Efficient preparation of carbon nanospheres-anchored porous carbon materials and the investigation on pretreatment methods

Highlights

• A carbon nanospheres-anchored porous carbon material was prepared from sawdust.

• 1980 m²/g SSA was obtained in a very short activation time at 450 °C.

• H₃PO₄ hydrothermal pretreatment is essential for material properties.

• The prepared material showed good adsorption performance for various pollutants.

Abstract

Manufacturing high-performance activated carbon (AC) materials from abundant biomass at low temperature and short activation time is targeted by the green and sustainable chemical industry. Here, a 1980 m²/g of carbon nanospheres-anchored porous carbon material (PHAC) derived from waste sawdust was prepared by a method of H₃PO₄ hydrothermal combined with fast activation at 450 °C within 2.8 min. It is found that H₃PO₄ hydrothermal pretreatment could promote the dehydration of carbohydrates to form more unstable C = O structures, which were decomposed in the subsequent fast activation to form pore structures. In addition, this process is also conducive to the formation of carbon nanospheres, increasing the degree of graphitization and producing more graphite defects. The prepared PHAC showed good adsorption performance for different types of pollutants. This work provides a new insight for the preparation of high performance biomass based carbon materials under mild conditions.

Introduction

As the demands of sustainable development and environmental protection, there is a growing momentum toward utilizing renewable biomass materials instead of fossil coal to produce activated carbon (AC), which is widely used in the fields of pollutant adsorption (Li et al., 2020, Wang et al., 2020), catalysis/electro-catalysis (Ghaith et al., 2020, Wang et al., 2019) and supercapacitor (Kim et al., 2020, Zhou et al., 2021). At present, the preparation of AC, especially those with high performance, usually requires high temperature and/or long activation time, which is accompanied by significant consumption of energy (Fu et al., 2020). Therefore, exploring low-temperature and short-time activation methods is of great interest for achieving more energy-efficient preparation of AC materials.

The H₃PO₄ activation method has great potential to achieve environmentally friendly preparation of AC, due to its low heating temperature, easily recovered activator, and high carbon yield (Gao et al., 2020). Researchers have carried out a lot of research on the preparation of biomass-based ACs with high performance by H₃PO₄ activation (Liu et al., 2021, Ma et al., 2019, Mokhati et al., 2021, Sarwar et al., 2021). Silva et al. (2021) prepared AC by hydrothermal carbonization of banana tree pseudostems following by H₃PO₄ impregnation and then activation at 500 °C (5 °C/min) for 60 min. The specific surface area (SSA) of the obtained AC reached 1975 m²/g, much higher than the material prepared by the direct impregnation method (1436 m²/g). By activating eucalyptus chips impregnated with H₃PO₄ at 500 °C (10 °C/min) for 60 min, AC with an SSA of 1265 m²/g and a Cr(VI) adsorption capacity of 112.3 mg/g was prepared by Lu et al. (2021). Chen et al. (2020) pre-carbonized H₃PO₄-loaded jute sticks at 200 °C and activated at 500 °C (5 °C/min) for 120 min, the obtained AC (1741 m²/g) had rich acidic functional groups and its Cr(VI) removal efficiency exceeded 98%. Though excellent ACs have been reported to be prepared by various pretreatment methods, which influence the properties of AC by changing the physicochemical properties of the precursor, there are few researches that systematically study the effects of pretreatment methods on the properties of carbon materials. Moreover, current researches are mainly based on slow heating activation, requiring the precursor to undergo a long-time programmed heating and activation process, which significantly increases the heating time and energy consumption.

Fast activation method that sends the precursor directly to the high-temperature heating zone has the potential to shorten the heating time (Yang et al., 2021). However, at present, only a few studies have tried to prepare ACs by this method. Gao et al. (2018) carried out H₃PO₄ activation of pine wood sawdust in a spouted bed and they reported that the SSA could reach more than 1500 m²/g under the fast activation treatment for only 5 min at 800 °C. Zhang et al. (2020a, 2020b) used fast activation method to activate H₃PO₄-loaded coconut shells at 800 °C for 120 min, and the SSA of AC reached about 1800 m²/g, which could be further improved under the co-activation of CO₂. Although AC with a high SSA was obtained, a very high activation temperature exceeding 800 °C was required, which was much higher than the traditional slow activation temperature using H₃PO₄ activator.

In this work, a high-SSA carbon nanosphere anchored porous carbon material was successfully prepared from wood sawdust using a hydrothermal pretreatment in H₃PO₄ and then fast activation at low temperature for a very short time. When the pretreatment product was fast activated from room temperature to 450 °C within 2.8 min, the carbon material with an SSA of 1980 m²/g was obtained. To explore the effect of H₃PO₄ in the hydrothermal and activation process, a series of comparative experiments with different pretreatment methods were carried out. The physical and chemical properties of carbon materials under different preparation conditions were studied, and the adsorption capacities of different types of pollutants, including carbamazepine antibiotic, heavy metal ion Cu(II) and methylene blue dye were explored.