Microporous Silica-supported Poi yoxometal ate Catalyst H3PW12O400/SiO2 with High Activity for Aceta

CHEM. RES. CHINESE U.2002. 18(2). 223-225Microporous Silica- supported Pol yoxometal ate CatalystH3PW12O4o/SiO2 with High Activity for Acetal R eactionbetween Benzaldehyde and Ethylene Glycol '06 AYANG Yu-lin12, WANG Yong -hui', HU Chang wen'"" , WANG En-bo' and FENG Shou-hua2(1. Institute of Polyorometalate Chemistry, Faculty of Chemistry, Northeast Normal University，Changchun 130024，P. R. China; 2. State Key Laboratory of Inorganic Synthesisand Preparative Chemistry, Jilin University, Changchun 130023, P. R. China)Received (xt.11. 2001Keywords Silica -supported l2-tungstophosphoric acid. Catalysis . Acetal reactionArticle ID 1005-9040(2002) 02-223-03Keggin-type polyoxometalates (POMs) whosein a Pyrex flask (50 mL) containing suspended cata-representative is HsPW:2Oo(PWl2) exhibits a uniquelyst powder (60 mcsh)， benzaldelyde(0. 14 mol ),combination of physical and chemical properties.ethylene glycol (0.21 mol) and cyclohexane (0. 07They have been widely used as acid catalysts in homo-mol). The amounts of the catalyst used were 1.25 ggeneous and heterogeneous reaction systems, Howev-for PW:2/SiOz (8.1% PWl2， mass fraction) wither, the catalytic activity of PW:2 is so limited due to0.1g PW12 included, 0.1 g for PW:z●SO; /ZrO2,its very low specific surface area(≤9 m'/g )0] thatNafion-H and H-ZSM-5，respectively. The silica-polar molecules can only react on its surface. There-supported PWlz was dried in vacuumat 150 C for 3h .fore, many attempts have been made at enhancingprior to use for catalytic reaction. The product PDOthe specific surface arca of PW1g. Acidic or neutralwas detrmined by gas chromatography on a quartzsubstances such as SiO2, active carbon, acidic ion-ex-capilary column(30 mX0.25 mm).change resins and layered double hydroxides (LDHs)’Table 1 indicates that the primary Keggin struc-with POMs intercalated2, etc. are suitable for beingture of the PW:z is retained after it was entrapped intoused as supports, among which SiO2 is most oftenthe silica matrix. The FTIR characteristic bands ofused]. Basic solids like MgO tend to decomposethe PW:2/SiO2 show somc shifts of ca. 30 cm-' com-POMs53]. Acetalization is a general method of pro-pared with those of the parent PWrz(e.g. W一Oa .tecting the aldehyde group in orgainc synthetic reac-and W- -O%- -W vibration peaks). Therefore, it istions，which can be implemented by using HCI,confirmed that a strong chemical interaction, not ap-toluene sulfonic acid, FeCls, ZnCl2， montmoril-simple physical adsorption, existed between the PWrzlonite clays, lanthanides!4J and Mg-Al or Zn-AI hy-and the silica network. Since the -OH groups ofdrotalctes5s.6]. We synthesized H3PW 12O4/SiO2 withsilanol were protonated in the acidic medium of PWr2，a load o{ 8. 1% PWlg (mass fraction) and a microp-it was supposed that one proton of PW:z reacted withorous structure according to a modlfied literaturethe - -OH of silanol, forming a SiOH; group whichmethod based on the time and temperature of the should act as a counter ion for the polyanion. Thepreparation reaction having been changed?. PW2/formula of the polyanion in such an environment canSiO2 thus obtained catalyzed the acetal rcaction be-be written as (=SiOHI )(H2PW i20o的)[8]. Thus, thetwen benzaldehyde and ethylene glycol. The conver-reaction of the polyanion with SiO: is not described assion of benzaldehyde was 93.8% for a reaction periodan ion exchange but as an acid-base reaction betweenof2 h.the silanol group(acting as the base) and the polyan-Catalytic reaction: the synthesis of 2-phenyl-ion (acting as a Bronsted acid). This interaction re1.3-dioxolane(PDO) was performed at 380 K for 2hsults中国煤化工of the PW:/SiOz。Supported by the Natural Science Foundation of China(No. 2007C N M H Gersity Key Teacher ofthe Ministry of Education of China.. # To whom correspondence should be addressed. E-mail: huchw@nenu. edu. cn224CHEM. RES. CHINESE U. .Vol. 18compared with those of the parent PW12. PW12 hasand H3PW1 VO,/SiOz as oxidative type of catalystsbeen verified to be a pure B-acid by MisonoLo. Simi-in liquid-solid systerms[1o].lar results were also observed for H3PMo2O4o/SiO2Table 1 Data of ICP-AES. UV and IR spectra for PW n/SIOzCatalystLod(%)OMCT band/nmi/em-1W=Osw-O.-wPW nz/SiO028.2661086. 0952. 9799. 3788.9PWuz102601079.2983. 2893.1812.2s;O;1086. 6---.__* SiO; was prepared by hydrolysis of TEOS at pH= 1(pH was adjusted with HxCrOn).The PW 12/SiO2 showed a maximum UV absorp-BET surface areas and median pore diameters aretion peak at 263- -266 nm, similar to that of the par-summarized in Table 2. The presence of microporesent PWIr， ttriluted to the oxygen- to-tungstenfor PW:2/SiO2 was indicated by the sharp increase inchangc- transfer transition for the Keggin anions.the amount of nitrogen adsorbed at a very low rela-This indicates that the primary Keggin structure wastive pressure, i.e.，p/po≤0. 01. The sharp peak ofretained after PW:2 was entrapped into the silica ma-the pore size distribution curve indicates that thetrix. The TGA indicated that the supported PW1z inpores in the PWrz/SiO2 are mainly within a pore di-the silica matrix began to decompose at a temperatureameter of ca. 0. 57 nm, suggesting that the microp-of 350 C.ores in the PW12/SiO2 are uniform. The SEMThe nitrogen adsorption isotherms of the PWl2/showed that the PW 12/SiO2 formed relativcly uniformSiO2(8. 1% PW:z) and SiO2 at 77.5 K and their porenanormeter particles with the diameter less than 20size distributions were measured, respectively. TheTable 2 Surface characterization and catalytic activities of various solid acids for acetal renctionAcid amount[12]/Surface aren/Median diameter/nmConversion'( %)10k*/(mmol'.g-1)(m2.g-)_(mol-1. L. min-I)PWr2/SiO20.08350. 50. 5793.87. 86PWYz1. 049.093. 37.33S0i- /ZrO20. 2093. 0-90.13.8Nflon-H0. 0288.62.35H-ZSM-50.39403. 00.5586.2 .1.33SiC216.20. 390a. Conversion of the various solid ncids for benzaldehyde during the reaction period of 2 hat 380 K. Benzaldehyde/ethylene gvco/cyclohexane;0.14 mol/O. 21 mol/0. 07 mol; PWu2/SiO2(1.25 g). PW12(0.1 g), SO1- /ZrO2<(0.1 g), Nafion-H(0.1 g), H ZSM-5(0.1 g), SiOz(1. 15 g),b. Second -order reaction rate constent 乱t 298 K. c. PW1z in the homogeneous system.The mesoporosities of the silica gels are con-catalytic activity of PWr2/SiO2 than homogeneousfirmed by the pore size distribution data for the purePW12 was also observed for this acetal reaction.SiO2(Table 2). The median pore diameter of the sili-PWl2/SiOz appeared to exist as a concentrated solu-cais ca.3.8 nm. During the course of hydrolysis oftion of benzaldehyde and ethylene glycol having beenTEOS in the presence of PW:2, the Keggin anionsentrapped into the pores of the silica network. Such awere entrapped into the silica network, resulting inconcentrated PW 1: solution probably catalyzes the ac-the decrease of the pore sice and the formation of theetal reaction more effectively than the diluted homo-PW2/SiOz composite with micropores. The ergeneous system because the reaction rate dependes ontrapped Keggin anion appeared to be a highly concen-the concentrations of both the proton and the het-trated solution in the silica network and the silica net-eropoly anion. PW12 is a typical strong Bronsted acidwork seemed to be narrow enough to prevent the re-ike perchloric and sulfuric acids. In the polar sol-moval of the PW:z molecules from the silica network.vents, PW:2 works as an efficient acid catalyst for ac-Fig. 1(A) and Table 2 indicate that PW12/SiO2etalization, esterification ，hydrolysis. The activationcatalyzed acetal reaction more eficiently than theenergies of the POM-catalyzed reactions are remark-typical solid acids such as SO: /ZrOz, Nafion-H andably中国煤化工tion of the cationicH-ZSM-5. The reason for this is that the acidteFroploy anions. Izu-amount of the PW:2/SiOz per unit is much highermi .YHcNMHGitedahigheracidthan those of the other solid acids. The higher strength(+ 1.5≤Ho≤+ 3.3) than H-ZSM-5No. 2YANG Yu-lin et al.225(+6.8≤Ho) in alcoholC7. The PW12 dehydrated al-integral formula of the second-order kinetics, 1/(1-so exhibited a higher acid strength(- 13. 16≤Ho)r)-1=kCot， where 工，k, Co and t are the conversionthan H-ZSM-5(- 12. 5≤Ho101]. These facts maybe of benzaldehyde, the second- order reaction rate con-account for the higher catalytic activity (stant, the initial concentration of benzaldehyde andPW :2/SiO2.reaction time, respectively. Therefore, the acetal re-action between benzaldehyde and ethylene glycol cat-W1:/SiOz(A)alyzed by the various solid acids exhibited a model ofsecond-order kinetics.40古9号In summary, silica-supported 12-tungstophos-SIOphoric acid, a new type of solid acid with a microp-30F口orous structure and large surface areas, has the ad-vantages of insolubility, ready separation, rcusabilityin oxygen-containing polar solvents. It catalyzcs ac-e-0-。etal reaction more effectively than the typical solidacids such as SO:- /ZrO2, Naflon-H and H-ZSM-5.(B.5The conversion is 93. 8% for benzaldehyde. It is ex-pected that PW12/SiO2 will find wide application toF0.4various types of acid-catalyzed liquid-phase organic8reactions in highly polar reaction media as an environ-二0.3-mentally friendly catalyst.芳等李言一10152025Reterences[1] Hu Changwen, Hashimoro M.. Okuhara T.. et al. J.Fig.1 Time courses(A) and the second-order plots(B)Caral., 1993, 143, 437for acetal reaction of benzaldehyde with ethylene[2] Hu Changwen, Zhang Xu, Xu Lin, et al. . Applied Clay Sci-glycol over various solid acids.ence, 1998， 13, 495After the reaction being finished, the PW12/Kozhevnikov. . Chem. Rev.. 1998. (1). 175SiO2 was separated from the reactant solution by fil-[4] Johi M. V.，NarasimhanC. s.. J. Catul. 1989, 120. 282[5] RaoK. K.. Gravelle M. . ValenteJ. S.. et al.. J. Catul.，tration, washed with alcohol and served for the next1998，173. 115acetal reaction. The PW12/SiO2 kept its initial activi-Hu Changwen, He Qinglin. Xu Lin. e al., Chin. Chem.ty during three times of reuse and the leakage of the7] lzumi Y.. Hisano K. , Hida T.. Appl. Catal. A, 1999. 181.Keggin anion was hardly analyzed, as determined by2771CP-AES, whereas the rate catalyzed by SO? /ZrO2[8] Guo Yihang, Wang Yuanhong, Hu Changwen et al.. Chem.was decreased, after it was repeatedly used once, toMaler.. 2000. 12, 3 501that less than half that of the 1st run.Fig. 1(B) indiciates that the linear relationship[10] Peng Ge, Wang Yonghui, Hu Changwen. et al，Appl.Catal. A, 2001，218, 91 .between the 1/co(1- x) and time(t) was in agree-[11] Misono M., Okuhara T.. Chemtech, 1993. 11, 23ment with the second-order kinetics law at 298 K.[12] Chen Xin, Xu Zheng. Toshio Okuhara, et ul. Appl. Catul.The conversion data ilustrated by the curves of vari-A.999, 180. 261ous solid acids in Fig. 1 were analyzed by using the中国煤化工MYHCNMHG