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一锅法合成金鸡纳碱相转移催化剂

Catalysis Science &Technology

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Cite this:DOI:10.1039/c4cy01518e

Received 18th November 2014,Accepted 7th January 2015DOI:10.1039/c4cy01518e https://www.wendangku.net/doc/f718716947.html,/catalysis

Facile one-pot fabrication of a silica gel-supported chiral phase-transfer catalyst —N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium salt ?

Dandan Feng,a Jinghan Xu,a Jingwei Wan,a Bing Xie *b and Xuebing Ma *a

A novel type of silica gel-supported cinchona alkaloid-based quaternary ammonium salt was prepared by available one-pot synthesis for the first time through the free radical addition of the sulfhydryl group of 3-mercaptopropyltrimethoxysilane to an exocyclic carbon –carbon double bond in N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide and subsequent hydrolysis of trimethoxysilane.In the α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester with alkyl halides,it was found that the various substituted benzyl bromides,both with electron-withdrawing (–CF 3and –F)and electron-donating (–CH 3)substituents,

afforded the corresponding α-alkylation products with moderate to excellent enantioselectivities (76.0–96.9%ee)in high yields (80–96%).However,allyl bromides gave poor yields (10–50%)and enantio-selectivities (52.0–67.1%ee).After completion of the α-alkylation reaction,the silica gel-supported chiral phase-transfer catalyst was readily recovered in quantitative yield by filtration and reused for five consecu-tive runs without significant loss in the catalytic performance.

Introduction

Due to its use of simple experimental procedures,mild reac-tion conditions,inexpensive and environmentally benign reagents and solvents,phase-transfer catalysis (PTC)has long been recognized as a versatile method to achieve a wide vari-ety of transformations in the field of organic synthesis.1Vari-ous structurally well-defined non-natural and natural phase-transfer catalysts (PTCs),such as cinchona alkaloid-derived quaternary ammonium salts and N -spiro ammonium salts,2have been developed,particularly in the last 20years.Among them,some privileged chiral PTCs have been successfully and widely used in various asymmetric syntheses such as alkylation,3conjugate addition,4Mannich reaction 5and Aldol reaction.6However,some practical limitations of the PTC method are the difficult recovery of homogeneous PTCs and ease of formation of stable emulsions.From the viewpoint of green chemistry,many endeavors have been devoted to

solving these problems.One of these is the immobilization of PTCs on various polymer supports such as Merrifield resin,7poly(ethylene glycol)8and polystyrene.9It is worthwhile to note that these polymer supports have low specific surface areas and obvious shrinkage characteristics in aqueous/organic systems,resulting in decline in the catalytic performance,compared to their corresponding homogeneous analogues owing to diffusional retardation.

It is well-known that inorganic materials are excellent supports for the heterogenization of stereoselective organo-catalysts due to their excellent thermal and chemical stabil-ity,large specific surface areas,well-defined tunable pores and adjustable hydrophobic or hydrophilic character.10Silica gel is one of their outstanding representatives.11However,the immobilization of homogeneous chiral PTCs on supports with an inorganic backbone such as silica gel has seldom been reported.12Particularly,in the enantioselective α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester,strong bases such as concentrated KOH,NaOH or CsOH solu-tions are required and could result in serious corrosion of the inorganic backbone of the catalyst support.This may be the reason why there is no report on inorganic material-supported chiral PTCs used in strong basic medium.

In this paper,we developed a novel type of recoverable silica gel-supported cinchona alkaloid-based quaternary ammonium salt,N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide,through available one-pot synthesis for the first time,in which the inorganic backbone of silica was meticu-lously wrapped by alkyl organic moieties,protecting it

Key Laboratory of Applied Chemistry of Chongqing Municipality,

School of Chemistry and Chemical Engineering,Southwest University,Chongqing,400715,PR China.E-mail:zcj123@https://www.wendangku.net/doc/f718716947.html,;Fax:+862368253237;Tel:+862368253237b

School of Chemistry and Environmental Science,Guizhou Minzhu University,Guiyang,550025,PR China.E-mail:bing_xie1963@https://www.wendangku.net/doc/f718716947.html,;Fax:+868513610278;Tel:+868513610278

?Electronic supplementary information (ESI)available:1H and 13C NMR spectra of the α-alkylation products;HPLC spectra of racemic and enantioselective alkylation products;and XRD diffraction of SiO 2@CDPTC.See DOI:10.1039/c4cy01518e

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from alkali corrosion (Scheme 1).In the enantioselective α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester with various o ,m ,p -substituted benzyl bromides,moderate to excellent yields (80–96%)and enantioselectivities (76.0–96.9%ee)for α-alkylation products and satisfactory tolerance of the supported PTCs were achieved under the optimal conditions.

Experimental

General methods

All chemicals were purchased and used without further puri-fication.N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide (CDPTC)was synthesized using cinchonidine (CD)as the starting material according to the reference and ascertained by 1H NMR.13

Reaction monitoring was accomplished by TLC on silica gel PolyGram SILG/UV254plates.FT-IR spectroscopy was performed on a Perkin-Elmer model GX spectrometer using the KBr pellet method with polystyrene as the standard.Thermogravimetry-differential thermal analysis (TG-DSC)was carried out on a SBTQ600thermal analyzer at a heating rate of 20°C min ?1from 40°C to 900°C in air using N 2as protective gas (100mL min ?1).Elemental analysis of C,H,N,O and S in the catalyst was performed using a vario Micro cube elemental analyzer instrument.The surface morphologies of the samples were determined using a Tecnai G2F20transmission electron microscope operated at 200kV.X-ray powder diffraction patterns were analyzed using an XRD-7000S/L instrument:Cu-K αradiation,X-ray tube settings of 40kV/30mA and a step size of 2°min ?1in the 10–100°(2θ)range.N 2adsorption –desorption analysis was carried out at 77K using an Autosorb-1apparatus (Quantachrome),in which the sample was degassed at 120°C for 12h before measurement.The specific surface area and the pore diameter were calculated by the BET method and the BJH model,respectively.The enantiomeric excess (%ee)of the α-alkylation products was determined with an Agilent LC-1200HPLC using Phenomenex Lux 5u Amylose-2and Daicel Chiralpak OD-H 4.6mm ×25cm columns ?n -hexane/2-propanol =95/5)under the conditions of 20°C,254nm and 0.5mL min ?1.

One-pot preparation of the silica gel-supported PTC catalyst A sealed round-bottom flask (100mL)was charged with N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide (265.3mg,0.5mmol),?3-mercaptopropyl)trimethoxysilane (3-MPTS,392.7mg,2.0mmol)and AIBN (16.4mg,0.1mmol),flushed three times in an Ar atmosphere.Then CHCl 3(30mL)was added using a syringe,and the reaction mixture was refluxed for 72h at 80°C with TLC monitoring.During the catalytic reaction,AIBN (16.4mg,0.1mmol)was added once per 24hours.The reaction mixture was concentrated under reduced pressure,ethanol –water solution (0.4mL,v/v =1/1)containing 0.25mol L ?1hydrochloric acid was added,and then stirred at room temperature for 36h and at 90°C for 5h.The pale yellow solid formed was filtered,washed with CHCl 3(2mL ×3),water (2mL ×3)and ethanol (2mL ×3),and dried overnight in vacuo to afford the silica gel-supported PTC catalyst SiO 2@CDPTC (550.0mg).Anal.found:C,48.72;H,6.18;N,3.65;O,1.39;S,9.87.

General enantioselective α-alkylation reaction

To a mixture of SiO 2@CDPTC (0.11g,18.8mol%),N -?diphenylmethylene)glycine tert -butyl ester (0.15g,0.51mmol),50wt%KOH aqueous solution (1.0mL,13.4mmol)and toluene (4.0mL),o ,m or p -substituted benzyl bromide (2.5mmol)was added at ?40°C and vigorously stirred for 72h.After the complete consumption of N -?diphenylmethylene)glycine tert -butyl ester,the reaction mixture was diluted with water (5mL)and extracted with ether (5mL ×3).The catalyst SiO 2@CDPTC was recovered by filtration and reused directly.The combined organic layers were dried over anhydrous Na 2SO 4and concentrated under reduced pressure to afford the crude product,which was purified by gradient chroma-tography on alumina gel using petroleum ether/ethyl acetate ?v/v =60/1→40/1)as the eluents to afford the pure α-alkylation product.

Results and discussion

Chemical composition of the catalyst SiO 2@CD/PTC

As shown in Scheme 1,the silica gel-supported N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium salt SiO 2@CDPTC was prepared conveniently by a one-pot method through the free radical addition of sulfhydryl in 3-MPTS to an exocyclic carbon –carbon double bond in CDPTC and subsequent hydrolysis of trimethoxysilane.The free radical addition to the exocyclic carbon –carbon double bond was verified by the 1H NMR spectra of an analogue of CDPTC,which was prepared instead of 3-MPTS with mercaptan (see the ESI ?).The covalent attachment of homogeneous CDPTC to the inor-ganic backbone of silica gel is clearly corroborated by FT-IR spectroscopy (Fig.1).The C –H stretching vibrations of CH,CH,CH 2and the C C characteristic vibration bands of the quinoline ring were confirmed,respectively,at 3056,2925,2866cm ?1and in the 1600–1456cm ?1range.In particular,the stretching vibrations of the νOH and νSi –OH bands with

high

Scheme 1One-pot synthesis of the silica gel-supported cinchona alkaloid-derived PTC catalyst.

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relative intensities are very informative.The very strong νOH stretching vibrations in the 3600–3200cm ?1region are attributed to the hydrogen-bonded silanol groups and adsorbed water on internal and external surfaces,which elu-cidated that the catalyst is hydrophilic.The peaks positioned at 1253and 692cm ?1indicate the stretching vibrations of the C –S –C and C –Si bonds,respectively.Furthermore,the charac-teristic asymmetric stretching (νas ),symmetric stretching (νs )and bending modes of Si –O –Si 14located at 1029,766and 466cm ?1,respectively,show the formation of the inorganic backbone of silica.Therefore,it can be concluded that the homogeneous CDPTC was successfully anchored to the back-bone of silica.

The loading capacity of homogeneous CDPTC in the back-bone of silica could be evaluated from the content of nitro-gen in SiO 2@CDPTC determined by elemental analysis.Based on the content of nitrogen (3.65%),the loading capacity of CDPTC was calculated to be 0.87mmol per gram of SiO 2@CDPTC,which elucidated that 95.7%of CDPTC was immobilized on the backbone of silica through the one-pot method.Furthermore,according to the contents of nitrogen (3.65%)and sulfur (9.87%),the molar ratio of free to CDPTC-attached 3-mercaptopropyl organic moieties on the surface of silica was calculated to be 3.54.

Thermogravimetric analysis (TGA)was performed in order to further offer insight not only into the chemical composi-tion,but also into the thermal stability of CDPTC attached to SiO 2@CDPTC.From the TGA curves (Fig.2),it was found that the thermal decomposition of SiO 2@CDPTC occurs in three main steps:the first one with a 2.4%weight loss between 25°C and 100°C corresponds to the surface-bound or inter-calated water adsorbed in the pores;the second one with a sharp weight loss of 37.4%between 100°C and 400°C,accompanied by an endothermic peak in the DSC curve,is related to the initial decomposition of grafted organic moie-ties;and the third one with a similar sharp weight loss (39.7%)in the temperature range of 400–730°C is attributed to the further decomposition of the organic fragments.More-over,the total thermal weight loss (77.1%)in the 100–730°C

range is in perfect accordance with the total mass (76.8%)of C,H,N,O,S and Br elements in SiO 2@CDPTC obtained by means of elemental analysis and using the precipitation method using AgNO 3as a precipitant.Surface morphology

Taking into account the compact relationship between the surface properties of the supported catalyst and its catalytic performance,SEM and TEM studies were necessary to eluci-date the surface morphology,particle size and pore struc-ture of SiO 2@CDPTC.After being well-dispersed in ethanol (2–3mg of the sample in 5mL of ethanol)under ultrasonic irradiation for 10min,sputtered over copper wire and dried under infra-red irradiation,the TEM images were observed at an acceler-ating voltage of 200keV.Fig.3presents the typical scanning electron microscopy (SEM)and transmission electron micros-copy (TEM)images of SiO 2@CDPTC.From the SEM image,SiO 2@CDPTC has a micrometer-sized honeycomb-like shape (Fig.3a).The TEM image further shows that SiO 2@CDPTC possesses spherical particles with a uniform diameter of approximately 0.3–0.5um (Fig.3b),whereas the HRTEM image clearly shows the more delicate surface morphology with irregular ribbing at distances of 0.5–1.5nm (Fig.3c).Furthermore,the electron diffraction pattern (inset of Fig.3c)exhibits the amorphous structure feature of SiO 2@CDPTC,which is also supported by X-ray diffraction measurements (see the ESI ?

Fig.1Comparative FT-IR spectroscopy of homogeneous CDPTC and the supported catalyst SiO 2

@CDPTC.

Fig.2Thermogravimetric curves of the supported catalyst

SiO 2

@CDPTC.

Fig.3The surface morphologies of the supported catalyst

SiO 2@CDPTC:(a)SEM,(b)TEM and (c)HRTEM images.

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Porous structure

The nitrogen adsorption –desorption isotherms of SiO 2@CDPTC correspond to the classic definition of a Type II isotherm,which matched well with the normal form obtained with a non-porous or macroporous adsorbent (Fig.4).The pore size distribution obtained using the BJH algorithm on the desorption isotherm showed uneven microporous (<2nm)and mesoporous (2–30nm)structures.The BJH curve clearly displayed four peaks with the micropores centered at about 0.9,1.2,1.4and 1.8nm and three peaks with the mesopores centered at about 3.6,4.8and 7.1nm (inset of Fig.4).Regrettably,from the BET analysis,the surface area and pore volume of SiO 2@CDPTC were calcu-lated to be 0.56m 2g ?1and 2.72×10?3cc g ?1,respectively.Catalytic performance

The catalytic efficiency of SiO 2@CDPTC was evaluated by the model enantioselective phase-transfer α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester,which could produce a large number of natural and non-natural optically active α-amino acids.15

In order to screen out the optimal reaction conditions,an initial attempt was made using a three-variable screening experiment.As can be seen in Fig.5,the used amount of SiO 2@CDPTC,reaction time and temperature have different degrees of influence on the enantioselectivity,among which the reaction temperature is a key factor that affects the enantioselectivity.The enantioselectivity increased sharply when the reaction temperature decreased from 10°C to ?40°C.SiO 2@CDPTC (18.8mol%)gave 87.1–90.1%ee,93.0–94.1%ee and 94.1–96.0%ee values,respectively,at 0°C,?20°C and ?40°C.However,no remarkable difference in enantioselectivity was observed for the different amounts of SiO 2@CDPTC in the whole reaction.Furthermore,it was found that the reaction temperature has a significant effect on the yields of the product (Table 1).At 0°C,all enantio-selective α-alkylations gave excellent yields (>98%)for the different amounts of the catalyst used.However,with the

decrease in the reaction temperature,the yields sharply decreased.At ?20and ?40°C,only the α-alkylation reaction catalyzed by 18.8mol%SiO 2@CDPTC produced the α-alkylation product in an excellent yield (>98%).Therefore,by considering the two factors of enantioselectivity and yield,the experimental parameters of ?40°C for 72h with 18.8mol%SiO 2@CDPTC were chosen as the optimized reaction condi-tions to be applied in the following catalytic experiments.Encouraged by the remarkable catalytic results of α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester with benzyl bromide,the substrate scope was extended to various o ,m ,p -substituted benzyl bromides under the optimal catalytic conditions and the results are summarized in Table 2.It was found that the various substituted benzyl bromides,both with electron-withdrawing (–CF 3and –F)and electron-donating (–CH 3)substituents,could produce the cor-responding alkylation products with high

enantioselectivities

Fig.4N 2adsorption –desorption isotherms of SiO 2@CDPTC and pore size distributions obtained by BJH analysis of the desorption

isotherm.

Fig.5The enantioselectivities of α-alkylation products in the three-variable screening experiments.

Table 1The yields of enantioselective α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester under different

conditions

Entry Used cat.(mg/mol%)Temp.(°C)Isolated yield (%)150/8.5094270/11.90>98390/15.40>984110/18.80>98550/8.5?2052670/11.9?2065790/15.4?20918110/18.8?20>98950/8.5?40501070/11.9?40651190/15.4?408112

110/18.8

?40

>98

Reaction conditions:72h,N -?diphenylmethylene)glycine tert -butyl ester (0.15g,0.51mmol),BnBr (0.44g,2.55mmol),50%aq.KOH (1.0mL,13.4mmol),toluene (4.0mL).

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(92.8–96.9%ee)in good to excellent yields (82–96%)(entries 1–10).Moreover,when the benzyl bromides bearing o -CF 3,o -F and o -CH 3substituents were employed as electro-philes,slightly lower yields and higher enantioselectivities were obtained probably due to the sterically hindered and confined interaction between the o -substituents of the electro-philes and glycine tert -butyl ester controlled by the catalyst SiO 2@CDPTC (entries 3,6and 9).15Unfortunately,the enantio-selective α-alkylation of glycine tert -butyl ester with benzyl bromides bearing the strong electron-withdrawing NO 2group (entries 14–16)afforded moderate to good enantio-selectivities (76.0–85.6%)in high yields (90–95%).It was rather disappointing that the allyl bromides gave poor yields (10–50%)and unsatisfactory enantioselectivities (52.0–67.1%ee)(entries 11–13).

Compared with the catalytic efficiency of the homogeneous organocatalyst CDPTC,SiO 2@CDPTC afforded the same excel-lent enantioselectivities for various benzyl bromides bearing –CF 3,–F and –CH 3substituents at o ,m ,p -positions,whereas poorer enantioselectivities were obtained in the heterogeneous catalysis for the allyl bromides (Table 2).Moreover,in order to achieve similar yields to those in homogeneous catalysis,the reaction time in heterogeneous catalysis was prolonged from 7h to 72h,which resulted in the dramatic decrease of the catalytic rates for all electrophiles owing to mass transfer of reactants and the embedded effect of active sites resulting from the immobilization of CDPTC on silica.

Table 2The enantioselective α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester using different electrophiles

Entry Product Time (h)Yield a

(%)%ee

728093.0(S )10

95.3(S )d

728293.7(S )c 1094

96.2(S )d

728595.2(S )109597.0(S )d

729593.1(S )10

95.6(S )d

729193.3(S )109595.9(S )d

729595.0(S )109696.7(S )d

729095.7(S )79696.3(S )d

728592.8(S )79596.3(S )d

728296.9(S )79697.0(S )d

729693.6(S )109695.2(S )d

721057.8(S )79396(S )e

721652.0(S )79597(S )e

Table 2(continued)

Entry Product Time (h)Yield a (%)%ee b

725067.1(S )7

98(S )e

489076.0(S )79398(S )e

489585.6(S )59594.9(S )d

489582.1(S )59592.7(S )d

Reaction conditions:20mol%SiO 2@CDPTC,?40°C,substituted benzyl bromide (2.5mmol),N -?diphenylmethylene)glycine tert -butyl ester (0.15g,0.51mmol),50%aq.KOH (1.0mL,13.4mmol),toluene (4.0mL).a Isolated yield.b Determined by chiral HPLC with a Phenomenex Lux 5u Amylose-2chiral column.c Daicel Chiralpak OD-H column.d Homogeneous catalysis by CDPTC.e The cited %ee from ref.13catalyzed by CDPTC at ?20°C.

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The recovery and reuse of the catalyst

After completion of the α-alkylation reaction,the catalyst SiO 2@CDPTC was readily and quantitatively recovered from the reaction mixture by filtration and directly reused five times without an appreciable drop in the yield and enantioselectivity.Although SiO 2@CDPTC was exposed for a long time in a strong alkaline environment during the reaction,it is exciting that the high enantioselectivity (91.6%ee)was retained in the sixth run (Fig.6).However,the yield of the α-alkylation product signifi-cantly decreased to 80%yield in the sixth run.In order to find out the reasons why the catalytic activity of SiO 2@CDPTC decreased,several methods such as elemental analysis,SEM,HRTEM,TGA and nitrogen adsorption –desorption were used to monitor the change in the chemical composition,pore struc-ture and surface morphology of the recovered SiO 2@CDPTC in the sixth run (Fig.6).

Compared with the fresh catalyst SiO 2@CDPTC,the recov-ered SiO 2@CDPTC lost its original micrometer-sized honey-comb-like shape as observed from the SEM image,and the TEM image also showed that the fresh regular and spherical particles with a uniform diameter of 0.3–0.5μm turned into irregular particles with a diameter of about 0.1–0.2μm.The porous structure and surface area of the recovered SiO 2@CDPTC were further monitored using N 2adsorption –desorption isotherms.Its surface area and pore volume increased from 0.56m 2g ?1and 2.72×10?3cc g ?1to 4.54m 2g ?1and 1.47×10?2cc g ?1,respectively,whereas the average pore diameter decreased from 9.7nm to 6.5nm.From the BJH curve,the pore size distribution showed that the micro-pores were little altered and similar micropores remained cen-tered at about 0.6,1.0,1.2and 1.9nm.However,the meso-porous structure exhibited a great change,including the disappearance of the mesopores at 4.8nm and 7.1nm,with only one remaining mesopore centered at 3.3nm (inset of Fig.7).Furthermore,a nonuniform microporous structure was obtained after deep hydrolysis of the recovered SiO 2@CDPTC in ethanol –water solution containing 0.25mol L ?1hydrochloric acid (see the ESI ?).Unfortunately,an unsatisfactory enantio-selectivity (82.2%ee)was obtained.Based on the results

described above,it could be concluded that the backbone of amorphous silica collapsed to some extent,and the regularity of the porous structure and the disappearance of the meso-pores resulted in the decrease in enantioselectivity due to their steric and confinement effects.On the other hand,the active catalytic sites played a dominant role in the catalytic activity.In principle,the increased surface area and pore volume of the recovered SiO 2@CDPTC,monitored by N 2adsorption –desorption isotherms,were beneficial to improve its catalytic activity.However,the weight loss of organic moieties in the tempera-ture range of 150–900°C decreased from 77.1%to 72.7%from the TG curve,which indicated the loss of CDPTC in the catalytic process.Therefore,it is conjectured that the loss of the organocatalyst CDPTC in strong alkaline medium is responsible for the decrease in the catalytic activity.

Conclusions

A novel type of amorphous silica gel-supported PTC catalyst,N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide,was prepared through a simple and available one-pot synthesis.The effective immobilization of N -?2-cyanobenzyl)-O ?9)-allyl-cinchonidinium bromide was confirmed by means of elemen-tal analysis and TGA.This catalyst is the first example of an inorganic backbone-supported PTC catalyst applied to the enantioselective α-alkylation of N -?diphenylmethylene)glycine tert -butyl ester with a variety of substituted benzyl bromides,leading to the corresponding α-alkylation products in good to high yields as well as excellent enantioselectivities.The catalyst could tolerate serious corrosion of a strong base;it is also easily recovered by filtration and can be reused six times with a slight decrease in enantioselectivity.

Acknowledgements

We are grateful to the National Science Foundation of China (21362005,21071116)and the Chongqing Scientific Foundation (CSTC,

2010BB4126).

Fig.6Reusability of SiO 2@CDPTC under the optimized reaction

conditions.

Fig.7The SEM,TEM images,TGA and nitrogen adsorption –desorption isotherms of the recovered SiO 2@CDPTC in the sixth run.

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