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
Graphical abstract
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 CH 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.
References (58)
- et al.
The contribution of homogeneous and non-oxidative side reactions in the performance of vanadyl pyrophosphate, catalyst for the oxidation of n-butane to maleic anhydride, under hydrocarbon-rich conditions
Catal. Today.
(2005) - et al.
Some selectivity criteria in mild oxidation catalysis: V- P- O phases in butene oxidation to maleic anhydride
J. Catal.
(1979) Crystallochemistry of V-P-O phases and application to catalysis
Catal. Today
(1987)Reactivity and crystal chemistry of VPO phases related to C4-hydrocarbon catalytic oxidation
Catal. Today
(1988)- et al.
Synthesis and characterization of new VPO catalysts for partial n-butane oxidation to maleic anhydride
J. Catal.
(1991) Vanadium phosphorus oxides, a reference catalyst for mild oxidation of light alkanes: a review
C. R. Acad. Sci. IIC-Chem.
(2000)- et al.
Mosaic crystals of vanadyl pyrophosphate obtained by oriented nucleation and growth
J. Solid State Chem.
(1998) - et al.
Physicochemical study of structural disorder in vanadyl pyrophosphate
J. Catal.
(1993) Nature of the active and selective sites in vanadyl pyrophosphate, catalyst of oxidation of n-butane, butene and pentane to maleic anhydride
Catal. Today
(1993)- et al.
Selective oxidation of n-butane in the presence of vanadyl pyrophosphates synthesized by intercalation–exfoliation–reduction of layered VOPO4·2H2O in 2-butanol
J. Catal.
(2004)
The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane
Catal. Today
On the topotactic dehydration of VOHPO4·0.5H2O into vanadyl pyrophosphate
J. Solid State Chem.
VPO catalysts for oxidation of butane to maleic anhydride: influence of (VO)2H4P2O9 precursor morphology on catalytic properties
Appl. Catal.
Systematic control of crystal morphology during preparation of selective vanadyl pyrophosphate
Solvothermal synthesis of vanadium phosphate catalysts for n-butane oxidation
Chem. Eng. J.
Influence of alcohol solvents on characters of VOHPO4·0.5H2O prepared from V4O9 and ortho-H3PO4
Appl. Catal. A Gen.
Preparation of catalyst precursors for selective oxidation of n-butane by exfoliation–reduction of VOPO4·2H2O in primary alcohol
Catal. Today
New trends in V-P–O solids
Curr. Opin. Solid State Mater. Sci.
Chemistry of vanadium-phosphorus oxide catalyst preparation
Appl. Catal. A Gen.
Nature and mechanism of formation of vanadyl pyrophosphate: active phase in n-butane selective oxidation
J. Catal.
Solvothermal synthesis of vanadium phosphates in the form of xerogels, aerogels and mesostructures
Mater. Res. Bull.
Flame generation of two new precursors of vanadyl pyrophosphate
J. Solid State Chem.
Evolution of a VPO catalyst in n-butane oxidation reaction during the activation time
J. Catal.
Transient reactivity of vanadyl pyrophosphate, the catalyst for n-butane oxidation to maleic anhydride, in response to in-situ treatments
Appl. Catal. A Gen.
A comparison of the reactivity of “Nonequilibrated” and “Equilibrated” V-P–O catalysts: structural evolution, surface characterization, and reactivity in the selective oxidation of n-butane and n-pentane
J. Catal.
Activity of surface species of (VO)2P2O7 in the oxidation of n-butane
Appl. Surf. Sci.
A study by in situ laser Raman spectroscopy of VPO catalysts for n-butane oxidation to maleic anhydride I. Preparation and characterization of pure reference phases
J. Catal.
Oxidation of butane and butene on the (100) face of (VO)2P2O7: a dynamic view in terms of the crystallochemical model of active sites
J. Catal.
XPS investigations of VPO catalysts under reaction conditions
Surf. Sci.
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