## 1 Introduction  
In the European Action Plan, the Circular Economy has been defined as an economic system in which \"the value of products, materials and resources is maintained in the economy for as long as possible, and the generation of waste minimised\", while the Bioeconomy has been defined as \"the production of renewable biological resources and the conversion of these resources and waste streams into value-added products, such as food, feed, bio-based products and bioenergy\" [1]. The conversion of biological waste streams into value-added products, in the definition of the bioeconomy, represents an important principle of the circular economy itself and, consequently, for its realization and fulfillment. In bioeconomy, the general principle of the cascading use of biomass and waste streams provides the definition of priority actions and then secondary approaches [2, 3, 4]. Following this principle, the generation of the higher-value products such as food and animal feed should have the priority; then, it firstly should be considered the sustainable reuse/recycling of byproducts, wastes, and raw materials together with bioproduct manufacturing, and, finally, the implementation of energy-yielding technologies [5, 6]. This approach following the cascading use principle is in agreement with the waste minimization concept defined as \"a zero waste programme\" [7] and can be considered as a crucial strategy for circular economy [4, 8, 9]. An ideal goal for any production sector is to modify its product chain,improving the efficiency, enabling reuse of all byproducts in the value chain, and reducing the waste production, trying to fulfill the zero waste criteria. The circular bioeconomy finds in agriculture one of the best sectors in which to play a key role in increasing the sustainability of the sector, given the countless possibilities of transforming crop residues and so recycling this precious biological resource [10]. In fact, agricultural waste--or agri-waste--can be considered the unwanted or unsalable materials deriving from agricultural processes. Similarly to food-waste, agri-waste can represent an environmental issue, but promoting their use and valorization, they can represent a benefit for several existing production processes. The implementation of these residues as novel raw material for production processes can promote the circularity and sustainability of these processes. In particular, crop wastes discarded directly on the field during harvesting are usually comprised of a cellulose backbone and, depending on their source, they may possess high carbon content and multi-functional groups. These properties have been researched upon the past and have led to employment of food and agri-waste in effluent management and biodiesel generation, as phytochemicals or catalysts [11; 12]. Recently, agricultural waste management and its processes are considered more and more interesting topics for researchers [13]. Nevertheless, in common practice, agricultural waste is discharged to the surrounding environment without treatment or, in some situations, is burnt off, becoming an environmental loads. In this last scenario, the production of energy comes with the production of fly ash, which can nowadays be valorized in environmentally friendly methods. In fact, due to the high silica content, the fly ash can be considered a potential raw material for the synthesis of nanoporous materials, such as zeolites or mesoporous silica [14; 15]. Agricultural wastes can be categorized in waste generated in the fields, after crop harvesting, (i.e., straw, stalk or leaves) [12] and waste generated after the crops are processed (i.e., husk, bagasse, peel or stover.) [16]. In the maize grain production process, the residues, such as corn stover--the above-ground portion of the plant including the cob remaining in the field after the grain harvesting--are very abundant, with an estimated residue yield of 8.9 t/ha with an averaged production of 85 Mt/year, at the European level [17].