Insights into the Effect of a Microwave Field on the Properties of Modified γ-Alumina: A DFT Study

Abstract

γ-Alumina is often used as a support for hydrodesulfurization catalysts due to its excellent performance. During the catalytic reaction, the strong surface acidity of γ-alumina can induce a strong interaction between the active phase and the support. The reaction activity of the catalyst can be affected by changing the present mode of the active phase on the surface of the support. The (110) crystal plane, acting as the strongest acidity plane of γ-alumina, was selected for modification. The supports modified with boron and phosphorus were successfully constructed, and the acid strengths were quantified by simulating the adsorption of the relevant probe molecules: pyridine in correlation with surface electronic properties via density functional theory. The surface adsorption energy calculation shows that the boron-modified surface is able to moderately reduce the adsorption capacity of alumina, while that of the surface modified by phosphorus is found to be enhanced over the sites of a tetrahedral coordination structure; however, at the other unsaturated Al sites, this is obviously reduced. The results of introducing electric fields imply that applying horizontal electric fields changes the surface acidity of alumina under the premise of a stable structure. With the enhancement of the horizontal electric fields, the adsorption capacity of tetra-coordination sites on the original surface gradually decreases, while those of the others gradually increases. However, for the boron-modified surface, introducing horizontal electric fields can reduce the adsorption capacity of all sites. Hence, microwave-electric-field-assisted modification of B further reduces the surface acidity of alumina, making it beneficial for deep hydrodesulfurization reactions.

1. Introduction

The exploitation and utilization of fossil energy have promoted the rapid development of the social economy while aggravating pollution and damaging the environment. The development and promotion of clean fuels are the focus of the petrochemical industry. The catalytic hydrodesulfurization (HDS) process plays an important role in removing sulfur from petroleum fractions as well as petrochemicals. The most widely used catalyst is the alumina-based Co-Mo-S catalyst; among the alumina structures, γ-Al₂O₃ is broadly used as a support due to its operable pore structure and satisfactory catalytic activity, providing an uphold for the active component to maintain a stable cluster structure on the catalyst surface. There are Lewis acid and Brønsted acid on the surface. The center of the former is aluminum ions with surface unsaturated coordination, and the latter is related to the degree of surface hydroxylation [1]. These acidic and basic sites have been looked upon as the active catalytic centers of γ-alumina [2].

When the active component attaches to γ-Al₂O₃ with a single or lower layer firmly, it induces lower catalytic activity. On the contrary, high layers of the active component stacks on the support are beneficial to accelerate the reaction process [3,4]. Therefore, weakening the interaction between the active component and support surface and reducing the surface acid strength are critical to maintaining the high activity of the catalyst.

Introducing modified elements, such as boron [5,6,7,8], phosphorus [9,10] and lanthanum [11], is effective to adjust the surface acidity of γ-Al₂O₃. The additives may improve the acidity of the γ-Al₂O₃ surface and the dispersion of active components. It also helps prolong the service life of the catalyst and improves the selectivity of HDS [12,13,14].

Microwave irradiation is advantageous in the field of materials due to its selective heating and non-thermal effects [15,16], including HDS technology [17,18]. Several pieces of research reveal that the active components of the catalyst prepared by the microwave method are uniformly distributed, and the load is significantly increased. It is also found that the HDS catalyst prepared by microwave has lower metal–support interaction (MSI) and higher reaction performance. Deep desulfurization requires a thorough understanding of the mechanism of how microwave affects the acidity of pure or modified surfaces and improves HDS efficiency.

The results of previous studies [19] on boron-modified Mo/γ-Al₂O₃ catalyst prepared by the microwave-assisted method show that the introduction of microwaves effectively weakens the interaction between the active component and the support on the modified catalyst and thus increases the stacking layer of active components. Moreover, the activities of the three- and four-layer active phases significantly increase from 14% and 1%, respectively, to 38% and 22%, and obviously, those of the monolayer and bilayer phases decrease. As the number of stacked layers being improved, the number of adsorbed active sites is effectively enhanced, which contributes to the enhancement of the rate and selectivity of the HDS reaction [20].

The density functional theory (DFT) method has been widely used to investigate solid acids [21]. The change of surface acidity can be effectively explained theoretically by judging the adsorption energy and charge transfer amount of probe molecules on clean and modified surfaces. At present, acid–base sites are usually detected by the adsorption of carbon monoxide [5] and pyridine [22,23]. Carbon monoxide is a weak Lewis base and is preferentially adsorbed. Pyridine is a stronger base, less sensitive to the strength of the site, and is used to detect Lewis or Brønsted acid sites [24].

Therefore, this work screened stable boron- and phosphorus-modified γ-A1₂O₃ surface models via simulation methods, and a systematic and in-depth study of the electronic properties, acidity, and surface hydroxylation characteristics of the γ-A1₂O₃ surface were conducted through DFT methods, so as to reveal its surface structural characteristics; moreover, the relationship between microstructure and macroscopic properties was analyzed. At the same time, the influence of a microwave electric field on the surface properties of alumina and modified alumina was discussed logically.

2. Computational Details and Methods

2.1. Details of Molecular Modeling

Numerous studies [25,26,27,28] confirm that the highest proportion of (110) surface is the most active plane in γ-Al₂O₃. In the present work, the γ-A1₂O₃ model proposed by Digne et al. [24] was adopted, and the γ-A1₂O₃ (110) surface was cut based on the principle of atomic integrity, with a periodic model of the five-layer structure established. The most stable configuration after optimization with the lowest energy was then selected as the research subject of investigation and analysis, as shown in Figure 1.

The pyridine molecule was used as the probe in Figure 2, showing the Mulliken charge of each atom in detail. It is clear that the molecule is electrically neutral. When pyridine adsorbs on the surface, the electrons of pyridine transfer to γ-A1₂O₃, displaying positive electricity, which verifies the presence of acidic sites on the (110) surface.

2.2. Quantum Chemical Calculations

Molecular models were constructed through Visualizer, and all data were carried out using DMol³ via the Materials Studio package (MS).

A plane-wave basis set with medium energy cutoff was set. The generalized gradient approximation (GGA) with the Perde–Burke–Ernzerhof (PBE) exchange-correlation functional and the basis set using double numerical plus polarization (DNP) with an orbital cutoff of 4.4 Å in real space were utilized to produce highly accurate results [22,23]. And the spin state of the electrons was considered, but the structural symmetry was not used. The integration accuracy of Hamiltonian was set as Medium. And the self-consistent field (SCF) tolerance was set to Medium (10⁻⁵). The k-point was Medium with 2 × 1 × 2. All electrons in the calculation system were treated. Direct inversion of the iterative subspace (DIIS) size of 6 and thermal smearing of 0.005 Ha were employed for accelerating convergence. Moreover, the mixing charge density was set to 0.2 and the mixing spin density to 0.5. The convergence tolerances of energy, maximum force, and maximum displacement applied in geometry optimization were 210⁻⁵ Ha, 410⁻³ Ha/Å, and 0.005 Å, respectively.

(1)

The adsorption energy

The adsorption energy (E_ads) is defined as the energy released by the adsorption of molecules or atoms from the gas phase on a solid surface [3]. The adsorption energy of the probe molecule on the surface was calculated by the following relationships:

$${∆}_r{E}_{ads}=E_{\left(surf+probe\right)}-E_{\left(surf\right)}-E_{\left(probe\right)}$$

where $E_{\left(surf\right)}$ stands for the total energies of the surface, $E_{\left(probe\right)}$ for the isolated gas-phase molecule, and $E_{\left(surf+probe\right)}$ for the adsorbed molecule on the surface. A negative value indicates stable adsorption, corresponding to an exothermic process.

(2)

The displaced energy

The displaced energy determines the difficulty of replacing Al atoms on the surface of γ-Al₂O₃ with B and P. If the displaced energy is positive, the stability of the displaced crystal model becomes worse. Conversely, negative displace energy means that the reaction is spontaneous. The larger the absolute value, the easier the displacement process. For instance, the calculation method of the energy of B-γ-Al₂O₃ is as follows:

$$E_{dope}=E_{B-A{l}_2{O}_3}+E_{Al}-E_{A{l}_2{O}_3}-E_B$$

where $E_{Al}$ and $E_B$ are the energies of single aluminum and boron atoms, and $E_{B-A{l}_2{O}_3}$ is the energy of the optimized B-γ-Al₂O₃.

3. Results and Discussion

3.1. Electronic Properties of the Modified Surfaces

Aluminum atoms were exposed in the upper two layers, including di-coordination, tetra-coordination, and penta-coordination, numbered Al1~Al8 (Figure S1). The coordination numbers of the aluminum atoms at various positions are given in Table S1. The doping energies of various sites were calculated to determine the best reaction sites. Sites with the lower doping formation energy are considered easy to replace. To investigate the effects of various sites and different coordination numbers, sites with the lowest doping formation energy were selected for research. As shown in Tables S2 and S3, the optimal doping sites for boron on the γ-Al₂O₃ (110) surface were Al1, Al3, and Al8 sites in turn, and those for phosphorus were Al1, Al2, and Al8. A comparison of the two tables indicates that B is more easily substituted for Al on the surface.

Mulliken population analysis refers to the charge density distributed on the atoms that make up the molecule.

Table 1. Mulliken charges of different atoms at substitution sites.

Substitution Sites	Mulliken Charge
	Al	B	P
Al1	1.615	0.774	1.164
Al2	1.699	—	1.292
Al3	1.712	1.022	—
Al8	1.727	1.014	1.419

displays the Mulliken charges of different atoms at substitution sites. The results indicate that electrons transfer from the aluminum, boron, and phosphorus to the adjacent oxygen. Comparison of the same site doped with different atoms demonstrates that Mulliken population charge is the largest for Al on the undoped surface, followed by P and B atoms, respectively, which suggests the high ionic character of Al-O bonds on the (110) surface. B (χ = 2.04) and P (χ = 2.19) presents a stronger electronegativity and ability to attract electrons than Al (χ = 1.61). The electron density at the modified site increased, which is not conducive to the adsorption of foreign molecules on it. Since the electronegativity of B is smaller than that of P, the adsorption capacity of the B modified surface is lower [29].

Moreover, partial density of states (PDOS) [30,31] curves for the γ-A1₂O₃ (110) surface and the surfaces of A1 sites replaced by B and P of γ-A1₂O₃ (110) were analyzed from −22.7 eV to 18.2 eV, as plotted in Figure 3.

For the γ-A1₂O₃ (110) surface, a narrow peak from −21 eV to −15 eV and a wide peak from −8 eV to 0 eV can be observed. The PDOS was further analyzed to define the formation of the peaks. The narrow peak is found to form due to the hybridization of O-2s and Al-3p orbitals. The contribution to the broader peak at 0 eV comes from the O-2p and Al-3s orbitals.

From Figure 3b, on B-γ-A1₂O₃ (110) surface, the narrow peak from −21 eV to −15 eV is composed of O-2s and B-3p orbitals, while the broad peak at −8 eV to 0 eV is composed of O-2p and B-2s orbitals. Between 5 eV and 17.5 eV, there is a relatively wide orbital composition of O-2p, Al-3p, and B-2s.

For the P-γ-A1₂O₃ (110) surface, the narrow peak from −21 eV to −15 eV is composed of O-2s and P-3p orbitals. The broad peak at −8 eV to 0 eV consists of O-2p and Al-3s and P-3s orbits. There is a relatively broad orbital composition of O-2s, Al-3p, and P-3p, ranging from 5 eV to 17.5 eV.

From the above PDOS analysis, B and P are found to have hybridized and bonded with atoms of the γ-A1₂O₃ (110) surface, indicating that the atoms of B and P were successfully displaced.

3.2. Effect of Acid Types

3.2.1. Lewis Acid Analysis

Pyridine, the basic probe molecule, was simulated for the adsorption on different γ-alumina acid sites. The adsorption energies were calculated to obtain the extrinsic acid strength at different surface sites. The Mulliken population charge reflects the electric property and quantity of the charge of the atom. The adsorption strength can be determined by analyzing the amount of transferred charge of the probe molecule.

Taking the adsorption of Al1 sites as an example, from Figure 4, the charges of pyridine are 0.075, 0.268, and 0.191, when adsorbed on the surfaces of pure γ-Al₂O₃, B-γ-Al₂O₃, and P-γ-Al₂O₃, respectively. This is because the electrons in pyridine molecules flow into the alumina surface during adsorption, indicating successful adsorption of pyridine on the alumina surfaces. A comparison of the charges at different active sites before and after adsorption confirms that pyridine electrons do flow into alumina, as shown in Table S4.

When pyridine is stably adsorbed on Al1 sites of pure, B-modified, and P-modified γ-Al₂O₃ (110) surfaces, the adsorption energies are −111.64 kJ/mol, −88.5 kJ/mol, and −165.85 kJ/mol, respectively. That is, pyridine is more easily adsorbed on P-γ-Al₂O₃ with stronger acidity. The adsorption capacity of the γ-Al₂O₃ (110) surface is weaker than that of the P-γ-Al₂O₃ (110) surface and stronger than the B-γ-Al₂O₃ (110) surface. This demonstrates that the B atom modification is more constructive to weaken the acidity over the Al1 adsorption site, whereas the P modification would enhance the acidity of the Al1 site, which is well in agreement with the reported literature [32,33].

Electronic transfers also occurred during the adsorption of pyridine over penta- and di-coordination sites, which again indicates that pyridine has successfully adsorbed on all three surfaces. Analysis of adsorption energy shows that the energy released by pyridine adsorption on the modified surface reduced, revealing that B and P modification is beneficial to reduce the acidity of the γ-Al₂O₃ (110) surface.

The adsorptions of pyridine on the surfaces were analyzed by replacing the penta-coordinated sites of Al atoms with B and P. From Table S8, the weakening ability via P modification is too strong, which would affect the adsorption of other substances. However, B can properly weaken the surface acidity without affecting the adsorption of other substances. The adsorption of pyridine at the di-coordination site is similar to the above rule.

Therefore, P modification enhances the acidity of the tetra-coordination sites and significantly reduces the acidity of the di- and penta-coordinated sites, resulting in too strong or too weak adsorption capacities. Conversely, B modification moderately reduces surface acidity.

3.2.2. Brønsted Acid Analysis

Since the aluminum surface is always hydroxylated under normal operating conditions, the presence of the hydroxyl group will change the acid–base properties of γ-alumina [34,35,36]. Hence, the structures of the hydroxyl group displaced on the original and modified γ-Al₂O₃ (110) surfaces at various adsorption sites were optimized to obtain stable configurations, and the adsorption energy of hydroxyl groups was calculated. The results are shown in

Table 2. Hydroxyl adsorption energy on the modified surfaces.

Absorption Sites	Adsorption Energy (kJ/mol)
	γ-Al2O3	B-γ-Al2O3	P-γ-Al2O3
1	−152.26	−134.30	−404.67
2	−185.42	--	−246.22
3	−267.59	−156.77	--
8	−265.58	−208.86	−338.98

, where the adsorption energies are less than −125.0 kJ/mol, indicating that they are of chemical adsorptions [37]. Additionally, hydroxyl is easy to adsorb on the P-γ-Al₂O₃ (110) surface, resulting in cover of the Lewis acid, and then the Brønsted acid site is provided. Therefore, only the properties of hydroxylated γ-Al₂O₃ and B-γ-Al₂O₃ (110) surfaces are analyzed below.

It can be seen from the data in

Table 3. Mulliken charges for different adsorption sites on various surfaces.

Absorption Sites	γ-Al2O3		B-γ-Al2O3
	Pure	Hydroxylated	Pure	Hydroxylated
1	1.707	1.783	0.774	0.854
3	1.838	2.036	1.023	1.804
8	1.853	2.018	1.013	1.806

that Mulliken charges of the adsorption site over the hydroxylated γ-Al₂O₃ surfaces are higher than those on the surface not hydroxylated, and the surface acidity is accordingly enhanced. B-OH had a higher acidity than Al-OH [33]. The main reason is that the electronegativity of the oxygen atom within the hydroxyl group is relatively large, and the adsorption of the hydroxyl group therefore induces the electron flow into the hydroxyl group and increases the number of the positive charges over the adsorption site. However, from

Table 4. Adsorption energy of pyridine on various surfaces (kJ/mol).

Absorption Sites	γ-Al2O3		B-γ-Al2O3
	Pure	Hydroxylated	Pure	Hydroxylated
1	−111.69	−107.40	−88.54	−25.58
3	−214.05	−60.60	−163.97	−125.12
8	−209.47	−15.76	−140.77	−96.50

based on the adsorption energies of pyridine on different surfaces, the opposite conclusion can be drawn: the acidity of hydroxylation surfaces is obviously weakened.

Therefore, the analysis of the surface structure of the hydroxylation species shows that the adsorption performance of the surface is different due to the change in Al-O bond length (In Figure 5). Mayer bond level analysis was performed to understand the Al-O and O-H bonds. It is clear that the shift of the hydroxyl group is the key to the decrease in surface acidity on the hydroxylated γ-Al₂O₃ surfaces.

It is concluded that the steric hindrance effect of the hydroxyl group affects the adsorption of pyridine, and Brønsted acid on the alumina surface is weaker. B effectively weakens the acidity of a hydroxylation surface and reduces its surface adsorption capacity.

Only when the surface of γ-Al₂O₃(110) is partially dehydroxylated, the Lewis acid site on the surface can appear, and its adsorption activity will be significantly improved.

3.3. Effects of the Electric Field

Compared to P, B-modified alumina has a better abatement in surface adsorption. Hereby, the effect of electric field on surface adsorption performance was explored.

The addition of the electric fields can change the internal charge arrangement of the alumina crystal system, which in turn would induce a change in the crystal structure. The electric fields were applied in different directions, including X, Y, Z, positive, and negative directions, as shown in Figure 6.

3.3.1. Pure γ-Al₂O₃ (110)

The calculations show that the surface of the alumina crystal is largely deformed by the perpendicular electric fields, which is not favorable for adsorption. Horizontal electric fields of different directions and intensities were applied to the γ-Al₂O₃ (110) surface, and Mulliken charge analysis was performed on the Al1, Al3, and Al8 sites of the surface. From Figure 7, the maximum field strengths for the stable pure crystal structure of alumina in the X and Y directions are 0.02 a.u. and 0.015 a.u. respectively.

When an electric field parallel to the X+ was applied, the charge of the Al1 remained almost constant, while those of the Al3 and Al8 sites changed in N-typed curves with increasing intensities of the electric field. The electric field in the Y- direction has an irregular effect on the sites’ charge, e.g., the charge at the Al1 site did not change significantly, while that over the Al8 site decreases and the Al3 site increases.

When the electric fields in the X- and Y+ directions are applied, it can be determined that the number of charges at different sites varies regularly with the electric field intensities. That is, the charge at the Al1 site decreases linearly, while the charges at the Al3 and Al8 sites increases linearly.

The decreases in the positive charge at the adsorption site is beneficial for reducing the adsorption capacity and vice versa for increasing it. Correspondingly, it is conjectured that the adsorption capacity of the Al1 site decreases and those of the Al3 and Al8 sites increases after applying the X- and Y+ horizontal electric fields. Figure 8 shows the structural model and Mulliken charge for the adsorption sites at the maximum field strength.

The adsorption energies of pyridine at Al1, Al3, and Al8 sites on the γ-Al₂O₃ surface were calculated under the electric fields of X- and Y+, as shown in

Table 5. Adsorption energy in the X- electric field.

Absorption Sites	Adsorption Energy (kJ/mol)
	EX− = 0	EX− = 0.005	EX− = 0.010	EX− = 0.015	EX− = 0.020
1	−111.64	−109.43	−107.14	−104.85	−103.26
3	−214.05	−216.97	−220.57	−223.92	−226.80
8	−209.47	−211.28	−214.76	−216.52	−218.95

and

Table 6. Adsorption energy in the Y+ electric field.

Absorption Sites	Adsorption Energy (kJ/mol)
	EY+ = 0	EY+ = 0.005	EY+ = 0.010	EY+ = 0.015
1	−111.64	−107.15	−101.36	−96.34
3	−214.05	−217.65	−219.95	−223.25
8	−209.47	−212.11	−214.83	−216.49

.

As the electric field is enhanced, the absolute value of the adsorption energy at Al1 site gradually decreases, indicating that the adsorption capacity is gradually weakened. However, the absolute values of adsorption energy for Al3 and Al8 are enhanced, and the adsorption capacities also clearly increase. The variation of the adsorption energy of the pyridine molecule at different sites as a function of the field strength in the Y+ direction followed the same pattern as above.

The change in adsorption energy corresponds to the change in the amount of charge at the adsorption site before adsorption, suggesting that the charge at the adsorption site affects the surface adsorption capacity. Thus, under the assumption of a stable structure, an increase in the strength of electric field weakens the adsorption capacity at the Al1 site of the pure γ-Al₂O₃ surface and simultaneously enhances it at the Al3 and Al8 sites.

3.3.2. B-γ-Al₂O₃ (110)

The maximum field strengths that B-γ-Al₂O₃ withstood in the X and Y directions are 0.020 a.u. and 0.015 a.u., respectively, which coincides with those for the non-modification surface.

Calculation of PDOS of B-γ-Al₂O₃ analyzes whether the B atom still exists in the alumina structure after introducing the electric field and whether it has a good bonding state with the surrounding atoms.

Taking the PDOS of Al1 site on the B-γ-Al₂O₃ under the electric field of E_X− = 0.020 a.u. and E_Y+ = 0.015 a.u. as examples, are compared with that of the Al1 site without an electric field.

As can be seen from Figure 9a,b, for instance, the relatively narrow peak appearing at around −20 eV to −15 eV in the Y+ under the applied electric field is a result of the hybrid bonding of the O-2s orbit and B-3p orbit. The dominant contributions to the broader peak at about −7.8 eV to 0 eV comes from the O-2p and B-2s orbitals. At about 0 eV to 17 eV, there is a very gentle broad peak consisting of B-3p and O-2s orbitals. Comparing the PDOS curves for the E_X− and E_Y+ electric fields, the peak positions are found to be essentially the same, indicating that applying different directions and intensities of electric fields does not change the bonding states of the boron and oxygen atoms.

According to Figure 9c, there is a peak range from 0 eV to 17 eV at the PDOS curve of boron without the electric field. However, after the introduction of the electric field, it is found that there are two peaks at this position, with the new peaks locating at about 13 eV to 17 eV. The corresponding curves are relatively flat, implying that the electron delocalization become stronger. The introduction of the electric field disrupts the charge balance in the system and favors the electrons movement. The PDOS curve changes no significantly for oxygen atoms, which is likely due to the high electronegativity of the oxygen atoms and the low degree of delocalization of the surrounding electrons. To sum up, the addition of electric field does not cause significant changes in the structure of B-γ-Al₂O₃.

The Mulliken charges at the adsorption sites for different field intensities and directions are shown in Figure S7. It is not difficult to judge that the electric fields in the X+ and Y- directions have no advantage in weakening the adsorption capacity of the B-γ-Al₂O₃ (110) surface, while that in X- and Y+ may be beneficial to weakening.

The adsorption energies of the adsorption sites are obtained and are listed in

Table 7. Adsorption energy at the X- electric field.

Absorption Sites	Adsorption Energy (kJ/mol)
	EX− = 0	EX− = 0.005	EX− = 0.01	EX− = 0.015	EX− = 0.02
1	−88.50	−83.97	−74.63	−68.14	−64.72
3	−163.97	−161.34	−157.75	−154.47	−153.29
8	−140.77	−137.92	−135.46	−133.15	−131.82

and

Table 8. Adsorption energy at the Y+ electric field.

Absorption Sites	Adsorption Energy (kJ/mol)
	EY+ = 0	EY+ = 0.005	EY+ = 0.010	EY+ = 0.015
1	−88.50	−76.72	−63.23	−54.20
3	−163.97	−161.47	−158.69	−156.53
8	−140.77	−137.15	−133.61	−129.39

. At increasingly strong electric fields, the energy released by pyridine adsorption become lower and the Al1 site changes dramatically. So, the X+ electric field weakens the adsorption ability of the B-γ-Al₂O₃ (110) surface, especially the Al1 site. A consistent conclusion is reached by analyzing the action of the Y- electric field.

The effects of X- and Y+ electric fields on pyridine adsorption on the γ-Al₂O₃ (110) and B-γ-Al₂O₃ (110) surfaces are compared in Figure 10. The positive half axis of the horizontal represents the electric field strength in the Y+ direction, and the negative half axis represents that in the X- direction.

It is easy to find that the electric fields in X- and Y+ directions weaken the adsorption capacity of the Al1 site on the γ-Al₂O₃ surface and enhance those of the Al3 and Al8 sites. Meanwhile, they effectively weaken the adsorption capacity of the adsorption site on the B-γ-Al₂O₃ surface, so as to weaken the surface acidity.

Because the direction of the microwave electric field is variable, the surface acidity can be effectively weakened when the horizontal electric field is applied to the B-γ-Al₂O₃ surface. For the pure γ-Al₂O₃ surface, the horizontal electric field has different effects on various active sites. Therefore, microwave-assisted B modification is beneficial to reduce the surface acidity of γ-Al₂O₃.

4. Conclusions

The DFT results show that the surface acidity modified by B has moderately better weakened acidity than that modified by P. The acidity of the hydroxylated surface is weaker, and Brønsted acid is much weaker than Lewis acid. The steric hindrance of hydroxyl affects the adsorption of pyridine. Similarly, the acidity of the hydroxylated surface is obviously reduced via B modification.

In addition, the introduction of electric fields effectively weakens the adsorption capacity of the B-γ-Al₂O₃ surface and has no advantage in the pure γ-Al₂O₃ surface. Through comprehensive comparison, it is found that microwave-assisted B modification is beneficial to reduce the surface acidity of γ-Al₂O₃.

In the future, molecular simulation and other means will be used to search for suitable and effective atoms in a wide range for the modification of γ-Al₂O₃. Finally, efficient HDS catalyst support will be prepared by experimental means and with microwave assistance.