RAKE Scheme Research Articles

Reprint of influence of catalyst preparation on the contribution of homogeneous and heterogeneous steps to the selectivity in nitroaliphatic compounds hydrogenation

2-Methyl-2-nitropropane hydrogenation proceeds along a rake scheme that may encompass homogeneous steps, reactions on the support and on the metallic phase as well. This complexity makes it useful as a test reaction. Reaction rates, selectivities and intermediate compounds accumulation provide experimental information on the modification of catalytic parameters such as support functionality and metallic dispersion. Experiments were carried out in homogeneous phase, on bare carbon supports, with mechanical mixtures of bare carbon supports and carbon-supported catalysts, and with catalysts prepared on modified supports. We have shown that hydrogen activation takes place on the metal and that organic reactants and intermediates may be activated on the metal and on the carbon support as well and may react with hydrogen available after spillover and jumpover migrations.

Influence of catalyst preparation on the contribution of homogeneous and heterogeneous steps to the selectivity in nitroaliphatic compounds hydrogenation

2-Methyl-2-nitropropane hydrogenation proceeds along a rake scheme that may encompass homogeneous steps, reactions on the support and on the metallic phase as well. This complexity makes it useful as a test reaction. Reaction rates, selectivities and intermediate compounds accumulation provide experimental information on the modification of catalytic parameters such as support functionality and metallic dispersion. Experiments were carried out in homogeneous phase, on bare carbon supports, with mechanical mixtures of bare carbon supports and carbon-supported catalysts, and with catalysts prepared on modified supports. We have shown that hydrogen activation takes place on the metal and that organic reactants and intermediates may be activated on the metal and on the carbon support as well and may react with hydrogen available after spillover and jumpover migrations.

Stochastic-Based Analyses of Noncoherent Code Tracking Loops Over Wireless Fading Circumstances

For band-limited pseudonoise signals, a noncoherent digital delay-lock loop (DDLL) equipped with a discrete automatic gain control algorithm under a RAKE scheme over frequency-selective fading channels is studied. Its associated statistical characteristics are analyzed based on the Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> stochastic process. The transition timing error probability density function (pdf) of the Chapman-Kolmogorov equation is evaluated to drive an analytical expression for the steady-state timing error pdf. The mean time to lose lock and the mean square tracking error with its corresponding Cramér-Rao lower bound (CRLB) under frequency-selective Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading environments are scrutinized, whereas their numerical results are confirmed by time-domain computer simulations. Moreover, the tracking performance of the DDLL in the presence of Doppler shifts is also examined.

Two-stage hybrid decision feedback equalization for DS-CDMA systems

This letter proposes a hybrid decision-feedback equalizer (HDFE) for DS-CDMA systems. The proposed HDFE is carried out in two stages to improve the accuracy of the feedback signals by exploiting the spreading gain in the feedback filter. The spread signals for a symbol are buffered and then detected, after which re-spreading is applied to generate feedback signals at the chip level. Through the use of symbol detection for feedback filtering, more accurate feedback signals are achieved. Simulation results demonstrate the superior error performance of the proposed scheme over the frequency domain linear equalization (FD-LE) and RAKE schemes in highly dispersive channels.

Transmitter-Side Multipath Preprocessing for Pulsed UWB Systems Considering Pulse Overlapping and Narrow-Band Interference

We analyze a prerake diversity combining scheme for pulsed ultrawideband systems to shift signal processing needs from the receiver to the transmitter. We consider the realistic case that received pulses carrying the same transmitted symbol could overlap with one another in optimizing the prerake scheme based on zero-forcing and eigenanalysis techniques. We show that in the presence of interpulse interference caused by pulse overlapping, the optimum prerake combining scheme in the sense of maximizing the received signal-to-noise ratio derived by using the eigenanalysis technique performs the same as a conventional rake with maximal ratio combining. We also analyze the different behaviors of prerake and rake schemes in the presence of tone interferers and provide numerical results to validate the conclusions in the presence of in-band-modulated interferers. We find that when the number of narrow-band interference (NBI) signals is small, the prerake scheme outperforms the rake scheme; as the number of NBI sources increases, however, this performance gap reduces.

Dehydrocyclization of n-heptane over a PtBa/Kl catalyst: reaction mechanism

The dehydrocyclization of n-heptane to toluene over a 1 wt.% PtBa/KL catalyst was studied in a fixed bed tubular reactor, at 723 K, 100 kPa and space–time in the range of 1.8–117.0 g h mol −1. The catalyst was prepared by incipient wetness impregnation of a KL zeolite, previously alkalized with BaO, using an aqueous solution of tetraammineplatinum(II) hydroxide as platinum precursor. The solid was successively calcined in an oxygen stream and reduced in flowing hydrogen, at 773 K. Reaction is highly selective towards toluene (>60%) at conversion levels even close to 100%, with benzene, heptenes, methylcyclohexane and ethylcyclopentane as major byproducts. From the product distribution and the dehydrocyclization results of the reaction products performed in separate experiments, a macroscopic mechanism is proposed. Essentially, n-heptane is adsorbed and transformed on the catalyst surface through an alkene-like intermediate (σC 7 ), following five possible parallel interconnected paths, involving hydrogenolysis, isomerization, dehydrogenation and cyclization reactions. Formation of toluene as a primary product is explained by a “rake scheme” in which the σC 7 intermediate is successively transformed, following two possible routes: (a) C 1–C 6 ring closure and subsequent further dehydrogenation; (b) successive dehydrogenation to heptatriene and then cyclization to toluene. These transformations occur in the adsorbed phase and on the same active site, in such a way that only a small fraction of the total adsorbed intermediates formed appear in the gas phase. The rest are not desorbed due to their high reactivity.

New antimultipath technique for spread spectrum receivers

The adaptive RAKE filter can be used in spread spectrum systems to combine the separate signal energies received due to multipath propagation effects. However, shorter length coding schemes are often subject to significant autocorrelation sidelobes, which when incorporated into the RAKE scheme cause further interference. A new approach to combining these signal energies is made possible using the adaptive multipath cancellation receiver and the improved RAKE filter is shown to avoid the sidelobe interference experienced with nonideal autocorrelation functions.

The photo-oxidation of propionic acid by oxygen in the presence of TiO2 and dissolved copper ions

The photo-oxidation of propionic acid by oxygen in aqueous solution has been studied in the presence of TiO2 and dissolved copper ions as catalyst. The kinetics results agree with a “rake scheme” which involves acetic acid as an intermediate between propionic acid and the final product CO2. The role in this scheme of carboxylato complexes of copper both in the dissolved state and in the adsorbed state is stressed.

Kinetic study of the oligomerization of isobutene over H-mordenite

Isobutene has been oligomerized in the liquid phase over H-mordenite in a batch reactor. The reactions were conducted at several temperatures between 313 and 413 K. At these temperatures, the reaction products were essentially dimer, trimer and tetramer. The initial rates showed that the oligomerization reaction followed a Rideal mechanism. The reaction scheme was found to be a rake scheme. The rate constants for the dimerization of isobutene and oligomers decreases with the carbon atom number of the olefin while the adsorption constant increases with the length of the hydrocarbon chain. A model for the oligomerization of the alkenes is proposed.