A Brief Review of Cu-Based Catalysts for the Selective Liquid-Phase Hydrogenation of Furfural to Furfuryl Alcohol

Abstract

With the rapid industrialization, excessive reliance on fossil fuels has resulted in energy crises and environmental pollution, driving the search for sustainable alternatives. Biomass-derived resources have emerged as promising candidates to replace fossil-based feedstocks. Among these, furfural (FF) serves as a key platform molecule that can be catalytically hydrogenated to various high-value chemicals, with furfuryl alcohol (FA) representing one of the most valuable products. Currently, Cr-based catalysts remain predominant for the selective hydrogenation of FF to FA. However, the severe environmental toxicity of Cr necessitates urgent development of alternative Cr-free catalytic systems. This study systematically reviews recent advances in FF hydrogenation to FA, providing an in-depth discussion of reaction mechanisms, including adsorption configurations, solvent effects, and side reactions, as well as a comprehensive analysis of structure–activity relationships, involving active metal, support, promoter, and preparation methods. Furthermore, we evaluate the application of the advanced characterization techniques for monitoring the reaction processes. Finally, we propose the future research directions: (1) designing efficient and stable non-noble metal catalysts and (2) elucidating reaction mechanisms via the combined in situ characterization and theoretical calculations. These efforts would facilitate the academic understanding and industrial implementation of the FF-to-FA conversion process.

1. Introduction

Since the Industrial Revolution, the utilization of fossil fuels has significantly contributed to societal development and improvements in material living standards. However, the excessive exploitation has led to severe energy and resource supply crises, as well as environmental pollution, which have caused global attention and concern [1,2,3]. To address these challenges, researchers must identify an energy alternative with cost-effective, clean, readily available, and renewable advantages. Fortunately, biomass satisfies these criteria and holds potential as a substitute for fossil fuels [4,5,6]. It encompasses a diverse range of organic materials produced through photosynthesis, with numerous types naturally occurring in the environment. Nevertheless, only a limited number of biomass resources can simultaneously fulfill all the aforementioned requirements for energy substitution. In April 2004, the U.S. Department of Energy identified 12 representative biomass-derived compounds known as platform molecules [7]. Among them, furfural (FF) is regarded as one of the most significant biomass-based chemicals and is considered a highly promising platform molecule for the sustainable production of fuels and chemicals in the 21st century [8,9]. FF contains several functional groups, such as furan cyclic, aldehyde, and diene ethers, which render it highly active. Leveraging these chemical properties, a range of high-value-added chemicals can be synthesized using FF as a starting material [10,11,12,13,14,15,16,17], as illustrated in Scheme 1.

Although there are numerous routes and products for FF conversion, the selective hydrogenation to produce furfuryl alcohol (FA) warrants priority discussion. As one of the most significant derivatives of FF, FA accounts for approximately 65% of FF’s annual consumption. As early as 1934, Miller explored the potential transformation of FF to FA and measured the activation energy of the reaction. Subsequently, the catalytic conversion of FF to FA was successfully demonstrated in the literature in 1947. Subsequent studies employed a reduced CuCr catalyst, achieving a yield greater than 90% under high-temperature and high-pressure conditions. Furthermore, after the addition of CaO, the yield of FA approached 98% at lower pressure [18]. However, the high toxicity of chromium renders it unsuitable for use in industrial catalytic applications. In light of the growing demand for FA, there is an urgent need to develop environmentally benign alternatives [19,20].

In addition to the composition and structure, the design of the novel catalysts should also take into account the operational conditions of hydrogenation reactions. FF hydrogenation can generally be categorized into gas-phase and liquid-phase processes [21,22,23]. The gas-phase hydrogenation is widely preferred in industrial settings due to the more straightforward separation of products and catalysts, particularly in fixed-bed reactor configurations. Compared to the gas-phase hydrogenation, the liquid-phase hydrogenation offers higher reaction efficiency, improved mass transfer rates, as well as higher selectivity and product yield. As a result, the current research efforts are predominantly focused on the liquid-phase hydrogenation process [24,25,26].

Therefore, to bridge the existing gap between the development of environmentally friendly and highly efficient catalysts and their industrial catalytic liquid-phase hydrogenation application, it is necessary to review the catalytic reaction mechanisms of FF hydrogenation to FA, as well as the composition of the catalyst, and other related issues. This will provide theoretical guidance for the design and preparation of novel and highly efficient catalysts.

2. Reaction Mechanisms

Theoretical calculations and surface science studies have demonstrated that the carbonyl group of FF can be adsorbed on the surface of the active metal in either a perpendicular or parallel manner [27]. The adsorption mode of FF on the surface of metals plays a critical role in determining the pathway of the hydroreduction reaction, and this mode is highly dependent on the properties of the metal. Combining detailed experimental and theoretical studies, two different reaction mechanisms based on Cu-based catalysts [28] and other transition metals-based catalysts (e.g., Fe, Co, Ni, Pd, Pt) [29,30,31] have been proposed.

2.1. η¹-(O)-aldehyde Configuration

As shown in Scheme 2 [27], for Cu-based catalysts, FF is adsorbed on the metal surface in a perpendicular configuration, bound in an η¹-(O) configuration by lone electron pairs of carbonyl O atoms [28]. Due to the partial overlap between the antibonding orbital of the furan ring and the 3d orbital of the Cu atom, there is a strong repulsion between the furan ring and the Cu(111) crystal plane [28,32]. This repulsion prevents the stabilization of a parallel adsorption configuration of FF on the Cu surface. Consequently, the H atoms generated from H₂ dissociation preferentially react with the adsorbed carbonyl group, thus limiting the hydroconversion of the furan ring and resulting in high selectivity for FA. A similar adsorption configuration is observed for FF on the Ag(110) crystal plane. The subsequent hydrogenation step follows two reaction mechanisms: (1) one H atom attacks the carbon atom of the carbonyl group to form an alcohol oxide intermediate, which is then attacked by a second H atom to produce FA; (2) the other H atom first attacks the carbonyl O atom to form a hydroxylalkyl intermediate, which subsequently combines with a second H atom to form FA. When the H atom initially attacks the carbonyl O, the electric charge in the C atom of the intermediate can be delocalized by the furan ring. This makes the intermediate more stable, reducing the activation energy of the reaction process, and more conducive to the formation of FA, so route (2) is more conducive to the occurrence of the reaction [28]. In addition, two configurations of the η¹-(O) binding mode exist: vertical and tilted. Among these, the tilted configuration is more stable than the vertical one, favoring the interaction between the aldehyde group and adsorbed H atoms, thereby restricting the hydrogenation of the furan ring [33]. As shown in Scheme 3a. In recent years, studies have also indicated that for moderately or weakly binding metals such as Cu and Ag, stepped (211) surfaces may exhibit higher activity than terrace (111) surfaces [34]. It can be noticed that the adsorbed furfural on the surface of Cu sites exhibits a unique mode, which is different from that on other metals (Scheme 3b), which will be discussed in detail in Section 2.2.

2.2. η²-(C,O)-aldehyde Configuration

Due to the strong interaction between the transition metals other than Cu (e.g., Fe, Co, Ni, Pd, Pt) and the π bonds in FF molecules, there is no repulsion between the furan ring and the metal surface. As a result, FF can be adsorbed on the metal surface in a parallel manner [30,35]. On these metal surfaces, FF adopts an η²-(C,O)-aldehyde adsorption mode, where both the O and C atoms of the carbonyl group, as well as the C atom of the furan ring, are all co-bound to the metal surface [31], as shown in Scheme 4 [27]. The hydrogenation of FF to produce FA occurs in two steps: (1) carbonyl oxyhydrogenation to form hydroxyalkyl intermediates; (2) hydrogenation of the C linked to the hydroxyl group to form FA (Route 3). These two steps might also be carried out simultaneously, utilizing the active H species adsorbed on the surface [36]. At low temperatures, the second step of the hydrogenation reaction mechanism occurs preferentially. However, at elevated temperatures, the adsorption configuration of FF will change from η²-(C,O)-aldehyde adsorption to η¹-(C)-acyl adsorption (Route 2). In this configuration, FF tends to undergo a decarbonylation reaction, leading to the formation of a byproduct, furan, resulting in a reduction in the selectivity of the product FA. In addition, when the adsorption active site is located on the oxygen atom (Route 1), the reaction follows a mechanism analogous to that of η¹-(O)-furfural, leading to the formation of adsorbed FA intermediates. However, owing to the strong adsorption affinity of metals for oxygen, the FA intermediate undergoes subsequent hydrogenation and deoxygenation reactions, ultimately resulting in its conversion to 2-methylfuran [37]. Both the furan ring and the aldehyde group interact with the metal, enabling hydrogenation reactions. The furan ring can also completely hydrogenate to tetrahydrofuran, and even the ring-opening reaction will occur, further affecting the product distribution and selectivity of FF hydrogenation.

An accurate understanding of the reaction mechanisms contributes to the rational design of the novel catalysts. Although numerous studies have been devoted to exploring the active sites of FF hydrogenation catalysts, significant controversy persists at present.

2.3. Solvent Effects

The aforementioned adsorption model focuses on the direct adsorption of H₂ on the catalyst surface. Although studies demonstrate that FF liquid-phase hydrogenation can proceed in solvent-free systems [38], the influence of solvents on catalytic performance remains significant. Wang et al. [39] revealed that solvents markedly affect FA selectivity. Specifically, FF is first hydrogenated to form FA; however, the incomplete or delayed desorption of FA may lead to its further hydrogenation to tetrahydrofurfuryl alcohol (THFA). When the alcoholic solvents are employed, the competitive adsorption between FA and the solvent molecules suppresses the overhydrogenation, thereby enhancing FA selectivity

In addition to the adsorption and activation of the substrate, the solvent also affects the activation of H₂. Current studies have employed solvents such as isopropanol, methanol, and ethanol for FF hydrogenation to FA using Cu-based catalysts, as summarized in

Table 1. Reaction conditions and results of FF liquid-phase hydrogenation to FA over the Cu-based catalysts.

Catalyst	Metal Loading (wt%)	Solvent	Temperature (°C)	Time (h)	H2 Pressure (bar)	FF Conversion (%)	FA Selectivity (%)	FA Yield (%)	Ref.
Cu/Zn/Al2O3 (CZAl)	10	H2O	100	4	10	100	~99	99	[40]
Cu/Zn/Al2O3 (CZAl)	10	Ethanol	100	4	10	99	~99	98.01
Cu/Zn/Al2O3 (CZAl)	10	Methanol	100	4	10	99	~95	94.05
Cu/Zn/Al2O3 (CZAl)	10	2-Propanol	100	4	10	19	~90	17.1
Cu/Zn/Al2O3 (CZAl)	10	DMF	100	4	10	98	~99	97.02
Cu/Zn/Al2O3 (CZAl)	10	Acetone	100	4	10	98	~80	78.4
Cu/Zn/Al2O3 (CZAl)	10	Toluene	100	4	10	77	~99	76.23
Cu/Zn/Al2O3 (CZAl)	10	Cyclohexane	100	4	10	4	~99	4.0
Cu/MgO	5	Isopropyl alcohol	109.85	2.3	20	99.9	99.9	99.98	[41]
Cu/Al2O3 (Acetate)	1	MeOH	50	7	1.5	24.2 ± 1.2	96.0 ± 4.8	23.23	[42]
Cu/Al2O3 (Acetate)	5	MeOH	50	7	1.5	47.7 ± 2.4	97.6 ± 4.9	46.56
Cu/Al2O3 (Sulfate)	1	MeOH	50	7	1.5	2.2 ± 0.1	5.1 ± 0.3	0.11
Cu/Al2O3 (Sulfate)	5	MeOH	50	7	1.5	7.8 ± 0.4	0.8 ± 0.1	0.062
Cu0.9Mg3AlOy	-	Isopropanol	120	1.5	16	99	98.4	97.42	[43]
CuMg3Al-R	20	i-PrOH	130	3	20	100	99.3	99.3	[44]
Cu/CeO2-R (nanorod)	5	γ-butyrolactone	100	4	20	97.5	96.2	93.8	[45]
Cu/η-Al2O3	5	Isopropanol	150	2	10	99.7	94	93.7	[46]
NPCu@SiO2	-	H2O	150	4	30	62	31	19.2	[47]
SiO2@Cu	7.6	H2O	150	4	30	84	17	14.3
PtCu@S-1 (S-1, silicalite-1)	0.8	THF	160	6	20	99.9	100	99.9	[48]
Cu@MFI	-	H2O	70	5	40	100	100	100	[49]
Co-Cu/SBA-15	10	Isopropanol	170	4	20	99	80	79.2	[50]
Cu/MCM-41	5	i-PrOH	120	4	1	100	100	100	[51]
Cu/AC-SO3H	16	2-propanol	105	2	4	100	100	100	[52]
[Cu2(L1)2·5DMF·4H2O]n	-	Methanol	140	24	50	76	100	76	[53]
Fe3O4/Cu@C	27.4	n-butanol	180	4	10	98.5	89.4	88.1	[54]
Fe/Cu@C	30.4	i-propanol	300	4	10	~60%	~90	54	[54]
Cu3Co1/MgOx	20	Isopropanol	110	2	20	100	99	99	[55]
Cu2Zn/SiO2	13	Deionized water	120	4	25	81.9	94.8	77.6	[56]
Na–Cu@TS-1	2.1	Isopropanol	110	2	10	93	98.1	91.2	[57]
Cu/SiO2-AE	5	Isopropanol	90	2	10	55.2	99.9	55.1	[58]
NiCoCuZnFe/C-800	-	Isopropanol	90	9	30	87.32	100	87.32	[59]
CuO#TiO2	12	Ethanol	140	1	20	99	99	99	[60]
ZJU-199-350	-	Isopropanol	130	3	10	97	99	96	[61]

. Regarding the role of solvents, Bordoloi et al. [40] systematically investigated FF dissolution using a Cu/Zn/Al₂O₃ (CZAl) catalyst (Table 1). The study revealed that the high hydrogen bond donor (HBD) capacity of H₂O enhances the reaction performance. Notably, H₂ solubility in solvents did not play a decisive role; instead, hydrogen bond acceptance (HBA) and HBD capacities were identified as key factors. FF conversion exhibited a positive correlation with HBA capacity, generally following the solvent trend: polar protic > polar aprotic > non-polar. Consequently, 100% FF conversion was achieved with H₂O as the solvent. While FA selectivity might be correlated with H₂ solubility and HBD capacity, further research was required to elucidate these relationships.

2.4. Side Reactions

The hydrogenation of FF to FA is highly susceptible to excessive hydrogenation, leading to various side reactions. These include hydrodeoxygenation to 2-methylfuran (MF), ring-opening to form 1,5-pentanediol, and ring-hydrogenation to tetrahydrofurfuryl alcohol (THFA), as shown in Scheme 5 [62]. As illustrated, the primary by-products of FF hydrogenation are THFA and furan derivatives, while other by-products originate from the subsequent reactions of FA, THFA, or furans. To suppress the side reactions, it is critical to (1) selectively hydrogenate the carbonyl group to produce FA and (2) prevent further hydrogenation of FA. Compared to other transition metal catalysts, Cu-based catalysts exhibit superior selectivity by preferentially adopting the η¹-(O)-aldehyde adsorption configuration, which avoids the interaction with the furan ring and effectively inhibits THFA formation. The formation of furan derivatives requires the decarboxylation of FF or FA, a process typically facilitated by precious metals (e.g., Pt, Pd) or Ni due to their strong CO adsorption capability [63]. Consequently, employing Cu-based catalysts and avoiding the incorporation of such active metal components can effectively suppress the generation of these by-products. However, the potential for further conversion of FA (e.g., to THFA or via ring-opening reactions) remains a challenge. In addition, there is a possibility of a humin reaction, which leads to catalyst inactivation [64].

Under the liquid-phase hydrogenation conditions, the formation of 2-methylfuran (2-MF) necessitates the hydrodeoxygenation of FA, a process requiring strong surface acidity [65]. Furthermore, the introduction of additional active metal components (e.g., Pt, Pd, or Ni) may promote this pathway, leading to increased 2-MF selectivity [66].

In conclusion, Cu-based catalysts effectively suppress the formation of major by-products during the liquid-phase hydrogenation of FF to FA. Furthermore, the prevention of FA overconversion can be achieved by selecting appropriate supports or additives that avoid introducing strong acidic sites, thereby minimizing undesired side reactions.

3. Catalyst

3.1. Active Metal

3.1.1. Noble Metal

In order to avoid the environmental damage caused by the use of Cr in the preparation and generation of Cr-containing wastes in the catalyst preparation and reaction, an increasing number of researchers are developing Cr-free catalysts with superior performance, including noble metals such as Pd, Pt, and Ru.

Noble metal Pd has very high activity, but its selectivity is usually low when directly used to catalyze the production of FA from FF. The selectivity of the hydrogenation product FA can be improved by reducing the Pd loading, reaction temperature, and H₂ pressure [67]. Wang et al. [68] prepared Pd/TiH₂ catalysts with different mass fractions, achieving a maximum conversion rate of 97.8% with 1 h of reaction time, which indicates that Pd has high activity in the liquid-phase hydrogenation of FF to prepare FA. However, its conversion is only 45.9%.

Noble metal Pt is usually used to catalyze α, hydrogenation of β-unsaturated aldehydes, obtaining a variety of chemicals and fuels with high energy density when applied to catalyze the FF hydrogenation reaction [69]. Papadogianakis et al. [70] employed the water-soluble platinum catalyst PtCl₂/TPPTS (where TPPTS denotes tris(3-sulfophenyl)phosphine trisodium salt, a sulfonated analog of triphenylphosphine widely used in industrial aqueous-phase catalysis) for FA hydrogenation. Under mild and neutral conditions (130 °C, 4 MPa H₂) in a green aqueous solvent, the system achieved a remarkable turnover frequency (TOF) exceeding 20,000 h⁻¹ and a furfuryl alcohol selectivity of up to 99%. Yang et al. [71] synthesized Pt/CeO₂-270 catalyst that achieved nearly 100% conversion within 1 h of reaction time, with 97.3% selectivity. These results demonstrate the high reactivity of Pt in this catalytic system. Although the selectivity is markedly superior to that of conventional precious metal catalysts, this enhancement is contingent on the synergistic role of Ce species.

Ru-based catalysts exhibit broad applicability, not only in the reduction of aromatic hydrocarbons [72], nitrile [73], CO methanation [74], α, β-unsaturated aldehydes [75] and hydrolysis of polyols to alkanes [76], but also in the reduction of FF under mild conditions. Yang et al. [77] prepared Al-MIL-53 and employed it as a support for Ru. The results indicate that the Ru/Al-MIL-53 catalyst with aromatic linker demonstrated superior performance, achieving a 100% yield of furfuryl alcohol at 20 °C and 0.5 MPa.

All these noble metal-based catalysts showed high activity and selectivity in the preparation of FA through liquid-phase hydrogenation of FF. Moreover, the use of noble metal-based catalysts effectively eliminates the need for Cr. However, the high cost of the noble metal catalysts does not make them suitable for large-scale industrial applications, and they are difficult to recycle. Therefore, the cheap transition metals have more potential for experimental research and industrial applications.

3.1.2. Fe, Co, and Ni

Catalysts with the active components of Fe, Co, and Ni have been increasingly developed in recent years for the hydrogenation of FF to produce FA [18,78,79,80]. Compared with noble metals, these transition metals can also exhibit high activity and stability in FF hydrogenation reactions but require more detailed design.

Fe is mainly used as an oxygenophilic metal promoter to enhance the hydrogen uptake activity of metals such as Pd, Cu, Ni, and Co [81]. Li et al. [82] synthesized a Ni₃Fe₁ bimetallic catalyst via a solvothermal method, which achieved over 98% furfural conversion and furfuryl alcohol selectivity under reaction conditions of 130 °C and 1 MPa H₂ for 1 h.

For the supported Co catalysts, Audema et al. [83] reported a monometallic Co/SBA-15 catalyst in which Co nanoparticles were encapsulated within the mesoporous structure of SBA-15, and the optimal Co nanoparticle size suppressed the generation of side reactions.

The metal Ni is capable of catalyzing the conversion of FF to a variety of products, including FA, cyclopentanone, tetrahydroFA, furan, tetrahydrofuran, and many other hydrogenation products. Meng et al. [84] reported two heterogeneous Ni catalysts loaded on mixed metal oxides prepared by the structural topological transformation of NiAl hydrotalcite precursor interlayer containing nitrate or carbonate to tune the hydrogenation product species, denoted as Ni/MMO-CO₃ and Ni/MMO-NO₃, respectively.

Fe, Co, and Ni are widely distributed in nature, which are cheap and easy to obtain, and they have an excellent ability to activate H₂ [85,86,87]. However, due to their distinct catalytic mechanisms compared to Cu-based systems (as discussed in Section 2.2), these metals might result in low selectivity toward FA [88]. Therefore, using these metals as the active sites still requires more careful design and precise control.

3.1.3. Cu

In recent years, studies have shown that Cu⁰ can facilitate the transfer of H species during hydrogenation reactions, such as CO₂ and methanol hydrogenation, while Cu⁺ is generally recognized as the primary active site for hydrogenation processes [89,90]. This dual role has also been observed in FF hydrogenation to FA [91]. Therefore, further investigation into the mechanism and structure–activity relationships of Cu-based catalysts in FF hydrogenation toward FA is essential. Among all catalysts with non-noble metals as active components, Cu-based catalysts have garnered extensive attention because of their unique activity toward C=O bonds and exceptional selectivity for FAs [92,93,94].

Zhang et al. [41] prepared a series of Cu/MgO catalysts reduced at 300, 350, 400, 450, and 500 °C, respectively (Table 1). Characterization results revealed that a reduction temperature of 350 °C facilitated the optimal dispersion and reduction in Cu on the basic interface of MgO. With a higher active surface area, the weak adsorption of FA on the catalyst surface area avoids its excessive hydrogenation, while simultaneously exposing more active sites. The synergistic interaction between the basic interface of MgO and active Cu species contributed to the excellent performance of Cu/MgO-350.

Mohammed et al. [42] prepared monometallic Cu/Al₂O₃ catalysts with loadings of 1.0 wt.% and 5.0 wt.%, respectively, using copper nitrate, copper acetate, and copper sulfate as precursors, respectively (Table 1). At a 5.0 wt.% loading, Cu formed well-dispersed nanoparticles on the surface of Al₂O₃. Whereas at 1.0 wt%, Cu mainly existed in the form of single atoms and dimers. Characterization results indicated that the Cu/Al₂O₃ derived from the three precursors were sensitive to the structure of FF hydrogenation. At low Cu loadings, the decarbonylation reaction dominated, while at higher loadings, the hydrogenation reaction became the primary reaction. The sulfur-containing Cu/Al₂O₃ catalyst derived from copper sulfate precursor exhibited substantial side reactions, resulting in a FA selectivity of merely 0.8%. In contrast, the catalysts prepared with copper acetate as precursors provide the best performance under all conditions, demonstrating that metal loading and metal precursor selection are key to achieving optimal catalytic activity.

Li et al. [43] prepared a series of Cu_xMg₃AlO_y (x = 0.3, 0.5, 0.7, 0.9, and 1.0) composite metal oxide catalysts using CuMgAl-LDHs as precursors. These catalysts were directly employed for liquid-phase furfuryl hydrogenation without H₂ pre-reduction. As shown in Figure 1a. Characterization results showed that Cu_0.9Mg₃AlO_y has the highest catalytic activity, as shown in Table 1. Notably, the catalyst maintained its activity without significant degradation after five reuse cycles. Further analysis revealed that Cu²⁺ on the surface of Cu_0.9Mg₃AlO_y was reduced in situ to Cu⁰ and Cu⁺ species during liquid-phase hydrogenation. The study of the structure–activity relationship demonstrated that Cu⁰ mainly facilitated activation and dissociation of H₂, while the coexistence of Cu⁰ and Cu⁺ enabled co-adsorption of carbonyl groups, thereby enhancing the selectivity of FA (Figure 1b,c). Using an LDH precursor, Liu et al. [44] successfully anchored Cu species onto the Mg-Al LDH surface and subsequently prepared the CuMg₃Al-R catalyst via high-temperature reduction (shown in Figure 2a). Characterization results revealed that the catalyst surface contained well-dispersed Cu⁰/Cu⁺ nanodots, which exhibited exceptional catalytic performance for FF hydrogenation to FA, achieving both conversion and selectivity exceeding 99%, as shown in Table 1. These findings demonstrate the following: (1) Cu species serve as highly active catalytic centers for this transformation, exhibiting remarkable intrinsic activity; (2) the catalyst’s structure and composition offer significant tunability through synthetic modifications. The study highlights the potential for optimizing catalytic performance through the rational design of Cu-based nanomaterials (Figure 1b,c).

In summary, Cu should maintain the highest possible dispersion, while having a high Cu⁺ content and an appropriate Cu⁰ content; its interaction mechanism requires further investigation. Cu is abundant in nature, inexpensive, easily available, highly selective, mechanistically well-defined, and can achieve high catalytic activity under mild reaction conditions, and has great potential for industrial application, and therefore is the best choice of the main active component for FF catalysts for furfuryl aldehyde selective direct hydrogenation to prepare FA.

3.2. Support

During the preparation and reduction of the catalysts, the active metal typically interacts with the support, which would further affect the activity and product selectivity of the catalyst [95,96]. Therefore, the selection of catalyst supports should be considered from multiple perspectives, such as specific surface area, pore structure, mechanical strength, and surface properties. In recent years, researchers have focused on optimizing the topography and surface characteristics of the support to enhance the performance of catalysts for the high-performance FF hydrogenation to produce FA. However, different supports exhibit unique characteristics; therefore, their selection and modification should be based on their structural and physicochemical properties in catalyst design.

3.2.1. Metal Oxide

In the design of catalysts for the hydrogenation of FF to FA, metal oxide supports play a crucial role and are extensively employed [97,98,99]. They not only support active metals but also actively participate in and optimize the entire catalytic process. Appropriate metal oxide supports can significantly enhance the activity, selectivity, and stability of the catalysts.

Liu et al. [100] using MgO-La₂O₃ mixed support to load Cu, which could promote the selective hydrogenation of FF to FA. The catalytic performance and characterization confirm that MgO effectively disperses Cu nanoparticles; meanwhile, La₂O₃ enhances the adsorption of the catalyst to FF. In addition, compared with single MgO, MgO-La₂O₃ support can effectively promote the reduction of Cu. Under optimal reaction conditions, the conversion rate of FF and the selectivity of FA exceeded 99.9%. The activity of the catalyst remains at about 97.6% after five cycles. This result demonstrates that suitable mixed metal oxide supports generate a synergistic effect, where the combined performance exceeds that of individual components.

Structural and morphological changes in metal oxides may lead to altered surface crystal exposure and increased oxygen vacancy formation, both of which are conducive to enhanced catalytic performance. Tan et al. [45] prepared three Cu-based catalysts supported on CeO₂ supports with distinct morphologies—nanorods, nanocubes, and nanopolyhedra—for the catalytic hydrogenation of FF. The results revealed that compared to the other two catalysts, Cu/CeO₂ catalysts with a nanorod (CeO₂-R) morphology exhibited superior performance (Table 1). Characterization results indicated that the main exposed surface of the rod-shaped CeO₂ support was the (111) crystal surface with more oxygen vacancies and higher Cu⁰ content of the reduced Cu/CeO₂-R catalyst. The cooperative interaction between Cu⁰ and oxygen vacancies led to the excellent performance of Cu/CeO₂-R.

The crystal phase of metal oxide supports also plays a critical role in determining catalytic performance. Liu et al. [46] employed Al₂O₃ with different crystalline phases to support Cu and catalyze FF hydrogenation for the production of FA. The results revealed that η-Al₂O₃ was more effective in dispersing Cu nanoparticles and promoting the highest surface concentrations of Cu⁺ and Cu⁰ species. This enhanced performance is attributed to the stronger FF adsorption and FA desorption capabilities of η-Al₂O₃ compared to other crystalline forms, which helps prevent hydrogenation of the furan ring and thereby achieves a high FF conversion and FA selectivity (Table 1).

In summary, metal oxides serve as promising catalyst supports and can be effectively utilized in the hydrogenation of FF to yield FA. However, metal oxide supports exhibit certain limitations, such as an undeveloped porous structure. Consequently, the choice of support material should consider appropriate modification strategies to mitigate these drawbacks.

3.2.2. SiO₂

Compared to metal oxides, non-metallic SiO₂ is also widely employed as a catalyst support [101,102,103]. It offers broader availability at lower cost, exhibits higher chemical inertness, and facilitates the formation of well-defined porous structures. Zhang et al. [104] modified granular SiO₂ with ethanolamine to adjust the surface functional groups and hydrophilic properties while keeping the skeleton structure unchanged, followed by impregnation with Cu. Further studies have shown that this support can effectively improve the dispersion of Cu and form an appropriate Cu⁺/Cu⁰ ratio, thereby improving the activity and stability of the catalyst at the same time. However, the experimental results also show that this kind of SiO₂ support cannot completely inhibit the sintering of carbon and reactive metals in the surface area of the catalyst.

Zhu et al. [105] selected nano-SiO₂ balls as the support to prepare PtCuCo/SiO₂ catalysts, and the reaction performance of FF hydrogenation to FA could reach 97.9% conversion and 95.4% selectivity. This is because the SiO₂ sphere with a regular structure and well-developed pores can not only effectively reduce mass transfer resistance but also promote the formation of alloys while dispersing a variety of active components on the surface. This suggests that some of the shortcomings of the SiO₂ supports can be compensated for by adjusting their structure.

Marco et al. [47] synthesized Cu/SiO₂, SiO₂@Cu, and nNPCu/SiO₂ catalysts, and found that the NPCu@SiO₂ and SiO₂@Cu of the core–shell structure can promote different degrees of bonding between copper and silica, thereby adjusting the metal-support interaction between copper and silica. At the same time, the study fully demonstrates that catalysts with weak metal–support interactions do not have a significant effect on the distribution of products even in anhydrous reaction systems.

To sum up, the tunable properties of SiO₂ render it a promising candidate as a support material for catalysts in the hydrogenation of FF to FA. Moreover, SiO₂ supports featuring regular structures and well-developed porosity, offering distinct advantages and warranting further investigation into compositionally analogous materials such as zeolites.

3.2.3. Molecular Sieve

As mentioned above, SiO₂ often suffers from underdeveloped porosity, a limitation that can be effectively addressed by molecular sieves [106]. Based on their pore structure, molecular sieves are generally classified into zeolitic and mesoporous types. Zeolite molecular sieves possess a well-defined topological framework with uniform micropores, whereas mesoporous molecular sieves feature ordered mesopores formed using templating methods [107,108].

Li et al. [48] synthesized PtCu@S-1 (S-1, silicalite-1) catalysts by the zeolite encapsulation method. As shown in Figure 3a, the rigid framework of S-1 effectively prevents the sintering of active metal particles, while its well-developed pore structure minimizes mass transfer limitations. PtCu@S-1 showed the remarkable activity in the hydrogenation of FF to FA (Table 1). The high performance of PtCu@S-1 is related to the high dispersion of active metals, while the developed pore structure and large specific surface area facilitate the dispersion of metals (Figure 3b).

Fu et al. [49] synthesized a Cu@MFI catalyst via a high-temperature rapid mixing hydrothermal method. Characterization results indicate that the MFI framework can coordinate with Cu to form a [Cu₂(μ-O)_x]²⁺ structure, enabling Cu to stably exist as single atoms within the 10-membered ring of the MFI zeolite. Compared to conventional impregnation methods, this Cu@MFI catalyst demonstrates excellent catalytic performance in aqueous-phase FF hydrogenation (Table 1). However, zeolites affect the FF hydrogenation reaction to produce by-products due to their acidic sites, and zeolites also have problems, such as a small pore size and low quantity. Therefore, special strategies are often required to mitigate the adverse effects of side reactions. It is worth noting that although the above results have fully demonstrated the feasibility of zeolites as catalyst supports in this reaction, some inherent problems remain. It is noteworthy that molecular sieves, characterized by well-developed pore structures and high specific surface areas, exhibit valence state distributions of Cu species that are highly sensitive to H₂ reduction [109]. Given that Cu⁺ species are more favorable for hydrogenation reactions, excessive reduction temperatures should be avoided. Furthermore, the strong Brønsted acid sites inherent to molecular sieves may promote side reactions, such as the formation of cyclopentanone [110]. Consequently, further research and a more precise design are essential for optimizing Cu-loaded molecular sieve catalysts in the liquid-phase hydrogenation of FF to FA.

Compared with zeolites, mesoporous molecular sieves have weaker acidity and larger pore channels. Parikh et al. [50] investigated a Co-Cu/SBA-15 catalyst. Its high surface-to-volume ratio reduces the diffusion barrier of reactants and products. It was found that Co-Cu/SBA-15 had higher performance than the amorphous silica catalyst (Table 1). However, mesoporous molecular sieves may exhibit structural instability at elevated temperatures and typically possess fewer surface functional groups, which hinders the effective anchoring of active sites and may result in compromised catalyst stability.

In addition to SBA-15, another mesoporous molecular sieve material, MCM-41, has also been utilized for the liquid-phase hydrogenation of FF to FA. Fang et al. [51] employed Cu-loaded MCM-41 for FF hydrogenation, achieving nearly 100% FF conversion and FA selectivity at only 1 bar H₂ pressure (Table 1). This exceptional performance is attributed to the unique spatial structure and high specific surface area of MCM-41, which facilitates FF diffusion and adsorption while enabling the stable and uniform dispersion of Cu⁰ and Cu⁺ sites on both internal and external surfaces.

In summary, molecular sieves offer advantages such as tunable structure and surface properties, as well as chemical inertness. However, zeolite molecular sieves suffer from strong surface acidity and limited pore size, whereas mesoporous molecular sieves often exhibit poor thermal stability and challenges in anchoring active sites. Therefore, effective modification of molecular sieve supports is essential to address these inherent limitations and enhance their performance as catalysts.

3.2.4. Carbon-Containing Material

In addition to the aforementioned supports, carbon-based materials offer advantages such as structural diversity and facile functionalization, making them widely employed in the synthesis of catalysts [111,112]. A carbon-coated Cu catalyst, CuO_x@NC-150, was prepared by Qiu et al. using a thermal decomposition strategy of Cu-EDTA complexes [113]. Cu nanoparticles with complex valence states are prone to agglomerate during the hydrogenation reaction, which leads to catalyst deactivation. The NC formed after EDTA decomposition effectively coats Cu nanoparticles, preventing their sintering. Meanwhile, the good conductivity of NC helps maintain its catalytic reactivity.

The advantage of activated carbon lies in its large specific surface area and abundant surface functional groups, which facilitate the effective dispersion of Cu nanoparticles and enable efficient surface modification. Gong et al. [52] developed a sulfonic acid-based grafted activated carbon-supported Cu-based catalyst denoted as Cu/AC-SO₃H, which had better catalytic performance than the unmodified Cu/AC (Table 1). That is, grafting -SO₃H onto the surface of activated carbon enhances the dispersion of Cu nanoparticles, promotes a higher reduction degree of Cu, and strengthens the adsorption capacity of FF. These improvements collectively contributed to the enhanced hydrogenation performance of the catalyst.

At present, research on carbon-containing material supports remains relatively limited; however, it is undeniable that such materials hold significant potential for Cu-based catalyst support.

3.2.5. MOF

Metal–organic framework (MOF) is a porous crystalline material with a periodic network structure formed by the self-assembly of metal ions or metal clusters (as nodes) and organic ligands (as connecting rods) via ligand bonding. Taking MIL-100 and MIL-101 as examples, the composition and structure of the MOFs are illustrated in Scheme 6 [114]. MOF has an ultra-high specific surface area and porosity, which provides a large number of anchoring sites for activated metal nanoparticles (NPs) to promote a high degree of dispersion of the metal NPs and to prevent agglomeration efficiently. So, MOF can be used as a support. Moyo et al. [53] prepared a novel Cu-MOF, which showed a high conversion rate for FF and a high selectivity for FA, as listed in Table 1. However, the organic ligands of MOFs are prone to decomposition when exposed to high temperatures for a long time, which directly leads to the collapse of the MOF’s skeletal structure, making large-scale usage of MOFs difficult. This can also be used to synthesize novel coated catalysts, and the relevant description will be elaborated in the following text.

The advantages and disadvantages of the aforementioned supports are listed in

Table 2. Comparison of the advantages and disadvantages of different support.

Support	Advantage	Disadvantage
Metal oxide	Low cost, easy to regulate, well-studied for application	Relatively undeveloped porous structure
SiO2	Low cost, high chemical inertness	Irregular structure, poor porosity
Microporous molecular sieves	Well-defined topological framework with uniform micropores, high hydrothermal stability	Strong surface acidity, limited pore size
Mesoporous molecular sieves	Ordered mesoporous structure, relatively facile preparation	Structural hydrothermal instability
Carbon-containing material	Structural diversity, facile functionalization	Severe side reactions
MOF	Developed pore structure, high surface area, and abundant metal anchoring sites	Expensive organic ligands, easy decomposability

. In summary, suitable supports can effectively enhance the dispersibility of active components, improving both the activity and stability of the catalysts. An ideal support should possess a well-developed mesoporous structure to facilitate reactant mass transfer, a large specific surface area to promote Cu dispersion, appropriate metal–support interactions to favor the Cu⁺ ratio, as well as good FF adsorption and FA desorption capabilities, along with the ability to promote H₂ adsorption, activation, and hydrogen species overflow. However, most supports fail to satisfy all these requirements simultaneously, making it necessary to introduce promoters or modify the preparation method to compensate for their limitations.

3.3. Promoter

The incorporation of a metal promoter can modify the electronic structure and surface adsorption capacity of the active phases, thereby enhancing the performance of the catalyst. Promoters include metal, metal oxide, and other promoters. According to the literature, oxygen-friendly metals such as Fe [54], Zn [115], Sn [116], Ir [117], Mn [118], Co [119] and Mg [43] et al. can promote the adsorption of carbonyl groups on metal surfaces. In addition, Other types of promoters have also been found to similarly enhance the activity of the catalysts [120]. Some promoters have anti-carbon deposition properties. So even if the promoter does not have catalytic activity or has weak catalytic activity, its modification effect on the active component or support can effectively compensate for the defects of the catalyst, improve the activity or stability of the catalyst, and is of great research value.

3.3.1. Metal Promoter

For Cu-based catalysts, metal promoters can combine with Cu to form mixtures with phase interfaces or even directly form solid solutions. This effect will regulate the dispersion and electron state of Cu, thereby improving its adsorption and activation ability for FF, while preventing Cu sintering. Zhao et al. [55] prepared a series of layered bimetallic hydroxide-derived Cu_nCo/MgAlO_x (n = 1, 2, 3, 4) catalysts with varying Cu/Co ratios to investigate their catalytic reactivity in FF hydrogenation. The results demonstrated that the synergistic effect between Cu and Co facilitated the formation of Cu₂O, which acted as a Lewis acid site, preferentially adsorbing C=O bonds and polarizing them to promote the formation of FA [50,121,122]. Among these catalysts, Cu₃Co₁/MgO_x exhibited the strongest synergy and achieved the highest activity (Table 1). Additionally, Zhang et al. [56] explored the impact of Zn doping on the activity of Cu/SiO₂ catalysts. They found that Zn not only promoted the reduction in CuO and improved the dispersion of metallic Cu but also effectively inhibited the decarbonylation reaction, enhancing the selectivity for FA. The Cu₂Zn/SiO₂ catalyst displayed the best catalytic performance (Table 1).

In summary, loading Cu onto different supports might lead to either overly strong or insufficient metal–support interactions, resulting in low Cu⁺ ratios or poor Cu dispersion. The use of various metal or metal oxide promoters can help modulate these interactions. Additionally, excessive acidity of the support may negatively affect the catalyst’s selectivity toward FA, which can be mitigated by incorporating basic promoters. Therefore, the rational selection of promoters based on the characteristics of the support is key to further enhancing catalytic performance.

3.3.2. Metal Oxide Promoter

Similarly to metal elements, metal oxide promoters are able to affect the activity of catalysts by adjusting the chemical environment of Cu, modulating some properties of the support. Zhu et al. [123] explored the effect of Al₂O₃ introduction during the synthesis of Cu/ZnO catalysts using the co-precipitation method. As shown in Figure 4a, XRD analysis revealed that these catalysts contained both ZnO and Cu phases, though with distinct crystallinity. H₂-TPR further demonstrated significant differences in the reduction temperature of Cu species. These findings indicate that the Zn–Al interaction simultaneously modulates the metal-support interaction and Cu dispersion (as illustrated in Figure 4b,c), ultimately changing the catalytic performance.

Li et al. [54] prepared Fe₃O₄/Cu@C by N₂ atmosphere roasting using Fe(NO₃)₃-loaded Cu-MOF as precursor. These Fe₃O₄ particles embedded in the middle of the C support can serve to reduce the electron cloud density at the neighboring Cu active sites and promote hydrogen overflow, thus improving the intrinsic activity and selectivity of the catalyst (Table 1). In contrast, the Fe/Cu@C catalysts generated after H₂ roasting showed lower activity because Fe(0) is more likely to promote deep hydrogenation and carbon buildup.

In summary, metal oxide promoters can simultaneously modulate the active components and supports, and there are many related studies. Therefore, they hold significant potential for further research and large-scale application.

3.3.3. Other Promoter

In addition to metal elements and metal oxides, there are some basic substances that can also serve as promoters, primarily for regulating the surface properties of supports. Researchers explored the use of inorganic salts such as K₂CO₃ as a promoter to improve the catalytic activity. Zhang et al. [124] reported an effective strategy involving K₂CO₃ assisted FF hydrogenation using a CuO#TiO₂ catalyst, in which the FA production rate of CuO#TiO₂ catalyst was as high as 24.2 mol_FOL/(mol_Cu·h) at 100 °C and 1.4 MPa H₂ pressure, which was 4 times that of CuO#TiO₂ catalyst without K₂CO₃ accelerator. The results showed that K₂CO₃ assistance has outstanding dual functions: (1) promoting the dissolution of gaseous H₂ molecules in solvents; (2) promoting the formation of Cu active sites on the surface.

Wu et al. [57] employed a Na ion-exchange process to modulate the zeolite microenvironment, obtaining a Na–Cu@TS-1 catalyst with enhanced activity and selectivity for the selective hydrogenation of FF to FA. The introduced Na species modulate the zeolite’s acid-base properties to suppress side reactions and promote an electron-rich state in the Cu species, thereby enhancing the FF hydrogenation. Catalyst deactivation primarily results from Na leaching into the liquid phase, because activity is nearly fully restored through a Na re-addition process.

In summary, loading Cu onto different supports may lead to either overly strong or insufficient metal–support interactions, resulting in low Cu⁺ ratios or poor Cu dispersion. The use of various metal or metal oxide promoters can help modulate these interactions. Additionally, excessive acidity of the support may negatively affect the catalyst’s selectivity toward FA, an issue that can be mitigated by incorporating basic promoters. Therefore, the rational selection of promoters based on the characteristics of the support is key to further enhancing catalytic performance.

3.4. Preparation Method

3.4.1. Supported Cu Catalysts

One of the most commonly used preparation methods for catalysts involves loading active components onto a support. This method has a wide range of applicability and is easy to operate. However, this method can easily cause changes in catalyst performance through changes in supports. Liu et al. [58] compared Cu/SiO₂ catalysts prepared via ammonia evaporation (AE) and impregnation methods. The results showed that the AE-synthesized sample formed a layered copper silicate species, which facilitated the generation of a higher concentration of Cu⁺ ions. Consequently, under reaction conditions of 90 °C and 1 MPa H₂, the AE catalyst achieved a turnover frequency (TOF) of 36.0 h⁻¹ for the hydrogenation of FF to FA.

The morphology, dispersion, and chemical state of the active components, as well as the interaction between the active metal and the support, will differ for supported catalysts obtained through different preparation methods, which will further affect the performance of the catalysts. Furthermore, this preparation method offers advantages such as tunable catalyst composition and straightforward fabrication, making it highly suitable for large-scale catalyst production at present. However, since the active metal sites are directly exposed on the support surface, challenges including metal sintering, surface carbon deposition, and leaching of soluble components might arise. Notably, in liquid-phase hydrogenation, the solvent’s influence on the catalyst cannot be overlooked. Thus, future efforts should not only focus on improving catalyst stability but may also require strategies for efficient catalyst regeneration.

3.4.2. Coated Cu Catalysts

Using support to encapsulate the active components will effectively avoid sintering of Cu, thereby improving the stability of the catalyst [125,126]. The core–shell structure catalyst is a special nanocomposite material consisting of an active component as the core and an outer shell as a support. In addition to regulating the electronic and geometrical structure of the catalyst, this structure can also effectively avoid the sintering of the active components and improve the life of the catalyst. Tu, R et al. [59] synthesized the novel ultra-dilute high-entropy alloys (HEAs) with multiple metal sites and core–shell structure for boosting the selectivity of FF hydrogenation to FA, as shown in Figure 5a. The results showed that the FF was converted to FA with a conversion of 87.32% and selectivity of 100% at 90 °C over ultra-dilute HEA (NiCoCuZnFe/C-800) (Table 1). This is because different metal sites play different roles in hydrogenation reactions, and the core–shell structure effectively stabilizes the structure of ultra-dilute HEAs (Figure 5b,c). The core–shell structure enhanced the stability of the catalysts during the hydrogenation reaction, and the porous carbon shell improved the transfer of FF and electrons. However, the core–shell structure is complicated to prepare, the process is demanding and costly, there are interface defects, and the shell may rupture during prolonged use, among other disadvantages.

Although core–shell catalysts can effectively protect and confine copper species, thereby mitigating active metal sintering and enhancing suitability for large-scale applications, their performance is inherently dependent on the stability of the shell structure. Furthermore, the shell must exhibit well-developed porosity to facilitate efficient mass transfer of reactants and products. Consequently, the design and synthesis of such catalysts entail greater complexity compared to traditional supported catalysts. Additionally, in liquid-phase hydrogenation processes, fluid-induced shear forces may compromise the structural integrity of the shell, necessitating further investigation into coating methodologies to ensure mechanical robustness under reaction conditions.

3.4.3. Integrated Cu and Support

In addition to the two methods of loading active metals onto supports and coating supports onto active metals, researchers have developed catalyst synthesis strategies in recent years to simultaneously prepare active metal particles and supports. MOF is a porous crystalline material formed by the self-assembly of metal ions or metal clusters with organic ligands through coordination bonds. This precursor can be thermally decomposed to form C-coated Cu. Yang et al. [60] reported a CuO#TiO₂ catalyst prepared by using Cu₃(BTC)₂ as the backbone, first loading TiO₂ support inversely into the interior of the MOF’s backbone, and then decomposing Cu₃(BTC)₂ by roasting. The particle size of the catalyst was smaller than that of the conventional loaded catalyst, and the catalyst had a superior activity (Table 1), and an FF conversion rate of 20.8 mol_FF/mol_Cu·h. While MOF as a precursor can effectively enhance the dispersion of the active components, the organic ligand is completely decomposed during the roasting process, thus requiring the addition of TiO₂ as an extra support. In fact, organic ligands have the potential to be converted into polymer supports. Qiao et al. [61] prepared a MOF (ZJU-199) with a ligand having an acrylate-like structure, which was subsequently pyrolyzed at a controlled roasting temperature to prepare the catalyst. As shown in Figure 6a. Under appropriate calcination temperature, the acrylate substituent on the organic ligand undergoes decarboxylation to form a C=C bond, which subsequently undergoes polymerization, leading to the formation of a cross-linked polystyrene network by the ligand; Cu²⁺ undergoes sintering and partial reduction during this process, ultimately forming a Cu/Cu₂O heterojunction (Figure 6b). The crosslinked polystyrene effectively wrapped and isolated the heterojunction, preventing it from further sintering, thus ensuring the activity and stability of the catalyst at the same time. The catalyst was prepared with a single MOF precursor without the need for additional supports.

The preparation method of integrated Cu and support may have potential advantages in terms of activity and stability. However, considering that the synthesis of MOFs may require some expensive organic ligands, it may not be suitable for large-scale production and industrial applications. Nevertheless, this method can be utilized for the directed synthesis of Cu-based catalysts with different valence state distributions and particle sizes, which is beneficial for further understanding the mechanisms.

In summary, for specific active components and supports, the catalyst structure and metal-support interaction will fully affect properties such as metal particle size and metal valence distribution, which are highly correlated with the performance of the catalyst. An ideal support should exhibit high (FF adsorption capacity and strong FA desorption capability, facilitate H₂ adsorption and activation, maximize Cu dispersion and the Cu⁺/Cu ratio, and maintain structural stability under reaction conditions. Therefore, in the design of Cu-based catalysts for the preparation of FA by FF hydrogenation, it is necessary to fully consider the synthesis mode of the catalyst based on the selected support.

4. Characterization Method

The development of efficient catalysts for the hydrogenation of FF to FA is critical, and the characterization methods are fundamental to understanding and optimizing catalyst performance. Traditional characterization includes XRD [40,127], N₂-physisorption [128,129], SEM [130,131] and TEM [53,132] only discloses = readily accessible structure and morphology information (Figure 7). The spectrum techniques, such as FT-IR [61,133], Raman [134,135], XPS [136,137], EPR [134], and the various temperature programmed (TP) techniques [131,138,139] could also provide further insights into the intrinsic surface chemical properties of the catalyst (Figure 8). For example, functional groups, elemental valence, surface acidity and alkalinity, and redox properties. It is particularly important to note that XPS measurements require careful consideration of testing conditions. Since electronic states under operational conditions (e.g., in H₂ atmosphere) differ significantly from those of air-exposed samples, XPS results may not fully represent the catalyst’s working state. Furthermore, XPS alone often cannot reliably distinguish between Cu⁰ and Cu⁺ species due to their similar binding energies. Therefore, Auger electron spectroscopy is frequently employed in conjunction with XPS to achieve a more accurate identification of copper valence states. Extensive literature has demonstrated that these analytical methods can effectively elucidate the structural and compositional characteristics of catalysts and establish the superficial structure–activity relationships. Furthermore, the development and application of more advanced characterization techniques may facilitate the acquisition of certain critical data more efficiently.

Defining the metal dispersion is crucial for unraveling the intrinsic activity and kinetic experiments. Conventionally, Cu dispersion is determined via N₂O pulse chemisorption [43], while Ni dispersion is generally calculated via H₂-TPD or chemisorption data [140,141]. CO pulse adsorption is a useful technique employed to determine metal dispersion and quantify the active sites on catalyst surfaces [142]. CO molecules exhibit strong chemisorption on the surfaces of certain transition metals (e.g., Ni, Fe), but this interaction is generally weak on Cu surfaces. However, at lower temperatures (e.g., 193 K), CO can undergo reversible chemisorption on Cu as well. In fact, even Cu⁺ species can effectively coordinate with CO to form Cu⁺(CO)₂ complexes [143]. Based on this principle, the total dispersion of an alloy can be calculated by combining data from CO pulse chemisorption with elemental analysis. Although this method cannot separately determine the dispersion of individual metals, it may still hold practical value, considering that not all exposed metal atoms are capable of chemisorption or catalytic activity. Liu et al. [144] synthesized a series of SiO₂-supported Cu-Ni bimetallic catalysts and systematically investigated the influence of the Cu/Ni ratio on FF hydrogenation to FA. Obviously, the larger the specific surface area of active metal particles and the higher the metal dispersion, the more favorable it is for the catalytic reaction to proceed. The metal dispersion and surface area of Cu₃Ni₁/SiO₂ were the largest, at 55.7% and 20.8 m²/g, respectively. Under the optimum conditions (333 K, 2.0 MPa, 3 h), Cu₃Ni₁/SiO₂ gives 99.9 % yield to FOL at 333 K in isopropanol. Compared to Ni/SiO₂, it has a yield of about 60% higher. CO pulse adsorption enables simultaneous measurement of both metals’ dispersion in Cu–Ni alloys, offering a convenient approach for establishing catalyst structure–activity relationships. Notably, the Cu₃Ni/SiO₂ catalyst exhibited the highest catalytic activity, correlating with its superior metal dispersion.

As a novel spectroscopic technique that provides deeper insight into the intrinsic properties of materials, X-ray absorption spectroscopy (XAS) consists of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Specifically, XANES can be used to study valence and electronic structure of the metal elements in the catalysts, while EXAFS can provide information about the coordination environments of the metal atoms and the particle size information [145,146].

Dalai et al. [147] prepared a series of Cu–Zn–Cr–Zr catalysts. Owing to the complex composition of metallic elements in the catalyst, identifying the presence of active Cu species on the catalyst surface after fresh and reduced states poses significant analytical challenges. However, as demonstrated in Figure 9a, XANES analysis clearly reveals the presence of CuO in the fresh catalyst and Cu⁰ in the reduced catalyst, indicating that catalyst deactivation is caused by Cu oxidation under reaction conditions. These findings corroborate the experimental results obtained through the conventional characterization techniques (Figure 9b).

It is well known that at equivalent metal loading, higher metal dispersion corresponds to a greater number of catalytic active sites. Single-atom catalysts (SACs) not only achieve the highest possible dispersion, but the isolated metal sites can also engage in strong metal–support interactions (SMSI) with the support, effectively modulating the electronic states of the metal atoms and enhancing their intrinsic activity [148]. SACs can effectively enhance the utilization of active metal components. However, due to the extremely small size of SACs, the conventional characterization methods struggle to directly observe the presence of metal single atoms. Mesa et al. [149] prepared a PdCu single-atom alloy supported on Al₂O₃ catalyst. They used the EXAFS technique to demonstrate that palladium atoms successfully replaced copper atoms on the nanoparticles while maintaining atomic dispersion, which confirms the formation of single-atom Pd, as shown in Figure 10a,b. TOF analysis clearly demonstrated that the optimal Pd₁Cu₂₁₆ SAA catalyst exhibited a copper site activity of 7.21 h⁻¹, representing an 85% increase compared to the pure Cu100 reference. The palladium site activity also increased significantly to 813 ± 81 h⁻¹, surpassing that of the monometallic Pd₁₀₀. This enhancement is attributed to the mutual reactivity promoted by the interaction between atomically dispersed Pd sites and the Cu surface. In fact, STEM and other advanced electron microscope techniques can offer a more direct visualization of single-atom sites; however, due to the high magnification involved, they cannot conclusively demonstrate that all active sites are uniformly dispersed as single atoms. The above results show that the rational use of XAS technique associated with electron microscope can help us to fully understand the structure of catalysts at the atomic level and thus associate the conformational relationships (Figure 10c,d).

The aforementioned characterization techniques can effectively provide static structural and compositional information about catalysts, but they are less capable of revealing dynamic changes that occur during actual preparation or reaction processes—information critical for tracing reaction pathways and investigating deactivation mechanisms. In situ experiments using conventional characterization methods can effectively address this limitation. For example, in situ XRD allows direct observation of phase transitions under specific conditions. Liu et al. [150] employed in situ XRD to monitor the crystalline evolution of Cu/Al₂O₃ during programmed-temperature calcination in different atmospheres. The results showed that calcination in N₂ effectively suppressed sintering of Cu species compared to that in air, leading to a more facile reduction in Cu active components to lower oxidation states in subsequent hydrogenation steps (Figure 11). The Cu/Al₂O₃-N₂-R with the smallest Cu particles exhibited excellent performance, achieving a FA yield of up to 99.9% after 2 h at 120 °C and 1 MPa of H₂ pressure. To investigate the valence state changes in Cu species, in situ XPS is a suitable technique. Zhang et al. [151] used in situ XPS to study the redox behavior of Cu in Cu/SiO₂ catalysts prepared using different loading methods during H₂ reduction. Their results indicated that the Cu loading method influenced the formation of lamellar copper phyllosilicate phases after calcination (Figure 12a), which in turn affected the valence distribution of Cu during the subsequent reduction process (Figure 12b). The Cu/SiO₂-EA and Cu/SiO₂-HDP catalysts exhibited significantly enhanced Cu dispersion after reduction, attributed to the formation of a lamellar copper phyllosilicate structure during synthesis. This structural feature resulted in the superior activity and stability compared to other catalysts.

While TP techniques are valuable for catalyst characterization, their application is limited for FF and FA due to the high boiling points of these compounds. In such cases, thermogravimetric analysis (TG-DTG) offers an effective alternative for qualitative and kinetic studies by monitoring mass changes and amplifying differential details. Furthermore, thermal gravimetric analysis of catalysts after FF adsorption can provide insights into adsorption behavior. For example, Liu et al. [46] synthesized a series of Cu-supported Al₂O₃ catalysts with different crystal phases and employed TG analysis to investigate FA adsorption/desorption properties. The results demonstrated that the Al₂O₃ crystal phase significantly influences FA adsorption strength. Notably, FA desorbed more readily from Cu/η-Al₂O₃ compared to other catalysts, which may account for its superior catalytic activity (FAL conversion: 99.7 %, FA selectivity: 94.0 %), highest TOF value (29.3 h⁻¹), and lowest apparent activation energy (66.3 kJ/mol).

Liu et al. [152] prepared a series of MgAlO_x-supported Cu or CuCo catalysts, and characterized them using in situ DRIFTS. Unsupported Cu₂O was employed as the reference (Figure 13a,b). In situ DRIFTS results show that on the Cu₃Co₁/MgAlO_x catalyst, the intensity of the characteristic spectral band at 1713 cm⁻¹ decreases rapidly, accompanied by an increase in the intensity of the characteristic spectral band at 1089 cm⁻¹ attributed to the C-O bond of alcohols, indicating that C=O is rapidly reduced. Based on the free energy of the FF hydrogenation process to form FA on different crystal faces (Figure 13c), the Cu₃Co₁ (111) crystal face is favorable for H* attacking C=O to form the -CH₂O⁺ intermediate, and also favorable for the subsequent hydrogenation steps of this intermediate. Another possible pathway, where H* attacks O to form C+, has relatively high activation energy (Figure 13d), resulting in a lower probability of this reaction pathway occurring. However, the activation energy for FF hydrogenation on the Cu₃Co₁ (111) crystal face still has a relative advantage.

In summary, the novel characterization method facilitates researchers to determine reaction paths more quickly and accurately, and analyze the interactions between different components more precisely and in detail. Novel in situ characterization methods enable real-time tracking of reaction paths and guide efficient interface design. The future trend is to focus on high temporal and spatial resolution in situ platforms (e.g., XAS-DRIFTS-MS [152]) and machine learning-assisted data parsing to promote FF hydrogenation catalysts towards the industrialization goal of “precise design, efficient conversion, and long-term stability”.

5. Conclusions and Perspectives

5.1. Summary and Significance

Cr-based catalysts have been widely employed in the industrial hydrogenation of FF to produce FA. However, concerns over the toxicity of Cr have driven efforts to develop non-toxic alternatives. Although extensive research has highlighted the potential of Cu-based catalysts as substitutes, their activity and stability remain inferior to those of Cr-based systems. While current studies have provided insights into the adsorption and activation mechanisms of FF on Cu-based surfaces, a detailed understanding of the overall reaction mechanism remains lacking. Furfural vapor-phase hydrogenation has advantages in terms of reaction efficiency and continuous production, but the gas-phase reaction is usually carried out at higher temperatures, which requires strict thermal stability and mechanical strength of the catalyst. The liquid-phase reaction conditions are relatively mild, which is conducive to the development and use of more diverse catalyst systems. In addition, since the liquid-phase method is easier to perform at the laboratory level, the reaction mechanism and the structure–activity relationship of the catalyst can be efficiently studied with the help of the liquid-phase method. Although liquid-phase direct hydrogenation of FF lacks the inherent capacity for continuous production characteristic of gas-phase processes, it retains significant potential for industrial application. This potential can be realized if catalysts achieve higher turnover TOF and superior stability under milder reaction conditions, thereby demonstrating a decisive performance advantage over gas-phase hydrogenation systems.

5.2. Composition and Preparation Method for Better Catalyst Design

Compared to noble metals or active transition metals such as Fe, Co, and Ni, Cu offers both low cost and high selectivity, making Cu-based catalysts promising alternatives to Cr-based systems. Current research suggests that maximizing the dispersion of Cu is essential for enhancing catalytic performance, with single-atom-dispersed Cu being particularly beneficial in this regard. Cu⁺ is known as a highly active site, while Cu⁰ may favor the adsorption of FF, implying that an optimal Cu⁺/Cu⁰ ratio could significantly improve catalytic activity. A support with a well-developed mesoporous structure and a large specific surface area facilitates better dispersion of Cu species. Additionally, supports that provide adsorption sites for FF or H₂ can further enhance catalytic performance. However, strong acidity on the support surface may promote undesirable side reactions. This drawback can be mitigated by incorporating promoters, which help regulate the oxidation state of Cu, improve its dispersion, and promote reduction through modulation of the metal–support interaction. Moreover, for the same support, variations in pretreatment methods and Cu loading techniques can influence the metal–support interaction, potentially suppressing or even preventing Cu sintering. Therefore, when designing Cu-based catalysts, it is crucial to carefully select suitable promoters and synthesis strategies that are tailored to the nature of the support.

5.3. Using Novel Characterization Technique

Conventional characterization methods are essential for understanding the structure–activity relationships of catalysts; however, advanced techniques are still required to probe their fine structural features and dynamic behavior under real reaction conditions. CO pulse adsorption is an effective tool for evaluating the dispersion of multi-metal active components. When combined with XAS and advanced electron microscopy, it enables a more intuitive analysis of Cu distribution, particularly in SACs. In situ characterization techniques offer significant advantages in monitoring structural and compositional changes during catalyst preparation, reduction, and reaction processes, as well as the dynamic evolution of reactants on the catalyst surface. TG can serve as an alternative to traditional TP experiments, providing more accurate insights into surface adsorption and desorption behaviors. Looking ahead, the development and application of novel characterization methods will further advance the understanding of structure–activity relationships and support the rational design of more efficient Cu-based catalysts.

5.4. Perspectives

In summary, well-designed Cu-based catalysts enable high activity and stability for the liquid-phase hydrogenation of FF to FA under relatively mild conditions, demonstrating potential for industrial implementation. However, unlike gas-phase processes, liquid-phase systems lack inherent advantages for continuous operation. Consequently, future research should prioritize achieving milder reaction conditions, higher TOF, and enhanced catalyst reusability. This direction implies that conventionally supported catalysts with simple preparation methods might be less competitive, while encapsulated catalyst architectures could offer greater promise for advancing FF hydrogenation research and applications.