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Stoichiometric reactions can be used to remove H2 and the associated kinetic and thermodynamic barriers that lead to low dehydrocyclodimerization selectivity during alkane reactions on cation-modified H-ZSM5. O2 co-reactants can form H2O in exothermic reactions that balance the enthalpy of endothermic dehydrogenation steps. O2 reacts preferentially with H2 via homogeneous and heterogeneous pathways, but also with hydrocarbons as H2 is depleted; thus, it must be gradually introduced as H2 forms in dehydrogenation reactions. Staged O2 protocols significantly increased aromatics yields during C3H8 reactions on unexchanged and on Ga- and Zn-exchanged H-ZSM5. On Ga/H-ZSM5, the maximum aromatic yields increased from 54% to 68% and aromatization/cracking selectivity ratios increased from 2.1 to 3.9 when O2 was introduced gradually into a gradientless batch reactor as H2 formed. O2 introduced was converted to H2O with >95% selectivity; an equivalent amount of O2 initially added with C3H8 led to low H2O selectivities (<60%). Similar effects of O2 addition and of staging protocols were observed for alkane reactions on H-ZSM5 and Zn/H-ZSM5. Staging strategies led to selective use of O2 to remove thermodynamic and kinetic bottlenecks and to unprecedented aromatics yields during alkane reactions on cation-exchanged H-ZSM5.

Hydrogen removal steps limit alkane dehydrogenation reactions on cation-exchanged H−ZSM5 and cause desorption bottlenecks and the formation of hydrogen-rich reaction intermediates and products. For this reaction, the catalytic surface acts for all kinetic purposes as if it were in equilibrium with a H2 pressure greater than in the prevalent gas phase. The hydrogen chemical potential within adsorbed intermediates is described rigorously by a virtual H2 pressure, defined as that required to achieve the prevalent surface hydrogen content if adsorption−desorption steps were equilibrated. These virtual pressures can be measured from the deuterium content in products formed from C3H8/D2 reactants. H2 virtual pressures are high during propane reactions on H−ZSM5, because recombinative desorption of hydrogen atoms formed in C−H activation steps is slow. H2 virtual pressures decrease as Zn cations replace protons in H−ZSM5, because cations catalyze hydrogen adsorption−desorption steps and provide a kinetic path for communication between H2 in the gas phase and propane-derived reaction intermediates. As a result, the addition of H2 to C3H8 reactants decreases propane reaction rates and aromatics selectivity on Zn/H−ZSM5, causing it to resemble kinetically H−ZSM5. The hydrogen chemical potential on these catalytic surfaces and the hydrogen content within reactive intermediates reflect the rate at which hydrogen is formed in either C−H bond activation steps or in the dissociative chemisorption of gas-phase H2. The reaction pathways for species derived from each of these two H-sources are kinetically indistinguishable. Both sources contribute hydrogen atoms or hydrogen-rich species to the prevalent pool of reactive intermediates, the hydrogen content of which determines the rate and selectivity of all surface chemical reactions.

Co/H-ZSM5 catalyzes propane dehydrogenation and aromatization reactions. Initial product selectivities, product site-yields, and the ¹³C content and distribution in the products of 2-¹³C-propane show that propane undergoes two primary reactions&sbd;dehydrogenation to propene and H2 and cracking to methane and ethene. Propene and ethene then form aromatics via oligomerization−cracking reactions and both ethene and propene hydrogenate to form ethane and propane, respectively, via both hydrogen transfer from coadsorbed intermediates and dissociative adsorption of H2. These reaction pathways resemble those occurring on H-ZSM5, but Co cations provide an alternate pathway for the removal of hydrogen atoms in adsorbed intermediates as H2. A kinetic model was used to describe experimental rates based on these observations and to obtain rate constants for individual reaction steps as a function of Co content and H2 concentration. H2 inhibits propane dehydrogenation to propene and alkene conversion to aromatics, and increases the rate and the rate constant for ethene hydrogenation. Rate constants were obtained for propane reactions on H-ZSM5 and Co/H-ZSM5 (Co/Al = 0.05−0.22). Propane dehydrogenation (k1), ethene hydrogenation (k3), and alkene dehydrocyclization (k4) rate constants increased with increasing Co/Al ratio, because Co cations catalyze both the recombinative desorption of H2 and its microscopic reverse, the dissociative adsorption of H2. Co cations increase the reversibility of hydrogen adsorption−desorption steps, and in this manner, they increase the deuterium content in all products of C3H8-D2 reactions. Propane cracking rate constants (k2) were not influenced by Co cations, because cracking occurs on acid sites and its stoichiometry and mechanism do not require hydrogen.

Co/H-ZSM5 catalysts with Co/Al ratios of 0.09−0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C6−C8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The selectivities to propene, aromatics, and H2 are also higher on Co/H-ZSM5 than on H-ZSM5. The rate of D2 exchange with OH groups increases with increasing Co/Al ratio, suggesting that Co cations catalyze D2 dissociative chemisorption steps that limit the rate of isotopic exchange. Co cations also catalyze hydrogen recombinative desorption steps, which limit the rate of propane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D2−OH isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D2−OH and infrared measurements showed that Co²⁺ cations replace 1.1−1.3 zeolitic protons, suggesting the predominant presence of Co²⁺−O−Co²⁺ dimers, with some Co²⁺ monomers, each bridging two next-nearest neighbor Al sites. The location and structure of exchanged Co cations were probed using X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). No H2 consumption was detected up to 1273 K during TPR in any of the Co/H-ZSM5 samples, consistent with the absence of CoOx crystallites, which reduce at ~800 K. In situ near-edge X-ray absorption studies confirmed that Co species remain as divalent cations during exposure to H2 or C3H8 at 773 K. Near-edge and fine structure analysis detected Co²⁺ cations with similar structure in all Co/H-ZSM5 samples (Co/Al < 0.22), and Co coordination changes from octahedral to tetrahedral upon sample dehydration at 773 K in He. Radial structure functions showed weak contributions from the first and second shells around Co. This reflects the nonuniform nature of the distance and orientation in Al−Al next-nearest neighbor sites in ZSM5.

Light microscopy studies on the fish cracker gel and expanded product (‘keropok’) emphasised the role of fish proteins in the starch expansion process. The addition of salt (20 g kg⁻¹) in the ‘keropok’ helped to distribute evenly the starch in the fish protein. Formation of thin fish muscle bundles assisted the expansion of ‘keropok’. At 700–900 g kg⁻¹ fish content, the fish muscle bundles formed a continuous network that caused a drop in the ‘keropok’ expansion. From the scanning electron microscopy study, ridges were found in samples (containing 600–900 g kg⁻¹ fish content) with 20 g kg⁻¹ salt at high magnification. © 1999 Society of Chemical Industry

The effect of steaming time on ‘keropok’ was studied. The morphology of the starch granules in keropok gel at different steaming times was examined using scanning electron microscopy (SEM). There was an optimum time for the steaming of keropok which gave the best linear expansion and hardest texture of steamed gel. This occurred when the starch granules were expanded to their largest sizes and before fragmentation of the granules. A steaming time of 20–30 min is sufficient to cook the keropok gel. Excessive steaming time will result in a poor-quality product as well as extra production costs. © 1999 Society of Chemical Industry

The Notch signalling pathway is involved in many processes where cell fate is decided. Previous work showed that Notch is required at successive steps during R8 specification in the Drosophila eye. Initially, Notch enhances atonal expression and promotes atonal function. After atonal autoregulation has been established, Notch signalling represses atonal expression during lateral specification. In this paper we investigate which known components of the Notch pathway are involved in each signalling process. Using clonal analysis we show that a ligand of Notch, Delta, is required along with Notch for both proneural enhancement and lateral specification, while the downstream components Suppressor-of-Hairless and Enhancer-of-Split are involved only in lateral specification. Our data point to a distinct signal transduction pathway during proneural enhancement by Notch. Using misexpression experiments we also show that particular Enhancer-of-split bHLH genes can differ greatly in their contribution to lateral specification.