Jingzhong Wang

Lewis acid sites on dehydroxylated zeolite HZSM-5 studied by NMR and EPR

Abstract Zeolite HZSM-5 calcined at different temperatures were studied by 29 Si MAS NMR and EPR methods. It is proposed that over a certain temperature range, dealumination did not necessarily occur simultaneously with framework dehydroxylation and thus framework Lewis acid sites may emerge. A two-step mechanism of dehydroxylation may be present, and it dominates the dehydroxylation and dealumination processes at different temperatures.

Synthesis and characterization of silicon and cobalt substituted mesoporous aluminophosphates

Abstract A variety of silicon and cobalt substituted mesoporous aluminophosphate materials with different incorporated contents were prepared in the presence of supramolecular template alkyltrimethylammonium cations. The characterization of these solid products by various techniques such as XRD, N 2 adsorption, DRS, and 27 Al, 31 P MAS NMR spectroscopies has been carried out to understand the local atomic arrangements and the coordination environment. Si and Co can be introduced into the framework of aluminophosphate-based mesoporous materials, respectively. About 41 wt.% of the templating surfactant can be extracted by acid alcohol, however this treatment can not efficiently enhance the thermal stability of these products.

Nonaqueous synthesis: Iron aluminosilicates with the ZSM-48 structure

Abstract A crystalline, iron-containing material having framework structure of the ZSM-48 zeolite has been synthesized in a nonaqueous system of triethylamine and ethylenediamine. The XRD patterns of the as-synthesized as well as the calcined FeZSM-48 matched well with that of ZSM-48 synthesized hydrothermally; replacement of Al 3+ by Fe 3+ in the zeolite lattice was confirmed by spectroscopic (XRD, framework i.r., and e.s.r.) results, and the M 3+ (M = Al, Fe)-rich composition of the ZSM-48 materials was determined. Framework i.r. bounds of ZSM-48 are shifted toward lower frequencies on incorporation of iron in the framework; the white color of the as-synthesized as well as that calcined samples of iron-subsituted ZSM-48 and e.s.r. (g = 4.3) data indicate that FeOFe interactions are absent.

Synthesis of mesostructured lamellar aluminophosphates in the presence of alkylpyridinium cationic surfactant

A series of mesostructured lamellar aluminophosphates and silicoaluminophosphates have been synthesized hydrothermally using alkylpyridinium bromide surfactant templating or mixed surfactants of alkylpyridinium and cetyltrimethylammonium bromide. The synthesis of present aluminophosphate-based mesostructured materials can be performed within the following molar gel composition: Al2O3:(0 8-1.9) P2O5:(0.3-1.0) CPBr:(0.8-4.2) TMAOH:(100-350) H2O:(0-1.0) SiO2 at various temperatures in the range of 80-150 degreesC for 0.5-4 days. The effect of various synthetic factors, such as P/Al ratio, TMAOH/Al ratio, water content, surfactant concentration, synthesis temperature, and reaction time, has been discussed. The addition of Si in the mixture gel requires more base content. The interlayer distance of materials decreases with either increasing water content in the reaction mixture or decreasing alkyl chain length of the templating surfactant alkylpyridinium cation. The mesostructured aluminophosphate-based materials were also prepared with templates mixtures of cetylpyridinium bromide and cetyltrimethylammonium bromide. and their interlayer distance of materials can be finely controlled by changing the CPBr/CTABr molar ratio. Al-27 and P-31 nuclear magnetic resonance spectra have been carried out to understand the local atomic arrangements and coordination environments of these solid products

A simulated annealing study of Si, Al distribution in the omega framework

Abstract The Si,Al framework distribution of zeolite omega is studied by three different models, in which the model 3 provides numerical results manifesting the fact that site T B , one of the two crystallographically unequivalent tetrahedral sites, is preferentially occupied by Al atoms in parent zeolite omega. The dependence of the partitioning ratio of Al atoms in the two crystallographically unequivalent tetrahedral sites, on the Si/Al is predicted correctly. The agreement between the calculated Si building units, {Si k ( n Al); k=A or B, n =0–4}, and that obtained from 29 Si MAS NMR is excellent.

A simulated annealing study of Si,Al distribution in the faujasite framework

Abstract Several important properties of faujasite are concerned with the Si,Al distribution in its framework. In the present study, a starting configuration of the distribution is generated with randomly positioned Si,Al. After the simulated annealing procedure, some orderings appear in the optimized configurations of the system and lead to a series of consequences, some of which have already been obtained by pioneer studies concerning ordering schemes. In optimized configurations, rations, a certain degree of disorder reasonably is maintained because of the use of standard Monte Carlo finite temperature techniques, which bring about a realistic employ of the numerical simulation. The calculated Al distribution on the first shell of the Si site is assessed by29SiMAS n.m.r. experiments to demonstrate the rationality of the present theory. The calculated Al distribution on the second shell of the Al site is related to the acidity, which leads to the proposition that the acid amount of faujasite is in proportion to the number of single Al four rings. The electrostatic energy calculation provides possible explanations of the observed discontinuity in the plot of unit cell parameter against Al content.

Studies of mordenite by the Monte Carlo method

Abstract A model on the distributions of Al atoms in the framework of parent modernites is suggested. The distribution of Al atoms in the framework of dealuminated mordenite is determined by both the parent sample and its follow-up dealumination. The continuous dealumination is simulated by a discrete stochastic process (Markov chain). The relative populations of the building units {Si(n-Al)} and the types of Al atom {Si(m-Al)} for parent mordenites with different Si Al ratios and the evolutions of the relative populations of {Si(n-Al)} and {Si(m-Al)} during the dealumination can then be calculated by Monte Carlo method. Since the relative populations of both {Si(n-Al)} and {Al(m-Al)} are obtained from the same sample space of the Monte Carlo simulation, their correlation is established from a reliable theoretical foundation. The former, calculated {Si(n-Al)}, can be compared directly with 29Si MAS n.m.r. observations of parent and dealuminated mordenites to prove the reasonability of the present theory. The latter, calculated {Al(m-Al)}, predicts the acidic properties of the parent and the dealuminated mordenites. The definition of the efficiency parameter of type acid site, which scales the contribution of a type acid site toward the acidity, is introduced. Based on this definition, the acid amount and the efficiency parameter αo (introduced by Beaumont and Barthomeuf for faujasites) in a mordenite sample can be expressed quantitatively. The present theory satisfactory explains the ammonia t.p.d. acidity measurement results for both the parent and dealuminated mordenite samples.

SIMULATED ANNEALING STUDY OF CATION DISTRIBUTION IN DEHYDRATED ZEOLITES

Abstract A simulated annealing method is employed to describe the cation distribution in dehydrated zeolites. The cation distribution in faujasite-type zeolites with 56 cations per unit cell as a function of temperature is simulated and the results are compared with that obtained by Van Dun et al. [J.J. Van Dun, K. Dhaeze, W.J. Mortier and D.E.W. Vaughan, J. Phys. Chem. Solids, 50 (1989) 469] who used the Bragg-Williams approximation and the quasi-chemical-equilibrium method in their statistical thermody namical model. The Na-ion distribution in faujasite-type zeolites can be accurately simulated through adjusting three para meters, i.e. eI − eI and eII − eI, the site energy level differences between the cations on the different cation sites, and w, the repulsive energy between cations on adjacent sites I and I′.

A simulated annealing study of cation distribution in dehydrated Na-exchanged faujasite zeolites

Abstract A simulated annealing method is employed to study the cation distribution in dehydrated Naexchanged faujasite zeolites. By use of the same three parameters, i.e. e I ′ - e I , e II - e I , the site energy level differences between the cations on the different cation sites I, I′ and II, and w , the repulsive energy between cation pairs located on adjacent sites I and I′, the cation distribution in dehydrated NaHY-type zeolites (with a Si/Al ratio of 2.46) with varying cation loading or temperatures could be accurately predicted. These parameters were determined by fitting the simulated results with experimental data report in the literature. It is found that the influence of the framework Al content, N AI , on the site energy level differences can be expressed as e I′ -e I = − 23.0490 + 0.5605 N AI KJ mol −1 and e II -e I = −22.4770 + 0.4437 N AI KJ mol −l . By use of these linear relations and a refined w = 40 KJ mol − l , the cation distribution of dehydrated Naexchanged faujasite with various Al content could be predicted excellently.