Conversion Of Biomass Into Formic Acid In NaVO₃-H₂SO₄ Aqueous Solution With Oxygen As Oxidant

Formic acid (FA) is an important material in chemical industry. Preparation of FA from biomass, the most abundant renewable carbon resource, can be regarded as a sustainable way for FA production. Nowadays, the most promising way to yield FA from biomass is the conversion in water with vanadium-containing aqueous solution with oxygen as oxidant. However, the understanding of the interaction between biomass and catalyst, the reaction mechanism and real biomass catalytic conversion is needed to be improved. Among these catalysts, NaVO3-H2SO4 system shows a high efficiency in catalytic reaction. In this work, the conversion of biomass into FA in NaVO3-H2SO4 system with O2 was studied systematically in order to gain insight into the reaction mechanism of biomass conversion to FA and to provide theoretical guidance for FA production in a large-scale application. The main research contents and conclusions are shown as follows.1. The relationships between various reactions occurring in catalytic system were studied with cellulose as substrate. A route from cellulose to byproduct was found:biomass was deep hydrolyzed and the hydrolysis products were further oxidized to acetic acid. The relationships between four reactions, initial hydrolysis, deep hydrolysis, catalytic oxidation and ordinary oxidation, were clarified. The catalytic oxidation to form FA and the deep hydrolysis to form byproducts were competitive. Based on these relationships, enhancing oxidation conditions can accelerate catalytic oxidation to yield more FA, while enhancing hydrolysis conditions can accelerate deep hydrolysis to form more byproducts. This research provides guidance for byproducts inhibition in the FA formation from biomass.2. The reaction mechanism of carbohydrate transformation in the catalytic system was studied by LC-MS and NMR spectrum with monosaccharide (glucose) and disaccharide (cellobiose) as model compounds. C1-C2 bond cleavage was found in the transformation of hexose (glucose). C1 part was combined with catalyst followed by FA formation with the help of O2. C2-C6 part formed corresponding pentose (arabinose). Pentose proceeded with similar bond breaking to form FA. Besides the hydrolysis to form monosaccharides for the followed C-C bond cleavage, disaccharide (cellobiose) can directly react with catalyst for C-C bond breaking to give CH2O unit to form FA and the disaccharides with one or more carbon less. This research is beneficial for the understanding of degradation of polysaccharides and bond-breaking mechanism.3. The conversion of lignin in the catalytic system was studied with organosolv lignin as substrate. This transformation yield limited FA (15 %) and acetic acid (6%). H2SO4 can accelerate lignin degradation to produce soluble aromatic substances followed by O2 degradation. The effect of oxidation can act directly on phenolic hydroxyl groups and primary alcoholic hydroxyl groups in lignin structure. Various oxidations and bond breakings can be found in lignin degradation in the catalytic system. Lignin can undergo different bond breakings, such as (3-O cleavage, α-β cleavage, (3-y cleavage, γ-O cleavage and a cleavage, to form hydroxylbenzoquinone ketones and phenol aldehydes (ketones).4. The conversion of lignocellulose into FA in catalytic system was studied with wheat straw as substrate. This transformation can yield 47% FA and 7.3% acetic acid. The sources of products were clarified:FA was from the catalytic oxidation of polysaccharides (cellulose and hemicellulose); acetic acid was from the bond breaking in acetyl groups of hemicellulose and from the oxidation of deep hydrolysis products of polysaccharides. The formation of acetic acid was independent of NaVO3 addition. A degradation order of "hemicellulose, lignin and cellulose" was found during the transformation of lignocellulose. This outside-to-inside order reflects the structure of lignocellulose. The catalytic system exhibits good reusability and reaction with multiple feeding of raw materials can obtain a high concentration of FA solution. Therefore, the cost reduction of FA-liquid separation can be possibly realized.5. The conversion of polyuronide into FA in catalytic system was studied with alginic acid as substrate. This transformation can yield 42% FA. The effects of NaVO3 and H2SO4 on the conversion were studied, and results show that these two catalysts have synergistic effect on the FA formation from alginic acid. NaVO3 can dissolve in H2SO4 aqueous solution as the form of VO2+, which acts as catalytic center. A high concentration of NaVO3 can lead to precipitate formation due to the polymerization of vanadium species to V2O5.