Neutron powder diffraction study of orthorhombic and monoclinic defective silicalite

The crystal structure of silicalite (SiO₂) with a substantial amount of structural hydroxyl groups [Si_(1−x)O_(2−4x)(OH)_4x, with 0.08 < x < 0.10] has been refined from neutron powder diffraction data measured using the HRPD instrument at the ISIS pulsed neutron source. Powder data were collected on the as-synthesized orthorhombic sample at 298 K, and on the deuterated monoclinic sample at 100 K. Preferential location of Si-atom vacancies was found on four out of 12 independent T sites in the orthorhombic silicalite [Si(6), Si(7), Si(10) and Si(11)], although the H atoms of the substituting hydroxyl groups could not be located because of the low statistical site occupancy on multiple sites. No significant population of D atoms or of Si vacancies was found in the tetrahedral sites of the monoclinic sample. The detected long-range order of adjacent Si atoms in defective orthorhombic [MFI] structures is compatible with a mechanism of Si vacancy clustering and with the model of hydroxyl nests assumed in the literature on the basis of IR spectroscopic evidence. Crystal data: orthorhombic, Pnma, Z = 8, a = 20.0511 (1), b = 19.8757 (1), c = 13.36823 (9) Å, V = 5327.62 (5) ų, D_x = 1.798 g cm⁻³, M_r = 721.01; monoclinic, P2₁/n, Z = 4, a = 19.8352 (2), b = 20.0903 (2), c = 13.3588 (1) Å, β = 90.892 (1)°, V = 5322.78 (6) ų, D_x = 1.799 g cm⁻³, M_r = 1442.02.

Computing details

(I)

Crystal data

H3.6O24Si11.04	c = 13.36823 (9) Å
Mr = 721.01	V = 5327.62 (5) Å^3
Orthorhombic, Pnma	Z = 8
Hall symbol: -P 2ac 2n	Dx = 1.798 Mg m^−3
a = 20.0511 (1) Å	Neutron radiation, λ = 1.22 Å
b = 19.8757 (1) Å

Data collection

h = →	l = →
k = →

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Si(1)	0.4245 (3)	0.0601 (3)	−0.3315 (4)	0.0256 (4)
Si(2)	0.3083 (3)	0.0281 (2)	−0.1860 (4)	0.0256 (4)
Si(3)	0.2779 (3)	0.0612 (3)	0.0341 (4)	0.0256 (4)
Si(4)	0.1182 (3)	0.0628 (3)	0.0283 (4)	0.0256 (4)
Si(5)	0.0749 (3)	0.0289 (3)	−0.1874 (5)	0.0256 (4)
Si(6)	0.1888 (3)	0.0607 (3)	−0.3272 (4)	0.0256 (4)
Si(7)	0.4249 (3)	−0.1723 (2)	−0.3265 (4)	0.0256 (4)
Si(8)	0.3087 (3)	−0.1321 (3)	−0.1842 (4)	0.0256 (4)
Si(9)	0.2721 (3)	−0.1754 (2)	0.0335 (4)	0.0256 (4)
Si(10)	0.1199 (3)	−0.1738 (2)	0.0320 (4)	0.0256 (4)
Si(11)	0.0723 (3)	−0.1292 (3)	−0.1809 (5)	0.0256 (4)
Si(12)	0.1858 (3)	−0.1732 (2)	−0.3233 (4)	0.0256 (4)
O(1)	0.3777 (3)	0.0495 (5)	−0.2359 (5)	0.0408 (4)
O(2)	0.3060 (5)	0.0631 (4)	−0.0782 (4)	0.0408 (4)
O(3)	0.1981 (3)	0.0601 (4)	0.0274 (4)	0.0408 (4)
O(4)	0.0989 (3)	0.0663 (4)	−0.0876 (4)	0.0408 (4)
O(5)	0.1161 (3)	0.0616 (5)	−0.2776 (5)	0.0408 (4)
O(6)	0.2487 (3)	0.0618 (5)	−0.2482 (5)	0.0408 (4)
O(7)	0.3745 (4)	−0.1628 (4)	−0.2343 (6)	0.0408 (4)
O(8)	0.3077 (5)	−0.1545 (3)	−0.0694 (4)	0.0408 (4)
O(9)	0.1963 (3)	−0.1505 (3)	0.0227 (5)	0.0408 (4)
O(10)	0.0876 (4)	−0.1665 (4)	−0.0772 (5)	0.0408 (4)
O(11)	0.1152 (3)	−0.1546 (4)	−0.2746 (6)	0.0408 (4)
O(12)	0.2437 (4)	−0.1531 (4)	−0.2462 (6)	0.0408 (4)
O(13)	0.3153 (4)	−0.0521 (2)	−0.1876 (4)	0.0408 (4)
O(14)	0.0840 (3)	−0.0501 (3)	−0.1678 (5)	0.0408 (4)
O(15)	0.4193 (4)	0.1321 (4)	−0.3842 (6)	0.0408 (4)
O(16)	0.4054 (4)	0.0006 (4)	−0.4069 (6)	0.0408 (4)
O(17)	0.4014 (4)	−0.1346 (4)	−0.4264 (6)	0.0408 (4)
O(18)	0.1906 (5)	0.1319 (3)	−0.3828 (5)	0.0408 (4)
O(19)	0.1899 (5)	0.0019 (3)	−0.4089 (5)	0.0408 (4)
O(20)	0.1959 (6)	−0.1323 (3)	−0.4251 (4)	0.0408 (4)
O(21)	−0.0021 (3)	0.0441 (4)	−0.2093 (5)	0.0408 (4)
O(22)	−0.0019 (3)	−0.1513 (5)	−0.2103 (6)	0.0408 (4)
O(23)	0.4287 (7)	−0.25	−0.3573 (9)	0.0408 (4)
O(24)	0.2025 (6)	−0.25	−0.3473 (8)	0.0408 (4)
O(25)	0.2881 (6)	−0.25	0.0722 (7)	0.0408 (4)
O(26)	0.1136 (7)	−0.25	0.0696 (9)	0.0408 (4)

Bond lengths (Å)

Si(1)—O(1/15/16)	1.600 (2)	Si(7)—O(7/17/23)	1.602 (2)
Si(2)—O(1/2/6)	1.601 (2)	Si(8)—O(8/12/13)	1.599 (2)
Si(3)—O(2/3)	1.603 (2)	Si(9)—O(8/9/25)	1.603 (2)
Si(4)—O(3/4)	1.600 (2)	Si(10)—O(9/10/26)	1.602 (2)
Si(5)—O(5/14/21)	1.601 (2)	Si(11)—O(10/11/14/22)	1.601 (2)
Si(6)—O(5/6/18/19)	1.600 (2)	Si(12)—O(11/12/20/24)	1.600 (2)

(II)

Crystal data

H3.6O24Si11.04	c = 13.3588 (1) Å
Mr = 1442.02	β = 90.892 (1)°
Monoclinic, P21/n	V = 5322.78 (6) Å^3
Hall symbol: -P 2yn	Z = 4
a = 19.8352 (2) Å	Dx = 1.799 Mg m^−3
b = 20.0903 (2) Å	Neutron radiation, λ = 1.22 Å

Data collection

h = →	l = →
k = →

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Si(1)	0.0622 (4)	0.4223 (4)	−0.3282 (6)	0.0114 (5)
Si(2)	0.0329 (4)	0.3195 (4)	−0.1641 (5)	0.0114 (5)
Si(3)	0.0605 (4)	0.2820 (4)	0.0600 (5)	0.0114 (5)
Si(4)	0.0628 (4)	0.1253 (4)	0.0358 (7)	0.0114 (5)
Si(5)	0.0307 (4)	0.0781 (4)	−0.1760 (6)	0.0114 (5)
Si(6)	0.0546 (4)	0.2006 (4)	−0.3115 (5)	0.0114 (5)
Si(7)	−0.1737 (4)	0.4250 (4)	−0.3190 (6)	0.0114 (5)
Si(8)	−0.1250 (4)	0.3098 (4)	−0.1726 (6)	0.0114 (5)
Si(9)	−0.1782 (4)	0.2730 (4)	0.0389 (5)	0.0114 (5)
Si(10)	−0.1777 (4)	0.1193 (5)	0.0334 (6)	0.0114 (5)
Si(11)	−0.1267 (4)	0.0715 (4)	−0.1714 (6)	0.0114 (5)
Si(12)	−0.1596 (3)	0.1897 (4)	−0.3167 (6)	0.0114 (5)
Si(13)	0.4458 (4)	0.4303 (4)	−0.3369 (6)	0.0114 (5)
Si(14)	0.4775 (4)	0.3144 (4)	−0.1889 (6)	0.0114 (5)
Si(15)	0.4395 (4)	0.2742 (4)	0.0222 (5)	0.0114 (5)
Si(16)	0.4358 (4)	0.1202 (4)	0.0417 (6)	0.0114 (5)
Si(17)	0.4739 (4)	0.0732 (5)	−0.1745 (6)	0.0114 (5)
Si(18)	0.4365 (3)	0.1871 (4)	−0.3115 (6)	0.0114 (5)
Si(19)	0.6710 (4)	0.4237 (4)	−0.3098 (6)	0.0114 (5)
Si(20)	0.6341 (4)	0.3099 (4)	−0.1709 (6)	0.0114 (5)
Si(21)	0.6646 (4)	0.2738 (4)	0.0454 (6)	0.0114 (5)
Si(22)	0.6685 (4)	0.1203 (4)	0.0347 (6)	0.0114 (5)
Si(23)	0.6333 (4)	0.0714 (4)	−0.1818 (6)	0.0114 (5)
Si(24)	0.6840 (3)	0.1956 (4)	−0.2963 (5)	0.0114 (5)
O(1)	0.0619 (6)	0.3829 (5)	−0.2232 (8)	0.0134 (4)
O(2)	0.0636 (5)	0.3100 (7)	−0.0528 (5)	0.0134 (4)
O(3)	0.0470 (5)	0.2036 (4)	0.0447 (8)	0.0134 (4)
O(4)	0.0718 (6)	0.1068 (6)	−0.0806 (7)	0.0134 (4)
O(5)	0.0444 (6)	0.1260 (5)	−0.2700 (8)	0.0134 (4)
O(6)	0.0519 (6)	0.2500 (5)	−0.2169 (8)	0.0134 (4)
O(7)	−0.1567 (6)	0.3732 (5)	−0.2300 (8)	0.0134 (4)
O(8)	−0.1703 (5)	0.2947 (6)	−0.0763 (6)	0.0134 (4)
O(9)	−0.1555 (5)	0.1963 (4)	0.0302 (8)	0.0134 (4)
O(10)	−0.1741 (6)	0.0909 (6)	−0.0791 (7)	0.0134 (4)
O(11)	−0.1511 (6)	0.1184 (5)	−0.2627 (7)	0.0134 (4)
O(12)	−0.1264 (6)	0.2472 (5)	−0.2479 (8)	0.0134 (4)
O(13)	−0.0470 (4)	0.3236 (6)	−0.1450 (8)	0.0134 (4)
O(14)	−0.0479 (4)	0.0859 (6)	−0.1505 (9)	0.0134 (4)
O(15)	0.1359 (4)	0.4230 (5)	−0.3771 (7)	0.0134 (4)
O(16)	−0.0015 (5)	0.3862 (6)	−0.3810 (7)	0.0134 (4)
O(17)	−0.1352 (5)	0.4090 (6)	−0.4215 (7)	0.0134 (4)
O(18)	0.1308 (4)	0.2066 (6)	−0.3499 (7)	0.0134 (4)
O(19)	0.0025 (5)	0.2116 (6)	−0.4035 (7)	0.0134 (4)
O(20)	−0.1256 (4)	0.1944 (7)	−0.4251 (7)	0.0134 (4)
O(21)	0.0486 (6)	0.0036 (4)	−0.2113 (9)	0.0134 (4)
O(22)	−0.1425 (6)	−0.0026 (4)	−0.2111 (8)	0.0134 (4)
O(23)	−0.2526 (4)	0.4293 (6)	−0.3478 (8)	0.0134 (4)
O(24)	−0.2391 (3)	0.2036 (6)	−0.3310 (8)	0.0134 (4)
O(25)	−0.2560 (4)	0.2763 (6)	0.0710 (8)	0.0134 (4)
O(26)	−0.2541 (4)	0.1160 (7)	0.0721 (8)	0.0134 (4)
O(27)	0.4592 (6)	0.3852 (5)	−0.2389 (7)	0.0134 (4)
O(28)	0.4584 (5)	0.3196 (5)	−0.0725 (6)	0.0134 (4)
O(29)	0.4292 (5)	0.1957 (4)	0.0025 (8)	0.0134 (4)
O(30)	0.4461 (6)	0.0820 (6)	−0.0628 (6)	0.0134 (4)
O(31)	0.4288 (5)	0.1200 (5)	−0.2468 (8)	0.0134 (4)
O(32)	0.4405 (6)	0.2513 (5)	−0.2394 (9)	0.0134 (4)
O(33)	0.6676 (6)	0.3775 (5)	−0.2115 (7)	0.0134 (4)
O(34)	0.6418 (5)	0.3167 (5)	−0.0512 (6)	0.0134 (4)
O(35)	0.6441 (5)	0.1967 (4)	0.0315 (7)	0.0134 (4)
O(36)	0.6588 (6)	0.0785 (6)	−0.0673 (6)	0.0134 (4)
O(37)	0.6726 (6)	0.1212 (5)	−0.2551 (8)	0.0134 (4)
O(38)	0.6750 (6)	0.2439 (5)	−0.2013 (8)	0.0134 (4)
O(39)	0.5562 (3)	0.2969 (6)	−0.2002 (8)	0.0134 (4)
O(40)	0.5536 (4)	0.0868 (6)	−0.1830 (9)	0.0134 (4)
O(41)	0.3761 (5)	0.4211 (6)	−0.3987 (8)	0.0134 (4)
O(42)	0.5025 (5)	0.4190 (6)	−0.4199 (7)	0.0134 (4)
O(43)	0.6317 (5)	0.3909 (6)	−0.4040 (8)	0.0134 (4)
O(44)	0.3669 (4)	0.1925 (7)	−0.3744 (7)	0.0134 (4)
O(45)	0.5021 (4)	0.1801 (5)	−0.3802 (6)	0.0134 (4)
O(46)	0.6346 (5)	0.2119 (6)	−0.3899 (7)	0.0134 (4)
O(47)	0.4625 (6)	−0.0041 (5)	−0.2021 (8)	0.0134 (4)
O(48)	0.6448 (6)	−0.0041 (4)	−0.2182 (9)	0.0134 (4)

Bond lengths (Å)

Si(1)—O(1/15/16)	1.611 (1)	Si(13)—O(27/41/42)	1.610 (1)
Si(2)—O(1/2/6/13)	1.610 (1)	Si(14)—O(27/28/32/39)	1.609 (1)
Si(3)—O(2/3)	1.609 (1)	Si(15)—O(29/29)	1.609 (1)
Si(4)—O(3/4)	1.610 (1)	Si(16)—O(29/30)	1.609 (1)
Si(5)—O(4/5/14/21)	1.609 (1)	Si(17)—O(30/31)	1.610 (1)
Si(6)—O(5/6/18/19)	1.610 (1)	Si(18)—O(31/32/44/45)	1.609 (1)
Si(7)—O(7/17/23)	1.610 (1)	Si(19)—O(33/43)	1.610 (1)
Si(8)—O(7/8/12/13)	1.609 (1)	Si(20)—O(33/34/38/39)	1.609 (1)
Si(9)—O(8/9/25)	1.610 (1)	Si(21)—O(34/35)	1.610 (1)
Si(10)—O(9/10/26)	1.610 (1)	Si(22)—O(35/36)	1.610 (1)
Si(11)—O(10/11/14)	1.610 (1)	Si(23)—O(36/37/40/48)	1.610 (1)
Si(12)—O(11/12/20/24)	1.610 (1)	Si(24)—O(27/38/46)	1.610 (1)