Metal oxides catalyst

A class of oxide catalysts which have been employed for combustion reactions, particularly hydrocarbon combustion are oxides with the perovskite structure, possessing the general formula ABO3 [60]. The activities of several unsubstituted component B oxides (BO3) have been compared with perovskite oxides for the catalytic oxidation of propylene [61], this is shown in figure 5. 

Catalytic activity is expressed in terms of the temperature at which the combustion of propylene occurred at a given rate. Catalysts below the straight line showed enhanced activity over the component oxide due to the formation of a perovskite structure. Conversely for catalysts above the line activity was reduced. Although there is a degree of scatter most catalyst are generally distributed close to the line indicating that the activity of the unsubstituted perovskite oxides are primarily determined by the nature of the B component oxide. The most active catalysts being based on the oxides of Co and Mn. 

The catalytic combustion of methane over perovskite type catalysts has been investigated by Arai et al [62]. Methane is one of the most stable alkanes and is relatively difficult to combust by virtue of the high strength of the C-H bond which must be activated. Studies were performed using relatively high space velocities in the range 45,000-50,000 h with a 2% methane feed in air. The catalytic activity, expressed as the temperature required for 50% conversion, is 

A more detailed study of the combustion of methane over a series of Mg doped LaCrOs perovskites has been reported by Saracco et al [63]. The catalysts prepared were LaCri.xMgxOs, where 0 X 0.5, and were synthesised by the method denoted as the citrate method , which briefly consisted dissolving the constituent metal nitrates in citric acid solution. After heating the solution the catalyst precursor was obtained and subsequently calcined at 1100°C to produce the final catalyst. Catalysts were screened for activity using 1.5% methane, 18% 

The combustion of other VOCs by perovskites, besides alkanes and alkenes, has also been investigated. Ling et al. [64] have studied LaNiOs catalysts for the combustion of ethanol and acetaldehyde, comparing activity of that for methane combustion. Oxidation of 1 vol.% VOC in air (total flow-100 ml min O.lg catalyst) followed the order for ease of combustion  

Another argument relating to the benefit of oxide additives is that the presence of brittle oxides, when combined with ball milling, simply causes structural refinement to help break down the hydride into smaller particles. 

Further support for the importance of variable oxidation state of the metal oxide is provided by titanium and vanadium oxide additive studies on MgH2. In a study of Mg-20 wt% rutile Ti02 prepared by RMA in a H2 atmosphere, it was found that complete H2 desorption occurred in 9 min at 350 °C in 0.1 MPa H2 (Wang et al, 2000). In the survey of a range of oxide materials by Oelerich et al. (2001), a 0.03 wt% Ti02 addition to ball milled MgH2 for 20h resulted in a material which took only 7 min for complete desorption at 573 K in vacuum, while 0.06 wt% V2O5 addition under the same conditions showed complete desorption in 5 min which is also an oxide of a multivalent metal. 

Both show similar reductions in activation energies for hydrogen desorption. 

Before getting over-excited about the importance of the electronic stmcture of the metal oxide catalyst it is worth noting the other mechanisms that may affect the kinetics of MgH2 such as the metal oxide catalysts acting as a milling aid, their high defect density, size and surface area effect, crystal structure and availability of sites for OH groups. It is therefore a complex system to which there is a temptation to oversimplify. 

Another complicating factor is the presence of physisorbed water and OH groups on the surface of the oxide additives affecting dehydrogenation of MgH2. Water reacts readily with MgH2 via the spontaneous reaction (Grosjean et al, 2006)  

Various kinds of oxide materials, including single oxides, mixed oxides, molybdates, heteropoly-ions, clays, and zeolites, are used in catalysis they can be amorphous or crystalline, acid or basic. Furthermore the oxides can be the actual catalysts or they can act as supports on which the active catalysts have been deposited. Silica and alumina are commonly used to support both metals and other metal oxide species. Amorphous silica/alumina is a solid acid catalyst, it is also used as a support for metals, when bifunctional (acid and metal) catalysis is required, e.g., in the cracking of hydrocarbons. Other acid catalysts are those obtained by the deposition of a soluble acid on an inert support, such as phosphoric acid on silica (SPA, used in the alkylation of benzene to cumene. Section 5.2.3). They show similar properties to those of the soluble parent acids, while allowing easier handling and fixed bed operation in commercial units. 

An important class of mixed oxides is constituted by zeolites. Zeolites were first defined to comprise only microporous crystalline aluminosilicates (micro-porous, pore diameter 20 A, mesoporous, 20-500 A, macroporous, 500 A). However today other microporous crystalline materials are included, such as 

Isomorphous substitution with a suitable transition metal, notably Ti, produces an oxidation catalyst. The larger ionic radius, coupled with the preference of Ti for octahedral coordination, produces strain in the silica lattice, favouring the splitting of SiO-Ti bonds by a protic molecule, e.g., H2O 

Both acidic and oxidation properties are obtained when Fe is inserted in the zeolite lattice. The deposition of discrete metal particles (e.g., Pd, Pt) on the surface leads to bifunctional catalysts with both acidic and hydrogenation properties. These are frequently used to slow down the deposition of carbonaceous residues that eventually deactivate the catalyst by blocking the pores ( pore plugging ). 

A number of commercial oxidation catalysts are based on V and Mo oxides. They are multicomponent materials, such as the mixed oxides Mo-Fe-O (oxidation of methanol to formaldehyde), V-P-O (oxidation of butane to maleic anhydride), Bi-Mo-O (propylene to acrylonitrile), Bi-Fe-Mo-O 

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Of little use commercially except as a route to anthraquinone. For this purpose it is oxidized with acid potassium dichromate solution, or better, by a catalytic air oxidation at 180-280 C, using vanadates or other metal oxide catalysts. 

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Processes have been developed whereby the oxygen is suppHed from the crystal lattice of a metal-oxide catalyst (5) (see Acrylonitrile Methacrylic acid AND derivatives). 

The vapor-phase reduction of acrolein with isopropyl alcohol in the presence of a mixed metal oxide catalyst yields aHyl alcohol in a one-pass yield of 90.4%, with a selectivity (60) to the alcohol of 96.4%. Acrolein may also be selectively reduced to yield propionaldehyde by treatment with a variety of reducing reagents. 

The elimination of alcohol from P-alkoxypropionates can also be carried out by passing the alkyl P-alkoxypropionate at 200—400°C over metal phosphates, sihcates, metal oxide catalysts (99), or base-treated zeoHtes (98). In addition to the route via oxidative carbonylation of ethylene, alkyl P-alkoxypropionates can be prepared by reaction of dialkoxy methane and ketene (100). 

Numerous patents have been issued disclosing catalysts and process schemes for manufacture of acrylonitrile from propane. These include the direct heterogeneously cataly2ed ammoxidation of propane to acrylonitrile using mixed metal oxide catalysts (61—64). 

Formaldehyde is readily reduced to methanol by hydrogen over many metal and metal oxide catalysts. It is oxidized to formic acid or carbon dioxide and water. The Cannizzaro reaction gives formic acid and methanol. Similarly, a vapor-phase Tischenko reaction is catalyzed by copper (34) and boric acid (38) to produce methyl formate ... 

Most of the world s commercial formaldehyde is manufactured from methanol and air either by a process using a silver catalyst or one using a metal oxide catalyst. Reactor feed to the former is on the methanol-rich side of a flammable mixture and virtually complete reaction of oxygen is obtained conversely, feed to the metal oxide catalyst is lean in methanol and almost complete conversion of methanol is achieved. 

Fig. 2. Flow scheme of a typical metal oxide catalyst process. S = steam CW = cooling water.

The process can be operated in two modes co-fed and redox. The co-fed mode employs addition of O2 to the methane/natural gas feed and subsequent conversion over a metal oxide catalyst. The redox mode requires the oxidant to be from the lattice oxygen of a reducible metal oxide in the reactor bed. After methane oxidation has consumed nearly all the lattice oxygen, the reduced metal oxide is reoxidized using an air stream. Both methods have processing advantages and disadvantages. In all cases, however, the process is mn to maximize production of the more desired ethylene product. 

In the vapor phase, acetone vapor is passed over a catalyst bed of magnesium aluminate (206), 2iac oxide—bismuth oxide (207), calcium oxide (208), lithium or 2iac-doped mixed magnesia—alumina (209), calcium on alumina (210), or basic mixed-metal oxide catalysts (211—214). Temperatures ranging... 

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. 

Catalytic alkylation of aniline with diethyl ether, in the presence of mixed metal oxide catalysts, preferably titanium dioxide in combination with molybdenum oxide and/or ferric oxide, gives 63% V/-alkylation and 12% ring alkylation (14). 

Isopropyl alcohol can be oxidized by reaction of an a,P-unsaturated aldehyde or ketone at high temperature over metal oxide catalysts (28). In one Shell process for the manufacture of aHyl alcohol, a vapor mixture of isopropyl alcohol and acrolein, which contains two to three moles of alcohol per mole of aldehyde, is passed over a bed of uncalcined magnesium oxide [1309-48-4] and zinc oxide [1314-13-2] at 400°C. The process yields about 77% aHyl alcohol based on acrolein. 

High Density Polyethylene. High density polyethylene (HDPE), 0.94—0.97 g/cm, is a thermoplastic prepared commercially by two catalytic methods. In one, coordination catalysts are prepared from an aluminum alkyl and titanium tetrachloride in heptane. The other method uses metal oxide catalysts supported on a carrier (see Catalysis). 

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

Chemical Properties. On thermal decomposition, both sodium and potassium chlorate salts produce the corresponding perchlorate, salt, and oxygen (32). Mixtures of potassium chlorate and metal oxide catalysts, especially manganese dioxide [1313-13-9] Mn02, are employed as a laboratory... 

Either gas- or hquid-phase reactions of ethyleneamines with glycols in the presence of several different metal oxide catalysts leads to predominandy cychc ethyleneamine products (13). At temperatures exceeding 400°C, in the vapor phase, pyrazine [290-37-9] formation is favored (14). Ethyleneamines beating 2-hydroxyalkyl substituents can undergo a similar reaction (15). 

The imida2olines can be dehydrogenated at high temperatures over metal oxide catalysts to give the corresponding imida2oles (46). 

Meta/ Oxides. The metal oxides aie defined as oxides of the metals occurring in Groups 3—12 (IIIB to IIB) of the Periodic Table. These oxides, characterized by high electron mobiUty and the positive oxidation state of the metal, ate generally less active as catalysts than are the supported nobel metals, but the oxides are somewhat more resistant to poisoning. The most active single-metal oxide catalysts for complete oxidation of a variety of oxidation reactions are usually found to be the oxides of the first-tow transition metals, V, Cr, Mn, Fe, Co, Ni, and Cu. 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

In this process ethylene, dissolved in a liquid hydrocarbon such as cyclohexane, is polymerised by a supported metal oxide catalyst at about 130-160°C and at about 200-500 Ibf/in (1.4-3.5 MPa) pressure. The solvent serves to dissolve polymer as it is formed and as a heat transfer medium but is otherwise inert. 

Methanol is converted into formaldehyde by catalytic vapour phase oxidation over a metal oxide catalyst. In one variation of the process methanol is vaporised, mixed with air and then passed over the catalyst at 300-600°C. The formaldehyde produced is absorbed in water and then fed to a fractionating column. A 37% solution of formaldehyde in water is removed from the bottom of the column with some methanol as a stabiliser whilst excess methanol is taken from the top of the column and recycled. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Dehydrogenation of ethylbenzene to styrene occurs over a wide variety of metal oxide catalysts. Oxides of Ee, Cr, Si, Co, Zn, or their mixtures can be used for the dehydrogenation reaction. Typical reaction... 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

As a final example of the application of gas-liquid-particle operation to a process involving a gaseous reactant and a solid catalyst, the possibility of polymerizing ethylene in, for example, a slurry operation employing a metal or metal oxide catalyst can be cited. It has been suggested that the good control of reaction conditions obtained in a slurry-type operation may be of importance in the production of certain types of polyethylene (Rl). 

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