## 1 Introduction  
The replacement of fossil fuels with more sustainable energy sources as well as the use of biomass wastes to obtain \"platform molecules\", intermediary compounds for the generation of diverse high value-added products, are essential actions for progress towards a circular and environmentally friendly economy. Agricultural residues are mostly lignocellulosic materials composed of cellulose (35-55%), hemicellulose (20-40%), and lignin (10-25%) [1], although other minor fractions can be distinguished such as proteins, oils, pectins, chlorophyll and ashes [2]. The main advantages for lignocellulosic material vaporization are centered on aspects such as their easy and wide availability, low cost, renewable nature and composition. The underspend of these green materials is unquestionable considering that only 8 billion tons of lignocellulosic biomass are used annually, out of a total of 182 billion tons produced worldwide [3, 4]. Therefore, in view of the rapid growth of these biomass resources and drawbacks of mismanagement, its valorization is essential to avoid problems of soil damage (leachate production and over-accumulation of nutrients) when they are burned or deposited on lands, atmospheric deterioration (waste burning produces greenhouse gases and suspended particles), and water pollution. Significant efforts are currently underway to separately recover the main biopolymer constituents of lignocellulosic biomass: cellulose, hemicellulose, and lignin. In order to achieve efficient biomass fractionation, several pretreatments have been implemented and classified into four different groups: physical (milling, sieving, microwave, ultrasonication), chemical (with acids, alkalis, organic solvents, ozone, and ionic liquids), physicochemical (steam explosion, hydrothermal pretreatment with hot water, wet oxidation, ammonia or carbon dioxide explosion), or biological (employing white, brown, and soft-rot fungi and bacteria) [5,6]. These pretreatments would modify the structure of the original biomass by fractionation, thus enhancing subsequent hydrolytic processes. The reasons are basically related to the increase in the surface area and the reduction of cellulose crystallinity. Olive-tree pruning (OTP) biomass, composed by trunks (25%), branches (50%), and leaves (25%) [7], is one of the most plentiful lignocellulosic feedstocks worldwide. Specifically, there are 9.15 million hectares cultivated with olive trees in more than 23 different countries, mainly in the Mediterranean region [8]. This implies that OTP production is in the range of 14.3-27.5 million tons per year, taking into account that, on average, the practice of pruning generates 1.5-3.0 tons of olive-tree biomass per cultivated hectares [7,9]. Unfortunately, even today, OTP continues being considered a waste because of its underutilization, so important efforts are being made to exploit this abundant biomass resource. Research for OTP biomass valorization, based on biorefinery framework studies, are focused on three main general processes:  
* Thermochemical: obtaining syngas by gasification [10, 11, 12, 13], bio-oil and solid-charcoal (biochar) by pyrolysis [14, 15, 16, 17], as well as other heating gases through combustion [18, 19, 20]. Particular cases include terfaction encompassing pyrolysis at mild conditions ranging from 230-300 \({}^{\circ}\)C [15, 21, 22], hydrothermal carbonization for hydrodrax production [15, 23, 24], and hydrothermal liquefaction for hydro-oil obtainment [25]. * Chemical: mainly hydrolysis methods leading to the formation of oligomers or monosaccharides [26], discarding research based on other chemical processes such as transfer-esterification to obtain biodiesel or glycerin due to the non-only nature of OTP biomass.