Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae
Abstract
:1. Introduction
2. Materials and Methods
2.1. Reagents
2.2. Polymerization and Quenching Reaction
2.3. Characterization
3. Results and Discussion
3.1. Polymerization Kinetics
3.2. Morphology of the Polymer/Catalyst Particles
3.3. Polymer Aggregation State in Nascent Polymer Particle
- Changes of active center concentration in the initial stage (0–10 min) clearly show that the lower rate of ethylene polymerization compared with that of propylene can be attributed to a much slower build-up of [C*] in the former system.
- Both polymerization systems experienced a similar degree of diffusion limitation in the first 0–3 min, as shown by the larger apparent propagation rate constant in ethylene polymerization and similar slopes of the kp versus mP/mCat curves. The porosity of PE/catalyst particles was larger than that of the PP/catalyst particles. The lower activity of ethylene polymerization cannot be attributed to its stronger diffusion limitation.
- The catalyst particles have rather compact solid structure, though they are composed of sub-particles with a size of about 200–500 nm, and there are cracks with widths ranging from 100 nm to 5 µm. Pore size distribution, determined by the nitrogen adsorption method, shows that the nanometer pores in the catalyst are concentrated in the range of 15–25 nm. There is a huge number of such small pores, which renders the catalyst a very large specific surface area (282 m2/g) and high porosity (0.32 cm3/g). These 20 nm pores should be uniformly scattered in the solid phase of the catalyst. Assuming that the sub-particles are dense cubes with edges of 200 nm and a density of 2.34 g/cm3 (density of MgCl2 crystal), their aggregate will have a specific surface area of 13 m2/g, which is far lower than the measured specific surface area. The measured value of 282 m2/g will correspond to MgCl2 crystallite size (length of cube edges) of about 9 nm. This size is quite close to that of MgCl2 crystallites (7 nm) in supported Z–N catalysts determined by E. Redzic et al. [53]. Therefore, the 200–500 nm sub-particles cannot be dense solid, but rather aggregates of smaller MgCl2 crystallites containing many nanopores. It is likely that there is a large number of pores and cracks of about 20 nm in the sub-particles.
- Because the sizes of PE lamellae formed by the growing polymer chains are far larger than the size of nanopores in the catalyst’s sub-particles, these lamellae cannot grow inside the nanopores, leaving the porous sub-particles basically intact during ethylene polymerization. Only the active sites exposed on the outer surface of the sub-particles can be activated and work as catalytic centers, but a large proportion of active site precursors is buried in the sub-particles and thus becomes unavailable to the polymerization reaction, resulting in a low [C*]/[Ti] ratio of ethylene polymerization. With the proceeding of polymerization, the PE layer covering the sub-particle will form a diffusion barrier that grows quickly with the increase of the mP/mCat ratio, and finally leads to ceasing of the polymerization.
- In the propylene polymerization system, the PP lamellae with a size of 6–11 nm can enter the 20 nm pores in the sub-particles. Growth of these PP lamellae in the pores can exert hydraulic forces strong enough to break up the sub-particles and release their buried active site precursors. Subsequently, the 20 nm pores in the sub-particles will disappear, and the exposed surfaces carrying active sites will be covered by PP chains. After a short period of time, the whole polymer/catalyst particle will become rather compact, and the texture of the particle becomes rather smooth. Though the PP layer covering the MgCl2 crystallites (carrier of the active sites) can also cause serious diffusion barrier, for the much higher density of active sites in this system compared with the ethylene polymerization, the dynamically renewed carrier surface can allow for the presence of tiny pores in the PP layer. This will enable slow but stable diffusion of monomer stream in the PP layer, and a stable polymerization rate supported by a high [C*]/[Ti] ratio and low apparent rate constant.
4. Conclusions
Supplementary Materials
Author Contributions
Funding
Conflicts of Interest
References
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Run | tpb (s) | mP/mCatc (g/g) | Activity (kg/g Ti·h) | Mwd (105) | Đ d | Rp (10−3 mol/L·s) | [C*]/[Ti] (%) | kp (L/mol·s) |
---|---|---|---|---|---|---|---|---|
E1 | 30 | 0.17 | 0.76 | 3.34 | 10.6 | - e | - e | - e |
E2 | 60 | 0.35 | 0.78 | 3.71 | 8.1 | 0.32 | 0.20 | 2035 |
E3 | 120 | 0.65 | 0.72 | 4.26 | 16.7 | 0.48 | 0.25 | 2410 |
E4 | 180 | 1.12 | 0.83 | 5.79 | 12.6 | 0.54 | 0.49 | 1380 |
E5 | 240 | 1.67 | 0.93 | 5.51 | 9.5 | 0.45 | 0.51 | 1103 |
E6 | 480 | 2.38 | 0.66 | 5.67 | 14.0 | 0.04 | 0.55 | 85 |
E7 | 600 | 2.39 | 0.53 | 6.21 | 10.8 | 0.01 | 0.59 | 17 |
P1 | 30 | 0.36 | 1.61 | 1.51 | 5.3 | 2.27 | 0.37 | 1433 |
P2 | 60 | 1.56 | 3.48 | 1.57 | 5.7 | 2.27 | 1.50 | 356 |
P3 | 120 | 5.44 | 6.05 | 1.59 | 6.2 | 2.27 | 3.10 | 172 |
P4 | 180 | 8.05 | 5.97 | 1.49 | 5.3 | 2.27 | 3.25 | 164 |
P5 | 240 | 11.17 | 6.20 | 1.24 | 5.6 | 2.27 | 3.35 | 160 |
P6 | 480 | 25.12 | 6.98 | 1.25 | 5.3 | 2.27 | 4.51 | 118 |
P7 | 600 | 30.34 | 6.74 | 1.27 | 5.5 | 2.27 | 4.87 | 110 |
Sample | Specific Surface Area (m2/g) | Total Pore Volume (cm3/g) | Average Pore Size (nm) |
---|---|---|---|
Cat. | 281.55 | 0.320 | 22.37 |
E3 | 34.84 | 0.065 | 37.95 |
E4 | 51.85 | 0.090 | 33.36 |
P1 | 2.79 | 0.013 | 95.80 |
P2 | 1.55 | 0.009 | 117.44 |
Run | Polymer | Tma (°C) | ∆Hf b (J/g) | Xcc (%) |
---|---|---|---|---|
E1 | PE | 139.5 | 228.8 | 79.4 |
E2 | 140.1 | 208.5 | 72.4 | |
E3 | 139.9 | 201.1 | 69.8 | |
E4 | 141.8 | 200.7 | 69.7 | |
P1 | PP | 161.0 | 86.7 | 56.3 |
P2 | 161.6 | 83.4 | 54.2 | |
P3 | 161.0 | 79.8 | 51.8 | |
P4 | 161.1 | 69.2 | 44.9 |
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Zhang, Z.; Jiang, B.; He, F.; Fu, Z.; Xu, J.; Fan, Z. Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae. Polymers 2019, 11, 358. https://doi.org/10.3390/polym11020358
Zhang Z, Jiang B, He F, Fu Z, Xu J, Fan Z. Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae. Polymers. 2019; 11(2):358. https://doi.org/10.3390/polym11020358
Chicago/Turabian StyleZhang, Zhen, Baiyu Jiang, Feng He, Zhisheng Fu, Junting Xu, and Zhiqiang Fan. 2019. "Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae" Polymers 11, no. 2: 358. https://doi.org/10.3390/polym11020358
APA StyleZhang, Z., Jiang, B., He, F., Fu, Z., Xu, J., & Fan, Z. (2019). Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae. Polymers, 11(2), 358. https://doi.org/10.3390/polym11020358