A core–shell Ni/SiO2@TiO2 catalyst for highly selective one-step synthesis of 2-

Hualiang An,Rui Wang,Wenhao Wang,Daolai Sun,Xinqiang Zhao*,Yanji Wang

Hebei Provincial Key Laboratory of Green Chemical Technology and Efficient Energy Saving,Tianjin Key Laboratory of Chemical Process Safety,School of Chemical Engineering and Technology,Hebei University of Technology,Tianjin 300130,China

Keywords: Reaction integration n-Pentanal 2-Propylheptanol Core-shell structure Reaction kinetics

ABSTRACT One-step synthesis of 2-propylheptanol (2-PH) from n-pentanal via a reaction integration of n-pentanal self-condensation and successive hydrogenation is of great significance for it can simplify process flow and reduce energy consumption.The key to promotion of 2-PH selectivity is to enhance the competitiveness of n-pentanal self-condensation with respect to its direct hydrogenation.For this purpose,a core–shell structured Ni/SiO2@TiO2 catalyst was designed and prepared.With the precise architecture of this core–shell structured catalyst, n-pentanal can be firstly in contact with TiO2 to 2-propyl-2-heptenal (2-PHEA) while the direct hydrogenation to n-pentanol can be effectively inhibited,and then 2-PHEA diffuses into the core of Ni/SiO2 and is hydrogenated to 2-PH.The spatial threshold of the core–shell catalyst significantly enhanced its catalytic performance;a 2-PH selectivity of 77.9% was reached with a 100% npentanal conversion.The 2-PH selectivity is much higher than that obtained by employing Ni/TiO2 catalyst.Furthermore,the reaction kinetics of one-step synthesis of 2-PH from n-pentanal catalyzed by Ni/SiO2@TiO2 was studied and its kinetic model was established which is useful for reactor design and scale-up.

1.Introduction

2-Propylheptanol (2-PH),an important plasticizer alcohol,can react with phthalic anhydride,trimellitic anhydride and adipic acid to synthesize a series of plasticizers.Among them,bis(2-propylheptyl) phthalate (DPHP) is a major 2-PH-derived plasticizer.Compared with the traditional plasticizer,dioctyl phthalate derived from 2-ethylhexanol,DPHP can meet the requirements for health,safety and environmental protection for its low toxicity and volatility [1,2].The industrial production process of 2-PH includes three reaction units:butene hydroformylation,npentanal self-condensation and 2-propyl-2-heptenal (2-PHEA)hydrogenation,suffering from some problems such as long process flow,high equipment and operation cost.An integration of multistep reaction is one of effective ways to solve these problems [3–5].Therefore,the two-step reaction integration ofn-pentanal self-condensation to 2-PHEA and subsequent hydrogenation to 2-PH was proposed and studied in this work.

At present,there are few reports on the integration reaction ofn-pentanal self-condensation and 2-PHEA hydrogenation.Our group[6]prepared a novel silica-immobilized nickel and acid ionic liquid (Ni-IL/SiO2) catalyst for one-pot sequential synthesis of 2-PH,i.e.n-pentanal self-condensation was performed without H2first and then hydrogenation of the self-condensation product was conducted,affording a 2-PH selectivity of 75.4% at thenpentanal conversion of 100% .Sharmaet al.[7] used Ru-HT (ruthenium supported on hydrotalcite)as a bifunctional catalyst for onestep synthesis of 2-PH fromn-pentanal in a stainless autoclave and found that the conversion ofn-pentanal was 100% but the selectivity of 2-PH was only 48% .Instead some researchers including our group have investigated a similar reaction integration ofnbutanal self-condensation and its condensation product 2-ethyl-2-hexenal hydrogenation.Lianget al.[8]realized one-step synthesis of 2-ethylhexanol fromn-butanal with Ni/Ce-Al2O3bifunctional catalyst,attaining a 2-ethylhexanol selectivity of 66.9% at anbutanal conversion of 100% .Patankaret al.[9] prepared a Cu-Mg-Al catalyst for this reaction integration and obtained a 2-ethylhexanol selectivity of 90% at a rather lown-butanal conversion of 30% .Miaoet al.[10] studied one-step synthesis of 2-ethylhexanol using a Cu-Mg-Fe catalyst and found that the conversion ofn-butanal and the selectivity of 2-ethylhexanol were 100% and 73.5% ,respectively.Our group investigatedn-pentanal selfcondensation and 2-PHEA/2-ethyl-2-hexenal (n-butanal selfcondensation product) hydrogenation extensively and found that TiO2and Ni-based catalysts showed excellent catalytic activities respectively for them [1,11–13].Based on this,we used a Ni/TiO2catalyst for one-step synthesis of 2-PH fromn-pentanal [13].The result showed that a roughly equal selectivity of 2-PH (45.4%)andn-pentanol (50.8%) was achieved at a completen-pentanal conversion,suggesting thatn-pentanal self-condensation and its direct hydrogenation took place almost equally.It can be seen that it is a challenge to attain a high yield of the desired product alcohol since there is a competitive relationship between selfcondensation and direct hydrogenation in the integration reaction ofn-butanal/n-pentanal self-condensation and its condensation product hydrogenation.

It has been reported that the core–shell structured catalyst can be used to regulate complex reactions[14–17].Thus we propose to use a core–shell catalyst for one-step synthesis of 2-PH fromnpentanal to enhance the competitiveness ofn-pentanal selfcondensation with respect to its direct hydrogenation.We designed a core–shell catalyst with SiO2-supported Ni as the core and TiO2as the shell (Ni/SiO2@TiO2) and hoped thatn-pentanal self-condensation occurs in TiO2shell to form 2-PHEA first and then 2-PHEA diffuses into the inner Ni/SiO2core to be hydrogenated to 2-PH.Silica was selected as a core material because of its highly mechanical and chemical stability,abundant pores,and easy incorporation of transition metals [17].Fig.1 illustrated schematically a comparison of the reaction integration ofnpentanal self-condensation and 2-PHEA hydrogenation separately on Ni/TiO2and Ni/SiO2@TiO2catalyst.

In this work,a series of core–shell structured catalysts consisting of a silica core,Ni metal particles and a TiO2shell were prepared,and their confinement effect on one-step synthesis of 2-PH fromn-pentanal was investigated.The Ni loading and the thickness of TiO2shell were further tested to improve the catalytic performance by help of several characterizations.On this basis,the one-step synthesis of 2-PH fromn-pentanal catalyzed by Ni/SiO2@-TiO2was investigated kinetically for a further reactor design and analysis.

2.Experimental

2.1.Preparation of SiO2@TiO2

In a typical synthesis,isopropanol (40 ml) and ammonium hydroxide (1.5 ml) were dissolved in deionized water (20 ml)and stirred for 20 min.Then tetraethyl orthosilicate (TEOS,2 ml)was added dropwise,followed by stirring for 2 h.The resultant precipitate was centrifuged and washed with deionized water and absolute ethanol successively,and dried under vacuum at 60 °C for 10 h to afford SiO2microspheres.The prepared SiO2(0.3 g)was dispersed in 100 ml of absolute ethanol under ultrasonication and then ammonium hydroxide (1.5 ml) was added into the mixture to obtain solution A.Tetrabutyl titanate (TBOT,2 ml) and absolute ethanol (100 ml) were added to a 250 ml flask to obtain solution B.Then the solution A was added quickly into the solution B,followed by stirring at 60 °C for 3 h.The suspension was centrifuged and washed with deionized water and absolute ethanol successively,and dried under vacuum at 60 °C for 10 h.After that,the resulting sample was calcined at 450°C in air for 1 h,leading to SiO2@TiO2microspheres.

2.2.Preparation of Ni/SiO2@TiO2

The process for preparation of Ni/SiO2@TiO2comprises four steps sequentially.(1) Preparing SiO2microspheres using the method described above.(2)Preparation of NiO/SiO2microspheres by excessive impregnation method.The prepared SiO2microspheres were dispersed in a nickel nitrate aqueous solution and ultrasonicated for 6 h.Then the suspension was evaporated at 70 °C under vacuum for 1 h and subsequently dried at 100 °C for 12 h.After that,the dried solid powder was calcined at 400 °C for 2 h to obtain NiO/SiO2microspheres.(3) Encapsulating NiO/SiO2with TiO2shell to prepare NiO/SiO2@TiO2.Its preparation procedure was similar to that of SiO2@TiO2but replacing SiO2with NiO/SiO2.(4) The Ni/SiO2@TiO2catalyst was successfully prepared after reducing NiO/SiO2@TiO2under the atmosphere ofVN2:VH2=60:40 at 450 °C for 3 h.

2.3.Characterization methods

The surface morphology the samples were characterized with a Quanta 450 FEG scanning electron microscope(SEM,FEI,USA).The transmission electron microscopy(TEM)images,element mapping and line scanning of samples were obtained on a JEM-2100F field emission transmission electron microscope (JEOL,Japan) operated at 200 kV with an EDAX energy disperse spectroscopy.X-ray diffraction (XRD) patterns of the samples were recorded with a Bruker D8 Discover diffractometer (Bruker AXS,Karlsruhe,Germany)using a Cu Kα radiation at 100 mA and 40 kV.The elemental content of the samples was measured by inductively coupled plasma optical emission spectrometry (ICP-OES) on an Optima 7300 V spectrometer (PerkinElmer,USA).

2.4.One-step synthesis of 2-PH from n-pentanal

The one-step synthesis of 2-PH fromn-pentanal was carried out in a 25 ml stainless steel autoclave.In a typical procedure,4 ml ofn-pentanal and 0.45 g of catalyst were added into the autoclave,and the air inside was replaced by flushing H2for three times.Then the mixture was heated to the required temperature and the reaction took place at 190 °C for 10 h under a 3.0 MPa of H2pressure with a magnetic stirring.After the completion of reaction,the mixture solution was filtered to remove the catalyst and the filtrate was analyzed on a SP 3420A gas chromatograph (Beijing Beifen-Ruili Analytical Instrument Co.,Ltd.)equipped with a flame ionization detector (FID) and a KB-1 capillary column.An internal standard method was adopted for quantification,and cyclohexanol was selected as the internal standard which was added into the reaction liquid prior to GC analysis.The relative mass calibration factors ofn-pentanal,pentanol,2-propyl-2-heptanal,2-propylheptanal and 2-propyl-1-heptanol compared with the cyclohexanol were 1.208,1.098,0.984,1.083 and 1.065,respectively.

Fig.1.Schematic illustration of one-step synthesis of 2-PH using (a) Ni/TiO2 and (b) Ni/SiO2@TiO2 core–shell catalyst.

The conversion ofn-pentanal (XP) is calculated as follows:

3.Results and Discussion

3.1.Characterization of catalyst

3.1.1.SiO2@TiO2

To clarify the effect of core–shell structure on the catalytic performance of TiO2forn-pentanal self-condensation,a reference sample,SiO2@TiO2,was prepared first with SiO2microspheres as template and TBOT as titanium source.SEM and TEM images of SiO2microspheres and SiO2@TiO2samples before and after calcination are presented in Fig.2.

The SiO2microspheres exhibit a very smooth particle surface morphology and a uniform particle size of about 250 nm.As for SiO2@TiO2,their particle sizes increase to about 340 nm and their surface becomes rough,indicating the TiO2layer was obtained on the surface of SiO2.After calcination,the particle sizes of the samples decrease slightly and the honeycomb mesopores are clearly observed on the external surface of SiO2@TiO2.To further determine the structure and element distribution of SiO2@TiO2,TEMEDS analysis was performed and the representative line scanning profiles are shown in Fig.S1.The results showed that Ti element(derived from TiO2) disperses mainly in the shell while Si element(derived from SiO2)locates in the core,demonstrating more clearly that SiO2cores are homogeneously encapsulated by TiO2shells and the shells remain intact during the calcination process.

Fig.3 shows the XRD patterns of SiO2and SiO2@TiO2before and after calcination.A weak broad peak can be found at 15°–35°,which is attributed to amorphous SiO2.The diffraction peak of SiO2@TiO2before calcination is basically the same as that of SiO2,indicating that SiO2@TiO2also has an amorphous structure before calcination.After calcination,the diffraction peaks of SiO2@TiO2appear at 25.4°,37.0°,48.1°,54.2°,55.0° and 62.9° corresponding to anatase TiO2,indicating that the crystalline phase of TiO2in SiO2@TiO2was transformed from amorphous to anatase and has a strong crystallinity.The diffraction peaks corresponding to the SiO2core are no longer observed in SiO2@TiO2due to the overlap of the high peaks attributed to TiO2,which is consistent with previous report by others [18].

3.1.2.Ni/SiO2@TiO2

The SEM and TEM images of Ni/SiO2@TiO2core–shell structured catalysts are presented in Fig.4.It can be seen that the Ni/SiO2@-TiO2microspheres are dispersed separately and evenly,basically with the same particle size of about 340 nm as SiO2@TiO2.The TiO2shell is not affected by loading Ni and remains still regular and continuous with a thickness of about 45 nm.

Fig.2.SEM and TEM images of samples:(a,d) SiO2;(b,e) SiO2@TiO2 before calcination;(c,f) SiO2@TiO2 after calcination.

Fig.3.XRD patterns of SiO2 and SiO2@TiO2 before and after calcination.

To further determine the structure and element distribution of Ni/SiO2@TiO2,TEM-EDS analysis was performed and the representative line scanning profiles are shown in Fig.5.The scanning lines in Fig.5(b) indicate that Ti element disperses mainly in two areas separately in the range of 35–100 nm and 340–400 nm while Si element locates in the range from 80 nm to 340 nm.This suggests that the sample shows a typical core–shell structure.Ni element has the same variation tendency as the Si element,indicating that the metal Ni was loaded on the surface and in the pores of SiO2as the core of the nanoparticles.Fig.5(c)–(e) present the elemental mapping images of Ni/SiO2@TiO2.The Si element is uniformly distributed in a spherical shape while the Ti element is mainly distributed in a ring shape whose inner diameter is basically equal to the diameter of SiO2core,indicating that TiO2was uniformly coated on the surface of SiO2microspheres.Most of Ni particles are concentrated in the core with a similar diameter to the SiO2core.However,a small amount of Ni particles are dispersed out of the SiO2core,probably because a few Ni particles migrated into the TiO2shell during the preparation process.Hence,the mapping results clearly show the formation of Ni/SiO2@TiO2core–shell structure with Ni/SiO2in the inner core and TiO2in the shell.

Fig.6 shows the N2adsorption–desorption isotherm and the pore size distribution of Ni/SiO2@TiO2.According to the classification of IUPAC[19],Ni/SiO2@TiO2shows type IV isotherm accompanied by a H1 hysteresis loop,indicating that a developed mesoporous structure exists in the sample.The specific surface obtained by BET method is 90.7 m2∙g-1and the pore volume by BJH method is 0.173 cm3∙g-1.The pore diameter distribution of the sample was also calculated by BJH method and a unimodal pore size distribution which is mainly centered at 5.7 nm was observed,indicating that the pore channel is relatively regular.

3.2.Activity evaluation of catalyst

3.2.1.SiO2@TiO2

An excellent catalytic performance of SiO2@TiO2forn-pentanal self-condensation is the prerequisite for one-step synthesis of 2-PH.Hence,we evaluated the activity of SiO2@TiO2forn-pentanal self-condensation and investigated the effect of core–shell structure.For benchmark comparison,the same reaction conditions as our previous work using TiO2catalyst were selected here [1] and the results are listed in Fig.7.

No reaction occurred in the absence of catalyst and the SiO2@-TiO2showed a similar catalytic performance to that of pure anatase TiO2.To confirm whether there existed a synergetic catalysis between SiO2and TiO2in the SiO2@TiO2,we measured the content of Si and Ti in SiO2@TiO2using ICP-OES first,and then evaluated the catalytic activities of pure SiO2microspheres and the mixture of SiO2and TiO2with the measured ratio.The ICP-OES results show that TiO2accounts for 38.7% and SiO2accounts for 61.3% of the total mass in the SiO2@TiO2.It can be seen from Fig.7 that both SiO2and the mixture of SiO2and TiO2had poor catalytic performance forn-pentanal self-condensation,indicating that there is a synergetic catalysis between SiO2and TiO2in SiO2@TiO2.The improved dispersion of TiO2in SiO2@TiO2is conducive to the contact between the reactants and the active centers,which may be another reason for its higher activity and catalytic efficiency.

3.2.2.Ni/SiO2@TiO2 with different Ni loadings

Metal loading is one of important factors affecting the hydrogenation performance[20].A series of Ni/SiO2@TiO2catalysts with different Ni loadings were prepared by changing the amount of nickel nitrate in the preparation process.SEM and TEM characterizations were performed for the prepared Ni/SiO2@TiO2samples first.As shown in Fig.S2,Ni/SiO2@TiO2spheres in all samples are separated and evenly dispersed with a particle diameter of about 340 nm,and the thickness of the uniform and continuous TiO2shell has no obviously difference.From the above results,the different Ni loadings should not affect the formation of TiO2shell.

The effects of different Ni loadings on the catalytic performance of Ni/SiO2@TiO2were evaluated and the results are shown in Table 1.When the Ni loading was 2.5% (mass),a rather low 2-PH selectivity of 27.5% was obtained at a reaction time of 5 h and there was a large amount of intermediate 2-propylheptanal left in the system.When the reaction proceeded for 10 h,2-propylheptanal was almost completely hydrogenated to 2-PH and the 2-PH selectivity reached 77.9% .These results demonstrate once again the hydrogenation of C=C bond of 2-propyl-2-heptenal to 2-propylheptanal is easier than that of C=O bond [21].With the increase of Ni loading,bothn-pentanol and 2-PH selectivity increased first because of the promotion to the ability of Ni hydrogenation.However,the direct hydrogenation ofn-pentanal was obviously promoted when the amount of Ni loading is too large,ruining the integration reaction effect.

Fig.4.SEM image (a) and TEM image (b) of Ni/SiO2@TiO2.

Fig.5.TEM-EDS line scan analysis and elemental mapping of Ni/SiO2@TiO2.

Fig.6.N2 adsorption–desorption isotherm and pore size distribution of Ni/SiO2@TiO2.

Fig.7.Catalytic performance of different catalysts for n-pentanal self-condensation(Reaction conditions:Mass percentage of catalyst=15% , T=190 °C, t=8 h).

As the TEM characterization results showed in Fig.S2,there are no obvious difference in the thickness of TiO2shell.So it could be predicted thatn-pentanal self-condensation should not be affected and the hydrogenation of 2-propyl-2-heptenal to 2-PH should be improved with the increase of Ni loading.However,the results in Table 1 are not the case.In order to analyze the reasons,the content of each element in Ni/SiO2@TiO2samples with different Ni loadings was measured by ICP-OES and the results are shown in Table 2.It can be seen that the mass ratio of Ti to Si element decreases gradually with the increase of Ni loading,suggesting that the increase of metal Ni on SiO2surface is detrimental to the formation of TiO2shell.As for the reason why the thickness of TiO2shell basically doesn’t change in Fig.S2,we think this is probably because more Ni sites on SiO2surface can slow the formation of Ti-O-Si down,leading to a lower density of TiO2.Kimet al.[22]also found that the formation of Ti-O-Si bonds was a key factor for TiO2shell coated on the surface of SiO2in the process of preparing SiO2@TiO2.

Table 1 Effect of Ni loading on the catalytic performance of Ni/SiO2@TiO2

Table 2 Element content in Ni/SiO2@TiO2 with different Ni loadings

3.2.3.Ni/SiO2@TiO2 with different TBOT dosages

The thickness of TiO2shell of Ni/SiO2@TiO2is another important parameter affecting its catalytic performance for one-step synthesis of 2-PH fromn-pentanal.Increasing the thickness of TiO2shell can increase the diffusion distance of the reaction component,effectively extend the reaction time ofn-pentanal selfcondensation,reduce or even avoid the direct hydrogenation ofn-pentanal ton-pentanol,and improve the integration reaction effect.Thus,we investigated the effect of TBOT dosage with a fixed Ni loading of 5% (mass).

Ni/SiO2@TiO2samples were prepared by varying the TBOT dosage,and their SEM images are shown in Fig.8.With increasing TBOT dosage,the particle size of the Ni/SiO2@TiO2microspheres increased,then the agglomeration between particles appeared and became serious gradually.The effect of TBOT dosage on Ni/SiO2@TiO2catalytic performance was investigated and the results are listed in Table 3.With an increase of TBOT dosage,the selectivity ofn-pentanol and 2-PH decreased while the selectivity of C10products (2-PHEA,2-PHA,and 2-PH) increased.Combining the SEM images in Fig.8,it can be seen that increasing the thickness of TiO2shell can really promoten-pentanal self-condensation reaction and suppress its direct hydrogenation.However,increasing the thickness of TiO2shell is also not conducive to the hydrogenation of intermediate 2-propylheptanal to 2-PH.Using Ni/SiO2@TiO2prepared with 4 ml TBOT,there was a 46.0% selectivity of 2-propylheptanal left after reaction for 5 h and the surplus 2-propylheptanal could be completely hydrogenated to the target product 2-PH at a reaction time of 10 h.With increasing TBOT dosage continuously,the hydrogenation activity of the catalyst became worse,not only the C10components selectivity increased obviously but also partial hydrogenation product 2-propylheptanal existed mainly.So too large thickness of TiO2shell in Ni/SiO2@TiO2is not suitable.

Fig.8.SEM images of Ni/SiO2@TiO2 prepared with different TBOT dosages:(a) 2 ml,(b) 3 ml,(c) 4 ml,(d) 5 ml.

Table 3 Effect of TBOT dosage and reaction time on the catalytic performance of Ni/SiO2@TiO2

Based on comprehensive analysis of the structure and catalytic performance of Ni/SiO2@TiO2,the suitable preparation conditions were determined as follows:the Ni loading was 2.5% (mass) and the mass ratio of Ni/SiO2to TBOT was 0.3:2.The prepared catalyst had good dispersion and catalytic performance.Then-pentanal conversion and 2-PH selectivity were 100% and 77.9% respectively,and the overall selectivity of C10+C5products reached 98.2% .

A comparison of this work with the reported results for the reaction integration ofn-pentanal self-condensation and 2-PHEA hydrogenation was made and shown in Table 4.One-step synthesis of 2-PH fromn-pentanal was separately realized over Ru-HT and Ni/TiO2catalysts[7,13],however the selectively of 2-PH was fairly low because of the competitiveness ofn-pentanal direct hydrogenation with respect to its self-condensation.For Ni-IL/SiO2catalytic system [6],the selectively of 2-PH was significantly increased.The competition ofn-pentanal self-condensation was enhanced by a sequential one-pot method,i.e.n-pentanal selfcondensation was performed without H2first and then hydrogenation of the self-condensation product was conducted,existing a problem of operating process relatively complex.Through comparison,it can be seen that the Ni/SiO2@TiO2catalyst prepared here has a good catalytic performance and application prospect.For a further scale-up,the integration reaction kinetics are required for reactor design and analysis.So the reaction kinetics for one-step synthesis of 2-PH fromn-pentanal were investigated below.

Table 4 Summary of catalysts for the reaction integration of n-pentanal self-condensation and 2-PHEA hydrogenation

Table 5 Kinetic parameters of one-step synthesis of 2-PH from n-pentanal catalyzed by Ni/SiO2@TiO2

3.3.Analysis of reaction kinetics

3.3.1.Elimination of external and internal diffusion influence

One-step synthesis of 2-PH fromn-pentanal over Ni/SiO2@TiO2catalyst is a gas–liquid–solid three-phase reaction and mass and heat transfer may influence the reaction.In order to establish the intrinsic kinetics,the influence of internal and external diffusion was investigated and the results are shown in Figs.S3 and S4.The conversion ofn-pentanal leveled off basically at about 91% under a stirring speed of 700 r∙min-1or greater,indicating theexternal diffusion was excluded in that range of speed.Furthermore,the catalyst particle size had no influence on the conversion ofn-pentanal in the range of catalyst particle sizes examined here.Consequently,the kinetic experiments were carried out at a stirring speed of 700 r∙min-1and a catalyst particle size of 125–150 μm in this work.

3.3.2.Establishment of kinetic model

The reaction equations for one-step synthesis of 2-PH fromnpentanal can be simplified as follows:

where A representsn-pentanal,B refers to 2-PHEA,C stands fornpentanol,D denotes 2-PHA,E indicates 2-PH,and W is H2O.

The respective reaction rate equations can be expressed as follows:

The reaction rate of each component can be expressed as follows:

Since H2is excessive in the reaction process,can be regarded as a constant (k′=).Then Eqs.(9)–(13) are changed as follows:

Substituting the Arrhenius equation into Eqs.(14)–(18),they change as follows:

whereA1+,A1-,A2,A3andA4separately represent pre-exponential factors of each reaction rate constant,L∙(kmol∙min)-1;CA,CB,CC,CDandCEseparately indicate the concentration of each component,mol∙L-1;Ea1+,Ea1-,Ea2,Ea3,andEa4separately stand for the activation energy of each reaction,kJ∙mol-1;m1,m2,m3,m4,n1andp1separately represent the reaction order with respect to each component;rA,rB,rC,rDandrEseparately indicate the reaction rate of each component,mol∙(L∙min)-1.

3.3.3.Kinetic experiments

The kinetic experiments were carried out on an autoclave controlled at a stirring speed of 700 r∙min-1using Ni/SiO2@TiO2catalyst with a particle size of 125–150 μm under a reaction pressureof 3 MPa.The reaction temperatures were selected at 140 °C,150 °C and 160 °C.The concentrations of each componentversusreaction time at different temperatures are shown in Fig.S5.

3.3.4.Estimation of parameters

The ode45()function and lsqnonlin()function in MATLAB software are called to estimate the parameters and the results are shown in Table 5.The activation energy ofn-pentanal selfcondensation to 2-PHEA is 48.65 kJ∙mol-1,much lower than that in our previous work (77.71 kJ∙mol-1over TiO2catalyst) [11].The above comparison indicates that it is more available fornpentanal self-condensation over Ni/SiO2@TiO2than TiO2due to the better dispersion of TiO2in Ni/SiO2@TiO2probably.

The kinetic equations for one-step synthesis of 2-PH fromnpentanal catalyzed by Ni/SiO2@TiO2are as follows.

3.3.5.Test of models

The relationship between the experimentally measured and the model-estimated concentrations of each component at 150 °C is shown in Fig.S6 in the supplementary material.It can be seen that the experimental data agree well with the estimated results,suggesting that the integration reaction kinetics equations are valid under the conditions examined.

In order to further verify the prediction accuracy of the kinetic models,the variance analysis and theF-test of the kinetic model were carried out and the results are shown in Table S1 in the supporting information.According to variance analysis theory,if the correlation indexR2>0.9 andF>10Fα,the model is considered significant at the level of α.In this work,the correlation index of each component is larger than 0.9,and the test value ofFis larger than 10Fα(F0.05(2,21)=3.467),indicating that the kinetic model obtained here is significant at the level of α=0.05.

4.Conclusions

(1) A core–shell structured catalyst Ni/SiO2@TiO2with highly catalytic performance for one-step synthesis of 2-PH fromn-pentanal was prepared successfully.Its suitable preparation conditions were determined as follows:the Ni loading was 2.5% (mass) and the mass ratio of Ni/SiO2to TBOT was 0.3:2.The characterization results reveal that the Ni/SiO2@-TiO2microspheres with a regular and continuous TiO2shell are dispersed separately and evenly.The 2-PH selectivity of 77.9% was obtained at an-pentanal conversion of 100% ,suggesting that the core–shell structure effectively enhances the competitiveness ofn-pentanal self-condensation to its direct hydrogenation.However,too large thickness of TiO2shell in Ni/SiO2@TiO2is not conducive to the hydrogenation of intermediate 2-propylheptanal to 2-PH.

(2) The reaction kinetics of one-step synthesis of 2-PH fromnpentanal catalyzed by Ni/SiO2@TiO2was established.The activation energy ofn-pentanal self-condensation is much lower than that obtained by employing TiO2catalyst,suggesting that Ni/SiO2@TiO2is more suitable for this reaction.The variance analysis and theF-test results show that the kinetic model is significant at the level of α=0.05.The establishment of the integration reaction kinetics can lay a foundation for reactor design and scale-up.

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 financially supported by the National Natural Science Foundation of China (21978066,21506046),Natural Science Foundation of Hebei Province (B2020202048,B2018202220),and Natural Science Foundation of Tianjin City(18JCYBJC42600).The authors are gratefully appreciative of their contributions.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.042.