Iron Pyrite Nanocrystals: A Potential Catalyst for Selective Transfer Hydrogenation of Functionalized Nitroarenes.

We report a solution-based synthetic method to produce shape-tunable iron pyrite (FeS2) nanocrystals using iron oxy-hydroxide (β-FeOOH) as a precursor and their application for selective reduction of functionalized nitroarenes to aniline derivatives with formic acid-triethylamine (HCOOH-Et3N) as a hydrogen donor system.

Introduction

Substituted aromatic amines and anilines are valuable compounds that are heavily used as precursors or reagents for a variety of commercial products in agriculture, pharmacology, and in materials science for the production of polymers, lubricants, surfactants, and dyes.[1−3] Anilines are generated via the hydrogenation of nitrobenzenes over noble metal catalysts, such as palladium (Pd), platinum (Pt), gold (Au), and ruthenium (Ru), in the presence of molecular hydrogen.[4−7] Major challenges in using such catalysts include the high cost of and low natural abundance of noble metals, their poor selectivity,[8−10] incomplete reduction,[11,12] the formation of byproducts such as hydroxylamines, hydrazos, azoarenes, or azoxyarenes,[13−16] and the generation of toxic waste.[13,17,18] The cost associated with purifying the final product from the heavy metal residue and other side products is also high;[19] spurring investigators to invest significant resources to identify cost-effective, highly selective, recyclable, and earth-abundant metal catalysts for the hydrogenation of nitrobenzenes in the last decade.[20]

Some alternatives to expensive noble metal catalysts include materials such as earth-abundant metal oxides,[1,21,22] metal–organic frameworks,[23,24] graphene-supported nitrogen,[25,26] carbon-doped metal nanomaterials,[27,28] and strategies that include diluting noble metal catalysts (e.g., Pt) by alloying them with earth-abundant metals.[29−31] Many of these catalysts generally require the use of hydrogen at high temperatures (100–150 °C) and pressures (10–50 bar).[1,31,32] Promising catalytic activity toward the hydrogenation of nitrobenzenes has been demonstrated with first row transition metal oxide nanomaterials with H2 pressures under 50 bar,[21,22] however, lingering concerns over the safety, the cost of transportation, and the storage of hydrogen have led to new endeavors to replace molecular hydrogen altogether. Other hydrogen donors such as sodium borohydride, hydrazine, isopropanol, formic acid, etc. have been explored to obtain anilines through catalytic hydrogen transfer reactions instead. These reactions mitigate some of the challenges in using molecular hydrogen and facilitate more controllable optimization procedures. Importantly, while noble metal catalysts are sometimes used for hydrogen transfer reactions,[33] there are a growing number of reports of earth-abundant transition metal-based catalysts that show promising activity toward hydrogen transfer reactions.[34−36]

In this work, we report a facile method for the synthesis of iron pyrite (FeS2) nanocrystals using iron oxy-hydroxide (β-FeOOH) nanoneedles as a precursor and their activity for the selective reduction of functionalized nitroarenes to aniline derivatives through the hydrogen transfer reaction. Iron pyrite, a nontoxic, prevalent material, has been a well-studied semiconductor for solar cells,[37,38] lithium-ion batteries,[39,40] and an electrocatalyst for the hydrogen evolution reaction.[41,42] Guo[43] and Schaak[10] have independently investigated the use of nanoscale and bulk iron pyrite, respectively, for the activation of molecular hydrogen in the hydrogenation of substituted nitrobenzenes. These reports use molecular hydrogen as the hydrogen source while our report highlights the use of formic acid (HCOOH) and triethylamine (Et3N).

Results and Discussion

Our synthetic methodology uses β-FeOOH nanorods as the iron precursor, sulfur powder as the sulfur source, and octadecylamine (ODA) as a capping agent, which also facilitates the formation of hydrogen sulfide (H2S) gas needed to produce FeS2 nanocrystals (see the Experimental Section). In a typical reaction, 0.05 g of the dried β-FeOOH nanoneedles and 0.053 g of the sulfur powder were dissolved in 7.42 mmol of ODA at 140 °C for 30 min and then the temperature was increased to 180 °C for 2 h.

β-FeOOH nanorods (Figure S1) were prepared by the precipitation of aqueous FeCl3·6H2O containing polyethyleneimine (PEI, MW = 750,000) at 80 °C for 2 h as reported in the literature.[44] A benefit of this solution-based synthetic protocol is that it can provide some control over the morphology of the resulting FeS2 nanocrystals compared to other methods, which require hot injection,[45] multiple reaction steps,[37] or high pressure equipment (autoclave).[46] Parameters such as the amount of alkylamine and the reaction time affected the phase and the dimensions of the resulting FeS2 nanocrystals. Figure shows transmission electron microscopy (TEM) images of the sizes and morphology of the FeS2 particles as a function of the amount of ODA. Varying the mole ratios of ODA: S from 4.5:1 to 2.25:1 and 1.12:1 resulted in cubes of varying sizes ranging from an average length of ∼50 to ∼65 and ∼70 nm, respectively, with decreasing amounts of ODA (Figure S2). The distinct lattice fringes (Figure a,b) of the particles revealed highly crystalline nanocubes even as the amount of ODA was reduced from 7.42 to 3.71 and 1.85 mmol. The XRD pattern (Figure d) shows the phase transformation from a mixture of FeS and FeS2 to pure FeS2 as the amount of ODA is increased. Under these synthetic conditions, we did not observe formation of the Fe3S4 phase as described by the Park group; however, the Park group used FeCl3 as the iron precursor and a greater amount of ODA.[47] In our case, the prevalence of the FeS phase decreased with an increasing amount of ODA suggesting that FeS is an intermediate phase en route to FeS2. We hypothesize that increasing the amount of ODA from 1.85 to 7.42 mmol allows for the production of sufficient quantities of H2S gas necessary for the formation of phase pure FeS2. A detailed mechanistic study on the formation of FeS2 was not performed here, but we propose the reactions below for the synthesis based on previous literature. In the first step, formation of H2S takes place on heating elemental sulfur with alkylamine and upon the reaction of RCSNH2 with moisture as described in eqs and 2.[47,48] The production of H2S gas was observed indirectly through the precipitation of PbS from the reaction of excess H2S with aqueous Pb(NO3)2 (Figure S3). Moisture can be produced in our system from the reaction of β-FeOOH with H2S as shown in eq ,[49] but moisture may also be present in the system because it is not rigorously eliminated.[50]

Figure 1

TEM images of FeS2 nanocrystals prepared using (a) 1.85 mmol, (b) 3.71 mmol, and (c) 7.42 mmol of ODA; (d) XRD patterns of the resulting nanomaterials. Reference XRD patterns for FeS2 (tick marks) and FeS (dots) are shown. Insets show high magnification TEM images of particles with a d-spacing corresponding to the (200) plane of the cubic phase of FeS2 with space group pa3̅̅ [205]) (JCPD 00-024-0076).

Produced H2S reacts with β-FeOOH to yield FeS as observed by XRD (eq ). Finally, the FeS intermediate can be converted to FeS2 in the presence of excess H2S gas (eq ).[50] ODA is a solid at room temperature and not readily soluble; as a result, the thioamide and ketoamide were not observed with solution NMR. Phase pure FeS2 was produced with 7.42 mmol of ODA (based on XRD) and, keeping this and other typical synthetic parameters constant, the effect of time on the phase and shape of the final product was studied further. Figure shows the TEM images of nanocrystals synthesized at 180 °C (7.42 mmol ODA) upon varying the reaction time (1–6 h). The nanoparticles were cubic in shape at shorter reaction times and later reorganized to have a spherical shape on prolonged heating (4–6 h). Moreover, the size of nanocubes reduced from ∼150 to ∼50 nm (Figure a,b) on lengthening the reaction time from 1 to 2 h. This may indicate that the growth of nanocrystals proceeds through a digestive ripening process where smaller particles form at the expense of bigger particles, which favors the formation of a monodisperse sample of spherical nanocrystals.[51−53] TEM images show clear and continuous lattice fringes with a d-spacing of 0.27 nm, which corresponds to the (200) plane of iron pyrite (inset, Figure ).[41] Nanocrystals obtained at a reaction time of 1 h contain mixed-phases of FeS (JCPD00-001-1247) (P63/mmc [194], hexagonal) and FeS2 (JCPD00-024-0076) (pa3̅ [205], cubic) (Figure ).

Figure 2

TEM images of iron pyrite (FeS2) nanocrystals synthesized at (a) 1 h, (b) 2 h, (c) 4 h, and (d) 6 h. Insets show lattice spacing d200 = 0.27 nm, which corresponds to the cubic phase of FeS2 (JCPD 00-024-0076).

Figure 3

XRD patterns of FeS2 nanocrystals collected at various reaction times (1–6 h). Reference XRD patterns for FeS2 (tick marks) and FeS (dots) are shown.

XRD patterns of FeS2 nanocrystals n class="Chemical">collected at various reaction times (1–6 h). Reference XRD patterns for FeS2 (tick marks) and FeS (dots) are shown.

XRD patterns in Figure illustrate that the product collected consists only of the FeS2 phase at ≥2 h as evidenced by the disappearance of diffraction peaks at 2θ values of 33.7, 43.9, and 53.5° corresponding to the (101), (102), and (110) crystal planes of FeS. To determine the core-level Fe 2p and S 2p binding energies, XPS spectra (Figure ) were collected for the FeS2 product obtained after 2 h of heating. The binding energy peaks of Fe 2p2/3 at 707.4 eV and Fe 2p1/2 at 720.1 eV can be assigned to the Fe2+ ion of iron pyrite, as reported in the literature.[41,43,47,54,55] A broad peak observed at a binding energy of 162.3 eV was attributed to the disulfide (S22–) of the pyrite phase.[56] Two peaks are typically observed at S 2p2/3 (162.4 eV) and S 2p1/2 (163.5 eV) for the disulfide (S22–) of FeS2 but were not deconvoluted into separate peaks here.[46,56,57] No characteristic peaks were observed from other phases such as marcasite (FeS2) (Fe 2p 707.7 eV, S 2p 164.1 eV) or pyrrhotite (Fe(1–S) (Fe 2p 708 eV, S 2p 161.2 eV).[58]

Figure 4

XPS spectra of FeS2 nanocubes synthesized at 2 h: (a) Fe 2p and (b) S 2p regions of FeS2.

XPS spectra of FeS2 nanon class="Chemical">cubes synthesized at 2 h: (a) Fe 2p and (b) S 2p regions of FeS2.

The use of iron pyrite as a reductant to remove the pollutants such as halogenated organic compounds, chromate, selenate, etc. has been reported.[59] This previous work served as the motivation to study the catalytic activity of cubic-shaped FeS2 nanocrystals toward the reduction of nitrobenzene, a typical benchmark substrate. Two hydrogen donor systems were explored for this reaction: ammonium formate (NH4HCO2) and formic acid–triethylamine (HCOOH–Et3N). Reduction of nitrobenzene to aniline was previously demonstrated using iron-gold nanoparticles and NH4HCO2 as the hydrogen donor in ethanol.[60] In this work, when FeS2 was used as the catalyst, conversion of nitrobenzene to aniline was not observed using NH4HCO2 in either a protic solvent (ethanol) or an aprotic solvent (acetonitrile, ACN) (Table S1, entry 1–3), which could be because of the poor solubility of NH4HCO2 in ACN. With HCOOH–Et3N as the hydrogen donor, reduction was observed in ACN, and the reaction conditions were optimized further by varying the temperature, the amount of FeS2 nanocrystals, reaction time, solvent, and the molar ratio of HCOOH to Et3N. Varying the temperature had a significant effect on the rate of reduction; at a lower temperature (40 °C), no product was detected even at a higher FeS2 loading while quantitative reduction increased from 57 to ≥99% on elevating the temperature from 60 to 80 °C (Table S1, entries 4–8). Similar amounts of FeS2 nanocrystals were used for optimization as reported by others in the literature.[1,21,34,43] When the nanocrystal loading was increased from 42 to 64 mg, conversion increased from 28 to ≥99% while no reduction was achieved with a 20 mg loading (Table S1, entry 4–8). At the largest loading of 84 mg of FeS2 nanocrystals, the reaction was completed in 7 h (Table S1, entry 9) instead of the typical 12 h with 64 mg. Several protic and aprotic solvents such as ethanol, isopropanol, dimethylsulfoxide, dichloromethane, dimethylformamide, tetrahydrofuran, and ACN were screened. With the exception of ACN, reduction was not observed in any of these solvents (Table S1, entry 10) even though ethanol, isopropanol, and THF have been utilized as solvents in the reduction of nitroarenes albeit with different reductants such as hydrazine and molecular hydrogen.[43,61,62] The catalytic activity of metals in hydrogenation reactions is reported to heavily rely on the ratio of HCOOH to Et3N;[63,64] thus, the proficiency of reduction was investigated by changing the molar ratio of HCOOH to Et3N from 1:1 to 1:8 (Table S1, entries 11–14), but no considerable difference was observed as a result. So, a 1:1 molar ratio of HCOOH to Et3N was chosen for all subsequent experiments. Conversion increased as a function of time from 27 to 55%, 69, and ≥99% for nitrobenzene at 3, 6, 9 and 12 h, respectively (Table S1, entries 15–18).

A variety of functionalized nitroarenes that include halogen, ketone, carboxylic acid, ester, nitrile, hydroxy, amines, olefin, and heterocyclic moieties were investigated to evaluate the substrate scope of the reaction. The following reaction conditions were used: 64 mg of FeS2, 12 h reaction time, 80 °C, and ACN as the solvent. Good selectivity was observed in the presence of halogen-substituted nitrobenzenes (Table , 2a–2c) as these undergo selective and complete reduction of the nitro functional group with no dehalogenated byproduct unlike palladium and platinum catalysts, which are prone to dehalogenation.[65−70] Importantly, selective reduction of para nitroacetophenone (3) and meta nitrostyrene (4) was observed without altering labile reducing moieties like olefin and ketone groups. Conventional platinum catalysts reduce the nitro group along with olefin and carbonyl groups.[71,72] For instance, Corma’s group reported the formation of a mixture of 3-aminostyrene, 3-nitroethylbenzene, and 3-ethylaniline in the reduction of 3-nitrostyrene with unaltered platinum.[8] Nitrobenzenes with electron-donating and electron-withdrawing functional groups such as hydroxy, carboxylic acid, nitrile, ester, and amines were also well tolerated. This selectivity is rare; for instance, nitrile groups are generally reduced with the use of sodium borohydride as a hydrogen transfer reagent.[73,74] In this system, there was a noticeable difference in conversion of the ortho-, para-, and meta-substituted nitroanilines (7a–7c) ranging from 96 to 67 and 27%, respectively, which could be because of lower basicity or differences in the electronic properties of ortho/para v. meta nitroaniline.[75,76] We also screened the reduction of the nitro groups in heteroaromatics such as pyridine and benzothiazole (11a–11b) and observed ≥99 and ≥85% conversion, respectively, which has not been reported before using the FeS2 catalyst. This approach could be a very attractive process if adapted by agrochemical and pharmaceutical companies.[34,77]

Table 1

Substrate Scope of Nitroarene Reduction via a FeS2-Assisted Hydrogen Transfer Systema

Reaction conditions: 0.5 mmol nitroarenes, a 1:1 ratio of 2.5 mmol of Et3N and 2.5 mmol HCOOH, 64 mg of the iron pyrite nanocrystals, 2 mL of acetonitrile, and 80 °C for 12 h. GCMS was used to determine the conversion, selectivity is shown in parentheses.

As described in the synthesis of the FeS2 nanocrystals, there is a tendency for the particles to change their shape upon extended heating (Figure ). The morphology of the nanocrystals changed from cubic to spherical during the course of the reduction reaction of nitrobenzene to aniline. To assess the catalytic efficacy of cubic v. spherical FeS2 nanocrystals, the reduction of nitrobenzene was carried out using spherical nanocrystals under the same conditions. Our initial assessment shows no apparent difference in the catalytic properties with respect to the shape (cube v. sphere) of FeS2 over a 12-h reaction period. This result is consistent given that the spheres appear to be a more stable shape. A kinetic study was not performed at shorter time points (<4 h) to fully assess the effect of nanocrystal shape on conversion. Because the particles undergo morphological rearrangement, it is not clear whether iron species are leached during the course of the reaction that may affect the reduction of nitrobenzene to aniline. Nonetheless, we observed the same crystal structure of FeS2 (cubic) after the reaction using a XRD and no other phases (Figures S4–S5). Given that the FeS2 nanocrystals are stabilized by ODA and contain some amounts of PEI, FeS2 was used in apparent stoichiometric amounts; however, these capping ligands likely affect the efficacy of the reaction and the ability to use it in truly catalytic amounts. Based on our initial assessment, ODA accounts for 10% and PEI accounts for 26% of total mass on rough estimates from thermogravimetric experiments. Control experiments were performed with those same mass percentages of ODA and PEI to determine their effect on the conversion of nitrobenzene to aniline and no conversion was observed under the same reaction conditions as those of FeS2. The FeS2 nanocrystals were recovered after the reaction with a magnet and their stability and recyclability were studied. A full assessment of the magnetic properties of the FeS2 nanocrystals is underway because not all synthetic methods are able to generate paramagnetic/superparamagnetic FeS2;[78] nonetheless, the material was attracted to a strong neodymium magnet and could be physically separated. The recycled FeS2 nanocrystals showed excellent hydrogenation of nitrobenzene after five successive cycles with no significant loss of efficiency (Figure S6).

Conclusion

In summary, an inexpensive, recyclable, and recoverable iron pyrite nanomaterial was synthesized that showed good efficiency in the transfer hydrogenation of nitroarenes to anilines. The material displayed chemoselectivity toward a diverse set of functionalized nitroarenes containing halogen, olefin, ketone, ester, and nitrile moieties. Further optimization and characterization of FeS2 and its potential as a catalyst toward the hydrogenation of nitroarenes is planned for the near future.

Experimental Section

Materials

Ferric chloride hexahydrate (FeCl3·6H2O, ≥98%), a 50% (w/v) poly(ethyleneimine) solution (PEI, MW 750,000), sulfur (reagent grade), oleylamine (C18H37N, technical grade), and ODA (C18H39N, 97%) were purchased from Sigma-Aldrich (St. Louis, MO). Anhydrous ethyl alcohol 200 proof (absolute, ACS/USP grade) and hexane (reagent grade, ACS) were purchased from Pharmco (Brookfield, CT). TEM Cu grids (carbon-coated, 200 mesh) were purchased from Electron Microscopy Sciences (Hatfield, PA).

Characterization

A JEOL JEM 2100 TEM operating at an accelerating voltage of 200 kV with a beam current 102 μA was used to analyze the morphology and size of the nanoparticles. The TEM samples were prepared by casting a dilute solution of nanoparticles in ethanol on a Cu TEM grid (carbon coated, square mesh, 200) and dried under vacuum. The X-ray diffraction (XRD) was performed on a Rigaku Smart lab X-ray diffractometer with a Cu Kα radiation source (λ = 1.54 Å). The wide scan angle was varied from 5 to 90° (2θ) at a scan rate of 1°/min. XPS measurements were performed in a UHV chamber with a base pressure <10–9 Torr at room temperature and the spectra were acquired with the Al Kα emission line (hν = 1486.6 eV) from a dual-anode X-ray source (Physical Electronics XR 04-548) operated at 400 W, an incident angle of 54.7°, and normal emission. The photoelectrons were collected and analyzed with an Omicrometer EA 125 hemispherical electron energy analyzer with a resolution of 25 meV. The hydrogenation reaction product was analyzed using a GCMS-QP2010S, SHIMADZU, Japan.

Methods

Synthesis of Iron Oxy-Hydroxide (β-FeOOH)

Iron oxyhydroxide nanoneedles were synthesized following a literature protocol.[44] In a 500 mL three-necked round bottom flask fitted with a condenser, solid FeCl3·6H2O (5.4 g, 20 mmol) was dissolved in 100 mL DI water (18.2 Ω/cm) under ambient conditions. A 600 μL volume of 33.3% v/v PEI solution was added to the reaction mixture to synthesize β-FeOOH nanoneedles (l = 87 ± 15, w = 56 ± 9). The resulting solution was stirred (400 rpm) using a Teflon-coated magnetic stir bar at 80 °C in an oil bath for 2 h. The brownish-yellow precipitate was collected by centrifugation at 8,000 rpm for 15 min, washed with ethanol five times, and dried in a vacuum desiccator (Nalgene) overnight.

Synthesis of Iron Pyrite Nanocrystals

In a typical synthesis, 0.05 g of the dried β-FeOOH nanoneedles, 0.053 g of the sulfur powder, and a variable amount of ODA were taken separately in a 100 mL three-necked round bottom flask fitted with a condenser. The amount of ODA was varied from 1.85 and 3.71 to 7.42 mmol to study the effect of the ratio of ODA/S on the size and morphology of the resulting nanoparticles. Argon gas was purged for 15 min to remove the oxygen from the flask. The reaction mixture was heated under an inert atmosphere with a glass stirring bar at 140 °C for 30 min. Then, the temperature was increased to 180 °C and heated for 2 h. After completion of the reaction, ∼2 mL of chloroform was added to the flask when the temperature dropped to 90 °C to prevent ODA from solidifying. After that, the mixture was cooled at room temperature and precipitated with ethanol. The black solid was separated by centrifugation (7500, 3 min), washed with hexane several times (until the supernatant was clear), dried in a vacuum desiccator (Nalgene) overnight, and stored in a polypropylene Eppendorf tube. From the XRD data, it was observed that the product obtained with 7.42 mmol of ODA was composed of phase pure FeS2 nanocrystals while the lower amounts of ODA (1.85 and 3.71 mmol) gave a mixture of two phases, FeS and FeS2. A time-dependent study was performed with 0.05 g of the dried β-FeOOH nanoneedles, 0.053 g of the sulfur powder, and 7.42 mmol of ODA (1, 2, 4, and 6 h).

Hydrogenation of Nitroarenes

Hydrogenation of functionalized nitroarenes was carried out in a 20 mL borosilicate glass vial capped with screw top closures. A typical reaction consisted of 0.5 mmol of the desired nitroarene, a mixture of Et3N and HCOOH in a 1:1 mol ratio (2.5 mmol Et3N and 2.5 mmol HCOOH), 2 mL acetonitrile, and 64 mg of the dried FeS2 nanocrystals obtained from a 2 h reaction time utilizing 7.42 mmol of ODA. The reactants were heated at 80 °C in an oil bath with magnetic stirring for 12 h. After completion, the product was separated with a neodymium magnet and ∼25 μL of the supernatant was diluted with DCM for GCMS analysis..