Regioselective Friedel-Crafts Acylation Reaction Using Single Crystalline and Ultrathin Nanosheet Assembly of Scrutinyite-SnO2.

Peculiar physicochemical properties of two-dimensional (2D) nanomaterials have attracted research interest in developing new synthetic technology and exploring their potential applications in the field of catalysis. Moreover, ultrathin metal oxide nanosheets with atomic thickness exhibit abnormal surficial properties because of the unique 2D confinement effect. In this work, we present a facile and general approach for the synthesis of single crystalline and ultrathin 2D nanosheets assembly of scrutinyite-SnO2 through a simple solvothermal method. The structural and compositional characterization using X-ray diffraction (Rietveld refinement analysis), high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and so on reveal that the as-synthesized 2D nanosheets are ultrathin and single crystallized in the scrutinyite-SnO2 phase with high purity. The ultrathin SnO2 nanosheets show predominant growth in the [011] direction on the main surface having a thickness of ca. 1.3 nm. The SnO2 nanosheets are further employed for the regioselective Friedel-Crafts acylation to synthesize aromatic ketones that have potential significance in chemical industry as synthetic intermediates of pharmaceuticals and fine chemicals. A series of aromatic substrates acylated over the SnO2 nanosheets have afforded the corresponding aromatic ketones with up to 92% yield under solvent-free conditions. Comprehensive catalytic investigations display the SnO2 nanosheet assembly as a better catalytic material compared to the heterogeneous metal oxide catalysts used so far in the view of its activity and reusability in solvent-free reaction conditions.

Introduction

Ultrathin two-dimensional (2D) nanomaterials are a class of material endowed with sheetlike nanostructure of atomic thicknesses and high area/thickness aspect ratios.[1−10] Owing to the geometric confinement in one of the dimensions, they have been attracting a lot of attention as a new generation advanced material with distinctive physical/chemical properties.[1−15] These compelling physical/chemical properties of 2D nanomaterials compared to bulk counterparts improve their applicability in various fields of catalysis, nanomechanics, gas sensors, and so on.[2−20] Particularly, the ultrathin nanosheets are most versatile one as nearly all the active atoms of the material lie at the surface of nanosheets maximizing the utility of such atoms.[2−25] Hence, the interaction between the weakly coordinated active surface atoms of the nanosheets and reactant species and the distinctive electronic property arising from 2D effects of nanosheets can tremendously change the activity of ultrathin nanosheets compared to conventional nanosheets/nanostructures.[1−4] The synthesis of ultrathin nanosheets from 2D graphene and graphene-type materials with layered structures that are bonded by weak van der Waals force are easy to prepare from their bulk structure.[10−12] However, achieving ultrathin nanosheets and their assembly of a structurally nonlayered material with high activity and structural stability is extremely difficult. Moreover, it is challenging to synthesize single crystalline ultrathin nanosheets and their assembly with high structural stability/durability.

Recently, 2D ultrathin metal oxide (such as Co3O4, Rh2O3, ZnO, TiO2, WO3, SnO2, and so forth) nanosheets with a microscopic level of thicknesses have been considered as advanced catalytic materials for heterogeneous catalytic reactions because of their improved catalytic activity and stability.[26−29] Among the synthetically demanding 2D metal oxide nanomaterials, scrutinyite tin(IV) oxide with the orthorhombic crystal structure plays an important role in various applications because of its low electrical resistivity (10–4–10–3 Ω cm), high sensitivity to atmospheric gas pollutants such as CO, CH4, NO, and so forth, improved capacitance by combining with other phases, and so on.[30−33] Normally, under ambient temperature and pressure, SnO2 forms its most stable tetragonal rutile-type crystal phase.[34−37] Conversely, the scrutinyite (α-PbO2)-type orthorhombic crystal phase is known to be stable only at high pressures and temperatures.[30,31] Still, a number of researchers have also claimed the formation of this crystal phase under ambient conditions. Moreover, scrutinyite-SnO2 with an orthorhombic crystal structure exhibits unique physiochemical properties compared to the stable tetragonal rutile-type phase that also motivate researchers to develop easy synthetic routes to obtain this phase.[30−33] However, because of stability concerns, the scrutinyite-SnO2 phase is seldom investigated and reported in the past few years. As a result, the formation of stable scrutinyite-SnO2 with the orthorhombic crystal structure under ambient conditions allows the exploration of new physical and chemical properties, opening its many possibilities for different applications.

The synthesis of the scrutinyite-SnO2 phase is mainly related to the method used and the experimental conditions such as temperature, pressure, and so forth.[31−33] Therefore, synthetic routes such as chemical vapor deposition, pulsed laser deposition, coprecipitation method, sol–gel method, and so forth are used so far. For example, epitaxial tin oxide film of the orthorhombic phase has been prepared by Kong et al. using the metal organic chemical vapor deposition (MOCVD) method.[38] Lamelas and Reid synthesized SnO2 nanoparticles of the orthorhombic crystal phase by the oxidation and postheating treatment of mechanically milled SnO powder.[39] A sequence of orthorhombic SnO2 thin films was also prepared by Chen et al. using pulsed laser deposition (PLD).[30] Carvalho et al. also observed the orthorhombic SnO2 phase in a composite of (Sn1–Ce)O2 synthesized using the sol–gel route.[33] Thus, all the above results indicate that the scrutinyite-type phase of orthorhombic SnO2 can be easily synthesized using various experimental methods. It is reasonable to use high pressures and/or temperatures or strain as these factors can be vital for the formation of the orthorhombic phase of SnO2.[30−32] However, till date, the use of solvothermal methods to prepare the orthorhombic phase of SnO2 has been rarely reported. Besides the innumerable importance of the α-PbO2-type phase of orthorhombic SnO2, morphology-controlled synthesis of the orthorhombic SnO2 nanostructure is scarce in reports. Furthermore, solvothermal synthesis and applications of single crystalline 2D ultrathin nanosheet or nanosheet assembly of scrutiniyte-SnO2 with an orthorhombic crystal structure have not been reported yet.

Again, the Friedel–Crafts (FC) acylation of aromatic substrates with acid halides is a fundamental and important conversion in organic synthesis.[40−42] In particular, regioselective FC acylation to synthesize aromatic ketones have potential significance in chemical industry as the ketones are involved in the preparation of various synthetic intermediates,[43] chemical feedstocks,[44] pharmaceuticals,[45] and fine chemicals.[46] Although the aromatic ketones have tremendous applications, yet conventional strategies to produce such ketones often suffer from major challenges of complicated synthetic procedures and environmental concerns.[47,48] The use of homogeneous catalysts, for example, protic acids,[49] soluble metal halides,[50] and substituted anhydrides[51] in stoichiometric amounts and other homogeneous catalysts, for example, Ln(OTf)3-LiClO4,[52] LiClO4-acyl anhydride complex,[53] (PhCN)2PtCl2/AgSbF6,[54] Re-Br(CO)5,[55] and so on in the catalytic amount spectacle certain progress in the reaction. However, the industrial application of these homogeneous catalysts is also associated with various shortcomings because of the noneconomical reaction process and environmental issues.[55,56] Therefore, design and development of a highly reactive and economical catalyst system with less environmental disputes is quiet challenging. Consequently, the effort has been focused on the synthesis of solid acid catalysts as certain surface properties of solids are not duplicated either in the solution or gas phase.[57] Various solid acid catalysts such as sulfated ZrO2 or Fe2O3,[58,59] sulfated Al2O3–ZrO2 (or TiO2),[60,61] acidic zeolite,[62] clays,[63] metal oxides,[64] heteropolyacid salts,[65] Nafion[66] and so forth have been used so far. Among them, mainly, the metal oxides have plenty of edge and corner sites for adsorption and activation of reactant molecules and are easily accessible, economically viable, and noncorrosive in nature.[67] At the present time, researchers have paid attention for synthesis and application of nanoscale metal or metal oxide catalysts for various organic transformations as catalysts in their nanometric regime provide improved reactivity and selectivity.[68,69] However, use of metal oxide catalysts for regioselective FC acylation in their nanoscale dimension is extremely less in earlier reports.

Accordingly, herein, we have demonstrated an easy route to directly synthesize single crystalline 2D ultrathin scrutinyite-SnO2 nanosheet assembly via a solvothermal method without using any external structure directing agent. Later, the single crystalline and ultrathin SnO2 nanosheet assembly is used for the regioselective FC acylation of aromatic substrates in solvent-free conditions. Finally, the synthesized scrutinyite-SnO2 nanosheets is represented as a better catalytic material compared to the conventional catalyst and heterogeneous catalysts used so far in the view of its activity/stability, reusability, and so forth.

Experimental Section

Synthesis of Scrutinyite-SnO2 Nanosheets

In a typical synthesis of scrutinyite tin(IV) oxide, 22.5 g/L of SnCl2·2H2O is dissolved in a mixture of distilled water and ethanol (1:3) to form a homogeneous solution. The pH of the solution is maintained at 12 by adding NaOH solution (1 M) to the solution and stirred for 1 h. The final mixture is transferred to a Teflon-lined autoclave, which is sealed and heated to 100 °C for 20 h. After the autoclave is cooled, the resulting precipitate is separated by centrifugation, washed several times, and finally dried overnight in an oven at 50 °C. Scheme illustrates the procedure for the synthesis of scrutinyite-SnO2 nanosheets.

Scheme 1

Schematic Representation of the Synthesis of Scrutinyite-SnO2 Nanosheets

Characterization

The synthesized SnO2 nanosheets are characterized by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (p-XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, Brunauer–Emmett–Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia (NH3-TPD) analysis, and FTIR spectroscopy of adsorbed pyridine (Py-FTIR). The products of the FC acylation reaction are characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS). Further details on the characterization techniques are provided in the Supporting Information.

Catalytic Experiments

The catalytic reaction is carried out in a 50 mL round-bottom flask equipped with a digital magnetic stirrer and a spiral glass condenser, immersed in a constant temperature oil bath. In this procedure, 10 mol % of catalyst and acid chloride (2 mmol) are mixed, followed by the addition of the aromatic substrate (4 mmol) to the mixture. The whole mixture is stirred at a desired temperature for a certain period of time until the reaction is complete (monitored by TLC). The reaction mixture is diluted with DCM, and the catalyst is separated from the reaction mixture by filtration. The DCM extract is washed with aqueous solution of sodium bicarbonate, and the organic layer is dried over anhydrous sodium sulfate. After solvent evaporation under reduced pressure, the product is purified by column chromatography. The isolated compound is identified and confirmed by different spectral analyses.

Results and Discussion

Characterization of SnO2 Nanosheets

Single crystalline and ultrathin nanosheets of scrutinyite-SnO2 have been synthesized using a single-step solvothermal route at 100 °C for 20 h using ethanol as a typical solvent. At first, the FTIR spectroscopic study of the as-synthesized sample confirms the formation of metal oxide with strong absorption bands located at mid and far regions of IR spectra (Figure S1). The mid-FTIR spectrum (Figure S1a) shows three intense bands at around 540.2, 585.6, and 692.0 cm–1 because of the stretching and bending vibration of Sn–O bonds in the orthorhombic SnO2 lattice. The presence of a broad band at 3415.1 cm–1 and a weak band at 1643.2 cm–1 may be due to the stretching and bending vibration of hydroxyl groups of Sn–OH or coordinated H2O molecules of the SnO2 lattice.[8] In the far-FTIR spectrum (Figure S1b), some weaker broad bands located around 150–300 cm–1 are observed that may be due to a few combination modes of bending and torsional vibrations of Sn–O bonds.[7] No other absorption and inverse absorption bands are observed in the spectrum, indicating the absence of defects associated with oxygen vacancies or tin interstitials and molecular SnCl2, respectively.[8] Thus, this FTIR study confirms the formation of SnO2 through the solvothermal treatment of tin chloride dihydrate with NaOH.

The powder XRD analysis is carried out for the as-synthesized sample, and results show the X-ray diffraction peaks at 2θ = 26.9, 29.7, 33.4, 37.2, 47.2, 50.4, 51.8, 57.1, 60.8, 62.0, 65.3, 70.1, and 78.1° that are assigned to diffractions of (112), (113), (022), (121), (117), (221), (130), (134), (208), (312), (233), (228), and (240) crystal planes, respectively, of orthorhombic, that is, the scrutinyite-SnO2 crystal structure correctly matches with JCPDS Card No. 78-1063 (Figure a). Rietveld refinements (Figure a) are also carried out using the same powder XRD data and found that all the diffraction peaks are indexed to the orthorhombic (scrutinyite) crystal phase of SnO2 with space group Pbcn (No. 60). Table S1 represents all the lattice and structure parameters of the scrutinyite-SnO2 sample. There are no oxygen vacancies in the SnO2 sample, confirming the complete formation of scrutinyite-Sn(IV)O.[7]

Figure 1

Results of the Rietveld refinement of the XRD pattern (a), corresponding unit cell (b), and crystallographic planes (c–f) of the scrutinyite-SnO2.

Again, the crystallite size of the SnO2 sample is calculated considering the full width at half-maximum (FWHM) of the first four high intense XRD peaks at 2θ = 26.9, 29.7, 33.4, and 37.2° corresponding to (112), (113), (022), and (121) crystal planes. The crystallite sizes of the (112), (113), (022), and (121) planes of the as-synthesized SnO2 sample are found to be 12.2, 26.9, 71.2, and 23.8 nm, respectively. The crystallite size of the (022) plane is found to be much larger than that of the other planes, suggesting the main growth of as-synthesized SnO2 crystallites in the [011] direction. The result means the main surface of the SnO2 sample is predominantly contributed by the (022) plane or/and parallel planes in the [011] direction. Consequently, the growth of crystallites of the SnO2 sample perpendicular to the [011] direction, that is, in the [100] direction, must be slow. This means that the planes in the [100] direction or/and the parallel planes of the (100) plane have minor contribution to the main surface of SnO2 sample.

The morphology and structural/compositional details of the synthesized scrutinyite-SnO2 have been studied by SEM/FESEM and TEM analyses. SEM images (Figure S1c,d), FESEM images (Figure a,b), and TEM images (Figure c,d) confirm the formation of SnO2 nanosheets and assembly of such nanosheets with almost dendritic-like morphology. More information about the SnO2 nanosheets and its assembly is collected by a series of TEM/HRTEM investigations (Figure a–h). The size of the SnO2 nanosheets is found to be nonuniform with longitudinal dimensions of almost 120–200 nm and lateral dimensions of 50–100 nm (Figure a). The thickness, that is, cross-sectional dimension of SnO2 nanosheets is determined by focusing TEM/HRTEM investigation on the edge of the nanosheet assembly. The completely transparent fringelike regions indicate planar sheets parallel to copper grid, and quite dark wirelike regions resemble the sheets perpendicular to the grid (Figure b). The measured widths of very dark wirelike regions are found to be 1.2–1.35 nm that confirms the formation of ultrathin nanosheets (Figure b). The planar sheet also clearly displays lattice fringes with a spacing of 0.26 nm corresponding to the (022) plane of scrutinyite-SnO2 (Figure b). Again, a particularly magnified area of the HRTEM image distinctively shows the lattice fringes (d022SnO = 0.26 nm) over the planar nanosheet of scrutinyite-SnO2 (inset in Figure b). Moreover, the nanosheets are formed by strong aggregation and oriented attachment of various tiny particles of a few nanodiameters (slightly visible on the top flat surface of the sheet) (Figure c). The average diameter of the particles is calculated and found ca. 2.47 nm (inset in Figure c).

Figure 2

FESEM (a, b) and low-magnified TEM (c, d) images of synthesized scrutinyite-SnO2 in different resolutions.

Figure 3

Morphological and topological characterizations for scrutinyite-SnO2 nanosheet assembly. Low-magnification TEM image (a) representing nanosheets, HRTEM image (b) of nanosheet assembly on the edge region (inset shows high magnified image of the yellow square box portion), low-magnification TEM image (c) of a portion of the nanosheet displaying the aggregation and attachment of tiny particles (slightly visible on the top flat surface of the sheet) (the inset shows the corresponding particle size distribution), HRTEM image (d) of a nanosheet having two different zonal area (yellow and red square box corresponding to [044̅ ] and [4̅44̅] zone axes, respectively), high magnified HRTEM image of yellow color square box corresponding to [044̅ ] (e) and red color square box corresponding to the [4̅44̅] (f) zone axis, HRTEM image (g) of a different portion of the nanosheet displaying [2̅22̅] zone axis, SAED analysis (h) of the nanosheet, AFM image (i) of the SnO2 nanosheet after strong ultrasonic treatment, corresponding height profiles (j) from the AFM analysis of the SnO2 nanosheet.

As of yet, the assembly of scrutinyite-SnO2 formed by ultrathin nanosheets that are made of oriented attachment of tiny particles have been found under TEM/HRTEM investigation. Thereafter, the high-resolution TEM investigation is performed for the collection of in-depth information on SnO2 nanosheets (Figure d–g). Two different zonal areas highlighted with yellow and red color square box corresponding to [044̅ ] and [4̅44̅] zone axis are observed on the surface of a nanosheet (Figure d–f). A distinctive set of planes matching with (022) and (200) lattice planes of scrutinyite-SnO2 at an angle of 90° in between having lattice fringes (d022SnO = 0.26 nm) and (d200SnO = 0.25 nm), respectively, in the [044̅ ] zone axis is clearly visible (highlighted yellow color square box of Figure d,e). Another set of plane matching with (022) and (220) lattice planes of scrutinyite-SnO2 making an angle of 60° in between with fringes (d022SnO = 0.26 nm) and (d220SnO = 0.18 nm), respectively, in the [4̅44̅] zone axis is also observed (highlighted red color square box of Figure d,f). Another zonal area with a set of planes matching with the (022) and (110) lattice planes of scrutinyite-SnO2 at an angle of 60° in between having lattice fringes (d022SnO = 0.26 nm) and (d110SnO = 0.38 nm), respectively, is also viewed in the [2̅22̅] zone axis of a different portion of the nanosheet (Figure g).

From the HRTEM investigation, the lattice plane (022) corresponding to the {022} facet of scrutinyite-SnO2 (d022SnO = 0.26 nm) in the [011] direction predominantly contributed to the main surface of the nanosheets along with two other exposed planes (220) and (110) in the [110] direction at an angle of 60° to the (022) plane. Moreover, a lattice plane (200) corresponding to the {200} facet in the [100] direction at an angle of 90° to the (022) plane also contributes slightly. These results are consistent with the XRD results by confirming the (022) lattice plane as the main exposed crystal plane of the scrutinyite-SnO2 nanosheets, suggesting predominant growth and orientation of the SnO2 crystallites in the [011] direction with least contribution of the (200) lattice plane in the [100] direction. Furthermore, SAED analysis of the nanosheet shows a spotty pattern, revealing the pure and single crystalline nature of the synthesized SnO2 nanosheets (Figure h). The lattice planes (004), (102), (112), (022), (220), (200), and (117) of scrutinyite-SnO2 are clearly indexed by the bright spots in the SAED pattern. The higher brightness of the (022) spot (red color circle) in the pattern confirms more exposure of the (022) lattice plane in the SnO2 nanosheets, and this is again in agreement with HRTEM and XRD observations. Again, the thickness of the SnO2 nanosheet can be further characterized by AFM analysis (Figure i,j). The AFM image depicts a thickness ca. 1.3 nm along with longitudinal and lateral dimensions of the nanosheet about 140 and 85 nm, respectively (Figure i). The corresponding height profiles show slight roughness in the surface of the nanosheet with the height in the range of 0.7–1.32 nm and a maximum height of ∼1.32 nm (Figure j). The result of AFM analysis strongly confirms the formation of ultrathin nanosheets, which is in good agreement with the HRTEM observation.

Furthermore, we have conducted Raman spectroscopic and XPS analysis on the single crystalline and ultrathin SnO2 nanosheet assembly (Figures S2 and ). Basically, Raman spectroscopy is interrelated with XRD. However, in contrast to XRD, the number of atoms per unit cell provides the possible phonon modes in Raman spectroscopy. As the primitive cell of scrutinyite-SnO2 contains 12 atoms, one should achieve a total of 33 normal modes of vibration. The vibrational representation of those optical phonon modes at the center of the Brillouin zone can be given by eq .

Figure 4

XPS spectrum of Sn 3d (a) and O 1s (b), BET isotherm (c), and pore size distribution (d) of the scrutinyite-SnO2 nanosheet assembly.

All of these nondegenerate vibrational modes are IR-active (B1, B2, and B3), Raman-active (A, B1, B2, and B3), and the A modes are silent. The vibrational modes on the spectrum do not match with the characteristic modes of tetragonal (rutile) and deformed rutile phases of SnO2, which is in good agreement with the XRD results. However, till date, no spectral analysis for the Raman spectrum of SnO2 in its orthorhombic (scrutinyite) phase has been reported yet. As there is scarce of data in the literature, it is difficult to completely assign the observed phonon modes. Therefore, we can confirm the formation of the orthorhombic phase of SnO2 in resemblance with the Raman spectrum of the α-PbO2 (scrutinyite) and metal oxides having a scrutinyite crystal structure.[31−33] The XPS analysis is carried out to investigate the chemical state of the SnO2 nanosheets (Figure a,b). The Sn 3d XPS spectrum displays two peaks at binding energies 486.1 and 494.5 eV for Sn 3d5/2 and 3d3/2 states, respectively (Figure a). The binding energies are very close to the standard data for Sn4+, and no peaks either for Sn2+ or Sn0 is detected. The energy difference between Sn 3d5/2 and Sn 3d3/2 states of 8.4 eV is in agreement with the reported value of energy splitting for Sn(IV)O2. Again, in the O 1s spectrum, two deconvoluted peaks located at binding energies 530.1 and 531.5 eV are observed (Figure b). The peak at 530.1 eV is ascribed to oxygen bound to Sn in the SnO2 crystal lattice. Another peak located at 531.5 eV is attributed to the adsorbed oxygen (O– and O2–) species (e.g., OH and H2O), respectively. Finally, the XPS spectra indicate that the synthesized nanosheets are Sn(IV)O2 rather than Sn(II)O or Sn(0) metals.[8−10]

The nitrogen adsorption–desorption analysis is also performed to determine the specific surface area and pore size of the as-synthesized SnO2 nanosheet assembly. A type IV isotherm characteristic of mesoporous material with hysteresis loop H3-type according to IUPAC is observed for the sample (Figure c). The BET surface area about to 71.7 m2 g–1 is found for the nanosheets, which is comparatively larger than the surface area of commercial SnO2 powder (Sigma-Aldrich; 38.3 m2 g–1) and quite reasonable for SnO2 nanomaterials.[7−10] Again, the pore size distribution based on the BJH model is determined from the desorption branch of the isotherm. The BJH pore volume and diameter 0.21 cm3 g–1 and 3.99 nm, respectively, is observed for the sample. The maximum of pore size distribution from the curve is observed at 3.99 nm, clearly indicating the mesoporosity of the sample (Figure d).

To evaluate the acidity of the synthesized SnO2 nanosheets, the temperature-programmed desorption of ammonia (NH3-TPD) is performed (Figure S3). The NH3-TPD profile shows a broad desorption peak in the range 250–550 °C. In general, the strength of acidic sites can be divided into weak (200–300 °C), medium (300–450 °C), and high (>450 °C) regions.[70,71] The measured NH3-TPD profile is deconvoluted into three peaks having maximal temperatures at ∼320, 361, and 446 °C corresponding to 23, 45, and 32% of the total desorbed amount of NH3. All the peaks can be assigned to the medium temperature region (300–450 °C), which indicates the presence of medium strength acidic sites in the SnO2 nanosheets.[72] Moreover, the nature and stability of the acidic sites in the synthesized SnO2 nanosheets are evaluated by performing Py-FTIR analysis at different temperatures, and the results are presented in Figure . The Py-FTIR spectra exhibit bands at 1610, 1575, and 1450 cm–1 corresponding to the vibrational modes of pyridine coordinated at Lewis acidic sites (L). The two bands at 1610 and 1450 cm–1 are characteristic of strong Lewis acidic site-bound pyridine, while another band at 1575 cm–1 corresponds to weak Lewis acidic site-bound pyridine.[73] Again, the bands at 1650, 1540, and 1517 cm–1 are attributable to pyridine molecules protonated at Bronsted acidic sites (B).[74,75] Besides, another less-intense band at 1485 cm–1 originates from pyridine associated with both Lewis and Bronsted acidic sites (L + B).[76] All the bands are thermally stable at 200 °C, which suggests that the acidic sites present in the nanosheets remain intact.

Figure 5

Py-FTIR spectra of synthesized SnO2 nanosheet assembly at different temperatures.

By following the experimental results, we can propose a reasonable mechanism for the formation of 2D SnO2 nanosheets. The 2D SnO2 nanosheets can be obtained by the solvothermal treatment of Sn(II) salts in alkaline aqueous solution in the presence of ethanol via hydrolysis, oxidation, and decomposition of Sn(II) salts/ions.[30−34] In general, the hydrolysis of Sn(II) salt may lead to the formation of layered/sheetlike hydroxide intermediates, which further oxidize and/or decompose to the sheetlike structure of SnO2.[7−10] Therefore, chemical reactions involved during the synthesis process can be described as follows:

As the solution is rich in ethanol, initially, Cl– is replaced from SnCl2·2H2O molecules by C2H5OH (eq ). Later, the H2O molecules may attack the Sn-OC2H5 bonds and replace the C2H5OH molecules to form Sn(OH)2 (eq ). Subsequently, the Sn(OH)2 may undergo oxidation to the precipitate as Sn(OH)4 (eq ). Afterward, the OH– ion from the alkaline (NaOH) solution interacts with Sn(OH)4 and forms Sn(OH)62– (eq ). Furthermore, the decomposition of Sn(OH)62– may take place forming the crystals of SnO2 (eq ). Finally, the formation and self-construction mechanism of 2D SnO2 nanosheets by oriented attachment from 0D nanocrystallites is schematically represented in Scheme .

Scheme 2

Schematic Representation of the Formation and Self-Construction Mechanism of 2D SnO2 Nanosheets (l ≈ 140 nm, b ≈ 85 nm, and h ≈ 1.32 nm)

Catalytic Activity of SnO2 Nanosheets

Nanoscale metal oxides are less frequently employed for FC acylation reaction. Therefore, in the present study, we have performed the FC acylation reaction over as-synthesized single crystalline and ultrathin SnO2 nanosheet assembly to evaluate its catalytic activity. To determine the standard experimental conditions, we have chosen the acylation reaction of anisole (aromatic substrate) and benzoyl chloride (acylating agent) as a model reaction. No product formation takes place without the catalyst in the reaction, suggesting the role of the catalyst (Table , entry 1). As the temperature has a significant role on a reaction therefore at first, we have tested the reaction at different temperatures and observed that the yield of product 4-methoxybenzophenone (3a) is considerably affected by the reaction temperature (Figure S4). The increase in the reaction temperature from room temperature (=27 °C) to 50 °C enhanced the yield of the product from 46 to 92%, respectively, perhaps due to the higher molecular thermal agitation and collision probability between the benzoyl cation and the arene.[77] Further increase in the reaction temperature does not alter the yield of aromatic ketone. Thus, the reaction temperature 50 °C is considered as a prime temperature for the major yield of aromatic ketones in the presence of as-synthesized SnO2 nanosheets.

Table 1

Effect of the Solvent on the Acylation of Anisole with Benzoyl Chloridea

entry	catalyst	solvent	time (min)	yieldb (%)
1	none	solvent-free	>120	0
2	SnO2	acetonitrile	120	46
3	SnO2	nitromethane	120	45
4	SnO2	ethyl acetate	120	trace
5	SnO2	tetrahydrofuran	120	trace
6	SnO2	dichloromethane	120	59
7	SnO2	solvent-free	50	92

Reaction Conditions: anisole (4 mmol), benzoyl chloride (2 mmol), solvent (0.5 mL), catalyst (10 mol %), and temperature (50 °C).

Isolated yield.

To achieve maximum yield of the product, the reaction is optimized by changing the amount of the catalyst (Figure S5). The figure shows that as the amount of the catalyst increases the yield of the product of the reaction increases because of the increase in the concentration of the benzoyl cation that accelerates the reaction,[77] and a maximum yield of the product is observed with 10 mol % of the catalyst. No further improvement of the reaction yield is observed on employing more than 10 mol % SnO2 nanocatalyst, presumably due to the fact that the reaction mixture becomes more viscous, leading to resistance in mass transfer. The observations suggest that a notable result is attained when the acylation reaction is carried out with 10 mol % of the SnO2 nanocatalyst at 50 °C.

Moreover, the effect of the solvent on the entitled reaction is observed in the presence of five different solvents (Table ). Although the reaction progress is smooth in the presence of solvents, miserable yields of the product are observed (Table , entries 2–6). A trace amount of the product is detected with solvents ethyl acetate and tetrahydrofuran having strong coordination ability (Table , entries 4 and 5).[78] In the same reaction, if carried out in nitromethane or acetonitrile, only 45 and 46% yields are detected, respectively. Noticeably, dichloromethane provides a better amount of product, yet less improvement (59% yield) is noticed (Table , entry 6). Meanwhile, the acylation of anisole is also performed under solvent-free conditions, resulting in a commendable yield of 92% (Table , entry 7). The observation suggests that there is no significant effect of solvents in the reaction, which is in agreement with literature reports, and a good result is obtained by employing aromatic ether as the self-solvent.[79]

To explore the substrate scope of the reaction, a range of aromatic compounds and acid chlorides are reacted under optimized conditions, and the subsequent results are presented in Table . It is observed that acylation occurs exclusively at the position para to methoxy (−OCH3) and methyl (−CH3) groups for all the substituted aromatic compounds furnishing corresponding products within 40–70 min (Table , entries 1–6). The presence of electron-releasing −OCH3 and −CH3 groups in these cases resulted in good to high yields (78–92%) of corresponding aromatic ketones at 50 °C. However, the acylation of benzene (Table , entries 7 and 8) proceeds after vigorous stirring, affording low yields of corresponding products due to its low activity.[80] The reaction proceeds well with five-membered heterocyclic compounds such as thiophene (Table , entries 9 and 10), yielding quantitative amounts of 2-acylated products. The SnO2 nanosheet assembly exhibits remarkable catalytic activity with excellent regioselectivity of aromatic ketones under solvent-free reaction conditions, which may be attributed to the ultrathin 2D confinement effect, acid site, and the redox behavior of the SnO2 nanosheets.

Table 2

Substrate Scope for FC Acylation Reaction over SnO2 Nanosheetsa

Reaction Conditions: substrate (4 mmol), acid chloride (2 mmol), catalyst (10 mol %), and temperature (50 °C).

Isolated yield.

Recyclability of the catalyst is a prime factor regarding environmental sustainability and economic industrial processes. To test the recyclability of the catalyst, six repeated cycles of catalytic reaction are performed and presented in Figure a. After each consecutive run, the catalyst is recovered from the reaction mixture followed by centrifugation, washing with ethanol, drying, activation, and redispersion in a freshly prepared identical reaction mixture. The high yield of aromatic ketone is retained without much loss of activity up to the sixth consecutive run, indicating the fair stability of the catalyst. The minute decrease in activity may be considered for the unavoidable mechanical loss of the catalyst during separation from every previous runs.

Figure 6

Recyclability test (a) and hot filtration test for leaching (b) of the SnO2 nanocatalyst.

Moreover, a hot filtration test is conducted to investigate the leaching of the SnO2 nanocatalyst (Figure b). After the first 10 min expedition of the model reaction between anisole and benzoyl chloride under optimized conditions, the catalyst is separated from the reaction mixture by simple filtration. The filtrated reaction mixture is further continued for additional 50 min. No obvious enhancement of the % conversion is noticed after separation of the catalyst, suggesting no significant leaching of the catalyst. After completion of the sixth run, the composition and morphology of the recovered SnO2 nanocatalyst is studied by using FTIR, XRD, and TEM/HRTEM analyses. The FTIR spectrum of the recycled catalyst exhibited no significant changes, implying that the functionalities of the catalyst remained unchanged even after six consecutive runs (Figure S6). The powder XRD pattern of the reused catalyst also shows inconsequential changes than that of the fresh catalyst, confirming the good structural stability of the catalyst (Figure S7a). TEM investigation of the reused catalyst reveals that the nanosheets of the SnO2 catalyst are very slightly broken and stacked compared to those of the fresh catalyst probably due to continuous stirring under heat (Figure S7b). However, the distinctly visible lattice fringes in the HRTEM images indicate that the crystallinity of the nanosheets is still intact in the recovered catalyst (inset in Figure S7b). The overall TEM observation confirms the high structural and morphological stability of the catalyst.

Proposed Reaction Mechanism for the FC Acylation over SnO2 Nanosheets

The plausible mechanism involved for the action of SnO2 nanosheets in acid-catalyzed FC acylation is shown in Scheme . FC acylation is an electrophilic aromatic substitution reaction, and the reaction is due to the generation of an acyl carbonium ion. Mainly, acidic sites of the nanocatalyst facilitate the formation of the acylium ion. At first, adsorption of acid chloride takes place on the acidic site of the SnO2 nanocatalyst that converts into acylium ions. This acylium ion acts as an electrophile and reacts with the nucleophilic aromatic substrate to generate the corresponding aromatic ketone.

Scheme 3

Schematic Representation of the Acid-Catalyzed Mechanism of FC Acylation over SnO2 Nanosheets (l ≈ 140 nm, b ≈ 85 nm, and h ≈ 1.32 nm)

Again, the reduction of Sn4+ to Sn2+ is relatively easy owing to the positive standard reduction potential for Sn4+ (aq) + 2e– → Sn2+ (aq) (E°Sn4+/Sn2+ = +0.13 V).[81] Therefore, along with the conventional acid-catalyzed mechanism for FC acylation, a redox mechanism also can be expected.[82,83] The steps involved in this pathway are mentioned below:

The acid chloride gets activated to 2R–CO–Cl+· with the reduction of Sn4+ to Sn2+ simultaneously (eq ), thereby generating the R–CO+ carbocation (eq ). Subsequently, the carbocation reacts with the aromatic substrate, resulting in the formation of the acylated product (eq ). Furthermore, Sn2+ is oxidized back to Sn4+ (eq ), followed by the liberation of HCl (eq ).

Finally, we compared some results obtained in this work with that of other metal oxide catalysts, and the results are listed in Table . In the case of ZrO2, Fe2O3, and Fe2O3/HY (Table , entries 1–3), although the reaction is performed in solvent-less conditions, it required prolonged time to furnish the products with comparatively lower yields than the present protocol. Although the results obtained with PANI/nano-ZnO and SnO2-SiO2 (Table , entries 4 and 5) are comparable to that of the present protocol, these methods require the preparation of the catalyst support.

Table 3

Comparative Study for FC Acylation of Aromatic Compounds with Benzoyl Chloride Using Different Metal Oxide Catalysts

entry	catalyst	substrate	conditions	time	yield (%)	refs
1	ZrO2	anisole	solvent-free/120 °C	5 h	65	(84)
2	Fe2O3	anisole	solvent-free/80 °C	120 min	32	(64)
3	Fe2O3/HY	toluene	solvent-free/110 °C	10 h	43	(85)
4	PANI/nano-ZnO	anisole	solvent-free/rt	10 min	95	(86)
5	SnO2-SiO2	anisole	CH3NO2/80 °C	30 min	86	(87)
6	TiO2	toluene	solvent-free/80 °C/MW	20 s	68	(88)
7	SnO2 nanosheets	anisole	solvent-free/50 °C	50 min	92	this work
8	SnO2 nanosheets	toluene	solvent-free/50 °C	60 min	84	this work

The FC acylation of toluene with TiO2 (Table , entry 6) completes quickly within 20 s. However, it requires higher reaction temperature (80 °C), microwave irradiation, and yields a lower amount (68%) of the corresponding product. These results clearly disclose that the synthesized SnO2 nanosheets can be well utilized as an alternative for the regioselective acylation of aromatic compounds.

Conclusions

In summary, we have reported a facile and robust solvothermal route for the synthesis of single crystalline and ultrathin nanosheet assembly of SnO2. The structural analyses have revealed that the SnO2 nanosheets are ultrathin and single crystallized in the scrutinyite-SnO2 phase with high purity. The ultrathin SnO2 nanosheets have exposed preferential growth in the [011] direction on the main surface with a thickness of ca. 1.3 nm. Furthermore, the SnO2 nanosheets have been employed for the regioselective FC acylation to synthesize aromatic ketones in solvent-free reaction conditions. The acylation reaction over the SnO2 nanosheets afforded predominantly para-substituted ketones with yields up to 92% under mild reaction conditions. The comprehensive catalytic study has shown that the SnO2 nanosheet assembly is a better catalytic material compared to other metal oxide catalysts used so far with respect to its activity and reproducibility. The 2D nanosheet assembly of scrutinyite-SnO2 should be a highly promising catalyst for specific applications in the acylation reactions and will also open up some opportunities for the integration of various organic- and liquid-phase reactions.