In Situ Growth of ZIF-8 Nanocrystals on the Pore Walls of 3D Ordered Macroporous TiO2 for a One-Pot Cascade Reaction

It is wise to mimic a bioinspired system to design a nanoreactor as a catalyst containing multiple components for a cascade reaction. Here, we report the uniform growth of well-dispersed nano-scale ZIF-8 crystals on the pore walls of 3DOM TiO2 via the TEA-assisted crystallization process. The UV-vis spectra indicate that the ZIF-8 photosensitizer can extend the visible-light absorption of 3DOM TiO2. The obtained nanoreactor can efficiently catalyze the one-pot aromatic alcohol oxidization and Knoevenagel condensation cascade reaction for larger molecules. This work offers an important strategy for preparing semiconductor–MOF multifunctional composites with a spatially separated compartmentation for the cascade reaction.


Introduction
As a special class of crystalline microporous materials, the metal-organic framework (MOF) is a promising candidate for creating a composite heterogeneous catalyst system due to its thermal stability, ultrahigh surface area, tunable pore size, and versatile architectures [1][2][3][4]. The diverse chemical and structural properties of MOF make them attractive in the field of catalysis [5,6]. A type of MOF, known as zeolitic imidazolate frameworks (ZIFs), has attracted much attention since their initial discovery [7]. However, there have been some major hurdles to their implementation. First, the micropores of bulk ZIF-8 make it difficult for the macromolecules to contact its active sites, although the ZIF-8 nanocrystal (NC), which can expose more active sites, is a dominant catalyst to improve the catalytic efficiency [8]. However, ZIF-8 NCs are difficult to employ in practical systems [9]. Furthermore, pure ZIF-8 exhibits a single function that impedes the use of their full potential [10]. The above hurdles significantly limit the handling, operation, and processing of ZIF-8 materials for wider application. Therefore, it is important to develop a new method for the integration of ZIF-8 with a porous support matrix, in which ZIF-8 NCs are selectively and uniformly formed and protected in a continuous macroporous material. Specifically, the integration of ZIF-8 NCs and the macroporous material can remarkably promote the mass transfer for bulky-molecule-involved reactions and expose inside active sites [11]. Moreover, the ZIF-8 composites not only can combine the advantages but also mitigate the shortcomings of both components [12].
Furthermore, multifunctional heterogeneous catalysts for a one-pot cascade reaction have been recently widely researched in selective organic synthesis due to their eco-friendly, energy sustainable, and easily recycled characters [13]. The cascade reaction can complete the synthesis of multiple new bonds in one reactor without separating intermediates. In a short time, it can synthesize complex organic molecules in a highly atomic and economical manner [14,15]. Moreover, in some reaction systems, the protection-deprotection process and purification process of the intermediate are omitted [16,17]. MOFs consist of compounds based on metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures that generate sites to anchor the catalysts [18][19][20]. Allowing metal nanoparticles to embed in MOF, achieving a catalytic cascade reaction, is a common strategy. Wang and co-workers recently reported a composite Au@Cu(II)-MOF catalyst that can gradually promote the benzyl alcohol oxidization and Knoevenagel condensation reactions [21]. Our group has designed a bifunctional catalyst integrated with a yolk-shell structure (IY-SO 3 H/Rh@S-ZIF-8) to accomplish cascade reaction, which provides an innovative idea for preparing MOF composites with a hierarchical framework [11]. Therefore, driven by the need to continuously optimize the properties of composite materials, particular efforts are devoted to the design and formation of special structures rather than random mixtures [22]. Artificial nanodevices that are rationally designed will facilitate many cascade reactions that occur simultaneously with excellent specificity and efficiency.
Recently, research on semiconductor nanostructures/MOF composites are growing promptly; however, the research of hybrid photocatalysts is still in the preliminary stage [23]. TiO 2 /ZIF-8 composites have been comprehensively reviewed as promising photocatalysts [24]. It has been proved that the properties of the ZIF-8 composite are better than that of the single component due to the synergistic effect of the semiconductor and ZIF-8 [23]. The charge transfer induced by the synergistic effect can appear between photoexcited inorganic TiO 2 and ZIF-8, which considerably overcome the electron-hole recombination in the TiO 2 and supplies long lifetime electrons for photocatalytic reaction [24].
In this work, a cascade catalyst that integrated ZIF-8 NCs on the pore walls of the three-dimensional ordered macroporous (3DOM) TiO 2 was constructed (ZIF-8@3DOM TiO 2 ), which is utilized for the one-pot aromatic alcohol oxidization-Knoevenagel condensation cascade reaction by illumination at room temperature. The metal and ligand salts of ZIF-8 were sequentially impregnated in the pore of 3DOM TiO 2 , and then ZIF-8 NCs were in situ grown on the pore walls of 3DOM TiO 2 through the triethylamine (TEA)-assisted crystallization process (Scheme 1). The obtained structure has nano-sized ZIF-8 NCs, monolithically interconnected macropores, and an integrated micro-nano structure. The ZIF-8 NCs located on the pore walls of 3DOM TiO 2 can improve the visible-light absorbance of TiO 2 to efficiently promote the photocatalytic oxidation of aromatic alcohols to aromatic aldehydes by 3DOM TiO 2 . Meanwhile, the ZIF-8 NCs can expose the active sites extremely well, which can then efficiently continue to exhibit high reactivity in the Knoevenagel condensation reaction of aromatic aldehydes at room temperature. Moreover, the monolithically ordered macropores can not only enhance the light absorption to improve their photoreaction efficiency and the integrated micro-nano structure, but also have excellent cycle stabilization.
Catalysts 2021, 11,533 2 of 14 economical manner [14,15]. Moreover, in some reaction systems, the protection-deprotection process and purification process of the intermediate are omitted [16,17]. MOFs consist of compounds based on metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures that generate sites to anchor the catalysts [18][19][20].
Allowing metal nanoparticles to embed in MOF, achieving a catalytic cascade reaction, is a common strategy. Wang and co-workers recently reported a composite Au@Cu(II)-MOF catalyst that can gradually promote the benzyl alcohol oxidization and Knoevenagel condensation reactions [21]. Our group has designed a bifunctional catalyst integrated with a yolk-shell structure (IY-SO3H/Rh@S-ZIF-8) to accomplish cascade reaction, which provides an innovative idea for preparing MOF composites with a hierarchical framework [11]. Therefore, driven by the need to continuously optimize the properties of composite materials, particular efforts are devoted to the design and formation of special structures rather than random mixtures [22]. Artificial nanodevices that are rationally designed will facilitate many cascade reactions that occur simultaneously with excellent specificity and efficiency.
Recently, research on semiconductor nanostructures/MOF composites are growing promptly; however, the research of hybrid photocatalysts is still in the preliminary stage [23]. TiO2/ZIF-8 composites have been comprehensively reviewed as promising photocatalysts [24]. It has been proved that the properties of the ZIF-8 composite are better than that of the single component due to the synergistic effect of the semiconductor and ZIF-8 [23]. The charge transfer induced by the synergistic effect can appear between photoexcited inorganic TiO2 and ZIF-8, which considerably overcome the electron-hole recombination in the TiO2 and supplies long lifetime electrons for photocatalytic reaction [24].
In this work, a cascade catalyst that integrated ZIF-8 NCs on the pore walls of the three-dimensional ordered macroporous (3DOM) TiO2 was constructed (ZIF-8@3DOM TiO2), which is utilized for the one-pot aromatic alcohol oxidization-Knoevenagel condensation cascade reaction by illumination at room temperature. The metal and ligand salts of ZIF-8 were sequentially impregnated in the pore of 3DOM TiO2, and then ZIF-8 NCs were in situ grown on the pore walls of 3DOM TiO2 through the triethylamine (TEA)assisted crystallization process (Scheme 1). The obtained structure has nano-sized ZIF-8 NCs, monolithically interconnected macropores, and an integrated micro-nano structure. The ZIF-8 NCs located on the pore walls of 3DOM TiO2 can improve the visible-light absorbance of TiO2 to efficiently promote the photocatalytic oxidation of aromatic alcohols to aromatic aldehydes by 3DOM TiO2. Meanwhile, the ZIF-8 NCs can expose the active sites extremely well, which can then efficiently continue to exhibit high reactivity in the Knoevenagel condensation reaction of aromatic aldehydes at room temperature. Moreover, the monolithically ordered macropores can not only enhance the light absorption to improve their photoreaction efficiency and the integrated micro-nano structure, but also have excellent cycle stabilization.

Characterization of the Microscopic Morphology of Catalyst
The overall synthesis process of ZIF-8@3DOM TiO 2 is shown in Scheme 1. First, the monodisperse CLPS microspheres with a diameter of around 210 nm were synthesized by emulsion polymerization. Subsequently, the highly ordered opal was formed through a self-assembled process of the CLPS microspheres by centrifuge at 2500 rpm (Figure 1a), which was used as a hard template for making a 3D interconnected network. The average macropore size of 3DOM TiO 2 is approximately 150 ± 5 nm (Figure 1b). Compared with the diameter of the CLPS microspheres, the macropores of 3DOM TiO 2 are smaller due to calcination. The periodic macroporous structure of 3DOM TiO 2 restricts the propagation of light at certain energies, causing stopband reflection due to coherent Bragg diffraction and increasing the path length of light through the slow proton effect, thus improving photocatalytic activity [25]. Besides, the interconnected macroporous structure of 3DOM TiO 2 facilitates the immobilization and dispersion of ZIF-8 NCs and reduces mass transfer limitations. Then, the Zn(NO 3 ) 2 ·6H 2 O and 2-MeIM of the ZIF-8 precursor were successively filled into the macropores of the 3DOM TiO 2 and further crystallized with the assistance of TEA vapor. The ZIF-8 NCs were gestated on the pore walls of the macropores without any need for surface modification of the pore walls. In Figure 1c, it can be observed that a lot of ZIF-8 NCs (about 20 nm, inset of Figure 1c) are scattered on the pore walls of 3DOM TiO 2 , which are not full of the macropores nor blocking the connected windows. The void space between the nested NCs and the TiO 2 skeleton can accommodate guest macromolecules participating in the cascade catalytic reaction. Furthermore, the multifunctional composite not only combines the merits but also mitigates the shortcomings of the components. On one hand, the ZIF-8 NCs scattered on the pore walls of 3DOM TiO 2 can improve the visible-light absorbance of 3DOM TiO 2 , which is typically a UV-active photocatalyst. On the other hand, the size of the ZIF-8 NCs anchored onto the 3DOM TiO 2 walls is much smaller compared to the bulk ZIF-8 (Figure 1d, larger crystals and heavily aggregated), which is more conducive to exposing more active sites and increasing the contact area between ZIF-8 and the reaction substrate. In addition, the in situ growth of ZIF-8 NCs on the 3DOM TiO 2 walls can avoid the problem of an uneven ZIF-8 distribution.

Characterization of the Microscopic Morphology of Catalyst
The overall synthesis process of ZIF-8@3DOM TiO2 is shown in Scheme 1. First, the monodisperse CLPS microspheres with a diameter of around 210 nm were synthesized by emulsion polymerization. Subsequently, the highly ordered opal was formed through a self-assembled process of the CLPS microspheres by centrifuge at 2500 rpm (Figure 1a), which was used as a hard template for making a 3D interconnected network. The average macropore size of 3DOM TiO2 is approximately 150 ± 5 nm (Figure 1b). Compared with the diameter of the CLPS microspheres, the macropores of 3DOM TiO2 are smaller due to calcination. The periodic macroporous structure of 3DOM TiO2 restricts the propagation of light at certain energies, causing stopband reflection due to coherent Bragg diffraction and increasing the path length of light through the slow proton effect, thus improving photocatalytic activity [25]. Besides, the interconnected macroporous structure of 3DOM TiO2 facilitates the immobilization and dispersion of ZIF-8 NCs and reduces mass transfer limitations. Then, the Zn(NO3)2·6H2O and 2-MeIM of the ZIF-8 precursor were successively filled into the macropores of the 3DOM TiO2 and further crystallized with the assistance of TEA vapor. The ZIF-8 NCs were gestated on the pore walls of the macropores without any need for surface modification of the pore walls. In Figure 1c, it can be observed that a lot of ZIF-8 NCs (about 20 nm, inset of Figure 1c) are scattered on the pore walls of 3DOM TiO2, which are not full of the macropores nor blocking the connected windows. The void space between the nested NCs and the TiO2 skeleton can accommodate guest macromolecules participating in the cascade catalytic reaction. Furthermore, the multifunctional composite not only combines the merits but also mitigates the shortcomings of the components. On one hand, the ZIF-8 NCs scattered on the pore walls of 3DOM TiO2 can improve the visible-light absorbance of 3DOM TiO2, which is typically a UV-active photocatalyst. On the other hand, the size of the ZIF-8 NCs anchored onto the 3DOM TiO2 walls is much smaller compared to the bulk ZIF-8 ( Figure 1d, larger crystals and heavily aggregated), which is more conducive to exposing more active sites and increasing the contact area between ZIF-8 and the reaction substrate. In addition, the in situ growth of ZIF-8 NCs on the 3DOM TiO2 walls can avoid the problem of an uneven ZIF-8 distribution.

Characterization of the Catalyst Composition
The compositions of the materials were analyzed via FT-IR, XPS, the Raman spectrum, elemental analysis, and zeta potential, seen in Table 1, Figure 2 and Figure S3. The FT-IR spectra of CLPS CCT, 3DOM TiO 2 , and ZIF-8@3DOM TiO 2 are demonstrated in Figure 2a.
In the spectrum of the CLPS CCT, the peaks in 1470, 765, and 700 cm −1 are characteristic absorption peaks of CLPS. In the spectrum of 3DOM TiO 2 , the characteristic Ti-O-Ti peak in the range of 400-800 cm −1 can be observed. In comparison, the peaks in the spectrum of ZIF-8@3DOM TiO 2 at 1578, 1145, 750, and 1387 cm −1 are assigned to the imidazole group [26]. In addition, the peak at 423 cm −1 is related to the Zn-N stretch mode. The peak at around 675 cm −1 is associated with the Ti-O-Ti bond in TiO 2 . In Figure 2b, the XPS wide-scan spectrum shows that ZIF-8@3DOM TiO 2 contains Ti, Zn, N, C, and O elements. The spectrum of Ti 2p depicts two bands with the binding energies of 457.9 and 463.6 eV assigned to Ti 2p 3/2 and 2p 1/2 , respectively (Figure 2c), which correspond to Ti 4+ in a tetragonal structure such as anatase titania [27]. In Figure 2d, the peaks located at 1021.8 and 1044.9 eV are assigned to the Zn 2p 3/2 and 2p 1/2 orbitals, respectively. The N 1s region consists of three peaks, as shown in Figure 2e. The three peaks at 399.9, 399.4, and 398.8 eV are attributed to the N-Ti-O, C-N, and C=N chemical bonds, respectively, indicating the formation of typical N-Ti-O bonds between ZIF-8 NCs and 3DOM TiO 2 [28]. The C 1s spectrum can be deconvoluted into two peaks at 284.4 and 285.9 eV ( Figure S2a), which correspond to the C-C bond and C-N bond, respectively [29]. In Figure S2b, the fitting peaks at 529.1 and 531.8 eV are attributed to Ti-O and Zn-OH chemical bonds [30]. Moreover, the structure of the TiO 2 /ZIF-8 composite was also analyzed via the Raman spectrum ( Figure 2f). The Raman active fundamental modes correspond to 142.65 (Eg), 398.34 (B1g), 512.64 (A1g), and 637.57 cm −1 (Eg) for the 3DOM TiO 2 [31]. It is worth noting that the peak (143.72 cm −1 ) ascribed to the Eg mode of the ZIF-8@3DOM TiO 2 composite has a redshift compared with the 3DOM TiO 2 (142.65 cm −1 ). The redshift of the Eg band can be attributed to the strain on the TiO 2 /ZIF-8 surface, which has arisen from the introduction of N-Ti-O bonds on the 3DOM TiO 2 [24]. In order to determine the content of ZIF-8 in ZIF-8@3DOM TiO 2 , according to the proportions of the C, H, and N elements in ZIF-8 and ZIF-8@3DOM TiO 2 , the prepared ZIF-8@3DOM TiO 2 is calculated containing around 60.93 wt% ZIF-8 and 39.07 wt% TiO 2 , respectively. As shown in Figure S3, the zeta potential values of the 3DOM TiO 2 , bulk ZIF-8, and ZIF-8@3DOM TiO 2 measured by a Malvern Zetasizer Nano-ZS90 are −22.7 mV, +23.2 mV, and +7.67 mV, respectively. Under these initial conditions, the surface of TiO 2 is negatively charged. After loading positively charged ZIF-8 on the pore walls, the potential value increased significantly, indicating that ZIF-8 NC has been successfully loaded on the 3DOM TiO 2 .

Characterization of Catalyst Crystal Phase, Nanoporous Structure, Optical Properties, and Thermal Stability
To further examine the crystal phase, nanoporous structure, and optical properties of the obtained catalyst, the XRD analysis, nitrogen adsorption-desorption isotherms, and optical absorption spectra of the catalysts were investigated, respectively. The XRD patterns of the 3DOM TiO 2 , ZIF-8, and ZIF-8@3DOM TiO 2 are shown in Figure 3a. The XRD pattern of 3DOM TiO 2 shows three peaks at 2θ = 25.3 • , 37.9 • , and 48.0 • , corresponding to the Catalysts 2021, 11, 533 5 of 13 crystal planes of (101), (004), and (200), respectively, which indicates that the TiO 2 sample adopts an anatase phase after calcination at 570 • C. In the XRD pattern of ZIF-8, the obvious peaks, including 011, 002, 112, 022, 013, and 222, are observed [11]. The diffraction pattern of ZIF-8@3DOM TiO 2 exhibits that the patterns include those of ZIF-8 and 3DOM TiO 2 , indicating that ZIF-8 NCs are dispersed in the 3DOM TiO 2 and this is consistent with the SEM results. Furthermore, Figure 3b shows the nitrogen adsorption-desorption isotherms of the ZIF-8@3DOM TiO 2 composite. The ZIF-8@3DOM TiO 2 showed a significant high uptake at very low relative pressure, suggesting the presence of a microporous structure, which can be attributed to the frameworks of ZIF-8 NCs. Compared with 3DOM TiO 2 , the specific surface area of the ZIF-8@3DOM TiO 2 increased from 52 to 235 m 2 ·g −1 , ascribed to the high specific surface area of ZIF-8 (427 m 2 ·g −1 , Figure S4).

Characterization of Catalyst Crystal Phase, Nanoporous Structure, Optical Properties, and Thermal Stability
To further examine the crystal phase, nanoporous structure, and optical properties of the obtained catalyst, the XRD analysis, nitrogen adsorption-desorption isotherms, and optical absorption spectra of the catalysts were investigated, respectively. The XRD patterns of the 3DOM TiO2, ZIF-8, and ZIF-8@3DOM TiO2 are shown in Figure 3a. The XRD pattern of 3DOM TiO2 shows three peaks at 2θ = 25.3°, 37.9°, and 48.0°, corresponding to the crystal planes of (101), (004), and (200), respectively, which indicates that the TiO2 sample adopts an anatase phase after calcination at 570 °C. In the XRD pattern of ZIF-8, the obvious peaks, including 011, 002, 112, 022, 013, and 222, are observed [11]. The diffraction pattern of ZIF-8@3DOM TiO2 exhibits that the patterns include those of ZIF-8 and 3DOM TiO2, indicating that ZIF-8 NCs are dispersed in the 3DOM TiO2 and this is consistent with the SEM results. Furthermore, Figure 3b shows the nitrogen adsorption-desorption isotherms of the ZIF-8@3DOM TiO2 composite. The ZIF-8@3DOM TiO2 showed a significant high uptake at very low relative pressure, suggesting the presence of a microporous structure, which can be attributed to the frameworks of ZIF-8 NCs. Compared with 3DOM TiO2, the specific surface area of the ZIF-8@3DOM TiO2 increased from 52 to 235 m 2 ·g −1 , ascribed to the high specific surface area of ZIF-8 (427 m 2 ·g −1 , Figure S4).
It is known that TiO2 has attracted attention because of its abundance, low price, and nontoxicity among many other photocatalysts [32]. However, the anatase TiO2, which has It is known that TiO 2 has attracted attention because of its abundance, low price, and nontoxicity among many other photocatalysts [32]. However, the anatase TiO 2 , which has a large bandgap, responds extremely only to UV-light irradiation, leading to the lower utility of light; this means that more strategic methods are needed to be proposed to improve the visible-light absorbance of TiO 2 [33]. The optical properties of the bulk ZIF-8, 3DOM TiO 2 , the random mixture (3DOM TiO 2 and bulk ZIF-8), and ZIF-8@3DOM TiO 2 were investigated and the results are presented in Figure 3c. The absorption spectrum of ZIF-8 shows a weak absorption peak in the range from 260 to 350 nm (green curve). 3DOM TiO 2 shows a strong absorption peak in the UV domain (200-405 nm) with a bandgap of 3.06 eV (black curve). The UV-vis spectrum of the random mixture shows slightly higher visible light absorption compared with 3DOM TiO 2 (blue curve). Interestingly, the ZIF-8@3DOM TiO 2 composite shows obvious absorption in the visible domain surrounded 400-600 nm (red curve). It is reasonable to explain that the N-Ti-O chemical bonding in the composite interface can easily improve the visible-light absorbance of TiO 2 [34,35]. In order to investigate the thermal stability of the obtained 3DOM TiO 2 , bulk ZIF-8, and ZIF-8@3DOM TiO 2 , the thermogravimetric curves are shown in Figure 3d. From 40 • C to Catalysts 2021, 11, 533 6 of 13 1000 • C, the 3DOM TiO 2 mass remains stable, indicating that it does not degrade in the temperature range. The TGA curves of ZIF-8 and ZIF-8@3DOM TiO 2 are also similar to the ZIF-8 crystals prepared in TEA [36]. The first step of weight loss of bulk ZIF-8 (40 • C to 230 • C) is due to the release of solvent methanol and other absorbed unreacted molecules. The sharp weight loss from 230 • C to 1000 • C is due to the structural degradation and decomposition of the organic ligand. From the thermogravimetric curves, it can be obtained that there is a total weight loss of about 67% for bulk ZIF-8 and about 42% for ZIF-8@3DOM TiO 2 in the heating range from 40 • C to 1000 • C. Thus, the approximate weight ratio of ZIF-8 in ZIF-8@3DOM TiO 2 can be calculated as 63%, which is consistent with the results obtained from the elemental analysis (Table 1).
3.06 eV (black curve). The UV-vis spectrum of the random mixture shows slightly higher visible light absorption compared with 3DOM TiO2 (blue curve). Interestingly, the ZIF-8@3DOM TiO2 composite shows obvious absorption in the visible domain surrounded 400-600 nm (red curve). It is reasonable to explain that the N-Ti-O chemical bonding in the composite interface can easily improve the visible-light absorbance of TiO2 [34,35]. In order to investigate the thermal stability of the obtained 3DOM TiO2, bulk ZIF-8, and ZIF-8@3DOM TiO2, the thermogravimetric curves are shown in Figure 3d. From 40 °C to 1000 °C, the 3DOM TiO2 mass remains stable, indicating that it does not degrade in the temperature range. The TGA curves of ZIF-8 and ZIF-8@3DOM TiO2 are also similar to the ZIF-8 crystals prepared in TEA [36]. The first step of weight loss of bulk ZIF-8 (40°C to 230 °C) is due to the release of solvent methanol and other absorbed unreacted molecules. The sharp weight loss from 230 °C to 1000 °C is due to the structural degradation and decomposition of the organic ligand. From the thermogravimetric curves, it can be obtained that there is a total weight loss of about 67% for bulk ZIF-8 and about 42% for ZIF-8@3DOM TiO2 in the heating range from 40 °C to 1000 °C. Thus, the approximate weight ratio of ZIF-8 in ZIF-8@3DOM TiO2 can be calculated as 63%, which is consistent with the results obtained from the elemental analysis (Table 1).

Analysis of Catalytic Performance
The benzylidenemalonitrile (BMN) and its derivatives (usually as the product of the aryl aldehydes Knoevenagel condensation reaction) can be used for synthetic reaction transformation and molecules of biological value [37]. Herein, the prepared ZIF-8@3DOM TiO2 was utilized as a micro-nano reactor to synthesize the BMN derivatives from aromatic alcohol via a cascade reaction at 25 °C (Table 2). First, the aromatic alcohol was oxidized to arylaldehyde by the catalysis of the 3DOM TiO2 skeleton under continuous illumination in O2, and then the arylaldehyde was further transformed into BMN derivative by the catalysis of ZIF-8 NCs located on the pore walls of ZIF-8@3DOM TiO2. Gener-

Analysis of Catalytic Performance
The benzylidenemalonitrile (BMN) and its derivatives (usually as the product of the aryl aldehydes Knoevenagel condensation reaction) can be used for synthetic reaction transformation and molecules of biological value [37]. Herein, the prepared ZIF-8@3DOM TiO 2 was utilized as a micro-nano reactor to synthesize the BMN derivatives from aromatic alcohol via a cascade reaction at 25 • C (Table 2). First, the aromatic alcohol was oxidized to arylaldehyde by the catalysis of the 3DOM TiO 2 skeleton under continuous illumination in O 2 , and then the arylaldehyde was further transformed into BMN derivative by the catalysis of ZIF-8 NCs located on the pore walls of ZIF-8@3DOM TiO 2 . Generally, it is difficult to oxidize aromatic alcohol to arylaldehydes with molecular oxygen under ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO 2 skeleton can extend the absorbance of the 3DOM TiO 2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO 2 catalyst constructed with ZIF-8 NCs and 3DOM TiO 2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique Catalysts 2021, 11, 533 7 of 13 structure of the ZIF-8@3DOM TiO 2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO 2 , the mixture of 3DOM TiO 2 and the bulk ZIF-8, and 3DOM TiO 2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. der ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. Table 2. Aromatic alcohol oxidization and Knoevenagel condensation cascade reaction catalyzed by different catalysts.

Entry
Substrates In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is der ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. Table 2. Aromatic alcohol oxidization and Knoevenagel condensation cascade reaction catalyzed by different catalysts.

Entry
Substrates In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is der ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. Table 2. Aromatic alcohol oxidization and Knoevenagel condensation cascade reaction catalyzed by different catalysts.

Entry
Substrates In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is der ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2.  In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is der ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. Table 2. Aromatic alcohol oxidization and Knoevenagel condensation cascade reaction catalyzed by different catalysts.

Entry
Substrates In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is ally, it is difficult to oxidize aromatic alcohol to arylaldehydes with molecular oxygen under ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2.  In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is ally, it is difficult to oxidize aromatic alcohol to arylaldehydes with molecular oxygen under ambient condition. This particular conversion usually requires inorganic oxidants, such as stoichiometric chromium, manganese oxides, or organic oxidants, resulting in a large amount of carcinogenic or toxic wastes being generated subsequently [38]. However, the ZIF-8 NCs located on the 3DOM TiO2 skeleton can extend the absorbance of the 3DOM TiO2 to the visible light domain for the photocatalytic oxidation reactions (Figure 3c). The ZIF-8@3DOM TiO2 catalyst constructed with ZIF-8 NCs and 3DOM TiO2 is beneficial to the cascade reaction, especially bulky-molecule catalysis. To certify the benefits of the unique structure of the ZIF-8@3DOM TiO2 involved in bulky-molecule catalysis, the catalytic performances of ZIF-8@3DOM TiO2, the mixture of 3DOM TiO2 and the bulk ZIF-8, and 3DOM TiO2 were tested in the aromatic oxidization and Knoevenagel condensation cascade reaction, as illustrated in Table 2. Table 2. Aromatic alcohol oxidization and Knoevenagel condensation cascade reaction catalyzed by different catalysts.

Entry
Substrates In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO2, the composite can effectively combine ZIF-8 and TiO2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO2 can promote the absorption of visible light by 3DOM TiO2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is In the conversion reaction of aromatic alcohols to aromatic aldehydes, ZIF-8@3DOM TiO 2 has the highest conversions (73.0-98.0%), followed by the random mixture catalyst (64.8-90.0%), and the 3DOM TiO 2 is the weakest among them (40-58.0%). For ZIF-8@3DOM TiO 2 , the composite can effectively combine ZIF-8 and TiO 2 so that the ZIF-8 NCs are uniformly loaded on the pore walls of the 3DOM TiO 2 (Figure 1c). It also can be seen from the XPS spectra of Figure 2e that 3DOM TiO 2 and ZIF-8 NCs are tightly combined through the N-Ti-O bonds. Such a unique structure is extremely beneficial to the conversion of aromatic alcohols: first, the ZIF-8 NCs supported on 3DOM TiO 2 can promote the absorption of visible light by 3DOM TiO 2 due to the existence of the N-Ti-O bond (Figures 2e and 3c), which is beneficial to the photocatalytic conversion of aromatic alcohols; second, the uniform dispersion of ZIF-8 NCs in the 3DOM TiO 2 allows the conversion products of aromatic alcohols (aromatic aldehydes) to easily contact the surface of ZIF-8 NCs. Accordingly, the ZIF-8 NCs can continue to convert aromatic aldehydes into BMN, so the presence of the ZIF-8 NCs is conducive to the forward conversion of aromatic alcohols to aromatic aldehydes. For a random mixture catalyst, the catalytic effect is weaker than that of ZIF-8@3DOM TiO 2 , but it is higher than that of 3DOM TiO 2 in the conversion reaction of aromatic alcohols to aromatic aldehydes. On the one hand, the lower visible light absorption of the random mixture compared with ZIF-8@3DOM TiO 2 leads to a lower light utilization efficiency, so that the yield is lower in the photocatalytic reaction. On the other hand, the bulk ZIF-8 cannot disperse evenly with 3DOM TiO 2 , causing the aromatic aldehydes to take a long reaction path to touch the bulk ZIF-8 in the random mixture. Moreover, a few active sites exposed in the ZIF-8 are not conducive to the further conversion of aromatic aldehydes. Therefore, the random mixture catalyst is less favorable to the forward conversion of aromatic alcohols to aromatic aldehydes than the ZIF-8@3DOM TiO 2 . As for 3DOM TiO 2 , the photocatalytic efficiency is the lowest because of the large intrinsic bandgap (3.06 eV), severely restricting its utilization of the visible light (Figure 3c).
In the Knoevenagel condensation reaction, among the three catalysts, the ZIF-8@3DOM TiO 2 still has the highest yields (60.0-98.0%), especially in the reactions of aromatic alcohols with multiple benzene rings ( Table 2). The random mixture catalyst takes second place (34.8-89.0%), and the 3DOM TiO 2 is inactive. This phenomenon is mainly due to the fact that only the ZIF-8 has reactivity in Knoevenagel condensation reactions. However, the 11.6 Å large pores connected by pores of 3.4 Å in diameter is the main characterization of the ZIF-8 structure [39]. Such a microporous structure makes them relatively inert to macromolecular reactions, which can only be catalyzed by the surface of the catalyst. Therefore, the bulk ZIF-8 can only provide fewer active centers and has a poor catalytic effect on reactions involving large molecules. Besides, the bulk ZIF-8 cannot be uniformly mixed with 3DOM TiO 2 because of the large agglomerate size of bulk ZIF-8, which extends the reaction route of the substance. On the contrary, the ZIF-8@3DOM TiO 2 composite catalyst is loaded with ZIF-8 CNs on the pore walls. Such a unique structure is beneficial to the reaction: (1) the nano-scale ZIF-8 can greatly increase the contacting area between the reaction substrate and the ZIF-8 NCs, thus improving the catalytic performance; (2) the diffusion path of the first step product (the intermediate product aromatic aldehyde) is shortened, which is more conducive to contact with the ZIF-8, thereby increasing the catalytic yield.

Catalyst Stability Test and Explanation of Catalytic Mechanism
The stability and the reusability of the ZIF-8@3DOM TiO 2 were also investigated in this study. After the cascade reaction was performed for 2 h, the solid catalyst ZIF-8@3DOM TiO 2 was filtered out and we continued stirring the mother liquor under the same experimental conditions. The findings show that the amount of 2-benzylidenemalononitrile was the same as the previous time, which reflects that the leaching was absent (Figure 4a). The effects of the reaction time from 1 to 10 h on the yield of benzylidenemalononitrile were also investigated, as shown in Figure 4a. Whether the catalyst is ZIF-8@3DOM TiO 2 or the random mixtures, the yield rapidly increases with the reaction time from 1 to 8 h until the reaction equilibrium appears for the rest of the time. Hence, the time of 8 h was selected as the optimal condition of the reaction time. The ZIF-8@3DOM TiO 2 was recovered by filtration and reused for the next run of the benzyl alcohol oxidization-Knoevenagel condensation cascade reaction. The results confirm that the ZIF-8@3DOM TiO 2 can be stably reused for at least 5 circulations with no significant decrease in activity (Figure 4b).
From the SEM as well as the XPS analysis ( Figure 5 and Figure S5), the morphology and the chemical bonds between ZIF-8 and 3DOM TiO 2 were well maintained after 5 times of reuse. Figure S6a shows that the ZIF-8 characteristic absorption peaks (423 cm −1 ) and TiO 2 characteristic absorption peaks (400-800 cm −1 ) did not change significantly after the catalytic cascade reaction of ZIF-8@3DOM TiO 2 , and no other spurious peaks appeared. The XRD pattern of ZIF-8@3DOM TiO 2 after recycling in Figure S6b shows that the peak positions and relative intensities of its diffraction peaks did not change significantly before and after use. Figure S6c shows the structural parameters obtained after N 2 adsorptiondesorption characterization of the recycled ZIF-8@3DOM TiO 2 catalyst. The results show that the specific surface area of the ZIF-8@3DOM TiO 2 catalyst decreased to some extent after 5 times of use. This may be caused by the residual reactants in the recycled catalyst, but the ZIF-8@3DOM TiO 2 still has a high specific surface area value after five cycles, and thus still has some catalytic activity. All the above analysis results indicated that the catalyst ZIF-8@3DOM TiO 2 was structurally stable and could be recycled. This can be attributed to the monolithically interconnected TiO 2 skeleton and the immobilized ZIF-8 NCs. The high performance of the ZIF-8@3DOM TiO 2 was explained through the reaction path in Figure 4c. The cascade reaction in micro-nano structures can shorten the reaction paths, and the aromatic alcohol first passes through the interconnected macropores and is catalyzed by TiO 2 to transform into intermediate arylaldehyde. Then, the intermediate arylaldehyde reaches the ZIF-8 NCs immobilized on the wall of 3DOM TiO 2 and is transformed into the final products. Specifically, multiple reactions are carried out in this nanoreactor because of the integration of the multiple micro-nano reactor units. The integration of the ZIF-8 NCs and the macroporous TiO 2 in one micro-nano reactor can lessen the diffusion distance of the substances and can be beneficial to the exposure of the active sites.
value after five cycles, and thus still has some catalytic activity. All the above analysis results indicated that the catalyst ZIF-8@3DOM TiO2 was structurally stable and could be recycled. This can be attributed to the monolithically interconnected TiO2 skeleton and the immobilized ZIF-8 NCs. The high performance of the ZIF-8@3DOM TiO2 was explained through the reaction path in Figure 4c. The cascade reaction in micro-nano structures can shorten the reaction paths, and the aromatic alcohol first passes through the interconnected macropores and is catalyzed by TiO2 to transform into intermediate arylaldehyde.
Then, the intermediate arylaldehyde reaches the ZIF-8 NCs immobilized on the wall of 3DOM TiO2 and is transformed into the final products. Specifically, multiple reactions are carried out in this nanoreactor because of the integration of the multiple micro-nano reactor units. The integration of the ZIF-8 NCs and the macroporous TiO2 in one micro-nano reactor can lessen the diffusion distance of the substances and can be beneficial to the exposure of the active sites.  Furthermore, the ZIF-8 NCs located on 3DOM TiO2 not only improve the photoresponse of 3DOM TiO2 to the visible light domain but also improve the conversion of aromatic alcohol by 3DOM TiO2 under the light. Because of the above phenomenon, a provisional mechanism for the photocatalytic performance of TiO2/ZIF-8 nanocomposite was proposed. The main reason is that the Ti-O-N bond between 3DOM TiO2 and ZIF-8 NCs can enhance the visible-light activation of 3DOM TiO2. The illustration in Figure 4d shows Furthermore, the ZIF-8 NCs located on 3DOM TiO 2 not only improve the photoresponse of 3DOM TiO 2 to the visible light domain but also improve the conversion of aromatic alcohol by 3DOM TiO 2 under the light. Because of the above phenomenon, a provisional mechanism for the photocatalytic performance of TiO 2 /ZIF-8 nanocomposite was proposed. The main reason is that the Ti-O-N bond between 3DOM TiO 2 and ZIF-8 NCs can enhance the visible-light activation of 3DOM TiO 2 . The illustration in Figure 4d shows the generation and conversion process of electron-hole pairs between the 3DOM TiO 2 and the ZIF-8 NCs interface. First, the electrons and holes are generated on the 3DOM TiO 2 under excitation by light. The ZIF-8 is conducive to charge separation and reduces the bandgap of 3DOM TiO 2 (Eg1 to Eg2) [20]. Under the circumstances, the photoelectrons transfer to the surface of 3DOM TiO 2 is easily achieved. The photogenerated electron can combine with oxygen molecules in the solution to form OH − . Simultaneously, the holes (h + ) first oxidize HOCH 2 -Ph to (HOCH-Ph) + , and then (HOCH-Ph) + is oxidized to Ph-CHO [40].

Preparation of 3DOM TiO 2
First, the cross-linked polystyrene (CLPS) microspheres were prepared according to the previous report [41]. Then, the product was collected by centrifugation and finally dried in a dryer to obtain CLPS colloidal crystal template (CCT).
The 3DOM TiO 2 was prepared by a sol-gel method using the CLPS CCT as the template. The precursor was prepared by mixing 1 mL of tetrabutyl titanate with 1 mL of ethyl alcohol and 1 mL acetic acid under magnetic stirring, then a mixture of 3 mL deionized water and 1 mL hydrochloric acid was added into the above-mentioned solution under continuous stirring until the solution became clear. Afterward, the precursor was injected into a tube loaded with CLPS CCT under vacuum and immersed for 5 min. After the excess liquid was taken out, the samples were aged in an air-dry oven at 60 • C for 2 h. The whole process was repeated 3 times. Finally, the TiO 2 /CLPS hybrid materials were calcined at 300 • C for 4 h and then 570 • C for 5 h in the air using a ramp rate of 4 • C/min to remove the CLPS CCT and form anatase TiO 2 , and the product 3DOM TiO 2 was obtained.

Preparation of ZIF-8@3DOM TiO 2
The ZIF-8 precursor solutions included Zn(NO 3 ) 2 ·6H 2 O and 2-MeIM methanol solution. Their concentrations were all 120 mg/mL, and the volume ratioV(Zn(NO 3 ) 2 ·6H 2 O)/V(2-MeIM) was 4/3. Initially, a Zn(NO 3 ) 2 ·6H 2 O methanol solution (2 mL) was added into a glass vial containing 200 mg of 3DOM TiO 2 , and the mixture was rotary evaporated under vacuum at 50 • C to remove the methanol solvent. Subsequently, the dried Zn(NO 3 ) 2 ·6H 2 O/3DOM TiO 2 was immersed into the 2-MeIM solution, and then rotary evaporated at 50 • C to obtain the Zn(NO 3 ) 2 ·6H 2 O/2-MeIM/3DOM TiO 2 composite. The whole filling process was repeated 2 times. Afterward, a small container containing TEA was placed in the glass flask and sealed at room temperature for 24 h (diagram as shown in Figure S1). Finally, the resulting composite was washed with methanol three times and dried at 60 • C overnight to obtain ZIF-8@3DOM TiO 2 . In comparison, the bulk ZIF-8 was synthesized by the same method as ZIF-8@3DOM TiO 2 as follows: Zn(NO 3 ) 2 ·6H 2 O (0.480 g) and 2-MeIM (0.360 g) were dissolved in 10 mL of methanol. Then, the mixture was rotary evaporated in a glass flask under vacuum at 50 • C to remove the methanol solvent. Subsequently, a small container containing TEA was placed in the glass flask and sealed at room temperature for 24 h. Finally, the product was washed with methanol three times and dried at 60 • C overnight.

Catalytic Applications in a Cascade Reaction
The one-pot cascade aromatic alcohol oxidization-Knoevenagel condensation reaction was carried out on Labsolar-6A system (Perfect Light Co., Bejing, China). The Labsolar-6A system mainly includes a 300 W Xenon-arc lamp and a reactor system. The Xenon-arc lamp is used as the photo source for the photocatalytic oxidation of aromatic alcohol. To remove the radiant heat, the circulating water flowing through the reactor is used to keep the temperature at 25 • C. During the reaction, 55 mg catalyst was added to 10 mL of acetonitrile in the reactor under continuous stirring. Subsequently, 0.5 mmol benzyl alcohol and 1 mmol malononitrile were introduced into the quartz glass reactor. Before illumination, the reaction reactor was vacuum treated and purged with 0.1 MPa high purity O 2 . The mixture was then illuminated using the Xenon lamp for 8 h and sampled once every hour. Then, the ZIF-8@3DOM TiO 2 was filtered out, washed with methanol, and reutilized in the next cycle. Finally, the product was quantified by gas chromatography (GC). For comparison, the catalytic conditions of the random mixture and 3DOM TiO 2 were the same as above.
is more conducive to improving the recovery of the catalyst. Particularly, the obtained ZIF-8@3DOM TiO 2 can catalyze the cascade reaction of aromatic alcohol oxidization by illumination and the Knoevenagel condensation reaction at room temperature. In the oxidation reaction of alcohol, the semiconductor-ZIF composite structure can make the TiO 2 more beneficial to the absorption of visible light. In the Knoevenagel condensation reactions, the interconnected porous structure and nano-scale ZIF-8 crystals are more conducive to the contact of macromolecular substances. This work provides a novel method for fabricating a multifunctional MOF/semiconductor composite.
Author Contributions: Software, T.K. and X.L.; methodology, Y.G.; resources, X.W.; writing-original draft preparation, J.C.; writing-review and editing, X.Z.; project administration, X.Z. and X.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China (grant nos. 51573038, 51403049, and 50903027) and the Natural Science Foundation of Hebei Province (nos. E2020202146 and E2020202133).

Data Availability Statement:
The data presented in this study are available.