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

Abstract

This work examines how engineered hierarchical (microporous− mesoporous) zeolites influence deactivation during hydrocarbon upgrading. Accumulations of carbonaceous deposits in microporous (MFI, MOR BEA), hierarchical (MFIh, MOR-h, BEA-h), and mesoporous (Al-MCM-41) aluminosilicates at differing crystal size regimes and pore topologies are evaluated as functions of overarching material descriptors like surface areas and pore volumes after liquid-phase, batch alkylation of 1,3,5-trimethylbenzene with benzyl alcohol. Thermogravimetric analysis (TGA) of spent catalysts shows that total foulant accumulation increases linearly with BET surface area (Pearson's r = 0.82), even absent considerations of physical nuances (mesopore connectivity and distribution) or chemical nuances (silanol and proton densities) among diverse hierarchical zeolites prepared by demetalation or recrystallization. Comparing calcination (O 2 ) to pyrolysis (inert) in TGA reveals two foulant classes that preferentially deposit in micropores or mesopores: low organics decomposing at 353−623 K and coke combusting at 623−1073 K. Mass spectrometry of foulants and Raman spectroscopy of catalysts show that low organics are strongly adsorbed reaction species from the alkylation network, while coking involves occlusion of undesired graphitic products of polyalkylation and dehydration. These investigations yield general insights into fouling behaviors of (hierarchical) zeolites having broad synthetic diversity.

Toward Intrinsic Catalytic Rates and Selectivities of Zeolites in the Presence of Limiting Diffusion and Deactivation

“…Our work considered a comprehensive suite of (hierarchical) zeolites varying in crystal size, mesoporosity, proton density, and parent architecture. 27,37 We contextualized the intrinsic reactivities and propensities for deactivation of hierarchical zeolites during reactions of (poly)substituted aromatics according to baseline diffusion barriers imposed by microporous zeolite crystal sizes and pore architectures.…”

“…For hierarchical zeolites, available D e,i are averages across extensive heterogeneities in (meso)pore distributions. Core−shell hierarchical zeolites offer the most extreme example 37 because stark separation of microporous and mesoporous phases leads to two distinct diffusion regimes. For example, we previously demonstrated that core−shell mesoporosity resulted from desilication of MFI zeolites having shells (within 20 nm of crystallite surfaces) with high Al density (Si/Al < 25), which prevented cleavage of Si−O−Si bonds by NaOH (Figure 2).…”

“…The notable difference in mesopore distribution, despite use of identical desilication parameters (0.2 M NaOH, 338 K, 30 min), manifested in different diffusion regimes for reaction moieties that preceded coke formation, consistent with different weight percent coke accumulation on each MFI-h (relative to dry catalyst masses) after 2 h of reaction. 37 Mesopore zoning has similarly been achieved with surfactant-mediated formation of core−shell structures with occluded mesopores as opposed to ones open to the external crystal surface. 46 Clearly, any estimated D e,i exclusively describes diffusion through one hierarchical zeolite derived from one postsynthetic treatment applied to one parent zeolite.…”

“…Commercially, low ϕ has been achieved predominantly using zeolite nanocrystals with L < 200 nm. 37 Small crystallites delay complete fouling by reducing the number of protons accessed by molecules traversing the micropore network before crystal egress, thus minimizing the number of undesired secondary turnovers that would otherwise form bulky coke molecules. However, as kinetic diameters of diffusing reactants approach or exceed the porelimiting diameters of micropores, the increased energy barriers for activated diffusion inflate ϕ and increase coking rates due to higher molecule residence times.…”

“…In contrast, hierarchical zeolites (i.e., nMFI-h) typically accumulated more coke within 120 min while giving significantly lower k D than their microporous parents (i.e., nMFI). 37 We hence deduced that hierarchical zeolites can accumulate more foulants before exhibiting measurable deactivation kinetics. The preferential location of coke deposition in hierarchical zeolites remains under active investigation and depends on the location of protons and the connectivity of each mesopore to micropores, other mesopores, and the zeolite crystal surface.…”