CeO2 Frustrated Lewis Pairs Improving CO2 and CH3OH Conversion to Monomethylcarbonate

Frustrated Lewis pairs (FLPs), discovered in the last few decades for homogeneous catalysts and in the last few years also for heterogeneous catalysts, are stimulating the scientific community’s interest for their potential in small-molecule activation. Nevertheless, how an FLP activates stable molecules such as CO2 is still undefined. Through a careful spectroscopic study, we here report the formation of FLPs over a highly defective CeO2 sample prepared by microwave-assisted synthesis. Carbon dioxide activation over FLP is shown to occur through a bidentate carbonate bridging the FLP and implying a Ce3+-to-CO2 charge transfer, thus enhancing its activation. Carbon dioxide reaction with methanol to form monomethylcarbonate is here employed to demonstrate active roles of FLP and, eventually, to propose a reaction mechanism clarifying the role of Ce3+ and oxygen vacancies.


Structural and spectroscopic characterizations
The IR spectrum of conv(650) material, measured after O 2 activation, presents a peculiar profile i.e., flat scattering profile parallel to a reverse band around 1000 cm -1 , in line with its low SSA making it less transparent to IR radiation, and few features that can be ascribed to some organic fragments present on the surfaces (dark cyan line in Figure S4a). As detailed in the experimental section 2.2, samples undergo O 2 /H 2 activation procedures to remove organic residuals and to obtain a betterdefined structure. A band at 2342 cm -1 , observed in MW(100) catalysts was assigned to ν 3 (CO 2 ) as indicating the presence of CO 2 produced during the synthesis (from urea decomposition) and trapped in the catalysts crystallites. 1 The absence of the band in MW(650) indicates that the calcination process removes trapped CO 2 increasing its catalyst SSA and cumulative pore volume (Table S1).
Part b) of Figure S4 illustrates the enlarged view of the ν(O-H) region. Whilst all the samples presented isolated terminal, bi-bridged and tri-bridged hydroxyls groups, identified from ν(O-H) at 3710, 3685 and 3657 cm -1 , respectively, the former was not observed on MW(100)-red ( Figure S4 red line), consequence of CeO 2 surface partial reduction. 2 To verify MW(100) structural integrity after reduction at 150°C, MW(100)-red PXRD pattern was measured from a sealed capillary and compared with MW(100). As shown in Figure S6, there is not a clear difference between the two diffractograms, confirming as the reduction process influences the catalyst surface without modifying its structure and crystallites size.

XPS beam damage evaluation and spectra fitting results
Beam damage of as-prepared MW(100) was evaluated for I) beam exposure during time and II) sample heating under vacuum. Considering dual anode beam damage ( Figure S7a,b) we observed as by collecting spectra with 5min/scan ( Figure S7a) the Ce 4+/3+ regions begin to vary after 30'. The first scan (5') presented Ce 3+ abundance of 11% whilst averaging the 6 spectra in Figure S7a led to Ce 3+ ≈14%. Since there is not a considerable difference between the two spectra and fit results, we proceeded by collecting spectra with 30' time/scan for 330', observing as Ce 3+ concentration increased to 15% after 60' of exposure while the sample temperature increased to 25°C. Eventually, Ce 3+ concentration increased up to 25% after 330' exposure and temperature grew up to 35°C. On the contrary, catalyst damage induced by temperature under UHV ( Figure S7b,c) showed a drastic increase of Ce 3+ concentration already at 150°C, reaching values >45% at 400°C. These results showed as: I) dual anode exposure increased Ce 3+ abundance of 4% in the first 60' and II) heating under vacuum had a stronger effect on Ce 3+ oxidation state. Since at 50°C we observed 27% Ce 3+ and after 330' of sample exposure to dual anode its temperature/Ce 3+ reached 32°C/23%, we can conclude that heating from the X-Ray beam caused most of the damage from its exposure. To minimize the beam exposure effect on Ce(3d) region and following the evidence in Figure S7a, we then measured this region as the first one limiting the measurement to 30' time/scan.   interaction can also donate an electron to CO unoccupied π orbitals (π-backdonation) and III) hydroxyl groups (Ce-OH) forming Bronsted acid sites, expected to interact with CO giving ν(CO)≈2153 cm -1 . 33 The latter interaction should perturb the ν(OH) vibration shifting it to lower and increasing its broadening. As observable in Figure S9, none of these effects were observed, hence confirming the absence of an interaction between CO and hydroxyl groups indicating as they might behave as weak bases rather than acid sites.

CH 3 OH adsorption
Methanol is well known to be adsorbed over CeO 2 surface by reacting with its hydroxyl groups forming methoxide species represented in Figure

CO 2 adsorption
Carbon dioxide is commonly adsorbed and activated over CeO 2 surface through formation of several carbonates (mono/bidentate/bridged), bicarbonates (bridged and bidentate) and formate (bridged and bidentate) species. As showed in the schematic representation in Figure S13 bicarbonates and formates can be easily distinguished from carbonates by the presence of vibrations related to -COH and -CH groups, respectively. Whilst a preliminary distinction between these three families is then straightforward, a fine identification of the precise formed member (mono/bi/tri-dentate and bridged) is not trivial. After CO 2 adsorption over all the materials ( Figure S14 and Figure S15) we observed formation of two bands in the OH/CH stretching region ( Figure S14a and Figure S15a) and six distinguishable bands in the carbonyls stretching regions ( Figure S15c). The former region contains two bands at 3618 and at 2860 cm -1 , undoubtedly associated to ν(OH) and ν(CH) from bicarbonates and formate species, respectively, the latter being absent/too weak to be detected on conv(650). 3,6 On the contrary in the low energy region, due to their frequency position and separation, bands at 1577 and 1293 cm -1 can be ascribed to asymmetric and symmetric ν(CO) vibration, respectively, from carbonate species. The band at 1217 cm -1 is well reported as related to bicarbonates δ(COH), leading us to associate the band at 1401 cm -1 to ν(CO) from the same bicarbonate. A less distinguishable shoulder and a weak band, observed around 1604 and 1512 cm -1 , have been assigned after 13 CO 2 adsorption (vide infra), to ν(CO) of bicarbonate and formate species, respectively. and ν(CH), b) Ce +3 electronic transition and c) ν(CO) spectral regions. Spectra are reported as difference by substracting the activated spectra showed in Figure S4a. 13 C shift allowed us to separate: I) a shoulder at 1604 cm -1 which position and chemical shift (Table   S4) can be assigned to h-CO 3ν(CO) vibration and II) two weak bands at 1512 and 1365 cm -1 related to formate ν(CO) as/sym . On the contrary, bands at lower energy can be parallelly associate to CO 3 = /HCO 3 and HCO 2 i.e., i) band at 857 cm -1 matches the 13 C shift ( Figure S16 and Table S4) of all the species and ii) band around 1025 cm -1 is considerably broad leading to an isotopic shift between 1 and 19 cm -1 , ascribable again to all the three mentioned species. Figure S16. FTIR spectra of MW(100) after absorption of CO 2 (black line) and 13 CO 2 (orange line).
Main identified vibrations are indicated. Spectra are reported as difference by substracting the activated spectra.     RT CH 3 OH adsorption.