Benzene Selectivity in Competitive Arene Hydrogenation: Effects of Single-Site Catalyst···Acidic Oxide Surface Binding Geometry

Abstract

Organozirconium complexes are chemisorbed on Brønsted acidic sulfated ZrO2 (ZrS), sulfated Al2O3 (AlS), and ZrO2-WO3 (ZrW). Under mild conditions (25 °C, 1 atm H2), the supported Cp*ZrMe3, Cp*ZrBz3, and Cp*ZrPh3 catalysts are very active for benzene hydrogenation with activities declining with decreasing acidity, ZrS ≫ AlS ≈ ZrW, arguing that more Brønsted acidic oxides (those having weaker corresponding conjugate bases) yield stronger surface organometallic electrophiles and for this reason have higher benzene hydrogenation activity. Benzene selective hydrogenation, a potential approach for carcinogenic benzene removal from gasoline, is probed using benzene/toluene mixtures, and selectivities for benzene hydrogenation vary with catalyst as ZrBz3(+)/ZrS(-), 83% > Cp*ZrMe2(+)/ZrS(-), 80% > Cp*ZrBz2(+)/ZrS(-), 67% > Cp*ZrPh2(+)/ZrS(-), 57%. For Cp*ZrBz2(+)/ZrS(-), which displays the highest benzene hydrogenation activity with moderate selectivity in benzene/toluene mixtures. Other benzene/arene mixtures are examined, and benzene selectivities vary with arene as mesitylene, 99%, > ethylbenzene, 86% > toluene, 67%. Structural and computational studies by solid-state NMR spectroscopy, XAS, and periodic DFT methods applied to supported Cp*ZrMe3 and Cp*ZrBz3 indicate that larger Zr···surface distances are present in more sterically encumbered Cp*ZrBz2(+)/AlS(-) vs Cp*ZrMe2(+)/AlS(-). The combined XAS, solid state NMR, and DFT data argue that the bulky catalyst benzyl groups expand the "cationic" metal center-anionic sulfated oxide surface distances, and this separation/weakened ion-pairing enables the activation/insertion of more sterically encumbered arenes and influences hydrogenation rates and selectivity patterns.

Single‐Face/All‐cis Arene Hydrogenation by a Supported Single‐Site d0 Organozirconium Catalyst

“…After each cycle,a ll volatiles were vacuumtransferred from the reactor to a À78 8 8Ct rap for further analysis.T he supported catalyst was then dried in vacuo for 1hbefore substrate addition for the next cycle.For each cycle (for 2 nd cycle data see Table 1, entry 2; for 3 rd cycle data see Table S4), negligible variation in product cis-selectivity (> 99 %) is observed, suggesting that one benzyl group remains bound to the Zr IV center. [13] Thes tructure of 13 C-enriched Cp*Zr( 13 CH 2 Ph) 2 /ZrS was investigated by 13 CC PMAS NMR before and after o-xylene hydrogenations ( Figure S13). [13] Thes tructure of 13 C-enriched Cp*Zr( 13 CH 2 Ph) 2 /ZrS was investigated by 13 CC PMAS NMR before and after o-xylene hydrogenations ( Figure S13).…”

“…[22] Kinetic measurements on Table 1, entry 1b etween 0a nd 45 8 8Ca t constant 7atm of H 2 with rapid (> 1450 rpm) stirring to minimize mass transfer effects (Table S5) combined with as tandard Eyring/Arrhenius analysis yields DH°= 4.8 AE 0.5 kcal mol À1 , DS°= À58.7 AE 1.9 eu, and E a = 5.3 AE 0.6 kcal mol À1 ,s uggesting ah ighly organized transition state (large negative DS°), typical of many d 0 /f n -centered catalytic processes, [23] and an intermolecular turnover-limiting stepthe first H 2 delivery.T he observed E a (5.3 kcal mol À1 )i s somewhat lower than those for conventional supported metal nanoparticle o-xylene hydrogenation catalysts (E a = 7.4-14.9 kcal mol À1 ). [13] TheZrS surface model consists of a42 slab (12.85 14.50 30) built starting from the (101) plane cut from the optimized bulk structure of tetragonal zirconia (See SI for more details). [13] TheZrS surface model consists of a42 slab (12.85 14.50 30) built starting from the (101) plane cut from the optimized bulk structure of tetragonal zirconia (See SI for more details).…”

“…For o-xylene hydrogenation ( Table 1, entry 1), as ingle product is observed, which, by comparison with commercially available dimethylcyclohexanes,i sd etermined to be cis-1,2-dimethyl-cyclohexane (Figure 2A and D). [12][13][14]17] Likewise, m-a nd p-xylene hydrogenated under identical conditions yield exclusively the respective cis-fully hydrogenated products (Table 1; Figure 2B and E, C and F). [12][13][14]17] Likewise, m-a nd p-xylene hydrogenated under identical conditions yield exclusively the respective cis-fully hydrogenated products (Table 1; Figure 2B and E, C and F).…”

“…Owing to the versatility of alcohols and ketones as synthetic intermediates,t hese products can then be modified in many ways to afford avariety of valuable substances. [11] Our group recently reported the kinetics and reaction pathways by which benzene and toluene hydrogenation are catalyzed by Cp*ZrR 3 complexes (Cp* = h 5 -C 5 (CH 3 ) 5 ;R= Me, [12] CH 2 Ph; [13] Figure 1A)chemisorbed on Brønsted acidic sulfated oxides (Cp*ZrR 2 /MS, M = Zr,Al). [10] In particular, electrophilic d 0 catalysts supported on "super" Brønsted acids with large percentages of catalytically significant sites (70 % À98 %) exhibit high activity in technologically relevant olefin polymerization and hydrogenation processes.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[413][414][415][416][417][418][419] Characterization of the generated species and computational analysis,a sw ell as polymerization studies, suggest that the surface anionic sites are charge-delocalized and weakly coordinating (Scheme 26). [417][418][419] Similar observations were found for reactions of organozirconium species with sulfated zirconia [414] and sulfated tin oxides. [417][418][419] Similar observations were found for reactions of organozirconium species with sulfated zirconia [414] and sulfated tin oxides.…”