Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO2 catalysts

Oxidation of renewable polyol/sugar into formic acid using molecular O2 over heterogeneous catalysts is still challenging due to the insufficient activation of both O2 and organic substrates on coordination-saturated metal oxides. In this study, we develop a defective MnO2 catalyst through a coordination number reduction strategy to enhance the aerobic oxidation of various polyols/sugars to formic acid. Compared to common MnO2, the tri-coordinated Mn in the defective MnO2 catalyst displays the electronic reconstruction of surface oxygen charge state and rich surface oxygen vacancies. These oxygen vacancies create more Mnδ+ Lewis acid site together with nearby oxygen as Lewis base sites. This combined structure behaves much like Frustrated Lewis pairs, serving to facilitate the activation of O2, as well as C–C and C–H bonds. As a result, the defective MnO2 catalyst shows high catalytic activity (turnover frequency: 113.5 h−1) and formic acid yield (>80%) comparable to noble metal catalysts for glycerol oxidation. The catalytic system is further extended to the oxidation of other polyols/sugars to formic acid with excellent catalytic performance.

demonstrated on (hydroxylate-)Al 2 O 3 , InO x , CeO x , MoO x and CoO x oxides [27][28][29][30][31][32][33][34][35] . We notice that the cascade oxidation of alcohol using O 2 may be regarded an acid-base synergistic reaction: the activation of O 2 into reactive oxygen species is accelerated by acid sites 36,37 , while C-H bond activation and C-C bond cleavage are known to be promoted by basic sites 38,39 . The characteristics of FLPs match the active site requirements for polyol/sugar oxidation [40][41][42][43][44] , but the saturated coordination structure in traditional MnO 2 catalyst shows limited surface oxygen vacancies and low surface electron density. Due to the tendency of the rigid lattice of traditional metal oxides to form saturated coordination M-O structures, it is still challenging to construct defective metal oxides with FLPs based on existing limited and cumbersome preparation strategies [45][46][47] . Inspired by the variable coordination structure of transition metals 48 , rational reduction of the coordination number of the central metal provides a feasible strategy to obtain defective MnO 2 with enhanced FLPs for polyol/sugar cascade oxidation to formic acid.
Herein, we present a strategy to construct low-coordinated, defective MnO 2 catalyst (MnO 2 -D) for polyol/sugar oxidation. Specifically, the defective Mn δ+ -O V structure with unsaturated tricoordinated Mn forms FLPs to allow spatially adjacent acid and basic sites to work cooperatively. Mn δ+ species associated with oxygen vacancy serve as Lewis acid to promote the activation of O 2 under the assistance of adjacent basic sites, whereas electron reconstructed O nearby Mn δ+ serves as Lewis base to facilitate the bond activation of organic substrate, in synergy with the Lewis acid site. In this manner, MnO 2 -D with intensified FLPs exhibits superior catalytic activity in converting various substrates into formic acid using O 2 , surpassing previous reports using heterogeneous catalysts.

Synthesis and characterization of defective MnO 2 -D with low Mn-O coordination
Two α-MnO 2 catalysts (MnO 2 -P and MnO 2 -D) were synthesized by hydrothermal methods, in which P and D refer to perfect and defective structures, respectively 49,50 . Significant differences in composition and crystal structure were observed on Rietveld refinement of powder X-ray diffraction (PXRD) and high-resolution transmission electron microscopy (HRTEM) images. Although only the Bragg peaks corresponding to the α-phase MnO 2 (I4/m space group) are present for both samples (Fig. 1a) [51][52][53][54] , the PXRD pattern of MnO 2 -D displays a broader diffraction peak than that of MnO 2 -P, indicating its smaller crystallite size [8.2(1) nm] compared with MnO 2 -P [36.5(1) nm]. The site occupancies of O and Mn were obtained by refining the PXRD patterns (Supplementary Tables 1 and 2), which were then taken as the starting model for Density functional theory (DFT) calculation described in later sections. The hydrothermal synthesis process was significantly affected by the nature of the reductant and temperature, resulting in a sharp difference in the amount of O and Mn ion defects in the two catalysts. In MnO 2 -P, Mn is distributed at the octahedral 8 h site with a 99(1)% occupancy, along with the existence of oxygen vacancy [5(2) Fig. 1), which manifests the Jahn-Teller (JT) effect [55][56][57] Fig. 1b shows that MnO 2 -P consists of short and thick nanowires with length of~500 nm and width of~30 nm, while MnO 2 -D exhibits longer and thinner nanowire morphology (width:~10 nm), which are consistent with the crystallinity results of PXRD, and the notion that MnO 2 -D exposes more defective sites on surface. The HRTEM images and selected area electron diffraction (SAED) pattern indicate that MnO 2 -P mainly exposes (211) and (200) planes, while MnO 2 -D mainly exposes (310) and (301) planes (Fig. 1b). The crystallinity results of PXRD also confirms that the ratio of (310) crystal plane to (211) crystal plane on MnO 2 -D [I (310) /I (211) = 0.78] is significantly higher than that on MnO 2 -P [I (310) /I (211) = 0.65].
To confirm the coordination structure and valence states of Mn, X-ray absorption fine structure (XAFS) were performed. The k 3weighted Fourier-transform Mn K-edge extended XAFS spectra (EXAFS) in Fig. 1c show that both MnO 2 -P and MnO 2 -D exhibit typical spectral features of α-MnO 2 phase with the intense FT peaks at approximately 1.52 Å, 2.85 Å and 3.40 Å, corresponding to Mn-O, edge-shared Mn-(O)-Mn, and corner-shared Mn-Mn shells, respectively 55,56,60,61 . Compared with MnO 2 -P, MnO 2 -D exhibits lower intensity of Mn-O shell and stronger intensity of corner-shared Mn-Mn shell, manifesting the increase of defective structure 49,50,62 . Wavelet transform (WT) analysis in Supplementary Fig. 2 shows that the peak with a maximum intensity of approximately 6 Å is observed for bulk α-MnO 2 , MnO 2 -P and MnO 2 -D in the k-space, ascribed to the Mn-O bond. The WT spectrum of MnO 2 -D also shows a maximum intensity of approximately 12 Å in the k-space and 3.5 Å in the R-space, corresponding to the Mn-Mn shell. EXAFS data fitting suggests that the coordination number of O for bulk α-MnO 2 and MnO 2 -P is 6.0 and 5.2, respectively, while that for MnO 2 -D is only 3.0, confirming the significant decrease of Mn-O coordination (Supplementary Table 3).  Table 3). In addition, we synthesized and characterized two sets of distinct phases, β-MnO 2 and γ-MnO 2 . Each set comprises a high coordination and a low coordination sample, respectively (Supplementary Table 3 and Supplementary Fig. 2). The intention behind these preparations was to establish a comprehensive comparison on glycerol oxidation efficiency across various MnO 2 samples bearing different coordination environment.
Probing the structure of the Frustrated Lewis Pairs in defective MnO 2 -D The Mn K-edge X-ray absorption near edge structure (XANES) shows that the absorption edges of MnO 2 -P and MnO 2 -D are similar to standard MnO 2 (Fig. 2a). Through the first-derivative of absorption edge in normalized XANES spectra, the Mn valence state is quantified as 3.89 and 3.56 for MnO 2 -P and MnO 2 -D respectively ( Supplementary Fig. 3), consistent with PXRD refinement results. The Mn 2p X-ray Photoelectron Spectroscopy (XPS) spectra (Fig. 2b) clearly demonstrates that the formation of Mn δ+ -O V defect structure leads to a significant decrease of the binding energy, confirming that the surface of MnO 2 -D possesses more electrons 63 . The XPS peak deconvolution analysis in Supplementary Table 4 shows a higher content of Mn 2+ and Mn 3+ is observed in MnO 2 -D. This electronic reconstruction caused by the reduction of coordination number induces Mn to be more prone to get electrons, while O more prone to lose electrons-an essential prerequisite for the formation of FLPs.
Notably, the generation of oxygen vacancy in the lowcoordination Mn δ+ -O V structure is crucial for FLPs formation: the reduced oxygen coordination leads to the exposure of the metal atom as Lewis acid sites, while the oxygen vacancies also induce stronger electronegativity of the residual oxygen, thus enhancing the Lewis basicity. As shown in Fig. 2c, the temperatureprogrammed desorption (TPD) of O 2 of the two catalysts reveals the presence of the surface oxygen species (~200°C, O sur ), the active oxygen species near the surface (~300°C, O sur-sub ) and the lattice oxygen species (~600°C, O lat ). MnO 2 -D exhibits a high O sur-sub content (77.6%) and a minimum O lat content (9.5%), indicating that the defective structure induces more active oxygen species (Supplementary Table 5). Low-temperature electron paramagnetic resonance (EPR) also shows that MnO 2 -D exhibits a stronger oxygen vacancy feature at a g-factor of 2.003 64,65 than MnO 2 -P (Supplementary Fig. 4). Consistent with O 2 -TPD and EPR analysis, the XPS spectra of O 1 s region in Fig. 2d and Supplementary Table 4 demonstrates that the MnO 2 -D catalyst has the highest oxygen vacancy content (27.6% O II ) with lower binding energy. Additionally, the existence of oxygen vacancy on the surface of MnO 2 -D is observed via aberration-corrected high-angle annular dark-field scanning TEM (Fig. 2e).
On these foundations, we analyzed the structure and Mulliken charge of penta-coordinated MnO 2 -P and tri-coordinated MnO 2 -D to provide direct evidence for the formation of FLPs. Figure 2f shows that the near saturated coordination O in MnO 2 -P creates a steric hindrance effect that obstructs the access of partially negatively charged reactant molecules to the Mn Lewis acid site. In sharp contrast, Mn in the lowcoordinated MnO 2 -D being highly unsaturated requires more molecules to coordinate (Lewis acid property). In parallel, the three O atoms in MnO 2 -D process more electrons due to the electron transfer from Mn to O, which could further transfer electrons to reactant molecule (Lewis base property). Hence, the behavior of defective Mn δ+ -O V structure in MnO 2 -D aligns well with the concept of FLPs 30,[66][67][68][69][70] . To probe the FLPs on MnO 2 -D, NH 3 -TPD, and CO 2 -TPD were further performed. Figure 2g and Supplementary Table 5 show that the MnO 2 -D exhibits sharply different acid-base properties from MnO 2 -P. For NH 3 -TPD, MnO 2 -D displays more medium strong and strong acid sites, while MnO 2 -P mainly contains weak acid sites with a small amount of strong acid sites. This indicates that the decrease of coordination 30 Table 6). The above results indicates that the saturated coordination structure of MnO 2 does not have sufficient activity in C-H and C-C bond activation, implying that an unsaturated structure is key for high activity. Of note, the difference of specific surface area is not the main factor affecting the activity and FA selectivity, as excluded by control experiments (Supplementary Fig. 6), while little activity was observed in the absence of a Mn-based catalyst (Supplementary Table 6). Through further optimization of reaction time, MnO 2 -D delivered an optimal 99.2% conversion and 83.2% FA selectivity in 6 h ( Supplementary Fig. 7). Ethanol pulse adsorption was further conducted to determine the exposed active sites ( Supplementary Fig. 8 Table 7). Moreover, MnO 2 -D exhibits excellent stability for glycerol oxidation, with almost no change in FA selectivity (>80%) and a slight decrease in glycerol conversion (<20%) after 6 catalytic cycles due to the coverage of some surface active sites by carboxylic acid products ( Fig. 3c and Supplementary Fig. 5). Characterizations of the spent catalyst using XRD, UV-Vis, XPS, NH 3 -TPD and CO 2 -TPD techniques prove that the defective Mn δ+ -O V structure and acid-base properties in MnO 2 -D were well-preserved during the reaction (Supplementary Fig. 5). After calcining the spent catalyst, the catalytic activity of MnO 2 -D was fully restored. MnO 2 -D is further extended to the selective oxidation of intermediate substrates derived from glycerol and other polyols/ sugars (Fig. 3d). In all cases, high conversion (>75%) and FA selectivity (>60%) were obtained. The excellent catalytic performance encourages us to further explore the structure-activity relationship on the defective Mn δ+ -O V structure with strength-intensified FLPs in the polyol/sugar oxidation to FA.

Activation of O 2 into hydroxyl radical on Lewis acid site in MnO 2 -D
In situ EPR spectra were collected to probe the evolution of oxygen vacancy of MnO 2 -D during catalytic oxidation of glycerol (Fig. 4a). The signal of oxygen vacancy could be regarded as the function strength of FLPs in the Mn δ+ -O V structure since the strength-intensified FLPs are originated from the rich oxygen vacancies induced by the lowcoordination structure. We observe a symmetrical EPR peak at g = 2.003 in MnO 2 -D, attributable to unpaired electrons associated with oxygen vacancies of metal oxides. Interestingly, the intensity of the peak shows a progressive increase with rising reaction temperature. Moreover, the peak intensity demonstrates a strong linear correlation with initial catalytic activity ( Supplementary Fig. 9), indicating that an increase in the oxygen vacancy signal corresponds to a proportional increase in catalytic activity. The sluggish O 2 activation is one of the important reasons that restrict the oxidation activity. Inferentially, the formation of strength-intensified FLPs promotes the activation of O 2 , and then linearly increases the oxidation activity. To confirm this point, free radical trapping agent was added during glycerol oxidation by MnO 2 -P and MnO 2 -D catalyst to provide insights on O 2 activation (Fig. 4b). Blank test confirmed that there is no signal of radical in the absence of catalyst. In sharp contrast, both catalysts (especially MnO 2 -D) display obvious hydroxyl radical signals. The hydroxyl radical quenching experiment, employing organic and inorganic radical scavengers, further established that the produced hydroxyl radical significantly impacts the oxidation reaction (Supplementary Table 6). It is plausible that O 2 mainly interacts with H 2 O to generate hydroxyl radicals that participate in the subsequent glycerol activation, aligning with previous studies [71][72][73] .
Due to the difference in electronegativity between Mn δ+ and O V , the d-π feedback of the anti-bonding orbital weakens the O = O bond, allowing O 2 to dissociate into OH* in the presence of H 2 O (Fig. 4c)

Enhanced C-C and C-H bond activation over Lewis base sites in MnO 2 -D
The reaction mechanism of polyol oxidation to formic acid over the Mn δ+ -O V pair was further investigated by in situ Fourier transform infrared reflection (in situ FTIR), DFT calculation and reaction kinetic studies. Figure 5a shows that the α interaction (1125-1075 cm −1 ) and γ interaction (1075-1000 cm −1 ), attributable to alkoxy bond between primary hydroxyl and metal oxides, gradually increase. ρ(OH) at 1370 cm −1 and δ(OH) at 1440 cm −1 belonged to glyceraldehyde gradually increase, suggesting the activation of primary hydroxyl group in glycerol 71,74 . Meanwhile, the β interaction at 1200-1125 cm −1 , and ω(CH 2 ) at 1250 cm −1 and τ(CH 2 ) at 1300 cm −1 belonging to dihydroxyacetone are also enhanced. These suggest MnO 2 -P activates primary and secondary hydroxyl groups indiscriminately. Compared with MnO 2 -P, τ(CH 2 ), ω(CH 2 ) and the β interaction corresponding to the activation of secondary hydroxyl groups are significantly lower on MnO 2 -D, thus the preferential activation of primary hydroxyl group and C-C bond cleavage promote the generation of formic acid.
It is found that MnO 2 -D exhibits low activation energies for continuous C-C bond cleavage ( Supplementary Fig. 10), but high activation energies for deep oxidation reactions (such as the dihydroxy oxidation and decarboxylation to CO 2 ). In contrast, the activation energies of C-H and C-C bonds activation on MnO 2 -P is significantly higher than that on MnO 2 -D, and furthermore, the activation energies of multiple C-C bond cleavage are close to that of decarboxylation to CO 2 on MnO 2 -P. These corroborate in situ FTIR findings that MnO 2 -P without Mn δ+ -O V pairs does not activate glycerol effectively and that the specific selectivity towards formic acid is low. ( Supplementary Fig. 11c) 75,76 . Benefiting from the enhancement of O electronegativity (Lewis base) in the Mn δ+ -O V pair, the binding ability of C and H is abnormally promoted. This provides a superior stable base for C-C bond cleavage and C-H bond activation, resulting in the low activation energies of C-C cleavage and C-H bond activation on MnO 2 -D (Tri). Combined with micro-kinetic analysis ( Supplementary  Fig. 11d), the MnO 2 -D (Tri) is located in the red region (i.e., high TOF) with~8.0 eV of E C and~6.0 eV of E O , close to several noble metal catalysts 77 .

Discussion
In summary, we developed a defective α-MnO 2 catalyst enriched with Frustrated Lewis Pairs (FLPs) to promote the cascade oxidation of various polyols and sugars to formic acid. The reduction of oxygen coordination generates abundant exposed Mn species as Lewis acid sites, and strengthens the electron donating properties of adjacent oxygen to serve as Lewis base sites. During catalytic tests, a positive correlation between the abundance of strength-intensified FLPs in the defective Mn δ+ -O V structure and catalytic activity was identified. Further characterization by various techniques provide evidence that the FLPs on defective MnO 2 -D catalyst promote the O 2 as well as C-H and C-C bond activation synergistically on adjacent Lewis acid and base sites. As a result, the low-coordination MnO 2 -D exhibits superior catalytic activity (TOF: 113.5 h −1 ) and formic acid yield (formic acid yield >80%) for glycerol oxidation, which are comparable to the performance of previously reported noble metal catalysts. The catalyst is also effective in converting ethylene glycol, 1,2-propanediol, 1,3-propanediol, erythritol, xylitol and sorbitol into formic acid in yields ranging from 51.5 to 94.8%. This work demonstrates that metal oxide catalysts with water tolerant FLPs are promising for the oxidative polyol/sugar transformation.

Catalyst preparation
The MnO 2 -P catalyst was prepared by a hydrothermal method 50 . In a typical process, 0.1 mol HCl and 0.25 mol KMnO 4 were added into 90 mL deionized water. Then, the mixed solution was transferred into a Teflon-lined stainless autoclave, which was hydrothermally treated at 140°C (12 h). The as-formed precipitate was centrifugated and washed to remove excess ions. After drying in air overnight (100°C), the sample was calcined to obtain the MnO 2 -P catalyst in muffle furnace at 400°C for 1 h. The MnO 2 -D catalyst was prepared by another hydrothermal method 50 . In a typical process, 1.58 g KMnO 4 was added in 30 mL deionized water, and 20 mL (NH 4 ) 2 C 2 O 4 ·H 2 O was further added in the above solution drop by drop. After 1 h of stirring, the mixture was transferred to a Teflon-lined stainless autoclave for 24 h at 180°C. The as-formed powder was washed and filtered repeatedly. After drying in air overnight (100°C), it was calcined to obtain the MnO 2 -D catalyst in muffle furnace at 400°C for 1 h.

Catalytic test
Catalytic oxidation of polyol/sugar was carried out in the 50 mL autoclave. In a typical process, 25 mL deionized, 0.5 g substrate and 0.1 g catalyst were added in the autoclave. Through several times of O 2 washing, the final pressure was maintained at 1.0 MPa. After this oxidation reaction, the liquid product was analysed by the high performance liquid chromatography (HPLC) equipped with refractive index (RID-10A) and UV detectors (Shimadzu LC-20AT). The Rezex ROA-Organic Acid H+ (8%) was used as the chromatographic column in the mobile phase (0.005 M H 2 SO 4 ). The gas product was detected by the Chromatography equipped with a FID and TCD detector (Scion 456-GC). The definitions of conversion (X), product selectivity (S), turnover frequency (TOF) and carbon mass balance (CMB) were calculated by the following formula: All energy correction terms are extracted from the normal mode analyses of transition state the optimized reactant at various temperatures. Gibbs free energies correction has contained the ZPE correction. Outputs from Dmol 3 calculations include corrections to consider bare Gibbs free energies and electronic energies. The activation barrier (ΔE), the activation barrier with zero-point vibrational energy correction (ΔE Z ) and free energy presented (ΔG) are obtained using the following formula: Where E R and E TS are the electronic ground state energies of the reactant and transition state, respectively. The ZPE TS and ZPE R are the respective zero point vibrational energy (ZPE) corrections. G R and G TS are the free energy corrections of the reactant and the transition state, respectively.

Characterizations
Rietveld refinements were carried out using TOPAS Academic 51,53,59 . Firstly, Pawley fitting was performed to refine the lattice parameters. The background was modeled using the Chebyshev function with 12 parameters. The peak shape was modeled via the Stephens peak shape function (an approach to spherical harmonics for hkl dependent peak shapes) considering the strain anisotropy broadening 3 . The initial atomic coordinates for the Manganese oxides with space group I4/m (87) were generated from the crystal structure in the Pearson's Crystal Data (#1102438) 4 . The refined parameters such as lattice parameters, background and sample specimen displacement from the Pawley fitting were kept the same for the Rietveld refinements. The z atomic coordinates of the Oxygen and Manganese ions were set to 0. Then, the following parameters were refined in sequence: (1) scale factor, (2) O, Mn atomic coordinates (x and y), (3) atomic (Oxygen and Manganese ions) occupancies and (4) the overall atomic displacement parameters. All the Rietveld refinements gave satisfactory agreement factors. Scanning electron microscopy (SEM) was used to obtain the morphology of the MnO 2 -P and MnO 2 -D catalysts on a Hitachi S-4800. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out on the Titan 80-300 scanning/ transmission electron microscope. X-ray photoelectron spectroscopy (XPS) was measured on the Thermo ESCALAB 250Xi with the correction of C 1 s binding energy of 284.8 eV, and other testing and analysis details were provided in Supporting Information. N 2 physisorption was measured on Micromeritics ASAP 2020. X-ray absorption spectroscopy (XAS) were measured on the Advanced Photon Source at Argonne National Laboratory (fluorescence mode on beamline 12-BM). The ATHENA module was employed to deal the data of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS). CO 2 temperature programmed desorption (CO 2 -TPD), H 2 -temperature programmed reduction (H 2 -TPR), NH 3 -TPD and O 2 -TPD were measured on the Micromeritics AutoChem II 2920. For the CO 2 -TPD, O 2 -TPD and NH 3 -TPD, 0.1 g sample was treated in the N 2 gas at 300°C for 1 h (60 mL/min). After the cool down of the sample (50°C), 5 vol% O 2 (NH 3 or CO 2 ) gas in 95 vol% N 2 was introduced for 1 h (30 mL/min). Then, the quartz tube was heated to 1000°C at a rate of 20°C/min. For H 2 -TPR, the sample was reduced in 10 vol% H 2 in Ar (60 mL/min) from 50°C to 1000°C with a heating rate of 15°C/min. All the signals were collected by TCD detector. UV-vis absorption spectras were measured on a UV-vis-NIR Cary 5 (Varian) spectrophotometer. NMR was conducted on a Bruker Ascend 400 MHz NMR spectrometer. Mass spectrometric analysis was conducted on a QMS 200 (Balzers) quadrupole mass spectrometer.
Electron paramagnetic resonance (EPR) spectra were measured to obtain oxygen vacancy on a Bruker EMX-6/1spectrometer at 298 K. For the in situ EPR experimental (detection of oxygen vacancy), an in situ cell was loaded by 10 mL mixed solution of glycerol and sodium hydroxide (0.05 mol/L). The system pressure drop was monitored in real time. The sample was heated using a Bruker EMX plus continuous flow temperature control system. During this process, pure O 2 (100 mL/min) was purged into the cell. The EPR spectra were collected between 2400 and 3600 G in 83 ms. The microwave frequency was 9.3 GHz with a power of 0.2 mW, and the field was modulated at 100 kHz and with an amplitude of 5 G. For the Operando EPR experimental (detection of hydroxyl radical), three kinds of contrast experiments were designed. Fully put MnO 2 -P and MnO 2 -D catalysts into the evaluation conditions for reaction (120°C for 4 h), and immediately put the samples after the reaction with DMPO into EPR for testing. The other group is a parallel blank experiment. Except that no catalyst is added, the other processes are completely consistent.
In situ, Fourier transform infrared reflection (in situ FTIR) was measured on the Thermo Scientific Nicolet iS50 FT-IR. For the in situ CO and CO 2 , the samples were pretreated by 50 mL/min of N 2 at 300°C for 1 h. After the sample cools to 80°C, 5 vol% CO or CO 2 in N 2 (40 mL/min) was purged for 20 min to realize the adsorption saturation. Then, 40 mL/min N 2 was performed to remove the gas phase CO or CO 2 . The spectra were collected using 128 scans in a resolution of