# Zirconium-Containing Metal-Organic Frameworks (MOFs) as Catalysts for Biomass Conversion
## 1 Introduction  
The pressing environmental demands for more sustainable energy sources and renewable raw materials, together with the continuous depletion of oil reserves worldwide, have strongly boosted the interest for developing new routes of biomass valorization, including the preparation of biofuels. The aim of this industrial transformation at a global level is to decrease the dependence on fossil fuels by substituting them with renewable resources and to reduce the emission of greenhouse gases and carbon footprint.  
Biomass comprises all the organic matter with a biologic or anthropogenic origin that can be used as feedstock for obtaining energy and chemical products of industrial interest, excluding that of a fossil origin used by the petrochemical industry [1]. According to this definition, this encompasses all the organic matter produced by plants and animals such as trunks, leaves, branches, peels and bones, animal dung, but also wastes or byproducts from the paper, wood, or food industries related with human activities. Other sources of biomass include the so-called energy crops, such as corn, beet, or sugar cane, produced for energy purposes, or marine biomass, including the production of microalgae.  
One of the most extended uses of biomass is as an energy source, either by direct transformation into biofuels by gasification to obtain _syn-gas_, or by direct combustion. Nevertheless, there is an increasing interest in recent years in biomass valorization, in which biomass is used as a starting material to produce chemical compounds of industrial interest, known as platform chemicals, which can then be used as intermediates for the synthesis of several bulk and fine chemicals, such as solvents, fuel additives, fragrances,points, or biopolymers [2]. Some of the main platform chemicals obtained from biomass include furfural, 5-hydroxymethyl furfural, ethanol, succinic acid, or levulinic acid, among others (see Scheme 1) [3].  
The use of heterogeneous catalysis is critical for developing efficient transformation routes for biomass valorization, since it allows easy separation, recovery, and reutilization of the catalyst. Moreover, with the synthetic tools available nowadays, it is possible to design materials with specific catalytic active sites, or even catalysts containing more than one type of sites (multifunctional catalysts) that can be used in cooperative catalysis or in multistep processes. Altogether, this allows us to develop novel transformation routes combining two or more sequential reactions in one single reactor (one-pot cascade reactions), thereby avoiding the isolation and purification of reaction intermediates and reducing the energy consumption of the overall process. In this sense, several solid materials have been considered so far as (multifunctional) heterogeneous catalysts to replace the homogeneous catalysts traditionally used. Those include supported metal catalysts and heteropolyacids, zeolites, metal oxides, and metal-organic frameworks (MOFs).  
In the last two decades, we have been intensively involved in our group in using MOFs as heterogeneous catalysts for developing new routes for biomass valorization from various sources. Throughout this review, we will present the most relevant results obtained in this research line by us and other groups. The article is structured depending on the origin of the biomass used, whether carbohydrates, lipids, or other sources. But first, we will briefly describe what are MOFs and why we want to use them as heterogeneous catalysts.