High-Pressure Chemistry in Nanoconfined Systems

High-pressure polymerization of simple molecules (C2H2, C2H4, C6H6, CO2, N2, etc.) is a hot topic in chemistry, condensed matter physics, and materials science [Chelazzi, Ceppatelli]. For instance, polyacetylene (PA) is the textbook system for monodimensional, conducting polymers, while polyCO (pCO) is regarded as a very promising high energy density material. In our studies, we have tried to exploit the effect of high density and highly confined conditions in controlled dimensionality to drive reactivity and to obtain stable, linear polymer chains embedded in protecting host systems with the purpose of synthesizing new materials with strong directional properties. We have selected two different synthetic, non catalytic and electrically neutral pure SiO2 zeolites, silicalite and ZSM-22 (TON). Silicalite has a 3D pore network, due to the mutual intersection of two different sets of nano-channels with a diameter of about 5.5 Å [Artioli]; ZSM-22 (TON) has a 1D structure, and a single set of elliptical shaped linear channels (4.6 x 5.7 Å) [Di Renzo]. Driving the polymerization of ethylene, acetylene and CO inside silicalite, using only relatively low pressure conditions (between 5 and 10 GPa) or mild UV laser irradiation (few milliwatts of 350 nm wavelength radiation), we obtained products termed PESIL (PolyEthylene filled SILicalite) [Santoro], PASIL (PolyAcetylene filled SILicalite) [Scelta] and pCOSIL (pCO filled SILicalite) [Santoro2]. Then we have studied the polymerization of acetylene and CO inside TON. From these reactions, we obtained two new materials called PA/TON (PolyAcetylene filled TON) and pCO/TON (polyCO filled TON), where the guest polymers have a strict planar conformation, which is most likely due to the elliptical shape of the host channels, and are mainly in the cis- configuration, although PA is affected by some degree of disorder within the plane. Remarkably, confined pCO is chemically ordered in contrast to bulk pCO observed so far, which is structurally and chemically disordered. On a general ground, our protocols indicate new chemical routes for synthesizing advanced nanocomposite materials with directional properties, where an inorganic scaffold drives the formation, supports, and protects from the atmosphere an embedded organic polymer with selected low dimensionality. We have performed our experiments using membrane Diamond Anvil Cells (mDAC), working with optical (FTIR, micro-Raman) and structural (x-ray diffraction, XRD) techniques. In order to investigate different regions in nanocomposite samples, we have exploited high spatial resolution techniques provided by synchrotron radiation facilities (micro-FTIR and micro-XRD). Computational methods (DFT based calculation) have been used to model the properties of our nanocomposites products. [Chelazzi] D. Chelazzi, et al. High-pressure synthesis of crystalline polyethylene using optical catalysis. Nat. Mater. 2004, 3, 470-475. [Ceppatelli] M. Ceppatelli, et al. Fourier trans-form infrared study of the pressure and laser induced polymerization of solid acetylene. J. Chem. Phys. 2000, 113, 5991-6000. [Artioli] G. Artioli, et al. Neutron powder diffraction study of orthorhombic and monoclinic defective silicalite. Acta Cryst. B, 56, 2-10, 2000. [Di Renzo] F. Di Renzo, et al. Crystallization kinetics of zeolite TON. Zeolites 1991, 11, 539-548. [Santoro] M. Santoro, et al. A. High-pressure synthesis of a polyethylene/zeolite nano-composite material. Nat. Commun. 2013, 4, 1557, DOI: 10.1038/ncomms2564. [Scelta] D. Scelta, et al. High Pressure Polymerization in a Confined Space: Conjugated Chain/Zeolite Nanocomposites. Chem. Mater. 2014, 26, 2249-2255. [Santoro2] M. Santoro, et al. High Pressure Synthesis of All-Transoid Polycarbonyl [--(C?O)--]n in a Zeolite. Chem. Mater. 2015, 27, 6486-6489.