One step synthesis of Mo-doped carbon microspheres for valorization corncob to levulinic acid

• The Mo-doped carbon microspheres (MoCMP) were synthesized by one step.

• The MoCMP catalyst displayed a peak levulinic acid yield of 33.02% from corncob.

• The improvement of Lewis acid sites was related by the Mo₂C and Mo⁶⁺ species.

• The Lewis acid site could promote glucose isomerization and 5-HMF rehydration to LA.

• The enhanced strong Brønsted acid sites were generated by carboxyl groups.

• The Brønsted acid sites could facilitate fructose dehydration to 5-HMF.

Abstract

This work presents the catalytic valorization agricultural waste into levulinic acid over Mo-doped carbon microspheres (MoCMP) in γ-valerolactone/water solvents. The MoCMP with Brønsted and Lewis acid were synthesized by hydrothermal carbonization of uncrystallized cellulose with phosphomolybdic acid in one step, which was in absence of any additional modification. The MoCMP catalyst displayed a peak levulinic acid yield of 33.02% at 195 ℃ for 90 min. The improvement of Lewis acid sites was related by the Mo₂C and Mo⁶⁺ species to promote the isomerization of glucose to fructose. The enhanced strong Brønsted acid sites were generated by carboxyl groups to facilitate the glucose dehydration to levulinic acid. MoCMP provided a high catalytic efficiency for production of LA from corncob. A simple and green method was proposed to synthesize Mo-doped carbon microspheres for valorization of agricultural biomass into valuable chemicals.

Introduction

Lignocellulosic feedstock was considered as a promising renewable bioresource for production of biofuels and chemicals, which mainly contains hemicellulose, cellulose and lignin (Wang et al., 2021, Yu et al., 2019b). Among the three components, cellulose is most abundant component, which is a polymer including glucose units connected by β-1,4 glycosidic bonds (Liu et al., 2021). Cellulose nanomaterials are widely used in industry, agriculture, medicine and other fields because of their renewable and degradable properties, as well as excellent chemical, mechanical and rheological properties (Liu et al., 2021a, Liu et al., 2021b, Liu et al., 2021c, Xu et al., 2021). Valorization of the cellulose to high value-added chemicals is a key aspect for the integral utilization of lignocellulose (Du et al., 2019). However, the intramolecular and intermolecular hydrogen bonds in lignocellulosic biomass render the two components difficult to be hydrolyzed. Cellulose is identified as a renewable raw material, which is used to produce sustainably valuable chemicals and fuels (Liu et al., 2022). However, the intramolecular and intermolecular hydrogen bonds in cellulose are not conducive to the degradation of cellulose. Therefore, many catalytic systems have been investigated for valorization of cellulose into chemicals, such as glucose, 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) (Li et al., 2019, Li et al., 2019, Li et al., 2020, Li et al., 2020). Among those chemicals, LA was listed in the twelve bio-based platform chemicals and have attracted researchers' great attention (Li et al., 2019, Li et al., 2019, Yu et al., 2015).

Ionic liquids as solvent used for the biomass conversion could improve the catalytic activities, which possess excellent biomass dissolving capacity, chemical and thermal stability (Lee et al., 2019, Peleteiro et al., 2016). However, their industrial utilization in the large-scale production of levulinic acid is hindered by the high cost and environmental problems. Recently, γ-valerolactone (GVL) has been applied intensively in the biomass conversion, which is a green and efficient biomass-derived solvent (Alonso et al., 2013, Li et al., 2017, Mellmer et al., 2014, Yang et al., 2017). In the presence of GVL/H₂O (w/w: 9:1) solvent, 69% of LA yield from cellulose was achieved over Amberlyst 70 catalyst, which much higher than the low yield of 20% in water (Alonso et al., 2013).

In addition to the solvent selection, another research focus to increase the levulinic acid yield is the development of heterogeneous solid catalysts. Sn-MMT/SO₄²⁻ solid acids were used in the LA production from bagasse by a two-step process, and Sn-MMT/SO4²⁻ have shown 62.1% of LA yield (Wang et al., 2018b). Amberlyst 36 was could efficiently enhance the valorization of paper towel waste to LA, and 30 mol% of LA was achieved in 20 min in the presence of the catalyst (Chen et al., 2018). The ferrous sulfide was loaded on the lignin, and was used as solid acid for the catalytic LA production from cellulose. The levulinic acid yield observed was 35.64% at 185 °C (Han et al., 2019). Fe₃O₄-SBA-SO₃H used as an acid solid catalyst to convert cellulose into LA directly, and the LA yield was 45% at 150 °C after 12 h (Lai et al., 2011). Among these solid acids, carbonaceous solid acids had several advantages such as high specific surface area, excellent hydrothermal stability and efficient functionalization (Wang et al., 2019). The sulfonated solid acid carbonaceous catalysts were prepared by carbonization and sulfonation of wheat straw. The catalysts were applied to convert waste biomass to levulinic acid, and 65.6 mg LA/g biomass was observed (Ozsel et al., 2019). The preparation of carbonaceous solid acid by two-step reaction is more complicated and time-consuming than the one-step. In this work, the Mo-doped carbon microspheres were synthesized by hydrothermal carbonization of cellulose with phosphomolybdic acid, which were applied in the catalytic hydrolysis of corncob to enhance the production of levulinic acid.