Preparação de carbonos porosos por moldagem sequencial

Abstract

Recebido em 1/5/08; aceito em 30/3/09; publicado na web em 26/8/09 PREPARATION OF POROUS CARBON BY SEQUENTIAL TEMPLATING. Porous carbons have received great attention because of their potential use as adsorbents, sensors, catalytic supports and others. Among the available methods for preparing these solids, the sequential templating emerges as an efficient way to tail carbon materials, with large specific surface areas, high porosity, controlled narrow pore size distribution and different functional groups. The template process is based on the infiltration of carbon precursors into the pores of inorganic frameworks. This work reviews the most important variables involved in the sequential template process to prepare micro, meso and macroporous carbon with tailored properties as well as their applications.Keywords: porous carbon; template process; applications. INTRODUÇÃODurante muitos anos os materiais tradicionais de carbono, tais como os carvões ativos, foram largamente utilizados em processos de adsorção e como suportes catalíticos.1-4 Entretanto, somente após a descoberta dos fulerenos em 1985 por Kroto e Smalley, 5 e dos nanotubos em 1991 por Iijima, 6 a comunidade científica voltou a estudar extensivamente esses materiais, enfocando os carbonos porosos nanoestruturados, os nanotubos e as nanofibras de carbono. Esses sólidos apresentam extraordinária resistência mecânica e térmica, além de elevada condutividade elétrica, tornando-os potencialmente atrativos para diversas aplicações. 7,8 Entre elas, destaca-se o seu uso como suporte e como catalisadores, 9-12 além de aplicações em célu-las a combustível, eletroquímica e em processos de adsorção.13-15 O controle da estrutura desses materiais levou ao desenvolvimento de carbonos porosos nanoestruturados com tamanho de poros variando desde microporos e mesoporos até macroporos, 16 através de diversos métodos, entre os quais se destaca o método da moldagem sequencial, normalmente utilizado para preparar materiais de carbono ordenados, permitindo o controle preciso da porosidade. 17Muitos tipos de carbonos porosos são conhecidos por exibir estrutura periódica de poros, que resulta do empilhamento uniforme das estruturas grafíticas e do arranjo periódico dos átomos dentro de suas estruturas.1 Os fulerenos e os nanotubos de paredes simples (SWNTs, Single-Wall Nanotubes) apresentam estruturas periódicas com espaços acessíveis a moléculas de adsorvatos. Entretanto, essa estrutura é mantida por interações fracas de van der Waals e, então, esses materiais não podem ser considerados como sistemas com porosidade ordenada permanente. 18,19 A utilização de materiais mesoporosos ordenados como moldes abriu novas oportunidades para a síntese de materiais de carbono com estruturas periódicas ordenadas, representadas por CMK-n (n=1-9), Carbon Mesoestructured from Kaist. Em particular, o uso da MCM-48 permitiu a obtenção do primeiro carbono poroso possuindo uma estrutura periódica de célula unitária, com tamanho de alguns nanômetros, chamado CMK-1. Esse material, também denomin...

Selecting Catalysts for Pyrolysis of Lignocellulosic Biomass

“…Sourcing the organic feedstock from sustainable and renewable origins produces biochar, eco-friendly charcoal used as a starting material for activated carbon. These solids are typically microporous, but some mesoporous carbons can also be found [57]. The morphology and physical-chemical properties of obtained carbonaceous materials significantly vary.…”

Green synthesis of templated porous carbons

“…[4] However, one way to enable precise adjustment of the pores is to use an existing chemical structure that can act as a template. [1] The use of templates to produce porous carbon materials has attracted considerable research attention in different areas. Materials such as zeolites, metal-organic frameworks (MOFs), carbon nanotubes (CNTs), metal hydrides, and intrinsically microporous polymers have been extensively studied for the synthesis of materials suitable for gas storage and adsorption processes.…”

“…Materials such as zeolites, metal-organic frameworks (MOFs), carbon nanotubes (CNTs), metal hydrides, and intrinsically microporous polymers have been extensively studied for the synthesis of materials suitable for gas storage and adsorption processes. [1,[8][9][10] In these synthesis methods, solid material is formed around the template by physical bonding between the template and the carbon precursor (hydrocarbons, sugars, and others). The main steps in the process are preparation of the template (which may be organic or inorganic), impregnation of the template with the precursor, carbonization of the organic material, and removal of the template.…”

“…The carbonization step can employ different heating rates and temperatures that are typically in the range of 973-1273 K. The procedure for removal of the template varies depending on the material used, with inorganic templates typically being removed using strong acids or bases while those of organic origin are generally removed by leaching or thermal decomposition. [1,9,10] When the template is impregnated using the liquid route, the pores of the material are filled with the carbon precursor present in an aqueous solution, forming a template precursor composite with the characteristics of a gel. The time during which this gel is allowed to rest (aging time) affects the textural properties (such as porosity and specific area) of the final product, leading to different types of materials.…”

“…The time during which this gel is allowed to rest (aging time) affects the textural properties (such as porosity and specific area) of the final product, leading to different types of materials. [1,11] There is a broad range of applications for this type of synthesis due to the ease of manipulating the pore characteristics of the material using different raw materials, carbonization conditions, and activation methods and agents. Consequently, porous materials can be produced that present different physical structures and chemical groups with varied acid-base characteristics depending on the conditions employed.…”