The Effect of Different Polyol Precursors on Disordered Spinel ZnAl2O4 Structure Prepared by the Polymeric Citrate Complex Method and the Corresponding Catalytic Behavior in the Glycerolysis of Urea

ZnAl2O4 is a cubic close‐packed oxide with tetrahedral and octahedral sites in the lattice structure. The catalytic activity of ZnAl2O4 is strongly connected with the surface properties caused by the partially inverse spinel ZnAl2O4 structure. In this research, ZnAl2O4 catalysts are prepared for the glycerolysis of urea via the polymeric citrate complex method using polymeric templates made of citric acid with a different polyol precursor. Depending on the polyol precursor, the partial inversion parameters representing the disordered bulk ZnAl2O4 structure are controlled, which results in different surface AlO4/AlO6 ratios and surface acidity. The specific surface areas of the prepared ZnAl2O4 catalysts are proportional to the molecular weight of the polyol precursor. The s–ZnAl2O4 (polyol precursor = sorbitol) prepared via two pathways from citrate complex and polymeric citrate complex shows the highest inversion parameter and surface acidity, leading to the highest glycerol carbonate (GC) yield and glycerol conversion in the glycerolysis of urea. A relationship between the GC yield and the surface properties, such as the acidity and inversion parameters, is established.

their surface properties. [12]In the partially inverse ZnAl 2 O 4 , some Zn 2þ cations substituted for Al 3þ cations in the octahedral sites, while some Al 3þ cations substituted for Zn 2þ cations in the tetrahedral sites.7e] In addition, employing polyols enables the conversion of the metal mixture into a covalent polymer network, effectively entrapping the metal.This approach facilitates a uniform dispersion of metals within the polymer network, as it allows for controlled growth.Subsequently, the gradual decomposition of the polymeric matrix provides enhanced control over the formation of the material. [13]To the best of our knowledge, despite numerous studies on preparing the spinel structure of ZnAl 2 O 4 , no previous research has explored the impact of different polyol precursors on the formation of the disordered spinel structure of ZnAl 2 O 4 catalyst, the corresponding alteration in the surface acidity, and the resultant catalytic activity in the glycerolysis of urea reaction.We believe that the catalytic performances of ZnAl 2 O 4 catalysts can be effectively controlled by manipulating the surface acidity through the interplay between the disordered ZnAl 2 O 4 structure and its influence on catalytic behavior.
In this study, we prepared disordered ZnAl 2 O 4 spinel catalysts via the polymeric citrate complex method using different polyol precursors.These catalysts were subsequently calcined at 550 °C to examine the impact of different polyol precursors on the formation of the disordered ZnAl 2 O 4 and the corresponding alteration in surface acidity, which was evidenced by X-Ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-Ray photoelectron spectroscopy (XPS), and NH 3 -temperature programmed desorption (TPD).The glycerolysis of urea for the prepared catalysts was conducted at 140 °C to measure their catalytic activity, and found that the polyol precursors strongly influenced the disordered ZnAl 2 O 4 structure, the surface acidity of the catalysts, and the catalytic behavior in the glycerolysis of urea.Finally, we confirmed the relationship between the inversion parameter in the ZnAl 2 O 4 structure, the surface acidity, and the glycerol conversion.

Characterization of Fresh Catalysts
Table 1 summarizes the results of the N 2 adsorption-desorption measurements that were used to evaluate the textural properties of the fresh catalysts.Figure S1, Supporting Information, plots the corresponding N 2 adsorption-desorption isotherms and pore size distributions, where all the ZnAl 2 O 4 catalysts exhibited a type IV isotherm and type H3 hysteresis loops, according to the IUPAC classification. [14]This behavior is typical for mesoporous materials in the presence of slit-shaped pores. [15]Furthermore, it was observed that as the length of the polyol precursor increased, the hysteresis loop became wider, accompanied by a higher Brunauer-Emmett-Teller (BET) specific surface area (S BET ) and a higher average pore volume.The order of these properties was found to be i-ZnAl 2 O 4 < eg-ZnAl 2 O 4 < pg200-ZnAl 2 O 4 < gl-ZnAl 2 O 4 < s-ZnAl 2 O 4 (i: without polyol, eg: ethylene glycol, pg200: polyethylene glycol 200, gl: glycerol, and s: sorbitol).Conversely, the average pore diameter showed the opposite trend.As anticipated, the S BET increased with the length of the polyol precursor.This could be explained by the presence of more interconnected voids in the gel network formed by longer-length polyols, as opposed to larger voids. [15]These findings are in good agreement with the experimental results presented in Table 1 and Figure S1B, Supporting Information.Specifically, it can be observed that the s-ZnAl 2 O 4 exhibited numerous small-sized holes, while the eg-ZnAl 2 O 4 displayed a smaller number of larger-sized holes (Figure S1B, Supporting Information).
Figure 1 presents the XRD patterns of the fresh catalysts.Typical peaks were observed at positions of 31.2°,36.8°,44.8°, 49.1°, 55.7°, 59.3°, and 65.2°, corresponding to the ZnAl 2 O 4 phase with the space group Fd3m (JCPDS No. 05-0669). [16]hese characteristic diffraction peaks associated with the ZnAl 2 O 4 phase were evident in the XRD patterns of all the fresh catalysts synthesized using different polyol precursors.This observation served as evidence supporting the successful synthesis of spinel ZnAl 2 O 4 through the polymeric citrate complex method, with the incorporation of polyols.In addition, the XRD analysis revealed the presence of characteristic peaks at positions 31.8°,34.4°, 36.3°,47.5°, 56.7°, and 62.9°, associated with the ZnO phase with the space group P63mc (JCPDS No. 36-1451). [17]Interestingly, the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 samples exhibited shoulder peaks at 31.8°and 36.3°, in comparison to the dominant peaks of the ZnAl 2 O 4 phase appearing at 31.2°and 36.8°(Figure1B).This behavior differed from that observed for the i-, eg-, and pg200-ZnAl 2 O 4 samples, where there was minimal formation of the ZnO secondary phase during the synthesis process.
The morphological structures of the ZnAl 2 O 4 spinel catalysts were monitored via cross-sectional transmission electron microscopy (CS-TEM) (Figure S2, Supporting Information).The particle size distributions and average sizes (d avg. ) clearly show the formation and homogeneous distribution of ZnAl 2 O 4 phase even though the different polyol precursors are used in the synthesis.The average sizes calculated from Figure S2 (Supporting  Measured via N 2 isothermal adsorption-desorption at 77 K; surface area was calculated with the BET theory; pore volume and pore size were obtained by the Barett-Joyner-Halenda method.Information) are similar to the values from the XRD results (Table 1).

The Influence of Polyol Precursors on the Formation of Partially Disordered ZnAl 2 O 4 Spinel Structure
In spinel structures with normal ordering, the intensity of odd reflections, such as (331), is typically higher than that of even reflections, such as (400).Figure 2A shows enlarged XRD patterns in the range of 44°-50°, displaying changes in the peak intensities of the (400) and (331) planes obtained from the use of different polyol precursors during the preparation process.These peak intensities were observed to undergo significant changes, which could be used to compare the degree of spinel disorder.The degree of the order parameter in the spinel samples was determined using the following equation. [12,18]der parameter ¼ where I o represents the intensity corresponding to odd reflections and I e represents the intensity corresponding to even reflections.The presence of a disordered spinel phase can occur when a fraction of Al 3þ cations occupies the tetrahedral site, Zn 2þ occupies the octahedral site, or the formation of secondary phase. [19] Figure 2B presents a variation in lattice parameters resulting from the utilization of different polyol precursors.An increase in the lattice parameter, particularly in the Al 3þ parameter, can be attributed to the distribution of cations between the tetrahedral and octahedral sites.The synthesized powders exhibited a higher degree of disorder in the distribution of cations, leading to an increase in configurational entropy.This increase in the configurational entropy promoted the formation of localized clusters, which subsequently reduced the lattice parameter. [18]In Table 1, the lattice parameter values followed the order: s-ZnAl 2 O 4 (8.0846)< gl-ZnAl 2 O 4 (8.0861)< eg-ZnAl 2 O 4 (8.0873)< pg200-ZnAl 2 O 4 (8.0910)< i-ZnAl 2 O 4 (8.1058).These values prove that s-ZnAl 2 O 4 and gl-ZnAl 2 O 4 have a more disordered structure than the other catalysts, which is consistent with the results mentioned earlier (Figure 2B).
Figure 3A shows the FTIR spectra of the fresh catalysts.The two vibration bands observed at 661 and 554 cm À1 were attributed to the stretching mode of octahedrally coordinated Al-O (AlO 6 ).Additionally, the peak at 495 cm À1 corresponded to the bending mode of AlO 6 .7e,20] Figure 3B shows the enlarged spectra from 600 to 1000 cm À1 focusing on the vibration modes of AlO 4 and AlO 6 .The relative intensities of AlO 4 and AlO 6 increased in the order: i-ZnAl 2 O 4 < pg200-ZnAl 2 O 4 < eg-ZnAl 2 O 4 < gl-ZnAl 2 O 4 < s-ZnAl 2 O 4 .The shoulder of the AlO 4 vibration in the s-ZnAl 2 O 4 and gl-ZnAl 2 O 4 samples was notably prominent and observable.This observation suggests a significant presence of tetrahedrally coordinated AlO 4 , indicating a partial inversion of the ZnAl 2 O 4 spinel structure.This finding from the FTIR analysis was also consistent with the inversion parameter obtained from XPS data, which will be further described later.The ZnAl 2 O 4 spinel structure normally consists of Zn 2þ cations at tetrahedral sites and Al 3þ cations at octahedral sites.However, in a partially inverse spinel structure, there is a disorder with Al 3þ cations occupying tetrahedral sites and Zn 2þ cations occupying octahedral sites.This disorder can extend to the catalyst surface.The inversion parameter, defined by Duan et al. [21] quantifies this inversion, and can be estimated using XPS measurements.The parameter represents the ratio of tetrahedrally coordinated Al 3þ (AlO 4 ) to the total Al 3þ (AlO 6 þ AlO 4 ) and the ratio of octahedrally coordinated Zn 2þ (ZnO 6 ) to the total Zn 2þ (ZnO 4 þ ZnO 6 ) on the sample surface.Table 2 displays the inversion parameters for all the catalysts, obtained from the XPS data, indicating the extent of disorder in the spinel structure.
Figure 4 presents the XPS results of Al 2p, O 1s, and Zn 2p 3/2 , along with their corresponding deconvoluted peaks, for all the fresh catalysts.Figure 4A shows the Al 2p XPS data.The peak at a binding energy of 71.0 eV was attributed to Al 3þ occupying tetrahedral sites (AlO 4 ), [22] while the peak around 74.2 eV was assigned to Al 3þ occupying octahedral sites (AlO 6 ). [22,23]In the Zn 2p 3/2 spectra, as depicted in Figure 4B, the larger peak at a lower binding energy (approximately 1022-1022.2eV) corresponded to Zn 2þ occupying tetrahedral sites (ZnO 4 ), while the smaller peak at a higher binding energy (around 1024.5 eV) was assigned to Zn 2þ occupying octahedral sites (ZnO 6 ). [21,22,24]The deconvolution of the O 1s peaks resulted in four distinct components.The peak labeled O a ,  appearing at the highest binding energy (around 543.5-534.87t,7u] The peak in the range of 531.4-531.7 eV was attributed to lattice oxygens in the ZnAl 2 O 4 phase (O c ).Finally, the peak at the lowest binding energy around 530.4-530.7 eV corresponded to lattice oxygen in the ZnO phase (O d ).The inversion parameter values followed the order: s which was consistent with the XRD and FTIR data mentioned earlier.It was demonstrated that the s-ZnAl 2 O 4 and gl-ZnAl 2 O 4 catalysts contained a higher degree of disorder, as indicated by their higher inversion parameter values.This disorder could be attributed to the presence of oxygen vacancies resulting from the substitution of Zn 2þ with Al 3þ to create Zn 2þ octahedral and Al 3þ tetrahedral sites, thus maintaining charge balance. [7e, 12,25] The number of oxygen vacancies (O v ) was estimated using the O b peak intensity derived from the deconvoluted results of O 1s XPS, which Table 2 lists.These values exhibited the same trend in the inversion parameter values.The values from XPS partly showed that the polyol precursors in the synthesis had a significant influence on the disordered structure of spinel ZnAl 2 O 4 .In particular, the use of polyols of longer length and with more hydroxyl groups next to each other increased the degree of disorder of the catalyst.
To investigate the presence of AlO 4 in the bulk ZnAl 2 O 4 , 27 Al nuclear magnetic resonance (NMR) measurements were performed.Figure S3, Supporting Information, shows that the NMR spectra exhibited four deconvoluted peaks in different chemical shift regions.The peaks in the range À12-10 ppm were attributed to Al 3þ tetrahedral positions (α 1 -α 3 ), while the peak in Intensity ratios of the AlO 4 (tetrahedral site) and AlO 6 (octahedral site) and intensity ratios of the ZnO 6 (octahedral site) and ZnO 4 (tetrahedral site).b) Inversion parameter NMR were obtained from the deconvolution of NMR 27 Al.  the range 23-25 ppm corresponded to penta-coordinated Al 3þ positions (β).Additionally, the peak in the range 70-82 ppm was assigned to Al 3þ octahedral sites (γ). [26]Comparing the bulk inversion parameter value (AlO 4 /(AlO 4 þ AlO 6 )) with the surface inversion parameter values obtained from the deconvolution of XPS, the bulk inversion parameter values were relatively small.This suggests that a majority of the disordered AlO 4 sites were located at the surface of the catalyst, rather than in the bulk structure.
Figure S4 (Supporting Information) shows Raman spectra of the prepared samples.A pronounced and intense peak at 660 cm À1 is ascribed to the high-frequency F 2g vibration mode.The next prominent peak at 420 cm À1 corresponds to the E g mode of vibration. [18,27]The principal Raman mode for the ZnAl 2 O 4 crystal structure is the high-frequency F 2g mode.In this material, the high-frequency mode at 660 cm À1 arises from the movement of oxygen atoms within the AlO 6 octahedral structure. [18,27,28]Any disordered material defects are usually discerned by observing some asymmetry in the peak positions. [28,29] strong peak at 720 cm À1 is identified as A* 1g .This peak serves as an indicator of structural disorder, arising from the vibrational motion of AlO 4 units associated with their breathing mode.[18] In Figure S4 (Supporting Information), the peaks corresponding to the F 2g and E g modes for the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 catalysts are asymmetric, indicating the presence of imperfections within the materials.That is, with the use of glycerol and sorbitol in the catalytic synthesis of ZnAl 2 O 4 , the A* 1g band at 720 cm À1 demonstrates the presence of disordered AlO 4 sites.This band almost does not appear in the ZnAl 2 O 4 -ordered structure of i-ZnAl 2 O 4 and pg200-ZnAl 2 O 4 .
Scheme 1 depicts the pathways involved in the formation of ZnAl 2 O 4 via the polymeric citrate complex method, utilizing different polyols.The complex citrate method, specifically the Pechini method utilizing polyols, is commonly used to synthesize ZnAl 2 O 4 spinel.Polyols convert the mixture into a covalent polymer network, allowing controlled material formation.13a,30] In this study, we employed various polyols with differing lengths and numbers of hydroxyl groups.The first group comprised linear polyols, including eg and pg200, which had two hydroxyl groups.The second group consisted of polyols with multiple adjacent hydroxyl groups, such as gl and sorbitol.
Scheme 1. Schematic of the pathways for the formation of ZnAl 2 O 4 by polymeric citrate complexes using different polyols.
In the absence of polyols during the synthesis, the formation of ZnAl 2 O 4 occurred solely through pathway 1, which involved the decomposition of the metallic citrate precursor complex. [31]In the synthesis method utilizing eg, apart from the formation pathway involving the metal citrate complex, an esterification process occurred to produce the polymeric citrate complex through pathway 2. Although both eg and pg200 were linear polyols, pg200 had a longer chain length.The polymeric citrate complex formed from pg200 had a bulkier structure.Consequently, the formation of ZnAl 2 O 4 was more challenging for pg200-derived polymeric citrate complex, due to an increase in difficulty caused by the bulkier structure during the calcination step.Therefore, the disorder of the pg200-ZnAl 2 O 4 sample was similar to that of the i-ZnAl 2 O 4 sample, because most of the ZnAl 2 O 4 formation follows the pathway 1 shown in Scheme 1.To investigate the influence of hydroxyl group number and arrangement, gl and sorbitol were utilized as polyol precursors in the Pechini method.In the case of gl, which contained three adjacent hydroxyl groups, the formation of polymeric citrate complexes via pathway 2 was comparable to eg.However, due to the presence of two possible binding sites between the carboxyl and hydroxyl groups (i.e., positions 1-2 and 1-3), multiple species of polymeric citrate complexes with diverse structures were formed, which were distinct from those obtained with eg.Furthermore, sorbitol, characterized by six contiguous hydroxyl groups, resulted in the formation of numerous polymeric citrate complex variants exhibiting varying degrees of structural bulkiness.These structural differences significantly impacted the extent of structural disorder observed in the ZnAl 2 O 4 spinel following the calcination process.
To substantiate the claims, we conducted a survey of the decomposition temperature and ZnAl 2 O 4 formation process using the TGA experiment.Figure S5, Supporting Information, presents the derivative thermogravimetric analysis (DTGA).The first decomposition stage occurred at a lower temperature range, typically below 200 °C, corresponding to the evaporation of surface water present in the ZnAl 2 O 4 xerogel precursors.The second decomposition stage, occurring around 150-250 °C (α peak), was attributed to the evaporation of structural water, which resulted from the decomposition of the acid citric component.The third stage, spanning from 250 to 420 °C (β peak), primarily involved the removal of the organic constituents present in the precursors.These organic components, existing in a polymeric network with a rigid structure, facilitated the homogeneous distribution of metal ions.The final decomposition stage, observed in the range of 420-600 °C (γ peak), was mainly associated with either carbonate decomposition or spinel formation. [32]The DTGA patterns provided valuable insights into the synthesis pathways and resulting structures of the ZnAl 2 O 4 catalysts (Figure S5, Supporting Information).In i-ZnAl 2 O 4 , where polyol was not utilized, a single peak (α peak) was observed, indicating the formation of a metallic citrate complex solely through pathway 1.Additionally, a significant presence of free citric acid was noted.For the eg-ZnAl 2 O 4 and pg200-ZnAl 2 O 4 samples, the appearance of β peak confirms the formation of polymeric citrate complexes via pathway 2. The intensity of the γ peak for pg200-ZnAl 2 O 4 was comparatively lower than that for eg-ZnAl 2 O 4 , suggesting a more challenging ZnAl 2 O 4 formation process through pathway 2 for pg200-ZnAl 2 O 4 .In the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 samples, the broader and overlapping peaks of β peak indicated the formation of diverse polymeric species with varying levels of structural bulkiness.Interestingly, the intensity of the γ peak for s-ZnAl 2 O 4 was higher than that for gl-ZnAl 2 O 4 , implying the significant influence of the six adjacent hydroxyl groups on the formation of the ZnAl 2 O 4 spinel.Furthermore, the shift of the γ peak to higher temperature (approximately 550 °C) necessitated a higher calcination temperature to achieve a normally ordered structure.This observation proved that when subjected to calcination at 550 °C, s-ZnAl 2 O 4 exhibited a more disordered structure, compared to the other catalysts.

Relationship between the Disordered Spinel Structure and the Surface Acidity
The presence of ZnO 6 and AlO 4 sites in the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 catalysts provides evidence for the partially inverse spinel structure of ZnAl 2 O 4 .The inversion parameter, calculated as AlO 4 /(AlO 4 þ AlO 6 ) from the XPS and 27 Al-NMR measurements, along with the order parameter from the XRD data and the intensities of AlO 4 shoulder peaks from FTIR, confirmed the degree of disorder in the ZnAl 2 O 4 spinel structure.The consistent trend observed in the inversion parameter, the order parameter, and the AlO 4 shoulder peak intensities demonstrated a significant formation of the partially inverse spinel structure in the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 catalysts.This inverse spinel structure contributed to surface acidity, as indicated by the XPS intensity of O a (H 2 O or O 2 adsorbed from the atmosphere).Notably, gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 exhibited higher O a peak intensities, signifying a greater structural disorder.These results emphasized the influence of the partially inverse spinel structure on the surface properties and disorder in the gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 catalysts, supported by the inversion parameter, the order parameter, and the AlO 4 shoulder peak intensities.
Figure S6, Supporting Information, shows the NH 3 -TPD profiles for the fresh catalysts, while Table 2 summarizes the acidic sites.The number of acidic sites followed the order: pg200- Figure 5A clearly demonstrates the correlation between the surface acidity of the catalysts and the inversion parameters.In the normally ordered ZnAl 2 O 4 spinel structure, Al 3þ occupies octahedral sites (AlO 6 ) characterized by lower acidity.7e,12] The higher Lewis acidity of AlO 4 compared to AlO 6 can be attributed to the lower energy acceptor orbital of the AlO 4 sites.When the AlO 4 sites come into contact with the catalyst surface, the removal of one of the four oxygen atoms results in the formation of three-coordinated Al, which exhibits stronger Lewis acidity. [33]he presence of AlO 4 sites is associated with medium, mediumstrong, and strong surface acidic sites, whereas AlO 6 sites are connected to weakly acidic sites [34] (Scheme 2).In this study, the positive correlation between the inversion parameter AlO 4 /(AlO 4 þ AlO 6 ) and the acidic sites confirmed that an increase in the degree of disorder resulted in a higher abundance of medium, medium-strong, and strong acidic sites.Consequently, the pronounced disorder in the spinel ZnAl 2 O 4 structure, as observed in gl-ZnAl 2 O 4 and s-ZnAl 2 O 4 , led to the formation of a greater number of strong acidic sites on the catalyst surface, compared to the other catalysts.
The increased surface acidity can also be attributed to the presence of oxygen vacancy sites.Substitution of Zn 2þ cations with Al 3þ cations in the octahedral position results in the generation of oxygen vacancies to maintain charge neutrality, which contributes to enhanced surface acidity.Figure 5B illustrates the relationship between the inversion parameter ZnO 6 /(ZnO 4 þ ZnO 6 ), and the O1s XPS intensity associated with oxygen vacancies.The concurrent increase in the inversion parameter and the oxygen vacancies, as determined from the XPS data, indicates an elevated level of disorder in the spinel ZnAl 2 O 4 structure.This suggests a greater substitution of Zn 2þ for Al 3þ in the octahedral positions, leading to the formation of additional oxygen vacancy sites.The formation of oxygen vacancies positively influences the surface acidity of the catalysts, as supported by the presence of surface hydroxyl groups observed in the XPS O a peaks and the OH vibration peaks in the range of 3000-3600 cm À1 in the FTIR spectra of the fresh catalysts [35] (Figure S7, Supporting Information).
Scheme 2 illustrates the conceptual structure of the partially inverse ZnAl 2 O 4 catalysts.This scheme conceptualizes the distinction between a normally ordered spinel lattice structure and a partially inverse spinel lattice structure resulting from the substitution of Zn 2þ and Al 3þ positions.In a normally ordered structure (Scheme 2A), the ZnAl 2 O 4 spinel comprises Zn 2þ cations and Al 3þ cations occupying tetrahedral and octahedral positions, respectively.In the partially inverse spinel structure (Scheme 2B), some Zn 2þ sites substitute Al 3þ in the octahedral position, resulting in the presence of octahedrally coordinated Zn 2þ (ZnO 6 , represented as Zn* in Scheme 2), tetrahedrally coordinated Al 3þ (AlO 4 , represented as Al* in Scheme 2), and oxygen vacancy (represented as O v in Scheme 2).This structural arrangement highlights the presence of AlO 4 sites, ZnO 6 sites, and oxygen vacancies, which collectively contribute to the partial inversion of the ZnAl 2 O 4 catalysts.The introduction of ZnO 6 sites and AlO 4 sites through the partial inversion of the spinel structure enhances the structural disorder and promotes the formation of surface acidity.The surface AlO 4 sites are connected with (ii) medium, (iii) medium-strong, and (iv) strong acidic sites in Scheme 2. The more disordered the spinel ZnAl 2 O 4 structure, the more the AlO 4 disordered sites, leading to an increase in the number of strong acid sites on the catalyst surface.The presence of oxygen vacancies further influences the surface properties, resulting in increased surface acidity and the formation of surface hydroxyl groups.These structural features play a crucial role in dictating the catalytic performance of the ZnAl 2 O 4 catalysts.

Catalytic Activity
Table 3 summarizes the reaction results of the glycerolysis of urea for all the catalysts.In addition to the main product, GC, by-products, such as chemicals (2), (3), and (5), were also formed during the reaction.Figure 6 shows the relationship between acidic sites and GC yield, as well as the number of basic sites (Figure S8, Supporting Information), and the selectivity toward by-product (2) for each catalyst.After a reaction time of 5 h, the s-ZnAl 2 O 4 catalyst exhibited superior performance in terms of glycerol conversion, GC yield, and GC selectivity, achieving the values of 78%, 41%, and 53%, respectively.The catalyst efficiency in the glycerolysis of urea followed the order: pg200- The activity level of the catalysts correlated with their degree of disorder and surface acidic sites, as evident in Figure 6.Increased disorder in the ZnAl 2 O 4 spinel structure led to higher surface acidity, thereby contributing to enhanced GC yield in the glycerolysis of urea reaction.
Table 3 shows that the surface basicity of the fresh catalysts exhibited a similar trend to the surface acidity.7j] The presence of oxygen vacancies in the disordered spinel ZnAl 2 O 4 structure could lead to an increased electronic density on the M δþ -O 2À sites. [36]Consequently, the s-ZnAl 2 O 4 catalyst contained the highest acidity and basicity, due to its disordered spinel structure.Moreover, Figure 6 shows the relationship between the number of basic sites and the (2) selectivity.An inverse relationship between the number of basic sites and the formation of the (2) by-product was clearly shown.A lower number of basic sites on the i-ZnAl 2 O 4 and pg200-ZnAl 2 O 4 catalysts resulted in an increase in the formation of (2) by-product, which adversely affected the yield of the main product GC.The results presented in Table 3 and Figure 6 demonstrate the positive impact of the basic sites on the GC yield by reducing the formation of by-product (2) during the glycerolysis of urea.The presence of a higher number of basic sites limited the formation of by-product (2), and thereby enhanced the production of the desired main product GC.
According to Yuqiao Li et al. [1] Zn possesses the capability of activating glycerol through the formation of Zn-GC, making Zn-based catalysts suitable for the alcoholysis reaction of urea with glycerol.In the reaction, the Lewis acid sites of Zn 2þ can coordinate with the Lewis basic carboxyl group in the urea molecule, thereby facilitating the reaction.Moreover, the presence of suitable Lewis acid-base sites on the surface of the Zn catalyst plays a crucial role in enhancing the reaction rate.Furthermore, Fernandes et al. [37] studied the simultaneous influence of acid and basic sites on the efficiency of glycerolysis of urea.The results show a positive effect of the number of acid and basic sites on GC yield, and limit the formation of by-product (2).Based on the information, the mechanism of the glycerolysis reaction on the x-ZnAl 2 O 4 catalysts could be described as follows (Scheme 3): initially, glycerol molecules diffuse from the bulk phase, and adsorb onto the basic catalytic sites (B site) on the catalyst surface.Proton abstraction takes place from the hydroxyl group of glycerol, leading to the formation of the glyceroxide anion.The highly reactive glyceroxide anion attacks the carbonyl carbon of urea on the acidic sites (A site), resulting in the formation of intermediate (2) (by-product (2)).Subsequently, another proton is abstracted from the secondary alcohol group of glycerol in the (2) by-product.This proton then attacks the carbonyl carbon of urea, releasing an additional ammonia molecule.This entire process occurs on the basic sites of the catalysts.Finally, a cyclic reaction occurs, leading to the production of GC.
Based on Scheme 3, the increased degree of disordered ZnAl 2 O 4 structure facilitates the formation of more acidic sites, which serve as active sites for the urea molecule in the reaction.Furthermore, the increased disordered ZnAl 2 O 4 structure of the catalyst also leads to a higher number of basic sites, which act as active sites for glycerol in the reaction.These basic sites both promote the degradation of intermediate (2), and contribute to the formation of the main product, GC.Therefore, the enhanced disordered ZnAl 2 O 4 structure of the s-ZnAl 2 O 4 catalyst positively influences both the acidic and basic sites, which play crucial roles in facilitating the glycerolysis of urea reaction and the subsequent formation of GC, resulting in the highest GC yield.

Conclusion
The catalytic activity of ZnAl 2 O 4 catalysts for the glycerolysis of urea was strongly influenced by the surface properties resulting from the partially inverse spinel structure.The use of polyol precursors with long lengths and high numbers of side-by-side hydroxyl groups in the synthesis process contributed to an increase of the disordered ZnAl 2 O 4 spinel structure.The disorder degree in the disordered ZnAl 2 O 4 spinel structure was characterized by the inversion parameter, the order parameter, and the presence of a ZnO second phase in the prepared ZnAl 2 O 4 catalysts.Among the ZnAl 2 O 4 catalysts, s-ZnAl 2 O 4 prepared from sorbitol exhibited the highest inversion parameter, resulting in the highest surface acidity with a significant presence of medium, medium-strong, and strong acidic sites.Furthermore, the highly disordered structure of s-ZnAl 2 O 4 also generated numerous basic sites in conjunction with the acidic sites, positively influencing the glycerolysis of urea.The superior performance of the s-ZnAl 2 O 4 catalyst, as evidenced by its high GC yield and low (2) selectivity of 41% and 25%, respectively, further highlighted the relationship between catalyst surface properties, and catalytic performance in the glycerolysis of urea.These findings provide valuable insights into the design and optimization of ZnAl 2 O 4 catalysts for various applications, contributing to the Table 3. Analysis of liquid products obtained from the glycerolysis of urea over various mixed oxide catalysts prepared by different polyol precursors (reaction temperature = 140 °C, reaction time = 5 h, reaction pressure = 3 kPa, glycerol/urea ratio = 1:1): (2): 2,3-dihydroxypropyl carbamate, (4): 4 (hydroxymethyl)oxazolidin-2-one, and (5): (2-oxo-1,3-dioxolan-4-yl) methyl carbamate.understanding of structure-property relationships in heterogeneous catalysis.

Experimental Section
Catalyst Preparation: The citrate complex ZnAl 2 O 4 catalysts were synthesized using a modified citrate complex technique. [38]Zn(NO 3 ) 2 .6H 2 O (Sigma-Aldrich Korea, Gyeonggi-do, South Korea) and Al(NO 3 ) 3 .9H 2 O (Sigma-Aldrich Korea, Gyeonggi-do, South Korea) in a molar ratio of 1:2 were dissolved in water at room temperature (RT).Citric acid powder (Sigma-Aldrich Korea, Gyeonggi-do, South Korea) was added to the solution with a citric acid to metal ratio of 2:1, followed by stirring using a magnetic bar.The polyol precursor was then added to the stirring solution with a polyol to citric acid ratio of 2:1.The mixture was stirred at 70 °C, until water evaporation resulted in the formation of a yellow viscous gel solution.The gel was further dried in an oven at 140 °C, until spontaneous solidification occurred due to the emission of NO x gases.Finally, the catalysts were calcined at 550 °C in a furnace for 4 h.The catalysts were denoted as x-ZnAl 2 O 4 , where x represents different polyol precursors (Sigma-Aldrich Korea, Gyeonggi-do, South Korea).
Reaction Test: The reaction was conducted in a 100 mL round-bottom three-neck flask, with one neck connected to a vacuum line through a water condenser.Glycerol (0.2 mol) was added to the flask at 80 °C under stirring to reduce its viscosity.The flask was then connected to a vacuum pump through an HNO 3 solution trap (to remove NH 3 ) and a cold trap (to remove other volatiles).After 10 min, urea (0.2 mol) was added to the flask and mixed with the glycerol, until complete dissolution and a transparent solution were obtained.A certain amount of catalyst (5 wt% relative to the initial glycerol) was added to the flask.The reaction was carried out under a vacuum pressure of 3 kPa at 140 °C, with constant stirring.
After the reaction, ethanol was added to the final products, and the liquid products were separated from the spent catalyst by filtration.The liquid product was quantitatively analyzed using gas chromatography (Acme 6100 GC, YL Instrument Co., Ltd., Dongan-gu, Anyang, South Korea) equipped with a flame ionization detector and a capillary column DB-Wax (30 m Â 0.25 mm Â 0.25 μm).The molar amounts of each component were calculated using the internal standard method, with tetraethylene glycol as the internal standard.Glycerol conversion, GC selectivity, GC yield, and by-product selectivity were calculated using appropriate equations, considering the mole units of each chemical.Catalyst Characterization: Thermogravimetric analysis (TGA) of the precursors (before calcination) was measured by a TGA Q50 apparatus (TA Instruments, New Castle, DE, USA).Surface characterizations were performed using N 2 adsorption/desorption isotherm analysis on a Micromeritics ASAP 2020 apparatus (Micromeritics, Norcross, GA, USA).The surface area was determined using the BET theory.A laser-Raman spectrum was acquired using the DXR Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA).This instrument was equipped with a 532 nm laser beam for the Raman spectroscopy analysis.XRD patterns of the fresh catalysts were collected using a Rigaku RAD-3C diffractometer (Rigaku Corp., Tokyo, Japan) with Cu-Kα radiation (λ = 1.5418Å) at a scanning rate of 2°min À1 , operating at 35 kV and 20 mA.The average crystallite size (D) of single-phase spinel samples was estimated using the Debye-Scherrer equation. [39]heme 3. Mechanistic pathway for the alcoholysis of urea with glycerol over X-ZnAl 2 O 4 catalysts.

Glycerol conversion
where D is the crystallite size, λ is the X-Ray wavelength, β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg angle.Morphological structures of ZnAl 2 O 4 spinel particles were analyzed via CS-TEM and high-angle annular dark-field scanning TEM using a JEM-ARM300F JEOL instrument (Tokyo, Japan).
The fresh catalysts were analyzed using a Thermo Scientific Nicolet iS5 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).XPS data was surveyed using a Thermo Scientific K-α XPS spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).To determine the number of acidic and basic sites, TPD-NH 3 /CO 2 measurements were conducted on a MicrotracBEL BELCAT-M instrument provided by MicrotracBEL Corp., in Osaka, Japan.In the analysis process, approximately 100-200 mg of the fresh catalysts was placed within the quartz sample tube of the instrument.The catalyst samples were initially subjected to a pretreatment phase, involving exposure to helium flow at a rate of 100 mL min À1 at a temperature of 600 °C for a duration of 1 h.Subsequently, the samples were cooled down to 50 °C and NH 3 or CO 2 gases were then injected into the system at a flow rate of 50 mL min À1 to enable chemisorption.Finally, the temperature was increased gradually to 600 °C, with a ramping rate of 1.5 °C min À1 .Desorbed species were eliminated from the system using helium flow at a rate of 30 mL min À1 , and subsequently analyzed using the TCD detector. 27Al magic angle spinning (MAS) NMR (Bruker Avance III 300, Germany) spectra were recorded using a 2.5 mm MAS probe at RT in a 7.04 T magnet.All spectra were acquired at 30 kHz (ν r ) with a rotor-synchronized Hahn-echo pulse sequence (90°-τ-180°-τ-acq), where τ = 1/ν r .A π/2 pulse width of 2.5 μs was used with a recycle delay of 0.5 s.For all samples, acquisitions of 1 K transients at RT were obtained.
c) Intensities were obtained from the NH 3 -TPD profiles.d) Intensity of oxygen vacancy (O b ) values were obtained from the deconvolution of XPS by using the Origin software (OriginPro 2021 9.8.0.200).
Scheme 2. A) The ordered lattice and B) disordered lattice of spinel ZnAl 2 O 4 .The acidic sites of ZnAl 2 O 4 spinel: i) the weak Lewis acid site, ii) the medium-weak Lewis acid site, iii) the medium-strong Lewis acid site, and (iv) the strong Lewis acid site.

Figure 6 .
Figure 6.The relationships between acidic sites versus glycerol carbonate yield and basic sites versus (2) selectivity.

Table 1 .
Textural properties, order parameters, and crystallite sizes of the ZnAl 2 O 4 for the fresh catalysts.