An effective route to improve the catalytic performance of SAPO-34 in the methanol-to-olefin reactio

Available online at www.sciencedirect.comJOURNALOFScienceDirectNATURAL GASCHEMISTRYELSEVIERJourmal of Natural Gas Chemistry 21(2012)431- 434www.elsevier. com/locate/jngcAn effective route to improve the catalytic performanceof SAPO-34 in the methanol-to- olefin reactionGuangyu Liul,2，Peng Tian'，Qinbua Xia3，Zhongmin Liul*1. National Engineering Laboratory for Methanol to Olefin, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian116023, Liaoning, China; 2. School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001,Henan, China; 3. Ministry of Education Key Laboratory for the Synthesis and Application of Organic FunctionalMolecules, School of Chemistry and Chemical Engineering, Hubei Universirt, Wuhan 430062, Hubei, China[ Manuscript received January 11, 2012; revised April 6, 2012 ]AbstractAn effective route to improve the catalytic performance of SAPO-34 in the methanol-to-olefin reaction by simple oxalic acid treatment wasinvestigated. The samples were characterized by XRD, SEM, N2 adsorption-desorption, XRF, TG, 29Si MAS NMR and NH3-TPD techniques.The results indicated that the external surface acidity of SAPO-34 was finely tuned by oxalic acid treatment, and the selectivity to C2H4 onSAPO-34 and the catalyst lifetime in the methanol-to-olefin reaction were greatly improved.Key wordsSAPO 34; methanol-to-olefin (MTO); oxalic acid; modification; acidity1. Introductionwould appear with the growth of crystal size during the crys-tallization [19]. This implies the unnecessary to crystallizeMethanol-to-olefin (MTO) reaction has been attracting a special SAPO-34 catalyst with low si content in order tomuch attention because it is the key step as a non-oil routeobtain good MTO results. In the present paper, we report ato produce ethylene and propylene. SAPO-34 is currentlysimple and effective post-synthesis method to corrode the ex-recognized as the desired catalyst for the MTO reaction,ternal surface of SAPO-34 crystals with 'higher' Si contentdue to its small pore, moderate acidity and excellent ther-by oxalic acid solution, and to tune the surface acidity further.ma/hydrothermal stability [1,2]. Up to now, researches on theInterestingly, very encouraging improvement of catalytic per-MTO reaction are mainly focused on the reaction mechanismformance has been achieved.and the effect of different synthesis parameters of SAPO-34on the reaction [3-11]. Few studies put emphasis upon the2. Experimentalpost-synthesis modification of SAPO-34 to improve its cat-alytic performance in the MTO reaction [12- 16].It has been recognized that the Si amount in SAPO-34 is2.1. Catalyst preparationthe unique factor for controlling the catalyst acidity, relativelyimportant for improving the catalyst lifetime and the selectiv-SAPO-34 was hydrothermally synthesized from the gelity of light olefins in the MTO reaction, for which SAPO-34with a molar composition of 1.5 diethylamine (DEA): 1.0with a low Si content is required [17,18]. However, our previ-Al2O3 :0.8 P2Os : 0.4 SiO2 : 50 H2O. The detailed synthesisous results showed that the crystallization yield of SAPO-34procedure has been described elsewhere [20]. Post-synthesisdropped with decreasing Si content in the synthesis gel, espe-treatment of as-synthesized SAPO-34 was carried out at roomcially in the range of low Si content. And, the Si distributiontemperature for 6h with a solution of oxalic acid (solu-in crystals was not homogeneous, showing a gradual increasetion/solid = 2 mL/g). The catalysts were obtained by calciningfrom the core to the shell; i.e., the Si environment was onlythe modified samples at 550。C in air for 5 h and designatedSi(4AI) in the initially formed core and other Si environmentsas SAPO-xI中国煤化工oxalic acid sluion).TYHCNMHG* Corresponding author. Tel/Fax: +86 411-84685510; E-mail: liuzm@ dicp.ac.cnCopyright@2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10. 1016/S 1003-9953(11)60387-3432Guangyu Liu et al./ Jourmal of Natural Gas Chemistry Vol. 21 No.4 20122.2. Catalytic activity measurementswith the temperature-programmed rate of 10 °C/min under airflow. The textural data were obtained by nitrogen adsorptionMTO reaction was carried out with a fixed-bed reactormeasurement using a Micromeritics 2010 analyzer. Beforeat atmospheric pressure. 1.5 g of catalyst (20~40 mesh) wasanalysis all samples were degassed at 350 °C under vacuum.loaded into the reactor. The sample was pretreated in a flowThe temperature-programmed desorption of ammonia (NH3-of dry nitrogen at 550 °C for 1 h and then the temperature ofTPD) was carried out with an Autochem 2910 equipment (Mi-reactor was reduced to 450°C. The mixture (the weight ra-cromeritics). Ammonia was injected in order to saturate thetio of CH3OH/H2O was 40/60) was pumped into the reactorsample surface at 100 °C. Prior to the injection, the sampleafter nitrogen was turned off. The weight hourly space veloc-(200 mg) was activated at 650 °C for 40 min (10。C/min) un-ity (WHSV) was 2h-' . The products were analyzed on-lineder He (20 cm/min). The measurement of the desorbed NH3by a Varian GC3800 gas chromatograph equipped with a FIDwas performed from 100。C to 650 °C (10 °C/min) under Hedetector and a PoraplotQ-HT capillary column.(40 cm/min).2.3. Characterization3. Results and discussionThe powder XRD pattern was recorded on a RigakuThe MTO reaction results on catalysts are listed in Ta-D/MAX-yB X-ray diffractometer with Cu Ka radiationble 1. Very clearly, the selectivity of C2H4 was notably in-(入=0.15406 nm). The chemical composition of the sam-creased over post-modified samples. Moreover, both the to-ple was determined with a Philips Magix-601 X-Ray Fluo-tal selectivity of (C2H4+C3H6) and the ratio of C2H4/C3H6rescence (XRF) spectrometer. The crystal morphology waswere higher over modified samples than those on the parentanalyzed by scanning electron microscopy (SEM, KYKY-SAPO-34. The best catalytic activity and selectivity were re-AMRAY-1000B).29Si MAS NMR spectroscopy measure-alized over the sample SAPO-0.2M modified by 0.2 M oxalicment was conducted at resonance frequencies of 79.41 MHz,acid solution. For example, the selectivity of C2H4 was in-using a Varian Infinity plus 400 WB spectrometer. The spin-creased from 41.6% to 45.7%, and the on-stream lifetime ofning rates of the samples at the magic angle were 4 kHz.catalyst was extended from 82 min to 130 min. However, aThe reference material for the chemical shift (in ppm) wasfurther increase of oxalic acid concentration led to an appre-2, 2-dimethyl-2-silapentane 5-sulfonate sodium salt (DSS).ciable drop of the on-stream lifetime of the treated sample toThe thermal analysis was performed on a TA Q600 analyzer66 min.Table 1. MTO results on samples modified by oxalic acidSelectivity (wt%)SampleTOS (min)CH4C2H4C2HoCH6C3HgCC5IC5G+C3SAPO-3420.33.20.440.74.416.93.60.8173.82*141.6.739.62.112.12.71.0581.2SAPO-0.1M34.60.541.14.9150.8475.71.44.30.838.42.410..21.1582.6SAPO-0.2M34.540.64.80.857545.737.91.810.1SAPO-0.4M35.10.640.5.314.0.87 .6*.1633Reaction conditions: WHSV=2h- I , T = 450 °C, 40 wt% methanol-water solution; * Lifetime: the reaction duration with >99% methanol conversionXRD results given in Table 2 show that the relative crys-Figure 2 ilustrates the N2 adsorption-desorptiontallinity of the samples decreased with increasing oxalic acidisotherms of different samples at 77 K. The N2 adsorption-concentration and SAPO-0.4M had the lowest value of 74%.desorption isotherms of SAPO-34 and SAPO-0.1M are ofFigure 1 presents the SEM images of SAPO-34 andtype I with a plateau at higher relative pressures and no distinctmodified samples. The crystal surfaces of the modifiedhysteresis loop, which is typical for a microporous materialsamples have became rough， especially for the samplewithout significant mesoporosity. Although the isotherm ofSAPO-0.4M.SAPO-0.4M is also of type I, it has a hysteresis loop betweenp/po = 0.5:arance of mesoporousTable 2. Relative crystallinity and chemical compoition of samples中国煤化工structure.erent samples are pre-Relative cysallinity (%)Mole compositionsented in'MHCNMHGumeandsurfacearea，00Alo.493Po.421Sio.086)5Alo.495Po.423Sio.082of samples dropped gradually after modification treatment,32Alo. 495Po.42sSio.80but accompanied by the appearance of mesopore volume and74Alo.497Po.429Si0.074_external surface area.Joumal of Natural Gas Chemistry Vol. 21 No. 42012433(b1000 fm1000 am10000 nm2SEVFigure 1. SEM images of (a) SAP0-34 (b) SAPO-0.IM (c) SAPO-0.2M and (d) SAPO-0.4Msamples gradually rose following up the increase of oxalicSAPO-34acid concentration until only Si(4AI) environment emergedin SAPO-0.4M. This means that the occurrence of desalina-tion phenomenon during the post treatment mainly removedSAPO-0.4MSi atoms from Si-rich areas, which is contradictory to onecommon sense that oxalic acid is usually used as a dealu-mination reagent. Considering rough external surface of thetreated samples and previous finding of‘Si enrichment on theSAPO- 34 surface' [19], we can come to a conclusion that 0x-alic acid treatment caused an entire corrosion of external crys-tal surface, not merely occurring on the Si sites.0..0Relative pressure (p/p,)Si(4AI)Figure 2. Nz adsorption desorption isotherms of samplesTable 3. Textural properties of samplesSurface area ((m-/g)Pore volume (cm*/g)Samplemicro Sexermal StotalVmicro Vmeso ViotalSAPO-34_SiZA)Si0AI)SAPO-34 55905590.28 00.28SAPO-0.1M 5385470.27 0 0.27三SAPO-0.IMSAPO-0.4M 463134760.230.02 0.25SAPO-0.2MTable 4 presents the thermal analysis results of samples.The template amount occluded inside the framework was-7(-90-100-110close to that of SAPO-34, implying the untouched interior ofChemical shit (ppm)the crystals. Therefore, it is concluded from the above infor-Figure 3.29Si MAS NMR spectra of dfferent samplesmation that the damage to the crystalline SAPO-xM samplesduring oxalic acid treatment only occurred on the external sur-Table 5. Distribution of Si environments obtained fromface of the crystals.deconvoluted 29Si MAS NMR spectraSilicon environment distribution (%)Table 4. Thermal analysis results of samplesSi(3AI)Si(2AI) S(OAI)H2O weight loss (%)Template weight loss (%)86.010.02.31(<185°C) I1(185- -450°C) m (>450°C) IISAPO-0.1M90.89.22.668.084.5912.6792.7.2.523.723.739.343.3712.71Element-:ct the change of the29Si MAS NMR was performed to investigate the Sisample comp中国煤化工Epresented in Table 2.environment in the samples. As one can see from FigSi content inYHC N M H Gously with the sever-ure 3, different Si environments co-exist in the framework ofity of the post treatment, but both the alumina and phosphateSAPO-34 and modified samples. The deconvoluted resultscontents increased a lttle, which is in agreement with the re-in Table 5 show that the percentage of Si(4A1) in the treated sults of SEM and 29si MAS NMR.434Guangyu Liu et al./ Jourmal of Natural Gas Chemistry Vol. 21 No.4 2012It seems that the variation in the catalytic performancewards the centre of the crystal leading to fast catalyst deactiva-may be related to the change of Si environment in the modifiedtion, suggesting that the acid sites close to the external surfacesamples. After oxalic acid treatment, the Si environment inplayed an important role in the reaction, in agreement wellthe crystals became more uniform and the acidity on the exter-with the present work. The drop in the lifetime ofSAPO-0.4Mnal surface of crystals decreased due to the removal of Si frommay be ascribed to its low crystallinity and surface area.Si islands. Results from NH3-TPD given in Figure 4 and Ta-ble 6 confirmed that both the acid strength and the acid num-ber of the samples decreased following oxalic acid treatment.The percentage of the strong acid sites in the total acid numberdecreased from 27% in SAPO 34 to 20% in SAPO-0.4M.According to the present results, the external acidity ofSAPO-34 crystal had important influence on the MTO reac-tion. The acidity arising from Si island would lead to the de-crease of catalyst lifetime and the drop of the selectivity of_SAP0-34C2H4, which was due to the higher acid strength and the acidSAPO-0.IMdensity. The change of MTO reaction in the present workwas also consistent with our previous investigation- SAPO-SAPO-0.2M34 synthesized hydrothermally with higher Si content in theSAPO-0.4Mframework shortened the catalyst lifetime and decreased the1030040000600C2H4 selectivity [20]. Mores et al. [21] once reported thatTemperature (C)large carbonaceous deposits formed in the cages at the edge ofthe SAPO -34 crystal prevented the reaction front to move to-Figure 4. NH3-TPD profiles of dfferenett samplesTable 6. Acidity of samples from NH3-TPDWeak acidityMedium acidityStrong aciditySampleTotal (%)T(Cpercentage (%)T(°C)T(C)SAPO-341873512242927100.0SAPO-0.1M8151343204212697 .53428422)493184_5(424. Conclusions[6] Arstad B, Kolboe s. J Am Chem Soc, 2001, 123(33): 8137[7] Cui Z M, Liu Q, Song W G, Wan LJ Angew Chem Int Ed, 2006,45(39): 6512SAPO-34 was modified by oxalic acid solution. The re-[8]WangYD,WangCM,LiuHX,XieZK.ChinJCatal(Cuihuasults indicate that the external surface of SAPO 34 crystalsXuebao), 2010, 31(1): 33is corroded and the relative crystallinity decreases after the[9] LiP, Zhang W P, Han X W, Bao X H. Chin J Catal (Cuiruamodification treatment. The micropore volume and surfaceXuebao), 2011, 32(2): 293area of SAPO-34 drops after modification, but accompanies[10] ZhangJC, ZhangH B, YangX Y, Huang Z, Cao W L. J Natby the appearance of mesopore volume and external surfaceGas Chem, 2011, 20(3): 266area. The modified samples exhibit higher selectivity of C2H4[1] LiuG Y, Tian P, Liu Z M. Chin J Catal (Cuihua Xuebao), 2012,and longer catalyst lifetime in MTO reaction than the parent33(1):174SAPO-34. The phenomena can be assigned to the decrease of12] Song W G, Haw J F. Angew Chem Int Ed, 2003, 42(8): 892acidity strength and acid site density on the external surface of13] MeesF DP, Van Der Voort P, Cool P, Martens L R M, JanssenSAPO 34 crystals induced by removing Si atoms from Si-richM J G, Verberckmoes A A, Kennedy G J, Hall R B, Wang K,Vansant E F. J Phys Chem B, 2003, 107(14): 3161areas. In summary, our results presented here indicate that the[14] Inui T, Kang M. 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