Quantum chemical study on non-thermal plasma degradation tar in gasification syngas

The bond dissociation energies (BDEs) of benzene, toluene, and naphthalene which are the main components of tar in fluidized bed gasification syngas have been investigated by B3LYP method at 6-31G (d, p), 6-31G+ (d, p), 6-31G++ (d, p) and 6-311G++ (d, p) basis setsof density functional theory (DFT).The structures of benzene, toluene, naphthalene, and corresponding radicals were optimized, and their total energiesand zero-point energies (ZPE) were calculated.The frequency analysis was carried out, then, the transition state and the high order saddle were excluded, and the influence of zero point energy scaling factors was neglected.The results showed thatthe BDEs decreased with increasing computational basis sets. The calculated results obtained by B3LYP/6-31G (d, p) are much closer to the results of the documents than the other three basis sets. This indicates that the basis set of B3LYP/6-31G (d, p) is much more credible in reproducing the BDEs. The lowest electron energy values for the ring opening of benzene, toluene and naphthalene are 8.05 eV, 7.45 eV, and 7.35 eV, respectively, indicating that when the energy of high-energy electrons reaches 8.05 eV, the ring of aromatic hydrocarbon can be broken.


Introduction
Lignite is one of thesignificantfossil fuels.It exceeds 2600 billion tons worldwide, sharing approximately 24.4% of the total global coal resources [1] .Gasification is considered to be a promising choice for the clean utilization of coal due to its high efficiency and flexibility [2] .Fluidized bed gasification technology with many technical advantages such as low gasification temperature, strong adaptability to coal, sulfur fixation and fewer nitrogen oxides, is one of the main methods and the widely concerned coal gasification [3] .However, it is also facing some problems.For example,the tar produced during the gasification process is frequently condensed in the downstream tunnel.This cannot be used in the combustion equipment and willlead to a waste of some energy.It easily combines with water, ash, etc., and adheres to the inner wall of the pipeline and equipment, affecting the stable operation of the equipment due to its blockage and corrosion.These unfavorable stability and economic performances as well as their adverse environmental and health effects have catalyzed increasing interest in the purification of tar produced in gasification.At present, the removal of tar can be classified into two major approaches, i.e., physical and thermochemical methods [4] .The use of physical methods only changes the physical state of tar, which is the transfer of the problem, rather than the real removal of the tar.The thermochemical method is to make the tar of macromolecules convert into useful gas of small molecules under certain reaction conditions.The thermochemistry method can remove the tar with effect.However,the equipment for removing tar is quite complex, with a huge up-front investment.A lot of efforts in syngas purification technology have been donein the world.Unfortunately, the technology is still imperfect,least of to meet the requirements of industrial and civil syngas quality.In recent years, a very hot technology dielectric barrier discharge (DBD) has been applied to the treatment of pollutants and has obtained a positive effect.For the physicochemical properties of tar,a scheme of purifying tar in syngaswas proposed by using DBD to produce non-thermal plasma (NTP).
The DBD unit is stable and can produce a large amount of NTP which has high energy and large density.Thereby, more active particles will be stimulated to apply to the object pollutants.This technology has some strong points, such asa wide treatment range, rapid degradation rate, and good effects.But beyond that, it can also be implementedatroom pressure and temperature.Therefore, for a long time, it has been a major concern in the industrial application of VOCs, desulfurization and denitrification, sewage treatment, and material modification [5][6][7][8][9][10][11] .However, the previous research is mainly focused on the electrical parameters and reactor parameters [12][13][14] .More details on the pollutants are limited to the concentration, species of pollutants, etc. in the macro analysis [15][16][17] .Furthermore, little micro on pollutants is reported from the available literature.And the fundamental reason for the degradation of pollutants can be decided by two processes including (a) direct degradation resulting from the collision of high-energy electrons with the target pollutants and (b) reaction between pollutant molecules and oxidative active radicals which were produced by high-energy electrons.It can be seen that the energy of electrons is decisive in degradation processes.
For a particular pollutant, the energy required to degrade is constant.When the electrical parameters of the DBD remain constant, the density and energy of the high-energy electrons are also constant.So, the energy of the high-energy electrons is suitable to degrade the target contaminants.It would be a waste whenthe energy is too high and the energy is too low to open the bonds of target pollutants.Therefore, it is necessary to study the dissociation energy of some important bonds in the target pollutant.Accurate predictions of the bonds' dissociation energy also show how much energy is needed for high-energy electrons, which in turn directs the design and optimization of DBD,resulting in a more efficient and energy-saving technological tool with better application for industrial production.The composition of the tar which contains more than 10,000 kinds of organic matter is quite complex [18] .In the current work, the focus is directed on the bonds' dissociation energy of benzene, toluene, and naphthalene.The selection of three objects is justified by previous work from which we can find that the components of tar in the gasification syngas of the lignite mainly include benzene, toluene, naphthalene, and anthracene.The analysis and calculation giving the quantitative results of the energy required for degradation were put into practice by the density functional theory (DFT) of quantum chemicals.It is announced how much energy is needed to degrade them under the action of non-thermal plasma.

Computational details
DFT is used to calculate the dissociation energy of the bond.It uses the functional (function as a variable) to solve the Schrodinger Equation.DFT methods compute electron correlation via the general function of the electron density, so it achieves significantly greater accuracy than Hartree-Fock (HF) theory at the acceptable computational [19,20] .A lot of efforts considering the DFT method have been done on the bond dissociation energies (BDEs) and molecular energies.Earlier, McMillen et al. reviewed the dissociation energies of various C-H, which laid a good foundation for future research [21] .Yaoet al. calculated BDEs of C-C bonds in a series of alkane and alkyl benzene by HF and DFT [22] .It was shown that the HF method provided results with high errors and DFT methods performed well, showing its high potential in BDE calculations.Jursic believed that for polar small molecules, the calculated energy is highly consistent with the experimental values by the hybrid B3LYP DFT method. [23,24].In another study, the experimental valuesare all greater thanthe calculated BDE values for some chemical systems by the B3LYP method.In the case of naphthalene, the dissociation energy of the C-H bond with two different positions has been determined experimentally by Reed and Kass [25] .Good fitness with B3LYP/6-31G+ (d) computations is found.
In this paper, the BDEs of benzene, toluene, and naphthalene have been investigated by the B3LYP method at6-311G++ (d, p), 6-31G++ (d, p), 6-31G+ (d, p)and6-31G (d, p)basis sets [26,27] .The constitutions of benzene, toluene, naphthalene, and corresponding radicals were optimized, and their single-point energy and zero-point energy (ZPE) were calculated.At the same time, frequency analysis is carried out to eliminate the transition state and high-order saddle in the optimized structure.We select GaussView5.0 and Gaussian09W as the computational tool.Taking into account the accuracy requirements of the study, this paper does not consider ZPE scaling factors.According to the definition of the BDE and the method shown in the literature, the following dissociation energy formula is adopted.
Where E is the total energy (or single point energy, SPE) of molecules or radicals, ZPE is the zero point energy of molecules or radicals.

Results and discussion
In this paper, benzene, toluene, and naphthalene are selected as the modeling objects.The BDEs of C-C, C-H, and delocalized π bond in the benzene ringarecalculated.When the energy of the high-energy electrons produced in the dielectric barrier discharge reactor is greater than the dissociation energy of a particular bond of an organic substance, the bond will be interrupted so that the organic matter can be dissociated.The units of total energy and ZPE ofmolecules and radicals in the following tables are Hartree, unless otherwise specified, the unit of BDE is kcal/mol.The conversion relation between different units is given below: Hartree=627.51 kcal/mol=27.21eV (2)

BDEs of benzene
Benzene is the simplest aromatic hydrocarbon in tar.Benzene molecule, 12 atoms in the same plane, is a planar molecule, in which 6 carbons and 6 hydrogens are equal.The atoms in the structure of benzene are arranged with a high degree of symmetry, shown in Figure 1.So, only one of the C-H and C-C in the benzene ring was calculated.The results of quantum chemical calculationsarelisted in Table 1.It is evident from the table, that with the basis set increases, thecalculation value of bond dissociation energy is gradually reduced.It is notable from the table, that the increase of the basis set from 6-31G (d, p) basis set to 6-311++G (d, p) basis set may result in the BDE decrease from 110.18 kcal/mol to108.89kcal/mol.The calculated results of the 6-31G (d, p) basis set are the most approach to the experimental values 113.07±0.72 kcal/mol, with the absolute error of 2.88±0.72kcal/mol.113.07±0.72 [36] When the high-energy electrons produced by the DBD reactor bombard one of the delocalized π bonds of benzene, the corresponding radical-like chain will be produced, labeled as L radical.According to Table 2, BDE follows the law: the larger the basis set, the smaller BDE.The BDE of the delocalized π bond calculated at 6-31 G (d, p) is 185.61 kcal/mol, which is 75.44 kcal/mol higher than that of C-H at the corresponding basis set.This also reflects the stability of the benzene structure.This means that the energy of the DBD reactor must be high enough to utterly destroy the structure of benzene.

BDEs of toluene
Toluene has a relatively high content in the tar component of gasification syngas, which is about 12.49%.Moreover, it has typical representativeness in chemical structure.The methyl group and the benzene ring include both the ring structure and the chain-like structure, shown in Figure 2.So toluene will be selected as a computational model.Table 3 shows the comparisons between the computedresults and the experimental data [23,36] .Interestingly, it is found that the computed results of the toluene BDE with 6-31G (d, p) basis set are the closest to the experimental values.And the absolute errors of C1-C7 and C7-H are 4.04±2.01kcal/mol, 2.64 kcal/mol, respectively.101.79±2.01 [23]9.87 [36] Toluenehasacertain symmetry.In the case that the chemical bond is bombarded by high-energy electrons, the delocalized π bond on the benzene ring may break in three positions, namely C1-C2, C2-C3 and C3-C4labeled as π1, π2 and π3 bond.From Table 4, it can beseenthat the average BDE of thedelocalizedπ bondlocated on the benzene ring is 176.96kcal/mol at 6-31G (d, p), which is slightly less than the isolated benzene.The dissociation energy of each bond can be plotted in Figure 3, which indicates that the maximum BDE of toluene is from the delocalized π bond, and the π1 bond which is the nearest to methyl is slightly smaller than that of the other two.This is due to the presence of methyl affects the distribution of the electron cloud in the benzene ring, and thus has a certain influence on the original symmetry structure of the benzene ring.Figure 4 shows the relationship between BDEs and bond lengths of toluene's delocalized π bond.The presence of methyl not only affects the values of the energy but also affects the length of the bond, which corresponds to each other.The larger the BDE value, the shorter the length of the bond.In addition to the delocalized π bond in the toluene, the C-H bond dissociation energy is larger and the numerical size is close to the C-C bond dissociation ofmethyl and benzene connected to second, C-H on methyl bond dissociation energy minimum.The unit of BDE (kcal/mol) will be converted to a unit of electron energy (eV).Note that it is comparatively simple to destroy the chemical structure of toluene molecules, only the DBD reactor can inject high-energy electrons whose energies reach 3.78eV.3.78 eV is the dissociation energy of C-H on methyl of toluene.However, to utterly destroy the cyclic structure of the benzene molecules, the high-energy electrons generated by DBD must be greater than or equal to 7.45eV, which serves as our reference minimum value to open the ring of toluene (dashed line in Figure 3).

BDEs of naphthalene
Naphthalene is one of the main components of tar produced in gasification syngas.The proportion of naphthalene in tar of syngas is about 7.94%.Naphthalene is the simplest polycyclic aromatic hydrocarbons (PAHs) composed of two benzene rings and of high aromaticity, it is difficult to open the ring because of the constitutionally stable structure, shown in Figure 5.At present, there is little research on the dissociation energy of naphthalene.The BDEs of αC-H and βC-H in naphthalene were evaluated by Reed et al. with the help of a Fourier transform mass spectrometer (FTMS) [27] .Furthermore, they calculated the BDEs of αC-H and βC-H at the B3LYP/6-31G+ (d) level of theory.The BDEs of αC-H is110.70 kcal/moland βC-H's is 110.60 kcal/mol.The above values are in goodconsistencywith our calculating results bythe B3LYP/6-31G (d, p) method (110.45 kcal/mol for αC-Hand 110.33 kcal/mol for βC-H, see Table 5).112.2±1.3 [27] 111.9±1.4 [27] In the topological structure of naphthalene, two types of bonds were calculated.One of the bondsis between carbonatoms both bearing hydrogens (labeled as π1 bond); the other bond is between carbon atomswhere only one of the carbons bears a hydrogen (labeled as π2 bond).At the 6-31G (d, p) basis set, the dissociation energies of π1 and π2 bonds were calculated to be 169.60kcal/moland 181.08 kcal/mol.Their average value of 175.34 kcal/mol was less than that of toluene by 1.63 kcal/mol.
It can be intuitively seen that the BDEs of each bond in naphthalene are shown in Figure 6.The dissociation energy of αC-H is not much different from that of βC-H.To destroy the structure of naphthalene, the energy of high-energy electrons produced by the DBD reactor must reach 4.79 eV.
To open the ring of naphthalene, it is necessary to further improve the energy of high-energy electrons to 7.35 eV.It serves as our reference minimum value to open the ring of naphthalene, and the dashed line is depicted in Figure 6.

Conclusions
Using the B3LYP method in density functional theory, the dissociation energies of benzene, toluene, and naphthalene are calculated at the four basis sets of6-31G (d, p), 6-31G+ (d, p), 6-31G++ (d, p), 6-311G++ (d, p), which also compared with the experimental values given in some works of literature.As is expected, this work contributes to the optimization and application of DBD technology.Based on the data acquired from our calculations, the following conclusions can be drawn: (1) Four basis set levels were used in this paper, the BDEs decrease with increasing computational basis set.Comparing the four basis sets, the one calculated by B3LYP/6-31G (d, p) is much closerto the results of the documents.That is to say, it is more credible.
( (3) The attempt to destroy the molecular structure of benzene, toluene, and naphthalene has been made relatively simple.However, if their ring structures are to be bombarded into chains or smaller molecules, it is necessary to adjust the reactor so that the generated electron energy is higher.The lowest electron energy values for the ring opening of benzene, toluene, and naphthalene are 8.05eV, 7.45eV, and 7.35eV.Simply put, when the energy of high-energy electrons reaches 8.05eV, the ring of theirs can be opened.
(4) When a lot of target pollutants are treated, the components will be calculated and analyzed one by one, so as to derive the energy of the components that are the most difficult to degrade.According to the energy of high-energy electrons, the input parameters of DBD can be adjusted.In the case of sufficient energy and density of high-energy electrons, all the target pollutants can be degraded.
) At the 6-31G (d, p) basis set, the BDE of the delocalized π bond of benzene is 185.61kcal/mol, and the dissociation energy of C-H is smaller.The average BDE of the delocalized π bond in the toluene is 176.96kcal/mol, and the BDEs of other bonds follow the laws: BDE[C(2-6)-H]>BDE(C1-C7)>BDE(C7-H).The average dissociation energy of C-C in naphthalene is 175.34kcal/mol, and the dissociation energies of αC-H and βC-H are 110.45kcal/moland 110.33 kcal/mol, respectively.

Table 1 .
Calculation of C-H dissociation energy of benzene.
. Calculation of dissociation energy of delocalized π bond of benzene.

Table 3 .
Calculation of C-H and C-C dissociation energy of toluene.

Table 4
Calculation of dissociation energy of delocalized π bond of toluene.

Table 5
Calculation of BDEs of naphthalene.