Jiyuan Li ,Mifen Cui ,Zhuxiu Zhang,,Xian Chen ,Qing Liu ,Zhaoyang Fei ,Jihai Tang,2,,Xu Qiao,2

1 State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),Nanjing 210009,China

Keywords:Sulfated zirconia Zinc oxide Isobutene Selectivity Chemical reaction Catalysis

ABSTRACT Isooctane attracts great interest in recent years because of its promising potential as friendlyenvironmental gasoline,which is obtained by dimerization of isobutene with a hydrogenation step.Herein,a solid acid catalyst sulfated zirconia modified by ZnO was prepared.The oligomerization of isobutene had been investigated over ZrO2-SO4 and ZnO(X)/ZrO2-SO4 catalyst in order to find efficient catalysts for the production of isobutene oligomers.The presence of ZnO obviously enhanced the dimerization of isobutene and ZnO(X)/ZrO2-SO4 exhibited the highest di-isobutene yield of 60%.Kinetic studies showed the higher trimerization-to-dimerization activation energy ratios of ZnO(X)/ZrO2-SO4 than those of ZrO2-SO4 from 353 to 393 K.In addition,reaction rate of dimerization was higher than trimerization over ZnO(X)/ZrO2-SO4.The high L/B ratio manifested the capability to enhance the selectivity of C8 in isobutene dimerization.Furthermore,ZnO(X)/ZrO2-SO4 exhibited stable conversion for the dimerization of isobutene.

1.Introduction

The isooctane (2,2,4-trimethylpentane) occupies about 20% the current global gasoline blending market because of its high-octane and low-sulfur characteristics [1–3].The market demand is expected to gradually grow year-by-year by 2026 [4–6].Up to now,alkylation of isobutane with butene and indirect alkylation of isobutene with subsequent process of hydrogenation have been respectively utilized to produce isooctane[7,8].The second process has attracted considerable attention because it effectively utilizes surplus isobutene and the existing MTBE production units [9–11].

The control of the isobutene oligomerization degree to achieve high selectivity of certain product is as challenging as other olefins oligomerization [12–14].Isobutene oligomerization is a complex reaction process including a sequence of parallel and consecutive reactions(Fig.1)[15-17].The C8olefins(containing two main isomers) produced via isobutene dimerization will be further converted to C12olefins (containing four main isomers) or high oligomers [9].Great efforts have been devoted to control the C4olefins oligomerization degree by adjusting the acid property of catalyst surface [18–20].C4olefins can be converted to C8olefins over either Br?nsted acid sites or Lewis acid sites,so that the judicious selection of acid category and the exquisite control of acid strength are supposed to be proper methods to realize the high selectivity of certain product [7].The strong Br?nsted acid sites can reduce deprotonation energies of cationic transition states such as tert-butyl cation,so that there is a positive correlation between the Br?nsted acid strength and the C4olefins conversion[21].However,the strong Br?nsted acid sites resulted in the formation of higher polymers or isomers[22].In comparison,the transition metal-based Lewis acid catalyst had been explored for the inhibition of higher oligomers because of the efficient desorption of the formed C8olefins [23],but the C4olefins conversion and the catalyst stability were unsatisfactory.Recently,Wang et al.outlined that the substitution of the Br?nsted acid sites for Lewis acid sites by elements exchange significantly enhanced the ability for butene oligomerization degree control [24].The control of Lewis acid and Br?nsted acid concentrations over Ca-incorporated zeolite exhibited better catalytic transformation of C4olefins [25].We conjectured that combining the advantages of both Br?nsted acid sites and Lewis acid sites was supposed to be a proper method to simultaneously realize the high selectivity of di-isobutene and high conversion of isobutene.

Fig.1.Reaction network of isobutene oligomerization (major isomers of C8 and C12 are listed) [15].

Modifying solid acid catalyst with metal-oxides has been regarded as an efficient method to control the ratio of Lewis-to-Br?nsted acid sites [26].The doping of NiO greatly enhanced the Lewis acid properties of sulfated zirconia catalyst surface,and furtherly realized the high selectivity of C8product in isobutene dimerization [27].Temvuttirojn outlined that hybrid sulfated zirconia catalyst modifying with CuO-ZnO was more active and stable than conventional sulfated zirconia for CO2hydrogenation to dimethyl ether [28].However,the effect of ZnO on acid property over sulfated zirconia in isobutene dimerization has not been clearly studied.Herein,we report the control of the Lewis-to-Br?nsted ratio over sulfated zirconia modified with ZnO to realize the high yield of di-isobutene and furtherly investigated the correlation between the acid properties and catalytic performance.The pyridine FT-IR showed variation of the concentrations of Lewis and Br?nsted acid sites with the loading of ZnO.The resulting compound,ZnO(X)/ZrO2-SO4,contained higher Lewis acid sites and lower Br?nsted acid sites.The ratio of L/B and catalytic performance were positive related.The result proved the capability of higher L/B ratio to improve the isobutene dimerization.

2.Experimental

2.1.Materials

All chemicals were obtained from commercial supplies and used directly as received.n-Propyl zirconate (Zr(OCH2CH2CH3)4,70% (mass)),zinc acetate dihydrate (Zn(CH3COO)2.2H2O,99.9%)were obtained from Aladdin Industrial Inc.(Shanghai,China).Sulfonic acid (H2SO4) was purchased from shanghai Lingfeng Chemical Reagent Co.Ltd (Shanghai,China).Ethanol and ammonia aqueous were gotten from Sinopharm Chemical Reagent Co.,Ltd.

2.2.Catalyst preparation

ZnO(X)/ZrO2-SO4:Zr(OCH2CH2CH3)4(4.30 g)and Zn(CH3COO)2-.2H2O was dissolved in ethanol (30 ml).After mixing evenly,ammonia aqueous was gradually added to the above solution until pH=10.Then the solution was stirred for 1 h and evaporated at 333 K.The obtained compound was washed,dried and dissolved in ethanol (30 ml) with H2SO4(0.55 g).The sample was collected via centrifuge and was heated in air at 823 K for 3 h to afford ZnO(X)/ZrO2-SO4(X=mole fraction of Zn to Zr).ZrO2-SO4was synthesized via the same procedure without adding the Zn(CH3COO)2.2H2O.

2.3.Catalyst characterization

Powder X-ray diffraction(PXRD)data were recorded on a SmartLab X-ray diffractometer at 40 kV,40 mA for CukR (λ=0.15418 nm),with a scan speed of 2(°).min-1and a step size of 0.05° in 2θ at room temperature.Nitrogen sorption isotherms were obtained at 77 K with a BEL SORP II analyzer.The FT-IR spectra and pyridine FT-IR spectra were undertaken on a Thermo-Nicolet AVATAR 360 IR spectrometer over the range of 4000–400 cm-1at a resolution of 4 cm-1.The relative content of Lewis and Br?nsted acid sites was calculated by the area of those two peaks at 1445 cm-1and 1540 cm-1with the method reported by Emeis[29,30].X-ray photoelectron spectroscopy (XPS) measurement was performed using a PHI 5000 VersaProbe spectrometer equipped with a monochromatic AlKα anode.The charging effects were corrected by using C 1s band at the binding energy of 284.8 eV.The Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was conducted on Optima2000DV.Elemental analyses were performed on Vario EL cube.

2.4.Evaluation of catalyst activity

All catalysts were activated using the same procedure used for gas sorption studies.In a typical experiment,activated catalyst(0.2 g) was added in a 50 ml high-pressure stainless-steel reactor.The isobutene (2 mol) was then injected into the reactor with the mass weighted by electronic scale.The reaction mixture was stirred at a certain temperature (353–393 K).During the reaction,a small portion of reaction mixture was collected from the reactor at set interval.Products were analyzed by SP-6800A gas chromatography equipped with SE-54 column and flame ionization detector(FID),during which toluene was used as an internal standard.The material balance was checked by weighing by electronic scale.

The isobutene conversion and the selectivity of C8and C12for dimers were calculated:

Where XIBand SC8respectively represent conversion of isobutene and selectivity of di-isobutene.nIB,inand nIB,outare the molar amount of actual-injected and unreacted isobutene,respectively.nC8and nC12respectively represent the molar amount of diisobutene and tri-isobutene.

2.5.Kinetic study

The kinetic study of dimerization of isobutene(Eq.(3))was performed over ZrO2-SO4and ZnO(X)/ZrO2-SO4at 353–393 K.The trimerization of isobutene is given (Eq.(4)),as following:

The rate of the reaction was based upon the pseudo first-order model as shown below (5) and (6):

where cA,cBand cCrespectively represent molar concentrations of C4,C8and C12,riand kiare the reaction rate,rate constant of the reaction,respectively.The correlation between the rate constant and temperature was calculated by the Arrhenius equation (7):

where k0is pre-exponential factor,Eais the activation energy(kJ.mol-1) and T is the temperature.

3.Results and Discussion

3.1.Catalytic performance

We controlled the ZnO loading in sulfated zirconia and prepared a series of catalysts named ZnO(X)/ZrO2-SO4.The reaction was conducted in batch-type high-pressure stainless steel at 373 K for 2 h.The catalytic data for the oligomerization of isobutene over various solid acid catalysts was listed in Table 1,including ZrO2-SO4,ZnO(X)/ZrO2-SO4,ZrO2and ZnO.A series of control experiments showed no conversion of the substrate isobutene over ZrO2or ZnO(Table 1,Entries 1–2),whereas ZrO2-SO4afforded 84%conversion of isobutene and 46% selectivity of C8(Table 1,Entry 3).The doping of ZnO almost unchanged the conversion of isobutene,but the C8selectivity increased to 59% (Table 1,Entry 4).The increased selectivity of C8(72%)and the slight decrease of conversion was observed over ZnO(10)/ZrO2-SO4(Table 1,Entry 5).Furtherly increasing the ZnO content obtained the highest selectivity of 76% (Table 1,Entry 6),but the conversion was reduced to 77%.The selectivity of C8over NiO(10)/ZrO2-SO4was increased to 63%and the conversion was 73% after 2 h reaction (Table 1,Entry 7).The lower conversion compared with ZnO(X)/ZrO2-SO4may be attributed to the weaker Br?nsted acidity[27].Moreover,Ni2+sites inhibited the formation of higher oligomers because of the desorption of the formed C8,but the fast deactivation of Ni2+sites caused the lower selectivity than ZnO(X)/ZrO2-SO4[27].

3.2.Kinetic study

In order to explore the difference of the catalytic performance,we performed the kinetic study of oligomerization of isobutene over ZrO2-SO4and ZnO(X)/ZrO2-SO4at 353 K,373 K and 393 K.The catalytic data was fitted by the pseudo-first-order model(Table 2,S1).As shown in Fig.2,activation energy (Ea) for both dimerization and trimerization over all ZnO(X)/ZrO2-SO4were larger than those over ZrO2-SO4.The result indicated the presence of ZnO simultaneously inhibited dimerization and trimerization over ZnO(X)/ZrO2-SO4.Moreover,the higher Earatio of trimerization to dimerization was beneficial to the formation of C8olefins in the isobutene oligomerization.There was a general uptrend in the ratio of Ea,trimer/Ea,dimerwith the increase of ZnO loading (Fig.2A),giving rise to the higher catalytic activity of dimerization than that of trimerization over ZnO(X)/ZrO2-SO4.We then calculated the rate constants (k) from the activation energy and the pre-exponential factor (A) according to the Arrhenius equation.As shown in Table S1,kdimerand ktrimervaried over ZnO(X)/ZrO2-SO4and ZrO2-SO4.The data showed the positive correlation existed between the ratio of kdimer/ktrimerand ZnO loading(Fig.2B).In addition,the presence of ZnO leaded to the increase of kdimer/ktrimerratio up to two order of magnitudes compared to ZrO2-SO4at all reaction temperatures (Table S1).The kinetic study proved the capability of ZnO to improve the reaction rate of the isobutene dimerization.

3.3.Effect of acid properties on the catalytic performance in isobutene dimerization

The bulk purity of ZnO(X)/ZrO2-SO4and ZrO2-SO4were verified by powder XRD.Upon loading ZnO onto the ZrO2-SO4support,new diffraction peaks appeared at 32°,34° and 36°,respectively corresponding to the (1 0 0),(0 0 2),and (1 0 1) planes of the ZnO(Fig.3A).The ZrO2peaks at 2θ=30°,35°,50° and 60° remained unaffected.The chemical composition of ZnO(X)/ZrO2-SO4and ZrO2-SO4were furtherly studied by XPS.The main peaks at 168.1,183.0,284.0,332.5,431.9 and 530.9 eV were ascribed to S 2p,Zr 3d,C 1s,Zr 3p,Zr 3s and O 1s,respectively.In addition,two peaks at 1021.9 and 1044.7 eV corresponding to Zn 2p1/2and Zn 2p3/2were observed.In HRTEM images of ZrO2-SO4and ZnO(X)/ZrO2-SO4(Fig.4A–D),the lattice fringes with the fringe space of 0.287 nm were consistent with the (1 0 0) crystal planes spacing of the ZnO,and the lattice fringes of ZrO2(1 1 1) were observed for all samples.The identical FT-IR spectra showed that surface functional groups did not change,and sulfated group was well anchored on zirconium (Fig.5A).The unchanged type of isotherm hysteresis loop indicated that ZnO did not change the pore structure of catalyst as shown in Fig.5B.

The concentrations of Br?nsted and Lewis acid sites were significantly influenced by doping of ZnO(Table 3).The main peaks were observed at 1540 and 1445 cm-1in the Py-FTIR spectrum of all samples,which were ascribed to Br?nsted and Lewis acid sites respectively (Fig.6A).The Br?nsted acid sites were provided by the zirconium hydroxyl group linked with sulfuric acid group,and the Lewis acid sites were derived from both Zr4+sites and Zn2+sites.The concentrations of Br?nsted acid sites slightly decreased from 0.59 to 0.53 (Table 3),owing to the reduction of zirconium hydroxyl group after ZnO doping.On the contrary,the Lewis acid sites distinctly increased with ZnO loading althoughthe concentration of Zr4+ion was same for all the samples(Table 3),because Lewis acid sites relative strength of Zn2+sites was larger than Zr4+sites.Furthermore,the effect of ZnO doping on Lewis acid sites was investigated via XPS.The binding energy of Zr 3d over ZnO(15)/ZrO2-SO4at 183.0 and 185.4 eV was slightly higher than that over ZrO2-SO4(Fig.6B).This increase of the binding energy gave rise to the generation of zirconium species with stronger Lewis acidity [31].

Table 1Survey of catalysts for the dimerization of isobutene

Table 2Kinetic parameters

Fig.2.Correlation between kinetic data and ZnO loading in catalysts of (A) activation energy and (B) rate constants.

Fig.3.(A) PXRD patterns and (B) XPS spectra of ZrO2-SO4 and ZnO(X)/ZrO2-SO4.

The change of single acid content cannot accurately reflect the effect on catalytic performance,so that comprehensive consideration of Br?nsted and Lewis acid sites is necessary.The ratio of L/B varied over ZrO2-SO4and ZnO(X)/ZrO2-SO4as shown in Table 3.Well negatively linear correlation existed in ratio of L/B and conversion of isobutene.In addition,the ratio of L/B and the selectivity of di-isobutene were positively related (Fig.7).The linear fitted Eqs.(8) and (9) are:

Table 3Physicochemical properties of the samples

Fig.4.HRTEM of (A) ZrO2-SO4;(B) ZnO(5)/ZrO2-SO4;(C) ZnO(10)/ZrO2-SO4 and (D) ZnO(15)/ZrO2-SO4.

Fig.5.(A) FT-IR spectra and (B) N2 sorption isotherms of the samples (a) ZrO2-SO4,(b) ZnO(5)/ZrO2-SO4,(c) ZnO(10)/ZrO2-SO4,(d) ZnO(15)/ZrO2-SO4.

Where V(L/B)is the value of L/B.Higher ratio of L/B gives rise to the higher di-isobutene selectivity and smaller isobutene conversion.ZrO2-SO4showed the L/B ratio of 1.66 with 84%conversion but only 46%selectivity.ZnO(5)/ZrO2-SO4exhibited L/B ratio of 2.17 and the selectivity increased to 59%with barely unchanged conversion.The L/B ratio of 2.67 was obtained over ZnO(10)/ZrO2-SO4,and the selectivity of di-isobutene slightly increased with decreased conversion.Higher L/B ratios were obtained with increased ZnO loading,leading to the decreased conversion of isobutene,while the selectivity of di-isobutene increased slightly.The reason is that the doping of ZnO covered the Br?nsted acid sites,so that conversion deceased.In other words,it would be fair to propose that there isthe synergistic effect between the conversion and the selectivity from the quantitative perspective.

Fig.6.(A) Pyridine FT-IR spectra of (a) ZrO2-SO4,(b) ZnO(5)/ZrO2-SO4,(c) ZnO(10)/ZrO2-SO4,(d) ZnO(15)/ZrO2-SO4 and (B) XPS spectra of Zr 3d.

Fig.7.Correlation between catalytic performance and surface acidity.

3.4.Catalyst reusability

The recyclability of the catalyst is an important factor for oligomerization catalysis.Fig.8 showed the isobutene conversion and products selectivity using the ZnO(10)/ZrO2-SO4catalyst for five runs,in series,batch reactions to test the catalyst reusability.The catalyst remained inside the reactor,without any type of treatment,and after each catalytic run,isobutene was added to carry out the reaction under identical experimental conditions.It can be observed that the catalyst maintains its activity after five catalytic runs in such a way,remaining the C8yield between 59%and 60% throughout the cycles.Therefore,the ZnO(10)/ZrO2-SO4catalyst can be successfully applied to isobutene oligomerization catalysis with good stability and recyclability.

Fig.8.Reusability study over the ZnO(10)/ZrO2-SO4 catalyst.

4.Conclusions

In summary,sulfated zirconia modified by ZnO was prepared to improve the selectivity of di-isobutene in oligomerization of isobutene.ZnO(X)/ZrO2-SO4exhibited the di-isobutene selectivity of 72%and the yield of 60%,which was much higher than ZrO2-SO4.The kdimer/ktrimerratios over ZnO(X)/ZrO2-SO4were higher than that over ZrO2-SO4from 353 to 393 K.The activation energy for both the dimerization and trimerization of isobutene over ZnO(X)/ZrO2-SO4were higher than those over ZrO2-SO4,but the ratios of Ea,trimer/Ea,dimerover ZnO(X)/ZrO2-SO4were much higher.The improved performance of ZnO(X)/ZrO2-SO4is presumably associated with higher ratio of Lewis acid sites to Br?nsted acid sites.The pyridine FT-IR showed that the L/B ratios increased with ZnO loading.The higher L/B ratio gave rise to higher selectivity of diisobutene.In addition,ZnO(10)/ZrO2-SO4exhibited stable conversion for the dimerization of isobutene.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21676141,21808104),the National Key Research and Development Program of China (2017YFB0307304),the Natural Science Foundation of Jiangsu Province(BK20170989),the Natural Science Foundation of Jiangsu Higher Education Institutions of China (17KJB530005),the Project “333” of Jiangsu Province(BRA2016418)and the Six Major Talent Peak Project of Jiangsu Province (XCL-017).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.039.