CATALYSIS REVIEWS Vol. 46, No. 1, pp. 53–110, 2004
Nickel Molybdate Catalysts and Their Use in the Selective Oxidation of Hydrocarbons
L. M. Madeira,1 M. F. Portela,2,* and C. Mazzocchia3
1LEPAE, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Porto, Portugal 2GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Quı´mica, Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa, Lisboa, Portugal 3Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Milano, Italy
CONTENTS
ABSTRACT ...... 54
1. Introduction ...... 54
2. Preparation of Catalysts ...... 55 2.1. Coprecipitation Techniques ...... 55 2.2. Other Techniques ...... 58 2.2.1. Molybdenum-Enriched Catalysts ...... 60 2.2.2. Nickel-Enriched Catalysts ...... 61 2.3. Supported and Doped Catalysts ...... 61
3. Thermal Activation—Transition of Phases ...... 63
4. Characterization of Catalysts ...... 66
*Correspondence: M. F. Portela, GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Quı´mica, Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal; Fax: þ351-21-8477695; E-mail: [email protected].
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DOI: 10.1081/CR-120030053 0161-4940 (Print); 1520-5703 (Online) Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com ORDER REPRINTS
54 Madeira, Portela, and Mazzocchia
4.1. Composition of Phases for Catalysts with Different Ni : Mo Ratios ...... 66 4.2. Other Physicochemical Characterizations ...... 67 4.2.1. Stoichiometric Nickel Molybdate ...... 67 4.2.2. Catalysts with Excess Molybdenum or Nickel ...... 71 4.2.3. Catalysts Prepared Using Organic Precursors and Sol–Gel Methods ...... 75 4.2.4. Doped and Supported Nickel Molybdates ...... 76 4.3. Characterization of the High Temperature b-Phase ...... 79
5. Applications of Ni–Mo–O Catalysts ...... 80 5.1. Oxidation of Hydrocarbons ...... 81 5.2. Oxidative Dehydrogenation of Light Alkanes ...... 85 5.2.1. Undoped Ni–Mo Catalysts ...... 85 5.2.2. Doped and Supported Catalysts ...... 89 5.2.3. Kinetics and Mechanism ...... 93 5.3. Nature of Active Sites ...... 97
6. Conclusions and Future Trends ...... 98
REFERENCES ...... 102
ABSTRACT
This paper reviews the preparation techniques, characterization, and use of nickel molybdate catalysts in the selective oxidation of hydrocarbons, particularly of light alkanes. Catalysts with different Ni : Mo ratios, unsupported and supported, undoped and doped, were considered. Particular attention is given to the thermal activation process for the transition of the low temperature a-phase into the metastable b-phase, which was shown to be more selective in some cases. Special reference is also made to the results of kinetic studies performed, to the mechanisms proposed for some important reactions, and to the nature of the active sites. Finally, after some general conclusions, future trends are analyzed.
Key Words: Nickel molybdate; Preparation; Characterization; Selective oxidation; Hydrocarbons; Oxidative dehydrogenation; Light alkanes.
1. INTRODUCTION
Olefins, aromatics, and many oxygenates are widely used as important raw materials in industrial processes,[1,2] and thus the strong pressure of international markets has led to constant optimization of production processes. Cost reduction can be achieved by using cheaper raw materials (for instance alkanes), combined in some cases with the use of more sophisticated catalysts. Indeed, in the last years a clear trend has been obser- ved for the use of light alkanes for the direct production of oxygenates—through ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 55 partial oxidation[3 –5]—or to manufacture olefins through dehydrogenation or oxidative dehydrogenation (ODH) processes,[5 –7] due to the ready availability and low price of natural gas. However, this is a challenging problem for the chemical industry because alkanes are less reactive than the products obtained, such as alkenes, dienes, or aldehydes and acids, which are easily totally oxidized at the high temperatures required to activate alkanes properly. Therefore, around the world much effort has been put into developing new catalytic systems providing selective oxidation of hydrocarbons, particularly light alkanes with useful yields. However, the search for better and more effective catalyst compositions, preparations, and processes continues and, up to now, few promising catalysts were found for these applications. For instance, metal molybdates were successfully employed in selective oxidation reactions and are quite versatile catalysts for important industrial processes.[5] Among them, nickel molybdates show very interesting potential for oxidation reactions, and particularly for ODH of light alkanes. A large number of papers and patents is found in the literature regarding these applications (mentioned throughout this text). But nickel (Ni)–molybdenum (Mo) catalysts are also very important for other processes, such as the hydrodesulfurization and hydrodenitrogenation of petroleum distillates;[8 –17] the water–gas shift reaction;[18] the steam reforming, hydrogenolysis, and cracking of n-butane;[19] the oxidative coupling of methane;[20] and other industrially important hydrogenation and hydrotreating reactions.[8,21 –24] Despite this large number of important industrial applications, a review that systematically analyzes the preparation techniques used, the more important characterization results, and the main catalytic studies performed for oxidation reactions with Ni–Mo–O catalysts, is not found in the open scientific literature. With this review we intend to fill this gap. We should remark that we will only consider reaction investigations involving selective oxidation of hydrocarbons.
2. PREPARATION OF CATALYSTS
2.1. Coprecipitation Techniques
During the preparation, through the coprecipitation method, of nickel–molybdenum catalysts with different Ni : Mo ratios, Andrushkevich et al.[25] found in 1973 that both the chemical composition and composition of phases of the obtained precipitate depend strongly on the precipitation conditions (concentration of reactant ions in solution, temperature, and duration of the aging process). The unsatisfactory aspects of the coprecipitation method, involving direct mixture of the solutions, and particularly the lack of reproducibility in the results, were eliminated by using an experimental setup that allowed continuous preparation of the catalysts by precipitation.[25] The nickel nitrate and þ ammonium paramolybdate solutions were mixed at constant flow rate at 868C. The NH4 2 ion concentration in the molybdenum-solution was equal to the NO3 ion concentration in the nickel solution. The Ni : Mo ratio in the solution was varied by changing the respective ratio in the original solutions. When steady-state conditions for precipitation were established, the pH in the reaction volume was 5.4. The obtained precipitates were air dried at room temperature and calcined at 5008C.[26] ORDER REPRINTS
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Andrushkevich et al. also knew that, for preparation of nickel molybdates, the pH of the medium during precipitation has a significant influence on the composition of the precipitates. Thus, they decided to investigate the problem thoroughly.[27] They found that by increasing the ammonia concentration in the paramolybdate, while the other precipitation conditions were kept constant, an increase in the nickel concentra- tion in the final precipitate was recorded due to solubilization of molybdenum with ammonia. After the 1980s, several works were published in which the NiO–MoO3 system was studied because of its use as a hydrodesulfurization catalyst. However, the preparation methods adopted varied slightly from one group to another:
[28] Vagin et al. prepared NiO–MoO3 samples with various compositions by coprecipitation of analytical salts [Ni(NO3)2 and (NH4)6Mo7O24], from the corresponding solutions at 908C and pH ¼ 6.0–6.5. The solutions containing the precipitates were then evaporated in a water bath, dried at 1108C, and calcined at 6008C for 6 hr. Brito et al.[29] also prepared a series of Ni–Mo mixed oxides by coprecipitation (either in continuous or discontinuous mode), always controlling the precipitation conditions in order to change the Ni : Mo ratio of the final product, namely by the pH of the medium. The hydrated precursor of the hydrodesulfurization catalysts[11] was synthesized by coprecipitation of nickel nitrate (pH ¼ 4.7) and ammonium heptamolybdate (pH ¼ 5.6) aqueous solutions. The methodology used to obtain the phase that is stable at high temperatures (b-NiMoO4) will be described later (see Section 3).
The investigations carried out at the Polytechnic of Milan, Italy, have helped, among other aspects, to clarify the experimental conditions that determine the formation of oxides with [30,31] different compositions in the NiO–MoO3 system. In two preliminary studies, special attention is given to the methodology that enables the precursor of the catalytically active phase to be obtained. The solvated precursor was prepared by mixing, with stirring, equimolar solutions of ammonium molybdate and nickel nitrate [Ni(NO3)2 . 6H2O] at pH 5.6 and at a temperature of 858C. The molybdenum solution was prepared by dissolution of molybdic acid (H2MoO4 . H2O) in an ammoniacal solution at 858C and pH of 6.2. The precursor obtained, partially crystalline and with a pale yellow color, was dried at 1208C and thermally activated at 5508C for 2 hr. It is noteworthy that the NiMoO4 prepared by coprecipitation was also patented;[32] however, nickel molybdate was also synthesized by [30,31] the dry mode, from NiO and MoO3. As shown by Mazzocchia et al.,[30 –34] the precipitation process can lead to different precursors with the general formula:
xNiO yMoO3 nH2O mNH3 (1)
Small changes in experimental conditions such as pH, precipitation temperature, H2MoO4/NH4OH ratio, filtration temperature, duration of aging of the precipitate in the mother liquor, and duration and temperature of drying may lead to precursors with different x, y, n, and m values. The effect of some of these parameters is shown in Fig. 1. ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 57
Figure 1. Effect of the precipitation parameters on the type of precursor obtained. Each arrow refers to the effect of a given parameter. (Information adapted from Refs.[30,33].)
Attention should be paid to the fact that each arrow refers to the effect of a given parameter on the type of precursor obtained. For formation of the precursors, the following equilibria are established between the different species:[33]
2 þ 6 7MoO4(aq:) þ 8H(aq:) !Mo7O24(aq:) þ 4H2O (2) 2þ 2 Ni(aq:) þ MoO4(aq:) !NiMoO4(s) (3) 2þ 6 10 6þ Ni(aq:) þ Mo7O24(aq:) !NiMo6O24(aq:) þ (Mo ) (4) 2þ Ni(aq:) þ 2OH(aq:) !Ni(OH)2(s) (5)
[31] The conditions required for preparation of pure a-NiMoO4 are highly critical. In order to avoid polymerization of the molybdate ions it is not sufficient to control the environmental conditions (Fig. 1). The rate at which the nickel solution is added is also a determining factor, probably because the rate of Eq. (4) is a critical condition. For formation of the precursor with Formula (1), the main reactions involved are the following:
1 x , , 1)Eqs. (3) þ (4); x ¼ y)Eq. (3); 6 y x 1 , , 1)Eqs. (3) þ (5) y ORDER REPRINTS
58 Madeira, Portela, and Mazzocchia
Thus, at pH 6 the G precursor is obtained (see Fig. 1), which becomes more green as pH is increased due to coprecipitation of nickel hydroxide [cf. Eq. (5)], yielding an x/y ratio higher than 1. In fact, in the patented preparation method, in which the temperature was maintained at 858C, but with a pH of 6, the final catalyst had the following composition: [32] Ni1.5MoO4.5. On the other hand, if room temperature is used, without changing the other experimental conditions, a pale blue (B) precursor is obtained for which y/x ¼ 6 (cf. Fig. 1). After thermal activation, the sample exhibits the infrared spectrum and x-ray diffraction (XRD) pattern characteristic of excess MoO3. The yellow precursor (S—of stoichiometric), where x ¼ y ¼ 1, is obtained in the conditions already mentioned. If, during filtration, the solution is allowed to cool (between 658C and 858C), a pale yellow precursor (E) is obtained with a ratio y/x . 1. For this precursor in particular, both the time of aging in the mother liquor and the temperature of the solution determine the y/x ratio. Using, for instance, a pH of 5.6 but mixing the solutions at 708C, a precursor is obtained, which after calcination (during 2 hr at 5508C) yields a catalyst with the formula [32] NiMo1.5O5.5. The stoichiometric catalyst has been used in several studies, but the preparation procedure employed was not always exactly the same. For instance, 0.5 M solutions of [35] Ni(NO3)2 . 6H2O and H2MoO4 were employed, with a final pH of 5.1 and at 858C. In other cases 0.25 M solutions were used, with precipitation at pH 5.2 and a temperature of 908C,[36] with filtration of the precipitate at 858C and drying for 4 hr at 1108C, followed by calcination for 2 hr at 5508C.[37] The parameters m and n in the precursor with Formula (1) are affected by slight variations in the temperature or in the amount of ammonia. For instance, in a study where the yellow precursor (S) was prepared by using 0.25 M solutions of molybdic acid and nickel nitrate, at 858C and pH 5.4, thermogravimetric analyses revealed the following [33] composition for the precursor: NiMoO4 . 3/4H2O . 3/4NH3. A green precursor (G) was also prepared with the following composition: Ni1þdxMoO4 . 1/3H2O . 5/3NH3, from a 0.25 M solution of ammonium heptamolybdate [(NH4)6Mo7O24 . 4H2O] at 858C, the pH of the solution being adjusted with ammonia in order to yield an NH3/Mo ratio of 1.5. The 0.25 M solution of nickel nitrate, at the same temperature, was added at a controlled rate (7 mL/min). The green precipitate was immediately formed and the pH dropped from its initial value of 8.48 to 7.09 in 1 hr.
2.2. Other Techniques
New techniques have been developed for preparation of catalysts that enable clarification of specific aspects in multicomponent catalytic systems. Better control of the contact between phases is achieved when compared with catalysts prepared by precipitation or impregnation. In these situations, it is also difficult to control the thickness and structure of the superficial layer. With these goals in mind, Zou and Schrader[38] used reactive sputtering, an advanced technique for materials processing, in order to produce catalysts of the NiMoO4 –MoO3 system with controlled compositions and structures, especially thin films with well-defined architectures. Another advantage of the samples prepared in this way, compared to materials obtained by precipitation, is that they are more easily characterized by several techniques. They prepared very thin films of MoO3,of NiMoO4, and of combinations of both oxides over different supports (including SiO2), ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 59 which were later characterized by various techniques. In the samples containing both components, the films were prepared through a sequential process, the NiMoO4 being deposited over the predeposited MoO3. More recently, they have examined in detail the deposition parameters for the reactive sputtering technique and found that the multilayer films of NiMoO4 on a-MoO3 include an interfacial material identified as b-NiMoO4, which was detected at relatively low temperatures in the bilayer structures.[39] The use of organic salts for preparation of active catalytic systems, such as Ni–Mo–O, has the advantage of providing a lower crystallization temperature. Using an oxalic precursor (a product that decomposes at a lower temperature than ammonia) and different thermal treatments, Mazzocchia et al. have obtained several catalysts with [34] different compositions. The NiC2O4 . 2H2O and MoOC2O4 . 4H2O mixture was prepared by adding ammonium heptamolybdate to 250 mL of a solution containing a large excess of oxalic acid. After dissolution, nickel nitrate is added at room temperature such that the Ni : Mo ratio is 1. The solution (0.14 M in Ni and in Mo) is then slowly warmed under vacuum to 408C. Precipitation starts immediately and increases as the water evaporates. The precursor is finally dried at 1208C for 15 hr. Another method of catalyst preparation that has been recently used resorts to natural substances or polymers. A polymeric network is created, containing the ionic compounds of the active catalyst inside the organic matrix. In this context, Anouchinsky et al. tested a new methodology for NiMoO4 preparation in which the precursor is an organic gel (agar- agar) containing the Ni and Mo ions in a 1 : 1 atomic ratio.[40] This approach offers several advantages: it is cheap and simple and multicomponent catalytic systems can be prepared by simple dissolution of the desired elements, at the appropriate concentrations, in the aqueous solution. In this way one may change, for instance, the Ni : Mo ratio with the simultaneous presence of promoters. The gel was prepared from 0.5 M solutions of nickel nitrate and ammonium heptamolybdate and mixed at room temperature with continuous stirring. A load of 1% (by weight) of powdered agar-agar is added and the solution warmed at 808C to solubilize the agar-agar. Rapid cooling of the solution yields the gel, which is subsequently dried by slow heating (108C/hr) from room temperature up to 1208C. This temperature is then maintained for 4 hr and finally the gel is calcined. The agents that control pH in the synthesis of precursors of mixed oxides must be easily removed from the precipitate. From this point of view, oxalic precursors have been shown to be advantageous since they crystallize at low temperatures.[34] It would then be expected that, for the Ni–Mo–O system, the use of the sol–gel technique could be beneficial. The sol–gel technique offers a low-temperature method for synthesizing materials that are either totally inorganic in nature or both inorganic and organic. The process offers many advantages, including the use of simple and inexpensive equipment, excellent control of the stoichiometry of precursor solutions, and ease of compositional modifications. Good control of the stoichiometry may be very useful for fine control in the preparation of Ni–Mo–O catalysts, as their composition is crucial for catalytic app- lications. With this goal in mind, as well as the fact that precursor decomposition can be achieved at low temperatures, several authors decided to apply the sol–gel route for preparation of nickel molybdate catalysts. Although papers on this subject are somewhat scarce, we should mention the work of Anouchinsky et al., who have prepared several catalysts by the sol–gel method.[40] As expected, homogeneous dispersion of the Ni and ORDER REPRINTS
60 Madeira, Portela, and Mazzocchia
Mo ions in the precursor was achieved, which led to formation of the NiMoO4 phases, whose crystallization occurs at temperatures lower than those prepared by coprecipitation. This process also leads to the stabilization of the b-phase at room temperature. Lezla et al.[41] have also adopted the sol–gel methodology to prepare the stoichiometric catalyst using citric acid (1 mol/Ni), which was added to a solution of nickel nitrate (0.4 M). Then a solution of (NH4)6Mo7O24 . 4H2O was added very slowly so as to avoid precipitation. The solution was evaporated until a gel and then a solid were obtained. Finally, the solid was ground and heated in air at 5008C for 24 hr.[41] The sol–gel technique also was used recently for the preparation of supported catalysts, with particular advantages in the case of the Ni–Mo–O system (see Section 2.3). Nanocrystalline NiMoO4, among other molybdates, was also recently prepared from the complete evaporation of a polymer-based metal-complex precursor solution.[42,43] These fine-grained materials (with particle diameters less than 100 nm) are expected to have potential applications in many technological areas.
2.2.1. Molybdenum-Enriched Catalysts
Catalysts containing excess MoO3 are often prepared by drying the final solution, after mixing the reactants in appropriate ratios. However, this method gives rise to numerous problems regarding the nature of the dried precursor, because the concentration of the solution changes during the drying process, leading to precipitation of hetero- polymolybdates or to a mixture of molybdate and molybdic acid.[44] Thus, Mazzocchia et al. decided to prepare several catalysts with excess Mo, all with the same composition but obtained from different precursors. They used decomposition of the heteropolymolybdate or dry-mixing of nickel molybdate and excess of molybdenum trioxide. A catalyst studied, derived from the heteropolymolybdate, was NiMoO4 . 5MoO3, obtained through thermal [44] decomposition of (NH4)4H6NiMo6O24 . 5H2O. In other works a certain excess of MoO3 was introduced by cooling the solution of the a-NiMoO4 precursor, with consequent coprecipitation of (NH4)4H6NiMo6O24 . nH2O (cf. Fig. 1).[31] Both the temperature and the cooling period depend on the excess of molybdenum desired. In a very interesting work published by Ozkan and Schrader, the synthesis of nickel molybdates containing an excess of molybdenum through several methods is described in detail.[45] The excess of Mo (relative to the stoichiometric) is present as a new phase, MoO3, substantially increasing the complexity of the system. The authors report the fol- lowing methods for incorporation of MoO3 into the catalyst: precipitation, solid state reaction, and impregnation. Nickel molybdates prepared through precipitation were obtained from aqueous solutions of ammonium heptamolybdate [(NH4)6Mo7O24 . 7H2O] and nickel nitrate [Ni(NO3)2 . 6H2O], the pH being changed with ammonium hydroxide or nitric acid solutions. In order to obtain pure nickel molybdate, pH during addition and reaction was kept at 6 (with a temperature of 638C). The catalysts with excess MoO3 are obtained by acidification of the medium during addition of the solutions, the pH depending on the excess of MoO3 desired. It should be stressed that with this procedure the catalyst composition is insensitive to both concentration and composition of the reactants, the pH of the medium during the precipitation being the key factor. The solid-state synthesis basically consists of heating NiO together with MoO3, or MoO3 mixed with NiMoO4. Nickel molybdate, obtained by precipitation, was also impregnated with ammonium ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 61 heptamolybdate to provide catalysts with an excess of MoO3 between 2% and 55%. Molybdenum trioxide was also impregnated with NiMoO4. The experimental procedure used for these syntheses has been described in detail.[45] More recently, Lezla et al. have also prepared Ni–Mo–O catalysts with Mo : Ni ratios between 0.90 and 2.15 using several methods, which include precipitation, evaporation to dryness, sol–gel, impregnation, and mechanical mixing. They analyzed the influence of the preparation method on the catalytic performances for propane ODH.[41]
2.2.2. Nickel-Enriched Catalysts
Catalysts with an excess of Ni have usually been prepared in two ways: (i) precipitation at 858C and pH 6.2, using 0.25 M solutions of H2MoO4 and Ni(NO3)2, which yields an Ni : Mo ratio of 1.40 and provides formation of NiO together with the a- and b-phases of NiMoO4; and (ii) mechanical mixing of NiMoO4 . H2O and Ni(OH)2, which provides Ni : Mo ratios in the range 1.1–1.6, followed by activation at various [46] temperatures. Chemical impregnation of a-NiMoO4, using an aqueous solution of nickel acetate, was also adopted for preparation of Ni-enriched catalysts.[47] Using moly- bdenum oxalate instead provided catalysts with excess molybdenum.[47]
2.3. Supported and Doped Catalysts
The use of supported nickel molybdate catalysts in ODH reactions is not very [48] [49 –52] common. Some exceptions are recent works in which TiO2 (anatase) and SiO2 were used to support the active phase. In the first case, the catalysts were prepared using two procedures: (i) wet impregnation of the support with an aqueous suspension of NiMoO4, and (ii) direct precipitation of NiMoO4 on the support surface at 858C, using [48] solutions of nickel nitrate and ammonium heptamolybdate. The SiO2-supported [52] catalysts were also prepared by wet impregnation or by direct precipitation of NiMoO4 on the support,[49,52] or even by sol–gel routes.[50,51] Nickel–molybdenum catalysts are frequently used as supported catalysts in important industrial processes like hydrodesulfurization or hydrogenation, so many works exist in the literature regarding these issues. Conventional methods of preparation of hydro- treatment catalysts usually consist of depositing transition-metal salts onto the support, usually g-Al2O3, followed by calcination to produce stable oxidic materials that must be sulfided either prior to or during the start-up of the hydrotreatment process. In a pioneering work, Laine et al.[12] found that it is advantageous to impregnate alumina with nickel before molybdenum. Later, Brito and Laine[53] prepared nickel–molybdenum catalysts supported over g-Al2O3 through impregnation of commercial g-Al2O3. First Mo was added—using an ammonium heptamolybdate solution—followed by drying (overnight at 1208C) and calcination (2 hr at 4008C). Portions of this solid were then submitted to dry impregnation with nickel nitrate solutions with the purpose of obtaining solids with several NiO loads. After drying, the samples were calcined at different temperatures between 4008C and 8008C. It is well known that for the preparation of silica-supported catalysts, the sol–gel route allows very good control of the composition, homogeneity, and textural properties of the final products. In fact, the nanoscale chemistry involved in sol–gel methods appears to ORDER REPRINTS
62 Madeira, Portela, and Mazzocchia be the most straightforward way to prepare tailored nanocomposites, including organic– inorganic hybrid materials. Moreover, sol–gel methods have been found to be effective for dispersing small metal oxide particles in nonmetallic matrices. With these features in mind, [51] Cauzzi et al. prepared NiMoO4/SiO2 composites by the sol–gel process via silicon alkoxides, involving Si(OMe)4 (tetramethoxysilane), Ni(NO3)2, and (NH4)6Mo7O24 as starting materials. The dried gels were treated at increasing temperatures until crystalline grains of nickel molybdate highly dispersed in the amorphous silica matrix were formed (6758C). Xerogels with different Ni : Mo ratios were synthesized and the preparation procedures are described in detail.[51] It is noteworthy that besides leading to the support of catalytic materials, the xerogel plays the important role of stabilizing b-nickel molybdate, which otherwise would turn into the a-phase at room temperature. Many other interesting investigations dealing with the preparation of Ni–Mo supported catalyst could be mentioned. However, they are directed for use in processes other than oxidation of hydrocarbons, which is outside the scope of the present review. It is nonetheless important to stress that different materials have been used as supports for Ni– Mo catalysts, namely alumina,[17,54,55] magnesia-alumina mixed oxides,[13] zeolites,[21,24] titania-alumina mixed oxides,[14,56] activated-carbon,[15,16,57] or zirconia.[58] In order to improve the catalytic performance of the Ni–Mo–O system in the selective ODH of alkanes, particularly n-butane, nickel molybdate was doped with several alkali (lithium, sodium, potassium, or cesium)[59] or alkaline-earth (calcium, strontium, and barium)[60] promoters. The samples were prepared through wet impregnation of the pure a-NiMoO4 material, using different loads of the respective nitrate solutions, followed by filtration, drying, crushing, and calcination in dry air for 2 hr at 5508C. For propane ODH, promoters such as K, Ca, and P were frequently used, the catalysts being prepared through an incipient wetness impregnation technique starting from an a-NiMoO4 calcined pure catalyst.[61 –63] Still for application in oxydehydrogenation processes, we should note the preparation of catalyst compositions that contain other elements like phosphorus, antimony, bismuth, or arsenic, and that are effective in converting paraffins or monoolefins to a higher degree of unsaturation.[64] Methods described therein include coprecipitation, impregnation, dry mixing, and similar methods the final catalyst compositions being unsupported or supported. While with the conventional impregnation method the doping element only stays on the catalyst surface, the sol–gel route simultaneously produces surface and structural modifications. In addition, better dispersion of the active species on the support can frequently be achieved, as well as appropriate compositional homogeneity. This led Soares et al.[65] to prepare, by the citric acid method, mixed Ni–Mg molybdates, which were calcined under air flow at 5508C for 8 hr. These catalysts were tested for n-butane ODH and exhibited a noteworthy catalytic performance.[65] Dopants such as tellurium (Te) and phosphorus (P) were also added to Ni–Mo catalysts, particularly for application in the direct oxidation of propane to acrolein and acrylic acids. Reported techniques for preparation of the Te-doped catalysts include the mechanical mixing of Te2MoO7 with NiMoO4–MoO3, the mixing of telluric acid with [66,67] NiMoO4 –MoO3, and the impregnation of nickel molybdate with ammonium telluromolybdate.[68] For the P-doped system, the incipient wetness technique was used, [66,67] with (NH4)2HPO4. The preparation of doped nickel–molybdenum catalysts can also be found in US Patent No. 3,968,054, by Cherry et al.,[69] who described an improved coprecipitation ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 63 method for the preparation of antimony-doped Ni–Mo catalysts, useful for oxidation of n-butane to maleic anhydride. Finally, Ferlazzo et al.[70] claimed a process for preparation of a complex molybdenum-based catalytic system, which is comprised by one or two crystalline phases (including beta nickel molybdate) and at least one modifying agent (promoter element), useful, for instance, for the selective conversion of unsaturated hydrocarbons into unsaturated aldehydes or diolefins.
3. THERMAL ACTIVATION—TRANSITION OF PHASES
The structure of some molybdates changes with temperature, while for others it remains unchanged. For instance an irreversible structural conversion in Bi2MoO6 was observed at temperatures higher than 5508C. Transformation is complete at 6408C.[71] A reversible conversion in CoMoO4 occurs at 5008C while for NiMoO4 a temperature of about 6908C is needed.[72,73] In fact, as long ago as in 1973 Plyasova et al.[26] identified two polymorphic forms in the nickel molybdate. One of them has a monoclinic crystalline network with the molybdenum with number of coordination six and is stable at room temperature (then named the P-phase). When heated to ca. 6508C, this form was converted into another (then named the N-phase)—isomorphic with a-MgMoO4 and a-MnMoO4—in which the molybdenum has number of coordination four, but which is not stable at room temperature. After cooling a transition into P-phase was observed. The thermograms of samples with Ni : Mo ratios close to 1 that were previously calcined at 5508C, showed an endothermic effect at ca. 6508C when the solid was heated and an exothermic one at 508C when the solid was cooled.[26] The former was attributed to conversion of the low temperature into the high temperature phase (P ! N), and the latter to the reverse transformation, i.e., N ! P. The transformations of phases that occur when the precipitates are heated were studied for the first time by Andrushkevich et al.[27] Differential thermal analysis showed an endothermal effect due to the removal of crystallization water at about 1808C and another at 4208C due to the decomposition of the hydrated molybdate. These results are in very good agreement with the data shown in Fig. 2, obtained recently by Za˜voianu et al.[48] We can see that the thermal analysis performed over the precursor of NiMoO4 shows a loss of weight below 473 K, which corresponds to the desorption of water and ammonia. The strong exothermic processes occurring at 723–773 K are attributed to the decomposition [48] of NH4(NiMoO4)2OH . H2O and ammonium nitrate present in the structure. According to Andrushkevich et al.,[27] in samples with excess Mo the peak that appears at 7808C coincides with the melting point of the molybdenum oxide present in the catalyst. In their studies they still concluded that the crystallization temperature of the fresh precipitate (4308C for Ni : Mo ratio ¼ 0.7) increases with the nickel content, probably because the nonstoichiometric molybdate or the formed solid solution crystallize at higher temperatures than the stoichiometric molybdate. Indeed, the infrared spectrum of a sample with an Ni : Mo ratio ¼ 2.3 heated at 6508C shows the characteristic bands of the N-phase (now named b-phase), showing that crystallization occurs at 6258C and is accompanied by the exothermal effect observed in the thermal analyses. It should also be noted that these authors found that in samples with a great excess of Ni (Ni : Mo ¼ 2.3) the N ! P transformation was not recorded when the sample was cooled. ORDER REPRINTS
64 Madeira, Portela, and Mazzocchia
[48] Figure 2. Thermal analysis of the precursor of NiMoO4. (Adapted from Ref. , with the kind permission of Elsevier Science.)
The Ni–Mo–O system indeed presents certain particularities. It is now well known that NiMoO4 can have three different structures, two of them stable at atmospheric pressure, while the other is observed at higher pressures. The two atmospheric pressure isomorphs are now commonly named the a-phase—stable at room temperature and with octahedral coordination of the Mo6þ ions—and the b-phase—high temperature phase, metastable, and with tetrahedral coordination of the molybdenum.[74] The b-phase is formed after heating the a-phase to ca. 7208C and undergoes reverse transition at low temperature on cooling to ca. 2008C.[63] Figure 3 shows the differential thermal analysis (DTA) cycle of phase transitions in the stoichiometric NiMoO4 system.
[63] Figure 3. The DTA cycle of stoichiometric NiMoO4 phase transitions. (From Ref. , with the kind permission of Kluwer Academic Publishers.) ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 65
The coordination of the molybdenum atoms in both phases was confirmed more recently by Rodriguez et al. using x-ray absorption near-edge spectroscopy (XANES), which has also shown that the Ni atoms are in octahedral sites.[75] However, in a subsequent paper these authors reported that in the a-phase the molybdenum exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 A˚ ).[76] Regarding the stability of the phases, calculations of first-principles density functional theory (DFT) have evidenced that the a-phase is 9 kcal/mol more stable than the b-phase, with an energy barrier for the a to b transition of 50 kcal/mol, while time- resolved XRD experiments point to an apparent activation energy of 80 kcal/mol.[76] The phase stable at high temperature, b-NiMoO4, is frequently formed by heating the precalcined a-NiMoO4 sample in situ, for instance at 7608C for 5 min. The sample is then quickly cooled to the desired temperature (which must always be higher than 2508C) for other treatments (e.g., sulfiding), characterization, or catalytic runs. It should be noted that when more severe treatments were applied for b-phase formation (temperatures higher than 7608C or for more than 5 min), after cooling to room temperature the sample exhibited not only the a-phase, but also a more complex diffractogram, with peaks characteristic of both phases. In this case the b-phase is stabilized at room temperature, which would be due, as detailed below, to an excess of NiO as a result of the decomposition of the mixed compound and sublimation of MoO3. When the normal treatment is applied for transition of phases, b ! a conversion is recorded when the sample is cooled to room temperature. To use the high temperature b-phase of NiMoO4 in catalytic runs, Mazzocchia et al. performed the a-tob-phase conversion in the reactor, by thermal activation of nickel molybdate. The reactor was usually heated in 25 min to 7008C under oxygen, and then this temperature was maintained for 5–15 min before cooling to the reaction temperature,[32,33,37] but always avoiding excessive cooling to prevent the b to a-phase transition. The temperature selected for transition of phases is in agreement with the high temperature XRD data that show that at 5958Ctheb-phase is already present, but a temperature of about 7008C is required to obtain full conversion into a pure b-phase.[37] The data found in the literature reveal some discrepancies regarding the temperature [36] for phase transition in the NiMoO4 system. According to Di Renzo and Mazzocchia, this is due to the strong influence of the preparation conditions of the sample. Therefore, it was decided to investigate, by differential thermal analysis, how thermal treatment of the precursor affects the transition of phases in NiMoO4. It was found that the transition temperature of the a- to the b-phase increases, and that for the b ! a transition it decreases, due to a temperature-induced relaxation. Thus, when the activation temperature of the sample is increased, the activation energy for the a-tob-phase transition is increased. It should, however, be noted that the endothermic peak that corresponds to this transition (a ! b) was not detected when the previous calcination thermal treatment was performed at a temperature below 5508C. The temperature at which the exothermic peak (relative to the b ! a conversion) began ranged between 2578C and 2008C, depending on whether the previous heating temperature was 7008C or 9008C, respectively.[36] Later, the use of a high temperature diffraction camera showed that when activating NiMoO4 at temperatures between 7008C and 9008C the temperature of the b to a transition at no point differed significantly from 1808C, but the transition rate in the sample heated to 9008C was slower than in the samples heated to lower temperatures. This effect was attributed to the loss of MoO3 in the NiMoO4 with formation of a nickel-rich solid solution and with the [46] structure of the b-phase of NiMoO4. ORDER REPRINTS
66 Madeira, Portela, and Mazzocchia
4. CHARACTERIZATION OF CATALYSTS
4.1. Composition of Phases for Catalysts with Different Ni : Mo Ratios
Plyasova et al. studied, by XRD and infrared spectroscopy, the composition of phases of the Ni–Mo–O system with Ni : Mo ratios from 0.2 to 2.0.[26] As described in the previous section, two polymorphic forms have been identified in nickel molybdate catalysts: one that is stable at room temperature (then named the P-phase), which when heated to about 6508C is transformed into another (then named the N-phase), Which is unstable at room temperature. After cooling, the transition to the P-phase is observed. It has been reported that the high temperature modification reacts with excess nickel (relative to the stoichiometric), forming a solid solution that is stable at room temperature. The existence of the N-phase at room temperature and the absence of peaks characteristic of the P-phase or of NiO in the x-ray diffractogram for samples with well-defined compositions indicates that, in certain conditions, nickel is dissolved in the structure of the N-phase, stabilizing it at room temperature, thereby forming a solid solution of nonstoichiometric composition.[26] It was subsequently found that in samples containing excess nickel, relative to the stoichiometric NiMoO4, the solid solution formed has a solubility limit in Ni in the range Ni : Mo ¼ 1.10– 1.20 (atomic).[77] A solid solution of the vacancy type is formed, i.e., the excess of dissolved Ni ions occupies the normal octahedral positions in the structure of the N-phase, while some tetrahedral positions of the Mo remain empty. Other investigations have also been carried out in which differential thermal analysis and XRD techniques were used to investigate the composition of the Ni–Mo–O system in a wide range of Ni : Mo ratios (from pure NiO up to pure MoO3). The four phases detected and identified were the following: nickel oxide, molybdenum trioxide, normal nickel molybdate, and nonstoichiometric nickel molybdate.[78] In all the samples, with the exception of the pure oxides, thermal analyses showed an irreversible exothermal effect at about 430–4408C (which corresponds to crystallization of the nickel molybdate with composition NiMoO4) and another at 620–6708C (which is presumed to be due to the transition of phases of nickel molybdate). It was found that samples with m , 1 (m ¼ Ni : Mo ratio) present two phases: MoO3 and NiMoO4; the species with m close to 1 are mainly composed of normal nickel molybdate; the species with m . 1 are mixtures of three phases: nickel oxide and normal and nonstoichiometric nickel molybdates.[78] However, for m . 1.6, only nickel oxide and the high temperature phase were detected.[26] This identification of the phases that are present in Mo- or Ni-rich catalysts was subsequently confirmed.[35] While in the stoichiometric catalyst, at room temperature, only the low temperature phase was detected (with Mo in octahedral coordination), in Ni-rich catalysts (Ni : Mo ¼ 1.0–1.3) a solid solution of nickel and both phases (high and low temperature) were found, and the presence of nickel oxide was not detected in the x-ray diffractograms or in the IR spectra. Therefore, the b-phase is stabilized at room temperature due to the excess of nickel in the crystalline lattice of the molybdate. In catalysts where Mo : Ni . 1, both the low temperature phase and MoO3 are present. For example, the com- pound [(NH4)4H6NiMo6O24], prepared in well-defined conditions (see Fig. 1), at the drying temperature presents very well-defined XRD diffraction patterns but at the thermal [31] activation temperature (5508C), only the a-NiMoO4 and MoO3 peaks were identified. For catalysts with excess nickel, in 1958 Corbet et al.[79] observed that, after heating to 5008C the precursor obtained by precipitation of the solution containing Ni2þ and Mo6þ ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 67 ions at pH higher than 6, a new phase was formed. This phase, then called the N-phase with Ni : Mo . 1, according to Di Renzo et al.[46] corresponds to the high temperature phase (b-NiMoO4), whose stabilization at room temperature is achieved by insertion of excess Ni in the NiMoO4 lattice. The formation of the nickel-rich solid solution (NiO in b-NiMoO4) entails an increase in the lattice parameters and leads to an increase in the reducibility of the system. In fact, a catalyst with composition Ni : Mo . 1.40 is more easily reduced than NiMoO4, which agrees with some results that will be described later concerning the reducibility of this system. The formation of the solid solution of NiO in NiMoO4 is demonstrated by the stabilization of the b-phase at room temperature when the nickel-rich samples are activated at temperatures around 550–7508C. The XRD data indicate that the solid solution is responsible for the enlargement of the lattice parameters of the structure of the [46] b-NiMoO4 phase compared with the parameters of the stoichiometric phase. The Ni-rich samples prepared by coprecipitation, when heated to 5508C, mainly showed the high temperature phase (b-NiMoO4), even when cooled to room temperature. The longer this treatment lasts, the higher the percentage of that phase, and the smaller the amount of crystalline NiO. However, when the temperature was increased, the percentage of b-phase stabilizing at room temperature decreased and the proportion of NiO increased. At very high temperatures the solid solution separates. Indeed, x-ray data showed that NiO precipitation reached a significant rate at 8008C, contraction of the b-NiMoO4 cell occurred at 9008C, and that after heating to 10008C the sample was composed, at room temperature, only of a-NiMoO4 and NiO. This was confirmed by the results of diffuse reflectance spectroscopy: the sample activated at 5508C showed the characteristic band of 21 the tetrahedral coordination of molybdenum (b-NiMoO4 phase) at 35,500 cm , while when activated at 9008C it presented the spectrum characteristic of stoichiometric 21 NiMoO4 with a band at 30,000 cm (typical of the octahedral coordination of molybdenum in the a-phase) with the additional band of NiO at 14,000 cm21.[46]
4.2. Other Physicochemical Characterizations
4.2.1. Stoichiometric Nickel Molybdate
Stoichiometric nickel molybdate has been characterized by several research groups. Usually the bulk composition of the catalyst is determined by inductively coupled plasma spectroscopy, for molybdenum, and atomic absorption, for nickel. A typical BET surface area for the stoichiometric catalyst is 44.1 m2/g.[59] Since the material is crystalline, XRD analysis (including high temperature XRD) has often been used to study its structure. A typical diffractogram for both phases is presented in Fig. 4, which shows the characteristic peak of the a-phase located at 2u ¼ 28.78 (JCPDS powder diffraction file card no. 33-948) and of the b-phase at 26.48. The structure of the solid has also frequently been analyzed by infrared spectroscopy because it provides useful information regarding, for instance, the stoichiometry of the material. The Fourier transform infrared (FTIR) spectra of a-NiMoO4 (Ni : Mo ¼ 1.00) is shown in Fig. 5, which is in good agreement with others found in the literature.[26,37] It is characterized by bands at 608, 934, and 958 cm21. When, through stabilization, the b-phase is also present, the spectrum at room temperature reveals a band at 950 cm21, and ORDER REPRINTS
68 Madeira, Portela, and Mazzocchia
Figure 4. X-ray diffraction patterns of a-NiMoO4 at 218C (A) and b-NiMoO4 at 7108C (B). (Adapted from Ref.[80], with the kind permission of Elsevier Science.) two new characteristic bands are also visible at 800 and 880 cm21 as a consequence of the change in the Mo coordination from 6 to 4. This is an important feature in order to ensure that the obtained nickel molybdate has a well-defined octahedral structure. Moreover, the 21 absence in the FTIR spectra of the characteristic MoO3 bands (at 980 cm —attributed to the vibration of the Mo–O bond—and at 870 and 812 cm21—attributed to the Mo–O bond),[37] and the absence of those characteristic of the b-phase, are a good indication that the prepared nickel molybdate does not contain excess of either Mo or Ni. Regarding the electrical conductivity (s) of the solid, it is known that a-NiMoO4 is an n-type semiconductor when prepared in quasistoichiometric conditions.[37,81] Studies performed with this catalyst have shown that when the oxygen partial pressure in the gas phase is lowered, at sufficiently high temperatures, the electrical conductivity s / P 21/5.8 [37] 2 / increases according to O2 . The value of the exponent is close to 1 6, thus demonstrating that the main surface defects are compatible with the model of doubly ionized vacancies,[82] whose formation can be described by the following equilibria:
1 (O ) ! O (g) þ V (6) O s 2 2 O o VO !VO þ e (7) o oo VO !VO þ e (8) ORDER REPRINTS
Nickel Molybdate Catalysts and Selective Oxidation of Hydrocarbons 69
[60] Figure 5. The FTIR spectra of a-NiMoO4. (Adapted from Ref. , with the kind permission of Academic Press, Inc.)
where
(OO)s ¼ surface anion;
VO ¼ anionic vacancy with the two electrons trapped (neutral entity); o oo VO and VO ¼ singly and doubly ionized anionic vacancies, respectively.
From the equilibria of Eqs. (6)–(8), and taking into account that the corresponding ¼ equilibrium constants follow Van’t Hoff’s law [Ki Koi exp (2DH i/RT)], it can be easily deduced (e.g., Ref.[83]) that: