Desorption Of Pyridine Research Articles

Synthesis and characterization of copper and aluminum salts of H 3PMo 12O 40 for their use as catalysts in the eco-friendly synthesis of chromanes

Acid catalysts based on aluminum (or copper) salts of molybdophosphoric acid (H 3PMo 12O 40) were prepared. They were synthesized from a heteropolyacid solution to which Al 2(SO 4) 3, Al 2O(CH3COO) 4 or CuSO 4 was added. The obtained salts were texturally characterized and a low specific surface area, between 1 and 9 m 2/g, was observed. The analysis of the prepared salts by ICP-AES spectrometry indicated that the molar content of Mo and P was that corresponding to the anion [PMo 12O 40] 3−. The presence of undegraded Keggin structure was confirmed by XRD, FT-IR, DRS and 31P MAS-NMR. The catalyst acidity was measured by means of potentiometric titration with a solution of n-butylamine in acetonitrile and by temperature-programmed desorption of pyridine. The AlPMo 12O 40 salt (from sulfate), which at 5 h of reaction showed the highest conversion of m-cresol (98%) and 75% and 25% selectivity to chromane 4 and 3, respectively, presented the highest total acidity and, simultaneously, acid sites with maximum acid strength very high. On the other hand, Cu 0.5H 2PMo 12O 40 catalysts at 5 h of reaction showed lower conversion of m-cresol than the former salt (90%) and also lower selectivity to chromane 4 and 3 (37% and 10%, respectively). In the latter case also 43% of open-chain compounds were observed. With the other salts, Al 0.5H 1.5PMo 12O 40, Cu 3(PMo 12O 40) 2 and AlPMo 12O 40 (from Al 2O(CH 3COO) 4), only the reaction intermediaries, open-chain compounds, were obtained because these salts presented low total acidity and acid sites with maximum acid strength very low.

Catalytic Properties of Pt−Re/Al2O3Naphtha-Reforming Catalysts Modified by Germanium Introduced by Redox Reaction at Different pH Values

Ge was deposited by catalytic reduction on Pt−Re/Al2O3−Cl base catalysts prepared by coimpregnation. The nominal amounts of Ge incorporated from various impregnation media (H2O, HCl, or NH3) were 0.1, 0.3, 1.0, and 2.0 wt %. The catalysts were characterized by means of the test reactions of cyclohexane dehydrogenation and cyclopentane hydrogenolysis. The catalysts were further characterized by FTIR spectroscopy of adsorbed CO and adsorbed pyridine and by temperature-programmed desorption of pyridine. Blank treatments performed on the Pt−Re/Al2O3 base catalyst in the different media (without Ge addition) showed that the catalytic reduction method modified the properties of the parent catalyst depending on the pH of the solution. The Bronsted acidity decreased as the impregnation pH was increased, whereas the Lewis acidity of the catalyst was not substantially modified by any of the impregnation media. Catalysts with higher Pt−Re interactions were obtained after treatment in a solution of high pH (NH3 mediu...

MTW zeolites for reducing cold-start emissions of automotive exhaust

Increasingly strict environmental legislations have led to the need for better control of vehicle cold-start emissions. In this research work, a series of one-dimensional channel molecular sieves with 12 oxygen ring apertures (12R) having MTW structure (MTW is the designation for ZSM-12 in the IZA nomenclature of zeolite structures), have been synthesized and characterized by different techniques such as: XRD, SEM, BET surface area, elemental analysis by atomic absorption spectroscopy, FTIR of adsorbed pyridine and temperature-programmed desorption (TPD). The synthesized samples were tested as hydrocarbons (HCs) trap adsorbents using toluene and ethylene as heavy (aromatic) and light (olefin) probe molecules present in the exhaust stream at engine cold-start. TPD tests were performed after adsorption under four different mixture conditions: binary (toluene–ethylene), ternary (toluene–ethylene–CO2), ternary (toluene–ethylene–H2O) and quaternary (toluene–ethylene–CO2–H2O). The results demonstrated that a silver exchanged MTW zeolite, Al-ZSM-12 (Ag) exhibited high and stable trapping capacities for both probe molecules in all mixtures investigated in this study, thus showing essentially no sensitivity to CO2 and/or H2O. Moreover, a high desorption temperature, particularly for toluene, was associated with the large amount of strong Lewis and Brönsted acid sites. Isomorphous substitution of Al by Fe in MTW structures did not lead to drastic changes in adsorption capacities and desorption temperatures of the two sorbates.

Catalytic properties of Ga–HMS- n materials in the tertiary butylation of phenol

The gallium-incorporated hexagonal mesoporous silica material (Ga–HMS- n, where n is the Si/Ga ratio in the precursor gel = 30, 60 and 90) was successfully synthesized at ambient temperature using hexadecylamine (HDA) as the template agent. The obtained materials were analyzed by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption to determine the structural order and the textural properties. It has been observed by 71Ga MAS-NMR spectroscopy that gallium atoms are majority in tetrahedral coordination for all samples. Temperature-programmed desorption (TPD) of pyridine showed that Ga–HMS-30 has a higher number of strong acid sites. The catalyst Ga–HMS-30 showed bigger performance in the acid-catalyzed tertiary butylation of phenol employing tert-butanol as the alkylation agent. In fact, a high phenol conversion of 75.3% is observed for this catalyst at a reaction temperature of 433 K. However, the 4- tert-butyl phenol (4-TBP) yield amounts to 41.8% and the 2,4-di- tert-butyl phenol (2,4-DTBP) yield corresponds to 30.3%. Furthermore, due to the large pore size, which allows a faster diffusion of reactants and products, catalyst deactivation of Ga–HMS-30 was not observed even after reaction for 5 h.

Preparation of the supported heteropolyacids catalyst by ultrasound-plasma treatment

Novel catalysts of phosphotungstic heteropolyacids (PW12) supported on neutral alumina were prepared by assistance of ultrasound and plasma treatment. The prepared catalysts were characterized by FT-IR pyridine adsorption (Py-IR), temperature programmed desorption of Pyridine (Py-TPD), BET and X-ray diffraction (XRD), and their catalytic performances were evaluated by the cationic polymerization of tetrahydrofuran. The results indicate that plasma treatment remarkably increases the surface acidity of the prepared catalyst while ultrasonic treatment induces PW12 to uniformly disperse on the support surface and expose more active sites for the acid catalytic reaction. A higher catalytic activity (69.7%) is obtained on the novel catalyst, which significantly outstripped that on the conventional sample (57.5%).

Analysis of coke deposition and study of the variables of regeneration and rejuvenation of naphtha reforming trimetallic catalysts

The deactivation, by coke deposition and sintering, and the regeneration of the metal function of PtReSn/Al 2O 3–Cl and PtReGe/Al 2O 3–Cl catalysts were studied. The variables affecting the rejuvenation of the metal function (oxychlorination at 480 °C with 300, 530 and 900 ppm, stabilization at 520 °C) and the coke burning-off (400–500 °C, 0.5, 2, 6 or 12 h) were assessed. The analysis of the carbon deposits and of the final state of the metal and acid functions were performed by means of temperature programmed oxidation, temperature programmed reduction and temperature programmed pyridine desorption (TPO, TPR and TPD-Py) respectively. The degrees of deactivation and activity recovery of the metal function were measured by means of the cyclohexane dehydrogenation and cyclopentane hydrogenolysis reactions. The deactivation or activity recovery of the acid function assessed with the n-pentane isomerization reaction. It was found that the PtReSn catalysts were more stable than the PtReGe ones. This was due to the lower amount of coke deposited on the surface of PtReSn. It was found that when the catalysts were oxidized with an oxygen stream more than 6 h were needed to eliminate the coke at 400 °C while at 500 °C the coke was eliminated in 30 min. Regeneration produced a segregation of the metal phase on both kinds of catalysts. Metal particle sintering at 650 °C modified the metal function severely and Pt was segregated from the other components. All the rejuvenation treatments (Cl, air and high temperature) were unable to restore the original state of the metal function.

Propane Oxidative Dehydrogenation on V–Sb/ZrO2 Catalysts

The catalytic properties of V–Sb/ZrO2 and bulk Sb/V catalysts for the oxidative dehydrogenation of propane were studied. Samples were characterized by nitrogen adsorption, temperature-programmed reduction, temperature-programmed pyridine desorption and photoelectron spectroscopic techniques. Vanadia promotes the transition of tetragonal to monoclinic zirconia and the formation of ZrV2O7. Surface V and Sb oxide species do not appear to interact among them below monolayer coverage, but SbVO4 forms above monolayer. Simultaneously the excess of antimony forms α-Sb2O4. Activity and selectivity show no dependence on the acidity of the catalysts. However, there is a strong dependence of activity/selectivity on composition; surface vanadium species are active for propane oxidative dehydrogenation and the presence of Sb, affording rutile VSbO4 phase makes the system selective to C3H6, this is believed to be related to the redox cycle involving dispersed V5+ species and lattice reduced vanadium site in the rutile VSbO4 phase.

Determination of the Acidity of MCM-41 with Different Si/Al Ratios by the Temperature Programmed Desorption of Pyridine

Mesoporous catalysts of MCM-41 type with different Si/Al ratios were synthesized by the hydrothermal method and characterized by X-ray diffraction, nitrogen adsorption and thermogravimetric analysis. Their acidic properties were determined via the temperature programmed desorption of pyridine. Vyazovkin kinetic models were used to correlate the total acidity as a function of the Si/Al ratio with the apparent activation energy for pyridine desorption. The results obtained indicated that the catalysts possessed weak and medium acidic sites. An increase in the Si/Al ratio led to an increase in the activation energy for pyridine desorption.

Comparative Study of Regioselective Synthesis of β-Aminoalcohols under Solventless Conditions Catalyzed by Sulfated Zirconia and SZ/MCM-41

Sulfated zirconia and SZ/MCM-41 were used as catalysts for the synthesis of beta-aminoalcohols via epoxide aminolysis. Sulfated zirconia was prepared by sol-gel and SZ/MCM-41 was obtained by impregnation. Solid catalysts were characterized by XRD, SEM-EDS, UV-Vis, FT-IR pyridine desorption and Nitrogen physisorption. Both acid materials were useful as catalysts, even when they were recycled several times. The beta-aminoalcohols were characterized by FT-IR, (1)H- and (13)C-NMR and GC-MS.

Activation and reactivity of epoxides on solid acid catalysts

The aminolysis of epoxides over novel solid catalysts (Brönsted-acidic SBA-15 functionalized with propylsulfonic acid and Lewis-acidic Ti-MCM-41) is reported. The acidic properties of these catalysts were determined by FTIR spectroscopy and temperature-programmed desorption of pyridine and NH 3, respectively. The mesoporous solid acids of the present study are reusable and exhibit significantly higher catalytic activities than known catalysts for opening of the oxirane ring with nitrogen (aromatic and aliphatic amines)-containing and oxygen (alcohols)-containing nucleophiles. A range of β-amino alcohols with high regioselectivity and stereoselectivity were synthesized. Adsorption studies as well as the sigmoid shape of the conversion-versus-time plots show that the epoxide and amine compete for adsorption on the acidic sites ( SO 3H or Ti 4+) on the catalyst surface. Epoxide adsorption and activation on acid sites are the more critical processes. Catalytic activity decreases with increasing basicity of the amines and/or the alcohol, as well as the dielectric constant of the solvent.

Friedel-Crafts acylation reactions using heterogeneous catalysts stimulated by conventional and microwave heating

The acylation of anisole with decanoic acid over a range of solid-acid catalysts has been studied under conventional heating and microwave stimulation. The microwave-irradiated experiments exhibited increased reaction rates, the extent of which was dependent upon the nature of the catalyst. The origin of this microwave enhancement has been attributed to the selective desorption of water from the surface of the catalyst. Pyridine adsorption has been used to quantify the number of acid sites present on the catalyst. The nature of the acid sites in terms of their Brønsted/Lewis acidity has been determined using infrared analysis of the adsorbed pyridine. The strength of the acid sites has been characterised by differential thermal analysis (DTA) of the pyridine desorption. This indicated that the sulphated zirconia and supported heteropoly acids contained significantly more strong acid sites than the cerium exchanged zeolite.

Novel use of mesoporous aluminas as supports for Cp2ZrCl2 and Cp*ZrMe3: Ethylene polymerization and formation of polyethylene nanofibers

AbstractMesoporous aluminas with average pore sizes of 4.3–7.8 nm were prepared by anodization of an aluminum film (AAO), and by a sol–gel templating method (TPL). In addition to a commercial alumina and sulfated TPL, the aluminas were used as supports for cyclopentadienyl zirconocene dichloride (Cp2ZrCl2) and trimethyl(η5‐pentamethylcyclopentadienyl)zirconium (Cp*ZrMe3) and tested in the polymerization of ethylene. The metallocenes supported on the alumina prepared with the templating method and its sulfated modification exhibited polymerization activities of 440 and 350 kgPE/(molZr × h × bar), respectively, comparable to that obtained with silica‐supported metallocenes (390 kgPE/(molZr × h × bar)). The acid site distribution of the aluminas was studied with FTIR and temperature programmed desorption (TPD) of pyridine, and also the amount of medium and strong acid sites was determined gravimetrically from the adsorption of pyridine. Relative to the surface area, AAO with the highest amount of acid sites (2.10 μmolpy/m) adsorbed Cp2ZrCl2 and Cp*ZrMe3 the most. Study of the polymers' morphology with a scanning electron microscope revealed polyethylene nanofibers in all the polymer samples, also in those obtained from the reference polymerizations with homogeneous Cp2ZrCl2 and Cp*ZrMe3. This finding suggests that a catalyst support with a tubular pore structure is not a prerequisite for the formation of polymer nanofibers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4002–4012, 2007

Sulfated zirconia grafted on a mesoporous silica aerogel: Influence of the preparation parameters on textural, structural and catalytic properties

Silica supported sulfated zirconia catalysts were synthesized via a new method by grafting sulfated zirconia on the surface of a silica aerogel previously prepared. The main parameters studied in this work were the S/Zr, Zr/Si molar ratios and the support nature. The synthesized solids were characterized using XRD, N 2 physisorption at 77 K, TG-DTA/SM, sulfur chemical analysis and adsorption–desorption of pyridine followed by infrared spectroscopy. These solids were tested in the n-hexane isomerization reaction. Two types of mesopores were observed on the silica aerogel. This mesoporosity was affected depending on the preparation parameters. The increase of the Zr/Si molar ratio induces the decrease of the size of zirconia particles deposed on the support. In this case, appreciable amounts of sulfur are retained with the presence of a relatively strong Brönsted and Lewis acid sites on the catalyst surface. A high density of Brönsted sites seems to be interesting in the n-hexane isomerization reaction.

Synthesis and characterization of mesoporous silica-supported nano-crystalline sulfated zirconia catalysts prepared by a sol–gel process: Effect of the S/Zr molar ratio

Abstract Mesoporous silica-supported nano-crystalline sulfated zirconia catalysts were prepared via the sol–gel process using an in situ sulfation. The parameter studied in this work was the S/Zr molar ratio. The synthesized solids were characterized using XRD, N 2 physisorption, TG-DTA/SM, sulfur chemical analysis and adsorption–desorption of pyridine, and tested in the gas-phase acid-catalysed isomerization of n -hexane. The gelation process is highly affected by the sulfate loadings. Two gelation mechanisms were evidenced depending on the S/Zr molar ratio. The first one observed when 0.15 ≤ S/Zr ≤ 0.5, is characterized by a relatively high gelation rate. This mechanism favours the formation of two types of mesopores and a low percentage of retained sulfur. The second gelation mechanism occurs for higher S/Zr ratios: 0.5 ≤ S/Zr ≤ 1.2. In that case, slower gelation rates are observed. This slower gelation process leads to materials with reduced BET surface area but the amount of retained sulfur is increased. Appreciable catalytic properties were observed for the sample prepared with the highest S/Zr ratio, which presents the smallest size of sulfated zirconia crystallites and shows both Bronsted and Lewis acid sites on its surface.

Isomerization Cracking of n-Octane and n-Decane on Regulated Acidity Pt/WOx−SO4−ZrO2 Catalysts

The effect of WO3 loading on the acidity and specific surface area of catalysts composed of Pt supported on ZrO2 and double-promoted with SO42- and WO3 catalysts (PtWSZ) was studied. The catalysts were tested in the isomerization cracking of heavy alkanes, and the objective was to assess their ability to produce branched shorter alkanes, contributing to the gasoline pool. As a consequence, the focus was put on the maximization of the yield of C4−C7 isomers (i-C4-7). The catalysts were characterized by several techniques. The crystalline structure was analyzed by XRD. The acidity of the catalysts was measured by thermal desorption of pyridine. A screening and first selection of the most promising catalysts was done by means of the reaction of n-octane at 300 °C, 0.1 MPa, WHSV = 1 and H2/nC8 = 6 mol/mol. A high yield of i-C8 was obtained with the Pt/WO3−ZrO2 catalyst. The incorporation of SO42- as a promoter increased the acidity and the cracking activity. Pt/SO42-−ZrO2 displayed very strong acid sites and generated the highest amount of light hydrocarbons. Catalysts of regulated acidity combining both promoters yielded the best results. The most promising PtWSZ catalyst was obtained with 5% W and 1.4% S. The test reaction of n-decane at near-industrial conditions (1.5 MPa, 300 °C, WHSV = 4, H2/n-C10 = 6 mol/mol) was used for a further assessment of the catalytic properties. This test confirmed that the double-promoted catalyst (PtWSZ, 5% W, 1.4% S) had high activity and stability and produced an isomerizate with the highest i-C4-7/i-Ctotal molar ratio in comparison to the sulfated zirconia (PtSZ) and tungstated zirconia (PtWZ) catalysts.

Metal−Support Interactions in Zeolite-Supported Noble Metals: Influence of Metal Crystallites on the Support Acidity

The metal-support interactions on a series of catalysts of different acidities, including platinum-modified zeolites and H-MCM-41, are investigated by means of XPS, CO and pyridine adsorption, and a model reaction (ring opening of decalin). The electronic properties of Pt are influenced by the acidity of the support, and the alteration of Pt properties increases with increasing acidity of the support, as can be seen from the changes in the Pt binding energy and stretching frequency of adsorbed CO. At the same time, the presence of platinum affects the acidic properties of the supports by reducing the strength of the acid sites. This is observed directly as the changes in desorption of pyridine from the acid sites and indirectly as the suppression of cracking reactions during the ring opening of decalin on the Pt-modified catalysts. The observed results are discussed in terms of the interatomic potential model.

Role of Sn in Pt–Re–Sn/Al 2O 3–Cl catalysts for naphtha reforming

The influence of Sn addition on the catalytic activity of Pt–Re/Al 2O 3 naphtha reforming catalysts was studied. Trimetallic Pt–Re–Sn catalysts supported on chlorided alumina were prepared by coimpregnation. The content of Re and Pt was 0.3%, while the Sn load was varied between 0.0 and 0.9% (weight basis). In order to make a comparison, Pt/Al 2O 3 and Pt–Re/Al 2O 3 catalysts were also prepared. The acid function of the catalysts was studied by means of temperature programmed desorption of pyridine and the reaction test of n-pentane isomerization. Temperature programmed reduction and cyclohexane dehydrogenation were used to characterize the activity of the metal function. n-C 7 dehydrocyclization was used as a test of the combined action of the acid and metal function. It was found that Sn decreases the amount of strong acid sites of chlorided alumina producing catalysts with lower cracking activity. In the case of the trimetallic Pt–Re–Sn catalyst, Sn addition to Pt–Re decreases the hydrogenolytic activity and increases both the isomerization activity and the stability. The best catalyst is the one with 0.1% Sn. The addition of Sn to Pt–Re catalysts also decreases the benzene/ i-C 7 ratio of the reformate, an important issue from an environmental point of view.

SiO 2 film growth at low temperatures by catalyzed atomic layer deposition in a viscous flow reactor

SiO 2 thin films were grown by catalyzed atomic layer deposition (ALD) at low temperatures in a viscous flow reactor using sequential SiCl 4 and H 2O exposures. Pyridine (C 5H 5N) was used as a catalyst for both reactant exposures. Micropulsing was employed to avoid possible film contamination by pyridinium chloride. Quartz crystal microbalance experiments measured a SiO 2 film growth rate of 1.35 Å per SiCl 4/H 2O AB cycle at 305 K with SiCl 4, H 2O and pyridine exposures of 10 3–10 4 Langmuir (1 Langmuir = 10 − 6 Torr s). This SiO 2 ALD growth rate was verified by ex situ X-ray reflectivity, spectroscopic ellipsometry and surface profilometry measurements. The SiO 2 ALD film growth rate decreased dramatically at higher temperatures. Transmission Fourier transform infrared spectroscopy studies revealed that the hydrogen-bonded pyridine coverage was correlated with the catalyzed SiO 2 ALD growth rates from 305 to 360 K. Larger pyridine pressures were needed for catalyzed SiO 2 ALD at higher temperatures to offset the larger pyridine desorption rates. The SiO 2 ALD films contained negligible C, N, or Cl impurities as determined by X-ray photoelectron spectrometry. The deposited SiO 2 films were also extremely smooth with a surface roughness identical to the initial Si(100) substrate.

Hydrodesulfurization of Selective Catalytic Cracked Gasoline

Hydrodesulfurization (HDS) reaction of catalytic cracked gasoline (CCG) on Co–Mo/γ-Al2O3 was investigated in detail to make clear the important factors for deep HDS of CCG. A CCG containing 229 ppm sulfur and 30.4 vol% olefins was used in this study. Eleven alkylthiophenes and 2 alkylbenzothiophenes, 3 alkylthiacyclopentanes, and 2 disulfides were identified in this CCG by means of GC-AED analyses. In the reaction at 220 °C and 1.6 MPa using a conventional flow reactor of bench pilot scale, these sulfur compounds were hydrodesulfurized, whereas thiols were produced from H2S and olefins. The reactions of thiophene HDS, isoolefin and n-olefin hydrogenation (HG) were studied to clarify the active sites on the catalyst. First, the effect of H2S on the reaction was examined. The HG of n-olefin as well as thiophene HDS was inhibited by H2S, while the HG of isoolefin was promoted. The effects of Co on these three reactions were also examined over the catalysts with different Co/(Co + Mo) ratios. Thiophene HDS was promoted by Co, while isoolefin HG was little affected and n-olefin HG was largely retarded. From these examinations, three types of active sites for thiophene HDS, isoolefin HG and n-olefin HG were proposed. Oligomers of isoolefins were found in the isoolefin hydrotreated product. The possibility of improving the HDS selectivity by carbonaceous deposit was investigated for HDS reactions of CCG and model compounds. The coking pretreatment was carried out on the catalyst and each reaction was examined. HDS selectivity (higher activity for HDS and lower activity for olefin HG) on CCGHDS was improved. Relative deactivation was in the following order, isoolefin HG > thiophene HDS > n-olefin HG. Pyridine modification (i.e. pyridine injection at 150 °C and partial pyridine desorption at 300 °C) was investigated on thiophene and olefins reaction. Thiophene HDS was little affected. Olefin HG and thiol production reaction were strongly inhibited. Improvement of HDS selectivity was observed in the reactions of CCG after pyridine modification. Improvement of HDS selectivity by pyridine modification was considered to result from the selective deactivation of the active sites for olefin reactions (hydrogenation and thiol production).

Nanoporous Reactor with Tunable Selectivity on Alkylation of Ethylbenzene

As nanoporous reactor, mesoporous silicates containing aluminum acidic site with n(Si)/n(Al) ratio up to 7 (AISBA-15) were prepared through a simple one-pot synthesis, which was recently developed by parts of us. The obtained materials were satisfactorily characterized by XRD and nitrogen adsorption/desorption and have regularly arranged hexagonal structures with pore diameters in the 9.6-11.8-nm range. The density and strength of the acid sites were determined using temperature-programmed desorption (TPD) of pyridine and controlled distribution between weak, moderate, and strong acid sites was demonstrated. The alkylation of ethylbenzene in a fixed-bed flow-type reactor using the AISBA-15 nanoporous reactor was carried out under various conditions such as temperature, feed ratio, flow rate, and time on stream. Interestingly, perfect regioselectivity between the m-isomer and the o-/p-isomers of diethylbenzene was achieved, where 100% selectivity of the m-isomer or the o-/p-isomers can be obtained by simply tuning the reaction temperature by using AISBA-15 catalyst with appropriate aluminum content.