Strong Solid Acid Research Articles

Titanium Niobate and Titanium Tantalate Nanosheets as Strong Solid Acid Catalysts

HTiNbO5, HTi2NbO7, and HTiTaO5 nanosheets obtained by exfoliation of protonated cation-exchangeable layered metal oxides are examined as solid acids. These nanosheets exhibit high catalytic activit...

Al- and Ga-promoted WO 3/ZrO 2 strong solid acid catalysts and their catalytic activities in n-butane isomerization

A series of Al- and Ga-promoted tungstated zirconia strong solid acid catalysts were prepared. The effect of Al 2O 3 and Ga 2O 3 on the structural, acidic and redox features of WO 3/ZrO 2 (WZ) has been characterized by means of X-ray powder diffraction (XRD), N 2 adsorption (BET), UV–visible diffuse reflectance spectra (UV–vis), infrared spectra (IR), NH 3 temperature-programmed desorption (NH 3-TPD) and H 2 temperature-programmed reduction (H 2-TPR). With respect to tungstated zirconia, the promoted catalysts stabilized the zirconia tetragonal phase, prevented crystalline WO 3 growth. Al or Ga addition to WZ had less effect on the strength of the acid sites, but effected reduction temperature of WZ. The isomerization of n-butane was investigated at 300 °C over tungstated zirconia and promoted catalysts. The promoted catalysts improved the isomerization conversion and stability in the presence of hydrogen. The best Al and Ga loading in these catalysts is around 0.5 and 1.0%, respectively, and the optimal calcination temperature is ∼850 °C. The experiment results showed that Ga was a more efficient promoter than Al. Pt addition to promoted catalysts showed great improvement on the performance of selectivity and stability.

Highly Efficient Deprotection of Acetals by Titanium Cation‐Exchanged Montmorillonite as a Strong Solid Acid Catalyst.

Deprotection of various kinds of acetals sufficiently occured in the presence of Ti4+-exchanged montmorillonite as a recyclable strong solid acid catalyst.

Structural insights into aluminum chlorofluoride (ACF).

The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlCl(x)F(3-x), x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, (19)F MAS NMR, and (27)Al SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of alpha-AlF(3) x 3H(2)O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF(3) prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF(6) octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AlF(3). Terminal fluorine atoms were detected in ACF by (19)F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl(3) phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.

Catalytic properties of heteropoly acid/zeolite hybrid materials: toluene disproportionation and transalkylation with 1,2,4-trimethylbenzene

A series of hybrid heteropoly acid (HPA)/zeolite catalysts has been prepared by supporting 12-tungstophosphoric acid (H3PW12O40) on a faujasitic matrix. It has been shown that the structure of a dealuminated zeolite Y is retained upon contact with a very strong solid acid. The catalytic properties of the hybrid catalysts were examined in the two gas-phase reactions: disproportionation of toluene and transalkylation of toluene and 1,2,4-trimethylbenzene (1,2,4-TMB). All the hybrid catalysts exhibit enhanced catalytic activity in both processes. Already the small amounts of heteropoly acid exert significant influence on the activity in both reactions studied. A pronounced shape-selectivity, expressed as p-xylene/o-xylene ratio, is evident both in toluene disproportionation and its transalkylation with 1,2,4-TMB at low temperatures, while equilibrium distribution of xylene isomers is typical at higher temperatures. As evidenced by 31P NMR, two types of the Keggin unit are present in the catalysts. They are located in the mesopores of zeolite and on the zeolite crystals, respectively. The Keggin units interacting strongly with the support, characterised by chemical shift at −13.3ppm and located in mesopores are probably the more active ones in title reactions.

Preparation and characterisation of chemisorbents based on heteropolyacids supported on synthetic mesoporous carbons and silica

The preparation of chemisorbents based on tungsto- and molybdophosphoric acids supported on two types of synthetic mesoporous carbons and two types of mesoporous silica is described. Strong solid acids with good accessibility to acid sites may potentially be effective adsorbents for the removal of basic molecular impurities, such as amines, from ultrapure manufacturing environments. Prepared materials were characterised by scanning electron microscopy, nitrogen adsorption, Fourier-transform infrared spectroscopy, powder X-ray diffraction, and equilibrium ammonia uptake. Composites of SBA-15 with heteropolyacids were synthesised. It was shown that the inclusion of HPAs into SBA-15 results in the loss of long range order. Adsorbents based on the HPAs impregnated into the supports with the open-pore morphology (Novacarb and SBA-15) were found to be promising materials. A composite of tungstophosphoric acid with sol–gel SiO 2 was found to have the highest ammonia uptake.

Highly Efficient Deprotection of Acetals by Titanium Cation-exchanged Montmorillonite as a Strong Solid Acid Catalyst

Abstract Deprotection of various kinds of acetals sufficiently occured in the presence of Ti4+-exchanged montmorillonite as a recyclable strong solid acid catalyst.

The Zeolite Micropore as a Unique Reaction Field for the Selective Catalytic Reduction of NO by Hydrocarbon

AbstractFor Abstract see ChemInform Abstract in Full Text.

Exfoliated nanosheets as a new strong solid acid catalyst.

Two-dimensional metal oxide sheets in HTiNbO(5) and HSr(2)Nb(3)O(10), cation-exchangeable layered metal oxides, were examined as solid acid catalysts. Exfoliation of HTiNbO(5) and HSr(2)Nb(3)O(10) in aqueous solutions formed colloidal single-crystal TiNbO(5)(-) and Sr(2)Nb(3)O(10)(-) nanosheets, which precipitated under an acidic condition to form aggregates of HTiNbO(5) nanosheets and HSr(2)Nb(3)O(10) nanosheets. Although esterification of acetic acid, cracking of cumene, and dehydration of 2-propanol were not catalyzed by original HTiNbO(5) because of the narrow interlayer distance, which prevents the insertion of organic molecules, HTiNbO(5) nanosheets functioned as a strong solid acid catalyst for the reactions. Nanosheets of HSr(2)Nb(3)O(10) exhibited no or slight catalytic activity for these reactions. NH(3) temperature-programmed desorption and (1)H magic-angle spinning nuclear magnetic resonance spectroscopy revealed that HTiNbO(5) nanosheets have strong Brønsted acid sites, whereas HSr(2)Nb(3)O(10) nanosheets do not.

Regioselective H/D isotope exchange and skeletal rearrangement reactions of propane over strong solid acids

The regioselectivity of the hydrogen/deuterium exchange reaction of propane over D 2O-exchanged ZSM-5 zeolite, unpromoted (SZ), and Al 2O 3-promoted sulfated zirconia (SZA) using a batch recirculation reactor was studied by means of 1H and 2H liquid state NMR spectroscopy. Low-temperature H/D exchange, below 100 °C, between acidic deuterium of the solid surface and propane was observed only on sulfated zirconia-based catalysts involving specifically the methyl protons. Detection of deuterium in the methylene position however occurred at higher temperatures as a consequence of skeletal rearrangement of the intermediate carbenium ion, evidenced by the 13C scrambling in propane 1- 13C as well as by the 2H scrambling in 1,1,1,3,3,3 propane- d 6 and 2,2-propane- d 2. The regioselectivity of the initial H/D exchange and the label redistribution observed at higher temperatures support the assumption that the exchange between propane and catalyst acidic surfaces proceeds via carbenium-ion-type intermediates and exclude possible pentavalent carbonium ion intermediates under these conditions.

The Zeolite Micropore as a Unique Reaction Field for the Selective Catalytic Reduction of NO by Hydrocarbon

Zeolite micropores offer unique reaction fields for catalytic reactions based on narrow pores, a wide variety of channel structure, ion exchangeability and strong solid acids. As a negative effect, narrow pore and channels may suppress the diffusivity of reactant molecules and reduce the catalyst effectiveness factor. This article focuses on the influence of the intracrystalline unique reaction fields of zeolite on the selective catalytic reduction of NO by hydrocarbons (HC-SCR) under extreme conditions, i.e. high space-velocity ratio and low content of reactants in exhausts. Zeolite diffusivity in the HC-SCR is affected by two types of diffusion: geometry-limited diffusion, which is determined by the relative size of diffusing molecule and zeolite pore-opening, and adsorption-controlled diffusion, which is determined by the strong interaction between diffusing molecule and intracrystalline cation.

Study of the alkylation of isobutane with n-butenes over WO 3/ZrO 2 strong solid acid. III. Effect of superfine ZrO 2 support on physico-chemical properties and catalytic behavior of the catalyst

Study of the alkylation of isobutane with n-butenes over WO 3/ZrO 2 strong solid acid. III. Effect of superfine ZrO 2 support on physico-chemical properties and catalytic behavior of the catalyst

Study of the Alkylation of Isobutane with n-Butene over WO3/ZrO2 Strong Solid Acid. 1. Effect of the Preparation Method, WO3 Loading, and Calcination Temperature

A series of strong solid acids composed of WO3/ZrO2 were prepared. Their crystal structure, surface state, and acidity were determined by the methods of X-ray diffraction, thermal gravimetric and differential thermal analysis, temperature-programmed reduction, laser Raman, and acidity measurement. The results revealed that ZrO2 in WO3/ZrO2 existed mainly in the tetragonal phase, the addition of WO3 plays an important role in stabilizing the tetragonal phase of ZrO2, and all of the samples possessed large surface areas. WO3 in WO3/ZrO2 is mainly monolayer dispersed, and a small amount crystallized on the ZrO2 surface and partly reacted with ZrO2 to form the bond of Zr-O-W, acting as the strong solid acid center. The catalytic properties of WO3/ZrO2 strong solid;acids for alkylation of isobutane with butene at different conditions were investigated. They had a better reaction performance than other strong solid acids; a parallel relationship could be drawn between the catalytic activity and the acid amounts as well as the acidic strength of the catalysts.

Probing the Surface Polarity of Various Silicas and Other Moderately Strong Solid Acids by Means of Different Genuine Solvatochromic Dyes

Reichardt's ET(30) as well as Kamlet−Taft's α (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and alumina-functionalized silica particles, are presented. The ET(30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorine-substituted derivative of 1a (1b) as surface polarity indicators. Kamlet−Taft's α and π* parameters for the various solid acids are analyzed by means of Fe(phen)2(CN)2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler's ketone [4,4‘-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarity indicators. The chemical interpretation of the α and π* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe ...

Catalytic Activities of Various Moderately Strong Solid Acids and Their Correlation with Surface Polarity Parameters

The specific rate constant k‘ of the surface-mediated hydride-transfer reaction of 1.4-cyclohexadiene with triphenylmethylium is strongly dependent on the nature of the solid acid catalyst used. Th...

Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes

Abstract Acid-catalyzed hydrocarbon reactions involve by far the largest amount of catalyst and the largest volume of transformation in oil refinery and chemical industry. However, despite the general agreement on the carbocationic nature of the reaction intermediates, the initial steps and the true nature of the cations on the surface are still open to debate. On these points our basic knowledge has gained enormously from landmark experiments in physical organic chemistry with spectroscopic observations using liquid superacids as solvents. As the range of superacidity covers over 11 logarithmic units of the Hammett acidity scale, the mechanistic behavior of small alkanes vary accordingly. This allows for comparison with strong solid acids such as zeolite or others used in industry. Isotopic labeling of the alkane and the acids is a powerful tool used to understand the nature of the intermediates, as well as the reaction mechanism in which carbenium and carbonium ions intervene.

Acid Strength of Sulfated Zirconia Inferred from Catalytic Isobutane Conversion

Abstract The kinetics of the catalytic conversion of isobutane under gas-phase fixed-bed flow conditions over sulfated zirconia (SZ), when compared with those of other strong solid acids of known acidity, suggests that SZ may be a very strong solid superacid; this conclusion disagrees with recent reports claiming that SZ is not a superacid, and supports older claims, based on acidity tests with Hammett indicators, that SZ may have H0 of < −16.

Electron energy loss spectroscopy (EELS) of nanocrystals of zirconia and sulfated zirconia strong solid-acid

The strong solid-acid of sulfated zirconia nanocrystals is capable of catalyzing the isomerization of n-butane to isobutane at room temperature. Oxygen atoms at the nanocrystal surface directly affect the rates of catalytic reaction. Electron energy loss spectroscopy results show the difference in the oxygen K-edges of the sulfated and nonsulfated zirconia, especially in the post-edge peaks. It is proposed that the ratio between the height of post-edge peak (h) and the height of K-edge peak (H) of oxygen may be used as an indicator of the acidity and reactivity of the sulfated zirconia.

Synthesis of[formula omitted]-α-Tocopherol Using Strong Solid Acids as Catalysts

The synthesis ofD,L-α-tocopherol starting from trimethylhydroquinone and isophytol using a heterogeneous solid acid catalyst, e.g., Nafion NR 50, is described. Advantages of this new procedure are high yield and selectivity, facile recovery of the catalyst, no waste problems, and mild reaction conditions.

A Solid Acidity Scale Based on the31P MAS-NMR Shift of Chemisorbed Triethylphosphine Oxide

The change in chemical shift of a chemisorbed probe molecule is an informative tool for the characterization of solid acids. This work demonstrates that the phosphorus-31 isotropic chemical shift of the probe triethylphosphine oxide (TEPO) can be used to identify the acidity of multiple acid sites on a surface. The difference in the phosphorus-31 chemical shift of chemisorbed TEPO with respect to physisorbed TEPO, Δδ, establishes a scale of solid acidity that can be validated by comparison to enthalpies of calorimetric methods. A linear correspondence is observed for the strong solid acid (silica-gel)nSbVCl3with four acid sites and with enthalpies of −66, −46, −33, and −21 Kcal/mol for interaction of the solid acid with pyridine in a cyclohexane slurry that correlate with Δδ values of 66, 48, 36, and 22 ppm, respectively.