Oxidative dehydrogenation of 1-butene over vanadium modified bismuth molybdate catalyst: an insight into mechanism

Abstract

BiMoV_x catalysts (x = 0–0.3) were prepared by co-precipitation method and investigated in the oxidative dehydrogenation (ODH) of 1-butene to 1,3-butadiene. The results show that the vanadium content has significant effects on the catalyst performance and BiMoV_0.15 exhibits the superior activity. X-ray photoelectron spectroscopy (XPS) and temperature programmed re-oxidation (TPRO) indicate that the oxygen mobility of catalysts is one of crucial factors determining catalytic performance in ODH reaction. The mechanism of oxygen migration in ODH reaction is discussed and proposed.

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Introduction

Recently, the oxidative dehydrogenation (ODH) of light paraffin has attracted much attention due to the growing demand for light olefins such as 1,3-butadiene (BD). It is an important raw material for manufacturing a large number of chemical products.^1,2 Several metal oxides have been investigated in ODH of n-butene to BD, including vanadium-containing catalyst,³ ferrite-type catalyst,⁴ manganese oxide molecular sieve,⁵ Cu–Mo catalyst⁶ and bismuth molybdate.^7,8 Among these catalysts, bismuth molybdate exhibited the promising catalytic performance.^7–11 The present investigation reveals that ODH of n-butene to BD over bismuth molybdate follows the Mars-van Krevelen mechanism.^12–15 That is lattice oxygen in the catalyst reacts with n-butene and produces water and BD firstly. And then molecular oxygen in the gas phase makes up oxygen vacancy in the catalyst.^16–18 This implies that oxygen mobility is one of the vital factors determining the catalytic performance of bismuth molybdate in ODH of n-butene.

As a result, much effort has been taken to enhance the oxygen mobility of bismuth molybdate by adding some promoters, which can be categorized into two groups. One is to substitute Bi in the catalysts with other metal atoms partially. The other one is to replace Mo atom in bismuth molybdates. Currently, the metal for Bi atom in bismuth molybdate includes Co, Fe and Ni, and etc.^10,14,19,20 While P was employed for the substitution of Mo atom.^10,21 Based on these studies, it is found that the enhancement of oxygen mobility is due to the formation of some new crystal phases between the promoter and Bi or Mo. The intrinsic mechanism is that lattice oxygen in the formed phases is easy to react with n-butene to produce oxygen vacancy. The oxygen mobility is strongly related to these vacancies.

In this regard, we have prepared La-modified bismuth molybdate and investigated its performance in ODH of n-butene previously.²² Our prepared catalysts showed good catalytic activity in the reaction due to the substitution of Bi with La. And La is well known for its capability to store and release oxygen and low-temperature reducibility.²³ Although extensive researches have been conducted aiming at the improvement of oxygen mobility of bismuth molybdate by the addition of the third metal.^8,14,19–22 But some important issues are still not clear. For example, the bulk structure modification of catalyst related to the promoter addition will influence the formation of oxygen vacancy inevitably. Some oxygen migration during ODH reaction will happen in the bulk due to the different bonding energy between metal and oxygen. Further insight into the mechanism of oxygen mobility can provide useful information for screening more effective promoter, which is still an interesting research topic in ODH reaction.

In this work, we employed vanadium as promoter to replace the Mo in bismuth molybdate and a series of BiMoV_x with different vanadium contents (x = 0–0.3) were prepared by co-precipitation method. Vanadium is the metal with mixed-valence state which shows good activity in some dehydrogenation reaction.^21,24–26 The prepared catalysts were investigated in ODH of 1-butene to BD. BiMoV_x catalysts were characterized by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed re-oxidation (TPRO). The relationship between the catalytic activity and vanadium content in the catalysts was discussed, the oxygen mobility as well. The route of oxygen migration in the catalysts for ODH reaction was proposed.

Experimental

Preparation of BiMoV_x catalysts

BiMoV_x catalysts (x = 0–0.3) were prepared by conventional co-precipitation method. The desired amount of ammonium meta-vanadate solution (NH₄VO₃) was dissolved in deionized water at 70 °C and added dropwise into ammonium molybdate solution ((NH₄)₆Mo₇O₂₄·4H₂O) under vigorously stirring. Afterwards, bismuth nitrate solution (Bi(NO₃)₃·5H₂O) acidified with 10% nitric acid was added dropwise into the above-mentioned mixture. The mixture was adjusted to pH 5 by aqueous 27 wt% NH₄OH and aged at room temperature for 4 h and then stirred to viscous solid at the water bath to remove the excess water. The resulting viscous solid was dried at 80 °C overnight and calcined at 550 °C for 2 h in an air atmosphere. The catalysts are designated as BiMoV_x catalysts (x = 0–0.3), where x is the molar ratio of vanadium in the catalyst.

Catalyst characterization

X-ray powder diffraction (XRD) patterns of BiMoV_x were recorded on XRD 6000 equipped with Cu Kα radiation at 40 kV and 30 mA. Data were collected in the 2θ range of 10–50° with a scan rate of 1° min⁻¹. Elemental composition of the prepared catalysts was determined by ICP-AES (Thermo iCAP6300) analyses. The BET surface area and pore volume of the catalysts were measured using N₂ as a sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP2020). The relationship between oxygen mobility and vanadium content was also investigated through the temperature-programmed re-oxidation (TPRO) on fixed-bed apparatus attached to a Hiden QIC-20 quadruple mass spectrometer. For the temperature programmed reduction by 1-butene, 0.2 g catalyst was pretreated at 440 °C for 1 h in flowing 1-butene (20 cm³ min⁻¹), held for 2 h at that temperature and then cooled to room temperature in flowing He (30 cm³ min⁻¹). Subsequent temperature programmed oxidation was performed in flowing 2% O₂/He (30 cm³ min⁻¹) from room temperature to 600 °C at 10 °C min⁻¹. X-ray photoelectron spectroscopy (XPS) spectra for BiMoV_x catalysts were conducted by using an Escalab 250Xi XPS system. An Mg Kα anode was used as the X-ray source. The binding energy of the electron, correcting charge effects, was calibrated with respect to the signal of the adventitious carbon (binding energy: 284.6 eV). Spectrum deconvolution was carried out using the spectra of X-ray satellite peak subtraction and background correction.

Catalytic measurement

Catalytic tests were performed in a tubular fixed-bed reactor (i.d. = 8 mm) with 1.5 g catalyst under atmospheric pressure. The prepared catalysts were pressed into 40–60 mesh for testing. Prior to the reaction, the catalyst was routinely activated with flowing air stream (50 cm³ min⁻¹) at 440 °C for 2 h. The water feed was continuously vaporized by passing through a pre-heating zone at 120 °C and fed into the reactor together with 1-butene and air. The feed composition was fixed at a ratio of 1-butene: oxygen: steam = 1 : 0.8 : 10 with total gas flow rate of 112.5 cm³ min⁻¹. The reactants and products were analyzed by on-line gas chromatography with two columns. Hydrocarbons were analyzed using a KB-Al₂O₃/Na₂SO₄ column with a FID detector, and CO and CO₂ by a TDX-01 column with a TCD detector.

Results and discussion

XRD patterns of BiMoV_x catalysts with different vanadium content are shown in Fig. 1. Three solid phases of α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, and γ-Bi₂MoO₆, coexisting with a minor amount of the BiVO₄ phase, can be detected in the prepared samples. Trifiro et al. demonstrate that Mo=O is related with the selectivity activity center of catalyst while Mo–O–Mo acts as total oxidation center in bismuth molybdate oxides.^27–29 In BiMoV_x catalysts, the structure and circumstance of Mo=O are modified by the incorporation of the component vanadium. V is in the tetrahedral VO₄ structure.^28–32 This probably implies that Mo in the lattice position is replaced partially by V in BiMoV_x oxide catalysts.

Fig. 1 XRD patterns of BiMoV_x catalysts with different V content; (a) x in BiMoV_x catalysts = 0, (b) 0.1, (c) 0.15, (d) 0.2, and (e) 0.3.

BET surface area, total pore volume and elemental composition of the BiMoV_x oxide catalysts are shown in Table 1. The surface areas of the catalysts are found to be very low (1.1–2.3 m² g⁻¹), as reported in the previous works.²¹ And there is no consistent trend with a variation of vanadium content. Elemental analysis by ICP-AES indicates that the actual content of each metal are in good agreement with that of the expected one experimentally. This result also confirms the successful preparation of BiMoV_x oxide in this work.

Table 1 Structural property of BiMoV_x catalysts

Catalyst	Atomic ratio^a			SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Oxygen consumption of first peak in TPRO (μmol g^−1)	Total oxygen consumption in TPRO (μmol g^−1)
	Bi	Mo	V
a Determined by ICP-AES.
BiMo	1	0.98	0	1.2	0.011	15.4	40.6
BiMoV0.1	1	0.97	0.09	1.4	0.014	13.2	44.8
BiMoV0.15	1	0.96	0.14	2.3	0.018	11.7	51.2
BiMoV0.2	1	0.99	0.21	1.5	0.019	12.5	46.5
BiMoV0.3	1	1.00	0.28	1.1	0.007	8.5	43.2

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The prepared catalysts were investigated in ODH of 1-butene at 440 °C for 2 h. Three types of compounds are detected including dehydrogenation products (BD), isomerization products (trans and cis-2-butene) and combustion products (CO and CO₂). The cracking products (CH₄, C₂H₄, C₂H₆, C₃H₆ and C₃H₈) are almost negligible. BD and CO₂ are dominating products. Fig. 2 plots the tendency of BiMoV_x oxides activity vs. vanadium content. One can see that the selectivity for CO₂ is increased with the increase of the vanadium content. However, it is noticeable that conversion of 1-butene and yield for BD show volcano curves with respect to vanadium content. BiMoV_0.15 shows the optimal catalytic performance in this work.

Fig. 2 Catalytic performance of BiMoV_x catalysts as a function of V content in ODH of 1-butene.

It has been reported that the oxygen mobility of bismuth molybdenum catalyst is one of crucial factors to catalytic performance in ODH reaction. In order to clarify the relationship between the catalytic performance and the oxygen mobility of BiMoV_x, XPS and TPRO analysis are conducted. XPS analyses are carried out to determine oxygen mobility of the catalyst by measuring the binding energies of O 1s of BiMoV_x catalyst. Fig. 3A depicts deconvolution of the O 1s spectrum of BiMoV_0.15 catalyst. Deconvolution of O 1s spectrum reveals that there are three types of surface oxygen species in the BiMoV_0.15 catalyst (denoted as type-I, -II, and -III in the order of increasing binding energy). As literature reported,^9–13,33 type-I oxygen with the lowest binding energy (530.3 eV) is attributed to be the oxygen species strongly bonded to the metal component in the catalyst, type-III oxygen with the highest binding energy (532.5–533.1 eV) corresponds to the oxygen species weakly bonded on the catalyst surface. type-II oxygen reflects the oxygen mobility of the multicomponent bismuth molybdate catalyst.³³ Table 2 summarizes the binding energy of deconvolution of O 1s spectrum of BiMoV_x catalysts. It is generally believed that only the binding energy of type-II oxygen is used as an index for the oxygen mobility of the multicomponent bismuth molybdates catalyst.³³ The higher binding energy of type-II oxygen is, the higher the oxygen mobility is.

Fig. 3 (A) Deconvolution of the O 1s spectrum of BiMoV_0.15 oxide catalysts and (B) TPRO profiles of partially reduced BiMoV_x catalysts.

Table 2 Binding energies of O 1s of BiMoV_x oxide catalysts

Catalyst	Binding energy (eV) of O 1s
	Type-I	Type-II	Type-III
BiMo	530.3	531.1	533.1
BiMoV0.1	530.2	531.6	532.8
BiMoV0.15	530.3	531.8	532.5
BiMoV0.2	530.3	531.7	533.1
BiMoV0.3	530.3	531.4	532.6

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TPRO analysis is also conducted to estimate oxygen mobility of the catalyst by testing the peak temperature in TPRO. Prior to TPRO analysis, catalysts were pretreated with flowing 1-butene at 440 °C for 3 h. Fig. 3B shows the TPRO profiles of BiMoV_x catalysts. The profiles for each catalyst consist of two oxidation peaks. It is widely believed that the peak temperature in TPRO indicates the capacity of oxygen mobility on bismuth molybdenum oxides. The lower the temperature is, the easier the oxygen mobility is. The total oxygen consumption in TPRO is related with catalytic performance and reflects the capability for oxygen make-up. The total oxygen quantity can be calculated by TPRO total peak area, as displayed in Table 1. The more the total oxygen consumption is, the easier the oxygen mobility is. BiMoV_0.15 with the total oxygen quantity of 51.2 μmol g⁻¹ exhibits optimal catalytic activity. These results are well consistent with the trend of binding energy of type-II oxygen in Table 2.

Fig. 4 demonstrates a comprehensive correlation between the yield for BD and the oxygen mobility of BiMoV_x catalysts. It is evident that the yield increases linearly with the increase of oxygen mobility. This result implies that the catalytic activity could be improved with increasing oxygen mobility of the catalyst. Therefore, the catalysts of BiMoV_0.15 displays the superior activity as result of its higher binding energy of type-II oxygen and lower TPRO temperature compared to the others prepared catalysts.^17–19,33

Fig. 4 A comprehensive correlation between catalytic performance and oxygen mobility of BiMoV_x in the ODH of 1-butene to 1,3-butadiene: x in BiMoV_x oxide catalysts = 0, 0.1, 0.15, 0.2, and 0.3.

To assign the oxygen consumption in TPRO profiles, the effluent gas during testing BiMoV_0.15 is analyzed by a Hiden QIC-20 quadruple mass spectrometer. The gas composition is mainly composed of O₂ and CO₂, as displayed in Fig. 5. In the low temperature at ca. 180.7 °C, there is no CO₂ formed indicating the composition of oxygen may be related to the oxidation of metal species partially reduced by n-butene. The second small peak is the combustion of coke on the catalyst surface, as some literatures reported.^19,21,33 And the O₂ consumption at the high temperature at ca. 410.7 °C may be correlated with the oxidation of metal species and the combustion of coke on the catalyst surface.

Fig. 5 MS signals of TPRO of BiMoV_0.15 oxide catalyst; m/z = 32 (˙O₂), 44(˙CO₂) were recorded during TPRO analysis.

From above analysis, the vanadium addition can improve ODH performance of bismuth molybdate significantly due to the enhancement of oxygen mobility. The assignment of oxygen consumption in TPRO tests indicate that the evolution of oxygen vacancy during the reaction, which may result from the oxygen migration. To elucidate the intrinsic mechanism, three typical samples, described in Table 3, at different stage of TPRO test were collected and characterized by XPS. As shown in Fig. 6, the shape of the Mo3d_5/2 spectrum remains unchanged from BiMoV_0.15-440 to BiMoV_0.15-200, however, the shape of the Mo3d_5/2 spectrum make a distinct change from BiMoV_0.15-200 to BiMoV_0.15-600. Deconvolution of the Mo3d spectrum in Fig. 6a produce three kinds of core spectra of Mo3d_5/2 = 232.5, 231.3, and 229.4 eV. The first core spectrum of 232.5 eV is related to Mo⁶⁺.^34–36 The second one is ascribed to Mo⁵⁺. The last one is accord with Mo⁴⁺ reported by Cimino and Angelis.³⁷ Grzybowska et al. have also reported the appearance of Mo⁴⁺ on bismuth molybdates treated in 470 °C at reducing atmosphere.³⁸ Nevertheless, deconvolution of the Mo3d spectrum in Fig. 6c produce only core spectra of Mo3d_5/2 = 232.5 eV, this means the catalysts is merely composed of Mo⁶⁺. Therefore, it can be inferred that the high temperature peak one in the range of 400–500 °C is ascribed to the oxidation process of molybdenum.^19,39,40 Fig. 7 shows V2p spectra obtained in different samples. The shape of the V2p_3/2 spectrum remains unchanged from BiMoV_0.15-440 to BiMoV_0.15-200, two kinds of V2p_3/2 can be observed, the first spectrum of 516.9 eV is related to V⁵⁺. The second spectrum of 515.9 eV corresponds to V⁴⁺.The similar phenomenon is reported in literature.^41,42 It is evident that the second spectrum of 515.9 eV disappears in BiMoV_0.15-600, it can be deduced that the valence state of V⁴⁺ is oxidized to V⁵⁺.Therefore, the high temperature peak may be attributed to the oxidation process of vanadium. Therefore, the oxygen consumption at high temperature peak in TPRO may be correlated with the oxidation process of both molybdenum and vanadium.

Table 3 Three typical samples used for XPS characterization

Sample	Description
BiMoV0.15-440	BiMoV0.15 pretreated by 1-butene at 440 °C for 2 h
BiMoV0.15-200	BiMoV0.15 oxidized in TPRO when the temperature reaches to 200 °C
BiMoV0.15-600	BiMoV0.15 oxidized in TPRO when the temperature reaches to 600 °C

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Fig. 6 Mo 3d signals of different sample (a) BiMoV_0.15-440; (b) BiMoV_0.15-200; (c) BiMoV_0.15-600.

Fig. 7 V 2p signals of different sample (a) BiMoV_0.15-440; (b) BiMoV_0.15-200; (c) BiMoV_0.15-600.

XRD characterization was used to analyze the bulk structure changes during TPRO test, as shown in Fig. 8. One can see that the patterns of the BiMoV_0.15 oxide catalysis change significantly from BiMoV_0.15-440 to BiMoV_0.15-200. For BiMoV_0.15-440, there is a clear diffraction peak of metal Bi, the characteristic diffraction angle of Bi is 27.2°, 37.9°, 39.6°. However, for BiMoV_0.15-200, no diffraction peaks of Bi are observed. These results suggest that the lattice oxygen bonded to Bi reacts with 1-butene during the pretreatment totally. Grzybowska et al.³⁸ have also reported that metallic bismuth or Bi⁰ is formed along with the reduced molybdenum cations in vigorous or excessive reduction. Thus, it is reasonable to deduce that the low temperature peak of TPRO profiles in the range of 150–200 °C ascribe to the oxidation of bismuth species. But for Mo and V species, no characteristic peaks can be observed. The reason may be due to the low diffraction of their oxides.

Fig. 8 XRD patterns of different sample (a) BiMoV_0.15-440; (b) BiMoV_0.15-200.

Based on our experimental results and references reported,^9–14,39,40 we can deduce the metal valence changes during the reaction, as Table 4 summarized. For fresh BiMoV_0.15, four solid phases including α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, γ-Bi₂MoO₆, and BiVO₄ can be found (Fig. 1), which indicates the valence of Bi, Mo, and V is +3, +6, and +5, respectively. After reduced by 1-butene, the significant changes occur. Bi⁺³ turns into metallic Bi because the oxygen coordinated is captured by 1-butene completely (Fig. 8). But in the case of Mo and V, the valence of former one changes to +5, +4 and the latter to +4 (Fig. 6 and 7). These suggest that not all of oxygen around Mo and V participate in the reaction. The energy of the metal component bonded to the oxygen is responsible for these differences, which will lead to the oxygen migration during the ODH reaction.

Table 4 Metal valence changes on BiMoV_0.15 during reaction

Sample	Bi	Mo	V
BiMoV0.15	+3	+6	+5
BiMoV0.15-440	0	+5, +4	+5, +4
BiMoV0.15-200	+3	+5, +4	+5, +4
BiMoV0.15-600	+3	+6	+5

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It is well known that the oxidative dehydrogenation of 1-butene over bismuth molybdate catalyst follows the Mars-van Krevelen mechanism.^19–22 According to this mechanism, oxygen in the catalyst directly reacts with 1-butene, and in turn, oxygen in the gas phase makes up oxygen vacancy in the catalyst. The transformation of oxygen between the gas phase and catalyst surface will lead to the valence changes of metal surrounding oxygen inevitably. The changes of metal valence on the catalysts during the different stage of reaction can give us an indirect understanding of oxygen migration in the reaction, as Fig. 9 proposed. 1-Butene reacts with surface oxygen atom bonding to metal on the catalysts firstly. But the oxygen around Bi may be captured preferentially due to the low energy between them. With the reaction taking place, oxygen vacancy is formed. Then there is the oxygen migration between Bi and Mo, V for the difference from the bonding energy. But if 1-butene is only react with oxygen atom in the surface and near-surface regions of catalysts, the peak of consumption of oxygen in TPRO will be not so obvious. Therefore, there must be the occurrence of oxygen migration in the catalysts. It can be evolved that the bulk lattice oxygen transfers into the surface to participate in ODH reaction following the direction of arrows. The Bi metal species in the bulk regions is also reduced. There are many oxygen vacancy formed in the catalysts with the oxidative dehydrogenation proceed. Afterward, oxygen vacancy in the surface and near-surface of catalysts will be filled by gas oxygen. In view of the difference of peak temperature in TPRO, the Bi metal species in the surface and near-surface of catalysts is oxidized firstly. As the reaction continues, the oxygen species around Bi enrich. Then the Mo and V metal species in the bulk regions is also oxidized. Consequently, the whole process is circulated repeatedly of ODH process and oxygen migration process. Our results demonstrate the introduction of V in BiMo catalysts strengthen the oxygen migration process and oxygen mobility subsequently.

Fig. 9 Reaction mechanism and oxygen migration process of this reaction.

Conclusions

In this work, BiMoV_x with different vanadium content have been tested in ODH of 1-butene to BD. Conversion of 1-butene and yield for BD over BiMoV_x catalysts show volcano-shaped curves with respect to vanadium content. The characterization results reveal that the catalytic performance could be enhanced by increasing oxygen mobility of the catalyst. This factor leads the better activity of BiMoV_0.15 than that of the others. The reason of temperature peak in TPRO profile is investigated, and the first peak of TPRO profiles could be assigned to the oxidation of bismuth species and the second peak is attributed to the oxidation process of molybdenum and vanadium. The oxygen migration between Bi and Mo, V take place during the reaction due to the difference from the bonding energy between metal and oxygen, the migration from bulk to surface as well. This work demonstrates that the introduction of element with high oxygen mobility can improve the performance of catalyst for ODH reaction effectively. It provides useful information for future catalysts designs. Moreover, BiMoV_x catalyst is expected to be applied in oxidative dehydrogenation of other hydrocarbons.

Acknowledgements

Financial supports from Zhejiang Provincial Natural Science Foundation (LZ13B060004) and the Program for Zhejiang Leading Team of S&T Innovation (2013TD07) are gratefully appreciated.

Notes and references

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