Henry C. Foley

Carbogenic molecular sieves: synthesis, properties and applications

Abstract Carbogenic molecular sieves (CMS) are microporous, high-carbon solids. Because of their microporosity, they share with zeolites the ability to separate molecules on the basis of size and shape. For example, nitrogen is produced from air by pressure swing adsorption over packed beds of CMS. Other shape-selective separations also can be accomplished over CMS. Despite these apparent similarities CMS are quite different from zeolites. Instead of being crystalline, they are globally amorphous having no long-range order. Despite their disorder the microporosity can be controlled through choice of precursor, additives, and thermal history. Highly variable surface chemistries—acidic, basic, neutral, and free radical—also set CMS apart from zeolites. Here the properties of CMS are reviewed in detail. Synthesis properties and applications, both separative and catalytic, are covered. Finally, microstructural features of CMS are put in context with recognized features of other carbon materials which feature minimal surfaces with high local curvature.

An LDA-based Community Structure Discovery Approach for Large-Scale Social Networks

Community discovery has drawn significant research interests among researchers from many disciplines for its increasing application in multiple, disparate areas, including computer science, biology, social science and so on. This paper describes an LDA(latent Dirichlet Allocation)-based hierarchical Bayesian algorithm, namely SSN-LDA (simple social network LDA). In SSN-LDA, communities are modeled as latent variables in the graphical model and defined as distributions over the social actor space. The advantage of SSN-LDA is that it only requires topological information as input. This model is evaluated on two research collaborative networkst: CtteSeer and NanoSCI. The experimental results demonstrate that this approach is promising for discovering community structures in large-scale networks.

Reproducible production of nanoporous carbon membranes

Inorganic membranes offer significant potential advansubstrate. In the present study, a solution of poly(furfuryl) tages over polymer membranes, but only if they can be alcohol in acetone (30 wt.% PFA and balance acetone) fabricated readily and reproducibly [1–4]. Nanoporous loaded into a syringe pump is sprayed through the ulcarbons (NPC) or carbon molecular sieves (CMS) are trasonic nozzle onto a porous stainless steel support (0.2 good candidate materials for selective membrane formation mm SS316L, Mott Corporation (Farmington, CT) or 2.0 [5–8]. These can be prepared by the controlled pyrolysis of mm SS316L, Pall Process Filtration Company (East Hills, polymers such as poly(vinyl chloride) (PVC), poly(acrylNY). The ultrasonic spray system can accommodate onitrile) (PAN) and poly(furfuryl) alcohol (PFA) [9,10]. tubular supports ranging from 7.62 to 38.1 cm in length Although amorphous [11], they are endowed with a regular and from 3.175 to 19.05 mm in outside diameter. While the nanostructure that leads to a pore network with narrowly tubular support rotates with a constant frequency (from 0.1 distributed pore dimensions between 0.3 and 0.6 nm [12– to 10 Hz), the tube also moves independently in the 15]. In free standing form the materials are too friable to forward and reverse axial directions with a specified 21 be useful [16,17]. However they can be grown on porous translational velocity (from 0.254 to 25.4 mm s ). Thus, stainless steel support media by brush-coating, spray-coatmultiple coatings are possible with the number of spray ing [5,18], or preferably, ultrasonic deposition of the passes ranging between 1 and 100. polymer resin precursor [19]. Previously, membranes were Ultrasonic nozzles employ high-frequency sound waves prepared by manual control of the ultrasonic deposition of (25 to 120 kHz) to produce atomization of a liquid. They PFA on the support. As with any manual process, this step are ideally suited for spraying thin films because they can was somewhat variable with respect to the degree of dispense extremely small amounts of liquid onto a subcoverage of the support by polymer. Since uniform coverstrate precisely and reproducibly. Compared with convenage is needed to prepare a uniform film a new automated tional pressure spray nozzles, ultrasonic nozzles deliver a 21 unit was designed to do the ultrasonic deposition. Herein, low-velocity spray (,5 cm s ), which is approximately we describe the variability in performance of a group of one hundredth the velocity produced by a standard presseven supported nanoporous carbon membranes (SNPCM) sure nozzle. In an ultrasonically produced spray, drop size prepared in this way under nominally identical conditions. is governed primarily by the operating frequency at which The automated ultrasonic spray system was designed the nozzle vibrates, and to a lesser extent by the surface as-built by Sono-Tek Corporation (Milton, NY). The basic tension and density of the liquid being atomized. The system consists of a broadband ultrasonic generator, a higher the frequency, then the smaller will be the median syringe pump to control the flow of a polymer solution, an drop size. In the present study, we chose an ultrasonic ultrasonic nozzle, an electronic controller with operator nozzle (Sono-Tek, model 06-04029) operating at 60 kHz. interface, and a slide and rotator assembly for the coating This nozzle produced a number-mean droplet diameter of 40 mm for liquid water. To control the spray beam flux, the liquid flow rate was metered with a syringe pump. *Corresponding author. Tel.: 11-814-865-2574; fax: 11-814However, the adjustable flow rate range was limited for 865-7846. E-mail address: hankfoley@engr.psu.edu (H.C. Foley). producing a stable and uniform spray beam. The accept-

Argon porosimetry of selected molecular sieves: experiments and examination of the adapted Horvath-Kawazoe model

The calculated micropore size of various molecular sieves has been examined using static argon porosimetry and a simple model for adsorption. The molecular sieves used in this study are NaY, ZSM-5, AlPO4-5, AlPO4-11 and VPI-5. Since zeolites and aluminophosphates have different interactions with argon, a lumped parameter for adsorptive interactions was found to be critical to the evaluation of micropore size using an area-averaged cylindrical pore model. The parameter of choice is the magnetic susceptibility of the adsorbent oxide ion. For NaY and ZSM-5, a magnetic susceptibility of χ=−1.3·10−29cm3 shows good results. With a value of χ=−1.9·10−29cm3, the calculated pore sizes of AlPO4-5 and AlPO4-11 are in good accord with the values from X-ray diffraction studies. For VPI-5, which has a larger pore, the monolayer cylindrical pore model is more adequate, but it requires inclusion of an adjustable factor to compensate for adsorbate-adsorbate interactions.

Local structure of nanoporous carbons

The local atomic structure of nanoporous carbons produced by pyrolysis of poly(furfuryl alcohol) at various temperatures has been studied using neutron diA raction. Atomic pair distribution functions (PDFs) were obtained from the neutron data. Structure models have been ® tted to the PDFs to understand the ® ne features of atomic ordering. It has been found that carbons produced at 800 and 12008C are made of stacked graphene sheets, that are more or less curved. The 4008C-processed carbon has a heavily distorted nonplanar structure. For each sample we have determined the extent of the structural coherence, the average density within the structurally coherent regions and the proportion of carbon atoms, which are trigonally coordinated. We have also produced microscopic models for the local structure of the nanoporous carbons investigated.

Simulation of nanoporous carbons: a chemically constrained structure

Abstract Nanoporous carbons (NPCs) are useful in adsorptive separations and catalysis, owing to their ability to discriminate between molecules on the basis of size and shape. This property arises from their narrow pore size distribution, which is typically centred at a size corresponding to 0.5 nm. Despite this level of nanoregularity. there is no long-range order within these materials. Structural coherence dissipates to extinction at distances longer than 1–1.2 nm. For this reason, these nanoporous materials are complex solids and offer an intriguing problem in structural simulation and modelling. We show that modelling the spatial complexity of NPCs can be overcome by their chemical simplicity. Recognizing that the structures are comprised of trigonal sp2 carbon and imposing chemical and physical constraints on the possible outcomes of the simulation provide a means to surmounting the modelling problem presented by the intrinsic disorder. By this approach, models of the solid can be arrived at that ma...

High-resolution nitrogen and argon adsorption on ZSM-5 zeolites: effects of cation exchange and SiAl ratio

Abstract ZSM-5 zeolites with Si Al ratios of 23, 72 and >400 were synthesized in the laboratory as Na+ and cation (H+, Co2+, Cu2+, Fe2+ and Zn2+)-exchanged forms and were examined by nitrogen and argon porosimetry. Na-ZSM-5 of any Si Al ratio and high-alumina ZSM-5 ( Si Al =23 ) zeolites of any exchanged cation display single transition adsorption isotherms (type I isotherm) for both nitrogen and argon. However, other cation-exchanged ZSM-5 zeolites of lower alumina contents display step-like isotherms, i.e. two transitions in adsorption, and the second transition occurs between a relative pressure ( P P o ) of 0.15 and 0.2 for nitrogen and between 1·10−3 and 2·10−3 for argon. Calculated micropore size distributions based on argon adsorption show that the Na+ form or high-alumina ZSM-5 samples have a single pore of 0.55 nm as expected, while the other ZSM-5 samples which have a second transition of adsorption display the excepted pore size of 0.55 nm but also an apparent larger pore at 0.77 nm, a non-physical result. The calculated pore size of 0.55 nm for all the ZSM-5 represents its true micropore structure. Variations in the Si Al ratio and exchanged cations have no effect on this calculated value.

Thermoelectric chemical sensor based on single wall carbon nanotubes

Thermoelectric properties of single wall carbon nanotubes (SWNT) are quite sensitive to gases in contact with the tube walls. This effect makes possible a thermoelectric chemical sensor. Large, reversible swings in thermoelectric power (S), sometimes even involving sign changes in S, have been observed. Even contact of the SWNTs with He and N 2 and H 2 result in easily detectable and reversible changes in S. Smaller, polar alcohol molecules stimulate a large thermoelectric response, although H 2 O has no effect. For adsorption of six membered ring molecules C 6 H n in SWNTs, the large thermoelectric response observed for Benzene (n=6) is seen to decrease as the π electrons in the molecule are removed, and the coupling between the molecules and the SWNT is thereby reduced. These effects are discussed in terms of the diffusion thermopower for a rope, and a new scattering channel associated with adsorbed molecules.

Bimetallic catalysts comprised of dissimilar metals for the reduction of carbon monoxide with hydrogen

A series of bimetallic catalysts containing Co—Mo, Ru—Mo, Rh—Mo, Co—W, Ru—W and Rh—W on γ-Al2O3 have appreciable activity for the reduction of carbon monoxide with hydrogen under mild conditions (150, 300°C and 101 to 407 kPa). Ru-W/γ-Al2O3 had the highest overall activity and propensity for chain growth, Co—Mo and Co—W/γ-Al2O3 generated the highest yields of alkeneproducts, while Rh—Mo and (methanol and ethanol). Alkali promotion of the Ru—W/γ-Al2O3 catalyst increased the alkene content of the products but decreased the overall activity for carbon monoxide hydrogenation. X-ray photoelectron spectroscopy indicated that both Ru—W and Ru—Mo contain ruthenium with a slightly higher binding energy after reduction than the monometallic ruthenium catalyst. Molybdenum and tungsten are not reduced. X-ray photoelectron spectroscopy of the fresh Rh—Mo/γ-Al2O3 catalyst indicates the presence of RhIII 2h, the rhodium is reduced, with a binding energy close to but slightly higher than that of bulk Rh0. Here too, the molybdenum binding energy is unaffected by reduction, and the rhodium-to-molybdenum atom ratio remains constant, suggesting little or no further sintering. High pressure (250°C, 1724–6689 kPa, hydrogen: carbon monoxide = 0.75:1.0) reduction of carbon monoxide with hydrogen by 3% Rh-2.8% Mo/γ-Al2O3 produced predominantly dimethyl ether and methane. This catalysts activity and selectivity are quite stable over prolonged period (>300 h). Transmission electron microscopy of used Rh—Mo/γ-Al2O3 shows that there is a bimodal distribution of particles (0.5 nm and 1.0—1.5 nm). Dedicated scanning transmission electron microscopy-energy dispersive spectroscopy of the particles indicates that the larger particles are enriched in rhodium (Rh-to-Mo>1.60) and the smaller in molybdenum (Rh-to-Mo<0.9). A structural model is proposed for Rh-Mo/γ-Al2O3 with two separate sites leading to oxygenates and hydrocarbons. Oxygenate production is attributed to chemistry at the smaller, molybdenum-rich site. It is believed that the role of molybdenum in this site is to act as a textural promoter, providing site-isolation of the rhodium.

Ultrafiltration membrane synthesis by nanoscale templating of porous carbon

A novel method for producing carbon membranes for ultrafiltration applications is presented using a spray deposition and pyrolysis of poly(furfuryl alcohol)/poly(ethylene glycol) mixtures on macroporous stainless steel supports. The poly(ethylene glycol) or PEG employed as a carbonization template creates a mesoporosity that leads to pores in the ultrafiltration range. Scanning electron microscopy (SEM) shows that the membranes consisted of 12- to 15-m thick carbon films. Gas permeation and water permeability data were used for the calculation of mean pore sizes, which were found to decrease with decreasing average molecular weight of the PEG template. Ultrafiltration of a polydisperse dextran solution was used to quantify the retention properties of the membranes. Molecular weight cutoffs determined from dextran retention data were shown to vary with template molecular weight: values of 2 × 10 4 ,3 .5 × 10 4 , and 6 × 10 4 gm ol −1 dextran were measured for respective templates of 2000, 3400, and 8000 g mol −1 PEG. For PEG molecular weights of 2000 or below, the templating effect was ill defined, membrane film cracking became more prominent, and membrane selectivity and reproducibility were adversely affected.