Elsevier

Journal of Catalysis

Volume 377, September 2019, Pages 638-651
Journal of Catalysis

Control of the crystal morphology of VOHPO4·0.5H2O precursors prepared via light alcohols-assisted solvothermal synthesis and influence on the selective oxidation of n-butane

https://doi.org/10.1016/j.jcat.2019.08.004Get rights and content

Highlights

  • Solvothermal preparation of {001} facets of VOHPO4.0.5H2O in C2-C4 alkanoic alcohols.

  • Metastable intercalated alkyl-phosphate was only formed when using linear alcohols.

  • The metastable phase was converted to platy VOHPO4·0.5H2O crystallites at 330 °C.

  • Pseudomorphic {2 0 0} facets of (VO)2P2O7 obtained via annealing or in situ activation.

  • Good ethanol-derived VPO catalyst with larger V5+ amount and thinner crystallites.

Abstract

The catalytic performance of vanadyl pyrophosphate (VPP) catalysts in the oxidation of n-butane to maleic anhydride (MA) depends strongly on the display of the active and selective {0 0 1} faces, that may be controlled during the preparation of VOHPO4·0.5H2O precursor by solvothermal technique using C2 to C4 alkanoic alcohols. Intercalated metastable vanadyl-alkyl-phosphates were formed when linear alcohols (ethanol, n-butanol) were used, at variance with iso-alcohols. {0 0 1} platelets of VOHPO4·0.5H2O yielded {2 0 0} platelets to (VO)2P2O7 in nitrogen or in situ in the reactor, as revealed by structural analyses. The catalysts prepared in ethanol or n-butanol in situ activated at 440 °C were more active and selective to MA than in iso-alcohols, but if the equilibration was performed at 380 °C the influence of alcohol was mitigated. The highest MA yield was obtained with the ethanol-derived catalyst, which exhibited the smallest platy crystallites of (VO)2P2O7, as well as surface V5+ species as seen by XPS.

Introduction

The selective oxidation of n-butane to maleic anhydride (MA) over vanadium phosphorus oxides (VPO) is an essential process in petrochemical industry with a huge annual productivity of greater than one million tons [1], [2]. The limit to about 60–70% of the MA yield in industry [3], [4] is due to several factors depending on the reaction itself (MA selectivity decreases when n-butane conversion increases), on the operating conditions and thus on the ability of the catalyst to respond correctly to these conditions. The catalyst design should be adapted to the high concentration of n-butane in the feed, which is now preferred in most industrial processes [5], but there is still room to improve its behavior in oxidizing conditions. The VPO catalyst contains mainly vanadyl pyrophosphate (VO)2P2O7) (here called VPP). Associated to VOPO4, VPP was first identified in the late seventies during the oxidation of 1-butene to MA on P/V = 1–1.6 catalysts [6]. VPP was further found as the major phase in the catalyst formula adapted to n-butane oxidation [4], [7], [8], [9], [10], because the reducing power of n-butane is stronger than that of butene. There has been considerable debate about the role of (VO)2P2O7 itself during the catalytic reaction. Several features of the solids like the actual surface P/V ratio, the presence or not of oxidized phases like α, β, δ, γ-VOPO4 [8], [9], [11], [12], [13], [14], the amorphous state of the surface, the microstructure [15], [16], [17], [18], [19], etc., were shown to exert a strong influence on catalytic performance. But generally (VO)2P2O7 is considered as the active phase of the n-butane selective oxidation. As supported by 40 years experiments, the clue of its performance was claimed to be its particular {1 0 0} surface framework exhibiting pairs of edge-sharing [VO6] octahedra linked to phosphate groups [8], [9], [11], [20], [21], [22], [23]. Some authors suggested that the high MA selectivity could also depend on the surface V5+ sites of VOPO4 species [7], [12], that would be formed due to the oxidation of (2 0 0) planes [2]. Recent DFT calculations even proposed that the initial Csingle bondH activation of n-butane proceeds on the phosphate group of VOPO4 [24] and not on these V5+ sites which are quite stable on the surface of VPP in oxidizing operating conditions. The particular crystal morphology of VPP is owed to the use of a particular precursor, vanadyl hydrogen phosphate hemihydrate VOHPO4·0.5H2O (here called HVP), whose structure is topologically related. The pseudomorphism observed between {0 0 1} platelets of VOHPO4·0.5H2O and {1 0 0} platelets of (VO)2P2O7 is owed to the topotactic mechanism, as demonstrated in 1984 by Bordes et al. [25] and other authors [26], [27], [28]. This peculiar kind of transformation was observed in inert atmosphere or after catalysis. In oxidizing conditions, HVP delivered two new VOPO4 phases, δ and γ, and not α or β [8]. Pseudomorphism between HVP and VPP particles was also observed at the nano (platelets) and micro (rosets) scales after in-situ activation in n-butane/air mixture by many authors [10], [17], [18], [29]. Therefore, the crystal morphology of VPP and its microstructure depend primarily on those of the HVP precursor, and a careful control of the synthesis conditions of the latter should lead to well-defined characteristics of VPP particles [23], [27]. As HVP is usually prepared by refluxing vanadium oxide and phosphoric acid in alcohols, their properties (boiling temperature, reducing power, type of function) are responsible for the final crystal size and morphology of particles [30], [31]. Other methods of preparation were proposed, such as hydrothermal and solvothermal techniques in which the temperature is imposed and not determined by the boiling temperature of the medium. In the first case a surfactant is required [32], [33], while it is not necessary with the solvothermal method. Taufiq-Yap et al. [34] experimented both, heating for example a mixture of vanadium oxide and phosphoric acid at 100–150 °C in n-propanol or n-butanol. The MA yield was increased from 21 mol% for VPP prepared by the common reflux method to 38 mol%. Starting from VOPO4·2H2O, they also explored the influence of different C4 to C10 alcohols that allowed to obtain well developed {0 0 1} facets of HVP, delivering consequently {1 0 0} facets of selective (VO)2P2O7. Again increases of MA yield up to 45 mol% vs. 21 mol% for conventional catalysts were observed [29], [32], [34]. Several authors took the opportunity of the layered structure of VOPO4·2H2O, α-VOPO4 or VOHPO4·0.5H2O to synthesize ionic or organic intercalates for various applications [1], [22], [32], [35], [36], [37], [38]. Indeed, the possible formation of intercalated molecules inside the layers could be a means to decrease the plate thickness after decomposition and dehydration. Hiyoshi et al. [22] obtained VPP samples differing by their microstructure by intercalation-exfoliation-reduction process of a VOPO4·2H2O suspension in 2-butanol, obtaining MA yield of ca. 50 mol% at 390 °C. However, the structure-activity relationship has not been significantly demonstrated for the catalysts synthesized by this method. Indeed, the alcohol molecules intercalated into the VOHPO4·0.5H2O precursor structure could favor the structural disorder of (VO)2P2O7, creating structural defects and possibly changing the redox properties [19], [39]. The overoxidation of n-butane to CO2 could also happen if the (VO)2P2O7 structure is highly defective [10], [16], [40].

Herein, a feasible pathway to control the synthesis of VOHPO4·0.5H2O precursor with predominantly exposed {0 0 1} facets was demonstrated in order to obtain (VO)2P2O7 phase with highly exposed {1 0 0} crystal facets. Ethanol and C3-C4 alkanoic alcohols, varying by their amount, their structure (linear or branched chain), and the alcoholic function (primary of secondary), were used as reducing agents of VOPO4·2H2O in solvothermal conditions. VOHPO4·0.5H2O was transformed to (VO)2P2O7 either by thermal treatment in nitrogen, or in situ activated during catalytic experiment (Scheme 1). The precursors and catalysts (including catalysts after experiments) were characterized by several methods of analysis (X-ray diffraction, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy), their composition measured by CHN analysis, their surface area was measured by BET and their reactivity evaluated by thermal analyses. The catalytic activity of the resulting VPO catalysts was evaluated in the gas-phase oxidation of n-butane to maleic anhydride in a fixed bed reactor in oxidizing conditions.

Section snippets

Catalyst preparation

Vanadyl phosphate dihydrate (VOPO4·2H2O) was prepared from V2O5 (Sigma Aldrich) and H3PO4 (Merck) by reflux method as reported by several authors [6]. Ten grams of V2O5 and 98 g of H3PO4 85% were mixed with 240 mL of distilled water. The mixture was heated to boiling temperature and stirred for 8 h under reflux condition. The obtained lemon-yellow precipitate of VOPO4·2H2O was filtered, sequentially washed with distilled water and acetone, and then dried at 90 °C for 24 h.

In a typical procedure

Structural properties of the alc-precursors

Fig. 1a shows the XRD diffractograms of the alc-precursors synthesized by using 10 mL of different alcohols per g of VOPO4·2H2O All patterns are typical of vanadyl hydrogen phosphate hemihydrate (VOHPO4·0.5H2O; JCPDS No. 01-074-3078). The structure of VOHPO4·0.5H2O (HVP) is orthorhombic with a = 7.416, b = 9.592, c = 5.689, Z = 4, Pmmn [27]. It is described as stacked a, b layers of [VOHPO4] held together by water molecules along c. Remarkably, crystalline HVP particles were obtained with the

Conclusion

Not commonly used to prepare catalysts, the solvothermal technique was used to prepare VOHPO4·0.5H2O precursors from VOPO4·2H2O in an excess of C2 to C4 alkanoic alcohols. These alcohols differed by their linear (C2, nC4) or branched (iC3, iC4) configuration, their boiling temperature, the primary or secondary alcoholic function and their reducing power, all these factors determining the crystal growth and thus the crystal morphology of the hemihydrate precursor. For the first time ethanol

Acknowledgements

This research is funded by Ministry of Education and Training, Vietnam under grant number KYTH-55.

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