Wm. Curtis Conner

Kinetics of furfural production by dehydration of xylose in a biphasic reactor with microwave heating

In this paper we report a kinetic model for the dehydration of xylose to furfural in a biphasic batch reactor with microwave heating. There are four key steps in our kinetic model: (1) xylose dehydration to form furfural; (2) furfural reaction to form degradation products; (3) furfural reaction with xylose to form degradation products, and (4) mass transfer of furfural from the aqueous phase into the organic phase (methyl isobutyl ketone - MIBK). This kinetic model was used to fit experimental data collected in this study. The apparent activation energy for xylose dehydration is higher than the apparent activation energy for the degradation reactions. The biphasic system does not alter the fundamental kinetics in the aqueous phase. The organic layer, which serves as “storage” for the extracted furfural, is crucial to maximize product yield. Microwave heating does not change the kinetics compared to heating by conventional means. We use our model to describe the optimal reaction conditions for furfural production. These conditions occur in a biphasic regime at higher temperatures (i.e. 170 °C) and short reaction times. We estimate that at these conditions furfural yields in a biphasic system can reach 85%. At these same conditions in a monophase system furfural yields are only 30%.

A GENERAL EXPLANATION FOR THE COMPENSATION EFFECT - THE RELATIONSHIP BETWEEN DELTA-S+/+ AND ACTIVATION-ENERGY

Abstract Changes in activation energy are often offset by a change in the pre-exponential factor for a group of similar reactions. I present a possible general explanation for this “compensation effect” by relating the entropy of transition to a change in the energy levels of the transition state. Reduction of the activation energy results in a “narrower” transition state. This energetic funneling dictates a decreasing (compensating) entropy.

In situ FTIR and surface analysis of the reaction of trimethylgallium ammonia66316

Using a combination of in situ FTIR spectroscopy and detailed surface analysis, we find that TMGa decomposes at the same rate in either hydrogen or nitrogen forT < 300° C. Although ammonia does not decompose under these conditions, mixing TMGa with ammonia increases the rate of methane formation. Reacting perdeutroammonia with TMGa shows that hydrogen from the ammonia is incorporated into the product methane (whereas deuterium in the gas phase is not incorporated into the gaseous product). TMGa and ND3 do react; however, nitrogen incorporation in the growing film is temperature dependent. Further, although the decomposition of TMGa occurs in the gas phase, the last steps of the decomposition/reaction occur on the substrate surface.

INTERPRETATION OF MERCURY POROSIMETRY DATA USING A PORE-THROAT NETWORK MODEL

A three-dimensional network model with pores and throats is used for the interpretation of mercury porosimetry data. The throat and pore size distributions are obtained from the experimental intrusion and extrusion curves. Application of this model to pore structures with unimodal and bimodal distributions is demonstrated. The present model is found to provide an improved description of the pore topology.

Searching for microporous, strongly basic catalysts: experimental and calculated 29Si NMR spectra of heavily nitrogen-doped Y zeolites.

Nitrogen substituted zeolites with high crystallinity and microporosity are obtained by nitrogen substitution for oxygen in zeolite Y. The substitution reaction is performed under ammonia flow by varying the temperature and reaction time. We examine the effect of aluminum content and charge-compensating cation (H(+)/Na(+)/NH(4)(+)) on the degree of nitrogen substitution and on the preference for substitution of Si-O-Al vs Si-O-Si linkages in the FAU zeolite structure. Silicon-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) and (1)H/(29)Si cross-polarization MAS NMR spectroscopy have been used to probe the different local environments of the nitrogen-substituted zeolites. Experimental data are compared to simulated NMR spectra obtained by constructing a compendium (>100) of zeolite clusters with and without nitrogen, and by performing quantum calculations of chemical shifts for the NMR-active nuclei in each cluster. The simulated NMR spectra, which assume peak intensities predicted by statistical analysis, agree remarkably well with the experimental data. The results show that high levels of nitrogen substitution can be achieved while maintaining porosity, particularly for NaY and low-aluminum HY materials, without significant loss in crystallinity. Experiments performed at lower temperatures (750-800 degrees C) show a preference for substitution at Si-OH-Al sites. No preference is seen for reactions performed at higher temperatures and longer reaction times (e.g., 850 degrees C and 48 h).

Measurement of the morphology of high surface area solids: hysteresis in mercury porosimetry

Abstract The cause of hysteresis between the intrusion and extrusion curves in mercury porosimetry has usually been attributed to (1) the void structure of the solid, (2) a shift in contact angle between intrusion and extrusion, or (3) a combination of both in specific cases. Our studies indicate that all forms of hysteresis can be attributed to structural factors. Intrinsic contact angle shifts in porosimetry are thermodynamically and experimentally inconsistent.

Measurement of the Morphology of High Surface Area Solids: Porosimetry of Agglomerated Particles

Abstract Mercury intrusion porosimetry is a primary method of characterizing the morphology of high surface area solids. Based on a series of pressed microspheres, we have developed a three-dimensional interconnected network model for the void structure. As contrasted to the conventional model involving nonintersecting cylindrical pores (which are neither), a new perspective on porosimetry is discussed. Intrusion is controlled by constrictions, “throats,” in the structure and extrusion is controlled by openings, “pores,” in the structure. Because porosimetry is sequential, there is statistical deviation between the actual and measured “throats” and “pores.” This comparison between scanning porosimetry data and the simulation of porosimetry provides a consistent method for interpreting the morphology of agglomerated particles.

A study of the OMVPE growth mechanisms using internal reflectance spectroscopy to examine adsorption of TMGa and NH 3 and surface reactions between them

Internal reflection spectroscopy spectra show that NH3 and ND3 chemisorb onto (100) and (111)A GaAs surfaces. Adsorption occurs by the formation of Ga—N bonds via Lewis acid-base reactions which are identified by an absorption band between 1325 and 1100 cm−1 with peaks near 1285, 1220 and 1150 cm−1. No NH3 absorption bands are detected when the (111)B surface is exposed. TMGa also chemisorbs onto the (100) GaAs surface. The adsorption spectra of NH3 + TMGa is a function of the order in which the reactants are introduced. When NH3 is introduced first, the reactivity is much greater as is evidenced by the almost total elimination of absorption peaks associated with N—H and CH3 peaks which suggests that the reactions are surface catalyzed methane elimination reactions. Implications of the requirement that the hydride be adsorbed and the methyls react with the hydrogen atoms from the hydride to ALE and MOMBE growth are discussed. Also, consistent explanations are presented for why growth on the (111)B surface is difficult, the growth rate is independent of the hydride partial pressure under normal growth conditions, the incorporation of C into GaAs has an orientational dependence, and As is more preferentially incorporated into GaAsP at the lower growth temperatures.

HYDROGEN SPILLOVER ON SILICA - ETHYLENE HYDROGENATION AND H2-D2 EXCHANGE

Abstract Using spiltover hydrogen, activation of silica Aerosil has been achieved utilizing a unique reactor similar to one developed in Lyons, France, by Teichner. Activation was achieved without direct contact between the oxide (spillover acceptor phase) and the supported metal (spillover source phase). The induced hydrogenation and exchange activity occurs independent of the metal. This induced catalytic activity is an activated process requiring high temperatures and long time periods. The mechanism of the hydrogenation reaction is similar to that on metal oxides in that the molecular identity of the reacting hydrogen is retained. Thus, dideuteroethane is the primary product in the reaction of deuterium with ethylene. Differences in activity from both metals and metal oxides are noted, such as in ethylene and H 2 -D 2 exchange reactions; it is unclear whether these are due to the higher temperatures and/or to changes in the relative rates of the mechanistic Steps.

Measurement of the morphology of high surface area solids: Effect of network structure on the simulation of porosimetry

Abstract A computer simulation of mercury porosimetry based on a pore/throat network model is tested for the effects of changing various structural parameters. These include lattice size, connectivity, and the shape of the pore and throat size distributions. Some general rules of thumb are presented for simplifying the interpretation of porosimetry data.