Experimental Evidence from H/D Exchange Studies for the Failure of Direct CC Coupling Mechanisms in the Methanol‐to‐Olefin Process Catalyzed by HSAPO‐34

Marcus, David M.; McLachlan, Kelly A.; Wildman, Mark A.; Ehresmann, Justin O.; Kletnieks, Philip W.; Haw, James F.

doi:10.1002/ange.200504372

Cited by 43 publications

(30 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…These methyl groups persisted in dynamic vacuum at 423 K for >1 h, consistent with the presence and stability of CH 3 * groups during carbonylation. 28 CH 3 OCH 3 -CD 3 OCD 3 -CO mixtures led to fast methyl scrambling via unproductive methoxylation of CH 3 * with DME (possibly via trimethyl-oxonium intermediates), without detectable isotopic dilution of methyl groups, as also found by Marcus et al 45 in their study of H-D exchange for CH 3 *-CD 3 * mixtures on SAPO-34 at 573 K.…”

Section: Persistent Species Adsorbed In Zeolitessupporting

confidence: 63%

“…On SAPO-34, H-D scrambling between equimolar CH 3 * and CD 3 * mixtures preformed by CH 3 OCD 3 dissociation was not detectable for periods over 2 h at 573 K, and subsequent reactions with H 2 O-D 2 O mixtures form DME with isotopically intact CH 3 * and CD 3 * groups, indicating that C-H activation in methyls did not occur during reactions with dimethyl ether, methanol, and water at these temperatures. 45 We have recently explored the carbonylation of DME at low temperatures (408 -453 K) on Brønsted acids. Methyl acetate forms via rate-determining CO insertion into CH 3 * groups; this step was probed by stoichiometric reactions of preadsorbed methyls with CO. All zeolites examined (H-MFI, H-MOR, H-FER) adsorbed 0.5 ( 0.05 DME/Al upon exposure to DME at 423 K, as expected from stoichiometric methylation of Al sites (CH 3 /Al ) 1.0 ( 0.1), with the concurrent evolution of acidic protons as water.…”

Section: Persistent Species Adsorbed In Zeolitesmentioning

confidence: 99%

See 1 more Smart Citation

A Link between Reactivity and Local Structure in Acid Catalysis on Zeolites

2008

View full text Add to dashboard Cite

T he extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Brønsted acid sites encapsulated within subnanometer channels.We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD 4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH 3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD 4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH 3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.

show abstract

Section: Persistent Species Adsorbed In Zeolitessupporting

confidence: 63%

Section: Persistent Species Adsorbed In Zeolitesmentioning

confidence: 99%

A Link between Reactivity and Local Structure in Acid Catalysis on Zeolites

2008

View full text Add to dashboard Cite

show abstract

“…Purin-Nucleoside wie [108,109] Adenosin (110) oder Inosin (111) wurden chemoselektiv in 2-und 8-Position deuteriert. [109] Mit CD 3 …”

Section: Methodsunclassified

“…[3] Nach einer Periode intensiver Forschung in den sechziger und siebziger Jahren des letzten Jahrhunderts war es zwischenzeitlich ruhiger um die H/D-Austauschreaktionen geworden. Erst Mitte der 1990er erlebten sie wegen des wachsenden Interesses an der katalytischen C-H-Aktivierung und des zunehmenden Bedarfs an isotopenmarkierten Substanzen als Referenzmaterialien eine Renaissance.…”

Section: Introductionunclassified

Die Renaissance des H/D‐Austausches

et al. 2007

View full text Add to dashboard Cite

Die steigende Nachfrage nach stabil isotopenmarkierten Verbindungen führt zu einem verstärkten Interesse an H/D‐Austauschreaktionen an Kohlenstoffatomen. Heutzutage werden Deuterium‐markierte Verbindungen als interne Standards in der Massenspektrometrie eingesetzt oder helfen dabei, mechanistische Theorien zu untermauern. Der Zugang zu diesen deuterierten Verbindungen erfolgt über einen Austausch von Wasserstoff gegen Deuterium im Zielmolekül deutlich effizienter und preiswerter als über die klassische Synthese. Schwerpunkt dieses Aufsatzes sind präparative Anwendungen der H/D‐Austauschreaktion zur Herstellung Deuterium‐markierter Substanzen. Die Fortschritte der letzten zehn Jahre werden zusammengefasst und kritisch bewertet.

show abstract

“…In contrast, less research and discussion has been carried out on the initial C À C bond formation from the starting C 1 compound, methanol. [1,5,8,9] Since the initial processes of MTO and MTCH would not be affected by the porous structure because of the small sizes of the products, reaction mechanisms on a single acidic OH group of zeolites should become significant rather than zeolite topologies.…”

mentioning

confidence: 99%