Phosphorus-Based Ionic Liquid as Dual Function Promoter Oriented Synthesis of Efficient VPO Catalyst for Selective Oxidation of n-butane

Abstract

Vanadium phosphorus oxide (VPO) catalysts promoted by phosphorus-based ionic liquids (ILs) as structure directing agent and promoters have been innovatively synthesized and investigated for selective oxidation of n-butane to maleic anhydride (MA). The catalytic performances showed that the IL addition notably improved the n-butane conversion and MA selectivity, during which the optimized 3%IL-VPO catalyst exhibited the maximum MA yield of 59.2% that is much better than that of blank VPO with 49.4% MA yield under the same reaction conditions. XRD, FI-IR, Raman, SEM, TG, BET, XPS and H2-TPR techniques were utilized combinatorically to elaborate the synergistic effect of cations and anions of ILs. Results demonstrated that IL-cations oriented synthesis of VPO precursor showing a vertically intercrossed slice structure morphology having smaller lamellar thickness. Correspondingly, it notably enhanced the specific surface area of the VPO catalysts and exposed the more active surface of (VO)2P2O7 after activation. Meanwhile, IL-anions as promoters largely modulated the P/V ratio, valence state of V and oxygen species amounts on the surface of VPO catalyst, etc. All of these influence factors were subsequently discussed in detail and correlated to the catalytic performance of VPO catalysts. Graphic Abstract

Influence of the Structural Characteristics of VPO Catalysts on the Temperature Sensitivity of the Oxidation Reaction of n-Butane to Maleic Anhydride

“…66−68 Therefore, to facilitate the n-butane oxidation reaction, the P/V ratio needs to be controlled within a reasonable range, and a lower P/V ratio is beneficial for improving the temperature sensitivity of the n-butane conversion rate. (2) The synergy between different positive pentavalent vanadium species and (VO) 2 P 2 O 7 is conducive to the smooth progress of the n-butane oxidation reaction. 69 However, different positive pentavalent vanadium species can also transform into each other at different temperatures, that is, in the whole process of different vanadium species during the n-butane oxidation reaction.…”

Mono-, Bi-, and Tri-Metallic DES Are Prepared from Nb, Zr, and Mo for n-Butane Selective Oxidation via VPO Catalyst

“…It can be concluded that this expanded crystalline plane is very crucial for C-H bond activation, which is required for initiating the catalyzed reaction. Researchers believe that the (001) plane in the precursor generates the (020) plane in the catalyst after precursor activation [12,33]. Thus, the main precursor phase (VOHPO 4 •0.5H 2 O) is topologically transformed into the main active phase (VO) 2 P 2 O 7 (Figure 5); herein, the relative integral intensity I (020) /I (204) ratio was used to evaluate the crystallinity of the VPO catalyst.…”

“…The presence of active domains on the precursor and catalyst surfaces was further assured from FT-IR characterization. Figure 6 shows the FT-IR spectra of mono-, bi-and tri-metallic precursors in the range of 900~1300 cm −1 [12]. After the FT-IR spectra were obtained, the precursor phases were compared with the unpromoted precursor (Blank-PVPO) with the following results: V υ(P-(OH) and V υ(V=O) emerged at 927 cm −1 and 979 cm −1 respectively, while asymmetric PO 3 absorption peaks appeared at 1203 cm −1 , 1105 cm −1 and 1045 cm −1 .…”

“…It has been commonly found that efficient VPO catalysts exhibit adequate surface characteristics such as surface area, structure morphology, active sites, V +4 /V +5 ratio, P/V ratio, valence state of vanadium, and surface and lattice oxygen [12]. In addition to these physiochemical properties, the electronic characteristics also affect the catalytic performance.…”

“…Moreover, the polyols-based DES acted as a structure-directing agent in VPO synthesis [27]. Importantly, the electronic properties of the VPO catalyst could be improved via iron-based ionic liquids (Fe-ILs) such as VPO-OMIMFeCl 4 and VPO-BMIMFeCl 4 [12]. All of these findings provoked us to prepare a novel and green modifier for the VPO catalyst, which could potentially act as structure-directing agent and electronic promoter.…”