Preparation of O rganic Nanoacid Catalyst for Urethane Form ation

Zi-jian ChenWei-qiang ZhongDong-lin TangGuang-zhao Zhang

SchoolofMaterials Science and Engineering,South China University ofTechnology,Guangzhou 510640, China

Preparation of O rganic Nanoacid Catalyst for Urethane Form ation

Zi-jian Chen,Wei-qiang Zhong,Dong-lin Tang?,Guang-zhao Zhang

SchoolofMaterials Science and Engineering,South China University ofTechnology,Guangzhou 510640, China

M etal-free catalysts are preferred during these days in organic synthesis or in polymerizations.Sulfonic acid is reported to be effi cient in catalyzing reactions between isocyanates and alcohols.In this work,synthesis of sulfonic acid immobilized organic nanoparticles (nanoacid)and its application in catalyzing urethane formation,are elaborated.The nanoacid can be sim p ly prepared by m iniemulsion polymerization w ith a reactive surfactant,nam ely sodium 4-((perfluoronon-8-en-1-yl)oxy)benzenesulfonate,followed by an acidification.From the images of scanning electron m icroscope,the nanoacid obtained is found to be narrow ly dispersed and the average diameter is around 90 nm.Themeasured sulfur content is 0.5%,from which the content of sulfonic acid in the nanoparticles is calculated to be 0.16mmol/g.W hen catalyzing urethane formation based on hexamethylene diisocyanate andn-butanol,the nanoacid catalyst exhibits considerable effi ciency.

Nanoacid,Catalysis,Urethane,M iniemulsion

I.INTRODUCTION

Polymer-supported solid acids(PSAs),such as Amberlyst-15,Nafion NR50 and Dowex 50W X 2 have been w idely utilized as heterogeneous catalysts in organic synthesis[1–7].Com pared w ith homogeneous acids,PSAshaveattracted environmentaland econom ic attention due to its non-corrosion,easy separation, and high effi ciency in catalyzing organic reactions,e.g., alkylation,etherification,hydration,hydrogenation,nitration,oxidation and esterification[8–15].However, the particle size of such traditional PSAsare inm illim eter scale so that they must be rem oved from the polymer,otherw ise itwould aff ect themechanicalproperties of the final products.Moreover,Nafion NR50 does not swell in a lot of organic solvents so that the acid groups are not active in reactions,and the low specific surface area(about 0.02m2/g)is another factor lim iting the catalytic effi ciency[11,16].To increase the specifi c surface area of PSA,one of them ost feasible way is to make it into nanoscale.It is feasible for nanoparticles to leave in polymermaterials since they are usually app lied as additives to enhance them echanical properties of polym er materials,and furtherm ore the particles do notm igrate during processing or in use.

Polyurethanes(PUs)are a versatile class of polym ers w ith carbam ate linkages in polymer backbone.Since it isconvenient to tailor thepropertiesof PUsby ad justing their chem icalandmorphologicalmakeup,these diverse materialsarew idely utilized as foam s,elastomers,coatings,adhesives,composite materials,etc.[17].Generally,PUs are primarily synthesized via polyaddition of diisocyanates and diols in which organotins are the most commonly-used catalysts for their remarkable catalytic activity.However,organotins are diffi cult to be rem oved from PU products and the residual tin causes adverse eff ects,such as unwanted toxicities and side reactions,thus it lim its the app lication of PUs in some sensitive areas,especially in biomedical app lications [18–20].Less toxic metal catalysts[21–23]and even metal-free organic com pounds[20,24]are therefore introduced to rep lace the organotins.Metal-free organic catalysts are preferable because of their high substrate tolerance and selectivity[25].Am ong the organic catalysts,organic acids,especially sulfonic acids,show considerable catalytic effi ciency in the polymerization of PUs[26].Moreover,organic acidsare better choices for base-sensitive PUs[18,27]and the residualorganic acid catalysts do not cause the degradation of PUswhile organic bases do[26].

In this work,we elaborate the preparation and the app lication of organic nanoparticles embedded w ith sulfonic acid group on the surface(abbreviated as nanoacid).Since there is no monomer transfer in m iniemu lsion system[28–30],nanoacid is proposed to be prepared bym iniemulsion polym erization in which a reactive surfactant bearing both carbon-carbon double bond(C=C)and sulfonate is possible to react on the surface of the particles.

II.EXPERIM ENTS

A.M aterials

Hexamethylene diisocyanate(HDI,99%)and styrene (St,99%)were purchased from A laddin Industrial Corporation(Shanghai,China).Divinylbenzene(DVB, 80%)and hexadecane(98%)were bought from Energy Chem ical Corporation(Shanghai,China).Sodium 4-((perfluoronon-8-en-1-yl)oxy)benzenesulfonate(OBS) was from Hongjia Technology Corporation(Jiangsu, China).A ll chem icals mentioned above were used as received w ithout further treatment.Azobisisobutyronitrile(AIBN)waspurchased from Fuchen Chem icalCorporation(Tianjin,China)and was purified by recrystallization in ethanol before use.Toluene(AR,from Guangzhou Chem ical Reagent Factory,Guangzhou, China)andn-butanol(AR,from Chinasun Specialty Products Corporation,Jiangsu,China)were dried over calcium hydride and distilledin vacuo.

B.Characterizations

1H NMR wasdetected at 25?C on a Bruker AVANCE III HD 600(600 MHz)spectrom eter(Germany)and chem ical shiftswere reported in ppm from tetramethylsilane w ith the solvent resonance as the internal standard(DMSO-D6:δ2.50 ppm).Elemental analysiswas carried out in an Elem entar Vario EL cube elemental analyzer(Germany).In situFourier transform infrared (FTIR)spectra were collected every 30 s during the m onitored reaction by a M ettler Toledo ReactIR 15 spectroscope w ith a DiCom p diamond probe.

C.Synthesis of nanoacid

OBS(3.60 g)was fi rst dissolved in 144 g deionized water.St(28.80 g),DVB(7.20 g),AIBN(0.72 g) and hexadecane(2.88 g)were m ixed into a homogeneous liquid and then was added dropw ise into the OBSwater solution.A fter stirring for 15m in,the‘presheared’emulsion was sub jected to probe-typed sonication(JY 98-IIIDN,800W,from Ningbo Xinzhi,China) for another 15m in in an ice bath(an on-pulse period of 10-10 seconds and an output power of 66%).The obtained m iniemulsion was then transferred into a threenecked flask equipped w ith an argon inlet,a gas outlet and an overhead mechanical stirrer.Under argon flow, the reaction was heated to 70?C for 4 h.A fter reaction,the formed m iniemulsion was cooled to room tem perature and freeze-dried for 48 h to get white powder.By acidification w ith HClsolution(0.1mol/L)and then by extraction in a Soxhlet extractor for 12 h,excessm onom ers and sodium chloridewere removed.The residuewas freeze-dried again to receive the final product.

D.Nanoacid-catalyzed reactions of HDIw ithn-butanol

n-Butanol(21 mm ol,1.56 g),nanoacid and 10 m L toluenewere added in a three-neck flask equipped w ith an argon inlet,a gas outlet and a DiComp probe of ReactIR under argon.Then the flask was heated at 75?C.HDI(10 mm ol,1.68 g)was added to start the reaction and the processwasmonitored byin situReactIR spectroscope.Sam p les were taken in the end of each reaction for1H NMR m easurem ents to determ ine the final conversion.

III.RESULTS AND D ISCUSSION

A.Synthesis and characterizations of nanoacid

In thiswork,organic acid functionalized nanoparticle (nanoacid)was synthesized throughm iniemulsion polymerization.St and DVB were used as com onomers in order to obtain cross-linking particlesso that the form ed particles are able to remain heterogeneous in any reaction system w ith good chem ical resistance.OBS(chemical structure see Schem e 1)is chosen to be a reactive surfactant in order to introduce and imm obilize theacid groups on the surface of nanoparticles.The hydrophobic fluorinated moieties of OBS prefer to embed in the hydrophobic monomer drop lets so that the C=C could copolym erize w ith the m onom ers while the moiety of sodium benzenesulfonate prefers staying on the surface of the drop lets.Azobisisobutyronitrile was used as the initiator and hexadecane was app lied as a costabilizer. A fterm iniemulsion polym erization,the form ed m ixture was freeze-dried to keep the original shape as nanoparticles.Cross-linking particles bearing sodium benzenesulfonate on the surfacewere obtained.By acidification w ith dilute hydrochloric acid solution and then by extraction in a Soxhlet extractor,the excess monomers and sodium chloride were sim p ly removed.The targeted nanoparticles w ith sulfonic acid can be obtained after another freeze-drying process.

The scanning electron m icroscope image(SEM, FIG.1)tells that the particles obtained are narrow ly dispersed and the average diameter is around 90 nm. The sulfur content of the obtained nanoacid m easured by elemental analysis is found to be 0.5%,from which the sulfonic acid content can be calculated as 0.16mm ol/g.Them easured content of sulfonic acid is slightly higher than its corresponding theoretical value (0.14mmol/g)since the conversion of themonomersdid not achieve 100%and the unreacted oneswere removed during the purification process.

B.Nanoacid-catalyzed urethane form ation

Schem e 1 Synthetic schem e for nanoacid via m iniemu lsion polymerization.

FIG.1 SEM image of the obtained nanoacid.

Asmentioned above,the obtained nanoacid is supposed to be app lied in catalyzing the synthesis of urethane from isocyanate and alcohol.To evaluate the catalytic effi ciency,nanoacid was then utilized in amodel reaction of HDI andn-butanol at 75?C under argon. The progress of the reaction wasm onitored byin situReactIR spectroscope(see FIG.S1?S4 in the supp lementary materials).As shown in FIG.2,in themodel reaction,as soon as the HDIwas added,new peaks at 1728(v(C=O)),1510(δ(NH))and 1234 cm?1(v(COC)) were observed in the IR spectra.It indicates that the reaction between the NCO and the OH groups takes p lace to form carbamate groups.In particular,v(C=O) (1728 cm?1)which demonstrates the formation of carbamate is chosen to monitor the conversion of the reaction.The relationship between the absorbance and the concentration of carbam ate product app roxim ately follows the Beer-Lambert Law,whichmeans they are in a positive correlation.Sam ples were also taken for1H NMR m easurem ents to determ ine the absolute value of final conversion of HDI(see FIG.3).Based on the data of bothin situReactIR and1H NMR,the conversion of HDIwere calibrated and the final results are shown in FIG.4.It is found that the reaction catalyzed by nanoacid runs faster than that w ithout any catalyst, especially in the fi rst hour.It took 90m in for the conversion of HDI to reach 50%w ithout any catalystwhile the tim e reduced to 33m in and even shorter when catalyzed by nanoacid.W hen the am ount of nanoacid increases,the reaction rate keeps sim ilar.To achieve a HDI conversion of 95%,it took 276,273 and 255 m in when the catalyst loading was 0.1m ol%,0.2m ol%,and 0.4 mol%,respectively.0.1 mol%of nanoacid is sufficient and effi cient enough for catalyzing the reaction of HDIw ithn-butanol.

FIG.2In situReactIRm onitoring the reaction ofn-butanol and HDI catalyzed by nanoacid at 75?C.([BO]/[HDI]=2.1, 0.1mol%nanoacid was used).

FIG.31H NMR spectra(DMSO-D6)of the products catalyzed by nanoacid(0.1 m ol%,0.2 m ol%,and 0.4 m ol%).

FIG.4 The conversion curvesof HDIcatalysed by nanoacid. ([BO]/[HDI]=2.1,at 75 cm?1).

IV.CONCLUSION

A novel organic nanoacid was synthesized via m iniemulsion polymerization in which styrene and divinylbenzenewere applied as comonomerswhile a reactive surfactant OBSwasutilized to offer and immobilize sulfonic acid groups on the surface of the formed particles.The size of the particles is found to be narrow ly dispersed and the average diameter is around 90 nm. The sulfonic acid content of the nanoparticles is calculated to be 0.16mmol/g.M onitored byin situReactIR, the nanoacid is proven to be an effi cient catalyst in the urethane formation from NCO and OH groups.Further investigation and development of utilizing nanoacid in other organic reactions,e.g.,(trans)esterifications,urea formation,Biginelli reactions,etc.,is underway in our group.

Sup p lem en tary m aterials:In situReactIR spectra m onitoring the reaction ofn-butanoland HDIat 75?C and1H NMR spectra of the urethane products catalyzed by nanoacid are shown.

V.ACKNOW LEDGM ENTS

Thiswork is supported by the National Natural Science Foundation of China(No.21404040),the Fundam ental Research Funds for the Central Universities (No.2015ZM 053)and the State Key Laboratory of Pulp and Paper Engineering(No.201538).

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ceived on March 29,2017;Accepted on April 25,2017)

?Author to whom correspondence shou ld be addressed.E-m ail: m sd ltang@scu t.edu.cn