Vapour-phase O-methylation of Catechol with Methanol on Ti-containing Phosphate Catalysts

Available online at www.sciencedirect.comSCIENCEdoinnor.CHEM. RES. CHINESE U.2006 ,224),533- -536V apour-phase O-methylation of Catechol with Methanolon Ti-containing Phosphate CatalystsZHU Xiao-meil , LI Xue-mei? , LIU Gang' , Z0U Xiu-jing'WANG Yan-li' , JIA Ming-jun'** and ZHANG Wen-xiang'1. College of Chemistry , Jilin University , Changchun 130021 ,P. R. China ;2. Shanghai Huayi Acrylic Acid Co. Ltd. , Shanghai 200137 ,P. R. ChinaReceived July 28 ,2005Ti-containing phosphat( Ti-P-0 ) catalysts with different molar ratios of P to Ti( 0- -2. 0 ) were synthesized andcharacterized by XRD , N2 -adsorption/ desorption , IR and temperature-programmed desorption( TPD ) methods. Thecatalytic properties of Ti-P-O samples in the vapor-phase O-methylation of catechol with methanol were also studied.The catechol conversion increases with the increase of the molar ratioof P to Ti in a range of0- -0. 33 ,while a furtherincrease in the P content leads to a decrease of the catalytic activity. Meanwhile , the selectivities of the catalysts tothe main produc( guaiacol ) increase gradually with the increase of the molar ratio of P to Ti. The presence of rela-tively strong Lewis acidic and/ or basic sites in the P-free catalyst should be responsible for the formation of C-alkyla-tion products. The weak acid-base characteristics of the catalysts are favourable for the mono-O-methylation of cate-chol. In comparison with the Lewis acidic sites , the Bronsted acidic sites on the catalysts are more active for the titlereaction.Keywords Phosphates ; Acid-base catalysis ; Guaiacol ; Catechol ; O-MethylationArticle ID 1005 _9040( 2006 )-04 -533-04Introductionincrease in basic intensity of the metal ions , the selec-Guaiacol is an important synthesis intermediate intivities of these catalysts to guaiacol decrease. Vish-the production of flavoring agents , fragrances , agricul-wanathan et al.'41 proposed that the presence of weaktural chemicals and pharmaceuticals' ". Vapor -phasebasic sites on supported caesium catalysts is importantalkylation of catechol with methanol is believed to be anfor the formation of guaiacol. In another work , Fu etalternative method for the synthesis of guaiacol , sinceal.!10] found that a ZnCl2/ y-Al2O3 catalyst with a cer-this process is more economical and more environmen-tain amount of Lewis acidic sites is very active and se-tally friendly than traditional synthetic methods' 2- 61.lective for the preparation of guaiacol. Calzolari and co-Several heterogeneous catalysts have been reported toworkers' 2 I reported that a co-operation effect betweenbe active for the reaction , including various oxides andbasic and acidic sites by the addition of potassium intosupported oxides ，aluminium phosphates and zeo-B/P/O catalysts could improve the catalytic perform-lites' 3-1. Recently , more attention has been drawn toance for the O-methylation of catechol. A similar con-studying the relationship between the acidic or basicclusion has recently been put forward in our work' 1 ! al-properties and the catalytic performance of a catalyst forso. We found that AI-P-Ti-O catalysts bearing relativelythe purpose of understanding the nature of active sitesweak acid-base characteristics show very high activitiesand to provide guidance for designing novel efficientand selectivities to mono-O-alkylation production with acatalysts.good中国煤化工Bal et al.t9y found that alkali supported SiO2 cata-CNMHGrelationship betweenlysts are active for the title reaction. However , with thethe aciu-baste pruperuies U1 ule catalysts and their cata-. Supported by the Development Project of Science and Technology of Jilin Province( Nos. 20050309-1 and 20040563 ) , the Spe-cialized Research Fund for the Doctoral Program of Higher Educatior( No. 20040183003 )，CNPO( No. JTGS 20040010 ) , and the Na-tional Natural Sciese Foundation of Chind( No. 20403006 ).* *ToWhomCorrespondence should be addressed. E-mail : zhwenx@ mail. ju. edu. cn ; jiamingjun@ email. jlu. edu. cn .534CHEM. RES. CHINESE U.Vol. 22lytic performances for the title reaction , we attemptedphate or titanium pyrophosphate. For the sample withto simplify the multi-ingredient catalysts to binary phos-n(P )r( Ti )=2. 00 ,the main crystal phase is titani-phate( Ti-P-0 ) catalysts. A series of Ti-P-O catalystsum pyrophosphate.with different molar ratios of P to Ti was prepared , and: TiO2the effects of the molar ratio of P to Ti on the texturev: TiP2O7and surface acid-base properties were studied by vari-ous methods. The vapor-phase O-methylation of cate-chol was investigated on the Ti-P-O catalysts.Experimental1 Catalyst PreparationTi-P-O catalysts with the molar ratio of P to Ti var-ied from 0 to 2. 00 were prepared according to a proce-103(5C28/()dure for synthesizing Ti-containing aluminium phos-Fig.1 XRD patterns of Ti-P-O catalysts withphates 13 . Typically , tetra-n-butyl titanate dissolved indifferent molar ratios of P to Ti.ethanol was hydrolyzed in distilled water at room tem-n(P):r(Ti):a. 0;b. 0.50;c. 1.00;d. 2.00.perature under vigorous stirring. After that , phosphoricThe textural properties of various Ti-P-O samplesacid was dropped into the mixture under reflux. Afterre listed in Table 1. Compared to the sample ofstanding for several hours , the mixture was heated atn(P )n( Ti)=0 ,the sample of n( P )n( Ti )=0.33363 K in open air with continuous stirring to removehas a higher BET surface area( 238 m'/g ) and a largerwater and all other volatiles. Then the resulting whitepore volume( 0. 50 cm3/g ). With the increase of the Psolid was dried at 383 K for 12 h and calcined at 673 Kto Ti ratid( from 0.33 to 2. 00 ) , both the BET surfacefor5hinair.area and the pore volume decrease obviously ，and a2 Catalyst Characterizationvery low specific surface area( 6 m'/g ) can be ob-XRD patterns were recorded on a Lab XRD-6000served in the sample of n( P )/n( Ti )=2.00. .X-ray diffractometer with nickel-filtered Cu Ko radia-Table 1 Textural properties of Ti-P-O catalysts withtion operated at 40 kV and 30 mA in a 20 range ofdifferent molar ratios of P to Ti5°- -50°.Nitrogen adsorption-desorption isothermsTi-P-OSET/d/were obtained via a Micromeritics an ASAP2010NCatalyst( m2. g~") ( cm3. g-')nmsorptometer. TPD was carried out with NH or CO2 as风P )Vn( Ti )=0.00820.30.3川( P)/r( Ti )=0.332380.506.9the probe moleculesIR spectra were recorded on an( P )Vn( Ti)=0. 50760.265.5Nicolet 410 FTIR spectrometer 14].(P)/n( Ti)=1.00600.1510.03 Catalytic Reaction(PY(Ti)=2. 006. 0112. 0The vapor-phase O-methylation of catechol withThe surface acidities of Ti-P-O phosphate catalystsmethanol was carried out in a fixed bed continuouswere studied by NH,-TPD and FT-IR. Fig. 2 shows thedown-flow reactor at atmospheric pressure' ”，The re-ammonia desorption curves. The P-free sample displaysaction conditions were : n( catechol ) n( methanol )=a remarkably broad peak in a range of 340- -670 K ,in-1/5 , reaction temperature = 553 K，flow rate = 0.5mL/h. The products were analyzed on a Shimadzu-8Agas chromatograph with a capillary column.Results and Discussion1 Catalyst Characterization ResultsThe X-ray diffraction patterns of the Ti-P-O cata-lysts with different molar ratios of P to Ti are displayed中国煤化工in Fig. 1. The sample of n( P )n( Ti )=0( TiO2 )shows an anatase crystal phase5. With the increase ofMYHCNMHG523T/Kthe molar ratio of P to Ti , the peak intensity of the ana-Fig. 2NH3-TPD patterns of Ti-P-O catalysts withtase phase decreases gradually ，and new diffractiondifferent molar ratios ofP to Ti.peaks appear , which can be assigned to various phos-n( P):/( Ti):a. 0.00 ;b. 0.33 ;e. 0.50;d. 1.00 ;phates lil雨 市数榍m phosphate , titanium hydrophos-e. 2.00.No. 4ZHU Xiao-mei et al.535dicating a very wide distribution of acid strength. Whenthat a great number of relatively strong basic sites existasmallamountofP[n(P)n(Ti)=0.33]isadded,on the surface of TiO2. Then the number of basic sitesthe area of the desorption peak increases. With a fur-considerably decreases with increasing the P content.ther increase of the P content , the area of the desorp-Meanwhile , the Tmas shifts from 500 to 400 K when thetion peak decreases gradually , and the position of themolar ratio of P to Ti changes from 0 to 2 , indicatingdesorption temperature( Tmx ) shifts to lower tempera-that the relative strength of the basic sites decreasesture range , indicating the decreases in the number ofwith the increase of the molar ratio of P to Ti.acidic sites and acidic strength.The FT-IR spectra of various Ti-P-O samples inthe - -0H region are shown in Fig. 3. Two weak bandsappear at 3715 and 3665 cm - in the spectrum of asample with n( P )n( Ti )=0 , which are attributed tcthe Ti-OH groups in anatase. For a sample withn( P )n( Ti)=0.33 , a sharp vibration peak( 3670x4,cm-' ) can be assigned to the non-bonded surface P一423523623OH groups517]. The intensity of the P- -0H band de-T/Kcreases with further increasing the molar ratio of P toFig.4 CO2-TPD patterns of Ti-P-O catalysts withTi , which may be caused by the formation of polyphos-different molar ratios ofP to Ti.phate due to the condensation of P- -OH groups. Thisn(P);( Ti):a. 0.00;b. 0.33 ;e. 0.50;d. 1.00 ;has already been confirmed by the XRD characteriza-e. 2.00.tion( Fig.1 ).These results suggest that the Ti-P-O catalysts bearbi-functional acid-base properties. It is worth notingthat the change of acidity( or basicity ) on the surfacesf_of the P-containing samples is similar to the change ofP-OH groups , which means there might be a direct re-lationship between the P- -OH groups and the acid-baseproperties. On the basis of the related literature18] ,itcan be concluded that P- -OH groups represent most ofBronsted acidic sites , and phosphorous( or titanium )4000380036003400atoms function as Lewis acid sites , while the exposed5/cm~1Fig.3 FTIR spectra of Ti-P-O catalysts in the一-OHoxygen atoms of P- -OH groups and bridged oxygen at-region.oms( e. g. P一0- -Ti ) should be responsible for the(P):n(Ti):a. 0.00;b. 0.33 ;c.0.50;d. 1.00;presence of basic sites.e. 1.50;f. 2.00.2 Catalytic ReactionThe basic properties of Ti-P-O catalysts were in-The catalytic properties of various Ti-P-O catalystsvestigated by CO2-TPD( Fig. 4 ). Each of the testedin vapor-phase 0-methylation of catechol are shown insamples shows a broad CO2 desorption peak , indicatingTable 2. A rather low catechol conversion of 13. 4%the presence of basic sites. The P-free sample presentswas obtained over a sample of n( P )n( Ti )=0 with athe highest degree of CO2 desorption , and the highestrelatively poor selectivity( 47.5% ) to guaiacol , whiledesorption temperature( Tm is ca. 500 K ) , suggestingthe selectivity to various C-methylation products is veryTable 2 Catalytic properties of various Ti-P-O catalysts"Selectivitg( % )Molar ratio of P to Ti in catalystsConv. of catechol( % )GuaiacolVeratroleOthers'(P)r( Ti)=0. 0013.4中国煤化工52.5r(PVr( Ti )=0.3378.641.7r(P)r( Ti )=0.5070.9MYHCNMHG.33.3n(P)Vn( Ti)=1.0062.777. 06.416.6r( P)r( Ti)=1.5039. 391.00.18.9以( P)( Ti)=2.0034.598.40.01. 6a. Reaction conditions :mcat =1.4 g , catechol )Vr( methanol )=1/5 ,flow rate=0.5 mL; h - 1 ,reaction temperature =553 K，reaction time=3- 4 h ;b.西方数掘s include mainly 3-methylceatechol ,4-methylcatechol , and 2 -methoxy-4-methy-phenol.536CHEM. RES. CHINESE U.Vol. 22high. With the increase of the P content , the activitymono-O-methylation of catechol and the existence ofincreases considerably and reaches a maximum of 78%stronger acidic and/ or basic sites in the catalysts with a .when the molar ratio of P to Ti is 0.33. Then the cata-higher Ti content should be responsible for the forma-lytic activity decreases gradually with a further increasetion of C-alkylation products. These results are inof the P content. Besides , the selectivity of the catalystagreement with our previous reports', providing fur-to guaiacol increases with the increase of the molarther evidence to support the acid-base co-operationratio of P to Ti in the whole range and a remarkablymechanism for the O-methylation of catechol. Besidesrigh guaiacol selectivity of 98. 4% was obtained atit can be also concluded that the Bronsted acid sites of34.5% catechol conversion for a sample with n( P )Vour tested catalysts are more active compared with then( Ti )=2.00.Lewis acid sites in the O-methylation of catechol with3 Discussionmethanol.From the reaction data combined with the charac-terization results of the catalysts , it can be seen appar-Referencesently that the conversion of catechol has a correlation[ 1 ] Dorothea G. , Barbara E. , Stephen H. ,et al. , Eds. , PhenolDerivatives , Ullmann's Encyclopedia of Industrial Chemistry , Vol.with the number of acidic sites of the catalyst. TheA19 , VCH , egagellschaft , Weinheim , 1991 ,354sample with n( P )r( Ti )=0.33 , which possesses the[ 2 ] Barbara A. , Stoochnorff N. , Benoiton L. , Tetrahedron Lett.biggest number of acidic sites , exhibits the highest ac-1973 ,1 ,21tivity( 78% conversion ). With the increase of the mo-[ 3 ] Kiwi-Minsker L. ,Jenzer G. , Pliasova L. ,et al. , Stud. Surf.lar ratio of P to Ti , the numbers of acidic sites as wellSci. Catal. , 1999 ,121 ,159[ 4 ] Vishwanathan V. , Ndou S. , Sikhwivhilu L. , et al. , Chem.as the catalytic activities of the catalysts decrease.Commun. ,2001 , 893However , it should be noted that the P- free sample[ 5 ] Porchet s. , Kiwi-minsker L. , DoepperR. ,et al. ,Chem. Eng.[ n( P )n( Ti)=0], which also has a relatively bigSei. ,1996 ,51 ,2933number of acidic sites( Lewis acidic sites ) , shows a[ 6] LiX. M. ,Zhang W. ,Liu G. ,et al. , React. Kinet. Catal.elt. ,2003 ,79 ,365very low catechol conversion. By analyzing the IR[ 7 ] PorchetS. ,Su S. , Doepper R. ,et al. , Chem. Eng. Tech.spectra ,it is known that the addition of P species can1994 ,17 ,108produce a certain number of P- -OH groups( Bronsted[ 8 ] Matsuzaki T. , Ohsuga K. , Sugi Y. ,et al. ,J Chem. Soacidic sites ). Therefore , we may suppose that not onlyJpn. ,1985 ,12 ,2331the number of acidic sites but also the acidic types[ 9 ] BalR. ,Tope B. B. ,SivasankerS. J. Mol. Catal. A ,2002,181 ,161( Lewis or Bronsted acidity ) should have important[10] FuZ. ,Yu Y. ,YinD. ,etal. ,J. Mol. Catal. A ,2005 ,232.effects on the catalytic performances of a catalyst.59In addition , the catalytic properties especially the[11] FischerE. ,Olaf S. , Gesine w.，Wiss Z. Uni. Rostock ,selectivity to guaiacol may also be influenced by the1990 ,39 ,67presence of basic sites on a catalyst. The P-free sample[ 12] Calzolari L. , Cavani F. , Monti T. , Solid State Chem. andCatal. , 2000 ,3 ,533[n( P )n( Ti )=0 ] bearing the strongest basicity[13] ZhuX. ,LiX. ,Jia M. ,et al. ,Appl. Catal. A ,2005 ,282 ,shows the lowest guaiacol selectivity. After P species is155introduced , both the number and the intensity of basic[ 14] JiaM. J. ,Zhang W. X. , WuT. H.，J Mol. Catal. A ,sites decrease , while the guaiacol selectivity increases2002 ,185 , 151obviously. A sample with n( P )n( Ti )=2.00 , which[ 15] BuscaG. ,CentiG. ,TifiroF. ,J Phys. Chem. , 1986 ,90 ,1337possesses the smallest number of basic sites and basic[16] Primet M. ,Pichat P. , Mathieu M. ,J Phy. Chem. , 1971 ，strength , shows a very high selectivity( 98.4% ) to75(9) , 1216guaiacol.[ 17] Bautista F. M. , CampeloJ M. , Garcia A. ,et al. ,J Mater.Chem. ,1999 ,9 ,827To summarize , we can conclude that the presenceof moderate numbers of both weak acidic and basic .[18]ClimentM.J.,CormnaA.,GarciaH.,etal.,J.Catal.sites on the surface of a catalyst is favorable for the中国煤化工YHCNMHG.