Dehydrogenation Of Propane To Propylene In The Presence Of Carbon Dioxide Over Novel Gallium Oxide Based Catalysts

Propylene is currently an important building block of the chemical industry, which is mainly produced from steam cracking and FCC. However the propylene production from these sources barely matches with its rapid increasing consumption. Therefore, transformation of the relatively cheap and abundant propane into propylene seems to be an alternative to slove this problem. Thermal dehydrogenation of propane to propylene (DHP), although industrialized, requires high reaction temperature due to the thermodynamics limits. This drawback makes the process not only extremely energy intensive, but also severe coking and consequent deactivation which restrict the further application of this technique. Meanwhile oxidative dehydrogenation of propane by molecular oxygen (ODHP), which allows the endothermic reaction to be exothermic, permits high conversion of propane at lower temperature. However, introduction of oxygen leads to side-reaction such as over-oxidation and insertion of molecular oxygen, significantly decreasing its selectivity to propylene. Employment of CO2as weak oxidant instead of O2would couple the DHP with reverse water-gas shift (RWGS), which consumes H2by CO2, leading to the enhancement of DHP equilibrium conversion. In contrast to ODH, by-products are remarkably suppressed, and selectivity to propylene is greatly promoted in the CO2-DHP. Besides, CO2can stabilize the catalysts via elimination of coke and as green house gas has been effectively utilized, CO2-DHP is recognized as a "green chemistry" route.In the field of CO2-DH of light alkanes, Ga2O3receives considerable attentions as a new class of catalysts. Although Ga2O3-based catalysts are reported to be active in some cases, they suffer heavily from coke, which renders the catalysts to deactivate rapidly in a few hours commonly. Over the years, a number of studies have established that the population of the catalytically relevant (cus) Ga species is a strong function of the surface area of Ga2O3among different Ga2O3polymorphs. In this sense, one may rationalize that simply increasing the surface area of Ga2O3can lead to an improvement in the PDH activity. Alternatively, biomass has been widely employed as a simple and convenient carbon template precursor in synthesis of high surface area metallic oxide, which could be a new alternative for the preparation of Ga2O3.Moreover, a new class of Ga2O3-Al2O3solid solution reported, with unique acidic and structural properties, has been frequently applied to the selective catalytic reduction (SCR) of NOx. In contrast to those of Al2O3, the total amount of acid site of the Ga2O3-Al2O3solid solution increases with the enhancement of Ga concentration, but the amount for strong acid site decreases accordingly. The solid solution is also fairly stable in crystalline structure against heat treatment. Previous work in our group indicated that the catalytic dehydrogenation of propane in the presence or absence of CO2over a series of Ga2O3-AlO3mixed oxides had favorable results. Therefore, we attempt to prepare high surface area Ga2O3-Al2O3mixed oxides materials using eco-friendly biomass as a non-surfactant template to improve the performance for catalyzing propane dehydrogenation in the presence of CO2.This dissertation is divided into two parts according to the catalyst systems studied in the current CO2-DHP process:β-Ga2O3catalysts and Ga2O3-Al2O3solid solutions. The activity, stability and regeneration behavior for the two catalyst systems have been systematically studied, and the structural-activity relationships have been carefully analyzed via detailed characterizations. The activity centers as well as the origin of the catalytic performance have been discussed in depth.The main content of this dissertation is as follows:1) Studies on β-Ga2O3for CO2-DHPBiomass-template samples with sucrose/Ga2O3molar ratio at1,2,4, and8were synthesized, denoted as β-Ga2O3-S1,0-Ga2O3-S2, β-Ga2O3-S4, and β-Ga2O3-S8, respectively. For comparison purpose, a conventional β-Ga2O3sample was prepared. The characteristic peaks of β-Ga2O3are observed for all samples in the XRD measurement, demonstrating that the most stable form of Ga2O3was achieved via the sucrose-template synthesis. The sucrose derived β-Ga2O3samples show obviously lowers crystalline, as reflected from their broadened peaks and decreased diffraction signals relative to that for conventional β-Ga2O3. Besides, the sucrose-template approach can afford the production of a series of mesoporous samples with favorable textural properties and enhanced surface areas. The SEM and TEM analysis reveals that conventional β-Ga2O3presents a solid bulk structure and smaller average particle size, while the sucrose-template samples display distinct sponge-like morphology and higher sucrose/Ga2O3ratio possesses higher porosity. Specifically, the total acidity of the four sucrose-derived samples is approximately1-2times higher than that of the conventional P-Ga2O3measured by NH3-TPD experiments.The dehydrogenation of propane over the β-Ga2O3samples in the presence/absence of CO2was investigated at773K. Activities in the presence of CO2are generally superior to those in the absence of CO2. In both conditions, the initial propane conversions follow the sequence:β-Ga2O3-S4>β-Ga2O3-S8> P-Ga2O3-S2> β-Ga2O3-S1>β-Ga2O3. Worth mentioning is that over β-Ga2O3-S4with the highest specific surface area, a conversion of propane (43.7%) approximately twice that of conventional β-Ga2O3(23.8%) can be achieved. As far as the stability is concerned, regeneration for P-Ga2O3-S4is tested, and after the third regenerations, the propane conversion maintains37.6%. It is important to highlight here that this approach is not limited to the use of sucrose as the carbohydrate-based carbon sources. The present findings may provide new opportunities for the rational design of new mesostructured gallium oxide catalyst systems for advanced applications.2) Studies on Ga2O3-Al2O3solid solution for CO2-DHPPrepare biomass-template Ga2O3-Al2O3solid solution with sucrose/Ga/Al ratio at0:1:1,2:1:1,4:1:1,8:1:1,12:1:1,8:1.6:0.4,8:0.4:1.6, denoted as SOGalAl1, S2GalAl1, S4GalAl1, S8GalAl1etc. XRD reveals the samples all formed Ga2O3-Al2O3solid solutions. The characteristic peaks of the samples are similar with γ-Ga2O3and γ-Al2O3except for S12GalAl1, which presents a β-Ga2O3crystalline phase style. Besides, the sucrose-template approach can afford the production of a series of mesoporous samples with favorable textural properties and enhanced surface areas that enlarge the non-template SOGalAl1(135m2/g) to S8-Gal-All (187m2/g). The SEM and TEM analysis reveals that series sucrose-template Ga2O3-Al2O3samples appear to a spongy porous morphology, and the average particle diameter is smaller than the non-template preparing Ga2O3-Al2O3solid solution. NH3-TPD experiment further suggests the number of weak acid sites increased initially and then decreased with the increment in sucrose content, and the maximum value was achieved over sample S8-Gal-All.The catalytic activities of the Ga2O3-Al2O3solid solutions were monitored on a continuous micro-reactor at773K and1atm. Activities both in the presence/absence of CO2were analyzed, showing composition effect has a great impact on the DHP activity. In both conditions, the initial propane conversions follow the sequence: S8-Gal-All> S4-Gal-All> S2-Gal-All> SO-Gal-All> S12-Gal-All. Activities in the presence of CO2are generally more stable than those in the absence of CO2, though higher initial propane conversions were achieved in the absence of CO2. Moreover, it was discovered that sample with lower Ga2O3content deactivates more slowly. Taking into account the application potential, the Ga2O3-Al2O3solid solution was accessed by50h on-stream activity test, and S12-Gal-All showed conversion of propane as high as19.3%at50h.