English

• Sulfoxides are widely used as valuable reagents in chemistry and as essential bioactive ingredients in pharmaceuticals, such as trifluoromethyl sulfoxides as reagents for metal-free C—H trifluoromethylthiolation and omeprazole as a proton pump inhibitor^[1-6]. A traditional methodology to synthesize sulfoxides from sulfides relies on strong oxidants such as hydrogen peroxide or m-chloroperbenzoic acid, often suffering from insufficient selectivity due to harsh reaction conditions or overoxidation^[7-11]. Therefore, the synthesis of sulfoxides in a mild and green manner is of great importance. Aerobic photocatalytic oxidation of sulfides into sulfoxides is regarded as one of the greenest pathways, which utilizes light for energy and atmospheric oxygen as an oxidant^[12-13]. A series of photocatalytic systems, such as polyoxometalates^[14], porphyrins^[15], and cyclometalated Ir(Ⅲ) complexes^[16], have been explored. However, their application raises issues regarding the usage of expensive metals and wearisome separation of catalysts from products under homogeneous conditions. Thus, it is highly demanded to develop efficient photocatalysts that are cost-effective and reusable.

  In this context, titanium-based metal-organic frameworks (Ti-MOFs) offer an interesting platform for photocatalysis, as they can integrate light-harvesting molecular units and catalytically active sites into a single matrix, resulting in improved charge separation and catalytic activity^[17-20]. Emerging Ti-MOFs have shown outstanding performance in oxidizing reactions^[21]. For example, Gascon and coworkers reported a visible-light-responsive Ti-MOF (ACM-1), which can be used for the selective oxidation of benzyl alcohol and cyclohexanol^[22]. Wang and Zhang′s group employed a sulfonated Ti-MOF (FIR-138) for photocatalytic oxidative phosphonylation of tertiary amines^[23]. Our group previously reported a porous Ti-MOF (Mn₂Ti-CPCDC) for photocatalytic oxidation of benzylamine in air^[24]. In spite of these advances, studies on photo-driven organic transformation by Ti-MOFs are relatively scarce. This could be ascribed to the significant synthetic challenges associated with crystalline Ti-MOFs, because Ti(Ⅳ) precursors could undergo fast hydrolysis, usually leading to the formation of amorphous phases. To overcome the circumstance, one possible method to enrich Ti-MOFs is a heterometallic approach, in which a second metal is introduced into the titanium-oxo cluster during MOF synthesis. For instance, Zhou et al. have successfully used the bimetallic [Ti₈Zr₂O₁₂(COO)₁₆] cluster to construct photoactive PCN-415 and PCN-416 Ti-MOFs, which showcase not only high stability but also photoactivity of the frameworks^[25]. Liu and colleagues have synthesized a bimetallic Ti-MOF (PFC-20-Co₂Ti), in which the presence of Ti(Ⅳ) cation in the node of the framework endows the material with ultrahigh photocatalytic activity for O₂ evolution^[26]. While attempting to generalize this heterometallic approach, we discovered that a combination of Ti(Ⅳ) and other metals into a bimetallic cluster would indeed bring new opportunities to diversify Ti-MOFs and their applications.

  Herein, we present a novel porous bimetallic Ti-MOF ([Mg₂Ti(μ₃-O)(ABTC)_1.5(H₂O)₃)]·2DMF·5H₂O, designated Mg₂Ti‑ABTC) constructed by an in‑situ generated trinuclear [Mg₂Ti(μ₃-O)(COO)₆(H₂O)₃], abbr. as [Mg₂Ti], cluster and a tetradentate ligand H₄ABTC (3, 3′, 5, 5′-azobenzene tetracarboxylic acid). Mg₂Ti-ABTC exhibited excellent stability and permanent porosity in the adsorption of N₂, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆, and showed n-type semiconductor behavior. Moreover, it demonstrates outstanding catalytic activity in the photocatalytic oxidation of benzyl sulfides, with both conversion and selectivity of thioanisole up to 99%. Importantly, Mg₂Ti-ABTC maintained good crystallinity and catalytic efficiency even after three consecutive recycles.

  1.   Experimental

  1.1   Materials and instruments

  Materials and physical measurements are shown in the Supporting information.

  1.2   Synthesis of Mg₂Ti-ABTC

  H₄ABTC (30.0 mg, 0.084 mmol), MgCl₂·6H₂O (15.0 mg, 0.074 mmol), Ti(ⁱOPr)₄ (23.0 μL, 0.072 mmol), N,N-dimethylformamide (DMF, 2.0 mL), and glacial acetic acid (1.1 mL) were added to a 23 mL Teflon-lined stainless steel reactor. After reaction at 120 ℃ for 48 h, pale-yellow cubic crystals (Mg₂Ti-ABTC) were obtained (Fig.S1). After washing with fresh DMF several times, the crystals were collected by centrifugation and dried in air. Yield: > 38%. Elemental analysis (mass fraction) for C₃₀H₃₉O₂₃N₅Mg₂Ti: Exp.: C 38.64%, H 4.50%, N 7.87%; Calcd.: C 38.57%, H 4.21%, N 7.50%. IR (KBr, cm^-1): 3 368 (br), 2 970 (m), 1 667 (s), 1454 (s), 1 335 (s), 1 086 (w), 1 043 (m), 929 (s), 879 (w), 781 (s), 714 (s), 536 (m), 479 (m).

  Mg₂Ti-ABTC can also be prepared under the same conditions except by replacing Ti(ⁱOPr)₄ with Cp₂TiCl₂ or TiCl₄. The successful synthesis of Mg₂Ti-ABTC can be determined by powder X-ray diffraction (PXRD, Fig.S2).

  1.3   Gas adsorption isotherms

  The nitrogen adsorption test was conducted under the condition of 77 K using ASAP 2460. Before the test, the MOFs were soaked in ethanol (EtOH) for three days (with EtOH changed three times a day), and then activated at 80 ℃ for 12 h. The tests for CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ adsorption were carried out at 273 and 298 K, respectively, with the activation conditions being the same as those for the nitrogen adsorption activation.

  1.4   General procedure for photocatalysis

  Firstly, 0.3 mmol aromatic sulphides and 2.0 mL methanol (MeOH) were added to a 10.0 mL light-induced tube. Then, 10 mg Mg₂Ti-ABTC was introduced into the above solution at room temperature and under normal pressure. The system was irradiated with blue light (10 W). After the reaction, the crystals of the catalyst were centrifuged, then thoroughly washed with MeOH and used for the next catalytic run. The conversion and selectivity of the reaction were determined by ¹H NMR.

  2.   Results and discussion

  2.1   Crystal structure description

  A suitable cubic crystal of Mg₂Ti‑ABTC was selected for a single-crystal X-ray diffraction analysis. The structural analysis indicates that Mg₂Ti-ABTC crystallizes in the P43n space group (Table S1) and has a three-dimensional open framework (Fig. 1), which is constructed from in‑situ generated [Mg₂Ti] (Fig. 1a) clusters and ABTC^4- ligands. The coexistence of Mg and Ti was further corroborated by elemental mappings and energy-dispersive X-ray spectroscopy (EDS) (Fig.S4). The assembly of H₄ABTC with different trimeric clusters has produced a handful of MOFs with soc topology, named individually as PFC-M₂Ti^[26], M-soc-MOF^[27], PCN-250^[28], and CPM-200^[29]. These highly porous MOFs serve as a valuable platform for applications in gas storage and CO₂ capture. Therefore, Mg₂Ti-ABTC can be viewed as a new member of the soc-MOF based on H₄ABTC. [Mg₂Ti] cluster is rarely reported in MOF structures. To our knowledge, Mg₂Ti-ABTC represents the second MOF structure built on [Mg₂Ti] cluster^[30]. As shown in Fig. 1a, each [Mg₂Ti(μ₃-O)(COO)₆(H₂O)₃] consists of three statistically occupied metal atoms (two Mg and one Ti) fused by one central μ₃-O atom, six carboxylate groups from six different ABTC^4- ligands, and three terminal H₂O molecules. The [Mg₂Ti] clusters function as triangular prismatic secondary building units (SBUs). Each ABTC^4- ligand is deprotonated to connect four [Mg₂Ti] clusters (Fig. 1b). As such, each [Mg₂Ti] cluster connects to six ABTC^4- ligands and each ABTC^4- ligand links to four [Mg₂Ti] clusters to form a highly open framework (Fig. 1c) with a 4, 6-connected soc network (Fig. 1d). The framework of Mg₂Ti-ABTC encloses cuboidal cages (0.96 nm in diameter) composed of eight [Mg₂Ti] clusters as vertices and six ABTC^4- ligands as faces of the cage (Fig. 1e, 1f). These cages are further interconnected by 1D channels with dimensions of about 0.53 nm. The free volume of Mg₂Ti-ABTC estimated by PLATON software was 57.6%, indicating that Mg₂Ti-ABTC has a microporous structure that is favorable for gas adsorption. Interestingly, the incorporation of redox-active titanium-oxo clusters and photo-responsive azo ligands into a single structure would enhance the photocatalytic performance of the framework. Furthermore, considering the rich abundance of Ti and Mg elements in the Earth′s crust, it is of industrial interest to scale up and make use of Mg₂Ti-ABTC.

  Figure 1

  

  Figure 1.  Structure of Mg₂Ti-ABTC: (a) trinuclear [Mg₂Ti(μ₃-O)(COO)₆(H₂O)₃] cluster and simplified triangular prismatic SBU; (b) H₄ABTC ligand and simplified rectangular SBU; (c) 3D framework; (d) soc topology; (e, f) cuboidal cage

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  2.2   Stability test and gas adsorption properties

  As shown in Fig. 2, the PXRD pattern of as-synthesized Mg₂Ti-ABTC solids is consistent with the simulated one, indicating good phase purity of the MOF. The scanning electron microscope (SEM) showed the uniform morphology of the Mg₂Ti-ABTC sample (Fig.S4). Element mapping reflected the uniform distribution of Mg and Ti on the material (Fig.S5). When Mg₂Ti-ABTC solids were immersed in different organic solvents, such as tetrahydrofuran (THF), MeOH, EtOH, dichloromethane (DCM), acetone (ACE), and acetonitrile (MeCN) for 48 h, after being filtered, the treated samples were subjected to PXRD experiments. The results showed that the PXRD patterns of the treated samples were in line with the simulated one, showing that Mg₂Ti-ABTC was stable in various solvents (Fig. 2). The TGA analysis showed a significant weight loss of about 22.4% before 213 ℃, which can be attributed to the removal of free guest water and DMF molecules. The continuous weight loss from 213 to 350 ℃ can be ascribed to the release of coordinated water (Fig.S6). There was no plateau between 213 to 350 ℃. This is possibly due to the decarboxylation reaction of ABTC^4- linkers at high temperature, which has been reported in literature^[31]. The above results verify that Mg₂Ti-ABTC has good chemical and thermal stability.

  Figure 2

  

  Figure 2.  PXRD patterns of the simulated, as-synthesized, and soaked Mg₂Ti-ABTC in different organic solvents

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  To investigate the porosity of Mg₂Ti-ABTC, a N₂ adsorption-desorption experiment was conducted at 77 K. Before gas adsorption, solvent exchange was performed using EtOH. The sample was then vacuum-dried at 80 ℃ for 12 h. As shown in Fig. 3a, the adsorption isotherm was a typical type-Ⅰ isotherm, indicating that Mg₂Ti-ABTC was microporous. The Brunauer-Emmett-Teller (BET) specific surface area was determined to be 1 170 m²·g^-1, with a pore volume of 0.462 4 cm³·g^-1. Non-local density functional theory (NLDFT) pore size distribution analysis revealed a predominant pore size centered around 0.91 nm (Fig. 3a), consistent with its crystal structure. PXRD analysis conducted on the sample of post-N₂ adsorption confirmed the retention of high crystallinity, thereby validating the permanent porosity and excellent stability of Mg₂Ti-ABTC (Fig.S7).

  Figure 3

  

  Figure 3.  (a) N₂ adsorption-desorption isotherm at 77 K (Inset: pore size distribution); (b) CO₂ adsorption isotherms at 273 and 298 K; (c) CH₄, C₂H₂, C₂H₄, and C₂H₆ adsorption isotherms at 273 K (hollow line) and 298 K (solid line); (d) Isothermal heats of CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ calculated by using the viral method

  In a and b: solid and hollow curves represent adsorption and desorption, respectively.

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  Due to the permanent porosity of Mg₂Ti-ABTC, the adsorption performance of CO₂, CH₄, C₂H₆, C₂H₄, and C₂H₂ was tested at 273 and 298 K. The experimental results showed that at 273 K and 1×10⁵ Pa, the adsorption capacities of CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ in Mg₂Ti-ABTC were 139.7, 30.6, 168.4, 105.9, and 123.5 cm³·g^-1, respectively; while at 298 K and 1×10⁵ Pa, the adsorption capacities of the above gases were 83.5, 15.3, 100.2, 75.8, and 100.1 cm³·g^-1, respectively (Fig. 3b and 3c). Based on these adsorption data, the standard adsorption enthalpies at zero loading (Q_st, ₀) of each gas at the two temperatures were calculated by the virial equation as follows: 22.6 kJ·mol^-1 for CO₂, 22.2 kJ·mol^-1 for CH₄, 40.0 kJ·mol^-1 for C₂H₂, 32.5 kJ·mol^-1 for C₂H₄, and 29.6 kJ·mol^-1 for C₂H₆ (Fig. 3d). All the above gas uptake values were impressive. It is interesting to compare the adsorption performances of Mg₂Ti-ABTC with those of other Mg-doping counterparts (CPM-200-M^Ⅲ/Mg; M=In³⁺, Ga³⁺, Fe³⁺, V³⁺, Sc³⁺)^[29]. As shown in Table S2, the CO₂, CH₄, and C₂H₂ uptake values of Mg₂Ti-ABTC were superior to those of CPM-200-Ga/Mg, V/Mg, and Sc/Mg^[29], but inferior to those of CPM-200-Fe/Mg and In/Mg under the same conditions. These results demonstrate that Mg₂Ti-ABTC is a highly robust and porous candidate for gas adsorption applications.

  2.3   Optical and photoelectric properties

  Solid-state ultraviolet-visible (UV-Vis) diffuse reflection test of Mg₂Ti-ABTC was conducted at 298 K. There was a broad absorption band in a range of 200-570 nm (Fig. 4a). This could be attributed to the ligand-centered absorption. The band gap was calculated to be 2.36 eV based on the Tauc plot (Fig. 4b). The Mott-Schottky curves recorded at frequencies of 500, 1 000, and 1 500 Hz were measured in a sodium sulfate solution (1.0 mol·L^-1) using a standard three-electrode system. All curves exhibited positive slopes, characteristic of n‑type semiconductor behavior^[32]. The flat band potential was determined to be -0.62 V (vs Ag/AgCl). Consequently, the conduction band (CB) potential of Mg₂Ti-ABTC was calculated at -0.42 V (vs NHE), and the valence band (VB) potential was calculated at 1.94 V (vs NHE) (Fig. 4c), which is very close to the value (2.07 V) obtained from VB-XPS (Fig. 4d).In addition, the obvious photocurrent response of Mg₂Ti-ABTC suggested that it had a good separation efficiency of photogenerated carriers (Fig.S8). Based on the high stability, porosity, and light response characteristics of Mg₂Ti-ABTC, we speculate that it might be a potential photocatalyst for organic reactions.

  Figure 4

  

  Figure 4.  (a) Solid-state UV-Vis diffuse reflection spectrum of Mg₂Ti-ABTC; (b) Bind gap calculation according to Tauc plot; (c) Mott-Schottky plots for Mg₂Ti-ABTC in 1.0 mol•L^-1 Na₂SO₄ (Inset: schematic diagram of the CB and VB energy levels); (d) VB-XPS curve

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  2.4   Photocatalytic oxidation of aromatic sulfides in air

  The photocatalytic activity of Mg₂Ti-ABTC was initially evaluated by the oxidation of aromatic sulfides. Using Mg₂Ti-ABTC as the photocatalyst, thioanisole was employed as a model substrate to screen optimum parameters, including solvent and light irradiation time, the reaction is shown in Reaction 1. The results were summarized in Table 1. It is well established that solvents play a crucial role in the photocatalytic oxidation of sulfides^[33-34]. To systematically investigate this effect, we examined the influence of various solvents on the photocatalytic oxidation of thioanisole under identical conditions. When a non-protonic solvent such as THF, MeCN, and DCM was used, the conversion of thioanisole was only 2%, 2%, and 46%, respectively. In contrast, when a protonic solvent like MeOH and EtOH was present, the conversion of thioanisole into methyl phenyl sulfoxide (MPSO) was up to 99% and 96%, respectively (entries 1-5, Table 1). Based on these findings, MeOH was selected as the optimal solvent for subsequent experiments due to its superior performance.

  (1)

  Table 1

  

  Table 1.  Photocatalytic oxidation of thioanisole by Mg₂Ti-ABTC under different conditions^a

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  Entry	Light	Gas	Catalyst	Time / h	Solvent	Conversionb / %	Selectivelyc / %
  1	Blue light	Air	Mg2Ti-ABTC	24	THF	< 2%	99%
  2	Blue light	Air	Mg2Ti-ABTC	24	DCM	< 2%	99%
  3	Blue light	Air	Mg2Ti-ABTC	24	MeCN	46%	99%
  4	Blue light	Air	Mg2Ti-ABTC	24	EtOH	96%	99%
  5	Blue light	Air	Mg2Ti-ABTC	24	MeOH	99%	99%
  6		Air	Mg2Ti-ABTC	24	MeOH	< 2%	99%
  7	Blue light	Air		24	MeOH	< 2%	99%
  8	Blue light	N2	Mg2Ti-ABTC	24	MeOH	< 2%	99%
  9	Blue light	Air	Mg2Ti-ABTC	10	MeOH	21%	99%
  10	Blue light	Air	Mg2Ti-ABTC	15	MeOH	48%	99%
  11	Blue light	Air	Mg2Ti-ABTC	20	MeOH	85%	99%
  12	Blue light	Air	Mg2Ti-ABTC	24	MeOH	99%	99%
  13	Blue light	Air	H4ABTC	24	MeOH	32%	99%
  14	Blue light	Air	H4ABTC+MgCl2	24	MeOH	46%	99%
  15d	Blue light	Air	Mg2Ti-ABTC	24	BQ	39%	99%
  16d	Blue light	Air	Mg2Ti-ABTC	24	DABCO	< 2%	99%
  17d	Blue light	Air	Mg2Ti-ABTC	24	KI	< 2%	99%
  a Reaction conditions: thioanisole (0.3 mmol), catalyst (10 mg), solvent (2.0 mL), gas (1×105 Pa), room temperature, 10 W blue light (400 nm); b The conversion was determined by 1H NMR; c The byproducts were the corresponding sulfones; dBQ, DABCO, and KI were dissolved in MeOH as scavengers of •O2-, 1O2, and h+.

  Then, we investigated the influences of other reaction parameters on the reaction. When any of the parameters, such as Mg₂Ti-ABTC catalyst, blue light, and air, were absent, it was difficult to oxidize thioanisole to MPSO even after 24 h (entries 6-8, Table 1). However, when Mg₂Ti-ABTC, MeOH, blue light, and air were all present simultaneously at room temperature, thioanisole could be oxidized to MPSO, with the yield gradually increasing from 21% to 99% with irradiation time extension from 10 to 24 h (entries 9-12, Table 1). This indicates that Mg₂Ti-ABTC, MeOH, blue light, and air were all indispensable in this catalytic process. Consequently, the optimized conditions for photocatalytic oxidation of anisole into MPSO were determined as follows: using Mg₂Ti-ABTC (10 mg) as the photocatalyst, air (1×10⁵ Pa) as the oxidant, MeOH (2.0 mL) as the solvent, and continuous blue light irradiation (10 W) as energy input, thioanisole (35 μL, 0.3 mmol) in MeOH was photooxidized at room temperature, affording 99% conversion of thioanisole and 99% selectivity of product MPSO.

  The recyclability of a catalyst is a critical parameter for assessing its catalytic performance. The recycling experiments demonstrated that Mg₂Ti‑ABTC maintained consistent catalytic activity after three consecutive cycles (Fig.S9). SEM images (Fig.S10), PXRD patterns (Fig.S11) of recycled Mg₂Ti-ABTC confirmed the integrity of Mg₂Ti-ABTC during the catalytic process. To further clarify the heterogeneous nature of the catalysis, control experiments were carried out. When an equivalent H₄ABTC ligand or physical mixture of H₄ABTC and MgCl₂ was employed as photocatalyst, only 32% or 46% conversion of thioanisole was obtained (entries 13 and 14, Table 1), much inferior to that using Mg₂Ti-ABTC as catalyst. These results suggest that Mg₂Ti-ABTC is a truly heterogeneous catalyst.

  With the optimal reaction conditions in hand, we expand the substrate to other aromatic sulfides. As seen in Table 2, Mg₂Ti-ABTC exhibited excellent catalytic activity towards phenyl methyl sulfides regardless of bearing electron-donating groups (e.g., —CH₃, —OCH₃, entries 2 and 3 in Table 2) or electron‑ withdrawing groups (e.g., —F, —Cl, —Br, —NO₂, entries 4-8 in Table 2). However, when the methyl group in sulfides is replaced by a larger ethyl or phenyl group, the conversion of the sulfides significantly decreases (entries 9 and 10 in Table 2). This might be due to the pore size effect of Mg₂Ti-ABTC on restricting the diffusion of larger substrates into the pores. The above results demonstrated that Mg₂Ti-ABTC had excellent catalytic performance and size selectivity for photocatalytic oxidation of small molecule sulfides, comparable to the best MOF-based photocatalysts^[35-38].

  Table 2

  

  Table 2.  Photocatalytic oxidation of various sulfide substrates by Mg₂Ti-ABTC^a

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  Entry	Substrate	Product	Conversionb / %	Selectivityc / %
  1			99	99
  2			90	99
  3			99	99
  4			82	99
  5			99	99
  6			91	99
  7			88	99
  8			75	99
  9			22	99
  10			59	99
  a Conditions: phenyl sulfides (0.3 mmol), Mg2Ti-ABTC (10 mg), MeOH (2.0 mL), air (1×105 Pa), room temperature environment with 10 W blue light (400 nm); b The conversion was determined by 1H NMR; c The byproducts were the corresponding sulfones.

  On account of the above catalytic activity, the catalytic mechanism of Mg₂Ti-ABTC on the photocatalytic oxidation of sulfides was investigated using thioanisole as a mode compound. As previously mentioned, light, a catalyst, and air were all required for the reaction (entries 6-12, Table 1). Generally, reactive oxygen species (ROS), such as superoxide radical (·O₂^-) and singlet oxygen (¹O₂), are vital for the aerobic photo-oxidation process. ROS was first identified by quenching experiments (entries 15-18, Table 1). When 1, 4-benzoquinone (BQ, 1 mmol, a scavenger for ·O₂^-) or 1, 4-diazabicyclo[2.2.2]octane (DABCO, 1 mmol, a quencher for ¹O₂) was individually introduced into the reaction system, the conversion of thioanisole decreased significantly to 39% or below 2%, respectively. To further monitor the generation of ¹O₂ and ·O₂^- on Mg₂Ti-ABTC, ¹O₂ trapping agent 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) and ·O₂^- trapping agent 2, 2, 6, 6-tetramethyl-1-piperinedinyloxy (TEMPO) were employed for electron paramagnetic resonance (EPR) spectroscopy. When the suspension of Mg₂Ti-ABTC in MeOH was exposed to illumination, the characteristic EPR signals of DMPO-·O₂^- and TEMP-¹O₂ were detected in EPR spectra (Fig. 5), confirming the involvement of ¹O₂ and ·O₂^- in the photocatalytic oxidation of thioanisole. Additionally, when KI (a scavenger for h⁺) was added to the reaction, the conversion of thioanisole decreased sharply to below 2%, suggesting that photoinduced holes would oxidize thioanisole to cationic radicals during the process^[39-40].

  Figure 5

  

  Figure 5.  EPR spectra of Mg₂Ti-ABTC to detect ¹O₂ (a) and •O₂^- (b) using the DMPO and TEMPO trapping agents with light on and off

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  Based on the experimental data and the open literature, we propose a plausible mechanism for the photooxidation of thioanisole on the Mg₂Ti-ABTC catalyst^[41-44]. As shown in Fig. 6, Mg₂Ti-ABTC is excited to be Mg₂Ti-ABTC* upon light irradiation, and meanwhile, electron-hole pairs are generated. The photogenerated holes oxidize the thioanisole (A) to cationic radicals (B), while the electrons undergo single-electron transfer to reduce O₂ to ·O₂^-. These two radicals subsequently combine to form per intermediates (C). Another molecule of thioanisole then reacts with these intermediates to produce sulfoxide (D). Additionally, under light irradiation, electrons in the LUMO orbital transition from the singlet sate to the triplet state, facilitating intersystem crossing and energy transfer. This energy transfer process transforms O₂ into ¹O₂. The reaction between thioanisole and ¹O₂ also leads to the formation of persulfoxide intermediates (C), which further react with another thioanisole molecule to generate sulfoxide.

  Figure 6

  

  Figure 6.  Possible mechanism of photocatalytic oxidation reaction of thioanisole by Mg₂Ti-ABTC in an air atmosphere

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  3.   Conclusions

  We have provided a new porous bimetallic Ti-MOF (Mg₂Ti-ABTC), which combines [Mg₂Ti(μ₃-O)(COO)₆(H₂O)₃] clusters with the tetradentate ligand H₄ABTC. It exhibited excellent stability and permanent porosity in the adsorption of N₂, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆, and shows n-type semiconductor behavior. The photocatalytic oxidation of benzyl sulfide demonstrated good photocatalytic activity, with a 99% conversion rate of thioanisole and a 99% selectivity of the product MPSO. Furthermore, Mg₂Ti-ABTC retained good crystallinity and catalytic efficiency after three consecutive recycle runs. Mechanism investigations reveal that ROS such as ·O₂^- and ¹O₂ are responsible for the excellent photocatalytic activity. Our work suggests that Ti-MOFs are promising photocatalysts for organic transformations.

  
  Acknowledgements: We thank the financial support from Shandong Provincial Natural Science Foundation (Grant No.ZR2022MB137), National Natural Science Foundation of China (Grants No.21501086, 22003003). Notes: The authors declare no competing financial interest.
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Structure of Mg₂Ti-ABTC: (a) trinuclear [Mg₂Ti(μ₃-O)(COO)₆(H₂O)₃] cluster and simplified triangular prismatic SBU; (b) H₄ABTC ligand and simplified rectangular SBU; (c) 3D framework; (d) soc topology; (e, f) cuboidal cage

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  Figure 2  PXRD patterns of the simulated, as-synthesized, and soaked Mg₂Ti-ABTC in different organic solvents

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  Figure 3  (a) N₂ adsorption-desorption isotherm at 77 K (Inset: pore size distribution); (b) CO₂ adsorption isotherms at 273 and 298 K; (c) CH₄, C₂H₂, C₂H₄, and C₂H₆ adsorption isotherms at 273 K (hollow line) and 298 K (solid line); (d) Isothermal heats of CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ calculated by using the viral method

  In a and b: solid and hollow curves represent adsorption and desorption, respectively.

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  Figure 4  (a) Solid-state UV-Vis diffuse reflection spectrum of Mg₂Ti-ABTC; (b) Bind gap calculation according to Tauc plot; (c) Mott-Schottky plots for Mg₂Ti-ABTC in 1.0 mol•L^-1 Na₂SO₄ (Inset: schematic diagram of the CB and VB energy levels); (d) VB-XPS curve

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  Figure 5  EPR spectra of Mg₂Ti-ABTC to detect ¹O₂ (a) and •O₂^- (b) using the DMPO and TEMPO trapping agents with light on and off

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  Figure 6  Possible mechanism of photocatalytic oxidation reaction of thioanisole by Mg₂Ti-ABTC in an air atmosphere

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  Table 1.  Photocatalytic oxidation of thioanisole by Mg₂Ti-ABTC under different conditions^a

  Entry	Light	Gas	Catalyst	Time / h	Solvent	Conversionb / %	Selectivelyc / %
  1	Blue light	Air	Mg2Ti-ABTC	24	THF	< 2%	99%
  2	Blue light	Air	Mg2Ti-ABTC	24	DCM	< 2%	99%
  3	Blue light	Air	Mg2Ti-ABTC	24	MeCN	46%	99%
  4	Blue light	Air	Mg2Ti-ABTC	24	EtOH	96%	99%
  5	Blue light	Air	Mg2Ti-ABTC	24	MeOH	99%	99%
  6		Air	Mg2Ti-ABTC	24	MeOH	< 2%	99%
  7	Blue light	Air		24	MeOH	< 2%	99%
  8	Blue light	N2	Mg2Ti-ABTC	24	MeOH	< 2%	99%
  9	Blue light	Air	Mg2Ti-ABTC	10	MeOH	21%	99%
  10	Blue light	Air	Mg2Ti-ABTC	15	MeOH	48%	99%
  11	Blue light	Air	Mg2Ti-ABTC	20	MeOH	85%	99%
  12	Blue light	Air	Mg2Ti-ABTC	24	MeOH	99%	99%
  13	Blue light	Air	H4ABTC	24	MeOH	32%	99%
  14	Blue light	Air	H4ABTC+MgCl2	24	MeOH	46%	99%
  15d	Blue light	Air	Mg2Ti-ABTC	24	BQ	39%	99%
  16d	Blue light	Air	Mg2Ti-ABTC	24	DABCO	< 2%	99%
  17d	Blue light	Air	Mg2Ti-ABTC	24	KI	< 2%	99%
  a Reaction conditions: thioanisole (0.3 mmol), catalyst (10 mg), solvent (2.0 mL), gas (1×105 Pa), room temperature, 10 W blue light (400 nm); b The conversion was determined by 1H NMR; c The byproducts were the corresponding sulfones; dBQ, DABCO, and KI were dissolved in MeOH as scavengers of •O2-, 1O2, and h+.

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  Table 2.  Photocatalytic oxidation of various sulfide substrates by Mg₂Ti-ABTC^a

  Entry	Substrate	Product	Conversionb / %	Selectivityc / %
  1			99	99
  2			90	99
  3			99	99
  4			82	99
  5			99	99
  6			91	99
  7			88	99
  8			75	99
  9			22	99
  10			59	99
  a Conditions: phenyl sulfides (0.3 mmol), Mg2Ti-ABTC (10 mg), MeOH (2.0 mL), air (1×105 Pa), room temperature environment with 10 W blue light (400 nm); b The conversion was determined by 1H NMR; c The byproducts were the corresponding sulfones.

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