# Effect of TiO2 on Acidity and Dispersion of H3PW12O40 in Bifunctional Cu-ZnO(Al)-H3PW12O40/TiO2 Catalysts for Direct Dimethyl Ether Synthesis

E. M. Ordóñez, N. M. Toledo, B. Revel, Olivier Lafon, R. M. N. Yerga

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E. M. Ordóñez, N. M. Toledo, B. Revel, Olivier Lafon, R. M. N. Yerga. Effect of TiO2 on Acidity and Dispersion of H3PW12O40 in Bifunctional Cu-ZnO(Al)-H3PW12O40/TiO2 Catalysts for Direct Dimethyl Ether Synthesis. Catalysts, 2024, Catalysts, 14, 10.3390/catal14070435. [https://doi.org/10.3390/catal14070435](https://doi.org/10.3390/catal14070435)

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# Effect of TiO₂ on Acidity and Dispersion of H₃PW₁₂O₄₀ in Bifunctional Cu-ZnO(Al)-H₃PW₁₂O₄₀/TiO₂ Catalysts for Direct Dimethyl Ether Synthesis

Elena Millán Ordóñez <fnref n="1" />,*, Noelia Mota Toledo <fnref n="1" />, Bertrand Revel <fnref n="2" />, Olivier Lafon <fnref n="3" /> and Rufino M. Navarro Yerga <fnref n="1" />,*

**Abstract:** The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H₃PW₁₂O₄₀, HPW) supported on TiO₂ combined with a Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. In this work, different types of TiO₂ were used as a support to study the effect of changes in the structure of the TiO₂ support on the acidity and dispersion of HPW. Various TiO₂ supports with different structural and surface characteristics have been studied and the results indicate that: (i) the crystallinity and crystallite size of the primary particles of the HPW units depend on the TiO₂ support; (ii) the pore size distribution of the TiO₂ support affects the surface segregation of the heteropolyacids; and (iii) changes in the supported HPW acid catalysts do not significantly alter the crystal structure of the CuO and ZnO phases after contact with CZA in bifunctional catalysts. The activity results indicate that the variation in the intrinsic activity of the Cu-ZnOₓ centers in the bifunctional catalysts for direct DME synthesis is minimal due to the limited alteration of the crystal structure of the centers.

**Keywords:** heteropolyacid; polyoxometalates; dimethyl ether; methanol; syngas; bifunctional

## 1. Introduction

Dimethyl ether (DME) is a very versatile compound used for chemical synthesis and as a fuel [1,2]. Industrially, DME is produced in a two-step industrial process, where methanol is first produced from syngas on a Cu/ZnO/Al₂O₃ catalyst and, after its purification, is dehydrated and converted to DME in a second reactor [2]. However, the main problems with this two-stage DME process are the low conversion and high operating cost. In contrast to this two-step industrial process, the direct synthesis of dimethyl ether (DME) from syngas is economically more favorable. This process occurs in a single reactor with bifunctional catalysts that have methanol synthesis and methanol dehydration functions. The design of these catalysts is challenging, as both functions must be maximized to achieve optimal cooperation [1,2]. The cutting-edge development of bifunctional catalysts for the direct synthesis of DME primarily utilizes Cu-ZnO(Al) catalysts for methanol synthesis and zeolites (mainly HZSM-5) as the acidic component for methanol dehydration. Heteropolyacids (HPAs) have emerged as highly promising acidic materials, offering an efficient alternative to HZSM-5 zeolites. HPAs facilitate the dehydration of methanol under milder conditions than conventional HZSM-5 acid catalysts, thereby addressing the limitations associated with the latter. The most studied HPAs for the methanol-to-DME process are those with a Keggin structure and the most acidic ones, namely H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀. The design of bifunctional catalysts for the direct synthesis of dimethyl ether

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**Citation:** Millán Ordóñez, E.; Mota Toledo, N.; Revel, B.; Lafon, O.; Navarro Yerga, R.M. Effect of TiO₂ on Acidity and Dispersion of H₃PW₁₂O₄₀ in Bifunctional Cu-ZnO(Al)-H₃PW₁₂O₄₀/TiO₂ Catalysts for Direct Dimethyl Ether Synthesis. *Catalysts* **2024**, *14*, 435. https://doi.org/10.3390/catal14070435

Academic Editors: Balasubramanian Viswanathan, Indra Neel Pulidindi and Thirukkallam Kanthadai Varadarajan

Received: 12 June 2024

Revised: 27 June 2024

Accepted: 2 July 2024

Published: 8 July 2024

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**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Instituto de Catálisis y Petrolequímica ICP (CSIC), 28049 Madrid, Spain;
elena.millan.ordonez@csic.es (E.M.O.); noelia.mota@icp.csic.es (N.M.T.)</footnote>

<footnote n="2">² Federation Chevreul, Univ. Lille, CNRS, Centrale Lille, Univ. Artois, FR 2638, F-59000 Lille, France;
bertrand.revel@univ-lille.fr</footnote>

<footnote n="3">³ UMR 8181—UCCS—Unité de Catalyse et Chimie du Solide, Univ. Lille, CNRS, Centrale Lille, Univ. Artois, F-59000 Lille, France; olivier.lafon@univ-lille.fr</footnote>

<footnote>* Correspondence: r.navarro@icp.csic.es; Tel.: +34-915-85-47-74</footnote>
(DME) based on Cu/ZnO(Al) (CZA), as a methanol synthesis function, and heteropolyacid H₃PW₁₂O₄₀ (HPW) supported on TiO₂, as an acidic dehydration function, constitutes a challenge, as both functions must be thoroughly optimized to achieve optimal synergy [1]. Considering that the degree of contact between the CZA and the supported HPW catalysts determines the activity and stability of the bifunctional catalyst, it must be optimized to ensure both functions work in complete synergy without negative interactions, thereby maximizing the DME production [2,3].

Because the accessibility of protons in pure H₃PW₁₂O₄₀ is limited due to diffusional constrains originating from its a very low surface area (5–10 m²/g) [4], the HPAs are typically studied in its supported forms. In this context, the characteristics of the TiO₂ support, such as its structure, textural properties, and hydroxyl groups density, affect the interaction between the heteropolyacid HPW and the support. These factors influence the coverage and dispersion of HPW, as well as its structure and acidity [4–7]. Changes in the structure and exposure of HPW units will influence their interactions with the methanol synthesis function in the bifunctional catalysts. Previous studies have explored how the structural and textural characteristics of the acid function determine the final characteristics of the catalytic performance of catalysts [8–19]. For example, Kraleva et al. [8] observed that Brønsted acid centers predominated in the heteropolyacid (HPA) supported on TiO₂ and ZrO₂, whereas Lewis acid centers were more prevalent in the HPA supported on Al₂O₃ and the ternary oxide. On the other hand, Kojima et al. [9] observed differences in the stability of HSiW·6H₂O depending on the support, which was stable on silica supports (Aerosil and SBA-15), while the destruction of the Keggin units occurred on γ-Al₂O₃, highlighting the importance of controlling the size and/or shape of the supported HSiW·6H₂O to regulate selectivity, whereas Peinado et al. [10] indicated that the catalytic performance of heteropolyacid-based catalysts for DME synthesis is strongly affected by the nature of the support (TiO₂, CeO₂, BN, ZrO₂, and Al₂O₃). A study on the Keggin- or Wells-Dawson-type heteropolyacids (HPAs) supported on SiO₂, TiO₂, and ZrO₂ [11] demonstrated that HPA deposition can cause distortions in the structure of the supported HPW particles depending on the presence of stronger (TiO₂ and ZrO₂) or weaker (SiO₂) basic centers in the support. Alsalme et al. [12] observed that compared to bulk HPW, which possesses strong Brønsted acid centers, the catalysts supported on TiO₂, ZrO₂, and Nb₂O₅ exhibited both Brønsted and Lewis acid centers, with the latter primarily originating from the support. The correlation between the activity and proton density on the surface of silica-supported HPA catalysts was observed by Al-Faze et al. [13].

TiO₂ is considered an excellent support for heterogeneous catalysts due to its high surface area, chemical stability, and acid-base properties, which stabilize heteropolyacids on its surface, attributed to its hydroxylated surface [14–18]. In particular, HPW supported on TiO₂ has demonstrated high activity in methanol dehydration, being more active than bulk HPW [15]. It was observed that a weak interaction between the support and HPA results in improved performance due to the enhanced mobility and desorption of water. Micek-Ilnicka et al. [16] studied the effect of different TiO₂ polymorphs (anatase (A), rutile (R), and a mixture of both phases (P25)) as supports for HPW, identifying in differences in their acidity and catalytic activity in the gas-phase conversion reaction for the dehydration of n-butanol.

Given that the structural and surface characteristics of the TiO₂ support significantly influence the dispersion of the Keggin units of heteropolyacids and, consequently, their catalytic activity, further investigation is needed to understand their impact on the contacts and structures when combined as an acidic function in bifunctional catalysts with Cu-ZnO(Al) systems for the direct synthesis of DME. Therefore, our objective in this work was to evaluate the influence of various TiO₂ supports, anatase (calcined at 250 and 500 °C (Ti-A-250 and Ti-A-500), anatase-rutile (Ti-AR), and mesoporous TiO₂ (Ti-M), on the dispersion and structure of the Keggin units of the heteropolyacids and their impact on the structure and activity of the bifunctional Cu-ZnO(Al)-HPW/TiO₂ catalysts for direct DME synthesis.
## 2. Results

### 2.1. Characterization of TiO₂ Supports

#### 2.1.1. X-ray Diffraction (XRD)

The crystal structure of the TiO₂ supports was studied using X-ray diffraction (Figure 1). Diffraction peaks corresponding to the anatase phase (JCPDS 21-1272) are observed in all the supports. Additionally, the Ti-AR support presents diffraction peaks corresponding to the rutile phase (JCPDS 75-1750). The differences in the intensity of the diffraction peaks of the anatase phase indicate differences in their degree of crystallinity, which increases in the following sequence: Ti-A-250 ≈ Ti-M < Ti-A-500 ≈ Ti-AR. Using the Scherrer equation, the crystallite size has been calculated, considering the reflection peak of the (1 0 1) planes of the anatase phase for each of the TiO₂ supports and the reflection peak of the (1 1 0) planes of the rutile phase for the Ti-AR support (Table S1). The crystallite size of the anatase phase decreases according to the following order: Ti-AR (20.1 nm) > Ti-A-500 (17.9 nm) > Ti-M (13.0 nm) > Ti-A-250 (11.0 nm). It is observed that the crystallite size of the Ti-A-500 support is significantly larger than that of Ti-A-250 due to the crystalline growth of the anatase phase after thermal treatment at 500 °C.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** X-ray diffraction profiles of TiO₂ supports (A) and their detailed region from 23° to 29° (B): Ti-AR (a), Ti-A-250 (b), Ti-A-500 (c), and Ti-M (d) (anatase (Δ) and rutile (♠)).</figcaption></figure>

#### 2.1.2. N₂ Adsorption–Desorption Isotherms

The textural properties of the TiO₂ supports were determined from their N₂ adsorption–desorption isotherms (Figure S1A). According to the IUPAC classification, the Ti-A-250 and Ti-A-500 supports exhibit type IVa isotherms, characteristic of mesoporous materials, with an H3 hysteresis loop associated with slit-shaped pores [17,18]. The Ti-M support also presents a type IVa isotherm, but with an H1 hysteresis loop, which is characteristic of the aggregates of spherical particles that have a certain degree of order and possess a narrow pore size distribution (formation of cylindrical pores) [19]. Lastly, the Ti-AR support exhibits a type IIb isotherm, characteristic of non-porous or macroporous solids, with a high-pressure hysteresis loop (P/P₀ > 0.9) indicative of interparticle condensation [19]. Table S2 shows the values of the BET surface area, total pore volume, and mean pore diameter of the different TiO₂ supports. The BET surface area decreases in the following order: Ti-A-250 (138 m²/g) > Ti-A-500 (74 m²/g) > Ti-M (55 m²/g) ≈ Ti-AR (48 m²/g). The decrease in the surface area between the Ti-A-250 and Ti-A-500 samples is related to the increase in the crystallite size, as determined previously by XRD (Table S1), which leads to a reduction in the porosity in Ti-A-500 and thus a decrease in the surface area. The pore size distribution of the TiO₂ supports has been calculated from the desorption branch of
the isotherms (Figure S1). The Ti-A-250 sample exhibits two main contributions to the pore size distribution, centered at an average pore size of 5 and 9 nm, while the Ti-A-500 sample shows two contributions centered at 9 and 14 nm (Figure S1B). However, the Ti-M sample presents a very uniform and symmetrical distribution of pores centered around 6 nm. In the Ti-AR sample, no pore distribution curve is observed, indicating the non-porous nature of this sample.

## 2.2. Characterization of the HPW/Ti Acid Catalysts

### 2.2.1. X-ray Diffraction (XRD)

Figure 2 depicts the X-ray diffractograms of acid catalysts made of HPW loaded on TiO₂ supports (HPW/Ti). In all cases, the diffraction peaks present in the TiO₂ supports are observed, corresponding to crystalline species of anatase (JCPDS 21-1272) and, in the case of the HPW/Ti-AR sample, additional species of rutile (JCPDS 75-1750).

<figure><img src="image_5.png" /><figcaption>**Figure 2.** X-ray diffraction profiles of HPW/Ti acid catalysts ((A) and (B)) (detailed region from 5 to 22.5 °): HPW/Ti-AR (a), HPW/Ti-A-250 (b), HPW/Ti-A-500 (c), and HPW/Ti-M (d) (anatase (Δ), rutile (♠), HPW-6H₂O (♦)).</figcaption></figure>

Additionally, diffraction peaks typical of crystalline species of HPW·6H₂O with a cubic structure (JCPDS 70-0351) are observed [20,21]. The intensity of the diffraction peaks of the crystalline species of HPW·6H₂O varies in the HPW/Ti acid catalysts, indicating a different degree of crystallization depending on the type of TiO₂ support used to disperse the HPW. Table S3 shows the intensity values of the (1 1 0) diffraction peaks of the HPW·6H₂O, as well as the relative intensity of this diffraction peak with respect to the HPW load, to compare the crystallinity degree of these species in the acid catalysts. The crystallite size of the HPW·6H₂O species obtained from the Scherrer equation is also provided. Depending on the TiO₂ support, differences in the crystallinity and crystal size of the HPW·6H₂O particles are observed (Table S3). The crystallinity of the HPW·6H₂O particles decreases, according to the variation in the relative intensity, in the following order: HPW/Ti-M ≈ HPW/Ti-A-250 > HPW/Ti-A-500 > HPW/Ti-AR. A similar sequence shows the decrease in the crystallite size of the HPW units: HPW/Ti-M > HPW/Ti-A-250 > HPW/Ti-A-500 ≈ HPW/Ti-AR.

The crystalline species of H₃PW₁₂O₄₀·6H₂O in the HPW/Ti-M, HPW/Ti-A-250, and HPW/Ti-A-500 acid catalysts have a crystallite size larger than the average pore diameter of the TiO₂ supports (Table S2), indicating their possible segregation on the surface of the supports. A relationship between the pore size distribution of the TiO₂ supports, the degree of segregation, and the crystal size of the HPW species is observed: the smaller the pore
diameter of the TiO₂ support, the greater the surface segregation of the heteropolyacid and, consequently, the larger its crystallite size.

### 2.2.2. N₂ Adsorption–Desorption Isotherms

The modification of the textural properties of the TiO₂ supports has been determined from the nitrogen adsorption–desorption isotherms of the HPW/Ti acid catalysts (Figure 3). The HPW/Ti-A-250, HPW/Ti-A-500, and HPW/Ti-M acid catalysts exhibit type IVa isotherms (Figure 3A), similar to those presented by their respective supports (Figure S1A), but with differences in the type and amplitude of the hysteresis loop and in the total volume of adsorbed N₂. In the case of the HPW/Ti-AR acid catalyst, its isotherm exhibits a shape similar to that of the Ti-AR support, which is as expected, considering its non-porous nature.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** N₂ adsorption–desorption isotherms (A) and pore size distribution (B) of HPW/Ti acid catalysts: HPW/Ti-AR (a), HPW/Ti-A-250 (b), HPW/Ti-A-500 (c), and HPW/Ti-M (d).</figcaption></figure>

Table S4 summarizes the BET surface area (S<sub>BET</sub>) and microporous area (S<sub>Microporous</sub>) of the HPW/Ti acid catalysts. The BET surface area of all the acid catalysts significantly increases compared to the normalized area, which accounts for dilution with varying HPW loading. The increase in the surface area is directly related to the development of microporosity in the acid catalysts, which was not previously observed in the TiO₂ supports. There is a direct relationship between the developed microporous area in the HPW/Ti acid catalysts and the HPW load, which indicates that the developed microporosity is related to the structuring of the segregated HPW units on the surface of the supports.

The pore size distribution of the HPW/Ti acid catalysts (Figure 3B) shows differences compared to the supports (Figure S1B). In the HPW/Ti-A-250 sample, there is almost complete blocking of the mesoporous structure of the Ti-A-250 support, while the HPW/Ti-A-500 catalyst blocks the 9 nm pores of the support but retains the porous structure of the larger 14 nm pores. The HPW/Ti-M sample also shows significant blocking of the porous structure of the Ti-M support. Finally, the HPW/Ti-AR sample shows slight porous modification with respect to the Ti-AR support, with a certain increase in the pore volume generated between its non-porous particles that may be related to the generation of new porosity between the aggregates of the HPW particles segregated on the Ti-AR support surface.
### 2.2.3. Attenuated Total Reflection Infrared Spectroscopy (ATR-FTIR)

The structural changes in the Keggin units of HPW when deposited on different TiO₂ supports were studied by ATR-FTIR (Figure 4A). In the 650–1200 cm⁻¹ region, characteristic bands of the Keggin structure of HPW appear at 750 and 890 cm⁻¹ (W–O–W, attributed to bridging (O_c) and terminal (O_b) oxygens, respectively), 975 cm⁻¹ (W=O_d, stretching of terminal oxygen) and 1075 cm⁻¹ (P–O_a, asymmetric stretching of oxygen) [22–24]. In the HPW/Ti acid catalysts, these bands exhibit an intensity proportional to the HPW loading. No significant shifts or broadenings are observed between the bands of bulk HPW and the supported HPW structures of the HPW/Ti acid catalysts. The exception is the band around 750 cm⁻¹ (W–O_c–W), which is observed at a higher wavenumber (≈785 cm⁻¹) in the acid catalysts compared to the bulk HPW. This shift could indicate a variation in the degree of hydration of the HPW structures when deposited on the TiO₂ supports compared to bulk HPW [23–25]. However, the frequencies most sensitive to changes in the degree of dehydration are the vibrations v (P–O_a) and v (W=O_d) [26], which do not show significant alteration in intensity and position. The results of the FTIR-ATR analysis corroborate earlier XRD findings, suggesting that there are no substantial alterations in the structure or hydration level of HPW units when supported on TiO₂.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** ATR-FTIR spectra (A) and TPD-NH₃ profiles (B) of the HPW (bulk) heteropolyacid (a) and the HPW/Ti acid catalysts: HPW/Ti–AR (b), HPW/Ti–A–250 (c), HPW/Ti–A–500 (d), and HPW/Ti–M (e).</figcaption></figure>

### 2.2.4. Temperature-Programmed Desorption of NH₃ (TPD-NH₃)

The desorption profiles of the adsorbed ammonia on the HPW/Ti catalysts are shown in Figure 4B. All the desorption profiles of NH₃ exhibit broad and low-intensity contributions in the temperature range of 150–400 °C, and a more intense contribution above 400 °C. The desorption peaks observed at lower temperatures are attributed to NH₃ adsorption on the Lewis acidic sites of the TiO₂ supports, as well as on the moderately strong acidic sites formed through the interaction between the protons of HPW and the surface OH⁻ groups of the TiO₂ support [22]. The desorption peak observed at higher temperatures (T > 400 °C) corresponds to the decomposition (Equations (1)–(3)) of the ammonium salt (NH₄)₃PW₁₂O₄₀ formed by the interaction between NH₃ and the acidic protons of HPW [5,27].

$$(NH_4)_3PW_{12}O_{40} \rightarrow 3NH_3 + H_3PW_{12}O_{40} \quad (1)$$

$$NH_3 \rightarrow 1/2 N_2 + 3/2 H_2 \quad (2)$$

$$H_2 + H_3PW_{12}O_{40} \rightarrow H_2O + PO_x + Mo_yO_x \quad (3)$$
In Table 1, the quantification of the TPD areas is provided for the bulk HPW and the HPW/Ti catalysts. In all the acid HPW/Ti samples, a reduction in the desorption contribution area is observed at temperatures above 400 °C compared to bulk HPW. The number of H⁺ per Keggin unit decreases in the following order: HPW/Ti-AR > HPW/Ti-M > HPW/Ti-A-250 ≈ HPW/Ti-A-500. This decrease could be related to a blockage of access to the protons of the Keggin units, but it is more probable that this decrease is attributable to the loss of H⁺ as a result of their interaction with the OH⁻ groups of the TiO₂ support [13].

<table><caption>**Table 1.** Quantitative data obtained from the integration of the TPD-NH₃ profiles of bulk HPW and HPW/Ti acid catalysts.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Area (a. u.)</th><th rowspan="2">H<sup>+</sup>/KU</th></tr><tr><th>150-400 °C</th><th>>400 °C</th></tr></thead><tbody><tr><td>HPW</td><td>-</td><td>215.5</td><td>3</td></tr><tr><td>HPW/Ti-A-250</td><td>145.9</td><td>134.7</td><td>1.88</td></tr><tr><td>HPW/Ti-A-500</td><td>159.5</td><td>130.7</td><td>1.82</td></tr><tr><td>HPW/Ti-AR</td><td>150.4</td><td>182.5</td><td>2.54</td></tr><tr><td>HPW/Ti-M</td><td>129.5</td><td>143.4</td><td>2.00</td></tr></tbody></table>

### 2.2.5. Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy

The ¹H MAS-NMR spectrum of the reference bulk HPW (Figure 5A) exhibits a main signal centered at 8.3 ppm, which is associated with the highly acidic protons of the HPW units interacting via hydrogen bonding with water molecules [28–30]. The profile of the bulk HPW (Figure 5A) also displays a small signal at 5.2 ppm related to coordinated or adsorbed water molecules in the Keggin units [28,31]. The signal corresponding to the protons of hydrated HPW·6H₂O units appears close to 8 ppm, indicative of moderate mobility and/or strong dipolar interactions.

<figure><img src="image_8.png" /><figcaption>**Figure 5.** ¹H MAS-NMR (A) and ³¹P MAS-NMR signals (B) of the bulk HPW (treated at 150 °C) (a) and the HPW/Ti acid catalysts: HPW/Ti-AR (b), HPW/Ti-A-250 (c), HPW/Ti-A-500 (d), and HPW/Ti-M (e).</figcaption></figure>

The ¹H MAS-NMR spectrum of the HPW/Ti acid catalysts (Figure 5A) shows, in all cases, a broad signal between 4 and 11 ppm, which is composed of several signals centered at approximately 5.2, 7.3, and 8.3 ppm. The signal at 8.3 ppm is similar to that observed in the bulk HPW [32] and is related to the H⁺ protons of the HPW units interacting with water
molecules, forming H⁺(H₂O)₂ clusters [30,32]. Meanwhile, the peak at 7.3 ppm may be associated with protons interacting with water molecules but with different coordination, probably H⁺(H₂O)ₙ clusters with an n > 6 [31–34]. The relative intensity of the main resonance peak at 7.3 ppm is significantly higher in the HPW/Ti-A-250 sample, which could be related to the higher water content in this sample [29,33,35]. The signal appearing in the HPW/Ti catalysts at 5.2 ppm is associated with adsorbed or coordinated water molecules on the TiO₂ surface and/or the HPW units [31,34,36–39]. This signal is more intense in the HPW/Ti-M and HPW/Ti-A-500 samples. Additionally, the resonance peak centered at 2.5 ppm is attributed to relatively mobile physisorbed water [36,39], while the signals at a lower chemical shift (0.8 and 1.4 ppm) correspond to water molecules interacting with the OH⁻ groups on the TiO₂ surface [33,40]. The relative intensity of signals corresponding to the OH⁻ groups of the TiO₂ supports interacting with water compared to the signals of the HPW units in the HPW/Ti acid catalysts decreases in this order: HPW/Ti-AR > HPW/Ti-A-500 > HPW/Ti-M >> HPW/Ti-A-250. This trend suggests that, after the incorporation of the HPW units, the number of free OH⁻ groups on the supports decreases according to this sequence.

The ³¹P MAS-NMR signal of the reference bulk HPW (Figure 5B) exhibits a strong signal centered at −15.6 ppm, which is associated with HPW units where the protons of the polyanions are bound by hydrogen bonds with water molecules, forming H⁺(H₂O)₂ clusters [30,41,42]. The spectrum of the bulk HPW shows small contributions at −15.5 and −15.4 ppm indicative of HPW species with a slightly lower degree of hydration [30,41]. The ³¹P MAS-NMR spectra of all the HPW/Ti acid catalysts are similar, with all of them exhibiting a strong signal around −15.6 ppm that is associated with the crystalline structure of HPW units where the protons of the polyanions are coordinated by H⁺(H₂O)₂ clusters [12, 41,42], similar to what is observed in the reference bulk HPW. Additionally, the acid catalysts show a broad signal ranging from −14.5 to −15.6 ppm, which is composed of the contributions of several overlapping peaks corresponding to species with a higher degree of dehydration [H₃-xPW₁₂O₄₀]ₓ⁻ or HPW species interacting with the support [12,28,41]. Possible differences associated with the degree of dehydration can be ruled out since the previous ¹H MAS-NMR signals (Figure 5A) did not show very significant differences in the dehydration degree among the HPW/Ti acid catalysts. Therefore, the chemical shifts observed in the ³¹P signal may be mainly associated with differences in the interaction of the HPW units with the TiO₂ supports. The percentage of HPW species interacting with the support compared to the bulk HPW decreases according to the following sequence: HPW/Ti-A-250 > HPW/Ti-A-500 > HPW/Ti-M ≈ HPW/Ti-AR.

The longitudinal relaxation times (T1) of ³¹P nuclei were measured to determine the degree of dispersion of the HPW Keggin units in the acid catalysts (Figure 6), because components with faster relaxation times are associated with dispersed HPW units, while components with a slower relaxation time correspond to species similar to the bulk HPW [43,44].

From the relative proportion of species with short/long relaxation times (w), the proportion of dispersed/bulk HPW units has been determined on the HPW/Ti acid catalysts (Table S5). The relative proportion of dispersed/bulk HPW species in the acid catalysts increases according to this sequence: HPW/Ti-AR ≈ HPW/Ti-A-M < HPW/Ti-A-500 < HPW/Ti-A-250. Comparing this sequence with the surface area of the supports used for dispersing HPW (Table S2), we notice that when the surface area of the TiO₂ supports increases, the relative percentage of species interacting with the support also increases.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** $^{31}$P MAS-NMR longitudinal relaxation spectra of the HPW/Ti acid catalysts: HPW/Ti-A-250, HPW/Ti-A-500, HPW/Ti-AR, and HPW/Ti-M (Arrow on the graphs used to guide in the relative displacement of the signals).</figcaption></figure>

### 2.2.6. X-ray Photoelectron Spectroscopy (XPS)

Surface analysis of the HPW/Ti acid catalysts was carried out by analyzing the energy levels of Ti 2p, W 4f, and O 1s (Figures 7A, 7B and 7C, respectively). The W 4f level spectrum of the HPW/Ti acid catalysts (Figure 7B) shows a main doublet of the W 4f$_{7/2}$ (35.5 eV) and W 4f$_{5/2}$ (37.7 eV) levels, characteristic of W$^{6+}$ ions in Keggin structures [45,46]. Additionally, the W 4f level exhibits a second doublet of the W 4f$_{7/2}$ (34.0 eV) and W 4f$_{5/2}$ (36.2 eV). There is some discrepancy in the assignment of these peaks [32], although they are generally attributed to interfacial Keggin species [47,48] related to the perturbation of Keggin units by their interaction with the support [11,49,50]. The intensity ratio between the W 4f$_{7/2}$ (34.0 eV) and W 4f$_{5/2}$ (35.5 eV) peaks, indicative of the relative contribution of these interfacial/Keggin W species, is shown in Table 2. Differences in the relative contribution of interfacial W species are observed, with the HPW/Ti-AR sample showing the highest relative percentage of interfacial species in the HPW units, while the HPW/Ti-M sample shows the lowest relative percentage (Table 2). With the exception of the HPW/Ti-M sample, the rest of the acid catalysts show an increase in the surface HPW concentration with a decrease in the relative percentage of interfacial species (Table 2), suggesting the similar development of the terminal W = O bonds of the Keggin units on the surface of the TiO$_2$ supports. The results of the interfacial/mass species ratio of the HPW units in the HPW/Ti catalysts determined by XPS (Table 2) contrast with previous NMR results (Figures 5 and 6). The discrepancy observed between XPS and NMR data could be due to the fact that the surface-level information obtained by XPS may not be representative of the dispersed/bulk species distribution at the mass level.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** XPS spectra of the Ti 2p (A), W 4f (B) and O 1s (C) levels of HPW/Ti acid catalysts: HPW/Ti-AR (a), HPW/Ti-A-250 (b), HPW/Ti-A-500 (c), and HPW/Ti-M (d).</figcaption></figure>

<table><caption>**Table 2.** Binding energy, atomic W/Ti ratios (XPS and bulk) and W 4f<sub>7/2</sub> (34.0 eV)/W 4f<sub>7/2</sub> (35.5 eV) ratio (interfacial/Keggin atomic W) of HPW/Ti acid catalysts.</caption><thead><tr><th rowspan="2"></th><th colspan="2">BE (eV)</th><th rowspan="2">W/Ti Superf.</th><th rowspan="2">W/Ti Bulk *</th><th rowspan="2">W 4f<sub>7/2</sub> (34.0 eV)/<br>W 4f<sub>7/2</sub> (35.5 eV)</th></tr><tr><th>Ti 2p<sub>3/2</sub></th><th>W 4f<sub>7/2</sub></th></tr></thead><tbody><tr><td>HPW/Ti-A-250</td><td>458.9</td><td>35.7</td><td>0.88</td><td>0.00297</td><td>0.13</td></tr><tr><td>HPW/Ti-A-500</td><td>458.9</td><td>35.6</td><td>0.73</td><td>0.00297</td><td>0.17</td></tr><tr><td>HPW/Ti-AR</td><td>458.6</td><td>35.4</td><td>0.88</td><td>0.00292</td><td>0.22</td></tr><tr><td>HPW/Ti-M</td><td>458.7</td><td>35.5</td><td>1.10</td><td>0.00291</td><td>0.11</td></tr></tbody></table>

From the intensities of the Ti 2p and W 4f levels, the surface atomic ratios W/Ti on the HPW/Ti acid catalysts were calculated (Table 2). The surface atomic ratio W/Ti on the HPW/Ti acid catalysts varies slightly with the type of support used. A decrease in the surface concentration of the HPW units is observed according to the following sequence: HPW/Ti-M > HPW/Ti-A-R ≈ HPW/Ti-A-250 > HPW/Ti-A-500. The surface concentration of the HPW does not match the HPW crystalline segregation determined by XRD (HPW/Ti-A-250 ≈ HPW/Ti-M > HPW/Ti-A-500 > HPW/Ti-AR, Table S3), which seems to indicate that there is a significant contribution by non-crystalline species that contribute to the XPS signal but are not visible by XRD.

## 2.3. Characterization of Bifunctional CZA-HPW/Ti Catalysts

### 2.3.1. Attenuated Total Reflectance Infrared Spectroscopy (FTIR-ATR)

In Figure 8, the FTIR-ATR spectra of the bifunctional CZA-HPW/Ti catalysts are plotted, along with the reference spectrum of the methanol synthesis CZA catalyst. For all the bifunctional catalysts, two contributions centered at 1415 and 1475 cm<sup>-1</sup> are observed, whose ratio and intensity do not vary significantly between the samples, indicating the permanence of the residual carbonates of the CZA catalyst without significant modification

<footnote>* Bulk ratio derived from the chemical composition normalized by the BET surface area of the TiO<sub>2</sub> support.</footnote>
after mixing with the HPW/Ti acid catalysts. The residual carbonates are crucial for maintaining the high mesostructure and interdispersion of the CuO and ZnO particles in the CZA catalyst [51].

<figure><img src="image_11.png" /><figcaption>**Figure 8.** FTIR-ATR spectra of the CZA catalyst (a) and the bifunctional CZA-HPW/Ti catalysts in the regions 1200–1650 cm⁻¹ (A) and 700–1200 cm⁻¹ (B): CZA-HPW/Ti-AR (b), CZA-HPW/Ti-A-250 (c), CZA-HPW/Ti-A-500 (d), and CZA-HPW/Ti-M (e).</figcaption></figure>

The spectra of the bifunctional catalysts in the region 700–1200 cm⁻¹ (Figure 8B) do not exhibit significant differences in the characteristic bands of the HPW Keggin structures. No significant shifts or broadenings are observed when compared to the HPW/Ti acid catalysts (Figure 4A), indicating no modifications to the structure of the Keggin units of the supported HPWs after combination with the CZA catalyst [26,52,53].

### 2.3.2. Temperature-Programmed Reduction with H₂ (TPR-H₂)

The reducibility of the methanol synthesis catalyst CZA after its combination with the acid catalysts in the bifunctional CZA-HPW/Ti catalysts was analyzed by TPR-H₂ (Figure 9). The reduction profile of the CZA catalyst shows an initial H₂ consumption (140–165 °C) and a main contribution centered around 175 °C. The initial H₂ consumption is associated with the reduction of the highly dispersed Cu²⁺ particles (in high temperature carbonates not decomposed in the calcination stage of the CZA precursor). This first reduction facilitates the subsequent reduction of CuO species in close contact with ZnO particles, which are also partially reduced to ZnOₓ, leading to the main H₂ consumption observed at 175 °C [22]. The bifunctional catalysts CZA-HPW/Ti present reduction profiles very similar to those of the reference catalyst CZA, suggesting that the combination of the CZA catalyst with the different HPW/Ti acid catalysts does not significantly modify the reducibility of the Cu species in contact with ZnO.
<figure><img src="image_12.png" /><figcaption>**Figure 9.** H₂ consumption profiles during temperature-programmed reduction of the CZA catalyst (a) and the bifunctional CZA-HPW/Ti catalysts: CZA-HPW/Ti-AR (b), CZA-HPW/Ti-A-250, (c) CZA-HPW/Ti-A-500 (d) and CZA-HPW/Ti-M (e).</figcaption></figure>

### 2.3.3. X-ray Diffraction (XRD)

Figure 10 shows the diffraction profiles of the bifunctional CZA-HPW/Ti catalysts and the methanol synthesis CZA catalyst in reduced state. All the reduced bifunctional CZA-HPW/Ti catalysts show diffraction peaks similar to those observed for the CZA catalyst, corresponding to Cu⁰ (JCPDS 85-1326) and hexagonal ZnO (JCPDS 75-0576) phases. Due to the low concentration of the HPW/Ti acid catalyst in the bifunctional catalysts, very-low-intensity diffraction peaks associated with the anatase phase (JCPDS 21-1272) and rutile phase (JCPDS 21-1276) are observed in the CZA-HPW/Ti-AR catalyst. A small diffraction peak is also observed at 26.3°, corresponding to the HPW·3H₂O phase with a cubic structure, resulting from the dehydration of the HPW·6H₂O units during the reduction process [54].

<figure><img src="image_13.png" /><figcaption>**Figure 10.** X-ray diffraction profiles of the CZA catalyst (a) and reduced CZA-HPW/Ti bifunctional catalysts ((A) and (B) (detailed region from 23 to 29°)): CZA-HPW/Ti-AR (b), CZA-HPW/Ti-A-250 (c), CZA-HPW/Ti-A-500 (d) and CZA-HPW/Ti-M (e) (anatase (Δ), rutile (♠), HPW (◊), ZnO (♣), Cu⁰ (o)).</figcaption></figure>
The relative intensity of the dehydrated supported HPW, as determined from the intensity ratio of the HPW·3H₂O/ZnO (1 0 0) diffraction peaks, is similar for all the bifunctional catalysts except for the CZA-HPW/Ti-M sample, which presents a slightly lower value (Table 3).

<table><caption>**Table 3.** Crystallite size of the Cu⁰ and ZnO phases and relative intensity of HPW with respect to ZnO in the reduced CZA-HPW/Ti bifunctional catalysts (from XRD) and Cu surface area and % Cu surface/bulk (from N₂O chemisorption data).</caption><thead><tr><th rowspan="2"></th><th colspan="2">Cu⁰ (1 1 1)</th><th colspan="2">ZnO (1 0 0)</th><th rowspan="2">HPW (1 0 0) Relative Intensity</th><th rowspan="2">Cu Surface Area (N₂O) (m²/gCZA)</th><th rowspan="2">Cu Surface/Bulk (%)</th></tr><tr><th>2θ (°)</th><th>(nm)</th><th>2θ (°)</th><th>(nm)</th></tr></thead><tbody><tr><td>CZA</td><td>43.04</td><td>5.0</td><td>31.71</td><td>4.7</td><td>-</td><td>46.2</td><td>13.4</td></tr><tr><td>CZA-HPW/Ti-A-250</td><td>42.96</td><td>4.9</td><td>31.68</td><td>4.2</td><td>0.21</td><td>52.3</td><td>15.2</td></tr><tr><td>CZA-HPW/Ti-A-500</td><td>43.32</td><td>5.7</td><td>31.94</td><td>4.7</td><td>0.19</td><td>53.5</td><td>15.5</td></tr><tr><td>CZA-HPW/Ti-AR</td><td>43.22</td><td>4.9</td><td>31.97</td><td>4.4</td><td>0.20</td><td>48.1</td><td>14.0</td></tr><tr><td>CZA-HPW/Ti-M</td><td>42.98</td><td>4.9</td><td>31.73</td><td>4.1</td><td>0.15</td><td>51.2</td><td>14.9</td></tr></tbody></table>

The crystallite size of the Cu⁰ and ZnO phases (Table 3) in the methanol synthesis catalyst CZA is 5.0 and 4.7 nm, respectively. These values are characteristic of catalysts with high interdispersion between the Cu⁰ and ZnO particles, which is necessary to achieve high activity for methanol synthesis [55]. The crystallite size of the Cu⁰ and ZnO phases in all the reduced bifunctional CZA-HPW/Ti catalysts is very similar to that of the CZA catalyst, indicating the absence of significant modifications of the Cu⁰ and ZnO structures after the combination of the CZA catalyst with the acid HPW/Ti catalysts.

### 2.3.4. N₂O Chemisorption

The copper surface area, surface defects of copper particles and contacts between copper particles and reduced ZnOₓ species [56] on the reduced bifunctional CZA-HPW/Ti catalysts were evaluated by N₂O chemisorption (Table 3). The N₂O chemisorption over the reduced bifunctional CZA-HPW/Ti catalysts shows slightly higher Cu surface area values and contacts with the reduced ZnOₓ species than those shown for the CZA catalyst. It is observed that the N₂O chemisorption capacity increases according to the following sequence: CZA-HPW/Ti-AR < CZA-HPW/Ti-M < CZA-HPW/Ti-A-250 < CZA-HPW/Ti-A-500. This variation may be attributed to: (i) differences in the physical blocking of Cu centers by HPW/Ti acid catalyst particles [57] and/or (ii) modifications to the Cu-ZnO interactions in the CZA catalyst associated with the migration of Cu²⁺/Zn²⁺ species from CZA to the HPW/Ti acid catalysts [58]. Considering that the chemisorption capacity on the bifunctional catalysts increases with respect to the CZA catalyst, the chemisorption data seem to indicate that there are differences in the relative migration of Cu²⁺/Zn²⁺ surface species with the protons of the supported HPW units. Considering the variation in the N₂O chemisorption data, the relative migration of Cu²⁺/Zn²⁺ surface species on the bifunctional catalysts increases according to the following sequence: CZA-HPW/Ti-AR < CZA-HPW/Ti-M ≈ CZA-HPW/Ti-A-250 < CZA-HPW/Ti-A-500. However, it should be noted that the overall migration of Cu²⁺/Zn²⁺ species on none of the reduced bifunctional CZA-HPW/Ti catalysts is very significant, considering the variation of the Cu surface area on the bifunctional catalysts with respect the CZA catalyst (from 4% on CZA-HPW/Ti-AR to 15% on CZA-HPW/Ti-A-500).

### 2.3.5. Temperature-Programmed Desorption of NH₃ (TPD-NH₃)

The acidic properties of the bifunctional catalysts CZA-HPW/Ti-AR and CZA-HPW/Ti-A-250 in a reduced state, as representative samples of the extremes of the series in terms of the HPW loading, have been studied by TPD-NH₃ experiments. The desorption profiles are plotted in Figure 11, which also includes the desorption profile of the reduced CZA catalyst
that shows a very weak NH₃ desorption signal (m/z = 15) and a more intense N₂ desorption signal (m/z = 28) from NH₃ decomposition. NH₃ decomposition occurs on Cu⁰ species in contact with ZnOₓ or Al₂O₃ that allows the cleavage of the N-H bonds of the metal amides formed by the adsorption of NH₃ on the (Cu-)ZnOₓ and (Cu-)ZnOₓ-Al₂O₃ centers [59,60]. Therefore, the two broad contributions centered at 260 and 420 °C are associated with that decomposition of NH₃ adsorbed on different Lewis acid centers associated with (Cu-)ZnOₓ and (Cu-)ZnOₓ-Al₂O₃ species, respectively [61].

<figure><img src="image_14.png" /><figcaption>**Figure 11.** NH₃ desorption and decomposition profiles (MS, m/z = 28 and m/z = 15) of the CZA catalyst (a) and reduced CZA-HPW/Ti bifunctional catalysts: CZA-HPW/Ti-AR (b) and CZA-HPW/Ti-A-250 (c).</figcaption></figure>

The reduced bifunctional catalysts show similar desorption profiles to components corresponding to NH₃ desorption (m/z = 15) and N₂ formation from NH₃ decomposition (m/z = 28). The NH₃ desorption signal presents a weak peak at a low temperature (200 °C) and a more intense peak around 350 °C associated with NH₃ adsorbed on the Lewis acid centers of the TiO₂ and the moderate strength acid centers generated by the interaction of the HPW protons with the surface OH⁻ of the TiO₂. The bifunctional catalysts also present the desorption peak corresponding to NH₃ decomposition over the Cu-ZnOₓ centers at a slightly higher temperature, around 275 °C [60,62]. The N₂ desorption peak at 420 °C, observed on the CZA catalyst, does not appear on the bifunctional catalysts. The disappearance of this peak in the bifunctional catalysts indicates the loss of the more acidic centers of the CZA catalyst after contact with the acidic HPW/Ti catalysts. The desorption profiles of the bifunctional catalysts present an N₂ peak at around 355 °C that may be associated with the decomposition on the Cu-ZnOₓ centers of the NH₃ adsorbed on the Lewis acid centers of the TiO₂ support and on the moderate strength acid centers of the HPW interacting with the support. Finally, the bifunctional catalysts present a high temperature (T > 400 °C) N₂ desorption peak corresponding to the decomposition of the ammonium salt (NH₄)₃PW₁₂O₄₀ formed by the interaction between NH₃ and the acidic protons of the bulk HPW units [5,27]. The relative intensity of this peak is also, in this case, lower than that observed for the corresponding HPW/Ti acid catalysts (Table 1), which would be in line with the loss of acidity of the protons of the bulk HPW units after exchange with the Cu²⁺/Zn²⁺/Al³⁺ surface species of the CZA catalyst.

Comparing the relative contribution of the desorption peaks at intermediate (355 °C) and high (545 °C) temperatures in the bifunctional CZA-HPW/Ti-AR catalyst (Figure 11) with respect to the HPW/Ti-AR acid catalyst (Figure 4B), a relative increase in the desorption peak at an intermediate temperature with respect to that at a high temperature
is observed. This increase is indicative of the transformation of the strong acid centers of the bulk HPW units into centers of moderate acidity and is probably associated with the exchange with surface Cu²⁺/Zn²⁺/Al³⁺ species. Comparison of the desorption profiles in the bifunctional samples indicates that the relative loss of acidity of the bulk HPW units is higher in the CZA-HPW/Ti-AR sample with respect to the CZA-HPW/Ti-A-250 sample. This higher relative loss of acidity in the CZA-HPW/Ti-AR sample may be related to a higher contact of the HPW units with the CZA catalyst particles derived from the lower relative ratio between CZA and HPW/Ti in this bifunctional catalyst and by its higher accessibility as the HPW units are deposited on the external surface of the support. This higher contact of the HPW/Ti catalyst with the CZA catalyst in the CZA-HPW/Ti-AR sample would counteract the higher initial acidity presented by the HPW/Ti-AR acid catalyst with respect to the HPW/Ti-A-250 catalyst (Table 1).

## 2.4. Measurements of Catalytic Activity in Direct Synthesis of DME

The activity results of the bifunctional CZA-HPW/Ti catalysts in the direct synthesis reaction of DME from synthesis gas are shown in Figure 12 and Table 4. The results also include, for comparative purposes, the activity of the methanol synthesis catalyst CZA.

<figure><img src="image_15.png" /><figcaption>**Figure 12.** CO conversion (A), DME production (B) and methanol production (C) from syngas (250 °C, 3 MPa) over the CZA catalyst (♦) and the bifunctional CZA-HPW/Ti catalysts in a reduced state: CZA-HPW/Ti-AR (▼), CZA-HPW/Ti-A-250 (◎), CZA-HPW/Ti-A-500 (●) and CZA-HPW/Ti-M (□).</figcaption></figure>
<table><caption>**Table 4.** Initial and final CO conversion and selectivity (DME, methanol, and CO₂) in direct DME synthesis tests over the bifunctional CZA-HPW/Ti catalysts.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">CO Conversion<br>(%)<br>(X<sub>i</sub>; X<sub>f</sub>)</th><th colspan="3">Selectivity (%)</th></tr><tr><th>DME<br>(S<sub>i</sub>; S<sub>f</sub>)</th><th>Methanol<br>(S<sub>i</sub>; S<sub>f</sub>)</th><th>CO<sub>2</sub><br>(S<sub>i</sub>; S<sub>f</sub>)</th></tr></thead><tbody><tr><td>CZA</td><td>27.3; 24.6</td><td>0</td><td>100</td><td>0</td></tr><tr><td>CZA-HPW/Ti-A-250</td><td>23.8; 21.1</td><td>52.0; 51.8</td><td>7.9; 9.2</td><td>40.1; 39.0</td></tr><tr><td>CZA-HPW/Ti-A-500</td><td>24.2; 20.7</td><td>52.3; 52.2</td><td>7.7; 9.4</td><td>40.1; 38.4</td></tr><tr><td>CZA-HPW/Ti-AR</td><td>24.3; 20.7</td><td>51.6; 51.5</td><td>7.8; 8.9</td><td>40.6; 39.6</td></tr><tr><td>CZA-HPW/Ti-M</td><td>25.0; 21.2</td><td>51.1; 50.5</td><td>9.8; 11.1</td><td>39.1; 38.3</td></tr></tbody></table>

The bifunctional catalysts exhibit similar initial CO conversion values (≈24%, Figure 12A) and high initial DME production (>400 µmol DME/min g<sub>cat</sub>, Figure 12B). All the bifunctional CZA-HPW/Ti catalysts show only methanol, DME, CO₂ and H₂O production, without hydrocarbon formation from a possible dehydrogenation of the DME. The bifunctional catalysts present similar values of DME/methanol selectivity (Table 4), except the CZA-HPW/Ti-M sample, which presents a lower value of selectivity to DME. The correspondence between the DME/methanol ratio and the acidity of the bifunctional catalysts is well-established in acid catalysts based on heteropolyacids [4]. Therefore, the DME/methanol ratio obtained is consistent with the similar acidity observed for the bifunctional catalysts. All the bifunctional CZA-HPW/Ti catalysts show deactivation, as a decrease in CO conversion is observed with the reaction time (Figure 12A and Table 4). The deactivation is similar for all the catalysts. In addition to the deactivation in CO conversion, a slight decrease in DME/methanol selectivity with the time in reaction is also observed (Table 4), but of a lesser magnitude. The parallelism between the evolution of CO conversion and DME production, without a significant increase in methanol production, indicates that the deactivation observed in the bifunctional catalysts is mainly related to an evolution of the CZA catalyst under reaction conditions (Cu and/or ZnO sintering, Cu oxidation, etc. [63–65]).

## 2.5. Characterization of the Used Bifunctional CZA-HPW/Ti Catalysts
X-ray Diffraction (XRD)

The structural changes in the bifunctional CZA-HPW/Ti catalysts after their use in the reaction were evaluated by X-ray diffraction (Figure 13). The bifunctional CZA-HPW/Ti catalysts used show the diffraction peaks observed in the reduced samples (Figure 10), corresponding to the Cu⁰ (JCPDS 85-1326), ZnO (JCPDS 76-0704) and TiO₂ anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276) phases. The small diffraction peak at 26.3°, corresponding to the HPW·3H₂O phase with a cubic structure, is also observed. The intensities of the diffraction peaks of the crystalline phases of Cu⁰, ZnO, TiO₂ and HPW in the used samples do not undergo significant variations with respect to the reduced samples (Figure 12), which indicates the absence of significant changes after the reaction.

The crystallite sizes of the Cu⁰ and ZnO phases (Table 5) show a similar Cu⁰ size in all the used catalysts, without significant variation with respect to that of the fresh reduced catalysts (Table 3). On the contrary, some increase in the crystalline size of ZnO is observed in the used bifunctional catalysts compared to the fresh reduced catalysts, which could be related to the deactivation of the methanol synthesis centers. In line with this argumentation is the parallelism observed in the crystalline growth of ZnO particles on the catalysts used (CZA-HPW/Ti-A-250 < CZA-HPW/Ti-A-500 < CZA-HPW/Ti-AR < CZA-HPW/Ti-M, Table 5), which coincides with the deactivation sequence detected on the bifunctional samples. The increase in the ZnO particle size on the bifunctional catalysts is higher than that observed on the CZA catalyst used in methanol synthesis, suggesting that the deactivation derived from ZnO growth is associated with the generation of water in
the reaction and/or by cation exchange of Cu²⁺/Zn²⁺ species with the H⁺ of HPW under reaction conditions that may facilitate the sintering and growth of ZnO particles.

<figure><img src="image_16.png" /><figcaption>**Figure 13.** X-ray diffraction profiles of the CZA (a) and CZA-HPW/Ti bifunctional catalysts used in the reaction ((A) and (B) (detailed region from 23 to 29 °)): CZA-HPW/Ti-AR (b), CZA-HPW/Ti-A-250 (c), CZA-HPW/Ti-A-500 (d), and CZA-HPW/Ti-M (e) (anatase (Δ), rutile (♠), HPW (♦), ZnO (♣), CuO (●) and Cu⁰ (◎)).</figcaption></figure>

<table><caption>**Table 5.** Crystallite size of the Cu⁰ and ZnO phases from the X-ray diffraction profiles of the bifunctional CZA-HPW/Ti catalysts used in the reaction.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Cu⁰ (1 1 1)</th><th colspan="2">ZnO (1 0 0)</th></tr><tr><th>2θ (°)</th><th>(nm)</th><th>2θ (°)</th><th>(nm)</th></tr></thead><tbody><tr><td>CZA</td><td>42.99</td><td>5.2</td><td>31.63</td><td>5.6</td></tr><tr><td>CZA-HPW/Ti-A-250</td><td>42.94</td><td>4.6</td><td>31.54</td><td>6.0</td></tr><tr><td>CZA-HPW/Ti-A-500</td><td>43.02</td><td>4.5</td><td>31.59</td><td>6.1</td></tr><tr><td>CZA-HPW/Ti-AR</td><td>43.17</td><td>4.4</td><td>31.68</td><td>6.2</td></tr><tr><td>CZA-HPW/Ti-M</td><td>42.94</td><td>4.7</td><td>31.58</td><td>6.4</td></tr></tbody></table>

### 3. Discussion

#### 3.1. Influence of the Characteristics of TiO₂ on the Final Properties of HPW/TiO₂ Acid Catalysts

The physicochemical characterization of the HPW/Ti acid catalysts has shown that the dispersion and structuring of the HPW Keggin units depend on the type of TiO₂ support used. The primary HPW crystalline particles of all the acid catalysts, with the exception of HPW/Ti-AR, present a crystallite size larger (20–30 nm, Table S3) than the average pore size of the corresponding TiO₂ supports (5–14 nm, Table S2), indicating a possible segregation of the HPW species outside the porous structure of the supports. Such segregation is related to the pore size distribution of the TiO₂ supports used, since the smaller the pore diameter of the supports, the greater the surface segregation of the crystalline HPW particles as the volume and available surface area decrease.

The TPD-NH₃ data of the HPW/Ti acid catalysts (Table 1 and Figure 4B) showed losses in the acidity of the HPW units compared to the bulk HPW. In this regard, the study by Newman et al. [66] demonstrated that the loss of acid strength for 6.2 wt.% HPW on an SiO₂ support was not due to the decomposition of the Keggin unit structure, but that the high dispersion of the units on the support resulted in an acidity loss due to
the inability to host crystallization H₂O between adjacent Keggin units. In our case, the integrity of the Keggin units of the heteropolyacids is maintained in all the HPW/Ti acid catalysts, as demonstrated by the ³¹P-MAS-NMR (Figure 5B) and FTIR-ATR (Figure 4A) experiments. Hence, the variation in the acidity of the HPW units in the HPW/Ti catalysts must be related to differences in the interaction of the units with the support and their degree of structuring. According to the literature [67], the interaction of HPW units with TiO₂ supports occurs through interaction with the surface hydroxyl groups via the [≡Ti(OH...H⁺)]₂[H⁺][PW₁₂O₄₀]³⁻ protons or via the [≡Ti(O)]₃[PW₁₂O₃₇] terminal oxygens of the HPW units. The ¹H-MAS-NMR spectra of the acid catalysts (Figure 5A) showed a decrease in the relative intensity of the OH⁻ groups of the TiO₂ supports with respect to the signals from the HPW units in the following order: HPW/Ti-AR > HPW/Ti-A-500 > HPW/Ti-M > HPW/Ti-A-250, indicating that after the incorporation of the HPW units, the number of free OH⁻ groups of the support decreases according to that sequence.

With regard to the surface exposure of HPW, the XPS analysis (Figure 7) showed that the relative contribution of interfacial W species differs among the HPW/Ti acid catalysts, with HPW/Ti-AR showing the highest relative interaction of HPW units with the support while HPW/Ti-M shows the lowest interaction (Table 2). In good agreement with the XPS, the longitudinal relaxation times of ³¹P in the HPW/Ti acid catalysts (Figure 6) also showed differences in the proportion of dispersed and bulk HPW heteropolyacid species. The observed proportionality between the sequence of dispersed/bulk HPW species (³¹P NMR) with the area of the supports used to disperse the different HPW units (HPW/Ti-AR < HPW/Ti-M ≈ HPW/Ti-A-500 < HPW/Ti-A-250) (Figure 6) shows that as the surface area of the TiO₂ supports increases, the relative percentage of species interacting with the support also increases.

## 3.2. Influence of the Catalyst Acidic Function on the Final Characteristics of CZA-HPA/TiO₂ Catalysts

The combination of the HPW/Ti acid catalysts with the methanol synthesis function to form bifunctional catalysts can lead to detrimental interactions between both functionalities [68,69]. The results of the characterization of the reduced bifunctional catalysts CZA-HPW/Ti using XRD (Table 3 and Figure 10) showed that the crystalline phases of Cu⁰ and ZnO in all the catalysts were similar to those of the reference CZA catalyst, indicating the absence of significant modifications after the combination of both functionalities. However, it is known that the migration and exchange of Cu²⁺/Zn²⁺/Al³⁺ ions from the methanol synthesis catalyst with the H⁺ ions of the supported heteropolyacids can occur at the surface level without a significant alteration of the crystalline properties of the Cu and Zn phases [70–73].

The bifunctional catalysts showed a slight increase in their N₂O chemisorption capacity compared to the CZA base catalyst according to the following sequence: CZA-HPW/Ti-AR < CZA-HPW/Ti-M ≈ CZA-HPW/Ti-A-250 < CZA-HPW/Ti-A-500. This variation would be due to the migration of the surface Cu²⁺/Zn²⁺/Al³⁺ species and their exchange with the protons of the supported HPW units. However, the overall migration of Cu²⁺/Zn²⁺ species in all the reduced CZA-HPW/Ti bifunctional catalysts is not very significant, as the Cu surface area in the bifunctional catalysts is not appreciably altered compared to the value of the CZA catalyst (4 to 15%). The acidity data also showed that the relative loss of acidity of the bulk HPW units on the bifunctional catalysts was related to the contact of the HPW units with the CZA catalyst particles. This loss was determined by the relative ratio between CZA and HPW/Ti and by the accessibility of HPW units on the surface of TiO₂ supports. Thus, the different contact and access of the HPW units of the HPW/Ti-AR and HPW/Ti-A-250 catalysts with the CZA catalyst particles in the bifunctional catalysts counteracted the differences in the initial acidity presented by the acid catalysts, making both bifunctional samples present a similar acid strength.

CO conversion over the bifunctional catalysts is considered to be controlled by the methanol synthesis step, which is directly related to the Cu surface area determined by N₂O chemisorption (reduced Cu species in close contact with ZnOₓ species and/or defects
in Cu particles derived from contact with ZnO) [74]. Furthermore, syngas conversion may also be controlled by the additional effect of the water formed in methanol dehydration, which may alter the active centers and/or the kinetics of methanol formation [63]. The ratio of the CO conversion to the Cu surface area determined by N₂O chemisorption decreases according to the following sequence: CZA-HPW/Ti-AR > CZA-HPW/Ti-M > CZA-HPW/Ti-A-250 ≈ CZA-HPW/Ti-A-500. This variation in the intrinsic activity of the copper centers is associated with the modification of the Cu-ZnOₓ sites in the bifunctional catalysts by the different migration of Cu²⁺/Zn²⁺/Al³⁺ surface species and their exchange with the protons of the HPW units. However, the variation in the intrinsic activity of the Cu-ZnOₓ centers in the bifunctional catalysts is not very significant (<11%), indicating that the alteration in the active centers by the exchanges is limited. In addition to the changes in the intrinsic activity of the active centers, a slightly different level of deactivation in CO conversion is also observed in the bifunctional catalysts. The deactivation of the bifunctional samples coincides with some increase in the crystalline size of ZnO in the used catalysts (CZA-HPW/Ti-A-250 < CZA-HPW/Ti-A-500 < CZA-HPW/Ti-AR < CZA-HPW/Ti-M, Table 4). The order of the increase in the ZnO crystallite size on the used bifunctional catalysts is parallel to the level of initial DME production, suggesting that the growth of ZnO is associated with the formation of water in the reaction [75]. All the bifunctional catalysts show similar DME/methanol selectivity values, except for the CZA-HPW/Ti-M sample, which shows a lower value. Although the HPW/Ti acid catalysts exhibited differences in their acidity, as in the case of the HPW/T-AR and HPW/Ti-A-250 samples, the different contact and access of the HPW units with the CZA catalyst particles could counteract the differences in the initial acidity of the acid catalysts in the bifunctional catalysts. This fact would justify the equal DME/methanol selectivity obtained on the bifunctional samples CZA-HPW/Ti-AR and CZA-HPW/Ti-A-250 despite the differences observed in the H⁺ concentration in their acid catalysts HPW/Ti-A-R (2.54 H⁺/KU) and HPW/Ti-A-250 (1.88 H⁺/KU). Similar argumentation could justify the lower DME/methanol selectivity obtained on the CZA-HPW/Ti-M sample, in which case the higher contact and access of the HPW units with the CZA catalyst particles could lower the H⁺ concentration in the HPW/Ti-M acid catalyst (2.00 H⁺/KU) with respect to the rest of the bifunctional catalysts in the series.

The obtained results have shown that the segregation of the HPW units outside the porous structure of the TiO₂ supports facilitates the contact and cation exchange of Cu²⁺/Zn²⁺/Al³⁺ between the CZA catalyst and the HPW units in the bifunctional catalysts. To avoid this, it is essential to use supports with a pore size significantly larger than the crystallite size of the primary HPW particles in order to obtain three-dimensional structures that are dispersed inside the porous structure of the supports. In this way, we can avoid the segregation of HPW units on the surface to prevent their contact and ionic exchange with the CZA catalyst, thus inhibiting the detrimental interactions between both functionalities.

# 4. Materials and Methods

## 4.1. *Preparation of Catalysts*

### 4.1.1. TiO₂ Supports and Acid Catalysts HPW/Ti

The TiO₂ supports used (Table S2) were (a) commercial anatase (TiO₂, Alfa Aesar (Ward Hill, MA, USA), 1/8" pellets), ground and sieved (particle size < 75 µm), and calcined in static air (2 °C/min, 4 h) in a muffle up to two different temperatures, 250 and 500 °C (Ti-A-250 and Ti-A-500); (b) commercial anatase–rutile (TiO₂ Alfa Aesar, particle size 32 nm) (Ti-AR); and (c) mesoporous TiO₂ prepared by the methodology described by Gajjela et al. [76] (Ti-M). Nomenclature and characteristics of the TiO₂ supports used are shown in Table 6. The acid catalysts based on H₃PW₁₂O₄₀ (HPW) supported on the different TiO₂ supports were prepared by incipient impregnation. According to our previous study on the effect of HPW loading on TiO₂ support with an anatase–rutile structure [54], the optimum amount of HPW on the surface of that support corresponds to 2.1 theoretical monolayers (considering an area of 1.13 nm² and 21 water molecules per unit of HPW).
For HPW impregnation on the supports, commercial HPW (H₃PW₁₂O₄₀·xH₂O, reagent grade, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in a volume of ethanol (Panreac, pure) corresponding to the pore volume of the TiO₂ supports and then added dropwise and homogeneously on each of the supports. The impregnated catalyst was dried for 24 h at room temperature and finally heat-treated in a muffle in two stages: 50 °C (1 °C/min, 2 h) and subsequent heating to 150 °C (2 °C/min, 2 h).

<table><caption>**Table 6.** Nomenclature and characteristics of the TiO₂ supports used.</caption><thead><tr><th>Nomenclature</th><th>Phase</th><th>Particle Size (μm)</th><th>Calcination (°C)</th><th>S<sub>BET</sub> TiO<sub>2</sub> (m<sup>2</sup>/g)</th></tr></thead><tbody><tr><td>Ti-A-250</td><td>Anatase</td><td>&lt;75</td><td>250</td><td>138</td></tr><tr><td>Ti-A-500</td><td>Anatase</td><td>&lt;75</td><td>500</td><td>74</td></tr><tr><td>Ti-AR</td><td>Anatase-Rutile</td><td>0.032</td><td>150</td><td>48</td></tr><tr><td>Ti-M</td><td>Anatase</td><td>-</td><td>450</td><td>55</td></tr></tbody></table>

### 4.1.2. Methanol Catalyst CZA

The methanol synthesis base catalyst Cu/ZnO(Al) (CZA), with the composition Cu/Zn/Al = 68/29/3, was prepared by the co-precipitation method optimized in our laboratory [77,78].

### 4.1.3. Bifunctional CZA-HPW/Ti Catalysts

The bifunctional CZA-HPW/Ti catalysts were prepared with a CZA:HPW/Ti mass ratio of 10:1 by the physical mixing method, which consists of manually mixing the HPW/Ti and CZA catalysts with a spatula for approx. 3 min until a homogeneous mixture of both catalyst components was obtained. According to our previous study on the effect of the mixing method on bifunctional catalysts [22], this method allows optimization of the distance and the interaction between both catalyst components.

## 4.2. Physicochemical Characterization

The crystalline structures of the supports and catalysts (in the oxidized, reduced-active and used states) were studied by X-ray diffraction (XRD), with a computerized X'Pert Pro PANalytical diffractometer (Malvern Panalytical, Worcestershire, UK), using Ni-filtered Cu Kα radiation (λ = 0.15406 nm, 45 kV, 40 mA). The crystallite sizes of HPW, TiO₂, CuO, ZnO, and Cu⁰ were calculated using the Debye-Scherrer equation.

The textural properties of the supports and catalysts were obtained from the analysis of the N₂ adsorption-desorption isotherms (−196 °C) using Micromeritics ASAP 2420 automatic equipment. Previously, the samples were degassed at 120 °C for 12 h. The specific surface area was calculated from the N₂ isotherms using the BET method within the range P/P₀ = 0.05–0.30 relative pressures. The desorption data were used to determine the average pore size distribution of the samples according to the Barrett-Joyner-Halenda (BJH) method.

The structural characteristics of the acid HPW/Ti and calcined bifunctional CZA-HPW/Ti catalysts were investigated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), with a Jasco FT/IR-6300 spectrophotometer equipped with a detector that comprises Hg, Cd and Te (MCT-M detector). All the spectra were recorded at 4 cm⁻¹ resolution and averaged over 200 scans.

The hydrogen temperature-programmed reduction (H₂-TPR) of the methanol synthesis Cu/ZnO(Al) and bifunctional CZA-HPW/Ti catalysts was conducted on a semiautomatic apparatus (PID Eng&Tech) equipped with a thermal conductivity detector (TCD) and interfaced to a data station. After loading into a U-shaped quartz reactor, the sample was treated under He flow (30 mL/min) at 120 °C for 20 min. After being cooled to room temperature in the same He flow, the catalyst was reduced in a 10 vol.% H₂/Ar gas mixture (50 mL/min) from room temperature to 200 °C (2 °C/min).
The specific copper surface area of the reduced Cu/ZnO(Al) and CZA-HPW/Ti catalysts was determined by N₂O chemisorption. The reduced samples were treated at room temperature with diluted N₂O/Ar (2 vol.%, 30 mL/min) for 5 s. Then, the physisorbed N₂O was removed under an He flow (30 mL/min, 30 min), and the samples were subsequently reduced under an H₂/Ar flow (10 vol.% H₂, 50 mL/min) up to 200 °C (10 °C/min). The hydrogen consumption corresponding to the reduction is equivalent to the amount of N₂O consumed during surface chemisorption, as shown in Equations (4) and (5):

$$2\text{Cu} (\text{surface atoms}) + \text{N}_2\text{O} \rightarrow \text{N}_2 + \text{Cu}_2\text{O} \quad (4)$$

$$\text{Cu}_2\text{O} + \text{H}_2 \rightarrow 2\text{Cu} + \text{H}_2\text{O} \quad (5)$$

For the calculation of the copper's specific surface area, a molar stoichiometry of Cu(surface)/N₂O = 2 and an average value of the surface density of metallic copper of 1.47 × 10¹⁹ copper atoms/m² were assumed [79].

The total acidity of the reduced HPW/Ti acid and bifunctional CZA-HPW/Ti catalysts was determined by NH₃-TPD experiments performed on a semiautomatic apparatus (Micromeritics TPD/TPR (Norcross, GA, USA)) coupled to a quadrupole mass spectrometer (OmniStar from Balzers Instruments (Oerlikon Balzers, Pfaeffikon, Switzerland)). Prior to the chemisorption experiments, the samples were pretreated under an He flow (30 mL/min, 120 °C for 20 min) and then reduced as described above for the TPR experiments. After reduction, the sample was purged under an He flow (50 mL/min, 240 °C (2 °C/min), 40 min). After lowering the temperature to 100 °C, the sample was saturated with diluted NH₃ (5 vol.%) at 100 °C for 30 min. After chemisorption, the physisorbed NH₃ was removed under He flow (50 mL/min, 1 h) and the chemisorbed ammonia was subsequently desorbed by heat treatment from 100 to 750 °C (10 °C/min).

The solid-state NMR experiments were carried out using the NMR facility of the advanced characterization platform of the Chevreul Institute. The 1D ¹H and ³¹P solid-state NMR experiments were carried out at 9.4 T, using a 400 MHz NMR spectrometer equipped with a 4 mm HXY probe used in double resonance mode with an MAS frequency of 12.5 kHz. The 1D ¹H NMR spectra of all the samples were recorded using the DEPH sequence. The radiofrequency (rf) nutation frequency of the ¹H channel was 90 kHz. The 1D ¹H NMR spectra resulted from averaging 16 transients with a recovery delay of 5 s. The ¹H isotropic chemical shifts were externally referenced to tetramethylsilane (TMS) (0 ppm) using the resonance of adamantane at 1.8 ppm as a secondary reference [80]. For the samples containing heteropolyacid, the 1D ³¹P NMR spectra were acquired using a single pulse sequence of the ³¹P channel, whereas ¹H decoupling was applied during the acquisition at room temperature. The rf nutation frequency of the ³¹P channel was 58 kHz. The 1D ³¹P NMR spectra resulted from averaging 16 transients with a recovery delay of 1500 s. The T1 values of ³¹P nuclei were measured using a saturation-recovery experiment. The 1D ³¹P NMR signal was integrated between -13 and -18 ppm and the build-up of the integrated intensity was simulated using Topspin software (https://www.bruker.com/en/products-and-solutions/mr/nmr-software.html (accessed on 1 July 2024)). The ³¹P isotropic chemical shifts were externally referenced to aqueous 85% H₃PO₄ solution (0 ppm) at room temperature.

The X-ray photoelectron spectroscopy experiments were performed on a VG Escalab 200R electronic spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a hemispherical electron analyzer, using a MgKα X-ray source (hν = 1253.6 eV, 1 eV = 1.603 × 10⁻¹⁹ J). After degassing at 10⁻⁶ mbar, the samples were transferred to the ion-pumped analysis chamber, where the residual pressure was kept below 4 × 10⁻⁹ mbar during the data acquisition. The binding energy (BE) of the C 1s peak at 284.6 eV was taken as the internal standard. The accuracy of the BE values was ±0.1 eV. The peak areas were estimated by calculating the integral of each peak after subtracting a Shirley baseline and fitting the experimental peak to a combination of Lorentzian/Gaussian curves in varying proportions, using the spectrum processing software and choosing the best
fit by minimization of the χ² function. Quantitative determination of the surface atomic composition (relative concentration of different atomic species) was performed by applying the atomic sensitivity factors [81].

### 4.3. Catalytic Tests

The catalyst activity was evaluated in a fixed-bed reactor (8.9 mm i.d.) with the bifunctional CZA-HPW/Ti catalysts diluted with SiC (1:3 vol.) to improve the thermal behavior of the catalytic bed. The final bifunctional catalysts were obtained after the activation step, consisting of thermal reduction under a flow rate of hydrogen diluted in Ar (2.2% vol. H₂) following the temperature schedule: 150 °C (2 °C/min) and 200 °C (1 °C/min, 2 h). After reduction, the syngas was fed to the reactor (75 mL/min, composition (mol %): 4.5% CO₂, 22.0% CO, 58.8% H₂, 14.7% N₂) to pressurize it up to 30 bar. The catalytic tests were conducted under high-space-velocity (GHSV = 30 Lsyngas gcat⁻¹ h⁻¹) to achieve low conversions, to exclude the effect of equilibrium, and to better visualize the differences in deactivation. The activity was measured at 250 °C and 30 bar, maintaining the reaction for 6 h after a catalyst stabilization time under syngas feed of 1 h. The reaction products were analyzed by gas chromatography on a Varian 450-GC apparatus (Varian, Palo Alto, CA, USA) equipped with a TCD detector and capillary columns connected in series (RESTEK-QS-bond: CO₂, CH₃OH, DME, and H₂O; and molecular sieve 5A: H₂, O₂, N₂ and CO). The activity results were reported according to the following Equations (6)–(11), where M is the molar flow rate of each component:

$$ \text{CO conversion: } X_{\text{COx}} (\%) = \frac{M_{\text{CO(i)}} - M_{\text{CO(f)}}}{M_{\text{COx(i)}}} \cdot 100 \quad (6) $$

$$ \text{DME selectivity: } S_{\text{DME}} (\%) = \frac{M_{\text{DME}}}{\sum M_{\text{products}}} \cdot 100 \quad (7) $$

$$ \text{Methanol selectivity: } S_{\text{Methanol}} (\%) = \frac{M_{\text{Methanol}}}{\sum M_{\text{products}}} \cdot 100 \quad (8) $$

$$ \text{CO₂ selectivity: } S_{\text{CO₂}} (\%) = \frac{M_{\text{CO₂}}}{\sum M_{\text{products}}} \cdot 100 \quad (9) $$

$$ \text{DME production: } P_{\text{DME}} = \frac{M_{\text{DME}} (\mu\text{mol})}{m_{\text{cat}} (\text{g}) \cdot \text{t (min)}} \quad (10) $$

$$ \text{Methanol production: } P_{\text{Methanol}} = \frac{M_{\text{Methanol}} (\mu\text{mol})}{m_{\text{cat}} (\text{g}) \cdot \text{t (min)}} \quad (11) $$

### 5. Conclusions

Based on the results obtained concerning the effect of the textural properties of the TiO₂ support on the dispersion of the HPW heteropolyacid and on the physicochemical characteristics of the bifunctional catalysts and their behavior in the direct synthesis reaction of DME from syngas, the following conclusions are drawn:

* The pore size distribution of the TiO₂ supports affects the surface segregation of the HPW, which increases with the decreasing volume and available surface area of the support.

* The ratio between the dispersed/bulk species of supported HPW increases as the surface area of the TiO₂ supports increases.

* Differences in the interaction and degree of structuring of the HPW units on the support explain the differences in their acidity: HPW/Ti-AR > HPW/Ti-M > HPW/Ti-A-250 ≈ HPW/Ti-A-500.

* Characterization of the bifunctional catalysts has shown that changes in the acid catalysts do not significantly alter the crystal structure of the CuO and ZnO phases after contact.

* There is a slight increase in the copper surface area of the bifunctional catalysts with respect to the CZA base catalyst due to the migration of Cu²⁺/Zn²⁺/Al³⁺ surface species and their exchange with the protons of the supported HPW units. However, the overall cation migration on all the reduced bifunctional catalysts does not occur to a large extent.
* The variation in the intrinsic activity of the Cu-ZnOₓ centers in the bifunctional catalysts is not very significant as a consequence of the limited alteration of the active centers by the contacts between the CZA catalyst and the HPW units of the acid catalysts.

* All the bifunctional catalysts present similar DME/methanol selectivity values, except for the CZA-HPW/Ti-M sample, which presents a lower value. The different contact and access of the HPW units with the CZA catalyst particles in the bifunctional catalysts counteracts the differences in the initial acidity presented by the acid catalysts, resulting in similar values of acid strength in the bifunctional catalysts.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14070435/s1, Figure S1: N₂ adsorption-desorption isotherms (A) and pore size distribution (B) of TiO₂ supports; Table S1: Crystallite size of the anatase and rutile phases of TiO₂ supports; Table S2: Textural properties (S<sub>BET</sub>, total volume, and pore diameter) of the TiO₂ supports calculated from the nitrogen adsorption-desorption isotherms; Table S3: Absolute and relative intensity (with respect to HPW loading), and crystallite size of the HPWx6H₂O phase in HPW/Ti acid catalysts determined by XRD; Table S4: Textural properties (BET surface and micropore (t-plot)) of HPW/Ti acid catalysts calculated from N₂ adsorption-desorption isotherms; Table S5: Best fit parameters Quantitative data obtained from longitudinal relaxation time (T1 and w of ³¹P nuclei f HPW (bulk) and HPW/Ti acid catalysts.

**Author Contributions:** Conceptualization, methodology and supervision, R.M.N.Y. and N.M.T.; investigation, writing—original draft preparation and writing—review and editing, R.M.N.Y., N.M.T., B.R., O.L. and E.M.O.; visualization, E.M.O.; project administration and funding acquisition, R.M.N.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the financial support provided by the Spanish Ministry of Science, Innovation and Universities through the research programs PID2022-1434800b-I00 and PID2019-111219RB-100. Elena Millán (BES-2017-080240) would like to acknowledge the FPI program of the Spanish Ministry of Science, Innovation and Universities for the research grant. The Chevreul Institute is thanked for supporting CPER projects funded by the "Ministère de l'Enseignement Supérieur et de la Recherche", the region "Hauts-de-France", the ERDF program of the European Union and the "Métropole Européenne de Lille".

**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.

**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

**Abbreviations**

<table><tr><td>CZA</td><td>Cu-ZnO(Al)</td></tr><tr><td>HPW</td><td>Tungstophosphoric acid</td></tr><tr><td>HPW/Ti</td><td>Tungstophosphoric acid supported on titania</td></tr><tr><td>Ti</td><td>Titania</td></tr><tr><td>Ti-A</td><td>Titania anatase</td></tr><tr><td>Ti-R</td><td>Titania rutile</td></tr><tr><td>Ti-M</td><td>Mesoporous titania</td></tr><tr><td>Ti-A-250</td><td>Titania anatase calcined at 250 °C</td></tr><tr><td>Ti-A-500</td><td>Titania anatase calcined at 500 °C</td></tr><tr><td>MCT</td><td>FTIR detector that comprises mercury, cadmium, and tellurium</td></tr><tr><td>ATR-FTIR</td><td>Attenuated total reflectance Fourier transform infrared spectroscopy</td></tr><tr><td>GHSV</td><td>Gas hourly space velocity</td></tr></table>
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