Tracer Chromatographic Study of Pore and Pore Mouth Adsorption of Linear and Monobranched Alkanes on ZSM-22 Zeolite

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

A quantitative multiscale perspective on primary olefin formation from methanol

The formation of the first C-C bond and primary olefins from methanol over zeolite and zeotype catalysts has been studied for over 40 years. Over 20 mechanisms have been proposed for the formation of the first C-C bond. In this quantitative multiscale perspective, we decouple the adsorption, desorption, mobility, and surface reactions of early species through a combination of vacuum and sub-vacuum studies using temporal analysis of products (TAP) reactor systems, and through studies with atmospheric fixed bed reactors. These results are supplemented with density functional theory calculations and data-driven physical models, using partial differential equations, that describe the temporal and spatial evolution of species. We consider the effects of steam, early degradation species, and product masking due to the inherent autocatalytic nature of the process, which all complicate the observation of the primary olefin(s). Although quantitative spectroscopic determination of the lifetimes, surface mobility, and reactivity of adspecies is still lacking in the literature, we observe that reaction barriers are competitive with adsorption enthalpies and/or activation energies of desorption, while facile diffusion occurs in the porous structures of the zeolite/zeotype catalysts. Understanding the various processes allows for quantitative evaluation of their competing energetics, which leads to molecular insights as to what governs the catalytic activity during the conversion of methanol to primary olefins over zeolite/zeotype catalysts.

Probing Zeolite Crystal Architecture and Structural Imperfections using Differently Sized Fluorescent Organic Probe Molecules

A micro‐spectroscopic method has been developed to probe the accessibility of zeolite crystals using a series of fluorescent 4‐(4‐diethylaminostyryl)‐1‐methylpyridinium iodide (DAMPI) probes of increasing molecular size. Staining large zeolite crystals with MFI (ZSM‐5) topology and subsequent mapping of the resulting fluorescence using confocal fluorescence microscopy reveal differences in structural integrity: the 90° intergrowth sections of MFI crystals are prone to develop structural imperfections, which act as entrance routes for the probes into the zeolite crystal. Polarization‐dependent measurements provide evidence for the probe molecule's alignment within the MFI zeolite pore system. The developed method was extended to BEA (Beta) crystals, showing that the previously observed hourglass pattern is a general feature of BEA crystals with this morphology. Furthermore, the probes can accurately identify at which crystal faces of BEA straight or sinusoidal pores open to the surface. The results show this method can spatially resolve the architecture‐dependent internal pore structure of microporous materials, which is difficult to assess using other characterization techniques such as X‐ray diffraction.

Cadmium-BINOL Metal-Organic Framework for the Separation of Alcohol Isomers

The large‐scale isolation of specific isomers of amyl alcohols for applications in the chemical, pharmaceutical, and biochemical industries represents a challenging task due to the physicochemical similarities of these structural isomers. The homochiral metal–organic framework cadmium–BINOL (BINOL=1,1′‐bi‐2‐naphthol) is suitable for the separation of pentanol isomers, combining adsorption selectivities above 5 with adsorption capacities of around 4.5 mol kg⁻¹. Additionally, a slight ability for separation of racemic mixtures of 2‐pentanol is also detected. This behavior is explained based on matching shapes, strength of host–guest interactions, and on the network of hydrogen bonds. The last of these explains both the relative success and shortfalls of prediction methods at high loadings (ideal adsorbed solution theory) or at low coverage (separation factors), which are therefore useful here at a qualitative level, but not accurate in quantitative terms. Finally, the high selectivity of cadmium–BINOL for 1‐pentanol over its isomers offers prospects for practical applications and some room for optimizing conditions.

Maximizing n-alkane hydroisomerization: the interplay of phase, feed complexity and zeolite catalyst mixing

The preferred hydroisomerization of heavy alkanes in parapur on Y–ZSM22 is counterbalanced by physisorption destabilization effects which emerge at micropore saturation.

Plane-wave density functional theory investigation of adsorption of 2,4,6-trinitrotoluene on Al-hydroxylated (0001) surface of (4 × 4) α-alumina

This article reports the results of the theoretical investigation of adsorption of 2,4,6-trinitrotoluene (TNT) on Al-hydroxylated (0001) surface of (4 × 4) α-alumina (α-Al2O3) using plane-wave Density Functional Theory. Sixteen water molecules were used to hydroxylate the alumina surface. The Perdew-Burke-Ernzerhof functional and the recently developed van der Waals functional (vdW-DF2) were used. The interaction of electron with core was accounted using the Vanderbilt ultrasoft pseudopotentials. It was found that hydroxylation has significant influence on the geometry of alumina and such changes are prominent up to few layers from the surface. Particularly, due to the Al-hydroxylation the oxygen layers are decomposed into sublayers and such partitioning becomes progressively weaker for interior layers. Moreover, the nature of TNT adsorption interaction is changed from covalent type on the pristine alumina surface to hydrogen-bonding interaction on the Al-hydroxylated alumina surface. TNT in parallel orientation forms several hydrogen bonds compared to that in the perpendicular orientation with hydroxyl groups of the Al-hydroxylated alumina surface. Therefore, the parallel orientation will be present in the adsorption of TNT on Al-hydroxylated (0001) surface of α-alumina. Further, the vdW-DF2 van der Waals functional was found to be most suitable and should be used for such surface adsorption investigation.

Pulse gas chromatographic study of adsorption of substituted aromatics and heterocyclic molecules on MIL-47 at zero coverage

The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure-property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role.

Effects of molecular sieving and electrostatic enhancement in the adsorption of organic compounds on the zeolitic imidazolate framework ZIF-8

In this work, the adsorption behavior of a range of organic vapors and gases on the zeolitic imidazolate framework, ZIF-8, is investigated using an inverse gas chromatography (IGC) methodology at the zero-coverage limit and elevated temperatures. The measured thermodynamic values and surface energies for the adsorption of n-alkanes on ZIF-8 are found to be reduced from those previously reported for IRMOF-1. This reduction is most likely an effect of the predominately organic accessible surface of ZIF-8 and the resulting weaker interactions in comparison to IRMOF-1. The pore aperture size of ZIF-8, which is significantly reduced from that of IRMOF-1, is seen to introduce molecular sieving effects for branched alkanes, aromatics, and heavily halogenated compounds. Deformation polarizabilities of the adsorbates were used to calculate the specific adsorption free energy, and it is determined that the specific effects account for around 1-5 kJ/mol, or between 10% and 70% of the total free energy of adsorption for the sorbates studied (at 250 °C). The importance of electrostatic forces was seen in the significantly enhanced adsorption of propylene and ethylene in comparison to their respective alkanes and in the direct correlation shown between the specific components of the free energy of adsorption and the adsorbate's dipole moment.

Understanding Adsorption and Interactions of Alkane Isomer Mixtures in Isoreticular Metal–Organic Frameworks

Novel metal-organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4-C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n-butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption.

n- and Isoalkane Adsorption Mechanisms on Zeolite MCM-22

Low-coverage adsorption properties (Henry constants, adsorption enthalpy, and entropy) of linear and branched alkanes (C3-C8) on zeolite MCM-22 were determined using the chromatographic technique at temperatures between 420 and 540 K. It was found that adsorption enthalpy and entropy of linear alkanes vary in a nonmonotonic way with carbon number. The adsorption behavior of alkanes was rationalized on the basis of the pore geometry. Short molecules prefer to reside in the pockets of the MCM-22 supercage, where they maximize energetic interaction with the zeolite. Longer molecules reside in the larger central part of the supercage. For carbon numbers up to six, singly branched alkanes are selectively adsorbed over their linear counterparts. This preference originates from the entropic advantage of singly branched molecules inside MCM-22 supercages, where these species have high rotational freedom because of their small length.

Influence of Pore Dimension and Sorption Configuration on the Heat of Sorption of Hexane on Monodimensional Siliceous Zeolites

Sorption of n-hexane on monodimensional pure silica SSZ-35, CIT-5, ZSM-12, and ZSM-22 zeolites with different pore dimension and on recently synthesized ITQ-29 was studied by IR spectroscopic and computational chemistry methods. Heats of sorption of n-hexane on these zeolites was determined experimentally from the temperature dependence of the intensity of IR bands of sorbed hexane as well as from theoretical calculations. Calculations have shown the different orientations of sorbed hexane molecules inside zeolite channels, which depend on the type of zeolite and loading. At high loadings, ordering of hexane inside the channels is observed due to optimization of sorbate-sorbate and sorbate-zeolite interaction energies. Such ordering is responsible for the increase of the sorption energy. A decrease of the sorption energy upon increasing the pore dimension of zeolite was observed, in agreement with results previously published in the literature. Effects of pore diameter of zeolites and ordering of molecules inside zeolite channels on the sorption energy of hexane are discussed.