Thermodynamic studies of donor binding to heterogeneous catalysts

Lim, Yau Yan; Drago, Russell S.; Babich, M.W.; Wong, Ngai M.; Doan, Peter E.

doi:10.1021/ja00235a026

Cited by 20 publications

(15 citation statements)

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“…Calorimetric Measurements. Acidity measurements by calorimetry , were carried out using 1 g of catalyst slurried in cyclohexane (dried over P 2 O 5 and distilled) with pyridine solution. Injections of known quantities of pyridine were made to the stirred catalyst in cyclohexane, and the heat evolved from the reaction was measured after each injection.…”

Section: Methodsmentioning

confidence: 99%

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

Kob

Drago

et al. 1997

J. Am. Chem. Soc.

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Solid state NMR, calorimetry, and density functional theory (DFT) all provide a consistent interpretation of the acidity of the solid acid catalyst (SG) n AlCl2, which is prepared by reacting aluminum chloride with conditioned silica gel. These studies firmly establish that the acid sites are Brønsted in nature and that their strength is significantly greater than those in zeolites. Proton NMR results, including experiments exploiting 1H−27Al dipolar couplings, demonstrate that the Brønsted acid sites have an isotropic 1H chemical shift of 5.7 ppm and a concentration of 0.58 mmol/g. The strongest sites on this solid acid, present at 0.03 mmol/g, have −ΔH av values of 52 kcal/mol for reaction with pyridine. A value of 44 kcal/mol is maintained for incremental addition of pyridine up to 0.1 mmol/g. In comparison, −ΔH av for the strongest sites in zeolite HZSM-5 is only 42 kcal/mol. 15N magic angle spinning (MAS) NMR studies of adsorbed pyridine and 31P MAS NMR of trimethylphosphine confirm the Brønsted nature of these acid sites. The 13C isotropic chemical shift of acetone-2-13 C on (SG) n AlCl2 (245 ppm) is identical to that measured in 100% H2SO4. 13C in situ NMR studies of ethylene and propene oligomerization show that the activity of (SG) n AlCl2 is far greater than that of zeolites. Cyclopentenyl carbenium ions are formed in significant yields in those reactions as well as during skeletal isomerization and cracking of cyclohexane at 433 K on (SG) n AlCl2. Local DFT calculations at the SVWN/DZVP2 level were used to provide predictions of the structure and energetics of the catalyst. The acidity (defined as the deprotonation energy corrected for zero-point and thermal contributions) obtained from these calculations ranges from 275.5 to 293.4 kcal/mol. Two of the three (SG) n AlCl2 models considered are more strongly acidic than a HZSM-5 cluster model treated at the same level of theory. The aggregate evidence from this study strongly supports classification of (SG) n AlCl2 as a catalyst with a Brønsted acid strength on the threshold of superacidity.

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Section: Methodsmentioning

confidence: 99%

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

Kob

Drago

et al. 1997

J. Am. Chem. Soc.

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“…However, in any calorimetric or equilibrium measure that involves gas-solid equilibria, the enthalpies contain contributions from a donor-acceptor interaction as well as a dispersion component. The cal-ad measurements are carried out in a hydrocarbon solvent whose molecules are close in molecular mass to those of the donor 18,19 to cancel out the dispersion component. Thus, the cal-ad analysis provides a complete characterization of the acid strength for different sites and determines the number of acid sites of each strength, providing the most complete characterization of the acidity of SZ reported to date.…”

Section: T H Imentioning

confidence: 99%

“…We report the use of results from a cal-ad titration of sulfated zirconia with pyridine to address this problem. Cal-ad differs from other solid acid characterization methods because it determines the number ( n i ), equilibrium constants ( K i ), and strengths (Δ H i , enthalpy) of different acid sites on solids. , The subscripts i refer to the above quantities, where i = 1 would be the first site describing the strongest sites, i = 2 the next site, and so on. In the general area of donor−acceptor bonding, it has long been accepted that the appropriate measure of the strength of binding is the enthalpy of adduct formation.…”

Section: Introductionmentioning

confidence: 99%

Acidity and Reactivity of Sulfated Zirconia and Metal-Doped Sulfated Zirconia

and

Kob

1997

J. Phys. Chem. B

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Since its discovery, sulfated zirconia and metal-doped sulfated zirconia have been the subject of numerous reports on their catalytic activity. Their ability to perform low-temperature hydrocarbon isomerizations has led to claims that sulfated zirconia and metal-doped sulfated zirconia are superacids or at least very strong acids. This has led to many investigations on the acid strength of sulfated zirconia and metal-doped sulfated zirconia producing varying results. We report the use of cal-ad (which uses combined information from calorimetry and adsorption of pyridine onto the solid acid) to determine the acidity of sulfated zirconia and metal-doped sulfated zirconia. We find that sulfated zirconia has two types of acid sites: 24 µmol of a strong site, which has a strength of 31.2 kcal/mol, and 52 µmol of a weaker site of 25.8 kcal/mol, which places the acidity of sulfated zirconia lower than that of HZSM-5 (41 kcal/mol) and about the same as that of HY (34 kcal/mol). Doping sulfated zirconia with 0.2 wt % Pt does not change the acidity, but doping with iron and manganese (which increases the catalytic activity) results in a lower measured acidity, indicating that iron and manganese have occupied the strongest acid sites.

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“…In using eq 2, the minimum number of processes, i , needed to obtain a good fit of the adsorption isotherm are used. As discussed in our earlier report of the cal−ad method, a very shallow minimum results in the solution of the series of simultaneous equations that arise from even a two-process fit of adsorption data for K 1,ads , K 2,ads , n 1 , and n 2 . Uncertain values of the quantities result.…”

Section: Resultsmentioning

confidence: 99%

A New Adsorption Model for Analyzing Gas−Solid Equilibria in Porous Materials

1996

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Equilibria involving three probe gases adsorbed on two porous carbonaceous supports are measured at various temperatures. A novel analysis of the data is offered which uses multiple process equilibria to calculate adsorption equilibrium constants for the interaction of the gas with the solid. Equilibria involving three distinct processes are found. The equilibrium constants (K 1,ads , K 2,ads , and K 3,ads ) are obtained as well as the capacity of the solid for each type of process (n 1 , n 2 , and n 3 ), in millimoles of adsorptive per gram of solid. The first process, K 1,ads , involves adsorption of the gas in the solid's micropores which are of molecular dimensions. The second process, K 2,ads , involves adsorption in the larger micropores. The third process, K 3,ads , involves adsorption by the remaining surface. Multilayer formation is likely involved in some processes. The temperature dependencies of the K ads 's produce the enthalpy of adsorption for these processes. This analysis is important for, in contrast to BET analyses, it provides thermodynamic data for different adsorptives that can be interpreted in terms of those molecular properties that facilitate probe-solid interactions and can provide a quantitative definition of solid reactivity.

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Thermodynamic studies of donor binding to heterogeneous catalysts

Cited by 20 publications

References 0 publications

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

Acidity and Reactivity of Sulfated Zirconia and Metal-Doped Sulfated Zirconia

A New Adsorption Model for Analyzing Gas−Solid Equilibria in Porous Materials

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