(89e) The Kinetics of Elementary Steps in Condensations Catalyzed By Solid Acids: Effects of the Size, Shape, and Charge of Intermediates, Transition States and Catalytic Structures

The efficient conversion of oxygen-rich feedstocks derived from biomass to valuable chemicals and fuels requires an understanding of the oxygenate chemistry that can lengthen carbon chains and decrease oxygen content. Aldol condensation of alkanones and alkanals provides a pathway to form new C-C bonds, while also removing O-atoms as H­₂O. In this study, condensation reactions are investigated experimentally, through spectroscopic, H/D isotopic, and kinetic measurements, in combination with density functional theory (DFT) treatments for a diverse range of solid Brønsted acid catalysts. These acids include H-FER, H-TON, H-MFI (with Al, Ga, Fe, and B at framework locations), H-Al-BEA, H-Al-FAU, H-Al-MCM-41, and supported Keggin polyoxometalate clusters (H₃PW₁₂O₄₀/SiO₂). Condensation rates on these solid acid catalysts decrease rapidly with time on stream as a result of secondary reactions of unstable α,β-unsaturated intermediates, leading to the formation of large, unsaturated products that block active sites. A hydrogenation metal function, present within the solid acids or as a separate component in physical mixtures, effectively scavenges these unsaturated species. In doing so, it (i) essentially eliminates deactivation; (ii) removes thermodynamic bottlenecks in C-C bond formation; (iii) avoids secondary β-scission reactions; and (iv) allows rigorous mechanistic studies and the elucidation of the consequences of acid strength and confinement within zeolite voids on the kinetically-relevant rate constants and transition states.

The selective titration of protons by non-coordinating bases (2,6-di-tert-butyl-pyridine) during acetone condensation, together with infrared (IR) spectra collected during catalysis, indicate that condensation reactions (473 K, 0.1-10 kPa acetone) occur on protons nearly saturated with H-bonded acetone, without detectable contributions by Lewis acid sites. These results are consistent with energies and frequencies calculated using periodic density functional theory (DFT) (T-12, Al-MFI unit cell, VASP, RPBE, D3BJ). These calculations show that acetone binds strongly (Î?G = -80 kJ mol^-1_, 473 K) as H-bonded species and confirm the origins of the observed shift in the O-H stretch in zeolites upon acetone binding from 3600 cm^-1to 2440 cm^-1. The condensation turnover rates are proportional to acetone pressure on all samples; the resulting first-order rate constant reflects the free energy differences between a bimolecular C-C bond-formation transition state and a proton saturated with H-bonded acetone and a gaseous acetone molecule. The kinetic relevance of the C-C bond formation steps on these samples is consistent with its DFT-derived free energy barrier, which lies at the highest value along the reaction coordinate at T12 sites in H-Al-MFI structures.

These first-order condensation rate constants can be compared on solid Brønsted acids with different acid strength and confining environments to determine their consequences on the stability of the kinetically-relevant transition states and their relevant precursors. The effects of acid strength are evident from comparisons among catalysts with different acid strength and either mesopores much larger than the relevant molecular structures (Al-MCM-41 and H₃PW₁₂O₄₀/SiO₂) or small voids similar in structure (MFI) but with different framework heteroatoms (Al-, Ga-, Fe-, B- isomorphous substituents). In both cases, rate constants increase exponentially with decreasing deprotonation energy (DPE), a theoretically-accessible measure of catalyst acid strength [1]. These measured effects of acid strength on rate constants reflect the stability of the conjugate anion, which influences the stability of the ion-pair transition states more strongly than for the less charged H-bonded acetone precursors. The DFT-derived free energy barriers for substituted MFI systems (T-12, Al-, Fe-, Ga-, B-MFI unit cell, VASP, RPBE, D3BJ, 473 K) agree well with these experimental measurements.

Microporous aluminosilicates with different framework structures provide protons of similar acid strength [2], but very diverse confining environments. Measured condensation rate constants reach a maximum value for voids similar in size to the condensation transition states (MFI, BEA), which preferentially stabilize these transition states over smaller H-bonded acetone precursors through van der Waals interactions. The relative size (and shape) of these species and their strong effects on reactivity require DFT methods that accurately account for attractive dispersion forces in energy minimizations and that can dissect, posteriori, the quantum mechanical and dispersion components of DFT-derived Gibbs free energies (RPBE, D3BJ). This is supplemented by screening methods developed here, which use Lennard-Jones potentials to determine interaction energies between each framework oxygen atom and all atoms in the organic moieties, thus rigorously capturing the consequences of both the size and the shape of the relevant structures and the inorganic containers of these active structures. These interaction energies are then ensemble-averaged for each transition state and precursor species over all crystallographically-distinct proton locations within each microporous framework. The resulting values act as rigorous descriptors of the â??fitâ? for all species and do so in a manner that accounts for the most favorable placement of all structures within their confining â??containersâ?. These descriptors extend those based only on the independent properties of the species (proton affinities) and the catalyst (DPE) and their ability to reorganize charge [3] to include parameters that describe how organic and inorganic components in transition state and precursors interact, which depends on the size and shape of both the organic and the inorganic moieties.

We have also addressed in this study the enduring challenge of deactivation and secondary reactions that break C-C bonds in metal-free solid acids [4]. All solid acids show significant selectivities to isobutene and acetic acid, which form via β-scission of the α,β-unsaturated products (mesityl oxide (MO) and isomesityl oxide (IMO), for acetone reactants). These secondary reactions depend strongly on the size of the void environment and are most effectively catalyzed in MFI. These reactions require the presence of protons (MO/IMO/H₂O mixtures do not react on pure-silica MFI), but β-scission selectivities at a given acetone conversion increase sharply as Al (and proton) densities decrease, even though acetone condensation turnover rates remained unchanged. These counterintuitive trends, in which lower densities of the purported active sites preferentially enhance secondary reactions, cannot reflect diffusional constraints and require the concurrent presence of protons and a vicinal confining environment that, even without active sites, can convert highly reactive intermediates into more stable β-scission products. A detailed examination of the effects of H₂O, which determines the relative concentrations of IMO/MO and their aldol precursors, shows that two β-scission routes with different site requirements are involved. One route involves the β-scission of aldol species; it occurs on all zeolites and is exclusively catalyzed by protons. A second and previously unrecognized route involves β-scission events of IMO/MO or their tautomer C₆-alkenol products. These pathways involve the formation of an initial product on protons and its subsequent reaction within vicinal confined spaces that provide stabilization for chain propagation transition states, even in the absence of a proton. In this manner, β-scission rates become proportional to both the number of initiation sites (protons) and of propagating locations (voids), a dependence that cannot be described by conventional bifunctional catalytic formalisms. Such â??physical catalysisâ? by voids free of active sites was recently shown to account for the reactivity of pure-silica zeolites in reactions of molecules with radical character [5]; its plausible involvement here is consistent with coupled cluster (CCSD) calculations of the free energy of formation of potential chain initiators from MO/IMO and their C₆-alkenol analogs.

[1] Jones, A., Carr, R., Zones, S., and Iglesia, E., Journal of Catalysis, 312 (2014) 58.

[2] Jones, A. and Iglesia, E., ACS Catalysis, 5 (2015) 5741.

[3] Deshlahra, P. and Iglesia, E., ACS Catalysis (submitted).

[4] Kubelková, L. and Nováková, J., Journal of Molecular Catalysis, 75 (1992) 53.

[5] Artioli, N., Lobo, R., and Iglesia, E., Journal of Physical Chemistry C, 117 (2013) 20666.