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The synthesis of molecular sieves with framework densities (FD) lower than those currently attainable is an ongoing challenge in molecular sieve science. Currently, the minimum FD is 12.5 for CoAPO-501 which translates to a void space that occupies approximately half the crystal volume. Brunner and Meier2 have shown that there is a correlation between the minimum FD and the smallest ring size within the structure [minimum ring (MINR)]. If the correlation of Brunner and Meier is correct, then structures with three-membered rings (3MR) must be synthesized in order to achieve porosities greater than 50%. We have initiated a project aimed at finding and exploiting a suitable gel chemistry for the synthesis of 3MR-containing materials. Beryllosilicates do promote 3MR formation as evidenced by the microporous mineral lovdarite3.4 and the dense minerals phenakite and euclase .5 A synthetic analogue of lovdarite has also been reported.6 However, many beryllium compounds are highly toxic and thus make it an undesirable element to work with. A synthetic aluminosilicate zeolite, ZSM-18, contains 3MR.7 Aluminosilicates, however, do not prefer the narrow T-0-T angles ca. 130" which are necessary to form 3MR. It is not likely that a general synthetic route to 3MR-containing frameworks will employ aluminosilicate chemistry. We have observed that zincosilicate analogues of euclase and phenakite, namely clinohedrite and willemite ,8 respectively, do exist. A synthetic non-microporous zincosilicate has also been reported.9 Thus, zinc appears to be a suitable substitute for beryllium in the formation of 3MRcontaining materials. We report here VPI-7$ the first microporous zincosilicate material to contain 3MR. VPI-7 is synthesized by heating a reaction mixture of composition 0.08 TEABr: 0.44 Na20: 0.28 ZnO: SiO2: 44 H 2 0 for 12 days at 200°C. The addition of TEABr (tetraethylammonium bromide) serves only to speed the rate of crystallization and is not incorporated into the product VPI-7. The Si02 : ZnO ratio of this sample is 3.3 : 1. VPI-7 crystals are typically in the form of fans composed of thin rectangular plates; the f_ans range in size from -10 to 50 pm. VPI-7 was exchanged with ammonium ions by refluxing in a 0.67 mol dm-3 ammonium hydroxide-0.33 mol dm-3 ammonium acetate solution for 4 h and the experimental X-ray powder diffraction pattern of NH4-VPI-7 is shown in Fig. l(a). The VPI-7 framework is tetragonal with maximum toplogical symmetry of Z4rn2. Ammonium exchanged VPI-7 has unit-cell dimensions of a = 7.179(1) and c = 40.62(1)A, and has a FD = 17.2. The c dimension of VPI-7 is the largest framework repeat in any known molecular sieve topology. The unit-cell dimensions of VPI-7 are similar to those of lovdarite (a = 39.72, b = 6.94, c = 7.15 A, orthorhombic basis).6 The Na cations present in as-synthesized VPI-7 are exchangeable, like in other molecular sieves, for such cations as Li+, K+ or NH4+. The VPI-7 framework exhibits reversible H20 adsorption. At PH20 = 20 Torr and T = 25"C, K-VPI-7 reversibly adsorbs H20 with an adsorption capacity of 0.19 g

VPI-7: the first zincosilicate molecular sieve containing three-membered T-atom rings