General Kinetic Modeling of the Selective Hydrogenation of 2-Methyl-3-butyn-2-ol over a Commercial Palladium-Based Catalyst

Zn-Promoted Selective Gas-Phase Hydrogenation of Tertiary and Secondary C4 Alkynols over Supported Pd

We have investigated the gas-phase (P = 1 atm; T = 373 K) hydrogenation of (tertiary alkynol) 2-methyl-3-butyn-2-ol (MBY) and (secondary) 3-butyn-2-ol (BY) over a series of carbon (C), non-reducible (Al₂O₃ and MgO), and reducible (CeO₂ and ZnO) supported monometallic [Pd (0.6-1.2% wt) and Zn (1% wt)] and bimetallic Pd-Zn (Pd:Zn mol ratio = 95:5, 70:30, and 30:70) catalysts synthesized by deposition-precipitation and colloidal deposition. The catalysts have been characterized by H₂ chemisorption, hydrogen temperature-programmed desorption (H₂-TPD), specific surface area (SSA), X-ray photoelectron spectroscopy (XPS), and transmission (TEM) and scanning transmission electron microscopy (STEM) analyses. Reaction over these catalysts generated the target alkenol [2-methyl-3-buten-2-ol (MBE) and 3-buten-2-ol (BE)] through partial hydrogenation and alkanol [2-methyl-butan-2-ol (MBA) and 2-butanol (BA)]/ketone [2-butanone (BONE)] as a result of full hydrogenation and double-bond migration. The catalysts exhibit a similar Pd nanoparticle size (2.7 ± 0.3 nm) but a modified electronic character (based on XPS). Hydrogenation activity is linked to surface hydrogen (from H₂ chemisorption and H₂-TPD). An increase in H₂:alkynol (from 1 → 10) results in enhanced alkynol consumption with a greater rate in the transformation of MBY (vs BY); H₂:alkynol had negligible effect on product distribution. Reaction selectivity is insensitive to the Pd site electron density with a similar response (S_MBE = 65 ± 9% and S_BE = 70 ± 8%) over Pd^δ- (on Al₂O₃ and MgO) and Pd^δ+ (on C and CeO₂). A Pd/ZnO catalyst delivered enhanced alkenol selectivity (S_MBE = 90% and S_BE = 96%) attributed to PdZn alloy phase formation (proved by XRD and XPS) but low activity, ascribed to metal encapsulation. A two-fold increase in the consumption rate was recorded for Pd-Zn/Al₂O₃ (30:70) versus Pd/ZnO with a similar alloy content (32 ± 4% from XPS), ascribed to a contribution due to spillover hydrogen (from H₂-TPD) where high alkenol selectivity was maintained.

Single-atom heterogeneous catalysts based on distinct carbon nitride scaffolds

Carbon nitrides integrating macroheterocycles offer unique potential as hosts for stabilizing metal atoms due to their rich electronic structure. To date, only graphitic heptazine-based polymers have been studied. Here, we demonstrate that palladium atoms can be effectively isolated on other carbon nitride scaffolds including linear melem oligomers and poly(triazine/heptazine imides). Increased metal uptake was linked to the larger cavity size and the presence of chloride ions in the polyimide structures. Changing the host structure leads to significant variation in the average oxidation state of the metal, which can be tuned by exchange of the ionic species as evidenced by X-ray photoelectron spectroscopy and supported by density functional theory. Evaluation in the semi-hydrogenation of 2-methyl-3-butyn-2-ol reveals an inverse correlation between the activity and the degree of oxidation of palladium, with oligomers exhibiting the highest activity. These findings provide new mechanistic insights into the influence of the carbon nitride structure on metal stabilization.

Hydrogenation of 2-methyl-3-butyn-2-ol over a Pd/ZnO catalyst: kinetic model and selectivity study

The three-phase hydrogenation of 2-methyl-3-butyn-2-ol has been studied over a Pd/ZnO catalyst. A mathematical model has been proposed to predict the behavior of the system in the typical operating ranges if industrial reactors.

Reactivity and mechanism investigation of selective hydrogenation of 2,3,5-trimethylbenzoquinone on in situ generated metallic cobalt

We successfully developed Co-based catalysts for efficient and selective hydrogenation of TMBQ. Metallic Co was proved to be responsible for TMBQ hydrogenation. The hydrogenation process was also investigated by DFT calculation.