1. Introduction
Lignin is a complex aromatic polymer of plant origin that provides rigidity and strength to plant cell walls and is, for example, a key component of wood (about 30%). It is considered the only renewable resource in nature and is regarded as important for its conversion into petroleum-based chemicals. Lignin is composed of a complex spatial network structure, with syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units being connected by abundant stubborn C–O/C–C bonds (e.g., β-O-4, α-O-4, β-5, β–β).
My note: I want to make it clear that this article is not intended to be a standalone scientific study. Through this blog, I hope to carefully share scientifically relevant information. If you’d like to explore this topic in greater depth, I invite you to refer to the links to the scientific research that I’m sharing here so you can analyze all the results.
2. Single-atom catalyst (SAC) Ru-N-C
To produce fuels and chemicals, it is necessary to induce the oxidative depolymerization of lignin using a single-atom ruthenium catalyst (Ru-N-C). Oxidative depolymerization causes the cleavage of C-C and C-O bonds. The Ru–N–C SAC prepared with 12 h of ball milling demonstrated high catalytic performance in the oxidative depolymerization of various β-O-4 model compounds and diverse lignin feedstocks. Heterogeneous noble-metal-catalysts such as Pd, Ru and Au are commonly used for lignin depolymerization, among which Ru has drawn extensive attention for its relatively cheaper price and good catalytic activity. In single-atom catalysts, metal centers (Ru) are dispersed as individual atoms rather than as larger particles on the support surface, which means the particles are smaller in size and cover a larger surface area than their nanostructured counterparts, thereby leading to lower production costs. In this case, isolated ruthenium atoms are anchored within carbon containing added nitrogen atoms.
3. Advantages of SACs
SACs can be considered a bridge between heterogeneous and homogeneous catalysis, as they can achieve yields equal to or greater than those of homogeneous catalysts while retaining thephysical properties of heterogeneous catalysts.
- Metals present in small quantities
- High yield and catalytic selectivity: reducing costs and environmental impact.
- Fine-tuning of the structure: the preparation of catalysts allows for fine-tuning of the active site structure, thereby improving the efficiency of catalytic reactions.
- Maximum dispersion: the catalytic sites are in direct contact with the reaction environment, maximizing atomic efficiency.
4. Techniques
The chemical states or coordination environments of Ru atom on Ru–N–C SACs were identified by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and other physicochemical properties by N2 adsorption–desorption, and CO2 temperature-programmed desorption (CO2-TPD). These studies have shown that Ru–N4 served as the major active sites in O2 activation and C–O/C–C bond cleavage. At these sites, one ruthenium atom is held in place by surrounding nitrogen atoms, creating a location where oxygen molecules can be activated. That oxygen activation is essential because ordinary oxygen is not reactive enough to dismantle lignin efficiently.
5. Results
Photo from Unveiling the Role of Ru–N4 on Ru–N–C Single-Atom Catalyst in C–O/C–C Bonds’ Oxidative Cleavage in Lignin | ACS Catalysis. (a,b) STEM–EDX elemental mapping analysis of Ru–N–C (12 h), HRTEM of (c) Ru–N–C (12 h), (d,e) XRD patterns and Raman shift of Ru–N–C (2, 6, 12, 24 h) and N–C (12 h).
Figure 1a,1b shows the STEM–EDX mapping images of the as-prepared Ru–N–C (12 h) catalyst, which confirmed the successful introduction of Ru within the material. The HRTEM image in Figure 1c shows smooth surfaces and the absence of metal aggregates in Ru–N–C (12 h),which is consistent with the XRD patterns depicted in Figure 1d, where no signal of Ru particles but the peak at ∼26° associated with the (002) facet of graphitic carbon was observed. The Raman spectra of the various catalysts illustrated in Figure 1e confirm the graphitic nature of the carbon supports, revealing an inverse correlation between ID/IG and ball-milling time, i.e., a greater degree of graphitization with longer milling times.
Photo from Unveiling the Role of Ru–N4 on Ru–N–C Single-Atom Catalyst in C–O/C–C Bonds’ Oxidative Cleavage in Lignin | ACS Catalysis. (a) Ru–K-edge XANES spectra; (b) R-space magnitudes of k2-weighted EXAFS of Ru–N–C (2, 6, 12, 24 h), Ru(acac)3, and Ru foil references.
The chemical state and coordination environment of the Ru atom within the catalytic materials were analyzed by XAS. X-ray absorption near-edge structure (XANES) spectra in Figure 2a show that the absorption edge position of Ru–N–C (2, 6, 12, 24 h) lies between that of Ru foil, Ru(acac)3, and RuO2, indicating that Ru is present as cationic species in all materials. Fourier transform of the extended X-ray absorption fine structure (EXAFS) in Figure 2b identifies a dominant scatter with the Ru–N–C series at ∼2.02 Å (i.e., 2.01 2.03, 2.00, and 2.01 Å for 2, 6, 12, and 24 h, respectively). This is attributed to a Ru–N single scattering path, consistent with Ru coordinated to N in its first shell. Furthermore, the absence of a scatter at 2.67 Å in all catalysts rules out the presence of metallic Ru nanoparticles. Thus, it is concluded that Ru presents as isolated atoms coordinated to N within the support matrix, i.e., Ru–N4, and thus Ru–N–C SACs are successfully prepared.
Photo from: Unveiling the Role of Ru–N4 on Ru–N–C Single-Atom Catalyst in C–O/C–C Bonds’ Oxidative Cleavage in Lignin | ACS Catalysis. Proposed mechanism of Ru–N4 and N–C in cleaving C–O/C–C bond in lignin.
Bibliographic sources For more info Unveiling the Role of Ru–N4 on Ru–N–C Single-Atom Catalyst in C–O/C–C Bonds’ Oxidative Cleavage in Lignin | ACS Catalysis.
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