Coupled Fluidized Bed Reactor for Pyridine Synthesis

To eliminate the feeding device coking deposit in a commercial pyridine synthesis reactor, a coupled fluidized bed reactor is proposed. The coupled reactor is composed of a feeding zone (FZ), a riser reaction zone (RRZ), and a fluidized bed reaction zone (FRZ). During 15 days of continuous operation, there is no coke deposit in the feeding device. Experimental results show that the product yield reaches as high as 75%. The selectivity is around 2.5 and is higher than that in commercial reactors, which is around 2.2. A core–annulus model and a dispersion model are proposed to model different zones of this coupled reactor, and the predicted average deviation from experimental data is 12%. The prediction results show that the main reactions take place at FZ and FRZ. RRZ contributes the least conversion, because of the limit by the mass transfer between the core and the annulus

Investigation of Pyridine Synthesis in a Fast Fluidized Bed Reactor

Pyridine has been generally synthesized by aldehydes and ammonia in a turbulent fluidized bed reactor. In this paper, a fast fluidized bed reactor was proposed for pyridine synthesis. Experiment results show that the yields of pyridine and 3-picoline decrease while the selectivity of pyridine over 3-picoline is increased. A model was proposed to predict the performance of the fast fluidized bed reactor; the average prediction deviation is 6%. The influence of mass transfer, heat transfer, and backmixing of the gas phase is represented by a modification factor, and the mean value of this modification factor is 0.75 within the experimental operating conditions. By model prediction, the reaction should be terminated when the critical point of R2 is reached to avoid over-reaction. To optimize the pyridine and 3-picoline product yield and minimize coke product yield, the reaction temperature should be kept around 723 K

Chiral Structure Determination of Aligned Single-Walled Carbon Nanotubes on Graphite Surface

Chiral structure determination of single-walled carbon nanotube (SWNT), including its handedness and chiral index (<i>n</i>,<i>m</i>), has been regarded as an intractable issue for both fundamental research and practical application. For a given SWNT, the <i>n</i> and <i>m</i> values can be conveniently deduced if an arbitrary two of its three crucial structural parameters, that is, diameter <i>d</i>, chiral angle θ, and electron transition energy <i>E</i>_<i>ii</i>, are obtained. Here, we have demonstrated a novel approach to derive the (<i>n</i>,<i>m</i>) indices from the θ, <i>d</i>, and <i>E</i>_<i>ii</i> of SWNTs. Handedness and θ were quickly measured based on the chirality-dependent alignment of SWNTs on graphite surface. By combining their measured <i>d</i> and <i>E</i>_<i>ii</i>, (<i>n</i>,<i>m</i>) indices of SWNTs can be independently and uniquely identified from the (θ,<i>d</i>) or (θ,<i>E</i>_<i>ii</i>) plots, respectively. This approach offers intense practical merits of high-efficiency, low-cost, and simplicity

Thermally Induced Transformation of Nonhexagonal Carbon Rings in Graphene-like Nanoribbons

Exploring the structural transformation of nonhexagonal rings is of fundamental importance for understanding the thermal stability of nonhexagonal rings and revealing the structure–property relationships. Here, we report on the thermally induced transformation from the fused tetragon-hexagon (4–6) carbon rings to a pair of pentagon (5–5) rings in the graphene-like nanoribbons periodically embedded with tetragon and octagon (4–8–4) carbon rings. A distinct contrast among tetragon, pentagon, hexagon, and octagon carbon rings is provided by noncontact atomic force microscopy with atomic resolution. The thermally activated bond rotation with the dissociation of the shared carbon dimer between the 4–6 carbon rings is the key step for the 4–6 to 5–5 transformation. The energy barrier of the bond rotation, which results in the formation of an irregular octagon ring in the transition state, is calculated to be 1.13 eV. The 5–5 defects markedly change the electronic local density of states of the graphene-like nanoribbon, as observed by scanning tunneling microscopy. Our density functional theory calculations indicate that the introduction of periodically embedded 5–5 rings will significantly narrow the electronic band gap of the graphene-like nanoribbons

Crystal structure of apo-Cas12g.

(A) Domain organization of Cas12g. The amino acid segment marked by the solid gray line represents the unresolved region. (B) Overall structure of apo Cas12g shown in one view (left) and rotated through 180° (right). (C) Surface representations of apo Cas12g in the same views as in (B).</p

RNA transcription template used in this study.

Cas12g is an endonuclease belonging to the type V RNA-guided CRISPR–Cas family. It is known for its ability to cleave RNA substrates using a conserved endonuclease active site located in the RuvC domain. In this study, we determined the crystal structure of apo-Cas12g, the cryo-EM structure of the Cas12g-sgRNA binary complex and investigated conformational changes that occur during the transition from the apo state to the Cas12g-sgRNA binary complex. The conserved zinc finger motifs in Cas12g undergo an ordered-to-disordered transition from the apo to the sgRNA-bound state and their mutations negatively impact on target RNA cleavage. Moreover, we identified a lid motif in the RuvC domain that undergoes transformation from a helix to loop to regulate the access to the RuvC active site and subsequent cleavage of the RNA substrate. Overall, our study provides valuable insights into the mechanisms by which Cas12g recognizes sgRNA and the conformational changes it undergoes from sgRNA binding to the activation of the RNase active site, thereby laying a foundation for the potential repurposing of Cas12g as a tool for RNA-editing.</div

Interaction between Cas12g and sgRNA.

(A) Ribbon diagram of sgRNA (left panel) and its interaction with the Helical 1, Helical 2 and RuvC domains (right panel). Close-up view of the opening on Cas12g guiding the guide crRNA into the central channel. (B) Recognition of the stem1 of sgRNA by the Helical 1 domain of Cas12g. (C) Recognition of the R:AR duplex 2 of sgRNA by the Helical 2 and RuvC domain of Cas12g. (D) RNA cleavage assay using wild-type Cas12g and Cas12g mutants in combination with sgRNA.</p

Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process

We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities

Circular dichroism (CD) spectra of wild-type and mutants of zinc finger motifs in Cas12g.

Circular dichroism (CD) spectra of wild-type and mutants of zinc finger motifs in Cas12g.</p

Detailed cryo-EM density map of the Cas12g-sgRNA complex with final atomic model fitted in.

(A) Cryo-EM map and model of the Cas12g-sgRNA complex with each domain of Cas12g color coded as in Fig 1(A). (B) Fitting of nucleic acids to the corresponding cryo-EM map. The atomic models are shown in stick with crRNA and tracrRNA. The crRNA strand and tracrRNA colored in orange and sky blue, respectively. (C) Fitting of the Helical 1 domain. Despite the unresolvable structure Helical 1 subdomain II (aa 232–355) region, its position in the Cas12g-sgRNA complex can be determined based on the density. (D-G) Fitting of the Helical 2 (D), WED (E), RuvC (F) and, Nuc domain (G). (TIF)</p