Nickel Molybdate Catalysts and Their Use in the Selective Oxidation of Hydrocarbons

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 Tećnico, Universidade Tećnica 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 Tećnico, Universidade Tećnica de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal; Fax: þ351-21-8477695; E-mail: [email protected]. 53 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 56 Madeira, Portela, and Mazzocchia 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 concentration 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

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