Dendrimer‐Encapsulated Pd Nanoparticles, Immobilized in Silica Pores, as Catalysts for Selective Hydrogenation of Unsaturated Compounds

Abstract

Heterogeneous Pd‐containing nanocatalysts, based on poly (propylene imine) dendrimers immobilized in silica pores and networks, obtained by co‐hydrolysis in situ, have been synthesized and examined in the hydrogenation of various unsaturated compounds. The catalyst activity and selectivity were found to strongly depend on the carrier structure as well as on the substrate electron and geometric features. Thus, mesoporous catalyst, synthesized in presence of both polymeric template and tetraethoxysilane, revealed the maximum activity in the hydrogenation of various styrenes, including bulky and rigid stilbene and its isomers, reaching TOF values of about 230000 h −1 . Other mesoporous catalyst, synthesized in the presence of polymeric template, but without addition of Si(OEt) 4 , provided the trans ‐cyclooctene formation with the selectivity of 90–95 %, appearing as similar to homogeneous dendrimer‐based catalysts. Microporous catalyst, obtained only on the presence of Si(OEt) 4 , while dendrimer molecules acting as both anchored ligands and template, demonstrated the maximum activity in the hydrogenation of terminal linear alkynes and conjugated dienes, reaching TOF values up to 400000 h −1 . Herein the total selectivity on alkene in the case of terminal alkynes and conjugated dienes reached 95–99 % even at hydrogen pressure of 30 atm. The catalysts synthesized can be easily isolated from reaction products and recycled without significant loss of activity.

Heterogeneous Dendrimer-Based Catalysts

“…Herein, depending on the dendrimer size and generation, as well as on the metal to dendrimer ratio, small dendrimer-encapsulated nanoparticles (DENs) of 1–2 nm can be obtained for Pd with the narrower size distribution or dendrimer-stabilized nanoparticles (DSNs) with the sizes of 1–4 nm for Pd, 3–12 nm for Au, and 4–20 nm for Ag ( Figure 4 ) [ 10 , 25 , 62 , 64 , 90 , 150 , 186 , 187 , 188 , 191 ]. The similar is also true for PPI, PAMAM, and other related poly(alkyl amine) dendrimers [ 60 , 115 , 116 , 156 , 168 , 192 , 193 ].…”

“…Depending simultaneously on the reaction type, dendrimer nature, and catalytic centers location, as well as on the overall weight of the stabilization or steric factors from the dendrimer ligand, the so called positive dendritic effect can be observed, appearing in the enhanced catalyst performance with the increase in the dendrimer generation [ 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 92 , 94 , 113 , 115 , 120 , 121 , 129 , 131 , 132 , 136 , 141 , 145 , 146 , 149 , 190 , 193 , 195 , 207 , 215 , 216 , 218 , 221 , 227 , 242 , 247 , 248 , 252 , 255 , 258 , 261 , 262 , 263 , 271 , 272 , 273 , 274 , 279 , 280 , 281 , …”

“…When Pd nanoparticle-cored Fréchet type poly(aryl ether) dendrimers were used, both covalently attached to the Pd surface [ 137 ] or stabilizing via phosphine focal point [ 129 ], C≡C triple bonds or conjugated C=C double bonds underwent exhaustive hydrogenation. Vice versa, alkynes and dienes could be selectively converted to corresponding monoenes in the presence of Pd catalysts (both mono- and bimetallic), based on triazol [ 150 ], PAMAM [ 131 , 148 , 159 , 160 , 207 , 241 ] and, especially, on PPI dendrimers [ 116 , 128 , 140 , 156 , 171 , 193 , 241 ], with the total selectivity on alkenes increased with the increase in the dendrimer generation [ 116 , 148 , 193 ]. It was due to the poisoning effects of pyridyl moieties at the dendron focal point and/or dendrimer node donor tertiary amino group, occupying a part of adsorption sites and thus enriching the nanoparticle surface with electrons, facilitating the de-adsorption of alkene formed and interfering with its possible further re-adsorption [ 4 , 128 , 131 , 171 , 177 , 193 , 207 , 285 , 286 , 287 ].…”

“…Vice versa, alkynes and dienes could be selectively converted to corresponding monoenes in the presence of Pd catalysts (both mono- and bimetallic), based on triazol [ 150 ], PAMAM [ 131 , 148 , 159 , 160 , 207 , 241 ] and, especially, on PPI dendrimers [ 116 , 128 , 140 , 156 , 171 , 193 , 241 ], with the total selectivity on alkenes increased with the increase in the dendrimer generation [ 116 , 148 , 193 ]. It was due to the poisoning effects of pyridyl moieties at the dendron focal point and/or dendrimer node donor tertiary amino group, occupying a part of adsorption sites and thus enriching the nanoparticle surface with electrons, facilitating the de-adsorption of alkene formed and interfering with its possible further re-adsorption [ 4 , 128 , 131 , 171 , 177 , 193 , 207 , 285 , 286 , 287 ]. Herein, the product distribution in semi-hydrogenation of alkynes and, especially, dienes, was strongly affected by metal composition of the catalyst active phase; the dendrimer nature, influencing the metal surface electron properties, as well as substrate and intermediate adsorption features; and, finally, the substrate structure (including the accessibility of C=C double bonds) and electronic properties (the presence of electron donor (EDG) or electron withdrawing groups (EWG)) [ 128 , 131 , 150 , 171 , 193 , 207 , 288 , 289 ].…”

Silsesquioxane Modified with PAMAM Dendrimer and a Bimetallic Complex for Electrochemical Detection of Ascorbic Acid

“…Dendrimers are macromolecules that have a highly branched, three‐dimensional structure [20]. The intrinsic characteristics of dendrimers make them promising for several applications, such as micelles [21, 22], the encapsulation of different substances [23–25] including liquid crystals [26, 27], electroanalysis [28, 29], sensors [30–32], electroluminescent devices [33, 34], catalysts [35, 36], and others [37–39].…”