13.7.2 Mechanism of Skeletal Isomerization of C5+ Alkanes with Acid Catalysts 2809

13.7Isomerization

Swan Tiong Sie∗

13.7.1Introduction

Probably the most important isomerization reaction inthe context of energy-related catalysis is the skeletal iso-merization of alkanes and alkenes. Other hydrocarbonisomerization reactions that play a role are the doublebond isomerization of alkenes, interconversion of alkyl-cyclopentanes and cyclohexanes, isomerization of bi- andpolycyclic saturated hydrocarbons and isomerization ofalkyl aromatics. These other isomerization reactions canoccur during acid- or bifunctionally catalyzed conversions,for instance in catalytic reforming, catalytic cracking,hydrocracking, polymerization and alkylation, but in gen-eral they are not carried out as dedicated conversionprocesses in the energy field.

Skeletal isomerization of acyclic hydrocarbons, bycontrast, is the basis for several important processesin the hydrocarbon processing industry. These includethe conversion of light linear alkanes such as n-pentaneand n-hexane into their branched isomers to improve theoctane quality of the gasoline in which they are present.Conversion of n-butane into isobutane is of importanceto increase the availability of the latter hydrocarbon foralkylation or for producing methyl tert-butyl ether (MTBE)via dehydrogenation to isobutene. Skeletal isomerizationof higher alkanes is the basis for the conversion of paraffinwax into lubricating base oils with excellent viscometricproperties and for improving the cold-flow behavior ofparaffinic oils. Skeletal isomerization of normal butenes isanother way of obtaining isobutene for MTBE production.

In view of the above, this discussion of isomerizationfocuses primarily on the skeletal isomerization ofalkanes. More specifically, we will discuss acid-catalyzedisomerization of alkanes and ignore metal-catalyzedisomerizations, since the acid-catalyzed route is by farthe most important one in practice.

Extensive reviews of isomerization reactions of purehydrocarbons in a broader sense have appeared in theliterature, for example the monograph by Egloff et al. [1]and the review by Condon [2], covering the literatureup to 1958 and dealing mainly with mechanisms,kinetics and thermodynamic aspects. In the presentreview, more recent concepts and data are discussed andattention is given to the technological aspects of alkaneisomerization.

∗ Corresponding author.

13.7.2Mechanism of Skeletal Isomerization of C5+ Alkanes withAcid Catalysts

13.7.2.1 Reaction Cycle in Isomerization of AlkanesAcid-catalyzed skeletal isomerization of alkanes occursvia carbenium (formerly called carbonium) ions asintermediates. It is a part of a chain reaction, that is,a reaction cycle involving chain initiation, carbeniumion rearrangement (the isomerization reaction proper)and chain propagation. Scheme 1 shows the conventionalmechanism of acid-catalyzed isomerization of n-pentane.Whereas the notion of a ‘‘free’’ carbenium ion asreaction intermediate is a very useful one to describethe phenomena in acid-catalyzed reactions, carbeniumions may not exist as such in these reactions, but thecharged intermediate species may be complexed with theacid catalyst. For example, in the case of an aluminosilicate(zeolite) catalyst having acidic hydroxyl groups, thecharged species may be an alkoxy intermediate, ratherthan a free carbenium ion. For the sake of simplicity,however, we will discuss the mechanism of acid-catalyzedisomerization in terms of ‘‘carbenium ions’’, since in thisdiscussion it is immaterial whether the ion is free orbound to the catalyst.

In the chain initiation reactions, carbenium ions areformed from neutral hydrocarbon molecules in contactwith the acid catalyst, from alkenes by addition of a protonsupplied by the acid catalyst and from alkanes by eithera combination of dehydrogenation and proton additionor by abstraction of a hydride ion. The latter ion can beaccepted by the acid catalyst, by combining with a protonto form molecular hydrogen.

In the chain propagation reaction, a carbenium ionreacts with a neutral feed hydrocarbon molecule wherebytransfer of a hydride ion from this molecule to thecarbenium ion results in a neutral hydrocarbon productmolecule and a new carbenium ion originating fromthe feed molecule. This new ion can in turn undergoisomerization, thus perpetuating the reaction cycle.

13.7.2.2 The Isomerization Reaction: Rearrangementof the Intermediate Carbenium IonA simple way to visualize skeletal isomerization is toassume an alkyl shift, such as a methyl shift. Thus,a methyl ion is detached from the carbenium ionchain and reattached at another position in the residualhydrocarbon chain. This simple mechanism is highlyunlikely, however, since the methyl ion is a high-energyspecies so that its detachment from the carbenium ionchain would involve a prohibitively high activation energy(see Table 1).

References see page 2828

2810 13.7 Isomerization

Chain initiation:

CH3 CH2 CH2 CH2 CH3 + H+

CH3 CH CH2 CH2 CH3 + H2

+

Carbenium ion rearrangement:

CH3 CH CH2 CH2 CH3

+

+ +CH3 CH CH2 CH2

CH3

CH3 C CH2 CH3

CH3

Chain propagation:

CH3 C CH2 CH3 + CH3

+

+CH3 CH CH2 CH3 + CH3

CH3

CH3

CH2 CH2 CH2 CH3

CH CH2 CH2 CH3

Scheme 1 Reaction cycle in isomerization of n-pentane.

Tab. 1 Heats of formation of alkylcarbenium ions [3]

Carbenium ion Type of ion Heat offormation/kJ mol−1

Methyl Primary 1080Ethyl Primary 942n-Propyl Primary 913n-Butyl Primary 883Isopropyl Secondary 812sec-Butyl Secondary 795tert-Butyl Tertiary 733

Another argument against the classical alkyl shiftmechanism is that this mechanism would favor theformation of isomers with larger side-groups than methylgroups. Detachment of primary ethyl, n-propyl and n-butylcations from a long-chain secondary carbenium ion wouldinvolve less energy than of a methyl ion, as can beseen in Table 1. Thus, such an alkyl shift mechanism isincapable of rationalizing the very strong preponderanceof methyl-branched isomers, as observed in the skeletalisomerization of n-alkanes.

The most likely mechanism of skeletal isomeriza-tion of the intermediate carbenium ion involves therearrangement of the classical secondary carbenium ioninto a non-classical carbonium ion, namely a proto-nated dialkylcyclopropane. This mechanism, originallyproposed by Condon [4] and Brouwer [5], is depicted inScheme 2.

The transformation of the classical secondary car-benium ion into the protonated dialkylcyclopropanestructure will not involve a high energy barrier since

the latter structure can be considered to be a hybrid of res-onance structures, as shown in Fig. 1. This resonance willcontribute to the stability of the protonated cyclopropanespecies and will compensate for the inherent strain ofthe three-membered ring. That resonance is capable ofgreatly enhancing the stability of this ring is demonstratedby the high stability of the cyclopropenyl cation, whichcan even display some aromatic character. For example,the triphenylcyclopropenylium cation is so stable that itssalts can be isolated [6].

Protonated cyclopropane structures as reaction inter-mediates have been deduced from the stereochemistryof Wagner–Meerwein-type rearrangements of norbornylderivatives and evidence for this mechanism has also beenobtained by experiments with carbon isotopes [7]. Sincethese rearrangements occur under mild conditions (be-low 50 ◦C with only moderately strong acids), it is unlikelythat high energy barriers are involved. Mass spectrometricexperiments have provided supporting evidence for theexistence of protonated cyclopropane in the gas phase andthe ionization potential for formation of this ion proved tobe not very much higher than that for a classical secondaryion [8].

C C

H H

R R’

CH H

H

C C

H H

R R’

CH H

H

C C

H H

R R’

CH H

H+

+ +

Fig. 1 Some resonance structures of protonated dialkylcyclo-propane. In addition to the corner-protonated structures shown,edge- and face-protonated structures are in principle also possible.

13.7.2 Mechanism of Skeletal Isomerization of C5+ Alkanes with Acid Catalysts 2811

CC C C C HHH

H

H

H

H

H

H

H

H

H

Hydride abstraction/transfer n ≥ 1

m ≥ 1

Linear alkane

Classicalcarbenium ion

CH H

H+Non-classicalion

Classicalcarbenium ionC

HHH

Hydride transfer

Isomerized product

n m

CC C C C HHH

H

H

H

H

H

+

H

H

H

n m

CC C C HHH

HHH

H

H

n m

CC C+

C HHH

HHH

H

H

n m

CHH

H

CC CH

C HHH

HHH

H

H

n m

Scheme 2 Isomerization via a protonated cyclopropaneintermediate.

On the basis of the above arguments, isomerization viaa dialkylcyclopropane intermediate seems an acceptablemechanism from an energy point of view. Furtherevidence for this mechanism is presented below.

13.7.2.3 Supporting Evidence for the ProtonatedCyclopropane (PCP) Isomerization Mechanism

13.7.2.3.1 Reactions with n-Butane Strong evidencefor the PCP isomerization mechanism is the findingof Brouwer and Oelderik [9, 10] that n-butane is notisomerized by the superacid HSbF6 at room temperature,whereas n-pentane and n-hexane are rapidly converted totheir isomers. This can be understood from Scheme 2.Since for n-butane the value of m is zero, the ruptureof the only bond in the cyclopropane ring that wouldlead to the isobutane structure involves the formationof an energetically unfavorable primary carbenium ion.Breakage of the two other bonds is possible, but will inboth cases result in a carbenium ion with a straight chain.

The heats of formation of primary carbenium ionsare about 100 kJ mol−1 higher than those of the

corresponding secondary ions, as can be seen fromTable 1. Since a difference in activation energy of thismagnitude at a temperature of 100 ◦C corresponds to a dif-ference in reaction rates by a factor of 1014, the breakage ofthe one cyclopropane bond that leads to isomerization isso strongly disfavored compared with breakage of the twoother bonds that its occurrence can be neglected.

The breakage of these other two bonds leads to either theoriginal n-butane molecule or another n-butane moleculein which two carbon atoms have exchanged their positionin the chain of carbon atoms (Fig. 2). Using 13C-labeledn-butane, Brouwer [5] was able to show that this exchangeindeed occurred and that the rate of isotope scramblingwas comparable to the rate of skeletal isomerization ofn-pentane under similar conditions (Fig. 3).

Similar evidence for the PCP isomerization mechanismwas collected by Chevalier et al. in later experiments with13C-labeled n-butane over a Pt/silica–alumina catalystat 300 ◦C [11]. Under these conditions, which are morerepresentative for isomerization of alkanes in practicalprocesses, they observed very little skeletal isomerizationof n-butane, but scrambling of 13C occurred. Moreover, inexperiments with different catalysts the rate of scramblingproved to correlate very well with the transformation ofn-pentane into isopentane, as can be seen in Fig. 4.

13.7.2.3.2 Effect of Chain Length on the Relative Ratesof Isomerization of n-Alkanes As discussed above, n-alkanes larger than n-butane can be isomerized by the PCPmechanism. Since the number of possible cyclopropanestructures increases for longer chains, the reactivity forisomerization also increases with chain length. With thesimplifying assumption that all cyclopropane structuresinvolved in isomerization form and react with equalchances, it can be deduced that the isomerizationreactivity of the n-alkanes with carbon number N shouldbe proportional to N –4. The experimental data ofMaslyanskii et al., as reported by Weisser [12], are inline with this prediction, as can be seen from Table 2.

13.7.2.3.3 Location of Branching in Isomerized AlkanesA detailed study of the isomer distribution of theproducts obtained in the isomerization of n-alkanes atlow conversion levels has also provided evidence forthe PCP mechanism. According to this mechanism, theformation of the 2-methyl isomer will be less likely thanthat of the 3-methyl isomer for alkanes with relatively longchains. This has been observed in actual isomerizationexperiments, for example, by Steijns et al. [13] and byWeitkamp [14]. The abnormally low abundance of the2-methyl isomer in the product of primary isomerization

References see page 2828

2812 13.7 Isomerization

C

HH

HC

C

H H

H2

C

H H

H1 3

4

C

HH

H C

C

H H

HC

H

H

H+

+

C

C

C

HH

H

H H

HC

HH

H +

C C

H H

CH

C

HHH

H

H

H+1

2 4

3

a

a

b

b

Fig. 2 Possible and forbidden rearrangements of the sec-butyl cation.

0.7

0.6

0.5

0.4

0.3

0.20.1

00 0.90.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

Rate C4 scrambling

Rate C5 isomerization= 0.25

Observed/equilibrium conversion of pentane

Obs

erve

d/eq

uilib

rium

con

vers

ion

of b

utan

e

Fig. 3 Conversion of n-butane[1-13C] to n-butane[2-13C] compared with isomerization of n-pentane to isopentane. Experiments with amixture of n-pentane and labeled n-butane over HSbF6 catalyst at 0 ◦C [5].

30

20

10

10 20 30 400

0

T = 300 °C0.2% Pt

0.5% Pt

1.2% Pt

Catalyst:Pt/SiO2Al2O3

Scr

ambl

ing

of n

-But

ane

[1-13

C]/%

Coversion of n-Pentane/%

Fig. 4 Correlation between scrambling of n-butane[1-13C] and skeletal isomerization of n-pentane [11].

13.7.2 Mechanism of Skeletal Isomerization of C5+ Alkanes with Acid Catalysts 2813

Tab. 2 Relative rates of isomerizationof n-alkanes over a tungsten disulfidecatalyst [12]

Alkane Relative rate

Experimental PCP theory

n-Pentane 1 1n-Hexane 2.0 2n-Heptane 3.1 3n-Octane 4.2 4

of n-alkanes, especially those of higher carbon number,can be seen in Fig. 5.

Figure 6 compares the experimentally observed distri-butions of monomethyl-branched isomers from isomer-ization of n-alkanes with distributions predicted from thePCP reaction scheme according to a simplified model asproposed by Weitkamp [14]. In this model, the chancesof formation of protonated cyclopropane structures areconsidered equal for all possible structures, while thelikelihood of breaking the bonds in the cyclopropane ringis supposed to be the same for all bonds. It can be seen thatthere is reasonably good agreement between theory andexperiment. However, some systematic deviations can beobserved in Figs. 5 and 6: for the hexanes, heptanes andoctanes there is an excess of 2-methyl isomers, whereasthere is a slight underproduction of these isomers for do-decane and higher alkanes. These systematic deviationshave been ascribed to some difference in the rates of thering-opening modes forming the 3-methyl-2-n-alkyl and2-methyl-3-n-alkyl cation, related to the relative stability ofthese cations, as suggested by Martens and Jacobs [15].

The latter authors also suggested that protonatedcycloalkanes with larger rings are involved in orderto explain the formation of isomers with larger side-groups than the methyl group. However, compared

0

80

60

40

20

2-M

e is

omer

in m

ethy

lis

omer

frac

tion

/%

6 8 10 12 14

Carbon number of feed alkane

Estimated for equilibrium and calculatedfor classical mechanism

Experimental (X iso = 1 to 3%)

Calculated for PCP mechanism

Fig. 5 Formation of 2-methyl isomers in the isomerization ofn-alkanes of different chain length [14].

60

40

20

0A

bund

ance

/ %

of m

ethy

l iso

mer

s 2–Me 3–Me

Predicted

Experiment

6 8 10 12 14 6 8 10 12 14

Carbon number

40

20

0

4–Me 5–Me 6–Me 7–Me

Carbon number

8 10 12 14 10 12 14 12 14 14 15

(a)

(b)

Fig. 6 Distributions of monomethyl branched isomers fromisomerization of n-alkanes of different chain length [15].

with the methyl-branched isomers, the isomers withethyl, propyl and butyl branches are generally formedin minor amounts only; for example, in the case ofn-heptane isomerization only 3% ethylpentane was foundamong the isomers, the remaining 97% being methyl-branched alkanes [14]. Isomerization via a protonatedcyclobutane intermediate leading to the ethyl-branchedisomer is apparently much less favored than by the PCProute, which may be explained by the lower stabilityof protonated cyclobutanes. Protonated cyclobutane hasbeen found to be 130 kJ mol−1 less stable than protonatedmethylcyclopropane [16].

13.7.2.4 Chain Termination Reactions: CatalystDeactivationIf there were no chain termination reactions, the reactioncycle for isomerization, once initiated, would continueindefinitely, given ample supply of reactants. In practice,

References see page 2828

2814 13.7 Isomerization

however, chain termination reactions are responsible foracid consumption or catalyst deactivation.

Chain termination occurs when a carbenium ionundergoes a hydride transfer reaction with an alkenemolecule, instead of with an alkane molecule as in chainpropagation. Hydride transfer between a carbenium ionand an alkene gives rise to an alkane and an unsaturatedcarbenium ion. The latter ion will be an allylic carbeniumion, which can be considered as a protonated diene that isa much more stable species than the original carbeniumion. In a similar way, a carbenium ion may be involvedin a hydride transfer with a conjugated diene to form analkane and the protonated form of a conjugated triene,and so on. The triene molecule may cyclize to form anaromatic molecule.

The highly unsaturated molecules are much more basicand bind the protons much more strongly than simplemonoalkenes. They are incapable of participating in thereaction cycle and give rise to a loss of effective acidity orof catalyst activity.

Another way in which polyalkenic species can beformed is by cracking, which occurs particularly withcarbenium ions having longer chains originating fromheavy feed molecules or from oligomerized light alkenes.Cracking gives an alkanoic and an alkenic fragment.If the latter fragment already contained a double bondthat was present in the original feed molecule, a dieneis formed. Thus, conditions which are inducive tooligomerization/cracking (disproportionation) of alkenesare likely to give rise to catalyst deactivation.

13.7.2.5 Acid-catalyzed Cracking and Isomerization: Effectof Chain Length on Selectivity for IsomerizationSince cracking and isomerization are both catalyzedby similar catalysts, it is plausible that cracking mayaccompany isomerization, thus decreasing isomerizationselectivity. This is particularly true for higher alkanes.

Acid-catalyzed cracking of alkanes also proceeds withcarbenium or carbonium ions as intermediates. Theclassical, generally accepted mechanism assumes thatin a classical secondary carbenium ion the C–C bond inthe β-position to the charge center is broken (β-scission).However, from energetic considerations, β-scission israther unlikely in the case of a secondary ion with a straightchain. Many other facts related to experimentally observedcharacteristics of acid-catalyzed cracking also stronglyargue against this β-scission cracking mechanism [17].

Another mechanism that has been proposed assumes anon-classical carbonium ion, namely the same protonateddialkylcyclopropane species as in isomerization, to bethe reaction intermediate in acid-catalyzed cracking [17].For energetic reasons, namely the avoidance of C–Hdissociation in primary carbon atoms, it follows that

cracking may occur if the hydrocarbon has seven or morecarbon atoms.

The relationship between cracking and isomerizationover bifunctional catalysts in the presence of hydrogen(see below) is depicted in Scheme 3. This scheme suggeststhat hydroisomerization and hydrocracking may occuras parallel reactions, in addition to sequential reactions(cracking of pre-isomerized feed molecules and post-isomerization of cracked fragments).

Since skeletal isomerization requires a carbon chainof more than four atoms whereas cracking needs atleast seven carbon atoms in the chain, pentanes andhexanes are the only members of the homologousseries of alkanes that can be easily isomerized, butnot easily cracked. Indeed, in practice highly selectiveisomerization of pentanes and hexanes and their mixturesis readily achievable. For higher alkanes, crackingusually accompanies isomerization, resulting in a lowerisomerization selectivity.

The clear difference in the behavior of n-pentaneand n-hexane versus n-heptane is illustrated by thedata in Table 3, obtained over the same catalyst in

n ≥ 1

m ≥ 3m ≥ 1

n-Alkane

Classicalcarbenium ion

Non-classicalion

C C HH

Isomerized productHH

H HH+

Cracked products

CC C C C HHH

H

H

H

H

H

H

H

H

H

n m

CC C C C HHH

H

H

H

H

H

+

H

H

H

n m

CH H

H+

CC C C HHH

HHH

H

H

n m

CHH

H

CC C C HHH

HH

H

H

H

H

n m

CHH

H

CC CH

HH CHHH

H

H

n m-2

Scheme 3 Isomerization and cracking via a protonated cyclo-propane intermediate.

13.7.3 Mechanism of C4 Alkane Isomerization 2815

Tab. 3 Isomerization of n-pentane, n-hexane and n-heptane over Pt/H-mordenite [18](P = 2.4–3.0 MPa, weight hourly space velocity = 1 g g−1 h−1, H2 : HC = 2.5 : 1)

Parameter n-Pentane (N = 5) n-Hexane (N = 6) n-Heptane (N = 7)

Temperature/ ◦C 250 250 210Cracked

products(<CN)/wt.%

0.7 2.1 55.3

BranchedCN/wt.%

60.4 70.0 24.3

UnconvertedCN/wt.%

38.9 27.9 20.4

Iso-CN/totalCN/%

60.8 71.5 54.4

Approach toequilibrium/%

86 85 59

experiments by Kouwenhoven and Wagstaff, as reportedby Sie [18]. n-Pentane and n-hexane are very selectivelyisomerized: nearly 100% selective isomerization at arather close approach to thermodynamic equilibrium.By contrast, for n-heptane as feed, the isomerization se-lectivity with the same catalyst is considerably lower:only 30% at a degree of isomerization still far below theequilibrium limit. Extensive cracking has accompaniedisomerization, notwithstanding the much lower temper-ature applied.

13.7.3Mechanism of C4 Alkane Isomerization

As discussed before, the PCP mechanism is only operativefor n-alkanes having at least five carbon atoms and istherefore not directly applicable for skeletal isomerizationof butanes. Under conditions where pentanes and hexanesundergo skeletal isomerization, n-butane is generally leftunconverted. This is illustrated by the data in Table 4,obtained with a Pt/mordenite catalyst. Under much moreforcing conditions, however, isobutane formation fromn-butane can be observed over this catalyst, albeit at arelatively low selectivity (Fig. 7).

This formation of isobutane occurring under muchmore severe conditions than required for pentane andhexane isomerization must therefore take place by adifferent mechanism. A possibility is the involvement ofC8 species as intermediates, which can undergo skeletalisomerization followed by cracking. Such a reactionmechanism (Scheme 4) assumes that n-butene, formedby dehydrogenation of n-butane or by deprotonation of abutyl cation and present in small amounts in the steadystate, reacts with a tert-butyl cation abundantly present inthe steady state. Thus, an isooctyl(dimethylhexyl) cation isformed, which has a sufficiently long backbone chain to be

Tab. 4 Isomerization of n-butane mixed with C5/C6 light naph-tha over a Pt/H-mordenite catalyst (T = 250 ◦C, P = 2.5 MPa,H2 : HC = 2.5 : 1)

Component Feed Producta

<C4/wt.% 0.0 0.41Iso-C4/wt.% 0.0 0.80b

n-C4/wt.% 8.28 8.48Iso-C5/wt.% 15.8 26.6n-C5/wt.% 26.0 15.8Iso-C6/wt.% 23.3 35.0n-C6/wt.% 19.7 8.1Others/wt.% 6.9 4.8Iso-C4/total C4 alkanes – 0.09 (0.53)Iso-C5/total C5 alkanes – 0.63 (0.73)n-C6/total C6 alkanes – 0.19 (0.16)

aValues in parentheses are equilibrium data.bIn a comparative experiment without n-butane in the feed,0.94 wt.% isobutane was found in the product resulting fromhydrocracking.

isomerized to a trimethylpentyl cation. The latter cation,for example the 2,2,4-trimethylpentyl cation depicted inScheme 4, can be easily and selectively cracked to twofragments, both of which have the isobutane skeleton,namely an isobutene molecule and a tert-butyl cation. Thehigh stability of the this cation is the reason why the highlybranched octyl cation cracks so readily. The isobuteneis converted to isobutane by reaction with hydrogen orprotonated to form another tert-butyl cation. tert-Butylcations can start a new reaction cycle or be converted toisobutane by hydride transfer.

The above scheme is an alkylation/dealkylation mecha-nism, with an intermediate skeletal rearrangement. Thenet result of this reaction cycle is the same as for a

References see page 2828

2816 13.7 Isomerization

70

60

50

40

30

20

10

0

wt.%

360300 310 320 330 340 350

Temperature / °C

Conversion ofn-butane

Selectivity toisobutane

Fig. 7 Conversion of n-butane over Pt/H-mordenite (P = 2.5 MPa;H2 : HC = 2.5 : 1).

CH2 CH2 CH3 CH CH2 CH3 + H2

CH3

C+

H3C H2C

CH3

+ H3C

CH3

CH3C

CH3

CH2 CH CH2 CH3

CH3

CH3C

CH3

CH2 C CH3

CH3

++

CH3

C

C

H3C

H3C

CH3

+ + H2C C

CH3

CH3

+ H2

CH3

H

CH3

Scheme 4 Conversion route from normal butane to isobutane.

simple skeletal isomerization, namely the transformationof n-butane into isobutane.

13.7.4Isomerization Catalysts

There are in principle two types of catalysts for skele-tal isomerization of alkanes via carbenium or carbonium

ions as intermediates: monofunctional acidic catalysts andbifunctional catalysts that combine the acidic isomeriza-tion function with the hydrogenation–dehydrogenationfunction of a metal.

13.7.4.1 Monofunctional Acidic CatalystsA variety of acids have been tested for the skeletal isomer-ization of alkanes. The most prominent representativeof this class is aluminum chloride. This acid of theFriedel–Crafts type is not used as such, but in combina-tion with HCl to give HAlCl4, which is the Brønsted acidactive in isomerization. The HCl/AlCl3 system has beenextensively applied in older industrial processes, as willbe discussed later.

The role of HCl in producing active centers forisomerization by interaction with AlCl3 is illustratedin Fig. 8, showing the increase in hexane isomerizationactivity with increasing HCl pressure for AlCl3 supportedon alumina [19].

Among liquid acid catalysts, the superacid HSbF6 is ofinterest insofar as it allows skeletal isomerization of lightalkanes to be carried out at temperatures as low as roomtemperature [10].

Monofunctional acidic catalysts suffer from the dis-advantage that chain termination reactions give rise tocatalyst deactivation, as discussed before. Therefore, inprocesses that use monofunctional catalysts, replenish-ment with fresh acid during operation is required tocompensate for loss of acidic activity. This is the mainreason why older processes based on monofunctional cat-alysts have been superseded by processes based on the useof bifunctional catalysts, since the need to supply freshacid and to dispose of spent catalyst are disadvantagesfrom both an economic and ecological point of view.

The inevitable decline of the activity of a monofunctionalcatalyst of the HCl/AlCl3 type is illustrated in Fig. 9 for

0.05

0.04

0.03

0.02

0.01

0

1 / k

/ (g

Fee

d)−1

g A

lCl 3

h

50 1 2 3 4

1 / PHCl / MPa−1

Fig. 8 Langmuir–Hinshelwood plot showing the increase in theactivity of alumina-supported AlCl3 with increasing pressure ofHCl [19].

13.7.4 Isomerization Catalysts 2817

2520151050

20

10

k / g

Fee

d (g

AlC

l 3 h

)−1

86

4

2

1

Wt. % H2O on dry carrier:1.202.402.613.03

Run hours

Fig. 9 Decline of n-hexane isomerization activity of alumina-supported HCl/AlCl3 catalyst [19].

hexane isomerization over AlCl3 supported on hydratedalumina [19].

13.7.4.2 Bifunctional CatalystsIn this class of catalysts, the acid is combined witha metal having hydrogenation/dehydrogenation activity.Examples of such catalysts include chlorided aluminacombined with platinum, silica–alumina combined withplatinum and acid forms of zeolites combined withplatinum or palladium. Sulfides of molybdenum andtungsten, optionally promoted with nickel and supportedon (halogenated) alumina or silica–alumina, can also beconsidered as bifunctional catalysts.

The main advantage of using bifunctional catalysts isthe possibility of stable isomerization under a sufficientlyhigh hydrogen pressure (this isomerization is oftenreferred to as hydroisomerization). The improved catalyststability can be understood from the mechanism ofchain termination as discussed before: formation ofpolyunsaturated compounds by hydride transfer reactionsbetween a carbenium ion and an alkene and by crackingof higher alkenes (oligomerized light alkenes) will bereduced when the alkene concentration is kept low bysaturation with hydrogen.

The hydrogen pressure will also affect the steady-state concentration of carbenium ions by the equilibriumreaction

R+ + H2 −−−→←−−− RH + H+

so that the rate of isomerization will also be reduced.However, the effect of hydrogen pressure on the chaintermination reactions is much stronger, since theseinvolve bimolecular reactions between reaction partnersthe concentrations of which are both affected by thehydrogen pressure. Thus, by operating under hydrogenpressure in the presence of a hydrogen-activating catalyticfunction, chain termination can be made insignificantrelative to chain propagation.

The effect of hydrogen and a hydrogen activatingfunction can be seen from Fig. 10. In the absence ofa metal function, a very high activity is initially observedfor isomerization of n-pentane over bare H-mordenite (anacidic zeolite), but the activity decreases very rapidly.In contrast, a perfectly stable operation is possibleunder a hydrogen pressure of 3 MPa with platinumpresent on the catalyst. Under the latter conditions theselectivity is nearly 100%, whereas in the absence ofplatinum the isomerization selectivity is much lowerowing to the formation of products lighter and heavierthan pentanes. These disproportionation products areindicative of cracking and oligomerization reactions thatplay a role in chain termination.

In the industrial isomerization of light alkanes, the mostimportant bifunctional catalysts are platinum supportedon an alumina which is chlorided by passing HCl (eitheras such or obtained by decomposing an organochlorinecompound under operating conditions) over the catalystand platinum supported on aluminosilicates.

Crystalline aluminosilicates (zeolites) have largely re-placed amorphous silica–aluminas used in classicalprocesses, mainly because their much higher activityallows lower operating temperatures with less thermody-namic constraints for isomerization (see below). A zeolitewhich has found wide application in light alkane isomer-ization is mordenite in the hydrogen form. The choice ofthis zeolite can be understood by considering that easymovement of branched hydrocarbons in the intracrys-talline space calls for zeolites with wider pores (that is,determined by 12-membered oxygen rings) rather than

80

60

40

20

0

Con

vers

ion

/ %

0 10 20 30

H–mordenite + Pt

H–mordenite withoutPt

(97%)

(70%)

(Selectivity toi -C5)

Run hours

Fig. 10 Effect of Pt on the stability of H-mordenite for isomeriza-tion of n-pentane in the presence of hydrogen [20].

References see page 2828

2818 13.7 Isomerization

the so-called narrow- and medium-pore zeolites (8- and 10-membered rings) and that have a high acidity. Figure 11compares the acidities of known 12-ring zeolites, and itcan be seen that mordenite ranks high within this class.

The activity of mordenite for light alkane isomerizationmay be further increased by partial dealumination,effected by leaching with aqueous HCl, as can be seenin Fig. 12. Recent studies by van Donk et al. [23] haveprovided evidence that the beneficial effect of acid leachingcan be attributed to facilitated transport of reactants andproducts, as a result of the formation of mesopores byleaching away alumina [24, 25]. Figure 12 shows that thereis an optimum silica : alumina ratio; beyond this optimumthere is a loss of activity which is presumably caused by adecrease in the number of acidic sites associated with theloss of aluminum atoms.

13.7.5Isomerization of Pentanes and Hexanes for OctaneEnhancement

The most important application of alkane isomerizationis the improvement of the octane quality of lightfractions of petroleum (so-called tops or light naphtha,consisting mainly of C5 and C6 hydrocarbons). Asdiscussed in a previous section, isomerization of pentanesand hexanes can be very selective with minimalcracking. Several review papers have appeared in thecourse of time that discussed advances in the field ofisomerization [26–35].

13.7.5.1 Octane Numbers and ThermodynamicEquilibrium Concentrations of Isomeric Pentanesand HexanesAs can be seen from Table 5, branched alkanes havehigher (blending) octane numbers than n-alkanes. Theisomerization of light n-alkanes is a reaction which islimited by the thermodynamic equilibria between theisomeric alkanes. Figure 13 shows the equilibria betweenn-butane and isobutane and between n-pentane andisopentane in the gas phase. Figure 14 shows the gas-phase equilibrium concentrations of the different hexaneisomers.

From these figures, it can be seen that the concen-trations of isomers with high octane numbers, namelyisopentane and 2,2-dimethylbutane, decrease with in-crease in temperature, whereas the concentrations ofthe low-octane n-alkanes increase. Therefore, the attain-able octane number of the isomerized product decreaseswith increasing reaction temperature, as can be seen inFig. 15. Accordingly, the activity of the catalyst used playsan important role in determining the octane ceiling.

3650

3625

3600

nO

H /

cm−1

∆nO

H /

cm−1

TO

F /

105

s−1

FaujasiteMordeniteLinde type LMazziteOffretite

320

315

25

20

15

10

5

3.8 4.0 4.2 4.4

Sanderson electronegativity

(a)

(b)

(c)

Fig. 11 Correlation of the electronegativity parameter of Sander-son with some indicators for the acid strength of some 12-ringzeolites. (a) Frequency of the hydroxyl stretching vibration; (b) shiftof the OH vibration frequency caused by benzene adsorption;(c) turnover frequency (TOF) for dehydration of 2-propanol at425 K. Adapted from Dwyer [21].

0.2

0.4

0.6

0.8

Iso

–C

5 / t

otal

C5

3025201510

SiO2 / Al2O3 molar ratio

Fig. 12 Effect of silica–alumina ratio on pentane isomerizationactivity of acid-leached mordenite samples [22].

13.7.5 Isomerization of Pentanes and Hexanes for Octane Enhancement 2819

Tab. 5 Blending octane numbers of C5 and C6hydrocarbons. RON-0 = clear research octanenumber; MON-0 = clear motor octane number

Hydrocarbon RON-0 MON-0

Isopentane 93 90n-Pentane 62 61Cyclopentane 102 852,2-Dimethylbutane 94 952,3-Dimethylbutane 105 1042-Methylpentane 74 753-Methylpentane 75 76n-Hexane 31 30Methylcyclopentane 96 85Cyclohexane 84 77Benzene >100 >100

90

80

70

60

50

40

30

Iso

/ iso

+ n

orm

al /

%

4000 100 200 300

Temperature / °C

C4

C5

Fig. 13 Thermodynamic equilibria between n-butane and isobu-tane and between n-pentane and isopentane in the gas phase.Calculated from thermodynamic data collected in API Project 44 [36].

13.7.5.2 Processes for Isomerization of Pentanesand HexanesIndustrial processes for the isomerization of C5 and C6

alkanes in light straight-run naphtha can be classifiedaccording to the type of catalyst used:

(i) processes using HCl/AlCl3 as a monofunctionalcatalyst

(ii) processes using a noble metal on chlorided aluminaas a bifunctional catalyst

(iii) processes using a noble metal on amorphoussilica–alumina as a bifunctional catalyst

(iv) processes using a noble metal on an acid form of azeolite as a bifunctional catalyst.

60

50

40

30

20

10

Isom

er in

tota

l hex

anes

/ %

4003503002500 50 100 150 200

Temperature / °C

2–MP

3–MP2,2–DMB

2,3–DMBn–C6

Fig. 14 Thermodynamic equilibria of isomeric hexanes in the gasphase. Calculated from thermodynamic data collected in API Project44 [36].

90

85

80

75

70

RO

N-0

4003503002502001500 50 100

Temperature / °C

C5gas

C6gas

C6liquid

C5liquid

Fig. 15 Octane numbers of equilibrium mixtures of pentanes andhexanes.

The most important processes in these categories arebriefly discussed below.

13.7.5.2.1 Processes Using Aluminum Chloride/Hydrochloric Acid as Monofunctional Catalyst A pro-cess belonging to this category of older processes is thelight naphtha isomerization process of Standard Oil Co.of Indiana [37]. The reaction is carried out in fixed beds of

References see page 2828

2820 13.7 Isomerization

a solid, supported catalyst. The feedstock used is purifiedto remove contaminants, and HCl is added to it as cat-alyst promoter. Because of catalyst deactivation, severalreactors are used, which allows periodic regeneration ofa reactor without interrupting the overall operation. HClis removed from the reactor effluent by distillation orstripping and is recycled.

Use of a liquid instead of a solid catalyst allows easiermaintenance of catalyst activity since the deactivatedcatalyst can be removed and replenished during operation.This concept was applied in the Isomate process of theStandard Oil Co. of Indiana [38]. Purified naphtha, withdissolved HCl as catalyst promoter, is fed to the bottomof a reactor containing a pool of molten catalyst. Catalystactivity is maintained by periodically injecting a slurry of

fresh aluminum chloride in naphtha. Effluent from thetop of the reactor is cooled and flashed for separation ofentrained catalyst. The liquid product is stripped fromhydrogen chloride, which is recycled. A flow scheme ofthe Isomate process is shown in Fig. 16.

The operating temperature in an isomerization pro-cess with liquid catalyst can be lowered by employingAlCl3 in combination with SbCl3 in the form of a low-melting eutectic mixture. This catalyst system was usedin Shell’s liquid-phase isomerization process [39–41], aflow scheme of which is shown in Fig. 17. The cold lightnaphtha feed is dried and contacted countercurrently witha stream of catalyst from the reactor to extract active com-ponents from the latter stream, leaving a waste stream ofspent catalyst. After addition of recycled HCl, the naphtha

Feed

AlCl3slurry Spent

catalyst

Causticwash

Waterwash

Water

MakeupHCI C1–C4

Recycle HCI

Hydrogen

Gas tofuel

Isomateproduct

Caustic

Dei

sope

ntan

izer

Drie

r

HC

I abs

orbe

r

Rea

ctor

HC

I str

ippe

r

Settler

Sta

biliz

er

Fig. 16 Flow scheme of the Isomate process for light naphtha isomerization [38].

Drying Catalystscrubbing Reaction Catalyst

recoveryGas

scrubbingHCL

strippingSoda

treating

Product

Water

Ventgas

Caustic

Catalystrecycle

Spentcatalyst

Feed

Recycle gas

Fig. 17 Flow scheme of the Shell liquid phase isomerization process [39].

13.7.5 Isomerization of Pentanes and Hexanes for Octane Enhancement 2821

stream enters the reactor, where isomerization takes placeat a temperature between 60 and 100 ◦C. The reactor ef-fluent containing dissolved catalyst is passed to a columnwhere catalyst is recovered as a bottoms stream which isrecycled to the reactor. Fresh catalyst is added to the latterstream for activity maintenance. The isomerized naphthastream is freed from HCl by stripping and HCl is recycled.

Typical data obtained in isomerization of a mixed C4–C6feed with Shell’s liquid-phase isomerization process arelisted in Table 6. The data show the relatively highproportion of isoalkanes, notably isopentane in thepentanes fraction and 2,2-dimethylbutane in the hexanesfraction, which is consistent with the relatively lowoperating temperature (see Figs. 13 and 14). The dataalso show the relatively low proportion of isobutane in theproduct, illustrating the difficulty of isomerizing n-butaneunder conditions where pentanes and hexanes are readilyisomerized to concentrations close to equilibrium (seeSection 13.7.2).

13.7.5.2.2 Processes Using Bifunctional Catalysts Consist-ing of Pt Supported on Chlorinated Alumina The stableoperation possible with bifunctional catalysts operat-ing in the presence of hydrogen have rendered alkaneisomerization processes using monofunctional catalystswith their complications arising from catalyst deactiva-tion practically obsolete. Among the earlier processesfor isomerization of pentanes and hexanes with platinumsupported on chlorinated alumina is the BP isomerizationprocess of British Petroleum [42–46]. A flow scheme ofthis process is shown in Fig. 18.

Tab. 6 Isomerization of mixed C4 –C6 feeda by the Shellliquid-phase isomerization process [41]

Component Concentrationin isomeri-zate/wt.%

Relativeproportion in

carbon numberfraction/%b

C3 and lighter 0.7 –Iso-C4 3.8 27.9 (72.3)n-C4 10.3 72.1 (27.7)Iso-C5 49.6 78.7 (81.0)n-C5 13.5 21.3 (19.0)2,2-Dimethylbutane 5.0 35.7 (41.3)2,3-Dimethylbutane 1.4 10.0 (9.9)2-Methylpentane 4.0 28.6 (27.0)3-Methylpentane 2.2 15.7 (14.0)n-C6 1.4 10.0 (7.8)Others 8.1 –

aStraight-run feed contained 0.7 wt.% propane, 13 wt.% butanes,62 wt.% pentanes, 14 wt.% hexanes and 10 wt.% C5 and C6cycloalkanes.bValues in parentheses are equilibrium data.

The light naphtha feed is purified to remove waterand sulfur compounds and freed from aromatics byprehydrogenation over a nickel catalyst prior to passageover the isomerization catalyst consisting of platinum onan alumina support, which is activated by chlorinatingwith HCl (added as such or as an organic chloride whichgenerates hydrogen chloride in situ). The process is carriedout between 100 and 160 ◦C at a pressure of about 2 MPaand at a hydrogen : hydrocarbon molar ratio of about 2.Hydrogen is recovered from the effluent and recycled.

Notwithstanding the use of predried feed, somechloride is stripped from the catalyst so that continuousreplenishment of chloride is necessary. Hydrogenchloride stripped from the catalyst is recovered fromthe product and recycled via an absorber system. In alater version, the BP process was simplified considerablysince it was found that the level of chloride as catalystactivator can be lowered considerably to the point whereHCl recovery facilities can be eliminated. It was alsorecognized that prehydrogenation over a separate catalystis not needed, since the platinum on the isomerizationcatalyst in the inlet part of the reactor can perform thesame function [47]. This simplified version of the BPprocess has much in common with the Penex process tobe discussed below.

The Penex process of Universal Oil Products (UOP)is among the oldest processes based on the use ofplatinum on chlorided alumina as catalyst and is verywidely used [48–54]. A flow scheme of the Penex processis shown in Fig. 19. Desulfurized C5, C6 or C5 –C6 feed ispassed through a molecular sieves drier, combined withrecycle hydrogen and passed through the isomerizationreactor or two isomerization reactors in series. The catalystconsists of platinum on alumina which, as in the BPprocess, is activated with chloride. Hydrogen is separatedfrom the reactor effluent, combined with make-uphydrogen to compensate for hydrogen losses (consumedin aromatics saturation, in hydrocracking and solution inproduct) and recycled. The liquid product is stabilized,and the stabilizer overhead vapors containing HCl (fromdecomposition of an organic chloride compound addedto the feed) are scrubbed with caustic solution to removethe acid.

Continuous addition of chloride activator to the feedensures maintenance of catalyst activity. A life of upto 4 years has been claimed for the I-8 catalyst [54].The operating temperatures and pressures are typically120–170 ◦C and 2–7 MPa, at a hydrogen : hydrocarbonmolar ratio of 1 : 2. Due to the relatively low isomerizationtemperature, a product with a relatively high degreeof branching can be obtained and an octane number

References see page 2828

2822 13.7 Isomerization

Desulfurization

Cat reformeroff-gas

Rawfeedstock

H2 + H2S

FurnaceNi catalyst

Recyclehydrogen

H2S/H2Oremoval

Make-uphydrogen

Activator

Dearomatization Isomerization

Activatorrecovery

Product

Recyclehydrogen

Fig. 18 Flow scheme of the BP isomerization process [42].

Make-uphydrogen

Dryer

Recycle gas

Reactors

C5/C6charge

Dryer

Separator

Stabilizer

Isomerate

Fresh/spentcaustic

Scrubber

Gas tofuel

Fig. 19 Flow scheme of the Penex process of UOP [54].

of about 84–85 (RON-0) is attainable in once-throughisomerization of a C5 plus C6 feedstock.

A later version of the Penex process applies hydro-gen in once-through operation. In this HOT (hydrogenonce-through) Penex process, several major items of plantequipment can be omitted, namely the recycle compres-sor, the product separator and associated heat exchangers,resulting in equipment cost savings of about 15% [31].

13.7.5.2.3 Processes Using a Noble Metal on AmorphousSilica–Alumina as Bifunctional Catalyst The use ofsilica–alumina as a solid catalyst eliminates the drawbackof continuous chloride addition, removal of HCl fromeffluent streams and the precautions needed in thedesign and operation to avoid corrosion problems. Anexample of this class of processes is the Pentafiningprocess of Atlantic Refining [55–57]. However, due to the

13.7.5 Isomerization of Pentanes and Hexanes for Octane Enhancement 2823

relatively low acid strength of amorphous silica–alumina,a relatively high operating temperature is required(between 424 and 480 ◦C) at an operating pressure inthe range 2–5 MPa [55].

Because of unfavorable thermodynamic equilibriumat these high temperatures (see Figs. 13 and 14), theoctane levels attainable are relatively low (see Fig. 15).For this reason, C5 and C6 isomerization processesusing amorphous silica–alumina have become obsoleteand have been replaced by processes using chlorinatedalumina (see above) or processes applying zeolites (seebelow).

13.7.5.2.4 Processes Using a Noble Metal on Zeolite asBifunctional Catalyst The most prominent example inthis category is the Hysomer process, which is offeredfor license by Shell and by UOP (formerly by UnionCarbide) [60–62]. The zeolite used as acidic component,H-mordenite, is a much stronger acid than amorphoussilica–alumina and allows operation at temperatures inthe range 240–280 ◦C. With platinum as the hydrogen-activating component on the catalyst, the process operatesunder hydrogen at a total pressure between 0.8 and 3 MPa.

An important advantage of zeolitic catalysts overchlorinated alumina-based catalysts is their much greaterrobustness. In particular, water in the feed can betolerated up to fairly high concentrations, obviating theneed for expensive drying facilities. Continuous additionof a chloride activator, removal of HCl from effluentstreams and precautions against chloride corrosion areunnecessary. The Hysomer catalyst can also toleratea fair amount of sulfur in the feedstock, renderinghydrotreatment of the feedstock superfluous in mostcases.

The tolerance of the Pt/H-mordenite catalyst of theHysomer process for water and sulfur is shown in Figs. 20and 21. A flow scheme of the Hysomer process is shownin Fig. 22, which illustrates the much greater simplicityof this process compared with processes using aluminumchloride.

The ruggedness of the Hysomer catalyst is alsodemonstrated by its long life: catalyst charges have beenused for up to 7 years in commercial operation. A catalystdeactivated by operational mishaps can in most cases beregenerated by a simple carbon burn-off.

Table 7 lists some representative data on the isomeriza-tion of a C5 plus C6 feed with the Hysomer process. Dueto the higher operating temperature, the octane levelsof the products are somewhat lower than for the Penexprocess (RON-0 81–82 versus 84–85).

The Hysomer process, which saw its first commercialapplication in 1970, is now employed in a large number ofplants. More recently, a rather similar process based on a

95

94

93

92

100 98 96 94 92

RO

N -

3 m

l TE

L/U

SG

10 ppm H2O in gas

200 ppm H2O in gas

C5+ yield / wt.%

∆T = 5°C

Fig. 20 Effect of water on the isomerization of light naphtha overPt/H-mordenite.

95

94

93

RO

N -

3 m

l TE

L/U

SG

100 98 96 94 92

C5+ yield / wt.%

No S added

14 wt. ppmadded to feed

Fig. 21 Effect of sulfur on the isomerization of light naphtha overPt/H-mordenite.

modified zeolite impregnated with platinum (ProcatalyseIS-632) has been developed by the Institut Francais duPetrole (IFP) [63]. Another recent development is thatof the Hysopar catalyst, which reportedly is a zeolitecatalyst containing noble metals and which is applied in anewly developed process, the CKS (Cepsa–Kellogg–Sud-Chemie) Isom process [64].

13.7.5.3 Combining C5 and C6 Alkane Isomerizationwith Physical SeparationsDue to the thermodynamic equilibrium limitationsdiscussed earlier, complete conversion of n-alkanes intoisoalkanes is not achieved in once-through operation. To

References see page 2828

2824 13.7 Isomerization

C5/C6 feed H2

Product

ReactorRecycle

Fig. 22 Flow scheme of the Shell Hysomer process.

Tab. 7 Typical compositions of feed and products of the Hysomerand TIP processes

Component Feed Hysomerproduct

TIPproduct

C4 and lighter/wt.% 0.7 1.8 2.8Iso-C5/wt.% 29.3 49.6 72.0n-C5/wt.% 44.6 25.1 2.02,2-Dimethylbutane/wt.% 0.6 5.0 5.52,3-Dimethylbutane/wt.% 1.8 2.2 2.52- and 3-Methylpentane/wt.% 13.9 11.3 13.4n-C6/wt.% 6.7 2.9 0.1Cyclo-C5 and -C6/wt.% 2.4 2.1 1.8RON-0 73.2 82.1 90.7C5+ yield/wt.% – 97.5 96.8

obtain complete isomerization, the isomerization processcan be combined with a physical separation processwhich allows the isolation of residual n-alkanes fromthe isomerization product for recycling and conversion toextinction.

Since isoalkanes boil at lower temperatures than theirstraight-chain isomers, this separation can be effectedby distillation. In the case of pentane isomerization,n-pentane can be isolated from the isomerate as bottomproduct in a deisopentanizer column. Similarly, forhexane isomerization a deisohexanizer column can beinstalled.

A disadvantage of separation by distillation is thatthe proportion of residual n-alkanes in the isomerizedproduct is generally rather low, whereas the capacity ofthe fractionator is determined by the bulk of isoalkanesto be distilled off as overhead product. In the caseof isomerization of mixed C5 and C6 feed, completeisomerization becomes quite involved.

Particularly in the last case, iso/normal separationby selective molecular sieve adsorption becomes ofinterest. Zeolite 5A selectively adsorbs n-alkanes sinceisoalkanes have a too large molecular diameter to enterthe intracrystalline pores. Since the required capacity ofthe adsorption unit is mainly determined by the amountof n-alkanes to be adsorbed, molecular sieve adsorptionis advantageous at relatively low residual contents ofn-alkanes.

Examples of iso/normal separation using zeolite 5A ina pressure-swing adsorption process are the BP pressureswing process of British Petroleum [65] and the Isosivprocess of Union Carbide [66–68]. An alternative molec-ular sieve iso/normal separation process is the Molexprocess of UOP [69–71]. In contrast to the BP and Isosivprocesses, which operate at relatively high temperaturesin the gas phase, the Molex process operates at a rela-tively low temperature in the liquid phase. Desorptionis effected by using a light n-alkane as desorption aid,and this desorber is recovered from the extract phaseby distillation. A special feature of the Molex processis the use of a single cylindrical adsorbent vessel pro-vided with multiple inlet/outlet ports connected to amultiport rotary valve. Thus, with a stationary adsor-bent column, a continuous chromatographic separationis simulated.

The BP isomerization process has been combinedwith the BP pressure swing n-alkane adsorption processto achieve complete isomerization of n-alkanes anda commercial plant was built in the mid-1970s [72].The Penex–Molex combination was commercialized in1990 [31].

A special advantage of a zeolite-based isomerizationprocess such as Hysomer is that within the windowof feasible operating conditions (temperature, pressureand hydrogen flow-rate), these can be chosen so asto fit the requirements of an Isosiv process modewith desorption by purging with hydrogen instead ofby pressure reduction. Thus, rather than by applyingtwo separate processes, isomerization and separationmay be closely integrated in a single process withsignificant savings. This concept was realized in the totalisomerization package (TIP) process [73–78].

Figure 23 shows a flow scheme of the TIP process,in which the Hysomer reactor is situated upstreamof the Isosiv adsorbers (Hysomer lead option). Thisis the preferred configuration if the feed is rich inn-alkanes. However, if the feed is rich in isoalkanesand cyclic hydrocarbons, the reverse sequence will bemore advantageous (Isosiv lead option). These alternativeconfigurations are shown in Fig. 24.

The application of molecular sieve separation andrecycle of n-alkanes not only leads to a higher octanenumber of the product, but also decreases the effect of

13.7.5 Isomerization of Pentanes and Hexanes for Octane Enhancement 2825

Hysomer Isosiv

Feed

H2

Product

Fig. 23 Flow scheme of the TIP process (Hysomer lead option).

a: Hysomer lead

Feed

Feed

Hysomer

Hysomer

Isosiv

Isosiv

n-alkanes recycle

iso- + n-alkanes recycle

Sta

biliz

erS

tabi

lizer

C4−

C4−

Isoalkanesproduct

b: Isosiv lead

Fig. 24 Alternative configurations of the TIP process.

isomerization temperature on product quality. Therefore,the disadvantage of the zeolite-based isomerizationprocesses with their higher operating temperature thanthe processes using chlorided alumina largely disappearsin recycle operation, as can be inferred from Fig. 25.Table 7 compares the product compositions obtained inisomerization of light naphtha with the Hysomer processin the once-through mode and with the TIP process. TheTIP process was commercialized in 1975 and since thattime an appreciable number of plants have been built.

A more recently proposed extension of the TIP processis its integration with UOP’s SafeCat technology. ThisSafeCat isomerization scheme is shown in Fig. 26. Insteadof installing a stand-alone hydrotreater to remove sulfurfrom the feed, the hydrotreater reactor is incorporatedin the same gas circuit as the Hysomer and Isosivunits. Because of the similar pressures in hydrotreating,isomerization and adsorption, a simple line-up is possiblewith a single gas loop and a minimum of heating andcooling equipment.

Once through

With n-alkanes recycle

Pt/Cl /Alumina Pt/H-MOR

92

90

88

86

84

82

8050 100 150 200 250 300 350

RO

N -

0

Operating temperature / °C

Fig. 25 Effect of temperature on attainable octane numbers inisomerization without and with recycle of n-alkanes. Feed: 60%pentanes, 30% hexanes, 10% cyclics [63].

Another process that combines isomerization andphysical separation of n- and isoalkanes is the morerecently developed Ipsorb Isom process of IFP. Theisomerization reactor product is separated in a molecularsieve separation section of the vapor phase, pressureswing adsorption type (Ipsorb). A first unit was put intoservice around 1996 [79, 80].

Probably the closest integration of isomerization andnormal/iso separation of alkanes is the application ofreactive distillation or a membrane reactor. Conceptualschemes for membrane catalysis in alkanes isomerizationare shown in Fig. 27 for the production of eitherisoalkanes or n-alkanes [81]. Recently, some experimentalproof of this concept has been obtained by Gora andJansen in a laboratory reactor containing a silicalite-1membrane and a chlorided Pt/alumina catalyst [82].However, the development of an industrial process on

References see page 2828

2826 13.7 Isomerization

Hydrogen recycle

H2 makeup

Feed

Safecat

DE

S

Gas

HT

AD

S

ISO

M

Isomerate

HT = HydrotreatingADS = AdsorptionDES = DesorptionISOM = Isomerization

Fig. 26 SafeCat isomerization scheme [31].

Selective membrane

Isomerization catalyst

Isoalkanes

Normal alkanes

i + n Feedi + n Feed

i + n

H2

H2

H2

n + H2

n

n

n

n

n

n

n

n

(a) (b)

Fig. 27 Conceptual schemes for membrane catalysis. (a) For production of n-alkanes; (b) for production of isoalkanes.

this basis will be far from easy and still represents a greatchallenge for the future.

13.7.6Processes for Isomerization of Higher Alkanes

For reasons explained earlier, selective isomerization ofalkanes higher than hexanes is difficult since crackinggenerally accompanies isomerization. For this reason,

isomerization of heptanes with the objective of octaneenhancement has found little application, since crackingto gaseous hydrocarbons represents a loss of productvalue. Another reason is that the octane values of the mainmonobranched isomers are still relatively low, namelyRON-0 values of 45 and 65 for 2- and 3-methylhexane,respectively.

The isomerization of higher n-alkanes that may besolid at room temperature is a way to lower the pour

13.7.7 Processes for Conversion of n-Butane to Isobutane 2827

point. Thus, lubricating oils can be produced fromwaxy oil fractions by isomerization in the presence ofhydrogen using bifunctional catalysts. In this application,some hydrocracking can be accepted, since the crackedfragments that still boil in the desired range do not leadto loss of product yield but merely to some reduction inaverage molecular weight. Moreover, smaller fragmentsboiling below the main product range can still be valuableby-products as long as they are liquid.

A process for converting slack wax to lubricating baseoil of very high viscosity index (>145) by hydrocrack-ing/isomerization has been developed by Shell and isapplied commercially as part of a complex to producelubricating base oils by catalytic hydroprocessing of waxydistillates and deasphalted oils [83].

A more recent process in the field of wax isomerizationfor lube oil production is the Isodewaxing process ofChevron [84]. The Isodewaxing catalyst typically containsa hydrogenation component on an intermediate poresilicoaluminophosphate molecular sieve (SAPO). Theintermediate pore size suppresses the formation ofhighly branched structures, which is favorable sincemonobranched structures are more desirable componentsas they combine a relatively low pour point with a highviscosity index. A Pt/SAPO-11 catalyst was found capableof isomerizing linear alkanes of long chain length witha higher selectivity than can be achieved with a catalystconsisting of platinum on amorphous alumina. This canbe seen from Table 8, which shows a comparison of thesetwo catalysts in the isomerization of n-hexadecane [85].

Isomerization of heavier alkanes also plays a role inthe conversion of the product of the Fischer–Tropschsynthesis step of the Shell middle distillate synthesis(SMDS) process. This Fischer–Tropsch product, whichconsists mainly of long-chain n-alkanes, is hydrocrackedto produce hydrocarbons boiling in the kerosene andgasoil ranges. In this hydrocracking step, isomerizationof n-alkanes which accompanies the cracking is essential

Tab. 8 Isomerization of n-hexadecane at 6.9 MPa, 3.1 weighthourly space velocity, H2 : HC = 30 and 96% conversion [85]

Parameter Pt/SAPO-11catalyst

Pt/silica–aluminacatalyst

Temperature/◦C 340 360Isomerization selectivity/wt.% 85 64n-C16 in C16 product/wt.% 4.7 6.0Methyl-C15 in C16 product/wt.% 53.3 21.6Dimethyl-C14 in C16 product/wt.% 29.8 37.8Other C16 in C16 product/wt.% 12.2 34.6Pour point/◦C −51 −28

to meet the requirements on cold flow properties of theproducts [86–89].

A similar application of hydroisomerization of a waxyFischer–Tropsch product is the conversion of this productto a liquid that can be easily shipped by pipeline or ina conventional crude tanker, as part of Exxon’s advancedgas conversion technology [90, 91].

13.7.7Processes for Conversion of n-Butane to Isobutane

As mentioned before, n-butane is hardly isomerized underthe conditions of C5 and C6 isomerization, but may beconverted to isobutane under more severe conditions bythe alkylation–isomerization–dealkylation mechanism,using the same type of catalysts.

As can be seen in Fig. 13, the conversion of n-butane into isobutane is limited by the thermodynamicequilibrium, even more so than the isomerization ofhigher alkanes. Therefore, isomerization of n-butaneis generally coupled with iso/normal separation in adeisobutanizer or isobutane is allowed to react selectivelyin the isomerized mixture, for example by reaction withalkenes in the alkylation process.

Processes for the manufacture of isobutane fromn-butane either use AlCl3 activated with HCl as amonofunctional catalyst or Pt supported on chloridedalumina as a bifunctional catalyst. Monofunctionalcatalysts based on AlCl3/HCl have been applied inolder processes developed before and shortly after WorldWar II.

An example of this type of process is the catalyticisomerization process of Phillips Petroleum [92, 93]. Thisuses a solid catalyst consisting of AlCl3 sublimed onto bauxite. Dried n-butane is vaporized, superheatedand mixed with recycled HCl vapor and passed overthe catalyst in a number of fixed-bed reactors. HCl isremoved from the condensed product by stripping andrecycled. A similar catalyst is used in the butane vapor-phase isomerization process of Shell [94, 95]. This processoperates at temperatures of 100–140 ◦C and pressuresin the range 1–2 MPa. The liquid-phase isomerizationprocess of Shell, discussed earlier in the context of C5and C6 isomerization, has also been used for n-butaneisomerization in a number of commercial units [39–41,95]. Butane isomerization is typically carried out at90–110 ◦C and about 2–3 MPa.

As in the case of C5 and C6 isomerization, mono-functional catalysts suffer from deactivation in n-butaneprocessing and catalyst is consumed in the process. Cat-alyst deactivation is minimized by using bifunctionalcatalysts operating under hydrogen pressure. Among the

References see page 2828

2828 13.7 Isomerization

butane isomerization processes using bifunctional cata-lysts, the most prominent example is the Butamer processof UOP [96, 97]. This process, which was actually a prede-cessor of the Penex process, uses a similar platinum onchlorided alumina catalyst, which can be operated for along period in the presence of hydrogen. Another processwhich uses platinum on chlorided alumina is the BP C4

isomerization process. Typical conditions are a pressurebetween 1.5 and 3 MPa and a temperature in the range150–210 ◦C [98].

13.7.8Role of Alkane Isomerization in the HydrocarbonProcessing Industry

Isomerization of light alkanes has been applied com-mercially for many years in the hydrocarbon processing(petroleum refining) industry. Particularly during WorldWar II, demand for alkylate as component for aviationgasoline caused a rapid increase in butane isomerization:from the first commercial unit coming on stream in 1941to nearly 40 units by the end of the War.

The replacement of high-octane gasoline by keroseneas the main aviation fuel has stopped the growth ofbutane isomerization, although the process still retainsan important place in hydrocarbon processing. In the1990s, interest in isobutane manufacture revived becauseof the rapid growth in demand for methyl tert-butylether (MTBE) as an octane-boosting component ingasoline, spurred by the advent of unleaded gasolineand the specified presence of oxygenated compoundsin so-called reformulated or green gasoline. Becausethe availability of isobutene for MTBE manufacturefrom the traditional source (catalytic cracker gases)is limited, production of isobutene by isomerizingn-butane followed by dehydrogenation gained interest.An alternative possibility is the skeletal isomerization ofn-butenes present in catalytic cracker gases and a suitablezeolitic catalyst for this conversion has been found [99].

Isomerization of C5 and C6 alkanes commencedtowards the end of World War II, to provide additionalblending stock for aviation gasoline. The advent ofcatalytic reforming over platinum-type catalysts hasreduced the need for isomerization of light naphtha foroctane enhancement in the production of gasoline forcars. In the 1980s, however, the drive to remove leadadditives from gasoline led to a rapid growth in theneed for C5/C6 isomerization capacity. This is illustratedby Fig. 28, showing the increase in Hysomer and TIPcapacity in that period.

With the increasing global demand for gasolineand environmental concerns (spreading to developingcountries as well) that led to a drive for a reduction in

1970 75 80 85 90 95 2000

5

10

15

Mill

ion

tons

per

yea

rYear

Fig. 28 Growth of capacity of installed Hysomer units, either asstandalone unit or integrated in TIP.

aromatics in gasoline and of MTBE, isomerization of lightnaphtha remains of interest in recent times.

The total capacity of light alkane isomerizationhas been estimated at more than a million barrels(160 000 m3) per day in 1993. This capacity is aboutequally split between butane isomerization, light naphthaisomerization over Pt/Cl/alumina and light naphthaisomerization over Pt/zeolite [31] and implies a largenumber of plants. UOP, the leading licensor inC5/C6 isomerization technology, was reported to havecommissioned 188 C5/C6 isomerization units as of thesecond quarter of 2002 [100].

Isomerization of higher alkanes in the wax range is ofgrowing importance as the moderately branched alkanesproduced are excellent components for lubricating oils.Hydrocracking/hydroisomerization of synthetic heavywax produced by a Fischer–Tropsch process providesa possibility for the manufacture of high-quality fuelsand other hydrocarbon products from sources other thanpetroleum, such as natural gas and coal. This optionmay well become more important in the future whendemands on the quality (cleanliness) of fuels increasewhile petroleum resources diminish.

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13.8Alkylation of Isobutane with Light Alkenes on SolidCatalysts

Yvonne Traa and Jens Weitkamp∗

13.8.1Introduction

The alkylation of isobutane with light alkenes to pro-duce higher hydrocarbons in the gasoline range has beenknown for a long time. Light alkenes such as propene,butenes and pentenes can be used as alkylating agents. Incommercial practice, however, mostly butenes are appliedand this is why we shall often refer to isobutane/butenealkylation in this chapter. In industry, the reaction is cat-alyzed by liquid HF or H2SO4. Therefore, the alkylationof isobutane with alkenes is a classic example of old liquidacid-catalyzed processes where environmentally benignsolid catalysts have been sought for a long time. However,until recently, this search had not been successful, andconsiderable research efforts are still being undertaken.The importance of the topic is documented by recent re-view articles focusing on new research in this field [1–4].The literature up to about 1995 was reviewed in the firstedition of this Handbook [5]. The current review will focuson the main lines of development using solid catalystssince then.

13.8.2Stoichiometry of the Alkylation of Isobutane with LightAlkenes

In chemistry, alkylation is the generic term for a broadvariety of reactions which have in common that an alkyl

∗ Corresponding author.