Effect of hydrogen on the molecular weight of polypropylene with Ziegler-Natta catalysts

Summary Polymerization of propylene was conducted at 40 °C with the catalytic system of TiCl₄/MgCl₂/ Al (C₂H₅)₃/ethyl benzoate using hydrogen as a chain transfer agent, and the effect of hydrogen on the molecular weight was examined separately with the soluble and insoluble fractions in boiling n-heptane. It was found that the chain transfer reaction of the atactic and isotactic polymers took place via an atomic hydrogen and a molecular hydrogen, respectively. The result strongly supports the previous mechanism that there exist two types of polymerization centers, one having two vacancies which gives the atactic polymer and the other having one vacancy which gives the isotactic polymer.     

Reaction intermediate over mordenite-type zeolite catalysts for no reduction by hydrocarbons

An infrared spectroscopic study has been made over mordenite-type zeolite catalysts prepared by the ion exchanging method to observe a surface species during the selective reduction of NO by hydrocarbons with and without H₂O. The strong absorptions at 2,274 and 2,325 cm_-1 were observed over HM and CuHM as well as over CuNZA catalysts, respectively after the reaction without H₂O, regardless of the types of reductant employed. It may be attributed to the isocyanate (-NCO) species formed on the catalyst surface which may be one of the most probable reaction intermediates for this reaction system. When H₂O was added to the feed gas stream, its formation on the synthetic mordenite catalysts such as HM and CuHM was significantly suppressed, but not for CuNZA catalyst It agrees well with the fact that CuNZA catalyst exhibits a strong water tolerance for this reaction system. It also reveals that the formation of the -NCO species on the catalyst surface plays a crucial role for the maintenance of NO removal activity when H₂O exists in the feed gas stream.     

Hydrocarbon absorption in tetrachloroaluminate catalysts effects of catalyst cation, hydrocarbon polarity and charge transfers via molecular modeling

Absorption levels were obtained for five hydrocarbons of increasing polarity in three tetrachloroaluminate catalysts of increasing cation size. Obtained absorption levels were found to correlate with cation size and hydrocarbon polarity. Molecular modeling was used to calculate charge transfers from absorbed hydrocarbons to the catalysts. Obtained charge transfers were also found to correlate with hydrocarbon absorption levels. Lastly, polarities calculated for absorbed free-radicals suggest a mechanism for previous findings. In the previous effort, resid conversion was found to go through a maximum with increasing hydrogen tetrachloroaluminate contents in a sodium tetrachloroaluminate catalyst.     

Exceptional activity for NO x reduction at low temperatures using combinations of hydrogen and higher hydrocarbons on Ag/Al2O3 catalysts

Topics in Catalysis - The effect of the addition of hydrogen on the SCR of NO x with a hydrocarbon reaction was investigated. It was found that hydrogen had a remarkable effect on the temperature...     

Selective reduction of nitrogen oxides by hydrocarbons on hydrotalcite Co and Ni catalysts

Hydrotalcitelike Co-Al and Ni-Al catalysts of different compositions (with the atomic ratio M ²⁺/Al³⁺ = 0.5–3.0) were studied in the reaction of selective reduction of NO by propane, propylene, and n-decane in the presence of O₂. The higher activity of the catalysts with M ²⁺/Al³⁺ = 0.5 is connected with high dispersity of Ni or Co cations stabilized by a significant amount of Al³⁺ ions. Propylene was shown to be the most efficient reducing agent for nitrogen oxide. The highest degree of conversion to the extent of 90–99% was attained at 400 and 420–440°C for Ni-Al and Co-Al samples, respectively. When propane and decane were used as reducing agents, the conversion of both catalysts was characterized by the volcano-shaped dependence on temperature due to the fact that the catalyst took part in the concurrent reaction of hydrocarbon (reducing agent) oxidation. Hydrotalcitelike materials are promising representatives of inexpensive bi- and multicomponent systems. The design strategy for new active catalysts for processes of purification of gas exhausts from NO _x , that are stable in the presence of water and sulfur oxides, may be based on usage of hydrotalcites with modified ions introduced into them.     

氢气选择性催化还原氮氧化物催化剂的研究进展

综述了H_2-SCR的反应机理以及近年来国内外氢气选择性催化还原氮氧化物催化剂的研究进展。重点介绍了Langmuir-Hinshelwood和双功能两种反应机理。机理不同,其中间产物也不相同。对不同负载金属(如Pt、Pd)对H_2-SCR反应的影响进行了介绍。之后对不同类型载体(传统氧化物、复合氧化物、钙钛矿型氧化物、分子筛、其它类型载体)对H_2-SCR反应催化性能的影响进行了阐述。最后对氢气选择性催化还原氮氧化物的发展方向进行了展望。     

Using vanadium catalysts for the oxidation of hydrogen chloride with molecular oxygen

Regeneration of chlorine by oxidation of hydrogen chloride is an important problem in the production of chloroorganic products. The known catalysts for this reaction are insufficiently active and typically not stable enough, while data on the use of V catalysts for this process are absent. Here we report on our study of the stability and catalytic activity of the industrial sulfuric acid production sulfate-promoted vanadium catalyst IK-1-6 in the process of oxidation of hydrogen chloride with molecular oxygen. Under conditions of low conversion (less than 15%) with the reaction in the external diffusion region, the catalyst activity attained 660 g/(kg cat h) at 400°C, and the mass loss rate of the catalyst was (due to the formation of volatile vanadyl chloride) 4.6 % per hour, based on vanadium. Under high conversion conditions (over 60%), the vanadyl chloride formed in the top layer of the catalyst was hydrolyzed and precipitated on subsequent layers as the conversion of the reaction mixture increased, leading to a redistribution of vanadium over the catalyst bed height and hindering its removal form the reactor. The stable operation of the catalyst can be ensured by intermittently changing the flow direction of the reactant gas in the catalytic reactor or using an array of several reactors connected in series, intermittently changing their places in the inlet-outlet chain. Our results show that the industrial sulfate-promoted vanadium catalysts for the oxidation of sulfur dioxide are more active and stable than all known catalysts of the Deacon process (except for ruthenium catalysts) and could be used for catalytic oxidation of hydrogen chloride.     

Selective catalytic reduction of nitrogen oxide by hydrocarbons over mordenite-type zeolite catalysts

The reduction of NO by hydrocarbons such as C₂H₄, C₂H₆, C₃H₆, and C₃H₈ has been investigated over mordenite-type zeolite catalysts including HM, CuHM, NZA (natural zeolite), and CuNZA prepared by an ion-exchange method in a continuous flow fixed-bed reactor. NO conversion over CuNZA catalyst reaches about 94% with 2000 ppm of C₃H₆ at 500°C. As reductants, alkenes seem to exhibit a higher performance for NO conversion than alkanes regardless of the catalysts. No deterioration of the catalytic activity due to carbonaceous deposits for CuNZA was observed above 400°C even after 30 h of on-stream time, but SO₂ in the feed gas stream causes a severe poisoning of the CuNZA catalyst. The effect of H₂O on NO conversion was significant regardless of the catalysts and the reductants employed in this study. However, CuNZA catalyst shows a unique water tolerance with C₃H₆. The reaction path of NO to N₂ is the most important factor for high performance of this catalytic system. NO is directly reduced by a reaction intermediate, C_n′H_m′(O) formed from hydrocarbon and O₂, N₂O is another reaction intermediate which can be easily removed by C_n′H_m′(O).     

Oxidation of hydrocarbons by hydrogen peroxide over Ti catalysts: Kinetics and mechanistic studies

n-hexane conversion to hexanols and hexanones as well as the conversion of 2- and 3-hexanol to 2- and 3- hexanone, by reaction with aqueous hydrogen peroxide on titanium silicalite were the objects of kinetic investigations. The choice of these reactions is justified on the basis of an analysis of literature dealing with partial oxidation reactions performed on Ti sites in silica environments. The kinetic analysis based on equations derived using the mathematical theory of linear graphs allows several conclusions on the nature of surface intermediates and on the reaction mechanism. These conclusions are discussed in terms of the partial oxidation of organic materials by hydrogen peroxide over Ti catalysts. This review paper relies mostly on work by the authors and it intends to make an integrated presentation of the kinetics of some critically significant catalytic oxidation reactions. The content was the subject of a plenary lecture at the 5^th symposium of the Romanian Catalysis Society.     

Enhanced activity of in and Ga-supported sol-gel alumina catalysts for NO reduction by hydrocarbons in lean conditions

Nitrogen monoxide was reduced efficiently by hydrocarbons in the presence of oxygen over sol-gel alumina supported indium, gallium, cobalt and tin catalysts. The support alumina prepared by a sol-gel method had high surface area and accordingly active alumina sites for the reaction. Particularly indium/alumina showed a high activity to reduce NO preferably by propene, propane and ethene but also by alcohols in the absence and the presence of water vapor. The activities of alumina supported cobalt, silver and tin catalysts were increased when calcinating the catalysts at 800°C instead of 600°C. In the case of gallium/alumina, NO₂ has higher reactivity than NO to nitrogen when propene was used as a reductant, proving the significance of the oxidation step of NO to NO₂. The step of NO oxidation was promoted by preparing a physical mixture of 5 wt% Mn₃O₄ with indium/alumina or gallium/alumina. The NO conversion to nitrogen was increased from 58 to 84% with the manganese oxide promotion over indium/alumina in the presence of water. The reaction mechanistic differences between the alumina supported catalysts and Cu/ZSM-5 were also discussed.     

Molecule sieving catalysts for NO reduction with hydrocarbons in exhaust of lean burn gasoline and diesel engines

NO oxidation catalysts with small pores and low hydrocarbon oxidation activity owing to molecular sieving were prepared by dispersing platinum in the pores of ferrierite and chabazite zeolites. These small pore platinum zeolite crystals were physically mixed with large pore silver mordenite zeolite crystals and evaluated as HC-SCR catalysts for selective catalytic reduction of NO in a synthetic gas mixture doped with octane or isooctane, mimicking exhaust from lean burning engines with unburned hydrocarbons. A synergetic effect on N₂ formation and hydrocarbon efficiency was obtained in these physical zeolite mixtures. In the pores of the small pore zeolite where the diffusion of NO, O₂ and NO₂ molecules with small kinetic diameters was rapid, NO was effectively oxidized by the platinum into NO₂, while the small window apertures suppressed the diffusion and reaction of the hydrocarbon. The silver plated large pore zeolite catalyzed the reaction between the NO₂ formed in the small pore zeolite and the hydrocarbon. The importance of molecular sieving was demonstrated in experiments with permutation of pore sizes and catalytic functions.     

Membrane functionalization using bisamide-based organic frameworks for molecular weight cutoff reduction

Functional polymers or copolymers have been added to separations membranes by incorporating them in the membrane dope prior to casting, by in situ polymerization, and by postsynthesis surface modification of existing membranes. Here, a postsynthesis membrane functionalization that targeted decreasing the molecular weight cutoff (MWCO) and increasing the hydrophilicity without significantly decreasing the operating flux was studied. Hybrid bisamide molecules with added amine and carboxylic acid functionalities as end groups were synthesized to form a selective layer on membrane surface via covalent attachment to the membrane. Fourier transform infrared spectroscopy analysis showed the functional groups corresponding to bisamide molecules were present on modified membranes. Furthermore, modified membranes displayed MWCO of 400 Da as compared to 1000 Da MWCO of unmodified membranes, along with an increase in the hydrophilicity of modified membranes. Modified membranes showed an improvement in divalent salt rejection and percent flux recovered after reverse-flow filtration as compared to unmodified membranes. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48327.     

A review of the effect of the addition of hydrogen in the selective catalytic reduction of NO x with hydrocarbons on silver catalysts

Research is progressing fast in the field of the hydrogen assisted hydrocarbon selective catalytic reduction (HC-SCR) over Ag-based catalysts: this paper is a review of the work to date in this area. The addition of hydrogen to the HC-SCR reaction feed over Ag/Al₂O₃ results in a remarkable improvement in NO _x conversion using a variety of different hydrocarbon feeds. There is some debate concerning the role that hydrogen has to play in the reaction mechanism and its effect on the form of Ag present during the reaction. Many of the studies use in situ UV–Vis spectroscopy to monitor the form of Ag in the catalyst and appear to indicate that the addition of hydrogen promotes the formation of small Ag clusters which are highly reactive for NO _x conversion. However, some authors have expressed concern about the use of this technique for these materials and further work is required to address these issues before this technique can be used to give an accurate assessment of the state of Ag during the SCR reaction. A study using in situ EXAFS to probe the H₂ assisted octane-SCR reaction has shown that small Ag particles (containing on average 3 silver atoms) are formed during the SCR reaction but that the addition of H₂ to the feed does not result in any further change in the Ag particle size. This points to the direct involvement of H₂ in the reaction mechanism. Clearly the addition of hydrogen results in a large increase in the number and variety of adsorbed species on the surface of the catalyst during the reaction. Some authors have suggested that conversion of cyanide to isocyanate is the rate-determining step and that hydrogen promotes this conversion. Others have suggested that hydrogen reduces nitrates to more reactive nitrite species which can then activate the hydrocarbon; activation of the hydrocarbon to form acetates has been proposed as the key step. It is probable that all these promotional effects can take place and that it very much depends on the reaction temperature and feed conditions as to which one is most important.     

The mechanism of the selective reduction of nitrogen oxides by hydrocarbons on zeolite catalysts

The mechanism of the selective catalytic reduction (SCR) of nitrogen oxides over 3d transition metal zeolites has been investigated in a variety of ways. The initial step is the abstraction of hydrogen from the hydrocarbon by adsorbed NO₂ species which is rate determining with methane but not with isobutane. The subsequent path appears to involve nitroso and/or nitro compounds. Comparative studies of the reactions of such compounds indicate that nitromethane is more likely to be an intermediate than nitrosomethane during the methane-SCR reaction over Co-MFI although the latter cannot be ruled out entirely. In both cases the predominant route to N₂ is an initial decomposition to carbon oxides and ammonia followed by the NH₃-SCR reaction. The isobutane-SCR reaction over Fe-MFI produces substantial amounts of hydrogen cyanide which disappears only at temperatures where all the hydrocarbon has been consumed. Hydrogen cyanide appears to arise from isobutyronitrile, the expected dehydration product if an initially formed nitroso compound undergoes tautomerism to an oxime. HCN is converted to N₂ largely by reaction with NO₂ which is fast well below 300°C in the absence of isobutane. The corresponding isobutane-SCR reaction over Cu-MFI gives rise to cyanogen (C₂N₂) rather than HCN. The general path is probably the same in the two systems with the difference arising from variation in the relative reactivity of HCN. The copper-containing catalyst is very effective at forming and dimerising adsorbed cyanide groups while the iron catalyst has higher activity for the oxidation of NO to the NO₂ needed to convert adsorbed cyanide to N₂. The difference between the apparent involvement of a nitro route in methane-SCR with Co-MFI, and a nitroso one with isobutane, is similarly explainable. The former reaction proceeds with simultaneous production of NO₂ which can participate in the intermediate chemistry that follows. However, the NO₂ concentration is low during the latter reaction over Cu-MFI and Fe-MFI as long as any hydrocarbon remains. This is due to the blocking of sites for NO oxidation by deposits and the recycling of NO₂ back to NO during hydrocarbon oxidation. Thus only NO is available and the nitroso route prevails. The extent to which this picture applies with other catalysts and other hydrocarbons remains to be established.     

Selective reduction of nitrogen oxides with hydrocarbons over solid acid catalysts in oxygen-rich atmospheres

Highly selective reduction of nitrogen oxides to dinitrogen occurs to a high level in oxygen-rich atmospheres by using a small amount of propane as a reducing agent over alumina, silica-alumina, titania and zirconia catalyst. Judging from the data of activity and ammonia TPD measurement on a series of silica-alumina catalysts, acidity is suggested to be one of the main factors that determine catalytic activity.     

富氧条件下氢气选择性催化还原NOx催化剂研究进展

着重介绍了NOx脱除技术中的选择性催化还原技术,分析了以不同还原剂(NH3、H2、CO、HC)选择性催化还原NOx技术的优缺点,认为H2-SCR脱硝技术具有较高的研究价值和实际的应用前景。从催化剂的活性组分、助剂及载体的角度分析了不同催化剂在氢气选择性催化还原脱硝技术中的应用及其所达到的效果,得出负载贵金属Pt与Pd、添加酸性助剂以及采用复合载体的催化剂性能更佳,并对今后氢气选择性催化还原脱硝技术的研究方向进行展望。     

Selective reduction of NOx by hydrocarbons in excess oxygen by alumina- and silica-supported catalysts

A range of alumina- and silica-supported metal catalysts have been investigated for the selective reduction of NO_x by propene in excess oxygen. Platinum, in particular, has been found to have a high activity and selectivity. There is a close correlation between the activity for NO_x reduction and propene combustion. For platinum group metals, it is found that silica-supported catalysts are more active than alumina-based materials. For Pt/SiO₂ catalysts, it is found that the specific activity for NO_x reduction decreases with decreasing metal dispersion. NO is found to inhibit the oxidation of propene on Pt-based catalysts     

A study of molecular weight regulation in polyethylene by titanium diolate catalysts

A series of homogeneous titanium(IV) complexes derived from cyclohexanediol were synthesized and characterized. These catalysts, formulated as [Ti–(O^∩O)(OR)₂], [Ti–(O^∩O)OPrⁱCl] and [Ti–(O^∩O)₂], were found to be highly active in the polymerization of ethylene at high temperature in combination with ethylaluminium sesquichloride (Et₃Al₂Cl₃, EASC) as co‐catalyst. The resultant polyethylene had a low polydispersity index indicating the formation of single‐site catalytic species with molecular weight in the range 639–1036 g mol^?1 which was analysed using ¹H NMR spectroscopy and found to be linear, suggesting that a chain transfer to the aluminium of EASC occurs. © 2013 Society of Chemical Industry     

Sialon-supported catalysts for deep oxidation of carbon monoxide and hydrocarbons

Catalysts supported on a new class of supports—β-Sialons Si_{6 ? z} Al _z O _z N_{8 ? z} (z = 1, 3, 4) modified with transition-metal oxides—were synthesized and tested in the complete and partial oxidation of hydrocarbons and CO. These Sialons were prepared by self-propagating high-temperature synthesis (SHS). Catalytic activity in the test reactions appeared in systems that contain unary, binary, or ternary mixtures of metal oxides of the second half of the 3d row. Cobalt oxide-based catalysts showed the highest activity. Methods were proposed for activating Sialon supports with a considerable enhancement of the activity of a supported catalyst. More precise investigative tools are required for refining new data and understanding the nature and operation mechanism of Sialons. On account of their high thermal stability and wear resistance, Sialon-supported catalysts are proposed for use in deep oxidation processes in abrasive and acid solutions and at elevated temperatures.