Cubic platinum nanoparticles capped with Cs₂[closo-B₁₂H₁₂] as an effective oxidation catalyst for converting methane to ethanol

Highlights

• Cs₂[closo-B₁₂H₁₂] forms Pt nanoparticles by reduction under facile conditions.

• The Pt nanoparticle sizes can be controlled by changing the reaction conditions.

• Pt nanoparticles directly oxidize methane to ethanol and methanol with H₂O₂ and O₂.

• The methane oxidation is free-radical-mediated.

Abstract

Direct conversion of methane to alcohol remains a key challenge. Here, we report a novel aqueous catalyst, cubic platinum (Pt) nanoparticles capped with Cs₂[closo-B₁₂H₁₂], capable of direct oxidation of methane to ethanol and methanol with H₂O₂ and O₂ under facile conditions. The selective conversion to ethanol exceeded 97% with a yield reaching 148.41 mol·kg_cat⁻¹·h⁻¹ at 50 °C. Experiments with 5,5′-dimethy1-1-pyrroline-N-oxide (DMPO) and electron paramagnetic resonance (EPR) tests revealed that methane oxidation occurred via free-radical chain reactions involving CH₃, CH₃CH₂, and HO radicals. Theoretical analysis suggested that the {1 0 0} surface of the Pt nanoparticles was capped with Cs₂[closo-B₁₂H₁₂] mediated by Pt–B bonds with a binding energy of −278.6 kcal/mol. This promoted the growth of particles along the direction of the (1 0 0) facets and finally formed a cubic structure. The EPR tests combined with theoretical calculations confirmed the hypothesis that methane–ethane–ethanol conversion was mediated by the catalyst by employing the features of nano-platinum and Cs₂[closo-B₁₂H₁₂], on which only C₁ and C₂ products were generated, while no products with three or more carbon atoms were detected in the reaction systems described above.

Introduction

Methane is a main component of natural gas, which is an important clean fuel and raw material for producing syngas and many chemical products. The conversion of natural gas to liquid fuels and chemical products is thought to be one of the most promising routes [1], [2], [3]. However, the CH bond is too stable to be broken at ambient temperature or under relatively mild conditions, rendering direct catalytic methane conversion a serious challenge for many years [4], [5], [6].

Methane conversion has been primarily realized through indirect routes by turning methane to syngas and other chemical products at high temperature (200–500 °C). These processes consume a lot of energy [7]. However, methane can be converted under mild conditions by liquid catalysts, such as Hg and Pt complexes, but the catalysts are easily consumed during the reaction which still requires high temperatures (180 °C) and strongly acidic media [8], [9]. Using water as the oxygen source, copper-exchanged mordenite zeolites (CuMOR) are employed as catalysts for converting methane to methanol with high selectivity at 200 °C [10]. Under milder conditions, hydrogen peroxide can be used as an oxidant with Fe complexes, gold chloride, and Fe-ZSM-5 catalysts; however, the conversion gives low yields and selectivity [11], [12], [13], [14]. Methane Conversion to methanol and ethanol by oxygen was recently achieved using NiO/CZ as a catalyst, but this required a minimum temperature of 450 °C [15]. Photocatalysis using copper-modified polymeric carbon nitride as a catalyst is a more energy-conserving and environmentally friendly, with a maximum ethanol productivity of 106 μmol·g_cat⁻¹·h⁻¹, but it has low selectivity [16]. Noble metal nanoparticles (Au-Pd nanoparticles) were reported for the oxidation of methane to methanol under mild conditions with high selectivity [17], [18]. Pt salts promote oxidation of methane more efficiently than do Au and Pd [9], and CO₂ is not detected when reacting CH₄ with H₂O₂ in the presence of H₂PtCl₆ [13]. Thus, the nano-platinum could be considered as a more efficient catalyst that can further increase the yield and selectivity to methanol and even ethanol under facile conditions. Thus, refining the method for forming platinum (Pt) nanoparticles is important.

Boron clusters with a closed icosahedral structure were used as an inert agent for boron neutron capture therapy in anti-cancer therapy. Boron clusters form bivalent anions ([closo-B₁₂H₁₂]²⁻) by themselves or with other metals. Furthermore, boron clusters and their salts can function as reductants to form nano-metal materials capped with them [19], [20], [21], [22], [23], [24]. In our recent work, we reported a cesium salt of a boron cluster (Cs₂[closo-B₁₂H₁₂]), which could be used as a new bifunctional reductant to form Au, Pd, Pt, and Ag nanoparticles under mild conditions [25], [26], [27].

In this work, we synthesized cubic Pt nanoparticles capped with Cs₂[closo-B₁₂H₁₂] through an efficient one-pot method in aqueous solution at 80 °C. The cubic Pt nanoparticles capped with Cs₂[closo-B₁₂H₁₂] could directly oxidize methane to ethanol and methanol with H₂O₂ and O₂ under mild conditions with the high selectivity to ethanol.