A Density Functional Theory Probe of the Hydrolysis of Heavy

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Catalysis and Kinetics

A Density Functional Theory Probe of the Hydrolysis of Heavy Hydrocarbon Structural Moieties in Supercritical Water Hassan Alasiri, and Michael T Klein Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01852 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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A Density Functional Theory Probe of the Hydrolysis of Heavy Hydrocarbon Structural Moieties in Supercritical Water Hassan Alasiria and Michael T. Kleinb, c a

b

Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, USA c

Center for Refining and Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

ABSTRACT

The thermochemistry and kinetics of hydrolysis in supercritical water were probed using density function theory (DFT). Four molecules (propane, dimethyl ether, 1,3-diphenylpropane, and dibenzyl ether) were selected for this study to compare the reactivity of molecules with and without a heteroatom on a saturated carbon. We found that the activation energy for compounds with a heteroatom attached to saturated carbon was lower than that for fully hydrocarbon systems. The fastest reaction among the four molecules was that for dibenzyl ether with water. The activation energy and pre-exponential factor of the dibenzyl ether reaction with water is rationalized in the context of experimental values.

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Introduction: The gap between the demand and supply of energy is closing which lends urgency to research to finding and developing alternative primary resources. This development of new energy resources must be sustainable, in order to avoid impacts on air pollution, the risk of supply disruption and global climate change. In this context, the upgrading of heavy oils has been the focus of intense study. A promising component of this research is the use of supercritical water (SCW) as a reaction medium.

Early results [1–3] suggest that both reaction selectivities and fouling

considerations are improved in this reaction medium. Since previous experimental data [4–10] involving the reaction of heavy hydrocarbon model compounds in SCW at varying water density have suggested that SCW acts not only as a solvent but also, in certain circumstances, a reactant, we sought a more rigorous theoretical basis for the potential benefits of reactions in SCW. Supercritical fluids have been used in many applications because of their remarkable physical properties. In the supercritical region just above the critical temperature, water transforms from a highly polar medium into a less polar medium with relatively small changes in pressure and thus density. Thus supercritical water can be a medium favorable to the intimate mixture between water and organic compounds that might otherwise be immiscible in liquid water [11]. This has motivated interest in the use of SCW as a reaction medium. In the context of the possibility of the reaction between SCW and heavy oil structural moieties, there are thousands of potential reaction sites and there is thus a need for a framework to measure, predict and organize the rate constants for these reactions [12,13]. This motivated our interest in using density functional theory (DFT) as a method to predict the rate constants for supercritical water reaction.


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Table 1 summarizes the key experimental findings that underly our interest in the reaction of heavy hydrocarbons with SCW. Shown are examples of the reaction of heavy hydrocarbon model compounds in supercritical water in terms of the reactants and products that resulted from pyrolysis (no water present) and hydrolysis. In general, both pyrolysis and hydrolysis reactions take place simultaneously, the formal products of each resulting from rates resulting from the process conditions. Pyrolysis is favored by higher reaction temperatures and is well known to proceed via a free radical mechanism. On the other hand, the hydrolysis reaction was more selective, as its occurrence depended not only on the concentration of water but also the details of the structure of the model compound.

Although the reaction mechanism is not fully

understood, it appeared to involve SN2 nucleophilic substitution at a saturated carbon with a heteroatom-containing leaving group with water acting as the nucleophile [10]. Our interest in this paper to study the transition state of the hydrolysis reaction, determine the kinetic parameters, and probe the heteroatom-related structural requirement just noted.


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Table 1: Some experimental examples of pyrolysis and hydrolysis in supercritical water (SCW). Reactant

Products Hydrolysis

Pyrolysis

Reference

[6]

[8]

XXXX

CH4

[8]

CH3OH

[9]

[14]

Our ability to exploit this hydrolysis reaction for practical purposes in enabled through the knowledge of how much energy is released or absorbed, how fast the reactions take place, and which structural factors influence the reaction rate. Density functional theory (DFT) can provide insight into these issues. Thus the specific aim of the present work is to use DFT calculations to determine, for a set of probe molecules listed in Table 2, the enthalpy and Gibbs free energy of reaction and to model the transition states of the hydrolysis reactions. Rate constants as a function of temperature can thus be obtained, which will allow estimation of the associated Arrhenius parameters. The probe molecules of Table 2 were selected to focus on the essence of the nucleophilic substitution hypothesis. Propane and dimethyl ether represent the irreducible structural increments of dibenzyl ether (DBE) and 1,3-diphenylpropane (DBP), which are more representative of the heavy hydrocarbon moieties and, also, entries in Table 1. Collectively, 4 ACS Paragon Plus Environment

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these four probe molecules provided a starting point for our analysis of the possibility of hydrolysis in the upgrading of heavy oils in SCW. Table 2: Summary table of reactants and products considered in the present analysis. Name of the system

Molecule react with water

CCC+W (propane) COC+W (dimethyl ether)

Products

(CC)

(CCC)

(COC)

(CO)

PhCPh+W (1,3-diphenylpropane)

(PhCPh)

PhOPh+W (dibenzyl ether)

(PhOPh)

(PhO)

(PhO)

(CO)

(CO)

(PhC)

(PhO)

Methodology: The DFT calculations were aimed at estimation of the rate constants as a function of temperature, the Arrhenius parameters, and the nature of the transition states of reactions in Table 2. More details about the density functional theory (DFT) method used are available [15]. All calculations were performed with the Dmol program by using material studio [16]. The equilibrium geometries of the reactants, transition state, and products were fully optimized using m-GGA and M11-L functionals level with larger and more accurate DNP+ (Double Numerical basis) basis set. The m-GGA stands for meta generalized gradient approximation [17] and M11-L is a dualrange local for meta-GGA [18].


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At the optimized geometry, the vibrational frequencies were calculated to predict the thermochemistry data and to calculate the enthalpy of reaction ∆ and Gibbs free energies of reaction ∆ from the following equations [15], ∆ () = ∑( + ) − ∑( + )

(1)

∆() = ∑( + ) − ∑( + )

(2)

where is the total electronic energy, and is correction to the enthalpy due to internal energy, and is the correction to the Gibbs free energy due to internal energy. Transition state optimization using Synchronous Transit-guided Quasi-Newton (QST2) method [19] has been used for locating the transition structures of reactions. This method uses a quadratic synchronous transit approach to get closer to the quadratic region of the transition state and then uses a quasi-Newton or eigenvector-following algorithm to complete the optimization. QST2 requires the reactants and products molecules as input. The order of the atoms must be identical within all molecule specifications. The rate constants for reactions were estimated using the conventional thermodynamic formulation of the transition state theory [15], as in Eq. 3, () =

∗ !/#

∆

(3)

where $ is the Boltzmann constant, T is temperature, ℎ is Planck’s constant, R is the gas constant, and ∆∗ is the Gibbs free energy of activation, which was computed according to Eq. 4: ∆∗ () = ∑( + )&'()*+&+,) *&(&- − ∑( + ) (4)


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The heat of reaction ∆ () was calculated as a function of temperature by using Kirchoff’s law,

∆ () = ∆ ° (2982) + 3 456 (789:;) − 56 (8?)A B

(5) where Cp is the heat capacity, T is the temperature, and T0 is the reference temperature. The heat capacities of the reactants and products are function of temperature as commonly expressed in polynomial form, 56 (C = Molecule) = ? + J + CCC. The estimated Arrhenius parameters are listed in Tables 5. For the activation energy (Ea), the large values obtained for the propane pathway (130 kcal/mol) and the 1,3-diphenylpropane pathway (107 kcal/mol) explain the slow reaction rates compared with DME (77 kcal/mol) and DBE (61 kcal/mol). For the pre-exponential factors A (s-1), different values were found corresponding to the size of molecules. The DFT-calculated values of the Arrhenius parameters for the reaction of dibenzyl ether with water are compared with the experimental values in Table 5. The calculated values are higher than the experimental values, as follows. Since the DFT calculations were run in vacuum and the experiments were conducted at water density is 387 kg mm3, the former did not include the solvent effects that accelerated the kinetics of the latter.

Since the transition state of the

hydrolysis reaction is more polar than the ground state of the reactants, increasing the solvent polarity increases the rate constant for the reaction [6,8–10,14]. In essence, the DFT calculations were for the limiting case of hydrolysis in the absence of water [21]. Our continuing research is exploring the effects of both water density and substituent effects on these reactions, which will provide more obvious information about the reactions of real systems at supercritical water conditions.


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-30 -50 -70 -90 ln(k)

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COC+W

-110

CCC+W

-130 -150

PhOPh+W

-170

PhCPh+W

-190 -210 0

0.5

1

1.5

2

2.5

3

3.5

4

1000/T (K-1)

Figure 6 Arrhenius plot for the calculated rate constants for hydrolysis. Table 5 Estimated parameters of the Arrhenius equation for the reactions in Table 2. Note: Values in brackets for PhOPh+W are experimental values from [22]

Parameter

COC+W

A (s-1)

4.25E+06

Ea/R (K)

38654

CCC+W

PhOPh+W PhCPh+W 3.35E+05 1.86E+05 8.74E+05 (1.22E+5) 30977 65601 53978 (11200)

Conclusion: DFT calculations of models of the transition states of the hydrolysis reactions of COC, CCC, PhCPh, and PhOPh revealed kinetics that are consistent with the experimentally observed trends. Specifically, the DFT results indicated that the rate of reaction followed the order of PhOPh ˃ COC ˃ PhCPh > CCC, which confirms that the molecules with oxygen attached to a saturated carbon react faster reaction than molecules without a heteroatom. For heavy oil moieties, then, the key structural features for reactions with SCW will be a saturated carbon to which is attached a heteroatom-containing leaving group. 15 ACS Paragon Plus Environment

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This work provides a basis to study hydrolysis in SCW more fully. It is anticipated that these types of predictions of kinetic parameters from DFT calculation can be used as initial estimates for process models of the reactions in SCW of actual heavy hydrocarbon kinetics that can be tuned and refined for specific systems [23–25].

Acknowledgement Michael T. Klein acknowledges collaborations with and support of colleagues via the Saudi Aramco Chair Program at KFUMP and Saudi Aramco.


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[1]

Yan T, Xu J, Wang L, Liu Y, Yang C, Fang T. A review of upgrading heavy oils with supercritical fluids. RSC Adv 2015;5:75129–40. doi:10.1039/C5RA08299D.

[2]

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[4]

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[5]

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[7]

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[8]

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[25] Andrade LA, Batista FRX, Lira TS, Barrozo MAS, Vieira LGM. Characterization and product formation during the catalytic and non-catalytic pyrolysis of the green microalgae Chlamydomonas reinhardtii. Renew Energy 2018;119:731–40. doi:10.1016/j.renene.2017.12.056.


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A Density Functional Theory Probe of the Hydrolysis of Heavy

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