Ralph M. Dessau

Spectroscopic evidence for the presence of internal silanols in highly siliceous ZSM-5

Abstract FT i.r. and MAS n.m.r. evidence is presented which establishes the presence of internal silanols in ZSM-5. The concentration of internal silanols is shown to increase as aluminium content decreases and they are believed to be responsible for the phenomenon of excess ion exchange previously reported.

On the H-ZSM-5 catalyzed formation of ethylene from methanol or higher olefins

Abstract The distributions of olefins formed in the HZSM-5 catalyzed total conversions of methanol and heptene-1 were examined at low partial pressures as a function of contact time. The results clearly indicated that ethylene was only obtained at long contact times, and then via secondary reequilibration of the primary kinetic olefinic products, propylene and butenes.

Aluminum zoning in ZSM-5 as revealed by selective silica removal

Abstract Treatment of ZSM-5 crystals with an aqueous base results in partial dissolution of the sample with preferential removal of silica. The treated zeolites, which now have lower silica/alumina ratios, exhibit higher exchange capacities and enhanced catalytic activities. Scanning electron micrographs of base-treated large-crystal ZSM-5 samples revealed that highly selective dissolution of the interior of the crystals had occurred, while leaving the exterior surface relatively intact. These results provide dramatic evidence for aluminum zoning in large-crystal ZSM-5 synthesized in the presence of quaternary directing agents.

On the mechanism of methanol conversion to hydrocarbons over HZSM-5

Abstract Studies of methanol conversion over HZSM-5 using 13 C-labeled methanol in the presence of various olefins and aromatics revealed the importance of desorption limitation of reaction intermediates, as evidenced by extensive multiple labeling of polymethylbenzenes produced from unlabeled toluene. These studies also indicated that most of the ethylene produced from methanol is not a primary C 1 coupling product, but rather a secondary product derived from methyl attack on C 3 + olefins, as evidenced by the predominant formation of singly labeled ethylene. A classical carbenium ion mechanism incorporating these findings is presented for the conversion of methanol to hydrocarbons over HZSM-5.

Framework topology of AIPO4-8: the first 14-ring molecular sieve

The framework topology of AIPO 4 -8, a synthetic aluminophosphate molecular sieve, has been determined via model building followed by DLS-refinement and powder pattern simulation. AIPO 4 -8 has been shown to possess a unidimensional 14-ring channel system with pore dimensions 7.9×8.7 A.

Base- and acid-catalysed cyclization of diketones over ZSM-5

Abstract The acid-catalysed cyclization of acetonylacetone at 350°C over a 300:1 silicon/aluminum HZSM-5 yielded > 97% 2,5-dimethylfuran. In contrast, the calcined, excess sodium-containing, basic form of this same catalyst produced an entirely different cyclization product, 3-methyl-2-cyclopenten-1-one, in greater than 89% yield under identical reaction conditions. This base-catalysed cyclization reaction can be used diagnostically to determine the presence of basic sites in zeolitic catalysts.

On the presence of internal silanol groups in ZSM-5 and the annealing of these sites by steaming

In a recent interesting letter Dessau et al. presented FTIR and 29Si (CP) MAS NMR results providing further evidence for the presence of internal silanol groups in ZSM5. The decrease in the silanol groups during a steaming procedure accompanied by a dramatic enhancement of the resolution of the @ signal in 29Si MAS NMR spectra was reported ( I). Recently, we described a very similar ordering of the lattice and decrease in internal silanol groups that we observed during prolonged heat treatment in laboratory air of high-silica ZSM-5 in the presence of alkaline ions. By means of XPS the Na+ or K+ ions were found to concentrate at the external surface of the zeolite particles during this procedure (2). Depending on the synthesis procedure we always find considerable amounts of Na+ or K+ ions (typically 0.75 to 1.3 ions per unit cell) in high-silica ZSM-5 even after thoroughly washing with water. They are located in terminal siloxy groups that can be converted into silanol groups upon ion exchange with diluted hydrochloric acid. Regrettably, the amounts of alkaline ions in the samples studied by Dessau et al. are not described very precisely. Only once do they mention an ion exchange with ammonium ions in order to remove Na+. But the corresponding FTIR spectrum does not exhibit the typical v(NH) bands which should indicate that the ion exchange was effective (3). Therefore, we think it to be possible that sodium ions were still present during steaming in that case. However, it is interesting to conclude from Dessau’s work that heat treatment at an increased vapor pressure of water probably accelerates the process of lattice improvement. While the evidence for internal silanol groups in ZSM-5 is rather convincing we would like to propose a different view concerning the arrangement of silanol groups and the mechanism of rearrangement in the lattice. Silylation of silanol groups in our ZSM-5 with trimethylchlorosilane (TCS) and subsequent analyses of the products by means of 29Si CP MAS NMR clearly indicate that silanol groups are not isolated but rather clustered (2, 4). Isolated silanol groups as suggested by Dessau er al. should yield exclusively trimethylsiloxysilanes. Instead, we find the secondary and tertiary products (dimethyldisiloxysilanes and methyltrisiloxysilanes) which can be formed only if further neighboring silanol groups are present. The ease of the formation of tertiary silylation products as compared with the silylation of, e.g., silica gels, indicates that in the ZSMJ samples silanol groups are in arrangements favoring the insertion of TCS. We think that high-silica ZSM-5 typically contains clusters of four terminal groups each (e.g., silanol groups) where T atoms are missing in the lattice. During prolonged heat treatment (or steam-

Observations, modeling and optimization of yield, selectivity and activity during dehydrogenation of isobutane and propane in a Pd membrane reactor

Dehydrogenation of isobutane and propane was carried out in a membrane reactor made of a PdRu (or PdAg) tube packed with a supported Pt catalyst. The shell side was swept by a stream of nitrogen or its mixture with hydrogen. Significant gains in yield were achieved by separating the hydrogen through the selective Pd membrane: up to 76% butene at 500°C (compared with 32% in equilibrium) and 70% propene at 550°C (23% at equ.). The attained yields, however, were limited at low feed rates by suppressed catalyst activity in the absence of hydrogen. To avoid low activity and fast aging, hydrogen concentration should be kept at about 2% by adjusting the shell or tube flow rates. Fast deactivation was observed with high ratios of shell to tube flow rates. The degree of cracking and of isomerisation increases with conversion. Temperature should be kept below 500°C, during butane dehydrogenation, to avoid cracking and fast aging. Yields under high pressures (18 psi for isobutane and 100 psi for propane) were similar to those obtained under atmospheric conditions. Operation under pressure may be advantageous as high purity hydrogen can be produced. The yield dependence on feed rate and on hydrogen shell-side pressure were adequately described (at 500°C) by a simple model, that incorporates a three-parameter rate expression, that accounts for the accelerating role of hydrogen pressure. The degree of cracking and isomerisation were adequately described by a single-parameter rate expression which assumes that the main and side reactions occur on the same sites. The model was optimized to determine the feed and shell flow rates which maximize the yield. The optimization suggests that, in the present design, the yield cannot be improved significantly beyond 90%, but that almost complete conversion could be achieved when the reactor profile of hydrogen pressure is optimized.

Aluminum incorporation into high silica zeolites

Abstract Strong shape-selective acidic sites were generated inside high silica/alumina ratio ZSM-5 and ZSM-11 zeolites by treatment with aluminum chloride, followed by hydrolysis and calcination. Incorporation of aluminum in the zeolites was indicated by catalytic and ion-exchange studies, and supported by 27Al MAS-n.m.r., FT i.r., and ammonia t.p.d.

Shape-selective reactions of zeolites. Selective metal-catalyzed hydrogenation and oxidation using ZSM-5

Abstract The ZSM-5 zeolite was shown to act as a shape-selective support for the platinum-catalyzed hydrogenation of olefins and for the copper-catalyzed oxidation of hydrocarbons.