1,3-butadiene (BD) is an important raw material with wide applications in chemical industry, e.g. synthetic elastomer manufactory, adiponitrile, chloroprene, etc., chemical productions1. Currently, it is mainly produced by steam cracking of hydrocarbon feedstock. However, the usage of natural gas and refinery waste gas as cracker feedstock led to a significant drop in BD production2. The vast reserves of shale gas will certainly increase this trend. In addition, the requirement of BD is expected to grow quickly. To meet the fast increasing BD demand, alternative processes are required to provide BD to the market.
Among the alternative processes for producing BD, catalytic oxidative dehydrogenation (ODH) of 1-butene, using air as oxidant, is the most widely investigated since 1-butene can be obtained from natural gas condensates and refinery waste gases3. The feasibility and advantages of applying soft oxidant (CO2) instead of air was demonstrated in our previous work4. After screening of active sites, systematic investigation was done to study key factors for catalytic performance. A preliminary reaction mechanism is proposed, followed by the further improvement of catalytic activities.
Materials and Methods
A fixed-bed continuous flow reactor equipped with quartz tube was used for feasibility and activity testing. Premixed 1-butene/CO2 with 1:9 molar ratios was used as reaction gas. Temperature-programmed surface reaction (TPSR) was applied with on-line MS to test the feasibility. Catalytic activity was evaluated with on-line GC at different reaction temperatures. Techniques, e.g., XRD, XRF, BET, XPS, Raman, solid NMR, SEM, TEM, NH3-TPD, TGA, etc, were applied to characterize the materials.
Results and Discussion
Blank test was carried out using empty quartz tube and TPSR-MS technique. With 1-butene/He feeding, BD was only formed at quite high temperature (770 °C), according to thermal cracking (1-butene → BD + H2). The BD formation peak slightly shifted to lower temperature (750 °C), with the observation of CO formation (1-butene + CO2 → BD + CO + H2O), when 1-butene/CO2 mixture was fed. It confirmed the feasibility of using CO2 as soft oxidant for ODH reaction. When the reaction mixture passed over Fe2O3 catalyst, BD was formed at even lower temperature (410 °C) but not sustainable (Fig. 1), which was suspected due to its poor redox capacity. As Fe2O3 was highly dispersed on Al2O3support, BD production became sustainable, suggesting the formation of continuous redox cycles.
The activity performance comparison of Fe2O3, Al2O3 and Fe2O3/Al2O3 confirmed their different capacity: Al2O3 mainly contributes 1-butene conversion, Fe2O3 supports BD selectivity, and Fe2O3/Al2O3 combines both advantages of them. The poor conversion of Fe2O3/SiO2 also suggested the important role of Al2O3. The main by-products, trans-2-butene and cis-2-butene, can give BD with a similar yield from 1-butene. Thus, excluding 1-butene conversion and BD selectivity, C4 selectivity (the sum of C4 products’ selectivity) is another important index in the activity evaluation.
Fig. 1. TPSR-MS pattern of Fe2O3 and Fe2O3/Al2O3catalysts.
Further activity screening on various transition metal oxides supported on Al2O3gave the similar 1-butene conversion. However, BD selectivity was significantly varied as different transition metal oxides to be used as the catalysts. Correlated with the characterization results, a brief reaction mechanism was hypothesis and shown in Fig. 2.
Fig. 2. Hypothesized reaction mechanism for ODH of 1-butene to BD.
In addition to supported transition metal oxide catalysts, a novel zeolite based catalysts were developed for 1-butene oxidative dehydrogenation to give butadiene5. The effect of reaction temperature was studied from 500 to 650 °C. The conversion and BD selectivity were enhanced as the temperatures increased.
As shown in Fig. 3, Zn-MCM-22 with MWW structure showed good BD yield above 30% when reaction temperature was higher than 600 oC. The butadiene yield was further increased to near 50% with the combination of noble metal to be used as the catalysts. However, the butadiene yield was decreased at 650 oC due to the significant formation of C1-C3 byproducts and coke. Such a good catalyst also showed much better stabilities compared to the transition oxide metal catalysts. Promoters were studied to improve BD yield and catalytic stability. Further improvement is still ongoing.
Fig. 3. Catalytic performance of Zn-MCM-22 based catalysts. Reaction condition: 250 mg catalyst; 30 ml/min 1:9 premixed 1-butene and CO2. Reaction temperature: varied (left); 600 °C (right).
3. López Nieto, J. M.; Concepción, P.; Dejoz, A.; Knözinger, H.; Melo, F.; Vázquez, M. I., Selective Oxidation of n-Butane and Butenes over Vanadium-Containing Catalysts. J. Catal. 2000, 189(1), 147-157.
5. Liu Y, Borgna A, Preparation and application of Zn-Si (ZS-1) zeolite with MWW structure. SG application No: 10201400 17T.
See more of this Group/Topical: Catalysis and Reaction Engineering Division