High Solubility and Partial Molar Volume of Hexyl Methyl Malonate in Supercritical Carbon Dioxide

Yang Haijian，Zhao Lu，Xiang Li，Cai Zhuofu

(College of Chemistry and Materials Science，South-Central University for Nationalities，Wuhan 430074，China)

Supercritical carbon dioxide(scCO2)has been receiving increasing attentions for many applications as reactant in the field of chemistry and chemical industry due to its unique properties such as liquid-like density and solventpower，gas-like transportproperties，sensitivity of the properties with critical constants，and its benignity to environment［1-3］.

However，the non-polarity of CO2limits the further utilizations by the need for high CO2pressure to dissolve even smallamounts ofpolar，siliconefunctional amphiphilic，organometallic，or highmolecular-masscompounds［4，5］.And compounds which have a perfluoroalkyl polyether(PFPE)tails showing high solubilities in supercritical CO2，but this type of compounds are very expensive and toxic［6］.

Generally，the solubility of compounds depends on their groups of molecular structure，especially the end groups and molecular weight.Sothedesign and synthesis of highly CO2-soluble compounds via end group modification would greatly benefit the potential applications of CO2as a solvent.According to the literature and based on our research results，hydrocarbons substituted with carbonyl groups as CO2-philes have appeared economically，and the carbonyl group，ether group，and alkyl group with suitable length are so-called CO2-philic groups［7-9］.So，we designed and synthesized a new compound——hexyl methyl malonate via end group modification，which has carbonyl group，and alkyl group with suitable length.The solubilities of compound in scCO2over the pressure ranging from 8.5 MPa to 13.6 MPa and at temperatures 313 K，333 K and 353 K were determined.The tested results were also correlated by Bartle and Chrastil models.And the partial molar volume was calculated according to Kumar-Johnston theory.

1 Experimental

1.1 Reagents and Apparatus

Methyl malonyl chloride(w=0.97，mass fraction，CAS Registry No.37517-81-0)，1-hexanol(w=0.99，mass fraction，CAS Registry No.111-27-3，Alfa Aesar Chem.Co.)Triethylamine(w=0.99，mass fraction，CAS Registry No.121-44-8，Acros Organics Chem.Co.).CO2(w=0.9999，mass fraction，Wuhan Steel Co.).Dichloromethane(TianjinKemel Co.Ltd.).CO2deliverypump(JASCO PU-CO2)，back pressure regulator(JASCO BP-1580-81)，NMR experiments(JEOL Al-600MHz).

1.2 Synthesis of hexyl methyl malonate

The target compound hexyl methyl malonate was synthesized according to the method shown as Fig.1［10］.

1-Hexanol(2.6 mL，20.702 mmol)was added into a 100 mL flask under N2atmosphere，then a CH2Cl2solution(30 mL)of methyl malonyl chloride(1 mL，9.407 mmol)and triethylamine(2.8 mL，20.17 mmol)was added dropwise to the flask.The mixture was stirred at room temperature for a whole night，then the reaction mixture was washed twice with aq.HCl(0.05 mol/L)，saturated aq.NaHCO3，and deionized water.The organic phase was collected and dried over anhydrous Na2SO4.After filtration and evaporation under vacuum，the residue was purified by silica gel column chromatography with ethyl acetatepetroleum ether(volume proportion is 1 to 5)as eluent to give a yellow oil(GC purity＞99%)with 83%yield.1H NMR(CDCl3)δ:4.131～4.120(m，J=4.4，2H，CH2O)，3.730(s，3H，OCH3)，3.365(s，2H，COCH2CO)，1.789～1.290(m，J=64.8，8H，CH2)，1.029～0.722(t，3H，CH3).13C NMR(CDCl3)δ:166.71，70.06，65.65，52.36，41.34，31.30，28.36，25.38，22.45，13.89.Elemental Anal:C10H18O4found C 59.41%，H 8.91%，O 31.68%;required C 59.43%，H 8.89%，O 31.68%.

2 Results and Discussion

2.1 Solubility results

The solubility data of Hexyl methyl malonate tested at different conditions of pressures(8.5 MPa to 13.6 MPa)and temperatures(313 K，333 K and 353 K)in scCO2were listed in Tab.1.As shown in Tab.1，the solubility of compound increased with the increase of pressure at the same temperature，while at the same pressure，the solubility decreased with the increase of temperature，mainly because the solvent power of CO2varied with the change of CO2density at different pressures or temperatures.

2.1.1 Bartle model

The experimental solubility data for the compound were correlated using the following equation.

where

where x is the mole fraction of the solutes;P is the pressure;Prefis 0.1 MPa;ρ is the density of pure CO2at the experimental temperature and pressure;ρrefis 700 kg·m－3;and A，C，a and b are constants.At the initial stage，ln(xP/Pref)values were plotted against(ρ－ ρref)(Fig.2)，and the values were fitted with a straight line by least-squares regression to estimate the C and A parameters.The C values，obtained from the slopes of the corresponding plots，were averaged for each compound(Tab.2).When C was held atitsaverage value，the experimental solubility data were used to evaluate A values at various temperatures.The plots of A versus 1/T were fitted to a straight line(Fig.2)from which the intercept and the slope(a and b)were obtained.The resulting a and b values were shown in Tab.2.The a，b，and C values were used to predict solubility using Eq(1)and Eq(2).In this model，the parameter b is related to the enthalpy of sublimation of the solute，ΔsubH，by the expression ΔsubH=－Rb，where R is the gas constant［11］.

The average absolute relative deviation(AARD)was used to testthe correlation results.Itwas calculated with the following Eq(3).

where n was the number of experimental points，and xcaland xexpwere the calculated and experimental data respectively.The values of AARD were in the range of 3.28%～58.65%.

2.1.2 Chrastil model

In this model，the experimental solubility data for the compound were correlated by Eq(4).

where the solubility(S)was calculated by Eq(5).

where x is the molar fraction of the solute，M1and M2are the molecular weights of CO2and the solute.The constants α，β and k can be estimated from the experimental solubility data in scCO2.ρ is the density of the pure scCO2;S the solubility of the solid in the supercritical phase;T the temperature in K;k，α and β are the adjustable parameters of the model.The constant k is the association number，α a constant，defined as-ΔH/R (where ΔH is the sum of the enthalpies of vaporization and solvation of the solute and R the gas constant)and β depends on the molecular weights of the solute and solvent.The Chrastil model suggests that plots of lnS for several temperaturesare straightlines whose slopes are identical and equal to k.The parameters，k，α and β are obtained performing a multiple linear regression on the experimental solubility data［12，13］.

The values of calculated constants for the Hexyl methyl malonate in scCO2systems were presented in Tab.2.The quality of the correlation is expressed in terms of σ2and AARD between experimental and calculated solubility S.The consistency of the model with measured data can be seen from Fig.4 and the values of AARD at different temperatures，which are between 0.0068%～27.7%.The results exhibited the good agreement between the tested and calculated data.

2.1.3 Estimation of the partial molar volumes of the solutes

According to Kumarand Johnston［14］，the dependence of the solubility x of the solute with its partial molar volume in the vicinity of the critical density of the SCF，can be expressed by the following equation.

where x represents the equilibrium mole fraction of the solute in the SCF;andthe vapor pressure and molar volume of the solid solute;R the universal gas constant;2the partial molar volume of the solute in the SCF phase;κT=［(1/ρ)(?ρ/?)T，x］and ρr= ρ/ρcthe isothermal compressibility and reduced density of the phase;T is the operating temperature.

The partial molar volumes of the solute in the SCF phase()，are much larger than the molar volume of the solute().The 3rd term in Eq(6)was considered as constant in the region of interest.Eq(6)may thus be simplified as

Eq(7)implies that in the approximate density interval 0.5≤ρr≤2.0，log of the mole fraction of the solubility of the solute in a SCF varies linearly with log of the density of SCF phase.The slope of this line is the ratio of the partial molar volume of the solute in SCF phase to the isothermal compressibility of the fluid phase.This ratio is considered as independent of ρr，thus the knowledge of the value of and the slope of the ln x versus ln ρrat this temperature permits the estimation ofunder these conditions.As demonstrated in Fig.4，the systems investigated display linearity when plotted as lnx versus lnρr.This linearity was not observed when lnx values were plotted versus ρr.The slopes of the line ln x versus lnρrwere computed by linear squares fit for the hexyl methyl malonate in scCO2systems at different temperatures.The quality of the linear correlation is expressed in terms of σ2.Partial molar volumes were then deduced from the determined slopes and the values of κTfor CO2at the appropriate P－T conditions.

The results obtained are recapitulated in Tab.3.As shown in Tab.3，the partial molar volume for each solute decreases as temperature increases.The partial molar volumes2of the hexyl methyl malonate in the vicinity of the critical point of the solvent，which are difficult to measure experimentally，are then estimated by following the theory developed by Kumar and Johnston.As reported by these authors［15］，the data calculated for naphthalene+CO2and naphthalene+ethylene systems according to this theory were in good agreementwith experimentaldata.However，the calculation results of this work necessitate a confrontation with experimental measurements by other scientists.

3 Conclusion

In this work，a new CO2-philic compound，hexyl methyl malonate was designed and synthesized via simple procedures with high yield.The solubilities of the compound in scCO2were measured at 313，333，353 K and in the pressure range of 8.5 to 13.6 MPa.The compound showed good solubility at high temperature in supercritical CO2(2.62 ×10－3mol/L at 8.5 MPa and 313 K)and represented the same character as the low temperature.The measured data were correlated with(Bartle and Chrastil models and showed good agreement with the correlated results AARD was 3.28%～58.65%and 0.0068%～27.73%respectively.Solubility data were also used to estimate the partial molar volume2(－ 5708.79，－1665.36，－707.43 cm3/mol at 313，333，353 K)for the compound by Kumar and Johnston This work mightprovide basic information to design and synthesize new low-cost，nonfluorous CO2-philic compounds.

Acknowledgements

We are gratefulto NationalNaturalScience Fundation of China(No.2067031)，the finacial support of Beijing National Laboratory for Molecular Sciences(BNLMS)and the valuable help of Prof.Buxing Han.

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