superacidBCl3/silica–alumina

Marek Marczewski,* Marzena and Hanna MarczewskaDeÓ bowiak

W arsaw University of T echnology, Chemistry Department, Noakowskiego 3, 00-662 W arsaw,Poland

Received 17th November 2000, Accepted 19th January 2001First published as an Advance Article on the web 27th February 2001

The solid superacid of type was studied. It was found that a suitable carrier for superacidBCl3/silicaÈaluminasynthesis with vapours should possess OH groups with strong acid properties situated in the proximity ofBCl3strong Lewis centres. Superacid centres are formed as a result of the reaction between adjacent OHBro� nstedgroups (at least one of which should be of high acid strength) of the carrier and BCl3 .

Superacids are potentially very interesting catalysts for acidÈbase transformations of organic compounds. Di†erent reac-tions of alkanes such as isomerization, alkylation or crackingproceed in their presence at room temperature.1 Classic super-acid solutions are mixtures of strong protic acids like FSO3Hor HF with Lewis acids : or Such liquids,SbF5 , TaF5 BF3 .2besides their high acid strength, are corrosive and difficult torecover which considerably limits their large-scale use. Theapplication of solid superacids could solve these problems.Such a class of catalysts has already been obtained. It includeschlorinated metal oxides,3,4 metal oxides modiÐed with Lewisacids,4h14 the products of metal salts reactions with metalchlorides and bromides,15,16 complexed cationicAlCl3resins,17,18 Lewis acids intercalated in graphite,19h21 sulfatedmetal oxides22 and NaÐon HÈthe commercial perÑuorinatedsulfonic resin from DuPont.23 All the above systems fulÐl atleast one of the two generally accepted criteria of super-acidity : acid strength higher than that of H2SO4 (HO \ [12)or the ability to catalyse low temperature isomerization of n-alkanes. The great variety of solid superacids, in which

centres,8h10,14,17,18,23 Lewis sites6,11h13 or redoxBro� nstedsites24h26 are responsible for the catalytic activity, explainswhy there is no uniform theory explaining the rules for theirsynthesis or the mechanism of their catalytic action. Somegeneralisations can only deal with a particular class of super-acids.

ModiÐcation of oxide carriers with vapours of metal halidesis a convenient method for the synthesis of solid super-acids.9h14 Surface hydroxy groups of a carrier react with ahalide, forming potential acid centres whileMX

n, Bro� nsted

surface electron-donating oxygen anions complex with MXnmolecules diminishing the nucleophilic character of the

surface. Examples of the reactions that proceed on an aluminasurface are depicted below.

We have formulated an approximate rule which helps topredict the nature of such synthesised superacids.27 If the acidstrength, expressed in terms of the partial positive chargelocalised on hydrogen, of a hypothetical acid is[MX

n`1]~H`

higher than that of sulfuric acid, then the centresBro� nstedobtained possess superacid character. Metal oxides modiÐedwith or vapours9,10 are examples of such systems.SbF5 BF3On the other hand, the action of metal halides like AlCl3 ,11

or and for which the acidity ofZrCl412 TiCl4 HfCl4 ,13molecules is lower than for results in[MX

n`1]~H` H2SO4 ,the formation of catalysts having Lewis-type superacid activecentres :

The strength of the Lewis site depends on the acceptorproperties of the metal halide used. The above rule is illus-trated in Fig. 1 where partial charges localised on hydrogen in

hypothetical molecules, calculated according to[MXn`1]~H`

Sanderson methodology,28 are compared with the experimen-tally determined nature of the acid centres of MX

n/carrier

superacids. The approximate nature of the rule is seen particu-larly for the systems obtained by the reaction of metal halidesfor which the acid strength of is close to the[MX

n`1]~H`acidity of Similar examples have been observed forH2SO4 .

and reactions with alumina and HZSM-5, respec-ZrF4 BCl3tively. In spite of the high acid strength of higher[ZrF5]~H`,than for the system obtained is characterised by theH2SO4 ,presence of Lewis superacid centres.13 Similarly, although themolecule is less acidic than there is spec-[BCl4]~H` H2SO4troscopic evidence for the existence of superacid sitesBro� nstedin the structure of HZSM-5/BCl3 .29,30

In our previous work14 we presented preliminary resultsconcerning a catalytic system. It was shown thatBCl3/carrierthe reaction of with silicaÈalumina (87% results inBCl3 SiO2)the formation of superacid centres able to catalyse n-pentaneisomerization at 333 K. On the contrary, the reaction of BCl3

Fig. 1 Acidity of hypothetical acids of type expressed inMXn~H`

terms of partial positive charge localised on H atom.

DOI: 10.1039/b009256h Phys. Chem. Chem. Phys., 2001, 3, 1103È1107 1103

This journal is The Owner Societies 2001(

Publ

ishe

d on

27

Febr

uary

200

1. D

ownl

oade

d by

Uni

vers

ity o

f M

ichi

gan

Lib

rary

on

28/1

0/20

14 0

3:40

:54.

View Article Online / Journal Homepage / Table of Contents for this issue

vapour with alumina, silica and others silicaÈaluminas resultsin preparations of low or no catalytic activity. We concludedthat acid centres of SiÈOHÈAl type participate inBro� nstedsuperacid site formation.

The aim of this work was to study the structure and natureof the superacid sites formed and to determine the necessaryconditions which a carrier should fulÐl in order to form super-acid centres in the reaction with To this end we exam-BCl3 .ined di†erent catalysts possessing a controlledBCl3/carrierquantity of SiÈOHÈAl centres on the carrier surface (silicaÈalumina with 87% calcined at 373 and 753 K and theSiO2silicaÈalumina with the OH groups blocked by exchange withNa`). The results of both surface analyses (acidity, Cl content)and test reactions (n-pentane and n-hexane low temperatureisomerization and pent-1-ene double bond shift) were thebasis of our study.

Experimental(100 m2 g~1), (87 and (370 andc-Al2O3 SiO2 53%)ÈAl2O3130 m2 g~1) and (260 m2 g~1) supplied by Strem Chemi-SiO2cals and (250 m2 g~1) obtained by calcina-SiO2 (94%)ÈAl2O3tion at 823 K of the respective hydroxides prepared from a

propan-2-ol solution of aluminium isopropoxide and siliconethoxide (Fluka), of 1.02È1.2 mm grain fraction were used ascarriers in the superacid syntheses. HZSM-5 zeolite of

ratio equal to 35 (Institute of Industrial Chem-SiO2/Al2O3istry, Warsaw) was used as the reference catalyst with welldeÐned acid centres. Catalyst synthesis was per-Bro� nstedformed in a batch reactor (60 ml) in three steps : carrier calcin-ation (1 g, 753 K, 1.3 ] 10~2 Pa, 3 h) followed by cooling thesystem to 333 K, reaction with (Aldrich) at saturatedBCl3vapour pressure in 273 K (63.595 kPa) for 10 min and evac-uation (5 s). After the synthesis 0.4 ml of n-pentane, n-hexaneor pent-1-ene (all supplied by Aldrich) was injected into thereactor. Analysis of the reaction products was performedusing a gas chromatograph Ðtted with a squalane capillarycolumn. Product identiÐcation was based on comparison ofreactant retention times with those of standards.

For Cl analysis the catalysts were boiled with water and theresultant solutions were then analysed by Volhard titration.

Acid centre concentration was measured using a baseadsorption and ionic exchange methods. Freshly preparedcarrier and catalyst samples (1 g) were contacted with : 0.1 Mtoluene solution (20 ml) of n-butylamine at 298 K; 0.1 Maqueous solution (20 ml) of sodium acetate at 298 or 373 K(only carrier) ; 0.1 M aqueous solution of NaOH at 298 K(only carrier). After 20 h the decanted solutions were analysedby titration with hydrochloric acid (n-butylamine), sulfuricacid (NaOH) and sodium hydroxide (NaOAc). The amount ofbutylamine and NaOH used was taken as a measure of the

quantity of acid centres. The exchange reaction with NaOHmay be accompanied by silica dissolution. We found thatunder the applied reaction conditions base consumption canbe interpreted in terms of Na` exchange. The amount ofacetic acid evolved during the NaOAc reaction was taken as adirect measure of the concentration of OH groups of appro-priate acid strength.

ResultsIn order to explain the mechanism of formation of activecentres by action on silicaÈalumina (87% theBCl3 SiO2)physicochemical properties of both the carrier and the respec-tive system were examined. The acidity of theBCl3-containingcarrier used was measured by means of ionic exchange with

(NaOAc) and NaOH solutions. AdditionalCH3COONaexperiments including n-butylamine adsorption from toluenesolution were also performed (Table 1).

The yield of surface OH groups exchanging protons withNa`, under di†erent reaction conditions, could be a measureof the quantities of the di†erent centres. The low temperatureexchange (at 298 K) proceeds with the strong sites.Bro� nstedThe strong and medium centres react at higherBro� nstedtemperature, 373 K, with NaOAc solution while all proticsites are exchanged at 298 K with the more basic NaOHsolution. All acid sites adsorb n-butylamine. The resultsindicate that the carrier studied possesses centres ofBro� nsteddi†erent strength. The higher yield of NaOH exchange thann-butylamine adsorption indicates that sodium hydroxidereacted with OH groups of lower acid strength than didn-butylamine. Independent measures of the acidÈbaseproperties are made by the use of model reactions. N-pentaneand n-hexane isomerization at 333 K was chosen for super-acid site detection and the double bond shift in pent-1-ene at333 K was chosen as a general test for acid/base properties.The results indicate that the silicaÈalumina carrier possessesno superacid centres. The acid sites present on the surface areonly able to catalyse the pent-1-ene to pent-2-ene reaction. Toverify the type of sites active in pent-1-ene isomerization thereaction was performed in the presence of silicaÈaluminas with

content equal to 94, 87 and 53% and LewisSiO2 (Bro� nstedacids) and with pure (a neutral solid). All silicaÈaluminasSiO2catalysed pent-1-ene isomerization with the initial rate beingdependent on the concentration of strong acid Bro� nstedcentres (measured by NaOAc exchange at 298 K). Theobserved linear correlation (Fig. 2) allows us to assign pent-1-ene double bond isomerization to the action of strong proticsites of silicaÈaluminas.

strongly inÑuences the properties of the carrier. OneBCl3can observe an increase in acidity as measured both by

Table 1 Acid properties of carriers and systemSiO2(87%)ÈAl2O3 BCl3/SiO2(87%)ÈAl2O3Properties

SuperacidityCatalyst alkane convc

Protic acidity/mmol g~1 (%)SiO2(87)ÈAl2O3 Cl content n-Bubcarrier treatment /mmol g~1 strong medium totala ads/mmol g~1 Pent Hex

Calc. 773 K, 0.00 0.23 0.21 1.47 1.21 0.0 0.0NaOAc exchange 298 K, calc. 773 K 0.00 0.00 0.21 1.26 0.0 0.0NaOAc exchange 373 K, calc. 773 K 0.00 0.00 0.00 1.03 0.0 0.0Calc. 773 K, BCl3 adsorption 2.32 1.73 57.3 66.8NaOAc exchange 298 K, calc. 773 K 2.63 Method not applicable 0.0 0.0

BCl3 adsorptionNaOAc exchange 373 K, calc. 773 K 3.08 0.0 0.0

BCl3 adsorption

a NaOH exchange. b n-Butylamine adsorption from toluene solution. c n-Pentane conversion into 2-methylbutane and 2-methylpropane, n-hexane conversion into methylpentanes, 2-methylbutane and 2-methylpropane. Reaction time 20 h. Temperature 333 K.

1104 Phys. Chem. Chem. Phys., 2001, 3, 1103È1107

Publ

ishe

d on

27

Febr

uary

200

1. D

ownl

oade

d by

Uni

vers

ity o

f M

ichi

gan

Lib

rary

on

28/1

0/20

14 0

3:40

:54.

View Article Online

Fig. 2 Dependence of : initial rate of pent-1-ene isomerization in thepresence of silicaÈalumina carriers (1), superacidsBCl3/silicaÈalumina(2) and initial rate of pentane isomerization in the presence of

superacids (3) on the concentration of strongBCl3/silicaÈaluminaacid centres on silicaÈalumina carriers. a, silicaÈaluminaBro� nsted

(87% b, silicaÈalumina (94% c, silicaÈalumina (53%SiO2) ; SiO2) ;SiO2).

n-butylamine adsorption (Table 1) and by the appearance ofactivity in n-pentane and n-hexane low temperature isomer-ization (Table 1). These results indicate that superacid centresare formed on the carrier surfaces under the inÑuence of BCl3 .It is worth noting that the samples obtained by reactionBCl3with the carrier having protic centres of only medium or lowerstrength (NaOAc exchanged preparation) did not possesssuperacid sites.

Introduction of onto (94, 87 andBCl3 SiO2 53%)ÈAl2O3surfaces results in a decrease in the pent-1-ene isomerizationrate (Fig. 2). This unexpected phenomenon occurs because ofthe increase in the rate of the competitive alkene reaction, i.e.,coking, with increasing acid strength of the active centres. Asin the case of the reaction catalysed by pure silica-aluminas alinear correlation exists between the initial rate of pent-1-enereaction and the quantity of strong centres presentBro� nstedin carriers (Fig. 2).BCl3

DiscussionBoron halides react easily with di†erent molecules due to thehigh reactivity of the BÈX bonds and the strong electron-acceptor nature of the molecule. Thus, for example, boronchloride reacts with compounds containing OH groups suchas alcohols31 and with molecules having atoms of a nucleo-philic nature such as oxygen in ethers32 or halogen in organichalides.33

The OH groups as well as the negatively charged latticeoxygens are present on the surfaces of such carriers asalumina, silica and silicaÈaluminas. They are, like the alcoholor ether functional groups, potential reactants for mol-BCl3ecules. Possemiers et al.34h36 showed that the surface groupsof silica undergo such reactions forming surface chlorobo-ranes :

The work of Rhee and Peglara37,38 conÐrmed that similarreactions should also take place with alumina. A detailedstudy of adsorption in the HZSM-5 structure29 allowedBCl3the authors to postulate that the OH group, being a strong

acid centre, forms a complex with boron chlorideBro� nstedgiving a site with superacid properties.

It should be noted that the above structure is not probable.Due to the high reactivity of the boronÈchlorine bond, BCl3readily undergoes chlorine exchange reactions. For example,in a solution of hydrochloric acid boron chloride does notform an analogue of the well known ÑuoroboraneBCl4~,anion, but hydrolyses to Analogously, in theB(OH)3 .39reaction with perÑuoroalkanesulfonic acids the

superacid is not created butRFSO2OH2`(BCl3RFSO2O)~another superacid in which all the chlorine atoms arereplaced by perÑuoroalkanesulfonic groups, i.e.,

is formed.40 From this pointRFSO2OH2`[B(RFSO2O)4]~of view it is reasonable to predict that the superacid centreof the system is not a molecule com-BCl3/HZSM-5 BCl3plexed by a bridged OH group but rather boron chloridepartial hydrolysis products stabilised by the strong electro-static Ðeld of the zeolite lattice. Formally it is equivalent to the

acid site (IVa) in which hydrogen chloride resultingBro� nstedfrom the reaction of with OH remains bonded to theBCl3surface :

Based on the above consideration one can assume that for thesystem studied sites of type I, II and III,BCl3/silicaÈaluminabeing potential Lewis acid centres, should be formed as well assites of type IVa which possess protic character. Our resultsconÐrmed the role of carrier OH groups in the formation ofactive centres (Fig. 2). The linear correlation obtained betweenthe number of acid hydroxy groups of the support and thecatalytic activity of the catalysts in n-pentaneBCl3/carrierisomerization indicates that the superacid centres are formedin the reaction involving acidic OH groups. In order toconÐrm this phenomenon, the silicaÈalumina with SiO2content equal to 87% after ionic exchange with NaOAc solu-tion at 298 K was used as support. It turned out thatBCl3such a preparation did not possess any superacid sites (Table1). During the exchange procedure 0.23 mmol g~1 of strong

acid centres were blocked by Na` while 0.21 mmolBro� nstedg~1 weaker protic acid sites remained unchanged. Thus, it isevident that either all strong acid centres or a part of themparticipate in the superacid site formation. This Ðndingexcludes type III sites as a source of superacid properties.Such a centre results from a reaction that does not use upsurface hydroxy groups.

Fig. 3 The inÑuence of n-butylamine on pentane isomerization inthe presence of superacid.BCl3/SiO2(87%)ÈAl2O3

Phys. Chem. Chem. Phys., 2001, 3, 1103È1107 1105

Publ

ishe

d on

27

Febr

uary

200

1. D

ownl

oade

d by

Uni

vers

ity o

f M

ichi

gan

Lib

rary

on

28/1

0/20

14 0

3:40

:54.

View Article Online

The number of superacid centres can be estimated in a cata-lytic experiment in which the n-pentane is mixed with con-trolled amounts of n-butylamine, a poison of acid sites (Fig.3). Introducing n-butylamine sufficient to poison ca. 0.20mmol g~1 of acid centres stopped pentane isomerization. Thisvalue is close to the number of strong acid sitesBro� nstedpresent on the carrier surface and means that from one strongacid OH group of one superacid centre isSiO2(87%)ÈAl2O3formed in the reaction with Such stoichiometry can beBCl3 .explained by the following elementary reactions :

which result in and Lewis site formation.Bro� nstedAdditional experiments were performed in order to estab-

lish the structure and nature of the surface sites. Carriers cal-cined at di†erent temperatures can be treated as model solidsdi†ering in the nature and concentration of surface OHgroups. Silica-alumina calcined at 373 K has a fully hydratedsurface with OH groups close together. On the other hand onthe surface of a partially dehydrated carrier (heated to 753 K)there are both adjacent and isolated OH groups. Both sup-ports were reacted with For calcinedBCl3 . SiO2(87%)ÈAl2O3at 753 K the reaction can be depicted as follows :

On the basis of these reactions it is possible to write a molecu-lar balance of OH groups and chlorine :

OH \ M ] 2D

Cl\ 3M ] 2D

where OH, M, D and Cl denote concentrations : of OHgroups, monodentate and didentate boron species and chlo-ride respectively. Based on the OH and Cl values presented inTable 1 one can calculate the number of M and D moietiespresent on the catalyst surfaces (Table 2).

The results indicate that boron species of two types coexiston the surface, i.e., having boron bonded through one (M) andtwo oxygens (D). The elimination of strong acid OH groups(ionic exchange with NaOAc at 298 K) results in a decrease inthe number of D-type species. Further reduction of the con-centration of protic sites (ionic exchange with NaOAc at 373K) leads to complete disappearance of the D centres. Suchsamples are characterised by the presence of the M sites inwhich boron is Ðxed to the surface by only one oxygen. The

Table 2 Quantity of monodentate (M) and didentate (D) boronspecies on system surfaceBCl3/carrier

Surface species/mmol g~1

PentaneBCl3 M D isom.a (%)

SiO2(87%)ÈAl2O3 (753 K) 0.41 0.53 12.3exchanged at 298 K 0.70 0.28 0.0exchanged at 373 K 1.03 0.00 0.0SiO2(87%)ÈAl2O3 (373 K) none 1.54 7.4

a n-Pentane conversion into 2-methylbutane. Reaction time 20 h.Temperature 333 K.

observed lack of isomerizing activity of the exchanged cata-lysts (Table 2) allows one to conclude that the active superacidcentre is of the D type and should involve, at least, oneoxygen originating from the strong acidic OH group of thecarrier. The above hypothesis was conÐrmed by an indepen-dent catalytic experiment where carrier wasSiO2(87%)ÈAl2O3calcined before adsorption at 373 K. ThermogravimetryBCl3indicates that such a pretreated carrier is totally hydrated. It isevident that on such a surface there are no isolated hydroxygroups, preferred substrates for the M type species. Hence, inthe reaction with only D groups of (B) andBCl3 Bro� nstedLewis (L) acid nature can be formed.

The observed catalytic activity of this material (Table 2) indi-cates that both type of D acid centres, i.e., and LewisBro� nstedcan be responsible for the superacid properties.

The results of pent-1-ene isomerization allow indirect con-clusions on the nature of the superacid centres.41 The reactionwas carried out in the presence of alumina, silicaÈaluminasand HZSM-5 zeolite, being the model solid(SiO2/Al2O3\ 35)

acid. The initial ratio of cis- to trans-isomers ofBro� nstedpent-2-ene formed is an important parameter, which is inÑu-enced by the nature of the active site.41 For silicaÈaluminacarriers the ratios are limited to the range 0.45È0.77 and forHZSM-5 zeolite it is equal to 0.50. Only alumina, the carrierwhose surface does not possess strong acid centresBro� nsted(exchange reaction with NaOAc was not observed) is charac-terised by the initial twofold excess of cis pent-2-ene overtrans-pent-2-ene (Fig. 4).

On the basis of the above Ðndings one can conclude thatthe low value of the initial pent-2-ene cis/trans ratio is charac-teristic of the reaction mechanism in which pentyl cation, i.e.,the product of pent-1-ene interaction with a centre,Bro� nstedis an intermediate state. The high value of the ratio observedfor alumina reÑects the alternative reaction mechanism. In thiscase Lewis acid sites and electron-donor surface oxygens playthe role of the active centres.42,43 Reaction of with theBCl3silicaÈalumina carriers did not change the initial pent-2-enecis/trans ratio observed for pure carriers. Only for the

system can one observe a decrease in the ratioAl2O3/BCl3from 2 to 1 (Fig. 4). These Ðndings suggest that pent-1-ene

Fig. 4 Initial cis/trans pent-2-enes ratio for : (1) alumina and=(2) silicaÈalumina (53% andBCl3/alumina K, SiO2) =

(53% (3) silicaÈalumina (94%BCl3/silicaÈalumina SiO2)/K, SiO2) =and (94% (4) silicaÈalumina (87%BCl3/silicaÈalumina SiO2) K, SiO2)and silicaÈalumina (87% related to the concentra-= BCl3 SiO2) Ktion of strong acid centres.Bro� nsted

1106 Phys. Chem. Chem. Phys., 2001, 3, 1103È1107

Publ

ishe

d on

27

Febr

uary

200

1. D

ownl

oade

d by

Uni

vers

ity o

f M

ichi

gan

Lib

rary

on

28/1

0/20

14 0

3:40

:54.

View Article Online

isomerization proceeds according to the same mechanism forsilicaÈaluminas, and alumina treatedBCl3/silicaÈaluminaswith boron trichloride. A similar phenomenon has alreadybeen observed for the catalysts obtained by impregnation of

and with strong protic agents such as 12-ZrO2 SiO2ÈTiO2tungstophosphoric heteropolyacid and sulfuric acid.44,45 Inboth cases acid impregnation was accompanied by a changein the initial ratio of cis/trans but-2-enes from 1.4 to 0.9 andfrom 1.3 to 0.5, respectively. These changes were interpreted interms of a reaction mechanism change caused by the appear-ance of new strong acid centres not present on theBro� nstedcarriers before impregnation. Thus, we can also assume thatthe new superacid centres formed upon the action of onBCl3the support are of type.Bro� nsted

It is interesting to consider why the reaction of withBCl3silicaÈalumina results in the formation of superacid centreswhile the same reaction with alumina and silica along doesnot produce such sites. The proposed superacid centre struc-ture is similar to that characteristic for acid sites inBro� nstedperfect, i.e., crystalline silicaÈaluminasÈzeolites.

The high acid strength of the latter is explained by the strongelectron-withdrawing action of the adjacent strong electronacceptor centre, i.e. Lewis acid site. The same situation isobserved in the case of treated silicaÈalumina where twoBCl3Lewis centres interact with a site. Pure silica pos-Bro� nstedsesses no Lewis sites to activate proton-donating groups. Inthe case of alumina the situation is di†erent. On the surfacethere are strong Lewis centres but the lattice oxygens ofalumina are of a higher basicity than those of silica and silicaÈalumina. For example, the partial charge on the O atom, cal-culated according to the Sanderson method28 is [0.225 e in

and [0.358 e in The higher negative chargeSiO2 Al2O3 .localised on the alumina surface diminishes the ability of theproton to react with weak organic bases such as alkanes.This is equivalent to a substantial decrease in acid strengthof the corresponding acid centres. Our calculation of thepartial positive charge localised on hydrogen in the structureof the acid sites originating from the reaction ofBro� nsteddi†erent carriers with led to the expected acidityBCl3sequence :27 0.196 0.164BCl3/silicaÈalumina e[BCl3/silica0.146 e, which supports the resultse[BCl3/aluminaobtained. In view of the presented results we propose that :

acid centres are responsible for the superacidBro� nstedproperties of the system.BCl3/SiO2(87%)ÈAl2O3A superacid centre is formed in the reaction of withBCl3two adjacent OH groups of the carrier. At least one of thereacting OH groups should possess high acid strength.

A suitable carrier for superacid system synthesis with BCl3vapours should possess OH groups with strong acid proper-ties (able to react with NaOAc at 298 K) situated in the prox-imity of strong Lewis centres.

AcknowledgementThis work was partially sponsored by a grant from the PolishState Committee for ScientiÐc Researches (grant no. 3 TO9B034 17).

References1 G. A. Olah and A. M. Molnar, Hydrocarbon Chemistry, Wiley,

New York, 1995, pp. 105È110.

2 G. A. Olah, G. A. Prakhash and J. Sommer, Superacids, Wiley,New York, 1985.

3 A. G. Goble and P. A. Lawrance, in Proceedings of the 3rd Inter-national Congress on Catalysis, North-Holland, Amsterdam,1965, p. 320.

4 E. Garbowski, J. P. Candy and M. Primet, J. Chem. Soc.,Faraday T rans., 1983, 79, 835.

5 K. Tanabe and H. Hattori, Chem. L ett., 1976, 625.6 H. Hattori, O. Takahashi, M. Takagi and K. Tanabe, J. Catal.,

1981, 68, 132.7 S. Namba, K. Ihara, Y. Sakaguchi and T. Yashima, J. Jpn. Petr.

Inst., 1980, 23, 142.8 A. Roumegous and J. Le Page, Stud. Surf. Sci. Catal., 1980, 7,

1018.9 M. Marczewski, H. Marczewska and K. Bull. Soc.Witos¡awski,

Chim. Fr., 1991, 128, 366.10 M. Marczewski, H. Marczewska and K. J. Mol.Witos¡awski,

Catal. A, 1995, 97, 101.11 A. Krzywicki and M. Marczewski, J. Chem. Soc., Faraday T rans.

I, 1980, 76, 1311.12 W. Juszczyk and M. Marczewski, J. Chem. Soc., Faraday T rans.,

1992, 88, 3591.13 M. Marczewski and W. Juszczyk, J. Chem. Soc., Faraday T rans.,

1998, 94, 719.14 M. Marczewski, M. and K. Przerwa, Polish J. Chem.,DeÓ bowiak

1998, 72, 2115.15 N. Kitajima, S. Sakura and Y. Ono, J. Chem. Soc., Perkin T rans.

2, 1980, 1201.16 Y. Ono, T. Tanabe and N. Kitajima, J. Catal., 1979, 56, 47.17 V. L. Magnotta and B. C. Gates, J. Catal., 1977, 46, 266.18 G. A. Fuentes and B. C. Gates, J. Catal., 1982, 76, 440.19 N. Yoneda, T. Fukuhara, T. Abe and A. Suzuki, Chem. L ett.,

1981, 1485.20 H. Imamura and S. Tsuchiya, Z. Phys. Chem. Neue Folge., 1981,

125, 251.21 G. A. Olah and J. Kaspi, J. Org. Chem., 1977, 42, 3046.22 X. Song and A. Sayari, Catal. Rev.-Sci. Eng., 1996, 38, 329.23 G. A. Olah, in Acidity and Basicity of Solids, NATO ASI Series,

Ser. C, vol. 444, ed. J. Fraissard and L. Petrakis, Kluwer, Dor-drecht, 1994, p. 305.

24 P. L. Fabre, J. Devenyck and B. Catal. Rev., 1982, 82,Tre� millon,591.

25 D. A. Ghenciu and L. J. Qi, J. Catal., 1996, 158, 116.Fa‘ rcasÓ iu,26 M. T. Tran, N. S. Gnep, G. Szabo and M. Guisnet, Appl. Catal.

A, 1998, 171, 207.27 M. Marczewski, H. Marczewska and W. Juszczyk, Polish J.

Chem., 1999, 73, 509.28 R. T. Sanderson, Chemical Periodicity, Reinhold, New York,

1960.29 W. P. Fletcher, C. S. Gilbert and A. I. Biaglow, Catal. L ett., 1997,

47, 135.30 W. P. Fletcher and A. I. Biaglow, Catal. L ett., 1998, 54, 217.31 H. A. Nguyen and J. R. Kennedy, Polym. Bull., 1981, 6, 47.32 A. Scheurer, P. Mosset and R. W. Saalfrank, T etrahedron : Asym-

metry, 1997, 8, 1243.33 R. Russell, M. Moreau, B. Chorleaux, J. R. Vairon and K. Maty-

jaszewski, Macromolecules, 1998, 31, 3782.34 K. Possemiers, K. C. Vrancken, P. Van Der Voort and E. F.

Vansant, J. Chem. Soc., Faraday T rans., 1995, 91, 2173.35 K. Possemiers, P. Van Der Voort and E. F. Vansant, J. Chem.

Soc., Faraday T rans., 1996, 92, 679.36 K. Possemiers, N. R. E. N. Impens and E. F. Vansant, J. Chem.

Soc., Faraday T rans., 1997, 93, 2309.37 K. E. Rhee and R. Basila, J. Catal., 1968, 10, 243.38 R. J. Peglar, F. H. Hambleton and J. A. Hockey, J. Catal., 1971,

20, 309.39 P. Pascal, Nouveau de Chimie Masson, Paris,T raite� Mine� rale,

1963, vol. VI, p. 109.40 G. A. Olah, K. Looli and O. Farooq, J. Org. Chem., 1984, 49,

4591.41 J. Medema, J. Catal., 1975, 37, 91.42 M. Guisnet, J. L. Lemberton and G. Perot, J. Catal., 1977, 48,

166.43 G. A. Olah and A. M. Molnar, Hydrocarbon Chemistry, Wiley,

New York, 1995, pp. 115È118.44 E. J. G. Ma. A.Lo� pez-Salias, Herma� ndezÈCorte� z, Corte� z-

J. Navarrete, Ma.E. Llanos, A. H.Ja� come, Va� zquez, Armenda� rizand T. Appl. Catal. A, 1998, 175, 43.Lo� pez,

45 P. Iengo, G. Aprile, M. Di Serio, D. Gazzoli and E. Santacesaria,Appl. Catal. A, 1999, 178, 97.

Phys. Chem. Chem. Phys., 2001, 3, 1103È1107 1107

Publ

ishe

d on

27

Febr

uary

200

1. D

ownl

oade

d by

Uni

vers

ity o

f M

ichi

gan

Lib

rary

on

28/1

0/20

14 0

3:40

:54.

View Article Online