Lignin represents an untapped resource in lignocellulosic biomass, primarily due to its recalcitrance to depolymerization and its intrinsic heterogeneity. In Nature, microorganisms have evolved mechanisms to both depolymerize lignin using extracellular oxidative enzymes and to uptake the aromatic species generated during depolymerization for carbon and energy sources. The ability of microbes to conduct both of these processes simultaneously could enable a Consolidated Bioprocessing concept to be applied to lignin, similar to what is done today with polysaccharide conversion to ethanol via ethanologenic, cellulolytic microbes. To that end, here we examine the ability of 14 bacteria to secrete ligninolytic enzymes, depolymerize lignin, uptake aromatic and other compounds present in a biomass-derived, lignin-enriched stream, and, under nitrogen-limiting conditions, accumulate intracellular carbon storage compounds that can be used as fuel, chemical, or material precursors. In shake flask conditions using a substrate produced during alkaline pretreatment, we demonstrate that up to nearly 30% of the initial lignin can be depolymerized and catabolized by a subset of bacteria. In particular, Amycolatopsis sp., two Pseudomonas putida strains, Acinetobacter ADP1, and Rhodococcus jostii are able to depolymerize high molecular weight lignin species and catabolize a significant portion of the low molecular weight aromatics, thus representing good starting hosts for metabolic engineering. This study also provides a comprehensive set of experimental tools to simultaneously study lignin depolymerization and aromatic catabolism in bacteria, and provides a foundation towards the concept of Lignin Consolidated Bioprocessing, which may eventually be an important route for biological lignin valorization. Electronic Supplementary Information (ESI) available: Additional resultsdescribing the HPAE-PAD findings, growth curves of the bacteria over the 7-day experiments, the full set of ligninolytic enzyme profiles, and genomic analyses for ligninolytic enzymes and aromatic catabolic enzymes. See

We report a synthesis–structure–function relation describing how different routes to crystallize single tetrahedral-site (T-site) zeolites of fixed composition lead to different arrangements of framework Al atoms and, in turn, of extraframework proton active site ensembles that markedly influence turnover rates of a Brønsted acid-catalyzed reaction. Specifically, synthetic routes are reported that result in systematic changes in the arrangement of aluminum atoms (Al–O­(−Si-O) x –Al) in isolated (x > 2) and paired (x = 1, 2) configurations within chabazite (CHA) zeolite frameworks of effectively fixed composition (Si/Al = 14–17). Precursor solutions containing different structure-directing agents and aluminum sources crystallize CHA zeolites with one organic N,N,N-trimethyl-1-adamantylammonium cation occluded per CHA cage, and with amounts of occluded Na+ cations that increase linearly with paired framework Al content (0–44%). Ammonia and divalent cobalt ion titrations are used to quantify total and paired Brønsted acid sites, respectively, and normalize rates of methanol dehydration to dimethyl ether. First-order and zero-order methanol dehydration rate constants (per H+, 415 K) systematically increase with the fraction of paired protons in CHA zeolites and are ∼10× higher at paired protons. Such behavior reflects faster dissociative (surface methoxy-mediated) pathways that prevail at paired protons over slower associative (methanol dimer-mediated) pathways at isolated protons, consistent with in situ infrared spectra. These findings demonstrate that zeolites of fixed elemental composition, even when crystalline frameworks contain one unique T-site, can exhibit catalytic diversity when prepared via different synthetic routes that influence their atomic arrangements.

Research efforts in zeolite catalysis have become increasingly cognizant of the diversity in structure and function resulting from the distribution of framework aluminum atoms, through emerging reports of catalytic phenomena that fall outside those recognizable as the shape-selective ones emblematic of its earlier history. Molecular-level descriptions of how active-site distributions affect catalysis are an aspirational goal articulated frequently in experimental and theoretical research, yet they are limited by imprecise knowledge of the structure and behavior of the zeolite materials under interrogation. In experimental research, higher precision can result from more reliable control of structure during synthesis and from more robust and quantitative structural and kinetic characterization probes. In theoretical research, construction of models with specific aluminum locations and distributions seldom capture the heterogeneity inherent to the materials studied by experiment. In this Perspective, we discuss research findings that appropriately frame the challenges in developing more predictive synthesis−structure−function relations for zeolites, highlighting studies on ZSM-5 zeolites that are among the most structurally complex molecular sieve frameworks and the most widely studied because of their versatility in commercial applications. We discuss research directions to address these challenges and forge stronger connections between zeolite structure, composition, and active sites to catalytic function. Such connections promise to aid in bridging the findings of theoretical and experimental catalysis research, and transforming zeolite active site design from an empirical endeavor into a more predictable science founded on validated models.

We combine experiment and theory to investigate the cooperation or competition between organic and inorganic structure-directing agents (SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites and to rationalize the effects of SDA siting on biasing the framework Al arrangement (Al–O­(−Si–O) x –Al, x = 1–3) among CHA zeolites of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized using mixtures of TMAda+ and Na+ contain one TMAda+ occluded per cage and Na+ co-occluded in an amount linearly proportional to the number of 6-MR paired Al sites, quantified by Co2+ titration. In contrast, CHA zeolites crystallized using mixtures of TMAda+ and K+ provide evidence that three K+ cations, on average, displace one TMAda+ from occupying a cage and contain predominantly 6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis media containing more than 10-fold higher inorganic-to-organic ratios with K+ than with Na+ before competing crystalline phases form, providing a route to decrease the amount of organic SDA needed to crystallize high-silica CHA. Density functional theory calculations show that differences in the ionic radii of Na+ and K+ determine their preferences for siting in different CHA rings, which influences their energy to co-occlude with TMAda+ and stabilize different Al configurations. Monte Carlo models confirm that energy differences resulting from Na+ or K+ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements, respectively. These results highlight opportunities to exploit using mixtures of organic and inorganic SDAs during zeolite crystallization in order to more efficiently use organic SDAs and influence framework Al arrangements.

The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, developing robust synthesis−structure relations for MFI zeolites is difficult because of the complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space of Al−Al site pairs to exhaustively model by density functional theory (DFT) and quantify by experiment. Here, we develop an experimental protocol to reproducibly quantify Co 2+ -titratable Al−Al site pairs in MFI with saturation uptakes validated by corroborating spectroscopic and cation site balance data. Using tetrapropylammonium (TPA + ) as the sole SDA, MFI zeolites were crystallized with varying Al contents (Si/Al = 37−185; 0.52−2.52 Al per unit cell) within a composition range consistent with charge density mismatch theory and the occlusion of one TPA + per channel intersection with fractions of paired Al (0.0−0.34) that increased with bulk Al content. DFT calculations performed using a 96 T-site MFI unit cell containing either an isolated Al site (all 96 configurations) or various Al−Al site pairs (1773 out of 13 680 total configurations), charge balanced by one or two TPA + , respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA + and the anionic lattice charge on Al siting energies. Together with DFT calculations of Co 2+ exchange energies at Al−Al site pairs, theory predicts that two TPA + cations confined within adjacent channel intersections can form many Al−Al site pair ensembles that are Co 2+ -titratable, rationalizing the considerable presence of paired Al sites in MFI samples crystallized using only TPA + . The use of TPA + and Na + as co-SDAs in the synthesis gel, while varying the Na + /TPA + ratio (0−5) at a constant SDA/Al ratio ((TPA + + Na + )/Al = 30), crystallized MFI with a similar bulk Al content (Si/Al ≈ 50) but varying fractions of Al in pairs (0.12−0.44). Separate crystallization experiments performed using charge-neutral organic SDAs, either pentaerythritol or a mixture of 1,4-diazabicyclo[2.2.2]octane and methylamine, together with Na + to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/Al ≈ 50) but with lower fractions of Al in pairs (<0.14). Among MFI samples crystallized with an organic SDA and Na + as a co-SDA, the number of paired Al sites formed generally increased with the co-occluded Na + content on the zeolite, a synthesis−structure relation that resembles our prior observations on CHA zeolites. The combined theoretical and experimental approach used here provides a microscopic model to define and quantify Al−Al site pairs in MFI, which can be adapted to do so for other framework topologies. These findings highlight how such Al siting models can be exercised to quantitatively characterize zeolite materials to develop synthetic strategies that can predictably vary their framework Al arrang...

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A robust sample workup protocol is described that allows quantification of acidic components in complex biomass-derived process streams. This protocol is shown to have application in the field of lignin conversion.

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites.T urnover rates (per H +)f or methanol and ethanol dehydration increase with the fraction of H + sites sharing sixmembered rings of chabazite (CHA) zeolites.D ensity functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density.C ertain Al distributions yield rigid arrangements of anionic charge that stabilizec ationic intermediates and transition states via H-bonding to decrease barriers.T his is ak ey feature of acid catalysis in zeolites olvents,w hichl ack the isotropyo fl iquid solvents.T he sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.