Introduction

Acetylene (C2H2) is an important raw material for the coal chemical industry in China. The naturally occurring fossil fuel can be obtained from abundant coal reserves, especially in the western region. With the rise of petroleum prices and the increase in demand for chemical industry sites, developing sustainable acetylene production methods is an important focus of researchers [1,2,3,4,5]. Furthermore, acetylene is an important hydrocarbon source for large-scale preparations of oxygenated products, such as acetone, acetic acid, and acetaldehyde [6,7,8]. Among them, acetaldehyde is an important organic chemical intermediate, which can be used in the production of acetic acid, acetate, pentaerythritol, crotonaldehyde, trichloroacetaldehyde, glyoxal, vinyl acetate, and pyridine compounds and has a wide range of applications in pesticides, medicine, food, and feed additives. As such, there is a great demand for acetaldehyde in industry, exceeding a global production of 106 tons per year [9,10,11,12,13].

Since 1916, industrial acetylene has been utilized in the production of its hydration products, indicating the importance of acetylene as a chemical raw material [7]. The two main routes for producing acetaldehyde are the direct oxidation of ethylene and the hydration of acetylene. In many developed countries, the direct oxidation of ethylene to acetaldehyde is an attractive approach. However, water is widely used in organic synthesis owing to its low price and nontoxicity [14, 15], and green, environmentally benign processes that maximize economic benefits are among the goals of chemical technology. Thus, the hydration of acetylene has received increased attention as the preferred route for the production of acetaldehyde in China and other countries that would benefit from abundant coal resources [7, 16].

The hydration of acetylene was first reported in 1881 by Kutscheroff [17]. Since this seminal work, many researchers have investigated the use of transition metals as active catalysts in acetylene hydration reactions (e.g., Cu2+, Cd2+, Hg2+, Ag+, Ru2+, and Zn2+) [18,19,20,21]. In these studies, cationic sites have been shown to be more active for acetylene hydrations than negatively charged species. In another study, Dai et al. [22] investigated the reactivity of a Cu(I)–S catalyst with DFT calculations; however, these calculations were limited to the relative energy of the intermediates. Other works have shown that Hg2+, Cu2+, and Ag+ are more likely to be reduced to the respective zero-valent metals by acetylene, causing a reduction in the catalyst’s activity. In addition, the high toxicity and volatility of cadmium make it harmful for both industrial workers and the entire ecosystem [23]. Therefore, considering reactivity and economic and environmental factors, active zinc has shown promise as a suitable, active component to replace cadmium in acetylene hydration processes. Wang et al. [24,25,26] demonstrated the use of zinc chloride (ZnCl2) as a catalyst for acetylene hydration, which displayed high catalytic activity (96.1% C2H2 conversion). The researchers characterized the surface acid sites of the catalysts and proposed that Zn(OH)+ may serve as an active compound in the transformation via XPS analysis. Despite these experimental findings, the authors did not theoretically investigate the reaction mechanism at the molecular level, limiting our full understanding of the reaction.

In the present work, we elucidate the reaction mechanisms of Zn catalysts in acetylene hydration reactions via computational approaches. It is worth noting that theoretical studies on acetylene hydration are rare. Cooksy et al. [27] presented a computational model based on DFT calculations for the hydration of acetylene to form acetaldehyde on the organotransition metal fragment CpRu(PMe2Im′)2+ (where Me = methyl and Im′ = 1,4-dimethylimidazol-2-yl). The researchers predicted a ten-step reaction mechanism, and the results showed that the highest predicted activation barrier was 28.4 kcal mol−1. In another report, Najafian et al. employed computational techniques to study acetylene hydration by six bioinspired catalytic models [28]. Despite these advances, atomistic-level insights on processes occurring at the surface of Zn catalysts remain scarce. As such, we sought to design and investigate acetylene hydration reactions over Zn-based catalysts, i.e., ZnCl2, a new catalyst Zn(OH)Cl, and Zn(OH)2. We anticipate that a new understanding on the mechanism of Zn-catalyzed acetylene hydrations would help to inform the design of promising industrial catalysts.

Computational methods

All DFT calculations were carried out via the Gaussian 09 software package [29]. Geometric optimizations of the co-adsorption species, intermediates (Ims), transition states (Ts), and de-adsorption species were calculated with the B3LYP hybrid functional, which combined Becke’s three-parameter exchange functional (B3) and the Lee–Yang–Parr (LYP) functional in conjunction with the Los Alamos effective core pseudo-potential basis set LANL2DZ for zinc atoms, and the 6-31++G** basis set for carbon, hydrogen, oxygen, and chlorine atoms [30,31,32,33,34]. The relative energies of all optimized structures along the reaction pathways were zero-point energy corrections at the same level of optimization. We optimized single-adsorption and co-adsorption structures with correction for basis set superposition error (BSSE) [35]. No symmetry constraints were imposed on the geometry optimizations. For the Hessian matrix calculation, all of the stationary points were characterized as minima (no imaginary frequency) or transition states (one imaginary frequency). Intrinsic reaction coordinate (IRC) [36,37,38,39] calculations were performed to confirm that the correct products were linked to the corresponding reactants. Transition states were characterized using frequency calculations implemented at the same level of theory by analyzing the vibrational modes.

Fukui function [40] is a very important concept in the conceptual density functional theory, we calculate and visualize it in prediction of reactive sites and adsorption sites in this work. The condensed Fukui function [41] was calculated to analyze the nucleophilicity and electrophilicity of Zn atom. After the condensed Fukui function analysis, we predicted the electron acceptor and donor in the reactions. In order to confirm the reliability of the Fukui function analysis, Hirshfeld population data [42] were cited to analyze electron transfer behavior between the adsorbates and substrates. Adsorption energy (Eads) and co-adsorption energy (Eco-ads) calculations were carried out to reveal the adsorption behavior of the catalysts and are defined as follows:

$$ {E}_{\mathrm{ads}}={E}_{{\mathrm{C}}_2{\mathrm{H}}_2/{\mathrm{H}}_2\mathrm{O}+\mathrm{catalyst}}-{E}_{{\mathrm{C}}_2{\mathrm{H}}_2/{\mathrm{H}}_2\mathrm{O}}-{E}_{\mathrm{catalyst}} $$
(1)
$$ {E}_{\mathrm{co}-\mathrm{ads}}={E}_{{\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{catalyst}}-{E}_{{\mathrm{C}}_2{\mathrm{H}}_2}-{E}_{{\mathrm{H}}_2\mathrm{O}}-{E}_{\mathrm{catalyst}} $$
(2)

The EC2H2/H2O+catalyst is the whole energy of the system that H2O or C2H2 is adsorbed on the catalyst. The EC2H2+H2O+catalyst is the total system adsorption energy that both of the reactants are adsorbed on the catalyst. Furthermore, EC2H2, EH2O, Ecatalyst are the energy of the isolated catalyst and the reactants, respectively. The Multiwfn 3.7 program [43] was used to calculate all the Fukui function, condensed Fukui function, and Hirshfeld charges. All reactions were carried out in the gas phase. Fukui function and condensed Fukui function were also calculated in the gas phase, ignoring the effect of solvents [44]. In this work, we chose the Hirshfeld’s stockholder approach [45] to calculate condensed Fukui function.

Results and discussion

Geometries for reactants

Reactants used for these calculations included an isolated C2H2 molecule, an H2O molecule, a ZnCl2 molecule, a Zn(OH)Cl molecule, and a Zn(OH)2 molecule. The most stable optimized geometries for the reactants are shown in Fig. 1.

Fig. 1
figure 1

Optimized structures of C2H2, H2O, ZnCl2, Zn(OH)Cl, and Zn(OH)2. Distances are in Å and angles in degrees. Carbon, hydrogen, chlorine, oxygen, and zinc atoms are depicted in gray, white, green, red, and blue, respectively

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Adsorption of C2H2 and H2O onto catalysts

Figure 2 shows the calculated f+and f− of the Fukui function for the ZnCl2, Zn(OH)Cl, and Zn(OH)2 catalysts. The f+and f− of Fukui function for these catalysts were employed to predict their adsorption sites and reactive sites. Table 1 shows the condensed Fukui functions for ZnCl2, Zn(OH)Cl, and Zn(OH)2, as well as the Hirshfeld charges for all atoms of Zn catalysts. With the addition of hydroxyl groups, the positive charge of Hirshfeld charges of Zn atoms increases, indicating that the electrophilicity of Zn atoms is enhanced. The condensed Fukui function fA+ of Zn catalysts were bigger than those of fA−, indicating that the Zn atom was the acceptor of the electron and it was easy to obtain the electron, which was a nucleophilic attack site. Likewise, the value of fA+ and fA− of the O atom in Zn(OH)Cl and Zn(OH)2 molecule indicated that O atom was an electrophilic attack site. The presented data show that the fA+ of ZnCl2 (0.5217) was smaller than Zn(OH)Cl (0.5286) and Zn(OH)2 (0.5540), suggesting that the hydroxyl group enhanced the chemical reactivity of the Zn catalyst.

Fig. 2
figure 2

The calculated Fukui function of ZnCl2 (a(fA+), b(fA−)), Zn(OH)Cl (c(fA+), d(fA−)), and Zn(OH)2 (e(fA+), f(fA−)) (isovalue = 0.002)

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Table 1 The Hirshfeld charges, condensed Fukui functions for the Zn catalysts
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The Hirshfeld charges of Zn catalysts and Zn atom for the C2H2/H2O–Zn catalysts adsorption complexes in Table 2 illustrated that electrons could be transferred from the reactant to the Zn catalyst. It further confirmed the veracity of the condensed Fukui function. We also calculated the electrostatic potential (ESP) of Zn catalysts (Fig. S1), showing that the charge population of the Zn catalysts consistent with the conclusion of condensed Fukui function.

Table 2 The Hirshfeld charges of Zn catalysts for the C2H2/H2O–Zn catalysts adsorption complexes
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In order to find the best adsorption configurations for the reactions, we constructed all possible C2H2 and H2O adsorption sites on the ZnCl2, Zn(OH)Cl, and Zn(OH)2 species manually. The most stable catalyst–C2H2 and catalyst–H2O complexes as well as their configurations are depicted in Fig. 3.

Fig. 3
figure 3

a–f the most stable adsorption configurations for C2H2/H2O on the following Zn catalysts: a C2H2–ZnCl2, b H2O–ZnCl2, c C2H2–Zn(OH)Cl, d H2O–Zn(OH)Cl, e C2H2–Zn(OH)2, f H2O–Zn(OH)2

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The adsorption energy values of C2H2 and H2O show a lower barrier for the adsorption of H2O than C2H2 on the Zn catalyst surface (Table 3). These data indicate that H2O is preferentially adsorbed onto the Zn catalysts over C2H2. We provided the ESP of single-adsorption of C2H2/H2O–Zn catalysts in the Fig. S2, in order to find the best optimal co-adsorption (co-ads) configuration. ESP analyses of the catalyst–H2O complexes were carried out to investigate the interactions between C2H2 and the adsorption complexes, making it possible to obtain the co-adsorption configurations of the catalysts with H2O and C2H2. In Fig. S3, the ESP of the co-adsorption of reactants and Zn catalysts showed that the color of Zn atom was lighter than in single adsorption; in other words, Zn atom accepted electron from the reactants, The Hirshfeld charges of co-adsorption configuration in Table 4 further confirmed the veracity of the ESP. The co-adsorption energies were shown in Table 4. The Eco-ads of ZnCl2–H2O–C2H2 was − 16.93 kcal/mol. The Eco-ads of the first and the second reaction pathway of Zn(OH)Cl–H2O–C2H2 were − 16.00 kcal/mol and − 18.86 kcal/mol, respectively. Likewise, the Eco-ads of Zn(OH)2 were − 14.77 kcal/mol and − 17.02 kcal/mol.

Table 3 Optimal adsorption energies (kcal mol−1) of C2H2 and H2O adsorbed separately and co-adsorbed on the catalyst
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Table 4 The Hirshfeld charges of Zn atom for co-adsorption configuration
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Reaction mechanisms of acetylene hydration on catalysts

ZnCl2

All possible reaction pathways were systematically investigated to better understand the catalytic mechanism of acetylene hydration. The hydration of acetylene using ZnCl2 as the catalyst proceeded via a concerted reaction mechanism. Figure 4 illustrates the reaction pathways of ZnCl2, which originate from C2H2 and ZnCl2–H2O co-adsorption structures. All geometries of the various stationary points located on the potential energy surface are depicted in Fig. 5.

Fig. 4
figure 4

The energy profiles for the reaction pathway ZnCl2

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Fig. 5
figure 5

Optimized structures of stationary points for reaction channel ZnCl2

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The most stable adsorption states of the catalyst during activation of the reactants were determined. During the formation of the co-adsorption structure with the ZnCl2–H2O complexes, the bond length of H2O was largely unchanged, indicating that the H2O molecule was not activated in the co-adsorption structure. The co-adsorption structure was then transformed into Ims1 via Ts1. At Ts1, the H4–O bond broke, and its bond length increased from 0.97 to 1.52 Å. Simultaneously, the H4 atom approached the C=C bond, and the C1–H4 distance decreased from 2.35 to 1.22 Å. The distance between the O atom and C2 atom decreased from 3.11 to 2.24 Å during this transformation. In the transition state structures, the H4–O bond of H2O was substantially lengthened, leading to direct H2O elimination, which indicated the complete activation of H2O. Our calculations also revealed distortion in the configuration of acetylene in which the angle of H1–C1–C2 decreased from 177.28° to 143.62°. This distortion suggests that there are interactions between C2H2 and the H4 atom.

The only imaginary frequency (− 1302.96 cm−1) was obtained from the vibrational analysis of Ts1, indicating that it is a valid transition state. This frequency was associated with the stretching movement of the H4 atom. Since the energy barrier of Ts1 was calculated to be 42.78 kcal mol−1, this transformation was assigned as the rate-determining step.

To ensure that this transition state connected the appropriate reactant and product, IRC calculations were performed and confirmed that Ts1 connects the co-adsorption structure and Ims1. At Ims1, the C1 atom was bonded to the H4 atom, and the length of C1–H4 was shortened to 1.087 Å, which was almost equal to the C1–H4 bond length (1.097 Å) of the isolated acetaldehyde molecule. In addition, the O atom is bonded to the C2 atom in this state with a bond length of 1.39 Å. It is worth noting that this bond length is shorter compared with Ts1. During the final step, the organic product acetaldehyde was produced via tautomerization of vinyl alcohol, which was calculated to be an exothermic process.

Zn(OH)Cl

The acetylene hydration reaction pathway catalyzed by Zn(OH)Cl was then studied through DFT computations. We found two reaction pathways for the hydration reaction using Zn(OH)Cl as the catalyst which was very distinct from ZnCl2. All geometries and their corresponding energy values in the first kind of the reaction pathway are depicted in Figs. 6 and 7. Owing to the stronger adsorption of H2O compared with C2H2, H2O was initially adsorbed onto Zn(OH)Cl, followed by adsorption of C2H2 onto the H2O–Zn(OH)Cl complex to form the co-adsorbed intermediate. The adsorbed C2H2 molecule was nearly identical in length to the O2–Zn bond in Zn(OH)Cl. The bond lengths of C1–C2 and H4–O1 were identical within 0.05 Å, indicating that molecular H2O was not activated in this state. During the first step, a barrier of 11.26 kcal mol−1 was initially overcome, and the co-adsorption structure was transformed into the intermediate state (Ims1) via transition state, Ts1. At Ts1, the Zn(OH)Cl molecule was distorted, with a decrease in the Cl–Zn–O2 angle from 153.50° to 141.91°. Simultaneously, distortion of C2H2 was detected, and the C1=C2 and O2–Zn bonds were slightly lengthened. These calculations revealed that there was an interaction between C2H2 and Zn(OH)Cl. Specifically, the C2 and C1 atoms of C2H2 approached the O2 and Zn atoms of the catalyst, with the C2–O2 and C1–Zn distances decreasing from 3.02 Å to 2.06 Å and 2.74 Å to 2.24 Å, respectively.

Fig. 6
figure 6

The energy profiles for the first reaction pathway Zn(OH)Cl

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Fig. 7
figure 7

Optimized structures of stationary points for the first reaction channel Zn(OH)Cl

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It was determined that Ts1 had only one imaginary frequency (− 386.39 cm−1), indicating that it was a valid transition state, and an energy barrier of 11.26 kcal mol−1 was predicted. The IRC calculation of this transition state confirmed that Ts1 connects the co-adsorption intermediate to Ims1, suggesting that no additional intermediates were involved in the first reaction step. At Ims1, the Zn–O2 bond was broken, followed by bonding of the O2 and Zn atoms to the C2 and C1 atoms, respectively, with lengths of 1.40 Å (O2–C2) and 2.01 Å (Zn–C1). The lengths of the bonds in Ims1 were shortened in comparison to Ts1. In this step, the two carbons in C2H2 transformed from a triple bond to a double bond.

Ims1 was shown to lead to Ims2 via Ts2. The pathway to Ts2 was calculated to involve the participation of an H2O molecule. Specifically, the O1–H3 bond of H2O was substantially elongated from 0.98 Å in Ims1 to 1.24 Å in Ts2, indicating the complete activation of H2O. Then, the H3 atom dissociated from the O1 atom and bonded to the adjacent C1 atom, and the corresponding H3–C1 bond length was shortened to 1.40 Å. Simultaneously, the Zn–C1 bond was broken, and the O1 atom bonded to the Zn atom. In this scenario, the length of Zn–C1 was elongated from 2.01 Å to 2.19 Å. At Ts2, the newly generated Zn(OH)Cl catalyst was identified. The length of the Zn–O1 bond (2.01 Å) was 0.1 Å longer than that of the Zn–O2 bond (1.91 Å) in the co-adsorption state. Moreover, the angle of O1–Zn–Cl (140.11°) was 13.39° smaller than that of O2–Zn–Cl (153.50°) in the co-adsorption state. To improve the efficiency of the reaction, the enolic group was rotated to make it nearly perpendicular to the Zn(OH)Cl catalyst. However, other significant bonds, such as C1=C2 and C2–O2, were barely changed during this transformation. The only imaginary frequency (− 1335.21 cm−1) was obtained from the vibrational analysis of the Ts2 structure, which is associated with the stretching movement of the H3 atom. To access Ts2, Ims1 indicated an energy barrier of 21.14 kcal/mol.

IRC calculations were performed again to confirm that Ts2 could convert to Ims2. In Ims2, the newly generated H2O molecule from Ims1 was determined to serve as an acid by transferring a proton to C1 of the vinyl anion to form a vinyl alcohol intermediate. The Zn–O1 bond was shortened to 1.85 Å during this transformation, and the Cl–Zn–O1 angle was increased from 140.11° in Ts2 to 152.41° in Ims2, indicating that the newly formed Zn(OH)Cl catalyst became more stable in the latter state. It is worth noting that the length of the O1–H3 bond increased by 3.14 Å from Ts2 (1.24 Å), and the length of the C1–H3 bond decreased from 1.40 to 1.09 Å. When employing Zn(OH)Cl as the catalyst, the energy barriers for the two steps of the reaction were calculated to be 15.46 and 21.14 kcal mol−1, respectively, with the second step being the rate-determining step. The final step of the reaction was tautomerization of the vinyl alcohol to generate acetaldehyde, which was calculated to be an exothermic process.

Then, we found the second reaction pathway of the Zn(OH)Cl catalyst which is distinct from the first one. All geometries and their corresponding energy values in the second kind of the reaction pathway are depicted in Figs. 8 and 9. In contrast to the reaction pathways catalyzed by ZnCl2, H2O and the Zn(OH)Cl catalyst both participated in the first step of the reaction. One proton from the H2O molecule and one hydroxyl group from Zn(OH)Cl were predicted to simultaneously react with the C≡C triple bond of acetylene. In this step, the co-adsorption structure transformed into an intermediate state (Ims1) via a transition state (Ts1). At Ts1, the Zn(OH)Cl molecule was distorted, with a decrease in the O2–Zn–Cl angle from 155.27° to 124.43°. Simultaneously, distortion of the C2H2 molecule was detected, and the O1–H3 and O2–Zn bonds were slightly lengthened. In this state, the O1–H3 bond was broken, demonstrating that the H2O molecule was definitively activated. The H3 and O2 atoms approached the C1 and C2 atoms, and the C2–O2 and C2–H3 distances decreased from 2.96 to 1.97 Å and from 2.42 to 1.62 Å, respectively. Identification of distorted acetylene suggested that there were strong interactions between the acetylene molecule, H3 proton, and Zn(OH)Cl. Only one imaginary frequency was found in Ts1, indicating that it is a valid transition state. The energy barrier for this step was calculated to be 18.11 kcal mol−1 relative to the co-adsorption state and was assigned as the rate-determining step.

Fig. 8
figure 8

The energy profiles for the second reaction pathway Zn(OH)Cl

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Fig. 9
figure 9

Optimized structures of stationary points for the second reaction channel Zn(OH)Cl

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IRC calculations were then carried out and confirmed that Ts1 connected the co-ads state to Ims1. In Ims1, vinyl alcohol was formed, and the Zn(OH)Cl catalyst was regenerated. The O2–Zn–Cl angle increased from 124.43° to 160.42°, and the C1 and C2 atoms were bonded to the separated H3 and O2 atoms. The lengths of the C1–H3 and C2–O2 bonds were 1.40 Å and 1.08 Å, respectively, and both bonds were shortened in comparison to the previous transition state (Ts1).

The final step of this reaction was similar to the previously studied catalysts and involved the release of acetaldehyde following tautomerization of vinyl alcohol. However, in contrast to the previous two catalytic systems, this step was calculated to be an exergonic process for Zn(OH)Cl.

Zn(OH)2

Figures S4, S5, S6, and S7 display two kinds of relevant energy and structures of the acetylene hydration reaction catalyzed by Zn(OH)2. The reaction pathways are similar to the reaction pathways catalyzed by Zn(OH)Cl. We provide only a brief analysis of the two reaction pathways below.

In the first kind of reaction pathway, the reaction converted into three steps. During the first step, a barrier of 10.90 kcal mol−1 was initially overcome, and the co-adsorption structure was transformed into the intermediate state (Ims1) via transition state, Ts1. The IRC calculation of this transition state confirmed that Ts1 connects the co-adsorption intermediate to Ims1, suggesting that no additional intermediates were involved in the first reaction step. Ims1 was shown to lead to Ims2 via Ts2. The pathway to Ts2 was calculated to involve participation of an H2O molecule. The only imaginary frequency (− 1318.75 cm−1) was obtained from the vibrational analysis of the Ts2 structure. To access Ts2, Ims1 indicated an energy barrier of 20.73 kcal/mol. IRC calculations were performed again to confirm that Ts2 could convert to Ims2. The final step of this reaction was similar to the previously studied catalysts and involved the release of acetaldehyde following tautomerization of vinyl alcohol.

In the second kind of reaction pathway, the reaction pathway started from the co-adsorption structure, which was converted into acetaldehyde in two steps: a distinct vinyl alcohol formation step and vinyl alcohol isomerization to form acetaldehyde. One proton from the H2O molecule and one hydroxyl group from Zn(OH)2 were predicted to simultaneously react with the C≡C triple bond of acetylene.

In this step, the co-adsorption structure transformed into an intermediate state (Ims1) via a transition state (Ts1). Only one imaginary frequency was found in Ts1, indicating that it is a valid transition state. The energy barrier for this step was calculated to be 19.51 kcal mol−1 relative to the co-adsorption state and was assigned as the rate-determining step. IRC calculations were then carried out and confirmed that Ts1 connected the co-ads state to Ims1. In Ims1, vinyl alcohol was formed, and the Zn(OH)2 catalyst was regenerated. The final step of this reaction was similar to the previously studied catalysts and involved the release of acetaldehyde following tautomerization of vinyl alcohol.

Conclusions

In the present work, the hydration of acetylene to acetaldehyde catalyzed by a series of Zn catalysts (ZnCl2, Zn(OH)Cl, and Zn(OH)2) was theoretically studied using DFT methods. The Fukui function, condensed Fukui function, Hirshfeld charges, and ESP of the reaction processes enabled the prediction of the catalytic active sites and electron transfer information. The real acetylene hydration reaction was gas and solid phases process, we neglected the limitations of their gas-phase calculations. Some research confirmed the validity of the negligible effect of solvents on the Fukui function. We calculated different reaction pathways for the three Zn-based catalysts, providing insights into their distinct reaction mechanisms and catalytic performances.

To summarize, the catalytic performance of ZnCl2 was inferior to Zn(OH)Cl and Zn(OH)2. Our calculations predict that in all catalytic systems, when H2O was added to acetylene, a vinyl alcohol intermediate was formed. Subsequently, isomerization of the alcohol afforded acetaldehyde as the desirable product. For the first kind of reaction pathway catalyzed by Zn(OH)Cl and Zn(OH)2, acetylene reacted with the catalyst via interaction of its carbon atom with Zn to form a vinyl–metal complex. Meanwhile, the proton of the H2O molecule reacted with the complex via electrophilic addition. Then, vinyl alcohol was formed, and the Zn(OH)Cl or Zn(OH)2 catalyst was regenerated. Finally, tautomerization of the vinyl alcohol yielded acetaldehyde. For the second kind of reaction pathway catalyzed by Zn(OH)Cl and Zn(OH)2, our calculations suggested that it undergoes a concerted mechanism. The hydroxyl group from the Zn(OH)Cl or Zn(OH)2 catalyst and the hydrogen in the H2O molecule reacted with acetylene at the same time, forming the vinyl alcohol. The isomerization of vinyl alcohol to acetaldehyde occurred in the last step.

The IRC calculations were consistent with the proposed transformations. The activation barrier for the reaction of acetylene hydration was 42.78, 21.14 (18.11), and 23.22 (19.51) kcal mol−1 for ZnCl2, Zn(OH)Cl, and Zn(OH)2, respectively. In these three reactions, we found that the O–H bond in H2O was relatively easy to break, lowering the activation barrier of the reaction. We anticipate that this reactivity could be exploited when designing a new acetylene hydration catalyst. We predict that Zn single atomic photocatalytic hydrolysis catalyst may be a promising catalyst applied to this reaction. Taken together, our calculations reveal that Zn(OH)Cl and Zn(OH)2 are promising as catalysts for the hydration of acetylene.