n-butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal...

Applied Catalysis A: General 210 (2001) 151–164

n-Butane isomerization over transition metal-promoted sulfatedzirconia catalysts: effect of metal and sulfate content

J.A. Moreno, G. Poncelet∗Unité de Catalyse et Chimie des Matériaux Divisés, Université Catholique de Louvain,

Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium

Received 9 February 2000; received in revised form 10 September 2000; accepted 10 September 2000

Abstract

n-Butane isomerization has been investigated over transition metal-promoted sulfated zirconia catalysts. Fe alone was a moreefficient promoter than Mn and Cr, and mixtures of Fe–Mn, Fe–Cr, and Fe–V. Promotion of a commercial sulfated zirconia(4.8% sulfate) with iron amounts in excess of 2% depressed the conversion and increased the deactivation rate. Increasingreaction temperatures improved the conversion and decreased the induction period. At 70◦C and above, the induction periodwas suppressed. The influence of activation temperature was studied over a Fe-sulfated zirconia catalyst (with 2% Fe and4.8% SO4

2− pre-calcined at 600◦C). The best conversion was achieved when the catalyst was activated at 350◦C in air. Thistemperature was apparently sufficient to generate the redox sites active at low reaction temperature. Activation in flowinghelium depressed the catalytic activity and increased the induction period. The sulfate content had a significant effect on thecatalyst performance. For 2% Fe-promoted zirconia, a maximum conversion was found for 7.5% SO4

2−, probably relatedwith a better balance (or synergism) of the redox and acid sites involved in a bimolecular mechanism. The time required toreach a maximum of conversion (induction period) decreased with increasing total acidity, i.e. with sulfate content. The seriesof catalysts with different amounts of sulfate has been characterized by X-ray diffraction, nitrogen adsorption–desorptionisotherms, TGA, ammonia-TPD, DRIFT, Raman, and X-ray photoelectron spectroscopy. © 2001 Elsevier Science B.V. Allrights reserved.

Keywords: n-Butane isomerization; Fe, Mn, Cr, V-promoted sulfated zirconia; Activation temperature; Sulfate content; DRIFT, Raman, XRD,XPS

1. Introduction

Sulfated oxides, in particular sulfated zirconias,are considered as potential alternative catalysts toenvironmentally harmful mineral acids such as HF,H2SO4, and Pt/chlorinated alumina catalysts used forbutene–iso-butane alkylation, andn-butane isomer-ization, respectively. Sulfated zirconia impregnated

∗ Corresponding author. Fax:+32-10-472005/3649.E-mail address:[email protected] (G. Poncelet).

with 1.5% Fe and 0.5% Mn, more active by threeorders of magnitude than the non-promoted form inbutane isomerization, was first reported by Hollsteinet al. [1]. The high activity at low temperatures is ofsignificant interest for thermodynamic reasons [2].Sulfated zirconias and modified compositions havealso been evaluated in several other reactions withindustrial relevance [3]. These catalysts have been(e.g. [4,5]), and still are [6] considered as super-acidsolids, although activity results obtained at differ-ent laboratories merely suggest a combined effect of

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152 J.A. Moreno, G. Poncelet / Applied Catalysis A: General 210 (2001) 151–164

moderately strong acid sites in close vicinity with re-dox sites, related with sulfate and transition metal(s)(e.g. Fe and/or Mn), respectively [7,8].

The acidity, undoubtedly generated by the sulfateanions with still uncertain configuration at the sur-face, as illustrated by the various models proposed inthe literature [3], involves both Brønsted and Lewiscenters, as inferred from the dependence of the cat-alytic activity on several parameters such as sulfatecontent, calcination, and activation temperature, allintervening in the development of acidity [9]. In par-ticular, Morterra et al. [10] observed a maximum ofactivity coinciding with a maximum of strong Lewisacid sites and a minimum of Brønsted sites.

Extensive research has been devoted to the eluci-dation of the nature of the redox sites of Fe- and/orMn-promoted sulfated zirconia catalysts. The roleof the transition metal(s) has been associated witha larger number of catalytic centers and a higherstrength of the acid sites than in non-promoted sul-fated zirconia [2,4], a result contradicted by some au-thors who found no difference in nature and strengthof the acid sites between FMSZ and SZ catalysts [7].Other authors concluded that the remarkable activ-ity of FMSZ catalysts was not due to exceptionallystrong acid sites, but merely to the stabilization of thetransition state complex at the surface involved in abimolecular reaction mechanism, with the redox sitesintervening in the formation of olefinic intermediatesvia oxidative dehydrogenation [7,8,11–13]. Althoughthe nature of the redox sites still remains obscure,the transition metals would form aggregates of theparent oxide(s) (i.e. Fe2O3 and MnO) supported ontetragonal zirconia, some sulfate groups forming asurface complex (possibly S2O7

2− species) [14,15].Recent XAFS, XRD and Raman results establishedthat in those catalysts, Fe is trivalent and inside thebulk, forming interstitial-type solid solution and lo-cated at the center of distorted oxygen octahedra,whereas Mn is present as MnSO4 [16]. On the otherhand, a three-site model was proposed in order toaccount for the catalytic performance of FMSZ cata-lysts with, in addition to an acid site common to bothSZ and FMSZ catalysts, two redox sites ascribed,the one to an iron oxy species tentatively assignedto an iron(IV), and the second one to an iron(III),responsible for the isomerization activity near roomtemperature (30◦C) and at 100◦C, respectively [7].

The possibility that the metal promoters could playa non-catalytic role as initiators has been suggested[17] and experimentally verified [18]. However, adifferent interpretation of the role of Fe has beenproposed [16].

Several research groups investigated similar cata-lyst compositions, regardless of the apparently arbi-trary metal composition adopted for their preparation.Although it has been generally admitted that ironand manganese behaved similarly for the creation ofthe active redox sites, some authors reported that aMn-promoted sulfated zirconia was a less active cata-lyst than a non-promoted sulfated zirconia [19]. Otherones found a strong dependence of the promoting ef-fect of Fe and/or Mn on the preparation and reactionconditions [17].

The influence of promoter and sulfate content onthe catalyst activity has been addressed in a limitednumber of studies. A sulfate content of 4.0% wasfound to be optimal for a catalyst with 1.5% Fe and0.5% Mn [5], whereas at the opposite, no relationshipwas observed between activity and sulfate content inFe-promoted sulfated zirconias [20]. Coelho et al. [19]investigated a series of FSZ catalysts and observed anoptimal Fe content of 0.2% for samples with 8% nom-inal sulfate content. For similar catalysts, but underdifferent reaction conditions, Song et al. [21] reportedan optimum activity for 4% Fe without specificallyconsidering the effect of the actual sulfate content oftheir catalysts. Since transition metal-promoted sul-fated zirconias behave as bifunctional catalysts [7,8],one would expect an optimum activity when the bal-ance (synergism) between the acid and redox functionsis best achieved.

Another controversial parameter is the temperatureat which the catalyst is activated prior to the catalytictest. Some authors found a maximum of conversionat two activation temperatures for FMSZ catalysts [7],whereas other ones noticed only one [22].

This contribution compares in a first part the resultsof screening tests performed over sulfated zirconiaspromoted with different metal and mixed-metal ions,with the aim of selecting the most efficient catalyst.The second part is devoted to the influence of the sul-fate content of Fe-promoted sulfated zirconia catalysts,the effect of the activation and reaction temperatureon n-butane isomerization, and the characterization ofthese catalysts.

J.A. Moreno, G. Poncelet / Applied Catalysis A: General 210 (2001) 151–164 153

2. Experimental

2.1. Preparation of catalysts

A first series of catalysts was prepared by wet im-pregnation of a sulfated zirconium hydroxide (MELChemicals, XZO 682/01) with appropriate amounts ofaqueous solutions of Fe3+ and Mn2+ in order to havea total nominal metal content of 2 wt.%. Briefly, thestarting sulfated hydroxide previously dried at 100◦Cwas mixed with the Fe3+ solution (Fe(NO3)3·9H2O,UCB) in a proportion of 1 ml solution per gram solid.The slurry was stirred for 1 h and oven-dried for24 h at 100◦C. A second impregnation with Mn2+(Mn(NO3)2·4H2O, Merck) was performed, followedby drying at 100◦C. In a similar way, the commercialsulfated zirconium hydroxide was impregnated withthe required amount of the Fe(III) nitrate solution inorder to have 1.5% Fe and, after drying at 100◦C, withammonium metavanadate (0.5% V) or chromium(III)nitrate solutions (0.5% Cr). A sample with 2% Crwas also prepared. The same commercial sulfated zir-conium hydroxide (dried at 100◦C) was impregnatedwith amounts of Fe3+ in the range 2–8%. The sulfatecontent of the starting sulfated zirconium hydroxidecalcined at 600◦C was 4.8 wt.%.

In order to investigate the influence of the sulfatecontent, a series of catalysts has been prepared witha commercial zirconium hydroxide (MEL Chemicals,XZO631/01) previously dried at 100◦C and impreg-nated with ammonium sulfate (UCB), with nominalsulfate contents between 2.5 and 10%. The sulfatedsolids were dried at 100◦C and impregnated with theFe3+ solution (2% Fe) and dried again at 100◦C.

All the catalysts were calcined for 3 h at 600◦C instatic air, with a ramp of 5◦C min−1 between roomtemperature and 600◦C.

The effect of reaction temperature (40–100◦C),and activation temperature (300–600◦C for 1.5 h) wasinvestigated over a Fe3+-promoted sulfated zirconia(2% Fe) prepared by impregnation of the commercialsulfated zirconium hydroxide.

2.2. Characterization methods

The series of Fe-promoted catalysts (2% Fe)with sulfate contents between 2.5 and 10% werecharacterized with several methods and techniques.

The sulfate contents were determined with a LECOanalyzer (HF-400 from LECO Corp.). X-ray diffrac-tion (XRD) patterns were recorded between 5 and65◦ 2θ at a scanning rate of 0.03◦ s−1 with a SiemensD-5000 equipment, using nickel-filtered Cu Ka radia-tion. The BET specific surface areas were determinedfrom nitrogen adsorption–desorption isotherms estab-lished at 77 K with a Micromeritics ASAP 2000 sorp-tometer. The pre-calcined samples were outgassedfor 6 h at 200◦C prior to the sorption measurement.Thermogravimetric analyses (TGA, 30 mg of sample)between RT and 950◦C were carried out in flowingair (100 ml min−1) with a ramp of 10◦C min−1 usinga 2960 SDT analyzer (TA Instruments). The totalacidity was obtained by back-titration of the ammoniadesorbed between 100 and 500◦C (10◦C min−1) andcollected in 0.2 M boric acid from samples (200 mg)previously treated at 500◦C under helium flow(65 ml min−1) and saturated with successive pulses ofammonia at 100◦C, followed by helium purge.

Diffuse reflectance Fourier transform infrared spec-tra (DRIFT) were obtained with a Bruker IFS88spectrometer equipped with a DTGS detector anda Spectra-Tech environmental chamber. The spectra(200 scans with a resolution of 4 cm−1) were recordedat room temperature and after heating the samplesup to 350◦C followed by cooling at 50◦C, either inflowing air or helium (50 ml min−1). The gases werepassed through a water trap (Hydro-purge II, Alltech)prior to their admission in the analysis cell.

Samples were analyzed at ambient conditions withRaman spectroscopy using a LABRAM II Dilor-JobinYvon-Spex spectrometer equipped with a He–Ne laser(632.8 nm, 25 mW). The slit width was 200mm andthe spectra were taken after three scans with an accu-mulation time of 60 s each.

The O 1s, Zr 3d, Fe 2p and S 2p X-ray photo-electron spectroscopy (XPS) peaks were recordedwith a SSI X-Probe (SSX-100/206) photoelectronspectrometer (Surface Science Instruments) equippedwith a mono-chromatized micro-focused Al X-raysource. The residual pressure in the analysis chamberwas below 5× 10−7 Pa. The spot size was approx-imately 1.4 mm2 with the energy pass set at 150 eV.In these conditions, the resolution determined by thefull width at half maximum of the Au 4f7/2 peak ofa standard gold sample was around 1.6 eV. A floodgun set at 6 eV and a Ni grid placed 3 mm above the


sample surface allowed for charge compensation. Thebinding energies of the different peaks were refer-enced to the C–(C, H) component of the C 1s peak ofadventitious carbon fixed at 284.8 eV. Decompositionof the peaks was performed with the least squaresfitting program provided with the equipment, usinga Gauss/Lorenz ratio of 85/15 and after non-linearbaseline subtraction. The atomic concentration ratioswere calculated from the peak areas normalized onthe basis of acquisition parameters and sensitivityfactors provided by the manufacturer.

2.3. Catalytic activity

The reactions were carried out in a U-shaped quartzmicro-reactor (8 mm i.d.) operated at atmosphericpressure. The catalyst (typically 0.5 g) previouslycalcined for 3 h at 600◦C and sieved to collect the0.2–0.315 mm fraction was introduced in the reac-tor between quartz wool plugs, and activated for1.5 h in flowing dry air (50 ml min−1, Air Liquide)at given temperatures (TA). Cooling at the reactiontemperature (40–100◦C) was also done under airflow. The reactor was purged with helium prior tothe admission of the reaction mixture. The feed con-sisted of a mixture of 1 ml min−1 of n-butane (AirLiquide, 99.95%) and 4 ml min−1 of He (Air Liq-uide, 99.995%), corresponding to a WHSV of 0.29 gbutane per gram catalyst per hour. The gases werepurified with a water-trap (Hydro-purge, Alltech), andoxygen impurities were removed with an oxy-trap(Alltech). It was verified that under the reactionsconditions, diffusional limitations were absent. Thereaction products were analyzed in a gas chromato-graph (Hewlett-Packard 5880A) equipped with a TCdetector, using a HP-Plot/Al2O3 capillary columnto separate the products. Conversion and selectivitywere calculated on the carbon number basis.

3. Results and discussion

3.1. Effect of promoting metal and metal content

Fig. 1 (inset) shows the conversion of butane at5 min as a function of the Fe/(Fe+ Mn) ratios, for atotal metal content of 2%. The reaction was performedat 100◦C on catalysts activated in situ at 600◦C. The

Fig. 1. Inset: conversion at 5 min vs. Fe/Fe+ Mn ratio of sulfatedzirconias. Activation: 1.5 h at 600◦C in air; reaction temperature:100◦C. Main graph: conversion vs. time at 100◦C over; (topcurves): Fe- (2 and 8%) and Mn-promoted sulfated zirconias;(bottom curves): over sulfated zirconias promoted with Fe, Cr,Mn, Fe–V, Fe–Cr.

results show that although Mn also had a beneficial ef-fect on the activity (the conversion at 5 min was<1%for a non-promoted sulfated zirconia tested undersimilar conditions), the Mn-promoted catalyst deacti-vated faster and was less active than the Fe equivalent(FSZ) (main graphs in Fig. 1). The conversion overthe MFSZ catalysts was intermediate between thoseof MSZ and FSZ catalysts, with small but significant


differences according to the relative proportion of thepromoters. Similar results at 100◦C over FSZ andMSZ catalysts have been reported in the literature[17]. As mentioned earlier, some authors reported thatMn and Fe had a similar promoting effect [7], while atthe opposite, other ones found that a Mn-promoted SZwas even less active than a non-promoted SZ [19], aninconsistency which could be attributed to the higherpartial pressure of butane (0.25 versus 5× 10−3 bar)[17]. Although in this study the partial pressure ofbutane was 0.2 bar, the results did not reproduce thoseof Coelho et al. [19], owing perhaps to the apparentlymuch higher space velocity used by these authors.

Fig. 1 (bottom) compares the results obtained at100◦C over the sulfated zirconia catalysts promotedwith different transition metals and mixtures thereof,for a total metal content of 2%. The Cr-promotedsample was the least active one. All the mixed-promoted sulfated zirconias were less active thanthe Fe-promoted one. The selectivities toiso-butanewere in the range 96% (Fe)–100% (Mn). Underrather different reaction conditions (closed recircu-lation reactor), Miao et al. [23] observed a substan-tial improvement of the catalytic activity for a V-,Fe-containing sulfated zirconia (total metal: 2%) pre-pared by co-precipitation, an effect which was notobserved in this study.

The conversion versus time curves obtained at100◦C over sulfated zirconias containing 2 and 8% Fe(4.8% SO4

2−) are compared in Fig. 1. Intermediatecurves (not shown) were observed for the catalystswith 4 and 6% Fe. All the catalysts exhibited a highinitial activity, but the deactivation rate gradually in-creased with increasing Fe content. The catalyst with2% Fe was more active than those with higher con-tents. The selectivity toiso-butane varied between 90and 96%. C3 andiso-C5 were the main side products,with the former one being more abundant. For similarFSZ catalysts, Song et al. [21] found an optimal Fecontent of 4% for a unspecified sulfate content. Inview of these results, sulfated zirconia promoted with2% Fe was selected for further investigation.

3.2. Influence of reaction temperature

It is known that the conversion versus time profileover FMSZ catalysts is affected by the reaction tem-perature with, below 100◦C, a regular increase of the

Fig. 2. Influence of reaction temperature on conversion over FSZcatalyst (2% Fe and 4.8% SO4

2−). Activation: 1.5 h at 400◦C inair.

conversion with time until a maximum is attained (in-duction period), followed by a more or less rapid de-activation, depending, among others, on the reactiontemperature [13,24,25]. Since in Fig. 1, the reactionswere carried out at 100◦C, only the deactivation profilewas observed. When performing the reaction at 60◦Cover a Fe-impregnated (2% Fe) catalyst, the typicalinduction period was followed by a slow deactivation.However, the maximum conversion was significantlylower than values reported in the literature over cat-alysts with similar metal content and under compara-ble reaction conditions. Carrying out the reaction at60◦C over a fresh catalyst activated at 400◦C instead of600◦C increased the conversion at the end of the induc-tion period by a factor 3. The effect of activation tem-perature will be further discussed in the next section.

The results obtained over a catalyst with 2% Feand 4.8% SO42− (activated at 400◦C) in the temper-ature range 40–60◦C are compared in Fig. 2. Withincreasing reaction temperature, the induction periodshortened and the maximum of conversion increased.After about 3 h on stream, all the curves superim-posed. At 70◦C and above, the induction period wassuppressed, and only a fast deactivation of the catalystoccurred. Up to 70◦C, the selectivity toiso-butanewas 100%. It decreased at higher temperatures (95%at 80◦C and 88% at 100◦C). The induction period has


been accounted for by the formation and accumula-tion of olefinic intermediates at the catalyst surface[13], their stability being improved in the presence ofa promoter (Fe and Mn) [25]. The comparable cat-alytic performances of the FSZ catalysts with thosereported in the literature for FMSZ catalysts allowone to infer a similar reaction pathway.

3.3. Influence of activation temperature (TA)

In reported works on FMSZ catalysts, the activa-tion prior to the measurement of the isomerizationactivity was performed between 450 and 500◦C un-der inert atmosphere (N2), after a previous calcina-tion in static air at temperatures between 500 and725◦C [2,4]. The catalytic results obtained at 50◦Cover Fe-promoted sulfated zirconias (2% Fe and4.8% SO4

2−) pre-calcined at 600◦C and activated attemperatures between 300 and 500◦C are shown inFig. 3. It is seen that the induction period shortenedas TA increased from 300 to 400◦C, with an opti-mal conversion for a TA of 350◦C. For TA of 500◦Cand above, the time to reach maximum conversiondid no longer change, but the conversion decreased(curves not shown). No second optimum of TA wasobserved. Independent of the activation temperature,the selectivity toiso-C4 was always total.

Fig. 3. Influence of activation temperature on conversion at 50◦Cover FSZ catalyst (2% Fe and 4.8% SO4

2−).

As mentioned earlier, a maximum of conversionwas found by some authors at two activation tempera-tures [7] with a first one at 350◦C, as for SZ catalysts,attributed to the active sites generated by a treatmentunder air or helium of a FMSZ catalyst pre-calcined at650◦C and ambient re-hydrated, and a second one, forTA above 450◦C, only when the catalyst was treatedin air (development of ‘oxy’ species, proposed to beresponsible for the catalytic activity at temperaturesbelow 100◦C). For Morterra et al. [26], a calcinationtemperature (TC) higher than 600◦C, and a TA be-tween 400 and 450◦C in air were necessary to observethe promoting effect of Fe and Mn. The effect of theactivation and calcination temperature on the catalystefficiency was clearly illustrated in a recent studyshowing, for a SZ catalyst, a maximum of activityfor TC = 610◦C and TA = 250◦C, compared withTC = 650◦C and TA= 450◦C for a FMSZ catalyst, adifference of activity which was attributed to the dif-ferent degree of synergism between the redox and theacid sites of these two catalysts [22]. The lower op-timal TA (350◦C) found in this work over FSZ couldpossibly be related to the different ex situ calcinationtemperatures used, 600◦C in this study versus 650◦C,namely, in the range where an important effect of TCwas observed [22]. It may also indicate that FMSZ andFSZ catalysts require different activation temperaturesin order to ensure the formation of the redox sites, andprobably also an adequate balance of the two catalyticfunctions. Alternatively, it could be due to the lessereffectiveness of the promoting effect of Fe in thepresence of Mn, as observed by some authors [21].

It should be noted that confrontation with publishedwork is often delicate owing to the diversity of thepreparation methods and the reaction conditions formeasuring the activity, in agreement with other authors[14,17]. A clear illustration is provided by the oppo-site results of Hino and Arata [20] and Song et al. [21]with respect to the activity of FSZ catalysts. There-fore, a valuable comparison of different results shouldsometime restrict to particular cases.

3.4. Influence of sulfate content

The reaction has been carried out at 50◦C over 2%Fe-promoted sulfated zirconias with sulfur contentsbetween 2.5 and 10% and activated in flowing air at350◦C for 1.5 h. The conversion versus reaction time


Fig. 4. Conversion vs. time curves at 50◦C over FSZ catalysts:influence of sulfate content. Activation: 1.5 h at 350◦C in air.

curves are shown in Fig. 4. Increasing sulfate contentfrom 5 to 10% decreased the induction period from120 to 40 min. The sample with 7.5% SO4

2− was themost active one, whereas the one with 2.5% SO4

2−was inactive. Selectivities of 100% were obtained forthe catalysts with 5 and 10% SO4

2−, and 98% forthe one with 7.5% SO42−. Lin and Hsu [5] found anoptimum sulfate content of 4% for a FMSZ catalystwith 1.5% Fe and 0.5% Mn. For similar total metalcontents (2%), this optimum was lower than the valuefound in this study over FSZ catalysts, owing perhapsto the lower effectiveness of Fe induced by the pres-ence of Mn [21], and consistently with the results ofFig. 1.

The influence of the activation atmosphere wasexamined over the most active FSZ catalyst (2% Fe,7.5% SO4

2−) activated at 350◦C under flowing heliuminstead of air. The conversion versus time curve at50◦C is compared, in Fig. 5, to the one obtained overa same catalyst activated in air. Obviously, the FSZcatalyst activated in helium was less active (maximumconversion was 15% versus 25% for the one activatedin air), and the induction period was increased by afactor of about 5 (264 min versus 56 min), thus veri-fying air to be a preferred activation atmosphere thanhelium, probably reflecting a deficiency of redox siteswhen activation is done under helium [7].

Fig. 5. Effect of activation atmosphere (helium, air) on conversionat 50◦C over FSZ catalyst (2% Fe and 4.8% SO4

2−). Activation:1.5 h at 350◦C.

3.5. Catalyst characterization

Characterization results relative to the series ofFe-promoted zirconias (2% Fe) with different sulfateamounts are presented in Table 1. The sulfate con-tents of the calcined catalysts established by chem-ical analysis corresponded to the nominal quantitiesintroduced at the preparation step. Assuming thateach sulfate group occupied a surface equivalent to25 Å2 [27], an amount of 2.5% SO42− correspondedto 30% of a monolayer, while for the sample with10% SO4

2−, the coverage exceeded by 15% the the-oretical monolayer. Sulfate contents of 5 and 7.5%ensured 55 and 77.5% of the monolayer capacity,respectively.

The XRD patterns of the calcined catalysts (600◦C)(Fig. 6) showed the predominance of the reflectionsof the tetragonal ZrO2 phase, in agreement with theliterature (e.g. [16,28]). Small reflections of the mon-oclinic phase were observed particularly for the sam-ples with 2.5 and 5% SO42−. The lower intensitiesof the reflections for the sample with 10% SO4

2− (inexcess of the monolayer) suggest a lesser crystallinitycompared with the samples with 5 and 7.5% SO4

2−.Lin and Hsu [5] observed that the presence of sulfateretarded the transformation of amorphous hydrouszirconium oxide to the tetragonal phase, and an excess


Table 1Characterization of 2% Fe/sulfated zirconias: sulfate contents, BET specific surface areas, weight losses, decomposition temperature (Td),and total acidity

Sample % SO42− SBET (m2 g−1) TGAa Total acidity(mmol g−1)Nominal Chemical analysis % SO4

2− Td (◦C)

FSZ56 2.5 2.6 129 2.15 817 128FSZ57 5.0 4.9 140 4.35 733 259FSZ58 7.5 7.5 153 6.23 671 333FSZ59 10.0 9.6 129 8.51 614 360

a % SO42− calculated from weight loss at 950◦C; Td: temperature of sulfate decomposition.

of sulfate in FMSZ catalysts resulted in a decrease ofthe catalytic activity which is in line with our results.

The values obtained from the TGA weight losses(assuming the evolution of gaseous SO3 [29]) be-tween the decomposition temperature (Td) and 950◦C

Fig. 6. X-ray diffraction patterns of FSZ catalysts with different amounts of sulfate. M: monoclinic; T: tetragonal.

(Table 1) were smaller than those found by chemi-cal analysis, but it is clear from Fig. 7 that the lossof sulfate continued above 950◦C. Furthermore, theonset temperature (Td) decreased from 817 to 614◦Cwith increasing sulfate content with, for the sample


Fig. 7. Thermogravimetric analysis of FSZ catalysts with different sulfate contents.

with 10% SO42−, two weight losses between 614 and

950◦C. These observations suggest that the sulfate ismore strongly held on the surface at lower than athigher sulfate content.

DRIFT spectra were recorded on the series ofcatalysts with different sulfate contents heated inflowing air at increasing temperatures up to 350◦C(optimal activation temperature), and after coolingat 50◦C (conditions reproducing those used for thecatalytic measurements). In the region between 2000and 800 cm−1, the spectra of the unheated samplesshowed bands at 1678 (H3O+) and 1630 cm−1 (H2O),and several other ones between 1250 and 850 cm−1

due to the sulfate anion. The complexity and in-tensity of the absorption mass if between 1300 and1000 cm−1 increased with the sulfate content. Succes-sive treatments between 100 and 350◦C brought abouta progressive diminution of the bands at 1668 and1630 cm−1, a upward shift of a band near 1250 cm−1

by about 30–40 cm−1 and loss of intensity up to totalremoval at 350◦C. A new band appeared after heatingat 100◦C, at wavenumbers depending on the sulfatecontent: 1339 cm−1 for 2.5%; 1355 cm−1 for 5%;and 1368 cm−1 for 7.5 and 10%. The intensity ofthis band slightly increased with temperature and, at350◦C (Fig. 8a), it was located, at 1370 (2.5%), 1385(5%), and 1394 cm−1 (7.5 and 10%), respectively. Forthe samples with 7.5 and, particularly, 10% SO4

2−,a new broad band developed near 1205 cm−1.

Different authors observed a band at 1390 cm−1

(e.g. [30,31]) which has been assigned to the S=Ostretching of adsorbed SO3 [32]. Bensitel et al. [15]noticed in the case of monoclinic SZ with low sul-fate content, bands at 1352 and 1365 cm−1 (S=Ostretch of isolated sulfate species). With increasingsulfate content, the band at 1350 cm−1 vanished, andthe one at 1365 cm−1 shifted at 1392 cm−1 givingrise, at still higher sulfate content, to a new bandat 1403 cm−1, apparently developing at the expenseof that at 1392 cm−1 and tentatively attributed to a(S2O7

2−)-type species (one coupled S–O–S bridgeand two isolated S=O oscillators). The first two bandswere assigned to the S=O stretching mode of terminalS=O of (ZrO)3–S=O surface species, possibly on dif-ferent crystal faces of zirconia. A similar assignmentwas proposed by Haase and Sauer [33] for tetragonalSZ. A band at 1250 cm−1 (observed at low tempera-tures in this study) was assigned to the symmetricalS=O stretch of adsorbed H2SO4 [32].

The spectra of the samples heated at 350◦Crecorded in the OH stretching region are shown inFig. 8b. Bands were observed at 3740, 3650–3620,3560 and 3300–3200 cm−1 (very broad and weaklyintense). The band at 3740 cm−1 (appearing at 100◦C)was more developed for the sample with 2.5% SO4

2−(inactive in butane isomerization), and barely visiblefor the one with 7.5% SO42− (the most active one).The higher intensity for the sample with 10% SO4

2−


Fig. 8. (a) DRIFT spectra of the sulfate region of FSZ catalysts with different amounts of sulfate heated in flowing air at 350◦C. (b)DRIFT spectra in the hydroxyl region of FSZ catalysts with different amounts of sulfate heated in flowing air at 350◦C.

(in excess of the quantity necessary to achieve amonolayer) compared with that with 7.5% SO4

2− mayreflect a partial aggregation of sulfates groups (per-haps as di-sulfate species), possibly related with thepresence of a band at 1205 cm−1. The intensity of theband at 3560 cm−1 increased with increasing sulfate

content. The band at 3652 cm−1 for the sample with2.5% SO4

2− shifted at 3631 cm−1 for that with 5%SO4

2−, and at 3620 cm−1 for those with higher sulfatecontents. Similar shifts were assigned to an increasinginteraction between bridging OH and sulfate groups,traducing an increasing Brønsted acidity [34,35].


Bands at 3740 and 3650–3620 cm−1 have been as-signed to isolated OH groups bound to single cations(Zr4+) (type I), and to bridging OH (type II) coordi-nated to more than one cation, respectively [34,36,37].A band near 3560 cm−1 was attributed by Kustov et al.[34] to weakly hydrogen-bonded OH groups of ZrO2(or to another type of bridging OH groups). These au-thors also noticed a substantial diminution of the OHband at 3740 cm−1 upon sulfation of ZrO2. Ward andKo [38] observed that only zirconia supports exhibit-ing a band at 3740 cm−1, disappearing after sulfateimpregnation, were active in butane isomerization;only these strongly interacting sulfate species givinglocalized Zr–O–S bonds would be capable of formingstrong acid sites on SZ catalysts.

The catalytic and DRIFT results obtained in thisstudy seem to indicate that at least two types of sulfateare formed depending on the sulfate content: the oneforming at low sulfate content (2.5%), characterizedby a S=O stretching band at 1370 cm−1 and leav-ing the band at 3740 cm−1 unaffected. The secondspecies, at higher sulfate content (higher surface cov-erage), would involve Zr–O–S bonds formed at theexpenses of the type I OH groups (disappearance ofthe band at 3740 cm−1), with a S=O stretching bandat 1394 cm−1. It appears from the catalytic results,in agreement with the cited authors [38], that thesulfate species exhibiting a band at 1394 cm−1 wouldconstitute the active sites capable of isomerizing bu-tane. The fact that the sulfate species giving a bandat 1370 cm−1 (inactive catalyst) started to decomposeat higher temperature than those characterized by aband at 1394 cm−1 which decomposed at lower tem-peratures allows to infer two different sulfate species(with different interactions or covalent character ofthe sulfate groups). The existence of two species hasbeen related with sulfate groups on different crystalfaces of ZrO2 [15,32,39], or distinct acidities [40]. ForMorterra et al. [39], the isolated sulfates withνS=O ≤1375 cm−1 are ascribable to sulfate groups formed inmore energetic configurations and corresponding tocrystallographic defects, whereas those withνS=O inthe range 1380–1400 cm−1 are mainly formed on theless regular patches of low-index crystal planes.

Cooling the samples at 50◦C in flowing air causeda partial re-hydration, (reappearance of the band at1620 cm−1), the simultaneous shift of the S=O band at1394 cm−1 back at 1372 cm−1 for the samples with 7.5

and 10% SO42− (from 1370 and 1385 to 1342 cm−1

for those with 2.5 and 5.0% SO42−), and the resur-gence of the band at 1250 cm−1. In fact, the spectrain the region 2000–800 cm−1 nearly reproduced, forall the samples, those recorded after heating at 100◦C,showing the reversible nature of the sulfate-surfaceinteractions, as also observed by other authors (e.g.[9,32,41]). Since the pre-treatment conditions used forthe DRIFT analyses and those for the activation of thesamples prior to running the catalytic test were sim-ilar, the real configuration of the surface active cen-ters in the catalytic measurements at low temperaturecould differ from that prevailing at high temperatures(and in most cases, under outgassing conditions). It isinteresting to mention that the spectra obtained overthe FSZ catalyst with 7.5% SO42− activated at 350◦Cin flowing air or helium were identical, although thecatalyst activated under helium was substantially lessactive (Fig. 5) which, as proposed by Wan et al. [7],could reflect a deficiency of redox sites.

Raman spectroscopy was used to complement theXRD and IR results. It allowed to verify the type ofthe crystalline ZrO2 phase (monoclinic, cubic, tetrag-onal), and provide some information on the sulfate.Concerning the structural aspect, the Raman spectrain the region below 800 cm−1 confirmed (Fig. 9) thatall the FSZ samples were mainly tetragonal (bands at630, 466, 451, 306, and 267 cm−1), with a small frac-tion of monoclinic phase. A total of 13 bands (at 632,612, 553, 533, 496, 472(s), 374, 341, 326, 301, 217,186 and 174 cm−1) was found for the pure monoclinicZrO2, consistently with reported works [14,37,42,43].The sample with 2.5% SO42− contained the tetrago-nal phase along with a small amount of the monoclinicphase, (doublet at 174 and 182 cm−1, and contribu-tions at 374 and 470 cm−1). This phase was muchreduced in the samples with 7.5 and 10% SO4

2−. Themetastable cubic phase (single band at 490 cm−1 [44])was absent.

Raman bands due to the S–O and S=O stretchingmodes of the surface sulfate species show up near1000 and 1400 cm−1, respectively [42,45]. A shoulderat 1382 cm−1 and a doublet at 1000–1030 cm−1 wereassigned to two sulfate species with slightly differentdistortions [42]. As seen in Fig. 9, the samples exhib-ited a doublet at 990 and 1020 cm−1 (S–O stretching)the intensity of which increased with increasing sulfatecontent. For the sample with 10% SO4

2−, there was a


Fig. 9. Raman spectra of FSZ catalysts with different sulfate contents.

broad asymmetrical band centered around 1020 cm−1.Since the Raman spectra were recorded on hydratedsamples, the Raman band around 1400 cm−1 (S=Ostretching) was absent.

The XPS spectra did not show significant differ-ences of the binding energies of the O 1s, Zr 3d, Fe2p, and S 2p peaks of the FSZ catalysts with respectto the sulfate content. The Fe 2p peak had a bindingenergy of 710.9 eV, as in Fe2O3, in agreement withthe literature [1,12]. The O 1s peak was asymmetri-cal and could be decomposed in two components, theone with BE of 530.2 eV assigned to O2− in ZrO2,and the second one, less intense, with BE of 532.1 eV,due to oxygen belonging to sulfate species. The S 2pdoublet had components with BE values of 169.1 and170.3 eV (169.6 and 170.8 eV in zirconium sulfate).

Quantitative XPS analysis (Table 2) gave nearlyidentical Fe/Zr atomic ratios, except for the sam-

Table 2XPS atomic ratios of FSZ catalysts with different sulfate contents

Sample % SO42− a Fe/Zr S/Fe S/Zr S/(Zr+ Fe)

FSZ56 2.5 0.012 6.14 0.07 0.07FSZ57 5.0 0.015 8.77 0.13 0.13FSZ58 7.5 0.013 13.31 0.17 0.17FSZ59 10.0 0.019 8.87 0.17 0.16

a Nominal sulfate content.

ple with 10% SO42− which exhibited a significantlyhigher value. The S/Fe and S/Zr atomic ratios in-creased with increasing sulfate content, with similarS/Zr ratios for the catalysts with 7.5 and 10% SO4

2−.The lower S/Fe ratio and the higher Fe/Zr ratio for thesample containing 10% SO42− (above the monolayercoverage) could reflect a heterogeneous distributionof the sulfate species at the surface of the catalyst,possibly indicative of di-sulfate groups, in line bothwith the IR features (higher absorption and complex-ity of the spectra between 1400 and 950 cm−1 andthe band developing around 1205 cm−1), the lowersulfate decomposition temperature, and the lowercatalytic activity compared with the sample with7.5% SO4

2−.The BET surface areas (Table 1) increased with in-

creasing sulfate content up to 7.5% SO42− with, as

the XPS S/Fe atomic ratio, a lower value for the sam-ple with 10% SO42−. The most active catalyst hadthe highest surface area and S/Fe ratio, as shown inFig. 10. The total acidity (mmol NH3 g−1) regularlyincreased with the sulfate content, with closer valuesfor the samples with 7.5 and 10% SO4

2− (Table 1). Asimilar trend was observed by other authors [46,47].As shown in Fig. 11, there was a nearly linear rela-tionship between the total acidity normalized to thesurface area (mmol NH3 m−2) and the sulfate content.


Fig. 10. Relationship between maximum conversion and S/Fesurface ratio, and sulfate content.

For all the samples, themmol NH3/mmol SO42− ratio

did not exceed 0.5, corresponding to an average of atleast two sulfate groups per acid site. This ratio wasthe lowest (0.36) for the sample with the highest sul-

Fig. 11. Total acidity (from ammonia-TPD) and acidity normalizedto the specific surface area vs. sulfate content.

fate content, owing probably to the sulfate in excessof the monolayer coverage.

From the results of Fig. 4 and Table 1, the timeto reach maximum conversion (induction period)linearly decreased with increasing total acidity. Ithas been proposed that an active catalyst requires aproper balance of the acid and redox functions [21].Since no significant difference of the XPS Fe 2p peakwas observed for the catalysts with different sulfatecontents, one may assume similar contents of redoxsites and, accordingly, the better results obtained overthe sample with 7.5% SO42− would traduce a morefavorable balance (better synergism) of the catalyticfunctions. It is not excluded that by tuning the sul-fate content in order achieve complete monolayercoverage (somewhere between 7.5 and 10%), theconversion would be further improved. The inactivityof the sample with 2.5% SO42− clearly indicated thatthe acidity alone was not sufficient to impart catalyticactivity.

4. Conclusions

Sulfated zirconias (4.8% SO42−) promoted with 2%Fe provided more active catalysts compared with Mnand Cr, or mixed Mn–Fe, Cr–Fe and V–Fe-promotedones, all with 2% total metal content. Promotion withmore than 2% Fe gave less efficient catalysts. For acatalyst with 2% Fe and 4.8% SO4

2− pre-calcined at600◦C, the maximum conversion and induction periodwere influenced by the activation temperature, with anoptimal activation temperature of 350◦C.

The sulfate content affected both the maximum ofconversion and induction period. The most active cat-alyst with 2% Fe was obtained for 7.5% SO4

2−. Thiscontent corresponded to the nearly complete consump-tion of the isolated OH groups (band at 3740 cm−1) inthe formation of Zr–O–S bonds and characterized bya S=O stretching band at 1394 cm−1, probably gener-ating stronger acid sites than for a sample with lowsulfate content (2.5%), inactive, and where the S=Oband was located at 1370 cm−1, leaving the isolatedOH groups little affected. The most active catalyst alsoexhibited the highest specific surface area and S/Fesurface atomic ratio, but not the highest acid content,probably due to a better balance (or synergism) of theredox and acid sites.


Acknowledgements

J.A. Moreno gratefully acknowledges Colciencias(Colombia) for financial support.

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n-butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal...

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