Structural characterization and catalytic properties of bis(1,1,3,3-tetramethylguanidinium) dichromate Johannes Due-Hansen a,⇑ , Kenny Ståhl a , Soghomon Boghosian b , Anders Riisager a , Rasmus Fehrmann a,⇑ a Department of Chemistry, Centre for Catalysis and Sustainable Chemistry, Technical University of Denmark, 2800 Lyngby, Denmark b Department of Chemical Engineering, University of Patras and FORTH/ICE-HT, GR-26500 Patras, Greece article info Article history: Received 29 July 2010 Accepted 10 December 2010 Available online 22 December 2010 Keywords: Structure XRD Catalysis Tetramethylguanidinium Dichromate abstract The structure of bis(1,1,3,3-tetramethylguanidinium) dichromate was determined from powder X-ray diffraction data. The compound crystallizes in the monoclinic system (space group P2 1 /n) with a = 10.79714 (15) Å, b = 11.75844 (16) Å, c = 8.15097 (11) Å, b = 109.5248 (6)°. The structure consists of dichromate anions (Cr 2 O 7 2 ) stabilized by tetramethylguanidinium cations ([H 2 NC(N(CH 3 ) 2 ) 2 ] + or [TMGH] + ). Phase transitions of [TMGH] 2 Cr 2 O 7 were determined by differential scanning calorimetry, thermal gravimetric analysis and in situ Raman spectroscopy, where the decomposition of the matrix into CrO x was found at 171–172 °C. Further heat treatment to above 400 °C resulted in formation of the ther- modynamically stable Cr 2 O 3 , most likely with the [TMGH] + cation as reductant. The catalytic activity of [TMGH] 2 Cr 2 O 7 supported on TiO 2 anatase in the selective catalytic reduction (SCR) of nitrogen oxide was also investigated, however only moderate activity was observed in the temperature range 100–400 °C compared to the activity of e.g., vanadia supported on titania. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. 1. Introduction Selective catalytic reduction (SCR) plays an important role in the removal of nitrogen oxides from flue gases generated by fuel combustion in stationary sources. The traditional, and widely com- mercialized, catalyst for the SCR reaction is composed of V 2 O 5 and WO 3 or MoO 3 supported on TiO 2 which is operated in the temper- ature window of 300–400 °C [1]. Due to the relatively high operat- ing temperatures the catalyst unit is often placed upstream of cleaning units (e.g., desulfurizer, electrostatic particle remover) to avoid costly reheating of the flue gas. This environment, how- ever, promotes deactivation of the catalyst because of high concen- trations of dust. Placing the SCR unit downstream of the desulfurization and dust removal units would thus solve the deac- tivation, but also lower the temperature of the flue gas and conse- quently the activity of the catalyst, since the low-temperature activity of the traditional SCR catalysts is negligible. Accordingly, efforts have been made to develop catalysts operating in the range 80–280 °C [2–8], which would render placement of the SCR unit possible in the tail end after the desulfurizer and particulate con- trol installations. Recent studies have shown that chromium and/or manganese oxides, such as MnO x [9], MnO x /TiO 2 [10], Fe–Mn–TiO x [8], MnO x –CeO 2 [11], exhibit good low-temperature SCR potential, but their N 2 -selectivity and sulfur resistance need to be improved. Sreekanth et al. [12] investigated the NO conversion over a range of mono and bimetallic transitions metals, namely Cr, Mn, Cu, Mn–Cu and Mn–Cr, and found that the monometallic catalysts, Cr, Mn and Cu yielded better SCR performance than the mixed oxides, espe- cially at low-temperature (100 °C). Schneider et al. [13] reported higher selectivity to N 2 (>90%) for a range of titania-supported chromia catalysts (i.e., CrO 2 Cr 2 O 3 , CrOOH), but the catalytic perfor- mance, such as N 2 O selectivity, was very sensitive to the phase of the chromium oxide. In this paper, we have examined the low-temperature SCR activity of a newly characterized compound of chromium, tetram- ethylguanidinium dichromate, which was first reported by Kim et al. [14] for oxidation of alcohols. The physicochemical properties of 1,1,3,3-tetramethylguanidine are well documented, e.g., by Anderson and Hammer [15], but the bis(1,1,3,3-tetramethylguan- idinium) dichromate salt has not yet been structurally character- ized. The crystal structure of the unsubstituted guanidinium dichromate has been reported by Wajsman et al. [16]. However, addition of four methyl groups to the arrangement alters the resulting structure, which is reported in the following study. 2. Materials and methods 2.1. Synthesis [TMGH] 2 Cr 2 O 7. was synthesized by mixing 1,1,3,3-tetramethyl- guanidine, TMG (Aldrich, 99%) with absolute ethanol in the molar 0277-5387/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.12.013 ⇑ Corresponding authors. Tel.: +45 4525 2363 (J. Due-Hansen), tel.: +45 4525 2389 (R. Fehrmann). E-mail addresses: jdh@kemi.dtu.dk (J. Due-Hansen), rf@kemi.dtu.dk (R. Fehr- mann). Polyhedron 30 (2011) 785–789 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly