Kinetics of Catalyst Poisoning during Capillary Condensation of Reactants

The influence of the capillary condensation of reactants on the poisoning of Pt/SiO₂ catalysts by thiophene is studied experimentally for p-xylene hydrogenation at T = 60 and 80°C. The poisoning kinetics is independent of a catalyst and its rate decreases with a decrease in temperature. Poisoning during capillary condensation is 1.5–6 times slower than that in the gas phase, depending on the fraction of surface platinum in the pores filled with a liquid. The poisoning of the catalyst active sites in the pores filled with a liquid requires less sulfur at the same deactivation degree. The number of sulfur atoms per one platinum atom necessary for the complete poisoning of platinum in the gas phase is higher than that in the case of capillary condensation by a factor of 1.4–1.5.     

Diffusion and reaction in polyester melts

An understanding of the physical and chemical processes involved in the melt polymerization of polyesters in the higher inherent viscosity ranges is of fundamental importance in polyester preparation. For example, the volatile condensation product must diffuse to a polymer–vapor interface before polymerization can take place. Thus, the rate of polymerization of a polyester may be dependent not only upon the chemical kinetics of the polymerization reaction but also upon the diffusion of the condensation product through the polymer melt. The objective of the work presented in this paper was to determine to what degree diffusion or reaction kinetics, or both, limit the melt polycondensation of poly(ethylene terephthalate). Degrees of polymerization in melts between 0.0285 and 0.228 cm in depth at 270°C were measured for various reaction times and were compared with the predictions of mathematical models. The polycondensation rates under these conditions depend upon both the polycondensation rate constant k₁ and the diffusivity D of ethylene glycol through the melt. Estimates of the values to these parameters are: k₁ = 0.0500 (moles/mole of repeat unit)^?1 sec^?1; D = 1.66 × 10^?4 cm²/sec.     

On the LPCVD-Formed SiO2 Etching Mechanism in CF4/Ar/O2 Inductively Coupled Plasmas: Effects of Gas Mixing Ratios and Gas Pressure

An investigation of etching mechanism of low-temperature SiO₂ thin films in CF₄/Ar/O₂ inductively coupled plasmas at constant input power (900 W) and bias power (200 W) was carried out. It was found that that the variations of Ar/O₂ mixing ratio (0–50 %) at constant 50 % CF₄ fraction as well as the change in gas pressure (4–10 mTorr) resulted in non-monotonic SiO₂ etching rates. The zero-dimensional plasma model with Langmuir probe diagnostics data provided the detailed information on formation-decay kinetics for plasma active species. The model-based analysis of etching kinetics showed that these effects were not connected with the non-monotonic change of fluorine atom density (as was found in several works for the binary CF₄/O₂ system), but resulted from the decrease in reaction probability and with the transition from neutral-flux to ion-flux-limited regimes of ion assisted chemical reaction.     

A new study on the kinetics of Stöber synthesis by in-situ liquid 29Si NMR

Liquid-state ²⁹Si NMR was used to investigate the hydrolysis and condensation kinetics of ammonia-catalyzed tetraethoxysilane (TEOS) in methanol system. The reactive rate constants were calculated by applying first-order reaction approximation and the steady state approximation theory. The reaction orders with respect to TEOS, ammonia and water were derived, as well as the activation energies and the Arrhenius constants. It was found that the formation of intermediate species Si(OH)(OEt)₃ was the rate-limiting step and its reaction rate equation was r _TEOS=7.41×10⁻³[TEOS][NH₃]^0.333[H₂O]^0.227. Higher reactive temperature benefited the hydrolysis of TEOS. The results presented here indicated quantificationally that the formation of colloidal SiO₂ particles was controlled by the initial hydrolysis of TEOS.     

The nature of autocatalysis in the Butlerov reaction

The effect of the nature of an initiator on the kinetics of formaldehyde consumption and on product composition in the Butlerov reaction was studied in a stirred flow reactor and a batch reactor. It was found that, under flow conditions, the kinetics and the product composition of this reaction are independent of the nature of the initiator. The reaction schemes proposed previously for an autocatalytic process mechanism based on the formation of glycolaldehyde from two formaldehyde molecules are incorrect. A correlation between the initiating activities of various monosaccharides and the rates of their conversion into an enediol form was found with the use of a batch reactor. Solid enediol complexes with Ca²⁺ ions were isolated for glucose, fructose, ribose, and sorbose; the initiating activity of these complexes was found to be much higher than the initiating activity of pure monosaccharides. A self-consistent mechanism was proposed for Butlerov reaction initiation. The formation of the enediol forms of monosaccharides followed by degradation to lower carbohydrates plays a key role in this mechanism. In turn, the initiating activity depends on the position of the carbonyl group in the monosaccharide molecule. The condensation reactions of glycolaldehyde, glyceraldehyde, and dihydroxyacetone with each other were studied. Based on data on the condensation products of lower carbohydrates, a scheme was proposed for the Butlerov reaction. According to this reaction scheme, C₂ and C₃ carbohydrates mainly undergo an aldol condensation reaction with formaldehyde, whereas the formation of higher monosaccharides occurs by the aldol condensation of lower C₂–C₃ carbohydrates with each other.     

High-temperature thermal analysis study of the reaction between magnesium oxide and silica

The reaction between SiO₂ and MgO at temperatures up to 1500°C was studied using thermal analysis, with X-ray diffraction being used to identify reaction products. The reaction is slow and results in the formation of Mg₂SiO₄ and MgSiO₃, with minor amounts of SiO₂·nH₂O and residual amounts of unreacted SiO₂ and MgO. Complete reaction of the starting materials to form Mg₂SiO₄ can only be achieved by maintaining the mixture at 1500°C for extended periods of time (>1 h).     

Lead silicate—potassium carbonate solid—solid reaction kinetics

The kinetics and mechanism of the reaction between 4PbO·SiO₂+K₂CO₃ and PbO·SiO₂ + K₂CO₃ were studied using isothermal weight change determinations and thermal gravimetry (TG) to monitor the fraction of the reaction completed as a function of time. The reactions were found to be nuclei growth controlled with one-dimensional diffusion controlled growth having procedural activation energies of 85 and 108 kcal/mole respectively.The results from the TG methods of Test, Weidemann and Vaughan and Sativa were compared with the isothermal data to determine their relative merits. These methods were found to be, with reservations, suitable for determining the procedural activation energy. However, the method of Sativa was found to be lacking in sufficient sensitivity to allow determination of the reaction mechanism.     

An NMR Study of MeTMS Based Coatings Filled with Colloidal Silica

Coatings were prepared by mixing MeTMS and an aqueous colloidal silica. Mixing of an MeTMS hydrolysis mixture with the aqueous colloidal silica is only possible, without flocculation of the colloidal silica particles, within a certain time window. ²⁹Si NMR was used to follow the hydrolysis/condensation reactions of MeTMS, whereas ¹H NMR was used to monitor the reaction of the MeTMS monomers and oligomers with the silica surface in the coating liquid. The reaction of MeTMS with the surface of the SiO₂ particles is determined by the oligomer size. Typical SiO₂ surface coverage is less than 3 molecules/nm² (approximately one monolayer). Large MeTMS oligomers and/or cyclic species do not react with the SiO₂ surface. These species are probably too apolar to react or absorb at the SiO₂ surface. Flocculation of the aqueous colloidal silica occurs due to the low polarity of the hydrolysis mixture. The water content and the degree of condensation of the MeTMS determine the width of the time window.     

Ethylene/hexene copolymerization with the heterogeneous catalyst system SiO2/MAO/rac‐Me2Si[2‐Me‐4‐Ph‐Ind]2ZrCl2: The filter effect

The main focus of this study is the ethylene/hexene copolymerization with the silica supported metallocene SiO₂/MAO/rac‐Me₂Si[2‐Me‐4‐Ph‐Ind]₂ZrCl₂. Polymerizations were carried out in toluene at a reaction temperature of 40°C–60°C and the cocatalyst used was triisobutylaluminium (TIBA). The kinetics of the copolymerization reactions (reactivity ratios r_E/H, monomer consumption during reaction) were investigated and molecular weights M_w, molecular weight distributions MWD and melting points T_m were determined. A schematic model for the blend formation observed was developed that based on a filtration effect of monomers by the copolymer shell around the catalyst pellet.     

Cross-aldol Condensation of Cycloalkanones and Aromatic Aldehydes in the Presence of Nanoporous Silica-based Sulfonic Acid (SiO2-Pr-SO3H) under Solvent Free Conditions

The aromatic aldehydes underwent cross aldol condensation with cycloalkanones in the presence of a catalytic amount of nanoporous silica-based sulfonic acid (SiO2-Pr-SO3H) under solvent-free conditions to afford the corresponding α,α’-bis(substituted benzylidene)cycloalkanones in excellent yields with short reaction time without any side reactions. This method is very general, simple and environmentally friendly in contrast with other existing methods. SiO2-Pr-SO3H was proved to be an efficient heterogeneous solid acid catalyst, which could be easily handled and removed from the reaction mixture by simple filtration, and also recovered and reused without loss of reactivity.     

Water Permeation of SiO2-CH3SiO3/2 Thin Films Modified with Trimethylsilyl Groups on Nylon-6 Substrates

SiO₂-CH₃SiO_3/2 thin films coated on nylon-6 substrates by the sol-gel method were modified with trimethylsilyl (TMS) groups and their water permeability was evaluated. The water permeability coefficient of the nylon-6 substrates coated with TMS-modified SiO₂-CH₃SiO_3/2 thin films was smaller than that of unmodified ones. The wettability for water of SiO₂-CH₃SiO_3/2 thin films modified with TMS was smaller than that of unmodified ones. The decrease in the wettability by the modification with TMS resulted in the decrease in water permeability. The measurements of pore size distribution and the water permeability coefficient of coating films with different pressures of upstream side suggested that the mechanism of permeation was governed by the capillary condensation flow. This mechanism was also supported by the result that the water permeability coefficient was decreased with a decrease in wettability.     

Studies on preparation of metalloporphyrin‐functionalized PVI and PVI/SiO2 via axial coordination reaction

Cobalt tetraphenylporphyrin (CoTPP) was bond on the side chains of poly(N‐vinylimidazole) (PVI) and immobilized on PVI/SiO₂ via axial coordination reactions, respectively, leading to the soluble metalloporphyrin‐functionalized PVI (denoted as CoTPP–PVI) and the particles CoTPP–PVI/SiO₂. The above two target products were characterized using spectroscopy methods such as infrared spectrum, electronic absorption spectrum, and fluorescence emission spectrum. And the effects of various factors on the bonding reaction and immobilization reaction of CoTPP were studied in detail. The experimental results show that CoTPP–PVI and CoTPP–PVI/SiO₂ can be prepared favorably through axial coordination reaction of CoTPP with the side imidazole groups of PVI and PVI/SiO₂. In addition, the soluble CoTPP–PVI has all the spectral characteristics of CoTPP. As compared to CoTPP, the adsorption spectra of CoTPP–PVI exhibit red shift obviously. In the immobilization process of CoTPP on PVI/SiO₂, the higher the concentrations of CoTPP and the reaction temperature, the greater the immobilization amount of CoTPP is. Copyright © 2009 John Wiley & Sons, Ltd.     

Phosphomolybdic Acid (PMA)–SiO2 as a Heterogeneous Solid Acid Catalyst for the One‐Pot Synthesis of 2H‐Indazolo[1,2‐b]phthalazine‐triones

Phosphomolybdic acid (PMA)–SiO₂ was found to be an efficient catalyst for the three‐component condensation reaction of phthalhydrazide, 1,3‐diketone, and aldehydes to produce 2H‐indazolo[1,2‐b]phthalazine‐triones in excellent yields. The catalyst can be recovered and reused without significant loss of activity.     

Finite-difference model for the reaction sintering of oxide ceramics by direct oxidation of intermetallic powder compacts

A kinetic model for the reaction sintering of oxide ceramics in the system Al₂O₃–SiO₂–ZrO₂ using mixtures of intermetallic compounds is presented. A 2D finite-difference model is developed to describe the exothermic gas-solid reactions taking place during the firing of ZrAl₃/ZrSi₂ powder compacts. The model accounts for the oxidation kinetics of the powder particles, as well as the consumption and diffusion of gaseous oxygen through the porous matrix. Additionally, possible changes in the pore structure of the green body due to the oxidation reactions and sintering effects are incorporated in the model. The resulting differential equations are coupled with a two-dimensional Fourier heat balance equation leading to a system of nonlinear partial differential equations, which is solved by the numerical method of lines. The influence of different processing parameters like sample composition and heating cycle on the reaction sintering process is investigated and the model-predicted reaction behaviour is compared to experimental results.     

Knoevenagel condensation of aldehydes with active methylene compounds catalyzed by MgC2O4/SiO2 under microwave irradiation and solvent-free conditions

MgC₂O₄/SiO₂ catalyzes the efficient Knoevenagel condensation of aldehydes with active methylene compounds in solvent-free conditions under microwave irradiation to give alkenes derivatives in excellent yields. MgC₂O₄/SiO₂ can be reusable for Knoevenagel condensation. However, ketones have been found to be unsatisfactory in the reaction under the same conditions.     

Temperature-response polymer coating for in-tube solid-phase microextraction coupled to high-performance liquid chromatography

The silica nanoparticle (SiO₂ NP)-deposited capillary fabricated by liquid phase deposition (LPD) was bonded by 3-(triethoxysilyl) propyl methacrylate and then modified with poly(N-isopropylacrylamide) (PNIPAAm) by polymerization. The resulting PNIPAAm modified SiO₂ NP-deposited capillary was applied to in-tube solid-phase microextraction coupled to high-performance liquid chromatography (in-tube SPME-HPLC). To investigate the extraction performance of the prepared capillary, diethylstilbestrol (DES) with moderate polarity was selected as the model analyte. Results demonstrate that PNIPAAm modified SiO₂ NP-deposited capillary exhibited obvious temperature responsive character. Finally, the PNIPAAm modified SiO₂ NP-deposited capillary was applied to the analysis of three synthetical estrogens from milk samples. The detection limit of the method was found to be in the range 1.2-2.2 ng/g, and recovery was 71.7-98.9% with relative standard deviations in the range of 2.8-12.6%.     

Three Component Reaction: An Efficient Synthesis of 3,4-Dihydropyrimidin-2(1H)-ones and Thiones Using Heterogeneous Catalyst

Fluoroboric acid supported on silica gel (HBF₄‐SiO₂) catalyzes efficiently the three component condensation of an aldehyde, a 1,3‐dicarbonyl compound and urea or thiourea to afford the corresponding 3,4‐dihydropyrimidin‐ 2(1H)‐ones and thiones in high yields. The ambient conditions, fast reaction rates, and excellent product yields are the important characteristics of this reaction.     

Fe3O4@SiO2@Im‐bisethylFc [HC2O4] as a novel recyclable heterogeneous nanocatalyst for synthesis of bis‐coumarin derivatives

Nanomagnetic bisethylferrocene‐containing ionic liquid supported on silica‐coated iron oxide (Fe₃O₄@SiO₂@Im‐bisethylFc [HC₂O₄]) as a novel catalyst was designed and synthesized. The described catalyst was recycled and used without change in the time and efficiency of the condensation reaction. The Fourier transform‐infrared spectroscopy (FT‐IR), scanning electron microscopy images, X‐ray diffraction patterns, energy‐dispersive X‐ray spectroscopy, transmission electron microscope and vibrating‐sample magnetometer results confirmed the formation of Fe₃O₄@SiO₂@Im‐bisethylFc [HC₂O₄] magnetic nanoparticle. The novel bis‐coumarin derivatives were identified by ¹H‐NMR, ¹³C‐NMR, FT‐IR and CHNS analysis.     

IDM-1: A Zeolite with Intersecting Medium and Extra-Large Pores Built as an Expansion of Zeolite MFI

IDM-1 is a new silica zeolite with an ordered and well-defined framework constructed by alternating pentasil layers and interrupted layers, giving rise to an intersecting system of straight medium pores and undulating extra-large lobed pores. This unique structure was solved by rotation electron diffraction and refined against synchrotron powder X-ray diffraction data. Despite the presence of both Si(OSi)₃(OH) and Si(OSi)₂(OH)₂ sites, this new zeolite presents high thermal stability, withstanding calcination even to 1000 °C. The location of defects at specific sites of the structure results in alternating hydrophobic SiO₂ and hydrophilic SiO_(2−x)(OH)_2x intracrystalline regions. This peculiar combination of intersecting medium and extra-large pores and alternating regions of different chemical character may provide this zeolite with unique catalytic properties.     

NaHSO4•SiO2催化合成β-烯胺酮(酯)

NaHSO₄•SiO₂作催化剂, 1,3-二羰基化合物和伯胺在室温下反应合成了一系列β-烯胺酮(酯), 该法反应条件温和, 产率高, 催化剂能回收再利用.