Round 1

Reviewer 1 Report

The manuscript reports comprehensive investigation into the thermodynamics of promising organic hydrogen storage materials. The manuscript is well written, the data are extensive, and the underlying correlations that the authors pointed out are more than sufficient to support their conclusions. I have only a few minor comments.
questions: the manuscript reviews the LOHC and their hydration/dehydration energetics from the enthalpic point of view. What about the entropy of this process? Is it about the same for all LOHC concerned, or does it play an important role in the comparative dehydration efficiency of LOHC? Have there been any studies into the entropic contribution? Were they extensive? It may be beneficial to add a few words in this regard to introduction or conclusions.
abstract: from 'experimental methods such as combustion calorimetry and transpiration based measurement of enthalpy of vaporisation measurements have been combined with quantum-chemical approaches to obtain a validated, consistent data set' it could be (mis)understood that the manuscript presents new combustion calorimetry data, while in fact the said data were taken from the literature, mostly from the authors' previous works, in contrast with the transpiration measurements and calculation results, which are original. I suggest clarifying these things in the abstract.
Fig 1: there's a gray background behind the reaction arrows
introduction: 'riled' is a typo (relied?)
supplementary: 'volume...was readied from...sensor'
supplementary: what is meant by 'three samples were taken during the sample flashing at'?
supplementary, transpiration method: what thermostat was used to maintain the saturator temperature? How was the standard uncertainty of 0.1 K determined?
results: what is 'were estimated to account with 30 % to the total adjustment'? to be within 30% of the adjustment value?

Author Response

please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Review

Vapor pressures of several liquid/solid iso-propyl and tert-butyl substituted benzenes have been measured at different temperatures, using three different procedures, in this work. From the obtained values, sublimation/vaporization enthalpies and entropies could be calculated and compared to reported values at a reference selected temperature. Fusion enthalpies have also been calculated for solid iso-propyl/tert-butyl substituted benzenes. The obtained vaporization enthalpies were correlated with three related parameters (Kovats retention indices, solution enthalpies, and solvent accessible surfaces). Formation enthalpies of the interest molecules were re-evaluated as in improvement of previous measurements.

The aim of the work was to calculate the hydrogenation/dehydrogenation reaction energy for a new set of branched benzenes to carefully analyzed the effect of branching and it has been achieved after huge amount of experimental work and data work out.

Some corrections (the original text is given before the arrow, whereas the correction proposed is placed after the arrow) and comments are provided below:

1.      Abstract :

For this purpose, experimental methods such as combustion calorimetry and transpiration based measurement of enthalpy of vaporisation measurements have been combined with quantum-chemical approaches to obtain a validated, consistent data set.

 à For this purpose, experimental methods such as combustion calorimetry and transpiration based measurement of enthalpy of vaporisation measurements have been combined with quantum-chemical approaches to obtain a validated, consistent data set.

2.      The standard molar reaction enthalpies, ∆r?m o , are important fundamentals for process optimization. They are calculated according to the Hess´s Law, using the liquid-phase standard molar enthalpies of formation, ∆f?m o (liq), of the reactants. The enthalpies of the hydrogenation/dehydrogenation reactions are desired to be as low as possible to decrease energy demand of hydrogen release [5]

 à The standard molar reaction enthalpies, ∆r?m o , are important fundamentals for process optimization. They are calculated according to the Hess´s Law, using the liquid-phase standard molar enthalpies of formation, ∆f?m o (liq), of the reactants and products. The enthalpies of the hydrogenation/dehydrogenation reactions are desired to be as low as possible to decrease energy demand of hydrogen release (without compromising the stable interaction of hydrogen with the organic carrier) [5].

3.      Section “3.2. Consistency of phase transitions enthalpies of alkylbenzenes”

To provide more confidence, weighted average values of of ∆cr l ?m o (Tfus) was calculated using experimental uncertainty as a weighting factor.

à To provide more confidence, weighted average values of of ∆cr l ?m o (Tfus) was calculated using experimental uncertainty as a weighting factor.

4.      Sections “3.3.1. Empirical correlations: vaporisation enthalpies versus Kovats retention indices » and “3.3.2. Empirical correlations: vaporisation enthalpies versus solution enthalpies. »

Absortion/desorption involve the interaction between two different species (the benzene derivative and the stationary phase) whereas, vaporization only involves interactions among the benzene derivative itself. I had doubts about the correctness of correlating Jx-values and vaporisation enthalpies. However, molecule retention on the stationary phase involve the breakage of intermolecular interactions among molecules. This also happens for the vaporization of the liquid, so it makes sense to correlate Jx-values and vaporisation enthalpies. Besides, authors rely on reference [11].

A similar reasoning could be done for the correlation between vaporization and solution enthalpies.

Authors could add some reasoning to help readers an easier understanding of these two correlations.

5.      Section “3.3.5. Quantum chemistry: theoretical enthalpies of formation of iso-propyl- and tertbutyl-benzenes”

It was found, that the enthalpies of formation calculated by AT are slightly but systematically more negative for both method.

à It was found, that the enthalpies of formation calculated by AT are slightly but systematically more negative for both methods.

6.      Section “3.3.6. Liquid-phase enthalpies of formation and thermodynamic analysis of the hydrogenation/dehydrogenation of the alkyl-benzenes based LOHC systems”

Authors mention (1)-(4) reactions in the introduction section, but they are not easy to find.

Author Response

please see attachment

Author Response File: Author Response.pdf