Effect of steaming on the defect structure and acid catalysis of protonated zeolites

The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. A new picture of the formation and evolution of mesopores during hydrothermal treatment has emerged from recent electron microscopy studies on hydrothermally dealuminated USY materials. Laboratory steam treatments give rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are observed among different USY grains as well as within single grains. In regions with high defect concentration, mesopores “coalesce” to form channels and cracks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boundaries. High-silica Y materials, having little or no nonframework Al, exhibit poor catalytic activity. The results of recent studies provide compelling evidence that the critical nonframework Al species are (1) highly dispersed, and (2) quite possibly exist as cationic species in the small cages of dealuminated H-Y. Investigations of Lewis acidity in mildly dealuminated zeolites indicate that the origin of the high catalytic activity is a synergistic interaction between Brønsted (framework) and highly dispersed Lewis (nonframework) acid sites. The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity, as, for example, by calorimetry, or by NMR spectroscopy. The enhanced activity of mildly steamed protonated zeolites is not due to an increase in acidity of the bridging hydroxyl or Brønsted sites.     

The influence of steric and electronic effects on the preparation of cyclic acetals catalyzed by sulfated metal oxides

Sulfated metal oxides (SMO) were able to catalyze the reaction of carbonyl compounds with ethylene glycol (EG), producing cyclic acetals at near ambient temperature. Sulfated hafnium oxide containing only Lewis acid sites (indicated by IR/pyridine adsorption experiments) was found active for the reaction. However, these Lewis acid sites might not be the active sites for the reaction, because they were most likely transformed into Bronsted acid sites by water produced in the reaction. Both electronic and steric effects played important roles in this reaction. The sulfate group coordinating to the active acid site had exerted a steric influence on the reaction. Therefore, the less bulky cyclic acetals were more readily formed. The carbonyl group conjugated with a double bond delocalized the positive charge on the carbocation reaction intermediate, resulting in lower cyclic acetal yields.     

Hierarchical ZSM-5 zeolite designed by combining desilication and dealumination with related study of n-heptane cracking performance

The effect of the basic (NaOH) and/or acid (citric acid and EDTA-2Na) treatment of ZSM-5 zeolite has been studied comparing the structural and acidic features and their catalytic performance in n-heptane cracking. The properties of the catalysts have been elucidated using XRD, N₂ low-temperature sorption, ²⁷Al and ²⁹Si NMR, pyridine adsorbed FTIR, NH₃–TPD, SEM and TEM analysis. The results showed that the degree of desilication and dealumination of ZSM-5 zeolites was greatly dependent on the agents. NaOH obviously created new mesopores on parent ZSM-5 zeolites by desilication. Citric acid contributed to the removal of nonframework Al species, causing the increase of micropore surface area. EDTA-2Na promoted desilication and simultaneously converted part of removed framework Al species into nonframework Al species. The treatment of ZSM-5 combined with those three agents was very effective to obtain a hierarchical structure with partial breakdown of the crystallites and high acid amounts of both Brönsted and Lewis acid sites. Catalytic tests showed that the post-treated ZSM-5 catalysts had higher activity and stability than parent ZSM-5 catalyst at the same reaction temperature. The synergetic effect of Brönsted acid and Lewis acid of ZSM-5 catalyst (Z5-ACE) probably facilitated n-heptane conversion, while more clean micropore and newly created mesopores facilitated the slight increase of olefin selectivity and suppressing the formation of coke deposition in its inherent micropores to some extent.     

Thermogravimetric and microcalorimetric studies of ZSM-5 acidity

The combination of thermogravimetry, microcalorimetry and infrared spectroscopy studies of pyridine adsorption has been used to characterize the acidity of a ZSM-5 catalyst. The majority of the acid sites are Brønsted acid centers associated with framework Al species, with heats of pyridine adsorption equal to 140 kJ/mol. Non-framework Al species in the zeolite sample of this study eliminate an approximately equal number of Brønsted acid sites. These nonframework Al species also produce strong Lewis acid sites with pyridine adsorption heats greater than 140 kJ/mol, as well as weak adsorption sites (e.g., weak Bransted acid sites or hydrogen bonding sites) with heats equal to 90–140 kJ/mol.     

The use of pyridine and piperidine hdn as probe for activity of molybdenum-based hydrotreatment catalysts. The role of the nickel (part II)

The reactions of pyridine and piperidine HDN have been used as a probe for testing the activity and the selectivity of eleven hydrotreating catalysts in the absence of sulfur: these comprised pure alumina, doped alumina, MoO₃/Al₂O₃, NiMo and CoMo/AI₂O₃. The piperidine HDN reaction needs only single Bronsted or Lewis acidic sites while the pyridine reaction needs at least a bifunctional site with a reduced centre and an acidic centre. The intervention of a trifunctional site, two reduced centres and an acidic one, explains the promoting effect of the nickel. The presence of this new site can be correlated with the existence of a phase where the nickel, in the reduced state, “decorates” the Mo oxide layer. This new phase will have as a precursor the xNiO molybdate phase proposed by Knozinger et. al. [11].     

Characterization of γ-alumina doped with Li and K by infrared studies of CO adsorption and27Al-NMR

The influence of alkali metals on the Lewis acid sites of gamma-alumina has been studied with NMR and IR of adsorbed CO. The physical properties of the alumina are slightly changed by the addition of low alkali metal concentrations. Li and K are adsorbed selectively on tetrahedral Lewis Al³⁺ sites of the alumina surface. These effects on the tetrahedral cations are more marked with increasing molar concentration and ionic radius of the alkali metal added to alumina. Results demonstrate that the influence of the alkali metals is not only due to steric factors but also to possible electronic modifications.This work was supported by a grant from CONICET, Argentina.     

New redox deposition-precipitation method for preparation of supported manganese oxide catalysts

A new preparation method has been developed to produce homogeneous coatings of manganese compounds on alumina-coated monoliths by redox deposition-precipitation using acetone as solvent. The deposition is produced by reduction of manganese permanganate with ethanol. The Lewis acid sites on the alumina catalyze this reaction. Thus, the precipitation is produced preferentially on the surface of the monolith and not in the bulk of the solution. Monoliths prepared by this method are very active for the complete oxidation of oxygenated volatile organic compounds (VOCs). These monoliths consist of a mixture of complex phases including aluminum, sulfated alumina, potassium sulfate and manganese oxide.     

CO hydrogenation over Al2O3: pseudo-hydrothermal and basic alkali metal compounds pretreatment effect

Aluminas pretreated with pure water (pseudo-hydrothermal treatment) and very dilute basic aqueous solutions were used as catalysts for CO hydrogenation. Pseudo-hydrothermal treatment results in an apparent enhancement of ethylene selectivity at elevated temperature (>673 K). The active sites seem to be coordinatively unsaturated metal and oxygen counterions (Lewis acid and base pair), which are very probably located on the most external surface of the alumina and can be coordinatively saturated by calcination in air. The precursors of the active sites probably originate from a very thin layer of boehmite beyond XRD detecting ability. They can be generated by both pseudo-hydrothermal treatment and dilute basic aqueous solution treatment of alumina.     

On the dehydrogenation reaction of n-butane with the deactivation of alumina-chromia catalyst

An experimental study on the dehydrogenation reaction of n-butane to n-butene with alumina-chromia catalyst has been performed using a differential reactor. N-butene produced and coke deposit on the catalyst were continuously measured and the mechanisms of the reactions of dehydrogenation of n-butane and coke deposition have been investigated. According to the assumptions that there are two types of active sites effective for the dehydrogenation reaction, Lewis acid site and Brönsted acid site, and that the coke deposition takes place only at Lewis acid sites, the rate equations of these reactions have been obtained.     

The influence of gaseous chlorine on the chemical and catalytic properties of alumina

The influence of gaseous chlorine on the chemical and catalytic properties of alumina was investigated. Cl₂ reacts both with surface OH groups and with surface oxygen anions possessing electron-donating power, forming new surface sites. When such chlorine treatment is performed at a low temperature (293 K), both Lewis and Bronsted acid centres are formed. High temperature chlorination leads exclusively to Lewis acid sites. It is proposed that such Lewis sites are responsible for olefin isomerization reactions.     

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO₂, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm^?1 suggests that besides structural Lewis sites on the surface of ZrO₂, strong Lewis sites are made from chemisorbed SO₃. Upon adsorption of water, SO₃ is converted, partially, into a surface sulfated species which may act as strong Brønsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO₄. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Brønsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.     

FCC催化剂基质组分直链烯烃反应研究

制备了一系列SiO2-Al2O3基质材料,采用吡啶吸附—红外光谱法对其酸性特征进行了表征。并且以n-C^=7-1为模型化合物考察了反应产物分布与基质材料酸中心特性的关联,提出了n-C^=7-1的裂化反应网络,对n-C^=7-1的异构化、氢转移、芳构化等二次反应进行了探讨。结果表明,异构化反应过程同时以单分子与双分子两种机理进行,但双分子反应选择性较低;氢转移反应不仅与基质的酸中心密度有关,对酸类型的影响更为敏感,在只有L酸的基质上,氢转移以Rideal机理进行,活性随酸密度呈线性增加;在既有B酸又有L酸的基质上,氢转移更倾向于L-H机理,活性随酸密度呈指数增加;烯烃二次反应之间选择性的竞争很大程度上取决于B酸与L酸在总酸量中的分配。     

Catalytic aminolysis of epoxide by alumina prepared from amine-protected Al precursor

The catalytic activities of alumina prepared from an Al alkoxide-amine adduct monomer for the reaction of cyclopentene oxide with piperidine was determined after various pretreatments, including calcination and exposure to moisture. They were compared with the activity of alumina prepared by the conventional hydrolysis method. It was found that the as-prepared sample from the alkoxide-amine monomer preparation was five times more active than a conventional preparation, suggesting that it has a higher density of surface Lewis acid sites. However, its activity was much more severely suppressed by exposure to moisture.     

A new sol‐gel route alumina for selective oxidation of H2S to sulphur

In this study, a new synthesis method was developed for the production of modified sol‐gel alumina (SG‐M) for the selective oxidation of H₂S to elemental sulphur. The catalytic activity of this modified alumina without any active metal incorporation was then compared with the activity of commercial alumina (alumina‐com) for H₂S selective oxidation. The N₂ adsorption‐desorption isotherm showed that the SG‐M alumina synthesized in this work has a mesoporous structure with well‐defined hysteresis loops. Both alumina materials showed a γ‐Al₂O₃ crystalline phase with an amorphous structure in their crystal structure. The surface acidity of the alumina materials was determined using pyridine‐adsorbed FTIR analyses, and both alumina showed Lewis acid sites on their surfaces. The catalytic activity tests were performed at 250°C using a feed ratio of O₂/H₂S:0.5. The complete conversion of H₂S over SG‐M was achieved during 400 minutes of reaction time. However, the commercial alumina lost its activity at earlier reaction times. Lewis acid sites and surface hydroxyl groups caused the alumina to be active in H₂S selective catalytic oxidation, and the formation of Al‐S bonds, observed when the H₂S conversion fell, caused a decrease in the catalytic activity of the alumina materials. A high sulphur yield (≥95%) was obtained over SG‐M, even though there was no active metal incorporation and even in the presence of excess oxygen. Considering the catalytic activities, the new sol‐gel alumina synthesized in this work is superior to commercial alumina. It was concluded that, as a catalyst without any active metal, SG‐M is a promising catalyst in H₂S selective oxidation to sulphur.     

Effect of lithium addition upon γ-Al2O3 for isopropanol dehydration

The effect of lithium addition to γ-Al₂O₃ on the acidity and on the catalytic behaviour in the isopropanol dehydration reaction is studied. Results show that there is an important blocking or poisoning effect of lithium on the Lewis Al³⁺ sites of the alumina surface; these sites play an important role in the isopropanol dehydration reaction to di-isopropylether and propylene. The di-isopropylether formation is drastically reduced by lithium addition, while the propylene formation can be maintained, even at a high lithium content, by increasing the reaction temperature. Results are interpreted by considering that the ether would be produced by a concerted mechanism (E₂), and the olefin formation could be carried out by both E₂ or E₁ mechanisms.     

Preferential occupation of molybdenum on the thin alumina film: Characterization by CO titration

A thin alumina film, the alumina model surfaces modified by the metallic molybdenum and by the MoO₃ are titrated by CO chemisorption. Two types of CO adsorption sites, namely octahedrally and tetrahedrally coordinated Al³⁺ sites, are present on the thin alumina film. The thin alumina film prepared under UHV conditions can be used to simulate the conventional high-surface-area alumina supports in real catalysis. The deposited metallic molybdenum preferentially occupies octahedrally coordinated Al³⁺ site and suppresses CO chemisorption on this site, and oxidation of the surface molybdenum species enhances this suppression.     

Hydrolytic decomposition of CF4 over alumina-based binary metal oxide catalysts: high catalytic activity of gallia-alumina catalyst

The hydrolytic decomposition of CF₄ has been conducted on gallia promoted alumina (Ga-Al oxide) catalysts at 803–903 K and 0.1 MPa. Steady state activity on 20% Ga-Al oxide was 15 times of that on Ce10%-AlPO₄ catalyst, on which the highest activity has been reported. The catalytic activity was further improved by incorporation of sulfate anion in Ga-Al oxide and by applying sol–gel method in preparation of the catalyst. XRD spectra of Ga-Al oxides showed a shift of diffraction peaks assigned to γ-alumina toward lower angles, indicating the formation of gallia-alumina solid solution. In situ FT-IR of pyridine adsorption spectra of Ga-Al oxides showed peaks solely attributable to Lewis acid (L-acid) sites at 1445–1455, ca. 1495, 1577, ca. 1594, and ca. 1622 cm⁻¹. The steady state catalytic activities increased with increasing peak areas at 1446 or 1622 cm⁻¹ of Ga-Al oxides with various Ga%, suggesting participation of Lewis acid sites into the reaction. It is demonstrate from surface area measurements of Ga-Al oxide catalyst before and after the reaction that not only higher catalytic activity but also higher catalyst stability were observed on Ga-Al oxide, Ga-Al oxide with sulfate, and Ga-Al oxide prepared by sol–gel method than on their parent oxides of alumina, Ga-Al oxide, and Ga-Al oxide prepared by incipient wetness method, respectively.     

Effects of water density on acid-catalytic properties of TiO2 and WO3/TiO2 in supercritical water

The effects of water density on the acid-catalytic properties of TiO₂ and WO₃/TiO₂ catalysts in supercritical water at 400 °C were investigated by using the kinetic analysis of the dehydration reaction of glycerol. The reaction selectivity of TiO₂ and WO₃/TiO₂ catalysts and the apparent-reaction orders for water indicated that the acid-catalytic properties of these two catalysts show different dependence on water density. In the reaction using TiO₂, the contribution of Lewis acid sites in TiO₂ was large at a low water density, while the contribution of Brönsted acid sites in TiO₂ increased with increasing water density. On the other hand, the reaction using WO₃/TiO₂ was mainly catalyzed by Brönsted acid sites in WO₃/TiO₂ even at a low water density, and the nature of Lewis/Brönsted acid sites in WO₃/TiO₂ was not influenced by the water density.     

不同类型氧化铝对FCC轻汽油芳构化性能影响

针对第六阶段汽油国家（国Ⅵ）标准大幅度降烯烃同时保辛烷值的生产需要，本文通过改变氧化铝基质类型，制备了系列催化裂化（FCC）轻汽油芳构化催化剂，并对催化剂酸性质和织构性质进行调变。采用X射线衍射（XRD）、N₂吸附脱附、扫描电子显微镜（SEM）、红外光谱（IR）和吡啶红外光谱（Py-IR）等手段对催化剂的物理性质进行表征，并以工业FCC轻汽油为原料对催化剂芳构化性能进行评价。实验结果表明，不同类型氧化铝的引入未对ZSM-5分子筛晶型产生影响，但可以显著调变ZSM-5分子筛的表面酸性，增加L酸中心数量。具有均一和相对较小比表面积和孔容的氧化铝材料更有利于轻汽油芳构化性能，轻汽油产品中，烯烃大幅下降（体积分数减少18.18%），异构烷烃（体积分数提高10.51%）和芳烃（体积分数提高2.75%）增幅较为明显，辛烷值损失（-5.1）较小并且可控。     

The nature of the active sites in cumene cracking on amorphous silica aluminas

We present evidence which indicates that the nature of the active sites is the same for amorphous catalysts as it is for crystalline alumina silicate cracking catalysts. The only differences we find in the various cracking catalysts lie in the total concentrations of active sites and in the ratio of Bronsted to Lewis sites. In view of this, and because even zeolites have only a fraction of their cationic sites active in cumene cracking, we suggest that it is possible that a silica alumina catalyst could exist that is more active by as much as an order of magnitude than any catalyst presently available for cumene cracking.