CLAY-ACTWATED ISOMERIZATION~ REAC11ONS

F. J. A. Kellendonk,J. J. L. Heinerman,and R. A. van Santen

Koninklijke/Shell-Laboratorium, AmsterdamShell Research B.V.1003 AA Amsterdam, The Netherlands

I. INTRODUCTION

Natural clays were among the earliest solid acid catalysts used in the oilindustry to promote cracking and isomerization reactions. Not long aftertheir introduction in the late 1930s, they were replaced in large part bysynthetic amorphous silica—aluminas, which gave more consistent results.During the 1960s the amorphous catalysts were superseded by the moreactive and shape-selective microporous crystalline aluminosilicates knownas zeolites. However, there has been a renewed interest in the use of claysas catalysts because zeolites are not as suitable for the conversion ofrelatively large molecules. This interest resulted in the synthesis ‘of highlyactive clays and in the development of the pillared clays, in which theuseful surface has been increased and shape selectivity created by separating the layers with the aid of largecationic clusters (pillars).

It is well known that most of the observed catalytic properties of claysare due to their acidic character. Therefore, this chapter starts with a shortsummary of methods to determine these acidic properties and the mainresults of these methods. Then the various steps to obtain an active catalystare discussed, such as synthesis, purification, and activation. This isfollowed by a discussion of the mechanism of the isomerization reactionand a review of the use of clays as isomerization catalysts.

455

PREPARATIVE CHEMISTRY Copyright © 1987 by Academic Press. Inc.USING SUPPORTED REAGENTS Alt rights of reproduction in any torm reserved.

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II. ACIDIC PROPERTIES OF CLAYS

For details of structures of clay minerals and their characterizationby x-ray diffraction, we refer to Chapter 11 in this volume. The othercharacterization methods are fully covered in the monographs of Weaverand Pollard (1973), Theng (1974, 1982), and van Olphen (1977).

The acidity of solid surfaces can be determined by using the well-knownHammett indicators (Tanabe, 1970). This simple technique gives onlyqualitative information about the number and distribution of acidic sites.More quantitative data can be obtained by monitoring at various temperatures the adsorption of a base such as ammonia, trirnethylamine, andpyridine. In combination with IR spectroscopy it is even possible todistinguish between Brønsted and Lewis acidity. For example, the JRspectrum of pyridine adsorbed on Lewis sites differs considerably from thatof the pyridinium ion that is formed on Brønsted sites (Aldridge et al.,1973; Forni, 1973).

It has been shown conclusively that the presence of water influences theacidic properties of clays. For cation-exchanged clays, Brønsted acidity ismainly due to the dissociation of adsorbed water. This dissociation isinduced by the polarizing power of the cation (e.g., Al3~, Fe3~), whichdepends on the size and the charge of the ion. When the water content of aclay is reduced to less than —5% by weight, the acidity increases, becausepolarization by the cation is less dissipated (Theng, 1982). Removing all ofthe water from the clay, by calcination above —200°C, results in a markedreduction in Brønsted acidity. The clay assumes a much more pronouncedLewis acid character, which is due to the transformation of BrØnstedacidity into Lewis acidity.

A different kind of acidity is due to H~ located at exchange positions.The positive charge compensates for the negative charge of the layersinduced by isomorphous substitution with lower-valence cations. Theeffect of calcining H~ (exchanged) clays depends on the type of layersilicate. In the case of octahedrally substituted clays (montmorillonitetype), the protons will migrate into vacancies in the octahedral sheet,where they associate with lattice oxygens (Wright et al., 1972; Davidtz,1976). In general, these protons are not accessible to catalysis, because thefree of diameter of the hexagonal hole in the tetrahedral sheet is only0.26 nm (Barrer, 1978). After calcination the acidic sites left will besituated at the edges.

However, for tetrahedrally substituted clays, calcination leads to protons attached to surface oxygens of the tetrahedral sheet. This results inacidic sites that are similar to the Brønsted acidic sites on Y zeolite(Uytterhoeven et al., 1965; Barrer, 1978). The work of Plee et al. (1985)

with pillared montmorillonite and beidellite (a tetrahedrally substitutedclay) corroborate nicely the previously mentioned features. They observedthat the acidity of calcined beidellite compared well with Y zeolite, whilethe calcined montrnorillonite-based catalyst was much less acidic. With 1Rspectroscopy it was shown that on beidellite mainly Brønsted sites occurred, while montmorillonite exhibited a much lower acidity which wasmainly of the Lewis type.

In conclusion, it emerges that the acidic properties are strongly dependent on the type of clay, the exchanged cation, and the H2° content.

III. METHODS OF CATALYST PREPARATION

A. Purification of Natural Clays

Natural clays often contain quite large amounts of impurities, whichshould be removed to prevent interference with the catalytic function. Asthe variation in amount and type of impurities is as large as the variation inclay types, it is not possible to give here a specific purification recipe.Therefore, we limit ourselves to some general remarks and refer to theliterature for experimental details (van Olphen, 1977).

Each purification method should involve separation of coarse mineralimpurities from the clay. This is best achieved by suspending the sample inwater and allowing the large paTticles to settle. The clay particles can thenbe siphoned off (van Olphen, 1977). If the sample contains a relativelylarge amount of organic material, the dry material can be brought incontact with concentrated hydrogen peroxide. Several methods exist forthe removal of inorganic impurities. For instance, carbonates and freeferric oxide can be removed by a subsequent treatment with sodiumacetate buffer solution and sodium dithionite/sodium citrate (Tzou. 1983).

B. Activation Procedure

The first step in the activation procedure, which is required for bothnatural and synthetic clays, is to convert the clay into the desired ion form.In the case of metal ions, this can be done with relative ease by ionexchange using a concentrated solution of a suitable salt. The excess saltcan be removed by repeated washings employing a centrifuge, followed bydialysis. As an alternative, one can use ion-exchange resins in the suitableion form. In contrast, it is not as easy to prepare the H~ form of a clay viaion exchange with an acid or a resin in the H~ form. The acid attacks theclay structure and part of the aluminum in the structure is transferred to

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ion-exchange sites. This effect occurs even under relatively mild conditions. For example, van Olphen (1977) found that the conversion ofWyoming bentonite (montmorillonite) with 0.01 N HCI at room temperature led to the occupation of 60% of the exchange sites by Al. The best wayto overcome this acid leaching problem is to prepare the NH4~ form via ionexchange, followed by a heat treatment at 550°C (Wright etal., 1972). TheNH4~ ion will decompose, leaving an H~ ion on the clay. By performingthe heat treatment in air all of the residual organic material will be burntoff, which results in a ready-to-use acidic catalyst.

C. Synthetic Clays

The main incentive for using synthetic clays is that several interestingmaterials are either not available in sufficient quantities in their naturalform (beidellite) or do not exist in nature. Most clays can be synthesizedvia hydrothermal methods (Barrer, 1982). Here we discuss briefly thesynthesis of clays that have been used for isomerization reactions.

Koizumi and Roy (1959) have prepared in a pressure vessel at 300°C and1000 bar, starting with a stoichiometric gel, 100% beidellite with thecomposition Nao,33Al2(Si3.67Alo33O ~ (OH)2. The very high pressureposes a problem for large-scale preparation of the clay. However, Diddamset a!. (1984) claim that the reaction also proceeds at 300°C and autogenouspressure.

Granquist (1966) described the synthesis at —300°C of a novel high-surface-area (140—160 m2/g) clay with randomly interstratified mica-and beidellite-type layers having the following approximate formula:Nao69Al2(Si336Alo64)O~(OH)2. This clay has become known as syntheticmica montmorillonite (SMM), although it does not contain any montmorillonite but only beidellite. It was found that partial incorporation ofnickel into the octahedral positions of the SMM structure resulted in adramatic increase in acidity. This became apparent only after reduction ofthe nickel to the zero-valent state (Swift, 1977; Heinerman et a!., 1983;Robschlager et a!., 1984). In the original NiSMM synthesis (Granquist,1974) a relatively large excess of H20 had been used, which resulted in aninefficient use of the high-pressure autoclave. Heinerman (1985) reportedan improved synthesis, which involved the use of (commercial) amorphoussilica—alumina as starting material and minimum quantities of water. Thisresulted in a yield that was three times higher per reaction mixture volume.

Synthesis of trioctahedral clays, where all octahedral positions areoccupied with cations, is, in general, somewhat less difficult than thepreparation of the dioctahedral materials, where two out of three octa

hedral positions are filled with cations. For example, chrysotile (trioctahedral) can be prepared by heating a mixture of the appropriate oxidesand an excess of H20 at 250°C for 24 h (Robson, 1973, 1974), whereaspreparation of dioctahedral isomorphous kaolinite requires more severeconditions (Barrer, 1982). Furthermore, Sohn and Osaki (1980) haveclaimed that nickel—talc {—Ni3Si4O1o(OH)21 can be synthesized by precipitation from an acidic solution of a nickel salt—sodium silicate mixture attemperatures as low as 60°C.

Several interesting materials, known as fluorine micas, have beensynthesized by employing salt melts as crystallization medium or via asolid-state reaction (Shell and Ivey, 1969; Kitajima et a!., 1984). Oneexample is a material with the structural formula NaMg25L105(Si4Oio)F2,which is swellable in water (Kitajima and Daimon, 1975). Here the chargeimbalance is not due to isomorphous substitution but to vacancies in theoctahedral layer. Morikawa et a!. (1983a) have shown that this material inthe Na form is inert, in contrast with other alkali-neutralized clay materialswhich exhibit some residual acidity. It is, therefore, ideal to study theintrinsic activity of metals and metal ions.

D. Pillared Clays

Pillared clays are materials tHat are modified by exchanging the interlamellar cations for bulky ions designed to provide increased access to theinterior region and shape selectivity at temperatures where water or othersolvents are thermally removed. Work on pillaring started by ion exchanging with organic cations such as ethylene—diammoniurn. This was onlypartially successful because the ion intercalated with its long axis parallel toclay layers which resulted in a layer separation of only 0.28 nm (Barrer andMacLeod, 1955). A clear advance was the use of the cage-like molecule1,4-diazobicyclo[2.2.2joctane (DABCO), which resulted in an interlayerdistance of 0.53 nm (Mortland and Berkheiser, 1976; Shabtai etal., 1977).However, the thermal stability of these materials appeared to be unsatisfactory. Therefore, attention shifted to inorganic materials as pillars. Thebest results were obtained with polymeric cationic complexes of aluminum.Several preparation methods have been reported, all of which resulted insamples with a surface area of 200—400 m2/g and a layer separation of0.9 nm. All procedures start with the preparation of well-dispersed clayslurries using purified <2-gm materials. The main difference between themethods is the concentration of the clay suspension. For instance, Lahavand Shani (1978) and Lahav et al. (1978) added dropwise aged aluminumhydroxide solutions, prepared with AlCl3 and NaOH (OH/Al = 1.85), to

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stirred montmorillonite suspensions of only 50—200 mg/L. After flocculation occurred, the resulting material was allowed to settle for several hoursand was collected by centrifugation. The main disadvantage of thisprocedure is the necessity to handle large volumes of suspension. Vaughanet a!. (1979, 1981; Vaughan and Lussier, 1980) brought much higher clayconcentrations (>27 g montmorillonite or beidellite/L) in contact withsolutions containing aluminum hydroxy oligomers at temperatures of theorder of 60°C for about 30 mm. The most convenient oligomer source is acommercial aluminum chlorhydroxide known as Chlorhydrol (supplied byReheis Chemical Company). After pillaring the material should be washedthoroughly in order to obtain an active material. Comparable methodshave been reported by Gaaf and van Santen (1984) and Gaaf ci’ a!. (1983)for NiSMM pillaring, and P1cc eta!. (1985) for beidellite pillaring. Similarclay concentrations have been used by Jacobs et a!. (1984), who achievedpillaring by dialyzing clay—aluminum hydroxy slurries at room temperaturefor 4 days. After the actual pillaring it is necessary to give the material aheat treatment at about 550°C, which probably covalently binds thepillaring agents to the clay layer. The material can now be brought in thedesired (active) ion form. Alternatively, one can try to pillar the active ionform of the clay. However, this is not always possible because the swellingdepends on the type of interlamellar ion (Barrer, 1978).

Novel pillars with interesting properties have been described in theliterature. Chromium oxide pillars, where chromium oxide has intrinsichydrogenation activity, have been claimed to give larger pore size (Tzou,1984; Pinnavaia et a!., 1985). Other novel pillaring materials are siliconoxide (Lewis eta!., 1985), zirconium oxide (Yamanaka and Brindley, 1979;Vaughan eta!., 1979, 1981; Kikuchi eta!., 1985), titanium oxide (Vaughanci’ a!., 1979), and nickel oxide (Tamanaka and Brindley, 1978). Siliconpillars appear to have better hydrothermal stability and have been used inthe preparation of an isomerization catalyst.

IV. ISOMERIZATION REACTIONS

A. Reaction Mechanism

The isomerization of hydrocarbons over solid acidic catalysts has beenstudied extensively. It is now generally accepted that the reaction mechanisms involve carbonium ions. The factors that determine the selectivity ofthe reaction are well known as well. Therefore, we give here only a shortreview of the relevant principles.

The first step is formation of a surface carbonium ion. This intermediatecan undergo rearrangements by hydrogen-atom and/or carbon shifts. The

former operates in the case of double-bond isomerization. The latterresults in skeletal isomerizations of the type given below (Brouwer andHogeveen, 1972):

C~ C

____ /0” ____c—C—c——c—C c—c—C——c C—c—c—C+ +

TERTIARYCARBONIUM ION

The driving force is the relatively high stability of the tertiary carboniumion compared with the other ions.

Carbonium ions can be formed via several routes:

(1) protonation of unsaturated bonds by BrØnsted acidic sites (Weisz,1970; Kouwenhoven, 1973);

(2) abstraction of a hydride (H ) ion by a Lewis acidic site (Nace,1969); and

(3) protonation of saturated compounds at BrØnsted sites has beenclaimed to give five-coordinated carbon atoms (nonclassical carboniumion) followed by the formation of molecular hydrogen and a classicalcarbonium ion (Olah, 1973).

The first route, protonation of unsaturated hydrocarbons, proceeds withrelative ease compared with the other mechanisms. For that reason lowertemperatures can be used, which gives less (thermal) cracking and therefore higher selectivities.

To achieve similarly high selectivities with saturated compounds, theconcept of bifunctional catalysis has been developed, in which a (de)hydrogenation function (for instance, a noble metal) is combined withan acidic support (Sinfelt eta!., 1960). The dehydrogenation function produces alkenes resulting in a lowering of the reaction temperature. According to Coonradt and Garwood (1964), the highest selectivities are obtainedwhen there is a balance between the (de)hydrogenation activity and theacidity of the catalyst. Fast dehydrogenation (relative to the isomerizationreaction) results in a relatively high steady-state concentration of unsaturated compounds. These compounds will displace the isomerized carbonium ions from the acid sites by competitive adsorption, thus limiting theresidence time on the catalyst, which reduces the chances of ~-scission ofbranched carbonium ions. Furthermore, fast hydrogenation of the desorbed rearranged alkenes prevents readsorption and hence removesthe possibility of j3-scission. The validity of this balance concept has been

SECONDARYCARBONIUM ION

PROTONATEDCYCLOPROPANEINTERMEDIATE

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experimentally demonstrated for several types of acidic supports. Forinstance: Pt/Si02—A1203 (Coonradt and Garwood, 1964), Pt ultrastableY zeolite (Jacobs et at., 1980), and NiSMM (Robschlager et at., 1984).

In the preceding mechanism, isomerization and 13-scission (cracking) areconsecutive reactions. It is well known in the field of reactor engineeringthat the highest yield of the intermediate (in this case isomerized) product isobtained in a reactor configuration where mixing of product is kept to aminimum, i.e., plug-flow operation (Levenspiel, 1972), while mixing in abatch-type operation leads to an increased contribution of consecutivereactions.

B. Aikene Isomerizations

In this section we only deal with gas-phase double bond isomerization ofbutenes. Other alkene reactions will be discussed by J. Adams in Chapter 27. This rearrangement of butenes is a typical test reaction to detecteven the weakest (BrØnsted) acidic sites.

The importance of the presence of H2O in the formation of Brønstedsites in layered silicates without tetrahedral substitution (no intrinsicBrønsted acidity) has been shown by Sohn and Ozaki (1980) for anickel—talc-like structure [Ni3Si4O1{(OH)2]. They found that the activityfor 1-butene isomerization (at a temperature as low as 20°C) peaks at apretreatment evacuation temperature of 100°C. The activity ran closelyparallel with the Brønsted surface acidity as measured by n-butylaminetitration in benzene. At an evacuation temperature of 400°C the catalystwas quite inactive and hardly any Brønsted sites could be detected.

That there are still Brønsted sites present in a tetrahedrally substitutedSMM clay (NL Industries, Baroid division) even after activation at 530°Cin vacuum has been demonstrated by Hattori et al. (1973). They performedtracer studies with deuterated SMM and nondeuterated butene in a pulsereactor at ambient temperatures under water-free conditions. They inferred from the appearance of deuterium in the reaction products thatBrønsted sites are responsible for the isomerization reaction.

The isomerization of cis-2-butene at 150°C over 1,4-diazobicyclo-[2,2,2}octane (DABCO)-exchanged Zn-montmorillonite has been used byStul et al. (1983) to show that pillaring could make more sites accessible tocatalysis. They observed a fourfold increase in initial activity due to thepillaring.

Morikawa et al. (1983b) used a conventional pulse reactor to convert1-butene to cis- and trans-2-butene (at 380°C/He flow) over fluorotetrasilicic micas (TSM, Topy Industries) exchanged with several metal ions suchas Mg2~, Al3~, Pt2~, etc. The Pt2texchanged sample was the most active

with 70% conversion and 85% selectivity. From the relatively low ratio oftrans-2- to 1-butene (—1) in the products after cis-2-butene isomerization itwas concluded that no BrØnsted acid site was involved in the reactions.Furthermore, no relation could be found between known Lewis strengthsof the ions and the activity. The use of hydrogen instead of helium as acarrier gas resulted in a marked increase in activity, which stronglyindicates that the reaction proceeds on the metal ions via the type ofhalf-hydrogenated species well known in the field of metal and metalcomplex catalyses (Anderson, 1973).

C. Paraffin IsomerizatiOnS

The skeletal isomerization of n-paraffins is an important reactionespecially for the oil industry. For instance, isomerization of paraffins inthe light gasoline boiling range results in an increase in octane number.Most work on this reaction has been done with zeolites and y-A12O3 dopedwith F and/or Cl (to induce acidity). Clay like materials, i.e., synthetic andpillared clays, have been evaluated as substitution for zeolites to overcomethe pore diffusion and steric occlusion problems encountered with micro-porous materials.

In Table I we have summarized the catalysts that have been used forparaffin conversion, together with the relevant data regarding conversionand conditions. The following points deserve further comment:

(1) A relatively high hydrogen pressure has been applied (except in twocasses). The rationale is prevention of deactivation due to coke formationon the catalyst surface.

(2) The most active clay catalyst, i.e., NiSMM doped with Pd or Pt, canhave an activity comparable to that of the very active 1% Pd—H mordenite(compare nos. 7, 8, and 11).

(3) The high activities are combined with very high selectivities whichindicates a bifunctional mechanism.

(4) The very poor performance of natural montmorillonite can beimproved markedly by pillaring with an Al oligomer prepared in this casevia dialysis (compare nos. 14 and 15). Whether the activity of the pillaredclay is comparable with Pt—NiSMM (no. 13) is questionable becauseJacobs et a!. (1984) used a very low H2 pressure. This normally results in arelatively high initial activity (high alkene concentration) followed by arapid (coke) deactivation.

The best area for clays as catalysts is the conversion of large moleculesas demonstrated by the work of Shabtai et a!. (1981) on cracking ofperhydrophenanthrene over a Ce-exchanged, Al-pillared montmorillonite.

N ~t~

C)~ ‘—~ra:~)__“ °°~‘—,a~—~ ~

~

~Ct >-~—~~ ~,C~C ~

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C)C)C,,-”C— CCC

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CH3

They observed a markedly higher activity of the pillared clay as comparedto Y zeolites, while the opposite was found for smaller molecules.

0. a-Pinefle IsomeriZatiOn to CamphefleThe effectiveness of various clays for the production of camphene, a

raw material for camphor productiofl~ has been studied since the 1930s.TischcheflkO and RudakoV (1933) used an unspecified clay activated with10% hydrochloric acid for the a-pinene (I) rearrangement to camphene(II) and obtained a 62% yield:

CH~H3C

~ Wagner—Meerwetn

CH3 -.

\ rearrangementCH3

I II

Nazir et al. (1976) reported a more detailed investigation with china(kaolin-type) and fire clay samples from Pakistan. The best yield (60%)was obtained with a china clay treated with hot concentrated sulfuric acid.The reaction was carried out at 110°C with 2% by weight catalyst in astirred round-bottom flask. Other clays catalyzing camphene productionare H- and Al~montmorilloflitC (Dupont and Dolou, 1948; Ovcharenko,1982), H-vermiculite, H-kaolinite, and H-hydrous mica (Battalova et al.,1977). in all cases yields of around 60% were obtained.

E. CholeStefle Rearrangement

During studies concerning the origin of polycyclic compounds present insediments, shales and crude oils by the group of Albrecht at Strasbourg(Sieskind and Albrecht, 1985) it was discovered that cholest-5-ene (III)could be converted quantitatively to ~~ckbone~rearranged 20-R and 20-S-cholest-13(17)efles (IV) at room temperature in the presence of dry K-10montmorillomte (FLUKA)

C-~v~‘C C, CC N ‘C CC ‘C N — ‘C — CCC) CC CC ‘C N In N N N C’ CC

N’C’C ‘C’C’C’C’C ‘COfl’CCC CC“C CC) CC CC CC SC ‘C CC CC CC SC CC C, ‘SrN N N N N N N N N N N N N N

“C ‘C ‘C ‘C — — — ‘C CC ‘C N ‘C — —“C ç•) C) N N N N N CC CC

CCCC‘C’HN N ~ h”? —

— ‘C N N N C, N ‘C — NN NN

N ~N N N ‘C SC‘ N N N N ‘C ‘C

‘C’C’C ‘C’CCCC,’C CC’C’C’C ‘CCCCCN N”C,C,N CC’CCCN’C ~

(“C ‘‘ (“C (‘S

C,‘C00 C

~a~ “a~ E-~ C IC~“i ~&~ a~ ~‘—z I I‘C ‘C ‘C ‘C ‘C ‘C - ,~ ‘C ≥ - -C..0C. ~~ti~C.C,~ ~~CC~N flCflCC,flCC~CflI,I

~ ‘C’C’C’CC~ ,-.‘CC,C.Q,

C) C)

~ a~ ‘C C)C) C)

C.

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aC.

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‘-CCC’C aC)C .0.00 “.0‘~.C 0.0

.C~ 00

~‘C

~ •~>a~,. 0CC’—

~~ao C_~C>CC —‘--a02. ‘C0~C)-~~ C)C)~~

~ C.~SO a~C .~ s’ z .0 “~O-CC~ —-aa a

-a a .a a~~’-’a

-II ‘. C — C’)II CC C) C)

> ~ ~ aCC ta C. ~, II ‘C

ii C- C

17

IvO — N N C, ‘C) ‘C N CC ‘C — N N C,z

III

I.,. ~ _. I iou 0111 iOu I, Oi 11.1 rt I-’u. VctII OctuIL~lI LO. l.~,Iay-ni_LivQtci..J 101.11101 IL.aLIuJI I I IUCIt.LIIJ1 Ia

Unfortunately, the “dryness” was not specified. A typical experiment wasdone in the following way: 750 mg of reactant was added to the sameamount of clay suspended in 6 mL of dry cyclohexane, and the mixture wasstirred for 1 h. Without the clay catalyst this reaction proceeds only undersuperacid conditions, e.g., p-toluenesulfonic--acetic acid at refiux temperature. Furthermore, the reported yields are only of the order of 50%(Blunt et al., 1969; Kirk and Shaw, 1970). So we have here a clear exampleof a very effective clay catalyst.

F. Oleic Acid Isomerization

Montmorillonites are industrially used as catalysts for the dimerizationof unsaturated fatty acids to dicarboxylic acids (Johnston, 1944). Sometimes considerable amounts of isomerized products can be found in thereaction products. Nakano et al. (1985) observed that heating oleic acid at230°C for 3 h with a montmorillonite clay (Alabama Blue Clay) gave abranched chain isomer yield of 27%. Adding water increased the yield by10% (higher Brønsted acidity). On (CH3)4N pillared montmorillonitemainly cis—trans isomerizations took place, which was probably due toinduced shape selectivity (Weiss, 1981).

V. CONCLUSIONS

We did an extensive literature search on clay-catalyzed isomerizationreactions. It appears that clays have only scarcely been used in syntheticorganic chemistry reactions, while much more effort has been devoted tothe industrially important acid-catalyzed paraffin isomerization.

We conclude that Brønsted acidic properties play an important role inclay-catalyzed isomerization reactions. Also, clay selection, pretreatment,and activation procedures are important parameters in obtaining anoptimum acidity. This does not seem to be fully recognized in the syntheticorganic chemistry literature, where details on clay catalyst preparation,characterization, and activation are often lacking.

In addition, the present review indicates that to obtain H~-exchangedclay catalysts which combine a high thermal (~-500°C) stability with ahigh Brønsted acidity, the negative charge on the clay layers should arisefrom isomorphous substitution in the tetrahedral sheets, such as is the casefor beidellite. This results in active sites with acid strength comparable tothose of zeolites. Therefore, clays may be advantageously used either inapplications where zeolites are more expensive or in reactions of relativelylarge molecules that are unable to enter the zeolite pore structure.

REFERENCES

Aidridge, L. P., McLaughlin, J. R., and Pope, C. G. (1973). J. Catal. 30, 409.Anderson, J. R. (1973). Adv. Catal. 23, 1.Barrer, R. M. (1978). “Zeolites and Clay Minerals as Sorbents and Molecular Sieves.”

Academic Press, New York.Barrer, R. M. (1982). “Hydrothermal Synthesis of Zeolites.” Academic Press, New York.Barrer, R. M., and MacLeod, D. M. (1955). Trans. Faraday Soc. 51, 1290.Battallova, S. B., Pak, N. D. and Mukitayeva, T. R. (1977). Izv. Akad. Nauk Kaz. SSR 2,

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~A OXIDATIONS AND CATALYTIC~ OXIDATIONS

A. McKiIIop D.W. Clissold

School of Chemical SciencesUniversity of East AngliaNorwich NR4 7TJ, England

I. INTRODUCTION

Adsorption of a number of oxidizing agents on clays not only induceschanges in their reactivity and selectivity but also simplifies commonly encountered workup and isolation problems associated with these reagents.

Foremost among the clay minerals used for this purpose has been thehydrous aluminosilicate— montmorillonite. Its adsorptive behavior towardsmall organic molecules (Theng. 1974) and organic polymers (Theng,1979) has been described in some detail. The ability of montmorillonite tointercalate a large range of guest molecules (see Chapter 11) is belated toits ability to undergo extensive expansion of its lamellar structure (Theng,1979) as indicated in Fig. 1. The lamellae are composed of aluminosilicatesheets consisting of aluminium ions in an octahedral environment sandwiched between two layers of Si04 tetrahedra. In the naturally occurringclay the main interlayer cations are sodium and calcium; however, cationexchange can be effected by treatment with solutions of other ions such asLi~, Mg2’, Al3~, and NH4’~ (Theng, 1974). Isomorphous replacement ofAl3~’ ions by Mg2~ and Fe2~ and of Si4~ ions by Al3~ results in the layerscarrying a permanent negative charge of 0.25—0.6 per formula unit. Thischarge is balanced by exchangeable counterions which, apart from thoseassociated with the external clay surface, occupy the interlamellar regions

PREPARATIVE cHEMIsTRyUSING SUPPORTED REAGENTS

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