Effects of morphology and composition on

Applied Catalysis A:General 240(2003)263–271Effects of morphology and composition on catalytic performance of double metal cyanide complex catalystYi Jun Huang a ,Guo Rong Qi a ,∗,Lin Shen Chen baDepartment of Polymer Science and Engineering,Zhejiang University,Hangzhou 310027,PR Chinab Analysis and measurement center of Zhejiang University,Hangzhou 310028,PR China Received 19November 2001;received in revised form 8March 2002;accepted 1August 2002AbstractDouble metal cyanide (DMC)complexes based on Zn 3[Co(CN)6]2were prepared using different zinc salts and complexing agents and They were characterized by elemental analysis,IR and XRD.The catalytic properties of DMC complexes were determined in polymerization of propylene oxide.Great differences in morphology of DMC complexes was were correlated with preparation route as well as with the kind of zinc salts employed.The catalytic activity was strongly dependent on themorphology of DMC complexes and composition of catalysts.The study revealed that the amorphous catalyst made from ZnCl 2and t -BuOH had the highest activity (up to 26kg polymer/g catalyst).The DMC catalyst was non-stoichiometric.Polymerization of propylene oxide (PO)catalyzed by DMC proceeded in a controlled manner.The molecular weight of each polymer was controlled by the monomer/initiator ratio,the values of molecular weight distribution (MWD)were in the range of 1.23–1.48.The polymer has an atactic structure and a head-to-tail regiosequance.©2002Elsevier Science B.V .All rights reserved.Keywords:Double metal cyanide complex;Morphology;Catalytic activity;Propylene oxide;Polyether polyol1.IntroductionPolyether polyols are widely used as precursors in the manufacture of polyurethanes.These polymers are currently prepared by anionic polymerization of alkene-oxide (mainly propylene oxide),catalyzed by an alkaline base such as KOH.As a result of trans-fer reaction to the monomer,the molecular weight is limited to a relatively low value (equilvalent weight <2000)and the molecular weight distribution (MWD)is broadened.Moreover,the polyols obtained have a high unsaturation that results in a loss of hydroxylfunctionality [1].∗Corresponding author.Fax:+86-571-87951773.E-mail address:qiguorong@ (G.R.Qi).Aluminum porphyrins are excellent catalysts for polymerization of propylene oxide,affording the cor-responding polymers of controlled molecular weight with narrow MWD [2].However,the high cost of these catalysts prevents for an industrial application.Other coordination catalysts such as AlR 3–H 2O [3],ZnR 2–H 2O [4],Bu 2SnO–Bu 3PO 4condensate [5],alumoxane-propylene oxide complex [6]and rare earth metal compounds [7]have been used to prepare high molecular weight (Mn up to 105)polyoxypropy-lenes,which contain a fraction of atactic polymers in addition to isotactic polymers.Moreover,the molec-ular weight of polymers prepared by these catalysts is not predictable.Double metal cyanide complexes (DMC)are well-known catalysts for alkene-oxide polymerization.Recent improvements have made0926-860X/02/$–see front matter ©2002Elsevier Science B.V .All rights reserved.PII:S 0926-860X (02)00452-无定形的聚醚多元醇羟基部分264Y.J.Huang et al./Applied Catalysis A:General240(2003)263–271DMC catalysts much more attractive for commercialmanufacture of polyoxypropylene polyols[8–11].Thecatalysts are highly active,and give polyether poly-ols that have low unsaturation and narrow molecularweight distribution compared with similar polyolssynthesized using conventional KOH catalysis.DMC catalysts are usually prepared by react-ing aqueous solutions of water-soluble metal salts(M1X w)and metal cyanide salts to form a precipitateof DMC compound.A water-soluble organic com-plexing agent(L)is included in the preparation ofeach catalyst.The resulting catalyst has the generalformula:M1u[M2(CN)6]v·x M1X w·y L·z H2O.In fact,it is found from patent reports that the mostwidely used DMC catalysts which are of importancein industry applications are based on Zn3[Co(CN)6]2.However,DMC catalysts are very versatile,and evenif prepared with the same starting material,they mayshow great differences in catalytic activity.In ad-dition to patent reports,few open studies on DMCand DMC catalyzed polymerization of alkene-oxideare reported.The present work was mainly aimed atstudying the interrelation of differences in morphol-ogy and composition with catalytic activity.2.Experimental2.1.MaterialK3Co(CN)6(ACROS95%)was recrystallized be-fore use,ZnCl2,ZnSO4·7H2O,zinc acetate(Zn(Ac)2), dioxane,tetrahydrofuran(THF),dimethoxyethane (DME)are all analyticallg pure grade reagents. Tert-butyl alcohol(98%)was used without further pu-rification.Propylene oxides(>99.8%pure)were used as received.PPG-400was a polyoxypropylene diol (Mn≈400)made with KOH and was a commercial product.2.2.Catalyst preparationIn order to investigate the dependence of catalytic activity on preparation conditions,several different methods were adopted to prepare DMC catalysts. (a)A solution of3g K3Co(CN)6in40ml of H2O and8ml of complexing agent(solution1)was addeddropwise with stirring to a solution of0.08mol Zn2+in40ml water and15ml complexing agent (solution2).The resulting white suspension was pressurefiltered.DMC solids were isolated,and were resuspended with vigorous stirring in a solu-tion of complexing agent and H2O(v/v=3/1).The solids were again isolated by pressurefil-tering.This process(alternatefiltration and re-suspending)was repeated several times.Finally, the solids were resuspended in neat complexing agent,and isolated as described above.The result-ing DMC catalyst solids were dried under vacuum at50◦C for more than8h.(b)The preparation was similar to method(a),exceptthat volumes of initial complexing agent included in solutions1and2were4and8ml,respectively.(c)A solution of3g K3Co(CN)6in40ml of H2Owas added dropwise with stirring to a solution of0.08mol Zn2+in40ml water at30◦C.Then theDMC solids were isolated by pressurefiltration, and were resuspended in50ml H2O.This process was repeated several times.Finally,the solids were vacuum-dried.(d)DMC catalysts prepared with method(c)were re-suspended in neat complexing agent again,and specified amounts of ZnCl2(dissolved in a mix-ture of H2O and complexing agent)were added with stirring.The resulting suspensions were di-rectly dried under vacuum.(e)A solution of3g K3Co(CN)6in40ml of H2Owas added dropwise with stirring to a solution of0.08mol Zn2+in40ml water at30◦C.After theaddition,23ml of complexing agent was added to the suspension.Stirring continued for another1h, then the DMC solids were isolated.The subse-quent steps were similar to those in method(a).(f)The preparation was similar to method(e),ex-cept that after the addition of K3Co(CN)6,the resulting suspension was heated for12min at 100◦C.All these above-prepared catalysts are insoluble in monomers,polyether polyols and in all common or-ganic solvents.Thus these DMC catalysts seem to be heterogeneous.Interestingly,unlike normal heteroge-neous catalysts,most of the DMC catalysts in this study became soluble(the reaction mixture became clear)after polymerization started.四氢呋喃二甲氧基乙烷Y.J.Huang et al./Applied Catalysis A:General 240(2003)263–2712652.3.Polymerization experimentPolymerizations with one-step and semi-continuous addition of monomer were carried out as described previously [12].A polyoxypropylene diol was used as an initiator.A clear liquid product was ob-tained.Herein the catalytic activity is defined as the weight of finished polymer per weight of Zn 3[Co(CN)6]2.2.4.MeasurementWide-angle X-ray diffraction (W AXD)patterns of the DMC catalyst were obtained by means of a Ru-gaku D/MAX-B X-ray diffractometer using Cu K ␣ra-diation.The scan rate was 3◦/min and the scan range is 30–70◦.The elemental analysis of catalysts was performed by using a Hitachi 180–50atomic absorp-tion spectrophotometer and a ThermoQuest EA 1110instrument.13C NMR spectra of the polymer were obtained on a Avance DMX 500spectrometer with CDCl 3as a solvent.Infra-red spectra of catalysts were measured with a VECTOR-22IR spectrometer.Num-ber average molecular weight of the polymers was de-termined by using a KNUER vapor phase osmometer (THF as solvent).Molecular weight distributions were measured using a Waters 150-C instrument,operated at 25◦C,with a set at 104,103and 500Åcolumns.Table 1Effect of preparation method of DMC catalyst on catalytic activity a Catalyst Method CompositionK +(mg/kg)Activity (kg/g)1a Zn 3[Co(CN)6]2·1.0ZnCl 2·2.5t -BuOH ·4H 2O 30514.12a Zn 3[Co(CN)6]2·0.85ZnCl 2·2.2t -BuOH ·5H 2O 27914.63a Zn 3[Co(CN)6]2·1.2ZnCl 2·2.5t -BuOH ·5H 2O 29613.84b Zn 3[Co(CN)6]2·0.94ZnCl 2·2.5t -BuOH ·5H 2O 21713.95b c Zn 3[Co(CN)6]2·8H 2O850.126c c Zn 3[Co(CN)6]2·1.2ZnCl 2·6H 2O 980.417d c Zn 3[Co(CN)6]2·1.3t -BuOH ·5H 2O950.268d Zn 3[Co(CN)6]2·0.22ZnCl 2·1.6t -BuOH ·4H 2O 90 6.59d Zn 3[Co(CN)6]2·0.65ZnCl 2·2.5t -BuOH ·6H 2O 987.210d Zn 3[Co(CN)6]2·1.6ZnCl 2·4.1t -BuOH ·3H 2O 125 6.811e Zn 3[Co(CN)6]2·0.86ZnCl 2·3.8t -BuOH ·3H 2O 1808.212fZn 3[Co(CN)6]2·1.1ZnCl 2·3.8t -BuOH ·4H 2O95–aOne-step polymerization:catalyst level =55mg/kg (concentration in a mixture of monomer and initiator on a solid Zn 3[Co(CN)6]2weight basis),PO/PPG-400(w /w )=40/1,temperature =100±3◦C,reaction time =6h.b Catalyst level =3500mg/kg.c Catalyst 6was prepared by treating catalyst 5with ZnCl 2,catalyst level =2500mg/kg.d Catalyst 7was prepared by treating catalyst 5with BuOH,catalyst level =2500mg/kg.Polystyrene standards with low polydispersities were used to generate a calibration curve.The unsaturation of each polymer was determined according to ASTM D4671–D87.3.Results and discussion3.1.The non-stoichiometric nature of the DMC catalystUsing K 3Co(CN)6,ZnCl 2and t -BuOH as starting materials,we prepared various catalysts by different methods.Table 1shows their compositions detected according to elemental analysis and the correspond-ing polymer yields in polymerization of PO.From Table 1,it is seen that catalysts 1–3,which were pre-pared by the same method had different composition formulas (existence of t -BuOH and its coordination to Zn 2+had also been confirmed by IR spectra (omit-ted)).The reason for such differences in composi-tion may be unavoidable variations during operation or post-treatment.Fortunately,these differences had no effect on activity.Catalysts 8–10,in which dif-ferent amounts ZnCl 2were added deliberately,also had similar catalytic activities.There was no ZnCl 2retained in the catalysts prepared with method (c),though excess ZnCl 2was used in the preparation,and266Y.J.Huang et al./Applied Catalysis A:General 240(2003)263–271the catalysts in which ZnCl 2and t -BuOH were ab-sent showed rather low activity.Probably retention of certain amounts of ZnCl 2in these DMC catalysts is associated with use of t -BuOH during pared with catalyst 4,introducing ZnCl 2(cata-lyst 6)and t -BuOH (catalyst 7)raised the activity of Zn 3[Co(CN)6]2by factors of about 3.4and 2.1,re-spectively.Surprisingly,a combination of these two agents (catalyst 8–10)resulted in a great increase in catalytic activity of Zn 3[Co(CN)6]2(up to 50times).This result indicatesthat ZnCl 2and t -BuOH (complex-ing agent)were both required as important promoter for high activity of DMC catalyst,and that the consid-erable variations of the content of ZnCl 2and t -BuOH had no significant effect on catalytic activity.Thus the DMC catalyst is of non-stoichiometric nature.3.2.Effect of morphology of DMC on catalytic activityAs shown in Table 1,DMC catalysts prepared by different methods exhibited great differences in cat-alytic activity.The catalytic activity of these catalysts decreased in order of preparation method:a ∼b >e >d >f.Fig.1.XRD patterns of DMC catalysts prepared with different methods.Fig.1shows XRD patterns of catalysts prepared by different methods.From Fig.1,the crystalline phase of all samples was Zn 3[Co(CN)6]2since there were no peaks related to ZnCl 2.Catalyst 1was low crys-talline or substantially amorphous,lacking sharp lines in the XRD pattern,whereas catalyst 11exhibited a crystalline pattern,which ascribed to a cubic lattice structure (d value:5.12,3.62and 2.56Å).Indeed,cat-alysts prepared by methods (c)and (d)also exhibited a cubic lattice structure.Catalyst 10had a rhombo-hedral lattice structure (d value:9.03,6.54,5.42and 4.51Å).In contrast to catalyst 1,catalyst 4exhibited a crystalline pattern.Unfortunately,this crystal struc-ture modification of Zn 3[Co(CN)6]2can not be identi-fied using the standard JCPDS.These results indicate that the morphology of each catalyst was largely de-pendent on preparation conditions.On the other hand,it can be concluded that the morphology had a signif-icant effect on catalytic activity.3.3.Effect of complexing agent on catalytic activity Table 2shows the catalytic activities of DMC cat-alysts prepared by methods (a)and (b)using vari-ous complexing agents.The complexing agent clearlyY.J.Huang et al./Applied Catalysis A:General 240(2003)263–271267Table 2Effect of complexing agent on catalytic properties of DMC catalyst a Catalyst Preparation method Complexing agent Catalyst level (mg/kg)Reaction time (h)Conversion (%)Activity (kg/g)1a t -BuOH 5056813.613a Dioxane 505469.214a DME 100543 4.315a THF1505––16a t -BuOH/dioxane b 5056212.517a t -BuOH/DME b 5055811.618e t -BuOH 100565 6.519e DME5055310.620e t -BuOH/DME 502551120et -BuOH/DME5055711.2a Other polymerization conditions as described in Table 1.bMixed ratio =1/1(v/v).played an important role.Catalytic activities of cat-alysts prepared by method (a)decreased as follows:t -BuOH >dioxane >DME >THF.However,when prepared with method (e),DME was a better candi-date than t -BuOH.Interestingly,if a mixture of DME and t -BuOH was used as complexing agent,the cata-lyst prepared with method (e)could provide the high-est rate of polymerization,though the polymer yield was not the highest.These results indicate the exis-tence of an optimum combination of preparation and complexing agent regarding catalytic activity.3.4.Effect of water-soluble metal salts (M 1X w )on catalytic activityAs mentioned in the introduction of this paper,on the one hand M 1X w was used as cation source inTable 3Effect of M 1X w on catalytic activity of DMC catalyst a Catalyst M 1X w Catalyst level (mg/g)Time (h)Conversion (%)Activity (kg/g catalyst)1ZnCl 25557814.121ZnSO 44005––22+HCl b 3005––23+ZnCl 2c 3005––24Zn(Ac)22005––25+HCl b 5556511.826+ZnCl 2c 5556010.627CoCl 22006––28NiCl 22006––29NiCl 22006––a Other polymerization conditions as described in Table 1.bCatalysts 21and 24were prepared by treating catalysts 20and 23with 1.0ml HCl (37%)+10ml t -BuOH,respectively.c Catalysts 22and 26was prepared by treating catalysts 20and 24with 0.4g ZnCl 2+0.6ml H 2O +10ml t -BuOH,respectively.formation of precipitate,while on the other hand it was retained or intentionally added to the finished catalyst as a promoter.Table 3shows the catalytic activities of DMC catalysts prepared by method (a)using various M 1X w .Table 2shows that the catalyst prepared with ZnCl 2had the highest activity,whereas those prepared with ZnSO 4(catalyst 21)and Zn(Ac)2(catalyst 24)had low activity.Interestingly,after being treated with HCl or ZnCl 2,catalysts prepared with Zn(Ac)2exhibited considerable higher activity.The reason may be that Zn(Ac)2retained in the catalyst was changed to ZnCl 2.However,either treated with HCl or with ZnCl 2,no improvement of activity of the catalyst prepared with ZnSO 4was observed.We therefore investigated the morphology of the above samples.Fig.2shows XRD patterns of catalysts268Y.J.Huang et al./Applied Catalysis A:General 240(2003)263–271Fig.2.XRD patterns of DMC catalysts prepared using different M 1X w .prepared by method (a)using Zn(Ac)2and ZnSO 4as M 1X w .The morphology of these catalysts differed ob-viously from that prepared using ZnCl 2.The catalyst prepared with ZnSO 4exhibited a rhombohedral lat-tice structure (9.01,6.54,5.41and 4.50Å).This crys-tal structure and the corresponding low activity were consistent with that of the catalyst mentioned above.The catalyst prepared with Zn(Ac)2also exhibited a highly crystalline pattern which can not also be identi-fied using the standard JCPDS.Other kinds of transi-tion metal salts,such as CoCl 2,NiCl 2and CdCl 2had been used instead of ZnCl 2to prepare catalysts.How-ever,such catalysts exhibited a rather low activity.The above results indicate thatZnCl 2was by far the most suitable candidate as cation source and as an impor-tant promoter in DMC catalyst,and the morphology of DMC catalysts was largely dependent on the kind of M 1X w employed.3.5.Characteristic features of DMC-catalyzed polymerization of propylene-oxideFig.3shows a plot of propylene oxide consump-tion versus reaction time.The polymerization reac-tion exhibited a very short induction period (severalminutes).This is in contrast to the long induction pe-riod (several hours)of the typical DMC catalyst as reported by some patents [9,10].It can be seen that the reaction rate decreased gradually with incremental monomer addition and finally,further consumption of the monomer was limited,which means the presenceFig.3.Polymerization of propylene oxide with different amounts of catalyst (40g PPG-400as initiator,at 120±2◦C,with semi-continuous addition of monomer):(᭿)0.038g;(᭹)0.024g;(᭡)0.012g.Y.J.Huang et al./Applied Catalysis A:General240(2003)263–271269 Table4Polymerization of PO catalyzed with DMC catalyst aRun P/I(w/w)b M n(VPO)M w/M n M n(calculated)c Unsaturation(meq/g)1 4.31985 1.2321200.0082 5.52329 1.2526000.01137.73326 1.2834800.012 4176522 1.4872000.01257.53515 2.8134000.009616.26258 3.4768800.011a PPG-400(40g)as initiator,run1–4,polymerization with semi-continuous addition of monomer,at120±3◦C,catalyst 1=0.05g;run5and6polymerization with one-step addition of monomer:at100±3◦C,catalyst concentration=55mg/kg.b P/I:weight ratio offinished polymer to initiator.c M n(calculated)=(P/I)×M n(initial)=(P/I)×400.of a termination process.Both polymer yield and re-action rate decreased with clearly decrease of catalyst amounts.Table4shows the experimental results of poly-merization of propylene oxide.All polymers had a low level of unsaturation without respect to molecular weight,such levels demonstrated the relative absence of side reactions such as transfer reaction to monomer. Interestingly,polymers prepared by semi-continuous addition of monomer exhibited a narrower MWD than that prepared with prepared by one-step addition of monomer.Furthermore,molecular weight of polymers formed was consistent with the calculation based on monomer to initiator ratio(M/I),not with that based on the monomer to catalyst ratio(M/C),which in-dicates that every molecule of the initiator partici-pated in initiation and propagation steps.Considering the fact that the moles of initiator exceeded greatly the moles of catalyst,a rapid exchange must exist be-tween dormant species and active species.From the practical viewpoint,this control behavior of molec-ular weight is highly desirable.In our forthcoming paper[12],partial living polymerization characteris-tics of DMC system will be described in -pared with DMC catalyst based on Zn3[Fe(CN)6]2 [13],DMC catalyst prepared in this work(based on Zn3[Co(CN)6]2)provides high activity.In the case of polymerization with semi-continuous addition of monomer,catalytic activity up to26kg polymer/g cat-alyst was reached.This catalytic activity is approxi-mately100times that of aluminum porphyrins[2].In addition,the GPC traces of all polymers synthesized in this work exhibited a single and symmetrical peak, which is evidence that only one type of active centre existed in this system.It is well known that polyether oligomer produced with ZnCl2showed very low de-gree of polymerization.As mentioned in the previ-ous section,neat Zn3[Co(CN)6]2alone was also active for ROP of propylene oxide and polymers of medium molecular weight could be generated.Thus it can be concluded that the active site was on Zn3[Co(CN)6]2. However,the precise structure of the active species can’t be determined in situ because the catalyst level was very low.Fig.4shows the13C NMR spectrum of product prepared by DMC catalyst.The methyl,methylene, and methine carbon resonances can be easily assigned: a-CH(74.6–75.5ppm),b-CH2(72.6–73.5ppm),c-CH3 (16.9–17.2ppm).The simplicity of signal c indi-cates that the polymer chain consists exclusively of head-to-tail sequence,as typically observed for an-ionic polymerization of propylene oxide.Two signals of methylene carbon observed at72.0and73.1ppm are ascribed to meso(m)and racemic(r)diads re-spectively,and three signals of methine observed at 75.2,75.0and74.8ppm are ascribed to isotactic(i), heterotactic(h)and syndiotactic(s)triads,respec-tively.The similar intensity of there three signals(a) indicates that the polymer has a random distribution of the configurational sequences[14].In fact,the mi-crostructure of the product was similar to that of the one synthesized by base-catalyzed polymerization, and polyether polyols with this kind of microstruc-ture are very suitable candidates as precursors in the manufacture of polyurethanes.In conclusion,the preparation route and condition such as temperature,the kind and the concentration of starting material have significant effect on the mor-phology of DMC complexes.The results of the present work demonstrate that catalytic activity values were strongly dependent on composition as well as mor-phology of DMC catalyst and that the DMC catalyst is non-stoichiometric.The choices of suitable prepa-ration condition and starting materials are very im-portant to obtain DMC catalysts with high activity.So far,the substantially amorphous DMC catalyst made from ZnCl2and t-BuOH have the highest activity.The DMC catalysts are very active for polymerization of propylene oxide;high-quality polyether polyols thatY.J.Huang et al./Applied Catalysis A:General240(2003)263–271271have very low unsaturation,and narrow molecu-lar weight distributions can be prepared under mild conditions.References[1]C.P.Smith,J.W.Reisch,J.M.O’Connor,J.Elastoplast.24(1992)305.[2]M.Akatsuka,T.Aida,S.Inoue,Macromolecules27(1994)2820.[3]R.O.Colough,K.Wilknson,J.Polym.Sci.C4(1963)311.[4]N.Oquni,S.Watanabe,M.Maki,Macromolecules6(1973)195.[5]K.Miura,T.Kitayama,K.Hatada,T.Nakata,Polym.J.25(1993)655.[6]B.Wu,J.Harlan,R.W.Lenz,A.R.Barron,Macromolecules30(1997)316.[7]Y.S.Zheng,L.Q.Ying,Z.Q.Shen,Polymer41(2000)1641.[8]Le-Khac.Bi.EP0755716(1997).[9]K.G.McDaniel,M.J.Perry,J.E.Hayes,WO9914258(1999).[10]J.Hofmann,P.Gupta,H.Pielartzic,EP0892002(1999).[11]R.M.Wehmeyer,WO0104177(2001).[12]Y.J.Huang,G.R.Qi,Y.H.Wang,J.Polym.Sci.Part A:Polym.Chem.40(2002)1142.[13]Y.J.Huang,G.R.Qi,J.W.Qian,L.X.Feng,Acta PolymericaSinica2(2001)210.[14]F.C.Schilling,A.E.Tonelli,Macromolecules19(1986)1337.。