Thermophysical Properties of Furfural Compounds - ACS Publications

Article pubs.acs.org/jced

Thermophysical Properties of Furfural Compounds Laura Lomba,† Beatriz Giner,*,† Ma Carmen Lopéz,‡ Luis Aldea,† and Carlos Lafuente‡ †

Grupo de investigación GIMACES, Facultad de Ciencias de la Salud, Universidad San Jorge, Villanueva de Gállego, 50830, Zaragoza, Spain ‡ Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009, Zaragoza, Spain S Supporting Information *

ABSTRACT: A physicochemical study of 5-methylfurfural and tetrahydrofurfuryl alcohol (density, speed of sound, refractive index, surface tension, static permittivity, and viscosity) has been carried out under atmospheric pressure at temperatures from (278.15 to 338.15) K, while the vapor pressure was determined over a temperature range from (303.15 to 453.15) K. Furthermore, the pρT behavior has also been assessed over a temperature range from (283.15 to 338.15) K and a pressure range from (0.1 to 60.0) MPa. Results indicate that although values of speed of sound, molar free volume, and viscosity of tetrahydrofurfuryl alcohol are higher than for the rest of the furfural family, the free volume/molar volume ratio is smaller for 5-methylfurfural, indicating a better packing. Furthermore, VOC character of the studied compounds is smaller than some traditional solvents.

1. INTRODUCTION One of the Green Chemistry principles described by Anastas and Warner1 is related to solvents, because they are greatly used in the chemical industry not only for many chemical reactions but also for cleaning equipment, regulating temperatures, isolating and purifying compounds by recrystallization or extractions, and assisting structural characterization. However, it is well-known that some of these solvents can result in serious environmental and health problems. Accordingly, it is important to develop new less toxic and hazardous solvents.2 A good alternative to traditional harmful solvents would be processes without solvent use, especially when one of the substrates or the product is a liquid that can be used as the solvent of the reaction. Nevertheless, this is not possible in most cases because solvents are still really important in the vast majority of reactions. For this reason, it is necessary to look for different kinds of environmentally friendly solvents. There are some candidates for alternates to traditional solvents, for example solvents from biomass, which have received much attention as renewable organic resources.2,3 Biomass is defined as any organic matter that is available on a renewable basis, including dedicated energy crops and trees, aquatic plants, animal wastes, agricultural food and feed crop residues, wood and wood residues, and other waste materials.4 With the use of biomass, some solvents can be obtained by fermentation, enzymatic, or esterification processes. These kinds of compounds are classified as green solvents and can be used for a variety of industrial processes. Some of these chemicals are fatty acid esters, furfural, levulinic acid, terpenes, glycols and their esters, low molecular weight alcohols, and lactates.5 Furfural is an important compound accessible from biomass because it is the only unsaturated large-volume organic chemical © XXXX American Chemical Society

prepared from carbohydrate resources and is a key derivative for the production of important chemicals that cannot be obtained from petroleum.6,7 The chemistry of furfural has been developed during the past years.7 One important chemical coming from furfural is furfuryl alcohol which presents several applications in the chemical industry such as lubricants, adhesives, and precursor. Moreover, it is mainly used in the manufacture of resins, as a starting material for the production of tetrahydrofurfuryl alcohol, and it is also an important chemical intermediate for the manufacture of fragrances, vitamin C, and lysine.8 On the other hand, 5-methylfurfural can be obtained from traditional balsamic vinegar which is produced from acetic acid fermentation under specific conditions.9 This chemical can give an especial flavor to this product. However, its presence in this vinegar is regulated in order to evaluate its organoleptic properties and potential fraudulent commercial use.9 Nevertheless, the use of these chemicals is very limited due to the poor knowledge about them, and for this reason it is important to know as much information as possible about their molecular behavior and how these types of molecules interact with other chemicals. In addition, through the study of several physicochemical properties, the applicability of these chemicals in different industrial processes can be assessed; moreover, the thermophysical properties will probably validate the goodness of these compounds from an environmental point of view and as a potential substitute for traditional solvents. A review of the literature shows that the thermophysical properties of furfural derivatives are not complete. Regarding Received: August 29, 2013 Accepted: December 23, 2013

A

dx.doi.org/10.1021/je400783h | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

dry air. The final uncertainty of density and speed of sound can be estimated in ± 5·10−6 g·cm−3 and ± 0.05 m·s−1 respectively. Refractive indices at 589.3 nm sodium D wavelength, nD, were measured using a high precision automatic refractometer Abbemat-HP from DR. Kernchen, whose temperature was internally controlled at ± 0.01 K. The uncertainty of the measurements is ± 5·10−6. Previous results of density and refractive index of tetrahydrofurfuryl alcohol11 corroborates our results; deviation of 0.07 % for density and 0.06 % for refractive index. Surface tensions, σ, were determined using a drop volume tensiometer Lauda TVT-2.17 The temperature was kept constant within ± 0.01 K by means of an external Lauda E-200 thermostat. The uncertainty of the surface tension measurement is ± 0.05 mN·m−1. Kinematic viscosities, ν, were measured using an Ubbelohde viscosimeter with a Schott-Geräte automatic measuring unit model AVS-440. The temperature was kept constant at ±0.01K by means of a Ct52 Schott-Geräte thermostat. The viscosimeter was calibrated with ultrapure water supplied by SH Calibration service GmbH. The estimated uncertainty of kinematic viscosity determinations, expressed as percentages, is ± 1 %. Kinetic energy corrections were applied to the experimental data. The dynamic viscosity, η = ρ·v, has been obtained from density and kinematic viscosity measurements, and the estimated uncertainty in dynamic viscosity is ± 1%. Static permittivities, ε, at a frequency of 2 MHz were obtained using a capacitive measurement method, the capacitances were measured by means of an Agilent 4263BA precision LCR meter connected to a four terminal Agilent 16452A liquid dielectric test fixture through Agilent 16048A test leads. The uncertainty of permittivity measurements is less than 0.5 %. During all measurements, the temperature of the cell was controlled at ± 0.01 K by means of a CT52 SchootGeräte thermostat. Vapor pressures were determined using an all-glass dynamic recirculating still, Fischer-Labodest which was equipped with a Cottrell pump. The temperatures were measured by means of a thermometer model F25 with a PT100 probe from Automatic Systems Laboratories, and the pressures in the still were measured with a Digiquartz 735-215A-102 pressure transducer from Paroscientific equipped with a Digiquartz 735 display unit. The uncertainty in the temperature and pressure equilibrium measurement in the still is 0.01 K and 50 Pa, respectively. Values of log P have been calculated using the ALOGPS 2.1 method (available on http://www.vcclab.org/lab/alogps/).12−14

mixtures of 5-methylfurfural and other chemicals, the activity coefficients of water in 5-methylfurfural at infinite dilution have been measured, while the azeotropic information for the system 5-methylfurfural water has also been studied.10 Furthermore, in the case of tetrahydrofurfuryl alcohol, there is a report in which the density and refractive index at T = 293.2 K has been published.11 Nevertheless, as far as we know, 5-methylfurfural or furfuryl alcohol has not been studied within all its perspectives. In view of that, we have completed the available information for this family of chemicals through a comprehensive thermophysical study. To achieve this objective, density, speed of sound, refractive index, surface tension, static permittivity, and dynamic viscosity of 5-methylfurfural and tetrahydrofurfuryl alcohol have been obtained at atmospheric pressure from (278.15 to 338.15) K, while the vapor pressure was determined over a temperature range from (303.15 to 453.15) K. Furthermore, the pρT study has been carried out over a temperature range from (283.15 to 338.15) K and a pressure range from (0.1 to 60.0) MPa for the studied chemicals. Experimental density data were fitted to the TRIDEN and from the fitted equation, related properties, such as isobaric thermal expansion, α, and isothermal compressibility, kT, have been obtained. On the other hand, in order to achieve an impression of the molecular hydrophobicity, the log P (logarithm of 1 − octanol/water partition coefficient) has been calculated making use of the AGLOPs method.12−14 The results have been analyzed in terms of molecular interactions. Finally, a comparative study for the furfural family (furfural, 5-methylfurfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol) was performed to assess how the small differences in their structure can affect their thermophysical behavior.

2. EXPERIMENTAL SECTION 5-Methylfurfural and tetrahydrofurfuryl alcohol have been provided by Sigma with a mass purity of 99 %. Information of the studied chemicals is summarized in Table 1. These purities Table 1. Purities and Source of the Solvents chemical name

CAS

source

purity/mass fraction

5-methylfurfural tetrahydrofurfuryl alcohol

620-02-0 97-99-4

Sigma Sigma

99 % 99 %

have been confirmed by GC chromatography, and no further purification was considered to be necessary. To obtain pρT behavior, a high pressure and high temperature Anton Paar DMA HP cell connected to an Anton Paar DMA 5000 densimeter has been used. The density of the sample was determined by measuring the oscillation period of the U-shaped tube made from Hastelloy C-276 gold. The cell temperature is controlled at ± 0.01 K by means of an integrated Peltier thermostat. The required pressure was created by a hand pump 750.1100 from Sitec, Switzerland, and measured by a pressure transducer US181 from Measuring Specialties, USA. The uncertainty in the pressure measurement is ± 5 kPa. For evacuating the whole apparatus a vacuum pump was employed. Details of the calibration and procedure have been reported in a previous paper,15,16 the estimated uncertainty of our density measurements is ± 5·10−5 g·cm−3. Densities, ρ, and speed of sounds, u, of the pure compounds were determined simultaneously with an Anton Paar DSA 5000 vibrating tube densimeter and sound analyzer, automatically thermostatted at ± 0.005 K. The calibration was carried out with ultrapure water supplied by SH Calibration service GmbH and

3. RESULTS The density values as a function of temperature and pressure were collected and are detailed in Table 2. The experimental values of density, refractive index, speed of sound, surface tension, kinematic and dynamic viscosity, static permittivity, vapor pressure, and derived properties such as molar refraction, isentropic compressibility, and enthalpy of surface formation for the studied chemicals were gathered and presented in Table 3. Moreover experimental values of vapor pressure are listed in Table 4. The measured densities have been correlated with temperature and pressure using the three-dimensional pρT correlating model TRIDEN.18 In this model the Tait equation19 for isothermal compressed densities was combined with a modified Rackett equation20 for isothermal compressed densities. For the B



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Table 2. Experimental Densities, ρ, as a Function of Temperature and Pressurea ρ/kg·m−3 at p/MPa T/K

0.1 MPa

2.5 MPa

5.0 MPa

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15

1118.02 1113.09 1108.15 1103.20 1098.25 1093.29 1088.32 1083.33 1078.33 1073.32 1068.30 1063.25

1119.83 1114.82 1110.03 1104.82 1100.71 1095.01 1090.31 1085.42 1080.39 1075.43 1070.05 1065.43

1121.37 1116.39 1111.50 1106.62 1101.81 1096.90 1092.06 1087.05 1082.17 1077.24 1072.29 1067.33

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15

1062.84 1058.59 1054.34 1050.06 1045.78 1041.48 1037.17 1032.83 1028.47 1024.08 1019.68 1015.24

1064.37 1060.15 1055.94 1051.72 1047.47 1043.19 1038.92 1034.57 1030.30 1025.88 1021.52 1017.04

1065.68 1061.47 1057.30 1053.11 1048.90 1044.66 1040.41 1036.11 1031.87 1027.49 1023.18 1018.75

7.5 MPa

10.0 MPa

20.0 MPa

5-Methylfurfural 1122.65 1124.02 1129.41 1117.87 1119.23 1124.75 1112.96 1114.40 1120.15 1108.34 1109.63 1115.43 1103.11 1104.89 1110.81 1098.15 1100.06 1106.17 1093.64 1095.27 1101.50 1088.71 1090.41 1096.72 1083.95 1085.56 1092.07 1079.03 1080.70 1087.38 1074.10 1075.84 1082.66 1069.06 1070.99 1077.96 Tetrahydrofurfuryl Alcohol 1066.97 1068.24 1073.16 1062.80 1064.09 1069.11 1058.64 1060.00 1065.10 1054.48 1055.84 1061.08 1050.31 1051.69 1057.06 1046.09 1047.48 1052.96 1041.89 1043.41 1048.93 1037.62 1039.11 1044.81 1033.41 1034.93 1040.79 1029.13 1030.63 1036.60 1024.84 1026.39 1032.49 1020.45 1022.06 1028.31

30.0 MPa

40.0 MPa

50.0 MPa

60.0 MPa

1134.53 1129.99 1125.46 1120.97 1116.44 1111.86 1107.37 1102.74 1098.22 1093.64 1089.10 1084.52

1139.44 1134.99 1130.57 1126.15 1121.72 1117.24 1112.86 1108.39 1104.01 1099.55 1095.12 1090.69

1144.14 1139.79 1135.40 1131.12 1126.87 1122.38 1118.10 1113.73 1109.47 1105.14 1100.86 1096.51

1148.58 1144.31 1140.08 1135.84 1131.66 1127.34 1123.12 1118.89 1114.68 1110.48 1106.23 1102.02

1077.84 1073.88 1069.97 1066.04 1062.11 1058.12 1054.21 1050.15 1046.28 1042.20 1038.22 1034.16

1082.28 1078.41 1074.62 1070.75 1066.94 1063.05 1059.19 1055.24 1051.44 1047.50 1043.64 1039.68

1086.55 1082.72 1078.97 1075.22 1071.49 1067.61 1063.91 1060.12 1056.41 1052.52 1048.73 1044.89

1090.57 1086.83 1083.18 1079.49 1075.79 1072.09 1068.44 1064.68 1061.07 1057.30 1053.60 1049.83

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.050 MPa, and the combined expanded uncertainty Uc is Uc(ρ) = 0.1 kg·m−3 with 0.95 level of confidence (k ≈ 2). a

⎛ ⎜1 RMSDr (%) = 100⎜ n ⎝

liquid saturation densities, the Rackett equation used here is a further modification of the modified form as suggested by Spencer and Danner:21 ρ0 =

ρ=

AR BR [1 + (1 − T /C R )DR ]

ρ0 ⎛ B +p ⎞ 1 − C T ln⎜ B T + p ⎟ ⎝ T 0⎠

⎛ T ⎞3 ⎛ T ⎞2 ⎛T ⎞ BT = b0 + b1⎜ ⎟ + b2⎜ ⎟ + b3⎜ ⎟ ⎝ ET ⎠ ⎝ ET ⎠ ⎝ ET ⎠

2⎞ ⎛ρ − ρi ,corr ⎞ ⎟ i ,exp ⎟ ∑ ⎜⎜ ⎟⎟ ρi ,exp ⎠⎠ i=1 ⎝

1/2

n

(4)

where n is the number of experimental data. TRIDEN parameters along with the corresponding deviation were collected in Table 5. The deviations between experimental and correlated densities are close to the uncertainty of the experimental densities and randomly distributed. Isobaric expansibility, αp, and isothermal compressibility, κT, have been calculated. The temperature and pressure derivatives of density have been evaluated using the TRIDEN equation. The calculated values of these properties over the wide temperature and pressure ranges can be found in the Supporting Information, Tables S1 and S2. A linear dependence of several properties such as refractive index, speed of sound, surface tension, and static permittivity with temperature in the range of measurements under study has been found. These experimental data have been correlated with the following equation:

(1)

(2)

(3)

where ρo is the density at atmospheric pressure and ρ is the density. For the Rackett equation parameters AR, BR, CR, and DR are fitting parameters and T is the absolute temperature. All parameters can be simultaneously fitted to temperaturedependent experimental density data at atmospheric pressure (p0 = 0.1 MPa). In the case of the Tait equation and eqs 2 and 3, p is the pressure expressed in MPa and the fitting parameters are b0, b1, b2, b3, and ET in eq 3 and the parameter CT in eq 2 that was treated as temperature independent. The relative root mean-square deviations, RMSDr, between experimental and correlated density values were used as statistical values for TRIDENT fits:

Y = AT + B

(5)

where Y is the studied property and A and B are adjustable parameters. The best linear fitting parameters and relative rootmean square deviations between experimental and correlated values are gathered in Table 6. Values of density as a function of temperature and pressure and its derivatives, isobaric expansibilities, and isothermal compressibilities are plotted in Figures 1 and 2. The isobaric C



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Table 3. Experimental and Calculated Physicochemical Properties at Atmospheric Pressure as a Function of Temperaturea T

ρ

u

κS

K

kg·m−3

m·s−1

TPa−1

278.15 280.65 283.15 285.65 288.15 290.65 293.15 295.65 298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15

1122.936 1120.495 1118.023 1115.557 1113.091 1110.623 1108.153 1105.676 1103.203 1100.728 1098.250 1095.770 1093.288 1090.804 1088.316 1085.822 1083.329 1080.833 1078.332 1075.829 1073.322 1070.811 1068.296 1065.776 1063.252

1521.03 1511.82 1502.55 1493.23 1483.97 1474.72 1465.52 1456.28 1447.02 1437.84 1428.70 1419.57 1410.46 1401.41 1392.34 1383.28 1374.29 1365.29 1356.09 1347.33 1338.36 1329.45 1320.54 1311.63 1302.77

384.92 390.47 396.18 402.03 407.96 414.01 420.16 426.46 432.91 439.44 446.08 452.86 459.77 466.79 473.97 481.31 488.75 496.35 504.28 512.05 520.14 528.38 536.79 545.40 554.15

278.15 280.65 283.15 285.65 288.15 290.65 293.15 295.65 298.15 300.65 303.15 305.65 308.15 310.65 313.15 315.65 318.15 320.65 323.15 325.65 328.15 330.65 333.15 335.65 338.15

1067.066 1064.958 1062.839 1060.717 1058.593 1056.467 1054.338 1052.201 1050.064 1047.925 1045.782 1043.633 1041.480 1039.327 1037.166 1034.999 1032.828 1030.650 1028.467 1026.279 1024.084 1021.883 1019.676 1017.463 1015.243

1556.44 1547.59 1538.61 1529.70 1520.76 1511.85 1503.00 1494.16 1485.32 1476.53 1467.78 1459.02 1450.27 1441.56 1432.84 1424.13 1415.43 1406.75 1398.06 1389.40 1380.75 1372.13 1363.47 1354.83 1346.20

386.85 392.06 397.44 402.89 408.46 414.12 419.86 425.70 431.66 437.71 443.85 450.12 456.51 463.00 469.63 476.39 483.28 490.29 497.46 504.75 512.19 519.77 527.53 535.44 543.51

nD

Rm

σ

ΔHσ

v

η

cm3·mol−1

mN·m−1

mN·m−1

mm2·s−1

mPa·s

ε

43.02 42.75 42.37 42.13 41.80 41.46 41.20 40.90 40.61 40.22 39.95 39.66 39.32 39.04 38.75 38.44 38.13 37.78 37.51 37.26 36.91 36.54 36.21 35.92 35.57

77.34 77.38 77.31 77.37 77.35 77.32 77.37 77.38 77.40 77.31 77.35 77.37 77.34 77.37 77.39 77.38 77.38 77.34 77.38 77.44 77.40 77.34 77.31 77.33 77.29

2.8176 2.6537 2.4718 2.3559 2.1949 2.0685 1.9630 1.8594 1.7654 1.6835 1.6000 1.5281 1.4615 1.4000 1.3424 1.2913 1.2417 1.1950 1.1510 1.1171 1.0720 1.0370 1.0038 0.9745 0.9426

3.164 2.974 2.764 2.628 2.443 2.297 2.175 2.056 1.948 1.853 1.757 1.674 1.598 1.527 1.461 1.402 1.345 1.292 1.241 1.202 1.151 1.110 1.072 1.039 1.002

45.2 44.5 43.7 43.0 42.3 41.6 40.9 40.2 39.4 38.7 38.0 37.2 36.4 35.8 35.1 34.4 33.7 33.6 32.4 31.7 31.1 30.4 29.8 29.1 28.4

39.54 39.29 39.03 38.78 38.53 38.27 38.02 37.77 37.51 37.26 37.01 36.75 36.50 36.25 35.99 35.74 35.48 35.23 34.98 34.72 34.47 34.22 33.96 33.71 33.46

67.75 67.76 67.72 67.75 67.76 67.72 67.77 67.72 67.74 67.72 67.73 67.72 67.79 67.85 67.88 67.81 67.78 67.75 67.68 67.70 67.63 67.80 67.74 67.74 67.77

9.7379 8.9079 8.1709 7.5147 6.9286 6.4036 5.9322 5.5076 5.1242 4.7773 4.4625 4.1763 3.9155 3.6774 3.4594 3.2597 3.0762 2.9074 2.7518 2.6081 2.4752 2.3521 2.2380 2.1319 2.0332

10.391 9.487 8.684 7.971 7.335 6.765 6.254 5.795 5.381 5.006 4.667 4.359 4.078 3.822 3.588 3.374 3.177 2.996 2.830 2.677 2.535 2.404 2.282 2.169 2.064

15.2 15.0 15.0 14.9 14.8 14.7 14.6 14.5 14.4 14.4 14.3 14.2 14.1 14.0 13.9 13.8 13.8 13.6 13.5 13.5 13.4 13.3 13.2 13.1 13.0

5-Methylfurfural

1.536401 30.729 1.535291 30.744 1.533931 30.746 1.532694 30.755 1.531443 30.763 1.530189 30.771 1.528904 30.778 1.527652 30.786 1.526400 30.794 1.525132 30.801 1.523855 30.808 1.522567 30.815 1.521298 30.822 1.520000 30.829 1.518716 30.835 1.517421 30.842 1.516145 30.849 1.514857 30.856 1.513565 30.863 1.512288 30.870 1.510996 30.877 1.509721 30.885 1.508422 30.892 Tetrahydrofurfuryl Alcohol

1.456462 1.455486 1.454522 1.453559 1.452588 1.451616 1.450627 1.449652 1.448668 1.447686 1.446715 1.445720 1.444730 1.443746 1.442742 1.441757 1.440721 1.439698 1.438711 1.437718 1.436683 1.435673 1.434661

26.144 26.148 26.152 26.157 26.161 26.166 26.169 26.174 26.177 26.182 26.186 26.190 26.194 26.198 26.202 26.207 26.209 26.212 26.216 26.221 26.224 26.228 26.232

Standard uncertainty u is u(T) = 0.01 K. and the combined expanded uncertainties Uc are Uc(ρ) = 0.05 kg·m−3. Uc(u) = 0.05 m·s−1. Uc(nD) = 5· 10−5. Uc(σ) = 0.05 mN·m−1. Uc(η) = 1 %. Uc(ε) = 0.5 % with 0.95 level of confidence (k ≈ 2).

a

expansibilities, αp, and isothermal compressibilities, κT, at T = 313.15 K and at atmospheric pressure are presented in Table 7.

In Figure 3, experimental refractive index, and calculated molar refraction values are shown. The molar refraction, Rm, of the studied compounds has been calculated from experimental data D



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Table 4. Experimental Vapor Pressures as a Function of Temperaturea

Table 6. Fitting Parameters and Relative Root-Mean Square Deviations for the Measured Properties

p/kPa

a

5-methylfurfural

tetrahydrofurfuryl alcohol

303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15 378.15 383.15 388.15 393.15 398.15 403.15 408.15 413.15 418.15 423.15 428.15 433.15 438.15 443.15 448.15 453.15

0.10 0.15 0.20 0.30 0.40 0.54 0.74 0.97 1.30 1.66 2.17 2.76 3.52 4.40 5.52 6.79 8.43 10.20 12.31 15.00 18.00 21.27 25.33 30.01 34.95 40.57 47.43 55.14 63.39 72.37 83.40

0.09 0.15 0.21 0.31 0.43 0.63 0.86 1.15 1.55 2.11 2.72 3.53 4.51 5.72 7.22 9.11 11.18 13.68 16.64 20.17 24.22 28.63 34.22 40.19 47.32 56.31 65.02 74.02 84.98 97.93

nD u/m·s−1 s/mN·m−1 h/mPa·s p/kPa e nD u/m·s−1 s/mN·m−1 h/mPa·s p/kPa e

5-methylfurfural


771.51 0.73780 354.68395 0.94211 0.07842 344.6906 12.23527 −53.31163 8.33967 103.69167 0.00582

662.54 0.71555 421.02707 0.79258 0.07490 279.9662 34.36228 −39.36321 4.77683 90.97230 0.00317

nD2 + 2

Vm

149.31 190.15

117.68 152.464

RMSDr/% 0.00 0.03 0.08 0.29 0.87 0.30 0.00 0.02 0.08 0.06 0.87 0.12

⎛ ∂σ ⎞ ΔSσ = −⎜ ⎟ ⎝ ∂T ⎠ p

(8)

⎛ ∂σ ⎞ ΔHσ = σ − T ⎜ ⎟ ⎝ ∂T ⎠ p

(9)

Experimental values of surface tension have a linear dependence with temperature; for this reason, entropy of surface formation per unit surface area is constant in this range of temperature. The entropy of surface formation values at T = 313.15 K are collected in Table 7. The dynamic viscosity of the studied compounds as a function of temperature is shown in Figure 4. The values for this property decrease as temperature increases. The temperature dependence of dynamic viscosity values reveals convex shape functions, therefore, the data have been fitted using the Vogel−Fulcher−Tamman equation:22−24 η = η0 exp[B /(T − T0)]

(10)

where η0, B, and T0 are adjustable parameters, which are provided along with the corresponding relative root mean square deviations in Table 6. The static permittivities are graphically shown in Figure 4. From experimental data, the dipolar moment at T = 313.15 K has been calculated through the Onsager equation.25

of density and refractive index making use of the Lorentz− Lorenz relation. nD2 − 1

C

The calculated values of isentropic compressibility for the chemicals studied are plotted in Figure 3. The temperature dependence of surface tension of studied chemicals can be found in Figure 4. From experimental surface tension measurements, the entropy and enthalpy of surface formation per unit surface area have been calculated using the following equations:

Table 5. Parameters of the TRIDEN Equation and Relative Root Mean Square Deviations, RMSDr, between Experimental and Correlated Densities

Rm =

B

5-Methylfurfural −5.16096·10−4 1.681017 −3.63968 2532.59 −0.12338 77.36 0.084 467.5 6.205 1585.824 −3·10−1 123.046 Tetrahydrofurfuryl Alcohol −3.96282·10−4 1.568766 3.50178 2529.74 −0.10142 67.75 0.027 953.0 5.797 1246.500 −4·10−2 25.077

compressibility has been obtained from density and speed of sound experimental values, thorough Newton−Laplace equation: 1 κs = ρ ·u 2 (7)

Standard uncertainties u are u(T) = 0.01 K; u(p) = 50 Pa.

AR/kg·m−3 BR CR/K DR CT b0/MPa b1/MPa b2/MPa b3/MPa ET/K RMSDr/%

A

property

T/K

μ2 = (6)

2 2 9κTM (ε − nD)(2ε + nD) 4πNAρ ε(nD2 + 2)2

(11)

where κ is the Boltzmann constant, NA is the Avogadro constant and M is the molar mass. The values of the dipolar moment are shown in Table 7.

where Vm is the molar volume. The experimental speeds of sound are shown in Figure 3. Assuming that ultrasonic absorption is negligible, isentropic E



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Figure 1. Density, ρ, isobaric expansibility, αp, and isothermal compressibility, κT, as a function of temperature and pressure for 5-methylfurfural: ○, experimental densities; , calculated values using TRIDEN equation.

Figure 2. Density, ρ, isobaric expansibility, αp, and isothermal compressibility, κT, as a function of temperature and pressure for tetrahydrofurfuryl alcohol: ●, experimental densities; , calculated values using TRIDEN equation.

where t is temperature in Celsius degrees and A, B, and C are adjustable parameters that are gathered in Table 5. The experimental data are represented in Figure 5. From the slope of vapor pressure curves, the enthalpy of vaporization can be determined using the Clausius−Clapeyron equation, values for this property at T = 373.15 K are shown in Table 7. Finally, log P has been calculated (logarithm of 1-octanol/ water partition coefficient) making use of the AGLOPs method to know the molecular hydrophobicity which is an important characteristic in catalytic reactions.12−14 AGLOPs values found for studied compounds are given in Table 7.

Table 7. Calculated Properties at Atmospheric Pressure; Isobaric Expansibility, αp, Isothermal Compressibility, κT, Entropy of Surface Formation, ΔSσ, and Dipolar Moment, μ, at T = 313.15 K, Enthalpy of Vaporization, ΔHvap, at T = 373.15 K, and log P property/unit

5-methylfurfural


αp/kK−1 κT/TPa−1 ΔSs/mN·m−1·K−1 μ/D ΔHvap/kJ·mol−1 log P

0.9145 626.25 0.123 4.21 49.73 0.69

0.8357 590.85 0.101 2.69 49.98 −0.28

4. DISCUSSION In this section, a comparison of the thermophysical behavior of furfural compounds (furfural, 5-methylfurfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol) has been carried out.16,26 Some values for several thermophysical properties analyzed beforehand have been collected and are presented in Table S3 of the Supporting Information.16,26 To assist in the understanding of

The vapor pressure of the studied solvents has also been determined. The results have been correlated using the Antoine equation: B log p = A − (12) C+t F



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Figure 3. Refractive index, nD, molar refraction, Rm, speed of sound, u, and isentropic compressibility, κS, as a function of temperature for the studied solvents: ■, furfural; □, furfuryl alcohol; ○, 5-methylfurfural; ●, tetrahydrofurfuryl alcohol.

because of the presence of a hydroxyl and a keton group, but only tetrahydrofurfuryl alcohol is a protic solvent. Furthermore, dispersive interactions between alkyl chains also must be taken into account.16,26 The density plots as a function of pressure and temperature are very similar for the studied chemicals; the density values decrease with temperature at constant pressure due to the thermal expansion and the weakening of molecular interactions; moreover, at a fixed temperature, density increases with pressure, as expected. Density values (Figures 1 and 2), in almost all the ranges of temperatures and pressures analyzed, follow the order furfural > furfuryl alcohol > 5-methylfurfural > tetrahydrofurfuryl alcohol.

this discussion, we have included information regarding (furfural and furfuryl alcohol) in Figures 3 to 5. The analysis of a complete set of thermophysical properties such as volumetric spectroscopic, superficial, or transport properties is a common way to study the molecular behavior of chemical compounds regardless of their molecular characteristics.26−29 To get as much information as possible, it is important to revise basic information about molecular behavior of the studied compounds as a starting point for the analysis. In the case of 5-methylfurfural and tetrahydrofurfuryl alcohol, we can justify the experimental behavior taking into account energetic and structural phenomena at the molecular level. As for the rest of the furfural compounds, they are polar molecules G



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Figure 4. Surface tension, σ, dynamic viscosity, η, and static permitivitty, ε, as a function of temperature for the studied solvents: ■, furfural; □, furfuryl alcohol; ○, 5-methylfurfural; ●, tetrahydrofurfuryl alcohol.

knowledge about molecular structure of the studied chemicals, isentropic compressibility has been obtained. The results indicate that tetrahydrofurfuryl alcohol shows the highest isentropic compressibility value followed by 5-methylfurfural, furfuryl alcohol, and furfural. The refractive index also decreases with temperature and it reveals that the highest value is obtained for 5-methylfurfural (Figure 3). The trend is as follows: 5-methylfurfural > furfural > furfuryl alcohol > tetrahydrofurfuryl alcohol. Again, to get further information about the structural behavior, the molar refraction, Rm, of the studied compounds has been obtained from the experimental data of density and refractive index (Figure 3). In all cases this property increases with temperature. Molar refraction is considered the hard core volume of a mole of molecules; therefore, free molar volume (unoccupied part of the molar volume)17 can be derived easily. The free volume is strongly related to properties such as solute solvation or gas solubility.30 The results obtained for molar refraction indicate that the chemical that presents highest value are 5-methylfurfural followed by tetrahydrofurfuryl alcohol, furfural, and furfuryl alcohol. The calculated free volume for the chemicals studied is 57.88 cm3·mol−1, 69.03 cm3·mol−1, 61.98 cm 3 ·mol −1 , and 71.09 cm 3 ·mol −1 for furfural, 5-methylfurfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol, respectively. Moreover, calculated free volume associated to molar refraction shows the ratio free volume/molar volume at T = 298.15 K is around 0.70 for all furfural chemicals.26 Taking into account the volumetric properties, we can get some interesting information regarding to the molecular structure of the chemicals; considering that the isoentropic compressibility values for the compounds derived from furfural are quite similar, we have to pay specific attention to the free volume/molar volume relation, which was found to be lower for furfural and 5-methylfurfural. On the other hand, the tendency can also be explained by taking into account isothermal compressibility results. Low values for this property indicate good packing of the molecules; the values for this property for furfural are smaller than for 5-methylfurfural. The same trend is followed by the alcohols; values for furfuryl alcohol are smaller than tetrahydrofurfuryl alcohol.

Figure 5. Logarithm of pressures, p, as a function of the inverse of the temperature for the studied solvents: ■, furfural; □, furfuryl alcohol; ○, 5-methylfurfural; ●, tetrahydrofurfuryl alcohol.

The calculated isobaric expansibilities and isothermal compressibilities (Figures 1 and 2) for all the studied compounds decrease with pressure when temperature is constant following the general behavior of liquids. On the other hand, the αp and κT values increase with temperature when pressure is constant. For isobaric expansibilities, 5-methylfurfural shows higher values followed by furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol. However, for isothermal compressibility the trend changes, the values being higher for tetrahydrofurfuryl alcohol, followed by furfural, 5-methylfurfural, and furfuryl alcohol. In the case of the speed of sound behavior related to temperature (Figure 3), values for the studied chemicals decrease with temperature, as expected. The tetrahydrofurfuryl alcohol presents the highest value for this property, followed by 5-methylfurfural, furfuryl alcohol, and furfural.26 To get further H



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The logarithm of 1-octanol/water partition coefficient, log P, has been calculated making use of the AGLOPs method to know the molecular hydrophobicity, which is an important characteristic in catalytic reactions and environmental processes.12−14 Knowledge of this property gives an idea of how the chemical will behave in the environment. Furthermore, this parameter and the solubility of the substance are directly related. Water solubility will also affect the potential routes of exposure to the substance, its bioavailability and its potential for biodegradation.34 AGLOPS values found for 5-methylfurfural and tetrahydrofurfuryl alcohol is log P = 0.69 and −0.28, respectively; these values are slightly different from furfural with P = 0.43 and furfuryl alcohol at P = 0.25.26

This experimental data corroborate the relation found between values of the isothermal compressibility and the extent of hydrogen bonds that can be established between molecules of fluids.31 As expected, surface tension of 5-methylfurfural and tetrahydrofurfuryl alcohol decreases linearly as the temperature increases (Figure 4). Surface tension is a property caused by the cohesive forces between liquid molecules. The bulk of liquid molecules of a pure compound are completely surrounded by other molecules so that the forces of attraction between adjacent molecules are equal in all the directions. On the contrary, the molecules of the surface of the liquid at the liquid/gas interface present unbalanced forces, resulting in an inward attraction. As it is well-known the surface tension is the force per unit length along the surface film of a liquid in the liquid/gas boundary which causes the film to behave like an elastic sheet. Because of surface tension, the molecules in a liquid/gas interface are in tension and tend to contract in a minimum surface area.32 This property shows the extent of the intermolecular interactions in the bulk and therefore, it is a consequence of the structure of the molecules on the surface. In this case the higher surface tension values are presented by furfural followed by 5-methylfurfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol. The static permittivity of 5-methylfurfural and tetrahydrofurfuryl alcohol decreases with temperature, as furfural and furfuryl alcohol do (Figure 4). The highest static permittivity value is shown by furfural, followed by 5-methylfurfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol.26 Furthermore, from experimental data, dipolar moment has been calculated. The dipolar moment of 5-methylfurfural and tetrahydrofurfuryl alcohol at T = 313.15 K is 4.21 D and 2.69 D, respectively. These two properties, static permittivity and dipolar moment, can explain the values obtained for other properties that reflect mainly intermolecular interactions (surface tension for instance). This means that despite the fact that alcohols are protic solvents that can form hydrogen bonds between their molecules, furfural and 5-methylfurfural show higher values for static permittivity that must make polar interactions of these compounds quite important. Regarding the transport properties, the dynamic viscosity values for studied chemicals decrease with temperature (Figure 4). Furthermore, dynamic viscosity increases when the alkyl chain does. It is noticeable that the dynamic viscosity for tetrahydrofurfuryl alcohol and furfuryl alcohol is much higher than for furfural and 5-methylfurfural.26 The stronger the intermolecular forcers are, the higher the viscosity is. However, it is important to highlight that viscosity does not only depend on the interactions, but it also mimics the internal organization of the molecules and their ability to flow.33 Values for viscosity increase if the molecules entangle with one another. In this case, the effect of intermolecular interactions seems to be predominant, as the values obtained for dynamic viscosity follow the same trend as surface tension or polar character. Another important property that has to be studied in order for solvents to fall within the green denomination is vapor pressure because it determines the volatile organic compounds (VOCs) character. The studied chemicals present low vapor pressures26 at T = 383.15 K. They followed the sequence: 5-methylfurfural < tetrahydrofurfuryl alcohol < furfuryl alcohol < furfural (Figure 5). These results indicate that the vapor pressures for the furfural family are lower than some traditional solvents such as chloroform with values of 392.403 kPa, dichloromethane with 733.011 kPa, benzene with 237.806 kPa, or toluene with 91.289 kPa in toluene; therefore, they are expected to have a lower VOC character.

5. CONCLUSIONS New data about the physicochemical behavior of 5-methylfurfural and tetrahydrofurfuryl alcohol (density, speed of sound, refractive index, surface tension, static permittivity, and dynamic viscosity) have been measured as well as pρT behavior, while the vapor pressure has also been determined. All these values obtained for 5-methylfurfural and tetrahydrofurfuryl alcohol have been compared with other chemicals of the furfural family such as furfural and furfuryl alcohol which have been reported before. The results give information about several important characteristics of these compounds and their potential applications as green chemicals. For instance, tetrahydrofurfuryl alcohol shows lower values of density, surface tension, and dipole moment than the rest of chemicals within the family, while values of properties such as speed of sound and viscosity are higher. The study concludes that molar free volume for this chemical is the highest within the family. However, the free volume/molar volume ratio is smaller for 5-methylfurfural. Regarding some environmental characteristics, the results indicate that the vapor pressures for the studied chemicals are much lower than that for some traditional solvents; therefore, these chemicals should have a lower VOC character.

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ASSOCIATED CONTENT

* Supporting Information S

Additional tables as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 34 976 060100. Notes

The authors declare no competing financial interest. Funding

Research groups GIMACES (Grupo Consolidado de Investigación Aplicada, E02) and PLATON (Grupo Consolidado, E54), acknowledge financial support by Gobierno de Aragón and Fondo Social Europeo “Construyendo Europa desde Aragón”.

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Thermophysical Properties of Furfural Compounds - ACS Publications

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