Generation Of Acid Sites Research Articles

Unusual acceleration of acid-catalyzed reactions by water in the presence of Mo/Zr mixed oxides calcined at high temperatures

Various acid-catalyzed reactions, such as hydrolysis of ethyl acetate, esterification of acetic acid with ethanol, dehydration of 2-butanol, isomerization of 1-butene, and skeletal isomerization of n-butane, in the presence of Mo–Zr mixed oxides and other solid acids have been studied. MoO3/ZrO2 exhibited high activities only for the hydrolysis and esterification reactions, in which these high catalytic activities could not be explained by the acidic property of MoO3/ZrO2 alone, as shown by NH3–temperature programmed desorption (TPD) studies. It was found that the addition of water significantly accelerated the esterification and dehydration in the presence of MoO3/ZrO2 (calcined above 973 K), wheras SiO2–Al2O3 was greatly deactivated by water under similar reaction conditions. Furthermore, in the presence of water vapor, the activity of MoO3/ZrO2 for the isomerization of 1-butene did not decline, whereas SiO2–Al2O3 became inactive completely. IR measurements revealed that the water adsorbed on MoO3/ZrO2 (calcined at 1073 K) gave a sharp peak at 1628 cm−1, and that this peak gradually decreased at 423 K, while ZrO2 only or MoO3/ZrO2 (calcined at 673 K) did exhibit well-defined peaks. Since the adsorption-desorption behavior of water was in good agreement to the catalytic activity for dehydration of 2-butanol, the acceleration by water was presumably a result of the generation of acid sites by the dissociation of water molecules that are coordinated on the Mo species of MoO3/ZrO2.

Framework cobalt and manganese in MeAPO-31 (Me=Co, Mn) molecular sieves

Pure phases of CoAPO-31 and MnAPO-31 were synthesized hydrothermally using di- n-propylamine as a structure-directing agent. The incorporation of manganese(II) and cobalt(II) into framework aluminum sites of AlPO 4-31 was suggested from elemental, thermogravimetric and X-ray powder diffraction analysis. Isomorphous aluminum substitution with cobalt(II) was confirmed from static 31P NMR spectra. UV–VIS and XANES spectra revealed a partial oxidation of framework cobalt(II) and manganese(II) into cobalt(III) and manganese(III) in the calcined MeAPO-31 and thus the presence of redox centers in the products. The generation of acid sites (Brønsted and Lewis) in MeAPO-31 was supported by IR measurements of pyridine and by ammonium adsorption/desorption. The strength of the acid sites in the catalysts studied decreased in the following order: MnAPO-31>CoAPO-31>AlPO 4-31.

Silica–alumina catalysts prepared in sol–gel process of TEOS with organic additives

Amorphous silica–alumina catalysts were prepared in the sol–gel reaction of tetraethoxysilane in the presence of aluminium nitrate and various organic additives, and the effects of the additives on both pore formation and acid-site generation were investigated. Macropores with bicontinuous morphology are formed in the system with poly(ethylene oxide) (PEO) with an average molecular weight of 10 0000, when a transitional structure of spinodal decomposition is fixed by sol–gel transition of inorganic components. In the systems with other organic additives with low molecular weight, such as ethylene glycol oligomers and citric acid, silica–alumina is provided with only mesopores. Although their mesopore structures are not affected by organic additives, the catalytic activity varies depending on the kind of organic additives. It is found that organic additives with the ability to increase the interaction between silica oligomers and aluminium cations increase the dispersion of Al in the silica network, resulting in high catalytic activity in the cracking of cumene. PEO interacts with both silica and aluminium cations in the sol–gel solution, so that macroporous silica–alumina prepared in the presence of PEO shows excellent catalytic activity.

Unprecedented Acceleration Effects of Water on Acid-Catalyzed Reactions Over Molybdena-Zirconia Catalysts

Effects of water on various acid-catalyzed reactions have been studied mainly using MoO3/ZrO2. While the reaction rate of gas-phase esterification of acetic acid with ethanol and the dehydration of 2-butanol over SiO2-Al2O3 decreased significantly by the addition of water vapor in the feed gas, these reactions over MoO3/ZrO2 were found to be greatly enhanced by the presence of water. The enhancement by water was reversible and was remarkable when MoO3/ZrO2 was calcined at a temperature range of 773-1073 K. IR spectroscopy of H2O adsorbed on MoO3/ZrO2 suggests that the acceleration effect of water would be due to the generation of acidic sites from water coordinated to Mo species on the surface of MoO3/ZrO2.

TiO 2–SiO 2 mixed oxide modified with H 2SO 4: II. Acid properties and their SCR reactivity

The acid properties of a series of titania and silica mixed metal oxide samples modified by sulfate ions have been studied by FT-IR adsorbed pyridine. The highest concentration of Brönsted acid sites per unit surface area is obtained in the case of 90 mol% TiO 2–SiO 2/SO 4 2− and pure TiO 2/SO 4 2−, while the number of Lewis acid sites per unit surface area is proportional to TiO 2 contents. It is suggested that the interaction of SO in sulfate complex with bulk TiO 2 phase effectively accommodates the electron from the neighbor Ti–O–Ti bond. But, considering that the Brönsted acid sites on 56 mol% TiO 2–SiO 2/SO 4 2− are decreased as compared with unsulfated sample, it could be suggested that the effect of sulfation on the generation of acid sites also depends on the charge balance of the support and the valence unit in Ti–O–Si bond generated during the formation of mixed oxide. The activity obtained in SCR reaction at 450 °C suggests that the oxygen in Ti–O–Ti easily participates in the NH 3 activation as compared with Ti–O–Si and Si–O–Si bonds and the addition of sulfate can lead to the high reactivity of SCR, due to the acidity increase.

N-Hexane isomerization over Pt–Na(H)Y catalysts obtained by different preparation methods

Catalysts containing 0.5 wt.% of Pt on Na(H)Y zeolite with potentially high acidity were prepared alternatively by ion exchange and wetness impregnation. Their acidity was altered by altitude of calcination temperature and by the order of calcination and Pt–acid decomposition. The catalysts were tested in n-hexane isomerization reaction. It was shown that optimum selectivity with the maximal isomer yield was obtained for the catalyst where calcination and Pt–acid decomposition were hold simultaneously at high temperature. This was attributed to very well-balanced acidic and metallic sites due to optimal metal–support interaction. When the sequence of generation of acid sites was followed by Pt–acid decomposition in a separated temperature sequence, the catalyst with poor activity and selectivity was obtained. This is explained by the lower acidity and poorer metal dispersion as the result of different pretreatment procedure.

Generation of acid sites on silica-supported rare earth oxide catalysts: Structural characterization and catalysis for α-pinene isomerization

Silica-supported rare earth oxide catalysts (Ln/SiO2; Ln=La, Ce, Pr, Sm, Eu, Tb, Yb and Y), loading amounts of which were 3.4 mmol g (support)-1, were characterized by α-pinene isomerization, temperature-programmed desorption (TPD), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), thermogravimetric-differential thermal analysis (TG-DTA) and Raman spectroscopy. In the lanthanoid series, the catalytic activity increased with atomic number from 57La to 70Yb, except for Ce. All the Ln/SiO2 catalysts, except for Ce, were amorphous. On the surface of the catalyst, Ln–O–Si and Ln–O–Ln linkages formed, the ratio of which varied with the loaded element. The ratio of Ln–O–Si linkage increases with stronger affinity among LnOn units and SiO4 tetrahedra, and the affinity depends on the size of Ln3+. With increasing ratio of Ln–O–Si to Ln–O–Ln linkage, the catalytic activity increases. Silica-supported yttrium oxide catalyst, the trivalent ion radius of which is quite similar to that of ytterbium, exhibited the same activity as that of Yb/SiO2. Raman spectroscopic characterization revealed that excess loading of Yb atoms on SiO2-support block Yb–O–Si linkage to form Yb2O3 fine particles. When Yb/SiO2 was pretreated at 1273 K, fine ytterbium silicate crystallites formed. Ln–O–Si linkage without a long-ranged ordering structure was the active site for α-pinene isomerization. Introduction

Acidic and Catalytic Properties of SiO 2-Ta 2O 5 Mixed Oxides Prepared by the Sol-Gel Method

The acidic and catalytic properties of a series of silicon-tantalum mixed oxides containing between 0 and 30 at% tantalum were characterized by temperature-programmed desorption of ammonia, by FTIR spectra of adsorbed pyridine, and by the test reaction of 1-butanol dehydration at 250-300°C. Probe molecule adsorption and catalytic testing show an acid site generation in silicon-tantalum mixed oxides compared to pure tantalum oxide. Both Brønsted and Lewis acid sites are present on the mixed oxide surface. Brønsted and Lewis acid sites seem to be weaker in SiO 2-Ta 2O 5 mixed oxide than in analogous SiO 2-Al 2O 3, since a greater reaction temperature is required to achieve the same conversion in the above test reaction. However, a great advantage of this new solid acid is the production of butenes with 100% selectivity.

Acidity studies of titania-silica mixed oxides

A series of titania-silica mixed oxides were prepared by co-precipitation and sol-gel methods. The mole fraction of titanium was varied from 0 to 1. The effects of preparation method and chemical composition on the physical properties and acidity profiles of mixed oxides were examined. While pure silica was non-acidic and titania contained only Lewis acid sites, Bronsted acidity was created by the Ti-O-Si interaction in the binary oxides. Samples containing a titania: silica mole ratio of 9 ∶ 1 possessed the largest amount of Bronsted acidity. The generation of acid sites and the stability of the oxide was a strong function of preparation method. With all preparation methods, higher calcination temperatures increased crystallinity and decreased the total number of acid sites.

Footprint Catalysis VII. Reinvestigation of the Imprinting Procedures for Molecular Footprint Catalytic Cavities: The Effects of Imprinting Procedure Temperature on the Catalytic Characteristics

Abstract The authors’ imprinting procedures for “molecular footprint” catalytic cavities on a silica (alumina) gel surface were reinvestigated. The increases of temperature during the incorporation of aluminum ions into the silicate matrix and during the footprint-imprinting with a template remarkably affected the catalytic characteristics of the resulting cavities. A change from room temperature to 75—80 °C increased the catalytic specificities (kcat/Km) by a factor of nearly 50. The results also provided experimental proof for the speculated mechanism of footprint formation. That is, the imprinting comprises precedent Lewis acid site generation on a silicate surface and subsequent Lewis acid–template complexation. The resulting acid–template complexes give rise to the footprint cavities upon removal of the template.

Generation of acid sites by SiO 2 deposition on groups IVB metal oxides

An ultra thin silica layer was prepared by chemical vapor deposition of Si(OCH 3) 4 onto titania and zirconia. The relationship between surface structure and acidity was studied by measurement of coverage, IR spectroscopy of probe molecules, and test reactions. Silica was deposited to form a 1 : 1 complex with the oxide cation (-MSi-), until more than 90% of monolayer coverage was obtained. The silica overlayer showed weak Brønsted acidity only sufficient to catalyze the isomerization of 1-butene and the dehydration of tert-butyl alcohol. The acidity could not be correlated with the difference in coordination number or valence of cations but seemed to correlate with the interaction between silica and metal oxide.

Reversibility in the transformation between cationic and metallic silver and the generation of Brønsted-acid sites in the silver salt of dodecatungstophosphoric acid

The 1H FTNMR technique was applied to the characterization of protons in Ag3PW12O40, which had been partially reduced with hydrogen. The intensity of the 1H NMR peak varied reversible with variation of the pressure of hydrogen in the gas phase. The evidence of reversible transformation between Ag+ cations and silver metal particles was also revealed by XRD measurements of silver metal particles and IR measurements of CO chemisorbed on Ag– cations. The catalytic activity of Ag3PW12O40 supported on silica and partially reduced with hydrogen, for isomerization of hexane, was enhanced by the presence of hydrogen in the gas phase at 423 K. These results indicate that the reversibility in the catalytic activity is caused by the generation of acid sites with concomitant reduction of Ag+ cations into silver metal particles; the whole process can be reversed by eliminating hydrogen from the system.

Skeletal isomerization of hydrocarbons over zirconium oxide promoted by Platinum and Sulfate Ion

The genesis of the high activity of zirconium oxide promoted by platinum and sulfate ion (Pt/SO 4 2−--ZrO 2) for skeletal isomerization of butane and pentane in the presence of hydrogen is studied in terms of the interaction of the catalyst with molecular hydrogen. For skeletal isomerization of pentane at 523 K, Pt/SO 4 2−-ZrO 2 showed activity only in the presence of molecular hydrogen, and its activity persisted for a long period. For skeletal isomerization of butane at 523 K, the catalyst showed activity in the absence of hydrogen, but the activity was markedly enhanced in the presence of hydrogen. For pentane and butane skeletal isomerization, the products consisted exclusively of 2-methylbutane and 2-methylpropane, respectively. For a typical acid-catalyzed reaction of cyclopropane ring opening at 373 K, the presence of hydrogen enhanced the activity, but the hydrogen enhancement effect was small. The products consisted exclusively of propene even in the presence of hydrogen: hydrogenation of propene scarcely occurred. Infrared spectroscopic study of adsorbed pyridine showed that by heating the catalyst in the presence of hydrogen in the temperature range 423–623 K, protonic acid sites were formed with concomitant decrease in the number and strength of Lewis acid sites, demonstrating that the protonic acid sites originate from molecular hydrogen. The mechanisms of protonic acid site generation are discussed. It is suggested that molecular hydrogen dissociates on the platinum to hydrogen atoms which undergo spillover on the SO 4 2−-ZrO 2 and convert to an H+ and an e − or H −. The H+ acts as catalytic site for acid-catalyzed reactions.

Preparation and characterization of silica overlayers deposited on magnesia surface by chemical vapor deposition technique.

The silica-overlayer deposited on MgO (SiOx/MgO) is prepared by the chemical vapor deposition of tetramethoxysilane. The surface property of the resulting oxides is characterized by XPS, IR, TPD and titration techniques. The deposition of silica overlayer brings about a change in acid-base property of the surface or the concentration of surface hydroxyls. The generation of Lewis acid sites on SiOx /MgO is evidenced by 1R spectra of adsorbed pyridine. The amount and the strength of acid of SiOx /MgO are found to be greater than those of silica gel. With increasing silanization period (the amount of deposited SiOx), the amount and the strength of acid increase at coverages less than monolayer, while the multilayer deposition of SiOx layer decreases the surface acidity. XPS study shows that the chemical state of Si atoms in SiOx overlayers is different from that in silica gel. It is concluded that the generation of acid sites on SiOx/MgO results possibly from the electronic and structural effects from the MgO substrate through Mg-O-Si bond formed at the interface.

Generation of acid sites on finely divided TiO 2

The CO2 utilization for coupling reaction with methane for the production of C1 building block chemicals was studied. The influence of surface oxides of zirconia-titania catalysts and CO2/methane gas ratios were investigated to promote methane activation and coupling with CO2 for the production of value-added chemicals of acetic acid, ethane and ethylene. The synthesized catalysts by three impregnation methods assisted by microwave irradiation, exhibited different nanoscale crystal sizes and were characterized by various techniques of XRD, TEM, BET, NH3-TPD and H2-TPR. The anatase and tetragonal phases of ZrO2/TiO2 framework were identified and the synthesized catalysts showed an increase of surface acidity with distinct H2 reduction patterns for each catalyst as result of Zr oxide interaction with Ti oxide. Methane activation was observed and the coupling reaction produced ethane and ethylene and was affected via modified catalysts. Despite ongoing reaction, CO2 coupling reaction formulated acetic acid and consumed methyl surface species over catalyst then dimerized to form ethane and ethylene. It was found that the obtained products from CO2 coupling were altered by the variation in methane dissociation due to changing CO2/methane ratios, at which higher CO2/methane ratio (90%:10%) showed the highest methane dissociation and CO2 coupling reaction.

Generation of acidic sites in metal salts of heteropoly acids

Generation of acidic sites in metal salts of heteropoly acids

The Effect of the Difference in the Oxidation State of the Second Component Added to MoO3 on the Generation of the Acidic Property

Abstract The effect of the difference in the oxidation state of the second component added to MoO3 on the generation of acidic sites was studied with regard to Sn2+—Sn4+, Fe2+—Fe3+, Cr3+—Cr6+, and Cu+—Cu2+. It was found that a markedly lager amount of acidic sites is generated by the combination of Sn2+, Fe2+, or Cr3+ with MoO3 than by that of Sn4+, Fe3+, or Cr6+ respectively. As a general feature of metal-oxide combination, it is proposed that the addition of a metal ion in a lower oxidation state to MoO3 brings about a markedly more significant acidity generation than that of a metal ion in a higher oxidation state. An explanation to account for the results was attempted in relation to the reducing function of the second component.

Isomerization and disproportionation of olefins over tungsten oxides supported on various oxides

The isomerizations of 1-butene and cyclopropane and the disproportionations of 1-butene, propylene and ethylene were investigated over tungsten oxides supported on MgO, Al 2O 3, SiO 2, TiO 2 and ZrO 2. A combination of tungsten oxide with TiO 2, in which the amount of tungsten oxide was varied from 1 to 20 mol%, resulted in a generation of acid sites. The acid properties were well correlated with the catalytic activity in the isomerization of 1-butene. The production of acid sites also caused a marked decrease in the cis trans ratio in 2-butene formed. The order of the activity of various supported catalysts for the isomerization of cyclopropane was WO 3Al 2O 3 > WO 3ZrO 2 > WO 3TiO 2 > WO 3 ⪢ WO 3SiO 3 = WO 3MgO = 0, whereas that for the disproportionation of propylene was WO 3TiO 2 ≳ WO 3ZrO 2 > WO 3Al 2O 3 > WO 3SiO 2 > WO 3MgO = WO 2 = 0. An optimum activity was obtained for 5 mol% tungstem oxide on WO 3TiO 2. During the disproportionation reaction of propylene over WO 3TiO 2, the formation of i-butene was found. Production of propylene was also found in the reaction of ethylene over WO 3TiO 2. The formation i-butene from propylene and propylene from ethylene suggests that the reaction takes place via carbene intermediates.

The acidic properties of TiO2-SiO2 and its catalytic activities for the amination of phenol, the hydration of ethylene and the isomerization of butene

Abstract The generation of strong acid sites was observed on the binary oxide, TiO2-SiO2, which was obtained by a coprecipitation method. The acidity and acid strength of the mixed oxide of a certain composition were estimated to be higher than those of SiO2-Al2O3 containing 20 wt% of Al2O3 by n-butylamine titration and by the adsorption of basic molecules on the surface. The acidity depended both on pretreating temperature and the catalyst composition. The highest acidity per unit weight of catalyst was observed when TiO2-SiO2 (mole ratio = 1) was heated at 500 °C. The binary oxide showed high catalytic activity and selectivity for the amination of phenol with ammonia to produce aniline, but low activity and selectivity for the hydration of ethylene. The isomerizations of butenes were also catalyzed; the products ratio of cis-/trans-2-butene was 1–2. The catalytic action for three reactions is well interpreted in terms of the acidic property. The mechanism of the generation of acid sites by mixing TiO2 with SiO2 is discussed.