(165h) Simulating the Fate of Carbon Precursors in Mesoporous Silica Material Using Reactive Molecular Dynamics

Mesoporous silica materials have attracted considerable attention in the past for their applications in catalysis, sorption, separation, hydrogen storage, oil & gas extraction, etc. and as host matrices for optically and electronically active components [1]. In the recent past, mesoporous silica materials (MSMs) have also shown tremendous potential in drug delivery technologies. Their superior thermophysical, mechanical, and chemical properties such as variable pore size, pore morphology, and surface functionalizability create avenues for optimized interactions between drug molecules and carriers. The small pores present in MSMs serve the purposes of diffusion and adsorption processes in general and therefore, it is important to keep the small pores intact.

MSMs can be used at very high temperature for processes like oil extraction and storage. These materials are synthesized by calcinating polymeric carbonaceous materials at a very high temperature to create small pores. A complete carbonization of these materials will lead to distinct pores in the materials suitable for the synthesis process. To keep the small pores intact, therefore we choose a scheme to fill the small pores with various polymeric materials. The polymeric materials are carbon precursors which needs to be chosen carefully to maximize the products formed due to high temperature carbonization. We consider three different carbon precursors for the carbonization process inside the silica pore: polyethylene, cellulose and lignite. The presence of O containing groups within the precursors definitely initiates the carbonization process faster. Therefore, we consider blends too for our study purposes as they can be a promising alternative for the carbonization.

We use ReaxFF [2] molecular dynamics tool to study the carbonization process as it can study covalent bond making and breaking. The study is done at three different temperatures: 2200 K, 2500 K and 2800 K. The density of the hydrocarbons is considered to be within the range of 0.50 – 2.00 g/cm³. Figure 1a shows a system configuration for high temperature carbonization of precursors insides silica pore. We analyze the carbonization of hydrocarbons in the silica pore on the basis of formation of carbon ring and volatile gases (Figure 1b) and their correlations with the evolution of O containing groups. We inspect the functionalities of O-containing groups at atomistic scale which enable us to reveal the mechanisms of their evolutions from functional groups to volatile gases. Particularly, O-containing groups are more efficient for initiating the carbonization reactions, hence they are responsible for the formations of large graphitic ring networks after the carbonization initiation. In particular, we calculate the basal plane and edge carbon of the graphitic structures formed from carbonization. In order to examine the morphology of the silica pore at high temperature, we analyze the Silicon-Carbon linkages at the surface of the pore and find the void distribution during the process. We aim to investigate the best carbon precursor for the high temperature carbonization inside silica pores on the basis of pre-oxidation process, carbon ring formation and fast conversion rate.

References

[1] Asefa and Tao, Can. J. Chem., 2012, 90, 1015.

[2] van Duin et al., JPC-A, 2001, 105, 9396.