## 1 Introduction
* Biochemical: including fermentation, yielding both alcohols like ethanol and xylitol [27, 28, 29] and organic acids [30], as well as anaerobic digestion [31, 32] involving sequential stages for the organic matter decomposition to generate biogas or biofertilizers, innovating in this last case by incorporating OTP pretreated by pyrolysis into the starting organic material [33]. The above processes can be complemented with physical methods based on particle size reduction or feedstock separation [5, 34]. Other OTP biomass applications, contextualized in the refinery concept, would encompass antioxidant production [35, 36], pulp for paper production [37], bioplastics [38], adsorbent materials [39], cement based matrices [40], particleboards [20], and supercapacitors [41]. Most pretreatments proposed in the last ten years to fractionate and leverage the potential of OTP biomass, have been focused on obtaining products like bioethanol through hydrolytic and fermentation processes, considering preliminary treatments such as liquid hot water or steam explosion [42, 43, 44], calcium dihydro or sodium hydroxide [30, 45, 46], steam explosion with phosphoric acid [29], sulphuric acid pretreatment [28], or hydrogen peroxide [47]. Other processes aim to depolymerize OTP for lignin recovery using organoslov or soda [48], innovative deep eutectic solvents [35], or phosphoric acid steam explosion [49]. Specifically, the use of organic solvents offers important advantages for lignocellulosic biomass fractionation, as they could selectively dissolve lignin and hemicellulose to obtain cellulose-rich materials with lower recalcitrance than the original biomass, and they could theoretically be retrieved easily as they usually have low boiling points [50]. However, one of the main challenges today regarding the use of organoslov agents in treatments of lignocellulosic biomass is focused on solvent recovery, as studies on efficient separation processes for these solvents are currently very scarce [51]. This solvent recovery would reduce the environmental impact caused by waste flows containing organic chemicals, and make processes more cost-effective at an industrial level. Recently, it has been shown that pretreatment efficiency could be improved by integrating organoslov with other physical systems, such as microwave radiation. Microwave heating has been shown to be a technique with which high temperatures can be reached easily, safely and reproducibly, and it has the characteristics of fast heating, high efficiency, green energy saving and easy control, allowing shorter reaction times compared to conventional heating methods [52]. Apart from the above-mentioned characteristics, microwave heating represents an enormous intensification of the process, while at the same time reducing the environmental impact. In recent years, it has been successfully applied in industrial processes. Currently, some researchers are giving much prominence to cellulose obtainment from residual biomasses, especially at the nanometer level. The main reasons for this interest are centered on its properties, both in terms of its environmentally friendly production and biodegradability as well as its structural versatility. The recovery of waste solids with a high cellulose content is in the spotlight not only for biofuels production, but also for nanomaterials manufacture, expanding the field of applications to other sectors such as biomedicine, electronic engineering, and the purification of liquid and gaseous effluents, among others [53; 54]. One of the main challenges is still the separation of the hemicellulose and lignin fractions that interact with the cellulose to obtain a high quality material. In this sense, the conditions of each specific pretreatment are closely linked to the end use proposed for OTP biomass, which requires a detailed analysis of the experimental variables and knowledge of their impact on the treatment response.