# Green and Efficient Processing of Wood with Supercritical CO2: A Review
## 6 Wood Extraction  
### Natural Component of Wood  
Wood extraction with scCO\({}_{2}\) is an effective method to obtain various high-value chemical products. ScCO\({}_{2}\) extraction has a simple process, fast mass transfer rate, high extraction efficiency, and is a green separation technology [121]. ScCO\({}_{2}\) is a low-polar solvent, in which the solubility of polar compound is very low, and the extraction efficiency can be improved by adding a polar co-solvent, such as ethanol or ethyl acetate [48]. However, excessive addition of organic solvents will affect the environment. A brief overview of wood extraction with scCO\({}_{2}\) is shown in Table 5.  
Candeia (_Eremanthus erythromycinopus_) wood is a Brazilian tree species with a high commercial value, and the chemical components of its oil can be used to make cosmetics and drugs, especially the active ingredient, s-bisabolol, which has sedative, anti-allergy, and anti-bacterial characteristics [122]. [PERSON] et al. [123] used scCO\({}_{2}\) to extract candeia wood oil at different temperatures and pressures. The results showed that the highest extraction rate of mahogany oil was 1.42% and the residue was 0.41% at 70 \({}^{\circ}\)C and 240 bar. Queiroz and Cajaiba [124] extracted chemical substances from candeia wood using scCO\({}_{2}\) with ethanol. The extraction rate of this method was higher than steam distillation. [PERSON] et al. [125] conducted dynamic scCO\({}_{2}\) extraction of candeia wood, and the kinetic curve showed that supercritical extraction and phase behavior were the main factors influencing the extraction efficiency. Pine extract is important in the fields of food, medicine, and health [126]. [PERSON] et al. [127] extracted _Pinus brutia_ bark under different scCO\({}_{2}\) extraction conditions by adding 3% ethanol and performing ultrasonication. The results showed that 46.8% (-)-catechin was extracted, and almost 100% (-)-catechin gallate and (-)-epicatechin were extracted. [PERSON] et al. [128] extracted low-molar-mass phenolics and lipophilic compounds from maritime pine (_Pinus pinaster_) wood with scCO\({}_{2}\). The maritime pine wood extracts had strong antibacterial effects. Ethanol was used as a co-solvent, and the products extracted accounted for 4.1 wt% of the dried wood. [PERSON] et al. [129] used scCO\({}_{2}\) to extract maritime pine bark, and achieved a maximum extraction of 84%, which was rich in catechin and epicatechin. The extract content was improved after the addition of ethanol solvent during extraction. An aromatic essential oil can be extracted from _Eucalyptus robusta_. This oil is secreted by eucalyptus leaves and can kill insects, repel mosquitoes, reduce inflammation, and is used for sterilization [130,131]. [PERSON] et al. [132] analyzed the extraction conditions for volatile oils from _Eucalyptus grandis \(\times\) Eucalyptus urophyllla_ by scCO\({}_{2}\). The extraction temperature was 80 \({}^{\circ}\)C, the extraction pressure was 40 MPa, and the extraction time was 8 h, reaching a maximum extraction rate of 7.86%. Chemical components were analyzed by gas chromatography-mass spectrometry. The content of eucalypt and \(\alpha\)-pinene were 45.57% and 24.78%, respectively. [PERSON] et al. [133] used supercritical CO\({}_{2}\) to extract eucalyptus globulus bark, with added ethanol, and showed an optimal extraction rate and high selectivity for flavonoids. [PERSON] et al. [134] treated _Eucalyptus grandis \(\times\) Eucalyptus globulus_ bark with dichloromethane for 7 h, and then used scCO\({}_{2}\) to extract lipophilic extractives from eucalyptus bark after drying. They detected lipophilic extractives by gas chromatography-mass spectrometry, with beta-sitosterol in the bark interior and triterpene in the bark exterior. In the latest studies, some scholars have used scCO\({}_{2}\) to extract cedarwood oil from eastern red cedar (_Juniperus virginiana_ L.) to protect the wood from termites and fungi.