Arturo J. Hernández-Maldonado

Desulfurization of Transportation Fuels by Adsorption

Abstract This paper is a review on sorbents for desulfurization of transportation fuels (gasoline, diesel, and jet fuel). Since the π‐complexation sorbents are the most promising, they are the focus of the discussion. During π‐complexation, the thiophenic compounds can bind selectively to the sorbents, especially the substituted ones. The later remain highly unreacted in hydrodesulfurization (HDS) (i.e., “refractory” sulfur). Molecular orbital (MO) calculations and experiments have shown that these refractory compounds [(e.g., 4‐methyldibenzothiophene and 4,6‐dimethyldibenzothiophene (DMDBT)] bind strongly with the π‐complexation sorbents because of a better electron donation/back‐donation ability. The sorbents reviewed include Ag‐Y, Cu(I)‐Y, Ni(II)‐Y, and Ni(II)‐X zeolites prepared using various ion‐exchange techniques. The techniques included vapor and solid‐state ion exchanges, which are suitable for obtaining high loadings of transition metals. The best sorbent, Cu(I)‐Y [vapor‐phase ion‐exchanged (VPIE)], is capable of producing almost 38 cm3 of desulfurized fuel per g of sorbent with a sulfur concentration of less than 0.2 ppmw. Using these π‐complexation sorbents in layered bed matrices further increases the desulfurization capacity.

Selective Adsorption of Organosulfur Compounds from Transportation Fuels by π‐Complexation

Abstract Ab initio molecular orbital (MO) calculations were performed on the adsorption bond energies between the main sulfur compounds and the Cu+ on CuCl and CuY zeolite, for desulfurization of transportation fuels by π‐complexation sorbents. The relative adsorption bond energies of these compounds were measured by the elution order, based on the breakthrough curves of these compounds, from a column of CuY zeolite using commercial diesel and gasoline as the influents. The order from the elution followed: 4,6‐dimethyldibenzothiophene ≥ dibenzothiophene > benzothiophene ≥ 2‐methylthiophene > thiophene. The order is in agreement with that predicted from the calculations. The calculated values for benzene and thiophene were also in excellent agreement with the measured values that we reported earlier. The methyl and benzo groups have an electron‐donating effect on the aromatic rings that undergo π‐complexation. For the sulfur compounds, the thiophene ring was bonded to Cu+. For π‐complexation on both CuY and CuCl, the amount of electron forward donation was more than that of electron back donation for thiophenic adsorbates, but the reverse was true for benzene and toluene.

Transition metal modified and partially calcined inorganic-organic pillared clays for the adsorption of salicylic acid, clofibric acid, carbamazepine, and caffeine from water.

Pharmaceutical and Personal Care Products (PPCPs) are considered emerging contaminants, and their efficient removal from water is going to be a challenging endeavor. Microporous adsorbent materials, including pillared clays, could offer a potential solution if tailored properly. Although pillared clays have been employed previously for the removal of organics, the effective removal of PPCPs will only be possible if their surface and textural properties are manipulated from the bottom-up. This work presents the use of modified inorganic-organic pillared clays (IOCs) for the adsorption of salicylic acid, clofibric acid, carbamazepine, and caffeine. The IOCs have been modified with Co(2+), Cu(2+), or Ni(2+) to induce complexation-like adsorbate-adsorbent interactions at ambient conditions, in an attempt to provide an efficient and yet reversible driving force in the sub-ppm concentration range. Furthermore, the IOCs were partially calcined to increase effective surface area by an order of magnitude while preserving some hydrophobicity. In general, the Ni(2+) IOCs exhibited the greatest interaction with salicylic and clofibric acids, respectively, while the Co(2+) adsorbents excelled at adsorbing caffeine at low concentrations. All of the metal-modified IOCs showed comparable adsorption capacities for the case of carbamazepine, probably due to the lack of availability of particular functional groups in this adsorbate.

Single and multi-component adsorption of salicylic acid, clofibric acid, carbamazepine and caffeine from water onto transition metal modified and partially calcined inorganic-organic pillared clay fixed beds.

Fixed-beds of transition metal (Co(2+), Ni(2+) or Cu(2+)) inorganic-organic pillared clays (IOCs) were prepared to study single- and multi-component non-equilibrium adsorption of a set of pharmaceutical and personal care products (PPCPs: salicylic acid, clofibric acid, carbamazepine and caffeine) from water. Adsorption capacities for single components revealed that the copper(II) IOCs have better affinity toward salicylic and clofibric acid. However, multi-component adsorption tests showed a considerable decrease in adsorption capacity for the acids and an unusual selectivity toward carbamazepine depending on the transition metal. This was attributed to a combination of competition between PPCPs for adsorption sites, adsorbate-adsorbate interactions, and plausible pore blocking caused by carbamazepine. The cobalt(II) IOC bed that was partially calcined to fractionate the surfactant moiety showcased the best selectivity toward caffeine, even during multi-component adsorption. This was due to a combination of a mildly hydrophobic surface and interaction between the PPCP and cobalt(II). In general, the tests suggest that these IOCs may be a potential solution for the removal of PPCPs if employed in a layered-bed configuration, to take care of families of adsorbates in a sequence that would produce sharpened concentration wavefronts.

Single and multi-component adsorptive removal of bisphenol A and 2,4-dichlorophenol from aqueous solutions with transition metal modified inorganic-organic pillared clay composites: Effect of pH and presence of humic acid.

Pillared clay based composites containing transition metals and a surfactant, namely MAlOr-NaBt (Bt=bentonite; Or=surfactant; M=Ni(2+), Cu(2+)or Co(2+)), were prepared to study selectivity and capacity toward single and multiple-component adsorption of bisphenol A (BPA) and 2,4-diclorophenol (DCP) from water. Tests were also performed to account for the presence of natural organic matter in the form of humic acid (HA). Equilibrium adsorption capacities for single components increased as follows: NaBt<Al-NaBt<AlOr-NaBt<MAlOr-NaBt. The observed equilibrium loadings were ca. 5 and 3mgg(-1) for BPA and DCP, respectively, at neutral pH conditions and ambient temperature, representing an ordered of magnitude increase over the unmodified pillared clay capacities. Inclusion of the transition metal brought an increase of nearly two-fold in adsorption capacity over the materials modified only with surfactant. The MAlOr-NaBt adsorbents displayed remarkable selectivity for BPA. Multi-component fixed-bed tests, however, revealed competition between the adsorbates, with the exception of the CuAlOr-NaBt beds. Inclusion of HA, surprisingly, enhanced the phenols adsorption capacity. Preliminary regeneration tests suggested that the adsorbent capacity can be recovered via thermal treatment or by washing with alkaline solutions. The former strategy, however, requires surfactant replenishment. More complex schemes would be needed to deal with absorbed HA.

Long range structural and textural changes in [Zn(bdc)(ted)0.5] upon spontaneous dispersion of LiCl and hysteretic adsorption and desorption of carbon dioxide and hydrogen

A LiCl containing Zn(bdc)(ted)0.5 metal–organic framework (MOF) was prepared via an innocuous impregnation and thermal spontaneous dispersion process. The dispersion of the LiCl and crystallinity of the modified microporous framework were verified via standard and in situ high temperature X-ray diffraction (XRD). Pure component CO2 and H2 equilibrium adsorption–desorption analyses were performed on Zn(bdc)(ted)0.5 and (LiCl)[Zn(bdc)(ted)0.5] at 25 °C. The latter exhibited unexpected adsorption–desorption hystereses for both adsorbates and, according to XRD data, these were due to what appears to be a framework or pore contraction. Pore volumes estimated via a fit of desorption data with a Modified Dubinin–Astakhov model indicated that the LiCl containing material framework voids were at least 74% smaller as compared to the apohost. XRD patterns also indicated that the (LiCl)[Zn(bdc)(ted)0.5] structure expanded back to the apohost original state upon a nearly complete desorption of the CO2 adsorbate, suggesting that the apparent phase transition was reversible for this case. The overall adsorptive behavior of the (LiCl)[Zn(bdc)(ted)0.5] materials suggests that these could be suitable for gas capture applications under ambient conditions. For instance, the resulting average H2 heat of adsorption value is among the highest reported so far for a MOF.

Fabrication of Porous and Nanoporous Aluminum via Selective Dissolution of Al-Zn Alloys

Porous and nanoporous aluminum have been fabricated via selective dissolution. Al-Zn alloys were dealloyed in an aqueous solution of nitric acid to selectively dissolve zinc. Fast solidification methods permitted to adjust the precursor microstructure of the parent alloy in order to induce supersaturation of zinc in aluminum. An electric potential applied during corrosion affected the final morphology of porosity. Electronic imaging evinced the presence of regions with less than 100 nm diameter pores. This nanoporosity was only present in electrochemically dealloyed samples. We observed two types of porosity in dealloyed samples: a primary porosity resulting from the selective removal of zinc-rich interdendritic phases and a secondary porosity resulting from nanoporosity evolution inside zinc-supersaturated dendrites.

UPRM-5 titanium silicates prepared using tetrapropylammonium and tetrabutylammonium cations: framework stability, textural properties, and carbon dioxide adsorption

UPRM-5 is a flexible titanium silicate first prepared using tetraethylammonium (TEA+) and that exhibited improved structural and adsorption properties when compared to other titanium silicates. In order to further tailor these properties, we have employed tetrapropylammonium (TPA+) and tetrabutylammonium (TBA+), as structure directing agents (SDAs), respectively. Analysis of the local-range structure using 29Si magic angle spinning nuclear magnetic resonance spectroscopy suggested silicon environments corresponding to Si(2Si, 2Tiocta) and Si(3Si, 1Tisemi-octa), as expected for a flexible titanium silicate. A quantitative analysis, however, revealed that the amount of semi-octahedral titanium centers was greater in the variant prepared with TPA+ suggesting that the nature of the NR4+ cation plays an important role in the formation of framework faulting. Both UPRM-5 variants were detemplated and modified to include extraframework Sr2+ and produce materials for carbon dioxide adsorption. Their thermal stability and pore contraction were first investigated by means of in situ high-temperature X-ray powder diffraction and nitrogen porosimetry. Materials prepared with TBA+ showcased better thermal stability when compared to variants prepared with TPA+ and even TEA+, probably due to the relative low level of structural faulting. All variants, however, displayed a pore contraction process associated with the release of tenacious water. Carbon dioxide uptakes varied considerably depending on the choice of SDA employed and the isosteric heat of adsorption profiles correlated with a heterogeneous surface. The results suggest that Sr2+–UPRM-5 (TPA) materials could be tailored for purification applications, whereas Sr2+–UPRM-5 (TBA) materials could be tailored for bulk-level separation applications.

Unveiling Adsorption Mechanisms of Organic Pollutants onto Carbon Nanomaterials by DFT Computations and pp-LFER Modeling

Predicting adsorption of organic pollutants onto carbon nanomaterials (CNMs) and understanding the adsorption mechanisms are of great importance to assess the environmental behavior and ecological risks of organic pollutants and CNMs. By means of density functional theory (DFT) computations, we investigated the adsorption of 38 organic molecules (aliphatic hydrocarbons, benzene and its derivatives, and polycyclic aromatic hydrocarbons) onto pristine graphene in both gaseous and aqueous phases. Polyparameter linear free energy relationships (pp-LFERs) were developed, which can be employed to predict adsorption energies of aliphatic and aromatic hydrocarbons on graphene. Based on the pp-LFERs, contributions of different interactions to the overall adsorption were estimated. As suggested by the pp-LFERs, the gaseous adsorption energies are mainly governed by dispersion and electrostatic interactions, while the aqueous adsorption energies are mainly determined by dispersion and hydrophobic interactions. It was also revealed that curvature of single-walled carbon nanotubes (SWNTs) exhibits more significant effects than the electronic properties (metallic or semiconducting) on gaseous adsorption energies, and graphene has stronger adsorption abilities than SWNTs. The developed models may pave a promising way for predicting adsorption of environmental chemicals onto CNMs with in silico techniques.

Removal of Carbon Dioxide from Light Gas Mixtures using a Porous Strontium(II) Silicoaluminophosphate Fixed Bed: Closed Volume and Portable Applications

A Sr2+ -SAPO-34 bed was assembled to study CO2 dynamic adsorption under conditions that emulate those found in closed volume and portable applications. Although the surface area was reduced by 7% during pelletization, adsorption capacities estimated from breakthrough curves compared well with static volumetric adsorption data. Modeling of the breakthrough adsorption was achieved using a Linear Driving Force mass transfer rate model, showing good agreement with the experimental data and confirming fast kinetics and efficient use of the bed. Fast kinetics were also evidenced by the length of the unused section of the bed as calculated from a Mass Transfer Zone model. Adsorption capacity degradation was not observed after multiple regeneration cycles. Apparent and equilibrium adsorption isotherm data estimated from the bed and static volumetric experiments at 25° C were compared to that of 5A Zeolite. These showed that Sr2+ -SAPO-34 is a superior adsorbent for CO2 removal in the low partial pressure range (<1500 ppm). CO2 and H2 O multicomponent adsorption breakthrough curves were also gathered for a CO2 inlet concentration of 1000 ppm and dew points of −5 and 8° C. The addition of moisture resulted in a decrease in total processed gas volume by 31 and 47%, respectively.