RETRACTED ARTICLE: High throughput computations of the effective removal of liquified gases by novel perchlorate hybrid material

Abstract

The utilization of hybrid materials in separation technology, sorbents, direct air capture (DAC) technology, sensors, adsorbents, and chiral material recognition has increased in the past decade due to the recognized impact of atmospheric pollutants and hazardous industrial gases on climate change. A novel hybrid material, perchlorate hybrid (PClH), has been proposed in this study for the effective sensory detection and trapping of atmospheric pollutants and industrial hazardous gases. The study evaluated the structural properties, adsorption mechanism, electronic sensitivity, and topological analysis of PClH using highly accurate computational methods (M062X-D3BJ/def2-ccpVTZ and DSDPBEP86/def2-ccpVTZ). The computational analysis demonstrated that PClH has considerable adsorption energies and favorable interaction with CO2, NO2, SO2, COCl2, and H2S. PClH is more suitable for detecting liquefiable gases such as COCl2, CO2, and SO2, and can be easily recovered under ambient conditions. Developing such materials can contribute to reducing hazardous gases and pollutants in the atmosphere, leading to a cleaner and safer environment.

Introduction

Organic–inorganic hybrid composites are a rapidly growing area of research in advanced-functional material science¹. These hybrid structures, which combine organic and inorganic materials, have shown promise in adsorbing hazardous environmental and health-attacking gases². The properties of organic–inorganic hybrid materials depend not only on their constituent materials but also on their morphology and interfacial characteristics, making them highly versatile for various applications^{3, 4}. Organic–inorganic hybrid materials offer unique properties that are absent in their precursor materials, making them highly versatile and suitable for a wide range of applications such as high-energy radiators, shielding materials, absorbers, polarizers, and hydrogen storage systems^{5, 6}. Their conductive ability, stability, high-temperature operation ability, and processability make them ideal for meeting the current industrial standards⁷.

Recently, the impact of anthropogenic environmental pollution has been brought to light through cumulative impact mapping, which uses data on environmental conditions and demographics. Air pollution is a significant risk factor for premature death and is caused by the burning of fossil fuels without sequestration⁸. The combustion process of natural gases leads to the emission of acidic air pollutants, such as H2S, NO2, COCl2, SO2, CO, and CO2, which have a magnifying molecular structure and a high carbon ratio. This highlights the need for effective methods to detect and trap these hazardous gases. The emission of acidic gases such as SO2, NO2, CO2, and COCl2 during the combustion of natural gases has resulted in global environmental degradation, causing acidic rain and ocean acidification. These gases are responsible for lowering the PH of water, which has a detrimental effect on human health and leads to the raising of atmospheric temperature on the earth’s surface^{5, 9}. The reduction of these gases is critical to mitigating the negative impacts of air pollution on human health and the environment.

In response to this environmental and atmospheric degradation, several efforts to find novel and eco-friendly techniques for the adsorption of these hazardous gases have surged. Promisingly, the current and trending state-of-the-art in chemical adsorbent for gas adsorption has been reported in studies by Sanchora et al.¹⁰, and Kaushik et al.¹¹. Efficient gas adsorption concerning Organic–Inorganic hybrid nanomaterials has been reported to continually enhance gas sensing performance¹². Blair et al. and Xie have carried out and illustrated the gas sensing ability of organic conducting polymers (such as; Polyaniline (PANI), and polypyrrole (PPy), which were reported to exhibit an improved gas sensing ability, due to their desired functionality and ability¹³. However, due to the high flair of conducting polymers on volatile organic compounds (VOC) and moisture present in the compound’s environment¹⁴, they were reported to be sometimes unstable and thus exhibit poor sensitivity towards the gases⁵. More so, it was reported that gas sensors that are of Inorganic metal oxides, such as ZnO, SnO₂, SiO₂, etcetera exhibited improved sensing characteristics upon adsorption of hazardous gases because of changing the O₂ stoichiometry and electrical active surface charge^15,16,17,18. As reported, despite the high sensitivity, the use of such inorganic metal-oxide gas sensors for practical application is not yet prominent. More interestingly, the adsorption of small gaseous molecules such as H₂O, H₂, O₂, CO, and CO₂ on graphene/g-C₃N₄ nanocomposite which was coordinated with eight (8) transition metals were reported to possess physisorption activity¹⁹. Hence, a very minute study has been harnessed on the adsorption ability of hybrid materials. As a result, issues related to low conductivity, poor stability of organic material²⁰, and the complicated processability of inorganic materials, hinder their effective utilization in gas adsorption and sensing ability²¹. Profoundly, the use of Organic–Inorganic hybrid material may give a well-proficient adsorption ability for potential trapping cum adsorption of hazardous gases. With concerns to this, hybrid materials have intrinsically aroused extensive interest in gas-adsorption and separation technologies and express positive prospects for the establishment of sustainable processes required for the future process industry.

Based on the above findings, the gas adsorption potential of organic–inorganic hybrid material as an efficient adsorbent for environmental fitness and sequestration is proposed in this study. To effectively quantify the suitability and applicability of the synthesized hybrid material for adsorption purposes, high-level computational simulations were deployed to this effect. The highly parameterized Minnesota functional (M062X) incorporated with D3 dispersion of Grimme’s with Becke-Jonhson damping was utilized in comparison with the highly accurate double hybrid functional (DSD-PBEP86) which incorporates spin-component scaled MP2 and dispersion corrections. Adsorption capacities, structural geometry, and electronic properties such as conductivity and variations in energy gap which are key parameters in assessing the electronic behavior of adsorbent surfaces prior to adsorption was considered. The susceptibility and nature of intermolecular interactions by virtue of adsorption are quantified by the famous quantum theory of atoms in molecules (QTAIM), non-covalent interaction (NCI), electron localization function (ELF), and electron density delocalization (EDD). Also, atomic densities of state together with thermodynamic assay have been used to espy the spontaneity and deformation of the material during the adsorption. These objectives were wholistically used to address our curiosity which is highlighted in the following research questions: {1} Does the synthesized perchlorate hybrid material possess the potential and suitable attributes to be employed in adsorption and separation technology? {2} If yes, what is the affinity of the hybrid material towards the selected gases of interest, {3} what adsorption configuration and mechanism is adopted by the material towards the considered gases {4} what structural deformation is imposed on the surface during the adsorption process? {5} How do energy gap and conductivity vary with the preferential adsorption of the selected gases, and {6} Does the hybrid material express any selectivity towards any of the selected gases? To address these questions, several adsorption configurations, and orientations of the respective gases on the hybrid material were tested via high-level computations. Please note that the detailed synthesis and characterization of the hybrid material reported here has been published elsewhere²² and is not in the scope of the work reported herein. The novelty of this work lies in the focus on organic–inorganic hybrid materials as a promising solution for the adsorption of hazardous environmental gases, which pose a significant threat to human health and the environment. This article highlights and explores the multifaceted properties of PClH material that make it suitable for selective adsorption of liquified gases such as hydrogen sulfide and carbonyl chloride, which contribute to global warming and environmental degradation. Based on our search, this material has not been reported for adsorption and detection purposes.

Methods

Experimental

Synthesis and crystallization

The PClH Compound was obtained as colourless crystals, after a few days, by slow evaporation from an aqueous mixture of 2,4-dichloroaniline (Sigma-Aldrich) and 10% aqueous HClO₄ (70%, Aldrich) solution in a molar ratio of 1:1 at room temperature.

X-ray data collection

The single crystal structure of PClH was examined under a polarizing microscope. The crystallographic data of PClH was collected on a Mercury CCD System (Rigaku) at room temperature employing MoKα radiation source (λ = 0.71075 Å). The intensity data collections were corrected for Lorentz and polarization effects as well as empirical absorption based on multi-scan technique. Crystallographic details and processing data of the elaborated compounds are gathered in Table S1. Crystallographic data for the structural analysis has been deposited at the Cambridge Crystallographic Data Centre, CCDC No 2151547. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK: fax: (+ 44) 01223-336-033; e-mail: deposit@ccdc.cam.ac.

Spectroscopic analysis

A Nicolet Impact 410 FT-IR spectrophotometer apparatus was employed to study the infrared spectra of PClH compound in the 4000–400 cm⁻¹ range as shown in Fig. S1 of the supporting information.

Computational methodology

Gaussian 16 program was utilized for all molecular and geometry optimizations²³. Geometry optimizations of all complexes were performed within the meta-generalized gradient framework and spin component scaled double hybrid at the M062X-D3(BJ)/cc-pVTZ and DSD-PBEP86/aug-cc-pVTZ level^{24, 25} without imposing any symmetry constraints. The exchange–correlation functional (M062X) based on the meta-generalized gradient approximation and DSD-PBEP86 were chosen to ensure the accuracy of computations. These functionals were coupled with the third (D3) and fourth (D4) versions of Grimme’s atomic pair-wise dispersion correction combined with Becke-Jonhsons’ damping to empirically account for all long-range dispersion interactions including Van der Waals effects. Also, the DSD-PBEP86 has emerged as a highly accurate and robust method for ab-initio computations from benchmark studies^26,27,28. This method incorporates spin-component scaled MP2 and dispersion corrections which tackle all interatomic and Van der Waals effects efficiently. The optimized geometries were further utilized for all computations of intermolecular and topological investigations without further optimizations. Basis set superposition error (BSSE) was considered for all interaction energy computations based on the counterpoise method²⁹. The adsorption energies for the respective interactions were computed in line with the following equation as the difference between the final and initial states.

$$\Delta {E}_{{\varvec{a}}{\varvec{d}}{\varvec{s}}}= {E}_{(complex)}-\left({E}_{\left(adsorbent\right)}-{E}_{\left(\mathrm{adsorbate}\right)}\right),$$

where $\Delta {E}_{{\varvec{a}}{\varvec{d}}{\varvec{s}}}$ is the adsorption energies of the studied systems, while the incorporated terms ${E}_{(adsorbent)}$ and ${E}_{(adsorbate)}$ connotes the total energies of the PClH and gases respectively. To further quantify the extent of deformation upon adsorption of each respective gases, the deformation energy was calculated as:

$${\text{E}}_{{{\text{def}}}} = {\text{ E}}_{{{\text{sp}}}} \left( {{\text{complex}}} \right) \, {-}{\text{ E}}_{{{\text{tot}}}} \left( {{\text{complex}}} \right),$$

where E_sp and E_tot (complex) are the single point energy and total energies of the complexes respectively.

Thermodynamics considerations which are crucial parameter for the quantification of adsorption feasibility was also, invoked to analyze the feasibility of the adsorption process as well as the amount of gas adsorbed onto the surface of the studied material. The thermodynamic parameters include Enthalpy (H), entropy (S), and Gibb’s free energy (ΔG*). Topological investigations for the quantification of adsorption strengths were computed based on Bader’s quantum theory-of-atoms-in-molecule (QTAIM). However, due to the limitation of QTAIM hypothesis, non-covalent interaction (NCI) based on the promolecular electron density and electron localization function (ELF) as well as Electron density difference (EDD) were employed to further reveal the type of interactions existing between the surface and adsorbed gases respectively. The multiwfn software³⁰ was deployed to this effect. All isosurface plots were rendered via visual molecular dynamics (VMD) software and Iboview^31,32,33.

Results and discussion

Structural geometry of the modeled systems

It is kingpin to have a lucid understanding of the structural geometry of the studied systems with the inclusion of the adsorbed gases. Thus, the hybrid material (PClH) in its distinctive study is striated with C1 symmetry. Figures 1 and 2 illustrates the atomic enumeration scheme and the optimized structures of the gas molecules (CO₂, COCl₂, H₂S, SO₂, and NO₂), and the PClH@gas optimized structures respectively. The information on the bonds is shown in the preceding Tables S1 and S2 of supporting information. CO₂ is a linear molecule with a C–O bond length of 1.15 Å and 1.16 Å before and after adsorption, maintaining a bond angle of 180 degrees before adsorption to the PClH surface. Hence, a slight minimal decrease was observed in the bond angle of the CO₂ gas after adsorption with a degree of 179.95°. This minimal C–O–O bond angle decrease can be attributed to the presence of lone pairs and the electronegative adsorption site of the CO₂ gas, which causes more repulsion on the bond pairs, thus, the bond pairs tend to come closer on adsorption of CO₂ gas. Therefore, resulting in a more stable adsorption structure and an increase in the bond length of the C–O bond. Phosgene (COCl₂) whose bond angle follows a linear geometry with an angle of 180 degrees because of the two Chlorine atoms repelling each other and are driven apart. It was observed that the geometrical bond angle of COCl₂ after adsorption decreased to a bond angle of 177.85°. Also, this decrease in degree can be attributed to the interaction with the negative adsorption site of the PClH surface. More so, an increase was also observed in the C–Cl bond length in COCl₂@PClH as depicted in Table S2. In furtherance, an increase in bond angle was observed for the H₂S gas molecule from 92.25° to 92.55° after adsorption onto the PClH surface with an increment in the C–Cl bond distance. The respective bond angles of SO₂ and NO₂ with normal bent shapes were observed to have a decrement in their bond angles from 118.60° and 134.04° to 115.89° and 132.84° with NO₂ having the least deviation in bond angle and an obviously increased bond length observed for the S–O bond from 1.180 to 1.430 Å. It is worth ascertaining that the usual increase in bond length observed in the hybrid material after adsorption depicted that the bond length increases in proportion to the size of the adsorbate. As such, the bigger the atoms, the greater the bond lengths deviated from regular geometry. This increased bond length affirms the presence of visible interaction between the PClH surface and the respective gases and, gives a clear idea about the distribution of bonded electron pairs around the atom.

Figure 1

Optimized geometry of the hybrid material and gases at the DSD-PBEPB86/cc-pVTZ level.

Figure 2

Relaxed geometry of the studied hybrid material enumerating the various interactions with gases at suitable distances.

Adsorption energy and fraction of electron transfer

A recent study suggests that Organic–inorganic hybrid materials exhibit good adsorption efficacy for the rapid removal of hazardous gases, especially due to their high surface area and a great number of active interaction sites³⁴. The adsorption energy is an important parameter to determine the effectiveness of a medium to be an excellent surface or an adsorbent, where a more negative E_ads value corresponds to stronger adsorption³⁵. In this study the adsorption energy for the interaction between the perchlorate systems (PClH-systems) and the adsorbed gases is presented in Table 1, using Eq. (1). The result indicated adsorption energies (E_ads) of the titled systems. Hence:

$${E}_{ads} = {E}_{int}-\left({E}_{gas}+ {E}_{Pcl}\right).$$

(1)

Table 1 Adsorption energy, fraction of electron transfer, and adsorption distances for the interaction of the studied gases with PClH material computed at the M062X(D3BJ) and DSD-PBEP86/cc-pVTZ level.

Similarly, the fraction of electron transfers between the interacting molecules of gas and the PClH surfaces was calculated using Eq. (2), to evaluate the amount of negative charge depleted or accumulated during the interactions.

$$\Delta N = \frac{{\chi_{PClH} - \chi_{gas} }}{{2 \left( {\eta_{PClH} - \eta_{gas} } \right)}},$$

(2)

where the E_int is the total energy of the interaction between the gas and the hybrid systems, E_gas is the energy of the gases alone and E_PClH is the energy of the studied perchlorate hybrid. The E_ads range from − 1.918 to − 11.790 kcal/mol. The magnitude of the adsorption energy for SO₂@PCl, was observed to be − 11.790 kcal/mol accompanied by a high amount of fraction of electron transfer compared to other interactions; CO₂@PCl with adsorption energy of − 8.382 kcal/mol and fraction of electron transfer of − 4.681, compared to CO₂@PCl with the smallest E_ad of − 1.918 kcal/mol with similar magnitude of fraction of electron transfer of − 4.681 and COCl₂@ PCl obtaining adsorption strength of − 7.215 kcal/mol and ∆N of − 2.165, respectively. H₂S@PCl exhibited an E_ad of − 6.855 kcal/mol and Fraction of electron transferred was obtained to be 3.304. The lowest E_ad for all the interactions was observed for CO₂ regardless of its atom that is involved in the interaction. In Pcl@CO₂, E_ad of − 1.918 kcal/mol and ∆N of − 4.681 was indicated, displaying that the adsorption is physisorption, and quite limited adsorption ability of PClH to CO₂ gas. More so, on benchmarking with a double hybrid correctional functional, more negative adsorption energies were obtained, which depicts more adsorption efficiency than the M062X(D3BJ) functional. As such the adsorption energies obtained for the calculated PCl-hybrid are − 4.420, − 12.130, − 8.049, − 18.430, and − 4.174 kcal/mol for CO₂@PClH, COCl₂ @PClH, H₂S@PClH, SO₂@PClH and NO₂@PClH, respectively. This affirms coherently with the calculated adsorption energies obtained with the double-hybrid (DSD-PBEP86) functional. In replication with the functionals, the COCl₂, SO₂, CO₂, and sparingly the H₂S gases adsorbed on the hybrid material were observed to have the more negative and more proficient adsorption energies. In relation, the variation in the interacting atom of the gas has no effect on the fraction of electron transfer but significantly affected the adsorption energy. Thus, for the adsorption energies analysed, the more negative adsorption energy was observed for SO₂@PClH likewise the fraction of electron transfer, which indicates a stronger adsorption than the related studied systems. As such, the variation in the adsorption strength of the material intrinsically explains the adsorption capacity of the referenced hazardous gases on the studied surfaces. More so, the theoretically calculated adsorption energy discloses a physisorption mechanism for the interaction of hazardous gases on the Organic–Inorganic hybrid material. In addition, the theoretically obtained value of E_ads for SO₂ adsorption on consideration, is in the more negative similitude of latterly reported work³⁶ on ionic liquids, with respect to the anionic Cl counterpart. From Fig. 2 where the various gas molecules adsorption is shown, it is quite obvious that the bond distance between the PClH surfaces to the adsorbed gases (CO₂, COCl₂, H₂S, SO₂, and NO₂) dramatically enlarged up to 3.723 Å for the system in which H₂S gas was adsorbed, demonstrating that there is an iota of physical adsorption mainly by the weak Van der Waal force interaction for the H₂S molecule. Thus, this phenomenon was followed also by the SO₂ and CO₂ molecules with adsorption distance of 3.558 Å, drawing an obvious inference of strong interactions. In consonant to the adsorption distance of the NO₂ and COCl₂ gases, in which their adsorption bond distance was observed to have a minute adsorption bond length of 1.447 Å and 1.521 Å, respectively. This shorter bond length to the PClH material indicates concurrently a physical adsorption mechanism occurring in the system, which denotes a strong interaction between them, because of a large number of transferred electrons. This phenomenon is in close agreement with their respective adsorption energies, as the farther the bond distance, the more negative and exothermic the reaction occurring in the systems. Succinctly, in conclusion, adsorption energy analysis suggests that the synthesized hybrid material has a great potency for the adsorption of CO₂, COCl₂, H₂S, SO₂ and NO₂, and adsorption strength in the ascending range of SO₂@PCl > COCl₂@PClH > H₂S@PClH > CO₂@PClH.

Electronic properties

Conductivity analysis

Electronic densities and energy levels gives a clearer illustration of the adsorption mechanism and intermolecular rearrangement occurring during adsorption process. Thus, to enliven the electronic remodeling of the perchlorate hybrid material and the gases, because of adsorption, the distinctive highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the energy gap (Eg) contingent on electronic frontier molecular orbital theory were scrutinized as projected in Fig. 3 and Table 2. A minimal change was observed for the HOMO and LUMO electronic distribution on the adsorption of the gaseous molecules. The HOMO distribution for the PClH system occurred within the energy range of − 8.42 to − 8.93 eV, whereas their LUMO energy took a minimal range of − 0.89 to − 1.56 eV. Hence, on interaction with the gaseous molecules, the HOMO and LUMO energies witnessed a minute decline in energy, which exhibited a small deviation before adsorption as illustrated in Table 2. This energy disparity upon adsorption of the gases can be attributed to repositioning of electrons in the systems, during the adsorption process. To substantiate these energetic differences, the iso-surface electron density mapping of the HOMOs and LUMOs of the PClH material are evaluated in Fig. 4. Thus, their distribution exhibited an overlap to a large extent. Consequently, the adsorption of the gases on PClH material, resulted in an electronic shift of the HOMO on the gaseous molecules. This occurs because of many electronegative atoms that makes the adsorbed gases and the PClH-surface electron rich. Thus, the gases and the perchlorate being electronegative will tend to retain these electrons and as such, the iso-surface mapping densely located on the PCl-hybrid material and the gaseous molecule without spreading out.

Figure 3

HOMO–LUMO isosurface maps of the studied gases.

Table 2 Molecular electronic orbital parameters: HOMO and LUMO energies, energy gap, and global reactivity descriptors of the studied systems computed with the M062X-D3BJ and DSD-PBEP86 functionals.

Figure 4

Projected densities of states for the studied systems. A, b, c, d, e, and f corresponds to the DOS of PClH, COCl₂@PClH, NO₂@PClH, SO₂@PClH, and H₂S@PClH respectively. Figures are plotted at 0.5 isosurface value respectively.

More so, the energy gap, which is directly related to conductivity, indicated a sparingly high HOMO energy for the interacted systems. With regards to orbital calculation, the LUMO energies decreased in gas@PClH geometries as compared to the bare system (PClH), with a minimal increase observed for the CO₂ and H₂S adsorbed systems, as depicted in the preceding table. The adsorption of the gaseous molecules slightly stabilizes the HOMO, except for the NO₂@PClH system, distinctively, the highest adsorption energy calculated for the studied systems discloses a greater charge transfer through sharing of the frontier molecular orbitals. Hence, the vigorous change in the energy of COCl₂@PClH, CO₂@PClH, and SO₂@PClH gas during adsorption on PCl surface to a greater extent regards to a substantial charge transfer, leading to a stronger charge transfer, higher and more negative adsorption energies. More so, as energy gap have an agreeable relationship with the electronic conductivity. As such, the adsorbed systems with the lower energy gap show superior conductance than the systems with higher energy gap values and in this work, the electronic conductivity of CO₂@PClH, SO₂@PClH, and H₂S@PClH is much lower than the bare PClH. As a result, the bare PClH material is less conductive than its gas adsorbed counterparts, especially in the aforementioned. With respect to benchmarking of the system of study as depicted in Table 2 with a double-hybrid functional, more stabilized energy gap was observed. Thus, as shown in the tabulated values, the E_g observed for the PClH-material falls within the value 7.68 eV, which upon adsorption of the gases, a decrement in energy gap was observed which portrayed more stability and shift in density upon adsorption, which is a key criterion for sensitivity. Obviously, the dramatic energy gap transition of the gas adsorbed systems as observed depicts that the considered PCl surface has a considerably large propensity to adsorb the investigated gases (CO₂, H₂S, COCl₂, and NO₂), with much adsorption tendency on COCl₂, CO₂, and SO₂ gases, with more credits to their liquefiable nature. In furtherance, the mechanism of COCl₂, CO₂, NO₂, and SO₂ adsorption on the perchlorate hybrid material is also analyzed from the global indices of reactivity. The electronic properties such as chemical hardness (η), chemical potential (μ), electrophilicity (ꞷ), and the maximum electric charge index (ΔN_max), as also illustrated in Table 4 were analyzed. More so, chemical hardness was investigated as descriptor variable which estimated the hardness and softness of the studied structures. With respect to that, a hard chemical system has a large energy gap, which is a reverse for a soft chemical system which obviously has a small energy gap. As lucidly stated from the table, the chemical hardness of PClH system decreased drastically upon adsorption of the considered gases from the range of 7.53 to 7.09 eV, with the H₂S@PClH system having the higher softness identity. As such, it can be depicted that, the structure of PCl has become more chemically softer upon the adsorption of the gaseous molecules. Electrophilicity index and maximum electronic charge index (ΔN_max) were also tabulated. Hence, the electrophilicity (ꞷ) being a measure of the electron loving power of a compound on reaction with a relative compound. As such, one acts as an electrophile while the other behaves as a nucleophile. Hence, the studied systems with higher electrophilicity index demonstrates more electrophilic character. Whereas its positive electron charge can be attributed to the affinity of charge flows to the studied systems with dramatic electron accepting ability observed for the PClH@gas systems. Consequently, it can obviously be established that the investigated hybrid material has a unique electronic key parametric identity to adsorb the gaseous molecules (COCl₂, CO₂, NO₂ and H₂S) studied.

Re-hybridization of atomic orbitals and density of state (DOS)

The mathematical function exclaimed as the density of states (DOS), gives the distribution of all the possible quantum states per unit energy of molecular systems³⁷. It intensely explains the states of density proportion occupied by a system at each energy level. As such, the density of state parametric key electronic property is termed a continuous molecular property³⁸. In analyzing the density of state of the systems under study, an isolated system whose atoms or molecules in the gas phase possess discrete distribution of density like spectra lines ought to be put into exemplified consideration. Hence, the first frontier axis on the uppermost of the respective DOS plots represents the total density of state (TDOS), while the second intense and other spectra elucidate the partial density of state (PDOS) spectra of the various contributing fragments of the studied PClH-material and its interaction with gases. The various electronic density property of each orbital in the investigated systems are unveiled in distinct energy range as shown in the various DOS plots in Fig. 4. It could be deduced from the plots, that the molecular orbital spin (alpha and beta-spin) for all the six (6) studied systems are symmetric with no focus on the magnetic behavior on the respective systems. In furtherance, from the TDOS plot, the state contributions from the perchlorate system (PClH) for the entire systems are mainly around − 0.31 eV. it is also observed that at the energy levels of − 0.40 to − 0.70 a.u, the benzene fragment of the PClH material displayed a more dominant orbital contribution to the α-HOMO’s while at the LUMO, the ClO₄⁻ exhibited a dominant orbital contribution from 0.1 to − 0.15 a.u. Hence, accrediting the electronic charge transfer between the benzene fragment of the PClH-material and the ClO₄⁻ fragment. For the COCl₂@PClH system, the major HOMO orbital fragment contribution as illustrated from the DOS plot was associated with the ClO₄⁻ fragment, followed by the overlapped of the COCl₂ gas and the PClH fragment having an α-HOMO energy in the range of − 0.24 a.u, whereas, the PClH orbital contributed effectively to the LUMO as a result of the unoccupied orbital of the chlorine atom constituent of the ClO₄^- fragment. As such, for the CO₂ gas adsorbed system, the α-HOMO energy was dominated with contributions at − 0.254 a.u, emanating majorly from the ClO₄⁻ participating atoms, due to electron transfer from the benzene fragment of the PClH-material to the ClO₄⁻ fragment therefore, contributing to the electron deficiency of the PClH-material as observed in the plotted DOS diagram. It is also clearly observed that the CO₂ fragment had a minimal density of states, owing to its electronegative carbon and oxygen atomic constituent, likewise the H₂S and other studied gases (SO₂@PClH, and NO₂@PClH). Interestingly, it can be depicted from the various DOS plots for the investigated systems, that the unoccupied orbital contribution of the PClH-atomic constituents, is because of its ionic nature and charge transfer. Hence, contributing to its unoccupied orbital character. More so, this finding corroborates with the strong electron hybridization between the PClH-system and the respective atoms that forms the stable chemical bonds and interaction upon adsorption of the gases. As such, these observable changes in the TDOS peak intensities and energy levels were observed in the virtual orbital (LUMO) of the investigated PClH-system and the PClH@gas adsorbed systems. This is in probable consistent with the conductivity (FMO) analysis, where the highest change in energy gap of the systems under study is observed due to minimal change in the respective LUMO energies.

Stabilization energy analysis

Natural bond orbital (NBO) analysis was utilized to obtain lucid insight into the stabilization energy of the organic–inorganic hybrid material (Perchlorate) and its interaction with the gases of interest. NBO analysis is efficient for this study, in understanding the delocalization of the electron densities, and hyper-conjugation electronic effects. Thus, it is proficient in measuring intermolecular and intramolecular interactions because of the chemical bonds in the systems of study³⁹. Hence, PCl hybrid material being a neutral referenced system have equivalent number of electropositive and electronegative atoms and more of a distinct electronic structural behavior. However, the electronic behavior of the PClH system is sparingly tampered upon adsorption of COCl₂, CO₂, NO₂, SO₂ and H₂S gaseous molecules. The orbital parametric stabilization energy for each donor (i) and acceptor (j) orbitals can be genuinely ascribed in terms of electronic second-order perturbation energy, otherwise referred to as stabilization energy (E²). It is paramount to understand that the higher the energetic stabilization of the systems, the greater the electronic interactions existing between the respective donor and acceptor orbitals.

Meticulously, the NBO analyzed results on the organic–inorganic hybrid material (PClH) are represented in Table 3. In distinction to Table 5, it is observably spotted that, the highest stabilization energies of the COCl₂@PClH, CO₂@PClH, NO₂@PClH, SO₂@PClH, H₂S@PClH systems occurred with the major orbital transition of $\pi$* → $\pi$* bond and LP → $\pi$*, with highest perturbation energy of 132.94, 137.50, 242.15, 63.41 and 169.30 kcal/mol for COCl₂@PClH, NO₂@PClH, SO₂@PClH, and H₂S@PClH systems respectively, with major atomic contributions from the electronegative atoms. Distinctively, Carbon, Nitrogen and Oxygen, and major atomic contribution emanating from the carbon atoms of the benzene fragment of the PClH (i.e., from the C₃ → C₄ donor orbital and C₅ → C₆ acceptor orbital). Interestingly, from the stabilization energy study, it was observed that upon adsorption of the studied gases onto the PClH-material, a perturbation in energy was also observed for the COCl₂, SO₂, and the CO₂ adsorbed systems. Thus, decreasing the energy of the bare PClH system from an E² value of 163.03 kcal to 132.94, 137.50 and 63.42 kcal/mol, respectively, owing to their supposed favorable interaction and adsorption onto the hybrid material. This result is also, in alignment with the energy gap analysis and other electronic studies, whereas the interactions with NO₂ and H₂S were sparingly perturbed energetically. Hence, from the above energetic orbital analysis, it can be evidently inferred that, proficient interaction occurred between the PClH-material structure and the diverse gas adsorbed systems. Thus, suggesting PClH-material to be more efficient in trapping COCl₂, CO₂, and SO₂ gas molecules.

Table 3 Distinctive intramolecular donor–acceptor interactions and NBO stabilization energies of the studied systems calculated with the M062X-D3BJ/cc-pVTZ theoretical model.

Topological studies

Atoms in molecule (AIM) analysis

Quantum theory of atom in molecule (QTAIM) is a theoretical analysis based on electron density, which is the most explicit and established method for analyzing bonding interaction characteristics such as bond sites, bond critical points (BCPs), atom critical points (ACPs), ring critical points (RCPs), and bond strength⁴⁰. As such, for the purpose of this study, the non-covalent interactions within the studied PClH-material as well as the interacted complex (gas@PClH), were lucidly analyzed through Bader’s quantum theory of atoms in molecule (QTAIM). The key parametric quantities used to elucidate the bond critical points in AIM analysis are the; density of all electrons (${\rho }_{(r)}$), Laplacian of electron density (${\nabla }^{2}\rho$), Lagrangian kinetic energy (G_(r)), potential energy density (V), and total electron density (H_(r)). The ideal values of the bond critical points (BCPs) extracted for the studied PClH and gas@PClH adsorbed systems, are given in Table 4 and Fig. 5 correspondingly.

Figure 5

Image depicting the interaction of the gases on the PClH material and their bond critical points (BCP).

It is worthy to note that, closed-shell interactions have varying ${\rho }_{bcp}$ values from 0.002 to 0.0162 a.u for weak Vander Waals interactions and from 0.0024 to 0.0547 a.u for strong electrostatic hydrogen bonding. More so, ${\nabla }^{2}{\rho }_{bcp}$ and ${\rho }_{(r)}$ > 0, result from weak hydrogen bond interaction with small and positive values in the range (0.0040 to 0.0498 a.u). Whereas the hydrogen bonding for the ${\nabla }^{2}{\rho }_{bcp}$ and ${\rho }_{(r)}$ falls within the range of 0.0091 to 0.0643 a.u. Hence, it is worthy to note that, ${\nabla }^{2}{\rho }_{bcp}$ < 0 denotes a covalent bond (shared orbital interaction) while ${\nabla }^{2}{\rho }_{bcp}$ > 0 designates hydrogen bonding, ionic bonding, and Vander Waals interactions, which are explicitly regarded as closed-shell interactions. Interestingly, using the Espinosa approach (${E}_{HB}=\frac{1}{2}{V}_{BCP}$)⁴¹, hydrogen bonding was characterized by the studied systems’ individual interaction energies laying in the range 2.59–10 kcal/mol, while their respective binding energies were characterized using W.F Bader binding energy equation; Binding energy (BE) = $({\rho }_{\left(r\right)*-223.08+0.7423})$. Conventionally, the values of G_(r) and V_(r) always remain positive and negative at their respective BCPs. It is worth noting that, the values of H_(r) are the sum of G_(r) and V_(r). The obtained values of H_(r) (i.e., H_(r) < 0) for the studied systems show the presence of covalent and non-covalent properties for H_(r) > 0. In corresponding cases, ${\nabla }^{2}{\rho }_{bcp}$ > 0 < H shows the existence of a partial covalent character (i.e. medium hydrogen bonding). As such, hydrogen bonding can be characterized by the values of ${\nabla }^{2}{\rho }_{bcp}$ and H_(r) for both non-covalent interactions (H_(r);${\nabla }^{2}{\rho }_{bcp}>0$), whereas, a covalent existence is illustrated by ${\nabla }^{2}{\rho }_{bcp}<0$. Tentatively, for a close shell interaction and covalent interactions, the values of ${V}_{(r)}/{G}_{(r)}$ are < 1 and > 2, respectively. Thus, as ${\rho }_{(r)}$ is relational to bond strength while the interaction types are determined by the Laplacian of electron density (${\nabla }^{2}{\rho }_{bcp}$) and total electron energy density (H_(r)), the theoretically obtained values at different BCPs for the studied PClH systems and gas@PClH system provides standardized information about the interaction strength of ${\text{ClO}}_{4}^{-}$ fragment and the benzene fragment of the PClH and also between the adsorbed gases and the PClH-material.

In the case of the PClH material, three (3) BCPs were observed (as illustrated in Table 4). The value range of the electron density (${\rho }_{(r)}$) and the Laplacian of electron density (${\nabla }^{2}{\rho }_{bcp}$) are within 0.0099 to 0.0668 a.u and 0.0347 to 0.1551 a.u, respectively. Strong non-covalent interaction was predicted and observed between H₁₄ of the benzene fragment and O₁₅ of the anionic ${\text{ClO}}_{4}^{-}$ fragment as a result of their highest electron density values and Laplacian of electron density of 0.0668 and 0.1551 a.u, respectively. Illustratively, the other topological key parametric values (G_(r), V_(r), H_(r), ${V}_{(r)}/{G}_{(r)}$) of PClH also denote the non-covalent nature of the studied PClH material.

Table 4 Topological parametric values of the BCP’s of the studied complexes obtained from the QTAIM analysis.

Table 5 Key parametric descriptors of the sensing mechanism and recovery time of the studied systems computed at the M062X (D3BJ)/cc-pVTZ level of theory.

For the COCl₂@PClH system, 9 (nine) BCPs were observed. The ${\rho }_{(r)}$ and ${\nabla }^{2}{\rho }_{bcp}$ values obtained for H₁₄….O₁₆ are 0.0697 and 0.1573 a.u, with the ${\rho }_{(r)}$ values higher than the electron densities of the other studied systems (see Table 6). Whereas the Cl_X…. H_Y (X = 21, 22; Y = 7, 8), H_X…. O_Y (X = 14, 8; Y = 16, 19), O₁₉….Cl₂₁ have ${\rho }_{(r)}$ and ${\nabla }^{2}{\rho }_{bcp}$ values of 0.0053, 0.0071, 0.0043, 0.0697, 0.0091, 0.0064, 0.0181, 0.0221, 0.0140, 0.1573, 0.0334, 0.0219 a.u. Hence, the larger value of ${\rho }_{(r)}$ as observed in H₁₄….O₁₆ bond, indicates that the COCl₂ is strongly interacting with the perchlorate hybrid material, when compared with other studied systems. This is also in tandem with the adsorption energy described in “Computational methodology” section, extenuating the basic trend explanation of adsorption energy of the titled system. Thus, the bond critical points (BCPs) of ${V}_{(r)}/{G}_{(r)}$, E_binding energy, E_interaction energy are also in agreement with the electron density and the Laplacian of electron density, which confirms the non-covalent nature of the studied systems. The bond critical points observed in CO₂@PClH and NO₂@PClH systems are intrinsically four (4) in number (Table 4). The bond critical point parametric values observed for these systems are in maximum agreement with the generally accepted and attested range for non-covalent interactions. The ${\rho }_{(r)}$ values range from 0.0072 to 0.0675 a.u for the CO₂@PClH system and 0.0059 to 0.0071 a.u for the NO₂@PClH system, whereas the ${\nabla }^{2}{\rho }_{bcp}$ for CO₂@PClH and NO₂@PClH systems falls within the observed range of 0.0316 to 0.1570 a.u and 0.0226 to 0.0250, respectively. The highest ${\rho }_{(r)}$ and ${\nabla }^{2}{\rho }_{bcp}$ values were observed for O₁₆….H₁₄, for the two studied systems. Complementarily, other of their AIM tabulated key parameters show a relational agreement to their non-covalent nature.

Table 6 Calculated results of standard thermodynamic parameters (${\Delta }_{f}{H}^{\circ },{\Delta }_{f}{G}^{\circ },{\Delta S}^{\circ },{C}_{v}$) of the studied systems at the M062X(D3BJ)/cc-pVTZ theoretical level.

Obviously, in the case of the SO₂@PClH and H₂S@PClH systems, the lower electronegative tendency of the S atom relative to the O atom resulted in their interaction with the benzene fragment in the positively charged ring (cationic ring), H_X…. S₂₀ (X = 8, 9) for the H₂S@PClH system and the ${\rho }_{(r)}$ and ${\nabla }^{2}{\rho }_{bcp}$ values obtained for the two studied structures showed a high Laplacian of electron density for the H₂S system at H₇….S₂₀. As such, the highest ${\nabla }^{2}{\rho }_{bcp}$ value of the H₂S@PClH system substantiates the high adsorption energy values of the H₂S@PClH surface, in conjunction with other gas-adsorbed systems. Hence, this also depicts a stronger interaction between the SO₂-adsorbed system and the PCl-material. More so, the AIM topologically obtained parameters such as; ${\rho }_{(r)}$ and ${\nabla }^{2}{\rho }_{bcp}$ are in the range of Hydrogen bonding (H-bonding). This was explicitly explained by the low and negative individual interaction and binding energies (< 2.95 kcal/mol) required for H-bonding.

The result of the QTAIM analysis theoretically obtained is superior, in that, they attested to some bond interactions that were not able to be confirmed structurally. Hence, for all the studied systems, the calculated values of ${\rho }_{(r)}$ indicate that viable bonding strength exists within the studied systems, and this concept is in line with the adsorption behaviors of the systems. Conclusively, it can be depicted from the AIM analysis that, the gases (COCl₂, CO₂, NO₂, SO₂, H₂S) interacted primarily with the PClH-material due to its cationic nature, which is in tandem with the electronic structural analysis.

Non-covalent interaction (NCI) analysis

Due to the limitations of QTAIM hypothesis to wholistically predict the type/origin of interactions in molecular species, the NCI approach is further utilized to comprehend the nature and type of interactions. The NCI based on promolecular electron density is utilized to this effect, these has the advantage of quantifying interactions based on pairwise partition of electron density between two interacting systems thereby providing a contribution of individual atoms to non-covalent interactions^{42, 43}. The iso-surface mapping for the various systems under consideration is representatively shown in Fig. 6. The green iso-surface appearance in the 3D plot of the studied systems reveals intermolecular interactions within the systems. Similarly, the projection of sparse-shattered green spikes in the 2D-RDG graph, of the value λ2 $\le 0$ for the studied systems gives affirmation to a weak van der Waal interactions. More so, the wide patches of green colouring surfaces between the atoms of the studied surfaces are relatively large compared to that of the PClH structure, depicting a stronger interaction of the gases with the PClH-material. This is illustrated lucidly from the 2D-RGD graph, where the plotted green spikes fall off in the negative region with eigen value (λ2)$\rho \cong$ − 0.02 a.u, compared to the spikes of the PClH-material and also as a result of the greater number of interacting atoms between the PClH and the studied gases. Therefore, it is affirmed that the dominant stabilization interactions occurring within the studied hybrid material and, the interaction complex is the Van daar Waals interaction and hydrogen bond interaction through the NH₂ group linking the aromatic ring and the ClO₄⁻ fragment. Significant steric repulsions are also, observed within the ring and between the NH proton and chlorine atoms due to bulkiness of the chlorine atoms attached to the PClH material.

Figure 6

Noncovalent interactions plot revealing the type of interactions within the studied gases and the hybrid material. The figures are plotted at 0.5 au isosurface value respectively.

Electron density delocalization (EDD)

The electron density difference is depicted as the difference between the molecular electronic density and the superimposed densities of the constituent non-interacting atoms⁴⁴. Thus, for the purpose of this study, the analysis of electron density difference was utilized to interpret the electron redistribution upon the formation of the various studied systems, which was calculated at the DSD-PBEP86/cc-pVTZ level of theory. And, in cognizance with the other molecular characterization techniques, EDD greatly foster the characterization of the diverse interactions between the adsorbent and the adsorbate molecules. Thus, the EDD plot obtained from the density difference are shown in Fig. 7. The iso-surface EDD mapping contains purple and green coloration at the various interacting sites of the adsorbed gases and the PClH-material, which indicates electronic charge shifts upon interactions. The green iso-surface EDD map portrayed the depletion of the electron density, whereas the purple mapping depicts the lack of electronic density. Thus, the green iso-surface mapping seen on the Nitrogen atom, oxygen atom, and hydrogen atoms (from the EDD plot) of studied systems shows notable polarization activity of the Nitrogen, Oxygen, and hydrogen atoms of the systems under study. Hence, this aids in the extraction of charges from the electron rich benzene fragment (chlorinated) cavity of PClH material.

Figure7

Electron density difference map of the studied compounds. The plots are obtained at 0.3 a.u isosurface value.

Consequently, the chlorine atoms attached to the PClH surface on interaction with the respective atoms of the adsorbed gas molecules are covered with purple isosurface with minute contribution of the green isosurfaces. The mixed isosurfaces was obviously observed to appear on the highly electronegative atoms of the systems (especially oxygen, Nitrogen and Chlorine), indicating their donating and retro-donating effect of these electronegative atoms. Hence, from the EDD plot of the studied systems, in the six studied system, a very large amount of electron leaves the PClH region, and at the same interval newly mapped electron occurred around the ClO4- and the gas systems, whereas the carbon atoms of the benzene fragments of oxygen atom which, are purple in color are deficient in electron. Therefore, the visibility of each gas-adsorbed system, indicate electronic charge exchange between the system of study. Thus, it is obvious that EDD analysis verified the outcome from NBO analysis which exhibited more charge transfer, in the case of PCl@CO₂ and PCl@SO₂ with electronic charge of 0.500é. Whereas, for the H₂S and NO₂ adsorbed system, the charge transfers sparingly occurred from the gas surfaces to the PClH surfaces as depicted in Table 4. Intrinsically, the greater charge transfer was observed in the case of COCl₂, CO₂, NO₂ systems which amount to 0.500é. Thus, this higher charge transfer can be attributed to the electronegativity of chlorine and oxygen, resulting in a better adsorption of the studied gases. Accordingly, the hybrid material can be utilized efficiently to trap the target gases.

Electron localization function (ELF)

The electron localization function (ELF) values are depicted as a measure of the likelihood of finding an electron located at a given point and with the same spin orientation. Thus, physically measuring the extent of spatial localization of a reference electron. In essence, ELF analysis was utilized to give a finite explanation of bonding characterization, specifically between the studied gases (COCl₂, CO₂, NO₂, SO₂, H₂S) and the bare PClH-material⁴⁵. Thus, Fig. S2 Portrayed the colour-sparse electron localization function mapping of the various studied systems. The colour shades in the ELF mapping reveal the existence of both bonding and non-bonding electrons, as shown in the ELF plot coded with high and low ELF regions. Hence, from the ELF plots, it can be inferred that the bonding and non-bonding electron localization fall specifically within the calibrated interval of 0.5–1.0, whereas the delocalized electrons are obviously distributed below the interval range of 0.5 with light cum dark blue colouration. Thus, with the clear observation of the colour-filled mapping for the studied systems, the regions within the molecules with dense localized bonding and non-bonding electrons are the hydrogen and chlorine group, which depicted a high ELF index and with yellow–red colour mapping for all the studied systems. It is also expedient to note that, the blue-coloured regions at the benzene ring centers and the gas fragments with the inclusion of the carbon atoms represent regions of delocalized electron density, while the red-mapped coloured regions between the carbon atoms and hydrogens depict a region of high covalency. In furtherance, it is observed that the obtained ELF range of 0.5, demonstrates its non-covalent interaction⁴⁶.

Therefore, the result of the electron localization mapping which, affirms the molecular electrostatic potential (MESP) analysis gives credit to carbon, hydrogen, and chlorine atoms as predominant sites of high electron locality. Overall, all these mapping parameters, lucidly suggest that the PClH-surface might be a preferred candidate for trapping and adsorbing the studied gases; COCl₂, CO₂, NO₂, SO₂, and H₂S, with the least adsorption ability towards NO2 and H₂S, as depicted from the ELF mapping in Fig. S2 with few localizations of electrons and bond interaction possibly due to greater extent of structural deformation during the adsorption process.

Molecular electrostatic potential (MESP)

Molecular electrostatic potential (MESP) mapping has been computed to obtain discernment into the liaison between the studied PClH structures and the reactivity of the molecules of interest in a three-dimensional assay. Figure 8 depicts the MESP distribution of the perchlorate (PClH) surface and the gas-adsorbed complex of the PClH structure, visualizing variably charged regions of the molecules and charge-related properties of molecules. Representatively, in the bare PClH-material, it is easy to see that it is optically active (i.e. a stereo structure) and its charge density distribution is in tandem with its electrostatic potential surface mapping at sites near the anionic group regions. Thus, for all the studied systems, it is obviously observed that the highly negative regions (with blue mapping) were located away from the benzene cationic fragment of the surfaces. Hence, stronger electrostatic potential energetic activity was exhibited on the anionic electronegative atoms; Chlorine (Cl), Oxygen (O), and in its entirety the ClO⁻₄ polar group. The perchlorate cationic ring, of which the chlorine group is a major contributor to the potential activity of the PClH-surface as chlorine is a strong electron-withdrawing group. As such, withdrawing electrons from the surrounding carbon atoms, because of the inductive effect. Thus, attributing to the denser character of the chlorine group of the Perchlorate (PCl-system), in regards, to possessing highly positive regions (red ESP mapping). In accordance with this assertion, it is worthy to note that, the ESP characteristic distribution of nucleophiles, of which chlorine as an anion is one seems to be competitively unique, as its ESP minimum point is within the range of 0.2 kcal/mol. More so, the wide-ranging minimum ESP mapping point on the ClO₄^- anion is densely uniform because of its tetrahedral geometry (as seen in Fig. 7). For the gas-adsorbed PCl-systems, the minimum ESP mapping of the anionic groups does not significantly differ from one another. In the case of the studied COCl₂@PClH, CO₂@PClH, NO₂@PClH, SO₂@PClH, and H₂S@PClH systems, the red mapping was also observed for the chlorine atom on the aromatic fragment, owing to the electron delocalization on the chlorine atom. Therefore, the low electrostatic potential energetic region is designated in blue colour and highly dispersed also on the ClO₄⁻ surface as a factor of its electronegativity. Thus, the MESP iso-surface molecular preference for nucleophilic or electrophilic attacks can be seen using the different ESP-mapped surface density(s).

Figure 8

Molecular electrostatic potential isosurface maps of (a) PClH, (b) COCl₂@PClH, (c) CO₂@PClH, (d) NO₂@PClH, (e) SO₂@PClH and (f) H₂S@PClH. Isosurface value is between 0.1 to 0.5 a.u.

Interestingly, in all the studied gas-adsorbed surfaces, the red colour on the surfaces indicates the region with high electron density. As such, nucleophilic attacks are likely to occur in this specific region, while the white-mapped part on the studied surfaces indicates the hydrogen groups. In essence, as observed; the electron density mapping for the studied gas@PClH adsorbed systems are intensely dense compared to the PClH material, which is in order; PClH > COCl₂@PClH > SO₂@PClH > H₂S@PClH > NO₂@PClH, representatively. Thus, these results can withhold the finding of the analyzed molecular parametric properties, which also suggest proficient interactions between the gas molecules and the PCl-surface. The presence of the most viable negative potentials of the adsorbed gas molecules results in stronger interaction between the gas molecules and the PClH-material. As depicted by quantum descriptors, with reference to ionization potential (− E_HOMO) and electron affinity (− E_LUMO). Thus, for all the studied systems, it was observed that the electron density dispersion mapping illustrated by the ESP iso-surface agrees with the calculated ionization potential and electron affinity of the investigated systems. Here, the affinity of strong interaction between the PClH-material and the gas molecules can be explained in terms of their interactive potential energies^{47, 48}. Therefore, the higher the − E_HOMO and − E_LUMO energy, the broader the electron density (red mapping) and the electrophilic mapping (blue mapping). Consequently, in relation to the quantum descriptors data of the PClH-material and the studied gas@PClH systems, the obtained MESP analysis obviously explains the interaction between the hybrid (PClH) material with all the studied gases efficaciously.

Sensing mechanism and recovery time

The sensitivity of the studied perchlorate (PClH) system towards the studied hazardous gases (COCl₂, CO₂, NO₂, SO₂, and H₂S) specifically depends on two parametric keys; Work function (ϕ) and energy gap (E_gap). Thus, the preceding equation is used in determining the sensitivity of the Organic–Inorganic materials toward the referenced gases.

$$\sigma ={AT}^\frac{3}{2} {e}^{\left(\frac{{E}_{gap}}{2KT}\right)},$$

where $\sigma$ is the electrical conductivity, A is the Richardson’s constant (253.2 A cm⁻² k⁻²)⁴⁹, T is the standard working temperature and K is Boltzmann’s constant (1.987 kcal$\times {10}^{-3}$/mol K⁻¹). According to the given equation, it is stated that a chemical sensor works based on the electrical conductivity change, upon adsorption of an adsorbate^50,51,52,53. Hence, with respect to the equation, a smaller energy gap at a given temperature leads to higher electrical conductivity. More so, the chemical effect of adsorption of the gases onto the studied material (PClH) with regards to fermi energy (${E}_{f}$) and work function (ϕ) were also investigated. From the calculated results in Table 5, it can be deduced that a minimal energy change was observed for the studied systems upon the adsorption of the considered gases, likewise the change in HOMO–LUMO energy. Consequently, in the COCl₂@PClH, NO₂@PClH, and SO₂@PClH systems, a minimal decrease was visibly observed in the HOMO and LUMO energy, from − 8.93 and − 1.56 eV for the bare PClH material to − 8.98 and − 1.54 eV, − 8.98 and − 1.54 eV, − 8.93 and 1.66 eV, representatively. Thus, initiating a minimal energy gap by 0.67%, 0.95%, and − 1.36%, respectively. However, in the case of the NO₂@PClH and H₂S@PClH systems, a much defined ΔE_gap (%) was observed, which can be attributed to the deformation imposed as a result of gas adsorption onto the surface of the investigated PClH system. Hence, the noticeable change in ΔEg after gas adsorption dynamically affects the electrical conductivity of the systems and as such, the examined Organic–Inorganic hybrid structure displays a maximal tendency to work as a chemical sensor for trapping these gaseous molecules under study, with the highest adsorption preference for COCl₂, CO₂, and SO₂. More so, from the analysed parametric sensing properties, the PClH-material has the affinity of trapping COCl₂, CO₂, and SO₂ gas more efficiently than the NO₂ and H₂S. Hence, the adsorption of COCl₂, CO₂, and SO₂ gas onto the PClH-surface affected the Fermi level and work function of each of the systems minimally and maximally respectively, which further affirmed that the PCl surface is a work function-based sensor and towards the gases. The calculated E_F is the mid-point of the HOMO and LUMO, while the ϕ is described as the required energy needed to take an electron from the Fermi level⁵¹. Theoretically, Fermi energy and work function can be calculated using the following illustrated equations.

$${E}_{f}=\left(\frac{HOMO+LUMO}{2}\right),$$

$${\Phi = -E}_{f}.$$

Table 5, illustrates that, the ϕ calculated values increases for the COCl₂, CO₂, and SO₂@PClH-systems, whereas a decrement in ϕ was observed for the NO₂@PClH and H₂S@PClH systems among all the gas@PClH systems studied. Tentatively, the tabulated negative values of E_ads have earlier suggested a stronger interaction and an exothermic reaction occurring in the investigated systems. Consequently, various interaction strengths of the gaseous molecules with the PClH-material affected the sensing assay of the studied systems^{51, 52}. Since, the tendency of desorption occurring in the system could be difficult and thus, the proposed studied systems may suffer from long recovery time. At the interim, it is prominent to calculate the recovery time of a chemical sensor, which reflects its usability and stability. The theoretical method used to determine the recovery time of the systems is through transition state theory, using the following equation.

$$\tau ={(\nu }^{-1}{e}^{{-E}_{ads}/KT}).$$

Here, $\tau$ is the recovery time,$\nu\, is\, the\, attempted\, frequency,$ ${E}_{ads}$ is the adsorption energy, K is Boltzmann’s constant, and T is the temperature in Kelvin, respectively. Thus, the recovery time is stated in Table 5. The calculated $\tau$ of the studied systems from the bare PClH-material is long and minimal because, the calculated ${E}_{ads}$ of the various PClH@gas systems are moderate enough to hinder the recovery of the PClH-chemical sensor. These analysed parametric values portray the PClH-material to be a useful chemical sensor material. In furtherance, depicting that the PClH@gas adsorbed systems benefited from a moderate recovery time ($\tau$) as a gas sensor for the respective gases (COCl₂, CO₂, NO₂, SO₂, and H₂S), with much adsorption preference to COCl₂, CO₂, and SO₂ liquefiable gases.

Thermodynamic consideration

Attributable to the fact that the purpose of this study is to investigate the adsorption of selected hazardous gases onto the surface of perchlorate hybrid material (PClH), and to elucidate its chemical, energetic, and sensing mechanism. Chemical thermodynamics provides a relationship between the macroscopic properties of a compound under investigation and the individual properties of its constituent molecules and atoms. Through the help of thermodynamic studies, the feasibility of the reaction and adsorption between the studied PClH-material and the PClH@gas adsorbed systems were investigated. According to the second law of thermodynamics which is focus on the determination of entropy generation because of the irreversibility of the system⁵², it is possible to ascertain if the reacting system is thermodynamically feasible or not for a certain adsorption condition. Energetic differences can be of vital use in evaluating suitable adsorption conditions and the reacting system performance by carrying out appropriate comparisons of the efficiency of the processes as well as reacting species before and after the reaction^{54, 55}. Thermodynamic properties such as entropy(S), constant temperature, enthalpy(H), Gibbs free energy (G), heat capacity (Cv) etc. for the formation of PClH, and adsorption of COCl₂, CO₂, NO₂, SO₂ gases by PClH material as laid out in Table S3 (of the supporting information) has been carefully calculated using density functional theory methods for equilibrium geometry state for the formation of C₆H₅Cl₂NClO₄ (PClH) and of the gases being adsorbed by the PClH-material with a temperature of 298.150 K.

$$\Delta {\text{H}}^{ \circ } \left( {{298}\;{\text{K}}} \right) \, = \Sigma \, \Delta_{{\text{f}}} {\text{H}}^{ \circ } \;{\text{product at}}\left( {{298}\;{\text{K}}} \right) \, {-} \, \Sigma \, \Delta_{{\text{f}}} {\text{H}}^{ \circ } \;{\text{reactants }}\left( {{298}\;{\text{K}}} \right),$$

$$\Delta_{{\text{f}}} {\text{H}}^{ \circ } \left( {{298}\;{\text{K}}} \right) \, = \, \Sigma \, \left( {{\text{E}}_{0} + {\text{ H}}_{{{\text{corr}}}} } \right){\text{ product }}{-} \, \Sigma \, \left( {{\text{E}}_{0} + {\text{ H}}_{{{\text{corr}}}} } \right){\text{ reactants}}.$$

Δ_fH⁰(298 k) and ΔH° (298 K) represent the standard enthalpy of the formation of the product and summation of the total standard enthalpy of the reacting species, Σ(E₀ + H_corr) product and Σ(E₀ + H_corr) reactants show the summation of electronic energy EE $\cong$ (E₀) and that of electronic energy (EE) + thermal enthalpy correction (H_corr) of the product and reactants respectively.

$$\Delta_{{\text{f}}} {\text{G}}^{ \circ } \left( {{298}\;{\text{K}}} \right) \, = \, \Sigma \left( {{\text{E}}_{0 \, + } {\text{G}}_{{{\text{corr}}}} } \right)\;{\text{product }}{-} \, \Sigma \left( {{\text{E}}_{0} + {\text{ G}}_{{{\text{corr}}}} } \right)\;{\text{reactants}}.$$

Δ_fG° (298 K) represents the standard Gibbs free energy of reaction, while Σ (E₀ + G_corr) product, Σ(E₀ + G_corr) reactants, denotes the summation of electronic energy (EE) and electronic energy + thermal free energy correction for the product and reactants respectively. In Table S4 the formation of perchlorate is said to be highly spontaneous and feasible following the high standard free energy of formation(Δ_fG°) of 750,723.82 kcal/mol (PClH), whereas the enthalpy (Δ_fH°) value of − 3915.97 kcal/mol suggest a more spontaneous and endothermic reaction for the formation of C₆H₅Cl₂NClO₄(PClH). Moreover, other cases of adsorption also disclosed a spontaneous and feasible adsorption process by exhibiting negative standard enthalpy of − 2,705,832.70 kcal/mol (CO₂@PClH), − 4.542 kcal/mol (NO₂@PClH), − 21.95 kcal/mol (SO₂@PClH), − 13.48 kcal/mol (COCl₂@PClH) respectively, while disclosing a positive enthalpy of 2.12 kcal/mol for the PCl@H₂S system, indicating that on comparison with other studied compounds, its formation will require more energy. Consequently, its adsorption formation is an endothermic process, this is affirmed by the lesser adsorbing tendency of the H₂S gas on the surface of the studied PClH material. Obviously, the calculated Δ_fG° of the PClH material signifies the non-spontaneity of the surface, which was achieved by the positive standard Gibb’s free energy of formation obtained. However, on adsorption of the studied gases (COCl₂, CO₂, NO₂, SO₂, and H₂S) by the PClH material, negative standard Gibbs free energy of − 304.98 kcal/mol, − 2,705,886.38 kcal/mol, − 8.32 kcal/mol, − 10.08 kcal/mol, and − 37.35 kcal/mol were obtained for the adsorption of the gases by the PClH material respectively. Thus, this affirms that the adsorption reaction is feasible and highly spontaneous. Therefore, the relationship between Gibbs free energy and enthalpy as a spontaneous reaction is one in which the standard enthalpy of formation(Δ_fH°) and standard Gibbs free energy (Δ_fG°) is negative, and a non-spontaneous reaction is one in which the standard enthalpy (Δ_fH°) and standard Gibbs free energy (Δ_fG°) is positive. However, scrutinizing the computed enthalpy values for the respective systems, the adsorption of CO₂ by PClH material is more spontaneous and feasible following its high outstanding standard enthalpy and Gibbs free energy value of − 2,705,832.701 kcal/mol and − 2,705,886.378 kcal/mol respectively. The adsorption of COCl₂ by PClH is more feasible and spontaneous than NO₂ and SO₂ from the enthalpy Δ_fH° (298 K) value of − 13.48 kcal/mol which is comparably higher than that of NO₂ and SO₂ from Table 6. In tandem with this, the free energy (ΔG) of COCl₂ adsorption with an energy value of − 304.98 kcal/mol by the PClH is comparably spontaneous than that of NO₂ (8.32 kcal/mol) with a positive energy value, likewise CO₂ (− 10.08 kcal/mol) and H₂S (− 37.35 kcal/mol) respectively.

The theoretically calculated standard entropy and specific heat capacity give more assertion to the spontaneity of the studied systems and the heat per unit mass required to raise the temperature of the studied systems by 1 °C. As such the positive values of the systems as presented in Table S3, affirms the entropy of the studied systems, thus increasing the system’s spontaneity. Representatively, grand correlational values of ${\Delta S}^{\circ }$ were obtained for the systems under study, with values ranging from 123.494 to 155.768 cal/mol K⁻¹. With regards to the specific heat capacity of the studied systems, a lesser value of specific heat capacity shows a greater energetic property. Intrinsically, the energetic property of the PClH material of interest is greater than its gas-adsorbed derivatives. It is worth noting that the increased heat capacity value which was observed for the studied gas-adsorbed systems was a result of the increase in molecular weight observed upon the adsorption of the studied gases, which is illustrated in Table 6. Therefore, the sensitivity of PClH to heat decreased on the adsorption of the gases, as high specific heat capacity requires a greater amount of heat to increase its temperature ^56,57,58.

Conclusion

A novel organic–inorganic hybrid material (PClH) has been synthesized and its efficacy for selectively adsorbing hazardous atmospheric gases has been investigated in this work using DFT computations. The hybrid material showed a higher affinity for SO2, COCl2, and CO2 gases compared to other investigated gases. Recovery time for the chemical sensor was observed to be within the range of 2.55 × 1013S to 4.44 × 1020S (at 298 K), indicating easy recovery under ambient conditions. The PClH material is projected as a potent chemical sensor for trapping hazardous liquefiable gases (COCl2, CO2, and SO2) for an efficient sustainable, and green environment. The thermodynamic assay revealed that the parameters of enthalpy, free Gibbs energy, standard entropy, and heat capacity values provided more evidence of the spontaneous formation of the studied systems, except for the NO2@PClH and H2S@PClH systems which displayed significant discrepancy in standard enthalpy and free Gibbs energy. The results of the interaction energy, binding energy, DOS, NCI QTAIM, ESP, ELF, NBO, FMO, and thermodynamic analyses are all consistent and support the conclusion that the novel organic–inorganic hybrid material PClH is selective towards COCl2, CO2, and SO2 gases. Overall, the results offer a substantial prospect for the utilization of the studied hybrid material towards the development of efficient and sustainable technologies for controlling hazardous liquefiable gases in the environment.