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Journal of Colloid and Interface Science 272 (2004) 378–383www.elsevier.com/locate/jcis

Sulfated nanozirconia: an investigation on acid–base propertiesandn-butane isomerization activity

H.K. Mishra,a A.K. Dalai,a,∗ D.D. Das,a K.M. Parida,b and N.C. Pradhana

a Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan,105 Maintenance Road, S7N 5C5 Saskatoon, Canada

b Environment Management and Inorganic Chemicals Department, Regional Research Laboratory, Bhubaneswar, 751 013 Orissa, India

Received 11 June 2003; accepted 4 November 2003

Abstract

Hydrated zirconia was synthesized by an organo–inorganic route employing surfactant and was sulfated using aqueous ammsulfate, followed by drying at 110◦C. The sample thus obtained was calcined at 600◦C to obtain sulfated zirconia and was characterizedseveral physicochemical methods. Crystallite sizes of sulfated zirconia were calculated from X-ray line broadening using the Debyequation and were found to be in the range of 25 nm. When pretreated in air, the catalyst was found to exhibit butane isomerizatiat a temperature as low as 35◦C under atmospheric pressure. It showed conversion as high as 37% at 100◦C under normal pressure whepretreated in air, whereas nitrogen-pretreated catalyst showed zero activity under similar conditions. NH3 and CO2 temperature-programmedesorption studies on air- and helium-pretreated samples indicated that the catalyst surface changes appreciably during air pResults on butane isomerization in conjunction with TPD studies suggest that zirconium–oxy sites play an important role in butaneduring the reaction. 2003 Elsevier Inc. All rights reserved.

Keywords: Sulfated zirconia; Pretreatment effect; Acidity; Basicity, zirconium–oxy species; Butane isomerization

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1. Introduction

Solid acids such as alumina, silica–alumina, and titafind vast applications in pharmaceuticals and refineriecatalysts, and also as supports for other active phasesSulfate-modified metal oxides, which are a class of strsolid acid catalysts, are found to be active for alkane isomization at relatively lower temperatures than the commerchlorided–platinated–alumina catalyst [3]. Among variosulfated metal oxides, sulfated zirconia has attracted mattention and has been extensively investigated duringlast two decades [4,5]. A large number of publications dwith its preparation and physicochemical characterizatiwhich could reveal the unique characteristics of this maial. However, the major concern of this research still focuon the acidity, in terms of types and strength.

The method of temperature-programmed desorp(TPD) of a base such as ammonia has been widely use

* Corresponding author.E-mail address: dalai@engr.usask.ca (A.K. Dalai).

0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2003.11.003

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the determination of the number as well as strength ofsites [6,7]. It was proposed that ammonia TPD was not sable for determining acidity of sulfated zirconia, as ammodecomposes sulfate at elevated temperatures [8]. Howmany researchers have used this technique in recent yeathe determination of the acidity of zirconia, zirconia–titanand their sulfated compositions [9,10]. On the other hathe basic property of sulfated zirconia is rarely reportedthe literature. In a recent work, Bolis [10] carried out C2TPD on both pure and sulfated zirconia and found thatsorption of CO2 modifies the surface with carbonate- andbicarbonate-like species. Based on their observationsconcluded that sulfation does not entirely change the bcharacteristics of zirconia.

Sulfated zirconia catalyst, promoted with iron and mganese, has shown much higher activity and could isomen-butane at 35◦C under normal pressure in a continuoflow recirculation tank reactor. Also, it was proposed tiron–oxy species were the active sites [11]. No work sohas claimed that sulfated zirconia can isomerizen-butane atrelatively lower temperatures (∼35◦C) under atmospheri

H.K. Mishra et al. / Journal of Colloid and Interface Science 272 (2004) 378–383 379

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Fig. 1. Schematic representation of then-butane isomerization experimental setup: (1) 20%n-butane in nitrogen; (2) carrier gas; (3) pretreatment g(4, 5, 6) needle valves; (7, 8) mass flow meters; (9, 10, 11, 12, 15) check valves; (13) mixing chamber; (14, 28) gas to fume hood; (16) furnacture controller; (17) reactor temperature indicator; (18, 20) thermocouples; (19) rubber septum for injection; (21) electric furnace; (22) catalyst; (23) quartzwool; (24, 27) two way valve; (25) bypass line; (26) glass sampler.

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pressure in a continuous-flow tubular reactor. In additthere are speculations that sulfated zirconia is unablactivate butane at room temperature and atmosphericsure [12].

In this work, attempts have been made to change theface characteristics of sulfated nanozirconia by pretreatmwith air and nitrogen and to study butane isomerizationder mild reaction condition. Results of these experimestrongly indicated that air treatment of sulfated nanozircogenerates reactive zirconium–oxy species that act astemperature butane activation sites.

2. Materials and methods

Sulfated nanozirconia was prepared by wetness impnation of zirconium hydroxide with ammonium peroxdisulfate, followed by drying at 110◦C and calcination a600◦C for 3 h under static air. The hydroxide was prepaby mixing a saturated solution of zirconium oxychloriwith a solution containing ethanol, sodium dodecyl sfate (SDS), acetyl acetone, and ammonium hydroxide.whole content was autoclaved at 110◦C for 6 h under stirringconditions, cooled to room temperature, filtered, waswith water followed by ethanol (to make it free from SDas confirmed by FTIR), and dried overnight in an oven110◦C.

Surface area, porosity, and pore size distributions of600◦C-calcined sample were measured in an automnitrogen adsorption–desorption apparatus (ASAP 2000,

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cromeritics) at liquid nitrogen temperature (−196◦C) usingnitrogen as the adsorbate. Before measurement, theple was degassed at 200◦C and 10−4 Torr. The oxygenchemisorption study, at liquid nitrogen temperature, walso performed in the same apparatus. Elemental anaof the 600◦C calcined sample was carried out usingICP technique at the Saskatchewan Research Council (SCanada.

Ammonia TPD of the calcined sample was carried oua TPD analyzer (CHEM-BET 3000, Quanta Chrome CIn a typical experiment, about 0.15 g of a calcined sulfazirconia sample was heated to 450◦C in helium (or air) andcooled to room temperature. Then 1 mol% ammonia introgen was allowed to flow through the sample for 30 mThen helium gas was passed over the catalyst for 30to expel any physisorbed or weakly adsorbed ammonia.catalyst was then heated to 800◦C in helium, and the released gas was analyzed in a thermal conductivity deteSimilar experiments were also done using carbon dioxidan adsorbate.

A butane isomerization reaction was carried out in anfixed-bed tubular reactor (id 8 mm) at atmospheric pressFig. 1 shows the schematic of the experimental setup. Pto reaction, the catalyst was pretreated with air (or nitrogat 450◦C for 3 h. Reactions were carried out in the tempature range from 35 to 250◦C and at a WHSV of 0.45 h−1

using nitrogen as the carrier gas. It may be noted herethe butane gas used contained 20% butane in nitrogenproduct gases were analyzed in a Carl GC; the detailsdescribed in our previous report [13].

380 H.K. Mishra et al. / Journal of Colloid and Interface Science 272 (2004) 378–383

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Fig. 2. X-ray diffraction pattern of 600◦C calcined sulfated nanozirconshowing tetragonal (T) and monoclinic (M) phases.

Fig. 3. BJH pore size distribution pattern of sulfated zirconia calcine600◦C.

3. Results

The calcined sulfated zirconia was characterized byemental analysis and was found to contain 1.4 wt% su0.05 wt% iron, 0.1 wt% nickel, and 0.2 wt% sodium (sTable 1). The tetragonal crystallite size, determined fromX-ray diffraction study (cf. Fig. 2) using the Debye–Scheequation, was 25 nm. The BET surface area and poreume are 162 m2/g and 0.39 cc/g, respectively. The BJH [14pore size distribution (cf. Fig. 3), calculated from the dorption branch of the isotherm, shows a broad distribupattern with an average pore diameter of 96 Å. The Leand Brønsted acidities of zirconia (determined by the FPAS method) [15] were 9.0 and 10.9, respectively. The ltemperature oxygen chemisorption value was 18.24 cc/g atSTP. The improved structural and textural characteristicsattributed to the highly porous zirconia network. The Tpatterns of the sample pretreated with air showed anpreciably higher adsorption capacity toward ammoniacarbon dioxide than the ones pretreated with nitrogen.

Table 1Physicochemical properties of sulfated zirconia calcined at 600◦C

BET surface area (m2/g) 162Pore radius (Å) 48.0Pore volume (cc/g) 0.39Lewis zirconiaa 9.0Brønsted zirconiaa 10.9Sulfur content (wt%) 1.45Nickel (wt%) 0.1Iron (wt%) 0.05

a Measured from FTIR photoacoustic study of adsorbed pyridine.

Fig. 4. Effects of air and nitrogen pretreatment on butane isomerizatiotivity of sulfated nanozirconia at 100◦C.

Table 2Effects of reaction temperature on butane conversion and carbon depo

Reaction Maximum conversion Carbontemperature (◦C) (mol%) (wt%)a

100 37 0.0775 7.5 0.3750 2.0 0.3935 1.4 0.42

a Carbon content in the catalyst after 5 h of time-on-stream butanemerization study.

Time-on-stream butane isomerization experiments wcarried out using 600◦C calcined catalyst samples. The sfated zirconia catalyst pretreated in nitrogen at 450◦C for3 h showed no activity at 100◦C, whereas air-pretreatecatalyst under identical conditions gave 37 mol% consion (cf. Fig. 4). The activity of the air-pretreated calyst was found to decrease drastically at temperatbelow 100◦C and only 1.4 mol% conversion could bobtained at 35◦C (cf. Table 2). The carbon contentsthe spent catalysts were found to be quite high folow-temperature isomerization process and surprisinglycreased with decreases in reaction temperature below 10◦C(cf. Table 2).

H.K. Mishra et al. / Journal of Colloid and Interface Science 272 (2004) 378–383 381

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4. Discussion

Activation of butane on the sulfated zirconia catalsurface was reported to occur at different acid sites sas Brønsted and Lewis acid sites [16,17]. Interestinthe role of basic sites was never invoked in the previliterature, although a great amount of research on mepromoted sulfated zirconia dealt with the possible involment of oxy species on the catalyst [11]. For instanlow-temperature butane isomerization activity over iromanganese-promoted sulfated zirconia was reportedlier and was attributed to iron–oxy species, which wformed during air activation of the catalyst [11]. Zirconand sulfated zirconia were also found to have zirconium–species such as Zr+4O−2, Zr+4O−, and Zr+4O−

2 [18,19].However, to the best of our knowledge no report hasfar claimed that these species can activate butane at 3◦Cunder atmospheric pressure. Rather, it was believedzirconium–oxy sites could not activate butane near rotemperature [12]. In an earlier work Moro-Oka [20] pointout that nucleophilic oxy ions such as O2−, O−

2 , and O−are highly reactive and can abstract hydrogen from srated hydrocarbons at room temperature to generate alkMoreover, these oxy species do not oxidize alkenes [From these reports and our results on butane isometion over air- and nitrogen-pretreated sulfated zirconiaFig. 4), we propose that air activation of the catalyst gerates zirconium–oxy species, which are responsiblebutane activation at low temperature (�100◦C). However, atthis point, it is hard to predict which of the above-proposoxy species of zirconium is most responsible for butanetivation or if all of these species are equally responsiblemay be noted here that the catalyst used in our work con0.1 wt% nickel and therefore one can argue that butanetivation takes place on the nickel–oxy sites. This confusis eliminated based on the earlier report that pure sulfzirconia was active for butane isomerization at 100◦C whenpretreated in air [22].

In earlier investigations, Chen and co-workers [19] pposed that high-temperature air activation of sulfated zinia produces oxygen ion via one-electron oxidation of Z3+sites as

(1)Zr3+ + O2 Zr4+ + O−2 .

This fact is further supported by some recent electron pmagnetic resonance studies showing that air-activatedfated and pure zirconias contain Zr3+, Zr4+O−

2 species [18].It has also been reported that oxygen species having ntive charges behave as nucleophiles. In addition, the reabetween O−2 and RX, where RX is an alkyl halide, wareported as a nucleophilic substitution reaction [20]. A psible mechanism involving these nucleophilic species is

(2)C4H10 + Zr4+ + O−2 C4H•

9 + Zr3+ + OOH,

(3)C4H• + Zr4+ + O− C4H8 + Zr3+ + OOH.

9 2

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t

.

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Fig. 5. Effects of pretreatments on NH3 and CO2 TPD of sulfated zirconiacalcined at 600◦C: (A) air-pretreated; (B) helium-pretreated.

To complete a catalytic cycle the reduced zirconium spehas to reoxidize, which may be described by the followequation:

(4)2Zr3+ + 2OOH 2Zr4+ + 2O−2 + H2.

Evolution of dihydrogen duringn-butane isomerization wareported [23].

To support our view that air pretreatment of the calyst generates zirconium–oxy species, carbon dioxideammonia TPD were carried out on both air- and heliupretreated sulfated nanozirconia. It was observed thaammonia adsorption capacity of the air-pretreated samphigher than that of the one pretreated in helium (cf. Fig. 5)a recent study, Bathos et al. [6,7] carried out ammonia Tmeasurement on H-ZSM-5, SO−2

4 /ZrO2, and SO−24 /ZrO2–

TiO2 and, based on the results and a frequency respspectroscopic study of ammonia-adsorbed samples,concluded that ammonia is adsorbed onto the surfacesites, in addition to the Brønsted and Lewis acid sites ofcatalysts. It was also mentioned that ammonia from thoxy sites was released at a lower temperature than thossorbed on the Lewis acid sites. They proposed that hydroin ammonia interacts with the basic oxy sites through hydgen bonding. In our TPD experiments (cf. Fig. 5, patternammonia desorption from the air-pretreated sample staaround 250◦C, whereas it started around 360◦C in the caseof the helium-pretreated catalyst (cf. Fig. 5, pattern B). Tobservation agrees well with the one reported by Bathoal. [6] and, therefore, low-temperature ammonia desorp

382 H.K. Mishra et al. / Journal of Colloid and Interface Science 272 (2004) 378–383

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Fig. 6. Effects of carbon dioxide on time-on-stream butane isomerizati200◦C over sulfated zirconia. “Fresh” in the figure (inset) represents buisomerization results over an air-pretreated fresh catalyst, whereasgenerated” represents isomerization results over air-regenerated suzirconia catalyst.

Scheme 1.

from the air-pretreated sample is attributed to the ammattached to zirconium oxy sites, that are formed duringpretreatment. Formation of zirconium oxy species duringpretreatment of the catalyst was further confirmed byCO2 TPD, which showed a large peak (cf. Fig. 5, patternin the temperature range 250–450◦C due to CO2 desorptionfrom the weakly basic oxy sites. The peak centered at 55◦Cis attributed to the formation of surface carbonate spe[24,25].

To further confirm that these oxy species participate inbutane isomerization reaction, a time-on-stream studyperformed in which a freshly air-pretreated catalyst wasposed to a mixture ofn-butane, nitrogen, and carbon dioxiat 200◦C; the details of the experimental procedure aresented in Scheme 1. It may be noted here that the timstream “0” in the case of regenerated catalyst (cf. Figinset) refers to the 14.5th hour in Scheme 1. The catashowed negligible conversion (∼2 mol%) for a period of9 h and the activity even went down when CO2 was re-

d

moved, whereas the catalyst on regeneration in air at 45◦Cshowed a conversion of 24 mol% (cf. Fig. 6, inset). Tlower activity of the air-regenerated catalyst compared toair-pretreated fresh catalyst may be due to the presencarbonate species, which are not decomposed at 450◦C, asshown in the CO2 TPD (cf. Fig. 6, pattern A). These obsevations strongly support the involvement of zirconium osites in butane isomerization. We also propose that buactivation at low temperature (�100◦C) probably occurs bythe abstraction of hydrogen from the butane moleculethese active oxy species.

One can ask why, if zirconium–oxy sites of the sulfazirconia cluster are responsible for low-temperature buisomerization, the conversion is only 1.4 mol% at 35◦C,whereas iron–manganese-promoted catalyst is more aunder similar reaction conditions [11]? To account forlower activity of sulfated zirconia catalyst we propose tbutane conversion depends not only on active sites buton the release of surface-adsorbed reaction intermediatthe strength of the active sites is very high then they canmanently adsorb the reactant molecule, thus hinderinthe subsequent processes. To support our view that thtane molecules get adsorbed on the active sites of sulzirconia, we studied butane isomerization in the tempture range of 35–100◦C and estimated carbon contentthe spent catalysts. Interestingly, we found that carbontent increased drastically below 100◦C (cf. Table 2). Basedon these results we suspected that high carbon contethe spent catalyst could be due to adsorption of hydrobon onto the catalyst surface. The source of carbon inspent catalyst could also be from coke deposition. Hever, this possibility is eliminated based on the fact tduring alkane isomerization coke deposition over an acatalyst (sulfated zirconia in this case) is most likely dto cracking of hydrocarbons [26]. Moreover, the crackprocess, being endothermic, becomes highly unfavorablow reaction temperatures. Therefore, we conclude thatbon deposition in the spent catalysts is due to adsorptiobutane onto the zirconium–oxy sites, most likely as butspecies [27].

5. Conclusions

The experimental facts suggest that nanosized sulfzirconia with high surface area and mesoporosity can betained by following a surfactant-based synthesis route,ammonium persulfate as the source of sulfate can beto generate high acidity on the catalyst surface. Stabof the reaction intermediates on the catalyst surface dmines the low-temperature activity of the sulfated zircocatalyst and surface oxy species play an important rolthis regard. Knowledge of coordination between the acbase functionalities of the sulfated zirconia catalyst wobe helpful in designing stable catalysts for alkane transmation.

H.K. Mishra et al. / Journal of Colloid and Interface Science 272 (2004) 378–383 383

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Acknowledgments

Financial support from Petro-Canada Ltd. and the Nural Sciences and Engineering Research Council of Ca(NSERC) for this research is acknowledged.

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