氧化脱氢

Chemistry and Technology of Fuels and Oils, Vol. 33, No. 3, 1997 REVIEW CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF BUTENES AND BUTANE TO BUTADIENE A. N. Vasil'ev and P. N. Galicia UDC 541.128.3 One of the most important products of large-scale petrochemical production is butadiene, which is the starting material in the manufacture of rubbers, synthetic fibers, plastics, film-forming additives for fuels and oils, and other valuable products. One of the most attractive methods for the synthesis of butadiene is oxidative dehydrogenation of butenes or butane. Oxidative dehydrogenation of butenes to butadiene was accomplished for the first time by Shell Oil (USA) at the end of the 1950s, using catalysts based on bismuth molybdates, phosphates, and tungstates. Somewhat later, reports appeared on the possibility of oxidative dehydrogenation of n-butane. These processes are distinguished by irreversibility, low energy consumption as a result of exothermic side processes of combustion, and long-term stable operation of the catalysts before regeneration is required. The oxidative dehydrogenation of butenes proceeds with particular ease. In the case of n-butane, as a consequence of its low reactivity, higher process temperatures are required. In the present review we have systematized data on catalysts for the oxidative dehydrogenation of butenes and n-butane that have been published subsequent to the period covered by the monograph [1]. CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF 1-BUTENE Ferrites are among the most widely used of these catalysts. The scientific principles for obtaining these materials were set forth in [2]. Commercial catalysts developed on the basis of ferrites include K-24, K-28, and K-28i (Russia) and G-64c, G-48, and Shell 105 (USA and Western Europe). The G-64c and K-28 exhibit the broadest range of catalytic properties [3]. Pure iron oxide is relatively inactive in the oxidative dehydrogenation reaction. Its catalytic properties can be improved considerably by modification with various promoters. The influence of antimony and tin oxides and bismuth phosphate on the properties of catalysts based on ZnFe204, FeSbO 4, Fe(MoO4) 3, and others were investigated in [4]. Complex catalysts are obtained by coprecipitation or by steaming nitrate solutions of the corresponding compounds. The catalysts Zn.Fe204, Bi2Mo3012, and FeSbO 4 are active and selective in the formation of butadiene; iron, cobalt, and nickel molybdates are relatively inactive; simple oxides are considerably less active than any of these compounds. The addition of Sb204 has a promoting effect on FeSbO 4 and Bi2Mo3OI2, has practically no effect on nickel and cobalt molybdates, and inhibits the formation of carbon dioxide. The addition of SnO 2 has an adverse effect on the activity of all of the complex catalysts with the exception of ZnFe204. The addition of BiPO 4, like the addition of Sb204, inhibits the formation of carbon dioxide and increases the activity and selectivity of ZnFeO 4, and also that of iron and nickel molybdates. It has been suggested that the mechanism of action of the oxides consists of modification of the active centers of the ferrites as a result of oxygen spillover. The activity and selectivity of Fe-MgO catalysts promoted with potassium were investigated in [5]. As a result of such promotion, the activation energy is reduced from 194 to 156 kJ/mole. The active phase KFeO 2 is not reduced in the course of the reaction; however, commercial catalyst containing the phase KFellO17 is reduced to iron(II,II_l) oxide. It is interesting that at a comparable conversion level, the selectivity of butadiene formation over the promoted catalyst is higher than over the commercial catalyst. Institute of Bioorganic Chemistry and Petroleum Chemistry, National Academy of Sciences of Ukraine. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 3, pp. 44-48, May-June, 1997. 0009-3092/97/3303-0185518.00 �9 Plenum Publishing Corporation 185 A study has been made of the oxidative dehydrogenation of 2-butene over film oxide-iron catalysts supported on an inactive quartz plate. The film catalysts are prepared by distributing finely divided particles of iron oxide (maghemite) on the plate; hemmatite [sic] is formed on the surface of the particles, increasing the selectivity of butadiene formation [6]. In a study of the catalytic system MgFe204-MgO, obtained by impregnating magnesium oxide with solutions of organometallic complexes of iron(HI) and subsequent calcination, it was found that the magnesium ferrite particles are reduced, forming a solid solution of FeO in MgO in the volume of the catalyst, and Fe304 on its surface [7]. Catalyst deactivation was noted as a result of deposition of carbon particles on the surface. According to [8], the reasons for catalyst deactivation are carbonization of the surface, an increase of the spinel crystal size, and a decrease of the density of active centers. A great number of publications have been devoted to the kinetics and mechanism of butene dehydrogenation over ferrite catalysts. The kinetics of oxidative dehydrogenation in the presence of MgFe204 catalyst were examined in [9]. The reaction order with respect to oxygen was found to vary from 0 to 1, depending on the oxygen partial pressure. Oxidative dehydrogenation proceeds not only in the presence of oxygen, but even in its absence on an oxidized catalyst. The reaction proceeds very slowly on a reduced surface of the catalyst. It was established by the use of labeled atoms that the large isotope effect (Kn /K D --. 2) is related to rupture of a C-H bond in the limiting stage. It was concluded that the Rennard-Massoth mechanism is realized, including a redox cycle between Fe 2+ and Fe 3+, and it was also concluded that C4H7, C4H6, and OH species are present on the catalyst surface. In an investigation of the influence of diffusion within particles of an Fe-Zn-Mg catalyst with the spinel structure, it was established that under isothermal conditions, the catalyst efficiency coefficient in a rate equation of the Langmuir-Hinshelwood type may be greater than unity. The activation energy of the reaction in the region of intraparticle diffusion is the arithmetical mean value of the activation energies for 1-butene diffusion and for the chemical reaction in the kinetic region [10]. The kinetics of oxidative dehydrogenation of 2-butenes over an Fe-Te-MoO catalyst were investigated by a pulse microcatalytic method in a reactor with a vibrofluidized bed [11]. It was established that the rate of butadiene formation depends on the concentration of only the butenes, while the rates of formation of carbonyl compounds and products of exhaustive oxidation depend on the concentrations of both butenes and butadiene. These products are formed in a parallel-consecutive scheme, and the differences between 1-butene and 2-butenes in reactivity are explained by the weaker C-H bond in 1-butene, which is broken in the limiting stage of the process-in dissociative adsorption of oleflns with the formation of a normal allyl surface compound. There are two types of centers on the surface of a ferrite catalyst [12]. Some of these centers are active in the dehydrogenation reaction forming butadiene; others provide total oxidation of 2-butene to form carbon dioxide and water. The nature of the active surface centers on various ferrite catalysts was studied in a procedure involving chemisorption and temperature-programmed desorption of oxygen and n-butene. Iron(HI) oxides adsorb oxygen considerably better than does ZnFe204; however, the latter material adsorbs n-butene more readily. Mixed catalysts are capable of adsorbing both reactants [13, 14], thus playing the role of sorbents and oxygen transfer agents. As a result of a kinetic study of the oxidative dehydrogenation of butene to butadiene over an iron-containing oxide catalyst, a process scheme was proposed, with parallel--consecutive formation of carbon monoxide and dioxide. It was noted that the cations of the oxide catalyst should be those that will readily change their degree of oxidation. This is true for iron ions, since the change of Gibbs free energy in the transition o~-Fe203 -- Fe304 is relatively small [15]. The kinetic relationships of the process have also been studied for a complex magnesium-manganese ferrite catalyst. The main products from this process are butadiene and carbon dioxide, with oxygen-containing compounds as a by-product (0.2-0.4 %), these compounds having practically no influence on the oxidative dehydrogenation process. With increasing partial pressure of oxygen, the rate of butadiene formation remains very nearly unchanged, while the rate of carbon dioxide formation increases [16]. The catalytic activity of pure molybdenum trioxide in this reaction is low, even though the oxidation proceeds through the same mechanism as in the presence of molybdates, i.e., adsorption of butene on an Mo 6+ cation with the formation of an allyl complex through abstraction of a hydrogen atom by a surface oxygen ion [17]. In the oxidative dehydrogenation of butenes, high activity has been registered for catalysts based on oxides of molybdenum and bismuth. The most active catalyst is c~-Bi203"MoO 3. The molybdates Bi203-3MoO 3 and "y-Bi203-2MoO 3 have intermediate activities in this reaction, while Bi4MoO 4 is inactive [18]. In order to elucidate the role of the oxygen in complex oxides, the rate of diffusion of lattice oxygen was determined on the basis of exchange with 1802. It was established that the 186 catalytic activity corresponds to the rate of oxygen exchange and that the concentration of cation vacancies is not the decisive factor determining the catalyst activity [19]. The results of experiments with multicomponent catalysts have made it possible to expand the "bath model" for the lattice of oxides with mobile oxygen ions that has been described in the literature, and to propose a "hydroponics" model. According to this model, oleffms are adsorbed from the gas phase on active surface centers and are oxidized by mobile oxygen ions from the oxide lattice. The dynamics of surface change of Bi-Mo catalysts in the course of the reaction has been followed by measuring their electrical conductivity. It was shown in [20] that the Bi/Mo ratio on the catalyst surface differs from that in its volume. It is suggested that the oxygen first passes from the surface layers into the gas phase, after which its ions migrate from the volume to the surface. According to [21], the oxidation centers of the catalyst and of the butene are different: The oxygen enters the catalyst on certain centers and leaves, interacting with the substrate, through other centers. In addition to oxidative dehydrogenation of 1-butene on the catalyst surface, it is also isomerized to 2-butene (c/s and trans forms); also, exhaustive oxidation takes place. The process of oxidative dehydrogenation is strongly retarded in the presence of butadiene [22]. The product yields (butadiene, carbon dioxide, and cis-2-butene) increase linearly with increasing partial pressure of oxygen. The butadiene selectivity decreases with increasing temperature in the 350-400~ interval; the carbon dioxide selectivity increases with increasing oxygen pressure [23]. The activity of Bi-Mo catalysts increases upon the introduction of promoting additives. High values of the process indexes were noted in [24] for the dehydrogenation of butenes to butadiene over an Mo-Bi-Fe catalyst containing 30% Fe 2+ ions in place of Bi 3+ ions. Also effective was modification with the ions Cr 3+, Ni 2+, Pb 2+, and K + [25]. A series of communications [26-28] described effective catalysts with a complex composition: Mol_~BiCr3Ni~ K~ ,Pb0.5--SiO z, Mo~ _,N i, 5Co4Fe BiP%.,~TIo ~Si~sO4s.6, Mol2Sbo.2_2oBiu 2_2uFeo.2_t, ~ --Coc~.2_~.,X,.u~_~Y:~ o~_~ O, (where X is potassium, rubidium, cesium, or thallium; Y is selenium, zinc, niobium, vanadium, or ruthenium), etc. It was established in [29] that the structure and degree of oxidation of molybdenum in compounds with a crystallographic shift (MoO 3, Mo18Os2, Mo8032, Mo4011 ) have a significant influence on the selectivity of oxidative dehydrogenation of bntene, and do not affect the catalyst activity. In a comparative study of the kinetics and mechanism of oxidative dehydrogenation of butenes over bismuth-molybdenum and ferrite catalysts, it was established that over the bismuth-molybdenum catalysts, the oxidation rate of 1-butene was higher than that of 2-butene. When using the ferrite catalyst, the order of the reaction of normal oxidation relative to the pressures of butene and oxygen was found to be 1 and 0.25, respectively. It was concluded that isomerization over the bismuth-molybdenum catalyst proceeds through a stage of formation of carbonium ions, but over the ferrite catalyst through dissociation of allyl [30]. Molybdenum compounds had been used as the base in developing many other catalysts for butadiene synthesis, in particular catalysts for the ICC process (Chinese People's Republic). This catalyst contains molybdenum and tin. The process can be accomplished without catalyst regeneration. The butadiene selectivity is 90.1%, conversion of butenes 69.3% [31]. Significant activity is exhibited by catalysts of the Cu-Mo-AI203 type, prepared by either coprecipitation or impregnation. Catalysts prepared by impregnation have the widest range of catalytic properties; in the opinion of Tiwari et al. [32], this is related to the higher concentration of active centers in these catalysts. Considerably more active are catalysts based on phosphomolybdic acid [33, 34]. Over such a catalyst, from a mixture of butenes, steam, and air, a mixture of methacrolein and butadiene is obtained. A catalyst of this type, promoted with potassium in a ratio K/Mo = 1/12 has the highest activity. A number of publications have been devoted to the interaction of l-butene with V-Mg-Mo catalysts. The influence of vanadium oxide on the activity of Mg-Mo catalysts in a medium of steam was investigated in [35-37]. Modification of such catalysts with vanadium oxide (0.5-10%) increases their activity in the oxidative dehydrogenation when using steam as a diluent. The relationships in oxidative dehydrogenation of butene when using nitrogen or argon as diluents in place of steam have been reported in [38]. The Mg-Mo catalyst manifests the highest activity when inert diluents are used. In steam, the activity is lower by a factor of 1.5-2. 187 Certain processes of oxidative dehydrogenation of butenes are based on the use of catalysts containing vanadium. Thus, Union Oil has developed a process using a catalyst prepared by depositing compounds of alkali metals, vanadium, phosphorus, and tin on crystalline silicon dioxide [39]. In preparing these catalysts, potassium, sodium, or cesium phosphate may be used, and as the source of tin either SnCI 2, SnO, SnSO 4, or Sn(AcO) 2. These compounds are mixed in an acidic aqueous medium with the addition of crystalline silicon dioxide. The conversion of 1-butene over such a catalyst amounts to 70%, with butadiene selectivity 75%. When the support is replaced by zeolites of the type of ZSM or mordenite, the catalysts that are obtained exhibk higher activity with respect to butadiene, up to 91.8% [40]. The state of vanadium in V-Mg oxide catalyst has been studied. It was established by means of ESR that the V 4+ ions are tetrahedrally coordinated. Even when using a mixture of oxygen with 1-butene in a i000/1 ratio, reduction of the catalyst takes place, and the V 4+ ions have a coordination corresponding to the vanadyl ion VO 2+. The reaction proceeds through a redox mechanism. Taking part in the reduction stage is lattice oxygen bound to VO 3+ and V 5 + [41]. Upon hydration with water vapor, the catalyst activity is increased, with no change in butadiene selectivity [42]. Also used for the oxidative dehydrogenation of butenes is an Sn-P-Li oxide catalyst. In an investigation of the kinetic relationships for the conversion of butenes over this catalyst, it was established that the reaction rate constants decrease in the following order: Oxidative dehydrogenation of 1-butene > skeletal isomerization > selective oxidation of 2-butene > isomerization of the position of the double bond > total oxidation of 2-butene and butadiene > total oxidation of 1-butene. The rate of oxidative dehydrogenation of butene is proportional to [C4H8]~ 02, and the rate of its total oxidation is proportional to [C4H8]~ 0"7 [43]. A method was proposed in [44] for obtaining butadiene in the presence of the Sn or Fe form of fluorinated tetrasiIicate micas. The catalyst is obtained by treating the mica in the Li, Na, K, H, or Ca form with solutions of chlorides or nitrates of tin or iron. When the process is carried out over this catalyst, there is no coke deposition. Over the tin-containing catalyst, the 1-butene conversion amounts to 10%, butadiene selectivity 78.1%; over the iron-containing cat,qt.lyst, the respective values are 21% and 45%. Data on the catalytic activity of aluminum metaphosphate are reported in [45]. In an investigation of the properties of a platinum catalyst deposited on the walls of a reactor prepared from a mixture of yttrium and zirconium oxides, it was found that an oxide layer is formed on the platinum surface in the presence of oxygen. Butene is partially oxidized on the platinum surface to butadiene; when it interacts with the oxide surface, carbon monoxide and dioxide are formed, and also water [46]. Catalytic dehydrogenation of butene has also been reported when a palladium membrane is used [47]. CATALYSTS FOR THE OXIDATIVE DEHYDROGENATION OF n-BUTANE Free iron(HI) oxide is quite inactive in the process of oxidative dehydrogenation of butane, displaying activity only at temperatures above 430~ Catalysts based on ferrites are considerably more active. By impregnating magnesium oxide with solutions of complexes of iron(III), followed by calcining in air, a high-activity MgFe204 catalyst is obtained [48]. Promotion of this catalyst by potassium carbonate gives a mixed oxide KFeO 2, which inhibits the reduction of iron oxide to the flee metal [49]. However, this catalyst is unstable in air, as it interacts with carbon dioxide and water. Catalysts based on iron and antimony have been reported. A method for increasing their activity by a factor of 6 by adding dibromoethane to the reaction mixture is described in [50]. This effect is explained by the intermediate formation of Br" radicals, which, reacting with the butane, form HBr and butene. Highly active in the oxidative dehydrogenation of butane are catalysts with the formula BiaO3.nMoO3-A1PO 3 [51]. An approximately 13% yield of a mixture of butenes and butadiene is obtained at temperatures of 400-500~ with a C4H10/O 2 ratio from 0.5 to 2. By the interaction of bismuth and molybdenum oxides with surface aluminum phosphate in the process of calcination, phases are formed on the catalyst, the most effective of which are bismuth molybdates with n equal to 1 or 2. At the optimal temperature of 450~ the BiaO3.2MoO3-A1203 catalyst is considerably more effective than the Bi203.MoO 3 catalyst [52]: The yield of butadiene over the former catalyst is 7.25%, with a 23% selectivity. A study has been made of the influence of composition of a catalytic system containing nickel and molybdenum on its activity in the process of oxidative dehydrogenation of the butane. The highest yield and selectivity are obtained with a catalyst 188 consisting of nickel oxide and normal or nonstoichiometric nickel molybdate. It was suggested in [5