Catalytic Conversion Of Cellulose To 5-hydroxymethyl Furfural Using Ionic Liquid As The Solvent And Catalyst

In this work, (1, 4)-dimethoxy-β-D-glucose (Glc) andβ-D-glucose (Glu) were chosen as the model for cellulose. First of all, DFT theory of quantum chemistry, AIM theory, NBO theory and IR spectroscopy were performed to investigate the mechanism of cellulose dissolution and regeneration in 1-ethyl-3-methylimidazolium acetate ([emim]Ac). The theoretical results show that the interaction of [emim]Ac with Glc or [emim]Ac with Glu is stronger than that of Glc with Glc, Glu with Glu,and [emim]Ac with [emim]Ac. This means that Glc and Glu are easier to combine with [emim]Ac, we can therefore speculate that [emim]Ac can dissolve cellulose. Further studies indicate that the anion acetate of [emim]Ac forms strong H-bonds with hydroxyl groups of Glc and Glu. It is also observed that the H-bonds between [emim]Ac and Glc or Glu are weakened or even destroyed by the addition of water. The experimental results prove that cellulose can be readily reconstituted from the [emim]Ac-based cellulose solution by the addition of water. In addition, both the original and regenerated cellulose samples were characterized with FT-IR, XRD, TGA and SEM, and the crystalline structure of cellulose is converted to cellulose II from cellulose I in the original cellulose.Then, the interaction of [emim]Ac with H2O was systematically studied at the same theoretical level. It is found that [emim]Ac interacts with water molecules mainly via H-bonds, and the anionic part of [emim]Ac plays a major role in the interaction with H2O. The energies of H-bonds are estimated from spectral shifts of hydroxy antisymmetric stretching vibration. Moreover, the experimental results also indicate that hydroxy of water mainly interacts with the COO—of [emim]Ac. Further studies indicate that the intensity of hydroxy stretching vibrations tend to be stronger with the increase of the concentration of water. In addition, the frequency of hydroxy stretching vibrations shows clearly red-shift, and the COO—vibrational frequency gradually shifts to the lower wavenumber region, which are indicative of extended hydrogen bonded network.Finally, we investigated the catalytic conversion of microcrystalline cellulose (MCC) to 5-hydroxymethyl furfural (HMF), the process consists of hydrolysis of cellulose toβ-glucose, isomerization ofβ-glucose to fructose, and fructose dehydration to HMF. Because acidic ILs with metal ions can synergetic catalytic conversion ofα-glucose toβ-glucose and isomerization ofβ-glucose to fructose, while cellulose hydrolysis and fructose dehydration can be simply achieved at acidic condition. Herein, we investigated the conversion of MCC to HMF with several acidic ILs and metal salts in the solvent of [emim]Ac under mild conditions. A series of acidic ILs were synthesized and tested in conversion of MCC to HMF. The effect of reaction conditions, such as reaction time, temperature, catalyst dosage, metal salts, water dosage, Cu²⁺ concentration and various acidic ILs were investigated in detail. The results show that CuCl₂ in 1-(4-sulfonic acid) butyl-3-methylimidazolium methyl sulfate ([C₄SO₃mim]CH₃SO₃), is found to be an efficient catalyst for the catalytic conversion of MCC to HMF, and 69.7% yield of HMF is obtained. Furthermore, a mechanism to explain the high activity of CuCl₂ in the [C₄SO₃mim]CH₃SO₃ is proposed. To our knowledge, this report first proposes that the Cu²⁺ and [C₄SO₃mim]CH₃SO₃ show better catalytic performance in the conversion of MCC to HMF.