Mechanistic insights into the catalytic transfer hydrogenation of furfural with methanol and alkaline earth oxides

Journal of Catalysis 372 (2019) 61–73

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Mechanistic insights into the catalytic transfer hydrogenation of furfural with methanol and alkaline earth oxides Maria S. Gyngazova a,b, Lorenzo Grazia a, Alice Lolli a, Giada Innocenti a, Tommaso Tabanelli a, Massimo Mella c,⇑, Stefania Albonetti a, Fabrizio Cavani a,⇑ a b c

Dipartimento di Chimica Industriale ‘‘Toso Montanari”, Università degli Studi di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Institut für Technische und Makromolekulare Chemie, RWTH Aachen, Worringerweg 2, 52074 Aachen, Germany Dipartimento di Scienza ed Alta Tecnologia, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy

a r t i c l e

i n f o

Article history: Received 9 August 2018 Revised 17 February 2019 Accepted 20 February 2019

Keywords: Meerwein-Ponndorf-Verley reduction H-transfer MgO CaO SrO Furfural Methanol Mechanism modeling

a b s t r a c t DRIFT characterization and DFT calculation were carried out to clarify the previously unexplored use of methanol as a H-transfer agent for the liquid-phase Meerwein-Ponndorf-Verley reduction of biomassderived furfural using alkaline earth oxide catalysts (MgO, CaO, SrO). Methanol adsorption mechanism has been studied in detail and the energy correlated to the process has been theoretically calculated for each of the prepared catalyst to investigate the relative performances of the three basic oxides. Although, the higher-surface-area MgO displayed an exceptionally high activity for the H-transfer process at low temperatures, CaO and SrO were found to be the catalysts with the highest specific productivity per unit surface area and unit basic site. The different specific productivities of the three catalysts was explained by DRIFT with different adsorption mode selectivities (3 different modes for MgO versus only 1 for CaO and SrO, with the production of only the active methoxide), which may indicate a different methanol activation with regard to the H-transfer toward the carbonyl moiety of FAL. Furthermore, higher SrO than CaO productivity can be explained by the different basicity, which in turn leads to differences in the main methanol activation pathways. DFT calculations make it possible to gain further insight into the role of the basic strength on methanol activation and H-transfer reaction suggesting the increased ability of activating the alcohol via formation of the methoxide ion being the key factor in modulating the catalyst activity rather than the polarization of the aldehydic carbonyl group due to the coordination onto the M3C site. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Catalytic hydrogenation using supported precious metal catalysts and high H2 pressure continues to be the main hydrogenation technique used today [1–3]. An alternative approach is that of reducing carbonyl groups using alcohols as the hydrogen donor, i.e. the Meerwein-Ponndorf-Verley (MPV) reaction. This process is characterized by a higher selectivity and usually requires accessible, inexpensive, and stable catalysts. Generally, Al, Ti, B, La, or Zr alkoxides are used as homogeneous catalysts for MPV reactions [4]. However, with these materials the reaction requires a large excess of alkoxide in order to reach acceptable yields, and ends up with non-reusable compounds obtained in the catalyst recovery process. ⇑ Corresponding authors. E-mail addresses: [email protected] (M. Mella), Fabrizio.cavani@ unibo.it (F. Cavani). https://doi.org/10.1016/j.jcat.2019.02.020 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

Thus, in order to overcome this problem, the application of solid-base catalysts for reactions of this type has been investigated, mostly based on alkali and alkaline earth oxides (MgO, CaO) [5–13], mixed oxides (Mg/Al, Mg/Ga, Mg/In, Ga/Al, Mg/Al/Zr, Co/ Al, Ni/Al, Cu/Al) [14–20], and amphoteric oxides (Al2O3) [21,22], as well as on zeolites or mesoporous materials, sometimes incorporating metal ions acting as Lewis acid sites (Al3+, Zr4+, Sn4+) [23–45]. In particular, MgO, CaO, and SrO are solid-base catalysts used for a number of base-catalyzed reactions: the isomerization of alkenes and alkynes, aldol condensation, Knoevenagel condensation, nitroaldol condensation, Michael addition, conjugate addition of alcohols, nucleophilic ring opening of epoxides, oxidation reactions, and Si-C bond formation [46]. The surface basic strength follows the order MgO < CaO < SrO. Among the latter, MgO has been studied most extensively, probably because samples of definite structure with high surface area are prepared much more easily by thermal pretreatment than are CaO and SrO samples [47].

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The MPV reaction can be conducted in solutions using substrates such as citral, cyclohexanone, acrolein, acetophenone, hexanone, mesityl oxide, acetone, benzaldehyde, crotonaldehyde, furfural, and other aldehydes and ketones. The most frequently used hydrogen donor is isopropanol, while in some cases ethanol, cyclopentanol, cyclohexanol, 1,4-butandiol, or 2-butanol was used. Among the various carbonyl-bearing substrates, furfural (FAL) was chosen as a substrate, being an important renewable building block and a key precursor for biofuel and chemical production. Furfural can be upgraded by the hydrogenation of carbonyl groups to form furfuryl alcohol (FFA). FFA is used as a solvent, modifier for resins, and chemical intermediate to produce lubricants, adhesives, wetting agents, and for the synthesis of lysine, vitamin C, and tetrahydrofurfuryl alcohol. More frequently, furfural was hydrogenated using 2-propanol as a H-transfer reagent, while using Pd/Fe2O3 [48], Cu/Pd [49], Al2O3-C [50], Cu/AC-SO3H [51], c-Fe2O3@HAP [52] and Ru/Carbon [53] as catalysts. Cu/Mg/Al/O was used for FAL hydrogenation when ethanol, propanol, and 2-propanol [54] were used as hydrogen donors. Moreover, ruthenium-based catalysts were used with primary and secondary alcohols [55] and benzyl alcohol [56] for a MPV reduction of furfural carbonyl compounds. Lastly, this process was also investigated in supercritical methanol in the presence of Al/Mg hydrotalcites [57]. Nevertheless, the use of these alcohols can lead to liquid co-product formation, i.e. the corresponding aldehyde or ketone. With the aim of reducing co-product formation, our group reported on the previously unexplored use of methanol as a Htransfer (HT) agent for the MPV reaction of aromatic aldehydes and aryl ketones using high-surface-area MgO as an inexpensive and reusable catalyst [58]. CH3OH activation is a real challenge, but its use as a hydrogen donor makes it possible to facilitate coproducts separation from the reaction mixture, since formaldehyde degradation compounds are transferred into the gas phase. In the present study, we report the preparation, characterization, and catalytic activity of alkaline earth metal oxides (CaO, SrO) different from MgO. These oxides are known to be interesting basic catalysts; nevertheless, their catalytic activity was never tested in the MPV liquid-phase furfural reduction using methanol as a H-donor. CaO and SrO are generally considered materials with stronger basicity respect to MgO and this property could significantly change both their interaction with methanol and the mechanism of the reaction. Therefore, the main aim of this work was to investigate the correlation between the catalytic performances of the materials, prepared in different conditions to obtain different active species and surface area, and the mechanisms for methanol adsorption and activation. In particular, an in-depth DRIFT study and the calculation via DFT of the energy correlated to the process, has been completed for each of the three basic oxides.

2. Experimental 2.1. Preparation of MgO, CaO, SrO MgO was prepared via the thermal decomposition of brucite. Mg(OH)2 was synthesized by precipitation, by adding dropwise an aqueous solution containing the corresponding metal nitrate, Mg(NO3)2
6H2O (Sigma-Aldrich) into a solution containing 1 M Na2CO3 (Sigma-Aldrich). While the brucite precipitated, the slurry was maintained at 55 °C and pH 10.5. At the end of precipitation a 1 h aging treatment was carried out to increase the crystallinity of the formed phase. The obtained solid was then filtered and washed with 2 L of warm water (40 °C) per gram of precipitate. Lastly, brucite was dried overnight at 120 °C in static air. CaO and SrO were prepared by means of the thermal decomposition of CaCO3 and

SrCO3 respectively, following the procedure described above for MgO. Ca(NO3)2
4H2O (Sigma-Aldrich) and Sr(NO3)2
4H2O (SigmaAldrich) were used as metal precursors. All catalyst precursors were later calcined at different temperatures in the range 350– 1200 °C in static air for 5 h with the heating rate of 10 °C/min. The prepared catalysts were stored under nitrogen to avoid the adsorption of H2O and CO2 over the catalyst surface. 2.2. Characterization methods Both the precursors and final oxides were characterized by means of X-ray diffraction. XRD powder patterns of catalysts were recorded with Ni-filtered Cu Ka radiation (k = 1.54178 Å) on a Philips X’Pert vertical diffractometer equipped with a pulse height analyzer and a secondary curved graphite-crystal monochromator. The specific surface areas of powders were measured by means of the BET single-point method (nitrogen adsorption-desorption in flowing N2 at the temperature of liquid nitrogen), using a Sorpty 1750 Fison Instrument. A 0.4 g sample was typically used for the measurement, and it was outgassed at 150 °C before N2 adsorption. The samples characterized by very low surface area were analyzed also by means of nitrogen physisorption using the Micromeritics ASAP 2020 instrument. All the samples were first pretreated for 30 min at 150 °C and under a pressure of 30 lmHg (2 °C/min ramp rate from room temperature); samples were then heated up to 250 °C (10 °C/min ramp rate). After pretreatment, nitrogen was added and the surface area was determined. The TPD of CO2 was performed to determine the basicity of MgO, CaO, SrO catalysts using a POROTEC Chemisorption TPD/R/ O 1100 automated system. In order to remove adsorbed H2O and CO2 from the catalyst surface prior to adsorption, the samples were pretreated in N2 for 10 min (flow rate 20 mL min 1) at their calcination temperature. Then the samples were cooled down to 80 °C and the catalyst surface was saturated with CO2 for 1 h. Physically adsorbed CO2 was removed by flushing the samples in a stream of N2 for 10 min. The temperature-programmed desorption of CO2 was carried out at a constant rate of 20 °C min 1 from 40 °C to 1100 °C in a stream of He (25 mL min 1). The total basicity of solid bases was examined via the irreversible adsorption of acrylic acid [59–61]. Prior to basicity measurements, all catalysts were calcined at their optimal calcination temperatures in static air for 5 h with a heating rate of 10 °C min 1. 10–20 mg of catalysts were placed into the stoppered bottle, and then 20 mL of a standard solution of organic acid (acrylic acid) in cyclohexane were added. The resulting suspension was stirred for 4 h at room temperature. In all experiments, 4 h were found to be sufficient to achieve the adsorption equilibrium. The solution was then filtered using syringe filters (Chromafil, PTFE, pore size 0.45 mm). The concentration of the non-adsorbed organic acid in the solution was determined spectrophotometrically (StellarNet Inc.) at k 255 nm. Diffuse reflectance FTIR spectra (DRIFTS) of methanol adsorbed on MgO, CaO, and SrO samples were measured on a Bruker Vertex 70 IR spectrometer in the 4000–650 cm 1 range (resolution 4 cm 1, 128 scans per spectrum) using a Pike Diffus IR cell attachment. The mass spectrometer was an EcoSys-P from European Spectrometry Systems. Spectra were recorded using a mercurycadmium-telluride detector. The sample was first thermally pretreated at 450 °C for 30 min (10 °C min 1 heating rate from room temperature) in a He stream (8 mL min 1), and then cooled down to 30 °C. A background spectrum was collected at each adsorption/desorption temperature tested during the analysis. Methanol was adsorbed by injecting it into a heated line (90 °C) with the DRIFT cell at 30 °C. During the adsorption process a spectrum was collected every minute for 30 min. To gain some information on the strength of methanol-catalyst interaction, the temperature

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in the DRIFT cell was increased up to 450 °C, with several temperature steps, to promote the desorption of methanol. An IR spectrum was recorded with the subtracted background collected at the same temperature, at each temperature step, to follow up the desorption process; at the end of the temperature ramp the sample was cooled down again to 30 °C. 2.3. Catalyst testing All the chemicals used were purchased from Sigma-Aldrich (furfural, furfuryl alcohol, 2-methylfuran, methanol). The hydrogenation of FAL was carried out in a Parr Instrument 4561 autoclave reactor (300 mL). Reactions were carried out in methanol, using the appropriate amount of catalyst. If not otherwise indicated, each test was conducted for 3 h with the following amounts of reagents: 50 mL methanol, 1.21 mmol FAL, 1 g catalyst, and 1 bar of nitrogen. After loading the reactor with methanol, FAL, and catalyst, the autoclave was purged 3 times with N2 (20 bar) and then pressurized at 1 bar N2. The reaction mixture was heated up to the desired temperature and the stirring speed was adjusted to 400 rpm for the time needed. At the end of the reaction, the reaction mixture was cooled down in an ice bath, the gas was sampled at room temperature for the analysis, and the catalyst was separated by filtration. Liquid products were analyzed by means of HPLC (Agilent Technologies 1260 Infinity instrument equipped with a DAD UV–Vis detector), using a 120 50  4.6 mm C-18 core-shell column using a gradient elution starting with 90% H2O and 10% acetonitrile as the mobile phase: after 4 min. the result was 50% H2O and 50% acetonitrile, while after 8 min. it was 30% H2O and 70% acetonitrile. For compound identification, GC–MS (Agilent Technologies 6890 N, provided with a 30 m  0.25 mm HP5 column) and ESI-MS (Waters micromass ZQ 4000) analyses were also performed. 2.4. Theoretical modeling Gas phase electronic structure calculations were carried out employing the Gaussian09 suits of programs and exploiting the B3LYP density functional [62,63] to describe correlation and exchange energies. The basis sets used were the 6-31++G(d,p) set for the H, C, O, Mg and Ca atoms, and the dgdzvp set for the Sr atom. Similar levels of theory has been previously found able to rationalize change in catalyst activity upon changing the metal ion involved [64,65], and to semi-quantitatively describe intermolecular interactions between charged species [66–69]. The reactive sites for all oxides were modeled employing a previously used model for the Mg3C corner site on the MgO oxide [58]; we found

possible to employ the minimalistic model M10O10 (M=Mg, Ca or Sr) for all oxide clusters involved in the title reactions, as this was previously found able to minimize over-polarization effects when the reactive steps take place in the close vicinity of the Mg3C site [70,71]. The geometrical parameters of the MgO (cubic, dMgO = 2.1084 Å), CaO (cubic, dCaO = 2.406 Å [72], and SrO (cubic, dSrO = 2.586 Å [73] clusters were kept frozen in all calculations, as minor effects would be expected for them [71] and for energetic quantities. Geometries for reactants, products and transition states (TS’s) were fully optimized and characterized by means of frequency calculations. Putative structures for energy minima and TS’s were built using data from our previous works, followed by a complete geometrical relaxation, keeping the MO clusters constrained. For a direct comparison between reactions taking place over different oxides, the common energy zero represented by gas phase reactants and isolated clusters was chosen. With this choice, the relative positioning of intermediates and TS’s can be straightforwardly used to analyze the catalysts relative performances. 3. Results and discussion 3.1. Catalyst characterization In order to determine the influence of calcination temperature on the catalytic activity, MgO, CaO, and SrO precursors synthesized from Mg(OH)2, CaCO3, SrCO3, were calcined at different temperatures in the range 350–1200 °C. Specific surface areas were measured by means of nitrogen physisorption (Table 1). Moreover, TGA-DTA analyses of the catalyst precursors were performed (Figs. S1–S3), while dried and calcined materials were characterized by means of X-ray diffraction (Figs. 1–3). A summary of the crystalline phases present in each sample is reported in Table 1. MgO pretreated at temperatures between 350 and 500 °C has a high specific surface area of 200 m2 g 1. MgO pretreated at 600 °C, however, shows a specific surface area of 127 m2 g 1, whereas the surface area of material pretreated at 700 °C and 900 °C is only 30 and 16 m2 g 1, respectively. CaO and SrO have a low specific surface area (2 m2 g 1) at all calcination temperatures. TGA shows the temperatures at which the catalyst precursors decompose when heated in an air flow. The thermal decomposition of Mg(OH)2 is presented in Fig. S1 ESI. Water is removed from the precursor at a temperature between 40 °C and 250 °C, with physically adsorbed water eliminated at 40–100 °C (the observed mass loss accounts for around 5%) and crystalline water released at higher temperatures. The layer dehydroxylation is observed between 250 and 400 °C. These thermal phenomena are related

Table 1 Specific surface area (SBET) and total basicity (determined by acrylic acid absorption) of catalysts calcined at different temperatures. Catalyst name

Alkalineearth metal

Precursor

Calcination temperature (°C)

Crystalline phase

SBET (m2/g)

Total basicity (mmol/g)

Base sites/surface area (mmol/m2)

MgO

Mg

Mg(OH)2 (brucite)

350 500 600 700 900

MgO (periclase)

200 200 127 30 16

6.12 7.05 5.67 2.37 1.98

0.031 0.035 0.045 0.079 0.124

CaO

Ca

CaCO3 (vaterite + calcium carbonate)

500 600 700 900

CaCO3 (calcium carbonate)

1.4 1.3 2.4 2.3

0.22 0.18 2.57 1.10

0.157 0.377 1.071 0.478

500 700 900

SrCO3 (strontianite)

1.1 1.0 2.5

0.04 0.09 0.91

0.036 0.090 0.365

2.6

0.60

0.232

SrO

Sr

SrCO3 (strontianite)

1200

CaO (lime) and Ca(OH)2 CaO (lime)

SrCO3 (strontianite), Sr(OH)2 and Sr(OH)2(H2O) Sr(OH)2 and Sr(OH)2(H2O)

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Fig. 1. XRD patterns of MgO precursor as synthesized and after calcination at different temperatures.

Fig. 2. XRD patterns of CaO precursor as synthesized and after calcination at different temperatures.

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Fig. 3. XRD patterns obtained for SrO precursor as synthesized and calcined at different temperatures.

to the corresponding endothermic peaks on the DTA curve. The thermal decomposition of CaCO3 (Fig. S2) shows two major mass loss steps at 40–100 °C and 550–770 °C. The mass losses observed at those temperatures are <1% and 42%, respectively. At the first step, the adsorbed water is released, while at the second step CO2 is released due to carbonate decomposition. Both steps are related to the corresponding endothermic peaks on the DTA curve. TGA and DTA data for SrCO3 are shown in Fig. S3. The adsorbed water is lost between 40° and 100 °C, showing the corresponding endothermic peak in the DTA curve. The next considerable mass loss takes place at 800–1050 °C, where CO2 release occurs. In this case, also, the corresponding endothermic DTA peak is shown. XRD diffraction patterns of MgO before calcination and after pretreatments at different temperatures are shown in Fig. 1. The uncalcined catalyst precursor showed the typical reflections of brucite (Ref. Code: 00-044-1482). The thermal pretreatment of brucite caused a change in the XRD pattern, which is due to the loss of water and dehydroxylation of the starting material. Patterns of samples activated at temperatures >350 °C show reflections that can be attributed to MgO (periclase, Ref. code 01-075-0447). The catalysts calcined at temperatures lower than 600 °C present low crystallinity (broad reflections), whereas those treated at 600 °C, 700 °C and 900 °C are more crystalline and are characterized by a more ordered structure (sharper, more intense peaks). At temperatures lower than 300 °C, however, calcination does not lead to the formation of a pure MgO phase. XRD data correlate well with the results of TGA-DTA, showing that the formation of the oxide phase occurs at temperatures lower than 500 °C. As a matter of fact, in the sample calcined at 500 °C the oxide phase is already formed, and still preserves a high surface area (Table 1). Fig. 2 shows the XRD patterns of the CaO precursor calcined at different temperatures. The precursor dried at 120 °C shows the most complicated XRD patterns. Indeed, two different polymorphic structures of CaCO3 are present: vaterite (Ref. Code: 00-033-0268) and calcium carbonate (Ref. Code: 01-085-1108). The latter is the more stable compound, while vaterite is the least stable polymorph of calcium carbonate. The different stabilities of the two polymorphs are in keeping with the experimental results; indeed,

the thermal treatment of the precursor at 500 °C causes the transformation of vaterite into calcium carbonate. XRD diffraction patterns confirm the presence of a single phase that totally corresponds to the theoretical pattern of calcium carbonate as shown in Fig. 2. Even the material calcined at 600 °C shows reflections typical of calcium carbonate. The structural change of vaterite into calcium carbonate occurring when the temperature is increased is also in agreement with the results of the TGA/DTA analysis. No significant weight loss appears up to 600 °C, thus confirming that only a structural change occurs. The materials calcined at 700 °C and 900 °C show sharp and intensive peaks typical of CaO (lime, Ref. Code 01-077-2010). Therefore, materials pretreated at temperatures lower that 700 °C are still in the carbonate form, in line with the TGA. Fig. 3 illustrates the XRD patterns of the SrO precursor, as synthesized and calcined at temperatures from 500 to 1200 °C. The main reflections of the non-calcined sample can be attributed to strontianite (Ref. Code 01-084-1778). The diffraction patterns of the materials pretreated at 500 °C and 700 °C show the same main reflections, however considerably sharper and more intense, due to the better crystallinity. Patterns after calcination at 900 °C exhibit reflections of strontianite and other reflections which can be attributed to Sr(OH)2 (Ref. Code 00-019-1276) and to Sr(OH)2(H2O) (Ref. Code 01-077-2336). The presence of hydrated compounds is explained by the very high reactivity of SrO toward water [74], which is easily adsorbed from the air. This effect is more evident with the sample calcined at 1200 °C, when only the crystalline phases of hydroxydes are observed. Thus, it is reasonable to state that the sample calcined at 900 °C consists of both strontianite and SrO, which is actually hydrated. Compared to CaO, the SrO precursor requires higher calcination temperatures to decompose carbonate. In fact, when treated at temperatures lower than 1000 °C, Sr carbonate is the main compound (Fig. 3, Fig. S3). Basic sites play a major role as active centers for hydrogen transfer reaction. CO2-TPD is the method most frequently used to measure the number and strength of basic sites [33]. This technique was then used for the evaluation of the strength of basic sites, which is correlated to CO2 desorption temperature

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(Fig. S4; experiments were conducted on samples after in-situ total decarbonation, at temperatures necessary for completely removing CO2; see Figs. 1–3). The obtained results highlighted the fact that, in the case of MgO, the majority of the basic sites are of the weak and medium-strength types (desorption temperatures in the range 150–500 °C), with also strong sites (desorption in the range 700–900 °C). CaO shows the predominance of strong basic sites (desorption in the range 600–800 °C). In the case of SrO, CO2 desorption starts only at temperatures over 1000 °C, in agreement with the decarbonation tests and XRD patterns of the calcined samples. This confirms that SrO has mainly very strong basic sites, confirming the well-known basicity strength order for alkaline earth oxides: SrO > CaO > MgO. Another method for investigating the basicity of catalysts is the measurement of the irreversible adsorption of non-interacting organic acids, such as acrylic acid. The basicity of samples is summarized in Table 1. Both acrylic acid adsorption (Table 1) and CO2-TPD results (Fig. S4) highlight that the number of basic sites per unit weight increases in the order SrO < CaO < MgO. These results are in good agreement with literature results for solid base catalysts [75]. Therefore, we can conclude that – as expected – all prepared catalysts have weak and strong base sites; SrO has a considerably lower amount of total basic sites per unit weight than MgO and CaO which, however, are much stronger. When dividing the total number of strong basic sites by the surface area of each sample, it is apparent that MgO is characterized by a much lower concentration of sites, compared to CaO and SrO. The following catalytic tests will elucidate which is the most important feature needed to enhance catalytic activity.

Fig. 4. Catalytic performance as a function of the pretreatment temperature for MgO at T = 160 °C, after 3 h reaction (j FAL conversion, solid line; d FFA yield, broken line).

3.2. Catalytic tests The liquid phase hydrogenation of FAL using methanol as the hydrogen donor (Scheme 1) was performed over the solid bases MgO, CaO, and SrO. The materials calcined at different temperatures were tested at different reaction temperatures (160°– 210 °C) and times. Figs. 4–6 show FAL conversion and FFA yield at 160 °C and 210 °C after a 3 h reaction as a function of the calcination temperature for the three catalysts. The changes observed in the XRD patterns (Fig. 1) and TGA-DTA (Fig. S1) of MgO coincide with a change in the catalytic activity (Fig. 4). MgO calcined at Tcalc = 450 °C, a poorly crystalline highsurface-area catalyst, was found to exhibit a superior catalytic activity in the reduction of FAL. A lower catalytic activity is observed when the calcination temperature of MgO is either <350 °C or >500 °C. In fact, the pretreatment of the catalyst at temperatures <350 °C did not lead to the formation of MgO. Moreover, by-product formation is enhanced; in fact, a high carbon loss is registered. An increased calcination temperature, up to above 500 °C, leads to a decrease in catalyst activity as MgO becomes more crystalline and the specific surface area decreases (Table 1). These results agree with literature [9], describing the influence of the pretreatment temperature on the catalytic activity of MgO in the liquid phase Meerwein-Ponndorf-Verley reaction of benzaldehyde with 2-butanol.

Fig. 5. Catalytic performance as a function of the pretreatment temperature for CaO (red) and SrO (blue) at T = 160°, after 3 h reaction (j FAL conversion, solid line; d FFA yield, broken line).

Fig. 6. Catalytic performance as a function of the pretreatment temperature for CaO (red) and SrO (blue) at T = 210°, after 3 h reaction (j FAL conversion, solid line; d FFA yield, broken line).

Scheme 1. Hydrogenation of furfural (FAL) to furfuryl alcohol (FFA).

The results of XRD (Figs. 2, 3) and TGA-DTA (Figs. S2, S3) for CaO and SrO, pretreated at different temperatures, can also be correlated to their catalytic activities (Figs. 5, 6). Both CaO and SrO show good activity for FAL hydrogenation, despite their very low specific surface area (Table 1).

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As far as CaO is concerned, at reaction temperatures of 160 and 210 °C, the highest conversion is shown when it is calcined at 700 °C, even though some by-products are formed; at this temperature, full decarbonation of the material is achieved. However, different values for FFA selectivity are shown. While the samples calcined at 500 and 600 °C give 100% selectivity to the desired product, samples calcined at 700 and 900 °C show opposite trends for FFA selectivity when different reaction temperature are used. In fact, at 160 °C (Fig. 5) the calcination temperature which resulted in the lowest FFA selectivity is 700 °C; on the contrary, when the reaction was carried out at 210 °C (Fig. 6) the sample calcined at 900 °C showed the lowest FFA selectivity value. Regarding SrO, the calcination temperature of 900 °C, at which the phase transition from strontianite to Sr(OH)2 takes place, is the best one for promoting catalytic activity. Catalytic performances can be improved by increasing calcination temperature, although with a significant formation of by-products. As a matter of fact, all reactions carried out using SrO show a poor carbon balance; the greatest carbon loss is observed when the catalyst is calcined at 500 °C, since this catalyst is characterised by a lower amount of basic sites (Table 1). It is worth noting that when the carbon balance is less than 100%, no other products other than FFA are detected by means of both HPLC and GC–MS. However, the color of the solutions after reaction (Figs. 4–6) makes it possible for us to conclude that when carbon balance is low, solutions are darker than those obtained with catalysts calcined at their optimal temperature. This suggests that, under non-optimized conditions, the formation of complex heavy molecules characterized by the presence of chromophore groups (e.g. conjugated CAC double bonds) takes place together with the formation of FFA. Catalytic tests showed that the activity of catalysts correlates well with the measured number of total basic sites, the latter being a function of the calcination temperature (Fig. 7). On the other hand, it is also evident that both CaO and SrO are more active than might have been expected based on the number of basic sites. This behaviour might be explained considering the CO2-TPD results (Fig. S4), where it is shown that CaO and SrO are mainly characterised by medium and strong basic sites. The effect of reaction time on FFA yield was reported in our previous work [58], using a MgO catalyst pretreated at 450 °C. The yield of FFA increased rapidly up to 66% at a reaction time of 30 min and Tr = 160 °C. After 3 h, FAL yield exceeded 90% with almost 100% conversion. Time course tests were also carried out with CaO (Tcalc = 700 °C) and SrO (Tcalc = 900 °C) (Figs. S5 and S6, respectively). However, for

Fig. 7. Correlation of catalytic activities with the number of total base sites for MgO (black), CaO (red) and SrO (blue) at T = 160 °C, after a 3 h reaction (solid lines: FAL conversion; broken lines: total base sites).

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both CaO and SrO the C balance is very poor during the first hours of reaction. Once again, no by-products are detected by means of both HPLC and GC–MS, but the color of the reaction mixtures – see Figs. S5 and S6; note that the reaction mixture after reaction with SrO is not as dark as the one obtained with CaO, in agreement with the better C balance observed with the former catalyst - is a clear indication of the formation of heavy by-products. Remarkably, for longer reaction times the C balance improves and becomes close to 100%, while the corresponding reaction mixture is colorless and transparent. This suggests that heavy compounds are formed in parallel with FFA, but the reaction leading to their formation is fully reversible, and as long as the reactant is consumed, the heavy compounds formed for short reaction times are transformed back to FAL. In the end, FFA is the only product formed with very high selectivity and almost a 100% C balance. ESI-MS analysis of the brownish reaction mixture after 15 min reaction time (Figs. S7, S9) made it possible for us to confirm the presence of high-molecular-mass fragments which, conversely, were no longer present in clear mixtures after a 3 h reaction time (Figs. S8, S10). A blank test was carried out without a catalyst at 210 °C to evidence possible reactivity of furfural at this temperature. Nevertheless, after 30 min, the reaction mixture was colorless and no products were formed (Fig. S11), indicating that the catalyst promotes FAL conversion to both FFA and heavy compounds, and that methanol needs a basic catalyst to be activated as H-donor. Fig. 8 compares the specific productivity referred to the number of basic sites (number of mmol FFA produced)/(reaction time * specific number of basic sites * mass of the catalyst) at 160 and 210 °C for the three oxides. It is shown that at both reaction temperatures the TOF follows the order SrO > CaO > MgO; therefore, despite its lower surface area and lower number of basic sites, SrO is the most active catalyst. This result can be tentatively attributed to the basicity strength rank, which follows the same order as for the calculated TOF: SrO > CaO > MgO. DRIFT in-situ experiments with methanol and DFT calculations will provide further information on how both activity and reaction mechanism are affected by the catalysts’ basic strength. 3.3. Adsorption of methanol followed by DRIFT Figs. 9, 10, and 11 show DRIFT spectra recorded after methanol adsorption at different temperatures for the three catalysts, calcined at their optimal temperatures: MgO (450 °C), CaO (700 °C), and SrO (900 °C).

Fig. 8. Specific productivity (TOF) of catalysts referring to the number of specific basic sites in FAL reduction with methanol at Tr = 160 °C ( ) and Tr = 210 °C ( ). Catalysts were calcined at the optimal reaction temperature. The total number of basic sites was inferred from acrylic acid absorption (Table 1).

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Absorbance (AU)

2760 2840 2910

2784 1640

450 °C 300 °C 200 °C

3200

4000

MgCO3

1604 1369

3500

2940 2828

3000

100 °C 60 °C 2657

2500

1117 1052 1080

1500

RT

1000

-1

Wavenumber (cm ) Fig. 9. DRIFT spectra registered after methanol adsorption on MgO at different temperatures.

Fig. 10. DRIFT spectra registered after methanol adsorption on CaO at different temperatures.

Fig. 11. DRIFT spectra registered after methanol adsorption on SrO at different temperatures.

The MgO room temperature spectrum shows three different methanol adsorption modes: undissociated methanol, monodentate, and bidentate methoxy species. The broad bands at 3200 cm 1, 2600 cm 1, and 1500 cm 1, together with the one at 1052 cm 1, characterize the undissociated methanol. The first two bands are due to a resonance between the first overtone 2d (OH) and the fundamental m(OH), the third is correlated to an interaction between the d(CH3) and d(OH) modes, while the latter is due to m(CO). On the other hand, m(CO) methoxy vibrations are detectable at 1117 cm 1 and 1080 cm 1, corresponding to monodentate and bidentate species, respectively. As far as the C-H stretching region is concerned, the bands at 2940 cm 1 and 2828 cm 1 are attributed to undissociated methanol. Conversely, 2910 cm 1 and 2784 cm 1 are related to methoxy species. Above 3500 cm 1, it is possible to identify m(OH) modes of both catalyst terminal OH and newly formed surface OH as negative and positive bands, respectively. Anticipating some of the theoretical results discussed in the following Section, we find noteworthy the fact that the patterns of experimental and theoretical vibrational frequencies for m(CH) and m(CO) of the undissociated and dissociated methanol closely match, supporting the usefulness of our theory level for the interpretation of the spectra. In fact, the theoretical results gave 3134/3087/3032 cm 1 and 2947/2984/3003 cm 1, respectively, for the C-H stretching, and 1060 cm 1 or 1145 cm 1 for the C-O stretching. Besides, the relative intensities of the lowest C-H stretch for the two species predicted by DFT, respectively 99 and 174 KM/mol, nicely agree with the experimental results, which indicate lower dipole strength for the undissociated species. When increasing the temperature, a band at 1640 cm 1 related to formaldehyde m(C@O) appears. At 200 °C, the undissociated methanol bands vanish while the intensities of the methoxy one increase, thus suggesting that part of undissociated MeOH has been transformed into the dissociated species. At this temperature, two other bands related to formate mas(COO) and ms(COO) appear at 1604 cm 1 and 1369 cm 1, respectively, together with the bands at 2840 cm 1 and 2760 cm 1 due to formate m(CH) and ms(COO) + m (CH), respectively [76]. Both formate and bidentate methoxy are stable on the catalyst surface even at 450 °C. It is important to note, for the sake of comparison with the other two catalysts, that no MgCO3 band [77] (dotted spectrum line in Fig. 9) is detectable in the spectrum. CaO spectra are shown in Fig. 10; at room temperature it is possible to note bands associated with both formaldehyde and methoxy species. The bands at 1660 cm 1 and at 2825 cm 1 are associated with formaldehyde m(C@O) and mas(CH), respectively, while the bands at 1080 cm 1, 2922 cm 1 and 2779 cm 1 are related to bidentate methoxy m(CO), mas(CH), and ms(CH). Over 3500 cm 1, it is possible to appreciate the presence of both catalyst terminal OH and newly formed OH group m(OH). On CaO surface just one kind of methoxy is present, while undissociated MeOH is absent even at low temperatures. These observations might explain the greater catalytic activity of CaO compared to MgO. Formaldehyde m(CO) vanishes at 100 °C, while formate mas(COO) and ms(COO) start to be detectable at 1581 cm 1 and 1371 cm 1. A further temperature increase up to 450 °C leads to methoxy band disappearance, while formate bands undergo a small shift. This shift might be due either to a change in the formate coordination or to their transformation into carbonates. In the absence of a clear change in the band shape, however, it is not possible to distinguish with certainty between the two species. Nevertheless, it is possible that the formate coexists with carbonates on the catalyst surface, since new bands appear at 704 cm 1, 853 cm 1, 1770 cm 1, and 2490 cm 1, which are in perfect agreement with aragonite vibrations (CaCO3, whose ATR spectrum [77] is indicated with the dotted line in Fig. 10). Also worthy of note is the appreciable

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negative m(OH) band (between 3500 and 3600 cm 1) associated with a diminution of catalyst terminal OH; this suggests that part of CaO was actually Ca(OH)2, in agreement with the XRD of the CaO sample calcined at 700 °C. As far as SrO is concerned, the spectra are shown in Fig. 11: at room temperature it is possible to detect a weak band at 1082 cm 1 that might be attributable to bidentate methoxy m (CO). Apart from this, the SrO surface seems to be the most reactive of the three tested, probably thanks to its higher basicity. Supporting this idea, there is the fact that the analysis of the C-H stretching region seems to indicate that methanol can certainly be adsorbed as a methoxy species, but that it may also decompose into CH3 (probably CH+3) and OH on the catalyst surface [78,79]. The possible decomposition of methanol as CH3 cation and OH on MgO was originally postulated by Di Valentin et al [78], although, in the end, the authors discarded such possibility for the present situation as the computed CH+3 + OH energy was slightly higher than for CH3O + H+, which resulted in the latter as the most populated adsorbed form. The relative population of the two species could, however, be closer to one in the SrO case as the lower acidity of the coordinating cation (Sr(II)) can impact more on the energy of CH3O than OH as the former has a more negative oxygen. In principle, the band at 2816 cm 1 may also be ascribed to m(CH) of formaldehyde which presence can be further suggested by the broad and very weak peak at 1600 cm 1 (m(C@O)). However, considering the relative intensity of the two peaks of formaldehyde it is reasonable to discard this molecule presence from catalyst surface since the C@O stretching is usually strong. In fact, the DFT calculations on adsorbed aldehydes invariably suggest that the aldehydic C-H stretching holds a far less intense adsorption (8– 10 times lower) than the C@O one. Beside, the theoretically computed shift in m(CSH) of adsorbed formaldehyde upon moving from CaO to SrO surfaces (40–60 cm 1) is far too large compared with the one obtained (10 cm 1) assuming that the band at 2816 cm 1 on SrO is due to such species. We thus suggest that the band can be related to the CH+3 presence, an assignation supported by the results shown in Fig. S12, which shows the DFT simulated spectra for CH+3 and CH3O adsorbed onto SrO (vide infra for the energetics). Exploiting the DFT computed relative intensities, it would thus be possible to attribute the band at 2918 cm 1 to CH3O mas(CH3), while the bands at 2816 cm 1 and 2774 cm 1 may be attributed to CH+3 mas(CH3) and ms(CH3), respectively (see also Fig. S13). The possible presence of this adsorbed species is de facto supported also by the DFT energetics, which show a negative adsorption Gibbs’ energy for CaO and SrO (vide infra Table 4 in Section 3.4). At 100 °C, two small bands start to rise at 1558 cm 1 and 1375 cm 1, which might be associated with formate species. Carbonate bands appear upon further increasing the temperature; the latter species might be generated via a subsequent oxidation of formate moieties (see Scheme 2) owing to the dehydrogenating

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nature of the oxide. At 450 °C, only carbonates are present on the surface of the catalyst, the bulk of which, however, seems to be largely carbonated, as shown by the agreement with the characteristic transitions of strontianite (SrCO3, whose ATR spectrum is indicated with the dotted line in Fig. 12). Not surprisingly, it seems that SrO is more susceptible to carbonation with respect to CaO, a characteristic that can be attributed to its higher basicity. In other words, while for CaO it was sensible to suggest that only surface carbonation took place, it seems likely that even the bulk of the SrO catalyst may be partially carbonated. In retrospect, the DRIFT experiments highlighted an easier transformation of MeOH (e.g. into CH3O /H+ or CH+3/OH ) when CaO and SrO are used as the catalyst; this is evident, for instance, from the fact that both SrO and CaO are selective toward a single methanol adsorption mode. Thus the higher productivity obtained with these materials should be correlated with their enhanced basicity and, consequently, a greater capability for dissociating methanol, which may correspond to an increase in methanol conversion (for a more quantitatively detailed discussion of the differences in reactivity, vide infra Section 3.4). As a consequence, the catalytic performances of those two catalysts appear comparable in terms of yield and conversion. To conclude the analysis of experimental data, mass spectra were also collected at the various temperatures studied; these data are shown in Fig. S14. Among the latter, the trend in CH4 concentration is quite interesting, as it shows that this molecule can be produced even at low temperatures on SrO. As for the pathway leading to the generation of methane, in principle it might form via methanol disproportionation, perhaps beginning with the simultaneous production of formaldehyde as an oxidized product (i.e. 2CH3OH ? CH4 + CH2O + H2O); this reaction should occur more easily on strong basic sites, which also ought to facilitate the further oxidation of formaldehyde. Some support for this idea comes from the behavior of CH4, CO2, and H2O production with regard to temperature, reaching their peaks at low temperatures on SrO. The direct disproportionation of alcohols to alkane and aldehyde with water co-production was also recently reported for Fe-V-O [80–82] and Co-Fe-O [83] catalysts. In addition, the presence of CH+3 species on the catalyst surface at low temperatures might also be responsible for CH4 formation. Such a process might involve methanol directly as the reducing agent since, in fact, H2 is released by all catalysts only after 300 °C, when the alcohol starts to decompose [84]. This notwithstanding, H2 could also be produced at low temperatures when MeOH is converted into H2CO in the presence of a high concentration of defective sites and could only be released from the catalyst at high temperatures [78]. A similar phenomenon has been reported for ethanol partial oxidation to acetaldehyde [83]. What has just been discussed with regard to MeOH activation pathways is summarized in Scheme 2, where the possible methanol activation for SrO and CaO is shown in black and blue, respec-

Scheme 2. Possible methanol activation pathways. (a) = adsorbed, (s) = solid, (g) = gas phase.

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Fig. 12. Minimum energy structures for two methanol molecules adsorbed onto the MgO, CaO, and SrO model crystals.

tively. Although the path followed on CaO (blue) may also be active on SrO, it is reasonable to consider its contribution as quantitatively limited, at least judging from infrared transition intensities. Likewise, it is currently not possible to completely rule out the dissociation of methanol as CH3 and OH on CaO surface, despite the fact that IR intensities suggest a reasonably easy oxidation of methanol on the latter oxide. In conclusion, the different specific productivities of the three catalysts might be explained with different adsorption mode selectivities (3 different modes for MgO versus only 1 for CaO and SrO, with the production of only the active methoxide), which may indicate a different methanol activation with regard to the Htransfer toward the carbonyl moiety of FAL. Furthermore, higher SrO than CaO productivity can be explained by the different basicity, which in turn leads to differences in the main methanol activation pathways (Scheme 2). DFT calculations will make it possible to gain further insight into the role of the basic strength on methanol activation and H-transfer reaction. 3.4. Theoretical results on catalysts properties and activities As a first step to investigate the relative performances of the three catalysts, we opted to gauge the adsorption energy of methanol onto the three-coordinated metal cation corner site (M3C, with M = Mg, Ca or Sr) of our oxide models. In order to, at least partially, mimic the liquid phase, it was decided to co-adsorb two alcohol molecules instead of only one as commonly done (for the energy data associated with the adsorption of a single methanol molecule, vide infra and SI). Table 2 and Fig. 13 report energies and structures for the optimized species. As it can be noticed from Table 2, the adsorption is much more exo-ergonic (roughly, 40.2 kJ/mol) when CaO or SrO are the adsorbing substrates; such marked difference emerges due a reduced entropy loss upon adsorption onto CaO and SrO compared with MgO, as it can be inferred contrasting DEads values with DGads ones. Also worth of notice, it is the fact that the stepwise adsorption energy of a single methanol molecule ought to depend markedly on the number of already adsorbed molecules, as demonstrated previously for MgO (for a single methanol molecule,

Table 2 Change in electronic (E) and Gibbs (G) energies due to the adsorption of two methanol molecules onto the M3C site of oxide models with respect to the common energy zero (see Section 3.4). Catalyst MgO CaO SrO

DEads (kJ/mol) 58.3 to 244.0 66.6278.8 65.5274.2

DGads (kJ/mol) 31.6132.3 43.6182.5 44.0184.2

Fig. 13. TS optimized structures for the MPV-mediated HT from methanol to furfuraldehyde on the MgO, CaO, and SrO model crystals.

DEads = 169.1 kJ/mol) [71]. The same conclusion emerges also for CaO and SrO, as DEads (DGads) for a single methanol molecule is, respectively, 168.7 ( 112.2) and 157.4 ( 109.3) kJ/mol (Fig. S15). Thus, the values quoted in Table 2 should be considered representative of an averaged behavior involving, at least, two alcohol molecules. Interestingly, Fig. 12 shows that only one of the two methanol molecules dissociate to produce a proton and the methoxide anion after adsorbing onto the Mg3C site, while it happens to both alcohol molecules when adsorbed onto the heavier metal oxides. With this result in mind, one could attribute the aforementioned reduced entropy loss upon methanol adsorption onto CaO and SrO compared to MgO to the dissociation of both adsorbed molecules onto the heavier metal oxide, which lead to the softening of a few vibrational modes. Of course, we checked this unexpected result by re-optimizing two fully dissociated methanol molecules placed onto MgO with various initial geometries; invariably, we found that one of the two molecules reformed the undissociated alcoholic group. In retrospective, it would seem natural to correlate such different behaviors with the experimentally determined difference in basicity highlighted at the end of Section 3.1, as the capability of the metal oxide to strongly bind the dissociated protons should represent a key contribution to the driving force for the alcohol adsorption. De facto, this observation correlates well with the difference in gas phase proton affinity (PA) of the oxides; as shown in Fig. S16, the protonation of CaO and SrO releases, respectively, 115.1 and 171.6 kJ/mol more than in the case of MgO, suggesting a fairly stronger basicity for the heavier metal oxides. Of course, one would also expect a decrease in the interaction energy between the methoxide and the corner cation upon increasing its mass, as the ionic radius also increases (respectively, 72, 100 and 118 pm). However, the change in radius is more marked on going

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from Mg to Ca than from Ca to Sr, so that the decrease in interaction energy in the latter case should be smaller. In turn, this supports our conclusion on the key role played by the PA in defining the oxide basic behavior with respect to methanol. The result just discussed also bear relevance to the DRIFT-based finding of the presence of both dissociated and non-dissociated methanol species present on MgO, while only dissociated methanol is seen adsorbed onto CaO and SrO (vide supra, Section 3.3). As hinted earlier, this suggests that all methanol molecules adsorbed onto defective sites of the heavier oxides are de facto activated to undergo the MPV-mediated HT, whereas only a fraction of the MgO-adsorbed alcohol molecules are amenable for the reaction. In other words, a contribution to the higher effectiveness of CaO and SrO in catalyzing the MPV reduction seem to derive by a simple mass effect due to a higher concentration of activated (i.e. dissociated) methanol molecules. Turning to aspects with more relevance for the reactivity induced by the three metal oxides, Fig. 13 shows the optimized structure for the TS’s involved in the HT via the MPV reactions taking place onto the three oxides, while Table 3 provides the results for the relative positioning of the TS’s for the same processes; the results for the MgO case were obtained in a previous study on the MPV-mediated HT onto aromatic and aliphatic aldehydes [58]. As it can be judged from Fig. 13, the geometries for all TS’s are very much alike, suggesting that similar changes in the electronic structure of the reactants must happen while the HT takes place. Turning to the relative activities, one notices, instead, that the energetic location of the TS for the MPV-mediated HT markedly depends on the catalyst, the TS for the process catalyzed onto SrO being roughly 9.6 kJ/mol lower than the one onto CaO, which is roughly 27.2 kJ/mol below the one onto MgO. A similar relative ordering is also found for the barrier heights estimated from the reactants adsorbed onto the metal oxides, a finding suggesting that the nature of the cation strongly modulates the energetics of the HT. Such relative positioning seems to anti-correlate with the energy released upon adsorption of the reactants onto the M3C site ( 225.6, 234.0 and 203.0 kJ/mol for, respectively, MgO, CaO and SrO; see Fig. S17), and thus, once again, with the relative basicity of the three materials investigated (vide infra for further details). Exploiting TST with energy changes estimated from the common energy zero, the latter results can be transformed into relative rate constants (see the rightmost column in Table 3); employing as reference the case of MgO, the following order of reactivity emerges: MgO < CaO < SrO. The latter agrees nicely with the results on the specific productivity per basic site shown in Fig. 8, thus sugTable 3 Electronic (E) and Gibbs (G) energy location for the TS’s of the MPV-mediated HT between methanol and furfural adsorbed onto the M3C site of oxide models with respect to the common energy zero (see Section 3.4), or from the adsorbed reactants (between brackets). jMO/jMcO provides a relative comparison for the reaction rate constant onto the MO and MgO species as estimated via Transition State Theory (TST). Catalyst MgO CaO SrO

DE (kJ/mol)

DG (kJ/mol)

110.1 (115.1) 137.3 (94.2) 146.9 (56.5)

102.6 138.1 155.3

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gesting that it is the increased ability of activating the alcohol via formation of the methoxide ion that is mainly involved in modulating the catalyst activity rather than the polarization of the aldehydic carbonyl group due to the coordination onto the M3C site. With a higher degree of detail, one may also add that an important role in defining the relative reactivity should also be attributed to the lower interaction energy between the methoxide and the heavier cations due to their larger ionic radius, which is expected to leave a higher negative charge on the polarizable anion. This idea is clearly supported by the analysis of Mulliken partial charges of methanol adsorbed onto the three oxides. Specifically, the methoxide ion bears a total charge of 0.411, 0.711, and 0.748 in units of electronic charge when adsorbed, respectively, onto MgO, CaO and SrO. To conclude the discussion on the theoretical results, we report the data obtained for the alternative methanol activation pathway leading to surface adsorbed CH+3/OH , whose energetics is shown in Table 4 (optimized structures are shown in Fig. S18). Comparing these data with the energetics of methanol adsorption generating the methoxide, it clearly emerges that the methyl cation can be formed as metastable species on both CaO and SrO. Albeit such species should convert into the thermodynamically more stable adsorbed methoxide, the process may not be particularly fast at low temperature due to the necessity of (at least, partially) detaching the cation from the oxide lattice to reform methanol. 3.5. Reuse of the catalysts In a final set of experiments, the catalysts were recovered to check for the reusability. The spent catalyst was washed with acetonitrile, dried at 120 °C and the quantities of substrate and methanol were rescaled to maintain the original concentrations and recycling experiments were performed at the same reaction conditions. Catalysts were investigated for 5 successive uses. The reusability of MgO in gas-phase furfural reduction was reported previously [85]. Figs. 14 and 15 show the results of recycling experiments. CaO exhibited very good reusability, whereas the activity of SrO dropped drastically after the second run and resembles the activity of SrO calcined at non-optimal temperatures. To confirm this hypothesis, we characterized by TGA and XRD the 3times reused SrO (Figs. S19, S20). The diffraction pattern of the used SrO was similar to that one of SrO calcined at 500 °C, as also confirmed by TGA (Fig. S19). The used catalyst showed a first weight loss of 2–3% centered at 300 °C that could be related to the removal of some carbon deposit formed during the reaction. The same sample showed a second weight loss of 30% centered at 900 °C. Overall, the TGA curve of the spent SrO catalyst resembles the TGA curve of the un-calcined SrCO3. Therefore, that CO2

jMO/jMcO – 4000 28000

Table 4 Electronic (E) and Gibbs (G) energy location for the optimized CH+3and OH species adsorbed onto the M3C site of oxide models with respect to the common energy zero (see Section 3.4). Catalyst MgO CaO SrO

DE (kJ/mol) 57.8 115.9 110.9

DG (kJ/mol) 4.2 57.4 60.7

Fig. 14. Reuse of CaO catalyst, Tr = 210 °C. Legend: FAL conversion ( ) and FFA yield ( ).

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Fig. 15. Reuse of SrO catalyst, Tr = 210 °C. Legend: FAL conversion ( ) and FFA yield ( ).

generated during the reaction forms SrCO3. However, it was possible to recover almost completely the activity of the catalyst with a regeneration treatment carried out by calcination at 900 °C (Fig. 15).

4. Conclusions The basic strength of the synthesized MgO, CaO, and SrO played a key role in the catalytic transfer hydrogenation of furfural (FAL) with methanol as the hydrogen source. The studied catalysts, obtained after calcination of the precursors prepared from the co-precipitation process, had different surface areas and basic site numbers and/or strengths. Despite the larger surface area and higher number of basic sites characterizing MgO, it was not the best catalyst in H-transfer from methanol to FAL. In fact, MgO was not very active in MeOH dehydrogenation to formaldehyde, adsorbing the alcohol as both an undissociated molecule and monodentate and bidentate methoxy species, as revealed by DRIFT spectroscopy. Conversely, CaO and SrO had very small surface areas and hence low numbers of basic sites. However, the greater basicity strength promoted methanol conversion and the specific productivity in the FAL reduction. DRIFT experiments highlighted an easier transformation of methanol (e.g. into CH3O /H+ or CH+3/ OH ) with both CaO and SrO; the increased methanol conversion is thus correlated with both a greater capability of dissociating methanol and the enhanced basicity which characterizes these materials. DFT calculations confirmed that the methyl cation may be formed as a metastable species on both CaO and SrO, a species that should be converted into the thermodynamically more stable adsorbed methoxide, especially when the temperature is increased. Another interesting aspect to be considered is the much more exo-ergonic adsorption when CaO or SrO are the adsorbing substrates; this marked difference is attributable to a reduced entropy loss upon adsorption onto CaO and SrO compared to MgO, which, in turn, is connected with the complete dissociation of the alcohol upon adsorption. Lastly, reusability tests highlighted the stability of these inexpensive materials, which can be used numerous times and recovered completely after calcination when adsorbed CO2 and by-products poison catalyst active sites, as is the case with SrO. Acknowledgements This work was funded by SINCHEM Joint Doctorate ProgrammeErasmus Mundus Action (framework agreement No. 2013-0037; specific grant agreement no. 2015-1600/001-001-EMJD).

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.02.020.

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