Indian Journal of ChemistryVol. 17A, March 1979, pp. 270-272

Hydroisomerization of Normal Paraffins .Over Synthetic ZeolitesSHAWKY M. HASSAN*

Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt

and,

SARA M. SOLIMANEgyptian Petroleum Research Institute, Cairo, Egypt

Received 4 May 1978; revised 24 .I uly 1978; accepted 19 August 1978

The hydro isomerization of n-pentane, rr-hexane and rr-heptane 'has been studied over thefaujasite and mordenite types of zeolite. The hydroisomerization activity of zeolites in the caseof n-pentane increases to a limiting value with the increase in 8i02: A1203 mole ratio from 2·5 to10·57. Ca2+ ions increase the hydro isomerization activity of m.ordenite, while Li+ and K+ ionsdecrease it. The optimum concentration of Pt in HM type of zeolite for the hydro isomerizationis 0·5 wt %. The hydroisomerization and hydrocracking of n-paraffins are parallel reactionsand proceed on different active sites. The nature of hydro cracking products indicates carbo-nium ion mechanism.

ISOMERIZATION of n-paraffins has been in-vestigated over different zeolites>". Unliketypical bifunctional c~t<;tly?ts, hydrogen .as

well as cation-exchanged faujasite and mordemteare active for isomeriza:tion of n-paraffins7-1o.Addition of Pt or Pd greatly increases n-hexaneisomerization activity of HYll, while addition ofPt, Pd or Ni to HM does not increase its n-pentaneor n-hexane isomerization activity12-H. Additionof Ni to cation-exchanged mordenite increases itsn-pentane isomerization activity". However littleis mention ed in the literature concerning the effectof metal concentration in zeolites on their hydro--isomerization activityt-.

The present paper reports ~he results o~ studieson the quantity of Pt required for optimum. n-pentane isomerization activity. Other factors l~kethe Si02: Al20a ratio, type of exchanged c~tlonand paraffin chain length have been also studied.

Materials and MethodsCatalyst preparation - The Na-form zeolites used

were NaX and NaY, which are Linde MolecularSieve samples SK-20 and SK-40 respectively. NaMshowed the following chemical an alysis (wt %):Si02, 69·82; AI20a, 11·23; Na20, 6·36; Si02jAI~Oa,10·57 (mole); Na20jAI20a, 0·93 (mole). Cation-exchanged mordenites were prepared by treatment

.of the sodium mordenite with neutral aqueoussolution of the corresponding metal chloride (IN)in a dynamic system. The percent. ~xchange. ofsodium was determined by flame emISSIOn StudIES.

Ammonium mordenite was calcined in a streamof dry air at 4000 for 4 hr to obtain the hydrogenform (HM).

The PtjHM catalysts were p~epared by imp:e9'-nation of HM with aquecus solution of chloroplatinicacid.

All catalyst tests were performed under atmo-spheric pressure using 10 ml of catalyst m a

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fixed bed reactor of con ventional design as describedelsewhere-e. The paraffin was charged at 3000

using a micro-dosing pump. Space velocity of4 ml n-paraffin (ml catalystj! hr'! and hydrogenflow rate of 3·5 litre hr '! were used. .

Results and Discussion

Silica-to-alumina ratio - The' effect of Si02 :AI20amole ratio of zeolite on the isomerization ofn-penhne was studied using NaX, NaY and NaMtype of zeolite catalysts. The n-pentane conversioninto isopentane increases to a limiting value withthe increase of Si02:AlzOa ratio from 2·5 to 10·57.Similar results were reported for the zeolites A,X and Y in the oligomerization reaction ofpropylene=- 16. Also the catalytic activities forisomerization of o-xylene-? and cracking ofcumene-s were found to increase with the increase inSi02: Al20a ratio in the faujasite type of zeolites.

The increase in the isomerization activitv ofzeolites with the increase in Si02: Al203 ratio' maybe related to the silicon-aluminium ordering changesin the ze.ol.ite framework-s and consequent change?f the aCId~ty20. Eberly et al.21 found that acidityIS the dominant factor for the hydroisomerizationof n-pentane over PdjHM catalysts.

Type of cation ._- Although all exchanged mor-denit es were prepared under similar conditionsthe extent of ion-exchange is not the same inall the. cases (Table 1). Among all exchangedmordemtes HM possesses the highest isomerizationactivity. Although conversion was lower for theother cation .e::changed mor~enites the followingorder of activity was obtained: Ni>Ca>Mn =Na>K>Li. Both Ni2+ and Ca2+ increase thehydroisomerization activity of the parent N aMwhile the monovalent K+ and Li+ decrease it.

Platinum concentration - Three catalysts contain-ing 0'3, 0·5. and 0·7 wt % Pt were prepared andcompared with the parent HM catalyst with regard

HASSAN & SOLIMAN: HYDROISOMERIZATION OF PARAFFINS

TABLE 1 - ION EXCHANGE AND ISOMERIZATION RESULTSIN THE CASE OF n-PENTANE

Catalyst Ion Conversionexchange to

% isoperitane%

NaM 3'2EM 97'99* 5·6LiM 51·68 0·4KM 89·38 2'2CaM 54·31 4·2NiM 18'53 4·4MnM 41·38 3'2

*% exchange to ammonium ions.

to the hydroisomerization activity of n-pentane.The products of n-pentane conversion over all thesecatalysts contain Cl-C4 hydrocarbons in addition toisopentane. This indicates that the hydroisomeriza-tion of n-pentane is accompanied by a hydrocrackingside reaction. As the platinum concentration in-creases to 0·5 wt ,%, the hydroisomerization as wellas the hydro cracking activity of the catalyst increase(Fig. 1). Further increase in the platinum con-centration (to 0·7 wt %) decreases only the hydro-isomerization activity, while hydrocracking continuesto increase continuously. Consequently, at thisplatinum concentration, the catalyst undergoes somedeactivation process decreasing the number of onlyhydroisomerization active sites. This behaviour canbe explained on the basis of the active ensemblesand multiplet theories of catalysis=.

On the basis of the data of the present workit can' be concluded that the platinized hydrogen

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Fig. 1 - Variation in yields of iso-pentane and c,.-C4hydrocarbons with Pt content in Pt/HM catalyst

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mordenite catalysts of platinum content up to0·5 wt % have a relatively large number of catalyticactive sites which are more stericallv conformableto the isomerization of n-pcntane. F~rther increasein the platinum content on the hydrogen mordenitemay lead to. a decrease in the number of theseactive sites but may lead to conditions conduciveto the occurrence of the side reaction such ascracking.

Chain length of n-paraffins -- n-Pentane, n-hexaneand n-heptane have been used within the tem-perature range 150-3750 and space velocity of 4 rnln-paraffin (ml catalyst):" hr=! using the 0·5% PtjHMcatalyst.

As can be seen from Fig. 2, the total conversionfor all the n-paraffins increases continuously withincrease in the reaction temperature. Below 2500

O~~~~~~~~ __ ~ __ ~-J150 200 250 300 350 400

Temperature; C

Fig. 2 - Effect of temperature on the conversion ofn-paraffins

0.~--~--~---L--77~~ ~100 150 200 250 300 350 400Temperature,Oc

Fig. 3 - Effect of chain length on the yields of iso-paraffinsat different temperatures

;;-0+-~,20c.2'"..••>cou 10

0..•..°12+-~"'·10c:f(! BaIo'" 6+-o-0

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INDIAN J. CHEM., VOL. 17A, MARCH 1979

-s°+-~ 20c0

+=u~.•.. ,•..'"~!?

10-.-0C5 I"0

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Fig. 4 - Plot of hydro cracking (%) versus temperature

the conversion of n-pentane is the highest butabove 2500 n-hexane" and n-heptane are convertedin larger amounts than n-pentane.

Analysis of the conversion products of n-paraffi'1sindicates also that two reactions, hydroisomerization(Fig. 3) and hydrocracking (Fig. 4) take placeover the PtjHM catalyst. The hydroisomerizationis the predominant reaction in case of n-pentane,while the hydrocracking is the main reaction in thecase of n-hexane "and n-heptane.

The hydroisomerization ability of n-paraffins de-creases "with the increase in chain length (Fig. 3),while their hydrocracking ability increases with theincrease in chain length (Fig. 4).

Comparison between the curves of Fig. 3 andFig. 4 indicates that on increasing the reactiontemperature the hydroisomerization activity of thecatalyst reaches a maximum value thereafter it~ecreas.es, while the hydrocracking activity goes onincreasmg.

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It is worth noting that the products of hydro-cracking of presently studied n-paraffins containvery low yields of C1 and C2 products as comparedto C3 and (4 products. They also contain highyields of isobutane indicating carbonium ion mecha-nism of the cracking reaction.

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