Depolymerization of Polytrimethylene Terephthalate in Sub-Critical Water

Phase behavior of polytrimethylene terephthalate (PTT) in sub-criticalwater with and without catalyst was investigated in a fused silica capillaryreactor (FSCR). Depolymerization of PTT in sub-critical water with andwithout catalyst was studied in a batch autoclave reactor. Thedepolymerization products of PTT were analyzed qualitatively andquantitatively by fourier-transform infrared spectroscopy (FT-IR), gaschromatography mass spectrometry (GC-MS), liquid chromatography massspectrometry (LC-MS), gas chromatography (GC), and high performanceliquid chromatography (HPLC). The influence of reaction conditions on thedepolymerization yield of PTT and yields of main products terephthalicacid (TPA) and1,3-propanediol (1,3-PDO) was investigated. According tothe analysis of depolymerization products and the results of phase behavior,the depolymerization mechanism of PTT in sub-critical water with andwithout catalyst was proposed, respectively.Phase changes of PTT in water and aqueous zinc acetate solution wereinvestigated in FSCR. The results showed that in the two reaction mediums,with reaction temperature and time increasing, PTT gradually melted and dissolved in water, ultimately became a homogeneous aqueous solution.Meanwhile, in FSCR, the vapor-liquid-solid phases turned intovapor-liquid-liquid phases and finally became vapor-liquid phases withincreasing reaction temperature and time. In the cooling process, TPAgradually separated out from the water and the size and number of TPAcrystals increased gradually in the FSCR. The results in the FSCR providea basis for the choices of reaction conditions and the studies of mechanismof PTT depolymerization in sub-critical water with and without catalyst.Depolymerization of PTT in sub-critical water was studied under theconditions of mass ratio of water to PTT (4.0-12.0), reaction temperature(240-320℃), reaction pressure (3.1-10.6MPa) and reaction time (5-60min).The results showed that the depolymerization of PTT were mainlyinfluenced by reaction temperature and time, while it had little relationshipwith the mass ratio of water to PTT. Under the optimal reaction conditions,i.e., water/PTT ratio of8:1(w/w), reaction temperature300℃, and reactiontime15min, complete depolymerization was achieved, and the yields ofTPA and1,3-PDO reached90.5%and69.0%, respectively. The results ofkinetic analysis showed that the activation energy of PTT depolymerizationwas111.5kJ·mol-1.Depolymerization of PTT in sub-critical water was studied under theconditions of mass ratio of water/PTT (8.0), reaction temperature (230-260℃), reaction pressure (2.5-4.5MPa), reaction time (5-60min). and mass of sodium hydroxide or zinc acetate (0-0.075g). The results showed that zincacetate had the advantage in depolymerization of PTT in sub-critical watercompared to sodium hydroxide. The depolymerization yield of PTT andproducts yields mainly depended on catalyst, reaction temperature and time.Under the optimal reaction conditions, i.e., water/PTT ratio of8:1(w/w),the mass of zinc acetate0.045g, reaction temperature240℃, and reactiontime60min, PTT could be totally depolymerized and the yields of TPA and1,3-PDO attained90.1%and75.2%, respectively. The results of kineticanalysis showed that the activation energy of PTT catalyticdepolymerization was96.9kJ·mol-1. It meaned that zinc acetate wasbeneficial to the depolymerization of PTT in sub-critical water.