hydroisomerization of normal hexadecane with platinum catalysts

8/12/2019 Hydroisomerization of Normal Hexadecane With Platinum Catalysts

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Applied Catalysis A: General 193 (2000) 155171

Hydroisomerization of normal hexadecane with platinum-promotedtungstate-modified zirconia catalysts

Shuguang Zhang, Yulong Zhang, John W. Tierney, Irving Wender Department of Chemical and Petroleum Engineering, 1249 Benedum Hall, University of Pittsburgh, Pittsburgh, PA 15261, USA

Received 6 May 1999; received in revised form 3 September 1999; accepted 7 September 1999

Abstract

Thepresentworkexaminestheactivity,selectivityandlong-termstabilityofplatinum-promotedtungstate-modifiedzirconia

(Pt/WO3/ZrO2) catalysts for the hydroisomerization of long-chain linear alkanes under relatively mild conditions, usingn-hexadecane as a model compound. The preparation of catalysts is described. A trickle-bed reactor is used to compare theactivities and selectivities of three Pt/WO3/ZrO2 catalysts prepared by different methods and to investigate the effects oftungsten loading and of reaction conditions for hydroisomerization ofn-hexadecane. A run of 93.5 h was conducted usingthe most highly active Pt/WO3/ZrO2catalyst which contains 0.5 wt.% well-dispersed Pt and 6.5 wt.% W. Reaction conditionswere manipulated and five shutdown (feed stopped, reactor temperature lowered but H 2 flow rate maintained) and restartoperations were carried out. This catalyst showed high activity and stability. Considerable success has also been achieved inconverting n-hexadecane to isohexadecanes for 100h at temperaturesof about 220CandunderH2pressure as low as 160 psig(1 psig= 6.895 kPa). Our best results (highest i-C16yield) are a 79.1 wt.%n-C16conversion, 89.9 wt.% i-C16selectivity and71.1wt.% i-C16yield at 218C, 160psig H2, H2/n-C16mole ratio= 2 and weight hour space velocity (WHSV)= 1 h1. ThePt/WO3/ZrO2catalyst is rugged and has properties which allow one to propose that it may attain commercial use after furtherstudy. 2000 Elsevier Science B.V. All rights reserved.

Keywords:Tungstate-modified zirconia; Long-chain linear alkanes; Hydroisomerization; Isohexadecanes; Platinum

1. Introduction

The acid-catalyzed isomerization of alkanes is ofgrowing importance in determining the nature oftransportation fuels, including environmentally cleanhigh octane gasoline, high cetane diesel fuel, lowpour point jet fuel and lubricant base stocks. How-ever, isomerization of long-chain alkanes generallyprecedes cracking, leading to extensive undesiredcracking reactions; this tends to limit catalytic paraffinisomerization to C4-C6 alkanes [1,2]. Alkanes above

Corresponding author.E-mail address:[email protected] (I. Wender).

C7+ crack easily [3]; they have a large number ofmethylene units that, in the presence of Lewis acids,can form carbenium ions by abstraction of hydrideions [4] or, in the presence of strong Bronsted acids,can form carbonium ions [5]. It is highly desirableto obtain environmentally clean catalysts that can beused to isomerize long-chain paraffins with minimumcracking. Catalysts with a certain optimal balance ofmetal and acid functions at suitable reaction condi-tions are generally needed to suppress cracking inorder to achieve high isomerization selectivity for

long-chain paraffins [68].Efforts [912] have been made to avoid the use

of present commercial catalysts for the isomerization

0926-860X/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S0 926-860X (99)0042 5-1


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156 S. Zhang et al. / Applied Catalysis A: General 193 (2000) 155171

of alkanes: HF is particularly dangerous while cat-alysts containing halides such as AlCl3 or sulfuricacid are corrosive and pose significant environmen-tal challenges including disposal of wastes [2]. Re-cently, there have been efforts by industrial [1315]and academic researchers [1618] to obtain solid acidcatalysts which are environmentally more suitable foralkane isomerization with minimal cracking. The ob-

jective is to combine high activity at low temperaturesand pressures with catalyst stability and regenerability.

Considerable interest has been focused on the useof strong solid acids based on anion-modified zirco-nium oxide catalysts [1620]. Sulfated zirconia wasfound to be effective in catalyzing butane isomeriza-tion at room temperature, a thermodynamically diffi-cult reaction that does not take place in 100% sulfuricacid [21]. Farcasiu et al.have furnished evidence thatthe high activity of sulfated zirconia for the isomer-ization of paraffins is associated with a redox process

which produces ion radicals leading to the formationof carbenium ions [22,23]. A commercial process fornaphtha isomerization using sulfated zirconia has beenreported [24].

Hino and Arata [25] found that the substitution ofsmall amounts of WO3for SO42 to give ZrO2/WO3resulted in a strong solid acid. The ZrO2/WO3 is ofsomewhat lower acid strength than ZrO2/SO4[2,4,26].A number of researchers then found [2729] that,as with ZrO2/SO4 [30], addition of Pt to ZrO2/WO3was, in the presence of hydrogen, an active and se-lective catalyst forn-alkane isomerization. Iglesia andco-workers [2,31] found that Pt/WO3/ZrO2was more

selective than Pt/SO4/ZrO2in the hydroisomerizationof n-heptane, partly the result of increased hydridetransfer rates which limit the lifetime of adsorbed car-benium ions. Iglesia and co-workers [1,2,31] proposedthat platinum particles dissociate molecular hydrogeninto hydrogen atoms which spill over to acid sites onthe catalyst surface to form protons and hydride ions.The state of Pt on the surface of anion-modified zir-conia is still in controversy [3237].

The hydroisomerization and/or hydrocracking ofn-hexadecane, FischerTropsch (FT) waxes andpolyolefins in microautoclaves have been success-fully carried out in this laboratory using Pt/SO4/ZrO2

and Pt or Ni promoted WO3/ZrO2 catalysts [3840].Pt/WO3/ZrO2 was more selective for n-hexadecanehydroisomerization (300C, 500 psig [cold] H2,

20 min) and more stable in the hydrocracking ofFT wax and polyolefins (375C, 1500psig [cold]H2, 25min) than was Pt/SO4/ZrO2. Preliminaryresults from XANES analyses on fresh and usedPt/WO3/ZrO2 catalysts indicated that platinum andtungsten existed as zerovalent platinum and hexava-lent tungsten, in both materials [38]. Hydroisomer-ization of n-hexadecane using Pt/SO4/ZrO2 catalystshas also been studied by Davis et al.in a trickle bedcontinuous reactor [41]. The results indicated thathigh isomerization selectivity was difficult to achieveeven at low n-hexadecane conversion. There havebeen few studies [14,42] of the promising catalyst,Pt/WO3/ZrO2, for hydroisomerization of long-chainlinear paraffins (>C7) in a continuous reactor system.

The objective of this study, carried out in a smallcontinuous trickle-bed reactor, is to examine the pos-sibility of the use of Pt/WO3/ZrO2 as a catalyst forthe isomerization, better termed hydroisomerization,

ofn-alkanes above C7, usingn-hexadecane as a modelcompound. The activity, selectivity and long-term sta-bility of this catalyst is investigated. A later paper willapply this catalyst to the hydroisomerization of longern-alkanes in FT waxes and in various polymers, suchas polyethylene and polypropylene.

It should be pointed out that Chevron has introduceda silicoaluminophosphate catalyst, SAPO-11, in theirIsodewaxing process [8]. Metal-promoted SAPO-11is more selective than zeolites for long-chain paraffinisomerization [43]; however, it is generally used athigh pressure (1000psig), high hydrogen to feed moleratios (30/1) and at about 340C, a relatively high

reaction temperature. Isodewaxing seems to be aimedmainly at producing lubricant oil, although transporta-tion fuels can be produced from waxy feedstocks.The Pt/WO3/ZrO2 catalyst appears to be environ-mentally benign, stable, robust and likely regenerableand could be a viable alternative to Isodewaxing forlong-chain alkanes.

2. Experimental

2.1. Catalyst preparation and characterization

In their study of the isomerization of n-butane toisobutane, Larsen and coworkers [28] compared two


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S. Zhang et al. / Applied Catalysis A: General 193 (2000) 155171 157

Pt promoted tungstated zirconia catalysts which hadthe same Pt and tungstate loadings. The first cata-lyst was prepared by adding Pt to WO3/ZrO2 whichwas already calcined at a high temperature (823C);it had a high Pt dispersion after another calcination at500C. The second one was prepared by calcinationat 823C of a zirconia to which W and Pt had beenadded by impregnation, resulting in a catalyst with alow Pt dispersion. They found that the first catalystincreased selectivity toward hydrogenolysis productsin n-butane hydroisomerization while hydrogenoly-sis was suppressed on the second catalyst. However,Figoli et al.claimed that addition of Pt to a calcinedtungstated zirconia, followed by a second calcination,generated a catalyst with better catalytic activity andstability for n-hexane hydroisomerization than a cata-lyst from a one step calcination (830C, 3h) of a Ptand tungstate promoted zirconia, on which there wereless accessible Pt particles [44].

We studied the effects of two different prepa-ration methods on the activity and selectivity ofPt/WO3/ZrO2 catalysts for n-hexadecane hydro-isomerization. Two Pt (0.5wt.%)/WO3/ZrO2 cata-lysts (I and II) were prepared. They had the samecomposition, 0.5 wt.% Pt and 6.5 wt.% W. Bothstarted with zirconium hydroxide which had beenprecipitated by addition of ammonium hydrox-ide to zirconium chloride, but they differed in the

Fig. 1. Three Pt/WO3/ZrO2 catalysts prepared by different methods.

following ways. There were two calcinations in thepreparation of I, the first at 700C after zirconiumhydroxide was modified by impregnation of ammo-nium metatungstate ((NH4)6W12O39xH2O) solutionand the second at 500C after the 700C calcinedtungstated zirconia was impregnated with a platinumsalt (H2PtCl66H2O); For II, there was only one cal-cination at 700C after co-impregnation of zirconiumhydroxide by the solution of tungsten and metal salts.Based on our previous results, 700C was chosen asa calcination temperature to obtain WO3/ZrO2 withhighest activity after Pt promotion. We used 500C,for the other calcination, as did Larsen et al. andFigoli et al.

A third catalyst was prepared in a different way.A zirconium hydroxide sample to which WO3 hadalready been added was obtained from Magne-sium Elektron, Ltd. (MEL). In our laboratories, thistungstated zirconia (6.7 wt.% W) sample was calcined

at 700

C and then was promoted by impregnationwith Pt, followed by another calcination at 500C.The resulting Pt (0.5wt.%)/WO3/ZrO2catalyst waslabeled as PtWZ(MEL). The details of the prepara-tion procedures for these three catalysts are shown inFig. 1.

Comparison of I and II (Fig. 2) showed that I wasmore active forn-C16hydroisomerization. A series ofsix catalysts with different W but with the same Pt


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Fig. 2. Comparison of conversion and selectivity for hydroisomerization of n-C16 using I and II [Reaction conditions: 300C, 300psig,H2/n-C16 mole ratio=2, 6th hour reaction results].

loadings were then prepared under thesame conditions

used for I so as to study the effects of tungsten loadingand reaction variables.BET specific surface areas (by nitrogen adsorp-

tion) and platinum dispersions (by carbon monox-ide adsorption) were measured using a Micromerit-ics ASAP 2010 instrument. X-ray diffraction exper-iments were carried out on a Phillips XPert X-raydiffractometer; a nickel-filtered Cu K radiation at40kV and 30mA was used. Scanning electron mi-croscopy (SEM) characterizations were carried outusing a Phillips SEM equipped with a XL30 FieldEmission Gun. Temperature-programmed desorptionof ammonia (NH3-TPD) experiments were conducted

using Altamira Instruments AMI-1. The catalyst waspretreated in 30 cc/min He at 450C for 1h. Ammo-nia was adsorbed at 100C for 10min and then He(30cc/min) was used to flush the catalyst for 1h toremove physisorbed ammonia. Then the temperaturewas increased to 700C at a rate of 10C/min in a he-lium flow (30 cc/min). TCD was used to monitor thedesorption process of ammonia.

2.2. Reactor system, operating procedure and

product analyses

Catalytic activity and selectivity tests were carriedout in a continuous trickle bed with a 0.305in. i.d.and 17 in. long stainless steel reactor. Reaction tem-

peratures were controlled by a computer and system

pressures by a back-pressure regulator.n-Hexadecane(99wt.% n-C16, from ICN Biomedical Inc.) was de-livered from a feed tank into the reactor by a syringepump at a constant rate of 6.6 ml/h. Because of thelow flow rate, flow through the back-pressure regula-tor was intermittent and there was a pressure surge of10 psig every 2 min. Total gas flow rate (pure hydro-gen or a mixture of hydrogen and helium) was keptconstant for catalyst tests. Input rates of hydrogen andhelium (make-up gas to keep gas flow rate constantwhen studying pressure effects) were each controlledby a separate mass flow meter.

The catalysts were crushed to 4060 mesh pellets

and placed into the center of the reactor after mixingwith an equal volume of quartz (5070 mesh). Thequartz had been washed with 10 wt.% hydrochloricacid and double distilled deionized water and then cal-cined at 700C for 3 h before use. Blank runs showedthe pretreated quartz had no catalytic activity for n-C16conversion. It was also used as packing at each end ofthe reactor. The length of catalyst bed was about 6.3 in.when weight hour space velocity (WHSV) was 1h1.Since the flow rates of gas and liquid feed were main-tained constant, variations in WHSV were made bychanging the amount of catalyst so that possible heatand mass transfer or channeling problems would be

same for all runs. Before reactions, the catalysts wereactivated at 450C with 20 ml/min of air for 1h. Gen-erally, a steady state was reached on a fresh catalyst


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within 5h at 300C on stream (TOS) but at longertimes at low reaction temperatures.

Two of the long runs included shutdown operationsand restarts. The shutdown procedure was as follows:feed (n-C16) was halted but H2flow and pressure weremaintained, oven temperature was lowered to roomtemperature and the H2 flow decreased to 10 cc/min.To restart a reaction, the H2 flow was adjusted toreach the desired flow rate and pressure, the oven tem-perature was raised and n-hexadecane addition com-menced. Catalysts experienced no deactivation duringa series of changes of reaction conditions (T, P andH2/n-C16 mole ratio) and five shutdown and restartoperations or during a run of 100 h at about 220C,160 psig, WHSV=1 h1 and H2/n-C16mole ratio= 2.In one case, the reaction was arbitrarily stopped andthe used but still active catalyst was calcined at 450Cin 20 ml/min of air for 3 h to determine its activity af-ter high temperature treatment.

Liquid products were collected in an ice-watercooled vial for analysis using an HP-5980 GC. Thecolumn was an HP-1 (Cross-linked Methyl SiliconGum, 25 m 0.2mm 0.33m). The GC oven tem-perature increased from 40C to 280C at a rate of5C/min and then maintained for 10 min. The injectortemperature and the FID detector temperature were320C and 280C, respectively. Helium was used as a

Fig. 3. NH3-TPD patterns of Catalysts I and II.

carrier gas at a rate of 50 cc/min. Cracking products,normal hexadecane and isohexadecanes with only onemethyl group were identified using a GC (HP-5890II)-MS (HP-5970). The column used in the GC-MSwas an HP-1 (Cross-linked Methyl Silicon Gum,50 m 0.2mm 0.33m). The temperature programfor the GC-MS oven was the same as that for the GCexcept that the rate was 3C/min. Identification of C16isomers with two or more branches was achieved bycomparison with spectra found in the literature [7,45].

3. Results and discussion

n-Hexadecane conversion was defined as the dif-ference between n-C16weight percentage in the feed(100 wt.%) and that in the reaction products; the i-C16selectivity was calculated by dividing the i-C16weightpercentage in the products by n-C16 conversion; the

i-C16yield was the weight percentage of branched C16isomers in the products. To obtain comparable n-C16conversions with different catalysts, activity and se-lectivity tests were carried out at the same tempera-ture, usually at 300C. Reactions at lower tempera-tures, from 210 to 250C, using our most active cata-lyst (I) were investigated and the influence of reactionpressure and H2/n-C16mole ratios were studied.


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Fig. 4. Distribution of branched C16 isomers versus n-C16 conversion at various WHSV. Catalyst: I, 300C, 300psig, H2/n-C16moleratio=2, 6th hour results.

3.1. Comparison of catalysts I, II and PtWZ(MEL)

Catalysts I and II were tested at 300C, 300psig,H2/n-C16mole ratio= 2 and various WHSVs. Resultsat steady states are shown in Fig. 2. n-Hexadecaneconversion decreased and i-C16 selectivity increasedfor both catalysts when the WHSV was increased,but the n-C16 conversion with catalyst II declinedmore rapidly. At the same level ofn-C16 conversion,90 wt.% for instance, the i-C16 selectivity with I wasabout 20 wt.% higher than that obtained with II. Thesetwo catalysts had similar acidity, as shown by their

TPD patterns in Fig. 3. A possible explanation isthat the high temperature calcination (700C), afterplatinum salt impregnation used in the preparation ofII, resulted in a low degree of platinum dispersion.These two catalysts have similar surface areas, about67.5m2/g, but carbon monoxide chemisorption exper-

Table 1Surface properties of PtWZ(MEL), I and II

Catalyst I II PtWZ(MEL)

Pt (wt.%) 0.5 0.5 0.5W (wt.%) 6.5 6.5 6.7Surface area (m2/g) 67.5 67.6 49.2

Pore volume (cm3/g) 0.16 0.19 0.05Pore diameter () 88.2 113.2 44.0Platinum dispersion (%) 71.0 9.8 54.5

iments show the platinum dispersion of I to be 71.0%,much higher than that of II (9.8%) (Table 1). Met-als such as platinum [1], palladium or nickel [38]can provide hydride ions by dissociation of hydrogenmolecules and enhance acidity by hydrogen spillover[4648]. Lack of available hydride ions due to lowplatinum dispersion may result in low activity or longresidence times for reaction intermediates and increasethe opportunity for cracking to occur.

The distribution of branched hexadecane isomersobtained from the hydroisomerization ofn-C16usingcatalyst I varied at different n-C16 conversions (Fig.

4). At high conversions, more normal hexadecaneswere converted to monobranched isomers by alkyl(mainly methyl) shifts; monobranched i-C16 isomer-ized by further alkyl shifts so that the percentageof multibranched isomers increased. The latter aremore easily cracked since the chances of -scissionincreases when there are more tertiary carbon atomsin isomers, but -scission was probably inhibitedby fast hydride ion transfer on this type of catalyst[2,31]. Cracked products increased markedly as theconcentration of multibranched hexadecanes grew.As intermediates between monobranched and multi-branched isomers, the amount of dimethyltetrade-

canes remained fairly constant. A typical GC-MSspectrum of hexadecane isomers is shown in Fig. 5.A similar product distribution was obtained by Girgis


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Fig. 5. A typical GC-MS spectrum of hexadecane isomers in the products of n-C16 hydroisomerization.

and Tsao [7], who studied the hydroisomerization ofn-hexadecane using a Pt promoted proprietary zeolite.They developed a reaction network for this catalyst.In the network, monobranched isomers were proposedto be the primary product, the multibranched isomers(isomers with two or more branches) were the sec-ondary products; cracked products were formed fromthe multibranched isomers.The catalytic activities ofPtWZ(MEL) and I forn-C16hydroisomerization werecompared at 220C, 300psig, WHSV= 1 h1 andH2/n-C16mole ratio= 2. The results were essentiallythe same (Table 2), although I had a higher surfacearea (Table 1). It is interesting that these two catalystsdiffered greatly in their XRD patterns (Fig. 6). Thepeaks of tetragonal WO3 were only observed in theXRD pattern of I. A possible explanation is that thetungsten oxide in PtWZ was so well dispersed thatthe WO3 clusters were too small to be identified byXRD. If the tungsten oxide was well dispersed, thehigh activity of PtWZ(MEL) would not be surprising

Table 2Comparison of two catalysts having similar compositionsa

Catalyst n-C16conversion (wt.%) i-C16 selectivity (wt.%) i-C16 yield (wt.%)

PtWZ (MEL) 53.8 96.5 51.9I 53.8 97.6 52.5

aReaction conditions: 220C, 300psig, H2/n-C16 mole ratio= 2/1, WHSV= 1 h1. Results at steady state.

although its surface area was not as large as I. Almostall zirconia in PtWZ(MEL) was in the tetragonalphase, while there was much more monoclinic zirco-nia in I. Both monoclinic and tetragonal zirconia hadactivity for n-C16hydroisomerization. As determinedby SEM, PtWZ(MEL) consists entirely of particles ofabout 2m which are much larger than particles in I(Fig. 7 and Fig. 10a).

3.2. Effect of tungsten loading on catalytic activity

BET surface areas, pore sizes and pore volumes ofsix catalysts with different amounts of tungsten areshown in Table 3. These catalysts were tested at thesame temperature (300C), pressure (300 psig) and hy-drogen to n-C16 ratio (2/1mol/mol) (Fig. 8). A rel-atively high temperature was chosen so that steadystates could be reached within several hours (generally5h) and n-C16 conversions over each catalyst were


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Fig. 6. XRD profiles of PtWZ(MEL) and I.

high enough for comparisons of activity and selectiv-ity. Catalysts with 6.5 and 8 wt.% tungsten have highhydroisomerization selectivities at high conversions.Generally, about 8 wt.% tungsten loading is consideredas the amount which forms a monolayer coverage overtungsten modified zirconia [31,49]. For Pt/WO3/ZrO2with different tungsten loadings, both the surface areasand the relative amount of tetragonal zirconia shownin Fig. 9 increased with increasing tungsten contents.

Fig. 7. SEM patterns of PtWZ(MEL).

The 15 wt.% W catalyst, which has the largest surfacearea and the largest amount of tetragonal zirconia, wasnot the most active catalyst for hydroisomerization ofn-hexadecane under these test conditions. Our opti-mum tungsten loading range, 6.58 wt.%, is consistentwith that obtained inn-butane isomerization by Larsenand Petkovic, who pointed out that inactive tungstenoxide species could be formed above a 6 wt.% W limit[29]. It has been suggested that a two layer coverage on


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Table 3Physical properties of Pt (0.5wt.%)/WO3/ZrO2with different tung-sten contents

Catalysta A B C D E F

Tungsten content (wt.%) 3 4.5 6.5 8 10 15Surface area (m2/g) 42.5 56.4 67.5 74.9 90.0 99.1Pore diameter () 125.1 111.8 96.8 88.2 87.3 72.5

Pore volume (cm3

/g) 0.14 0.16 0.16 0.16 0.20 0.18a Prepared with same procedure for I.

tungstated zirconia occurred at 16 wt.% W, generatinga maximum activity for n-pentane isomerization [49].Parera et allfound that the optimum tungsten loadingwas 15wt.% forn-butane isomerization [50]. The dif-ference with tungsten loadings to achieve various ac-tivities is likely the result of differences in preparationmethods, starting materials or test reactions.

At 50 wt.% isomerization selectivity, overall iso-merization and cracking rates are equal. From Fig.

8, a 95wt.% conversion of n-hexadecane with a50 wt.% isomerization selectivity could be achievedusing the Pt/WO3/ZrO2 (6.5 wt.% W) catalyst atWHSV= 8.5h1 (220C, 300 psig, and H2/n-C16mole ratio= 2). Above this conversion, crack-ing will exceed isomerization so that more than50 wt.% of convertedn-hexadecane will form cracked

Fig. 8. Effect of tungsten content (315wt.%) on n-C16 conversion () and i-C16 selectivity (-- - - ). [Catalysts prepared by same manneras that for I; see Table 3 for details of A, B, C, D, E, F] reaction conditions:300C, 300psig, H2/n-C16 mole ratio=2, 6th hour results.

products. Since the conversion at 50 wt.% selec-tivity using catalyst I is high, it appears to be avery good catalyst for the hydroisomerization ofn-hexadecane.

In the XRD patterns of Pt (0.5wt.%)/WO3/ZrO2with different tungsten loadings (Fig. 9), the peak rep-resenting tetragonal zirconia at 2= 30 was slightlyshifted toward a lower 2value with increased tung-sten loading. Peaks representing tetragonal tungstenoxide (WO3) had the greatest area at 6.5 wt.% W andthen diminished at higher tungsten loadings, a likelyindication of strong interaction between tungstatespecies and the zirconia support. The stabilization oftetragonal zirconia by tungstate has been shown byothers [44,51]. Effects of variables during impregna-tion, such as the pH values of the salt solutions andthe pore size of zirconium hydroxide, on catalyticactivity for butane isomerization have been studiedby Parera and coworkers [50].

The amount of platinum was too small to be iden-tified by XRD. The SEM characterization shows thatthere is little difference in texture between catalystscontaining 6.5 and 15wt.% W (Fig. 10). No Pt orWO3 entities were observed in SEM characteriza-tion, probably because their domain sizes were toosmall.


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Fig. 9. XRD patterns of Pt (0.5wt.%)/WO3/ZrO2 with different tungsten contents. Catalyst prepared using the method for I (see Fig. 1).(a) 6.5 wt.% W; (b) 15wt.% W.

TPD experiments using ammonia as the adsorbatewere carried out to obtain a measure of the acidities ofthese Pt/WO3/ZrO2 (4.515wt.% W) catalysts. TPDexperiment was also carried out with an H-ZSM-5

Fig. 10. SEM patterns of Pt (0.5wt.%)/WO3/ZrO2.

sample (SiO2/Al2O3 = 30, Na= 0.02 mol%) for com-parison (Fig. 11 ). The amounts of ammonia adsorbedand the temperatures of ammonia desorption increasedwith higher tungsten loadings. However, compared


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Fig. 11. NH3-TPD patterns of HZSM-5 and Pt (0.5wt.%)/WO3/ZrO2 with different W loadings.

Fig. 12. Effect of operations of calcination, shutdown and restart on the activity of catalyst I for n-C16 hydroisomerization.


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Fig. 13. Effects of reaction variables: T, P and H2/n-C16 mole ratio. Catalyst: I, WHSV=1 h1 n-C16 conversion () and i-C16 selectivity(----) .

with the HZSM-5 sample, Pt promoted tungstated zir-conia has fewer acid sites and a weaker acid strength.The acidity of these Pt (0.5 wt.%)/WO3/ZrO2catalystsmay not be the principal cause for high activity forhydroisomerization.

3.3. Effect of high temperature treatment

A 72 h run started with fresh catalyst I (Fig. 12).It took more than 20h to reach a steady state at230C, 300 psig, WHSV= 1 h1 and H2/n-C16 moleratio= 2. The temperature was varied to obtain botha high n-C16 conversion and a high i-C16 selectiv-ity. After 34 h, the reaction was arbitrarily stoppedalthough the catalyst did not deactivate. The cata-lyst was calcined in the reactor at 450C, 3h in a20cc/min air flow to see if the calcination would

affect its activity. When operated again at the sameconditions, the induction time was 9 h, shorter thanthat with the fresh catalyst. Compared with the fresh

catalyst, n-C16 conversion and i-C16 selectivity withthe calcined catalyst were about 3wt.% lower and9 wt.% higher, respectively. Operations of shutdownand restart had little effect on activity and selectiv-ity. In the presence of hydrogen, no deactivation was

observed in our experiments.

3.4. Performance of catalysts under various

operations

The performance of catalyst I at different operatingconditions was investigated during a run of 93.5hTOS (Fig. 13). There were five shutdown and restartoperations. Reaction variables, including T, P andH2/n-C16 mole ratios, were manipulated at differentlevels. Steady states may not have been reached atsome points, but the results showed the response of

n-C16 conversion and i-C16 selectivity to changesof reaction conditions and the rugged nature of thiscatalyst.


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Fig. 14. Effect of reaction temperature on n-C16 conversion and i-C16 selectivity. Reaction conditions: 300 psig, H2/n-C16 mole ratio= 2,WHSV=1 h1.

Table 4Comparison of two sets of reaction conditionsa

Reaction 300C, WHSV= 24 h1, 230C, WHSV=1 h1,conditions 300 psig, H2/n-C16 mole 300 psig, H2/n-C16 mole

ratio= 2 ratio=2

n-C16 conversion (wt.%) 82.8 85.9i-C16 selectivity (wt.%) 74.3 83.1i-C16 yield (wt.%) 61.5 71.4

a Catalyst: I, steady-state results.

In the temperature range from 210 to 250C,lowering the temperature resulted in lower conver-sion but higher selectivity (Fig. 14). An 85.9wt.%n-C16 conversion with an 83.1 wt.% i-C16 selectivitywas achieved at 230C, WHSV= 1 h1. Compara-ble n-C16 conversion (82.8wt.%) was also obtainedat higher temperature and higher WHSV (300C,WHSV= 24 h1), but the i-C16 selectivity was only74.3 wt.% (Table 4). This suggests that larger i-C16yields will be obtained at low temperatures and alow WHSV, rather than at high temperatures and a

high WHSV. For a once-through process, operation at230C is desirable because of the high i-C16yield of71.4 wt.%.

An increase of total reaction pressure but with con-stant hydrogen partial pressure resulted in low conver-sion with high selectivity (Fig. 15). The hydrogen ton-C16 mole ratio has an effect similar to that due tototal pressure (Table 5). At low H2/n-C16mole ratios(low hydrogen partial pressures), high n-C16 conver-sions and low i-C16selectivity were obtained, indicat-ing a negative reaction order with respect to hydrogen.It is likely that H2and n-C16compete for adsorptionon the catalyst. A similar effect of hydrogen partialpressure on conversion was observed by Larsen et al l

in butane isomerization [28] and by Iglesia et al. inheptane isomerization [31] using Pt/WO3/ZrO2. Forisomerizations of short chain paraffins (


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Fig. 15. Effect of reaction pressure on n-C16 conversion and i-C16 selectivity. Reaction conditions: 230C, H2/n-C16 mole ratio= 2,WHSV=1 h1.

sulfated zirconia with or without Pt, both positiveand negative effects of H2 pressure were reported[19,52].

Catalyst I lost activity in the absence of hydrogenat 180C, but the activity could be restored by resum-ing hydrogen addition. Essentially the same results(85.9wt.% n-C16 conversion and 82.74wt.% i-C16

Table 5Effect of H2/n-C16 mole ratio on n-C16 conversion and i-C16 selectivity

T(oC) 230 200 180

TOS (h) 31 61.5 66.5 72.5 77.5 78.5 82.5H2/n-C16 mole ratio 2 1 1 0.5 0.5 0 0n-C16 conversion (wt.%) 85.9 96.5 28.9 36.1 10.1 21.0a 6.1b

i-C16 selectivity (wt.%) 83.1 52.2 99.4 99.5 95.0 100.0a 94.1b

i-C16 yield (wt.%) 71.4 50.4 28.7 36.0 9.6 21.0a 5.7b

i-C16 distribution (wt.%)Multibranched i-C16 26.6 38.9 3.8 5.3 0 2.4 0Dimethyltetradecanes 42.8 42.8 30.0 31.7 19.8 28.6 14.07- and 8-methylpentadecane 6.4 4.2 15.7 13.3 17.8 16.7 35.16-methylpentadecane 5.7 3.0 13.6 14.4 c 14.3 c

5-methylpentadecane 4.6 2.8 10.8 10.3 12.5 11.4 14.04-methylpentadecane 4.1 2.4 9.0 8.3 10.4 9.5 10.53-methylpentadecane 4.5 2.8 9.0 8.6 12.5 9.5 17.52-methylpentadecane 3.9 2.6 6.6 6.4 8.3 8.1 8.83-ethyltetradecane 0.8 0.4 1.4 1.7 0 0 0

a The result was obtained at 1st hour after changing H2/n-C16 ratio from 0.5 to 0.b The result was obtained at 5th hour after changing H2/n-C16 ratio from 0.5 to 0.c The peak of 6-methylpentadecane was not separated from that of 7- and 8-methylpentadecane.

selectivity at TOS= 31 h, 84.64 wt.% n-C16 conver-sion and 84.12 wt.% i-C16selectivity at TOS= 93.5h)were obtained when reaction conditions were switchedback to 230C, WHSV= l h1, 300psig, H2/n-C16mole ratio= 2 after shutting off hydrogen for 6 h. Itis likely that hydrogen cleans the catalyst surface andrestores hydroisomerization activity.


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Fig. 16. XRD patterns of fresh, washed and dried, washed and calcined catalysts (Washing, drying and calcination were carried out onCatalyst I used for 93.5h).

At the end of the 93.5 h TOS run, the catalyst wastaken out of the reactor and washed with n-pentane.Half of the used washed catalyst was dried at 60Cin air for 1 h and was labeled as ; the other half wascalcined at 450C for 3h in air in an oven and waslabeled as . Characterizations using XRD showedlittle difference among the fresh catalyst,and(Fig.16). Catalyst contained 0.99 wt.% carbon.

Fig. 17. Performance of catalyst I at a low pressure (160psig).

Catalyst I was operated at 160psig for 100h ata temperature of about 220C with no shutdowns,restarts or high temperature calcinations during therun (Fig. 17). No sign of deactivation was observedat 100 h TOS, indicating a possibility of operation ateven lower pressure. At this point (218C,160psigH2,H2/n-C16mole ratio= 2 and WHSV= 1 h1), the bestresults (highest i-C16 yield) were a 79.1 wt.% n-C16


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conversion, 89.9 wt.% i-C16selectivity and 71.1wt.%i-C16yield.

Future work will study the selective hydrocrackingand hydroisomerization of high molecular weight ma-terials, largely paraffinic, such as FT waxes, naturaloils and polyolefins using Pt promoted tungstated zir-conia and similar catalysts in a continuous unit.

4. Conclusions

Calcination of a tungstate-modified zirconium hy-droxide at 700C followed by impregnation withH2PtCl66H2O solution and another calcinationat 500C results in a highly active and selectiveplatinum-promoted tungstate-modified zirconia cat-alyst (Pt/WO3/ZrO2) for the hydroisomerization ofn-hexadecane. The optimum range of tungsten load-ing to achieve high isomerization selectivity at highn-hexadecane conversion is between 6.5 and 8 wt.%.

Reaction conditions can be adjusted to obtain maxi-mum i-C16yields or desirable branched isomers. Thiscatalyst is robust under proper operations, sustainingchanges of reaction conditions, shutdown and restartand high temperature calcination. It retains a stableactivity and selectivity for n-C16 hydroisomerizationwhen operated at a temperature of about 220C anda hydrogen pressure as low as 160psig. A catalystprepared from an MEL tungstated zirconia has hy-droisomerization activity similar to the best catalystprepared in our laboratories.

Acknowledgements

We thank the U.S. Department of Energy forthe financial support of this work (Grant No.DEFC2293PC93053). We are also grateful toProf. J. R. Blachere of the Materials Science andEngineering Department, University of Pittsburgh forhis help on XRD and SEM analyses and discussions,to Magnesium Elektron, Ltd. for providing catalystsamples and to Magnesium Elektron, Inc. for carbonanalyses of a used catalyst.

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hydroisomerization of normal hexadecane with platinum catalysts

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