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

Dehydrogenation of propane to propylene in the presence of carbon dioxide over novel gallium and indium oxide based catalystsPropylene is currently an important building block of the chemical industry, which is mainly produced from steam cracking and FCC. The steam cracking maximizes ethylene yield and in the FCC plant propylene is produced as a by-product, so propylene production from these sources barely matches with its rapidly increased consumption. Therefore, transformation of the relatively cheap and abundant propane into propylene seems to be an alternative in solving this problem. Thermal dehydrogenation of propane to propylene (DHP), although industrialized, requires high reaction temperature due to the thermodynamics limits. This drawback not only makes the process extremely energy intensive, but also brings about drawbacks in terms of thermal cracking, severe coking and consequent deactivation which restrict the further application of this technique. In order to explore new alternatives that permit high conversion of propane at lower temperature, people resort to oxidative dehydrogenation of propane by molecular oxygen (ODHP), which allows the endothermic reaction to be exothermic. 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 CO2 as weak oxidant instead of O2 would couple the DHP with reverse water-gas shift (RWGS), which consumes H2 by CO2, leading to enhanced DHP equilibrium conversion. In contrast to ODH, by-products are remarkably suppressed, and selectivity to propylene is greatly promoted in the CO2-DHP. Besides, CO2 can stabilize the catalysts via elimination of coke. Since CO2 as green house gas has been effectively utilized in this process, CO2-DHP is recognized as a "green chemistry" route.In the field of CO2-DH of light alkanes, Ga2O3 receives 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. Thus, new approach in circumvent this problem is desirable. Recently, a new class of Ga2O3-Al2O3 solid 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 acid site amount of the Ga2O3-Al2O3 solid 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. By utilizing these extraordinary merits and submitting the Ga2O3-Al2O3 solid solution into CO2-DHP, marked improvement in DH activity as well as stability can be anticipated.From the viewpoint of mechanism, the activities of Ga-based catalysts for both DHP and SCR derive from their unique activation abilities toward C-H bonds in hydrocarbons. Beside Ga-based materials, another important type of NOx SCR catalysts is In2O3-Al2O3 mixed oxide materials. Nevertheless, In2O3-Al2O3 mixed oxide has never been systematically studied in the dehydrogenation reaction of short chain alkenes. Comparing with In2O3, In2O3-Al2O3 mixed oxide is with greater surface acidity, remarkably larger surface area, and superior properties in activation of alkanes. All of these are extremely favorable for promotion of DHP performance over indium-based catalysts. Based on the similarity of In2O3-Al2O3 and Ga2O3-Al2O3 solid solution in NOx SCR reactions, application of In2O3-Al2O3 mixed oxide into the DHP would allow in-depth and comprehensive study of the mixed oxide fromⅢA family elements in the dehydrogenation field.This dissertation is divided into three parts according to the three catalyst systems studied in the current CO2-DHP process:Ga2O3-Al2O3 solid solutions, In2O3-Al2O3 mixed oxides and supported In2O3 catalysts. The activity, stability and regeneration behavior for the three 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 with depth.The main content of this dissertation is as follows:1) Studies on Ga2O3-Al2O3 solid solution for CO2-DHPPrepare Ga2O3-Al2O3 solid solution with Ga/Al ratio at 4:1,1:1 and 1:4 respectively via alcoholic ammonia coprecipitation, denoted as Ga8Al2O15, Ga5Al5O15 and Ga2Al8O15 respectively.γ-Ga2O3 andγ-Al2O3 have been synthesized accordingly and were used as references. XRD reveals the three mixed oxide samples all formed Ga2O3-Al2O3 solid solutions.71Ga MAS NMR suggests the Ga has two locations in the Ga2O3-Al2O3 solid solution, namely four coordinated sites (GaⅣ) and six coordinated sites (GaⅥ). By increasing the Al content, the percent of GaⅣkeeps increasing while the total Ga2O3 content decreases. Pyridine-IR indicates that there are exclusively Lewis acid sites other than Bronst acid sites exist on the surface of Ga2O3-Al2O3 solid solution. NH3-TPD experiment further suggests that increment in Al2O3 content results in enhancement of total Lewis acid site density, but the weak acid site density increased initially and then decreased, and the maximum value was achieved over sample Ga8Al2O15. It was discovered that the amount of GaⅣshowed linear relationship with the weak acid site density, suggesting GaⅣprovides mainly weak Lewis acid sites for the solid solution.The catalytic activities of the Ga2O3-Al2O3 solid solutions were monitored on a continuous micro-reactor at 773 K and 1 atm. Activities both in the presence/absence of CO2 were analyzed, showing composition effect has a great impact on the DHP activity. In both conditions, the initial propane conversions follow the sequence: Ga8AlO15＞γ-Ga2O3> Ga5Al5O15> Ga2Al8O15.γ-Ga2O3 deactivates the most rapidly during 8 h on stream. It was discovered that sample with lower Ga2O3 content deactivates more slowly. Activities in the presence of CO2 are generally more stable than those in the absence of CO2, though higher initial propane conversions were achieved in the absence of CO2. Perfect correlation of the weak acid site density and the initial activities suggests the GaⅣto be the possible active center for dehydrogenation.Taking into account the application potential, the Ga2O3-Al2O3 solid solution was accessed by 50 h on-stream activity test and 2 round deactivation-regeneration tests. Ga5Al5O15 showed conversion of propane as high as 22.5% at 50 h, while y-Ga2O3 totally deactivated. After regeneration, the Ga5Al5O15 showed no loss in the initial conversion of propane, whereasγ-Ga2O3 decreased by c.a.9%. This is closely related to the thermal stability of the Ga2O3-Al2O3 solid solution against heat treatment. XRD characterization for the regenerated samples reveals that Ga5Al5O15 maintained the originalγphase whileγ-Ga2O3 was converted to theβpolymorph. 2) Studies on In2O3-Al2O3 mixed oxide for CO2-DHPIn2O3-Al2O3 mixed oxide samples with molar percent of In at 40%,20% and 10 % were synthesized via ammonia coprecipitation in a mixed alcoholic/aqueous solution, and were symbolized as In-Al-40, In-Al-20 and In-Al-10 respectively. Simple oxide In2O3 and Al2O3 were prepared through the same route as references. Characterization of XRD reveals that In2O3 in mixed oxides all exists in a cubic crystalline form. In2O3 and Al2O3 did not form solid solutions but render better dispersions for each other. TPR confirmed the existence of highly dispersed In2O3 species, which was reduced in the low-temperature range (473-773 K). The proportion of the highly dispersed In2O3 species increased with the enhancement of the alumina content. Further experiment by XPS evidenced that metallic In0 derived from highly dispersed In2O3 can not be re-oxidized by CO2 at 873 K.In the activity tests at 873 K and 1 atm, all In2O3-Al2O3 mixed oxide samples far exceed simple oxide In2O3 and Al2O3 in terms of propane conversion both in the presence/absence of CO2. Activities in the presence of CO2 are generally superior to those in the absence of CO2. In-Al-20 shown to be the best sample exhibits maximum propane conversion of 35.7% and selectivity to propylene of 76.5% in the presence of CO2 at 3 h on stream. After continuous reaction lasting 30 h, conversion> 25% was still maintained for In-Al-20. It was noteworthy that an obvious induction period has been experienced for all mixed oxide samples. Namely, in the induction period, yield of propylene increased initially and then gradually decreased, and maximums were achieved at 3 h for in the presence of CO2. However, In-Al-20 pre-reduced by H2-Ar at 773 K has not undergone such induction period. This implies the metallic In0 species derived from highly dispersed In2O3 should be the intrinsic active site for dehydrogenation, as was further confirmed by the positive correlation between its amount and the corresponding dehydrogenation activity data.CO2-H2 temperature-programmed tests were employed to measure the RWGS performance for the In2O3-Al2O3 mixed oxide samples, suggesting the bulk In2O3 as the essential active species for RWGS. Therefore, In2O3-Al2O3 mixed oxide samples actually comprises double active centers, and ideal dehydrogenation activity is prompted by balanced composition of bulk as well as highly dispersed In2O3 in the alumina matrix. 3) Support effect in CO2-DHP over supported In2O3 catalystsSiO2, Al2O3 and ZrO2 were employed as three oxide supports for impregnating In2O3, and samples with two loadings (3 and 10%) were prepared on each support, marked as In(3)/M and In(10)/M respectively. Studies by XRD reveal SiO2 supported samples are with the poorest dispersion of In2O3, while much better dispersion of In2O3 is obtained over Al2O3 and ZrO2. As was further evidenced by H2-TPR, the majority of In2O3 exists in a bulk form, while the proportion of highly dispersed In2O3 is much higher over Al2O3 and ZrO2.Acid-base properties of the In2O3-Al2O3 mixed oxide samples were evaluated by NH3-TPD和CO2-TPD tests, which shows the zirconia supported samples are with the highest acid as well as basic site density.Activity tests at 873 K and 1 atm reveals, in the presence of CO2, SiO2 supported samples are with the poorest dehydrogenation performance. In(10)/Si has a initial propane conversion of c.α.10% and totally deactivated in 8 h. ZrO2 supported samples have the optimal propane conversion (maximum of 27.5% for In(10)/Zr at 3 h, in contrast, those for Al2O3 supported samples are slightly lower. On the other hand, the stabilized selectivities to propylene all exceed 80% for In/Al comparing to 60-70 % for In/Zr. The conversions of CO2 are highest for In/Zr, apparently superior to those over In/Si and In/Al. Conversion of propane dropped obviously as atmospheric condition was switched to that in the absence of CO2 for all samples, particularly dramatic for In/Zr, and the order of propane conversion ranked:In/Al> In/Zr> In/Si.Works in part II already reveals that the metallic In0 species derived in-situ from highly dispersed In2O3 acts as the intrinsic active center for dehydrogenation, namely the dehydrogenation performance depends largely on distribution of In2O3 on supports. Excellent correlation has been obtained between propane conversions in both presence/absence of CO2 and their corresponding amount of the highly dispersed In2O3, suggesting outstanding dehydrogenation results over In/Al and In/Zr could be attributed to the superior ability in dispersing In2O3 of Al2O3 and ZrO2. Activity contrast for In/Zr between two atmospheric conditions in conjunction with its dramatically high conversion of CO2 indicates that its dehydrogenation performance was facilitated by RWGS to a large extent. The RWGS over In/Zr was promoted by its extraordinarily high surface basic site density. High acidic site density is negative for the desorption of hydrocarbons, which explains the lower selectivities for In/Zr in contrast to those for In/Al.