. The structure-performance relationships developed and steady-state kinetics acquired over different catalyst families hinted that C-H bond activation in oxybromination proceeds in the gas phase via homolytic dissociation mediated by free radicals. However, due to their short lifetimes and corrosive reaction environment, the direct detection of these highly reactive intermediates has been the long-standing challenge in oxyhalogenation. This thesis has demonstrated that operando photoelectron photoion coincidence spectroscopy presents a powerful method to accomplish this goal. Namely, not only this method has provided the evidence for the formation of methyl (CH3●) and bromine (Br●) radicals over the benchmark EuOBr and VPO catalysts, but it also uncovered a strong correlation between the concentrations of the radicals and the production of bromomethanes. In addition, complemented with operando prompt gamma activation analysis that revealed extremely low coverage of bromine on VPO, it also provided unequivocal proof of the mechanism comprising surface catalyzed oxidation of HBr generating Br● and Br2 that then induce the gas phase bromination of CH4 as rationalized by molecular modeling. The aforementioned concepts and findings provide the basis for efficient halogen looping in bromine-mediated natural gas upgrading and give momentum to the future developments in the halogen-based functionalization of alkanes