H.K. Mishra a, A.K. Dalai a,∗, K.M. Parida b, S.K. Bej a
a Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering,University of Saskatchewan, Saskatoon, Sask., Canada S7N 5C9
b Inorganic Chemicals Division, Regional Research Laboratory (CSIR), Bhubaneswar 751013, Orissa, India
Received 21 December 2000; received in revised form 3 April 2001; accepted 6 April 2001
Abstract
A study on n-butane isomerization reaction over persulfated zirconia was performed under atmospheric pressure varyingpretreatment gas, butane diluents, concentration of butane, contact time and reaction temperatures. It was found that pretreat-ment of the catalyst in various atmospheres such as nitrogen, hydrogen and air altered the properties of the catalyst as wellas its performance towards n-butane isomerization. Air, as a pretreatment gas enhanced the initial activity of the catalyst;whereas, hydrogen had a negative effect. Among the pretreatment gases used, air and hydrogen were the most and the leastdeactivating carrier gases, respectively. Increase in conversion with increasing butane concentration, the reaction productdistribution, and the induction phenomenon together strongly supported the existence of a surface stabilized C8
Sulfated zirconia (SZ) catalyst was extensivelystudied because of its high acidity and ability to cat-alyze isomerization of linear alkanes [1–6]. Manyresearchers have worked on its preparation, character-ization of physico-chemical properties and catalyticactivities for various reactions [7,8]. Despite all theseintensive investigations there is no universally ac-cepted theory as to what type of sites account forthe unique isomerization properties of this material.There were many hypotheses proposed on the activesites of SZ. For example, it was initially consideredsuper-acidic. However, based on the work of Hall and
coworkers [9,10], the acidity was found comparableto that of 100% H2SO4. It is now almost accepted thatSZ catalyst is not super acidic. Attentions also focusedon the redox properties [11,12]; however, not muchwork has been done in this direction. Iron–manganesepromoted SZ were three-fold more active than SZand were able to catalyze butane isomerization atroom temperature. The enhanced activity of promotedSZ was attributed to iron-oxy species [13]. Recently,Hong et al. [14] from their density functional studysuggested that alkane isomerization over SZ possiblyoccurred through participation of Zr–O bond.
Adeeva et al. [15] using IR spectroscopic techniqueon adsorbed pyridine, CO and NH3, and temperatureprogram desorption study of adsorbed NH3 showedSZ, and Fe–Mn promoted SZ were of comparableacidity. In addition, Tabora and Davis [16], and Wanet al. [17] reported that both SZ and iron manganese
264 H.K. Mishra et al. / Applied Catalysis A: General 217 (2001) 263–273
promoted SZ possessed similar acid strength and acidsite densities. However, butane isomerization activitiesof promoted and unpromoted SZ were quite differentwhile using hydrogen as a butane carrier [18]. Isomer-ization activity of SZ and related catalysts were alsoexplained on the basis of physico-chemical propertiesgiving much emphasis, particularly, to super-acidity[2]. Adeeva et al. [15] for the first time proposedthat super acidity might not be a necessary conditionfor alkane isomerization. Rather, a surface stabilizedspecies could well be realized to account for the mech-anism of butane isomerization.
Earlier research on SZ concentrated on its prepa-ration and evaluation of physico-chemical propertiesthat were correlated to the isomerization activity.However, no systematic work on n-butane isomer-ization over SZ has been done to explore the effectof different gases such as air, nitrogen and hydrogenunder various pretreatment and reaction conditions.In the present work, the focus was to study n-butaneisomerization using a persulfate-modified zirconiaunder various pretreatment and reaction conditions.The results obtained suggest the possibility of a redoxcouple and a surface stabilized species that causes theisomerization reaction to occur.
2. Experimental
2.1. Material preparation
The zirconium hydroxide, used for preparationof SZ was synthesized by adding aqueous solutionof zirconium oxychloride to a solution (at pH 10)containing sodium dodecyl sulfate, ethanol, acetylacetone and ammonium hydroxide. This mixture wasautoclaved at 110◦C for 6 h. The hydroxide thusproduced was washed with water and followed byethanol, and finally dried at 110◦C in an oven understatic air condition. Details of the hydroxide prepa-ration are reported elsewhere [19]. The dried hy-droxide was treated with ammonium peroxy-disulfateby wetness impregnation technique to produce sul-fated zirconium hydroxide, which was calcined at600◦C for 3 h in an electric furnace under static aircondition to obtained SZ. The sample thus obtainedwas kept in a glass vial, and used throughout thisstudy.
Table 1Physico-chemical properties of sulfated zirconia calcined at 600◦C.
XRD phasea Tetragonal + less monoclinic
BET surface area (m2/g) 162Pore radius (Å) 48.0Pore volume (ml/g) 0.39Lewis zirconiab 9.0Brønsted zirconiab 10.9Sulfur content (wt.%) 1.45
a Ratio of tetragonal to monoclinic phase intensity is approxi-mately 7.
b Measured from photoacoustic study of adsorbed pyridine.
2.2. Physico-chemical characterizations
2.2.1. Elemental analysisSaskatchewan Research Council (SRC, Canada)
analyzed the calcined SZ for sulfur content by induc-tively coupled plasma method (ICP) and the valueis given in Table 1. The physico-chemical propertiessuch as surface area, pore volume, pore radius, acidsites and catalytic activities of the sample were in-vestigated using various instrumental techniques asdescribed below.
2.2.2. XRD, FTIR and surface acidity studiesPhase identification of calcined sample was done
in a powder X-ray diffractometer (XRD7, Rich. Seit-ertand Company; Freiberg, Germany). Bonding ofsulfate in SZ was studied by FTIR analysis of thecalcined sample, recorded in a Perkin-Elmer spec-trophotometer (Paragon 500). The nature of acidsites (Brønsted zirconia, BZ and Lewis zirconia, LZ)was determined by FTIR photoacaustic spectroscopicmeasurements using pyridine, adsorbed on calcinedSZ sample. The details of the acid site characteriza-tion procedure are reported elsewhere [20].
2.2.3. BET surface area, pore volume and pore sizedistribution studies
The BET surface area, pore volume and pore sizedistributions of the calcined sample were measuredfrom nitrogen adsorption–desorption studies using aBET surface area analyzer (ASAP 2000 from Mi-cromeritics) at liquid nitrogen temperature (77 K).Prior to analysis, the sample was degassed at 200◦Cand 10−4 Torr pressure.
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2.2.4. Ammonia temperature programmeddesorption (TPD) studies
Ammonia TPD of the calcined sample was car-ried out in a TPD analyzer (CHEM-BET 3000,Micromeritics). In a typical experiment, 0.1 g of thesample was taken and was degassed at 450◦C for1 h under nitrogen flow followed by cooling to icetemperature. About 1 mol% ammonia in nitrogen wasthen flowed through the sample for 1 h. The ammoniaadsorbed catalyst sample was slowly heated to 100◦Cand kept at that temperature till steady state wasattained. The sample was then heated from 100 to800◦C at a heating rate of 10◦C/min. A TCD analyzerdetected the ammonia desorbed during the process.
2.2.5. Temperature programmed reduction (TPR)studies
The TPR study of the calcined SZ was carried outin a TPR analyzer (CHEM-BET 3000, Micromerit-ics). About 0.1 g of sample was taken and degassedat 450◦C for 1 h under nitrogen flow and then cooleddown to 100◦C. It was heated under a reducing atmo-sphere (3 mol% hydrogen in nitrogen) at a flow rate of30 ml/min to 800◦C and a heating rate of 10◦C/min.The effluent gas was passed through Pb(II) solutionto detect evolution of oxides (SOx) and hydride (H2S)of sulfur.
The time-on-stream (TOS) n-butane isomerizationreaction was carried out over the catalyst in a fixedbed SS tubular reactor with an internal diameter of8 mm. About 0.6 g of the catalyst was used in each ex-periment. Two thermocouples, one inside the reactorand the other attached to outer surface of the reactor,were used to monitor the reaction temperatures. Thereaction was studied by varying different pretreatmentconditions, butane diluents, butane concentration,contact time and reaction temperatures. During thereaction, products were collected at 5, 15, 30, 60, 90,150 and 300 min of time-on-stream in air-tight gassamplers and were analyzed in GC (Carle, GC Series500), fitted with a stabil-wax capillary column hav-ing 0.2 mm internal diameter and a length of 30 m,and operated in FID mode. An HP GC (Series 5890)equipped with a carbosieve column was used to an-alyze hydrogen and oxides of carbon in TCD mode
of operation. The n-butane used in this study was amixture of 20 mol% butane and 80 mol% nitrogen.The feed (20 mol% n-butane in nitrogen and carriergas) and reaction products were analyzed for butene.However, it was not detected. This implies that olefinlevel in the feed, if at all present, could be very lowand therefore, below the detection limit of GC. Thegases used were, unless otherwise stated, 99.5% pureand supplied by Praxair Canada.
3. Results
3.1. Physico-chemical properties
The results on XRD, surface area, pore volume, poreradius and the acid sites analyses of the calcined sam-ple are presented in Table 1. The XRD pattern of thesample (not presented) shows that the sample containsboth tetragonal and monoclinic phases with tetragonalas the predominating phase. The surface acidity datashows the presence of both Lewis and Brønsted acidsites. From porosity and surface area analyses, it isfound that the sample preserved its mesoporosity andhigh surface area even after calcination at 600◦C.
The qualitative investigation of acidity of SZ cata-lyst using ammonia TPD, presented in Fig. 1, showsa broad spectrum ranging from 200 to 650◦C. Twodistinct hoods at around 200 and 450◦C are alsoobserved.
In order to understand the nature of reducible sulfurspecies in the SZ catalyst hydrogen TPR experimentwas done, which is shown in Fig. 2. The patternshows a strong peak at around 600◦C correspondingto hydrogen consumption. No peak is seen below thistemperature.
Fig. 1. Profile for temperature program desorption of ammoniafrom calcined sulfated zirconia.
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Fig. 2. Temperature program reduction profile of calcined sulfated zirconia.
3.2. Isomerization of n-butane
To study the effects of various pretreatment envi-ronment on the catalytic activity of SZ, the calcinedcatalyst was treated under different atmospheres suchas nitrogen, hydrogen and air at 450◦C for 3 h atflow rate of 15 ml/min. Pretreatment temperatureplays an important role in butane isomerization overSZ. For example, the pretreatment temperature foroptimum isomerization activity was 350◦C whenthe iron–manganese promoted SZ was heated un-der helium atmosphere [13]. In an earlier study onmicropulse butane isomerization over SZ [21], theoptimum pretreatment temperature with nitrogen was450◦C. Based on these studies and for comparisonof the various pretreatment data, the catalyst waspretreated at 450◦C. However, this temperature maynot be optimum under all pretreatment environments.The effects of various pretreatments on butane con-version are shown in Fig. 3. With air pretreatment,the catalyst shows highest activity in the initial stageof reaction and deactivates rapidly within 60 min oftime-on-stream. When the catalyst is pretreated withnitrogen, initial activity is much less. However, after15 min of TOS its activity is 50% higher with nitro-gen than with air. Interestingly, when the catalyst ispretreated with hydrogen it becomes almost inactivefor butane conversion.
Investigations on the effect of butane carriers, on thenitrogen pretreated catalyst are quite interesting (cf.Fig. 4). Under similar reaction conditions, nitrogen
Fig. 3. Effect of pretreatment gases on the time-on-stream butaneisomerization over calcined sulfated zirconia at 250◦C with a feedhaving C4H10:N2:H2 molar ratio 1:5:4.
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Fig. 4. Effect of carrier gases on butane isomerization over sulfatedzirconia at 250◦C. Feed composition: 20 mol% butane in nitrogen(10 ml/min) + carrier (10 ml/min) and catalyst pretreatment underN2.
shows much better initial activity than hydrogen. How-ever, hydrogen as a carrier gas is more effective thannitrogen for a long run butane transformation. Apartfrom this, an induction period is also observed withhydrogen. Butane isomerization in presence of air ascarrier shows high initial conversion. However, thecatalyst deactivates rapidly and the activity becomesalmost zero after 90 min of time-on-stream.
The effect of butane concentration in nitrogen onbutane isomerization was studied by varying it inthe range of 6–15 mol%. As observed from Fig. 5,increase in butane concentration increases the initialbutane conversion, and also increases the rate of cata-lyst deactivation. In addition, rapid deactivation of thecatalyst occurs only during the first 30 min of TOS.
Effect of contact times (i.e. 0.97–3.84 hour) on bu-tane isomerization and product selectivity, which wasstudied by varying the catalyst amount and keepingall other parameters constant, is presented in Fig. 6.It is observed that the increase in contact time greatlyimproved initial conversion and also affected the prod-uct selectivity. Moreover, the isobutane selectivity
Fig. 5. Effect of butane concentration (mol%) on its conversionover sulfated zirconia at 250◦C. Catalyst pretreated under air.
Fig. 6. Effect of contact times on butane conversion and prod-uct selectivity at 250◦C, and a time-on-stream of 5 min oversulfated zirconia. Feed composition: 20 mol% butane in nitrogen(10 ml/min) + nitrogen (10 ml/min), catalyst pretreated under air.
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Fig. 7. Effect of reaction temperature on butane conversion attime-on-stream of 30 min over sulfated zirconia. Pretreatment andreaction conditions are same as in Fig. 6.
decreased whereas the selectivity of propane isincreased with increase in contact time.
Butane conversion at various reaction temperaturesis presented in Fig. 7. It is observed that catalyticactivity at 150◦C is maximum. In addition, an incuba-tion phenomenon exists up to a reaction temperatureof 200◦C.
4. Discussions
4.1. Physico-chemical properties
Existence of tetragonal phase as the major con-stituent of SZ sample was already reported [13]. Itwas also reported that tetragonal to monoclinic phaseratio influenced the isomerization activity of SZ [22].In the present case, this ratio in the calcined sample,measured at diffraction angle (2θ ) 30.32 and 28.27◦,respectively, is approximately 7. Therefore, it is pro-posed that tetragonal phase plays a major role inbutane isomerization.
The result on acidity (Table 1) shows that thecalcined sample contains appreciable amounts ofBrønsted as well as Lewis acid sites. Prior to aciditymeasurements, the catalyst was pretreated at 150◦Cunder nitrogen flow. However, prior to butane isomer-ization the catalyst was pretreated at 450◦C under dif-ferent pretreatment environments. It may be noted thatcatalyst pretreatment condition significantly changedthe acidity data and hence activity. Therefore, acidityof the catalyst, after pretreatment at 450◦C may notmatch with the data presented in Table 1. However,the data is to show the presence of both Lewis as wellas Brønsted acid sites, which are taken into accountin the proposed mechanism for butane isomerization.
As observed from Table 1, pore radius of the cal-cined sample is 48 Å. The latest reported mesoporouszirconia by Larsen et al. [23] was stable only up toa temperature of 575◦C and the pore radius was inthe range of 20–30 Å, which is quite less comparedto that used in the present study. The XRD pattern ofthe sample calcined at 400◦C (pattern not provided)shows hood in the lower range of 2θ but no such hoodis seen in the sample calcined at 600◦C. In general, formesoporous sample, XRD peaks are observed in thelower range of 2θ , as in the case of MCM and pillaredlayered solids [24]. However, in the present study,though the material is mesoporous, no such peak isseen. This implies that the pore formation mechanismin the present case is different, and does not necessar-ily indicate the presence of two- or three-dimensionalX-ray detectable ordering.
The method of temperature programmed desorptionof ammonia has been widely used for the determina-tion of number as well as strength of the acid sites[25]. Some of the earlier work [26] proposed thatammonia TPD for determination of acidity of SZ wasnot suitable, because ammonia decomposed sulfateat elevated temperature. However, for qualitative in-vestigation of the strength of acid sites this process isstraightforward. For example, if sulfate is decomposedby adsorbed ammonia at elevated temperature thismeans ammonia is present on the catalyst at that tem-perature and hence, the sites responsible for ammoniaretention must be strong. In general, high desorptiontemperature corresponds to greater strength of the acidsites. The observed TPD profile (Fig. 1) shows a broadspectrum in the temperature range of 150–650◦C,which suggests that the solid is highly acidic. The
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hoods, present at 200 and 450◦C, indicate that thecatalyst contained acid sites ranging from moderate tohigher strength and the surface were inhomogeneous.A similar observation was reported elsewhere [25].
Hydrogen TPR is quite informative and is used toassess the reducible species in the solid. In the TPRprofile (see Fig. 2), which is a plot of temperatureversus hydrogen consumption, a strong peak above600◦C is observed agreeing with the report of Xu andSachtler ([27] and references therein). No significantchange in the hydrogen consumption below 500◦Cindicates that sulfate in SZ is probably not reducedbelow this temperature. The TPR experiment alsoindirectly confirmed the evolution of sulfur oxidesby giving white precipitate when the effluent, duringTPR run, was passed through lead(II) solution. Fromthe earlier TPR work on SZ and platinum promotedSZ [28], it was found that hydrogen reduced sulfatemostly to S−2 and S0 state. In another TPR study, Xuand Sachtler ([27] and references therein) reported thatunder a hydrogen environment and at elevated temper-ature (600◦C) sulfate of SZ was completely reducedto SO2, SO3, and surface adsorbed S2−. They also re-ported that reduction of sulfate to SO2 was favored atcomparatively lower temperature than that of reduc-tion to S2−. In addition, there were reports [29] thatclaimed reduction of sulfate in SZ depended on sulfurcontent and not necessarily gave SOx during TPR;instead it produced H2S. Based on these results andour own findings, it is proposed that sulfate in SZ wasreduced in different steps producing oxides of sulfurand surface adsorbed S2−. However, the exact mech-anism of sulfate reduction by hydrogen in SZ is notyet known.
4.2. n-Butane isomerization
Time-on-stream n-butane isomerization is an exten-sively studied reaction over SZ. However, no compar-ative results on the influence of various pretreatmentgases on this catalyst have so far been reported. Fromthe results of the effects of the pretreatment gases(see Fig. 3), one can argue that pretreatment underoxidizing atmosphere makes the catalyst more activethan under inert atmosphere, and pretreatment underreducing atmosphere makes it almost inactive.
The most probable chemical changes under airpretreatment may be oxidation of the Zr3+, which can
be written as follows:
Zr3+O2 → Zr4+ + O2−1
Presence of Zr3+ in SZ was reported earlier [30].The possibility of dihydrogen dissociation over SZ
was reported in [14]. If we extend the idea of di-hydrogen dissociation over the catalyst used in thisstudy, then formation of surface adsorbed hydrideis not surprising on hydrogen pretreatment of thecalcined catalyst at 450◦C. Moreover, the oxygen ofsulfate can stabilize H+, formed during the pretreat-ment. There is also a possibility that oxidation stateof sulfur may reduce during hydrogen pretreatment.However, the TPR pattern (see Fig. 2) does not revealhydrogen consumption at 450◦C implying that sulfateis probably not reduced at this temperature. Thus,the interaction of hydrogen with the catalyst duringpretreatment at 450◦C may be represented as
Zr4+ + H2 → Surface adsorbed hydride of zirconium
It was reported that nitrogen treatment of cal-cined SZ swept out any surface contaminant such asadsorbed moisture [31]. It may be noted here thatprior to the pretreatment, the catalyst was calcinedex situ at 600◦C in static air during which a part ofthe sulfate was decomposed. Therefore, pretreatmentof the calcined catalyst at 450◦C in nitrogen wouldnot decompose sulfate further. Hence, from all thethree pretreatment studies and catalytic activities (cf.Fig. 3) we propose that a redox site, most probably aZr3+/Zr4+ site coupled with sulfate, may be involvedas the high temperature active site for n-butane iso-merization reaction. It was reported that the ratio ofZr3+/Zr4+ in sulfated zirconia was reduced duringisomerization of butane [30]. Moreover, during theTPR (cf. Fig. 2) a strong peak is observed above600◦C, and no indication of hydrogen consumptionat 450◦C implies that hydrogen most probably re-duces the Zr4+ of Zr3+/Zr4+ species to Zr3+ and notsulfates in the sulfated zirconia. In addition, it wasalso reported that pure zirconia contained Zr3+ andZr4+O2
−1 species [32]. However, unsulfated zirconiais not at all active for butane isomerization. Thesefindings further support the proposition that redoxsites, coupled with sulfate acts as the activation sitesfor butane isomerization at elevated temperature.
In isomerization of n-butane it was proposedthat butene reacted with the butyl carbenium ion to
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generate octyl carbenium ion, which after rearrange-ment produced the desire products [33]. From thework of Parera and coworkers [34] and our findings,we propose that butene is formed by a redox processinvolving sulfate-coupled Zr3+/Zr4+ as the active site;whereas, the butyl carbenium ion is generated by theprotonation of butane involving Brønsted proton (seeTable 1). So, the mechanism of butane transformationover sulfated zirconia may be written as follows:
C4H10 + Zr4+ + SO42−
↔ C4H9• + Zr3+ + HSO4
2− (1)
C4H9• + Zr4+ + SO4
2−
↔ C4H8 + Zr3+ + HSO42− (2)
2[Zr3+ + HSO42−] ↔ 2Zr4+ + 2SO4
2− + H2 (3)
The overall reaction is written as follows:
C4H10 → C4H8 + H2 (4)
Evolution of dihydrogen in butane isomerization wasreported earlier [14]. Carbenium ion formed by proto-nation of Brønsted acid site of the catalyst reacts withbutene to form octyl carbenium ion:
C4H10 + H+ ↔ C4H9+ + H2 (5)
C4H8 + C4H9+ ↔ C8H17
+ (6)
This octyl carbenium ion after methyl shift, hydridetransfer and � scission gives the desired products.In this mechanism, butene is treated as one of theintermediate products whose transformation to octylcarbocation is most likely a fast process and thus notdetected in the product stream. A similar argument,that butene formed during oxidation of butane israpidly transformed into other reaction intermediates,was also forwarded by Centi et al. [35] in their studieson transformation of butane to maleic anhydride.
Effects of carrier gases on butane transformation canbe well explained on the basis of above proposed bi-molecular mechanism. Nitrogen as carrier shows muchhigher conversion as compared to the result reportedin the literature. For instance, Vera et al. [34] carriedout a similar study in which substantially lower butaneconversion was observed with nitrogen as the butanecarrier. This discrepancy may be due to the fact that
sulfated zirconia used in their study was prepared bycalcining the sulfated zirconium hydroxide at 620◦Cin the reactor tube under flowing air. Moreover, thereaction temperature was 200◦C compared to 250◦Cin their case. Out of hydrogen and air (cf. Fig. 4),hydrogen has a negative effect on butane transforma-tion and air shows rapid decrease in catalyst activity.The reason for the negative effect of hydrogen maybe the following: when a mixture of hydrogen and bu-tane was passed through the catalyst, as per the mech-anism proposed, dehydrogenation of butane becamea less favored process. Thus, presence of hydrogenin the feed stream probably reduces the initial buteneconcentration, and subsequently the octyle carboca-tion thereby reducing butane conversion. Moreover,any mechanism that removes hydrogen from the react-ing system (see Eq. (4)) can facilitate butene formationand subsequently butane conversion. It was found (cf.Fig. 4) that air as carrier increases the initial butaneconversion by almost two-fold as compared to hydro-gen. This implies oxygen in the system possibly helpsin withdrawing hydrogen. One can ask at this point,which oxygen of the two, i.e. the oxygen of feed gasor the oxygen adsorbed on the catalyst surface is in-volved, if at all, in withdrawing hydrogen? Since, anormal combination of oxygen and hydrogen at 250◦Cis not feasible, it may be the surface adsorbed oxy-gen that withdraws hydrogen, producing water. Sur-face oxygen of the catalyst can also oxidize carbon toform CO2 and under such conditions catalyst deacti-vation should decrease. However, in the present studywe have observed rapid deactivation of the catalyst inpresence of air. Moreover, no CO2 was detected dur-ing the isomerization. This implies that surface oxy-gen did not oxidize carbon to CO2 and thus, did nothelp in reducing catalyst deactivation. It was reportedthat water formed during alkane transformation actedas a catalyst poison [36]. This implies the active sitesare poisoned by the water formed and if this happensthen the catalytic activity of sulfated zirconia wouldrapidly decrease in the presence of air. Our experimen-tal results (cf. Fig. 4) clearly show rapid deactivationof the catalyst and the activity becomes zero after acertain period of time. All the results discussed above,however, strongly support the proposed bimolecularmechanism.
During long run, catalytic activities are muchdifferent with different carrier gases (cf. Fig. 4). This
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may be due to different degrees of deactivation of thecatalyst under different reaction environments. Ear-lier studies on alkane isomerization considered thatdeactivation of sulfated zirconia catalyst were dueto coking [37]. However, we propose coking is notthe sole cause of catalyst deactivation rather hydridetransfer process [38], related to coking, possibly playsan important role.
The observation of an induction period is quiteinteresting when hydrogen is used as a carrier gas(cf. Fig. 4). It was earlier proposed; the induc-tion phenomenon in n-butane isomerization overiron–manganese promoted sulfated zirconia was dueto the stability of the transition state complex (TSC),and the higher the stability of the TSC, the higherwould be the induction period [18]. To make this cleara two-step process is assumed in which the first step isconsidered as the step of formation of a transition statecomplex (TSC, i.e. C8
+ species) and the second isthe transformation of the TSC to products as follows:
C4k1→TSC
TSCk2→Products
where k1 and k2 are the rate constants. During theinduction period, it is probably the k1 that dominatesover k2. However, after the catalyst surface becomessaturated with TSC, k2 probably exceeds k1. FromEq. (4) it is clear that abstraction of hydrogen fromthe surface adsorbed reaction intermediate is less fa-cilitated in presence of hydrogen, as compared to airor nitrogen, as the carrier gas. This means the reactionintermediate is stabilized more in the presence of hy-drogen than in the presence of air or nitrogen leadingto induction. This is probably the reason why hydro-gen in the feed exhibited induction and not nitrogenand air.
As per the bimolecular mechanism, increase inbutane concentration should increase the butane iso-merization. We indeed obtained the expected resultsby changing butane concentration (cf. Fig. 5). Thisis another evidence in support of the above-proposedbimolecular mechanism. The data in Fig. 5 showsdifferent degrees of deactivation of the catalyst withtime. However, increase in catalyst deactivation withincreasing butane concentration, believed to be due tocoking, is a subject of much contradiction. For exam-
ple, Dumesic and coworkers [37] studied the deacti-vation behavior of sulfated zirconia and H-mordeniteduring butane isomerization and found that coke for-mation during butane and isobutane isomerization isone of the key factors causing catalyst deactivation.In addition, Li and Gonzalez ([38] and referencestherein) proposed that it is the coke that deactivatedsulfated zirconia. However, Vera et al. [30], on the ba-sis of coke deposition during isomerization of butaneon sulfated zirconia proposed that the amount of cokein a fully coked catalyst was just sufficient to cover afraction of the total active sites (10%) and coke de-position process was a localized phenomenon. Theyalso argued that coking might not be the main causeof catalyst deactivation. It was reported that crackingis associated with hydride transfer [39]. We thus pro-pose that in addition to coking, catalyst reduction andhydride transfer related to cracking, may also be thecauses of deactivation. To support the above proposal,we measured coke (using a CHN analyzer) containedin the spent catalysts, used for butane isomerizationat a reaction temperature of 250 and 100◦C. Interest-ingly, it is found that 0.34 wt.% of coke depositionat a reaction temperature of 250◦C decreases the cat-alyst activity by 90 mol% whereas the activity at areaction temperature of 100◦C decreased by 65 mol%with a coke deposition of only 0.07 wt.%. Such adrastic decrease in activity at 100◦C and with verylow amount of coke deposition suggests that it is notonly coking that deactivates the catalyst but also someother process, probably reduction associated with hy-dride transfer prevails during the reaction. Reductionof sulfated zirconia during butane isomerization wasreported earlier [12,15,30].
As expected, with increase in contact time, theconversion of n-butane increases (cf. Fig. 6). How-ever, the product selectivity is quite interesting. Theproduct distribution suggests that with an increase incontact time the selectivity of the cracking product,such as propane, gradually increases and the isomericbutane selectivity decreases. Cracking is an associ-ated process during butane isomerization over acidcatalysts that produce propane and methane [40]. Bu-tane can also undergo thermal decomposition yieldingequivalent amount of methane and propane. However,the product distribution in the present study indicatesa catalytic phenomenon, in which butane is mostlycracked over sulfated zirconia yielding propane as
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one of the products. Based on the earlier work [40]and our own results we expect that higher contacttime provide sufficient time to the surface stabilizedC8
+ intermediate, which after methyl shift, hydridetransfer and finally � scission produces propane. Thismay be the reason why propane selectivity is in-creased with an increase in contact time. The decreasein isobutane selectivity and simultaneous increase inpropane selectivity (cf. Fig. 6) may be an indica-tion of parallel and consecutive process in which theisobutane and propane are believed to be producedfrom a common intermediate, the C8
+.To account for the effect of temperature on the
catalytic conversion of n-butane, processes such assurface adsorption and stabilization of the hydro-carbon intermediate, and deactivation are taken intoconsideration. As observed, the isomerization activ-ity at 150◦C is maximum, and above and below thistemperature the activity decreases. One of the ear-lier studies on sulfated zirconia reported maximumisomerization activity above 300◦C [41]. However,catalytic performance of sulfated zirconia dependsnot only on reaction temperature but also on sur-face acidity and reaction environments. Recently, Liand Gonzalez ([38] and references therein) reportedisomerization activity of sulfated zirconia maximumbutane conversion was obtained below 200◦C. Abovethis temperature cracking dominated over isomeriza-tion. Furthermore, it is believed that isomerizationand cracking reactions occur simultaneously over anacid catalyst [42]. Based on these facts and our re-sults, we conclude that 150–200◦C is the optimumtemperature range for maximum isomerization ac-tivity. Beyond this it would be cracking dominatingover isomerization. At lower temperature it could bethe induction that continues and hence, the activity islow. In fact, induction phenomenon is observed in allthe cases below a reaction temperature of 200◦C.
5. Conclusions
From the above study it is concluded thatbutane isomerization over sulfated zirconia involvesa redox process in which the possibility of existenceof Zr3+/Zr4+ coupled with sulfate group will playa vital role. The induction phenomenon suggeststhe formation of a surface stabilized species, most
probably the C8+ carbenium ion whose stability on
the catalyst surface depends on the nature of the car-rier gas. The reaction product distribution indicatesthat the methyl shift, hydride transfer and � scissionare the most probable processes involving a surfacestabilized C8
+ species, and also strongly supports thebimolecular mechanism of butane isomerization. It isalso concluded that besides coking, catalyst reductionassociated with hydride transfer phenomena could beresponsible for catalyst deactivation.
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