Hua Song*,Lele Zhao,Na Wang

1College of Chemistry&Chemical Engineering,Northeast Petroleum University,Daqing 163318,China

2Provincial Key Laboratory of Oil&Gas Chemical Technology,College of Chemistry&Chemical Engineering,Northeast Petroleum University,Daqing 163318,China

1.Introduction

The increasing interest in improving environmental protection and promoting efficiency of automotive motors encourage the design of environmentally benign catalysts and development of new processes for gasoline production.In view of the branched paraffin has higher octane numbers than linear alkanes,the use of gasoline that contains higher proportions of these compounds is an alternative way in obtaining clean fuel[1].Usually,the branched paraffins are obtained by isomerization reactions,and C4–C6n-paraffin isomerizations are the most common used reactions.The isomerization was usually conducted on liquid and solid acid catalysts.Traditional industria lliquid acid catalysts,such as H2SO4,HF and SbF5/HSO3F have undesired drawbacks which attributed to their severe corrosivity and difficulty of recover and reuse[2].The solid acids employed are either chlorided alumina or Pt-mordenite catalysts.For chlorided alumina,constant addition of chloride is necessary and its disposal would cause a serious pollution to the environment.For Pt enhanced zeolites,the disadvantage is the relatively high operating temperature which leads to poor RON results[3].Sulfate-modified metal oxides,which are a class of strong solid acid catalysts,are found to be active for alkane isomerization at relatively lower temperatures than commercial chloride-platinated-alumina catalyst[4].Among various sulfated metal oxidies,the sulfated zirconia(SZ)(SZ) has attracted much attention and has been extensively investigated. However, the isomerization activity over SZ catalyst is still not high enough [5]. To enhance the isomerization activity, monomet allic and bimetallic modified SZ catalysts are studied, and the bimetallic modified SZ catalysts exhibit a better isomerization activity.

Rare earth(RE)metals are well known catalyst additives for many reactions[6–9],such as selective catalytic reduction(SCR)of NOxwith ammonia, fluid catalytic cracking(FCC)of hydrocarbons,and fatty acid methyl ester synthesis andn-paraf fins isomerization.Generally,RE metals are added to disperse and interact with other transition metal oxides to generate increased active sites,enhance the strength and density of the acid sites,and improve the thermal stability of the catalysts.

In the present work,a series of RE metals(Ce,Yb,Pr)were introduced into Ni–/ZrO2–Al2O3(Ni–SZA)catalyst.The physicchemical properties of various RE-modified SZA catalysts was comparative studied,andn-pentane isomerization as a probe reaction was used to investigate the catalytic performance of prepared catalysts.

2.Experimental

2.1.Catalysts preparation

The ZrO2–Al2O3binary oxide was prepared by a homogeneous coprecipitation method.For this purpose,an aqueous solution containing the requisite quantities of ZrOCl2·8H2O and Al(NO3)3·9H2O were prepared.This solutionw ashydrolyzed with dilute ammonium hydroxide with vigorous stirring until the pH of the solution reached to 9–10.At this pH,a white precipitate was formed and the precipitate was allowed to settle for 24 h.The suspension was then filtered off and washed with deionized water to eliminate the Cl-ions,and finally dried at 120 °C for 12 h.The powdered ZrO2–Al2O3was immersed in 0.75 mol·L-1of(NH4)2S2O8solution for 6 h to incorporate withions.After that,the suspension was centrifuged.The resulted solid sample was dried overnight at 110°C and then calcined at 650°C for 3 h,denoted as SZA.The as-prepared SZA were divided into four parts,and each of them was impregnated with Ce(NO3)3·6H2O,Yb(NO3)3·6H2O and Pr(NO3)3·6H2O solution for 6 h,respectively.The resulted precipitations were collected and dried at 120°C.Subsequently,the dry powders and SZA were impregnated with Ni(NO3)2·6H2Osolution for 6 h,respectively,and then dried overnight at 110°C.The obtained samples were finally calcinated at 650°Cfor 3 h.The prepared catalyst with 1.0 wt%Ni is denoted as Ni–SZA,and the RE metals modified catalysts contained1.0wt%RE(Ce,Yb,Pr)and1.0wt%Niaredenoted as Ce–Ni–SZA,Yb–Ni–SZA and Pr–Ni–SZA,respectively.

2.2.Catalysts characterization

X-ray powder diffraction(XRD)patterns were recorded on a Rigaku D/max 2200pc diffractometer(40 kV,20 mA) fitted with CuKα radiation(=0.15404 nm).The scan range was from 2θ =10°to 80°.The crystallite sizeDcof the samples was calculated using the Debye–Scherrer's relationship,

whereDcis the crystallite size,λ is the incident X-ray wavelength,β is the full width at half-maximum,and θ is the diffraction angle.

The specific surfaces(BET)of the supports and catalysts were obtained from nitrogen adsorption isotherms using Micro meritics adsorption equipment of NOVA2000e.All the samples were outgassed at 200°C until the vacuum pressure was 800 Pa.The adsorption isotherms for nitrogen were measured at-196°C.

Fourier transform infrared spectroscopy(FT-IR)of the catalysts was acquired on a Bruker Tensor 27 FT-IR spectrometer.The FT-IR samples were mixed with KBr and compacted into a thin pellet,the contents of the samples in the KBr wafers were maintained 1 wt%in all FT-IR experiments.The spectra were recorded at room temperature.

IR spectra of adsorbed pyridine(Py-IR)were recorded on a Spectrum GX Fourier transform infrared spectrometer at 4 cm-1resolution.Samples,in the form of self-supported wafers,were pretreated in vacuumfor1hat400 °Candexposedtopyridineat50 °C.Afterdesorptionat 150,300 and 350°C,the spectra were recorded.

H2temperature-programmed reduction(H2-TPR)experiments were carried out on a Builder PCA-1200 instrument with a H2/N2flow of 40 ml·min-1from 50 to 700 °C with a ramp of 10 °C·min-1.

2.3.Catalytic activity measurements

The isomerization reaction ofn-pentane was performed under hydrogen atmosphere in a fixed-bed flow reactor.Prior to the reaction,the catalyst was activated in a hydrogen stream at 300°C for 3 h,and then cooled to a reaction temperature.A dose ofn-pentane was passed over the 5 g of activated catalyst under the following reaction conditions:reaction pressure of 2.0 MPa,molar H2/n-pentane ratio of 4:1,and weight hourly space velocity(WHSV)of 1.0 h-1.The products were flash-evaporated into an online FL9790 gas chromatograph equipped with an FID detector.

3.Results and Discussion

3.1.XRD

Fig.1.XRD patterns of Ni–SZA and RE–Ni–SZA catalysts.

XRD patterns of various catalysts are illustrated in Fig.1.All catalysts exhibit the similar XRD patterns.More concretely,all catalysts show the diffraction peaks of the tetragonal crystal of ZrO2at 2θ of 30.2°,35.4°,50.4°,60.3°and 63.3°.Whilst,no diffraction peaks assigning to Ni,Ce,Yb and Pr are observed,implying that all metal particles have a better dispersion on the SZA supports respectively.In addition,their crystal sizes are not large enough to be detected byXRD[10].The mean particle diameters(Dc)based on the tetragonal crystal of ZrO2calculated from Scherrer's equation for various catalysts are shown in Table 1.Compared with Ni–SZA catalyst,Pr–Ni–SZA catalyst shows similarDc.However,Ce and Yb modified Ni–SZA catalysts exhibit largerDc.This indicates that the addition of Ce and Yb could promote the growth of ZrO2tetragonal crystal particles.

Table 1The textural and structural properties of Ni-SZA and RE-Ni-SZA catalysts

3.2.BET

Table 1 lists the data on BET surface areas(SBET),pore volumes(Vp)andporediameter(dp)ofallcatalysts.TheSBET,Vp,anddpofNi-SZAcatalystis95.1 m2·g-1,0.089 cm3·g-1,and3.73nm,respectively.However,compared with Ni-SZA,all the RE metals modified Ni-SZA catalysts show slightly lowerSBETanddpwith similarVp.In other words,both bimetallic and monometallic modified SZA catalysts show similar pore structures,which indicates that the addition of bimetal could not lead to the serious blockage of pores.

3.3.FT-IR

FT-IR spectra of samples with different RE metals in the region of 4000–450 cm-1are shown in Fig.2.The band found at 3414 cm-1and1630cm-1are the O–Hbondvi bration.The spectrum of all samples exhibited three bands at1270cm-1,1151cm-1and1072cm-1,attributed to S=OandS–Ovi bration modes of surfacespecies[11].The characteristic vibration peaks for asymmetric and symmetric stretching frequencies of S=OandS–O bonds can prove that the formation of solid superacid structure[12].The partially ionic nature of the S=O bond is responsible for the Bronsted acid sites in catalysts[13].The intensity and splitting extent of the vibration peaks reflect the proportion of acid sites linked to the catalyst.In comparison with Ni–SZA,Yb–Ni–SZA catalyst shows stronger vibration peak for S=O bond,in addition,the peak at 1151 cm-1was sharper,indicating the formation of stronger acid on the Yb–Ni–SZA catalyst.Nevertheless,the Ce–Ni–SZA and Pr–Ni–SZA catalysts exhibit worse splitting extent over three vibration peaks,suggesting that acidity become weaker after introducing Ce and Pr,respectively.

Fig.2.FT-IR spectra of Ni–SZA and RE–Ni–SZA catalysts.

3.4.Py-IR

Py-IR analysis is used to discriminate acid types andacid amounts in the catalysts,and the acid distribution of each catalyst quantitatively calculated from the IR spectra of pyridine desorption at 150 °C,300 °C and 350°C are listed in Table 2.For all the catalysts,the total acidity,as well as Bronsted and Lewis acidity decrease with the increase of desorption temperature,and the amount of Br?nsted acid sites is lower than that of Lewis acid sites.Therefore,the acidity of catalysts mainly derives from Lewis acid sites.

Table 2Acidity data of Ni–SZA and RE–Ni–SZA catalysts from Py-IR spectra

In comparison with Ni–SZA catalyst,a distinct increase in Lewis and Br?nsted acid sites areobserved for all RE metals modified catalysts[14,15].This indicates that the acid strength of catalysts is improved,and the amounts ofloaded on the catalysts are increased.The compensation effects of the RE3+cations with unsaturated coordination result in the increasing amount of Lewis acid sites[16].The RE3+combine with H2O molecule to form hydrated RE cations during the impregnation.Then hydrolysis of the hydrated RE cations takes place(Eqs.(1)and(2)).Meanwhile,the formed protons would generate Br?nsted acid sites[17–20].Thereby,the amount of Br?nsted acid sites is increased.Generally,the acid sites measured at 150°C were assigned to weak acid sites.Those at 300°C were moderately strong acid sites.Whereas those at 350°C were assigned to superacid sites[15].Yb–Ni–SZA exhibits the largest amounts of Bronsted and Lewis acid sites at desorption temperature of 150 °C and 300 °C,indicating the largest amounts of weak,moderately strong acid sites over Yb–Ni–SZA catalyst.The Ce–Ni–SZA catalyst shows largest amount of total acid sites at 350°C.

3.5.H2-TPR

TheH2-TP Rpro files of various catalysts are displayed in Fig.3.Ascan be seen,two consecutive reduction peaks are observed for all catalysts.The low temperature peak at 490–521 °C is attributed to the reduction of Ni2+to metallic nickel[21],and the high temperature peak at 499–542 °C is attributed to the reduction of sulfate ions[22].Generally,the reduction peak of Ni2+to metallic state is observed at around 320°C.The shift of Ni oxide reduction peak towards higher temperature results from the strong interaction between Ni2+with the support[23].What's more,no peak ascribed to RE3+(Ce3+,Yb3+,Pr3+)for each catalyst is detected,which dues to low RE3+loading or RE3+is hardly reduced under experimental condition of TPR measurement[24–26].For Ni-SZA catalyst,the reduction peak of Ni2+to metallic state is at 520°C.However,the addition of appropriate amount of RE3+to the Ni–SZA catalyst leads to decrease in reduction temperature,meaning that active Ni species are more readily to participate in the isomerization reaction.Meanwhile,the lower reduction temperature indicates that the interaction between Ni and support is weakened,and the dispersion ofon the surface of the catalyst is improved.As a result,the reducibility of catalysts is enhanced.Among these modified catalysts,Yb–Ni–SZA exhibits the lowest reduction temperature and the best redox performance.

Fig.3.H2-TPR pro files of Ni–SZA and RE–Ni–SZA catalysts.

3.6.Activity test

Fig.4.Isopentane yield over Ni–SZA and RE–Ni–SZA catalysts(p=2.0 MPa,molar H2/n-pentane ratio=4:1,WHSV=1.0 h-1).

The influence of RE metals on the isomerization performance of the Ni-SZA catalysts was studied usingn-pentane isomerization as a probing reaction.The results are presented in Fig.4.TheNi–SZA catalyst exhibits an isopentane yield of 57.5%at its optimum temperature of 180 °C.Compared with Ni–SZA catalyst,the optimum temperatures for Ce–Ni–SZA and Pr–Ni–SZA catalysts are increased by about 20 °C,showing a decreased isomerization catalytic performances at lower temperature.However,Yb–Ni–SZA shows a highest isopentane yield at lower temperature,which achieves a maximum isopentane yield of 61.7%at 160 °C.The Py-IR characterization results show that Yb–Ni–SZA catalyst showed the strongest acid properties and the largest amounts of weak and moderately strong acid sites,which is benefit for the reaction processes on acid sites.Wamet al.reported[27]that the redox properties of the transition metal loaded on the catalyst could initiate the alkane conversion.According to the H2-TPR results,among the modified catalysts,Yb–Ni–SZA shows the best redox properties,improving the hydrogenation process of pentene intermediates on the metallic sites.This maybe another factor leading to the best isomerization catalytic performance for Yb–Ni–SZA.Above all,the optimum isomerization catalytic performance of catalysts decreased in the order of Yb–Ni–SZA＞Pr–Ni–SZA＞Ni–SZA＞Ce–Ni–SZA.The abovecatalytic performance of Yb–Ni–SZAcatalystcan beinterpreted by the traditional bifunctional reaction mechanism for isomerization of pentane,which involves hydrogenation–dehydrogenation on metal sites,isomerization and/or cracking on acid sites,and diffusion of the olefinic intermediates between acid and metal sites.In addition,the synergistic interaction between metal sites and acid sites plays an importantrole in isomerization.The improvement effect of Yb may be associated with the facilitated reducibility of Ni species(Fig.3),which is favorable to form metal sites for the hydrogenation-dehydrogenation procedure and the increased weak and moderately strong acid sites(Table 2).

Fig.5showed the isopentane yield versus time on stream over Ni–SZA and Yb–Ni–SZA catalysts at a pressure of 2.0 MPa,a hydrogen/hydrocarbon molar ratio of 4:1,and a MHSV of 1.0 h-1.The reaction temperature chosen was 180 °C for Ni–SZA catalyst,and 160 °C for Yb–Ni–SZA catalyst.Within 1700 min,the isopentane yield of the Ni–SZA catalyst maintains above 57%.However,isopentane yield fell sharply from 57%to 30%within 1700–2500 min.Within 1700 min,the isopentane yield of Yb–Ni–SZA maintains above 62%.And the isopentane yield fell from 62%to 43%within 1700–2500 min.As compared with Ni–SZA catalyst,the drop in isopentane yield was slower for Yb–Ni–SZA at lower reaction temperature.This shows that the activity and stability of Ni–SZA catalyst was improved upon intrducing the Yb.

Fig.5.Stability test of Ni–SZA and Yb–Ni–SZA catalysts for n-pentane isomerization.(Reaction conditions:p=2.0 MPa;H2/oil=4;WHSV=1.0 h-1).

4.Conclusions

A series of rare earth metals modified Ni–SZA catalysts have been prepared by impregnation method.The acid analysis revealed that the addition of RE could modify the acidity properties of Ni–SZA,and the Yb–Ni–SZA catalyst showed the strongest acid properties and the largest amounts of weak and moderately strong acid sites.The Ce–Ni–SZA catalyst showed largest amount of total acid sites at 350°C.Meanwhile,H2-TPR analysis con firmed that RE could lead to a better dispersion of persulfate ions on the surface of the catalyst.As a result,more acid sites were provided.Yb–Ni–SZA catalyst exhibited the best redox properties,which could provide enough metallic sites.Rich metallic sites and acid sites are essential for then-pentane isomerization reaction on catalysts.The synergistic interaction between them resulted in the best isomerization catalytic performance over Yb–Ni–SZA catalysts.The optimum isomerization catalytic performance of catalysts decreased in the order of Yb–Ni–SZA ＞ Pr–Ni–SZA ＞ Ni–SZA ＞ Ce–Ni–SZA.

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