ARTICLE

# Impact of NH₄OH treatment on the ion-exchange and pore characteristics of metakaolin-based geopolymer

Jing Li,*<sup>abc</sup> Sarah Mailhiot,<sup>b</sup> Mohammad I.M. Alzeer,<sup>a</sup> Tero Luukkonen,<sup>a</sup> Anu M. Kantola,<sup>b</sup> Ville-Veikko Telkki<sup>b</sup> and Paivo Kinnunen<sup>a</sup>

Received 00th January 20xx,
Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

This paper investigates the viability and influence of NH₄OH post-synthetic treatment on the pore characteristics of geopolymers. Geopolymers are a class of materials with amorphous aluminosilicate three-dimensional frameworks, regarded as amorphous analogues of zeolites. Similar to zeolites, when geopolymers are used in catalysis or adsorption applications, post-synthetic treatments such as ion-exchange with NH₄⁺ salts (e.g., NH₄Cl, NH₄NO₃) and desilication (using strong bases such as NaOH) are necessary to introduce active sites and modify their pore structure, respectively. Recently, it has been shown that treatment with NH₄OH combines these two steps, in which it introduces acidic sites and modifies the pore structures of zeolites simultaneously. Considering the increasing interests of geopolymers in catalysis and adsorption applications, understanding the impact of such treatment on the structure of geopolymers is needed. Our diffuse reflectance infrared fourier-transforms spectra show the NH₄⁺ exchange Na⁺ in geopolymer and both laser diffraction with scanning electron microscopy images show that the particle size of powdered geopolymer decreases after NH₄OH treatment. N₂ sorption isotherms, ¹²⁹Xe and ³H NMR measurements reveal information about the changes in interparticle pore structures: micropores enlarge to mesopores and inborn mesopores increase in diameter, thereby reducing the surface area to volume ratio. However, the pore accessibility and pore connectivity are not altered by NH₄OH treatment. Since solid-state NMR and X-ray fluorescence revealed the desilication, these changes in particles size and pore characteristics are considered to be due to desilication caused by NH₄OH treatment.

## 1. Introduction

Geopolymers are a class of amorphous aluminosilicate inorganic polymers synthesized by alkali-activation of aluminosilicate precursors (e.g., natural clays or low-Ca slags).¹ They could be described as the amorphous analogues of zeolites because of their disordered three-dimensional frameworks.² Compared with synthetic zeolites, geopolymers are cheaper and more easily synthesized at low temperatures (< 100 °C).³ In our recent works, we showed that the pore structures of geopolymers include inborn and interconnected micro- and mesopores.⁴,⁵ Geopolymers have shown good performance as catalysts and adsorbents in some processes, such as NOₓ reduction,⁶ oxidation of volatile organic compounds (VOCs)⁷ and water purification.⁸,⁹

For catalysis applications, acidic sites within geopolymer framework have been generated by ion-exchange of the charge-balancing alkali cations (i.e., Na⁺ or K⁺) with NH₄⁺ followed by thermal treatment which releases gaseous NH₃ forming H⁺-geopolymer.¹⁰ This is typically done by the treatment of a pre-formed geopolymer with a solution of ammonium salt (e.g., NH₄Cl, NH₄NO₃). In addition, ion-exchange procedures have also been used to incorporate catalytically reactive metals or metal oxides (e.g., Ni, Fe, Ce, Co, Cu, Pt etc.) within the geopolymer framework.² Such cations have been shown to be more efficiently incorporated into NH₄⁺ contained geopolymer compared to originally synthesised Na⁺/K⁺-geopolymer.²,¹¹,¹² In such applications, the pore characteristics (i.e., surface area and pore volume) of geopolymer catalysts are essential. This has been shown to be significantly improved by applying post-synthetic treatments such as dealumination (treatment with a weak acid) and desilication (treatment with a strong base).¹³ Such post-synthetic treatments are routinely carried out on zeolite catalysts, in which they generate additional interconnected and accessible mesopores.¹⁴ Similar procedures have been shown to be applicable on geopolymer catalysts.¹³ The effect of such treatments on the geopolymer properties have been

<footnote><sup>a</sup> Fibre and Particle Engineering Research Unit, Faculty of Technology, P.O.Box 4300, FIN-90014, University of Oulu, Oulu, Finland. Email: jing2.li@cea.fr</footnote>

<footnote><sup>b</sup> NMR Research Unit, Faculty of Science, P.O.Box 3000, FIN-90014, University of Oulu, Oulu, Finland.</footnote>

<footnote><sup>c</sup> NIMBE, CEA, CNRS, Université de Paris Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France.</footnote>

<footnote>Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x</footnote>
studied to some extent by diffuse reflectance infrared fourier-
transforms (DRIFT) spectroscopy for monitoring the success of
the NH₄⁺ ion-exchange,¹⁵ and ²⁷Al and ²⁹Si magic angle spinning
nuclear magnetic resonance (MAS NMR)¹⁶ which provide
information about structural changes caused by the treatment.
However, more information about the impact of such
treatments on modifying the chemical structure, particle
morphology, and pore properties, importantly pore-
connectivity, is still needed.

When probing the pore structures of geopolymers, N₂
sorption is a widely used method to provide information about
mesopores.¹⁷ Previously, ¹²⁹Xe and ¹H NMR have been shown
to be efficient methods to investigate the pore structures of
geopolymers⁴,⁵ as well as zeolites, silica gels and cementitious
materials.¹⁸⁻²¹ By introducing Xe and H₂O as probe fluids into
pores, three of the most relevant porous characteristics, pore
accessibility, pore connectivity and pore sizes, can be
determined. ¹²⁹Xe NMR spectra give information about the
pore size and accessibility, as ¹²⁹Xe chemical shift (δ) is
inversely proportional to the pore size²² and variable
temperature ¹²⁹Xe spectra enable the determination of the
mesopore sizes.²³ ¹²⁹Xe spin-spin relaxation time (T₂), spin-
lattice relaxation time (T₁) and exchange rates of Xe gas
between different pores (k)²⁴ reflect the pore connectivity.⁵ ¹H
T₁ and T₂ relaxation times of H₂O also reveal information about
the pore structure, such as pore size, pore connectivity, and
information about the pore surface characteristics.⁴,¹⁹ Two-
dimensional (2D) relaxation experiments, such as T₂−T₂²⁵ and
T₁−T₂²⁶,²⁷ experiments, provide higher resolution and more
detailed exchange information than 1D measurements.⁴ Pore
size distributions can be measured by 1H NMR
cryoporometry,²⁸,²⁹ which relies on the fact that the melting
point of water in a small pore is inversely proportional to the
pore size. The amount of unfrozen water is detected as a
function of temperature by ¹H spin echo or Carr-Purcell-
Meiboom-Gill (CPMG) experiments. The derivative of the
signal intensity is converted to pore size distributions using the
Gibbs-Thompson equation.³⁰,³¹

In this work, we report, for the first time, the impact of mild
NH4OH treatment on the chemical and physical properties of
geopolymers. We investigate the dual role of NH4OH; the ion-
exchange of Na+ with NH4+ simultaneously with modifying the
pore structure of geopolymer by desilication (due to the
basicity of NH4OH). The impact of NH4OH treatment on zeolite
catalysts have been reported,³² but as far as we are aware it
has not been reported on geopolymers previously. Modified
geopolymer catalysts with post-synthetic treatments have
been previously produced via multistep synthesis procedures
involving initial NH4+ ion-exchange step, followed by
dealumination, desilication and then an additional NH4+ ion-
exchange step. Using NH4OH produces desilicated geopolymer
in its NH4+ form in an one-pot process without the need for an
additional ion-exchange step. Herein, the NH4OH (0.02 M)
treatment was done on a metakaolin based geopolymer,

previously treated with acetic acid (0.1 M). The chemical
structure and particle morphology of the modified geopolymer
were thoroughly investigated with different NH4OH treatment
durations. Special attention was paid on studying the impact of
NH4OH treatment on the intraparticle pore structure of
geopolymer via N2 physisorption combined with 129Xe and 1H
relaxometry NMR techniques.

2. Experimental methods

2.1. Sample preparation

An alkali activating solution was first prepared by mixing 37.4 g sodium silicate solution (27% SiO₂, 8% Na₂O and 65% H₂O) (VWR, BDH, Frankenwald, South Africa) with 5.6 g NaOH pellets (VWR, BDH, Frankenwald, South Africa). The mixing procedure was done by a shear mixer (IKA, T 25 digital ULTRA–TURRAX, Staufen, Germany) using 3000 rpm speed for 5 min. Then 31.55 g metakaolin (MetaMax, BASF, Ludwigshafen, Germany) was dissolved into the alkali activating solution, with 3.85 mL H₂O. The mixture was placed into silicone molds and sealed into a plastic bag for curing at room temperature. After 24 h of curing, the solid samples were ground into powders by a vibratory disc mill (Retsch RS200, RETSCH GmbH, Haan, Germany). The oxide molar ratio of this geopolymer were: Na₂O: Al₂O₃: SiO₂: H₂O = 1.0: 1.1: 3.8: 13.6.

Before the NH₄OH treatment, a low concentration acetic
acid treatment was performed. 12.5 g of the geopolymer
powder were added into 100 mL of 0.1 M acetic acid (VWR,
Merck, Radnor, United States) and shaken on an orbital shaker
(Advanced d3500 Orbital Shaker, VWR, Radnor, United States)
for 10 mins. The powders were then filtered out, while the pH
of the filtered liquid was measured (Accumet model 20, Fisher
scientific, Vantaa, Finland). The geopolymer powders were
then shaken with 0.1 M acetic acid and filtered repeatedly
until the pH value of filtered liquid became 7 to 8.

The four sets of 12.5 g of geopolymer powders were
subjected to 500 mL of 0.02 M NH₄OH (Sigma-Aldrich, Munich,
Germany) solution treatment in a beaker, and stirred using a
magnetic stirrer (Mixdrive 6, 2mag AG, Muenchen, Germany)
with 600 rpm for 0 min, 15 min, 3 h and 24 h, respectively.
These four geopolymers treated samples were named N1, N2,
N3 and N4, respectively. Before four samples were sent for
characterization, they were dried overnight at 100°C.

Here we would like to note that we used the low
concentration (0.02 M) NH4OH solution in the in order to avoid
spoiling the pore structure, which is expected to happen at
higher concentration. However, the 0.02 M concentration was
too low to replace efficiently Na⁺ ions by NH4⁺ ions and thus it
was hard to observe the effect of the NH4⁺ treatment.
Therefore, an acetic acid treatment was applied before the
NH4⁺ treatment in order to help to exchange some original Na⁺
ions in geopolymer by H⁺ ions, thus facilitating the introduction
of NH₄⁺ ions into pores.
Preparation of amorphous Si-Al-geopolymer

<figure><img src="image_1.png" /></figure>

NH₄OH treatment with four durations

Figure 1. The procedure of the sample preparation.

## 2.2. DRIFT spectra

DRIFT spectra in the range of 500 – 4000 cm⁻¹ were collected for powder samples by a Bruker Vertex v80 spectrometer (Bruker, Rheinstetten, Germany).

## 2.3. X-ray fluorescence

X-ray fluorescence (XRF) measurements were used to determine the chemical composition of the samples. Scans were performed on a XRF spectrometer, PANalytical AXIOSmAX fitted with a Rh X-ray tube, which has a maximum power of 4 kW (Malvern PANanalytical, Malvern, UK).

## 2.4. Solid-state magic angle spinning (MAS) NMR

²⁷Al and ²⁹Si MAS NMR spectra were measured with Bruker Avance III 7.05 T spectrometer (Bruker, Billerica, MA, USA). Samples were packed into 7 mm zirconia rotors and a 7 kHz spinning frequency was applied. For collecting ²⁷Al MAS spectra, a single pulse sequence was used. The length of the excitation pulse was 1 µs. The number of scans was 1024 with the relaxation delay of 2 s. The experiment time was about 0.5 h. The ²⁷Al spectra were collected for all four samples and referenced to external reference of Al(NO₃)₃ at 0 ppm. The ²⁹Si MAS spectra were also collected with the single pulse sequence. The pulse length was 4 µs. The spectra were accumulated for 8192 scans. The relaxation delay was 3 s and the experiment time was about 7 h. The ²⁹Si spectra were referenced to tetramethylsilane (TMS) at 0 ppm.

## 2.5. Particle sizes

Particle size distributions were measured when geopolymer samples were dispersed into NH₄OH treatment solution by using a laser diffraction particle-size analyser (LS 13320, Beckman Coulter, Inc., Brea, CA, USA). The induced optical model was 'Fraunhofer. rfd'.

## 2.6. SEM images

The SEM images were recorded by using a Zeiss Ultra Plus field emission scanning electron microscope (FESEM) (Zeiss, Jena, Germany). The powder geopolymers were carbon coated before scanning.

## 2.7. N₂ sorption

The N₂ sorption measurements were performed at -196 °C with Micromeritics ASAP 2020 (Micromeritics, Norcross, US). The four samples were degassed under vacuum at 100°C for overnight. The pore size distributions of mesopores were obtained using the Barret–Joyner–Halenda (BJH) method³³ and the pore volumes of micropores were obtained using the t–plot method.³⁴ The surface areas were analysed by Brunauer–Emmett–Teller (BET) method.³⁵

## 2.8. ¹²⁹Xe NMR

To control for the particle size effect on the ¹²⁹Xe NMR results, the samples were sieved with an air jet sieve (HOSOKAWA ALPINE 200LS, Hosokawa Alpine AG, Augsburg, Germany) before the experiments. The particles in the range of 90 – 250 µm were selected. However, this is not the actual particle size as this sieve cannot separate the aggregated small particles. The actual particle size distributions were measured while dispersed into iso-propanol by using a laser diffraction particle-size analyser (see Section 2.4).

The sample powders were loaded into 5 mm NMR tube and 5–6 atm of ¹²⁹Xe gas was then condensed into each sample using liquid nitrogen and a vacuum system. After 5–6 atm of ¹²⁹Xe gas was condensed into each sample using liquid nitrogen and a vacuum system, ¹²⁹Xe NMR spectra were collected on Bruker Avance III 7.1 T spectrometer (Bruker, Billerica, MA, USA) with a 10 mm BBO probe. The ¹²⁹Xe dynamic experiments, including measurements of exchange rates and relaxation times (T₂ and T₁), were performed for samples N1, N2 and N3 on Bruker Avance III 14.1 T spectrometer (Bruker, Billerica, MA, USA) with a 5 mm BBO probe.

The ¹²⁹Xe NMR spectra were measured using a single pulse sequence. The data was collected using 64 scans and a 60 s relaxation delay between each scan. The experiment time of one ¹²⁹Xe spectrum was about 1 hour. The variable temperature ¹²⁹Xe spectra were collected for 11 temperature points in the range of 212 – 324 K. For 1 K of temperature
increase, the sample was left for 5 min for temperature stabilization.

The exchange rates (k) were measured with ¹²⁹Xe selective inversion recovery (IR) experiments²⁴ at 298 K.⁵ The recovery time was varied from 10 µs to 1 s by 36 log spaced steps. The length of the selective sinc pulse was 436 µs with 7 W power and the pulse was centred on the peak at 42 – 53 ppm (labelled with IP in Fig. 6). The number of scans was 128 and the relaxation delay was 10 s. The experiment time was about 15 h.

$T_1$ of ¹²⁹Xe was measured by the IR pulse sequence³⁶ at 298 K. The recovery time increased from 100 µs to 1 s with 25 log spaced steps. The number of scans was 64 and the relaxation delay was 10 s. The experiment time was about 6 h.

$T_2$ of ¹²⁹Xe was measured by the CPMG pulse sequence³⁷ at 298 K. The echo time (2τ) was 160 µs. The number of echoes varied from 2 to 50 with 14 log spaced steps. The number of scans was 256 and the relaxation delay was 10 s. The experiment time was about 10 h.

The *k*, $T_1$ and $T_2$ data fits were shown in Section. S2.

## 2.9. ¹H NMR experiments

Before the ¹H NMR experiments, the geopolymer samples were water saturated for 14 days. The ¹H $T_2$ and $T_1$ measurements, as well as 2D ¹H $T_2-T_2$ and $T_1-T_2$ experiments were performed on Magritek Spinsolve 43 MHz NMR spectrometer (Magritek, Aachen, Germany) at 298 K.

The spectrally resolved CPMG experiments were conducted to acquire $T_2$ distributions. The echo time was 150 µs and 1000 echoes were collected in a single scan. The number of scans was 4 and the relaxation delay was 14 s. The experiment time was about 16 h.

The $T_1$ distributions were acquired with the saturation recovery (SR) pulse sequence.³⁸ The recovery time was varied from 1 ms to 2 s with 64 log spaced steps. The number of scans was 4 and the repetition time was 14 s. The experiment time was about 1 h.

The $T_2-T_2$ experiments²⁶ were performed with an echo time equal to 150 µs and 1000 echoes were acquired in the direct dimension. In the indirect dimension, echo number was varied from 2 to 2000 in 64 log spaced steps. The number of scans was 128 and the relaxation delay was 6 s. The experiment time was about 14 h. The experiments were repeated with four different mixing times of 0.01, 0.02, 0.2 and 1 ms.

The $T_1$ SR-$T_2$ pulse sequence²⁶,²⁷ was used for collecting $T_1-T_2$ data. The echo time was 150 µs and 1000 echoes were collected in a single scan. The recovery time varied from 1 ms to 5 s with 64 log spaced steps. The number of scans was 128 and the relaxation delay was 6 s. The experiment time was about 14 h.

The ¹H NMR cryoporometry experiments³⁰ were performed on Bruker Avance III 11.7 T spectrometer (Bruker, Billerica, MA, USA) with a 10 mm BBO probe. The cryoporometry experiments contained 56 variable temperature CPMG experiments over a temperature range of 170 to 276 K. A temperature stabilization delay of 5 min/K was used between experiments. The CPMG echo time was 200 µs and the number of echoes was 1000. The relaxation delay was 5 s and the number scans was 64. The total experiment time for each sample was about 16 h. The data fits were shown in Section. S3.

$1D T_2$ and $T_1$ as well as 2D $T_2-T_2$ and $T_1-T_2$ distributions were obtained using the Laplace inversion implemented in MATLAB R2017b (Mathworks, Natick, Massachusetts, United States of America), provided by the research group of late prof. P. Callaghan.³⁰,³⁹,⁴⁰

# 3. Results and discussion

## 3.1. Ion-exchange and structure

DRIFT spectra were collected to detect the changes of the chemical structure of the geopolymer due to the NH₄OH treatment. As visible in Fig. 2a, the geopolymer without NH₄OH treatment shows three main peaks at 3452, 1649 and 1254 cm⁻¹, which are assigned to stretching H–O, bending H–O, and asymmetric stretching Si–O–T groups (T is Si or Al) in the geopolymer framework, respectively.⁴¹,⁴² After the geopolymer was treated with NH₄OH for 15 min, 3 h and 24 h, three peaks appear at 3246, 3037 and 2848 cm⁻¹ (Figs. 2b–d), representing the appearance of anti-symmetric N–H stretching bands, as well as another new peak at 1454 cm⁻¹, which is ascribed to the anti-symmetric bending H–N–H.⁴³,⁴⁴ These peaks is an evidence that NH₄⁺ went in pores after NH₄OH treatment and the NH₄⁺–geopolymer formed. This is corresponding to the XRF measurement results: the weight percentage of Na₂O decreased from 6.3 to 5.1 (Table 1), which is because the Na⁺ is continuously exchanged by NH₄⁺ during NH₄OH treatment, although the acetic acid (0.1 M) resulted larger level of exchange, decreased from 11.6 to 6.3. The NH₄⁺–exchanged geopolymer enables the preparation of material with high acidity after calcination at temperature of 550–600 °C.¹⁴,¹⁷
<figure><img src="image_2.png" /><figcaption>Figure 2. DRIFT spectra of four geopolymers with different NH₄OH treatment times, (a) N1, (b) N2, (c) N3 and (d) N4.</figcaption></figure>

<table><caption>Table 1. Weight percentage of Na₂O and SiO₂/Al₂O₃ molar ratio of the geopolymer before acetic acid treatment and four geopolymers after acetic acid and NH₄OH treatment, N1, N2, N3 and N4 measured by XRF analysis.</caption><thead><tr><th></th><th>Na₂O (%)</th><th>SiO₂/Al₂O₃ molar ratio</th></tr></thead><tbody><tr><td>Geopolymer before acetic acid treatment</td><td>11.6</td><td>3.54</td></tr><tr><td>N1</td><td>6.26</td><td>3.13</td></tr><tr><td>N2</td><td>5.61</td><td>3.16</td></tr><tr><td>N3</td><td>4.14</td><td>3.14</td></tr><tr><td>N4</td><td>5.13</td><td>3.12</td></tr></tbody></table>

The local Al and Si chemical environment changes after the NH₄OH treatment were also characterized using ²⁷Al and ²⁹Si MAS NMR. In ²⁷Al NMR spectra (Figs. 3a-d), the peak at 55 ppm is assigned to Al in tetrahedral coordination and the small peak at 2 ppm to six-fold coordinated Al, arising from a small amount of non-framework Al.⁴⁵ The treatment does not change the positions of the peaks, but the peak at 2 ppm gradually vanishes as NH₄OH treatment time increase to 24 h. This may be because the extra-framework-Al was removed via NH₄OH treatment. The similar result was also found in zeolites.³² In ²⁹Si MAS NMR spectra (Figs. 3e-h), two partially overlapping peaks are observed. The smaller peak at -110 ppm corresponds to SiO₄(0Al), which mainly comes from unreacted metakaolin.⁴⁶ The other peak is a typical broad geopolymer peak at -90 to -92 ppm. Normally, this peak contains four species, SiO₄(4Al), SiO₄(3Al), SiO₄(2Al) and SiO₄(1Al), arranged from low to high frequency.⁴⁷ This peak shifts from -91.7 ppm (Fig. 3e) to -90.7 ppm (Fig. 3h) with increasing NH₄OH treatment time. This small shift was also found for zeolite Y after NH₄OH treatment in a previous study, which is ascribed to desilication impact of NH₄OH.³² Removal of Si from the framework leads to an increased chemical shift. The decreasing input Si/Al ratio in geopolymer framework was also previously found to lead to increasing chemical shift.⁴⁷ The desilication was also found from XRF measurements, where the decreased SiO₂-to-Al₂O₃ molar

<figure><img src="image_3.png" /><figcaption>Figure 3. (a–d) ²⁷Al and (e–h) ²⁹Si MAS NMR spectra of (a, e) N1, (b, f) N2, (c, g) N3 and (d, h) N4.</figcaption></figure>

ratio from 3.16 to 3.12 as the NH₄OH treatment time increased from 15 min to 24 h (Table 1).

## 3.2. Particle size

Particle size distributions of the geopolymer powders were measured to investigate the effect of the mild NH₄OH treatment on geopolymer particles. The particle size distributions in Figs. 4a–d show that the NH₄OH treatment lasting for 0–3 h (N1, N2 and N3) does not alter the particle size. However, the NH₄OH treatment lasting for 24 h (N4) leads to a decrease in particle size. The particles with similar sizes are visible in the SEM images (Figs. 4e–h). The reason for the decreased particle size is hypothesized to be the desilication caused by the NH₄OH treatment. As the Si–O bonds break, the large particles become smaller while breaking into multiple smaller pieces. The formation of crystal fragments of ZSM-5 zeolite due to desilication was also observed in a previous

<figure><img src="image_4.png" /></figure>

study.⁴⁸

<caption>Figure 4. (a–d) Particle size distributions and (e–h) SEM images of four geopolymers with different NH₄OH post-treatment</caption>
times: (a, e) 0 min (N1), (b, f) 15 min (N2), (c, g) 3 h (N3) and (d, h) 24 h (N4).

### 3.3. Pore structure

The effect of NH₄OH treatment on the pore structure of the geopolymer were studied by N₂ physisorption, ¹H cryoporometry, ¹H NMR relaxometry and ¹²⁹Xe NMR methods as described below.

#### Pore volume, pore surface area and pore size

The pore volumes, the average pore sizes, and the surface areas of the four samples were measured by N₂ sorption isotherms (Table 2). The micropore volume decreases and the mesopore volume increases as the NH₄OH treatment time increases. The average mesopore size is also found to increase. This means that NH₄OH treatment enlarges some micropores to mesopores and already existing mesopores to bigger mesopores. The average pore surface area acquired from both BET and t-plot analysis in the system decreases as a function of treatment time. This, as expected, shows the impact of the desilication caused by NH₄OH treatment on enlarging micropores to mesopores and perhaps introducing additional mesopores within the geopolymer framework.

The pore size distributions of the samples were determined by N₂ sorption³³ and ¹H NMR cryoporometry³⁰,³¹ measurements. The N₂ pore size distribution obtained by the BJH method (desorption curve) are limited into the mesoporous range, while the NMR cryoporometry can probe both micro- and mesopores.⁵¹ The pore size distributions (Fig. 5a) fitted from NMR cryoporometry (Fig. S3) include two peaks for all the samples, one from micropores and another from mesopores. The mesopore peak is centred around 7 nm, being in a good agreement with the single peak observed by N₂ sorption (Fig. 5b). The micropore peak is centred around 1.7 nm. According to the N₂ sorption data shown in Table 2, the micropore volume decreases while mesopore volume increases with longer NH₄OH treatment time.

<table><caption>Table 2. Pore volumes, pore sizes and surface areas of the four geopolymers measured by N₂ sorption.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Pore volume (cm³/g)</th><th>Pore size (nm)</th><th colspan="2">Surface area (m²/g)</th></tr><tr><th>Micropore (t-plot)</th><th>Mesopore (BJH)</th><th>Mesopore (BJH)</th><th>Average (BET)</th><th>Micropore (t-plot)</th></tr></thead><tbody><tr><td>N1</td><td>0.00385</td><td>0.256</td><td>6.54</td><td>130</td><td>14.1</td></tr><tr><td>N2</td><td>0.00321</td><td>0.261</td><td>6.61</td><td>126</td><td>12.5</td></tr><tr><td>N3</td><td>0.00213</td><td>0.264</td><td>6.66</td><td>123</td><td>10.1</td></tr><tr><td>N4</td><td>0.00130</td><td>0.274</td><td>7.43</td><td>112</td><td>7.89</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Figure 5. Pore size distributions of the four geopolymers derived from (a) ¹H cryoporometry NMR data and (b) N₂</figcaption></figure>

desorption data analysed with BJH method.

#### Pore accessibility and exchange detected by ¹²⁹Xe NMR

The pore accessibility was probed by ¹²⁹Xe NMR, as the ¹²⁹Xe chemical shift is able to distinguish intra and interparticle pores.¹³ ¹²⁹Xe in a small pore has larger chemical shift than ¹²⁹Xe outside the pores.²²

The ¹²⁹Xe spectra were collected at variable temperatures from 212 to 324 K to study pore accessibility (Fig. S1). As seen in Fig. 6, two peaks are observed for samples N1, N2 and N3 at low (212 K) and high (297 K) temperatures. The tall peak labelled as IP around 110 ppm at 212 K is assigned to Xe in small pores inside the particles. The chemical shift and its relatively small temperature dependency implies that the IP signal arises predominantly Xe atoms in micropores.⁵,²² The small peak labelled as BP around 6 ppm is attributed to Xe between larger particles (55 to 250 µm).⁵,²² Due to the large interparticle void spaces, the chemical shift of the BP peak is close to free gas shift and almost independent of temperature. In contrast, a single, very broad peak is observed for the N4 sample in the chemical shift range of 50 and 130 ppm at 212 K. This is interpreted to be a consequence of smaller particle size of the N4 sample. According to Fig. 4d, contrary to other samples, N4 sample lots of particles in the range of 0.04 to 55 µm. Due to the decreased particle size, exchange is faster between the intraparticle and interparticle pool, resulting in partially exchange averages signals (intermediate exchange region).

Exchange rates *k* of Xe between the intraparticle and interparticle pores of N1, N2 and N3 were determined by selective inversion recovery experiments (Fig. S2).²⁴ The observed exchange rates (Fig. 6i) are in the range of 140 – 270 s⁻¹, and within the error bars they are independent of the NH₄OH treatment time.

¹²⁹Xe *T*₁ and *T*₂ relaxation times (Figs. 6j and 6k) also reflect the dynamics of Xe in the geopolymer samples. Within the error bars, *T*₁ relaxation times (1.5 – 3 s) of the IP and PB peaks are equal due to exchange averaging, as the relaxation rates
<figure><img src="image_6.png" /></figure>

(0.3 – 0.7 s⁻¹) are much smaller than the exchange rates (140 to 270 s⁻¹). T₁ is longer (3 s) for the NH₄OH treated samples N2 and N3 than for the untreated N1 sample (1.5 s). This may be a consequence of changed surface interactions due to the partial Na⁺ – NH₄⁺ ion exchanges. T₂ relaxation times (0.5 – 3.5 ms) are much shorter than T₁ relaxation times. According to relaxation modelling, fluctuations in the isotropic chemical shift are known to be dominating T₂ relaxation mechanism of ¹²⁹Xe in porous materials because of large chemical shift between the exchanging sites.⁵² Because T₂ rates (300 – 2000 s⁻¹) are higher than the exchange rates (140 – 270 s⁻¹), T₂ values are not exchange averaged; T₂ of ¹²⁹Xe in the intraparticle site is longer (about 3 ms) than in the inter particle site (about 0.7 ms).

Pore types and connectivity detected by ¹H NMR relaxation

The 1D ¹H T₂ and T₁, as well as 2D T₂–T₂ and T₁–T₂ measurements of water absorbed in the geopolymer samples

N1 (0 min)

N2 (15 min)

N3 (3 h)

N4 (24 h)

<figure><img src="image_7.png" /></figure>

Figure 7. (a–d) ¹H T₂–T₂ exchange spectra of water in the geopolymer samples measured with mixing time of 0.2 ms. (e–h) ¹H T₁–T₂ correlation spectra. T₂ and T₁ distributions measured by CPMG and saturation recovery experiments are shown on the top and right of the 2D plots.

were performed to investigate how the pore types and connectivity change with the NH₄OH treatment. When water is inside the geopolymer, T₂ and T₁ relaxation times reflect the pore size.⁴,¹⁷

Four peaks are observed in the T₂ distributions shown on the top of T₂–T₂ maps in Figs. 7a–d. The shortest T₂ peak (label S) around T₂ = 1 ms is interpreted to arise from water in the 1.7 nm micropores. The peaks (label B) in the region of 6–36 ms are assumed to represent water in the 7 nm mesopores. The longest T₂ peak (label BP) is assigned for water in large voids in between the particles. The BP signal of the N4 sample has smaller amplitude and shorter T₂ than the other samples. This is because the smaller particles of N4 leave less space for the free water between the particles.

T₁ distributions are shown on the right of the T₁–T₂ maps in Figs. 7e–h. T₁ values are systematically longer than T₂ values. The T₁ distributions include two peaks around 5 and 50 ms,
which are attributed to the water inside the intraparticle pores (IP) and the water inside the interparticle pores (BP). The micropore and mesopore peaks S and B observed in the $T_2$ distributions are expected to fuse in the $T_1$ distributions due to exchange; this implies that the micropore–mesopore exchange rate of water is between the $T_1$ and $T_2$ relaxation rates, i.e., in the range of 200 – 1000 s⁻¹. The cross-peak S–BP visible in the $T_2–T_2$ exchange maps (Figs. 7a–d) show that there is also exchange between the intraparticle and interparticle pools. The $T_1–T_2$ maps show the correlations between the $T_1$ and $T_2$ relaxation time values and confirm the above-mentioned interrelated assignments of the peaks in the $T_1$ and $T_2$ distributions.

## 4. Conclusions

This work studies the effect of mild $NH_4OH$ treatment to a metakaolin-based geopolymer in terms of ion-exchange, chemical structure, particle size and pore structure.

1) The obtained DRIFT, accompanied with XRF results show that ion-exchange of charge balancing alkali cations (i.e., $Na^+$) with $NH_4^+$ was successfully achieved by this treatment.

2) The solid-state NMR spectra and XRF data proved the $NH_4OH$ treatment removed the extra-framework-Al out of geopolymer structure and also lead to the desilication of geopolymer framework.

3) The particle size reduction resulted from $NH_4OH$ treatment were found by both laser diffraction particle-size analysis and SEM.

4) After the $NH_4OH$ treatment, the pore volume and pore size were enlarged, but the pore accessibility and pore connectivity were not altered, as revealed by $N_2$ sorption isotherms, $^{129}$Xe and $^{1}$H NMR.

The changes in particles size and pore characteristics are considered to be due to desilication caused by $NH_4OH$ treatment.

In this study the $NH_4OH$ treatment was carried out under mild conditions (0.02 M at room temperature) to ensure maintaining the structural integrity of geopolymers. This resulted in limited ion-exchange efficiency as well as limited impact on the pore characteristics of the prepared materials. This work provides a new approach of simplifying the synthesis of ion-exchanged geopolymers with modified porosity as low-cost functional materials for catalysis and adsorption applications. Further investigation on the impact of $NH_4OH$ treatment under harsher conditions (for instance, $NH_4^+$ concentration ≥ 0.1M, and moderate temperature range (323 K–353 K)) would be of high interests.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

J. L. and P.K. are grateful for the support from the Kvantum Institute (University of Oulu), and The University of Oulu & The Academy of Finland Profi5 project 326291 and 329477. S.M. is grateful for support from Academy of Finland (grant no. 321701) and Marie Sklodowska-Curie Actions (grant no. 896824). V.-V.T. is grateful support from European Research Council (grant no. 772110), Academy of Finland (grant no. 340099) and the Kvantum Institute (University of Oulu). J. L. acknowledges for the financial supports from the French National Research Agency (ANR) (Projet-ANR-20-CE30-0021 HELPING), Finnish Foundation for Technology Promotion as well as Otto A. Malm Foundation. Part of the work was carried out with the support of the Centre for Material Analysis, University of Oulu, Finland. J. L. is particularly grateful for the help from Professor Patrick Berthault for working in CEA-Saclay.

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<figure><img src="image_8.png" /><figcaption>Abstract figure</figcaption></figure>