Pyrolysis of tire powder: influence of operation variables on the composition and yields of gaseous product

The pyrolysis of tire powder was studied experimentally using a specially designed pyrolyzer with high heating rates. The composition and yield of the derived gases and distribution of the pyrolyzed product were determined at temperatures between 500 and 1000 °C under different gas phase residence times. It is found that the gas yield goes up while the char and tar yield decrease with increasing temperature. The gaseous product mainly consists of H₂, CO, CO₂, H₂S and hydrocarbons such as CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, C₄H₈ and C₄H₆ with a little other hydrocarbon gases. Its heating value is in the range of 20 to 37 MJ/Nm³. Maximum heating value is achieved at a temperature between 700 and 800 °C. The product distribution ratio of gas, tar and char is about 21:44:35 at 800 °C. The gas yield increases with increasing gas residence time when temperature of the residence zone is higher than 700 °C. The gas heating value shows the opposite trend when the temperature is higher than 800 °C. Calcined dolomite and limestone were used to explore their effect on pyrolyzed product distribution and composition of the gaseous product. It is found that both of them affect the product distribution, but the effect on tar cracking is not obvious when the temperature is lower than 900 °C. It is also found that H₂S can be absorbed effectively by using either of them. About 57% sulfur is retained in the char and 6% in the gas phase. The results indicated that high-energy recovery could not be achieved if fuel gas is the only target product. In view of this, multi-use of the pyrolyzed product is highly recommended.     

Thermal decomposition of trichloroethylene under a reducing atmosphere of hydrogen

The thermal reaction of trichloroethylene (TCE: C₂HCl₃) has been conducted in an isothermal tubular flow reactor at 1 atm total pressure in order to investigate characteristics of chlorinated hydrocarbons decomposition and pyrolytic reaction pathways for formation of product under excess hydrogen reaction environment. The reactions were studied over the temperature range 650 to 900 °C with reaction times of 0.3–2.0 s. A constant feed molar ratio C₂HCl₃: H₂ of 4: 96 was maintained through the whole experiments. Complete decay (99%) of the parent reagent, C₂HCl₃ was observed at temperature near 800 °C with 1 s reaction time. The maximum concentration (28%) of C₂H₂Cl₂ as the primary intermediate product was found at temperature 700 °C where up to 68% decay of C₂HCl₃ occurred. The C₂H₃Cl as highest concentration (19%) of secondary products was detected at 750 °C. The one less chlorinated methane than parent increased with temperature rise subsequently. The number of qualitative and qualitative chlorinated products decreased with increasing temperature. HCl and dechlorinated hydrocarbons such as C₂H₄, C₂H₆, CH₄ and C₂H₂ were the final products at above 800 °C. The almost 95% carbon material balance was given over a wide range of temperatures, and trace amounts of C₆H₆, C₄H₆ and C₂HCl were observed above 800 °C. The decay of reactant, C₂HCl₃ and the hydrodechlorination of intermediate products, resulted from H atom cyclic chain reaction via abstraction and addition replacement reactions. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermochemical and kinetic principles, were suggested. The main reaction pathways for formation of major products along with preliminary activation energies and rate constants were given.     

The use of aliphatic alcohol chain length to control the nitrogen type and content in nitrogen doped carbon nanotubes

Nitrogen doped carbon nanotubes (N-CNTs) were synthesized using acetonitrile/alcohol mixtures as the nitrogen, carbon and oxygen sources using the chemical vapor deposition (CVD) method. XPS analysis of the CNTs produced from an acetonitrile/ethanol mixture using different CVD temperatures (700–1000 °C), revealed that nitrogen incorporation in N-CNTs decreased with an increase in CVD temperature and that the type of nitrogen species incorporated also varied. Molecular nitrogen and a low content of pyridinic nitrogen was obtained in N-CNTs grown at 700 and 800 °C, while quaternary nitrogen was noted in all N-CNTs grown. Use of 20% acetonitrile/ROH (R = CH₃, C₂H₅, C₄H₉, C₅H₁₁, C₇H₁₅ and C₈H₁₇) mixtures allowed the C/O ratio to be changed whilst the N content in the precursor mixture was kept constant. The N content in the N-CNTs grown at 850 °C increased with the alcohol chain length and also controlled the nitrogen species incorporated, an effect related to the oxygen content of the reactant mixtures.     

Comparison for thermal decomposition and product distribution of chloroform under each argon and hydrogen reaction atmosphere

Thermal reaction studies of diluted mixture (1%) of chloroform (CHCl₃) under each argon (Ar) and hydrogen (H₂) reaction atmosphere have been investigated to examine the effect of reaction atmosphere on decomposition of CHCl₃ and product distributions. The experimental results were obtained over the temperature range 525?C900 °C with reaction times of 0.3?C2.0 sec. at 1 atm by utilizing an isothermal tubular flow reactor. Complete destruction (>99%) of the parent reagent, CHCl₃ was observed near 675 °C under H₂ reaction atmosphere (CHCl₃/H₂ reaction system) and 700 °C under Ar reaction atmosphere (CHCl₃/Ar reaction system) with 1 sec reaction time. The CHCl₃ pyrolysis yielded more conversion in H₂ atmosphere than in Ar atmosphere. Major products in CHCl₃/Ar reaction system were C₂Cl₄, CCl₄, C₂HCl₃ and HCl over a wide temperature range. Hydrocarbon was not found in CHCl₃/Ar reaction system. Major products of CHCl₃/H₂ reaction system observed were CH₂Cl₂, CH₃Cl, CH₄, C₂Cl₄, C₂HCl₃, C₂H₂Cl₂, C₂H₃Cl and HCl at 600 °C with 1 sec. reaction time. Non-chlorinated hydrocarbons such as CH₄, C₂H₄ and C₂H₆ were the major products at above 850 °C. Product distributions were distinctly different in Ar and H₂ reaction atmospheres. The H₂ gas plays a key role in acceleration of reagent decay and formation of non-chlorinated light hydrocarbons through hydrodechlorination process. The important reaction pathways, based on thermochemical and kinetic principles, to describe the features of reagent decay and intermediate formation under each Ar and H₂ reducing reaction atmosphere were investigated.     

X-ray crystal structure of the dye 2-bromo-4-cyano-4′-N,N-diethylaminoazobenzene

The crystal structure of 2-bromo-4-cyano-4′-N,N-diethylaminoazobenzene has been determined from X-ray diffraction data: C₁₇H₁₇N₄Br, mol. wt = 357·1. Triclinic, Pī (No. 2), α = 13·162(5) Å, b = 7·516(3) Å, c = 8·496(4) Å, α = 101·63(4)°, β = 95·79(4)°, γ = 91·49(4)°, V = 818·10 ų, Z = 2, D_c = 1·45 g cm^?3, F(000) = 378, λ(Mo_Kz) = 0·7107 Å, μ(Mo_Kα) = 26·70 cm^?1. The structure was solved by direct methods and refined by full-matrix least-squares to R = 0·053 for 2081 independent reflexions. The molecule possesses an essentially planar azobenzene skeleton. The effects of substituents on the geometry of the azo group are discussed. Significant molecular parameters are: NN, 1·264(6) Å; ₁BrC, 1·904(5) Å; mean NC, 1·410(7) Å; NNC, 115·7(2)° and 113·0(2)°; NCC (cis relative to NN), 125·4(3)° and 123·1(2)°; CC(Br)C, 123·0(2)°.     

X-ray crystal structure of the dye 2-cyano-4-bromo-4′-N,N-diethylaminoazobenzene

The crystal structure and molecular conformation of 2-cyano-4-bromo-4′-N,N-diethylaminoazobenzene (C₁₇H₁₇N₄Br, mol. wt. = 357·2 a.m.u) has been determined from X-ray diffraction data; triclinic, P$\text{1}$ (No. 2), a = 10·132(11) Å, b = 12·216(16) Å, c = 6·966(11) Å, α = 104·21(9)°, β = 92·67(12)°, γ = 97·22(7)°, V = 826·5(9) ų, Z = 2, D_c = 1·436 g cm^?3, F(000) = 378, λ(MoKα) = 0·71069 Å, μ(MoKα) = 26·0 cm^?1. The structure was solved by the multiple solution direct method and refined by full-matrix least-squares to R = 0·059 for 1538 independent observed reflections. The azobenzene skeleton is planar to within 0·06 Å. Most significant bonding data are: NN, 1·290(8) Å; BrC, 1·866(6) Å; mean CN (azo) 1·380(8) Å; NNC, 113·6(4) and 115·3(4)°; NCC (cis relative to NN) 125·9(4)° and 126·7(4)°; NCC (trans) 116·8°(5)° and 116·1(4)°.     

Structure of the dye 2,6-dichloro-4′-N,N-diethylaminoazobenzene

The structure of 2,6-dichloro-4′-N,N-diethylaminoazobenzene has been determined from X-ray diffractometer data: C₁₆H₁₇Cl₂N₃, MW = 322·2, monoclinic, P2₁/n, a = 11·160 (2), b = 12·066 (2), c = 13·633 (3) Å, β = 116·46 (2)°, V = 1643·5 ų, Z = 4, D_c = 1·30 g cm^?3, F(000) = 672, λ(MoKα) = 0·71069 Å, μ(MoKα) = 3·94 cm^?1. The structure was solved by direct methods and refined to R = 0·073 for 1495 independent reflexions. The molecule is non-planar with a dihedral angle of 87·8° between the phenyl rings. The effects of substituents on the aromatic ring geometry are discussed. Significant molecular parameters are: NN, 1·164 (9) Å; mean ClC, 1·741 (6) Å; mean CN(azo), 1·487 (9) Å; NNC, 112·4 (2)° and 109·1 (2)°; NCC (cis relative to NN), 125·5 (3)° and 122·4 (2)°; NCC (trans relative to NN) 114·0 (3)° and 119·5 (3)°; mean CC(Cl)C, 122·3 (3)°.     

The conjugation and epoxidation of fish oil

Wilkinson's catalyst [RhCl(PPh₃)₃] has been used to conjugate fish oils in high yields under very mild reaction conditions. A catalyst load of 0.35 mol% of RhCl(PPh₃)₃, 0.43 mol% of (o-CH₃C₆H₄)₃P, and 0.87 mol% of SnCl₂·2H₂O in ethanol solvent at 60°C for 2 d produces 82% conjugated Norway fish oil affords 90% conjugated fish oil in 93% yield. The Sharpless epoxidation procedure has also been employed to epoxidize fish oils. Using 0.34 mol% of CH₃ReO₃, 8.15 mol% of pyridine, and 1.03 equivalents of aq. 30% hydrogen peroxide in methylene chloride solvent at 25°C for 6 h, the Norway fish oil ethyl ester can be 100% epoxidized in an 86% yield. The Capelin fish oil gives 100% epoxidized fish oil in a 72% yield. Decreasing the amounts of CH₃ReO₃ and pyridine used in the reaction results in partially epoxidized fish oils.     

Chlorination of Pt–Re/Al2O3 during naphtha reforming

The reforming of a paraffinic naphtha was studied in order to determine the influence of chlorination during the run. Experiments were performed at 505°C, 15 kg cm⁻², WHSV = 4, H₂: HC= 4, and with or without an 8 h initial period of deactivation at 1 kg cm⁻². A commercial Pt–Re/Al₂O₃ (0·3% Pt, 0·3% Re, 0·04% S, 0·15% Cl) catalyst was chlorinated using naphtha feeds with different H₂O/Cl ratios. A model of the chlorination kinetics was developed and represents adequately the experimental results. The acid controlled reactions such as C₂–C₄ and production of C₅ paraffins, disappearance of C₉ paraffins and production of aromatics increase in parallel to the chlorination of the catalyst and the increase is independent of the amount of coke deposited on the catalyst. The sites of chlorine adsorption are different from the sites of coke deposition.     

Si/B/C/N/Al precursor-derived ceramics: Synthesis,high temperature behaviour and oxidation resistance

The Si/B/C/N/H polymer T2(1), [B(C₂H₄Si(CH₃)NH)₃]_n, was reacted with different amounts of H₃Al·NMe₃ to produce three organometallic precursors for Si/B/C/N/Al ceramics. These precursors were transformed into ceramic materials by thermolysis at 1400 °C. The ceramic yield varied from 63% for the Al-poor polymer (3.6 wt.% Al) to 71% for the Al-rich precursor (9.2 wt.% Al). The as-thermolysed ceramics contained nano-sized SiC crystals. Heat treatment at 1800 °C led to the formation of a microstructure composed of crystalline SiC, Si₃N₄, AlN(+SiC) and a BNC_x phase. At 2000 °C, nitrogen-containing phases (partly) decomposed in a nitrogen or argon atmosphere. The high temperature stability was not clearly related to the aluminium concentration within the samples. The oxidation behaviour was analysed at 1100, 1300, and 1500 °C. The addition of aluminium significantly improved the oxide scale quality with respect to adhesion, cracking and bubble formation compared to Al-free Si(/B)/C/N ceramics. Scale growth rates on Si/B/C/N/Al ceramics at 1500 °C were comparable with CVD–SiC and CVD–Si₃N₄, which makes these materials promising candidates for high-temperature applications in oxidizing environments.     

Generation of hydrogen gas from polyethylene mechanically milled with Ni-doped layered double hydroxide

A process for generating hydrogen gas from polyethylene (PE) by milling and heating with Ni-doped layered double hydroxide (LDH), which was prepared also by a mechanochemical route of two-step milling operation, was reported in this work. A mixture of PE and the prepared Ni-doped LDH was first milled in a planetary ball mill for 1 h followed by heating the milled product to 700 °C under He/Ar gas environment for hydrogen emission. Characterizations by a set of analytical methods of X-ray diffraction (XRD), thermogravimetry-mass spectroscopy (TG-MS) and gas chromatography (GC) were performed on the milled and heated samples to monitor the process. Gaseous products obtained during heating mainly consisted of H₂, CH₄, CO, CO₂ with H₂ concentration over 80% between 450 and 550 °C. Such a process could be developed to treat hydrocarbon based solid wastes for hydrogen generation.     

Pyrolysis of palm oil wastes for enhanced production of hydrogen rich gases

A study on pyrolysis of palm oil wastes in a countercurrent fixed bed was carried out, aiming to characterize the hydrogen rich gas products in view of enhanced energy recycling. The effects of temperature, residence time and catalyst adding on the yields and distribution of hydrogen rich gas products were investigated. The main gas species generated, as identified by Micro-GC, were H₂, CO, CO₂, CH₄ and trace amounts of C₂H₄ and C₂H₆. With temperature increasing from 500 °C to 900 °C, the total gas yield was enhanced greatly and reached the maximum value (∼ 70 wt.%, on the raw biomass sample basis) at 900 °C with big portions of H₂ (33.49 vol.%) and CO (41.33 vol.%). Residence time showed a significant influence on the upgrading of H₂ and CO₂ yields. The optimum residence time (9 s) was found to get a higher H₂ yield (10.40 g/kg (daf)). The effect of adding chemicals (Ni, γ-Al₂O₃, Fe₂O₃ and La/Al₂O₃, etc.) on gas product yield was investigated and adding Ni showed the greatest catalytic effect with the maximum H₂ yield achieved at 29.78 g/kg (daf).     

Oxidative dehydrogenation of ethane over Co–BaCO3 catalysts using CO2 as oxidant: effects of Co promoter

Co–BaCO₃ catalysts exhibited high catalytic performance for oxidative dehydrogenation of ethane (ODE) using CO₂ as oxidant. The maximal formation rate of C₂H₄ was 0.264 mmol · min⁻¹ · (g · cat.)⁻¹ (48.0% C₂H₆ conversion, 92.2% C₂H₄ selectivity, 44.3% C₂H₄ yield) on 7 wt% Co–BaCO₃ catalyst at 650 °C and 6000 ml. (g · cat.)⁻¹. h⁻¹. Co–BaCO₃ catalysts were comparatively characterized by XRF, N₂ isotherm adsorption-desorption, XRD, H₂-TPR and LRs. It was found that Co⁴⁺–O species were active sites on these catalysts in ODE with CO₂. The redox cycle of Co–O species played an important role on the catalytic performance of Co–BaCO₃ catalysts. On the other hand, the co-operation of BaCO₃ and BaCoO₃ was considered to be one of possible reasons for the high catalytic activity of these catalysts.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

Insights into carbon nanotube growth using an automated gravimetric apparatus

The development of an automated chemical vapour deposition micro-reactor based on a thermogravimetric analyser is presented. This apparatus was used to investigate carbon nanotube (CNT) growth. Gas flow and reaction modelling highlighted problems with current systems and demonstrated why experimental results are currently apparatus specific. Modelling also indicated that the large-scale production of chiral selective CNTs cannot be achieved in current systems using hydrocarbons. The apparatus provided unprecedented insight into the synthesis of CNTs. Catalyst reduction does not reach completion, probably due to alumina stabilisation of iron oxides, and any discrete pre-reduction step is detrimental to final carbon yield. Any CNT nucleation period was solely attributable to carbon source or H₂ supply restrictions, and growth and final yield were highly dependent on deposition rates within the initial period. The apparatus high throughput capabilities were used for a two-stage optimisation protocol. An optimum carbon yield of >40 g_C/g_Fe·h was found at 700 °C with 20% C₂H₄ and 80% H₂, comparable with the highest reported literature values. A new method to selectively accelerate, slow, or even stop, CNT growth rates using ‘carrier’ gas modulation is also demonstrated.     

Chemical lithiation route to size-controllable LiFePO4/C nanocomposite

Chemical lithiation of amorphous FePO₄ with LiI in acetonitrile is performed to form amorphous LiFePO₄. The amorphous FePO₄·2H₂O precursor is synthesized by co-precipitation method from equimolar aqueous solutions of FeSO₄·7H₂O and NH₄H₂PO₄, using H₂O₂ (hydrogen peroxide) as the oxidizing agent. The nanocrystalline LiFePO₄/C is obtained by annealing the amorphous LiFePO₄ and in situ carbon coating with sucrose in a reducing atmosphere. The particle size of FePO₄·2H₂O precursor decreases with increasing reaction temperature. The final LiFePO₄/C products completely maintain the shape and size of the precursor even after annealing at 700 °C for 2 h. The excellent electrochemical properties of these nanocrystalline LiFePO₄/C composites suggest that to decrease the particle size of LiFePO₄ is very effective in enhancing the rate capability and cycle performance. The specific discharge capacities of LiFePO₄/C obtained from the FePO₄·2H₂O precursor synthesized at 75 °C are 151.8 and 133.5 mAh g^?1 at 0.1 and 1 C rates, with a low capacity fading of about 0.075 % per cycle over 50 cycles at 0.5 C rate.     

Synthesis and utilization of mesoporous alumina as a supporting material of Ce-promoted Ni-based catalysts in the methane reforming process

Due to the large applications of hydrogen as a feedstock of chemical industries and as an energy carrier, its production on large scales with low costs has attracted researchers. Steam reforming of methane (SRM) is the most common process for producing H₂-rich syngas over Ni/Al₂O₃ catalysts, which suffer from coke deposition and Ni particles agglomeration. For overcoming these issues, we have synthesized mesoporous alumina (MA) as a supporting material of Ni particles, structure, and activity, which were compared with the bulk alumina (BA) supported catalysts in the SRM process for the first time. Besides, cerium as an appropriate promoter for lowering deposited coke was added to all prepared catalysts. The reaction temperature (600–700°C), Ni loading (10–25 wt.%), and Ce loading (1–5 wt.%) were the parameters that were optimized for maximizing H₂ yield and CH₄ conversion. Prepared samples were characterized by various techniques before and/or after reaction. The results of TEM and XRD depicted the formation of nanocrystalline and mesoporous structure for Ni-MA catalysts compare to Ni-BA samples. The observations indicated that 20Ni-3Ce/MA had the highest catalytic performance, achieving a CH₄ conversion of 91.0% and H₂ yield of 92.8% at 700°C.     

Novel Silicon-Boron-Carbon-Nitrogen Materials Thermally Stable up to 2200°C

Three novel Si-C-B-N ceramic compositions, namely Si_2.9B_1.0C₁₄N_2.9, Si_3.9B_1.0C₁₁N_3.2 and Si_5.3B_1.0C₁₉N_3.4, were synthesized using the polymer-to-ceramic transformation of the polyorganoborosilazanes [B(C₂H₄Si(Ph)NH)₃]_n, [B(C₂H₄Si(CH₃)NH)₂–(C₂H₄Si(CH₃)N(SiH₂Ph))]_n, and [B(C₂H₄Si(CH₃)–N(SiH₂Ph))₃]_n, where Ph is phenyl (C₆H₅), at 1050°C in argon. The Si-B-C-N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000° and 2200°C, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200°C was as low as 1.4 wt%·h⁻¹ for Si_5.3B_1.0C₁₉N_3.4, 1.7 wt%·h⁻¹ for Si_2.9B_1.0C₁₄N_2.9, and 2.4 wt%·h⁻¹ for Si_3.9B_1.0C₁₁N_3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, β-Si₃N₄ and β-SiC were found exclusively in the samples annealed at 2200°C at 0.1 MPa in argon. For thermodynamic reasons, β-Si₃N₄ should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of β-Si₃N₄ in our materials indicated that carbon and boron kinetically stabilized the Si₃N₄-based composition.     

Thermal degradation of polyurethanes. Model compounds

An analysis was carried out of degradation products of model compounds polyethylene glycol and ethylene diphenylcarbamate. Pyrolysis has been effected at 350 and 400°C under nitrogen and in vacuo. The products were analyzed by gas chromatography. Aniline, CO₂, CO, C₂H₄, CH₃CHO, etc., were found.     

Oxidative coupling of methane and oxidative dehydrogenation of ethane over strontium-promoted rare earth oxide catalysts

Sr-promoted rare earth (viz. La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxide catalysts (Sr/rare earth ratio = 0·1) are compared for their performance in the oxidative coupling of methane (OCM) to C₂ hydrocarbons and oxidative dehydrogenation of ethane (ODE) to ethylene at different temperatures (700 and 800°C) and CH₄ (or C₂H₆)/O₂ ratios (4–8), at low contact time (space velocity = 102000 cm³ g⁻¹ h⁻¹). For the OCM process, the Sr–La₂O₃ catalyst shows the best performance. The Sr-promoted Nd₂O₃, Sm₂O₃, Eu₂O₃ and Er₂O₃ catalysts also show good methane conversion and selectivity for C₂ hydrocarbons but the Sr–CeO₂ and Sr–Dy₂O₃ catalysts show very poor performance. However, for the ODE process, the best performance is shown by the Sr–Nd₂O₃ catalyst. The other catalysts also show good ethane conversion and selectivity for ethylene; their performance is comparable at higher temperatures (≥800°C), but at lower temperature (700°C) the Sr–CeO₂ and Sr–Pr₆O₁₁ catalysts show poor selectivity. © 1998 SCI.