Gina Vanbutsele

Evidences for pore mouth and key–lock catalysis in hydroisomerization of long n-alkanes over 10-ring tubular pore bifunctional zeolites

Abstract The evidence for the pore mouth catalysis model for n-alkane methylbranching on Pt/H-ZSM-22 hydroisomerization catalyst is reviewed. It is based on adsorption equilibria at catalytic temperatures determined using tracer and perturbation chromatography, reaction product distributions obtained with nC8–nC24 n-alkanes and rival model screening for catalytic conversions. In the Henry regime, methylbranched isomers have lower adsorption entropy and enthalpy compared to n-alkanes explained by the enhanced rotational and translational freedom of methyl and methylene groups positioned outside the pore interacting with the external surface. Adsorption isotherms for isoalkanes are in agreement with dual site adsorption in pore mouths and on external surfaces, respectively. The hydroisomerization can be modeled with a bifunctional reaction scheme and adsorption on the external crystal surfaces and pore mouths. The selectivity for 2-methylbranching reflects the optimum van der Waals interaction of the n-alkane with the zeolite pore and methylbranching in that part of the chain that is located outside the first 10-ring of the zeolite pore to facilitate desorption. Very long n-alkanes (C12+) exhibit key–lock adsorptions and penetrate simultaneously with their two ends into two different pores. Key–lock physisorption leads to branching at more central C atom positions.

Nanoscale intimacy in bifunctional catalysts for selective conversion of hydrocarbons

The ability to control nanoscale features precisely is increasingly being exploited to develop and improve monofunctional catalysts. Striking effects might also be expected in the case of bifunctional catalysts, which are important in the hydrocracking of fossil and renewable hydrocarbon sources to provide high-quality diesel fuel. Such bifunctional hydrocracking catalysts contain metal sites and acid sites, and for more than 50 years the so-called intimacy criterion has dictated the maximum distance between the two types of site, beyond which catalytic activity decreases. A lack of synthesis and material-characterization methods with nanometre precision has long prevented in-depth exploration of the intimacy criterion, which has often been interpreted simply as ‘the closer the better’ for positioning metal and acid sites. Here we show for a bifunctional catalyst—comprising an intimate mixture of zeolite Y and alumina binder, and with platinum metal controllably deposited on either the zeolite or the binder—that closest proximity between metal and zeolite acid sites can be detrimental. Specifically, the selectivity when cracking large hydrocarbon feedstock molecules for high-quality diesel production is optimized with the catalyst that contains platinum on the binder, that is, with a nanoscale rather than closest intimacy of the metal and acid sites. Thus, cracking of the large and complex hydrocarbon molecules that are typically derived from alternative sources, such as gas-to-liquid technology, vegetable oil or algal oil, should benefit especially from bifunctional catalysts that avoid locating platinum on the zeolite (the traditionally assumed optimal location). More generally, we anticipate that the ability demonstrated here to spatially organize different active sites at the nanoscale will benefit the further development and optimization of the emerging generation of multifunctional catalysts.

Hydroisomerization of Emerging Renewable Hydrocarbons using Hierarchical Pt/H‐ZSM‐22 Catalyst

Last site standing: A new generation of hierarchical Pt/H-ZSM-22 zeolites is designed for the efficient processing of upcoming renewable feedstocks. The enhanced accessibility of the active sites is vital for the superior activity and exceptional selectivity in the hydroisomerization of model molecules such as nonadecane and pristane.

Modeling of adsorption and bifunctional conversion of n-alkanes on Pt/H-ZSM-22 zeolite catalyst

Abstract The hydroconversion of C6–C9 n -alkanes on Pt/H-ZSM-22 was investigated by measuring and modeling of the adsorption and catalytic properties of this strongly shape-selective zeolite catalyst. Henry adsorption constants and adsorption isotherms of C6–C9 n - and iso-alkanes on H-ZSM-22 were determined with chromatographic techniques and fitted to different models. While n -alkanes have access to the internal pore structure of ZSM-22, only the linear part of the iso-alkanes adsorbs in the pore entrances. Besides pore and pore mouth adsorption, external surface adsorption plays an important role. In the hydroconversion of C6–C9 n -alkanes, nonselective adsorption of n - and iso-alkanes on the external surface of the ZSM-22 crystals determines the concentration of the reacting species and the reaction orders. The skeletal rearrangement reactions occur exclusively in the pore mouths at the external surface of the zeolite crystals. The hydrocracking reactions were modeled with a lumped reaction scheme, accounting for adsorption on the different types of adsorption sites. The best agreement between model and experiments was obtained with an external surface adsorption model.

Aluminium atomic layer deposition applied to mesoporous zeolites for acid catalytic activity enhancement

Atomic Layer Deposition (ALD) of aluminium is a new method for enhancing acidity and acid catalytic activity in mesoporous zeolites and hierarchical materials.

Generation of acid catalytic activity in Si-VPI-5 by partial Si for P substitution during hydrothermal synthesis

Si-VPI-5 crystals containing Si atoms in Si(4Al) environments arc synthesized. The crystals contain occluded amine. It is protonated and neutralizes the residual negative lattice charges associated with the Si(4A1) sites. Calcined Si-VPI-5 is active in the isomerisation of decane and exhibits extra-large pore characteristics.

Catalytic activation of OKO zeolite with intersecting pores of 10- and 12-membered rings using atomic layer deposition of aluminium

Tetrahedral framework aluminium was introduced in all-silica zeolite -COK-14 using Atomic Layer Deposition (ALD) involving alternating exposure to trimethylaluminium and water vapour. The modification causes permanent conversion of the originally interrupted framework of -COK-14 to a fully connected OKO type framework, and generates catalytic activity in the acid catalysed hydrocarbon conversion reaction.

Catalytic activity and extra-large pores of germanosilicate UTL zeolite demonstrated with decane test reaction

Pyridine adsorption monitored by FTIR spectroscopy revealed germanosilicate UTL type zeolite to contain intrinsic Bronsted acidity required for catalytic activity. Germanosilicate UTL type zeolite was converted into a bifunctional catalyst and evaluated in n-decane isomerization and hydrocracking. To assess the stability of the framework during catalyst preparation and use, three different strategies of converting the zeolite into bifunctional catalyst were followed: incipient wetness impregnation with Pt(NH3)4Cl2 solution before and after evacuation of the template by calcination, and physical mixing of calcined UTL with Pt-containing amorphous silica. All three samples showed catalytic activity but to very different degrees. The possibility of structure degradation during catalyst preparation and catalysis was investigated by XRD and 29Si MAS NMR. The impregnation of as-made germanosilicate UTL zeolite with platinum and pretreatment in the reactor led to superior stability and activity. The n-decane test previously was used to probe zeolite micropore architectures with 8-, 10- and 12-membered rings. Its validity is now extended to the extra-large pore zeolites. The linear trend of increasing ethyloctanevs.methylnonane selectivity with increasing pore size has now been confirmed also for UTL with 14 membered rings.

CHARACTERIZATION OF LARGE AND EXTRA-LARGE ZEOLITE PORES WITH THE HEPTADECANE TEST

ABSTRACT A cage effect is observed in the hydrocracking of heptadecane on bifunctional zeolite catalysts of the FAU/EMT family. In zeolites containing the EMT phase, the formation of cracked products containing 6 and 11 carbon atoms is suppressed. The maxima at C 5 and C 12 in the cracked product distribution result from hydrocracking in the constrained environments of the hypocages. The contribution of hypocage catalysis in zeolites of the type Y, SAPO-37, EMC-1, CSZ-1, ZSM-2, ZSM-3, ZSM-20, and EMC-2 is evaluated based on the molar C 5 /C 6 reaction product ratio. The C 5 /C 6 criterion is particularly useful to monitor the generation of mesopores during dealumination of FAU/EMT zeolites.

Synthesis of MTT zeolite catalysts with surface Al depletion

MTT zeolite crystals with siliceous layer on their external surface were synthesized using a two step hydrothermal procedure in which cores prepared from an aluminosilicate gel were transferred into a siliceous gel for farther crystal growth. The thickness of the siliceous shell was varied by varying the proportion of the two gels. The samples were characterized using physicochemical methods (XRD, XPS, 27Al MAS NMR, SEM, N2 adsorption). The zeolites were converted into bifunctional catalysts and evaluated in hydroisomerization of decane.