Influence of Calcination Temperature on the Performance of Cu-Al-Ba Catalyst for Hydrogenation of Esters to Alcohols

YUAN Peng LIU Zhong-Yi SUN Hai-Jie LIU Shou-Chang

(Department of Chemistry,Zhengzhou University,Zhengzhou 450001,P.R.China)

Influence of Calcination Temperature on the Performance of Cu-Al-Ba Catalyst for Hydrogenation of Esters to Alcohols

YUAN Peng LIU Zhong-Yi SUN Hai-Jie LIU Shou-Chang

(Department of Chemistry,Zhengzhou University,Zhengzhou 450001,P.R.China)

Novel chromium-free Cu-Al-Ba catalysts were prepared by co-precipitation and were calcined at different temperatures.Their performance during the hydrogenation of palm oil esters to higher alcohols was evaluated in an autoclave.Results showed that the catalytic properties of the catalysts were greatly influenced by the calcination temperatures.The yield of higher alcohols showed three steps when the calcination temperature of the catalysts was raised from 150 to 750℃.The thermogravimetric(TG-DTG)curves of the precursor also exhibited three steps related to mass loss.X-ray power diffraction(XRD),X-ray fluorescence(XRF),transmission electron microscopy-energy dispersive spectrometry-selected area electron diffraction(TEM-EDS-SAED),N2-physisorption,and temperatureprogrammed reduction(TPR)characterization revealed that the catalysts were obtained from a malachite-boehmite-BaCO3precursor.After calcination at 300 or 550℃,the catalysts were found to be composed of crystalline CuO and BaCO3as well as amorphous Al2O3.Amorphous Al2O3has a large surface area which results in a high dispersion of CuO.Rod-like BaCO3helps in the provision of micropores.The formation of BaAl2O4at a calcination temperature of 750℃destroys the amorphous structure and causes a sharp decline in the surface area and pore volume of the catalyst and this causes CuO aggregation.An optimal higher alcohol yield of 92.3%was obtained over the Cu-Al-Ba catalyst that was calcined at 550℃due to its larger surface area,larger pore volume,and higher degree of CuO dispersion.

Hydrogenation;Ester;Higher alcohol;Cu-Al-Ba catalyst;Calcination temperature

Fatty alcohols(FOH)and their derivatives are widely used as surfactants,lubricants,solvents,synthetic detergents,antifoaming agents,perfumes,cosmetics,pharmaceuticals or additives in many industrial products[1].Naturally derived fatty alcohols ap-proximately account for 65%of the world′s steadily growing fatty alcohol demand[2].They are special products since they are only obtained from natural fats and oils by hydrogenation of fatty acids or fatty acid methyl esters(FAME)[3].Since nineteen fifties,natural-fatty-alcohol-based surfactants have gained growing significance in the detergent market due to their excellent washing properties and superior biodegradability.

On the other hand,plant oils,which are complex mixtures of fatty acid esters in triglyceride form,are very useful renewable sources for producing chemicals[4].During the last decade,the development of biodiesel has been emphasized.Biodiesel is defined as fatty acid methyl esters obtained by the transes-esterification route of renewable biological sources with methanol,such as soybean oil,cottonseed oil,rapeseed oil,various animal fats, and waste frying oil.The production of FAME has been commercialized in EU and USA,and has been offered abundant feedstock for the hydrogenation to obtain FOH[5].It has been frequently stated that the best method of converting an acid to the corresponding alcohol is through the esters,since esters are less corrosive and can be reduced to alcohols with considerably higher yields than in reducing the corresponding acids and esters can be normally obtained from acids in nearly quantitative yields.

Therefore,up to date,hydrogenation of FAME to the corresponding FOH(RCOOCH3+2H2=RCH2OH+CH3OH)is of great industrial importance.And,the concern of catalyst research is raised because catalyst is the key technology for effective production of FOH and successful utilization of natural fats.Commercial large scale production always relies on effective heterogeneous catalysts in slurry-phase or fixed-bed reactors[6].Cu-Cr based Adkins catalysts are typically used under very severe reaction conditions of 200-300℃and approximately 20-35 MPa[7]. Cu-Cr catalysts are currently the commercially most successful catalysts due to their adequate hydrogenation activity and adequate resistance to the fatty acids in the reaction mixture.However,because chromium compounds are toxic and because many of the hydrogenation products have final applications in household products(soaps,detergents,cosmetics,etc.),strong environmental restrictions are driving force behind the search for more efficient and less polluting processes.Meanwhile,severe reaction conditions are problematic from safe and energetic viewpoints.Therefore,hydrogenation of esters to the corresponding alcohols over chromium-free catalysts under mild reaction conditions becomes one of the most intriguing tasks from the aspect of“green and sustainable chemistry(GSC)”[8].It is also a great challenging subject due to the weak polarisability and intrinsic steric hindrance of the C═O bond of esters[7].

Fatty alcohol producers and catalyst researchers have attempted to develop Cr-free catalysts for many years.In the last decade,noble metals,such as Pt[9],Pd[4],Ru[3]and Rh[10],were dominantly studied to develop novel catalysts with special emphasis on the hope of less severe reaction conditions.However, most of the above studies were concentrating on the characterization of the catalysts or mechanism of the reaction yet Cr-free catalysts with equivalent performance to the Cu-Cr catalysts have not been developed to date.

Copper containing catalysts are considered suitable for hydrogenation of esters to alcohols,since they allow for selective hydrogenation of C—O bonds and are relatively inactive in C—C bond hydrogenolysis[11].Copper on itself,however,is usually not active,sinter-resistant or mechanically stable enough for industrial operation,so promoters are added to obtain the desired chemical and physical properties.Recently,the effects of Ba on the textural,structural and catalytic properties of Pt/Al2O3and Pd/ Al2O3systems[12-14]have been extensively studied for NOxstorage and reduction.However,no recent or precise report about Cu-Al-Ba catalysts for the hydrogenation of FAME has been found except our previous work concentrating on the preparation methods and operation conditions of the Cu-Al-Ba catalyst[15].It has been found that the calcination procedure has significant effect on catalytic performance of catalyst,but no detailed analysis has been provided.In this paper,slurry phase hydrogenation of natural palm oil esters was reported using Cu-Al-Ba catalysts under relatively mild conditions for producing higher alcohols. The influences of calcination temperatures of the catalysts are studied in detail by means of XRF,XRD,TEM,TG-DTG,H2-TPR characterizations and hydrogenation tests.

1 Experimental

1.1 Experimental materials

The methyl esters used in the experiments were produced via trans-esterfication of natural palm oil with methanol and were offered by Shangqiu Longyu Chemical Ltd.According to the analysis following the standard IUPAC methods[16],the substrates have an acid value of 0.81,intrinsic hydroxyl value of 1.12,and saponification value of 188.79.Therefore,the theoretical hydroxyl value is 210.0.

1.2 Preparation of catalysts

The catalysts were prepared via co-precipitation method in aqueous solutions.All of the chemicals(Kermel,AR)were commercially obtained and were used as received.Typically,a warm (80℃)solution of 0.3 mol·L-1Na2CO3was added dropwise into a warm(80℃)mixed solution of an appropriate amount of Cu(NO3)2·3H2O,Al(NO3)3·9H2O and Ba(NO3)2(0.3 mol·L-1of total metal ions)under rapid stirring,the pH value was tuned to 8.0.The reaction was lasted for 1 h to ensure the complete coprecipitation of the metallic ions in the solution.The resulting sample precipitates were filtered and washed thoroughly with large amount of deionized water until pH=7.After being dried in air at 90℃,the sample(referred to as precursor)was divided into several fractions.Each fraction was calcined at a certain temperature for 3 h in air atmosphere,and the resulting catalysts were ground into powders for laboratory tests.The samples obtained at 300,550,and 750℃were denominated as Cu-Al-Ba-300, Cu-Al-Ba-550,and Cu-Al-Ba-750,respectively.XRF chemical analysis of the calcined sample gave bulk compositions of CuO 58.24%(w,same as follows),BaO 16.42%,and Al2O323.49%.

1.3 Characterization of catalysts

Actual catalyst composition was determined by X-ray fluorescent spectrometry(XRF,Bruk S4 Pioneer,Germany).The structures and crystal sizes were investigated on a XRD diffractmeter (PANalytcal X′Pert PRO,Netherland)by using Ni-filtered Cu Kαradiation in the range of 10°＜2θ＜80°.The calcination process was examined in situ by a combined thermogravimetric analysis (STA 409 PC/PG NETZSCH,Germany)with air flow of 60 mL· min-1,the temperature was raised from ambient temperature to 800℃at a heating rate of 10℃·min-1.The surface area(Sg),pore volume(Vp),and pore distribution of catalysts were measured by Nova 1000e surface area and pore size analyzer(Quantachrome Instruments,US).N2physisorption isotherms were obtained at -196℃by using the BET equation for surface area and the BJH method for pore distribution calculations.The samples were previously degassed at 150℃for 90 min.TEM,EDS and SAED were performed on a JEM-2100 instrument using accelerating voltage of 200 kV.TPR profiles were obtained by heating about 30 mg of the sample in a H2/N2mixture(5%H2by volume,Praxair,99.999%pure)at a flow rate of 20 mL·min-1.During the experiment,the temperature was raised to 600℃at a rate of 10℃·min-1and the hydrogen consumption was measured by a thermal conductivity detector(TCD).

1.4 Apparatus and procedures

Slurry phase hydrogenation was carried out in a stainless autoclave(FYX,4thInstrument Factory of Dalian)equipped with a magnetic stirrer and a pressure regulator.In a typical experiment,50 mL of methyl esters and 2.5 g of the activated Cu-Al-Ba catalyst were charged into the reactor.The reactor was purged with hydrogen four times to expel air.After the desired temperature reached,a certain H2pressure was maintained and the stirring of 1000 r·min-1was commenced to exclude the diffusion effect.Each reaction was allowed to proceed for 10 h with sampling of a small portion of the reaction mixture every 1 h.The yield(Y)of alcohols was expressed in the formula:

wthere HVP,HVI,and HVTrepresent the hydroxyl value of the products,the intrinsic hydroxyl value of the starting material,and the theoretical hydroxyl value,respectively.The hydroxyl value of the products was obtained according to the standard IUPAC method[16].

2 Results and discussion

2.1 XRD analysis of catalysts

To determine the crystal phases in Cu-Al-Ba samples,XRD analysis is carried out,and the results are shown in Fig.1.Detailed information is listed in Table 1 as well.

In the precursor state,the sample presents only the peaks of BaCO3andCu2(OH)2CO3phases.After calcination at 300℃,CuO phases emerge,indicating that the reaction of Cu2(OH)2CO3thermaldecomposition(Cu2(OH)2CO3=2CuO+H2O+CO2)hashappened.After calcination at 550℃,the catalyst exhibits also BaCO3and CuO phases.The BaCO3phases and their crystal sizes are nearly unmodified,showing that the solid is stabilized.The pattern of Cu-Al-Ba-750 shows more pronounced and sharper peaks of CuO and BaAl2O4,indicating better crystallized phases.

The results reveal that the calcination temperature has significant influence on the structure of the catalysts.The CuO phase gives the strongest diffraction signal in Cu-Al-Ba-550 and Cu-Al-Ba-750.Obviously,the peaks of CuO phases in Cu-Al-Ba-550 were relatively broadened in comparison with that in Cu-Al-Ba-750.The crystallite size of CuO increases from 38.3 to 45.0 nm when calcination temperature rises from 550 to 750℃.Weak peaks of CuO are observed in Cu-Al-Ba-300,the crystal size is about 13.8 nm.Moreover,Cu-Al-Ba-750 shows apparently sheerer peaks and especially newly formed crystal phases which are attributed to BaAl2O4.Peden[17]and Li[13]et al.have obtained similar results and suggested that increasing the calcination temperature to 800℃led immediately to Ba-aluminate formation.It is worth noting that no Al containing phase is detected except Cu-Al-Ba-750,this is related to the amorphous phase of Al2O3confirmed by TEM-EDS-SAED studies below.Li et al[18]have obtained similar results that at temperatures up to 700℃,theAl2O3precursor patterns are ascribed to an amorphous phase.It was considered that amorphous precursors produced catalysts of high activity and stability[19].The patterns of Fig.2 and Fig.1(c) clearly indicate the good dispersion of CuO and BaCO3on the amorphous structure.It can be inferred that the intimate contact between Ba and Al cations exists in the catalyst calcined at 550℃,which leads to the formation of BaAl2O4species at higher calcination temperature of 750℃.

Table 1 Information of the crystal phases determined by XRD

2.2 TEM-EDS-SAED analyses

TEM micrographs and SAED pictures of the calcined samples are reported in Fig.2.In photograph(a),the deep-dark particles are copper oxide species and the light-gray subtransparent particles are species of Al2O3with the aid of EDS analysis.BaCO3particles are observed to be rod-like,which is in good accordance with Shen group′s report[20].The SAED pattern of Al2O3shows only a diffraction halo as exemplified in Fig.2(b),confirming the amorphous nature of the Al-containing particles.And, this is in responsible for the absence of Al-containing component peaks in Fig.1(b,c).As can be seen,the CuO species are highly dispersed on the amorphous structure of Al2O3with BaCO3embedded.Micropores can be observed.

In photograph(c),although deep-dark particles of copper oxide species are also observed,there are clear differences in the morphology.Obviously,in Cu-Al-Ba-750,copper oxide particles appear in larger agglomerates than those of Cu-Al-Ba-550. The species become more aggregated and concentrated on relatively smaller surface of the crystallized structure.The amorphous structure disappeared accompanied with the loss of micropores.The gray particles of crystal phase are ascribed to BaAl2O4according to EDS and XRD analyses.The SAED pattern in Fig.2(d),exhibiting scattered spots,reveals the crystallization of BaAl2O4,which is in accordance with the XRD pattern in Fig.1(d).Namely,copper oxide particles in Cu-Al-Ba-550 were relatively higher dispersed and isolated.

2.3 Thermal behavior

Fig.3 shows the TG-DTG curves of the precursor as a function of temperature.It can be concluded that the thermal decomposition of the precursor occurs in three steps.The first step exhibits three combined peaks with mass loss of 17.93%in total. The sharp peak at 146.0℃is attributed mainly to the loss of the condensation water.According to XRD analysis,sample calcined at 300℃exhibits only weak peaks of CuO,better crystal phase of CuO appears in the sample obtained at 550℃.The overlapping peaks at 201.1 and 274.6℃can thus be mainly attributed to the release of H2O and CO2from the decomposition of Cu2(OH)2CO3.The second step with mass loss of 5.20%,which occurs between 300 and 390℃,is sharp and can be ascribed to the loss of lattice water from Al2O3hydrates.It is responsible for the decomposition of bayerite to η-Al2O3[21-22]which suggests existence of bayerite in the procursor.This step caused the CuO crystals grow from 38.3 to 45.0 nm.The loss of 3.84%occurs above 550℃in the last step which is related to the release of CO2from the reaction of Al2O3and BaCO3to form BaAl2O4.The formation of the porous structure of the resulting catalyst occurs by elimination of both the hydroxyl groups and the carbonate ions as H2O and CO2from the catalyst precursor.Calcination temperature of 550℃is adequate for the dehydroxylation and decarbonation of the catalyst precursor.It is also low enough to avoid the damage of the amorphous structure of Al oxides and the form of BaAl2O4.

2.4 Physical properties

In Fig.4,the N2adsorption isotherms and pore diameters of the precursor and samples treated with different calcination temperatures are reported.There are hysteresis loops in the N2-adsorptiondesorption isotherms of the samples,indicating the existence of mesopores.All isotherms are of type IV according to the IUPAC classification.The hysteresis loops are similar to that of type H1defined by IUPAC,and it is considered that the pore structures are capillary or piled with homogeneous spheres.Pore distribution of Cu-Al-Ba-550 is more concentrated,indicating that the pores are well-proportioned.Calculated BET surface areas,micropore volumes as well as average pore diameters are reported in Table 2.

As expected,Cu-Al-Ba-550 exhibits a significant larger specific BET surface area.In comparison of the precursor,dehydroxylation and decarbonation of the thermal decomposition result in larger surface area and pore volume.The surface area increases with increasing the calcination temperature from 300 to 550℃mildly.A significant decrease of surface area and pore volume occurs with elevating the calcination temperature from 550 to 750℃.It has been revealed by XRD that the different heating conditions of 550 and 750℃produce samples with different phases.Higher calcination temperature leads to the reaction between Al2O3and BaCO3to form BaAl2O4which destroys the amorphous structure of the Al2O3phase and results in the loss of specific surface area and pore volume.

2.5 TPR measurements

The TPR profiles of samples calcined at different temperatures are reported in Fig.5.The TPR curve of CuO is also reported for comparison.The TPR profile of pure CuO shows a single peak with maximum at 311℃.All signals observed of the samples can be attributed to the reduction of CuO(CuO+H2=Cu+ H2O),because the reduction of Al3+and Ba2+has been proved difficult.Cu-Al-Ba-550 and Cu-Al-Ba-750 give single peak with maximum at 268.7 and 293.9℃,respectively.Reduction signals of the two catalysts were shifted to a lower temperature range in comparison with that of pure CuO.The reduction of Cu-Al-Ba-550 is easier than that of Cu-Al-Ba-750,which is in good accordance with their BET surface areas and TEM results,indicating that the reduction of CuO component in the catalysts was influenced by the dispersion.CuO with higher dispersion gives TPR signal at lower temperature.It has been found that the more difficult the reduction of the Cu-based catalyst,the less catalytic activity there is[23].Meanwhile,it is now widely accepted that metallic copper particles provide the active sites for the reaction[24-27]. Since the catalyst is activated at 300℃which is much higher than its reduction temperature before its application for the reaction at 240℃,metallic copper can be confirmed as active sites for the hydrogenation.

Table 2 Physical properties of the samples used in this work

2.6 Performance of catalysts

As can be seen in Fig.6,the yield of higher alcohols showed three steps with elevating the calcination temperature of the catalysts.In the first step of 150-300℃,a significant increase of the yield is observed.This step corresponds to the first step of thermal decomposition of the precursor in the TG-DTG curves,illustrating that the formation of metal oxide is vital to high activity of the catalyst.The catalysts are prepared by co-precipitation methods,via malachite-boehmite intermediates(confirmed by XRD and TG-DTG).After being calcined at 300℃,decomposition of the precursor leads to the development of small crystallites of CuO(confirmed by Fig.1(b)),simultaneously,porous structure and larger surface area are provided by elimination of both the hydroxyl groups and the carbonate ions,the dispersion of CuO is also favored and significant increase of the yield are observed. The second step is a mild increase from calcination temperature of 300-550℃.This is mainly caused by the increase of surface area and mespores which lead to a higher dispersion of CuO, Such dispersion would generate,after reduction,nanoparticles of metalliccopperintimatelydispersedamongBaandAl oxides[28-29]and,provide large amount of active sites exposed to the reaction mixture resulting in high yield of higher alcohols[30-31].Combined with BET and TPR analysis,in the catalyst calcined at 550℃, amorphous Al2O3offered large surface area for the high dispersion of CuO species.Al2O3has been reported to increase the BET surface area and the Cu dispersion and to decrease the sintering of the Cu particles which occurs under working conditions[27,30-31].Rod-like BaCO3helped to provide more micropores in the structure.The third step exhibits a sharp decline above calcination temperature of 600℃.According to BET,XRD and TEM study,this decline is mainly caused by the great loss of surface area and pore volume,the aggregation of CuO species also takes account.When calcined at higher temperature,the amorphous structure is destroyed and the sharp decline of surface area and pore volumes is observed due to the formation of BaAl2O4.Simultaneously,the CuO species gets closer to each other and sinters with the help of plenteous heat.In general,the catalytic activities of Cu-Al-Ba catalysts prepared at different calcination temperatures followed a similar sequence to that of the surface areas and Cu dispersions of these catalysts,which wasalsoproposedbyQiao′sgroup[32-34]intheirstudies of dimethyl maleate hydrogenolysis.

Fig.7 summarizes the catalytic performance for 10 time recycles of hydrogenation,for each time,the catalysts were separated and used for another run without regeneration.It was found that although higher alcohol yields fluctuated around 91%,no significant drop was observed,indicating good stability of the catalyst.

For comparison,commercially available Cu-Cr,Cu-Cr-Ba, and Cu-Cr-Mn catalysts were also employed in the hydrogenation of esters with normal activation.Cu-Al binary catalyst prepared via the same procedure of Cu-Al-Ba was investigated as well.

The results in Fig.8 show that the activity of Cu-Al-Ba catalyst is much higher than that of Cu-Cr,Cu-Cr-Mn,and Cu-Cr-Ba catalysts under the same reaction conditions.Typical reaction conditions for the commercial catalysts usually include high temperature of about 300℃and high pressure of above 20 MPa. In our experiment,they are insufficient for high yield under relatively milder reaction conditions of 240℃and 10 MPa.However,Cu-Al-Ba catalyst exhibits much higher yield of alcohols, which would be promising for commercial applications under milder reaction conditions.In addition,only 45.6%of the alcohol yield was obtained over Cu-Al binary catalyst,indicating that Ba played an important role as a promoter in the catalyst.

3 Conclusions

According to the three steps of thermal decomposition of their precursor,Cu-Al-Ba catalysts were prepared by co-precipitation method and calcined at 300,550,and 750℃,respectively.Their performances for the hydrogenation of natural palm oil esters to the corresponding higher alcohols showed that the optimal calcination temperature for Cu-Al-Ba catalyst is about 550℃,atwhich an oxide system of crystal CuO,BaCO3and amorphous Al2O3is formed.Amorphous Al2O3offers large surface areas for high dispersion of CuO while rod-like helps to form the micro pores.Lower temperature cannot facilitate the development of large surface areas and pore volume.Higher calcination temperature will decline the surface area of the catalyst.The formation of BaAl2O4destroys the amorphous structure of Al2O3and the rod-like morphology of BaCO3and thus causes the decline of surface areas and the aggregation of CuO,finally,the yield of higher alcohols declines.Higher alcohol yield of 92.3%was obtained over Cu-Al-Ba-550 catalyst under reaction conditions of 240℃and 10 MPa H2pressure.

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焙燒溫度對酯加氫制醇Cu-Al-Ba催化劑性能的影響

袁 鵬 劉仲毅 孫海杰 劉壽長*

(鄭州大學化學系,鄭州 450001)

通過共沉淀法在不同焙燒溫度下制備了新型無鉻Cu-Al-Ba催化劑.測試了其在高壓釜中將棕櫚油甲酯加氫制備高碳醇的反應性能.結果表明催化劑的焙燒溫度對催化性能有較大影響.在從150℃升至750℃的過程中,高碳醇的收率顯示了三個階段的變化,相應地,催化劑前驅體的熱重(TG-DTG)曲線也顯示了三個階段的失重.X射線衍射(XRD)、X射線熒光(XRF),透射電鏡-能譜分析-選區電子衍射(TEM-EDS-SAED)、N2物理吸附和程序升溫還原(TPR)表征表明,催化劑是由一種孔雀石-勃石-碳酸鋇前驅體制得的.在300或550℃焙燒后,催化劑組成為晶態的CuO、BaCO3和非晶態的Al2O3.其中,非晶態的Al2O3為CuO的高分散提供了大的比表面,桿狀的BaCO3組分有利于提供微孔結構.在更高的焙燒溫度750℃，新物相BaAl2O4的形成破壞了催化劑中的非晶態結構,導致其比表面積和孔容的急劇下降,并引起CuO物種的聚結.550℃焙燒的催化劑顯示了最高的高碳醇收率,達到92.3%，這歸因于其大的比表面積、大孔容和較高的CuO分散性.

加氫;酯;高碳醇;Cu-Al-Ba催化劑;焙燒溫度

O643

Received:January 11,2010;Revised:March 13,2010;Published on Web:May 13,2010.

*Corresponding author.Email:liushouchang@zzu.edu.cn;Tel:+86-371-67783384

?Editorial office of Acta Physico-Chimica Sinica