W. Curtis Conner

SPILLOVER OF SORBED SPECIES

Publisher Summary This chapter focuses on spillover of sorbed species. The exchange of species from one position to another, either on the surface or through the bulk, has been well established. More unique is the mobilization of a sorbed species from one phase onto another phase where it does not directly adsorb. This has been defined as “spillover. Spillover may result in a spectrum of changes in the nonmetallic phase onto which it occurs. In the weakest sense, the spiltover species is transported across the surface of this phase as a two-dimensional gas. It may exchange with similar surface species.The spiltover species may react with the surface, which can result in the creation of surface defects and/or active sites. Further, the bulk of the solid may be transformed into a different structure. In each of these cases, the second phase is no longer an inert. It is not serving to promote the inherent activity on the first phase. The second phase is participating directly in the transport, exchange, and reaction with the spiltover species. In some cases it is able to become catalytically active on its own and thereby to participate directly in subsequent catalysis. The chapter discusses the types of phenomena associated with spillover, the spillover of species other than hydrogen, the aspect of spillover with the most significant catalytic implications, the implication of spillover to catalysis and other heterogeneous processes, the mechanism of spillover, and the nature of the surface and spiltover species.

Depolymerization of lignocellulosic biomass to fuel precursors: maximizing carbon efficiency by combining hydrolysis with pyrolysis

In this paper we study the carbon efficiency of combining hydrolysis and pyrolysis processes using maple wood as a feedstock. A two-step hydrolysis of maple wood in batch reactors, that consisted of a thermochemical pretreatment in water followed by enzymatic hydrolysis, achieved an 88.7 wt% yield of glucose and an 85 wt% yield of xylose as liquid streams. The residue obtained was 80 wt% lignin. A combination of TGA and pyroprobe studies was used for the pyrolysis of pure maple wood, hemicellulose-extracted maple wood, and the lignin residue from the hydrolysis of maple wood. Pyrolysis of raw maple wood produced 67 wt% of condensable liquid products (or bio-oils) that were a mixture of compounds including sugars, water, phenolics, aldehydes, and acids. Pyrolysis of hemicellulose-extracted maple wood (the solid product after pretreatment of maple wood) showed similar bio-oil yields and compositions to raw maple wood while pyrolysis of the lignin residue (the final solid product of enzymatic hydrolysis) produced only 44.8 wt% of bio-oil. The bio-oil from the lignin residue is mostly composed of phenolics such as guaiacol and syringol compounds. Catalytic fast pyrolysis (CFP) of maple wood, hemicellulose-extracted maple wood, and lignin residue produced 18.8, 16.6 and 10.1 wt% aromatic products, respectively. Three possible options for the integration of hydrolysis with pyrolysis processes were evaluated based on their material and carbon balances: Option 1 was the pyrolysis/CFP of raw maple wood, option 2 combined hemicellulose extraction by hydrolysis with pyrolysis/CFP of hemicellulose-extracted maple wood, and option 3 combined the two-step hydrolysis of hemicellulose and cellulose sugar extraction with pyrolysis/CFP of the lignin residue. It was found that options 1, 2, and 3 all have similar overall carbon yields for sugars and bio-oils of between 66 and 67%.

Simulating infrared spectra and hydrogen bonding in cellulose Iβ at elevated temperatures

We have modeled the transformation of cellulose Iβ to a high temperature (550 K) structure, which is considered to be the first step in cellulose pyrolysis. We have performed molecular dynamics simulations at constant pressure using the GROMOS 45a4 united atom forcefield. To test the forcefield, we computed the density, thermal expansion coefficient, total dipole moment, and dielectric constant of cellulose Iβ, finding broad agreement with experimental results. We computed infrared (IR) spectra of cellulose Iβ over the range 300-550 K as a probe of hydrogen bonding. Computed IR spectra were found to agree semi-quantitatively with experiment, especially in the O-H stretching region. We assigned O-H stretches using a novel synthesis of normal mode analysis and power spectrum methods. Simulated IR spectra at elevated temperatures suggest a structural transformation above 450 K, a result in agreement with experimental IR results. The low-temperature (300-400 K) structure of cellulose Iβ is dominated by intrachain hydrogen bonds, whereas in the high-temperature structure (450-550 K), many of these transform to longer, weaker interchain hydrogen bonds. A three-dimensional hydrogen bonding network emerges at high temperatures due to formation of new interchain hydrogen bonds, which may explain the stability of the cellulose structure at such high temperatures.

SURFACE-ANALYSIS WITH FT-IR EMISSION-SPECTROSCOPY

The technique of infrared emission spectroscopy (IRES) is reviewed and further examined in this study as a surface analysis tool. A system has been designed which allows simultaneous kinetic and in situ infrared emission analysis of catalyst surfaces. IRES spectra of several gas mixture/solid systems are obtained in order to examine sample preparation and spectra processing issues; these systems include Pt/Al2O3 exposed to CO and CO-NO mixtures, an oxidized copper plate, and a zeolite exposed to inert atmospheres. For the temperature range of importance to catalysis (300–600 K), IRES is limited to frequencies less than 2500 cm−1. However, IRES is especially well suited for studying solid-state vibrational modes (<1000 cm−1). Moreover, IRES allows catalyst samples to be studied without dilution or extensive sample preparation. The thin samples required for IRES make it possible to study both surface adsorbate and the solid-state lattice vibrations simultaneously. This information can provide useful insight into the interpretation of kinetic data of reactions on metal oxide catalysts. However, samples which are too thick or are supported on a high-emissivity surface will not yield satisfactory spectra. Two correction techniques are examined which reduce background and sample-reflectance effects in the emission spectra. Some of the IRES data are compared to the corresponding spectra obtained by transmission and diffuse-reflectance spectroscopy. IRES is shown to be competitive with these more popular techniques for IR surface analysis.

Microwave Synthesis of Zeolites: Effect of Power Delivery

The effect of microwave power magnitude and pulsing frequency on the synthesis enhancement of zeolites, silicoaluminophosphate SAPO-11, silicalite, and NaY, was studied. Pulsing the microwave power compared to continuous delivery at the same averaged fed microwave power showed no effect on the nucleation and crystallization rates of SAPO-11, silicalite, or NaY. However, SAPO-11 synthesized with continuous microwave power delivery produced larger particles compared to pulsed microwave power with the same reaction time (3.77 microm for continuous versus 2.49 microm for pulsed 1 s on; 3 s off). Further, pulsed microwave power delivery used lower steady state power to maintain the same reaction temperature compared to continuous power delivery (55 W compared to 65 W, respectively). The microwave power used to heat the reaction precursors of SAPO-11 and silicalite was varied by applying cooling gas at various rates while maintaining the reaction temperatures. Significant enhancement of the crystallization rate for SAPO-11 was observed with increasing the fed microwave power (0.014 min(-1) at 65 W, 0.030 min(-1) at 130 W, and 0.066 min(-1) at 210 W), with little effect on the nucleation time. The crystallization rate to microwave power relation was found to obey a power curve (y = 0.4259x(2) - 0.2776x + 0.8517). Lower microwave power produced larger crystals but required longer reaction time to complete crystallization (3.77 microm at 65 W compared to 2.04 microm at 210 W). Conversely, silicalite synthesis at 150 degrees C was found to be independent of the magnitude of the applied microwave power.

An experimental approach to test sorption mechanisms in MCM-41

In order to evaluate the current theories employed to explain hysteresis in mesoporous solids, scanning sorption behavior was determined for several different MCM-41 silicates. Both nitrogen and argon sorption scans at 77 K were studied. Several different scanning behaviors were observed. No single current theory predicts the behavior observed.

Sorption of Gases on Microporous Solids: Pore Size Characterization by Gas Sorption

Abstract Characterization of micropores in zeolite crystals may be performed by automated, dynamic, high resolution adsorption. Of the systems considered only nitrogen over ZSM-5 silicalite at 77 K shows an anomalous hysteresis/transition in the micropore sorption isotherm. The presence of aluminum, steaming of the zeolite or the use of argon @ 77 K or CO 2 @ −60°C eliminates the transition and hysteresis. Framework aluminum tends to reduce pore volume and to broaden the pore size distribution. Steaming reduces pore volume, broadens pore size and generates a significant amorphous phase, presumably largely aluminum. The measured pore volume, but not necessarily the pore dimensions, depend on the equilibration time during adsorption.

Simulating microwave-heated open systems: tuning competitive sorption in zeolites.

We have developed a new grand canonical molecular dynamics (GCMD) algorithm to study microwave (MW) heating effects on competitive mixture sorption and have applied the method to methanol and benzene in silicalite zeolite. The new algorithm combines MW-driven molecular dynamics with grand canonical Monte Carlo (GCMC), the latter modeling adsorption/desorption processes. We established the validity of the new algorithm by benchmarking single-component isotherms for methanol and benzene in silicalite against those obtained from standard GCMC, as well as against experimental data. We simulated single-component and mixture adsorption isobars for conventional and MW-heated systems. In the case of the single-component isobars, we found that for dipolar methanol, both the MW and conventional heated isobars show similar desorption behavior, displaying comparable loadings as a function of molecular temperature. In contrast, nonpolar benzene showed no desorption upon exposure to MWs, even for relatively high field strengths. In the case of methanol/benzene mixtures, the fact that benzene is transparent to the MW field allows the selective desorption of methanol, giving rise to loading ratios not reachable through conventional heating.

Microwave assisted synthesis of silicalite—power delivery and energy consumption

Silicalite was synthesized by microwave heating and two conventional heating methods: an ethylene glycol bath or an electric oven. The effects of microwave and temperature ramp rate on induction time, crystallization rate, morphology of silicalite and energy consumption were investigated. In a microwave oven the reaction time was significantly reduced due to the rapid heating rate, however, the final yield decreased compared with the conventional methods. When the same temperature ramp rates were used in both microwave heating and conventional heating, the induction time and the crystallization rate were similar and the effect of microwave was found only to enhance the final yield of silicalite. Regardless of the irradiation of microwave the slower temperature ramp rate increased the final yield of silicalite. Energy efficiency was not always high in microwave heating. However, if the temperature ramp rate is carefully controlled, silicalite can be produced with less energy in a microwave reactor.

Microwave effects in exhaust catalysis

Many attempts have been made to employ electromagnetic energy as a selective coreagent in catalytic reactions. Photocatalysis employs visible light for a variety of oxidation reactions on a very limited series of catalysts, those employing TiO2. In contrast, attempts to excite specific atom-atom bonds of adsorbed species in the infrared by vibrational resonance have not proved to break the specific bonds selectively. The differences are significant. Many recent studies have suggested that microwave energy can be employed in catalysis and that the results differ from “conventional” heating of the systems studied. Although “photocatalysis” is well accepted, “microwave-catalysis” has not here-to-fore been accepted as an approach to change the selectivity or efficiency of catalysis in the presence of microwave energy. We have studied the influence of microwave energy on sorption and catalysis, particularly on automotive exhaust catalysis. The development of a new generation of automotive exhaust catalysts faces several significant challenges, which might be overcome by the use of microwave energy. The first challenge is to shorten the time required for catalyst “light-off,” since a disproportionate fraction of the pollutants are produced as the catalyst is heated up to operating temperature. The second challenge is the influences of sulfur contaminants that impede the catalytic activity, particularly during light-off. Based on preliminary experiments, we propose that indeed microwave energy can induce catalyst light-off more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst.