Three inter-linked active sites in the dehydrogenation of n-octane over magnesium molybdate based catalysts and their influences on coking and cracking side reactions

Ni, Co, Mg, and Al mixed-oxide solids, synthesized via the hydrotalcite route, were investigated in previous works toward the dry reforming of methane for hydrogen production. The oxide Co2Ni2Mg2Al2 calcined at 800 °C, Co2Ni2Mg2Al2800, showed the highest catalytic activity in the studied series, which was ascribable to an interaction between Ni and Co, which is optimal for this Co/Ni ratio. In the present study, Co2Ni2Mg2Al2800 was compared to a commercial catalyst widely used in the industry, Ni(50%)/Al2O3, and showed better activity despite its lower number of active sites, as well as lower amounts of carbon on its surface, i.e. less deactivation. In addition to this, Co2Ni2Mg2Al2800 showed stability for 20 h under stream during the dry reforming of methane. This good durability is attributed to a periodic cycle of carbon deposition and removal as well as to the strong interaction between Ni and Co, preventing the deactivation of the catalyst. The evaluation of the catalytic performances in the presence of toluene, which is an impurity that exists in biogas, is also a part of this work. In the presence of toluene, the catalytic activity of Co2Ni2Mg2Al2800 decreases, and higher carbon formation on the catalyst surface is detected. Toluene adsorption on catalytic sites, side reactions performed by toluene, and the competition between toluene and methane in the reaction with carbon dioxide are the main reasons for such results.

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Catalytic routes to fuels from C1 and oxygenate molecules

Faraday Discussions ◽

10.1039/c7fd00018a ◽

2017 ◽

Vol 197 ◽

pp. 9-39 ◽

Cited By ~ 7

Author(s):

Shuai Wang ◽

Iker Agirrezabal-Telleria ◽

Aditya Bhan ◽

Dante Simonetti ◽

Kazuhiro Takanabe ◽

...

Keyword(s):

Transition State ◽

Active Sites ◽

Aldol Condensation ◽

Bond Formation ◽

Transition States ◽

Solid Acids ◽

State Theory ◽

Side Reactions ◽

Oh Radicals ◽

Acid Base

This account illustrates concepts in chemical kinetics underpinned by the formalism of transition state theory using catalytic processes that enable the synthesis of molecules suitable as fuels from C1 and oxygenate reactants. Such feedstocks provide an essential bridge towards a carbon-free energy future, but their volatility and low energy density require the formation of new C–C bonds and the removal of oxygen. These transformations are described here through recent advances in our understanding of the mechanisms and site requirements in catalysis by surfaces, with emphasis on enabling concepts that tackle ubiquitous reactivity and selectivity challenges. The hurdles in forming the first C–C bond from C1 molecules are illustrated by the oxidative coupling of methane, in which surface O-atoms form OH radicals from O2 and H2O molecules. These gaseous OH species act as strong H-abstractors and activate C–H bonds with earlier transition states than oxide surfaces, thus rendering activation rates less sensitive to the weaker C–H bonds in larger alkane products than in CH4 reactants. Anhydrous carbonylation of dimethyl ether forms a single C–C bond on protons residing within inorganic voids that preferentially stabilize the kinetically-relevant transition state through van der Waals interactions that compensate for the weak CO nucleophile. Similar solvation effects, but by intrapore liquids instead of inorganic hosts, also become evident as alkenes condense within MCM-41 channels containing isolated Ni2+ active sites during dimerization reactions. Intrapore liquids preferentially stabilize transition states for C–C bond formation and product desorption, leading to unprecedented reactivity and site stability at sub-ambient temperatures and to 1-alkene dimer selectivities previously achieved only on organometallic systems with co-catalysts or activators. C1 homologation selectively forms C4 and C7 chains with a specific backbone (isobutane, triptane) on solid acids, because of methylative growth and hydride transfer rates that reflect the stability of their carbenium ion transition states and are unperturbed by side reactions at low temperatures. Aldol condensation of carbonyl compounds and ketonization of carboxylic acids form new C–C bonds concurrently with O-removal. These reactions involve analogous elementary steps and occur on acid–base site pairs on TiO2 and ZrO2 catalysts. Condensations are limited by α-H abstraction to form enolates via concerted interactions with predominantly unoccupied acid–base pairs. Ketonization is mediated instead by C–C bond formation between hydroxy-enolates and monodentate carboxylates on site pairs nearly saturated by carboxylates. Both reactions are rendered practical through bifunctional strategies, in which H2 and a Cu catalyst function scavenge unreactive intermediates, prevent sequential reactions and concomitant deactivation, and remove thermodynamic bottlenecks. Alkanal–alkene Prins condensations on solid acids occur concurrently with alkene dimerization and form molecules with new C–C bonds as skeletal isomers unattainable by other routes. Their respective transition states are of similar size, leading to selectivities that cannot sense the presence of a confining host. Prins condensation reactions benefit from weaker acid sites because their transition states are less charged than those for oligomerization and consequently less sensitive to conjugate anions that become less stable as acids weaken.

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Dehydrogenation of 3-carene in the vapour phase. II. Performance studies of chromia and chromia-alumina catalysts

Australian Journal of Chemistry ◽

10.1071/ch9801313 ◽

1980 ◽

Vol 33 (6) ◽

pp. 1313 ◽

Cited By ~ 4

Author(s):

V Krishnasamy

Keyword(s):

Water Vapour ◽

Performance Studies ◽

Active Sites ◽

Vapour Phase ◽

Progressive Increase ◽

Oxidation States ◽

Side Reactions ◽

Potassium Ions ◽

Alumina Catalyst ◽

Alumina Catalysts

The influence of contact time on the dehydrogenation of 3-carene over reduced chromia and chromia-alumina catalysts has been investigated at 450 and 400°C respectively. For the dehydrogenation, reduced chromia- alumina catalyst is more active than reduced chromia alone. It has been found that the dehydrogenating ability of reduced chromia is superior to that of oxidized chromia. The effect of water vapour on reduced chromia showed an initial suppression of its reactivity, but at intermediate values increased its activity. Progressive increase of water vapour was seen to modify the active sites of reduced chromia, an effect similar to that of the oxidized sample. �� The dehydrogenation of 3-carene, over chromia, follows second-order kinetics. The energy of activation is found to be greater for oxidized than for reduced chromia. Impregnation of chromia with potassium decreases the activation energy of reduced chromia and enhances its dehydrogenation ability by suppressing the side reactions. �� The observed experimental data are explained in terms of the acidity of the catalysts, oxidation states of chromium ions and the promoting influence of potassium ions.

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Modification of the Acyl Chloride Quench-Labeling Method for Counting Active Sites in Catalytic Olefin Polymerization

Catalysts ◽

10.3390/catal11060683 ◽

2021 ◽

Vol 11 (6) ◽

pp. 683

Author(s):

Haoyang Yang ◽

Biao Zhang ◽

Wentao Zhong ◽

Zhisheng Fu ◽

Zhiqiang Fan

Keyword(s):

Active Sites ◽

Olefin Polymerization ◽

Acyl Chloride ◽

Propylene Polymerization ◽

Molar Ratio ◽

Side Reactions ◽

Carbonyl Chloride ◽

Natta Catalyst ◽

Chain Transfer Reaction ◽

Labeling Method

The reliable and efficient counting of active sites in catalytic olefin polymerization has been realized by using acyl chloride as the quench-labeling agent. However, the molar ratio of acyl chloride to the alkylaluminum cocatalyst must be larger than 1 in order to completely depress side reactions between the quencher and Al-polymeryl that is formed via chain transfer reaction. In this work, a tetrahydrofuran/thiophene-2-carbonyl chloride (THF/TPCC) mixture was used as the quenching agent when counting the active sites of propylene polymerization catalyzed by MgCl2/Di/TiCl4 (Di = internal electron donor)-type Ziegler–Natta catalyst activated with triethylaluminum (TEA). When the THF/TEA molar ratio was 1 and the TPCC/TEA molar ratio was smaller than 1, the [S]/[Ti] ratio of the polymer quenched with the THF/TPCC mixture was the same as that quenched with only TPCC at TPCC/TEA > 1, indicating quench-labeling of all active sites bearing a propagation chain. The replacement of a part of the TPCC with THF did not influence the precision of active site counting by the acyl chloride quench-labeling method, but it effectively reduced the amount of acyl chloride. This modification to the acyl chloride quench-labeling method significantly reduced the amount of precious acyl chloride quencher and brought the benefit of simplifying polymer purification procedures after the quenching step.

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Taking Advantage of Promiscuity of Cold-Active Enzymes

Applied Sciences ◽

10.3390/app10228128 ◽

2020 ◽

Vol 10 (22) ◽

pp. 8128

Author(s):

Sondavid K. Nandanwar ◽

Shweta Bharat Borkar ◽

Jun Hyuck Lee ◽

Hak Jun Kim

Keyword(s):

Energy Cost ◽

Active Sites ◽

Catalytic Efficiency ◽

Industrial Applications ◽

Side Reactions ◽

Structural Flexibility ◽

Enzymatic Modification ◽

Cold Active Enzymes ◽

Cold Active ◽

Substrate Promiscuity

Cold-active enzymes increase their catalytic efficiency at low-temperature, introducing structural flexibility at or near the active sites. Inevitably, this feat seems to be accompanied by lower thermal stability. These characteristics have made cold-active enzymes into attractive targets for the industrial applications, since they could reduce the energy cost in the reaction, attenuate side-reactions, and simply be inactivated. In addition, the increased structural flexibility could result in broad substrate specificity for various non-native substrates, which is called substrate promiscuity. In this perspective, we deal with a less addressed aspect of cold-active enzymes, substrate promiscuity, which has enormous potential for semi-synthesis or enzymatic modification of fine chemicals and drugs. Further structural and directed-evolutional studies on substrate promiscuity of cold-active enzymes will provide a new workhorse in white biotechnology.

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Side-chain Alkylation of Toluene with Methanol, Modification and Deactivation of Zeolite Catalysts of the Reaction

Catalysis and petrochemistry ◽

10.15407/kataliz2021.31.017 ◽

2021 ◽

pp. 17-40

Author(s):

Yu.G. Voloshyna ◽

◽

O.P. Pertko ◽

Keyword(s):

Molecular Sieves ◽

Active Sites ◽

Metallic Copper ◽

Coke Formation ◽

Catalytic Efficiency ◽

Acid Sites ◽

Zeolite Catalysts ◽

Lewis Acidity ◽

Side Chain ◽

Side Reactions

The review deals with main aspects of the toluene methylation reaction on basic catalysts. The side reactions of decomposition of methanol to CO and H2 on strong basic sites and ring alkylation of toluene on Lewis acid sites (cations of high polarizing ability) hinder obtaining high yields of the target products – styrene and ethylbenzene. Both types of sites are necessary for the course of the target reaction. So optimizing their strength and quantity is an important prerequisite for the selectivity of the side-chain alkylation catalysts. The advantage of fojasite-based systems for this reaction was confirmed by the works of many researchers. However, the possibilities of use of zeolites of other structural types and representatives of a new generation of molecular sieves are being studied, as well as ways of modifying such materials to increase their catalytic efficiency. The main direction of modification is to regulate the balance of acidity and basicity. Effective charge of framework oxygen atoms, which determines basicity of zeolite framework, increases due to the introduction of guest compounds into the catalyst, and this effect is more significant than influence on basicity of ion exchange for cations of elements of low electronegativity. However, the role of this method of modifying in increasing the selectivity remains crucial due to potentiality to decrease the Lewis acidity of cations. Compounds of other elements and transition metals also are used for modification, as well as promotion with metallic copper and silver. Techniques are applied, but not widely, to deprive the external surface of crystallites of active sites. This method of modification is effective for slowing down their deactivation by coke. Acid sites, in particular BAS, are most often distinguished among the sites responsible for coke formation. The mechanism of coke formation in the absence of such centers is also proposed. On the whole, this issue not fully disclosed and requires a deeper study.

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Catalytic Performance of Oxidized Ni3Sn2 for Hydrogen Production from Methanol Decomposition

Journal of New Materials for Electrochemical Systems ◽

10.14447/jnmes.v17i4.399 ◽

2014 ◽

Vol 17 (4) ◽

pp. 251-255

Author(s):

Pan Wei ◽

Lingtong Zhou ◽

Wei Xia ◽

Zhujian Li ◽

Haifei Long ◽

...

Keyword(s):

Active Sites ◽

High Efficiency ◽

Catalytic Properties ◽

Catalytic Performance ◽

H2 Production ◽

Methanol Decomposition ◽

Side Reactions ◽

Isothermal Test ◽

Shift Reaction ◽

Water Gas

The catalytic properties of oxidized Ni3Sn2 powders were investigated for producing hydrogen from decomposing methanol in the temperatures ranging from 240 to 480 ºC. The oxidized Ni3Sn2 had much higher catalytic activity than that of Ni3Sn2 in the temperature range of 320~400 ºC. The results of an isothermal test performed at 320 ºC revealed that the oxidized Ni3Sn2 was spontaneously activated within 4 h of the reaction and slowly deactivated in the followed reaction time. The oxidized Ni3Sn2 suppressed side reactions such as methanation and water-gas shift reaction and showed a high efficiency for H2 production from methanol decomposition. Surface analysis revealed that the activity of oxidized Ni3Sn2 was attributed to the formation of Ni/SnO2 catalyst, which was supposed to serve as active sites for methanol decomposition.

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Mars van Krevelen Mechanism for the Selective Partial Oxidation of Ethane

International Journal of Chemical Reactor Engineering ◽

10.1515/ijcre-2018-0085 ◽

2018 ◽

Vol 17 (7) ◽

Cited By ~ 1

Author(s):

Paola Mora-Briseño ◽

Gladys Jiménez-García ◽

Carlos-Omar Castillo-Araiza ◽

Horacio González-Rodríguez ◽

Rafael Huirache-Acuña ◽

...

Keyword(s):

Partial Oxidation ◽

Active Sites ◽

Mixed Oxides ◽

Kinetic Mechanism ◽

Reaction Rates ◽

Raw Material ◽

Side Reactions ◽

Attractive Alternative ◽

Catalytic Dehydrogenation ◽

The One

Abstract Ethylene is the most important olefin in the petrochemical context, since it is the main raw material for the production of many polymers. Traditional production of ethylene via thermal cracking and catalytic dehydrogenation consumes large amounts of energy; hence selective partial oxidation of ethane has been considered as an attractive alternative production path. Recently, development of a promising catalyst for selective partial oxidation of ethane, which consists of multi-metallic mixed oxides of Mo, Te, V, and Nb, has been published. It is also noteworthy that this catalytic system starts to be active at temperatures below 400 °C, substantially lower than the one required by commercial thermal processes, >800 °C. In this work, a kinetic mechanism based on Mars van Krevelen formalism is proposed for the selective partial oxidation of ethane, considering the surface itself as active protagonist of the reaction. RedOx steps on active sites are considered as the controlling ones, and the rest of transformations are considered as pseudo-steady steps. It is noticed that there are side reactions, which produces CO and CO2 as combustion by-products. Additionally, there is competition for reduced sites on the catalytic surface, mainly between oxygen and water molecules, which adsorb strongly on these sites. Adjusted results by the mechanism proposed are in agreement with experimental observations of reaction rates diminishing proportionally to partial pressure of water, caused by competition of reduced sites on catalyst surface.

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What catalysis needs from the electron microscopist

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100105539 ◽

1988 ◽

Vol 46 ◽

pp. 694-695

Author(s):

Alexis T. Bell

Keyword(s):

Active Sites ◽

Heterogeneous Catalysts ◽

Catalyst Deactivation ◽

Surface Composition ◽

Internal Surface ◽

Active Phase ◽

Atomic Scale ◽

Metal Catalyst ◽

Internal Surface Area ◽

Porous Support

Heterogeneous catalysts, used in industry for the production of fuels and chemicals, are microporous solids characterized by a high internal surface area. The catalyticly active sites may occur at the surface of the bulk solid or of small crystallites deposited on a porous support. An example of the former case would be a zeolite, and of the latter, a supported metal catalyst. Since the activity and selectivity of a catalyst are known to be a function of surface composition and structure, it is highly desirable to characterize catalyst surfaces with atomic scale resolution. Where the active phase is dispersed on a support, it is also important to know the dispersion of the deposited phase, as well as its structural and compositional uniformity, the latter characteristics being particularly important in the case of multicomponent catalysts. Knowledge of the pore size and shape is also important, since these can influence the transport of reactants and products through a catalyst and the dynamics of catalyst deactivation.

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Aldehyde gold clusters for molecular labeling

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010014066x ◽

1995 ◽

Vol 53 ◽

pp. 858-859

Author(s):

James F. Hainfeld ◽

Frederic R. Furuya

Keyword(s):

Covalent Bond ◽

Aldehyde Group ◽

Gold Clusters ◽

Side Reactions ◽

Amino Groups ◽

Sodium Cyanoborohydride ◽

Schiff’S Base ◽

Protein Molecules ◽

Hydroxysuccinimide Esters ◽

Primary Amino

Glutaraldehyde is a useful tissue and molecular fixing reagents. The aldehyde moiety reacts mainly with primary amino groups to form a Schiff's base, which is reversible but reasonably stable at pH 7; a stable covalent bond may be formed by reduction with, e.g., sodium cyanoborohydride (Fig. 1). The bifunctional glutaraldehyde, (CHO-(CH2)3-CHO), successfully stabilizes protein molecules due to generally plentiful amines on their surface; bovine serum albumin has 60; 59 lysines + 1 α-amino. With some enzymes, catalytic activity after fixing is preserved; with respect to antigens, glutaraldehyde treatment can compromise their recognition by antibodies in some cases. Complicating the chemistry somewhat are the reported side reactions, where glutaraldehyde reacts with other amino acid side chains, cysteine, histidine, and tyrosine. It has also been reported that glutaraldehyde can polymerize in aqueous solution. Newer crosslinkers have been found that are more specific for the amino group, such as the N-hydroxysuccinimide esters, and are commonly preferred for forming conjugates. However, most of these linkers hydrolyze in solution, so that the activity is lost over several hours, whereas the aldehyde group is stable in solution, and may have an advantage of overall efficiency.

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