A Solvent Free Synthetic Route for Cerium(IV) Metal ...

doi.org/10.26434/chemrxiv.9199634.v2

A Solvent Free Synthetic Route for Cerium(IV) Metal-OrganicFrame-works with UiO-66 Architecture and Their PhotocatalyticApplicationMatteo Campanelli, Tiziana Del Giacco, Filippo De Angelis, Edoardo Mosconi, Marco Taddei, FabioMarmottini, Ferdinando Costantino

Submitted date: 07/10/2019 • Posted date: 07/10/2019Licence: CC BY-NC-ND 4.0Citation information: Campanelli, Matteo; Giacco, Tiziana Del; Angelis, Filippo De; Mosconi, Edoardo; Taddei,Marco; Marmottini, Fabio; et al. (2019): A Solvent Free Synthetic Route for Cerium(IV) Metal-OrganicFrame-works with UiO-66 Architecture and Their Photocatalytic Application. ChemRxiv. Preprint.

A novel solvent-free synthesis for Ce-UiO-66 metal-organic frameworks (MOFs) is presented. The MOFs areobtained by simply grinding the reagents, cerium ammonium nitrate (CAN) and the carboxylic linkers, in amortar for few minutes with the addition of a small amount of acetic acid (AcOH) as modulator (1.75 eq, o.1ml). The slurry is then transferred into a 1 ml vial and heated at 120°C for 1 day. The MOFs have beencharacterized for their composition, crystallinity and porosity and employed as heterogenous catalysts for thephoto-oxidation reaction of substituted benzylic alcohols to benzaldaldehydes under near ultraviolet lightirradiation. The catalytic performances, such as yield, conversion and kinetics, exceed those of similarsystems studied by chemical oxidation and using Ce-MOF as catalyst. Moreover, the MOFs were found to bereusable up to three cycles without loss of activity. Density functional theory (DFT) calculations gave anestimation of the band-gap shift due to the different nature of the linkers used and provide useful informationon the catalytic activity experimentally observed.

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Supporting Information

A Solvent Free Synthetic Route for Cerium(IV) Metal-Organic Frameworks with UiO-66 Architecture and Their

Photocatalyitic Application

Matteo Campanelli,# Tiziana Del Giacco,#,ǂ Filippo De Angelis,#,,║Edoardo Mosconi, Marco Taddei,§ Fabio Marmottini,#,ǂ Roberto D’Amato#,ǂ and Ferdinando Costantino#, ,*ǂ

#ǂDipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy e-mail: [email protected]

ǂCentro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123 Perugia, Italy

║CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

§Energy Safety Research Institute, Swansea University, Bay Campus, Fabian Way, Swansea, SA1, 8EN, UK

S-1

1.1Figures and Tables

Table S1. BET surface area, micropore volume of Ce-MOFs.

Sample BET surface area

(m2 g-1)

Micropore volume

(cm3 g-1)

Micropore volume

(cm3 g-1)a

Ce-UiO66-H 827 0.31 0.501

Ce- UiO66-Br 654 0.25 n.a.

Ce-UiO66-NO2 622 0.22 0.291

Ce-UiO66-4F 732 0.28 0.262

Ce-MiL140A-4F 279 0.10 0.112

Ce-UiO-66-PDC 547 0.20 0.243

avalues from literature reported for comparison.

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S-3

a)

b)

Figure S1. Thermal gravimetric analysis (TGA) plot for Ce-MOFs.

Table S2. Comparison of the thermogravimetric analysis of Ce-MOFs and theassociated mass losses.

Compound name Chemical Formula

(desolvated)

T/°C Theoretical

wt %

Measured wt %a

Ce-UiO-66-H Ce6O4(OH)4(BDC)6 220°C 195 207

Ce-UiO-66-Br Ce6O4(OH)4(Br-BDC)6 220°C 235 237

Ce-UiO-66-NO2 Ce6O4(OH)4(NO2-BDC)6 250 °C 213 Not measuredb

Ce-UiO-66-4F Ce6O4(OH)4(4F-BDC)6 200 °C 231 Not measuredb

Ce-MIL-140A-4F CeO(4F-BDC) 220 °C 237 238c

Ce-UiO-66-PDC Ce6O4(OH)4(PDC)6 200 °C 189 185a100 % is referred to the weight of 6·CeO2 (1·CeO2 in case of Ce-MIL-140A-4F) measured at the end ofthe analysis (700 °C). bThe vertical drop above 300 °C for Ce-UiO-66-4F and for Ce-UiO-66-NO2 (figureS1,b) is consistently observed over repeated analyses, suggesting that it is not an artifact. Similar behaviorhas been previously observed in lanthanide-based materials containing TFBDC as the linker and wasattributed to explosive decomposition, which expels sample from the crucible. Similarly, the samebehavior for Ce-UiO-66-NO2 was already reported by Biswas and Buragohain.4 cIf the analysis ends at1200 °C.

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Figure S2. FT-IR spectra of Ce-MOFs.

Figure S3. 1H-NMR spectra of digested Ce-MOFs.

S-5

Figure S4. Solid state absorption spectra for all Ce-MOFs.

Table S3. Photo-oxidation of a series of benzyl alcohol derivatives with Ce-UiO-66-PDC.a

Substrate tirr (min) Alcohol (%)b Aldehyde (%)b

MeO-BA 75 0 94MeO-BA(D)c 75 0 92BA 105 0 93CF3BA 360 75 19ClBAe 360 7.3 87

a [Substrate] = 1.00 × 10-2 M, [Ce-UiO-66-PDC] = 8.25 × 10-4 M, under Oxygen flux, with 355 ± 20nm. bYields refer to the initial amounts of substrate. The error is ±5%. c4-OCH3-C6H4CD2OH

S-6

Figure S5. XRPD patterns of the Ce-UiO-66-PDC MOF for each catalytic cycle

compared to synthesized and activated phase.

Table S4. ICP-OES analysis for all Ce-MOFs after photocatalytic experiments.a

Sample ×10-3Concentration

(mg/L)

% Leaching Ce

(%)

Ce-UiO66-H 3.29 0.355

Ce-UiO66-Br 3.26 0.352

Ce-UiO66-NO2 3.26 0.352

Ce-UiO66-4F 1.25 0.135

Ce-MIL140A-4F <LOD NA

Ce-UiO-66-PDC 3.10 0.335

Ce-UiO-66-PDCa 6.54 0.707

aCe-UiO-66-PDC was analyzed also after 3rd cycle.

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1.2 Analytical and instrumental procedures

Powder X-Ray Diffraction (PXRD). PXRD patterns were collected in reflection

geometry in the 4-40° 2θ range, with a 40 s step-1 counting time and with a step size of

0.016°on a PANalytical X’PERT PRO diffractometer, PW3050 goniometer, equipped

with an X’Celerator detector by using the Cu Kα radiation. The long fine focus (LFF)

ceramic tube operated at 40 kV and 40 mA.

Thermogravimetric analysis (TGA). TGA was performed using a Netzsch STA490C

thermoanalyzer under a 20 mL min-1 air flux with a heating rate of 10 °C min-1.

Nitrogen adsorption and desorption isotherms. N2adsorption/desorption isotherms

were performed using a Quantachrome Nova 2000e analyzer or a Micromeritics ASAP

2010 analyzer. Prior of the analysis, the samples were degased overnight under vacuum

at 120 °C. B.E.T. Analysis and t-plot analysis of the adsorption data were used to

calculate specific surface area and micropore volume respectively. Harksin and Jura

equation was used as reference for the statistical thickness calculation.

UV-VIS solid state absorption spectroscopy. A portable instrument for combined

reflectance, time-resolved and steady-state fluorescence based on the Time Correlated

Single Photon Counting (TCSPC) technique was used, with a Deuterium-Halogen lamp

(175x110x44 mm, 300 gr.) and CCD spectrometer excitation source and detector

respectively.5

Gas-Chromatography Analysis. The GC analysis was performed in anAgilent 6850

Series II GC, carried out in a Agilent J&W DB-35ms column (Phenyl Arylene polymer

virtually equivalent to (35%-Phenyl)-metlylpolysiloxane 50m, 0.32mm, 0.25m) with a

set point of 70°C, running from 70 °C to 260°C for 15 min. The front-inlet temperature

was 280 °C.

S-8

ICP-OES Analysis.The ICP-OES analysis was carried out using a Varian 700-ES

series. A calibration curve was performed with four standard Cerium solution (1,5,7,10

mg/L respectively).

NMR Analysis. 1H NMR spectra were recorded on aBrukerAvance III HD 400 MHz

Smart Probe spectrometer (400 MHz). The samples (tipically 15 to 20 mg) were

digested on 1 M NaOD at 120 °C for 5h, then formic acid as internal standard was

added with a final concentration of 0.05 M. The concentration of acetic acid was

estimated by dividing the integrated signal of 1H-NMR spectrum of the -CH3 group of

AcOHby three and multiplying it for the concentration of formic acid. By multiplying

the obtained concentration for the volume of the solution the absolute amount of acetic

acid was determined.

FT-IR spectra. FT-IR spectra were collected in ATR mode with a Jasco

4600LE, with a ZnSe Crystal, in the 4000-600 cm-1, spectral range and

a resolution of 0.7 cm-1.

Ionic chromatography Analysis. The determination of the nitrate ions content in the

samples was made by ion chromatography on a Dionex 500 apparatus with a CD20

suppressed conductivity module with the following procedure: about 30 mg of the

sample was dispersed in a solution obtained by adding 5 mL of NaOH 1 M to 30 mL of

water and refluxed for 2 hours. After reflux, the solution was diluted to 100mL by water.

The nitrate concentration in the resulting solution was eluted with a Na2CO3 1.8 mM /

NaHCO3 1.7 mM solution with a flux of 1.5 mL/min on a Dionex AS4SC column.

S-9

1.3Experimental Section

Chemicals

Cerium Ammonium Nitrate (CAN) Terephtalic acid (bdc), 2-Bromoterephtalic acid (Br-

bdc), 2-Nitroterephtalic acid (NO2-bdc), Tetrafluoroterephthalic acid (F4-bdc) Pyridine-

2,5 dicarboxylic acid (Pz-dc) ,Acetic Acid (99.5%) (AcOH), 4-Methoxybenzyl alcohol,

4-Chlorobenzyl alcohol, 4-(Trifluoromethyl)benzyl alcohol, Benzyl alcohol,4-Methoxy

benzaldehyde, 4-(Trifluoromethyl) benzaldehyde, Benzaldehyde, Bibenzyl, Acetonitrile,

Cerium standard solution (1000 mg/L) were purchased from Sigma-Aldrich. Nitric acid

(65%), N,N-Dimethylformamide (DMF) and Ethanol (EtOH) were purchased from

Carlo Erba. Benzyl alcohol-α,α-d2 was obtained by synthesis.6

Synthesis of Ce-MOFs

Ce-UiO66-H: Cerium Ammonium Nitrate (1 mmol, 548 mg) and terephthalicacid(1

mmol, 166 mg)were ground together in a mortar with pestle, after the addition of 90 µL

of acetic acid. The mixture is subsequently transferred in a 2.0 mL vial sealed with an

elastomeric plunger stopper and heated using a custom-made aluminium heater block at

120 °C for 24 h. After hot vial has reached room temperature, the off-white slurry was

recovered by adding 3 mL of DMF and centrifuged, and then re-dispersed the solid in

DMF and performing further 6 washes in a centrifuge(x15min, 5500 rpm), three with

DMF in order to remove the unreacted ligand, and three with acetone; the solid is placed

in an oven to eliminate the washing solvent, at 60 °C for 2 hours. Alternatively, it can be

left to air dry for 3 days.Yield: 89.5%, based on the ligand. ICP-OES: Ce calcd. for

CeO0.67(OH)0.67(C8H4O4) = 42.9%, obs. = 40.7%. ATR-FTIR (cm-1): 1650 (m, C=C),

1567 (m, CO asym.), 1502 (m, C=C), 1384 (s, CO sym.), 1180 (w), 814 (w), 745 (s).

S-10

Ce-UiO66-Br: Cerium Ammonium Nitrate (1 mmol, 548 mg) and 2-Bromoterephthalic

acid (Br-BDC) (1 mmol, 245 mg)were ground together in a mortar with pestle, after the

addition of100 µL of acetic acid. The mixture is subsequently transferred in a 2.0 mL

vial sealed with an elastomeric plunger stopper and heated using acustom-

madealuminium heater block at 120 °C for 20 h. After hot vial has reached room

temperature, the off-white slurry was recovered by adding 3 ml of DMF and

centrifuged, and then re-dispersed the solid in DMF and performing further 6 washes in

a centrifuge (x15min, 5500 rpm), three with DMF in order to remove the unreacted

ligand, and three with acetone; the solid is placed in an oven to eliminate the washing

solvent, at 60 °C for 2 hours. Alternatively, it can be left to air dry for 3 days. Yield:

79.3%, based on the ligand. ICP-OES: Ce calcd. for CeO0.67(OH)0.67(C8H3O2Br) =

37.5%, obs. = 33.7%. ATR-FTIR (cm-1): 1653 (m, C=C), 1584 (m, CO asym.), 1475

(w, C=C), 1384 (s, CO sym.), 1098 (w), 1038 (w), 816 (w), 766 (m), 661 (w, CBr).

This synthesis procedure has also been successfully tried in larger glass reactor (8 mL).

Cerium Ammonium Nitrate (1 mmol, 1.8 mL of 0.6 M aqueous solution of CAN) and 2-

Bromoterephthalic acid (Br-BDC) (1 mmol, 5.6 mL of 0.178 M DMF solution of

ligand) were place together in a 12 mL vial. The vial was sealed, heated, and stirred,

using a custom-made aluminum heater block at 100 °C for 30 min. Same procedure for

wash the suspension powder. Yield: 86%, based on the ligand.

Ce-UiO66-NO2: Cerium Ammonium Nitrate (1 mmol, 548 mg) and 2-Nitroterephthalic

acid (NO2-BDC) (1 mmol, 211 mg), were ground together in a mortar with pestle, after

adding 120 µL of acetic acid in a 2.0 mL vial sealed with an elastomeric plunger stopper

and heated using a custom-made aluminum heater block at120 °C for 12 h. After hot

vial has reached room temperature, the light-yellow slurry is recovered by adding 3 ml

S-11

of DMF and centrifuged, and then re-dispersed the solid in DMF and performing further

6 washes in a centrifuge(x15min, 5500 rpm), three with DMF in order to remove the

unreacted ligand, and three with acetone; the solid is placed in an oven to eliminate the

washing solvent at 60 °C for 2 hours. Alternatively, it can be left to air dry for 3 days.

Yield: 85%, based on the ligand. ICP-OES: Ce calcd. for CeO0.67(OH)0.67(C8H3O6N) =

37.7%, obs. = 39.5%. ATR-FTIR (cm-1): 1711 (w), 1650 (w, C=C), 1587 (m, CO

asym.), 1536 (m, N=O), 1494 (m, C=C), 1384 (s, CO sym.), 778 (w).

Ce-UiO-66-4F: Cerium Ammonium Nitrate (1 mmol, 548 mg) and 2,3,5,6-Tetrafluoro-

1,4-benzenedicarboxylic acid (4F-BDC)(1 mmol, 238 mg), were ground together in a

mortar with pestle, after the addition of100 µL of acetic acid. The mixture is

subsequently transferred in a 2.0 mL vial sealed with an elastomeric plunger stopper and

heated using a custom-made aluminum heater block at 110 °C for 72 h. After hot vial

has reached room temperature, the yellow slurry was recovered by adding 3 ml of

deionized water and centrifuged, and then re-dispersed the solid in water and

performing further 6 washes in a centrifuge(15min, 5500 rpm), three with water in

order to remove the unreacted ligand, and three with acetone; the solid is placed in an

oven to eliminate the washing solvent, at 60 °C for 2 hours. Alternatively, it can be left

to air dry for 3 days. Yield: 75%, based on the ligand. Ce calcd. for

CeO0.67(OH)0.67(C8O2F4) = 35.2%, obs. = 31.1%. ATR-FTIR (cm-1): 1615 (m, C=C),

1466 (m, C=C), 1392 (s, CO sym.), 1270 (m), 997 (m), 792 (w), 734 (s, CF).

Ce-MIL140A-4F: Cerium Ammonium Nitrate (1 mmol, 548 mg) and 2,3,5,6-

Tetrafluoro-1,4-benzenedicarboxylic acid (4F-BDC) (1 mmol, 238 mg),were ground

together in a mortar with pestle, after the addition of100 µL of acetic acid. The mixture

is subsequently transferred in a 2.0 mL vial sealed with an elastomeric plunger stopper

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and heated using a custom-made aluminum heater block at 110 °C for 24 h. After hot

vial has reached room temperature, the yellow slurry was recovered by adding 3 ml of

deionized water and centrifuged, and then re-dispersed the solid in water and

performing further 6 washes in a centrifuge (15min, 5500 rpm), three with water in

order to remove the unreacted ligand, and three with acetone; the solid is placed in an

oven to eliminate the washing solvent, at 60 °C for 2 hours. Alternatively, it can be left

to air dry for 3 days. Yield: 79%, based on the ligand. ICP-OES: Ce calcd. for

CeO(C8F4O4) = 35.7%, obs. = 28.5%. ATR-FTIR (cm-1): 1594 (s, C=C), 1577 (s, CO

asym.), 1556 (s), 1468 (s, C=C), 1392 (s, CO sym.), 1268 (w), 995 (m), 780 (w), 731

(s, CF).

The synthesis is carried out successfully, by replacing 100 µl of acetic acid with 100 µl

of water and heated using a custom-made aluminum heater block at 110 °C for 24 h

with a 88% yield.

Ce-UiO-66-PDC: Cerium Ammonium Nitrate (1 mmol, 548 mg) and 2,5-

Pyridinedicarboxylic acid (PDC) (1 mmol, 167.12 mg), were ground together in a

mortar with pestle, after adding 100 µL of 65 % w/w nitric acid. in a 2.0 mL vial sealed

with an elastomeric plunger stopper and heated using a custom-made aluminum heater

block at 110 °C for 20 h. After hot vial has reached room temperature, the colourless

yellow slurry is recovered by adding 3 ml of water and centrifuged, and then re-

dispersed the solid in water and performing further 6 washes in a centrifuge(x15min,

5500 rpm), three with water in order to remove the unreacted ligand, and three with

acetone; the solid is placed in an oven to eliminate the washing solvent at 60 °C for 2

hours. Alternatively, it can be left to air dry for 3 days. Yield: 88%, based on the ligand.

ICP-OES: Ce calcd. for CeO0.67(OH)0.67(C7H3NO4) = 43.5%, obs. = 42.8%. ATR-FTIR

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(cm-1): 1591 (s, CO asym.), 1487 (w, C=C), 1398 (s, CO sym.), 1362 (s, CN), 1182

(w), 1035 (w), 828 (w), 759 (w), 585 (w), 509 (m).

The synthesis is carried out successfully, by replacing 100 µl of nitric acid with 100 µl

of acetic acid and 50 µl of water, or 60 µl of water and 40 µl of nitric acid, with an 87%

yield.

Photocatalytic tests

MOFs were activated by keeping them in oven overnight at 120°C before use. A

solution of alcohol (0.08 nm in 8 ml) was placed in the reactor (15 ml-cylindric flask

equipped with cooled jacket) together with the MOF mmols (calculated per Ce atom)

corresponding to half of the substrate mmols. After vigorous stirring for about 30

minutes, in order to favor the adsorption of the substrate on the solid, the bubbled

oxygen mixture was irradiated by an Applied Photophysics multilamp apparatus with 6

phosphor-coated fluorescent lamps (15 W each) emitting at 355 nm (1/2= 20 nm). The

temperature was that of water circulating in the Pyrex jacket of the reactor. The reaction

mixture was analyzed, after filtration of the solid, by gas chromatography, adding an

internal standard (bibenzyl) in the case of quantitative analysis. The substrates and all

products formed were identified by comparison with commercially available authentic

specimens. In all cases, material recovery was satisfactory (>93%). All analyses were

made in duplicate and errors of <5% were obtained.

To check further photooxidation of aldehyde to acid, the mixture from the photo-

oxidation of MeOBA with Ce-UiO-66-PDC, carried out in the same conditions

described above, was analyzed after irradiation by 1H NMR (400 MHz, CDCl3), adding

bibenzyl as internal standard (Figure S6).The yields of 4-methoxy benzaldehyde,

referred to the initial amount of substrate, was 95 %. No reagent and further oxidation

S-14

products were detected.In addition to 4-metoxy benzaldehyde, hydrogen peroxide was

also recovered. The amount of H202 was determined by the standard method (titration by

iodide ion);6 the diluted H2O solution of the product mixture was treated with excess of

Nal and the amount of I3 formed was determined by the visible spectrum (max = 351

nm, = 1.7 104 M1 cm1),7 after subtraction of the absorption spectrum of the blank

solution, prepared similarly (Figure S7). The amount of H2O2 recovered was about 10

%, based on theinitial moles of MeOBA.

We carried out the photocatalysis in H2O and MeCN, but EtOH was the solvent in

which the oxidation of 4-methoxybenzyl alcohol was more efficient, as observed in

Figure S8.

Figure S6. 1H NMR spectrum of the reaction mixture of the photo-oxidation of MeOBA

with Ce-UiO-66-PDC.

S-15

300 350 400 450 500 550 600

0

1

2

3

abso

rban

ce

Wavelength (nm)

Figure S7. UV-visible absorption spectrum of triodide ions formed by reduction of H2O2

with sodium iodide in aqueous solution.

S-16

Figure S8. Photo-oxidation yields of 4MeOBA with Ce-UiO-66-H in various sovents.

The ethanol stability to oxidation was tested by carrying out the irradiation (for 360

min) of the more reactive MOF (Ce-UiO-66-PDC, 15. 5 mg) in ethanol (10 mL), in the

absence of benzyl alcohol, under the same experimental conditions used for photo-

oxidation. Taking into account the volatility of acetaldehyde, all operations were carried

out at temperature lower than 15 ° C. The absorption spectrum, recorded on the solution

obtained by filtration of the catalyst after irradiation, did not shown acetaldehyde traces,

as confirmed by comparison with an authentic sample (Figure S8)

S-17

.

250 300 350 400

0.0

0.2

0.4

0.6

0.8

1.0

abso

rbance

Wavelength (nm)

ca. 0.02 M acetaldehyde in EtOH irradiated Ce-UiO-66-PDC sospension in ethanol

Figure S8. Absorption spectra of Ce-UiO-66-PDC (15.3 mg) in 10 mL ethanol (red

line) recorded after irradiation and acetaldehyde (ca. 0.02 M) in ethanol (black line).

ICP analysis

The four standard Cerium solution for calibration curve was prepared with a several

dilution by the same matrix (2% of HNO3solution) used for the standard starting

solution. All samples were processed through an extraction (water: ethyl acetate) to be

analyzed.

1.4 Computational Detalis

All calculations have been carried out with VASP program package.8,9 The core−valence

electron interactions were described using the projector augmented wave (PAW)

method8with electrons from H1s; C 2s2p; N 2s2p; O 2s2p; Ce 5s5p6s5d4f shells

explicitly included in calculations. Dispersion interaction were described with DFT-D3

approach.10,11 Geometry optimization were carried out using the fixed experimental cell

parameters relaxing only the atomic position using PBE exchange correlation functional

S-18

with a kinetic energy cutoff of 400 eV. Single point calculation on the relaxed

geometries has been carried with HSE06 functional.8,9 The models were extracted from

the crystallographic data made up of 456 atoms for the Ce-UiO-66-H (24 Ce, 128 O,

192 C, 112) and of 432 atoms for Ce-UiO-66-PDC (24 Ce, 128 O, 168 C, 88 H, 24 N).

All calculations were carried out at Г point.

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download fileview on ChemRxivrevised_ESI_Campanelli_ChemRXiv.docx (2.87 MiB)

A Solvent Free Synthetic Route for Cerium(IV) Metal-Organic Frameworks with UiO-66 Architectureand Their Photocatalytic Application

Matteo Campanelli,# Tiziana Del Giacco,#,ǂ Filippo De Angelis,#, ,║Edoardo Mosconi,

Marco Taddei,§ Fabio Marmottini,#,ǂRoberto D’Amato#and Ferdinando Costantino#, ,*ǂ

#ǂDipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123Perugia, ItalyǂCentro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, ItalyComputational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123 Perugia, Italy║CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

§Energy Safety Research Institute, Swansea University, Bay Campus, Fabian Way, Swansea, SA1, 8EN, UK

KEYWORDS :Metal-Organic Frameworks, Cerium, Photocatalysis, DFT Calculations, Green Syntheses.

ABSTRACT: A near solvent-free synthetic route for Ce-UiO-66 metal-organic frameworks (MOFs) ispresented. The MOFs are obtained by simply grinding the reagents, cerium ammonium nitrate (CAN) andthe carboxylic linkers, in a mortar for few minutes with the addition of a small amount of acetic acid(AcOH) as modulator (1.75 eq, o.1 ml). The slurry is then transferred into a 1 ml vial and heated at 120°Cfor 1 day. The MOFs have been characterized for their composition, crystallinity and porosity andemployed as heterogeneous catalysts for the photo-oxidation reaction of substituted benzylic alcohols tobenzaldaldehydes under near ultraviolet light irradiation. The catalytic performances, such as selectivity,conversion and kinetics, exceed those of similar systems studied by chemical oxidation using similar Ce-MOFs as catalyst. Moreover, the MOFs were found to be reusable up to three cycles without loss ofactivity. Density functional theory (DFT) calculations were used in order to fully describe the electronicstructure of the best performing MOFs and to provide useful information on the catalytic activityexperimentally observed.

IntroductionMetal-organic frameworks (MOFs) are a class ofporous crystalline materials constituted ofmetallic clusters and organic linkers connected inan ordered fashion and containing accessiblechannels to gases and solvents. They haveattracted increasing attention during the pastdecade due to their high porosity, stability,tunable structures and controllable surfacefunctionalities. Therefore, in view of potentialindustrial application the intensive research onlarge scale and sustainable MOFs syntheses isevery day growing up.1-6However, the most part

of MOFs still in their infancy stage and have notyet been brought into the market place. In 2008,the catalysis section at University of Oslopublished UiO-66, the first zirconium-based MOF(Zr-MOF) reported, built from Zr6O4(OH)4

12+ nodesand 1,4-benzene-dicarboxylate (BDC) linkers,which exhibits exceptional chemical and thermalstability compared to other known MOFs.7

Furthermore, the UiO-66 series can be tuned bypartial/total linker or node functionalization.8-10 Alarge number of Zr-MOFs based on differentclusters and with variable structure and porosityhas been recently reported.11-13 At the same time,the chemistry of Ce(IV) based MOFs has rapidly

grown because the use of Ce(IV) allows to getMOFs isostructural to Zr- homologues withremarkable stability and with potential interestin the field of redox and photo-catalysis. Stockand co-workers reported in 2015 the DMFsynthesis of Ce-UiO-66 MOFs with severaldicarboxylic ligands demonstrating the stronghomologies between the Zr and Ce basedhexanuclear clusters.14 Other topologies, basedon the same building units were reported lateron.15-17Finally, the syntheses of mixed Zr-CeMOFs and water based syntheses of Ce-MOFwere recently presented.18 Photocatalyticapplications of MOFs have also been widelyanalysed due to their semiconductorproperties.19-20 When a semiconductor isirradiated with energy equal to or greater thantheir band gap, charge separated states aregenerated, with the electrons on the valenceband (VB) excited to the conduction band (CB),leaving positive holes (h+) in the VB. Thesephotogenerated charge carriers (electrons andholes) migrate separately to the surface of thesemiconductor particles and react with surface-adsorbed substrates to promote photocatalyticreactions.21

The rational design and modification of MOFsenables control on the chemical and physicalproperties of photocatalysts at molecular level.So far, Silva and co-workers firstly synthesizedUiO-66-NH2 by replacing H2BDC by 2-NH2-terephthalic acid in the synthesis22 and Wu andco-workers studied the photocatalytic selectiveoxidation of alcohols to their correspondingaldehydes and reduction of aqueous Cr(VI)utilizing this MOF as the photocatalyst.23

Following this research direction, a series ofmixed-linker Zr-based MOFs have beensynthesized in one pot reactions for the visible-light photocatalytic oxidation of alcohol by Gohand co-workers.24 The research on novelefficient photocatalysts focussed our attentionon cerium based MOF. A recent paper ofGagliardi and co-authors reported theapplication of Ce-MOF in photocatalysis from atheoretical point of view.25 In light of this, itsforecast encourages the synthesis andexperimental application in several reactionsdriven by light. The exchange of metal centerof MOFs does not show any effect on thephotocatalytic activity.21 In Ce-MOFs, ligand-to-metal charge transfer (LMCT) can bring aboutthe separation of photogenerated charges,promoted by the low-lying empty 4f orbitals ofCe4+. Another important structural variationthat can be used to design a promising UiO-66(Ce) photocatalyst, is linkerfunctionalization.25 Combining these ideas,herein we present a new green, solvent free,synthetic methodology to get crystalline Ce-MOF with several substituted dicarboxylicligands, as well as the electronic structureobtained by DFT calculations and the

application in photo-oxidation reaction ofsubstituted benzyl alcohols to aldehydes.

Results and discussion

Synthesis ad characterizationThe linkers used for the synthesis of Ce-MOFs

are shown in Scheme 1. The correspondingMOFs are named as Ce-UiO-66-X (with X =H,Br,NO2), Ce-UiO-66-4F, Ce-MiL140A-4F andCe-UiO-66-PDC.

Scheme 1. Molecular structure of the linkersused.

For obtaining Ce-UiO-66, a near solvent freestrategy is here proposed. This new synthesis,request a multiple combined techniques toobtain MOFs. The first step consists in anenergetic grinding of the metal salts and linkerswith a small amount of acetic acid (0.1 ml, 1.75eq), followed by heating at temperaturescomprised in the 110-120 °C range in smallvials (1 mL) in a custom-made aluminiumreactor block. Syntheses performed with largervolume vials generally lead to products withlow cristallinity. The method here presented canbe considered a Liquid-Assisted Grinding (LAG)if the ratio Liquid/Solid reagents is 0.1 or slurryif the ratio Liquid/Solid reagents is around 1,according to recent definitions in the field ofsolid state syntheses of MOFs.26 The synthesishere reported is solvent-free (except forwashes) and it uses a low amount of acetic acid(from 1.75 to 8.75 eq. respect to Ce) to obtainthe products, unlike recently reportedworks.27,28 Besides, to the best of ourknowledge, Ce-UiO-66-Br was obtained here forthe first time. X-Ray Powder Diffraction (XRPD)patterns (Figure 1) show the typical face-centered UiO-66fcu phase peak positions atabout 7.1° and 8.2°of 2θ, which correspond tothe lattice planes (111) and (002), respectively.In addition to the UiO-66 phases the use of F4-BDC also afforded the F4_MIL-140A(Ce) phasewhich was recently reported via other syntheticprocedures.17

Figure 1. XRPD patterns of the Ce-MOFs.

The N2adsorption-desorption isotherms at 77K obtained with the Ce-MOF samples are shownin Figure 2. From the B.E.T. and t-plot analysesof the adsorption data the specific surface areaand micropore volumes were calculated andreported in Table S1. The micropore volumevalues indicate that all the samples aremicroporous. In the case of Ce-UiO-66-H andCe-UiO-66-PDC the micropore volumes werefound to be 20 and 35% smaller than thevalues of analogous MOFs reported in theliterature, whereas for the MOF with the tetra-fluoroterephthalate linker, they matches quitewell with the analogous material obtained bysolvothermal syntheses.14,17,29 This is probablydue to the formation of a certain percentage ofnon-porous impurities such as cerium oxide,although the XRPD patterns show only thepeaks belonging to the MOF structure. We canspeculate that the solvent very likely plays afundamental role on the crystallization processand on the formation of pores.

Figure 2. N2 adsorption isotherms for Ce-MOFs.

Figure 3 displays the SEM images of thesynthesized MOFs. They have irregular

octahedral shape with size ranging between 30and 150 nm, as reported in previousstudies.14,17,29

Figure 3. FE-SEM images of Ce-MOFs: a) Ce-UiO66-H; b) Ce-UiO66-Br; c) Ce-UiO66-NO2;d)Ce-UiO66-4F ; e) Ce-MIL140A-4F; f) Ce-UiO-66-PDC.

The morphology of the Ce-MIL140A-4F MOF,here observed for the first time, is plate-like asin the case of the isoreticular analogue of Zr.30

Focusing on this image, UiO impurities are, verylikely, present in the sample as smalloctahedral crystals (Figure 3, e). Regarding thestability, TGA analysis (Figure S1) shows thatthe weight loss of all Ce-MOFs is the same tothose already reported.14,17,29 However, Ce-UiO66-H and Ce-UiO66-Br are more stable thanthe other functionalized MOFs. It is knownthatCe-UiO66-4F undergoes explosivedecomposition, which leads to ejection ofpowder from the sample holder, as evidencedby the vertical loss in the TGA curve of thisMOF.17 The same behaviour was also observedfor Ce-UiO-66-NO2 sample which exhibited avertical drop of mass at T higher than 250 °C.In order to detect the presence of impurities orunreacted reagents ionic chromatography (IC)was also performed on digested samples. Thepresence of nitrates was detected only in caseof Ce-UiO-66-PDC sample (1.5 mmol/g). Verylikely, nitrate ion is present as counter anion ofthe protonated pyridine group. FT-IR spectraare reported in Figure S2. 1H-NMR spectra(Figure S3) were also recorded on digestedsamples in order to check the presence of

acetic acid as monocarboxylic substituent.Notably, no AcOH was detected in all samplessuggesting that the MOFs could be defect free.Table 1 reports the max and Emax values of thesolid state absorption spectra of Ce-MOFs. Thespectra (Figure S4) show a single broadabsorbance band with the maximum in thenear VIS region. The corresponding Eonset

provides the estimated Band-Gap values for allthe samples.

Table 1. max, and Emax obtained from solidstate absorbance measurements.

Sample onse

t

(nm)

Eons

et

(eV)

max

(nm)

Emax

(eV)

Ce-UiO66-H 434 2.86 301 4,12

Ce-UiO66-Br 457 2.71 316 3,92

Ce-UiO66-NO2

420 2.95 320 3,87

Ce-UiO66-4F 458 2.71 300 4,13

Ce-MIL140A-4F

448 2.77 305 4,06

Ce-UiO-66-PDC

460 2.70 324 3,82

Photocatalytic testsAll MOFs were activated at 120°C for 1h

before use. This treatment was used in order toremove water molecules on the catalystsurface, which can compete with the adsorptionof the substrate. At first, in order to test thecatalytic efficacy, the oxidation of 4-methoxybenzyl alcohol (MeOBA) by Ce-UiO66-H(pristine) was chosen as a probe reaction. Weselected this alcohol because its redoxpotential is lower than that of other benzylalcohol derivatives. The oxidation of MeOBA inethanol (EtOH) as solvent, photocatalyzed byCe-UiO66-H and carried out under 355 nm lightirradiation (wavelength absorbed only by thecatalyst), produced the correspondingbenzaldehyde (Scheme 2) in quantitative yieldwithin 90 min. Ethanol proved to be an optimalsolvent because it is polar enough both todissolve the substrate and to stabilize the ionicintermediates known to form in the photo-oxidation reaction (see below).The use of thissolvent was possible because it was inert tooxidation; in fact no acetaldehyde trace wasfound among the products (see SupportingInformation). We also carried out thephotocatalysis in H2O and MeCN, but EtOH wasthe solvent in which the oxidation of 4-methoxybenzyl alcohol was more efficient (seeFigure S8).

Scheme 2. Photo-oxodation of MeOBA to 4-methoxy benzaldehyde.

Oxidation experiments performed in the dark,or in the absence of O2 (under Ar), or with CAN(a potential strong oxidant under irradiation31)instead of MOF, did not produce any product inall cases. The second step was that to examinethe substituent effect, such as Br, NO2 and F, ofthe BDC linker on the photo-oxidation yield ofMeOBA. As observed from the data reported inTable 2 and compared to those of the pristineMOF, in case of Ce-UiO66-NO2 a significantlower conversion was observed, obtainingquantitatively the aldehyde after 150 min. Ce-UiO66-4F was even less reactive, producingonly 2% of oxidation product after 90 min. Noaldehyde was detected when the MIL-140_Aanalogue was employed. A possible explicationfor this behavior can be due to the small poresof both F4-BDC derivatives which do not allowan efficient diffusion of alcohols moleculesinside. A slightly higher yield than that ofpristine MOF was achieved with Ce-UiO-66-Br,which reached the maximum value of productyield within 80 min. From these results it isevident that the weak electron-withdrawingsubstituents favor the photocatalysis. Anyway,the highest reactivity was shown by the Ce-UiO-66-PDC MOF. Indeed, aldehyde completeconversion was obtained just after 75 min,without any evidence of the corresponding acid(see Figure S6). High selectivity was observedin all reactions, contrary to what reported byvarious previous studies,32-35 reporting that thephotocatalysis of aromatic alcohols is often anot selective process.36

Table 2. Photo-oxidation of MeOBA.a

Photocatalyst

tirr

(min)

Alcoholb

(%)

Aldehydeb

(%)

CAN 60 90 0

Ce-UiO-66-Hc

90 94 0

Ce-UiO66-H 90 0 95

Ce-UiO66-Br 80 0 94

Ce-UiO66-NO2

150 0 94

Ce-UiO66-4F 90 93 2

Ce-MIL140A-4F

60 95 0

Ce-UiO-66-PDC

75 0 94

a[MeOBA] = 1.00 × 10-2 M, [Photocatalyst]=8.25 ×1 0-4 M, under Oxygen flux, with =355 ± 20 nm. bYields refer to the initialamounts of substrate. The error is ±5%.cExperiment under Argon condition.

Moreover, in our case the reaction times arein general rather short compared to otherexamples of photocatalyzed oxidations byMOFs already reported.23 All these results wererationalized with the mechanism schematizedin Scheme 3, which was formulated on thebasis of that already known for theheterogeneous photocatalysis with titaniumdioxide.36

Scheme 3. Mechanism of photo-oxidation ofbenzylic alcohols with MOFs.

The first stage involves the absorption of lightby the MOF, in particular by the linker, withconsequent charge separation involving theelectron located on the metal and the positivehole on the linker.19,21,22 The alcohol, by electrontransfer from the aromatic ring to the positivehole, forms the corresponding radical cation(eq. 2); this intermediate, being more acidicthan the corresponding neutral substrate,deprotonates to 4-methoxy--hydroxy benzylradical (eq. 3), which evolves, by reaction withmolecular oxygen, to the final oxidation product(eq 3, path a).36,37 Another plausible reactionpath of the radical could be its oxidation byMOF to the corresponding 4-methoxy--hydroxybenzyl cation, which leads to the formation ofaldehyde by subsequent proton loss. As regardsthe reduced metal, a back electron transferwith O2 is hypothesized, which regenerates the

MOF in its initial oxidation state, together withthe superoxide anion (eq. 4), which thenevolves to hydrogen peroxide and oxygen inthe presence of acid (eqq. 5 and 6). ). Indeed,the hydrogen peroxide was recovered in thereaction mixture, as described in SupportingInformation (Figure S7). Obviously, the reactionefficiency of the hole with alcohol will beconditioned by the rate of its recombinationwith the electron to reform the initial MOF (backreaction of eq. 1).

The experimental data point out that thereactivity is exactly the opposite of thetheoretical oxidative power (BDC-NO2> BDC-H>BDC-Br), expressed by their vacuum alignedHOCO (highest occupied crystal orbitals)energy, equal to –7.57, -6.73 and –6.19 eV,respectively.25 The reason of this trend is ratherto be sought on the effects that influence thelifetime of the oxidizing hole. It is known that,when Ce-MOFs are excited, the photogeneratedelectrons undergo LMCT to form charge-separated states, so preventing the rapidrecombination of the photogenerated charges.The kinetics of this process can be related withthe LMCT energy, ELMCT, which is defined as theenergy change upon transferring thephotogenerated electron from the photoexcitedlinker orbital to the lowest unoccupied metalorbital. This means that the substituents on thelinker not only influence the oxidative ability ofMOF, but also the efficiency of separation of thephotogenerated charges. In particular, ELMCT willbe all the more negative, i.e. the electron-holeseparation will be the more favored, as thesubstituent is less electron-withdrawing.Accordingly, also the electron-holerecombination rate, i.e. the competitivereaction with the photo-oxidation, will slowdown. ELMCT values for Ce-BDC-Br, Ce-BDC-H andCe-BDC-NO2 are -1.42, -1.43 and -1.33 eV,respectively,25 which are perfectly in line withthe similar reactivity of the first two catalystsand with the lower reactivity of the third one. Inthe case of Ce-UiO-66-4F, the presence of fourelectron-withdrawing substituents as fluorineon the linker ring could increase the ELMCT value,favoring further the electron-holerecombination and this is in agreement with thelowest reactivity observed for this MOF.25 Use ofpyridine-2,5 dicarboxylates as a linker producesa MOF (Ce-UiO-66-PDC) able to convertquantitatively MeOBA to the correspondingaldehyde in 75 min, a time shorter than thatwith Ce-BDC-Br. Reasonably, the electron-withdrawing effect of nitrogen of the pyridinecould be similar to that of a bromide on abenzene ring, but a contribution due to theHOCO energy cannot be excluded.

With the aim of providing more quantitativeinformation on the reactivity, a systematicstudy was performed on the photo-oxidation ofa series of benzyl alcohol derivatives (benzyl

alcohol, BA, 4-trifluoromethylbenzyl alcohol,CF3BA, and4-chlorobenzyl alcohol, ClBA) by thebest performing MOF, i.e. Ce-UiO-66-PDC.Interestingly, the only product obtained fromthe Ce-UiO-66-PDCphotocatalyzed oxidation ofeach alcohol, in oxygenated EtOH, was thecorresponding benzaldehyde (Table S3), asalready observed for MeOBA, proving that theprocess is always highly selective. All alcoholsinvestigated were found to be less reactivethan MeOBA. In fact, while the completeconversion of 4-MBOH occurred within 75 min,that of BA was quantitative within 105 min.Increasing the electron-withdrawing ability ofthe substituent, i.e. going from Cl to CF3, thereactivity further decreased: after 360 min,87% of 4-chlorobenzaldehyde and only 19% of4-trifluoromethylbenzaldehyde were formed.Thus, the reactivity decreases with the increaseof the oxidation potential, which extends therange from 1.5 V for MeOBA to 2.7 V for CF3BA(oxidation potentials determined in MeCN vs.SCE).35 These evidences lead to suppose that,on the basis of the previously discussed photo-oxidation mechanism (Figure 5), the electrontransfer step (eq. 2) is certainly ratedetermining. In addition, the reactivity ofMeOBA with Ce-UiO-66-PDC was identical tothat of the corresponding deuterated alcohol onthe benzylic position (4-OCH3-C6H4CD2OH). Thelack of kinetic isotopic effect of deuterium leadsus to conclude that the slowest step is not theradical cation deprotonation (eq. 3), but theelectron transfer step. It is interesting tocompare our results with those obtained byShen et al,23 who carried out the photo-oxidation of some benzylic alcohols catalyzedby Zr-NH2-UiO-66. Firstly, the Zr-NH2-UiO-66system showed much lower selectivity for thealdehyde with electron withdrawing substitutedalcohols compared with that obtained with ourMOFs. Furthermore, the reactivity of the benzylalcohols increased with the electronwithdrawing ability of the substituent and thusthe deprotonation reaction at the benzylicposition as rate-determining step washypothesized. The different behavior may bedue to the lower reactivity of our catalystcompared to that of Shen. Anyway, torationalize the diverse oxidative reactivity ofthe two systems is complex, considering themany factors involved in heterogeneousphotocatalysis.

Recycling attempts, up to three times, werecarried out by using Ce-UiO-66-PDC in thephoto-oxidation reaction of MeOBA. After eachreaction, the catalyst was separated from thereaction mixture by filtration, washed withCH2Cl2, heated at 100°C for 1h in order toremove CH2Cl2 and then washed again withH2Oand, finally, dried under vacuum at roomtemperature. For each cycle, X-ray diffractionpatterns were collected (Figure S5). Thephotocatalyst showed to be successfully

recyclable; indeed, after 75 min, the aldehydeyield was of 98% after each photo-oxidation.On the basis of these results, we can state thatthe MOF is subject to low leaching phenomena,covering of the active phase or sintering,despite not having been subjected to anytreatment between one use and an other.Release of Ce in solution was investigated usingInductively Coupled Plasma spectroscopy (ICP-OES). The results reported in Table S4 showthat Ce leaching after three photocatalyticexperiments is very low.

Theoretical calculationsTo gain insight into the electronic and optical

properties of these materials, DFT simulationshave been carried out to evaluate the densityof states (DOS) of Ce-UiO-66-H and Ce-UiO-66-PDC. Following a similar approach proposed byTruhlar and cooworkers,25 the electronicproperties were simulated with the HSE06hybrid functional as implemented in VASPprogram package38,39 (see SupportingInformation for theoretical details). As alreadyreported HSE06 nicely reproduces band gaps ofvarious functionalized UiO-66(Zr) showingquantitative agreement with the experimentalvalues.40 For Ce-UiO-66-H,we calculated a bandgap of 2.99 while for Ce-UiO-66-PDC we obtaina value of 2.79 eV in excellent agreement withthe absorption onset (λonset) reported in Table 1.As we can see from the DOS analysis in Figure4, the organic molecules mainly contribute tovalence band (VB) states while the CB is mainlyconstituted by the Ce states. To compare therelative VB and CB energy levels, the Ce-UiO-66-Hand Ce-UiO-66-PDC DOS was aligned tothe Ce signal present at around -15 eV andwere set to zero at the Ce-UiO-66-HHOMOlevel.Interestingly,wefound that thedecreasing of the band gap found for Ce-UiO-66-PDC is essentially related to an upshift of VBof about 0.2 eV due to the presence of anadditional band associated to the to thepyridine states (see in Figure 4b the Ncontribution in blue). In particular, while for Ce-UiO-66-Hwe found one main intense band atthe VB edge, for Ce-UiO-66-PDC we observetwo different bands: one at around -0.1 eVcharacterized by the C and O contribution andone at 0.2 eV showing the contribution of N, Oand C. As a matter of fact, in the Ce-UiO-66-PDC system the pyridine moiety introduces anadditional occupied band that is responsible ofthe upshift of the VB leading to the decreasingof the band gap.

Figure 4. DOS analysis of (a) Ce-UiO-66-H and(b) Ce-UiO-66-PDC in black. Separatedcontribution of the atomic species to the DOSare also reported in gray (Ce), red (O), green(C) and blue (N).

ConclusionsIn summary, a new quick and easy synthetic

solvent-free methodology for obtaining Ce-UiO-66 MOFs is here presented. The proceduremakes use of a very small amount of aceticacid as modulator and no further solvents areneeded. The MOFs were obtained by usingdifferent substituted BDC linkers and pyridinedicarboxylic acid. Their optical properties wereinvestigated both from the theoretical and theexperimental point of view. The effect of band-gap shift of the materials upon near UV light isdue to the molecular structure of the linkerwhich induced different charge separationstates referred to LMCT. The MOFs display goodphotocatalytic properties towards the photo-oxidation of substituted benzylic alcohols toaldehydes. The activity of the photocatalyticsystems depends on the electron withdrawingeffects of the aromatic substituent. Furtherinvestigation will be devoted to fine tuning theband gap of the Ce-MOFs by choosingappropriate linkers for other key reactions to betested upon photoirradiation.

ASSOCIATED CONTENT

Supporting Information available. Syntheticand analytical procedures, experimental andinstrumental details, TGA curves, UV-Vis, FT-IR and1H-NMR spectra, additional catalytic data,additional XRPD patterns, theoretical calculationsdetails. This material is available free of chargevia the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author*[email protected].

PresentAddresses†If an author’s address is different than the one given in the affiliation line, this information may be included here.

Author ContributionsThe manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. /

ACKNOWLEDGMENT

The present work was supported by Fondo Ricercadi Base (FRB-2017) and the Project AMIS,Department of Excellence University of Perugia.M.T. acknowledge the European Union’s Horizon2020 research and innovation programme underthe Marie Skłodowska-Curie grant agreement No663830, and the Engineering and PhysicalSciences Research Council (EPSRC) for fundingthrough the First Grant scheme EP/R01910X/1.

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