Study on Highly Active Catalysts and a Once-Through Process for Methanol Synthesis from Syngas

Journal! of Natural Gias Chemistry 12(2003)49 -55SCIENCE PRESSStudy on Highly Active Catalysts and a Once-Through Processfor Methanol Synthesis from SyngasXin Dong ,Bingshurn Shen,Hongbin Zhang*,Guodong Lin，Youzhu YtuanDeparlment of Chemistry, Slate Key 1.aborotory of Phyxical Chemstry for Solid Surfaces,Xurenr C'nioersily, Xiamen 861005, ChinaManuscript received February 10, 2003; revised March 16. 203)Abstract: Highly active CNT-promoted co-precipitated Cu- ZnO- Al2O3 catalysts, symbolized aCu,ZlnzA1-x:%CNTs, were prepared. and their catalytie activily for once-through Inethanol synthesis fromsyugas was investigated. The results ilustrated tlat. under the reaction conditions (at 493 K, 5.0 MPa; thevolume ratio of H2/CO/CO2/N2= 62/30/5/3, GHSV= 4000 h-1), the observed single pass CO- (CugZngAli- 10.0%CNTs) >ox.ygen able t0 elenisorb at tlese sites.(iugZn3Ah- 15.0%CNTs) > (Cu6Zng Al-0%CNTs),so was the sequence of increasing surface concentra-tion of hydrugen -adspecies on the functioning cata-lysts. This was in line with the observed sequenceof catalytic activity of these catalysts for melanolsyntlesis.I1ss4)Pigure3. TEM image (x10) or the CNTe catalyt-ically grown. frura CH4 decomposition.(3)Our recent H2-TPD investigation[14] showed thatlydrogen adsorption on the CNTs can occur at roomn21temperature and atmnospheric pressure. Furthermore,the desorbed product wns almost exclusively gaseoushydrogeu at temperaturcs lower tha 693 K and in-volved CH1, C2H4 and C2H2: in additiun to H2, at1)693 K and above. implying that H2 adlsorption on theCNTs nlay be in tWo forms: associative (nnolecnlarstate) and dissociative (atomic state), This has alsobeeu denoustrated by our LRS characterization of the0060070H2/MIWCNTs adsorption system [15].TIKFigure 4 shows the TPD spectra taken ou theFigure 4. H2-TPD spectra on the prc-reduced cat-pre-rcduced catalysts adsorbing H2 (9999% purity)alystsat 433 K followed by the cooling to room temnper-(1) CugZngAl 0%CNTs; (2) CugZn3 A1-10.0%CNTs;ature. Overall, each spectrum contaiued a lower-(3) CueZn3AI1-12.5%CNTs; (4) CugZn3 A1-15.0%C:NI'stenperature peak (peak-I) at 373 K and a higher-temperature peak (peak-1I) centered around 553 K.Using u 2H2/CO rmixture in place of H2 as theThe lower- temperature peaks originated from theadsorbate in the above expcriments, the observeddesorption of weakly adsorbed l,ydrogen specics, andTPD- TPSR (temperaturc-programmed surface reac-the higher-trmperature peaks were duc to the des-tion) spectra are shown in Figure 5. On the sampleorption of strongly adsorbed hydrogen-species. per-of CugZn3A1-12.5%CNTs pre adsorbing the 2H2/COhaps dissociatively chenisurlbed hydrogen H(a). Itmixturc. pcaks were present at 383. 448. and 568 K. .is conceivablc that, at methanol synt hcsis tenper-The lower-temperature peak (peak-I) at 383 K orig-atures (453 -563 K in this experiment). the surfaceinated from the desurption of H2(a) and weakly ad-concentration of hydrogen adspecics associated withsorbed CO(a): the peak at 448 K (peak-II) was duepeak-I wns very low, and most of the hydrogen ad-mainly to desorption of rhemisorbed CO(a)-species:species it the surface of the functioning catalysts中国煤化工ontained coutri-YHCNMHG54Xin Dong et al,/ Journal of Natural Gas Chenistry Vul. I2 No.1 2003bu ions from the desorption of strongly chemisorbedconcentration of hydrogen-adspccics at the surfacehydrogen H{(a) and CO(a) as well as a mior anountuof the functioning catalyst and thus favor a seriesof the CO hydrogenation produet. CIIxOII.of surface hydrogenation reactions in the process ofC0/CO2 bydrogenation to methanol. Alternatively,the uperating tenperature for methanol synthesis ofthe catalysts appropriately prunoted with a ninorarnout of CNT's can be 10- 20 K lower than that ofthe corresponding CNT-free system. 'This would con.tribnte to an increase in equilibrium CO-conversion383and CHzOH-yield. The results of the prcsent studyindicate that the CNTs could serve as an excellent\ (4pronoter, and that their uniquc ability to adsorb H2may play an important role in effectively promotingmethanol synthcsis.4. Conclusions\13Appropriate incorporation of a minor amount of\ (2CNTs into Cu,Zn,A]k can significantly increase thecalalyst activity for methanol synt hesis from sylugas.Ou the prepared CNT prormoted Cu ZnO-Al2O3 cat-adlyst. the single pass conversion of syngas can reachlevels of over 60%, with the corresponding singl-passmethanol yicld at about 1200 mg/(h.g). nder reac-3004000080tion conditions of 493 K, 5.0 MPa. the volume ratio ofT:KH2/C0/CO2/N2= 62/30/5/3 and GHSV= 4000 h-'.Figure 5，2Hz/CO-TPD-TPSR spectra on the pre-Measurements of the apparent activation eergyreduced catalystspredict that doping of a minor amount of CNTs into(1) Cixs'2njA1-0%CNTs; (2) CuzZn3A1-10.0%CNTS:the Cu;ZnjAlx does not alter the reaction pathway of(3) CueZunyA1-12.5%CNTs: (4) CneZngA12-15.0%CNTslydrogenation of CO/CO2 to form nethanol. How-Similar TPD-TPSR behaviors were also obscrvedever. this did lcad to a considerable increase in the aC-011 the other three catalysts, but their adsorptiontive Cu surface arca of the catalyst annd a pronouncedcapacities of the 2H2/CO mixturc were relativelyerhancement of the stationary-state concentration oflow, especially on the CNT-free catalyst, CugZn3Al1.active hydroge-adspecies on the surface of the func-The sequence of adsorption rapacity of these cala-tioning catalyst. In addition, the optimm opcrat-ing tempcrature required for methanol synthesis dropslysts wward the 2H2/CO mixture is:CueZIn3Al-10- -20 dgrees. All these factors greatly contribute to12.5%CNTs > CuoZn;Ah- 10.0%CNTs > Cu6Zn3Al-the increase in methanol synt hesis reaction activity.15.0%CNTs > CugZn;A1-0%CNTs and is again il .line with the observed scquence of their catalytie ac~-rivity for mcthanol synthesis.ReferencesFron a chemical catalysis point of view, the[1j MacDougall L V. 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