No abstract

Ensemble-averaged deprotonation energies (DPE) derived from periodic density functional theory models are insensitive to the location of isolated Al atoms and associated protons and similar among microporous aluminosilicates (i.e., zeolites) with different crystalline frameworks (MFI, BEA, FER, MOR, CHA, FAU). These DPE values are 1201 ± 11 kJ mol–1 after correcting for systematic artifacts of periodic DFT methods, which vary with framework density, and averaging over the four distinct proton locations at each Al atom. These energies rigorously reflect the strength of the acid sites in these important catalytic solids. Thus, the stability of the conjugate anions and the acid strength of these materials merely reflect the presence of Al atoms within the silicate framework, and not their specific siting or local confining environment. DPE values did not show any systematic trends with the vibrational frequency or length of O–H bonds, with Si–O–Al bond angles, or with NH3 adsorption enthalpies, properties that are frequently but inaccurately used as experimental indicators of acid strength. Such properties may reflect or bring forth confinement effects that do not influence acid strength, but which can stabilize the relevant ion-pair transition states and adsorbed intermediates through dispersion forces. These findings confirm that the different shape and size of the confining voids near Al atoms and their associated protons, instead of any differences in their acid strength, give rise to the remarkable diversity of acid forms of zeolites in the practice of catalysis.

Mechanisms of tolerance initiated in the thymus are indispensable for establishing immune homeostasis, but they may not be sufficient to prevent tissue-specific autoimmune diseases. In the periphery dendritic cells (DCs) play a critical tolerogenic role, extending the maintenance of immune homeostasis and blocking autoimmune responses. Here we review these essential roles of DCs in orchestrating mechanisms of peripheral T cell tolerance that were determined by methods of targeted delivery of defined antigens to DCs in vivo in combination with various genetic modifications of DCs. Further, we discuss how the functions of DCs empowered by specific delivery of T cell antigens could also be harnessed for tolerance induction in clinical settings.

Mechanistic interpretations of rates and in situ IR spectra combined with density functionals that account for van der Waals interactions of intermediates and transition states within confining voids show that associative routes mediate the formation of dimethyl ether from methanol on zeolitic acids at the temperatures and pressures of practical dehydration catalysis. Methoxy-mediated dissociative routes become prevalent at higher temperatures and lower pressures, because they involve smaller transition states with higher enthalpy, but also higher entropy, than those in associative routes. These enthalpy-entropy trade-offs merely reflect the intervening role of temperature in activation free energies and the prevalence of more complex transition states at low temperatures and high pressures. This work provides a foundation for further inquiry into the contributions of H-bonded methanol and methoxy species in homologation and hydrocarbon synthesis reactions from methanol.

The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the strength) of the protons that act as Brønsted acids. First-order rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton locations; first-order rate constants also depended exponentially on these enthalpies. In contrast, zero-order rate constants, which reflect the stability of transition states relative to protonated CH3OH dimers similar in size, depended weakly on dispersive stabilization, whether measured from experiment or simulations, because dispersive forces influence species similar in size to the same extent. These results, taken together, demonstrate the preeminent effects of confinement on zeolite reactivity and the manner by which the local voids around protons held within diverse intracrystalline environments give rise to the unique behaviors that have made zeolites ubiquitous in the practice of catalysis. Enthalpic stabilization of relevant transition states prevail over entropic losses caused by confinement at low temperatures in a manner reminiscent of how catalytic pockets and solvents do so in catalysis by molecules or enzymes.