جستجوی مقالات مرتبط با کلیدواژه "selectivity" در نشریات گروه "شیمی"

The Mizoroki-Heck coupling reaction has been carried out using Pd(OAc)2 and bisimidazolium (1,1′-methylene-3,3′-bis[(N-(isopropyl)imidazol-2-ylidene]dibromide) for aryl bromides as well as less reactive chlorobenzene with mono- and di-substituted olefins. Only 0.025 mol% of Pd(II) shows a high activity toward the coupling reaction with turnover number (TON) ≈ 600-3960. Further, this catalyst system, Pd(OAc)2/Bisimidazolium, exhibits the excellent selectivity for the trans-isomer coupling products and the terminal arylation coupling products for mono- and di-substituted olefins, respectively. The nature of olefins (n-butyl acrylate, styrene, and 1,1-disubstituted olefins) has a significant contribution on the yield of the coupling products.

A co-impregnation method was applied to the Ni/Zr-HMS/HZSM-5 catalyst (with various amounts of zirconium) during the hydrogenation of benzene. The physicochemical properties of the prepared nickel catalyst were characterized using X - ray diffraction, X - ray fluorescence, Fourier transform infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, H2 chemisorption, N2 adsorption - desorption, and thermogravimetric analysis. The catalytic performance was assessed on a fixed-bed reactor (reaction temperature between 130 °C and 190 °C). The results indicated that the nickel catalyst with Si/Zr = 35 exhibited better catalytic performance and stability than others, so providing a better selectivity in long-term performance.

The influence of NaNO2, HNO3 and H2SO4 on the structure and morphology of gamma-alumina was investigated by XRD, BET, SEM and FT-IR spectroscopy. Selected amounts of NaNO2, HNO3 and H2SO4 were added to a solution of aluminum isopropoxide (as a precursor for the boehmite synthesis). The boehmite samples were calcined at 350 and 600 ℃ to form the semi-crystalline high surface area gamma-Al2O3. The reactivity and selectivity of the modified gamma-alumina catalysts were examined using dehydration reaction of 2-octanol. There was a good correlation between reactivity, selectivity and the amounts (and type) of the modification. The dominant product was cis-2-octene for all of the catalysts. Low conversion and high selectivity were obtained for dehydration of 2-octanol over alumina modified with NaNO2. High conversion, low selectivity and excessive isomerization were found for gamma-alumina catalyst modified with H2SO4.

Ni catalysts supported on Nano porous catalysts were prepared by the impregnation method and tested for vapor phase hydrogenation of benzene. The textural and physico-chemical properties of Ni catalysts were characterized by the X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscope and N2 adsorption-desorption analysis. The catalytic evaluation revealed that the best selectivity to benzene (> 84%) and high stability with the low coke deposition (< 1.1%) are related to Ni/Folded Sheets Mesoporous Materials-No. 16. The kinetics of benzene hydrogenation has been examined as a function of benzene and hydrogen pressures at various temperatures. Two kinetic models based on power law and Langmuir-Hinshelwood mechanisms were developed for the reaction and compared with the obtained experimental data. The apparent kinetic parameters were estimated using the multiple regression analysis. Both these models present the good results.

Ni/Al-HMS/HZSM-5 catalysts with varying amounts of Si/Al ratios were prepared via the impregnation method and evaluated for the hydrogenation of benzene at 130−190 °C. To study the catalyst characterization, various methods were used as X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, H2 chemisorption, N2 adsorption-desorption, and thermogravimetric analysis. The effect of reaction temperature and residence time on catalytic performance was considered and kinetic of this reaction was investigated under different pressures. The results demonstrated that the composite catalysts have high benzene conversion (~80%), medium to low toluene and xylene conversion (16–80%), moderate benzene selectivity (~50%) in the mild reaction condition, and good catalytic stability against the coke deposition.

This study aimed to investigate the adsorption of CH4, CO2, H2S at a temperature of 298.15 K and pressurerange of 0.1 to 30 atm, and compare the results with experimental data for MIL-47 using GCMC. Themaximum CH4, CO2 and H2S adsorptions were 3.6, 10.45, and 12.57 mol.kg-1, respectively. In addition, theselectivity for binary mixtures of CH4/CO2 and CH4/H2S was calculated. The results for CH4/CO2 mixturesat 10 atm showed that: 1) MIL-47 only adsorbed CO2 in a 0.05 CH4/ 0.95 CO2 mixture, and 2) by increasingthe mole fraction of CH4, the selectivity toward CO2 decreased. The results for H2S/CH4 mixture at 10atm showed that: 1) H2S was adsorbed only in mole fractions of 0.95, 0.75, and 0.50 of H2S, and 2)the observed selectivity was about 132.7 and 63.2 at H2S mole fractions of 0.25 and 0.05, respectively.The MD simulations and RDF analyses were used to investigate 0.5 CH4/0.5 CO2 and 0.75 CH4/0.25 H2Smixtures. The results showed that the adsorption mostly occurs on the metallic part of MIL-47. Wefound that V and O atoms were the active adsorption sites in MIL-47. H2S and CH4 showed to have thehighest and lowest levels of self-diffusions, respectively. The MD simulations were used to study the selfdiffusionfor mixtures across all mole fractions. In the binary mixture of 0.95 CO2/0.05 CH4, the maximumself-diffusion was 1.49×10-12 m2s-1 for CO2. The maximum self-diffusion for H2S in the mixture of 0.05CH4/0.95 H2S was 2.62×10-10 m2s-1.

Pb2+ ion was investigated by voltammetry selectivity using TiO2-Ionophores hexa(p-tert butyl)hexa(ethyl ester)calix[6]arene (BEK6) composite to increase the selectivity and sensitivity of carbon paste electrode (CPE). The BEK6 was synthesized by using hydrolysis method, meanwhile the TiO2-P25 and graphite powder were mixed as an electrode composite. The Scanning Electron Microscopy (SEM) shows that the CPE doped TiO2-BEK6 (CPE/TiO2-BEK6) have morphology likes a rough, non-porous, composed of fine particles, and homogeneously. The presence of Pb2+ ion in pH 5 citrate buffer electrolyte causes the peaks appearance on Epa and Epc were 0.17 V and 0.37 V with  the peak potential current generated of 15.15 μA and -12.0 μA, respectively. The 0.005 g BEK6 was increased significantly the peak current of the Pb2+ ion with the LOD value of 0.1756. In addition, the presence of TiO2-BEK6 composite shows the measurement current stability by %RSD of 0.83.

The total oxidation of toluene at a wide temperature range (200–500 oC) over micro/mesoporous platinated catalysts has been investigated about activity, selectivity to CO2 and CO, catalyst's stability versus coke deposition and reaction kinetics. Kinetic of toluene oxidation was measured under various oxygen and toluene pressures and also the effect of the reaction conditions on the catalytic performance was studied. For more study, two kinetic models have also been selected and tested to describe the kinetics for this reaction. The results show that the Langmuir-Hinshelwood model provides a good fit for the experimental data. The obtained results showed that Pt/HZSM-5(30)-HMS have a better ability than other catalysts for the oxidation reaction of toluene, such as maximum toluene conversion (>97%), high selectivity to CO2 (100%), good catalytic stability against coke deposition and appropriate kinetic parameters

A new acetate selective poly (vinyl chloride) membrane sensor based on newly synthesized nickel complex of Schiff base, N,N'-benzene-1,2-diylbis[1-(1H-pyrrol-2- yl)ethanimine] as an ionophore was prepared. The electrode show Nernstian slope of 31.4±1 mV/decade with improved linear range of 1.6×10-7 to 1.0×10-2 M, with a lower detection limit of 1.0×10-7 M between pH range of 3.0-9.5, giving a relatively fast response within (5s) and can be used for at least 2 months without any divergence in potential. The selectivity coefficient was calculated using matched potential method. The proposed electrode was successfully applied as an indicator electrode for the titration of arsenate ion with sodium thiocynide. It was also be used for the determination of arsenate ion in different water samples.

Bagasse as a real biomass was converted to hydrogen rich gas via catalytic supercritical water gasification process. To find the effect of Cu on selectivity of products, Cu promoted Ni-γAl2O3 catalysts were prepared with 1 to 20wt% Ni and 0.5 to 10wt% Cu loadings via impregnation method. Catalysts were characterized by ICP, BET, XRD, H2 chemisorption and TEM technique as well CHNS analysis was carried out for elemental analysis of bagasse. The biomass supercritical water gasification process was performed in a batch micro-reactor at 400oC and 240 bars. The total gas yield increased with increasing in Ni and Cu loadings up to 15wt% Ni and 7.5 wt% Cu and became approximately constant. The catalytic process increased hydrogen yield to 9.5, CO yield to 2.2 and light hydrocarbons to 0.59 mmol/g bagasse. Total H2 selectivity (THS) ratio increased with increasing in Ni and Cu loading reached a maximum at 15%wt Ni and 7.5%wt Cu and then began to decrease.

1,1’-[benzene-1,4-diyldimethylylidene]bis(3-phenylurea)) was synthesized and tested for the fabrication of aluminum selective membrane electrode. The membrane electrode with dimethyl phthalate (DMP) as plasticizer works satisfactorily in the linear concentration range of 1.3×10-6 to 1.0×10-1 M with best optimized composition of DMP: PVC: ionophore: NaTPB of 60:35:3:2 (%, w/w). The membrane electrode has a fast response time (5 s) and wide pH range (2.5–8.5). The selectivity coefficients were calculated with fixed interference method. The membrane sensor was also used as an indicator electrode for the titration of Al(III) with EDTA and Na3PO4.

An extensive study of Higher Alcohols Synthesis (HAS) from syngas using alkalized MoS2 catalysts supported on Carbon Nanotubes (CNTs) is reported. Up to 30wt.% of Mo and 8wt.% K are added to the CNTs by impregnation method. The catalysts were characterized by different methods and the performance of the catalysts was assessed in a micro-reactor. TEM images showed that most of the metal particles were homogeneously distributed inside the tubes and the rest on the outer surface of the CNTs, with particle sizes in the range of 3 to 16 nm. Temperature Programmed Reduction (TPR) tests showed that increasing the amount of Mo decreased the first TPR peak from 518 to 503oC and increased the second TPR peak temperature from 782 to 825oC. The diffraction peaks representing the characteristic K-Mo-O phase (these species can enhance formation of higher alcohols) were observed in the XRD of catalysts. The catalyst with 20 wt.% Mo and 8 wt.% K showed the highest conversion. The total alcohol selectivity reached a maximum of 45.3 wt.% on the catalyst with 15 wt.% Mo. The catalyst with 15 wt.% Mo exhibited selectivity of 35.3 wt.% towards higher alcohols.

An extensive study of Higher Alcohol Synthesis (HAS) from synthesis gas using cobalt (Co) promoted alkalized MoS2 catalysts supported on Carbon NanoTubes (CNTs) is reported. Up to 5wt.% of Co is added to the 15wt.% Mo-8wt.%K/ CNTs by incipient wetness impregnation method. Most of the metal particles were homogeneously distributed inside the tubes and the rest on the outer surface of the CNTs. The catalysts are extensively characterized by different methods and the activity and selectivity of the catalysts were assessed in a fixed bed micro-reactor. Temperature Programmed Reduction (TPR) tests showed that addition of cobalt decreased the second TPR peak temperature from 801 to 660oC. The diffraction peaks that represent the characteristic K-Mo-O phase (i.e. K2Mo2O7, K2MoO4, K2Mo7O20, KMo4O6, and K0.33MoO3; these species can enhances formation of higher alcohols) were observed in the X- Ray Diffraction (XRD) patterns of unpromoted Mo-K/CNTs and to a greater extent in the XRD patterns of Co-promoted Mo-K/CNTs catalysts. Co addition to Mo-K/CNTs not only increased the number of surface sites, but also decreased the average active metal particle sizes from 7.53 to 5.33 nm and increased the percentage dispersion from 51.1 to 68.2%. Among the catalysts with different Co loadings, catalyst with 5 wt.% Co showed the highest %CO conversion of 38.8%. The total alcohol selectivity reached a maximum of 59.7 wt.% on the catalyst promoted with 3 wt.% cobalt. The catalyst with 3 wt.% Co exhibited selectivity of 41.65 wt.% towards higher alcohols.

The three chelates 1,5-bis(o-aminobenzoate)-3-thiapentane (L1), 1,5-bi (omethoxybenzoate)- 3-thiapentane (L2), 1,5-bis(o-chlorobenzoate)-3-thiapentane (L3) have been synthesized and explored as a neutral ionophores for preparing poly(vinyl chloride) based membrane sensors selective to Er3+. The addition of sodiumtetraphenylborate and various plasticizers, viz., o-NPOE, TBP, DBBP, TEP and DOS has been found to substantially improve the performance of the sensor. The best performance was obtained with the sensor no.6 having membrane of ionophore (L2) with composition PVC: o-NPOE: ionophore (L2): NaTPB (%w/w) of 35:56:2:7. This sensor exhibits Nernstian response with slope 22.1±0.3 mV decade-1 of activity in concentration range 3.0×10-9 to 1.0×10-2 M Er3+, performs satisfactorily over wide pH range (1.5-6.7), with a fast response time (8 s). The sensor was also found to work satisfactorily in partially non-aqueous media up to 20% content of methanol, ethanol or acetone and could be used for a period of 5 months without any change in response characteristics. The Er3+ selective sensor was used as an indicator electrode for the titration of 25 ml 1×10-3 M EDTA solution and for the determination of Er3+ ion in different synthetic samples. The proposed electrode no. 6 was also used for the determination of fluoride ion in two mouth wash sample.

Novel polymeric membrane electrode based on o-hydroxyacetophenone carbohydrazone (OHAC) as a neutral ionophore carrier has been prepared and explored as silver(I) ion selective electrode. The addition of lipophilic anion excluder (NaTBP) and various plasticizers viz. o-nitrophenyl octyl ether (o-NPOE), dibutyl phthalate (DBP) and butyl acetate (BA) have found to improve the performance of membrane sensor which have a composition of I:NaTBP:NPOE:PVC in the ratio 3:2:62:33 (w/w; mg). The electrode exhibit Nernstian slope for silver (I) ions over wide concentration ranges from 1.0×10-7 M to 1.0×10-2 M with limits of detection 1.6×10-7 M. The response time was found to be 10 s. The potentiometric responses were independent of the pH of the test solutions in the pH range of 3.0 to 8.0. The proposed electrode revealed good selectivity over a wide variety of other cations including alkali, alkaline earth, transition and heavy metal ions. The proposed PVC electrode was used as an indicator electrode in the potentiometric titration of Ag+ ions with KI and NaCl solutions and in direct determination in different water samples and urine sample of peoples working as jewelry maker.

Copolymer resin (8-HQ5-SAOF-IV) synthesized by the condensation of 8-hydroxyquinoline5-sulphonic acid and oxamide with formaldehyde in the presence of acid catalyst, was proved to be selective chelating ion-exchange copolymer for certain metals. The chelating ion-exchange properties of this copolymer were studied for Fe (III), Cu (II), Ni (II), Co (II), Zn (II), Cd (II) and Pb (II) ions in the form of their metal nitrate solutions. A batch equilibrium method was employed in the study of the selectivity of metal ion uptake involved in the measurements of the distribution of a given metal ion between the copolymer sample and a solution containing the metal ion. The study was carried out over a wide pH range, shaking time and in media of various ionic strengths of different electrolytes. The copolymer showed a higher selectivity for Fe (III), Cu (II), and Ni (II) ions than for Co (II), Zn (II), Cd (II) and Pb (II) ions. Distribution ratios (D) of metal ions were found to be increased by increasing pH of the solutions; hence the resin can be used to recover certain metal ions from waste solutions and used for the purpose of purification of waste water and removal of iron from boiler water. The ion-exchange capacity of metal ions has also been determined experimentally and compare with other commercial resins. The morphology of the copolymer was studied by scanning electron microscopy (SEM); showing amorphous nature of the resin therefore can be used as a selective ion-exchanger for certain metal ions.