Lloyd Abrams

Properties of boron-substituted ZSM-5 and ZSM-11 zeolites

Using NMR and IR spectroscopy boron-substituted pentasil zeolites are found to contain four-coordinated B in the hydrated state and three-coordinated B in the dehydrated state. In addition a new tetrahedral B site assigned to (HO)2B(H2O)(OSi) was characterized by its NMR quadrupole coupling constant and asymmetry parameter. Ammonia adsorption microcalorimetry gave heats of adsorption of ~65 kJ/mol for HBZSM-11 and showed that B-substituted pentasils have only very weak acidity. Calcination at 800 °C increased the heat of NH3 adsorption to ~170 kJ/mol by creation of strong Lewis acid sites. The lack of strong Bronsted acid sites in HBZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation With methanol.

Selective synthesis of dimethylamine over small pore zeolites

Abstract Zeolite H-RHO is highly selective for the synthesis of dimethylamine (DMA) from methanol and ammonia. Shallow-bed dry calcination of NH4-RHO at temperatures from 400 to 700 °C results in progressive deammoniation, dehydroxylation, and dealumination and in dimethylamine (DMA) and trimethylamine (TMA) selectivities that progressively change from 50 to 67% and 25 to 5%, respectively. Concurrent changes in dealumination and internal and external acid sites suggest a process whereby DMA selectivity arises from destruction of nonselective sites on impurity phases such as pollucite and/or from hindered TMA diffusivity by either nonframework Al species (NFA) or NFA/methylamine adsorption complexes. Shallow-bed steam calcination at temperatures from 400 to 700 °C results in higher degrees of dehydroxylation and dealumination than shallow-bed dry calcination and in DMA and TMA selectivities that change from 50 to 71% and 20 to 2%, respectively. The higher DMA selectivities result from the higher degree of dealumination and/or the more effective deactivation of impurity phases. Increased activity of shallow-bed steamed H-RHO is correlated with destruction of most of the highly acidic 3610 cm−1 Bronsted sites and to their replacement with weakly acidic 3640 cm−1 OH groups. High dimethyl ether yields observed in samples calcined above 650 °C are associated with an OH band at 3680 cm−1 and attributed to the condensation of nonframework Al species.

Dynamic Quantum Molecular Sieving Separation of D2 from H2–D2 Mixture with Nanoporous Materials

Quantum molecular sieving separability of D(2) from an H(2)-D(2) mixture was measured at 77 K for activated carbon fiber, carbon molecular sieve, zeolite and single wall carbon nanotube using a flow method. The amount of adsorbed D(2) was evidently larger than H(2) for all samples. The maximum adsorption ratio difference between D(2) and H(2) was 40% for zeolite (MS13X), yielding a selectivity for D(2) with respect to H(2) of 3.05.

Designing zeolite catalysts for shape-selective reactions: Chemical modification of surfaces for improved selectivity to dimethylamine in synthesis from methanol and ammonia

The relative contributions of external and intracrystalline acidic sites of small pore H-RHO zeolite for the selective synthesis of methylamines from methanol and ammonia have been studied. Nonselective surface reactions which produce predominantly trimethylamine can be eliminated by “capping” the external acidic sites with trimethylphosphite (TMP) and other reagents, thus improving the selectivity toward the formation of dimethylamine. For small pore zeolites, neither the zeolite pore size nor the internal acidic sites is significantly affected by this treatment. In situ infrared and MAS-NMR studies show that TMP reacts irreversibly with the zeolite acidic sites via a modified Arbusov rearrangement to form surface-bound dimethylmethylphosphonate.

Selective synthesis of dimethylamine over small-pore zeolites. IV: Effects of SiO2 and Al2O3 coatings

Al2O3 and SiO2 coatings are effective in increasing dimethylamine (DMA) and decreasing trimethylamine (TMA) selectivities of small-pore zeolites HRHO and HZK-5 used as methylamine catalysts. The HRHO catalysts typically contain chabazite or chabazite and pollucite impurities. SiO2 is more effective than Al2O3 for improving DMA selectivity. SiO2 coatings from monosilicic acid (MSA) reduce dimethyl ether (DME) yields over shallow-bed nitrogen (SBN)-calcined HRHO, whereas Al2O3 coatings and SiO2 coatings [from tetraethylorthosilicate (TEOS)]do not. Correlations between thickness of SiO2 (TEOS or MSA) coatings and DMA selectivity as well as a decrease in the n-hexane rate of sorption suggest a physical hindrance to egress of TMA from RHO channels and cages to the product stream. Coating deep-bed calcined H-pollucite with either Al2O3 or SiO2 from TEOS reduces activity and increases DMA selectivity through deactivation of nonselective surface sites. Improvement of DMA selectivity by coating DB-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) deactivation of external acid sites of HRHO and H-pollucite and secondarily from deactivation of H-chabazite. Improvement of DMA selectivity by coating SBN-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) further deactivation of external acid sites of HRHO and amorphous H-chabazite and secondarily from the deactivation of H-pollucite. SiO2 coatings on HZK-5 increased DMA selectivity and decreased DME yields, but reduced activity.

Selective synthesis of dimethylamine over small-pore zeolites: III. HZK-5

HZK-5 is a highly selective catalyst for dimethylamine (DMA) synthesis in the reaction of methanol and ammonia. Deep-bed (DB) calcination of NH4ZK-5 containing 4 Csuc results in DMA selectivities of 60–75% but with 15–30% dimethyl ether (DME) yields. Deep-bed or shallow-bed steam calcination of NH4ZK-5 with only 1 Csuc gives similar high DMA selectivities but lower DME yields of 5–10%. Shallow-bed calcination of NH4ZK-5 results in DMA selectivities of ~45%. The activity of HZK-5 in the methylamine synthesis reaction is lower than that of zeolite HRHO.

Synthesis of dimethylamine by zeolite Rho: A rational basis for selectivity

Abstract The sorption of trimethylamine by zeolite rho was studied to provide a rational basis of catalyst selectivity for the methylamines synthesis reaction from ammonia and methanol. Sorption kinetics, obtained for the range 150–250°C, yield an activation energy of 9 kcal/mole. Mass spec data indicate that trimethylamine is both physisorbed and chemisorbed with relative amounts of 10 and 90%, respectively. The physisorbed entity desorbs intactly while, above 300°C, the chemisorbed trimethylamine pyrolyzes forming mono- and dimethylamines plus some residue. For a given zeolite rho sample, the yield of trimethylamine in the methylamines synthesis reaction was found to correlate with the quantity of physisorbed trimethylamine. For zeolite rho catalysts, an activation energy of 42 kcal/mole was determined for the appearance of trimethylamine in the product stream.

Selective synthesis of dimethylamine over small-pore zeolites: catalytic selectivity and sorption behavior

Abstract Sorption measurements of alcohols were used to rationalize the performance of zeolites used as catalysts for the synthesis of methylamines via the sequential reaction of methanol and ammonia. Low methanol absorption corresponds to low catalytic activity while high isopropanol absorption corresponds to zeolites producing an equilibrium distribution of methylamines. Generally, zeolites, with sorption values for methanol (or ethanol) of 10–30 w o and little or no isopropanol sorption, selectively produce mono- and dimethylamines versus trimethylamine. Mineral chabazites, while having similar activities, surprisingly provide a wide range of product selectivities. The Geometric Selectivity Index, GSI, denned as the ratio of methanol sorption to the sorption of n -propanol, was found to correspond to the observed catalytic selectivity of the mineral chabazites.

Flexibility of the RHO framework: A comparison of the Rb-exchange zeolite and the novel composition Rb24Be24As24O96·3.2 D2O

Abstract the structure of D 2 O-exchanged partially dehydrated Rb 24 Be 24 As 24 O 36 ·3.2 D 2 O was refined using a combination of neutron and X-ray powder diffraction data coupled with interatomic distance constraints. This material crystallizes with the framework of zeolite RHO [ I23, a = 14.01(1) A ]. The 24 Rb + are distributed as follows: The six double 8-ring sites (D8R) are fully occupied, two single 6-ring sites (S6R) are occupied by an average of 15 Rb + , and the remaining Rb + are randomly distributed over the 12 single 8-ring sites (S8R). The structure of the analogous D 2 O-exchanged, dehydrated aluminosilicate RHO, Rb 1 ]Al 10 Si 36 O 96 [ I 43 m, a =14.374(1) A ] was refined using neutron powder diffraction data. The Rb + ions are located, in order of preference, at the D8R and at only one of the S6R-sites at ( x,x,x ); x = 0.191(1). The relative framework distortion parameters for the elliptical 8-rings, Δ/ a , are 0.14 and 0.22 for the aluminosilicate and berylloarsenate, respectively. This variation of distortion with the unit cell parameter follows the same trend as that established for a number of materials with the RHO framework.

Selective synthesis of dimethylamine over small-pore zeolites: II. Effects of impurities on catalytic properties of H-RHO

Abstract Typical RHO preparations can contain impurities such as unreacted gel, chabazite, P c , and pollucite. These impurities were synthesized in pure form to investigate their effects on the behavior of RHO as a dimethylamine (DMA)-selective catalyst in the methanol-ammonia reaction. Gel and p c , after exchange and calcination, were amorphous and inactive at typical reaction temperatures of 300–325 °C. Chabazite, always present in RHO preparations at 5–10% levels, is active and DMA selective after deep-bed calcination. After shallow-bed calcination, it loses much of its crystallinity and becomes inactive. Pollucite, appearing in the later stages of RHO crystallization, loses activity at 325 °C after shallow-bed calcination. However, after deep-bed calcination, it is an active and nonselective methylamine catalyst. Thus, the presence of pollucite can be a primary cause of lower DMA selectivity of deep-bed calcined RHO catalysts. Steaming, one of the most effective methods of reducing the activities of impurities, destroys pollucite crystallinity and/or nonselective Surface sites.