Propane Oxidative Dehydrogenation Under Oxygen-free Conditions Using Novel Fluidizable Catalysts: Reactivity, Kinetic Modeling and Simulation Study

Propane oxidative dehydrogenation (PODH) was studied using VOx/γAl2O3 and VOx/ZrO2-γAl2O3 (1:1 wt.%) catalysts, as well as consecutive propane injections under oxygen-free conditions. These catalysts were synthesized with 2.5, 5 and 7.5 wt.% vanadium loadings, and prepared using a wet saturation impregnation technique. Different characterization techniques were used to establish catalyst properties including NH3-TPD, pyridine FTIR and NH3-TPD kinetics. As well, PODH runs in the CREC Riser Simulator were developed under oxygen free atmospheres at 500-550°C, close to 1 atm., 10-20 s and 44.0 catalyst/propane weight ratio (g/g). Propylene selectivity obtained were up to 94%, at 25% propane conversion. Using this data, a “parallel-series” model was established based on a Langmuir-Hinshelwood rate equation. Adsorption constants were defined independently, with this leading to a 6-independent intrinsic kinetic parameter model. These parameters were calculated via numerical regression with reduced spans, for the 95% confidence interval and low cross-correlation coefficients. A larger 2.82×10-5 mol.gcat-1s-1 frequency factor for propylene formation versus the 1.65×10-6 mol.gcat-1s-1 frequency factor for propane combustion was obtained. The calculated energies of activation (55.7 kJ/mole for propylene formation and 33.3 kJ/mole for propane combustion) appeared to moderate this effect, with the influence of frequency factors prevailing. Furthermore, propylene conversion in COx oxidation appeared as a non-favored reaction step, given the 98.5 kJ/mole activation energy and 4.80×10-6 mol.gcat-1s-1 frequency factor. This kinetic model was considered for the development of a scaled-up twin fluidized bed reactor configuration. For this, a hybrid computational particle-fluid dynamic (CPFD) model featuring either “Particle Clusters” or “Single Particles” was employed. Results obtained in a 20-m length downer unit showing a 28% propane total conversion and a 93% propylene selectivity using the “Single Particle” model. However, and once the more rigorous particle cluster flow was accounted for, propane conversion was limited to 20%, with propylene selectivity staying at 94% level. Thus, the obtained results show that a PODH simulation using CPFD requires one to account for “Particle Clusters”. This type of comprehensive model is needed to establish unambiguously the PODH downer reactor performance, being of critical value for the development of down-flow reactors for other catalytic processes

Oxygen-Free Propane Oxidative Dehydrogenation Over Vanadium Oxide Catalysts: Reactivity and Kinetic Modelling

Propane conversion to propylene has been the subject of intensive researches. This is due to the increasing demand for propylene. Current propylene production processes suffer from several limitations. Oxidative dehydrogenation (ODH) is a promising alternative technology for propylene production overcoming the drawbacks of current processes. However, selectivity control in ODH is still a challenge preventing it from an industrial application. This is due to the formation of undesired carbon oxides. Thus, the development of a selective catalyst is crucial for the commercialization of ODH. Vanadium oxide catalysts have been proposed as the most active and selective catalyst for propane ODH. Moreover, new reactor concepts such as fluidized-bed might also help to make the ODH a feasible alternative for olefins productionas, offering some outstanding advantages in comparison to conventional reactors. This dissertation provides fundamental understanding of structure-reactivity relationship of vanadium oxide catalyst for propane ODH in a fluidized-bed reactor using the lattice oxygen of vanadium oxide catalysts in the absence of gas-phase oxygen. Supported vanadium oxide catalysts with different vanadium loadings (5-10 wt %) supported on γ-Al2O3 is used. The prepared catalysts are characterized using several techniques such as BET surface area, H2-TPR, NH3-TPD, O2 Chemisorption, Laser Raman Spectroscopy, Pyridine FTIR and XRD. Characterization of the catalysts reveals that monomeric VOx species are predominant at low vanadium loadings while polymeric VOx species increase with higher loadings until monolayer surface coverage is reached. Moreover, the catalysts display moderated acidity compared to that of the bare alumina due to the relative increase in the number of Brønsted acid sites. Successive-injections propane ODH experiments in the CREC Riser Simulator over partially reduced catalyst show good propane conversions (12%-15%) and promising propylene selectivity (68-86%) at 475-550 0C. Product selectivities are found to augment with the catalyst’s degree of reduction suggesting that a certain degree of catalyst reduction is required for better propylene selectivity. Compared to average propylene yields of 5% and 15% obtained in FCC and steam cracking technologies, respectively, promising value of 7% was obtained in the present propane ODH study over vanadium oxide catalyst and under oxygen free conditions. Such result would encourage further investigation of propane ODH in the absence of molecular gas oxygen as promising alternative/supplementary technology for the production of propylene. A kinetic model relating reaction rate to the catalyst’s degree of oxidation is proposed. Non- linear regression leads to model parameters with low confidence intervals, suggesting the adequacy of the proposed model in predicting the ODH reaction under the selected reaction conditions

Catalytic light alkanes selective conversion through ammonia-assisted reforming

The fact that hydrogen is a clean and versatile fuel offers an attractive carbon-free source of energy and leverages the U.S. economy toward long-term sustainable economic growth. At an industrial scale, hydrogen production is mostly relying on methane steam reforming producing stoichiometric amounts of carbon oxides (CO and CO2), which imposes economic and environmental concerns. To mitigate the issue, we propose NH3 assisted anaerobic reforming of natural gas liquids (ethane and propane) as an alternative approach to produce COx free hydrogen. Here, in the first chapter, through comprehensive performance evaluation, characterization, and transient kinetic studies, it is shown that the atomically dispersed Re-oxo grafted into framework Al of the HZSM-5 zeolite are highly active and stable for the ammonia reforming of ethane and propane at temperatures comparable to steam reforming ≤ 650 °C. In the second chapter, an alternative non- noble Ni/Ga intermetallic compound (IMC) with various Ni to Ga ratios is synthesized through the solvothermal synthesis by forming the oxalate MOF precursor. The result indicates that while Ni-rich samples form pure Ni3Ga IMC with promising catalytic performance, the Ga rich catalyst consists of segregated phases of Ni/Ga IMC and Ga2O3 with ill-defined structure showing lower stability despite the high activity. In chapter 3, a bifunctional Ni/Ga supported ZSM-5 is successfully developed in ethane aromatization. Influence of metal function in early-stage and steady-state activity and stability as well as structure reactivity relation was investigated applying comprehensive characterization, performance test, deactivation modeling, and transient studies. The results suggest that a tandem reaction mechanism between Ni3Ga intermetallic compound, Ga cation, and Bronsted acid sites of zeolite is responsible for the superior performance of bimetallic catalysts compared to their monometallic counterpart. In the last chapter, applying transient kinetic technique, the mechanism of ethane aromatization over Pt and Zn supported ZSM-5 model catalysts was precisely explored. The results reveal that despite mechanistic differences between these catalysts, ethane amortization on both catalysts follows a hydrocarbon pool mechanism

Kinetics of Methyl Ketone and Levulinic Acid Oxidative Scission Over Supported Vanadium Oxide Catalysts

Levulinic acid (LA) is a platform chemical derived from lignocellulosic biomass. Among the various applications LA has in industrial commodities and specialty chemicals, we previously reported a novel pathway that converts LA to maleic anhydride (MA) in high yields through methyl a-carbon oxidative scission over supported vanadium oxide catalysts. However, the high selectivity of methyl scission during LA oxidation appears to be unexpected according to the trends observed during the analogous oxidative scission of methyl ketones (e.g., 2-butanone and 2-pentanone). The impacts of vanadium oxide structures, metal oxide substrates, and methyl ketone molecular structures on both selectivity and reactivity of the oxidative scission were investigated in order to understand the origins of the high MA yield resulting from LA oxidation. Surprisingly, none of the aforementioned significantly increased the selectivity of methyl scission. However, further analysis of the oxidation route from LA to MA identified a new reaction intermediate—protoanemonin—and clarified its significance for the observed high MA yield. Reactivity data demonstrated that the rate-limiting step for the oxidative scission of LA over supported vanadium oxide is the methyl scission of protoanemonin to MA. A mechanism study of analogous methyl ketone scission was carried out to investigate the fundamentals of LA oxidation. The observed mechanistic insights suggested that the oxidative scission of methyl ketones involves both Eley-Rideal and Mars van Krevelen mechanisms. Accordingly, this joint mechanism is proposed for the first time. The active site for the methyl ketone oxidative scission was identified using pyridine poisoning, NH3 poisoning, and water co-feeding. The results suggest that adsorbed methyl ketones on acidic sites (Lewis and Bronsted) and redox sites (V-O-M) are able to react with gas-phase oxygen, cleaving into two fragments. The alkyl fragment can form ketones or aldehydes with the oxygen of either. In contrast, the carbonyl fragment can form surface acetate with lattice oxygen and desorb as acetic acid only if the lattice oxygen is in the V-O-M bond bridge

Chemical Looping for Selective Oxidations

This Dissertation describes the development of chemical looping for selective oxidations. Chemical looping is a reactor technology that achieves simultaneous reaction and separation. For a large subset of reactions (viz. abstraction or insertion of oxygen), this technology is based upon the use of oxygen carriers. These materials, typically metal oxides, reversibly store and release oxygen, and there is growing interest in using these materials for selective oxidations. This Dissertation describes work on the development of oxygen carriers for selective oxidations, including foundational work on a method for analysing periodic non-catalytic gas-solid reactions, of which chemical looping selective oxidations are a subset. The oxygen chemical potential of Ca2Fe2O5 was exploited to improve the efficiency of the steam-iron process to produce hydrogen. The ability of reduced Ca2Fe2O5 to convert a higher fraction of steam to hydrogen than chemically unmodified Fe was demonstrated in a packed bed. This demonstrates how the oxygen chemical potential might be manipulated and exploited for chemical looping reactions. The oxygen chemical potential determines the selectivity in thermodynamically-controlled selective oxidations, and, depending on the reaction mechanism, kinetically-controlled selective oxidations. A generic method for enhancing the oxygen-carrying capacity of oxygen carriers for use in selective oxidations is presented, where one material that is selective in the reaction is deposited on the surface of a second material acting as a reservoir of oxygen and as a support. The presence of ceria in the support was found to supply lattice oxygen additional to that provided by the bismuth oxide, without affecting the selectivity of bismuth oxide. The surface chemistry was decoupled from the bulk properties of the support, thus simplifying the design and formulation of composite oxygen carriers. Building upon the concepts of oxygen chemical potential and composite oxygen carriers, chemical looping epoxidation was demonstrated for the first time. The oxygen carrier was composed of Ag, for its unique catalytic properties, and SrFeO3 as the support, for its high oxygen chemical potential at low temperatures. A reaction mechanism was proposed based on the observations. Nonlinear frequency response theory was used to analysis a periodic non-catalytic gas-solid reaction. Generalised frequency response functions (which are higher order analogues to traditional, linear transfer functions) were derived to obtain the nonlinear frequency response of the archetypal reactor. Such a method lies between the traditional frequency response theorem and numerical methods in terms of accuracy and speed. A niche application was proposed for the analysis of experimental kinetics, avoiding convolution of measurements with the response time of measuring equipment. In summary, this Dissertation describes how materials might be formulated for selective oxidations in chemical looping mode. This was demonstrated for an industrially-significant reaction for the production of ethylene. A novel application of nonlinear frequency response theory was also demonstrated for chemical looping reactions.EPSRC Doctoral Training Gran