Effect of copper loading on texture, structure and catalytic performance of Cu/SiO2 catalyst for hyd

Available online at www.sciencedirect.comJOURNALOFScienceDirectNATURAL GASCHEMISTRYELSEVIERJoumal of Natural Gas Chemistry 21(2012)563 -570www.elsevier. com/locatejingcEffect of copper loading on texture, structure and catalyticperformance of Cu/SiO2 catalyst for hydrogenationof dimethyl oxalate to ethylene glycolBo Zhang'，Shengguo Huil，Suhua Zhang'，Yang Ji2，Wei Lil，Dingye Fangl*1. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China;2. Pujing Chemical Industry (Sha) Limited, Shanghai 200231, China[ Manuscript received November 28. 2011; revised Deccember 21.2011 ]AbstractCu/SiO2 catalysts prepared by a convenient and eficient method using the urea hydrolysis deposition precipitation (UHDP) technique havebeen proposed focusing on the effect of copper loading. The texture, structure and composition are sytematically characterized by ICP, FT-IR, N2-physisorption, N2O chemisorption, TPR, XRD and XPS. The formation of copper pllosiliace is observed in Cu/SiO2 catalyst byadopting UHDP method, and the amount of copper pylosilicate is related to copper loading. It is found the structure properties and catalyticperformance is profoundly affected by the amount of copper plosilicate The excellent catalytic activity is atributed to the synergetic effetbetween Cu' and Cu*. DM0 conversion and EG sectivity are deternined by the amount of Cu0 and Cu+, respetively. The proper copperloading (30 wt%) provides with the highest ratio of Cu+ /Cu0. giving rise to the highest EG yield of 95% under the reaction conditions ofp= 2.0 MPa, T = 473 K, H2/DMO = 80 and LHSV= 1.0h-1.Key wordsCu/SiO2; copper loading; dimethyl oxalate; ethylene glycol; copper phyllosilicate1. IntroductionSilica-supported copper catalysts were then found widespreadapplication in this area, which exhibited excellent activity inoxalates hydrogenation [5-7].In recent years, synthesis of ethylene glycol (EG) fromGenerally, silica support is considered as inert, for in-coal-derived syngas has attracted significant attention becausestance, as a mere dispersant of the active phase. How-it provided an alternative route to gain EG. EG is widely usedever, researchers revealed that silica may react with theas anti-freezer, polyester fibers, alkyd resin in polyester manu-metal precursors and form silicates during catalyst prepara-facture and solvents [1,2]. At present, direct hydration for thetion. Silica-supported phyllosilicates are known to be formedsynthesis of EG from ethylene oxide (EO) is widely adoptedduring the preparations of Ni/SiO2 [8- 10], Co/SiO2 [11,12],in large-scale production plant.and Cu/SiO2 [13- 16] by cation exchange and/or deposition-It is well-known that catalytic hydrogenation of dimethylprecipitation. The advantages of high dispersion, poor crys-oxalate (DMO) to EG is one of the most important parts intllinity, and high thermal stability of the supported metalthe coal-based EG synthesis process [3]. Catalysts for hydro-phyloilicate, even at elevated metal loadings, make themgenation have drawn an increasing research interest since it ispromising materials for catalysts.one of the key technologies during the coal-to- EG commer-Recently, Cu/SiO2 catalysts with different copper load-cialization process. Copper-based catalysts were employedings synthesized by urea hydrolysis method have been inves-for hydrogenation of DMO to EG in heterogeneous phase. Attigated, but the effects of copper loading have not been dis-the very beginning, copper-chromium catalyst has been usedcussed [17]. Lin et al. [18] prepared an array of CwSiO2in hydrogenation process for its relatively high catalytic sta-catalysts by sol-gel method, but the actual copper loadingsbility and long lifespan [4]. Nevertheless, the toxic chromiumwere inadequate to control, yet trace of copper phyllosilicatecontained in copper-chromium catalyst constrains its practi-was observedIrpddition r rie if haxagonal mesoporouscal applications due to the increasing environmental pressures.silica (HMS中国煤化工21 sppoted copper* Coresponding author. Tel: +86-21-64251002; Fax: +86-21-64251002; E-mail: dyfang@ecuYHCNM HGCopyrightO2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10. 1/100309911)60405-2564Bo Zhang et al/ Jounal of Naural Gas Chemistry Vol. 21 No.5 2012catalysts were proposedwith high catalytic activity, and they2.2. Catalyst characterizationfocused on incorporating copper or copper oxide into meso-porous materials. However, the preparation process of theThe copper loadings were determined by the inductively: support is complicated and time consum-coupled plasma method (ICP, thermo E. IRIS). FT-IR spectraing, which is beyond the scope of this paper.were recorded on a Nicolet Protege 460 spectrometer. TheMany researchers have been devoted to study thesamples were finely ground, dispersed in KBr, and then pel-influence of preparation parameters on the formation 0letized. The spectral resolution was 4cm ', and 32 scanscopper plylosiliates in silica supported copper catalysts.were recorded for each spectrum.Toupance et al. [15] obtained copper phylosilicate by cationNtrogen adsorption-desorption isotherms at 77K wereexchange, and they found the amount of copper phylosilimeasured with a Micromeritics ASAP 2020 instrument andcate depended on the pH value of the precursor solution andthe samples were outgassed at 423 K before each measure-increased with the solution/silica contact time. Chen et al.ment. The specific surface areas were calculated following[16] found ammonia evaporation termperature also affectedtheBET method. Pore size distribution were calculated by BJHamount of copper phylloilicate. However, the efets of cop-method according to the desorption isotherm branch. Theper loading on the formation of copper phyllosilicate have notdispersion and metllic copper surface areas of thecatalystsbeen investigated and the roles of copper phyllosilicate playedwere determined by N2O chemisorption at 333 K with a Mi-in catalytic performance are ambiguous.cromertics Autochem II 2920 equipped with aTCD.In the present work, we report a convenient andThe reducility of the calcined sample was determinedefficient method via a urea hydrolysis deposition-precipitationby H2 temperature programmed reduction (TPR) on a Mi-(UHDP) technique for the preparation of silica-supported cop-cromeritics Autochem II 2920 instrument connected to a Hi-per catalyst with copper phlosilicate. The efcts of copperden Qic-20 mass spectrometer (MS).loadings on the texture and structure of Cu/SiO2 catalysts, es-X-ray powder dffaction (XRD) paterms of catalystspecially on the formation of copper phyllosilicate, are system-were recorded using an X -ray dffractometer (Rigaku D/Maxatically studied with a series of physico-chemical characteri-2500VB/PC) operated at 40kV and 100mA, using Cu Kazations. To better understand the role of copper component,(入= 0.15056 nm) radiation to determine the crystalucturethe evaluation of the catalytic performance off Cw/SiO2 cata-and crystallinity of the catalyst particles. For the reduced cat-lysts has been investigated. Furthermore, the asignments ofalyst, pure Ar was used to protect the sample from oxidation.active site are discussed and crrelate with catalyst texture andThe surface species were detected by X ray photoelec-catalytic performance.tron spectroscopy (XPS, Thermo ESCALAB 250). Thespectrum was recorded with Al Ka line as the excitation2. Experimentalsource (hv = 1486.6 eV). The binding energy (B.E.) valueswere referenced to the C 18 peak of contaminant carbon at2.1. Catalyst synthesis284.6- 285.2eV.Cu/SiO2 catalysts were prepared by urea hydrolysis2.3. Catalytic evaluationdpsion-pecipitation (UHDP) method. CuNO323H2O(A.R.. Sinopharm Chemical Reagent Ld,) was used as copperThe catalytic tests were caried out in a U-type stain-source, slia sol (JA-30, Qingdao Haiyang Chemical Ltd.) asless steel tubular reactor located in a salt bath with vigor-silicon source, and urea (A.R., Sinopharm Chemical Reagentously sirred. Typically, 4.0 mL (2.0 g) catalyst (40-60 mesh)Ltd,) as preipitant. A requiste amount of Cu(NO32:3H20was sandwiched with quartz sand and packed i(depending on the desired copper loading of the catalyst)with an inner diameter of 4 mm. The catalyst was reducedwas dssolved in deionized water (200 mL). A stoichiometricwith pure H2 atmosphere at 493 K for 18 hn, with gas-phaseamount of urea was added and strred until the urea dissolvedspace velocity(S.V,)= 1500h-'. After cooling to the reac-absolutely. Subsequently, silica sol mixed with 100 mL deion- tion temperature of 463 K, 15 wt% DMO (purity >99.5%)ized water was added into the solution moderately, and the pH in methanol and H2 was preheated, then fed into the reac-value of the mixture was adjusted to 2-4 by nitric acid. Then,tor at a H2/DM0 molar ratio of 80 and a system pressurethe suspension was vigorously sirred at 363 K in an oil bath,of 2.0 MPa. The room- temperature liquid hour space veloc-to alow for the hydrolyzing of urea and the increase of pH andity (LHSV) of DMO was 1.0h- I . The condensed productsconsequently the precipitation of silica and the deposition ofwere analyzed off line by an Agilent 6890GC equipped withcopper species on slia. When the pH value of the suspensiona fame-ionization detector (FID).increased to 6- -7, the heating process was terminated. Themixture was separated by vaccum filtration, then the filtrate3. Resultswas washed with deionized water three times, dried at 393K中国煤化工for 12- 14h, and calcined in air at 723 K for4h. The cal-3.1. FormatTYHCNMH Gcined samples were crushed and sieved to 40- -60 mesh. Thecatalyst samples were denoted as x-Cu, where x represents theFT-IR spectra of SiO2 and calcined catalyst samples arecopper content.Journal of Natural Gas Chemisty Vol. 21 No. 5 2012565demonstrated in Figure 1. In the spectrum of SiO2, an ab-and almost vanished when copper loading reached 40 wt%.sorption band at about 1100cm-' and a large shoulder nearThe vibration frequency of Usio band shifted from 1100 to1225 cm~ 1 were observed, which were assigned to Si-O -Si1024 cm- 1 could be ascribed to the incorporation of copperasymmetric stretching vibrations. Additinally, the band at ions into the silica, which identified as the red shft, imply-800 cm-' assigned to Si- 0- Si symmetric stretching vibrationing there was a strong interaction between copper species andcould also be observed [23]. The broad absorption band insilica. Simultaneously, a δoH band at about 670 cm~ 1 was ob-the range of 3600- -3200cm-1 is due to the overlapping ofserved as copper loading higher than 5 wt%. It is reported thatOH stretching of adsorbed water and silanols [24]. And thethe formation of copper phyllosilicate is identified by the co-band near 1640 cm- 1 corresponds to the bending mode of OH existence of the δoH band at about 670 cm-' and the vsio bandgroups of adsorbed water [25].at about 1024 cm -1 [14]. According to the FT-IR spectra, itis clearly that copper phylosilicate can be formed during thepreparation process by UHDP method.It is summarized that the relative amount of copper phyl-losilicate in the calcined Cu/SiO2 samples can be evaluatedby considering the integrated intensity of the δoH band at670cm-1 normalized to the integrated intensity of the sym-、 40-Cumetric Vsio band of amorphous silica at 800cm~ , whichdefined as I7o/I8oo [14]. In the present work, I670/ I800 ra-tio increased with the increase of copper loading, and it was30-Cuclearly that copper phylosilicate became predominant in cata-20-Culyst as copper loading higher than 10 wt% and 40-Cu catalystcontained the most abundant of copper phylosilicate..10-Cu5-Cu3.2. Physicochemical properies of catalysSiO2」The copper loadings of the calcined catalysts are listed in000 3500 3000 250020001500 1000Table 1. The actual copper loading in the catalysts was closeWavenumber (cm ')to the pre-set value determined by ICP, and the filtrate wasalmost colorless, implying the preparation process of UHDPFigure 1. FT-IR specta of SiO2 and calcined catalystsmethod was reasonable under control. The copper dispersionsIt is obviously that the spectrum of 5-Cu catalyst sampleand copper surface areas of Cu/SiO2 catalysts determined bywas almost the same as that of SiO2. A shoulder at aboutN2O chemisorptions are summarized in Table 1. The Cu dis-1024 cm-1 appeared in 10-Cu catalyst, and the shoulder be-persions decreased with the increase of copper loading, whilecame apparent with increasing copper loading, and the inten-the copper surface areas increased firstly and maximized atsity of the band attained a maximum in 40-Cu catalyst. In con-20 wt% copper loading, with a value of 28.0 m2.g-' , and thentrast, the v's;o band at 1100 cm-1 gradually became weakerdeclined mildly with further increase in copper loading.Table 1. Physicochemical properties of the calcined CuSiO2 catalyst samplesCu loadingCu dispersionVdounOdauCatalysts(wt%)"(%)(m2.g-1)(nm)(cm3:g-)(nm)f(nm)d5.4037.712.2194.213.050.7810-Cu .9.0023.015.0232.38.850.5919.0021.628.033.36.870.64 ;30.8013.326.0387.235.850.66 .3._40-Cu39.6024243396.10.8.a Cu loading conducted by ICP; b Cu dispersion and maeallic copper surface area determined by N2O surface oxidation;“Cu2O crytallite size calculatedby the Scherter formula; d Cu crytallite size calculated by the Scherrer formulaN2 adsorption-desorption isotherms of the calcinedThe BET surface area increased from 194.2 to 396.1 m2:g 1Cu/SiO2 samples and their pore size distribution curves arewith increasing copper loading, which was consistent with theillustrated in Figure 2. AlIl of them exhibited a type IV Lang-decrease of the average pore diameter. However, the variationmuir isotherm according to the IUPAC classification, and theof the averag中国煤化工; which maximized athysteresis loop changed from H1-type to H3-type with in-0.66 cm3.g2NMLC-Cu catalyst with ancreasing copper loading, implying that mesopores dominatedaverage pMHCNMHGin these Cu/SiO2 catalyst samples and the pore shape changedThe pore size distribution curves derived from the desorp-from spherical to slit-like channels [26]. The physicochemi-tion branch (Figure 2b) show that, at relatively lower coppercal properties of the calcined catalysts are shown in Table 1.loading (≤10 wt%), the contribution of pores to the total pore566Bo Zhang et al/ Joumal of Natural Gas Chemistry Vol. 21 No.5 2012volume pore size, herein, we called the most probable porecompared with bulk CuO [27]. Zhao et al. [28] reported thatsize, were at 16.0 and 12.4 nm for 5-Cu and 10-Cu catalystthe H2-TPR peak for bulk CuO (prepared from Cu(NO3)2 de-samples, respectively. The bimodal distributions of the mostcomposition at 623 K) is at the temperature about 653 K. Chenprobable pore size appeared in the calcined catalyst sampleset al. [29] reported that the lack of reduction peaks at temper-with further increase in copper loading, which might be re-atures lower than 473 K indicated the absence of oxocationssponsible for the large specific surface area. van der Grift et(Cu-O-Cu)2+, while the lack of peaks at temperatures higheral. [13] proposed that deposition-precipitation of copper ionsthan 533 K indicated the absence of a copper crystal phase. Inonto the silica support resulted in a significant enlargementaddition, some literatures indicated that the reduction temper-of the specific surface area with increasing copper content.ature of a well-dispersed (3- -5 nm) CuO/SiO2 catalyst pre-Moreover, the increase of BET surface area with the copperpared by an ion-exchange method can be below 523 K [30].loading can be attributed to that silica support is consumed inIt is reported an increased porosity of catalysts resulting inthe formation of chrysocolla-like structure.more difficult to reduce, to some extent, that means relativelyhigher reduction temperature is requested for porous catalysts.In the present work, with the increase of copper loading, thereduction temperature of the catalyst samples increased froma)482K to 512K. It is in line with the N2-adsorption results.This shift of reduction temperature inferred that the peaks ofthe catalyst samples can be assigned to the reduction of well-dispersed cupreous species.30-Cu|5-Cu0.20.0.81.0Relative pessre (p/Po)40-Cu20-Cub)10-Cu. 40-Cu.5-Cu203.337373Temperature (K)125Figure 3. TPR profiles of the catalyst precursors12.4It can be seen that H2 consumption increased with the10-Cu.increase of copper content, particularly evident in 40-Cu cat-16.0alyst sample. It was documented that reduction of cupreousspecies with H2 involved the processes as following: Cu2+toCu', Cu2+ to Cu+ and Cu+ to Cu'. Van der Grift et al.[31] identified only one reduction peak at ca.510 K in cal-Pore diameter (nm)cined copper phylosilicate, suggesting the identical reductionFigure 2. N2 adsorption-desorption isotherms (a) and pore size distributiontemperature for copper phyllosilicate to Cu+ and for the well-curves (b) calculated by BJH equation in desorption branch of the calcineddispersed CuO to Cu'. Chen et al. [16] reported the reductioncatalystsofCu2+ to Cu+ for Cu-0 -Si species and copper phyllosili-cate occurred at about 520 K, which strongly overlapped withthe peak for the reduction of small particles of CuO to Cu0.3.3. Reduction behavior of CwSiO2 catalystsFurther reduction of Cu+ to Cu0 for Cu-O-Si species requireda temperature above 873 K [30]. Because of the strong inter-TPR characterizations were carried out to investigate theaction between copper species and support in catalysts withreducibility of the catalyst samples from 303 K to 773 K. Asthe structurer^1 a相c 2u2+ is hard to reduce中国煤化工seen from Figure 3, all of the catalyst samples displayed adirectly toon termperature was notnarrow and almost symmetrical peak. This finding inferredhigh enougtYHC N M H Ghus, the strong reduc-that the particle size distribution was homogeneous. It is welltion peak of the catalyst samples consisted of the reduction ofknown that highly dispersed copper oxide is more easily re-copper phyllosilicate to Cu+ and well-dispersed CuO directlyduced, which means it can be reduced at lower temperature asto Cu.Joumal of Natural Gas Chemistry Vol. 21 No.5 20125673.4. Crystalline phase of CuSiO2 catalyststhe most abundance of CuO species which is also corroboratedby TPR. Moreover, agglomeration of copper species occurredXRD patterns of the catalyst samples including bareon the surface, resulting in a poor dispersion of copper speciessupport (SiO2) after calcination are shown in Figure 4. .in 40-Cu catalyst sample, which was consistent with the N2ODiffraction peaks of crytaline phase of copper species weretitration measurements. It was worthwhile to note that thereobserved very rarely and the diffraction peaks morphologywas no detection of Cu' phase when copper loading lowerof catalyst sarmples were almost identical to the bare supportthan 30% in the XRD patterns due to its amorphous phase orwhen the copper loading lower than 10 wt%. When the cop-very small particle size.per loading was above 10 wt%, diffraction peaks at ca.31.20and 35.80 became apparent, which can be attributed to copperphylolillicate with poor crystallinity, indicating that copperI SiO,species was in good dispersion [15,32]. The diffraction peak●Cu.0▲Cuiat ca. 22.00 was atrbuted to SiO2, and it could be seen fromFigure 4 that the diffraction peaks of SiO2 were broad anddiffuse, implying that SiO2 was amorphous. The diffractionpeaks attributed to silica diminished as the increasing coppercontent. It should be noted that there was no detection of cop-官per species phase when copper loading was lower than 10 wt%in the XRD patterns due to its amorphous phase or very smallparticle size.S-CiSiO2CuO10204(608(20/(° )Figure 5. XRD patterns of the reduced catalysts3.5. Surface chemical state of the reduced CuSiO2 catalysts20-CuThe surface chemical compositions of the reduced cat-alyst were evaluated by XPS. In section 3.4, XRD pattermsdemonstrated that the valence of copper species was +2 in thecalcined catalyst samples. Generally, the Cu 2p3/2 binding en-ergy of CuO is found at around 933.5 eV, and the binding en-SiO,ergy for well-dispersed Cu2+ species strongly interacting with2(40507080silica supports is detected at above 934.9eV [13,33]. Com-pared with the calcined catalysts, in Figure 6, the Cu 2p3/2Figure 4. XRD patterns of SiO2 and calcined catalystsB.E. of the reduced catalysts shifted to 932.7 eV, which canbe ascribed to the reduction of Cu2+ to Cu0 or/and Cu+ at .XRD patterns of the catalyst samples after reduction are493 K. It is known that it is hard to discriminate the coppershown in Figure 5. After reduction, diffraction peak of cop-per species was still invisible when copper loading lower than10 wt%. However, with the continuous increase of copperCu2p932.7loading, the diffraction peaks of cupric species vanished alongwith the appearance of the peaks at ca.36.80 from Cu2O.Additionally, diffraction peaks at 43.3°, 50.40 and 74.1° at-tributed to Cu' were observed in the reduced 40-Cu catalyst40-Cu。sample whereas very lttle Cu0 diffraction peak was found in30-Cu20 Cu and 30-Cu catalyst samples. Among the reduced cat-alyst samples, the 30-Cu catalyst sample exhibited the mostdistinct Cu2O (111) diffraction peak. While in the 40-Cu cat-10-Cualyst sample, the crystalline phase of Cu' (111, 200, 220)中国煤化工appeared obviously after reduction, together with broad andYHCNMHGo30weak Cu2O diffraction peak. The copper species crystalliteBinding energy (eV)sizes calculated by Scherrer formula are also listed in Ta-ble 1. This finding implied 40-Cu catalyst sample containedFigure 6. Cu 2p XPS spectra of the reduced catalyst samples568Bo Zhang et al./ Journal of Natural Gas Chemitry Vol. 21 No. 52012species with low valence, since the B.E. of Cu0 and Cu+Cu), the Cu+ surface area was estimated according to the Cu0species is almost the same. Conventinally, Cu0 and Cu+surface area and the surface Cu (Cu+ +Cu") LMM intensityspecies can be distinguished only by their different kineticratio [16]. As listed in Table 2, inconsistent to the Cu0 surfaceenergies in the XAES Cu LMM line position or the modi-area, Cu+ surface area gained the maximum in 30-Cu catalystfied Auger parameters aCu [34]. It is reported thata' is ca.sample, with a value of 42.9 m2.g-1.1851.0eV for Cu0 and 1849.0 eV for Cu+ [35].In Figure 7, all the Cu LMM spectra possessed the asym-metrical and broad peaks, implying the coexistence of Cu0 andCu+ in the catalysts surface. Deconvolution of the original CuLMM peaks was performed, obtaining two symmetrical peakscentered at near 916 and 918 eV, corresponding to Cu+ and_40-CuCuP species, respectively.The deconvolution results are listed in Table 2. Itis clearly that copper loadings have significant effects onthe surface distributions of Cu+ and Cu' species. The30-CuCu+/(Cu'+Cu+) intensity ratio increased with the increaseof copper loading, and maximized at 30-Cu sample, with avalue of 62.3%. This finding was in line with the XRD pat-告tems. The large amount of Cu+ species in the reduced cata-lyst samples indicated the interaction between copper speciesand supports was strong, hindering the reduction of Cu2+ tol0-CuCu0. IR results showed that copper phyloilicate was prc-dominant in Cu/SiO2 catalysts, in which the reduction behav-ior was constrained by the unique structure, causing the re-s-Cuduction of Cu4+ ceased at Cu+. The simultaneous existenceof Cu+ and Cu0 inferred one more copper species on calcined910Cu/SiO2 catalysts, which was identified by TPR results.Binding energy (eV)Assuming that Cu+ ion occupies the same area as that ofCu' atom, and has the same atomic sensitivity factor as that ofFigure 7. Cu LMM XAES spectra of the reduced catalyst samplesTable 2. Surface Cu species of the reduced catalyst samples derived from Cu LMM XAES spectraK.E (eV)a (eV)Catalystsb.E. of Cu 2p3/2 (eV)Xo+ %Sou+ (m2:g门)Cu+Cu5-Cu918.01848.71850.7932.735.26.10-Cu916.0918.2.1848.7.1850.940.710.320-Cu916.11848.848.226.11848.8 .1850.8932.862.342.940-Cu916.2918.11848.952.627.03.6. Catalytic activityIn hydrogenation of DMO to EG, it is known that DMOC2HsOH + (CH2OH)2一+ HOCH2CH(OH)C2Hs + H2Ohydrogenation firstly forned methyl glycolate (MG), then fur-4)ther hydrogenation of MG to EG, while EG can dehydrate toIn Figure 8, it is obvious that at least 20 Wt% of cop-ethanol (ET). Additionally, EG and ET can also dehydrate toper loading is required for Cu/SiO2 catalyst to achieve ac1, 2-butanol (BDO). MG might be known as partial hydro-credited catalytic performance. Under the reaction conditionsgenation product, whereas ET and BDO were called the prod-specified in Figure 8, 5-Cu catalyst showed no activity to EG,uct of excess hydrogenation. DMO hydrogenation proceedsonly MG was obtained during the hydrogenation process, andDMO conversion was also low. For 10-Cu catalyst, all theaccording to the following reactions:converted DMO had been turned into MG and EG, EG se-(COOCH3)2 +H2 - -+ CH20HCOOCH3 + CH3OH (1lectivity reached 25%, the other was MG, DMO conversionwas still P中国煤化Inded rapidly to 100%CH2OHC00CH3 +H2一(CH20H)2 + CH3OH (2when coPPIHCNMH Gwt%, and EG selectiv-ity reachedalydrogenation productappeared. DMO conversion was maintained at 100% in 30-HOCH2CH2OH +H2 - -→CH3CH2OH +H2O (3Cu and 40-Cu catalysts, and EG selectivity attained a maxi-Joumal of Natural Gas Chemistry Vol. 21 No.52012569mum at 95% in 30-Cu catalyst, and dropped to 92% in 40-Cucatalyst. The over hydrogenation product became evident as100copper loading higher than 20 wt%, attributable to sufficientquantity of active sites in catalyst. 30-Cu catalyst exhibitedthe highest hydrogenation activity with DMO conversion of80一DMO conversion100% and EG selectivity of 95% among the set of Cu/SiO2十MG seletivitycatalysts.一EG slectivity十ET slectivity+ BDO setivity100天50 E40- DMO conversionEG selectivity0.4.8.01.220 tET selectivityLHSV(h")MG selectivity8- BDO seletivityFigure 9. Effects of DMO LHSV on catalyic performances of 30-Cu cata-lyst. Reaction conditions: p= 2.0MPa,T =473 K, H/DMO= 806It is reported that more amount of copper phyllosilicategave rise to the higher catalytic activity [16]. Obviously, inthis article, the most abundant of copper phyllosilicate wasfound in 40-Cu catalyst sample. Interestingly, though it exhib-ited good activity in oxalate hydrogenation, the partial hydro-Cu content (wt%)genation product (MG) was non-neglectable, and EG selec-Effects of Cu loadings on catalytic performances. Reaction condi-tivity was still not high enough compared with 30-Cu catalysttions: p= 2.0MPa, T = 473 K, H2/DMO = 80, LHSV= 1.0h-sample. How to explain this finding? It is reported that coppersurface specific area profoundly affected the catalytic perfor-In Figure 9, a typical set of results of the effect of DMOmance of copper-based catalysts [36]. According to the N20liquid hourly space velocity (LHSV) on catalytic performancechemisorpotion results (as shown in Table 1), copper surfaceover 30-Cu catalyst were demonstrated. The conversion ofarea of 40-Cu sample was lower than that of 30-Cu sample.DMO was kept on at 100% when LHSV was increased fromThe biggest copper surface specific area among the series of0.4 to 1.6h-'. The seletivity to EG increased with the in-Cu/SiO2 catalysts belonged to 20-Cu sample, which lookedcrease of LHSV when it raised to 1.0h~1, then decreasedlike to obtain the highest EG yield. However, it is demon-gradually when LHSV varied from 1.0to 1.6h-'. Ata low setstrated that 30-Cu catalyst sample gained the best catalyticof LHSV ranged from 0.4- 1.0h- 1, almost no MG yielded,performance. This phenomenon indicated there were somebut the formation of ET and BDO were apparent, especiallyfactors else related to the catalytic performance besides thewhen LHSV was lower than 0.8h 1. At a high set of LHSVcopper surface area. The asymmetrical and broad peaks of theranged from 1.2-1.6h-1, the selectivity to MG increasedCu LMM spectra implied the coexistence of Cu0 and Cu+ onslightly and reached 6% at a LHSV of 1.6 h 1, and the selec-the catalysts surface. By deconvolution of Cu LMM peaks,tivity to excess hydrogenation product declined to lower thanit could be obtained Cu+/(Cu'+Cu+ ) intensity ratio increased1%. When the reaction conducted at a low LHSV, the resi-with the increasie of copper loading, maximized at 30 wt%,dence time of the reactant was relatively long, implying theand then dropped at 40 Wt%. Thus, Cu+ may play a significantreactant had adequate chance to contact with the active siterole in oxalate hydrogenationreaction. Li et al. [37] suggestedon the catalyst. Consequently, DMO converted completelythat Cu+ determines the conversion of oxalate and Cu0 is re-and partial hydrogenation product was rare, but excess hy-lated to the selectivity to EG. In methy acetate hydrogenation,drogenation product appeared, causing reltively low EG se-Poels and Brands [38] reported Cu0 dissociatively adsorbs H2,lectivity. Besides, at a low LHSV, the heat transfer was bad,and Cu+ stabilizes the methoxy and the acyl species. Simi-therefore the temperature of the inner catalyst might be higherlarly, in hydrogenation of DMO, accordingly, we propose Cu'than the bulk reaction temperature. Since high temperaturedissociates H2 and Cu + activates DMO.favors the excess hydrogenation reaction, thus, low LHSVTo our knowledge, DMO hydrogenation reaction is a cas-was inevitable to form excess hydrogenation product. At acade reaction, DMO proceeds via MG to EG. As copper load-high LHSV, in contrast, the formation of partial hydrogena-ing was relatively low (≤10 wt%), the amount of Cu0 anction product was apparently due to the short residence time ofCu+ surface snerific area was low rsulting in a poor DMOthe reactants, and the selectivity to EG was also relatively low.conversion a中国煤化Livity to MG than EG.It was notable that, in hydrogenation of DMO to EG, appro-When catalyCNMHGntabove20wt%,thepriate LHSV was very important to gain the optimal productCu' surface area was larger tnan 24 m*.g -I , DMO was con-distribution. As a result, in the present work, the best LHSVverted completely. But EG selectivity changed with Cu+ sur-was 1.0h-1.face area, the biggest Cu+ surface area gave rise to the highest570Bo Zhang et al./ Joumal of Natural Gas Chemistry Vol. 21 No. 52012EG selectivity. Therefore, we tentatively conclude that DMO[12] Trillano R, Lambert J F, Louis C. 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