José A.C. Silva

Single and multicomponent sorption of CO2, CH4 and N2 in a microporous metal-organic framework

Abstract Single and multicomponent fixed-bed adsorption of CO2, N2, and CH4 on crystals of MOF-508b has been studied in this work. Adsorption equilibrium was measured at temperatures ranging from 303 to 343 K and partial pressures up to 4.5 bar. MOF-508b is very selective for CO2 and the loadings of CH4 and N2 are practically temperature independent. The Langmuir isotherm model provides a good representation of the equilibrium data. A dynamic model based on the LDF approximation for the mass transfer has been used to describe with good accuracy the adsorption kinetics of single, binary and ternary breakthrough curves. It was found that the intra-crystalline diffusivity for CO2 is one order of magnitude faster than for CH4 and N2.

Separation of branched hexane isomers using zeolite BEA for the octane improvement of gasoline pool

Abstract A sorption study of single, binary, ternary and quaternary mixtures of hexane (C6) isomers n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA (BEA structure), covering the temperature range between 423 K and 523 K and partial pressures up to 0.3 bar. From these data, single and multicomponent adsorption equilibrium isotherms were collected. An extended tri-site Langmuir model (TSL) was developed to interpret accurately the equilibrium data, and a dynamic adsorption model was developed and tested predicting with a good accuracy the behaviour of multicomponent fixed bed experiments. At the partial pressures studied, the sorption hierarchy in the zeolite BETA is: nHEX>3MP>23DMB>22DMB. BEA structure demonstrates a significant selectivity between C6 isomers, especially at low coverage, giving a good perspective regarding future work.

Separation of Hexane Isomers on Rigid Porous Metal Carboxylate-Based Metal-Organic Frameworks

Screening separation studies with equimolar quaternary mixtures of hexane isomers, namely, n-hexane/3-methylpentane/2,3-dimethylbutane/2,2-dimethylbutane (n-HEX/3MP/23DMB/22DMB), in the temperature range of 343–423 K and partial pressures up to 10 kPa have been performed using three rigid porous metal (III/IV) carboxylate-based metal–organic frameworks (MOFs) with different composition and topology. These carboxylate-based MOFs are denoted as MIL-100(Cr), MIL-125(Ti)-NH2 and MIL-127(Fe). Adsorption study results revealed that all the isomers were adsorbed on MIL-100(Cr) and MIL-125(Ti)-NH2 with adsorption amounts reaching 24 and 12 wt%, respectively, and the order of adsorption being similar to the normal boiling point of the compounds, that is, n-HEX > 3MP > 23DMB > 22DMB, with selectivity of adsorption never exceeding three (i.e. selectivities between the extreme compounds n-HEX and 22DMB). It is worth noting that adsorption of branched isomers is practically excluded on MIL-127(Fe), which exhibits a strong molecular sieve effect of the adsorbed n-HEX with the loading capacity reaching a maximum of 7 wt%. Finally, an alternative strategy, which involved probing the effect of post-synthetic functionalization on the coordinatively unsaturated metal sites of the MIL-100(Cr) using functional group ethylamine or N-methylethylenediamine showed no significant differences in terms of separation.

Separation of branched hexane isomers on zeolite BETA

An experimental study of the single, binary, ternary and quaternary fixed bed adsorption of hexane isomers onto zeolite BETA was performed covering the temperature range between 423 K and 523 K and partial pressures up to 0.3 bar. Adsorption equilibrium isotherms were collected from breakthrough experiments. Based on an analysis of sorption events at the molecular level, a Tri-Site Langmuir model (TSL) was developed to interpret the equilibrium data with good accuracy. At the partial pressures studied, it was found that the degree of branching was related to the affinity; the sorption hierarchy was most favourable towards the linear isomer and least favourable towards the di-branched ones. A mathematical model based on a linear driving force (LDF) was developed and used to test the experimental data. It was found that the model predicted the behaviour of the fixed bed experiments with good accuracy. Zeolite BETA demonstrated significant selectivity between branched C6 isomers (especially at low coverage), thereby suggesting a means of solving this difficult separation problem.

Dynamics of a Fixed Bed Adsorption Column in the Kinetic Separation of Hexane Isomers in MOF ZIF-8

A fixed bed adsorption mathematical model has been developed to describe the kinetic separation of hexane isomers when they flow through a packed bed containing the microporous Metal-Organic Framework (MOF) ZIF-8 adsorbent. The flow of inert and adsorbable species through the fixed bed is modeled with fundamental differential equations according to the mass and heat conservation laws, a general isotherm to describe adsorption equilibrium and a lumped kinetic mass transfer mechanism between bulk gas phase and the porous solid. It is shown that a proper combination of two characteristic times (the residence time of the gas in the fixed bed, \(\tau _{fb}\) and the characteristic time of diffusion of solutes into the pores \(\tau _{dif}\)) can lead to very different dynamics of fixed bed adsorbers where in a limiting case can gives rise to a spontaneous breakthrough curves of solutes. The numerical simulations of an experimental breakthrough curve with the developed mathematical model clearly explain the complete separation between linear n-Hexane (nHEX) and the respective branched isomers: 3-Methyl-Pentane (3MP) and 2, 2-Dimethyl-Butane (22DMB). The separation is due to significant differences in the diffusivity parameters \(\tau _{dif}\) between 3MP and 22DMB and the residence time of the gas mixture \(\tau _{fb}\) within the fixed bed. This work shows the importance of mathematical modelling for the comprehension and design of adsorption separation processes.

Modelling the fixed bed adsorption dynamics of CO2/CH4 in 13X zeolite for biogas upgrading and CO2 sequestration

The sorption of \(\mathrm{CO}_{2}\) and \(\mathrm{CH}_{4}\) in binderless beads of 13X zeolite has been investigated between 313 and 423 K and total pressure up to 0.5 MPa through fixed bed adsorption experiments. Experimental selectivities \(\mathrm{CO}_{2}/\mathrm{CH}_{4}\) range from 37 at a low pressure of 0.0667 MPa to approximately 5 at the high temperature of 423 K. The breakthrough curves measured show a plateau of pure \(\mathrm{CH}_{4}\) of approximately 6 min depending of the operating conditions chosen. A mathematical model was developed and tested predicting with good accuracy the behaviour of the fixed bed adsorption experiments being a valuable tool for the design of cyclic adsorption processes for biogas upgrading and \(\mathrm{CO}_{2}\) capture using 13X zeolite.