Functionalization of zeolites
1. A method for functionalizing a zeolite, comprising:
- (a) providing a zeolite;
(b) mixing the zeolite into a solution of a phosphonic acid in a first organic solvent to form a slurry;
(c) stirring and centrifuging the slurry and decanting a liquid fraction thereof to form a modified powder;
(d) washing or rinsing the modified powder with a second organic solvent; and
(e) removing the second organic solvent to obtain the functionalized zeolite.
Functionalized zeolites, including a zeolite substrate and a self-assembled monolayer of a phosphonic acid on a surface of the zeolite substrate, are disclosed, as are methods of making and using the functionalized zeolites. The disclosed methods and compositions have various applications, including in the use of molecular sieves to separate small-molecule gases from mixtures thereof. Gas adsorption selectivities and diffusion rates of the functionalized zeolites may be tuned or selected according to the disclosed methods.
- 1. A method for functionalizing a zeolite, comprising:
(a) providing a zeolite; (b) mixing the zeolite into a solution of a phosphonic acid in a first organic solvent to form a slurry; (c) stirring and centrifuging the slurry and decanting a liquid fraction thereof to form a modified powder; (d) washing or rinsing the modified powder with a second organic solvent; and (e) removing the second organic solvent to obtain the functionalized zeolite.
- View Dependent Claims (2, 3, 4, 5)
- 6. A functionalized zeolite, comprising:
a zeolite substrate; and a self-assembled monolayer of a phosphonic acid, disposed on a surface of the zeolite substrate.
- View Dependent Claims (7, 8, 9, 10, 11, 12, 13, 14)
- 15. A method for separating a first chemical compound from a mixture comprising the first chemical compound and a second chemical compound, the method comprising:
contacting the mixture with a functionalized zeolite to selectively sorb the first chemical compound, the functionalized zeolite comprising a zeolite substrate and a self-assembled monolayer of a phosphonic acid disposed on a surface of the zeolite substrate.
- View Dependent Claims (16, 17, 18, 19, 20)
This application claims the benefit of U.S. Provisional Patent Application 62/664,763, filed Apr. 30, 2018, the entirety of which is incorporated herein by reference.
This invention pertains generally to the functionalization of zeolites, and specifically to the tuning of gas adsorption selectivity and diffusion rates into zeolites by use of phosphonic acid monolayers.
The separation of mixtures by distillation to yield high-purity components accounts for 10-15% of the world'"'"'s energy consumption. Among these separations, alkene/alkane separations are among the most difficult and energy-intensive. For example, propylene (C3H6) is typically produced via thermal and fluid catalytic cracking of propane (C3H8), resulting in propylene/propane mixtures; these mixtures are then separated by distillation to obtain polymer-grade propylene, which requires a purity of 99.5%. One estimate suggests that 0.3% of global energy consumption is used to separate propylene and ethylene (C2H4) from their alkane analogues. More broadly, energy demands are high for many separations of light gases, including ethylene/ethane (C2H6), ethane/propane/butane (C4H10), and oxygen (O2)/nitrogen (N2) separations. High demand and the energy-intensive nature of conventional separation technologies have spurred significant research efforts into alternative separation technologies, including selective adsorption, amine adsorption (which is economically and energetically inefficient), and the use of membranes.
Application of nanoporous molecular sieves in moving bed or pressure swing adsorption systems has been found to have the potential to significantly decrease energy requirements for propane/propylene separation and other light gas separations. Zeolites have received particular attention as molecular sieves because of their uniform and near-molecular diameter pore sizes. Though zeolites may have a range of pore diameters, commercially available zeolites do not have diameters that are continuous across the range of light gases, and substantial effort has been devoted to creating materials with more tunable pore diameters or chemistries. One approach has been to change the bulk crystal structure or cations within the zeolite; other studies have functionalized the external surface of zeolites with an additional diffusion layer, e.g. by reacting the zeolite with silanes; and hybrid approaches have been used to modify zeolites by functionalizing the pores and external surface with specific functional groups, e.g. amines for binding acids. Recently, zeolite 5A, whose pore diameter of 0.43 nm is close to the molecular diameters of propylene and propane, has been functionalized with titania-based molecular layer deposition (MLD); this increased the propylene/propane ideal adsorption selectivity from 1.2 to 6.0.
One approach that has recently been used to improve the specificity of interactions between organic molecules and porous materials has been in the application of organic self-assembled monolayer (SAM) coatings to heterogeneous catalysts. SAMs are formed using amphiphilic monomers with hydrophilic head groups, which deposit on the substrate surface in solution, and long-chain hydrocarbon tails that align to assemble the monolayer; the self-assembly of the monolayer is spontaneous due to thermodynamically favorable van der Waals interactions between tail groups. Monomer head groups commonly used to form SAMs include silanes, thiols, and phosphonic acids. By tuning the functionality of the organic ligands in SAM coatings, the binding and reaction of organic species on the catalyst can be tuned by effects ranging from steric repulsion to specific non-covalent interactions between the SAM and the reactant.
Because adsorption/desorption is a key phenomenon linking membrane or sorbent separations and catalysis, it has been hypothesized that the application of organic modifiers based on SAM chemistry to zeolites may be applicable to control selectivity in binding for separations; in principle, organic modifiers can alter diffusion characteristics of zeolites through a “gatekeeping” function at the external surface of zeolite particles, or by inducing changes to the pore structure itself. However, self-assembly of monolayers of phosphonic acids—which are particularly desirable head groups due to their high thermal stability—on zeolites has not previously been successful, and the effect of phosphonic acid-based monolayers on zeolites is poorly understood.
There is thus a need in the art for tunable methods and systems for depositing phosphonic acid-based SAMs on zeolites, as well as tuned, functionalized zeolites produced by such methods and systems.
It is one aspect of the present invention to provide a method for functionalizing a zeolite, comprising (a) providing a zeolite; (b) mixing the zeolite into a solution of a phosphonic acid in a first organic solvent to form a slurry; (c) stirring and centrifuging the slurry and decanting a liquid fraction thereof to form a modified powder; (d) washing or rinsing the modified powder with a second organic solvent; and (e) removing the second organic solvent to obtain the functionalized zeolite.
In embodiments, step (a) may comprise calcining and subsequently cooling a zeolite material.
In embodiments, the zeolite may be selected from the group consisting of zeolite 5A, SAPO-34 zeolite, MFI zeolites, chabazite, Y zeolite, faujasite, ferrierite, mordenite, SSZ-13 zeolite, and ZSM-5 zeolite.
In embodiments, the organic tail of the phosphonic acid may comprise between one and eighteen carbon atoms.
In embodiments, the method may further comprise, between steps (c) and (d), annealing the modified powder.
It is another aspect of the present invention to provide a functionalized zeolite, comprising a zeolite substrate; and a self-assembled monolayer of a phosphonic acid, disposed on a surface of the zeolite substrate.
In embodiments, the zeolite substrate may be annealed.
In embodiments, the zeolite substrate may comprise at least one of a powder and a pellet.
In embodiments, the zeolite substrate may comprise at least one zeolite selected from the group consisting of zeolite 5A, SAPO-34 zeolite, MFI zeolites, chabazite, Y zeolite, faujasite, ferrierite, mordenite, SSZ-13 zeolite, and ZSM-5 zeolite.
In embodiments, the organic tail of the phosphonic acid may comprise between one and eighteen carbon atoms.
In embodiments, the zeolite substrate may comprise zeolite 5A and the phosphonic acid may be methylphosphonic acid.
In embodiments, at least a portion of the monolayer may be disposed on or within a pore of the zeolite substrate.
In embodiments, at least one of the following may be true: (i) the functionalized zeolite has a propylene/propane ideal adsorption selectivity of at least about 45; (ii) the functionalized zeolite has an ethane/propane ideal adsorption selectivity of at least about 44; and (iii) the functionalized zeolite has an ethane/n-butane ideal adsorption selectivity of at least about 13.
In embodiments, an organic tail of the phosphonic acid may comprise at least one of an alkyl chain having between one and eighteen carbon atoms, a branched alkyl group, an unsaturated hydrocarbon including at least one carbon-carbon double (alkene) or triple (alkyne) bond, a haloalkane, a haloarene, an alcohol, an ether, an aldehyde, a ketone, a carboxylic acid, a carboxylate, an ester, an acyl group, an acyl halide, an acid anhydrides, an amine, an amide, a nitrile, an aromatic group, a nonaromatic cyclic group, an organosulfur, an organophosphorus, and an organometallic
It is another aspect of the present invention to provide a method for separating a first chemical compound from a mixture comprising the first chemical compound and a second chemical compound, the method comprising contacting the mixture with a functionalized zeolite to selectively sorb the first chemical compound, the functionalized zeolite comprising a zeolite substrate and a self-assembled monolayer of a phosphonic acid disposed on a surface of the zeolite substrate.
In embodiments, at least one of the first chemical compound and the second chemical compound may be selected from the group consisting of carbon dioxide, methane, ethylene, ethane, ethanol, propylene, propane, propanol, butadiene, 1-butene, 2-butene, isobutene, n-butane, isobutane, water, oxygen, and nitrogen.
In embodiments, the first chemical compound may be propane and the second chemical compound may be propylene or vice versa, the zeolite substrate may comprise zeolite 5A, and the phosphonic acid may be methylphosphonic acid.
In embodiments, the functionalized zeolite may have a propylene/propane ideal adsorption selectivity of at least about 45.
In embodiments, the zeolite substrate may comprise at least one zeolite selected from the group consisting of zeolite 5A, SAPO-34 zeolite, MFI zeolites, chabazite, Y zeolite, faujasite, ferrierite, mordenite, SSZ-13 zeolite, and ZSM-5 zeolite.
In embodiments, an organic tail of the phosphonic acid may comprise at least one of an alkyl chain having between one and eighteen carbon atoms, a branched alkyl group, an unsaturated hydrocarbon including at least one carbon-carbon double (alkene) or triple (alkyne) bond, a haloalkane, a haloarene, an alcohol, an ether, an aldehyde, a ketone, a carboxylic acid, a carboxylate, an ester, an acyl group, an acyl halide, an acid anhydrides, an amine, an amide, a nitrile, an aromatic group, a nonaromatic cyclic group, an organosulfur, an organophosphorus, and an organometallic.
While specific embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise configuration and components described herein. Various modifications, changes, and variations which will be apparent to those skilled in the art may be made in the arrangement, operation, and details of the methods and systems of the present invention disclosed herein without departing from the spirit and scope of the invention. It is important, therefore, that the claims be regarded as including any such equivalent construction insofar as they do not depart from the spirit and scope of the present invention.
The advantages of the present invention will be apparent from the disclosure contained herein.
As used herein, “at least one,” “one or more,” and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B, and C,” “at least one of A, B, or C,” “one or more of A, B, and C,” “one or more of A, B, or C,” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B, and C together.
It is to be noted that the term “a” or “an” entity refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more,” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising,” “including,” and “having” can be used interchangeably.
The embodiments and configurations described herein are neither complete nor exhaustive. As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below.
As used herein, terms of the form “CxPA” refer to a phosphonic acid (PA) having an organic tail that is x carbon atoms in length, i.e. “C1PA” refers to methylphosphonic acid, “C2PA” refers to ethylphosphonic acid, “C3PA” refers to propylphosphonic acid, “C18PA” refers to octadecylphosphonic acid, and so on. C1PA may also be referred to as “MPA,” and C18PA may also be referred to as “ODPA.” Where necessary, a leading letter may indicate a structural isomer of a particular phosphonic acid, e.g. “nC4PA” refers to n-butylphosphonic acid while “tC4PA” refers to tert-butylphosphonic acid, but where no structural isomer leading letter is provided, the linear isomer (n-butyl, n-octadecyl, etc.) may be assumed. It is to be expressly understood that phosphonic acids suitable for use in the present invention may have organic chains or tails other than or in addition to simple linear alkyl hydrocarbon tails, including, by way of non-limiting example, branched alkyl (e.g. tert-butyl) groups, unsaturated hydrocarbons including at least one carbon-carbon double (alkene) or triple (alkyne) bond, haloalkanes and haloarenes, alcohols, ethers, aldehydes, ketones, carboxylic acids, carboxylates, esters, acyl groups, acyl halides, acid anhydrides, amines, amides, nitriles, aromatic and other cyclic groups, and groups including other heteroatoms not previously described (e.g. organosulfur, organophosphorus, and organometallic groups).
As used herein, unless otherwise specified, the term “zeolite” refers to any material having a zeolitic structure, including but not limited to zeolites and zeotypes.
The present inventors have modified zeolites via the deposition of phosphonic acids, and specifically via the deposition of self-assembling monolayers (SAMs) of phosphonic acids. The present inventors have discovered that the size or length of the alkyl tail of the phosphonic acid and other deposition parameters may be utilized as tools to pattern the phosphonic acid on specific locations of the zeolite, allowing the inventors to tune total loadings of certain gases and diffusion rates; this ability results in zeolites capable of efficiently separating molecules of nearly identical size. Without wishing to be bound by any particular theory, it is believed that the size or length of the phosphonic acid tail may govern, at least in part, the extent to which phosphonic acid SAMs penetrate pores of the zeolite, and thus the effective pore size and surface structure of the zeolite may be controlled by choosing a zeolite and a phosphonic acid suitable for a given application. Aspects of the present invention are thus directed to methods and systems for controllably depositing phosphonic acid SAMs on zeolites, and to the tuned, functionalized zeolite compositions themselves. Other aspects of the present invention are directed to methods and systems for separating a mixture of two or more chemical compounds into one or more higher-purity component fractions by utilizing tuned, functionalized zeolites as highly selective molecular sieves. It is to be expressly understood that any zeolite having a pore size and structure suitable for use in a given application may suitably be used in the practice of the present invention, including, by way of non-limiting example, zeolite 5A, SAPO-34 zeolite, WI zeolites, chabazite, Y zeolite, faujasite, ferrierite, mordenite, SSZ-13 zeolite, and ZSM-5 zeolite. Zeolites may also be provided in any suitable physical form, including but not limited to powders and pellets.
The disclosure now being generally described will be more readily understood by reference to the following examples, which are included merely for the purposes of illustration of certain aspects of the embodiments of the present disclosure. The examples are not intended to limit the disclosure, as one of skill in the art would recognize from the above teachings and the following examples that other techniques and methods can satisfy the claims and can be employed without departing from the scope of the claimed disclosure.
Zeolite 5A was functionalized by calcining 750 mg of zeolite 5A powder for 4 hours at 400° C., cooling the calcine to room temperature, and then adding the calcine to 180 mL of a 5 mM solution of a phosphonic acid in tetrahydrofuran (THF). For this Example, six phosphonic acids were used: C1PA, n-C4PA, n-C8PA, n-C10PA, n-C14PA, and n-C18PA. The resulting slurry was stirred overnight and centrifuged, and the solvent was then decanted; the resulting powder was annealed in air at 120° C. for 6 hours. The annealed powder was then cooled to room temperature and extensively washed with tetrahydrofuran to remove all physisorbed phosphonic acids. As used in Examples 1-5, unless otherwise noted, the term “native material” refers to zeolite 5A that has been subjected to the process described in this paragraph, except that the liquid to which the calcine was added was pure THF (i.e. did not contain phosphonic acid).
Powder X-ray diffraction (XRD) spectra were obtained with an Inel CPS 120 powder XRD system with a monochromated Cu Kα radiation source that was calibrated with silicon and silver behenate standards, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed using a Harrick Praying Mantis diffuse reflectance accessory (samples were run at atmospheric conditions, using a mirror as a background, with 100 scans at a resolution of 4 cm−1).
As indicated by the XRD spectra illustrated by
Brunauer-Emmett-Teller (BET) surface area measurements were performed on each powder using nitrogen adsorption in triplicates after pretreating samples at 200° C. for 3 hours in an atmosphere of 30% nitrogen (N2) and 70% helium, and phosphorus loadings were measured by inductively coupled plasma mass spectrometry (ICP-MS). The results are presented in Table 1.
The BET surface area measurements and elemental analysis presented in Table 1 are consistent with most of the phosphonic acid being deposited on the external surface of the zeolite, with similar surface areas measured for native and functionalized materials. However, zeolites modified with the smallest phosphonic acid, C1PA, had a lower surface area than the native material and much higher levels of phosphorus than any other material. Although C1PA has been found to form somewhat denser coatings on the surface of other substrates, e.g. aluminum oxide, the increase in phosphorus loading (approximately an order of magnitude relative to other samples) suggests more extensive modification of the zeolite.
Scanning transmission electron microscopy (STEM) images and energy dispersive X-ray spectroscopy (EDS) maps of the C1PA- and C18PA-functionalized zeolite powders described in Example 1 were collected on an FEI Talos F200X device operated at 200 kV; sample powders were dispersed onto a carbon film 300 mesh copper grids. Cross-sectioned samples were prepared by microtoming; powdered samples were mixed into epoxy resin and polymerized overnight in a 60° C. oven, then thin (100 to 150 nm) sections were cut with a diamond knife on a Leica UCT Ultramicrotome and picked up onto copper grids for analysis. Quantification of the chemical content of the zeolite materials was averaged across all imaged areas in each sample; the results of the quantification are presented in Table 2.
As illustrated by
To complement these techniques, low-energy ion scattering (LEIS) analysis was carried out by IonTOF on a Qtac100 instrument using a 5 keV 4He+ beam and an analysis current of 2 nanoamperes; to establish peak positions for the elements aluminum, silicon, and phosphorus, aluminum oxide (Al2O3), silicon dioxide (SiO2), and calcium pyrophosphate (Ca2P2O7) were used as reference materials. The LEIS spectra (not shown in the Figures) indicated, for the C18PA-coated zeolites, only surface carbon could be detected (no surface phosphorus, silicon, or aluminum was present), which is consistent with extensive modification of the outer zeolite surface with a dense layer of alkyl ligands. For the C1PA-coated zeolite, substantial carbon signal was also detected (not shown), but signals from phosphorus, aluminum, and silicon were prominent as well; as discussed in greater detail below, LEIS experiments following calcination of the samples suggests that phosphorus content was significantly higher in C1PA-coated samples than in C18PA-coated samples.
Pressure decay adsorption and isotherm measurements of the functionalized zeolite powders described in Example 1 were performed on an Autosorb-1 instrument equipped with a custom LabVIEW-based data acquisition system. Pressure decay adsorption was performed at a manifold pressure of 100 kPa after samples had been pretreated for 3 hours at 200° C. under vacuum.
As illustrated by
The clear outlier to the above-referenced trends was zeolites functionalized with C1PA. The initial diffusion rates of carbon dioxide (34.9% of native) and methane (9.3% of native) were both much lower than the other functionalized materials, but the total achievable loading was roughly identical to that of the native material; by contrast, total loading of n-butane on the C1PA-functionalized zeolite is only about 1% that of the native material.
Given the dramatic differences in adsorption performance between the C1PA-modified zeolite and the other modified zeolites, adsorption of ethane, propylene, and propane on the C1PA-modified zeolite was also studied using similar techniques. As illustrated by
Referring now to
C1PA- and C18PA-functionalized zeolites as described in Example 1 were calcined at 400° C. for 2 hours, and the initial diffusion rates and loadings thereon of carbon dioxide, methane, and n-butane were measured according to the procedure of Example 4. The loadings and diffusion rates of the calcined, functionalized zeolites were generally lower for all gases than for their uncalcined counterparts, as illustrated by
As Examples 1 through 4 illustrate for zeolite 5A, C1PA modifiers have a much larger effect on diffusion and adsorption of light gases than phosphonic acids having alkyl ligands containing four or more carbon atoms. Without wishing to be bound by any particular theory, it is believed that these differences result from the smaller molecular size of C1PA relative to other phosphonic acid modifiers, which allows C1PA molecules to access sites in the near-surface region (perhaps at the pore entrance) of the zeolite particle; this hypothesis is consistent with the smaller BET surface area, higher surface phosphorus content, and much smaller propane and n-butane uptake for C1PA-modified zeolites than for zeolites modified with other phosphonic acids.
Projected minimum diameters of various SAM and adsorbate molecules were calculated using the online software package Chemicalize™. The results are presented in Table 3.
As shown in Table 3, the projected minimum diameter of C1PA is estimated to be approximately 35% smaller than the projected minimum diameter of C18PA. Without wishing to be bound by any particular theory, the observed sensitivity of diffusion to molecular size suggests that C1PA either partially blocks the zeolite pore entrance or occupies some of the internal volume at positions close to the external surface.
Phosphorus nuclear magnetic resonance (NMR) spectra were collected in a 7 T Bruker Avance IIIHD 300 spectrometer measuring at 121.5 MHz; the spectrometer was calibrated using ammonium dihydrogen phosphate (ADP) set to 0 ppm. Samples were prepared by placing approximately 100 mg of hydrated zeolite into a 4 mm zirconia rotor and spinning at 10 kHz, and spectra were collected with proton decoupling at a frequency of 300.2 MHz. Measurements of the C1PA-modified zeolite were made using a 1-microsecond pulse with a 5-second delay between pulses over a total of 256 scans, and measurements of the C18PA-modified zeolite were made using a 1-microsecond pulse with a 1-second delay between pulses over a total of 1,280 scans.
As illustrated by
Examples 1-5 above demonstrate that the inventors have been able to select phosphonic acid modifiers to selectively tune diffusion and loadings of small molecules in zeolites. Adding C1PA to zeolites can increase the ideal adsorption selectivities in ethane/propane, ethane/n-butane, and propylene/propane separations by factors of 40 to 80; particularly, the propylene/propane ideal adsorption selectivity of 59 is of great interest and value given the difficulty and significant energy requirements of conventional propylene/propane separations. Without wishing to be bound by any particular theory, C1PA appears to partially penetrate into a zeolite during deposition of the monolayer, so as to dramatically change adsorption and diffusion characteristics of some molecules without affecting the zeolite'"'"'s bulk crystalline structure.
SAPO-34 zeolite powders were synthesized by both microwave and non-microwave synthesis, using a reactant mixture of 1.0 part aluminum oxide, 2.0 parts phosphorus pentoxide (P2O5), 0.6 part silicon dioxide, 4.0 parts tetraethylammonium hydroxide ((C2H5)4N(OH), “TEAOH”), and 75 parts water (H2O). 200 mg of powder was mixed with a volume of a 10 mM phosphonic acid-in-toluene solution (stirred for 2 to 3 hours prior to addition of powder to facilitate dissolution) sufficient to provide a 10-fold stoichiometric excess of the phosphonic acid monomer. This mixture was vigorously stirred overnight (600 to 700 rpm for 16 to 24 hours), centrifuged (7000 rpm for 5 minutes), and decanted, and the resulting powder was then annealed in an oven at 120° C. for 4.5 hours. On the day after annealing, the annealed, functionalized zeolite powder was broken with a spatula and then rinsed (by adding liquid, mixing with vigorous shaking and 10 seconds in a vortex mixer, centrifuging, and decanting) three times with toluene and once with THF. As used in this and all subsequent materials, unless otherwise noted, the term “native material” refers to SAPO-34 that has been subjected to the process described in this paragraph, except that the liquid to which the calcine was added was pure toluene (i.e. did not contain phosphonic acid).
As illustrated by
Referring again to the IR spectra of
The mass that the monolayer contributes to the functionalized zeolite was measured by thermogravimetric analysis (TGA) and compared to the calculated mass of a complete monolayer to determine if physisorbed layers or other impurities may be present. Microwave-synthesized SAPO-34 particles have a cubic structure and an average width (a) of 300 nm; thus, the particles have a volume a3 of 2.7·107 nm3 and a surface area 6a2 of 5.4·105 nm2. The framework density of SAPO-34, equal to the density of silicon and aluminum atoms, is 15.1 atoms per cubic nanometer, and from this it can be calculated that the density of the zeolite is 4.06·1021 g/nm3; multiplying this by the volume gives an approximate mass of one zeolite particle of 1.10·10−13 g. To calculate the mass of the monolayer, the packing density was estimated. C18PA molecules bind to hydroxyl groups on the surface of the zeolite particle, but little is known about the density of the hydroxyl groups or how zeolite bonds terminate at the surface of the particle. To get an estimate of maximum coverage, the monomers were approximated as perfectly packed squares with a width equal to the kinetic diameter of the phosphonic acid molecule, which (because the alkyl tail is the widest part of the molecule) is approximated as equal to the diameter of the alkyl tail, in this case 0.43 nm. Dividing the calculated surface area of the zeolite particle (5.4·105 nm2) by this value gives an approximation of the number of monomer molecules per particle as 2.92·106; multiplying by the molecular weight of C18PA gives a total mass of C18PA per zeolite particle of 1.62·10−15 g, or 1.5% of the mass of the functionalized zeolite. TGA, however, measured the monolayer as making up 5% of the particle mass; this deviation from the calculated value, combined with the IR results illustrated by
Samples of the functionalized zeolite described in Example 6 were loaded into a bulbous sample cell by funneling the powder into the cell using weigh paper and then forcing the sample into the spherical part of the cell with a pipe cleaner. A glass rod was added to the sample cell, and the sample cell was attached to a Quantachrome Autosorb-1 device using the given fittings, with the O-ring lightly greased with vacuum grease; the Autosorb was operated in manual mode to evacuate the sample cell, first with the fine vacuum valve and then with the coarse vacuum valve after the pressure inside the cell dropped below 30 torr. Once the pressure in the cell dropped below 10 torr, a heating jacket was placed around the spherical portion of the cell, and the sample was outgassed at 200° C. overnight, after which the heating jacket was removed and the cell cooled to room temperature. The volume of the sample cell was then determined by filling the manifold of the sample cell with helium, allowing the helium to expand into the sample cell, and recording the pressure drop due to the expansion; the sample cell was then evacuated by coarse vacuum at room temperature for 20 minutes to remove any helium.
To test gas adsorption of the functionalized zeolite, the manifold was filled with the adsorbate gas to a pressure of 200 to 220 torr (for carbon dioxide or methane) or 1000 to 1040 torr (for propane or n-butane), monitored via the pressure display on the Autosorb device or a LabVIEW computer program. The initial adsorbate pressure in the manifold was recorded by LabVIEW, with the input still on the manifold pressure gauge. The pressure was then allowed to equilibrate (typically less than 1 minute for carbon dioxide or methane and about 10 minutes for propane or n-butane), and, with the LabVIEW program now monitoring the sample cell pressure gauge, the valve between the sample cell and the manifold was opened for just enough time to allow the two volumes to equilibrate in pressure and then immediately closed. The pressure over time in the sample cell was then recorded on the LabVIEW every 0.5 seconds, or every 5 seconds for samples with slower adsorption rates, until the pressure reached and maintained a constant value.
Samples were then outgassed for 1 hour, or longer if the pressure increased in the sample cell when the valve was closed after 1 hour, to remove adsorbed gases. The sample cell was then cooled to room temperature and removed from the Autosorb by filling to atmospheric pressure with helium. Once removed from the Autosorb, the cell was immediately weighed. Sample cell volume was calculated based on the ideal gas law and the manifold pressure reported by the Autosorb; the sample cell and manifold volumes were then used to calculate the change in adsorbate pressure from expansion based on the initial adsorbate pressure. The decrease in pressure beyond that expected from expansion was then converted to a molar quantity via the ideal gas law, where the volume is that of the sample cell (for propane or n-butane) or the combined volume of the manifold and sample cell (for carbon dioxide or methane). The sample mass was calculated as the net mass difference between the mass of the bulb and glass rod measured before testing and the mass of the outgassed sample, bulb, and glass rod measured after testing, from which the adsorption capacity on a mass basis (moles absorbed per gram) was calculated. The adsorption rate was quantified by the slope of the initial linear region (approximated as beginning when adsorption equals zero) of the adsorption-over-time curve; this regime often lasted minutes for treated samples, but unmodified samples adsorbed gases quickly and a rough estimate of slope could be gathered from the first few time points. Some adsorption testing was carried out at elevated temperature (although the helium testing was always done at room temperature to obtain accurate sample volume), although not all of a sample may have been at the elevated temperature because the heating jacket used did not cover the entire volume of the sample cell.
As illustrated by
The lack of effect on carbon dioxide adsorption rate is particularly important because high carbon dioxide permeability is critical for separating carbon dioxide from natural gas; carbon dioxide decreases the energy contact of natural gas and is corrosive to pipelines in the presence of water, and current amine adsorption processes for removing the carbon dioxide are economically and energetically inefficient. Methane diffusion is slowed, but to a lesser extent than the longer hydrocarbons. The weight of the functionalized zeolite was adjusted for the monolayer mass as determined by TGA so that adsorption values are based on weight of the zeolite alone. Total methane and propane adsorption capacities for the C18PA-coated zeolite closely match that of the untreated zeolite, but n-butane and carbon dioxide adsorption capacities are 13-16% lower.
N-butane was also adsorbed on SAPO-34 powder that was exposed to toluene. As illustrated by
As previously discussed, gas adsorption rates were estimated from the initial slope of adsorption over time, but this curve is not, or at least not in all cases, linear over the initial time range.
Zeolite samples were prepared according to the procedure described in Example 6, except that some samples were subjected to a different anneal time to investigate the effect of this parameter on gas adsorption.
N-butane was adsorbed onto C18PA-functionalized samples annealed for 4.5 hours, 8 hours, and 12 hours. As illustrated by
As shown in Table 4, and without wishing to be bound by any particular theory, longer anneal times may result in more organized and dense packing of both the monolayer and any physisorbed multilayers. More densely packed multilayers interact more strongly with the monolayer to which they are physisorbed, and increasing anneal time may therefore create multilayers that are too strongly physisorbed to be effectively washed off by toluene or THF, resulting in multilayers on the final sample. The formation of such multilayers would cause a greater increase in weight and, potentially, a further decrease in n-butane adsorption rate, due to the presence of more material at the surface. IR spectra of these samples are illustrated by
Zeolite samples were prepared according to the procedure described in Example 6, except that some samples were subjected to a different concentration of phosphonic acid in the toluene solution to investigate the effect of this parameter on gas adsorption.
N-butane was adsorbed onto native material and onto zeolite samples functionalized in 1 mM, 5 mM, and 10 mM C10PA solutions. As illustrated by
Zeolite samples were prepared according to the procedure described in Example 6, except that the solvent was THF rather than toluene, using three different phosphonic acids—C4PA, C10PA, and C18PA—to investigate the effect of alkyl tail length on gas adsorption. As illustrated by
Subsequent to this test, the present inventors discovered that use of THF as a solvent rather than toluene tends to decrease total adsorption (see Example 12 below); while all samples whose test results are illustrated by
C18PA-modified zeolite samples were prepared according to the procedure described in Example 6, and n-butane was adsorbed onto these samples and native material at a series of temperatures, as illustrated by
The data more closely fit a model with two trendlines, where the slope changes at approximately 100° C. (373 K), as illustrated by
Some zeolite samples were prepared according to the procedure described in Example 6, except that THF rather than toluene was used as the solvent. These samples were generally characterized by decreased gas adsorption relative to samples functionalized in toluene solutions; as illustrated by
Without wishing to be bound by any particular theory, one possible explanation for this phenomenon is that the specific zeolite used in this Example (SAPO-34) is known to be a catalyst for dimethyl ether conversion, a reaction that is typically carried out at temperatures of about 500° C. (773 K) but that may, due to the presence of the THF ether bond, occur at low levels at the temperatures involved in this test, particularly during annealing at 120° C. (393 K). Comparison of n-butane adsorption on C18PA-modified zeolite to n-butane adsorption on native material exposed only to THF reveals that THF slows adsorption by a factor of 36, but the addition of the C18PA monolayer slows the uptake rate by a factor of 210.
Zeolites functionalized with C18PA according to the procedure of Example 6 were exposed to water; the contact angles resulting from this exposure are illustrated by
Examples 6-13 indicate that surface modification with a phosphonic acid monolayer decreases the rate of adsorption of propane and n-butane into zeolites, has a small effect on methane adsorption, and has little or no effect on adsorption of carbon dioxide or water. Without wishing to be bound by any particular theory, these results suggest that alkane molecules may have an affinity for the monolayer due to van der Waals forces, causing the alkanes to spend more time in the monolayer before entering the pores of the zeolite. These results are consistent with previous studies that demonstrated the affinity between alkanes and hydrocarbon monolayers.
The van der Waals effect would be greater for longer hydrocarbon adsorbates, as each additional carbon atom contributes more potential for van der Waals interactions. This is consistent with the stronger effect seen on longer hydrocarbons. The same principle applies to increasing the alkyl tail length of the monolayer; as the alkyl tail length increases, so too does the number of sites for van der Waals interaction that the adsorbate passes through before reaching the pore opening. Thus, the alkane would take more time to pass through a longer-tailed monolayer, which is consistent with results from testing coatings of monomers of different tail lengths. However, longer tails can also create a more densely packed monolayer; since van der Waals attraction between the monomer tails is the driving force for monolayer packing, self-assembly is greater for longer chains.
Although the mechanism of van der Waals forces is consistent with the results, other possibilities exist. The diameter of SAPO-34 pores, as used in Examples 6-13, is barely wide enough to fit molecules of linear alkanes having three or more carbon atoms. Adding a monolayer may crowd the surface enough to make it harder for these longer molecules to fit through the pore openings, slowing adsorption.
The invention illustratively disclosed herein suitably may be practiced in the absence of any element which is not specifically disclosed herein. It is apparent to those skilled in the art, however, that many changes, variations, modifications, other uses, and applications of the invention are possible, and also changes, variations, modifications, other uses, and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is limited only by the claims which follow.
The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. In the foregoing Detailed Description of the Invention, for example, various features of the invention are grouped together in one or more embodiments for the purpose of streamlining the disclosure. The features of the embodiments of the invention may be combined in alternate embodiments other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description of the Invention, with each claim standing on its own as a separate preferred embodiment of the invention.
Moreover, though the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations, combinations, and modifications are within the scope of the invention, e.g. as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable, and/or equivalent structures, functions, ranges, or steps to those claimed, whether or not such alternate, interchangeable, and/or equivalent structures, functions, ranges, or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.