The Effect of Surfactant P₁₂₃ on the KCuLaZrO₂ Catalysts in the Direct Conversion of Syngas to Higher Alcohols

Abstract

The direct conversion of coal-based syngas to HA (higher alcohols) is of great significance, but it remains challenging stemming from the complexity of the reaction and the difficulty in regulating the alcohol distribution. P₁₂₃, as a structure-directing agent, is of great significance for the preparation of mesoporous materials with specific pore sizes and pore structures. Therefore, a series of KCLZ-xP (KCuLaZrO₂-xP₁₂₃) catalysts with varying P₁₂₃ contents were prepared via the coprecipitation method and applied for HA synthesis. The KCLZ-30P catalyst exhibits a high CO conversion of 63.1%, a C₂₊OH/MeOH ratio of 0.98, and a comparable STY_ROH (space-time yield of total alcohol). Notably, it can selectively form linear alcohols in HAS while suppressing the formation of i-C₄ (branched alcohols). The results show that P₁₂₃ remarkably boosts catalytic activity through enlarging the specific surface area and facilitating the generation of t-ZrO₂. Simultaneously, P₁₂₃ suppresses the formation of i-C₄ alcohols by reducing the number of basic sites and weakening the strength of high-strength basic sites. Remarkably, the abundant CH_x and non-dissociated CO adsorbed on Cu⁰ facilitate the CO insertion process, thereby enhancing the C-C chain growth capability in linear alcohols, particularly favoring the formation of ethanol. These findings may offer the potential designing efficient catalysts for HAS (higher alcohol synthesis) from syngas.

1. Introduction

Coal, as an irreplaceable primary energy source, plays a role in providing a backup for the construction of a new energy system. The clean and efficient utilization of coal is of great significance in promoting the high-quality development and green low-carbon transformation of the coal industry in the new era [1,2,3]. HA (C₂₊OH) has drawn significant attention due to its widespread use as a synthetic fuel, alternative fuel additive, hydrogen carrier, and pharmaceutical [4,5,6]. The direct thermal catalytic conversion of coal-based syngas (CO/H₂) into higher alcohols is one of the effective ways to alleviate the shortage of energy resources and increasingly serious environmental problems. Nevertheless, there are still significant challenges in developing highly efficient and stable catalysts for industrialization due to technical difficulties such as complex synthesis reactions and multiple by-products.

As is well known, the HAS is directed from syngas via the CO insertion mechanism, in which the insertion of undissociated adsorbed CO* into the C_nH_x* formed by CO dissociation is the most critical step. Although the HAS process is thermodynamically favorable, the formation of HA will be kinetically controlled due to the presence of many different chemical reactions [7,8,9]. Until now, five types of heterogeneous catalysts have extensively explored for HAS: (a) Rh-based catalysts, (b) modified methanol synthesis catalysts (high-temperature ZnCr catalysts and low-temperature Cu-based catalysts), (c) modified F-T synthesis catalysts, (d) Mo-/MoS₂-based catalysts, and (e) tandem catalysts [10,11,12,13]. Among them, the supported non-precious metal Cu/ZrO₂ catalysts are of particular interest, owing to the high activity, the great total alcohol selectivity (lower hydrocarbons), and a superior cost advantage. In the Cu/ZrO₂ catalyst systems, ZrO₂ with high thermal stability plays a role in supporting and dispersing active metals, and it can also generate unique interactions with Cu species. Typically, the copper active sites are responsible for dissociating the adsorbed H₂ and non-dissociating adsorbed CO* and transferring the adsorbed species to ZrO₂, where the alcohol is generated [14,15]. Scotti and co-workers observed that highly dispersed Cu species are generated on a high surface area ZrO₂, which achieved a superior catalytic activity [16]. Wu et al. prepared Fe promoted CuZrO₂-based catalyst showed higher total alcohol STY and isobutanol selectivity, which were ascribed to the greatest quantity of weak CO adsorption and basic sites [17]. They also proposed that the addition of La enhances the Cu-Zr synergistic interaction, leading to significantly improved catalytic activity. This enhancement is attributed to both the increased reducibility of the catalyst and the higher dispersion of Cu species [18]. Some studies have revealed that the Cu-La interface is conducive to the formation of stable Cu⁺ species, which played a crucial role in the formation of C₂₊ alcohols [19,20]. In brief, the formation of C₂₊ alcohols strongly depends on the Cu-Zr interaction. Despite these efforts, Cu/ZrO₂ catalysts still encounter some significant challenges, including lower alcohol yield, high selectivity for CO₂ and CH_x, and poor catalyst stability.

Notably, an important strategy in the synthesis of higher alcohol from syngas is to add non-metallic surfactants to regulate the electronic and geometric structure of catalysts [11,21,22]. P₁₂₃ (PEO₂₀PPO₇₀PEO₂₀—EO represents ethoxy, and PO represents propoxy) is a triblock non-ionic surfactant that can be used as a template for preparing catalysts with mesoporous structures. The effect of P₁₂₃/Cu-Zn-Zr molar ratio on the conversion of CO₂ to methanol was recently studied by Marcos et al. [23]. It was demonstrated that a higher surfactant ratio can optimize the Cu^+/0 and ZnO-ZrO₂ physicochemical properties. Liu et al. designed ordered mesoporous NiMo-Al₂O₃ catalysts using P₁₂₃ as a structure-directing agent to regulate its textural properties, resulting in different metal–support interactions [24]. Ganiyu et al. observed that the P₁₂₃-assisted preparation of the Ti-SBA-15 HDS catalyst exhibited improved dispersion, textural properties, and surface acidity, which are mainly accounted for in the outstanding catalytic activities [25]. Han et al. utilized ionic surfactant P₁₂₃ as the template to regulate the morphology and construct prism-like silica tubes with mesoporous membranes [26]. Nonetheless, to the utmost extent of our awareness, there is scarce research on the impact of P₁₂₃ as an additive on the pore structure and active components of Cu/ZrO₂ catalysts.

Herein, we aimed to improve the specific surface area and regulate the alcohol distribution by introducing template P₁₂₃ during the coprecipitation process. For this purpose, a series of KCuLaZrO₂ catalysts with different contents of P₁₂₃ were designed and applied for HAS from syngas. Combining multiple characterizations, the structure and chemical state of KCLZ-xP catalysts and their impact on alcohol distribution were systematically studied. Furthermore, our comprehensive characterization provided valuable insights into the active sites within the KCLZ-xP catalyst, as well as the crucial factors that influence the reaction routes toward HA. This research will enhance the level of catalyst technology for the synthesis of HA from syngas.

2. Results

2.1. Catalytic Performance

To study the catalytic performance of the KCLZ-xP catalysts, HAS was evaluated at 350 °C and 5 MPa with syngas (H₂/CO = 2.5), and the results are shown in

Table 1. Catalytic performance of different KCLZ-xP catalysts in gas-phase CO hydrogenation reaction ¹.

Catalyst	XCO/%	Selectivity/%				STYROH g/(L·h)	STYgas g/(L·h)	C2+OH/MeOH	Distribution of Alcohols/(wt)%
		ROH	CO2	CHx	CH4				C1	C2	C3	C4	i-C4	C4+
KCLZ	19.3	37.2	36.3	12.9	12.1	40	77	0.51	66.4	12.0	10.1	2.2	9.3	0
KCLZ-10P	48.3	7.7	49.5	18.9	23.7	30	411	0.78	56.1	20.7	13.5	3.0	6.8	0
KCLZ-20P	58.3	7.6	47	20.6	24.5	38	522	0.89	52.9	25.5	13.0	3.3	4.4	0.9
KCLZ-30P	63.1	6.5	45.6	24.6	23.0	33	528	0.98	50.5	27.2	13.1	3.7	3.1	2.4

¹ Reaction conditions: T = 350 °C, P = 5 MPa, GHSV = 3000 h⁻¹, H₂/CO = 2.5.

. Figure 1A,B intuitively present the alcohol distribution and CO conversion, i.e., C₂₊OH/MeOH of the KCLZ-xP catalysts, respectively. A gradual increase in P₁₂₃ leads to a significant increase in CO conversion (X_CO) from 19.3% to 63.1%. Meanwhile, the impact of P₁₂₃ on STY_ROH is minimal and almost unchanged. Compared with the KCLZ catalyst without P₁₂₃, all KCLZ-xP (x = 10–30) catalysts exhibited lower ROH selectivity. However, the space-time yield of gaseous products (STY_gas) increases sharply from 77 to 528 g/(L·h). This is mainly due to the fact that the selectivities of both CH_x and CH₄ have been increased to nearly twice, indicating that the introduction of P₁₂₃ is conducive to the formation of a CH_x intermediate. In the upcoming discussion, we will examine the reasons that give rise to this phenomenon. In the distribution of alcohols, the selectivity of branched-chain alcohols, namely isobutanol (i-C₄), decreased monotonously from 9.3 to 3.1 wt%, while the selectivity of straight-chain alcohols increased gradually. It can be seen intuitively from Figure 1 that the addition of P₁₂₃ leads to an increase in C₂₊OH/MeOH from 0.51 to 0.98, notably boosting ethanol selectivity from 12.0 to 27.2 wt%. Figure 2 shows the ASF plot of alcohols and the corresponding chain-growth probability (α) of the KCLZ-xP catalysts. It is also found that the distribution of alcohol products is closer to classical ASF distribution with the increase in P₁₂₃ content [27]. It is noteworthy that, compared to the KCLZ catalyst (0.28), the chain-growth probability (α) of KCLZ-30P is as high as 0.32. These results strongly indicated that the KCLZ-xP catalysts with higher P₁₂₃ content can effectively suppress the formation of CH₃OH and exhibit an enhanced carbon chain growth capability.

2.2. Physicochemical Properties of Catalysts

To investigate the influence of the surface structure on the reaction, we further calculated the N₂ physical adsorption of all the catalyst powders and summarized the results in

Table 2. Textural properties of the KCLZ-xP catalysts.

Catalyst	Surface Area SBET (m2/g)	Pore Volume Vp (cm3/g)	Pore Size Dp (nm)	XCu+ (%)
KCLZ	82.58	0.10	3.15	63
KCLZ-10P	88.53	0.11	3.65	60
KCLZ-20P	91.65	0.11	3.57	57
KCLZ-30P	72.75	0.10	3.79	60

. As the amount of P₁₂₃ increases, the S_BET significantly increased from 82.58 m²/g to the highest 91.65 m²/g, and then decreased to 72.75 m²/g by further increasing the P₁₂₃ to 30 g. This may be attributed to the removal of P₁₂₃ during the calcination process of the catalysts, which resulted in the formation of more porous structures. As the P₁₂₃ content increased to 30 g, the S_BET significantly reduced, which is possibly because the collapse of some pores caused by the continuous decomposition of higher P₁₂₃ during the calcination process. After the introduction of P₁₂₃ into the KCLZ-xP catalysts, no change in the pore volume was observed, and it remained at 0.1 cm³/g. In addition, the average pore size of the KCLZ-xP catalyst slightly increases from 3.15 to 3.79 nm. As depicted in Figure 3A, a type IV adsorption isotherms with an H2-type hysteresis loops are observed in all KCLZ-xP catalysts, indicating the preservation of the typical characteristics of “inkbottle” structure and mesoporous materials [28]. The narrow pore size distribution concentrated at 3–4 nm was exposed in all KCLZ-xP catalysts, as shown in Figure 3B. In other words, the large specific surface area and mesoporous properties facilitate the generation of more active sites, enabling reactant molecules to quickly adsorb on the active sites and allowing the formed molecules to depart promptly.

2.3. XRD

Crystallinity and phase patterns were performed to investigate the effect of P₁₂₃ on HAS. XRD patterns of calcined and reduced KCLZ-xP catalysts are shown in Figure 4A and Figure 4B, respectively. As displayed in Figure 4A, a relatively broad diffraction peak in the range of 28–38° for the calcined KCLZ-xP catalysts are unambiguously indexed to amorphous ZrO₂ (am-ZrO₂) [29,30]. Remarkably, it can be seen that the KCLZ-30P catalyst has distinct diffraction peaks around 30.3°, 50.3°, and 60.2°, which were attributed to tetragonal ZrO₂ (t-ZrO₂, JCPDS 50–1089) [17]. In other words, as the content of P₁₂₃ increases, the presence of am-ZrO2 in the calcined KCLZ-xP catalyst is accompanied by the emergence of t-ZrO₂, indicating that the introduction of P₁₂₃ promotes the formation of t-ZrO₂. No phases attributed to CuO and La₂O₃ were observed in the calcined KCLZ-xP catalysts, which may be due to the high dispersion of CuO and the low loading amount of La₂O₃.

For reduced KCLZ-xP catalysts (Figure 4B), ZrO₂ mainly exhibited amorphous form when the P₁₂₃ content is less than 20 g, similar to the calcined KCLZ-xP catalysts. Interestingly, new visible peaks related to t-ZrO₂ appeared completely in the KCLZ-30P catalyst, which again proved that P₁₂₃ above 20 g was conducive to the formation of t-ZrO₂. Since the specific surface area of am-ZrO₂ (100–300 m²/g) is significantly higher than that of t-ZrO₂ (10–50 m²/g), which accounts for the changes in S_BET observed in Table 2 [12,31]. All reduced KCLZ-xP catalysts displays the distinct peaks around 2θ of 43.5°, 50.4°, 73.9°, characteristic of metallic Cu⁰ (JCPDS 04–0836) [10]. It might be considered that the CuO crystallite transforms into metallic Cu after the reduction. In addition, the diffraction peaks assigned to metallic Cu⁰ became stronger as the amount of P₁₂₃ (0–20 g) increases, indicating that the Cu0 particles size increased with the increase of P₁₂₃. By contrast, the diffraction peak intensity of metallic Cu⁰ is significantly weakened when the P₁₂₃ content is 30 g. It is further validated that metallic Cu⁰ is highly dispersed on the surface of the KCLZ-30P catalyst, which expectedly improved the activation of H₂ and the formation of HA [32,33].

2.4. H₂-TPR

To investigate the reduction process of Cu species on the surface, the KCLZ-xP catalysts were explored by H₂-TPR, as shown in Figure 5. It should be explained that ZrO₂ and La₂O₃ will not be reduced under reduction conditions, so we believe that the hydrogen consumption peaks only represent the reduction of Cu species. In view of KCLZ catalyst, three overlapped reduction peaks are indicative of the presence of three Cu species with different chemical environments. Among them, the reduction peaks at the lower temperatures of 107 °C and 117 °C were attributed to the reduction of well-dispersed and dispersed CuO species [34,35]. The broad reduction peaks located around 155 °C were assigned to a consecutive reduction of bulk CuO to Cu⁰. As the P₁₂₃ content increased to 10 g, three hydrogen consumption peaks still persist and all shift towards higher temperatures. When P₁₂₃ is further increased from 10 g to 30 g, a single low-temperature reduction peak emerges, and all reduction peaks shift towards higher temperatures. Previous studies have shown that the interaction (Cu-O-Zr) between finely dispersed CuO species and am-ZrO₂ promotes the reduction of CuO [36,37]. According to H₂-TPR, the reduction temperature gradually shifted toward higher temperature with an increase in P₁₂₃ content, which means that the reduction of CuO is more difficult. Based on the XRD results, this may be due to the decrease in am-ZrO₂ caused by P₁₂₃, which reduces the Cu-O-Zr interaction between finely dispersed CuO and am-ZrO₂, making the reduction of CuO more difficult [12].

2.5. CO, CO₂, NH₃-TPD

The CO-TPD was carried out to investigate the adsorption and desorption behavior of CO over the reduced KCLZ-xP catalysts. As shown in Figure 6, all KCLZ-xP catalysts display two wide CO desorption peaks around 262 °C and 486 °C, corresponding to non-dissociative adsorption CO and dissociative adsorption CO, respectively [17]. The KCLZ catalyst possesses a significantly higher density of non-dissociative adsorption CO sites compared to the other three catalysts. The content of different CO adsorption sites in reduced KCLZ-xP catalysts is shown in

Table 3. Distribution of CO adsorption sites, base amount and acid amount over KCLZ-xP catalysts.

Catalyst	CO Adsorption (mmol/g)			Basic Amount (mmol/g)				Acid Amount (mmol/g)
	Weak	Strong	Total	Weak	Medium	High	Total	Weak	Medium	High	Total
KCLZ	0.081 (70%)	0.035 (30%)	0.116	0.142	0.288	0.056	0.486	0.092	0.341	0.09	0.553
KCLZ-10P	0.056 (53%)	0.050 (47%)	0.106	0.273	0.180	0.039	0.492	0.010	0.242	0.08	0.400
KCLZ-20P	0.072 (56%)	0.057 (44%)	0.129	0.193	0.191	0.046	0.430	0.012	0.237	0.079	0.386
KCLZ-30P	0.059 (56%)	0.047 (44%)	0.106	0.181	0.193	0.041	0.415	0.007	0.241	0.076	0.367

. It is distinctly observable that when the content of P₁₂₃ increases from 0 to 10 g, the proportion of strong CO adsorption sites significantly increases from 30% to 47%. However, the proportion of strong CO adsorption decreases slightly to 44% when P₁₂₃ increases to 30 g. Additionally, the introduction of P₁₂₃ does not induce any temperature shift in the CO desorption peaks, indicating that P₁₂₃ has no obvious effect on the adsorption strength of CO.

The surface acidity and basicity of the catalyst are of great significance in the growth of carbon chains and the formation of alcohols in the process of CO hydrogenation. As displayed in Figure 6B,C, the surface acidity and alkalinity of the KCLZ-xP catalysts were detected by CO₂-TPD and NH₃-TPD, respectively. Table 3 lists the contents of three different basic sites on the KCLZ-xP catalyst surface, including weak-strength basic sites (50–200 °C), medium-strength basic sites (300–500 °C) and high-strength basic sites (580–700 °C). Compared with KCLZ, the KCLZ-xP catalyst with P₁₂₃ showed an increase density of weak-strength basic sites. By contrast, the content of medium-strength basic sites over KCLZ-xP catalyst with P₁₂₃ is lower than that of KCLZ. Notably, as the P₁₂₃ increases, the high-temperature desorption peak of CO₂ (622 °C) gradually shifts toward lower temperatures, indicating that P₁₂₃ weakened the strength of high-strength basic sites. Altogether, the number of high-strength basic sites and total basic sites decreased with the increase in P₁₂₃.

Figure 6C shows the NH₃-TPD profiles of different catalysts. All KCLZ-xP catalysts display three NH₃ desorption peaks, corresponding to weak-strength acid sites (124 °C), medium-strength acid sites (378 °C), and high-strength acid sites (583 °C). It was clearly observed that the KCLZ catalyst exhibited a significantly higher density of weak-strength acid sites than that of the other three catalysts. Additionally, the amounts of medium-strength acid sites decreased slightly with the increase in P₁₂₃. Compared with KCLZ catalyst, the NH₃ desorption peak around 583 °C shifts to higher temperature after the introduction of P₁₂₃, accompanied by a decrease in density. It indicates that P₁₂₃ enhances the acidity of high-strength acid sites, which may be caused by the high lack of electron Zr⁴⁺ or Cu⁺. The highly electron-deficient Zr⁴⁺ or Cu⁺ species facilitate CO adsorption and activation, which is responsible for explaining why the significant enhancement in catalytic activity with the increase in P₁₂₃. These reports were in great coincidence with the CO-TPD obtained here.

2.6. XPS

We used a surface sensitive technique XPS to obtain surface information about the composition, chemical state, and electronic structure on the reduced catalyst. The XPS patterns of Zr 3d, O 1s, Cu 2p, and Cu LMM regions of the catalysts were shown in Figure 7. As presented in Figure 7A, the Zr 3d_3/2 and Zr 3d_5/2 of KCLZ-xP catalysts locate at approximately 184.2 and 181.8 eV, respectively, with a disparity of 2.4 eV, which are in good accordance with the reported data of Zr⁴⁺ [38,39,40]. At the same time (Figure 7B), the oxygen species with binding energy at 531.3 eV was attributed to the hydroxyl species (O_H), and the oxygen species at a lower binding energy of 529.9 eV was attributed to the lattice oxygen species (O_L) [41]. The surface OH can react with CO to generate C1 species (CH_xO), which is also an important intermediate in the process of alcohol formation [17]. And then, as shown in Figure 7C, the Cu 2p spectra presented two prominent peaks at around 954.7 and 934.5 eV, which corresponded to Cu 2p_1/2 and Cu 2p_3/2, respectively. Moreover, the lack of satellite peaks within the binding energy range of 940–945 eV indicated that all Cu²⁺ species had been reduced to Cu⁺ or Cu⁰ at 360 °C. To gain further insights, the Cu LMM XAES spectra were obtained (Figure 7D). It was observed that the broad and asymmetrical peak centered at around 916.8 eV and 913.5 eV, which was assigned to Cu⁰ and Cu⁺ species, respectively [42]. The proportions of Cu⁺ species (X_Cu+) obtained after peak deconvolution are summarized in Table 2. Combined with the XRD results, Cu⁰ and Cu⁺ species coexisted on the surface of reduced KCLZ-xP catalysts.

2.7. In Situ CO-DRIFTS

The aim for a better understanding of the surface chemistry on the KCLZ-xP catalysts with different P₁₂₃ contents, in situ CO-DRIFT spectroscopy was exploited. Figure 8 and Figure 9 show the in situ CO-DRIFT spectra in the high-frequency range of 2175–2025 cm⁻¹ and the low-frequency range of 1850–1150 cm⁻¹, respectively. Compared with Cu⁰ and Cu²⁺ species, CO-Cu⁺ has stronger interaction and thermal stability [43]. Therefore, the stable peak approximately at 2104 cm⁻¹ for the KCLZ-xP catalysts is likely to be related to the existence of Cu⁺-CO carbonyls, which act as the predominant species [39,44], while the bands at 2004 cm⁻¹ are considered to be due to the CO adsorbed on Cu⁰ [45]. In the case of KCLZ catalyst, the peak at 1640 cm⁻¹ is ascribed to the bridged bicarbonate species (b-HCO₃⁻), which serves as evidence that CO had reacted with the surface OH groups, leading to the formation of C1 species [17,39]. Another peak appearing at 1578 cm⁻¹ could be possibly linked to the O-C-O asymmetric stretching vibration of bidentate formate species (b-HCOO⁻) [30,46,47]. Additionally, the peak at 1308 cm⁻¹ has been identified as corresponding to the result of the overlap of formate (b-HCOO-Cu, 1350 cm⁻¹) and bridged bidentate carbonates (b-HCO₃⁻, 1250 cm⁻¹) [30,39], whereas the absorption peak at 1067 cm⁻¹ is ascribed to C-O stretching vibrations of methoxyl species [48]. Upon increasing the P₁₂₃ content to 20 g, the intensity of the absorption peaks linked to HCO₃⁻ and HCOO⁻ increased significantly. However, for the KCLZ-30P catalyst, the intensity of the absorption peaks associated with HCO₃⁻ and HCOO⁻ decreased significantly, especially for the HCOO⁻ species. At the same time, a new absorption peak appeared at about 1520 cm⁻¹ of the KCLZ-30P catalyst, which was the characteristic peak of monodentate carbonate species (m-CO₃²⁻) [39]. According to the above results, we found that increasing P₁₂₃ caused the characteristic peak of all species on the catalyst surface increased first and then decreased, which may be related to the change in the specific surface area of the catalyst (S_BET result).

3. Discussion

According to the catalytic performance evaluation, an increase in P₁₂₃ content led to CO conversion increasing linearly. The BET analysis showed that when the content of P₁₂₃ increased from 0 to 20 g, the S_BET gradually increased. This suggests that the number of active sites exposed on the catalyst surface increases, which is conducive to the improvement in CO conversion. Although the S_BET of KCLZ-30P catalyst decreased slightly, XRD demonstrated that KCLZ-30P catalyst exhibited obvious t-ZrO₂ compared with other catalysts, which is an effective active site for the formation of CH_x intermediates [49]. On the other hand, in situ DRIFTS showed that the content of the CO₃²⁻ species on the KCLZ-30P catalyst was significantly increased. Due to the good stability of the CO₃²⁻ species, they are prone to undergoing excessive hydrogenation, leading to the formation of hydrocarbons, which is also a reason for the improved CO conversion. In other words, when the addition amount of P₁₂₃ is 0–20 g, the large specific surface area is the main reason for the activity improvement, whereas when P₁₂₃ is 30 g, the t-ZrO₂ and highly stable CO₃²⁻ species become the key factor for the further enhancement of activity.

Regarding the selectivity for total alcohols, it shows a decreasing trend with the introduction of P₁₂₃, while the selectivity for CO₂ and hydrocarbons (CH_x and CH₄) increase significantly. In particular, the selectivity for hydrocarbon products increases from 25% to 47.6%. There are the following three points to explain this phenomenon: Firstly, the XRD of the reduced catalyst confirms that the introduction of P₁₂₃ significantly increases the particle size of metal Cu, leading to the formation of hydrocarbons. Some researchers have proposed that a smaller Cu particle size was beneficial for the dissociation and adsorption of H₂, while a larger Cu particle size was beneficial to the C-O bond dissociation for CH_x formation [33,37]. Secondly, the CO-TPD results indicate that the introduction of P₁₂₃ increases the amount of strongly adsorbed CO on the catalyst surface, which tends to undergo over-hydrogenation, resulting in C-O bond cleavage to form CH_x species, thus facilitating the generation of hydrocarbon products. Thirdly, NH₃-TPD also revealed that the introduction of P₁₂₃ significantly increased the content of medium-strength acid sites and notably enhanced the acidity of high-strength acid sites. The strong acidity will enhance the dissociation adsorption of CO and further hydrogenation ability, thereby increasing the selectivity of hydrocarbons [17,50].

From the alcohol product distribution, the KCLZ catalyst without P₁₂₃ exhibits a relatively high selectivity for branched alcohols (i-C₄), reaching 9.3 wt%. As the P₁₂₃ content increased, the selectivity for i-C₄ gradually decreased, while the selectivity for linear alcohols progressively increased. Notably, the selectivity for ethanol increased linearly from 12 wt% to 27.2 wt%, and the C₂₊OH/MeOH ratio rose from 0.51 to 0.98, indicating that the introduction of P₁₂₃ favors carbon chain growth in linear alcohols. On the one hand, the decline in i-C₄ selectivity can be ascribed to the decline in the number of basic sites and the weakened strength of strong basic sites, which leads to the obstruction of the formation of i-C₄ alcohols (via the condensation reaction pathway) [50]. On the other hand, CO-TPD and NH₃-TPD indicate that the addition of P₁₂₃ stimulates the formation of CH_x species. Meanwhile, in situ DRIFTS reveals the enhanced adsorption of non-dissociative CO on Cu⁰ species. This means that the process of CO insertion into CH_x is facilitated, thereby promoting the formation of linear alcohols. In addition, according to the results of XRD and XPS, both Cu⁺ and Cu⁰ species coexist on the reduced catalysts and the synergistic effect of Cu⁺-Cu⁰ is crucial for CO hydrogenation to ethanol [10]. Meanwhile, He et al. found that the presence of t-ZrO₂ facilitates the formation of ethanol [49]. Excessive P₁₂₃ tends to form overly dense micellar networks, which may lead to pore channel collapse or disordering after calcination, resulting in reduced specific surface area and metal agglomeration. Additionally, the incomplete decomposition of excessive P₁₂₃ can leave residual carbon deposits covering the pore surfaces. In conclusion, the introduction of P₁₂₃ into the CuZr-based catalyst not only significantly influences the structure and surface properties but also alters the alcohol product distribution by reducing the formation of i-C₄ alcohols while promoting the generation of linear alcohols (especially ethanol).

4. Materials and Methods

4.1. Catalyst Preparation

All chemical reagents used in the preparation process of catalysts are analytically pure. A series of KCuLaZrO₂ catalysts with different P₁₂₃ content were facilely prepared via the coprecipitation method. Firstly, Cu(NO₃)₂·3H₂O, La(NO₃)₃·6H₂O and ZrOCl₂·8H₂O with a molar ratio of n(Cu):n(La):n(Zr) = 1:0.1:4.5 were dissolved in deionized water, and the resulting acidic solution was labeled as solution A. Meanwhile, an aqueous solution of KOH (1 mol/L) was prepared, and the obtained alkaline solution was labeled as solution B. Next, different weights of P₁₂₃ were dissolved in 1000 mL distilled water, followed by adding the above mixed solution A and B dropwise under vigorous magnetic stirring (60 °C) at a pH of around 11. At the end of the coprecipitation reaction, the suspension was kept to age at room temperature for 3 h. Then, the precipitate was washed and filtered off several times with deionized water until there were no Cl⁻ ions in the filtrate. Afterwards, the obtained blue precipitate was dried overnight at 120 °C and calcined at 400 °C for 4 h. Finally, the synthesized KCuLaZrO₂-xP₁₂₃ catalysts were abbreviated as KCLZ, KCLZ-10P, KCLZ-20P, and KCLZ-30P catalysts, representing the weight of P₁₂₃ as 0, 10, 20, and 30 g, respectively. All the calcined catalysts were granulated, crushed, and sieved to 30–40 mesh for catalytic activity tests.

4.2. Catalyst Characterization

The N₂ physisorption experiments were carried out using a Micromeritics Tristar 3000 physisorber (Shanghai, China). The specific surface area (S_BET), pore volume (V_p), and pore size distributions were determined by the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) method using the desorption branch of the isotherms, respectively.

X-ray diffraction (XRD) patterns of tested catalysts were obtained on a Rigaku SmartLab diffractometer using Ni-filtered Cu-Kα (40 kV and 100 mA) radiation (Tokyo, Japan). The catalysts were scanned in the range of 2θ = 5–80° with a scanning speed of 4°/min.

Temperature-programmed desorption (H₂-TPR, CO, CO₂, NH₃-TPD) patterns were recorded on a BELCAT-B instrument (BEL JAPAN, Inc., Osaka, Japan). H₂-TPR: Prior to each test, the catalyst was purged in Ar at 350 °C for 30 min to remove impurities adsorbed on the surface. After cooling to 50 °C in Ar, the catalyst was heated from 50 to 800 °C, with a heating rate of 10 °C/min under 10% H₂/Ar flow. CO, CO₂, NH₃-TPD: The catalyst was treated in the flow of 10% H₂/Ar at 350 °C for 30 min to ensure complete surface reduction, followed by flushing with Ar to remove the residual H₂ on the surface. After cooling to 50 °C in Ar, the catalyst was exposed to adsorbed gas (CO, CO₂, and NH₃) until full saturation. Finally, increasing the temperature to 800 °C at a heating rate of 10 °C/min under Ar flow. The thermal conductivity detector (TCD) signal was recorded automatically to track adsorbed gas consumption.

X-ray photoelectron spectroscopy (XPS) measurement was conducted on a K-Alpha apparatus (Thermo Fisher Scientific, Waltham, MA, USA) with an Al Kα (hv = 1486.6 eV, 12 kV and 6 mA) radiation. The binding energy (BE) of C 1s (284.6 eV) was used to calibrate all XPS spectra.

In situ CO diffuse reflectance infrared Fourier transform (in situ CO-DRIFTS) measurements were executed on a Bruker Tensor 27 infrared spectrometer (Karlsruhe, Germany). Prior to measurement, the reduction process was performed on each catalyst by passing 10% H₂/N₂ at a flow rate of 15 mL/min. Later on, it was switched to Ar to purge the in situ cell, and we recorded the background signal at 350 °C. After that, the corresponding spectrum was collected every 3 min in CO atmosphere. The temperature was reduced from 350 to 50 °C, and the spectra were collected again under Ar stream for 30 min.

4.3. Catalytic Performance Evaluation

The catalytic performance of the KCLZ-xP catalysts (5 mL, 30–40 mesh) was evaluated in a continuous flow fixed-bed reactor (Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan City, Shanxi Province, China). The mass flow controller was set to control the syngas flow rate, while the wet flowmeter was utilized to regulate the tail gas flow rate. In each operation, the catalyst was loaded into a stainless-steel tubular reactor and activated in 10% H₂/N₂ at 350 °C and atmospheric pressure for 6 h. After reduction, the temperature was kept at 350 °C, and the gas was switched to syngas (H₂/CO = 2.5), with WHSV of 3000 h⁻¹ at the 5 MPa. After stabilizing for 10 h, the products were collected after passing through a cold trap. H₂, CO, CH₄, and CO₂ in the feed gas and tail gas were quantified by carbon molecular sieve column with a thermal conductivity detector (TCD, GC4000A) (Beijing Changliu Scientific Instruments Co., Ltd., Beijing, China), while the hydrocarbons, DME, and CH3OH were detected by GDX-403 column with a flame ionization detector (FID, GC4000A) (Beijing Changliu Scientific Instruments Co., Ltd., Beijing, China). A GDX-401 column (GC4000A) with a TCD was utilized for analyzing the H₂O, methanol, and alcohol in effluent products. A Chromosorb 101 column (GC-2010) with an FID was utilized for detecting alcohol products.

CO conversion (X_CO), product selectivity on carbon basis (S_i) and the mass selectivity of alcohol i in alcohol products (W_i) can be calculated based on the following formula [11,19].

$${\mathrm{X}}_{\mathrm{CO}}={\displaystyle \frac{{\mathrm{n}}_{\mathrm{CO},\mathrm{in}}-{\mathrm{n}}_{\mathrm{CO},\mathrm{out}}}{{\mathrm{n}}_{\mathrm{CO},\mathrm{in}}}}\times 100\%$$

(1)

$${\mathrm{S}}_{\mathrm{i}}={\mathrm{v}}_{\mathrm{i}}\times {\displaystyle \frac{{\mathrm{n}}_{\mathrm{i},\mathrm{out}}-{ \mathrm{n}}_{\mathrm{i},\mathrm{in}}}{{\mathrm{n}}_{\mathrm{CO},\mathrm{in} }-{\mathrm{n}}_{\mathrm{CO},\mathrm{out}}}}\times 100\%$$

(2)

$${\mathrm{W}}_{\mathrm{i}}={\displaystyle \frac{{\mathrm{R}}_{\mathrm{i},\mathrm{s}}{\mathrm{f}}_{\mathrm{i},\mathrm{m}}}{\sum _{\mathrm{i}}{\mathrm{R}}_{\mathrm{i},\mathrm{s}}{\mathrm{f}}_{\mathrm{i},\mathrm{m}}}}\times 100\%$$

(3)

The n_CO,in and n_CO,out represent the moles of CO at the inlet and outlet, respectively. S_i and v_i stands for the selectivity of C mol and carbon number of the products, such as hydrocarbons, DME and CO₂. R_i,s and f_i,m represent the area ratio of alcohol i on the chromatogram and quality correction factor, respectively.

The Anderson–Schulz–Flory (ASF) model is used to describe the relationship between chain growth probability and the number of carbons in alcohol product distribution. The chain-growth probability (α) of product was calculated according to the following equation:

$$\ln ({\displaystyle \frac{{\mathrm{W}}_{\mathrm{n}\mathrm{i}}}{\mathrm{n}}})=\mathrm{n} \ln \mathsf{\alpha }+2 \ln ({\displaystyle \frac{1-\mathsf{\alpha }}{\sqrt{\mathsf{\alpha }}}})$$

where n and W_ni represented the carbon number of product i and the mass fraction of product i, respectively.

5. Conclusions

In this work, the structure–activity relationship of CO hydrogenation on CuZr catalyst was constructed by studying the effect of introducing P₁₂₃ on the surface properties and chemical environment of KCLZ-xP catalyst. When the content of P₁₂₃ increases to 20 g, the specific surface area of the catalyst increases. Further increasing the content of P₁₂₃ to 30 g promotes the formation of t-ZrO₂. All of these are beneficial for improving the CO conversion. The decrease in total alcohol selectivity and the increase in CO₂ and hydrocarbon selectivity are due to the introduction of P₁₂₃, which leads to the appearance of large particles metal Cu, an increase in strong CO adsorption sites, and an enhancement of the acidity of strong acid sites, all of which are conducive to the formation of more CH_x species. In the distribution of alcohols, due to the decrease in basic sites and the weakening of the alkalinity of strong basic sites, the branched alcohol (i-C₄) selectivity decreases. This is also the most unique point compared to the previous K-CuZnZrO₂, KFeCuZrO₂, and Cu-ZrO₂ catalysts [12,17]. The enhanced non-dissociated adsorption of CO^* species on Cu⁰, combined with abundant CH_x species, promoting the insertion of non-dissociated CO^* into CH_x, resulting in increased selectivity for linear alcohols (especially ethanol). In summary, the strategy of regulating KCuLaZrO₂ catalysts through surfactant P₁₂₃ provides a new approach for designing high-performance HAS catalysts.