Confining Gold Nanoparticles in Preformed Zeolites by Post-Synthetic Modification Enhances Stability and Catalytic Reactivity and Selectivity.

Confining Au nanoparticles (NPs) in a restricted space (e.g., zeolite micropores) is a promising way of overcoming their inherent thermal instability and susceptibility to aggregation, which limit catalytic applications. However, such approaches involve complex, multistep encapsulation processes. Here, we describe a successful strategy and its guiding principles for confining small (<2 nm) and monodisperse Au NPs within commercially available beta and MFI zeolites, which can oxidize CO at 40 °C and show size-selective catalysis. This protocol involves post-synthetic modification of the zeolite internal surface with thiol groups, which confines AuCl x species inside microporous frameworks during the activation process whereby Au precursors are converted into Au nanoparticles. The resulting beta and MFI zeolites contain uniformly dispersed Au NPs throughout the void space, indicating that the intrinsic stability of the framework promotes resistance to sintering. By contrast, in situ scanning transmission electron microscopy (STEM) studies evidenced that Au precursors in bare zeolites migrate from the matrix to the external surface during activation, thereby forming large and poorly dispersed agglomerates. Furthermore, the resistance of confined Au NPs against sintering is likely relevant to the intrinsic stability of the framework, supported by extended X-ray absorption fine structure (EXAFS), H2 chemisorption, and CO Fourier transform infrared (FT-IR) studies. The Au NPs supported on commercial MFI maintain their uniform dispersity to a large extent after treatment at 700 °C that sinters Au clusters on mesoporous silicas or beta zeolites. Low-temperature CO oxidation and size-selective reactions highlight that most gold NPs are present inside the zeolite matrix with a diameter smaller than 2 nm. These findings illustrate how confinement favors small, uniquely stable, and monodisperse NPs, even for metals such as Au susceptible to cluster growth under conditions often required for catalytic use. Moreover, this strategy may be readily adapted to other zeolite frameworks that can be functionalized by thiol groups.

Dynamic adsorption/desorption of p-xylene on nanomorphic MFI zeolites: Effect of zeolite crystal thickness and mesopore architecture.

Zeolites have attracted great interest as an adsorbent for the removal of volatile organic compounds. However, they suffer from low adsorption capacities due to severe diffusion limitations. Here, the effects of zeolite thickness and mesopore architecture on dynamic adsorption of p-xylene have been examined with a number of MFI-type zeolites with different crystal thicknesses and mesopore openings (i.e. open mesopore, constricted mesopore), which were prepared via hydrothermal synthesis with various organic structure-directing agents and post-synthetic desilication. The results showed that the breakthrough time of MFI zeolite could be improved by more than 2.3 times by reducing the crystal thickness of zeolite to a single-unit-cell dimension (∼2 nm). The time improvement can be attributed to the short diffusion path length that results in easy access of p-xylene to intracrystalline micropores and a large external crystal surface area. In the case of mesopore openings, the presence of constricted mesopores caused the mass transfer of p-xylene into zeolite adsorbents to slow down while open mesopores did not. Furthermore, mesopore opening is an important factor for the desorption behavior of p-xylene. Adsorbed p-xylene by mesoporous zeolites could be desorbed at lower temperatures only when facile diffusion to the exterior through mesoporous channels was possible.

Electrocatalytic Stability of Tin Cathode for Electroreduction of CO2 to Formate in Aqueous Solution.

The electrocatalytic stability of tin (Sn) nanoparticle for electrochemical reduction of CO2 to formate was measured using an H-type cell during electrolysis for 40 h. The Faradaic efficiency (FE) and partial current density (PCD) of formate formation reduced as much as 10% and 13% of the maximum values, respectively. To elucidate the decrease in FE and PCD, the changes in the morphology, chemical composition, the crystalline structure were investigated. The spherical Sn nanoparticles were pulverized after electrolysis. Furthermore, the crystal structure of the fresh Sn electrocatalyst was collapsed and changed into amorphous phase after 40 h electrolysis. The decrease in FE and PCD of formate production on the Sn/CFP electrode could be mainly originated from the reduction of the SnOx to Sn on the cathode surface during electrolysis.

CO2 absorption and sequestration as various polymorphs of CaCO3 using sterically hindered amine.

One aspect of the attempt to restrain global warming is the reduction of the levels of atmospheric CO2 produced by fossil fuel power systems. This study attempted to develop a method that reduces CO2 emissions by investigating the absorption of CO2 into sterically hindered amine 2-amino-2-methyl-1-propanol (AMP), the acceleration of the absorption rate by using the enzyme carbonic anhydrase (CA), and the conversion of the absorption product to stable carbonates. CO2 absorbed by AMP is converted via a zwitterion mechanism to bicarbonate species; the presence of these anions was confirmed with (1)H and (13)C NMR spectral analysis. The catalytic efficiency (kcat/Km), CO2 absorption capacities, and enthalpy changes (ΔHabs) of aqueous AMP in the presence or absence of CA were found to be 2.61 × 10(6) or 1.35 × 10(2) M(-1) s(-1), 0.97 or 0.96 mol/mol, and -69 or -67 kJ/mol, respectively. The carbonation of AMP-absorbed CO2 was performed by using various Ca(2+) sources, viz., CaCl2 (CAC), Ca(OOCCH3)2 (CAA), and Ca(OOCCH2CH3)2 (CAP), to obtain various polymorphs of CaCO3. The yields of CaCO3 from the Ca(2+) sources were found in the order CAP > CAA > CAC as a result of the effects of the corresponding anions. CAC produces pure rhombohedral calcite, and CAA and CAP produce the unusual phase transformation of calcite to spherical vaterite crystals. Thus, AMP in combination with CAA and CAP can be used as a CO2 absorbent and buffering agent for the sequestration of CO2 in porous CaCO3.

Carbonic anhydrase promotes the absorption rate of CO2 in post-combustion processes.

The rate of carbon dioxide (CO2) absorption by monoethanol amine (MEA), diethanol amine (DEA), N-methyl-2,2'-iminodiethanol (MDEA), and 2-amino-2-methyl 1-propanol (AMP) solutions was found to be enhanced by the addition of bovine carbonic anhydrase (CA), has been investigated using a vapor-liquid equilibrium (VLE) device. The enthalpy (-ΔHabs) of CO2 absorption and the absorption capacities of aqueous amines were measured in the presence and/or absence of CA enzyme via differential reaction calorimeter (DRC). The reaction temperature (ΔT) under adiabatic conditions was determined based on the DRC analysis. Bicarbonate and carbamate species formation mechanisms were elucidated by (1)H and (13)C NMR spectral analysis. The overall CO2 absorption rate (flux) and rate constant (kapp) followed the order MEA > DEA > AMP > MDEA in the absence or presence of CA. Hydration of CO2 by MDEA in the presence of CA directly produced bicarbonate, whereas AMP produced unstable carbamate intermediate, then underwent hydrolytic reaction and converted to bicarbonate. The MDEA > AMP > DEA > MEA reverse ordering of the enhanced CO2 flux and kapp in the presence of CA was due to bicarbonate formation by the tertiary and sterically hindered amines. Thus, CA increased the rate of CO2 absorption by MDEA by a factor of 3 relative to the rate of absorption by MDEA alone. The thermal effects suggested that CA yielded a higher activity at 40 °C.

Carbonic anhydrase immobilized on encapsulated magnetic nanoparticles for CO2 sequestration.

Bovine carbonic anhydrase (BCA) was covalently immobilized onto OAPS (octa(aminophenyl)silsesquioxane)-functionalized Fe(3)O(4)/SiO(2) nanoparticles by using glutaraldehyde as a spacer. The Fe(3)O(4) nanoparticles were coated with SiO(2), onto which was grafted OAPS, and the product was characterized using SEM, TEM, XRD, IR, X-ray photoelectron spectroscopy (XPS), and magnetometer analysis. The enzymatic activities of the free and Fe(3)O(4)/SiO(2)/OAPS-conjugated BCA (Fe-CA) were investigated by hydrolyzing p-nitrophenylacetate (p-NPA), and hydration and sequestration of CO(2) to CaCO(3). The CO(2) conversion efficiency and reusability of the Fe-CA were studied before and after washing the recovered Fe-CA by applying a magnetic field and quantifying the unreacted Ca(2+) ions by using ion chromatography. After 30 cycles, the Fe-CA displayed strong activity, and the CO(2) capture efficiency was 26-fold higher than that of the free enzyme. Storage stability studies suggested that Fe-CA retained nearly 82 % of its activity after 30 days. Nucleation of the precipitated CaCO(3) was monitored by using polarized light microscopy, which revealed the formation of two phases, calcite and valerite, at pH 10 upon addition of serine. The magnetic nanobiocatalyst was shown to be an excellent reusable catalyst for the sequestration of CO(2).

Immobilization of carbonic anhydrase on spherical SBA-15 for hydration and sequestration of CO2.

Bovine carbonic anhydrase (BCA) was immobilized on spherical SBA-15 through various approaches, including covalent attachment (BCA-CA), adsorption (BCA-ADS), and cross-linked enzyme aggregation (BCA-CLEA). The spherical SBA-15 was characterized by XRD, BET, and FE-SEM analysis. (29)Si CP-MAS NMR was used to confirm the 3-aminopropyltriethoxysilane grafting (an intermediate step in the immobilization technique), and the immobilization of BCA was confirmed by FT-IR spectrum. The catalytic activities for hydration of CO(2) were calculated for free and immobilized BCA with and without buffer. The K(cat) values for free BCA, BCA-CLEA, BCA-CA and BCA-ADS were 0.79, 0.78, 0.58 and 0.36 s(-1), respectively, indicating that BCA-CLEA showed a comparatively higher hydration of CO(2) than BCA-CA and BCA-ADS, which was nearly the same as free BCA. The amount of CaCO(3) precipitated over free BCA, BCA-CLEA, BCA-CA and BCA-ADS were 140, 138, 135 and 130 mg, respectively. Performance studies, including assays on reusability, thermal stability and storage stability, were also carried out for BCA-CLEA. The results confirmed that BCA-CLEA is reusable, thermally stable and, withstands storage, and is thus a suitable candidate for use in hydration and sequestration of CO(2).