Crystal Structure and Catalytic Activity of Poly[bis(3-bromo-2-hydroxybenzaldehyde)-2-aminopyrimidinemagnesium(II)] for Hydrogenation of 1,3-Butadiene

A new six-coordinated Mn(II) coordination polymer, [Mn(L1)(L2)2]n (L1 = 2-aminopyrimidine, HL2 = 3-bromo-2hydroxybenzaldehyde) was synthesized by 3-bromo-2-hydroxybenzaldehyde, NaOH, 2-aminopyrimidine and manganese(II) acetate dihydrate. The Mn(II) coordination polymer was structural characterized by elemental analysis and single crystal X-ray diffraction. The results show that each Mn(II) ion is six-coordinated with two phenolic hydroxyl O atoms from two 3-bromo-2-hydroxybenzaldehyde ligands (O1 and O4), two formyl group O atoms from two 3-bromo-2-hydroxybenzaldehyde ligands (O2 and O3), and two N atoms from two 2-aminopyrimidine molecules (N1A and N2), and forms a distorted octahedral coordination geometry. The Mn(II) coordination polymer displays a 1D chained structure by the bridge effect of 2-aminopyrimidine N atoms. The catalytic activities of Mn(II) coordination polymer and Pd@Mn(II) coordination polymer for hydrogenation of 1,3-butadiene have been investigated. The Pd@Mn(II) coordination polymer catalyst shows the good catalytic activity and selectivity in the hydrogenation of 1,3-butadiene. The 1,3-butadiene conversion is 61.3% at 70 °C, and the selectivity to total butene is close to 100%. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
In the petrochemical industry, the thermal cracking of petroleum fractions usually produces the byproduct 1,3-butadiene (1,3-BD), which must be removed below 10 ppm for polymerization processes because they poison the catalysts and degrade the product quality [1]. The hydro-genation of 1,3-butadiene is an effective strategy to remove 1,3-butadiene in the in the petrochemical industry. Pd supported catalyst is mostly used in the selective hydrogenation of 1,3-butadiene due to the excellent catalytic activity, selectivity, and stability [2,3]. Manganese coordination polymers exhibit excellent activity in many fields, such as: catalytic property [4][5][6][7], magnetic properties [8][9][10][11], solvent adsorption [12], oxidative dehydrogenation [13], antitumor and antibacterial activities [14,15], lumines-cence properties [16][17][18][19], electrochemical property [20], and nonlinear optical property [21] and so on. So the researches on the manganese coordination polymer have attracted the attention of scientists. The synthesis of coordination polymers is influenced by ligands, metal centers, solvent, pH and reaction temperature [22]. In addition, the mixed ligands has become an impactful factor in the formation of coordination polymers.
However, there are few studies on hydrogenation of 1,3-butadiene catalyzed by manganese coordination polymers. Our research group has studied the catalytic activity of some metal complexes [23][24][25][26][27][28]. To further explore the catalytic activity of metal complexes, in this paper, a novel Mn(II) coordination polymer has b e e n s y n t h e s i z e d b y 3 -b r o m o -2h y d r o x y b e n z a l d e h y d e , N a O H , 2aminopyrimidine and manganese(II) acetate dihydrate. The structure of Mn(II) coordination polymer has been determined by elemental analysis and single crystal X-ray diffraction. The catalytic activities of Mn(II) coordination polymer and Pd@Mn(II) coordination polymer for hydrogenation of 1,3-butadiene have been investigated. The Pd nanoparticles were loaded on the surface of Mn(II) coordination polymer, and the catalytic active center of Pd can be completely exposed to the reactants of 1.0 vol%1,3-butadiene/N2 and H2. Furthermore, the catalytic activity is not affected by the size of reactants. Interestingly, the Mn(II) coordination polymer catalyst exhibits very low catalytic activity with 1,3-butadiene conversion less than 1% toward the 1,3-butadiene hydrogenation, however, the Pd@Mn(II) coordination polymer catalyst shows the higher conversion (61.3% at 70 ºC) than Mn(II) coordination polymer.

Synthesis of Pd@Mn (II) Coordination Polymer
The Mn(II) coordination polymer-supported Pd catalysts were synthesized using impregnation method according to the procedures in the literature [29]. First, 0.0074 g of Pd(CH3COO)2 were melted in 0.5 mL of ethanol. Then, the solution of Pd(CH3COO)2 was added into Mn(II) coordination polymer (0.070 g). After sonicating for 1 h, the suspension liquid was sustained at room temperature for 12 h. The final homogenous mixture was further dried at 50 °C for 7 h in air dry oven.

Crystal Structure Determination
A suitable single crystal of poly[bis(3bromo-2-hydroxyb enzaldehyde)-2-a mi no-Pyrimidinemagnesium(II)] (0.30 mm × 0.20 mm × 0.10 mm) was chosen to collect data on a Bruker Smart APEX CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K. The structure was solved by direct method using SHELXL program [30] and refined by full-matrix least squares on F 2 by means of the program OLEX2 [31]. The crystallographic data of Mn(II) coordination polymer are summarized in Table 1.

General Procedure for the Hydrogenation of 1,3-Butadiene
The selective hydrogenation of 1,3butadiene was performed in a quartz fixed bed at the atmospheric pressure. 0.020 g of the catalyst mixed with 0.480 g quartz sand (40-80 mesh), and then packed into the reactor. Prior to catalysis, the catalyst was treated for at 50 °C for 1 h in a reducing environment with the H2 (99.999 %) flow (at 10 mL/min). Then the reactants of 1.0 vol% 1,3-butadiene in 99.0 vol% N2 (13.0 mL/min) and 99.999 vol% H2 (6.5 mL/min) flowed through the Pd@Mn (II) coordination polymer catalyst bed. The effluent from the reactor was collected and analyzed using an online gas chromatography (GC-6890, Purkinje General Instrument Co., Ltd., China) equipped with a Al2O3 capillary column.

Structural Description of Mn(II) Coordination Polymer
Single-crystal X-ray diffraction shows that the Mn(II) coordination polymer 1 crystallizes in the monoclinic system with the P21/n space group. The asymmetric unit of Mn(II) coordination polymer 1 is shown in Figure 1. The selected bond lengths (Å) and angles (°) for Mn(II) coordination polymer 1 are given in Table 2. As shown in Figure 1, the asymmetric unit of Mn(II) coordination polymer is made up of one Mn(II) ion, one 2-aminopyrimidine ligand and two 3-bromo-2-hydroxybenzaldehyde ligands. Each Mn(II) ion is six-coordinated with two phenolic hydroxyl O atoms from two 3-bromo-2-hydroxybenzaldehyde ligands (O1 and O4), two formyl group O atoms from two 3-bromo-2hydroxybenzaldehyde ligands (O2 and O3), and two N atoms from two 2-aminopyrimidine molecules (N1A and N2), and forms a distorted oc-   (Mn1-N2), which are comparable to other Mn(II) coordination polymers [32,33]. The asymmetric unit of Mn(II) coordination polymer are connected by 2-aminopyrimidine to generate a 1D infinite chain ( Figure 2). Finally, the isolated 1D chains assemble an extended -stacking interactions (Figure 3). It is worth mentioning that 2-aminopyrimidine as a bridge ligand plays an important role in the formation of coordination polymer.

Synthesis Strategy of Pd@Mn(II) Coordination Polymer Catalyst
Pd@Mn(II) coordination polymer catalysts were synthesized using impregnation method. First, the solution of Pd(CH3COO)2 was added into Mn(II) coordination polymer. After sonicating for 1 h, the suspension was stored for12 h at room temperature and dried in air dry oven at 50 °C for 7 h. Finally, the catalysts were in-situ reduced at 50 °C for 1 h under H2 flow at rate of 10 mL/min before catalysis in a quartz fixed bed. Liu et al. [29] prepared Pd/MIL-101(Cr) catalysts by impregnation using Pd(CH3COO)2 as precursor, and followed by H2 reduction at 50 °C for 2 h.    peaks were obtained in the Pd 3d XPS spectra of Pd/MIL-101(Cr) at the binding energy of 341.1 eV and 335.6 eV which were attributed to Pd 0 3d3/2 and Pd 0 3d5/2, respectively [29]. Taking into account that Pd@Mn(II) coordination polymer catalysts were prepared by a similar method to that of Pd/MIL-101(Cr), we may infer that Pd in the Pd@Mn(II) coordination polymer catalysts mainly exist in the metallic palladium (Pd 0 ).

Conclusions
In summary, we synthesized a new six-