Hot-spots during the calcination of MCM-41: A SAXS comparative analysis of a soft mesophase Lidia López Pérez, M.J. Ortiz-Iniesta, Hero Jan Heeres, Ignacio Melián-Cabrera n University of Groningen, Institute of Technology & Management, Chemical Reaction Engineering, Nijenborgh 4, 9747 AG Groningen, The Netherlands article info Article history: Received 16 July 2013 Accepted 22 November 2013 Available online 1 December 2013 Keywords: Surfactant-templated mesoporous materials MCM-41 Calcination Structural shrinkage Hot-spots Glow effect abstract The phenomenon of microscopic hot-spots, during the calcination of MCM-41, was investigated by quantifying the magnitude of the temperature increase during the calcination of a soft MCM-41 mesophase using a SAXS comparative study. This was performed by thermally treating a soft material that was detemplated via Fenton chemistry followed by equilibrating and drying in a low-surface- tension solvent (n-butanol or N,N-dimethylformamide); these samples have limited structural shrinkage. The resulting samples were thermally treated at increasing temperatures, and the structural shrinkage was compared with that of the directly calcined material. By comparing the structural shrinkage, it was found that the microscopic temperature increase would fall between 190 and 250 1C, as deduced from N, N-dimethyl-formamide and n-butanol. The order of magnitude of the temperature increase appears to be consistent with the well-known glow effect. It is, however, substantially higher than the experimentally determined macroscopic temperature increase. Several aspects are discussed to interpret this difference. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Exothermal reactions, such as combustions, partial oxidations, alkylations, hydrogenations and oxychlorinations, are problematic in heterogeneous catalysis [1–3]. The limited thermal conductivity of catalytic materials, together with the irregular distribution of catalyst pellets in the reactor, can lead to an uneven temperature distribution, also called a macroscopic hot-spot, with an eventual thermal runaway. In oxidation reactions, the actual temperature can be several hundreds of degrees higher than the nominal reaction temperature [4]. Various approaches have been proposed to overcome such macroscopic hot-spots, including the use of multitubular reactors, quenching, intermediate cooling, slurry reactors, fluidised beds and ceramic foams. From a materials perspective, supports with enhanced thermal conductivity have also been proposed to circumvent hot-spots [5–9]. At the microscopic level, the phenomenon of hot-spots has been less documented and almost exclusively restricted to the “glow” phenomenon of colloidal hydrous oxides [10], which produces a sudden temperature increase during their dehydroxylation or during the thermal activation of promoted sulphated zirconia [11,12], which is necessary to produce the final active phase. Hot-spots occur due to severe exothermic transformations. In the case of sulphated zirconia, this exothermal transformation produced a rapid overheating of the material to as high as 300 1C, above the nominal furnace tempera- ture, and this overheating resulted in a conversion increase in the investigated reaction: the isomerisation of n-butane. A similar phenomenon can be expected during the activation of surfactant-based mesoporous materials. The final step of activat- ing these materials consists of combusting the organic template by calcination [13], during which the porosity is developed. The amount of organic template varies, but it is considerably high; MCM-41, MCM-48 and SBA-15 mesophases typically contain on the order of 45–60 wt% of organics [14] and up to ca. 80 wt% for mesoporous aluminas [15,16]. During calcination, network con- traction is usually observed, which is associated with the forma- tion of Si–O–Si bonds. This formation of bonds provides stability to the network, but some textural features, such as pore volume and surface hydroxylation, are reduced. These effects are more proble- matic when calcination of the mesophase is conducted in large volumes in a deep-bed configuration without appropriate heat dissipation. In addition to the elevated amount of organics and the deep bed, the low thermal conductivities of silica (0.015– 1Wm 1 K 1 ) and alumina (1–8Wm 1 K 1 ) [5] do not consider- ably contribute to dissipating heat from the combustion reaction. Thus, the development of a local temperature increase (micro- scopic hot-spots) during the calcination of these mesophases could certainly occur. In this communication, we have indirectly investigated this phenomenon by quantifying the magnitude of the temperature increase during the calcination of a MCM-41 mesophase through a SAXS comparative study. 2. Experimental Material synthesis, detemplation and thermal treatments: Synth- esis of the soft silica mesophase: The synthesis procedure is based Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$- see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.091 n Corresponding author. Tel.: þ31 5036 342 67; fax: þ31 5036 344 79. E-mail address: i.v.melian.cabrera@rug.nl (I. Melián-Cabrera). Materials Letters 118 (2014) 51–54