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In Situ Natural Abundance O and Mg NMR Investigation of Aqueous Mg(OH) Dissolution in the Presence of Supercritical CO 2

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Mary Y. Hu, Xuchu Deng, Kanchana Sahan Thanthiriwatte, Virgil E Jackson, Chuan Wan, Odeta Qafoku, David A Dixon, Andrew R. Felmy, Kevin M. Rosso, and Jianzhi Hu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03443 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

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Environmental Science & Technology

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In Situ Natural Abundance 17O and 25Mg NMR Investigation of Aqueous Mg(OH)2 Dissolution in the Presence of Supercritical CO2

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Mary Y. Hu,a Xuchu Deng,a K. Sahan Thanthiriwatte,b Virgil E. Jackson,b Chuan Wan,a Odeta Qafoku,a David A. Dixon,b Andrew R. Felmy,a Kevin M. Rosso,a and Jian Zhi Hua*

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a

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b

Pacific Northwest National Laboratory, Richland, Washington 99354 USA Department of Chemistry, The University of Alabama, Shelby Hall, Tuscaloosa, AL, 35487

ABSTRACT: We report an in situ high pressure NMR capability that permits natural abundance 17O and 25Mg NMR characterization of dissolved species in aqueous solution and in the presence of supercritical CO2 fluid (scCO2). The dissolution of Mg(OH)2 (brucite) in a multiphase water/scCO2 fluid at 90 atm pressure and 50 °C was studied in situ, with relevance to geological carbon sequestration. 17O NMR spectra allowed identification and distinction of various fluid species including dissolved CO2 in the H2O-rich phase, scCO2, aqueous H2O, and HCO3-. The widely separated spectral peaks for various species can all be observed both dynamically and quantitatively at concentrations of as low as 20 mM. Measurement of the concentrations of these individual species also allows an in situ estimate of the hydrogen ion concentration, or pCH+ values, of the reacting solutions. The concentration of Mg2+ can be observed by natural abundance 25Mg NMR at a concentration as low as 10 mM. Quantum chemistry calculations of the NMR chemical shifts on cluster models aided in the interpretation of the experimental results. Evidence for the formation of polymeric Mg2+ clusters at high concentrations in the H2O-rich phase, a possible critical step needed for magnesium carbonate formation, was found.

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INTRODUCTION Capture and storage of carbon dioxide in geologic formations represents one of the most promising options for mitigating the impacts of greenhouse gases on global warming, owing to the potentially large capacity of these formations and their broad regional availability.1-6 Mineral-fluid interactions are of prime importance for reservoir permanence through prospective carbonation reactions that trap CO2 in the form of mineral phases such as metal carbonates.7-11 As a result, the reactivity of aqueous solutions saturated with scCO2 with different mineral phases including portlandite, anorthite, wollastonite, and olivine has been examined.12-21 These studies show that the primary reaction products 1

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are divalent metal carbonates (e.g., calcite and magnesite)22, 23 and, for the initial phases containing silica, the formation of either amorphous silica or at least a mineral surface enriched in silica. The nominal mechanism for the formation of the metal carbonate phases involves nucleation and growth in the aqueous phase. In an aqueous phase reaction, the concentrations of Mg2+, dissolved CO2 species such as HCO3-, and the hydrogen ion concentration (pCH+) are key quantities for understanding the detailed reaction pathways as these will define the saturation state with respect to various metal carbonate solid phase products. Furthermore, mineral dissolution rates that supply Mg2+ for these carbonation reactions are also controlled by the pCH+.14, 16, 24, 25 Nuclear magnetic resonance (NMR) spectroscopy, a quantitative, non-destructive and atom specific analytical tool, is ideally suited for such investigations.26-31 In particular, 17O NMR is attractive due to its large chemical shift range, i.e., a few thousand ppm, which provides for high sensitivity to subtle structural changes such as speciation between HCO3-, CO2 and H2O, etc. 17O NMR investigations are relatively scarce, because the only NMR-active isotope of oxygen, 17O, is the least abundant (0.037%) of all of the three natural isotopes (16O - 99.76%, 18O - 0.2%), making 17O a challenging nuclide to observe by NMR spectroscopy.32, 33 Most previously reported 17O NMR studies used isotopic enrichment for enhanced signal to noise ratio (SNR), or average the signal over a very long time. This limits 17O NMR investigations as isotope enrichment can be prohibitively expensive, and very long time data averaging at natural abundance is often not practical. Similar to 17O NMR, 25Mg NMR at natural abundance is also difficult to detect due to its low gyromagnetic ratio and its natural abundance of only 10.13%. In the current work, a novel large-sample-volume, in situ high pressure NMR probe was developed for a high field of 21.1 T (900MHz system), with new designs for both the RF coil and the high pressure sample cell. The high sensitivity enabled due to the combined high field and the large sample volume allows both 17O and 25Mg NMR observations at natural abundance. With this powerful capability, the dissolution dynamics of brucite, i.e., Mg(OH)2, in scCO2 saturated H2O was investigated, where the concentrations of HCO3-, dissolved CO2, H2O, Mg2+, and pCH+ values are obtained in situ as a function of the reaction time for a pressure of 90 atm scCO2 at 50 °C, values consistent with scCO2 stabilized at a subsurface depth of approximately 2 km. The results offer new insights into the reaction mechanism of aqueous phase dissolution of Mg(OH)2 in the presence of supercritical fluid CO2 that would otherwise be difficult to obtain, including obtaining evidence for the formation of polymeric Mg2+ clusters at high concentrations in the H2O-rich phase, a possible critical step needed for magnesium carbonate formation.

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EXPERIMENTAL SECTION NMR experiments. The 17O and 25Mg NMR experiments were performed on a 63 mm Varian-Inova Wide bore 900 MHz NMR spectrometer, operating at a magnetic field of 21.1 T and located in the Environmental Molecular Sciences Laboratory. The net bore size after the room temperature shim is 51 mm. The corresponding 17O and 25Mg Larmor frequencies are 122.04 and 55.100 MHz, respectively. All of the spectra were acquired using a specially designed large-sample-volume high pressure NMR probe described in detail below. A single pulse sequence with an approximately 45° pulse angle for liquids was used for acquiring each spectrum for both 17O and 25Mg NMR experiments. The spectra were referenced to 98% D2O (0 ppm) for 17O and 1 M MgCl2 in D2O for 25Mg. The DMFIT program was used to simulate the spectra and fit the peaks.34 High pressure NMR probe. The key components of the high pressure NMR probe are illustrated in Figure 1. The probe consists of a high pressure sample cell unit and a special RF coil. The components 2


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of the high pressure cell unit include a sample tube with OD of 15 mm and ID of 10 mm that was made of high mechanical strength zirconium dioxide (ZrO2) (labeled as “1”); an O-ring (“2”); an adaptor (“3”) made of alumina that fits into the upper heating stack of the spectrometer and guides the hot gas to the space around the sample tube when assembled for variable temperature experiments; a high pressure screw fitting (“4”) that links 1/6 inch OD high pressure PEEK tubing between the adaptor (“3”) and a high pressure ISCO syringe pump (not shown) for delivering the high pressure gas into the sample cell. “4” is mounted into the adaptor “3” from the bottom as shown at the left assembled figure, and a plastic screw sleeve (“5”) that holds the sample tube “1” and seals “1” into “3” when screwed in place by compressing the O-ring (“2”). The advantage of mounting “4” at the bottom of “3” makes it convenient to connect the high pressure capillary tube from the bottom of the probe to the high pressure ISCO pump. Note also that the top of the sample tube has a T shape. This arrangement makes it very easy for “5” to hold “1” and seals “1” when the screw is in place.

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Figure 1. Key parts of the in situ high pressure NMR probe. 1: High pressure sample tube; 2: O-ring; 3: Aluminum support; 4: High pressure fitting; 5: Screw fitting; 6: The hybrid NMR Coil.

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The RF coil is a hybrid Alderman-Grant and saddle RF coil design, consisting of a one turn AldermanGrant35 and a two turn saddle coil in series for improved field homogeneity compared with using only the saddle coil due to the better field homogeneity of the Alderman-Grant Coil and for easy tuning to the 25 Mg resonant frequency of 55 MHz due to the increased inductance from the saddle coil. The winding directions of the Alderman-Grant and the saddle coils are shown in Figure 1 and arranged such that the B1 fields generated by both coils are adding together. The one turn Alderman-Grant coil is a free standing coil made of oxygen free copper with OD of 18 mm and ID of 15.8 mm, coil length of 22 mm. The arch of the two opening windows is 120° that are symmetrically arranged around the circle with the height of the windows 14 mm. Copper wire with 0.5 mm diameter that was placed inside a 1.5 mm OD and 0.6 mm ID Teflon tube for insulation was used for winding the saddle coil. A 90° degree pulse with of 20 µs was obtained using 220 W input RF power for 17O and 362 W for 25Mg, respectively. Quantum Chemical Calculations. Computational modeling was carried out using the Amsterdam Density Functional (ADF-2013) package36-38 and Gaussian0939. The first set of ADF calculations were done with the generalized gradient approximation (GGA) based Becke-Lee-Yang-Parr functional40, 41 with dispersion correction (BLYP-D) employed for geometry optimization.42 All the calculations were carried out using the TZ2P basis set (triple Z, 2 polarization function, all-electron) with the Slater type 3



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functional implemented in the ADF program.43 NMR calculations were performed based on the geometry optimized structures at the same level of the theory and with the same basis set as those used in geometry optimization, to evaluate the chemical shielding for each atom. An additional set of calculations with ADF was performed with just the BLYP functional and with the ZORA scalar relativistic treatment with the TZ2P basis set44-46. The calculations with Gaussian09 were done with the B3LYP functional40, 47 and Ahlrichs VTZP basis set48. The NMR calculations in Gaussian09 were done in the GIAO approximation49, 50.

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RESULTS AND DISCUSSION Figure 2 demonstrates the excellent performance of this new large-sample-volume probe, where Fig. 2a highlights a natural abundance 25Mg NMR spectrum obtained on a 10 mM MgCl2 solution H2O and Fig. 2b shows a natural abundance 17O NMR spectrum acquired in 1.5 ml D2O.

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Figure 2. Performance test of the large sample volume probe. (a) 25Mg NMR for 1.5 ml 10 mM MgCl2 in H2O. Spectrum acquired using an acquisition time of 60 ms, 8540 scans with a recycle delay time of 1s. The resultant total experimental time was 2.5 h, S/N≈60 and the half linewidth (∆υ1/2) is 21 Hz with 5 Hz Lorentz line broadening bore FT. (b) Natural abundance 17O NMR spectrum acquired for 1.5 ml D2O using 128 scans, a recycle delay time of 0.5 s, 25 ms acquisition time for FID. The total experimental time was 67 s. An S/N of 650 was obtained with a half linewidth of 110 Hz and a Lorentz line broadening of 10 Hz before FT. In situ investigation of the aqueous phase dissolution of Mg(OH)2 in the presence of supercritical CO2. Natural abundance 17O NMR. Figure 3 shows natural abundance in situ 17O NMR spectra collected under 90 atm CO2 and 50 °C with and without the presence of H2O and Mg(OH)2. Note that for all of the in situ measurements discussed below, the 90 atm CO2 pressure was maintained at constant using an ISCO 260D programmable high pressure syringe pump via a capillary tube with ID of 200 µm and 4 m long. CO2 in scCO2(g) resonates at about 66.4 ppm with reference to H2O (0 ppm) (Fig. 3a). Molecular CO2 dissolved in H2O, i.e., CO2(aq), is located at about 77.9 ppm (Fig. 3b) which is well separated from that of CO2 in scCO2(g) as shown in Fig. 3C, where the interface of scCO2(g) and H2O was intentionally placed at the middle of the sensitivity region of the RF coil. At the water/scCO2 interface, the CO2 peak is split into two partially overlapped peaks, indicating diffusion and exchange of CO2 between the scCO2 phase and the aqueous phase occurring at the interface. The bicarbonate, i.e., HCO3-, peak is not 4


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observable in either Fig. 3b and Fig. 3c due to its low concentration. However, in the presence of brucite (i.e., Mg(OH)2), the HCO3- peak located at 171.7 ppm is clearly observed in Fig. 3d as a result of brucite dissolution and the resulting increase in pH which yields HCO3- and CO32-, according to reactions [1] to [3]. It should be pointed out that the NMR time scale is milliseconds in terms of NMR spectroscopy peaks. Combining with various kinds of relaxation studies, the time scale can be extended to pico seconds to seconds. For simple chemical exchange between two species, such as bicarbonate and carbonate where the predicted 17O NMR peak are very similar (0 to a few ppm) based on quantum chemistry calculations (Table 1 of the manuscript to be discussed below), a single peak would have been observed if the exchange time is shorter than about 1.0 ms.51-53 Given the possible fast exchange between bicarbonate and carbonate, i.e., Eq. [3], it is difficult to for 17O NMR to differentiate between bicarbonate and carbonate. In order to be consistent with prior NMR investigations in similar systems29-31, we favor assigning the 171.7 ppm peak in 17O NMR as the bicarbonate. Mg(OH)2+2H+ ↔ Mg2++2H2O

[1]

CO2 (aq)+ H2O ↔ HCO3- + H+ HCO3- ↔ CO32- + H+

[2] [3]

pH is approximately the negative of the logarithm to base 10 of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion.30, 54 The pH values can be estimated within experimental error (± 0.1 units) in situ using Eq. [4] and [5] because the concentrations of CO2(aq), H2O and HCO3- can all be determined from a single 17O NMR spectrum. [HCO3-][H+]/[CO2] = 10-pK* [H+]= 10-pK* [CO2]/ [HCO3-] pH~- log [H+] = pK* – log ([CO2]/ [HCO3-]).

[4] [5]

A more rigorous calculation on pH using concentration-dependent equilibrium constant via activity corrections on similar reaction systems has been reported previously.30 Since at the in situ conditions 17 O T1 for HCO3- is very short, shorter than 1 ms, 17O T1 for H2O is also short, i.e., less than 5-10 ms, while the longer component of 17O T1 for CO2 (aq) is shorter than about 60 ms (see Supporting Information Figure S1), the 17O NMR spectra acquired using a recycle delay time of 0.3 s and a 45° pulse angle is quantitative. Note the 17O NMR carrier frequency was set at approximately the CO2 (aq) peak position so that the offset effects for a 10 µs pulse was minimized (Figure S3). By taking advantage of the reliable quantitation from 17O NMR, the concentration of H2O at 50 °C and 90 atm scCO2 in the presence of brucite can be determined. Prior to the dissolution of CO2 in H2O, the density of H2O is 988 g/liter at 50 °C, meaning, that for the first 17O NMR spectrum, the density of H2O is 988g/liter = 54.9 M (mole/liter), as this spectrum was acquired without pumping scCO2 into the sample cell space. Pumping scCO2 into the sample cell at the beginning of the 2nd spectrum required about 1 minute which is much shorter than the data acquisition time for acquiring each spectrum, i.e., 5.56 minutes, meaning that all the subsequent spectra from the 2nd spectrum were acquired at 90 atm scCO2 pressure.

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Figure 3. In situ high pressure natural abundance 17O NMR spectra at 50 °C. Each spectrum was acquired using an acquisition time of 25 ms, a recycle delay time of 0.3 s and 1024 scans except (d) which required 2048 scans. (a) Pure CO2 at 90 atmospheric (atm) scCO2; (b) 1.5 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 7.3 hours, where all of the sensitive region of the RF coil was filled with sample; (c) 0.8 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 3.88 hours, where the interface of H2O and scCO2 was at approximately the center of the RF coil; (d) 0.7 M brucite (i.e., 60.9 mg Mg(OH)2) + 1.5 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 15 hours. The inserts show spectral segments that are vertically expanded 6-fold. The asymmetric lineshapes for peaks in (c) are due to the asymmetry of the sample—a partially-solidfilled coil. The difference in linewidths of the HCO3- peak versus the linewidth of the CO2(aq) linewidth in (d) is presumably due to a combination of the asymmetry of the localized EFG on the oxygen and the fast exchange between HCO3- and H2CO3.

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To establish the baseline for comparison of reaction dynamics in the presence of brucite, the system in the absence of brucite, i.e., the system of 1.5 ml H2O + 90 atm scCO2 at 50 °C, was evaluated first. The normalized area of the H2O peak as a function of reaction time (Figure 4a) decreases with increasing reaction time initially and then reaches a plateau. There are two reasons for the decrease of H2O peak area as a function of reaction time. The first is the volume expansion due to the dissolution of CO2 in H2O; and the second is the signal loss due to the decrease in the quality factor (Q) of the RF coil as a result of increased hydrogen ion concentration based on the Equations [2] and [3] after CO2 reacts with H2O. It is known11 that at 50 °C and 90 bar scCO2, the CO2 mole fraction in the H2O-rich phase is about 2%. As the experimental conditions were kept the same during the acquisition of the series of spectra as a function of reaction time, and if we further assume that Q of the RF coil does not change with reaction time, the observed NMR peak area should be directly proportional to the number of molecules, so the apparent concentration of the H2O can be determined by multiplying the normalized peak area in Fig. 4a by the initial H2O concentration, 54.9 M. Figure 4b summarizes the apparent concentrations of H2O as a function of the reaction time after 90 atm of CO2 was in contact with pure H2O at 50 °C. The apparent concentration of H2O drops from the initial 54.9 M (prior to pumping 90 bar scCO2 into H2O) to the plateau value of about 52.7 M. The drop is 4%, which is 2% higher than the literature reported value of 2% under the same experimental conditions. This calibration shows that the additional 2% is due to the 6


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signal loss from changes of the Q of the RF coil due to the changes of the bulk magnetic susceptibility arising from the increased proton and Mg2+ concentration to be discussed later. Note that the electronics of the probe remain the same, i.e., the Q change is not due to probe electronics. Changing in Q-factor increases the effective 90 degree pulse width, accounting for the major part of the signal loss, in particular if the probe is not retuned during the course of the measurement. A reasonable way to scale back this signal loss is to determine the difference between 54.9 M and the calculated H2O concentration in Figure 4b and then add 0.5 times of this value to Figure 4b, which is equivalent to using the formula: I(0) - (I(0) - I(t)) × 2%/4%=I(t) + 0.5 × (I(0) - I(t)), where I(0) is the initial H2O peak area and I(t) the H2O peak area at reaction time t). The Q loss corrected H2O concentration is presented in Figure 4c. Since in the same spectrum, the 17O peak area is proportional to the number of oxygen atoms, the concentration of CO2 dissolved in the H2O-rich phase can thus be obtained.

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Figure 4. (a) Normalized peak area of H2O; (b) Concentration of aqueous phase H2O with the apparent H2O concentration in solid dots and the true H2O concentration (after a correction is applied: see text) in open circles; and (c) Aqueous phase CO2 as a function of the reaction time obtained by in situ 17O NMR on the system of 1.5 ml H2O + 90 atm scCO2 at 50°C.

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The concentration of CO2(aq) reaches an equilibrium concentration of about 1.38 ± 0.02 M after only about 90 minutes. Note that the time to pressurize the cell with 90 atm CO2 in the sample chamber was 7



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very fast, i.e., within a minute, while the time to acquire each data point (or each spectrum) is 5.56 minutes. As CO2 is absorbed into the solution, the concentration of H2O drops from its initial value of 54.9 M to an equilibrium concentration of about 53.8 ± 0.02 M after 90 minutes of reaction time. A drop in H2O concentration from 54.9 M to 53.8 M corresponds to a volume expansion of about 2.0% (see Table S1).

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Figure 5. Results obtained from in situ natural abundance 17O NMR spectra of 0.7 M brucite + 1.5 ml H2O + 90 atm scCO2 at 50 °C as a function of the reaction time. (a) Normalized peak area of H2O; (b) Concentration of H2O with the apparent H2O concentration denoted by solid dots and the true H2O concentration denoted by open circles; (c) Concentrations of CO2(aq), HCO3- and the sum of CO2(aq) + HCO3-; (d) Estimated pH values. Note that the pH values cannot be determined due to the low HCO3concentration for reaction times less than about 44 minutes since the characteristic HCO3- peak in in situ 17 O NMR spectra cannot be observed.

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The results obtained for the system of 0.7 M brucite (i.e., 60.9 mg Mg(OH)2) + 1.5 ml H2O + 90 atm scCO2 are presented in Figure 5a, which shows that the normalized H2O peak area decreases monotonically with reaction time with a dramatic drop during the first half hour when scCO2 was injected into H2O. At a reaction time of 33 minutes, the H2O peak area has already dropped from its initial 1.0 to 0.8, which is equivalent to an approximate apparent 20.0% H2O volume expansion. At a reaction time of about 900 minutes, the normalized H2O peak area drops by approximately 40% compared with its initial value. This dramatic drop cannot be explained by simple dissolution of the scCO2 and the 0.7M added Mg(OH)2. As an example, the density of a 0.7 M MgCl2 solution at 50 °C is about 1.04g/cm3, which gives a water concentration of about 54 M only about 1.6% lower than that of pure H2O. It has been discussed above that under the condition of 50 °C and 90 bar of scCO2, the dissolution of scCO2 in the H2O-rich phase only causes the H2O concentration to drop by 2%. Adding the contributions from both the dissolved brucite and scCO2, the maximum H2O is approximately 3.6%. Although the dissolution of the Mg(OH)2 could result in a solution with a different density owing to reactions with dissolved CO2 to form bicarbonate, such reactions could not possibly account for the large differences in H2O peak area (or the corresponding apparent water concentration) observed in both the initial and the final stages of reaction. It should also be noted that the loss due to H2O dissolved in scCO2 was very small, giving the weak solubility of water in scCO2, ~0.4%, under similar conditions.11, 18, 20, 56-58 The empty space above the aqueous H2O in the high pressure cell as well as the volume of the capillary tube was less than 2 ml. Thus, < 8 µl of H2O was dissolved into the scCO2, and this effect can 8


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be ignored as compared to the 1.5 ml of initial H2O present. Furthermore, a batch reaction under the same experimental conditions as the in situ NMR studies was carried out, where a transparent window made of quartz allowed the direct observation of any volume changes. No noticeable volume expansion was observed as a function of the reaction time. Therefore, the remaining 36.4% of H2O peak area drop at long reaction time is due to the loss arising from the changes associated with the Q value of the NMR probe (i.e., the reduced skin depth and the shift of the probe tuning frequency) due to the significantly increased Mg2+ concentration and the changes in hydrogen ion concentration to be discussed below. In fact, we have carried out extensive in situ 17O NMR investigations by periodically monitoring the changes of probe tuning during the in situ NMR measurement as well as by comparing the H2O peak area at long reaction times before and immediately after releasing scCO2. We found that the center resonant frequency of the probe shifted noticeably with the reaction proceeding and the H2O peak area did not change between the spectra acquired prior to and immediately after the 90 bar scCO2 was released as the Q of the probe prior to and immediately after releasing the scCO2 were the same. These results confirm that the apparent large H2O peak area drop is due to the Q changes of the NMR probe. This effect needs be corrected before calculating the H2O concentration in situ. A simple strategy is to scale the H2O peak area based on I(0) - (I(0)-I(t))×3.6%/40%, where I(0) is the initial H2O peak area, 3.6% is the correct drop of the H2O peak area in percentage and 40% is the experimentally observed H2O peak area drop in percentage. The calculated H2O concentration as a function of the reaction time after this correction is given in Figure 5b. The concentrations of CO2(aq) and HCO3- can be determined by the following equations.

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[[HCO3-] = ((PAHCO3-/3)/(PAH2O)) × H2O concentration

[6]

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[CO2] = ((PACO2/2)/(PAH2O)) × H2O concentration

[7]

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where PA indicates peak area. The lowest concentration of HCO3- detected by natural abundance 17O NMR using a 5.6 minute data acquisition time was 20 mM by evaluating the spectra at 44 minutes of

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reaction time. The concentration ratio of [CO2]/ [[HCO3-] = (PACO2/2)/(PAHCO3-/3) is independent of the H2O concentration. This gives an accurate and robust way of determining pCH+ values via in situ 17O NMR. An advantage of in situ 17O NMR over in situ 13C NMR29, is that in the in situ 17O NMR spectra of CO2 in the aqueous phase and CO2 in the supercritical fluid phase are widely separated (Fig. 3) and no isotope enrichment is needed. Figure 5c summarizes the concentrations of CO2(aq) and HCO3- as a function of the reaction time. Figure 5d shows the corresponding pCH+ as a function of the reaction time. In contrast to the H2O concentration, the concentration of dissolved CO2(aq) rapidly increases from zero to a peak value of 1.06 M at a reaction time of 60.5 minutes. At this reaction time, the concentration of HCO3- is only about 31 mM. The concentration of H2O further drops to 53.67 M, giving rise to an equivalent volume expansion of 2.3% (see Table S1). The data in Fig. 5d show that the estimated pH is 4.8 ± 0.1, indicating the solution is acidic at a reaction time of about 44 minutes. The pH value then increases as a function of the reaction time and reaches an estimated value of about 5.6 ± 0.1 at a reaction time of 290 minutes, and leveled off afterwards, indicating that the solution has become more basic due to the continued dissolution of Mg(OH)2. The observed pH as a function of the reaction time is consistent with that reported previously using in situ 13C NMR.30 Changes in 17O NMR chemical shifts. For the H2O + scCO2 system at 50 °C, we observe (Figure 6a) that the 17O chemical shift of H2O drops monotonically from its initial value of 0 ppm (without CO2 injected into H2O) to a minimum of -0.25 ppm at a reaction time of 100 minutes. Concurrently, the 17O chemical shift of the dissolved CO2 drops from the first observable value of 78.05 ppm to a minimum of 9



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77.83 ppm at a reaction time of about 100 minutes. For both H2O and CO2(aq), only a slight increase in the 17O chemical shifts are observed with further increase of the reaction time, reaching -0.2 ppm for H2O and 77.87 ppm for CO2 (aq) at a reaction time of about 450 minutes. For the brucite + H2O + 90 atm scCO2 system at 50 °C, the 17O chemical shifts for both the CO2 (aq) and the H2O (Figure 6b) are similar to those observed in the system of H2O + 90 atm CO2 at 50 °C albeit with more pronounced “v” character. The 17O chemical shift of HCO3- on the other hand decreases monotonically with increasing reaction time. As described below, we have used quantum chemical calculations of the chemical shifts to identify model structures that can simultaneously explain the observed chemical shift trend for 25Mg, and of the 17O chemical shift of scCO2, HCO3-, and H2O.

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Figure 6. 17O chemical shifts as a function of the reaction time obtained by in situ 17O NMR. (a) System of 1.5 ml H2O + 90 atm scCO2 at 50 °C. Top: dissolved scCO2; and Bottom: H2O. (b) System of 0.7 M brucite(Mg(OH)2)) + 1.5 ml H2O + 90 atm scCO2 at 50°C. Top: HCO3-; Middle: dissolved scCO2; and Bottom: H2O. The considerable localized noise in (b) for the cases of HCO3- and CO2 (aq) are mainly due to the inherent error of integration of the peaks associated with the lower S/N of the corresponding peaks as shown in Figure 3d compared with H2O, where apparently the S/N associated with the HCO3is lower than that of CO2 (aq), thus even more noise for the case of HCO3-.

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Natural abundance 25Mg NMR. Figure 7 summarizes the results obtained from in situ 25Mg NMR studies of the brucite (Mg(OH)2) plus H2O plus CO2 at 50°C, where the 25Mg NMR spectra were acquired as a function of the reaction time. A stack of 92 spectra, each acquired using 0.22 hrs consecutively, are displayed in Fig. 7a. Essentially a single peak close to zero ppm with respect to the 1.0 M MgCl2 standard aqueous solution was observed for all of the spectra in the series, indicating the generation of aqueous Mg2+. A shoulder peak centered at approximately 0.5 ppm is noticeable across the spectral series and is fairly obvious at short reaction times of e.g., less than 100 minutes. The assignment of the 0.5 ppm peak is given below based on quantum chemistry calculations of the chemical shifts. Determining the concentration of Mg2+ by in situ 25Mg NMR is not straightforward and our strategy is now explained. In brief, 0.1 M aqueous MgCl2 was used initially as a concentration standard. The 10


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concentrations of Mg2+ in a system of 0.1 M Mg(OH)2-H2O-CO2 at 50 °C were estimated by comparing the absolute integrated peak with respect to that obtained for the standard of 0.1 M MgCl2 at 50 °C. The estimated Mg2+ concentration (See Figure S4 in supporting Information) increases with the reaction time and levels off at a concentration of 0.13 M. Furthermore, based on the in situ 17O NMR results for the same system, the volume of the H2O is expanded by about 2.3% at equilibrium. This would result in a scaling factor of 0.752, calculated as 0.13(V+0.023V) × scaling = 0.1V, to obtain the correct Mg concentration, with V the volume. Applying the scaling factor of 0.752, the in situ determined Mg2+ concentration is plotted in Figure 7b. Comparing the data in Fig. 7b for 25Mg with those in Figure 5c for 17O on the same reaction system (but two different runs), the following results are obtained. (i) At any instant of the reaction time sequence, the concentration of HCO3- is about twice the concentration of Mg2+. This is exactly what is expected from reactions [1] and [3]. The concentration of CO2(aq) quickly increases as a function of the reaction time, reaching a maximum value of about 1.05 M at a reaction time of about 60-100 minutes and then decreases to about 0.8M at a reaction time of 310 minutes. The CO2(aq) then decreases at a much slower rate after about 310 minutes, while the Mg2+ concentration continues to increase (Figure 7c). This slow decrease in CO2(aq) after 310 minutes is at least partially a result of the increasing Mg2+ and HCO3concentrations. Increasing Mg2+ concentrations tend to decrease the equilibrium solubility of a neutral species such as CO2(aq). In fact, Harvie et al.59 report a positive Pitzer model coefficient for CO2(aq)Mg2+ of 0.183 which lowers the concentration of CO2(aq) in Mg2+ containing solutions. This lower solubility of a neutral species with increasing Mg2+ concentration is a special case of the widely known salting out effect. In fact, thermodynamic modeling calculations using the OLI software package55 as a function of ionic strength show that the CO2(aq) decreases from 1.1 M to 0.93 M at a Mg(HCO3)2 concentration of 0.3 M, which is in excellent agreement with the experimental observations (i.e. 1.05 to 0.8 M). The concentrations of HCO3- and Mg2+ continually increase monotonically as the Mg(OH)2 dissolves. At a reaction time of 900 minutes, the concentration of HCO3- is approximately 0.66 M which is, again, in excellent agreement with the Mg2+ concentration of about 0.325 M in accordance with reactions [1] and [3]. In terms of chemical shifts, the major observations from the in situ 17O NMR experiments are (i) the peak center of CO2 in scCO2 is located at about 66 ppm, aqueous CO2 is at about 78 ppm, HCO3- is at about 172.2 ppm, and H2O (the standard) is at 0 ppm at the beginning of the reaction; (ii) There is a slight upfield (i.e., toward lower ppm) shift with increasing reaction time, i.e., about a maximum of 0.25 ppm for H2O and about -0.5 ppm for HCO3- over the course of the reaction time studied (see Fig. 6). The major findings from the in situ 25Mg NMR (Figs. 7a and 7b) are that (i) the major Mg2+ peak gradually shifts to a negative ppm, i.e., from 0 ppm (the standard is aqueous Mg2+) at time zero to -0.26 ppm at a reaction time of 799 minutes. It should be emphasized that the trend of shifting to negative ppm with increasing the reaction time is a valid observation as the magnetic field drift has been carefully calibrated by using the standard reference sample, i.e., 1.0 M MgCl2, immediately before and immediately after the completion of the in situ 25Mg NMR experiment; and (ii) there is a positive shoulder peak with a peak center located at about 0.5 ppm that is of significant intensity relative to the 0 ppm peak at short reaction time (up to about 53 minutes). Electronic structure calculations of 17O and 25Mg NMR chemical shifts. There are two equivalent ways for interpreting the change of chemical shifts. One is at the molecular level, where any chemical shift can be related to the detailed molecular bonding surrounding the nucleus of interest, i.e., the detailed electronic structures. The other way is using the concept of bulk magnetic susceptibility variations, where any additional magnetic field created by the solvents or the interfaces will superimpose a small additional magnetic field onto the main field at the site of the nucleus of interest, 11



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thus causing a chemical shift to the observed nucleus. In our case, the solid brucite particles are at the bottom of the NMR sample tube and are outside the NMR detection coil. The NMR signals observed are from the dissolved species in H2O (a homogeneous system). Since NMR is sensitive to local structure changes (mainly the structure changes associated with the 1st and 2nd solvation shells), cluster computational modeling of NMR chemical shifts is valid as the longer range interaction has a minimal effect on the chemical shifts.52, 53, 60 Liquid H2O is used as the experimental reference standard, so an appropriate value from first principles calculations for the chemical shielding of H2O was needed. Because of hydrogen bonding, the absolute chemical shielding of H2O moves downfield (i.e., with increasing chemical shift values) when the number of water molecules increase (See Tables S2-S4). The absolute chemical shift for an isolated gas phase H2O molecule is 323, 325, and 328 ppm at the BLYP-D, BLYP/ZORA, and B3LYP levels, respectively. For (H2O)16, the average value is 278, 282, and 294 ppm and the most down-shifted values are 270, 275, and 285 ppm at the BLYP-D, BLYP/ZORA, and B3LYP levels, respectively, showing the downfield shift. In order to provide a best estimate of the chemical shift in liquid H2O, we extrapolated the most down-shifted chemical shifts using an exponential fit. The calculated chemical shifts for bulk H2O are 268, 268, and 278 ppm at the BLYP-D, BLYP/ZORA, and B3LYP levels, respectively. Use of the average values gives 277, 276, and 293 ppm at the BLYP-D, BLYP/ZORA, and B3LYP levels, respectively. Since bulk H2O is used as the 17O NMR chemical shift reference (set to 0 ppm), the following equation is used to convert the calculated 17O chemical shielding to the experimental chemical shift scale: δobs = δH2O(calc) δcal = 268 - δcalc ppm for the BLYP-D and BLYP/ZORA values.61 It is appropriate to use a value of 278 ppm for the B3LYP values. The predicted 17O NMR chemical shifts for pure CO2 clusters containing 1 to 20 CO2 molecules range from 63 to 71 ppm at the BLYP-D level (see Table 1), in excellent agreement with the experimental value of 66.4 ppm for CO2 molecules in the scCO2 phase at 50 °C (see Fig. 3a). When a CO2 molecule is interacting with 1 to 60 H2O molecules, the average chemical shift is predicted to be between 61 and 90 ppm (see Table 1). The experimental value of ~ 78 ppm for CO2 in the H2O-rich phase(Fig. 6b) is in this range and we note that this is consistent with 10 to 12 H2O molecules solvating one CO2 molecule. Although there are smaller (H2O)n clusters solvating a CO2 that give a similar chemical shift, notably n = 3 and 6, this does not represent a full solvation shell. An isolated HCO3- or one HCO3- solvated by one to five H2O molecules are predicted to have an average 17O chemical shift between 172 to 174 ppm. These results are in agreement with the experimental range of 171.7 to 172.2 ppm for HCO3- observed in the system with added brucite at 50°C (see Fig. 6b). However, we note that this average represents a broad range from 128 ppm for the C-O(H) to near 200 ppm for the C-O moieties. Larger solvation shells give an average value near 200 ppm. The results suggest that the proton is exchanging faster than the time-scale for the NMR measurement to make all of the oxygen nuclei equivalent. We note that the chemical shift for solvated CO32- is also predicted to be in this range. Experimentally, an upfield shift of -0.2 ppm for CO2 and about -0.25 for H2O is observed at a reaction time of about 100 minutes (see Fig. 6a). Taking the value of 71 ppm for 20 CO2 as our estimate of bulk CO2, we predict an initial upfield shift for the O in CO2 bonded to a small number of water molecules which then changes to a downfield shift for larger numbers as we approach the molarity of 1 M CO2 in H2O (on average one CO2 is interacting with 54 H2O molecules). However, the magnitudes of the shifts are much larger than experimentally observed. We note that the average H2O shift for ~ 1M CO2 (60 H2O) is -2 ppm. We assume quick molecular exchange among the H2O molecules in the cluster but no exchange with the bulk H2O phase. The results suggest that the initial decrease in the CO2 chemical shift at short times when CO2 initially dissolves into H2O is due to the fact that only a small number of 12


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440 441 442


H2O molecules are interacting with one CO2 molecule. As the reaction time increase, the number of H2O molecules interacting with a CO2 molecule increases leading to a change in the chemical shift.

443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462

Figure 7. In situ natural abundance 25Mg NMR on the system of brucite(Mg(OH)2)) + H2O + 90 atm scCO2 at 50°C as a function of the reaction time. (a) Stack plot of the in situ spectra containing 92 spectra, each was acquired using 0.22 hrs with 720 scans and a recycle delay time of 1s and an acquisition time of 0.11 s. The reason for choosing 1 s for recycle delay is because the longer component of the T1 is about 200 ms (see Figure S2 in Supporting Information). (b) Selected in situ 25 Mg spectra from (a), highlighting the existence of a shoulder peak at about 0.5 ppm and the upfield shift of the major peak. (c) Deconvoluted spectra selected from (b). (d) The concentration of Mg2+ in the solution as a function of the reaction time. The calculations predict that when HCO3- is interacting with 0-5 water molecules, the average 17O chemical shift in HCO3- are effectively independent of the number of H2O molecules. However, as the number of interacting H2O molecules increases, there is a significant increase in chemical shifts driven by both increases in the C-OH and C-O chemical shifts. In contrast, it is shown in Figure 6b that experimentally, the chemical shift values of HCO3- decreases linearly from an initial value of about 172.2 ppm to about 171.7 ppm at a reaction time of 900 minutes for the brucite + 90 atm scCO2 + H2O at 50 °C. Thus, HCO3- interacting with H2O alone cannot explain the decreased 17O chemical shifts with the increase of the reaction time. The 0 ppm peak is assigned to Mg2+•6H2O as this is the solvated structure of Mg2+ associated with MgCl2 in aqueous solution, with the six H2O molecules arranged octahedrally about the Mg2+. The 13



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Page 14 of 30

chemical shielding predicted for Mg2+•6H2O is 565 ppm. The calculated 25Mg chemical shielding can be converted to the experimentally observed scale by δobs = 565 - δcalc ppm. Table 2 summarizes the predicted 25Mg and 17O chemical shifts for different models where various combinations of HCO3- and H2O interacting with one or more Mg2+ were explored. An even more extensive set of the interactions of a single Mg2+ with varying amounts of H2O, OH-, HCO3-, and CO32are given in Table S5 in the Supporting Information. In all cases, with a single Mg2+, the chemical shifts are predicted to be positive. If one H2O is displaced by HCO3- to generate (MgHCO3)+(H2O)5, the predicted chemical shifts are 6 ppm for 25Mg,at the BLYP-D level and 4 ppm (SI) at the BLYP/ZORA level. The calculated values at the BLYP/ZORA level for Mg(HCO3)2(H2O)4 is 5 ppm, and for [Mg(HCO3)3(H2O)3]- is 4ppm. The corresponding BLYP-D values for the two structures of Mg(HCO3)2(H2O)4 are 10 ppm This suggests that the 0.5 ppm 25Mg NMR peak observed at earlier reaction time in Figure 7b can be explained by using a fast exchange model where most of the Mg2+ is coordinated with 6 H2O molecules while ~ 10% has form 1 to 3 HCO3- bonded to the Mg2+. Likewise, the H2O molecules bonded to a Mg2+ bonded to some number of HCO3- also can undergo fast exchange with those H2O molecules in the bulk. The average chemical shifts of the waters are -35 ppm (-33 ppm for BLYP-D), -27, and -20 ppm for (MgHCO3)+(H2O)5, Mg(HCO3)2(H2O), and [Mg(HCO3)3(H2O)3]respectively at the BLYP/ZORA level. Assuming that this is only 10% of the coordinated Mg2+ gives average 17O chemical shifts of -2 to -3 ppm, which are larger than the experimental value by about a factor of 10. However, all of the models containing a single Mg2+ center in the cluster cannot simultaneously explain the increased upfield (negative) 17O chemical shift for HCO3- in Figure 6b and the negative 25Mg chemical shift in Figure 7b of the major 25Mg NMR peak as a function of the reaction time. The chemical shifts for models containing two to four Mg2+ molecules with at least one HCO3between each pair of Mg2+ are summarized in Table 2, i.e., [2Mg-HCO3-10H2O]3+, [3Mg-2HCO314H2O]4+, and [4Mg-4HCO3-16H2O]4+ can simultaneously produce negative 25Mg chemical shifts and negative 17O shifts for both HCO3- and H2O. Note that a model with lower charge, [2Mg-2HCO38H2O]2+, does not produce a negative shift. By assuming fast molecular exchange between the magnesium in these multiple models with magnesium in Mg2+(H2O)6, the slight upfield 25Mg shift observed in Figure 7b for the major peak can be explained. Similarly, by assuming fast molecular exchange for the H2O and the HCO3- associated with these models and the bulk, the upfield shifts of 17O for HCO3- and H2O can be explained. Therefore, only when there are multiple Mg2+ in a single complex with a high positive charge is an upfield 25Mg shift (i.e. with negative chemical shift) predicted. It can be concluded based on the experimentally observed chemical shift changes for both the 17O and 25Mg combined with the calculated NMR chemical shifts that polymeric Mg2+ clusters are formed at high concentrations of Mg2+ in H2O saturated with scCO2, a critical step needed for magnesium carbonate formation. This is an important result as the formation of polymeric Mg2+ clusters containing H2O and HCO3- is a critical step needed for precipitation of magnesium carbonate phases such as magnesite or hydromagnesite. Table 1. Predicted 17O chemical shifts of CO2 interacting with a variable number of H2O molecules, HCO3- interacting with variable number of number of H2O molecules, and CO32interacting with variable number of H2O molecules.a System

O1

CO2 O2

CO2 2 CO2

H2 O Average Average 63 64

14


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3 CO2 4 CO2 6 CO2 12 CO2 20 CO2 CO2-H2O CO2-2H2O CO2-3H2O CO2-4H2O CO2-6H2O CO2-8H2O CO2-10H2O CO2-12H2O CO2-40H2O CO2-60H2O

66 62 78 65 82 72 75 85 89 92

HCO3HCO3--H2O HCO3--2H2O HCO3--3H2O HCO3--4H2O HCO3--5H2O HCO3--6H2O HCO3--12H2O

O1 156 131 128 128 128 128 148 160

CO32CO32-- H2O CO32--2H2O CO32--3H2O CO32--4H2O CO32--6H2O CO32--9H2O 505 506 507 508 509 510 511

a

64 67 69 70 71 62 61 77 64 77 70 74 79 85 90

59 61 77 62 72 67 74 74 81 88 HCO3O2 O3 Average 177 183 172 180 204 172 186 204 172 191 198 172 190 201 173 196 197 174 209 215 190 225 226 204 2CO3 Average 162 172 173 176 180 187 201

-54 -54 -37 -44 -27 -24 -21 -17 -4 -2

-28 -33 -35 -31 -28 -17 -16

-22 -27 -28 -26 -24 -20

Using H2O as reference by subtracting the calculated absolute shift from 268 ppm.

We can calculate the displacement of H2O from Mg(H2O)62+ by HCO3- by taking the gas phase free energy plus the solvation free energy correction plus an additional standard-state thermodynamic correction which is needed to correctly model the solution chemistry and the results are summarized in Table S6 of Supporting Information.62-66 The gas phase ∆G298 values are applicable at a pressure of 1 atm. In the liquid phase (new standard state), water molecules are present at a concentration of 55.5 M, 15



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Page 16 of 30

yielding a pressure of 1354 atm (from P = ρRT). Each water molecule has less translational freedom, as the translational entropy partition function is pressure dependent. The result is that a correction factor to the overall reaction free energy of +4.3 is needed for each H2O molecule as a product and a value of -4.3 kcal/mol for each H2O molecule as a reactant. The solvation free energy is calculated using the selfconsistent reaction field COSMO approach as implemented in ADF. Thus reaction [8] Mg(H2O)62+ + HCO3- ↔ Mg(H2O)5(HCO3)+ + H2O [8] is predicted to be exothermic by -3.1 kcal/mol at 298 K and reaction [9] Mg(H2O)62+ + 2HCO3- ↔ Mg(H2O)4HCO3)2 + 2H2O [9] is predicted to be exothermic by -6.0 kcal/mol, showing that the addition of a second HCO3- is favorable as well. The displacement of 3 or 4 H2O molecules by 3 HCO3- anions are also exothermic processes by -3.1 kcal/mol for the former and by -9.3 kcal/mol for the latter as shown by the following reactions [10] and [11] Mg(H2O)62+ + 3HCO3- ↔ Mg(H2O)3(HCO3)3- + 3H2O [10] Mg(H2O)62+ + 3HCO3- ↔ Mg(H2O)2(HCO3)3- + 4H2O [11] In addition to the displacement reaction [8]), we also studied the condensation of solvated bicarbonate with Mg(H2O)62+ as shown in reaction [12] Mg2+(H2O)6 + HCO3-(H2O)7 ↔ Mg2+(H2O)6HCO3-(H2O)7 [12] This reaction is predicted to be exothermic by -5.2 kcal/mol to form a separated ion pair and slightly endothermic by 1.6 kcal/mol to form an inner shell ion pair (See Figure 8). The latter value of 1.6 kcal/mol is consistent with the value of -3.1 kcal/mol for reaction [12] given the different approaches and inherent errors in the solvation energy predictions as well as the gas phase free energies. We note that the predicted 25Mg NMR chemical shifts for either the separated ion pair (BLYP/ZORA = 4 ppm) or for the inner sphere structure (BLYP/ZORA = 6 ppm) are still slightly positive and are consistent with the discussion given above.

536 537 538 539 540 541 542 543 544 545 546 547 548

Figure 8. Optimized structures of the inner shell and solvent separated ion pairs for Mg2+ with HCO3-. Environmental Implications. By taking advantages of the high sensitivity resulting in from the use of both a high magnetic field and a large sample volume, the in situ high pressure NMR capability overcomes the detection challenge associated with natural abundance 17O and 25Mg NMR, enabling characterization of dissolved species, including determining their concentrations as a function of the reaction time, in aqueous solution and in the presence of supercritical CO2 fluid (scCO2) with relevance to geologic carbon sequestration in the deep subsurface. 17O NMR allowed identification and distinction of various fluid species including dissolved CO2 in the H2O-rich phase, scCO2, aqueous H2O, and HCO3- simultaneously, also allowing an in situ estimate of pH of the reacting solutions. With the help of 25Mg NMR, it can be confirmed that at any instant of reaction time, the concentration of HCO3is twice that of Mg2+ in solution, as expected from charge balance and the H+ concentration. Quantum 16


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chemistry calculations of the NMR chemical shifts on cluster models strongly support interpretation of the experimental results. Evidence for the formation of polymeric Mg2+ clusters at high concentrations in the H2O-rich phase, a critical step needed for precipitation of magnesium carbonate phases, was found. Table 2. Quantum chemistry predicted 25Mg, interacting with HCO3- and H2O clusters.

17

O chemical shifts on different models of Mg2+ 17

System

25

Mg Average Chemical Shift

a

O Average Chemical Shifts b HCO3-

H2O

6.0

173

-32

9.9

173

-23

10.2

170

-26

0.6

167

-34

-3.2

162

-34

-3.9

162

-34

(MgHCO3)+(H2O)5

Mg(HCO3)2(H2O)4-a

Mg(HCO3)2(H2O)4-b

2Mg-2HCO3-8H2O

2Mg-HCO3-10H2O

3Mg-2HCO3-14H2O

17



-2.2

Page 18 of 30

162

-34

4Mg-4HCO3-16H2O 556

a

Using Mg2+-6H2O as reference by the calculated absolute shift from 565 ppm.

557

b

Using H2O as reference by subtracting the calculated absolute shift from 268 ppm.

558 559

ASSOCIATED CONTENT

560 561 562 563

Supporting Information Optimized geometries for the calculated models BLYP-D/TZ2P/ADF and B3LYP/aug-ccpVTZ(C,O,H)/cc-pVTZ(Mg)/Gaussian09 are listed in Table S7 and S8 of Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

564

AUTHOR INFORMATION

565

Corresponding Author

566 567 568 569

* Tel: (509) 371-6544; fax: (509) 371-6546; e-mails: [email protected]. Notes The authors declare no competing financial interest.

570 571 572 573 574 575 576 577 578

ACKNOWLEDGMENTS This material is based upon work supported by the Geosciences Research Program in the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences & Biosciences Division, through its Geosciences program at Pacific Northwest National Laboratory (PNNL). A portion of this research was performed using EMSL, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at PNNL. PNNL is a multi-program national laboratory operated for DOE by Battelle. DAD thanks the Robert Ramsay Chair Fund of The University of Alabama for support.

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Figure 1. Key parts of the in situ high pressure NMR probe. 1: High pressure sample tube; 2: O-ring; 3: Aluminum support; 4: High pressure fitting; 5: Screw fitting; 6: The hybrid NMR Coil. 16x21mm (300 x 300 DPI)



Figure 2. Performance test of the large sample volume probe. (a) 25Mg NMR for 1.5 ml 10 mM MgCl2 in H2O. Spectrum acquired using an acquisition time of 60 ms, 8540 scans with a recycle delay time of 1s. The resultant total experimental time was 2.5 h, S/N≈60 and the half linewidth (∆ν1/2) is 21 Hz with 5 Hz Lorentz line broadening bore FT. (b) Natural abundance 17O NMR spectrum acquired for 1.5 ml D2O using 128 scans, a recycle delay time of 0.5 s, 25 ms acquisition time for FID. The total experimental time was 67 s. An S/N of 650 was obtained with a half linewidth of 110 Hz and a Lorentz line broadening of 10 Hz before FT. 26x20mm (300 x 300 DPI)


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Figure 3. In situ high pressure natural abundance 17O NMR spectra at 50 °C. Each spectrum was acquired using an acquisition time of 25 ms, a recycle delay time of 0.3 s and 1024 scans except (d) which required 2048 scans. (a) Pure CO2 at 90 atmospheric (atm) scCO2; (b) 1.5 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 7.3 hours, where all of the sensitive region of the RF coil was filled with sample; (c) 0.8 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 3.88 hours, where the interface of H2O and scCO2 was at approximately the center of the RF coil; (d) 0.7 M brucite (i.e., 60.9 mg Mg(OH)2) + 1.5 ml H2O + 90 atm scCO2 acquired after the reaction was carried out for 15 hours. The inserts show spectral segments that are vertically expanded 6-fold. The asymmetric lineshapes for peaks in (c) are due to the asymmetry of the sample—a partially-solid-filled coil. The difference in linewidths of the HCO3- peak versus the linewidth of the CO2(aq) linewidth in (d) is presumably due to a combination of the asymmetry of the localized EFG on the oxygen and the fast exchange between HCO3- and H2CO3. 206x185mm (72 x 72 DPI)



Figure 4. (a) Normalized peak area of H2O; (b) Concentration of aqueous phase H2O with the apparent H2O concentration in solid dots and the true H2O concentration (after a correction is applied: see text) in open circles; and (c) Aqueous phase CO2 as a function of the reaction time obtained by in situ 17O NMR on the system of 1.5 ml H2O + 90 atm scCO2 at 50 °C. 166x262mm (72 x 72 DPI)


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Figure 5. Results obtained from in situ natural abundance 17O NMR spectra of 0.7 M brucite + 1.5 ml H2O + 90 atm scCO2 at 50 °C as a function of the reaction time. (a) Normalized peak area of H2O; (b) Concentration of H2O with the apparent H2O concentration denoted by solid dots and the true H2O concentration denoted by open circles; (c) Concentrations of CO2(aq), HCO3- and the sum of CO2(aq) + HCO3-; (d) Estimated pCH+ values. Note that the pCH+ values cannot be determined due to the low HCO3concentration for reaction times less than about 44 minutes since the characteristic HCO3- peak in in situ 17O NMR spectra cannot be observed. 203x167mm (72 x 72 DPI)



Figure 6. 17O chemical shifts as a function of the reaction time obtained by in situ 17O NMR. (a) System of 1.5 ml H2O + 90 atm scCO2 at 50 °C. Top: dissolved scCO2; and Bottom: H2O. (b) System of 0.7 M brucite(Mg(OH)2)) + 1.5 ml H2O + 90 atm scCO2 at 50 °C. Top: HCO3-; Middle: dissolved scCO2; and Bottom: H2O. The considerable localized noise in (b) for the cases of HCO3- and CO2 (aq) are mainly due to the inherent error of integration of the peaks associated with the lower S/N of the corresponding peaks as shown in Figure 3d compared with H2O, where apparently the S/N associated with the HCO3- is lower than that of CO2 (aq), thus even more noise for the case of HCO3-. 198x190mm (72 x 72 DPI)


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Figure 7. In situ natural abundance 25Mg NMR on the system of brucite(Mg(OH)2)) + H2O + 90 atm scCO2 at 50 °C as a function of the reaction time. (a) Stack plot of the in situ spectra containing 92 spectra, each was acquired using 0.22 hrs with 720 scans and a recycle delay time of 1s and an acquisition time of 0.11 s. The reason for choosing 1 s for recycle delay is because the longer component of the T1 is about 200 ms (see Figure S2 in Supporting Information). (b) Selected in situ 25Mg spectra from (a), highlighting the existence of a shoulder peak at about 0.5 ppm and the upfield shift of the major peak. (c) Deconvoluted spectra selected from (b). (d) The concentration of Mg2+ in the solution as a function of the reaction time. 294x228mm (72 x 72 DPI)



Figure 8. Optimized structures of the inner shell and solvent separated ion pairs for Mg 143x73mm (72 x 72 DPI)


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2+

with HCO3-.

In Situ Natural Abundance 17O and 25Mg NMR ... - ACS Publications

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