Sol-Gel Preparation of Highly Active Sulfated Zirconia Supported by Alumina Catalysts

A series of sulfated mixed oxides of alumina and zirconia having a relative composition of 5% and 10% of ZrO₂ was prepared by means of sol-gel methods using zirconium propoxide or zirconium acetylacetone as precursor. The characterization of the physicochemical properties was carried out using ²⁷Al NMR, XRD, N₂ adsorption at 77 K, thermogravimetry, FTIR analysis of adsorbed pyridine, ²⁷Al NMR-MAS and XPS. The catalytic properties were studied by means of isomerization of n-hexane at 250°C. Results obtained allowed to propose that the use of Zr(acac)₄ as a zirconium precursor leads to a better retention of sulfate species which seems to form polymeric superficial sites. The symmetry of aluminium undergo an increase from tetrahedral to octahedral coordination and Zirconium atoms seems to be located in the second coordination sphere of Al. XRD analysis indicated an amorphous structure of obtained solids calcined at 650°C. The sulfated solids presented both Lewis and Brönsted acidic sites. Catalytic results showed that both activity and selectivity towards isomerization products were better using Zr (acac)₄ as precursor. Furthermore, the increase of the Zr loading affected considerably the catalytic properties of sulfated zirconia supported by alumina.     

Solvent-free preparation of nanosized sulfated zirconia with Brønsted acidic sites from a simple calcination

Nanosized sulfated zirconia with Br?nsted acidic sites has been prepared by a simple calcination in the absence of any solvent. XRD patterns reveal that the sulfated zirconia mainly consists of tetragonal crystalline zirconia with average size of about 7 nm, which is further confirmed by TEM images. N2 adsorption data show that the nanosized sulfated zirconia has high surface area (165-193 m2/g) and exhibits uniform pore distribution aggregated by zirconia nanoparticles. IR spectra of samples show that the sulfur species in the nanosized sulfated zirconia is a little different from that in conventional sulfated zirconia. Furthermore, IR spectra of adsorbed pyridine indicate that the nanosized sulfated zirconia contains relatively more Br?nsted acidic sites than conventional sulfated zirconia. Catalytic tests show that the nanosized sulfated zirconia exhibits much higher activity than conventional sulfated zirconia in catalytic esterification of cyclohexanol with acetic acid.     

Nature of the Ftir Band in Acidic Zeolites

The nature of a FTIR band (1490 cm ^-1 ) in the spectrum of zeolite HZSM-5 after pyridine adsorption was investigated. It is shown that the source of this band is not a combined contribution from Brönsted and Lewis acidic sites as widely believed.     

Acid activated montmorillonite and vermiculite clays as dehydration and cracking catalysts

Treatment of montmorillonite and vermiculite with hydrochloric acid results in an overall increase in acidity and acidic sites. IR and DSC studies of base adsorbed catalysts have revealed the heterogeneity of surface acidity on the activated catalysts. Clay catalysts have been found to dehydrate 1-butanol to 1-butene, which directly isomerizes tocis- andtrans-2-butenes, confirming the presence of Lewis and Brönsted acid sites. Cumene cracking studies using the clay catalyst also confirm the presence of Brönsted acid sites. The temperature and concentration of acid have been found to have a significant influence on the catalytic activity.     

Investigation of catalytic property in the t-butylation of 1,2-dihydroxybenzene using FT-IR and XPS study

Catalytic properties of HZSM-5s with three different Na⁺ ion-exchange levels and SiO₂ /Al₂O₃ ratios used in tert-butylation of DHB (1,2-dihydroxybenzene) are interpreted through pyridine adsorbed FT-IR and XPS study. The DHB conversion decreases as increment of degree of Na⁺ ion-exchange level and of Si content in HZSM-5. Catalytic properties with respect to Na amount in ZSM-5 are more sensitive than those of HZSM-5s with different SiO₂/Al₂O₃ ratios. But selectivity for 4-TBC (4-t-butylcatechol) is not changed significantly. Acidic properties, i.e. acid strength and acid density are characterized by pyridine adsorbed FT-IR and XPS study. Based on FTIR and XPS analyses, DHB conversion and selectivities for DTBC (3,5-di-t-butylcatechol) and 3-TBC (3-t-butylcatechol) depend on type and strength of acid sites, with the result that strong Brønsted acid rather than weak Brønsted or Lewis acid sites are more closely related to the conversion. Furthermore, t-butyl alcohol is selectively adsorbed on the Brønsted acid site of FT-IR band at 3612 cm^-1, which signifies that the Brønsted acid site is the active site. The mechanism for t-butylation of DHB is suggested based on the FT-IR results of adsorption/desorption of reactants.     

Temperature-programmed desorption of n-propylbenzene from HNaZSM-5 and Na(Li, K, Rb, Cs) ZSM-5 type zeolites

Adsorption of n-propylbenzene on ZSM-5 type zeolites occurs on both Brönsted and Lewis type acid sites. Alkali cations strongly bind hydrocarbons. One particle of hydrocarbon is adsorbed on one Na⁺ ion. The strength of the bond depends on the cation radius and decreases in the order: Li>Na>K>Rb>Cs.     

Evidence for Lewis and Brønsted Acid Sites on MgO Obtained by Sol-Gel

Traditionally, magnesium oxide has been considered a typical basic catalyst, catalyst carrier and/or adsorbent. In this study MgO was prepared using Mg-ethoxide dissolved in ethanol and hydrolyzed with various aqueous inorganic acids and bases. We have found that it is possible to induce Brønsted and Lewis acid sites depending on the method of preparation and, more specifically, on the type of inorganic acids used in the hydrolysis stage. In the FTIR spectra of adsorbed pyridine on MgO obtained using aqueous HCl (pH = 3), the bands corresponding to Lewis (1603, 1496 and 1444 cm–1) and Brønsted (1550 cm–1) acid sites are observed. The fact that upon vacuum and high temperature treatment, i.e., at 773 K, the above mentioned bands are clearly distinguishable indicated that the acid sites are very strong. On the other hand, the MgO materials obtained using aqueous acetic acid as a hydrolysis catalyst (pH = 5), and that without any catalyst (pH = 7) showed only Lewis acid sites which decreased markedly upon higher temperature treatment. When aqueous NH4OH (pH = 9) was used as the hydrolysis catalyst, the MgO obtained showed only Lewis acid sites. In all cases the number of Lewis acid sites was greater than that of Brønsted sites.     

Influence of phosphate modification on the surface properties of sulfated titania

The effect of increasing phosphate loading on the bulk and surface properties of sulfated titania was investigated by thermogravimetry, X-ray diffraction, laser Raman spectroscopy and FT-IR spectroscopy. The acid properties of the material were probed by low-temperature FT-IR spectroscopy of adsorbed CO. Monophosphate species in varying symmetries and protonation states were detected on the surface of the sulfated titania at low loadings. These condensed with formation of surface polyphosphates with increasing loading. Phosphate blocks weak Lewis acid sites and partially displaces sulfate groups. While the acid, properties of phosphate-free sulfated titania are dominated by Lewis acid sites, the phosphated samples also developed Brønsted acid sites due to the presence of POH groups on the surface.     

A Study of Acidic Sites in H[M]-ZSM-5 Zeolites by XPS, IR and TPD

The present paper covers an X-ray photoelectron spectroscopic (XPS) method proposed for identifying Bronsted and Lewis acidic sites in zeolites and estimating acidic strength of these sites. It was found that the adsorption of pyridine on different acidic sites of H[M]-ZSM-5 zeolites exhibited three peaks: A (402.20-401.30 eV), B (400.95-400.40 eV) and C (399. 40-399. 10 eV). The peaks A and B are assigned to the N(?) level of pyridine adsorbed on Bronsted and Lewis acidic sites, and peak C is assigned to the N(?) level of pyridine adsorbed on a weak Lewis site and/or strongly physisorbed pyridine, respectively. The comparison of the ratio of the Bronsted and Lewis acidic sites determined from the relative intensities of the N(?), peaks with IR spectroscopic data shows that there is an inhomoge-neous distribution of Bronsted and Lewis acidic sites in H[M]-ZSM-5 zeolites. The N(?), binding energies of pyridine adsorbed on these zeolites directly reflect acidic strength, and these results are in good agre     

Selective Deposition of Biocompatible Sol-Gel Materials

Sulphated zirconias with various atomic S/Zr ratios were prepared by sol–gel method. The acidity of these solids has been studied through qualitative and quantitative XPS studies, surface potential measurements and isopropanol dehydration reaction. The Correlation between corresponding results show that when zirconia is doped by sulfate groups the surface becomes more acidic in term of Lewis acidity and that the oxygen species in the sample exist in many types, which one is related to solid acidity. This type of oxygen species, probably in the hydroxyl groups, is other than the oxygen species of zirconia network and the oxygen sulphate groups. Consequently, acidity of sulphated zirconia is mainly due to a strong Lewis nature of the surface, which can give a Brönsted acidity by water or reactant chemisorptions     

应用原位漫反射红外-质谱技术研究CuO/Al2O3催化剂表面酸性及其反应性能

应用原位漫反射红外-质谱联用、程序升温和暂态响应技术研究了CuO/Al₂O₃催化剂表面酸性及其反应性能. 实验结果表明, CuO/Al₂O₃催化剂表面呈Lewis酸性, 硫化不仅可增强CuO/Al₂O₃催化剂的Lewis酸性, 而且可产生新的Brønsted酸性位; 吸附于Lewis酸性位的NH₃具有选择性催化还原(SCR)活性. 而在硫化样Cu8(400S)中Lewis和Brønsted酸性位同时存在的情况下, 吸附于Lewis和Brønsted酸性位的氨均具有SCR活性, 且后者较前者弱; CuO/Al₂O₃催化剂上的SCR反应遵循Eley-Rideal机理, 即SCR反应发生于吸附态NH₃与气相NO之间.     

SO42-/ZrO2-Al2O3催化剂表面酸性质对正丁烷异构化反应性能的影响研究

采用“沉淀-浸渍”法制备一系列不同硫酸负载量的SO₄^2-/ZrO₂-Al₂O₃催化剂,利用N₂吸附-脱附、Py-FTIR、XRD等手段对催化剂进行表征。在常压、200 ℃、H₂：C₄=2：3和质量空速为3 h^-1的反应条件下,在固定床微型反应评价装置上考察了硫酸负载量对SO₄^2-/ZrO₂-Al₂O₃催化正丁烷异构化反应性能的影响。Py-FTIR结果表明,硫酸化处理为催化剂表面提供了丰富的Brønsted酸性位,其中,强Brønsted酸性位在正丁烷异构化反应中起重要作用,因此,硫酸化处理可显著提高正丁烷异构化活性,而Lewis酸性位与之没有直接关系。     

单斜及四方晶相ZrO2催化CO加氢反应性能的比较

研究了以纯单斜（ｍ）和四方（ｔ）晶相ＺｒＯ２为催化剂的ＣＯ加氢反应．尽管两种晶相催化剂均有较高的低碳烯烃的选择性,但是也发现了两者催化性能的显著差别．ｍ－ＺｒＯ２催化剂对异丁烯有突出的选择性,而ｔ－ＺｒＯ２催化剂,则只有乙烯和丙烯,几乎没有Ｃ４烯烃的选择性．室温下ＣＯ吸附的原位ＩＲ谱测试可见,只有在ｔ－ＺｒＯ２催化剂上观测到不可逆含氧吸附物种．吡啶吸附的Ｒａｍａｎ谱显示出它们之间表面性质的差别,ｍ－ＺｒＯ２催化剂表面存在等强度的Ｌｅｗｉｓ和Ｂｒｏｎｓｔｅｄ酸中心,而在ｔ－ＺｒＯ２催化剂表面几乎只有Ｂｒｏｎｓｔｅｄ酸中心．催化过程的一些模型分子电子结构计算也表明了ＺｒＯ２催化剂对低碳烯烃选择性内在的电子结构条件．我们推测ＣＯ在ｍ－ＺｒＯ２催化剂表面的孪式吸附物种可能是导致异丁烯产物的根源     

H-MCM-22沸石分子筛中Brnsted/Lewis酸协同效应的~1H 和~(27)Al双量子魔角旋转固体核磁共振研究

采用各种固体核磁共振 (NMR) 技术详细研究了 H-MCM-22 分子筛中 Brnsted/Lewis 酸的协同效应. 二维 1H 双量子魔角旋转 (DQ-MAS) NMR 结果表明, 在脱铝 H-MCM-22 分子筛中 Brnsted 酸位 (骨架桥式羟基) 和 Lewis 酸位 (非骨架铝羟基) 之间是空间邻近的, 暗示着可能存在 B/L 酸协同效应. 二维 27Al DQ-MAS NMR 结果揭示了各种铝物种之间的空间邻近性, 表明 B/L 酸协同效应优先发生在 H-MCM-22 分子筛超笼中的骨架 T6 位铝和非骨架铝物种之间. 2-13C-丙酮探针分子实验发现, 因 B/L 酸协同效应而导致脱铝 H-MCM-22 分子筛酸性明显增强, 氘代吡啶探针分子实验也证实在 H-MCM-22 分子筛的超笼中发生了 B/L 酸协同效应. 上述结果将有助于我们理解在脱铝 H-MCM-22 分子筛上发生的多相催化机理.     

Influence of the Br?nsted and Lewis Acid Sites in Platinum/Solid Acid Catalysts on the Hydrogenation of Benzene and Toluene

The hydrogenation of benzene and toluene was investigated over US-SSY, -Al₂O₃, and Ha1-MCM-41 supported platinum catalysts. The acidity of catalyst supports was measured by IR spectra of pyridine adsorption. On the Brönsted acid supported catalyst, both the hexacyclic saturation product and the pentacyclic one, the hydroisomerization product, were observed, and these two products were revealed to be primary reaction products. The two products were proposed to be formed competitively on Brönsted acid sites with carbonium ions as the reaction intermediate. By contrast, no hydroisomerization product was found on Lewis acid supported catalysts. It was inferred that the electron-deficient reaction intermediate formed on Lewis acids could not undertake any skeletal isomerization process to give a pentacyclic saturation product.     

Neutralization of HZSM-5 Br?nsted acid sites by shaping with boehmite

The reduction of the concentration of strongly acidic Brönsted sites of HZSM-5 by neutralization due to added boehmite is reported. Strongly acidic Brönsted sites are neutralized by [AlO·(Al₂O₃)_x]⁺ isopolyoxocations proceeding during the calcination treatment.     

Synthesis and FT–IR study of the acido–basic properties of the V2O5 catalysts supported on zirconia

In this paper we describe the synthesis and characterization of the acido–basic properties of catalysts containing varied amounts of vanadium supported on ZrO₂. The preparation of the zirconia was carried out using a hydrolysis method and the vanadium was introduced by impregnation with a porous volume in several stages, followed by calcinations under air at a temperature of 723 K. The obtained samples are characterized by adsorption–desorption of nitrogen and infrared spectral analysis of different species formed by acidic and basic probes. This adsorption on the surface of these compounds has been studied in order, in the hand to investigate information on their surface acidity and in the other hand to know particularly the nature and strength of acidic and basic sites. Among the molecular probes, we used carbon monoxide, carbon dioxide, pyridine and 2,6-dimethylpyridine. The adsorption of CO has shown that contrary to pure zirconia and oxidized V₂O₅/ ZrO₂, the reduced V₂O₅/ ZrO₂ samples favour the formation of CO co-ordinated on Lewis acidic sites of reduced V₂O₅ species (CO on V⁴⁺ or V³⁺). We also observe the creation of Brønsted acidic sites by means of the incorporation of vanadium.     

Effect of gamma-irradiation on the catalytic activity and selectivity of gamma-alumina

Methanol conversion reaction was carried out in contact with a poorly crystalline -alumina pre-irradiated with different doses of -rays. The reaction was conducted at 140–440°C using a flow technique under atmospheric pressure. The results obtained revealed that -irradiation of Al₂O₃ resulted in drastic modifications of its activity and selectivity in methanol conversion reaction. The dose of 15 Mrad was sufficient to suppress completely the formation of dimethyl ether (DME) and stimulated the formation of methane, which started at 200°C instead of 300°C in the case of the unirradiated alumina specimen. However, the rate of CH₄ formation was found to decrease as a function of the dose employed. When the dose reached 140 Mrad, DME was reproduced with a rate comparable to that measured for the unirradiated catalyst sample. These results permitted us to conclude that DME is produced on the weak acidic sites (Brönsted acidity of Al₂O₃) and is not necessarily an intermediate compound for methane formation that takes place directly from methanol on strong acidic sites (Lewis acidity). The doses of 15–75 Mrad expelled completely the Brönsted acidic sites from Al₂O₃ surface, and the doses above this limit brought about a transformation of Lewis acidic sites into Brönsted acidity that is responsible for dimethyl ether formation. This transformation occurs by the action of liberated water from the dehydration of methyl alcohol.     

On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Lewis acidic aluminum in zeolites, particularly acidity that is inherent to the framework, is an indeterminate concept. A fraction of framework aluminum changes geometry to octahedral coordination in the proton form of zeolite mordenite. Such octahedrally coordinated aluminum is the precursor of a Lewis acid site and its formation is accompanied by a loss in Brønsted acidity. Herein, we show that such Lewis acid sites have a preferred location in the pore structure of mordenite. A greater proportion of these Lewis acid sites resides in the side-pockets than in the main channel. By reverting the octahedrally coordinated aluminum back to a tetrahedral geometry, the corresponding Brønsted acid sites are restored with a concomitant loss in the ability to form Lewis acid sites. Thereby, reversible octahedral–tetrahedral aluminum coordination provides a means to indirectly switch between Lewis and Brønsted acidity. This phenomenon is unique to Lewis acidity that is inherent to the framework, thereby distinguishing it from Lewis acidity originating from extra-framework species. Furthermore, the transformation of framework aluminum into octahedral coordination is decoupled from the generation of distorted tetrahedrally coordinated aluminum, where the latter gives rise to the IR band at 3660 cm⁻¹ in the OH stretching region.

Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.     

Molecular water on exposed Al3+ cations is a source of acidity in silicoaluminas

Evidence about the remarkable acidic nature of molecular water adsorbed at the surface of mesoporous MCM-41 silicoaluminas (MSA) with a ratio of Si/Al = 25 is reported, coming from both infrared spectroscopy and ab initio calculations. By using CO as a probe, OH stretching and HOH bending modes of water adsorbed on coordinatively unsaturated Al ions (Lewis sites) at the surface of MSA have been detected for the first time. CO adsorption causes OH stretching frequencies to suffer a bathochromic shift of about -200/-250 cm(-1), whereas the HOH bending frequency undergoes a hypsochromic shift by about +10/+20 cm(-1). B3LYP/6-31+G(d,p) calculations on model clusters designed to mimic both Br?nsted and Lewis sites validate the assignments, showing that water adsorbed on the surface Lewis site, when interacting with CO, shows an acidity comparable to that of a classical zeolitic Br?nsted site.