Controlling Reaction Routes in Noble‐Metal‐Catalyzed Conversion of Aryl Ethers

Abstract Hydrogenolysis and hydrolysis of aryl ethers in the liquid phase are important reactions for accessing functionalized cyclic compounds from renewable feedstocks. On supported noble metals, hydrogenolysis is initiated by a hydrogen addition to the aromatic ring followed by C−O bond cleavage. In water, hydrolysis and hydrogenolysis proceed by partial hydrogenation of the aromatic ring prior to water or hydrogen insertion. The mechanisms are common for the studied metals, but the selectivity to hydrogenolysis increases in the order Pd<Rh<Ir<Ru≈Pt in decalin and water; the inverse was observed for the selectivity to hydrolysis in water. Hydrogenolysis selectivity correlates with the Gibbs free energy of hydrogen adsorption. Hydrogenolysis has the highest standard free energy of activation and a weak dependence on H2 pressure, thus, the selectivity to hydrogenolysis is maximized by increasing temperature and decreasing H2 pressure. Selectivity to C−O bond cleavage reaches >95 % in water and alkaline conditions.


Additional information
The reductive conversion of diphenyl ether (Ph2O) was evaluated from low to higher conversion in decalin and water over three commercially available 5 wt.% carbon supported (Ru, Rh, and Pd) catalysts. The average particle sizes (3~6 nm) and dispersion (23 %~31 %) of the metals were characterized by TEM and H2 chemisorption measurements ( Figure S6 and Table S12). Ph2O was fully converted after 6 hours in water or decalin with low dosage of the catalysts (substrate:metal ≈ 8000:1) ( Table S13). The selectivity to hydrogenolysis on the metals scale as Ir (25%) > Ru (20%) ≈ Pt (19%) > Rh (10%) > Pd (2%) in decalin and water (Table S13). For hydrolysis, Pd showed the highest selectivity (84%) at full conversion and Ru and Rh were similar (42%) while Pt showed the lowest selectivity (18%) ( Table S13). The overall selectivities of different reaction pathways changed during the reaction ( Figures S4-S9) because the primary product from hydrogenation of Ph2O, cyclohexyl phenol ether was further hydrogenated or cleaved. Thus, to focus on the primary reactions, conversions of Ph2O were maintained below 20% by varying the reaction time and catalyst loading for different catalysts in water and decalin.

Derivation of the rate equations and fitting
We derive the kinetic expressions for the proposed mechanisms of hydrogenolysis in the main text based on the conventional Langmuir-Hinshelwood model with the following assumptions: H2 adsorbs dissociatively on the metal surface.
(ii) The coverage of reaction intermediates and products are negligible at low conversions, so the most abundant compounds on the active sites are the reactants.
1) Within the mechanistic framework proposed in Scheme 1 (main text), we first assume adsorption for reactants are quasi-equilibrated: H2 adsorption: H2(g) + 2* ⇄ 2H* (Kads,H2) Assuming any of the above steps to be rate-determining does not lead to rate expressions consistent with the observed reaction orders.
2) Applying site balance to all surface species, we have: where [*]0 is the total number of exposed metal sites, and Σ[Intermediates*] is the sum of the coverage of reaction intermediates and products. For ether adsorption on *n, because reaction orders in ether concentration are consistently zero order, we assume that the fraction of sites covered by adsorbed ether is at its maximum (i. 3a) Hydrogenolysis on Rh, Pd and Pt (reaction order in H2 is 0.5) The first hydrogen addition is quasi-equilibrated: where k1 and k-1 are the forward and reverse rate constants for the first H addition step. The first and second hydrogen additions are quasi-equilibrated: where ki and k-i are the forward and reverse rate constants for the first or second H addition step. 4a) Hydrogenation on Rh, Pd, Ir, and Pt (reaction order in H2 is 1) The first hydrogen addition is quasi-equilibrated: The second hydrogen addition is rate determining: 4b) Hydrogenation on Ru (reaction order in H2 is 1.5) The first and second hydrogen additions are quasi-equilibrated: The third hydrogen addition is rate determining:  Figure S1: Experimental dependence of the reaction rates on the H2 pressure in decalin at 150°C and fits (solid lines) with the rate equations derived in Section 3 using H2 adsorption equilibrium constants KH2,ads.
at reaction temperatures (Table S1)   We used the Langmuir equation for dissociative adsorption to fit the experimental chemisorption data to obtain equilibrium adsorption constants KH2,ads. at 35°C for H2: V corresponds to the volume adsorbed and V∞ to the volume absorbed at full coverage of the metal surface.
[H2] is the partial pressure of H2.
We used an adapted version of the van't Hoff equation to calculate H2 equilibrium adsorption constants KH2,ads. at reaction temperature (150°C): ∆rH θ is the standard reaction enthalpy (in this case the heat of H2 adsorption) and R is the ideal gas constant.