Adsorption of Biomass-Derived Products on MoO3: Hydrogen Bonding Interactions under the Spotlight.

We performed a computational study on the interaction of O-containing compounds coming from biomass with a catalytic surface of MoO3. The addition of H atoms on the metal oxide surface mimics different scenarios of its exposure to the ambient or protons coming from biomass. Representative compounds from fatty acids (from triacylglycerides) and aromatics (from lignin) were adsorbed on the metal oxide surfaces. We covered the complete H surface coverage, and the adsorbed molecules showed structural changes due to the interactions in turn. The driven force interactions in this process is hydrogen bonding, which reveals the complexity in biomass processing. H-bonds were fully characterized by the electron density and its Laplacian where bond critical points are present. These topological properties allow us to understand the correlation between the adsorption energies and the strength on each adsorption site. We also computed the relative Gibbs energies and harmonic oscillator model of aromaticity index of the adsorbed molecules to get more insights into their stability.

Introduction

Transformation of biomass feedstocks into more profitable commodities is an alternative for providing energy and high value added chemicals for the new society generations, looking for green and sustainable technologies.[1,2] The direct utilization of biomass is not possible because of its complex chemical properties. Several research studies would overcome its intrinsic limitations. For instance, biomass feedstocks cannot be used immediately as fuels because of their high oxygen content, which leads to low heating value, immiscibility with fossil fuels, a tendency for polymerization, thermal instability, and high viscosity.[3] High oxygen content in these molecules is a feature that needs their transformation into O-free molecules. The overall challenge with biomass conversion is how to efficiently remove oxygen from the biomass feedstocks and produce a molecule that has a high energy density and good combustion properties.[4]

The aviation sector is facing mounting pressures to reduce its greenhouse gas emissions, where biomass-derived jet fuel can provide a solution to this industry.[5,6] Vegetable oils, such as palm oil, have been good candidates to develop environmental-friendly and high-quality fuels.[7] Hydrocarbon chains coming from such oils, after the conversion of fatty acids, provide mixtures that can blend with jet fuel. On the other hand, today, most of the global bioenergy production and final use comes from lignocellulosic biomass. Unlike cellulose, with a well-defined sequence of monomeric units that are linked by regular β-1,4-glycosidic bonds, lignin is characterized by a variety of distinct and chemically different bonding motifs, each demanding different conditions for cleavage when selective depolymerization is targeted.[8] The aromatic compounds present in lignin make this feedstock very attractive to provide aromatic compounds for several high-value industries.

Catalytic upgrading of biomass is used to eliminate oxygen contained in the organic molecules by hydrodeoxygenation (HDO) and cracking reactions in the presence of a heterogenous catalyst.[9,10] The HDO process takes place at higher H2 pressures and temperatures, and the catalysts consist typically in MoS2-based active phases supported on metal oxides.[11−14] These catalysts were adapted from the petroleum industry, where they are being used for many years in the elimination of sulfur and nitrogen atoms from crude oils. Till date, several research efforts have shown the ability to use other active phases more appropriate to deal with water environments and be able to eliminate oxygen from biomass feedstocks. In this line, the use of MoO3 as an active phase is a promising catalytic system because it is the precursor from MoS2 and its use in the HDO of biomass is very much reliable. Results pointed out that MoO3 is a catalytically active species in the production of linear hydrocarbon from ketones and cyclic ethers.[15] It was also proved effectively in the HDO of aromatic compounds showing the ability to produce O-free hydrocarbons.[16,17] The MoO3 as an active phase has several differences in its physical and chemical properties than the MoS2 or metallic active phases.[18−20] For instance, MoO3 can be hydrated or protonated under water environments because of the Mo=O or Mo–O–Mo surface species, and this might affect its catalytic performance in biomass conversion.[21,22] Even its exposure to the atmosphere changes its electronic properties, where the reversible hydration produces changes in color related to different surface species.[23,24]

Computational studies have been used for a better understanding of biomass transformation. They have been applied to explain ketonization reactions over oxide catalysts,[25,26] adsorption of aromatic O-containing molecules on sulfide catalysts,[27] as well as the catalytic and thermochemical routes of fatty acids.[28,29] On the other hand, using molecules with high molecular weight together with periodic crystal models with an accurate method is always an issue for getting useful information; then small molecules and cluster models have been used computationally.[30,31] The presence of the oxygen atoms with phenolic or acidic functionalities in the biomass-related products contributes with hydrogen-bonding interactions. Understanding the H-bonds in catalytic processes is very important for a better interpretation of the adsorption phenomena as well as stabilization of substrates or intermediates. The interaction of these molecules with a catalytic surface of MoO3, where H-bonds are present, needs to be investigated. Here, we studied the effect of the hydroxylated species of the MoO3 surface interacting with biomass model compounds. Our main aim is to get atomistic insights into the adsorption process by computational calculations. We employed analytical tools from the quantum theory of atoms in molecules (QTAIM) for a better understanding of the interactions taking place in biomass conversion. We calculated the adsorption energies as well as the topological properties for characterizing the H-bonds formed between the organic molecules and the protonated MoO3 surface catalysts.

Results and Discussion

The optimized geometries for the MoO3 surface with H surface coverage (σH) at 0 and 100% are shown in Figure . After saturation, slight changes occur mostly on the surface atoms for all the σH. The addition of H produces hydroxylated species by a covalent O–H bond on the surface. The Mo–O bond length of the dehydrated surface is 1.720 Å, which can be ascribed to a Mo=O double bond. After protonation, this bond becomes larger (2.023 Å). The H atoms in the fully saturated structure are oriented to the closer oxygen atom. The orientation of all the saturated H is driven by the intramolecular interaction through hydrogen bridges. The density of states (DOS) for those structures is depicted in Figure . The saturation of the surface with H atoms produces changes in the electronic structure of MoO3. An additional peak was found for the hydroxylated surface at about −10.2 eV because of the hydroxyl σ bonding O–H states. Those results pointed out that surface modifications caused by the saturation of the surface with H atoms affect the electronic energy levels of MoO3, considering several possible effects of the added H atoms on the MoO3 layer.[21] The protonation of the MoO3 catalysts are possible experimentally because of several reasons that are not the main aim of this study. One reason is the exposure of this material to water from the atmosphere that can produce this protonated or hydroxylated surface species. Once this catalyst is in touch with water below the isoelectric point of the surface using water as a solvent, for example, electrocatalytic processes must produce surface protonated species.[32] The contact of this catalyst with H donating species such as organic acids might produce protonated species too. We can also produce this surface species by changing the oxidation states of Mo in the MoO3 catalysts, which have been demonstrated to play a major role in different supported species.[17] Therefore, the presence of protonated species in this catalytically active phase is of a paramount importance, given the characteristics of the biomass-derived products, where water is released from its chemical reactions and the presence of several molecules with acidic properties is predominant.

Figure 1

DOS for MoO3 with surface H coverage at (a) 0 and (b) 100%. Optimized geometries for the surface with coverage of H at 0% (c) side view and (e) top view; optimized geometries for the fully saturated MoO3 surface (d) side view and (f) top view. Partial DOS of s (blue), p (green), d (red), and sum (black).

Palmitic Acid

We studied the adsorption of palmitic acid (PA) on the MoO3 surfaces at different degrees of H surface coverage. Optimized geometries at σH = 0 and 100% are shown in Figure . The rest of the optimized geometries are shown in Figure S1. The interaction of PA with the surface atoms take place through H-bonds. In the first case, the H atom from the carboxylic moiety of PA is interacting with the O atom from the catalysts. Increasing the degree of hydroxylation provides a larger number of H-bonds at the catalysts’ surface and structural modification in the whole interacting system is noticeable. The optimized geometry for PA and MoO3 at σH = 100% showed the interaction of the H atom from the PA with the O atom at the surface. Another type of interaction was found because of the hydroxylated species at the surface, the O atom from the carbonyl groups interacts with the H atom from the MoO3 surface. The adsorption energy for the different degree of H surface coverage is depicted in Figure . We found the maximum adsorption energy (−62 kJ mol–1) at σH = 11%, whereas the maximum is found for the nonsaturated surface. This indicates that the presence of hydroxylated species on the surface of MoO3 provides adsorption sites with higher exothermic values. There are a major number of hydroxylated species on the catalysts’ surface at higher σH, but the correlation with the adsorption energy is not linear. It is important to note that these adsorption energy values are correlated directly with the H-bond interactions between the PA molecule and the MoO3 surface. It seems that the presence of hydroxylated species is a positive effect for the adsorption of PA on this material.

Figure 2

Optimized geometries for the adsorption of PA on MoO3 catalysts at (a) 0 and (b) 100% of H surface coverage (σH), (c) adsorption energy values of PA on MoO3 as a function of σH.

We employed the QTAIM for a further explanation of the interactions that take place upon the adsorption of O-containing molecules on MoO3. Briefly, the topology of electron density (ρ) leads to a partitioning scheme, which defines atoms inside a molecule or a molecular aggregate via the gradient vector field, ∇ρ. This vector field is a collection of gradient paths, which are curves in space that follow the direction of steepest ascent in ρ. Therefore, a gradient path has a physical meaning; it always starts and finishes at points where ρ vanishes. These points are called critical points (CPs). The CPs in ρ are special and useful points for a given molecule.[33] The Laplacian of the electron density (∇2ρ) is the sum of the eigenvalues. It has been applied for the characterization of covalent and ionic bonds; and also used to describe hydrogen bonds and van der Waals interactions, where ∇2ρ is positive.[34]

We found two main sites where the molecule interacts with the catalyst’s surface. One is found at the OH group from PA and the O atom at the surface of MoO3, this interaction takes place for all the σH. On the other hand, we found another site between the O atom from the carbonyl group of PA and the OH at the MoO3 surface. In some cases, both sites are present and they form a cyclic species between the PA and the oxide surface as we can appreciate on Figures and S1. We showed the values for the electron density and the Laplacian of the electron density in Table . We studied the main bond CP (BCP) and identified them as PA (H) and MoO3 (H). The ρ values for the interaction between the OH groups in PA and the O atom on the surface of the MoO3 showed slight changes for the different H surface coverages, and the values are about 0.1. More drastic changes are observed on the interaction between the O atom from the carbonyl group in the PA and the H atom from the catalyst’s surface. In this interaction, we found the highest values (ρ = 0.302) at σH = 11%, which is located at the higher adsorption energy. We also calculate the ∇2ρ for both adsorption sites between PA and MoO3. The values for the ∇2ρ > 0 for all the surface coverages, which is in good agreement for the characterization of hydrogen bonds by this topological property. The values showed that ∇2ρ from the interaction between OH of PA has a major value (1.837) for σH = 11%. The ∇2ρ for the other adsorption site also showed the maximum value (5.504) at the same H surface coverage. The values for the topological properties for both adsorption sites point out the importance of the hydrogen bonds formed by varying the number of surface OH groups. Both sites contribute to the adsorption energy and the electronic density involved in such interactions evinced the complexity of the adsorption processes of O-containing molecules on oxide materials. We could appreciate that the interaction is driven by the OH groups on the catalyst’s surface, while the OH group at the PA molecules interacting with the O atoms at the surfaces contribute less to the adsorption energy and almost constant for the complete range of H surface coverage.

Table 1

Topological Properties for the Noncovalent Interactions Established upon Adsorption of Palmitic Acid on the Surface of MoO3 with Different Degrees of H Coverage

	ρb		∇2ρc
σHa	PA (H)	MoO3 (H)	PA (H)	MoO3 (H)
0	0.098		1.508
11	0.117	0.302	1.837	5.504
33	0.086	0.061	1.132	1.094
56	0.099	0.165	1.501	4.050
78	0.121	0.140	1.052	3.513
100	0.120	0.289	1.785	4.924

Surface coverage (%).

Electron density calculated at the bond critical point (BCP).

Laplacian of the electron density at the BCP.

m-Cresol

The optimized geometries for the adsorption of m-cresol on the MoO3 are shown in Figure . We chose the dehydrated and the H fully saturated surfaces to depict the effect of the H-bonds on the interaction with m-cresol. The O atom from the MoO3 surface interacts with the H atom from the OH in the m-cresol, and these H-bonds lead to negative values for its adsorption energy. The fully saturated surface is plenty of H-bonds, where the H atom from the organic molecule interacts with an O atom at the surface, while several H atoms from the surface changed their orientation toward the O atom of the phenol group. The optimized geometries for the other σH were also analyzed, with H playing a major role in the adsorption of m-cresol by H-bonds (Figure S2). The adsorption energy for m-cresol on MoO3 at different degrees of H surface coverage is also shown in Figure . The presence of hydroxyl groups on the MoO3 surface has a positive effect on the adsorption energies, as it is more exothermic because of the H-bonds. It appears to present two adsorption regimes for σH = 11–60% with an average adsorption energy of −38 kJ mol–1 and the other between σH = 78–100% with an average adsorption energy about −52 kJ mol–1. This feature is related to the interaction of hydroxyl groups at the surface with the phenolic group from the organic molecule. We can point out that the increment on the number of hydroxyl species on the surface of MoO3 produces more sites for the adsorption of m-cresol.

Figure 3

Optimized geometries for the adsorption of m-cresol on MoO3 catalysts at (a) 0 and (b) 100% of H surface coverage (σH), (c) adsorption energy values of m-cresol on MoO3 as a function of σH.

The interaction of m-cresol with the MoO3 surface exhibits several adsorption sites because of the type/size of the molecule and its planar structure. Similar to PA, the m-cresol molecule can interact with the surface by its O atom within the phenol group. This OH group can interact also through its H atom with the O atoms at the MoO3 catalyst. It means that the adsorption sites seem to be more localized than the ones for PA. We studied further those adsorption sites by the QTAIM to get deeper insights into their type and magnitude. Table enlists the topological properties for these two adsorption sites. We found only one site from the H atom in the phenolic moiety interacting with the O atom in the material surface, while in some cases, we detected the presence of two H atoms from the MoO3 surface interacting with the O atom in the m-cresol molecule. The electron density for both interaction sites depends on the degree of saturation. In general, these values are larger for H surface atoms than the ones for the H atoms within m-cresol. For this molecule, the trend is not linear for the H surface atoms in some σH values. For instance, at σH = 11%, ρ = 0.175 for m-cresol (H) and ρ = 0.232 for MoO3 (H), while at σH = 100%, ρ = 0.210 for m-cresol (H) and ρ1 = 0.103 and ρ2 = 0.094 for MoO3 (H). The presence of two positions for the adsorption of this organic molecule must contribute to the adsorption energy, but the electron density within QTAIM at different sites would not be a simple sum of the contribution of each site. We also calculated the Laplacian of the electron density for all the sites identified. These have positive values, which are in good agreement with the H-bond interactions. This topological property is an appropriated descriptor for this type of interactions. We found that ∇2ρ follows a similar trend than the electron density. The highest value for the Laplacian of the electron density was found at σH = 100% for the m-cresol (H), ∇2ρ = 2.912, while the highest value for the MoO3 (H) is located at σH = 33%, ∇2ρ1 = 2.266 and ∇2ρ2 = 0.157. It is important to mention that the two adsorption sites have different values depending on the position and the chemical environment. In all the cases, one showed higher values than the other, the reason being the major proximity or higher electron density involved in such interaction.

Table 2

Topological Properties for the Noncovalent Interactions Established upon Adsorption of m-Cresol on the Surface of MoO3 with Different Degrees of H Coverage

	ρb		∇2ρc
σHa	m-cresol (H)	MoO3 (H)	m-cresol (H)	MoO3 (H)
0	0.122		1.837
11	0.175	0.232	1.828	2.063
33	0.173	0.233, 0.072	1.735	2.266, 0.157
56	0.121	0.190	1.536	2.140
78	0.168	0.157, 0.082	1.467	1.971, 0.298
100	0.210	0.103, 0.094	2.912	1.673, 0.319

Surface coverage (%).

Electron density calculated at the BCP.

Laplacian of the electron density at the BCP.

Guaiacol

The adsorption of guaiacol on the MoO3 was also studied by varying the H surface coverage. Selected optimized geometries for this process are depicted in Figure . The rest of the optimized geometries are shown in Figure S3. This molecule has two substituted oxygen atoms in the benzene moiety, one in a phenolic group and the other one in a methyl ether group. Both groups can interact with the H atoms on the oxide surface. This molecule exhibited a complex behavior because of the presence of hydrophobic (−OCH3) and hydrophilic (−OH) groups in closer proximity that restrict or promote the molecule for geometrically well-defined interactions. At the σH = 0%, the acidic proton of the phenol group interacts through H-bonds with the O atom at the surface, while the −OCH3 group is moving because of the repulsion forces from the methyl group. The orientation of both groups varies depending on the degree of the surface coverage. We observe that at σH = 100%, the interaction of the −OH groups on the catalytic surface with guaiacol is driven by hydrogen bonds with the O atom in either phenol or ether groups. The chemical structure of guaiacol changed drastically compared to the one adsorbed at zero H surface coverage. The methyl group is rotated to allow the interaction of the oxygen atoms in this ether group with the H surface atoms. The phenol group interacts toward its H atom with the O atoms at the surface, while the O atom in this functional group forms two H-bonds at the surface. The adsorption energy profile for guaiacol on MoO3 as a function of the surface coverage is displayed in Figure . We observed this value depending on the surface coverage due to the competition between the attraction and repulsion forces in this molecule due to H-bonds and van der Waals interactions. As we mentioned above, the phenol group in this molecule can form attractive interactions with the surface, while the ether group provides repulsion contribution from its methyl group and attraction from the O atom in this functional group. On the other hand, the preference to establish one type of interaction would be a consequence of the orientation of the molecule upon adsorption. The largest adsorption energy for guaiacol on MoO3 was obtained for the fully hydroxylated surface, while the lowest is reached at about 80%. Indeed, the profile of the adsorption energy of this molecule is related to the presence of both hydrophobic and hydrophilic groups in the molecule and the configuration changing upon adsorption. We can suggest that the hydroxylated surfaces might promote important structural changes by rotating the substituting groups in benzene.

Figure 4

Optimized geometries for the adsorption of guaiacol on MoO3 catalysts at (a) 0 and (b) 100% of H surface coverage (σH), (c) adsorption energy values of guaiacol on MoO3 as a function of σH.

The topological properties for the adsorption of guaiacol on the surface of MoO3 was further studied by the QTAIM. Table depicts the values for the ρ and the ∇2ρ at the BCP. Similar to the other oxygenated molecules analyzed in this work, we found two distinct adsorption sites, one for the −OH at the oxide surface and other for the −OH group in guaiacol. The surface at 0 % coverage showed only one site for adsorption, and it is formed by the −OH group in guaiacol and the O atom in the MoO3 surface with ρ = 0.118 and the Laplacian of 1.834. These values increased for higher σH, and the hydroxylated species contribute with more adsorption sites, the increasing number of adsorption sites and their larger values showed a correlation with the adsorption energies. The smallest adsorption energy was observed at σH = 78%, and we could appreciate that oxygen from the −OCH3 and −OH groups are the atoms that interact with the H surface atoms and it showed the lowest values for (−OCH3) ρ1 = 0.103 and ∇2ρ1 = 1.548 and (−OH) ρ2 = 0.112 and ∇2ρ2 = 1.068, while the hydrogen atom from the phenolic group does not interact with any oxygen atom at the surface. This null interaction of the proton from the phenol group is a consequence of the optimized adsorption geometry where the guaiacol molecule needs to be accommodated on the MoO3 surface. On the other hand, the topological properties of the adsorption sites changed drastically at full coverage of the oxide surface σH = 100%. We also found the largest value for the adsorption energy at this surface coverage. The values for the proton from the phenolic group are ρ = 0.276 and ∇2ρ = 3.160, which are the highest for this adsorption site in the guaiacol adsorption. The values for the interaction of the oxygen atoms in the organic molecule with the H atoms at the oxide surface showed also higher values for the topological properties. For instance, the electron density for this BCP for the −OCH3 and −OH are 0.158 and 0.308, respectively, while the values for the Laplacian of the electron density are also higher (−OCH3) 1.991 and (−OH) 3.233. With the help of these analytical tools provided by the QTAIM, we can understand the nonlinear profile of the adsorption energy of the guaiacol on the surface of MoO3. We can correlate the adsorption energies with the type and strength of the interaction of the different atoms and functional groups involved on the adsorption sites. Guaiacol exhibits a very complex behavior upon interaction with a hydroxylated MoO3 surface. This is just a model compound from biomass. If we think about the complexity of the functional groups present in lignin-based biomolecules, the interaction with an oxide surface would become much more complex. Therefore, the catalytic transformation of those biomass feedstocks must consider the importance of the hydroxylated degree of the catalysts for its adsorption and further transformation.

Table 3

Topological Properties for the Noncovalent Interactions Established upon Adsorption of Guaiacol on the Surface of MoO3 with Different Degrees of H Coverage

	ρb		∇2ρc
σHa	guaiacol (H)	MoO3 (H)d	guaiacol (H)	MoO3 (H)d
0	0.118		1.834
11	0.185	0.235	1.910	2.989
33	0.169	0.181, 0.075	1.785	2.306, 0.233
56	0.218	0.132, 0.077	1.771	1.614, 1.516
78	none	(−OCH3) 0.103, (−OH) 0.112	none	(−OCH3) 1.548, (−OH) 1.068
100	0.276	(−OCH3) 0.158, (−OH) 0.308	3.160	(−OCH3) 1.991 (OH) 3.233

Coverage (%).

Electron density calculated at the BCP.

Laplacian of the electron density at the BCP.

Values of the topological properties of H from the surface with O for the phenolic group and cases in which these groups do not interact are explicitly labeled.

Structural Modification of the Organic Molecules

We analyzed the interaction of representative model compounds from biomass, namely, PA (fatty acids, vegetable oils), m-cresol, and guaiacol (aromatics and lignin) with a catalytic surface of MoO3 at different H surface coverages for a better understanding of the factors that direct their adsorption and further transformation. We found important differences on the adsorption energies by varying the H surface coverage, which were further studied by the QTAIM. The organic molecules changed their structures because of the hydrophobic or hydrophilic character of the surface. Selected structural parameters of the ground state and the adsorbed molecules are shown in Tables S1–S3. The bond length and the angles for the atoms are responsible for the adsorption process, and the closer ones suffered more changes compared to the ground-state structures. These changes might play an important role for its transformation or stability on the catalytic surface. We calculate the Gibbs energy for the organic molecules adsorbed on the MoO3 at the different hydroxylated degrees. The relative Gibbs energy for all the molecules is plotted in Figure . Because of the structural changes, the relative energy of all the molecules is positive compared to its ground state. This means that the molecules adsorbed on the MoO3 surface are less stable than the ground state. However, this instability depends strongly on the hydroxylated degree of the catalytic surface. For instance, PA and m-cresol adsorbed on the oxide surface seem to be relative stable at σH = 0%. It is important to mention that the structural changes take place to accommodate the organic molecules adsorbed on the surface and this is related to the H bonding formed on the system. These instable structures might be easier to be converted on the catalytic surface upon adsorption. PA showed more regions of higher relative instabilities ranging from 15 to 25 kJ mol–1 at σH = 11–100%, while m-cresol did not change its structure drastically, and its relative Gibbs energy varies from 5 to 10 kJ mol–1 from all the σH ranges. On the other hand, similar to the analysis of the adsorption energies, the behavior of the structural stability for guaiacol is complex. The regions of less instability are located between σH = 11–80% mostly, while it is more instable at low and higher H surface coverages. The presence of the hydroxylated species on the MoO3 surface favors the destabilization of the organic molecules, which can lead to their further transformation.

Figure 5

Relative Gibbs energy for (■) PA, (●) m-cresol, and (⧫) guaiacol adsorbed on MoO3 as a function of H surface coverage.

Organic molecules in lignin are composed by linked aromatic rings, with a very stable character for its transformation. Therefore, the two aromatic model compounds in this study were further studied by the harmonic oscillator model of aromaticity (HOMA) calculations, an aromatic index. It was calculated for the benzene rings for m-cresol and guaiacol structures adsorbed on the oxide surfaces in the whole hydroxylated scenarios (Table ). The HOMA values for the ground-state molecules for both m-cresol and guaiacol are close to 1, which is in agreement with an aromatic molecule. The HOMA values decreased (losing aromaticity) for both aromatic molecules adsorbed on the MoO3 surfaces because of their structural and electronic changes in this process. In general, the aromatic character for guaiacol decreased more than m-cresol at the different degree of σH. This result agrees with the structural changes detected upon adsorption; guaiacol suffered more changes because of its structure adsorbed on the oxide surface. This feature of the O-containing molecules is a driven force for the adsorption processes that might be considered for a better understanding in the biomass transformations.

Table 4

HOMA Values on the Benzene Ring for m-Cresol and Guaiacol Adsorbed on MoO3 at Different Degrees of Surface Coverage

	m-cresol	guaiacol
ground state	0.996	0.993
σHa
0	0.866	0.801
11	0.914	0.886
33	0.925	0.890
56	0.919	0.899
78	0.918	0.910
100	0.912	0.902

Surface coverage (%).

A comparison with experimental data is possible mostly for the aromatic molecules in this study. For guaiacol and m-cresol, their conversion over the MoO3 catalyst has been reported to be about 74 and 49%, respectively. The adsorption energies for guaiacol are more exothermic than m-cresol in the range of σH studied, which might be related to their experimental conversion using this catalytically active phase. Higher conversion is achieved because of a more favorable interaction of guaiacol with this catalytic surface. We also observed the interaction of the −OCH3 group from guaiacol in different surface coverages with important contributions in the bonding parameters (specially at σH ≥ 78%). These results are also related to the reaction network of guaiacol in the HDO, where demethylation reactions have been reported.[35] We can state that the protonated surfaces of the MoO3-based catalysts are more likely active for demethylation reactions, and these can be produced for water releasing or changes in acidic properties of solvent or reaction media. On the other hand, the stability and aromaticity of guaiacol decreased much more than the values for m-cresol in all the protonated regimes. Both results are also in good agreement with the higher reactivity of guaiacol compared with m-cresol. The interaction of m-cresol with the different MoO3 catalytic surfaces maintains a more stable aromatic molecule that must be low reactive than its adsorbed counterparts from guaiacol.

Conclusions

The interaction of the products derived from biomass showed different scenarios depending on the degree of H surface coverage of the MoO3. Fatty acids interact mainly by one site with the carboxylic groups attached by hydrogen bonds on the MoO3 surface. The phenol-type compounds showed more than one site for adsorption leading to more complex interacting structures. The main driven force for the interaction of O-containing molecules with this catalytic surface is hydrogen bonding. These H-bonds showed different adsorbed structures on the MoO3 catalysts, where the molecules are feasible to interact under hydrogen-rich surroundings. The characterization of them by the topological properties sheds light into the strength, and some correlations with adsorption energies were established. The electron density together with the Laplacian of the electron density showed numerical values for direct correlations in atom to atom interactions. These topological properties are well correlated to the adsorption sites. The molecules present in biomass contain several electronegative atoms (oxygen mostly), which will be targeted for their transformation. Therefore, the use of the analytical tools provided by the QTAIM could be extensively used. Additionally, we calculated the HOMA index and stability of the adsorbed molecules on the MoO3 surface. These results explained the differences in reactivity of the two aromatic molecules, where m-cresol is more stable than guaiacol. In the context of catalytic processes, these results might be of interest for the community to fully characterize specific atomic interactions and the further transformation within a specific site of the molecule.

Computational Details

We chose representative O-containing molecules from biomass feedstocks. PA is a model compound of fatty acids (coming from triacylglycerides), whereas m-cresol and guaiacol are aromatic model compounds obtained from lignin depolymerization. The three organic compounds were adsorbed on a catalytic surface of MoO3. We studied the effect of the H surface coverage (σH) and the interaction of those molecules in a complete range of hydrogen coverage.

The surface model was represented with a 3D periodic slab model taking from the crystal structure of MoO3. This model consists of 72 atoms. Geometry optimization and electronic properties were carried out using the revised version of the PBE[36] functional that has shown excellent performance for chemisorption in catalysis,[37] using double numerical with polarization basis sets. Atomic coordinates of the internal atoms were fixed and the surface atoms were allowed full optimization. The adsorbed organic molecules were fully optimized in all the calculations. The threshold of density matrix convergence was set to 10–6. A Fermi smearing of 0.005 Ha and a real-space cutoff of 4.9 Å were also used to get the computational convergence for this system. Adsorption energies were obtained at the same level of theory. These were calculated as ΔE = Emol/cat – (Emol + Ecat), where Emol = energy of the molecule, Ecat = energy of the MoO3, and Emol/cat = adsorbed molecule on the MoO3. The calculations were performed with the DMol3 software.

The QTAIM[38] was used to study the topological properties on the adsorption sites. We found different sites of interaction depending on the molecule and the degree of surface coverage. More specifically, we identified hydrogen bonding interactions due to the presence of electronegative atoms (O) either in the organic molecules or in the oxide surface. These sites were fully characterized by their topological properties such as electron density (ρ) and the Laplacian of the electron density (∇2ρ).[39] Given the complexity of studying topological properties in the periodic system, we found AIM-UC[40] software very suitable for the analysis of the DMol3 files.

We took the optimized structures of the adsorbed organic molecules and calculated the single-point energy on each geometry, and we carried out the computation of vibration analysis to get the values of Gibbs energy at the M06-2X level of theory using the triple zeta basis set with polarization. These calculations were carried out with NWChem software.[41]

Aromaticity studies were conducted on the single-point structure of the m-cresol and guaiacol molecules adsorbed on the different hydroxylates surfaces. We employed a structure-based index of aromaticity, the so-called HOMA. Its definition iswhere n is the number of bonds taken into summation and α is an empirical constant fixed to give HOMA = 0 for a Kekulé structure of the aromatic benzene and equal to 1 for the system with all bonds equal to the optimal value Ropt.[42] Aromaticity studies were carried out in the multifunctional wavefunction analyzer (Multiwfn).[43]