Ranajit Saha

Modelling of pyrolysis of coal–biomass blends using thermogravimetric analysis

The primary objective of this work was to develop an appropriate model to explain the co-pyrolysis behaviour of lignite coal-biomass blends with different proportions using a thermogravimetric analyzer. A new parallel-series kinetic model was proposed to predict the pyrolysis behaviour of biomass over the entire pyrolysis regime, while a kinetic model similar to that of Anthony and Howard [Anthony, D.B., Howard, J.B., 1976. Coal devolatilization and hydrogasification. AIChE Journal 22(4), 625-656] was used for pyrolysis of coal. Analysis of mass loss history of blends showed an absence of synergistic effect between coal and biomass. Co-pyrolysis mass-loss profiles of the blends were predicted using the estimated kinetic parameters of coal and biomass. Excellent agreement was found between the predicted and the experimental results.

Modelling of pyrolysis of large wood particles

A fully transient mathematical model has been developed to describe the pyrolysis of large biomass particles. The kinetic model consists of both primary and secondary reactions. The heat transfer model includes conductive and internal convection within the particle and convective and radiative heat transfer between the external surface and the bulk. An implicit Finite Volume Method (FVM) with Tridiagonal Matrix Algorithm (TDMA) is employed to solve the energy conservation equation. Experimental investigations are carried out for wood fines and large wood cylinder and sphere in an electrically heated furnace under inert atmosphere. The model predictions for temperature and mass loss histories are in excellent agreement with experimental results. The effect of internal convection and particle shrinkage on pyrolysis behaviour is investigated and found to be significant. Finally, simulation studies are carried out to analyze the effect of bulk temperature and particle size on total pyrolysis time and the final yield of char.

Comparative Study on the Noble-Gas Binding Ability of BeX Clusters (X = SO4, CO3, O)

Ab initio computations are carried out to assess the noble gas (Ng) binding capability of BeSO4 cluster. We have further compared the stability of NgBeSO4 with that of the recently detected NgBeCO3 cluster. The Ng-Be bond in NgBeCO3 is somewhat weaker than that in NgBeO cluster. In NgBeSO4, the Ng-Be bond is found to be stronger compared with not only the Ng-Be bond in NgBeCO3 but also that in NgBeO, except the He case. The Ar-Rn-bound BeSO4 analogues are viable even at room temperature. The Wiberg bond indices of Be-Ng bonds and the degree of electron transfer from Ng to Be are somewhat larger in NgBeSO4 than those in NgBeCO3 and NgBeO. Electron density and energy decomposition analyses are performed in search of the nature of interaction in the Be-Ng bond in NgBeSO4. The orbital energy term (ΔE(orb)) contributes the maximum (ca. 80-90%) to the total attraction energy. The Ar/Kr/Xe/Rn-Be bonds in NgBeSO4 could be of partial covalent type with a gradual increase in covalency along Ar to Rn.

A coupled-cluster study on the noble gas binding ability of metal cyanides versus metal halides (metal = Cu, Ag, Au).

A coupled‐cluster study is carried out to investigate the efficacy of metal(I) cyanide (MCN; M = Cu, Ag, Au) compounds to bind with noble gas (Ng) atoms. The MNg bond dissociation energy, enthalpy change, and Gibbs free energy change for the dissociation processes producing Ng and MCN are computed to assess the stability of NgMCN compounds. The Ng binding ability of MCN is then compared with the experimentally detected NgMX (X = F, Cl, Br) compounds. While CuCN and AgCN have larger Ng binding ability than those of MCl and MBr (M = Cu, Ag), AuCN shows larger efficacy toward bond formation with Ng than that of AuBr. Natural bond orbital analysis, energy decomposition analysis in conjunction with the natural orbital for chemical valence theory, and the topological analysis of the electron density are performed to understand the nature of interaction occurring in between Ng and MCN. The NgM bonds in NgMCN are found comprise an almost equal contribution from covalent and electrostatic types of interactions. The different electron density descriptors also reveal the partial covalent character in the concerned bonds.

Exploring the Nature of Silicon-Noble Gas Bonds in H3SiNgNSi and HSiNgNSi Compounds (Ng = Xe, Rn)

Ab initio and density functional theory-based computations are performed to investigate the structure and stability of H3SiNgNSi and HSiNgNSi compounds (Ng = Xe, Rn). They are thermochemically unstable with respect to the dissociation channel producing Ng and H3SiNSi or HSiNSi. However, they are kinetically stable with respect to this dissociation channel having activation free energy barriers of 19.3 and 23.3 kcal/mol for H3SiXeNSi and H3SiRnNSi, respectively, and 9.2 and 12.8 kcal/mol for HSiXeNSi and HSiRnNSi, respectively. The rest of the possible dissociation channels are endergonic in nature at room temperature for Rn analogues. However, one three-body dissociation channel for H3SiXeNSi and one two-body and one three-body dissociation channels for HSiXeNSi are slightly exergonic in nature at room temperature. They become endergonic at slightly lower temperature. The nature of bonding between Ng and Si/N is analyzed by natural bond order, electron density and energy decomposition analyses. Natural population analysis indicates that they could be best represented as (H3SiNg)+(NSi)− and (HSiNg)+(NSi)−. Energy decomposition analysis further reveals that the contribution from the orbital term (ΔEorb) is dominant (ca. 67%–75%) towards the total attraction energy associated with the Si-Ng bond, whereas the electrostatic term (ΔEelstat) contributes the maximum (ca. 66%–68%) for the same in the Ng–N bond, implying the covalent nature of the former bond and the ionic nature of the latter.

On the stability of noble gas bound 1-tris(pyrazolyl)borate beryllium and magnesium complexes

An in silico study is performed to assess the noble gas (Ng) binding ability of 1-tris(pyrazolyl)borate beryllium and magnesium cationic complexes (TpBe+ and TpMg+). The Be and Mg centers in these complexes are found to bind heavier Ng atoms quite effectively. Both the zero point energy and basis set superposition error corrected dissociation energy values for the bonds between Ar–Rn and metal atoms range within 5.8–10.2 kcal mol−1 for Be and within 5.2–9.9 kcal mol−1 for Mg. The dissociation of the Kr–Rn bound analogues of TpBe+ and Ar–Rn bound analogues of TpMg+ into the individual Ng atoms and TpBe+ or TpMg+ complexes is endergonic in nature at room temperature. The remaining lighter Ng bound complexes would be stable at lower temperatures. The nature of Be–Ng or Mg–Ng bonds is explored via Wiberg bond indices computation, atoms-in-molecules and energy decomposition analyses. The degree of covalent character in the Be/Mg–Ng bonds increases gradually in moving from He to its heavier congeners. The Be–Xe/Rn and Mg–Xe/Rn bonds could be categorized as being of the partial covalent type. The contribution from the orbital term is at the maximum towards the total attraction. The magnitude of this term becomes gradually larger from He to Rn, implying a larger degree of covalent character for heavier Ng atoms.

Noble gas supported B3+ cluster: formation of strong covalent noble gas–boron bonds

The stability of noble gas (Ng) bound B3+ clusters is assessed via an in silico study, highlighting their structure and the nature of the Ng–B bonds. Ar to Rn atoms are found to form exceptionally strong bonds with B3+ having each Ng–B bond dissociation energy in the range of 15.1–34.8 kcal mol−1 in B3Ng3+ complexes with a gradual increase in moving from Ar to Rn. The computed thermochemical parameters like enthalpy and free energy changes for the Ng dissociation processes from B3Ng3+ also support the stability of Ar to Rn analogues for which the corresponding dissociation processes are endergonic in nature even at room temperature. The covalent nature of the Ng–B bonds is indicated by the localized natural Ng–B bond orbitals and high Wiberg bond indices (0.57–0.78) for Ng–B bonds. Electron density analysis also supports the covalency of these Ng–B bonds where the electron density is accumulated in between Ng and B centres. The orbital interaction energy is the main contributor (ca. 63.0–64.4%) of the total attraction energy in Ng–B bonds. Furthermore, the Ng–B bonding can be explained in terms of a donor–acceptor model where the Ng (HOMO) → B3Ng2+ (LUMO) σ-donation has the major contribution.

A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12– Clusters (M = Co, Rh, Ir)

Strong binding of carbon monoxide (CO) and dinitrogen (N2) by MB12- (M = Co, Rh, Ir) clusters results in a spinning umbrella-like structure. For OCMB12- and NNMB12- complexes, the bond dissociation energy values range within 50.3-67.7 kcal/mol and 25.9-35.7 kcal/mol, respectively, with the maximum value obtained in Ir followed by that in Co and Rh analogues. COMB12- complex is significantly less stable than the corresponding C-side bonded isomer. The associated dissociation processes for OCMB12- and NNMB12- into CO or N2 and MB12- are highly endergonic in nature at 298 K, implying their high thermochemical stability with respect to dissociation. In OCMB12- and NNMB12- complexes, the C-O and N-N bonds are found to be elongated by 0.022-0.035 Å along with a large red-shift in the corresponding stretching frequencies, highlighting the occurrence of bond activation therein toward further reactivity due to complexation. The obtained red-shift is explained by the dominance of L←M π-back-donation (L = CO, OC, NN) over L→M σ-donation. The binding of L enhances the energy barrier for the rotation of the inner B3 unit within the outer B9 ring by 0.4-1.8 kcal/mol, which can be explained by a reduction in the distance of the longest bond between inner B3 and outer B9 rings upon complexation. A good correlation is found between the change in rotational barrier relative to that in MB12- and the energy associated with the L→M σ-donation. Born-Oppenheimer molecular dynamics simulations further support that the M-L bonds in the studied systems are kinetically stable enough to retain the original forms during the internal rotation of inner B3 unit.

Ligand-Supported E3 Clusters (E=Si-Sn)

The interaction among E3 (E=Si, Ge, Sn) clusters and different ligands (L) encompassing five carbon-based donors (cyclic (alkyl)(amino)carbene (cAAC), N-heterocyclic carbene (NHC), saturated NHC (SNHC), mesoionic carbenes (MIC1, and MIC2)), two nitrogen-based donors (trimethylamine and pyridine), and two phosphorous-based donors (phosphinine and trimethylphosphine) in E3 (L)3 complexes is explored through DFT computations. Although all carbenes form very strong bonds with E3 clusters, cAAC makes the strongest bond with Si3 and Ge3 clusters, and MIC1 with the Sn3 cluster. Nevertheless, other ligand-bound complexes are also viable at room temperature. This finding indicates that experimentalists may make use of them to synthesize the desired clusters based on precursor availability. The nature of the interaction in E-L bonds is analyzed through natural bond orbital analysis; energy decomposition analysis, in combination with the natural orbital for chemical valence; and adaptive natural density partitioning analysis. The L→E σ-donation and L←E π-back-donation play important roles in making contacts between L and E3 clusters favorable; where the former is significantly more dominant over the latter.

Modeling of 1-D Nanowires and analyzing their Hydrogen and Noble Gas Binding Ability

AbstractThe theoretical calculation at the M05-2X/6-311+G(d,p) level reveals that the B–B bond length in [N4-B2-N4]2− system (1.506 Å) is slightly smaller than that of typical B=B bond in B2H2 (1.518 Å). These systems interact with each M+ (M = Li, Na, K) ion very strongly with a binding energy of 213.5 (Li), 195.2 (Na) and 180.3 (K) kcal/mol. Additionally, the relief of the Coulomb repulsion due to the presence of counter-ion, M+, the B–B bond contracts to 1.484–1.488 Å in [N4-B2-N4]M2. We have further extended our study to [N4-B2-N4-B2-N4]4− and [N4-B2-N4-B2-N4-B2-N4]6− systems. The B–B bond length is found to be 1.496 Å in the former case, whereas the same is found to be 1.493 Å and 1.508 Å, respectively, for the two B–B bonds present in the latter one. The M + counter-ions stabilize such negatively charged systems and thus, create a possibility to design a long 1-D nanowire. Their utilities as probable hydrogen and noble gas (Ng) binding templates are explored taking [N4-B2-N4-B2-N4]Li4 system as a reference. It is found that each Li center binds with three H2 molecules with an average binding energy of 2.1 kcal/mol, whereas each Ng (Ar–Rn) atom interacts with Li center having a binding energy of 1.8–2.1 kcal/mol. The H2 molecules interact with Li centers mainly through equal contribution from orbital and electrostatic interaction, whereas the orbital interaction is found to be major term (ca. 51–58%) in Ng-Li interaction followed by dispersion (ca. 24–27%) and electrostatic interaction (ca. 17–24%). Graphical AbstractThe B-B bonds in [N4-B2-N4]2-, [N4-B2-N4-B2-N4]4- and [N4-B2-N4-B2-N4-B2-N4]6- possess some degree of triple bond character. These anionic systems can be stabilized by combining with an adequate number of M+ (M = Li, Na, K) ions. It opens up the possibility of designing a long 1-D nanowire. The Li centers of this nanowire can bind hydrogen molecules and noble gas atoms effectively.