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The first commercial shape-selective process, Selectoforming, was developed by Mobil and allowed the selective cracking of the low octane n-alkane components of light gasoline over a natural zeolite erionite 6. Afterwards, the synthesis of various new zeolites, especially ZSM5 MFI, , the discovery of new shape selective transformations such as the accidental discovery of the remarkably stable and selective conversion of methanol into gasoline range hydrocarbons over HZSM5 7 , the development of post-synthesis treatments of zeolites, During the last 20 years, great progress was made in the synthesis of molecular sieves Table 1.

In this book, the main catalytic processes in oil refining and petrochemicals are reviewed with special emphasis on environmental issues; they play a historic role in catalysis and illustrate nicely the interplay between chemistry, processes and products. The immense potential hardly exploited of zeolites in the clean synthesis of fine chemicals is demonstrated with various examples. This introductory chapter should serve as a guide for young researchers in the field of zeolites.

It highlights briefly what these materials are, what makes them so particular and suited for molecular separations and highly selective catalytic transformations. A list of reference books covering various aspects of zeolite science structure, synthesis, characterization, applications is given at the end of this chapter and should be consulted by the serious reader.

It is also generally worth paying a visit to the original publications to better grasp the context of many new concepts and technological innovations. Structurally, zeolites are crystalline polymers based on a three-dimensional arrangement of T0 4 tetrahedra Si0 4 or A" connected through their oxygen atoms to form subunits and finally large lattices by repeating identical building blocks unit cells. The structural formula of zeolites i. However, zeolite-like materials with ultralarge pores such as Cloverite 20 T, 0.

A comparison between the pore openings of zeolites and the kinetic diameter of guest molecules shows clearly that zeolites can be used for molecular sieving. It should however be stressed that these values are temperature-dependant: temperature increases both the flexibility of the guest molecules and the breathing motions of the host zeolite pore mouth and framework.

Zeolite structures are designated by a 3 capital letter code according to rules set by the Commission of the International Zeolite Association IZA. For instance, FAU stands for the faujasite structure to which the well-known X and Y zeolites belong.

Regular updates are found on the website of the IZA. A very useful short notation is used for the description of the pore system s , each porous network is characterized by the channel direction, the number of atoms in bold type in the apertures, the crystallographic free diameter of the aperture in A , the number of asterisks indicating whether the system is one-, two- or three- dimensional.

However, this short notation does not indicate whether the pore system is made of interconnected cages or uniform channels. The IZA coding of the pore systems of some commercially available zeolites is given in Figure 1. Moreover, various compounds can be introduced or even synthesized within the zeolite pores ship in a bottle synthesis. This explains why zeolites can be used as acid, base, acid-base, redox and bifiinctional catalysts, most of the applications being however in acid and in bifiinctional catalysis.

Other examples of processes using acid zeolite catalysts will be examined in this book, like Methanol to Olefins Chapter 12 , Acetylation Chapter 14 etc. In zeolites, they are associated with bridging hydroxyl groups attached to framework oxygens linking tetrahedral Si and Al atoms : Al OH Si. The maximum number of protonic sites is equal to the number of framework aluminum atoms, the actual number being smaller due to cation exchange, dehydroxylation and dealumination during activation at high temperatures.

The number and density of protonic sites can therefore be adjusted either during the synthesis or during post synthesis treatments of the zeolite : dealumination, ion-exchange, etc. In order to design zeolite catalysts, the parameters controlling the other features of the protonic sites and particularly their strength Figure 1.

This is for instance evidenced by the higher heats of adsorption of nitrogen bases. To explain this stronger acidity, Mortier 10 proposed the existence of an enhanced donor-acceptor interaction in zeolites. A relation exists between the T-O-T bond angles and the acid strength of the associated proton in the zeolites The greater the angle, the stronger the sites. Again, the protonic sites bond angles of the zeolites are influenced by the basicity of the reactants and the temperature will also play a role.

The synthesis of metallosilicates containing trivalent elements in the framework other than Al B, Ga, In, Fe is of interest in designing the acid strength. The ranking in acid strength drawn from theoretical calculations is in relatively good agreement with acidity measurements As could be expected, the greater the exchange level, the stronger the protonic sites.

However at high exchange degree, there is not only creation of very strong protonic sites but also an increase in the strength of the protonic sites already present in the zeolite 9. Barthomeuf 15 improved this idea by using topological densities to include the effects of layers 1 to 5 surrounding the Al atom.

Extraframework aluminum species created by mild steaming were shown to increase the catalytic activity of zeolites. This increase in activity was ascribed to the creation of sites exhibiting "enhanced" acidity through interaction of bridging hydroxyl groups Bronsted sites and neighboring small extraframework aluminum species Lewis acid sites The creation by mild steaming of "enhanced acidity" sites resulting from the interaction of protonic sites and EFAL species was demonstrated in the case of HFAU 18 by the appearance in their IR spectra of two additional bands at and cm"1.

These bands result from a bathochromic shift of the high and low frequency framework hydroxyl bands of the parent zeolite OH located in supercages and in hexagonal prisms, respectively. This bathochromic shift is in agreement with an electron withdrawal from the bridging hydroxyls by EFAL species, leading to a weakening of the OH bond, hence an increase in acid strength.

The accessibility of the protonic sites also plays a significant role in the catalytic activity of zeolites. Obviously this accessibility depends both on the location of the OH in the zeolite and on the size of the reactant molecules.

Thus, the portion of protonic sites of HFAU zeolites located in the supercages is accessible to many organic molecules whereas the others, located in the hexagonal prisms, are inaccessible to all the organic molecules.

With HMFI, all the protonic sites being located at the channel intersections are equally accessible or inaccessible to reactant molecules. The same can be said for HERI, the protonic sites of which are located in large cages with small apertures, hence only accessible to linear organic molecules.

Finally, it is increasingly clear that molecules confined in the zeolitic nanocavities see their electronic properties modified. It has been shown for instance 19 that the dipole moment of acetonitrile increases significantly upon its introduction in the side pockets of MOR compared to the linear channels of the same zeolite. The guest molecule is made more basic and is easily protonated in such a confined environment.

Zeolites also act as solid solvents and the anionic framework acts as the conjugate base of the proton thereby stabilizing some charged intermediates along concerted catalytic pathways. Its remarkable efficiency for selective oxidation of various functional groups with dilute aqueous hydrogen peroxide can be attributed 24 to: - the isolation of titanium sites preventing the undesired decomposition of H 2 0 2 requiring two adjacent sites for its catalysis; - the hydrophobicity of the lattice enabling the preferential adsorption of the hydrophobic substrate in the zeolite micropores in the presence of water.

The discovery of TS1 led rapidly to the development of a process for phenol hydroxylation Unfortunately, the use of TS1 as well as TS2 discovered in by the group of Ratnasamy 27 in catalytic oxidations is restricted to the relatively small substrates able to enter the pores of these zeolites apertures 0. Other transition metal zeolites were also synthesized and tested in oxidation; one of the main problems of these systems is the release of redox cations in liquid phase A more detailed description of titanium zeolite catalysts and their applications can be found in ref.

One exception, however, is the selective aromatization of n-alkanes e. Indeed the PtLTL catalyst used commercially does not present any protonic sites. PdHFAU hydrocracking catalysts or deposited on alumina e. Thus n-hexane hydro isomerization involves successively n-hexane dehydrogenation in n-hexenes metal catalyzed , skeletal isomerization of n-hexenes into isohexenes over protonic acid sites followed by the metal catalyzed hydrogenation of isohexenes into isohexanes Figure 1.

The activity, stability and selectivity of bifunctional zeolite catalysts depend mainly on two parameters: the balance between hydrogenating and acid functions 29 and the zeolite pore structure 30, In most of the commercial processes, the balance is in favor of the hydrogenating function, hence the reaction is limited by the acid function. CaA also cracked n-hexane but not 3-methyhexane exclusively into linear products 4. These results demonstrated that reactions occurred inside the pores of CaA 0.

The first shape selective catalytic process Selectoforming, was based on this simple and elegant concept of shape selectivity by sieving 6. Table 1. However, it was the discovery of the synthesis of intermediate pore size zeolites and especially ZSM5 that led to an explosive development of research in shape selective catalysis and to an extraordinary expansion of the catalytic applications of zeolites It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels.

Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend slightly or significantly on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions.

Generally, this leads to a selectivity decrease, the external surface lacking the shape selective properties of the inner pores.. However, recent results show that reactions which can occur only on the external surface of zeolites or just within the pore mouth are very selective, suggesting a shape selective influence of external surface depending on the nature of the substrate Table 1. The simplest forms of shape selectivity come from the impossibility of certain molecules in a reactant mixture entering the zeolite pores reactant shape selectivity or of certain product molecules formed inside the pore network exiting from these pores product shape selectivity.

In practice, reactant and product shape selectivities are observed not only when the size of molecules is larger than the size of the pore apertures size exclusion but also when their diffusion rate is significantly lower than that of the other molecules. Differences of diffusivities by 2 orders of magnitude are required to produce significant selectivities between reactant species At this juncture several remarks are worth making: i As is the case for all heterogeneous catalysis reactions, the selectivity is not only determined by differences in diffusivities between reactant molecules, but also by differences between their reactivities i.

In this diffusion regime, called conflgurational diffusion, even subtle differences in the molecule or pore dimensions can result in large changes in diffusivities. Thus in CaA zeolite, the diffusivity of trans 2-butene was found to be at least times larger than that of the cis isomer even though the two molecules differ in size by only 0.

This will significantly affect the relative adsorption of polar and non polar molecules Reactant shape selectivity was the basis of the Selectoforming process previously mentioned. The n-alkanes of light gasoline essentially n-pentane, n- hexane enter the pores of the erionite catalysts and are transformed into propane and n-butane, whereas the branched alkanes are excluded from the pores and do not react Figure 1.

Product shape selectivity plays a key role in various processes developed by Mobil for the selective synthesis of para dialkylbenzenes 32 : selective toluene disproportionation, STDP toluene alkylation by methanol TAM or by ethylene PET etc. In all these processes, the bulkier ortho and meta isomers are also formed in the pores, but their exit is hampered by their slower diffusion; this selective removal of a product para isomer allows the bulkier isomers ortho and meta to be isomerized into the less bulky species para.

Above thermodynamic yields are thus possible by the simple application of LeChatelier's principle Figure 1. Thus, the diffusion coefficient of paraxylene in ZSM5 zeolite modified by coking at high temperature in toluene disproportionation, is of several orders of magnitude higher than those of ortho and metaxylene. It should be emphasized that in the case of dialkyl-benzene synthesis, the situation is ideal. Indeed, the ortho and meta isomers trapped in the pores transform rapidly into the para isomer.

In other reactions, the molecules trapped in the pores transform only very slowly into desorbable molecules, their accumulation and their transformation into larger molecules 'coke' leading to a fast deactivation of the zeolite catalyst. Venuto et al. This reverse shape selectivity, related to molecular sieving, has, of course, a negative effect; coke molecules limiting or inhibiting the access of reactant molecules to the active sites hence causing catalyst deactivation.

As stated above, shape selectivity due to molecular sieving depends on the relative rates of diffusion and reaction, hence on the respective sizes and shapes of molecules and pores and on the characteristics of active sites e. Obviously the diffusion rate, hence the selectivity depend also on the length of the diffusion path i.

For instance, in toluene disproportionation the selectivity to paraxylene of the MFI zeolite is significantly increased by deposits of silica, magnesia, coke etc, on the outer surface which limit the desorption of ortho and meta isomers However, this positive effect is also partly due to the selective deactivation of the non selective external surface sites.

This spatioselectivity depends on the size and shape of cages, channels and channel intersections. This type of selectivity was first proposed by Csicsery 34 to explain the absence of 1, 3, 5- trialkylbenzenes in the disproportionation products of dialkyl-benzenes transformation over H-mordenite although these trialkylbenzenes could diffuse in the zeolite channels.

The space available in these channels was not sufficient to accommodate the diphenylmethane intermediates involved in the formation of 1, 3, 5-trialkyl benzenes; they are bulkier than those involved in the formation of 1, 2, 3 and 1, 2, 4 trialkylbenzenes Figure 1. Contrary to molecular sieving, spatioselectivity does not depend on the relative rates of diffusion and reaction, hence both can be easily distinguished: changing the crystallite size has no effect on spatioselectivity whereas it increases selectivity by molecular sieving.

However, the two types of selectivity may act simultaneously as was shown by Song et al. As many organic compounds may transform simultaneously through mono molecular intramolecular and bimolecular intermolecular processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one.

This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI: the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites Indeed, coke formation involves various bimolecular steps condensation, hydrogen transfer that, as indicated above, are very sensitive to steric constraints.

Therefore, the rate of coke formation will greatly depend on the size and shape of cages, channels and their intersections. However, as discussed earlier in 1. The reactants' concentration in zeolite micropores is therefore considerably higher than in the gas phase with a significant positive effect on the reaction rates.

This effect is all the more pronounced as the reaction order is greater, favoring more the bimolecular over the monomolecular reactions. The concentration of reactant molecules in the zeolite micropores is largely responsible for the observation that zeolite activities of zeolites are much higher than that of more conventional catalysts 11, 46, It is the case in catalytic cracking FCC where REHY zeolites were found, depending on the hydrocarbon reactant, 10 to 10, times more active than amorphous silica alumina.

Moreover, the selectivity was completely different, the gasoline fraction in the cracked product being richer in aromatics and alkanes at the expense of naphthenes and alkenes on zeolites than on silica alumina. This drastic change in selectivity is due to different ratios between the rates of hydrogen transfer bimolecular reaction and cracking monomolecular reaction , much higher on zeolites than on silica alumina The effect is dramatic as far as the gasoline yield is concerned: paraffins and aromatics being more 'refractory' towards cracking, the 'zeolitic' gasoline is less prone to secondary cracking and yields but not octane are much higher on zeolites than on amorphous silica aluminas.

The use of zeolites spread so rapidly in FCC for this very reason. In other cases however, the high concentration of some reactants leads to undesired side reactions. In the alkylation of isobutane with butenes, zeolites are very efficient catalysts but lack stability because olefins are more strongly adsorbed than the paraffin and over a matter of minutes, their oligomerization takes over the alkylation reaction and deactivates the catalyst by pore blocking.

Introduction to Zeolite Science and Technology 21 1. Thus, when a nC22 alkane is cracked over erionite, there are two maxima in the size distribution of the product molecules at carbon numbers of 4 and 11 and a minimum at carbon number of 8. The diffusivities of n-alkanes also change in a similar periodic manner by over two orders of magnitude between the minimum at Cg and the maxima. This shows that for diffusion, and hence for shape selective effects, not only the size but also the structure of the reactant and product molecules need to be considered.

The concept of molecular traffic control was proposed by Derouane and Gabelica 49 to explain the unexpected absence of counter-diffusion effects during methanol conversion over zeolites such as MFI presenting interconnected channels with different sizes and tortuosity: the smallest molecules e. The diffusion of molecules in the channels of monodimensional zeolites may present peculiarities, accounting for shape selective effects. A first type of shape selectivity may be influenced by the diffusion in single file single file diffusion 50 preventing bimolecular transformations 51, Another type of shape selectivity called 'tunnel shape selectivity' has been recently proposed 53 to explain the high selectivity to orthoxylene during metaxylene isomerization over MCM 41 mesoporous molecular sieves.

The xylene molecules entering the non interconnected monodimensional channels of these molecular sieves undergo successive disproportionation and transalkylation reactions with consequently a purely bimolecular mode of xylene isomerization. Fraenkel et al. Acid sites located in the half cavities on the external surface of HMFI would be responsible for the selective formation of 2,6- and 2,7-dimethylnaphthalene during naphthalene methylation nest effect.

This explanation was afterwards rejected on the basis of adsorption experiments. However, a nest effect was recently proposed to be responsible for the shape selective properties of the MCM MWW zeolite and its delaminated analog ITQ-4 in aromatics alkylation The possibility of selective reactions on the external surface of zeolites, more exactly at the pore mouth, was recently addressed by Martens et al.

Pore mouth catalysis could also be responsible for the selective isomerization of n- butene into isobutene observed over aged HFER samples. However, in this case carbonaceous compounds trapped in the pores in the vicinity of the external surface were proposed to be the active species Furthermore, the second branching of n-alkanes over PtHTON was shown to occur only in positions determined by the distance between pore apertures at the surface of the zeolite crystallites.

This type of selectivity was considered to be due to a key-lock catalysis 57 i. In the case of zeolite catalysts, the approach can be based on more scientific concepts available e. Moreover, various well-known methods exist for tailoring the pores and the active sites of zeolites: ion exchange, dealumination, grafting or deposition of elements on the external surface of crystallites, etc.

Even emerging concepts such as those related to the shape selectivity of the external surface of the crystallites nest effect, pore mouth catalysis, The development of methods for obtaining large external surfaces such as the synthesis of nanocrystallites 62, 63 , delamination 64, 65 , mesoporous zeolites 66 , Of course, molecular modeling will play an ever increasing role in the rational design of zeolite based catalysts: zeolites are indeed prime candidates for thought or virtual experiments as well as computer screening of their properties for specific reactions.

Whereas zeolite catalysts have led to the development of many environmentally friendly processes in the refining and petrochemical industries, only few are used commercially in the field of fine chemicals Table 1. Many research reports have however demonstrated that acid zeolite catalysts and mesoporous molecular sieves , were active and selective in the synthesis of organic compounds and could substitute the highly polluting and corrosive acids A1C13, H 2 S0 4 , H3PO4 still often in use.

The relatively fast deactivation of zeolite catalysts during the synthesis of organic compounds is another important reason. Their deactivation is often due to heterogeneity in the acid strength of the active sites as well as to severe diffusion limitations of bulky and polar reactants or products.

The consequence is that secondary transformations into bulkier products ending up trapped in the zeolite micropores, block the access to incoming reactant molecules 'coke' formation. Therefore, the choice of operating conditions, reactor type and zeolite characteristics smaller crystallites, mesopores, delamination, etc. All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on "The Future Trends in Zeolite Applications".

Special emphasis was placed on the role played by the sites at the external surface pockets, etc. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts.

It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. Chlorotoluene Metachlorotoluene? Dichlorotoluene Metachlorotoluene? Atlas of Zeolite Framework Types.

J, and Higgins J. References 1. Cronstedt A. Stockholm, 18 Dyer A. Weisz P. Chen N. Chang CD. Baerlocher Ch. Guisnet M. Sherrington D. Mortier W. Zeolite Conference, Ed. Olson D. Rabo J. Barthomeuf D. Martens J. Ertl et al, 1 Wiley, Pines L. Wachter W. Mirodatos C.

Wang Q. Khabtou S. Smirnov and Thibault-Starzyk F. Chem B. Taramasso M. Introduction to Zeolite Science and Technology 27 Notari B. Vayssilov G. Clerici M. Sheldon R. Esposito A. Bellussi G. Ertl G. Reddy J. S, Kumar R. Weitkamp J. Song C , Garces J. Csicsery S.

Weisz P B. Venuto P. Derouane E. A: Chemical Gates B. Fraissard J. Gorring R. Karger J. Kopelmann R. Adeeva V. Fraenkel R. Congress Catal, 4 Dechema, Frankfurt am Main, Corma A. Andy P. Stostak R. Marcilly, Ch. Catal Schoeman B. Camblor M. Jacobsen C. Tanabe K. A : General At present, the oil refining industry is faced with important challenges, such as the processing of heavier and more contaminated crudes, the increasing demand for higher quality transportation fuels with reduced emissions of contaminants, and the need for more petrochemical feedstocks e.

In this context, there is no doubt that zeolites and related molecular sieves can help refiners to achieve the new goals. Recent advances in zeolite synthesis and post-synthesis modifications are expected to contribute to the development of improved catalysts and processes. In most of the processes, zeolites are involved in acid-catalyzed reactions that proceed through the formation of carbocation-like intermediates. Therefore, the chemistry of the catalytic processes on zeolite catalysts is closely related to the chemistry of the carbocations with the particularities imposed by the restricted microporous environment.

On the other hand, the availability of liquified petroleum gas LPG in the refinery is expected to increase as the FCC units are operated at higher severities in order to maximize the production of butenes, isobutane and propylene. Therefore, there is a clear incentive in converting the low value LPG into aromatics. Aromatization of short alkanes can be carried out on a purely acidic HZSM-5 zeolite. In this case, activation of the alkane is thought to occur by protonation on the zeolite acid sites, with formation of a penta-coordinated carbonium ion 1.

However, aromatization on purely acidic HZSM-5 produces large amounts of methane and ethane, thus limiting the formation of aromatics. The Chemistry of Catalytic Processes 31 The rate of alkane dehydrogenation and the yield of aromatics are significantly enhanced when a metal, such as Ga, is present in close proximity to the zeolite acid sites, as shown in Table 2. A significant advantage of the Cyclar process that makes it economically attractive is that significant amounts of hydrogen, a refinery deficitary and high valuable product, are also produced during the aromatization reaction.

Yield of aromatics and hydrogen of ca. These yields are little affected by the relative proportions of propane and butane in the feed stream. The catalyst becomes deactivated after a relatively short time on stream, and the process is carried out in a series of adiabatic stacked reactors with continuous regeneration 3. Table 2. Gallium can be introduced by impregnation, ion exchange or directly incorporated in framework positions during the synthesis of the ZSM-5 zeolite 4, 5.

In the latter case, most of the Ga is removed from the framework during calcination, resulting in a more active and selective catalyst than the original sample. It is, however, difficult to determine the exact nature of the active extraframework Ga species. However, besides aromatization, Pt also catalyzes other undesired reactions, such as hydrogenolysis, hydrogenation and dealkylation that leads to excessive formation of methane and ethane, and limits the selectivity to aromatics.

Therefore, Ga- and Zn-ZSM-5 catalysts are preferred over Pt-ZSM-5 except, perhaps, in the case of the more refractory ethane, in where a higher dehydrogenating function is needed to activate the reactant. The results obtained on the purely acidic H- ZSM-5 are also included in the table. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step.

Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. Some of the oligomers produced can also crack on the protonic sites to form lighter olefins. The oligomers are further dehydrogenated on the metal sites and the dienes formed undergo cyclization on the acid sites.

Finally, dehydrogenation of the cyclic olefins will lead to the formation of aromatics. The alkylaromatics formed can also undergo dealkylation, transalkylation, and isomerization reactions on the acid sites of the HZSM-5 zeolite. The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond Double bond cis- trans isomerization usually occurs on the acid sites before skeletal isomerization.

The general reaction mechanism for branching isomerization is depicted in Fig. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane PCP structure. H I Double-bond! Unfortunately, at low temperatures the selectivity to isoolefms decreases due to the competing olefin oligomerization reactions.

The extent of oligomerization can be decreased by using high reaction temperatures and low olefin partial pressures. However, at higher temperatures other undesired reactions, such as cracking of the dimers producing mainly C3 and C5 fragments from octenes , hydrogen transfer and coking, leading to catalyst deactivation, also occur Zeolites have been studied for this reaction with promising results.

Medium pore are preferred over large pore zeolites because bimolecular dimerization- oligomerization and hydrogen transfer reactions leading to coke precursors are diminished within the restricted space in the narrower channels of the former. Among the medium pore zeolites, better results were obtained for those having a monodirectional channel structure, such as ZSM, Theta-1, and ZSM This was ascribed to the particular structure of ferrierite having intersecting 10MR 4.

Such a mechanism is illustrated in Fig. The dimerization-cracking mechanism as the prevailing mechanism for isobutene formation has been recently questioned on the basis of isotopic experiments using l3 C-labelled molecules 29 and kinetic considerations opposite effect of reaction temperature and n-butene partial pressure on isobutene and by-products formation 30, In order to explain the beneficial effects of coke deposits on the isobutene selectivity of Ferrierite, Guisnet et al.

In this mechanism the active sites are carbenium ions attached to the coke molecules at the pore mouth of the zeolite Fig. Skeletal isomerization of n-paraffins is an acid-catalyzed reaction that is thermodynamically favored at lower temperatures. Therefore, acid catalysts with strong acidity have to be used in order to perform the reaction at temperatures as low as possible. The process is carried out in the presence of hydrogen and a bifunctional catalyst, which typically consists of a noble metal Pt supported on an acidic carrier.

Then the carbenium ion can suffer branching isomerization through the formation of a protonated cyclopropane PCP intermediate 34, 35 , or it can undergo cracking by 3-scission to produce an olefin and a smaller carbenium ion. According to Guisnet et al.

Under these conditions, the rearrangement of the carbenium ion becomes the controlling step of the process. The first generation of LSR isomerization catalysts consisted of Pt supported on chlorinated or fluorinated alumina.

In this zeolite small and controlled amounts of EFAL are generated, which can have a synergetic effect on the Bronsted acid sites of the zeolite associated with framework Al FAL , increasing their acid strength It would be highly desirable to extend the isomerization reaction to n- paraffins larger than C6.

In this case, high isomerization yields, especially of higher octane multibranched isomers, has to be achieved. It has to be considered, however, that the cracking tendency of the branched paraffins increases with an increase in the degree of branching and the length of the hydrocarbon chain.

As observed in Fig. This is explained by the combination of Bronsted acid sites of a lower acid strength than mordenite and a faster diffusion of the branched isomers through the small crystallites nm of the nanocrystallite Beta sample, thus decreasing the probability of recracking.

The Chemistry of Catalytic Processes 39 2. In this case, opening of the cyclic intermediate at side a leads to a n-butyl cation in which a terminal and a non- terminal carbon atom have exchanged positions C scrambling , while opening of the intermdiate at side b would lead to the tert-butyl cation branching isomerization through a highly unstable primary carbenium ion. These authors proposed a new mechanism wherein the protonated methyl-cyclopropane ring is the transition state for the carbon scrambling reaction, and the isomerization of the linear n-butyl cation into the branched tert-butyl cation occurs through a primary cation, as shown in Figure 2.

The activation energies calculated assuming this mechanism are in very good agreement with those obtained experimentally. This can be rearranged into a trimethylpentyl cation for example 2,2,4-trimethylpentyl cation , which can then be easily cracked giving isobutene and a highly stable rerr-butyl cation. Isobutene can then be hydrogenated to form isobutane, or protonated to give a rerr-butyl cation, which can react with isobutene to continue the chain mechanism, or converted to isobutane by hydride transfer.

In order to carry out the reaction at low temperatures, very strong acid sites are required. These catalysts, however, were rapidly deactivated with time on stream. The isomerization selectivity and the stability of sulfated zirconia catalysts can be incerased by the introduction of Pt and by carrying out the reaction in the presence of H2.

Higher catalytic activities were obtained when Pt was impregnated after the impregnation of zirconia gel with 0. Sulfated zirconia promoted with Fe or Mn showed an even higher activity than unpromoted SZ for the low temperature isomerization of n-butane Zeolites could be, in principle, good candidates for this reaction, and thus zeolite Y, either in the protonic form or exchanged with di- or trivalent cations, has been widely studied 54, 55, 56, 57, 58, On the basis of the nature of the products formed on both crystalline aluminosilicates and H 2 S0 4 Table 2.

The Chemistry of Catalytic Processes 43 Table 2. The main reactions competing with alkylation are the oligomerization of the olefins, and particularly the dimerization of the butenes, and the cracking of larger isoalkylcations that leads to a mixture of isoparaffms with less than 8 carbon numbers.

Dimerization of 2-butene leads to the formation of dimethylhexanes DMH which have a lower octane number than trimethylpentanes TMP. This can be clearly seen in Table 2. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite Moreover, the product obtained on ZSM-5 and ZSM contained more light compounds C5-C7 , and the C8 fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products.

Essentially, catalytic cracking involves the C-C bond rupture of hydrocarbons contained in the feedstock typically a vacuum gasoil to produce more valuable low molecular weight hydrocarbons including light olefins for petrochemistry, gasoline, and diesel.

The actual technology involves the formulation of multifunctional cracking catalysts which are composed of different amorphous and crystalline acid functions, and a series of additives for metal passivation, SOx removal, promotors for total combustion, and octane enhancing additives. Among them, zeolite Y is the main component controlling the activity and selectivity of the cracking catalysts. From the point of view of the mechanism, the acid sites of the zeolite are believed to be the catalytic active sites.

When the cracking of paraffins is considered, there is still some controversy regarding the intiation step and the nature of the acid sites involved. The following possibilities have been suggested: a abstraction of a hydride ion from the paraffin by a Bronsted acid site, giving H2 as a product 71, 72, 73 b abstraction of hydride by a Lewis acid site from the catalyst 74, 75, 76 c protonation of olefins that are either present in the feed or formed by thermal cracking on the zeolite Bronsted sites 77 More recently, it has been proposed 78, 79 that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state.

Then, once a smaller carbenium ion is left on the catalyst surface, cracking can continue by one of the following routes: a protolytic cracking, if the carbenium ion desorbs and regenerates the original Bronsted site, and b P-scission, if hydride transfer from a reactant molecule occurs.

The relative extension of protolytic to P-scission cracking has a clear impact on the final product distribution observed during catalytic cracking. Indeed, protolytic cracking favors the formation of undesired Cj and C2 products as compared to p-scission. On the other hand, hydride transfer is believed to be responsible for the saturation of olefins, with the corresponding increase in gasoline stability and selectivity, but producing a decrease of the research octane number RON of the gasoline.

The ratio of protolytic monomolecular to P-scission bimolecular cracking depends on reaction conditions temperature, hydrocarbon partial pressure , as well as on the characteristics of the zeolite chemical composition and pore dimensions. The medium pore ZSM-5 zeolite is also used as a cracking additive in small amounts 0. Due to the pore dimensions, ZSM-5 increases the octane rating of the gasoline by selectively upgrading low octane gasoline components into lower molecular weight compounds with a higher octane.

It mainly acts by cracking low-octane linear and monobranched paraffins and olefins in the gasoline range, which can access to the acid sites. Linear and branched olefins in the upper gasoline range are thus removed before they are converted into low octane paraffins and aromatics by hydrogen transfer reactions on the Y zeolite component of the cracking catalyst. The Chemistry of Catalytic Processes 51 An aromatics concentration effect is also responsible for the increase of octane produced by ZSM-5 addition.

Gasoline yield slightly decreases while C3-C4 gases, mainly propylene, increase Production of light olefins propylene, n-butenes and isobutene will be one of the main targets of FCC untis in the near future. These olefins can be fed to alkylation and etherification units to produce additional high octane environmentally acceptable gasoline components, or used as petrochemical feedstock. The large pore tridirectional zeolite Beta has also been evaluated for catalytic cracking 86, While zeolite Beta was found more active than USY for cracking n-heptane, it was less active for gasoil cracking, indicating some problems of accessibility for the more bulky molecules present in the gasoil.

However, zeolite Beta was less active for hydrogen transfer reactions, which should be reflected in a higher gasoline octane and more olefinic C3-C4 products. Moreover, when zeolite Beta is used as an additive instead of the medium pore ZSM-5, it produced higher yields of desirable C4 olefins Haag W.

Catal, 2 Berlin, Doolan P. Martindale D. Gricus-Kofke T. Simmons D. Giannetto G. Price G. Dooley K. Meitzner G. Mole T. Kanai J. Meriaudeau P. Butler A. Today 18 Basini L. Szabo J. Houzvicka J. A Seo G. Asensi M. The Chemistry of Catalytic Processes 53 Soc, Faraday Trans.

Grandvallet P. Mooiweer H. Soc, Chem. Condon F. Emmet, ed. Brouwer D. Pays Bas 87 Zhang A. Today 1 Koradia P. Voorhies A. US Pat. Eberly P. ACS Petrol. Alvarez F. Chica A. Boronat M. Arata K. Hino M. A 3. Garwood W. Minachev Kh. Kirsch F. IT World Petroleum Congress, 3 Olah G.

Khadzhiev S. Unverricht S. Chu Y. Guczi L. Wojciechowski B. Dekker, New York, Planelles J. Abbot J. Poustma M. Tung S. Nace D. Borodzinski A. Greensfelder B. Gianetto G. Pappal D. Madon R. Huehler C. Occelli M. Elia M. Johnson T. Bonetto L. In general, the physical form of the catalyst is governed by the needs kinetics, mass transport, heat transfer and hydrodynamics of the particular industrial process la.

Catalyst deactivation lb plays, for instance, a dominant role in the catalyst and reactor designs. The fast deactivating lifetime measured in seconds FCC catalysts are shaped as micron sized spheres to allow a short contact time during the reaction and a prompt transfer to and from the regenerator. The morphology of the catalyst can make a difference not only on the pressure drop within the reactor but also on the diffusional speed of the reagents and products.

The latter can have a significant impact on the selectivity of certain reactions. In petrochemical and oil refining operations, the zeolite is primarily responsible for the catalyst's activity, selectivity and stability catalytic, thermal and hydro thermal. The fluid catalytic cracking process FCC is the most widely used of the oil refining process and is characterized by the use of a finely divided catalyst, which is moved through the processing unit.

The catalyst particles are of such a size about 70 urn that when aerated with air or hydrocarbon vapor, the catalyst behaves like a liquid and can be moved easily through pipes. Spray drying is the most widely used industrial process involving particle formation and drying. It is highly suited for the continuous production of dry solids from pumpable suspensions.

Operating conditions and dryer design are selected according to the drying characteristics of the product and powder specifications. The discussion will be centered on spray drying principles as well as the different types of equipment commonly used in commercial applications. Other catalyst forming techniques will be discussed more briefly. The reader will notice that contrary to other aspects of zeolite science and technology, the source of open information is scant.

This feature underlines the central economic role played by catalyst preparation and may give the impression this aspect of zeolite science and technology is very empirical. Advances in process control and quality programs have, however, made catalyst preparation remarkably efficient and a source of ever diminishing waste streams.

A useful introduction is the subject of Chapter 1 of this book and more detailed information can be found in the references therein. By extension, molecular sieves are defined as microporous crystalline structures with a variable elemental composition leading to frameworks with variable charge as outlined in Table 1.

As far as the catalytic and adsorption properties are concerned, zeolite molecular sieves are further classified according to their pore openings, defined by the number of O2" anions delineating their pore mouths. The most useful zeolites are those containing 12, 10 or 8 of these O2" and are commonly referred to as , , and 8- ring structures. Three key parameters will influence their stability: their Al content the lower the more stable the zeolite , crystal size the larger the more stable the zeolite and Na level the lower the more stable the zeolite.

All zeolites are produced by hydrothermal synthesis 2 under alkaline conditions OH- mineralizing ion. These syntheses unfortunately suffer from two economic drawbacks, i. The two most discussed processes are based on the gas phase SiCl4 or the liquid phase NH4 2SiF6; they suffer however from the environmental penalty associated with the use of halogenated reactants. The mesoporosity created during the process is beneficial to the diffusion of the large molecules of oil.

The FAU zeolite can then be fine- tuned to cover most of its applications in PCC gasoline yield with high unit cell size materials, octane yield with low unit cell size materials and Hydrocracking high middle distillate yields with low unit cell size materials. Most of the unit operations encountered in catalyst manufacturing are gathered in the FCC catalyst preparation: powder preparation, wetting, mixing and blending of the additives, particle forming, drying, ion-exchanging, calcining, impregnating with active metals, further drying and calcining.

FCC catalyst preparation is therefore an excellent case study. The zeolite is at the core of the catalyst, but the role of the matrix is also crucial and parameters such as its surface area, porosity, composition and acidity need to be controlled very precisely 4. A general scheme for preparation of FCC catalysts is depicted in Figure 3. First, along the manufacturing scheme left to right in Figure 3.

This requirement implies repeated washing steps. Figure 3. The major differences are found in the first two steps, namely the process of synthesizing the zeolite and the mixing with the matrix. Another method is the in-situ crystallization of the zeolite in the matrix Engelhard process. Both techniques are used commercially. It is of note that the oldest technology, using Si0 2 as a binder, is the one leaving the most Na on the catalyst 5.

This step is often overlooked in the open literature on zeolite preparation and few authors describe details on this operation. Preparation of Zeolite Catalysts 65 3. These droplets fall through gravity in a flow of hot air following a spiral downward movement. The particles with the desired size are collected at the bottom of the spray dryer and the fines are recovered with the aid of cyclones and recycled.

The height of a commercial spray dryer is measured in meters and such a process is difficult to downsize, therefore explaining partly the lack of academic research in the field of zeolite catalysts spray drying 6. Atomization The heart of the spray dryer is the atomizer where the spherical droplets are initially produced.

The atomizers can be classified into three different categories: - Two Fluid Nozzles: the spray is created by contacting two fluids, namely the feed slurry and compressed air. The compressed air provides the atomization energy. The device has the advantage of being relatively simple, uses a simple feed pump and affords a wide particle size distribution.

On the minus side however, the consumption of compressed air is relatively high 0. Such a device produces a narrow particle size distribution of FCC catalyst. The feed pump is more complex high pressure is required than in the nozzle described above and is susceptible to plugging; the materials of construction have to be abrasion resistant and the capacity is generally limited to 2 gallons per minute, but multiple atomizing devices can be used in the spray dryer - Rotary Atomizers: typical devices and their principles are illustrated in Figure 3.

A narrow particle size distribution is obtained, very close to the one observed with the Pressure Nozzles. There is, however, only a single atomizing device fed by a single pump. The atomizing machine is more complex because it requires high rotation rates to produce droplets of the desired size.

It is, however, not easily plugged, does not require construction materials as abrasion resistant as the pressure nozzles and has a virtually unlimited capacity. Large differences in particles size distribution are observed between these types of nozzles as outlined in Figure 3. The initial contact between the spray droplets and the drying air will control the evaporation rates and the product temperature.

In order to have a smooth operation, the largest particles must be dry before hitting the chamber wall to avoid sticking to them. Three drying modes are currently encountered and illustrated in Figure 3. It is seen that in order to achieve the particle size around 70 um of interest in FCC catalysts, the drying chamber diameter is between meters.

Such dimensions make the spray drying process difficult to downscale for a detailed study and modeling of this unit operation. Extrusion and Oil-Drop are the most sophisticated of the other techniques commonly used in zeolite catalysts shaping 7. Extrusion is by far responsible for the largest volume of other zeolite catalysts paraffins and aromatics isomerization, hydroprocessing, During their fall in the hot oil, the NH3 released by the decomposition of, say urea, changes the pH of the alumina sol and provokes its precipitation and the formation of the alumina binder.

The spheres are then collected, washed, aged, dried and further processed. It is also possible to oil drop with other binders such as Si0 2 , Zr0 2 , One example of such a process is described in 11 ; the zeolite produced by such an in-situ technique is generally well mixed and distributed homogeneously throughout the binder.

The other shaping techniques are used when specific requirements hydrodynamics, pressure drop, Their industrial colleagues report their results in often difficult-to-read patent examples or keep them as trade secrets. The problem is made unappealing to the academic researcher because of the empirical image of catalyst preparation and even more shaping. In addition, some shaping processes spray drying are difficult to downscale to allow proper laboratory study and modeling.

This is however an area for a fruitful cooperation between industry and academia: catalyst shaping is more than physically mixing of an inactive 'binder' and an active zeolite. It has been demonstrated 14, 15 that important chemical changes leading to more stable zeolites for instance take place during these processes and also during subsequent treatments steaming for instance of finished particles: matrix bound zeolites are for instance more steam stable than the corresponding bare powders 16, This need for more science and engineering in catalyst shaping is unfortunately counterbalanced by the confidentiality requirements of a core aspect of the catalyst manufacturers' business.

If academia and industry find a suitable cooperation framework in this area, the common benefit will be an even more rational design of catalysts. Today, 37 Gilson J. Szostak R. Von Ballmoos R. Ertl et al. Wiley, Woltermann G. Masters K. Doesburg E. Surf Sci. Preparation of Zeolite Catalysts 73 8. Richardson J. LePage J. Stiles A. Patent 4,,, assigned to UOP, Moulijn J.

Farrauto R. Gelin P. Kubicek N. Zeolite-enabled processes are clearly cleaner technologies than their predecessors from the viewpoint of environmental impact, particularly with regard to air quality. A major environmental objective of the 21 st century is to further improve air quality by reducing the generation of greenhouse gases and the vehicle emissions that cause air pollution. In refinery operations, improving the efficiency of processes that transform petroleum into useful products will reduce greenhouse gases because less energy is consumed.

Requiring refined fuels to meet pertinent quality standards will minimize harmful emissions from vehicles. The composition of exhaust gas from a vehicle is dependent on its fuel properties and the vehicle's emission control technology. Table 4. The distillation curve also affects VOC emissions during starting and warming of the engine.

Toxic emissions are formed from the combustion of benzene, other aromatics and olefins. Sulfur indirectly contributes to toxic emissions by interfering with the emission control components of a vehicle. NOx emissions can be curtailed by equipping vehicles with modern catalytic converter technology. However, sulfur oxides in the exhaust reduce the efficiency of NOx conversion.

Sulfur oxides also impede diesel particulate traps. Both SOx and NOx contribute to acid rain, causing other adverse environmental consequences. For gasoline, the trend is to maintain high octane, while meeting stricter requirements for RVP, benzene, sulfur, distillation, aromatics and olefins.

For diesel, a high cetane number must be achieved, along with low levels of sulfur and aromatics. Gasoline specifications derive from studies that correlate reductions in emissions with changes in gasoline properties. Emission reductions calculated from the U. A reduction of RVP from 8. Sulfur and olefins mostly affect NOx emissions, with sulfur obstructing the performance of catalytic converters and oxygen sensors.

Toxic emissions are primarily reduced by decreasing RVP, benzene, sulfur and aromatics and by adding oxygenates to gasoline. Other benefits resulting from adding oxygenates to gasoline include high blending octanes and clean combustion products. Methyl-tertiary-butyl-ether was added to gasoline in various areas of the U.

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Step 3. Crossed Aldol Condensation An aldol condensation between two different carbonyl compounds so called crossed aldol condensation is not always useful as a mixture of four different possible products may be obtained. Intramolecular Aldol Condensation When a compound has two carbonyl groups, it can undergo intramolecular aldol condensation in the presence of dilute base if a-H atoms are present in the compound.

Related Chapters Aldol condensation. Benzoin Condensation. Beckmann Rearrangement. Cannizzaro Reaction. Clemmensen Reduction. Claisen condensation. Friedel-Crafts alkylation. Friedel Crafts Acylation. Fries Rearrangement. Gattermann-Koch Reaction. Grignard Reagent. Hell-Volhard-Zelinsky Reaction. Hunsdieker reaction. Hoffmann Bromamide Degradation. Jones reagent. Kolbes Reaction. Knoevenagel Reaction.

Meerwein-Ponndorf-Verley Reduction. Perkin Condensation. Pinacol-Pinacolone Rearrangement. Reformatsky Reaction. Reimer-Tiemann Reaction. Schimdt Reaction. Schotten Baumann Reaction. Figure 2 presents the 1 H NMR spectrum of p-anisaldehyde, clearly showing the resonances for the aldehyde and oxymethyl proton environments.

Inductive effects of the substituents are considered to assign the two remaining resonances that belong to the 1,4-disubstituted benzene ring. Figure 2. Figure 3. The diagnostic methyl resonances at 3. The remaining resonances between 6. Assigning the chemical shifts and multiplicities of these resonances is a complicated process due to their overlapping.

Figure 4. The 2D J-resolved experiment involves mapping of J-coupling constants against the proton chemical shift, thereby allowing accurate measurement of the chemical shifts of proton resonances through f2 dimension examination and their coupling constants in the fl dimension. Spin-coupling partners, typically over bonds, are identified in the COSY experiment. Figure 5 presents the COSY of Product A, showing the correlation among the same proton pairs determined in the 2D J-resolved experiment by interpreting cross-peaks in the diagonal of the 2D spectrum.

The proximity among these resonances is further validated by these correlations. Figure 5. A solution of p-anisaldehyde 0. A potassium hydroxide 0. The potassium hydroxide solution is then gradually added to the solution in the round bottom flask over two minutes with continuous stirring for 30 minutes.

The solid is extracted by vacuum filtration, followed by water washing. The resulting solid is then dried and recrystallized from ethanol Figure 6. Figure 6. Recrystallized 1,5-bis 4'-methoxyphenyl penta-1,4-dienone Product B. Figure 7 depicts the 1 H NMR spectrum of 1,5-bis 4'-methoxyphenyl penta-1,4-dienone, confirming the loss of the reactive methyl centre in Product A following the reaction with p-anisaldehyde.

Figure 7. The J-resolved spectrum Figure 8 differentiates 8. It also identifies a

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The butyric acid formed in the Cannizzaro reaction neutralizes the basic catalyst. device is small and the investment for it is correspondingly low. Active basic catalysts reported in the literature include aqueous NaOH () High reactivities of both the basic cobalt aluminate for. A one-pot method for jet fuel based on biomass-derived ketone platform compound has been developed through aldol condensation and hydrogenation over.