Synthesis of 2-Methylpyrazine Using Crude Glycerol over Zn-Cr-O Catalyst: A Value Addition Process for the Utilization of Biodiesel By-Product

Mixed oxides of ZnO and Cr2O3 with varied mole ratios were synthesized, characterized, and evaluated for the dehydrocyclization of crude glycerol for the production of 2-methylpyrazine (2-MP). The Zn-Cr-O composition was optimized using the bulk and surface properties of the catalysts rationalized by BET-SA, XRD, XPS, H2-TPR, O2 pulse chemisorption, and Raman spectroscopic techniques to achieve a high rate of 2-MP.

Comparison of Support Effects on Phillips and Metallocene Catalysts

Both metallocene and Phillips chromium catalysts are used in the commercial manufacture of polyethylene. Unlike most other commercial metallocene systems, the Chevron Phillips Chemical (CPC) platform does not use methylaluminoxane or fluoroorganic boranes. Instead, the support itself serves to activate (ionize) the metallocenes, which then polymerize ethylene at high activity. Most of these solid acid supports can also be used to anchor Cr to make a Phillips catalyst. This provides an interesting opportunity to compare the polymerization responses by these two disparate systems, Phillips Cr and CPC metallocene, when supported on the same solid acid carriers. In this study, both chromium oxide and several metallocenes were deposited onto a variety of solid oxides, under a variety of conditions, and the resulting support effects were observed and compared. Although using seemingly different chemistries, the two catalyst systems exhibited a surprising number of similarities, which can be attributed to the acidity and porosity of these diverse supports.

Mesoporous Organo-Silica Supported Chromium Oxide Catalyst for Oxidative Dehydrogenation of Ethane to Ethylene with CO2

Chromium oxide supported on mesoporous organo-silica (MOS) was synthesized with different Cr loading by an incipient method. The catalytic performance of a Cr(x)/MOS catalyst for CO2-based ethane dehydrogenation was investigated. The synthesized catalysts were characterized by XRD, BET, TEM, SEM, XPS, FTIR, and UV–Vis DR measurements. The textural properties of the prepared samples showed that the mesoporous nature of MOS sample was not disturbed by chromium impregnation. Among the prepared samples, Cr(8)/MOS catalyst exhibited good distribution of chromium species along with superior concentration of Cr6+ and the highest recorded Cr6+/Cr3+ ratio. The results revealed that the superior catalytic performance was reached at Cr(8)/MOS, with 50.4% and 90.1% of ethane conversion and ethylene selectivity, respectively. The catalytic activity decreased slowly over reaction time; it declined approximately 22% after 10 h of stream operation. The roles of CO2-based ethane dehydrogenation were also studied, where carbon dioxide can be a source of lattice oxygen and as a hydrogen consumer in reverse water–gas shift (RWGS) reaction. The effect of various catalytic factors, such as catalytic temperature, reaction time, space gas velocity, and CO2 partial pressure on the conversion of ethane, yield, and selectivity to ethylene, were investigated as well.

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 Et2 AlCl 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 Et2 AlCl co-catalyst, with the more active materials portraying a larger degree of activation.

Dehydrogenation of Ethane to Ethylene by CO2 over Highly Dispersed Cr on Large-Pore Mesoporous Silica Catalysts

A series of large-pore mesoporous silica (LPMS)-supported CrOx catalysts were synthesized by hydrothermal and impregnation methods and tested for ethane dehydrogenation in the presence of CO2 as an oxidant. To assess the effect of hydrothermal temperature treatment on the characteristics of LPMS support, different hydrothermal temperatures (100–160 °C) were studied and optimized. The optimum support was then loaded with different amounts of chromium (0, 2, 4, 8, and 11 wt % Cr). The obtained catalysts were characterized by different techniques such as XRD, BET, TEM, SEM, XPS, FTIR, and diffuse reflectance UV-Vis spectroscopy. The characterization results indicated that the sample hydrothermally treated at 130 °C exhibited the highest pore volume, a narrow pore size distribution, and a moderate BET surface area. Chromium species with various oxidation states including Cr3+, Cr6+, and α-Cr2O3 were detected in all synthesized Cr(y)/LPMS-130 catalysts. A lower Cr content resulted in the formation of Cr6+, whereas a higher Cr content dominated the α-Cr2O3 on the surface of the catalyst. Among the synthesized catalysts, the Cr(4)/LPMS-130 catalyst showed the highest Cr6+/Cr3+ ratio, indicating a good dispersion of chromium species along with a fine particle size. The ethane conversion and ethylene selectivity were 50.5 and 91.1% for Cr(4)/LPMS-130, respectively. Carbon dioxide was believed to supply enough lattice oxygen to maintain the Cr species at a higher oxidation state and to consume the hydrogen resulting from ethane cracking by a reverse water gas shift reaction.

Synergetic Impact of Secondary Metal Oxides of Cr-M/MCM41 Catalyst Nanoparticles for Ethane Oxidative Dehydrogenation Using Carbon Dioxide

Oxidative dehydrogenation of alkanes to alkenes by a mild oxidant such as carbon dioxide is an active area of research. A series of MCM41-supported bimetallic oxide catalysts containing chromium oxide in addition to metal oxides (Ce, Co, Zn, V, Nb, and Mo) has been prepared. The binary catalysts have Cr metal oxide incorporated into MCM41 structure while the other oxides are either incorporated with Cr or impregnated on the MCM41 surface. The synthesized catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 sorption, scanning electron microscopy (SEM), hydrogen temperature programmed reduction (H2-TPR), and Diffuse reflectance UV–vis spectroscopy (DRS). The catalytic activity of Cr(4)-M(4)/MCM-41 catalysts in the dehydrogenation of ethane with CO2 was investigated. The textural properties of the synthesized samples showed that the addition of the bimetallic oxides did not disturb the mesoporous structure of MCM41 and the prepared catalysts exhibited a high BET surface area; however, the lowest surface area was recorded for Cr(4)-Mo(4)/MCM41 catalyst at 701 m2/g. Among the prepared catalysts, H2-TPR profile of Cr(4)-Ce(4)/MCM41 revealed the increase in the concentration of Cr6+ species which interacted with the framework of siliceous support. On the other hand, H2-TPR profiles of Cr(4)-Co(4)/MCM41 showed wide reduction peaks centered at 400 °C which is ascribed to reduction of Cr6+ to Cr3+ species and Co3O4 to metallic Co. At the same time, Cr(4)-Mo(4)/MCM41 and Cr(4)-V(4)/MCM41 exhibited higher temperature reduction peaks, indicating these two catalysts require higher activation temperatures. The synergy between the Cr with Zn or Nb metals reduced the concentration of Cr6+ species which is reflected in their catalytic performance. Cr(4)-Ce(4)/MCM41 recorded the highest catalytic activity toward ethylene production where the ethane conversion and ethylene yield were 37.9% and 35.1%, respectively.

Probing the nature of active chromium species and promotional effects of potassium in Cr/MCM-41 catalysts for methyl mercaptan abatement

The introduction of K enables a large number of CrO₄²⁻ active species to be anchored and dispersed on the surface of Cr-based catalysts.

Catalytic Behavior of Chromium Oxide Supported on Nanocasting-Prepared Mesoporous Alumina in Dehydrogenation of Propane

Mesoporous alumina with narrow pore size distribution centered in the range of 4.4–5.0 nm and with a specific surface area as high as 270 m²·g⁻¹ was prepared via the nanocasting approach using a CMK-3 carbon replica as a hard template. Based on this support, a series of catalysts containing 1, 5, 10, 20 and 30 wt % of chromium was prepared by incipient wetness impregnation, characterized, and studied in the dehydrogenation of propane to propene (PDH). Cr species in three oxidation states—Cr(III), Cr(V) and Cr(VI)—were found on the oxidized surface of the catalysts. The concentration of these species varied with the total Cr loading. Temperature-programmed reduction (H₂-TPR) and UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) studies revealed that Cr(VI) species dominated at the lowest Cr content. An increase in the Cr loading resulted in an appearance of an increasing amount of Cr(III) oxide. UV-Vis-DRS measurements performed in situ during the PDH process showed that at the beginning of the catalytic test Cr(VI) species were reduced to Cr(III) redox species. A crucial role of the redox species in the PDH process over the catalysts with the low Cr content was confirmed. The stability test for the catalyst containing 20 wt % of Cr showed that this sample exhibited the reproducible catalytic performance after the first four regeneration–dehydrogenation cycles. Moreover, this catalyst had higher resistance on deactivation during the PDH process as compared to the reference catalyst with the same Cr loading, but was supported on commercially available alumina.

Insights into Cr/SiO2 catalysts during dehydrogenation of propane: an operando XAS investigation

In situ and operando XAS spectroscopic methods were applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO_2-DHS catalyst during propane dehydrogenation under non-oxidative and different oxidative conditions.

High-performance Cr–Zr–O and Cr–Zr–K–O catalysts prepared by nanocasting for dehydrogenation of propane to propene

Controlling the porosity and surface chemistry of Cr–Zr–O and Cr–Zr–K–O catalysts by nanocasting improved the yield of propene and their stability in the dehydrogenation of propane to propene.

Structure–performance relationships of Cr/Ti/SiO2 catalysts modified with TEAl for oligomerisation of ethylene: tuning the selectivity towards 1-hexene

Ethylene oligomerisation properties of TEAl-modified Cr/Ti/SiO₂ ethylene polymerisation Phillips-type catalysts studied with in situ UV-vis-NIR diffuse reflectance spectroscopy, mass spectrometry and gas chromatography.

Relationship between basicity, reducibility and partial oxidation properties of chromium containing MCM-41

MCM-41 mesoporous solids were modified with chromium species using Cr(NO₃)₃ and CrO₃ as precursors.

The charge percolation mechanism and simulation of Ziegler-Natta polymerizations, part VI: Mechanism of ethylene polymerization by supported chromium oxide

Despite intensive research over the last 50 years, many questions concerning ethylene polymerization by supported chromium oxide are still unanswered. Hence, the very fundamental issues of this polymerization are discussed in this paper. It is shown that a charge percolation mechanism (CPM) of olefin polymerization by Ziegler-Natta transition metal complexes, recently proposed by us, can give the answers in this case, too.