Lattice oxygen transfer induced active phase transition of VPO catalysts in cross condensation of acetic acid and formaldehyde

doi:10.1016/S1872-2067(25)64844-3

Abstract

Abstract:

Vanadium phosphorus oxide (VPO) catalyst is a promising candidate for the condensation reaction of formaldehyde (FA) and acetic acid (HAc) to produce acrylic acid (AA). However, the complexity of the active phases and their dynamic interconversion under redox conditions has led to controversies regarding the actual active phase in this reaction. To address this, this study systematically investigates the phase transition and underlying mechanism of VPO catalysts under reaction conditions. X-ray diffraction (XRD) patterns, Raman spectra, transmission electron microscopy images and X-ray photoelectron spectroscopy collectively demonstrated that the V⁴⁺ phase (VO)₂P₂O₇ retained the bulk phase structure throughout the reaction, with only minor surface phase transition observed. In contrast, the V⁵⁺ phase underwent reduction to other phases in both bulk and surface regions. Specifically, the δ-VOPO₄ phase rapidly transformed into the α_II-VOPO₄ phase, which could reversibly convert into the R1-VOHPO₄ phase (V⁴⁺). Controlled variable experiments, H₂-temperature programmed reduction and in-situ XRD experiments in a hydrogen atmosphere further demonstrated that these phase transitions were primarily attributed to the loss of lattice oxygen. The presence of V⁴⁺ phase in VPO catalysts enhanced the selectivity of acrylic acid, while the existence of V⁵⁺ phase promoted the activation of acetic acid. This work elucidates the redox-driven phase evolution of VPO catalysts and offers valuable insights for designing efficient catalysts for FA-HAc cross-condensation by balancing phase stability and activity.

Key words: Vanadium phosphorus oxide, Phase transition, Acetic acid, Formaldehyde, Cross condensation, Acrylic acid

Yinhong Niu, Zhen Shi, Zhiquan Yu, Qiang Guo, Junju Mu, Yafei Liang, Zhixin Zhang, Sheng Wang, Feng Wang. Lattice oxygen transfer induced active phase transition of VPO catalysts in cross condensation of acetic acid and formaldehyde[J]. Chinese Journal of Catalysis, 2025, 79: 112-126.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64844-3

https://www.cjcatal.com/EN/Y2025/V79/I12/112

Figures/Tables 20

Fig. 1. Preparation of four single-phase VPO catalysts.

Fig. 2. TEM images of VPO-1-F ((VO)2P2O7) (a), VPO-2-F (β-VOPO4) (b), VPO-3-F (δ-VOPO4) (c), and VPO-4-F (ω-VOPO4) (d) catalysts.

Fig. 3. Phase identification of VPO-1-F ((VO)2P2O7), VPO-1-U and VPO-1-R catalysts with XRD patterns (a) and Raman spectra (b).

Fig. 4. Phase identification of VPO-2-F (β-VOPO4), VPO-2-U and VPO-2-R catalysts with XRD patterns (a) and Raman spectra (b).

Fig. 5. Phase identification of VPO-3-F (δ-VOPO4), VPO-3-U and VPO-3-R catalysts with XRD patterns (a) and Raman spectra (b).

Fig. 6. (a) XRD patterns of VPO-3-F (δ-VOPO4) catalyst at different catalytic reaction times. (b) TEM images of VPO-3-AR2h and VPO-3-AR3h catalysts. AR = after reaction; R = regenerated.

Fig. 7. Corresponding catalytic performance of VPO-3 catalyst after different run times. (HAc represents acetic acid, AA represents acrylic acid, COx represents carbon oxides, MA represents methyl acrylate, L.O. represents other liquid byproducts. The specific substances represented by L.O. and their respective selectivity are listed in Table S1. X-run refers to the x-th catalytic process in the evaluation cycle. Reaction conditions: catalyst (1.20 g, 40-60 mesh), N2 as carrier gas (30 mL·min-1), 340 °C, WHSV = 2.2 h-1. The molar ratio of feedstocks is HCHO: CH3COOH: H2O = 1:1.5:1.6.

Fig. 8. XRD patterns of the VPO-3-F (δ-VOPO4) catalyst treated in a pure nitrogen for 5 h or an aqueous nitrogen atmosphere for 3 h (a) and treated in aqueous solutions of individual reactants HAc or FA or the selective product AA at the reaction temperature (340 °C) for 2 h (b).

Fig. 9. Phase identification of VPO-4-F (ω-VOPO4), VPO-4-U and VPO-4-R catalysts with XRD patterns (a) and Raman spectra (b). (c) The typical XRD and Raman peaks of VPO-4 catalysts.

Fig. 10. Curve fitting analysis of XPS V 2p3/2 spectra for the VPO-1 catalyst (a) and VPO-3 catalyst (b) at different catalytic reaction times. Curve fitting analysis of XPS O 1s spectra for the VPO-1 catalyst (c) and VPO-3 catalyst (d) at different catalytic reaction times. Curve fitting analysis of XPS P 2p spectra for the VPO-1 catalyst (e) and VPO-3 catalyst (f) at different catalytic reaction times. The original data are presented in the form of blue hollow circle curve, and the superimposed values after peak deconvolution are represented in the form of green solid lines in the figure.

Table 1 XPS and ICP-OES results of VPO catalysts.

Catalyst	V^5+/V⁴⁺^a	V_OX^a	P/V^a	Lat-O/Sur-O^a	V content ^b (wt%)	P content ^b (wt%)	P/V^b
VPO-1-F	0.048	4.04	1.54	0.80	35.49	20.26	0.94
VPO-1-U	0.018	4.02	1.58	0.80	36.04	20.60	0.94
VPO-1-R	0.223	4.18	1.64	0.85	35.18	20.37	0.95
VPO-3-F	4.786	4.83	1.69	0.87	24.98	14.37	0.95
VPO-3-AR2h	0.609	4.39	1.77	0.82	32.85	18.60	0.93
VPO-3-AR5h	0.043	4.04	1.61	0.79	31.95	18.52	0.95
VPO-3-AR2h-R	6.000	4.86	1.83	0.80	28.00	16.23	0.95
VPO-3-AR2h-R-AR2h	0.022	4.02	1.64	0.65	32.82	18.85	0.95

Table 2 The XRF result of R1 phase.

Catalyst	V content (wt%)	P content (wt%)	O content (wt%)	V/P/O
VPO-3-AR5h (R1-VOHPO₄)	31.46	19.13	49.41	1/1/5

Fig. 11. (a) The phase transition processes of four single-phase VPO catalysts. (b) Schematic of the transition from δ-VOPO4 to αII-VOPO4 and R1-VOHPO4 Phase. Legend: purple, O; green, V; blue, P; Red hollow circle, Lost oxygen atoms.

Fig. 12. (a) H2-TPR profiles of VPO catalysts with different phase: (VO)2P2O7, β-VOPO4, δ-VOPO4, αII-VOPO4, R1-VOHPO4, ω-VOPO4. (b) XRD patterns of the δ-VOPO4 catalyst and the catalyst obtained by treating δ-VOPO4 at 340 °C in hydrogen for 1 h (main phase: αII-VOPO4).

Fig. 13. The schematic diagram of H2-TPR profile integration for δ-VOPO4, αII-VOPO4, R1-VOHPO4 (a), and their integration results (b).

Fig. 14. Normalized in-situ XRD patterns of the δ-VOPO4 catalyst under a 50% H2/N2 atmosphere as a function of temperature.

Fig. 15. Catalytic performance of VPO catalysts with varying phase compositions. (a) HAc conversion and AA selectivity. (b) Specific HAc conversion rate. Reaction conditions: catalyst (1.20 g, 40-60 mesh), N2 as carrier gas (30 mL·min-1), 340 °C, WHSV = 2.2 h-1. The molar ratio of Feedstocks is HCHO: CH3COOH: H2O= 1:1.5:1.6. Legend: VPP represents (VO)2P2O7 phase, R1 represents R1-VOHPO4 phase, αII and β represent αII-VOPO4and β-VOPO4 phases.

Fig. 16. The condensation reaction mechanism on VPO (V5+) catalyst.

Fig. 17. Generation pathways of each by-product in the VPO-catalyzed condensation reaction of FA and HAc. COx represents CO and CO2, and the CO2 is the main product.

Fig. 18. Catalytic performance of VPO-1-F ((VO)2P2O7) catalyst as functions of WHSV at 340 °C with a formaldehyde to acetic acid molar ratio of 1/1.5 (a), reaction temperature at a fixed WHSV of 1.1 h-1 with the same molar ratio (b), acid-aldehyde feed ratio (c) and carrier gas (N2) flow rate at 340 °C and 1.1 h-1 WHSV (d). L.O. represents other liquid byproducts.

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