Hydrogenation of olefins and polymerization of ethene over chromium oxide/silica catalysts III. Hydrogenation of olefins and H/D exchange

Wittgen, P.P.M.M.; Groeneveld, C.; Zwaans, P.J.C.J.M.; MORGENSTERN, H. J. B.; Heughten, A.H. van; Heumen, C.J.M. van; Schuit, G.C.A.

doi:10.1016/0021-9517(82)90179-8

Cited by 33 publications

(5 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Cr/Al 2 O 3 , Cr/SiO 2 , and Cr/MgO have also been studied by other authors for the hydrogenation of propene and H 2 −D 2 equilibration. ,,− Selwood suggested that Cr 3+ ions are the active sites for catalytic hydrogen−deuterium equilibration over α-Cr 2 O 3 , whereas Indovina et al have shown that the activity of Cr/SiO 2 catalysts is linked to the presence of Cr 2+ ions . However, Wittgen et al have shown that exhaustively reduced Cr/SiO 2 catalysts, which contain predominantly Cr 2+ , are inactive and Cr 3+ is proposed as the active site. , More recently, Indovina and co-workers have studied the performances of Cr/ZrO 2 catalysts in propene hydrogenation as a function of the reduction treatment. − The catalytic activity increases when the average oxidation state of Cr decreases from 5.5 to about 3. Furthermore, reduced Cr/ZrO 2 catalysts were 3.6 to 100 times more active than Cr/SiO 2 and Cr/Al 2 O 3 catalysts, and mononuclear Cr 3+ , formed from the reduction of isolated Cr 5+ , is proposed as the active center.…”

Section: B Hydrogenation−dehydrogenation Reactionsmentioning

confidence: 99%

“…Chromium on silica (Cr/SiO 2 ), as illustrated in Figure , is the famous Phillips catalyst for the polymerization of ethylene at relatively low pressures. − This catalyst is the basis for the Phillips particle form process in the production of high-density polyethylene (HDPE), one of the most extensively used polymers. Other important catalytic activities are hydrogenation−dehydrogenation, oxidation, isomerization, aromatization, and DeNO x reactions. − The basis for the activity of Cr in such a wide spectrum of reactions lies in the variability of oxidation states, of coordination environments, and of degree of polymerization of Cr oxide species. This variability is especially pronounced on the surface.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

1996

View full text Add to dashboard Cite

Section: B Hydrogenation−dehydrogenation Reactionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

1996

View full text Add to dashboard Cite

“…To provide the “extra” hydrogen, some proposals abstract a proton from the silica surface, − even though this is seemingly in conflict with another piece of the puzzle, that is, significantly or even fully dehydroxylated surfaces provide the most active catalysts. Still other proposals have a permanent ligand being formed on the Cr from the first ethylene molecules adsorbed, such as an allyl or a diene group, from which the extra hydrogen is donated …”

Section: Introductionmentioning

confidence: 99%

Identification of the Starting Group on the First PE Chain Produced by the Phillips Catalyst

et al. 2019

View full text Add to dashboard Cite

Upon contact with ethylene, the Phillips Cr/silica catalyst must selfalkylate before polymerization can begin. How this happens has been a mystery for over 60 years, being not only of scientific interest but also of commercial importance. Consequently, many controversial schemes have been proposed, including some that hold the initial PE chain to be different from all subsequent chains, starting with a vinyl group, instead of the usual methyl group. In this study, polymer chains produced by the Cr(II)/silica−titania catalyst at low yield were analyzed in an effort to identify the starting group on the first chain produced. The PE/Cr yield was deliberately kept low to avoid diluting that initial chain with other chains produced afterward. The polymer was then characterized by gel permeation chromatography and 13 C NMR (natural abundance). Strong signals from methyl end groups were present, but terminal vinyl groups were not detected. Likewise, analysis by proton ( 1 H) NMR also indicated a general lack of vinyl chain ends. An attempt was also made to identify the first species on the initial chain through isotopic labeling. Deutero-ethylene (C 2 D 4 ), deutero-propylene (C 3 D 6 ), or 13 Cethylene was first "preadsorbed" onto the divalent catalyst at −78 °C, where polymerization does not occur. After evacuating the excess isotopically labeled monomer, polymerization was conducted with unlabeled ethylene (C 2 H 4 ) at higher temperatures. Deuterium ( 2 H) NMR analysis of the polymers again indicated a strong methyl signal but no resonance from deuteriumcontaining vinyl chain ends. Comparison of the different NMR methods indicated a high degree of deuterium enrichment in the methyl chain ends. These results indicate that the initial chain in these experiments began with a methyl group, not a vinyl group as has sometimes been contemplated. Although helpful, this information still leaves the mechanism of site alkylation unclear because there is not enough hydrogen in ethylene to produce a methyl-initiated chain, without some other (still undefined) reaction also being involved to furnish the additional hydrogen, such as the creation of a permanent ligand.

show abstract

“…In contrast, studying the mechanism of the reverse reaction, alkene hydrogenation, is easier because of the favored thermodynamics and the use of lower operating temperatures. To elucidate the mechanism in hydrogenation, classical approaches such as isotopic labeling and kinetic studies have been used . Parahydrogen-induced polarization (PHIP) can give insight into the pathways of hydrogen transfer similar to that of isotopic labeling .…”

Section: Introductionmentioning

confidence: 99%

X–H Bond Activation on Cr(III),O Sites (X = R, H): Key Steps in Dehydrogenation and Hydrogenation Processes

et al. 2016

View full text Add to dashboard Cite

We synthesized isolated Cr(III) sites on SiO2−Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr(OSi(OtBu)3)3(THF)2 and Cr(Al(OiPr)4)3 on alumina. CrO x /Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination, as carried out for the synthesis of industrial Cr-based dehydrogenation catalysts. These materials were characterized by IR, EPR, XAS, and by the adsorption of the probe molecules CO and pyridine, and the results were compared to previously reported isolated Cr(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 °C, where higher TOFs were obtained for Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3 than for CrO x /Al2O3 and Cr(III)/SiO2. For mechanistic studies the reverse reaction, propene hydrogenation, was studied. Here, the order of reactivity was opposite that of dehydrogenation, with CrO x /Al2O3 and Cr(III)/SiO2 being more active in hydrogenation than Cr(III)/SiO2-Al2O3 and Cr(III)/Al2O3. Kinetic analysis and labeling studies with D2 showed that the rate law is in all cases first order in H2 partial pressure but had different dependence on propene partial pressure from catalyst to catalyst. We found small normal kinetic isotope effects of 1 ≤ KIE ≤ 2, activation enthalpies up to 40 kJ mol–1, and large negative activation entropies between −100 and −194 J K–1 mol–1 for the different Cr catalysts. We also performed parahydrogen-induced polarization (PHIP) experiments, which showed that H2 addition to propene proceeds, at least in part, via a pairwise mechanism. The experimental data for propene hydrogenation suggests adsorption/desorption pre-equilibria of H2 (or D2) and propene followed by H2 activation and insertion of propene. DFT computations for propane dehydrogenation and propene hydrogenation on Cr(III) on a periodic alumina model show that a mechanism involving X–H activation (X = R, H) is energetically feasible with activation enthalpies and entropies that are comparable to the experimentally determined values.

show abstract

Hydrogenation of olefins and polymerization of ethene over chromium oxide/silica catalysts III. Hydrogenation of olefins and H/D exchange

Cited by 33 publications

References 16 publications

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

Identification of the Starting Group on the First PE Chain Produced by the Phillips Catalyst

X–H Bond Activation on Cr(III),O Sites (X = R, H): Key Steps in Dehydrogenation and Hydrogenation Processes

Contact Info

Product

Resources

About