Joaquín Martínez-Triguero

A large-cavity zeolite with wide pore windows and potential as an oil refining catalyst

Crude oil is an important feedstock for the petrochemical industry and the dominant energy source driving the world economy, but known oil reserves will cover demand for no more than 50 years at the current rate of consumption. This situation calls for more efficient strategies for converting crude oil into fuel and petrochemical products. At present, more than 40% of oil conversion is achieved using catalysts based on faujasite; this zeolite requires extensive post-synthesis treatment to produce an ultrastable form, and has a large cavity accessible through four 0.74-nm-wide windows and thus limits the access of oil molecules to the catalytically active sites. The use of zeolites with better accessibility to their active sites should result in improved catalyst efficiency. To date, two zeolites with effective pore diameters exceeding that of faujasite have been reported, but their one-dimensional pore topology excludes use in oil refining. Similarly, zeolites with large pores and a three-dimensional pore topology have been reported, but in all these materials the pore openings are smaller than in faujasite. Here we report the synthesis of ITQ-21, a zeolite with a three-dimensional pore network containing 1.18-nm-wide cavities, each of which is accessible through six circular and 0.74-nm-wide windows. As expected for a zeolite with this structure, ITQ-21 exhibits high catalytic activity and selectivity for valuable products in preliminary oil refining tests.

Methanol to olefins: activity and stability of nanosized SAPO-34 molecular sieves and control of selectivity by silicon distribution

Nano-SAPO-34 molecular sieves synthesized in a microwave environment with 20 nm crystal size showed a longer lifetime than SAPO-34 prepared by the conventional hydrothermal method in the reaction of methanol to olefins. It has been found that silicon distribution strongly affects the lifetime and selectivity. Thus, silicon at the border of the silicon islands gives a higher lifetime and lower C2/C3 ratio. This change in activity and selectivity is better explained in terms of different silicon distribution than by preferential diffusion of ethene through the 8MR pores and agrees with transition-state selectivity. The effects of equilibrium of olefins and deactivation by coke were isolated, showing that after full formation of the hydrocarbon pool, selectivity is independent of deactivation by coke.

Synthesis of nano-SSZ-13 and its application in the reaction of methanol to olefins

Nanosized SSZ-13 has been obtained from a one-pot synthesis procedure with the addition of CTAB to the synthesis precursor solution. Nano-SSZ-13 zeolite showed high intracrystalline mesoporosity and compared to standard SSZ-13 presented a much longer lifetime and higher conversion capacity for the reaction of methanol to olefins. The improved properties were attributed to a more efficient utilization of micropores by easier diffusion of reactants and products and slower deactivation by coke. A higher C2/C3 ratio was found for nano-SSZ-13, pointing to a lower deactivation of the aromatics cycle of the hydrocarbon pool.

Acidity and accessibility studies of desilicated ZSM-5 zeolites in terms of their effectiveness as catalysts in acid-catalyzed cracking processes

The structural, textural and acidic characteristics of hierarchical ZSM-5 (Si/Al = 18–32), obtained with two desilication approaches, and the effect of these treatments on the reactivity in various cracking reactions of variable feedstock size and severity have been investigated. Emphasis is given to understanding the accessibility of acid sites; this was investigated by textural analysis, FTIR probe molecules (pyridine, trimethylacetonitrile and 2,4,6-trimethylpyridine) and reactions involving n-decane, 1,3,5-triisopropylbenzene (TIPB), and low and high-density polyethylene, LDPE and HDPE, respectively. Higher surface areas and a narrower pore size distribution were obtained for NaOHT both parameters are linearly dependent on the pivalonitrile and collidine accessibility factors, for LDPE and HDPE. The T5% for HDPE is more influenced by the accessibility factors than it is for the LDPE. This is interpreted to be the result of the branching degree of HDPE and LDPE; linear HDPE is more sensitive to the enhanced number of pore mouths of ZSM-5 channels on the mesopores. At high conversion, the influence on the T50% of the accessibility factors for HDPE and LDPE is weaker, suggesting that the cracking at this stage involves intermediate molecules of smaller size with fewer diffusional limitations. With respect to our own prior work, the chosen zeolite and the cracking of polyolefins gave more pronounced differences for the hierarchical ZSM-5.

Improving the catalytic performance of SAPO-18 for the methanol-to-olefins (MTO) reaction by controlling the Si distribution and crystal size

The physico-chemical properties of the small pore SAPO-18 zeotype have been controlled by properly selecting the organic molecules acting as organic structure directing agents (OSDAs). The two organic molecules selected to attempt the synthesis of the SAPO-18 materials were N,N-diisopropylethylamine (DIPEA) and N,N-dimethyl-3,5-dimethylpiperidinium (DMDMP). On the one hand, DIPEA allows small crystal sizes (0.1–0.3 μm) to be attained with limited silicon distributions when the silicon content in the synthesis gel is high (Si/TO2 ∼0.8). On the other hand, the use of DMDMP directs the formation of larger crystallites (0.9–1.0 μm) with excellent silicon distributions, even when the silicon content in the synthesis media is high (Si/TO2 ∼0.8). It is worth noting that this is the first description of the use of DMDMP as OSDA for the synthesis of the SAPO-18 material, revealing not only the excellent directing role of this OSDA in stabilizing the large cavities present in the SAPO-18 structure, but also its role in selectively placing the silicon atoms in isolated framework positions. The synthesized SAPO-18 materials have been characterized by different techniques, such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), N2 adsorption, solid state NMR, and ammonia temperature programmed desorption (NH3-TPD). Finally, their catalytic activity has been evaluated for the methanol-to-olefin (MTO) process at different reaction temperatures (350 and 400 °C), revealing that the SAPO-18 catalysts with optimized silicon distributions and crystal sizes show excellent catalytic properties for the MTO reaction. These optimized SAPO-18 materials present improved catalyst lifetimes compared to standard SAPO-34 and SSZ-39 catalysts, even when tested at low reaction temperatures (i.e. 350 °C).

Synthesis of Al-MTW with low Si/Al ratios by combining organic and inorganic structure directing agents

A rationalized combination of alkali cations and bulky dicationic organic structure directing agents (OSDAs) has allowed the synthesis of the Al-rich MTW zeolites with Si/Al ratios of ∼12 and large pore accessibility. 27Al MAS NMR spectroscopy indicates that most of the aluminum atoms are in tetrahedral coordination in framework positions, and in situ infrared pyridine adsorption/desorption spectroscopy reveals strong Bronsted acidity after cationic exchange for the Al-rich MTW. In addition, another MTW material with a Si/Al ratio of 30 has been synthesized under alkali-free conditions using a bulky dicationic molecule such as OSDA, the lowest Si/Al ratio being achieved for a MTW zeolite synthesized in the absence of alkali-cations in the synthesis media. The catalytic activity of these MTW materials has been tested for the n-decane cracking reaction, achieving higher catalytic activities and olefin yields than other related large pore zeolites.

Synthesis of reaction‐adapted zeolites as methanol-to-olefins catalysts with mimics of reaction intermediates as organic structure‐directing agents

AbstractCatalysis with enzymes and zeolites have in common the presence of well-defined single active sites and pockets/cavities where the reaction transition states can be stabilized by longer-range interactions. We show here that for a complex reaction, such as the conversion of methanol-to-olefins (MTO), it is possible to synthesize reaction-adapted zeolites by using mimics of the key molecular species involved in the MTO mechanism. Effort has focused on the intermediates of the paring mechanism because the paring is less favoured energetically than the side-chain route. All the organic structure-directing agents based on intermediate mimics crystallize cage-based small-pore zeolitic materials, all of them capable of performing the MTO reaction. Among the zeolites obtained, RTH favours the whole reaction steps following the paring route and gives the highest propylene/ethylene ratio compared to traditional CHA-related zeolites (3.07 and 0.86, respectively).Methanol-to-olefins (MTO) conversion over zeolites is a promising route for the production of light olefins. Now, Corma and co-workers show that using mimics of reaction intermediates as structure-directing agents allows the synthesis of highly selective zeolite MTO-catalysts.

Synthesis of cocrystallized USY/ZSM-5 zeolites from kaolin and its use as fluid catalytic cracking catalysts

A series of samples of cocrystallized USY/ZSM-5 zeolites were synthesized from kaolin and silica following a sequential two-step procedure with varying content of ZSM-5 (5–25 wt%). The presence of the ZSM-5 phases was confirmed by XRD and 29Si-NMR. The samples were stabilized by steaming and tested as FCC catalysts in the cracking of vacuum gasoil. The results obtained show that effectiveness of ZSM-5 as a propylene booster is enhanced when zeolites USY and ZSM-5 were synthesized in the same kaolin material, instead of using merely the physical mixtures of the two zeolites. This enhancement is attributed to the higher ability of ZSM-5 to crack larger olefins and suppress hydrogen-transfer to the gasoline fraction when the zeolites are grown together.

Making Nanosized CHA Zeolites with Controlled Al Distribution for Optimizing Methanol‐to‐Olefin Performance

From theoretical calculations and a rational synthesis methodology, it has been possible to prepare nanocrystalline (60-80 nm) chabazite with an optimized framework Al distribution that has a positive impact on its catalytic properties. This is exemplified for the methanol-to-olefin (MTO) process. The nanosized material with the predicted Al distribution maximizes the formation of the required MTO hydrocarbon pool intermediates, while better precluding excessive diffusion pathways that favor the rapid catalyst deactivation by coke formation.