Influence of Pore Structure and Metal-Node Geometry on the Polymerization of Ethylene over Cr-Based Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have received increasing interest as solid single-site catalysts, owing to their tunable pore architecture and metal node geometry. The ability to exploit these modulators makes them prominent candidates for producing polyethylene (PE) materials with narrow dispersity index (Ð) values. Here a study is presented in which the ethylene polymerization properties, with Et2AlCl as activator, of three renowned Cr-based MOFs, MIL-101(Cr)-NDC (NDC=2,6-dicarboxynapthalene), MIL-53(Cr) and HKUST-1(Cr), are systematically investigated. Ethylene polymerization reactions revealed varying catalytic activities, with MIL-101(Cr)-NDC and MIL-53(Cr) being significantly more active than HKUST-1(Cr). Analysis of the PE products revealed large Ð values, demonstrating that polymerization occurs over a multitude of active Cr centers rather than a singular type of Cr site. Spectroscopic experiments, in the form of powder X-ray diffraction (pXRD), UV/Vis-NIR diffuse reflectance spectroscopy (DRS) and CO probe molecule Fourier transform infrared (FTIR) spectroscopy corroborated these findings, indicating that indeed for each MOF unique active sites are generated, however without alteration of the original oxidation state. Furthermore, the pXRD experiments indicated that one major prerequisite for catalytic activity was the degree of MOF activation by the Et2AlCl co-catalyst, with the more active materials portraying a larger degree of activation

Ethylene Polymerization over Metal-Organic Framework Crystallites and the Influence of Linkers on Their Fracturing Process

The physical properties and morphologies of polymers are pivotal for their manufacturing and processing at the industrial scale. Here, we present the formation of either fibers or micrometer-sized polyethylene beads by using the MIL-100(Cr) and MIL-101(Cr) zeotypes. The MOF structures have been used for ethylene polymerization with diethylaluminum chloride (DEA) as a cocatalyst, resulting in very different activities and morphologies. In situ DR UV–vis–NIR and CO-probe FT-IR spectroscopy revealed the formation of different types of Cr species for each catalyst material, suggesting that the linker (for the same metal and topological structure) plays a crucial role in the formation of Cr olefin polymerization sites. Activity in ethylene polymerization in toluene at 10 bar and 298 K was related to the observed spectra, corroborating the presence of different types of active sites, by their different activities for high-density polyethylene (HDPE) formation. SEM micrographs revealed that although MIL-100 and MIL-101 exhibit identical zeolitic MTN topology, only the latter is able to collapse upon addition of DEA and subsequent ethylene insertion and to fracture forming polymer beads, thus showing noticeable activity in HDPE formation. We ascribed this effect to the higher pore volume and, thus, fragility of MIL-101, which allowed for polymer formation within its larger cages. MOFs were compared to the nonporous chromium(III) benzoate [Cr3O(O2CPh)6(H2O)2](NO3)·nH2O complex (1), in order to study the effect of the embodiment in the porous framework. The properties of the polymer obtained under identical reaction conditions were comparable to that of MIL-101(Cr) but very different morphologies were observed, indicating that the MIL-101(Cr) structure is necessary to impart a certain architecture at the microscale. This work clearly shows that MOFs can be used as catalytically active morphology regulators for ethylene polymerization. Moreover, even for an identical topology and metal in a MOF structure, the linker and the pore structure play crucial roles and have to be carefully considered in the design microporous coordination polymers for catalytic purposes

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal-alkyl co-catalyst. However, metal-alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal-alkyl co-catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in-situ UV-Vis-NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri-ethyl borane (TEB) or tri-ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal-alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics

Ethylene Polymerization over Metal-Organic Framework Crystallites and the Influence of Linkers on Their Fracturing Process

The physical properties and morphologies of polymers are pivotal for their manufacturing and processing at the industrial scale. Here, we present the formation of either fibers or micrometer-sized polyethylene beads by using the MIL-100(Cr) and MIL-101(Cr) zeotypes. The MOF structures have been used for ethylene polymerization with diethylaluminum chloride (DEA) as a cocatalyst, resulting in very different activities and morphologies. In situ DR UV–vis–NIR and CO-probe FT-IR spectroscopy revealed the formation of different types of Cr species for each catalyst material, suggesting that the linker (for the same metal and topological structure) plays a crucial role in the formation of Cr olefin polymerization sites. Activity in ethylene polymerization in toluene at 10 bar and 298 K was related to the observed spectra, corroborating the presence of different types of active sites, by their different activities for high-density polyethylene (HDPE) formation. SEM micrographs revealed that although MIL-100 and MIL-101 exhibit identical zeolitic MTN topology, only the latter is able to collapse upon addition of DEA and subsequent ethylene insertion and to fracture forming polymer beads, thus showing noticeable activity in HDPE formation. We ascribed this effect to the higher pore volume and, thus, fragility of MIL-101, which allowed for polymer formation within its larger cages. MOFs were compared to the nonporous chromium(III) benzoate [Cr3O(O2CPh)6(H2O)2](NO3)·nH2O complex (1), in order to study the effect of the embodiment in the porous framework. The properties of the polymer obtained under identical reaction conditions were comparable to that of MIL-101(Cr) but very different morphologies were observed, indicating that the MIL-101(Cr) structure is necessary to impart a certain architecture at the microscale. This work clearly shows that MOFs can be used as catalytically active morphology regulators for ethylene polymerization. Moreover, even for an identical topology and metal in a MOF structure, the linker and the pore structure play crucial roles and have to be carefully considered in the design microporous coordination polymers for catalytic purposes

Influence of Pore Structure and Metal-Node Geometry on the Polymerization of Ethylene over Cr-Based Metal–Organic Frameworks

Metal–organic frameworks (MOFs) have received increasing interest as solid single-site catalysts, owing to their tunable pore architecture and metal node geometry. The ability to exploit these modulators makes them prominent candidates for producing polyethylene (PE) materials with narrow dispersity index (Ð) values. Here a study is presented in which the ethylene polymerization properties, with Et2AlCl as activator, of three renowned Cr-based MOFs, MIL-101(Cr)-NDC (NDC=2,6-dicarboxynapthalene), MIL-53(Cr) and HKUST-1(Cr), are systematically investigated. Ethylene polymerization reactions revealed varying catalytic activities, with MIL-101(Cr)-NDC and MIL-53(Cr) being significantly more active than HKUST-1(Cr). Analysis of the PE products revealed large Ð values, demonstrating that polymerization occurs over a multitude of active Cr centers rather than a singular type of Cr site. Spectroscopic experiments, in the form of powder X-ray diffraction (pXRD), UV/Vis-NIR diffuse reflectance spectroscopy (DRS) and CO probe molecule Fourier transform infrared (FTIR) spectroscopy corroborated these findings, indicating that indeed for each MOF unique active sites are generated, however without alteration of the original oxidation state. Furthermore, the pXRD experiments indicated that one major prerequisite for catalytic activity was the degree of MOF activation by the Et2AlCl co-catalyst, with the more active materials portraying a larger degree of activation

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal-alkyl co-catalyst. However, metal-alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal-alkyl co-catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in-situ UV-Vis-NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri-ethyl borane (TEB) or tri-ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal-alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics

Influence of Metal-Alkyls on Early-Stage Ethylene Polymerization over a Cr/SiO2 Phillips Catalyst: A Bulk Characterization and X-ray Chemical Imaging Study.

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization since its invention in 1953. The uniqueness of this catalyst is related to its ability to produce broad molecular weight distribution (MWD) PE materials as well as that no co-catalysts are required to attain activity. Nonetheless, co-catalysts in the form of metal-alkyls can be added for scavenging poisons, enhancing catalyst activity, reducing the induction period, and tailoring polymer characteristics. The activation mechanism and related polymerization mechanism remain elusive, despite extensive industrial and academic research. Here, we show that by varying the type and amount of metal-alkyl co-catalyst, we can tailor polymer properties around a single Cr/SiO2 Phillips catalyst formulation. Furthermore, we show that these different polymer properties exist in the early stages of polymerization. We have used conventional polymer characterization techniques, such as size exclusion chromatography (SEC) and 13 C NMR, for studying the metal-alkyl co-catalyst effect on short-chain branching (SCB), long-chain branching (LCB) and molecular weight distribution (MWD) at the bulk scale. In addition, scanning transmission X-ray microscopy (STXM) was used as a synchrotron technique to study the PE formation in the early stages: allowing us to investigate the produced type of early-stage PE within one particle cross-section with high energy resolution and nanometer scale spatial resolution

Genesis of MgCl2-based Ziegler-Natta Catalysts as Probed with Operando Spectroscopy

Ziegler‐Natta catalysts for olefin polymerization are intrinsically complex multi‐component systems. The genesis of the active sites involves several simultaneous and sequential steps, making the individual steps and interconnections difficult to be unraveled in an unambiguous manner. In this work, we combine X‐ray diffraction and spectroscopy to probe each step of the birth and life of a MgCl2‐based Ziegler‐Natta catalyst, namely the formation of high surface area MgCl2 by dealcoholation of an alcoholate precursor, the TiCl4 grafting, and the subsequent activation by triethylaluminum as co‐catalyst. The so‐prepared catalyst was tested towards ethylene polymerization, leading to the production of mainly crystalline high‐density polyethylene. The use of operando characterization techniques allowed probing the transient details that are difficult to be dissected in the aftermath, but can radically affect the overall catalytic process