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Skeletal Isomerization of Butene in Fixed Beds. 1. Experimental Investigation and

Structure-Performance Effects

Matias Kangas, Narendra Kumar, Elina Harlin, Tapio Salmi, and Dmitry Yu. Murzin*,

Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, bo AkademiUniVersity, FI-20500 Turku/bo, Finland, and Neste Oil, P.O. Box 310, FI-06101 PorVoo, Finland

An experimental investigation of structure-performance effects in zeolite catalyzed skeletal isomerizationhas been carried out. Two structurally different, medium-pore-size zeolites (H-TON and H-FER) with similaracidities were compared in butene skeletal isomerization. While both catalysts proved to be efficient in thetest reaction, their deactivation behavior differed substantially. H-FER exhibited significantly higher initialisobutene yields and selectivities, and the catalyst performance was also more stable with time-on-stream.H-TON, on the other hand, needed prolonged operation times in order to achieve product distributionscomparable to those of H-FER. The predominant route to isobutene was found to be the monomolecular one,with the bimolecular paths of butene largely responsible for byproduct formation. H-TON was more selectivetoward disproportionation and, owing to its slightly larger pore dimensions, hydrogen transfer products. H-TONwas also observed to be less sensitive to feed reactant than H-FER was, confirming previous theoreticallycalculated structure effects. Coke belonging to both aliphatic and aromatic families was detected over bothzeolites, although the coke formed on H-FER was overall heavier and more aromatic in nature. The tested

zeolites could be regenerated by burning of the carbonaceous deposits in air, and both catalysts regainedalmost all of their initial activities.

1. Introduction

Most of the oil refining processes in operation today involvethe use of solid acid catalysts, and the vast majority of thesecatalysts belong to the zeolite family. The major zeolite-catalyzed processes found in todays refineries include fluidcatalytic cracking (FCC), hydrocracking, hydroisomerization,and dewaxing of distillates and lubrication oils.1 Since theirintroduction some 40 years ago, the application of zeolites havehelped to increase many refinery process efficiencies throughimproved product yields and selectivities, while at the same time

meeting ever-increasing demands on product quality. Opportuni-ties for further commercialization of zeolite-based processes canbe found, for instance, in the transformation of light olefins, byoligomerization into fuels and lubes, or by isomerizing linearC4 and C5 olefins into their branched, and more valuable,counterparts.

The production of refinery-deficient isobutene by skeletalisomerization of linear butenes has received a lot of attentionin the past decade. The reaction has gathered industrial interestbecause of an increasing demand for isobutene in the productionof methyl tert-butl ether (MTBE), a fuel additive that improvesthe properties of gasoline. Although the use of MTBE as anoctane booster is decreasing, the demand for isobutene is still

high as the isobutene feedstocks formerly used in the productionof MTBE are finding new uses as raw materials in alternativeoctane-enhancing components for gasoline.2 The academicdiscussion of skeletal isomerization has mainly centered on thereaction mechanism315 and the beneficial effect of coking onreaction selectivity.35,8,13,1520

Zeolites with channel-type structure and pore diametersbetween 4 and 5.5 have been shown to be suitable materialsfor catalyzing the skeletal isomerization of n-butene into

isobutene.7 This range of pore diameters is found in 10-membered ring (10 MR), also known as medium-pore-size,zeolites. Among the medium-pore zeolites, ZSM-22 (TON)21

and ferrierite (FER)22 have both been shown to be selectiveand stable in skeletal-isomerization applications.

The scope of the present work is to experimentally comparethe activities of two proton form zeolites with TON and FERstructures and to study their deactivation behavior with time-on-stream (TOS). The study also aims at collecting data forkinetic modeling of skeletal isomerization and any relevant sidereactions.

2. Experimental Section

2.1. Synthesis and Characterization of Catalysts. K-ZSM-22 zeolite was synthesized from two solutions, according to themethod described by Byggningsbacka et al.23 The first solutionwas prepared by diluting the silica source, Ludox AS-40, indistilled water, and the second solution was obtained bydissolving the aluminum source, Al2(SO4)2 18 H2O (Merck),in distilled water. The organic template 1,6-diaminohexane(Fluka) and KOH (Merck) were added to the aluminum-containing solution. The solutions were subsequently stirred for15 min, after which the solutions were combined underconditions of vigorous stirring. Mixing the two solutions atambient temperature resulted in the formation of a white gel,which was stirred for an additional 30 min. The actual synthesisof K-ZSM-22 was carried out in a Teflon-lined autoclave at433 K for 72 h. The synthesized material was filtered, washedwith distilled water, and dried at 383 K for 12 h, after whichthe organic template was removed by calcination of the sampleat 823 K for 15 h. The ammonium form of the zeolite wasobtained by ion-exchange of K-ZSM-22 with 1 M NH4Cl(Merck) solution. The ion-exchanged zeolite was then washedfree of chloride ions and dried for 12 h at 323 K, and a final

* To whom all correspondence should be addressed. Fax: 358 2 2154479. E-mail: [email protected].

bo Akademi University. Neste Oil.

Ind. Eng. Chem. Res. 2008, 47, 540254125402

10.1021/ie800061q CCC: $40.75 2008 American Chemical SocietyPublished on Web 06/25/2008


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calcination of the sample at 823 K for 10 h was carried out ina muffle oven to obtain the proton form H-ZSM-22 (H-TON)zeolite.

NH4-FER was obtained from Zeolyst International (CP 914)and was transformed into proton form (H-FER) by a stepcalcination procedure in a muffle oven. The structure and phasepurity of the tested zeolites were determined by an X-raydiffractometer (Phillips PW 1820), and crystal shape and sizewas studied using scanning electron microscopy (Cambridge

Leica 360). The bulk silicon-to-aluminum ratio was determinedby X-ray fluorescence (Siemens SRS 303), and the specificsurface areas were measured by the nitrogen adsorption method(Sorptometer 1900, Carlo Erba Instruments). The catalysts wereoutgassed at 473 K prior to the measurements, and the Dubininmethod was used to calculate the specific surface area.

The acidity of the investigated catalysts was measured byinfrared spectroscopy (ATI Mattson FTIR) using pyridine(>99.5%, a.r.) as a probe molecule. Samples of the zeoliteswere pressed into thin self-supporting wafers (8 mm) ofapproximately 10 mg. Pyridine was adsorbed at 373 K for 30min and then desorbed by evacuation at different temperatures(523, 623, and 723 K) to obtain the distribution of acid site

strengths. The spectra were recorded at 373 K using a spectralresolution equal to 2 cm-1. Spectral bands at 1545 and 1450cm-1, respectively, were used to identify Brnsted (BAS) andLewis acid sites (LAS). Quantitative results were calculated fromthe integrated absorbance areas of the corresponding peaks usingthe integrated molar extinction coefficients of Emeis.24

The acidity was also measured by temperature-programmeddesorption of NH3 (NH3-TPD). The sample was adsorbed withammonia at 473 K and later flushed with helium at the sametemperature. A heating rate of 20 K/min was used.

2.2. Isomerization Experiments. A fixed-bed minireactoroperating in the vicinity of atmospheric pressure connected toa gas chromatograph (GC), equipped with an online gas-phase

autosampler, was used in the study. The reactor was packedwith roughly 0.5 g of catalyst particles (pelletized, crushed, andsieved to 100-125 m), and the bed was kept in place withquartz wool and sand. These, together with a packing of glassbeads in front of the catalyst bed, provided a uniform flow ofgases to the bed and preheated the reactants. The reactor wasplaced in an oven, and feedback for heating control was providedby a thermocouple inside the catalyst bed. The effluent fromthe reactor was passed through a heated line to the gaschromatograph (Agilent Technologies 6890N), equipped witha flame-ionization detector and a capillary column (HP-PLOTAl2O3 50 m 530 m 15 m). The GC column was able toseparate all components up to, and including, pentenes. Theheavier compounds could not be sufficiently separated and were

grouped according to carbon number into hexenes, heptenes,and octenes when calculating the results. The grouping limitswere determined by injecting a mixture of n-paraffins andassuming that the straigh-chain alkane is the first peak to appearfor each group.

The catalysts were activated in situ for 2 h at 773 K in airfollowed by drying for 2 h at 723 K in a nitrogen atmosphere.When the catalyst bed had reached reaction temperature, thereactant and nitrogen were introduced and the first sample wastaken after 10 min on stream. The experiments were carriedout with reactant partial pressures varying from 0.1 to 1.0 atm,temperatures ranging from 523 to 673 K, and weight hourlyspace velocities (WHSVs) between 6 and 88 h-1.

Regeneration of the used zeolite catalysts was done by insitu burning of the deposited coke. After completed reaction,

the reactant flow was switched to synthetic air and thetemperature was raised from the reaction temperature to 773 Kwith a heating rate of 1 K/min. The catalyst was kept at 773 Kfor 4 h and was then dried in nitrogen atmosphere at 723 K foran additional 1 h before the reactor was ramped down by 10K/min to reaction temperature.

2.3. Coke Characterization. The nature of the coke formedon the catalysts used in this work was studied by dissolution ofthe zeolite structures and extraction of the soluble part of the

coke. The structures of the coked zeolites were destroyed bytreatment in 2 mL of 40% hydrofluoric acid (Merck) for every0.25 mg of used catalyst. Following dissolution of the zeolitestructure, approximately 10 mL of dichloromethane (J.T. Baker)was added to recover the soluble part of the coke. After filtrationand separation of the phases, analysis of the organic phase withGC-MS (Agilent Technologies 5973N) was made. Quantitativemeasurements of the carbon content were obtained by oxidationof the carbonaceous deposits in either Strohlein CS-5000 or LecoCHN 2000 elemental analyzers.

2.4. Cracking Experiments. By adding an evaporator to theexperimental setup, the cracking behaviors of the linear 1-octeneand the heavily branched diisobutene isomers were also studied.The cracking experiments were performed at a temperature of623 K with an octene-to-nitrogen molar ratio of approximately1-5 and WHSV varying between 2.4 and 40 h-1.

3. Results and Discussion

3.1. Catalyst Structures and Characterization. Ferrierite(FER) is a two-dimensional zeolite containing two perpendicu-larly intersecting channel systems.25 One channel consists of10-membered rings (10 MR) with dimensions of 4.2 5.4 ,and the other consists of eight-membered rings (8 MR) withdimensions 3.5 4.8 . The TON structure is unidimensionaland has 10 MR pores with dimensions of 4.6 5.7 .25 Themorphology of the two zeolites was studied with scanning

electron microscopy (SEM), and flakelike crystals were observedfor FER, whereas the crystals of TON were shaped like needles,typical of 1-D crystal structures. The typical crystal sizesapproximated from SEM micrographs were 1 1 0.1 mfor FER and 1 0.1 0.1 m for TON.

Graphical representations of the two structures are shown inFigure 1. The figure depicts the 10 MR pores of the frameworksviewed along [001]. Isobutene has been placed inside themicropore space of the zeolites, and parts of the host structuresand the guest molecule are represented by 70% of their CPKvolumes, illustrating the limited space available to the reactantsand products.

The results of the characterizations are summarized in Table

1. Surprisingly, the Brnsted acidities measured by Py-FTIRfor the two zeolites were very similar (180 vs 178 mol/g),although their bulk Al contents differ quite a lot. However, thechoice of pyridine as a probe molecule for ferrierite typematerials has been challenged.2628 The theory of severelydiffusion-controlled transport of pyridine into the zeolite poreswas, therefore, tested by increasing the adsorption temperatureand time. Performing the adsorption at 473 K and prolongingthe adsorption time from 0.5 to 20 h revealed an additional 80mol/g (30%) of medium-strength Brnsted sites in H-FER,while the amount of BAS stayed mostly unchanged for H-TON.Both catalysts saw a dramatic increase in the amount of LASfound when saturated at elevated temperatures. The overallLewis acidity was increased by 50% for H-TON and almost

200% for H-FER, suggesting that most of the extra frameworkAl is located in the 8 MR pores of H-FER.

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5403


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The amount of acid sites measured by NH3-TPD for the twotested catalysts were also very close (590 and 560 mol/g, forFER and TON, respectively), although the quantitative resultsby NH3-TPD differed significantly from the ones obtained byPy-FTIR. The majority of the ammonia desorbed between 573and 773 K, for both catalysts, and they both exhibited a NH3desorption maximum at 723 K, indicating that the samples havesimilar distribution acid site strengths.

The FER zeolite used (Zeolyst CP 914) has also beenstudied using other acidity measurement techniques, such astemperature-programmed desorption-thermogravimetric analy-sis (TPD-TGA) with n-propylamine,29,30 and total Brnsted

acid site concentrations between 350 and 530 mol/g havebeen obtained, indicating that the use of Emeis extinction

coefficients for FER results in too low concentrations. Acomparison between reported extinction coefficients in arecent publication31 also suggests that the coefficients arenot portable from one material to another or from one IRtechnique to another. Another explanation is that NH3 andthe linear propylamine diffuse much faster inside the zeolitecrystals than pyridine and that they can access sites unavail-able to pyridine.

3.2. Isomerization of n-Butene. Although 1-butene is thereactant initially introduced into the reactor, n-butene (i.e.,1-butene, cis-2-butene, and trans-2-butene) can be considered

the reactant because double-bond migration in 1-butene issignificantly faster than skeletal isomerization or any other

Figure 1. Graphical representation of isobutene in the 10 MR channels of FER and TON: (a) FER and (b) TON.

5404 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008


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chemical reaction involving 1-butene. A constant equilibriumamong the linear butene isomers gives the following definitions:

Conversion)(1-butene)in- (n-butene)out

(1-butene)in 100% (1)

Yield)(product)out(1-butene)in

100% (2)

Selectivity) (product)out(1-butene)in- (n-butene)out

100% (3)

3.2.1. Isobutene Formation. Three main reaction mecha-nisms for skeletal isomerization of linear butenes into isobutenehave been proposed in the literature; the monomolecular,4,7,8,12,13,15

the pseudomonomolecular,3,5 and the bimolecular6,911,14 reac-tion routes. The monomolecular route involves only onen-butene molecule and converts it exclusively into isobutene,and 100% selectivity is, thus, theoretically possible. Thebimolecular mechanism proposals can be further subdivided intotwo separate routes; the classical bimolecular6,14 and thecodimerization911 paths. In the classical bimolecular proposal,n-butene dimerizes into dimethylhexene that undergoes further

isomerization and finally cracks into one isobutene and onen-butene. In the codimerization scheme, one linear and onebranched butene combine to form a trimethylpentene, whichcracks into two isobutene molecules. The codimerization route,also known as the autocatalytic route, is potentially the moreselective of the two bimolecular pathways. Guisnet et al.5 andAndy et al.3 suggested a pseudomonomolecular mechanism,somewhat similar to the codimerization route, in order to explainthe apparent change in mechanism with increasing time-on-stream observed in many studies. The sites for this, supposedlyvery active and selective, route are docked carbenium ionslocated inside the zeolite pores. The mechanism avoids thethermodynamically unstable primary product carbenium ion,

which is usually associated with monomolecular branching viaopening of the protonated cyclopropane transition state. Nev-ertheless, the pseudomonomolecular reaction mechanism hasbeen heavily criticized.8,12,32,33

Skeletal isomerization of pentene is much easier to catalyzethan branching of butene,6,12 and there is a general consensusthat branching of molecules with five or more carbons occurmonomolecularly. As shown in Figure 2, equilibrium amongthe pentene isomers was always reached for the experimentalconditions employed in this work. It is, moreover, improbablethat the pseudomonomolecular reaction or bimolecular codimer-ization mechanisms would operate only for skeletal isomeriza-tion of butene but not for pentene. Both routes involvecarbenium ions as active sites and, thus, avoid formation of

unstable primary carbenium ion-like reaction intermediates ortransition states but proceed instead through energetically more

favorable secondary carbenium ions only. In line with previousstudies,6,12 n-pentene was in this study isomerized much fasterthan n-butene, precluding the pseudomonomolecular and codimer-ization routes as the predominant ones for both tested zeolites.

If the main route to isobutene is the classical bimolecularone, an increase in reactant partial pressure should result insignificantly higher rates of isobutene formation. The possiblepressure effect on product yield and selectivity was, therefore,studied over H-FER. The initial rate ofn-butene conversion andisobutene formation versus butene relative partial pressure is

illustrated in Figure 3. Although an increase in conversion withincreasing n-butene partial pressure is observed, the apparentrate of isobutene formation over nondeactivated H-FER isactually decreasing. The simultaneous increase in conversionand byproduct selectivity indicates that skeletal isomerizationis less sensitive to reactant partial pressure than byproductformation via oligomerization-cracking reactions and/or thatformed isobutene readily transforms into byproducts in consecu-tive bimolecular reactions. At high n-butene partial pressures,probably both monomolecular and bimolecular mechanisms areoperating. The amount of isobutene formed monomolecularlyover fresh FER has been estimated to 70%34 and that over freshTON has been estimated to between 80 and 90%.8,35 Inagreement with the findings of Domokos et al.,4 the initial rate

of isomerization in this work is almost constant (zero order inbutene) for highly diluted feeds, and as the 1-butene partial

Table 1. Characterization of the Zeolites Used

H-FER H-TON

FTIR of Adsorbed Pyridine (mol/g)BAS LAS BAS LAS

weaka 10.3 3.6 11.4 10.7mediumb 60.5 2.1 74.7 6.1strongc 108.7 2.2 91.4 2.7sum 179.4 8.0 177.6 19.5

X-ray FluorescenceSi/Al 30 40

Nitrogen Physisorptionspecific surface area, (m2/g) 432 295micropore volume, (mL/g) 0.141 0.085

a Desorbing between 523 and 623 K. b Desorbing between 623 and723 K. c Remaining after desorption at 723 K.

Figure 2. n-Pentene-isopentene equilibrium, H-TON and H-FER: 2, 573K; O, 623 K; 9, 673 K; WHSV ) 6-44 h-1; pHC ) 0.1-1.0 atm.

Figure 3. Rates ofn-butene conversion (9) and isobutene formation () asa function of reactant relative partial pressure (pn-butene/ptot); closed symbols,TOS ) 10 min; open symbols, TOS ) 46 h; H-FER, T) 623 K; WHSV) 12 h-1; ptot ) 1.0 atm.



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pressure approaches zero (the dashed lines in Figure 3 areextrapolated lines), the overall rate of butene conversion andisobutene formation meet. The initial rate of 4 g/(gcat h)corresponds to 71 mmol/(gcat h), which is close to previouslyreported values of 644 and 686 mmol/(gcat h) for FER and similarexperimental conditions. As the catalyst deactivates, the overallconversion decreases, causing an increase in the apparent rateof isobutene formation and selectivity, especially in cases wherethe thermodynamic equilibrium between butene isomers isestablished. However, the apparent rate of isobutene formationwith a 1:9 dilution is continuously decreasing with time-on-stream with hardly any increase in selectivity. This is a strong

indication that carbonaceous deposits are not necessary forhighly selective operation over the tested H-FER catalyst andmight even have an adverse effect on isobutene yield.

The relative isobutene yield, defined as the yield at time tdivided by the initial yield measured at TOS ) 10 min, for theexperiments carried out with a WHSV value of 12 h-1, is shownin Figure 4. The figure illustrates the differences in time-on-stream behavior with regard to isobutene yield for the two testedzeolites. With the exception of the runs performed at 523 K,the absolute yields of isobutene for both catalysts continuouslyincrease during the initial stages of reaction and reach ap-proximately 35 mol % after 25 h TOS. It is clear that H-FERis the more stable of the two zeolites and that H-TON needslonger operation times to achieve high isobutene selectivities.

At 673 K, the isobutene yield over H-TON more than doubles,from 17 to 36 mol %, during the first 25 h, while the initial

yield over H-FER for the same conditions is already close to30 mol %. The apparent activation energy for H-FER seems tobe significantly lower than that for H-TON, as the initial yieldat 523 K for H-FER is already 16 mol %, whereas only 4 mol% isobutene is observed over H-TON. Both zeolites exhibitdecreasing trends in isobutene yields with TOS at the lowesttemperature. A direct comparison of isobutene yield at 573 and673 K is shown in Figure 5. The initial yield of isobutene isagain always higher for H-FER than for H-TON. The yield ofthe desired product at 573 K also stays higher for H-FERthroughout the experiments, whereas H-TON exhibits higherisobutene yields at longer times on stream at 673 K. The

different deactivation behavior is also, once more, evident fromFigure 5. A small but rapid increase during the first hours,followed by an almost linear change in isobutene yield, is typicalfor H-FER, whereas the change in yield with time-on-stream iseven more for H-TON. The differences in deactivation behaviorcan be attributed to the presence of 8 MR channels in FER(causing the initial rapid change) and the differences in crystalmorphologies between the two zeolites. The needlelike crystalsof TON have larger relative contribution of external (nonselec-tive) sites than the flakelike crystal structure of FER, whichcould be the reason that prolonged reaction times are neededfor highly selective operation of H-TON.

3.2.2. Olefinic Byproduct Formation. Beside the desired

isobutene, large amounts of unsaturated byproduct are alsoformed. The unwanted reactions yielding olefinic byproduct

Figure 4. Relative isobutene yield, WHSV ) 12 h-1, pHC ) 0.5 atm: (a) H-FER; (b) H-TON; square, 673 K; circle, 623 K; upward-facing triangle, 573 K;left-facing triangle, 523 K.

Figure 5. Evolution of isobutene yield: (a) T) 573 K, (b) T) 673 K. open circle, H-TON, WHSV ) 44 h-1; open square, H-TON, WHSV ) 6 h-1; solidcircle, H-FER, WHSV ) 44 h-1; solid square, H-FER, WHSV ) 6 h-1. pHC ) 0.5 atm.



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include disproportionation of butene into propene and pentene

and various dimerization and oligomerization reactions. Theside-reactions taking place in butene skeletal isomerization overH-FER were studied by Kangas et al.,36 and the proposedreaction network is reproduced in Scheme 1.

Byproduct formation was the most pronounced in theexperiments with long residence times and elevated reactiontemperatures. The product distributions for the experimentscarried out with a WHSV value of 6 h-1 are given in Tables 2and 3. The main behavior with regard to product distributionand time-on-stream trends was roughly the same for bothcatalysts, with increases in butene and octene yields with TOS,while the rest of the components in the system exhibiteddecreasing tendencies in yields with prolonged operation time.For fresh catalysts, H-FER has 5-8 mol % higher yields of

both the reactant and product butenes, translating into superiorinitial selectivites compared to those of H-TON. However, the

isobutene selectivities for H-TON increase significantly withtime-on-stream, and in the experiments carried out at 623 and673 K, H-TON exhibits higher isobutene selectivites and yieldsthan H-FER after 46 h. The dramatic increases in isobuteneyields and selectivites coincide with similar decreases fordisproportionation products. Propene and pentene were alwaysthe most abundant byproducts, in some cases accounting formore than one-third of the total yield. The yield of penteneversus the yield of propene for the n-butene experiments over

H-TON is illustrated in Figure 6a. The bending of the curvefor 673 K can be seen as an indication of one or more additionalpaths for the formation of propene at higher reaction temper-atures. H-FER exhibited similar trends with the exception thatthe pentene yields at 623 and 673 K presented maxima,36 whichimplies that pentene cracks into propene and ethene and thatthis reaction has a much faster rate of deactivation than otherroutes yielding propene. Figure 7 further illustrates the differ-ences in pentene and propene yield for the two catalysts at thehighest reaction temperature of 673 K. The pentene-to-propeneratios for fresh catalysts are approximately the same for bothzeolites, but while it stays rather constant for H-TON, the ratiois initially increasing for H-FER and approaches a constant

value, slightly higher than that for H-TON, as the catalystdeactivates. The maximum amounts of ethene obtained in thiswork were 4 mol % for H-FER and 7 mol % for H-TON,suggesting that pentene cracking occurs also on H-TON butthat the deactivation behavior of the reaction does not noticeablydeviate from the other reactions taking place over H-TON.Cracking of pentene involves a primary carbenium ion-likeintermediate and might, thus, be more difficult to catalyze thanthe rest of the reactions represented in Scheme 1. However,monodimensional 10 MR zeotypes and zeolites, includingproton-form FER, have been found to catalyze pentene crackingvia monomolecular -scission,37 and propene and ethene havebeen observed as primary products over FER when introducing1-pentene as a reactant under similar reaction conditions.6 Theyields of hexene (Figure 6b) and heptene (Figure 6c), whichare dimers of propene and codimers of propene and butene,respectively, are both typically around 1-7 mol % each,although they behave quite differently with regard to propene.The plots of hexene versus propene yields exhibit a clearbending at higher conversions, which suggests an additionalreaction involving propene and hexene. Heptene, on the otherhand, displays nearly linear dependencies with regard topropene, especially at 673 K. This indicates that heptene israpidly formed from propene and butene, with a reaction orderof close to zero with regard to butene. For H-FER, the linearrelationship between heptene and propene, shown in Figure 8,is even clearer. The yield of octene as a function of propene is

shown in Figure 6d, and it behaves differently compared to theother pseudocomponents. The fact that the curves go through amaximum shows that octene and propene are involved inconsecutive reactions with at least part of the propene formedfrom octene.

For modeling purposes, which will be discussed in detail in anaccompanying paper, the main reaction routes need to be identified.The reaction network was checked by estimating the fraction ofhexene produced via cracking of surface-bound nonene. Scheme1 shows that hexene can be formed either from dimerization ofpropene or from cracking of an adsorbed C9 species. As propenetakes part in most of the reactions yielding byproduct, the C3)-to-C5) ratio is a good test of the reaction network. If pentene and

propene were primarily produced by cracking of octene, the molarratio between them would be around 1. As evident from Figures

Scheme 1. Proposed Reaction Scheme: MH ) Methylheptene,DMH ) Dimethylhexene, and TMP ) Trimethylpentene

Table 2. Product Distribution (in mol %) for H-FER, WHSV ) 6h-1, pHC ) 0.5 atm

temperature 573 K 623 K 673 K

TOS 10 min 46 h 10 min 46 h 10 min 46 h

LHCa 0.1 0.0 0.1 0.0 0.3 0.0

ethene 0.5 0.0 2.2 0.3 4.1 0.5propane 1.0 0.1 2.1 0.2 2.7 0.1propene 11.0 5.4 18.3 13.7 21.1 13.8isobutane 1.3 0.4 2.0 0.7 1.7 0.6n-butane 2.2 0.6 2.4 0.5 2.2 0.5n-butene 27.2 40.4 23.2 32.5 24.4 36.2isobutene 28.7 37.8 21.7 29.9 20.7 30.3pentenes 13.7 6.2 16.4 13.8 13.5 12.3hexenes 4.8 1.7 5.6 3.0 5.6 2.1heptenes 6.5 3.6 5.0 3.7 2.9 2.3octenes 3.1 3.9 1.0 1.8 0.9 1.2

a LHC ) methane + ethane.

Table 3. Product Distribution (in mol %) for H-TON, WHSV ) 6h-1, pHC ) 0.5 atm

temperature 573 K 623 K 673 K

TOS 10 min 46 h 10 min 46 h 10 min 46 hLHCa 0.1 0.0 0.2 0.0 0.5 0.0ethene 0.6 0.1 2.9 0.4 6.8 1.1propane 1.6 0.4 4.6 0.9 6.7 0.8propene 12.9 5.5 19.7 10.6 25.1 12.1isobutane 1.3 0.4 2.2 0.4 1.9 0.3n-butane 3.9 3.6 5.0 3.9 5.5 3.5n-butene 22.7 40.0 17.0 33.4 16.5 37.3isobutene 23.5 32.9 16.1 32.4 13.6 32.0pentenes 16.0 5.2 16.9 9.5 12.2 8.1hexenes 6.0 1.0 7.5 1.8 6.0 1.5heptenes 8.0 4.6 6.4 4.4 4.2 2.5octenes 3.5 6.2 1.6 2.3 0.9 0.7

a LHC ) methane + ethane.



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6a and 7, the observed ratios can differ significantly from this andthe completeness of the reaction network can be assessed bychecking under which conditions the sum of molecules originatingfrom propene and pentene are equal. The sum of C3 and C5molecules is defined in the following manner,

C3 )C3)+C3-C2

)-xC6

)+ 2(1-x)C6

)+C7

) (4)

C5 )C5)+C2

)+xC6

) (5)

where x denotes the fraction of hexene formed from nonene(Scheme 1). The fraction of hexene produced from nonene as

a function of temperature and time-on-stream is shown in Figure9. For weight hourly space velocities g 44 h-1, the fractionalways exceeds 1, which means that cracking of hexene is fasterthan dimerization of propene and that part of the formed hexenefurther cracks into two propene molecules. For H-FER, thefraction is very close to 1 and almost independent of temperatureor TOS, suggesting that no changes in predominant routes tobyproduct occur with elevated temperature or prolonged opera-tion time. For H-TON, the fraction is steadily increasing withtime-on-stream and it exhibits an obvious dependence onreaction temperature, with larger portions of the hexene formed

from nonene at lower temperatures. For lower weight hourlyspace velocities (not shown), and consequently longer residence

Figure 6. Major byproduct distribution, H-TON; 2, 573 K; O, 623 K; 9, 673 K; T) 573-673 K, WHSV ) 6-44 h-1, pHC ) 0.5 atm.

Figure 7. Pentene yield versus propene yield for H-FER (2) and H-TON(9): T) 673 K, WHSV ) 6-44 h-1, pHC ) 0.5 atm.

Figure 8. Heptene yield versus propene yield, H-FER; 2, 573 K; O, 623K; 9, 673 K; WHSV ) 6-44 h-1; pHC ) 0.5 atm.



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times, the fraction of hexene produced from nonene crackingdecreases for both zeolites.3.2.3. Hydrogen Transfer Reactions. In solid acid catalyzed

reactions of hydrocarbons, deactivation occurs mainly throughformation of coke that covers the active sites and/or blocks thezeolite channels. The creation of coke is usually considered toinvolve many consecutive steps; oligomerization of smallerolefins to form larger ones, cyclization, transformation intomonoaromatics through bimolecular hydride transfer, alkylationof the formed monoaromatic, and further ring closure of thealkyl-substituted monoaromatic into a bicyclic compound thatin turn undergoes further hydrogen transfer reactions to finallyproduce a biaromatic molecule. The formed biaromatic coke

can then be further transformed into triaromatics, etc. Ac-companying the formation of each aromatic ring are threebimolecular hydrogen transfer reactions, in which propene andbutene act as hydrogen acceptors and transform into theirsaturated counterparts. Hydrogen transfer involves a net transferof H2 from one molecule to another, producing hydrogenatedand dehydrogenated products. The reaction consists of proto-nation, hydride transfer, and deprotonation elemementarysteps.38 Hydride transfer of branched molecules is faster thanthose involving linear isomers and has been shown to be verysensitive to pore size and structure in skeletal isomerization,7

aromatization,39 and cracking reactions.40,41

Hydride transfer is generally thought of as the rate-limitingstep in hydrogen transfer, and the bimolecular elementary step

exhibits sterical constraints in many zeolites. The hydrideabstraction step has been reported to be sterically more hindered

than olefin addition,42

and the correlation between paraffins andolefins is, therefore, also a convenient way to evaluate pontentialshape-selectivity effects. Examples of the olefin-to-paraffin ratiosfor the C3 and C4 molecules in H-FER and H-TON are shownin Figure 10. Although TON has larger pores than FER, hydridetransfer to isobutene seems more restricted in H-TON than inH-FER. While H-FER exhibits the expected order of the ratiosif no severe spatial restrictions are present, the ratio betweenisobutene and isobutane for H-TON is considerably higher thananticipated with values approximately 1 order of magnitudelarger than those of nonbranched hydrocarbons. This indicatesthat the transition states in bimolecular hydride transfer involvingisobutene are significantly more constrained in the channels in

TON than in FER. At the intersection between the 8 MR and10 MR pores in FER, spherical cavities of 6-7 are formed,16

and this seems sufficiently large to accommodate at least someof the transition states needed for hydrogen transfer to isobutene.Self-hydrogenation of isobutene in these cavities is still spatiallyrestricted as the transformation from isobutene to isobutane overFER has been reported to be 84 and more than 206 times slowerthan the corresponding n-butene to n-butane reaction.

A somewhat more accurate estimation of sterical constraintsin butene isomerization is the ratio of isobutane to n-butane(i/n). If the self-hydrogenation is controlled thermodynamically,the ratio should be around 0.8 for the conditions used in thiswork.16 If the hydrogen transfer reactions are kineticallycontrolled, and n-butene and isobutene are present in sufficient

amounts, the ratio should be above the thermodynamic value,due to the much faster reaction of branched molecules. Houz-

Figure 9. Fraction of hexene produced through nonene cracking: (a) H-FER and (b) H-TON; 2, 573 K; O, 623 K; 9, 673 K; WHSV ) 44 h-1; pHC ) 0.5atm.

Figure 10. Olefin-to-paraffin ratios, T) 673 K, WHSV ) 6 h-1, pHC ) 0.5 atm: (a) H-FER and (b) H-TON; 9, C3; O, n-C4; 2, iso-C4.



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vicka and co-workers7 studied shape selectivity in skeletal

isomerization of butene over a wide range of zeolites anddiscovered that the ratio was clearly below the thermodynamicvalue for small-pore (8 MR) catalysts, while hydrogen transferto isobutene was a lot faster than that to n-butene for large-pore (12 MR) zeolites, giving ratios well-above 0.8. Medium-pore (10 MR) molecular sieves were found to be the mostselective and stable for 1-butene skeletal isomerization, and theratio of branched to linear butane varied around the thermody-namic one. The values for TON and FER zeolites wereapproximately the same as those obtained in this study (Table4). For fresh zeolites, the i/n ratio obtained in this work forH-FER is approximately twice as high as that for H-TON,although the total amount of hydrogen transfer products forH-TON is 2-fold that of H-FER. Surprisingly, the isobutane-to-n-butane ratio increases with time-on-stream for H-FER,although the relative concentration of isobutene among thebutene isomers is decreasing. If the main effect of coking isincreasing the steric constraints for reaction and diffusion, anarrowing of the micropores by coke deposits should slow downhydrogen transfer to isobutene more than to n-butene,16 thusdecreasing the i/n ratio. This peculiarity can be explained bydifferent deactivation rates of the acid sites in 10 MR and 8MR pores in FER. It has been proposed that the 8 MR channelsdeactivate faster than 10 MR ones;17,43 since 8 MR porestructures have been shown to be far more selective towardn-butane than isobutane,7 and as long as a significant part ofthe n-butane is formed in the 8 MR channels, an increase in

the i/n ratio with TOS seems logical. The ratios of isobutane ton-butane for H-FER exhibited maxima after 10-20 h TOS inall the experiments. For H-TON, the ratio was always decreasing.

3.2.4. Deactivation and Regeneration. Deactivation bycoking always accompanies butene skeletal isomerization, atleast for conditions close to those employed industrially. Formost 10 MR zeolites, coking has proven beneficial, in the sensethat it usually results in higher isobutene selectivities. The exactrole of the carbonaceous deposits and their mode of formationand location are still subjects of discussion.35,8,13,1517,19,20,32,35

The chemical nature of the coke formed in this study wasinvestigated by dissolution of the zeolite structure, extractionwith dichloromethane, and analysis by GC-MS. No clear trends

in the nature of the carbonaceous deposits as a function oftemperature were observed, although higher butene partialpressures resulted in more aromatic and less aliphatic coke.Overall, the coke formed on H-FER was heavier and morearomatic in nature compared to that formed on H-TON. Thedistribution of coke components was estimated by approximatingthe area of the chromatographic peaks to the amount ofcorresponding compounds. The aliphatic coke family, mainlyconsisting of long linear olefins, made up roughly 50% of thedeposits on H-TON and between 20 and 40% of those onH-FER. For H-TON, the aromatic compounds were distributedequally among species containing one, two, or three aromaticrings, whereas components with two and four aromatic ringsseemed to be favored in H-FER. The amounts of lighter species,

such as toluene and xylenes, are probably underestimated, sincethey possess high volatilities and parts of them most likely

evaporate with the dichloromethane. Toluene and xylenes havealso been observed directly by GC in the reactor effluent (seeScheme 1).44 The most abundant coke species on both H-TONand H-FER were alkyl-substituted naphthalenes.

Quantitative measurements of the coke content were obtainedby temperature-programmed oxidation, and for H-FER, theamount of carbon after approximately 50-70 h time-on-streamvaried between 6 and 9 wt %, which is in agreement withprevious reports for FER catalysts.3,4,14,43 The amount of

deposits on H-TON was roughly one-half that of H-FER,ranging from 3 to 4.5 wt %. Coke content increased withincreasing temperature for both catalysts. High-temperature cokewas also less soluble in dichloromethane than deposits formedat lower temperatures.

The tested zeolites could be regenerated by burning thecarbonaceous deposits in synthetic air, and both catalystsregained almost all of their initial activity. The results of repeatedreactions with fresh and one, three, and five times regeneratedzeolites are depicted in Figure 11. After five regenerations, bothzeolites have lost roughly 5 mol % in conversion and isobuteneyield, although the loss in activity per regeneration is decreasing.

3.3. Cracking of Octene Isomers. There exists 15 structural

isomers of octene, and the isomers chosen in the present studyrepresent the two extremes. 1-Octene has a fully linear skeletonand lacks any side -chains, whereas diisobutene contains threemethyl groups, including a geminal pair. As such, they behavedifferently when cracked and when diffusing through the narrowzeolite channels of the medium-pore zeolites.

A summary of the experimental findings is presented in Table5, together with the results from 1-butene isomerization undercomparable conditions. Overall, H-FER is more sensitive toreactant octene isomer as far as product distribution is concernedand always exhibited higher yields of isobutene. As expected,the closest to desired product distribution was obtained whenfeeding diisobutene. Cracking of 1-octene produced the least

amounts of isobutene, and feeding 1-butene yielded a distribu-tion of products in between the two. The sum of C3-C4 hydridetransfer products for H-TON is, for all tested reactants,approximately 2-fold that of H-FER. H-TON is also moreselective toward disproportionation.

When cracking 1-octene, the olefin product distribution (Table5) was almost the same for H-FER and H-TON, and the reactionproduced mostly propene and pentene. Although isobutene cannotbe formed as a primary product from n-octene cracking, equilibriumbetween the skeletal isomers of butene was observed for H-FER.The deactivation was the most severe for 1-octene cracking witha decrease in conversion for both zeolites in excess of 10 mol %during the first hour of reaction. In the same time frame, losses inactivity of approximately 2-3 mol % were observed when eitherdiisobutene or 1-butene were fed as reactants.

Diisobutene cracking is usually considered the most facile ofthe octene isomer -scission reactions and is theoretically 100%selective to isobutene. Still, the disproportionation products accountfor one-third of the yield when cracking diisobutene over freshH-TON. This indicates that geminally branched isomers experiencesevere mass transfer problems in the zeolite pores or that formedisobutene rapidly undergoes further transformations producingmostly propene and pentene. There is an ongoing discussion inthe scientific literature on the shape-selective effects,45 stericconstraints,18,46 and diffusional resistance47 of mono- and di-branched hydrocarbons in TON. Because of the steric constraintsof the branched molecule in the tested catalysts, the cracking

reaction of diisobutene might also predominantly take place onthe outer surface of the zeolite crystals.

Table 4. Isobutane-to-n-Butane Ratios, WHSV ) 6 h-1, pHC ) 0.5atm

H-FER H-TON

10 min 46 h 10 min 46 h

673 K 0.75 1.36 0.35 0.08623 K 0.81 1.34 0.43 0.11573 K 0.58 0.63 0.33 0.10



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The isomerization efficiency, defined as IE ) iso-C4)/(C3) +C5)), for the two zeolites and all the reactants is compared in Table5. For 1-octene cracking and 1-butene isomerization, the IE valueof H-FER is only slightly higher than that for H-TON, but for the

case of diisobutene cracking, the efficiency of H-FER is close todouble that of H-TON. The isomerization efficiency also more thandoubled during the first hour on stream, for both tested zeolites,when introducing diisobutene into the reactor. At the same time,the conversion mostly stayed unchanged, which supports the theoryof consecutive reactions of isobutene forming propene and pentene.Although both tested catalysts deactivated the most when 1-octenewas fed as reactant, no changes in isomerization efficiencies wereobserved in this case.

Taken together, the cracking experiments support the conclusionsof Domokos et al.48 Using applied molecular simulations over FER-and TON-type zeolites in siliceous forms, they found that thedocking energies for C8 isomers that can potentially be cracked

into isobutene were more favorable for FER, whereas potentiallynonselective C8 isomers were equally stabilized in the twostructures.

4. Conclusions

Both the H-TON and H-FER zeolites used in this study haveproven to be efficient catalysts in n-butene skeletal isomerization.Although the acidities and pore dimensions of the two zeolites weresimilar, their catalytic behavior differed drastically. H-FER exhib-ited significantly higher initial isobutene selectivities compared toH-TON, and it was also the more stable with time-on-stream ofthe two catalysts. For the conditions used in this study, isobutenewas formed mainly via the monomolecular route, while the

bimolecular paths of butene were largely responsible for byproductformation. The predominant route to byproduct was dispropor-

tionation, yielding propene and pentene. Of the two zeolites usedin this study, H-TON was more selective toward disproportionation.The observed product distribution of H-TON was also less sensitiveto feed reactant than that of H-FER, confirming previous theoreti-

cally calculated structure effects. The steric constraints in the twozeolite structures were assessed from the ratio of paraffins to olefinsand the ratio of isobutane to n-butane. Because of the slightly largerpore dimensions of H-TON, it was more selective toward hydrogentransfer products, with total paraffin yields approximately twicethat of H-FER. FER, on the other hand, has channel intersectionslarge enough to accommodate some of the transition states yieldingisobutane, and the hydrogen transfer reaction of isobutene was,therefore, 1 order of magnitude faster in H-FER than in H-TON.From the ratios of isobutane to n-butane, and pentene to propene,the different deactivation rates of the 8 and 10 MR channels inH-FER could also be observed. Both zeolites experienced deactiva-

tion through coking during the experiments, with roughly 10-

30%losses in n-butene conversion after 2 days time-on-stream. Cokebelonging to both aliphatic and aromatic families was detected.Overall, the coke formed on H-FER was heavier and more aromaticin nature. The tested zeolites were easily regenerated by burningof the carbonaceous deposits in synthetic air, and both catalystsregained almost all of their initial activity.

Acknowledgment

This work is part of the activities at bo Akademi ProcessChemistry Centre within the Finnish Centre of ExcellenceProgram (2000-2011) by the Academy of Finland. Financial

support from Graduate School in Chemical Engineering forM.K. is gratefully acknowledged.

Figure 11. Repeated reactions with fresh (solid symbols) and 1-5 times regenerated (open symbols) catalysts; 2, isobutene yield; , conversion. T) 623K, WHSV ) 44 h-1, pHC ) 0.5 atm: (a) H-FER and (b) H-TON.

Table 5. Comparison of Main Product Distributions over the Fresh Zeolites for the Cracking and Isomerization Experiments, T) 623 K

reactant diisobutenea 1-octeneb 1-butenec

catalyst H-FER H-TON H-FER H-TON H-FER H-TON

Yields (mol %)n-butene 26 22 19 17 23 17isobutene 36 27 18 15 22 16C3) + C5) 22 32 46 41 35 37C3-C4 alkanes 2 4 1 3 7 12conversion 96 98 96 88 77 83n-C4)/iso-C4) 0.73 0.80 1.06 1.18 1.07 1.06(C3) + C5))/C4) 0.35 0.64 1.26 1.28 0.77 1.11IE 1.66 0.87 0.39 0.36 0.63 0.44

a TOS ) 15 min, pC8 ) 0.25 atm, WHSV ) 7.5 h-1. b TOS ) 15 min, pC8 ) 0.15 atm, WHSV ) 7.5 h-1. c TOS ) 10 min, pC4 ) 0.50 atm,WHSV ) 6.0 h-1.



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ReceiVed for reView January 15, 2008ReVised manuscript receiVedApril 23, 2008

AcceptedMay 7, 2008

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