Heat Of Ammonia Adsorption Research Articles

Shape selectivity in acidic zeolite catalyzed 2-pentene skeletal isomerization from first principles

• The “dimethylcyclopropane” pathway is dominant for 2-pentene isomerization. • The ring-opening step is the rate-determining step for the isomerization reaction. • Van der Waals interactions would change the scaling relations in zeolite catalysis. Periodic density functional theory (DFT) calculations have been performed to examine the monomolecular reaction mechanism for 2-pentene skeletal isomerization catalyzed by acidic zeolite Beta, Mordenite, and ZSM-5. The use of periodic models allows consideration and analysis of zeolite steric constraints that occur within zeolite microspores. Three different reaction pathways, the “dimethylcyclopropane” (DMCP), methyl shift, and ethyl shift mechanisms, have been considered. The DMCP mechanism which includes rearrangement of protonated cyclopropane is proposed as the dominant reaction pathway, and the ring-opening step determines the overall reaction rate. Comparison of the effective energy barriers for 2-pentene isomerization over the three different zeolites reveals that large 12-ring channels of Beta and MOR have little steric hindrance effect on 2-pentene isomerization while small 10-ring of ZSM-5 significantly increases the reaction activation energy. Calculated results also indicate that the adsorption heat of ammonia is a good descriptor for the reactivity of zeolites without taking into account long-range dispersive forces. In contrast, linear scaling relations cannot be obtained by using the BEEF-vdW functional because van der Waals interactions between zeolites framework and guest molecules affects the energetics of the isomerization reaction and changes the transition state energy scaling relation in zeolite catalysis.

Correlation of the cracking activity with solid acidity and adsorption property on zeolites

Activities for catalytic cracking of octane and hexane were measured on various zeolites, and the dependence of the turnover frequency and the activation energy on the characterization parameters was studied. The heats of ammonia and alkanes adsorption were measured to be correlated with the catalytic activities. From the relations between the heats of alkane and ammonia adsorption, zeolites were divided into two Groups I and II. In the zeolites of Group I on which a close relation between heats of alkane and ammonia adsorptions was observed, a compensation relation between the heat of alkane adsorption and activation energy was observed, but the so-called constant intrinsic activation energy was not confirmed. In the zeolites of Group II on which the relation between the heats of adsorption was not observed, the compensation relation did not exist. Thus, the dependence of the cracking activity upon characterization parameters was summarized: i.e., the heat of alkane adsorption cannot be utilized as a parameter to elucidate the activities of a few zeolites in Group II: on the other hand, the heat of ammonia adsorption can be totally correlated with these activities. Thus, the prevailing role of the solid acidity in the alkane cracking is proven. Finally, the cracking activity is discussed based on the solid acidity.

Transesterification of rapeseed oil with ethanol over heterogeneous heteropolyacids

Abstract Heterogeneized heteropolyacids, H3PW12O40/SiO2 and Cs2HPW12O40 were shown to catalyze rapeseed oil transesterification with ethanol in mild conditions. The number and the strength of the acid sites were evaluated by calorimetry. Over anhydrous H3PW12O40 and Cs2HPW12O40, acid sites of homogeneous strength with heat of ammonia adsorption near 200 kJ mol−1 were measured. By contrast, H3PW12O40/SiO2 showed sites of lower strength with a heterogeneous distribution. The catalytic activity did not correlate the acid strength of solid catalysts. Indeed, equivalent TOF were determined on H3PW12O40 and silica supported H3PW12O40 while lower TOF were measured on microporous Cs2HPW12O40. The TOF values were interpreted in terms of leaching of the active phase in the case of H3PW12O40/SiO2 while reduced acid sites accessibility might explain the lower TOF measured on Cs2HPW12O40.

Superacidity and Catalytic Activity of Sulfated Zirconia

The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm-2, and the adsorption heat of ammonia was ca. 200 kJ mol-1. This corresponds to −19 of the H0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm-2), an adsorption heat of ca. 160 kJ mol-1, an H0 of −12, and Bronsted nature. The former was active for the Friedel−Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butan...

Characterization of the Acid Properties of Tungsten/Zirconia Catalysts Using Adsorption Microcalorimetry and n-Pentane Isomerization Activity

Ammonia adsorption microcalorimetry was conducted on various solid acid tungsten/zirconia catalysts prepared by different techniques. The calorimetric data were compared to catalytic test results using n-pentane isomerization as a measure of acid activity. The results show that (1) the co-precipitation method of making the tungsten/zirconia catalyst produces a greater number of acidic sites than impregnating tungsten on hydrous zirconia, resulting in a more active catalyst, and (2) the addition of small amounts of iron to the tungsten/zirconia catalyst increases the acid site strength as determined by ammonia adsorption and improves the paraffin isomerization activity. The calorimetry data indicate that the acid site strength of the tungsten/zirconia materials is similar to or slightly higher than that found in zeolites or sulfated zirconia and is comparable to sulfuric acid. However, the paraffin isomerization activity results suggest that the acid sites of the tungsten/zirconia catalyst should be about four orders of magnitude more active than that of zeolite β on the basis of turnover frequency. Our experimental results indicate a lack of correlation between the heat of ammonia adsorption with catalytic activity. Comparisons of catalytic activity between materials based entirely on acid strength may not be valid, and kinetic probes would be more appropriate.

Tungsten Oxide Monolayer Loaded on Zirconia: Determination of Acidity Generated on the Monolayer

The benzaldehyde-ammonia titration (BAT) method clarified that the tungsta impregnated on zirconia formed a monolayer and almost completely covered the surface with ca. 4 W nm-2. Generation of acidity on the monolayer was shown by the water vapor treatment method of ammonia TPD (temperature-programmed desorption). Strength of the acid site was calculated according to the theoretical equation to be ca. 130 kJ mol-1 in adsorption heat of ammonia, which does not correspond to superacidity. The generated acid site was active for skeletal isomerization of butane.

Acidity Differences between Inorganic Solids Induced by Their Framework Structure. A Combined Quantum Mechanics/Molecular Mechanics ab Initio Study on Zeolites

The influence of the zeolite framework type (FAU, CHA, MOR, MFI) and the crystallographic position on the acidity of zeolites is investigated. The most stable Brønsted acid sites of the high-silica frameworks are considered: O1−H (FAU), O1−H (CHA), Al4−O2(H)−Si (MOR), and Al7−O17(H)−Si4 (MFI, sinusoidal channel). The latter is compared with the less stable Al12-O24(H)-Si12 position (MFI, channel intersection). Both the heat of the deprotonation and the heat of ammonia adsorption are considered as measures of acid strength. A novel hybrid computational scheme is used that combines the quantum mechanical cluster description (QM) of the active site with interatomic potentials (Pot) for the periodic zeolite framework. Specifically, the Hartree−Fock method (QM) is combined with ab initio shell model potentials (Pot) for the zeolite framework and its interaction with ammonia and ammonium ions. Complete relaxation of the framework is possible within this scheme and long-range corrections to the reaction energies are obtained from the shell model potentials. The total QM-Pot reaction energies are remarkably stable with increasing cluster size. The calculated heats of deprotonation suggest the acidity sequence Y (1171 kJ/mol) > CHA (1190 kJ/mol) > MOR (1195 kJ/mol) > ZSM-5 (1200 kJ/mol), which is neither explained by local structure effects nor by crystal potential effects alone. The calculated heats of NH3 adsorption suggest the sequence MOR > CHA ≈ Y > ZSM-5. The different order is caused by specific interactions of NH4+ with the negatively charged catalyst surface. The predicted heats of NH3 adsorption are −119, −114, −113, and −109 kJ/mol, respectively. Comparison is made with microcalorimetry and TPD data.

Hydroisomerization and Hydrocracking of Alkanes: 7. Influence of the Balance between Acid and Hydrogenating Functions on the Transformation ofn-Decane on PtHY Catalysts

The transformation ofn-decane was studied at 473 K, 101 kPa andpH2/pn-decane=9 on a series of PtHY catalysts containing from 0.02 to 1.5 wt% platinum and with Si/Al atomic ratios of 3, 9, or 35. The ratio between the number of accessible Pt atoms and the number of acid sites on which the heat of ammonia adsorption is greater than 100 kJ mol−1(nPt/nA) was chosen for characterizing the balance between the hydrogenating and the acid functions. The activities, stabilities, and selectivities of the catalysts are definitely governed by this balance. For low values ofnPt/nA(<0.03), the activity per acid site is low, the deactivation is rapid andn-decane leads directly to all the isomerization and cracking products. For high values (≥0.17) the activity per acid site is maximal, the deactivation is very slow andn-decane transforms successively into monobranched isomers, dibranched isomers, and tribranched isomers plus cracking products. In this latter case the catalyst can be considered as anidealbifunctional catalyst, namely a catalyst on which only one transformation of the alkene intermediates on the acid sites can occur during their diffusion from the platinum sites on which they are generated to those on which they are hydrogenated. Therefore the reaction scheme ofn-decane transformation matches the reaction scheme of olefinic intermediates. From the product distribution established on ideal catalysts the mechanism of the transformation of olefinic intermediates can be deduced. Branching isomerization occurs through protonated cyclopropane and cyclobutane intermediates. The cracking products result from the β-scission steps of tribranched decenes which involve two tertiary carbenium ion intermediates (mode A) and from the β-scission steps of dibranched decenes which involve one tertiary and one secondary carbenium ion intermediate (mode B). The participation of the β-scission of mono, di, and tribranched decenes through mode C (via two secondary carbenium ion intermediates) is negligible. The rate constants of the various steps involved inn-decane transformation are estimated. They can be classified in the following order: A cracking>Methyl shift>Branching>B cracking>>C cracking.

Selective Poisoning and Deactivation of Acid Sites on Sulfated Zirconia Catalysts forn-Butane Isomerization

Reaction kinetics measurements, selective poisoning, and microcalorimetry were used to study the activity, selectivity, and deactivation of acid sites on a sulfated zirconia catalyst forn-butane isomerization at 423 K. The sulfated zirconia catalyst has a distribution of acid site strengths, containing 50 μmol/g of strong acid sites characterized by heats of ammonia adsorption from 125 to 165 kJ/mol. The strongest acid sites (heats from 145 to 165 kJ/mol) are responsible for the high initial activity of the catalyst, but these sites deactivate rapidly under reaction conditions. The acid sites exhibiting heats from 125 to 145 kJ/mol are less active than the stronger sites, but deactivate more slowly. Weaker sites have low activity and deactivate more rapidly after the intermediate sites are deactivated or poisoned. Brønsted acidity is necessary for extended catalytic activity, although Lewis acid sites may play a role in generating the initial high activity. Acid sites with heats of ammonia adsorption from 125 to 165 kJ/mol show good selectivity for production of isobutane (ca. 93%), while sites with heats from 120 to 125 kJ/mol show lower isobutane selectivity (ca. 80%).

Effect of Si to Al substitution at next-nearest neighbor sites on the acid strength: ab initio calculation of the proton affinity and the heat of ammonia adsorption

The acidity variations upon Si to Al substitution at next-nearest-neighbor (NNN) sites were studied by ab initio molecular orbital calculations. Proton affinity and heat of ammonia adsorption were calculated when one NNN Si was replaced by Al. There are three cases out of nine Si to Al substitutions at NNN sites in which the acid strength increased, contrary to our expection. The analysis revealed that the acidity variation was derived from the change of the electric field. The disagreement between the heat of ammonia adsorpttion and proton affinity implies that they are not the equivalent measure of acidity.

Comparative Calorimetric Studies of the Acidity of Zeolites by Static and Temperature-Programmed Methods of Ammonia Adsorption and Desorption

A combined TG-DSC 111 apparatus from Setaram has been used for the characterization of HZSM5, H-Y and H-Ferrierite zeolites. Starting from the NH 4 forms it was possible to determine the weight loss under vacuum at increasing temperature up to 863 K, the subsequent weight gain upon ammonia adsorption at room temperature and finally the weight loss at increasing temperature. The heats of ammonia adsorption and desorption during the same experiments could also be determined. The experimental results indicate clearly a great heterogeneity in acid strengths. These results are compared to those obtained by a static method using a classical gas-handling line linked to a heat-flow microcalorimeter.

Superacid sites in zeolite H-mordenite

The presence of superacidic sites in fluorinated H-mordenite has been proved by calorimetric measurement of the differential heat of ammonia adsorption, catalytic testing for paraffin conversion at low temperatures and by means of the indicator test.

Role of structural factors and acidity in conversions of alkylaromatic hydrocarbons on high-silica zeolites

1. On high-silica zeolites of the pentasil family, xylenes are converted to isomerization products with high selectivity. In the presence of wider-pore zeolites, the isomerization is accompanied with a considerable degree of disproportionation, the contribution of which increases with increasing diameter of the zeolite pores. 2. The hydroxyl cover in the pentasils has a far higher thermal stability than in the mordenites; and as a consequence, the pentasils retain high catalytic activities even when heat-treated at 700°C. 3 The acidic centers of zeolites of the pentasil and mordenite types are similar in heats of ammonia adsorption and in their specific activity in xylene conversion.

Heats of adsorption of CO2 and NH3 on synthetic zeolites of various structural types

1. Heats of ammonia adsorption on sodium and lithium type A, G, L, X, and Y zeolites have been measured. 2. In the low-coverage region, the heats of ammonia adsorption by Li zeolites are substantially higher than those for adsorption by Na zeolites. The basic features of the variation of heats of adsorption are preserved when the change is made from Na to Li zeolites. 3. The results that we have obtained are explained on the basis of concepts of possible nonidentity of the effective charges of the Na+ and Li+ cations in the zeolites, and also on the basis of an analysis of the different positions of the cations in their crystal lattices.