Thermophysical Studies of Dibenzyltoluene and Its Partially and Fully

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Thermophysical Studies of Dibenzyltoluene and Its Partially and Fully Hydrogenated Derivatives Rabya Aslam,*,†,‡ Muhammad Hashim Khan,† Muhammad Ishaq,† and Karsten Müller‡ †

Institute of Separation Science and Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg, Bavaria, Germany 91058 Institute of Chemical Engineering & Technology, University of the Punjab, 54590 Lahore, Pakistan

‡

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S Supporting Information *

ABSTRACT: Liquid organic hydrogen carriers (LOHC) are an interesting and promising option for the storage and transport of hydrogen with reasonable energy density via a reversible hydrogenation reaction. The commercially available heat transfer oil dibenzyltoluene emerged as a potential carrier for the LOHC technology due to its availability, thermal stability, and reasonable hydrogen storage capacity. In this work, thermophysical property data (viscosity, density, surface tension, and refractive index) are measured for dibenzyltoluene and its partially and fully hydrogenated derivatives, namely, hexahydro-dibenzyltoluene, dodecahydro-dibenzyltoluene, and octadecahydro-dibenzyltoluene over a wide range of temperatures. Correlations for the temperature dependence are proposed for the measured properties. Moreover, the excess molar volume is reported within the temperature range from 293.15 K to 358.15 K for the binary mixtures of dibenzyltoluene (fully dehydrogenated form) and octadecahydro-dibenzyltoluene (fully hydrogenated form). Positive values for the excess molar volumes are observed over the entire concentration range.

1. INTRODUCTION Liquid organic hydrogen carriers (LOHCs) represent a very interesting option for the storage and transportation of hydrogen at ambient conditions.1 This concept is based on the reversible cycle of hydrogenation of LOHC compounds (usually an aromatic2) where the unsaturated LOHC (hydrogen-lean compound) can chemically store reasonable amounts of hydrogen (4−7.5 mass %) in the form of the saturated LOHC (hydrogen-rich compound).1,3−8 The fully saturated LOHC can be stored and transported at ambient conditions.5 The advantage of storage and transportation at ambient conditions makes LOHC a very favorable choice in comparison to other hydrogen storage technologies.1,5 Dibenzyltoluene (H0-DBT; often sold under the brand name Marlotherm SH), a commercially employed heat transfer oil, can be a potential candidate for the LOHC technology due to its low cost, nontoxic nature,9 thermal stability, reasonable hydrogen storage capacity (6.2 mass %), and safe handling.3 Each H0-DBT molecule can store 9 molecules of hydrogen in the form of octadecahydro-dibenzyltoluene (H18-DBT). The hydrogenation reaction is performed at temperatures around 180−230 °C and pressures around 20−50 bar.3 A simplified reversible reaction for dibenzyltoluene is shown in Figure 1. Dibenzyltoluene is not a single compound but can be found as a mixture of its structural isomers (GC chromatogram of dibenzyltoluene is provided in the Supporting Information, Figure S1 and Table S1). The (de)hydrogenation reaction proceeds via a series of steps, and stable intermediates are formed during the reaction. Additionally, the number of species is further increased by fact that H0-DBT is not a single © XXXX American Chemical Society

compound. More than 24 stable intermediates are observed in partially hydrogenated reaction mixtures. These compounds can be further classified as a function of degree of hydrogenation into four major classes, namely, dibenzyltoluene (H0-DBT), hexahydro-dibenzyltoluene (H6-DBT), dodecahydro-dibenzyltoluene (H12-DBT), and octadecahydro-dibenzyltoluene (H18-DBT). Each fraction represents the isomeric mixture of same degree of hydrogenation (i.e., none, one, two or all three rings hydrogenated). To develop a LOHC technology based on the dibenzyltoluene system, reliable thermophysical property data are required as these play a vital role in understanding, simulating and designing of the chemical process.10,11 Recently, some of the physical property data for fully dehydrogenated and hydrogenated fractions (H0-DBT and H18-DBT) have been published by our group.12 However, almost no data are available for its partially hydrogenated fractions, i.e., H6-DBT and H12-DBT. In this work, data for density, viscosity, surface tension, and refractive index are reported over a wide range of temperature for H0-DBT, H6-DBT, H12-DBT, and H18-DBT. Moreover, data for the excess molar volume are reported for the binary mixtures of H0-DBT and H18-DBT over the entire concentration range within the temperature range from 293.15 to 358.15 K. Received: July 26, 2018 Accepted: November 23, 2018

A

DOI: 10.1021/acs.jced.8b00652 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Reversible cycle of hydrogen storage using dibenzyltoluene

Table 1. Purity Data for Chemicals Used in This Work chemicals

chemical formula

ξdoh (%)a

purity (mass %)

analysis method

CAS no.

dibenzyltoulene (H0-DBT) hexahydro-dibenzyltoluene (H6-DBT) dodecahydro-dibenzyltoluene (H12-DBT) octadecahydro-dibenzyltoluene (H18-DBT)

C21H20 C21H26 C21H32 C21H38

0 33 67 100

>98 >98 >99 >99

GC-MSb GC-MSb GC-MSb 1 H and 13C NMRc GC-MSb

26898-17-9 − − −

a

Degree of hydrogenation. bGas chromatography-mass spectrometry. cNuclear magnetic resonance spectroscopy.

2. MATERIALS AND METHODS

ij ∂ρ y jj ΔT zzz + (σρ)2 + (σm)2 T ∂ k { 2

u(ρ) =

2.1. Materials. Dibenzyltoluene was obtained from Sasol with a purity >98%. The partially hydrogenated reaction mixture was synthesized at the Institute of Chemical Reaction Engineering, FAU Erlangen-Nürnberg, Germany, by catalytic hydrogenation of dibenzyltoluene as described in previous work.13 DBT-derivatives (H6-DBT, H12-DBT, and H18DBT) were separated from the reaction mixture using semipreparative scale reversed phase high-pressure liquid chromatography. The details are reported elsewhere.14 The purity of the chemicals was determined using GC-MS and NMR analysis. Details are listed in Table 1. 2.2. Methods. 2.2.1. Density Measurements. The densities of the pure fractions (H0-DBT, H6-DBT, H12DBT, and H18-DBT) and binary mixtures of H0-DBT and H18-DBT were measured using a vibrating tube density meter (Anton Paar DMA 5000) within a temperature range from 288.15 to 358.15 K. The density meter was initially calibrated with two-point calibration of air and double distilled degasified water at 293.15 K. Due to the high viscosity of the DBTfractions, samples were degasified for 1 h in a sonicator (BENSON 1501) just before measurement. Binary solutions of H0-DBT and H18-DBT were prepared from degasified samples using a balance (Mettler Toledo) with a precision of ±0.10 mg. Around 1 mL of substance was filled into the utube, and the density was measured three times at a fixed temperature. Every measurement was then repeated three times (with a new injection) so that in total at least 9 measurements were performed for each temperature to check the reproducibility. At atmospheric pressure, the overall uncertainty in density measurement was determined using eq 1 by taking into account the uncertainty in temperature (as provided by manufacturer), density measurement (as provided by the manufacturer), and standard deviation of the measurement.

(1)

where u(ρ) is the uncertainty in the density measurement at atmospheric pressure, ΔT is the uncertainty in temperature measurement (0.01 K), σρ is standard deviation calculated from three measurements, and σm is the uncertainty in the measurement provided by the manufacturer (1 × 10−5 g cm−3). 2.2.2. Viscosity Measurement. The dynamic viscosity was measured using a rotational rheometer (Anton Paar, MCR 102) equipped with a coaxial double gap stainless steel cup (DG26.7/T200/SS) and built in Peltier thermostat. Temperature was controlled with an accuracy of ±0.1 K. The reliability of measurement was checked by comparing viscosity data for water and toluene at 293.15 K. The measured data were found consistent in comparison of the literature value within ±1.0% deviation (comparison data of measured and literature values are provided in the Supporting Information, Table S2). For the DBT system, each measurement was repeated at least three times in series for each temperature value. Moreover, each measurement was repeated for three samples (with new injection) to check reproducibility. In total, at least 9 measurements were taken for each data point, and standard deviation was calculated using these 9 measurements at a single temperature value. 2.2.3. Refractive Index Measurement. The refractive index for the DBT-fractions was measured using a digital refractometer (high precision-DT6100, Krü ss Optronics, Germany) within the temperature range from 293.15 to 343.15 K with a step size of 5 K. Temperature was controlled with a built in Peltier thermostat with an accuracy of ±0.01 K. Experimental values of refractive index for water and toluene were compared with values reported in the literature and were found consistent within 0.10% deviation at 293.15 K and 298.15 K15,16 (Supporting Information, Table S3). For the dibenzyltoluene system, each measurement was repeated three B



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in the shape of the molecules. The dehydrogenated forms are polyaromatics while the hydrogenated forms are polycycloalkanes. Aromatics are planar molecules. These molecules can be packed more efficiently in a certain volume as compared to alicyclic molecules which form boat like structure due to sp3 hybridization.20 This effective packing results in higher densities for the dehydrogenated LOHCs as compared to the hydrogenated LOHCs even though the molar mass of the dehydrogenated forms is slightly lower. The density data were correlated to temperature with a linear expression as given by eq 2.

times to check reproducibility and overall standard uncertainty in refractive index measurement was found to be 0.0005. 2.2.4. Surface Tension Measurement. The surface tension data were measured using a drop volume tensiometer (DVT50, Krüss GmbH). Temperature was controlled by circulating water outside the measuring cell of the tensiometer with a RM 6B compact thermostat (Lauda, Germany) within ±0.2 K. The system was controlled with Labdisk tensiometer software (Krüss). The measurement was validated by comparing experimental values for water and toluene with data reported in the literature17−19 at 298.2 and 303.2 K (Supporting Information, Table S4). The results were in accordance with the literature data with an average deviation less than ±1.0%. For the dibenzyltoluene system, each measurement was repeated five times in series for each temperature value and the obtained surface tension data constitutes the average of these five measurements. Moreover, each measurement was repeated at least three times (with new injection of samples) to ensure reproducibility. The relative error in reproducibility was found to be less than 1%.

ρ /g·cm−3 = A + B(T /K)

where A and B are regression parameters obtained by fitting eq 2 to experimental data. The regression parameters A and B are listed in Table 4, and the comparison of experimental and calculated values is shown in Figure 2. By definition, a thermodynamic excess property is the difference between the experimental value of a property of a mixture (actual value) and the value that would be obtained if the mixture behaves ideally at the same temperature and pressure conditions. The excess molar volume (difference between actual and ideal molar volume of a mixture) is helpful in understanding the interactions between molecules within the system. The positive excess molar volume can be attributed to strong interactions between similar molecules, while a negative value represents the strong interactions between different molecules.22,23 The excess volumes (vE/cm3 mol−1) for binary mixtures of H18-DBT and H0-DBT (Table 3) were calculated from the density data of the respective mixtures using eq 3.

3. RESULTS 3.1. Density and Excess Volume Correlations. Density data for the DBT-fractions (H0-DBT, H6-DBT, H12-DBT, and H18-DBT) within the temperature range from 288.15 to 358.15 K are listed in Table 2. The data for fully hydrogenated Table 2. Density Data for Dibenzyltoluene and Its Derivatives at P = 0.1 MPa and T = 288.15−358.15 Ka density/g·cm−3 T/K

H0-DBT

H6-DBT

H12-DBT

H18-DBT

288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15

1.0480 1.0443 1.0414 1.0379 1.0343 1.0308 1.0272 1.0237 1.0202 1.0166 1.0131 1.0095 1.0060 1.0024 0.9989

1.0012 0.9977 0.9940 0.9905 0.9869 0.9835 0.9800 0.9770 0.9733 0.9700 0.9668 0.9635 0.9600 0.9569 0.9535

0.9529 0.9496 0.9463 0.9430 0.9398 0.9366 0.9334 0.9301 0.9268 0.9236 0.9203 0.9171 0.9138 0.9106 0.9073

0.9155 0.9122 0.9090 0.9058 0.9026 0.8994 0.8962 0.8930 0.8898 0.8866 0.8834 0.8802 0.8770 0.8738 0.8706

(2)

v E/cm 3·mol−1 =

x1M1 + x 2M 2 xM xM − 1 1 − 2 2 ρm ρ1 ρ2

(3)

where the subscripts 1 and 2 represent H18-DBT and H0DBT, respectively. x is composition in mole fraction, M represents the molar mass in g mol−1, ρ1and ρ2 are the densities of pure fractions, and ρm denotes the density of binary mixture at a particular temperature. The experimentally determined excess molar volumes were correlated using Redlich−Kister correlation24−26 as given by eq 4. The parameters have been fitted using the minimization of standard deviation approach.24 k

vcE/cm 3·mol−1 = x1·x 2 ∑ (aj(x1 − x 2) j ) j=0

a

The standard uncertainties u are u(T) = 0.01 K and u(ρ) = 0.0020 g· cm−3, average standard deviation = 8.1 × 10−5 g·cm−3.

(4)

vEc

where is the calculated excess molar volume and aj are the Redlich−Kister equation coefficients. Initially, the number of coefficients were decided on the basis of minimum value of standard deviation (σ(vE))24 as given by eq 5.

and dehydrogenated fractions were compared with the data reported by Müller et al.12 and was found in good agreement with the reported data with less than 0.2% deviation. The density data for a binary mixture of H18-DBT and H0DBT over the entire composition range as a function of temperature are presented in Table 3. The density of the DBTfractions decreases, as expected, with increasing temperature. Additionally, the fully dehydrogenated fraction exhibits approximately a 15% higher density than the fully hydrogenated form. Partially hydrogenated derivatives exhibit density values in between fully hydrogenated and fully dehydrogenated fractions. The higher densities of the dehydrogenated fractions can be attributed to the difference

σ(v E) =

vEe and

∑ (veE − vcE)2 n−N

(5)

vEc

where are the experimental and calculated values for excess molar volumes, respectively. n is the number of data points, and N represents the number of Redlich−Kister coefficients. Initially four coefficients (a0, a1, a2, and a3) were taken with average standard deviation of 0.0181 cm3 mol−1. The final evaluation of fitted coefficients were made by analysis of statistical significance using t-values and 95% confidence interval.21 The t-values, the ratio of the parameter value to the C



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Table 3. Density and Excess Volume Data for Binary Mixture of Octadecahydro-Dibenzyltoluene and Dibenzyltoluene within the Temperature Range of 293.15−358.15 K at 0.1 MPaa x1 − 0.000 0.117 0.293 0.480 0.685 0.884 1.000 0.000 0.117 0.293 0.480 0.685 0.884 1.000 0.000 0.117 0.293 0.480 0.685 0.884 1.000 0.000 0.117 0.293 0.480 0.685 0.884 1.000 0.000 0.117 0.293 0.480 0.685 0.884 1.000

ρ

vE −3

cm ·mol 3

g·cm

T = 293.15 1.0450 1.0255 0.9985 0.9724 0.9468 0.9242 0.9122 T = 308.15 1.0343 1.0149 0.9881 0.9623 0.9370 0.9146 0.9026 T = 323.15 1.0237 1.0044 0.9778 0.9522 0.9271 0.9049 0.8930 T = 338.15 1.0131 0.9939 0.9676 0.9421 0.9172 0.8952 0.8834 T = 353.15 1.0024 0.9835 0.9573 0.9320 0.9073 0.8855 0.8738

ρ

x1 −1

−

K 0.0000 0.2907 0.5223 0.6629 0.5679 0.2813 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3007 0.5342 0.6707 0.5440 0.2539 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3055 0.5427 0.6877 0.5596 0.2657 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3050 0.5536 0.7031 0.5714 0.2690 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3048 0.5607 0.7169 0.5796 0.2729 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

K

K

K

K

g·cm

vE −3

x1 −1

cm ·mol 3

T = 298.15 1.0414 1.0219 0.9950 0.9690 0.9436 0.9210 0.9090 T = 313.15 1.0308 1.0114 0.9847 0.9589 0.9337 0.9114 0.8994 T = 328.15 1.0202 1.0009 0.9744 0.9488 0.9238 0.9016 0.8898 T = 343.15 1.0095 0.9905 0.9641 0.9387 0.9139 0.8919 0.8802 T = 358.15 0.9989 0.9800 0.9539 0.9286 0.9040 0.8823 0.8706

−

K 0.0000 0.2908 0.5206 0.6536 0.5361 0.2524 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.2980 0.5387 0.6776 0.5508 0.2590 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3063 0.5434 0.6901 0.5658 0.2678 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

0.0000 0.3019 0.5511 0.7078 0.5759 0.2703 0.0000

0.000 0.117 0.293 0.480 0.685 0.884 1.000

K

K

K

ρ

vE −3

cm ·mol−1 3

g·cm

T = 303.15 1.0379 1.0183 0.9916 0.9657 0.9403 0.9178 0.9058 T = 318.15 1.0272 1.0079 0.9813 0.9555 0.9304 0.9081 0.8962 T = 333.15 1.0166 0.9974 0.9710 0.9454 0.9205 0.8984 0.8866 T = 348.15 1.0060 0.9870 0.9607 0.9354 0.9106 0.8887 0.8770

K 0.0000 0.3170 0.5265 0.6572 0.5303 0.2459 0.0000 K 0.0000 0.3051 0.5380 0.6867 0.5581 0.2670 0.0000 K 0.0000 0.3013 0.5486 0.6968 0.5662 0.2679 0.0000 K 0.0000 0.3034 0.5578 0.7111 0.5761 0.2720 0.0000

K 0.0000 0.3068 0.5648 0.7211 0.5841 0.2733 0.0000

x1 is the mole fraction of octadecahydro-dibenzyltoluene, the standard uncertainties u are u(T) = 0.01 K, u(ρ) = 0.0020 g·cm−3, and u(vE) = 0.005 cm3·mol−1.

a

The Redlich−Kister coefficients (a0 and a2) for a temperature range from 293.15 to 358.15 K are given in Table 5.

standard deviation, were expected to be positive. Coefficients with negative t-values were set to zero. Also, if the 95% confidence interval of any parameter included zero, that parameter was also set to zero.21 It was observed that the a1 and a3 coefficients of eq 4 have negative t-values and include zero for 95% confidence interval (statistically nonsignificant) for all temperatures. Therefore, these two coefficients were eliminated and excess molar volume was represented by eq 4a. The final regression was performed to find the values of coefficients a0 and a2 of eq 4a on the basis of minimization of standard deviation approach. The average standard deviation between experimental and calculated excess volumes with two coefficients were found to be 0.0184 cm3 mol−1, which was comparable with four coefficients equation.

vcE/cm 3·mol−1 = x1·x 2[(a0(x1 − x 2)0 + a 2(x1 − x 2)2 )] (4a)

Positive values for excess molar volumes are observed for binary mixtures of H18-DBT and H0-DBT over the entire composition range with a maximum value of 0.73 cm3·mol−1 for the equimolar binary mixture (x1 = 0.5). The excess molar volume as a function of composition is shown in Figure 3. 3.2. Viscosity Correlations. Viscosity data for the pure fractions of the dibenzyltoluene derivatives are listed in Table 6 within the temperature range from 283.15 to 423.15 K. The fully hydrogenated fraction of dibenzyltoluene (H18-DBT) was observed to be more viscous over the entire range of D



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Table 4. Regression Parameters for Density, Viscosity, and Refractive Index for Dibenzyltoluene and Its Derivatives with Statistical Parameters H0-DBT

A1 B1 Adj. R2a Q2b c ARD/ %

A B C Adj. R2a Q2b ARD/ %c

A B Adj. R2a Q2b ARD/ %c

H6-DBT

H12-DBT

Density Data Correlation (eq 2) Temperature Range = 283.15−358.15 K 1.2498 1.1965 1.1401 −0.00070 −0.00068 −0.00065 0.999 0.999 0.999 0.999 0.999 0.999 0.003 0.005 0.002 Viscosity Data Correlation (eq 6) Temperature Range = 283.15−423.15 K −275.593 −441.778 −463.825 16793.05 26446.19 29039.18 39.122 62.888 65.505 0.999 0.999 0.999 0.987 0.981 0.987 4.31 5.30 3.34 Refractive Index Correlation (eq 7) Temperature Range = 293.15−343.15 K 1.7295 1.6859 1.6435 −0.00043 −0.00042 −0.00039 0.999 0.999 0.999 0.999 0.999 0.999 0.01 0.01 0.01

Table 5. Redlich−Kister Coefficients for Binary Mixture of Octadecahydro-Dibenzyltoluene and Dibenzyltoluene within the Temperature Range of 293.15 to 358.15 Ka

H18-DBT

1.0999 −0.00064 0.999 0.999 0.007

−430.448 25938.81 61.22 0.999 0.985 4.61

T

a0

a2

σ(vE)

K

−

−

cm3·mol−1

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15

2.601 2.552 2.544 2.612 2.641 2.664 2.674 2.687 2.712 2.738 2.754 2.768 2.788 2.789

0.202 0.048 0.192 0.029 0.002 0.058 0.046 0.050 −0.032 −0.035 −0.087 −0.078 −0.098 −0.108

0.017 0.017 0.027 0.017 0.018 0.020 0.019 0.018 0.017 0.018 0.018 0.017 0.018 0.018

a

The standard uncertainties u are u(T) = 0.01 K and u(vE) = 0.005 cm3·mol−1.

1.6045 −0.00037 0.999 0.999 0.01

a

Adjusted determination coefficient. bCross validated determination coefficient. cAverage relative deviation, statistical parameters to determine the quality of fit.21

Figure 3. Excess molar volume for binary mixture of octadecahydrodibenzyltoluene and dibenzyltoluene within the temperature range from 293.15 to 358.15 K (▲ 293.15 K, Δ 308.15 K, ◊ 323.15 K, + 338.15 K, ■ 358.15 K).

Figure 2. Density of dibenzyltoluene and its derivatives and comparison with predicted values (■ H0-DBT, ▲ H6-DBT, × H12-DBT, ● H18-DBT, ···eq 2).

mixtures, intermolecular interactions between molecules might give rise to some abnormal behavior20,27 as observed in the case of H12-DBT, which exhibits approximately 5 times higher viscosity than H18-DBT at room temperature. The effect of temperature on viscosity is correlated using eq 6 as suggested by Andrade et al.28,29 B ln(μ/mPa ·s) = A + + C·ln(T /K) (6) T /K

temperature as compared to the fully dehydrogenated fraction. This may again be attributed to the planar structure of polyaromatics which could be helpful for the ease of molecular motion as compared to polycycloalkanes resulting in the lower viscosities of dehydrogenated fractions.20,27 Viscosity of H6-DBT was observed to be closer to the fully hydrogenated fraction (H18-DBT). However, H12-DBT shows unlike behavior and exhibits the highest viscosity among all fractions. Since the DBT-fractions are isomeric

where A, B, and C are the regression parameters of the Andrade equation obtained by fitting experimental data to eq 6. The parameters are given in Table 4. 3.3. Refractive Index Correlation. The experimental data for refractive indices of dibenzyltoluene and its derivatives are presented in Table 7 within the temperature range from 293.15 to 343.15 K. Analogous to density, refractive index decreases with increasing temperature for all DBT-derivatives. The trend is found consistent with the literature30,31 and can be expected E



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dehydrogenated form (H0-DBT) exhibits the highest value, and the fully hydrogenated form (H18-DBT) shows the lowest value at a given temperature. 3.4. Surface Tension Correlation. The surface tension for dibenzyltoluene system was measured within the temperature range from 293 to 346 K as described in section 2.2.4. The surface tension of all the DBT fractions was found to decrease with increasing temperature because cohesive forces between molecules decrease with temperature resulting in a decrease in surface tension. Furthermore, at a specific temperature, surface tension of dehydrogenated forms (H0DBT) at a specific temperature was found to be approximately 1.2 times higher than hydrogenated forms (H18-DBT). The experimental data (Table 8)) were correlated with temperature using the linear expression (eq 8) as suggested by Gittens et al. and Sánchez et al.18,32

Table 6. Viscosity Data for Dibenzyltoluene and Its Derivatives at 0.1 MPaa dynamic viscosity/mPa·s T/K

H0-DBT

H6-DBT

H12-DBT

H18−DBT

283.15 293.15 298.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 383.15 393.15 403.15 413.15 423.15

108.9 51.5 37.5 28.0 17.2 11.5 8.3 6.2 4.8 3.9 3.1 2.7 2.3 1.9 1.7 1.5

988.1 308.9 187.6 119.0 55.0 29.6 17.9 11.8 8.2 6.1 4.7 3.7 3.0 2.5 2.1 1.8

5277.1 1544.9 900.7 530.0 205.0 90.5 46.1 26.2 16.4 11.1 7.8 5.7 4.4 3.5 2.8 2.4

1189.5 362.1 219.7 140.0 64.9 35.0 20.9 13.7 9.5 7.0 5.3 4.2 3.4 2.8 2.4 2.0

γ = a + b·T/K

(8) −1

where γ is the surface tension in mN·m , T is the temperature in K, and a and b are the regression parameters obtained by fitting the experimental data to eq 8. These parameters with statistical parameter are listed in Table 9.

a

The standard uncertainty in temperature u(T) = 0.1 K and standard deviation in viscosity measurement is below 3%.

Table 9. Regression Parameters for Surface Tension Correlation with Temperature (eq 8)

Table 7. Refractive Index Data for Dibenzyltoluene and Its Derivatives at 0.1 MPaa

temperature range

nD/− T/K

H0-DBT

H6-DBT

H12-DBT

H18-DBT

293.15 298.15 303.15 308.15 313.15 323.15 333.15 343.15

1.6024 1.6007 1.5985 1.5964 1.5942 1.5900 1.5855 1.5809

1.5633 1.5609 1.5588 1.5566 1.5547 1.5505 1.5464 1.5421

1.5299 1.5279 1.5260 1.5241 1.5222 1.5183 1.5145 1.5104

1.4950 1.4932 1.4913 1.4895 1.4876 1.4839 1.4802 1.4763

LOHCs H0-DBT H6-DBT H12DBT H18DBT

regression parameters (eq 8)

statistical parameters

T

a

b

Adj. R2a

K

mN·m−1

K−1

−

−

%

293−346

76.124 71.131 61.781

−0.116 −0.110 −0.095

0.993 0.996 0.996

0.991 0.992 0.992

1.01 0.79 0.91

55.438

−0.077

0.999

0.992

0.45

Q2b

ARDc

a

Adjusted determination coefficient. bCross validated determination coefficient. cAverage relative deviation, statistical parameters to determine the quality of fit.21

a

The standard uncertainties u are u(T) = 0.01 K and u(nD) is 0.0005.

since refractive index is related to the density. The effect was correlated using a linear expression as given by eq 7. nD/− = A + B(T /K)

(7)

4. CONCLUSIONS In the present contribution, new physical property data (density, viscosity, surface tension, and refractive index) for a potential LOHC system dibenzyltoluene are presented for its fully dehydrogenated, partially hydrogenated, and fully hydro-

where A and B are the regression parameters obtained by fitting eq 7 to experimental data. The obtained parameters are listed in Table 4. The refractive index for DBT systems varies as a function of degree of hydrogenation. The fully

Table 8. Experimental Data of Surface Tension for Dibenzyltoluene and Its Derivatives at 0.1 MPaa H0-DBT

H6-DBT

H12-DBT

H18-DBT

T

γ±σ

T

γ±σ

T

γ±σ

T

γ±σ

K

mN·m−1

K

mN·m−1

K

mN·m−1

K

mN·m−1

± ± ± ± ± ± ±

295.0 303.4 308.1 312.8 318.9 322.4 332.1 345.3

± ± ± ± ± ± ± ±

294.6 303.4 308.1 313.1 322 332.1 345.0

± ± ± ± ± ± ±

296.4 303.4 312.5 321.9 332.1 345.5

297.0 303.4 307.9 312.8 322.4 332.3 344.8

41.63 41.07 40.53 40.08 38.74 37.65 36.23

0.18 0.14 0.15 0.16 0.14 0.13 0.17

38.70 37.65 37.15 36.81 36.04 35.68 34.58 33.12

0.11 0.13 0.15 0.11 0.13 0.14 0.12 0.11

33.83 32.95 32.52 31.85 31.13 30.35 28.95

0.13 0.11 0.12 0.13 0.14 0.11 0.12

32.66 32.03 31.36 30.64 29.93 28.87

± ± ± ± ± ±

0.18 0.14 0.16 0.14 0.14 0.18

The standard deviation σ is calculated from 15 measurements for each data point.

a

F



Article

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genated forms over a wide range of temperature (293.15 to 358.15 K). The fractions under consideration are dibenzyltoluene, hexahydro-dibenzyltoluene, dodecahydro-dibenzyltoluene, and octadecahydro-dibenzyltoluene. The experimental data for density, refractive index, and surface tension have been measured and correlated as a function of temperature using a linear expression. The viscosity data have been correlated using the Andrade equation with three parameters. Detailed statistical analysis for all the reported correlations have been performed and values of the adjusted determination coefficient, cross validated determination coefficient, average relative deviation are reported. Additionally, density and excess molar volume data for binary mixtures of dibenzyltoluene and octadecahydro-dibenzyltoluene are reported over the entire concentration range within the temperature range from 293.15 to 358.15 K. Positive values of excess volume are observed over the entire concentration range which indicates the presence of the repulsive forces between the fully hydrogenated and fully dehydrogenated molecules. This can be attributed to the partially polar and nonpolar behavior of fully dehydrogenated and fully hydrogenated fractions of dibenzyltoluene, respectively. The excess molar volume data have been correlated using the classical Redlich−Kister equation with two parameters. The coefficients are reported as a function of temperature. The obtained correlations can be used in detailed simulations, process development, and design of the hydrogen storage technology using liquid organic hydrogen carriers based on the dibenzyltoluene system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00652.

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GC analysis of dibenzyltoluene isomeric mixture (Figure S1) and its isomeric data (Table S1); and measurement method validated by comparing data with literature reported data (Tables S2−S4) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +92 (0) 99234062. Fax: +92 (0) 99231159. E-mail: [email protected]. ORCID

Rabya Aslam: 0000-0001-6505-6687 Funding

This work has been done within the framework of the Bavarian Hydrogen Center and has been funded by the state of Bavaria. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Prof. Wolfgang Arlt and Prof. Peter Wasserscheid for valuable discussions. REFERENCES

(1) Müller, K.; Arlt, W. Status and Development in Hydrogen Transport and Storage for Energy Applications. Energy Technology 2013, 1, 501−511. (2) Müller, K.; Völkl, J.; Arlt, W. Thermodynamic Evaluation of Potential Organic Hydrogen Carriers. Energy Technology 2013, 1, 20− 24. G



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Thermophysical Studies of Dibenzyltoluene and Its Partially and Fully

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