Abstract
Precious metal catalyst has been prepared by conventional wet impregnation method followed by precipitation and reduction with hydrogen finally passivated with water in air. The magnetically recoverable catalyst has been prepared starting from a stoichiometric Fe3O4 and ZrO2–Fe3O4 as supports prepared following a sequential precipitation procedure. Precious metal catalysts supported on carbon, alumina, magnetite and zirconia-magnetite nanocomposite has been used in the reduction of nitrobenzenes and acetophenone by using sodium and potassium formate as reducing agent in the presence and in absence of an aqueous phase. In addition, the same catalysts has been tested in CO2 and NaHCO3 hydrogenation, for verifying their potentiality in the CO2 as hydrogen carrier for hydrogenation processes.
Introduction
The question is: can be the climate change related to the increase of the atmospheric CO2? The answer could be yes, but in any case, CO2 recycling in the industrial processes is becoming almost a moral problem, because of the continuous forest depletion, which is the natural regulator of the CO2 in the environment [1]. From this point of view, the catalytic conversion of CO2 is not only a way for producing valuable chemicals and fuels, but also a good practice to decrease fossil fuels consumption, thus giving a contribution to diminish the anthropic CO2 creation [2].
The productions of commodities such as methanol, acetic acid, hydrocarbons and polyoxymethylene (polyketone) are based on the chemistry of CO, which is, however, obtained by steam and/or autothermic reforming. These processes are highly demanding from an environmental point of view, because of the high operating temperature obtained by hydrocarbon combustion thus contributing to increase the greenhouse gasses [3]. Starting from this evidence, the use of CO2 to achieve valuable organic product will become of paramount importance for the future especially in the view of further future limitation on greenhouse gas emission [4].
The production of chemicals starting from CO2 is thermodynamically disadvantaged because of its stability, but the needs of a decrease of the CO2 emission will give raise the importance of processes in which such a compound is used [5]. For instance, the reverse water gas shift reaction where CO and water is produced, may contribute, both to employ the surplus of H2 deriving from virgin naphtha cracking simultaneously to a reduction of CO2 emission by producing CO as intermediate [6].
In the production of fine chemicals, hydrogenation is of paramount importance, but sometimes the large flash point interval of the hydrogen may cause industrial concerns requiring special reactor and complex procedures to achieve safe productions. Furthermore, direct catalytic hydrogenation may have selectivity problems especially in the presence of the highly active heterogeneous precious metal catalysts [7], [8]. It is well known, that CO2 can be catalytically reduced by H2 to various products such as hydrocarbons, carboxylic acids, aldehydes and alcohols by homogeneous or heterogeneous way [9], [10], [11], [12].
Among them, formates and formic acid are valuable products because of their reducing properties. In fact, these compounds can be easily employed in productions of the fine chemistry, where the reduction by using liquid and solid stoichiometric reagents, in batch non-pressurized vessels, is the normal practice yet [13]. Particularly interesting is the use of formates as a substitute of the hydrogen in catalytic reduction of several organic compounds such as ketones, aldehydes and nitro-compounds [14]. In this way, formates could be the base for a cycle, where CO2 is utilized as hydrogen carrier.
From environmentally point of view, the best solvent where a reaction may take place, is water, for this reason water soluble formates obtained from bicarbonate reduction appears to be the most interesting way to achieve this cycle [15].
Starting from this consideration sodium bicarbonate hydrogenation in aqueous media is an intriguing reaction, since both CO2 utilization and H2 storage are obtained, simultaneously. In Fig. 1 a complete cycle of hydrogen storage and CO2 utilization for production of chemicals is shown.
CO2 as Hydrogen carrier for aqueous solvent catalyzed reduction processes.
In addition, formic acid, its esters and salts are versatile reagents because of their solubility in both aqueous and non-aqueous solvent. Finally, formic acid is a Brønsted acid, whose properties, both as proton donor or as active solvent, may influence several homogeneous catalyzed reactions enhancing and/or directing both activity and selectivity [16].
Catalytic hydrogen transfer from a hydrogen donor to a substrate has been well documented from long time, however, the study of new system are in continuous development in order to achieve both higher activity and selectivity. Hydrogen transfer reactions generally employ precious metal catalysts and the most active are normally homogeneous ones. However, separation and recovery of homogeneous catalysts are always an issue, especially when precious metal catalyst is used [17]. Moreover, specialties are chemicals, whose needs of purity are even more stringent than commodities particularly in the production of pharmaceutics. For these reasons, heterogeneous highly active and selective precious metal catalysts will be the target for greener and highly sustainable fine chemistry processes in the future [18].
The reduction of nitro compounds to amine is an important reaction and there are a large number of procedures to achieve such a reaction [19]. However, the use of formates are of paramount importance because of its potentiality in attaining selectivity to carbamates and ureas, similarly to what is observed employing CO as reducing agent [20].
Heterogeneous catalyst preparation is an art based on the well-known procedures of oxides and precipitation, metal reduction and its passivation, however, the control of the operative variables of preparation affects the final product, deeply [21]. Even more important is the development of a reproducible procedure when the magnetic properties of the catalyst are important, because of the needs of controlling a further feature of the catalyst. For this reason, a simple procedure of synthesis, which gives a reproducible catalyst with reproducible catalytic and magnetic performance, is of paramount importance for a sustainable chemistry [21], [22]. In fact, separation of heterogeneous catalysts appears less critics than heterogeneous ones; however, filtration and centrifugation are operation unit complex and expensive both from economical and from an energetic point of view.
This paper describes a complete cycle of CO2 utilization, employing supported precious metal catalyst, as hydrogenation catalyst of CO2 or NaO(C=O)OH to formates, and the same in hydrogen transfer from formates to ketones and nitro compounds for producing various chemicals of synthetic interest. The synthetized catalysts are ferrimagnetic and this property is evaluated by their capacity to be separated from a solution by a permanent magnet with a strong magnetic field. The catalytic performance of the home synthetized and commercial materials are compared in an integrated cycle of CO2 utilization as hydrogen carrier to be used in green catalytic reduction of test substrates.
Experimental
Materials
Nitrobenzene, aniline, 1-phenyl ethanol, acetophenone, and diphenyl urea were all Aldrich products, their purity were checked by the usual methods (melting point, TLC, HPLC, GC, GC-MS and NMR), ethanol BDH solvent grade.
Commercial catalysts used were materials supplied by Engelhard (now BASF catalysts): Pd/C 5%: Escat 10, Pt/C 3%, Ru/C 5% Escat 40 and Rh/C 5% Ru/Al2O3 5%, Pd/Al2O3.
Equipment and analyses
Products were identified by gas chromatography (GC), gas chromatography coupled mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). Gas phase analysis were carried out by a GC Agilent 6890 equipped with a Restek micropacked OD 530 μm 2 m long column and a TCD detector. Helium was employed as carrier under the following conditions: injector 523 K, detector 543 K, flow 1 mL min−1, oven 333 K for 3 min 523 K 15 K min−1 and 523 K for 15 min.
Analyses of the hydrogen transfer reactions were carried out with a Agilent 7890A equipped with FID or MS detector [Agilent 5975C and a HP 5 column (I.D. 320 μm, 30 m long). Helium was employed as carrier under the following conditions: injector 523 K, detector 543 K, flow 1 mL min−1, oven 333 K for 3 min 523 K 15 K min−1 and 523 K for 15 min. Calibration with standard solutions of the pure products allows the calculation of yield and selectivity. In order to verify the presence of thermo-labile substances some samples were analysed by HPLC (Perkin Elmer 250 pump, LC 235 diode array detector and a C 8, 5 μm, 4 mm i.d. 25 cm long column using CH3CN–H2O as mobile phase, in isocratic 70% of CH3CN at 1 mL min−1].
1H and 13C NMR measurements were carried out in a Brucker Avance 400 II at 400 MHz and 100 MHz, respectively, in CDCl3 or DMSO D6 as the solvent. The quantitative analysis of bicarbonate hydrogenation were obtained by13C NMR in H2O–D2O mixtures (90–10%), in order to obtain reliable concentration data a calibration curve has been obtained from the integral of the peak obtained with a no NOE pulse program with a relaxation time of 4 s and 4000 acquisition cycles.
Electronic microscopy measurements (SEM) were carried out on a Jeol JSM7401F at 15 kV, equipped with a Bruker Quantax 400 EDX.
X-ray powder diffraction (XRPD) spectra were recorded with a Philips X’Pert powder diffractometer (Bragg−Brentano parafocusing geometry). A nickel-filtered Cu Kα1 radiation (λ=0.15406 nm) and a step-by-step technique e (step of 0.05° 2 h) with collection times of 10 s/step were employed.
Relative magnetic permittivity were calculated by measuring the variation of the complex impedance of an air inductor solenoid charged with the ferrimagnetic solid, using a Solartron 1260 gain phase analyzer. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were carried out in a Perkin Elmer Optima 5300 DV.
Catalyst composition were obtained after digestion and solubilization of the catalyst was obtained in a microwave digestion labstation (Ethos 1600, Milestone) in Teflon bombs by treating with nitric acid, hydrochloric acid hydrofluoric acid and potassium permanganate . After dissolution, all samples were analyzed by ICP-OES in a Perkin Elmer Optima 5300 DV. The complete analysis results are reported in the Supplementary Information.
Brunauer Emmet Teller model (BET) surface area, Barret Joiner Halenda model (BJH) desorption pore size distribution and total pore volume (relative pressure of p/p0=0.98) of the catalysts have been determined by N2 adsorption and desorption isotherm at 94 K, by using an automatic adsorption unit (Micromeritics ASAP 2010C) [6]. Each sample is subjected to a thermal pretreatment at 453 K for 20 h under vacuum, in order to desorb the substance adsorbed (mainly water) on the catalyst surface. The temperature of pretreatment has been verified to give an effective desorption from catalyst surface as well as to ensure the maintenance of catalyst porosity [7].
Catalyst preparation and characterization
Supports preparation
Ferrimagnetic supports employed in the preparation of the magnetically recoverable precious metal catalysts were synthesized by precipitation of iron oxides from solutions of precursors. A Cowles mixer ensured an effective agitation resulting in a homogeneous slurry during the precipitation step. In a typical preparation, two filtered solution of FeCl3 and of FeCl2 were mixed together and added to a solution of 15% of NH3 (after addition pH>12.5) under N2 in order to avoid Fe(II) oxidation. The suspension was heated to 353 K and stirred for 2 h. Finally, the solid was separated magnetically and washed with plenty of water in order to remove NH3 and salts adsorbed on the solid surface. The black ferrimagnetic solid is a mixture of Fe(II) and Fe(III), whose composition is mainly determined by the starting initial composition of the precursors. The Fe(II)/Fe(III) molar ratio of the starting solution of precursors is 1/2 corresponding to the composition of the iron spinel magnetite (Fe3O4). The drying process is carried in air at 383 K for 24 h giving a brown solid (probably partial oxidation of the Fe(II) with consequent formation of ferrimagnetic γFe2O3), the oxides employed in the catalyst are likely a mixture, in agreement with a XRPD analysis of a sample of a ferrimagnetic iron oxides reported in the Supplementary Information.
A second type of ferrimagnetic material a Ni(II)–Fe(III) mixed oxide started from the precipitation of Fe(III) oxides at pH 9. The solid, which does not present ferrimagnetic properties (it is paramagnetic), is recovered by centrifugation and washed to remove residual salts. To the solid a solution of NiCl2 was added and after 2 h of stirring the solvent removed by a rotary evaporator. Then the material is oven dried at 383 K, finally the solid was crushed and calcined in air at 723 K for 4 h.
After this treatment, the material presents evident ferrimagnetic properties, qualitatively observed by the attraction of the solid to a permanent magnet.
ZrO2/FeOx supports was prepared starting from a dispersion of Fe(II)–Fe(III) mixed oxides in C2H5OH dispersed by a Cowles turbine. To the slurry a solution of Zr(C3H8O)4 in C2H5OH was added drop-wise. After the addition, the suspension was left under vigorous stirring for 15 min, A solution NaOH is than added drop-wise starting the precipitation of the ZrO2 precursor. The slurry was left at 80°C under vigorous agitation for 2 h. Then the solid was separated magnetically and washed with plenty of water until the washing solution is at pH 6–7, the surnantant liquid were checked in order to verify the presence chloride.
Dispersion of the precious metal on the support
The conventional precipitation method allows the dispersion of the precious metal into the pores of the ferrimagnetic support, in the desired percentage. In a typical preparation, the desired amount of support is dispersed in a water (1 g of support 20 mL of water) and the precious metal precursor was added and left under agitation for 1 h at room temperature. After this stage, a 1 mol L−1 solution of KOH was added to the slurry under vigorous agitation and the temperature raised at 343 K. Finally, after 1 h under agitation the solid was, left in digestion for 20 h separated magnetically, washed with plenty of water and dried under vacuum.
Analysis and characterization
The ratio of the Fe(II)/Fe(III) in the iron oxides are calculate by permanganate titration under nitrogen after dissolution of the oxide with HCl 4 mol L−1 under nitrogen at room temperature. Complete dissolution is achieved in 20–44 h. Dissolution of Ni–Fe mixed oxides occurs under more severe conditions, that is 363 K in concentrated mixtures of HCl, HNO3 and H2SO4, after 20 h.
The total metal composition of each catalyst was determined after dissolution of the solid, by ICP-OES analysis; the complete method was described elsewhere [23].
The geometrical properties of the nanoparticles, dimensions, size distribution, and elemental surface composition were investigated using a scanning electron microscope equipped with an energy dispersive probe (SEM-EDX). Measurements were performed at 5–15 keV electron beam energy and sampling current of 5–10 μA. The used conditions allowed of effectively measuring structures at few tens of nanometers level, thus avoiding any parasitic charging of the material.
Hydrogen transfer reactions
All the operations were carried out under nitrogen (charged at atmospheric pressure) in a jacketed sealed glass reactor (25 mL) at several temperatures and autogenous pressure (max. 5 bar). The course of reactions was checked sampling the liquid phase by a syringe at established intervals. All the analysis were carried out by GC, GC-MS, HPLC and NMR depending of the needs. In a typical experiment, a glass reactor was charged with 1 mmol of substrate 10 mL of an aqueous solution of NaO(C=O)H and 50 mg of catalyst. Analysis where carried after extraction with diethyl ether of a weighted amount of the reaction mixture by GC or GC-MS. Comparison with HPLC measurements allow to exclude thermal decomposition or side reaction into the GC injector, by comparing reagent and products with commercial chemicals.
Formates synthesis from CO2 and HO(C=O)ONa hydrogenation
The hydrogenation runs were carried out in a well-stirred autoclave reactor equipped with a magnetically driven self-aspirating impeller, thermostated by circulation bath in the range 363–433 K, using H2O or C2H5OH as a solvent. In CO2 hydrogenation, typically, the autoclave were charged with a solvent, a catalyst (normally 50 mg) and purged with N2, than the temperature raised to 318 K and pressurized with 0.4 MPa of H2. After few minutes, the temperature was raised, and equilibrated at the working conditions, then left under agitation for 1 h in order to activate the catalyst. CO2 was added and equilibrated at the desired pressure (typically 1 MPa), then H2 pressure was regulated until reaching the desired total pressure (range 2–5 MPa) by a membrane regulator. The procedure for HO(C=O)ONa hydrogenation are similar but a solution of the salt was added by a charging autoclave just before reaction start. Hydrogen consumption was measured by the pressure drop in the H2 reservoir by an electronic pressure gage. Analyses of the solution were obtained by 13C NMR in H2O/D2O (D2O 10%) mixtures, the latter added directly in the NMR tube.
The reaction rate of H2 consumption is not limited by diffusion phenomena, being the rate of each interphase transfer step almost 20 time faster than that measured [7], [24].
Catalyst recovery
A neodymium magnet (SC 35M: remanence 1.2 T, coercitivity 860 kA/m) is employed to recover the ferrimagnetic supported catalyst, while conventional supported ones are recuperated by filtration.
Results and discussion
The work deals mainly with the reactivity of catalysts in various reactions of industrial interest rather than the catalyst characterization, which will be the subject of future studies, however, the main behavior of the materials are reported in the section in order to give some information on the catalysts.
Catalyst characterization
In Table 1 the properties of the catalysts employed in hydrogenation and hydrogen transfer reaction are reported. Apparent density is necessary for the evaluation of the rate of diffusion of reagent and products, which is not object of this work, however, it is an important technological parameter necessary in the kinetics measurements [24]. In any case, it is likely that all reactions rate are not affected by diffusion limitation. Such a hypothesis is based on the results of a previous work, where the kinetics of hydrogenation in much faster reaction was not affected by diffusion limitation by using the same Pd/C catalyst [9], [16].
Properties of commercial precious metal catalysts supported on various support (more details in Supplementary Materials).
| Support, catalyst metal composition | Support | Apparent density (g mL−1) | Granule diametera (μm) | BET surface area (m2 gcat−1) | Pore diameterb (nm) | Relative permittivity |
|---|---|---|---|---|---|---|
| Fe(II)/Fe(III)=0.39 | FeOx | 0.97 | 25 | 80 | 20 | 1.15 |
| Ni/Fe=0.5 | NiFeOx | 0.81 | 40 | 2.1 | NDc | 1.08 |
| FeOx/ZrO2 60% | FeOx/ZrO2 | 0.38 | 30 | 210 | 7 | 1.06 |
| Pt/C 3% | AC | 0.32 | 50 | 920 | 2 | 1 |
| Pd/C 5% | AC | 0.38 | 50 | 880 | 2 | 1 |
| Rh/C 5% | AC | 0.37 | 50 | 890 | 2 | 1 |
| Pd/C 10% | AC | 0.38 | 50 | 905 | 2 | 1 |
| Ru/C 5% | AC | 0.37 | 55 | 915 | 2 | 1 |
| Ru/Al2O3 5% | Al2O3 | 1.1 | 25 | 180 | 5.5 | 1 |
| Pd/Al2O3 5% | Al2O3 | 1.1 | 25 | 190 | 5.4 | 1 |
| Pd/FeOx 5% | FeOx | 0.98 | 20 | 95 | 12 | 1.15 |
| Ru/FeOx 5% | FeOx | 0.99 | 20 | 98 | 12 | 1.15 |
| Pd/NiFe 5% | NiFeOx | 0.86 | 40 | 8 | 45 | 1.08 |
| Ru/NiFe 5% | FeOx | 0.85 | 40 | 8 | 45 | 1.08 |
| Pd/ZrO2–Fe 5% | ZrO2–FeOx | 0.39 | 30 | 190 | 21 | 1.06 |
| Ru/ZrO2–Fe 5% | ZrO2–FeOx | 0.41 | 30 | 205 | 21 | 1.06 |
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aEvaluated by SEM. bAverage obtained by BJH model. cLow porosity-microporous solid.
In Table 1 we report main behaviors of the ferrimagnetic supports, that is the FeOx support, NiFeOx support, and the nanostructured FeOx/ZrO2 mixed oxide. FeOx and FeOx/ZrO2 show a mesoporous structure with a surface area of 80 and 210 m2 g−1, respectively (see Figs. 2 and 3). Figure 4 shows the adsorption isotherm of NiFeOx prepared by calcination. As expected, it has small surface area (c.a. 2 m2 g−1) and a non-porous structure with a small amount of micropores. The absence of the hysteresis in the desorption branch suggest the absence of mesopores, thus no BJH plot is reported. FeOx is a mixture of iron oxides and accordingly with its ferrimagnetic properties it is likely that the solid could be composed of maghemite (γ-Fe2O3) and magnetite (Fe3O4), both ferrimagnetic. The ferromagnetic properties of NiFeOx is likely due to formation of the spinel NiFe2O4 (trevorite) [25]. Commercial catalysts, namely, active carbon and gamma alumina supported precious metal catalyst show, as expected, surface area and pore distribution compatible with those observed in commercially available carbon black and gamma alumina employed typically as supports [21].
N2 adsorption isotherm and pore size distribution, obtained from BJH model applied to desorption branch, of FeOx.
N2 adsorption isotherm and pore size distribution, obtained from BJH model applied to desorption branch, of ZrO2/FeOx.
N2 adsorption isotherm and pore size distribution, obtained from BJH model applied to desorption branch, of Ni/Fe support.
The synthetized supported metal catalyst, shows a small decrease of the surface area after the deposition of the precious metals and a negligible variation of the pore size distribution. Such a behavior can be ascribed to the metal load, which may block a part of pores thus reducing the overall area without changing the distribution. As regard the magnetic properties of the catalysts, in all cases, were verified by measuring the inductance of a solenoid in the absence or in the presence of a core constituted by catalyst particles, vibrated until no variation of volume was observed. The ratio between the values of inductance gives the relative permittivity of the solid. In this way, the apparent density is also measured. The pure iron oxides (FeOx support) supported catalyst show a higher relative permittivity resulting in an easy magnetic separation. However, also those ZrO2–FeOx support (mixed oxides) show a sufficient variation of the relative permittivity thus allowing a magnetic separation by a strong magnet, such as that employed in this work.
SEM/EDX measurements (reported in Supplementary Materials) suggest that catalysts and supports have a non-uniform granulometry. A uniform surface composition is observed for the FeOx support. Conversely, much less uniform is the surface composition of the support ZrO2/FeOx in which, both zones where Zr is present and zones that cannot be spotted. As regard, supported metal catalysts show a quite homogeneous external deposition of the precious metal, suggesting that the precipitation of the precursor occurs mainly on catalyst surface. As a matter of fact, both Ru and Pd appears to be homogeneously distributed on the granules surface. This is likely due to the faster precipitation rate on the granules respect to the solution, because of the nucleation effect of the support [26].
Nitrobenzene reductions with H(C=O)ONa in aqueous biphasic condition
The reduction of nitrobenzene in biphasic conditions by using aqueous H(C=O)ONa as reductant shows the formation diphenyl urea in practically 100% of selectivity as reported in Scheme 1. The reason of such high selectivity is due to the precipitation of the diphenyl urea thus avoiding side reaction and achieving high selectivity. Figure 5 shows a typical time vs. concentration profile of nitrobenzene disappearing. The reaction proceed smoothly giving high conversion and almost quantitative yield in about 20 h of reaction. Similar results were obtained by using CO as reductant in the presence of homogeneous Pd catalyst [20].
Nitrobenzene reduction to diphenyl urea, reaction stoichiometry.
Reaction profile of nitrobenzene disappearing in hydrogen transfer reaction. Run conditions: T 130°C, autogenous pressure, reaction volume 60 mL, substrate/reductant=1/10, catalyst 50 mg (Pd/ZrO2–Fe 5%), reaction time 5 h.
In Fig. 5 the reaction profile of nitrobenzene disappearing is reported, it shows a monotone decreases, which stops after 20 h of reaction with a conversion almost complete. The kinetics of reaction is beyond the aim of this work, where a comparison of catalysts activity. In the aqueous reaction mixture, 13C NMR measurements show the presence of NaOCOOH and NaOCHO. GC and GC-MS analysis does not allow a simple identification of the substance, because of the extensive decomposition of the compound. On the contrary, 1H and 13C NMR measurements of the reaction product (see ESI) allow a clear identification of diphenyl urea.
Figure 6 shows a comparison of the catalyst activity. It appears that all the Pd catalyst are highly active in nitrobenzene reduction in aqueous media, in particular, it is noteworthy the Pd/ZrO2–Fe 5% catalyst is active almost as the highly active commercial Pd/C catalysts, but the former has in addition the possibility of being recuperated magnetically. It is also interesting to note that also Pt catalyst are highly active in hydrogen transfer reaction in aqueous media. Ru catalysts are less active, probably because of the less ability to form surface hydride with H2 (compared to Pd), which is actually the reducing agent of the nitrobenzene.
Nitrobenzene reduction in biphasic aqueous solvent in the presence of various supported precious metal catalyst. Run conditions: T 130°C, autogenous pressure, reaction volume 60 mL, substrate/reductant=1/10, catalyst 50mg, reaction time 5 h.
Acetophenone reduction with H(C=O)ONa in aqueous biphasic condition
The biphasic reduction of acetophenone by NaOCHO to 1-phenylethanol (see Scheme 2) is an interesting reaction because of hydrogenation of ketones to alcohol could be poorly selective especially if alcohol are benzylic [7]. As a matter of fact, the reaction gives only 1-phenylethanol, and no traces of alcohol reduction. Despite of the presence of two liquid phase, reaction arrive to completeness in 20 h suggesting an activity of the catalyst comparable to those observed in the hydrogen transfer from NaOCHO nitrobenzene catalyzed by Pd catalyst, reported in the previous section, and also in reductive carbonylation of nitrobenzene with Pd catalyst [20].
Acetophenone reduction by H(C=O)ONa.
Table 2 shows the activity of various catalyst in acetophenone reduction to 1-phenylethatol. It appears that Ru catalysts are less active than Pd ones. The reason of such a behavior could be ascribed to the different ability of the metal to form hydride from the formate anion, which is the responsible for the acetophenone reduction. This is in agreement with what observed in nitrobenzene reduction. Reactions catalyzed by commercial Pd catalyst appear to reach higher conversion. The reason of such a behavior could be likely ascribed to the better availability of Pd active sites due to a higher dispersion of the metal on the commercial catalysts.
Acetophenone reduction by NaOCHO catalyzed by Pd and Ru supported catalysts.
| Catalyst | Conversion (%) | Alcohol selectivity (%) | Reaction time (h) |
|---|---|---|---|
| Pd/C 5% | 42 | 99 | 20 |
| Pd/C 5% | 99 | 97 | 20 |
| Pd/Al2O3 5% | 38 | 99 | 4 |
| Pd/Al2O3 5% | 99 | 97 | 20 |
| Ru/C 5% | 2 | 99 | 20 |
| Ru/Al2O3 5% | 2 | 99 | 20 |
| Pd/FeOx 5% | 10 | 99 | 4 |
| Pd/FeOx 5% | 60 | 98 | 20 |
| Ru/FeOx 5% | 1 | 99 | 20 |
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Run conditions: T 100°C, acetophenone 0.1 mol L−1, NaO(C=O)H 0.5 mol L−1 H2O 10 mL, catalyst 25 mg.
CO2 hydrogenation to ethyl formate in C2H5OH
The reaction of CO2 hydrogenation is a complex reaction whose selectivity derives both by the thermodynamic and the kinetics of the reaction. Scheme 3 shows a typical reaction behavior of CO2 hydrogenation, which corresponds with what we found experimentally. In fact, the measure of H2 consumption does not give a reliable value to compare catalysts activity, because of the CO2 and H2 consumption is partially counterbalanced by the formation of the gaseous byproducts (CO and CH4). This phenomenon does not allow sufficient negative variation of the internal reactor pressure thus causing a weakening of H2 flow compared with its real consumption. For this reason, the complex mixture of the vapor phase of the reactor change continuously and the partial pressure of H2 cannot be constant and decreases accordingly. It is evident the system does not allow simple measurements of the reaction kinetic since it is necessary a precise quantification of all the gaseous specie, which is beyond the scope of the present work. As a matter of fact, we employ the ethylformate productivity as a simple and reliable parameter for comparing catalysts to be used in the integrated cycle of CO2 utilization.
CO2 catalyzed hydrogenation in C2H5OH.
Figure 7 shows the catalytic activity of some Pd and Ru catalysts. It appears that commercial Pd catalyst are more active than Ru ones. However, the supported Pd and Ru catalyst on both FeOx and ZrO2/FeOx appears to be almost active in CO2 hydrogenation as the commercial catalysts. The Productivity of the catalyst to ethyl formate is related in any case, not only to its activity but also to its selectivity. It appears from Fig. 8 that is evident the necessity of the presence of the inorganic carbonate as co-catalyst, since ethylformate productivity decreases noticeably in absence of this carbonate. Among the various carbonates employed as co-catalyst, K2CO3 appears to be the best. Besides, Cs2CO3, shown a good activity to ethylformate formation, suggesting a specific action of the cation, however a direct correlation between the basicity of the salt and its action in the reaction is not straightforward even though Ammonium carbonate and lithium carbonate are both less active and less basic. Solubility in ethanol of the salts is negligible except that of (NH4)2CO3 which is sparingly soluble (ca. 2.68% http://periodic-table-of-elements.org/SOLUBILITY/ammonium_carbonate), this evidence suggests that there are no correlation between ethylformate productivity and the amount of the various cation in solution.
Influence of the type of catalyst in CO2 hydrogenation. Run conditions: T 163°C, P 70 bar, catalyst 50 mg, reaction time 4 h, solvent EtOH 60 mL, co-catalyst K2CO3 80 mg.
Influence of the type of co-catalyst in CO2 hydrogenation. Run conditions: T 163°C, P 70 bar, Pd/Al2O3 50 mg, solvent EtOH 60 mL, co-catalyst 80 mg.
In Fig. 9 the influence of temperature on ethylformate productivity is shown. it appears a neat increase of the productivity as temperature raises. This agrees either with favorable thermodynamic contribution of the endothermic formation of H(C=O)OH in gas phase starting from CO2 and H2, as well as with an increase of the kinetics of CO2 hydrogenation. This is only a simple qualitative evaluation because of the complexity of such a reaction where solvation of ion in water greatly determine the thermodynamic of the reaction. A detailed kinetic and thermodynamic study is, however, beyond the aim of the present paper.
Influence of the temperature in CO2 hydrogenation. Run conditions: P 70 bar, solvent EtOH 60 mL, Pd/C 50mg, co-catalyst 80 mg K2CO3.
Sodium bicarbonate hydrogenation to sodium formate in water
In Scheme 4 the reaction of HO(C=O)ONa hydrogenation is reported. In addition, we take into account bicarbonate decomposition, which contributes to the total pressure into the reactor, together with the consecutive reaction of formate decomposition.
HO(C=O)ONa hydrogenation to H(C=O)NaO.
Figure 10 reports a typical time vs. concentration profile of HO(C=O)ONa hydrogenation in water catalyzed by a heterogeneous precious metal catalyst. It appears formation of sodium formate parallels hydrogen consumption, however mass balance is not as simple as appear in the Fig. 1 since the gas phase of the reactor changes continuously since CO is produced continuously by formate decomposition. In this way, H2 consumed cannot be replaced by fresh one. In fact, as already discussed in the previous section, formation of CO does not allow pressure decreases into the reactor stopping the inlet of fresh H2. The reaction, proceed, by consuming H2 (being in large excess) into the autoclave and after 20 h of reaction formates is consumed to CO. Analysis of the gas phase confirm that gaseous products are CO and CO2, no relevant amount of hydrocarbons is found (except only small traces of CH4) thus suggesting that both Fischer–Tropsch and methanation reactions are inhibited.
Time vs. hydrogen, HO(C=O)ONa consumption and H(C=O)ONa formation. Run conditions: T 160°C, P 5 MPa, reaction volume 60 mL, HO(C=O)ONa concentration 0.2 mol L−1, Reaction volume 60 mL, solvent H2O, Pd/C 50 mg.
Table 3 shows conversion of HO(C=O)ONa and selectivity toward H(C=O)ONa catalyzed by several commercial and synthetized catalysts. It appears the best results is obtained with commercial Pd/C and Pd/Al2O3 catalysts, However, Pd/FeOx shows good conversion of bicarbonate with a selectivity of 12% in the formate. Ru catalysts appear less active and selective in comparison with Pd ones. However, Pd/ZrO2–Fe 5% and Ru/ZrO2–Fe 5% show good activity and non-negligible selectivity suggesting the possibility of increases the performance of the Ru catalysts at the same level of those of Pd. In any case, the research is in progress and new heterogeneous magnetically separable catalysts will be synthetized and tested.
Activity and selectivity of various catalysts in HO(C=O)ONa conversion to H(C=O)ONa.
| Catalyst | HO(C=O)Ona conversion (%) | H(C=O)NaO selectivity (%)a |
|---|---|---|
| Pd/C 5% | 66 | 49 |
| Pd/C 10% | 68 | 45 |
| Pd/Al2O3 3% | 65 | 41 |
| Ru/Al2O3 5% | 58 | 9.3 |
| Pd/FeOx 5% | 47 | 12 |
| Ru/FeOx 5% | 39 | 0 |
| Pd/NiFe 5% | 32.3 | 7.1 |
| Ru/NiFe 5% | 45 | 0 |
| Pd/ZrO2–Fe 5% | 22.5 | 9.5 |
| Ru/ZrO2–Fe 5% | 20 | 8.7 |
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aCO, CO2 and traces of CH4 are the only byproduct detected in gas phase.
Run conditions: T 160°C, P 5 MPa, reaction volume 60 mL, HO(C=O)ONa concentration 0.2 mol L−1, reaction volume 60 mL solvent H2O, time of reaction 240 min.
Catalyst recovery and reuse
The recovery of the catalysts with ferrimagnetic properties has been carried out by using a magnet. The reuse of the catalyst in the reaction has been repeated three times showing almost the same activity both in hydrogen transfer reactions and in the hydrogenation of bicarbonate.
Figure 11 shows a typical exemplum of catalyst separation (detail of catalyst recovery in Supplementary Materials).
Magnetic recovery of a catalyst.
4. Conclusions
In this work we shows some results relating the use of formates as green reagents to be employed as reductant for nitrocompounds and ketones promoted by Pd and Ru catalysts. The magnetic properties of the synthetized catalysts give a further improvement of the catalytic process, especially if carried out in batch reactors, because of the simplification of the catalyst separation operation. In addition, we showed the ability of the Pd catalysts (Ru catalysts give selectively CO) to catalyze the hydrogenation of CO2 to CO and formates. This step is the hydrogen source in a CO2 utilization-storage process. In any case, the formation of a large excess of CO suggests its possible employment in other production where CO is largely utilized such as in production of polymer such as polyketone whose practical employment may contribute to diminish the overall CO2 emission (Hyosung start a 50 000 ton/y plant in 2015) [27]. Another significant point is the magnetic properties of the catalysts, which will result, especially in the hydrogen transfer reaction, in a simplification of the catalysts separation process.
Article note
A collection of invited papers based on presentations at the 6th International IUPAC Conference on Green Chemistry (ICGC-6), Venice (Italy), 4–8 September 2016.
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Funding: Università Ca’ Foscari di Venezia (Grant/Award Number: ‘ADIR’), Ministero dell‘Istruzione, dell‘Università e della Ricerca (Grant/Award Number: ‘2010NRBMTP_010’).
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Supplemental Material
The online version of this article offers supplementary material (https://doi.org/10.1515/pac-2017-0704).
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