Breaking scaling relationships in alkynol semi-hydrogenation by manipulating interstitial atoms in Pd with d-electron gain

Pd catalysts are widely used in alkynol semi-hydrogenation. However, due to the existence of scaling relationships of adsorption energies between the key adsorbed species, the increase in conversion is frequently accompanied by side reactions, thereby reducing the selectivity to alkenols. We report that the simultaneous increase in alkenol selectivity and alkynol conversion is achieved by manipulating interstitial atoms including B, P, C, S and N in Pd catalysts. A negative linear relationship is observed between the activation entropies of 2-methyl-3-butyn-2-ol and 2-methyl-3-buten-2-ol which is highly related to the filling of d-orbital of Pd catalysts by the modification of p-block elements. A catalyst co-modified by B and C atoms has the maximum d charge of Pd that achieves a 17-fold increase in the turn-over frequency values compared to the Lindlar catalysts in the semi-hydrogenation of 2-methyl-3-butyn-2-ol. When the conversion is close to 100%, the selectivity can be as high as 95%.


Synthesis of phenolic resins.
The carbon precursors (low molecular weight, soluble phenolic resins) were prepared from phenol and formaldehyde 1 .
In a typical synthesis, 16.0 g of phenol, and 3.4 g of a 20 wt% aqueous sodium hydroxide (NaOH) solution were mixed in a round bottom flask, and heated to 45 °C. After 15 min, 28.2 g of 37 wt% formaldehyde was added. The mixture was then heated at 70 °C for 1 h with 360 rpm stirring. Next, the pH value was adjusted to ~7.0 with a 2 M HCl solution, and the superfluous water was removed by vacuum rotary evaporator below 45 °C. The soluble phenolic resins were dissolved in ethanol (20 wt%) for further use.

Synthesis of B-OMC.
B-doped ordered mesoporous carbon (B-OMC) was synthesized by a solvent evaporation induced self-assembly Small-angle X-ray scattering.
SAXS patterns were taken on a Bruker Nanostar U SAXS system using Cu Kα radiation (40 mV, 35 mA).

Thermogravimetry-mass spectrometry.
The TG-MS analyses were performed on a Rigaku Thermo Mass Photo TG-DTA instrument. First, the as-made products (ca. 150 mg) were dried at 100 C for 1 h and then cooled to 40 C in flowing high purity Ar (>99.999%).
Next, the sample were calcined in an Ar atmosphere at 350 C, and held at 350 °C for 160 mins. The heating rate was 5 °C min -1 . Analysis of the outlet gases from the decomposition of as-made products a by mass spectrometer.

N2 adsorption-desorption isotherms.
N2 adsorption-desorption isotherms were measured at 77 K with a Micromeritics TriStar II 3020 instrument. The

CO pulse chemisorption.
The dispersion of supported Pd catalyst was measured by pulsing CO adsorption on a Micromeritics Auto Chem II 2920 system. Catalysts were pretreated in 10 vol.% H2/Ar at 100 °C for 1 h then cooled to room temperature by pure He flowing at a of 20 mL min -1 . Analyses were taken by pulsing research grade CO on the pretreated catalyst. Ten pulses were recorded per analysis and referenced to a blank run obtained without adding any solid catalyst. The loop volume used was 0.5 mL with pulse decay set to 15 min. A 1:1 CO to metal stoichiometric factor for dispersion calculations was used.

Electrochemical H adsorption experiment.
Electrochemical experiments were carried out in a traditional three

Kinetics calculations.
The TOF for the all studied Pd catalyst was calculated on the basis of the estimated number of exposed palladium atoms, at a conversion below 20%.
where nSub is the molar amount of the substrate, X is the conversion, nPd is the molar amount of Pd, t is the reaction time, and τ is the exposed surface atom dispersion. τ is measured by CO pulse adsorption.
Apparent activation energies (Ea) were calculated according to the Arrhenius equation: where k is the reaction rate constant, A is the apparent pre-exponential factor, R is the universal gas constant and T is the reaction temperature. Taking into account the approximate 1/2-order reaction kinetics for H2, the reaction rate constant (k) was calculated according to the rate equation for the chemical reaction: where r0 is the rate of reaction, H 2 is the saturated concentration of H2 in ethanol.
The mole fraction XH 2 under different pressure of H2 was estimated from Henry's law 5 : where PH 2 is the hydrogen pressure, and K is the Henry constant. At 298 K, the value of K is 513 Mpa 6 .
Under same pressure of H2, the mole fraction XH 2 at different temperature was estimated as follows 7 : where X1, X2 are the mole fraction of H2 in ethanol at T1 and T2 temperature, respectively.
The entropy of activation (ΔS 0* ) was determined as follows.
The TOF value was expressed in the Eyring form 8,9 : where kB, h, ΔS 0* , and ΔH 0* is the Boltzmann constant, Planck constant, entropy of activation, and enthalpy of activation, respectively.
The apparent activation energy Ea was related to ΔH 0* by the Temkin equation: E a = ΔH 0* + ∑ n i ΔH i (8) where ΔHi and ni are the adsorption enthalpies and the reaction order of reactant i, respectively. 29 Taking into account the approximate 1/2-order reaction kinetics for MBY or MBE. The entropy change in the activation step of the chemical reaction was closely related to the thermodynamics of the rate constant, which results in the following equation: ΔS 0* = R ln ( Ah k B Te 1 2τ ) (9) Calculation of the d electron gain of the Pd nanocatalysts.
The difference in the number of 4d electrons (d charge) between the samples and reference Pd catalyst was evaluated from the Pd L3-edge XANES using the following equation (12)  10.45 (10) where A is the peak area of the white lines at the L3-or L2-edge, and 10.45 is the absorption cross-section per hole in 4d band of each Pd atom. In order to eliminate the size effect, Pd/OMC was used as the Pd reference catalyst. The ratio of L3 peak area to L2 peak area was determined to be about 2.5 according to the literature for simplification 11 .