Comparison of mesoporous fractal characteristics of silica-supported organocatalysts derived from bipyridine-proline and resultant effects on the catalytic asymmetric aldol performances

Three kinds of the bipyridine-proline chiral ligands as highly active species were successfully introduced on Zn-modified mesoporous silica nanomaterials (BMMs, MCM-41, and SBA-15) via the covalent attachment and coordination methods. Their microstructural features and physicochemical properties were extensively characterized via XRD patterns, SEM/TEM images, TGA profiles, FT-IR and UV-Vis spectra. In particular, their fractal features, the pair distance distribution function, and the Porod plots were evaluated thoroughly on the basis of the SAXS data. Meanwhile, their catalytic performances for asymmetric aldol reactions between p-nitrobenzaldehyde and cyclohexanone were evaluated. The results indicated that the bimodal mesoporous BMMs-based samples with short worm-like mesoporous channels possessed both mass and surface fractal features, whereas the MCM-41- and SBA-15-based samples with long-range ordered structures only showed surface fractal features. The influences of various reaction parameters, including the textures of the mesoporous silicas, the structures of the used chiral ligands, and the molecular volumes of aldehydes, on the catalytic activities (yield) and stereoselectivities (dr and ee) were investigated thoroughly. The results showed satisfactory activities (yields) and better stereoselectivity (dr and ee) in comparison with the homogeneous catalytic system using Z as the catalysts. In particular, the 3rd recycle catalytic performances of the Z-immobilized heterogeneous catalysts retained high catalytic yields (around 80%) and ee values of 28%. These phenomena were well interpreted by the essential relationships between the fractal characteristics of these heterogeneous catalysts and their catalytic activities.


Characterizations
The SAXS experiments were carried out using synchrotron radiation at the 1W2A Xray beamline at Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the incident X-ray was 0.154 nm. The sample-to-detector distance for SAXS was 1.59 m, calibrated with the diffraction ring of a standard sample. The scattering vector Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2022 magnitude q ranged from 0.08 to 3.05 nm -1 for the experiment reported in this paper. The intensity of scattering, I(q) was measured as a function of q, where q was considered as 4πsinθ/λ. The sample was loaded into a sample cell and sealed with scotch tape on a groove. The thickness of the sample cell was approximately 1 mm. The scattering image was collected with an exposure time of 5 min by the single-frame mode with a 'multi-read' of 2 times. The two-dimensional SAXS images were transferred to one-dimensional data by using the Fit2D software (http://www.esrf.eu/computing/scientific/FIT2D), 1 and further processed with the S program package. 2 In this study, a slit-collimated beam was used, for which the mass fractals (D m ) or surface fractals (D s ) were calculated according to the power law. Besides, the powder XRD patterns were recorded on a D6 Advance X-ray diffractometer (Beijing Pu Analysis General Instrument Co., Ltd, Beijing, China) using Cu-Kα radiation (λ = 0.154056 nm, 36 kV, 20 mA) at a scanning speed of 1 o and 4 o per min in the 1 -10 o and 10 -50 o 2θ range for the experiments, respectively. The textural properties of related samples were determined from N 2 sorption isotherms measured at 77 K using JWGB JW-BK300 analyzer. Prior to each adsorption measurement the samples were outgassed under vacuum at 353 K for at least 5 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated from the isotherm using the Barrett-Joyner-Halenda (BJH) model. Besides that, the total pore volume was determined from the N 2 adsorbed amount at a relative vapor pressure (P/P 0 ) of approximately 0.99. The SEM images were performed on a Hitachi field-emission scanning electron microscope (S-4800) with an acceleration voltage of 15.0 kV. The morphology and elemental composition of the materials were acquired with a TEM microphotograph (JEOL, JEM-2100F, Japan) coupled with EDX spectroscopy. The TGA was carried out on a PerkinElmer Simultaneous Thermal Analyzer (STA-8000) instrument from 30 to 900 o C at a heating rate of 10 o C/min under the N 2 atmosphere with a flow rate of 20 mL/min. The content of nitrogen was measured by vario MICRO cube elemental analyzer and the metal Zn concentrations were determined by ICP-OES using Optima 8300, PerkinElmer. The FT-IR spectra were measured recorded on a Bruker Tensor 2 spectrometer (Bruker Optik GmbH, Germany) via the KBr tablet method, in which the spectral resolution was 4 cm -1 , and 32 scans were recorded for each spectrum. The UVvis DR spectra were carried out in the wavelength range of 200-800 nm with a Shimadzu UV-2600 spectrophotometer (Shimadzu, Japan). The dr and ee values were determined by HPLC analyses on a Agilent Technologies 1200 system equipped with a photodiode array detector, using Chiralpak AD-H column (25 cm × 0.46 cm) by Daicel Chemical Ind., Ltd. Besides that, the chemical identity of representative chiral ligands (Z 1 , Z 2 , and Z 3 ) and all aldol products has been confirmed by nuclear magnetic resonance ( 1 H-or 13 C-NMR) in our previous report. 3       Based on the experimental results of this study and our group previous literature data, 4 major advantages for these heterogeneous catalysts (Z 1 -, or Z 2 -, or Z 3 ZnBMMs, Z 1 -, or Z 2 -, or Z 3 ZnMCM-41, and Z 1 -, or Z 2 -, or Z 3 ZnSBA-15) by comparison with that of the hybrid catalyst (Z 0 -BMMs), which were prepared by Tang et al. via grafting Z 0 onto the surface of bimodal mesoporous silicas (BMMs). Taking Z 2 ZnBMMs as an example, as shown in Table S2.

The comparison between current research and literature
In particular, for the purpose of comparison, Z 2 grafted BMMs was prepared without Zn by post treatment in this work. As can be seen in Table S2, from the perspective of their catalytic activity, Z 2 ZnBMMs as catalyst showed an excellent reusability with high activity (yield of 85%, Table S2, entries 4), however, Z-BMMs, if reused more than 2 times, showed rapid catalytic deactivation (yield from 80 to 10%, Table S2, entries 3 and 5).
In addition, on the basis of the N elemental data, Z 2 -grafted amounts could be roughly estimated in Z 2 -BMMs and Z 2 ZnBMMs, corresponding to 5.54 and 12.50 wt% (Table S2, entries 1 and 2), respectively. Obviously, further confirming that the Z 2 were successfully immobilized on the mesoporous surfaces.
In summary, we found that the immobilization of Z 2 on BMMs by coordination bonding (Table S2, entries 2) was more stable than that of hydrogen bonding without Zn (Table S2, entries 1). (2)

TON and TOF results
Where n 1 denote the moles of the product molecules, n 2 denote the moles of the involved in catalysis Z, m 1 denote the weight of the used catalyst, and t denotes the whole catalytic reaction time, respectively. The turnover frequency (TOF) and turnover number (TON) can be described in detail as Equations 1 and 2.
As shown in Table S3, taking Z 1 -, Z 2 -, Z 3 ZnBMMs-100 as an example, comparison of TON and TOF under above catalytic systems. The TOF values presented in Table  S3 almost remained the same as around 0.14 -1.54 [(gh) -1 ], and the TON values in the range of 0.50 -4.85. In addition, the aldol reaction with electron-donating substituent pmethoxybenzaldehyde as a reactant by comparison with p-nitrobenzaldehyde were carried out, but the results were not good presumably due to the inappropriate catalyst and substrate. Unfortunately, the bands of aldol products were almost not reflected on TLC plates, as shown in Fig. S6.