Facile Synthesis of COF-Supported Reduced Pd-Based Catalyst for One-Pot Reductive Amination of Aldehydes

: Dibenzylamine motifs are an important class of crucial organic compounds and are widely used in ﬁne chemical and pharmaceutical industries. The development of the efﬁcient, economical, and environmentally friendly synthesis of amines using transition metal-based heterogeneous catalysts remains both desirable and challenging. Herein, we prepared the covalent organic framework (COF)-supported heterogeneous reduced COF-supported Pd-based catalyst and used it for the one-pot reductive amination of aldehydes. There are both Pd metallic state and oxidated Pd σ + in the catalysts. Furthermore, in the presence of the reduced COF-supported Pd-based catalyst, many aromatic, aliphatic, and heterocyclic aldehydes with various functional groups substituted were converted to their corresponding amines products in good to excellent selectivity (up to 91%) under mild reaction conditions (70 ◦ C, 2 h, NH 3 , 20 bar H 2 ). This work expands the covalent organic frameworks for the material family and its support catalyst, opening up new catalytic applications in the economical, practical, and effective synthesis of secondary amines.


Introduction
Dibenzylamine motifs are presented as a class of crucial organic compounds with extensive applications in rubber compounds, constituting attractive intermediates for fine chemical industries, corrosion inhibitors, and drug formulations [1][2][3][4][5][6][7]. The development of a highly efficient selective synthesis method has attracted great attention in organic chemistry. In general, conventional methodologies for their production involve either direct base-promoted mono-N-alkylation or alkylative amination [8][9][10]. Although significant research efforts have proceeded, problems such as tedious workup process, poor product yield or selectivity, and large byproduct formation of wasteful salts need to be solved according to the environmental and sustainable issues. Reductive amination of carbonyl compounds using transition-metal catalyst has become a highly versatile and robust method for various transformations in the C-N bond construction due to its obvious advantages, such as mild reaction conditions, wide availability, and inexpensive reagents [4,7,10]. Moreover, the activation of H 2 and the catalytic reduction of unsaturated compounds are fundamentally promising environmentally friendly processes. However, in the reductive amination of benzaldehyde using H 2 and ammonia, for example, the controllable selective process to either 100% benzylamine or 100% dibenzylamine is very challenging. It is generally assumed that the reaction conversion and final product composition mainly rely on the catalyst metal component. In this respect, various transition metal systems, such as Raney Ni, Pd-based, Pt-based, etc., have been applied widely in reductive amination for the synthesis of amines [11][12][13][14][15][16]. Raney Ni was used widely and proved to be very suitable for the reductive amination of benzaldehyde with NH 3 . However, due to With the increasing problems concerning environmental and sustainable chemistry, many classes of transition metals like Pd, Rh, etc., -based heterogeneous catalysts had been developed and widely used. They showed excellent activities and selectivities for the product amine synthesis. As is well-known, the catalyst supports can control the active metal nanoparticles' size, distribution, surface electronic properties, which significantly affected the metal-supported catalyst's intrinsic activity and selectivity [19][20][21][22][23].
Covalent organic frameworks (COFs) have become one important class of the most promising porous support materials since they were first reported by Cote et al. [24]. They have been extensively studied due to their fascinating features, including low density, high and regular porosity, high stability, tunable pore size, strong covalent bonds, and easy design of functional groups. Recently, COFs were applied to construct core-shell structure composite materials (graphene, carbon nanotubes, Fe3O4, and alumina, etc.) via heterogeneously nucleating and growing on the surface of different matrices [25][26][27][28]. The nanosized core's aggregation can be effectively impeded via the incorporation of COF merits and nanosized components for core-shell structure nanocomposite synthesis. In this case, the surface modification could be more facile. Compared with other kinds of porous materials, COFs can be applied as more suitable scaffolds for fabricating core-shell structured noble nanocomposite, based on the feasibly tuned properties by the in-built covalent bond architecture. Until now, a few transition metals immobilized in COF materials have been applied in different chemical reactions such as Suzuki coupling, Knoevenagel condensation, nitroarenes reduction, and so on [29]. However, COF-based transition metal catalysts have been seldom explored for the preparation of secondary amines. Generally speaking, the catalytic transformation of primary amides to the corresponding secondary imines requires expensive amines, and the primary amines are mostly obtained from the reductive amination reactions of carbonyl compounds with NH3. So, it would be an ideal, green, and promising choice for the the secondary imines to be obtained through the direct reductive amination of aromatic aldehyde via a one-pot tandem process. Herein, a facile synthesis strategy of a Pd nanoparticles-anchored COF matrix was developed (Scheme 1d). We also evaluated its catalytic activity in the one-pot tandem reductive amination of benzaldehyde to amines. This system requires NH3 as the nitrogen source and H2 gas as the source of hydrogen. This process offered not only an atom-economical and environmentally friendly process for synthesizing amines but also Scheme 1. Reported methodologies [3,6,17] and this work for the selective production of amines via various transition metals based catalytic systems.
Covalent organic frameworks (COFs) have become one important class of the most promising porous support materials since they were first reported by Cote et al. [24]. They have been extensively studied due to their fascinating features, including low density, high and regular porosity, high stability, tunable pore size, strong covalent bonds, and easy design of functional groups. Recently, COFs were applied to construct core-shell structure composite materials (graphene, carbon nanotubes, Fe 3 O 4 , and alumina, etc.) via heterogeneously nucleating and growing on the surface of different matrices [25][26][27][28]. The nanosized core's aggregation can be effectively impeded via the incorporation of COF merits and nanosized components for core-shell structure nanocomposite synthesis. In this case, the surface modification could be more facile. Compared with other kinds of porous materials, COFs can be applied as more suitable scaffolds for fabricating core-shell structured noble nanocomposite, based on the feasibly tuned properties by the in-built covalent bond architecture. Until now, a few transition metals immobilized in COF materials have been applied in different chemical reactions such as Suzuki coupling, Knoevenagel condensation, nitroarenes reduction, and so on [29]. However, COF-based transition metal catalysts have been seldom explored for the preparation of secondary amines. Generally speaking, the catalytic transformation of primary amides to the corresponding secondary imines requires expensive amines, and the primary amines are mostly obtained from the reductive amination reactions of carbonyl compounds with NH 3 . So, it would be an ideal, green, and promising choice for the the secondary imines to be obtained through the direct reductive amination of aromatic aldehyde via a one-pot tandem process. Herein, a facile synthesis strategy of a Pd nanoparticles-anchored COF matrix was developed (Scheme 1d). We also evaluated its catalytic activity in the one-pot tandem reductive amination of benzaldehyde to amines. This system requires NH 3 as the nitrogen source and H 2 gas as the source of hydrogen. This process offered not only an atom-economical and environmentally friendly process for synthesizing amines but also minimized the workup and purification steps. What is more, this Pd/COF catalyst is also applicable for more than 20 aromatic, heterocyclic and cyclic aldehydes, and tolerates the presence of various functional groups with ortho-, meta-, and para-position substitutes under mild conditions (70 • C, 2 h, 20 bar of H 2 ).

Characterization of COF Support and Reduced COF-Supported Pd Catalyst
As shown in Figure 1a, for both SJ02 and reduced COF-supported Pd-SJ04 FTIR spectra ( Figure S4), the peaks at 1573 cm −1 (-C=N bond) in the COF support SJ02 and 1569 cm −1 (-C=N bond) in reduced COF-supported Pd-SJ04 catalyst could be detected, which confirmed the formation of a Schiff base in the COF synthesis and the COF-supported catalyst. The peaks around 1251 and 1573 cm −1 can be assigned to C-N and typical C=C stretching vibration, respectively. Powder X-ray diffraction (PXRD) was performed to obtain the insight crystalline structure of the COFs. Figure 1b shows the PXRD analysis of the spherical COFs SJ02 and reduced COF-supported Pd-SJ04 ( Figure S5). It is shown that the signals of reduced COF-supported Pd-SJ04 show no significant changes in comparison to the COF support SJ02, indicating that the palladium salt introduction does not damage the overall crystallinity and retains the framework integrity of isolated COFs SJ02. Additionally, as shown in the reduced COF-supported Pd-SJ04 catalyst, the peak at 6.4 • is derived from COF support SJ02 originally, and the relatively wide signal at around 26.3 • agrees well with the (001) reflection. This phenomenon was ascribed to the π−π stacking between the ordered adjacent layers of the COF sheets [30].
applicable for more than 20 aromatic, heterocyclic and cyclic aldehydes, and tolerates presence of various functional groups with ortho-, meta-, and para-position substitu under mild conditions (70 °C, 2 h, 20 bar of H2).

Characterization of COF Support and Reduced COF-Supported Pd Catalyst
As shown in Figure 1a, for both SJ02 and reduced COF-supported Pd-SJ04 FT spectra ( Figure S4), the peaks at 1573 cm −1 (-C=N bond) in the COF support SJ02 and 15 cm −1 (-C=N bond) in reduced COF-supported Pd-SJ04 catalyst could be detected, wh confirmed the formation of a Schiff base in the COF synthesis and the COF-suppor catalyst. The peaks around 1251 and 1573 cm −1 can be assigned to C-N and typical C stretching vibration, respectively. Powder X-ray diffraction (PXRD) was performed to tain the insight crystalline structure of the COFs. Figure 1b shows the PXRD analysis the spherical COFs SJ02 and reduced COF-supported Pd-SJ04 ( Figure S5). It is shown t the signals of reduced COF-supported Pd-SJ04 show no significant changes in compa son to the COF support SJ02, indicating that the palladium salt introduction does n damage the overall crystallinity and retains the framework integrity of isolated CO SJ02. Additionally, as shown in the reduced COF-supported Pd-SJ04 catalyst, the peak 6.4° is derived from COF support SJ02 originally, and the relatively wide signal at arou 26.3° agrees well with the (001) reflection. This phenomenon was ascribed to the π stacking between the ordered adjacent layers of the COF sheets [30]. At the same time, as illustrated in Figure 2 ( Figure S1, Figure S2, Figure S7, Table S reduced COF-supported Pd-SJ04 exhibits an irregular morphology with an average s of about 100 nm. Furthermore, the elements of reduced COF-supported Pd-SJ04 were f ther analyzed by the EDS mapping images. As depicted in Figure 2, other than C, N, a O, the Pd element is observed, suggesting that the Pd species is uniformly incorpora into the COF support SJ02 matrix. At the same time, as illustrated in Figure 2 (Figures S1,S2,S7, Table S3), reduced COFsupported Pd-SJ04 exhibits an irregular morphology with an average size of about 100 nm. Furthermore, the elements of reduced COF-supported Pd-SJ04 were further analyzed by the EDS mapping images. As depicted in Figure 2, other than C, N, and O, the Pd element is observed, suggesting that the Pd species is uniformly incorporated into the COF support SJ02 matrix. Nitrogen adsorption/desorption isotherms of the SJ02 and reduced COF-supported Pd-SJ04 are also displayed in Figure 3 ( Figure S6, Table S2). Both the COF support and reduced COF-supported Pd-SJ04 catalyst samples showed a type IV reversible adsorption isotherm curve, indicating the existence of mesopores. The Brunaue-Emmett-Teller surface area of SJ02 and reduced COF-supported Pd-SJ04 are calculated to be 559.531 and 347.076 m 2 ·g −1 , respectively. The Vpore of SJ02 and reduced COF-supported Pd-SJ04 dropped from 0.322 cm 3 g −1 to 0.254 cm 3 g −1 when incroporating of Pd in the framework, which could be ascribed to the fact that the Pd nanoparticles filled in the original pores of the COF matix. As shown in Figure 3b, based on the quenched solid density functional theory (QSDFT) model, the pore size distribution of both two samples revealed main peaks in the range of approximately 8 to 60 nm. In comparison with SJ02, the large decrease in the surface area and little change in the pore size distribution for reduced COFsupported Pd-SJ04, indicating that the presence of Pd species would not block the cavities of reduced COF-supported Pd-SJ04 but could enhance the weight significantly. This pore distribution behavior and specific surface area might make COF-supported Pd materials to be tempting options for potential catalysis applications. Nitrogen adsorption/desorption isotherms of the SJ02 and reduced COF-supported Pd-SJ04 are also displayed in Figure 3 ( Figure S6, Table S2). Both the COF support and reduced COF-supported Pd-SJ04 catalyst samples showed a type IV reversible adsorption isotherm curve, indicating the existence of mesopores. The Brunaue-Emmett-Teller surface area of SJ02 and reduced COF-supported Pd-SJ04 are calculated to be 559.531 and 347.076 m 2 ·g −1 , respectively. The V pore of SJ02 and reduced COF-supported Pd-SJ04 dropped from 0.322 cm 3 g −1 to 0.254 cm 3 g −1 when incroporating of Pd in the framework, which could be ascribed to the fact that the Pd nanoparticles filled in the original pores of the COF matix. As shown in Figure 3b, based on the quenched solid density functional theory (QSDFT) model, the pore size distribution of both two samples revealed main peaks in the range of approximately 8 to 60 nm. In comparison with SJ02, the large decrease in the surface area and little change in the pore size distribution for reduced COF-supported Pd-SJ04, indicating that the presence of Pd species would not block the cavities of reduced COF-supported Pd-SJ04 but could enhance the weight significantly. This pore distribution behavior and specific surface area might make COF-supported Pd materials to be tempting options for potential catalysis applications.
As shown in Figure 4, the XPS spectra of the COF support SJ02 and reduced COFsupported Pd-SJ04 catalyst were presented to further investigate surface structures and chemical states ( Figure S3). The binding energies were corrected for specimen charging using C1s as the reference at 284.8 eV in the XPS analysis. It is clear to see the signals of Pd3d, N1s, C1s, and O1s in the reduced COF-supported Pd-SJ04 catalyst, and the signals of N1s, C1s, and O1s in the SJ02 support. The corresponding high-resolution C 1s spectrum in Figure 4a shows two peaks at 284.8 and 285.1 eV, which matched with sp 3 -coordinated hybridized carbons and C-N bond. Furthermore, the N1s spectrums of COF support SJ02 and reduced COF-supported Pd-SJ04 catalyst can be fitted into two peaks, which belonging to sp 2 hybridized nitrogen in the form of C-N=C and the C-N bond, respectively. C=O peak at 532.4 eV can be observed in the peak-fitted O 1s XPS spectra of COF support SJ02 and reduced COF-supported Pd-SJ04 catalyst. According to the high-resolution Pd3d XPS spectrum of reduced COF-supported Pd-SJ04 catalyst, the Pd3d 5/2 was fitted into states at bonding energies of 335.8 eV and 338.4 eV, which can be ascribed to the presence of metal Pd 0 and Pd 2+ species, respectively. Moreover, the Pd3d 3/2 was also fitted into states at bonding energies of 341.6 eV and 343.8 eV, and these were ascribed to the presence of metal Pd 0 and Pd 2+ species, respectively. As is well known, Pd 0 , like Pd/C, has a specific dissociation power for hydrogenation reaction; thus, the presence of Pd σ+ could affect catalytic hydrogenolysis activity and further contribute to suppressing the over-hydrogenation reaction. As shown in Figure 4, the XPS spectra of the COF support SJ02 and reduced COFsupported Pd-SJ04 catalyst were presented to further investigate surface structures and chemical states ( Figure S3). The binding energies were corrected for specimen charging using C1s as the reference at 284.8 eV in the XPS analysis. It is clear to see the signals of Pd3d, N1s, C1s, and O1s in the reduced COF-supported Pd-SJ04 catalyst, and the signals of N1s, C1s, and O1s in the SJ02 support. The corresponding high-resolution C 1s spectrum in Figure 4a shows two peaks at 284.8 and 285.1 eV, which matched with sp 3 -coordinated hybridized carbons and C-N bond. Furthermore, the N1s spectrums of COF support SJ02 and reduced COF-supported Pd-SJ04 catalyst can be fitted into two peaks, which belonging to sp 2 hybridized nitrogen in the form of C-N=C and the C-N bond, respectively. C=O peak at 532.4 eV can be observed in the peak-fitted O 1s XPS spectra of COF support SJ02 and reduced COF-supported Pd-SJ04 catalyst. According to the highresolution Pd3d XPS spectrum of reduced COF-supported Pd-SJ04 catalyst, the Pd3d 5/2 was fitted into states at bonding energies of 335.8 eV and 338.4 eV, which can be ascribed to the presence of metal Pd 0 and Pd 2+ species, respectively. Moreover, the Pd3d 3/2 was also fitted into states at bonding energies of 341.6 eV and 343.8 eV, and these were ascribed to the presence of metal Pd 0 and Pd 2+ species, respectively. As is well known, Pd 0 , like Pd/C, has a specific dissociation power for hydrogenation reaction; thus, the presence of Pd σ+ could affect catalytic hydrogenolysis activity and further contribute to suppressing the over-hydrogenation reaction.

Reduced COF-supported Pd Catalyst Reductive Amination
With the ideal heterogeneous COF supported reduced COF-supported Pd-SJ04 catalyst in hand, we investigated its catalytic activity for the direct reductive amination of benzaldehyde using NH3. Generally speaking, H2 pressure is a key factor, which can influence the reaction selectivity for the reductive reaction. As shown in Table 1 ( Figure S9, Figure S10), in this case, when the hydrogen pressure increased from 10 to 30 bar, the yield The survey spectrum of SJ02 is also included as a reference sample.

Reduced COF-Supported Pd Catalyst Reductive Amination
With the ideal heterogeneous COF supported reduced COF-supported Pd-SJ04 catalyst in hand, we investigated its catalytic activity for the direct reductive amination of benzaldehyde using NH 3 . Generally speaking, H 2 pressure is a key factor, which can influence the reaction selectivity for the reductive reaction. As shown in Table 1 ( Figure  S9,S10), in this case, when the hydrogen pressure increased from 10 to 30 bar, the yield of dibenzylamine did not change dramatically and remained at about 80%. The reaction temperature had a great effect on the product distribution. When the reaction temperature was varied from 50, 70, 90, to 110 • C under 20 bar of hydrogen pressure, the yield of dibenzylamine increased first to 85.1% and then dropped to 72%. When the reaction was carried out over a long period of time from 2 to 12 h, this was not good for the selectivity of dibenzylamine; the product yield dropped from 83.8 to 72.3%. With the optimized reaction conditions in hand, we tested its great potential practical applicability for the reductive amination of the aldehyde with various functional substituted groups. As listed in Table 2, many different functional aromatic, heterocyclic, and cyclic aldehydes were well-tolerated under the optimized mild reaction conditions (70 • C, 2 h, 20 bar of H 2 ), which demonstrated its general applications in the field of reductive amination of aldehydes to target secondary imines ( Figures S11-S33). For the para-substituted aromatic aldehydes, when the substituent group changed from methyl, methoxyl, chloro, fluoro to amine, the reductive amination provided the desired product in good to excellent selectivity (3a to 7a). However, the substrates having amide, nitrile, and ethoxyl on the para-position were not well tolerated in this reaction condition (8a, 9a, 10a). Generally speaking, the position of the aromatic substrates in the substituted group influenced its catalytic reactivity and product selectivity. As it shown in Table 2, benzaldehydes with meta-Cl, meta-OMe, and meta-F substituent group had much higher selectivity (11a: 66%; 13a: 75%; 14a: 67%) for corresponding amine than those in the ortho-position (17a: 43%; 18a: 35%; 20a: 53%). This phenomenon could be ascribed to the synergy of electronic nature and steric effects. To our excitement, the biomass platform compounds can be converted to their corresponding amines in excellent activity and good desired product selectivity (21a: 70%; 22a: 65%). The aliphatic substrate aldehydes with either linear aldehydes or branched-chain aldehyde seemed to have substantial difficulty realizing the one-pot reductive amination and gave the desired product 24a in 29% selec- tivity, 25a in 16% selectivity, 26a in 40% selectivity, and 27a in 19% selectivity, respectively. The cyclic aldehyde 23, instead, can tolerate well and provided the desired amine product in full conversion and 67% selectivity. In conclusion, the heterogeneous COF-supported reduced COF-supported Pd-SJ04 catalyst showed promising potential results for the onepot reductive amination of aromatic, aliphatic, and heterocyclic aldehydes with various functional groups substituted, and many corresponding amines products were obtained in good to excellent selectivity under mild reaction conditions (70 • C, 2 h, NH 3 , 20 bar H 2 ).
A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH 3 and H 2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As depicted in Scheme 2, the starting material benzaldehyde was firstly reacted with one molecule of ammonia and dehydrated to form the reactive intermediates benzylimine under the reduced COF-supported Pd-SJ04 catalyst. During the reaction process, we did not observe the benzylimine product since it is very reactive in the reaction and prone to hydrogenate rapidly to 1c. As was reported in the previous literature [31], H 2 and NH 3 adsorb completely on the same active metal sites and affect the reductive amination process dramatically. Moreover, a variation of the ratio for Pd 2+ and Pd 0 species based on the XPS characterization analysis could greatly affect the hydrogenation and amination process. It was no wonder that the Pd 0 species also facilitated the 1d formation and 1a formation. Since there is a competitive process between 1c and 1d formation, precise controlling of the active sites of Pd 2+ and Pd 0 species and its responding reactivity for selective amination process as well as reductive amination to form 1c and reductive hydrogenation to form 1d is very important. In the meantime, the 1c was further reacted with another molecule of benzaldehyde and dehydrated to yield 1b over reduced COF-supported Pd-SJ04 catalyst. Then, the 1b was further hydrogenated to the desired product 1a using the reduced COF-supported Pd-SJ04 catalyst, mainly relying on the Pd 0 species under H 2 . All in all, this indicated that a suitable amount of NH 3 , H 2 , and the precise regulation of Pd 2+ and Pd 0 species for the reduced COF-supported Pd-SJ04 catalyst, were of great significance for the high selectively transformation and efficiency conversion to the desired product, dibenzylimine. A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As depicted in Scheme 2, the starting material benzaldehyde was firstly reacted with one molecule of ammonia and dehydrated to form the reactive intermediates benzylimine under the reduced COF-supported Pd-SJ04 catalyst. During the reaction process, we did not observe the benzylimine product since it is very reactive in the reaction and prone to hydrogenate rapidly to 1c. As was reported in the previous literature [31], H2 and NH3 adsorb completely on the same active metal sites and affect the reductive amination process dramatically. Moreover, a variation of the ratio for Pd 2+ and Pd 0 species based on the XPS characterization analysis could greatly affect the hydrogenation and amination process. It was no wonder that the Pd 0 species also facilitated the 1d formation and 1a formation. Since there is a competitive process between 1c and 1d formation, precise controlling of the active sites of Pd 2+ and Pd 0 species and its responding reactivity for selective amination process as well as reductive amination to form 1c and reductive hydrogenation to form 1d is very important. In the meantime, the 1c was further reacted with another molecule of benzaldehyde and dehydrated to yield 1b over reduced COF-supported Pd-SJ04 catalyst. Then, the 1b was further hydrogenated to the desired product 1a using the reduced COF-supported Pd-SJ04 catalyst, mainly relying on the Pd 0 species under H2. All in all, this indicated that a suitable amount of NH3, H2, and the precise regulation of Pd 2+ and Pd 0 species for the reduced COF-supported Pd-SJ04 catalyst, were of great significance for the high selectively transformation and efficiency conversion to the desired product, dibenzylimine.                  A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As de- A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As de- A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As depicted in Scheme 2, the starting material benzaldehyde was firstly reacted with one molecule of ammonia and dehydrated to form the reactive intermediates benzylimine under A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As depicted in Scheme 2, the starting material benzaldehyde was firstly reacted with one molecule of ammonia and dehydrated to form the reactive intermediates benzylimine under A possible catalytic reaction pathway for the benzaldehyde to dibenzylamine with NH3 and H2 gas has been proposed according to the results from the one-pot reductive amination of benzaldehyde and compared with the previously reported literature. As depicted in Scheme 2, the starting material benzaldehyde was firstly reacted with one molecule of ammonia and dehydrated to form the reactive intermediates benzylimine under qualitative and quantitative analysis of materials, especially qualitative analysis of the functional group characteristic absorption frequency. IR spectra were recorded on a Bruker Tensor 27 spectrometer with dried KBr (Berlin, Germany). The N 2 adsorption-desorption isotherms were measured with a Tristar 3010 isothermal nitrogen sorption analyzer (Micromeritics, Florida, USA) at 77 K after the samples were degassed in a vacuum at 120 • C for 10 h.

General Procedure for the Catalytic Reductive Amination Reaction
The reductive amination of aldehydes was carried out in a stainless-steel autoclave reactor (Anhui Kemi Machinery Technology Co., LTD, China) with six channels. In a standard run for the catalytic activity test of this reduced COF-supported Pd catalyst Pd-SJ04, aldehyde substance (0.5 mmol), reduced COF-supported Pd-SJ04 catalyst (5 mg), and 5 mL 2.0 mol/L ethanol solution of ammonia were added into the reactor with a Teflon coated stir bar. The reactor was sealed and then purged three times with hydrogen, followed by filling with H 2 to 1-3 MPa and heating to the target temperature of 50 to 110 • C and stirred vigorously for a settled time (2 to 12 h). We cooled down the reaction autoclave to room temperature and separated the catalyst by a 0.22-µm filtering membrane. The mixture was analyzed by 1 H NMR spectra spectroscopy or GC. The yield of the reaction was studied by the internal standard method (1, 3, 5 trimethoxybenzene as an internal standard).

Conclusions
In summary, we have designed and successfully synthesized the COF-supported material and the COF-supported heterogeneous reduced COF-supported Pd-SJ04 catalyst. The latter showed high reactivity and selectivity of amines for the facile and efficient one-pot reductive amination from aldehydes for the first time. The Pd nanoparticles were anchored on the COF supports successfully. It was found that both the metallic state of Pd and oxidated Pd σ+ existed in the catalysts based on the XPS analysis. In particular, the reduced COF-supported Pd-SJ04 catalyst could catalyze many aromatic, aliphatic, and heterocyclic aldehydes with various functional groups substituted to their corresponding amines products in good to excellent selectivity under mild reaction conditions (70 • C, 2 h, NH 3 , 20 bar H 2 ). This work makes the synthesis of secondary amine more economical and practical through the one-pot reductive amination process, since there are generally many undesired products, such as phenylmethanol, phenylmethanamine, and (E)-N-benzylidene-1-phenylmethanamine, occurring in the reductive amination of benzaldehyde. Moreover, further studies are in progress to design the more stable and precise regulation of Pd-based catalyst and investigate the detailed mechanism of COF-supported Pd catalyzed reductive amination.