Specific-Ion Effects Unveil a Route for Modulating Oxidatively Triggered Acid Systems for Reservoir Applications.

On-demand in situ preparation of industrially relevant organic acids, namely, methanesulfonic acid, triflic acid, and trifluoroacetic acid, is demonstrated in this study. Sodium and potassium bromate were found to selectively oxidize a series of ammonium salts NH4X, where X = OMs, OTf, or OTFAc, with characteristic clock reaction behavior. The redox system undergoes rapid acid formation following an extended induction time at 150 °C and is identified as a potential candidate for high-temperature oil field chemistry applications where on-demand acid placement is required. Although the reaction kinetics for acid formation from NH4X salts where X = Cl, Br, F, or SO42- follows a pKa trend, the rates of formation of the organic acids are much slower and deviate from this trend. Furthermore, we demonstrate that the rate of acid formation can be modulated by the addition of alkali metal salts, with the strongest effect observed from LiBr. Spectroscopic studies implicate the formation of lithium bromate ion pairs that slow or altogether inhibit the oxidation of NH4+. Additionally, the presence of Br- alters the reaction path, eliminating the clock behavior and creating a pathway for Li+ to strongly inhibit the redox reaction. From these studies, a method for slowing ammonium oxidation under reservoir conditions to sufficiently delay acid formation until the precursors are placed in the zone of interest is identified.

Bromate Oxidation of Ammonium Salts: In Situ Acid Formation for Reservoir Stimulation.

A redox chemistry approach has been employed to synthesize an assortment of acids in the subterranean environment for the purpose of enhancing productivity from hydrocarbon-bearing rock formations. Experimental studies revealed that bromate selectively oxidizes a series of ammonium salts NH4X where X = F-, Cl-, Br-, SO42-, and CF3CO2- to produce 5-17 wt % HX. Importantly, the in situ method allows strategic placement of the acid in the zone of interest where the fluid is heated, and the reaction is triggered. Ammonium counteranions are shown to influence the kinetics of the bromate-ammonium reaction, and the conditions are tailored to promote oxidation of ammonium at reservoir temperatures. The reaction is observed to be acid-catalyzed, where the formation of bromous acid (HBrO2) is involved in the rate-limiting step. As a result, an induction period that scales with the p Ka of the acid being formed is followed by rapid formation of the reaction products.

Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation.

The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal-organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF(2-)(6)) anions enables a 'sweet spot' of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

Potential of metal-organic frameworks for separation of xenon and krypton.

CONSPECTUS: The total world energy demand is predicted to rise significantly over the next few decades, primarily driven by the continuous growth of the developing world. With rapid depletion of nonrenewable traditional fossil fuels, which currently account for almost 86% of the worldwide energy output, the search for viable alternative energy resources is becoming more important from a national security and economic development standpoint. Nuclear energy, an emission-free, high-energy-density source produced by means of controlled nuclear fission, is often considered as a clean, affordable alternative to fossil fuel. However, the successful installation of an efficient and economically viable industrial-scale process to properly sequester and mitigate the nuclear-fission-related, highly radioactive waste (e.g., used nuclear fuel (UNF)) is a prerequisite for any further development of nuclear energy in the near future. Reprocessing of UNF is often considered to be a logical way to minimize the volume of high-level radioactive waste, though the generation of volatile radionuclides during reprocessing raises a significant engineering challenge for its successful implementation. The volatile radionuclides include but are not limited to noble gases (predominately isotopes of Xe and Kr) and must be captured during the process to avoid being released into the environment. Currently, energy-intensive cryogenic distillation is the primary means to capture and separate radioactive noble gas isotopes during UNF reprocessing. A similar cryogenic process is implemented during commercial production of noble gases though removal from air. In light of their high commercial values, particularly in lighting and medical industries, and associated high production costs, alternate approaches for Xe/Kr capture and storage are of contemporary research interest. The proposed pathways for Xe/Kr removal and capture can essentially be divided in two categories: selective absorption by dissolution in solvents and physisorption on porous materials. Physisorption-based separation and adsorption on highly functional porous materials are promising alternatives to the energy-intensive cryogenic distillation process, where the adsorbents are characterized by high surface areas and thus high removal capacities and often can be chemically fine-tuned to enhance the adsorbate-adsorbent interactions for optimum selectivity. Several traditional porous adsorbents such as zeolites and activated carbon have been tested for noble gas capture but have shown low capacity, selectivity, and lack of modularity. Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are an emerging class of solid-state adsorbents that can be tailor-made for applications ranging from gas adsorption and separation to catalysis and sensing. Herein we give a concise summary of the background and development of Xe/Kr separation technologies with a focus on UNF reprocessing and the prospects of MOF-based adsorbents for that particular application.

Tunable Rare Earth fcu-MOF Platform: Access to Adsorption Kinetics Driven Gas/Vapor Separations via Pore Size Contraction.

Reticular chemistry approach was successfully employed to deliberately construct new rare-earth (RE, i.e., Eu(3+), Tb(3+), and Y(3+)) fcu metal-organic frameworks (MOFs) with restricted window apertures. Controlled and selective access to the resultant contracted fcu-MOF pores permits the achievement of the requisite sorbate cutoff, ideal for selective adsorption kinetics based separation and/or molecular sieving of gases and vapors. Predetermined reaction conditions that permitted the formation in situ of the 12-connected RE hexanuclear molecular building block (MBB) and the establishment of the first RE-fcu-MOF platform, especially in the presence of 2-fluorobenzoic acid (2-FBA) as a modulator and a structure directing agent, were used to synthesize isostructural RE-1,4-NDC-fcu-MOFs based on a relatively bulkier 2-connected bridging ligand, namely 1,4-naphthalenedicarboxylate (1,4-NDC). The subsequent RE-1,4-NDC-fcu-MOF structural features, contracted windows/pores and high concentration of open metal sites combined with exceptional hydrothermal and chemical stabilities, yielded notable gas/solvent separation properties, driven mostly by adsorption kinetics as exemplified in this work for n-butane/methane, butanol/methanol, and butanol/water pair systems.

Quest for highly connected metal-organic framework platforms: rare-earth polynuclear clusters versatility meets net topology needs.

Gaining control over the assembly of highly porous rare-earth (RE) based metal-organic frameworks (MOFs) remains challenging. Here we report the latest discoveries on our continuous quest for highly connected nets. The topological exploration based on the noncompatibility of a 12-connected RE polynuclear carboxylate-based cluster, points of extension matching the 12 vertices of the cuboctahedron (cuo), with 3-connected organic ligands led to the discovery of two fascinating and highly connected minimal edge-transitive nets, pek and aea. The reduced symmetry of the employed triangular tricarboxylate ligand, as compared to the prototype highly symmetrical 1,3,5-benzene(tris)benzoic acid guided the concurrent occurrence of nonanuclear [RE9(µ3-OH)12(µ3-O)2(O2C-)12] and hexanuclear [RE6(OH)8(O2C-)8] carboxylate-based clusters as 12-connected and 8-connected molecular building blocks in the structure of a 3-periodic pek-MOF based on a novel (3,8,12)-c trinodal net. The use of a tricarboxylate ligand with modified angles between carboxylate moieties led to the formation of a second MOF containing solely nonanuclear clusters and exhibiting once more a novel and a highly connected (3,12,12)-c trinodal net with aea topology. Notably, it is the first time that RE-MOFs with double six-membered ring (d6R) secondary building units are isolated, representing therefore a critical step forward toward the design of novel and highly coordinated materials using the supermolecular building layer approach while considering the d6Rs as building pillars. Lastly, the potential of these new MOFs for gas separation/storage was investigated by performing gas adsorption studies of various probe gas molecules over a wide range of pressures. Noticeably, pek-MOF-1 showed excellent volumetric CO2 and CH4 uptakes at high pressures.

Tunable rare-earth fcu-MOFs: a platform for systematic enhancement of CO2 adsorption energetics and uptake.

A series of fcu-MOFs based on rare-earth (RE) metals and linear fluorinated/nonfluorinated, homo/heterofunctional ligands were targeted and synthesized. This particular fcu-MOF platform was selected because of its unique structural characteristics combined with the ability/potential to dictate and regulate its chemical properties (e.g., tuning of the electron-rich RE metal ions and high localized charge density, a property arising from the proximal positioning of polarizing tetrazolate moieties and fluoro-groups that decorate the exposed inner surfaces of the confined conical cavities). These features permitted a systematic gas sorption study to evaluate/elucidate the effects of distinctive parameters on CO2-MOF sorption energetics. Our study supports the importance of the synergistic effect of exposed open metal sites and proximal highly localized charge density toward materials with enhanced CO2 sorption energetics.

Synthesis and integration of Fe-soc-MOF cubes into colloidosomes via a single-step emulsion-based approach.

Bottom-up fabrication of complex 3D hollow superstructures from nonspherical building blocks (BBs) poses a significant challenge for scientists in materials chemistry and physics. Spherical colloidal silica or polystyrene particles are therefore often integrated as BBs for the preparation of an emerging class of materials, namely colloidosomes (using colloidal particles for Pickering stabilization and fusing them to form a permeable shell). Herein, we describe for the first time a one-step emulsion-based technique that permits the assembly of metal-organic framework (MOF) faceted polyhedral BBs (i.e., cubes instead of spheres) into 3D hollow superstructures (or "colloidosomes"). The shell of each resultant hollow MOF colloidosome is constructed from a monolayer of cubic BBs, whose dimensions can be precisely controlled by varying the amount of emulsifier used in the synthesis.

Highly monodisperse M(III)-based soc-MOFs (M = In and Ga) with cubic and truncated cubic morphologies.

In this work, we carry out an investigation on shape-controlled growth of In(III)- and Ga(III)-based square-octahedral metal-organic frameworks (soc-MOFs). In particular, controllable crystal morphological evolution from simple cubes to complex octadecahedra has been achieved, and resultant highly uniform crystal building blocks promise new research opportunities for preparation of self-assembled MOF materials and related applications.

A cubic, 12-connected, microporous metal-organometallic phosphate framework sustained by truncated tetrahedral nodes.

A rare example of a microporous metal-organic phosphate, [Co(12)(L)(6)(µ(3)-PO(4))(4)(µ(3)-F)(4)(µ-H(2)O)(6)][NO(3)](2) (1), is synthesized by the reaction of a [(η(5)-C(5)H(5))Fe(II)](+)-functionalized terephthalate ligand with Co(NO(3))(2)·6H(2)O and phosphate and fluoride ions generated from the in situ hydrolysis of hexafluorophosphate. 1 is a cubic, 12-connected, face-centered cubic framework sustained by the linear connection of unprecedented, dodecanuclear truncated tetrahedral coordination clusters.

The quest for modular nanocages: tbo-MOF as an archetype for mutual substitution, functionalization, and expansion of quadrangular pillar building blocks.

A new blueprint network for the design and synthesis of porous, functional 3D metal-organic frameworks (MOFs) has been identified, namely, the tbo net. Accordingly, tbo-MOFs based on this unique (3,4)-connected net can be exclusively constructed utilizing a combination of well-known and readily targeted [M(R-BDC)](n) MOF layers [i.e., supermolecular building layers (SBLs)] based on the edge-transitive 4,4 square lattice (sql) (i.e., 2D four-building units) and a novel pillaring strategy based on four proximal isophthalate ligands from neighboring SBL membered rings (i.e., two pairs from each layer) covalently cross-linked through an organic quadrangular core (e.g., tetrasubstituted benzene). Our strategy permits the rational design and synthesis of isoreticular structures, functionalized and/or expanded, that possess extra-large nanocapsule-like cages, high porosity, and potential for gas separation and storage, among others. Thus, tbo-MOF serves as an archetypal tunable, isoreticular MOF platform for targeting desired applications.

Zeolite-like metal-organic frameworks (ZMOFs) based on the directed assembly of finite metal-organic cubes (MOCs).

Two zeolite-like metal-organic frameworks (ZMOFs) with lta- and ast- topologies, zeolitic nets that can be interpreted as augmented edge-transitive 8-connected nets, are targeted through directed self-assembly of metal-organic cubes (MOCs) as supermolecular building blocks (SBBs).

Tuning Gas Adsorption Properties of Zeolite-like Supramolecular Assemblies with gis Topology via Functionalization of Isoreticular Metal-Organic Squares.

A strategy based on metal-ligand directed assembly of metal-organic squares (MOSs), built-up from four-membered ring (4MR) secondary building units (SBUs), has been employed for the design and construction of isoreticular zeolite-like supramolecular assemblies (ZSAs). Four porous Co-based ZSAs having the same underlying gis topology, but differing only with respect to the capping and bridging linkers, were successfully isolated and fully characterized. In this series, each MOS in ZSA-3-ZSA-6 possess an ideal square geometry and is connected to four neighboring MOS via a total of 16 hydrogen bonds to give a 3-periodic porous network.To systematically assess the effect of the pore system (size and functionality) on the gas adsorption properties, we evaluated the MOSs for their affinity for different probe molecules such as CO2 and light hydrocarbons. ZSA-3-ZSA-6 showed high thermal stability (up to 300 °C) and was proven highly porous as evidenced by gas adsorption studies. Notably, alkyl-functionalized MOSs were found to offer potential for selective separation of CO2, C3H6, and C3H8 from CH4 and H2 containing gas stream, such as natural gas and refinery-off gases.

A facile solvent-free synthesis route for the assembly of a highly CO2 selective and H2S tolerant NiSIFSIX metal-organic framework.

The development of materials for CO2 capture with high selectivity and high tolerance to H2S is of prime importance for various industrially relevant gas streams (e.g. natural gas and biogas upgrading as well as pre-combustion capture). Here, we report the successful fabrication of a MOF with combined exceptional CO2 capture properties and H2S tolerance, namely the NiSIFSIX-based MOF using both solvothermal and solvent-free methodologies.

Versatile rare earth hexanuclear clusters for the design and synthesis of highly-connected ftw-MOFs.

A series of highly porous MOFs were deliberately targeted to contain a 12-connected rare earth hexanuclear cluster and quadrangular tetracarboxylate ligands. The resultant MOFs have an underlying topology of ftw, and are thus (4,12)-c ftw-MOFs. This targeted rare earth ftw-MOF platform offers the potential to assess the effect of pore functionality and size, via ligand functionalization and/or expansion, on the adsorption properties of relevant gases. Examination of the gas adsorption properties of these compounds showed that the ftw-MOF-2 analogues, constructed from rigid ligands with a phenyl, naphthyl, or anthracene core exhibited a relatively high degree of porosity. The specific surface areas and pore volumes of these analogs are amongst the highest reported for RE-based MOFs. Further studies revealed that the Y-ftw-MOF-2 shows promise as a storage medium for methane (CH4) at high pressures. Furthermore, Y-ftw-MOF-2 shows potential as a separation agent for the selective removal of normal butane (n-C4H10) and propane (C3H8) from natural gas (NG) as well as interesting properties for the selective separation of n-C4H10 from C3H8 or isobutane (iso-C4H10).

Porous organic polymers with anchored aldehydes: a new platform for post-synthetic amine functionalization en route for enhanced CO2 adsorption properties.

A novel porous organic polymer has been synthesized using the molecular building block approach to deliberately encompass aldehyde functionalities amenable to post functionalization. The resultant porous framework allows a facile, one-step quantitative and post-synthetic functionalization by amines, permitting enhanced CO2 sorption properties.

Discovery and introduction of a (3,18)-connected net as an ideal blueprint for the design of metal-organic frameworks.

Metal-organic frameworks (MOFs) are a promising class of porous materials because it is possible to mutually control their porous structure, composition and functionality. However, it is still a challenge to predict the network topology of such framework materials prior to their synthesis. Here we use a new rare earth (RE) nonanuclear carboxylate-based cluster as an 18-connected molecular building block to form a gea-MOF (gea-MOF-1) based on a (3,18)-connected net. We then utilized this gea net as a blueprint to design and assemble another MOF (gea-MOF-2). In gea-MOF-2, the 18-connected RE clusters are replaced by metal-organic polyhedra, peripherally functionalized so as to have the same connectivity as the RE clusters. These metal-organic polyhedra act as supermolecular building blocks when they form gea-MOF-2. The discovery of a (3,18)-connected MOF followed by deliberate transposition of its topology to a predesigned second MOF with a different chemical system validates the prospective rational design of MOFs.