I think this is the best answer here https://tex.stackexchange.com/a/25926/120007, using mdframed
.
But previously, improving upon other answers I've defined a new command which works in single column mode and two column mode:
\documentclass[twocolumn]{article}
\usepackage{lipsum}
\usepackage[]{mdframed}
% draw a frame around given text
\newcommand{\framedtext}[1]{%
\par%
\noindent\fbox{%
\parbox{\dimexpr\linewidth-2\fboxsep-2\fboxrule}{#1}%
}%
}
\begin{document}
\lipsum[1]
\section{This uses mdframed}
Lorem ipsum dolor sit amet, consectetuer adipiscing
elit. Ut purus elit, vestibulum ut, placerat ac, adi-
piscing vitae, felis.
\begin{mdframed}
A long sentence with multiple words and a sample equation $e^{i\pi}+1=0$.
\end{mdframed}
Lorem ipsum dolor sit amet, consectetuer adipiscing
elit. Ut purus elit, vestibulum ut, placerat ac, adi-
piscing vitae, felis.
\section{This uses the new command}
Lorem ipsum dolor sit amet, consectetuer adipiscing
elit. Ut purus elit, vestibulum ut, placerat ac, adi-
piscing vitae, felis.
\framedtext{A long sentence with multiple words and a sample equation $e^{i\pi}+1=0$.}
\lipsum[4-7]
\end{document}

See also:
mdframed
package is suggested.