Hydroisomerization of n-butane over sulfated zirconia catalysts promoted by alumina and platinum

Applied Catalysis A: General 227 (2002) 279–286

Hydroisomerization ofn-butane over sulfated zirconiacatalysts promoted by alumina and platinum

Weiming Hua1, Jean Sommer∗Laboratoire de Physico-Chimie des Hydrocarbures, UMR 7513, Institut de Chimie, Université Louis Pasteur,

4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France

Received 13 August 2001; received in revised form 30 October 2001; accepted 30 October 2001

Abstract

The isomerization ofn-butane over Pt-promoted/Al2O3-promoted sulfated zirconia (Pt/SZA) catalyst in comparison withSZA was studied in the presence of H2 at 250◦C. Catalysts prepared using 1N H2SO4 are slightly more active than thoseprepared using 2N H2SO4. The optimum calcination temperature for Pt/SZA was found to be 650◦C, same as for SZA catalyst.The presence of platinum improves the catalytic stability and isomerization selectivity of SZA catalyst, particularly at lowH2/n-butane ratio. Comparing the maximum activity displayed by both catalysts, Pt/SZA is more active than SZA forn-butaneisomerization, especially at the steady state. Pre-treating the Pt/SZA catalyst in H2 at 350◦C before the reaction results in asignificant loss in activity. The mechanical mixture of Pt/Al2O3 and SZA exhibits much higher activity than Pt/SZA catalyst,probably due to a change of reaction mechanism from monofunctional over the latter catalyst to bifunctional over the former.The initiation step ofn-butane isomerization over SZA catalyst is discussed. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Butane activation; Pt-promoted SZA; Strong acids; Initiation step; Protolysis; H/D exchange

1. Introduction

Skeletal isomerization ofn-butane is of significantimportance in the petroleum refining industry dueto growing environmental constraints. The reactionproduct (isobutane) is a valuable precursor for theproduction of alkylated gasoline and oxygenates usedas octane number boosters, such as methyltert-butylether (MTBE) and ethyltert-butyl ether (ETBE). In-dustrial processes for the isomerization ofn-butanehave been carried out using Pt/chlorinated-Al2O3 cat-

∗ Corresponding author. Tel.:+33-3-90-24-14-86;fax: +33-3-90-24-14-87.E-mail address: [email protected] (J. Sommer).

1 On leave from Department of Chemistry, Fudan University,Shanghai 200433, PR China.

alyst in the 150–300◦C range, which require the con-tinuous addition of toxic and corrosive Cl2 additivesto restore chloride species that leach slowly during thereaction.

In 1962, Holm and Bailey [1] first reported thestrong acidity and catalytic properties of a zirconiagel modified by sulfate groups and of Pt crystallites.But, this type of material did not attract much at-tention until about 20 years later, when Arata andco-workers [2] reported that sulfated zirconia (SZ)was active forn-butane isomerization at low tem-perature. Since then, numerous investigations on thismaterial have been published. Several regularly pub-lished reviews reflect researchers’ great interest onit [3–9]. Recently, industrial application of SZ forisomerization of C5–C6 alkanes has been reported[10].

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(01)00945-0

280 W. Hua, J. Sommer / Applied Catalysis A: General 227 (2002) 279–286

Despite of the capability of SZ for catalyzingn-butane isomerization at low temperature, fast deac-tivation was always observed, primarily because ofcoke deposition [7,11–15]. To improve the life timeof SZ catalyst, the presence of hydrogen and/or ad-dition of a small amount of Pt was suggested [16].Hsu and co-workers [17,18] first discovered that SZdoped with 1.5 wt.% Fe and 0.5 wt.% Mn (SFMZ)was two to three orders of magnitude more activein n-butane isomerization at low temperature thanunpromoted SZ. Coelho et al. [19] reported a compa-rable enhancement upon the addition of Ni to SZ. Gaoand co-workers [20,21] found that sulfated oxidesof Cr–Zr, Fe–Cr–Zr and Fe–V–Zr were two to threetimes more active than SFMZ forn-butane isomeriza-tion. Unfortunately, these transition metals-promotedSZ catalysts deactivated rapidly during the reaction[22–26]. The marked promoting effect of these transi-tion metals disappeared if the reaction was performedat high temperature (e.g. 250◦C) in the presence ofH2 [26–28].

Recently, Gao et al. [27] have first reported thatincorporating small amounts of Al2O3 into SZsystem (SZA) enhances substantially the catalyticactivity and stability, if n-butane isomerization isperformed at 250◦C in the presence of H2. Theoptimum Al2O3 content is 3 mol%. These remark-able properties of SZA catalysts are most likelydue to a different distribution of acid sites strengthand to an enhanced number of acid sites with in-termediate strength, which is very different fromthe promoting mechanism of transition metals,such as Fe, Mn, and Ni [27,28]. The positive ef-fect of the main group element Al on SZ catalystfor n-butane isomerization was later confirmed byPinna and co-workers [29]. Similar phenomena werealso observed for other acid-catalyzed reactions,such as benzoylation of toluene with benzoyl chlo-ride [30] and alkylation of isobutane with 2-butene[31]. More recently, the influence of the additionof small amounts of Al2O3 on the structure andmicrostructure of ZrO2 phases has been reported[32].

In this work, we report our investigation ofn-butaneisomerization over Pt-supported SZA (Pt/SZA) cata-lyst in comparison with SZA at 250◦C in the presenceof H2. The initiation step of butane isomerization onSZA catalyst is discussed.

2. Experimental

2.1. Preparation of samples

SZA catalyst was prepared referring to the proce-dures in the literature [27]. Appropriate amounts ofZrOCl2·8H2O and Al(NO3)3·9H2O were dissolved indistilled water to make mixed solution. An aqueoussolution of ammonia was added dropwise under vigor-ous stirring to this mixed solution until the final pH=9–10. The precipitate was then filtered and washedwith distilled water. After drying the mixed hydroxideat 110◦C for 24 h, it was immersed in a 1 or 2Ndilutesulfuric acid at a ratio of 15 ml g−1 of hydroxide for 1 hwith continuous stirring at room temperature. SulfatedAl(OH)3–Zr(OH)4 was again filtered without washing,dried and calcined at 650◦C in dry air (30 ml min−1)for 3 h.

Pt/SZA catalyst (0.5 wt.% Pt) was prepared in themost typical way. Sulfated Al(OH)3–Zr(OH)4 wasimpregnated with an aqueous solution of H2PtCl6.After drying at 110◦C for 24 h, it was calcined atdifferent temperatures in dry air (30 ml min−1) for3 h. Pt/Al2O3 catalyst (0.5 wt.%) was prepared byimpregnating�-Al2O3 with an aqueous solution ofH2PtCl6, followed by calcination in dry air at 450◦Cfor 3 h and reduced in dry H2 at 500◦C for 3 h.The mechanical mixture of Pt/Al2O3 and SZA (1/1mass ratio) was obtained by mixing Pt/Al2O3 andSO4

2−/Al(OH)3–Zr(OH)4 physically, followed bycalcination at 650◦C in dry air (30 ml min−1) for 3 h.

2.2. Surface area and sulfur content

BET surface areas of the samples were measuredon a Micromeritics ASAP 2000 system. Titration ofsulfur in the catalysts is based on combustion and IRdetection of SO2 on a LECO SC/44.

2.3. Deuteration of the catalyst

Deuteration of the catalyst was performed in anall-glass grease-free flow system described earlier[33]. The catalyst was first activated in dry air at450◦C for 2 h to eliminate hydrocarbon contamina-tion. Then it was pre-treated in dry N2 at the sametemperature for an additional 1 h. The temperature

W. Hua, J. Sommer / Applied Catalysis A: General 227 (2002) 279–286 281

was lowered to 200◦C for deuteration. The catalystdeuteration was performed by sweeping D2O with N2(40 ml min−1, ca. 3 mol% D2O in N2) for 1.5 h. Ex-cess D2O was then removed by flushing the catalystat 450◦C with dry N2 for 1 h.

2.4. Brønsted acid sites

The measurement of Brønsted acid sites in the cata-lyst was described in detail elsewhere [34]. Briefly, 1 gof above deuterated catalyst was contacted at 200◦Cfor 1.5 h with 40 ml min−1 N2 (ca. 3 mol% H2O inN2) in the same glass system. Excess water was thenremoved by flushing the catalyst at 450◦C for 1 hwith dry N2. During the H/D exchange and flush-ing, the partially exchanged water (HxODy) was col-lected in a cold trap. An excess of trifluoroacetic an-hydride was then added to transform trapped HxODy

into CF3COOH and CF3COOD. The acid solution,thus obtained, was analyzed by 400 MHz1H- and2H-NMR, after addition of a CDCl3/CHCl3 mixtureused as reference. The Brønsted acid sites present onthe catalyst were calculated based on the H/D ratio de-termined by NMR and the weight of HxODy trapped.

2.5. Activity test

n-Butane isomerization was carried out at 250◦C ina flow-type fixed-bed reactor under ambient pressure.A gas mixture ofn-butane and H2 (different molarratios) was passed over the catalyst.n-Butane weighthour space velocity (WHSV) was 1.5 h−1. In the caseof mechanical mixture, this refers to SZA. Prior to thereaction, the catalyst was pre-treated in situ at 450◦Cin dry air for 3 h, unless otherwise stated. The mechan-ical mixture of Pt/Al2O3 and SZA as well as Pt/Al2O3catalyst were activated in situ at 450◦C in dry air for3 h, followed by reduction in dry H2 at 300◦C for1 h. Hydrocarbon products were analyzed on a Girdel300 with FID detector using a 2 m packed columnHAYESED R and the oven temperature was kept at130◦C.

2.6. Hydrogen detection

Hydrogen was analyzed with an Intersmat IGC112M chromatograph equipped with a 2 m column

filled with a 5A molecular sieve and a thermal con-ductivity detector. The rate of carrier gas (Ar) was14 ml min−1 and the oven temperature was constantat 50◦C.

3. Results and discussion

3.1. Catalyst characterization

The physicochemical properties of both Pt/SZA andSZA catalysts are listed in Table 1. It can be seen thatthe presence of Pt does not change the specific surfacearea, sulfur content and Brønsted acid sites.

It is noteworthy that these data presented here forSZA sample are somewhat smaller than our previouslyreported ones which were 134.4 m2 g−1, 1.47 wt.%and 0.121 mmol g−1, respectively [35,36]. This can beassigned to the different calcination conditions em-ployed for sulfated hydroxide. In this study, sulfatedAl(OH)3–Zr(OH)4 was calcined at 650◦C in flowingdry air (30 ml min−1) for 3 h, whereas, in our previouswork the sample was calcined in static air at the sametemperature for 3 h.

3.2. Effects of sulfuric acid concentration andcalcination temperature

The typical time course ofn-butane isomerizationover Pt/SZA and SZA catalysts at 250◦C in the pres-ence of H2 is shown in Fig. 1. A rapid deactivationtook place during the initial 1 h, followed by a slow de-cline in activity. After then both catalysts tend to reacha quasi-steady state. The main reaction product isisobutane. A cracking process (formation of methane,ethane and propane) as well as disproportionation (for-mation of propane and pentanes) were also observed

Table 1Physicochemical properties of both Pt/SZA and SZA catalystsa

Catalyst Surface area(m2 g−1)

Sulfur content(wt.%)

Brønsted acidsites (mmol g−1)b

Pt/SZAc 113 1.35 0.104SZA 117 1.32 0.108

a Immersed in 1N H2SO4.b Measured following [34].c Calcined at 650◦C in dry air (30 ml min−1) for 3 h.


Fig. 1. Activity of Pt/SZA and SZA catalysts forn-butaneisomerization as a function of time at 250◦C. (�) Pt/SZA,H2/n-butane= 1/1; (�) SZA, H2/n-butane= 3/1. Preparativeconditions: immersed in 1N H2SO4; calcined at 650◦C.

during the catalytic reaction. At the steady state, theselectivity to isobutane for all catalysts exceeded 90%.

It is well known that the acidic and catalytic prop-erties of SZ catalyst depend strongly upon its prepar-ative conditions. One of the factors that affect thecatalyst performance is the concentration of dilute sul-furic acid used to immerse amorphous zirconium hy-droxide (i.e. sulfation process). In the literature, someauthors utilized 2N H2SO4 [37,38], while the mostoften employed concentration was 1N [2,7,16,39–42].Table 2 presents the initial (10 min on stream) andsteady state activities of Pt/SZA and SZA catalystsprepared using a 1 or 2Nsulfuric acid forn-butaneisomerization at 250◦C. Catalysts prepared using 1NH2SO4 are slightly more active than 2N for boththe initial and steady state activities. On the basis

Table 2Initial and steady state activities of Pt/SZA and SZA catalystsprepared using a 1 or 2Nsulfuric acid forn-butane isomerizationat 250◦Ca

Catalyst H2SO4 (N) Conversion (%)

Initialb Steady state

Pt/SZA 1 34.0 31.1Pt/SZA 2 30.5 29.8SZA 1 40.9 30.0SZA 2 36.6 27.1

a Catalysts calcined at 650◦C. Reaction condition:H2/n-butane= 3/1.

b 10 min on stream.

of this result, we have decided to choose 1N H2SO4to sulfate Al(OH)3–Zr(OH)4 mixed hydroxide in thefollowing experiments.

To generate strong acidity, sulfated zirconium hy-droxide must be calcined in air at high temperaturesranging from 500 to 800◦C. Calcination tempera-ture plays a crucial role in its acidity and catalyticproperties. To make SZ catalyst extremely acidic andhighly active for smalln-alkanes isomerization atlow reaction temperature, the calcination tempera-ture generally employed was 650◦C [23,32,43–47].Nevertheless, to avoid sintering of Pt particles andmeanwhile to render the catalyst strongly acidic, atemperature of 600◦C was usually used to calcinatePt/SZ catalyst [43,48–52].

Fig. 2 depicts the variation of steady staten-butaneisomerization activity of Pt/SZA catalyst with calci-nation temperature. Pt/SZA catalyst is very inactive at550◦C and only 1.8% conversion was observed. Rais-ing the calcination temperature from 550 to 650◦Cresults in a large increase in catalytic activity. Increas-ing further the temperature leads to a small decreasein activity. The maximum value was found at 650◦C,which is the same as that for SZA catalyst, as recentlyreported by Pinna and co-workers [29]. This revealsthat the presence of Pt does not change the optimumcalcination temperature of SZA catalyst forn-butaneisomerization. We, therefore, infer that 600◦C maynot be the optimum calcination condition for Pt/SZcatalyst, at least for the isomerization ofn-butane. Inthe following experiments, we selected 650◦C to cal-cinate Pt/SZA catalyst.

Fig. 2. Steady state activity of Pt/SZA catalyst calcined at differ-ent temperatures forn-butane isomerization at 250◦C. Reactioncondition: H2/n-butane= 3/1.


Table 3Activity of Pt/SZA and SZA catalysts forn-butane isomerizationat 250◦C

Entry Catalyst H2/n-butane Conversion (%)

Initiala Steady state

1 Pt/SZA 1/1 45.9 38.82 Pt/SZA 3/1 34.0 31.13 Pt/SZA 10/1 23.0 22.04 SZA 1/1 36.4 18.35 SZA 3/1 40.9 30.06 SZA 10/1 27.7 22.67 Pt/SZAb 3/1 32.5 29.88 Pt/SZAc 3/1 25.4 21.89 Pt/Al2O3 + SZAd 3/1 45.4 41.3

a 10 min on stream.b Catalyst activated in dry air at 450◦C for 3 h and followed

in dry H2 at 300◦C for 1 h.c Catalyst activated in dry air at 450◦C for 3 h and followed

in dry H2 at 350◦C for 1 h.d Mechanical mixture and catalyst activated in dry air at 450◦C

for 3 h and followed in dry H2 at 300◦C for 1 h.

3.3. Comparison between Pt/SZA and SZA catalystsfor n-butane isomerization

Table 3 reports the activity of both Pt/SZA and SZAcatalysts forn-butane isomerization at 250◦C underdifferent H2/n-butane ratios, keeping the space ve-locity of n-butane constant. For Pt/SZA catalyst (en-tries 1–3), the activity diminishes with an increase inH2/n-butane ratio. The same trend was observed forSZA catalyst at higher H2/n-butane ratios (from 3 to10). The lower conversion at H2/n-butane ratio of 1is caused by the faster deactivation of SZA catalystprior to the initial activity measurement. In effect, ahigher loss in activity after 10 min on stream was alsoobserved in this case. Comparison between the initialand steady state activities shows that for SZA cata-lyst the conversion is decreased by one half at thislower H2/n-butane ratio. The decline in activity withan increment of H2/n-butane ratio can be interpretedby the inhibiting effect of H2, as reported by severalgroups forn-butane isomerization over SZ-based cat-alysts [38,53–56], on one hand and is related with thedecrease in the partial pressure ofn-butane, on theother hand.

At higher H2/n-butane ratios (i.e. 3 and 10), the ini-tial activity of Pt/SZA catalyst is clearly lower thanthat of SZA, suggesting that Pt/SZA catalyst displays

lower intrinsic activity forn-butane isomerization thanSZA under the same reaction conditions, probably be-cause of the mutual influence between Pt particles andproton centers. A similar observation was reported byLarsen et al. [57] forn-butane isomerization over un-promoted and Pd-promoted SZ catalysts in the pres-ence of H2. The steady state activity for both catalystsis similar. Compared to Pt/SZA, SZA catalyst shows,nonetheless, much lower activity at H2/n-butane ratioof 1. This can be ascribed to the faster deactivationof SZA catalyst within the first 10 min of reaction un-der lower H2/n-butane ratio. If we compare the max-imum activity displayed by both catalysts, Pt/SZA isclearly more active than SZA, especially at the steadystate.

Comparison between the initial and steady stateactivities shows that for SZA catalyst the conver-sion is decreased by 50, 27 and 18%, respectively, atH2/n-butane ratio of 1, 3 and 10, whereas, for Pt/SZAcatalyst it is 15, 9 and 4%, respectively. These dataindicate that the presence of Pt improves the catalyticstability of SZA sample whenn-butane isomeriza-tion was performed in the presence of H2 at hightemperature, primarily because Pt hydrogenates cokeprecursors, thus, keeping the catalytic activity. Atcomparable activity, the selectivity to isobutane forPt/SZA catalyst is higher than SZA. For example, ataround 30% conversion the isobutane selectivity isabout 94% for the former catalyst, while about 90%for the latter. This could be understood as follows.As earlier proposed by Iglesia et al. [48], H2 can actas a hydride source in the presence of Pt, possiblyby dissociation to H+ and H− species catalyzed byplatinum, thus, decreasing the residence time and con-centration of carbenium ion intermediates via hydridetransfer, which causes the desorption of isomerizedcarbocations before cracking. Another reason could beassociated with a change of reaction mechanism frompredominantly bimolecular over Pt-free SZ-basedcatalysts to predominantly monomolecular overPt/SZ-based catalysts, as recently reported by Suzukiand Okuhara [58] using double13C-labeledn-butane(13CH3–CH2–CH2–13CH3).

The chemical state of Pt on the surface of SZ isstill in controversy. Earlier studies by Ebitani et al.[59,60] showed that Pt on SZ was predominantly inthe cationic state after reduction in H2 at tempera-tures below 400◦C. Most authors [61–64] reported


that Pt was already in the metallic state after aircalcination of Pt/SZ catalyst at temperatures above600◦C, possibly because of the reduction of platinumspecies by SO2 evolved as result of decomposi-tion of surface sulfate. Hence, the reduction step ofPt/SZ-based catalysts at higher temperatures beforecatalytic reactions could be ignored. As it can beseen from Table 3 (entries 2,7 and 8), pre-treatmentof Pt/SZA catalyst in H2 at 300◦C brings about aslight decrease in activity, but a substantial declinewas observed after reduction at 350◦C. This is dueto the reduction of surface sulfate, which is vitalfor the occurrence of catalytic activity. The sulfurcontent of Pt/SZA catalyst decreases from 1.35 to1.25 wt.% after pre-treatment in H2 at 350◦C for1 h. A similar phenomenon was reported by Sig-noretto et al. [55] forn-butane hydroisomerizationover platinum-promoted SZ catalyst prepared by aone-step aerogel procedure. These data indicate thatpre-treatment in H2 at higher temperatures or abovethe reaction temperature would be detrimental toPt/SZ-based catalysts, thus, resulting in loss in activ-ity, at least this is the case for the isomerization ofn-butane.

Typical Pt/SZ sample behaves quite differentlyfrom the traditional Pt-supported bifunctional cat-alysts. Reduced Pt/SZ catalyst showed negligibleH2 and CO chemisorption at room temperature[48,59,65–67]. This kind of catalyst also displayedpoor or null properties for hydrocarbon hydrogenationand dehydrogenation [66,68,69]. The loss of metal-lic character appears to be due to sulfur poisoningor strong interaction with the support [65,67,70,71].These results clearly show that Pt supported onSZ-based solid acids does not function as a classicalbifunctional catalyst. Monofunctional or collapsed(compressed) bifunctional mechanism was proposedfor these catalysts [65,72,73]. Compared to Pt/SZAcatalyst, mechanical mixture of Pt/Al2O3 and SZAexhibits much higher activity forn-butane isomer-ization (Table 3, entries 2 and 9). Under identicalreaction conditions, Pt/Al2O3 is almost inactive. Only0.23% conversion (<2% selectivity) was observedat the steady state. These data could be understoodby a change in mechanism from monofunctionaloperated for Pt/SZA catalyst to bifunctional forthe mechanical mixture, as proposed by Yori et al.[65].

3.4. Initiation step

Concerning hydrocarbon conversions over acidiccatalysts, such as catalytic cracking, isomerizationand alkylation, it is generally agreed on the role ofcarbocations as reaction intermediates or transitionstates. Nevertheless, there is lack of information ontheir mode of formation. Isomerization of butane overSZ-based catalysts can be considered as a surfacechain reaction comprised of initiation, propagationand termination steps. A key question is how the reac-tion is initiated, viz., how carbenium ions are formedduring the initial step of reaction.

As shown in Table 4, traces of H2 was formed dur-ing the initial period of butane isomerization over SZAcatalyst. Moreover, the amount of hydrogen producedduring isobutane isomerization is more thann-butaneisomerization, which is in agreement with the resultreported by Hong et al. [74] for butane isomeriza-tion over SZ catalyst. These authors have suggestedthat hydrogen is formed via dehydrogenation of bu-tane over Zr–O sites. Quantum chemical calculationssupport this feasibility. Hence, the dehydrogenationof butane may be responsible for the initiation pro-cess during butane isomerization over SZ. It is wellknown that butene is easily protonated by the acidsites to give carbenium ions. In fact, earlier studiesshowed that pure ZrO2 can catalyze the hydrogena-tion of olefins [75–77]. Our result clearly shows thatH2 can be produced over sulfate-free Al2O3–ZrO2via dehydrogenation of butane, but not in the initialperiod. Therefore, we suggest that the initiation stepfor butane isomerization over SZA and SZ catalystsis protolysis of C–H bond. Butane is protonated to

Table 4Hydrogen produced during the initial period (10 min on stream)of butane isomerization over SZA catalyst at 250◦Ca

Reactant Hydrogen (�mol g−1)

SZA Al2O3–ZrO2

n-Butane 0.249 0 (0.104)b

Isobutane 0.720 0 (0.272)b

a Re-circulation reactor (50 ml). Reaction conditions: 1.5 g cat-alyst; 15 mln-butane or isobutane; 30 ml min−1 re-circulation rate;catalyst activated in situ at 450◦C in dry air for 3 h.

b The number in the parenthesis is the amount of H2 formedafter continuous re-circulation for 30 min.


the penta-coordinate carbonium ion type transitionstate which is easily converted into the more reactivetrivalent carbenium ion with evolution of hydrogen.Since, SZ-based catalysts possess both strong acidityand oxidation ability, oxidation of alkane followedby electron transfer resulting in the formation ofcarbenium ions may be possible for the initiation ofcatalytic reaction, as earlier proposed by Ghenciu andFãrcasiu [78] and Fãrcasiu et al. [79]. More recently,using EPR and IR spectroscopies Knözinger andco-workers [80] have also proposed redox initiationfor n-pentane isomerization over tungstated-zirconiacatalyst.

4. Conclusions

In the present study, we have shown that the ac-tivity of both Pt/SZA and SZA catalysts depends ontheir preparative conditions. Use of 1N H2SO4 to im-merse hydroxide is somewhat preferable to 2N H2SO4.The presence of platinum improves the catalytic sta-bility and isomerization selectivity of SZA catalystfor n-butane isomerization in the presence of H2 at250◦C, particularly at low H2/n-butane ratio, but doesnot change the optimum calcination temperature of thecatalyst. Pt/SZA catalyst shows the higher maximumactivity than SZA, especially for the steady state ac-tivity. Pre-treating the Pt/SZA catalyst in H2 at highertemperatures will deteriorate the catalyst, leading to aloss in activity.

Mechanically mixing Pt/Al2O3 and SZA increasesthe catalytic activity substantially, possibly becausethe monofunctional mechanism operated over Pt/SZAcatalyst, while bifunctional over the mechanical mix-ture. Based on the initial formation of hydrogenover SZA catalyst during butane isomerization, wesuggest that the initiation step is protolysis of C–Hbond. However, oxidation of alkane followed byelectron transfer resulting in the formation of car-benium ions could not be excluded at the presentstage.

Acknowledgements

We kindly acknowledge the financial support fromthe Loker Hydrocarbon Institute, Los Angeles.

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Hydroisomerization of n-butane over sulfated zirconia catalysts promoted by alumina and platinum