A New Pressure‐induced Mechanism of Polyvinyl Chloride Pyrolysis with Formation of Nanodiamonds

Abstract

Application of high pressure to molecular compounds with high compressibility can initiate chemical reactions, unexpected under normal conditions. Here we present the evidence of pressure‐induced change in the mechanism of dehydrochlorination at polyvinyl chloride (PVC) pyrolysis. PVC pyrolysis was investigated at pressures from 2 to 9 GPa and temperatures up to 1600 °C. At critical pressures of 8–9 GPa, decomposition of PVC starts at 500 °C and results in formation of ultrasmall nanodiamonds. Based on density functional theory calculations, we show that intermolecular linking by single C−C bonds during dehydrochlorination is responsible for the scenario of carbonization, leading to nanodiamonds formation. At decreased pressure, carbonized product consists mainly of graphite‐like carbon, implying that high‐pressure intermolecular dehydrochlorination of PVC becomes inferior to “normal”, intramolecular one. Our study suggests that at low pressures, random intermolecular linking of PVC with formation of single HCl molecules may trigger “normal”, HCl‐catalyzed dehydrochlorination.

Size-Dependent Thermal Stability and Optical Properties of Ultra-Small Nanodiamonds Synthesized under High Pressure

“…Firstly, we would like to discuss possible mechanism leading to easy formation of nanodiamonds from halogenated adamantanes in comparison to pure adamantanes. As it was shown before for the case of PVC molecules (in fact short fragments of PVC molecules) [ 21 ] the reaction involves several steps of pyrolysis accompanied by dehydrohalogenation of initial molecules, their polymerization, and further halogenation of 2D/3D polymers. This promotes subsequent dehydrohalogenation and even higher degree of polymerization.…”

“…Combined consideration of X-ray diffraction and mass loss data and taking into account well-studied pyrolysis of polyvinyl chloride (PVC) at normal pressures [ 21 ] suggest that the decomposition of chloradamantane C H Cl at temperatures above 500 C could start from dehydrochlorination, subsequent intermolecular linking, resulting in formation of single C–C bonds allowing nucleation of the smallest stable 1 nm nanodiamonds. However, the reaction of chloroadamantane molecule is probably more complicated in comparison to PVC, where mass loss during synthesis indicates the simple dehydrohalogenation route with no loss of carbon atoms.…”

“…It requires breaking of only 3 C–C bonds per two adamantane molecules and reconstruction of C–H and C–Cl bonds. It seems that it is the rupture of a strong C–C bond that hinders the kinetics of this reaction and requires higher temperatures in comparison to the PVC case, which involves only dehydrohalogenation-mediated pyrolysis [ 21 ].…”

“…Recent breakthroughs in nanodiamond synthesis in halogenated hydrocarbon systems at high static pressures (HPHT) enabled precise control of the ND size and made possible the growth of nanodiamonds with desired dimensions starting from values as small as 1 nm [ 17 , 18 , 19 , 20 , 21 ]. This provides an excellent opportunity for the refinement of thermal, structural, optical, and other properties of diamond on the ultra-small scale, which is important for the understanding and revealing of their prospects in future quantum and biomedical technologies.…”

“…In the current paper we present a study of the size-dependent thermal stability/graphitization and optical properties of nanodiamonds synthesized in the growth medium comprising C-H-Cl. A distinguishing feature of nanodiamonds produced by this approach is their surface hydrogenation [ 17 , 20 , 21 ]. We propose that, contrary to the recently published preprint [ 22 ], the surface of the ultra-small nanodiamonds grown from hydrocarbon precursor is largely H-free, is reconstructed, and consists of contiguous network of sp hybridized carbons with patches of sp hydrogenated surface carbon atoms.…”