SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

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1 11 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY

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3 11.1 xidation processes Gas phase oxidation processes Introduction Selective oxidation processes, in particular those that make use of solid catalysts (heterogeneous oxidation processes), play a fundamental role in the petrochemical industry. About 50% of the principal chemical products and over 80% of monomers are synthesized by means of at least one stage of selective heterogeneous catalytic oxidation. Table 1 contains a list of the main selective oxidation processes for hydrocarbons using solid catalysts, with an indication of the conversion and selectivity values obtained. In many commercial selective oxidation processes there is still scope for a significant margin of improvement in performance. For example, the potential increase in selectivity in the two main processes of selective oxidation (ethylene to ethylene oxide and propylene to acrylonitrile) could result in annual savings in reagent costs of around 800 million of euro. The action of solid catalysts in oxidation processes had already been noted by the beginning of the Nineteenth century, but it was only towards the middle of the Twentieth century that a systematic study of selective oxidation processes using solid catalysts and of their industrial applications was begun. The first processes to be developed industrially were: oxidation and ammonia oxidation (oxidation in the presence of ammonia) of propylene to produce acrolein and acrylonitrile respectively, oxidation of ethylene to ethylene oxide and the oxidation of aromatics to form anhydrides (maleic and phthalic anhydrides). The development of these processes, which was also driven by the growing demand for these types of products, led to the development of fundamental research, with a synergistic effect on both the development of new applications and the improvement of those already on the market. An example of this is the ammonia oxidation process of propylene using air and ammonia, which quickly replaced the previous process based on the reaction between acetylene and CN, both because of the lower raw material cost and the reduced safety issues. This made it possible to produce acrylonitrile with a significant reduction in costs, which resulted in a rapid expansion in the market for it between n the other hand, the success of this product stimulated the development of research into the catalysts being used (mixed Bi and Mo based oxides), resulting in their gradual improvement. The first generation catalysts, based on supported Bi 9 PMo 12 52, gave a yield of 55%, which increased to 65% with the development of second generation systems containing iron as the redox element and to about 75% with the development of third generation multi-component catalysts. The current fourth generation catalysts, containing up to 25 elements, allow yields in excess of 80% to be obtained. The development of new catalysts has brought about a comparable evolution in the type of catalytic reactors used, initially fixed bed, then bubbling fluid bed and finally braked fluid bed. In the period from development and innovation in the sector was instead driven by the growing importance attached to environmental and safety issues. owever, in the last decade of that period the introduction of new processes was heavily influenced by the reduction of investment in petrochemicals resulting from the restructuring taking place in businesses throughout the sector. Below is a summary of the principal lines of development during that time (Centi and Perathoner, 2003b). Use of new raw materials and alternative oxidizing agents. There has been an increasingly wider use of alkanes as raw materials, instead of aromatics and alkenes; for example, the synthesis of acrylonitrile from VLUME II / REFINING AND PETRCEMICALS 617

4 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY Table 1. Principal processes of selective oxidation of hydrocarbons using solid catalysts and typical results obtained (Arpentinier et al., 2001; Centi et al., 2002) REAGENT Principal product Types of catalysts Conversion * Selectivity* (%) (%) Methane/ 2 /N 3 CN Lattice of Pt-Rh C 4 or (C 2 ) x / 2 Syngas (C/ 2 ) Supported Rh or Ni Methanol/air Formaldehyde Ag on a-al 2 3, or Fe-Mo oxides Ethylene/ 2 /acetic acid Vinyl acetate Pd-Cu-K on a-al ** 92 Ethylene/ 2 Ethylene oxide Ag-K-Cl on a-al ** Ethylene/air or 2 /Cl 1,2-dichloroethane xychlorides of Cu-Mg(K) on g-al Ethanol/ 2 Acetaldehyde Ag, Cu ** Propylene/air Acrolein Bi-Mo-Fe-Co-K supported oxides Propylene/air/N 3 Acrylonitrile Bi-Mo-Fe-Co-K supported oxides Acrolein/air Acrylic acid V-Mo-W oxides n-butane/air Maleic anhydride V-P oxides n-butane/air Butenes/butadiene Bi-Mo-P oxides tert-butyl alcohol Methacrolein Bi-Mo-Fe-Co-K oxides Isobutene/air Methacrolein Bi-Mo-Fe-Co-K oxides Methacrolein/air Methacrylic acid V-Mo-W oxides Benzene/air Maleic anhydride V-Mo oxides o-xylene/air Phthalic anhydride xides of V-P-Cs-Sb on Ti Naphthalene/air Phthalic anhydride xides of V-K on Si * Conversion of the reagents and selectivity of the products compared with the hydrocarbon ** In the processes in which the operation includes recycling of the unconverted reagent, the conversion figure is for a single pass propane instead of propylene and the synthesis of maleic anhydride from n-butane instead of benzene, aimed at reducing costs and/or improving the eco-sustainability of the process. New processes that use alternative oxidants are being researched. An example is the direct synthesis of phenol from benzene (instead of the multi-stage processing of benzene with cumene as an intermediate), using N 2 as the oxidizing agent instead of 2. This is in order to reduce the complexity and the risks associated with the process, to avoid the co-production of acetone and to make use of a by-product such as N 2 (thereby also reducing its disposal costs). Development of new classes of catalysts and processes. The processes that use solid (heterogeneous) catalysts are increasingly replacing the homogeneous type, in order to reduce separation costs and the environmental impact and/or to use new raw materials, for example in the direct synthesis of acetic acid from ethane. The processes for oxidative dehydrogenation of alkanes are increasingly more competitive than those for dehydrogenation of alkenes. New processes are also being investigated which will enable the reduction or elimination of the formation of co-products and/or the formation of toxic or dangerous intermediates. An example is the synthesis of methacrylic acid through direct isobutane oxidation, as an alternative to the commercial acetone cyanohydrin process, which uses CN as a reagent and co-produces ammonium sulphate. 618 ENCYCLPAEDIA F YDRCARBNS

5 XIDATIN PRCESSES Conversion of processes based on the use of air into processes based on the feeding of pure oxygen. These processes enable a reduction in polluting emissions; as examples there are the synthesis of formaldehyde from methanol, the epoxidation of ethylene and the oxychlorination of ethylene to 1,2-dichloroethane. Improvement of the productivity of the processes. This is the result of the development of new generation catalysts with improved properties and/or improvements in the engineering of the reactors (for example, the introduction of a monolithic reactor in the synthesis of formaldehyde, or of structured-bed reactors in the synthesis of phthalic anhydride). Moreover, during the period from there was a significant increase of interest in the development of new reactor technologies (such as, for example, membrane reactors), which made it possible to achieve savings in processing even for small-medium scale production (scale-down of the processes; Centi and Perathoner, 2003a). The goal was to decentralize production and reduce its environmental impact, in contrast with the trend typical of the Twentieth century of achieving savings in processing costs through increases in scale and high integration in large petrochemical facilities. This came about due to the high environmental impact and the strong public opposition to the latter approach, as well as due to problems linked to a sluggish market with large fluctuations in demand. Selective catalytic oxidation processes can be divided into three categories. The first relates to oxidation of inorganic molecules (for example, oxidation of ammonia to N and of 2 S by sulphur). The second class relates to synthesis of basic chemical products (for example, ammonia oxidation of methane by CN or the partial oxidation of methane by syngas; C/ 2 mixtures). Finally, the third category relates to conversion of hydrocarbons by processing in the liquid phase (principally in the homogeneous phase even if there is a growing interest in the use of heterogeneous catalysts) and processing in the gas phase, which is the most commonly used industrially (see again Table 1). It should be pointed out that this last class of processes uses air or 2 as the oxidant (other than the cited process of direct hydroxylation of benzene by phenol with N 2 ), while in the liquid phase processes, in addition to 2, extensive use is also made of other oxidizing agents such as alkyl peroxides and 2 2 (Centi and Perathoner, 2003b). The different categories of gas phase selective oxidation processes (over solid catalysts) and the related principal industrial reactions are summarized in Table 2 (Arpentinier et al., 2001; Centi et al., 2002). Some important classes of reactions, which are not mentioned in the table, since they are not yet used commercially, include: oxidative dehydrogenation of C 2 -C 5 alkanes to their corresponding olefins; selective oxidation of alkanes such as synthesis of phthalic anhydride and maleic anhydride from n-pentane, of acrylic acid from propane and of methacrolein or methacrylic acid from isobutane; and ammonia oxidation of propane into acrylonitrile. The catalysts used for these reactions can be classified on the basis of their characteristic reaction mechanisms. Allylic oxidation. For these reactions catalysts based on mixed oxides of transition metals are used. These catalysts are capable of selectively extracting a hydrogen atom by breaking a C bond in the allyl position and if necessary replacing it with an oxygen atom. Industrial catalysts are generally multi-component (for example, Bi-Mo oxides, used in the synthesis of acrylonitrile from propylene, contain various promoters such as Fe, Cu, W, Te, Sb and K), but typically a principal phase can be identified (Bi-molybdate) which is able to catalyse different reactions, such as: the synthesis of acrolein from propylene, the ammonia oxidation of propylene to acrylonitrile, the dimerization of propylene to cyclohexene and the oxidative dehydrogenation of butenes to butadiene. These reactions are characterized by a common first stage of allylic oxidation (Fig. 1), where the extraction of a hydrogen atom in the allyl position gives rise to a chemisorbed p-allylic complex on the transition metal. The nature of the subsequent stages determines the type of reaction and product that is obtained. xidation and ammonia oxidation of a side chain of alkyl aromatics (for example, the oxidation of toluene to benzaldehyde or benzonitrile respectively) in principle follow a similar reaction mechanism, but the interaction of the aromatic ring with the surface is different and therefore different types of catalysts are used, such as vanadium oxides supported on Ti 2 or catalysts based on molybdate of Fe-(V, P, K). Nucleophilic oxidation to the C group (oxidative dehydrogenation of alcohols and oxidation of aldehydes to acids). Although this type of reaction has similarities to the mechanism previously described, there are various types of substrates such as alcohols (methanol) or aldehydes (acrolein or methacrolein) which interact too strongly with the surface of the catalyst when catalysts belonging to the first category are used. In the conversion of methanol into formaldehyde, the catalyst most often used on an industrial level is iron molybdate (which also contains other components in small quantities), while multi-component catalysts, based on Mo-V oxides or heteropolyacids of P-Mo-V, are used for the conversion of aldehydes into their corresponding acids. Electrophilic insertion of an oxygen atom. The catalysts for this category of reaction are highly specific. Examples are the systems based on Ag/a-Al 2 3 for the VLUME II / REFINING AND PETRCEMICALS 619

6 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY C C C C C C C C C Me Me Me Me Me Me Me Me anionic vacancy Bi Mo Bi Mo Bi Mo p-allylic complex Fig. 1. General outline of the allylic oxidation mechanism and example of the oxidation of propylene on bismuth molybdate to obtain acrolein. synthesis of ethylene oxide from ethylene (this catalyst, for example, when applied to the synthesis of propylene oxide from propylene, is not selective) and Fe/ZSM-5 for the hydroxylation of phenol with N 2 as the oxidant. xidation (or ammonia oxidation) of alkanes. In this case the slow stage is the initial selective activation of the alkane, for example for the concerted extraction of a hydrogen atom by a surface Lewis site (a transition metal) and of a second hydrogen atom by a base site (oxygen atoms) to give an alkene, which is immediately converted into an oxygenated product through oxidation or allylic ammonia oxidation mechanisms. Catalysts with properties which differ from those of catalysts belonging to the first reaction category are necessary Table 2. Different classes of gas phase selective oxidation processes (on solid catalysts) and the relative industrial reactions (Arpentinier et al., 2001; Centi et al., 2002) Allylic oxidation Type of reaction xidative dehydrogenation Examples propylene to acrolein or acrylic acid isobutene to methacrolein or methacrylic acid Synthesis of the acids can be carried out in a single stage from the alkene, but commercially it is preferred to use two stages for the best possible selectivities butenes to butadiene and isopentenes to isoprenes methanol to formaldehyde isobutyric acid to methacrylic acid Electrophilic insertion of an oxygen atom epoxidation of ethylene to ethylene oxide with 2 direct synthesis of phenol from benzene with N 2 Acetoxylation synthesis of vinyl acetate from ethylene and acetic acid xychlorination synthesis of 1,2-dichloroethane from ethylene and Cl in the presence of 2 Ammonia oxidation Synthesis of anhydrides propylene to acrylonitrile isobutene to methacrylonitrile a-methylstyrene to atroponitrile n-butane to maleic anhydride o-xylene to phthalic anhydride 620 ENCYCLPAEDIA F YDRCARBNS

7 XIDATIN PRCESSES because of the weak interaction of the substrate with the surface, and the activation mechanism. For example, catalysts based on vanadyl pyrophosphate are used for the oxidation of n-butane to maleic anhydride, or those based on vanadium antimonates are used for the ammonia oxidation of propane. In the latter case, antimony oxide is active in the ammonia oxidation of propylene, but is not able to activate the propane molecule; the addition of V gives the system the capability of oxidizing the alkane. Wacker-type oxidation mechanism. Vinyl acetate is produced through acetoxylation of ethylene with acetic acid in the presence of oxygen, with catalysts based on supported Pd/Au. Pd supported on V 2 5 /Al 2 3 or V 2 5 /Ti 2 is selective in the gas phase synthesis of acetaldehyde from ethylene or of methylethylketone from 1-butene, with a similar reaction mechanism. xychlorination. 1,2-dichloroethane is produced commercially from ethylene, C1 and 2 on supported copper chloride based catalysts. The mechanism consists of a direct addition of chlorine atoms by the catalyst onto the olefin, rather than in oxidation of hydrochloric acid to molecular chlorine, followed by chlorination of the double bond. Addition of oxygen to the aromatic nucleus, with ring opening. The electrophilic attack of oxygen on hydrocarbon substrates typically leads to the formation of carbon oxides, however in the case of the oxidation of benzene, selective oxidation to maleic anhydride is obtained. This process, which employs catalysts based on mixed vanadium and molybdenum oxides, has been partially replaced by the synthesis by oxidation of n-butane. Similar catalysts are used in the selective oxidation of polyaromatic compounds. Non-classic oxidation mechanisms. Ethylbenzene can be oxidatively dehydrogenated, with high selectivity, to styrene on various catalysts such as oxides and phosphates, but the active phase is constituted by the formation of a thin surface layer of carbon containing the active sites of the reaction. Recently even some types of carbon and carbon nanotubes have shown high selectivity in oxidative dehydrogenation of ethylbenzene to styrene. Another example is the ammoximation of cyclohexanone (to cyclohexanone oxime) over amorphous silica. Characteristics of gas phase oxidation processes General aspects Although the petrochemical industry uses selective oxidation processes both in the gas phase and in the liquid phase, those in gas phase are more widespread. Typically, oxygen (or air) is used as the oxidizing agent, although the species involved in the selective oxidation reaction are usually made up, not of adsorbed oxygen on the catalyst, but rather of the structural oxygen of the catalyst (typically mixed oxides, see again Table 1). The 2 oxygen ion removes the hydrogen atoms from the hydrocarbon with the subsequent formation of water or, if inserted into the molecular structure of the reagent, it gives rise to the formation of oxygenated compounds (see again Fig. 1). Instead, the gaseous oxygen intervenes in the reoxidation mechanism of the reduced catalyst, known as the Mars-van Krevelen mechanism (Fig. 2). The oxidation of the catalyst by 2 comes about through the formation of intermediate oxygen species such as 2 and, which have electrophilic characteristics and tend to make an addition to the M 2 m reduced catalyst oxide (catalyst) oxidized catalyst M 1 n oxidized product hydrocarbon Fig. 2. Mars-van Krevelen mechanism of selective oxidation of hydrocarbons on oxide based catalysts. A B M (n1) M n 2 M n M n 2 M (n1) M n 2 electrophilic oxygen species ( 2,,...) 2 activation nucleophilic oxygen species ( 2 ) C C C C 2 2 products with rupture of C C bond and formation of C x products of selective oxidation (e.g. aldehydes) Fig. 3. A, schematic mechanism of the incorporation of oxygen into oxide based catalysts; B, outline of different types of attack on the hydrocarbon by nucleophilic and electrophilic species of oxygen (Centi et al., 2002). VLUME II / REFINING AND PETRCEMICALS 621

8 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY unsaturated molecule, breaking the double bond and ending with the formation of carbon oxides; in contrast, the structural oxygen of the catalyst ( 2 ) has nucleophilic characteristics (Fig. 3). ence, in order to be selective, a catalyst must not only possess activation sites for the hydrocarbon and for selective insertion of the oxygen on the substrate, but must also be rapidly reoxidizable. This is so as to prevent the non-selective chemisorbed oxygen species from having a lifetime long enough to allow combustion reactions to take place. This mechanism is generally accepted for the oxidation of alkenes on mixed oxides, but there are doubts about its validity in the case of oxidation of other substrates, such as alkanes. A general characteristic of selective oxidation processes of hydrocarbons is the complexity of the reactions involved. For example, the oxidation of n-butane to maleic anhydride is a reaction which involves 14 electrons, the removal of 8 hydrogen atoms and the insertion of 3 oxygen atoms on the substrate, with the involvement of another 4 oxygen atoms of the catalyst to form 4 molecules of water. Notwithstanding the complexity of the transformation, the reaction takes place without the formation of products with intermediate levels of oxidation; selectivity of between 70 and 85% is achieved, depending on the reaction conditions. Therefore the catalyst, made up of a mixed oxide of V and P with the composition (V) 2 P 2 7, possesses characteristics such as to avoid both the desorption of the reaction intermediates and their non-selective transformation into carbon oxides. Finally, another characteristic of selective oxidation catalysts is their multi-functionality, which is necessary for the transformation of the hydrocarbon into the final product; in fact, to bring about the complex mechanism described above, it is necessary that the catalyst be capable of actuating different types of transformations on the substrate (Arpentinier et al., 2001). Moreover, it is necessary for the different stages involved in the transformation to have similar rates. Different relative rates could lead to the desorption of intermediate products or an increase in the rate of parallel reactions, with a reduction in the selectivity for the desired product. This is well illustrated in the selective oxidation of n-butane (Fig. 4). Combined design of the catalyst and the reactor ptimizing the yield, productivity and selectivity of selective oxidation reactions requires not only a detailed knowledge of the nature of the catalyst and the mechanism of the interaction of the reagents and products with the catalyst itself, but also the optimization of the reactor used. Recently, new reactor solutions (which enable, for example, the separation of the two n-butane maleic anhydride C x, 2 concerted abstraction of 2 atoms isomerization allylic abstraction 1,4-oxygen insertion allylic dehydrogenation and/or allylic oxygen insertion oxygen insertion Fig. 4. utline of the reaction mechanism in the selective oxidation of n-butane to maleic anhydride on catalysts based on (V) 2 P 2 7, showing the multifunctional character of the catalyst. stages of interaction of the catalyst with the hydrocarbon and with oxygen) have led to the development of new classes of catalysts. The two aspects, involving the development of the catalyst and the engineering of the reactor, are therefore closely correlated. Industrial reactors used in the petrochemical industry for highly exothermic reactions, such as those for selective oxidation, are typically either of the multi-tubular fixed-bed or fluid-bed type. Nevertheless, there is a growing interest in the development of new reactor solutions, such as for example, the circulating fluid-bed reactor, recently applied by DuPont to the synthesis of maleic anhydride from n-butane. The uncoupling of the two redox reactions, of oxidation of the hydrocarbon by the catalyst and the reoxidation of the latter by oxygen (see again Fig. 2), makes it possible to increase the selectivity towards maleic anhydride compared with the reaction carried out in the simultaneous presence of both the hydrocarbon and oxygen. ther advantages of this type of reactor are isothermicity and a reduction of the risk of explosion. Nevertheless, a limiting factor is its low productivity; in fact it is necessary to circulate large quantities of the catalyst (equal to about 1 kg per g of maleic anhydride produced) between the two reactor vessels, each of which is adapted to one of the reaction stages. Another example of a new reactor configuration, adopted for petrochemical processes, is the monolithic type of reactor; these reactors combine the advantages of the possibility of autothermic conduction of the reaction 622 ENCYCLPAEDIA F YDRCARBNS

9 XIDATIN PRCESSES and a reduction in the loss of pressure. The new generations of processes for oxidation of methanol to formaldehyde use a final adiabatic stage (post-reactor), with a catalyst structured in the form of a monolith. Very interesting results have been obtained using reactor with extremely short contact times (on the order of milliseconds, compared with times measured in seconds in conventional reactor), where the catalyst configuration is also of a non-conventional type (for example, in a grid form). Given the high spatial rates used (that is the high ratio between the input rate of the reagents and the quantity of the catalysts) and the type of mechanism involved, it is possible to avoid subsequent oxidations and therefore to obtain high selectivity of the intermediate products (for example, in the oxidative dehydrogenation of alkanes into alkenes). Finally, it is worth remembering the developments in the field of catalytic membrane reactors, which allow the continuous removal of one of the products or the differential addition along the catalytic bed of one of the reagents (for example, oxygen). This makes it possible to maintain the optimum hydrocarbon/ 2 ratio along the entire profile, to limit the formation of hot spots and to control the state of oxidation of the catalyst. Nevertheless, one of the current limiting factors is its low productivity, apart from the high cost of the membrane itself. Even conventional fixed-bed reactors can be improved through greater integration of the design of the catalyst and that of the reactor. In the process of the synthesis of phthalic anhydride from o-xylene, specialized catalytic beds are used, that is, containing different layers of catalysts each having a different composition, in order to optimize the axial activity and selectivity profile of the catalyst itself. A new reactor technique being developed consists of systems in which the flow is periodically reversed; this makes it possible to make the activity and temperature profile in the reactor more uniform, even though there are still some significant problems involving the difficulty of managing non-stationary operations and their potential danger. Also in this case, the design of the catalyst is different from that for operations in stationary conditions. Use of air and pure oxygen as oxidizing agents Currently, air is the most widely used reagent in gas phase oxidation processes, but there is a growing interest in the use of pure 2 as a means of increasing the productivity and reducing pollutant emissions and energy consumption. Table 3 illustrates an example of the emissions from the process of oxychlorination of ethylene, where air and oxygen are used as the oxidizing reagents. The significant reduction in the environmental impact of the second type of process can be seen. The following gas phase processes use pure 2, or air enriched with oxygen, as an alternative to air: a) partial oxidation (to syngas) of heavy fractions from the distillation of petroleum; b) oxidation of methanol to formaldehyde (air or enriched air); c) oxidation of ethylene to ethylene oxide (air or oxygen, the latter particularly in new plants); d) oxychlorination of ethylene to 1,2-dichloroethane (air or oxygen, the latter particularly in new plants); e) acetoxylation of ethylene to vinyl acetate (oxygen); f ) oxidation of n-butane to acetic acid (air or oxygen); g) oxidation of ethylene to acetaldehyde (air or oxygen); h) oxidation of acetaldehyde to acetic anhydride (air or oxygen); and i) ammonia oxidation of propylene to acrylonitrile (oxygen enriched air). Table 3. Composition of the emissions from the process of oxychlorination of ethylene where air and oxygen are used (Arpentinier et al., 2001). DCE: 1,2-dichloroethane; VCM: vinyl chloride monomer Component Process using air Process using 2 Content (vol%); flow (m 3 /h) 2 +Ar 4-8; 400-2, ; 25 Ethylene ; ; 50 C x (C 2 /C=3-4/1) 1-3; ; 300 DCE and chlorinated compounds ; ; 10 N 2 remainder remainder Waste (m 3 /h)* 10,000-30,000 1,000 *Approximately m 3 per t of VCM produced VLUME II / REFINING AND PETRCEMICALS 623

10 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY Selective oxidation catalysts for hydrocarbons Characteristics of oxidation catalysts xidation catalysts belong to a wider class of materials having redox or oxidoreductive type characteristics; systems which catalyse reactions of hydrogenation, dehydrogenation, halogenation and dehalogenation also belong to this class. The most important catalysts in the field of petrochemicals for the oxidation of hydrocarbons for processes carried out in the gas phase are listed in Table 1. In addition to these, it is worth mentioning catalysts used for the oxidation of inorganic compounds, such as those employed for the oxidation of S 2 to S 3 (based on supported vanadium oxide), of ammonia to N (based on Pt/Rh) and of hydrogen chloride to molecular chlorine (based on supported copper chloride). Below is a list of the principal characteristics of oxidation catalysts for gas phase reactions. Presence of a transition metal as the principal active component (V, Mo, Cu, Fe, Pd, Pt, Rh, Ag). ften in these cases, a second element is also present which can be transition or post transition (for example, P, Sb or Bi), which contributes to establishing the reactive characteristics of the catalyst. This effect can be explained by the formation of a mixed oxide (that is, of a specific compound, such as for example Bi 2 Mo 2 9, possibly only on the surface of another oxide, of a solid solution or of an oxide doped with the other element), with reactive characteristics different from those of the single elements, if present in distinct phases. In some cases the element is initially present in a metallic form, but under reaction conditions it can generate the corresponding oxide (or chlorides or oxychlorides). Presence of small quantities of promoter (or doping ) elements. The purpose of these elements is to optimize the performance of the principal active elements. The nature of the promoters can vary and they can therefore play different roles in the transformation of the reagents. The active elements and the promoter elements, constitute the active phase, that is the phase directly involved in the transformation of the reagents into products. Presence of a support (usually silica, alumina or titanium oxide). This support in the catalyst s formulation can fulfil a variety of tasks. A primary task is that of dispersing the active elements, conferring a larger surface area to the active phase compared with what would have existed in the absence of the support. It is clear, therefore, that the support must have surface area characteristics suitable for the reaction of interest. In selective oxidation, where the selectivity in the formation of the partially oxidized product is heavily dependent on the subsequent reactions to undesired products (for example, carbon oxides, which are thermodynamically favoured), a support is needed with a surface area which is not too large. In this way the rate of the undesired secondary reactions, which are also dependent on the time needed by the product to diffuse from the active centre into the gas phase, is limited. A further task of the support is that of providing the resistance of the active phase to phenomena which can cause abrasion or disintegration, especially for those applications which involve particular mechanical stresses on the catalyst (for example, in fluidized-bed reactors), in addition to avoiding powdering during the loading of the catalyst into packed fixed-bed reactors. Finally, in some cases the support serves to alter the characteristics of the intrinsic chemical reactivity of the active phase, through the effects of the interaction between the latter and the support itself. This comes about when the support presents functional groups on its surface which can lead to the formation of chemical bonds with the elements of the active phase, or it takes place as a result of particular crystallographic similarities between the surface and the support. These interactive effects can be positive for the reactivity of the catalyst itself, altering its oxidoreductive characteristics or reducing its volatility; the undesirable effects of loss by sublimation of components of the active phase are thus reduced. The most suitable combination of the type and number of active phases (including the promoters) and the type of support is dependent on the characteristics of the reaction and the type of reactors used. In particular, below are listed the factors that have the greatest influence on the formulation and the morphology of the catalyst used for oxidation reactions. Type of chemical transformation involved and mechanism through which it takes place. With increasing complexity of the transformation the composition of the catalyst also becomes the more complex, in terms of the number of elements making up the active phase, or of the structural complexity (the formation of crystalline phases having multi-functional characteristics). For example, catalysts used for oxidation or allylic ammonia oxidation always contain Mo as the principal element for the active phase, while catalysts for the synthesis of anhydrides or of acids almost always contain V. ptimization of the redox characteristics or of the acidity or basicity properties of the catalyst. The promoters (or doping agents) can play a fundamental role in the control of these properties. Promoters with base-type characteristics (alkaline or alkaline earth metal oxides) can reduce the surface acidity of the active phase, with a consequent improvement of selectivity through the suppression of the acid-catalysed reactions (cracking, formation of oligomers of unsaturated compounds). Promoters with acid-type characteristics can reduce the interaction between the active phase and 624 ENCYCLPAEDIA F YDRCARBNS

11 XIDATIN PRCESSES intermediates of the reaction which have acid-type characteristics, thus favouring their desorption into gas phase and limiting the contribution of the subsequent undesired reactions. ther promoters can optimize the oxidoreductive properties of the active phase, by a modification of the overall electronic properties of the solid. Reaction scheme. The presence of consecutive reactions (typically, combustion reactions of the desired product, or reactions which lead from the reagent to the desired product through the formation of intermediate products with an increasing state of oxidation) involves the use of a catalyst with characteristics such as to limit (or, alternatively, to favour) the contribution of these reactions. This can be achieved not only by control of the intrinsic activity of the catalyst, but also by a modification of the porosity of the active phase (and therefore of the support, if present). igh surface area and porosity values entail effective intra-particle residential times which are much higher that those calculable from the feeding capacity of the reactor, and therefore a significant contribution from the consecutive reactions for a given conversion of the reagent. This can have a considerable influence on the selectivity of the desired product. Reaction heat levels. ighly exothermic reactions involve the need both for fluidized-bed catalytic reactors, which are more efficient in removing heat than multitubular reactors, and for catalysts capable of operating in conditions of high mechanical stress. In these cases fluidizable supports are used, which feature particles with an average diameter of between 50 and 150 mm, resistant to abrasion and with an appropriate density. For medium-low heat levels, tubular or tube-bundle (multi-tubular) reactors can be used. In these cases the catalysts have a characteristic morphology for these applications and are produced in the form of extrusions (or pellets). When possible, supports with a high heat conduction capacity are used, such as SiC, in order to assist the dissipation of the heat from the reaction. Spatial rates in the reactor. igh spatial rates in packed catalytic beds can lead to a high loss of pressure, and therefore to the need for heavy compression of the flow upstream of the reactor. It is possible to minimize the loss of pressure by increasing the vacuum level in the catalytic bed, through the use of special structures of the catalyst particles. This can be instrumental in conditioning the performance of the process, as in the case of oxidative dehydrogenation of methanol to formaldehyde. In this case, the use of cylindrical pellets with an axial hole enables the loss of pressure to be reduced and therefore, the linear rate in the reactor to be increased for a given rate of feeding. This involves shorter contact times, better reaction temperature control and reduced catalyst deactivation effects. xidation catalyst mechanisms in gas phase The principal catalytic oxidation mechanisms are listed in Table 2. The redox type mechanism is the one that is used in the majority of oxidation reactions. It works through a series of successive stages, which include: the adsorption of the reagent (the substrate to be oxidized) on the active centre; the transfer of electrons from the reagent to the active centre and the simultaneous transfer of oxygen ions from this to the reagent (the oxygen is incorporated into the substrate, or alternatively returns in the formation of co-produced water); and finally, the desorption of the product. The same sequence of stages involves the molecule of oxygen for the catalyst reoxidation stage: co-ordination at the metallic centre; transfer of electrons (up to 4 for each oxygen molecule); dissociation of the molecule into two atomic species in ionic form; finally incorporation of the oxygen in its ionic form within the active phase. ne or more active centres may be involved for each reagent molecule, depending on the following factors: a) the overall number of electrons transferred and therefore of oxygen ions involved in the oxidoreductive process; b) the ionic and electronic conduction capacity of the solid, and therefore of the surface active phase under reaction conditions; c) the level of cover of the active phase by the adsorbed molecules (reagents and products); d) the surface mobility of the reaction intermediates; and e) the number of active centres close to the one in which the activation of the hydrocarbon took place. n the basis of the redox model, the selectivity of the process, that is the relationship between the quantity of the product formed and the total quantity of reagent transformed, can be traced back to two different situations. First and foremost the selectivity depends on the nature of the oxygen ions present as species adsorbed on the active phase and on the interaction between them and the reagent or the reaction intermediates. As previously stated, the 2 species, incorporated in the lattice of the oxide, is considered to be the selective species, while the 2 and species have electrophilic characteristics and are considered to be non-selective species. Since the formation of the first species comes about by the intermediate formation of the electrophilic species, it is clear that the transformation rate of each of them and their reactivity with the reaction intermediates obtained through activation of the substrate determine the selectivity of the process (Bielanski and aber, 1991). Moreover, the selectivity of the oxidation process is traceable to the concentration of 2 species incorporated in the metal oxide lattice, and therefore directly to the average state of oxidation of the catalyst (Grasselli, 2002). A strongly oxidized catalyst has a high density of active centres capable of receiving electrons VLUME II / REFINING AND PETRCEMICALS 625

12 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY from the substrate and of releasing 2 ions, and therefore it is able to transform that substrate into molecules with a high state of oxidation (for example, into combustion products). In contrast, a catalyst made up of a partially reduced oxide has a modest oxidizing capacity, and therefore is potentially more selective for the partially oxidized products. According to the redox model, the state of oxidation of a metal oxide in a stationary state is dependent on the conditions of the reaction; this implies that the selectivity in its turn is dependent on the operating parameters, such as the composition of the feed (that is the relationship between the substrate to be oxidized and the oxidizing agent) or the reaction temperature. Both models have been experimentally verified for different oxidation reactions, and still remain valid today for the explanation of the selectivity of oxidation processes involving reactions with redox type mechanisms. Principal industrial processes and relevant applications xidative dehydrogenation of methanol to formaldehyde Formaldehyde (C) is among the top twenty chemical compounds produced on a world scale, and is used in the synthesis of various resins (ureaformaldehyde, phenol-formaldehyde, and polyacetals) which find applications in the construction, automotive, textile and paper sectors. Methanol can be converted into formaldehyde both by direct oxidative dehydrogenation: C C 2 =155 kj/mol or by dehydrogenation combined with oxidation of the 2 product: C 3 C 2 = 84 kj/mol =238 kj/mol The two processes differ in their operating conditions and type of catalysts. In the first process low concentrations of methanol are used in the feed, in order to avoid the formation of explosive mixtures and to control the temperature of the reaction. Commercial catalysts are based on iron molybdate, but also contain an excess of molybdenum (Fe 2 (Mo 4 ) 3 Mo 3 ), since the presence of molybdenum oxide is a necessary condition for high selectivity. Typically a ratio of Mo/Fe within the range of is used; occasionally oxides of Co and Cr are added as promoters. The excess of molybdenum is also necessary because the sublimation of the oxide (particularly at the points of greatest overheating) cause the progressive depletion of the Mo in the catalyst and the condensation of Mo 3 in the coldest parts of the reactor. This induces, not just the deactivation of the catalyst, but also a progressive increase in pressure loss. Reaction temperatures are typically within the range of C, with conversions in excess of 98% and selectivity equal to 92-95%. Multi-tubular fixed-bed reactors are generally used. A recent development has seen the introduction of a final (post-reactor) adiabatic stage. In dehydrogenation combined with partial combustion of 2 (the overall process turns out to be partially exothermic) an understoichiometric oxygen current is fed in, to operate in the upper region at the limit of flammability. Due to the thermodynamic limits of dehydrogenation, it is necessary to operate at higher reaction temperatures than those for oxidative dehydrogenation. In this process use is made of Ag based catalysts supported on alumina with a low surface area, typically in spherical form with a diameter of 1-5 mm. If operating at temperatures in excess of 600 C (particularly C), it is possible to obtain an almost total conversion of the methanol, while at lower temperatures ( C) the conversion is less efficient (65-75%) and it is necessary to recycle the methanol which failed to react. Moreover, it is necessary to use short contact times in order to avoid decomposition of the formaldehyde. Selectivity to formaldehyde of 98-99% is obtained, with the formation of the following by-products: dimethylether ((C 3 ) 2 ), whose formation is due to the presence of acidic sites in the catalyst; methyl formate (CC 3 ) obtained through disproportionation of the formaldehyde on basic sites; carbon oxides, derived from both parallel and serial reactions. To limit the formation of carbon oxides, rapid cooling of the reaction products is necessary when they leave the catalyst bed. igh selectivity is achieved through the optimization of the acid-base properties of the catalyst, limitation of the oxidation of the formaldehyde to formic acid (a product which decomposes easily) and control of the redox properties of the catalyst. Fig. 5 illustrates the reaction mechanism in the case of direct oxidation of methanol on oxide based catalysts. The methoxy species is the first chemisorbed species that is formed by the contact of methanol with the catalyst; its subsequent transformation depends both on the reaction conditions and on the properties of the catalyst. If the concentration of methanol is high and the rate of the subsequent oxidation of the methoxy species is low, a condensation reaction takes place that leads to dimethylether (typical of the acidic oxides containing non-reducible cations, such as alumina). xidation of the methoxy species (extraction of an atom of and transfer of an electron) leads to co-ordinated formaldehyde, which is in equilibrium with the 626 ENCYCLPAEDIA F YDRCARBNS

13 XIDATIN PRCESSES C 3 C 3 C 3 2 dimethoxymethane C 3 C 3 C 2 C 2 formaldehyde C C 3 2 C 2 C 2 2C 3 [] metoxy [] dioxymethylene methylformate 2 disproportionation formic acid C CC 3 C x C 2 C 3 formate oxygen vacancy Fig. 5. Reaction mechanism of the oxidation of methanol on oxide based catalysts (Centi et al., 2002). dioxymethylene species. The reaction requires a nucleophilic attack by the catalyst s structural oxygen. If the Me bond is of a covalent type, the equilibrium between the dioxymethylene and the co-ordinated formaldehyde is shifted towards the latter species, which in its turn is in equilibrium with the gas phase formaldehyde. Ionic oxides, instead, favour the formation of the dioxymethylene species. This gives rise to the methoxy and formate species by Cannizzaro disproportionation; the formate can also form through direct oxidative dehydrogenation from the dioxymethylene. By reactions with methanol, dioxymethylene forms dimethoxymethane, which can desorb into gas phase. The formate species can also react with methanol to give methyl formate. The formation of this product requires that the desorption rate of the formaldehyde be low and that the oxidation/disproportionation rate be relatively high. Both these reaction pathways contribute to the formation of the formate species. Their relative rates depend on the degree of surface coverage of the dioxymethylene species and on the reaction conditions. Nevertheless, the formate species can also be converted and therefore the selective formation of methyl formate requires that the conversion rate of the formate species be low and that the concentration of methanol be relatively high. A simplified outline of the commercial processes for converting methanol to formaldehyde using iron molybdate and supported Ag as the catalysts is shown in Fig. 6. xidative dehydrogenation of methanol on iron molybdate based catalysts is carried out in a cooled multi-tubular reactor (see again Fig. 6 A). Due to the progressive deactivation of the catalyst (the lifespan is from 1-2 years), it is necessary to gradually increase the temperature of the reactor in order to keep the productivity at a constant level. The formaldehyde yield is about 95-96%. The oldest processes operate with a feed of 6% of methanol in air (a concentration which is less than the lower limit of flammability), but in this case productivity is low, the purity of the formaldehyde is poor because of the formation of formic acid, the catalyst s lifespan is limited and it is necessary to operate with high volumes of inert gas. For this reason, about half of the plants have been converted to a feed with reduced levels of oxygen (10%), and higher concentrations of methanol (8.2%). n account of the high amount of heat generated, it is necessary to dilute the catalyst and improve the efficiency of the reactor s cooling system. Some plants also use steam as a diluent. When the gas leaves the reactor vessel, it undergoes a heat recovery stage, after which it is sent to an absorber column, where water is used as a solvent. The formaldehyde solution, from the bottom of the absorber column, has a concentration of from 50-60%; it is then sent to an ionic exchange column for removal of the formic acid. n the other hand, the gas from the top of the column is recycled; the waste stream keeps the composition in the reactor constant. The process using supported Ag as the catalyst (see again Fig. 6 B) has the advantage, in comparison with the direct oxidation method, of producing a waste stream which can be sent directly for incineration, because it contains 2, methanol and formaldehyde, as well as small quantities of N 2 and C 2. The combustion of this flow produces the majority of the steam used in the process. The outline of the process is similar to that discussed previously, however a purer methanol feed is VLUME II / REFINING AND PETRCEMICALS 627

14 SYNTESIS F INTERMEDIATES FR TE PETRCEMICAL INDUSTRY recycle vent 2 using chlorohydrin, which is no longer used, and direct oxidation of ethylene with oxygen over supported Ag based catalysts A methanol B air methanol waste steam 2 air recycle catalyst formaldehyde solution cooling water formaldehyde solution vent Fig. 6. Simplified outline of the process of the synthesis of formaldehyde from methanol: A, direct oxidative dehydrogenation with catalysts based on iron molybdate; B, a process using catalysts based on supported Ag (Arpentinier et al., 2001). 2 required (there must be no iron carbonyls or sulphur compounds which would contaminate the catalyst), and a heat exchanger is required to heat the methanol. Furthermore, the reactor must operate with very short contact times and have a rapid cooling system (cooling times for the discharge flow from the catalyst bed of less than 0.02 s), in order to avoid consecutive reactions of the formaldehyde, and finally two absorber columns in series should be used. Epoxidation of ethylene to ethylene oxide Ethylene oxide is an intermediate product that is synthesized on a large scale, being used in the production of ethylene glycol and polyglycols, of ethanolamine and non-ionic detergents, and of esters of ethylene glycol. There are two technologies: a process C 2 C CC kj/mol CsCl and BaCl 2 are used as promoters (the concentration of these doping agents is of 0-10 ppm), since both the alkaline metals and the chloride ions promote the selectivity. In addition to C 2, the by-products are acetaldehyde and formaldehyde, but in concentrations no greater than 0.1%. Although there are still conflicting opinions on the matter, a significant body of data indicates that the active catalysing species is made up of Ag, with electrophilic characteristics, while oxygen bridging atoms between the silver atoms (AgAg) have a nucleophilic character and are non-selective, contrary to what happens in the case of selective oxidation on oxides (see again Fig. 3 B). In the past it was held that the selective species in epoxidation consisted of Ag 2, which after the insertion of oxygen into the ethylene left an adsorbed atom and Ag on the surface, which in turn was responsible for the non-selective oxidation of ethylene to C 2 and 2. n the basis of this model, a maximum selectivity of 85.7% was hypothesized, whereas current commercial processes operate with greater selectivity (at low conversion), in the range of 88-94%. The reaction scheme is of a triangular type, with two parallel reactions of the transformation of ethylene into ethylene oxide and C 2, and a consecutive reaction of ethylene oxide to C 2. The principal factors influencing the reaction kinetics and hence the choice of the optimal reaction conditions are summarized here. The ethylene oxide formation rate increases as the partial pressure of the oxygen increases, while a maximum rate is reached with respect to the concentration of ethylene, as a result of the competition between the ethylene and the ethylene oxide for the same catalytic sites. The ethylene oxide formation rate decreases on increasing the concentration of the chloride ions used as a doping agent (nevertheless, above a certain value traces of chlorinated compounds such as vinyl chloride and dichloroethylene form as by-products) and the concentration of C 2. From a practical point of view, the concentration of ethylene is determined essentially by the limits of flammability. The ratio of the rate of the two parallel reactions of formation of ethylene oxide and combustion, that is the initial selectivity, rises with the concentration of chloride ions, with the content of alkaline metal promoters and with the partial pressure of the ethylene. The processes employed for oxidation of ethylene operate with air or with pure oxygen. The former are still 628 ENCYCLPAEDIA F YDRCARBNS