The industrial production of isotactic polypropylene (i-PP), which now exceeds 60 million metric tons per year, is dominated by MgCl2-supported Ti-based Ziegler–Natta catalysts (ZNC). Despite 60 years of history and a stunning success, these heterogeneous catalyst formulations, which also include several organic electron donor compounds, remain black-box systems, and progress still relies on trial and error, due to the great complexity of the chemical and physical variables and the heavy nonlinearity of their effects on overall catalytic performance.
Recently, High Throughput Experimentation (HTE) approaches have been developed to determine the relationships between the formulation of ZNC and stereo-selectivity. Measuring regio-selectivity is at least as important, but much more difficult because all ZNC are exceedingly regioselective – in typical cases >99.8% in favor of 1,2 (‘primary’) propene insertion – and the few regiodefects (i.e., isolated 2,1 monomeric units) are below detectability by means of standard 13C NMR spectroscopy.
On the other hand, it is well-recognized that even tiny amounts of regiodefects in i-PP chains have dramatic effects on the catalytic reaction, because they dictate the so-called ‘hydrogen response’, that is the ease with which the growing i-PP chains are cleaved by molecular hydrogen (H2) added to the reaction phase. As a matter of fact, it has been demonstrated that 2,1 last-inserted monomeric units are poorly reactive towards propene insertion, and therefore have a ‘dormant’ character in chain propagation, whereas they are amenable to fast chain transfer by H2 and thus represent the main chain cleavage points. Therefore, measuring regioselectivity as a function of catalyst system formulation is a very important target.
Researchers at Federico II University of Naples: Antonio Vittoria, Alessio Mingione, Raffaele Andrea Abbate, Professor Roberta Cipullo, and Professor Vincenzo Busico disclosed a novel and conceptually straightforward HTE method to solve this long-standing problem. The work is currently published in the research journal, Industrial & Engineering Chemistry Research.
In brief, the proposed method combines the advantages of parallel HTE mini-reactors and the sensitivity of the latest high-temperature cryoprobe 13C NMR technology in order to quantify H2-generated chain ends of i-PP samples produced at increasing H2/propene pressure ratio, and determine the upper limit of the function which is demonstrated to coincide with catalyst regioselectivity. The method was successfully benchmarked with three representative industrial ZNC.
The study will pave the way for rapid and accurate measurements of Quantitative Structure-Activity Relation (QSAR) databases of regioselectivity and hydrogen response for ZNC as a function of their formulation, and the subsequent use thereof to train statistical models with predictive ability. A first application of the described protocol to a vast set of industrial catalysts has already been reported in a paper published in ACS Catalysis (2020, 10, 644-651).
In a statement to Advances in Engineering, Professors Busico and Cipullo are confident that their study will contribute to a better understanding of this class of catalysts, aiming to further advance the industrial production of i-PP-based materials.
Vittoria, A., Mingione, A., Abbate, R., Cipullo, R., & Busico, V. (2019). High Throughput Experimentation Protocol for Quantitative Measurements of Regioselectivity in Ziegler–Natta Polypropylene Catalysis. Industrial & Engineering Chemistry Research, 58(32), 14729-14735.