(621ci) Oxidative Dehydrogenation of 1-Butene to 1,3-Butadiene Using CO2 As Soft Oxidant

Introduction

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).

References

1.  Sun, H. N.; Wristers, J. P., Butadiene. In Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc.: 2000.

2.  Weissermel, K.; Arpe, H. J., 1,3-Diolefins. In Industrial Organic Chemistry, Wiley-VCH Verlag GmbH: 2008; ; pp 107-126.

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.

4.  Yan, W.; Kouk, Q. Y.; Luo, J.; Liu, Y.; Borgna, A., Catalytic oxidative dehydrogenation of 1-butene to 1,3-butadiene using CO2. Catal. Commun. 2014, 46 (0), 208-212.

5.  Liu Y, Borgna A, Preparation and application of Zn-Si  (ZS-1) zeolite with MWW structure. SG application No: 10201400 17T.