Static phase transfer catalysis for Williamson reactions: Pickering interfacial catalysis

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10.1039/C9CY00620F.

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DOI: 10.1039/C9CY00620F

ARTICLE

Static Phase Transfer Catalysis for Williamson Reactions: Pickering

Interfacial Catalysis

Qianqiang Zhao ^ab, Xiao Zhao ^{a b}, Hui Peng ^ab, Yang Liu ^ab, Lihui Yang ^ab, Jie Sun ^abc, Lei Yang

*^ab, Yifeng Shen ^ab

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Pickering interfacial catalysis (PIC) systems are regarded as an innovative platform for designing efficient static biphasic

catalytic reaction. In this paper, a static PIC system with high catalytic activity was constructed for phase transfer catalysis

system of Williamson reaction. The silica particle modified with the cation surfactant – DTAB (Dodecyl trimethylammonium

bromide) was used as the Pickering emulsifier, and the stable Pickering emulsion was constructed. The droplet size of the

emulsion is 21μm and has no change in size and distribution with standing for 24h. Because of the competitive adsorption

of DTA⁺between silica particle and phenolate anion, the second layers of DTA⁺adsorbed on the modified particle is in the

form of ion-pair with phenolate anion, and the ion-pair also can enter into the firs layer. This modified silica particle is

arranged on the interface of the aqueous phase and the oil phase, and adsorbed DTA⁺can transfer phenolate anion to

catalyze the PTC reaction. Tetrabutylammonium bromide (TBAB) can be used as a cocatalyst to enhance the catalytic

reaction. The conversion rate of benzyl bromide derivates reaches more 90% within the PIC system, demonstrating the

potential applications in the industrial synthesis.

method to enhance the mass transportation efficiency and

reduce energy consumption is the biggest obstacle in further

improving the efficiency of the PTC system.

Introduction

Phase transfer catalysis (PTC), which offers a number of

important advantages, namely: high rate, high conversions or

yields, few by-products and elimination of the need for

expensive or dangerous solvents, is one of highly advantageous

synthetic technology and has emerged as a widely used

technology throughout the domains of fine chemistry and

materials chemistry.^1-4The generation of enough high

interfacial region is the essential factor for the PTC reaction to

increase the mass transfer rate, thus most of PTC reactions are

conducted under vigorous stirring. For example, in the liquid-

liquid biphasic system, the droplets are generated by

mechanical agitation. However, the droplets are often large in

size, and the average droplet sizes no longer decrease after

downsized to ca. 200-300 μm.^{5, 6}Moreover, the droplets are

very unstable and prone to coalescence, driven by the high

interfacial tension between the organic phase and the aqueous

Pickering interfacial catalysis (PIC) has been developed for an

innovative platform for designing efficient static biphasic

catalysis systems and observed excellent efficiency.^9-11

Commonly, the catalytically active micro-/nanoparticles are

used to stabilize emulsion that is the Pickering emulsion (PE) for

interfacial reactions. In the PE, each emulsion droplet can be

considered as a micro-reactor, which provides a large interface

for reaction and enhances the opportunity for the collision of

catalysts with reactant molecules. In comparison to

conventional biphasic catalysis systems, PEs are more stable

against coalescence even at high temperatures and without

stirring due to the high adsorption energy of particles at

oil/water interfaces. Moreover, PEs allow their droplet sizes to

tune and the active catalyst can be recycled.^12-14PIC has been

successfully applied in the metal particle catalysis,^15-17the

enzymic catalysis,¹⁸the interface-active acid/base catalysis,^{19, 20}

the Metal-organic frameworks (MOF) catalysis.²¹The involved

reactions ranges from biotransformations,^{22, 23}oxidation,^{24, 25}

8

phase.^7,Therefore, the vigorous stirring is required in the

whole reaction process and the mass transfer rate in the PTC

system is still limited, which cause higher energy consumption

and lower reaction efficiency, particularly in industry. The

amination,²⁶

reduction,²⁷

enzymatic

catalysis,²⁸

tandem/cascade reactions^{29, 30}to flow reactions.^{31, 32}And the

PIC reaction system also include solid-gas,³³liquid-liquid, ionic

35

liquid^34,

and solvent free systems.²⁰To the best of our

^a.Engineering Research Center for Eco−Dyeing and Finishing of Textiles, Ministry of

Education, Zhejiang Sci−Tech University, Hangzhou 310018, China

^b.Key Laboratory of Advanced Textile Materials and Manufacturing Technology,

Ministry of Education, College of Materials and Textiles, Zhejiang Sci−Tech

University, Hangzhou 310018, Zhejiang, China

knowledge, there have been only few reports on the application

of PIC in the PTC system.²⁶

Silica (SiO₂) nanoparticle is the most popular solid

nanoparticle for constructed the PE. ³⁶Bare silica nanoparticles

are too hydrophilic due to a large amount of hydroxyl group at

^c.Zhejiang Province New Textile Research & Development Emphasized Laboratory,

Hangzhou 310009

Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x

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the surface and commonly require a partial hydrophobic As shown in Figure 1, the zeta potential of silica p_Va_ir_et_wic_Al_re_tics_leis_On-_l3_in8_e

DOI: 10.1039/C9CY00620F

coating. Chemical grafting of organic molecules and the mV without DTAB and 4-methylphenol sodium (0.14 M). As the

adsorption of different types of (macro)molecules are the addition of DTAB, the zeta potential increases gradually and

preferred strategies for the modification of the bare silica changes sign. However, the zeta potential of the silica particles

particles.³⁷For chemical grafting, the organic grafts are tightly is -26 mV in the presence of 4-methylphenoxide anion. As the

attached to the surface with chemical bonds. However, usage of DTAB increases, the zeta potential of the particle

chemical grafting reactions are often performed on the dry gradually increases and reaches the equilibrium value (-6mV).

powder or a suspension of the particle in dry organic solvent,

because most chemical reactions are not compatible with an

aqueous medium. The redispersion of the chemically modified

silica particle into the water medium may be quite difficult.

Adsorption of different types of (macro)molecules before the

emulsification is a quite easy implementation. Surfactants,

surfactant-like cationic molecules, polymers are the common

adsorbent modifier.³⁸The bare silica particle modified with the

cationic surfactant was used to construct PIC system for PTC

reaction in this study.

PTC reaction is catalysed by the transfer of the catalytic

cation between two phases. The cation can enhance the

transfer rate of the reactant and the collision of the reactant

molecules. With silica particle modified by surfactant cation as

the emulsifier, the cations are adsorbed on the surface of the

Figure 1. Zeta potential of 1 wt.% SiO2 nanoparticle dispersed in aqueous CTAB solution

silica particle to provide the proper hydrophilic, resulting in the

partial wetting by water and oil.³⁹Therefore, cation should play

roles of silica modifier and phase transfer catalyst in the PIC

system for PTC system, which means the unique mechanism is

generated. Meanwhile, the adsorption of the cationic

(square) and in CTAB + 4-methylphenol sodium solution (circle) as a function of initial

DTAB concentration.

Monolayer and bilayer adsorptions are involved in the

adsorption process of cationic surfactants on silica

nanoparticle.⁴¹The cationic surfactant molecules adsorb at the

particle surface via electrostatic interactions. Initially, the

cationic surfactant molecules form a monolayer at the surface

with their hydrophobic tails exposed toward water. With the

further increase in the usage of cationic surfactant, the second

layer is formed with their hydrophilic tails exposed toward

water. When the bilayer adsorption is complete, the adsorbed

amount and zeta potential reach the plateau value. The effect

of DTAB concentration on the zeta potential of particle indicates

that the adsorption process of DTAB in presence of 4-

methylphenoxide anion is different from that without 4-

methylphenoxide anion:

(1) The increasing zeta potential of the particle with the addition

of DTAB demonstrates that DTAB molecules are adsorbed at the

surface of the particle and the negative charges on the particle

surfaces is neutralized. Meanwhile, with the addition of 4-

methylphenoxide anion, the ionic strength of the solution

increases. The electric double layer of the silica nanoparticle is

compressed, and the absolute value of zeta potential decreases.

(2) The zeta potential reaches the equilibrium value with 1.43

mM (1% to mole of the 4-methylphenoxide anion, 0.5 mmol/g

to the weight of SiO₂) of DTAB means the existence of saturation

adsorption. With cetyltrimethylammonium bromide (CTAB) as

the cationic modifier, the equilibrium monolayer adsorbed

amount to silica nanoparticles is 0.44 mmol/g. In this aqueous

phase with 4-methylphenoxide anion, saturation adsorption is

generated with 0.5 mmol/g of DTAB. Meanwhile, the increasing

zeta potential with the usage of DTAB demonstrates that the

bilayer adsorption is not complete with 5 wt.% of DTAB. It can

be concluded that this saturation adsorption belongs to the

surfactant at the surface of the silica nanoparticle is in

40

equilibrium for the PE.

Whether the transfer of cation

between two phases has a considerable influence in this

equilibrium of the adsorption is the main factor to construct the

PIC system.

Herein, the aim of this research is to develop the convenient

and facile PIC system for PTC Williamsian reaction. Initially, the

adsorption process of cationic modifier on silica particle was

explored in the presence of the reactant anion. Subsequently,

factors that influence the stability of the Pickering emulsion

were studied in detail, and the true catalyst for PTC reaction in

the PIC system was investigated. Furthermore, Williamsian

reaction of phenol derivates with benzyl bromide derivates was

carried out in the PIC system.

Results and discussion

Adsorption of DTAB on silica particles

According to the reported literature,^{38, 40}the cationic modifier

can be adsorbed on the surface of the silica particles and the

zeta potential of the nanoparticle increases. However, in the

PTC system, the transfer of the reactant from the aqueous

phase to the organic phase also requires the help of the cation

molecule. In this PIC system for PTC reaction, there are two

types of the anion, resulting two adsorption process. The effect

of the addition of dodecyl trimethylammonium bromide (DTAB)

on the zeta potential of the silica particles was investigated in

the absence or presence of 4-methylphenoxide anion.

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monolayer adsorption. However, when a second layer begins to further increase in the concentration of CTAB. Without 4-

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adsorb on the first layer through hydrophobic interactions of methylphenoxide anion in PEs, DTA⁺was adsorbed on the

DOI: 10.1039/C9CY00620F

the chains, the zeta potential value of the particle keeps second layer with their hydrophilic tails exposed toward water.

invariable. These results indicate the molecule adsorbed in the The hydrophilicity of the modified particle was increased,

bilayer is electroneutral.

resulting in the decreased emulsifying capacity. In this PTC

system, the ion-pair of DTA⁺with the 4-methylphenoxide anion

also is generated due to the interaction, which is electroneutral

and amphiphilic, and can be adsorbed on the surface of the

nanoparticle. In the other words, DTA⁺in the second layer of the

modified particle also can adsorb the 4-methylphenoxide anion

due to electrostatic interactions. The ion-pair of DTA⁺and 4-

methylphenoxide anion is also amphiphilic. Therefore, the

emulsifying capacity of the modified nanoparticle remains

unchanged.

Figure 2. The water contact angle of SiO2 nanoparticle under the oil phase as a function

of initial CTAB concentration.

The contact angle of aqueous phase on the modified silica under

oi phase is shown in Figure 2. The contact angle for unmodified

SiO₂is only 44.7°, and for modified SiO₂contact angle increased

to more than 80°. With the further increase in the usage of

DTAB, lithe change in the contact angle was observed (only from

84.6° to 87.3°). The contact angle of the Pickering particle at the

ion-water interface is crucial in determine the type and stability

of PEs. Less than 90°of contact angle causes that the type of the

emulsion is W/O, which is also certified in the drop-test

experiment. It is seen that the contact angle remains invariable

after the saturation of the monolayer adsorption. However,

after the bilayer formation in the surface of the silica particle,

the contact angle would decrease without 4-methylphenoxide

anion.³⁸These results also mean that the novel bilayer

adsorption has no effect on the emulsifying ability of the

modified particle.

Effect of the concentrations of cationic modifier on the stability of

the Pickering emulsion

Figure 3. Photographs of diisopropyl ketone-in-water (1:1) emulsions stablized by

mixtures of 2 wt % silica and CTAB at different concentration taken at (a) 0.5h and (b)

1h, (c) 3h, (d) 5h after preparation. Concentratin of DTAB from left to right: 0.5%, 1%,

2%, 3%, 4%, and 5%.

The effect of DTAB concentration on emulsifying stability were

studied with the concentration of DTAB varied from 0.5% to 5%.

Photographs of the PEs with standing for 0.5h, 1h, 2h and 5h are

presented in Figure 3. The aqueous phase is separated from the

emulsion. The height of the separated aqueous phase has little

change with the standing time except for that with 0.5% wt. of

DTAB, indicating that the stable PEs are generated. Moreover,

the height of the water layer decreases with the concentration

of DTAB. With the increase in the usage of DTAB, the modified

silica exhibits a higher emulsifying ability.

The effect of the modifier usage on the emulsifying capacity of

the nanoparticle is different from that without 4-

methylphenoxide anion. In the previous literature,⁴⁰the

emulsifying ability of the modified particle reached the

maximum value with the monolayer saturation adsorption, and

the height of the separated water phase increased with the

Effect of cationic modifier structure on the stability of the

Pickering emulsion

For exploring the effect of cationic modifiers on emulsion

stability, tetraethylammonium bromide (TEAB), benzyl

triethylammonium bromide (BTEAB), tetrabutylammonium

bromide (TBAB), methyltrioctyl ammonium bromide (TOMAB),

CTAB and stearyltrimethylammonium bromide (STAB) were

used as the modifier. Photographs of PEs after standing 5h are

shown in Figure 4a. It can be seen that the emulsions are

unstable with the common PTC catalysts. Two liquid phases are

completely separated, while no flocculation is generated. Only

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for cationic surfactants with long-chain, stable emulsions are

generated. The height of the water phase for standing for 5h

increases with the length of long chain. The average particle size

distributions of stable PEs are shown in Figure 4b and 4c. The

statistics parameters for the distribution of emulsion size are

shown in Table 1. The average emulsion size of the PE with DTAB

is 21μm and is 17μm for that with CTAB or STAB. Although the

average size of the emulsions with STAB or CTAB is lower than

that with DTAB, absolute values for skewness and kurtosis are

much higher. The distribution of the droplet size for CTAB or

STAB is more spread out than that for DTAB.

Table 1. Statistics parameter for the distribution of emulsion droplet size

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DOI: 10.1039/C9CY00620F

Average (μm)

22.3

Skewness

Kurtosis

-0.072

-1.025

-0.744

DTAB

CTAB

STAB

0.118

0.485

0.632

17.2

17.4

Effect of the amount of silica particle on the stability of the

Pickering emulsion

Figure 5. Photographs of diisopropyl ketone-in-water (1:1) emulsions stabilized by

mixtures of silica and 5% molof DTAB. Concentratin of SiO₂from left to right: 0%,0.5%,

1%,1.5%,2%.

Effect of the amount of silica nanoparticles on the stability of

the PIC system was studied with the amount of silica particle

varied from 0.5% to 5%. The pictures of the Pickering emulsion

standing for 30min and 5h are shown in Figure 5. It can be seen

that the stability of the emulsion gradually increases with the

amount of SiO₂particles increased from 0.5% to 2%. Both the

organic phase and the aqueous phase are separated from the

PTC system, except with 2% of SiO₂particle used. In the PIC

system, DTAB, silica particle modified with DTAB and the ion-

pair of DTAB and the 4-methylphenoxide anion are amphiphilic

and can emulsify the organic phase and the aqueous phase.

Without silica particle, DTAB and the ion-pair of DTAB and the

4-methylphenoxide anion exist in the PIC system. However, the

limited emulsifying capability causes the rapid separation of

two liquid phases. With the addition of silica particle, Pickering

emulsifiers (silica particle modified with DTAB) are generated

and the stable emulsion is formed. The amount of the Pickering

emulsifier in PIC system increases with the usage of silica

particle, inducing the increase in the volume of the emulsion.

Effect of the cation concentration on the conversion rate of

Williamson reaction

Figure 4 a) Photographs of diisopropyl ketone-in-water (1:1) emulsions stabilized by

mixtures of 2 wt % silica and 5% mol of the cation modifier taken at 5h after preparation.

Cation modifier from left to right: TEAB, BTEAB, TBAB, DTAB, CTAB, STAB and TOMAC.

Optical micrographs (b) and size distribution (c) of diisopropyl ketone-in-water (1:1)

emulsions stabilized by mixtures of 2 wt. % silica and 5% mol of the cation modifier taken

at 5h after preparation, Cation modifier from left to right, DTAB, CTAB, STAB

For PTC reaction, the concentration of salt in the aqueous phase

is relatively high, and the modified silica particle with a

relatively high emulsifying capability is required to stabilize the

emulsion. The common PTCs can distribute into the organic and

the aqueous phase, while do not exhibit a high surface activity.

These cations also can be adsorbed at the surface of silica

particle. However, the lipophilicity for modified particle with

TEAB, BTEAB and TBAB is slightly increased. As a result, these

modified particles cannot emulsify these two phases. TOAB is

more lipophilic than TBAB and cannot dissolve in the aqueous

phase. Silica particle modified with TOAB would be too lipophilic

to emulsify these two phases. For long-chain surfactants, the

hydrophilic end is adsorbed at the surface of the silica particle,

and hydrophobic end can offer the enough high lipophilic

ability. Because the bilayer adsorption is generated in the

modified particle, the emulsifying capability of the modified

particle is more sensitive to the structure of the modifier. With

the increase in the chain length of the modifier, lipophilicity of

the modified particle is increased, and hydrophilicity is

decreased, resulting the decrease in the emulsifying capacity.

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anions in the aqueous phase are adsorbed again _Va_ien_wd_Art_th_icle_{e O}n_ne_lix_net

DOI: 10.1039/C9CY00620F

catalytic cycles are initiated. Therefore, the true phase-transfer

catalyst is the modified silica particle.

Figure 6. Effect of the concentrations of DTAB on the conversion rate of Williamson

reaction in the diisopropyl ketone-in-water (1:1) emulsions stabilized by mixtures of 2%

wt. silica and DTAB..

Figure 8. The adsorption process of the modifier (CTAB cation anion) on the surface of

the nanoparticle.

The effect of the DTAB concentration on the conversion rate of

Williamson reaction is shown in Figure 6. The reaction rate

gradually increased with the concentration of DTAB ranged

from 0.5% to 5%. The increasing magnitude of the reaction rate

with the concentration of DTAB ranged from 3% to 5% is much

lower that with the concentration of DTAB ranged from 0.5% to

3%.

Interestingly, with 0.5% of DTAB in the system, the relatively

high reaction rate was observed. These results be ascribed to

the adsorb of ion-pair of phenoxide anions in the first layer of

the particle. Relatively high zeta potential value of the particle

without DTAB demonstrated that phenoxide anions was

adsorbed in the stern layer of the particle. The competitive

adsorption of DTAB between silica particle and phenoxide anion

exist in the whole process of the PTC reaction. Although DTA⁺

adsorbed in the first layer is not saturated with 0.5% of DTAB,

the ion-pairs of DTA⁺and phenoxide anion are quickly

generated and can also enter in the first layer (Figure 8). The

particle also can be adsorbed at the interface and the ion-pair

can be transferred into the organic phase. The PTC reaction also

is initiated by the modified particle. As the increase in the usage

of DTAB, the bilayer absorption of the ion-pair was generated.

The ion-pairs in both the first layer and the second layer initiate

the catalytic cycle. Like other PIC system without phenoxide

anion, the adsorbed amount also reaches a plateau value.

Therefore, the content of the ion-pair in the modified particle

reaches the maximum value and the catalytic activity of the true

catalyst would also reach the maximum value. The extremum of

the reaction rate with 3% mol of DTAB means the bilayer

saturation adsorption reached. With the further increase in the

usage of DTAB, excess DTAB also can form the ion-pair with

phenoxide anion and catalyse the PTC reaction. In this case, the

increase in the reaction rate with the concentration of DTAB

ranged from 3% to 5% is induced by the catalytic role of DTAB.

Figure 7. The Catalytic mechanism for Williamson reaction in the PIC system.

For the conventional PTC system, the intermediate for the

whole reaction was the ion-pair of the catalyst and anion

reactant. The reaction rate always was in direct proportion to

the concentration of the catalyst. However, the reaction rate

firstly intensively increased, then slightly increased. These

results demonstrate that DTAB is not the true catalyst for the

PTC reaction and PTC reaction involves a novel mechanism

(Figure 7). In this PIC system, the orientation arrangement of

the modified particle is generated in the interfacial region

between the aqueous phase and the organic phase. One side of

the modified particle faces to the organic phase and the other

faces to the aqueous phase. In this way, the ion-pairs on the side

faced to the organic phase can enter the organic phase and

react with benzyl bromide. With the completion of the bond-

forming reaction, the ion-pairs are transformed into the sodium

salt. This side turns to face the aqueous phase. The phenoxide

Effect of the co-catalyst on the conversion rate of Williamson

reaction

It can be seen from Figure 4 the conversion rate reaches 93%

with 5% of DTAB. With the saturation of the bilayer adsorption,

the usage of CTAB only has a subtle influence on the catalytic

activity. The dynamic viscosity of the PEs is much higher than

that of the common emulsion⁴², and the true catalyst is fixed at

the interface of two immiscible phases. The effect of mass

transfer resistance on the reaction rate is more noticeable in the

second half of the reaction process. For intensifying the reaction

rate, the common PTC catalyst (TBAB, TEAB and 18-Crown-6)

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were also used as the cocatalyst in the PIC system. The be adsorbed on the surface of the particle. The relatively high

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+

DOI: 10.1039/C9CY00620F

conversion rates of p-nitro benzyl bromide for 3h and 6h are volume might cause the adsorbed amount of DTA decrease.

shown in Figure 9a, optical micrographs of PEs are shown in The amount of the active site in the modified particle decreases

Figure S2. The mean size of the droplet is characterized and and the catalytic activity is lower than that only with DTAB.

shown in Figure 9b. It can be observed that the reaction rate However, the adsorption of the 18-crown-6 complex of sodium

increases only with TBAB as cocatalyst, and the conversion rate cation can increase the lipophilicity of the particle, inducing the

reaches 96%. Meanwhile, the emulsion stability is not decrease in the droplet size for PE.

influenced by the addition of the cocatalyst. Droplet size of PEs

with cocatalyst is less than that without cocatalyst. The

sequence of droplet size for PEs is: 18-Crown-6 < TEAB< TBAB<

Effect of TBAB Concentration on the conversion rate of

Williamson reaction

None.

The effect of the concentration of TBAB on emulsion stability

and catalytic reactivity is shown in Figure 10. With the increase

of TBAB concentration from 2% to 6%, the conversion rate of 4-

nitrobenzyl bromide in the same reaction time is gradually

increased. The plateau value achieves with 6% of TBAB used.

The mean value of Droplet size with 2% of TBAB is 12 μm, and

droplet sizes firstly increase and keep invariable with TBAB

usage ranged from 2% to 8%.

(a)

(b)

Figure 9. a) Effect of phase-transfer catalyst on the conversion rate of willamiosn reaction

of p-nitro and phelinic. b) Effect of phase-transfer catalyst on the average droplet

diameter of the diisopropyl ketone-in-water (1:1) emulsions stabilized by mixtures of 2%

wt. silica and 5% molof DTAB.

In the PIC system, each droplet in the emulsion can be

considered as a micro-reactor, which provides a large interface

for reaction and enhances the mixing of catalysts and reactants.

With the decrease in the droplet size, the area of the interfacial

region increases, and the reaction rate increases. With the

addition of TBAB and TEAB, the ion-pairs of phenoxide anion

and TBA⁺or TEA⁺are also generated and can be adsorbed in the

first and second layers of the silica particle. The adsorption of

these ion-pairs can further increase the hydrophilicity, resulting

the increase in the emulsifying capability. Thus, the smaller

droplet size was formed. The volumes of the TEA⁺ion-pair and

TBA⁺ion-pair are less than that for DTA⁺ion-pair, which induces

that these ion-pair can enter the gap between DTA⁺ion-pairs in

the surface of the particle. Therefore, the adsorbed amount of

TEA⁺ion-pair is higher than those for TBA⁺ion-pair. The increase

in the emulsifying capacity of the modified particle for TEAB is

higher than that for TBAB. However, the reactivity of the benzyl

group with the DTA⁺ion-pair is higher than that with TEA⁺ion-

pair, and lower than that with TBA⁺ion-pair, resulting in the

increase in the reaction rate with the addition of TBAB. The

complex of 18-crown-6 with sodium cation is generated and can

Figure 10. a) The conversion rate of Williamson reaction and Average droplet diameter

in the diisopropyl ketone-in-water (1:1) emulsions stabilized by mixtures of 2% wt. silica

and 5% molof DTAB as a function of TBAB concentration.

With the addition of TBAB, TBA⁺is adsorbed on the surface of

the particle and complexes with phenoxide anion. The

competitive adsorption between TBA⁺ion-pair and DTA⁺ion-

pair. TBA⁺ion-pair can enter the first layer and the lipophilicity

is intensively increased with adding a very small amount (2

mol%) of catalyst. With the further increase the amount of TBA⁺

ion-pair, the adsorbed amount of CTA⁺ion-pair decreases and

the adsorbed amount of TBA⁺ion-pair increases. Because the

lipophilicity of TBA⁺ion-pair is much lower than that of CTA⁺ion-

pair, the emulsifying ability of the modified particle decreases,

and the increase in the droplet is generated for the PIC system.

The catalytic activity is increased, because the catalytic activity

of TBA⁺ion-pair is higher than that of CTA⁺ion-pair. Moreover,

the excess TBAB also can catalyse the PTC reaction. Finally, the

competitive adsorption between these two ion-pair reach the

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equilibrium value and the catalytic activity and the emulsifying dissolubility in the water phase. The undissolved s_Vu_ieb_wst_Aa_rtn_iclc_ee_Oc_nla_inn_e

ability of the particle attain the maximum value.

be found in the PIC system. The stabili^Dty^OI:o¹f^0.t¹h⁰e^39/e^Cm^9Cu^Yls⁰i⁰o⁶n²⁰i^Fs

intensively decreased, and the organic phase also separated.

With the separation of the organic phase, the interfacial region

Williamson reactions in the PIC system for various reactants

Williamson reactions for various reactants were carried out in decrease in magnitude and the transfer of ion-pair is hindered.

the PIC system. Photographs of PIC systems and the conversion Therefore, the rate of the bond-forming reaction and the final

rate of benzyl bromide derivates are shown in Figure S4 and conversion rate are decreased to a great extent. Temperature

Table 2, respectively. It can be seen that stable PEs were and organic solvent are the key factors to influence the ratio of

generated except for the reaction of vanillin. Both the introduce O-alkylation to C-alkylation product.^{43, 44}Because the enough

of the electron-withdrawing group to benzyl bromide derivate high interfacial region was generated in PIC system, O-

and the introduce of the electron-donating group to phenols alkylation reaction can quickly proceed at 30°C. The formation

derivate can increase the conversion rate of benzyl bromide of O-alkylated product is also favoured with the aprotic solvent

derivates. The conversion rate is higher than 90%, except - diisopropyl ketone used. Therefore, no C-alkylation product

Williamson reaction of vanillin and p-tert-butylphenol. was observed in Williamson reactions.

Meanwhile, no C-alkylation product was generated in all

reactions.

Experimental

Materials

Silica nanoparticle (primary diameter 20nm, 30 wt.% silica and

70 wt.% water, pH = 7.1) was provided by Zhejiang Yuda

Chemical Co. Ltd. Dynamic light scattering analysis (DLS) and

transmission electron microscopy (TEM) images of the particles

Table 2 Williamsian reaction of various reactants derivates in PIC system

are shown in Figure S1. DTAB, CTAB, TOMAB, TBAB, BTEAB,

t

[h]

5

Conversion

rate [%]

98.6

TEAB, 18-crown-6, 4-methylphenol, 2-methylphenol, phenol, o-

chlorophenol, vanillin, p-tert-butylphenol, 4-methoxyphenol, 2-

methoxypheno, 4-nitrobenzyl bromide and diisopropyl ketone

(98%) were purchased from Maclean Biochemical Technology

Co., Ltd, (Shanghai, China) and used without further

purification. Other reagents were all analytical grade and

ultrapure water with a resistance of 18.2 MΩ cm was used.

Entry

Substrates

R₁=2-CH₃

T (°C)

1

2

30

R₁=4-OCH₃

R₁=2-OCH₃

5

97.0

96.8

3

30

4

5

6

R₁=2-Cl

R₁=H

30

5

91.8

93.9

R₂=4-NO₂

R₁=4-C(CH₃)₃

66.00

Preparation of Pickering emulsions.

R₁=2-CHO,6-

OCH₃

7

30

5

80.00

Phenol (0.001mol), NaOH (0.002mol) and water (5mL) were

placed into a glass bottle of volume 25 cm³(65mm×25mm) and

treated under the ultrasonic wave (53kHz) for 10min. The silica

aqueous solution (1w.t.%) and the cationic modifier solution (5

mol%) were successively added. Diisopropyl ketone (6mL) was

added into the bottle after the volume of the aqueous phase

was adjusted to 7mL with deionized water. This mixture was

placed overnight at 30°C. Subsequently, the solution of p-nitro

benzyl bromide (0.01mol) in 1mL of diisopropyl ketone was

added into the reaction mixtures. The mixture was

homogenized at 11800rpm for 2min using an IKA ULTRA-

TURRAX T-18 homogenizer with a 1cm head. The emulsion was

placed at 30°C. Silica and the cationic modifier are expressed by

weight percent and molar ratio to phenolate anion in water,

respectively.

After the completion of the reaction, the organic phase and the

aqueous phase were separated from the reaction mixture with

centrifugation. The organic phase was washed with HCl solution

(pH=5) and dried over magnesium sulphate. Solvent was

removed under reduced pressure and the residue was

recrystallized from n- hexane. NMR spectra of the products

were recorded on a 400 MHz spectrometer.

8

9

10

11

R₂=2-NO₂

R₂=H

R₂=2-CN

R₂=2,6-2Cl

30

5

8

7

6

93.0

90.0

92.1

91.3

R₁=4-CH₃

Reaction conditions: 1 mmol of phenols derivate; 0.7 mmol of benzyl bromide

derivates; 5 mol % of DTAB; 6 mol % of TBAB; 2 wt.% of SiO₂; organic solvent (7

mL); water (7 mL); 30°C.

Williamson reactions of the alkoxide ion with a primary alkyl

halide proceed with an SN₂reaction mechanism. With the

introduce of the electron-donating group, the density of

electron cloud density in the oxygen anion increase, inducing

that the reactivity of phonic anion increased. Similarly, with the

introduce of electron-withdrawn group, the density of electron

cloud density in the carbon atom for the methylene group

decrease, inducing that the reactivity of the benzyl group

increased. Therefore, the conversion rates of 4-nitrobenzyl

bromide in Williamson reactions of o-cresol, 4-methoxyphenol,

2-methoxyphenol are higher than that in Williamson reactions

of O-chlorophenol and phenol, and the conversions rate of

benzyl bromide are lower than that for benzyl bromide

derivates with electron withdrawing group. However, for p-

tertbutyl phenol and vanillin, the anion exhibits a low

Characterization

J. Name., 2013, 00, 1-3 | 7

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Catalysis Science & Technology

Page 8 of 10

ARTICLE

Journal Name

The type of the emulsion was confirmed using the “drop” test. emulsifying ability of the modified particle was not influenced

View Article Online

Optical microscope photographs of the emulsion droplets were by the bilayer adsorption. The existence^Do^Of^I:t¹h⁰e^.10io³⁹n^/-^Cp⁹a^Cir^Y0in⁰⁶t²h⁰e^F

obtained using a Lv100N POL microscope system (Nikon Co.). surface of the particle caused that the modified particle

Microscope Images were analysed with ImageJ software participated the catalytic process and was the true catalyst. In

(National Institutes of Health, USA) to obtain the size of the the catalytic cycle, the ion-pair adsorbed on the surface could

droplets. The distribution function was obtained by the react with benzyl group. DTA⁺was generated and the next

treatment of experimental data with log-normal function.

catalytic cycle was initiated. TBAB could be used as the

The average size of silica particles was measured using a cocatalyst to increase the conversion rate and exhibited a subtle

Zetasizer Nanosizer (Nano ZS; Malvern). Light-scattering cells of influence on the emulsion stability. The conversion rate was

10mm were used. Particle morphology observed by more than 90%, except for the reaction of benzyl bromide

transmission electron microscopy (JSM-1200EX T20, JEOL). The derivates with vanillin or p-tert-butylphenol. These

Zeta potential of the aqueous dispersion of silica/DTAB was characteristics highlight that the PIC system for PTC reaction can

performed by a Zeta PALS potential analyser (Brookhaven, be used as promising green chemical catalytic agents that

USA).

strongly encourage further studies on the various high-

The conversion of benzyl bromide derivates for Williamson performance PIC system.

reaction was determined by high-performance liquid

chromatography (HPLC) analysis. Samples (about 50 μL) of the

reaction mixture were withdrawn at a regular time interval and

Conflicts of interest

put into a test tube. Subsequently, 0.2 mL of 10% (v/v) HCl

solution was added to quench the reaction, and the solution

was diluted to 10 mL with 50% (v/v) aqueous acetonitrile

solution. The contents of benzyl bromide derivates were

determined by HPLC analysis (HPLC) was performed using a

Zorbax Eclipse XDB-C18 column (150mm×0.46mm, 0.5μm) on

an Agilent 1260 system attached with a DAD detector. The

compositions of the sample were estimated with an internal

standard method and diphenyl was used as internal standard.

HPLC analysis was operated with a mobile phase consisting of

40:60 (v/v) water/acetonitrile and a flow rate of 1.0 mL/min.

Detection wavelength was set at 260nm and the column

temperature was 30°C.

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural

Science Foundation of China (21606206), Public Welfare

Technology Application Research Project of Zhejiang Province

(2017C31033, 2017C31089, LGG18B060001), the Science

Foundation of Zhejiang Sci−Tech University (ZSTU) under Grant

No. 16012058−Y, Zhejiang Provincial Top Key Academic

Discipline of Chemical Engineering and Technology of Zhejiang

Sci−Tech University (CETT 2016001)

The apparent contact angles of aqueous surfactant solution on

silica under oil phase were measured using an optical drop

shape analyser. The PIC system was prepared. The modified

silica and the aqueous phase were separated from the reaction

mixture with centrifugation. The modified particle was dried at

60 °C in a vacuum oven for 24 hours. The obtained power was

pressed into disks. The disk was put in a glass box, and oil phase

was added using a pipet along the wall of the box. A sessile drop

of aqueous phase (3μL) was released under the liquid level. The

image of the sessile drop was recorded, and the contact angle

was measured using software.

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PIC system for PTC reaction was successfully prepared and

exhibited enhanced catalytic activity. Cationic surfactant with

the long chain was the efficient modifier for silica particle to

construct the stable PE for Williamson reaction. With 2% wt. of

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DOI: 10.1039/C9CY00620F

Static PIC system with silica particle was prepared for PTC reaction and

exhibited enhanced catalytic activity

Article Doi

Static phase transfer catalysis for Williamson reactions: Pickering interfacial catalysis

DOI: 10.1039/c9cy00620f

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c9cy00620f

Full text of DOI:10.1039/c9cy00620f

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