Metal–organic frameworks HKUST-1 embedded in amino-functionalized SBA-15: Efficient catalytic reduction of 4-nitrophenol to 4-aminophenol

Abstract The amino-functionalized SBA-15 silicon carrier (PMO-pr-NH2) was prepared by the one-pot method, and then HKUST-1 functional composites were synthesized by adsorption combined with in-situ preparation technology (HKUST-1@PMO-pr-NH2). A series of characterization techniques were used to analyze composites, such as BET, FTIR, TEM, XPS, TG, SEM, and XRD. The analysis showed that HKUST-1 was successfully prepared in the pores of the PMO-pr-NH2 silicon-based carrier. The catalytic reduction activity and thermal stability of the composite material were investigated using the reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). After 4 cycles, the catalytic performance of the composites was maintained without any obvious change.


Introduction
Much interest has been focused on the development of highly efficient and low-cost catalysts to degrade organic pollutants. [1]Organic pollutants in wastewater, such as 4nitrophenol (4-NP), pose considerable risks to the ecological environment.The corresponding reduction product 4-aminophenol (4-AP) is a significant fine organic chemical intermediate, which is widely used in dye and pharmaceutical industries, and can also be used in the preparation of agents, antioxidants, and petroleum additives.Therefore, the catalytic reduction of 4-NP to 4-AP is an effective strategy for solving increasing environmental problems. [2,3]ecently, metal-organic framework materials (MOF) and mesoporous silica-based materials have received much importance. [4]There are several advantages to MOFs, including high specific surface area, high porosity, open metal positions, and strong chemical modification, thus, they are extensively used in the fields of energy, catalysis, and drug release. [5,6]At the same time, the high specific surface area and a variety of functional active adsorption sites of MOF promote the reduction of 4-NP and improve the reduction efficiency. [7]Among many MOFs, HKUST-1 is one of the MOFs and is also used in the field of catalysis. [8]owever, MOFs are low in stability because most MOFs will collapse under higher temperatures and some even cannot be stable in aqueous solution or at ambient temperature. [9,10]Also, the high production cost is unfavorable for their practice applications, because expensive organic ligands are usually adopted during the synthetic process. [4,11]ecently, it has been proposed to use MOFs in combination with various functional materials to enhance their properties.MOFs can interact with a variety of materials to synthesize MOF composite materials, including metal nanoparticles, silica, oxides, polymers, graphene and carbon nanotubes.[14] Among these materials, Santa Barbara Amorphous type material (SBA), especially mesoporous silica SBA-15, are good carriers because of its high stability and structural tunability.[17] It can not only serve as a carrier for MOFs, but also improve the stabilization of the material via covalent bonding or hydrophobic interactions.However, there are many hydroxyl groups on the surface of mesoporous silica SBA-15, which limits the use of mesoporous silica SBA-15 as a carrier.Therefore, it is necessary to modify the surface of mesoporous silica SBA-15.To our knowledge, most functionalized SBA-15 have been used for adsorption or loading of metal ions, and no one has yet used aminofunctionalized SBA-15 for loading MOFs.
In our earlier research, we prepared MOF-199 matrix composites by hydrothermal treatment with carboxyl-functionalized OMS as the carrier. [18,19]Based on this method, we prepared amino functionalized SBA-15 silicon carrier (PMO-pr-NH 2 ) by one-pot co-condensation, and a new composite HKUST-1@PMO-pr-NH 2 was successfully prepared by adsorption combined with in-situ preparation technology.Subsequently, with the adsorption of Cu 2þ inside the PMO-pr-NH 2 pore, HKUST-1 was synthesized in situ inside the PMO-pr-NH 2 .The composite HKUST-1@PMOpr-NH 2 had a highly ordered two-dimensional hexagonal mesoporous structure and high specific surface area.During the reaction of 4-NP reduction to 4-AP, it also demonstrated good catalytic reduction properties and high thermal stability.

Characterization of the composite materials
In Figure 1a, we can see that the XRD patterns of the samples HKUST-1@PMO-pr-NH 2 (0.1) and HKUST-1@SBA-15 are basically consistent with those of pure HKUST-1. [20]KUST-1@PMO-pr-NH 2 (0.1) has a significantly higher characteristic peak intensity than HKUST-1@SBA-15.This is because the amino group on the surface of PMO-pr-NH 2 adsorbs more copper ions through coordination bonds, thus forming more HKUST-1.Figure 1b shows the small-angle X-ray diffraction of the sample.As displayed in the figure, the composite has three significant diffraction peaks, corresponding to the diffraction peaks on the crystal surfaces of (100), (110), and (200), showing the two-dimensional hexagonal mesopore structure as the main pore structure of the composite.It can be concluded that the ordered mesopore structure of the composite HKUST-1@PMO-pr-NH 2 (0.1) is not changed.[21] As displayed in Figure 1c, HKUST-1 exhibits type I adsorption behavior, while the adsorption-desorption isotherm of PMO-pr-NH 2 (0.1) exhibits typical type IV adsorption behavior with H1 hysteresis loop.The adsorption-desorption isotherm of HKUST-1@PMO-pr-NH 2 (0.1) shifted down significantly, but it still exhibited typical mesoporous properties.The BET surface area of PMO-pr-NH 2 (0.1) was 595 m 2 /g and the pore volume was 0.75 cm 3 /g.In addition, the BET surface area of HKUST-1@PMO-pr-NH 2 (0.1) was 1322 m 2 /g and the pore volume was 0.54 cm 3 / g (Table S1).The reduction in pore volume of HKUST-1@PMO-pr-NH 2 (0.1) indicated that the HKUST-1 had been incorporated into the pores of the PMO-pr-NH 2 (0.1).The mesopore and micropore size distributions of the samples are plotted in Figure S1 (Supplemental Materials), and the decreased of the pore volume of HKUST-1@PMO-pr-NH 2 (0.1) indicated that HKUST-1 grew in the pores of PMO-pr-NH 2 (0.1). [22,23] The FTIR spectra of HKUST-1, SBA-15, PMO-pr-NH 2 (0.1) and HKUST-1@PMO-pr-NH 2 (0.1) are shown in Figure 1d.For the PMO-pr-NH 2 (0.1) the symmetric bending vibration peak of NH can be observed at about 1510 cm −1 , whereas the emerging bands at 2937 and 2978 cm −1 are attributed to C-H stretching vibrations, indicating that amino functionalized SBA-15 has been successfully synthesized.[18] The absorption peak of HKUST-1 at 1446 cm −1 is attributed to the change of C ¼ C in the benzene ring, and the absorption peaks of 1372 and 1643 cm −1 correspond to the asymmetric and symmetric tensile vibration of C-O-C in the carboxylic acid group in the BTC, [24] and the absorption peaks at 730 cm −1 correspond to the stretching vibration of Cu-O.The absorption peaks at 1110 cm −1 and 1042 cm −1 are associated with the tensile vibration of Cu-O-Cu.[25] The infrared spectroscopy of HKUST-1@PMO-pr-NH 2 (0.1) showed the characteristic absorption peaks of PMO-pr-NH 2 (0.1) and HKUST-1, indicating the successful synthesis of both materials.[22] Figure S2 (Supplemental Materials) shows that we were studied the thermal stability of HKUST-1@PMO-pr-NH 2 (0.1) and HKUST-1.From the thermogravimetric curve, HKUST-1@PMO-pr-NH 2 composite decomposes at about 322 � C, which is higher than the homogeneous HKUST-1 decomposition temperature of 293 � C.This is due to the strong coordination between the carrier PMO-pr-NH 2 and the active center HKUST-1.[7] Figure 2a and b show SEM images of HKUST-1@PMOpr-NH 2 (0.1) and HKUST-1.The HKUST-1 is octahedral, and HKUST-1@PMO-pr-NH 2 (0.1) shows unregulated morphology, indicating that the addition of silica has some effect on its morphology growth.[1] But the smooth surface of the composite material confirms that HKUST-1 grows in the pore of PMO-pr-NH 2 , as displayed in the TEM images in Figure 2c and d.The TEM images showed that the ordered pore structure and hexagonal channels of SBA-15 remained good after the addition of HKUST-1.The corresponding elemental mapping images of HKUST-1@PMOpr-NH 2 (0.1) shown in Figure S3, it could be found that both Cu element originated from HKUST-1 and Si, N elements related to PMO-pr-NH 2 existed in the hybrid material.Furthermore, these elements were highly distributed in the composite, as revealed by the elemental mapping, suggesting that the HKUST-1 nanocrystals were well arranged around the molecular sieve.[26] The survey XPS spectra of HKUST-1@PMO-pr-NH 2 (0.1) are shown in Figure 3a. Figure 3b shows that the Cu 2p 1/2 and Cu 2p 3/2 peaks of HKUST-1@PMO-pr-NH 2 (0.1) are located around 954.3 and 934.2 eV, respectively.[27] The binding energy of HKUST-1@PMO-pr-NH 2 was slightly higher than the HKUST-1.It was mainly due to the strong coordination interaction between PMO-pr-NH 2 and HKUST-1 in the composite, resulting in the Cu 2þ central electron density changes.[22] The ICP plot of the HKUST-1@PMO-pr-NH 2 (x) is displayed in Figure S4, where x is 5%, 10%, 12.5% and 15% respectively.With increasing APTES usage, the quality fraction of N in HKUST-1@PMO-pr-NH 2 (x) increases, but the mass score of Cu first increased and then slightly decreased. Thiss because excessive Cu 2þ causes the pore blockage of PMO-pr-NH 2 , which then affects the synthesis of HKUST-1 in HKUST-1@PMO-pr-NH 2 (0.1), which is why the reduced Cu content in HKUST-1@PMO-pr-NH 2 (0.15) occurs.

Catalytic activity test
This paper used the reduction of 4-NP to 4-AP as a model experiment to investigate the catalytic activity of the HKUST-1@PMO-pr-NH 2 (x) composites.In the Supplementary Material, specific experimental procedures are described.
In general, the maximum UV-visible maximum absorption peak of the 4-NP aqueous solution occurs at a wavelength of 368 nm, However, when the excess reducing agent sodium borohydride is added to the system, the maximum absorption wavelength of the 4-NP absorption peak is redshifted to a wavelength of 401 nm, and the color of the reaction system solution gradually changes from light yellow to yellow-green.As the catalyst was added to the mixed solution, the color of the entire reaction solution gradually turned lighter.When the reduction reaction was complete, the reaction solution system became colorless and transparent.During the reaction, as the reaction time was extended, the maximum absorption peak intensity at 401 nm was gradually decreased, while a weaker absorption band appeared at 296 nm due to the formation of the reduction product 4-AP, indicating that the reduction reaction has occurred.Figure 4 is the UV-visible spectrogram.Depending on the catalyst type, the reduction reaction time varied from 4 to 10 min, while the PMO-pr-NH 2 (0.1) spectrum was unchanged.The results showed that PMO-pr-NH 2 (0.1) had no effect in response to 4-NP, and that the reduction of 4-NP was not promoted in the presence of NaBH 4 .The catalytic activity of Cu@PMO-pr-NH 2 (0.1) (PMO-pr-NH 2 treated with copper nitrate) was evaluated by reducing 4-NP to 4-AP under the same conditions.As shown in Figure S5, Cu@PMO-pr-NH 2 (0.1) also possessed certain catalytic activity, but its kinetic constant was only 0.12 min −1 .Thus, the activity of Cu@PMO-pr-NH 2 (0.1) is much lower than that of all HKUST-1@PMO-pr-NH 2 (x) series composites (kinetic constants of 0.16-0.29 min −1 ).
As shown in Figure 4a, the homogeneous HKUST-1 had the best catalytic activity and a kinetic constant of 0.36 min −1 .However, the pure HKUST-1 had instability and readily decomposition, limiting its application.In a series of different composites, the catalytic efficiency of HKUST-1@PMO-pr-NH 2 (x) increased first and then decreased with  the loading of HKUST-1.HKUST-1@PMO-pr-NH 2 (0.1) had a relatively high catalytic efficiency, with a kinetic constant of 0.29 min −1 .The kinetic constant of HKUST-1@SBA-15 was 0.12 min −1 , lower than all HKUST-1@PMO-pr-NH 2 (x) indicating that the addition of the amino group forms more HKUST-1.The kinetic constants (k) for the different samples are shown in Table 1.
The following factors affect the reaction catalytic efficiency.Firstly, with its large BET specific surface area and well-ordered mesoporous channels, PMO-pr-NH 2 could be utilized as an accelerating cavity to facilitate the transport of reactants.Secondly, the amino group of PMO-pr-NH 2 surface enhances the strong coordination interaction between mesopore HKUST-1 and SBA-15, thereby increases the active site of Cu 2þ .Thus, the in-situ growth of HKUST-1 within the vector was promoted.
The comparison of various research for reduction of 4-NP to 4-AP was list in Table 2. Taking into account the concentration of 4-NP and NaBH 4 , the dosage of catalyst and the amount of MOF supported in the composite material, HKUST-1@PMO-pr-NH 2 (0.1) has better catalytic activity than the studies reported in this literature.
To investigate the recovery and stability of the composite HKUST-1@PMO-pr-NH 2 (0.1), five cycles of the experiment were repeated under the same experimental conditions.As displayed in Figure 4I, the catalytic reduction properties of HKUST-1@PMO-pr-NH 2 (0.1) were not significantly reduced after four cycles.However, the stability of HKUST-1@PMOpr-NH 2 (0.1) in the reduction reaction can be further verified by the insignificant changes in the XRD profile back and forth the reaction (Figure S6).The reduction of activity is  HKUST-1@PMO-pr-NH 2 (0.1) 0.29 5 HKUST-1@PMO-pr-NH 2 (0.125) 0.19 6 HKUST-1@PMO-pr-NH 2 (0.15) 0.17 7 HKUST-1@SBA-15 0.13 8 Cu@PMO-pr-NH 2 (0.1) 0.12 due to degradation of its crystallinity and not to degradation of its chemical composition.The steady of catalyst before and after the reaction was studied by ICP method.As displayed in Figure S5, the HKUST-1 in the HKUST-1@PMO-pr-NH 2 (0.1) composite material was studied using the Cu elements.The quality fraction of Cu in the new catalyst was 8.05%, which remained essentially unchanged after the reaction, indicating in the catalytic reduction reaction the good stability of HKUST-1@PMO-pr-NH 2 (0.1).

Synthesis of HKUST-1, SBA-15 and PMO-pr-NH 2
HKUST-1 [32] and SBA-15 [33] were prepared from the previous literature.In the Supplementary Materials, the detailed experimental procedures are described.PMO-pr-NH 2 was synthesized by an improved one-pot co-condensation method.In addition to the following two points: Firstly, after adding TEOS, add a certain amount of sodium chloride solid and then add a specific amount of 3-aminopropyl-triethoxysilane; [34] Secondly, the template agent was removed by extraction in 380 mL ethanol solution at 80 � C for 24 h.The product is named as PMO-pr-NH 2 (x), where x is the mole percentage of APTES and TEOS.

Table 1 .
Kinetic constants of different sample.