Iranica, Vol. 14, No. 4, pp 297{302 Scientia

c

Sharif University of Technology, August 2007

y-Supp orted Quaternary Ammonium and Cla

Cations in Triphase Catalysis and Phosphonium

E ect of Cosolvent in Catalytic Activity the

1 1 1 

B.L. Cutts D. Dutko and S. Khazaeli , Shab estary , N.

this research, a naturally o ccurring clay mineral, hectorite, was used as the supp ort for several In

ry ammonium and phosphonium cations to measure and compare their catalytic activity quaterna

a triphase catalytic system. The intercalation of the catalysts in the clay has the advantage in

easy catalyst recovery; the catalyst can b e removed by a simple separation technique, such of

ltration or centrifugation, up on completion of the reaction. The rate of conversion of n- as

yl bromide to n-butyl chloride was measured in the presence of two classes of phase transfer but

Quaternary ammonium and quaternary phosphonium cations. The rate of the reaction catalyst:

as measured for the biphase reactions (no supp orting clay) and for the triphase catalytic system w

supp orting clay). The results have shown that quaternary phosphonium catalysts ar e (with

more reactive than the corresponding quaternary ammonium catalysts. It wa s also somewhat

that the intercalated catalysts could be used several times b efore losing their catalytic found

y. Also, a remarkable increase in catalytic activity has b een observed using a co-solvent. activit

wever, it app ears that there is a limit for the co-solvent concentration to b e e ective. Ho

Based on this technique, synthetic metho ds phases. ODUCTION INTR

ve b een developed for aqueous-organic phase reac- ha

application of Phase-Transfer Catalysis (PTC) in The

using solid catalysts, such as synthetic p olymers tions

past few decades has b ecome prevalent in academic the

various clay minerals. TC shows considerable p oten- or

h, as well as in commercial pro cesses [1-4]. researc

for commercial application. Several commercially tial

of the advantages that this technique o ers Some

vailable ion-exchange resins, such as Dowex resins, a

increased reaction yield, requirement of only mild are

taining quaternary ammonium groups, h ave b een con

conditions, safety, requirement of inexp ensive reaction

as triphase catalysts [9]. Also, quaternary am- studied

ents and reagents and the ability to conduct reac- solv

salts immobilized on insoluble p olymers and monium

on a commercial scale. However, the recovery tions

oxides, such as silica and alumina, have b een inorganic

recycling of relatively exp ensive phase transfer and

ely explored in TC. However, silica, alumina extensiv

catalysts from the liquid phase has b een a challenge for

p olymers are relatively unstable in strong acidic and

technique. The treatment of the reaction mixture this

alkaline environments and they normally su er from or

ciated with soluble catalysts imp oses limitations on asso

oor physical strength. In contrast, clays h ave b et- p

applicability and usefulness of PTC as an industrial the

physical strength, b etter resistance towards alkali ter

hnique of synthesis. Recently, Triphase Catalysis tec

t and are relatively inexp ensive. Recently, treatmen

has b een introduced by Regen [5-8] as a unique (TC)

investigations have app eared on the application some

yp e of heterogeneous catalysis, in which the catalyst t

clays as solid supp orts for quaternary salts [10-16]. of

each of a pair of reactants are lo cated in di erent and

the present investigation, a study on the use In

clay of the smectite family, hectorite, is rep orted as of

ort for quaternary ammonium and phosphonium supp

Corresponding Author, Department of Chemistry, Col- *.

salts. Smectite clay minerals are among members of

ge of Arts and Sciences, Southern Il linois University le

arious layered silicates with 2:1 mica-like layer lattice v

at Edwardsville, Il linois, P.O. Box 62026-1652, USA.

Naturally o ccurring negative charges on structures.

1. Department of Chemistry, Col lege of Arts and Sciences,

Southern Il linois University at Edwardsville, Il linois,

clay layers are balanced by interlayer cations that the

P.O. Box 62026-1652, USA.

ccupy intercalated p ositions b etween the interlayers. o

N. Shab estary, S. Khazaeli, D. Dutko and B.L. Cutts 298

so dium acetate bu ered to pH 5 with Adding o ccurring alkali and alkaline earth cations in Naturally

acid converts calcium carb onate to carb on diox- acetic pristine clay minerals can b e replaced by almost any the



10 g which is removed when heated to 70 C. ide, cation. Cation exchange in this type of clay desirable

of clay were used to remove carb onates and samples is relatively rapid. Smectite or swelling clay mineral

salts. First, 100 mL of 1 N so dium acetate soluble a great anity towards water, as well as organic has

to pH 5 with acetic acid was added to the clay bu ered ts. This unique wetting prop erty makes the clay reagen

a blender. The clay susp ension was then heated for in attractive for catalysis. Earlier work [17,18] has quite



with o ccasional stirring. The solution hour at 70 C 1 that alkyl ammonium ions intercalated demonstrated

as then centrifuged and the sup ernatant was analyzed w smectite clay pro duce ordered structures, in which in

an atomic absorption sp ectrophotometer (Varian on alkyl chains and the ammonium groups form a the

ectra AA-10) for the amount of calcium. Standards Sp eci c orientation, with resp ect to the clay layers. sp

ere prepared from 1000 ppm Fisher standards. The w us, these structures dep end on the length of the alkyl Th

steps were rep eated until the amount of previous hains and the charge density of the clay layers. Chi-Li c

was less than 2 ppm. calcium et al. [19] h ave indicated that onium ions within Lin

ree iron oxides are removed by the addition of a F clay have an emulsion-forming ability, which, most the

dium citrate/so dium bicarb onate solution and with so ely, is related to the amphiphilic nature of onium ions lik

80 mL t by so dium hydrosul te (Na S O ). treatmen the external surfaces of the clay tactoids. Thus, on

2 2 4

ere 0.3 N so dium citrate and 10 mL of 1N NaHCO w of y adopting parallel or p erp endicular orientations at b

3

to the clay susp ension in water. The mixture was added clay-liquid interface, the onium ions may show the



and 2 g of so dium hydrosul te heated to 75-80 C then ydrophilic or organophilic wetting prop erties. h

wa s slowly added. Care was taken not to heat a b ove The rate of conversion of n-butyl bromide to



to prevent FeS precipitation. The clay susp ension 80 C -butyl chloride wa s measured in the presence of n

as heated for 15 minutes. The resulting solution was w wo classes of phase transfer catalysts: Quaternary t

trifuged and the sup ernatant was analyzed for the cen and quaternary phosphonium salts. The ammonium

t of iron. A typical atomic absorption calibra- amoun of the reaction was measured for the biphase cat- rate

curve was constructed to measure the amount of tion system (no supp orting clay) and for the triphase alytic

in ppm. iron system (with supp orting clay). The results catalytic

p eroxide wa s employed to digest or- Hydrogen ve shown that quaternary phosphonium catalysts ha

matter in the clay. A 30% solution of hydrogen ganic more e ective than the corresp onding quaternary are

eroxide (150 mL for 10 g clay) wa s mixed with the p catalysts. It wa s also found that the ammonium

y slurry for 15 minutes at ro om temp erature. Then, cla tercalated catalysts could be recycled several times in

clay wa s saturated with so dium by addition of the signi cantly losing their activities. without

dium chloride (10% solution, 200 mL). The mixture so

as stirred for another 15 minutes, then centrifuged. w

AL EXPERIMENT

clay was then washed four to 5 times until free of The

hloride ions (test with silver nitrate solution). After c stated otherwise, all reagents were obtained Unless

pro cess, the clay wa s freeze-dried using a VirTis this and used without further puri cation. commercially

ench top freeze-dryer. Typically, the freeze-drying is b

overnight. done

Hectorite Natural

o ccurring so dium hectorite, with particle size Naturally

Preparation of Clay Intercalates

2 m, was obtained in the precentrifuged and spray- <

tercalation reactions of all cations were p erformed In form. The idealized anhydrous unit-cell formula dried

similar conditions, since all the quaternary under hectorite is Na of (Si [Li Mg ] )O (OH,

:66 :66 :34 0 :00 0 5 8 20

salts were quite stable and also soluble in water. onium the exp erimentally determined cation-exchange F) ;

4

a typical exp eriment, a 1.0 wt% aqueous susp ension In y is ab out 73 meq/100 g and the surface area is capacit

2

was added to 0.1 g of the mineral in 10 mL H O) (e.g. of air-dried clay. The clay contains two out 750 m /g ab

2

vigorously stirred aqueous solution of the quaternary a jor impurities that may interfere with the catalytic ma

salt containing ab out 1.2 meq/meq of clay. The onium These are calcium carb onate and free (non- reactions.

mixture was stirred for over an hour. After an reaction iron oxides, which can inhibit intercalation of lattice)

time of 24 hours, the clay was rep eatedly equilibration clay, b ecause they act as cementing agents, which the

washed with deionized water and then collected by prevent o cculation from o ccurring. The impurities

trifugation. The resulting homoionic quaternary cen also interfere with the determination of the Cation-

cation-exchanged forms of the minerals were onium Exchange Capacity (CEC) of the clay. These two ma jor

or freeze-dried, as desired, and stored in air-dried impurities are removed prior to intercalation, in order

over anhydrous CaCl . desiccators to promote o cculation of the freeze-dried clay. 2

riphase Catalysis 299 T

1. Schematic representation of onium cation exchange reaction. Lines represent the negatively charged clay layers. Figure

ted as follows: of Biphase and Triphase-Catalyzed represen Kinetics

t Reactions Displacemen

+ Onium Salt ! Hectorite

reactions of alkyl bromides with NaCl under The

+

head cation) ][X ] [Hectorite][(Onium

conditions were carried out as follows: Air triphase

+

(0.037 meq [hectorite][(onium head cation) ][X ] dried

clay intercalate: Hectorite

quaternary onium salt as catalyst) wa s disp ersed of

3.0 mL aqueous 3.3 M NaCl in a 15  150 mm in

types of intercalates were used to study triphase These

Pyrex culture tub e tted with a Te on-lined screw

To prepare the triphase catalysts, onium catalysis.

and magnetic stir bar. The mixture wa s stirred cap

were added in excess of the clay Cation Exchange salts

a few hours until a homogeneous susp ension wa s for

y (CEC) and the clay intercalates were washed Capacit

To the susp ension wa s added 1.0 mmol obtained.

eral times with water to remove the excess onium sev

the appropriate alkyl bromide in 2 mL of toluene. of

(see Figure 1). Earlier investigations [17,19] have salts

tub es were sealed with a Te on-lined screw cap The

wn that, dep ending on the ty p e and size of the sho

vigorously shaken for a minute. The kinetics and

chain of the onium salt, the thickness of the clay alkyl

eriment was then started by placing the tub e in an exp

yer expands b eyond a monolayer of onium ions. This la

bath maintained at the desired temp erature, while oil



in a basal spacing of ab out 18 or more by x-ray results A

reaction mixture was constantly stirred using a stir the

owder di raction, dep ending on the size of the alkyl p

No signi cant change in the rate of reaction was bar.

since the va n der Waals thickness of the clay group,

ed due to change in the stirring rate. Perhaps observ



transfer is not a limitation for this reaction. mass

. Since the quaternary onium yer is only ab out 9.6A la

reaction wa s followed by withdrawing 1-uL The

are sandwiched b etween the clay interlayers, cations

of the organic phase at di erent times (no samples

larger cation will create a larger lateral surface. In a

than 30 minute intervals) and monitoring the less

a cation that creates a larger lateral surface general,

earance of the reactant by gas chromatography. disapp

is exp ected to result in higher catalytic activity. area

or sampling, the tub e was removed from the oil bath, F

the present study, various quaternary am- In

en, quickly co oled by ice water to b elow ro om shak

and phosphonium salts h ave b een used as monium

erature, op ened, resealed and returned to the temp

as well as triphase, catalysts for side by biphase,

after sampling with a small syringe. The overall bath

comparison. Schematic representations of biphase side

cess to ok less than one minute. The temp erature pro

triphase catalytic systems are given in Figures 2 and

the oil bath used for the kinetic exp eriments wa s of

3. All the catalysts are used in nucleophilic and



trolled to  1 with the aid of a temp erature con C

t reactions converting n-butylbromide to displacemen

troller. The pro duct mixture was analyzed by gas con

-butylchloride. This halogen ion displacement wa s n

chromatography. liquid

TS AND DISCUSSION RESUL

Catalytic Reactivity of two Comparing

t Onium Ion Hectorite Intercalates Di eren

Naturally o ccurring so dium within the smectite clay

2. Schematic representation of biphase Figure

interlayer is rapidly replaced by onium salt cations

t reaction. displacemen

in aqueous solution. The exchange reaction may be

N. Shab estary, S. Khazaeli, D. Dutko and B.L. Cutts 300

reactions at the aqueous liquid/organic liquid catalytic

terface. In fact, the emulsi cation b eing facilitated in

y the clay intercalates helps to bring the nucleophile b

the aqueous phase into contact with the substrate, from

h is contained in the organic phase. It is antic- whic

that a higher degree of emulsi cation leads to ipated

catalytic activity. It is imp ortant to mention higher

the emulsion formation is not a limitation for that

triphase system, since the emulsions can b e easily this

en through a simple separation technique, such as brok

or centrifugation. It seems that the degree ltration

emulsi cation dep ends on the nature of the onium of

and the characteristics of the intercalates, as well cation

the organic solvents. as

is interesting to note that once the onium It

are intercalated, their exchange with so dium cations

in the aqueous layer is not favored, due to the great ions

y that clay exhibits for these kinds of catalyst. anit

this phenomenon prohibits biphase catalytic Therefore,

within the solution, since little or no leaching reactions

3. Schematic representation of the triphase Figure

of quaternary onium cations out of the clay intercalates

system using quaternary onium clay intercalates as solid

phase.

ccurs, despite the high concentration of so dium ions o

the reaction mixture (so dium chloride concen- within

in 50 mL culture tub es, using the pro cedures conducted

in the aqueous phase is ab out ten times higher tration

ed in the exp erimental section. So dium chloride describ

n-butylbromide). than

as added in high concentration (several times more w

eral quaternary onium catalysts h ave b een Sev

alkyl bromide) to remove the dep endency of the than

(see Table 1) for intercalation, to investigate the used

ed rate constant (K for the displacement observ )

obs

of alkyl chain length, size and shap e of these e ects

on the chloride concentration. Also, no reaction

and, also, to observe a di erence b etween catalysts

t change: On the rate constant was observed signi can

and phosphonium catalysts, according to ammonium

y the rate of stirring. Rates of reaction were mon- b

equation b elow. Fo r all of the catalysts used, a the

by following the disapp earance of the starting itored

rst order reaction wa s observed. Since [Cl ] pseudo

halides from the organic phase, using gas liquid alkyl

[1-bromobutane], the complete kinetic equation >>

hromatography, every half hour. c

b e written in the following form. The reaction rate can

the clay catalytic system, alkyl halide and In

ts are given in Table 1. constan

anion from the organic phase and the aqueous halide

! R-Cl (org) + Br (aq) (org) + Cl (aq) R-Br

resp ectively, are transferred to the interface of phase,

solid clay intercalate where they can react. These the

bromobutane]: d[1 - bromobutane]/dt = k [1

obs

onium ion hectorite intercalates help to quaternary

emulsi cation of the organic aqueous mixture stabilize

comparison, somewhat lower catalytic activity has In

as a result, help to improve the reactivity and and,

een observed for tetrabutylammonium or phospho- b



able 1. Pseudo rst-order rate constants at 90 for the chlorination of n-butyl bromide in the presence of a quaternary T C

cation-hectorite intercalates as triphase catalysts and also under biphase reaction conditions. onium

yp e of Quaternary Onium orted Biphase y Supp orted Triphase T Unsupp Cla

Used in the Catalytic System System Salt Catalytic Catalytic

1 1

hr hr Reactions k , k ,

obs obs

0.420 0.260

rido decylmethylammonium chloride T

0.600 0.550

ylphosphonium bromide Hexadecyltribut

0.065 0.032

Tetrabutylammoniumchloride

0.260 0.057

Tetrabutylphosphoniumchloride

0.740 0.430

Tetraoctylammonium bromide

0.880 0.610 Tetraoctylphosphonium bromide

riphase Catalysis 301 T

h they are soluble. Smectite clays have dual char- whic intercalates, while somewhat higher catalytic nium

b oth hydrophilic and hydrophobic. This acteristics, y has b een observed in the biphasic reactions activit

prop erty is even enhanced when the clay exceptional Table 1). Due to the relatively large size of onium (see

intercalated with onium ions and helps to stabilize is it is fair to assume that there are no interlayer ions,

droplets of three-phase emulsion, which serve small available for catalytic reactions. Thus, the surfaces

an aid to the rate of the catalytic reaction. To as are o ccurring only at the lateral surfaces of reactions

stabilize the emulsion formation of the clay further clay intercalates rather than inside the interlayers. the

tercalates, a relatively p olar cosolvent (ethylene gly- in the onium catalysts are small, such as tetrabutyl When

was introduced into the triphase catalytic system. col) or phosphonium bromide, they are unable ammonium

exp ectation was to observe an enhancement in the The separate the clay layers to a great extent and, as a to

activity. In this study, tetrao ctylammonium catalytic the lateral surfaces decrease considerably. This result,

y intercalate wa s used as the solid phase to carry cla y directly a ect the rate of the reactions. ma

nucleophilic displacement reactions converting n- out tetrao ctylammonium bromide with Comparing

ylbromide to n-butylchloride. The results of this but ctylphosphonium bromide catalysts, somewhat tetrao

are given in Table 2, where three di erent study activity has b een observed with tetrao ctylphos- higher

trations of the cosolvent are used. As the results concen b oth in biphase and triphase catalytic sys- phonium,

catalytic activity is increased more than three indicate, Despite the fact that the non-symmetrical tems.

when only 5% of water is replaced by ethylene times onium salts of ammonium and phospho- quaternary

as a cosolvent. This remarkable increase in glycol have di erent alkyl chain length and anion, Hex- nium

reactivity due to cosolvent may be related catalytic ylphosphonium bromide has still shown adecyltribut

to b etter emulsion formation in the triphase system. higher activity than Tridodecylmethylammonium chlo-

wever, it is interesting to note that when the amount Ho The higher catalytic activity of phosphonium ion ride.

cosolvent wa s subsequently increased by two - and of comparison with the ammonium ion counterpart, has in

the catalytic activity somewhat dropp ed four-fold, een observed for biphase catalytic reactions [20]. It b

than increased. This reaction wa s rep eated rather interesting that a similar trend has b een observed is

few times to make sure that the decrease in the a the triphase catalytic system, using clay-supported for

ed reaction rate, up on increasing the cosolvent observ and ammonium cations. phosphonium

tration, were not an exp erimental error. concen few of these clay intercalates have b een recycled A

this investigation, it seems that certain con- In eral times after b eing washed with distilled water sev

tration mixtures create a metastable solution that cen no signi cant catalytic activity loss has b een and

the demolsibility rather than the emulsi cation helps ed. This indicates that these types of catalyst are observ

t of the system, which is necessary to in- enhancemen stable and robust. However, all of the quaternary quite

interfacial surfaces available for reaction. Thus, crease cations used in this study, when intercalated, onium

app ears that there is an optimum concentration of it w lower activity than in a biphasic system. A loss sho

ent that can contribute to emulsion stability. co-solv catalytic activity up on onium ion intercalation is of

results in a maximum catalytic activity in the This general feature of triphase catalysis and is not an a

y-supp orted triphase catalytic system. Since clay cla phenomenon [19]. uncommon

hydrophobic, as well as hydrophilic, characteristics, has

app ears that a ne concentration balance needs to it

of Cosolvent in Triphase Catalytic E ect

e kept to stabilize emulsion formation within the b

System

reaction mixture. Thus, the choice of a prop er catalytic

a triphase catalytic system, reagents from two In

ent or cosolvent is critical to the catalytic activity solv

liquid phases are transferred to the interface immiscible

the clay-supported triphase system. In this regard, of

a solid (in this case, intercalated clay catalyst), of

erhaps the Henson Solubility Parameter [21] can be p

the reaction o ccurs. The reaction pro ducts at where

prop er guide in choosing a right solvent mixture to a

interface are then returned to the liquid phases, in the

hieve the maximum catalytic activity. Investigations ac

able 2. E ect of co-solvent in the rate of nucleophilic displacement reaction. T

Co-Solvent of Reaction % Rate

ent System Solv

1 

hr k , ylene Glycol) 90 C (Eth at

obs

Toluene/water 0 0.43

oluene/water/ethylene glycol T 5 1.20

oluene/water/ethylene glycol T 10 0.56

oluene/water/ethylene glycol T 20 0.44

N. Shab estary, S. Khazaeli, D. Dutko and B.L. Cutts 302

Danao, S.P., Thorat, R.T. and Nageshwar, G.D. 9. various solvent p olarities in the triphase system and of

of oxidation of phenethyl alcohol by triphase \Kinetics

e ect on catalytic activity are in progress. their

Indian Chem. Engr., Section A, 47(3), pp catalysis",

(2005). 156-160

CKNOWLEDGMENTS A

Monsef-Mirzai, P. and McWhinnie, W.R., Inorg. 10.

authors would like to acknowledge supp ort from The

Acta, 52, p 211 (1981). Chim.

NSF \Increasing the Minority Scientist Pool", the

Tundo, P. , Venturello, P. and Angelletti, E., J. Am . 11.

School, Department of Chemistry, and Senior Graduate

Soc., 104, p 6547 (1982). Chem.

t Fund, SIUE. Assignmen

Choudary, B.M., Rao, S. and Prasad, B.P., Clays and 12.

Miner., 39(3), p 329 (1991). Clay

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