Chemical vapor deposition of trimethylaluminium on dealuminated faujasite zeolite

Chemical vapor deposition of trimethylaluminum (TMA) was explored as an approach for the preparation of model faujasite-type catalysts containing extraframework aluminum. The decomposition of the grafted organoaluminum species was investigated in hydrogen and oxygen atmosphere. The process of grafting Al-containing species and the associated changes of the zeolite hydroxyl groups were followed by in situ FTIR spectroscopy. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by 1H, 29Si and 27Al MAS NMR, COads IR, H/D exchange of acidic hydroxyl groups with perdeuterobenzene and propane cracking. Reaction of an extraframework aluminum-free high-silica faujasite zeolite with TMA leads to nearly complete substitution of the bridging hydroxyl groups with Al species. The reaction, however, does not produce uniform homogeneously distributed species. Because of the high reactivity of TMA, the zeolite lattice is partially decomposed resulting in its partial dealumination and formation of stable Si-CH3 moieties. The exact conditions of post-CVD treatment influence strongly the chemical and catalytic properties of the zeolites. The strongest increase of the propane conversion rate was observed when grafted TMA species were decomposed in H2 at high temperature. Such zeolite displays much higher activity per Brønsted acid site in propane cracking than a commercial ultrastabilized Y zeolite. It is proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Brønsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted TMA complexes

Structure and reactivity of ZnIZSM-5 catalysts

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Structure and reactivity of Zn-modified ZSM-5 zeolites : the importance of clustered cationic Zn complexes

A novel route for the introduction of well-defined zinc species into ZSM-5 zeolite via chemical vapor deposition of dimethylzinc (CVD(DMZ)) is explored. The structural properties and catalytic reactivity of the synthesized material (Zn/ZSM-5-CVD(DMZ)) are investigated against a set of Zn/ZSM-5 catalysts prepared by incipient wetness impregnation (IWI), ion exchange (IE), and high-temperature reaction with zinc vapor (CVD(m)). The materials are characterized by a range of physicochemical methods including temperature programmed reduction (TPR), in situ FTIR, magic-angle spinning (MAS) NMR, and X-ray absorption spectroscopy (XAS). The catalysts are tested for their activity in the dehydrogenation of propane. Catalysts prepared by IE and IWI exhibit a high degree of heterogeneity of extraframework zinc species. These include, besides isolated Zn2+ cations, multinuclear oxygenated zinc clusters and bulk zinc oxide aggregates. The CVD(m) method results in quantitative replacement of all Brønsted acid protons by isolated Zn2+. In CVD(DMZ) the Brønsted acid sites (BAS) react stoichiometrically with dimethylzinc Zn(CH3)2 (DMZ) yielding grafted [Zn-CH3]+ species, which can further be transformed to isolated Zn2+ ions by reduction in hydrogen. The presence of zinc in ZSM-5 enhances the rate of alkane dehydrogenation. The initial activity of Zn/ZSM-5 prepared by IWI and IE correlates with the Zn content. The samples with a more heterogeneous distribution of extraframework Zn species are more active than the samples with isolated Zn2+. The activity of reduced Zn/ZSM-5-CVD(DMZ) containing predominantly isolated Zn2+ ions can be substantially increased by oxidation prior to the reaction. However, the resulting oxygenated complexes easily decompose during the reaction. Propane dehydrogenation and catalyst stability of Zn/ZSM-5-CVD(DMZ) can be improved by addition of steam to the hydrocarbon feed. This rate enhancement is ascribed to an increase of the steady-state concentration of the reactive oxygenated sites

Structure and reactivity of Zn-modified ZSM-5 zeolites : the importance of clustered cationic Zn complexes

A novel route for the introduction of well-defined zinc species into ZSM-5 zeolite via chemical vapor deposition of dimethylzinc (CVD(DMZ)) is explored. The structural properties and catalytic reactivity of the synthesized material (Zn/ZSM-5-CVD(DMZ)) are investigated against a set of Zn/ZSM-5 catalysts prepared by incipient wetness impregnation (IWI), ion exchange (IE), and high-temperature reaction with zinc vapor (CVD(m)). The materials are characterized by a range of physicochemical methods including temperature programmed reduction (TPR), in situ FTIR, magic-angle spinning (MAS) NMR, and X-ray absorption spectroscopy (XAS). The catalysts are tested for their activity in the dehydrogenation of propane. Catalysts prepared by IE and IWI exhibit a high degree of heterogeneity of extraframework zinc species. These include, besides isolated Zn2+ cations, multinuclear oxygenated zinc clusters and bulk zinc oxide aggregates. The CVD(m) method results in quantitative replacement of all Brønsted acid protons by isolated Zn2+. In CVD(DMZ) the Brønsted acid sites (BAS) react stoichiometrically with dimethylzinc Zn(CH3)2 (DMZ) yielding grafted [Zn-CH3]+ species, which can further be transformed to isolated Zn2+ ions by reduction in hydrogen. The presence of zinc in ZSM-5 enhances the rate of alkane dehydrogenation. The initial activity of Zn/ZSM-5 prepared by IWI and IE correlates with the Zn content. The samples with a more heterogeneous distribution of extraframework Zn species are more active than the samples with isolated Zn2+. The activity of reduced Zn/ZSM-5-CVD(DMZ) containing predominantly isolated Zn2+ ions can be substantially increased by oxidation prior to the reaction. However, the resulting oxygenated complexes easily decompose during the reaction. Propane dehydrogenation and catalyst stability of Zn/ZSM-5-CVD(DMZ) can be improved by addition of steam to the hydrocarbon feed. This rate enhancement is ascribed to an increase of the steady-state concentration of the reactive oxygenated sites

Chemical vapor deposition of trimethylaluminium on dealuminated faujasite zeolite

Chemical vapor deposition of trimethylaluminum (TMA) was explored as an approach for the preparation of model faujasite-type catalysts containing extraframework aluminum. The decomposition of the grafted organoaluminum species was investigated in hydrogen and oxygen atmosphere. The process of grafting Al-containing species and the associated changes of the zeolite hydroxyl groups were followed by in situ FTIR spectroscopy. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by 1H, 29Si and 27Al MAS NMR, COads IR, H/D exchange of acidic hydroxyl groups with perdeuterobenzene and propane cracking. Reaction of an extraframework aluminum-free high-silica faujasite zeolite with TMA leads to nearly complete substitution of the bridging hydroxyl groups with Al species. The reaction, however, does not produce uniform homogeneously distributed species. Because of the high reactivity of TMA, the zeolite lattice is partially decomposed resulting in its partial dealumination and formation of stable Si-CH3 moieties. The exact conditions of post-CVD treatment influence strongly the chemical and catalytic properties of the zeolites. The strongest increase of the propane conversion rate was observed when grafted TMA species were decomposed in H2 at high temperature. Such zeolite displays much higher activity per Brønsted acid site in propane cracking than a commercial ultrastabilized Y zeolite. It is proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Brønsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted TMA complexes

Chemical vapor deposition of trimethylaluminium on dealuminated faujasite zeolite

Chemical vapor deposition of trimethylaluminum (TMA) was explored as an approach for the preparation of model faujasite-type catalysts containing extraframework aluminum. The decomposition of the grafted organoaluminum species was investigated in hydrogen and oxygen atmosphere. The process of grafting Al-containing species and the associated changes of the zeolite hydroxyl groups were followed by in situ FTIR spectroscopy. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by 1H, 29Si and 27Al MAS NMR, COads IR, H/D exchange of acidic hydroxyl groups with perdeuterobenzene and propane cracking. Reaction of an extraframework aluminum-free high-silica faujasite zeolite with TMA leads to nearly complete substitution of the bridging hydroxyl groups with Al species. The reaction, however, does not produce uniform homogeneously distributed species. Because of the high reactivity of TMA, the zeolite lattice is partially decomposed resulting in its partial dealumination and formation of stable Si-CH3 moieties. The exact conditions of post-CVD treatment influence strongly the chemical and catalytic properties of the zeolites. The strongest increase of the propane conversion rate was observed when grafted TMA species were decomposed in H2 at high temperature. Such zeolite displays much higher activity per Brønsted acid site in propane cracking than a commercial ultrastabilized Y zeolite. It is proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Brønsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted TMA complexes

Influence of steaming on the acidity of and methonol conversion reaction of HZSM-5 zeolite

The influence of steaming at varying temperatures on the physicochemical properties of a HZSM-5 zeolite (Si/Al = 27) was investigated by X-ray diffraction, 27Al MAS NMR, Ar physisorption and IR spectroscopy of adsorbed pyridine, 2,4,6-trimethylpyridine and CO. The catalytic activity of the zeolites was evaluated in propane and methanol conversion reactions. Mild steaming did not result in removal of framework Al atoms. Instead, evidence was found that some extra-framework Al atoms present in the parent zeolite were inserted into the framework at defect sites. This resulted in higher propane conversion rates. Severe steaming resulted in a strong decrease in the framework Al content and agglomeration of extra-framework Al atoms. This caused a strong decrease in the Brønsted acidity probed by pyridine and CO IR. The rate of propane conversion was consequently adversely affected. The steaming procedures did not result in the formation of noticeable mesoporosity. By IR spectroscopy of adsorbed 2,4,6-trimethylpyridine, however, indications were found for structural damage at the outer region of the zeolite crystals, resulting in increased accessibility of the Brønsted acid sites (BAS). For methanol conversion at 350 °C, the concentration of BAS governs the catalytic performance. For zeolites with a high BAS density (parent and mildly steamed zeolites), the rate of deactivation is high. The total amount of methanol converted per BAS is relatively low because of the high rate of consecutive reactions. These reactions involve the conversion of the dehydration product of methanol, dimethyl ether, to products with carbon–carbon bonds and the formation of carbonaceous deposits, which deactivate the zeolite catalyst. Decreasing Brønsted acidity by severe steaming results in an increased amount of methanol converted per BAS because of the lower coke formation rate. As a result, the total amount of methanol converted per BAS increases strongly with decreasing BAS density. However, it also causes lower rates of conversion of dimethyl ether to useful products. In terms of the total amount of methanol converted per BAS to light olefins, the set of zeolites shows optimum performance at intermediate BAS density (HZSM-5 severely steamed at 450 °C; concentration of BAS ~0.2 mmol g-1)

Influence of extraframework aluminium on the Brønsted acidity and catalytic reactivity of faujasite zeolite

A series of faujasite zeolites was modified by extraframework Al (AlEF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of AlEF and zeolite acidity were investigated by 27Al MAS NMR and COads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH4)2SiF6. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in COads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by AlEF only resulted in a small fraction of cationic AlEF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less AlEF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic AlEF species. The results of MQMAS 27Al NMR spectroscopy confirmed the presence of sites perturbed by AlEF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although COads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from AlEF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic AlEF species

Influence of extraframework aluminium on the Brønsted acidity and catalytic reactivity of faujasite zeolite

A series of faujasite zeolites was modified by extraframework Al (AlEF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of AlEF and zeolite acidity were investigated by 27Al MAS NMR and COads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH4)2SiF6. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in COads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by AlEF only resulted in a small fraction of cationic AlEF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less AlEF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic AlEF species. The results of MQMAS 27Al NMR spectroscopy confirmed the presence of sites perturbed by AlEF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although COads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from AlEF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic AlEF species