Synergy Effects of the Mixture of Bismuth Molybdate Catalysts with

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Synergy Effects of the Mixture of Bismuth Molybdate Catalysts with SnO2/ZrO2/MgO in Selective Propene Oxidation and the Connection between Conductivity and Catalytic Activity Minh Thang Le,*,† Van Hung Do,† Duc Duc Truong,† Els Bruneel,‡ Isabel Van Driessche,‡ Anders Riisager,§ Rasmus Fehrmann,§ and Quang Thang Trinh∥ †

School of Chemical Engineering, Hanoi University of Science and Technology, No. 1 Dai Co Viet Road, Hanoi, Vietnam Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium § Department of Chemistry, Technical University of Denmark, Kemitorvet, Building 207, 2800 Kongens Lyngby, Denmark ∥ School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459 Singapore ‡

S Supporting Information *

ABSTRACT: Bismuth molybdate catalysts have been used for partial oxidation and ammoxidation of light hydrocarbons since the 1950s. In particular, there is the synergy effect (the enhancement of the catalytic activity in the catalysts mixed from different components) in different phases of bismuth molybdate catalysts which has been observed and studied since the 1980s; however, despite it being interpreted differently by different research groups, there is still no decisive conclusion on the origin of the synergy effect that has been obtained. The starting idea of this work is to find an answer for the question: does the electrical conductivity influence the catalytic activity (which has been previously proposed by some authors). In this work, highly conductive materials (SnO2, ZrO2) and nonconductive materials (MgO) are added to beta bismuth molybdates (β-Bi2Mo2O9) using mechanical mixing, impregnation, and sol−gel methods. The mixtures were characterized by XRD, BET, XPS, and EDX techniques to determine the phase composition and surface properties. The conductivities of these samples were recorded at the catalytic reaction temperature (300−450 °C). Comparison of the catalytic activities of these mixtures showed that the addition of 10% mol SnO2 to beta bismuth molybdate resulted in the highest activity while the addition of nonconductive MgO could not increase the catalytic activity. This shows that there may be a connection between conductivity and catalytic activity in the mixtures of bismuth molybdate catalysts and other metal oxides. of Mo at the surface of the alpha phase,21 or the spillover of oxygen.23,24 However, no decisive conclusion on the origin of the synergy effect has been obtained. Actually, adding some additives to bismuth molybdate catalysts could result in many unprecedented performance enhancements in the activity and selectivity by modifying the electronic properties of active sites, generating the “remote control effect” to facilitate the redox Mars and van Krevelen mechanism, or could even lead to

1. INTRODUCTION Bismuth molybdates are popular catalysts for partial oxidation and ammoxidation of light hydrocarbons such as propylene, butylene, dimethylethylene, etc. They have been the subject of many studies since the 1960s.1−8 Recent publications show that ongoing research has resulted in the discovery of new properties and applications of these mixed oxides.9−17 The synergy effect (the enhancement of the catalytic activity in the catalysts mixed from different components) in bismuth molybdate based catalysts has been observed and studied since the 1980s.18−23 This synergy effect was interpreted differently by different research groups, such as the elimination of excess Bi at the surface of the gamma phase, the elimination © XXXX American Chemical Society

Received: January 3, 2016 Revised: April 7, 2016 Accepted: April 12, 2016

A

DOI: 10.1021/acs.iecr.6b00019 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research reconstruction of the surface structure during the reaction.25−27 The discussion about the catalytic activities of bismuth molybdate based catalysts seemed to cease in the early 2000s. Since the synergy effects of different compositions of gamma bismuth molybdate (γ-Bi2MoO6) in the mixture with beta bismuth molybdate (β-Bi2Mo2O9) were reported by Vieira Soares et al.22 and followed by our group,28,29 the discussions on the synergy effect in bismuth molybdate based catalysts were continued with more distinct interpretations.30 It is now generally agreed that the synergy effect occurs in mixtures containing the gamma phase of bismuth molybdate, which is known as the phase with more mobile oxygen atoms, in combination with the beta or alpha phasethe phases which provide more absorption sites for hydrocarbons. The mechanism of propene on metal oxides catalyst was proposed to follow the Mars and van Krevelen’s mechanism.17,31,32 Detailed theoretical studies showed that the exposure surface lattice oxygen atoms acted as active sites for the hydrogen abstraction33−37 and were incorporated into the product in the later oxidation step, generating surface oxygen vacancy.34−36 An isotopic labeling study using deuterium7,38 and 13carbon8 reported that the rate-determining step was the initial abstraction of hydrogen facilitated by lattice oxygen generating allyl radical. That allyl intermediate then coupled with the lattice oxygen to form the product acrolein, since it was formed even without the presence of oxygen gas atmosphere,39 and 18O labeling studies also confirmed that oxygen in the product acrolein indeed came from the bulk oxygen of the catalyst.38,40−42 Particularly, Getsoian et al.35 revealed that the bismuth perturbed molybdenyl MoO oxygen was indeed the active site for the initial, ratedetermining hydrogen abstraction step and that the second molybdenyl oxygen later combined with the generated allyl to form an allyl alkoxy intermediate following by the second hydrogen abstraction from this intermediate to produce acrolein. Bader charge analysis33,35,43,44 also showed that only molybdenum Mo6+ centers were reduced during the reaction, while bismuth centers provided an indirect role in changing the electronic property of the active site. Therefore, the oxidation mechanism of propene involves the electron transfer to facilitate oxidation state changes during the reaction (provided by the valence band of the oxides), and lattice oxygen anions diffusion to facilitate the reoxidation of the cation and lattice oxygen incorporation to the reactant molecule.32,45−47 However, both the electron transfer and oxygen mobility could be reflected by using the electrical conductivity measurement.48−50 In our previous work,29 we have shown that gamma bismuth molybdate possesses a higher conductivity than the beta and alpha phases; therefore, its ability to transport lattice oxygen from the bulk to the surface to reconstitute the reduced active sites for hydrocarbon absorption is higher. With a content increase of the gamma phase in the mixture with the beta phase, the conductivity of the mixture increased accordingly. The pure gamma phase sample had a higher conductivity compared to all its mixtures with the beta phase. This is an indication that the high conductivity of gamma bismuth molybdate is the reason for the synergy effect in its mixtures and agrees with the proposal in the earlier study.51 The influence of the conductivity on the activity of the catalysts was also widely announced for other oxidation processes, e.g. the oxidative dehydrogenation of ethane on Nb-doped NiO catalysts,32 the oxidative coupling of methane over mixed metal oxides,52,53 and natural gas conversion.54 Recent studies also

reported linear correlations between the measured catalytic apparent activation energies for propene oxidation to acrolein and the band gap energies for several different families of multicomponent, mixed metal oxides.55,56 Moreover, that band gap property, which correlates to the conductivity of the materials,57 was also reported as an accurate descriptor for the energy to generate oxygen vacancy on metal oxides58,59 and is very relevant for the incorporation of lattice oxygen into the reactant molecule. Therefore, those pieces of evidence further support our assumption that the conductivity could be one of the factors that reflect the catalytic activity in the oxidation of propene to acrolein. However, in the case of bismuth molybdate based catalysts, the catalytic activity only increases in the mixtures of gamma and beta bismuth molybdates within a certain composition range of the gamma phase (about 30− 70%).28,29 If the sample contains more than 70% gamma bismuth molybdate, the catalytic activity of the mixture decreases, since gamma bismuth molybdate possesses much lower catalytic activity than that of the beta phase. If the sample contains less than 30% gamma bismuth molybdate, there is also no increase of activity, since there is no increase in conductivity. It has been shown that the number of active sites to abstract the α-hydrogen, which are mainly provided by the beta phase, is more important factor.28,29,60 Thus, only a very small increase in conductivity of the mixed bismuth molybdate samples is enough to result in a remarkable increase in their catalytic activity. Therefore, it was concluded that an increase in conductivity of a sample can only increase the catalytic activity if the number of active sites to abstract the α-hydrogen maintains high. The above explanation lead us to suggest a new series of experiments in which the oxygen mobility in the catalyst mixture was increased by the addition of highly conductive materials such as SnO2 or ZrO2 instead of by the gamma phase into mixtures with beta bismuth molybdatethe phase with more active sites for propylene absorption. In order to explore if the increase in the conductivity is one of the reasons for the increase in catalytic activity (if any), a nonconductive material MgO was also added with beta bismuth molybdate. This paper, therefore, will focus on the catalytic and conductive properties of mixtures of SnO2, ZrO2 and MgO with beta bismuth molybdates. Different degrees of homogeneity, including mechanical mixing and chemical mixing via the sol−gel method or the impregnation method, will be studied to evaluate the correlation between the conductivity and the catalytic activity.

2. EXPERIMENTAL PROCEDURE Beta bismuth molybdate (β-Bi2Mo2O9) was prepared by the sol−gel method as described in our previous work.60 Mechanical mixtures of beta bismuth molybdates with SnO2 (Merk, p.a.) or ZrO2 (Merk, p.a.) were prepared by physical mixing and then grinding to the suitable particle size, which was repeated several times. Chemical mixtures containing Bi, Mo, and other components (Sn, Zr, or Mg) were also prepared by the sol−gel method mentioned above. The precursor solutions were aqueous solutions of 0.14 M (NH4)6Mo7O24·4H2O (Merk, p.a.), 0.67 M Bi(NO3)3·5H2O/HNO3 (Riedel-De Haen, p.a.), 0.1 M SnCl4·5H2O (Sigma-Aldrich, p.a.), 0.1 M ZrOCl2·8H2O (Sigma-Aldrich, p.a.), and 2 M Mg(NO3)2·6H2O (Sigma-Aldrich, p.a.). The as prepared solutions were mixed with suitable volumes, corresponding to the desired ratios of metal atoms in the mixtures. During this preparation, concentrated HNO3 was continuously added in order to B


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parameters of percent conversion were not used to ensure a precise comparison of catalytic activity per one mass unit or surface area unit of the catalyst. Analysis of the propylene, oxygen, and acrolein was performed using an on-line Trace GC Ultra-Thermo Electron Cooperation gas chromatograph with a column of 80/100 chromosorb and carbowax 20M. The products of the reactions on bismuth molybdate based catalyst are acrolein as the main product, while the byproducts involve acetaldehyde and COx. Kinetics study of propene oxidation on bismuth molybdate catalyst has reported that the rates of formation of the main product acrolein and byproducts CO and CO2 are all nearly first order, corresponding to propene, and therefore the selectivity toward acrolein, CO, and CO2 remains almost constant.15,56 Since the selectivity of acrolein (85−90%) was observed to be significantly higher than those of acetaldehyde (0−5%) and COx (5−10%),12,15,17,28,29,56 therefore, the reaction rates of other by products (acetaldehyde and COx) are not mentioned in detailed in this study. The catalytic activity, instead, is expressed by the number of moles of acrolein formed per gram of catalyst (rw, mol/g·s) or per m2 of surface area of the catalyst per second (rs, mol/m2·s) using the following formula:

prevent precipitation. Citric acid was then added as a solution of 10%w citric acid monohydrate (Merk, p.a.) so as to obtain a ratio of citric acid per bismuth ions of 2.5. The obtained solutions were gellified at 60−80 °C. The transparent yellow gels were then dried at 110 °C for 2 h, and the spongy solid precursors obtained were crushed. Powders obtained after the gelation were directly calcined in an air flow at 580 °C for 2 h with a heating rate of 10 °C/min. Mixtures of beta bismuth molybdate and tin oxide, or zirconia were also prepared by impregnation, in which the available tin oxide, or zirconia powders was added into a solution of beta bismuth molybdate precursor under vigorous stirring in order to obtain a wellmixed slurry. The gelation, drying, and calcination were performed afterward as described above. Obtained catalysts were characterized using XRD diffraction on a D8-ADVANCE-BRUKER diffractometer using Cu Kα radiation over a 2θ range between 10° and 60°. The composition of the beta and gamma bismuth molybdate was determined from corresponding XRD data as A% =

IA × 100 ∑ (IA + IB + ...)

(1)

where A, B represent a given phase (beta or gamma) and I represents the intensity of the strongest XRD reflection. The strongest XRD reflection of the gamma phase is at 2θ = 28.08°, and that of the beta phase is at 2θ = 27.72°. The morphologies of the bismuth molybdates were examined using a JEOL LSM 6360 LV scanning electron microscope. Specific surface area of powders was measured by the BET method using nitrogen gas with a Micromeritics Gemini device. The conductivity was measured as described previously.29,61,62 In brief, tablets of synthesized materials 0.3 cm in diameter and about 5 mm thick were prepared by a hydraulic press. Compact samples prepared by the sol−gel method could only be obtained when pressed at 50 kN/cm2 and calcined for 30 h, as was shown in SEM images of the samples pressed at different pressures and calcined at different times (Supporting Information, Figure S1). Contact electrodes on both surfaces of the tablets were made by painting with gold paste.29 The conductivity measurements were performed by an impedance technique in air atmosphere. A HP4192A impedance analyzer was used. The impedance measurements were made in the frequency range 5 Hz−13 MHz and in a temperature range from room temperature to 460 °C. The obtained data were analyzed by a Zview simulation program in order to calculate the resistance R, and the conductivity σ was calculated from the resistance R using the following equation: σ = d/SR

rw(C3H4O) =

YC3H4OoutFtot m

where rw(C3H4O) is the rate of acrolein formation per weight of the catalyst per second (mol/g·s), YC3H4Oout is molar fraction of acrolein in the flow coming out of the reactor, Ftot is the total molar flow rate (mol/s), and m is the weight of the catalyst (g). rw(C3H4O) is therefore proportional to the conversion of propylene and the selectivity to acrolein. As was shown in previous studies, the usage of reaction rate is a very convenient choice in evaluating the catalytic activity.15,28,29,32,56

3. RESULTS AND DISCUSSION 3.1. Properties of tin oxide−bismuth molybdate mixtures. Catalytic properties of different mechanical mixtures of tin oxide with beta bismuth molybdate with different mole percentages of SnO2 are shown as the reaction rate for acrolein formation in Figure 1. It is clear that pure beta bismuth molybdate has good ability to form acrolein while pure tin oxide has almost no activity to form acrolein. There is a significant increase in the reaction rate for acrolein formation (both based on catalyst mass and surface area) of the mechanical mixed sample with 10 mol % of SnO2. The other mixtures of tin oxide and beta bismuth molybdate do not have much higher reaction rates for acrolein formation (based on catalyst mass) than that of pure beta bismuth molybdate (Figure 1a). These mixtures even have a lower reaction rate for acrolein formation than that of pure beta bismuth molybdate when the catalyst surface area was taken into account, as presented in Figure 1b, since pure SnO2 has much higher surface area (9.4 m2/g) than pure beta bismuth molybdate (1.3 m2/g). The conversion of propene and the selectivity of the main products for pure oxides, beta bismuth molybdate, and the highest catalytic activity mixtures of beta bismuth molybdate with each oxide are presented in Table 1. It is important to demonstrate that although pure SnO2 has almost no selectivity toward acrolein, it possesses very high conversion of propene (the conversion of propene is even higher than that of beta bismuth molybdate) (Table 1). However, instead of producing acrolein, propene is converted into undesired

(2)

where σ is conductivity, d is the thickness of the tablets, S is the area of the contact electrode, and R is the resistance. Catalytic activities were measured in a conventional fixed-bed quartz tube reactor with an internal diameter of 0.4 cm. 0.2 g of catalyst (particle sizes are within the range 200−400 μm) was used with a total gas flow rate of 0.04 mmol/s at a pressure of 1 atm. The influence of mass transfer was ignored in this study and will be the subject of future study, either by using smaller particle size or a larger diameter reactor. The volume composition of the gas flow was C3H6/O2/N2 = 2.5/2.5/95 (%), and the reaction temperatures were maintained at 375, 400, and 425 °C, respectively. The reactor was operated differentially by keeping the conversion generally low (below 10%);15,56 thus, the conventional catalytic performance C


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bismuth molybdate may therefore increase significantly the catalytic activity of the catalyst. Since the study of the catalytic activity of mechanical mixtures of beta bismuth molybdate and tin oxide showed that the sample with 10% mol of tin oxide exhibits the highest activity, it would be interesting to know if more homogeneous mixtures with the same composition prepared by a chemical method, such as the sol−gel method and impregnation, could exhibit an ever higher activity. XRD patterns of these samples show that the major phase present is beta bismuth molybdate (Supporting Information, Figure S2). The presence of the relatively small amounts of tin oxide is hardly detected. However, using the XPS tec hnique, the presence of Sn4+ was clearly proven (Figure 2). XPS analysis also showed that the

Figure 1. Reaction rates for acrolein (C3H4O) formation based on (a) catalyst mass (rw, mol/g·s) and (b) catalyst surface area (rs, mol/m2·s) of the different mechanical mixtures of tin oxide and beta bismuth molybdate at different temperatures (375−425 °C).

Figure 2. XPS spectra of mixtures of beta bismuth molybdate and tin oxide prepared by the sol−gel method.

oxidation states of bismuth (Bi3+) and molybdenum (Mo6+) were not altered with the addition of SnO2 or ZrO2. In the sample prepared by the sol−gel method, a larger amount of gamma bismuth molybdate (20%, Table 2) could also be

Table 1. Conversion of Propene (%) and Selectivity of Acrolein and COx (%) on 0.2 g of the Catalysts at 425 °C

Pure β-Bi2Mo2O9 Pure SnO2 Pure ZrO2 Pure MgO 10% mol SnO2 chemical mixed 10% mol ZrO2 chemical mixed 15% mol MgO chemical mixed

Conversion of Propene (%)

Acrolein Selectivity (%)

COx Selectivity (%)

4.20 6.30 1.47 0.31 6.24

81.90 0 78.33 0 92.00

9.15 28.45 11.14 32.17 7.00

4.63

90.59

8.21

4.45

80.13

9.83

Table 2. Specific Surface Area and Beta Bismuth Molybdate Content of Chemical Mixtures of Beta Bismuth Molybdate with Other Oxides (in mol %) Samples 10% mol SnO2 (chemical mixed) 10% mol SnO2 (impregnated) 10% mol ZrO2 (chemical mixed) 10% mol ZrO2 (impregnated) 15% mol MgO 25% mol MgO 30% mol MgO

products (C1, C2, CH3OH, C2H5OH, COx, ...) on SnO2. The good conversion of propene on pure SnO2 may be an indication of the good adsorption of propene, which will be subjected to future study. This will be discussed further when comparing with other oxides (ZrO2, MgO) and in explaining the synergy effect of SnO2 with beta bismuth molybdate later. If comparing the measured reaction rate for acrolein formation of the mixtures of tin oxide and beta bismuth molybdate and the calculated value based on their corresponding compositions (the straight dotted line connecting between the reaction rate of pure beta bismuth molybdate and pure tin oxide), it is clear that there is a synergy effect in the mixtures with a narrow range of percentage of tin oxide (% mol SnO2 around 10%), since the experimental reaction rate of these samples is higher than calculated. Adding only a suitable amount of tin oxide into

SBET (m2/g)

mol % beta bismuth molydate compared to gamma bismuth molybdate

1.3

80.0

1.6

100.0

1.6

85.0

1.5

78.0

1.5 1.4 1.2

56.0 28.4 9.8

detected. The catalytic activities of all mixtures with 10 mol % tin oxide are presented in Figure 3. These data clearly show that the sample prepared by the sol−gel method (specific surface area of 1.3 m2/g) exhibited the highest activity (chemical mixture), followed by the sample prepared by the impregnation method (specific surface area of 1.6 m2/g), and the mechanical mixture exhibited the lowest activity. However, the presence of 10 mol % SnO2 in those mixtures still makes those catalysts have much higher activity than the pure beta bismuth molybdate (Figure 3). The higher activity of the samples prepared by the sol−gel and impregnation methods than the D


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properties of the active sites55,66−68 and the conductivity of the mixtures.14,46,47,69,70 For the mixture prepared by the sol−gel method, since the homogeneity is much higher, and part of the SnO2 phase could be directly incorporated into the lattice of the bismuth molybdate phase, therefore the conductivity of the sample will be changed with stronger magnitude than the mechanical mixture and sample prepared by the impregnation method.14,29,62 The Arrhenius curves representing the correlation between the logarithm of the conductivities and the inverse temperatures are shown in Figure 4a. It could be seen that pure tin oxide exhibited the highest conductivity, even higher than that of pure Bi2MoO6the phase that is commonly known to exhibit the highest conductivity compared to other bismuth molybdate phases.61 The conductivity of beta bismuth molybdate Bi2Mo2O9one component in the mixture with tin oxideis much lower than that of pure tin oxide. However, a mixture of 10% mol tin oxide and beta bismuth molybdate exhibited comparable conductivity with that of pure Bi2MoO6 at low temperature but much higher at high temperature (425 °C). If comparing the conductivity of the 10% tin oxide and beta bismuth molybdate chemically mixed (prepared by the sol−gel method) and the calculated value based on their corresponding compositions, which represents the conductivity of the mechanical mixture (the straight dotted line in Figure 4b),66 it is clear that there is a synergy effect in this mixtures, aligned with the increase of the catalytic activity. With the temperature increases, the synergy effect is more significant, since the movement of ions and electrons in the oxide is faster at higher temperature.16,29,32 The conductivity increase in the mixture of tin oxide and beta bismuth molybdate might explain the higher activity compared to pure beta bismuth molybdate due to faster movement of lattice oxygen to compensate for the oxygen at the surface used for the selective oxidation, thus increasing the reaction rate. It is also interesting to explain why the catalytic enhancement occurs at only the narrow range around 10 mol % SnO2; we correlated the rate of acrolein formation with the corresponding conductivity for the mechanical mixture of SnO2 with β-Bi2Mo2O9 at 375 °C (the conductivities for the mixture of metal oxides are calculated based on their composition content, which is always the reasonable choice62,66,67,71) in Table 3. Since SnO2 has much higher conductivity than the active phase β-Bi2Mo2O9, adding SnO2 to β-Bi2Mo2O9 will increase the conductivity of the catalyst and facilitate the oxygen diffusion from the bulk to the

Figure 3. Reaction rate rs, mol/m2·s of acrolein formation for mixtures of 10 mol % SnO2 and β-Bi2Mo2O9 synthesized using different methods.

mechanical mixtures may be explained by the higher homogeneity of the samples, especially obtained for the sample prepared by the sol−gel method from a homogeneous solution of the starting components. In this case, the interaction of bismuth molybdate and tin oxide increases compared to the mechanical mixture. The results presented here in this study are also very consistent with the observation in the study of Zhang et al.,53 wherein the activity of the CaO−CeO2 mixture in the form of a solid solution was much higher than the activity of the mechanical mixture in the oxidative coupling of methane. The better homogeneity of the solid solution CaO−CeO2 and the higher mobility of lattice oxygen (represented via the oxygen ion conductivity) in that catalyst were claimed as the reasons that had increased the catalytic activity. The higher activity of the sample prepared by the sol−gel method than by the impregnation method also could be deduced from the homogeneity. The sol−gel method is widely reported to give higher homogeneity than the impregnation method, and resulted in the higher activity of the catalyst.63,64 Since the sample with 10 mol % tin oxide prepared by the sol−gel method exhibited the highest activity, the conductivity of the sample was measured at different temperatures and compared to the pure samples to see the relation between conductivity and catalytic activity (Figure 4). For the mechanical mixtures and sample prepared by the impregnation method, the spillover of oxygen from SnO2 toward bismuth molybdate24,65 and the interaction at the interfaces of mixed oxides were stated as the reasons to change the electronic

Figure 4. (a) Arrhenius conductivity plots, and (b) conductivities for different contents of SnO2 in the mixture of pure and mixed SnO2 with βBi2Mo2O9 samples in the temperature range of the catalytic reaction (375−425 °C). E


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Table 3. Calculated Conductivity Based on Composition and Corresponding Rate of Acrolein Formation Measured at 375 °C for a Mechanical Mixture of SnO2 with β-Bi2Mo2O9 at Different Compositions Content of SnO2 (% mol)

Calc conductivity of the mixture, σ × 107 (S/cm)

Meas reaction rates for acrolein formation based on catalyst surface area, rs × 109 (mol/m2·s)

0 5 10 14 18 46 100

0.75498 2.17612 3.59726 4.73418 5.87109 13.82949 29.17784

5.01729 5.13754 13.6199 5.68115 4.60617 1.71276 0

surface and enhance the activity. However, from the Table 1, we can see that although the conversion of reactant propene on SnO2 is also very good, the selectivity toward acrolein is very poor (almost zero selectivity). Therefore, the presence of too large amounts of SnO2 will not reduce the conversion of propene but significantly reduce the selectivity toward the desired product and resulted in the lower rate of acrolein formation accordingly. There is an optimum content of SnO2 to ensure obtaining the high selectivity of acrolein while the conductivity has reached the effective level. Introducing too small content of SnO2 might not be enough to enhance the conductivity of the mixture to an effective level, but adding too much SnO2 will reduce the selectivity toward the desired product acrolein. As we could see in Figure 1 and in Table 3, the amount of 10 mol % of SnO2 compromises between both of those requirements and resulted in a significant enhancement in the rate of acrolein formation. 3.2. Properties of zirconium oxide−bismuth molybdate mixtures. Another high conductive oxide which was chosen to be added to bismuth molybdate catalyst is zirconium oxide.66,72 The catalytic activities of the mechanical mixture of zirconium oxide (surface area of 7.8 m2/g) and beta bismuth molybdate (surface area of 1.3 m2/g) are shown in Figure 5. Different from pure SnO2, pure ZrO2 exhibited much lower conversion of propene but much higher acrolein selectivity (Table 1), which means that the ZrO2 ability to adsorb propene may be poor but the ZrO2 structure may be convenient for the formation of allyl intermediates to produce acrolein. Therefore, contrary to SnO2, adding ZrO2 might not influence the acrolein selectivity but will significantly reduce the propene conversion. The catalytic activities (reaction rate of acrolein formation based on catalyst surface area) of the mixtures are not higher than for pure beta bismuth molybdate, which means that there is no synergy effect. The conductivity of zirconium oxide is measured in the temperature range of the catalytic reaction and shown in Figure 4a, which exposes that the conductivity of zirconium oxide at low temperature, the temperature range of the catalytic reaction, is even lower than that of pure gamma bismuth molybdate and much lower than that of tin oxide, although zirconium oxide is well-known as a high conductive material. Therefore, the addition of zirconium oxide to beta bismuth molybdate catalysts could not improve catalytic activity. Besides the fact that the conductivity of the mechanical mixture with ZrO2 was not efficient enough, the oxygen spillover effect from ZrO2 toward bismuth molybdate is also much weaker than in the SnO2 case and, therefore, could not change the catalytic properties of the mixture via the mechanical mixing only.65,66,71−74 Since it was observed from the previous section that homogeneous samples prepared by a chemical method

Figure 5. Reaction rates for acrolein formation based on (a) catalyst mass (rw, mol/g·s) and (b) catalyst surface area (rs, mol/m2·s) of the different mechanical mixtures of zirconium oxide and beta bismuth molybdate at different temperatures (375−425 °C).

exhibited better activity than that of mechanical mixed samples, the mixtures of beta bismuth molybdate with 10% mol zirconium oxide prepared by sol−gel synthesis (chemical mixture) and impregnation (previously synthesized zirconium oxide was impregnated by beta bismuth molybdate) were also tested and the data is shown in Figure 6. The XRD patterns of the samples with 10 mol % zirconium oxide prepared by sol−

Figure 6. Reaction rate of acrolein formation for mixtures of ZrO2 (10 mol %) and β-Bi2Mo2O9 synthesized using different methods. F


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Industrial & Engineering Chemistry Research gel and the impregnation method show that the samples contain mostly beta bismuth molybdate but there is also some amount of gamma bismuth molybdate present (Supporting Information, Figure S3). The ratio of gamma bismuth molybdate phase relative to the beta bismuth molybdate phase is 15:85 and 22:78 for the sample prepared by the sol− gel and impregnation method, respectively (Table 2). The homogeneous mixtures of zirconium oxide and beta bismuth molybdate prepared by sol−gel and impregnation methods again exhibited much higher activity than the less homogeneous ones. The catalytic activities of these chemical mixtures of zirconium oxide and beta bismuth molybdate are also significantly higher than that of pure beta bismuth molybdate. This higher activity may not only be due to the addition of zirconium oxide in a homogeneous way but also due to the presence of a higher amount of gamma bismuth molybdate in the samples (Table 2 and Figure S3, Supporting Information). It is known from our previous work that when the content of gamma bismuth molybdate in the mixture with beta bismuth molybdate reached a certain percentage, the activity of the mixture increased due to the synergy effects.29 Although the conductivity of the sample prepared by the sol−gel method is the calculated value, their corresponding compositions are not much different (data is not shown), the significantly higher activity of chemical mixed and impregnated beta bismuth molybdate-zirconium oxide samples observed here shows that when zirconium oxide is added homogeneously, it also contributes to increasing the catalytic activity of the sample, possibly via enhancing the formation of the synergistic phase gamma bismuth molybdate and improving the dispersion of the active beta bismuth molybdate phase while still keeping the BET surface area high (Table 2). 3.3. Properties of magnesium oxide−bismuth molybdate mixtures. Opposite to tin oxide and zirconium oxide, magnesium oxide is a less conductive material.66,71 Therefore, it was chosen to be added to beta bismuth molybdate catalyst to explore if the catalytic activity increases when the mixture is less conductive. Since it appears from the previous section that homogeneous mixtures exhibited higher activity, mechanical mixtures of beta bismuth molybdate were not tested. Instead, the catalytic activities of chemical mixtures of beta bismuth molybdate and magnesium oxide synthesized by the sol−gel method are measured as shown in Figure 7. The specific surface areas of these samples are also shown in Table 2. It can be seen that the surface areas of these samples are very similar. The XRD patterns of these samples also show that the samples contain a mixture of beta and gamma bismuth molybdate (Supporting Information, Figure S4). There are significant presences of gamma bismuth molybdate with different contents (Table 2), which may influence the activity of the samples. The presence of the small amount of magnesium oxide is hardly detected by XRD, but by EDX measurements, Mg is clearly detected in the samples. EDX analysis of the sample with 30% MgO shows a mole ratio of Bi/Mo = 1.3 and a mole ratio of Mg/Bi = 0.16, in good accordance with the stoichiometric calculation where mole ratios of Bi/Mo = 1 and Mg/Bi = 0.15 are expected. The catalytic activities of the chemical mixture of magnesium oxide and beta bismuth molybdate show that the reaction rates of acrolein formation based on the catalyst surface areas of the mixtures are not higher than that of pure beta bismuth molybdate, indicating that there is no synergy effect, as expected (Figure 7). Although some samples (15 and 25%

Figure 7. Reaction rates for acrolein formation based on the (a) catalyst mass (rw, mol/g·s) and (b) catalyst surface area (rs, mol/m2·s) of the different chemical mixtures of magnesium oxide and beta bismuth molybdate at different temperatures (375−425 °C).

MgO) contain a mixture of beta and gamma bismuth molybdate, their activities are not significantly higher. Pure MgO also exhibited very low propene conversion and also no selectivity toward acrolein (Table 1).Thus, the addition of a nonconductive material to beta bismuth molybdate does not increase the activity of the sample. This observation supports our starting assumption that the increase of conductivity might be one of the reasons for the synergy effect because the increase of conductivity helps to transport bulk oxygen ions rapidly to reoxidize oxygen ions, which have been used for the oxidation of propylene. However, some other factors, such as the ability to absorb reactants (propylene or oxygen), which is indeed observed to be strong for tin oxide, as seen from the propene conversion of pure SnO2, may also influence at the same time. Meanwhile, pure ZrO2 and MgO exhibited low propene conversion (Table 1), thus showing no synergy effect for the mixtures of ZrO2/MgO with bismuth molybdate. Pure ZrO2, nevertheless, exhibits good acrolein selectivity compared to MgO; therefore, the chemical mixtures of ZrO2 and bismuth molybdate still exhibit good activity for the formation of acrolein. However, within the context of this paper, this is difficult to explain in detail. In our continuous study, the influence of several other factors will be taken into account. A comparison of the catalytic acitvities of different chemical mixtures of beta bismuth molybdate with various oxides (tin oxide, zirconium oxide, magnesium oxide, gamma bismuth molybdate-Bi/Mo = 1.3, 54% mol beta bismuth molybdate) in Figure 8 shows that the mixture of beta bismuth molybdate with tin oxide results in the highest activity, since tin oxide is the most conductive oxide. The catalytic activity of the mixture of beta bismuth molybdate with zirconium oxide is approximately the same as that of the mixture of beta bismuth molybdate with gamma bismuth molybdate, since the conductivities of zirconium oxide and gamma bismuth molybdate are very similar (Figure 4a). The addition of G


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Figure 8. Comparison of the catalytic activities (reaction rates for acrolein formation based on catalyst surface area, rs, mol/m2·s) of different mixtures of beta bismuth molybdate with other oxides.

Notes

magnesium oxide to the mixture with beta bismuth molybdate does not lead to any increase of activity compared to that of pure beta bismuth molybdate. Thus, to improve the activity of the catalysts, about 10 mol % of tin oxide should be added to beta bismuth molybdate.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.03-2011.16. M.T.L. also acknowledges a grant from Erasmus Mundus Action 2 TECHNO II project, which allowed her to visit Ghent University for useful discussions.

4. CONCLUSIONS This paper exposed that, by adding a suitable amount (10% mol) of SnO2, the catalytic activity of beta bismuth molybdate is improved significantly to an unprecedented high level for any known single bismuth molybdate catalyst phase or other highly active bismuth molybdate mixtures (bismuth molybdate with Bi/Mo ratio of 1.3). When adding ZrO2 to beta bismuth molybdate, the catalytic activity of the sample increases slightly and only when the contact between ZrO2 and beta bismuth molybdate particles is intimate, prepared by impregnation and chemical mixing methods. In contrast, adding MgO to beta bismuth molybdate does not increase the catalytic activity of the samples. SnO2 was found to exhibit much higher conductivity than ZrO2, leading to mixtures of SnO2 and bismuth molybdate also exhibiting high conductivity, while MgO is a nonconductive material. This is evidence that the increase of the conductivity of the mixture of bismuth molybdate and SnO2 could be one of the reasons for the increase of their catalytic activity and the synergy effect in the catalytic activity of the mixture. Although beta bismuth molybdate with the addition of 10 mol % SnO2 exhibited the highest activity among all investigated samples, its surface area was still found to be low. In our continuous study, this active catalyst will be supported on high surface area supports to improve the activity and investigate further the promotional effect.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00019. SEM images of the samples pressed at different pressures and calcined at different times. XRD patterns of mixtures of beta bismuth molybdate with different metal oxides prepared by different methods (PDF)

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REFERENCES

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Synergy Effects of the Mixture of Bismuth Molybdate Catalysts with

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