“…Most of the catalysts are based on supported group VIII metals, and various factors controlling the intramolecular selectivity were studied. An increase of selectivity toward unsaturated alcohol can be obtained by the nature of the individual metal, , by the presence of a second metal in ex situ prepared bimetallic catalysts − and from metal salt additives in liquid-phase hydrogenations, − electron-donating ligand effects by supports such as basic zeolites , and graphite acting as macroligand for platinum and ruthenium particles, , steric constraints in the metal environment, ,− strong metal−support interactions (SMSI), − selective poisoning, − substitutents on the CC bond, ,, and solvent − and pressure effects. , 1 …”
Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO 2 and Ag/ TiO 2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silver particle size on activity and selectivity toward the unsaturated alcohol. Nanostructural features of the catalysts, as their silver particle size distribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity to the unsaturated alcohol was found to be (59 ( 3)%, independent of the particle size in the range of 3.7 nm e d h Ag e 6.3 nm. Moreover, the specific activities were similar in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO 2 catalysts appears to be structureinsensitive. Titania-supported silver catalysts reduced in hydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), however, showed a quite different behavior. These silver particles exhibit rather narrow size distributions and very low mean particle sizes (d h Ag ) 2.8 ( 1.9 nm for Ag/TiO 2 -LTR, d h Ag ) 1.4 ( 0.5 nm for Ag/TiO 2 -HTR), i.e., an exceedingly high dispersion (D Ag ) 0.46 and 0.69, respectively). The LTR catalyst gave a higher selectivity to crotyl alcohol (53%) than the ultradispersed HTR catalyst (28%). This pronounced change in selectivity suggests the hydrogenation of crotonaldehyde over these Ag catalysts to be qualified as structure-sensitive with the rate-determining step depending critically on the silver particle size and thus on the silver surface structure. If hydrogenation of the CdO group of the R,β-unsaturated aldehyde is favored by face atoms, most likely the increased fraction of Ag(111) planes of the larger silver particles will give higher formation rates of the desired unsaturated alcohol.
“…Most of the catalysts are based on supported group VIII metals, and various factors controlling the intramolecular selectivity were studied. An increase of selectivity toward unsaturated alcohol can be obtained by the nature of the individual metal, , by the presence of a second metal in ex situ prepared bimetallic catalysts − and from metal salt additives in liquid-phase hydrogenations, − electron-donating ligand effects by supports such as basic zeolites , and graphite acting as macroligand for platinum and ruthenium particles, , steric constraints in the metal environment, ,− strong metal−support interactions (SMSI), − selective poisoning, − substitutents on the CC bond, ,, and solvent − and pressure effects. , 1 …”
Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO 2 and Ag/ TiO 2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silver particle size on activity and selectivity toward the unsaturated alcohol. Nanostructural features of the catalysts, as their silver particle size distribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity to the unsaturated alcohol was found to be (59 ( 3)%, independent of the particle size in the range of 3.7 nm e d h Ag e 6.3 nm. Moreover, the specific activities were similar in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO 2 catalysts appears to be structureinsensitive. Titania-supported silver catalysts reduced in hydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), however, showed a quite different behavior. These silver particles exhibit rather narrow size distributions and very low mean particle sizes (d h Ag ) 2.8 ( 1.9 nm for Ag/TiO 2 -LTR, d h Ag ) 1.4 ( 0.5 nm for Ag/TiO 2 -HTR), i.e., an exceedingly high dispersion (D Ag ) 0.46 and 0.69, respectively). The LTR catalyst gave a higher selectivity to crotyl alcohol (53%) than the ultradispersed HTR catalyst (28%). This pronounced change in selectivity suggests the hydrogenation of crotonaldehyde over these Ag catalysts to be qualified as structure-sensitive with the rate-determining step depending critically on the silver particle size and thus on the silver surface structure. If hydrogenation of the CdO group of the R,β-unsaturated aldehyde is favored by face atoms, most likely the increased fraction of Ag(111) planes of the larger silver particles will give higher formation rates of the desired unsaturated alcohol.
“…Although in homogeneous catalysis Ir(I) compounds [52] and heterogenized Ir complexes [53] were successfully applied for the relatively simple selective hydrogenation of α, βunsaturated aldehydes with bulky substituents at the C=C group (cinnamaldehyde), the latter seems to be a prerequisite for the formation of unsaturated alcohols in the case of heterogeneous Ir catalysts. An experimental hint for the formation of allyl alcohol with Ir under technological aspects (high yields) can be found in much earlier work [54] where a yield of 60% was reported over a 5 wt.% Ir/C catalyst in liquid-phase hydrogenation (solvent EtOH) at 0.1 MPa and 298 K. However, a reexamination of acrolein hydrogenation in the liquid phase over different supported Ir catalysts clearly showed that the preferred hydrogenation of the C=O group is not possible (selectivity <15%, saturated aldehyde is the main product) [55]. with acrolein as educt, maximum selectivities of 27-37% towards allyl alcohol were observed over Ir/Al 2 O 3 but at very low conversions (0.2-0.5%) [43].…”
The sections in this article are
Generalities in Regioselectivity with Heterogeneous Catalysts
Definitions
General Factors Determining Selectivity
Reaction Engineering and Selectivity
Selective Hydrogenation of α, β‐Unsaturated Aldehydes
Physicochemical Basics and Reaction Mechanism
Metal Specificity and Structure Sensitivity
Influence of Support Materials, Zeolites and Surface Modifiers
Influence of a Second Metal on Selectivity
Regioselective Hydrogenation in the Presence of Green Solvents
Kinetic Modeling
Selective Hydrogenation of Polyenes
Selective Hydrogenation of Ketones
Selective Hydrogenation of Polycyclic Aromatics and Derivatives
Selective Hydrogenation of Nitroaromatics
“…Somit konnten die Ergebnisse der bereits erwähnten älteren Arbeit [7], nach denen bei der Flüssigphasenhydrierung von Acrolein im Lösungsmittel Ethanol eine 60%ige Ausbeute an Allylalkohol in Gegenwart von Ir-Katalysatoren erhalten worden sein soll, nicht bestätigt werden. Das dort gewählte Lösungsmittel führt zu Acetalisierungsreaktionen, die auch von anderen Autoren bei der Hydrierung von a,b-ungesättigten Aldehyden beobachtet wurden und im Falle von Acrolein neben der Bildung des gesättigten Aldehyds dominieren [11,15].…”
Section: Hydrierung Von Acrolein Und Crotonaldehydunclassified
“…In der heterogenen Katalyse fanden G I R O I R -F E N D L E R et al bei der Flüssigphasenhydrierung von Zimtaldehyd an Irund Pt-Katalysatoren auf Graphit bzw. Der einzige experimentelle Beleg für die Hydrierung von Acrolein unter chemisch-technologisch verwertbaren Aspekten (hohe Ausbeute) findet sich in einer älteren Arbeit, nach der ein Ir/C-Katalysator (5 Ma.-% Ir) in der Flüssigphasenhydrierung nach 70 min bei 0,1 MPa und 298 K eine Ausbeute an Allylalkohol von 60 % liefern soll [7]. Bei der Hydrierung von Acrolein an monometallischen Träger-Katalysatoren wurden generell geringe Selektivitäten zum Allylalkohol beobachtet, wobei die von C O Q et al angegebenen maximalen Werte von 27 ± 37 % (Ir/Al 2 O 3 ) bei sehr geringen Umsätzen (0,2 ± 0,5 %) erhalten wurden [6].…”
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