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In the crystal structure of the title complex, C6H6O2·C6H6N2O3, the mol­ecules are linked by intermolecular hydrogen bonds between the O—H and N—O groups. The O atom of the N‐oxide group of 3‐methyl‐4‐nitro­pyridine 1‐oxide acts as an acceptor for hydrogen bonds from O—H groups of two symmetry‐related hydro­quinone mol­ecules [O⋯O 2.732 (2) and 2.810 (2) Å]. The angles between the rings of the 3‐methyl‐4‐nitro­pyridine 1‐oxide and two hydro­quinone mol­ecules are 6.1 (3) and 0.6 (3)°. The crystal structure exhibits overlap between the aromatic rings of the mol­ecules in the [11] direction.

In the title co-crystal, C 7 H 5 NO 4 ÁC 5 H 4 N 2 O 3 , the two components are linked by an intermolecular hydrogen bond between the OÐH and NÐO groups [OÁ Á ÁO 2.577 (3) A Ê ]. The interplanar angle between the planes of the rings of the molecules is 5. 3 (2) . The rings are stacked in the crystal with a mean interplanar distance of 3.279 (3) A Ê . CommentOver several years, a systematic study of molecular complexes of 4-nitropyridine N-oxide (NPNO) with diverse hydrogenbond donors (Moreno-Fuquen et al., 1996) has been carried out. Some molecular complexes associated with NPNO show potential for application in non-linear optics; for example, the NPNO and 3-nitrophenol molecular complex has a satisfactory second harmonic generation (SHG) response (MorenoFuquen et al., 1995). The synthesis and characterization of the title NPNO and 4-nitrobenzoic acid (PNBA) complex, (I), have a threefold purpose: (a) to add to the crystallographic information available on compounds based on NPNO, (b) to analyse the type of hydrogen bond in the title complex and (c) to explore its possible application in non-linear optics. Fig. 1 shows the NPNO±PNBA adduct, (I), with the adopted atom-numbering scheme. The molecular complex is held together by an intermolecular hydrogen bond between O1 of the N-oxide group of NPNO and OÐH of the carboxyl group of PNBA, with an O1Á Á ÁO5 distance of 2.577 (3) A Ê and an O1Á Á ÁHO5ÐO5 angle of 161 (3) . The O5ÐHO5 and O1Á Á ÁHO5 distances are 0.95 (4) and 1.67 (4) A Ê , respectively. From the OÁ Á ÁO bond length value, the strength of the hydrogen bond can be classi®ed as strong (Emsley, 1984).The dihedral angle formed by the planes which essentially contain the rings of the molecules is 5.3 (2). If one compares the values of bond lengths and angles in the title complex with the values in the free molecules , CCDC references (Allen et al., 1991) NBZOAC06 (Tonogaki et al., 1993) and NTPYRO1-1 (Wang et al., 1976), the C6ÐC7 and C6ÐO5 bond lengths change from 1.488 (4) and 1.324 (4) A Ê in (I) to 1.500 (9) and 1.289 (9) A Ê in the free molecule, while the C1Ð N1, N1ÐC5 and O1ÐN1 bond lengths change from 1.353 (4), 1.348 (4) and 1.314 (3) A Ê in (I) to 1.369, 1.370 and 1.291 A Ê in the free molecule. There are no signi®cant differences for the other parameters.The molecules of (I) are stacked in the crystal with the NPNO ring at a mean interplanar distance of 3.279 (3) A Ê from the PNBA ring at Àx, 1 À y, 1 À z. The presence of a centre of symmetry in the crystal inhibits an SHG response. ExperimentalCrystals of (I) used in the present work were obtained by slow evaporation from an equimolecular solution of NPNO and PNBA in absolute ethanol. The sublimation point is 414 (1) K. A ZORTEP (Zsolnai, 1995) plot of the asymmetric unit of (I) showing the atomic numbering scheme. Displacement ellipsoids are plotted at the 50% probability level and H atoms are shown as spheres of an arbitrary radius. Crystal data

In this work a geopolymeric material was synthesized by alkaline activation of calcined coal gangue at 700 °C using a solution 1:9 of NaOH:Na2SiO3, as a sorbent to remove rhodamine B from aqueous solutions. X-ray diffraction, fourier-transform infrared spectroscopy and scanning electron microscopy were used to characterize the material structurally and morphologically; removal efficiency and rhodamine B sorption capacity were evaluated by using ultraviolet-visible spectrophotometry. Sorption tests were performed to evaluate the effect of the dose of geopolymeric material and the contact time of this material with rhodamine B solution in a range from 0.5 to 30 hours, interval in which the best sorption was observed. Removal efficiencies and sorption capacity to 80.4% and 1.035 mg/g respectively were found, these values show the usefulness of the material for the removal of rhodamine B in aqueous solution, making it an interesting alternative for water decontamination and object of study for future works. Removal efficiency increased by increasing geopolymer dosage and contact time.

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