Cu/ZSM5-Geopolymer 3D-Printed Monoliths

Cu/ZSM5-Geopolymer 3D-Printed Monoliths: Comparison

Please note this is a comparison between Version 4 by Jason Zhu and Version 3 by Jason Zhu.

Geopolymer-based monoliths manufactured by direct ink writing, containing up to 60% by weight of presynthesized ZSM5 with low Si/Al ratio, were investigated as structured catalysts for the NH3-SCR of NOx.

structured catalysts
additive manufacturing
SCR
hierachical porosity

1. Introduction

DeNO_x processes for both stationary and mobile sources are mostly based on selective catalytic reduction (SCR) using ammonia as a reducing agent ^[1]. This technology has been extensively applied to NO_x control in the exhaust products of fossil fuel-fired power plants using V₂O₅–WO₃/TiO₂ catalysts ^[2].

Zeolites show excellent catalytic properties for NH₃-SCR: in particular, ZSM5 has the suitable pore size and acidic character that, coupled to its good thermo-chemical resistance, make this zeolite one of the most used materials in SCR processes when exchanged with suitable active cations ^[3]. Fe and Cu are the most active metals providing good catalytic performance in a wide operating temperature range ^[4][5].

Pure zeolite-type technical catalysts are currently limited by the difficulty in producing self-supported monoliths or foams. Zeolite-based structured catalysts are generally produced as a zeolite washcoat deposited on a ceramic substrate, with a consequent low catalyst loading per unit volume of the system and/or risk of loss of the active phase due to poor adhesion and unmatched thermal expansion with the substrate, which are all negative features—especially in mobile applications ^[6][7].

Geopolymers (GPs) are considered amorphous analogues of zeolites. They are inherently macro/mesoporous materials with a high thermal stability and limited degradation of the structural network up to 800 °C ^[8]. GPs show good adsorption properties in many applications—typically water purification from organic molecules ^[9] or dissolved ions ^{[10][11][12][13]}—but they have been also investigated for SCR reactions both as NH₄- and Cu-GP ^[14], and Fe-promoted in combination with activated carbon ^[15]. The possibility of varying their pore architecture in a wide range of values and the close similarity with zeolites concerning ion exchange, make GPs good candidates for the production of composite zeolite–GP materials ^[9]. For this reason, the integration of the zeolite and geopolymers can take advantage of specific features from both components, such as the high surface area, pore volume, adsorption capacity and catalytic activity of zeolites, and the strength, permeability and support of the geopolymer matrix ^[16]. These composites have been mostly used as adsorbents, in both gaseous and aqueous systems ^[17].

The properties of geopolymer–zeolite composites depend on the quantities of zeolites formed in the matrix. The hybrid material can be obtained starting from the GP ^[18][19] or from the zeolite ^[20] through suitable hydrothermal treatment, inducing the formation of the other phase, as reviewed in ^[17]. Unfortunately, the formation of a zeolite fraction is limited to certain structural types, in most cases excluding zeolites with interesting catalytic properties such as ZSM5.

Additive manufacturing (AM) enables the fabrication of ceramics with morphologies not achievable using traditional processing technologies ^[21]. Among all the techniques, direct ink writing (DIW, an extrusion-based approach) permits the ease of design and rapid manufacturing of ceramic-based materials in complicated geometries ^[22] and, therefore, it is particularly advantageous for lab-scale production and development of structured catalysts ^[23].

The incorporation of 37% commercial ZSM5 into a GP matrix and its final shaping as a monolith by DIW, was first reported in ^[23] to obtain a structured zeolite catalyst with a hierarchical structure and suitable mechanical properties.

2. Catalyst Characterization

SEM images of a GP monolith and of the two GP–Z(x) monoliths are shown in Figure 1, where the deposition of continuous filaments with regular diameters following a 0–90° sequential build-up, creating a 3D network, was well visible for all samples. The diameters of filaments ranged from 845 to 880 μm, slightly increasing with zeolite load, consistent with the diameter of the conical nozzle used for printing. The inner section of the GP monolith showed the significant presence of some large pores with dimensions of some tens of microns, attributable to air trapped during the preparation of the ink, which tended to disappear in the composite materials that possess a denser and more homogeneous structure. Images at higher magnification confirmed the absence of needle-like particles associated with residual sodium compounds in all acid-treated samples and showed roughly rounded particles that were generally larger for the pure GP in comparison to the GP–Z composite monoliths.

Figure 1. SEM images of the cross-section of the struts of acid-treated GP (a,b), GP–Z(37) (c,d) (adapted from ^[23]) and GP–Z(60) (e,f) monoliths.

The compressive strength of GP–Z(37) and GP–Z(60) monoliths after the acid treatment was 1.02 ± 0.2 and 0.31 ± 0.15 MPa, respectively: the statistical error is associated to the number of specimens tested (>10 for each composition), some of which can show structural defects. For comparison purposes, a pure self-supported ZSM5 zeolite monolith foam previously prepared by hydrothermal synthesis with a polyurethane foam template revealed a compressive strength equal to 0.31 MPa ^[24], that was high enough to stand extensive SCR testing in lab scale reactor. These values are lower than that measured for pure GP monoliths without zeolite that weren't preliminarily treated with acid solution, which was as high as 5.23 ± 0.87 MPa ^[11]. As expected, the mechanical resistance decreased by increasing the zeolite fraction in the composites, due to the reduction in the amount of GP matrix as well as the disruption of its continuity. A depolymerization of the GP network caused by the acid attack can further decrease the mechanical strength of the monoliths ^[25].

Figure 2 shows the XRD patterns of the original ZSM5 and GP materials as well as those of the composite monoliths. The higher contribution of the typical ZSM5 signals up to 25° was easily detectable for Cu/GP–Z(60). The amorphous halo centered at 27–29°, typical of the geopolimeric matrix ^[26], is evident in both fresh GP–Z samples (not shown), whereas it is significantly reduced in the Cu-exchanged samples, mainly due to the preliminary acid treatment which eliminates (part of) the sodium from the geopolymer matrix by ion exchange, partially degrades the geopolymer network (see later), and dissolves the alkaline compounds present on the surface. The removal of these compounds, which can be attributed to the formation of sodium carbonate by reaction with atmospheric CO₂, was also consistent with the disappearance of the needle-like structure observed by SEM analysis, previously reported in ^[23]. However, the acid treatment did not affect the crystalline structure of the zeolite ^[23], as confirmed by the XRD patterns of Cu-exchanged materials. No other signals assignable to different zeolite-like phases were detected, thus excluding the possible zeolitization of the GP matrix—different to what was reported when zeolites such as 13X were introduced as a matrix filler ^[27].

Figure 2. XRD patterns of as synthesized GP and Cu/GP–Z(x) monolith catalysts. Legend: z = ZSM5; * = CuO.

No signals associable to copper oxide were detected in the composite samples, whereas weak signals at 35.5 and 38.7°, attributed to CuO, appeared in the XRD pattern of the Cu/GP monolith, suggesting that some oxide segregation occurred at the GP surface.

Table 1 reports the actual copper load (measured by ICP–MS) in the Cu-exchanged pure and composite geopolymer/zeolite materials. The actual amount of copper in the ZSM5 powder was 3.2 wt%, corresponding to ca 90% of the total cation exchange capacity (CEC = 0.62 mmol/g for bivalent copper cation), evaluated on the basis of the actual Si/Al ratio in the zeolite.

Table 1. Copper content (by ICP–MS), fractional cation exchange capacity, BET specific surface area, micropore and total pore volumes for Cu-exchanged composite monoliths and their acid-treated precursors compared with pure Cu/GP and Cu/Z (powder).

Sample		Cu Load wt%	Exchange Capacity %	S_BET m² g⁻¹	V_micro cm³ g⁻¹	V_tot cm³ g⁻¹
H-ZSM5		-		430	0.17	0.25
20	Cu/Z		3.2	90
Cu/Z Foam ^a	Monolith	415	0.16	0.24
40	74.6	3.4	3.4	GP–Z(60)	Acid-treated	-		264	0.11	0.17
Cu/GP–Z(60)	Monolith	13	65.1	6.0	10		Cu-exchanged	2.9		253	0.11	0.19
Cu/GP–Z(37)	Monolith	14	68.1	2.8	7.6	GP–Z(37)	Acid-treated	-
Cu/GP–Z(37)	133	0.05	0.14
Powder	200	68.1	2.8	7.6		Cu-exchanged	1.5		122	0.04	0.11
GP		-		11	-	0.13
Cu/GP		3.0	20	27	-	0.18

Considering an Al content of 5 mmol/g (dry basis) for the pure GP, its theorethical copper exchange capacity should be as high as 2.5 mmol Cu/g ^[28], but the actual amount introduced in the monolith by cation exchange was much lower, accounting for ca 20% of the total CEC. This could be caused by the hydrolytic degradation of the Si–O–Al framework ^[29] which could be the consequence of the electrophilic attack by acid protons on polymeric Si–O–Al bonds, resulting in the ejection of tetrahedral aluminum from the aluminosilicate framework ^[30][31] and consequent formation of separate domains of SiO₂ and Al₂O₃.

Composite monoliths displayed a copper content that was lower than expected from the linear combination of the values for the corresponding pure materials, and increased along with the zeolite fraction in the monolith. This suggests that the metal was mostly located in the ZSM5 crystals and ion exchange was not precluded by the GP matrix. It is worth mentioning that previous studies have indicated that the ion exchange method for structured (honeycomb) ZSM5 catalysts is less effective than for powder samples, with a double-exchange proceess achieving only ca half the theoretical CEC ^[24], even in the absence of any other component such as the GP matrix.

Figure 3a reports the N₂ adsorption isotherms at 77 K for Cu-exchanged monoliths, compared with that of the pure Cu/Z powder, showing the typical type I isotherm of microporous solids. The shape of the isotherms was largely preserved in the composite materials, although a clear reduction in N₂ adsorption capacity was observed due to the poor contribution of the GP. Indeed, the Cu/GP monolith showed a predominant mesoporosity (type IV isotherm with a H3 hysteresis loop between adsorption and desorption branches) that was also reported for GP with Si/Al = 1.2 ^[27], and was further accompaneid by some macroporosity. Table 1 reports the values of BET surface area, micro- and total pore volumes for composite monoliths after acid pre-treatment and after Cu-exchange, compared with the values of the pure Cu/Z and Cu/GP. The commercial ZSM5 after the preliminary deammoniation had a surface area of 430 m² g⁻¹ and a total porosity equal to 0.25 cm³ g⁻¹. Copper exchange induced only a slight decrease in both figures. In contrast, the Cu/GP monolith showed a lower specific surface area value of around 27 m² g⁻¹, which was somehow enhanced by the removal of alkaline residues upon preliminary acid pre-treatment and Cu exchange steps ^[23]; the characteristic dimension of mesopores in the Cu/GP monolith was around 50 Å. Therefore, the BET surface area of the composite samples increased with the zeolite load, and the final Cu-exchange step had only a weak impact on the textural properties of the monoliths, as also observed for the pure Cu/Z sample. Significantly, the BET values of the Cu-exchanged monoliths corresponded well to those expected on the basis of the zeolite and GP contents, especially for the Cu/GP–Z(60) sample. This indicates that the structural properties of the starting materials were mostly preserved upon slurry preparation and its subsequent DIW, giving rise to composite monoliths with hierarchical textural features, as shown by the pore volume distribution analysis presented in Figure 3b for the exemplificative case of the Cu/GP–Z(37) sample. Papa et al. ^[27] reported lower values of specific surface area for their composite GP-13X foam composites, likely due to the lower fraction of zeolite in the material, but also a higher pore volume, which reflects the larger microporosity of 13X compared to ZSM5.

Figure 3. (a) N₂ adsorption/desorption isotherms at 77 K over pure Cu/GP and composite Cu/GP–Z(x) monoliths and reference Cu/Z powder; (b) Cumulative pore volume as a function of the characteristic pore width for the Cu/GP–Z(37) monolith.

3. NH₃-SCR Tests

NO and NH₃ conversions, N₂O formation and N₂ selectivity measured during the NH₃-SCR tests on monolith catalysts are reported in Figure 4a–d as a function of the reaction temperature. A blank run using an “as-prepared” GP monolith, i.e., without performing any acid pre-treatment and subsequent copper exchange, was carried out in order to show that the SCR reaction does not take place in the absence of copper; ammonia oxidation occurs only at T > 350 °C (Figure 4b), likely homogeneously, leading to the formation of N₂O, NO₂ and, supposedly, N₂—being NO production negligible (Figure 4a).

Figure 4. NO (a) and NH₃ (b) conversion, N₂O production (c) and N₂ selectivity (d) as a function of the reaction temperature during the NH₃-SCR of NO over GP and Cu/GP-Z(x) exchanged monoliths.

Copper deposition on the GP monolith provided a detectable catalytic activity. Both NO and NH₃ started to be converted at temperatures as low as 200 °C (Figure 4a,b). A remarkable effect of the introduction of copper on SCR performance was also reported in ^[14] for NH₄Cu geopolymers. Similarly, it has been reported that the introduction of active iron in geopolymer/active carbon composites strongly improved SCR performance ^[15]. In agreement with Sazama et al. ^[14], ammonia oxidation took place over the whole range of temperatures in addition to the SCR reaction, as shown by the larger NH₃ conversion compared to that of NO. NO conversion achieved a maximum at about 250 °C; thereafter, it declined and eventually reached negative values for temperatures above 350 °C. This clearly indicates that NO is one of the products of ammonia oxidation (prevailing at high T) and is formed together with some N₂O (Figure 4c), causing a progressive decline of N₂ selectivity with increasing temperature (Figure 4d).

The introduction of ZSM5 into the GP matrix dramatically changed its catalytic behavior. NO conversion approached 100% at about 200 °C for Cu/GP–Z(37) and at as low as 150 °C for Cu/GP–Z(60) (Figure 4a). NO conversion started to decrease for both monoliths above 250 °C due to the simultaneous oxidation of ammonia. Nevertheless, both ZSM5-containing monoliths provided a high value of selectivity to N₂, which stayed close to 100% up to about 300 °C—slowly decreasing beyond this temperature, but keeping values above 90% over the whole range of temperatures explored (Figure 4d). N₂O formation over composite monoliths was very limited (Figure 4c), reaching maximum levels below 20ppmv at temperatures of around 220 °C, in good agreement with previous data reported for self supported Cu-ZSM5 foam catalysts ^[24].

Catalytic tests were also repeated at a shorter contact time using ground and sieved composite monoliths as well as mechanical mixturess of pure components with the same nominal composition (Table 2).

Table 2. Apparent activation energy (E_a) and first-order kinetic constant values at 150 °C (k_w, k_wz: per unit mass of catalyst or zeolite fraction, respectively) for the NO removal rate by NH₃-SCR on composite Cu/GP–Z(x) and reference catalysts calculated from conversion data collected under the specified experimental conditions.

Catalyst	Form	F/W_cat dm³g_cat⁻¹h⁻¹	E_a kJ mol ⁻¹	k_w (150 °C) cm³g_cat⁻¹s⁻¹	k_wz (150 °C) cm³g_zeolite⁻¹s⁻¹
Cu/Z	Powder	200	64.8	20
Cu/GP+Cu/Z(37)
^b
Powder	200	65.0	1.7	4.6
Cu/GP	Monolith	15	48.8	0.2	-

Data reported in Figure 5 for the case of Cu/GP–Z(37) indicated that both NO conversion and N₂ selctivity were sistematically higher for the ground monolith rather that for its mechanical mixture counterpart, suggesting that the negative contribution of Cu/GP to the overall catalytic behaviour is still detectable when the two materials are simply mixed, whereas the catalytic features of Cu/Z prevail when a more intimate contact is estabilshed during the production and printing of the composite monolith catalysts.

Figure 5. (a) NO conversion and (b) N₂ selectivity as a function of the reaction temperature during the NH₃-SCR of NO on a powdered Cu/GP–Z(37) sample and on a mechanical mixture of pure Cu/GP and Cu/Z powders with identical nominal composition.

All catalytic results were elaborated on the basis of integral reactor conversion data, assuming an ideal isothermal plug flow behaviour and a first-order dependency on NO concentration. In Figure 6, the Arrhenius plots for the NO consumption rate on Cu/GP–Z(x) monoliths are compared to corresponding data on GP and Cu/GP monoliths, as well as on the reference Cu/Z powder. The domain of a kinetic regime is confirmed by the linear trend of the experimental data obtained for each catalyst in the low to moderate conversion range; however, when using bulk monolith samples, this corresponds to operation in a lower temperature range because of the correspondingly high mass of catalyst (low GHSV). In fact, SCR activity tests on the Cu/GP–Z(37) composite were also repeated using a fixed bed of powdered monolith catalyst obtained by grinding and sieving part of the original monolith. As shown in Figure 6, data points for the two sets of experiments ran at significantly different contact times collapsed into a single line after correction for the mass of catalyst, thus confirming that the reaction proceeded under kinetic control without significant mass transfer limitations, as was also the case in monolith catalysts.

Figure 6. Arrhenius plots for the NO consumption rate per gram of catalyst during the NH₃-SCR of NO on pure Cu/GP and composite Cu/GP-Z(x) monoliths and reference (powder) catalysts.

In Table 2, the values of activation energy (E_a) are reported together with the kinetic constant for the NH₃-SCR of NO estimated at 150 °C. Cu/GP–Z(x) composite monoliths displayed an activation energy of 65–68 kJ mol⁻¹, which was equal to that evaluated for the reference Cu/Z powder catalyst, and compares with the value (61–75 kJ/mol) previously reported for Cu/ZSM5 self-sustained foam monoliths ^[24].

4. Conclusions

Geopolymer-based monoliths were produced by an additive manufacturing method (direct ink writing) with the introduction of up to 60% by weight of a pre-synthetized ZSM5 into the extrusion ink. Features of the Cu/ZSM5 were largely preserved in the composite monoliths. In particular, copper is preferentially located at the typical active exchange sites in the zeolitic structure.

The monolith catalysts with hierarchical texture and satisfactory mechanical resistance can be potentially used for the NH3-SCR of nitrogen oxides in industrial applications.

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