CO₂ emission reduction and resource utilization are one of the major challenges in the 21st century due to an energy crisis in the foreseeable future and increasing environmental pressure [1]. As an economic, safe, and sustainable C₁ resource, the transformation of CO₂ to high value-added products through modern technology has attracted significant attention [2-9]. However, the highly symmetrical molecular structure of CO₂ and the high oxidation state of the carbon atom are responsible for its extremely stable chemical properties, making the activation and conversion of CO₂ difficult under mild conditions. Typically, the conversion of CO₂ involves three steps: adsorption, activation, and directed transformation. Therefore, an ideal catalytic material for CO₂ transformation should exhibit multifunctional characteristics and be able to adsorb, activate, and selectively convert CO₂ to the desired product. Introducing an organic ligand containing active elements is a common means of achieving the transformation [3]. By targeted interaction with the active metal center [10-16], the ligand could suppress or increase the ability of the active metal center in certain aspects to finally achieve the accurate mediation of the catalytic performance. Although good results have been achieved by the above methods, the high cost and complicated synthetic processes of the organic ligands are undesirable from the standpoint of industrial application. Therefore, exploring new multifunctional materials with excellent catalytic performances in CO₂ resource utilization is important.

Pd has been proven to be active in the reductive formylation of CO₂ with H₂ as the reducing agent [17-25], and two examples of homogeneous catalytic systems [24, 25] and seven examples of heterogeneous versions [17-23] have been reported thus far. In these systems, good product yields were obtained when aliphatic secondary amines were used as the starting materials, while only low to moderate product yields were obtained in the case of aliphatic primary amines [22, 23]. Recently, Liu and Han et al. [17-19] showed that Mg-Al layered double hydroxide (Mg-Al LDH) and N-doped carbon (NC) supported Pd (Pd/LDH and Pd/NC) as well as mesoporous imine-based organic polymer coordinated Pd (Imine-POP@Pd) were active for the amine formylation with CO₂ and H₂, although a high reaction temperature (140 ℃) and/or CO₂ pressure (3 MPa), as well as high Pd loading (8.1 wt%) were required. Based on the continuous efforts focused on the transformation of CO₂ [26, 27], our group [20] demonstrated that the hydroxyl group-regulated nano-Pd/C catalyst could catalyze the N-formylation of amines with CO₂ and H₂, although the reaction conditions were still relatively harsh and the procedure for preparing the carbon support was complicated. Therefore, developing a general heterogeneous catalyst for the amine formylation with CO₂ and H₂ is still of significance.

Palygorskite (PAL), also known as attapulgite (APT), is a naturally available, one-dimensional nanoscale hydrated magnesium aluminum silicate clay mineral with abundant zeolite-like channels, a high surface area, and moderate cation exchange capacity [28]. Besides, PAL possesses multiple types of acid and basic active sites due to the existence of interlayer water molecules, surface adsorbed oxygen, and hydroxyl groups, as well as some cations, such as Si, Al, and Mg. Therefore, PAL could provide a reaction environment containing multiple functional sites and be a promising catalyst support candidate for the preparation of multifunctional catalytic materials. Herein, we present the first example of PAL supported heterogeneous Pd catalysts for the general and efficient amine formylation with CO₂ and H₂, under mild reaction conditions (Fig. 1). Both secondary and primary amines can be converted into the corresponding formamides with good to excellent yields, at < 100 ℃ with 1 MPa CO₂.

Initially, a series of supported Pd-based catalysts with different supports were prepared by the reductive deposition method, in which the metal Pd was reduced and deposited on or in the PAL support with H₂PdCl₄ as a metal precursor and hydrazine hydrate as the reducing agent (see Supporting Information). The catalysts were characterized by different analytical techniques and tested for formamide synthesis by the reaction of piperidine with CO₂ and H₂ (Table 1). Obviously, PAL was better as a catalyst support than other inorganic oxides (entries 1–8), and 24% yield of piperidine-1-carbaldehyde was obtained in the presence of Pd/PAL catalyst (entry 1). Considering the unique structure and cation exchange capacity of PAL, we speculated that the excellent catalytic performance of the Pd/PAL catalyst might have originated from the synergistic effect of the intrinsic acid and basic sites in the PAL support and the metal Pd species inside the PAL support. Basically, the introduction of Pd species inside the PAL support might be favorable for the formation of the desired formamide. To verify the assumption, a Pd/PAL-12 catalyst was prepared by extending the ion exchanging time to 12 h. As expected, the Pd/PAL-12 catalyst exhibited excellent catalytic performance and the formamide yield was increased to 35% (entry 9). Subsequently, the effect of base additives was studied, and different bases such as KOH, NaOH, K₂CO₃, Na₂CO₃, and NaF were tested and their effects were compared (Entries 10–14). Consequently, K₂CO₃ was proven to be the most suitable base and 60% yield of piperidine-1-carbaldehyde could be obtained by introducing 50 mol% K₂CO₃ (entry 12). Relatively low yields were observed for other bases (entries 10, 11, and 13). By extending the reaction time to 24 h, the yield of piperidine-1-carbaldehyde could be increased to 90% (entry 15). The formamide product was not observed when the bare PAL support or metallic Pd⁰ was applied, which implied that the supported Pd⁰ was active for the N-formylation of amine with CO₂ and H₂ (entries 16 and 17). As a weak acidic gas, CO₂ was easily adsorbed on basic sites, while basic amine was easily adsorbed on acid sites [29, 30]. Considering the existence of multiple functional acid and basic sites on and in the PAL, the support might be mainly responsible for the adsorption of amine and CO₂. When commercial Pd/C was applied under identical reaction conditions, a low yield of formamide was obtained (entry 18).

To explore the correlation between the structure and activity, the prepared catalysts were extensively characterized. The N₂ adsorption-desorption analysis revealed that the BET surface area of the active Pd/PAL-12 catalyst was smaller than that of Pd/PAL (Table 2, entries 1 and 2), which suggested that more metal Pd migrated into the support, leading to the blockage of some pores. The Pd/SiO₂ catalyst had the largest BET surface area (entry 5), and the smallest BET surface area was obtained in the case of Pd/AlSiO_x (entry 8). Notably, the active catalyst, Pd/PAL-12, exhibited the highest average pore radius (8.4 nm, Table 2, entry 1 and Fig. S1), suggesting that the large pore size might be favorable for the formation of the desired product. The Pd contents of the prepared catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and it varied in the range of 0.33 wt%–0.40 wt% (Table 2).

The powder X-ray diffraction (XRD) patterns are shown in Fig. 2. Only the major reflections of the supports were observed in the Pd/PAL-12, Pd/PAL, and Pd/SiO₂ catalysts; no signal indicative of the Pd species was observed, which suggested that the Pd species might be highly dispersed or amorphous. Contrary to the above cases, an obvious diffraction peak at ~18° was observed for all other catalysts, which could be attributed to the formation of metallic Pd species [31]. The Pd particles were not observed in the transmission electron microscopy (TEM) and high resolution (HR)-TEM images of Pd/PAL-12, which might be due to the low metal loading or the formation of Pd species with very small size inside the support (Figs. S2a–b). A similar phenomenon was observed in the TEM image of the used Pd/PAL-12 catalyst (Figs. S2s–t) and no obvious differences were observed upon comparing the fresh and used samples.

The surface properties of the catalysts were determined by X-ray photoelectron energy spectroscopy (XPS). The deconvolution of the Pd 3d spectrum for the Pd/PAL-12 catalyst revealed two components with binding energies (BEs) at approximately 335.6 and 338 eV (Fig. 3), which were similar to those for the metallic Pd [32]. The second component with a BE of 338 eV could be ascribed to the peak for Mg KL3 auger, which could be confirmed by comparing the XPS spectra of all other Mg-containing catalysts (Fig. S3). In similarity to the case with the Pd/PAL-12 catalyst, a typical metallic Pd BE could be observed in the XPS spectra of other catalysts (Fig. S3). In other words, Pd on the surface of all catalysts existed in its metallic Pd⁰ form. Thus, it is difficult to explain the difference in catalytic performances only by the state of the surface Pd. The above speculation that some Pd might migrate into the support, might be the reason for the relatively high activity with PAL as support. Therefore, the amount of Pd in the support was evaluated by comparing the surface and bulk Pd/M (M = Mg, Al or Si) ratio of the catalysts (Table 3). Noticeably, the surface Pd/Mg ratio in the Pd/PAL-12 catalyst was much lower than that in the bulk, which meant that most of the Pd migrated into the support. This is in good agreement with the results for N₂ adsorption-desorption analysis. A similar phenomenon was observed for Pd/PAL, although it was found that the amount of Pd inside the support should be less than that in the case of the Pd/PAL-12 catalyst by comparing the two surfaces and bulk Pd/Mg ratio, which might be the reason that the Pd/PAL catalyst exhibited lower catalytic activity than that of the Pd/PAL-12 catalyst. For the other Mg-containing catalysts such as Pd/MgO, Pd/MgAlO_x, Pd/MgSiO_x, and Pd/MgAlSiO_x, more Pd species were observed in the supports.

Summarizing the above discussions, the excellent catalytic performance of the Pd/PAL-12 catalyst could be attributed to the relatively high amount of metal Pd that migrated into the PAL support. The PAL support has a unique chain-layer structure and contains multiple acid and basic sites. If the metal Pd migrates into the support, it can easily interact with more acid and basic sites in comparison to the surface metal Pd. Moreover, the local environment consisting of multiple functional sites, including metal Pd, acid, and basic sites provides favorable reaction conditions for the adsorption and activation of amine, CO₂, and H₂.

Next, the scope and limitations of the Pd/PAL-12 catalyst were explored with the reaction of different amines and CO₂/H₂ (Table 4). Cyclic secondary amines could be converted into the desired formamides in excellent yields. For example, 90%–99% yields were obtained when piperidine and its derivatives were used as the substrates (entries 1–3). Similar results were obtained in the cases of morpholine, 1-methylpiperazine, and pyrrolidine (entries 4–6). The Pd/PAL-12 catalyst is also active for the reaction of normal secondary amine and CO₂/H₂. An important industrial molecule, i.e., DMF, could be obtained in 86% yield when dimethylamine aqueous solution was used as the starting material (entry 7). A nearly identical result was obtained for dibutylamine (entry 8). The tolerance for N-benzylethanamine was also good and could be converted into the corresponding formamide in 83% yield (entry 9). Besides, primary amines with diverse structures also proved to be active substrates and could easily react with CO₂/H₂ to synthesize the desired formamides. When butan-1-amine, 2-methylpropan-1-amine, butan-2-amine, and heptan-1-amine were used as the substrates (entries 10–13), 83%–96% yields were obtained. Cyclohexanamine and cyclopentanamine were converted into the corresponding formamides in 88% and 76% yields, respectively (entries 14 and 15). When benzylamine and 4-methyl-benzylamine were applied as the starting materials, 81-86% yields of the desired products were obtained (entries 16 and 17). Notably, good yield was obtained in the case of 1-phenylethanamine (entry 18). Typically, a 90% yield of the corresponding formamide was obtained when 3-phenylpropan-1-amine reacted with CO₂/H₂ (entry 19).

Finally, the reusability of Pd/PAL-12 catalyst was tested for the reaction of piperidine with CO₂/H₂ (Table 4, Entry 1). After each reaction, the catalyst was recovered by simple centrifugation, washed, dried, and reused without further treatment. The catalyst could be recycled for 3 runs without obvious deactivation and 84% product yield could be still maintained at the 3^rd run. Further Pd leaching test was performed to demonstrate the stability and heterogeneity of the catalyst. After reacting for 5 h, the catalyst was removed, and the filtrate was analyzed by ICP-AES. As a result, no Pd was detected in the filtrate and the formamide product was not generated after removing the catalyst, which further confirmed the stability of the catalyst during the reaction and the supported Pd as the active species.

To assess our catalytic system, the catalytic performance of the Pd/PAL-12 catalyst for the N-formylation of amine with CO₂ and H₂ was compared with that of the supported Pd-based catalysts reported in the literature. Considering the wide use of morpholine as a typical starting material, the N-formylation of morpholine was used as the model reaction to evaluate the catalytic performance of the different catalysts (Table 5). The results showed that our catalyst system had the lowest Pd loading and reaction temperature, but the highest yield of formamide for the same reaction time, although a catalytic amount of base was required, which confirmed that our catalyst was more active.

To gain insights into the reaction mechanism, we traced the reaction of piperidine and CO₂ by GC-MS. The analysis results of the reaction mixtures revealed the formation of methyl formate. Based on the results and early reports [18], a possible reaction mechanism was proposed. First, CO₂ and H₂ are adsorbed and activated by the Pd/PAL catalyst, followed by the reaction with methanol as the reaction medium to in situ generate HCOOCH₃ intermediate with the aid of K₂CO₃. Subsequently, the generated HCOOCH₃ reacts with piperidine to form the targeted product.

In summary, a general and efficient supported Pd catalyst was developed for amine formylation with CO₂ and H₂. The catalyst was prepared by a simple reductive deposition method with the natural palygorskite containing multiple functional sites as support. Utilizing this catalyst, N-formylation of a series of secondary and primary amines with CO₂ and H₂ could be performed under mild conditions and good to excellent yields were obtained. The analysis by a combination of characterization techniques revealed that most of the Pd in the active Pd/PAL-12 catalyst migrated into the PAL support, and the interaction of Pd and acid/basic sites inside the support was the key to synthesizing the desired formamides. The study of the reaction mechanism revealed that the amine formylation might proceed via methyl formate intermediate pathway.