Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein–Ponndorf–Verley Reduction

The development of highly active acid–base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid–base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein–Ponndorf–Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)–OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).


Preparation of hydrotalcites and their derivatives; preparation of hydrous ZrO 2
Ce-containing magnesium-aluminum layered double hydroxides (hydrotalcites, LDHs) were synthesized by a simple co-precipitation method (named in the text as one-step procedure).
MgCl 2 ×6H 2 O (3 mmol), (1-x) mmol AlCl 3 ×6H 2 O, and x mmol CeCl 3 ×7H 2 O (x = 0.01-0.15mmol) were first dissolved in 15 ml water.This mother liquor was then quickly added to 20 ml of freshly prepared NaOH solution of 0.40 M under N 2 atmosphere.After vigorous stirring for 45 min, the slurry obtained was centrifuged (4750 rpm for 5 min), washed twice with water and dispersed again (in 20 ml of water) and centrifuged (4750 rpm at 4°C for 15 min after the first step and for 30 min after the second step).The prepared gel-like product was then thoroughly redispersed in 30 ml of water and stored at room temperature for 3 days.Thereafter, the solidified final product was separated by centrifugation (4750 rpm at 4°C for 15 min) and then dried at 75°C in vacuo for 16 h.The LDHs obtained are labeled as MgAlCe x , where x is the initial Ce(III) : Al(III) molar ratio: MgAlCe 0.01 -MgAlCe 0.15 .Pure hydrotalcite (Mg 3 Al-LDH; denoted as MgAl) was prepared in the same method in the absence of cerium salt, using 10 ml of mother liquor and 20 ml of 0.4 M NaOH solution.
The one-step method described above was used to synthesize phase-pure products, but the method was severely limited in terms of the amount of cerium centers.To overcome this limitation, a second, two-step preparation method was introduced.In this method, exactly the same reaction steps as described above were repeated until the final suspension was obtained.
In this case, before the final step (separation/drying), the entire batch of cerium-containing slurry suspended in water (30 ml) was placed in a Teflon-lined stainless-steel autoclave with a capacity of 50 ml and then heat-treated at 110°C for 16 h, followed by the final separation procedure.This method allowed the incorporation of cerium in a slightly extended concentration range (up to a cerium content of 15% compared to the Al(III) centers).
During the FT-IR study (see below), a sodium carbonate treated MgAlCe 0.05 structure was synthesized and analyzed for comparison.For this purpose, a portion (0.2 g) of the hydrotalcite thus prepared was suspended in a 0.1 M sodium carbonate solution and then stirred at room temperature for 2 h.The solid product was separated by centrifugation and then dried at 60°C for 2 hours.
To investigate the role of hydroxyl groups and water content of LDHs during the catalytic reactions, a subsequent heat treatment was carried out leading to the formation of the partially or completely dehydrated/decarboxylated LDHs/mixed oxide.For this purpose, in each case, a 3-h heat treatment was carried out in a tube furnace under N 2 protection.The temperatures applied were the following: 110, 125, 150, 175, 200, 250, 300, 400, and 500°C.For comparison for the Raman study, both the physical mixture of CeO 2 and MgAl (CeO 2 -MgAl-LDH mixture) and MgAl-LDH supported CeO 2 (CeO 2 -MgAl-LDH composite) were synthesized.For the first product, CeO 2 and freshly prepared MgAl were combined in a mortar at a molar ratio of 1:20.The product was molded for 15 min using physical force.A wet impregnation method was used for the composite material.First, colloidal suspensions of CeO 2 in ethanol (5 mg/ml) were prepared.The desired amount of solution (CeO 2 from 5-15 wt%) were added to a portion of MgAl (0.1 g) and the mixture was sonicated at room temperature for 2 h.The obtained gray intermediate was separated by filtration, washed thoroughly several times with ethanol and water, and dried overnight in an oven at 60 °C.This solid was then heattreated at 200 °C for 8 h to obtain the products.
Hydrous ZrO 2 was used in a comparative study to show the catalytic performance of some benchmark catalysts.This was prepared by a simple procedure of partial dehydration of Zr(OH) 4 at 120°C for 12 h in air.

Preparation of CeO 2 derivatives for mechanistic control experiments
Hydrate nanoceria was produced with precipitation method using the cerium (+3) nitrate hexahydrate (Ce(NO 3 ) 3 × 6H 2 O, purity 99.5%) dissolved in a volume of distilled water.The nitrate solution was mixed and stirred for 2 h at room temperature.Ammonium hydroxide (NH 4 OH) of 25% was added to the solution in order to adjust pH (pH=10).At this step, a light-brown precipitate was formed in the solution.This precipitate was filtered, washed with distilled water in order to remove residual NH 4 + .This initial phase was subjected to drying process at 80 °C for 10 h to deliver the partially hydrated nanoceria sample.Finally, this precursor powder was thermally treated for 6 h at 600 °C under air to give the final product.
Solid FLP-like CeO 2 was prepared by a two-step hydrothermal process.Initially, 5 mL of 0.8 M Ce(NO 3 ) 3 solution was added into 75 mL of 6.4 M NaOH aqueous solution in a 100 mL Pyrex bottle.After a 30 min reaction, the mixture was aged at room temperature for 1 h.Then, the mixture in the Pyrex bottle was reacted for 24 h at 100 °C.After cooling to room temperature, the precipitates were washed with deionized water and ethanol alternatively for three times.After drying at 60 °C, the 2 mg mL−1 of precursor solution was prepared and treated hydrothermally at 180 °C for 12 h.Finally, the product was collected by centrifugation and dried at 60°C overnight.device as above at 175° scattering angle in disposable plastic cuvettes (VWR).Electrophoretic mobility was measured with the same device equipped with a 40 mW laser source operating at 658 nm wavelength.Disposable plastic omega-shaped capillary cells (Anton Paar) were used for the measurements.The obtained mobilities were then converted to zeta potentials.

Powder
The thermal behavior of the as-prepared layered composites was studied on a TGA/DSC 1 STAR e System (Mettler-Toledo Ltd., AU).The instrument worked under constant air flow, and the heating rate was 1 °C/min.The samples, between 30 and 35 mg, were placed into highpurity alpha-alumina crucibles.
The amount of Ce, Mg and Al components in the nanoparticles was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a Varian Vista Pro instrument.Before measurements, few milligrams of the samples measured by analytical accuracy were dissolved in 1.0 mL of concentrated nitric acid in 8 hours, and then, they were diluted with distilled water to 50 mL and then filtered.Ce(III)-content and the Ce(III):Ce(IV) actual molar ratios were determined by fluorescence spectroscopy (FLS) measurements using a SHIMADZU RF-5301PC spectrometer with excitation and emission slits of 5 nm.Prior to the measurements, one portion of the as-prepared hydrotalcites were completely dissolved in cc.
HCl and then diluted to the appropriate concentration.1-cm quartz cells were used for all measurement.The measurements were carried out at room temperature upon using λ ex = 255 nm and λ em = 355 nm.For determining the actual molar ratios, during the preparation procedure, sodium sulfite in excess to the theoretical maximum of cerium-content was added into the solution of LDHs to reduce Ce(IV) centers, which do not have fluoresce activity, to Ce(III).
Nitrogen sorption isotherms of samples were obtained using Quantachrome Autosorb-1 analyzer at 77 K. Prior to the measurement, the samples were de-gassed at 120 °C for at least 3 h.The Brunauer-Emmett-Teller (BET) specific surface areas were calculated using adsorption data at a relative pressure range of p/p 0 = 0.05-0.25.The morphologies of the samples prepared were studied by scanning electron microscopy (SEM).The SEM images were recorded on an FEI Quanta 650 FEG at an acceleration voltage of 20.0 kV.
The instrument for taking the Fourier-transform infrared (FT-IR) spectra was a Nicolet 5700 FT-IR spectrometer (Thermo Electron Corporation) with 2 cm -1 resolution in ATR mode (ATR-FT-IR).The 4000-600 cm -1 wavenumber ranges were recorded, averaging 256 scans for each spectrum.During the adsorption measurements for analyzing the surface acidity/basicity of the solids, the same FT-IR equipment was used for recording the corresponding spectra.The surface acidity of LDHs was determined by adsorption of pyridine on self-supporting wafers (10 mg/cm 2 ).FT-IR spectra of adsorbed samples were recorded in the range 1700-1400 cm -1.
The self-supporting wafer was placed between two CaF 2 windows.Before the adsorption, the wafers were degassed at 130°C for 2 h at 10 -4 N/m 2 .The pyridine was then adsorbed on the surface of the samples at room temperature for 1 hour followed by outgassing at room temperature for 2h at 10 -4 N/m 2 .The FT-IR measurements were carried out at room temperature.To correctly interpret the results, the adsorption-desorption procedure was repeated on using outgas temperature of 125°C after adsorption.The determination of the surface basicity of LDHs was performed by adsorption of methanol in a similar way as it was above mentioned for pyridine adsorption.A minor change was that a lower outgas temperature (T = 45°C) was used during the desorption.
Raman spectra were recorded with a portable IM-52 Raman Microscope (Snowy Range Instruments).The 785 nm laser wavelength with a laser power of 70 mW was used for excitation of Raman scattering.Raman spectra were obtained at 10 s integration time using a 50X microscope objective.
The first coordination sphere and oxidation state of the transition metal ions was established by using an X-ray photoelectron (XP) spectroscopic mapping.X-ray photoelectron spectra (XPS) were recorded with a SPECS instrument equipped with a Kratos Axis Supra Plus XPS, under a main-chamber pressure in the 10 −9 -10 −10 mbar range.The analyzer was run in the fixed analyzer transmission (FAT) mode with 20 eV pass energy.The Al Kα radiation (hν = 1486.6eV) of a dual anode X-ray gun was applied as an excitation source.The gun was operated at 210 W power (14 kV, 15 mA).The binding energy scale was corrected by setting the main C1s component to 284.1 eV in all cases. 27Al (cross-polarization magnetic angle spinning) solid state-(SS)-NMR measurements were carried out on a Bruker Avance III spectrometer with a 300 MHz magnet equipped with a 4 mm double air bearing, magic angle spinning probe.During the measurements, a zirconia rotor with a Kel-F cap was used.The samples were rotated at 5 kHz.
O 2 /NH 3 /CO 2 -TPD (temperature programmed desorption) was conducted using a BELCAT-A apparatus with a thermal conductivity detector (TCD) to assess the surface acidity and basicity of catalysts, respectively.The sample was pretreated under a He flows at 300°C for 1 h.Subsequently, the sample was cooled to 50°C, and subjected to a mixture of O 2 /NH 3 /CO 2 -He gas flow for 1 h, followed by flushing with pure He gas to remove physically adsorbed O 2 /NH 3 /CO 2 .Finally, TPD data were collected from 50°C to 600°C.

Cyclohexanone transfer hydrogenation to cyclohexanol in 2-PrOH
Scheme S1.The applied cyclohexanone-to-cyclohexanol transfer hydrogenation test reaction.(alcohol: EtOH, 2-propanol, 2-butanol) The Meerwein-Ponndorf-Verley(MPV) reactions of cyclohexanone, which were used as test reactions to describe the catalytic performance of the as-prepared solids, were carried out in a batch reactor at a constant reaction temperature of 65°C under a N 2 atmosphere.A solution of cyclohexanone (c = 0.5 M) and the appropriate amount of 2-PrOH (2-4 ml) was stirred under the above mentioned reaction conditions in the presence of a chosen hydrotalcite derivative (50-200 mg) for an appropriate reaction time (1-360 min).When the reaction was completed, the obtained slurry was cooled down to room temperature and then centrifuged at 10,000 rpm for 10 min to remove the catalyst.The obtained mixture was then evaporated under reduced pressure and re-dissolved in 2-PrOH.The reaction parameters were optimized by Box-Behnken design (BBD).The chosen reaction parameters can be seen in Table S1.(X 1 : Catalyst loading; X 2 : Quality of H-source; X 3 : Quantity of solvent (H-source)) Table S1.Coded factor levels for a Box-Behnken design of a three-variable system.
During the recycling tests, after the reductive transformations, the active catalyst was separated from the reaction mixture by centrifugation followed by thorough washing with ethanol and water.After that, the catalyst was re-activated at 200°C and then reutilized in the following runs under the optimized reaction conditions.To determine the structural integrity of the composites after each run, ex-situ XRD study on the spent catalysts was performed.To ascertain the heterogeneous nature of the reactions, the hot filtration test was carried out as follows.The catalytic composite was filtrated from the reaction slurry before completion of the transformation and then the filtrate was further treated under unchanged reaction conditions.To present the versatility of the catalysts, a short scope of the catalytic reaction was also introduced.During these tests, transfer hydrogenation of cinnamaldehyde, crotonaldehyde, furaldehyde, benzaldehyde and acetophenone was investigated after a shortened optimization procedure.These were implemented as the transfer hydrogenation of cyclohexanone.During scope, the catalytic markers were determined via the same method as above presented applying the below listed NMR signals and UV-Vis absorbances (Table S2).

CONTROL EXPERIMENTS
N-Alkylation of aniline with alcohols (benzyl alcohol, 1,2-hexanediol and diphenylmethanol) was carried out in a batch reactor.Typically, aniline (0.75 mmol), the corresponding alcohol (0.5 mmol) and the corresponding catalyst (100 mg) were added successively to toluene of 5ml.
Then the reaction mixture was stirred (600 rpm) at 60 °C under air or an N 2 atmosphere for 16 hours.Thereafter, the reaction vessel was immediately cooled down in ice water.The reaction mixture was analysed using a Hewlett-Packard 5890 Series II gas chromatograph (GC) equipped with flame ionization detector, using an Agilent HP-5 column and the internal standard (hexane) technique.Alcohol oxidation/dehydrogenation reactions were carried out in the same way, without adding any acceptor molecule (e.g.aniline) to the system.

STATISTICAL ANALYSIS
All experiments were conducted at least in triplicate with the data expressed as mean ± standard error of the mean.The student t-test was used to test the significant difference between the experimental groups.NS: no significant difference when p > 0.05; *: p < 0.05; **: p < 0.01; ***: p < 0.001; and ****: p < 0.0001.920 910 900 890 880 Intensity (a.u.) Binding energy (eV) A 920 910 900 890 880 Intensity (a.u.) Binding energy (eV) X-ray diffraction (XRD) patterns of the solids were recorded on a Bruker D8 Advance powder XRD instrument by applying CuKα radiation (λ = 0.15418 nm) and 40 kV accelerating voltage at 40 mA in the range of 2θ=5-80°.The characteristic reflections in the normalized diffractograms were identified on the basis of JCPDS-ICDD (Joint Committee of Powder Diffraction Standards-International Centre for Diffraction Data) database.Dynamic light scattering (DLS) was used to measure the hydrodynamic size of the dispersed particles.The measurements were carried out with the same Nanosizer (Malvern) Cyclohexanone conversions were determined by ultraviolet-visible (UV-Vis) spectrophotometry in 2-PrOH on using the absorption band maximum of the cyclohexanone in the UV region (λ max = 282 nm) associated with the n → π* transition (FigureS1, left).The UV-Vis absorbances were detected in a SHIMADZU UV-2450 spectrophotometer.For determining the cyclohexanol yields, 1 H-NMR measurements were introduced (FigureS1B,C). 1 H-NMR spectra were recorded at room temperature on a Bruker AV-500 in DMSO-d6.The actual cyclohexanol yield could be calculated by determining the actual ratio of the integrated peak areas (FigureS1C) of -OH proton shift (FigureS1B/orange, 4.41 ppm) from cyclohexanol and α-CH 2 protons shift from cyclohexanone (FigureS1B/light blue, 2.26 ppm).

Figure S2 .Figure S3 .Figure S4 .Figure S5 .
Figure S1.(A) UV-Vis spectrum of variable amounts of cyclohexanone in the wavelength region of 240-360 nm.(B) 1 H-NMR spectrum of (a) cyclohexanone after heating in 2-PrOH at 65°C for 5 in the absence of any catalyst under a N 2 atmosphere; (b) cyclohexanone : cyclohexanol mixture of 1:1 molar ratio (equal to 50mol% cyclohexanone conversion with 50 mol% cyclohexanol yield); (c) reaction mixture after catalytic reaction of cyclohexanone (0.5M) in 2-PrOH of 3 ml in the presence of MgAlCe 0.075 catalyst (pre-treated at 175°C) at 65°C for 3h under a N 2 atmosphere (cyclohexanol yield of 39% with 100% cyclohexanol selectivity).(C) Calibration curve for determining the actual cyclohexanol yield.Ratio of the integrated peak areas of 4.41 ppm and 2.26 ppm chemical shifts as a function of molar ratio of cyclohexanol: cyclohexanone in the calibration line.

Table S2 .
Fingerprint-like NMR shifts and UV-Vis transition states used for following the catalytic reactions during scope.

Table S3 .
BET surface area, average particle size and zeta potential of the as-prepared layered double hydrixides.: determined by N 2 sorption measurements using BET calculation method; b: determined by DLS a