Spectroscopic Fingerprints of MgCl2/TiCl4 Nanoclusters Determined by Machine Learning and DFT

Maddalena D’Amore,Fabien Pascale,Gentoku Takasao,Toshiaki Taniike,Hiroki Chikuma,Toru Wada,Anna Maria Ferrari

doi:10.1021/acs.jpcc.1c05712

Abstract

Understanding the structure and properties of MgCl2/TiCl4 nanoclusters is a key to uncovering the origin of Ziegler–Natta catalysis. In particular, vibrational spectroscopy can sensitively probe the morphology and active species of MgCl2/TiCl4. Here, we determined vibrational spectroscopic fingerprints of 50MgCl2 and 50MgCl2/3TiCl4 which were obtained by nonempirical structure determination based on an evolutionary algorithm and DFT. The adsorption of CO, TiCl4, and Ti2Cl8 dimers was also modeled on each of the coordinatively unsaturated Mg2+ sites available for binding including so-called defect sites, which are likely present at the surface of activated MgCl2 nanocrystals and plausible sites for strong TiCl4 species adsorption. The outcomes of thermodynamical and vibrational analysis were compared to results on ideal surfaces of MgCl2. Vibrational analysis (IR and Raman) on plausible models of TiCl4/ MgCl2 nanoclusters revealed that IR response is useful for distinguishing between the different ways of binding of TiCl4 on different sites of adsorption, whereas Raman response provides a clear fingerprint of supported TiCl4 species.

Highlights

The structural unit of the Ziegler−Natta catalyst (ZNC) is nanostructured and disordered MgCl2 whose lateral surfaces are capped by TiCl4 and Lewis bases
More recent DFT studies have claimed the crisis of those models, warning about the critical thermodynamic stability of TixCly species on flat and regular MgCl2 surfaces and moving toward a much more complex morphology particles.[3−11] Recent theoretical calculations of the δ-MgCl2 on MgCl2 bulk and surface structure by means of periodic density functional theory methods including dispersion (DFT-D*) predicted that well-formed MgCl2 crystals in the absence of adsorbates predominantly feature the (104) surfaces; the situation drastically changes in the presence of adsorbates
DFT-D simulations[12,13] demonstrated that MgCl2 crystals mainly expose the (110) surfaces when they are formed in the presence of Lewis bases or in the presence of alkoxysilanes adopted as external donors

Summary

INTRODUCTION

The structural unit of the Ziegler−Natta catalyst (ZNC) is nanostructured and disordered MgCl2 whose lateral surfaces are capped by TiCl4 and Lewis bases (called internal donors). Going beyond the too simplified surface models present in literature, here we adopted the well assessed DFT methodology previously employed for surfaces while stepping forward to a more realistic model of MgCl2 nanoclusters by means of the nonempirical structure determination On these nanostructures we carried out vibrational far-IR and Raman analysis to discuss to what extent exposed tetra- and penta-Mg sites are distinguishable and, even more interestingly, the possibility of recognizing (i) the effect of adsorption of different precatalytic complex TixCly on IR and/or Raman bands, (ii) to what extent it is possible to distinguish between. With this target in mind, we identified spectroscopic IR features and more Raman bands specific to each adduct as possible fingerprints of Ziegler−Natta catalysts

COMPUTATIONAL MODELS AND DETAILS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES