Carbon-supported Zn-HPW ligand catalysts for acetylene hydration†

Zhen Chen, Dingjie Luo, Qinqin Wang*, Long Zhou, Yufan Ma, Fangjie Lu* and Bin Dai*
School of Chemistry and Chemical Engineering, Shihezi University/State Key Laboratory Incubation Base for Green Processing of Chemical Engineering, Shihezi 832000, China. E-mail: wqq_shzu@sina.com; lfj_echo@shzu.edu.cn; db_tea@shzu.edu.cn

First published on 20th August 2024

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1. Introduction

Acetaldehyde is an important chemical raw material and intermediate, which is widely used in pharmaceuticals, plastics, textiles, dyes and fragrances.¹ With the continuous development of industry, the global demand for acetaldehyde will gradually increase in the future.² At present, acetaldehyde is mainly produced by ethylene and acetylene processes.^3–5 In general, countries with abundant oil resources prefer the ethylene method, but the production cost of this method is relatively high. On the other hand, the production of acetaldehyde by the acetylene method with abundant raw materials still has high research value.

The acetylene process for the production of acetaldehyde consists of two catalytic systems: the gas–liquid phase and gas–solid phase. The gas–liquid phase catalytic system is mainly focused on the study of Hg-based and Cu-based catalysts under strong acid conditions. In 2014, Trotus et al.⁶ found that the prepared strong acid mercury catalyst was easily reduced in the reaction, leading to the rapid inactivation of the catalyst. Yee et al.⁷ prepared a Hg and –SH complex Zr(IV)-based metal–organic framework (MOF) with only 1% load by the ion exchange method, which can catalyze acetylene hydration at room temperature. In addition, mercury is volatile and can seriously harm human health and pollute the environment. Yang et al.⁸ added a thiomalate ligand to Cu-based catalysts, which significantly improved the conversion of acetylene and the selectivity to acetaldehyde. However, the gas–liquid phase catalytic system has still suffered from low conversion, difficult catalyst recovery, high production cost, and environmental pollution to date.

Studies have shown that gas–solid phase catalytic systems can well avoid the above disadvantages, and the current research mainly focuses on metal catalysts (Zr, Cu, Zn, etc.), metal carriers and the reaction mechanism of zinc-based catalysts. Wang et al.⁹ attempted to modify ZSM-5 zeolite with Zr, and the acetylene conversion was found to be more than 96% in the experiment. Zhang et al.^10,11 introduced etidronic acid (HEDP) and an ionic liquid (IL) into activated charcoal (AC) preparation of a Cu/AC catalyst, which effectively improved the dispersion of the Cu active component on the AC support, inhibited carbon accumulation, and achieved an acetaldehyde selectivity of more than 90%. Wang et al.^12,13 used an amino-modified MCM-41 support, which improved the dispersion of the Zn metal, increased the force between Zn and the support, and increased the electron cloud density of the Zn component. In addition, they tried to impregnate Zn on a Ti-doped MCM-41 support with 90% selectivity to acetaldehyde.¹⁴ Liu et al.¹⁵ used heteroatom doping of AC followed by loading of Cu components. It was found that the acetylene conversion was up to 90% when the N and P doping was 0.40% and 2.61%, respectively. Li et al.¹⁶ investigated the mechanism of the acetylene hydration reaction by DFT calculations, and they found that the energy required to break the O–H bond of water determines the activation energy of the catalytic process, while Zn(OH)₂ with –OH and ZnOHCl with lower reaction energy barriers have better catalytic properties than ZnCl₂. Although some progress has been made in the study of gas–solid phase catalytic systems, there are still some shortcomings, such as Cu is easily reduced and although Zn-based catalysts have high initial activity, the stability is poor.

Through literature research, we found that some acid ligands (such as HEDP, methanesulfonic acid (MSA), etc.) can improve metal dispersion, inhibit carbon accumulation and provide more acid sites, which can facilitate the reaction. Moreover, it was also shown in relevant mechanistic studies that the Zn species with hydroxyl groups (HO–Zn²⁺(H₂O)) is the active center of the reaction, therefore we expect to increase its content by ligands and specifically analyze the coordination environment of Zn. Zhang et al.¹⁷ immobilized phosphotungstic acid (HPW) on the surface of Ni nanomaterials and found that it was able to form metal–acid centers with improved stability and HPW provided abundant acid sites with an increased B-acid site content. Talib et al.¹⁸ chose a phosphotungstate anion (PW₁₂O₄0₃⁻) as a support for anchoring single metal atoms and found that the metal could be coordinated with surface O atoms in the ligand to form different coordination configurations.

In this study, a green and non-polluting solid heteropolyacid ligand HPW was used to improve the active sites of Zn-based catalysts and the strong coordination anchor was used to improve the metal dispersion and reduce the loss of Zn. A series of Zn-HPW/AC catalysts were successfully prepared by the impregnation method, and the performance was tested under specific reaction conditions, and the reaction temperature and HPW content were optimized to achieve the best catalytic performance. With the assistance of structural characterisation tools and DFT calculations, the coordination structure, catalysis performance, and catalytic mechanism were explored. In addition, this work could provide new ideas for research on the development of green and non-polluting catalysts for acetylene hydration.

2. Results and discussion

First, the effects of different temperatures on the performance of Zn-HPW/AC catalysts in the acetylene hydration reaction were investigated, as shown in Fig. S1.† The 0.05HPW/AC catalyst is almost inactive as the reaction proceeds, while the catalytic activity of the Zn/AC catalyst increases and then decreases rapidly. The catalytic activity of Zn-HPW/AC shows an increasing trend with increasing temperature, while the activity is better at a higher temperature of 260 °C. Fig. S1(B)† shows that the effect of HPW ligands on the selectivity to acetaldehyde is more significant at higher temperatures. The above indicates that the HPW ligand has a facilitating effect on the acetylene hydration reaction catalyzed by Zn-based catalysts.

To investigate the optimal addition of HPW, the catalytic performance of the Zn-xHPW/AC (x = 0.03, 0.04, 0.05, 0.06) catalysts was tested for the acetylene hydration reaction at a reaction temperature of 260 °C, and the results are shown in Fig. 1. The Zn/AC catalyst has poor catalytic stability. However, with the increase of the HPW ligand content, the catalytic activity was significantly increased and the stability was improved; in particular, the Zn-0.05HPW/AC catalyst showed an acetylene conversion of more than 90% and the selectivity to acetaldehyde was maintained above 70%. In addition, the addition of more HPW ligands may cover the active site and thus lead to a decrease in catalytic performance, indicating that the optimal addition amount is 0.05. The test results indicate that the introduction of HPW ligands can effectively improve the catalytic performance of the Zn catalysts for the acetylene hydration reaction. According to our research, the Zn-0.05HPW/AC catalyst displayed relatively excellent catalytic performance among the Zn-based catalysts in the acetylene hydration.

For the characterization analysis of a series of HPW catalysts with different acid amounts, the specific surface area of the catalyst decreased, and obvious characteristic peaks appearing between 810 and 1078 cm⁻¹ were increased, indicating that the HPW ligand was successfully introduced (Fig. S2 and S3†). Meanwhile, XRD characterization (Fig. S4†) showed that the diffraction peaks of Zn(H₂PO₄)₂ were generated in the catalyst with the increase of the HPW ligand content, which could be attributed to the formation of Zn(H₂PO₄)₂ by the binding of some of the phosphoric acid groups in HPW with Zn. To further investigate the chemical state and coordination environment of Zn in fresh Zn-0.05HPW/AC catalyst, the X-ray absorption spectroscopy (XANES) was performed. Fig. S5† showed that there is no Zn–Zn coordination in the catalyst sample, but only Zn–O coordination. In addition, it can be seen from the k³ weighting of fresh Zn-0.05HPW/AC catalyst and the K-side EXAFS spectra of Zn in Fig. 2 that the sample fitting curve is in good agreement with the experimental curve, indicating that the fitting results are credible. After analysis, the fitting result of the Zn–O coordination number of the sample in Table 1 is 4.8, which is close to the Zn–O₄ coordination. This is because in the fitting of organic matter and single atom C and N elements, due to the disorder of the structure and the adsorption of small molecules, generally, the fitting result of the coordination number will have an error of 0.2–0.8. Therefore, the coordination number fitting results of the catalyst are within the acceptable range.

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In order to investigate the dispersion of Zn species in the fresh catalyst from a microscopic point of view, TEM characterization of fresh and used catalysts was performed and the results are shown in Fig. 3 and S6.† Larger metal particles appeared in the fresh Zn/AC catalyst and showed a large amount of agglomeration, whereas the metal species particles in the catalyst after the addition of ligands gradually decreased while the agglomeration was alleviated (Table 1).

As can be seen from the TEM images of the catalysts after the reaction in Fig. S6,† the Zn/AC catalysts showed further agglomeration of Zn species after the reaction, while the particles of Zn species did not further increase in the reacted Zn-HPW/AC ligand catalysts. The results of the above analysis indicate that the HPW ligand can promote the dispersion of the active component and inhibit the agglomeration of the Zn species. Combined with the results of the catalytic performance tests, it can also be seen that the HPW ligand has a significant positive effect on the acetylene hydration reaction.

In order to investigate the influence of surface acidity and acid strength of the catalyst, we conducted NH₃-TPD experiments, and Fig. S7† shows the NH₃-TPD graph. From Fig. S7,† it can be observed that the desorption peak of the catalysts at 100–400 °C after the addition of the Zn component indicates the presence of weak and medium strong acid sites in the catalysts. The acid sites and acid content on the catalyst surface generally influence its catalytic performance. To explore the variation in the acid site type and acid content, Py-FTIR spectroscopy was performed on fresh catalysts, and the results are shown in Fig. 4 and Table 2. The literature suggests that the absorption peak at 1450 cm⁻¹ corresponds to the Lewis (L) acid site, the peak at 1540 cm⁻¹ corresponds to the Brønsted (B) acid site, and the absorption peak at 1490 cm⁻¹ corresponds to the Lewis and Brønsted (L + B) acid sites.^19–22 Fig. 4 illustrates that the catalyst displays four absorption peaks at 1446 cm⁻¹, 1492 cm⁻¹, 1548 cm⁻¹, and 1610 cm⁻¹, which correspond to the L-acid site, (L + B)-acid site, B-acid site, and strong acid site, showing that the introduction of HPW could increase the acid site of the catalyst.

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By combining these findings with previous studies, we infer that the acid sites have a degree of influence on the acetylene hydration reaction. While it is true that the acid site can boost the activity of the acetylene hydration reaction to a certain extent, it is also the active site for the alcohol–formaldehyde condensation reaction.²³ Contrasting the acid site content changes of the catalysts, Table 2 illustrates that HPW exhibits robust B-acid acidity. A gradual decrease in the B-acid content of the catalyst was observed as the ligand was introduced. Furthermore, based on relevant literature that states that Zn²⁺ is capable of exchanging protons with strongly acidic –OH groups,^21,24 it is speculated that the B-acid site may promote the formation of (ZnOH)⁺ species. This proposition is supported by the evidence that the B-acid content of the ligand catalyst also shows a diminishing trend.

X-ray photoelectron spectroscopy (XPS) characterization was conducted on fresh and used Zn-xHPW/AC (x = 0.03, 0.04, 0.05, 0.06) catalysts to gain a deeper understanding of the chemical state of Zn. In Fig. 5, it could be observed that the fitted curves of the Zn 2p_3/2 peaks show clear peaks corresponding to both Zn species. Two peaks at 1021.9 eV and 1022.6 eV were assigned where the high binding energy corresponds to the (ZnOH)⁺ species, whereas the low binding energy corresponds to the (ZnH)⁺ species. The introduction of the HPW ligand resulted in a slight red shift in the Zn 2p_3/2 binding energy. This shift implies that Zn gained electrons and that the electron cloud density around the active Zn species increased, which is favorable for the reaction.²⁵ Moreover, this shift indicates coordination between Zn and HPW.

Previous studies indicate that Zn²⁺ with partial hydroxyl groups, or (ZnOH)⁺ species, is generated on the surface of Zn catalysts following water vapor treatment.²⁶ Considering the analysis results in Table 3, it can be observed that the (ZnOH)⁺ species accounted for only 34.6% of the Zn/AC catalyst, leading to the assumption that the quantity of (ZnOH)⁺ species produced by water vapor treatment was low. Table 3 displays that the inclusion of HPW ligands led to a gradual increase in (ZnOH)⁺ species within the catalyst, with the Zn-0.05HPW/AC catalyst showing 59.9% of (ZnOH)⁺ species. Moreover, substances containing (ZnOH)⁺ are unstable and prone to dehydroxylation reactions when subjected to high temperatures or reducing gases.²⁶ Since acetylene functions as a reducing gas, (ZnH)⁺ species will likely be generated through dehydroxylation of (ZnOH)⁺. The Py-FTIR analysis revealed that the Zn-0.05HPW/AC catalyst had the minimum B-acid content and a greater number of (ZnOH)⁺ species compared to the other catalysts. This observation supports the notion that the B-acid site is conducive to the formation of (ZnOH)⁺ species. The decrease in the (ZnOH)⁺ species content in all the catalysts after the reaction compared to their pre-reaction states suggests that (ZnOH)⁺ species were utilized during the reaction, leading to a decrease in reactivity. This outcome is consistent with the catalytic performance test results for the Zn-0.05HPW/AC catalyst.

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To investigate the loss of Zn fraction from the catalysts, we conducted ICP-OES characterization on both the fresh and used catalysts. The results are presented in Table S2.† The Zn component loss rate in the Zn/AC catalyst is exceptionally high, reaching 11.31%. Based on the test results of catalytic performance, we speculate that the loss of Zn component during the reaction process may have led to the rapid deactivation of the Zn/AC catalyst. In comparison with Zn/AC, the Zn fraction loss in the Zn-0.05HPW/AC catalyst was only 5.29%, suggesting that the coordination between Zn and HPW successfully prevented the loss of Zn fraction. This may have also contributed to the improved performance of the ligand catalysts.

In general, over-adsorption of acetylene gas and strong adsorption of acetone, butenal and other by-products could aggravate catalyst coking and carbon deposition. This work aimed to examine the impact of HPW ligands on carbon deposition in the reaction of Zn-based catalysts. TG characterization was performed on the Zn-xHPW/AC (x = 0.03, 0.04, 0.05, 0.06) catalysts, and their results are demonstrated in Fig. S8(B)† and Table 4. The considerable mass loss of the samples between room temperature and 150 °C is attributed to the evaporation of water adsorbed by the catalyst sample surface in contact with air.²⁷ The amount of carbon deposition was calculated by determining the difference in weight loss between the fresh and used catalysts at temperatures ranging from 160 °C to 400 °C. The TG and DTG curves for the fresh and used catalysts demonstrated similar trends with weight loss between 400 °C and 800 °C. This could be attributed to the decomposition of ZnCl₂ and dehydroxylation of the (ZnOH)⁺ species. The results in Table 4 indicated that the carbon deposition of the Zn/AC catalyst was 3.12%. However, the addition of the HPW ligand significantly decreased the carbon deposition to only 1.05% for the Zn-0.05HPW/AC catalyst. It is hypothesized that the formation of more (ZnOH)⁺ species facilitates the reaction and alleviates carbon deposition. The results show that the ligand catalysts demonstrated superior thermal stability and resistance to carbon buildup. Consequently, they display efficient performance in the acetylene hydration reaction.

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In this study, we performed the density functional theory (DFT) method using the Gaussian 09 (ref. 28) software package with the M06L²⁹ functional for the calculations. The LANL2DZ³⁰ relativistic effective core potential (RECP) for all metal atoms and the standard 6-31G(d)³¹ basis set for all main group atoms were used.

As shown in Fig. S9,† the energy of the configurations with different H atom positions in the catalyst was calculated and analyzed using DFT calculations. It was found that the configuration containing Zn–OH was the most stable. Meanwhile, the adsorption energy calculation in Fig. S10† indicates that water molecules preferentially adsorb onto the catalyst, followed by acetylene adsorption onto the catalyst.

As shown in Fig. 6 and 7, two reaction pathways were constructed. The first reaction pathway involves the participation of Zn–OH in the catalyst. In the co-adsorption state, water molecules dissociate into H⁺ and OH⁻ through the first transition state ts1. OH⁻ bonds with C₂H₂ to form intermediate im, and H⁺ bonds with Zn ortho oxygen. Then, in the ts2 state, the H atom in the original catalyst bonds with the intermediate product to form ethylene alcohol. The generated ethylene alcohol is de-adsorbed from the catalyst surface and isomerized to form acetaldehyde. In the first step, the dissociated H of H₂O plays a role in reducing the catalyst and achieving catalytic cycling. The rate determining step of the reaction in this pathway is the dissociation of water molecules and the binding of OH and C₂H₂, with a required energy barrier of 22.41 kcal mol⁻¹. Another pathway of the reaction is the direct reaction between water molecules and acetylene after H₂O dissociation. The dissociated OH and H bond with two C atoms in C₂H₂ to form ethylene alcohols, with a reaction energy barrier of 33.11 kcal mol⁻¹.

3. Experimental

Raw materials

AC (40–60 mesh) is coconut shell activated carbon, which was provided by Tangshan Union Carbon Technology Co. Zinc chloride (ZnCl₂, 98%) and phosphotungstic acid (HPW) were provided by Shanghai Maclean Biochemical Technology Co. Nitrogen (N₂, 99.99%), hydrogen (H₂, 99.99%) and acetylene (C₂H₂, 98%) were purchased from Shanghai Wei Chuang Gas Co.

Catalyst preparation

A series of zinc-based catalysts (with a theoretical loading of 15 wt% of ZnCl₂) were prepared by a simple over-volume impregnation method. The procedure was as follows: about 20 mL of deionized water was weighed in a beaker, 0.6222 g of zinc chloride (ZnCl₂) was added and stirred at room temperature until completely dissolved, and then 0.5259 g of phosphotungstic acid (HPW) was added, sealed and continued to stir at room temperature for 30 min until completely dissolved. Finally, 3 g of AC support was added to the above solution and stirring was continued for 12 h. The beaker was then sealed with tin foil and placed in a constant temperature oven at 80 °C for 10 h to obtain the desired catalyst named Zn-0.05HPW/AC (n(ZnCl₂):n(HPW) = 1:0.05). Zn-xHPW/AC (x = 0.03, 0.04, 0.05, 0.06) catalysts were made in the same way as above.

Catalytic performance evaluation

The acetylene hydration reaction was carried out in a home-made fixed-bed reactor (Φ10 × 500 mm), and 2 mL of catalyst sample was placed in the center of the reaction tube. The air was removed from the reaction tube by flushing with N₂ for 30 min. Then the heating was started by adjusting the temperature of the condensation circulation pump to about −2 °C. When the reaction tube was heated to 240 °C and the temperature of the reaction water was 160 °C, the catalyst was activated with water vapor using a peristaltic pump (about 0.06 g min⁻¹ for 30 min). Then N₂ was turned off and C₂H₂ was turned on, and acetylene and water vapor were injected into the reactor according to GHSV (C₂H₂) = 90 h⁻¹ and n(H₂O)/n(C₂H₂) = 4. Finally, the product content was detected by manual injection every hour by gas chromatography (GC-2014C).

Conclusions

In summary, we successfully prepared Zn-HPW/AC catalysts with uniformly dispersed high density of (ZnOH)⁺ active sites, among which the Zn-0.05HPW/AC catalyst exhibited the best catalytic performance in the acetylene hydration reaction (acetylene conversion of more than 90%, selectivity acetaldehyde above 70%). TEM, Py-FTIR, XPS and ICP-OES characterization analyses showed that the incorporation of HPW improved the dispersion of Zn while providing additional acid sites to promote the (ZnOH)⁺ species, inhibited the loss of Zn species and effectively enhanced the stability of the metal active species. XAFS showed that Zn existed mainly as a Zn–O₄ coordination structure in the catalyst. DFT calculations suggested that OH dissociated from water molecules was added to C₂H₂, while H reduced the catalyst, and the original H atoms in the ligand catalyst were further involved in the reaction to realize the catalytic cycle; meanwhile, the activation energy of Zn–OH involved in the reaction was significantly lower than that of the direct reaction, and the stable (ZnOH)⁺ structures played a key role in improving the acetylene conversion. This provides a new idea for the development of green catalysts for acetylene hydration.

Data availability

Data will be made available on request.

Author contributions

Zhen Chen: investigation, formal analysis, data curation, writing – original draft, writing – review & editing. Dingjie Luo, Long Zhou, Yufan Ma: formal analysis, data curation; Fangjie Lu and Qinqin Wang: visualization, investigation, supervision, resources, writing – review & editing. Bin Dai: conceptualization, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Scientific Research Project of Shihezi University (No. CXFZ202205), the Science and Technology Project of Xinjiang Bingtuan supported by the Central Government (2022BC001) and the Tianshan Talents Training Program of Xinjiang (Science and Technology Innovation Team, 2022TSYCTD0021).

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Footnote

† Electronic supplementary information (ESI) available See DOI: https://doi.org/10.1039/d4cy00806e

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