Selective Hydrodeoxygenation of Lignin‐Derived Vanillin via Hetero‐Structured High‐Entropy Alloy/Oxide Catalysts

The chemoselective hydrodeoxygenation of natural lignocellulosic materials plays a crucial role in converting biomass into value‐added chemicals. Yet their complex molecular structures often require multiple active sites synergy for effective activation and achieving high chemoselectivity. Herein, it is reported that a high‐entropy alloy (HEA) on high‐entropy oxide (HEO) hetero‐structured catalyst for highly active, chemoselective, and robust vanillin hydrodeoxygenation. The heterogenous HEA/HEO catalysts were prepared by thermal reduction of senary HEOs (NiZnCuFeAlZrOx), where exsolvable metals (e.g., Ni, Zn, Cu) in situ emerged and formed randomly dispersed HEA nanoparticles anchoring on the HEO matrix. This catalyst exhibits excellent catalytic performance: 100% conversion of vanillin and 95% selectivity toward high‐value 2‐methyl‐4 methoxy phenol at low temperature of 120 °C, which were attributed to the synergistic effect among HEO matrix (with abundant oxygen vacancies), anchored HEA nanoparticles (having excellent hydrogenolysis capability), and their intimate hetero‐interfaces (showing strong electron transferring effect). Therefore, our work reported the successful construction of HEA/HEO heterogeneous catalysts and their superior multifunctionality in biomass conversion, which could shed light on catalyst design for many important reactions that are complex and require multifunctional active sites.


Introduction
Lignin, a three-dimensional polymer with abundant methoxyphenylpropane units as shown in Scheme 1, is the second largest renewable biomass and often can be converted into value-added chemicals.However, lignin pyrolysis generally leads to products with high-oxygen content; if they are employed as biofuels, they have drawbacks such as low-energy density, unstable combustion, and excessive corrosion.Selective hydrodeoxygenation (SHDO) provides a potential route to remove partial oxygen from lignin and retains the benzene ring structure for obtaining value-added aromatics products. [1,2]or example, vanillin (VAN) is a typical derivative of lignin depolymerization or pyrolysis, [4,6] and its SHDO product 2-methoxy-4-methyl phenol (MMP) is an important intermediate in the synthesis of value-added drugs and fragrances. [5]Nevertheless, undesired byproducts like p-methylcyclohexane or vanillyl alcohol are inevitably generated due to over-hydrogenation or insufficient hydrogenation.8] Multimetals catalysts, such as the combination of transition metals (Ni, Co, Fe, Mo, W) and their oxides, were widely utilized in selective hydrogenation reactions [9] due to their unique coordination of different metals that leads to high-activity similar to the noble metal catalysts but with a much lower cost. [10]Particularly, high-entropy alloy (HEA) catalysts, a novel type of multielement materials (usually ≥5), have attracted a great amount of attention in recent years. [11]The unique multielement mixing leads to broadened and nearly continuous adsorption spectrum in HEA catalysts, which have the potential to obtain desired surface properties for various reactions, particularly complex ones. [12]Similarly, HEO also attracts lots of attention for its unique high-entropy structure and unexpected properties like excellent thermal stability [13,14] and has found applications in electronics, catalysis, and energy storage. [13,15,16]owever, the harmonious combination of HEA and HEO together to tackle complex catalytic reactions is seldom reported, largely due to the challenges in combing so many elements into yet different structures precisely at the nanoscale.
The challenge comes from the fact that HEA is often prepared in a reductive atmosphere, while HEO preparation requires an oxidative atmosphere.A novel strategy is thus imperative for obtaining the heterogeneous and harmonious integration of the HEA/HEO catalysts.Dragos [17] provided a potential route for combining metal and oxide catalysts, where they proposed an exsolution mechanism to obtain catalysts with metallic Ni particles embedded in the The chemoselective hydrodeoxygenation of natural lignocellulosic materials plays a crucial role in converting biomass into value-added chemicals.Yet their complex molecular structures often require multiple active sites synergy for effective activation and achieving high chemoselectivity.Herein, it is reported that a high-entropy alloy (HEA) on high-entropy oxide (HEO) hetero-structured catalyst for highly active, chemoselective, and robust vanillin hydrodeoxygenation.The heterogenous HEA/HEO catalysts were prepared by thermal reduction of senary HEOs (NiZnCuFeAlZrO x ), where exsolvable metals (e.g., Ni, Zn, Cu) in situ emerged and formed randomly dispersed HEA nanoparticles anchoring on the HEO matrix.This catalyst exhibits excellent catalytic performance: 100% conversion of vanillin and 95% selectivity toward high-value 2-methyl-4 methoxy phenol at low temperature of 120 °C, which were attributed to the synergistic effect among HEO matrix (with abundant oxygen vacancies), anchored HEA nanoparticles (having excellent hydrogenolysis capability), and their intimate hetero-interfaces (showing strong electron transferring effect).Therefore, our work reported the successful construction of HEA/HEO heterogeneous catalysts and their superior multifunctionality in biomass conversion, which could shed light on catalyst design for many important reactions that are complex and require multifunctional active sites.multimetal perovskite oxide.The in situ exsolved Ni particles are not only more uniformly dispersed but also partly immersed (or socketed) into the surface of the host oxide, thus making the exsolved particles considerably better anchored and more resistant to degradation.So far, several reports have confirmed the effectiveness of the exsolution strategy in synthesizing metal/oxide catalysts and improving the stability of the catalyst in different reactions. [3,18]et, HEA/HEO catalysts are still rarely reported.
Inspired by the exsolution mechanism, herein, we report heterostructured HEA on HEO catalysts and their highly active, chemoselective, and robust catalysis toward vanillin hydrodeoxygenation.We first prepared a porous HEO matrix (NiZnCuFeAlZrO x ) via a double hydrolysis process coupled with the multicomponent replacement.Then, in situ, exsolution strategy was employed to obtain the heterogeneous HEA/HEO catalysts (NiZnCuFeAl/NiZnCuFeAlZrO x ).The effect of thermal reduction temperatures on the composition and structure of HEA/ HEO catalysts was investigated and optimized.The HEA/HEO catalyst exhibits excellent catalytic performance toward vanillin hydrodeoxygenation (100% conversion and 95% MMP selectivity), whose origin was also analyzed systematically.We found that the superior catalytic performance is a result of the synergistic effect among the HEO matrix (with abundant oxygen vacancies), anchored HEA nanoparticles (having excellent hydrogenolysis capability), and their intimate heterointerface (showing a stronger electron transferring effect).This work can inspire the future design of multielement catalysts with a heterogenous structure toward many important and complex reactions.

Synthesis of the High-Entropy Hetero-Structured Catalyst
Firstly, the porous HEO was synthesized through double hydrolysis coupled with the multiple-replaced method as illustrated in Scheme 2. Typically, an organic-inorganic double hydrolysis process was carried out to obtain a highly dispersed nickel hydroxide precipitation.Subsequently, the other nitrate solutions (Zn 2+ , Cu 2+ , Al 3+ , Fe 3+ , Zr 4+ ) were added into the suspension successively to replace the Ni 2+ and form multiple-metals hydroxides (NiZnCuAlFeZr(OH) x ). [19]NiZnCuFeAlZ-rO x was obtained after 600 °C calcination in air.After the exsolution, the sample NiZnCuFeAlZrO x was transformed to NiZnCuFeAl/NiZnCu-FeAlZrO x , which shows that many particles exsolved out and evenly anchored on the NiZnCuFeAlZrOx.Then, the high-entropy hetero-structured catalyst was prepared.

Composition and Structure Characterization of Catalyst
To analyze the composition and structure high-entropy hetero-structured catalyst, the prepared NiZnCuFeAlZrO x was investigated first.As shown in Figure 1a, the energy-dispersive X-ray (EDX) elemental mapping image exhibits that Ni, Zn, Cu, Fe, Al, Zr, and O elements dispersed in the oxide uniformly.Moreover, the X-ray diffraction (XRD) (Figure S1, Supporting Information) of NiZnCuFeAlZrO x shows the typical peaks of spinel (CSD#80-0072), indicating that the oxide belongs to the spinel structure.BrunauerÀEmmettÀTeller specific surface area test results (Figure S2, Supporting Information) further exhibit that the adsorption isotherm of the NiZnCuFeAlZrO x belongs to type IV along with type H4 hysteresis loop, indicating that the NiZnCuFeAlZrO x has narrow fracture pores, which are mostly found in the adsorption materials mixed with micropores and mesopores. [20]Further analysis of the pore size distribution confirms the hierarchical porous structures, the average pore size of micropores and mesopores is 1.2 and 5.6 nm, respectively.Thus, the prepared NiZnCuFeAlZrO x is a kind of spineltype HEO with hierarchical porous structures. [21]hen above HEO sample was thermally reduced at 400 °C to prepare exsolved HEO (i.e., HEA/HEO) whose EDX elemental mapping was shown in Figure 1b.Obviously, many particles exsolved out and evenly anchored on the HEO after exsolution.In addition, the elements Ni, Zn, Cu, Fe, and Al are also assembled on the exsolved particle, indicating that these particles consist of the five metals. [22]The area was further observed via high-resolution transmission electron microscopy (HRTEM) as shown in Figure 1c.The detected d-spacings of the exsolved particle and the HEO were 0.20 and 0.19 nm, respectively, which proves the exsolved particle has its crystal structure and the interacted HEO exhibits (331) plane.These results reveal that the exsolution strategy successfully promoted the formation of the hetero-structure, confirming the design is in effect.
Since the exsolution strategy successfully achieved the synthesis of hetero-structure catalysts, several high-entropy catalysts were prepared at 400 °C (Re400), 500 °C (Re500), and 600 °C (Re600), respectively.The composition of high-entropy catalysts was analyzed via XRD.As shown in Figure 2f, a peak shift from 43 to 44°can be observed after exsolution, while the peaks around 29 and 52°were enhanced gradually (from Re400 to Re600).These new peaks at 44 and 52°were a good match with the alloy FeNi (CSD #65-3244)/CuNi (CSD #65-9047)/ AlNi (CSD #50-1294), which is also consistent with the HEA features reported in several previous works. [23]Moreover, the exsolved particles of each catalyst were analyzed via EDX mapping (Figure S3, Supporting Information), which further proved that these exsolved particles were composed of the five exsolvable metals (Ni, Cu, Fe, Zn, and Al). [24]hese results confirm that the exsolution particles existed as HEA. [23]esides the feature peak of HEA, another peak at 29°observed in Re600 shows a good match with ZrAlOx (CSD #65-9047), which suggests that the HEO may convert to ZrAlOx after suffering a dynamic evolution along with most of the metal exsolution.Due to the exsolution temperature may also induce a dynamic variety to the exsolved HEA, STEM-HAADF images were applied in determining the size distributions of exsolved HEA particles (Figure 2a-d).A modest increase in average Scheme 1.The model structure of lignin.
Energy Environ.Mater.2024, 7, e12638 particle size from 10 AE 2 nm (Re400) to 50 AE 5 nm (Re600) was observed after exsolution temperatures increased from 400 to 600 °C, which also showed consistent with the results calculated from XRD, providing a whole field investigation of the sample compared to TEM analysis.These results confirm that the exsolution temperature is a significant parameter in controlling the exsolution extent, while high temperature would cause notable agglomeration of the exsolved HEA.
XPS results show the presence of Ni, Cu, Fe, Zn, Al, and Zr signals in HEO and the high-entropy catalysts (Figure 2g).[27][28] Moreover, due to the gap between Zn 2+ and Zn 0 being <0.2 eV, thus the observation that a peak shifting from 1021.25 to 1021.35 ev in the Zn 2p spectrum (Figure S6e, Supporting Information) proves the conversion from Zn 2+ to Zn 0 . [29]These results provide direct evidence to prove these metals exsolution.By contrast, in the case of the Zr 3d spectrum (Figure S6f, Supporting Information), the local intensity maximum was changed from 182.0 to 182.3 eV (ZrO 2 ). [30]The shifting from sub-oxides to oxides of Zr reveals that the high-entropy stable status was broken with many metals exsolving out, which may also decrease the number of oxygen vacancies introduced by an unsaturated oxygen coordination environment.In comprehension, the exsolution strategy is feasible in obtaining a high-entropy catalyst from HEO.However, the essence of the exsolution of the highentropy catalysts is a system entropy-decreasing process driven by temperature, therefore, the extent of exsolution can be controlled by exsolution temperature.As shown in Figure 2e, although the higher temperature can promote more HEA exsolved out from HEO, the aggregation of HEA and the irreversible transformation from HEO to ZrAlOx are unavoidable.Thus, proper temperature is significant to obtain a clearer and purity HEA/HEO hetero-structure catalyst.

Evaluation of Catalytic Performance
Vanillin selective hydrogenation reaction was selected as a probe reaction to investigate the performance of the synthesized high-entropy catalysts.For comparison, these catalysts were tested at 100, 120, 140, and 180 °C SHDO experiments (Figure S8, Supporting Information), respectively.At the reaction temperature of 100 °C, the catalysts Re-500 and Re600 show poor catalytic activity on VAN conversion, while Re400 still obtained 90% vanillin conversion with 23.4% and 76.6% selectivity of HMP and MMP, respectively.For the Re500 and Re600, it is evident that the accumulated HMP did not be further converted, while the formed HMP inhibited the conversion of VAN conversely.Thus, a synergy catalytic effect in the selective hydrogenation reaction is significant in converting VAN to MMP. [31] Higher reaction temperature further enhanced the catalytic activity (Figure 3b).Even though Re500 and Re600 have obtained 68.7% and 59.5% MMP yield at 120 °C, Re400 obtained 100% VAN conversion and 95% MMP selectivity, which has been higher than parts of noble metal catalysts reported in the previous literature (Table 1).
As the reaction temperature increased to 180 °C (Figure S8, Supporting Information), the hydrodeoxygenation products of MMP p-methyl phenol and p-methyl cyclohexanol were detected in all three catalysts.p-methylphenol is the methoxy removal product of MMP and p-methylcyclohexanol is the ring-opening product of pmethylphenol. [32]Thus, the three catalysts also exhibited their high-temperature catalytic potential.Due to its excellent performance, the Re400 catalyst was further utilized in the vanillin hydrodeoxygenation cycling test for evaluating the stability of the high-entropy catalyst.Notably, Re400 exhibited excellent stability, in which the catalytic performance was almost consistent during the stability test even after five cycles (Figure 3c).In addition, the similar XRD patterns (Figure S9, Supporting Information) between fresh and life test samples further proved the stability of the catalyst.
Figure 3d illustrates the conversions of VAN and the selectivity toward HMP and MMP as a function of reaction time over Re400, Re500, and Re600, respectively.All samples show a short-term accumulation stage of HMP before obtaining MMP, while the intermediate accumulation is a typical feature of the tandem reaction.So the hydrogenation (VAN?HMP) and deoxidation (HMP?MMP) steps constitute a whole reaction from VAN to MMP. [39] Since the accumulation stage of HMP is different in these catalysts, illustrating the catalysts may exhibit various activities in each step of the tandem reaction.Therefore, the activation energy (Ea) of the HMP and VAN conversion reaction with these catalysts was calculated via Arrhenius plots (Figure S10, Supporting Information), respectively.Figure 3e shows that Re500 exhibited the lowest Ea in the VAN conversion, as the Ea of three catalysts followed the order Re600 > Re500 > Re400 in the HMP conversion, revealing that more exsolved HEA may be a benefit for the first hydrogenation step, as the further conversion of HMP was limited.Therefore, although a higher exsolution temperature can promote the HEA formation to accelerate the hydrogenation step, the rate-limiting step was also changed to the next deoxidation step at the same time.Moreover, these combined observations also suggest that Re400 may be a potential hydrogenation catalyst at high temperatures.Therefore, the catalyst Re400 was compared with commercial 5% Pd/C in the lignin decomposition reaction.Figure 3f shows that Re400 exhibited higher selectivity to the aromatic monomer without any ringopening products being detected, indicating its higher selectivity for the Aromatic products.The distribution of the products also reveals that Re400 exhibited activity in cleaving the b-O-4 bond, as the hydroxyls of the branched chain are also removed partly (Figure 3g), which inhibits the possibility of further polycondensation. [9]

Activity Characterization and Reaction Mechanism 9
The previous analysis has proved that the exsolved HEA plays an important role in the catalytic reaction of VAN.Since HEA can be oxidized in the air at high temperatures, thus TGA was employed to investigate its exsolution extent, as the increased mass after oxidation can reflect the HEA content of these high-entropy catalysts.As shown in Figure 4a, the results showed that the oxidation weight gain of Re400, Re500, and Re600 was 3.3%, 7.5%, and 10.0%, respectively.Meanwhile, to explore the up-limit of the exsolution amount, catalyst Re800 was prepared and detected, and its oxidation weight gain was 12.0%.Taking Re800 as a complete exsolution state, it could be calculated that the exsolution extent of Re400, Re500, and Re600 would be 27.5%, 62.5%, and 83.5%, respectively.These results indicate the exsolution extent can be controlled via temperature, while a higher exsolution temperature would promote more metals to exsolve out, resulting in a higher exsolution extent.
Oxygen vacancy is the important adsorption site for most oxygenated reactants, so the evolutionary power reactor (ERP) was employed to investigate the oxygen vacancies distribution of the high-entropy catalysts.As shown in Figure 4b, all catalysts show a peak at g = 2.0025, which is derived from the resonance absorption peak induced by electron capture by oxygen vacancies, [40][41][42] indicating that oxygen vacancy existed in all of the catalysts.However, the number of oxygen vacancies in these catalysts decreased gradually with the increase in exsolution temperature, whose change trend is coincident with the valence change of the metals observed in XPS.As HEO holds all the metal in the structure via a high-entropy system, each metal would compete for coordination oxygen, inducing the oxygen coordination of most metals that may be defective.After most metal exsolved, the system entropy decreased, and the residue metal could obtain sufficient coordination oxygen.Therefore, the HEO shows the highest oxygen vacancies concentration, while the ZrAlO x exhibits little oxygen vacancies.The result is also consistent with the relative content of lattice oxygen in these materials as investigated in XPS O1s (Figure S11, Supporting Information).In addition, the oxygen specie distribution of the high-entropy catalysts was further tested by H 2 -TPR.As shown in Figure S12, Supporting Information, The high-temperature reduction peak (>400 °C), which belongs to lattice oxygen, [43] increased from 417.9 to 700. 4  °C with the increase in exsolution temperature, the low-temperature hydrogen reduction peak (<300 °C) attributed to oxygen-vacancy adsorbed oxygen showed the same variation trend. [44,45]However, the intensity of both peaks decreased, indicating the surface oxygen specie was diminished after exsolution.
After the adsorption site exploration, adsorption studies were employed to reveal the adsorption behaviors of these high-entropy catalysts.The H 2 adsorption was tested via H 2 -TPD and shown in Figure 4c, whereas the hydrogen desorption peaks of the catalysts were divided into low-temperature desorption peak (<220 °C), moderatetemperature desorption peak (<520 °C), and high-temperature desorption peak (≥520 °C).Yan et al. [46] demonstrated that the lowtemperature desorption peak could be attributed to hydrogen species weakly chemisorbed on the surface of metal species.Alberto et al. [47] suggested that moderate-temperature desorption peaks belong to the spillover hydrogen that is activated via metals and chemisorbed on the support.It's worth noting HEO exhibited a broad hydrogen desorption peak from 300 to 800 °C without active metals, which is different from most catalysts.Therefore, the result was further verified via H 2 -TPD-MS (Figure S13, Supporting Information) and excluded the possibility that the broad desorption peak was the signal of H 2 O derived from HEO reduction.So, the low-temperature desorption peak is associated with exsolution HEA, and the high-temperature desorption peak can be attributed to HEO for its complex electronic structure and multielement coordination environment.Then moderate-temperature desorption peak can be attributed to the spillover of hydrogen species from HEA to support.Notably, the moderate-temperature desorption peaks in the Re400 and Re600 curves show different temperatures, illustrating the support before and after evolution may obtain different adsorption strengths to the spillover hydrogen.HEO exhibited stronger adsorption strength to the spillover hydrogen than ZrAlO x for its higher desorption temperature.Since the spilled-over hydrogen migrates and readily reacts with the yellow WO 3 to form dark blue HxWO 3. Therefore, WO 3 as an indicator was mixed with Re400 physically to conduct the VAN hydrodeoxygenation reaction to investigate whether activated H* was spilled to the support.As seen in Figure S14, Supporting Information, the WO 3 alone in the reaction system exhibited an unchanged white color under the same hydrogenation conditions.In contrast, WO 3 was reduced visibly as the mixture solution of catalyst and substrate changed its color into bluish gray, indicating the occurrence of hydrogen spillover.
Although the exsolved HEA endowed the catalyst with hydrogen dissociation and transfer function, the adsorption of the reactant is also significant for the reaction.UV-vis diffuse reflectance absorption spectra were employed to investigate the adsorption behaviors of catalysts to the reactants.As shown in Figure 4d, both HEO and Re400 show a higher adsorption tendency for vanillin than Re500 and Re600.Therefore, we inferred that the efficient adsorption of the C=O group in the vanillin primarily occurred on the OVs due to the higher oxygen vacancies concentration in HEO and Re400.In the HMP adsorption test (Figure S15a, Supporting Information), the adsorption trend of HMP was the same as the VAN, while the adsorption gap was smaller between the different catalysts.Moreover, there was no observable adsorption in all these catalysts during the MMP adsorption test (Figure S15b, Supporting Information).The result means that the target product MMP desorbs easier, leading to the effective re-exposure of the active sites.
Overall, the chemical adsorption properties seem to be the same variety trend as the adsorption site.The relationship between the adsorption of reactant (VAN, HMP, and H 2 ) with the exsolution extent (HEA content) and EPR signal (OVs concentration) was summarized, respectively.As shown in Figure S16a, Supporting Information, the adsorption of H 2 shows a positive relationship with the HEA content, while the VAN and HMP adsorption shows a positive relationship with the OVs concentration.Therefore, the chemical adsorption properties of the high-entropy catalysts show evolution with the structure evolution as shown in Figure S16b, Supporting Information.During the HEO suffering the evolution from NiZnCuFeAlZrO x to ZrA-lO x at the exsolution, the exsolved metals grew to bigger particles, as the surface oxygen vacancies decreased notably.As a result, although the activated H 2 was sufficient, the reactant adsorption was limited by the diminished oxygen vacancies.Therefore, the effect of HEA, HEO, and the hetero-structure form a synergistic system, which is necessary to achieve high-catalytic activity.

Discussion
Considering all the experimental and characterization data presented in this study, it is possible to summarize that the excellent activity of the highentropy catalyst can be attributed to the hydrogenolysis ability derived from HEA, the transferring effect provided by the hetero-interface, and the activation and adsorption function of HEO.However, the adsorption of hydrogen and reactant showed a contrary variety trend as the exsolution extent increased.Thus, controlling the exsolution at a proper level is necessary to obtain a suitable reaction environment with enough activated hydrogen and reactant.As previously mentioned, although the introduction of the exsolution strategy induced the evolution of the HEO in structure and composition, the basic reason for the HEO evolution should be attributed to the variety of entropy raised by the reduction.Before the evolution, reduction promotes the migration of the surface and lattice oxygen, which directly decreases the system entropy of the HEO and starts the evolution.Firstly, the metals with high electronegativity may migrate and exsolve out to the surface because of the gradually diminished O restriction and broken stable high-entropy system.As the reduction extent went to a deepen level, more lattice O migrated to the surface, while the vacancy provides space for more metal exsolving, resulting in the other metal with lower electronegativity exsolving and forming HEA with the metal migrated before.Since Al and Zr show the lowest electronegativity and less coordinated oxygen environment in oxygen tetrahedron units, therefore, the two metals formed a more stable M-O bond, resulting in the formation of ZrAlO x .Based on this mechanism, controlling the system entropy in an appropriate range through temperature can directly limit the exsolution extent to achieve a suitable HEA/HEO heterostructure.
Based on the analysis results, the reaction mechanism of vanillin hydrodeoxygenation under the high-entropy catalysis system was demonstrated in Figure 5.The high-entropy hetero-structure catalyst hetero-structure catalyst exhibited a complete reaction system, the effect of HEA, HEO, and the hetero-interface were combined harmoniously through proper exsolution strategy.As the results of H 2 -TPD and UV-vis, H 2 and Vanillin were absorbed and activated by HEA and HEO, respectively.Moreover, TEM results showed the hetero-structure obtained an obvious interface structure between HEA and HEO, while the interface provides a high-speed channel for the hydrogen protons, which were activated on the HEA to spill over to HEO.For the adsorbed Vanillin molecule, the O of C=O may be adsorbed on the OVs.Because of the interaction from different sites, the C=O would be activated and react with hydrogen protons.The similar pathway further promotes the C-O dissociation to obtain the aim product MMP.

Conclusion
In summary, the (NiZnCuFeAlZrOx) HEO with porous and spinel structure was prepared via double hydrolysis coupled multiple displacement method.The introduction of an exsolution strategy further helps to synthesize the HEA/HEO hetero-structure catalyst.The XRD, XPS, and TEM characterization results exhibit that the exsolution can be controlled via temperature.As the temperature increases, HEO would suffer a structure evolution, in which its porous structure would disappear gradually along with its main phase changing from NiZnCuFeAlZrOx to ZrAlOx.Meanwhile, the chemical adsorption properties would change correspondingly, the hydrogen adsorption would be enhanced with more HEA exsolved out, while the adsorption of oxygen species would decrease.After comparison, 400 °C has been considered as a suitable exsolution temperature.The catalyst Re400 not only kept the abundant oxygen vacancies of HEO but also obtained excellent hydrogen dissociation capacity from the exsolution HEA.The proper hydrogen dissociation capacity and oxygen vacancies concentration result in its excellent catalytic performance in the low temperature (120 °C) vanillin hydrodeoxygenation reaction (100% VAN conversion and 95% MMP selectivity).This high-entropy catalysts preparing strategy may inspire many unique performance catalysts soon.
The preparation of HEO: A solution of NaOH (4 g) in 100 mL DI was introduced to a beaker containing 12.4 g guaiacol.The mixture was stirred at 300 rpm for 30 min before the solution of Ni (NO 3 ) 2 (14.5 g) in 50 mL DI was dropped into the dark brown mixture solution.The suspension solution was stirred for 30 min before the precipitates were washed and filtered to obtain organic Ni(OH) 2 precursor.Next, all of the organic Ni(OH) 2 precursor was dispersed in 100 mL DI before the other five nitrate solutions were orderly introduced dropwise.(a. 25  The mixed solution was stirred for 30 min before the precipitates were washed and dried to yield an organic multielement hydroxide precursor.Finally, the sample was thermally treated in a tube furnace at 600 °C with a heating rate of 10 °C min À1 for 1 h in a stream of air to yield HEO. The preparation of exsolution high-entropy catalyst: The exsolution highentropy catalyst was prepared via HEO reduction.Typically, 0.5 g HEO was settled in a quartz tube (25 cm 9 Φ1.5 cm (wall = 1 mm)), and each side of the HEO was filled with quartz cotton to fix the HEO sample in the middle of the quartz tube.The quartz tube was purged with 10%H 2 /90%N 2 gas for 10 min before it was thermally treated at the aimed temperature (400, 500, and 600 °C) with a heating rate of 10 °C min À1 for 1 h to yield these catalysts reduced at 400 °C (Re400), 500 °C (Re500), and 600 °C (Re600).
Catalytic reaction: In total, 50 mg catalysts, 20 mL isopropanol, and 50 mmol VAN were mixed in a stainless-steel batch reactor before the reaction.Typically, the experiment was carried out at aim temperature with a stirring These experiments of Re400, Re500, Re600 were carried out under conditions using catalyst 50 mg with 20 mL Isopropanol [37,38] and 50 mg Vanillin.
Energy Environ.Mater.2024, 7, e12638 speed of 400 rpm under 2 MPa H 2 pressure for 4 h.The cycles test was the same as the normal reaction, while the spent catalyst was washed (three times with isopropyl alcohol) and recycled after each experiment.

Scheme 2 .
Scheme 2. The synthesis procedure of hetero-structured high-entropy catalyst.

Figure 2 .
Figure 2. The TEM patterns of a) HEO, and its reduced sample, b) Re400, c) Re500, d) Re600, e) structure evolution of the HEO exsolution process, f) the XRD patterns, and g) XPS analysis results of these fresh catalysts.

Figure 3 .
Figure 3. a) Reaction pathway from VAN to MMP, b) Products distribution of different catalysts at 120/140 °C, c) The stability test of Re400, d) Time-dependent the related content of VAN, HMP, and MMP in hydrodeoxygenation of vanillin at 140 °C with different catalysts (2Mpa H 2 pressure, 50 mg catalyst, 20 mL Isopropanol and 50 mg Vanillin), e) The E a of the different catalysts in VAN and HMP converting reaction, f) The product distribution of the lignin decomposition experiment, and g) its conversion scheme.

Figure 4 .
Figure 4. a) The TGA (air) analysis of high-entropy catalysts, b) The Oxygen vacancies analysis EPR, c) The surface H 2 adsorption analysis of high-entropy catalysts with H 2 -TPD, d) UV-vis diffuse reflectance absorption spectra of VAN, and e) The scheme of the chemical adsorption properties evolution with the structure evolution.

Figure 5 .
Figure 5.The SHDO reaction mechanism of the exsolution high-entropy catalyst.

Table 1 .
Summary of the HDO reactions of vanillin over various types of catalysts. a)