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Synthesis of propylene glycol methyl ether over amine modifiedporous silica by ultrasonic techniqueXuehong Zhang a,b, Wenyu Zhang a,b, Junping Li a, Ning Zhao a, Wei Wei a, Yuhan Sun a,*a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, ChinaReceived 16 May 2006; received in revised form 10 July 2006; accepted 12 July 2006Available online 21 July 2006AbstractAmine modified porous silica were synthesized by ultrasonic technique under mild The samples, which were characterizedby BET, 29Si NMR spectra, element analysis and indicator dye adsorption, exhibited promising catalytic properties towards the synthesisof propylene glycol methyl ether from methanol and propylene They had both high yields and reusability in the reaction, indicatingthat ultrasonic technique was effective for the preparation of organically modified silica Furthermore, the possible reactionmechanism was proposed for the synthesis of propylene glycol methyl ether over such type of 2006 Elsevier BV All rights Keywords: Modified porous silica; Ultrasonic technique; Propylene oxide; Methanol; Propylene glycol methyl IntroductionThe attempt to heterogenize homogeneous catalyst asalternatives to more traditional reagents and catalysts hasbeen one area of research that has seen increasing Much recent work was focused on the preparation oforganically modified solid bases to heterogenize homogeneousamine The modification process was generallyoperated by stirring, heating, refluxing, [1–3]Recently, the interest in synthetic sonochemistry reactionshas grown [4] The ultrasonic technique has been widelyapplied in two-phase systems due to its advantages, suchas high accuracy and Most of these reactions haveinvolved a heterogeneous chemical interaction [5] In thearea of porous materials functionalized by organic groups,however, only limited applications of ultrasound have beenexplored [6,7] In the present work, an alternative syntheticroute for the formation of amine modified silica was developedby using ultrasonic energy, which can produce chemicalmodifications on solids by cavitation phenomenon [8]The amines modified silica with ‘‘single site’’ base strengthwere promising catalysts for a variety of reactions [9]The synthesis of glycol ether over base catalysts is animportant kind of reaction in organic There areseveral methods for the synthesis of propylene glycol ether[10,11] Among them the propylene oxide method is mostlyconvenient and industrial Generally, propyleneoxide reacts with fatty alcohol via acid or base The catalysts used in this process include earlier homogenousbase or acid (NaOH, alcoholic sodium and BF3), latersolid acid and However, few studies have reported theuse of amine modified silica as catalysts for the synthesis ofpropylene glycol methyl ether, though inorganic solid basiccatalysts have performed good activity in the Theimmobilization of the amino functions on a mesoporoussupport, which were with ‘‘single site’’ base strength, couldafford an achieving this kind of In the present work, amine functionalized silica catalysts,including NH2/SiO2, NH(CH2)2NH2/SiO2, TAPM/SiO2 (2,4,6-triaminopyrimidine/SiO2) and TBD/SiO2(1,5,7-triazabicyclo[4,4,0]dec-5-ene/SiO2), were preparedwith 3-aminopropyltrimethoxysilane (APTMS), N-[3-(tri-methoxysilyl)propyl]ethylenediamine (EDPTMS) and 3-chloropropyltrimethoxysilane (CPTMS) as the couplingagents by ultrasonic technique under mild At the same time, in order to confirm the meritsof ultrasonic technique, NH2/SiO2 was also prepared bythe conventional method in order to understand the effectivebehaviour of ultrasonic technique in the preparationof functionalized porous In addition, the catalyticactivity of the organic solid base catalysts was evaluatedby the synthesis of propylene glycol methyl ether frommethanol and propylene Furthermore, the possiblereaction mechanism was proposed for the synthesis of propyleneglycol methyl ether over such type of E Synthesis of catalytic materialsThe amine functionalized silica catalysts could beachieved in two ways under similar conditions as reportedearlier [7]Aminopropylsilyl-functionalized SiO2 was prepared asfollows: 0 g SiO2 was preheated for 12 h at 473 K invacuum to remove all adsorbed moisture but surfaceOH-groups, and than cooled down to room temperaturein vacuum and transferred into a 250 mL conical After mixed with 0 mL cyclohexane and 0 mLAPTMS, the mixture in conical flask was put into the ultrasonicbath for 2 h (Sheshin, Japan, operating power 60 W)at ambient The catalyst was then obtained byextracting with toluene in a soxhlet extractor over a periodof 24 h and drying at 333 K in The same methodwas used for the preparation of NH(CH2)2NH2/SiOTBD/SiO2 was prepared by two steps: silica was firstlymodified by 3-chloropropyltrimethoxysilane via the samemethod as that of aminopropylsilyl-functionalized SiO2,and chloropropylsilyl-functionalized SiO2 was then reactedwith TBD (0 g) in cyclohexane (0 mL) The resultantwas treated by ultrasonic vibration for 1 Afterwards,the catalyst was obtained by extracting with toluene in aSoxhlet extractor over a period of 24 h and drying at333 K in The same method was used for the preparationof TAPM/SiO CharacterizationThe content of carbon, nitrogen, and hydrogen in all thesamples was determined using a Vario EL Thespecific surface area, total pore volume and average porediameter were measured by N2 adsorption–desorptionmethod on a Micromeritics ASAP-2000 instrument (Norcross,GA) The surface areas were calculated by BETmethod, and the pore size distribution was obtained byapplying the BJH pore analysis to the nitrogen adsorption–desorption 29Si NMR spectra were recordedon a Bruker MSL-400 The base strength ofsamples was detected by hammett Catalytic testThe catalytic properties were measured in a 75 ml batchreactor with mol ratio of methanol and propylene oxidebeing 5: After running at 403 K for 10 h under magneticstirring, the reactor was cooled down to room The product was then filtered and analyzed by a gas chromatographwith a flame ionization detector after centrifugalseparation from the The catalysts werewashed with solvent and used for recycling Results and Modification of porous silica with amine groupsThe content of carbon, nitrogen, and hydrogen inamine-free porous silica and all the modified samples werecarried out by elemental W and N% wereobtained from N%, C% and H% (see Table 1) The resultsshowed that there were no carbon and nitrogen in theamine-free porous As a result, carbon and nitrogenin modified samples ought to be from the The element analysis showed that N% of the graftedorganic groups achieved by conventional methods knownfrom the literature [12] was 13 mmol/g, which was farlower than that of the sample prepared by ultrasonic technique(00 mmol/g) (see Table 1) This should be due tothe application of ultrasonic energy to solids and liquids,which could provide the changes including cavitation(bubble formation in a liquid) and chemical reaction (accelerationof chemical reaction), [13] As a result, processesincluding particle size modification, cleaning ofsurfaces or the formation of fresh ones [14,15] could beobtained in heterogeneous media at a solid liquid As to the organic modification of porous silica, cavitationphenomena brought by ultrasound could speed up theliquid transferring velocity in the hole of porous materialsand the liquid–solid As a result, the liquidorganosilanes could be well contacted with silanol groupson the inner wall of the porous silica to react with themin a short time, while stiring could not reach such Therefore, the modification process finished simply andspeedily by The 29Si NMR spectra in solid state indicated that thecovalent bond formed between silylant agents and silanolgroups on the silica surface (see F 1) Two resonancesat 109 and 99 ppm could be attributed to 29Si nucleihaving four Si–O–Si linkages (Q4) and 29Si nuclei havingthree Si–O–Si linkages and one OH (Q3) [16], The resonances at 58 and 67 ppm were assignedto RSi(OSi)(OH)2 and RSi(OSi)3, respectively [17], whichillustrated the successful organo functionalization of poroussilica by the organic groups via covalent C/Nvalue (molar ratio) could also reflect the degree of graftingreaction between silanol groups and organosilanes [18]NH2/SiO2, NH(CH2)2NH2/SiO2 and TBD/SiO2 showedthe C/N = 3–5, 5–0 and 3–6, Theresults also suggested the anchorage of amine groups bySi–O–Si This agreed with the result of the 29SiNMR Structure and basicity of samplesF 2 displays N2 adsorption isotherms for the The functionalized samples displayed type IV isothermswith clear hysteresis loops associated with This indicated that the materials remainedmesoporous before and after functionalization and themodification by various organosilanes hardly changed theisotherm The BET surface area and pore volumeshowed a gradual reduction as the N% of graftedorganic groups increased (see Table 2) This could beattributed to the presence of functional A part ofamino groups grafted onto the microporous also led to adecrease in the BET surface The effect of the organicgroups on the pore diameter of the samples was slight forthe samples NH2/SiO2 and NH(CH2)2NH2/SiO But asfor the sample TBD/SiO2 and TAPM/ SiO2, perhaps dueto the big framework of (CH2)3/TAPM and (CH2)3/TBD groups, the average pore diameters of the samplesdecreased to 90 and 82 However, the average porediameter was not decreased seriously due to the low N%values of the The base strength H of a solid surface is defined as theability of the surface to convert an adsorbed electricallyneutral acid into its conjugate When an electricallyneutral acid indicator is adsorbed on a solid base from anonpolar solution, the color of the acid indicator is changedto that of its conjugate base, provided that the solidhas the necessary base strength to impart electron pairsto the acid [19] A solid with a large positive HH hasstrong basic Grafting with different functional groupscould result in different base As shown in Table3, TBD/SiO2 had the highest base strength of H 0,while NH2/SiO2 and NH(CH2)2NH2/SiO2 only had thebasicity of H 3 and 3 < H < 0, Compared to the other modified samples, TAPM/SiO2had weakest basicity of H < Thus, the basic strengthof the samples was in the order of TBD/SiO2 > NH(CH2)2NH2/SiO2 > NH2/SiO2 > TAPM/SiO Catalytic performanceThe catalytic activity was tested in the synthesis of propyleneglycol methyl ether from methanol and propyleneoxide (see Table 3) As shown in Table 3, PO conversionand isomer selectivity (the ratio of 1-methoxy-2-propanol/total propylene glycol methyl ether) reached 3 3% without the presented catalysts, Amongthe catalysts, the amine-free porous silica showed the lowcatalytic activity due to the weak acid strength of the surfacesilanol For anchored amino groupsNH(CH2)2NH2/SiO2 and NH2/SiO2 catalysts were foundto be more active and selective than other catalysts to 1-methoxy-2-propanol after 10 h of TAPM/SiO2catalyst had low propylene oxide conversion (0%) withthe isomer selectivity of 6% TBD/SiO2,NH(CH2)2NH2/SiO2 and NH2/SiO2 catalysts all showedhigh propylene oxide conversion (>94%), but different NH(CH2)2NH2/SiO2 and NH2/SiO2 withweaker base strength had higher isomer selectivity(>82%), while TBD/SiO2 with moderate base strengthshowed lower isomer selectivity (7%) As for solid basecatalysts, catalysts with moderate base strength shouldhave good isomer selectivity [20] The lower isomer selectivityof TBD/SiO2 could be due to the big framework ofTBDThe catalysts were easily recovered by filtration, andsubjected to utilization for seven cycles with constant conversionof propylene oxide >89% and the utilization formany recycles hardly changed the isomer selectivity under403 K (see Table 4), indicating that amine groups graftedonto silica surface were stable under the experimental The reusability of other samples was similar to thatof NH2/SiO Possible mechanismInorganic solid base catalysts have been extensively usedfor the synthesis of propylene glycol methyl ether frommethanol and propylene oxide [31], in which, the methoxideion and proton was absorbed on acidic and basic siteson the catalyst surface, respectively, and then the methoxideion attacked the C(1) In the present case, however,the samples used for the reaction were characteristicof single site, , the catalyst with unique basic site similarto the homogeneous Because there was no Lewisacidic site on the catalysts, the mechanism should be differentfrom those involving bifunctional The plausiblemechanism of 1-methoxy-2-propanol formation onNH2/SiO2 is illustrated in Scheme There was H-bondsformation between methanol and amine groups in path In path 2, because of the sterically hindered CH3 of PO, theO atom in methanol attacked the C(1) position and protonwas absorbed on the basic sites of the catalysts, and thenC(1)–O band cracked, followed by pick up the proton toform 1-methoxy-2-It seems reasonable to consider that the higher activityof NH(CH2)2NH2/SiO2, NH2/SiO2 and TBD/SiO2 wasdue to the appropriate base strength, which could not onlyform H-bond but also crack it TAPM/SiO2 withvery weak base strength could only form more unstableH- Thus, the activity of TAPM/SiO2 was lower thanthe other On condition that such mechanism isreasonable, the big framework of the organic groups probablycould affect the attacking position of the O atom As a result, TBD of the big framework led tolower isomer Thus, both appropriate basestrength and simple framework of the organic groups wereimportant for the high conversion and good selectivity to1-methoxy-2- ConclusionsThe results presented above led to the following conclusions:(1) the efficient ultrasonic technique could successfullyprepare the amine functionalized porous silicacatalysts, (2) the characterization indicated that the aminegroups were grafted onto the silica surface by covalentbond, (3) appropriate base strength and simple frameworkof the organic groups were important for the high conversionand good selectivity to 1-methoxy-2-propanol, (4) thecatalysts could be recovered by filtration and were subjectedto utilization for many cycles with constant

丙二醇甲醇以太结束胺物综合由超音波技术Xuehong张a， b， Wenyu张a， b， Junping李a， Ning赵a，韦・韦a， Yuhan太阳a修改了多孔硅土， *煤炭转换，学院煤炭化学，中国科学院，太原030001，中国b研究生院状态钥匙实验室中国科学院，北京100039，中国接受了2006年5月16日; 接受以修改过的形式2006年7月10日; 接受2006年7月12日