Selective Gas-phase Research Articles

Highly selective gas-phase oxidation of benzyl alcohol to benzaldehyde over silver-containing hexagonal mesoporous silica

Silver-containing hexagonal mesoporous silica (Ag-HMS) catalysts with different Si/Ag ratios were synthesized by one-pot hydrothermal method for gas-phase selective oxidation of benzyl alcohol to benzaldehyde. The samples were characterized by nitrogen adsorption, X-ray diffraction, scanning electron micrograph, transmission electron micrograph, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectra. It was found that the Ag-HMS catalysts with different Ag loadings (0.55–3.50 wt.%) and different Ag particle sizes (5–32 nm) showed a similar level of catalytic property because they possess a similar Ag surface area. The Ag-HMS catalyst with a Ag loading of 2.81 wt.% exhibited excellent catalytic properties at 583 K with a high benzyl alcohol conversion of near 100%, benzaldehyde selectivity of around 96.0%, and benzaldehyde yield of about 96.0%, superior to those of other M-HMS catalysts (M = Co, Ce, La, Cu, Sr, Cd, Ni, Mn, V, and Fe). The enhanced catalytic performance could be attributed to the presence of the Ag surface oxygen species generated via oxygen spillover process. The work would be helpful for the development of novel Ag catalysts for selective oxidation of benzyl alcohol to obtain high quality of benzaldehyde and understanding the catalytic mechanism.

An Attempt to Selectively Oxidize Methane over Supported Gold Catalysts

The potential of supported gold catalysts for the selective gas-phase oxidation of methane to methanol with molecular oxygen was investigated. A broad range of supported gold-based catalyst materials was synthesized using reducible and non-reducible support materials. Although the formation of small gold nanoparticles was established for all catalyst materials, only a very low activity for the total oxidation of methane was observed, at temperatures >250 °C. Since no traces of partial oxidation products, such as methanol, formaldehyde, formic acid, methyl formate, dimethyl ether and CO, were observed it was concluded that supported gold catalysts are not able to selectively oxidize methane to methanol under these experimental conditions. A broad range of gold-based catalyst materials was screened for the selectively oxidation of methane into partial oxidation products. The formation of small gold nanoparticles was established for all catalyst materials, however solely a low catalytic activity for the total combustion of methane was accomplished and no traces of partial oxidation products were observed.

Deactivation and regeneration studies of a PdSb/TiO 2 catalyst used in the gas-phase acetoxylation of toluene

A PdSb/TiO 2 catalyst was subjected to selective gas-phase acetoxylation of toluene (Tol) to benzyl acetate (BA). The catalyst showed low initial activity, which increased with time and displayed maximum activity after 6 h on-stream. The catalyst exhibited stable performance for the next 25 h on-stream and then started to deactivate rapidly. Carbon analysis pointed to coke deposition as a main reason for the loss of catalytic activity. Regeneration at 250 °C could restore the catalyst activity but showed rapid deactivation again in a subsequent run. Regenerative treatment at 300 °C was much more effective not only in restoring the performance but also in maintaining relatively good stability of the catalyst. However, treatment at 350 °C and higher was found to be ineffective in restoring catalytic activity. This surprising behavior of the catalyst was studied in detail with various techniques such as XRD, TEM, and XPS analysis. XRD data give evidence on shifting of Pd reflection to lower 2 θ values and the splitting into two parts pointing to different types of Pd species. The surface ratio of Pd 0/PdO of 1 seems to be essential for better performance as revealed by XPS analysis. Surprisingly, this ratio is significantly changed in the deactivated catalyst. Moreover, only metallic Pd species were found on the catalyst surface of this deactivated solid. In addition, regeneration at 300 °C could restore PdO concentration to a greater extent, while the one regenerated at 350 °C could not restore the PdO proportion. This behavior is also believed to be one of the reasons for the unproductive regeneration of deactivated catalyst at 350 °C. Furthermore, Pd 0, PdO, and Sb synergy is needed for stable and high performance of the catalyst.

Efficient and Selective Gas-Phase Monomethylation versus NH Bond Activation of Ammonia by “Bare” Zn(CH3)+: Atomic Zinc as a Leaving Group in an SN2 Reaction

Rally from C to N: The goal of the race concerns connecting carbon and nitrogen. While the SN2-type mechanism constitutes a one-way road to produce CH3NH3+ from Zn(CH3)+ and NH3, activation of the CH bond of methane in the Zn(NH2)+/CH4 system is a blind alley (see scheme).

Selective Gas-Phase Conversion of Glycerol to Acetol Over Promoted Zirconia Solid Acid Catalysts

The vapour phase selective conversion of glycerol to acetol at normal atmospheric pressure in a fixed-bed micro reactor was investigated over SO4 2� and WOx promoted ZrO2 and M-ZrO2 (M = Al2O3 or TiO2) solid acid catalysts. The investigated sulfate and tungstate ion promoted zirconia-based catalysts were prepared by both coprecipitation and impregnation methods and calcined at 923 K. The synthesized catalysts were characterized by means of X-ray powder diffraction, X-ray photoelectron spectroscopy, BET surface area and ammonia temperature-programmed desorption methods. Characterization results suggest that SO4 2� promoter strongly influences the physicochemical properties of the support oxides than WOx. However, the WOx promoted catalysts exhibited better catalytic activity than SO4 2� promoted catalysts. Among various catalysts investigated, the WOx/Al2O3-ZrO2 catalyst exhibited stable catalytic activity with the highest glycerol conversion of 99% and acetol selectivity of 74%.

Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature

Gold (Au) is an interesting catalytic material because of its ability to catalyze reactions, such as partial oxidations, with high selectivities at low temperatures; but limitations arise from the low O2 dissociation probability on Au. This problem can be overcome by using Au nanoparticles supported on suitable oxides which, however, are prone to sintering. Nanoporous Au, prepared by the dealloying of AuAg alloys, is a new catalyst with a stable structure that is active without any support. It catalyzes the selective oxidative coupling of methanol to methyl formate with selectivities above 97% and high turnover frequencies at temperatures below 80 degrees C. Because the overall catalytic characteristics of nanoporous Au are in agreement with studies on Au single crystals, we deduced that the selective surface chemistry of Au is unaltered but that O2 can be readily activated with this material. Residual silver is shown to regulate the availability of reactive oxygen.

High activity and selectivity of Ag/SiO2 catalyst for hydrogenation of dimethyl oxalate

Ag/SiO(2) prepared by a sol-gel process is highly effective for selective gas-phase hydrogenation of dimethyl oxalate to corresponding alcohols. The catalysts are of great potential as industrially viable and novel catalysts for the production of methyl glycolate and ethylene glycol.

Selective Gas-Phase Conversion of Glycerol to Acetol Over Promoted Zirconia Solid Acid Catalysts

Selective Gas-Phase Conversion of Glycerol to Acetol Over Promoted Zirconia Solid Acid Catalysts

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al2O3, TiO2, Fe2O3 and CeO2) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO2 bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO2 catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.

Pd-promoted selective gas phase hydrogenation of p-chloronitrobenzene over alumina supported Au

The gas phase hydrogenation of p-chloronitrobenzene has been investigated over alumina supported Au (ca. 1%, w/w) prepared by deposition-precipitation with urea (DP) and impregnation in excess solvent (IMP). Both catalysts were 100% selective in terms of –NO 2 group reduction, resulting in the sole formation of p-chloroaniline. Au–DP exhibited a smaller mean Au size (2.9 nm) compared with Au–IMP (4.5 nm) and delivered a higher (by a factor of 14) specific hydrogenation rate. Bimetallic Pd–Au/Al 2O 3 catalysts have also been prepared by DP and IMP with Au/Pd mol/mol = 8, 20 and 88. Catalyst activation by temperature programmed reduction has been monitored and the activated catalysts characterized in terms of H 2 chemisorption, TEM analysis and DRIFTS. DRIFTS measurements using CO as a probe molecule suggest the presence of bimetallic particles and surface Au–Pd interaction. A significant increase (by up to a factor of 3) in activity was observed for Pd–Au/Al 2O 3 (Au/Pd ⩾ 20) compared with Au/Al 2O 3 where the exclusive conversion of p-chloronitrobenzene to p-chloroaniline was maintained. At a lower ratio (Au/Pd = 8), nitrobenzene was produced as a result of a Pd catalyzed hydrodechlorination step. Under the same reaction conditions, Au/Al 2O 3 + Pd/Al 2O 3 physical mixtures (Au/Pd = 20) delivered higher reaction rates but with the formation of nitrobenzene and aniline, i.e. products of hydrodechlorination and hydrogenation. We attribute the enhanced and exclusive production of p-chloroaniline over the supported bimetallics to a surface Pd–Au synergism. Our results establish the viability of Pd-promotion in the selective continuous gas phase catalytic hydrogenation of p-chloronitrobenzene over supported Au.

Alternative Reagents for Chemical Noise Reduction in Liquid Chromatography–Mass Spectrometry Using Selective Ion–Molecule Reactions

Reduction of ionic chemical background noise based on selective gas-phase reactions with chosen neutral reagents has been proven to be a very promising approach in liquid chromatography-mass spectrometry (LC-MS). In this study further investigations on alternative reagents including the disulfides (dimethyl disulfide, diethyl disulfide, methyl propyl disulfide), dimethyl trisulfide, ethylene oxide, and butadiene monoxide, for example, have been carried out. Tandem mass spectrometric studies of ion/molecule reactions indicate that-besides dimethyl disulfide-ethylene oxide and butadiene monoxide also exhibit very efficient reactions with background ions. Furthermore, it is confirmed that the reactions are very selective according to the test with some analyte ions. In contrast to its rapid reactions with background ions, ethylene oxide does not react, or reacts much less, with these analytes. Therefore, it can be used as an alternative reagent for noise reduction. Although reactions of the other tested neutral reagents with background ions are evaluated, they are generally not suitable as reagents for this purpose because of lack of reactivity or dramatic ion losses during reactions.

Supported gold catalysts for selective hydrogenation of 1,3-butadiene in the presence of an excess of alkenes

Supported gold catalysts were investigated in the selective gas phase hydrogenation of 1,3-butadiene in an excess of propene (0.3% butadiene, 30% propene and 20% hydrogen), in order to simulate the process required for the purification of industrial alkenes streams to prevent poisoning of the polymerisation catalysts used for polyalkene production. Gold catalysts containing small gold particles (between 2 to 5 nm in average) are less active than commercial palladium catalysts, but they are much more selective. Under our experimental conditions, 100% of butadiene can be converted at ≈170°C into 100% butenes with 1-butene as the main product, and with only very small amount of alkanes formed (≈100 ppm). The absence or presence of propene does not drastically modify the rate of hydrogenation of butadiene.

Automated Neutral Loss and Data Dependent Energy Resolved “Pseudo MS3” for the Targeted Identification, Characterization and Quantitative Analysis of Methionine-Containing Peptides

A strategy involving the fixed-charge sulfonium ion derivatization, stable isotope labeling, capillary high- performance liquid chromatography and automated data dependent neutral loss scan mode tandem mass spectrometry (MS/MS) and "pseudo multiple mass spectrometry (MS3)" product ion scans in a triple quadrupole mass spectrometer has been developed for the "targeted" gas-phase identification, characterization and quantitative analysis of low abundance methionine-containing peptides present within complex protein digests. Selective gas-phase "enrichment" and identification is performed via neutral loss scan mode MS/MS, by low energy collision-induced dissociation of the derivatized methionine side chain, resulting in the formation of a single characteristic product ion. Structural characterization of identified peptides is then achieved by automatically subjecting the characteristic neutral loss product ion to further dissociation by data dependent product ion scan mode pseudo MS3 under higher collision energy conditions. Quantitative analysis is achieved by measurement of the abundances of characteristic product ions formed by sequential neutral loss scan mode MS/MS experiments from "light" (12C) and "heavy" (13C) stable isotope encoded fixed-charge derivatized peptides. In contrast to MS-based quantitative analysis strategies, the neutral loss scan mode MS/MS method employed here was able to achieve accurate quantification for individual peptides at levels as low as 100 fmol and at abundance ratios ranging from 0.1 to 10, present within a complex protein digest.

Selective gas-phase capture of explosives on metal β-diketonate polymers

A variety of metal β-diketonate polymers were assessed for gas-phase selective retention of nitro aromatic, nitrate ester, and peroxide explosives. The La(III) complex of p-di(4,4,5,5,6,6,6-heptafluoro-1,3-hexanedionyl)benzene [La(dihed)] showed 13–42 times the retention for the nitro aromatic compounds compared to a control column (identical column but lacking the 5% loading of the metal β-diketonate polymer). Nitrate esters, the peroxide explosive triacetone triperoxide, and the taggant 1,4-dimethyl-1,4-dinitrobutane were too strongly retained to elute from the La(dihed) column; however, these compounds could be eluted from the less retentive Cu(dihed) or Zn(dihed) columns. A Kováts index of 2124 for 2,4,6-trinitrotoluene (TNT) on the La(dihed) column compared to 1662 on the control illustrates the excellent discrimination against nonpolar hydrocarbons, the principal matrix interference expected in air samples. A proof-of-principle experiment demonstrated analysis of an extrapolated 47 part-per-trillion (ppt) (v/v) of TNT in an air extract concentrate.

Evolutionary Screening of Biomimetic Coatings for Selective Detection of Explosives

Susceptibility of chemical sensors to false positive signals remains a common drawback due to insufficient sensor coating selectivity. By mimicking biology, we have demonstrated the use of sequence-specific biopolymers to generate highly selective receptors for trinitrotoluene and 2,4-dinitrotoluene. Using mutational analysis, we show that the identified binding peptides recognize the target substrate through multivalent binding with key side chain amino acid elements. Additionally, our peptide-based receptors embedded in a hydrogel show selective binding to target molecules in the gas phase. These experiments demonstrate the technique of receptor screening in liquid to be translated to selective gas-phase target binding, potentially impacting the design of a new class of sensor coatings.

Collision induced dissociation of deprotonated guanine: Fragmentation of pyrimidine ring and water adduct formation

This investigation of collision induced dissociation of deprotonated guanine was conducted by using sequential ion trap tandem mass spectrometry and isotopically labelled guanine analogs to clarify the complex dissociation reactions of pyrimidine ring of deprotonated guanine. The fragmentation patterns confirmed that the gas-phase dissociation processes are initiated from the pyrimidine ring through charge site controlled reactions involving charge redistribution, proton transfer and nucleophilic attack to generate the three primary fragment products by the elimination of ammonia, cyanamide and isocyanic acid. Our findings shed light on the process of pyrimidine ring opening and closure prior to the decomposition of deprotonated guanine and therefore the identity of N1 and exocyclic N 2 is lost in deprotonated guanine as a result of scrambling. Intriguing association products of water addition to fragment ions have been structurally characterized to establish the role of ketene group in the selective gas-phase reaction of water addition and the formation of water adducts. The elimination of CO 2 from negatively charged water adducts provides evidence for the covalent attachment of water to the ketene moiety. The established mechanisms of the dissociation reactions of pyrimidine ring should provide a basis for the structural elucidation of guanine relevant species modified on its active sites. To lend support for our proposals, collision induced dissociation study of 8-phenyl-2′-deoxyguanosine adduct was performed.

Clean production of chloroanilines by selective gas phase hydrogenation over supported Ni catalysts

Abstract We have established for the first time 100% selectivity in the continuous gas phase hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline for reaction over a series of oxide and carbon supported Ni catalysts (6 ± 2%, w/w) under mild reaction conditions (T = 393 K, P = 1 atm). Catalyst activation by temperature programmed reduction (TPR) is addressed, BET area and H2 uptake measurements provided and mean metal particle sizes evaluated by transmission electron micrographic (TEM) analysis. The following activity sequence has been determined: Ni/Al2O3 > Ni/SiO2 > Ni/Activated Carbon > Ni/graphite. Pd/Al2O3, as an alternative catalyst, delivered an appreciably higher activity but with the production of nitrobenzene (principal product) and aniline (secondary product), i.e. hydrodechlorination with subsequent –NO2 reduction prevailed. Exclusive formation of the corresponding haloaniline is also demonstrated for the hydrogenation of o-chloronitrobenzene, m-chloronitrobenzene and p-bromonitrobenzene over Ni/Al2O3. A lower hydrogenation rate is established for p-CNB relative to nitrobenzene, consistent with a halogen substituent deactivation effect. While the Ni catalysts suffered a loss of activity with time-on-stream, exclusive selectivity to the haloamine product was maintained. These preliminary results can serve as a basis for the development of a cleaner, high throughput production of commercially important haloamines.

Functional group selective ion/molecule reactions: mass spectrometric identification of the amido functionality in protonated monofunctional compounds.

A mass spectrometric method was developed for the screening of the amido functionality in monofunctional protonated analytes. This method is based on selective gas-phase derivatization of protonated analytes by (N,N-diethylamino)dimethylborane in a Fourier transform ion cyclotron resonance (FT-ICR) and triple quadrupole mass spectrometer. Examination of a series of protonated analytes demonstrated that only the compounds containing the amido functionality react with the aminoborane by the derivatization reaction. The mechanism involves proton transfer from the protonated analyte to the borane, followed by addition of the amide to the boron center, which leads to the elimination of neutral diethylamine. The derivatized analytes are readily identified on the basis of a shift of 40 m/z units relative to the m/z value of the protonated analyte and characteristic boron isotope patterns. Collision-activated dissociation was used to provide support for the structures assigned to the derivatized analytes. The structural information gained from this gas-phase derivatization method will aid in the functional group identification of unknown compounds and their mixtures.

Structured fiber supports for ionic liquid-phase catalysis used in gas-phase continuous hydrogenation

Structured supported ionic liquid-phase (SSILP) catalysis is a new concept with the advantages of ionic liquids (ILs) used as solvents for homogeneous catalyst and the further benefits of structured heterogeneous catalysts. This is achieved by confining the IL with the transition metal complex to the surface of a structured support consisting of sintered metal fibers (SMFs). In an attempt to improve the homogeneity of the IL film, the SMFs were coated by a layer of carbon nanofibers (CNFs). The IL thin film immobilized on CNF/SMF supports presents a high interface area, ensuring efficient use of the transition metal catalyst. The regular structure of the support with high porosity (>0.8) allows a low pressure drop and even gas-flow distribution in a fixed-bed reactor. The high thermoconductivity of the CNF/SMF support suppresses the formation of hot spots during exothermic hydrogenation reactions. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene over a homogeneous Rh catalyst immobilized in IL supported on CNF/SMF was used as a test reaction to demonstrate the feasibility of the SSILP concept. The catalyst [Rh(H) 2Cl(PPh 3) 3/IL/CNF/SMF] showed a turnover frequency of 150–250 h −1 and a selectivity of >96%. High-pressure 1H NMR and 1H{ 31P} NMR spectroscopy was used to provide insights into the nature of the active catalytic species.

Comparison of Selective Gas Phase‐ and Liquid Phase Hydrogenation of (Cyclo‐)Alkadienes towards Cycloalkenes on Pd/Alumina Egg‐Shell Catalysts

AbstractThe hydrogenation of dienes such as 1,3‐butadiene, cyclooctadiene, and of acetylenic hydrocarbons on Pd catalysts shows high reaction rates and consequently, a strong influence of mass transfer on the selectivity of the intermediate alkene or cycloalkene product. 100 % selectivity towards (cyclo)‐alkene hydrogenation is achieved for the gas phase when the Thiele modulus is $ \varphi = L {\sqrt{ {k_1 c_{\rm H2}} \over {D_{\rm eff} c_{\rm diene}}}} \le {\sqrt{2}} $, where L is the thickness of the active layer and Deff is the effective diffusion coefficient of the diene. The interdependencies expressed by this formula were studied in detail using model catalysts with regular pores of uniform length and diameter and perpendicular to the surface. These catalysts were prepared by anodic oxidation of aluminium wires and immobilization of the active Pd. For the liquid phase procedure of selective hydrogenation, a reaction mass transfer model has been derived in order to compare the gas phase and liquid phase procedures, in particular with respect to the selectivity. The hydrogenation of 1,3‐cyclooctadiene and of 1,3‐butadiene were studied for both procedures employing the same catalyst. The rate of hydrogenation can be represented for both cases by the identical kinetic equation r1 = k1 cH2. This result is interpreted by assuming that the access of hydrogen to the surface through the dense layer of adsorbed diene is the rate determining step.