Synthesis of zsm-5 crystals with improved morphology (ExxonMobil)

CATEGORY: ZEOLITES 
Synthesis of zsm-5 crystals with improved morphology (ExxonMobil)

Type	Patent
Inventor	Wenyih Frank Lai
Inventor	Nicholas S. Rollman
URL	Free Full Text Source:  http://www.google.com/patents/US20140162867
Assignee	Exxonmobil Research And Engineering Company
Patent Number	US20140162867 A1
Issue Date	Jun 12, 2014
Abstract	Methods are provided for synthesizing crystals having a ZSM-5 framework structure from synthesis mixtures suitable for synthesis of ZSM-12 framework structure crystals in the absence of seed crystals with a ZSM-5 framework structure, such as in the absence of any seed crystals. For synthesis mixtures with a sufficiently high XO2:Y2O3 ratio, seeding a synthesis mixture with ZSM-5 seed crystals can result in production of pure phase ZSM-5 crystals with a new morphology. The morphology can include rod-like primary crystallites with a length of about 0.5 μm or less that are organized/aggregated into larger secondary crystal structures. The aggregation of the primary crystallites into a secondary crystal structure can allow for an unexpectedly large mesoporous surface area for the ZSM-5 crystals. BACKGROUND OF THE INVENTION Zeolite crystal structures have found a wide range of applications within refinery processes and other processes for manipulating petroleum streams. Some zeolite applications are catalytic in nature, while other applications focus on the ability of zeolites to selectively adsorb molecules within a gas stream. One type of zeolite structure used for catalytic processing of petroleum streams is ZSM-5. ZSM-5 is an MFI framework zeolite. U.S. Pat. No. 3,702,866 provides an example of synthesis conditions for forming ZSM-5 crystals. The conditions include use of a tetrapropylammonium salt as a structure directing agent for forming ZSM-5 crystals. Ratios of SiO2:Al2O3 of 5-100 are generally disclosed for the synthesis mixture, with ratios of 10-60 being described as preferred. Another type of zeolite structure used for catalytic processing is ZSM-12, which is a zeolite having an MTW framework type. U.S. Pat. No. 3,832,449 provides an example of synthesis conditions for forming ZSM-12 crystals, which include use of a tetraethylammonium salt as a structure directing agent. Ratios of SiO2:Al2O3 of 40-200 are generally disclosed for the synthesis mixture, with ratios of 85-125 being described as preferred. U.S. Pat. No. 4,452,769 provides another example of synthesis conditions for forming ZSM-12 crystals, including use of a methyltriethylammonium salt. Ratios of SiO2:Al2O3 of 40 or higher are generally disclosed for the synthesis mixture (including mixtures with no added alumina), with ratios of 80 or higher being described as preferred. U.S. Pat. No. 5,672,331 describes method for synthesizing ZSM-5 and other MFI zeolites using various tetraalkylammonium salt structure directing agents. The structure directing agents include tetrapropylammonium salts and tetraethylammonium salts. Some preference is indicated for tetrapropylammonium salts for synthesis of ZSM-5 with a high SiO2:Al2O3 ratio. A ratio of OH−:SiO2 of 0.1 or less is described as preventing nucleation independent of seed crystals, which allows for production of crystals with a narrower size distribution. For synthesis mixtures with OH−:SiO2 of 0.1 or less, the number of seeds are used to control the crystal size. All of the synthesis examples include the use of a tetrapropylammonium salt as a structure directing agent. One group of examples included an SiO2:Al2O3 ratio of 340, which led to formation of spherical ZSM-5 crystals. A second group of mixtures had no added alumina but did include fluoride ions. For this second group of mixtures, the crystallites were described as having a “coffin” shape. At a seed crystal weight percentage of about 0.12 wt %, “coffin” shaped crystals with a length of 0.6 μm were reported. The reaction conditions for all examples appeared to include low ratios of other reactants relative to silica. The synthesis conditions were reported as producing uniform particle sizes, but no evidence of organization of the uniform particles was reported. U.S. Pat. No. 6,180,550 describes methods for synthesizing ZSM-5 zeolite crystals. The ZSM-5 crystals are formed from synthesis mixtures with SiO2:Al2O3 ratios of 10-25. In at least some examples, ZSM-5 crystals are characterized as having an average size of 0.05 μm or less. The crystals are also described as having high surface area, including mesoporous surface area greater than 45 m2/g.