Huisheng Lü,Zhi Wang ,Zhongfeng Geng ,Yonghui Li ,*,Shuangyan Liu ,Jiatao Liu

1 Key Laboratory for Green Chemical Technology of Ministry of Education,R&DCenter for Petrochemical Technology,Tianjin University,Tianjin 300072,China

2 Collaborative Innovation Center of Chemical Science and Engineering,Tianjin 300072,China

Keywords:Adsorption Supercritical carbon dioxide Paeoni fl orin Albi fl orin Isotherm model

ABSTRACT Adsorption equilibria of paeoni fl orin and albi fl orin on a cyano-silica column(CN column)from the solution of supercritical carbon dioxide(scCO2)modi fi ed w ith ethanol w ere studied.The adsorption capacity at 308.15 K,313.15 K,318.15 K and 323.15 K under pressures corresp onding to carbon dioxid e/ethanol densities from 0.347 g·cm-3 to 0.662 g·cm-3 were determined using the elution by characteristic point method(ECP).The effectsof temperature and pressure on the solute loading were investigated.The resultsshowed that the low er the temperature,the higher the adsorption capacity.With the decrease of density of scCO2,the adsorption capacity strengthens.The maximum adsorption capacity of paeoni fl orin(albi fl orin)on the CN column w as 15.24 mg·ml-1(31.14 mg·ml-1)in the range of 0-1.84 mg·ml-1(0-1.67 mg·ml-1)of paeoni fl orin(albi fl orin)standard solution.The adsorption capacity of albi fl orin w as tw ice as much asthat of paeoni fl orin under the same conditions.Adsorption data of paeoni fl orin and albi fl orin could be well described by the Langmuir model and Freundlich model.Compared with the two model fi tting results,the adsorption of paeoni fl orin and albi fl orin belonged to the monolayer adsorption under conditions of 308.15-323.15 Kand 10-17 MPa.

1.Introduction

Red peony root of Paeonia lacti fl ora,cultivated in temperate zone w orldw ide,has attracted great attention for their bene fi cial roles in human health.It has been w idely used in many Traditional Chinese Medicine formulations to treat symptoms such as dizziness,costal part pain,stomach ache,irregular menses and spontaneous perspiration[1,2].Paeoni fl orin and albi fl orin are tw o main active components in red peony root of P.lacti fl ora[3].These tw o monoterpeneglycosidesexhibit numerous pharmacological properties such as anti-in fl ammatory effect,cognition enhancement,antioxidation and neuromuscular blocking[4-7].Therefore,it issigni fi cant to re fi ne these active constituents into medicines.

Now adays,numerous separation techniques for separation of paeoni fl orin and albi fl orin have been reported.Chen et al.investigated the separation of paeoni fl orin from P.lacti fl ora Pall.using high-speed counter-current chromatography[8].Zhang et al.studied separation of albi fl orin and paeoni fl orin by macroporous resin column simulated moving bed chromatography adsorption[9].Wang et al.studied preparative puri fi cation of paeoni fl orin and albi fl orin from peony rhizome by the macroporous resin and medium-pressure liquid chromatography[1].Whereas,those separation techniques have several disadvantages such astime-consuming,low yields,solvent w astage and poisonous residual solvents.In recent years,supercritical fl uid chromatography(SFC)using supercritical carbon dioxide(scCO2)as the mobile phase has attracted grow ing interest in separation and re fi ning processes[10,11].SFC has many advantages such as high column ef fi ciency,mild operation conditions,high analysis speed and environmental friendliness.It has not only been applied for analytical separations,but also been used at a preparative scale in a variety of industries,such as pharmaceuticals,foods,cosmetics,agrochemicals,petrochemical and natural products[12-15].Moreover,carbon dioxide,as the most frequently used supercritical fl uid,also has several advantages like the low critical temperature,non-toxic characteristics,low cost,abundant supply and environmental friendliness[16-18].Considering the advantagesand wideapplications,SFCcan bean attractive method for separation and puri fi cation of paeoni fl orin and albi fl orin.

Adsorption isotherms and related mathematical models are essential for the feasibility,optimization and engineering design of SFCseparation.Compared w ith the adsorption from gas or liquid solution,adsorption of solutes from supercritical fl uids is a complex process[19].Due to special physicochemical properties of supercritical fl uids,the adsorption process involves the complex interaction betw een molecules and unique molecular aggregation.Therefore,adsorption isotherms from scCO2are hard to theoretically predict and have to be determined experimentally.Up to now,only afew studieshave been reported on adsorption isotherms from scCO2such asα-tocopherol[20],Vitamin D3[21],β-carotene[22],furfural[23],and fatty acid[24,25].To date,to our know ledge,there is no published w ork that investigates adsorption isotherms about paeoni fl orin and albi fl orin from scCO2/ethanol on cyanopropyl-bonded silica.Therefore,it is signi fi cant to gain the adsorption isotherm data and describe the adsorption processes by appropriate adsorption models.And the research results can be applied for the engineering design and optimization of separation of paeoni fl orin and albi fl orin with pre-SFC.

In this study,an enrichment method of paeoni fl orin and albi fl orin w as developed based on the preparative SFC(pre-SFC)for the fi rst time.In our previous w ork[26],w e studied the separation of paeoni fl orin and albi fl orin from radix paeoniae rubra by pre-SFC,and found out the optimum operation conditions.As a continuation of our previous study,the adsorption isotherms of paeoni fl orin and albi fl orin from supercritical carbon dioxide/ethanol on a cyano-silica column w ere determined by pre-SFC.The effects of temperature and density of scCO2on the governing adsorption processat supercritical conditions w ere analyzed.The elution by the characteristic point method(ECP)w as adopted to determine the adsorption isotherm of paeoni fl orin and albi fl orin.Adsorption isotherm data w ere corrected w ith the Langmuir model and Freundlich model.

2.Experimental

2.1.Materials and experimental set-up

Carbon dioxide(≥99.0 w t%)w as supplied by Tianjin Liufang Industrial Gas Distribution Co.,China.Paeoni fl orin standard(≥99.3 w t%)and albi fl orin standard(≥98.0 w t%)w ere offered by Nantong Feiyu Biotechnology Co.Ethanol(analysis grade)w as purchased by Shanghai Macklin Biochemical Co.

The schematic diagram of experimental facilities applied in this study w as show n in Fig.1.A SFC-200 preparative supercritical fl uid chromatography device(Thar Company,USA)w as applied to the measurements of adsorption equilibrium capacity under different experimental conditions.A Zorbax SB-CN column(9.4 mm×250 mm i.d.,5μm)w as provided by Agilent,USA.Agilent Zorbax CN is a polar bonded-phase column packing.This packing is produced by chemically bonding cyanopropylsilane groups to Zorbax SILparticles,follow ed by an endcapping reaction w ith trimethylsilane.The uniform,spherical,Zorbax CNparticles have a controlled pore size of nominally 6 nm.The characteristics of the column are listed in Table 1.

2.2.Theoretical fundamentals

2.2.1.Elution by characteristic point method(ECP)

For SFC,Martin et al.analyzed the adsorption isotherm data generated by the different methods and validated by comparing computer simulated elution pro fi les.It w as found that the methods based on elution pro fi les,i.e.,elution by characteristic points(ECP),the inverse method(IM)and the retention time method(RTM),w ere able to accurately predict overloaded experimental elution pro fi les[27].Compared with RTM and IM,ECPhasmany advantagessuch asitssimplicity,highspeed data acquisition and small sample requirement[28,29].Moreover,ECPcan give raw adsorption isotherm data which make it suitable for conducting fundamental adsorption studies[27].

ECPisderived from the ideal model assuming that column ef fi ciency isin fi nite[30].In reality,theef fi ciency of a column is fi niteand thusthis model w ill lead to some error in theadsorption isotherm determination[29].However,this error will decrease with column ef fi ciency increasing.In SFC,the high column ef fi ciency makes the error being usually of minor concern,but this needs to be veri fi ed prior to calculations[27].ECPis based on the use of a simple equation giving the diffuse part of an overloaded elution band.The adsorption isotherm is calculated from the rear diffusive part of an eluted and overloaded peak.The adsorption capacity q(C)at the concentration C can be calculated by the following equation:

w here V isthe retention volume of adsorbate at the characteristic point,V0is the hold-up volume and Vais the volume of adsorbent.

Fig.1.Schematic diagram of experimental facilities.1—CO2 cylinder,2—stop valve,3—CO2 condenser,4—mass fl owmeter,5—CO2 feeding pump,6—modi fi er,7—modi fi er feeding pump,8—pre-heater,9—sample,10—sample feeding pump,11—six-way valve,12—constant temperature system,13—chromatogram column,14—automatic back pressure adjustment valve,15—pressure regulating valve,16—collection devices(CS1-4).

Table 1 Characteristics of ZorbaxSB-CN column

2.2.2.Adsorption isotherm models

The Langmuir isotherm model can be used to describe monolayer adsorption,w hile the Freundlich isotherm model can be used to describe monolayer adsorption as w ell as multilayer adsorption[1,31].The Langmuir isotherm model is based on the monolayer adsorbate assumption.The model assumes that one adsorption site can only adsorb one adsorbate molecule and there is no interaction betw een adsorbate molecules[31,32].The nonlinear form of the Langmuir isotherm equation is shown below:

w here C is the equilibrium concentration of adsorbate,q is the equilibrium adsorption capacity of adsorbent,qmis the theoretically calculated maximum adsorption capacity of adsorbent and KLis the adsorption equilibrium constant[33].

The Freundlich isotherm model is one of the fi rst empirical equationsused for correlating experimental data[34].Monolayer adsorption or multilayer adsorption can be described by the model.In the Freundlich equation,KFis the Freundlich constant that indicates the adsorption capacity,and n is an empirical constant.The equation is as follow s:

2.3.Experimental procedure

2.3.1.Calculation of the number of theoretical plates

As described above,the ef fi ciency of a column directly affects errors of adsorption isotherm measured by the ECPmethod.Therefore,the accuracy of the ECPmethod can be determined by calculating the number of theoretical plates.In the Langmuir case,the theoretical plate number should be more than 2000,w hich makes an error of less than 3%[35].Based on the elution pro fi les,the calculation of the theoretical plate number w as calculated according to the follow ing equation[19]:

In this equation,N is thenumber of theoretical plates,tRistheretention time,and Y1/2is the half-w idth of the chromatographic peak.

2.3.2.Calibration of Cvs.S

In the SFCchromatogram,the relationship of the detector signal(S)and the retention time(t)could be obtained directly.Before data processing by the ECPmethod,the relationship of S and t should be converted to the relation of the solute concentration in the mobile phase(C)and t.The calibration of S and t w asperformed by the static method described in Tan's w ork[36].There is a linear relationship betw een S and C,i.e.,C=k×S.In this equation,k is the response factor of the detector w hich isdepended on thetemperature and pressure.The value of k can be considered as a constant as the temperature and pressure changed in a speci fi c range.And the value of k can be obtained by the least squares method.

Thus,at 313.15 K,12 MPa and 20 g·min-1of CO2fl ow rate,k1w as measured by injecting 20 μl of different concentrations(0.09 g·L-1,0.18 g·L-1,0.89 g·L-1,1.84 g·L-1)of paeoni fl orin standard solution into the chromatographic system.The w avelength of UV detection w as set at 230 nm.And the series of elution pro fi le w as recorded in Fig.2.As a result,k1=0.2512 w as calculated w ith an R2of 0.9990,and thus the relationship of the concentration of paeoni fl orin standard solution and the response value of the UV detector is as follow s:C1=0.2512×S1.In the same way,k2=0.6760 for albi fl orin was calculated w ith an R2of 0.9994.The relationship of C2=0.6760×S2could be obtained,in w hich C2is the concentration of albi fl orin standard solution.

Fig.2.Elution chromatograms of different concentrations of paeoni fl orin at 313.15 Kand 12 MPa in SFC:(a)0.09 g·L-1;(b)0.18 g·L-1;(c)0.89 g·L-1;(d)1.84 g·L-1.

Fig.3.Chromatograms of paeoni fl orin with different densities of the mobile phase at 313.15 K:(a)0.398 g·cm-3;(b)0.508 g·cm-3;(c)0.577 g·cm-3;(d)0.626 g·cm-3.

Fig.4.Langmuir adsorption isothermsof paeoni fl orin w ith different densities of mobile phase at de fi nite temperatures:(a)308.15 K,(b)313.15 K,(c)318.15 K,(d)323.15 K.Solid lines,Langmuir model.

2.3.3.Measurement of adsorption capacity

Adsorption capacity of paeoni fl orin and albi fl orin w ere measured at different temperatures and pressures.Temperatures and mean pressures in the column w ere varied from 308.15 to 323.15 K and from 10 to 17 MPa,respectively.Solubility of paeoni fl orin and albi fl orin in scCO2w asenhanced by the addition of a modi fi er.In this study,ethanol w as selected by the solubility parameter criteria.The density of scCO2and ethanol binary mixed fl uid w as calculated by using the Peng-Robinson equation of state.The amount of ethanol added into scCO2w as 13.64%in mass fraction and the fi xed fl ow rate of scCO2/ethanol w as set to 20 g·min-1.20 μl of paeoni fl orin standard solution(1.84 mg·ml-1)and 0.02 μl of albi fl orin standard solution(1.04 mg·ml-1)w ere injected in pre-SFC,respectively.The wavelength of UVdetection w asset to 230 nm.Based on theelution pro fi les,adsorption capacity q at different densities of the mobile phase and the temperatures w ere calculated by Eq.(1).The densities of the scCO2and ethanol(13.64 w t%)binary mixtures at different pressuresand temperatures w ere listed in Table S1.Each experiment w as conducted in triplicate,and the average value w as taken w ith the error in 3%.

3.Results and Discussion

3.1.Adsorption isotherms

Fig.3 show ed the typical elution pro fi les of paeoni fl orin w ith different densitiesof the mobile phase at 313.15 K.The number of theoretical platesw as calculated by Eq.(4),and all of them w ere higher than 3000.Adsorption isotherm dataof paeoni fl orin and albi fl orin on a cyano-silica column from scCO2/ethanol at 308.15,313.15,318.15 and 323.15 K w ere determined at different densities of scCO2and ethanol binary mixed fl uid.

Adsorption isotherms of paeoni fl orin and albi fl orin on the CN column with different densities of scCO2/ethanol at de fi nite temperatures w ere show n in Figs.4 and 5,respectively.The adsorption capacity of paeoni fl orin or albi fl orin decreased as the density of scCO2/ethanol increased.As the density of scCO2increases,molecules of paeoni fl orin(or albi fl orin)are surrounded by more CO2molecules.The interaction betw een CO2molecules and paeoni fl orin(or albi fl orin)molecules may increase,w hich enhances the solubility of paeoni fl orin(or albi fl orin)in scCO2.In addition,the CO2competing sorption for the adsorption sites may be one of the reasons for the adsorption capacity decrease.

Fig.5.Langmuir adsorption isotherms of albi fl orin w ith different densities of mobile phase at de fi nite temperatures:(a)308.15 K,(b)313.15 K,(c)318.15 K,(d)323.15 K,solid lines,Langmuir model.

Fig.6.Adsorption data of paeoni fl orin and albi fl orin at similar densities(0.595 g·cm-3 and 0.597 g·cm-3).

Fig.6 demonstrated that the adsorption capacity of paeoni fl orin and albi fl orin declined asthe temperaturesincreased at the similar densities(0.595 g·cm-3and 0.597 g·cm-3)of scCO2/ethanol.The results suggested that adsorption capacity of paeoni fl orin(or albi fl orin)at constant density decreasesastemperatureincreases.In theprocessof adsorption,there w ere several kinds of interaction betw een paeoni fl orin(or albi fl orin)and the cyanopropyl-bonded silica stationary phase,such as theelectron donor-acceptor interaction and thehydrogen bond interaction.Rising temperatures enhance the molecular motion,w hich may cause the interaction between paeoni fl orin(or albi fl orin)moleculesand cyanopropyl-bonded silica molecules to w eaken.Furthermore,rising temperature accelerates the thermal motion of paeoni fl orin(or albi fl orin)and CO2molecules,w hich led to the improvement of the elution capacity of the mobile phase.It w ould be one of the reasons theadsorption capacity of paeoni fl orin(or albi fl orin)at constant density decreases as the temperature increases.

Fig.7.Adsorption data of paeoni fl orin and albi fl orin at 313.5 K.

The amount of solute adsorbed on the stationary phase is also related to the characteristics of the solute itself.The adsorption capacities of paeoni fl orin and albi fl orin on the CN column with densities of 0.508 g·cm-3and 0.577 g·cm-3at 313.15 K,w ere show n in Fig.7.Under the same conditions,the adsorption capacity of albi fl orin w as more than that of paeoni fl orin at the same equilibrium concentration in scCO2/ethanol.This is mainly because the molecule of albi fl orin has one more carbonyl group than paeoni fl orin,w hich makes the interaction betw een the molecule of albi fl orin and the-CN group on the stationary phase stronger.In this w ork,the maximum adsorption capacity of paeoni fl orin on the CN column w as 15.24 mg·ml-1in the range of 0-1.84 mg·ml-1of paeoni fl orin standard solution.When the concentration of albi fl orin standard solution w as in the range of 0-1.67 mg·ml-1,the maximum adsorption capacity of albi fl orin on the CNcolumn was 31.14 mg·ml-1,w hich was twice as much as that of paeoni fl orin.

3.2.Modeling of the adsorption isotherm data

Adsorption isotherm data of paeoni fl orin and albi fl orin at different temperatures(305.15,313.15,318.15 and 323.15 K)and different densities of binary mixed fl uid w ere correlated by the Langmuir and Freundlich models(Eqs.(2)and(3)),respectively.In Figs.4 and 5,Langmuir adsorption isotherms w ere indicated by solid lines.And the Freundlich adsorption isotherms w ere show n as dotted lines in Figs.8 and 9.The parameters of the models were listed in Tables 2 and 3,w hich w ere obtained by fi tting the adsorption data.As the results shown in Figs.4,5,8 and 9,the two models achieved a satisfactory fi t w ith the calculated correlation coef fi cients(R2)of both models higher than 0.9993.

The Langmuir model can be used to describe a monolayer adsorption.As show n in Table 2,the range of monolayer adsorption capacity(qm)was from 102.48 mg·ml-1to 248.90 mg·ml-1.From the results of the Langmuir equation,the theoretical maximum qmof paeoni fl orin w as 248.90 mg·ml-1under the condition of 308.15 K and 10 MPa.As show n in Table 2,the range of qmw as from 371.73 mg·ml-1to 664.47 mg·ml-1and the theoretical maximum qmof albi fl orin determined from the Langmuir equation w as 664.47 mg·ml-1under the condition of 308.15 K and 10 MPa.The parameter KLis an indicator of the stability of the combination betw een adsorbates and adsorbent surfaces.The parameter KLof paeoni fl orin or albi fl orin decreased as the temperature or the density of scCO2/ethanol increased.The trend of KLwasconsistent with the trend of adsorption capacity q in Figs.4,5 and 6.

Fig.8.Freundlich adsorption isothermsof paeoni fl orin with different densitiesof mobile phaseat de fi nite temperatures:(a)308.15 K,(b)313.15 K,(c)318.15 K,(d)323.15 K,dotted lines,Freundlich model.

Fig.9.Freundlich adsorption isotherms of albi fl orin w ith different densitiesof mobile phase at de fi nite temperatures:(a)308.15 K,(b)313.15 K,(c)318.15 K,(d)323.15 K,dotted lines,Freundlich model.

For the Freundlich model,the constant KF,w hich indicates the adsorption capacity,decreased with the increase of the density of scCO2/ethanol at a certain temperature or pressure in Table 3.The result demonstrated that the elution ability of the mobile phase w as enhanced w hen the density of the mobile phase increased.Thus,the adsorption capacity of paeoni fl orin and albi fl orin on the CNcolumn wasw eakened.

The R2of the Langmuir equation w as higher than 0.9993 in Table 2.For the Freundlich equation,the valueof R2w asabove 0.9999 in Table 3.The Freundlich model can be used to describe not only monolayer adsorption but also multilayer adsorption.The Langmuir model is only used to describe monolayer adsorption.Compared with the tw o model fi tting results,the Langmuir model can also w ell describe the adsorption behavior asw ell as the Freundlich model.Thus,the adsorption process of paeoni fl orin and albi fl orin on the CN column belonged to monolayer adsorption under the experimental conditions.

Table 2 Parameters in Langmuir model for paeoni fl orin and albi fl orin at different pressures and temperatures

Table 3 Parameters in Freundlich model for paeoni fl orin and albi fl orin at different pressures and temperatures

4.Conclusions

Adsorption capacity of paeoni fl orin and albi fl orin in the CN column w ith scCO2modi fi ed by ethanol w as measured using the ECPmethod.The maximum adsorption capacity of paeoni fl orin(albi fl orin)on the CN column w as 15.24 mg·ml-1(31.14 mg·ml-1)in the range of 0-1.84 mg·ml-1(0-1.67 mg·ml-1)of paeoni fl orin(albi fl orin)standard solution.The same solute concentration corresponding to the adsorption capacity of albi fl orin w as tw ice as much as that of paeoni fl orin under the same experimental conditions.Adsorption isotherms w ere determined at different temperatures(308.15-323.15 K)and pressures(10-17 MPa).The results showed that the adsorption of paeoni fl orin or albi fl orin w as more favored at low temperatures and low densities of scCO2.Adsorption data of paeoni fl orin and albi fl orin w ere fi tted w ell by both the Langmuir and Freundlich models.Compared w ith the tw o model fi tting results,the adsorption of paeoni fl orin and albi fl orin belonged to monolayer adsorption.The monolayer saturation adsorption capacity qmof paeoni fl orin(albi fl orin)w as in the range of 102.48 to 248.90 mg·ml-1(371.73 to 664.47 mg·ml-1)under conditions of 308.15-323.15 K and 10-17 MPa.Acquired data and obtained models can be used for the design and simulation of adsorptive separation processes of paeoni fl orin and albi fl orin w ith pre-SFC.

Supplementary Material

Supplementary material to thisarticlecan befound onlineat https://doi.org/10.1016/j.cjche.2018.05.026.