545405 Deciphering the Reaction Network of the Selective Oxidation of Butane to Maleic Anhydride over (V1-xWx)OPO4 Catalysts

Monday, June 3, 2019: 4:24 PM
Texas Ballroom A (Grand Hyatt San Antonio)
Jingxiu Xie1, Felix Pohl1, Christian Schulz1, Knut Wittich2, Ralph Kraehnert1, Benjamin Frank1, Robert Glaum2 and Frank Rosowski1,3, (1)BasCat – Unicat BASF JointLab, Berlin, Germany, (2)Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität, Bonn, Germany, (3)Process Research and Chemical Engineering, BASF SE, Ludwigshafen, Germany

Selective oxidation of lower alkanes (ethane, propane, and n-butane) converts these less reactive molecules to valuable chemicals in a resource-efficient and environmentally friendly approach. The selective oxidation of n-butane to maleic anhydride (MAN) is an important industrial process with a worldwide capacity of more than 1 million metric tons per year. The industrial vanadyl pyrophosphate (VPP) catalyst reaches a MAN yield of ~60% and research thus far focused on adding electronic promoters to improve its catalytic performance. To investigate potential catalysts beyond the extensively studied and optimized VPP, novel classes of catalysts with different crystal structures to VPP need to be designed.

 We recently identified a new class of catalysts based on the tungsten-doped αII‑VOPO4-structure. Single phase microcrystalline powders of (V1–xWx)OPO4 are accessible by solution combustion synthesis . X-ray powder diffraction before and after catalytic testing confirmed that these catalysts were phase-stable during catalytic testing (C4H10/O2/H2O = 2/20/3 vol-%, GHSV = 2000 h‑1, TOS = 1000 h). By variation of the V/W ratio the V0.8W0.2OPO4 was found to be most active and selective towards MAN. A parameters field study with V0.8W0.2OPO4 was subsequently carried out to obtain mechanistic insights and its reaction network was compared with those of highly selective reference VPP and non-selective V2O5, respectively. Parameters variation included feed concentrations (n‑butane and oxygen), gas hourly space velocity, temperature, and co-feeds (ethene, CO, and H2O).

 For all catalysts, MAN, CO, and CO2 selectivities totaled more than 95% but the product distributions were different, as shown in Figure 1a. At 420 °C, 1 bar, C4H10/O2/H2O = 2/20/3 vol-%, GHSV = 2000 h-1, the selective VPP showed 62% MAN selectivity at 95% n-butane conversion and CO/CO2 = 1.2. Instead, non-selective V2O5 showed negligible MAN selectivity at 22% n-butane conversion and CO/CO2 = 2.1. V0.8W0.2OPO4 displayed similarities to both VPP and V2O5, showing 18% MAN selectivity at 32% n-butane conversion and CO/CO2 = 3. The high CO/CO2 ratio of V0.8W0.2OPO4 suggests that COx is formed by primary oxidation of n-butane instead of secondary oxidation of MAN (Figure 1b). Minor products also showed different trends for the catalysts, suggesting that (V1–xWx)OPO4 and VPP have different reaction networks. For instance, acrylic acid is a significant byproduct for VPP but not for V0.8W0.2OPO4. To confirm reaction pathways identified by steady state parameter field studies for V0.8W0.2OPO4 and VPP catalysts, transient pulse experiments were carried out at 420 °C, 1 bar, C4H10/O2/H2O = 2/20/3 vol-%, GHSV = 2000 h-1. Important minor products including furan, 1-butene, acetaldehyde, acrylic acid, acetic acid were individually pulsed through the reactor. The identified reaction pathways give a deeper insight into the reaction network of each catalyst. The comparison of reaction networks of V0.8W0.2OPO4 with selective VPP and non-selective V2O5 catalysts is expected to lead to further understanding towards catalysts for the selective oxidation of n-butane to MAN.

Figure 1. (a) Product distribution of V2O5, αII-V0.8W0.2OPO4, and VPP at 1 bar, 420 °C, 2 %v C4H10, 20 %v O2, 3 %v H2O, 2000 h-1 and (b) classic triangle reaction network of n-butane to maleic anhydride.

 


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