Investigation of the Suzuki-Miyaura cross-coupling reaction on a palladium H-beta zeolite with DFT calculations.

Metal or metal cluster-doped zeolites catalyse a wide variety of reactions. In this work, a coupling reaction between bromobenzene and phenylboronic acid to yield biphenyl with the Pd-H-Beta zeolite catalyst was investigated with density functional theory (DFT) calculations. Utilizing a model system with tetrahedral Pd4 clusters within the H-Beta zeolite, it was demonstrated that the catalyst exhibited notable reactivity by effectively reducing the activation energy barrier for the reaction. Our investigation revealed that the zeolite framework facilitated electron transfer to the Pd cluster, thereby increasing the reaction activity. The coupling reaction was shown to be exothermic and comprise three main steps: oxidative addition of bromobenzene (C6H5Br), transmetallation with phenylboronic acid (C6H5B(OH)2), and reductive elimination of biphenyl (C12H10). Specifically, in the transmetallation step, which was the rate-determining step, the C-B bond breaking in phenylboronic acid (C6H5B(OH)2) and the phenylboronate anion (C6H5B(OH)3-) were compared under neutral and basic conditions, respectively. This comprehensive study clarifies the mechanism for the reaction with the modified Pd zeolite catalyst and highlights the essential role of the zeolite framework.

Combined Experimental and Theoretical Study of the Synthesis of 5,7-Dihydroxy-4-methylcoumarin via a Pechmann Condensation in the Presence of UiO-66-SO3H Catalysts.

An efficient synthesis of 5,7-dihydroxy-4-methylcoumarin from phloroglucinol with ethyl acetoacetate in the UiO-66-SO3H metal-organic framework is reported. The potential of UiO-66-SO3H as a solid catalyst was determined through optimized-condition experiments and quantum molecular calculations. The optimal conditions for the synthesis of 5,7-dihydroxy-4-methylcoumarin with UiO-66-SO3H were as follows: phloroglucinol/ethyl acetoacetate molar ratio = 1:1.6, reaction time = 4 h, and temperature = 140 °C, for which the reaction yield reached 66.0%. The reusability of UiO-66-SO3H catalysts for Pechmann condensation was examined. The activation energy of the reaction occurring on a sulfonic group of the UiO-66-SO3H catalyst was 12.6 kcal/mol, which was significantly lower than 22.6 kcal/mol of the same reaction on the UiO-66 catalyst. To comprehend the reaction mechanism, density functional theory with the ONIOM approach was applied for the synthesis of coumarin on the UiO-66-SO3H and UiO-66 clusters. A possible reaction mechanism was proposed involving three steps: a trans-esterification step, an intramolecular hydroxyalkylation step, and a dehydration step. The rate-determining step was suggested to be the first step which acquired an activation energy of 15.7 and 29.5 kcal/mol, respectively. Information from this study can be used as guidelines to develop more efficient catalytic metal-organic frameworks for various organic syntheses.

Hydroxylation of UiO-66 Metal-Organic Frameworks for High Arsenic(III) Removal Efficiency.

Zirconium clusters of UiO-66 have been hydroxylated with NaOH to generate strong binding sites for As(III) species in wastewater treatment. Hydroxylated UiO-66 provides high adsorption capacity over a wide range of pH from 1 to 10 with a maximum uptake of 204 mg g-1, which is significantly enhanced compared to those of pristine UiO-66, acid-modulated UiO-66, and other adsorbents for use in a wide pH range of treatment processes. The local structure of hydroxylated sites and As(III) adsorption mechanism are determined by extended X-ray absorption fine structure combined with density functional theory calculations.

Intrinsic Hole Mobility in Luminescent Metal-Organic Frameworks and Its Application in Organic Light-Emitting Diodes.

Most metal-organic frameworks (MOFs) lack charge mobility, which is crucial for realizing their use in optoelectronic applications. This work proposes the design of a MOF using triarylamine-based ligands (Zr-NBP) as the lone pair electron spacer to enhance the hole mobility in the MOF while maintaining its luminescent properties. Zr-NBP has strong fluorescence with a good hole mobility of 1.05×10-6  cm2 V-1 s-1 , which is comparable to organic materials used in optoelectronic devices. We also employed a Zr-NBP nanofilm in the pure phase as both a non-doped emissive layer and a hole-transporting layer within organic light-emitting diodes (OLEDs). The obtained OLED device produced a bright green light with a low turn-on voltage of 3.9 V. This work presents an advance in developing the electronic properties of MOFs by modifying the chemical properties of its building blocks, and will likely inspire further design of MOF materials as active layers in optoelectronic devices.

Adsorption and dehydration of ethanol on isomorphously B, Al, and Ga substituted H-ZSM-5 zeolite: an embedded ONIOM study.

Dehydration reactions are important in the petroleum and petrochemical industries, especially for the feedstock production. In this work, the catalytic activity of zeolites with different acidities for the dehydration of ethanol to ethylene and diethylether is investigated by density functional calculations on cluster models of three isomorphous B, Al, and Ga substituted H-ZSM-5 zeolites. Both unimolecular and bimolecular mechanisms are investigated. Detailed reaction profiles for the dehydration reaction, assuming either a stepwise or a concerted mechanism, were calculated by using the ONIOM(MP2:M06-2X) + SCREEP method. The adsorption energies of ethanol are -21.6, -28.1, and -27.7 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the rate-determining step of the unimolecular concerted mechanism for the ethylene formation are 48.5, 42.6, and 43.6 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the ethoxy formation as the rate-determining step for the bimolecular formation of diethylether are 42.3, 40.0, and 41.1 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The results indicate that the catalytic activities for the dehydration of ethanol decrease in the order H-[Al]-ZSM-5 ~ H-[Ga]-ZSM-5 > H-[B]-ZSM-5. Besides the acid strength, the zeolite framework affects the reaction by stabilizing the reaction intermediates, leading to more stable adsorption complexes and lower activation barriers.

Incorporation of Al3+ Sites on Brønsted Acid Metal-Organic Frameworks for Glucose-to-Hydroxylmethylfurfural Transformation.

5-hydroxylmethylfurfural (HMF) is a bio-based chemical that can be prepared from natural abundant glucose by using combined Brønsted-Lewis acid catalysts. In this work, Al3+ catalytic site has been grafted on Brønsted metal-organic frameworks (MOFs) to enhance Brønsted-Lewis acidity of MOF catalysts for a one-pot glucose-to-HMF transformation. The uniform porous structure of zirconium-based MOFs allows the optimization of both acid strength and density of acid sites in MOF-based catalysts by incorporating the desired amount of Al3+ catalytic sites at the organic linker. Al3+ sites generated via a post-synthetic modification act as Lewis acid sites located adjacent to the Brønsted sulfonated sites of MOF structure. The local structure of the Al3+ sites incorporated in MOFs has been elucidated by X-ray absorption near-edge structure (XANES) combined with density functional theory (DFT) calculations. The cooperative effect from Brønsted and Lewis acids located in close proximity and the high acid density is demonstrated as an important factor to achieve high yield of HMF.

Theoretical study of methane adsorption and C─H bond activation over Fe-embedded graphene: Effect of external electric field.

The effect of an external electric field (EF) on the methane adsorption and its activation on iron-embedded graphene (Fe-GPs) are investigated by using the M06-L density functional method. The EF is applied in the perpendicular direction to the graphene in the range of -0.015 to +0.015 a.u. with the interval of 0.005 a.u. The effects of EF on the adsorption, transition state and product complexes of the methane activation reaction are revealed. The binding energies of methane on Fe site in Fe-GPs are increased from -12.9 to -15.3, -18.1 and -21.5 kcal/mol for the negative EF of -0.005, -0.010 and -0.015, respectively. By applying positive EF, the activation barriers for methane activation are reduced in range of 3-8 kcal/mol (around 12-31%) and the reaction energies are more exothermic. The positive EF kinetically favors the reaction compared to the system without EF. The adsorption and activation of methane on Fe-GPs can be easily tuned by adjusting the external electric field for various applications. © 2019 Wiley Periodicals, Inc.

Construction of 5-Aminotetrazoles via in Situ Generation of Carbodiimidium Ions from Ketones Promoted by TMSN3/TfOH.

A novel synthetic approach for the synthesis of 5-aminotetrazoles has been developed by employing simple ketones as substrates. This methodology involved the N2-extrusion/aryl migration of azido complexes as the key step for the in situ generation of carbodiimidium ion, which could further react with hydrazoic acid and cyclize intramolecularly to provide 5-aminotetrazoles in good to excellent yields. In addition, the regioselectivity of the reaction was studied and rationalized by density functional theory calculations.

Theoretical study of CO2 hydrogenation into formic acid on Lewis acid zeolites.

Conversion of carbon dioxide (CO2) to more valuable chemicals is nowadays receiving increasing attention from an environmental and industrial point of view. Herein, we computationally investigated CO2 hydrogenation to formic acid on Lewis acid zeolites by means of density functional theory (DFT) with the M06-L functional. The reaction proceeds in two steps, hydrogenation of CO2 to form the formate intermediate and hydrogen-abstraction to form formic acid. A defect zeolite seems to be favored over a perfect one, leading to its low rate determining step barrier of 5.2 kcal mol-1. We also considered the effect of the zeolite frameworks and found that the catalytic activities are in the order Sn-ZSM-5 > Sn-BEA > Sn-FAU. Finally, we performed catalytic activity screenings of tetravalent metals (Ge, Zr and Hf) substituted into the defect Sn-ZSM-5 zeolite. The order Hf > Zr > Sn > Ge was found based on the rate determining step activation energy. The difference in activation energy can be explained by the difference in charge transfer from the catalytic site to the reacting molecules.

Theoretical study on the reaction mechanism of hydrogenation of furfural to furfuryl alcohol on Lewis acidic BEA zeolites: effects of defect structure and tetravalent metals substitution.

Furfural acquired from agricultural sources is receiving extensive attention in the petrochemical industry as it offers an alternative route to generate more valuable hydrocarbon compounds. Herein, we investigate the furfural hydrogenation to furfuryl alcohol catalyzed by Lewis acidic BEA zeolites at the molecular level by means of the M06-L density functional theory. The mechanistic pictures in the catalytic procedure are revealed. The possible reaction pathways are considered to proceed via either concerted or stepwise mechanisms. With the contribution of zeolite oxygen bridging for the H-H splitting, the rate determining step activation barrier for the stepwise mechanism is 14.7 kcal mol-1 lower than that for the concerted mechanism. The stepwise reaction therefore seems to be favored compared to the concerted one. The catalytic effect of the defect zeolite framework on the stepwise mechanism is also investigated. The activation energy for the stepwise rate-determining step over this site is significantly lower than the corresponding step over the perfect one by 14.1 kcal mol-1. Finally, the catalytic activity of tetravalent metal centers (Sn, Ge, Zr and Hf) substituted in BEA is also preliminarily compared and it is found to follow the order of Hf > Zr > Sn > Ge based on activation energies and the reaction rate. The difference in the activation energy can be traced back to the difference in the charge transfer from the catalytic site to the adsorbed molecules.

Metal-porphyrin: a potential catalyst for direct decomposition of N(2)O by theoretical reaction mechanism investigation.

The adsorption of nitrous oxide (N2O) on metal-porphyrins (metal: Ti, Cr, Fe, Co, Ni, Cu, or Zn) has been theoretically investigated using density functional theory with the M06L functional to explore their use as potential catalysts for the direct decomposition of N2O. Among these metal-porphyrins, Ti-porphyrin is the most active for N2O adsorption in the triplet ground state with the strongest adsorption energy (-13.32 kcal/mol). Ti-porphyrin was then assessed for the direct decomposition of N2O. For the overall reaction mechanism of three N2O molecules on Ti-porphyrin, two plausible catalytic cycles are proposed. Cycle 1 involves the consecutive decomposition of the first two N2O molecules, while cycle 2 is the decomposition of the third N2O molecule. For cycle 1, the activation energies of the first and second N2O decompositions are computed to be 3.77 and 49.99 kcal/mol, respectively. The activation energy for the third N2O decomposition in cycle 2 is 47.79 kcal/mol, which is slightly lower than that of the second activation energy of the first cycle. O2 molecules are released in cycles 1 and 2 as the products of the reaction, which requires endothermic energies of 102.96 and 3.63 kcal/mol, respectively. Therefore, the O2 desorption is mainly released in catalytic cycle 2 of a TiO3-porphyrin intermediate catalyst. In conclusion, regarding the O2 desorption step for the direct decomposition of N2O, the findings would be very useful to guide the search for potential N2O decomposition catalysts in new directions.

Glycine peptide bond formation catalyzed by faujasite.

The catalysis of peptide bond formation between two glycine molecules on H-FAU zeolite was computationally studied by the M08-HX density functional. Two reaction pathways, the concerted and the stepwise mechanism, starting from three differently adsorbed reactants, amino-bound, carboxyl-bound, and hydroxyl-bound, are studied. Adsorption energies, activation energies, and reaction energies, as well as the corresponding intrinsic rate constants were calculated. A comparison of the computed energetics of the various reaction paths for glycine indicates that the catalyzed reaction proceeds preferentially via the concerted reaction mechanism of the hydroxyl-bound configuration. This involves an eight-membered ring of the transition structure instead of the four-membered ring of the others. The step from the amino-bound configuration to glycylglycine is the rate-determining step of the concerted mechanism. It has an estimated activation energy of 51.2 kcal mol(-1). Although the catalytic reaction can also occur via the stepwise reaction mechanism, this path is not favored.

Structures and reaction mechanisms of glycerol dehydration over H-ZSM-5 zeolite: a density functional theory study.

The initial stage of glycerol conversion over H-ZSM-5 zeolite has been investigated using density functional theory (DFT) calculations on an embedded cluster model consisting of 128 tetrahedrally coordinated atoms. It is found that glycerol dehydration to acrolein and acetol proceeds favourably via a stepwise mechanism. The formation of an alkoxide species upon the first dehydration requires the highest activation energy (42.5 kcal mol(-1)) and can be considered as the rate determining step of the reaction. The intrinsic activation energies for the first dehydration are virtually the same for both acrolein and acetol formation, respectively, suggesting the competitive removal of the primary and secondary OH groups. A high selectivity to acrolein at moderate temperatures can be attributed to the selective activation of the stronger adsorption mode of glycerol through the secondary OH group and the kinetically favoured subsequent consecutive steps. In addition, the less reactive nature of acrolein relative to acetol precludes it from being converted to other products upon conversion to glycerol. In accordance with typical endothermic reactions, the forward rate constant for glycerol dehydration significantly increases with increasing reaction temperature.

Oxidative dehydrogenation of propane over a VO2-exchanged MCM-22 zeolite: a DFT study.

The adsorption and the mechanism of the oxidative dehydrogenation (ODH) of propane over VO(2)-exchanged MCM-22 are investigated by DFT calculations using the M06-L functional, which takes into account dispersion contributions to the energy. The adsorption energies of propane are in good agreement with those from computationally much more demanding MP2 calculations and with experimental results. In contrast, B3LYP binding energies are too small. The reaction begins with the movement of a methylene hydrogen atom to the oxygen atom of the VO(2) group, which leads to an isopropyl radical bound to a HO-V-O intermediate. This step is rate determining with the apparent activation energy of 30.9 kcal mol(-1), a value within the range of experimental results for ODH over other silica supports. In the propene formation step, the hydroxyl group is the more reactive group requiring an apparent activation energy of 27.7 kcal mol(-1) compared to that of the oxy group of 40.8 kcal mol(-1). To take the effect of the extended framework into account, single-point calculations on 120T structures at the same level of theory are performed. The apparent activation energy is reduced to 28.5 kcal mol(-1) by a stabilizing effect caused by the framework. Reoxidation of the catalyst is found to be important for the product release at the end of the reaction.

Effects of the zeolite framework on the adsorptions and hydrogen-exchange reactions of unsaturated aliphatic, aromatic, and heterocyclic compounds in ZSM-5 zeolite: a combination of perturbation theory (MP2) and a newly developed density functional theory (M06-2X) in ONIOM scheme.

The confinement effect on the adsorption and reaction mechanism of unsaturated aliphatic, aromatic and heterocyclic compounds on H-ZSM-5 zeolite has been investigated by the four ONIOM methods (MP2:M06-2X), (MP2:B3LYP), (MP2:HF), and (MP2:UFF). The H-ZSM-5 'nanoreactor' porous intersection, where chemical reactions take place, is represented by a quantum cluster of 34 tetrahedral units. Ethene, benzene, ethylbenzene, and pyridine are chosen to represent reactions of various adsorbates of aliphatic, aromatic and heterocyclic compounds. Among the four combined methods, (MP2:M06-2X) outperforms the others. The results confirm that the method that takes weak interactions, especially the van der Waals interaction, into account is essential for describing the confinement effect from the zeolite framework. The effects of the infinite zeolitic framework on the cluster model are also included by a set of point charges generated by the embedded ONIOM model. The energies for the adsorption of ethene, benzene, ethylbenzene, and pyridine on H-ZSM-5 from an embedded ONIOM(MP2:M06-2X) calculation are predicted to be -14.0, -19.8, -24.7, and -48.4 kcal/mol, respectively, which are very close to available experimental observations. The adsorption energy of pyridine agrees well with the experiment data of -47.6 kcal/mol. We also applied the same computational methodology on the systematic investigation of the H/H exchange reaction of benzene and ethylbenzene with the acidic H-ZSM-5 zeolite. The H/H exchange reaction was found to take place in a single concerted step. The calculated apparent activation energies for benzene and ethylbenzene are 12.6 and 4.9 kcal/mol, which can be compared to the experimental estimates of 11.0 and 6.9 kcal/mol, respectively. The confinement effect of the extended zeolite framework has been clearly demonstrated not only to stabilize the adsorption complexes but also to improve their corresponding activation energies to approach the experimental benchmark.