A microwave-free process for preparing nanoparticles of porous crystalline Fe-, Al- or Ti-based MOF carboxylate material with low polydispersity index, comprising steps of: A) mixing in an aqueous solvent system: (i) at least a first inorganic metallic precursor in the form of a metal M, a salt of a metal M or a coordination complex comprising the metal ion M z+ selected from Fe 2+, Fe 3+, Ti 3+, Ti 4+ or Al 3+; (ii) at least one precursor ligand L' having the structure R 1 -(C(=0)-R 3) q wherein: q represents an integer from 2 to 6; preferably q represents 2, 3 or 4; each occurrence of R 3 is independently selected from a halogen atom, - OH, -OR 4, -O-C(=O)R 4 or -NR 4 R 4', wherein R 4 and R 4', identical or different, represent C 1-12 alkyl radicals; R 1 independently represents: (a) a C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl radical; (b) a fused or non-fused monocyclic or polycyclic aryl radical, comprising 6 to 50 carbon atoms; (c) a fused or non-fused monocyclic or polycyclic heteroaryl, comprising 4 to 50 carbon atoms; R 1 optionally bearing one or more substituents independently selected from a halogen atom, -OH, -NH 2, -NO 2 or C 1-6 alkyl; and B) allowing the mixture obtained in step A) to react at a temperature below the boiling point and reflux temperature of the aqueous solvent system; so as to obtain the said nanoparticles; wherein the process is carried out in the absence of additives aimed at solubilizing the ligand precursor L' in the solvent system. A) mixing in an aqueous solvent system: (i) at least a first inorganic metallic precursor in the form of a metal M, a salt of a metal M or a coordination complex comprising the metal ion M z+ selected from Fe 2+, Fe 3+, Ti 3+, Ti 4+ or Al 3+; (ii) at least one precursor ligand L' having the structure R 1 -(C(=0)-R 3) q wherein: q represents an integer from 2 to 6; preferably q represents 2, 3 or 4; each occurrence of R 3 is independently selected from a halogen atom, - OH, -OR 4, -O-C(=O)R 4 or -NR 4 R 4', wherein R 4 and R 4', identical or different, represent C 1-12 alkyl radicals; R 1 independently represents: (a) a C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl radical; (b) a fused or non-fused monocyclic or polycyclic aryl radical, comprising 6 to 50 carbon atoms; (c) a fused or non-fused monocyclic or polycyclic heteroaryl, comprising 4 to 50 carbon atoms; R 1 optionally bearing one or more substituents independently selected from a halogen atom, -OH, -NH 2, -NO 2 or C 1-6 alkyl; and (i) at least a first inorganic metallic precursor in the form of a metal M, a salt of a metal M or a coordination complex comprising the metal ion M z+ selected from Fe 2+, Fe 3+, Ti 3+, Ti 4+ or Al 3+; (ii) at least one precursor ligand L' having the structure R 1 -(C(=0)-R 3) q wherein: q represents an integer from 2 to 6; preferably q represents 2, 3 or 4; each occurrence of R 3 is independently selected from a halogen atom, - OH, -OR 4, -O-C(=O)R 4 or -NR 4 R 4', wherein R 4 and R 4', identical or different, represent C 1-12 alkyl radicals; R 1 independently represents: (a) a C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl radical; (b) a fused or non-fused monocyclic or polycyclic aryl radical, comprising 6 to 50 carbon atoms; (c) a fused or non-fused monocyclic or polycyclic heteroaryl, comprising 4 to 50 carbon atoms; R 1 optionally bearing one or more substituents independently selected from a halogen atom, -OH, -NH 2, -NO 2 or C 1-6 alkyl; and q represents an integer from 2 to 6; preferably q represents 2, 3 or 4; each occurrence of R 3 is independently selected from a halogen atom, - OH, -OR 4, -O-C(=O)R 4 or -NR 4 R 4', wherein R 4 and R 4', identical or different, represent C 1-12 alkyl radicals; R 1 independently represents: (a) a C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl radical; (b) a fused or non-fused monocyclic or polycyclic aryl radical, comprising 6 to 50 carbon atoms; (c) a fused or non-fused monocyclic or polycyclic heteroaryl, comprising 4 to 50 carbon atoms; (a) a C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl radical; (b) a fused or non-fused monocyclic or polycyclic aryl radical, comprising 6 to 50 carbon atoms; (c) a fused or non-fused monocyclic or polycyclic heteroaryl, comprising 4 to 50 carbon atoms; R 1 optionally bearing one or more substituents independently selected from a halogen atom, -OH, -NH 2, -NO 2 or C 1-6 alkyl; and B) allowing the mixture obtained in step A) to react at a temperature below the boiling point and reflux temperature of the aqueous solvent system; so as to obtain the said nanoparticles;