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

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.

scholarly journals Synthesis of SAPO-34 Nanoplates with High Si/Al Ratio and Improved Acid Site Density

Nanomaterials ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 3198
Author(s):  
Syed Fakhar Alam ◽  
Min-Zy Kim ◽  
Aafaq ur Rehman ◽  
Devipriyanka Arepalli ◽  
Pankaj Sharma ◽  
...  
Keyword(s):  
Phosphoric Acid ◽  
Molecular Sieves ◽  
Active Sites ◽  
High Performance ◽  
Hydrothermal Process ◽  
Acid Sites ◽  
High Density ◽  
Structure Directing Agent ◽  
Transfer Resistance ◽  
Low Mass

Two-dimensional SAPO-34 molecular sieves were synthesized by microwave hydrothermal process. The concentrations of structure directing agent (SDA), phosphoric acid, and silicon in the gel solution were varied and their effect on phase, shape, and composition of synthesized particles was studied. The synthesized particles were characterized by various techniques using SEM, XRD, BET, EDX, and NH3-TPD. Various morphologies of particles including isotropic, hyper rectangle, and nanoplates were obtained. It was found that the Si/Al ratio of the SAPO-34 particles was in a direct relationship with the density of acid sites. Moreover, the gel composition and preparation affected the chemistry of the synthesized particles. The slow addition of phosphoric acid improved the homogeneity of synthesis gel and resulted in SAPO-34 nanoplates with high density of acid sites, 3.482 mmol/g. The SAPO-34 nanoplates are expected to serve as a high performance catalyst due to the low mass transfer resistance and the high density of active sites.

scholarly journals Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

2020 ◽  
Author(s):  
Konstantin Khivantsev ◽  
Ja-Hun Kwak ◽  
Nicholas R. Jaegers ◽  
Miroslaw A. Derewinski ◽  
János Szanyi
Keyword(s):  
Active Sites ◽  
No Reduction ◽  
Catalytic Reduction ◽  
Molecular Nitrogen ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Catalytic Cycle ◽  
Rate Limiting Step ◽  
Catalytically Active

<p>Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH<sub>3</sub>)<sub>2</sub> with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO<sup>+</sup> in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N<sub>2</sub>H<sup>+</sup> cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO<sup>+</sup> reacts, “freed up” Bronsted acid site gets occupied by NH<sub>3</sub> to form NH<sub>4</sub><sup>+</sup> – and so the catalytic cycle stops because NO<sup>+</sup> does not form on NH<sub>4</sub>-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO<sup>+</sup> by the reaction: Cu(II) + NO à Cu(I) + NO<sup>+ </sup>which we further confirm spectroscopically. The NO<sup>+</sup> then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.</p>

C-methylation of organic substrates: A comprehensive overview. Part iiia. Methanol as a methylating agent. A case of catalysis versatility

2021 ◽  
Vol 18 ◽  
Author(s):  
Saad Moulay
Keyword(s):  
Transition Metal ◽  
Metal Complexes ◽  
Transition Metal Complexes ◽  
Molecular Sieves ◽  
Acid Sites ◽  
Lewis Base ◽  
Side Chain ◽  
Organic Substrates ◽  
Solid Catalysts ◽  
The Impact

: The present account surveys the results of the myriad of works on the C-methylation of organic substrates with methanol as an eco-friendly methylating agent. The innumerable reports on this issue reveal the widespread use of a set of solid catalysts such as molecular sieves, zeolites, metal phosphates, metal oxides and transition metal complexes to accomplish such methylation. One related facet was the impact of the numbers of Brønstëd acid sites, Lewis acid sites, and Lewis base sites present in solid catalysts, such as zeolites, ratios, and strengths that affect the distribution of the methylation products and their selectivities. Moreover, specific surface area and porosity for some solid catalysts, such as zeolites, play additional roles in the overall reaction. Not only do these catalyst properties influence the methylation outcome, but the temperature, space velocity (WHSV, LHSV, GSHV), weight of catalyst per reactant flow rate (W/F), time of stream (TOS), and methanol/substrate molar ratio also do. The treated substrates herein discussed were aromatic hydrocarbons (benzene, biphenyls, naphthalenes, toluene, xylenes), alkenes, phenolics (phenol, cresols, anisole), N-heteroarenes, carbonyls, alcohols, and nitriles. Methylation of benzene affords not only toluene as the main product but also polymethylated benzenes (xylenes, pseudocumene, hexamethylenebenzene, and also ethylbenzene as a side-chain product). Furthermore, toluene is sensitive to the reaction conditions, giving rise to ring methylation and side-chain one (ethylbenzene and styrene), besides the formation of benzene as a disproportionation product. A number of results from the methylation of phenolic compounds bear witness to the interest of different investigators in this special research. As to these phenolics, concurrent O-methylation inevitably parallels the C-methylation, and the selectivity of the latter one remains dependent on the above-cited factors; ortho-cresol and 2,6-xylenol have been the main C-ring methylated phenols. Methylation of olefins with methanol over solid catalysts, leading to higher olefins, is of great interest. The chemistry involved in the methylation of N-heteroarenes, such as pyridines, indoles, and pyrroles, is significant. Application of the methylation protocols, using methanol as a reagent and transition metal complexes as catalysts to ketones, esters, aldehydes, nitriles, and alcohols, ends up with some important molecules such as acrylonitrile (a monomer) and isobutanol (a biofuel).

scholarly journals Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

2020 ◽  
Author(s):  
Konstantin Khivantsev ◽  
Ja-Hun Kwak ◽  
Nicholas R. Jaegers ◽  
Miroslaw A. Derewinski ◽  
János Szanyi
Keyword(s):  
Active Sites ◽  
No Reduction ◽  
Catalytic Reduction ◽  
Molecular Nitrogen ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Catalytic Cycle ◽  
Rate Limiting Step ◽  
Catalytically Active

<p>Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH<sub>3</sub>)<sub>2</sub> with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO<sup>+</sup> in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N<sub>2</sub>H<sup>+</sup> cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO<sup>+</sup> reacts, “freed up” Bronsted acid site gets occupied by NH<sub>3</sub> to form NH<sub>4</sub><sup>+</sup> – and so the catalytic cycle stops because NO<sup>+</sup> does not form on NH<sub>4</sub>-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO<sup>+</sup> by the reaction: Cu(II) + NO à Cu(I) + NO<sup>+ </sup>which we further confirm spectroscopically. The NO<sup>+</sup> then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.</p>

Mildly Acidic Aluminosilicate Catalysts for Stable Performance in Ethanol Dehydration

2020 ◽  
Author(s):  
Ales Styskalik ◽  
Vit Vykoukal ◽  
Luca Fusaro ◽  
Carmela Aprile ◽  
Damien Debecker
Keyword(s):  
Sol Gel ◽  
Coke Formation ◽  
Strong Acid ◽  
Acid Sites ◽  
High Specific Surface Area ◽  
Ethylene Oligomerization ◽  
Zeolite Catalysts ◽  
Activity Levels ◽  
Ethanol Dehydration ◽  
Silica Alumina

Ethanol dehydration is effectively catalyzed by strongly acidic zeolite catalysts which are known, however, to exhibit poor time on stream stability. Alumina and silica-alumina on the other hand are relatively stable but reach only low activity levels. Here, a series of aluminosilicate catalysts (Si:Al ratio = 16) was prepared by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of activity, between the HZSM-5 zeolite and a commercial silica-alumina. Importantly, the best samples, were very stable with time on stream. Unlike HZSM-5, which also catalyzes ethylene oligomerization due to its strong acid sites and is therefore prone to coking, NHSG prepared catalysts did not produce any traces of ethylene oligomers and did not show any trace of coke formation. Characterization (ICP-OES, N<sub>2</sub> physisorption, TEM, XPS, IR coupled with pyridine adsorption, Raman spectroscopy, solid state NMR spectroscopy) reveal that the unconventional synthetic method presented here allowed to prepare mesoporous aluminosilicate materials with a remarkable degree of homogeneity. It is this thorough dispersion of Al in the amorphous silicate matrix which is responsible for the formation of acid sites which are intermediate (in terms of strength and nature) between those of commercial silica-alumina and HZSM-5 zeolite. The texture of the best NHSG catalyst – mainly mesoporous with a high specific surface area (800 m² g<sup>−1</sup>) and pore volume (0.5 cm³ g<sup>−1</sup>) – was also unaffected after reaction. To overcome deactivation issues in ethanol dehydration, this study suggests to target amorphous aluminosilicate catalysts with open mesoporosity and with an intimate mixing of Al and Si.

scholarly journals Modulating Inherent Lewis Acidity at the Intergrowth Interface of Mortise-Tenon Zeolite Catalyst

2021 ◽  
Author(s):  
Huiqiu Wang ◽  
Boyuan Shen ◽  
Xiao Chen ◽  
Hao Xiong ◽  
Hongmei Wang ◽  
...  
Keyword(s):  
Single Crystal ◽  
Lattice Mismatch ◽  
Methanol Conversion ◽  
Scanning Transmission Electron Microscope ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Lewis Acidity ◽  
Light Olefins ◽  
Local Structures ◽  
Catalytic Performances

Abstract The tunability of local structures determines various catalytic performances of zeolite catalysts. The acid sites in zeolite catalysts are important local structures to control the products in methanol conversion. However, it remained still a great challenge to precisely design the acid sites, since there is a lack of controllable methods to generate and identify the acid sites with a high resolution. Here, we use the lattice mismatch of zeolite intergrowth to enrich the inherent Lewis acid sites (LASs) at the interface of a mortise-tenon ZSM-5 catalyst (ZSM-5-MT) showing a 90° intergrowth structure. ZSM-5-MT is formed by two perpendicular blocks that can be atomically resolved by the integrated differential phase contrast scanning transmission electron microscope (iDPC-STEM). It can be revealed by various methods that more framework-associated Al (AlFR) LASs are generated in ZSM-5-MT than single-crystal ZSM-5 catalyst. Combining with the iDPC-STEM results, we demonstrate that the partial missing of O atoms at interfaces results in the formation of inherent LASs in ZSM-5-MT. According to the catalytic performances, LAS-enriched ZSM-5-MT shows a higher selectivity of light olefins than the single-crystal ZSM-5 catalyst in methanol conversion. These results provide an efficient strategy to design the Lewis acidity in zeolites for tailored catalytic functions via interface engineering.

scholarly journals Strikingly Distinctive NH3-SCR Behavior over Cu-SSZ-13 in the Presence of NO2

2021 ◽  
Author(s):  
Yulong Shan ◽  
Guangzhi He ◽  
Jinpeng Du ◽  
Yu Sun ◽  
Zhongqi Liu ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Catalytic Reduction ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Nh3 Scr ◽  
Highly Active ◽  
Small Pore ◽  
Standard Scr ◽  
Catalyst Systems

Abstract Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the selective catalytic reduction (SCR) of NOx with NH3, but distinct from other catalyst systems, their activity is unexpectedly inhibited in the presence of NO2. Here, we combined kinetic experiments, in-situ/operando X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as fixed framework Cu2+ species (fw-Cu2+), which impede the formation of dynamic binuclear Cu+ species that serve as the main active sites for the standard SCR (SSCR) reaction. As a result, the SSCR reaction is significantly inhibited by NO2 in the zeolite system, and the NO2-involved SCR reaction occurs with an energy barrier higher than that of the SSCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BAS) rather than the fw-Cu2+ sites. This work clearly explains the strikingly distinctive selective catalytic behavior in the zeolite system.

scholarly journals Taking Advantage of Promiscuity of Cold-Active Enzymes

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.

Mildly Acidic Aluminosilicate Catalysts for Stable Performance in Ethanol Dehydration

2020 ◽  
Author(s):  
Ales Styskalik ◽  
Vit Vykoukal ◽  
Luca Fusaro ◽  
Carmela Aprile ◽  
Damien Debecker
Keyword(s):  
Sol Gel ◽  
Coke Formation ◽  
Strong Acid ◽  
Acid Sites ◽  
High Specific Surface Area ◽  
Ethylene Oligomerization ◽  
Zeolite Catalysts ◽  
Activity Levels ◽  
Ethanol Dehydration ◽  
Silica Alumina

Ethanol dehydration is effectively catalyzed by strongly acidic zeolite catalysts which are known, however, to exhibit poor time on stream stability. Alumina and silica-alumina on the other hand are relatively stable but reach only low activity levels. Here, a series of aluminosilicate catalysts (Si:Al ratio = 16) was prepared by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of activity, between the HZSM-5 zeolite and a commercial silica-alumina. Importantly, the best samples, were very stable with time on stream. Unlike HZSM-5, which also catalyzes ethylene oligomerization due to its strong acid sites and is therefore prone to coking, NHSG prepared catalysts did not produce any traces of ethylene oligomers and did not show any trace of coke formation. Characterization (ICP-OES, N<sub>2</sub> physisorption, TEM, XPS, IR coupled with pyridine adsorption, Raman spectroscopy, solid state NMR spectroscopy) reveal that the unconventional synthetic method presented here allowed to prepare mesoporous aluminosilicate materials with a remarkable degree of homogeneity. It is this thorough dispersion of Al in the amorphous silicate matrix which is responsible for the formation of acid sites which are intermediate (in terms of strength and nature) between those of commercial silica-alumina and HZSM-5 zeolite. The texture of the best NHSG catalyst – mainly mesoporous with a high specific surface area (800 m² g<sup>−1</sup>) and pore volume (0.5 cm³ g<sup>−1</sup>) – was also unaffected after reaction. To overcome deactivation issues in ethanol dehydration, this study suggests to target amorphous aluminosilicate catalysts with open mesoporosity and with an intimate mixing of Al and Si.

Modelling of catalysts and its relation to experimental problems

1992 ◽  
Vol 341 (1661) ◽  
pp. 255-268 ◽  
Keyword(s):  
Small Molecules ◽  
Active Sites ◽  
Thermal Activation ◽  
Porous Metal ◽  
Acid Sites ◽  
Zeolite Catalysts ◽  
Before And After ◽  
Porous Metal Oxides ◽  
And Diffusion

We illustrate the role of both computer simulation and the evaluation of electronic structure in the study of solid heterogeneous catalysis by reference to recent work in this laboratory on ( a ) microporous materials (that have a spatially uniform distribution of accessible active sites) and ( b ) non-porous metal oxides. Computational methodologies may be used to model, first, the structure of the uniform catalysts both before and after thermal activation, second, the docking and diffusion of molecules in solids and on their surfaces; and, third, the reaction pathways of molecules at the active site. We highlight recent successes in modelling (i) the structures of zeolitic solids, (ii) the sorption of hydrocarbons within them, (iii) the protonation of small molecules at the Bronsted acid sites in uniform solid acid (zeolite) catalysts, and (iv) the reactions of small molecules on CeO 2 and MgO surfaces.

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