Development of Ambient Nanogibbsite Synthesis ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

Development of an ambient nanogibbsite synthesis and incorporating the method for embedding ultrafine nanoAl(OH)3 into channels and partial alumination of MCM-41 Viktor Dubovoy, Michael Stranick, Laurence Du-Thumm, and Long Pan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01791 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 11, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Development of an ambient nanogibbsite synthesis and incorporating the method for embedding ultrafine nano-Al(OH)3 into channels and partial alumination of MCM-41 Viktor Dubovoy, Michael Stranick, Laurence Du-Thumm, Long Pan* Colgate-Palmolive Company, 909 River Road, Piscataway, USA ABSTRACT: A ultrafine aluminum hydroxide nanoparticle suspension was prepared via controlled titration of [Al(H2O)6]3+ with L-arginine to pH 4.6. The prepared material, predominantly 10-30 nm in diameter, was purified by GPC technique and subsequently identified as gibbsite (or hydrargillite) polymorph via FTIR, powder XRD, and elemental analysis. Chemical environment and morphology were probed using 27Al/1H NMR, FTIR, ICP-OES, TEM-EDS, XPS, XRD, and N2 adsorption experiments. Furthermore, by incorporating the newly developed synthetic route, Al(OH)3 was partially loaded inside the mesopores (2.7nm) of MCM-41. EDS and NMR analysis indicates both tetrahedral and octahedral Al (Oh/Td = 1.4) is incorporated at 11% w/w total Al and Si/Al of 2.9 indicating part of Al embedded into Si framework. In addition, differences in elemental composition between surface XPS and bulk EDS analysis provided insight into the distribution of Al within the material. Higher ratio of Si/Al was observed on the external surface (3.6) of MCM-41 compared to the internal (2.9) cavities. Estimated O/Al ratios suggest predominantly Al(O)3 and Al(O)4 motifs present near the core and external surface, respectively. This novel methodology produces Al-MCM-41 with relatively high Al content while preserving the ordered SiO2 framework and can be used in lateral applications where incorporating hydrated or anhydrous Al2O3 is desired.

INTRODUCTION Aluminum hydroxide exhibits desirable characteristics for a variety of industrial applications including catalysis1, water treatment2, cosmetics3, and pharmaceuticals4. It is capable of absorbing a considerable amount of heat during thermal decomposition pathway to the ubiquitous Al2O3 catalyst, making it a useful flame retarding agent.5 The known hydrated alumina polymorph minerals with the stoichiometry Al(OH)3 are gibbsite (also known as hydrargillite), bayerite, nordstrandite, and doyleite. Since aluminum hydroxide is an important industrial material, both experimental and theoretical investigations have been carried out to elucidate the intricate structure and formation of the four phases.6 A number of recent studies focused on developing methodology for the controlled synthesis of nanometer size gibbsite. Nanogibbsite particles with diameter on the order of 100nm can be readily synthesized under various conditions.7-14 However, preparation of aluminum hydroxide particles with smaller particle size is a challenge. Only a few cases of uniform gibbsite particles on the scale of 50nm have been reported14-17 while none have been observed less than 50nm to the best of our knowledge. The challenge

of synthesizing such small particles, especially in polar protic solvents, is to overcome their propensity to aggregate due to electrostatic instability and high probability of hydrogen bond formation between discrete colloidal particles. Herein, aqueous aluminum hydroxide aggregation was inhibited in the presence of L-arginine buffer. Stabilization of aqueous nanoparticles via buffer doping has previously demonstrated promising results (eg. CdS nanoparticles stabilized by surface polyphosphate adsorption).18 In this article, through extensive characterization, we identified that the guanidinium-containing amino acid arginine prevents metal hydroxide aggregation to stabilize Al(OH)3 nanoparticles with average hydrodynamic diameter 10-30nm. It is proposed that the amphoteric and zwitterionic nature of arginine effectively shielded the surface charge of Al(OH)3 nanoparticles during the mild hydrolysis, yielding an environment disfavoring extensive aggregation. Although arginine itself could not further reduce particle size in this “open” system, taking advantage of cage-effect confinement in a mesoporous material MCM-41, we detect ultrafine metal hydroxide nanoparticles formed within mesopores as well as framework alumination of MCM-41.

ACS Paragon Plus Environment

Crystal Growth & Design EXPERIMENTAL SECTION

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Al(OH)3 Nanoparticle Synthesis and Purification. Reagent grade aluminum chloride hexahydrate (AlCl36H2O), aluminum hydroxide (Al(OH)3), and Larginine (C6H14N4O2) were supplied by Alfa Aesar (Ward Hill, MA), Sigma Aldrich (St. Louis, MO), and BioKyowa (Cape Girardeau, MO), respectively. All materials were used as received by manufacturer. Sample preparation entailed addition of arginine to aluminum chloride with subsequent dilution, sonication, mixing, and thermal treatment at 50°C for up to a week. Arginine to aluminum molar ratios were varied in 0.61M AlCl36H2O solutions. Synthesis optimization was monitored using SEC equipped with a differential refractive index (dRI) detector. Separation was carried out using a Protein Pak 125 column by Waters (Milford, MA) with 20 minute run time and 1mL/min flow rate. Mobile phase consisted of deionized water acidified with 1.01% w/w HNO3 to pH 2.3. A gel permeation chromatography (GPC) column equipped with dRI detector and Bio-Rad P-4 packing was used for purification. The purified material was used for all subsequent analysis. Loading Al Nanoparticles. MCM-41 was activated in a 120°C oven under vacuum for at least 3 hours to remove excess water and other atmospheric contaminants. The aforementioned nano-Al(OH)3 synthesis was carried out in the presence of MCM-41 powder. 0.7g of activated MCM-41 was added to 50.0g of 0.61M AlCl36H2O solution under vigorous magnetic stirring at 50°C. Adequate mixing time (>1 hour) was allowed for homogeneity of AlCl3 diffused throughout MCM-41 channels. Arginine was added into the mixture to reach an Arg/Al ratio of 2.75. All ratios provided herein are molar unless otherwise stated. The solution was subsequently heated for 72 hours at 50°C, filtered, and washed with excess deionized water. Characterization. 27Al NMR spectra was obtained, in D2O solutions, on a Varian 400MHz Inova instrument with 66.7 mM NaAl(OD)4 internal standard. Chemical offset was set on a reference standard of 0.1M Al(NO3)39H2O in D2O solution to 0 ppm. Samples were equilibrated at 90°C for 20 minutes prior to data collection. Solid state 27Al MAS NMR was obtained under ambient conditions. The small-angle x-ray diffraction patterns (SAXRD) were obtained by using a Bruker HiStar multi-wire area detector on a rotating-anode x-ray generator (Nonius FR571) equipped with a 3-circle Azlan goniometer (Bruker), a 0.25 mm pin-hole collimator (Bruker) and a Rigaku Osmic parallel-mode (e.g. primary beam dispersion less than 0.01 deg in 2θ) mirror monochromator (Cu Kα; λ = 1.5418Å) operating at 40 kV and 50 mA. Data were collected at room temperature (20 °C). Samples were placed on the outside of a 1 mm special glass capillary (Charles Supper, Inc.). The sample to detector distance was 28 cm. Flood-field correction and spatial calibrations were performed at these distances prior to data collection. X-ray data was collected while rotating the sample 1 deg in ω without any rotation of φ. The detector angle 2θ was fixed at 0 deg. Scan time was 300 sec. For each data analysis, a χ integration in conic section 0 < χ < 360 deg was made for the 1 < 2θ < 7.5 deg

Page 2 of 10

region with intensities integrated using the binsummation method (GADDS software) using a 0.03 step size in 2θ. FTIR-ATR analysis was collected using an extended range Spectrum One Perkin Elmer system featuring a CsI beam splitter, DTGS detector, and single-bounce diamond KRS-5 ATR crystal. ESCA (XPS) analysis was carried out using PHI 5000 VersaProbe II Scanning XPS Microprobe instrument with a monochromatic Al Kα xray source (1486.6 eV) and 200 µm beam diameter. The sample was pressed into a 3 mm well on a stainless steel mount and analyzed at an angle of 75° with respect to the electron energy analyzer. PHI MultiPak software was used for subsequent data analysis. TEM micrographs were collected using FEI Tecnai G2 F20 X-TWIN Transmission Electron Microscope equipped with field-emission gun (FEG), scanning unit (STEM) with bright field/dark field detector, x-ray detector (EDS) for compositional analysis and CCD camera. Gas adsorption measurements were performed on a 3Flex volumetric sorption analyzer (Micromeritics Instrument Corporation). Liquid nitrogen was used as coolant to achieve cryogenic temperature (77K). Ultrahigh purity (99.999%) nitrogen was used for the measurements. Prior to each measurement, about 150 mg sample was outgassed under high vacuum. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to calculate the specific surface area and pore size or volume, respectively. Zeta potential, size distribution, conductivity, and electrophoretic mobility measurements were obtained using a Malvern Zetasizer Nano ZS. RESULTS AND DISCUSSION Nanogibbsite Synthesis. Size-exclusion chromatography (SEC) is an indispensable technique for particle size characterization of partially hydrolyzed aluminum systems. Typically, SEC can resolve the wide array of antiperspirant clusters into five domains, arbitrarily designated peaks 1-5.3 Prior to this report, peak 1 species were presumed as zirconium clusters while peaks 2-5 were assigned to aluminum species. The small, highly acidic salts (eg. Al3+) elute under peak 5 with relatively larger elution time. A wide array of soluble aluminum hydroxide clusters19, including various Baker-FiggisKeggin and flat structures, elute within peaks 2, 3, and 4 with intermediate elution times. To our best knowledge, no report exists pertaining to synthesis or characterization of the enormous Al species that elute under SEC peak 1 with the smallest elution time. In this study, we report that Al(OH)3 nanoparticles ranging 10-30 nm in diameter are a component of possibly several discrete molecules that elute under SEC peak 1. Preliminary SEC and XPS analysis suggested that the prepared transparent peak 1 suspension is an enormous polyoxometallic cluster; however, upon further structural (PXRD) and spectroscopic (27Al NMR and FTIR) analysis, it was unambiguously assigned to a Al(OH)3 structure with cationic impurities exhibiting [Al(OH)xCl3-x] stoichiometry (where x is the molar hydrolysis ratio ranging 0-3). SEC


Page 3 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


data demonstrates that the nanoparticles can be synthesized at a conversion rate of 82%. The hydrolysis ratio ([OH-]/[Al3+]) is a critical parameter for the controlled hydrolysis of [Al(H2O)]3+ to Al(OH)3. A variety of base addition mechanisms have been proposed to hydrolyze Al3+ ranging from slow addition of strong base (eg. NaOH) to soft hydrolysis via ion exchange resin or thermal decomposition of urea.20-21 Utilization of a strong base inevitably leads to a high local concentration of hydroxide ions yielding undesired premature precipitation of insoluble Al(OH)3, which has a low propensity to redisperse in the solution and form the valuable acidic nanoparticles. The formation of precipitation can be typically detected visually as a reduction of solution clarity, which was not noticed in the prepared transparent suspension of nanogibbsite. It is reported herein that arginine provided a mild equilibriumdependent alkalinity that suppressed bulk Al(OH)3 formation yielding a transparent and stabilized nanogibbsite at Arg/Al molar ratio 2.75. Arginine displays zwitterionic acid-base properties, exhibiting carboxylic acid (pKa= 2.17), α-amino (pKa= 9.04), and guanidinium (pKa= 12.48) functional groups with isoelectric point 10.76, allowing it to be used as a functional amphoteric buffer and relatively mild source of OH-.22 Due to the complexity of the argininealuminum dynamic system and a lack of kinetic constants for the numerous known hydrolyzed Al compounds, the exact hydrolysis ratio achieved by arginine was not predicted theoretically. It has been demonstrated that gibbsite structure forms at pH 5.8 or lower, while bayerite and nordstrandite prefer higher pH values.23 In the current work, nordstrandite and bayerite formation was suppressed by stopping the titration at pH 4.6 (see Fig. 1). Increasing Arg/Al ratio to 3 results in a drastic pH increase, suggesting formation of bulk Al(OH)3 phase. The stepwise hydrolysis of trivalent aluminum aqua acid is initiated by the following deprotonation equilibrium of the Al hydration shell:3

Al3+ yielding a transparent dispersed nano-Al(OH)3 suspension with 10-30 nm average hydrodynamic diameter at Arg/Al 2.75. The results were confirmed using a number of well-established spectroscopic (27Al NMR, FTIRATR), chromatographic (SEC, GPC), microscopic (TEM, SEM-EDS), and x-ray (XRD, XPS) experiments. Characterization. Vibrational spectroscopy, including infrared and Raman spectroscopy, is an invaluable analytical tool to discern gibbsite phase from remaining polymorphs.26-27 FTIR-ATR spectrum (see Fig. 3) of postGPC powder is consistent with the Al(OH)3 structure. Three regions of the characteristic Al(OH)3 spectrum have been assigned: the O-H stretch (2900-3700 cm-1), the AlOH bending (800-1300 cm-1), and the octahedral Al-O vibration (< 800 cm-1).14 In such materials, the O-H stretch region is isolated and therefore quite useful as a diagnostic tool to discern bayerite from gibbsite. Bayerite and gibbsite both crystallize in the monoclinic P21/n space group with nearly identical unit cell lattice parameters, if a and b are interchanged. However, gibbsite can be distinguished from bayerite based on the high frequency OH stretch at 3620 cm-1 (G) or 3650 cm-1 (B).6,14

Figure 1. Measured pH values at varied Arg/Al ratios.

⥂ (1) Subsequently, the partially neutralized monohydroxy species easily condense to yield the µ-bis(hydroxy) bridged [Al2(OH)2(H2O)8]4+ dimer building block. At approximately [OH-]/[Al3+] of 2.5, hydrolysis-dependent condensation yields polycationic clusters such as [Al13O4(OH)24(H2O)12]7+ (Al13-mer) and its dimer Al30-mer with crystalline size on the order of 1-2 nm, which were observed at 63 and 70 ppm in the Arg/Al 2.25 spectra (see Fig. 2).24 Currently, the largest reported aluminum polyoxometallic cluster, Al30-mer, exhibits dimensions on the order of 2 nm as determined by single crystal x-ray diffraction.24-25 However, the inherent difficulty of crystallizing such inorganic polymer structures has prevented any reports of aluminum clusters larger than 2 nm. To date, enormous polyaluminum clusters3 with more than 50 aluminum atoms have been proposed and tentatively characterized via a variety of state of the art instruments and techniques. In this research, we report that arginine promoted the hydrolysis of the inner hydration shell of

Figure 2. Liquid 27Al NMR of samples with Arg/Al 0 (a), 2.25 (b), and 2.75 (c)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. FTIR-ATR absorption spectrum of purified Al(OH)3 powder

Figure 4. TEM micrograph of purified nano-Al(OH)3

The IR bands observed in the OH stretch (3617, 3523, 3453 cm-1) and AlOH bending region (1023, 970, 918 cm1) are characteristic of gibbsite.23,28-29 Arginine infrared bands were not observed suggesting effective purification and a lack of coordination between arginine and aluminum. At Arg/Al ratio of 2.75, distribution of the gibbsite particles was 10-30 nm average diameter (see Fig. S1) as determined via static light scattering method. Crystalline size was determined by applying the Scherrer equation3031 (see Equation 2) to the powder x-ray diffraction spectrum (see Fig. S2),

(2)

where L is the mean crystalline size, K is the dimensionless shape factor, λ is the wavelength of the incident radiation, b is the additional broadening, and θ is the Bragg angle. Approximation of K = 1 yielded a crystallite size of ca. 8nm. The difference between particle and crystallite size is attributed to the crude spherical approximation of K as well as low signal to noise ratio caused by significant line broadening of nanoparticles.

Page 4 of 10

TEM (see Fig. 4) images show particles with diameters ca. 5-15 nm. 27Al (MAS) NMR analysis was performed to elucidate the Al environment and coordination modes in the prepared material. The aluminum nucleus has a spin number 5/2 and is largely limited to structures with octahedral (Oh) and tetrahedral (Td) arrangements in solution. Five-coordinated aluminum has also been proposed in certain circumstances.32 Contrary to our expectation, the dispersed nano-scale particles exhibit an 27Al NMR signal in aqueous environment, indicating this phase can be directly probed using common aqueous analysis techniques due to substantial interaction at the solid-liquid interface. In Figure 2, NMR spectra are plotted versus Arg/Al molar ratio. Aluminum chloride monomer (Al3+) exhibits a characteristic sharp NMR peak at 0 ppm. 27Al NMR data demonstrates some tetrahedral aluminum Keggin clusters, specifically Al13-mer and Al30-mer, were formed in the sample with Arg/Al 2.25 and exhibited characteristic 27Al NMR tetrahedral Al signals at 63 and 70 ppm, respectively (see Fig. 2b). As arginine content increased from Arg/Al 0-2.25, the concentration of Al13type compounds increased until reaching a maximum at 2.25. Maximum turbidity was between Arg/Al ratio 2.02.5 suggesting electrostatic instability. At Arg/Al ratio above 2.25, suspended Al(OH)3 nanoparticles were produced with the concentration reaching a maximum at 2.75. At the climax of nanoparticle concentration, liquid NMR spectrum exhibits a single Gaussian-type resonance at 8ppm. Structures with exclusively Oh Al atoms cannot be elucidated using NMR alone due to broad peaks caused by a large electric-field gradient.19 The relatively sharp nature of the NMR peak suggests presence of only a single Oh specie as well as good symmetry of Al environments. GPC technique was implemented to isolate nanoAl(OH)3 from the synthetic byproducts (i.e. arginine and soluble cationic Al compounds). Elution from the polyacrylamide packing produced two distinct peaks with adequate resolution for separation. The Al(OH)3 suspension is collected with 99% purity, exhibits pH of 6.7, and undetectable amounts of arginine (via ICP-OES). Zeta potential, often conducted parallel to potentiometric or conductivity studies, is a common technique imperative for investigating solid-liquid interface in heterogeneous systems. Moreover, zeta potential provides unique insight into adsorption behavior of charged particles to colloidal surface.33 The purified nanoparticle suspension exhibits a zeta potential of +8.9mV with electrophoretic mobility of 0.7 µmcm/Vs and conductivity of 0.7 mS/cm. The positive zeta potential can be explained by the presence of cationic aluminum species indicated by a small concentration of Cl- from XPS analysis (see Fig. S3). Chloride is present at an Al:Cl ratio of 35:1, suggesting presence of minor aluminum (hydroxide) chloride impurities. Based on XPS data, Al binding energy was 74.3 eV for both purified material and reagent grade Al(OH)3. Stoichiometric ratios of aluminum to oxygen (1:3.3) were in agreement with Al(OH)3 (see Table 1). Furthermore, zeta potential and electrophoretic mobility show only a slight deviation from deionized water. The large difference observed in conductivity is attributed to the cationic aluminum species. Initial hypothesis of aluminum


Page 5 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design Table 1. Summary of Elemental Composition Data (XPS) Atomic Concentration (wt.%) Sample

Al(OH)3 Control MCM-41 L-Arginine Nano-Al(OH)3 Al-MCM-41

Al

Si

O

C

N

Cl

O/Al

O/Si

Si/Al

21.80

-

61.51

-

-

-

2.8

-

-

-

30.10

68.33

-

-

-

-

2.3

-

-

-

15.61

58.73

25.66

-

-

-

-

22.34

-

74.46

-

-

0.63

3.3

-

-

6.13

21.75

66.36

4.04

1.53

-

2.8

2.3

3.6

hydroxide formation was formulated upon freeze drying the GPC-purified solution containing Arg/Al 2.75. It is noteworthy that the dehydrated Al(OH)3 powder was insoluble in water suggesting complete neutralization of Al3+. Precipitating Al(OH)3 with NaCl. Investigation into the stability of nano-Al(OH)3 suspension yielded intriguing observations. The dilute transparent solution eluted from GPC column was stable, in terms of flocculation, and transparent at room temperature for at least 3 months. However, addition of inorganic salts (eg. NaCl) induced colloidal instability leading to immediate precipitation of a white solid. A 1% w/w solution of NaCl was added dropwise to 10 mL of purified 1% nano-Al(OH)3 solutions. Precipitation was observed immediately. The white precipitate was collected and exhibited an FTIR spectrum identical to Figure 3. The destabilization effect of various electrolytes on metal oxide nanoparticle colloidal systems is commonly observed as a direct result of the salt shielding of surface repulsion forces.34 After successfully synthesizing the ultrafine Al(OH)3 nanoparticles eluting within SEC peak 1, all subsequent attempts to reduce particle size even further failed due to the limitation of arginine’s stabilization effect. However, we proposed that by incorporating a rigid mesoporous material to confine the aforementioned synthesis, via cage-effect, we could obtain Al(OH)3 with reduced particle size, which can be tailored by rational choice of porous framework. Al-MCM-41 Synthesis and Characterization. Mobil Composition of Matter (MCM-41) is a family of (alumino)silicate molecular sieves exhibiting an ordered array of uniform 1D channels in the mesoporous regime (1.6-10 nm). Herein, purely siliceous MCM-41 with 2.7 nm pore size was selected to investigate the interaction of MCM-41 with Al(OH)3. MCM-41 was activated in a vacuum oven at 120°C to remove atmospheric contaminants prior to aqueous aluminum chloride addition. Adequate (1h) magnetic stirring of heterogeneous system ensured maximum Al3+ concentration within MCM-41 void space. Local flocculation was observed during the slow addition of arginine, which was allowed to dissipate prior to further arginine addition. Cage-effect confinement of Al(H2O)3+ hydrolysis within mesoporous framework is able to produce ultrafine Al(OH)3 nanoparticles smaller than 2.7nm. Product formation in the bulk solution was carefully monitored via SEC and 27Al NMR. Results indicate the Al3+ reactant has been effectively converted to predominantly Al(OH)3 phase. We predicted the cage effect of the rigid mesoporous framework would constrain the Al(OH)3 particle growth. 27Al MAS NMR

Figure 5. 27Al MAS NMR spectrum of prepared nanogibbsite (a) and prepared Al-MCM-41 (b)

Figure 6. N2 sorption isotherms of MCM-41 and Al-MCM41. Inset is the corresponding pore size distribution.

Table 2. BET surface area analysis via N2 adsorption Sample BET Pore Pore Surface Volume Size Area (m2/g) (cm3/g) (nm) MCM-41 Al-MCM-41


997 742

0.932 0.649

2.7 2.1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. SAXRD spectra of MCM-41 (a) and Al-MCM41 (b)

Page 6 of 10

Figure 9. TEM micrograph of MCM-41

Figure 10. TEM micrograph of Al-MCM-41 Figure 8. 1H MAS NMR spectra of Al-MCM-41 (a) and MCM-41 (b) of the guest Al molecules demonstrates presence of both octahedral (ca. 2 ppm) and tetrahedral (ca. 57 ppm) Al environments (see Fig. 5). Mokaya et al. reported incorporation of tetrahedral aluminum into the MCM-41 framework by grafting with aluminum chlorohydrate (ACH).35 In addition to Al(OH)3 incorporated into the MCM-41 channels, consistent with their results, our NMR data suggests a portion of the Al was incorporated into the SiO2 framework. However, our synthesis led to a significantly higher content of octahedral Al (Oh/Td = 1.4) compared to Mokaya and coworkers (Oh/Td < 1).35 Al loading into MCM-41 void space was additionally confirmed via BET N2 adsorption method. Figure 6 shows the amount of adsorbed nitrogen at 77 K versus the relative pressure. The adsorption isotherm is type IV, which is typical for highly ordered mesoporous materials.36 A well-defined step occurs at approximately 0.300.40 and 0.25-0.35 P/P° for MCM-41 and Al-MCM-41, respectively, which is characteristic of capillary condensation. Incorporation of Al(OH)3 nanoparticles within the pores significantly reduced nitrogen physisorption capacity within the mesoporous SiO2 matrix. The

structure of unmodified MCM-41 exhibited BET surface area of 997 m2/g with a 0.932 cm3/g pore volume and 2.7 nm pore size, which is in agreement with literature values.37 After the Al grafting, reduction of BET surface area (20.4%), pore volume (30.4%), and pore width (22.2%) is observed (see Table 2). Moreover, the maximum adsorbed nitrogen was reduced from 602 to 419 cc/g. The drastic change in measured internal morphology suggests the channels within MCM-41 contained aluminum particles. ICP analysis of HNO3-digested AlMCM-41 suggests Al is present at ca. 10% by weight. EDS analysis (see Table 3) confirmed Al was present at 11 wt. % with Si/Al 2.90. Small angle x-ray diffraction (SAXRD) pattern of MCM-41 before and after Al insertion was obtained and indexed based on apparent hexagonal symmetry (see Fig. 7). The presence of 100 (2.2°), 110 (3.9°), 200 (4.4°), and 210 (5.8°) lattice d-spacings in both samples indicates that long range structural ordering is maintained in the Al-MCM-41 sample despite having a relatively high Al content (Si/Al ratio 2.9). Upon Al grafting, the crystallite size reduced from 50 to 40nm as determined by Jade software calculation using the Scherrer equation. The most significant difference is observed at the 4.4° 2θ peak where MCM-41 (19.86 Å) exhibits a smaller spacing


Page 7 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compared to Al-MCM-41 (19.96 Å). 1H MAS NMR spectrum of starting SiO material and 2 prepared Al-MCM-41 is illustrated in Figure 8. Incorporation of Al caused a downfield shift (ca. 1 ppm) for the predominant 3.1 ppm peak observed in the MCM-41 spectrum. Furthermore, a sharp isolated signal emerges at 0.9 ppm in Al-MCM-41 sample. Since this proton peak is not observed in the starting material and exhibits relatively stronger shielding, we assign the 0.9 ppm peak to hydroxyl groups coordinated to aluminum atoms. Previous 1H MAS NMR studies of acidic zeolites have tentatively assigned the ca. 0.9 and 2.8 ppm signals to terminal Al-OH groups with octahedral or tetrahedral coordination, respectively.38-40 High resolution transmission electron microscope (HRTEM) micrographs, as shown in Figures 9 and 10, suggest MCM-41 maintained its integrity of highly ordered hexagonal channels during internal Al(OH)3 nanoparticle formation. Even though the bulk of the functional surface area is contained within the pores, it should be noted that Si-OH groups are also present on the external surface which may have immobilized some Al(OH)3. Therefore, we predicted to observe aluminum particles anchored on the outside Si-OH groups as observed by Zhang et al. for Ag-MCM-41. Large Al(OH)3 particles were not observed near the external surface of MCM-41, which also provides further evidence that the aluminum is contained predominantly within the channels. Attempts to directly observe the host particles inside the channels were ineffective via TEM method, due to the weak contrast between the silica framework and aluminum hydroxide. After conducting XRD analysis on the prepared Al-MCM-41 solid, we could not confirm Al(OH)3 inside due to significant line broadening of nanoparticles that may have reduced the signals to an intensity indistinguishable from instrument noise even if the material structure was crystalline. Incorporating ZnO and Fe2O3 particles within mesoporous silica matrix also exhibited poor contrast.41-42 Although, consistent with similar work, some color contrast, brightness variation, and pore grid distortions were visible upon closer examination of TEM images.42 This was attributed to nonhomogeneous distribution of guest molecules. EDS analysis, shown in Table 2, was conducted on an area of the prepared solid and detected significant amounts of Al, Si, and O. Elemental composition was 8.02% Al, 23.26% Si, and 68.70% O. Surface elemental composition, measured via XPS, consists of 6.13% Al, 21.75% Si, and 66.36% O. Aluminum was not detected in the original MCM-41 material. Consistent with Al(OH)3 stoichiometry, the bulk O/Al atomic ratio, corrected for SiO2, is 2.77. However, the corrected surface O/Al ratio (3.83) suggests a high concentration of Al(O)4 tetrahedrons as also evidenced by Td NMR resonance at 57 ppm. Si/Al ratio was 2.9 and 3.6 as determined by EDS and XPS, respectively. A higher ratio of Si/Al measured using XPS versus EDS indicates more Al penetrated into the pores versus building up on the surface. Chloride was not detected in stoichiometric concentrations using either method. Other amino acids were also evaluated for capability to prepare a comparable material. Out of the evaluated amino acids (glycine, lysine, and arginine), only arginine

and lysine promoted formation of nano-Al(OH)3 phase. However, a significantly higher concentration of lysine was required, the yields couldn’t reach that of arginine, and the produced solution was bright yellow. The lower basicity of lysine may have had a large effect on the ability to neutralize Al3+ to form Al(OH)3 nanoparticles. Table 3. Comparison of EDS and XPS elemental analysis Element

EDS

XPS

Atomic Concentration (wt.%)

(wt.%)

O (K)

68.70

66.36

Al (K)

8.02

6.13

Si (K)

23.26

21.75

Si/Al

2.90

3.55

O/Al

2.77

3.73

Nevertheless, it is proposed that at concentrations relevant for environmental conditions, such interactions may arise with a number of natural amino acids which can influence bioavailability or toxicity for pertinent aquatic life forms.

CONCLUSION In this research, we prepared aluminum hydroxide nanoparticles with 10-30 nm average diameter via arginine-stabilized technique from aluminum aqua acid precursor. A transparent suspension of Al(OH)3, exhibiting neutral pH, without detectable byproduct (ie. arginine) was isolated by gel permeation chromatography column purification. Using this mild synthesis methodology, Al(OH)3 was loaded into the 2.7 nm mesoporous cavities of MCM-41 resulting in significant reduction of surface area and pore size, as determined by N2 adsorption experiments. After the post-synthesis alumination of MCM-41, Al was detected at 11.00 wt. % with Si/Al 2.90. By utilizing this method, a high loading of Al is achieved without toxic solvents and additives. The current work makes a progressive stride in furthering our understanding of the intricate and dynamic aqueous speciation of Al3+ by exposing a missing link between large hydrated polyoxometallic aluminum clusters and insoluble Al(OH)3 phase. Identification of arginine as an excellent stabilizer for controlled aluminum hydrolysis paves the way for further investigations into the mechanism as well as lateral applications for hydrated metal hydroxide systems. Furthermore, by implementing this novel mild hydrolysis it is possible to incorporate Al(OH)3 nanoparticles into a variety of porous frameworks with subsequent calcination to yield a porous material impregnated with Al2O3.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally. (match statement to author names with a symbol)

Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The authors extend their appreciation to Dr. Thomas J. Emge and Wei Liu of Rutgers University for their analysis and expertise in small angle x-ray diffraction, and powder xray diffraction. Furthermore, the authors acknowledge Hao Wang for his support with N2 adsorption experiments.

REFERENCES (1) Jackson, D.S.; Hargreaves, J.S.J. Metal Oxide Catalysis; Wiley-VCH: Weinheim, 2009. (2) Bratby, J. Coagulation and Flocculation in Water and Wastewater Treatment, 2nd ed.; IWA Publishing: London, 2006. (3) Laden, K. Antiperspirants and Deodorants, 2nd ed.; Marcel Dekker, Inc.: New York, 1999. (4) O’Hagan, D.T. Vaccine Adjuvants: Preparation Methods and Research Protocols; Humana Press Inc.: Totowa, 2000. (5) Mittal, V. Cambridge University Press: New York, 2011. (6) Demichelis, R.; Noel, Y.; Ugliengo, P.; Zicovich-Wilson, C.M., Dovesi, R. J. Phys. Chem. C 2011, 115, 13107-13134. (7) Wierenga, A.M.; Lenstra, T-A.J.; Philipse, A.P. Colloids Surf., A 1998, 134, 359-371. (8) Phambu, N.; Humbert, B.; Burneau, A. Langmuir 2000, 16, 6200-6207. (9) Rosenqvist, J.; Persson, P.; Sjoberg, S. Langmuir 2002, 18, 4598-4604. (10) Wijnhoven, J.E.G.J. Seeded growth of monodisperse gibbsite platelets to adjustable sizes. J. Colloid Interface Sci. 2005, 292, 403-409. (11) Shen, S.; Chow, P.S.; Chen, F.; Feng, S.; Tan, R. B.H. J. Cryst. Growth 2006, 292, 136-142. (12) Coelho, A.C.V.; Santos, H.S.; Kiyohara, P.K.; Marcos, K.N.P.; Santos, P.S. Materials Research 2007, 10, 183-189. (13) Liu, Y.; Ma, D.; Blackley, R.A.; Zhou, W.; Han, X.; Bao, X. J. Phys. Chem. C 2008, 112, 4124-4128. (14) Kumara, C.K.; Ng, W.J.; Bandara, A.; Weerasooriya, R. J. Colloid Interface Sci. 2010, 352, 252-258. (15) Zhu, L.; Pu, S.; Lu, F.; Liu, K.; Zhu, T.; Li, J.; Li, J. Mater. Chem. Phys. 2012, 135, 979-984. (16) Louaer, S.; Wang, Y.; Guo, L. ACS Appl. Mater. Interfaces 2013, 5, 9648-9655.

Page 8 of 10

(17) Shimpi, N.G.; Sonawane, H.A.; Mali, A.D.; Mishra, S. Polym. Bull. 2014, 71, 515-531. (18) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649-5655. (19) Casey, W.H. Large Aqueous Aluminum Hydroxide Molecules. Chem. Rev. 2006, 106, 1-16. (20) Vogels, R.J.M.J.; Kloprogge, J.T.; Geus, J.W. J. Colloid Interface Sci. 2005, 285, 86-93. (21) Shafran, K.L.; Deschaume, O.; Perry, C.C. J. Mater. Chem. 2005, 15, 3415-3423. (22) Nelson, D.L.; Cox, M.M. Lehninger Principles of Biochemistry, 4th ed.; W.H. Freeman and Company: New York, 2005. (23) Elderfield, H.; Hem, J.D. Mineral. Mag. 1973, 39, 8996. (24) Allouche, L.; Gerardin, C.; Loiseau, T.; Ferey, G.; Taulelle, F. Angew. Chem. Int. Ed. 2000, 39, 511-514. (25) Rowsell, J.; Nazar, L.F. J. Am. Chem. Soc. 2000, 122, 3777-3778. (26) Farmer, V.C. The infrared spectra of minerals; Mineralogical Society: London, 1974. (27) Ruan, H.D.; Frost, R.L.; Kloprogge, J.T. J. Raman Spectrosc. 2001, 32, 745-750. (28) Wang, S.; Johnston, C.T. Am. Mineral. 2000, 85, 739. (29) Balan, E.; Lazzer, M.; Morin, G.; Mauri, F. Am. Mineral. 2006, 91, 115. (30) Scherrer, P. Gottingen 1918, 26, 98-100. (31) Langford, J.I.; Wilson, A.J.C. J. Appl. Cryst. 1978, 11, 102-113. (32) Swaddle, T.W.; Rosenqvist, J.; Yu, P.; Bylaska, E.; Phillips, B.L.; Casey, W.H. Science 2005, 308, 1450-1453. (33) Lutzenkirchen, J.; Kupcik, T.; Fuss, M.; Walther, C.; Sarpola, A.; Sundman, O. Appl. Surf. Sci. 2010, 256, 5406-5411. (34) Peng, Y.; Tso, C.; Tsai, Y.; Zhuang, C.; Shih, Y. Sci. Total Environ. 2015, 530, 183-190. (35) Mokaya, R.; Jones, W. J. Mater. Chem. 1999, 9, 555-561. (36) Brunauer, S.; Deming, L.S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723-1732. (37) Kresge, C.T.; Leonowicz, M.E.; Roth, W.J.; Vartuli, J.C.; Beck, J.S. Nature 1992, 359, 710-712. (38) Zeng, Q.; Nekvasil, H.; Grey, C.P. J. Phys. Chem. B 1999, 103, 7406-7415. (39) Kao, H.-M.; Grey, C.P. J. Phys. Chem. 1996, 100, 51055117. (40) DeCanio, E.C.; Edwards, J.C.; Bruno, J.W. J. Catal. 1994, 148, 76-83. (41) Zhang, W.; Shi, J.; Wang, L.; Yan, D. Chem. Mater. 2000, 12, 1408-1413. (42) Froba, M.; Kohn, R.; Bouffaud, G. Chem. Mater. 1999, 11, 2858-2865.


Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information is available that includes: 1.

Static light scattering (SLS)

2.

Powder x-ray diffraction (PXRD)

3.

X-ray photoelectron spectroscopy (XPS)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 10

For Table of Contents Use Only

Development of an ambient nanogibbsite synthesis and incorporating the method for embedding ultrafine nano-Al(OH)3 into channels and partial alumination of MCM-41 Viktor Dubovoy, Michael Stranick, Laurence Du-Thumm, Long Pan* Colgate-Palmolive Company, 909 River Road, Piscataway, USA

Synopsis

Aqueous ultrafine aluminum hydroxidsuree nanoparticle suspension was prepared via controlled titration of [Al(H2O)6]3+ with L-arginine to pH 4.6. By using these ambient synthetic conditions, colloidal nanogibbsite aggregation was suppressed yielding a particle size distribution of 10-30 nm. The novel synthesis method was subsequently employed to incorporate nano-Al(OH)3 into channels of MCM-41.


Development of Ambient Nanogibbsite Synthesis ... - ACS Publications

Recommend Documents