TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Process: The heating step is performed in the presence of hydrogen, steam, and an inert atmosphere preferably nitrogen. The thermolysis is performed in the presence of recycled gaseous products, preferably carbon monoxide or carbon dioxide. The catalyst system is formed by calcining: at least one compound comprising an element including alkaline earth element X (calcium or magnesium) and a metal element Y2 (aluminum); or a clay comprising at least one alkaline earth element and aluminum. The catalyst system is formed from at least two compounds, where the first compound comprising the element X2 is a first metal oxide precursor and the second compound comprising the metal element Y3 is a second metal oxide precursor. The compounds are mixed to form an intermediate mixture that is homogeneous. At least one compound is calcined at a temperature of at least 900 degrees C. The clay is impregnated with a calcium- and/or magnesium-containing compound forming an intermediate mixture which is calcined at 1000 degrees C. The catalyst system is formed from at least a first, a second and a third compound, where the first compound is a calcium source, the second compound is an magnesium source and the third compound is an aluminum source. The catalyst system: is formed on a refractory support; and acts as a catalyst and as a heat carrier. The process further comprises adding a heat carrier to the cellulosic biomass. The cellulosic biomass is contacted with the catalyst system in a pretreatment step that comprises forming a biomass-catalysts system mixture and treating the mixture by thermal treatment, hydro-thermal treatment, torrefaction, mechanical treatment, swelling, steam explosion and/or precipitation. The step of treating the mixture is performed at an elevated temperature and an elevated pressure. The mechanical treatment: comprises milling, grinding and/or kneading; and is performed at an elevated temperature and an elevated pressure. Preferred Components: The clay is: an anionic clay, which is hydrotalcite or hydrotalcite-like compound; or a cationic clay, which is smectite, bentonite and/or kaolin. The catalyst system comprises mixed metal oxides calcium oxide-magnesium oxide-(magnesium aluminate spinel). The first and second metal oxide precursors are each metal oxide, metal hydroxide, metal hydroxycarbonate and/or metal carbonate. The catalyst system comprises spinels. The spinels are characterized by: X3 to Y4 molar ratio that is 1:1-10:1; a X-ray diffraction peaks at a 2- theta angle of 35-68 degrees . The first compound is a magnesium source and the second compound is an aluminum source or vice versa. The aluminum source is bauxite, gibbsite or boehmite. The catalyst system comprises transition aluminum. The magnesium source is magnesite ore, brucite, dawsonite, bentonite or dolomite. EXAMPLE - No suitable example given.