Silicate Fiberglasses Modified with Quaternary ... - ACS Publications

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Silicate Fiberglasses Modified with Quaternary Ammonium Base for Natural Gas Desulfurization Bair S. Bal’zhinimaev,*,† Evgenii V. Kovalyov,† Alexei P. Suknev,† Eugenii A. Paukshtis,†,‡ Dzhalil F. Khabibulin,†,‡ Irina S. Batueva,§ Aleksey N. Salanov,† and Mark G. Riley*,∥ †

Boreskov Institute of Catalysis, Prospekt Ak. Lavrentieva 5, Novosibirsk, 630090, Russia Novosibirsk State University, Pirogova St. 1, Novosibirsk, 630090, Russia § Buryat State University, Smolina St. 24a, Ulan-Ude, 670000, Russia ∥ Honeywell UOP, 25 E. Algonquin Road, Des Plaines, Illinois 60017, United States ‡

S Supporting Information *

ABSTRACT: A new gel-like film was generated on the surface of silicate fiberglass (FG) under hydrothermal treatment with tetramethylammonium hydroxide (TMAH) water solution. By means of scanning electron microscopy (SEM)/high resolution transmission electron microscopy (HRTEM), 1H NMR magic angle spinning (MAS), and diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), we revealed this film is a phase with density less than pristine FG, where TMA species, as the H2S adsorption sites, are confined. Indeed, FGs modified in this way exhibited a rather high dynamic adsorption capacity which was proportional to concentration of TMA cations bonded with very basic oxygen of Broensted acid residue. The N-modified FG sorbents showed good regenerability when in the presence of water, and the adsorbed hydrogen sulfide on the TMA+−−O−Si ion pair was easily desorbed at room temperature. This gives grounds to conclude that the process of hydrogen sulfide sorption on N-modified FGs is reversible and proceeds without a loss of adsorption capacity. Indeed, N-modified FGs remained stable during several adsorption−desorption cycles.

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INTRODUCTION Hydrogen sulfide, being the main impurity in natural gas (up to 25%), is detrimental for the environment and makes the industrial processes involving H2S hazardous. Thus, its concentration should be lowered to the content below 3 ppm which is required for pipeline transportation of natural gas.1 Natural gas is commonly purified via H2S absorption by aqueous solutions of amino alcohols. The main drawbacks of this technology are the low absorption/desorption rates and accordingly the large sizes of scrubbers as well as the high energy consumption for regeneration. The adsorption is the most appropriate method for purification of gas streams due to high H2S capacity of modern sorbents, high selectivity to methane, and the possibility of regeneration. Such adsorbents are represented by activated carbon,2,3 in particular modified with alkali,4 and oxides of metals, mostly Zn, Cu, and Fe,5−7 which efficiently remove hydrogen sulfide from natural gas and other gas streams via its chemical bonding. As the latter process is based on chemical conversion of metals into their sulfides, high temperatures are necessary here, especially for the subsequent oxidative regeneration. The adsorption temperature can be lowered substantially (down to room temperature) by increasing the © XXXX American Chemical Society

dispersion of metal oxides to some nanometers via their deposition on various supports, such as activated carbon, zeolites, mesoporous silicas, and others.7 Note that this not only increases the adsorption capacity of metal oxides but also decreases the regeneration temperature. Indeed, at room temperature, the adsorption capacity of ZnO/SiO2 and ZnO/ rGO (the reduced graphite oxide) was found to increase 2-fold in comparison with bulk zinc oxide,8,9 and supercapacity for H2S (ca. 0.7 g/g) was reached on supported Fe2O3/SBA-15.10 A recently synthesized ZnO−SiO2 composite with the ordered macroporous 3D structure and a high ZnO content11 showed not only a high capacity at room temperature but also a good stability in multiple adsorption/regeneration cycles. High efficiency in the low-temperature H2S removal was observed also for copper oxides CuOx supported on titanosilicate12 and on high silica zeolites of MSU-1 and Y types.13−15 Sorbents containing Cu(I) (Cu(I)Y, CuCl/MCM-41, and CuCl/SBA15) were shown to be completely regenerable in an inert or Received: April 4, 2017 Revised: August 8, 2017 Accepted: August 21, 2017

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DOI: 10.1021/acs.iecr.7b01389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

tetrahedra randomly bonded by the bridge oxygen) that alternate with less dense regions having a large amount of hydroxyl groups and molecular water (ca. 12 wt %22,23). Such a structure predetermines substantial differences in physicochemical properties of glass and solids and makes the properties of glass close to those of very viscous liquids. Thus, in comparison with solids, glasses have lower density and higher solubility (mobility) of various molecules in the glass matrix of, e.g., water.24 Fiberglass materials have found application as promising supports for preparation of the highly active and selective Pt and Pd catalysts for VOC removal25 and selective hydrogenation of acetylene.26 High performance is based mostly on the ability of glass to confine Pt(II) and Pd(II) cations in its bulk, which are then reduced into active highly dispersed metal species. By analogy with catalysts, we tried to develop efficient sorbents for hydrogen sulfide removal via the introduction of cations of organic bases into the bulk of fiberglasses by their treatment with tetramethylammonium hydroxide (TMAH). As follows from scanning electron microscopy (SEM)/high resolution transmission electron microscopy (HRTEM), FTIR, and NMR studies, the TMAH treatment of FGs under hydrothermal (HT) conditions results in appearance of the film of a new gel-like phase where Ncontaining species that are able to absorb hydrogen sulfide are confined. The N-FG sorbent demonstrated good adsorption performance and stability in adsorption−regeneration cycles.

oxygen atmosphere. Among these sorbents, Cu(I)Y has the greatest capacity and selectivity (the CH4/H2S ratio) for hydrogen sulfide. Irrespective of their good adsorption performance, supported iron, copper, and zinc oxides can be reduced in the course of adsorption,9 which in some cases deteriorates their capacity. In addition, the process of oxidative regeneration, which consists of the inverse chemical conversion of sulfide nanoparticles into oxide ones, is accompanied by a release of acidic gases and requires elevated temperatures. In this case, sintering of such particles and formation of inactive sulfates are possible. More promising in terms of energy consumption, primarily for regeneration of adsorbent, are mesoporous materials functionalized with amines.7,16−21 In this case, desulfurization of natural gas occurs due to the acid (H2S)−base (amine) reactions with the formation of weak salt-like species of the NH3+HS− type, which rapidly decompose upon slight heating with a release of hydrogen sulfide. Unlike the commercial amine technology for removing a high level of H2S, the adsorbents functionalized with amines can be used for fine purification of natural gas from hydrogen sulfide. In this case, NH2-functionalized sorbents are similar to supported metal oxides but seem to be regenerable. Functionalization of mesoporous silicas with amines was performed by their grafting onto the surface of MCM-48 via the reaction of aminoalkyltriethoxysilanes with hydroxyl groups of silica.16 As a result, 3-aminopropyl-functionalized MCM-48 showed quite a high adsorption capacity (0.3 mmol/g) and selectivity (H2S/CH4 > 100). It should be emphasized that the desorption of hydrogen sulfide took place at a temperature of 60 °C, which is much lower than in the case of supported nanoparticles of metal oxides. In addition, the FTIR study actually revealed that the adsorption of hydrogen sulfide takes place due to the reversible reaction of R-NH2 + H2S ⇄ R−NH3+HS−. Similar results were obtained with the silicate mesomaterials (SBA-15, MCM-41) whose pores are filled with polyethylenimine (PEI), the so-called “molecular basket” sorbents.17−19 However, due to involvement of bulk adsorption site of PEI, the capacity for hydrogen sulfide increased ca. 6-fold as compared to aminopropyl-functionalized MCM-48. PEI/ SBA-15 retained its adsorption performance for at least 8 adsorption/regeneration cycles. It seems interesting that these “molecular baskets” have good adsorption performance not only for H2S but also for CO2, which gave grounds to propose an original two-step process for deep purification of gas streams from carbon dioxide and hydrogen sulfide.18 Even a greater adsorption capacity was shown by triamine-grafted poreexpanded MCM-41, which contained more basic −(CH2)3− NH−(CH2)2−NH−(CH2)2−NH2 amines.20 The adsorption heat of H2S was found to not be high (ca. 35 kJ/mol), which makes it possible to desorb hydrogen sulfide by evacuation at 75 °C. The adsorbent showed a good stability during several adsorption−desorption cycles. A much lower capacity for H2S was observed for the MDEA/SBA-15 sample, which was synthesized by impregnation with a solution of methyl-diethyl amine.21 This indicates that grafting of amine species onto the surface of mesoporous silicate materials produces more efficient adsorbents than the impregnation method. In this work, silicate fiberglass (FG) materials are presented for the first time as efficient H2S sorbents. Thermodynamically, glass is a metastable state generated by the phase transition from liquid to solid and back. The molecular structure of leached silicate glasses is characterized by the short-range order and consists of dense regions (chains of silicon−oxygen

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EXPERIMENTAL SECTION Materials and Chemicals. The adsorbents were prepared using a commercial (Steklovolokno Ltd., Russia) leached aluminosilicate (Al−Si) fiberglass material containing 96.6% SiO2, 3.1% Al2O3, and the rest Na, Ca, Fe, and other impurities. N-containing basic sites in the glass were generated using a 2.5% aqueous solution of tetramethylammonium hydroxide (Acros Organics). The adsorbents were synthesized not only with Al−Si but also with REM-Si fiberglasses containing 13% of rare-earth metals (the La−Ce fraction) and Zr−Si fiberglasses with 12% of zirconium. The fiberglass material was delivered as a fabric woven of 1 mm threads that consist of twisted elementary fibers 6−7 μm in diameter. Specific surface area of the pristine fiberglass did not exceed 0.5 m2/g, which is close to its geometrical surface. The column breakthrough measurements were made with methane and hydrogen sulfide gases of 99.9% purity. Glycerol for simulation of NMR line broadening was of analytically pure grade (Laverna Ltd., Russia). Hydrothermal Treatment of Fiberglasses with TMAH. FGs were treated at 80, 110, 125, and 150 °C under hydrothermal conditions. For this purpose, 60 mL of a 2.5% TMAH solution in deionized water with 3 g of fiberglass was poured into a 100 mL autoclave equipped with a Teflon insert. The 2.5% TMAH water solution was used for hydrothermal treatment of FG materials. The duration of HT treatment was varied from 4 h to several days; after that, the sample was separated from the solution, washed with three portions of deionized water by roller wringing of the wetted FG fabric, and dried in air. FG treatment at 80 °C was carried out in a 450 mL Parr reactor (4567, Parr Instrument Company, USA) equipped with titanium vessel and high-speed stirrer. For this purpose, a 6 g FG textile was fixed in a special cartridge made of stainless steel gauze. This cartridge was loaded into 120 mL of a 2.5% TMAH water solution preheated to 80 °C. Stirring was then turned on (a rotation speed of 800 min−1), and the sample was treated for B


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experiments were performed at room temperature and ambient pressure; to improve a signal-to-noise ratio, the number of scans was equal to 1000. The external TMS standard served as a reference chemical shift. To eliminate the signal from rotor in 1 H MAS spectra of fiberglasses, the FID of a free rotor was recorded under the same conditions and then subtracted from the FID of rotor + sample. To simulate the width of 1H NMR lines, water−glycerol TMAH solutions were recorded at 5−25 °C. In addition, the spin−spin relaxation time T2 for 1H was measured using a spin−echo sequence of 90-τ-180-acq, where τ was varied from 0.1 to 5 ms. DRIFT spectra were recorded at room temperature on a Shimadzu 8300 spectrometer equipped with a DRS-8000 diffuse reflectance device, in the region of 400−4000 cm−1 with a resolution of 4 cm−1. The IR cell was an open metallic cup 2 mm in depth and 4 mm in diameter where 3−5 mg FG threads (as received) were placed. The spectra were transformed into Kubelka−Munk units with F(R) = (1 − R)2/2R, where R is the reflection factor. The integrated intensities of bands were determined after subtraction of the base spectrum. To quantify DRIFT data, the reference samples with the known TMAH concentration were used. HRTEM (high resolution transmission electron microscopy) images were obtained with a JEM-2010 electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV and a lattice resolution of 0.14 nm. The samples to be examined by HRTEM were prepared by fiberglass suspension in ethanol followed by their ultrasonic treatment. Then, they were deposited onto perforated carbon film mounted on a copper grid. Column Study of N-Modified FGs. H2S breakthrough measurements were carried out in tubular adsorbers with diameters of 3 and 6 mm and a length of 300 mm at 25 °C. The 89 ppmv H2S + CH4 feed gas at atmospheric pressure was fed into adsorber via a flow mass controller (Quarta Ltd., Russia) maintaining a volumetric rate of 5 L/h, which corresponds to GHSV = 2500 h−1 for a 1.0 g sorbent sample and 25.000 h−1 for a 0.1 g sample. The concentration of H2S was monitored continuously in the range of 0.5−150 ppm using an ELAN-H2S (ECHO-Intekh Ltd., Russia) electrochemical gas analyzer having a time constant of 20 s and an accuracy of 0.1 ppmv. Separate threads of a FG sorbent (1.0 or 0.1 g) were densely pulled through the adsorber and cut to length so that the pressure drop did not exceed 100 and 170 Pa for wide and narrow tubes, respectively. Prior to adsorption, the adsorber and all communication pipes were purged with an inert gas at room temperature for 30 min, and then, the feed gas was admitted. The dynamic capacity of hydrogen sulfide QBT was found as the amount of supplied hydrogen sulfide multiplied by the breakthrough time (tBT), i.e., the time of H2S appearance at the adsorber outlet with the concentration of 1 ppmv. The total amount of adsorbed H2S or the saturation capacity (Qsc) was calculated as the area over the response curve CH2S(t). In most cases, the Qsc value could not be determined due to a long saturation period. To carry out the adsorption−desorption cycles, the CH4 + 89 ppm of H2S dry flow with a flow rate of 5 L/h was fed into adsorber loaded with 0.1 or 1 g of sorbent which was also preliminary blown with He for half hour at room temperature. In the case of sorbent with a low capacity, the adsorption process was completed when outlet H2S concentration was matched with the inlet one. In the case of sorbent with a high

different times. It should be noted that such treatment in an open system allowed us to keep the pH value constant by periodic addition of TMAH. After that, the sample was separated from the solution, washed with three portions of deionized water by repeated roller wringing of the wetted fabric, and dried in air. All prepared sorbents are listed in Table 1. Table 1. List of Sorbents Prepared by Hydrothermal Treatment hydrothermal treatment no.

sorbent

T, °C

duration, h

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Al-Si-80-4 Al-Si-110-144 Al-Si-150-4 Al-Si-150-15 Al-Si-150-48 Al-Si-150-144 REM-Si-80-4 REM-Si-110-48 REM-Si-110-144 REM-Si-125-48 REM-Si-125-96 Zr-Si-125-72 Zr-Si-125-144 Zr-Si-125-288

80 110 150 150 150 150 80 110 110 125 125 125 125 125

4 144 4 15 48 144 4 48 144 48 96 72 144 288

Characterization Techniques. To estimate textural characteristics of the sorbents, we tried to measure their adsorption isotherms on an ASAP-2400 instrument (Micrometrics, USA). However, even a long-term (up to 7 days) evacuation did not give stable p/ps values because after shutting down the evacuation system the pressure grew continuously owing to water desorption from the bulk of fiberglasses (Section S2). Thus, we estimated specific surface area by the single-point BET method using a Catacon sorptometer (Russia); the measurements were made in a flowing gas mixture (Ar/He) at ambient pressure. For this purpose, 2 g samples were evacuated in flowing argon at 200 °C for 40 min. After that, argon was adsorbed at 77 K and desorbed at −100 °C, and volume of the adsorbed gas was measured. Standard alumina samples with the known SBET = 1.29 and 13.1 m2/g served as the reference samples. The surface morphology of FG samples was characterized by scanning electron microscopy (SEM) using a JSM-6460 LV (Jeol) microscope. Fiberglasses were deposited on a conductive carbon Scotch tape and covered with a gold film having a thickness of 5, 10, or 20 nm. SEM images were obtained in the secondary electron mode at a beam energy of 18 keV. The chemical composition was determined by energy dispersive Xray spectroscopy (EDX) using an INCA-Energy-350 (Oxford Instruments) spectrometer. The composition of pristine glass fibers was determined with X-ray fluorescence (XRF) using a VRA-30 X-ray fluorescence analyzer with a Cr-anode X-ray tube (Carl Zeiss, Germany). 1 H NMR spectra of fiberglasses were recorded at room temperature (as received) on an Avance (Bruker) spectrometer using a 9.4 T magnetic field and a 400 Hz frequency. Magic angle spinning (MAS) of the sample at 10 kHz was employed for line narrowing. The spectra were obtained by Fourier transform of the free induction decay (FID) excited by a singlepulse sequence. The π/2 pulse equal to 5 μs was used. The C


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Figure 1. SEM images of pristine Al−Si (A), Al-Si-80-4 (B), Al-Si-150-4 (C), Al-Si-150-48 (D), and Al-Si-150-144 (E, F) samples (inset: images at higher resolution).

surface of smooth fiberglasses (Figure 1B). Upon temperature elevation to 110 °C, essential changes in the surface morphology took place only after a very long treatment (144 h). When the temperature was raised to 150 °C, the larger species with a thickness of tens of nanometers appeared on the surface already after a 4 h treatment (Figure 1C). A long-term (48 and 144 h) hydrothermal treatment at this temperature produced a continuous 100−150 or 200−250 nm thick film, respectively (Figure 1D,E). Therewith, the specific surface area increased by more than an order of magnitude and reached S = 16 m2/g. At the same time, we have no such clear picture for REM-Si fiberglass. Most probably, it is related to their readiness to be modified, so that we could not image the early stages of a new phase formation. The easier modification of REM-Si is caused most likely by structural differences between glasses, particularly by sizes of the loose regions where hydroxyl groups are located. Although all types of glasses have a similar

capacity, the adsorption process was stopped when outlet H2S concentration achieved 5 ppm. Then, hydrogen sulfide flow was stepwise replaced with 2.7% H2O + CH4 humid flow with the same flow rate at room temperature to desorb the hydrogen sulfide. The amount of H2S desorbed (Qdes) was calculated from the area under its desorption curve. After termination of the desorption process, the humid gas flow was switched immediately over to dry feed gas in order to start the next adsorption−desorption cycle.

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RESULTS AND DISCUSSION N-Modification of FGs with TMAH under Hydrothermal Conditions. The hydrothermal treatment of pristine Al−Si fiberglass leads to significant changes in its surface morphology. The full SEM picture of morphological changes with temperature and duration for Al−Si fiberglass is presented in Figure 1. After a 4 h treatment, very small inhomogeneities, which are seen only at a very high resolution, appeared on the D


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Figure 2. HRTEM image of REM-Si-125-48 (A) and REM-Si-125-96 (B). Inset: the microstructure of the loose gel-like phase on the fiberglass surface.

concentration of OH groups, in REM-Si, their density in such regions happened to be much lower as compared to Al−Si and Zr−Si.27 In other words, the size of such regions is much greater in the glasses modified with rare-earth metals. It is natural to expect that in this case the effect of TMAH will be reached more easily and under milder conditions. Indeed, a noticeable change in the REM-Si morphology occurred already at 80 °C, whereas Al−Si and Zr−Si fiberglasses are more stable and similar modifications require more severe conditions. Note that the morphology of the REM-Si-125-48 (96) samples was very similar to that of Al-Si-150-144. SEM data agree well with the results of the HRTEM study. As seen from Figure 2, the clear contrast between the newly formed layer with a thickness of 100−200 nm (light) and pristine FG (dark) is observed. This confirms that density of the gel-like phase is actually lower than that of pristine fiberglass. The image with a great magnification (the inset) shows that this phase is characterized by a loose structure with the short-range order. The presence of dark and light regions indicates that this phase has a porous structure with the average size of ca. 5 nm. The hydrothermal treatment is accompanied by a partial dissolution of fiberglasses; weight losses vary from ca. 3−5% under mild conditions to ca. 30% under harsh conditions. This undesirable process increases with temperature and results in noticeable losses of the mechanical strength of fiberglass. Therefore, to minimize the dissolution of the fiberglass, we restricted the temperature (125 °C for REM-Si and 150 °C for Al−Si) and increased the duration of HT treatment so that a loss of sample weight did not exceed 30%. In this case, the glass fibers kept up the acceptable mechanical strength. Due to readiness of REMSi to be modified, the morphology of REM-Si-110-48 sample treated at 110 °C for 2 days was close to that of Zr-Si-125-288 treated at 125 °C for 12 days and Al-Si-150-144 treated at 150 °C for 6 days. According to SEM, the film thickness for these samples was around 200 nm. Table 2 lists the data of EDX and X-ray fluorescence elemental analysis of N-modified REM-Si-125-96 and pristine REM-Si fiberglasses, respectively. It is seen that the total composition of the fiberglass was not changed as much under HT treatment because the contribution of a thin gel-like film layer of 400 nm to a thick fiber of 6 μm is insignificant. Note, in N-modified sample, silicon and oxygen content slightly decreased due to partial dissolution of silicate framework. It seems interesting that rare-earth elements do not transfer into solution and remain on the external surface or inside of the

Table 2. Chemical Composition of Pristine and N-Modified REM-Si-125-96 FG concentration,a % wt

a b

element

pristine REM-Si (XRF)

N-modified REM-Si-125-96 (EDX)

O Si N Al Ce La

47.9 41.0 0 0.4 3.7 1.4

46.7 37.9 507 kJ/mol) will not be compensated by electrostatic attraction. However,

Table 3. H2S Sorption Capacities of FG Sorbents vs Specific Surface Areaa no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

sorbent Al-Si-80-4 Al-Si-110-144 Al-Si-150-4 Al-Si-150-15 Al-Si-150-48 Al-Si-150-144 REM-Si-80-4 REM-Si-110-48 REM-Si-110-144 REM-Si-125-48 REM-Si-125-96 Zr-Si-125-72 Zr-Si-125-144 Zr-Si-125-288

tBT, min 1.37 35.1 9.4 20.2 46.3 343 29.8 438 1364 777 2098 52 170 493

QBT, % wt 0.002 0.04b 0.01b 0.02b 0.05b 0.4 0.03b 0.5 1.4 0.8 2.2 0.05b 0.2 0.6

b

SBET, m2/g 1.0 2.6 1.6 2.5 11 16 3.5 n.d. n.d. 38 149 3.0 6.0 33

F


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Figure 5. (A) The sorption capacity for H2S vs the TMA content in FGs; (B) the sorption capacity for H2S vs the specific surface area of FGs with and without TMA.

H2S dissociation precedes its adsorption on the coordinatively unsaturated zinc cations which significantly increases the proton acidity.31 Therefore, the proton from the H2S molecule transfers easily to neighboring basic oxygen. In case of the (CH3)4N+···−O−Si acid−base pair, the proton detachment takes place at once mostly due to high basicity of oxygen. On the other hand, a clear correlation between sorption capacity for H2S and specific surface area of N-FGs also takes place (Figure 5B). Again, the larger the BET surface, the higher is the capacity. It looks like hydrogen sulfide adsorbs on the internal surface of the new phase where TMA species can be located. This contradicts the sorption kinetics for samples with a high amount of new phase and, accordingly, with high capacity for hydrogen sulfide (Figure 4, curves 3 and especially 4). Moreover, it is well-known that the materials of silicate origin reveal poor performance in H2S adsorption.17−19 Indeed, after decomposition of TMA species at high temperatures, the samples completely lose their performance in H2S sorption though their specific surface area remains the same. The TPD study of the N-modified fiberglass showed that at temperatures above 150 °C tetramethylammonium species are decomposed into trimethylamine and methanol, likely, via their intermediate hydrolysis to yield tetramethylammonium hydroxide (see Section S1). At first, the surface TMA/TMAH species are decomposed, then the bulk ones are desorbed, and the desorption process is limited by mass transfer of the hydrolysis products to the surface. As a result, the broad asymmetric peak shifts to higher temperatures. Note that the fraction of surface species is not high and does not exceed 20% of the total TMA content. Nevertheless, we carried out the additional studies using NMR spectroscopy to separate the surface and bulk sorption of the hydrogen sulfide on N-modified FGs. Relationship between H2S Sorption by the Surface and Bulk of New Phase. 1H NMR spectra of fiberglasses are represented by the wide lines that cannot be resolved even by magic angle spinning (Figure 6). As follows from Table 4, the spectrum of pristine fiberglass has three peaks, which correspond to molecular water, OH groups H-bonded to water, and terminal silanol groups.33,34 In addition, a weak line at 6.1 ppm is assigned to NH4+ that formed due to partial TMAH deep hydrolysis during long hydrothermal treatment at a high temperature. After TMAH treatment, a decrease in the width of lines attributed to water is observed. To elucidate the

Figure 6. 1H MAS NMR spectra of the pristine Al−Si (1), Al-Si-150144 (2), and 2.5% aqueous solution of TMAH (3).

Table 4. Chemical Shifts (δ), Assignment of Different Peaks in the 1H NMR Spectra, and Concentration of the Identified Species in FGs fiberglass

δ, ppm

concentration, mmol/g

assignment

pristine Al−Si

6.9 4.5 1.7 6.9 6.1 4.4 3.3 3.3 1.7

2.9 1.7 0.38 0.51 0.13 1.4 0.028 0.11 0.33

OH···H2O H2O Si−OH OH···H2O NH4+ H2O TMAHads TMA Si−OH

Al-Si-150-144

nature of this phenomenon, spin-echo 1H NMR spectra were measured. It was found that the TMAH treatment increased relaxation time T2 for water from 0.51 to 1.44 ms, which according to the literature data,33 indicates an increase in the domain size of liquid water from 2 to 5 nm. Since water in pristine glass is located in a narrow space between dense silicon−oxygen regions,23 the indicated increase in the domain size can be interpreted as the formation of a porous structure in the fiberglasses treated with TMAH. This agrees well with the 5 G


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does not contribute so much to the H2S adsorption capacity but can affect the sorption kinetics, increasing the total flow of hydrogen sulfide molecules to the gas−gel-like phase interface. The adsorption of hydrogen sulfide on the N-FG ionic pair indicates the H2S adsorption on NH2-modified mesoporous silica materials.18−20 Indeed, the desulfurization of natural gas takes place due to formation of weak salt-like NH3+HS− species which decompose at rather low temperatures with a release of H2S. In spite of lower H2S capacity compared to aminemodified MCM-41 and SBA-15, the N-FG sorbent revealed an excellent ability to be regenerated (see below). Regenerability and Stability of N-FGs. An important characteristic of adsorbent is its regenerability. As shown by the adsorption−desorption cycles performed at room temperature, H2S desorption under dry methane is incomplete. Thus, the amount of desorbed hydrogen sulfide in the REM-Si-80-4 sample was three times lower as compared to the adsorbed one. The imbalance between the amounts of H2S adsorbed and desorbed was noticeably greater for the samples with a higher capacity for H2S. The incomplete desorption may be caused mostly by a strong interaction of H2S molecules with ionic pairs confined in the gel-like phase as well as by their diffusion in the bulk in a rather thick film of this phase. Thus, even an increase in the desorption temperature to 80 °C did not produce a noticeable growth of hydrogen sulfide desorption. The situation changed dramatically when we replaced the dry methane flow with a humid one: the addition of water into methane stream sharply increased the desorption rate and produced a distinct peak of the evolved hydrogen sulfide (Figure 7). The amount of desorbed H2S increased sharply and

nm size of light regions in the HRTEM images (Figure 2). A sharp drop in intensity of the line at 6.9 ppm, which corresponds to the interaction of OH groups with water, is caused by desorption of water molecules from the bulk of fiberglass at elevated temperatures (see Section S2). It is one of the basic properties which differs glass from solid.22,23 This process is governed by thermodynamics, does not depend on the concentration of ambient water, and therefore does not affect the formation of the new phase. Overall, it can be concluded that the formation of the new phase is accompanied by a substantial (2.5-fold) decrease in the amount of molecular water only, although a minor decrease in the concentration of Broensted sites takes place, probably due to the interaction with organic base. Two new peaks with the 3.3 ppm chemical shift appear in spectrum 2. Both peaks can be attributed to protons of the methyl group in TMAH, because a similar peak is observed in the spectrum of tetramethylammonium hydroxide solution (spectrum 3). The narrow peak can be assigned to the adsorbed TMAH molecules where methyl groups are free to rotate. We believe that it is not TMAHads but the bulk TMA species that are exposed at the surface. It should be noted that TMA species are tetramethylammonium cations ionically bonded to the acid residue of Broensted sites. Since the concentration of surface species for the Al-Si-150-144 sample was estimated to be 0.028 mmol/g and its specific surface area is equal to 16 m2/g, the coverage is not high, ca. 10%. The broad peak can be assigned to the TMA species located in the bulk of the new phase where the rotation of methyl groups is strongly hampered. Indeed, it is well-known that a substantial anisotropic broadening related to the mobility of atoms in the condensed medium like glass takes place.35,36 The higher the medium viscosity, the broader is the NMR spectrum. Simulation of the width of the 1H NMR line at 3.3 ppm by variation of viscosity of a water−glycerol solution made it possible to find the viscosity of the new phase (ca. 7 kP), which was approximately 20 times higher than that of neat glycerol and almost 4 orders of magnitude higher as compared to water (see details in Section S3). Taking into account the low viscosity of the new less dense phase (relative to mother glass), we called it the gel-like phase. The concentration of bulk TMA species for the Al-Si-150-144 sample is estimated as 0.11 mmol/g, which is close to the dynamic sorption capacity for H2S (QBT = 0.1 mmol/g). It is important to know that the fraction of surface TMA species is only around 20%, which is very close to the estimate based on the TPD study. This is additional evidence of the crucial role of the bulk TMA contribution to H2S sorption over N-modified FGs. Thus, SEM/HRTEM, DRIFT, and NMR data allow one to conclude that the long-term treatment with TMAH at elevated temperature produces quite a loose gel-like phase of silicate origin confining the quaternary N-base cations. In an alkaline medium, both the cleavage of Si−O−Si bonds and the formation of a new silicate framework take place. Note that the formation of the gel-like layer over the silicate glass surface during its dissolution was discussed in detail previously.37 However, the oligomerization of (−Si−O)n− chains can terminate due to the interaction with TMAH with the formation of (CH3)4N+···−O− Si≡ ionic pairs, which play the key role in the sorption of hydrogen sulfide. A formation of the new gel-like phase is accompanied by generation of the developed surface area. Due to the silicate origin and a low TMA surface concentration, the amount of adsorbed hydrogen sulfide is also low. Therefore, the increased surface area virtually

Figure 7. H2S adsorption−desorption profile over REM-Si-80-4 for desorption under dry and humid methane flows (GHSV = 25 000 h−1).

became close to the amount of the adsorbed one. Thus, one can conclude that water stimulates H2S desorption, thus ensuring a good regenerability of FG sorbent. Similar results were obtained over Al-Si-150-15 and Zr-Si-125-288 samples, which have a higher sorption capacity (Figure 8). In this case, a good material balance between absorbed and desorbed H2S took place. One can suggest that since water is a more polar molecule than H2S, its adsorption on adjacent sites can weaken H2S bonds with TMA species and thus facilitate their desorption. In fact, the displacement of hydrogen sulfide with water proceeds, likely, according to another mechanism. Due to high proton affinity of water (PA = 1600 kJ/mol29), the interaction of the (CH3)4N+···−O−Si ionic pair with H2O does H


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the 1H NMR MAS study, the mobility of various species in the bulk of new phase should be higher than in pristine fiberglass. Supposing that the diffusion coefficient is 1 order of magnitude higher than in pristine fiberglass, it is easy to estimate that diffusion time is hundreds of seconds, which is much greater than absorption or desorption times at a very low contact time (25 000 h−1). Of course, to fit experimental data, the assumption that desorption rate increases due to water effect should be taken into consideration.

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CONCLUSIONS The SEM/HRTEM, DRIFTS, and NMR studies have demonstrated that the fiberglass surface can be strongly modified by hydrothermal treatment with an aqueous solution of TMAH. This is accompanied by the dissolution of glass with the formation of a new silicate framework on the fiberglass surface. Supposedly, the growing silicate chain terminates owing to the reaction of the end −Si−OH groups with the TMAH molecule producing the distorted tetramethylammonium cations ionically bonded with Broensted acid residue. As a result, a gel-like N-containing phase is formed; the density of this phase is lower in comparison with the pristine fiberglass due to the presence of some mesopores. Specific surface area of fiberglasses subjected to a long-term treatment with TMAH can reach 150 m2/g. The (CH3)4N+···−O−Si ionic pairs confined in the bulk of the gel-like phase were proposed to be the adsorption sites for H2S removal. Most probably, strong adsorption of hydrogen sulfide molecules proceeds via the proton interaction with strong basic oxygen leading to a sharp weakening or even cleavage of the H−SH bond. A clear correlation was found between adsorption capacity and concentration of these sites. N-modified FGs demonstrate quite a good dynamic adsorption capacity for H2S, which exceeds 2 wt %. However, the N-FG cannot be regenerated due to strong bonding of the hydrogen sulfide with ionic pairs, so that they do not desorb even at elevated temperatures. However, in a flow of humid methane, the desorption rate sharply increases and all the hydrogen sulfide adsorbed earlier is completely removed from the adsorbent. The hydrogen sulfide desorption stimulated by water can be attributed to the adsorbed H2S replacement with water without ionic pair destruction. This gives grounds to conclude that the absorption process is reversible and proceeds without a loss of sorption capacity. Indeed, N-modified FGs remained stable during three to four absorption−desorption cycles.

Figure 8. H2S adsorption−desorption profile over the Al-Si-150-15 sample. GHSV = 2500 h−1.

not lead to proton detachment as in the case of hydrogen sulfide. Energy of the electrostatic interaction of the TMA cation with hydroxyl is significantly higher than with hydrosulfide due to its smaller size. Therefore, water readily displaces hydrogen sulfide adsorbed on the ionic pair without its destruction to form weak H-bonding with the adsorption site. Obviously, the FGs do not lose their absorption performance because the next feeding of dry H2S + CH4 flow completely removes the adsorbed water. Therefore, the N-modified FG kept its absorption performance for at least four absorption−desorption cycles (Table 5). This proves that H2S adsorption is a reversible Table 5. H2S Absorption−Desorption Cycles on REM-Si-804a

a

cycle

tBT, min

Qsc, % wt

Qdes, % wt

1 2 3 4

2.7 5.1 5.4 5.1

0.026 0.031 0.033 0.030

0.022 0.031 0.031 0.028

GHSV = 25 000 h−1.

process and N-FGs are stable under absorption−desorption cycling. Similar results were also obtained for FGs with a much higher adsorption capacity, like REM-Si-125-96 sorbent. Due to high sorption capacity and long H2S absorption (high breakthrough times), we could not carry out the permanent adsorption−desorption cycles in a lab setup even at the shortest contact time. The amount of released H2S was so high that its desorption peak was cut due to the upper limit of the analyzer. However, after termination of the desorption process, under further H2S adsorption−regeneration, the cycle was reproduced. Obviously, the presence of porous structure and a rather thick layer of the new phase provides for reversible adsorption on the surface and diffusion of H2S into the bulk of the gel-like film layer. We believe that the rate-limiting step in both absorption and desorption processes is diffusion of the hydrogen sulfide or water molecules in the bulk of the new gel-like film layer having several hundred nanometers in thickness. Let us roughly estimate the characteristic time of mass transfer in this phase. Earlier, we estimated the water diffusion coefficient in the pristine fiberglass at room temperature to be as high as 10−12 cm2/s.38 As follows from

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01389. Thermostability of the N-modified fiberglass sorbent; temperature-programmed desorption of water from the pristine Al−Si fiberglass; 1H NMR spectra of 2.5% TMAH in glycerol−water (10:1) solution at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. I


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Mark G. Riley: 0000-0002-5951-8053 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to Dr. V.I. Zaikovskii for the HRTEM study. This work was partially supported by budget project No. 0303-2016-0006 for Boreskov Institute of Catalysis and project No. 15C0882 for Honeywell UOP.

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