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DOI: 10.1039/D5TA04265H (Paper) J. Mater. Chem. A, 2025, Advance Article

Mesostructure control in 3D-printed monoliths using industry-grade SiO2 powder for CO2 capture†

David Vogelsang*, Kenny Wyns and Steven Mullens
VITO, Unit Sustainable Materials, Boeretang 200, BE-2400 Mol, Belgium. E-mail: david.vogelsang@vito.be

Received 27th May 2025 , Accepted 21st July 2025

First published on 21st July 2025

Abstract

Direct ink writing (DIW) enables the fabrication of silica monoliths with hierarchical porosity, offering potential for liquid and gas adsorption applications. Although silica and silanized silica are widely employed in such processes, control over mesoporous architecture in DIW-printed structures remains limited. In this study, mesoporosity was tuned using inks formulated from industry grade silica powders and a colloidal binder. Rheological behavior relevant to printability was characterized, and the porous architecture of extruded filaments was evaluated via nitrogen sorption. Pore size distributions ranging from 4 to 50 nm were achieved by adjusting powder characteristics. Narrow distributions were observed in inks with high colloidal silica content, while broader distributions resulted from bimodal particle blends. Depending on the particle size and agglomeration state, porosity arose either from intrinsic powder features or from interparticle voids formed during ink preparation. Post-printing silanization with 3-aminopropyl(triethoxysilane) consistently reduced both specific surface area and pore volume. A direct correlation was identified between surface area and the density of amine groups. CO2 adsorption measurements confirmed functional group accessibility across all formulations, with an average uptake of 0.2 mmol CO2 per mmol NH2. This work provides a predictive framework linking ink formulation to the mesostructure and surface reactivity in printed silica monoliths.

1. Introduction

Adsorption processes are often based on the interaction of a gas or liquid with a porous sorbent material in a packed bed configuration. The shape and porous architecture of the packed bed material play a significant role in the operation and efficiency of the process, as they impact the flow pattern through the column, pressure drop and axial dispersion.1–3 Conventional packed bed materials include granulated and extrudates, leading to a variety of structures and geometries. However, the progress in additive manufacturing technologies and the large expansion in materials have opened new possibilities in controlling and tailoring the porous architecture of packed beds, avoiding preferential paths when a fluid passes throughout the monolith.4

Direct ink writing (DIW) is one of the preferred AM techniques due to its generic nature with regard to material selection and capability to achieve structures with resolutions mostly limited by the nozzle size.5 Among the ceramic materials used in DIW, silica based structures were previously explored for production of optical lenses, glass objects, CO2 adsorbents and Pd adsorbents.6–10 Depending on the conditions of the application, different silica sources are used. Quartz, glass, and sol–gel feedstocks have been preferred for optical applications,6,7,11 colloidal silica has been used as a binder and also in applications requiring thin features,10,12 aerogels in powder form had been implemented for research teams targeting thermal insulation,8,13 and other silicas such as fumed silica, or precipitated silica, were chosen for catalysis and adsorption.9,10,14

Current research into silica monoliths printed by DIW is focused on the formulation of the ink, the viscoelastic and flow behavior of the inks, and the application of the structure. However, one important characteristic is the pore architecture of the resultant hierarchical porous structure, comprising the specific surface area (SSA), the pore volume, and the pore size distribution (PSD). The porous architecture was discussed and controlled in few studies as this is a by-product of the ink formulation. Among these studies, the pore architecture was controlled by preparation of an ink using alkoxysilane precursors and a polymeric template which produced a hierarchical porous silica structure.7 In other relevant investigations, silica based monoliths were printed and their SSA, pore volume, and PSD were analyzed after different functionalization procedures.9 Easily scalable printing processes rely on silica powders as they are widely available materials with diverse intra-particle porosities, particle sizes, particle shapes and surface chemistries.8,10,15 Selection of the silica powder to be additively manufactured is therefore done based on the end application of the monolith. However, the pore and surface characteristics of the available silica powders could be altered during additive manufacture and post-processing of the monolith.

The goal of this research is to tailor the mesoporous architecture of extruded filaments by selecting different industry grade silica powder sources to fabricate hierarchical porous monoliths, to evaluate how these structural variations influence surface functionalization with 3-aminopropyl(triethoxysilane) (APTES), and to assess the accessibility of the grafted amine groups through CO2 sorption. Following Scheme 1, silica powders with distinct particle morphologies, specific surface areas (SSAs), and intraparticle porosities were combined with a colloidal silica suspension to prepare a series of printable inks. The flow behavior and viscoelastic properties of these formulations were characterized by rheological experiments. Cylindrical monoliths were subsequently fabricated via direct ink writing (DIW), followed by drying and thermal treatment. Liquid-phase silanization was performed to investigate the extent of functionalization as a function of the porous architecture. The microstructure, porosity, and SSA were characterized for the raw powders, the thermally treated monoliths, and the silanized materials. Finally, the influence of SSA and pore accessibility on surface reactivity was probed using CO2 as a model adsorbate to determine the adsorption capacity of the functionalized monoliths.


image file: d5ta04265h-s1.tif
Scheme 1 Workflow for the proposed research starting with a mixture of a silica powder and a colloidal silica sol, followed by DIW of the paste, and further analysis of the pore architecture within the filaments of the monolith before and after silanization.

2. Experimental

2.1. Materials

The colloidal silica Ludox® CLP (CLP), with an average particle size of 22 nm and an average solid content of 41%, was supplied by Grace. The fumed silicas Aerosil® OX50 (OX50) and Aerosil® 200 (A200) from Evonik were supplied by Safic-Alcan; A200 was dry-sieved and the fraction collected between 50 μm and 100 μm was used for the mixing. The silica gels Sunsphere® H31 (H31) and Sunsphere® NP30 (NP30) were supplied by AGC. Lanoline was used as a rheology modifier and drying additive and it was supplied by Leneka. Toluene and triethylamine (Et3N) were obtained from VWR. Toluene was stored with 4 Å molecular sieves to reduce water concentration. 3-Aminopropyl triethoxysilane (APTES) was purchased from Fisher Scientific; vinyl trimethoxysilane (VTMS) and N-(6-aminohexyl)aminomethyl triethoxysilane (6AHATES) were supplied by Gelest. The drying aid agent 1,3-propylenediol (Pr(OH)2) was acquired from Sigma–Aldrich and few drops were used in the formulations. All products were used as received unless indicated. A table indicating the name codes used in this work is annexed to the ESI file as Table S1.†

2.2. Methods

2.2.1. Preparation of the inks. Powder characteristics determine to a large extent the final rheological properties of the final ink. Therefore, the desired composition of the inks was achieved by adding the powder to a fixed amount of colloidal silica and lanoline, until the paste showed an acceptable viscoelastic behavior 24 hours after the ink was mixed. The inks developed for DIW were prepared by addition of lanoline (5.2 g) to aid flow and viscoelastic behavior, CLP (25.0 g) as a source of nanoparticles which can act as a binder material, and the solid OX50 (25.0 g), A200 (5.8 g), H31 (11.3 g), or NP30 (54.0 g) in a mixing flask to make the inks iOX50-C, iA200-C, iH31-C, and iNP30-C respectively. For all the formulations the silica powders were added in two or more instalments. This helps prevent water evaporation caused by viscous heating while mixing. The ink was mixed in a planetary mixer (SpeedMixer DAC 1400-1000 VAC) 3 times for 1 min at 1600 rpm. The ink was filtered by forcing it through a 60 μm mesh. The filtration was required to avoid clogging when printing. After filtration, the ink was transferred to an extrusion barrel and conditioned for 24 hours in the printer room at 25 °C and a relative humidity of 50%. The bulk density of the inks was measured by filling a 1 mL syringe with the inks and weighing the syringe before and after filling.
2.2.2. Printing of monoliths. Cylindrical monoliths with diameters of 0.8 cm and heights of 1 cm were printed with gap-spanning infills on an nScrypt 3Dn-500 3D printer. The printer has a Smartpump™ system to start and stop the flow of ink between printing and traveling steps. Plastic conical nozzles of 200 μm (Nordson) were used to extrude the filament. Monolith structures were formed by depositing the filaments with a 200 μm interfiber distance (IFD) and layers with a height of 120 μm. The printing velocity was selected between 5 and 10 mm s−1 and flow of the ink was controlled by pressurized air. The infill characteristics of the monoliths were selected to produce homogeneous density of APTES after 24 hours of reaction based on a previous report of a system composed of OX50 and CLP.10 Once printed, the samples were dried at 25 °C and 80% RH in a humidity chamber for 1 week. These drying conditions avoided the cracking of the monoliths. The dried monoliths were subjected to a thermal treatment from room temperature to 600 °C using a heating rate of 60 °C h−1; the sample was then held for 4 h at 600 °C and finally cooled down to room temperature at an approximate rate of 120 °C h−1. A video showing the printing of the monoliths can be seen in the ESI files.†
2.2.3. General functionalization procedures with alkoxysilanes. Functionalization was performed according to a previously reported protocol, in which covalent bonding of the silane to the silica surface was confirmed by solid-state 29Si NMR spectroscopy. That study employed a similar silica composition and identical silanization conditions, thereby supporting the effectiveness of the procedure applied here.10 To minimize water uptake prior to silanization, the calcined monoliths and glassware were stored in an oven at 75 °C and only removed immediately before use. Although this step helped reduce surface moisture, no direct measurement of residual water content was performed, as silica is highly hygroscopic and can rapidly adsorb atmospheric moisture during sample handling. Additional precautions, including vacuum treatment while warm and nitrogen purging, were taken to further limit moisture exposure. The silica structures had weights between 1.4 g and 2.9 g depending on the silica source. A theoretical hydroxyl density of 2.1 #OH nm−2 was used based on a literature report for materials treated at 600 °C.16 The reactions were conducted using a solution of toluene (200 mL) with 7 mol of the alkoxysilanes (ASs) APTES, VTMS, or 6AHATES per each theoretical mol of hydroxyl groups on the surface. TEA was added with a molar ratio of AS[thin space (1/6-em)]:[thin space (1/6-em)]TEA of 1[thin space (1/6-em)]:[thin space (1/6-em)]3. The specific quantities used in the experiments can be seen in Table S2.† The solution was mixed into a container under a N2 stream. A 60 mL syringe was then filled with the reaction mixture. Simultaneously, the samples and glassware were removed from an oven at 75 °C. The sample with known mass was placed in a two neck round bottom flask with a vacuum line in one of the necks and a rubber septum over the remaining neck. Vacuum was induced in the flask for 3 to 5 minutes before the valve was closed. The rubber septum in the two-neck flask was poked with the syringe containing the reaction mixture. The reaction mixture was then sucked by action of the vacuum quickly impregnating the monolith. Once the addition was finished, the vacuum line was briefly opened to ensure full impregnation of the reaction mixture. The vacuum was broken with a nitrogen stream and the remaining amount of solution was added. The reaction proceeded under stirring at 50 rpm for 24 h. The samples were taken out of the solution, flushed with nitrogen, and washed with acetone, followed by a washing with a 0.01 M HCl solution and a final washing with acetone. Once washed, the sample was flushed again with a N2 stream followed by mild drying at 75 °C for 24 h. Once treated, the samples were stored in closed vials at room temperature.
2.2.4. Characterization. Particle size distribution of the raw silica powders was determined by laser diffraction on a Microtrac S3500 with a PartAn instrument using 3 minutes of sonication in the sample addition chamber to disperse the particles. N2 sorption was performed on an Autosorb iQ2 MP instrument (Quantachrome GmbH). All samples were degassed at 200 °C under vacuum for 16.3 hours. Pore size distribution was calculated using non-local density functional theory (DFT) using a model for silica with cylinder-like pores adsorbing nitrogen at 77 K (−196.2 °C). The specific surface area was calculated using the method of Brunauer–Emmett–Teller (BET). Determination of macroporosity was performed by Hg intrusion porosimetry using a PASCAL 240 porosimeter.

Rheology of the inks was analyzed on a Haake Mars 60 rotational rheometer (Thermo Scientific). 35 mm parallel plates were used with a gap of 1 mm between the plates and a solvent trap. Rotational and oscillatory measurements were conducted at 25 °C. Rotational experiments were run from 0.1 s−1 to 10 s−1 shear rate after a stabilization step at 0.1 s−1 for 5 minutes. Viscoelastic behavior and thixotropy were determined by oscillatory experiments. Viscoelastic behavior was evaluated by a strain swap from low to high shear stress values. Thixotropic behavior was evaluated by applying a strain amplitude of 0.001 followed by 1.0, the latter corresponding to the crossover point of G′ and G′′ observed in oscillatory rheology experiments. All samples were preconditioned at this strain before yield stress was assessed through rotational measurements.

SEM-EDS images were acquired on a cold field emission scanning electron microscope (FEGSEM) Nova Nano SEM 450 (FEI, USA) operating with an acceleration voltage of 5 kV. Uniaxial compression tests were performed using an Instron mechanical testing machine equipped with a 1 kN load cell, by applying a constant displacement rate of 1 mm min−1. Each test was conducted on dry monolith cylinders, and the maximum compressive stress was recorded. Up to four replicates were tested per formulation. TGA experiments were performed on a NETZSCH STA 449C instrument. The program for hydroxyl quantification in the powders was performed based on the work of Mueller et al.16 In this, under a nitrogen atmosphere, the sample was heated from 25 °C to 120 °C at 10 °C min−1 and then the sample was held at 120 °C for 10 min, followed by a heating ramp from 120 °C to 800 °C at 20 °C min−1 to finally maintain the sample at 800 °C for 10 min. Carbon dioxide capacity (QECO2) was determined by TGA. The sample was initially heated from 25 °C to 120 °C at a rate of 10 °C min−1 and held for 3 h at 120 °C under a nitrogen flow. The system temperature was then reduced to 25 °C at a rate of 40 °C min−1 and further maintained at 25 °C for 3 h. After the equilibration 10% of CO2 was added to the gas stream and maintained for 3 h. The CO2 desorption was done by flushing only N2 throughout the TGA chamber for 3 h. Three adsorption–desorption cycles were performed in each experimental run.

Quantification of C and N in the samples was performed by elemental analysis using a vario EL cube (Elementar). This instrument has a detection limit of 0.05 wt% and the results have an uncertainty of 0.02 wt%. ξ-Potential was measured on a Malvern ξ-sizer by crushing the monoliths and suspending 20 mg of the crushed powder in 10 mL of a 10 mM KCl solution. The pH of the suspensions was adjusted with 0.01 M, 0.1 M and 1 M solutions of NaOH or HCl. The isoelectric points were calculated by interpolation.

3. Results and discussion

3.1. Characterization of starting materials

Different silica powders were mixed with the colloidal silica CLP to produce inks for DIW. These powders were selected because they have different particle sizes, specific surface areas, and intra-porosities. These properties are required to achieve different formulations and therefore different mesoporous systems. From Fig. 1a it can be observed that OX50 has a particle size distribution between 0.01 μm and 200 μm which is broader than that of A200 between 6 μm and 300 μm. Although both materials are fumed silicas, OX50 seems to deagglomerate easily compared with A200. On the other hand, the particles of H31 are distributed between 1 μm and 20 μm and the particles of NP30 are distributed between 1 μm and 30 μm. The adsorption isotherms in Fig. 1b have two main shapes. The first one for OX50 and A200 is typical for non-porous nano-sized powders; these curves do not stabilize as the nitrogen atoms keep filling the chamber as these are microporous materials. The second shape for H31 and NP30 is typical for materials with intraparticle mesoporosity where multilayer adsorption of the gas happens within the pores. NP30 is typically used as a non-porous material. However, it is clear from the adsorption isotherm that some pores are present in these dense spheres. The experimental SSA values calculated using the BET model from the nitrogen sorption isotherms in Fig. 1b are very similar to those reported by the powder suppliers with the largest SSA for H31 = 875 m2 g−1, followed by A200 = 175 m2 g−1 and OX50 = 49 m2 g−1, and the lowest for NP30 = 37 m2 g−1. After thermal treatment of the powders at 600 °C the SSA for H31 was 640 m2 g−1, A200 = 175 m2 g−1, OX50 = 51 m2 g−1, and NP30 = 38 m2 g−1. H31 is highly affected by the thermal treatment in comparison with the low SSA particles of A200, OX50, and NP30. The SSA of A200 and OX50 remained similar after thermal treatment, which indicates that these are dense particles forming aggregates and the nitrogen is condensing within the pores of the aggregates. The PSDs of the powders calculated from the DFT model and plotted in Fig. 1c agree with the supplier specifications for precipitated silicas H31 and NP30. In addition, the PSDs of A200 and OX50 are very broad as expected from aggregated materials where the PSD is given by the interparticle voids. After thermal treatment, the PSDs of H31 and NP30 do not have significant differences when compared with the native powders.
image file: d5ta04265h-f1.tif
Fig. 1 Characterization of the SiO2 powders used to mix the inks. Particle size distribution of the native powders (a), N2 adsorption isotherms for the native powders and the powders treated at 600 °C; the inset shows a magnified view to observe the isotherm of NP30 (b). PSD calculated using DFT for the native powders and the powders treated at 600 °C; the inset shows a magnified view to observe the pore size distribution of NP30 (c). Relation between the mass of SiO2 native powder added to the ink and the SSA of the powder used (d). The colors represent each powder used, A200 = black, OX50 = red, H31 = blue, and NP30 = green. For (b and c), the dotted line with the open symbols is attributed to native materials and the straight line with the solid symbols is attributed to thermally treated powders.

3.2. Preparation of inks

Production of hierarchical porous monoliths by DIW requires the preparation of a paste like material, or ink, to be extruded through a nozzle. The ratio of additives (colloidal silica and lanoline) was kept constant across all formulations, while the content of SiO2 powder was incrementally adjusted to reach the target formulation range defined by a figure of merit. Minor modifications to the composition were made following printability testing, and the printing parameters, specifically pressure and speed, were fine tuned for each formulation to ensure consistent filament formation and high print fidelity. The ratio between the mass of powder and the mass of colloidal silica as well as the density of each ink can be observed in Table 2. From these values and the relation between SSA and content of solids in the ink plotted in Fig. 1d, it was observed that inks of similar characteristics made with low SSA powders required higher solid contents. It is evident that to prepare a relatively similar ink, a larger amount of OX50 was required compared to that of A200. On the other hand, a larger amount of NP30 was required in comparison with that of H31 indicating that the amounts of silica powder added depend on the SSA and on the nature of the native silica powder as fumed silicas cannot be compared with precipitated silicas.

iA200-C has the lowest concentration of solids among the evaluated systems. A200 can be deagglomerated but it is difficult to deaggregate as relatively large particle sizes are observed after deagglomeration using sonication with an amplitude of 70 in ethanol for 1 minute as seen in Fig. 2a. However, since the primary particles of A200 are only a few nanometers in size, the material has low bulk density and high specific surface area, it does not require much of the aggregates to reach the required paste-like consistency. In contrast, iOX50-C is a system that can afford higher amounts of powder and is very homogeneous after mixing as the OX50 clusters deagglomerate into smaller aggregates or even into their base particles which have diameters of approximately 50 nm as seen in Fig. 2a after 1 minute of sonication in ethanol. The iNP30-C and iH31-C systems have more than 50% solid content. However, iNP30-C has 26% more solids than iH31-C showing again that to reach printable inks using powders with smaller surface areas larger solid contents will be required.


image file: d5ta04265h-f2.tif
Fig. 2 SEM images of the native powder after dispersion for 1 minute by sonication in ethanol (a), optical microscopy of the monoliths (b), optical microscopy of the top infill of the monoliths (c), and SEM images of the surface of the monoliths (d). (a–d) are organized from left to right as materials made with A200, OX50, H31, and NP30. The scale bars are b = 2000 μm, c = 200 μm, and d = 2 μm.

3.3. Flow and self-supporting behavior of the inks

Printable inks are reported as shear thinning fluids that can be extruded and once deposited can support the weight of the layers printed on top without deformation.17 From the steady state experiments it was determined that all the inks have shear thinning behavior as seen in Fig. S1.† In addition, the oscillatory experiments in Fig. S2† showed that under deformations below 0.01% the elastic modulus (G′) is always higher than the viscous modulus (G′′) indicating the solid-like behavior where the fiber is deposited. On the other hand, the liquid-like behavior can be achieved with shear stresses (τ) in the range between 1000 Pa and 10[thin space (1/6-em)]000 Pa. The Feilden figure of merit (ϕ), defined as the ratio between G′ in the linear viscoelastic region and τ when G′ = G′′ or the flow point, was used to assess the printability of the inks. This ratio was always in the printable region of the chart presented in Fig. S3.† Despite having acceptable parameters from the steady state and the oscillatory tests the inks were not responsive in the printing segments when the flow was stopped for traveling and resumed for printing. In consequence, it was decided to evaluate the thixotropy by the three interval thixotropy test.17–19 It was found that at a constant deformation, selected after the flow point, the G′ decreased with time. Therefore, all the inks showed thixotropy as seen in Fig. S4.† The thixotropic behavior is a serious problem for extrusion systems driven by air pressure as they use a constant stress. The problem was overcome by adapting the opening and the closing velocity of the Smart-pump™ valve. A deeper discussion of the rheology results can be found in the ESI file.†

3.4. Additive manufacturing of monoliths

The monoliths used to characterize the physical properties and to be functionalized were manufactured by DIW, followed by a drying step and a thermal treatment to reinforce the green bodies. All the inks were printed initially with a speed of 10 mm s−1. However, the printing speeds, the printing time, and the pressure of the system were adjusted with the values recorded in Table 1. The printing times differ between formulations; it is much faster for the inks with low solid content (iA200-C and iH31-C). These two inks have in common a large G′ vs. G′′ crossover point. The 3D printer used in this investigation can withstand 100 psi; it is believed that with a larger pressure the speeds can be increased and the process can be intensified. The monoliths did not exhibit visible cracks after drying after optical microscopy, Fig. 2b and c, and SEM, Fig. S5.† This effective drying is attributed in part to the 80% relative humidity inside the chamber.20 Drying and thermal treatment of the monoliths printed with the inks iA200-C, iOX50-C, iH31-C, and iNP30-C produced the solid structures A200-C, OX50-C, H31-C, and NP30-C respectively. Optical microscopy images observed in Fig. 2b contain the dimensions of the monoliths after thermal treatment. The shrinkage of the samples calculated from the original dimensions (diameter = 8 mm and height = 10 mm) and the dimensions after thermal treatment revealed that A200-C, OX50-C, H31-C, and NP30-C shrunk 18%, 7%, 8%, and 7% respectively in the radial direction and 12%, 5%, 9% and 4% respectively in the axial direction. The shrinkage is therefore proportional to the content of solids in the formulations described in Table 1. A graph showing the shrinking behavior can be seen in Fig. S6.† From the structures printed, only H31-C monoliths have a homogeneous ratio from bottom to top with no apparent defects. Despite the shrinkage, H31-C has high fidelity of the geometry printing compared with the CAD design. A200-C and OX50-C were acceptable meaning that there were variations in the ratio but these were not dramatic, and NP30-C had variations in the flow caused by reduction in the nozzle opening due to drying of the ink and accumulation of the same in the edges of the nozzle during printing affecting the fidelity of the shape. However, the infill of the monoliths observed from the top surface of the monolith is homogeneous and with high fidelity as seen in Fig. 2c. SEM images of the surface of the monoliths in Fig. S5† showed that the systems do not have cracks after thermal treatment. A visual inspection of the configuration of the powders and the colloidal silica in the monoliths was conducted by analyzing the surface with the SEM images as seen in Fig. 2d. The surface of the monolith OX50-C, in Fig. 2a or Fig. S7,† shows two different particle sizes homogeneously dispersed. The particles of smaller size are those of CLP and the particles with larger size are those dispersed after deagglomeration and deaggregation of OX50 powder while the ink was mixed.10 Fig. 2a and d for OX50 are contradictory to the particle size distribution plot achieved without intensive mixing in Fig. 1a. This indicates that deagglomeration and deaggregation are possibly favored by the intensive mixing process to prepare the ink. Fig. 2d for the system NP30-C is remarkable as it clearly showed the CLP particles on the surface of NP30 spherical powder. Additionally, CLP particles can be observed working as a binder between the NP30 spherical powder. This configuration can be observed as the ratio between the mass of colloidal particles from CLP and the mass of NP30 particles is largely in favor of NP30. Fig. 2d and S5† contain the surfaces of the systems A200-C and H31-C which are mainly occupied by the CLP particles surrounding the A200 or the H31 particles as expected from the mixing ratios in Table 1.
Table 1 Characteristics of the inks prepared in this work. All inks were prepared using 25 g of a CLP suspension which has a 40% solid content and 5.2 g of lanoline. Printing conditions used for every ink produced
Ink Mass of powder (g) Powder[thin space (1/6-em)]:[thin space (1/6-em)]CLP solids (weight ratio) Total solids (%) Bulk density (g mL−1) Printing pressure (psi) Printer speed (mm s−1)
iA200-C 5.8 1[thin space (1/6-em)]:[thin space (1/6-em)]1.7 43.89 1.19 33 10 = 28 min
iOX50-C 25.0 2.5[thin space (1/6-em)]:[thin space (1/6-em)]1 63.41 1.58 75 6 = 45 min
iH31-C 11.7 1.2[thin space (1/6-em)]:[thin space (1/6-em)]1 50.22 1.35 40 10 = 28 min
iNP30-C 54.7 5.5[thin space (1/6-em)]:[thin space (1/6-em)]1 76.21 1.84 85 7 = 39 min

To evaluate the suitability of the printed monoliths for use in adsorption columns or as catalytic supports, uniaxial compression tests were performed. The results in Fig. S8† show that OX50-C exhibited the highest compressive strength (6.45 MPa), which aligns with the stronger interaction between the two silicas that are homogeneously dispersed, followed by H31-C (3.26 MPa), A200-C (1.53 MPa) and NP30-C (1.38 MPa). The observed values indicate that the mechanical stability of the monoliths is strongly dependent on the formulation. These results support the potential of the printed structures for use in packed or structured reactors where moderate mechanical load is expected.

3.5. Specific surface area and pore size distribution

Nitrogen sorption was performed to collect more information about the pore architecture, to characterize the SSA, and the pore size distribution of the monoliths constituted by two particle sources. The adsorption isotherms for the monoliths can be observed in Fig. 3a. The shapes of the adsorption isotherms obtained for the monoliths in this work can be classified as type IV isotherms; the resultant monoliths therefore are mesoporous materials with high energy of adsorption.21,22 The mesoporosity in the monoliths can come from two different sources, the first is from the voids between aggregates of nanoparticles and the second one comes from the intraparticle porosity which is valid for the highly porous H31 and for the dense NP30 as these are the only particles with intraparticle porosity as seen from the hysteresis in Fig. 1b.
image file: d5ta04265h-f3.tif
Fig. 3 Pore architecture characterization of the DIW monoliths, nitrogen adsorption isotherms (a), scheme of the pore architecture of H31-C based on the analysis made with the nitrogen adsorption isotherms, mercury intrusion, and SEM images (b), PSD calculated using DFT (c), and PSD measured from mercury intrusion (d). The inset of (a) corresponds to a magnified view of the NP30-C isotherm. For (a, c and d), A200-C is represented by black squares, OX50-C by red circles, H31-C by blue triangles, NP30-C by green inverse triangles, and the thermally treated CLP fragment at 600 °C by violet diamonds.

Analysis of A200-C and OX50-C which are composed of aggregates of nanoparticles was performed to comprehend the effect of these silica sources over the pore configuration. Considering the large amount of solids from CLP in these samples, CLP was cast into a solid piece, and after drying, the resultant fragments were thermally treated exactly as the monoliths to compare the isotherm of CLP with that of the monoliths as seen in Fig. 3a. The shape of the CLP adsorption isotherm is very similar to the shape of the adsorption isotherm of A200-C. Therefore, the main contribution to mesoporosity for A200-C can be attributed to the addition of a significantly larger portion of CLP compared with the amount of A200 as described in Table 1. The system OX50-C is composed of a larger fraction of OX50 solids compared with the solid fraction from CLP. Remarkably the isotherm of this system still looks similar to that of the CLP fragment. It is clear that for systems composed of nanoparticles or aggregates of nanoparticles as a source of solids together with a colloidal binder, the porosity is given by the voids between the nanoparticles.

The case of NP30-C represents the combination of dense silica spheres in the micrometer range, with no significant porosity inside the particles and the CLP nanoparticles. The monolith NP30-C does not stabilize at the end of the adsorption isotherm depicted in Fig. 3a; this behavior could be associated with a type II isotherm for non-porous particles or systems with microporosity.9,23 This result is consistent with the internal configuration of the monolith observed in Fig. 2d and S5† and with the ratio between CLP and NP30 as previously stated. Remarkably, the isotherm of the monolith NP30-C has hysteresis given by mesopores derived from the voids between CLP particles forming the clusters that bind the larger NP30 particles as seen in Fig. 2d and in Fig. S5.† The system NP30-C has a step-like shape in the desorption branch of the isotherm. This shape indicates the occurrence of two different diameters of pores typically defined as bottle-neck pores.23–25 Additionally, as the adsorption branch of the isotherm is also a step trace as seen in Fig. 3a, it can be concluded that the pore architecture is the product of two independent porosities combined in a single isotherm. In other words, the two different pore sizes merge only in the boundaries of the bigger mesoporous spheres resulting in two independent pore domains. The system H31-C composed of highly porous H31 spheres in the micrometer range and CLP has a similar shape to the isotherm of NP30-C; the main difference is related to the amount of CLP in this sample as it seems to be filling the voids completely between the H31 spheres, removing the macropores as seen in Fig. 2d and presented in Fig. 3b. Therefore, this sample is completely mesoporous, and the porosity is a combination of the H31 intraparticle porosity and the CLP interparticle volume. In consequence a two-step adsorption and desorption isotherm representing the two different pores was obtained.

To further verify the behavior of the isotherms it was decided to theoretically combine the individual isotherms of the powders heated to 600 °C and the CLP thermally treated at 600 °C in a similar way to how it was done in a previous report.23 These theoretical adsorption isotherms for A200-C, OX50-C, H31-C and NP30-C monoliths were calculated using eqn (1). In this, Vpowder is the volume of nitrogen adsorbed at a reduced pressure for the powder heated to 600 °C, Vbinder is the volume of nitrogen adsorbed at the same reduced pressure selected for the powder at every reduced pressure of the CLP silica binder, x is the mass fraction of powder in the monolith, and y is the mass fraction of solids from the CLP binder in the monolith. The resultant theoretical isotherms for the systems NP30-C and H31-C shown in Fig. S9† have similar shapes to the measured isotherms, therefore confirming that two different pores are present in the mentioned systems, one from the intra-particle porosity of the H31 spherical powder or to a lower extent in the denser NP30 particles and the second one which is given by the inter-particular porosity between the CLP particles. In contrast the theoretical isotherms for A200-C and OX50-C do not match with the experimental results confirming the idea of a single porosity given by the voids between the different nanometer size particles and aggregates. The hysteresis of the adsorption isotherms observed in Fig. 3a could be classified as type H1 for the systems A200-C, OX50-C, and the CLP control; this is found in materials with narrow PSD. However, for the systems H31-C and NP30-C, the shape of the hysteresis is similar to that of the type H2. The H2 hysteresis is characteristic for complex porosity where multiple pore sizes and shapes could be coexisting in the monoliths.

 
VTh = Vpowderx + Vbindery (1)

The SSAs of the silica structures presented in Table 2 were calculated using the BET method by selecting p/p0 between 0.05 and 0.3.21 The SSA of each system was expected to be in agreement with the ratio of each silica source in the mixture. The theoretically expected SSA (SSATh) was calculated based on the SSA of each individual component thermally treated at 600 °C which is listed in the methodology for the powders and 106.2 m2 g−1 for CLP, and the fractions (θ) of each component using eqn (2). θpowder and θCLP solids were calculated from the mass of the powder and the mass of the solids coming from CLP described in Table 1 and they are constrained using eqn (3). The SSAs calculated by the BET method are in the range of the theoretically expected values presented in Table 2. The difference between SSA and SSATh is below 10% for all the systems. Therefore, an acceptable estimation of the SSA can be done based on the initial formulation of the ink and the SSA of the individual components thermally treated at the same temperature as the monolith.

 
SSATh = SSAPowder × θpowder + SSACLP solids × θCLP solids (2)
 
θpowder + θCLP solids = 1 (3)

Table 2 Specific surface area (SSA) of silica monoliths after thermal treatment at 600 °C and after functionalization with APTES (SSAAPTES) calculated by the BET method
Sample SSA (m2 g−1) SSATha (m2 g−1) Errorb (%) SSAAPTES (m2 g−1) SSARdc (%)
a Expected SSA from the contribution of each silica source in the mixture.b Difference between the theoretical and the measured values.c Reduction of SSA after silanization expressed as a percentage.
A200-C 129 134 4 104 19
OX50-C 60 60 0 49 18
H31-C 417 455 9 179 57
NP30-C 42 42 0 37 12

From the nitrogen adsorption isotherms, it was observed that all the samples evaluated in this work have mesopores, between 2 nm and 50 nm, as observed in the pore size distribution graphs in Fig. 3c. However, their distributions are different. The system OX50-C has broader PSD comprising all the mesopore range with most of the pores located around 17 nm. OX50-C is composed of two types of nanoparticles with different sizes as observed from SEM images in Fig. 2d. The particles close to 22 nm are from CLP and the particles between 50 nm and 80 nm proceed from the deagglomeration of OX50 into aggregates and its primary particles. The combination of the mentioned nanoparticles in a heterogeneous matrix resulted in the diversity of pore sizes shown in Fig. 3c. The system A200-C has a PSD with a major peak at approximately 12 nm attributed to the interparticle space between CLP spheres and A200 aggregates. This peak is narrower than that of OX50-C with pores between 5 nm and 30 nm. The narrower PSD in the system A200-C may be related to similar sizes between CLP and the primary particles of A200. H31-C and NP30-C have a bimodal PSD. For these samples there is a peak between 10 nm and 15 nm which can be assigned to the pores between CLP particles, and a second peak close to 5 nm which is attributed to the multiple pores inside H31 particles and the limited number of pores in the denser NP30 particles as can be observed in Fig. 1c.

The pore size distribution measured by mercury intrusion in Fig. 3d allows part of the mesoporosity and the pores larger than 50 nm, which are considered macropores, to be studied. The samples A200-C and OX50-C do not have microporosity. However, the mercury intrusion results confirm the pore size distribution calculated from nitrogen sorption experiments. On the other hand, the system NP30-C has macropores between 400 nm and 1000 nm. These macropores are formed by the void spaces between the NP30 polydisperse spheres as seen in Fig. S5d† and 2d where the macropores within the same range as the results of mercury intrusion are visible. Additionally, the PSD for H31-C after mercury intrusion has a small peak between 400 nm and 1000 nm; it is possible that a limited number of voids between H31 spheres may not be filled by CLP resulting in few macropores inside the H31-C structures.

3.6. Pore architecture after surface functionalization

The monoliths A200-C, OX50-C, H31-C, and NP30-C resulted in the functionalized monoliths A200-CA, OX50-CA, H31-CA, and NP30-CA respectively after solvent based silanization with APTES. Functionalization was performed to evaluate possible changes in the pore architecture of the monolith. All the samples were reacted to completion based on isoelectric points higher than 9 measured using ζ-potential in the ground monolith as seen in Fig. S10.† To assess the spatial distribution of functional groups within the monolith, elemental analysis was performed on three different cross-sectional zones (top, middle, and bottom) of the functionalized H31-CA sample. The results (Fig. S11†) revealed a clear gradient, with higher carbon and nitrogen content detected near the outer regions (zones 1 and 3) compared to the central zone (zone 2). This observation indicates a lower degree of silane grafting in the monolith core, which is attributed to diffusion limitations of the reagent during the liquid-phase functionalization process. These results highlight the challenge of achieving homogeneous surface modification in printed monoliths with complex internal geometries.

The shapes of the adsorption isotherms for the materials silanized with APTES, shown in Fig. 4a, are relatively similar to that of the materials before silanization as compared in Fig. S12.† It could be concluded that the pore shape remains the same. However, the pore volume after silanization in all the systems is lower than the pore volume of the monolith before silanization. This reduction in volume is more critical for H31-C as it lost approximately 100 cm3 g−1 after silanization apparently from the pores around 4 nm as seen in the inset of Fig. 4b. Previous literature reported reductions close to 50% in the nitrogen uptake after silanization of silica with APTES. This result is similar to the values reached in this work and it is attributed to shrinkage of the pore channels caused by the grafting of APTES.9,26 Therefore, the reduction of the pore diameter can make some pores no longer accessible to the nitrogen molecules, which in consequence will reduce the pore volume.


image file: d5ta04265h-f4.tif
Fig. 4 Pore architecture characterization after silanization of the monoliths with APTES. Nitrogen adsorption isotherms (a), pore size distribution calculated using DFT (b), percentage of carbon (solid symbols) and nitrogen (open symbols) related to the SSA after functionalization with APTES (c), and concentration of carbon dioxide adsorbed on the functionalized surface as a function of the concentration of amine groups in the sample (d). The inset of (a) corresponds to a magnified view of the NP30-C isotherm. The inset of (b) compares the pore size distribution before (dotted line) and after (solid line) functionalization. The dotted line in (c) is a linear fitting for the percentage of carbon while the dashed line is a linear fitting for the percentage of nitrogen. The dotted line in (d) is a linear fitting of the collected data. For (a–d) A200-C is represented by black squares, OX50-C by red circles, H31-C by blue triangles, and NP30-C by green inverse triangles.

The pore size distributions in Fig. 4b show peaks at the same positions for silanized and non-silanized monoliths. Additionally, the intensity of the peaks with smaller diameters after functionalization is lower than the intensity of the non-silanized monolith peaks. The only exception is NP30-CA which has similar peak intensities to NP30-C. Another parameter that was affected after functionalization was the SSA reported in Table 2. The SSA of the monoliths after silanization (SSAAPTES) was lower than the SSA of the untreated monoliths. Additionally, it was identified that the monoliths with smaller SSA values have a tendency to lose a lower percentage of the SSA, while the monoliths made with H31, with larger SSA values, have more significant losses, probably a product of pore blocking after functionalization at the entrance of the smaller pores.27

The percentages of carbon (% C) and the percentages of nitrogen (% N) in the samples were obtained by elemental analysis and are reported in Table 3. The sample with the highest content of carbon and nitrogen is H31-CA. However, the density of functional groups in the same sample is the lowest among the functionalized monoliths. In contrast, the samples containing fumed silica, A200-CA and OX50-CA have the largest density of functional groups. Therefore, the relation between the SSA and the extent of functionalization is not linear, and it is dependent on the porous architecture of the monolith. Fig. S13† shows the carbon and nitrogen percentages as a function of the SSA of the unfunctionalized monoliths. These values were fit within a logarithmic regression with an R-square of 0.97. Therefore, the regression equations in Fig. S12† can predict the amount of nitrogen and carbon that can be bonded from APTES, based on the SSA of the starting material, under the conditions reported in this manuscript and using the silica materials thermally treated at 600 °C. The percentages of carbon and nitrogen after silanization of the monoliths with APTES were plotted as a function of the SSA measured in the silanized samples as shown in Fig. 4c. Linear fitting of the data revealed R-squared values higher than 0.99 for carbon and for nitrogen. Therefore, the correlation between the results of elemental analysis and the SSA of the samples before and after silanization confirms that the grafting with APTES is blocking a portion of the pores, as previously discussed. APTES has a theoretical molecular volume of 230.92 A3 which is equivalent to a molecular diameter of 0.8 nm. Simulated APTES with some distances after optimization of the structure can be seen in Fig. S14.† The APTES molecules may react in the mesopores but these would reduce their size or even become inaccessible for other molecules trying to diffuse in the system during the synthesis or during the adsorption measurements. Silanization experiments performed on H31-C with silanes of different moieties showed that smaller alkoxysilanes showed larger extents of silanization on the surface as seen in Table S5.† This is an indication of possible obstruction of the pores by bulkier functional groups.

Table 3 Carbon and nitrogen weight percentage as determined by elemental analysis of the functionalized structures and calculated propylamine (PrNH2) density on the surface of the structures
  C (wt%) N (wt%) Propylamine (#PrNH2 per nm2)
A200-CA 3.38 1.30 4.59
OX50-CA 1.62 0.55 4.44
H31-CA 6.26 2.30 2.55
NP30-CA 1.05 0.37 4.08

3.7. CO2 sorption capacity

Materials functionalized with amine groups on the surface are known to be excellent adsorbents of CO2.28 Part of the goal of this research is to evaluate the effect of the pore architectures of the silica monoliths after functionalization with APTES in applications such as CO2 sorption. The CO2 adsorption capacity of the monoliths silanized with APTES, determined from the TGA CO2 loopings in Fig. 5, is one order of magnitude lower than that in previous reports.28 However, it is common for functionalization in dried toluene to form thin functional layers on the silica surface. From the density of aminopropyl groups described in Table 3 it can be concluded that not more than 3 APTES molecules were bonded to each hydroxyl on the surface, therefore limiting the possible amount of CO2 that can be adsorbed. Consequently, the amount of CO2 adsorbed is linearly related to the surface area of the monoliths. Also, the pore size distributions of the monoliths after silanization do not have a notable effect on the adsorption capacity.
image file: d5ta04265h-f5.tif
Fig. 5 TGA loops using a 10% stream of CO2 at 25 °C to evaluate the adsorption capacity of the functionalized monoliths A200-CA (black line), OX50-CA (red line), H31-CA (blue line), and NP30-CA (green line).

The CO2 adsorption capacity is linearly proportional to the concentration of amine groups in the sample as seen in Fig. 4d. The adsorption efficiency, or the slope of the linear fitting, is 0.20 mmol CO2 per mmol NH2 which indicated that one CO2 molecule was adsorbed per five NH2 groups on the surface under the experimental conditions used. Typical values reported for CO2 capacities are in the range between 0.10 mmol CO2 per mmol NH2 and 0.6 mmol CO2 per mmol NH2.9,26,29 Despite the low adsorption capacity, the adsorption efficiency is within the range of the reports using silica functionalized with APTES. Therefore, the SSA, the pore volume, and the pore size distribution of the silanized monoliths do not have a significant impact on the adsorption of CO2. Based on the observed adsorption capacity, the most effective sample in this study is H31-CA, which exhibited the highest surface area and, consequently, the greatest density of surface NH2 groups. While these results support the conclusion that surface area plays a dominant role in functional group density and overall capacity, a more comprehensive understanding of adsorption behavior will require further investigation. Future studies should examine adsorption kinetics and assess the influence of monolith geometry and filament dimensions on CO2 adsorption performance.

4. Conclusion

This study demonstrates a scalable strategy to control the mesoporous architecture of silica monoliths fabricated by direct ink writing (DIW), using industry grade silica powders with varying surface areas, morphologies, and intrinsic porosities. The pore size distribution (PSD) of the printed structures was shown to depend on both the intraparticle porosity of the selected powder and the interparticle voids formed with the colloidal silica binder. When the powder remained in the micron-sized regime, the resulting monoliths exhibited bimodal porosity derived from both components. In contrast, when the powder was effectively deagglomerated into nanoscale aggregates, the PSD was primarily governed by the interparticle arrangement. These results confirm that the porous structure of DIW monoliths can be rationally tuned through formulation design based on powder characteristics.

The inks formulated from these powders displayed rheological profiles suitable for DIW, with printability directly influenced by thixotropy and viscoelastic response. The specific surface area (SSA) of the monoliths could be reasonably predicted from the SSA of the raw components and their relative fractions. However, powders with higher initial SSA showed greater reductions after thermal treatment, indicating that initial porosity may not always be preserved during consolidation. Following silanization with 3-aminopropyl(triethoxysilane) (APTES), further decreases in SSA and pore volume were observed, particularly in samples with smaller pore diameters, suggesting partial pore blockage due to surface grafting.

The extent of functionalization, as quantified by elemental analysis, correlated with the SSA of the functionalized monoliths, confirming that accessible surface area plays a dominant role in determining the grafting density. CO2 adsorption experiments revealed a linear relationship between amine concentration and uptake, with adsorption efficiencies consistent with values reported for amine-functionalized silica. Notably, the PSD had no significant influence on the CO2 capacity, underscoring that functional group accessibility and surface chemistry are more critical than total pore volume in this regime.

Overall, this work establishes a methodology to predict and control the pore architecture and surface reactivity of silica monoliths through rational powder selection. Future studies will focus on optimizing functionalization protocols such as vapor-phase silanization which may offer improved homogeneity by addressing diffusion limitations associated with solvent-based methods. In solvent-mediated silanization, higher concentrations of functional groups are often observed near the monolith surface due to limited reagent penetration through complex internal geometries. Vapor-phase approaches have the potential to enable more uniform grafting by facilitating localized, stoichiometric reactions with surface hydroxyls, while reducing the likelihood of polymeric byproduct formation, expanding the range of tested molecules to further enhance sorption performance while preserving porosity. The approach presented here offers a practical platform for the development of structured materials tailored for adsorption, catalysis, and separation applications.

Data availability

All data supporting the findings of the manuscript, including raw and processed data from nitrogen and CO2 sorption measurements, as well as rheological and particle size analysis data (provided in Origin project files containing both raw data and plots), have been uploaded to an OpenAIRE-compliant Zenodo repository. The dataset is currently under review and will be made publicly accessible upon acceptance. A DOI has been reserved and can be cited at https://doi.org/10.5281/zenodo.15275853.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This research was funded solely by VITO. The authors gratefully acknowledge the technical assistance of A. De Wilde (TGA, N2 sorption, CO2 sorption, and Hg intrusion), R. Kemps (SEM), A. Vansant (ζ-potential) and J. De Wit (CN analysis).

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Footnote

† Electronic supplementary information (ESI) available: Table S1: name codes used in this work. Table S2: amounts of reagents used in the silanization of silica monoliths. Table S3: Ostwald–de Waele power law constants for the inks developed in this work. Table S4: viscoelastic properties of the inks evaluated by oscillatory measurements. Table S5: extent of functionalization after treatment with three alkoxysilanes. Fig. S1: steady state traces for the inks produced in this work. Fig. S2: viscoelastic behavior of the inks. Fig. S3: figure of merit to establish the printability of the inks. Fig. S4: thixotropy evaluation using the three interval thixotropy test. Fig. S5: SEM images obtained from the surface of the monoliths. Fig. S6: shrinkage after thermal treatment of the green body hierarchical porous cylinders as a function of the solid content of the ink. Open symbols represent the radial shrinkage while filled symbols represent the axial shrinkage of monoliths produced with OX50 (black squares), A200 (red squares), H31 (blue triangles), and NP30 (green inverse triangles). Fig. S7: surface of the OX50-C monolith magnified to verify the homogeneous dispersion of OX50 particles in the CLP matrix resulting in a structure with a bimodal particle size. Fig. S8: compressive stress of monoliths prepared as described in the Methodology section. Compression tests were conducted at a rate of 1 mm min−1 using a 1 kN load cell. The results represent the average of the four samples, except for NP30-C, for which only one sample was available due to the limited material and the complexity of fabrication. This single value is included for reference. Fig. S9: theoretical adsorption isotherms calculated from the adsorption isotherms of each thermally treated powder and the silica binder thermally treated at 600 °C, and the mass fraction used in the original mixture to prepare the monoliths. Fig. S10: ζ-potential collected from ground functionalized monoliths. Fig. S11: carbon and nitrogen content, determined by elemental analysis, at different positions within the functionalized H31-CA monolith. The results indicate a lower degree of functionalization in the monolith core compared to the outer regions. This gradient is attributed to diffusion limitations of the silane reagent through the macroporous structure. Fig. S12: sorption isotherms of powders thermally treated at 600 °C, monoliths after printing and thermal treatment, and monoliths functionalized with APTES. Fig. S13: percentage of carbon (solid symbols) and nitrogen (open symbols) related to the SSA of the monoliths. Fig. S14: APTES molecule drawn and optimized using ChemDraw 18 and further exported to Mercury software to measure approximate distances and volume. See DOI: https://doi.org/10.1039/d5ta04265h
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