Alkylation of poly-substituted aromatics to probe effects of mesopores in hierarchical zeolites with differing frameworks and crystal sizes

Abstract

Kinetic analysis of alkylation of 1,3,5-trimethylbenzene with benzyl alcohol and accompanying secondary reactions provides insight into reaction–diffusion–deactivation of bulky polyaromatic moieties in hierarchical zeolites.

Underpinnings of Carbonaceous Deposition among Structurally Diverse Hierarchical Zeolites during Hydrocarbon Upgrading

“…Causation, however, is more nuanced between V micro and coke accumulation (Figure b), because nearly every hierarchical zeolite enables greater coke accumulation than its microporous analogue despite reduction (or negligible change) in microporosity, in agreement with past findings. , Coke accumulation instead increases more unilaterally with V micro among hierarchical zeolites without regard for parent framework discrepancies. When deactivation rates are seemingly reduced to insignificance by sufficient mesopore volumes (as was previously found for all hierarchical samples except μMOR-h) such that diffusional artifacts no longer limit maximum conversion within the studied reaction time (2 h), micropore fouling continues to the maximum extent feasible by maximum S micro or V micro . A nuance, however, is apparent upon closer examination of coke accumulation as a function of V micro for nMFI-h1 and zMFI-h1, which are prepared from identical desilication treatments of nMFI and zMFI parents with zoned and uniform Al distributions, respectively.…”

“…Morphologies of hierarchical zeolites vary according to mesopore incorporation treatments, as well as structures and elemental compositions of parent frameworks. In all cases, previous powder X-ray diffraction verified that hierarchical zeolites retain the framework architectures and high crystallinity of their microporous parents after the post-synthetic treatments described in Section . Ambient-pressure desilication of nMFI (Figure a) following Section yields nMFI-h1 (Figure b) with a core–shell structure apparent in TEM.…”

“…In a typical batch kinetics experiment adapted from previous methods, the catalyst (0.1 g) was predried by heating statically for 2 h in an uncapped, two-neck, round-bottom flask submerged within an oil bath at 433 K. The bath was then cooled to 363 K, after which the reactor was fitted with a magnetic stirrer, connected through one neck to a reflux condenser, and capped at the second neck with a rubber septum. TMB (9.6 mL, 99%, Acros Organics) was then charged via injection through the rubber septum, after which the reactor was held isothermally for 90 min.…”

“…Micropores of molecular dimension also provide van der Waals (vdW) stabilization of transition states to overcome reaction free energy barriers otherwise prohibitively high in homogeneously catalyzed or uncatalyzed reaction systems . However, if kinetic diameters of reaction moieties approach zeolite PLDs beyond the maximum limits of effective vdW contacts achievable via feasible local distortions of the surrounding void frameworks, reactions are sterically hindered and enter diffusion-controlled regimes manifested by high Thiele moduli (ϕ ≫ 1). − Enhanced mass transport of bulky reaction moieties has been achieved with nanoparticle-sized zeolites having crystal diameters ( d crystal ) below 100 nm, two-dimensional/layered zeolites, , finned zeolites, pillared zeolites, and hierarchical zeolites containing auxiliary mesopores (PLDs = 2–50 nm). , The first four of these structures are almost exclusively prepared via bottom-up synthesis, but hierarchical zeolites may also be prepared through facile, post-synthetic dealumination and/or desilication of parent zeolites with aqueous acid (HNO 3 ) or base (NaOH). − Morphologies (mesopore sizes, locations, distributions, and connectivities), proton distributions, and defect densities of hierarchical zeolites vary as widely as the multitudes of post-synthetic procedures reported to synthesize them. Additional diversity in hierarchical zeolite properties can arise from identical leaching treatments applied to parent zeolites differing in Si/Al, defect densities, and crystallinity/polymorphism, hence further complicating investigations of their catalytic structure–function relations.…”

“…At low [BA]/[DBE], DBE was found to undergo a secondary reaction with TMB to form additional TM2B and BA (Figure 1). 2 Mesopores easily mitigated diffusion constraints of narrow micropores unless very high crystal diameters (>10 μm) severely inflated Thiele moduli beyond feasible compensatory increases in effective diffusivity. Extents of deactivation, via fouling and/or site blockage, 16 extracted from fitted deactivation rate constants, were less significant in hierarchical zeolites than in their parents.…”