Titanium MXenes as Heterogeneous Catalysts for the Styrene‐to‐Benzaldehyde Oxidation: Influence of the Etching Conditions

Ti3C2Tz MXenes are prepared under different etching conditions and tested as heterogeneous catalysts for the selective oxidation of styrene to benzaldehyde. A clear dependence is observed between the etching conditions (hydrofluoric acid concentration and etching time) and the surface chemistry, morphology and catalytic performance of the resulting MXenes. Under appropriate conditions, etching of the Ti3AlC2 MAX phase precursor with concentrated HF produces Ti3C2Tz MXenes with highly accessible accordion‐like structures and accessible Ti−O surface terminations that can act as active species for the catalytic process, keeping up with the best catalysts reported so far for this reaction. The present study represents one of the few existing reports on catalytic properties of MXenes under mild liquid‐phase conditions, paving the way for future developments of this new family of 2D materials for fine chemical applications.


Introduction
MXenes are a new family of two-dimensional transition metal carbides synthetized for the first time in 2011 by selective chemical etching of layered ternary carbide precursors known as MAX phases. [1]Various methods have been described to produce the etching of the MAX phase, but the most extensively reported use and aqueous solution of HF or other fluoride compounds, such as LiF/HCl, NH 4 HF 2 , NaHF 2 , KHF 2 , etc.However, given the elevated toxicity and corrosivity of HF, other etching methods are being considered, including electrochemical synthesis in the presence of a NH 4 Cl/tetramethylammonium hydroxide electrolyte, [2] alkali treatments with NaOH at high temperature, [3] or methods based on microwave heating. [4]owever, most of the methods studied so far present also their own drawbacks, such as low yields or the use of high temper-atures and concentrations, which limits also their practical application.
The first member of the MXenes family, viz.[7][8] The general formula of MXenes is M n + 1 X n T z where M is an early transition metal, X is carbon and/or nitrogen, and T z represents possible surface terminations (mainly hydroxyl À OH, oxygen À O and/or fluorine À F groups, and the subscript z indicates nonstoichiometric terminations).The possibility to obtain MXenes with different surface compositions, chemical and physical properties by varying MAX phase precursor, as well as etching or exfoliation conditions, make them very attractive materials for a large variety of applications.The 2D layered structure, hydrophilicity, tunable chemistry, high mechanical stability and outstanding electric conductivity of MXenes rapidly led to application of these materials in the fields of energy storage, particularly in hydrogen storage, [9,10] ion batteries [11,12] and supercapacitators. [13][16] Recently, MXenes have also been investigated as promising heterogeneous catalysts or supports in some relevant chemical processes concerning the activation of small molecules.In particular, outstanding performances of MXenes have been described for CO oxidation, [17] activation and conversion of CO 2 , [18] N 2 fixation, [19] water-gas shift (WGS), [20] direct dehydrogenation (DDH), [21] and hydrodeoxygenation (HDO) reactions. [22]Nonetheless, most of the examples of MXenes as heterogeneous catalysts reported in the literature are related to gas phase reactions that require harsh reaction conditions of temperature and pressure.However, to the best of our knowledge, there are only two examples of catalytic activity of MXenes in liquid phase reactions under low energy demanding conditions.
Zielińska and col.have reported on the catalytic performance of Ti 3 C 2 T z MXene in α-pinene isomerization to camphene. [22]ecently, Slot et al. have described the selective ring-opening of styrene epoxide using surface modified Ti 3 C 2 T z MXenes. [23]It was demonstrated that MXenes exhibit strong acid sites capable of catalyzing both ring opening and isomerization of epoxide, while a Ti 3 C 2 T z À TiO 2 sample modified by incorporation of TiO 2 crystals catalyze only the ring-opening reaction, thus increasing the selectivity to the mono-alkylated product.
However, to the best of our knowledge MXenes, and Ti 3 C 2type compounds in particular, have never been used as catalysts for oxidation reactions in liquid phase, despite having highly exposed titanium centers in their extended 2D surfaces as clearly potential active sites.Therefore, we wanted to evaluate herein the potential of Ti 3 C 2 -type MXenes as heterogeneous catalysts in liquid-phase selective oxidation reactions, using O 2 and/or (hydro)peroxides as oxidizing agents.In this line, selective oxidation of styrene to benzaldehyde has attracted large interest from academic and industrial points of view as a promising alternative green method to produce benzaldehyde.Benzaldehyde is a well-known industrially important chemical compound used in the synthesis of fine chemicals, drugs, perfumes, plasticizers and resin epoxides. [23,24]t is currently obtained in the industry by toluene chlorination followed by hydrolysis, or as a byproduct after oxidation of toluene to benzoic acid.Both of these methods possess important drawbacks, such as low selectivity to benzaldehyde, equipment corrosion, and severe environmental pollution. [25]In this context, selective oxidation of styrene to benzaldehyde in the presence of heterogeneous catalysts and environmentally friendly oxidants is considered one of the most promising alternatives to current industrial process.This alternative route allows decreasing the formation of toxic chlorine waste, avoids using complex separation process and reduces corrosion problems.Several heterogeneous catalysts have been studied for styrene oxidation to benzaldehyde, including metal oxides, nanoparticles supported on zeolites, MOFs and carbon materials as well as transitional metal coordination complexes, [26][27][28][29][30][31][32][33][34][35][36] (see Table S1).However, in most reported examples high selectivity to benzaldehyde was only achieved at low or moderate conversion of styrene.Therefore, there is still a big challenge to find active and selective catalytic systems for this reaction.
Herein, we have evaluated the performance of Ti 3 C 2 T z MXenes for selective liquid-phase oxidation of styrene to benzaldehyde under mild conditions.Special emphasis is placed on determining the influence of different etching conditions on surface chemistry, morphology and catalytic activity of MXenes.

Synthesis of Ti 3 C 2 T z -HF MXenes
Samples of MXenes were prepared by treating Ti 3 AlC 2 (MAX phase precursor, CARBON-UKRAINE Ltd) with HF under different synthesis conditions, following the method described by Mashtalir et al. [11] with slight modifications.These samples were labeled as Ti 3 C 2 T z -xHF-y, where x is the concentration of hydrofluoric acid and y is the time of etching (in hours).For example, to prepare Ti 3 C 2 T z -40HF-24 sample, 40 % HF aqueous solution (10 mL) was slowly added to 1 g of Ti 3 AlC 2 powder.The suspension was kept for 24 h under continuous stirring at ambient temperature.After that, the mixture was centrifuged at 8000 rpm and washed several times with distilled water until the pH of the supernatant reached the value of 6.Finally, the decanted solid was dispersed in water under Ar atmosphere and lyophilized.Ti 3 C 2 T z -40HF-6 sample was prepared using the same procedure as described above but reducing the synthesis time to 6 h.Ti 3 C 2 T z -10HF-24 sample was prepared using the same procedure as described above but using 10 % HF aqueous solution and 24 h of etching time.
Exfoliation of Ti 3 C 2 T z -40HF-24 was performed by intercalation of dimethyl sulfoxide (DMSO) into the interlayer space of Ti 3 C 2 T z -40HF-24, followed by sonication in an ultrasound bath.In particular, DMSO was added to Ti 3 C 2 T z -40HF-24 in a ratio of 1 mL DMSO per 60 mg of MXene.The suspension was stirred at ambient temperature for 18 h.After that, the solid was filtered to remove excess DMSO and introduced in an ultrasonic bath in distilled water (1 mL of water per mg of solid) for 6 h.Then, the mixture was centrifuged at 4000 rpm during 10 min and the solid deposited at the bottom of the tube was discarded (non-exfoliated fraction).The supernatant fraction was filtered, washed with water and dried by lyophilization.
All synthetized samples of MXenes were stored in a fridge at À 8 °C under nitrogen atmosphere until use.

Reaction procedure of styrene oxidation
The general procedure for styrene oxidation over MXenes using H 2 O 2 (50 %) as an oxidant was carried out as follows: 0.5 mmol of styrene, 10 mg of catalyst and 2 mL of acetonitrile were placed in a two-neck round-bottom flask connected to a reflux condenser.The reaction mixture was heated at 70 °C in an oil bath under nitrogen atmosphere.A constant stirring rate of 800 rpm was used to avoid external diffusion limitations.Then, 1 mmol of H 2 O 2 was stepwise added (10 μl aliquots added at 10 min intervals) to the reaction mixture.Reaction samples were taken periodically and analyzed by GC with a Varian 3000 instrument equipped with HP-5 column (30 m×0.25 mm×0.5 μm) and flame ionization detector (FID).Dodecane was used as internal standard.The oxidation products were identified by GC-MS technique and by comparing the retention times in the chromatograms with pure standards.

Characterization techniques
All synthetized MXenes were analyzed by powder X-ray diffraction (PANalytical CubiX Pro, Cu Kα radiation) to confirm the expected structure of materials.The morphology of MXenes was visualized by field emission scanning electron microscopy (FESEM) using a JEOL JSM-7001F.The chemical properties of MXene surface were studied by X-ray photoelectron spectroscopy analysis (XPS).XPS spectra were recorded using a SPECS spectrometer equipped with a mg Kα (1253.6 eV) X-ray source and a 150MCD-9 Phoibos detector.

Characterization of Ti 3 C 2 T z -xHF-y MXenes
Three different samples of MXenes were synthetized from the same Ti 3 AlC 2 MAX phase precursor by varying the concentration of HF and time of etching, as shown in Scheme 1.One selected sample (Ti 3 C 2 T z -40HF-24) was further modified by intercalation/ exfoliation with DMSO, as described in the Experimental section.
Ti 3 AlC 2 MAX phase precursor and the as-synthetized samples of Ti 3 C 2 T z -xHF-y were analyzed by X-ray diffraction (Figure 1).The XRD patterns of MXenes showed characteristic signals for these materials. [1,37,38]In particular, the XRD patterns of Ti 3 C 2 T z -xHF-y samples showed broadening and a downward shift of the (00 l) peaks at 2θ of 9.58°, 19.18°and 38.78°in the MAX phase precursor, reflecting the expansion of interlayer space after removing of Al, and leading to higher c lattice parameters, as indicated in Figure 1.
The results of inductively coupled plasma (ICP) analysis (See Table S2) confirm in all cases that Al was largely removed during the etching step, being the samples etched at 24 the ones with the highest Ti/Al molar ratio.Specifically, etching with diluted hydrofluoric acid is more selective for Al removal than when concentrated HF was used, affording a higher Ti/Al molar ratio in the resulting MXene sample: 23.5 vs 13.8 for samples Ti 3 C 2 T z -10HF-24 and Ti 3 C 2 T z -40HF-24, respectively.When the etching time was reduced from 24 h to 6 h, the treatment was less efficient, and a significant amount of aluminum remained (6.2 wt %), resulting in a lower Ti/Al molar ratio of 5.0 in the final MXene.
To study the morphology and structure of MXenes synthesized under different etching conditions, FE-SEM electron microscopy analysis was carried out.As shown in Figure 2 (C  and D), MXenes obtained in the presence of concentrated HF (40 %) exhibit the characteristic accordion-like layered structure typical of these materials.A spacing between the layers is evident, confirming the selective removal of aluminum layers present in the MAX phase (Figure 2-A).However, in the sample treated with diluted HF (10 %) (Figure 2-B), this accordion-like structure is not observed, and the layers are more densely packed and compacted in clusters, as in the case of the starting MAX phase precursor.This could indicate that the exfoliation process in the presence of 10 % HF produces less effective separation of the MXene layers compared to that achieved using more concentrated HF.
The use of XPS analysis provides valuable information on the surface chemistry of MXenes, enabling the correlation between the active Ti species present in MXenes and their catalytic activity.Based on the XPS survey spectra (Figure S1), the main components of MXene surface include titanium, carbon, oxygen, and fluorine, with their corresponding atomic compositions presented in Table S3.The absence of aluminum on the surface of the synthesized samples, as indicated by the survey spectra, suggests that it was effectively removed during the etching and washing procedures.
To identify the main atomic species in Ti 3 C 2 T z -xHF-y samples and MAX phase, the XPS spectra of O 1 s, C 1 s, Ti 2p, and F 1 s regions were deconvoluted (Figure S2).By comparing the XPS  spectra of as synthetized MXenes with MAX phase shown in Figure 3, the most relevant changes in the surface composition related to the different etching conditions were determined.
In all synthetized MXenes, the XPS spectra of Ti 2p region show four main doublets of Ti 2p 3/2 and Ti 2p 1/2 (see Figure S3 and Table S1) that were assigned to TiÀ C, Ti 2 + , Ti 3 + and TiO 2 components.This assignment corresponds to Ti atoms bonded to the carbon and terminal groups (À O, À OH or À F) according to the previous literature. [39,40]By comparing the XPS spectra of Ti 2p region among synthetized MXenes, an increase in the proportion of Ti 2 + and Ti 3 + species bonded to various terminal groups (À O, À OH and À F) was observed in contrast to the initial MAX phase (Figure 3).Meanwhile, the concentration of TiO 2 in all synthetized MXenes was significantly lower than in MAX phase (40 %), with the samples etched with 40 wt % HF having the lowest percentage of TiO 2 (see Table S3).
In the C 1 s region of the XPS spectra, four main peaks were detected and assigned to CÀ C (284.8 eV), CÀ TiÀ T z (281.5-281.9eV), CÀ O/CH x (286.5-287.6 eV) and C=O/COO (287.5-289.5 eV) carbon species in agreement with previously reported studies. [39,41]Clear predominance of CÀ C peak arising from carbonaceous species can be observed for MAX phase and MXene Ti 3 C 2 T z -10HF-24 etched with 10 wt % HF (Figure 3).Meanwhile, when the concentration of HF and time of etching increased, the concentration of oxidized carbon species (CÀ O/ C=O/COO) also increased, thus suggesting the intensification of oxidation processes under harsher etching conditions.
Finally, two main peaks were detected in XPS spectra of F 1 s region and assigned to CÀ TiÀ F x (685.3-686.8eV) and TiO 2- x F 2x (684.1-684.9eV) species.Comparative XPS spectra of F 1 s region confirm that the Ti 3 C 2 T z -10HF-24 sample with greater content of TiO 2 (based on Ti 2p spectra) has a higher percentage of TiO 2-x F 2x species (75.9 %) .Nevertheless, in samples treated with 40 wt % HF the percentage of F À bonded to CÀ Ti and TiO 2 species is quite similar (see Table S3).
Preliminary catalytic results discussed below showed that the Ti 3 C 2 T z -40HF-24 sample exhibited the highest catalytic activity among other tested materials.To increase the surface area of this MXene and the accessibility to active Ti centers, a post synthesis intercalation [5,37] and delamination [1,11] method was used.This method consists of intercalating different organic molecules on the MXene accordion-like structure to weaken the interlayer forces and promote easier delamination.Based on previously reported studies, DMSO [11] was chosen as an intercalation agent to facilitate the exfoliation of Ti 3 C 2 T z -40HF-24 MXene, as detailed in the Experimental Section.The XRD patterns of Ti 3 C 2 T z -40HF-24 sample before and after DMSO intercalation showed the appearance of a new diffraction peak at lower 2θ angle and a decrease of the intensity of the (002) peak.This indicates that only a fraction of the material was delaminated under the conditions used, producing an increase of the interlayer distance and a concomitant increase of the c lattice parameter of 4.7 Å (Figure 4-B).FESEM image of intercalated by DMSO sample (Figure 4-D) also showed appearance of very small (< 1 μm) defective few layered MXene particles.Any attempts to increase the fraction of material delaminated by changing the experimental conditions did not improve the results shown in Figure 4, and full delamination was never achieved.The XPS features of sample Ti 3 C 2 T z -40HF-24 (DMSO) are presented in Figure S2 and Table S3, while Figure S3 compares the XPS spectra of sample Ti 3 C 2 T z -40HF-24 before and after intercalation/delamination with DMSO.The most relevant changes produced by the DMSO treatment consist of a slight increase of the amount of TiO 2 , reflecting that a partial oxidation of the MXene surface takes place during the intercalation/delamination process.This is accompanied by a  small decrease of oxidized carbon species (CÀ O/C=O/COO), which are most likely removed during the washing steps of the sample.
In summary, it can be concluded that both the concentration of HF and the synthesis time affect the surface chemistry of MXenes.In the three HF treatment conditions studied, an increase in TiÀ C, Ti 2 + , and Ti 3 + species formed during the removal of the Al layer from the MAX phase and substitution of TiÀ Al bonds by TiÀ O, TiÀ OH, or TiÀ F terminations in the MXene layers is observed.These new Ti species on the surface of MXenes will determine their catalytic properties, which will be presented in the following section.However, the acid treatments carried out also produce other collateral effects, such as the decrease of TiO 2 impurities on the surface and the partial degradation and oxidation of carbon species.

Selective oxidation of styrene to benzaldehyde in presence of MXenes. The catalytic activity of the as-synthetized
MXenes was evaluated for the oxidation of styrene at 70 °C, using H 2 O 2 (50 %) as an oxidant and acetonitrile as a solvent.The results presented in Table 1 indicate that all synthetized MXenes catalyze this reaction, yielding benzaldehyde (2) as the main product.Additionally, other byproducts, including styrene oxide (3), phenylacetaldehyde (4) and 1-phenyl-1,2-ethanediol (5) were also detected by GC-MS analysis.
Clear differences in catalytic activity and selectivity to benzaldehyde were observed among MXenes etched under different conditions.Kinetic plots of conversion shown in Figure 5 show that the highest catalytic activity was achieved over Ti 3 C 2 T z -40HF-6 and Ti 3 C 2 T z -40HF-24 MXenes, reaching 71.7 % and 79.3 % conversion, respectively, after 6 hours of reaction.In contrast, the Ti 3 C 2 T z -10HF-24 catalyst only achieved 11.7 % of conversion after 6 h.Additionally, a progressive decline in the benzaldehyde selectivity was observed as a function of reaction time and conversion of styrene (Figure 5,  B).This decline can be attributed to the predominant formation of 1-phenyl-1,2-ethanediol as a byproduct at high styrene conversion.For comparison purposes, the reaction was carried out in the presence of MAX phase (entry 2, Table 1) and TiO 2 (Degussa P25, entry 8, Table 1).As shown in Table 1 the MAX phase (Ti 3 AlC 2 ) exhibited very low catalytic activity, achieving only 1.6 % conversion after 6 h.The low catalytic activity of MAX phase and Ti 3 C 2 T z -10HF-24 sample can be related to the stacked character of both structures, as observed by FESEM analysis (Figure 2), leading to low surface area and poor accessibility of reactants to active Ti centers.In contrast, highly accessible accordion-like structure of MXene samples treated with 40 %HF can contribute to their higher catalytic activity.Moreover, XPS analysis (Table S1) indicated a low proportion of accessible TiÀ O species (Ti 2 + and Ti 3 + ) on the surface of MAX phase, which may be responsible for its low catalytic activity.
On the other hand, TiO 2 show higher catalytic activity than MAX phase and Ti 3 C 2 T z -10HF-24 MXene, attaining 34.8 % styrene conversion after 6 h.However, its activity was still significantly lower than Ti 3 C 2 T z -40HF-6 and Ti 3 C 2 T z -40HF-24 MXenes, thus indicating that the superior catalytic activity of etched MXenes is not related to the presence of TiO 2 impurities on the surface (See XPS results, Table S3).
In summary, Ti 3 C 2 T z -40HF-24 sample exhibited superior catalytic activity among all tested catalysts, so that the effect of post synthesis DMSO intercalation as well as the amount of H 2 O 2 were studied using this MXene.

Effect of post synthesis DMSO intercalation and delamination of MXene.
As it was mentioned above, Ti 3 C 2 T z -40HF-24 sample was exfoliated by DMSO intercalation to increase the surface area of this MXene, as well as to enhance the accessibility to active Ti centers.However, in contrast to the results expected, the sample exfoliated with DMSO exhibited lower catalytic activity than the initial Ti 3 C 2 T z -40HF-24 MXene.Conversion of styrene was found to be 10 % lower, and the selectivity towards benzaldehyde was reduced by 13 %.(Entry 5 and 7, Table 1).Although intercalation with DMSO resulted in a more delaminated material (See FESEM image Figure 4-D), this was not reflected in an increase in its catalytic activity, contrary to expectations.This may be due to the increased amount of TiO 2 formed on the surface of MXene after the DMSO treatment (See XPS results in Table S2 and Figure S3), which could partially passivate the MXene surface and decrease accessibility to Ti active sites.

Effect of amount of H 2 O 2 on styrene oxidation reaction.
The effect of the amount of H 2 O 2 as an oxidant on the oxidation reaction of styrene was also explored.The amount of H 2 O 2 is a critical parameter since, during the course of the reaction, a part of this oxidant decomposes (producing water as a byproduct) and does not participate in the styrene oxidation reaction.To optimize the amount of H 2 O 2 , the reaction was carried out in the presence of two mmol of the oxidant instead of one mmol.As shown in Table 1 (Entry 6), the final conversion of styrene increases when more H 2 O 2 is introduced, reaching conversions of 98 % after 6 hours of reaction.However, kinetic curves in the Figure S4 (A) showed that the initial reaction rate does not increase with the increased excess of H 2 O 2 .Therefore, the lower final yield of benzaldehyde must be due to the unproductive consumption of H 2 O 2 through unproductive decomposition, limiting its availability at longer reaction time when a lower excess of the oxidant is used.On the other hand, the selectivity towards benzaldehyde was negatively affected by an increased excess of H 2 O 2 due to the higher production of 1-phenyl-1,2-ethanediol as a byproduct (Figure S4-B) at the same level of conversion.

Reaction mechanism.
The reaction mechanism of styrene oxidation to benzaldehyde in the presence of heterogeneous catalysts proposed by most authors involves the interaction of active metal species with the molecules of oxidant, generating reactive metal-oxide species. [24,30,33]Two different reaction pathways for styrene oxidation can be considered.On one hand, direct epoxidation of styrene to styrene oxide can occur, followed by a nucleophilic attack (HÀ OÀ OÀ H) of styrene oxide to form benzaldehyde and other byproducts (left path in Scheme 2).On the other hand, the formation of benzaldehyde by direct oxidative cleavage of C=C bond of styrene can proceed through a radical mechanism (right path in Scheme 2).Two types of nucleophilic radicals can be formed by decomposition of H 2 O 2 on the Ti-based surface: * OH and * OOH/O 2 *À radicals. [42]To determine whether radical or non-radical active species are involved (and to which extend) in the reaction mechanism, a series of additional experiments were performed.Thus, the styrene oxidation reaction was carried out in the presence of DMSO as a well-known * OH scavenger, or 1,4-benzoquinone (BQ) as a * OOH/O 2 *À scavenger.The results obtained are presented in the Table 2 and compared with the reaction in the absence of any additive.
When DMSO was added to the reaction mixture, the styrene conversion slightly decreased from 89.5 to 61.5 % (compare entries 1 and 2, Table 2), while the addition of BQ led to a stronger decrease in styrene conversion, down to 35.2 % (Entry 3, Table 2).In both cases, benzaldehyde was still the main reaction product, with a selectivity similar to that of the non-quenched reaction.These results demonstrate that both radicals (given the stronger effect of BQ).However, the reaction is not completely turned off by neither radical scavenger, which suggests that a radical and a non-radical reaction pathways coexist, as shown in Scheme 2.

Reusability of Ti 3 C 2 T z -40HF-24
MXene in oxidation reaction of styrene.To evaluate the reusability and stability of Ti 3 C 2 T z -40HF-24 catalyst under reaction conditions, the solid was used during four consecutive reaction cycles.After each reaction cycle the catalyst was washed with acetonitrile and dried under ambient temperature for 24 h and used directly in a consecutive cycle.As it can observed in Figure 6, the conversion decreases progressively with reuse, indicating a decline in the catalytic activity of MXene.This is probably due to the adsorption of reaction products on the surface of the catalyst, which cannot be completely removed during the washing stage, thus hindering the access of reactants to the reactive titanium species during consecutive reuses.However, this is partially compensated by a progressive increase of the selectivity towards benzaldehyde with reuses, so that the final yield does not decrease as significantly.Meanwhile, the XRD analysis of the solid recovered after the 4 consecutive uses (see Figure S5) shows a slight diminution of the intensity of the characteristic peaks of MXene at 9.6°, 19.2°and 28.9°2θ, but their position is maintained throughout the catalytic cycles, and thus also its crystalline structure.The decrease in intensity might be related with a decrease of the particle size due to physical attrition over the reuses, but also to the conditions of the analysis and the very low amount of sample used to measure the XRD pattern of the used catalyst.

Conclusions
In summary, Ti 3 C 2 T z MXenes prepared under different etching conditions were tested for the selective oxidation of styrene to benzaldehyde.The results clearly demonstrated that the concentration of HF as well as the time of etching strongly affect the surface chemistry, morphology and catalytic performance of MXenes.Particularly, Ti 3 C 2 T z -40HF-24 MXene prepared at harsher etching conditions showed superior catalytic activity among other synthetized MXenes.Highly accessible accordionlike structure of Ti 3 C 2 T z -40HF-24 MXene as well as the presence of accessible TiÀ O active species determine their higher catalytic activity.Interestingly, the performance of Ti 3 C 2 T z keeps up with the best catalysts reported so far for this reaction, including metal-incorporated zeolites, [43][44][45][46] metal oxides, [44,47,48] as well as metal-Schiff base complexes [49] and metalloporphyrins [50] homogeneous catalysts (see Table S1).Thus, MXenes emerge as a novel class of heterogeneous catalysts for the synthesis of benzaldehyde from styrene and oxidation reactions in general.The present study represents one of the few existing reports so far on catalytic properties of  MXenes under mild liquid-phase conditions, paving the way for future developments of this new family of 2D materials for fine chemical applications.

Figure 3 .
Figure 3. High resolution XPS spectra of C 1 s, Ti 2p, O 1 s, F 1 s for Ti 3 C 2 T z -xHF-y samples of MXene, MAX phase.

Figure 5 .
Figure 5. Catalytic results of styrene oxidation in presence of synthetized MXenes, MAX phase: A -Conversion of Styrene & time plot; B -Selectivity of Benzaldehyde & Conversion of Styrene plot.

Table 1 .
Summary of the catalytic results obtained for styrene oxidation over synthetized MXenes, MAX phase, TiO 2 .