Catalytic Properties of ZnZrO_x Obtained via Metal–Organic Framework Precursors for CO₂ Hydrogenation to Prepare Light Olefins

Abstract

The conversion of CO₂ into light olefins over bifunctional catalysts is a promising route for producing high-value-added products. This approach not only mitigates excessive CO₂ emissions but also reduces the chemical industry’s reliance on fossil fuels. Among bifunctional catalysts, ZnZrO_x is widely used due to its favorable oxide composition. In this work, ZnZrO_x solid solution was synthesized by calcining an MOF precursor, resulting in a large specific surface area and a small particle size. Characterization studies revealed that ZnZrO_x prepared via MOF calcination exhibited an enhanced CO₂ activation and H₂ dissociation capacity compared to that synthesized using the co-precipitation method. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO₂ adsorption on ZnZrO_x led to the formation of carbonate species, while HCOO* and CH₃O* intermediates were generated upon exposure to the reaction gas. When ZnZrO_x was combined with SAPO-34 molecular sieves under reaction conditions of 380 °C, 3 MPa, and 6000 mL·g_cat⁻¹·h⁻¹, the CO₂ conversion reached 34.37%, with a light olefin yield of 15.13%, demonstrating a superior catalytic performance compared to that of the co-precipitation method.

1. Introduction

Reducing CO₂ emissions is a global challenge, as excessive emissions exacerbate the greenhouse effect. Controlling CO₂ emissions or converting CO₂ into high-value-added products is therefore an urgent priority [1,2,3]. Light olefins (C₂⁼-C₄⁼) are essential raw materials that are used for industrial development. However, their production relies heavily on fossil resources, which are being rapidly depleted worldwide. Thus, a sustainable process is needed to address these challenges [4,5,6]. CO₂ hydrogenation for light olefin synthesis offers a dual benefit—it enables CO₂ utilization while mitigating fossil fuel dependence—making it a key research focus.

Currently, CO₂ hydrogenation to produce light olefins follows two main pathways—the Fischer–Tropsch route and the methanol-mediated route [6]. The Fischer–Tropsch process typically employs Fe- or Co-based catalysts, which exhibit good catalytic activity. However, product selectivity in this pathway is constrained by the Anderson−Schulz−Flory (ASF) distribution, resulting in a light olefin selectivity that is generally below 60% [7,8,9,10]. In contrast, the methanol-mediated route has gained attention due to its higher selectivity for light olefins [11,12,13,14]. This route employs bifunctional catalysts composed of a metal oxide and a zeolite molecular sieve, enabling the coupling of two distinct catalytic sites [15]. Initially, CO₂ undergoes hydrogenation on the metal oxide surface to form methanol intermediates, which are subsequently converted to light olefins on the acidic sites of the molecular sieve [16].

In recent years, oxides with spinel or solid solution structures, such as ZnCrO_x [15,17], InZrO_x [18,19,20], GaZrO_x [21,22], ZnZrO_x [23,24,25,26], etc., have attracted increasing research interest. Among these, ZnZrO_x not only exhibits a high activity and stability in high-temperature methanol synthesis, but also demonstrates an enhanced performance in CO₂ hydrogenation to olefins when coupled with SAPO-34, making it a promising candidate for industrial applications. Current research on ZnZrO_x primarily focuses on oxide modification [27,28]. The synthesis method significantly influences the microstructure and surface properties of the catalyst. Li et al. [29] first reported the preparation of ZnZrO_x via co-precipitation in 2017, prompting further investigations into its synthesis. Wang et al. [30] demonstrated that ZnZrO_x prepared using the aerogel method exhibits a larger specific surface area and a higher oxygen vacancy content. Sha et al. [31] proposed that ZnZrO_x synthesized via ammonia reflux contains more Zn-OH species, which enhance CO₂ and H₂ activation and adsorption, improving HCOO* formation and stability. Pinheiro Araújo et al. [32] employed flame spray pyrolysis to prepare ZnZrO_x, generating atomically dispersed Zn²⁺ sites at lattice positions on the ZrO₂ surface. Despite these advancements, ZnZrO_x still suffers from a low CO₂ conversion and a high CO selectivity. Therefore, further research is required to develop improved synthesis methods that enhance the catalytic performance of ZnZrO_x.

Metal–Organic Frameworks (MOFs) are a class of porous inorganic–organic hybrid materials composed of metal salts and organic ligands that are connected via coordination bonds. Compared to conventional materials, MOFs with a porous structure and a large specific surface area play a significant role in catalytic conversion [33,34]. In CO₂ hydrogenation systems, the most studied MOF-based materials include Zn, the Co-based ZIF series (ZIF-8 and ZIF-67), the Zr-based UiO series (UiO-66 and UiO-67), the MIL series (MIL-53 and MIL-101), and Cu-based MOFs (Cu-BTC) [35]. UiO-66 consists of Zr-containing ortho-octahedral Zr₆O₄(OH)₄ clusters that are linked to 12 terephthalic acid ligands, forming a three-dimensional microporous structure with an octahedral central pore cage and eight tetrahedral corner cages [36]. It has been shown that catalysts with higher specific surface areas and smaller particle sizes provide more active sites for CO₂ adsorption and generally exhibit a superior catalytic performance [30]. Oxides derived from UiO-66 precursors often inherit these advantageous properties, making UiO-66 a promising candidate for CO₂ hydrogenation. For example, Yu et al. [37] prepared Cu-Zn catalysts coated with UiO-66 via deposition–precipitation for CO₂ hydrogenation to methanol. Li et al. [38] synthesized Zr-based solid solutions using UiO-66 as a precursor for CO₂ hydrogenation catalysis. Tian et al. [39] developed tandem catalysts by combining metal oxides with UiO-66 for the direct hydrogenation of CO₂ to aromatics. However, the application of this approach to light olefin synthesis via CO₂ hydrogenation remains relatively unexplored, and the relationship between catalyst structure and performance requires further investigation.

Inspired by these studies, we synthesized Zn-modified UiO-66 and obtained ZnZrO_x through calcination. The structure and surface properties were analyzed using a series of characterization techniques. Compared to ZnZrO_x prepared via the conventional co-precipitation method, ZnZrO_x derived from MOF calcination exhibited smaller particle sizes and a larger specific surface area. When combined with SAPO-34 for CO₂ hydrogenation to light olefins, the ZnZrO_x-based catalyst achieved a CO₂ conversion of 34.37% and a light olefin yield of 15.13% under optimal conditions.

2. Materials and Methods

2.1. Chemicals and Catalyst Preparation

Zirconium nitrate [Zr(NO₃)₄·5H₂O; AR ≥98.5%], zinc nitrate [Zn(NO₃)₂·6H₂O; AR ≥98.5%], ammonium carbonate [(NH₄)₂CO₃; AR ≥98.5%], and N,N-dimethylformamide (DMF) [AR ≥99.5%] were purchased from Chengdu Cologne Chemical Co, Chengdu, China. 1,4-Dicarboxybenzene (H₂BDC) [AR ≥98.5%] was obtained from Fuchen Chemical Reagent Co, Fujian, China, and methanol [AR ≥99.5%] was purchased from Shanghai Titan Technology Co, Shanghai, China.

The preparation of UiO-66 was adapted from the literature with slight modifications [40]. The procedure was as follows: Zr(NO₃)₄·5H₂O and H₂BDC were dissolved in DMF under vigorous stirring. The resulting slurry was crystallized in an oven at 120 °C for 24 h. After crystallization, the solid was washed three times with DMF and methanol via centrifugation, before being dried overnight at 110 °C in an oven. The final product was UiO-66.

Zn-UiO-66 was synthesized using a similar method. Zn(NO₃)₂·6H₂O, Zr(NO₃)₄·5H₂O, and H₂BDC were dissolved in DMF under vigorous stirring, followed by crystallization at 120 °C for 24 h. After crystallization, the solid was washed three times with DMF and methanol via centrifugation, before being dried overnight at 110 °C. The final product was Zn-UiO-66.

The UiO-66 precursor was heated from 20 °C to 400 °C in a tube furnace at a rate of 2 °C/min and was calcined in air at 400 °C for 10 h to obtain ZrO₂-M. The Zn-UiO-66 precursor was treated under the same conditions to obtain ZnZrO_x-M.

For the preparation of ZnZrO_x-CP, the Zn:Zr molar ratio was set to 1:2. First, Zr(NO₃)₄·5H₂O and Zn(NO₃)₂·6H₂O were dissolved in 100 mL of deionized water. Then, (NH₄)₂CO₃ solution was added dropwise to the mixture while maintaining vigorous magnetic stirring at 70 °C. The pH was adjusted to 8, and the resulting mixture was aged, filtered, and washed to obtain a white filter cake. The cake was dried overnight at 110 °C and was then heated at a rate of 2 °C/min to 400 °C, followed by calcination for 5 h to obtain ZnZrO_x-CP.

The bifunctional catalyst was prepared by mixing and milling oxides and molecular sieves at a mass ratio of 1:1.6, followed by tablet granulation to 40–60 mesh. The molecular sieve used in this experiment was synthesized in-house by our research group.

2.2. Characterization of Catalysts

X-ray diffraction (XRD) was used to analyze the physical structure of the catalysts using a Rigaku Ultima IV X-ray powder diffractometer with a Cu Kα radiation source from Rigaku Corporation, Tokyo, Japan. The scanning parameters were set as follows: a 2θ range of 10–90°, an operating voltage of 40 kV, a current of 40 mA, a scanning rate of 8° min⁻¹, and a step size of 0.02°.

The textural properties were measured using N₂ sorption at −196 °C using a Micromeritics ASAP 2460 instrument from Micromeritics, Shanghai, China. The samples were degassed at 300 °C for 2 h prior to measurement. The total surface area was determined using the BET method. The pore volume was calculated from the desorption isotherm using the t-plot method, and the pore size distribution was analyzed using the BJH method. The Harkins–Jura equation was applied to calculate the statistical thickness of the adsorbed nitrogen layer for the t-plot method, while the Broekhoff–de Boer model was adopted for BJH pore size distribution analysis.

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed to examine the microstructure of the catalysts using a Thermo Scientific Talos F200S from Thermo Fisher Scientific Inc, Shanghai, China. The milled samples were dispersed in ethanol, sonicated for 20 min, and then dropped onto a microporous copper grid. Imaging was conducted at an accelerating voltage of 200 kV.

The catalyst surface properties were characterized using X-ray photoelectron spectroscopy (XPS) with a Thermo Fisher Scientific Escalab Xi+ from Thermo Fisher Scientific Inc, Shanghai, China. Oxygen vacancies were detected via electron paramagnetic resonance (EPR) spectroscopy using a Bruker A300.

H₂ temperature-programmed desorption (H₂-TPD) and CO₂ temperature-programmed desorption (CO₂-TPD) were conducted on a Priority TP-5080 chemisorption analyzer coupled with a Hiden Analytical mass spectrometer from Tianjin Xianquan Industry Trade Development Co, Tianjin, China. Typically, a 100 mg sample was pretreated at 300 °C for 1 h under a He atmosphere, cooled to 50 °C, and then exposed to a CO₂-He gas mixture (10% CO₂). Desorption was carried out under a He flow from 50 °C to 800 °C at a heating rate of 10 °C min⁻¹. The procedures for H₂-TPD and H₂ temperature-programmed reduction (H₂-TPR) were the same as those for CO₂-TPD.

The in situ Fourier transform infrared (FTIR) spectroscopy of CO₂ adsorption was performed on a Nicolet iS50 FTIR spectrometer from Thermo Fisher Scientific Inc, Shanghai, China. The samples were pretreated at 350 °C under a vacuum of 10⁻² Pa for 60 min, before being cooled to room temperature. CO₂ or raw gas was introduced into the in situ cell and was adsorbed for 30 min. Spectra were collected at different adsorption times over a range of 1000–4000 cm⁻¹.

2.3. Catalyst Evaluation

A high-pressure fixed-bed stainless steel tube reactor was used for all catalytic experiments from Chengdu Songsheng Measurement Control Technology Co, Chengdu, China. First, 0.4 g of the ZnZrO_x/SAPO-34 bifunctional catalyst was weighed and placed into a quartz tube with an inner diameter of 5 mm and a catalyst bed height of 2 cm. The quartz tube was then loaded into the fixed-bed reactor and pretreated with high-purity N₂ at 400 °C for 90 min. After pretreatment, the reaction gas mixture (H₂/CO₂/Ar = 72/24/4) was introduced into the reactor at a controlled flow rate. The reaction products were collected and analyzed online using an Agilent 8890 gas chromatograph.

Argon (4.0%) in the H₂/CO₂ gas mixture was used as an internal standard for data calculation. The CO₂ conversion, CO selectivity, and hydrocarbon distribution were determined based on carbon balance calculations, as follows:

$$\mathrm{Conversion} {\mathrm{CO}}_2\left(\%\right)={\displaystyle \frac{{\mathrm{CO}}_{2 \mathrm{inlet}} {- \mathrm{CO}}_{2 \mathrm{outlet}}}{{\mathrm{CO}}_{2 \mathrm{outlet}}}} \times 100$$

$$\mathrm{Selectivity} \mathrm{CO}\left(\%\right)={\displaystyle \frac{{\mathrm{CO}}_{\mathrm{outlet}}}{{\mathrm{CO}}_{2 \mathrm{inlet} }-{\mathrm{CO}}_{2 \mathrm{outlet}}}} \times 100$$

$$\mathrm{Hydrocarbon} \mathrm{distribution}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\left(\%\right)={\displaystyle \frac{\mathrm{n}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m} \mathrm{outlet}}}{{\mathrm{CO}}_{2 \mathrm{inlet}}-{\mathrm{CO}}_{2 \mathrm{outlet} }-{\mathrm{CO}}_{\mathrm{outlet}}}} \times 100$$

where ${\mathrm{CO}}_{2 \mathrm{inlet}}$ is the molar concentration of incoming CO₂ gas, ${\mathrm{CO}}_{2 \mathrm{outlet}}$ is the molar concentration of outgoing CO₂ gas, ${\mathrm{CO}}_{\mathrm{outlet}}$ is the molar concentration of reacted CO gas, ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m} \mathrm{outlet}}$ is the molar concentration of reacted hydrocarbons, and n denotes the carbon number.

3. Results and Discussion

3.1. Catalytic Performance Evaluation

ZnZrO_x exhibits an excellent catalytic performance for CO₂ hydrogenation and has been widely studied. By comparing the combination of ZnZrO_x and molecular sieves for CO₂ hydrogenation in different studies, as shown in

Table 1. Catalytic effectiveness of various metal oxide/zeolite tandem catalysts for CO₂ hydrogenation ^a.

Catalysts	CO2. Con %	CO. Sel %	Hydrocarbon Distribution/% (CO2 Free)				Reference
			CH4	C2=-C4=	C20-C40	C5+
ZnZrOx/SAPO-34	12.6	47	3	80	14	3	[29]
ae-ZnO-ZrO2/H-ZSM-5 b	16	34	/	/	/	/	[30]
13%ZnO-ZrO2/Mn0.1SAPO34	21.3	42.2	3.7	61.7	33.6	1.0	[25]
CZZ/MnSAPO34@Si-2	17.3	34.9	6.9	70	20.5	2.6	[41]
ZZ/4%MnSAPO-34	17.3	56.8	1.2	83.2	14.2	1.4	[42]
ZnZrOx/SSZ-13 b	9.1	32	/	89	/	/	[24]
ZnZrOx-CP/SAPO-34	26.6	38.9	1.3	80.9	11.9	5.9	this work
ZnZrOx-M/SAPO-34	34.3	46.1	1.6	81.8	10.7	5.9	this work

CO₂.Con: CO₂ conversion; CO.Sel: CO selectivity; C₂⁼-C₄⁼: light olefins; C₂⁰-C₄⁰: paraffins; the total sum of hydrocarbon distribution was 100%. All values are given in percentage (%). ^a The above data are taken from the best catalytic results reported in the literature, and the reaction conditions vary. ^b Only partially accurate data are available in the literature.

, we found that the catalysts in this work, ZnZrO_x-M and ZnZrO_x-CP, outperformed most reported catalysts in terms of CO₂ conversion.

The CO₂ hydrogenation performance of ZnZrO_x bound to SAPO-34, prepared using two different methods, was compared under the following conditions: 3 MPa, 380 °C, and 6000 mL·g_cat⁻¹·h⁻¹(shown in Tables S1 and S2). ZnZrO_x-M exhibited a higher CO₂ conversion, light olefin selectivity, and light olefin yield. It is noteworthy that there is a significant increase in CO₂ conversion, which may be attributed to the presence of more active sites for CO₂ on ZnZrO_x-M, which enhances the adsorption and activation of CO₂.

To further optimize the catalytic performance of ZnZrO_x-M, the effects of the oxide/molecular sieve mass ratio, reaction temperature, and reaction space velocity were investigated. As shown in Figure 1a, CO₂ conversion increased from 23.36% to 40.09% as the oxide content in the catalyst increased. This suggests that the oxide component plays a crucial role in CO₂ activation and conversion into reactive intermediates. Oxides provide additional active sites, enabling more CO₂ molecules to participate in the reaction. Conversely, as the molecular sieve content increased, the selectivity for light olefins improved from 48.22% to 81.97%, indicating that a higher molecular sieve content facilitates the rapid conversion of intermediates into light olefins. Therefore, optimizing the oxide/molecular sieve mass ratio is essential. At an oxide/molecular sieve ratio of 1:1.6, the bifunctional catalyst achieved both high CO₂ conversion and excellent light olefin selectivity, representing the optimal catalytic performance.

The catalytic performance of ZnZrO_x-M was also influenced by the reaction space velocity, as shown in Figure 1b. When the space velocity was increased from 3750 to 12,750 mL·g_cat⁻¹·h⁻¹, CO₂ conversion decreased from 37.27% to 26.74%. This decline was primarily due to the reduced contact time between the feed gas and the catalyst, limiting the number of activated CO₂ molecules. Meanwhile, light olefin selectivity gradually increased with higher space velocity. A higher space velocity facilitated the rapid removal of light olefins from the molecular sieve surface or cages, preventing further transformation into undesired byproducts. However, at excessively low space velocity values, the excessive hydrogenation of intermediates occurred, leading to an increase in alkane selectivity. Therefore, a space velocity of 6000 mL·g_cat⁻¹·h⁻¹ was identified as the optimal condition for maximizing light olefin production while ensuring efficient CO₂ activation.

Figure 1c illustrates the effect of the reaction temperature on the catalytic performance of ZnZrO_x-M. Between 360 °C and 400 °C, increasing the reaction temperature enhances molecular collisions, facilitating CO₂ activation and leading to a gradual increase in CO₂ conversion. However, the conversion of methanol to light olefins is an exothermic process. As the temperature continues to rise, light olefin formation becomes increasingly constrained by thermodynamic limitations, resulting in decreased light olefin selectivity. Additionally, the competing reverse water–gas shift (RWGS) reaction is endothermic. At higher temperatures, the RWGS reaction becomes more dominant, causing more CO₂ to be converted into CO rather than light olefins. The optimal catalytic performance was achieved at 380 °C, yielding 15.13% light olefins.

The ZnZrO_x-M/SAPO-34 bifunctional catalyst also exhibited good catalytic stability for the direct conversion of CO₂ to light olefins, as shown in Figure 1d. Under optimal reaction conditions, the catalyst maintained high catalytic efficiency for 55 h. These results demonstrate the significant potential of the ZnZrO_x-M/SAPO-34 bifunctional catalyst for further development.

3.2. Structure of Catalysts

The specific surface area, pore volume, and pore size of ZnZrO_x prepared using the two different methods were determined using N₂ adsorption–desorption isotherms, and the results are presented in

Table 2. Specific surface area, pore volume, pore size, and nanocrystal size of ZnZrOx prepared using different synthesis methods.

Catalyst	BET Surface Area a (m²/g)	Micropore Volume b (cm3/g)	Mesopore Volume c (cm3/g)	Pore Diameter d (nm)
ZnZrOx-CP	25	0.001	0.038	6.0
ZnZrOx-M	46	0.017	0.037	4.8

^a Specific surface area was calculated using the BET method. ^b Microporous pore volume was calculated using the t-plot method. ^c Neglecting the macropore pore volume, the total pore volume minus the micropore pore volume gives the mesopore pore volume. ^d The average pore size was calculated using the BJH method for the desorbed branch.

and Figure 2. The N₂ adsorption isotherms of both ZnZrO_x-M and ZnZrO_x-CP exhibit type IV characteristics with hysteresis loops, indicating that both oxides are mesoporous materials. ZnZrO_x-CP shows an H1-type hysteresis loop, while ZnZrO_x-M exhibits an H2-type hysteresis loop, suggesting that the pore structure of ZnZrO_x-M is more complex than that of ZnZrO_x-CP. ZnZrO_x-M possesses a significantly larger specific surface area and microporous volume compared to ZnZrO_x-CP, with the microporous volume being 14 times greater. A more developed pore structure facilitates the mass transfer of reaction intermediates to the molecular sieve, enhancing catalytic performance.

In Figure 3a, the XRD patterns of Zn-UiO-66 synthesized via the in situ method and UiO-66 show no significant differences, indicating that Zn incorporation did not alter the crystal structure of UiO-66. Upon the calcination of the Zn-UiO-66 and UiO-66 precursors, the characteristic peaks of UiO-66 disappeared, while new peaks corresponding to the tetragonal ZrO₂ phase emerged. This transformation is likely due to the decomposition of organic ligands supporting the UiO-66 framework under high-temperature conditions, leading to structural collapse and subsequent ZrO₂ formation. Figure 3b presents the XRD patterns of the two ZnZrO_x catalysts. Diffraction peaks at 2θ values of 30.22°, 35.27°, 50.21°, 59.27°, 60.20°, and 81.73° correspond to the (101), (110), (112), (103), (211), and (213) planes of tetragonal ZrO₂ (t-ZrO₂; PDF#79-1768). As shown in Figure S1, No distinct ZnO diffraction peaks were observed in either ZnZrO_x catalyst, suggesting that Zn may have been incorporated into the ZrO₂ lattice, forming a solid solution structure. [29] As shown in

Table 3. Parameters of ZnZrO_x-M, ZnZrO_x-CP, and ZrO_2.

Sample	Parameters
	c (Å)	V (Å3)	Diffraction Angles (°)
ZnZrOx-CP a	5.085 ± 0.003	65.51	30.46
ZnZrOx-M a	5.122 ± 0.004	66.61	30.35
t-ZrO2 b	5.184	67.04	30.22

^a Refinement is determined using the jade software. ^b Lattice parameters of t-ZrO₂ from PDF#79-1768.

, the lattice parameter c and cell volume of both ZnZrO_x catalysts are lower than those of pure t-ZrO₂ (PDF#79-1768), further supporting Zn²⁺ incorporation into the ZrO₂ lattice to form a solid solution. Notably, the diffraction angles of t-ZrO₂ in both ZnZrO_x catalysts exhibit a systematic shift toward higher angles compared to pristine t-ZrO₂ (PDF#79-1768), with ZnZrO_x-M displaying a less pronounced shift (Δ2θ = +0.13°) than ZnZrO_x-CP (Δ2θ = +0.24°). This suggests that ZnZrO_x-M contains a lower proportion of the solid solution structure compared to ZnZrO_x-CP, implying that some zinc species in ZnZrO_x-M may be highly dispersed on the surface.

To investigate the changes occurring during calcination, thermogravimetric (TG) analysis was performed on UiO-66 and Zn-UiO-66. As shown in Figure 4, both materials exhibit mass loss below 100 °C, which is attributed to the volatilization of residual water within the pore structure. The mass loss of UiO-66 between 100 and 300 °C corresponds to the dehydroxylation of zirconium clusters and the removal of some crystallization water, whereas in Zn-UiO-66, a similar mass loss occurs within the 100–250 °C range [43]. At higher temperatures, the organic linkers in UiO-66 begin to decompose, leading to a significant mass loss and the eventual conversion of UiO-66 into ZrO₂. In contrast, the TG curve of Zn-UiO-66 differs due to Zn incorporation, which partially replaces Zr and destabilizes the original UiO-66 framework. As a result, the thermal decomposition of the organic linkers occurs at lower temperatures, facilitating the formation of the ZnZrO_x solid solution structure. These findings further support the conclusions drawn from the XRD analysis.

Figure 5 presents the HRTEM images of the two ZnZrO_x catalysts. The lattice fringes of 2.92 Å in ZnZrO_x-M and 2.89 Å in ZnZrO_x-CP correspond to the (101) crystallographic plane of tetragonal ZrO₂ (t-ZrO₂) (Figure 5b,e). Both catalysts exhibit smaller lattice spacings than pure t-ZrO₂ (2.95 Å), indicating successful Zn incorporation into the ZrO₂ lattice to form a solid solution structure. Additionally, the crystal plane spacing of t-ZrO₂ in ZnZrO_x-CP is smaller than that in ZnZrO_x-M, which is consistent with the weaker diffraction angle shift observed for ZnZrO_x-M in the XRD results. This further confirms that ZnZrO_x-M contains a lower proportion of solid solution structure than ZnZrO_x-CP. As shown in the EDS mapping in Figure 5g, Zn is well dispersed on the surface of ZnZrO_x-M, leading to changes in its surface properties. ZnZrO_x-M primarily consists of spherical or ellipsoidal nanoparticles with an average size of approximately 6.7 nm and a relatively uniform distribution. In contrast, ZnZrO_x-CP consists mainly of irregular nanoparticles with an average size of about 12.5 nm (Figure 5c,f). Smaller and more uniformly distributed nanoparticles typically enhance mass transfer, facilitating the faster transport of reaction intermediates from CO₂ hydrogenation to the molecular sieve for further conversion.

3.3. Surface Properties of Catalysts

The surface oxygen vacancy concentrations of the oxides prepared using the two methods were analyzed using XPS O1s spectroscopy, as shown in Figure 6a. In general, the peaks at binding energies of 529.5–531 eV, 531–532 eV, and 532–533 eV in the O 1s spectrum correspond to lattice oxygen (O_lattice), oxygen defects (O_defect), and surface-adsorbed oxygen species, respectively. ZnZrO_x-M exhibited a higher concentration of surface oxygen defects. This high concentration of oxygen vacancies is not an accidental situation as shown in Figure S2. Combined with the catalytic performance analysis, oxygen defects showed a positive correlation with CO₂ conversion, suggesting that higher oxygen vacancy concentrations enhance the catalyst’s ability to activate CO₂ [44]. Oxygen vacancies in the oxides were further quantified using EPR spectroscopy, as shown in Figure 6b. Under visible light irradiation, ZnZrO_x exhibited an oxygen vacancy signal with a g-value of 2.003. The signal intensity for ZnZrO_x-M was significantly higher than that of ZnZrO_x-CP, which is consistent with the XPS results.

The reduction behavior of ZnZrO_x catalysts was further examined using H₂-TPR, as shown in Figure 7. Reduction peaks in the low-temperature region (100–300 °C) correspond to the reduction of dispersed ZnO. The higher surface Zn content in ZnZrO_x-M is evident from the intensity of this peak. ZnZrO_x-CP exhibited two H₂ reduction peaks above 600 °C—the peak at 604 °C corresponds to the reduction of the Zn-O-Zr solid solution, while the peak at a higher temperature is attributed to the reduction of bulk-phase ZrO₂. In contrast, ZnZrO_x-M showed reduction peaks at 393 °C and 629 °C, which both shifted to lower temperatures compared to ZnZrO_x-CP. The lower reduction temperatures indicate that ZnZrO_x-M has a stronger capacity for rapid H₂ dissociation and activation, which is beneficial for catalytic performance.

The H₂ adsorption capacity of the two ZnZrO_x catalysts was evaluated using H₂-TPD (Figure 8a). Peaks below 300 °C correspond to the physisorption and desorption of weakly chemisorbed H₂ molecules, while peaks above 400 °C indicate the desorption of strongly chemisorbed H₂ molecules. As shown in the figure, ZnZrO_x-M exhibits not only a greater number of strong adsorption sites but also a lower desorption temperature and a larger desorption peak area. These results are consistent with those from H₂-TPR, further confirming that ZnZrO_x-M can rapidly dissociate and activate H₂. The enhanced H₂ activation ability of ZnZrO_x-M is attributed to the higher surface concentration of Zn species.

The CO₂ adsorption capacity of the two ZnZrO_x catalysts was further examined using CO₂-TPD. Peaks below 300 °C correspond to physical adsorption and weak chemical adsorption, whereas peaks above 300 °C are attributed to moderately strong chemical adsorption on ZnZrO_x. ZnZrO_x-M exhibits a larger peak area in the high-temperature region compared to ZnZrO_x-CP, indicating its superior CO₂ adsorption and activation capacity. This finding is consistent with the XPS analysis, confirming that the higher oxygen vacancy concentration in ZnZrO_x-M provides more active sites for CO₂ adsorption and activation, thereby enhancing its catalytic performance.

3.4. Reaction Mechanism Study

To further understand the reaction mechanism of ZnZrO_x-M catalysts, in situ DRIFTS was used to monitor surface reactive species and investigate the kinetic behavior of these species under different atmospheres. First, CO₂ was introduced under vacuum conditions, as shown in Figure 9a. CO₂ adsorption on ZnZrO_x-M generated four main IR peaks. The peak at 1537 cm⁻¹ corresponds to the antisymmetric stretching vibration of the O-C-O group in monodentate carbonates, while the peak at 1402 cm⁻¹ corresponds to the symmetric stretching vibration of the O-C-O group in monodentate carbonates. Additionally, the peaks at 1325 cm⁻¹ and 1623 cm⁻¹ are assigned to bidentate carbonate species. These results indicate that CO₂ adsorption on ZnZrO_x-M leads to the formation of both monodentate and bidentate carbonates [45]. Moreover, as adsorption time increased, the intensity of these characteristic IR peaks gradually enhanced, suggesting that more CO₂ was adsorbed and converted into carbonate species.

The introduction of a CO₂/H₂ mixture simulated real reaction conditions. In the spectral range of 2800–3000 cm⁻¹, three IR peaks were observed at 2971 cm⁻¹, 2918 cm⁻¹, and 2865 cm⁻¹. The peaks at 2971 cm⁻¹ and 2865 cm⁻¹ were attributed to the C-H stretching vibration of formate (HCOO*) and the asymmetric O-C-O stretching vibration, respectively. The peak at 2918 cm⁻¹ was assigned to the C-H asymmetric stretching vibration of methoxy species (CH₃O*). Additionally, characteristic peaks at 1374 cm⁻¹ and 1592 cm⁻¹ were attributed to the symmetric and asymmetric stretching vibrations of the O-C-O group in formate (HCOO*) [31]. The intensities of the formate and methoxy peaks increased with increasing adsorption temperature, reaching their highest levels at 300 °C.

Based on the infrared spectral changes, the reaction pathway can be inferred as follows: CO₂ is initially adsorbed on ZnZrO_x, forming carbonate species. In the presence of oxygen vacancies, these carbonates are activated and converted into bidentate formate (HCOO*). Upon interaction with activated hydrogen, formate species (HCOO*) undergo further reduction to methoxide species (HCO₃*). The continued hydrogenation of methoxide leads to the formation of methanol (CH₃OH), which subsequently passes through the molecular sieve, ultimately yielding the desired C₂⁼-C₄⁼.

4. Conclusions

In this study, Zn-UiO-66 was synthesized by introducing Zn in situ into the MOF material UiO-66, followed by calcination to obtain ZnZrO_x-M with a solid solution structure. ZnZrO_x-M functioned as a bifunctional catalyst, with an oxide component facilitating the hydrogenation of CO₂ to light olefins.

• A comparison between ZnZrO_x-M, prepared via MOF calcination, and ZnZrO_x-CP, synthesized via co-precipitation, revealed that ZnZrO_x-M had a larger specific surface area and a smaller particle size due to the structural advantages of the MOF precursor. This enhanced mass transfer during the catalytic process. XRD and TEM analyses showed that ZnZrO_x-M contained less of a solid solution structure than ZnZrO_x-CP, with a portion of Zn being highly dispersed on the ZnZrO_x-M surface.
• The highly dispersed Zn on the surface improved the H₂ reduction and activation ability of ZnZrO_x-M. H₂-TPR and H₂-TPD results confirmed the superior H₂ dissociation and activation capacity of ZnZrO_x-M. XPS and CO₂-TPD analyses further indicated that ZnZrO_x-M had a high concentration of surface oxygen vacancies, which provided additional active sites for CO₂ adsorption and activation. These properties contributed to the high activity and excellent catalytic performance of ZnZrO_x-M.
• Under optimal reaction conditions (3 MPa, 380 °C, and 6000 mL·g_cat⁻¹·h⁻¹), the ZnZrO_x-M/SAPO-34 catalyst achieved a CO₂ conversion of 34.37%, a light olefin selectivity of 81.84%, and a light olefin yield of 15.13%. Additionally, the catalyst maintained catalytic stability for over 55 h. These findings indicate that ZnZrO_x synthesized from the MOF precursor system has promising potential for industrial applications and further development.