Amino functionalized SBA-15 Loaded Rh Catalyst for Asymmetric Hydroformylation of Styrene

. A recoverable Rh/N-SBA-15 heterogeneous catalyst for asymmetric hydroformylation of styrene was prepared by post grafting method. XRD, TG, 29 Si MAS NMR and ICP characterization, revealed that the introduction of amino silane and RhCl 3 had no adverse influence on the ordered mesoporous structure of the support. The Rh/N-SBA-15 catalyst was proved to be highly efficacious and stable for asymmetric hydroformylation of styrene. The experimental results demonstrated that amino silane improved the loading capacity of SBA-15 to RhCl 3 , and thereby improved its catalytic performance. The high efficiency and recyclability of Rh/N-SBA-15 catalyst demonstrated its great potential for application in asymmetric hydroformylation reactions.


Introduction
Asymmetric carbonylation of olefins is one of the most common methods to prepare optical activated carboxylic acids and aldehyde compounds. These chiral carboxylic acids, aldehydes, esters, and their derivatives are important chemical intermediates in the fields of organic synthesis, pharmaceuticals, materials, fine chemicals [1] . Asymmetric hydroformylation reaction (AHF) is an efficient and direct synthesis method for preparing chiral aldehydes, and is also one of the most challenging asymmetric catalytic reactions. The AHF reaction has been studied for more than forty years, many catalysts with high enantioselectivity, chemoselectivity and region-selectivity have been developed [2] . Although the catalysts exhibited excellent catalytic performance in homogeneous AHF reactions, its industrial application was hindered by the shortcomings of easy loss of active components, difficulty in separation and recovery of the catalytic systems .Loading noble metal catalytic species into the pores of porous materials has been proven to be one of the most common strategies for improving the recyclability of noble metal catalysts [3] . Mesoporous molecular sieves have become excellent supports for heterogeneous catalysts due to their high specific surface area, ordered mesoporous structure, tunable surface properties and great thermal stability [4] .
In this work, an amino-functionalized Rh/N-SBA-15 heterogeneous catalyst was successfully prepared by post grafting method (Scheme 1). The catalyst showed high activity and recyclability in the AHF of styrene. Besides, it was found that the coordination bond formed by amino groups and Rh stabilized the grafting of Rh on the SBA-15.

Synthesis
SBA-15 was prepared according to the previous literature report [5] . The functionalized SBA-15-supported Rh heterogeneous catalyst Rh/N-SBA-15 was synthesized starting from mesoporous silica SBA-15, 3aminopropyltrimethoxysilane (APTS), and RhCl 3 . Firstly, 0.5 g of SBA-15 and 0.25 g of APTS were added to 30 mL toluene, and then heated to reflux at 110 °C for 12 h. The obtained white powder was amino-functionalized N-SBA-15. 12 mg of RhCl 3 •nH 2 O was added to 15 mL of toluene suspension containing 0.1 g N-SBA-15, and then stirred overnight at room temperature. Finally, the obtained yellow solid was named Rh/N-SBA-15. In order to prove that the grafted amino silane can immobilize RhCl 3 more firmly on the support, a nonfunctionalized control sample was synthesized in the same way. The resulting sample was named Rh/SBA-15.

Scheme 2. AHF of styrene catalyzed by Rh/N-SBA-15
The activity of Rh/Imi-SBA-15 catalyst was evaluated by AHF of styrene as a probe reaction (Scheme 2). 0.1 g of catalyst, 1.0 g of styrene and (R)-BINAP chiral ligand were mixed with 10 mL of toluene solvent in a stainless steel autoclave. The reactor was flushed with syngas (CO/H 2 ) three times and pressurized to 2.0 MPa, followed by heating to 100 ℃ for 22 h. The mixture was separated by centrifugation after reaction, samples of the supernatant were analyzed by GC-MS, and the e.e. of chiral aldehyde was determined by automatic polarimeter. The effects of APTS modification and Rh loading on the structural framework of carrier were studied by small-angle XRD. As shown in Figure. 1a, the peak of pure SBA-15 appeared at 1.1° corresponded to the (100) reflection, indicating that SBA-15 has an ordered mesoporous channel structure [6] . In addition, compared with SBA-15, the (100) diffraction peak of N-SBA-15 and Rh/N-SBA-15 had shifted slightly, indicating that a slight local adjustment occurred after grafting.

Characterization
Direct evidence of the immobilization of APTS to the carrier was provided by 29 Si MAS NMR. As shown in Figure.  Obviously, the peak intensities of Q 2 and Q 3 site in SBA-15 sample were all higher than N-SBA-15 sample, indicating that massive Si-OH groups were consumed during the grafting process of APTS. Furthermore, two additional resonance signals at around -61 and -69 ppm appeared in the N-SBA-15 sample, which were ascribed to T 2 [RSi(OSi) 2 (OR)] and T 3 [RSi(OSi) 3 ] silica centers [7] . The appearance of these two new peaks also indicated that APTS was successfully grafted on the SBA-15. Valence bond composition of all samples was analyzed by FT-IR. As shown in Figure. 2, the peak at 3439 cm -1 was assigned to the -OH of silanol of SBA-15. The peaks at 1083 and 801 cm -1 arose from the vibrations of Si-O-Si groups. And the peak at 465 cm -1 corresponded to the bending vibration peak of Si-O groups. These absorption peaks were all derived from SBA-15. Moreover, a new absorption peak at 1650 cm -1 was in the spectra of N-SBA-15 and Rh/N-SBA-15 samples, which was assigned to N-H bond in the -NH 2 groups [8] . It indicated that APTS has been successfully grafted on the SBA-15.
The thermal stability of Rh/N-SBA-15 catalyst was illustrated by TG and DTG analysis. As displayed in Figure. 3, in the first step, a weight loss of 4.8 % at 30-230 ℃ was attributed to the desorption of H 2 O. In the second step, 19.77 % weight loss at 230-740 ℃ was ascribed to decomposition of APTS [9] .  As can be seen from the trend in the Figure. 4a, when the molar ratio was 1:2.4, the conversion, yield and e.e. reached the highest peaks (Xsty=100%, Yiso=89%, e.e.=52%). It illustrated that Rh needed to bind with an appropriate amount of chiral ligand (R)-BINAP to form the active component chiral bisphosphine complex [HRhBINAPCO] [10] . However, excessive chiral ligands will form larger molecular weight chiral Rh-P complexes with Rh, which will increase the steric hindrance of the reaction. Therefore, the optimal Rh/P molar ratio was 1:2.4. Figure. 4b showed the effect of different temperatures on the AHF of styrene. When the temperature was rose to 100 °C, Rh/N-SBA-15 exhibited higher activity (Conversion=100%, Yield=89%, e.e.=52%). Because the dissociation of CO was a key step in the AHF reaction, and required a higher temperature. But excessive temperature easily led to the racemization of the chiral aldehyde [11] .

Recyclability of Rh/N-SBA-15 catalyst
Under optimal reaction conditions (T=100 °C, P=2 MPa, Pd/S=1:210, Rh/P=1:2.4, 10 mL toluene as solvent, t=22 h), the stability of Rh/N-SBA-15 catalyst was tested. As shown in Figure. 5, after the Rh/N-SBA-15 catalyst was recycled seven times, it still maintained a high activity, the yield of isomerized aldehyde remained at 74 %, and the e.e. also remained at about 35%, indicating that the Rh/N-SBA-15 catalyst had good recyclability. Subsequently, the Rh content of catalysts before and after reaction was detected by ICP analysis. It was found that after 7 times of recycling, the Rh content decreased by 1.1 %, indicating that after the recycling reaction some rhodium would fall off from the carrier.

Conclusions
In conclusion, an amino-modified Rh/N-SBA-15 heterogeneous catalyst was prepared by post grafting method and applied to the asymmetric hydroformylation of styrene. Under the conditions of 100 ℃, 2.0 MPa, and 22 h, Rh/N-SBA-15 showed excellent catalytic performance for AHF reaction of styrene (89 % yield and 52 % e.e.). After repeated use for 7 times, Rh/Imi-SBA-15 still retained a high activity. Compared with Rh/SBA-15, the Rh/N-SBA-15 catalyst exhibited superior activity and stability, which indicated that the amino silane enhanced the loading capacity of the catalyst. It was suggested that the Rh/N-SBA-15 catalyst had immense potential for AHF of styrene.