The Hydrolyzation and Hydrolysis Rates of Chlorodifluoromethane during Catalytic Hydrolysis over Solid Acid (Base) MoO₃(MgO)/ZrO₂ Catalyst

Abstract

Chlorodifluoromethane (HCFC-22) is one of the air pollutants that destroy the ozone layer and warm the earth. In order to degrade HCFC-22, the catalytic hydrolysis method is currently the most effective environmental technology. Herein, we prepare solid acid MoO₃/ZrO₂ and solid base MgO/ZrO₂ catalysts for the degradation of HCFC-22, with a particular focus on revealing the effects of the catalyst preparation methods, calcination temperatures, and hydrolysis temperatures on the hydrolyzation and hydrolysis rates of HCFC-22. The catalysts are characterized by XRD, N₂ isothermal adsorption–desorption, and NH₃-TPD. The results show that the solid acid (base) MoO₃(MgO)/ZrO₂ catalysts have strong catalytic activity for the degradation of HCFC-22, while the catalytic activity of the solid acid MoZr is better than that of the solid base MgZr catalyst and the hydrolysis rate can reach more than 90%. The water vapor generates B-acid centers on the MoO₃(MgO)/ZrO₂ framework and promotes the hydrolysis of HCFC-22 to HF and HCl.

1. Introduction

Freon is a general term for chlorofluorocarbons, abbreviated as CFCs, which were first synthesized by American scientists led by Thomas Mickeli in 1928. Chlorodifluoromethane (HCFC-22) is one of the most widely used alternatives to CFCs; due to its stable chemical properties and low cost, it has been widely used in refrigerants [1], fire-extinguishing agents [2], and the foaming agents of foamed plastics [3]. However, as HCFC-22 can cause ozone depletion [4] and greenhouse effects [5], once discharged into atmosphere it can cause unpredictable disasters. Consequently, HCFC-22 decomposition has attracted widespread public attention [6]. Various processes for the harmless decomposition of HCFC-22 have been proposed. These include high-temperature decomposition, plasma treatment, supercritical hydrolysis, and plasma heating methods [7,8,9]. The above methods have many problems, such as the large energy consumption rates, cumbersome operation process, and contributions to environmental pollution. Compared with the above methods, catalytic hydrolysis seems to be the most practical and energetically favorable approach. It has many advantages, including its simple process, ease of establishment, and low reaction temperature, while the hydrolyzate will not cause secondary pollution to the environment [10,11]. It is well known that for catalytic hydrolysis reactions, the development of a highly efficient catalyst is the main challenge.

Thus, many scientists have done a lot of research on the catalytic degradation of CFCs. These studies found that the use of solid base and solid acid catalysts was thought to be most suitable process for the degradation of CFCs, because it was considered an economically benign decomposition method. Karmakar et al. [12] reported on the use of a titania catalyst for the decomposition of CFC-12 in the presence of water vapor. Niedersen et al. [13] found that sulfated zirconia was a very suitable catalyst for the catalytic hydrolysis of CFCs. Hino and Arata [14] prepared a loaded oxide solid super acid MoO₃/ZrO₂ catalyst. Liu et al. [15,16,17] studied a solid base Na₂O(CaO)/ZrO₂ catalyst for the catalytic hydrolysis of CFC-12. The results showed that CO₂ and CClF₃ were the main components. Huang et al. [18] studied the catalytic hydrolysis of HCFC-22 using a solid acid MoO₃/ZrO₂-TiO₂ catalyst. The results showed that the degradation rate of the HCFC-22 reached 96.21%. Zhao et al. [19] reported that a solid acid TiO₂/ZrO₂ catalyst showed good catalytic hydrolysis activity in the catalytic hydrolysis of CFC-12. Zhou et al. [20] studied the composite catalyst MoO₃-MgO/ZrO₂ for the catalytic hydrolysis of CFC-12, and the degradation rate of CFC-12 reached 98.43%. Based on the above, the catalytic hydrolysis of HCFC-22 over solid acid and solid base catalysts may be a promising approach.

In this paper, the catalytic performance of solid acid MoO₃/ZrO₂ and solid base MgO/ZrO₂ catalysts for the degradation of HCFC-22 was studied using the catalytic hydrolysis method due to the simple preparation method for the catalyst and the low price of the raw materials. The effects of the preparation method on the catalyst, calcination temperatures, and hydrolysis temperatures and on the hydrolysis products and hydrolysis rate of HCFC-22 were investigated. With nitrogen as the carrier gas, HCFC-22 was mixed with water vapor. The catalytic hydrolysis of HCFC-22 at a low concentration was achieved after a catalytic reaction bed was filled with the catalysts. Compared with the previous research results, this study proposed the MoO₃(MgO)/ZrO₂ catalyst for the catalytic hydrolysis of HCFC-22, which improved the degradation rate of HCFC-22 from 96.21% to 99.99%, without the generation of by-products, while HCFC-22 was almost completely degraded. This study can provide some theoretical references for the harmless treatment of low-concentration HCFC-22.

2. Results and Discussion

2.1. Characterization of Solid Acid MoO₃/ZrO₂ Catalyst

2.1.1. X-ray Diffraction Patterns of MoO₃/ZrO₂

In order to determine the effect of the phase structure of the catalyst on its performance, an X-ray diffraction pattern (XRD) analysis was carried out using a Bruker D8 Advance X-ray diffractometer in Karlsruhe, Germany with Ni-filtered Cu Kα radiation (λ = 0.154 nm). The diffraction curves were recorded from 10° to 90° with a scanning rate of 12°·min⁻¹. Having been calcinated at different temperatures, the XRD patterns of the catalysts are depicted in Figure 1a. The catalyst calcinated at 550 °C and 600 °C showed obvious characteristic peaks at 2θ of 30.3°, 35.3°, 50.4°m and 60.3°, which may represent tetragonal-phase ZrO₂. At calcination temperatures of 650 °C and 700 °C, there were extra peaks at 2θ = 23.1°, representing the Zr(MoO₄)₂ phase, which indicated that there was a chemical interaction between the metal and carrier [21,22]. It can be seen from Figure 1b that when the calcination temperature increased from 550 °C to 700 °C, the lattice parameters of the MoZr catalyst decreased first and then increased. When the calcination temperature was 600 °C, the lattice parameters of the MoZr catalyst decreased. No diffraction peak of MoO₃ appeared, which may imply that MoO₃ was highly dispersed on the support or penetrated the skeleton of ZrO₂ in an amorphous form [23,24].

2.1.2. N₂ Isothermal Adsorption–Desorption Characterization of MoO₃/ZrO₂

To explore the change in the surface structure of the MoZr catalyst, the S_BET and pore volume were characterized, and the results are shown in Figure 2. According to the IUPAC classification standard, the adsorption–desorption isotherm of the MoZr catalyst is a class-Ⅳ-type isotherm, which has the characteristics of typical mesoporous materials. In the range of P/P₀ = 0.3–0.8, there are obvious N₂ adsorption–desorption hysteresis loops, which can be classified as H2 hysteresis loops, and capillary condensation occurs at higher relative pressures.

To further understand the physical structures of the catalysts with different calcination temperatures, the specific surface area, pore volume, and average pore diameter statistics were measured (

Table 1. Structural properties of solid acid MoZr.

Calcination Temperature (°C)	Surface Area (m2/g)	Pore Size (nm)	Pore Volume (cm3/g)
500	197	2.41	0.22
600	135	4.37	0.22
700	74	7.21	0.18

); the specific surface area decreased more drastically from 197 m²/g at 500 °C to 74 m²/g at 700 °C, while the average pore size increased in sequence. These results indicate that the catalyst underwent a certain degree of sintering. With the increase in calcination temperatures, the relative pressure of the MoZr hysteresis ring increases, indicating that an increase in calcination temperatures will increase the pore diameter of the catalyst.

2.1.3. NH₃ Temperature-Programmed Desorption

NH₃ temperature-programmed desorption (NH₃-TPD) is an effective method to characterize the surface acidity of solid acid catalysts. NH₃-TPD can provide information such as the number of active acid centers, the strength of the acid centers, and the amount of acid corresponding to the acid strength [25]. It is generally believed that the desorption peak corresponds to the weak acid position below 200 °C, the desorption peak corresponds to the medium-strong acid position between 200–350 °C, and the desorption peak corresponds to the strong acid position between 350–500 °C.

The NH₃-TPD profile of the catalysts was studied, as shown in Figure 3 and

Table 2. Acid properties of the MoO₃/ZrO₂ catalyst.

NH3 Desorption at Acid Sites with Different Strengths (mmol/g)
Catalyst	Weak Acid Sites	Medium Acid Sites	Strong Acid Sites	Total Acid Sites
MoZr-500 °C	0.62	1.70	0.02	2.34
MoZr-600 °C	1.25	0	0	1.25
MoZr-700 °C	0.28	0.57	0	0.85

. The MoZr catalyst calcinated at 500 °C contained a weak acid desorption peak (α), a medium-strong acid desorption peak (β), and a strong acid desorption peak, respectively (γ) [26]. The NH₃ desorption temperatures were 100–150 °C, 200–230 °C, and 500–700 °C. The peak area was proportional to the adsorption capacity of the NH₃. The γ desorption peak was weak, indicating that the content of the strong acid was low. The calcination temperatures had a great effect on the acidity of the catalyst. The MoZr catalyst calcinated at 600 °C had the strongest α desorption peak and the weakest acid content. In contrast, the MoZr catalyst calcinated at 700 °C had the weakest α desorption peak but showed the lowest content of the weak acid. The results show that the MoZr catalyst calcinated at 600 °C has the best effect on the catalytic hydrolysis of HCFC-22, with the worst calcination result at 700 °C. In combination with the NH₃-TPD characterization, the weak acid sites also had strong catalytic activity on HCFC-22. When the catalysts were calcinated at 600 °C and 700 °C, the γ desorption peak disappeared, because the higher calcination temperatures were not conducive to the formation of strong acid sites. Table 2 reveals the amounts of acid in the catalysts, calculated according to the desorption peak area. The weak acid sites first increased and then decreased with the calcination temperatures, while the strong acid sites showed a downward trend. The calcination temperature was 600 °C, and the weak acid sites had the highest acid content and catalytic activity, indicating that the weak acid sites played a good role in promoting the catalytic activity of the catalyst [27].

2.2. Catalytic Performance of Solid Acid MoO₃/ZrO₂ Catalyst

The effects of the catalyst calcination temperatures and catalytic hydrolysis temperatures on the hydrolyzate and hydrolysis rates of HCFC-22 were investigated, and the results are shown in Figure 4. It was observed that the hydrolysis rate of HCFC-22 gradually increased with the increases in temperature due to the decomposition reaction of HCFC-22 being an endothermic reaction at medium and low temperatures, which causes the chemical equilibrium to shift to the right. The reaction is as follows:

CHClF₂ + H₂O→CO + HCl + HF

(1)

With the increases in catalytic hydrolysis temperature (100–400 °C), the hydrolysis rate of HCFC-22 increased gradually. With the increases in calcination temperature (500–700 °C), the hydrolysis of HCFC-22 firstly increased and then decreased. When the catalytic hydrolysis temperature was 250 °C, the catalyst calcined at 600 °C had the best effect in terms of hydrolyzing HCFC-22, and the hydrolysis rate reached 99.99%. The catalyst calcined at 700 °C had the worst effect on hydrolyzing HCFC-22. Therefore, too high or too low calcination temperatures are unfavorable for the catalytic hydrolysis reaction. Combined with the NH₃-TPD results (Figure 3), the calcination temperature was higher, the surface structure of the catalyst was destroyed, and the specific surface area of the catalyst and the catalytic activity of the catalyst were reduced. The results show that when calcinated at 600 °C, the MoZr catalyst showed a very high hydrolysis rate (almost 100%), which indicates that the specific surface area and pore size were moderate, the content of weak acids was large, and the catalyst had a higher hydrolysis rate for HCFC-22.

2.3. Characterization of Solid Base MgO/ZrO₂ Catalyst

2.3.1. X-ray Diffraction

The XRD patterns of the solid base MgO/ZrO₂ catalyst prepared via coprecipitation method are shown in Figure 5. The diffraction peaks at 2θ = 30.3°, 35.2°, 50.4°, and 60.3° are characteristic diffraction peaks of tetragonal zirconia (t-ZrO₂). The standard card is JCPDS 88-1007 corresponding to crystal planes of (101), (110), (112), and (211). Schuth et al. [28] believed that zirconia exists only in the monoclinic phase and tetragonal phase at 1170 °C and is metastable. With the calcination temperature rising from 500 °C to 800 °C, the tetragonal phase tends to change to the monoclinic phase. However, the tetragonal phase ZrO₂(t-ZrO₂) rather than the monoclinic phase was detected in this study. This phenomenon was attributed to the doping effect of Mg²⁺. More importantly, the zirconia crystallization was completed, and the Mg²⁺ ions could replace Zr⁴⁺ ions in the host crystal lattice to form the MgZr-C solid solution, generating a stable MgO-ZrO₂ solid solution. Mg²⁺ ions were introduced into the t-ZrO₂ carrier, indicating that there existed an amorphous form of Mg(NO₃)₂ • 6H₂O. It has been reported that MgO is added to ZrO₂ or stays on the surface of the ZrO₂ matrix, according to the preparation process [29].

Figure 6 shows the XRD patterns of the solid base MgZr-i catalysts, which were prepared using the impregnation method. The diffraction peak intensities of the MgZr-i catalyst vary greatly when prepared at different calcination temperatures, but there is no deviation to a certain extent. The calcination temperature of the catalyst was 500 °C, the crystal phase of ZrO₂ was a mixed state of the tetragonal phase and monoclinic phase, and its diffraction peak intensity was relatively weak. With the gradual increase in calcination temperatures, the monoclinic phase of ZrO₂ gradually increased and the diffraction peak became sharper and sharper, indicating that the catalysts were sintered to a certain extent, while the grain size became larger, which was consistent with the BET characterization. Meanwhile, the MgZr catalyst prepared using the impregnation method made MgNO₃•6H₂O uniformly distributed on the surface of the ZrO₂. Since MgO had not yet reached the detection threshold, no diffraction peak of MgO was found in the XRD pattern. As can be seen from Figure 7, when the calcination temperature was 700 °C, the lattice parameters of the MgZr catalyst decreased and the cell volume shrank. Because Mg²⁺ replaced the position of the Zr⁴⁺ particles in the lattice and the radius of Mg²⁺ was smaller than that of Zr⁴⁺, the lattice parameters decreased. The lattice displacement of ZrO₂ occurs due to the doping effect.

2.3.2. N₂ isothermal Adsorption–Desorption

The N₂ adsorption–desorption isotherms were applied to measure the pore structure of the catalysts. Figure 8 gives the N₂ adsorption–desorption isotherms and pore size distribution. According to the IUPAC classification standard, the isotherm of the catalysts is a class-Ⅳ-type isotherm. The stripping line is distributed in the pores of a relatively narrow mesoporous material, which indicates the existence of a mesoporous framework [30]. In the range of P/P₀ = 0.78–1.0, there were obvious hysteresis loops. These could be classified as H1 hysteresis loops, and the holes were mainly straight tube holes, which showed typical mesoporous characteristics. With the increase in calcination temperatures, the relative pressure at the close point of the MgZr hysteresis loop increases, indicating that an increase in calcination temperature will increase the pore diameter of the catalyst. To further understand the physical structure of the catalysts with different calcination temperatures, the specific surface area, pore volume, and average pore diameter statistics are listed in

Table 3. Specific surface area, average pore size, and total pore volume of the solid base MgZr.

Catalyst	Calcination Temperature (°C)	Specific Surface Area (m2/g)	Average Pore Size (nm)	Total Pore Volume (cm3/g)
MgZr-c	500	135	2.41	0.24
	600	60	4.63	0.22
	700	34	6.94	0.18
	800	15	14.1	0.16
MgZr-i	500	86	3.55	0.23
	600	52	6.06	0.20
	700	31	6.06	0.17
	800	20	9.21	0.17

.

The specific surface area and total pore volume of the catalyst gradually decrease with the calcination temperature increase, and the average pore diameter increases slowly. Compared with the impregnation method, the specific surface area of the catalyst prepared via the coprecipitation method has been greatly improved. The calcination temperature was 500 °C and the specific surface area was 135 m²/g. With the increase in calcination temperature to 800 °C, the specific surface area decreases dramatically to 15 m²/g, indicating that the catalyst has been sintered. As shown in Figure 6, as the characteristic signal values increase, the surface area of the catalyst prepared by impregnation decreases slowly. Combined with the XRD results (Figure 5 and Figure 6), with the increase in catalyst calcination temperature, the monoclinic-phase ZrO₂ dominates, the diffraction peak becomes sharper, the degree of crystallization increases, and the grain size becomes larger, exhibiting a gradual decrease in the specific surface area of the catalyst as the result of the joint action of the ZrO₂ crystal-phase transformation and catalyst sintering.

2.3.3. CO₂ Temperature-Programmed Desorption

In order to study the basic performance of the MgO/ZrO₂ catalyst, the CO₂-TPD profile was determined. Based on the characterization, the alkalinity strength and total alkalinity of the catalyst can be evaluated. The CO₂ desorption peak occurs at a lower temperature, the MgO/ZrO₂ catalyst forms a weak base site, and the strong base site is the opposite [31]. Namely, the higher the CO₂ desorption temperature, the higher the surface alkalinity.

The solid base MgO/ZrO₂ catalyst prepared via coprecipitation method was analyzed via CO₂-TPD, and the results are shown in Figure 9. The CO₂ desorption peaks of the MgZr-c catalyst were composed of α, β, and γ peaks at 50–100 °C, 100–200 °C, and 750–900 °C, respectively. The presence of α and β desorption peaks may be related to the ZrO₂ corresponding to the surface alkalinity of ZrO₂ [32], which is associated with the interaction between the weak base sites and hydroxyl groups on the surface. As the calcination temperature increased, the desorption amount of CO₂ decreased obviously and the desorption peak moved slightly in the direction of the low temperature. The desorption temperature of desorption peak γ is close to the desorption temperature of MgO, which can be attributed to the MgO desorption peak on the surface of the carrier ZrO₂, while the occurrence of γ desorption peaks is associated with metal–oxygen pairs (including Mg-O and Zr-O) and low-coordination oxygen atoms (O²⁻) [33].

A CO₂-TPD diagram of the solid base MgZr-i catalyst was prepared via impregnation method, as shown in Figure 10. The CO₂ desorption peak of the MgZr-i catalyst was only the CO₂ desorption peak of the ZrO₂. Due to the peak area and strength, we did not see the γ desorption peak. The CO₂ desorption peak was significantly reduced with the calcination temperature increase. Compared with the coprecipitation method, the α and β desorption peaks tend to move in the low-temperature direction. Combined with the XRD analysis, the crystalline phase of ZrO₂ prepared via impregnation method is dominated by the monoclinic phase, while that prepared via coprecipitation method is dominated by the tetragonal phase. The alkalinity of the tetragonal phase ZrO₂ was slightly higher than that of the monoclinic phase, indicating that the stronger the alkalinity, the higher the catalytic activity. The basic site distribution and total alkalinity of the solid base MgO/ZrO₂ catalyst are listed in

Table 4. Base properties of MgO/ZrO₂ catalysts.

CO2 Desorption at Basic Sites with Different Strengths (mmol/g)
Catalyst	Calcination Temperature/(°C)	Weak Basic Sites	Medium Basic Sites	Strong Basic Sites	Total Basic Sites
	500	6.56	3.68	8.69	18.93
MgZr-c	600	6.8	1.07	0	7.87
	700	2.59	1.04	0.23	3.86
	800	1.82	0	0	1.82
	500	2.58	7.75	0	10.33
MgZr-i	600	3.71	0	0	3.71
	700	2.42	0	0	2.42
	800	1.25	0	0	1.25

.

As can be seen from Table 4, as the calcination temperature increased from 500 °C to 800 °C, the amount of weak base in the solid base MgO/ZrO₂ catalyst gradually decreased. The NH⁴⁺ and NH₃ adsorbed on the catalyst are directly activated after participating in the reaction, while the NH⁴⁺ adsorbed on the Brønsted acid site in the catalyst can only be activated after migrating to the active site of the catalyst, then participating in the reaction [34]. The calcination temperature was 700 °C, and the synergistic effect between the NH₃ adsorption site and activation site might have an important effect on the catalyst, meaning that solid base MgO/ZrO₂ catalyst has a good catalytic performance for HCFC-22.

2.4. Catalytic Performance of Solid Base MgO/ZrO₂ Catalyst

The effects of the calcination temperatures and catalytic hydrolysis temperatures of the MgZr catalyst on the hydrolyzation and hydrolysis rates of HCFC-22 were studied and are shown in Figure 11. As shown in Figure 11a, as the catalytic hydrolysis temperature increased from 100 °C to 400 °C, the hydrolysis rate of HCFC-22 gradually increased. The catalytic hydrolysis temperature was 400 °C and the calcination temperature was 700 °C. The catalyst prepared via coprecipitation method had the best effect on the catalytic hydrolysis of HCFC-22, for which the hydrolysis rate of HCFC-22 reached 98.03%. From the XRD and BET analysis results, the sharp diffraction of ZrO₂ in the XRD image indicated that the catalyst had good crystallinity. The BET results showed that the catalyst had a moderate specific surface area and pore size, which may have been the reason for the good hydrolysis effect.

Figure 11b shows that the calcination temperature of the catalyst prepared via impregnation method was 700 °C and the catalytic hydrolysis temperature was 400 °C, while the hydrolysis rate of HCFC-22 reached 96.41%. Compared with Figure 11a, the solid base MgZr catalyst prepared via coprecipitation method had a better catalytic hydrolysis effect on HCFC-22. This may have been because the catalysts prepared via coprecipitation method mainly exist in the form of the tetrocheal-phase ZrO₂ (t-ZrO₂), while the ZrO₂ crystal phase in the impregnation method is the monocline phase. The basicity of tetragonal-phase ZrO₂ is slightly higher than in the monocline-phase, indicating that the stronger the basicity is, the higher the catalytic activity. Another aspect is attributed to the doping effect of Mg²⁺, as Mg²⁺ ions are introduced into the t-ZrO₂ carrier to generate a stable MgO-ZrO₂ solid solution, which improves the basicity of the oxygen vacancies on the catalyst’s surface and greatly improves the specific surface area of the catalyst.

2.5. The Contrast of the Catalytic Performance of Solid Acid (Base) MoO₃(MgO)/ZrO₂ Catalysts

We compared the catalytic performance of the solid acid MoO₃/ZrO₂ catalyst with the solid base MgO/ZrO₂ catalyst for HCFC-22, as shown in

Table 5. Catalytic performance of solid acid MoO₃/ZrO₂ and solid base MgO/ZrO₂ catalysts.

Catalyst	Calcination Temperature (°C)	Catalytic Hydrolysis Temperature (°C)	Hydrolysis Rate of HCFC-22 (%)
MoZr	500	400	92.16
	600	250	99.99
	700	400	86.09
MgZr-c	500	400	86.21
	600	250	94.83
	700	400	98.03
MgZr-i	500	400	86.28
	600	250	88.55
	700	400	96.41

. The catalytic performance of the two catalysts increased with the increase in catalytic hydrolysis temperatures. When the hydrolysis temperature of the solid acid MoZr catalyst was 250 °C and the calcination temperature was 600 °C, the hydrolysis rate of HCFC-22 reached 99.99%. When the hydrolysis temperature of the solid base MgZr catalyst was 250 °C and the calcination temperature was 600 °C, the hydrolysis rates of HCFC-22 reached 94.83% (coprecipitation method) and 88.55% (impregnation method).

At calcination temperatures of 500 °C and 600 °C, the catalytic performance of the solid acid MoZr catalyst is higher than that of the solid base MgZr catalyst. At the calcination temperature of 700 °C, the catalytic performance of the solid base MgZr catalyst is higher than that of solid acid MoZr catalyst. It may be that MgO is not evenly dispersed on the surface of the carrier ZrO₂ at the calcination temperature of 600 °C, which inhibits the doping effect of Mg²⁺, meaning that the catalyst cannot provide efficient catalytic performance. At the calcination temperature of 700 °C, MgO and ZrO₂ form a solid solution, and Mg²⁺ enters the lattice of ZrO₂ and replaces Zr⁴⁺ to form the Mg-O-Zr structure. Mg²⁺ is less electronegative than Zr⁴⁺, the electron cloud density and electronegativity are increased, and the surface of the solid base MgZr is strongly alkaline due to the lattice oxygen. The solid acid (base) MoO₃(MgO)/ZrO₂ catalysts had strong catalytic activity when HCFC-22 was catalyzed. The hydrolyzates of HCFC-22 are CO, HCl, and HF. Compared with the metal oxide catalysts reported in the previous literature, the MoO₃(MgO)/ZrO₂ catalyst has the advantages of high catalytic activity, simple operating conditions, a short reaction time, easy recovery of the catalyst, and no secondary pollutants. In this study, we achieved the catalytic hydrolysis of low-concentration HCFC-22, providing a certain theoretical basis for the practical application of the degradation of HCFC-22.

2.6. Mechanism of Catalytic Hydrolysis for HCFC-22

The NH₃-TPD and XRD results showed that MoO₃ may form strong acidic sites with ZrO₂ through strong interaction forces. The acid center model is shown below.

As can be seen from Figure 12, the solid acid MoO₃ has L-acid and B-acid centers, and the Mo anion clusters on the surface of the ZrO₂ (the weak acid properties of the Mo-OH-Mo or Zr-OH-Mo bridge hydroxyl groups) provide a lot of negative charges. During the catalytic hydrolysis of HCFC-22 by MoO₃/ZrO₂, the water vapor plays a role in forming B-acid centers on the MoO₃/ZrO₂ framework and promotes the hydrolysis of HCFC-22 into HF and HCl. As Cl^- is less electronegative than F^-, HCl is desorbed from the catalyst surface before HF and reacts with ZrO₂ in the catalyst to form ZrCl₂; the reason may be that ZrCl₂ is found via XRD detection but ZrF₂ is not. The formation of the B-acid center from Zr⁴⁺ is the main acid center for the decomposition of HCFC-22 into CO₂. The possible mechanism of the catalytic hydrolysis of MoO₃/ZrO₂ for HCFC-22 is shown in Figure 13.

Zirconium hydroxide exists in the form of tetramer [Zr₄(OH)₈(H₂O)₁₈]⁸⁺, which is dissolved in water and converted into tetramer ion [Zr₄(OH)₁₄(H₂O)₁₀]²⁺. Each zirconium atom contains 2–3 non-bridging hydroxyl groups. Zirconium tetramers are bonded by hydroxyl bridge bonds to form a polymer [Zr(O_x(OH) _4−2x• yH₂O]. The CO₂-TPD and XRD results show that the basicity of the surface lattice oxygen in the metal oxide solid base is closely related to the properties of the metal ions. The stronger the electron donor capacity of the metal ions, the stronger the basicity of the oxygen. MgO and ZrO₂ form a solid solution, and Mg²⁺ replaces the position of the Zr⁴⁺ particle in the lattice and forms a Mg−O−Zr structure, which makes the crystal ZrO₂ lattice shift. Moreover, the electronegativity of Mg²⁺ is less than that of Zr⁴⁺, and the formation of the Mg−O−Zr structure further increases the density of the lattice oxygen O²⁻ electron cloud and enhances the electronegativity. The change in O^2- electron cloud density caused by the embedding of Mg²⁺ into the lattice produces the basic potential. The initial decomposition step of HCFC-22 is hydrolytic. The water vapor is adsorbed and dissociated on MgO/ZrO₂, then the HCFC-22 is substituted with the adsorbed water vapor on the surface and bulk phase of the catalyst to generate CO₂, HF, HCl, and other gases. The mechanism of the catalytic hydrolysis for HCFC-22 by MgO/ZrO₂ is speculated and shown in Figure 14.

3. Experimental Section

3.1. Catalyst Preparation

The chemical reagents and solvents used in this research have a high purity percentage. The process of preparing the MoO₃/ZrO₂ catalyst using the impregnation method was as follows: Place 0.15 mol/L ZrOCl₂•8H₂O (Sino pharm group Chemical Reagents Company Limited) solution in a 250 mL beaker. Heat the solution to 60 °C in a water bath, stir to dissolve it completely, then slowly add 25% ammonia solution until the pH reaches 9~10 and continue stirring at 60 °C for 1 h. After this process, the reaction solution was sealed and stood at room temperature for 12 h, then it was washed until there was no Cl⁻. The filter cake was dried in an oven at 110 °C for 12 h. It was then impregnated with 0.5 mol/L (NH₄)₆Mo₇O₂₄•4H₂O (Tianjin wind boat Chemical Reagents Company Limited) solution for 4 h at 80 °C, then filtered and dried in an oven at 110 °C for 24 h. Finally, the samples were calcinated at 500 °C, 550 °C, 600 °C, 650 °C, and 700 °C for 3 h, and the samples were denoted as MoZr.

The MgO/ZrO₂ catalyst was prepared via coprecipitation and impregnation methods. Coprecipitation method: ZrOCl₂•8H₂O and Mg(NO₃)₂•6H₂O (Tianjin wind boat Chemical Reagents Company Limited) were mixed into an aqueous solution of 0.15 mol/L according to n(Mg):n(Zr) = 0.3:1. The water bath was heated to 60 °C and stirred to dissolve the solution. Then, 25% ammonia solution was added slowly until the pH reached 9~10. The filter cake was dried at 110 °C for 12 h and calcinated at 500 °C, 600 °C, 700 °C, and 800 °C for 6 h, respectively. Then, the samples were denoted as MgZr-c.

Impregnation method: First, 0.15 mol/L ZrOCl₂•8H₂O solution was prepared in a 250 mL beaker and heated to 60 °C in a water bath. The cake was dried at 110 °C for 12 h after standing at room temperature for 12 h. The cake was then drained and washed until there was no Cl⁻. The dried filter cake was impregnated for 12 h (n(Mg): n(Zr) = 0.3:1) in 0.5 mol/L Mg(NO₃)₂•6H₂O solution at 40 °C and filtered. The filter cake was dried at 110 °C for 12 h and calcined at 500 °C, 600 °C, 700 °C, and 800 °C for 6 h. The samples were denoted as MgZr-i.

3.2. Catalyst Characterization

The X-ray diffraction (XRD) patterns were assessed on a Bruker D8 Advance X-ray diffractometer in Germany using Cu Kα radiation, operating at 40 kV and 40 mA. The data were collected with 2θ values ranging from 10 to 90° using a step size of 0.01 °/s. The phase composition and structural degree of the samples were measured with a BELSORP-max Ⅱ gas adsorption instrument (McGee Bayer Co., Ltd., Durham, NC, USA) under liquid N₂ temperature using N₂ as the adsorbate. The surface acidity of the sample was measured on an American AutoChem II 2920 (Mike Company, Norcross, GA, USA) automatic chemical adsorption instrument. NH₃ temperature-programmed desorption (NH₃-TPD) was used to measure the NH₃ adsorption on Lewis and Brønsted acid sites in the catalysts. The following experimental steps were followed: (1) the samples were pretreated at 200 °C under helium flow, NH₃ was adsorbed at 200 °C, then the temperature was raised at a rate of 10 °C/min from 200 to 900 °C; (2) the amount of NH₃ desorbed was detected by gas chromatography; (3) the NH₃ desorption signal was detected by TCD. The surface alkalinity of the sample was measured using an American AutoChem II 2920 (Mike Company, USA) automatic chemical adsorption instrument. Here, 0.1 g sample was pretreated in an argon atmosphere at 700 °C for 2 h. After cooling to 30 °C, the CO₂ was adsorbed until reaching saturation and purged with argon for 0.5 h to drive off the CO₂ that had physically adsorbed on the surface. The temperature was increased to 900 °C at a rate of 10 °C/min, and the CO₂ desorption signal was detected via TCD.

3.3. Catalyst Evaluation

Here, 1.00 g of catalyst and 50 g of quartz sand were mixed evenly and placed in the quartz tube. The simulated gas composition (mol%) was 4.0 CFCs, 25.0 H₂O (g), while the rest was N₂. Silicagel was used as a desiccant and NaOH solution was used as the absorbent of acidic gases (HCl and HF). The samples were started 10 min after the temperature reached the target reaction temperature, and then qualitative and quantitative analyses of HCFC-22 in the reaction gas were carried out via gas chromatography-mass spectrometry (GC/MS). The hydrolysis rate of HCFC-22 was used to evaluate the catalytic hydrolysis effect. The detection conditions were as follows: the inlet temperature was 80 °C, the column temperature was 35 °C, with holding for 2 min; the carrier gas was high-purity He (He ≥ 99.99%), the column flow rate was 1 mL/min, with a constant current mode and split ratio of 200:1; the mass ion source was EI, the electron energy was 70 eV, the ion source temperature was 260 °C, the transmission line temperature was 280 °C, and the injection volume was 0.1 mL. The calculation formula was as follows:

[HCFC-22 hydrolysis rate] = ([HCFC-22]_import − [HCFC-22]_export)/[HCFC-22]_import × 100%

4. Conclusions

In conclusion, a solid acid MoZr catalyst and solid base MgZr catalyst were used to catalyze the hydrolysis of HCFC-22, which exhibited high catalytic activity. The catalytic hydrolysis effect of the solid acid MoZr was better than that of the solid base MgZr, and the hydrolysis rate reached more than 90%. The high dispersion of MoO₃ facilitates the transition of ZrO₂ from the monocline phase to the tetragonal phase, providing a weaker acid content. The electron cloud density and electronegativity could be promoted by the introduction of Mg²⁺, resulting in the formation of more oxygen vacancies, a stronger Brønsted acid, and larger specific surface area, which would increase the hydrolysis rate of HCFC-22. The optimum catalytic performance of the MoO₃(MgO)/ZrO₂ catalyst was determined in this study, and the degradation rate of HCFC-22 was improved. The experimental results show that the optimum catalytic conditions for the catalyst MoO₃/ZrO₂ to hydrolyze HCFC-22 were as follows: the calcination temperature was 600 °C, the catalytic hydrolysis temperature was 250 °C, and the hydrolysis rate of HCFC-22 reached 99.99% under this reaction condition. The optimum catalytic conditions for the catalytic hydrolysis of HCFC-22 by the catalyst MgO/ZrO₂ were as follows: the calcination temperature was 700 °C, the catalytic hydrolysis temperature was 400 °C, and the hydrolysis rates of HCFC-22 reached 98.03% (coprecipitation method) and 96.41% (impregnation method) under these reaction conditions. Compared with the functional ligand-based materials mentioned in this paper (zeolite, WO₃/M_xO_y, Na₂O/ZrO₂, CaO/ZrO₂), the MoO₃(MgO)/ZrO₂ catalyst has the advantages of high catalytic activity, simple operating conditions, a short reaction time, and easy recovery of the catalyst, providing a certain theoretical basis for the practical application of the degradation of low concentrations of HCFC-22.