A heterogenized alternative to the homogeneous precapture of CO2 with amines and subsequent hydrogenation to MeOH was developed using aminated silica and a Ru-MACHOTM catalyst. Commercial mesoporous silica was modified with three different amino-silane monomers and used as support for the Ru catalyst. These composites were studied by TEM and solid-state NMR spectroscopy before and after the catalytic reaction. These catalytic reactions were conducted at 155 degrees C at a H-2 and CO2 pressures of 75 and 2 bar, respectively, with the heterogeneous system (gas-solid) being probed with gas-phase infrared spectroscopy used to quantify the resulting products. High turnover number (TON) values were observed for the samples aminated with secondary amines.

The chemisorption of CO2 on aminated silica has a rich chemistry, and its details are important to research in relation to CO2 capture and catalytic chemistry. In this study, such chemisorption was investigated on aminated and diaminated silica with H-1 and C-13 solid state nuclear magnetic resonance (NMR) spectroscopy under dry and wet conditions. Fast magic-angle spinning (MAS) allowed us to obtain high resolution spectra. Porous silica was modified into a monoaminated version using (3-aminopropyl)triethoxysilane (APTS) and a diaminated one by using 3-(2-aminoethylamino)propyltriethoxysilane (AEAPTS). From the corresponding NMR spectra we could conclude that, under dry conditions, CO2 chemisorbed as carbamic acid and carbamate ammonium ion pairs and gave similar spectra for both directly-excited C-13 and under cross-polarization (CP) {H-1}C-13 MAS NMR. Under wet conditions, direct excitation and {H-1}C-13 CPMAS NMR showed that carbamate ammonium ion pairs formed along with bicarbonates (HCO3-). For the wet conditions, the C-13 and H-1 NMR spectra were especially well resolved, and we could detect two different types of carbamate ammonium ion pairs forming on the diaminated silica. We conclude that carbamate ammonium ion pairs and HCO3- moieties can be detected by {H-1}C-13 MAS NMR, at least qualitatively, in addition to the more time consuming direct excitation.

Self-aggregated colloids can be used for the preparation of materials, and we studied long rod-like aggregates formed on the evaporation of water from dispersed particles of colloidal hydrochar. The monodispersed hydrochar particles (100–200 nm) were synthesized by the hydrothermal carbonization ofglucose and purified through dialysis. During the synthesis they formed colloidal dispersions which wereelectrostatically stable at intermediate to high pH and at low ion strengths. On the evaporation of water,macroscopically large rods formed from the dispersions at intermediate pH conditions. The rods formedat the solid-water interface orthogonally oriented with respect to the drying direction. Pyrolysis renderedthe rods highly porous without qualitatively affecting their shape. A Cu-Si alloy was reactively infiltratedinto the in-situ pyrolyzed hydrochars and composites of tricopper silicide (Cu3Si)-silicon carbide(SiC)/carbon formed. During this process, the Si atoms reacted with the C atoms, which in turned causedthe alloy to wet and further react with the carbon. The shape of the underlying carbon template wasmaintained during the reactions, and the formed composite preparation was subsequently calcined intoa Cu3Si-SiC-based replica of the rod-like assemblies of carbon-based colloidal particles. Transmission andscanning electron microscopy, and X-ray diffraction were used to study the shape, composition, andstructure of the formed solids. Further studies of materials prepared with reactive infiltration of alloysinto self-aggregated and carbon-based solids can be justified from a perspective of colloidal science, aswell as the explorative use of hydrochar prepared from real biomass, exploration of the compositionalspace in relation to the reactive infiltration, and applications of the materials in catalysis. 

Global concerns regarding climate change and the energy crisis have stimulated, among other things, research on renewable and sustainable materials. In relation to that, hydrothermal carbonization of wet biomass has been shown to be a low-cost method for the production of hydrochars. Such hydrochars can be refined into materials that can be used in water purification, for CO2 capture, and in the energy sector. Here, we review the use of metal ions and particles to catalyze the formation of hydrochars and related hybrid materials. First, the effects of using silver, cobalt, tellurium, copper ions, and particles on the hydrothermal carbonization of simple sugars and biomass are discussed. Second, we discuss the structural effects of iron ions and particles on the hydrochars in conjunction with their catalytic effects on the carbonization. Among the catalysts, iron ions or oxides have low cost and allow magnetic features to be introduced in carbon-containing hybrid materials, which seems to be promising for commercial applications.

Introduced in the literature in 1913 by Bergius, who at the time was studying biomass coalification, hydrothermal carbonisation, as many other technologies based on renewables, was forgotten during the industrial revolution. It was rediscovered back in 2005, on the one hand, to follow the trend set by Bergius of biomass to coal conversion for decentralised energy generation, and on the other hand as a novel green method to prepare advanced carbon materials and chemicals from biomass in water, at mild temperature, for energy storage and conversion and environmental protection. In this review, we will present an overview on the latest trends in hydrothermal carbonisation including biomass to bioenergy conversion, upgrading of hydrothermal carbons to fuels over heterogeneous catalysts, advanced carbon materials and their applications in batteries, electrocatalysis and heterogeneous catalysis and finally an analysis of the chemicals in the liquid phase as well as a new family of fluorescent nanomaterials formed at the interface between the liquid and solid phases, known as hydrothermal carbon nanodots.

This chapter provides an overview of the most recent advances in the mechanistic study of hydrothermal carbonisation (HTC) and the strategies to improve the conversion by using carbon-based catalysts. HTC, although not a recent discovery, has lately been receiving increasingly attention by both academic and industrial sectors due to the possibility to exploit this process to perform a simple, green and inexpensive conversion of bio-derived waste material into valuable chemicals and advanced materials and, as such, this chapter will also look into the use of hydrochars formed in HTC and their application in catalysis, more specifically heterogeneous catalysis with a mention on electrocatalysis. The versatility and tuneability of these solids give rise to the great range of applicability in different fields. A detailed overview of the HTC process is presented and the main uses of hydrochars in catalysis is then shown, highlighting their use as solid acid catalysts, as pristine solid catalysts, as sacrificial agents in synthesis, since their removal through combustion is easy, and the niche application of these solids in electrocatalysis for future research perspective.