The Role of Mesopores in Intracrystalline Transport in USY Zeolite: PFG NMR Diffusion Study on Various Length Scales

In situ study on molecular diffusion phenomena in nanoporous catalytic solids

As an omnipresent phenomenon in nature, diffusion is among the rate-determining processes in many technological processes. This is in particular true for catalytic conversion in nanoporous materials. We provide a critical review of the possibilities of exploring diffusion phenomena over microscopic dimensions in such media by direct experimental observation. By monitoring the probability distribution of molecular displacements as a function of time, the pulsed field gradient technique of NMR (PFG NMR) records the rate of molecular re-distribution. By varying the observation time, PFG NMR is thus able to trace even hierarchies of transport resistances as occurring, e.g., in catalyst particles in the form of binder-compacted assemblages of zeolite crystallites. Alternatively, and complementary to this information, interference microscopy (IFM) and IR microscopy (IRM) are able to follow the evolution of intracrystalline concentration profiles during uptake and release. This allows, in particular, an accurate quantification of the transport resistances on the surface of the individual crystallites and of the probability that reactant molecules from the gas phase, upon colliding with the external surface, are able to penetrate through such "surface barriers" into the crystal bulk phase. Being able to distinguish between different molecular species, IRM is able to record the evolution of intracrystalline concentration profiles even during multi-component adsorption and catalytic reactions (169 references).

An overview of recent development in composite catalysts from porous materials for various reactions and processes

Catalysts are important to the chemical industry and environmental remediation due to their effective conversion of one chemical into another. Among them, composite catalysts have attracted continuous attention during the past decades. Nowadays, composite catalysts are being used more and more to meet the practical catalytic performance requirements in the chemical industry of high activity, high selectivity and good stability. In this paper, we reviewed our recent work on development of composite catalysts, mainly focusing on the composite catalysts obtained from porous materials such as zeolites, mesoporous materials, carbon nanotubes (CNT), etc. Six types of porous composite catalysts are discussed, including amorphous oxide modified zeolite composite catalysts, zeolite composites prepared by co-crystallization or overgrowth, hierarchical porous catalysts, host-guest porous composites, inorganic and organic mesoporous composite catalysts, and polymer/CNT composite catalysts.

In situ XAS and XRPD parametric rietveld refinement to understand dealumination of Y zeolite catalyst

Dealumination of NH(4)-Y zeolite during steaming to 873 K was investigated with in situ, time-dependent, synchrotron radiation XRPD and in situ Al K-edge XAS. Water desorption is complete at 450 K, and ammonium decomposition occurs between 500 and 550 K. Only a small fraction of Al(3+) species (5%) leaves the framework during heating from 710 to 873 K; these species occupy site I' inside the sodalite cage. This fraction increases up to 8% in the first 50 min at 873 K and remains constant for the following 70 min isotherm and during the high-temperature part of the cooling experiment. During cooling from 500 to 450 K, the electron density at site I' increases suddenly, corresponding to a fraction of 30-35% of the total Al, confirmed by ex situ (27)Al MAS solid-state NMR. At that temperature, in situ Al K-edge XAS indicates a change in Al coordination of a large fraction of Al, and thermogravimetric (TG) data show the first water molecules start to repopulate the pores. Such molecules drive the dislodgment of most of the Al from the zeolitic framework. Our data indicate that considerable structural collapse caused by steaming does not occur at the highest temperature; however, defects form, which lead to significant migration of framework Al(3+) to extraframework positions, which occurs only as water is able to enter the pores again, that is, at much lower temperature. Contrary to general opinion, these results demonstrate that zeolite dealumination is not primarily a high-temperature process. The standard Rietveld refinement approach failed to identify extraframework Al species. These new results were obtained by adopting the innovative parametric refinement [J. Appl. Crystallogr. 2007, 40, 87]. Treating all of the XRPD patterns collected during the evolution of temperature as one unique data set significantly reduces the overall number of optimized variables and, thus, their relative correlation, and finally results in a more reliable estimate of the optimized parameters. Our results contribute to a better understanding of the phenomena involved on the atomic scale in the preparation of ultrastable Y zeolites (USY). USY are employed in fluid catalytic cracking (FCC), which is the most important conversion process in petroleum refineries to convert the high-boiling hydrocarbon fractions of petroleum crude oils to more valuable products like gasoline and olefinic gases.

Heterogeneity of polyelectrolyte diffusion in polyelectrolyte-protein coacervates: a 1H pulsed field gradient NMR study

Proton pulsed field gradient (PFG) NMR was used to study the diffusion of poly(diallyldimethylammonium chloride) (PDADMAC) in coacervates formed from this polycation and the protein bovine serum albumin (BSA). Application of high (up to 30 T/m) magnetic field gradients in PFG NMR measurements allowed probing the diffusion of PDADMAC on a length scale of displacements as small as 100 nm in coacervates formed at different pH's and ionic strengths, i.e., conditions of varying protein-polycation interaction energy. Studies were carried out for a broad range of diffusion times and corresponding values of the mean square displacements. Several ensembles of PDADMAC polycations with different diffusivities were observed in the measured range of diffusion times. The existence of these ensembles and the pattern of their changes with increasing diffusion time support the hypothesis about the microscopic heterogeneity of PDADMAC-BSA coacervates and also provide evidence for the dynamic disintegration and reformation of dense domains.

Electrostatic-gated transport in chemically modified glass nanopore electrodes

Electrostatic-gated transport in chemically modified glass nanopore electrodes with orifice radii as small as 15 nm is reported. A single conical-shaped nanopore in glass, with a approximately 1 microm radius Pt disk located at the pore base, is prepared by etching the exposed surface of a glass-sealed Pt nanodisk. The electrochemical response of the nanopore electrode corresponds to diffusion of redox-active species through the nanopore orifice to the Pt microdisk. Silanization of the exterior glass surface with Cl(Me)(2)Si(CH(2))(3)CN and the interior pore surface with EtO(Me)(2)Si(CH(2))(3)NH(2) introduces pH-dependent ion selectivity at the pore orifice, a consequence of the electrostatic interactions between the redox ions and protonated surface amines. Nanopore electrodes with very small pore orifice radii (< approximately 50 nm) display anion permselectively at pH < 4, as demonstrated by electrochemical measurement of transport through the pore orifice. Ion selective transport vanishes at pH > 6 or when the pore radius is significantly larger than the Debye screening length, consistent with the observed ion selectivity resulting from electrostatic interactions. The ability to introduce different surface functionalities to the interior and exterior surfaces of glass nanopores is demonstrated using fluorescence microscopy to monitor the localized covalent attachment of 5- (and 6)-carboxytetramethylrhodamine succinimidyl ester to interior pore surfaces previously silanized with EtO(Me)(2)Si(CH(2))(3)NH(2).

Diffusion in Coiled Pores − Learning from Microrelease and Microsurgery

The anisotropic diffusion in coiled pore systems of SBA-3-type microparticles has been studied by the release of guest molecules. The diffusion turns out as an example of the influence of hierarchical structuring on physical properties. Two modes of diffusion, associated with transport along and across the mesopores, can be identified and measured using optical microscopy. Redistribution between the two modes has been achieved by mesopore opening using two methods of "microsurgery"-either focused ion beams (FIB) or mechanical tools. The particles trimmed by FIB have revealed risks of misinterpretation of sample preparation with this tool. Instead of pure pore opening, the cutting by FIB resulted in simultaneous sealing of the mesopores.

Direct Demonstration of Enhanced Diffusion in Mesoporous ZSM-5 Zeolite Obtained via Controlled Desilication

A 2 orders of magnitude gas transport improvement in a medium pore ZSM-5 zeolite has been achieved upon introduction of intracrystalline mesoporosity in gradient-free crystals by desilication post-treatment in alkaline medium.