DNP-SENS Formulation Protocols To Study Surface Sites in Ziegler–Natta Catalyst MgCl₂ Supports Modified with Internal Donors

Abstract

Heterogeneous Ziegler−Natta propylene polymerization catalysts are today responsible for the production of over 90% of polypropylene worldwide. These catalysts comprise an organic internal donor (ID) that coordinates to the MgCl 2 support and is key for the formation of active sites with high stereoselectivity. Yet, little is experimentally known about the details of the interaction between the internal donor and the support. Of various spectroscopic techniques, 13 C magic angle spinning solid-state NMR spectroscopy should be a method of choice. However, NMR spectroscopy displays intrinsic low sensitivity, which becomes more critical when addressing surface sites that correspond to only a fraction of the sample (1−2%), hence the difficulty to implement the powerful 2D NMR approaches that enable detailed structure determination. In order to increase the sensitivity of NMR and for surface sites in particular, dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) was developed to reveal the structural details of surface sites. Here, we use this approach to study the interaction of selected internal donors (1,3-dimethoxy-2,2-dimethylpropane, diisobutylphthalate, di-n-butylphthalate) with the MgCl 2 support. Toward this goal, we first investigated factors affecting DNP enhancements for signals corresponding to the internal donor on the support and developed so-called DNP formulation protocol− sample preparation procedure that leads to highest DNP enhancements. Scanning through the effects of internal donor loading, the number of −CH 3 groups in the internal donor and their deuteration, as well as MgCl 2 particle size allowed us to achieve surface DNP enhancements of 40 on a 600 MHz NMR spectrometer, which corresponds to 1600-fold experimental time savings. This opened up a possibility for remarkably fast 1D and 2D NMR experiments such as 1 H− 13 C HETCOR NMR, 2 H MAS NMR, and 1 H{ 35 Cl} REDOR MAS NMR. Analysis of the results has helped to reveal the NMR signatures of internal donors on the support and to obtain valuable information about the reorientation of the internal donor and its distance to the surface of the support.

Understanding the Role of Sulfonyl Amine Donors in Propylene Polymerization Using MgCl2-Supported Ziegler–Natta Catalyst

“…More importantly, the suitable internal donor and external donor pair can enhance the stereoregularity of the polymer product. The internal donor is typically added during the catalyst preparation, e.g., ethyl benzoate (EB) (third generation), dialkyl phthalate (fourth generation), 1,3-diether (fifth generation), and dialkyl succinate (sixth generation). − The external donor is added during the olefin polymerization process in combination with the cocatalyst AlR 3 , e.g., silane, and alkoxysilane. − The appropriate internal donor and external donor pair can control the regularity of the polypropylene and greatly increase the yield of the isotactic polypropylene. , In addition, the alkylation product AlR n Cl 3– n generated from AlR 3 could also be stably coordinated in the vicinity of the Ti active site, promoting the stereo- and regioselectivity of propylene insertion. , …”

The atomic defects on the (104) and (110) surfaces of the MgCl2-supported Ziegler–Natta catalyst: a periodic DFT study

“…So far, most studies have focused on the ideal (110) and (104) MgCl 2 surfaces. 19–34 However, it should be noted that surface defects, such as edges, corners, vacancies, steps, etc. , could be formed during the synthesis of the Ziegler–Natta catalyst under extremely drastic conditions.…”

“…So far, most studies have focused on the ideal (110) and (104) MgCl 2 surfaces. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, it should be noted that surface defects, such as edges, corners, vacancies, steps, etc., could be formed during the synthesis of the Ziegler-Natta catalyst under extremely drastic conditions. 16,18,[35][36][37][38][39][40][41] Experiments have confirmed the existence of edge crystallites on the MgCl 2 support by atomic force microscopy (AFM), scanning electron microscopy (SEM), and HR-TEM.…”

The Journal of Physical Chemistry C Virtual Special Issue on Advanced Characterization by Solid-State NMR and In Situ Technology and in Recognition of Michael Hunger’s 65th Birthday

“…The Fu group used high-resolution solid-state NMR to understand the mechanism of the dielectric transition and molecular dynamics of the MOF–dimethylammonium host–guest system . Copéret and coworkers achieved surface dynamic nuclear polarization (DNP) enhancements of 40 on a 600 MHz NMR, corresponding to 1600-fold experimental time savings for Ziegler–Natta catalyst MgCl 2 supports . Solid-state NMR was used by the Dubini group to describe the hydration-induced structural transitions in biomimetic tandem repeat proteins and the cis–trans isomerization of groups at the water–protein interface .…”

“…3 Copeŕet and coworkers achieved surface dynamic nuclear polarization (DNP) enhancements of 40 on a 600 MHz NMR, corresponding to 1600-fold experimental time savings for Ziegler−Natta catalyst MgCl 2 supports. 4 Solid-state NMR was used by the Dubini group to describe the hydrationinduced structural transitions in biomimetic tandem repeat proteins and the cis−trans isomerization of groups at the water−protein interface. 5 The stereoselective hydrogenation of propyne to propene over Pd−Ag/Al 2 O 3 catalysts has been studied by Xu and coworkers using parahydrogen-induced polarization (PHIP) NMR techniques.…”