1 Introduction
As the core of nitrogen-containing organic compounds, amine compounds (Amines) possess a unique lone pair electron distribution within their amino functional groups, endowing these molecules with exceptional Lewis basicity, nucleophilic reactivity, and coordination chelation capabilities with transition metals.
[1-6]These unique physicochemical properties enable them to demonstrate significant application value in cutting-edge interdisciplinary fields such as biomedicine
[7-11], functional materials
[12-16], and environmental engineering
[17-19]. In response to their diverse application demands, academia and industry have constructed diversified amine synthesis pathways
[20-21], covering nitrile hydrogenation reduction
[22-25], nitro group hydrogenation reduction
[26-28], alcohol amination
[29-32], phenol hydroamination
[33-34], alkyl halide amination
[35-36], and reductive amination of carbonyl compounds
[37]. However, against the backdrop of deepening concepts in sustainable chemistry, achieving both high atom utilization efficiency and broad substrate applicability simultaneously within complex reaction systems has become a key issue driving technological development. In this context, nitrile hydrogenation reactions, owing to their superior atom economy and excellent selectivity, have emerged prominently and are gradually becoming an important strategy for constructing green synthesis pathways for amine compounds.
[38].
From the perspective of atom economy, the nitrile hydrogenation reaction is accompanied only by a small amount of NH
3generation, with an atom utilization rate approaching 100%, and demonstrates extremely low waste intensity (E-factor) in life cycle assessment (LCA), embodying typical green synthesis characteristics. In terms of raw material sources, nitrile feedstocks can be derived from petrochemical by-products, coal-based materials, and bio-based platform molecules; this stable and diverse supply system provides a realistic foundation for their large-scale conversion. Additionally, the reaction system exhibits high chemical conversion flexibility; by regulating the active centers of the catalyst, directional selective synthesis of primary, secondary, and tertiary amines can be achieved, thereby meeting the customized demands of the fine chemical industry for structurally complex amine molecules
[39-43].
Although nitrile hydroamination offers significant process advantages, its research depth and industrial application level still lag behind the conversion processes of other unsaturated functional groups. This limitation primarily stems from the combined effects of molecular orbital constraints and kinetic energy barriers. From a thermodynamic perspective, the C≡N bond possesses high bond energy (891 kJ/mol) and strong molecular orbital stability, making it difficult to activate under mild conditions.
[44]; whereas at the kinetic level, nitrile hydrogenation involves a multi-step continuous transfer of hydrogen atoms. Intermediates such as imines and Schiff bases are densely distributed on the reaction potential energy surface, very easily leading to over-hydrogenation that triggers C—N bond hydrogenolysis to form alkanes, or causing a decline in target product selectivity due to intermediate condensation.
In the past decade, macroscopic catalysts represented by Raney nickel have become the mainstream catalysts for the hydrogenation of nitrile compounds to generate primary amines.
[45-48]. Such catalysts possess high metal site density and demonstrate good reaction performance in industrial processes. However, they are sensitive to air, typically require high hydrogen pressure conditions, and are difficult to finely tune regarding local coordination environments and electronic structures, thus exhibiting certain limitations in selectivity control and long-term stability.
[22]. Against this background, research in recent years has gradually shifted towards catalyst design strategies centered on microenvironment regulation. By synergistically employing methods such as regulating atomic coordination environments, interfacial field emphasis regulation, spatial geometric confinement, and comprehensive regulation, the electron density and coordination geometry of active metal sites are adjusted. This achieves microscopic intervention on the adsorption orientation and activation energy pathways of intermediates at the molecular scale, ultimately optimizing the selectivity and stability of nitrile hydrogenation reactions.
Based on reviewing the reaction pathways and mechanisms for the hydrogenation of nitrile compounds into various types of amine products, this paper focuses on catalyst microenvironment engineering to summarize and analyze strategies such as atomic coordination regulation, interfacial field enhancement regulation, spatial geometric confinement, and multi-dimensional synergistic regulation. It further deeply elucidates the intrinsic mechanisms by which these regulation methods improve the activity, selectivity, and stability of nitrile catalytic reduction, aiming to provide valuable theoretical basis and methodological guidance for the performance optimization and industrial scale-up application of heterogeneous metal-catalyzed nitrile hydrogenation systems.
2 Mechanism of nitrile hydroamination reaction
A deep understanding of the reaction mechanism for nitrile hydroamination is key to constructing efficient catalysts, and this knowledge has been continuously deepened and refined through long-term research. In 1923, Braun et al.
[49]first proposed a preliminary kinetic framework for the hydrogenation of cyano groups to produce secondary amines. Subsequently, Greenfield et al.
[50]revised and perfected this model, forming a classic multi-path cascade mechanism centered on the imine (R—CH＝NH) intermediate. As
Figure 2shows, this reaction model clarifies the dual-path conversion characteristics of the imine intermediate: In the first path (
Figure 2, Route 1), H
2is activated and first hydrogenates the cyano group to generate an imine intermediate; subsequently, R—CH＝NH undergoes further hydrogenation to yield a primary amine. The generated primary amine then undergoes nucleophilic addition with the imine intermediate to form an intermediate, which finally produces a secondary amine via hydrogenolysis and deamination. In the second path (
Figure 2, Route 2), the intermediate yields a Schiff base by eliminating NH
3, which is subsequently converted into a secondary amine via a hydrogenation step. Furthermore, the secondary amines generated by the above two paths may further react with the imine intermediate to yield tertiary amines. Although this classic mechanism has been widely applied, it is essentially a macroscopic kinetic description that fails to effectively address the quantitative correlation between the local electron density gradient at active sites and the microscopic potential energy surface, leaving significant limitations in the understanding of complex processes on catalyst surfaces.
Extensive research has focused on investigating the mechanism of nitrile hydrogenation on heterogeneous catalyst surfaces
[22,51-58]. Segobia et al.
[54]systematically investigated SiO
2-supported transition metal catalysts (Ni, Co, Ru, Cu, Pd, and Pt) for butyronitrile hydrogenation at 373 K and 13 bar H
2pressure, finding that primary amine selectivity exhibits significant metal dependence, following the order Ni > Co > Ru > Pt. This phenomenon not only indicates differences in macroscopic performance among different transition metals but also reflects at a deeper level that the activation pathway of the C≡N bond is influenced by the intrinsic electronic structure of the metal, stemming from differences in the interaction between the electron cloud distribution on the metal surface and the π orbitals of the nitrile group.
On this basis, Krupka et al.
[55]proposed a metal surface catalysis model, elucidating for the first time at the molecular level the decisive influence of the electronic structure of active sites on the adsorption configuration of intermediates. As
Figure 3shows, the adsorption mode of nitrile molecules on the catalytic surface directly determines the energy barrier distribution of subsequent transformation steps. Nitrile molecules (R—
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$) mainly exhibit two adsorption configurations on metal surfaces: M-N adsorption and π-bond adsorption, with the specific adsorption mode primarily depending on the electronic structure of the active metal. Wen et al.
[22]further confirmed that differences in the electronic structure of active metals not only significantly alter the adsorption configuration of reaction intermediates but also affect their thermodynamic stability. For transition metals represented by Ni and Co, their
dband center is closer to the Fermi level, enabling stronger interactions with the lone pair electrons of the cyano nitrogen atom, prompting nitrile molecules to adsorb vertically on the metal surface via the N atom, forming nitrene intermediates (nitrene, R—C＝N—M). After hydrogenation, these intermediates generate α-carbon saturated imine species; such intermediates possess high thermodynamic stability and are less prone to further condensation with primary amines, thus favoring the selective synthesis of primary amines. In contrast, for noble metals such as Pd and Pt, their
dband center is typically far from the Fermi level, tending more towards lateral interactions with the π orbitals of the C≡N bond to form a π-bond adsorption configuration, which upon hydrogenation generates α-carbon unsaturated imine intermediates. Meanwhile, Pd and Pt surfaces also readily adsorb the α-carbon of the cyano group, forming amino-carbene (amino-carbene, R—CH—NH
2—M) intermediates. Due to the high degree of unsaturation and reactivity of these intermediates, they are often more likely to undergo nucleophilic addition with primary amines to form new C—N bonds, thereby promoting the formation of secondary and even tertiary amines
[59].
Thus, the selectivity differences in nitrile hydrogenative amination cannot be simply attributed to macroscopic variations in metal types; their essence lies more in the microscopic regulation of adsorption configurations and intermediate stability by the electronic structure of active sites. The
delectron state distributions on different metal surfaces and differences in the density of states near the Fermi level alter the bonding mode between the cyano group and the metal, further influencing the formation pathways of key intermediates, hydrogenation energy barriers, and surface residence behavior, ultimately determining the competitive relationship between the primary amine generation pathway and the condensation amination pathway. Based on this understanding, performance optimization of nitrile hydrogenation catalytic systems should not merely stop at increasing metal exposure or adjusting loading morphology, but should further focus on the precise regulation of local electronic structures and adsorption potential energy surfaces to construct active sites with designability
[60]. Therefore, recent related work has gradually shifted towards microenvironment engineering-oriented catalyst design, achieving systematic optimization of the hydrogen donation process, intermediate lifetime, and side reaction channels based on the synergistic regulation of intrinsic metal properties. The following section will focus on strategies such as atomic coordination regulation, interfacial field enhancement regulation, and spatial geometric confinement (summarized in
Table 1)
[61-81], to further elucidate how their synergistic effects of electronic modulation and geometric constraints promote a better balance among activity, selectivity, and stability in nitrile hydrogenation systems.
3 Impact of microenvironment regulation strategies on nitrile catalytic hydrogenation
3.1 Atomic Coordination Regulation
Atomic coordination regulation achieves directional modulation of the electronic structure of metal centers by precisely designing the first coordination sphere of active sites, including parameters such as the type of coordinating atoms, coordination number, and bond length. Its essence lies in reconstructing the local charge distribution and orbital overlap status, thereby influencing the adsorption configuration and reaction energy barriers of cyano and imine intermediates on active sites at the molecular level
[82-86].
3.1.1 Heteroatom hybridization
Heteroatom hybridization is one of the most common and representative strategies in atomic coordination regulation. By introducing non-metallic elements such as N, P, and S into the carrier lattice or carbon skeleton, M-N
x, M-P
xand other local coordination structures can be constructed, or specific phases such as metal phosphides can be induced to form, thereby adjusting the electron density of the metal center and
dorbital energy level distribution. This type of regulation not only affects the H
2activation process but also alters the transformation behavior of key intermediates; therefore, in heterogeneous metal-catalyzed nitrile hydrogenation to primary amine systems, it is regarded as one of the key atomic-scale means to enhance activity, selectivity, and cycling stability
[87].
Coordination construction represented by nitrogen doping usually forms M-N
x local environments to enhance the ability of metal sites to regulate reaction pathways and improve catalytic activity. Liu et al.
[61] used widely available and low-cost chitin as a carbon and nitrogen source, and nickel acetate tetrahydrate as a metal precursor, to prepare a nitrogen-doped carbon-supported nickel catalyst, Ni-JKS-600, which was applied to the hydrogenation of benzonitrile to benzylamine. Under optimal reaction conditions, the catalyst exhibited excellent catalytic performance, with a benzonitrile conversion rate of up to 98.14% and a benzylamine selectivity of 97.08%. Furthermore, the catalyst demonstrated good applicability to various substituted aryl nitriles, such as 4-fluorobenzonitrile and 4-chlorobenzonitrile. Structural characterization revealed that Ni-JKS-600 possesses a high specific surface area and abundant micro-mesoporous structures, with Ni nanoparticles highly dispersed on the support surface, facilitating hydrogen dissociation and the continuous supply of active hydrogen species. In the nitrogen-doped carbon support formed after chitin pyrolysis, the ratios of pyridinic nitrogen and graphitic nitrogen were 36.37% and 63.63%, respectively. Among these, pyridinic nitrogen can interact with Ni's
d orbitals to form stable active centers; meanwhile, graphitic nitrogen regulates the electronic structure of Ni through electron transfer, effectively suppressing side reactions such as the self-condensation of imine intermediates, thereby achieving highly selective hydrogenation conversion of benzonitrile to benzylamine.
In dinitrile substrate systems that are difficult to reduce, nitrogen coordination regulation is also of great significance for enhancing deep hydrogenation capability. Yang et al.
[62]regulated the surface coordination environment of Co@C catalysts via a nitrogen-doping strategy, achieving efficient and selective hydrogenation of adiponitrile to hexamethylenediamine without added base. Using metal-organic frameworks (MOFs) as precursors, a series of Co-based catalysts were synthesized, among which Co@C-Z400, obtained by thermal reduction of ZIF-67 at 400 °C, exhibited the best performance. Characterization results indicated that the Co-N
xcoordination environment constructed on the surface of the Co@C-Z400 catalyst can stabilize Co
0species and optimize their electronic structure, thereby enhancing the adsorption of the semi-hydrogenated product 6-aminocapronitrile and its further hydrogenation capability. Under base-free conditions with 7 MPa H
2and at 100 °C, this catalyst achieved complete conversion of adiponitrile, with a hexamethylenediamine yield as high as 90.7% and a TOF of 2746 h
-1; after 5 cycles, the selectivity remained at 80.5%, and high primary amine yields were obtained for the hydrogenation of various aliphatic and aromatic nitriles.
In addition to enhancing deep hydrogenation capability, the M-N
xenvironment formed by nitrogen doping can also significantly improve the stability of metal active sites. Huang et al.
[63]reported an efficient heterogeneous Co catalytic system for nitrile hydrogenation to prepare primary amines. The catalyst was prepared via pyrolysis using biomass adenine as the carbon and nitrogen source. Among them, Co@NC-700 exhibited excellent catalytic performance at 110 °C and 1 MPa H
2conditions, achieving a nitrile hydrogenation yield of 80.1%~99.4%, and maintaining stable activity after five consecutive cycles. Studies show that pyridinic nitrogen (397.7 eV) and pyrrolic nitrogen (400.2 eV) generated from adenine during pyrolysis can effectively anchor Co particles and suppress their aggregation, thereby endowing the catalyst with superior dispersibility and structural stability. This is of great significance in practical reactions; for heterogeneous catalytic systems requiring long reaction times or cyclic use, migration and sintering of metal particles are often major sources of activity decay, whereas stable M-N
xcoordination environments can retain effective active sites while maintaining dispersibility, thus extending the catalyst's lifespan.
Unlike nitrogen doping, the phosphorization strategy primarily enhances catalyst performance toward H through phase structure reconstruction and electronic state modulation.
2activation ability. Mitsudome et al.
[64]A nano-cobalt phosphide catalyst named nano-Co was reported.
2P/HT, this catalyst can operate at a constant pressure of H under 1 bar.
2and efficiently catalyzes the selective hydrogenation of various nitriles to primary amines under mild conditions, with a turnover number (TON) exceeding 58,000, demonstrating superior intrinsic activity. Characterization and theoretical calculation results indicate that phosphidation can effectively regulate the electronic structure and crystal phase of the active metal Co.
Fig. 4aDisplay bulk Co (black curve), bulk Co
2P (blue curve) and Co
2The Co atoms on the P surface (red curve)
dProjected orbital density of states shows that Co
2The Co atoms on the P surface
dHigher electron density.
dThe increase in electron density enhances the interaction with nitrile molecules.
p*the reverse electron-donating ability of the orbital, thereby weakening
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$bond and promote substrate activation.
Fig. 4dSimulated H
2Molecules on the Co catalyst
2The dissociation process on the P surface shows that phosphidization significantly reduces H.
2activation energy barrier, making active H species easier to generate. Additionally, the activated
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$The bonds can then be further hydrogenated by dissociatively adsorbed H atoms on Co sites. Phosphidation not only enhances the activation ability of substrates but also improves H-donation efficiency; their synergistic effect enables nano-Co
2The P/HT catalyst exhibits high efficiency in catalyzing nitrile reduction under mild conditions.
Heteroatom hybridization provides an atomic-level design pathway for nitrile hydrogenation catalysts that differs from traditional component regulation. Existing research indicates that introducing heteroatoms into the support and constructing specific coordination units not only stabilizes metal species and optimizes electronic structures but also simultaneously regulates the adsorption and conversion behaviors of substrates and intermediates, suppresses side reactions such as condensation, thereby improving primary amine selectivity and cyclic stability. In some systems, heteroatom doping also induces phase structure reconstruction, further enhancing the adsorption and dissociation capability for H2, reducing the activation energy barrier and improving overall reaction efficiency from the hydrogen supply aspect. Overall, heteroatom doping provides crucial support for catalytic systems to achieve high activity, high selectivity, and long-term stable operation under mild conditions by synergistically regulating key reaction steps such as substrate activation, intermediate conversion pathways, and active hydrogen supply.
3.1.2 Synergistic multi-metal atoms
Polymetallic synergy typically constructs dual-atomic sites, heteronuclear metal bonds, or alloys to induce charge redistribution, forming spatially adjacent electron-rich and electron-deficient sites at the nanoscale. During nitrile hydrogenation, electron-deficient sites are more favorable for
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$bond and imine intermediate adsorption and activation, while electron-rich sites more readily promote H
2adsorption, dissociation, or hydrogen transfer supply. The synergistic effect of these two types of sites on the same catalytic interface can effectively lower the reaction energy barriers of key steps and regulate the evolution pathways of intermediates, thus holding promise for balancing activity, selectivity, and stability under mild conditions
[88-90].
Taking the Ni-based system as an example, the introduction of a second metal often enhances primary amine selectivity through the dual effects of electronic synergy and structural regulation. Ma et al.
[65]constructed a Re-modified Ni/SiO
2(Ni-Re/SiO
2) heterogeneous catalyst and applied it to the hydrogenation of nitriles for primary amine synthesis. In the hydrogenation of 3-hydroxypropionitrile (3-HPN) to synthesize 3-amino-1-propanol (3-AP), the catalyst achieved 100% conversion and over 96% selectivity, maintaining stability throughout a continuous operation of up to 1300 h. Furthermore, this system exhibited high activity and selectivity for aromatic nitriles substituted with —CH
3, —OCH
3, or —F groups, with conversions exceeding 99% and target product yields surpassing 97%. Studies indicate that the superior performance of this catalyst stems from the structural and electronic effects induced by Re incorporation. Re species are highly dispersed on the catalyst surface and partially embedded into the Ni lattice, reducing the Ni particle size from (15.5±1.4) nm on Ni/SiO
2to (10.9±1.1) nm on Ni-3.0Re/SiO
2, thereby effectively suppressing particle agglomeration and increasing the exposure of active sites; meanwhile, the introduction of Re promoted the reduction of Ni
2+to Ni
0and formed an electron-rich microenvironment at the local interface that is more favorable for hydrogenation reactions. Density functional theory (DFT) calculations further revealed that the Ni-Re surface of this catalyst exhibits high dissociation activity for H
2, while the acidic characteristics displayed by the Re sites facilitate the adsorption and activation of the substrate 3-HPN. The combined effects of structural regulation and electronic synergy ultimately lowered the reaction energy barrier for the hydrogenation process and suppressed side reactions.
Zhang et al.
[66]used NiZnAl layered double hydroxides (LDH) as precursors to prepare ZnAlO
x-supported Ni catalysts (Ni/ZnAlO
x). Among them, the optimal catalyst Ni/ZnAlO
x-600 could efficiently catalyze the selective hydrogenation of benzonitrile to benzylamine at room temperature and 10 bar H
2. After 14 h of reaction, the benzonitrile conversion reached 99.8%, and the benzylamine yield was 99.0%. Characterization and performance test results indicate that the excellent activity of this catalyst is mainly attributed to Ni nanoparticles of suitable size, which provide sufficient hydrogenation active sites; the formation of Ni-Zn alloys enhances the interaction between the metal and the support and induces electron-deficient Ni species, thereby significantly reducing the activation energy for H
2dissociation and
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$bond adsorption; the introduction of Zn increases the number and strength of Lewis acid sites on the catalyst surface, facilitating the directional adsorption of the nitrile N atom, thereby promoting primary amine formation and suppressing side reactions. Based on the synergistic effect of multiple active sites, the catalyst achieves efficient selective hydrogenation to produce primary amines at room temperature and exhibits good catalytic activity in the preparation of corresponding primary amines from various nitrile substrates.
When the target product shifts from primary amines to secondary amines, polymetallic synergy often needs to be further coupled with acidic sites on the support or intermediate stabilization mechanisms to guide the subsequent condensation pathway. Sun et al.
[67]reported a Cu-Ni alloy supported Cu
4.4-Ni
0.6/W
20O
58catalyst and investigated its catalytic performance using the hydrogenation of benzonitrile to prepare dibenzylamine as a model reaction. The research results show that both Cu and Ni in this catalyst exist in their metallic states, forming a uniformly distributed Cu-Ni alloy. Due to charge transfer between Ni and Cu, Cu exhibits electron-rich characteristics, while the Ni-W
20O
58interface displays electron-deficient properties. This catalyst demonstrates excellent activity and secondary amine selectivity in nitrile hydrogenation reactions; its activity stems from the metallic synergistic effect on the Cu-Ni alloy surface, while its selectivity arises from acidic sites generated by the Cu-Ni synergistic interaction. As
shown in Figure 5, mechanistic studies indicate that in this system, benzonitrile preferentially adsorbs at the Ni-W
20O
58interface, while H
2dissociates on the Cu surface; subsequently, benzonitrile undergoes hydrogenation with hydrogen atoms activated at Cu sites on the Ni-W
20O
58interface to generate benzylimine; benzylimine is further hydrogenated by hydrogen atoms activated at Cu sites to form benzylamine, which then migrates to adjacent Cu atoms; benzylamine undergoes a condensation reaction with benzylimine at Ni sites to produce N-benzylidenebenzylamine; N-benzylidenebenzylamine is hydrogenated to yield dibenzylamine; finally, dibenzylamine desorbs, regenerating the catalyst. Thus, the excellent performance of the Cu
4.4-Ni
0.6/W
20O
58catalyst mainly originates from the synergistic effect between Cu and Ni, wherein electron-deficient Ni species at the metal-support interface facilitate the adsorption and activation of cyano groups, while electron-rich Cu sites promote the adsorption and dissociation of H
2; meanwhile, weak to moderately strong acidic sites present on the W
20O
58support can stabilize imine intermediates, prolonging their surface residence time and promoting their condensation with benzylamine. The dual effect of metallic synergy and acidic site stabilization is key to the efficient generation of dibenzylamine in this catalytic system.
Guo et al.
[68]designed and synthesized a bimetallic CoCu@CN catalyst supported on nitrogen-doped carbon, using ammonia borane (NH
3BH
3) as the hydrogen source to achieve the selective conversion of benzonitrile to secondary amines. When reacted at 100 °C in methanol solvent for 12 h, the yield of the target product reached 94%, and the catalyst maintained high activity and stability after 6 cycles, demonstrating good reusability. Characterization results indicate that the active phases formed by Co and Cu, including Cu and CuCo
2O
4, effectively regulated the electronic structure and surface chemical environment of the catalyst, enhancing substrate adsorption capacity and active hydrogen transfer efficiency; meanwhile, the sheet-like structure of the nitrogen-doped carbon support increased the specific surface area, promoted uniform dispersion of metal components, and inhibited particle aggregation. For hydrogen transfer catalytic systems, the electronic synergistic effect of metal sites often determines whether the reaction can proceed smoothly, while the structural stabilizing effect of the support directly affects the lifespan and stability of the catalyst during cycling.
For metal systems that are thermodynamically difficult to mix, the heteroatomic charge polarization sites formed by random alloying can also regulate the selectivity and activity of nitrile reduction. The Katsutoshi team
[69]utilized a microwave-assisted rapid chemical reduction method to overcome the thermodynamic immiscibility limit between Pd and Pt, successfully preparing PdPt random alloy nanoparticle catalysts (PdPt-PVP/C). This catalyst achieves high-selectivity conversion of nitriles to secondary amines under ambient temperature and pressure conditions, without requiring additional heating, pressurization, or prolonged reaction times, with a maximum TOF value reaching 183 h
-1, and a secondary amine selectivity of 96%. As
Figure 6shows, mechanistic studies indicate that charge transfer between adjacent Pd and Pt atoms in the alloy forms Pd
δ+Pt
δ-heteroatomic sites. In the most stable benzonitrile adsorption configuration (
Figure 6a), the nitrogen and carbon atoms of the cyano group are located on Pd
δ+and Pt
δ-atoms, respectively, exhibiting distinct synergistic adsorption characteristics. Compared with monometallic catalysts, the PdPt alloy induces more significant valence electron changes during benzonitrile adsorption (
Figure 6), indicating that the electron-rich Pt
δ+Pt
δ-in the heteroatomic sites
δ-can activate the
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$bond through strong feedback electron transfer, thereby accelerating the rate-determining step. This system also demonstrates high conversion efficiency for aromatic nitriles containing electron-withdrawing or electron-donating substituents, as well as for heterocyclic nitriles, indicating that the heteroatomic polarization effect induced by random alloying possesses good universality.
In addition to typical metal-metal synergy, alkali metal promoters can also achieve multi-component synergy through charge transfer and acid-base regulation, thereby influencing the product distribution of nitrile reduction reactions. Liu et al.
[70]prepared Na-modified Ni/Al
2O
3catalysts and applied them to the hydrogenation of adiponitrile to hexamethylenediamine in a fixed-bed reactor. At 120 °C, 4 MPa, and 0.25 h
-1, the Ni-0.15Na/Al
2O
3catalyst achieved 100% adiponitrile conversion and 82.07% hexamethylenediamine selectivity. As
shown in Figure 7, the introduction of Na increased the proportion of weak acid sites on the support surface while reducing medium-strong acid sites. This reconstruction of the acid site distribution favors timely desorption after reactant adsorption, thereby suppressing condensation side reactions and improving selectivity. Furthermore, an appropriate amount of Na acting as an electron donor can increase the content of Ni
0species in the catalyst, enhancing the adsorption and dissociation capability of H
2. The authors further pointed out that in this system, the hydrogenation capability of the active metal is the primary factor determining whether catalysis proceeds smoothly, whereas acid sites more directly influence product distribution: weak acid sites favor moderate adsorption and timely desorption of substrates and intermediates, thereby enhancing primary amine selectivity; medium-strong acid sites are more likely to cause surface species retention, subsequently promoting condensation reactions and generating higher amines. For multi-step continuous hydrogenation systems represented by adiponitrile, this work holds significant process guidance value.
Thus, multimetallic synergy provides a functional division of labor driven by charge transfer and a selective design framework for regulating reaction pathways via interfacial coupling in nitrile hydrogenation. By constructing dual-atom sites, alloy structures, or heteronuclear metal bonds, electron redistribution can be achieved at local interfaces, forming active sites that are functionally complementary. In primary amine synthesis, introducing a second metal typically helps strengthen H2activation and$\mathrm{C}\equiv \mathit{ }\mathrm{N}$adsorption, thereby improving selectivity and long-term stability. In contrast, the directed formation of secondary amines relies more on the synergy between multimetallic sites, acidic sites on the support, and the stabilization effect of imine intermediates to enhance the controllability of subsequent condensation pathways. Heteroatomic polarized sites formed by random alloying, along with the dual regulation of acid-base and electronic effects introduced by alkali metal promoters, further expand the structural space and reaction dimensions of multicomponent synergistic control. Under the combined action of these multidimensional synergistic mechanisms, nitrile hydrogenation catalytic systems can achieve high activity, high selectivity, and long-term stability simultaneously under mild conditions.
3.1.3 Active metal size regulation
Active metal size regulation achieves directional control over the activity and product selectivity of nitrile hydrogenation by adjusting the geometric configuration and electronic structure distribution of metal sites across different scales, such as single atoms, atomic clusters, and nanoparticles, thereby altering the adsorption modes of reactants and intermediates as well as the generation and transfer pathways of hydrogen species.
[91].
Taking the Pd system as an example, Liu et al.
[71]constructed two types of atomically dispersed Pd catalysts using defect-rich nanodiamond-graphene (ND@G) as a support: single-atom Pd catalysts (Pd
1/ND@G) and Pd cluster catalysts composed of an average of three Pd atoms (Pd
n/ND@G).
Figure 8characterization results show that Pd
1/ND@G primarily exhibits isolated Pd-C/O coordination, whereas Pd
n/ND@G features both Pd-C/O and Pd-Pd coordination, indicating intrinsic differences in their local coordination environments and electronic structures. In the hydrogenation of benzonitrile using ammonia borane as the hydrogen source, Pd
1/ND@G tends to produce the secondary amine dibenzylamine with a selectivity exceeding 98%; in contrast, Pd
n/ND@G highly selectively produces the primary amine benzylamine under milder conditions, also with a selectivity exceeding 98%. DFT calculations further reveal that the size and structure of the Pd sites determine the dissociative adsorption mode of H
2and the subsequent transformation pathway of the benzylideneimine intermediate. On Pd
1sites, the activation of the second H
2molecule is thermodynamically and kinetically difficult, leading to an extended residence time of the benzylideneimine intermediate on the surface, which makes it more prone to condense with benzylamine to form
N-benzylidenebenzylamine, followed by further hydrogenation to yield dibenzylamine; whereas on Pd
nsites, the second H
2molecule dissociates more readily, promoting rapid hydrogenation of the benzylideneimine intermediate to benzylamine. Meanwhile, the weaker adsorption of benzylamine on cluster sites ensures that the hydrogenation pathway prevails over the condensation pathway.
In single-atom catalytic systems, the size effect often works together with the intrinsic metal adsorption matching relationship to determine the final product selectivity. Chen et al.
[72]prepared atomically dispersed Rh catalysts (Rh
1/ND@G) anchored on a defect-rich nanodiamond-graphene (ND@G) support using the deposition-precipitation method and applied them to the selective hydrogenation of nitriles. Using benzonitrile as a model substrate under mild conditions of 60 °C and 0.6 MPa H
2, the catalyst exhibited extremely high activity and selectivity, with a TOF as high as 2592 h
-1, and a selectivity towards secondary amines exceeding 99% (
Figure 9). To further reveal the structural basis of its highly efficient catalytic performance, researchers prepared Rh cluster catalysts (Rh
n/ND@G) and single-atom catalysts of Ru, Pd, Ir, Pt, etc. (M
1/ND@G), and combined experimental characterization with DFT calculations to systematically analyze the roles of size effects and metal-dependent effects in catalytic regulation. The authors employed Rh
1@G (single-atom model) and Rh
3@G (triatomic cluster model) to simulate the reaction pathways of Rh
1/ND@G and Rh
n/ND@G (
Figure 9e). Calculation results showed that the enthalpy change for the hydrogenation of benzylideneimine to benzylamine on the cluster model was 0.77 eV, higher than the 0.71 eV on the single-atom model; however, the corresponding reaction energy barrier for the cluster model was only 0.60 eV, significantly lower than the 0.97 eV for the single-atom model. Thus, Rh single-atom sites are thermodynamically and kinetically unfavorable for the formation of primary amine by-products, thereby favoring the highly selective formation of secondary amines. In addition, DFT calculation results indicated that the reaction activity of different M
1/ND@G catalysts is linearly correlated with the difference in adsorption energy between benzylideneimine and benzonitrile. Rh
1/ND@G exhibited moderate adsorption strength for benzonitrile (-0.59 eV) and benzylideneimine (-0.75 eV), which not only ensures effective activation of the substrate but also avoids excessive adsorption of intermediates, thereby achieving superior catalytic activity. In contrast, Pt
1/ND@G and Ir
1/ND@G exhibited negligible reaction activity due to poor adsorption energy matching. This result demonstrates that in single-atom catalytic systems, the metal type and size state are not independent of each other but jointly determine the adsorption matching relationship and subsequent reaction pathways.
Overall, the size regulation of active metals provides a selectivity design strategy for nitrile hydrogenation that differs from traditional component regulation. Single-atom sites, typically relying on discrete coordination environments and confined adsorption configurations, impose stronger constraints on intermediate transformation pathways, thereby favoring the targeted generation of specific products such as secondary amines. In contrast, small clusters or nanostructures, possessing more abundant adjacent adsorption sites and capable of providing a more continuous hydrogen supply process, often tend to promote the deep hydrogenation of imine intermediates, thus increasing the likelihood of primary amine formation. The size effect not only alters the geometric morphology of active sites but also profoundly influences the balance between intermediate residence time, hydrogen transfer efficiency, and competing reaction pathways, which is the fundamental reason for its ability to achieve selectivity regulation.
3.2 Interface field strength regulation
Interfacial field emphasis regulation centers on the contact interface formed between active metals and external phases (supports or coating layers), achieving precise modulation of the metal surface electronic structure and adsorption potential energy landscape through interfacial charge transfer, metal-support interactions, and reconstruction of the local acid-base environment
[86,91-92]. In nitrile hydrogenation reactions, this strategy not only affects H
2activation and the supply capacity of active hydrogen species, but also alters the adsorption configurations and residence behaviors of nitriles, imines, and amine intermediates at the interface, becoming a crucial means to regulate reaction activity and product selectivity.
The evolution of the support crystal phase can effectively regulate metal dispersion, thereby influencing product distribution. Zhang et al.
[73]systematically investigated the role of different alumina support crystal phases in the selective hydrogenation of benzonitrile to benzylamine over Pd/Al
2O
3 catalysts. The research team prepared γ, η, θ, and α alumina supports by calcining commercial pseudoboehmite and synthesized gibbsite, respectively, and loaded 0.5 wt% Pd onto their surfaces. Characterization results showed that as the alumina crystal phase transformed (γ→η→θ→α), the specific surface area of the catalysts decreased sharply from 265 m
2/g to 3.8 m
2/g, with a simultaneous reduction in the number of acidic sites, while the size of Pd metal nanoparticles increased from 3.0 nm to 36.9 nm. Catalytic performance tests revealed that all four catalysts achieved complete conversion of benzonitrile within 6 h, indicating that under the examined conditions, the conversion rate is primarily controlled by the hydrogen mass transfer rate, whereas the formation of the byproduct dibenzylamine depends on the Pd dispersion state determined by the support crystal facets. Comprehensive comparison showed that both Pd/γ-Al
2O
3and Pd/η-Al
2O
3exhibited excellent stability and catalytic performance, with benzylamine yields reaching approximately 90%. This result demonstrates that in interface regulation, the support crystal phase does not merely affect the metal loading method; rather, it regulates the reaction pathway through coupled changes in surface structure, acidic site distribution, and metal particle size.
The interfacial charge effect induced by metal-support interactions is another typical strategy for regulating substrate adsorption behavior and catalyst deactivation processes. Qu et al.
[74]constructed a Pd/Mg(OH)
2catalyst by loading Pd nanoparticles onto ultrathin porous sheet-like Mg(OH)
2. Under mild conditions of 30 °C and 1.0 MPa H
2without additives, this catalyst achieved highly efficient selective hydrogenation of benzonitrile, with benzonitrile conversion and benzylamine selectivity reaching 99.2% and 96.1%, respectively, significantly outperforming conventional Pd/C systems. Performance tests and structural characterization indicate that the high efficiency of this system mainly stems from the modulation of the Pd electronic environment by electronic metal-support interactions. XPS results show that in Pd/Mg(OH)
2, the Pd 3d
5/2binding energy is lower than that in Pd/C, and the proportion of Pd
0is higher; combined with the characteristics of Mg(OH)
2as an electron donor and the carbon support as an electron acceptor, it can be confirmed that electron transfer occurs from Mg(OH)
2to Pd, thereby increasing the electron density of Pd. FTIR and XPS further reveal that after treatment with benzylamine, distinct N—H, C—H, and N 1s characteristic peaks appear on the surface of Pd/C, accompanied by a shift in Pd 3d
5/2binding energy and a decline in catalytic activity; whereas no similar changes were observed for Pd/Mg(OH)
2after the same treatment, indicating that the electron-rich Pd surface can effectively weaken the strong adsorption of benzylamine, reducing the possibility of active site poisoning and deactivation. Reaction mechanism studies show that the Pd with higher electron density on the Pd/Mg(OH)
2catalyst not only facilitates the hydrogenation conversion of benzylimine to benzylamine, lowering the apparent activation energy of this step, but also weakens the strong adsorption of benzylamine on the Pd surface, avoiding active site poisoning and deactivation, and suppresses the condensation of benzylimine with benzylamine to form
N-benzylidenebenzylamine as a side reaction, thus achieving high activity and selectivity toward benzylamine.
In addition to carrier-induced interfacial charge effects, introducing an ultra-thin metal oxide coating layer on the active site surface can also achieve synergistic optimization of activity, selectivity, and stability by altering surface charge distribution, regulating metal exposure states, and suppressing particle loss. Wang et al.
[75]employed atomic layer deposition technology to construct ultra-thin NiO
2O
3coating layers on the surfaces of Pd/C, Pd/Al
xand commercial Pt/C catalysts for the selective hydrogenation of nitriles to synthesize secondary amines. Various characterization results indicate that NiO
xwas deposited on the surface of Pd nanoparticles in the form of approximately 1–2 monolayers, partially covering Pd sites and altering their surface reaction environment. In the hydrogenation of benzonitrile, the 3Ni-Pd/C catalyst achieved a dibenzylamine yield of up to 96% and almost completely suppressed the formation of the byproduct toluene (<1%). Meanwhile, the catalyst maintained stable activity and selectivity after six cycles. Characterization and mechanistic analysis reveal that the performance enhancement mainly stems from the triple synergistic mechanism of the ultra-thin NiO
xlayer: (1) NiO
xexhibits strong adsorption capacity for imine intermediates and primary amines, thereby promoting their condensation and improving secondary amine selectivity; (2) the ultra-thin NiO
xcoating layer can divide Pd clusters into smaller metal sites, effectively suppressing side reactions such as primary amine hydrogenolysis; (3) NiO
xacts as a protective layer to prevent metal particle loss, sintering, and the formation of metal hydrides, enhancing the structural stability and cycle life of the catalyst. Thus, the role of the coating layer is not limited to simple surface coverage but achieves multi-faceted regulation of the reaction through interfacial structure reshaping.
In addition to electronic effects, the acid-base microenvironment itself is also an important factor regulating the nitrile hydrogenation reaction pathway. Since both nitriles and amines are typical Lewis bases, they can interact with Lewis acid sites on the carrier surface, while surface hydroxyl groups on the carrier may also act as potential Brønsted acid sites participating in the adsorption process. Therefore, changes in interfacial acid-base properties will directly affect the surface enrichment state of substrates and intermediates and their subsequent transformation behavior
[70]. For example, in Ni-Re/SiO
2, Re can provide acidic sites, which is beneficial for the adsorption of the reactant 3-hydroxypropionitrile
[65]; in Ni/ZnAlO
x-600, the introduction of Zn increases the content of Lewis acid sites, which can act as electron acceptors to adsorb
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$the N atom in , thereby promoting benzonitrile activation
[66]; in the Cu-Ni/W
20O
58system, the acidic sites of the carrier are beneficial for the adsorption and stabilization of the intermediate imine
[67].
The synergistic interaction between metal sites and the acidic sites of the support further demonstrates the fine-tuning of selectivity by the interfacial acid-base microenvironment. Guan et al.
[76]employed Ni nanoparticles derived from NiAl layered double hydroxides embedded in Al
2O
3nanosheet catalysts (Ni/Al
2O
3-HT) for the catalytic hydrogenation of isophthalonitrile to prepare m-xylylenediamine. Without the addition of alkaline promoters, this catalyst achieved 100% isophthalonitrile conversion and 91.2% m-xylylenediamine selectivity at 6 MPa H
2, 90 °C, and a reaction time of 4 h; when NH
3was further introduced as a basic inhibitor, the m-xylylenediamine selectivity increased to 99%. Based on characterization and performance analysis, the researchers proposed that the reaction may proceed via the following process: isophthalonitrile and H
2molecules adsorb onto the catalyst surface under the synergistic action of Ni active sites and support acidic sites; after H
2dissociates on the Ni sites, the activated
$\mathrm{C}\equiv \mathit{ }\mathrm{N}$bonds combine with hydrogen atoms to generate imine intermediates, which are subsequently further hydrogenated to yield the target product, m-xylylenediamine. Furthermore, the Ni/Al molar ratio has a significant impact on the reaction pathway. When Ni/Al = 3:1, the catalyst surface possesses an appropriate amount of weak to medium-strong acid sites, effectively adsorbing imine intermediates and promoting the hydrogenation reaction; when Ni/Al = 2:1, excessive acid sites easily induce side reactions; whereas when Ni/Al = 4:1, insufficient acid sites fail to effectively adsorb intermediates, both leading to decreased selectivity. This indicates that acidic sites are not better in greater numbers; rather, they must match the hydrogenation capability of the metal sites to achieve optimal control of the reaction pathway.
Corresponding to the acidic microenvironment, the synergy between basic sites and the solvent provides another interfacial regulation mode for switching product selectivity. Li et al.
[77]Using a nitrogen-containing Co-MOF as a precursor, they directly pyrolyzed it under an inert atmosphere to prepare a bifunctional N-doped Co@C catalyst, Co@NC. Under alkali-free conditions, with isopropanol serving as both proton donor and solvent, they achieved controllable switching between primary amine and imine products during the transfer hydrogenation of nitriles. The most active Co@NC-900 consists of metallic Co nanoparticles uniformly dispersed or embedded within an N-doped graphitized carbon framework, along with N-derived basic sites retained from the nitrogen-containing ligands. Catalytic results indicate that at 80 °C under N
2atmosphere, when the amount of isopropanol is 1 mL, the catalyst highly selectively converts 2-methylbenzonitrile into 2-methylbenzylamine with selectivity exceeding 99%; when the isopropanol amount increases to 4 mL, the product switches to the corresponding imine with a selectivity of 95.5%. The authors suggest that the N-derived basic sites in this system not only promote proton transfer and enhance reaction conversion but also effectively control product distribution by regulating intermediate concentrations. As
Figure 10illustrates in the proposed mechanism, the reaction first involves the adsorption of nitrile and isopropanol on the catalyst surface. Subsequently, N-derived basic sites facilitate proton release from isopropanol and transfer it to the activated nitrile molecule, generating a primary imine intermediate, which can further undergo hydrogenation to form a primary amine. When the concentration of basic sites in the system is high, the reaction primarily stops at the primary amine formation stage; whereas when the isopropanol dosage increases and the relative concentration of basic sites decreases, the generated primary amine is more likely to react further with the primary imine intermediate, ultimately undergoing elimination of NH
3to yield the target imine. The above mechanism reveals that the interfacial basic microenvironment not only affects the reaction rate but also enables reversible switching of product selectivity by regulating the balance between intermediate accumulation and consumption.
Overall, the realization of interfacial field strength regulation is not the result of a single factor acting independently, but rather a comprehensive manifestation of the synergistic regulation of interfacial charge transfer, metal-support interactions, coating layer reconstruction, and acid-base microenvironments. The core of various regulation methods lies in the precise shaping of the local interfacial reaction environment. Combined with atomic coordination regulation strategies, it can be seen that interfacial field strength regulation extends reaction control from the optimization of single active sites to the overall design level of the interfacial microenvironment. This enables key reaction processes, such as active hydrogen supply, substrate activation, intermediate stabilization, and side reaction suppression, to be advanced synergistically within a unified framework. For nitrile hydrogenation reaction systems characterized by complex reaction pathways and evolutionary behaviors where product selectivity highly depends on intermediates, the interfacial field strength regulation strategy provides an effective solution path for their selectivity regulation.
3.3 Spatial geometric confinement
Spatial geometric confinement constructs physical boundaries around active sites through shell coating, channel or cavity constraints, and interlayer isolation, thereby regulating the adsorption configurations and diffusion behaviors of reactants and intermediates
[93-95]. In nitrile hydrogenation reactions, this strategy mainly manifests in two aspects: first, confined spaces can alter the surface residence time of intermediates and local collision probabilities, thereby influencing the tendency for side reactions such as condensation and self-polymerization, and ultimately enhancing target product selectivity; second, coating layers or skeleton structures can effectively restrict the migration, agglomeration, and leaching of metal nanoparticles, stabilizing particle size and the surrounding interfacial microenvironment, thus improving the cyclic stability of the catalyst
[96-98].
Sun et al.
[78]A series of carbon-encapsulated Cu@NC-ZrO catalysts were prepared by pyrolyzing the metal-organic framework PCN-222(Cu) at 500–800 °C.
2Among them, Cu@NC-ZrO obtained at a pyrolysis temperature of 600 °C
2-600 exhibited the optimal catalytic performance. Using ammonia borane as the hydrogen donor in a methanol system at 60 °C for 3 h, this catalyst achieved a 95% conversion rate of benzonitrile and 100% selectivity for benzylamine. Structural characterization results indicate that an appropriate pyrolysis temperature facilitates the formation of Cu-N
xcoordination structures, which can effectively activate ammonia borane and promote the generation of active hydrogen species, thereby accelerating the hydrogenation process; the synergistic effect between Cu-N
xsites and Cu nanoparticles further improves hydrogen activation efficiency; meanwhile, the carbon encapsulation layer formed by the pyrolysis of PCN-222(Cu) effectively prevents the agglomeration and leaching of Cu particles, maintaining the spatial distribution and structural integrity of active sites, enhancing the structural stability of the catalyst, and extending its cycle life.
Ma et al.
[79]Addressing the issue that traditional Cu-based catalysts often require the introduction of a second metal to achieve efficient nitrile hydrogenation, an amino-modified hollow mesoporous double-shell nanoreactor (AmHMSiO
2@C) was designed and prepared, and based on this, a Cu@AmHMSiO
2@C catalyst was constructed. Leveraging the confinement effect of the double-shell structure, Cu active sites are effectively encapsulated within a localized nanoreaction space, thereby forming a catalytic environment that combines activity regulation with mass transfer control. Using ammonia borane as the hydrogen source, the catalyst achieves highly selective transfer hydrogenation of benzonitrile under mild conditions of 60 °C for 0.5 h, reaching 100% benzonitrile conversion, 96% benzylamine selectivity, and a Cu TOF value of 199.75 h
-1. The excellent catalytic performance of this catalyst mainly stems from the synergistic effect between Cu nanoparticles and the AmHMSiO
2@C hollow double-shell structure. Cu nanoparticles provide electronic active sites; amino functional groups enhance the electron density on the Cu surface through electronic modulation, thereby improving substrate activation and hydrogen transfer processes; the confined space formed by the hollow double-shell structure increases the contact efficiency between substrates and active sites and shortens the residence time of intermediates on the surface, which is particularly crucial for suppressing further condensation of imine intermediates and enhancing primary amine selectivity. Furthermore, the catalyst maintains high conversion and selectivity after five recycling runs and demonstrates excellent applicability to various nitrile substrates containing electron-donating groups (—CH
3, —OCH
3) and electron-withdrawing groups (—F, —Cl). This indicates that the confined structure not only helps construct a stable local catalytic environment but also provides high adaptability for the selective transformation of substrates with different electronic properties.
Combining the above two Cu-based systems, it can be seen that spatial geometric confinement provides a design pathway for nitrile hydrogenation that is distinct from atomic coordination regulation or interfacial field enhancement regulation. The confined structure can not only influence the reaction process by controlling molecular diffusion and intermediate residence behavior, but also maintain the structural stability of active sites through encapsulation and skeleton constraints, thereby enhancing the selectivity and stability of the catalyst.
3.4 Comprehensive regulation measures
Comprehensive regulation aims to synergistically integrate multiple structural and chemical regulation dimensions into a single catalytic system, thereby effectively alleviating the inherent trade-off among activity, selectivity, and stability in nitrile hydrogenation reactions. Pure atomic coordination regulation struggles to simultaneously balance catalyst anti-deactivation capability and structural stability; while pure interfacial field strength regulation can suppress side reactions to a certain extent, it is often insufficient to significantly reduce the energy barrier of the main reaction. Meanwhile, single spatial confinement strategies are easily constrained by insufficient substrate contact area and low hydrogen supply efficiency. Therefore, the design of efficient nitrile hydrogenation catalytic systems typically requires the synergistic coupling of confinement stabilization, interfacial electronic effects, acid-base microenvironment regulation, and hydrogen source or reaction medium effects. Within comprehensive regulation approaches, the intensification of hydrogen supply, the modulation of intermediate lifetime, and the suppression of side reaction pathways can be achieved synchronously, thus obtaining more stable and highly selective conversion behavior in complex reaction processes. Compared with the aforementioned single strategies, comprehensive regulation is closer to the actual operating state of real catalytic systems and is more conducive to transitioning performance optimization toward engineering applications.
The coupling of zeolite confinement, bimetallic synergy, and basic site regulation is a typical model of multidimensional microenvironment synergistic regulation. Ma et al.
[80]reported a S-1 zeolite-encapsulated Pd-Ni bimetallic catalyst (Pd-Ni@S-1) and systematically investigated its performance using the hydrogenation of benzonitrile to benzylamine as a model reaction. The results indicate that the S-1 zeolite support effectively regulates the structure and electronic properties of the bimetallic nanoclusters, keeping the Pd-Ni nanoparticle size small and uniform; meanwhile, Ni species partially convert to Ni(OH)
2within the zeolite microporous environment, thereby introducing weak to moderate basic sites at the interface. These sites not only facilitate substrate activation but also modify the electronic structure of Pd through electronic regulation, achieving efficient synergistic catalysis. The coupling among confinement effects, bimetallic synergy, and basic sites enables the catalyst to achieve high-efficiency reactions without the need for additional alkaline additives. At 80 °C, 0.2 MPa H
2, and 2 h, the catalyst achieves highly selective hydrogenation of benzonitrile, with a benzonitrile conversion exceeding 99%, a benzylamine yield reaching 92%, and a TON value of 1667.
To simultaneously achieve high activity and high selectivity, it is equally important to incorporate the nature of the hydrogen source and the overall reaction environment into the design. Liu et al.
[81]designed Fe
3O
4@nSiO
2-NH
2-RhCu@mSiO
2core-shell-shell structured composite catalyst and applied it to the transfer hydrogenation of nitriles using formic acid (HCOOH) as the hydrogen source, achieving highly selective preparation of primary amines. In this catalyst, the Rh loading was only 49 ppm, yet the TOF reached as high as 6803 h
-1, while also exhibiting good magnetic response properties, enabling efficient recovery and maintaining no significant activity decay after 10 reuse cycles. In this system, comprehensive regulation strategies work synergistically: the RhCu alloy sites provide highly active hydrogenation centers, significantly enhancing the activation capability for cyano groups and imine intermediates; HCOOH, acting as the hydrogen donor, not only participates in the hydrogen supply process but also protonates the generated primary amines, reducing their nucleophilicity and suppressing further amination side reactions; the approximately 2 nm SiO
2mesoporous channels constructed on the outer layer of the catalyst restrict the formation of by-products through spatial confinement effects and regulate the diffusion pathways of reactants and intermediates. This triple synergistic mechanism enables the catalytic system to achieve efficient conversion under low metal loading conditions while significantly improving primary amine selectivity. Furthermore, the catalyst demonstrates good applicability to both aliphatic nitriles and aromatic nitriles containing functional groups such as amino and halogen groups, breaking through, to a certain extent, the limitations of traditional nitrile hydrogenation systems regarding low selectivity, high precious metal usage, and strong dependence on hydrogen sources.
4 Summary and Outlook
Hydrogenative amination of nitriles is an important pathway for converting basic chemical raw materials into high-value-added amine fine chemicals. This paper systematically reviews the development of heterogeneous metal catalysts from traditional macroscopic supported systems to precise regulation of atomic-scale microenvironments, and points out that microenvironment regulation strategies focusing on atomic coordination, interfacial field strength, and spatial confinement have become effective methods for enhancing the activity, selectivity, and stability of nitrile hydrogenation reactions. With the deepening of research, catalyst development in this field is gradually shifting from an experience-dominated screening mode to rational design based on structure-activity relationships. However, to further achieve research goals of verifiable mechanisms, scalable processes, and sustainable applications, it is still necessary to overcome the following core bottlenecks at both the levels of fundamental scientific cognition and engineering technology development.
(1) Evolution mechanism and operando characterization of dynamic active sites. The essence of nitrile hydrogenation is a dynamic reaction process, where active sites continuously change accompanied by processes such as metal valence evolution, reconstruction of local coordination structures, and redistribution of interfacial charge. Traditional offline characterization methods struggle to capture transient adsorption behaviors and pathway branching processes during the reaction. Future efforts need to develop in-situ and operando characterization technologies with high spatiotemporal resolution (such as time-resolved XAS, in-situ AP-XPS, and Operando IR), and combine them with isotope tracing, microkinetic modeling, and DFT calculations. This will enable real-time tracking of active site reconstruction, generation and migration of hydrogen species, and key hydrogenation pathways, thereby establishing a more reliable chain of mechanistic evidence for selectivity regulation.
(2) Establishment of quantitative prediction methods for structure-activity relationships. Although the academic community has extensively elaborated on the synergistic effects of electron-rich/electron-deficient sites, the regulation mechanisms of coordination environments on intermediates, and the suppression of side reactions by confinement effects, most related arguments remain limited to empirical explanations for specific systems. The lack of universal physical descriptors has become a major obstacle hindering quantitative predictions in this field. Future research should focus on establishing transferable quantitative models to linkdkey physical parameters such as the d-band center, coordination number, and Lewis acid/base strength directly with reaction step energy barriers and rate-determining steps. On this basis, aiming at structural differences of various substrates and requirements for target products, we should build a rational catalyst design platform oriented towards application scenarios, promoting the transformation of nitrile reduction catalyst development from the traditional trial-and-error screening mode to a prediction-driven precise creation mode.
(3) Synergistic intensification of catalyst macroscopic shaping and reaction engineering. Currently, high-performance catalysts are mostly validated in powder form and under batch reactor conditions, leaving a significant gap compared to the requirements of industrial continuous processes regarding mechanical strength, mass transfer efficiency, and long-term stability. During the catalyst shaping process, issues such as pore blockage, burial of active phases, and extended diffusion paths can easily lead to significant deviations between laboratory performance and industrial metrics. Future efforts need to focus on synergistic optimization across morphology control, interface modification, and structural reinforcement to systematically address problems caused by shaping, including loss of pore structure, reduced exposure of active sites, and increased mass transfer resistance. Meanwhile, catalyst design should be integrated with continuous-flow reaction systems; through the coordinated matching of bed structure, particle size, pore networks, and hydrogen supply methods, internal and external diffusion limitations can be alleviated, space-time yield improved, and stable operation cycles extended, thereby narrowing the gap between laboratory performance and industrial indicators.
(4) Development and process integration of green hydrogen supply systems. Compared with high-pressure hydrogen, sustainable hydrogen sources such as water, ammonia borane, and formic acid offer potential advantages in process safety and equipment cost control; however, their hydrogen supply kinetics and selectivity regulation still require further optimization. Future research could further combine microenvironment regulation strategies with efficient hydrogen transfer pathways, and promote their coupling with microchannel reactors or fixed-bed continuous flow processes to enhance the controllability, atom economy, and engineering adaptability of the nitrile reduction process.
In summary, constructing a new generation of nitrile hydrogenation catalytic systems that combine high selectivity, long-term stability, and industrial universality is a systematic engineering endeavor encompassing mechanistic understanding, material innovation, process engineering, and green energy applications. Only through collaborative innovation across the entire value chain can solid technical support be provided for the transformation of the fine chemical industry towards greater efficiency, precision, and low-carbon development.
National Natural Science Foundation of China(U24A20527)
National Natural Science Foundation of China(22508342)
Key Research and Development Program of Zhejiang Province(2023C01102)
Key Research and Development Program of Zhejiang Province(2023C01208)
Key Research and Development Program of Zhejiang Province(2024C01225)
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