Niobia-based magnetic nanocomposites: Design and application in direct glucose dehydration to HMF

Highlights

• Niobia-based magnetic nanoparticles (MNP) coated with Nb₂O₅ or Nb₂O₅-SiO₂ shells were prepared.

• Prepared catalysts may produce HMF selectively by glucose dehydration.

• High selectivity to HMF was correlated to pseudohexagonal niobium oxide (TT-Nb2O5) phase.

• High catalytic activity was correlated to the small nanoparticles size.

Abstract

Niobia-based magnetic nanocomposites were prepared by covering magnetite nanoparticle cores (Fe₃O₄, MNP) with either Nb₂O₅ or Nb₂O₅-SiO₂ shells using a two-step procedure. In the first step magnetite nanoparticles were prepared by the coprecipitation method. The second step involved their coverage with either Nb₂O₅ shells, through a precipitation method, or with Nb₂O₅-SiO₂ shells, through a sol-gel protocol followed by precipitation in the presence of the CTAB surfactant. The obtained materials were exhaustively characterised by X-ray diffraction, Mössbauer spectroscopy, magnetic measurements, ICP-OES, DRIFT and Raman spectroscopy, and CO₂ - and NH₃-TPD measurements, and investigated for glucose dehydration to HMF. The catalytic performances were directly correlated to the nature of the supported niobia phases, which, in turn, has been dictated by the niobia content and the preparation route. The high selectivity to HMF was correlated with to the large pseudohexagonal niobium oxide (TT-Nb₂O₅) phase while the catalytic activity was directly correlated to the small nanoparticles size. A proper combination of these features led to an optimum catalytic system for the selective production of HMF through glucose dehydration. A third important feature making the developed catalyst promising is its magnetic property, ensured by the magnetite nanoparticles core. This allowed its easy separation from the reaction products.

Introduction

In the context of the depletion of petroleum and the serious environmental pollution caused by the utilization of the fossil resources, the valorisation of the renewable biomass into sustainable fuels and fine chemicals became one of the extremely important research topics of the 21^st century aiming a carbon-neutral society.

Heterogeneous catalysis has a rich history in facilitating viable strategies for a sustainable chemical industry. Therefore, there is no wonder on the central role of the development of the solid catalysts to facilitate the design of efficient strategies for viable biomass valorization. Information about a rational design of such catalysts for valuable transformations is provided by a good number of review papers [1,2].

Besides the chemical nature, particle size and morphology, as important criteria for the evaluation of the performances of the solid catalysts for the biomass valorization, their tolerance to water is also essential [2]. Also, materials containing magnetically active nanoparticles cores, like magnetite (Fe₃O₄), may generate an efficient separation from the different mixtures by simply applying of an external magnetic field [1,3].

A few examples of efficient magnetic catalytic systems were recently developed in our group for the direct production of sorbitol and glycerol from cellulose (Ru(III)-SiO₂-Fe₃O₄) [4], lignin fragmentation (Co@Nb₂O₅@Fe₃O₄ catalysts composites) [[5], [6], [7]] or 5-hydroxymethylfurfural (HMF) upgrading [8]. Very important also, in the presence of oxygen, the ionic Ru(III)-SiO₂-Fe₃O₄ became an efficient catalyst for the conversion of levulinic acid [9] or glucose [10] to succinic acid and for the synthesis of furandicarboxylic acid (FDCA) from HMF [8].

Niobium compounds deposited over oxide supports (e.g., silica, titania, or alumina) have also attracted much attention in last years [11] being investigated in different transformations of the biomass derivatives in aqueous medium [[12], [13], [14], [15], [16], [17], [18]]. This interest is mainly due to the enhanced hydrothermal stability and acidity (which is maintained in water) of such materials. Lewis acidity was found for all of the supported niobium oxide systems, while Brønsted acid sites were only detected in niobium supported on alumina and silica. However, the stabilization of the active phase strongly depends on the characteristics of the support. The composition of a multi-component oxide is a very important variable since it affects the formation of different niobium species, including superficial and crystalline bulk phases. Their activity varies with the niobium oxide coverage [19]. Comparing to bulk niobia, the presence of dispersed Nb species in/on a high surface area and chemically inert host matrix, as silica, leads to surface expansions through Si-O-Nb linkages.

Obviously, niobium oxide forms different polyhedra structures and transforms its phase as a function of the calcination step. In the range 300–500 ̊C a pseudohexagonal niobium oxide (TT-Nb₂O₅) is formed while at 700–800 ̊C results in an orthorhombic niobium oxide (T-Nb₂O₅). Monoclinic niobium oxide (H-Nb₂O₅) is formed only at temperatures higher than 1000 ̊C [20]. However, increasing the calcination temperature, the degree of crystallinity also increases while its catalytic performances as a solid acid (Brønsted acidity) and surface area decreased [21]. Therefore, to preserve the Brønsted acidity and catalytic properties a modification of niobium oxide or supported niobium oxide with sulfate and phosphate has been recommended [22,23].

However, no clear relationships between the structure, acidity, and dispersion of niobium oxide in silica matrices and the influence of the preparation method and Nb loading were established [24]. For the specific example of the one-pot conversion of cellulose, it was shown that the core-shell Nb₂O₅-SiO₂@MNP catalysts are able to afford the depolymerisation of cellulose to α-hydroxy acids (AHAs) as the main products [25]. However, these catalysts displayed different characteristics as a function of the preparation methodology (ie, co-precipitation (CP) or sol-gel followed by precipitation (SGP)). The catalytic performances, expressed in terms of α-hydroxy acids (AHAs) yields, were directly correlated to the nature of the catalytic sites which, in turn, has been dictated by the niobia content and the preparation route. Also, bulk niobia shown activity for the dehydration of glucose to 5-(hydroxymethyl)furfural in the aqueous phase with a HMF yield of 20 % [26].

Based on this state of the art and with the aim to develop highly efficient niobia acid catalysts for glucose dehydration to HMF we focused our efforts on the rational design of niobium-based magnetic nanocomposites through the optimization of the Nb content and preparation methodology. For this purpose shell (ie, Nb₂O₅ or Nb₂O₅-SiO₂) covered magnetite nanoparticles with different chemical composition were prepared. The correlation of the catalytic properties with the structural characteristics of these materials allowed determining the most important catalytic features to reach a high productivity to HMF.