Based on ISO SWS+LWS observations, we identify IRAS 16594-4656 as a new member of the rare class of C-rich proto-planetary nebulae with the 21 μm dust feature in emission. In addition, the major PAHs emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 μm are detected, together with unusually strong features at 12.6 and 13.4 μm , very rarely observed, attributed to PAHs species with a high degree of hydrogenation. The shapes and relative intensities of the various features found are consistent with a mixed population of transient decomposition products of large hydrogenated amorphous carbonaceous (HAC) grains, consisting of fullerenes with a different degree of hydrogenation, which we suggest to be the main carriers of the strong 21 μm feature, and a combination of small and large partially dehydrogenated cationic PAHs molecules excited by the visible radiation coming from the central star. Additional features corresponding to crystalline silicates (mostly pyroxenes) are also detected. The combination of features corresponding to O-rich and C-rich dust grains found, if real, would suggest a recent change to a C-rich chemistry in the outer envelope of IRAS 16594-4656.