Werner Otto Haag

Chemical and physical properties of the ZSM-5 substitutional series

Abstract ZSM-5 is a new silica-rich zeolite that has been synthesized with greatly differing SiO 2 /Al 2 O 3 ratios; the aluminum content, while always small, can be varied by several orders of magnitude. ZSM-5 thus constitutes a substitutional series whose physical, chemical, and catalytic properties are reported and discussed in terms of their structural and compositional dependance. Ion exchange capacity, catalytic activity, and water sorption (at P P 0 = 0.006 ) are shown to vary linearly with aluminum content and extrapolate smoothly to the end member of the series, a pure silica ZSM-5. Other properties such as X-ray diffraction pattern, pore size and volume, framework density, and refractive index are primarily a result of the structural features and are largely composition independent. From a comparison of their properties, the recently reported “silicalite” appears to be a member of the ZSM-5 substitutional series.

Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

Monomolecular and bimolecular mechanisms of paraffin cracking: n-butane cracking catalyzed by HZSM-5

Abstract The cracking of n -butane catalyzed by the zeolite HZSM-5 has been characterized by measurements of the conversion determined with a flow reactor at temperatures of 426–523°C and n -butane partial pressures of 0.01–1.00 atm. The primary products, each formed in a first-order reaction, are H 2 + butenes; methane + propylene; and ethane + ethylene. In the limit approaching zero conversion, each compound in each stated pair was formed at approximately the same rate as the other. Propane and a small amount of isobutane were formed as secondary products in second-order reactions. The results are consistent with the occurrence of two simultaneous mechanisms: (1) a monomolecular mechanism proceeding through the pentacoordinated carbonium ion formed by protonation of the n-butane at the two position and (2) a bimolecular hydride transfer proceeding through carbenium ion intermediates. The former proceeds almost to the exclusion of the latter in the limit approaching zero n -butane conversion. The limiting product distribution characterizes the intrinsic selectivity of the collapse of the carbonium ion; at 496°C, the relative rates of decomposition of the carbonium ion to give H 2 + butenes, methane + propylene, and ethane + ethylene are 30 ± 6, 36 ± 4, and 34 ± 5, respectively, with the corresponding activation energies all being approximately 140 kJ/mol. These results provide the first demonstration of stoichiometric dehydrogenation accompanying paraffin cracking.

Synthesis and characterization of a pure zeolitic membrane

Abstract A new membranous material was synthesized, composed of a continuous intergrowth of 10–100 μm ZSM-5 crystals. The membrane was crystallized under hydrothermal conditions at 180°C on a variety of flat nonporous surfaces including Teflon, silver, and stainless steel and on the external surface of a porous Vycor disk. The permeability and selectivity of the membrane was measured for three bicomponent gas mixtures. The following permeability ratios were computed: O 2 N 2 : 1.07; H 2 CO : 1.62; n- C 6 H 14 CH 3 C(CH 3 ) 2 CH 2 CH 3 : 17.2. These results show that the material can discriminate between permeates at the molecular level because of shape-selective transport within the ZSM-5 pore system.

Aromatics, light olefins and gasoline from methanol: Mechanistic pathways with ZSM-5 zeolite catalyst

Abstract The conversion of methanol to hydrocarbons with zeolite ZSM-5 as catalyst provides a novel route to gasoline as well as to olefins and aromatics as chemical raw materials. The reaction is acid-catalyzed and involves alkylation of olefins and aromatics as major methanol conversion steps, accompanied by olefin isomerization, polymerization/cracking, cyclization and aromatization via hydrogen transfer. Shape-selective control of the aromatics produced results from the use of the medium pore size zeolite ZSM-5. It is shown that the true kinetic pathways are often disguised by diffusion/desorption effects. Ethylene is most likely the first olefinic hydrocarbon formed.

Rates of isobutane cracking catalysed by HZSM-5: The carbonium ion route

Low conversions (<1%) of isobutane catalysed by the zeolite HZSM-5 were measured with a steady-state flow reactor at 450–500 °C and an isobutane partial pressure of 7.98 kPa. The major products were methane and propylene, formed in equimolar amounts and indicative of cracking proceeding through a carbonium ion intermediate formed by protonation of isobutane. The rate of the reaction at 475 °C is 1.4 × 10−6 mol per g catalyst per s; the apparent activation energy is 57 kcal mol−1. Isobutylene was also formed as a primary product, at rates several-fold less than the cracking rate. Propane appeared to be a primary product, possibly formed in a hydrogen transfer reaction. Nonprimary products included ethylene, but-1-ene and but-2-enes.

SHAPE-SELECTIVE CATALYSIS IN AROMATICS PROCESSING

Shape selective catalysis by zeolites provides new insights into catalytic mechanisms as well as new capabilities for technology. Acidic zeolites provide a variety of highly selective aromatics transformations as well as new routes to their synthesis. Using ZSM-5 type catalysts, xylene isomerization is carried out with suppression of undesirable byproducts, and improved toluene disproportionation and ethyl-benzene synthesis processes have evolved. The kinetics and selectivities involved will be discussed. Furthermore, novel para-selective modes of alkylation or transalkylation reactions can be attained. They allow the synthesis of para-xylene, para-ethyl toluene, etc. Acidic ZSM-5 zeolites open up new routes to the synthesis of aromatics from a wide variety of non-aromatic molecules. In all these reactions, the formation and distribution of products is dominated by a few unifying mechanistic principles which follow classical carbenium ion theory. The active sites are the tetrahedral aluminum sites in the silicate framework. Most of the reactions are accomplished by a surprisingly small number of acidic sites.