J. Phys. Chem. C 2009, 113, 9845–9850
9845
Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70 Dan Xie,† Lynne B. McCusker,*,† Christian Baerlocher,† Lisa Gibson,#,‡ Allen W. Burton,‡ and Son-Jong Hwang§ Laboratory of Crystallography, ETH Zurich, CH-8093 Zurich, Switzerland, CheVron Energy Technology Co., Richmond, California 94802, The DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: April 24, 2009
A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an assynthesized sample (Pmn21, a ) 13.3167(1) Å, b ) 4.6604(1) Å, c ) 8.7000(1) Å) using a powder chargeflipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state 29Si, 11B and 1H NMR results are fully consistent with this ordered structure. Introduction In 2005, Dorset and Kennedy reported the crystal structure of the borosilicate zeolite MCM-70,1 whose synthesis had been patented 2 years earlier.2 However, the structure analysis was complicated by the fact that the peak shapes in the powder diffraction pattern exhibited anisotropic line broadening and were difficult to model. During the course of the Rietveld refinement, extra-framework species, whose positions were not easy to interpret, were found in the electron density maps generated for both as-synthesized and dehydrated samples. The geometries of the framework structures in both cases were distorted. There also appeared to be a large amount of K in the structure for the relatively small amount of B in the framework. Finally, although the 29Si MAS NMR spectrum was indicative of a nonrandom distribution of B in the framework structure, this was not apparent in the crystal structure analysis. With the aim of clarifying some of these details, an optimization of the synthesis of MCM-70 was undertaken. The improved sample was then examined using MAS NMR and synchrotron powder diffraction techniques. Experimental Section Synthesis. The MCM-70 sample was synthesized according to a procedure given in the original patent2 with only minor modification. In a 23-mL Teflon liner, 0.22 g of boric acid, 0.44 g of solid potassium hydroxide (88%), and 1.08 g of N,N′-diisopropyl-N,N′-dipropylbicyclo[2.2.2]oct-7-ene-2,3/ 5,6-dipyrrolidinium diiodide were dissolved in 7.0 g of * To whom correspondence should be addressed. Phone: +41-44-6323721. Fax: +41-44-632-1133. E-mail:
[email protected]. † ETH Zurich. ‡ Chevron. § California Institute of Technology. # Present address: Cambridge University Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW, U.K.
deionized water. Next, 3.50 g of Ludox-30 colloidal silica was mixed with the solution to create a uniform gel. The liner was then capped and placed within a Parr Steel autoclave reactor. The autoclave was fixed in a rotating spit within an oven heated to 160 °C for 11 days. The solid products were then recovered from the cooled reactor by vacuum filtration and washed with at least 250 mL of deionized water. The X-ray powder diffraction pattern was consistent with the data provided in the patent. Chemical Analysis. Chemical analysis was performed by Galbraith Laboratories, Inc. using inductively coupled plasma (ICP) methods. NMR. Solid state 1H, 11B, and 29Si MAS and CPMAS NMR spectra were recorded using a Bruker DSX-500 spectrometer (11.7 T) and a boron-free Bruker 4 mm CPMAS probe. The spectral frequencies were 500.23 MHz, 160.50 MHz, and 99.4 MHz for 1H, 11B, and 29Si nuclei, respectively. NMR shifts (in parts per million, ppm) for 1H and 29Si nuclei were externally referenced to tetramethylsilane (TMS), and those for 11B, to BF3 · O(CH2CH3)2. Powder samples were packed in a ZrO2 rotor under ambient conditions and spun at 2-15 kHz. A typical 11B MAS NMR spectrum was obtained after a 0.3 µs single pulse (
